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Jagdamba Singh(Pericyclic & Photochemistry)

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Contents
Preface to the Third Revised Edition
(v)
Preface to the First Edition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. (vi)
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. (viii)
CHAPTER
1.1
1.2
1.3
1.4
1.5
1.6
1. 7
1.8
1.9
CHAPTER
2.1
2.2
2.3
2.4
1 Pericyclic Reactions
!,.i.1
1-16
Introduction................ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Construction ofn Molecular Orbitals of Ethylene and 1, 3-Butadiene
Symmetry in n Molecular Orbital . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Filling of Electrons in n Molecular Orbitals in Conjugated Polyenes
Construction of Molecular Orbitals of Conjugated Ions and Radicals. . . . . ..
Frontier Molecular Orbitals
Excited States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Symmetries in Carbon-Carbon Sigma Bond
Theory of Pericyclic Reactions
Glossary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Electrocyclic Reactions
1
3
6
8
9
13
13
14
14
15
15
16
i
II
17-57
Introduction
Conrotatory and Disrotatory Motions in Ring-opening Reactions
Conrotatory and Disrotatory Motion in Ring-closing Reactions
2.3.1 Open Chain Conjugated System having 4nn Conjugated
Electrons
2.3.2 Open Chain Conjugated System having (4n + 2)n
Conjugated Electrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Frontier Molecular Orbital (FMO) Method
2.4.1 Cyclisation of 4nn Systems
2.4.2 Electrocyclic Ring-Opening in which Polyene has 4nn Electrons
2.4.3 Cyclisation of(4n + 2)n Systems
17
18
20
21
22
23
24
29
31
(Ix)
-- I
)1
r
Electrocyclic Ring-opening in which Polyene has (4n + 2)n
Electrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.5 Selection Rules and Microscopic Reversibility . . . . . . . . . . . . . . . . . .
Correlation Diagram
2.5.1 Correlation Diagram of the Electrocyclic Reaction in which
Polyene has 4mt Electrons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.2 Correlation Diagram of Electrocyclic Reaction in which Polyene
has (4n + 2)n Electrons
The Woodward-Hoffmann Rule for Electrocyclic Reactions
2.6.1 Woodward-Hoffmann Rule for Electrocyclic Thermal Reactions
2.6.2 Photochemical Electrocyclic Reactions
Hackel-Mobius (H-M) Method or Perturbation Molecular Orbital
(PMO) Method
Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Further Reading ..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Problems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.4
2.5
2.6
2.7
CHAPTER 3 Cycloaddition Reactions
34
37
39
39
I
\
1
43
46
46
46
50
52
53
53
58-110
3.1 Introduction
·
3.2 Theory of Cycloaddition Reactions: FMO Method
3.2.1 [2 + 2] Cycloaddition Reactions
3.2.2 [4 + 2] Cycloaddition Reactions
3.3 Correlation Diagrams of Cycloaddition Reactions
3.3.1 Orbital Symmetry in Cycloaddition
3.3.2 Correlation Diagram of [4 + 2] Cycloaddition Reactions
3.4 The Woodward-Hoffmann Rule for Cycloaddition Reactions
3.4.1 The Woodward-Hoffmann Rule in I 4 + 2 I Cycloadditions
3.4.2 Woodward-Hoffmann Rule in 12 + 21 Cycloadditions
3.5 Hiickel-Mobius Method
.3.6 Cycloreversion or Retrocycloaddition Reactions
3.7 [4 + 2 ] Cycloadditions of Cations and Anions
3.8 Cycloadditions involving more than [4 + 2] Electrons
3.9 Some Anomalous [2 + 2] Cycloadditions
3.10 Chelotropic Reactions
3.10.1 [2 + 2] Chelotropic Cycloadditions
3.11 Chelotropic Elimination
3.12 1, 3-Dipolar Cycloadditions
3.12.1 Stereochemistry of 1, 3-Dipolar Cycloadditions . . . . . . . . . . . . . . . ..
Glossary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Further Reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Problems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . ..
58
60
61
63
79
79
81
84
84
85
85
87
90
91
92
95
96
98
98
100
104
105
105
~
CHAPTER
4 Sigmatropic Rearrangement
111-152
4.1 Introduction and Classification
4.1.1 Classification of Sigmatropic Rearrangements
,
4.1.2 Name of the Rearrangement
4.2 Mechanism of Sigmatropic Rearrangement . . . . . . . . . . . . . . . . . . . . . . . . ..
4.2.1 Sigmatropic Shifts of Alkyl Group
4.2.2 Selection Rules for Sigmatropic Rearrangement. . . . . . . . . . . . . ..
4.3 Other Sigmatropic Shifts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
4.3.1 Cope Rearrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
4.3.2 Claisen Rearrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
4.4 [2, 3] Sigmatropic Rearrangements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
4.5 Some Other [m, n] Sigmatropic Rearrangements . . . . . . . . . . . . . . . . . . . ..
4.6 The Woodward-Hoffmann Rule for Sigmatropic Rearrangement
4.6.1 The Woodward-Hoffmann Rule for (m, n) Sigmatropic
Rearrangement where Migrating Group is not Hydrogen
4.6.2 The Woodward-Hoffmann Rule for (m, n) Sigmatropic
Rearrangement where Migrating Group is Hydrogen. . . . . . . . . ..
4.7 Huckel-Mobius Method in Sigmatropic Rearrangements
4.8 Modified and Degenerate Cope Rearrangement
4.9 Fluxional Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Glossary
Further Reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Problems
C~R
5 Group Transfer Reactions
111
111
112
115
120
124
125
125
130
134
136
137
137
139
141
143
146
147
148
148
153-159
I,
;1
5.1
5.2
CHAPI'ER
6.1
6.2
6.3
6.4
Ene Reactions
Group Transfer Reactions given by Diimide
Glossary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Problems
6 Introduction and Basic Principles of
Photochemistry
153
156
158
158
158
160-183
Energy of a Molecule
Photochemical Energy
6.2.1 Photochemical Excitation of the Molecule. . . . . . . . . . . . . . . . . . ..
Electronic Transitions
6.3.1 Types of Electronic Excitation and Molecular Orbital View
of Excitation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Spin Multiplicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
160
163
164
166
167
172
;J
6.5
6.6
6.7
6.8
6.9
Nomenclature of Excited States
The Fate ofthe Excited Molecule-Physical Processes: Jablonski Diagram
Photolytic Cleavage
Laws of Photochemistry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
6.8.1 Grotthurs-Drapper Law
6.8.2 Einstein's Law of Photochemical Equivalence. . . . . . . . . . . . . . . ..
Quantum Yield or Quantum Efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
6.9.1 The Reasons for High Quantum Yield
Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Problems
CHAPTER 7 Photochemistry of Carbonyl Compounds
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
8.1
8.2
184-228
a-Cleavage or Norrish Type I Process
7.1.1 Norrish Type I Process given by Acyclic Saturated Ketones
7.1.2 Norrish Type I Reaction of Saturated Cyclic Ketones
7.1.3 Norrish Type I Process given by Cyclopentanones
7.1.4 a-Cleavage given by Cyclobutanones
~-Cleavage Reaction
"
Intramolecular Hydrogen Abstraction (y-Hydrogen Abstraction)
Hydrogen Abstraction from Other Sites
7.4.1 ~-HydrogenAbstraction
7.4.2 8- and £-Hydrogen Abstraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
7.4.3 Hydrogen Abstraction from Distant Sites
"
Formatio~ of Photoenols or Photoenolisation . . . . . . . . . . . . . . . . . . . . . . ..
Intermolecular Hydrogen Transfer: Intermolecular Photo Reduction
Photocycloaddition Reaction (Patemo-Biichi Reaction)
7.7.1 Addition to Electron-Rich Alkenes
7.7.2 Addition to Electron Deficient Alkenes
"
7.7.3 Oxetane Formation with Dienes and Alkynes . . . . . . . . . . . . . . . ..
7.7.4 Intramolecular Paterno-Biichi Reaction
12 + 2/ Cycloaddition Reaction of Enones with Alkenes . . . . . . . . . . . . . . ..
Glossary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Problems .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
CHAPTER 8 Photo Rearrangements
174
175
180
182
182
182
182
183
183
183
184
184
190
196
198
203
207
212
213
213
214
215
216
218
218
219
221
223
224
226
226
227
229-261
Photo Rearrangement of Cyclopentenone
Cyclohexanone Rearrangements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
8.2.1 Lumiketone Rearrangement
8.2.2 Di-n-Methane Type Rearrangement
229
229
230
233
8.3
8.4
8.5
8.6
8.7
Rearrangement of Dienones
Photo Rearrangements of [3, y-Unsaturated Ketones
8.4.1 1,2-Acyl Shift (Oxa-Di-1t-methane Rearrangement) . . . . . . . . . . . ..
8.4.2 1,3-Acyl Shift. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Aza-Di-1t-Methane Rearrangement
Di-1t-Methane (DPM) Rearrangement
Rearrangements in Aromatic Compounds
Glossary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
CHAPTER 9 Photo Reduction and Photo Oxidation
9.1
9.2
9.3
9.4
235
243
244
245
250
251
257
259
259
260
262-268
Photo Reduction of Carbonyl Compounds
Photo Reduction of Aromatic Hydrocarbons
Photochemical Oxidations
~ . ..
Photo Oxidation of Alkenes and Polyenes
Glossary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
262
263
264
265
267
268
268
CHAPTER 10 Photochemistry of Alkenes, Dienes and Aromatic
Compounds
269-300
10.1 Photochemistry of Alkenes
10.2 Cis-Trans Isomerisation of Alkenes
10.2.1 Cis-Trans Isomerisation of Alkenes by Direct Irradiation
10.2.2 Sensitised Cis-Trans Isomerisation
10.3 Dimerisation of Alkenes . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
10.3.1 Intramolecular Dimerisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
10.4 Photochemistry of Conjugated Dienes
10.4.1 Photochemistry of Conjugated Dienes in Solution
10.5 Photoisomerisation of Benzene and Substituted Benzene
10.5.1 1,2-Alkyl Shift. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
10.5.2 Mechanism of 1, 3-Alkyl Group Shift
10.6 Photoaddition of Alkenes to Aromatic Benzenoid Compounds. . . . . . . . . ..
10.6.1 1,2-Cycloaddition Reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
10.6.2 1,3-Photoaddition of Benzene
10.6.3 1,4-Photoaddition of Benzenes
10.7 Addition of Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
J.u.~ Aromatic Photosubstitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
10.8.1 Type A Reactions
10.8.2 'Type B Substitutions
269
270
270
272
273
276
278
279
282
285
286
287
287
289
290
292
292
293
295
--
r
10.8.3 Radical Substitutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
10.9 Photochemistry of Diazo Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
10.10 Photochemistry of Azides
Glossary
"
Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
295
296
297
298
298
299
CHAPTER 11 Photo Substitution Reactions at sps Hybrid Carbon
Having at Least One Hydrogen
301-311
11.1
11.2
11.3
11.4
Introduction
Barton Reaction
"
The Hoffmann Loeffler Freytag Reaction. . . . . . . . . . . . . . . . . . . . . . . . . . ..
Photolysis of Hypohalites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Glossary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
301
301
306
309
310
310
310
1
I
I
i
)
I
l
l.
CHAPTER 12 Photochemistry in Natural Products
312-321
12.1 Photo Rearrangement or Isomerisation
12.2 Photochemistry of Natural Products Containing Bicyclo [3, 1,0]
Hexane-2-0ne Residue
12.3 Photochemistry of Natural Products having~, y-Unsaturated
Ketone Group
12.4 Photochemistry of Natural Products having Epoxy Ketone System
12.5 Dye-Sensitised Photooxygenations
; . . . . ..
12.6 Photochemistry of Saturated Ketones . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
CHAPTER 13 Photochemistry in Nature and Applied
Photochemistry
13.1
13.2
13.3
13.4
13.5
13.6
13.7
I
314
316
316
317
321
322-345
Photochemical Reactions in the Atmosphere
Chemistry of Vision
:
Photography...................................................
Light-Absorbing Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Photochromism.................................................
Photoinlaging..................................................
Photochemistry of Polymers
,
Problems
"
CHAPTER 14 Problems and Their Solutions
312
322
327
331
335
339
341
342
345
346-459
1
I,
CHAPTER
15 Quenching of Excited States and Investigation of
Reaction Mechanism
460-504
15.1 Introduction.................................................... 460
15.2 Quencher and Quenching of Excited States
461
15.2.1 Excimers
462
15.2.2 Exciplex................................................ 463
15.3 Quenching Processes and Quenching Mechanism. . . . . . . . . . . . . . . . . . . .. 465
15.3.1 Electron-transfer Quenching
466
15.3.2 Heavy Atom Quenching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 466
15.3.3 Quenching by Molecular Oxyzen (Triplet State) and Paramagnetic
Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 467
15.4 Electronic Energy Transfer . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . .. 467
15.5 Investigation of Reaction Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 470
15.6 Quantitative Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 470
15.6.1 Identification of Products
470
15.6.2 Intermediates........................................... 471
15.7 Isotopic Labelling
477
15.8 Lowest Energy Excited State. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 478
15.9 Reactive Excited States
479
15.10 Quantitative Methods
480
15.10.1 Quantum Yield. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 480
15.10.2 Experimental Determination of the Quantum Yield . . . . . . . . . . .. 482
15.10.3 Procedure of Determining Quantum Yield of a Photochemical
Reaction. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. 485
15.11 Lifetime ofthe·Reaction Intermediate for Lifetime of the Excited State .. 485
15.12 Rate Constants and their Relation with the Lifetime of Reactive Excited
States (Triplet State)
486
15.13 Rate Constants and their Relation with the Lifetime of Reactive Singlet
Excited States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 490
15.14 Determination of Rate Constants of Photochemical Reactions
493
15.14.1 Rate of Unimolecular Photochemical Reactions from Singlet
Excited State. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 494
15.14.2 Rate of Unimolecular Photochenl~\:.i:ll Reactions from the
Triplet State
. 495
15.14.3 Rate of Bimolecular Reaction from Triplet State
. 497
15.15 Effect of Light Intensity on the Rate of Photochemical Reactions
. 501
Problems
Index
504
505-509
1
CHAPTER
jcyclic Reacti
Pericyclic reactions are defined as the reactions that occur by a concerted cyclic shift of electrons.
This definition states two key points that characterise a pericyclic reaction. First point is that
reaction is concerted. In concerted reaction, reactant bonds are broken and product bonds are
formed at the same time, without intermediates. Second key point is pericyclic reactions involve
a cyclic shift of electrons. The word pericyclic means around the circle. Pericyclic word comes
from cyclic shift of electrons. Pericyclic reactions thus are characterised by a cyclic transition
state involving the 1t bonds.
The energy of activation of pericyclic reactions is supplied by heat (Thermal Induction),
or by UV light (Photo Induction). Pericyclic reactions are stereospecific and it is not uncommon
that the two modes of induction yield products of opposite stereochemistry.
We shall concern with four major types of pericyclic reactions. The first type of reaction
is the electrocyclic reaction: A reaction in which a ring is closed (or opened) at the expense of a
conjugated double (or triple bond) bond.
~ ~
0-
New "bond ojo,e, a ring
Loss of one 1t bond and
gain of one 0' bond
The second type of reaction is the cycloaddition reaction: A reaction in which two or
more 1t electron systems react to form a ring at the expense of one 1t bond in each of the
reacting partners.
New O'bond
C1
(
New O'bond
In this reaction formation of two new cr (sigma) bonds takes place which close a ring.
Overall there is loss of two 1t (Pi) bonds in reactants and gain of two cr (sigma) bonds in a product.
The third type of reaction is the sigmatropic rearrangement (or reaction); A reaction in
which a cr (sigma) bond formally migrates from one end to the other end of 1t (Pi) electron
system and the net -number of 1t bonds remains the same.
1
r
cr Bonded group moved from one end to the
other end of the 1t electron system
The fourth type of reaction is the group transfer reaction: A reaction in which one or
more groups or atoms transfer from one molecule to another molecule. In this reaction both
molecules are joined together by CJ (sigma) bond.
o
o
1
The three features of any pericyclic reaction are intimately interrelated. These are:
1. Actiuation: Pericyclic reactions are activated either by thermal energy or by UV light.
However, many reactions that require heat are not initiated by light and vice-versa.
2. The number of 1t (Pi) bonds involved in the reaction.
3. The stereochemistry of the reaction.
Consider the following three reactions:
CH,--M- CH,
HH
P
H C
3
...(1)
~CHa
...(2)
X
HaC
...(3)
CHa
First two reactions are thermal reactions activated by heat and third reaction is
photochemical reaction activated by light. The relationship between the mode of activation
and the stereochemistry is exemplified by a comparison of reactions (2) and (3). When starting
material is heated it gives cis product and when starting material is irradiated the product is
trans.
These results had been observed for many years but the reasons for them were not
known. Several theories have been developed to rationalise these pericyclic reactions. R.B.
Woodward and R. Hoffmann have proposed explanation based upon the symmetry of the
molecular orbitals ofthe reactants and products. The theory proposed is known as Conservation
ofOrbital Symmetry. K. Fukuii proposed another explanation based upon the frontier molecular
orbitals. The theory proposed is known as Frontier Molecular Orbital (FMO) method. The
Woodward-Hoffmann rule and Hackel-Mobius (H-M) methods are also used for the explanation
of pericyclic reactions. These four theories make the same predictions for pericyclic reactions.
These four are the alternate ways for looking at the same reaction.
In order to understand the theories of pericyclic reactions, we must first understand
molecular orbitals of compound containing 1t (Pi) bonds.
$"'lr···r~_tt:···rtr,nrtett·'tIIrfllrr1tttt*·tr'ft1frm'ttj""I'tr
We know that, the number of molecular orbitals is always equal to the number of atomic
orbitals that combine to form them. The same principle applies to 1t molecular orbitals. A 1t
electron system derived from the interaction of number m of p orbitals contain m molecular
orbitals, that differ in energy. Half of the molecular orbitals are bonding molecular orbitals
and remaining half are antibonding molecular orbitals. 1t molecular orbitals of ethylene from
the two p atomic orbitals of the two carbons can be constructed as follows:
Each p orbital consists of two lobes, with opposite phases of the wave function of the
two lobes. The plus and minus signs used in drawing these orbitals indicate the phase of the
wave function.
In the bonding orbital of ethylene, there is overlap of similar signs (+ with + and
- with -) in the bonding region between the nuclei. This reinforcement of the wave function is
called constructive overlap. In the antibonding orbital there is cancelling of opposite signs
(+ and -) in the bonding region. This cancelling ofthe wave function is called destructive overlap
(Fig. 1.1).
1t*
(antibonding);
\jI2*
r -~--------------------------------------------~~':~~~
Destructive overlap
1t
(bonding);
\jIl
Constructive overlap
Fig. 1.1 The combination of two p orbitals results in the formation of two molecular orbitals
In 1, 3-butadiene, we have a system of four p orbitals on four adjacent carbons. These
four p orbitals will overlap to produce four 1t molecular orbitals. We can get four new MOs in a
number of equivalent ways. One of the ways to obtain four new molecular orbitals is by linear
combination of two molecular orbitals of ethylene. Linear combination of orbitals is also known
as perturbation theory or perturbation molecular orbital (PMO) theory. Linear combination
always takes place between two orbitals (two atomic orbitals, two molecular orbitals or one
atomic and one molecular orbitals) having minimum energy difference. This means that we
need to look only at the results ofthe 1t ± 1t and 1t* ± 1t* interactions and do not have to consider
1t ± 1t* (Fig. 1.2).
1I
'1'4* = n:* ­ n:*
/
Antibonding ",,",,,c.ion \
Un:*
~
~
Un:*
~
Bonding interaction /
'1'3 * = n:* + n:*
I
'l'z=1t-1t
AnHbonding
"'teractio~
/
tl
~
Bonding interaction /
H·
~~~~
'l'1=n:+n:
Fig. 1.2 The schematic formation of the n: molecular orbitals of 1, 3-butadiene
from the 1t molecular orbitals of ethylene
I'
I
The lowest energy orbital ('lI1) of 1, 3-butadiene is exceptionally stable for two reasons:
There are three bonding interactions, and the electrons are delocalised over four nuclei (Fig. 1.3).
Bonding
Bonding
Bonding
r=r=r1
Fig. 1.3 ('1"1 =1t + 1t) Bonding interaction between two ethylene bonding MOs
The second molecular orbital 'lI2 of 1, 3-butadiene is obtained from the antibonding
interaction between two bonding molecular orbitals ofethylene. The 'lI2 orbital has two bonding
and one antibonding interaction, so we would expect it to be a bonding orbital (two bonding­
one antibonding = one bonding). Thus, energy of'll2 is more than that of'lli. 'lI2 molecular
orbital has one node between C2-C 3. A node is a plane where the wave function drops to zero
(Fig. 1.4).
(=1/------'R
Bonding
Antibonding
-
+
+
-
-
'
Bonding
-
­
+
+
=1t -1t (Antibonding interaction between two ethylene 1t molecular orbitals)
The third butadiene MO, 'lI3 * has two nodes. This molecular orbital is obtained from the
Fig. 1.4 \jf2
bonding interaction between n* and n* oftwo ethylene molecules. There is a bonding interaction
of the C2-C 3 bond and there are two antibonding interactions: One at C l -C 2 bond and the
other at the Ca-C4 bond. This is an antibonding orbital (one bonding - two antibonding = one
antibonding) having two nodes. Thus energy of this '113* orbital is more than the energy of '112
MO (Fig. 1.5).
R /---
Antibonding
~-
_,-m
-'­
Fig. 1.5 (\jf3*
Bonding
Antibonding
_/
+
-~
+
­
=1t* + 1t*) Bonding interaction between two antibonding MOs of ethylene
The fourth molecular orbital ('lI4*) is obtained from the antibonding interaction between
n* and n* of two ethylene molecules. This molecular orbital has three nodes and is totally
antibonding. This MO has the highest energy (Fig. 1.6).
Antibonding
Antibonding Antibonding
f 'f '1
~------
-------
------
"-~
Fig: 1.6 (\jf/ = 1t* - 1t*) Antibonding interaction between n* and n* of ethylene
1!
Following generalisations can be made to construct· the molecular orbitals of the
conjugated polyenes from the 1t molecular orbitals of ethylene and 1, 3-butadiene:
1. A 1t electron system derived from the interaction ofa number of m of p orbitals contain
m molecular orbitals (MOs) that differ in energy. Thus, the number of 1t MOs are
always equal to the number of atomic p orbitals. In 1, 3-butadiene, four p orbitals are
used in the formation ofthe 1t MOs, thus four 1t MOs results, which we shall abbreviate
as 'lf1' 'lfz' 'lfa and 'lf4 (or 1t1' 1tz' 1ta and 1t4)·
2. Half of the molecular orbitals (i.e., m/2) have lower energy than the isolated p orbit­
als. These are called bonding molecular orbitals (BMOs). The other half have energy
higher than the isolated p orbitals. These are called antibonding molecular orbitals
(ABMOs). To emphasise this distinction, antibonding MOs will be indicated with
asterisks. Thus, 1, 3-butadiene has two bonding MOs ('lf1 and 'lfz) and two antibonding
MOs ('Ifa* and 'Jf/).
\
3. The bonding MO of lowest energy 'lf1 has no node. Each molecular orbital of increas­
ingly higher energy has one additional node. Thus, in 1, 3-butadiene 'lf1 has zero
node, 'lf z has one node, 'lf3* has two nodes and 'If/ has three nodes.
4. The nodes occur between atoms and are arranged symmetrically with respect to the
centre of the 1t electron system.
The node in 'lfz * of ethylene is between two carbon atoms, in the centre of the 1t system
(Fig. 1.1). The node in 'lfz of 1, 3-butadiene is also symmetrically placed in the centre of the 1t
system. The two nodes in 'lfa * are placed between carbon-1 and carbon-2 and between carbon-3
and carbon-4 respectively-equivalent distance from the centre of the 1t system. Each of the
three nodes in 'If/, the orbital of highest energy, must occur between carbon atoms. Thus the
highest level has a node between each adjacent carbon pair (Fig. 1.2).
The next generalisation relates to the symmetry of the molecular orbitals. Various
molecular orbitals are classified according to their two independent symmetrical properties.
: : : : : ,:,::::::::::::::~::::::::::::::::::::=:::::• . :::
A 1t molecular orbital possesses either mirror plane symmetry or centre of symmetry. Both
symmetries are not present together in a given 1t molecular orbital.
m-Symmetry: Some molecular orbitals have the symmetry about the mirror plane (m)
which bisects the molecular orbitals and is perpendicular to the plane ofthe molecule (Fig. 1.7).
I
I
I
~ Mirror plane perpendicular
m
to the plane of the molecule
Fig. 1.7
Both orbitals in Fig. 1.7 are mirror images to each other hence in this MO there is
mirror plane symm.etry, abbreviated as m(S).
i
I
1
m
Fig. 1.8 (a>
In Fig. 1.8 (a) both orbitals are not mirror images to each other. Thus in this MO there
is mirror plane asymmetry, abbreviated as m(A).
C2-Symmetry: The centre of symmetry is a point in the molecular axis from which if
lines are drawn on one side and extended an equal distance on the other side, will meet the
same phases of the orbitals [Fig.1.8 (b)].
Centre of the molecular axis
Fig. 1.8 (b)
Both orbitals in Fig. 1.8 (b) are symmetrical with respect to centre of the molecular
axis. Thus in this MO there is centre of symmetry, abbreviated as C2(S).
m(S) means plane of symmetry.
C 2(S) means centre of symmetry.
Let us take examples for the purpose of symmetry properties in ethylene (Fig. 1.9) and
1, 3-butadiene (Fig. 1.10).
'1'*
2
r
Fig. 1.9 Symmetries in the
7t
molecular orbitals of ethylene
r
I
'P*
4
'P*
3
~ ~ ~
~ ~ ~
~ ~,
3
'P*
4
A
S
2
'P*
3
S
A
1
'P2
A
S
0
'PI
S
A
I
f"
I
,I
~ ~,
Fig. 1.10 Symmetries in the
.1
1t
molecular orbitals of 1, 3-butadiene
On the basis of the above two examples we can conclude the following very important
points for linear conjugated 1t systems:
1. The wave function
will have (n - 1) nodes.
2. When n is odd,
will be symmetric with m and asymmetric with C2 •
3. When n is even,
will be symmetric with C2 and asymmetric with m. Table 1.1.
"'n
"'n
"'n
Table 1.1 Symmetry elements in the orbital
,
"'n of a linear conjugated polyene
J
o or even
s
A
odd
A
s
_ m r••7 t f t ' _ t M " ' _ ' ' ' '
Conjugated polyenes always contain even number of carbon atoms. These polyenes contain
either (4n)1t or (4n + 2m conjugated electrons. The filling ofelectrons in the 1t molecular orbitals
of a conjugated polyene is summarised below:
1. Number of bonding 1t MOs and antibonding 1t MOs are same.
2. Number of electrons in any molecular orbital is maximum two.
­
,
J
l
I
--------, r,------­
Pericyclic Reactions
9
3. If a molecular orbital contains two electrons then both electrons are always paired.
4. Molecular orbitals follow Aufbau principle and Hund's rule.
5. Energy of the n molecular orbital is directly proportional to the number of the nodal
planes.
6. There will be no degenerate molecular orbitals in any energy level, i.e., each and
every energy level contains one and only one molecular orbital.
.
••,·_"'t'··l'ir,r<'!mttr''''tttff••'W'f'ff~f'l!r'!f!tf'!!:rr!'fJil.""tlttef.lff.
Conjugated unbranched ions and radicals have an odd number of carbon atoms. For example,
the allyl system (cation, anion or radical) has three carbons and three p orbitals-hence, three
molecular orbitals. We can get three new molecular orbitals by linear combination of one
molecular orbital of ethylene and two isolated p orbital. As already mentioned that the linear
combination always takes place between two orbitals having minimum energy difference. In
allylic system linear combination takes place between one ethylene MO and one p orbital.
This means that we need to look at the results of the n ±p and n* ±p interactions (Fig. 1.11).
fL5U)
~
U
1t*
(JT8~
=
'1'3*
1t* - P
f)
Antibonding interaction
~
E
Q
rn
n
Antibooding in,","otion
-,,;=n>+p
)
EthYlene~Onding interacti~
rn",
'1'1
=1t + P
Fig. 1.11
H
-
~- v;=-n-p +~
H
­
p
'1'2
+
10
Photochemistry and Pericyenc Reactions
n gives linear combination with the p orbital in a bond­
ing way and moving down in energy to give \111'
(ii) The ethylene antibonding orbital (n*) gives linear combination with p orbital in an
antibonding way and moving up in energy to give \113*'
(iii) The p orbital mixes with both the bonding and antibonding orbitals of ethylene. Thus,
there is double mixing for the p orbital. The lower energy ethylene bonding orbital (n)
mixing in an antibonding way to push the p orbital up in the energy (\112' = n - p) but
the ethylene antibonding orbital mixing in a bonding way to push to p orbital down
in energy (\112" = n* + p). Thus, the net result in the energy change is zero. Under this
situation a nodal plane always passes through the central carbon of the chain. This
means an electron in \112 has no electron density on the central carbon. Thus, the \112,
i.e., central molecular orbital must be non-bonding molecular orbital. Thus, the
molecular orbitals of the allylic system is represented as follows (Fig. 1.12).
(i) The ethylene bonding orbital,
rn
'If.; antibonding MO; m(s)
H
'1'2; non-bonding MO; C2(s)
-
+
fL!1L!f)
~
Fig. 1.12
1t
'Ill;
bonding MO;
m(s)
molecular orbitals of allyl system
From the example of allyl system following generalisation can be made to construct the
molecular orbitals of the conjugated open chain system (cation, anion and radical) containing
odd number of carbon atoms:
1. Number of conjugated atoms (or p orbitals) are always odd. For example, allyl system
has three orbitals and 2, 4-pentadienyl system has five orbitals.
2. These systems have always one non-bonding molecular orbital whose energy is always
equal to the unhybrid p orbital. Non-bonding molecular orbital is always central
molecular orbital of the system (Fig. 1.12).
3. If system has m (which is always odd) atomic p orbitals then it has:
(i) m - 1 bonding molecular orbitals,
2
(ii)
m - 1 antibonding molecular orbitals, and
2
(iii) one non-bonding molecular orbital.
4. In non-bonding molecular orbital all the nodal planes (i.e., n - 1 nodal planes for \lin
wave function) pass through the carbon nucleus (or nuclei) (Fig. 1.13).
5. For odd 'Vn ('V1' 'Vg, 'V5'
), all nodal planes pass between two carbon nuclei (Fig. 1.13).
), one nodal plane passes through the central carbon atom
6. For even 'Vn ('V2' 'V4' 'Vs'
and remaining nodal planes pass between two carbon atoms (Fig. 1.13).
Figure 1.13 illustrates schematically the forms of the molecular orbitals for chains up
to seven carbon atoms in length with symmetries and nodal planes.
'1'3*; m(s)
'1'3; m(s)
~ ~~
'V,; m(,)
'1'3; m(s)
'V,; C,(')
3C system
5C system
7C system
Fig. 1.13 7t molecular orbitals of allyl 2. 4-pentadieny! and 2, 4, 6-heptatrienyl system
12
Photochemistry and Pericyclic Reactions
Electron occupancy of allyl carbocation, allyl free radical and allyl carbanion is shown
in Fig. 1.14.
~
~
E
1
fLfL~
~
'1'3*; m(s)
'l'1;m(s)
cation radical anion
'--_----.. ,.--_-J/
Electron ~ccupancy
'--------------------­
Fig. 1.14 1t molecular orbitals with electron occupancy of the allyl system
Similarly, electron occupancy of 2, 4-pentadienyl systems are shown in Fig. 1.15.
~
~
~
~
1~
E
~~ ~
~ ~
~
~ ~
~ ~~
•
•
•
•
~
~
~
~
~
'1'5*; m(s)
'1'4*; C 2(s)
J,
J,j
J,j
H
J,j
J,j
J,j
J,i
'1'3; m(s)
'1'2; Cis)
'1'1; m(s)
,cation radical anion
v
"
Electron occupancy
Fig. 1.15 1t molecular orbitals with electron occupancy of the 2, 4-pentadienyl system
Pericyclic Reactions
13
Notice that cations, radicals and anions involving the same TC system have the same
molecular orbitals. For example, the MOs of the allyl system apply equally well to the allyl
cation, allyl radical and allyl carbanion because all the three species contain the same p orbitals.
These species differ only in the number of TC electrons, as shown in the "electron occupancy"
column of Fig. 1.14.
~'176~1f'()~StE'edt:lit_IYAtt;" ,14k*' tL,! u~, 4,4il."d¥,"';:;W,$ 43¥' LSihiJiiW\l;;1k,';;"MJ!i4k
~""ft,'t'tr-S'"C"m7e"f-".<It7mtzi"Wtrlr-
-- --~ 'ZntW1ft"'e'tfflM-'''61'--t o:nfmsf'" 'Ht'~'h¥ :'tt"i>;' "~"'t'r'r,,;.'t'-':"''-',c'r"",,;;i"'t""a,ri.tt1rN1t"to"W'W't-' 'iH,'"WVO''''%W,·{iW'''" '1f'""wt!tti'i;;-iP'',O@i,,-,,:,Lr,­
Two 1t molecular orbitals are of particular importance in understanding pericyclic reactions.
One is the occupied molecular orbital of highest energy, known as highest occupied molecular
orbital (HOMO). The other is the unoccupied molecular orbital of lowest energy known as
lowest unoccupied molecular orbital (LUMO). HOMO and LUMO of any given compound have
opposite symmetries (Fig 1.16). HOMO and LUMO are referred to as frontier molecular orbitals.
'V/
C2(s)
'lI3*
m(s)
LUMO or ground state LUMO
C2(s)
HOMO or ground state HOMO
'V2
'VI
J,j
J,j
m(s)
Fig. 1.16 Electronic state of ground state of 1, 3-butadiene
HOMO of the ground state species is also known as ground state HOMO. Similarly,
LUMO of the ground state of the species is known as ground state LUMO.
Why are the LUMO and HOMO so important in determining the course of a concerted
reaction? The electrons in the HOMO of a molecule are like the outer shell electrons of an
atom. They can be removed with the least expenditure of energy because they are already in a
highest energy level than any of the other electrons in the molecule. The LUMO of a molecule
is the orbital to which electrons can be transformed with the least expenditure of energy.
The higher is the energy ofHOMO ofa molecule, the more easily electrons can be removed
from it. The lower is the energy of the LUMO of the molecule, the more easily electrons can be
transferred into it. Therefore, the interaction between a molecule with a high HOMO and a
low LUMO is particularly strong.
In general, the smallest the difference in energy between HOMO of one molecule and
the LUMO of another with which it is reacting, the stronger is the interaction between the two
molecules.
The molecules and ions we have been discussing can absorb energy from electromagnetic
radiation of certain wavelengths. This process is shown schematically in Fig. 1.17 for
1, 3-butadiene. Let us refer to the normal electronic configuration of 1, 3-butadiene as the
ground state. When 1, 3-butadiene absorbs a photon of proper wavelength an electron is promoted
from the HOMO (\112) to the LUMO (\113*). The species with the promoted electron is an excited
state of 1, 3-butadiene. The orbital '113* becomes HOMO and '114* becomes LUMO of the excited
state. HOMO of excited state is termed as excited state HOMO or photochemical HOMO.
Similarly, LUMO ofthe excited state is termed as excited state LUMO or photochemical LUMO
(Fig. 1.17).
'1'/ excited state or photochemical LUMO
E '1'3*
r
'1'2
LUMO
n
n
hv
HOMO
~
+
+
n
l
I
I
~
i
\
'1'3* excited state or photochemical HOMO
'1'2
'1'1
excited state
'1'1
ground state
Fig. 1.17
Notice that the HOMO of ground state and excited state have opposite symmetries.
Similarly, LUMO of the ground state and excited state also have opposite symmetries.
::::;::":~:"~;J:~~:::,:::=:
The sigma orbital of a carbon-carbon covalent bond has a mirror plane symmetry, and since a
rotation of 180 0 through its midpoint regenerates the same sigma orbital, it also has a C2
symmetry. A sigma antibonding molecular orbital is asymmetric with respect to both m and
C2 (Table 1.2).
Table 1.2 Symmetric properties of cr and cr* molecular orbitals
cr*
A
A
s
s
J;f.::;:tDitlt:~tc:etmrmn;;;:::::::::::::,:::::::::!:':::;::::::::::!:::::=:':::::::
Pericyclic reactions are defined as reactions that occur by concerted cyclic shift of electrons.
According to the Woodward and Hoffmann symmetry ofthe molecular orbitals that participate
in the chemical reaction determines the course ofthe reaction. They proposed what they called
the principle of the conservation of orbital symmetry; in the concerted reactions. In the most
general terms, the principle means that in concerted pericyclic reactions, the molecular orbitals
of the starting ma~erials must be transformed into the molecular orbitals of the product in
1
Peric;yclic Reactions
15
smooth continuous way. This is possible only if the orbitals have similar symmetry, i.e., orbitals
of the reactant and product have similar symmetries.
In concerted reaction product formation takes place by formation of cyclic transition
state. The transition state ofpericyclic reactions should be intermediate between the electronic
ground states of the starting material and product. Obviously, the most stable transition state
will be one which conserves the symmetry ofthe reactant orbitals in passing to product orbitals.
In other words, a symmetric (8) orbital in the reactant must transform into a symmetric
orbital in the product and that an asymmetric (A) orbital must transform into an asymmetric
orbital. If the symmetries of the reactants and product orbitals are not the same, the reaction
will not take place in a concerted manner.
If symmetry is conserved during the course of the reaction then reaction will take place
and process is known as symmetry allowed process. If symmetry is not conserved during the
course ofthe reaction, the reaction is known as symmetry-forbidden process. The energy of the
transition state (i.e., energy of activation of the transition state) of symmetry allowed process
is always lower than the symmetry-forbidden process.
• The eatoon atom
e overlap: An overlap of orbitals
ally constructive
Del
A molecular orb
atomic
When filled; th
all the atomstnvolved:
Desb'Uctive
: An overlap of orbitals that contributes to antibonding. Overlap of lobes
opposite .ases is generally destructive overlap.
abbreviation for
. st occupied molecular orbital". In
state, this is'represented as
MO*.
L~O: An abbreviation for lowest unoccupied molecular orbital.
MoleCular orbi
. bitals that include more than()ne atom ina molecule. M.olet:ular orbital
caD. be bonding,
. nding, or nonbonding.
Bonding molecular orbitalsMOs that are lower in energy than the isolated atomic orbitals
,
from which they are made. Electrons in these orbitals serve to hold the atoms together:.
Antiboadia, molecular orbi
MOs that ~~menergytlWl the isOlated"atomic orbitals
frolll which they are made.ons in these 0
s tend to push the atoms apartc
,NOllbondin, meleeular orbitals: MOs that are similar in energy to the isolated atorilic orbitals
. from which they are made. Electrons in these orbitals have no effect on the bonding of the
atoms.
Pericyclic reactions: A reaction involving concerted reorganisation of electrons within a closed
loop of interacting orbitals.
FURTHER ,READING
1. RoB. Woodward and R. Hoffmann, The Conseroation of Orbital Symmetry, Academic Press,
New York, 1970.
r
16
Photochemistry· and Perioyelie. Reactions
2. T.L. Gilchrist and R.C. Storr, Orgp,nic Reactions and Orbital Syininetry, 2nd ed. C
University Press, Cambridge, U.K, 1979.
.
3. J.M. Coxon and B.Halton, Organic Photochemistry, Cambridge University Press, Cambridg~,
U.K.,1974.
.
PROBLEMS
1. Sketch the pi molecular orbitals of
(a) 1, 3-butadiene
(b) 1, 3, 5-hexatriene
2. Sketch the pi molecular orbitals of the allyl system. Give electron occupancy in allyl carbona­
tion, allyl free radical and allyl carbanion.
.
3. $ketch the pi molecular orbitals of2, 4-pentadienyl system. Also draw nodal points in molecu­
lar orbitals. Show electron occupancy in its carbbeation, f t p : a m o a .
4. Give symmetric properties of HLUMO of thefol
(0 Butadiene·
(ii) Exdteds~
(iii) 1, 3, 5-hexatriene·.
(tv)
5. • Show the ·electrQ .
8'1'0
properties of all
ofl, 3,
.
6. Show the electr 'e:&eited;s~teofB, 4-pebt8!di~t .
~ system;. ....
. ., .. ..
ric properties of
7. Draw molecular orbitals of 1, g.;butadiene from thelin.ea:r
two ethylene molecules.
.
.. ...•
.'.
8. Draw molecular orbitals of allyl
. by the use of the linen combination of molecular
orbital of one ethylene molecule
mic orbital.
.
than the lj/2?
9. Explain why lj/g of butadiene has
10. Explain why energy of 'lf5 of 1, 3, 5•eneis less. than the 'J1s?
11. What are HOMO and LUMO? Why these two orbitals are .~ impnrtant in pericyclic rea~tions?
12. Give classification of pericyclic reactions with one eJ(8mple each.
. ,. .,. .
13. (i) Draw the p-orbital array in the 1t molecular orbitals of the following ions (ii) Draw diagrams
showing the occupied orbitals·of the ground states and indicate the HOMOs.
e
\
•
\
I
j
·
e.
(0 CHg-CH=CH-CH-CHg
e
(ii) CH2=CH-CH-eH3
(iii) CH a-CH=CU:-0H=CH-CH-CHa.
14. How many approaches have been made to explain the
.
results 'of perieyclic reactions?
15. What do you understand from frontier orbital and orbi~ symmetry?
1
I'I
I,
I
,•
~
·f
l
Iit
j
2
CHAPTER
eetrocyclic Reactio
An electrocyclic reaction is the concerted interconversion of a conjugated polyene and a
cycloalkene. Electrocyclic reactions are induced either thermally or photochemically.
.1 or hv
)
(
o
.1 or hv
)
(
All electrocyclic reactions are reversible reactions. Open-chain partner of the reaction
is always a conjugated system whereas cyclic partner mayor may not contain conjugated
system.
In electrocyclic reactions either a ring is formed with the generation of a new cr bond
and the loss of a 1t bond (i.e., gain of one cr bond and loss of one 1t bond) or ring is broken with
the loss of one cr bond and gain of one 1t bond.
~
)
(
(kn - 2) electrons + one
cr bond (loss of one n bond
and gain of one cr bond)
krc electrons
(k == 4)
(
(krc - 2) electrons + one cr bond
kn electrons
(k =6)
(
1m electrons
o
)
)
(kn - 2)n electrons + one cr bond
17
1
~
)
(
(k + 2)1t electrons - one 0" bond
k1t electrons
.
-,
n
)
,,
(k + 2)1t electrons - one 0" bond
k1t electrons
Thus electrocycljc reactions can be classified into two categories:
(i) Electrocyclic opening of the ring, and
(ii) Electrocyclic closure of the conjugated system.
In electrocyclic closure of the ring (or ring closing electrocyclic reaction) if the n system
of the open-chain partner contains kn electrons, the corresponding cyclic partner contains
(k - 2)n electrons and one additional cr bond. In ring opening electrocyclic reaction if ring
partner contains kn electrons, the open chain partner will contain (k + 2}n electrons with the
~~~cr~~
.
There are two possible stereochemistries for the ring-opening afui' ring-closing of
electrocyclic reactions. They are:
1. Conrotatory process (or motion), and
2. Disrotatory process (motion).
_lrtmmrsnrT.tn1'r.,e_rr~trtrtrsirrn]iIt_
The most common example of the ring opening reaction is the conversion of cyClobutene to
1, 3-butadiene.
This conversion can only be possible if a cr (sigma) bond between C3~4 of cyclobutene
must break during the course of the reaction.
)
(
Ring opening reaction
This cr (sigma) bond may break in two ways. First, the two atomic orbital components of
the cr (sigma) bond may both rotate in the same direction, clockwise or counter-clockwise. This
.
process is known as conrotatory motion (Fig. 2.1).
Conrotatory
.
motion
)
Counter-clock- Counter-clock­
wise motion
wise motion
J
.
;,
Or
n
Conrotatory
.
)
mohon
+
­
C2 -Symmetry
ClockWise Clockwise
motiOn . .: motion
F~.
+
2.1 Conrotatory ring-opening
Second, the atomic 'orbitals, may rotate in opposite directions, one clockwise and the
other counter-clockwise.lI'his process of ring-opening is known as disrotatory motion
(Fig. 2.2).
-$
~
.
Disrotatory
motIon
)
Clock~ee _.' Counter-clock­
motion ".
mcSymmetry
wise motion
Or
(~)
Disrotatory
.
mohon
)
Counter.tfOckwi~ Clockwise
motion
n
n
. motion
m-Symmetry
Fig. 2.2 Disrotatory ring-opening
The substituents present on the carbons of the rotating orbitals may also rotate in the
direction of the rotating orbitals. Thus in the conrotatory motion substituents rotate in the
same direction (Fig. 2.3) and in disllOtatory motion substituents rotate in the opposite directions
(Fig. 2.4).
.~~. "-,
tf~
.• ·-.··.,.,.:·r;-.?~
H ....
~)
Conrotatory
.
motIon
)
~~-"~
Clockwise
: .-:Clockwise
. ..
'.~
Or
3C· +·.:~
.. ­
~
-
...,.
...
+ ..
.'.
.
:;~
.'
~"' ..
Counter.C-ounter­
clockwise' ";'clockwise
.-.',l"..... '.
Fig. 2.3 Conrotatbt.Y; ring-opening. orbitals and groups migrate in the same direction
20
Photochemistry and Pericyclic Reactions
H3C
~
-J)
~
~
+
~
Clockwise
motion
+
Disrotatory
.
)
motIon
~
Counterclockwise
motion
H~CH"
m-Symmetry
."
I
I
,
I
Or
Disrotatory
.
)
motIon
Counter­
clockwise
motion
~
H3C~HH3C~H
Clockwise
motion
i
m-Symmetry
1
Fig. 2.4 Disrotatory ring-opening, orbitals and groups migrate in the opposite directions
..
,
rr"rmt,
irllli~,l._'mi.f::Z'iWr''rrmiffJrt'ff'·f.mrrtmr·· .HClI' '!in7ttr m
i
1
When an electrocyclic reaction takes place, the carbon at each end of the conjugated 1t system
must turn in a concerted fashion so that the p orbitals can overlap (and rehybridised) to form
cr bond that closes the ring. This turning can also occur in two stereochemically distinct ways.
In a conrotatory closure the orbitals and groups of the two carbon atoms turn in the
same direction, clockwise or counter-clockwise (Fig. 2.5).
Conrotatory
.
)
motIon
~
H
Clockwise
motion
H
Clockwise
motion
Or
~
~
~"c~H)
Conrotatory
.
)
motion
Counter
Counter
clockwise
clockwise
C2 -Symmetry
Fig. 2.5 Conrotatory closure of the ring
In a disrotatory closure the orbitals and groups of the two carbon atoms turn in the
opposite direction, one clockwise and other counter-clockwise (Fig. 2.6),
Electrocyclic ~eacti.()ns
Disrotatory
.
motIon
Counter
clockwise
m-Symmetry
21
)
Clockwise
~
Or
Disrotatory
03c~~~aC~~
.
motIon
)
Counter
Clockwise
clockwise
m-Symmetry
Fig. 2.6 Disrotatory ring-closure
From these examples it is clear that orbitals having m-symmetry always give
disrotatory motion. (Fig. 2.4 and Fig. 2.6) whereas orbitals having C 2"symmetry give
conrotatory motion (Fig. 2.3 and Fig. 2.5).
The reason behind this rule can be easily understood by recalling that overlap of wave
functions of the same sign is bonding (and symmetry allowed reaction) whereas overlap of
wave functions of opposite sign is antibonding (and symmetry forbidden process).
Electrocyclic reactions are highly stereospecific. An intriguing feature about electrocyclic
reactions is that the stereochemistry of the product is dependent on whether the reaction is
thermally induced or photo-induced.
2.3.1 Open Chain Conjugated System having 4mt Conjugated Electrons
Let us consider the simplest example in which a cyclobutene derivative opens to a 1, 3-butadiene
derivative, i.e., open-chain conjugated system has 4n conjugated 1t electrons.
In thermal condition trans-3, 4-dimethylcyclobutene gives (2E, 4E)-2, 4-hexadiene. Thus,
this reaction is completely stereospecific.
In the photochemical condition the same substrate gives (2E, 4Z)-2, 4-hexadiene. In this
case too, the reaction is completely stereospecific. Thus the reaction can be performed thermally
or photochemically, and under either condition the reaction is completely stereospecific.
HaC(
H
i
~
CH 3
...(1)
trans-3,4-Dimethyl­
cyclobutene
H
. (2E, 4Z)-2, 4-Hexadiene
(2E, 4E)-2, 4-Hexadiene
...(2)
Stereochemistry of the thermal reaction-l (of the 4rm system) cfm only be explained if
process should be conrotatory.
I
Conrotatory
J).
\
)
Stereochemistry of the photochemical reaction-2 (of the '4n,
explained if process should be disrotatory.
1t
system) can only be
Disrotatory
hv
)
From the above two examples it is clear that thermally induced eIectrocyclic
reaction involving 4n1t conjugated electrons req~~re ~Drotatory motion and
photochemically induced electrocyclic reaction require.rotatory motion.
Conh..~rons
2.3.2 Open Chain Conjugated System having (4n + 2)n
The simplest example of this category is the ring-opebingcif '1, '3-cyclohexadiene into
1,3,5-hexatriene.
.
"
o
In thermal condition 5, 6-trans-dimethyl-l, 3-cyclohexadieneis converted exclusively to
(2E, 4Z, 6Z)-2, 4, 6-octatriene. In the photochemical condition the S;~e substrate is converted
"
exclusively to (2E, 4Z, 6E)-2, 4, 6 - o c t a t r i e n e . '
Q
~'
~
H,C
.'
" ",
""
,.-'
\
.'
#~,'
HH.C· ,....
... (3)
'''''.
...(4)
These two conversions are also highly stereospecific. Ste~eochemistry of these two
reactions (i.e., reaction-3 and 4) can only be explained if prOCess isdisrotatory in thermal
condition and conrotatory in photochemical condition.
.!
1
Ii
i
On the~is of these experimental results the stereochemistry of electrocyclic reactions
can be sununarised by noting. that thermally induced electrocyclic reactions involving 4nn
electron,s req1W'e conrotatory motion. Under similar conditions, electrocyclic reactions involving
(4n+ 2)n electrons follow disrotatory motion; Similarly, photo-induced electrocyclic reactions
involving 4nn electrons require disrotatory motion. Under similar conditions, electrocyclic
reactions involving (4n + 2)n electrons follow conrotatory motion.
A sumttiliry ofthe ~ype of motion to be expected from different polyenes under thermal
and photochemical conditions is shown in Table 2.1.
Table 2.1
(i) Thermal
Conrotatory
Disrotatory
(ii) Photochemical
4n
Disrotatory
Conrotatory
(i) Thermal
4n+2
(ii) Photochemical
The above experimental results can be explained by the four theories given for pericyclic
reactions.
~
::,::::::,;::=;:;::~,::::':;::
A methodology tor quickly predicting whether a given pericyclic reaction is allowed by examining
the symmetry of the highest occupied molecular orbital (HOMO) (in case of unimolecular
react\on) and, if the reaction is bimolecular, the lowest unoccupied moleculaiorbital (LUMO)
of the second, partner.
Thus, electrocyclic reaction is analysed by HOMO of the open. chain partner because
reaction is unimolecular reaction. The stereochemistry of an electrocyclic process is determined
by the symmetry of the highest occupied molecular orbital (HOMO) of the open chain partner,
regardless of which way the reaction actually runs. In thermal condition HOMO is always
ground state HOMO whereas in photochemical condition HOMO is always first excited state
HOMO.
Ifthe highest occupied molecular orbital has m symmetry, the process will be disrotatory.
On the otber hand,ifHOMO has C 2-symmetry then the process will be conrotatory (Table 2.2).
Table 2.2
m-Symmetry
Disrotatory
Conrotatory
, For.4DY electrocyclic reaction there are two conrotatory and two disrotatory modes of
rmg cleavase and ring closure. The two conrotatory modes, can give same or different products.
disrotatoIj modes can also give the same or different products.
.Similarly.,
.
th~ ~
'
24
Photochemistry and! P~ll'i~!clic R.Et8ctiiOJl~; ,.'.
Now we are in a position to consider specific examples of the application of the FMO
method.
2.4.1
Cyclisation of 4nn Systems
1. Electrocyclic ring-closure reaction given by butadiene: 1, 3-butadiene is the
first member of the conjugated polyene having 4nn electrons.
Thermal-induced cyclisation: When 1, 3-butadiene is heated, reaction takes place
from the ground state. The electrons that are used for the C1 (sigma) bond formation are in the
HOMO ('lf2 in this case). Pertinentp orbitals in ground state HOMO has C2-symmetry. For the
new C1 (sigma) bond to form, rotation must be conrotatory. Disrotatory motion would not place
the in-phase lobes together.
,
'I
r.::::-
n
~
Conrotatory
.
motion
~
i
Symmetry allowed,
C2 -Symmetry
in (J bond
Thermal; 'l'2; C2-Symmetry
l
)
Disrot~toI:Y
motIon
~
i
~
Symmetry-forbidden,
C2 -Asymmetric
Photo-induced cyclisation: In photo-induced cyclisation, the first excited HOMO of
1, 3-butadiene is "'3* which has m symmetry. For the new C1 (sigma) bond to form, rotation
must be disrotatory.
n
Excited state HOMO 'l'a*,
m-Symmetry
\.
j
1
Disrotatory
.
motion
)
~
i
i
Symmetry allowed,
m-Symmetry in (J bond
Let us return to (2E, 4Z)-2, 4-hexadiene to see why the cis-3, 4-dimethylcyclobutene
results from the thermal cyclisation and the trans-isomer from the photo cyclisation.
In the case of the thermal cyclisation the ground state HOMO is 'lf2 which has
C2-symmetry. Thu~, conrotatory motion is required for C1 (sigma) bond formation. Both methyl
1
Electrocyclic Reactions
25
groups rotate in the same direction, as a result they end up on the same side of the ring or cis
is the product.
H39E=\9H3
~
Conrotatory
motion
H
cis-3, 4-Dimethyl­
cyclobutene
Clockwise
Clockwise
H
(2E, 4Z)-2, 4-Hexadiene 'l'2'
C2 -Symmetry
Or
~
C\
(HC~~
~3
\::::!j-
Counter
clockwise
~
~
Conrotatory
.
)
motion
~ 1)
CH 3
CH 3
cis-3, 4-Dimethyl­
cyclobutene
Counter
clockwise
In the case of phot6cyclisation the excited state HOMO is 'lf3 * which has m-symmetry.
Thus, disrotatory motion is required for the (J bond formation. In disrotatory motion, one of
the methyl groups, rotates up and the other l.:>tates down. The result is that both methyl
groups a!"e trans in the product.
Disrotatory
hv
)
~
-
+
H
Excited state HOMO
'l'3*' m-Symmetry
+
­
CH 3
trans-3, 4-Dimethyl­
cyclobutene
2. Electrocyclic ring-closure given by allyl carbanion: Allyl carbanion is also a
4nn conjugated system.
yCH"
CH 2
CH 2
·h~'
6
l
e
~e
HOMO ofthe allyl carbanion in the ground state is 'lf2 which has C2-symmetry. Therefore,
conrotatory motion is the mode of cyclisation in the thermal condition.
e
CH 3-CH=CH-CH-CH 3
e
!
CH 3-CH-CH=CH-CH 3
1
III
Conrotatory
motion
In case of photocyclisation, the excited HOMO is 'lf3* which has m-symmetry. Thus,
disrotatory motion is required for cr (sigma) bond formation.
\
.'
hv
D"lsrotatory )
motion
3. Electrocyclic ring-closure reaction given by 1,3,5, 7-octatetraene: 1,3,5,7­
octatetraene and its derivatives contain 4nn conjugated electrons.
Consider the electrocyclic ring-closure of (2E, 4Z, 6Z, 8E)-2, 4, 6, 8-decatetraene in
thermal and photochemical conditions.
Thermal-induced cyclisation: The tetraene is a 4n polyene. Its ground state HOMO
is 'lf4 which has Co2-symmetry. Therefore, conrotatory motion is the mode of cyclisation.
1
Ii
Conrotatory
motion
)
CH~
trans-7, 8-Dimethyl-l, 3, 5-cyclotriene
Photo-induced cyclisation: In case of photo-induced cyclisation the excited state
HOMO is '115* which has m-symmetry. Thus disrotatory motion is the mode of cyclisation.
hv
D"lsrotatory )
motion
H
H
cis-7, 8-Dimethyl-l, 3, 5-cyclooctatriene
4. Nazarov cyclisation: Nazarov cyclisation is given by 1, 4-pentadiene-3-one and its
derivatives. The product of the reaction is cyclopentenone. The reaction is conrotatory
electrocyclic ring-closure under thermal condition and disrotatory ring-closure under
photochemical conditions.
o
o
H
Nazarov cyclisations require acid. Under acidic condition the substrate converts into
cation which has 47t conjugated electrons.
Ell
o
J\
R
0-H
R
J\
R
OH
R
J\
R
R
Under thermal condition, the motion is conrotatory and under photochemical conditions
the motion is disrotatory ])e{:ause the conjugated system has (4n)n conjugated electrons.
28
Photochemistry and Pericyclic Reactions
The reaction under thermal condition
..
H H
H H
I
OH
OH
(
Conrotatory
motion
~
l-~
o
OH
H
H
H
The reaction under photochemical conditions
Ell
1
OH
O-H
0
J
J
Ell
~
~
H H
H H
hv
Tautomerisation
H
motion
OH
OH
H
1Dis~otatory
Ell
~
H
H
H
1
'.
Elecfrocyclic Reactions
29
2.4.2 Electrocyclic Ring-Opening in which Polyene has 4nn Electrons
Conversion of cyclobutene to butadiene
(A) Thermal-induced ring opening: In the ring-opening reactions stereochemistry of
the product is determined by the symmetry of the ground state HOMO of the open-chain
partner. The ground state HOMO of butadiene and its derivative will be \lf 2 which has
C2 -symmetry. The cyclobutene ring must open in such a fashion that the (J bond orbitals
transform into the HOMO of the product having C2 -symmetry. To get C2 -symmetry in the
product HOMO, motion should be conrotatory in the ring opening of the reaction.
Conrotatory
motion
n
-
+
C,-Symmetry
Thus, if the open-chain polyene has 4mt electrons then the process is always conrotatory
in thermal condition whether the reaction is ring-closure or ring opening.
Let us take the stereochemistry of the ring opening of trans-3, 4-dimethy1cyclobutene.
There is possibility of two modes of conrotatory motion, counter clockwise and clockwise
motions.
conr~atory)
(i)
motIOn
a­
HaC
-
H
H
+ eH a
C2-Symmetry
Counter clockwise
(2E,4E)-2,4-Hexadiene
(Major product)
(ii)
)
HDcHHC:H
a
Clockwise
motion
3
(2Z, 4Z)-2,4-Hexadiene
(Minor product)
Thus, the thermal process is conrotatory with two products possible in the above case.
The conrotation in the second case (clockwise rotation) leads to severe steric interactions
between two methyl groups. This interaction is avoided in the first process (rotation is counter
clockwise) in which two methyl groups move away from each other, and this is the favoured
process.
(B) Photo-induced ring opening: The photo state HOMO of the open chain butadiene
and its derivative will be "'3* which has m-symmetry. Thus, the cyclobutene ring must open in
such a fashion that the (J bond orbitals transform into the excited state HOMO of the product
having m-symmetry. To get m-symmetry in excited state HOMO of the product, motion should
be disrotatory in the ring-opening of the reaction. There is also possibility of two modes of
disrotatory motion.
(i)
-~5?\~_­
(~)
~
~
Counter
clockwise
(f)I\f)
hv
)
Disrotatory
motion
H aC1}H H aC1}H
'I's*; Excited state HOMO
Clockwise
m-Symmetry .
(2E, 4Z)-2, 4-Hexadiene
\
Qa
(ii)
cs-
hv
)
~
~
Clockwise
Disrotatory
motion
(2Z, 4E)-2, 4-Hexadiene
Counter
clockwise
,
Thus, photo-induced process is disrotatory with either possible disrotation giving same
product.
Let us take the conversion of cis-3, 4-dimethylcyclobutene into 2, 4-hexadiene.
Thermal ring-opening
Qa
(i)
Q
a
._~
Q~
.1.
)
Conrotatory
motion
&
(ii)
cs-
~
~
Clockwise
+
H HaC
H
C2-Symmetry
~a
-~
-
(2E, 4Z)-2, 4-Hexadiene
Counter
Counter
clockwise
clockwise
C2 -Symmetry
Qa
n
HaC
1
.1.
)
Conrotatory
motion
1
0
H
'l
+ CH a H -
CHa
)
(2Z, 4E)-2, 4-Hexadiene
Clockwise
C2-Symmetry
Photo-induced ring-opening
~
a
(i)
Q-
a
-~
+ +
~
Counter
clockwise
~
Clockwise
hv
)
Disrotatory
motion
n
HaC
-
H
...'.
H
-
CHa
'I'a*; m-Symmetry
(2E, 4E)-2, 4-Hexadiene
(Major produl)t)
1
(.
hv
(ii)
)
Clockwise
~
Disrotatory
motion
'1'3*; m-Symmetry
Counter
clockwise
(2Z, 4Z)-2, 4-Hexadiene
(Minor product)
In case of cis-3, 4-dimethylcyclobutene, the thermal process is conrotatory with either
possible conrotation giving the same product. The photochemical process is disrotatory with
two possible products.
2.4.3
Cyclisation of (4n + 2)7t Systems
1. Electrocyclic ring-closure reaction given by 1,3, 5-hexatriene
1,3, 5-Hexatriene is the most common example ofthe polyene having (4n + 2)n conjugated
electrons.
n
o
~or
)
hv
1, 3, 5-Hexatriene
1,3-Cyclohexadiene
(A) Thermal-induced cyclisation: \lf3 is the ground state HOMO of 1, 3, 5-hexatriene
which has m-symmetry. Therefore, the thermal cyclisation proceeds by disrotatory motion.
Disrotatory
motion
)
t:::::b
l'
Symmetry-allowed
(m-Symmetry)
'1'3' Ground state. HOMO
m-Symmetry
(B) Photo-induced cyclisation: When an electron of 1, 3, 5-hexatriene is promoted by
photon absorption, \lf4 * becomes the HOMO. This excited state HOMO has C2-symmetry.
Therefore, photo-induced cyclisation proceeds by conrotatory motion.
Conrotatory
motion
)
~
~
l'
'1'/; Excited HOMO
(C2 -Symmetry)
Symmetry-allowed
(C2 -Symmetry)
32
Photochemistry and Pericyclic Reactions
Consider the stereochemistry of the thermal and photo-induced closure of (2E, 4Z, 6E)­
2, 4, 6-octatriene to 5, 6-dimethyl-l, 3-cyclohexadiene.
(A) Thermal-induced cyclisation: In the case of the thermal induced cyclisation, the
ground state HOMO is 'If3 which has m-symmetry. Thus, disrotatory motion is required for
sigma bond formation.
>
H
H
cis-5, 6-Dimethyl.l, 3-cyc1ohexadiene
'V3' Ground state HOMO
em-Symmetry)
(B) Photo-induced cyclisation: In case of photo-induced cyclisation, the excited state
HOMO is \114 * which has C2 -symmetry. Therefore, conrotatory motion is required for cr (sigma)
bond formation.
~
~
Conrotatory
motion
>
(H3C\SIH@~ CH0
~H3
H'\
H
CH3
trans-5, 6-Dimethyl-l, 3-cyc1ohexadiene
Problem 2: Whieh one of the following
concerted rru!Ch;anlr8"'~?
ele,J~tr:(wy(~lie
rea:etU)n8 sluJu/;tJ, occur
rp.lr.dij~V ~I)V j~
i
\
I
Reaction 2
2. Electrocyclic ring-closure reaction given by allyl carbocation
Allyl carbocation contains (4n + 2)1t conjugated electrons.
E9
CH 2=CH-CH2
E9
~
~ .......•...
CH2-CH=CH2= CH 2-CH-CH 2
(A) Thermal-induced cyclisation: HOMO of the allyl carbocation in the ground state
is \lfl which has m-symmetry. Therefore, disrotatory motion is the mode of cyclisation in the
thermal condition.
Disrotatory
)
motion
\lf2
(B) Photo-induced cyclisation: HOMO of the allyl carbocation in the excited state is
which has C2 -symmetry. Thus, conrotatory motion is required for cr (sigma) bond formation.
Conrotatory
motion )
l
I
2.4.4
Electrocyclic Ring-opening in which Polyene has (4n + 2}1t Electrons
1. Conversion of 1, 3-cyclohexadiene to 1, 3, 5-hexatriene system
The most common example ofthis class is the conversion of 1, 3-cyclohexadiene to 1, 3, 5­
hexatriene.
i
i
\
)
(
Let us take the example of the ring opening of a cis-5, 6-dimethyl-1, 3-cyclohexadiene
into 2, 4, 6-octatriene.
(
)
( '>
CHCH a CHCH a
(A) Thermal ring-opening: As mentioned earlier that in the ring opening reactions
stereochemistry of product is determined by the symmetry of the ground state HOMO of the
open-chain partner. The ground state HOMO ofthe triene will be \lfa which has m-symmetry.
The cyclohexadiene ring therefore, must open in such a fashion that cr bond orbitals transform
into the ground state HOMO of the prod.uct having m-symmetry. To get m-symmetry in the
HOMO of the product, motion should be disrotatory in the ring opening of the reaction.
\
I,
I
..\
I
The above results can be obtained in short as follows:
o
)
Ground state HOMO is '1'3 which
has m-symmetry. Therefore,
motion is disrotatory
Two modes of disrotatory motions can take place as follows:
Disrotatory
)
(i)
motion
Counter
clockwise
(2E, 4Z, 6E)·2, 4, 6·0ctatri~ne
Clockwise
(Major product)
Disrotatory
(ii)
)
motion
~H'C~
CH 3
Clockwise
(2Z, 4Z, 6Z)-2, 4, 6-0ctatriene
Counter
clockwise
(Minor product)
Thus, the thermal process is disrotatory with two possible products, one is major and
the other is minor due to the steric reasons.
(B) Photo-induced ring-opening: The photo state HOMO of the triene system is lj/4*
which has C2-symmetry. To get C2 -symmetry in the product, moticn should be conrotatory in
the ring opening reaction.
Consider the two modes of conrotatory motions:
Conrotatory
)
(i)
Counter
clockwise
(2Z, 4Z, 6E)·2, 4, 6-0ctatriene
Conrotatory
(ii)
,
)
motion
Clockwise
(2E, 4Z, 6Z)-2, 4, 6-0ctatriene
Clockwise
In this case either possible conrotation giving the same product.
2. Thermal electrocyclic reaction given by (2E, 4Z, 6Z, SE)-2, 4,6, S-decatetraene
The thermally induced electrocyclic reaction of (2E, 4Z, 6Z, 8E)-2, 4, 6, 8-decatetraene
provide elegant examples of electrocyclic reactions.
The starting tetraene forms a cyclooctatriene at room temperature. The tetraene is a 4n
polyene; therefore, conrotatory motion is the expected mode of cyclisation. Indeed the trans­
7, 8-dimethyl-l, 3, 5-cyclooctatriene is the product of this initial cyclisation. When this
cyclooctatriene is heated above room temperature, another electrocyclic ring closure occurs.
However, the cyclooctatriene is a (4n + 2)1t polyene; therefore, this thermally induced
electrocyclic reaction proceeds with disrotatory motion, and a cis ring junction is formed.
,
\
)
Conrotatory
4n,
1t
l
l
system, ground state HOMO is
V. which has C"symmetry.
I
Thus, motion is conrotatory
Disrotatory
motion )
H
1
.
\
(4n + 2)1t system
Ground state HOMO is Va which has m-symmetry.
Therefore, motion will be disrotatory
H
x
y
1
2.4.5 Selection Rules and Microscopic Reversibility
The selection rules in Table 2.1 are based on the orbital symmetry ofthe open-chain conjugated
polyene reactant. However, it is important to understand that these rules refer to the rates of
pericyclic reactions. These rules have nothing to say about the positions of the equilibrium
involved. Thus, the electrocyclic reaction of the diene (1) to give a cyclobutene favours the
diene at equilibrium because of the strain in the cyclobutene.
On the other hand, electrocyclic reaction of conjugated triene (2) favours the cyclic
compound because (J (sigma) bonds are stronger than 1t (Pi) bonds, and because six-membered
rings are relatively stable.
5 ~~CH
HC
3
3
It is also common for a photochemical reaction to favour the less stable isomer of an
equilibrium. For example, in the given reaction, the conjugated diene absorbs UV light, but
the bicyclic compound does not, hence, the photochemical reaction favours the latter.
H
hv
)
Disrotatory
Conjugated
q:>
\
1
:l!
\
\
H
Non-conjugated
tn summary, the selection rules (of Table 2.1) do not indicate which component of the
equilibrium will be favoured-only whether the equilibrium will be established at a reasonable
rate.
The principle of microscopic reversibility (i.e., any reaction and its reverse proceed by
the forward and reverse of the same mechanism) assures us that selection rules apply equally
well to the forward and reverse ofany pericyclic reaction, because the reaction in both directions
must pI:oceed through the same transition state. Hence, an electrocyclic ring-opening must
follow the same selection rules as its reverse, an electrocyclic ring closure. Thus, the thermal
ring-opening reaction of the cyclobutene (3), like the reverse ring closure reaction, must be a
conrotatory process.
~
.
I
."
11,.
'
\
)
Conrotatory
In the following electrocyclic ring opening reaction, the allowed thermal conrotatory
process would give a highly strain molecule containing a trans double bond within a small
ring.
1
,.
Conrotatory
Trans double bond
Although the selection rules suggest that the reaction could occur, it does not because of
the strain in the product (trans cycloalkenes with seven or fewer carbons have never been
observed). Thus, allowed reactions are sometimes prevented from occurring for reasons having
nothing to do with selection rules.
%~::::::::::::::::::::::::::::::::::::::::a:::::,:::
The diagram that shows the correspondence in energy and symmetry between relevant reactant
and product orbitals is called orbital correlation diagram or simply correlation diagram. This
method can be used for any pericyclic reaction, but is usually utilised only for electrocyclic
reactions. The relevant orbitals are those that undergo change during the reaction. Correlation
diagram is based on the fundamental rule of the conservation of orbital symmetry as proposed
by Woodward and Hoffmann. According to this rule orbital symmetry must be conserved
throughout the course of reaction in concerted reactions. Thus, as a concerted reaction proceeds
it must do so with conservation of orbital symmetry. This means that a symmetric orbital in
the starting material must transform into a symmetric orbital in the product and that an
asymmetric orbital must transform into an asymmetric orbital. The orbitals that correlate
(transform into each other) are connected by lines (Fig. 2.9) keeping in view that there is
correlation between orbitals of same symmetry having minimum energy difference. The diagram
is constructed as follows: in separate columns the relevant orbitals of reactant and product
are listed in their order of relative energies; each orbital is classified on the basis of the symmetry
elements retained at all points along the reaction coordinate; lines are drawn between the
reactant and product orbitals connecting the lowest energy orbitals of the same symmetry
type.
The following two conclusions can be drawn by correlation diagram:
(i) Thermal transformation is symmetry allowed reaction when the ground state orbitals
of the reactant correlate with ground state orbitals of the product.
(il) Photochemical transformation is symmetry allowed when first excited state orbitals
of the reactant correlate with first excited state orbitals of the product.
2.5.1 Correlation Diagram of the Electrocyclic Reaction in which Polyene has 4nn Electrons
We are now in a position to consider an electrocyclic reaction in terms of orbital symmetry. Let
us exemplify the above principle by analysing the cyclobutene-butadiene transformation.
1t
U
a
<
)
r
40
PhotO<?hemisiry and PericycliC'Reactiems.
The orbitals that undergo direct changes in cyclobutene are a, 7t and the related
antibonding orbitals, a* and 1t*; these orbitals pass on to the four 7t molecular orbitals of
butadiene, viz., lJII' lJI2' lJIs * and lJI4 *. For correlation diagram, all these orbitals are listed in order
of increasing energy alongwith their mirror plane (m) and C2-symmetric properties (Fig. 2.7).
r
u
~
E
\
I
Butadiene
Cyclobutene
Flg.2.7
Symmetric properties of molecular orbitals of cyclobutene and ?utadiene
Fig. 2.7 is shown in more simplified fashion as Fig. 2.8.
I
~
l
1
I
.•
I
r
,"'­
I'
\'
\
E
-1t; m(S), Cz(A)
Cyclobutene
Butadiene .
Fig. 2.8 Symmetric properties of molecular orbitals of cyclobutene and butadiene
Electrocycllc Reactions
41
We know that for bonding overlap (i.e., symmetry allowed reaction) disrotatory mode of
rotation is needed for orbitals having m-symmetry and conrotatory mode of rotation for
C2 -symmetry.
0
Disrotatory
(
)
m-Symmetry
~
0*; m(A)
r
- - '1'4*; m(A)
1t*; m(A)
-
'1'3*; m(S)
-
'1'2; m(A)
____ -
'1'1; m(S)
E
n; m(S)
o'~ m(S)
Fig.·2.9
Correlation diagram for disrotatory interconversion of cyclobutene-butadiene system
Let us analyse whether disrotatory mode ring-opening of cyclobutene to butadiene is
thermal allowed or photochemical allowed process in which m-symmetry is maintained
throughout the course of the reaction.
Inspection of correlation diagram (Fig. 2.9) shows that this process is thermally forbidden
because n in cyclobutene containing two electrons would pass to the antibonding orbital (\lf 3*)
of butadiene. In the correlation diagram (Fig. 2.9) the ground state cr orbital of cyclobutene
,:orrelate with the gi-ound state \If! orbital of butadiene. The ground state n orbital ofcyclobutene
does not correlate with the ground state 'V2 orbitals of butadiene. Instead it correlates with
\lf3 * which is an excited state and antibonding orbital.
Correlation diagram shows that the first excited state of the cyclobutene 0'2, 7t, n*
correlates with the first excited state of butadiene \If I, \lf2\1f3*' Thus, disrotatory process in
either direction is photochemically allowed. In the first excited state of cyclobutene, n* contains
an electron and it is transformed into bonding orbital ('V2) in butadiene.
cr2n1t*
First
excited
state
2
*
\If 1\lf2'lI3
First
excited
state
hv
f--
\lff\lf~
Ground
state
42
Photochemistry and· Pelicycllc .Reactions
Thus, following conclusions can be drawn from this correlation diagram:
(J21t2
-------4
upper excited
ground
state
\jJr\jJ~
WrWr
state
------}
ground state
(J2 n *2
Disrotatory thermal conver­
sion of cyclobutene to
butadiene is forbidden.
n in cyclobutene containing two
Disrotatory thermal conver­
sion of butene to butadiene
is forbidden.
"'2 of butadiene containing two
Disrotatory photochemical
coversion in either direction
is allowed.
electrons in the ground state
would pass to antibonding orbital
("'3*) of butadiene. An energy
barrier would have to be
overcome.
electrons in the ground state
would pass to antibonding orbital
(n*) of cyclobutene. An energy
barrier would have to be
overcome.
!
!
'<
First excited state of cyclobutene
(n*) containing an electron is
transformed into bonding orbital
('l12) of butadiene. Similarly, first
excited state of butadiene (\jJ3*)
containing an electron trans­
formed into bonding orbital (n) of
cyclobutene.
Now consider the conrotatory conversion of cyclobutene to butadiene in which a
C2-symmetry is maintained (Fig. 2.10).
Conrotatory .
(
)
,
i
't
."
r
E
Fig. 2.10 Correlation diagram for canrotatory interconversion of cyclobutene-butadiene system
Electrocyclic Reactions
43
The orbitals now correlate in such a way that the ground state of cyclobutene a 2n2
correlates with the ground state of butadiene \jff\jf:F- The thermal conrotatory process is thus
allowed in either direction.
2 2
an
Ground state
2
2
\jf1\jf2
Ground state
Inspection of correlation diagram shows that this process is photochemically forbidden.
The first excited state of cyclobutene (a 2nn*) correlates with the upper excited state (\jf;\jf2\jf/)
of butadiene thus making it a high energy symmetry-forbidden process. Similarly, the first
excited state of butadiene (\jff\jf2\jfa *) correlates with a high energy upper excited state (a 2 na*)
of cyclobutene. In other words, a photochemical conrotatory process in either direction is
symmetry-forbidden.
Thus, it becomes clear from the correlation diagrams that thermal opening of the
cyclobutene proceeds in a conrotatory process while photochemical interconversion involves a
disrotatory mode. These generalisations are true for all systems containing 4nn electrons.
2.5.2 Correlation Diagram of Electrocyclic Reaction in which Polyene has (4n + 2)n Electrons
A typical system of this category is the interconversion of 1, 3-cyclohexadiene and 1, 3, 5­
hexatriene.
In this transformation six molecular orbitals (\jf1' \jf2' \jfa' \jf/, \jf5 * and \jf6 *) ofhexatriene
and six molecular orbitals (four n molecular orbitals n I , n2 , n a* and n 4* and two a molecular
orbitals) of cyclohexadiene are actually involved and, therefore, need to be considered.
Consider the conrotatory conversion of cyclohexadiene to hexatriene in which a
C2-symmetry is maintained (Fig. 2.11).
-><­
--
1
--
/' -
'Jf4*;
C2 (S)
-~-----------------------
E
-><--
Cyclohexadiene
--
Hexatriene
Fig. 2.11 Correlation diagram for conrotatory interconversion
of cyclohexadiene-hexatriene system
44
Photochemistry and Pericycllc Reactions
(i) Inspection of the correlation diagram shows that this process is thermally forbidden
because ground state orbitals of the reactant does not correlate with the ground state orbitals
of the product.
cr2n 2n 2
=:.,
'Ir 2",2'lr 2
'"
'1'1'1'2'1'3
1 2
99i
In the correlation diagram, the ground state cr and nl' orbitals ofcyclohexadiene correlate
with the ground state 'JI1 and 'JI2 orbitals of hexatriene. The ground state orbitaln2 of
cyclohexadiene does not correlate with the ground state orbitals 'lis of hexatriene. Instead it
correlates with 'JI4 * which is an excited state and antibonding.
cr2n 12
2n 2 ~ 1Ir 211r 2", *
'1'1'1'2'1'4
Thus, in thermal condition n(n2) in cyclohexadiene containing two electrons would pass
to the antibonding orbital ('lI 4*) of hexatriene. An energy barrier would have to be overcome in
this process and thus process is forbidden.
(ii) Correlation diagram shows that the first excited state of cyclohexadiene cr2n~n2n3*
correlates with the first excited state ofhexatriene 'JIf'JI~'JI3'l1/. Thus, conrotatory process in
either direction is photochemically allowed.
0"
2
n121t22
Ground
state
hv
2
~
2
0" 1t 1 1t21t 3
*
~
First excited
state
2
2
'lI1 'lI2 'lI3'l14
*
First excited
state
~
2
2
2
'JI1'J12'l13
Ground state
I
1
1
In this case first excited state ofcyclohexadiene 1t3* containing an electron is transformed
into bonding orbital 'lI3 ofhexatriene. Similarly, first excited state ofhexatriene 'lI4 * containing
an electron also transformed into bonding orbital1t2 of cyclohexadiene. Thus, process is allowed
process.
0"21t21~1t3
*
-,----------+
+----------'-
2 2
'lI1 'lI2 'lI3'lI4
r
!
*
Now consider the disrotatory conversion of cyclohexadiene to hexatriene in which
m-symmetry is maintained (Fig. 2.12).
.
~
,I
0*; m(A)
x4*; m(A)
1
x/; m(S)
-><­
--
--
"\
'l'/; m(A)
j
•
E
'l'3; m(S)
_____ -
1
'l'1; m(S)
cr; m(S)
Fig. 2.12 Correlation diagram for disrotatory interconversion
of cyclohexadiene-hexatriene system
Electrocyclic Reactions
45
(i) Inspection of the correlation diagram shows that the orbitals correlate in such a way
that the ground state of cyclohexadiene cr 2n{nf correlates with the ground state of hexatriene
'II{'II:'II;. The thermal disrotatory process is thus allowed in either direction.
cr2n12n22
~
~
~
11(211(211(2
't'1't'2't'3
Inspection of the correlation diagram shows that this process is photochemically
forbidden. The first excited state of cyclohexadiene n3 * correlates with the upper excited state
'115 * of hexatriene. Similarly, the first excited state of hexatriene '114* correlates with a high
energy upper excited state of cyclohexadiene. An energy barrier would have to be overcome in
this process and thus process is forbidden.
Thus, it becomes clear from the above considerations that thermal opening of the
cyclohexa~ieneproceeds in a disrotatory process while photochemical interconversion involves
a conrotatory mode. There generalisations are true for all conjugated systems containing
(4n + 2)n electrons.
Thus Woodward-Hoffmann rules for electrocyclic reactions on the basis of correlation
diagrams may be summed up as in Table 2.3.
(ii)
Table 2.3 Selection rules for electrocyclic reactions
4n
4n + 2
Conrotatory
Disrotatory
Disrotatory
Conrotatory
The above predictions are in accord with experimental results. Thermal isomerisation
of cis-3, 4-dimethylcyclobutene gives (2Z, 4E)-2, 4-hexadiene and cis-bicyclo [6, 2, 0] deca-2, 9­
diene gives (lE, 3Z, 5Z)-1, 3, 5-cyclodecatriene.
)
Conrotatory
(2Z, 4'i:)-2, 4-Hexadiene
H
():)
(IE, 3Z, 5Z)-1, 3, 5-Cyclodecatriene
Conrotatory
1
Ll
46
Photochemistry and Pericyclic Reactions
Photochemical isomerisation of (2Z, 4Z, 6Z)-2, 4, 6-cyclooctatrione gives (2E, 4Z, 6Z)­
2,4, 6-cyclooctatrienone, which cyclises thermally in a conrotatory process to cis-bicyclo [4, 2, 0]
octa-3, 6-dien-2-one.
a
b
~
H
_
MI&t"aii;
'''1
····.·.,"'c,:_C• • •t'Mlflm'Wee'_.r:.''fIr'W'''W''m• •
"Wi'"
Fortunately, all the conclusions that can be drawn laboriously from correlation diagrams can
be drawn more easily from a pair of rules, known as the Woodward-Hoffmann rules. Rules
distil the essence of the idea into two statements governing all pericyclic reactions, one rule of
thermal reactions and its opposite for photochemical reactions. Correlation diagrams explain
why they work, but we no longer depend upon constructing such diagrams.
2.6.1
"
1
Woodward-Hoffmann Rule for Electrocyclic Thermal Reactions
A thermal (ground state) electrocyclic reactions is symmetry allowed when the total number
of (4q + 2)8 and (4r)a components is odd.
2.6.2 Photochemical Electrocyclic Reactions
An electrocyclic reaction in the first electronically excited state is symmetry allowed when the
total number of (4q + 2) a and (4r)8 is odd.
These rules need some explanation.
Component
A component is a bond or orbital taking part in a pericyclic reaction as a single unit. A double
bond is a rr;2 component. The number two is the most important part of this designation and
simply refers to the number of electrons. The prefix rr; tells us the type of electrons. A component
may have a~ number of electrons, for example 1, 3-butadiene is a rr;4 component. Component
may not have mixtures of rr; and cr electrons. Component either contains only cr electrons or
contains only rr; electrons. Designations (4q + 2) and (4r) simply refer to the number of electrons
in the component where q and r are integers (0, 1, 2, 3, 4, ...., n). An alkene is a rr;2 component
and so it is of the (4q + 2) kind where q = 0 while diene is a rri component and so it is of the (4r)
kind where r = l.
Suffix s and a
In electrocyclic reaction 8 means when upper (or lower) lobe of one frontier orbital overlaps
with upper (or lower) lobe of other frontier orbital and a means when upper lobe of one orbital
overlaps with lower lobe of other.
i
I
1
Let us start with hexatriene ring closure. As a preliminary, we would just note that
hexatriene is, of course, a 6n electrons (n 6 ) conjugated system and, on forming cyclohexadiene,
the end two orbitals have to form a cr bond.
So, now for the Woodward-Hoffmann treatment:
1. Draw the mechanism for the reaction
~
-----?
0
< - - Nowob'nd;,f,""ed
2. Choose the components. All the bonds taking part in the mechanism must be included
and no others.
3. Make the three-dimensional drawing of the way the components come together for
the reaction putting in orbitals at the ends of the components.
These orbitals are simple p-orbitals and do not make up HOMOs or LUMOs or any
particular MOs. Do not attempt to mix frontier orbitals and Woodward-Hoffmann
description of correlation of pericyclic reactions.
4. Join up the component(s) where new bond(s) are to be formed. Make sure you join
orbitals that are going to form new bonds.
In this case formation of new cr bond takes place by two possible cases:
Case I: When component is s. In this case overlapping will be possible if motion is
disrotatory.
Disrotatory
motion
)
o
Notice that we call the component's' because the upper lobes of the two p-orbitals were
joining together. If upper lobe of one orbital and lower lobe of another orbital is joined together
then component will be 'a'.
Number of (4q + 2) s component
=1
Number of (4r) a component
=0
Total = 1 (odd)
¥
48
Photochemistry and Pericyclic Reactions
Thermal: Allowed
Photochemical: Forbidden
Case II: When component is 'a'. In this case overlapping will be possible if motion is
conrotatory.
o
Conrotatory
)
Number of(4q + 2)a = 1
Number of (4r) s = 0
Total =1 (odd)
Thermal: Forbidden
Photochemical: Allowed
Similarly, take the example of conversion of cyclobutene to 1, 3-butadiene.
l
r
But for the use of Woodward-Hoffmann rule always consider the process in which open
system converts into cyclic system.
1. Draw the mechanism for the reaction.
/
\
2. Choose the components. All the bonds taking part in the mechanism must be included
and no others.
\
[
1
\
"I
1
n4
3. 'Make the three-dimensional drawing of the way in which the component comes
together for the reaction, putting the orbitals at the ends of the component.
n
,{
1
1
1\
1
•
I
49
4. Join up the component(s) where new bond(s) are to be formed.
5. Show bond formation by conrotatory as well as disrotatory motion. Label each
component s or a.
~
II
(i)
,;;:::­
Disrotatory
11111111111111111111111111
)
Number of (4q + 2) s component = 0
Number of (4r) a component = 0
Total =0
Thermally: Disallowed
Number of(4q + 2) a component = 0
Number of (4r) s component = 1
Total = 1 (odd)
Photochemically: Allowed
?
~
Conrotatory
(ii)
)
Number of (4r)a = 1
Number of(4q + 2) s = 0
Total
= 1 (odd)
Thermally: Allowed
Thus, we can conclude the following results from Woodward-Hoffmann rule for
electrocyclic reactions:
Woodward-Hoffmann Rules
Based on number of
electrons in TS
~
Thermal
~
(4n)
Conrotatory
Disrotatory
I
I
~
Photochemical
~
(4n + 2)
~
Thermal
~
(4n+ 2)
l
Photochemical
~
(4n)
r
•
_nmfrrm_:_r'ffrfrrr_=
Another method for quickly assessing whether a given pericyclic process is allowed is to examine
the cyclic array of orbitals at the transition state of the pericyclic reaction. This method was
popularised by H. Zimmerman and M.J.S. Dewar.
Huckel rule ofaromaticity states that a monocyclic planar conjugated system is aromatic
if it has (4n + 2) 1t conjugated or delocalised electrons and consequently stable in ground state.
Similarly, monocyclic planar conjugated system is anti-aromatic if it has (4n) 1t conjugated or
delocalised electrons. This system is unstable in ground state. However, further calculation
shows that these rules are reversed by the presence of a node in the array of atomic orbitals.
Thus, system with (4n + 2) 1t electrons and a node is antiaromatic while system with (4n) 1t
electrons and a node is aromatic.
i
Thus, system has no node then:
(4n +
j
2m electrons ---7 aromatic ---7 stable in ground state.
I
(4n)1t electrons ---7 antiaromatic ---7 unstable in ground state.
1
Similarly, system having a node then
(4n) 1t electron ---7 aromatic ---7 stable in ground state.
(4n + 2) 1t electrons ~ antiaromatic ---7 unstable in ground state.
If system has no node then it is called Huckel system and array is called Huckel array.
Similarly, if system has node then it is called Mobius system and array is called Mobius array.
Application ofthese rules to pericyclic reactions led to the generalisation that thermal reactions
take place via aromatic transition state [i.e., (4n + 2) 1t electrons having no node or (4n) 1t
electrons having one node] whereas photochemical reactions proceed via antiaromatic transition
state [i.e., (4n)1t electrons having no node or (4n + 2) 1t electrons having one nodel.
A cyclic transition state is said to be aromatic c;'isoconjugated with the corresponding
aromatic system if the number of the conjugated atoms and that of the 1t (Pi) electrons involved
are the same as in the corresponding aromatic system. Similarly, a cyclic transition state is
said to be antiaromatic or isoconjugated with the corresponding antiaromatic system if the
number of conjugated atoms and that of the 1t (Pi) electrons involved are the same as in the
corresponding antiaromatic system. We have only to consider a cyclic array of atomic orbitals
representing those orbitals which undergo change in the transition state and assign signs to
the wave function in the best manner for overlap. Then the number of nodes in the array and
number of electrons involved are counted.
Let us consider the following electrocyclic reaction (Fig. 2.13 and Fig. 2.14).
Cis-I, 3, 5-hexatriene
~
Cyclohexadiene
I
lI
I
\
~\QA
).;2/).;2/
~
d
-o.g
o:l.O
~
o
J1
S
i .g J1
§
-0 .
Positive
lobe interacts with
•
negatIve lobe, hence
there will be a node
Q
.~ S
c:l
.
Positive lobe will interact with
positive lobe, hence there
will be no node
r
Node in TS
61t electrons, one node, antiaromatic Mobius
system hence photochemically allowed
No node in TS
61t electrons, zero node, aromatic,
Hiickel system hence thermally allowed
Fig. 2.13 Array diagram for (4n + 2)n system
Similarly for
1, 3-butadiene
~
Cyclobutene
~
i-o".g
"-V
§ rpOSitive lobe interacts with
negative lobe, hence
there will be a node
§S
Q
,
.
~
j
~
r Positive lobe interacts
with positive lobe,
~ ~
hence there will be
.~
no node
~
.§
Q
;;:,
-...;;::~~
"$~t~~ 1Il1lllt.
Node u;. TS
4lt electrons, one node, aromatic
Mobius system thermally allowed.
Fig~
. 4lt electrons, zero nodeantiaro­
matie, Huckel system photochemi­
cally allowed.
2.14 Array diagram for (4n)n system
52
Photochemistry and Pe.ricyclic Reaetions
Thus, for the thermal reactions involving (4n + 2)1t electrons will be disrotatory and
involved Huckel type transition whereas those having (4n) 1t electrons will be conrotatory and
the orbital array will be of the Mobius type. Similarly, for photochemical reactions involving
(4n + 2) 7t electrons will be conrotatory and involved Mobius type transition whereas those
involving (4n) 7t eleCtrons will be disrotatory and the orbital array will be Huckel type.
Thus for convenience, the selection rules by this approach to electrocyclic reactions are
given in Table 2.4.
Table 2.4 Selection rules for electrocyclic reactions by H.M. method
I
4n
4n
(4n + 2)
(4n + 2)
.
zero
one
zero
one
antiaromatic
aromatic
aromatic
antiaromatic
GLOSSARY
disrotatory
conrotatory
disrotatory
-
~
conrotatory
I
.
Conrotatory motion: Rotation of bonds in the same direction, i.e., bothclockwise . Qt:.~~th
counterclock wise.
Disrotatory motion: Rotation of bonds in opposite directions, i.e., one clockwi~e
counterclock wise.
Electrocyclic .-eaction: Conversion of a conjugated polyene into an is(Jfm{~ri(~·.~f'tfjic.~lpD'~~tCil.::
The bond responsible for ring formation occurs between the two ends
of the starting material.
Photochemically allowed reaction: A reaction that will be concerted and Wii~1!j~o~~1~1~·
comparatively low energy of activation when initiated by light.
Symmetry allowed reaction: A reaction that Wiill occur by a concerted pr~lCel!~
involve radical or ionic intermediate. A symmetry-allowed process may beact:el~lra1t~;~j~~by
heat or by light.
Symmetry-forbidden reaction: A reaction that cannot occur by concerted process tmdera given.
set of conditions. Reactions that are "symmetry·forbidden'"do occur. However,when'theytdo
occur they are not concerted but, instead, involve classiqal intermediates (often. radie.)" They
usually have high energy of activation.
Thermally allowed reaction: A reaction that will be concerted and will possessacomPatativel$"
low energy of activation when initiated by heat.
Orbital correlation diagram: A diagram that shows the correspondence in en.
d~~metry
between relevant reactant and product orbitals in a pericyclicreaction. The...... )~tot~~a1s,
frequently called the basic set of orbitals, are those that undergo chaqge dl.lJ'ing th~teaetion.
Frontier-orbital approach: A methodology for qUicklypredicti~g whether ~giyextpeti<;yelic
reaction is allowed by examining the symmetry of HOMOl\Uld,<ifthereacti(t~;isbimf.l~ar,
the LUMO of the second partner.
.
The Woodward-Hoffmann rule: A series of generalised symmetry selection rules eiabq~~ by
R.B. Woodward and R. Hoffmann, based on arguments sueh as correlation diagra_, which
l
·1\)
l
t
Electrocyclic Reactions
53
'n be allowed under a given set of conditions. In its
rieyc1ic reaction is ground state allowed (excited
cial (4q + 2) and antarafacial (4r) reacting
r are any integer including zero). Note that the
prafacial (4r} c;omponents are not counted.
ic Rei:tiC:tio1ls, Academic Press, New York, 1980.
ThetJonseruation of Orbital Symmetry, Academic Press,
.J. Sunberg, Advanced Organic Chemistry, Part A and Part B, Plenum
Disrotatory
(
~
o
tructed, predict whether these transformations are allowed
o you arrive at the same conclusions using PMO method?
the fonowing electrocyelie transformation.
modesaDd.tell which one is preferred and
Conrotatory
)
H
CRll
::.. Sketch a suitable mechanistic scheme for the fo&wing transformations:
H
H
~
~.
H
1
(n0')
.~·H~
. CJfs
.
H
C
1
J
19.
1
I
------­
---------[',..;
I
51
21.
22.
....
3
CHAPTER
clcIition Re
~::=::::::::;::::::::::::'::,:;:::::;:=:':::;:=::::::::::::::::
A cycloaddition is a reaction in which two unsaturated molecules undergo an addition reaction
to yield a cyclic product. Formation of cyclic product takes place at the expense of one 1t (Pi)
bond in each of the reacting partner and gain of two 0- (sigma) bonds at the end of the both
components having 1t (pi) bonds. Thus, in this reaction there is loss of two 1t (Pi) bonds of the
reactants and gain of two 0- (sigma) bonds in the product.
181
hv
)
D
l
...(1)
Loss of two 7t bonds and
gain of two cr bonds
...(2)
Loss of two 1t bonds and gain of two cr bonds. Formation of
cr bonds at the end of the two components.
The cycloaddition reactions are classified with respect to three facts of the reaction:
(i) The number of electrons of each unit participating in cycloaddition.
(ii) The nature of orbitals undergoing change (1t or 0-).
(iii) The stereoche~ical mode of cycloaddition (supra, syn or antara, anti).
The reaction in equation (1) is a [2 + 2] cycloaddition reaction because the reaction
involves two el(')ctrons from one reacting component and also two electrons from the other.
The reaction in equation (2) is a [4 + 2] cycloaddition.
The stereochemical mode is given by a subscript s or a which indicates whether the
addition occurs in a supra or antara mode on each unit. A cycloaddition may in principle occur
either across the same face or across the opposite faces of the planes in each reacting component.
If reaction occurs across the same face of a 1t system, the reaction is said to be suprafacial with
respect to that 7t system. The suprafacial addition is nothing more than a syn addition.
58
l
Both lobes are above the
plane of the molecule or
both lobes are in same
plane, hence suprafacial
Both lobes are below the plane of
the molecule or both lobes are in
same plane, hence suprafacial
This addition reaction is thus [48 + 28] cyc1o~ddition reaction.
This can also be represented as follows:
~
t Supra t
)!~J(
Attack on
o
•
the same face of
the molecule
[48 + 28]
Supra
If the reaction bridges opposite faces of a 1t system, it is said to be antarafacial. An
antarafacial addition is just an anti addition.
Antarafacial hence 4a
.......
Suprafacial hence 28
This mode of addition reaction is thus [4(1 + 28] cycloaddition reaction. This can be
Lopresented as:
.
..
o
[4a
+ 28]
Antara
-
-~
H~\\\\\\\
~==-_____
or
t~
+
_t
t­
Antara
"'"
[4a + 2a] Cycloaddition reaction
In antarafacial, attack takes place with one bond forming to one surface but other bond
forming to other surface. It is rare, it does not occur in any reaction. Almost all cycloaddition
reactions are suprafacial on both components.
XJ::eIlItAt¢i
::::::::::::::
In order, for a cycloaddition to occur, there must be bonding overlap between p-orbitals at the
terminal carbons of each n-electron system, this is where the new cr bonds are formed. Let us
explain this point with a [4 + 2] cycloaddition. Let us suppose that diene (4n component)
behaves as electron donor and the dienophile (2n component) as the electron acceptor (or
vice-versa). What electrons will the 4n component donate? Obviously there will be its valence
electrons-the electrons in its HOMO ('lf2)' The 2n component will accept these electrons to
form the new bonds. The molecular orbital used to accept these electrons must be empty since
molecular orbital cannot contain more than two electrons. Therefore, the molecular orbital
which accepts electrons should be LUMO of the 211: component. Thus, one component uses its
HOMO and the other component uses its LUMO for overlapping. Simultaneously with the
merging of the n orbitals, these orbitals also undergo hybridisation to yield the new sp3 cr
bonds.
~
61
3.2.1
[2 +2] Cycloaddition Reactions
(A) Thermal Induced [2 + 2] Cycloaddition Reactions
Thermal induced [2 + 2] cycloaddition reactions are symmetry forbidden reactions.
When ethylene is heated, its 1t electrons are not promoted, but remain in the ground
state '111' If we examine the phase of the ground state HOMO of one ethylene molecule and the
LUMO of another ethylene molecule we can see why cyclisation does not occur by the thermal
induction.
U_~"LU/? 0_ ~"LUMO
08/~
H*~'HOMO H*~'HOMO
H
A
One molecule
~
Autibouding
Second molecule
LUMO of ono mol"ul'. C,-,ymm,t",
-+-
Bonding
H HOMO
of oth" mo!"ol'. m-Symm,t",
Phase wrong for overlap, symmetry forbidden.
For bonding to occur, the phase of the overlapping orbitals must be same. This is not
the case for the ground state HOMO and LUMO of two ethylene molecule or any other [2 + 2]
system. Because the phase ofthe orbitals are incorrect for bonding, a thermally induced [2 + 2]
cycloaddition is said to a symmetry forbidden reaction.
(B) Photo-Induced [2 + 2] Cycloaddition Reactions
When ethylene is irradiated with photon of UV light, a 1t electron is promoted from '111 to '112'" "
orbital in some,but not all, of the molecules. The result is a mixture of ground state and .
excited state ethylene molecules.
Thus photo-induced cycloaddition takes place between photochemical HOMO of one
molecule and ground state LUMO of other molecule.
Excited state HOMO or
photochemical HOMO
HH
LUMO, 'fI2 *
-
-
+
-
U
80#
HOMO,,,,,
+
G~
hv
---+
U
~#
One molecule of ethylene
U
ef(
'-
G;1;?­
U
80+
Another molecule
Ground state LUMO, C2 -symmetry
oIf:~:::~:g.Ymm_
n
Symmetry allowed,reactIOns
From the example it is clear that for cycloaddition reaction both HOMO and LUMO
should have same symmetry otherwise reaction will be symmetry forbidden. For symmetry
allowed reaction if HOMO has m-symmetry then LUMO should also have m-symmetry.
Similarly if HOMO has Cz-symmetry then LUMO should also have Cz-symmetry.
Stereochemistry
Stereochemical integrity is maintained in cycloaddition reaction because the reaction is a
concerted reaction,
H?C=C<:'
P~
.of;
CHa,........ CHa
'C-C""""
"CH a
CH a/ ' -
Opposite plane
Ph
H 1111""..... 1----(.
~,
CFf·"
-3
CHi
Opposite plane
H
H
Ca-
Phllll"".....
+
~, CH
a
CHa
'.
HIlIlIIIII ....
Ph
.
CH
a
1
+
Ph
H
H 1II1'"""··!----{
Ph 1II111""···!-----(
+
H
IIIIIII''' •.
CN 1I11111""··}-----(:
·r----(
H
CN
3.2.2 [4 + 2] Cycloaddition Reactions
Diels-Alder reaction is the best known [4 + 2] cycloaddition reaction. This reaction is a thermally
allowed reaction. Diels-Alder reaction is photochemically forbidden. Since Diels-Alder reaction
is the most common [4 + 2] cycloaddition reaction, let us first discuss the general description
of this reaction.
Diels-Alder reactions occur between a conjugated diene and an alkene (or alkyne),
usually called the dienophile.
(A) The Dienes
The diene of the Diels-Alder reaction is electron rich, while the dienophile is electron poor.
Some Diels-Alder reactions with electron-poor dienes and electron-rich dienophiles are also
known, but these are relatively rare. Simple dienes such as' 1, '~diene are sufficiently
electron-rich to be effective dienes for Diels-Alder reaction. The pree~nce'ofelectron releasing
groups such as alkyl groups, phenyl groups or alkoxy groups may further enhance the reactivity
of dienes.
The diene component of the Diels-Alder reaction can be open-chain or cyclic but it must
have s-cis conformation. Butadiene normally prefers the s-trans conformation with the two
double bonds as far away from each other as possible for steric reasons. The barrier to rotation
about the central cr bond is small (about 30 kJ/mole at 25°C) and rotation to the less favourable
but reactive s-cis conformation is rapid.
\
....
Favoured but cannot
give Diels-Alder reaction
(
Disfavoured but can
give Diels-Alder reaction
Cyclic dienes that are permanently in the s-cis conformation are exceptionally good for
Diels-Alder reaction. On the other hand, cyclic dienes that are permanently in the s-trans
conformation and cannot adopt the s-cis conformation will not give the Diels-Alder reaction at
all. (Why?) If the diene is in the s-trans conformation, the dienophile cannot 'reach' both ends
of the diene at the same time.
s-cis Conformation,
a requirement for the
Diels-Alder reaction
'.
s-trans Conformation
This explains why dienes such as those given below will not serve as dienes in the Diels­
Alder reaction.
co
(B) The Dienophile
The most common dienophiles are the electron-poor alkenes and alkynes. Since electron-poor
alkenes and alkynes are prone to react with a diene, these are called dienophiles (lover of
dienes). Thus the simple alkenes and alkynes such as ethylene and acetylene are not good
dienophiles. A good dienophile generally has one or more electron-withdrawing groups pulling
electron density away from the 1t bond.
1
Dienophiles that do undergo the Diels-!\lder reaction include conjugated carbonyl
compounds, nitro compounds, nitriles, sulphones, arylalkenes, arylalkynes, vinyl ethers, vinyl
esters, haloalkenes and dienes.
.
, ,··:,t
~',
.
.
~CN'
ryR
, .. C
~ O
I #
and">'''''
Some dienophiles for the Diels-Alder reaction
o
'1
!
1
.I
I~
1
(C) Mechanism of Diels-Alder Reaction
The mechanism of the Diels-Alder reaction is a simultaneous cyclic movement of six electrons:
four in the diene and two in the dienophile. The simple. representation of the mechanism
shown below is fairly accurate. This is called a concerted reaction because all the bond making
and bond breaking occurs simultaneously. For these three pairs of electrons to move
simultaneously, however, the transition state must have a geometry that allows overlap of the
two end p-orbitals of the diene with those of the dienophile (Fig. 3.1).
~(
X
B
Y
y
Fig. 3.1 Transition state
There are three major stereochemical features of the Diels-Alder l:eaction that are
controlled by the requirements of the T.S.
(i) s-cis conformation of the diene: This mechanism explains why the diene should
be in s-cis conformation.
Diels-Alder reaction is a one step reaction that proceeds through a cyclic transition
state. Bond formation occurs at both ends of the diene system, and the Diels-Alder transition
state involves a cyclic array of six carbons and six 1t electrons. For this the diene adopts the
s-cis conformation in the transition state.
1
,
Diene
H
1i
Dienophile
Overlap of
p-orbitals
':
Transition state
Adduct
(J
bond of two sp' orbitals
When the diene is in the s-trans conformation, the end p-orbitals· are too far apart to
overlap with the p-orbitals of the dienophile.
Structural feature that aid or hinder the diene in achieving the s-cis conformation affect
its ability to participate in the Diels-Alder reaction. Thus the diene with functional groups
that hinder the s-cis conformation react slower than butadiene. Dienes with functional groups
that hinder the s-trans conformation react faster than butadiene.
H
H
H
H
Much slower than butadiene
Similar to butadiene
o
Faster than butadiene
Much faster than butadiene
(iO syn stereochemistry; The Diels-Alder reaction is a syn addition with respect to both
the diene and dienophile. The dienophile adds to one face of the diene, and the diene adds to
one face of the dienophile. As you can see from the transition state in Fig. 3.1, there is no
opportunity for any of the substituents that are on the same side of the diene or dienophile
will be cis on the newly formed ring. Thus, the stereochemistry of the diene and dienophile are
preserved during the adduct formation.
(a) Stereochemistry of the adduct due to stereochemistry of the dienophile
The Diels-Alder reaction is stereospecific. Substituents that are cis in the dienophile remain
cis in the product; substituents that are trans in the dienophile remain trans in the product.
H
etCOOCH'
~
COOCHg
H
cis
cis
rrCOOCH,
H
(
~H
==
COOCH g
trans
trans
I
I
i
o
H.C~
+
o
H
H
H
(b)
0
Stereochemistry of the product due to the stereochemistry in the diene
I
I
l
I
i
I
The product stereochemistry is controlled by the geometry of the diene.
This is slightly more complicated as the diene can be cis, cis; trans, trans or cis, trans.
Terminal vinylic cat-bons (1 and 4 in butadiene) ofthe s-cis diene ,contain two substituents.
One is designated as outside group and other is designated as inside group of the diene. For
example,
Jc;:It
~ OutsiIOde "d'oup (hYdrohgendi)Ofthe die..
, R ~ nSI e group 0 f t e
~
ene
R ~ Inside group of the diene
H
cis-cis Diene
¢:~
H
R
~
Outside group of the diene
~
In.ide .ronp of the diene
Outside group of the diene
cis-trans Diene
¢:~
H
Inside gronp of the diene
R ~ Outside group of the diene
o:is-trans Diene
Outside groups at C-1 and C-4 are always trans to each other. Similarly, two iI)Side
groups at C-1 and C-4 are ds to each other.
.As mentioned earlier that the Diers-Alder reaction is stereospecific. Accordiug to the
structure of the transition state of the Diels-Alder reaction the outside substitr¢nts of the
diene are always below the plane of the new six-membered ring of the adduct.
/
Thus the inside substituents of the diene should always be above the pf~e of the new
six-membered ring ·of the adduct.
'
I
!
I
COOCHa
¢=
+
H
cis':.~diene
Q+
H
cis-cis diene
¢
Ph H
~
+
R
COOCHa
COOCH a
COOCHa
1
III
~
COOCH s
I
COOCH a
I
~
COOCHa
COOCHa
~Ph
COOCH s
I
H R
cis-trans diene
s
COOCHa
Ph
H
I
COOCH
"'I
trans-trans diene
~
~
COOCH s
H
¢
I
III
+
"'
I
COOCH
~
s
COOCHs
COOCH a
ri
j
I
I
./
I
We also need to consider the stereochemistry in the product of the substituents on the
diene relative to those on the dienophile. If trans 1, 3-pentadiene reacts with methyl propenoate
we know that in the product the methyl and ester groups could be either cis or trans. It is
found, in fact, that the cis product is formed from trans diene and the trans product is formed
from cis diene.
trans
CHa
Geometryt
~H
+
~
cis
Geometry
t
cis
H
V
CH
CHCOOCHa
a+
~
II
CH2
H
~
....",,'/ COOCH
a
trans
(iii) Endo and &0 stereochemistry: The Diels-Alder reaction will produce a bicyclic
ring system if the diene is monocyclic. Cyclic dienes are particularly reactive in Diels-Alder
reaction because the two double bonds are held in an s-cis conformation in five or six-membered
rings. This can be seen in the reaction of 1, 3-cyclopentadiene with methyl propenoate.
o
+
CH-COOCH3
II
CH2
~COOCH,
H
III
H
U~OOCH,
H
When a compound like this is synthesised, we introduce another stereochemical question.
What will be the relative stereochemistry of the methylene bridge and the substituent (in this
case COOCH3)? Will they be cis or trans to one another? Asking these questions is the same as
asking whether the substituent will be endo or exo in the product. An orientation is said to be
endo meaning that the substituent is trans to the bridge carbon or substituent projects into
I
I
f'
the cavity on the concave side of the bicyclic ring. Similarly, an orientation is said to be exo
meaning that the substituent is cis to the bridge carbon or substituent projects out of the
cavity on the concave side of the bicyclic ring.
Cavity on the --~
concave side
Concave side of
the bicyclic ring
Both are trans to each other.
Thus orientation is endo.
H )
COOCHa
t
This substituent is pointing towards cavity.
Thus orientation is endo.
Both are cis to each other.
. Jfus ori,ntation is exo.
fOOCH 3
H
"
This substituent is pointing
away from the cavity.
Thus orientation is exo.
The exo product is expected to be more stable than the endo product for sterle reasons.
In exo product, the exo substituent points away from the more congested part, i.e., bicyclic ring
but in endo product the endo substituent points towards the more congested part. Thus sterle
repulsion in endo is more than in exo. Due to this reason, exo is expected to be more stable
than endo.
But in practice it has been found that if the diene has a 1t bond in its electron-withdrawing
group then the endo product is more stable than the expected exo product. This stereochemical
preference for the endo position is often called the endo rule. The reaction of 1, 3-butadiene
with methyl propenoate produces three times as much as endo as exo product. Endo product is
major product due to the secondary interactions.
o
+
j
"!
+
H
86%
(endo)
COOCHg
COOCH g
24%
(exo)
H
I
1
]
(D) Orientation Effects in Diels-Alder Reaction
Even when the diene and dienophile are both unsymmetrically substituted, the Diels­
Alder product is usually a single isomer rather than a mixture. The product is the isomer that
results from orienting the diene and dienophile so that we can imagine a hypothetical reaction
intermediate with a "push-pull" flow of electrons from the electron-donating group to the
electron-withdrawing group.
Case I: When electron-donating group (D) is present on tlte middle carbon of the diene.
Ilw
W = electron with. drawing group
1, 4-Product (formed)
1, 3-Product (not formed)
l
I
Imaginary flow of electrons in the above case is as follows:
CH3Qf)"(~CH2
+ I
""- ~H
-----t
CH3-~~
CHa0'Q
I
-----t
')(-H
~~
Hypothetical intermediate
1, 4-Product
r-H
°
Thus, if electron-donating group is present either at C-2 or at C-3 then in this case
product is always 1, 4 and not 1, 3.
Case II: When an electron-donating group is present at terminal carbon of the diene.
D
(
~w
+
+
D
1, 2-Product (formed)
1, 3·Product (not formed)
Imaginary flow of electrons in this case is as follows:
~CH'+
OCH a
1,2·Product
Thus, if electron-donating group is present either at C-1 or C·4 then in this case the
product is always 1,2 and not 1, 3.
(E) Intramolecular Diels-Alder Reactions
When diene and the dienophile are part of the same molecule then such type of molecule
gives Diels-Alder reaction known as intramolecular Diels-Alder reaction.
Diene part
Product
The above example is an intramolecular Diels-Alder reaction.
In intramolecular Diels-Alder reaction the stereochemistry at fused ring carbons depends
on the nature of the substituent on the dienophile part of the molecule.
Case I: When electron-withdrawing group is not present on dienophile
I~ this case the two ring junctions are trans to each other. Thus the substituents (i.e.,
hydrogens) on both fused carbons are always trans to each other.
H
I~
H
Case II: When dienophilic part has electron-withdrawing group which is present on
inner carbon of the dienophile
, ....1 - - - -
Electron-withdrawing group
is present on the inner carbon
of the dienophile
In this case both bridge hydrogens are cis to each other and perpendicular to the plane
of the ring.
1~~cP
H
Case III: When electron-withdrawing group is present on terminal carbon of the diene
and geometry is cis
In this case a mixture of two products are formed. In one product both hydrogens of the
fused ring carbons are cis. In other product both hydrogens are trans. Electron-withdrawing
group is always below the plane of the ring and trans to the bridge hydrogen.
COOCH ...- - - Electron-withdrawing group
I
3
-
C:)
is present cm.the terminal
carbon and geometry is cis
d.~
y'
Stereospecific addition:
COOCH 3 and H are trans
in both products
1
H,coo,lH
+
H
H
(Both hydrogens are cis)
(Both hydrogens are trans)
(F) The Frontier Orbital Description of [4 + 21 Cycloadditions
.
The reaction condition of [4 + 2] cycloaddition reactions are different from [2 + 2] cycloadditton
reactions. [4 + 2] cycloaddition reaction is thermally allowed whereas [2 + 2] cycloaddition
reaction is photochemically allowed. To see why this is so, we will examine the HOMO-LUMO
interactions of only the p-orbital components that will form the new CJ (sigma) bonds ~n a
[4 + 2] cycloaddition. We will compare the HOMO-LUMO interactions for the ground a:!tate
(for a thermal induced reaction) and those for the excited state (for photo induced reaction).
(i) Thermal induced reaction: There are two possible interactions, HOMO (diene)·
LUMO (dienophile) and HOMO (dienophile)-LUMO (diene).
Case I: HOMO of diene and LUMO of dienophile
1
E
i
--±t- "'2'
--±t- "'1'
C2 (s), HOMO
m(s)
~
. HOMO (diene)
'1'2
C2-symmetry
-- "'1' m(s)
Dienophile
LUMO (dienophile)
'1'2*
C2-s¥mm~ry ,
\
I
j
HOMO of diene
'...,..........
~ondin~
"'H'"
,j"'.........
mteractlOn
~,"""
",~
+
Bonding interaction
LUMO of d'lenop h'l
1e
+
Phase correct for bonding
symmetry allowed process
Case II: LUMO of diene and HOMO of dienophile
LUMO (diene).,-HOMO (dienophile)
"'~
"'1
m-Symmetry
m-Symmetry
LUMO of diene
.
'....",...,
~ondin~
'H'
'Ii"'",,-
mteraetion
,...,..........
. . ,. . '. ~
Bonding interaction
HOMO ~f Dienophile
Phase correct for bonding
symmetry allowed process
The stronger ofthese two interactions will control the reaction. In this case, the stronger
interaction is between the HOMO and LUMO pair closer in energy. The strength of the orbital
overlap and the magnitude of the resulting stabilisation, depends on the relative energies of
the two orbitals. The closer the two are in energy, the stronger the interaction.
In the Diels-Alder reaction ofethylene and butadiene, the two HOMO-LUMO interactions
are of equal energy and orbital symmetry is same in both reactions. Both interactions involve
bonding overlap at the point of formation of the two new (J (sigmcW bonds. Accordingly this
reaction "is symmetry allowed reaction.
(ii) Photo induced reaction: In photo-induced cycloaddition reaction, interaction
always takes place between excited state HOMO(HOMO*) of diene and ground state LUMO
of dienophile and vice-versa. Thus, there are two possible interactions.
(i) Exdted state HOMO (diene) and ground state LUMO (dienophile), and
(ii) Excited state HOMO (die~ophile) and ground state LUMO (diene).
Let us take the example of excited state HOMO of diene and ground state LUMO of
dienophile.
When butadiene is excited by light, its HOMO becomes "'3* which has m-symmetry.
This MO cannot overlap with ground state LUMO of the ethylene which has Cz-symmetry.
+
\A.0
Excited state HOMO (Ijf,*)
-""
::H
",
"" ,,,,
"",
....
Phase is correct for overlapping
Phase is. not correct
Ground state LUMO (1jf2*)
forbondmg
Jl
+
Symmetry forbidden
Phase is not correct for overlapping symmetry forbidden process
Thus, photochemical cycloaddition of [4 + 2] system is symmetry forbidden reaction. On
the basis ofthe results we can obtain the following selection rules for the cycloaddition reactions.
Table 3.1 Selection rules for cycloaddition reactions
(iii) Endo orientation in bicyclic compounds: .AB mentioned earlier that the exo
product is more stable (i.e., it is thermodynamic product) than the endo product (i.e., it is
kinetically controlled product) in Diels-Alder reaction. Diels-Alder reaction always gives endo
product as a major product when dienophile has a 1t bond in its electron-withdrawing group.
This clearly confirms that transition state of endo product in this case is more stable
than the transition state of exo product due to some other factor, which overwhelms steric
considerations.
In endo orientation the electron-withdrawing group having a 1t bond of a dienophile is
directed to the inside of the cyclic ring, i.e., electron-withdrawing group is nearer to the
conjugated system of diene in the formation oftransition state. In this orientation the p-orbital
of the electron-withdrawing group approach the central carbon atoms (C-2 and C-3) of the
diene. This proximity results in a weak secondary overlap: An overlap of the p-orbitals of the
electron-withdrawing group with the p-orbitals of C-2 and C-3 of the diene. This fancy-phrase
simply means that there can be interaction between the back diene orbitals and orbitals on
the substituent only in the endo transition state. In exo orientation the electron-withdrawing
group of the dienophile is directed away from the cyclic diene conjugated system.
New bonds
Group is near to the conjugated system of
diene endo transition state
1
r
I
New bonds
Exo transition state has no possible secondary interactions
=:::::,::;:.:
3.3.1
Orbital Symmetry In Cycloaddltlon
Orbital symmetry arguments make useful predictions about concerted cycloaddition reaction
which is suprafacial-suprafacial. Consider the [1t2s + 1t2sl cycloaddition of ethylene molecules
in parallel planes approaching each other vertically (Fig. 3.2).
au or 1
'-------,1------'
au or 1
CH2
CH2
CH z
CH 2
ah or 2
ah or 2
Fig. 3.2 Two ethylene molecules approaching each other vertically
and approach is suprafacial-suprafacial
This system contains vertical and horizontal plane of symmetry (i.e., mirror plane of
symmetry) denoted by <JV (or 1) and ah (or 2) which are useful in characterising the orbitals. In
the transformation of ethylene molecules to cyclobutane we are mainly concerned with the
four 1t (Pi) orbitals of two ethylene molecules and the four a (sigma) orbitals of cyclobutane.
Let us first take the au and ah in two ethylene molecules
1
1
1
1
----/-----+------+-----+---- 2
(s, s)
(s, a)
"'f
"'~
(a, s)
"'3*
(A)
(1,2)
Now let us take the cru and crh in cyclobutane
(s, s)
(a, s)
(s, a)
(a, a)
2
01
o~
0*
3
0*
4
(1,2)
(B)
Fig. 3.3 Symmetry properties of (A) ethylene
7t
orbitals and (B) cyclobutane 0 orbitals
Note that more will be symmetry in the system less will be energy of the system
for 1t as well as cr orbitals. Energy of 11: (Pi) system is as follows in increasing order:
ss sa as aa
Energy of a (sigma) system is as follow in increasing order:
ss as sa aa
We can now construct the orbital correlation diagram. But before this we must classify
the symmetry of the orbitals twice over once for the plane bisecting the 1t bonds represented by
the vertical line, i.e., crv (or 1) (Fig. 3.3) and then for the plane between the two ethylene
molecules, the horizontal line, i.e., ah (or 2). Thus, the lowest energy orbital in the starting
materials is the bonding combination '1'1 (both '1'1 has mirror symmetry) ofthe bonding 1t orbitals.
This orbital has both symmetry (and hence represented as s, s). The next orbital up is the
antibonding combination of '1'2 (both '1'2 has m-symnietry) of the two bonding 1t orbitals. This
orbital has au symmetry and crh asymmetry so it is classified as sa.
The next orbital up is the bonding combination of'l'l (both'l'~ has C2-sym~etry)of the
two antibonding 1t orbitals. This orbital has au asymmetry and crh symmetry so it is classified
as as. The next orbital up is the antibonding combination of '1'4* (both '1'4* has C2-symmetry) of
the two antibonding 1t orbitals. This orbital has crv as well as ah asymmetry so it is classified as
aa. Thus, the orbitals of the interacting ethylenes are the result of forming bonding and
antibonding combination of 1t and 1t* orbitals of two ethylene molecules. The interacting a
orbitals are similar combination of the 0(01' S, s and O 2, a, s) and 0* (03*, sa and cr4*, 00)
(Fig. 3.3B).
We can now complete (Fig. 3.3) by correlating the energy levels, feeding the orbitals in
the starting materials into orbitals of the same symmetry in the product, ss to ss.
sa to sa
sa to sa
as to as
00 to 00.
On the basis of the above information, a correlation diagram (Fig. 3.4) may be drawn in
which the levels of like symmetry are connected by lines.
'\
I
I
1
J
)
Ii
J
I
j
I
t\
\
1
I
•
~
'j
I
.!
l
j
31
c=J
. Two ethylene
molecules
Cyclobutane
a, a'V/
0"3* S,
S,
a
a'V2
- - - - - - - - - - ­ 0"1 S, S
Fig. 3.4 Correlation diagram for cycloaddition of ethylene into cyclobutane
A close examination ofthe diagram leads us to the following two conclusions:
1. The ground state orbitals of ethylene correlate with an excited state of cyclobutane,
'JI;'JI! ~ 0[°3*2. Consequently, the combination of two ground state ethylene mol­
ecules cannot result in the formation of ground state cyclobutane while conserving
the orbital symmetry. Hence the thermal proce~s is symmetry-forbidden.
2. As there is correlation between the first ex(;ited state of the ethylene and cyclobutane,
'JI12'J12'J13* ~ °12(52°3* the photochemical.process is symmetry allowed.
3.3.2
Correlation Diagram of [4 +·2] Cycloaddition Reaction
As mentioned earlier correlation diagrams provide a complete explanation ofconcerted reaction in
which symmetry is preserved throughout the reaction. For correlation diagram of [4 + 2]
cycloaddition this can be done by the following steps:
1. Draw the concerted reaction and indicate the migration of electrons by curly arrows
for the backward arid forward reaction (Fig. 3.5).
Fig. 3.5
Any substituent(s) present in an~ reactant (in this case diene and dienophile)
symmetrical or unsymmetrical do not disturb the symmetry of the orbitals directly
involved.
2. Identify the orbitals undergoing change. The curly arrow helps to focus on what they
are, i.e., which orbitals are undergoing change. For the starting materials, they are
the 1t (Pi) orbitals ('JI1' 'JI2' 'JIa * and 'JI4 *) ofdiene unit and 1t (Pi) orbitals (1t and 1t*) of the
dienophile. For the product, they are the 1t bond (1t and 1t* orbitals) and two newly
formed (5 bonds «(51' 2 , (5a * and (5/).
°
r
3. Identify any symmetry elements maintained throughout the reaction, i.e., the same
symmetry should be present in reactants, cyclic transition state and product. The
symmetry may be of one or two types (C1V and oh).
In Diels-Alder reaction there is only one symmetry and that is vertical plane of
symmetry bisecting the bond between C-2 and C-3 ofthe diene and the double bond of
the dienophile.
au
q?"
4. Rank the orbitals in increasing order of energy (vertical on the paper) with the reac­
tants on the left and the product(s) on the right (Fig. 3.6).
-,..
...
5. Besides each energy level, draw the orbitals showing signs of the lobes of atomic
orbitals.
6. Identify the symmetry of the orbitals with respect to C1V or oh or both (which one is
possible) of reactant and product. In this case symmetry is C1V.
7. Construct an orbital correlation diagram. Following the assumption that an orbital
in the starting material must feed into an orbital of same symmetry in the product,
draw lines connecting the orbitals of the starting materials to those of the products
nearest in energy (i.e., energy difference between correlating orbitals of reactant and
product should be minimum and of the same symmetry. Thus 'l'l(S) connects to 1 (S),
7t(S) to 7t(s) and 'l'2(a) connect to 02(a) and similarly with unoccupied orbitals '1'3 *(S)
connects to ° 3*, 7t* (a) connects to 7t* (a) and '1'/ (a) connects to ° 4* (a).
Since ground state orbitals of reactants correlate with the ground state orbitals of the
product therefore Diels-Alder reaction is thermally allowed.
°
'l'r7t~; ----7 0120227t2
On the other hand, photochemical transformation is not possible as the first excited
state of the reactant does not correlate with the first excited state of the product. Rather it
correlates with the upper excited state of the product.
"'r7t2'1'2'1'a * ~ crrO;7t°3 *
1
l
48+ 28
o
a
H
+
It*
+
E
­
a-----+-------­
--------------------------------------
a
1t
H
8 - - - - - -......- - - - - - ­
8
Fig. 3.6 Correlation diagram of [4 + 2] cycloaddition reaction
WL,
:,.4i,Mki4;;&
"ltrf~"t'r'Y;t'tif··\1!7i"'e'!tii"if!1"""''"''e'it!i'~ ]'trntWtfii!"W'.f,"mFt'tr1rJiMfeetf'iiiitrrrmt
Thermal Reactions
A thermal (ground state) pericyclic change is symmetry-allowed when the total number of(4q + 2)8
and (4r) a components is odd.
l
Photo Chemical Reactions
A pericyclic change in the first electronically excited state is symmetry-allowed when the total
number of(4q + 2) sand (4r) a component is even.
3.4.1
The Woodward-Hoffmann Rule in I 4 + 2 I Cycloaddltions
1. Draw the general mechanism of the reaction
l
~(X
~
I
+
~~
.
2. Choose the components. All the bonds taking part in the mechanism must be included
and ignore all substituents which are not directly involved.
c
(4r)
(4q + 2)
3. Make a three-dimensional drawing ofthe way the components come together for the
reaction putting the orbitals at the ends of the components.
1
4. Join the components where new bonds are to be formed.
5. Label each component s or a depending on whether new bonds are formed on the
same side or opposite sides.
l
j
Ii
Cycloaddition Reactions
Hi" (=
4q + 2,
wh~, q =0)
6. Count the number of(4q + 2) sand (4r) a components.
Number of(4q + 2) s component = 1
Number of (4r) a component =0
Total = 1 (odd)
Thermally allowed.
3.4.2
Woodward-Hoffmann Rule in 12 + 21 Cycloadditions
IC(:II
~
~ ~
H
D
.'(40+2)=>.',
.'(4q+2)=>.',
Number of (4q + 2) s component =2
Number of(4r) a component =0
Total = 2 (even)
Photochemically allowed.
:t
:=:::,::::.:::::::::::::::::i:::::;::::=:::::::::::::;;:::
Huckel rule ofaromaticity states that a monocyclic planar conjugated system is aromatic ifit has
+ 2)1t conjugated or delocalised electrons and consequently stable in ground state. Similarly,
monocyclic planar conjugated system is anti-aromatic if it has (4n)n conjugated or delocalised
electrons. This system is unstable in ground state. However, further calculation shows that these
rules are reversed by the presence of a node in the array ofatomic orbitals. Thus system with (4n
+ 2)1t electrons and a node is anti-aromatic while system with (4n)1t electrons and a node is
aromatic.
If system has no node then it is called Huckel system and array is called Huckel array.
Similarly, if system has node then it is called Mobius system and array is called Mobius array.
Application ofabove rules to cycloaddition reactions led to the generalisation that thermal
reactions take place via aromatic transition state whereas photochemical reactions proceed
via antiaromatic transition state.
(4n
1!
I
Consider the [2 + 2] cycloaddition reaction:
I
I
~
i
28
[28 + 28]
(i)
l
28
4 Electrons, zero node.
Huckel system, antiaromatic, hv allowed.
(ii)
[28 + 28]
i
1
t1
J
4 Electrons, zero node.
Huckel system, antiaromatic hv allowed.
[2a + 28]
(iii)
4 Electrons, one node.
Mobius system, aromatic
~
allowed.
Now consider the [4 + 2] cycloadditions:
48
[48 + 28]
28
6 Electrons, zero node.
Huckel system, aromatic,
~
allowed.
j
j
48
[48 + 2a]
6 Electrons, one node.
Antiaromatic, Mobius system, hv allowed.
On the basis ofthe results we can obtain the following selection rules for the cycloaddition
reactions:
Table 3.2 Selection rules for cycloaddition reactions
4n
0
antiaromatic
4n
1
aromatic
4n + 2
0
aromatic
4n + 2
1
antiaromatic
supra-supra
antara-antara
supra-antara
antara-supra
supra-supra
antara-antara
supra-antara
antara-supra
:rc:
:..: :.: :
Cycloaddition reactions are, inthe()]:y and usually in practice, reversible reactions. The reverse of
cycloaddition reactions are known as cycloreversions or retrograde cycloadditions. Such reactions
follow same symmetry selection rules as does the cycloaddition itself. For example:
CH a
I
CHsC-c0
cH-h 1
a
°
...(1)
I
CRa
This reaction is represented as -[2 + 2] cycloreversion. A minus sign before the designation
indicates a cycloreversion reaction. The most common cycloreversion reaction is - [4"+ 2] cyclo
reversion. This reaction is commonly known as retro-Diels-Alder reaction.
Diels-Alder reaction is reversible, and on heating many adducts dissociate into their
components. In most of the cases retro-Diels-Alder reactions take place under quite mild
conditions. Retro-Diels-Alder reaction is very important in those cases where adduct obtained
in Diels-Alder reaction is chemically modified and than give new diene or dienophile by
retro-Diels-Alder reaction.
...(2)
It is not always the bonds formed in Diels-Alder addition is broken in retro-Diels-Alder
reaction
o
+C-CN~
III
~CN
~
1
CH
Retro-Diels-Alder R
~CN
o
n
...(3)
COOC 2 H s
I
III
C
180.C
~
Y
COOC 2H s
...(4)
1
I
...(5)
Retro-Diels-Alder reaction can also be used for the preparation of some reactive
compounds in vapour phase (iii. the absence ofsolvent and catalyst) that are difficult to prepare by
other methods.
o
CH-Cl
+ II
CH-Cl
r7l--(Cl
~
~Cl
[2 + 21
[4 +
211 H5C200C-C. ~COOC2H5
00C
HSC2
])r
Cl
Cl
H SC200C
This compound is highly reactive and
used as synthetic reagent for the pre­
paration of various compounds
(Highly reactive
compound)
...(6)
XL:lt1:'IC
::::::::=::':::::1::::::::'::
Conjugated ions like allyl cations, allyl anions and pentadienyl cations also give [4 + 2] cycloaddition
reaction. Allyl cation is a two electron system in a [4 + 2] cycloaddition reaction. Allyl carbanion
and pentadienyl cation is four electron system in a [4 + 2] cycloaddition reaction.
Examples of[4 + 2J cycloaddition reaction given by allyl carbocation.
(i) CFaCOOAg
(i)
(ii)
O
I 80
·)
I
2
...(1)
L.l
The reaction takes place as follows:
(B
~
CH2-C=CH 2
I
CHa
Allyl carbocation
40% yield
o
II
(ii)
...(2)
CHa-CH-C-CH-CHa
I
Br
I
Br
The above reaction takes place as follows:
o
II
o
(II
C
CHa-CH
I
Br
/~
CH-CH a
I
Br
CuINaI)
A
J
I
LCHa-CHCH-CHa
1? •
CHa-CH=G--CH-CHa
e
(Allyl cation which is stabilised by 0)
•
j
t
e
C?
CH,-ci: r-CH'
p
E9
[4 + 2]
Cycloaddition
+ Enantiomer
)
Example of[4 + 2] cycloaddition given by allyl carbanions
The above reaction takes place as follows:
Pr;NLi
e
)
CHr C---cCH2
I
CsH s
4-Electron system; allyl carbanion
CISHS~
' [ 4 + 2]
Cycloaddition
----~)
CH~~CH2
Hc--=CH-CsHs
Phll/ / ." ( ; / " Ph
e
Phl"""j_--(Ph
---~)
Ph
I
y
Ph
CsH s
2-Electron system
:.;,:::::
All the reactions described so far have used six electrons in the transition state. In some cycloaddition
reaction more than six electrons take part in cyclic transition state. The common examples are [8
+ 2] and [6 + 4] cycloadditions
[6 + 4] cycloaddition
~)8
r1
3 _
2 1
6n
9
5 6
~
10
4n
0
3 \
2
/8
9
1 10
0
0
4n
25°C
~
6n
~
92
Photochemistry and PericyClic Reactions
[8 + 2] cycloadditions
Ell
N-COOMe
~N-COOMe
II
N-COOMe
~~-COOMe
,
'"
21t
I
~
rvyCOOMe
~COOMe
ott
I
:D:IIt
::::::::~:::::::::::::::::
Let us first take some addition reactions of ketene with alkenes. The reaction has some
characteristics of pericyclic cycloaddition. The reaction is syn addition and geometry ofreactant
is maintained in the product.
H
o
W
Q=t""'' Cl
"II
C
C
"R
Cl/ "'Cl
&H+
trans
" (Hydrogens are trans)
II
II
C
Cl/ "'Cl
...(1)
Cl
Both hydrogens are cis
H
o
C
0
25°C
~
d=t~"Cl
R
Cl
Both hydrogens are trans
...(2)
r
f
In the given two examples stereochemistry of reactant is maintained in the product. These
two reactions are pericyclic [2 + 2] cycloaddition reactions and thermally allowed reactions. We
know very well that [2 + 2]cycloaddition is photochemical reaction and suprafacial-suprafacial. If
reaction is thermally allowed then reaction should be suprafacial-antarafacial reaction, i.e.,
[1t2s + 1t2a] cycloaddition (see Table 3.2). If this is [21ts + 21ta] then how does it overcome the
symmetry-imposed barrier?
One suggestion is that two molecules approach each other at right angles for overlapping
in an antarafacial sense of the ketene. Making the reaction the allowed [1t2s + 21ta] cycloaddition
that we have dismissed as being unreasonable. This is the most simplest explanation. The
[2 + 2] cycloaddition of ketenes being concerted is more likely to be a consequence of the fact
that ketenes have two sets of 1t orbitals at right angle to each other and overlap can be developed
to orthogonal orbitals (dashed lines) and in addition there is transmission of information from
one orbital to its orthogonal neighbour (heavyline) (Fig. 3.7).
Fig. 3.7 Overlapping orbitals of ketene and alkene
The vision identifies the reaction as an allowed [1t2s + 1t2a + 1t2s] cycloaddition reaction.
The FMO treatment shows that the bond formation between C-l and C-l' develops mainly
from the interaction of the LUMO of ketene (1t* of C=O) and HOMO of alkene (Fig. 3.7) and
that the bond between C-2 and C-2' develops mainly from the interaction of HOMO ketene [('11 2 )
the three atom linear set of orbitals analogous to the allyl anion] and LUMO of the alkene
(Fig. 3.8 and 3.9).
LUMO(+)
V
VI
o
~
Sup,.faciM
2/
C:---C..........
~
-
Ketene
~H"
2'
+
Alkene
+
HOMO
Fig. 3.8 Bond formation between C-1 of ketene and C-1' of alkene
r
2~~
1
HOMO C - - - C - - - C
&1
~
Antarafacial
~
~
~
~
H
l'
2'
-
LUMO
+
Fig. 3.9 Bond formation between C-2 of ketene and C-2' of alkene
The reaction can be represented as follows:
1
..
1
~----
xm::E°Jll¢JJlCtlQQtt=':':::::::·:;::::;:;::::·:::::::::::::::·:==::·:·::;::::
Chelotropic reactions are those reactions in which two <r bonds are formed on same atom or two <r
bonds are broken on same atom.
...(1)
Two
(J
bonds are formed on this carbon in the product.
...(2)
Two
(J
bonds are broken on nitrogen
...(3)
Two
3.10.1
(A)
(J
bonds are formed on sulphur
[2 + 2] Chelotropic Cycloadditions
Chelotropic Reactions ofAlkenes with Singlet Carbenes
Alkene reacts with carbene to fonn cyclopropane or substituted cyclopropane. This addition reaction
is stereospecific with singlet carbenes. Reaction is thennal allowed reaction.
HaC,
/CHa
C-C
H/ \ / "'H
...(4)
CH 2
Cis product
...(5)
The reaction is concerted reaction. An examination ofthe orbitals ofcarbene shows that a
bonding overlap between HOMO of a carbene and LUMO of an alkene or LUMO of carbene and
HOMO of alkene is possible.
In singlet carbene hy~ridisation of carbon is Sp2. It has three sp2 hybrid orbitals and one
empty p-orbital perpendicular to the plane defined by the carbon atom and the two substituents
on it. Out of three sp2 hybrid orbitals two are bonding and one is non-bonding having two
electrons in it (Fig. 3.10).
R ,
+
F
'~, ~
~_
R
Empty p-orbital, LUMO of carbene
Hybrid orbital containing pair of
electrons. It is HOMO of carbene
Sp2
Fig. 3.10 MO diagram of singlet carbene
Concerted cycloaddition reaction between alkene and carbene is [2 + 2] cycloaddition
reaction. This reaction is possible only if the carbene approaches the alkene sideways so that
the plane defined by the carbon atom and its two substituents parallels the plane of the alkene.
In this orientation, the empty p-orbital of the carbene pointing towards the electrons of the 1t
bond ofalkene [Figs. 3.11(a) and 3. l1(b)].
,,
I
i
CycloadditiOn Reactions
H-
97
Fig.3.11(a) Interaction between HOMO of alkene and LUMO of carbene
H,
~_
Bonding interaction
R
/H
+~
:::
:::
R~... ~~~...
::
~
R;"~
+-
LUMO of alkene (suprafacial)
R
Bonding interaction
HOMO of ,"",,~ (An_facial)
Fig.3.11(b) Interaction between LUMO of alkene and HOMO of carbene
Chelotropic reaction ofcarbene is [2 + 2] cycloaddition reaction. Thus, chelotropic reaction
of carbene with alkene is symmetry forbidden if both components interact suprafacially
(Fig. 3.11a). Antarafacial reaction of a simple alkene is sterically very unlikely, so that the
reaction is likely to involve the CH 2 antarafacially [Fig. 3.11 (b)].
(B) Chelotropic Reaction ofAlkene with 802
Result of 802 is identical to carbene.
~LUMO'fSO'
1~(:
HOMOofS0 2
. H­
The orbital interaction between 802 and alkene is as follow:
Bonding interaction ~
o
LUMO of alkene (Suprafacial)
. :
"%' ~ ~ ....8>'8
O/~
<±>
.
Bonding interaction
HOMO ofSO z (Antarafacial)
3:i1XM'eREl"m'Ru'l'fC'EttdfRlTje'ti4iU4LLIU"
teere- r
i
'
p
ZJlZJ+. 3Wi. tL:az;g;)Q. t XJM... ¥ tiit .iM';;;;:
""W't-tnt tr""rnfirr-re"Yttfd'-''t'ttrW'''t-f''ltrlttfh.itfr -:1" "t'··, 'wt '-~, ',- ff@""f""''''-'''''W'~t~,trirf-r-'''-~'-'''''"1X''f>r't "c7" "'Cr"'Y' -'~'''!t''1tttt ·tle,--"-"",~,,,-t<d"l--"f"--'~""'''''-t''''''We-'(~li,-",',;,~,- """ffW
't'
In chelotropic elimination, the atom X is normally bound to other atoms in such a way that
elimination will give rise to a stable molecule. The most common examples involves five
membered rings.
f \C
C
II
II +
C
C
X
A good example of a concerted chelotropic elimination is the elimination of diazene in
which elimination of nitrogen takes place.
t1
Use of substituted systems has shown that the reactions is completely stereospecific.
The groups on C-2 and C-5 of the pyrroline ring rotate in the disrotatory mode on going to
product. This stereochemistry is consistent with conservation of orbital symmetry.
Disrotatory
)
f'!,.
CH~H
H He
3
3
?:
I
Disrotatory
)
f'!,.
CH~CH
H H
3
Disrotatory
)
"r
3
CH~CH
H H
3
3
eve"coJt'BB1Tl8f!Jf
AkWW: lQUkJiii.X4\U4Z$4 £, . LiIt¢#£!2 #i A· $I M.#4t !IlIiI
'~12:'U13:'mifOr:ltA;
lIBel' ?i1t,d7:Jfmefttttt'!r: ltei'! rr! r nne,. trr'9tnnmnttHftm' tp en 'Trw_a CC'ITff'M'f uWH'
!
There is a large class ofreactions, known as 1, 3-dipolar cycloaddition reactions, that are analogous
to the Diels-Alder reaction in that they are concerted [41t + 21t] cycloadditions. These reactions
can be represented as in the given equation:
.
a
/b,
I
c
I
X--y
The species tz-b~ is called the 1, 3-dipolar molecule and X=Y is the dipolarophile. The
1, 3-dipolar molecules are isoelectronic with the allyl carbanion and have four electrons in a 1t
(Pi) system encompassing the 1, 3·dipole. AlII, 3-dipoles contain 41t (Pi) electrons in three
parallel p-orbitals of a, band c.
In 1, 3-dipolar species tz-b~
(0 a has six electrons in its outermost orbit. b has its complete octet having at least one
lone pair of electrons. c has its complete octet having negative charge.
(ii) a may be carbon, oxygen or nitrogen,
b may be nitrogen or oxygen, and
c may be carbon, oxygen or nitrogen.
(iii) If b is nitrogen then it has single or double bond.
If b is oxygen then it has single bond only.
Some typical 1, 3-dipolar species are given in Table 3.3.
Table 3.3 Some 1, 3-dipoles
(l)
Azoxy compounds
Nitrosoxides
Nitrous oxide
Carbonyl oxides
Nitrile oxide
Diazoalkanes
(l)
~
..
N-N-u
e
,,~
/C-O--O
..
e
O-Q--O
(l)~.
-C=N--Q:
-C
..
e
N-N
(l)
..
e/
N=N-C"
..
e
-N-Q--N­
(l)
Azides
(l)
e
..
Ell
e
-N=O-O
(l)
Nitrosoimines
••
(l)
••
e
-N--Q--O
(l)
Nitrile imine
e
-N=N-O
I
(l)
Ozone
••
-N-N-O
I
..
..
(l)
~
N=N-U
"/C=U-O
~ e
e
(l)
0=0--0
(l)
e
-C=N-O
(l)
e
-C=N-N
..
Ell
e/
N=N-C,
..
~
e
-N=U-N-
e
N=N-N­
Carbonyl ylides
Nitrones
Azomethine imines
Contd...
-
---,.------
- - --~~------
Azomethane ylide
Azimines
Nitro compounds
e
"
e
" e
..
e
-N-N-N­
I
e
e
-N=N-N­
I
o ~-8
O-N-u
I
I
Carbonyl imines
Carbonyl imides
Nitrile ylide
e
1
e/
"
r
-C=N-C,
The dipolarophiles (X=Y) are alkenes, alkynes, imines, nitriles and carbonyl compounds.
3.12.1 Stereochemistry of 1, 3-0ipolar Cycloadditions
r
The stereochemistry of 1, 3-dipolar cycloaddition reaction is analogous to that ofthe Diels-Alder
reaction and hence it is stereospecific syn addition.
l
Ii+ q../"'
----
b,"",,-£0-
----
J
-
X 1l1IillillllY
TS
Adduct
When both the 1, 3-dipole and the dipolarophile are unsymmetrical then there are two
possible orientations for addition (i.e., two isomeric products are formed). Both steric and
electronic factors play a role in determining the regioselectivity of the addition. The most
generally satisfactory interpretation of regioselectivity of dipolar cycloaddition is based on
Frontier orbital concepts. Ai;, with the Diels-Alder reaction, the most favourable orientation is
that which involves complementary interaction (secondary interaction) between the FMO of
the 1, 3-dipole and the dipolarophile in suprafacial-suprafacial manner. In most ofthe dipolar
addition LUMO of the dipolarophile interacts with the HOMO ofthe 1, 3-dipole. But in certain
cases relationship is reversed.
1, 3-Dipolar species has three p-orbitals having four electrons hence HOMO will be 'V2
which has c2-symmetry. The 1t orbitals of the 1, 3-dipolar species can be represented (Fig. 3.12)
as follows:
I
"
_
l
I
E
u
~ ~
H
+
+
'1'3* Antibonding
-#-
'1'2
#
V,
Nonbonding
­
~~~
Bon~
Fig. 3.12 MOs of 1, 3-dipolar species
III
,\
e
••
Suppose 1, 3-dipole is diazomethane (CH 2-N=N). Its HOMO will be "'2'
H
H
1
I
HOMO of 1, 3-dipole
Suppose dipolarophile is ethylene (CH2=CH2) its LUMO is '1'2*.
+
+
LUMO of dipolarophile; '1'2*
­
1, 3-dipolar addition between diazomethane and ethylene for example, can take place as
follows:
~/N",~
C
~
H
=
+
of
N
~
~
HUMO
)
LUMO
Symmetry allowed reaction
Some
the dipoles are stable compounds like ozone, diazomethane and suitably
substituted azides, nitrones and nitrile oxides, Others, like the ylides, imines and carbonyl
oxides are reactive intermediates that have to be made in situ. In most reactions the 1, 3-dipole
is not isolated but generated in situ in the presence of the dipolarophile. Some examples of
common 1, 3-dipolar cycloadditions are on the next page.
CH a CH a
/
C--O
"
~O
c--o
I
...(1)
HaC/ "CHa
Molozonide of alkene; a [4 + 2]
cycloaddition product
...(2)
1
Tautomerisation
...(3)
J::JH
Ph
N/
2-Pyrazoline
Ph
C6H s-CHO
)
CH3-NHOHIC6H6-CH=CH21~
Ph
~-CHa
-z.,O/N
2-Methyl-3, 5-diphenylisoxazolidine
Reaction (iv) takes place as follows:
1
E!3
••
e
C6H s-CH-N-O
I
CH a
N-Methyl-C-phenyl nitrone
1
C6H s-CH=CH2
Product
...(4)
l
ifI
104
.acticma
Photochemistry and Per~cYclic R•
I
':
I
I
I
CyoloadditionReaotions
105
FURTHER READING
1.
. Dougherty, The PMO Theory ofOrganic Chemistry, Plenum Press, New
the following supra-supra
. hemically?
cycloaddi~ion
o
e
106·
Photochemistry and Pericyclic Reactions
CH-CH=CH-JH2 +CH2
CH2
~
0
e
rocess and is therefore allowed under ph .
an
e alloWed under til
.undergoes thermal
Cycloaddition· Reactions
Cotnplete the given reaction sequence and givefuechanism of each step..
107'
108
Photochemistry and Psncyc1ic Reactions
I
Cycloaddition Reactions
109
n could be photo-induced if the dienophile, instead of the _
. Explain your answer.
were the excite
B2. Which oitha following types ofcycloadditions would you predict to proceed easily upon heating?
(aUG + ~}
(b) [6 + 4]
(c) f8 + 21
(ll) [8 + 4].
e hlllp ofFMO method show that the [2 + 21 cycloaddition reaction between alkene and
is thermal allowed reaction.
Give the mechanism of the ehelotropic eycloaddition reactions between
:e~and carbeue.
~ar species
in 1, 3~polar cycloaddition reactions?
on the basis of FMO method.
• ~BUpra:p~ js ~metri~a1lYpossible but symme-, .
.
. all()W but geometrically
j:ompOnents ishowevet~ Ii well .
r
I
110
Photochemistry and Pericyclic Reactions
I
I
I
I
l
l
I;
I
1
1
!
I
.
1
.t
,.
4
CHAPTER
~tropic Rearrang~
[1, 3)
... (1)
)
In sigmatropic rearrangement substrate can be divided in two parts: Alkenyl
(or polyalkenyl) chain and migrating group. All substrates have at least one allylic carbon in
alkenyl chain for sigmatropic rearrangement. Alkenyl chain is either a allyl group or a conjugated
system having at least one allylic carbon.
~
Migrating group
~
Alkenyl chain
Allylic carbon of alkenyl chain
Numbering of Alkenyl Chain
Numbering of alkenyl chain is always started from the allylic carbon and this carbon is
numbered-1
111
Pel'icy,clic Reactions
CHz-CHz-R
I
CHz-CH=CH z
1
2
3
Numbering for Migrating Group
Atom (H, C or heteroatom) of migrating group bonded with allylic carbon by cr (sigma) bond is
always given number-1
1
2
CH -CH -R
I z
z
CHz-CH=CH z
1
2
3
i.
4.1.2 Name of the Rearrangement
0,
In the example (1) atom-1 of the migrating group migrates on the atom-3 of the alkenyl chain,
Therefore, this rearrangement would be classified as a [1, 3] sigmatropic rearrangement. In
1, 3 [(i.e., i, j) i = 1 and j = 3] i indicates position of the atom in the migrating group and j
indicates position of the atom in the alkenyl chain.
~
C>
~
CHz-CH=CH-CH=CH-CH=CD z
1234567
[1,7]
)
Allylic carbon
Alkenyl chain
CHz=CH-CH=CH-CH=CH-CHD z
... (2)
The above reaction is an example of [1, 7] sigmatropic rearrangement.
It is not always the first atom of the migrating group that becomes bonded to the alkenyl
chain in the rearrangement. Consider the following example:
M~rating
group
Alkenyl chain
... (3)
In this rearrangement atom-3 of the migrating group migrates on the atom-3 of the
alkenyl chain. Therefore, this rearrangement is an example of [3, 3] sigmatropic rearrangement.
1­
i,
1.
The sigmatropic rearrangements can be divided into two classes:
Those where the migrating atom or group is bonded through the same atom in both
reactant and product.
3~
2~
[1,3]
~
...(4)
1
R is bonded to carbon in reactant as well as in product
...(5)
1, 5-shift of alkyl group, R is bonded to carbon in reactant as well as in product
...(6)
1, 7-shift of hydrogen. H is bonded to carbon in reactant as well as in product
2.
Those where the migrating atom or group is bonded through different atoms in reactant
and in product.
o
[3,3]
~IH
) l.J"'?H
2
... (7)
CH
II
CH 2
This is [3, 3] sigmatropic rearrangement in which carbon of allyl group is bonded to
oxygen in the reactant and carbon in the product.
[3,3]
)
...(8)
Allyl group
This is [3, 3] sigmatropic rearrangement of an allyl vinyl ether. Migrating group is
bonded to the oxygen in the reactant which is bonded to the carbon in the product.
I
Tol
I
2
(38-0)
1~~
2
Allyl group is
bonded to sulphur
o
0-8-Tol
[2,3]
)
...(9)
Allyl group is
bonded to oxygen
[2, 3] 8igmatropic rearrangement of an allyl sulphoxide
R
R .,I
e
1~~
...(10)
2
Allyl group is
bonded to nitrogen
Allyl group is
bonded to oxygen
[2, 3] 8igmatropic rearrangement of an amine oxide
!
'"
\
--~
r
:::,::::::::::::=:::L1::
FMO Method
Consider the following reaction:
4
3~5
2l
.6
nT
CD
2-D
1
)
[1,5]
1
1, 3-Pentadiene
When 1, 3-pentadiene is heated, it gives [1, 5] sigmatropic rearrangement. It is simple
to construct an arrow formalism picture of the reaction. The arrow could run in either direction,
clockwise or anticlockwise. That is not true for a polar reaction in which the conventiun is to
run the arrow from pair of electrons towards the electron deficient.
An arrow formalism description of the fl, 5] shift of deuterium in 1, 3-pentadiene.
In the given exa.mple deuterium of Sp3 hybrid carbon migrates on to the Sp2 hybrid
carbon (carbon-5). The given compound has also carbon-3 as Sp2 hybrid carbon. Thus this
compound can also give [1, 3] sigmatropic rearrangement on heating, but [1, 3] shift is not
observed.
[1,3]~)
~
CD 2
Not observed
An arrow formalism can easily be written and it might be reasonably argued that the
[1, 3] shift requiring a shorter path than the [1, 5] shift, could easier then why [1, 3] shifts not
observed on heating?
A second strange aspeetof this reaction comes from photochemical experiments. When
1, 3-pentadienes are irradiated, the product of the reaction includes the molecules formed
through [1, 3] shift but not those of [1, 5] shifts.
c
~
CD 2-D
e
Product of a [1, 3] shift
So any mechanism proposed must include an explanation of why thermal shifts are
[1, 5] whereas photochemically induced shifts are [1, 3]. We can use the frontier orbital approach
to analyse these reactions and see why this is so. Let us first consider the following thermally
induced sigmatropic rearrangement which is a [1, 3] shift.
l
I
i
l
For the purpose of analysing the orbitals, it is assumed that the 0' (sigma) bond
connecting the migrating group to its original position undergoes homolytic cleavage to yield
two free radicals (i.e., homolytic bond cleavage of 0' bond between position-1 of alkenyl chain
and position-1 of migrating group). This is not how the reaction takes place because reaction is
concerted. But this assumption does allow analysis of the molecular orbitals.
I
H·
Homolytic bond
)
cleavage
CH 2-CH=CD 2
Allyl free radical
The products of the hypothetical cleavage are a hydrogen atom and an allyl free radical,
which contains three p-orbitals. The 1t (Pi) molecular orbitals of allyl free radical are shown in
Fig. 4.1.
m
'l'!
+
E
Hv,
tH v,-#­
+HOMO
~
1
Fig. 4.1 Molecular orbitals of allyl free radical
The actual shift of hydrogen could take place in one of the two directions.
In the first case, the migrating group could remain on the same side of the 1t (Pi) orbital
system. Such a migration is known as a suprafacial process. In the thermal 1, 3 sigmatropic
rearrangement a suprafacial migration is geometrically feasible but symmetry forbidden.
l
Overlap is
bonding
'\. "..~ /
>'
~,
H
+
)
-
In the C-H bond the overlapping
lobes are of the same sign
Overlap is
antibonding
TS
­
+
Symmetry forbidden (phase
incorrect for overlapping)
Let us consider the second mode of migration for a symmetry allowed [1, 3] sigmatropic
shift to occur, the migrating group must shift by an antarafacial process-that is, it must
migrate to the opposite face of the orbital system.
~
+
-
H
­
+
TS
Overlap is bonding, symmetry
allowed but geometry difficult
While symmetry-allowed a [1, 3] antarafacial sigmatropic rearrangement of hydrogen
is not geometrically favorable. Why? The problem is that the Is orbital is smallest and cannot
effectively span the distance required for an antarafacial migration. In other words, size of Is
orbital of hydrogen is smallest and distance between two lobes of interacting p-orbitals of
carbon is maximum hence orbital of Is cannot interact effectively with p-orbitals at same time
in the formation of transition state.
Smaller size
Large distance
Symmetry allowed but
geometrically difficult
[1, 3] sigmatropic shifts take place in the presence of UV light but examples are rare.
Consider again what happens when a molecule absorbs a photon. LUMO of ground state will
become HOMO of excited state known as photochemical HOMO (Fig. 4.2).
+'II;
Photochemical
HOMO
I
.~
Ground state
HOMO
~
Photochemical HOMO
permits an easy
suprafacial migration
(photochemical reaction)
Antarafacial mode is
required for thermal reaction
Fig. 4.2 Suprafacial migration is possible in'll; photochemical HOMO of the reaction
[1, 5] Sigmatropic Rearrangement: The products of hypothetical cleavage in this
case are hydrogen free radical and pentadienyl radical. 1t MOs of pentadienyl radical is given
below:
+
Fig. 4.3
1t
HOMO
Molecular orbitals of the pentadienyl radical
1
!
119
[1, 5] Sigmatropic shift is thermally allowed and photochemically forbidden. If we again
assume a homolytic bond cleavage for purpose of analysis, we must consider the molecular
orbitals of pentadienyl radical (Fig. 4.3).
Ground state HOMO
TS
Suprafacial migration: The [1, 5} suprafacial shift is symmetry allowed and
geometrically feasible.
Consider photochemical [1, 5] sigmatropic rearrangement. In this case
photochemical HOMO.
)
,\\\\
\\\\
\\\\(f)
+
+
\\\\
TS
Photochemical HOMO
Antarafacial migration, symmetry allowed but geometrically difficult.
Stereochemistry of [1, 5] sigmatropic Rearrangement
Consider the following compound:
Compound has S-configuration at C-6 and E configuration at C-2.
Case I: Suppose migration is suprafacial
[1, 5]
)
Suprafacial
'lfi
will be
If migration is suprafacial then product has R-configuration at C- 7 and E-configuration
at C-3.
Case II: Suppose migration is antarafacial
SH
r?"""'CH,
~CH3
[1,5]
)
Antarafacial
E
C2H 5
If migration is antarafacial then product has S-configuration at C-7 and E-configuration
at C-3.
Experimentally it has been found that the product of the reaction is due to suprafacial
migration. Thus, the theory is confirmed by this experimental result.
4.2.1 Sigmatropic Shifts of Alkyl Group
Sigmatropic migration involving alkyl group shifts can also occur.
,
,
I
J
[1,3]
~) .)("1
R
1, 3-Alkyl shift
I
When an alkyl group migrates, there is additional stereochemical feature to consider.
The shift can occur with retention or inversion at the migrating centre. The allowed process
includes the suprafacial 1, 3-shift with inversion and the suprafacial 1, 5-shift with retention.
Thus, if the group that migrates is bonded to the backbone by a chiral carbon, then:
1. [1, 3] Suprafacial migration of the group proceeds with inversion of configuration at
the chiral centre.
2. [1, 5] Suprafacial migration of the group proceeds with retention of configuration at
the chiral centre.
b
c,,!/a
C
I
CH 2-C=CH 2
I
1
H
[I, 3] Sigma-,< ,hill
b
a,~/c
I
CH 2=C-CH2
I
H
(Inversion)
I
I
I
~
b
~a
c
§
. . . C="
I
CH 2
"­C=C-C=CH
I
H
I
I
2
H H
~
[1, 5] Sigmatropic shift
b
i
c.. . . . ~/
a
C
/
CH 2 =CH-CH=CH-CH2
(Retention)
As compared to a hydrogen atom which has its electron in a Is orbital that has only one
lobe, a carbon free radical (for imaginary TS) has its odd electron in a p-orbital which has two
lobes of opposite sign.
A consideration of the imaginary TS, shows that if in place of hydrogen one has carbon,
then during a thermal suprafacial [1, 5] process, symmetry can be conserved only provided the
migration carbon moves in a manner that the lobe which was originally attached to the 1t (Pi)
system remains attached to it (see figure given below)
1
Homolytic bond fission
)
A thermal suprafacial [1, 5] migration. Configuration is retained within the migrating group.
~
I
'I
'i
I
'[
The only way for this to happen is the retention of configuration within the migrating
group. However, a related [1, 3] thermal suprafacial would involve opposite lobes. Thus, if the
migrating carbon was originally bonded via its positive lobe, it must now use its negative lobe
to form the new C-C bond. The stereochemical outcome of such a process is the inversion of
configuration in the migrating group is shown below:
)
~
~
~
J5
!
1
r::1
+
(
+ ,~.L.-
""',\±,,·f--.=f"""\
T.S
A thermal supraficial [1, 3] migration. Configuration in the migrating group will be
inverted.
Compound (1) when heated at 300°C it gives compound (2). This is 1, 3 sigmatropic
shift with inversion at the migrating centre.
[1, 3] C-shift
~
i'
i'
H
D
(2)
Inversion of configuration
Let us use orbital symmetry to explain the observed result.
As the crucial bond between the alkyl system and the migrating carbon stretches, during
the course of formation of transition state, the phase relationship between two bonded lobes
must be mentioned.
Stretch )
C-Cbond
~
~
~
Developing two p-orbitals
!~
We also know the symmetry of HOMO of the developing allyl systems and can fill in the
lobes as in Fig. 4.4.
Bonding overlap
D
TS
D and AcO are trans
'/
Symmetry forbidden b~
geometrica]1y- feasilile
(a)
~
AcO
1
(TS)
Rotation
Further rotation
)
D and AcO are cis
(b)
Fig. 4.4
In this case migration occurs using the back lobe of migrating carbon, and now a bonding
interaction is created. The migrating carbon suffers inversion as reattachment takes place to
the position-3 of the allyl framework. In order to preserve bonding overlap with C-l and C-3
rotation must occur, and the trans starting material is thus converted into cis product.
4.2.2 Selection Rules for Sigmatropic Rearrangement
The stereochemistry (i.e., migration of group is suprafacial or antarafacial) of sigmatropic
rearrangement is a simply function of number of electrons involved (as with other pericyclic
reactions, the number of electrons involved is easily determined from the curved-arrow
formalism: Simply count the curved arrow and multiply by two). All suprafacial sigmatropic
reactions occurs when there are (4q + 2) electrons involved in the reaction-that is an odd
number of electron pairs or curved arrows.
Selection rule of [1, n] sigmatropic rearrangement when migrating group is hydrogen
atom.
If sigmatropic reaction of the order (m + n) (for hydrogen m =1) has m + n =4q + 2 then thermal
reaction is suprafacial and photochemical reaction will be antarafacial. However, for those
cases in which m + n = 4q then thermal reaction is antarafacial and photochemical reaction will
be suprafacial (Table 4.1).
Table 4.1 Selection rule for [1 + n] in which migrating group is hydrogen
supra
antara
antara
supra
4q
4q+ 2
Table 4.2 Selection rule for [1 + n] in which migrating atom is carbon
sr
al
ar
si
ar
4q
SI
4q + 2
sr
al
In the table s and a refer to supra and antara and rand i refer to retention and inversion
in the configuration of the migrating centre.
4.3.1 Cope Rearrangement
The most important sigmatropic rearrangement are the [3, 3] process involving carbon-carbon
bond. The thermal rearrangement of 1, 5-dienes by [3, 3] sigmatropy is called Cope
rearrangement. The reaction proceeds in the thermodynamically favoured direction.
.
[3,3]
~
3C<~1 CN
3
h
2
1
COOCzH s
(
New cr bond both ends
numbered 3
This particular reaction is called a [3, 3] sigmatropic rearrangement because the new a
bond has a 3, 3 relationship to the old cr (sigma) bond.
The equilibrium in this case is controlled by the conjugation present in the product. The
electron withdrawing substituents and electron donating substituents present at position-3
and/or at -4 favours the reaction in the forward direction. The rearrangement of the simplest
possible case, 1, 5-hexadiene, has been studied using deuterium labelling. For this reaction
activation energy is 33.5 kcallmole and the entropy of activation is -13.8 eu. The substantially
negative entropy conforms the formation of cyclic transition state.
g
C
~
j
D
D
Ea = +33.5 kcallmole
Conjugated substituents at C-2, C-3, C-4 or C-5 accelerate the rearrangement. Donor
substituents at C-2 and C-3 have an accelerating effect. The effect of substituents can be
rationalised in terms of the stabilisation of the transition state by depicting their different
effect on two interacting system (Fig. 4.5).
x
X
y~
(
4~
y~.
)'
1,4 Diradical
~
5
!x
x
y....,A
y~
I
I
1
I
~
Fig. 4.5
The transition state involvl;!s ~j.x partially delocalised electrons being transformed from
one 1, 5-diene system to another;' The transition state could range in character from a
1, 4-diradical to two nearly independent allyl radical, depending on whether bond making or
bond breaking is more advanced. The general framework for understanding the substituent
effects is that the reaction are concerted with relatively late transition state with well developed
C-I-C-6 bonds (Fig. 4.5 and 4.6)
) #'
z-..?
Two independent
allyl radical
1
j;::::1
AromaticTS
Fig. 4.6
1
The most advanced molecular orbital calculations support the idea ofan aromatic transition
state having six partially delocalised electrons. The net effect on reaction rate of any substituent
is determined by whether it stabilises the transition state or the ground state more effectively.
The aromatic concept of transition state predicts that it could be stabilised by conjugated
substituents at all positions.
In Cope rearrangement the migrating group is allyl radical. An analysis of the symmetry
of the orbitals involved shows why this reaction is a relatively facile thermal process but is not
com~only observed on photochemical activation. As we break the C(l)-C(l) bond (Fig. 4.7)
the phases of the overlaping lobes must be the same. The HOMO of the allyl radical is 1Jf2
(Fig. 4.1) and that information allows us to fill the symmetries of the two allyl radicals making
up of transition state (Fig. 4.7).
~3
-C
1
~3-
.03
2
Migration of an
allyl radical
Fig. 4.7
Reattachment at the two C(3) positions (Fig. 4.7) is allowed because the interaction of
the two lobes on the two C(3) carbons is bonding (Fig. 4.8).
+
+
­
§
~ _
H .
~
Symmetry
~=
:=
/
Symmetry
allowed
allowed
+
FlI.4.S Transition state: Two partially bonded allyl radicals-W2
is the HOMO of each
If i~t~action is ca~ried out in the presence of UV light then one electron is promoted
from the HOMO to the LUMO and LUMO will become photochemically HOMO (Fig. 4.9).
)AH
+
~
+
w2 HOMO
­
+-
Symmetry forbidden
~ w: Photochemical HOMO
Symmetrical
allowed ~
"fiIg. 4.1 ~betweeflHOMO· ground and
.
HOMO photochemical
. '~ forbidl:ten process
:.j
Stereochemistry of Cope Rearrangement
The Cope rearrangement usually proceeds through the chair like transition state. The
stereochemical features of the reaction can usually be predicted and analysed on the basis of a
chair transition state that minimises steric interactions between substituents. Rearrangement
of the meso diene through such transition state then would give the cis-trans isomer while in
the case or~ the rearrangement of the racemic mixture the trans-trans isomer is the major
product and this is actually the result.
H
Mesoforrn
Trans-trans (90%)
H
The above result establishes that chirality is maintained throughout the course of the
reaction. This stereospecificity is a general feature of [3, 3] sigmatropic shifts and has made
them valuable reactions in enantiospecific synthesis.
Preparation of Carbonyl Compounds from Cope Rearrangement
1, 5-Hexadiene-3-o1 on heating undergoes Cope rearrangement with formation of unsaturated
carbonyl compounds. Cope rearrangement given by such compounds is known as oxy-Cope
rearrangement.
HOlE(
Enol
Unsaturated carbonyl
compound
,
I
I
Si~mattopiC
Rearrangement
129
The reaction is accelerated when it is carried out in the presence of strong base. In the
presence of strong base allyl alcohol converts into alkoxide ion which is very stable. Cope
rearrangement given by anion of 1,5-hexadiene-3-01 is known as anionic oxy-Cope
rearrangement.
0Qf
e
H-OC
KH
~
Allyl alcohol
[3,3]
Anion of alcohol
e
O~
~
~
@
H 20lH
)
HO~
Tautomerisation
)
Enolate ion
0:J
Product
Alkoxide ion undergoes Cope rearrangement to give the product in which negative charge
is in conjugation to 1t (Pi) bond. This conjugation makes the anion very stable.
Iminium compound of type (I) also gives Cope rearrangement. Which is known as Aza
Cope rearrangement.
~
)II~
C'BNH
[3, 3]
R
R'
Compound (I) can be prepared from 4-aminoalkenes.
H 2N-CH 2-CH 2-CH=CH2
4-Aminoalkene
4-Aminoalkenes react with carbonyl compounds to give iminium compound of type (I).
H 2N-CH2-CH 2-CH=CH 2
1
CH 20 I
fI
ffi
CH2= NH-CH 2-CH2-CH=CH2
Iminium compound of type (I) is very useful when it is prepared from 4-(trimethylsilyD­
3-alkenylamines because in this case rearranged product undergoes cyclisation to give six
membered nitrogen heterocyclic compound. Thus, the overall reaction is as follows:
130
Photochemistry and Pericyclic Reactions
R-NH-CHz-CHz-C
=C-R
1
CHzOIH$·
,
EB
CH z = N-CHz-CHz-CH=C-R
I
I
I
I
Si(CH 3h
R'
III
[3, 3]; Aza-Cope rearrangement
)
Vinylsilane derivative
Allylsilane derivative
1
Cyclisation
y
R'-N
R
4.3.2 Claisen Rearrangement
Claisen rearrangement is the first sigmatropic rearrangement which was discovered. The original
sigmatropic rearrangement occurs when allyl phenyl ether is heated without solvent. The
product of the rearrangement is o-allylphenol.
OH
&
I
~
CH z
'CH
II
.~
CH z
o-Allylphenol
The above Claisen rearrangement is two step reaction. The first step in this reaction is
[3, 3] sigmatropic rearrangement
[3,3]
~
~
OCH
I/z
~(hCH
H
Ctl z
.....\
f
Sil;Jmattopic Rearrangement
131
This is one step mechanism without ionic intermediates. In this case numbering start
from the heteroatom oxygen having (J (sigma) bond and allylic carbon of the allyl group. The
second step in the reaction is a simple ionic proton transfer to regenerate aromaticity.
rt°
~H
CH 2-CH=CH 2
In this reaction allyl group turns inside out which is confirmed by unsymmetrical allyl
ether.
0
: :,. . I
0,
(0d
CH 2
)
I
(ii) Proton transfer
CH
II
C
H 3 C""'" 'CH 3
The conversion of allyl phenyl thioethers to a-allyl thiophenols, referred to as the thio­
Claisen rearrangement, is not possible as the latter compounds are unstable but they instead
give bicyclic compound.
)
r·l
Thio Claisen
rearrangement
GJ
~SHII
~
)
o-Allylthiophenol
H
But if the oxygen atom in allyl phenyl ether is replaced by nitrogen, then the normal
Claisen rearrangement takes place to afford the amino derivatives.
CCJ
~
.--::::­
~
)
13,31
OJ
~
I
H
1~~
~
((j
o-Allylaniline
.
.
Dl:
III
Do
C
Z
U
132
Photochemistry and Peri~lc<Reactions
Claisen rearrangement is also given by allyl vinyl ether and in this case rearrangement
is known as aliphatic Claisen rearrangement or Claisen-Cope rearrangement.
1
~
2
3
1
Old (J (sigma)
bond broken
1
[3,3]
~
2
3
o
N ew (J (sigma)
bond formed
The starting material of aliphatic Claisen rearrangement is allyl vinyl ether which can
be prepared with allyl alcohol and vinyl alcohol. Allyl alcohol is stable compound but vinyl
alcohol is not. Thus, instead of vinyl alcohol acetal of acetaldehyde and methyl alcohol is used.
i
When this acetal is treated with allyl alcohol, formation of vinyl allyl ether takes place.
[
Thus, overall reaction can be written as follows:
I
erR
y, Il-Unsaturated
aldehyde
In this reaction orthoesters and orthoamides or amide acetals can be used in place of
acetal or ketal and product of the reaction is y, 0 unsaturated ester and amide respectively.
\
j
-l
I
-,
~
11
Sigmafropic R$arrangemenf
OMe
Me0'toMe
+
OHyR
_ _C2_H_5C_O_O_H~
)
Meoll)R
I
,-:;;
Orthoester
133
~
~
[3,3]
y, a-Unsaturated ester
R,N~R
R,NlJR
~
~
[3, 3]
y, a-Unsaturated amide
Formation of y, 8-unsaturated ester from ortho ester and allyl alcohol is known as
Johnson-Claisen rearrangement, Similarly formation of y, 8-unsaturated amide from
orthoamides and allyl alcohol is known as Eschenmoser-Claisen rearrangement.
Allyl vinyl ether can also be prepared from allyl alcohols and acid chlorides.
Pyridine
---~)
°
II
R-CH=CH-CH 2-O-C-CH 3
1
LDA
~R
I
0y
~9
('8
E9
~(--~) R-CH=CH-CH 2-O-C....!CH 2Li
OLi
Lithium enolate
A combination of an oxygen atom in the chain and another (mt of the chain (at inner
vinylic carbon) is very powerful at promoting [3, 3] sigmatropic rearrangement. Such type of
compound can be used for the preparation of y, 8-unsaturated carboxylic acid via [3, 3]
sigmatropic rearrangement.
~R
~R
[3, 3]
~ )
0y
OLi
OLi
yR
OH
y, a-Unsaturated acid
Sometimes it is better to convert the lithium enolate into the sHyl enol ether before
[3, 3] sigmatropic rearrangement.
LDA
----7
THF
(CH:J3 SiCl
)
~ [3~31)
O-Si(CH 3)3
Silylenol ether
~R
0y
OH
y, a-Unsaturated acid
134
Photoohemlstryand
Cope rearrangement given by silyl enol ether is known as Ireland-Claisen rearrange­
ment.
a-Allyl imidates also give Claisen rearrangement known as Aza-Claisen rearrangement.
,
J,.
)
N-Allylamide
~-ketoesters can rearrange to give ~-ketoacids which will decarboxylate to give
y, o-unsaturated ketone. This rearrangement is known as Kimel-Cope rearrangement.
o
O~
V
/H
030
0
~ [~]
10 ~
1
~3
2
y, I)-Unsaturated ketone
Stereochemistry
These [3, 3] sigmatropic rearrangements happen through a chair-like six membered
cyclic transition state as in case of Cope rearrangement. This chair like transition state allows
us both to get the orbitals right and to predict the stereochemistry (if any) of the new double
bond in the product.
,)
I
The substituent prefers an equatorial position as the material reacts and substituent
retains this position in the product.
[3,3]
)
Thus the resulting double bond strongly favours trans (E) geometry.
,
-J.
All [3, 3] sigmatropic rearrangements have six-membered cyclisation transition state. It is no
accident that the size of the ring in transition state is given by the sum of the two numbers in
the square brackets as this is universally the case for sigmatropic rearrangements. In [2, 3]
sigmatropic rearrangement transition state should have five membered ring structure. [2, 3]
sigmatropic rearrangements have many variants depending upon which atoms are present in
I
1
-J.
Sigmatropic Rearrangement
135
the chain of five atoms. All atoms may be carbons but most have Y(= carbon} andX(= oxygen,
nitrogen, sulphur and carbon).
~0r
2
C~
y.----\
[2.3]
)
)
Y-X
-8
2
-8
TS
~
~
e(
BuLi
)
(0
Ph
[2, 3]
)
Ph
~o
~OH
)
Ph
Ph
Benzylallyl ether
In the given case
Y = Carbon and X = Oxygen.
If X is oxygen and Y is carbon then [2, 3] sigmatropic rearrangement is known as Wittig
rearrangement.
When X = Nand Y = C
The quaternary ammonium ion (A) is deprotonated adjacent to ester group to give the
nitrogen ylide which rearranges with ring expansion to give nine membered ring compound.
(A)
When X = Sand Y = C
Intramolecular alkylation of the sulphide (B) followed by deprotonation gives a sulphur
ylide which undergoes [2, 3] sigmatropic rearrangement.
~CI
S
,-
~
J
K 2 C0 3
)
SN z
(B)
)
0
0
~J
K 2 C0 3
[2,3]
)
.
D:
III
cd
Product
•
0
L
c(
Z
U
~
,
.­
r
1
[5, 5], Sigmatropic rearrangement
IOn
cV';
O)
I
2
I
3
[5, 5]
)
~
I
II
OH
)
H
J
I
1
1
I
Sigmatropic Rearrangement
137
[9, 9] Sigmatropic rearrangement
Benzidine is example of [9, 9] sigmatropic rearrangement
1
[4, 5] Sigmatropic rearrangement
The given quaternary ammonium ion gives [4, 5] sigrnatropic rearrangement.
~N/Me
p~./'--, j "Me
NaOMe
)
~~
w.,_erte'W'.:f'fWrMt{iW·#§n:m,titt',lifilli'''fWBiWt't"itli#r'eF'tiiit;,"'itf'~~f'i.
According to Woodward-Hoffmann rule a thermal (ground state) sigmatropic rearrangement
is symmetry allowed when total number of(4q + 2)8 component and (4r)a component is odd.
Similarly a sigmatropic change in the first excited state is symmetry allowed when
Itotal number of (4q + 2)8 and (4r)a component is even.
4.6.1 The Woodward-Hoffmann Rule for (m, n) Sigmatropic Rearrangement where Migrating
Group is not Hydrogen
\
'Here we will discuss [3, 3] sigmatropic rearrangement and in this case we will consider the
blaisen rearrangement.
.
1. Draw the mechanism for the reaction.
1
o
2~jl
3~jJ2
3
~
138
Photochemistry and Per!cyclic Reactions
2. Choose the components. Only the bonds taking part in the reaction mechanism must
be included.
.~:
1t
3. Make a three-dimensional drawing ofthe way the components come together for the
reaction, putting orbitals at the ends of the components.
4. Join up the components where new bonds are to be formed. Make sure you join orbit­
als that are going to form new bonds.
o
5. Label each component s or a. See below for the
1t
and cr bond symmetries.
(a) Whether 1t component is s or a
If both upper lobes or both lower lobes ofthe 1t (Pi) component are involved in the reaction then
the component will be s and it is label as 1t2s.
Ifone upper lobe and other lower lobe of the 1t (Pi) component are involved in the reaction
then the component will be a and it is label as 1t2a.
Upper lobe of
1t component
Jt2S
(4q
+ 2, q =0)
Upper lobe of
1t component
Upper lobe of
1t component
Lower lobe of
1t component
One lobe is upper and one lobe is
lower of this 1t component hence
this component is 1t2 a
Both lobes are upper of
this 1t component which are
taking part in the reaction
hence this 1t component is 1t2s.
1
Sigmatropic Rearrangement
(b) Whether
(J
component is
8
139
or a (when migrating atom is not hydrogen)
If sp3-hybrid orbital uses its large lobe for reaction then there will be retention or the
small lobe then there will be inversion.
Or
Iflarge lobe of one orbital ofthe (j bond (which is undergoing cleavage) interacts with
p-orbital of the adjacent atom (say atom-2 of allyl system) then at this end there will
be retention.
If small lobe of the other orbital of the (j bond (which is undergoing cleavage) inter­
acts withp-orbital of the adjacent atom (say atom-2 of vinyl system) then at this end
there will be inversion (Fig. 4.10).
(ii) If there is retention at both ends or inversion at both ends then (j component is (j2 s.
If there is retention at one end and inversion at other end then (j component is (j2 a
(Fig. 4.10).
(i)
Small lobe of orbital
which forms a (sigma) bond
Allylic carbon
0
............. "" +
.",
This small lobe of another
orbital of same a (sigma) bond
interacts with p-orbital of the
adjacent atom of the vinylic
system hence there is
inversion in this end
Vinylic
carbon
Large lobe of one orbital of cr
bond interacts withp-orbital
of the adjacent atom of the
allylic system hence there is
retention in this end
Large lobe of orbital
which forms (J bond
a orbital
which is
undergoing
cleavage
Fig. 4.10
Thus, (j component has retention at one end and inversion at another end hence
component is (j2 a .
6. Count the number of(4q + 2)s and (4r) a components.
Number of (4q + 2)s component = 1
Number of (4r)a component
= 0
Total
(j
= 1 (odd) thermally allowed.
Note: n 2 a (4q + 2)a and cr2a (4q + 2) components have irrelevant symmetry and are not counted.
4.6.2 The Woodward-Hoffmann Rule for (m, n) Sigmatropic Rearrangement where Migrating
Group is Hydrogen
Consider [1, 51Sigmatropic hydrogen shifts
1. Draw mechanism for the reaction.
H~
H
[1,5]
)
Suprafacial
flf»
H yy;\
H'
H H
140
·Photochemistry ·and Pencyclic Reactions
2. Choose the components. All bonds taking part in the reaction mechanism must be
included and no others
3. Make a three-dimensional drawing of the way the components come together for the
reaction, putting in orbitals at the ends of the components.
4. Join up the components where new bonds are to be formed. Make sure you join
orbitals that are going to form new bonds.
l
- .......... ,'\
~
II ':-----
1f======i:~ '~~
Retention
1
Retention
5. Label each components s or a. See below the (j (C-H) bond symmetry.
(i) The Is orbital is spherically symmetrical and has no node, so whenever you draw the
dotted line from Is orbital it always means retention.
Or
Orbital of hydrogen which forms (j (s!gma) bond is always treated as large lobe
~
T
.T
Large
lobe
Large
lobe
~
One cr (sigma) bond two
hybrid orbitals
One cr (sigma) bond two orbital,
one is hybrid and other is Is
Large Large
lobe lobe
t t
I
I
I
I
J
l
l
.~
Sigmatropic Rearrangem~nt
141
(ii) If large lobe of one orbital of 0 (C-H) bond (which is undergoing cleavage) interacts
with p-orbital of the adjacent atom (atom-2) then at this end there will be retention.
If small lobe of the one orbital of 0 (C-H) bond (which is undergoing cleavage) interacts
with p-orbital of the adjacent atom (atom-2) then at this end there will be inversion.
6. Count the number of (4q + 2)8 and (4r)a components.
(4q + 2) ~ 0 2 ~ 02S
(4r)
~
n4 ~
n 4s
Number of (4q + 2)s component = 1
Number of (4r)a component
=0
Total
= 1 (odd) ; thermally allowed.
According to Ruckel-Mobius rule thermal sigmatropic rearrangements take place via aromatic
transition state whereas photochemical sigmatropic rearrangements proceed via antiaromatic
transition state. Consider the following two rearrangements:
(i) [1, 3] Sigmatropic Rearrangement
Suprafacial [1, 3], 4 electrons,
zero node Huckel system, anti
aromatic, hv allowed
R"C-CH=CHo
R/I ~
~
~
H
Antarafacial, 4 electrons,
1 node aromatic, mobius
sYlitem, ~. allowed
142
Photochemistry and Pericyclic Reactions
(ii) [1, 5] Sigmatropic Rearrangement
R~
C>.
[1,5]
JC:>
RL-/
C-CH=CH-CH=CH 2
)
Suprafacial
6 Electrons, zero node, aromatic,
Huckel system, 6. allowed
R" C-CH=CH-CH=CH
~
~
R/
I
2
Antarafaclal
H
Node
6 Electrons. one node, antiaromatic,
mobius system, hv allowed
I
The selection rules for sigmatropic rearrangement of order (1, j) by this method are
summarised in table 4.3.
Table 4.3 Selection rules for sigmatropic rearrangement by H-M method
Number of electrons
involved (1 + j)
Number of nodes
4n
o
4n
1
4n + 2
o
4n + 2
1
antiaromatic
aromatic
aromatic
antiaromatic
supra, hv
antara, L!.
supra, L!.
antara, hv
1.
\,
I
(iii) [3, 3] Sigmatropic rearrangements
c
~
~llyl
~.•JAJlYl
The transition state for such processes is represented as two interacting allyl fragments as
mentioned earlier. In this there can be two stereochemical variations-suprafacial-suprafacial
(or antarafacial-antarafacial) and suprafacial-antarafacial as shown in the following transition
states:
!
~
1
Sigmatropic Rearrangement
143
r
I
I
I
i
Suprafacial-antara
facial
Supra-Supra
1I11l111J!l1I1l11I11I1
-Illllillfllllllllllll
6 Electrons, zero node, aromatic,
Hiickel system, tl. allowed.
+ _:--­
Node
6 Electrons, one node, antiaromatic,
mobius system, hv allowed.
~~xW1!!'Vi!'jJ~Wft:~ri#i#}!i';)#I¥t:.~• •i;.-.. 4MP3M,X1;4.,lhJI~"",'lA'i~$lM$k,.D(N'i'!',*~4';i,~·'ti1¥!g;,,&$w;~
,';.;:i~~~'f?~lh~flJf'!!fiMJlf~'j!J't}!f~·/fl!:fJ!
jtg~!!9Elf~~~!fRJls.q~~)~!5,~sg~~~~~~.~stiL_~,,~~~~
Cope rearrangement is the most general reaction of compounds having 1, 5-diene substructure
aCI
2
4
...(1)
~6
5
Ifwe add a 1t (Pi) bond between C-3 and C-4 then we will get cis 1,3, 5-hexatriene. This
compound can also be treated as 1, 5-diene substructure, therefore this compound will give
Cope rearrangement.
o
...(2)
Cope rearrangement given by 1, 5-hexadiene is known as degenerate Cope rearrange­
ment. Here degenerate means structure of reactant and product is same (Eqn. (1». Similarly,
rearrangement given by substructure of 1, 5-hexadiene, i.e., 1, 3, 5-hexatriene is known as
modified Cope rearrangement. In modified Cope rearrangement structure of reactant and
product are not same (Eqn. (2».
1,3, 5-Hexatriene give Cope rearrangement only when it has cis geometry. In the trans
isomer, the end of the triene system are too far apart for bond formation hence this does not
give modified Cope rearrangement.
Trans-I, 3, 5-hexatriene
o
::)imilarly, cis-I, 2-divinylcyclopropane undergoes Cope rearrangement but trans-1, 2­
divinylcyclorY'opane does not.
o
Cis-I,2-Divinylcyclopropane
@
1, 4-Cycloheptadiene
No Cope rearrangement
)
Trans-I,2-Divinylcyclopropane
These examples confirm that for Cope rearrangement both double bonds should have
cis geometry, i.e., both double bonds should be nearer to each other.
There is another stereochemical problem in this reaction. The most favourable
arrangement for cis-I, 2-divinylcyclopropane is shown in Fig. 4.11. The two vinyl groups are
directed away from the cyclopropane ring and away from the syn ring hydrogen. But this
stable form cannot undergo Cope rearrangement at all.
~
;~
H
H
H
H
®
Most stable form of
cis-I,2-divinylcyclopropane
)
Trans-trans Diene
(highly unstable, behaves as RI)
Fig. 4.11
Cis-I, 2-divinylcyclopropane undergo Cope rearrangement with less favoured coiled form
(Fig. 4.12). In this case geometry of two vinyl groups point back towards, the ring and are
closer to the offending syn hydrogens.
h
.i
H
:--:::: H
H
H
Cis-cis Diene
(stable compound)
Fig. 4.12 Less favourable, less stable coiled form
Homotropilidene undergoes degenerate Cope rearrangement rapidly.
Homotropilidene
Q
Still homotropilidene
Like 1, 2-divinylcyclopropane, the most stable extended form ofhomotropilidene is unable
to undergo the Cope rearrangement, and it must be the higher energy coiled arrangement
that is actually active 'in the reaction (Fig. 4.13).
\
,
l
~
....\
~
\
1
;
Sigmatropic Rearrangement
Extended form
145
Coiled form
Fig. 4.13 Two forms of homotropilidiene
From the above examples it has been confirmed that only those 1, 5 diene substructure
give Cope rearrangement whose geometry is cis and have coiled structure.
Compounds having these characteristics undergo Cope rearrangement very rapidly even
at room temperature and below room temperature.
Although such compounds undergo Cope rearrangement very rapidly, one might consider
how to make the reaction even faster for the reaction to occur, an unfavourable equilibrium
between more stable but unproductive extended, and the higher energy but productive coiled
form must be overcome. The activation. energy for the reaction includes this unfavourable
equilibrium (Fig. 4.14).
E a for Cope rearrangement
from extended form
Ea for Cope rearrangement
from coiled form
E
I
Extended form
-------i~~
Progress of reaction
Fig. 4.14 Activation energy of reaction for two forms of homotropilidene
If this unfavourable equilibrium could be avoided in some way, and the molecule
locked into a coil form arrangement, the activation energy would be lower (Fig. 4.14) andthe
reaction will occur faster.
Iftwo methylene groups ofhomotropilidene are bridged then the corresponding molecule
will exist only in coiled form and not in extended form. Since coiled form is responsible for
Cope rearrangement, obtained compound(s) should undergo Cope rearrangement rapidly
(Fig. 4.15).
Bridging between these two
methylenes will lock the molecule
only in coiled arrangement
Q
Extended form (more stable)
Coiled form (less stable)
)
The molecule will exist
only in coiled form
Fig. 4.15 Bridging between two methylene carbons which locks the molecule in coiled form
·146
Photochemistry and'Pericycllc Reactions
This modification greatly increases the rate of the Cope rearrangement as the activation
energy is decreased (coiled structure, higher energy, lower activation energy, Fig. 4.14). No
longer it is necessary to fight an unfavourable equilibrium in order for Cope rearrangement to
occur because the molecule obtained in this way exists only in coiled form.
Many bridged homotropilidenes are now known and are given in Table 4.4.
Table 4.4 Bridged homotropilidenes
Semibullvalene
Barbaralone
'tf"
Dehydrobullvalene
Barbaralene
Bullvalene
!!'~tu'Xi~oaMe'tt~aiii).J';;;
4:;zUi; W:;;;;...M.t;t;;;,;;; 4;4 .Nt. 14M M,IQ ··,'l!!";Il~9M1'#'i!!!.'''AA;;t&!§
:ikf• •rl1'Itt'.' IiSirlit'-"; -tttt" r-"'·w--,/:"-Wt':, '-'reo ,3rt"""@" YtfIFtttfe'~Wt&f¥tt:teWft'f-'-~fifft"--' -t r ·tYd'f f1 YfnfwaeW-'tW"rfiiYfw f,t'W"'yWf;rr '11" 'tt ;'!ft'7;''tWtf'('t'',·\"-",,
A number of compounds continually undergo rapid degenerate Cope rearrangement at room
temperature. One such compound is bullvalene which was first prepared in 1963 by G. Schroder
from cyclo-octatetraene as follows:
0
hv
~
g
I
I
~
I
l
C16H 16
lhv
g
+
0
ClOR IO
The [3, 3] sigmatropic rearrangements in bullvalene rapidly interconvert identical forms
of the molecule.
o(
>@ (
>
Q (
>M=yoth",
Bullvalene: Tricyclo [3, 3, 2, OJ deca 2, 7, tricyclo.
Sigmatropi~ Rearrangement
147
Ifthe carbons could be individually labelled, there would be 1,209,600 different structures
of bullvalene in equilibrium. Each one of these forms is interconverted into another at a rate
of about 2000 times per second at room temperature.
Molecules such as bullvalene that undergo rapid bond shifts are called fluxional
molecules. In fluxional molecules their atoms are in a continual state of motion associated
with rapid changes in bonding. The rearrangement may involve either bond reorganisation or'
atom (or group) migration.
Other neutral completely fluxional organic molecules have not appeared, although the
phenomenon of fluxionality appears to be rather more common in organic cations and
organometallic compounds. The fluxionality of the cr (sigma) bonded metal cyclopentadienide
involves atom migration.
~M
bd-'H
WH~
~
H~
M'bd_
M
Fluxionality is most readly ascertained by means of nuclear magnetic resonance
spectroscopy. Conversion of one structure into other in fluxional molecule is known as valence
tautomerism and isomers are known as valence tautomers.
nt within the
.attached to
clunent to the chain has
the
migrated from
to the nth position alan
[m. nl Sigmatropie shift (m '# 1): A sigmatropic s
rearrangement. The number m deno .
DlJDlbe
attached to-the migrating
. Th
asm {t, nl-shift specifies the
number ofthe atom in t
'grating group. The numbering ofatoms (m,
n) on both the chain and migra:
s at the atoms originally attached to the migrating
bond.
/
Cope real"l"angement: The thermal [3, 8l-sign1atropic shift in a I, 5-diene resulting in its
/ isomensation is known as Cope rearrangement.
Claisen.
induced rearrangement of an allylphenyl ether to an
occupied, the rearrangement gives the para isomer via
a subsequent Cope rearrangement. Claisen rearrangement is [3, 3] sigmatropic shift. The re.action
also proceeds with allyl .
.elding y, O-unsaturated carbonyl compound.
Fluxional molecules: !\fo
which undergo rapid degenerate rearrangement, that is,
rearrangement into indistinguishable molecules; the rearrangement may involve either bond
reorganisation' or atom (group) migration.
148
Photochemistry and Pericyclic Re$ctions
FURTHER READING
1. T.S. Stevens and W.E: Watts, Selected Molecular Rearrangements, Van Nostrand Reinhold,
London, 1973, Chapter 8.
2. H.J. Hasen, Mechanism of Moleculi#·Mig,.ati~'n$Ji!Vo~.<~,~ileYlntersciencej
New York.
3. A.P. Marchand and R.E. Lehr, Pericyclic ReOl!Jtion~, Vol. land.Il, Academic Press, New York,
1977.
4. F.A. Carey and R.J. Sundberg, Jid~ancedqrgqti~ C~mi,try,.4,th ed., Plenum Press, New
York, 2000.
5. J. Clayden, N. Greeves, S. Warren andP. Wothet.s, Organic Chemistry, 1st ed., Oxford
. University Press, Oxford, 2001.
PROBLEMS
I
1. . Which of the following sigmatropic rearrangements would proceed readily and which slowly?
Explain your answers.
)
IOQ
D
H.
[Hin~
This can proceed by a
[Hint: This can proceed by a {3, 31 sigmatropic shift. Thus thia can proceed readily.]
. 2. Classify each of the following siginatropic rearrangements as [1, 3], [3, 31, etc.
c:::
(i)!
6
6
(iii)
~
(X
~
I
>
l
)
CH-cn-CH-CH2
)
OD
l
I
1
Sigmatropic Rearrangement
149
3. Explain the following observatiOns:
0
HaC
H C/C····"tqCH
5
2
\
3
H
[Hint: [1,5] Migration is suprafacial.l
4. Give stereochemistry at the ehiral centres in the given sigmatropic rearrangements.
C2H5
Ph'~"CH3
I
CHa-CR C
[Hint: (i) [1, 3] Supr
of alkyl group proceeds with inversion of configuration
at the chir
(ii) [I,
p proceeds with retenJ~on of configuration
atth
6. Explain why in the ,th
geometrically fea~ible by ~ forbid
8. [1,3] Sigmatropic shift ofhYdrogenis~
7. Explain why fl, 5) sigmatropic shifl; ofh
8. .(i) Discuss sigmatropic; shift of ttlItYl
(ti) Discuss the Btereochemi~oitbei
backbone by a chiral carbon.
9•. With the help ofFMO method, explain
under thermal and photochemical
10. Give reasons for the equivalen
11. Explain why trans-divinylc:yclo
sible to synthesise the cis-iao
12. Write explanatory notes on
.(i) Fluxional tautomerism.
(iii Claisen rearrangement.
(iii) [I, 3) Sigmatropic reat;i .
13. Using PMO method prove,tha
aQgement occurs preferentially through a chair-like
rather than a bOllt like tr&isition state.
14. .Do y()u expect the [1, 3] shift shown below to occur when propene is heated? Explain, using a
molecUlar orbital argUment.~.
i
i
~
]
l
1
·SigmetropiC'Rearrangeme.,t·
-,:"iI,o ",-,. ;
-- ,
",,"
,.. ;-:-.":;.;,,., •
~
{Hint" 'This ,scrambling
28. The giV'en compound (
Explain.
'-~
-~,,-',;
c
1~1
"
I
I
l
l
\
C.HAPTER
p Transfer Reaaio
A pericyclic process involving the transfer of one or more groups or atoms from one molecule to
another is known as group transfer reaction. There are only a few reactions in this class. Ene
reaction is one ofthe most common group transfer reactions. Another well known group transfer
reaction is reduction of alkenes and alkynes by diimide.
Ene reaction involves the thermal reaction of an alkene (called ene) having an allylic hydrogen
with a compound having multiple bond (X=Y, X::::::Y), called enophile.
During the reaction, transfer of allylic hydrogen (1, 5 migration of hydrogen), shift of
allylic double bond and bonding between two unsaturated termini (one terminus of ene and
other terminus of enophile) takes place to form 1 : 1 adduct.
2~3>~4
1~~Y5
ene
In this reaction
X=Yis
C=C
C=O
C=S
C=N
N=O
-N=N­
0=0
("f
Y-H
enophile
... (1)
Adduct
X==Y is
C==C
C_N
In ene reaction hydrogen atom of allylic carbon moves from ene to enophile. In principle
atom other than hydrogen from allylic carbon can move from ene to enophile. In practice the
only elements other than hydrogen commonly employed in this kind of reaction are metals
like lithium, magnesium'or palladium. When metal moves the reaction, it is known as metalla­
ene reaction.
153
1,.1-----­
I
...(2)
Adduct
In ene reaction there is a loss of a 1t (Pi) bond and gain of two 0" (sigma) bonds. In this
reaction 1t (Pi) bond of enophile is replaced by two cr (sigma) bonds of ene, therefore, this
reaction resembles cycloaddition reaction. This reaction also resembles [1, 5] sigmatropic
rearrangement because hydrogen migrates on atom-5 (equation-I) but reaction is neither
sigmatropic nor cycloaddition reaction. This reaction is six electrons cycloaddition reaction.
In this reaction hydrogen moves from ene to enophile. Due to this reason this reaction is an
example of group transfer reaction. This reaction is like Diels-Alder addition. In Diels-Alder
addition all six electrons are 1t (Pi) electrons. In this reaction, out of six electrons, four electrons
are 1t (Pi) electrons but two electrons are cr (sigma) electrons. Thus, activation energy of this
reaction is greater than the Diels-Alder reaction. Due to this reason ene reactions take place
at higher temperature than Diels-Alder reaction.
i
200°C
(
+
)
...(3)
H
(
25°C
~o
)
...(4)
o
!
,.~.
Fortunately many ene reactions can be catalysed by Lewis acids. In the presence of
Lewis acids as catalyst reaction proceeds under milder conditions. As far as catalyst is concerned
the best result is obtained with alkyl aluminium halides.
CH2
II
CH/C
. . . . . CH 2 ..........
3
+
'CH
II 2
CH-COOCH 3
i.
)
230°C
H
1
I~COOCH"
C2HsAlC1 2 125°C
... (5)
Like Diels-Alder addition, ene reaction is also a reversible reaction. I-pentene gives
ethene and propene at 400°C.
1
J
l
....
i
...(6)
As mentioned earlier that enophile need not be alkene or alkyne derivatives; heteroeno­
philes are also known.
CH-COOC 4H g
+
II
... (7)
N-Ts
°II
+ C-H
I
CHs
H£b +~
Singlet oxygen
CH 2 0H
~CH,
-~)cb
... (8)
... (9)
O-O-H
Intramolecular ene reaction has great potential for the synthesis of cyclic compounds,
particularly for the synthesis of five membered ring compounds from 1, 6-dienes (or l-keto-6­
ene compounds).
...(10)
Thermal cyclisation of 1, 8-diene-3-one provides a useful method for the preparation of
cyclopentyl vinyl ketones by intramolecular ene reaction.
)
Enolisation
Similarly 7, 10-alkadiene-2-one gives intramolecular ene reaction.
156
I
.' Photochemistry and Pericyclic Reactions
350°C
-E-no-h-'sa-t-io-n~)
t
I
H3C~?H
V
~
9H3
c=o
1
Enolisation
<t' ~H'<:e7~
...(12)
The concerted mechanism is allowed by Woodward-Hoffmann rules. The transition
state involves the 1t (Pi) electrons of the ene and enophile and the cr (sigma) electrons ofthe
C-H bond of ene.
H
HOMO of allyl free radical
-
~
=
==
~
+
.
~~
@ 1s orbItal of hydrogen
~MO
of .nophil,
+
­
The ene reaction is bimolecular therefore a concerted ene reaction corresponds to the
interaction of a hydrogen atom with the HOMO of an allyl radical and the LUMO of the
enophile and is allowed.
I
:t#::::#
:::=:::.:':~:::.:::;::;:':
It has long been known that isolated carbon-carbon double bonds can be reduced with hydrazine
in the presence of oxidising agents. The actual reducing species in these reactions is in fact the
highly active species diimide, NH= NH, formed in situ by oxidation ofhydrazine. In fact diimide
is an ephemeral species which results from the decomposition with potassium azodicarboxylate,
anthracene-9, 10, diimine and hydrazine and its derivatives.
o
II
KO-C-N=NCOOK
AcOH
23°C
NH=NH
EtOH
5Q-60°C
Ii
"~
Y­
.'\ii
,~
Although this species has not been isolated, its transient existence has been proven by
mass spectroscopy and by its reactions in which it hydrogenates organic compounds with
concomitant evolution of nitrogen (Eqn. 1)
"C=C/ + NH=NH
... (1)
"
/
The reaction of diimide with alkenes is a concerted group transfer reaction proceeding
through a six-membered transition state. It results in predominant syn (cis) addition of hydrogen
and is therefore very useful for stereospecific introduction of hydrogen on to double bonds. The
mechanism explains the high stereospecificity of the reaction, and couples the driving force of
nitrogen formation with the addition reaction.
Concerted cis-transfer of hydrogen is symmetry allowed for the ground state reaction.
Applications of diimide are very rare and are mainly limited to hydrogenation of carbon-carbon
multiple bonds and azo compounds to hydrazo compounds.
H"C/COOH
II
COOH
I
ND 2-ND 210 2 1CuS0 4 EtOH I!!.
)
HOOC/C"H
H"C/COOH
"
H/C"COOH
H-C-D
I
D-C-H
I
COOH
+ Enantiomer
COOH
ND 2-ND 2 1°21 CuS0 4
I
)
EtOHI!!.
H-C-D
I
D-C-H
I
COOH
+ Enantiomer
Meso form
These two reactions confirm that ad~iti(;., is stereospecific syn addition reaction.
Partial reduction of alkyne into alkene by the reagent is also stereospecific
CH3-@--S02-NHNH2
Diglyoxime I!!.
)
Cis-stilbene
In this reaction no trans stilbene was detected.
In sterically hindered molecules, addition takes place to the less hindered side of the
double bond.
---~r
I
I
1'58
I
NH2-NH 2 1 0 2 1CuS0 4
)
EtOH/d
GLOSSARY
Ene reaction: A reaction of a double
syste
the olefinic) terminus of
the
reaction that is insensitive to thEl Stlbs"tittlents Ulvo][vel:!.
.
is
l
:.
FURTHER READING
1. W. Carruthers, Some Modern Methods of Organic Synthesis, Cambridge University Press,
1993.
2. W. Carruthers, Cycloaddition Reactions in Organic Synthesis, Pergamon Press, 1990..
PROBLEMS
1.
2.
S.
4.
Define group transfer reaction and give its el'amples.
.
In what way ene. reaction is related to {l, 5] sigmatropic shift; and Diels-Alder reaction.
Give mechanism of ene reaction.
Reduction ofalke
anism and ster
5.
alIa-ene
t is diffel\en(~e l:let'llVeEln
6.
the foll<lwing reacti9t1s and give
(a)
<::::::::
d
)J
IBint: This is ene reaction.}
[Hint:
~]
1
I
Group Transfer Reactions
159
6
CHAPTER
duction and Basic Prin
of Photochemistry
' 6.TAilE'NERe\,uartMOi:teUt1t,U!M''£tQ%4ji!iZJi,qii!tQQ"Zk
ittffl~11W";'tttttW&':lftli'i¥ff't!'"e'WW""'"'r""
"";"1'"'1''' :"C"''tWe''IW"ft'"'''-'''"'''''''h;'.''' -t""~'W"lftft--
,ali 44;;#[,2$&JI%1$ ,&14#4.)£ ,I44Jt ,zM'
'-tl1t;"""""""~f'tf'f'Yt-~-@''',r''k~ttw't¥''1''-~;''''t1r'lit'
f"'t""
'~'W,' -ttr"~K''''Yi;'''¥
Total energy of a molecule is sum of electronic energy, vibrational energy, rotational energy
and translational energy.
E Total = E e1ec + E Vib + E rot + E trans
The translational energy is not quantised whereas all other three energies are quantised
and furthermore
E e1ec » E vib » E rot» E trans
Translational energy is (only 4% of the total energy) negligible. Thus, the energy of
molecule is
ETotal = E e1ec + E vib + E rot
These energies depend on the property of the molecule that is size, shape, flexibility as
well as on the type of motion. These energies are quantised that is they can change only by
discrete sums (~E = hv),
The various energy levels in the molecule are shown in Fig. 6.1. Suppose A and Bare
the two electronic states indicated by electronic quantum number n (= 1, 2, 3, 4, .....). In each
electronic states there are vibrational energy levels indicated by vibrational quantum number
V (= 0, 1, 2, 3, 4, ....). Again for each vibrational state (energy levels), there exist several
rotational energy levels indicated by rotational quantum number J( = 0, 1,2,3,4,5, ....).
Translational energy is not quantised and changes in continuous manner.
There are two different means by which energy can be supplied to the molecules. First
changing the temperature produces a continuous increase in energy. Second a quantum of
energy may be absorbed by a molecule from electromagnetic radiation. These two modes of
energy input give rise to thermal chemistry and photochemistry.
Thermal Energy
When thermal energy is given to the molecules, molecules move more rapidly: that is
translational energy increases. A molecule can move with any velocity because translational
energy is not quantised. At a given temperature there will be an energy distribution among
the molecules, some molecules have higher velocity and some have lower velocity than the ~
average velocity of the molecule. Only a few molecules will have sufficient energy (known as
threshold energy) to react. At higher temperature a large fraction of molecules will have
threshold energy and the rate will increase (Fig. 6.2).
160
J
3
2
1
0
3
2
1
0
3
2
1
0
3
2
1
0
V=3
V=2
V= 1
V=o
B(n = 2)
J
3
2
1
Electronic transition
I1n = n2 - n1 = 1
o
V=3
Vibrational transition
I1V=Vs -V2 = 1
alongwith
rotational transition
3
2
1
o
V=2
3
2
1
o
V= 1
••
Rotational transition
I1J=3-2=1
V=O
A(n = 1)
3
2
1
o
Fig. 6.1 Different energy levels of the molecule
As the temperature is raised, the molecules can acquire additional vibrational and
rotational as well as translational energy. Vibrational and rotational energy are quantised,
that is they can change only by discrete sums (Fig. 6.1). This can be illustrated more simply
for a diatomic molecule by means of Morse curve (Fig. 6.3). If we imagine two atoms coming~
together from a distance they may eventually join to form a chemical bond and the energy of
the system will decrease and will be minimum. If the internuclear distance decreases below
equilibrium position (that is bond length between two atoms of the molecule) nuclear-nuclear
f
repulsion increase rapidly and energy rises. The molecule thus finds itself in a potential well
corresponding to chemical bond (Fig. 6.3).
:€;>­
:to)
en'"
0) 0)
-c:
:J 0)
~.Q
O'Q)
E .5
-~
0c:
"'0)
0)
>
.0 .­
EO)
:J[\l
z
Kinetic energy
Fig. 6.2 Energy distribution in molecules at two different temperatures
E
r
~~~~~~~;;:::::=T:B:o:nd~diissoCiafion
energy
:It-----,I'------------------. ------..
: --------- Zero point energy
,i
Bond length (re)
Internuclear distance
Fig. 6.3 Vibrational energy levels
Within the potential well molecule can occupy any of a number of discrete vibrational
energy levels. Solution of Schrodinger wave equation leads to the result that vibrational energy
levels are quantised with values
E = hv (V +
i)
..
(1)
where E is the vibrational energy and V is the vibrational quantum number.
These values are represented by equally spaced horizontal lines (Fig. 6.3). Note that the
minimum vibrational energy is not zero but
~ hv.
2
E= hv (V
+~)
--~
\
.1
...(2)
.
This energy ( Eo = %
hvo ) is called zero point energy. This confirms that molecule can
never be at rest. Thus even at absolute zero (- 273°C) when rotation stops, vibrational motion
is still present.
When energy is supplied to the molecule, higher vibrational states (VI' V2 , V3 , V4 etc.)
may become populated. Only the exact amount of energy needed to go from V o to VI' V2 , V3 or
some higher energy level may be absorbed. In typical organic molecules VI lies from 2 to 10
kcal/mole above Vo' Molecule at room temperature have an average thermal energy content of
about 0.6 kcal/mole. It is thus obvious that most molecules are present in their lowest vibrational
energy level (Vo) under these conditions. As the temperature is raised, some of additional
energy will go into populating higher vibrational levels. Many chemical reactions, specially
those that are intramolecular involve these higher vibrational energy levels.
At still higher temperature enough vibrational energy may be absorbed to bring about
rupture of bond. The minimum energy required to do this is known as bond dissociation energy
and is shown in Fig. 6.3. The amount of energy required to dissociate a bond varies widely
depending on the structure of the molecule and the nature of the atoms involved in th~ bond.
Another way of exciting the molecules involved absorption of electromagnetic radiation. The
amount of energy that such radiation contains depends upon its wavelength according to
relationship given below:
... (1)
E=hv
C
v=­
...(2)
A
Therefore
E
= hC
...(3)
A
where v = frequency of electromagnetic radiation
A= wavelength of electromagnetic radiation
C = velocity of light
h = Planck constant
From equation (3)
E= 2.62 x 10
4
...(4)
A
This energy supplied by light of A 254 nm equal to 113 kcal/mole (from equation (4)), an
energy sufficient to rupture most chemical bonds. (Table 6.1)
Table 6.1 Energy of some covalent single bonds and the
corresponding approximate wavelengths
_.-c' _
C-H
C-O
C-C
CI-Cl
0-0
95
88
83
58
35
300
325
345
495
820
Table 6.2 Type of electromagnetic radiation and energy
needed for different excitations
< 100 nm
> 286
Ultraviolet
(i) Vacuum
(ii) Quartz
100-200 nm
200-350 nm
286-143
143-82
Electronic
Electronic
Visible
350-800 nm
82-36
Electronic
0.8-2.0 Ilm
2-16 Ilm
16-300 Ilm
36-14.3
14.3-1.8
1.8-0.1
Vibrational
Vibrational
Vibrational
Microwave
1cm
10--4
Rotational
Radio frequency
meters
10~
Electron and
nuclear spin
transitions
Gamma radiation
X-rays, cosmic rays
Infrared
(I) Near infrared
(ii) Infrared
(iii) Far infrared
.•
Table 6.3 Symbols and definitions
uv
IR
t
Ilm
nm
Ultraviolet
Infrared
The unit angestron equal to 10--8 cm.
The unit micrometer 1 Ilm = 1O~ m.
The unit of nanometer 1 nm 1~ m.
=
6.2.1 Photochemical Excitation of the Molecule
Photochemistry begins with absorption of light in the 200-800 nm region of spectrum. In
order to know what wavelength of light we should employ in a particular photochemical
experiment, we must determine the UV absorption spectrum of the molecule we wish to study.
Such a spectrum measures the amount of incident light absorbed by the molecule as a function
of wavelength. The fraction of light absorbed
(~ Jis given by the Beer's-Lambert's law.
Lambert's law states that the fraction of the incident light absorbed by a compound is
independent of the intensity of the source. Beer's law states that the absorption of light is
proportional to the number of absorbing molecules. The absorption at a particular wavelerrgth
is defined by the equation
I
A = log -.J!.
I
1
"'
where A = absorbance
10 = intensity of the reference light
1 = intensity of the beam coming out of the sample cell
The absorbance by a compound at a parti~ular wavelength increases with increasing
number of molecules undergoing transitions. Therefore, absorbance depends upon the electronic
structure of the compound and also upon the concentration of the sample and length of the
sample cell. For this reason energy absorption is reported as molar absorptivity, E, also known
as molar excitation coefficient rather than actual absorbance. Often UV spectra are reported
to show E or log E instead of A. The log E value is specially useful when the value for E is very
large.
A
E
= CI
C = concentration of solution in molelL
I = length of tube in cm
where
5
Amax
10
E
t
0
II
CH 3-GH =CH-G-H
Amax
4
10
,,
,,
,,
,,,
,,,
3
10
2
10
10
1
A =215
! A =350
.. A
Fig. 6.4 UV spectrum of CH 3-CH=CH-CHO
Absorption maxima of some compounds are given in Table 6.4.
Table 6.4 General wavelength ranges for lowest energy absorption
band of some classes of photochemical substrates
Alkenes
199 - 200
Acyclic dienes
220 - 250
Cyclic dienes
250 - 270
Styrenes
270 - 300
Ketones
270 - 280
a, 13-Unsaturated ketones
310 - 330
Aromatic aldehydes and ketones
280 - 300
Aroqlatic compounds
250 - 280
_:
If we wish to excite these molecules we must irradiate them with light in regions where
they absorb. We must therefore match the emission of our source usually a mercury arc lamp
to the absorption of the compound. Mercury arc lamp has three principle emission lines are
253.7 nm, 313 nm, 366 nm.
Filters are available which permit selection of either of these lines. For example, if
system is constructed so that light must pass through borosilicate glass (Pyrex) only wavelength
longer than 300 - 310 nm will reach the sample 1:}ecause the glass absorbs below this wavelength.
Pure fused quartz which transmits down to 220 nm must be used if the 254 nm radiation is
desired. Other materials have cut off points between those of quartz.and pyrex. Filter solutions
that absorb in specific wavelength ranges can also be used to control the energy if light
reachening the sample.
:==:::::::::::]:::::::::::::=:;::;==:::
As mentioned earlier, molecule of any type is not only given in electronic state but also in a
given vibrational and rotational state. The difference between two vibrational levels is much
smaller than the difference between adjacent electronic levels, and the difference between
adjacent rotational levels is smaller still. A typical situation is shown in Fig. 6.5 in the form of
Morse potential curves for the ground state (EJ and first excited state (E 1) of polyatomic
molecule.
The excited state potential curve has minimum in it (VJ so the molecule will not break
(i.e., molecule will not fly apart on excitation). The minimum in the potential curve of the
excited state occurs at a larger inter nuclear distance (r' > r) than that of the ground state.
This is reasonable since the excited state has one electron in antibonding orbital and the
bonding will be weaker in the excited state. Secondly, the bond order of the molecule in excited
state is less than that of the ground state.
,
..
.,.­
i
£
\
\
E + IN
/V4
/V3
/V2
/V1
......... ~Vo
..
Vibratio nal
levels
Electronic
levels
I
,
r
Eo
/
V4
V3 Vibrational
IV
\
/V 2
levels
-----.... Vo 1
Internuclear distance
Fig. 6.5
i
~
~
r' > r
t
There is series of vibrational energy levels superimposed on potential curve for each
electronic state. Consider now the Morse curves for the ground state Eo and first excited state
E1 (Fig. 6.5). As mentioned earlier, at room temperature we have insufficient energy to populate
excited vibrational levels and most transitions will start from Va of the ground state Eo (or So),
The Frank-Condon principle tells us that during the electronic transition the interatomic
distance of molecule does not change because the time required for electronic transition
("" 10-16 s) is very-very short as compared to the time required for vibrational transitions
("" lO-13
B).
Or
The most probable electronic transitions are those in which separation (internuclear
distance) and kinetic energy do not change.
Thus, we have vertical excitation as shown in Fig. 6.5. Changes in molecular structure
(bond angles and bond lengths) will occur as the electronically excited molecule comes to thermal
equilibrium with its surrounding transition from Vo of ground state (Eo or So) may terminate
in any of several vibrational levels of excited state. This is the reason for band spectra rather
than sharp lines in UV spectra. The energy of electronic transition is measured from Vo of
ground state to Vo of the excited state i.e., t!J.E =E 1(Vo) - Eo(Vo) = hv when molecule absorbs a
quantum of light the electronic configuration changes to correspond to an excited state. Three
general points about this process should be emphasised.
1. The excitation promotes an electron from filled orbital to an empty orbital. In most
cases the promotion of electron is from the HOMO (highest occupied molecular orbital) to the
LUMO (lowest unoccupied molecular orbital i.e., antibonding MO).
2. At the instant of excitation, only electrons are reorganised (i.e., from HOMO to LUMO).
The heavier nuclei retain their ground state geometry according to Frank-Condon principle.
3. The electron does not undergo spin-inversion at the instant of excitation. Inversion
is forbidden by quantum-mechanical selection rules, which requires that there should be
conservation of spin during the eXcltation process. Although a subsequent spin state change
may occur, it is a separate step from excitation. Thus in the very short time (l0-16B) required
for excitation, the molecule does not undergo changes in nuclear position or in the spin state of
the promoted electron. Mter the excitation, these changes can occur very rapidly when excited
state comes in thermal equilibrium with its surrounding.
6.3.1 Types of Electronic Excitation and Molecular Orbital View of Excitation
Let us consider the different types of electronic excitations which take place on absorption of
light in UV and visible region. The ground state of organic compound contains valence electrons
7[*
n
1t
n -t 0* n -t 1t*
1t -t 1t*
Fig. 6.6 Different types of electronic excitations
in three principle types of molecular orbitals: Sigma (cr) MO's, pi (1[) MO's and filled but non­
bonding (n or p) orbitals. Both cr and 1[ MO's are formed from the overlap of two atomic or
hybrid orbitals. Each of these MO's therefore has an antibonding cr* and 1[* orbitals associated
with it. An orbital containing nonbonding electrons or lone pair of electrons does not have an
antibonding MO because it is not formed by the overlap between two atomic orbitals. Electron
transition involves the promotion of an electron from one of the three ground state (cr, 1[ or n)
to one of the antibonding (cr* or 1[*) MO's. The four important transitions are shown in Fig. 6.6.
The usual order of energy required for various electronic transitions is as follows:
cr ~ cr* > n ~ cr* > 1[ ~ 1[* > n ~ 1[*
Out of four excitations the 1[ ~ 1[* and n ~ 1[*are more important in organic photochemistry
than the other two cr ~ cr* and n ~ cr*.
Table 6.5 Types of excitation given by class of organic compounds
(i) cr -7 cr*
(ii) n -7 cr*
(iii) n -7 n*
(iv) n
-7
Alkanes which have only cr bonds
Alcohols, amines, ethers thioethers etc.
Alkenes, carbonyl compounds
Aromatic compounds etc.
Carbonyl compounds, acids and acid derivatives
n*
Understanding the electronic excitation in molecular orbital terms is possible only if we
consider bonding as well as antibonding MO's. The bonding and antibonding MO's of ethylene
and the electronic configuration of ethylene in the ground state and excited state are shown in
Fig. 6.7.
H.
2p - 2p = n* (antibonding)
,
~
~+<' '>+~
2p
\*/
2p
H
2p + 2p = 1t(bonding)
Fig. 6.78 Molecular orbital diagram of ethylene
n*---LUMO
hv
)
t.. max 165 nm. 173 kcallmole
n-i-t- HOMO
Ground state of CH 2=CH 2
Singlet state So
+
+
Excited state of CH 2=CH2
Singlet state 8 1
Fig. 6.7b Excitation of ethylene molecule
.,
Absorption of UV light similarly produces electronic transition in 1, 3-butadiene, a
conjugated multiple bond system. The lowest energy transition (HOMO-LUMO) occurs at
217 nm (132 kcal/mole). In this case also transition is n -} n* transition. Fig. 6.8 shows molecular
orbital diagram of 1, 3-butadiene,
HH
~
.
~
~
vtAntibOnding" MO - ­
14 Antibonding" MO
-­
*
*
V, Bonding" MO
V, Bonding" MO
Fig. 6.8 Ground state of 1, 3-butadiene
Fig. 6.8 shows relative energy and A max of the n MO's of 1, 3-butadiene.
---'1'/
- - ­ '1'3* LUMO
*
hv
"'max 217 nrn, 13 kcalJrnole
'1'2 HOMO
)
+
+
-it-
Ground state of
1, 3-butadiene
First excited state
of 1, 3-butadiene
Fig. 6.9 The HOMO-LUMO transition in 1, 3-butadiene
Thus conjugation decreases energy the difference between HOMO·LUMO and hence
Amax increases. This is a general phenomenon and it may be stated as follows:
• More will be the length of conjugation in the compound.
q
• Less will be energy difference between HOMO and LUMO.
• More will be Amax of the compound.
Absorption maxima of some conjugated polyenes is given in the Table 6.6.
Table 6.6 UV absorption of some conjugated polyenes
1, 3-Butadiene
217 nrn
1, 3, 5-Hexatriene
245 nrn
I, 3, 5, 7-Octatetraene
275 nrn
The carbonyl group presents some new features of interest in terms of molecular orbital
view examination. Carbonyl group is constructed from sp2-hybrid carbon and sp-hybrid oxygen,
we shall assume the (j (sigma) bond framework to be present, and we shall direct our attention
to the non-bonding orbitals and the 1t (Pi) molecular orbital. Before formation of 1t MO's we
have 2pz atomic orbitals on each carbon and oxygen. The 2pz orbital on more electronegative
oxygen atom is lower in energy than the 2pz orbital on a carbon atom; so the oxygen atom
contribute more to 1t than the more energetic carbon 2porbital. Thus the bonding 1t MO will be
constructed from more than 50% of oxygen 2p atomic orbital and less than 50% of the carbon
2p atomic orbital. Similarly we can see that 1t* will be made up of more than 50% of carbon 2p
orbital and less than 50% of the 2p orbital of oxygen. The remaining orbitals (2px and sp
hybrid orbital) are doubly occupied and are non-bonding orbitals (Fig. 6.10).
Nonbonding - ,
'f
orbital
2px
r
ffiJ
t L
This forms (J bond
2
with sp hybrid
orbital of carbon
28
Electronic configuration of oxygen
>
•
I
ltMO
2pz
1,
DJJIJ
Sp
Sp
)
\
,
This forms
rnm
sp hybridisation
1
.~
I
!
Nonbonding
orbital
--1';---­
Shape of C=O group
Fig. 6.10 A graphical construction of n, n* and two
non-bonding orbitals of carbonyl group
The graphical construction of 1t, 1t* and two non-bonding orbitals can be shown in short
as follows (Fig. 6.11):
- ­ n*LUMO
~
~
~
---#-n(2pJHOMO
r
---#-1t
---#-
~
n(sp)
Fig. 6.11 Energy diagram of 'C=O group
./
The non-bonding orbital, 2px' is a relatively high energy orbital and very important in
. photochemistry whereas the non-bonding orbital (sp) is a low energy orbital and this is not
+
--1t*
hv
hv
~
~
-#-n(2pJ
-#-
+
"
*
-#-1t
S2
+
+
So
Sl
n --77t*
First excitation (Sl)
7t --7 7t*
Second excitation (S2)
Fjg.6.12
important in photochemistry (because excitation always takes place between HOMO and
LUMO). In simple energy level diagram the n(sp) orbital therefore is ignored and this for
photochemistry point of view carbonyl group can be represented as in Fig. 6.12.
The n ~ 1t* transition is the lowest energy transition for most ketones and this is also
known as So ~ Sl (first excitation) transition. The 1t ~ 1t* is higher energy transition and this
is also known as So ~ S2 (second excitation) transition. So is the ground state whereas Sl and
S2 are excited states, these transitions can also be represented as follows:
--t-----.--- 8 1
8 0 ----+8 1
n----+1t*
--'------'--80
,
: ,
Spin multiplicity of any species = 2S + 1
where S = sum of spins of the electrons
Case 1: Suppose all electrons are paired then S =0 and spin multiplicity =2 x 0 + 1 =1.
If spin multiplicity is one then state of the species is known as singlet state. For example, methyl
carbocation has six electrons in its outer most orbit and all are paired hence it is in singlet
state.
Case II: Suppose all electrons are not paired. Take the example of methyl free radical
1 ~ 1) =0--~+1/2
+-+-1 ~ 1)
2
2
<lD- +-+-=0
~
2
t
+~ +t ~)=
0
2
-,,
Spin state
1
=28 + 1 = 2 x 2 + 1 = 2
Spin state of methyl free radical is doublet.
Now take the example of carbonyl group:
+1
~~ +1/2
QL:::::::;c<ID
Sum of spin =
t + t =1
°(J6 S~in state =28 + 1 =2 x 1 + 1 =3
Here spin state of this species is triplet.
There are even number of electrons in typical organic molecules and these electrons are
paired in ground state as demanded by the Pauli principle. Molecular states with all paired
electrons are thus called singlet states, 8 n , where n = 0, 1, 2, 3, 4.
Therefore, 8 0 = ground state singlet
8 1 = first excited state singlet
8 2 = second excited state singlet
Absorption of light occurs without spin inversion and the initial excited state produced
is a singlet excited state. In this case two electrons no longer share an orbital and the promoted
electron has the same spin as its former partner, i.e., ground state.
Both have opposite spin hence 8
\~=t-:::
--'1'3*
--'1'3*
~
8 0 (Ground state)
~
t t) =°
=+ t =
+
*"'1
'1'2
8 1 (First excited state)
Single excited state may undergo spin inversion giving a new excited state with two
electrons of same spin
8
(i. e., 8 =+ %+ ( + %) = 1J. Molecular state with such species having
= 1 is called triplet states (28 + 1 = 2 x 1 + 1 = 3), Tn' where n = 0,
Therefore, To = lowest energy state
1, 2, 3.
T 1 = higher energy than To
T2 = still higher energy than T 1 and To
The terms singlet and triplet originate from the fact that the singlet state does not split
in a magnetic field while the triplet state does as it has a magnetic moment and has three
possible energy states which are distinguishable. Electronic transitions between states of same
multiplicity i.e., singlet-singlet and triplet-triplet are spin allowed and transitions between
states of different multiplicity, i.e. singlet-triplet and triplet-singlet are spin forbidden.
The most important difference between the lowest excited singlet state and the lowest
excited triplet state is the difference in energy of the two states. The triplet state is lower in
energy as a result of the greater electronic repulsion in the excited singlet state. A systematic
representation of this is given in the figure which shows an approximate energy level diagram
for a carbonyl compound.
== ~
-
=
--- - ­ T2(7t ~ 7t*)
==
ISC
:::::!=:
T1(n~7t*)
Triplet states are not readily obtained directly by absorption of a photon since such a
absorption is strongly forbidden. Thus most observable spectra are singlet-singlet absorption
spectra. Some triplet states can be generated indirectly by absorption of light as a result of a
photophysical process referred to as intersystem crossing (ISC) from the initially generated
singlet state (see figure). A triplet state always has an energy lower than a singlet state with
the same electronic configuration but the singlet-triplet energy difference varies considerably.
This energy gap is much higher for n, n* states than for n, n* states and so it is not usual for a
molecule with a lowest energy singlet state, 81' that is nn* in nature have a lowest triplet
state T 1 that is nn*. The emphasis on the excited states of lowest energy arises because of the
observation formulated by Kasha. According to Kasha luminescene or photochemical reaction
normally occurs from the lowest energy singlet (81) or triplet (T1) excited state rather than
from a higher energy state (82 or T 2 etc.).
Fluorescence almost invariably occurs from the 8 1 state and phosphorescence from the
T1 state.
S
:::;:::;:::::;'::::::::::::::::::'::::::::
A excited state of molecule can be regarded as a distinct species, different from the ground
. state of the same molecule and from other excited states. It is obvious that we need some
method of naming excited states. One of the most common methods simply designates the
original and newly occupied orbitals with or without a subscript to indicate singlet or triplet.
Thus, the singlet state arising from promotion of a n to a n* orbital in 1, 3-butadiene or
ethylene would be the l(n, n*) state or the n, n* singlet state.
Another very common method can be used even in cases where one is not certain which
orbitals are involved. The lowest energy excited state is called 81' the next 8 2 etc. and triplet
states are similarly labelled Tl' T2, T3 etc. as mentioned earlier. In this notation the ground
state is 8 0 ,
Photochemistry occurs when a molecule is raised from its ground state to a higher state
which can differ in multiplicity, i.e., either singlet or triplet from ground state. The population
of such energy levels means that molecules are in states of shorter life time than usual and
have considerably more energy than the ground state from which they were formed. Thus the
molecule in the excited state are considerably more reactive in the excited state. Deactivation
of an excited state occurs by a photochemical or photophysical process, either
monomolecularily (intramolecularly) or bimolecularily (intermolecularly). There are several
paths by which deactivation can be achieved and it is these paths which will be discussed in
the text. The processes are:
1. Non-radiative processes between states.
2. Radiative processes between states.
3. Loss of energy by intermolecular energy transfer.
4. Chemical reactions.
_ ; I.... '.e';; lt"•••ttt'.".n ''Yftt
x• •
1t
t
When a molecule has been promoted photochemically to an excited state, it does not remain
there for long time. Most promotions are from the 8 0 to 8 1 state. Promotions to 8 2 and higher
singlet states (82 , 8 3 , .... ) take place, but in liquids and solids, these higher states usually drop
very rapidly to the 8 1 state because the life time of 8 2 , 8 3 etc. is usually less than 1O-1l to
10-13 sec. The energy lost when an 8 2 or 8 3 molecule drops to 8 1 is given to the medium. Such
a process is called an energy cascade. In a similar manner the initial excitation and the decay
from higher singlet states initially populate many of the vibrational levels of 81' but these also
cascade, down to the lowest vibrational level of 81' This cascade is known as vibrational
cascade. All these processes will occur in about 1O-1l to 10-13 sec. The thermally equilibrated
low-lying singlet excited state 8 1 (Va> has a relatively long lifetime (10-8 sec). Therefore, in
most cases, the lowest vibrational level (Va> of the 8 1 state is the only important excited singlet
state. This state can undergo various physical and chemical processes. Here we will describe
the physical pathways open to molecules in 8 1 and excited triplet states. These pathways are
shown in Jablonski diagram (Fig. 6.13).
1. A molecule in the 8 1 state can cascade down through the vibrational levels of the 8 0
ann HillS return to the ground state by giving up its energy in small increments to the
surrounding (i.e., environment), but this is generally quite slow because the amount of energy
is large between 8 0 and 8 1 , The process of cascade is called internal conversion (Ie). Because
it is slow, most molecules in the 8 1 state adopt other pathways.
F
=
=
~
ISC
VC
~
:+'-~f--.~~---f~-r---~~~~----
==. .'"
T
1
VC
J
"
Fig. 6.13 Jablonski diagram
Note: Internal Conversions: Decay of S2 to Sl or Sl to So is known as internal conversion and this
process in non-radiative and spin-allowed process. Internal conversion from S3' 8 2 to 8 1 is very-very
common whereas from 8 1 to So is not common.
2. The lifetime of Sl state is 10-9 to 10-15 sec. A molecule in the Sl state can drop to some
low vibrational level to the So (V2 , V 3 , V 4 but not Vo) state all at once by giving off energy in the
form of light. This process, when generally happens within 10-9 sec, is called fluorescence.
This pathway is not very common either (because it is relatively slow), except for smaller
molecules e.g., diatomic and rigid molecules (aromatic compounds are the main examples).
For compounds that show fluorescence, the fluorescence emission spectra are usually the
approximate minor images of the absorption spectra. This comes about because the fluorescing
molecules all drop from lowest vibrational level (Va> of Sl state to various vibrational levels of
So while excitation is from the lowest vibrational level (Va) of So to various vibrational levels of
Sl (Fig. 6.14).
I.
v4
V3
V2
V1
Vo
0-0
0-0
Fig. 6.14 Promotion and fluorescence between 51 and 50 states
,
!I
-"
The only peak in common is the one (called 0 - 0 peak) that results from transitions
between Vo (80) and Vo (8 1), In solution, even the 0 - 0 peak may be non-coincidental because
the two states are solvated differently.
.
Because of the possibility of fluorescence any chemical reactions of 8 1 state must take
place very fast, or fluoresct;nee will occur before they can happen.
3. Most molecules in the 81 state may drop to triplet state (T1). This is a slow process as
a change in multiplicity (spilt:~sion)is a formally forbidden process. However, if the singlet
state 8 1 is long lived, the '!1I ;;' T1 conversion occurs by a process called intersystem crossing
(lSC). It is an important phenomenon in photochemistry. For every excited singlet state there
exists a corresponding triplet state. Since transition from ground state singlet (8J to triplet
state (T1) is forbidden, ISC.t1J the main source of excited triplet state. This is on~ way of
populating the triplet state. 1'he efficiency in ISC depends on the difference in energy between
the low-lying singlet state (81) and triplet excited state (the 8 1 - T 1 energy gap). When this
energy difference is small the ISC is efficient. When energy difference is large, spin forbiddeness
is quite important and ISC, efficiency is low or zero. Generally speaking, ketones have high
ISC (benzophenone has l~;intersystem crossing) efficiencies, aromatic compounds have
intermediate to high ISC efficiencies and olefines have low ISC efficiencies. The presence of
heavy atoms (S, Cl, Br, I,etc.) in a molecule enhances ISC efficiency,
Since a singlet stat43~slly has a higher energy than the corresponding triplet, this
means that energy must b~~ ~p. One way for this to happen is for the 8 1 molecule to cross
to a T 1 state at a high vibr.~;1eveLthenfor the T 1 to cascade down to its lowest vibrational
level (Fig. 6.13). This casca~l.&veryrapid(10- 12 sec). When T2 or higher states are populated,
they too rapidly cascadetQ;"'iowest vibrational level of the T 1 state. Thus ISC is a nonradiative process.
.
4. Lifetime of T 1 sta~.,J.O:""> to 10-3 sec. A molecule in the T 1 state may return to the 8 0
state by giving up heat (lSC.nOD-radiative process) or light (this is called phosphorescence
radiative process). ISC and:Jbpsphorescence are very slow processes (10-3 to 10 1 sec). This
means that T 1 state generaUlhave much longer lifetime than 8 1 states (lifetime of 8 1 is 10-9
to 10-15 sec, lifetime of T 1 is If;Y6 to l(i3 sec). When fluorescence and phosphorescence occur in
same molecule phosphore~ce is found at lower frequencies than fluorescence (because of
the higher difference in enel'JY,between 8 1 and 8 0 than between T 1 and 8 0) and is longer-lived
(because of longer lifetimeof~;T1state).
5. For many mol~~t8C (81 ~ T 1) is not very efficient. If this is the only way to
produce T 1 then T 1 would'~.th&rvery·very limited.
Fortunately, triplet_ can be populated by the use of donor molecule. Donor molecule
is a substance whose ISC ~ is high. Donor molecule produces T 1 in high yield by ISC and
then this molecule transf~_~on energy to the second molecule (known as ac~eptor).
Excited state (8 1 orf\~'_donol' molecule transfers its large amount of energy to the
acceptor molecule and in "o~accePtor(A) species promoted tv excited state and donor
(D) species return to the ..~ ~ . .
.
. ':,-,,'"
::;:Hi~~L
-
.
"D*
+~ _~ ~ : .:
Although photons are absorbed by D, ji is A which becomes excited: This phenomenon i:,
known as photosensitisation and D i~ refel'!'"d '1S /'hotosensitiser.
The basic requirement for energy transfer is that the energy of donor molecule should
be 5 kcallmole more energy than the energy needed for excitation of acceptor molecule.
Thus there are two methods for molecule to reach an excited state, first by direct
absorption oflight and second by transfer of energy from excited donor molecule to the acceptor
molecule.
Transfer of energy from excited singlet state produces singlet and transfer from excited
triplet produces triplet. In other words, if the acceptor molecule possesses a state of same
multiplicity and lower in energy than that of donor molecule, energy transfer will occur from
donor to acceptor. This provides a versatile method of forming triplet by triplet-triplet energy
transfer
~t
D*
tt
[Be)
H
tt
D*
tt
H
D*+A~A*+D
tt
A*
Photochemical
----~)
Reaction
"
Products (Sensitisation)
!.
tt
D ~ Products (Quenching)
*If 3A gives the products of interest this is called sensitisation mechanism.
There are several ways in which energy transfer can occur but one of those phenomenon
of triplet-triplet energy transfer is the most common because lifetime is maximum for triplet
state.
The method of sensitisation has proved to be of enormous value as it provides a new
group of potentially reactive intermediates. Aldehydes and ketones are particularly employed
as sensitisor in a wide variety of photo reactions. The sensitiser should meet the following
critaria:
1. It must be excited by the irradiation to be used:
2. It must be present in sufficient concentration and absorb more strongly than the
other reactants under the conditions of the experiment so that it is the major light absorber.
3. The energy of triplet state of sensitiser (D* or 3D* or 3Sens *) must be greater than
that of the reactant. If this condition is not met, the energy transfer becomes endothermic.
4. It must be able to transfer energy to the desired reactant.
5. The sensitiser should possess high ISe, absorb at lower wavelength and does not
interfere with the analytical procedure.
Several photosensitisers in common use are given in Table 6.7.
.
:\
•
4
Table 6.7 Common sensitisers and their energy in triplet state
Benzophenone
69
Acetophenone
74
Fluorenone
53
Benzene
85
Naphthalene
61
Biacetyl
56
Benzil
53
Several useful features of using a photosensitiser are that
(i) The reaction proceeding via singlet state can be avoided and the quantum yield of
the 'products increased.
(ii) Substances which are unable to absorb light of conveniently longer wavelengths may
often be sensitised by additives which do. In terms of energetics, it implies that com­
mon compounds whose upper singlet (81) and (T1) are widely separated, thereby mak­
ing ISC difficult may be excited to their triplet states by sensitiser with appropriate
int€!rmediate energy levels.
Different physical processes undergone by excited molecules are given in Table 6.8.
Table 6.8 Physical processes undergone by excited molecules
Excitation
V
Sl
~ S1 + Heat
S1
S1~
S1~
T1 v
So+Heat
Internal conversion, IC
T 1v
Intersystem crossing, ISC
~
T1~
Fluorescence
So + hv
)0
Vibrational relaxation
T1 + Heat
So+Heat
Vibrational relaxation or Vibrational cascade
ISC
S1 + A(SO>
)0
So + A(S1)
Singlet-singlet transfer (photosensitisation)
T 1 + A(So)
~
So + A(T1 )
Triplet-triplet transfer (photosensitisation)
The subscript V indicates
Vibrationally excited state
Let us consider now a specific example which illustrates the use of sensitisation in
photochemistry. Direct irradiation of 1, 3-butadiene in solution gives cyclobutene and
bicyclobutane with minor amounts of dimers. Intersystem crossing efficiency in 1, 3-butadiE>jle
approaches zero and triplet derived products are not formed.
81
T --------­
47.64 kcal/mole
107.124 kcal/mole
80~
~
60 kcal/mole
hv
--~)
~)
[1,3-Butadiene]*
[[]
+
<I>
81
80
1, 3-Butadiene
Triplet excited butadiene produced by energy transfer from triplet-excited benzophenone
gives only dimers.
(C 6HJ2 CO
hv )
1[(C6HJ2CO]
366nm
~ 3[(C6HJ2CO]
l~
(C 6H s)sCO + 3[~J
3[~J +
~
)
~
+
cr
Quenching
Intermolecular interadion between a molecule in its excited state and a molecule in its ground
state can lead to deactivation of the electronically excited state and the generation of the
excited state of other molecule. This phenomenon is known as quenching. Thus bimolecular
deactivation is known as quenching.
M*+Q~M+Q*
Quenching process is reverse to sensitisation process. In sensitisation process a molecule
in the ground state is raised to its excited state by energy transfer from other excited molecule
(known as sensitiser).
M + Sens* ~ M* + Sens
The component that accelerates quenching is known as quencher and represented by Q.
There are basically two major routes of quenching: (i) Photochemical quenching and
(ii) photophysical quenching. In photochemical quenching the quincher transforms the
excitation energy into chemical energy and a product is formed. Photophysical quenching can
be divided into (a) self quenching or concentration quenching and (b) impurity quenching.
::::=::;:::::::$:;;;;:::::::;::::.:::.::::::::=::::
We have said that when a molecule absorbs a quantum of light, it is promoted to an excited
state. Actu'ally this is not the only possible outcome. Because the energy of visible and ultraviolet
-"
light is of the same order of magnitude as that of covalent bonds (Table 6.1), another possibility
is that molecule may cleave into two parts, a process is known as photolysis. There are three
situations that can lead to cleavage:
1. The excitation may bring the molecule to a vibrational level so high that it lies above
right hand portion of E I curve (line A, FIg. 6.15). In such a case the excited molecule cleaves at
its first vibration (absorbed energy is greater than bond dissociation energy).
2. Even where the promotion is to a lower vibrational level, one which lies wholly within
the E I curve (such as VI or V2 ), the molecule may still cleave. As Fig. 6.15 shows that equilibrium
distances are greater in excited states than in ground state. Promotion of an electron takes
place much faster than a single vibration. Therefore, when an electron is suddenly promoted,
even to low vibrational level, the distance between the atoms is essentially unchanged and the
bond finds itself in a compressed condition. This condition may be relieved by an outward
surge that is sufficient to break the bond.
\--
A
E1
E
r
--~~
r
~
Fig. 6.15
3. In some cases the excited state is entirely dissociative (Fig. 6.16), i.e., there is no
minima in the Morse curve. In such case there is no distance where attraction outweighs
repulsion and the bond must cleave.
E
t
Fig. 6.16 Promotion to a dissociative state results in bond cleavage
The photolytic cleavage can break the molecule into two smaller molecules or into two
free radicals. Once free radicals are produced by photolysis, they behave like free radicals
produced in any other way except that they may be in excited state and this can cause differences
in behaviour.
=::::::::::::::::: ::::::::::::::::::::::::::::::::::
The photochemical processes are governed by the following laws:
1. First law or Grotthurs-Drapper law.
2. Second law or Einstein-Stark law of photochemical equivalence.
6.8.1 Grotthurs-Drapper Law
When light radiations fall on a substance, only the fraction of incident light which is absorbed
can bring about a chemical change; reflected and transmitted light do not produce any chemical
change.
The law is purely qualitative and does not give any relationship between the amount of
light absorbed by the molecules and the number of molecules which have reacted.
6.8.2 Einstein's Law of Photochemical Equivalence
This law states that each molecule or atom activated by light, absorb only one quantum of
light, which causes activation.
Or
Each quantum oflight absorbed by a molecule, activates only one molecule in the primary
step of photochemical process or briefly one molecule one quantum.
AB
+
hv
~
AB·
One
molecule
One
photon
Activated molecule
(excited molecule)
Einstein's law of photochemical equivalence should not be interpreted to mean that one
molecule will react per quantum of light absorbed, but that only one molecule is activated by
each quantum of radiation.
:::::::::,::::=:::::;:::::::
The efficiency of photochemical process is often expressed in terms of quantum yield (cp) which
is defined as the number of molecules reacting per quantum of light absorbed.
Mathematically
Number of molecules reacting in given time
q> = Number of quantum of radiation absorbed in the same time
The energy E absorbed per mole of the reacting substance is called one einstein
E = NA hv = One einstein
The quantum yield of the product formation
Number of molecules of product formed
q> = Number of einstein of radiation absorbed
)
If the law is correct then the quantum yield should be unity. This however is very rare.
The quantum yield may be as high 10 6 or as low as 10-2 for several photochemical
reactions. The reason for low quantum yields are:
1. Excited molecules may get deactivated before they form products.
2. Collisions with excited molecules with non-excited molecules may cause the former
to lose their energy.
3. The primary photochemical process may get reversed.
4. The dissociated fragments may recombine to form the original molecule.
6.9.1 The Reasons for High Quantum Yield
1. The primary process of absorption of radiation produces excited free radicals. TQ.ey
undergo secondary processes. Each secondary process produces again excited free radicals.
This process continues unless it is checked. Thus by absorbing only one quantum of radiation, .
several reactant molecules undergo chemical reaction. Hence <p will be greater than unity.
2. Free radical gives chain reaction which increases quantum yield.
r
7
CHAPTER
Saturated ketones exhibit four main bands in their ultraviolet ah~ion spectra. These bands
are centered on 280, 195, 170 and 155 nm. The most important band is at 280 nm for the
photochemistry of carbonyl group. This band corresponds to the n -t n* transition. The
photochemical reactions of carbonyl group is initiated by n -t n* transition. Promotion of an
electron will lead to either a singlet state or a triplet state. Photochemical reactions given by
carbonyl group takes place either by singlet state or by triplet state or by both states.
According to the Kasha's rule, only the lowest excited s~~ will ~e involved in the
primary photochemical or photophysical processes of organicmoI."mes in solution.
Carbonyl compound's give four type of reactions. These reactions include:
(i) a-Cleavage.
(ii)
~-Cleavage.
(iii) Intramolecular and intermolecular hydrogen abstraction by carbonyl oxygen.
(iv) Addition of carbonyl oxygen atom to a carbon-carbon multiple bond.
In many cases the four processes are competitive and the major process followed is
sensitive to structural variations in the ketones and the choice c:ithe solventS'.
==
:t:::::=::::::::::::::::
Norrish type I process is given by three type of ketones:
(i)Saturated acyclic ketones.
(ii) Saturated cyclic ketones.
(iii) ~, y-Unsaturated ketones.
7.1.1 Norrish Type I Process Given by Acyclic Saturated K.t~.,
.
Saturated carbonyl compounds undergo photoinduced decarbonyltttion in the gas phase. This
process was first observed by R.G.W. Norrish and is known 8S Nomsh Type I or a-cleavage
184
process. Norrish Type I process is commonly encountered in the gas phase. The solution phase
reaction of this type is uncommon.
Primary Processes
Norrish Type I process is characterised by initial cleavage of the carbonyl carbon and
alpha carbon bond to give an acyl and an alkyl radical. This process is known as primary
photochemical process (Scheme 1).
o
II
R-CH -C-CH-CH -R"
I
2
2
R'
o
II
.
I
R-CH -C· + CH-CH -R"
2
2
R'
Scheme 1: Primary process
The initially formed acyl radical and alkyl radical is stabilised by one of the secondary
processes [(a) - (c)] shown in the Scheme 2. Similarly the alkyl radical can be stabilised by
recombination or disproportionation (Scheme 2).
Secondary Processes
(a) Decarbonylation of acyl radical to give carbon monoxide and an alkyl radical. This
alkyl radical can recombine to give an alkane or can undergo intermolecular hydrogen
abstraction to form an alkane and an alkene [Scheme 2(aH.
o
II
R-CH 2-C'
.
Decarbonylation
----~)
.
R-CH 2 + R -CH2
•
R-CH 2 + CO
--~)
R-CH 2-CH 2-R
(Recombination)
.
.
R-CH 2 + CH-CH2-R"--~) R-CH 3 + R'-CH=CH-R"
I
R'
Intermolecular hydrogen abstraction
Scheme 2 (8)
T
(b) Intermolecular hydrogen abstraction by the acyl radical from the alkyl radical to give
an aldehyde and an alkene [Scheme 2(b)].
o
II
.
R-CH -C' + R'-CH-CH 2 -R"
f3
a
2
1
o
II
R-CH2-C-H + R'-CH=CH-R"
Scheme 2 (b)
This process can only be possible if alkyl radical has atleast one ~-hydrogen.
(c) Intermolecular hydrogen abstraction by the alkyl radical from the a-carbon of the
acyl radical to form a ketene and an alkane [Scheme 2 (c)].
o
II
•
R-CH -C, + R'-CH-CH 2 -R"
a
2
f3
1
Scheme 2 (C)
The main secondary reaction of saturated acyclic ketones is decarbonylation.
Norrish Type I process is thus a two step radical mechanism. The first step is a primary
process and the second step is the secondary process. Formation of acyl and alkyl radicals can
be proved by trapping of these radicals by the use of suitable trapping agents.
2, 2, 4, 4-Tetramethylpiperidine-1-oxyl radical was used for the trapping and the radical
fragments produced by the fission of 1, 3-diphenylacetone were trapped as an ester and an
ether (Scheme 3).
o
II
c'Ho-CH'lhVCH,-C,H,
o
I
O-CH2~6H5
Ether
Scheme 3
Formation of ether and ester confirms that there should be the formation of acyl and
alkyl radicals..
The formation of radical intermediates is also readily demonstrated by photolysis of a
mixture of ketones (A) and (B) which give products from mixed radical combination.
O'
. II
0
II
Ph2-CH;C-':_:CHPhi + Ph-CH 2-C-CH 2-Ph
(A)
(B)
Ph 2-CH-CH-Ph2 + Ph-CH2-CH 2-Ph + Ph 2-CH-CH 2-Ph + CO
(Cross product)
(93%)
This cross over experiment also confirmed the two steps radical mechanism.
Norrish Type I process occurs from both the excited singlet and the triplet states of
n ~ n* transition. Photolysis of ditertbutylketone results in high yield of carbon monoxide (90%)
from both the excited singlet and triplet states. This clearly shows that the Norrish Type I
processes occur from both the excited states. The lifetime of the singlet state is 4.5 - 5.6 x 10-9
sec as compared with 0.11 x 10-9 sec for the excited triplet state. Since the reaction occurs
from both the singlet and trip)~t excited states, the Type I process must occur about 100 t~mes
faster frc~ triplet than from singlet excited state. Studies with triplet quenchers, such as
1, 3-cyclopentadiene, have also shown that Norrish Type I processes occurs from both triplet
and singlet excited states.
Norrish Type I cleavage is given mostly by those ketones whose n ~ n* state is the
lowest excited states. In most of the cases, the n ~n* state is the lowest excited state. However,
a cleavage in arylalkyl ketones and diaryl ketones is less efficient because n ~ n* excited state
is not the lowest excited state. In this case, there is a large barrier on the reaction coordinate
(Fig. 7.1).
--...,------ 52 (n, n*)
100 kcal/mole
51 (n, n*)
76 kcal/mole
- - - ' - - - - - ' - - - 50
Fig. 7.1 Energy of diphenyl ketone (CsHsCOCsHs)
In Norrish Type I reactions there is a preference for the formation of most stable alkyl
radical in case of unsymmetrical ketones.
Breaking of
a-bond
Breaking of
a'-bond
~
, f
In the above case, only a-bond undergoes cleavage. If both alkyl substituents are same
then there is little selectivity of bond cleavage.
The Norrish Type I process is mostly favoured by photolysis in the vapour phase and is
less pronounced for photolysis in the inert solvents. In inert solvent formation of solvent cage
takes place. Formation of solvent cage facilitates recombination ofthe initially generated radical
pair. Thus low quantum yield of products is obtained in inert solvents.
Norrish Type I process is efficient in liquid phase only if a stable radical is formed. The
stable radical includes allylic, benzylic, tert alkyl and acyl radicals.
o
C6Hs-CHz-C-CHz-C6Hs
"
1
hv, Liquid phase
7.1.2 Norrish Type I Reaction of Saturated Cyclic Ketones
Cyclic ketones, in contrast to the acyclic ketones, show a greater tendency to undergo a-cleavage
to furnish acyl-alkyl biradicals. Norrish Type I cleavage of acyclic ketones take place by singlet
as well as by triplet excited state. In cyclic ketones, the reaction takes place exclusively by the
triplet state. It is possible, however, that inter system crossing in cyclic ketones is so rapid
that reaction from the singlet state would not be observed. The triplet state is at least 100
times more reactive than the excited singlet state.
Available information indicates that the excited triplet state for Norrish Type I processes
is n ~ 1[* triplet state. This was demonstrated first for the irradiation of cyclopentanone in
both gases and in solution phases. Under both conditions, the product is 4-pentenal. Formation
of 4-pentenal takes place at 313 nm and 254 nm. The formation of the aldehyde can be quenched
by 1, 3-cyclopentadiene. This quenching experiment confirms the f'Ormation of the triplet state.
Norrish Type I process involves the initial cleavage of a carbonyl carbon-alpha carbon
bond. Subsequent secondary processes [(a) to (c)] correspond to those observed for acycliC
ketones.
(a) Decarbonylation of acyl-alkyl diradical to give carbon monoxide and a dialkyl radical.
The dialkyl radical can recombine to give a cycloalkane or it undergoes intramolecular hydrogen
abstraction to form an alkene. [Scheme 4 (a)].
/CH
2
r
.
r
"
(CH 2 )n
C=O
'--CH2-CH!
lhv
Sl~ T 1
1
?
/CH~C·
(CH 2 )n
'--cH~6H2
Acyl-alkyl diradical
1
DecarbonyIation
/CHt
(CH 2 )n
'--cH~6H2
Radical
recombination
Intramole­
cular
hydrogen
abstraction
/ C H 2\
(CH 2 )n
/ C H3
(CH 2 )n
CH 2
'--CHt/
'--CH=CH2
Cycloalkane
Alkene
Scheme 4 (a)
,
'y
(b) Intramolecular hydrogen abstraction by the acyl radical from the
alkyl radical to give an unsaturated aldehyde [Scheme 4 (b)].
~-carbon
of the
o
II
~CH2-C-H
(CH~n
"----CH=CH2
Unsaturated aldehyde
Scheme 4 (b)
(c) Intramolecular ~-hydrogen abstraction from the acyl radical by the alkyl radical to
produce a ketene [Scheme 4 (c)].
Scheme 4 (c)
The biradical of cyclic ketones can undergo one of the two hydrogen transfer processes
[(b) and (c)] via a cyclic transition state in which a hydrogen atom is transferred to one radical
centre from the atom adjacent to other radical centre.
Photolysis of cyclic ketones in gas phase gives decarbonylation as well as intramolecular
hydrogen abstraction. Intramolecular hydrogen abstraction mainly leads to the formation of
unsaturated aldehyde (Scheme 5).
O=o~
0=0
Decarbonylation
Intramolecular
hydrogen
abstraction
0;0
Unsaturated
aldehyde
Scheme 5
In solution phase biradical pair is not usually stabilised by decarbonylation. In this case,
biradical is mainly stabilised by intramolecular hydrogen atom transfer. This intramolecular
hydrogen atom transfer leads to the formation of either unsaturated aldehyde or a ketene or
both (Scheme 6).
0=0
hv
Solution
phase
0=0
Intramolecular
hydrogen abstraction by
carbonyl carbon radical
Intramolecular
hydrogen abstraction
by alkyl radical
0=0
Cr°
Ketene
Scheme 6
,
When photolysis is carried out in the presence of polar protic solvent then the main
species formed is ketene. This ketene then undergoes solvent addition to give carboxylic acid
(with water) or its derivative (ester with alcohol) as the only product.
0=0
a:
hv
ROH
)0=0
1
ROH
0=0
--.­
,
r
Ester
--
"""':0.-'
_
In case of unsymmetrical ketones, the a-bond that produces more stable alkyl radical
cleaves preferentially.
;.,..
According to the biradical mechanism, there is a possibility of a back recombination
reaction to reform the starting material.
Recombination
Use of suitably a-substituted cyclic ketones show that recombination reaction resulted
in epimerisation at the a-carbon if a-carbon is a chiral.
Epimer of (A)
(A)
Experimently it has been found that photolysis product formation via route [(a) - (c)]
is faster than the photochemical interconversion of the epimers. Since there is the
formation of epimeric mixtures, each epimer affords the same mixture of products.
hv
hv
&CH,
CH3
+
H'C'6r
HC
CH 3
I
6/CH,
''tJ
CH3
+
+
0
•
.I
Each epimer affords the same mixture of products. This clearly indicates that the
configurational integrity of the a-carbon is lost during the course of the reaction. This loss of
integrity is due to the free rotation about C2-C g bond in the diradical.
H'CYJCH'
o
hv
)
IJ
Rotation about
C2-C a single bond
Recombination
(
The recombination process is important even when hydrogen transfer reactions occur,
but loss of carbon monoxide in solution phase photochemistry is a major reaction pathway
only when the alkyl radical centres are stabilised by inductive effect, by B, y-unsaturation or
by cyclopropyl conjugation. This reflects an increase in the rate of loss of carbon monoxide
from the acyl-alkyl biradical in these systems. 2, 6-Dimethyl-cyclohexanone gives carbon
monoxide on photol,fsis in solution at room temperature. 2, 2, 6, 6-tetramethylcyclohexanone
gives carbon monoxide in a yield greater than 70%, 7, 7, 9, 9-tetramethylbicyclo [4, 3, 0] non­
1, 6-en-8-one gives 100% yield of carbon monoxide and hydrocarbon products.
o
o
>6<
hv
Solution
a
l-co
V
(Highly stable)
1
V
jF",---­
r
7.1.3 Norrlsh Type I Process Given by Cyclopentanones
Cyclopentanone also decarbonylates on irradiation in the gas phase at 147 nm. In this case
also a two step process is involved, affording a biradical which decarbonylates to another
biradical, which either fragments to ethylene or undergoes bond formation to cyclobutane.
Fission to ethylene is much more efficient in comparison to cyclobutane formation (Scheme 7).
i
o
o
o
hv
)
Gas phase
Q
-co
D
~
Scheme 7
In solution the loss of carbon monoxide from a cyclopentanone is a major path only
when the radical centres formed are stabilised by alkyl substitution, double bond or cyclopropyl
ring.
o
hv
)
Solution
~o
~
l~o
C¢~O¢
45%
(Highly stable)
/
o
Cyclopropyl
conjugation
hv
Solution
I-co
•
+
•
Minor product (13%)
(cis product)
Major product (80%)
(trans product)
(Both radicals aro conjugated
by cyclopropyl ring)
.;..;...
,
.
a-Cleavage is not given only by cyclic ketones. Other cyclic compounds such as lactones,
lactams and cyclic anhydrides undergo a-cleavage to give a biradical species on photolysis in
gas or solution phase.
61
6
o
o
.~
hv
254 nm )
•
.
0
~
~H
,
('
~~O
(Unsaturated
formate)
o
H
Intramolecular
hydrogen
abstraction
(Dialdehyde)
7.1.4 a-Cleavage Given by Cyclobutanones
Cyclobutanone also gives a-cleavage reaction. The efficiency of a-cleavage reaction of
cyclobutanone is ten times more than the cyclopentanone due to the angle strain. Angle strain
and steric strain increase the efficiency of a-cleavage. The photochemistry of a-cleavage of
~yclobutanonediffers significantly from t~e photochemistry of a-cleavage ofother cyclic ketones.
.
I
~
Unlike other ketones, cleavage occurs from 8 1 (n~1t*) and leads to the formation of aI, 4-acyl­
alkyl diradical. There are three following different pathways for stabilisation of the diradical:
(i) Loss of carbon monoxide and formation of 1, 3-diradical that undergoes either
recombination to cyclopropane or an hydrogen abstraction to form propene.
dO
hv
81
~
La
1, 4-Acyl-alkyl
diradical
l-co
L
"
';
•
~
L.
1
~
(ii) By a subsequent 13-cleavage and formation of ethylene and ketene.
~O
I 2 + 21-Cycloaddition
(iii) 1, 4-Acyl-alkyl biradical can undergo ring expansion by rebonding to oxygen to give
oxacarbene. This carbene can be trapped by polar protic nucleophile solvents. The over all
reaction is a ring expansion.
Ring
expansion
)
0:
Oxacarbene
ROH
~OyH
~I"OR
Acetal
Formation of 1, 4-acyl-alkyl radical can be proved by trapping experiment. 1, 4-acyl­
alkyl diradical can be trapped by 1, 3-butadiene at low temperature (-78°C) because low
temperature suppress the decarbonylation and I 2 + 2 l---cycloaddition reaction. Photolysis of
cyclobutanone at -78°C in the presence of trapping agent, 1, 3-butadiene gives 3-vinylcyclo­
hexanone.
-"
L
o
.
hv
~
CH 2
II .
• (CH-CH=CH 2
1
o
.{L
Q
3-Vinylcyclohexanone
Scheme 8: Trapping of 1,4-acyl-alkyl diradical by 1,3-butadiene
Ring expansion of unsymmetrical cyclobutanones is highly regioselective which indicates
the need for a more stable alkyl radical to attack the oxygen atom of the acyl radical. Indeed
alkyl substitution does increase the yield of ring expanded products. Both 2,2-dimethyl and
2,2,4, 4-tetramethyl-cyclobutanone form ring-expansion products predominantly.
L(
hv
f-*-
(Unstable alkyl radical)
d
hv
~
+
I,
-­
(Stable alkyl radical)
HoOR ROQH
0
G
1
1
ROH
Qo
Oxacarbene
ti e
hv
~
~OR
1
Ring expansion
ROH
yo
Photolysis of cyclobutanone is also stereospecific reaction. Stereochemistry at C-2 ill
retained during the rearrangement to the oxacarbene. Ring expansion reaction always exhLbit
:retention of configuration in methanol.
---~
o
OMe
hvlMeOH
H
Ii
'T
I
.
,
Ring expansion reaction is also possible in those cases where hydrogen transfer process
are inhibited by steric factor. Certain tricyclic ketones such as Fenchone gives ring expansion
due to the steric strain in the molecule.
0
.~O
~
1
H
OMe
MeOH
1
Some class of compounds have relatively weak Ca-C~ bonds which can undergo cleavage as a
result of electronic excitation of the carbonyl group. Cyclopropyl ketones are one such class,
and evidence for interaction between the carbonyl and cyclopropyl groups, which provides a
mechanism by which energy may be transferred from the carbonyl group to the bond which is
broken, is found in the UV spectrum.
The mechanism of the reaction has been shown to involve the formation of a 1,3-biradical
intermediate. Photolysis of acetylcyclopropane leads to the cleavage of the cyclopropane ring,
and this is followed by a 1,2-hydrogen shift or alkyl shift.
H
II
.~
. C-CH
(~
o
hv
"
•
.
Breaking of
C -C bond
.
•
~
CH
a
1, 2-Hydrogen
shift
(u)
(b)
o
o
II
CH -CH=CH-C-CH
'(Major)
"
"
CH =CH-CH -C-CH
2
,
2
3
(Minor)
In a similar way, bicyclo [4, 1,0] heptane-2-ones undergo cleavage of one of the cyclopropyl
C-C bonds.
o
" 2,Hydw"en ,hUt
))
)
1, 3- Diradical
~CH3
'1+
o
G
(ii)
1, 2-Hydrogen shift
(b)
(a)
In some cases the a-cleavage and 13-cleavage are often in competition as shown below:
0
0
0
&
hv
~
.~
II
U
+
.
.
_ (2)
(1)
In the above case, product (1) is formed due to the 13-cleavage and product (2) is formed
due to the a-cleavage.
a, 13-Epoxy ketones have also a relatively weak Ca-C~ bond which can be cleaved in the
excited state. Epoxy ketone reacts by way of 13-cleavage and alkyl migration on photolysis.
Mechanistically this reaction arises from a singlet nn* state and result is the fission of the
C-O bond. The migratory aptitudes shown withi~ such compounds is best explained via the
involvement of a biradical species formed by C-O bond fission. Some examples are:
o
(i)
0
~
Ph, /~\ II
hv
Ph,
. ~C-CRa~
Me~
Me
/ff)
eRa
11, 2-Methyl shift
"
o
.,
0
II
II
Ph-C-CR-C-CR s
I
CR a
...
i .
(ii)
1, 2-Methyl shift
)
Photochemi
;f«JiiiaiJiit
.
• n·...-,"IWt.,'j,MM",n'erl"!I'nttm• •'tfl''''·'''''I.$·",,!f'·
1, 3 (n, n*) excited carbonyl compounds having an accessible hydrogen atom in the y-position
undergo a characteristic 1, 5-hydrogen atom transfer by an intramolecular cyclic process with
the formation of ketyllike 1, 4-diradical.
o
H
)U:
R
hv
n ~ 1t*
)
8 1 or T 1
II
Cyclic transition state
1,4-Diradical
Depending on the conformation of the initially formed 1, 4-diradical, two different
pathways to stabilisation are possible:
(i) If only the sp-orbitals of the radical centres can overlap, a cyclobutanol is the product.
(ii) If the sp-orbitals of the radical centres are parallel to the ~-bond, they participate in
the formation of two double bonds (one in the enol and one in the alkene), a result of the
cleavage of the ~-bonds.
, I
f3-Bond
H
1
Cyclisation
C+OH
R
R
sp-Orbital
OH
I
R-C=CH 2
Jt
0
sp-Orbital
H
1
RACH'
Cleavage of f3-bond
CH 2
+
"
CH 2
.
The second process of this reaction is known as Norrish Type II process which leads to
the formation of alkene and alkenol.
Although the reaction occurs from both the singlet and the triplet states of n, 1t* transition,
the quantum yield from the singlet state is generally lower than that from the triplet state. In
case of aryl-alkyl ketones, the reaction occurs only with the triplet state because aromatic
ketones can undergo rapid intersystem crossing. Solvents also affect the efficiency of the
reaction. The singlet state reactions are unaltered in the presence of polar solvents. Polar
solvents such as alcohol, on the other hand, enhance the reaction from the triplet state.
The quantum yield of the reaction is poor since r,adiationless deactivation from the 8 1
and T1 states and reversal of the hydrogen transfer can comp~te with reactions proceeding to
products. The reversal process is confirmed by using the optical active ketone (1) having a
chiral y-carbon. Ketone (1) undergoes racemisation. Racemisation reaction confirmed that the
reaction intermediate is 1, 4-diradical. This also confirmed the back transfer of hydroge~
atom.
..~C2H5
CH
H
}U
'
(})
Back transfer of hydrogen atom i.e., photoracemisation can be quenched by the addition
of 1, 3-cyclopentadielle. This quenching experiment confirmed the formation of triplet state.
Participation of a 1, 4-diradical intermediate in the Norrish Type II reaction has been
confirmed by trapping experiments and spectroscopic.techniques. Formation of 1, 4-diradical
has also been proven chemically. Photoracemisation of a ketone with a y-chiral carbon atom
and loss of the chirality in the product was observed.
R2
~
oH
. :J
J
R
i'
R.
g~
~
R1
.
1
Rg
~
# 3
'g::r .)l~)R~
2
.
R
H
Reversal of
J:J0H
~ R
R1
·
1
Rg
+
(A)
OH
+
(B)
A
R 1·
(C)
~R3
+
.
~R 2
(D)
.
(A)
+ (B) + (C) + (D)
,
,
The y-hydrogen transfer to the oxygen atom has been shown to be intramolecular. The
transfer involves a six membered cyclic transition state. 5, 5-Dideuterohexan-2-one on
irradiation gives 2-deuteropropene and I-deuteroacetone. Formation of these products confirms
that the transfer of hydrogen takes place from y-carbon and process is intramolecular.
0
o
)
Ph
H
CHS
D
0
OD
CH s
+)lif'
>
CH,,--C~CH2
OH
/CH s
I
C
+ Ph-C=CH 2
II
+
CH 2
CH -CH=CH
D",
I
hv
+
1
s
2
When a molecule has two y-carbons both having hydrogens, transfer of hydrogen in the
Norrish Type II process is marked by a preference of cleavage of the weaker carbon-hydrogen
bond as in case of ketone (2).
o
II
CHs-C=CH-CHs + CHs-C-CH s
I
CH s
(Minor)
(Major)
Intramolecular hydrogen abstraction is not possible if y-carbon has no hydrogen.
For alkylaryl ketones the electron donating groups such as p-methyl and p-methoxy
substituents decreases the rate and quantum yield for Norrish Type II cleavage. Following
this trend p-hydroxy, p-amino and p-phenyl substituents inhibit the reaction completely. This
is because in such cases energy for n ~ n* excitation is less than that for the n ~ n* excitation.
The rate of radical recombination to give cyclobutanols compared with a, ~-bond cleavage
is often dependent on a-substitution.
o
II
R1
0
I
H
II. I
hv
Ar-C-?-CH 2-CHa ~ Ar-C-y-R 1+ CH2=CH2
R2
R2
lhv
-
OH
:,=P
R2
Substituents
Cyclisation
R1=R2=H
10%
R1=H, R 2=CH s
29%
R1=CHs' R2=CHs
89%
Thus substitution at a-position favours cyclisation reaction.
On the other hand, substitutions at f}-position favours Norrish Type II reaction. Thus
ketone (3) mainly gives Norrish Type II reaction i.e., elimination reaction. The rate of radical
combination to give cyclobutanol compared to f}-cleavage also depends on the stability of the
radical. If 1,4-diradical is stable then this favours cyclisation reaction and if 1,4-diradical is
unstable then this favours Norrish Type II reaction.
o
CH
•
OH
CH 2
I
CH a",,­
Ar-C-CH 2-C-CH a ~
I
t>I ;CH a ----+ Ar-C=CH 2 +
/C=CH 2
(3)
IX
131
"(
Ar~
CH a
CH a
CH a
II
I
a
OH
hv
The singlet state photoelimination (Norrish Type II process) reaction occurs with a high
degree of stereospecificity in threo and erythro form of ketone (4). Threo form of (4) gives trans
product whereas erythro form gives cis product. This elimination is syn elimination.
H
~
CH s
o
H
!XCOOC
If:
II
CH;r-C-CHs +
3
HsC
(4)
Threoform
H
~
CH s
MeooU
~
H
ells
(4)
Erythro form
o
hv
~
II
CH;r-C-CHs +
'\
Generally the yields of cyclisation product are only 10-20% of those of elimination in
normal ketones. But for certain compounds cyclisation is very efficient. Some of these are
systems where there are sterle factors promoting cyclisation, as with cyclododecanone.
hv
Pyrex
~
77% Yield
OR
Norborane also gives only cyclisation because alkene product is highly strained (highly
unstable).
.
.
~
"
..
hv
~
'-----+--OH
(Highly strained)
."
::::~;::::;:::::t:::':::::::'
While the normal reaction path of Norrish Type II process involves a six-membered cyclic
transition state, alternative reaction paths are fairly common. In these cases either larger or
small transition states provide diradicals which cannot fragment. Alternative routes are possible
when y-carbon has no hydrogen or biradical is stabilised by hetero atoms.
7.4.1 B-Hydrogen Abstraction
B-Hydrogen abstraction can only be possible if substrate has no y-hydrogen and the biradical is
stabilised by the presence of hetero atoms.
~
fr
.
lVJ
OH CH3
I I .
9-y-CH-NH-CH 3
CH 3
1
Cyclisation
7.4.2 0- and e-Hydrogen Abstraction
Intramolecular y-hydrogen abstraction by an excited carbonyl oxygen is approximately 20 times
faster than 0- and e-hydrogen abstraction. Nevertheless, the tendency of the half-occupied
n-orbital of the carbonyl oxygen to abstract a hydrogen atom from other positions is considerable.
A necessary condition is the planarity of the transition state. An important exalnple of this
class is cyclodecanone. In this case intramolecular hydrogen abstraction takes place from C-6.
co
5
4):
hv
Quartz
(Hydrogen abstraction
from £-position)
o
OR
There are other systems where hydrogen abstraction through a six-membered cyclic
transition state is not possible because there is no suitably placed hydrogen atom,
1-(8-benzylnaphthyl)-phenyl ketone is such a compound.
Ph[OOPh
RO
00
hv (366 nm)
(Hydrogen abstraction
from a-position)
7.4.3 Hydrogen Abstraction from Distant Sites
In the photolysis of alkyl esters of benzophenone-4-carboxylic acid, the planar transition state
is possible only for ~ 10, which leads to the formation of a ~ 1, 18 biradical whose recombination
produces a paracyclophane.
~
R-CR-(CH2)n\
OR
0----+
ph-b-fi--c/
'-~II
o
I, n- Diradical
R-CR-(CH2)n~
I
0
Ph-C-fi--C/
I~II
OR
-r
I
0
Paracyclophane
I
.
~
!
:::::::::::::::
The formation of photoenols arises by Norrish Type II process in ortho substituted aryl ketones
and are produced by the following mechanism:
o
Ph
" Ph
C-OR
(X. CH
I
C-.
,
~
I'
hv
~
S1 ~ T1
-~
(X
1
~
8
1
i
CH 2i
Spin
inversion
Ph
OR
(XCr-Ph
~
CR2
E·isomer
(XCr-OH
I
I
+
~
CH2
I
f--­
Ph
(X
~
I
V-OR
CH
•
2
Z-i80mer
I
,
I
,
216
The photoenolisation can be quenched by molecular oxygen. This confirmed the formation
of triplet biradical. The product of the reaction behaves as diene for Diels-Alder addition. Thus
photoenols can be trapped by addition of dienophile.
Ph
-;:/ I
((
~
Ph;
6=0
a6-0H
~ -;:/
~
CH3
~o
0
CH 2
o
Photo reduction of carbonyl compounds is one of the best known of photo reactions. In its
initial steps the mechanism involves transfer of a hydrogen atom to the oxygen atom of the
carbonyl excited state from a donor molecule which may be a solvent, an added reagent or
.ground state molecule of reactant. Ketone undergoes photo reduction in the presence of a
variety of hydrogen atom donors. Hydrogen atom donors are secondary alcohol, toluene and
cumene. This reaction is a bimolecular reaction.
hv
(i)
(C6H5)2-C=0
~
ISC
8 1 (n
~
Ph2-C-O + (CH3)2-CH-OH
•
•
•
1[*) ~ T 1 (n ~ 1[*) (C6H5)2-C- 0
~
II
-¢ + (CH3h-C-OH
Ph2
,.
I
o
(CH 3)2-C- OH + (C SH 5)2-C
OH
OH
~
I
(C SH 5)2V + (CH~2-C=0
(ii)
-
\
~
The above reaction takes place when concentration of benzophenone is very low
(10-4 m).
Other examples are:
a
ro---oo
OH
00
CHa"CH_OH
CH/
3
)
hv
a
©CCOOH
Il~COOH
r6'J ~~~~JJ
6
~
H
I
C-H
II
G-H
OH
0
.
H+
© C COOH
3
1
[ ©C~:j:)QJ]
o
COO~
COOH
1
©f~D--1~
aII
H
I
~
o 0
o NJQJ
I
H
(
aII
One of the first photocycloaddition reactions to be studied was the formation of oxetanes from
addition of carbonyl compounds to alkenes. This reaction is known as the Paterno-Biichi
reaction. For example, benzophenone with isobutene gives a high yield of an oxetane.
H,c,HcH,
C6 H 5
CH a
90% Yield
Paterno-Biichi reaction can be studied under two categories and the categories depend
on the nature of alkenes.
~
7.7.1 Addition to Electron-Rich Alkenes
Mechanistic studies have shown that the reaction pathway varies according to the type of
carbonyl compound and alkenes involved. Addition of simple aliphatic or aromatic ketones to
electron rich alkenes involves attack on ground state alkene by the n -m* triplet state of the
carbonyl compound in a non-concerted manner, giving rise to all possible isomers of the oxetane.
The reaction is non-concerted because the reactive excited state is a triplet state. The initial
adduct of this reaction is a triplet 1, 4-diradical, which must undergo spin inversion before
product formation is complete. Stereospecificity is lost if the intermediate I, 4-diradical
undergoes bond rotation faster than ring closure (Scheme 8).
o
II
C6H s-C-C 6H s
hv
~
81
ISC
~
T1
+
+
1
Spin
inversion
'p
C6H s
C
H5
H 3C
H 3C
C6H s
C6H s
HsCo-C-O
C6H s-C-0
I
I
t
I
I
i {.
+
H 3C"
/C--CH 2
H 3C
CH 2-C-CH 3
I
CH 3
1
90%
'4
C6H s
C
H5
CH 3
CH 3
10%
Scheme 9
"
Although the reaction is not stereospecific, there is a preference for one orientation of
addition, which can be rationalised in terms of initial attack on the alkene by the oxygen atom
of the excited carbonyl group to give a biradical intermediate. The existance of biradicals has
been confirmed by picosecond spectroscopy. The more energetically stable of the two possible
biradicals is formed more readily. Thus reaction is regioselective reaction. The consideration
of biradical stability is certainly applicable to the prediction of the major product of the
cycloaddition (see Scheme 9).
Two important rules for the successful synthesis of oxetanes have been put forward.
These rules are as follows:
(i) Only carbonyl compounds with a low-lying n ~ n* state will form oxetanes.
(ii) The energy of the carbonyl excited state must be less than that of the alkene to
prevent energy transfer from the carbonyl excited state to the alkene.
As far as the addition of aromatic carbonyl compounds is concerned only the triplet
state is reactive (because inter system crossing is very efficient in case of aromatic ketones)
and consequently a triplet biradical intermediate is produced.
The reaction of alkyl ketones can be complicaL~d by the less efficient inter system crossing
thus permitting reaction of both the singlet and the triplet state. Both singlet and triplet state
show equal reactivity for the reaction. The singlet state reaction is obtained at high
concentration of the alkene. On the other hand, triplet state reaction is obtained at low
concentration of the alkene.
7.7.2 Addition to Electron Deficient Alkenes
Photocycloaddition of aliphatic ketones to electron-deficient alkenes, particularly dicyano­
ethene, i:nvolves addition of singlet state (n ~ n*) excited ketone to ground state alkene. THe
reaction is stereospecific and the stereochemistry of the alkene is retained in the product,
oxetane.
CHa"
C=O +
CH/'
a
(CN
.I.
.
H':g
0
hv
313nm
CN
NC
fCN
CHa"
CH/
a
c=o +
,....
HaC
hv
313 nm
1
CN
H'C
)
NC
:::~g
a
CN
The course of the photocycloaddition of electron-deficient alkenes to ketones follows
certain rules. While oxetanes are formed only from 8 1 (n, n*) state, the T 1(n, n*) state
stereospecifically sensitises the cis-trans isomerisation of electron-deficient alkenes and does
not lead to oxetanes.
,.
o
II
CHa-C-CH a
T 1(n
~
x*)
(N
eN
CH
a~C=O
+
f
CN
CHa,
(CN
. . . . c=o +
NC
CH a
CN
Stereospecificity ofthe oxetane formation with electron-deficient alkenes can be explained
as follows:
It is suggested that in case of electron-deficient alkenes, oxetane formation takes place
via formation of exciplex. Exciplex formation takes place between the singlet excited state of
ketone and ground state of alkene. Exciplex is stabilised by charge transfer as well as energy
transfer between the constituent molecules.
CH a
h~/,,,
·.···6-0
,,-
"'C/
II
)
(a)
/C'"
r)c-o
\/
I
/'"
c+
Exciplex
r: * J
(
)
(b)
r;
J
+J
(a) Formation of an exciplex between a carbonyl excited state and an olefin ground state
(b) Charge transfer interaction in the formation of the carbonyl-olefin exciplex.
_.
,I
The exciplex (i.e., excited state cyclic complex) has considerable charge-transfer and the
stereospecific formation of the products is accounted for if both new bonds are formed
simultaneously in the complex, or if the second is formed after the first at a rate faster than
the rate of bond rotation. There is again a preference for one orientation of addition, but this
is opposite to that expected on the basis ofthe most stable biradical intermediate. The preference
reflects the preferred orientation in the exciplex which is governed by charge densities as
illustrated.
&t
o
)~
.&-
+
CN
'II::.
~
n --t1t*
7.7.3 Oxetane Formation with Dienes and.Alkynes
(A) With Dienes
Addition of carbonyl compounds to conjugated dienes is also feasible. The ETof dienes is usually
less than that of carbonyl compounds. However, the formation of oxetanes competes successfully
with excitation energy transfer because dienes quench the T 1 (n ~ n*) state. Thus formation
of oxetanes occurs from the 8 1 (n -4 n*) state of the carbonyl compounds. Therefore, dienes
give stereospecific reactions with dienes. Examples are as follows:
(i)
>C~O
+
0
~
:t:()
(ii»C~O <0 ~ J~
H
(iii)
)C=O +. J,~-
)
+
J~
)
H
hv
~
The third reaction takes place as follows:
o
A
II
i
'f
O-CH-CH=CH 2
~1.1
/"'. QH 2
(B) With Alkynes
Carbonyl compounds undergo photochemical cycloaddition reaction to alkynes to give oxetenes
which are usually not isolated but isomerises to a, ~-unsaturated carbonyl compounds in a
. subsequent thermal reaction.
\
lwarm
o
II
'
C6 H 5-CH=C-C-CH a
I
CH a
IX, ~.UnBaturated
carbonyl compound
1
7.7.4 Intramolecular Paterno-BOchi Reaction
Intramolecular Paterno-Biichi reaction is mainly given by y-S-enones. The efficiency of these
reactions can be attributed to the rapid rate of interaction between the excited C=O group
and the ground C=C group. This combination of substrates allows the formation of one
regioisomer. Thus yields are high and there is usually no byproduct(s). This reaction is highly
efficient and versatile method for the synthesis of a variety of compounds that are difficult or
impossible to be prepared by other methods.
Some examples are as follows:
1
=
R
R
otochemistry
One of the most important reaction of this class is the preparation of azulene as follows:
CCJ~CCJ----.OO
1
OJ
Azulene
Perhaps the most useful reaction of a, ~-unsaturatedenones is the I 2 + 2 I photocycloaddition
reaction with alkenes which affords cyclobutane derivatives. A simplified mechanism is given
below:
Alkene + Enone
hv
'"
>
[Enone]
1
lIse
[Enone]
3
lAlkene
Biradical
[Exciplex]
3
1
Product
The biradical can be formed by bonding at the a-carbon or the ~-carbon of the enone to
the alkene. The reaction is stereoselective at the fused junction. Cis-fused 4/5, 4/6 and 5/5
systems are common and are much more stable than their trans isomers. 5/6 can be cis or
trans. 6/6 can be cis or trans but prefers trans.
225
+
)~
I
r
l - - - ----­
hv
a-Bond
~-Bond
1
o
tv
r
--­
Biradical
1
1
~+~
H
H
65%
26%
$
H
+
+
o
lv
14%
The reaction is regioselective with respect to unsymmetrical alkenes. Electron rich alkenes
give head to tail adduct whereas electron-deficient alkenes form head to head adduct.
-
I
.
hv
~
+
Electron rich
alkene
~OMe
H
Head to tail adduct
0
6
+
I~N
Electron-deficient
alkene
hv
~
$CN
I.
H
Head to head adduct
8
CHAPTER
Rearrangem
:::~:::::::=:=
Cyclopentenone mainly gives a-cleavage reaction and I 2 + 2 I cycloaddition reactions.
5, 5-Disubstituted derivatives undergo rearrangement reaction to the cyclopropyl ketene. The
. rearrangement arises from the n ~ ft* singlet state and has to be carried out in dilute solution
to prevent dimerisation. The substrate first undergoes Norrish Type I cleavage leading to the
formation of a diradical. Recombination of this diradical leads to the formation of ketene.
Ketene reacts with protic nucleophilic solvent to give adduct.
0
6:
0
hv
n ~ 1t*
)
Sl~
Cf
I
R
I
0
)1 R
~R
I
0
)1
Recombination
I,
R
[jR
lROH
.' 1.-:
,
.
a:
0
229
I
i·
i'
:a:
::::::::::::=:::::::::::::;
a, ~-Unsaturatedcyclohexanones give two type of rearran"gements. These rearrangements are
possible when bimolecular reactions (cycloadditions and dimerisations) are suppresed by the
use of dilute solutions. The two prominent rearrangements are the di·n-methane type
rearrangement (path A) and the lumiketone rearrangement (path B). Both these rearrangements
.are examples of 1,2-shift with ring contraction.
PathB
Bicyclo [3,1,0]·
hexan-2-one compound
Migration of the
carbon atom of the
cyclohexene ring
°
Q
I
R
4r
k
Path A
-----~
Migration of aryl
group
R
Ar
Bicyclo [3,1. 0]·
hexane-2-one compound
Both type of rearrangements can be sensitised using propiophenone or quenched using
naphthalene. This shows that both reactions proceed via triplet excited species. The rearrangement
with ring migration occurs from a (n, n*) triplet excited state, whereas rearrangement with aryl
migration occurs from an (n, n*) triplet excited state. This is supported by the observation that
a 4-alkyl-4-aryl cyclohexenone undergoes rearrangement with aryl migration in benzene (i.e.,
non polar ,solvent) but with ring migration in aqueous methanol (polar protic solvent).
0
Ph
ey<
R
CH 3 0H/HOH
+-(- - - - ' ' - - - - - ­
3(lt .....
hv
It*)
8.2.1 Lumiketone Rearrangement
This rearrangement is given by 4,4·dialkyl-a,~-unsaturatedcyclohexenones. The rearrangement
involves a n,n* excited triplet state in the presence of sensitiser. The rearrangement is carried
t
out in the presence of polar protic solvents like BuOH, AcOH, H 2 S0 4 etc.
The Lumiketone rearrangement occurs through migration of ring atom (migration of
C-4 ring carbon to C-2 ring carbon i.e., bond formation between C-2, C-4 and bond breaking
t
between C-4, C-5). It is observed that n ~ n* state favours 1 adical intermediate in BuOH,
(Scheme 1) and Zwitter ion in acid (Scheme 2).
6
R
hv
--~~
BJOH
ISC
81
~
Q
T
,1
7t ~ 7t*
R
R
R
1
~.
C6
~
( Bond form,ation
betwe~n
CZ-C4
+ -Shift
( -of
--­
R
C4-CS bond
on Cg-carbon
•
R
R
R
Scheme 1
hv
6·1
Q.
~
-------+~
AcOH
ISC
81
~
T
1
7t ~ 7t*
e
e
o":::
'
+-(-­
If)
(i)
Unstable
e
(i»
A
~
AcOH,
~OAC
o
~O~
• I
232
Photochem'
e
0
e
0
~
(iii)
(f)
OR
AcOH )
~\
H e
[/OAc
0
~
Scheme 2
This rearrangement is stereospecific, Optical activity at C-4 is at least partially retained
in the product in some cases. This suggests that the bond-forming steps in the reaction sequence
occur at the same time as, or very shortly after, the bond breaking steps.
This reaction is quite general and also proceeds in the case of some 4-alkyl-4­
aryJcyclohexenones.
hv
BuOH'
)
8.2.2 Di-n-Methane Type of Rearrangement
This rearrangement is given by 4,4-diaryl-a,~-unsaturated cyclohexenones in the presence of
non-polar solvents like benzene and cyclohexane. In some cases reaction also takes place in the
presence of alcohol. In all the cases mechanism is radical mechanism (Scheme 3).
This rearrangement occurs through migration of a substituent at C4 . During the course
of the rearrangement aryl group first migrates from position-4 to position-3 (i.e., 1,2-shift) and
then formation of cyclopropyl ring (i.e., ring contraction from six to five membered) takes
place. The reaction is given by triplet state of n ~ n* transition. This reaction is carried out in
the presence of sensitiser.
The rearrangement is totally analogous to Di-n methane rearrangement except that the
enone rearrangement is brought by n ~ n* excitation which is not possible in the 1,4-diene.
6
0
¢
Ph
hv
)
CsH s
n ~1t*
81
ISC
T\
Q
Q.
H5 C6
Ph
C6 H5
I
~Ph
Ph
6
6)
0
f-­
~,
•
T:T
C6......
5
C6 H5
1,2·Shift
H5C6
C6 H5
Scheme 3
1, 2-shift in Scheme 3 t&kes place intramolecularly as follows in Scheme 4.
6
o
1,2-Shift
~Ph
)
Intramolecularly
Ph
Scheme 4
In compounds in which the two aryl groups are substituted differently, it is found that
substituents which stabilise radical character favour migration. Thus, the p-cyanophenyl
substituent migrates in preference to the phenyl substituent.
o
o
eN
One of the most important feature of the rearrangement is the remarkable stereospecificity
of the process with a 140 : 1 preference for the formation of the trans isomer rather than cis.
6 ~ ~~
Ph
Ph
~
Ph
+
O-.-Ph
!
Ph
6%
81%
.~
I
I
•
In some cases rearrangement takes place via formation of zwitterion intermediate
particularly in the presence of tert~butanol. .
,
This type of rearrangement is also given by C-4 vinyl substituted enones in which
migration of vinyl group takes place.
hv
(254 nm)
)
1­
[
I
6
o
1,2-Shift
~Ph
)
Intramolecularly
Ph
Scheme 4
In compounds in which the two aryl groups are substituted differently, it is found that
substituents which stabilise radical character favour migration. Thus, the p-cyanophenyl
substituent migrates in preference to the phenyl substituent.
o
o
CN
CN
One of the most important feature of the rearrangement is the remarkable stereospecificity
of the process with a 140 : 1 preference for the formation of the trans isomer rather than cis.
Q
Ph
Ph
o
0
~Ph C)l..""Ph
+
5
~
Ph
Ph
81%
6%
.~
I
,1
,
,
In some cases rearrangement takes place via formation of zwitterion intermediate
particularly in the presence of tert~butanol. .
This type of rearrangement is also given by C-4 vinyl substituted enones in which
migration of vinyl group takes place.
hv
(254 nm)
)
~
I
"'-..
I
::::::=::::::::::=:::::::::::::::
Cross conjugated dienones give type B rearrangement. Type B rearrangement takes place via
n ~ n* triplet state. The nature of intermediate of the rearrangement depends on the reaction
condition and the nature of the solvent.
Case-I. Rearrangement in the presence of polar protic solvent.
In polar protic solvent the rearrangement occurs in two steps. The first photochemical
step occurs with a high quantum yield (<jl = 0.85).
0
¢
Ph
Ph
0
O<Ph
hv
TypeB
rearrangement
Step 1
(1)
hv
OH
OPh
,ij
Ph
Ph
1
2
H
O step
Second
OH
+
~ C
+
,ij
Ph
Ph
~
~Ph
Ph
The efficiency of the second step is substantially low (<jl =0.013). The formation ofbicyclo
[3,1,0]-hex-3-en-2-one is formally analogous to a 1,2-acyl shift and occurs from the
T1 (n ~ n*) state. In all reported cases reaction proceeds from the n ~ n* triplet state. The
unsaturated acid arises from the 8 1 state of the bicycloenone (1). The phenols are produced
from the T 1 state of the bicycloenone (1).
Zimmerman found that 4, 4-diarylcyclohexa-2, 5-dienones undergo a rearrangement with
velocity four times more than the 4, 4-dialkyl-2-cyclohexenone. The reactive excited state is
the n ~ n* triplet state of the dienone. The reaction is thought to involve C-3,-C-5 bond
formation in the excited state followed by inter sy~t8m crossing and electron deactivation to
give Zwitterion intermediate. This Zwitterion intermediate then rearranges to the observed
bicyclic product. The electron deactivation process is known as demotion. The reaction takes
place in the presence of sansitiser. The reaction is quenched by the quencher. In this reaction
photophosphorescence phenomenon is observed. These results confirm that excited state is
. triplet and demotion process (n* ~ n) takes place during the course of the reaction. According
to Z~mmerman, the mechanism can be represented as follows (Scheme 5):
~
0
0
¢
hv
(S,>
> 310 nm
Ph
Ph
Ph
V
ISC
(TJ
Ph
Ph
.
0
Q'
.©
between C3-C 5
Ph
Ph
'.,
Ph
1.
~
Ph
Tj
Tj
Q,
0
0
Bond formation
1,
(C)
(B)
(A)
Ph
V
OJ
'.
"
Ph
Ph
Oxyallyl biradical
Tj
lIse
.
0
Q'
Ph
7t* --7
n
Demotion
\
(-
Ph
8j
(D)
l
o
a
2
o
2A6
6
3 1
5
4
~earrangement: bond breaking
of cyclopropane ring and
migration to carbocation
Ph
Ph
/Ph
~Ph
3
5
(F)
Scheme 5
4
The necessity of n* ~ n electron demotion step arises because the n (Pi) system in the
n ~ n* state is electron rich but the observed rearrangements are characteristic of migrations
to electron deficient centres. If demotion step does not take place, then Zwitterion has the
following structure:
(fl
o
9
8
Ph
Ph
Thus reactive ground state Zwitterion (E) is an intermediate of the rearrangement and
this Zwitterion rearranges to the observed product. Conformation of the participation of
Zwitterion intermediate in the rearrangement is provided by the thermal preparation of the
photoproduct by the reaction of the 2·bromoketone (G) (Scheme 6).
e
0
pRh
e
0Q
0
0
~
Ph
+--­
-
Q-Br
Ph
Ph
Ph
i
J
0
cA
Be
r
LDA
HgBr
0
Ph
Ph
Ph
Ph
(G)
Scheme 6
,.
Returning now to the example under discussion, we see that the product of the
rearrangement can itself react further since it still has enone system capable of undergoing
excitation. The following six structures represented the various states of electron distribution.
e
a
QPh
e
a
~
hv
n
~1t*
Ell
Ph
a
Ph
Ph
~P~
Ph
(I)
(II)
1
e
1
e
a
a
Q
Ph
Ph
~
OcPh
Ell
Ph
(IV)
(III)
1
e
1
a
~Ph
__
Ph
(V)
',\
e
0
~Ph
Ph
(VI)
Among these (I) and (II) were proposed to be precursors in the formation of the starting
material itself and they should now lead to new products.
(III) and (IV) can undergo phenyl migration leading respectively to 2, 3 and 3, 4-diphenyl
phenols. Both of these phenols have been isolated from the reaction mixture.
e
a
~
a
1,2-Shift
Ph
Ph
)
(III)
e
Cf
~
EIl:h
e
a
a
OPh
Ell
h
e
a
Ph
1,2-Shift
)
Q-Ph
Ph
(IV)
OH
Ph
6:
~
Ph
2, 3-Diphenyl phenol
OH
~
~
Ph
Ph
3, 4-Diphenyl phenol
When photolysis is carried out in aqueous dioxane, 2,3-diphenylphenol is the major
product and 3,4-diphenyl phenol is the minor product. In the presence of acetic acid equal
'~
amounts of 2, 3- and 3, 4-disphenylphenols are obtained. One possible reason for this difference
is the better electron delocalisation with (III) than with (IV) ia the dioxane. In acetic acid
protonation of oxygen occurs and delocalisation is less significant factor.
Structure ('IT) can rearrange to a ketene and forms an acid in aqueous medium but (VI)
cannot.
In terms of mechanism given by Zimmerman, the various steps can be represented in
Scheme 7.
0
a
0
0
Ph
Ph
hv
n ~lt+
)
.~Ph
Bond
alteration
Ph
)
6
~
Ph
Ph
l t+-
(A)
n
Demotion
e
0
OR
QPh
~
h
GC
~
Ph
(C)
e
0
1,2-Shift
6
~
Ph
Ph
Ph
(B)
Zwitterion
intermediate
Scheme 7
'.
The evidence for the Zwitterion (B) being an intermediate in the reaction follows from
the rearrangement of independently generated Zwitterion by non-photochemical means to the
same product (C).
e
APh
llA
Ph
~
~ Ph
e
6
0
hv
Ph
Ph
OPh
(B)
~
~Ph
Ph
---~~---
y.
Case-II. Rearrangement in gas phase or in the presence of non-polar aprotic solvent
The rearrangement in gas phase or in the presence of non-polar aprotic solvent occurs
via formation of biradical intermediates. The arrangement in gas phase resembles the di-1t­
methane rearrangement.
.
hv
Gas phase
)
c¢.
Rearrangement
)
Diradical
.~
A cyclohexan-2, 5-dienone with a good leaving group on C-4 undergoes a different reaction
in solvents with a readily abstracted hydrogen atom. Intermolecular hydrogen abstraction
followed by the elimination of the group from C-4 leads to a phenolic product (Scheme 8).
hv
Gas phase
h lHYdrogen donor solvent
v (C 2H S>2 0
1
.
CCls
CCla
+
1(C2H S>20
CHCla
Scheme 8
Radicill intermediates are also involved in the photochemical reaction of the
spirocyclopropyldienone. This also gives phenolic products in the solvent which is a good source
of hydrogen atom.
.Q
hv
1
.
~
Q
+
1
CH 3-CH-O-CH2-CH 3
CH s-CH-G-C 2H 5
1
Dimerisation
CH2
I
CH2
I
CH-CH3
I
o
I
C2H5
Natural products particularly terpenoids and steroids having cross-conjugated ketone
system undergo dienone rearrangement via formation of Zwitterion intermediate.
hv
R1
R1
Zwitterion intermediate
. In the presence of aqueous dioxane, Zwitterion converts into lumiketone.
R1
Lumiketone
Zwitterion intermediate gives two isomeric ketones (H) and (I) in acetic acid medium
(45% CH 3 COOH).
1
OH
o
o
(1)
(H)
Stability of intermediate cation (AI) and (A) determines the reaction path in acidic medium.
When R1 is electron donating group and R2 is H. then major product is fused ring ketone (H).
Similarly ifR 2 is electron donating group and R 1 = H than major product is spiran ketone (1).
Zi_:::::::;::::
....
~,y-unsaturationin a carbonyl compound promotes a-cleavage because of allylic stabilisation of
the radical produced. ~, y-Unsaturated ketones, in addition to undergoing normal photochemical
reactions of saturated ketones, undergo rearrangement reactions. Their irradiation induces
non-concerted sigmatropic reactions that are directed by the electronic configuration and
multiplicity of the excited state. The rearrangements are 1, 2-acyl shifts (oxa-di-n-methane
rearrangements) which occur from the lowest triplet state of n ~ n* transition and 1, 3-acyl,
shifts which occur from direct irradiation from the 8 1 or T 2 (n ~ n*) state with formation of an
acyl-alkyl radical pair. It has been suggested that, while at higher temperature the radical pair
is formed predominantely from the 8 1 state, at lower temperature the T2 state is involved.
Although both rearrangements involve biradical or biradical pair intermediates, they are generally
stereospecific.
Zwitterion intermediate gives two isomeric ketones (H) and (I) in acetic acid medium
(45% CH 3 COOH).
1
OH
o
o
(I)
(H)
Stability of intermediate cation (AI) and (A,) determines the reaction path in acidic medium.
When R1 is electron donating group and R2 is H then major product is fused ring ketone (H).
Similarly if R2 is electron donating group and R1 = H than major product is spiran ketone (I).
:
~, y-unsaturation in a carbonyl compound promotes a-cleavage because of allylic stabilisation of
the radical produced. ~, y-Unsaturated ketones, in addition to undergoing normal photochemical
reactions of saturated ketones, undergo rearrangement reactions. Their irradiation induces
non-concerted sigmatropic reactions that are directed by the electronic configuration and
multiplicity of the excited state. The rearrangements are 1, 2-acyl shifts (oxa-di-n-methane
rearrangements) which occur from the lowest triplet state of n ~ n* transition and 1, 3-acyl,
shifts which occur from direct irradiation from the 8 1 or T 2 (n ~ n*) state with formatfon of an
acyl-alkyl radical pair. It has been suggested that, while at higher temperature the radical pair
is formed predominantely from the 8 1 state, at lower temperature the T 2 state is involved.
Although both rearrangements involve biradical or biradical pair intermediates, they are generally
stereospecific.
8.4.1 1,2-Acyl Shift (Oxa-Di-n-methane Rearrangement)
Mechanistic studies of this rearrangement have shown that the excited state is low-lying
n -7n* triplet state. Because inter-system crossing probability is low in B, y-unsaturated ketones,
the triplet state reaction in general be initiated only by sensitiser. This rearrangement also
takes place in absence of sensitiser in those cases where intersystem crossing is efficient. This
rearrangement affords cyclopropane derivatives and referred to as oxa-di-n-methane
rearrangement. The mechanism of the rearrangement is analogous to the di-n-methane
rearrangement. 1,2-Acyl migration does not involve a-fission, but a stepwise process is involved
where the acyl group migrates to yield a biradical intermediate. In this rearrangement migration
takes place by formation of 1,4 biradical which than converts into 1,3-biradical due to 1,2-shift.
X(Ph
R
R
'y_
'l
0
Ph
~ Ph0
__ J
--+ 1t*
Ph
1
rr
R
R
;A~t'
~O
Ph
R~
Sens
1t
R
,Ph
~o
Ph
hv
Ph
Ph
~
Ph
0
;<.R Ph
I'
0
o
Ph
1, 4-diradical
/Ph
Ph~O
Ph
The product of the rearrangement is cyclopropyl alkyl (or aryl) ketone. Acyl group migrates
from position-I to position-2 on the alkenyl chain in this rearrangement. Thus, rearrangement
is 1,2-acyl shift.
R
R 0
3
2
I
II
hv
C6 H 5-C =CH-C-C-Ph
I
11
C6H 5
R
)
\
0
R~IIC-Ph
2
Ph
3
Ph
Product formation of the rearrangement in short can be represented as follows (not
based on the mechanism).
CH 3
I
-R
COCH3
CH
CH
I
I
C=O
I~
n
R-C-CH=CH 2
COCH 3
hv )
Sens
0/ \
R-C
I
R
CH2
0
I
)
/ \CH
R-C
I
R
2
\
8.4.2 1,3-Acyl Shift
13, y-unsaturated ketones on direct irradiation gives 1,3-acyl shift. This shift takes place by
formation of acyl radical and allyl radical due to a-cleavage of the substrate. 1,3-acyl shift
occurs from 8 1 or T 2 state of n --+ n* transition. It has been suggested that, while at higher
temperature the radical pair is formed predominantly from the 8 1 state, at lower temperature
the T 2 state is involved. Although intermediate is radical pair but the rearrangement is generally
stereospecific.
a
R
II I
CH a-C-C-CH=CH 2
I
R
l
CH a-
a
II
2]
v
+ .lCH= CH
I
.
R
1
(Radical pair)
Recombination
1,3-Acyl shift in short can be represented as follows:
CaCHa
I-----V
R-C-r-CH=CH 2
11~3
)
R
In this rearrangement acyl group migrates from position-1 to position-3 on the alkenyl chain.
Due to this reason, the rearrangement is known as 1, 3-aL-y1 shift. 1,3.acyl shift leads to migration.
2-cyclopentyl methyl ketones were the first e~amples of the rearrangement of ~,y-enones.
Photo CIDNP and radical trapping experiments indicate that the 1, 3-aeyl shift is a radical
process that proceeds predominantly via cage radical pair which also leads to a-cleavage
products.
Recombination
,
The stereospecificity of this reaction was shown by the following examples:
q:.COCH'
CDs
R(+)
S(-)
2-Cyclopentylmethyl ketones give 1, 2-acyl shift in the presence of triplet sensitiser.
hv
Sens
~
Similarly bicyclo [2, 2, 2] octenone gives 1, 3-acyl shift by direct irradiation. In the presence
of triplet sensitiser it gives 1, 2-acyl shift.
~
1
~23
hv
~
.6
4
5
o
Recombination
hv
Sens )
~ -0=9
.
0° - ffi ~ 0\9
7
5
8~O­
~
2
3
1
2
\p
o
8
4
1
6
5
7
Cyclic ~, y-unsaturated ketones undergo both 1, 2 and 1, 3-acyl shifts. 1, 2-acyl shift
leads to a cyclopropane derivative, while a 1, 3-shift leads isomerisation. Ring contraction
and/or isomerisation is main character of 1, 3-acyl shift.
Bicyclic ketone (1) gives isomerisation by 1, 3-shift.
0
0
d)
hv
~
0
d)
:pj
3
4
(I)
III
Bicyclo [3, 2, OJ-hept·6-en-2.one
0
pJ
0
ID
Recombination
1
4
On the other-hand compound (II) gives isomerisation as well as ring contraction reaction.
Cycloocta-3-enone
\J
(II)
1, 3-Shift mainly results isomerisation and/or ring contraction. Sometimes it can also
result in ring expansion. Compound (III) gives ring expansion 1, 3-shift.
o
)1..
n
~. ~
1
(III)
Recombination
o
o
R
The photolysis of homologous enones (4) and (5) are remarkable. Compound (4) gives
only 1, 2-acyl shift whereas compound (5) gives only 1, 3-acyl shift.
0
m
~
T1
)8(J
.
~
o
i
(4)
1, 2-Shift
y8
yO
f---­
0
"
0
hv
~
81~
f))
o
.
I
[{X)
0
f---­
fb
0
The above example shows that oxa-di-n-methane (ODPM) rearrangement is generally
much more efficient in rigid than in non-rigid systems. Similarly 1, 3-acyl shift is generally
much more efficient in non-rigid systems:
Since enone (4) has a much more rigid structUl:e than enone (5), the plane ofthe carbonyl
group and the double bond in (4) are separated. As a result, a-cleavage in the 8 1 state does not
compete efficiently with intersystem crossing to Tl' from which a-cleavage occurs with the
formation of a triplet biradical. Thus 1, 2-acyl shift occurs.
\
-
Enone (5) is a flexible molecule. Thus the overlap of carbonyl group and the double bond
in enone (5) increases the efficiency of the a-cleavage.- Thus it competes successfully with
intersystem crossing from the 8 1 state.
Generally seven membered enones (such as 5) in which double bond is present between
fused carbons are flexible and give 1, 3-acyl shift. On the other hand six-membered ring enones
(such as 4) in which double bond is present between fused ring carbons are rigid and give 1, 2­
acyl shift. Other types of enones give 1, 2 as well as 1, 3-acyl shifts.
1
,I
I
Some of the enones which gives only 1, 2-acyl shifts are as follows:
OR
OAc
(B)
(A)
OAc
(C)
(D)
(E)
Examples of enones which gives 1, 3-acyl shifts are as follows:
R
0(0
o
(F)
(H)
(G)
o
R
(I)
(J)
Xt::::=~:r:=::::~:=::::::
1, 2-Acyl shift is given only by ~, y-unsaturated ketones. This rearrangement is not given by
aldehydes. The failure of aldehydes to undergo 1, 2-acyl shift (i.e., oxa-di-1t-methane
rearrangement) can be overcome by conversion ofthe aldehyde to an imine. Thus when aldehyde
is converted to the imine and irradiated under sensitised conditions, the normal cyclopropane
forming process takes place. The formation of the cyclopropane by this path is known as aza­
di-1t-methane rearrangement (Scheme 9).
EB
CSH 5-CH 2-NH 2 1H
.C SH 5
CH a
I
I
) C6 H s-C = CH -J-J.H = N-:-CH 2-CJ:I5
CH a
1
hv (a type cleavage)
CH=N-CH2-C 6 H 5
1
C6--y
H CH
1/
C6 H5.
y
CH
.
/
Q,"",
CH
1
C -(1,2·Shift C =.11."-
aSS.
.' CHa
6
~
CH
I
.
Allyl radical
1
Scheme 9
a
f"H - C' +
Y
I'
CH a
~i
A similar path has also been shown to be involved with the oxime acetate.
The most remarkable photochemical reaction of nonconjugated dienes is the di-n-methane
rearrangement. This rearrangement is given by dienes having n (Pi) system separated by an
Sp3 hydridised carbon atom i.e., 1,4-pentadienes. The rearrangement is also given by
3-phenylalkenes in which one of the double bonds is replaced by benzene ring. Di-n-methane
rearrangement is also known as Zimmerman rearrangement or Zimmerman dian-methane
rearrangement. Irradiation of the substrate induces rearrangement to a vinylcyclopropane or
phenylcyclopropane derivative.
2
~
1
~
3
4
5
Vinylcyclopropane
Phenylcyclopropane
This rearrangement is found in both cyclic and acyclic dienes. The rearrangement of
monocyclic and acyclic dienes occurs solely from the singlet state. Sensitised reaction of these
substrates leads to cis-trans isomerisation or I 2 + 2 I cycloaddition.
Experimental results show that di-n-methane rearrangement is a cdncerted process,
obeys the Woodward-Hoffmann rules, retains configuration on atom-1 and -5 (a result of
disrotatory ring closure between C-3 and C-5), and inverts it on atom-3 (conrotatory mode). In
actual practice, the reaction is represented as one involving a 1, 4 and 1, 3-diradical
intermediates in case of 1, 4-pentadiene since it is easier to follow the various steps necessary
for the rearrangement (Scheme 10).
,.
Bond formation
between C2-C 4
4
)
5
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
1,4-Diradical
Bond breaking
between CZ-C g
or Cg-C 4
1
2
2
Ph
'/
4
Ph
4
Ph
Ph
Ph
Ph
1, 3-Diradical
Ph
Scheme 10
Thus the di-n-methane rearrangement involves a rearrangement of the pentadiene
skeleton with no alkyl or aryl group migration. Nevertheless, the reaction path that would
involve the more stable diradical is always followed (Scheme 11).
hv
R 5C2
CRa Ph
Ph
1,4-Diradical
'.'
Route-a
-'
Breaking of
bond between
Ph
C2-Cg
Ph
CR a
Ph
(More stable 1, 3-diradical)
1
I
Ph
(1)
(Sole product)
Ph
J:"
I
!
i
Route-b
Bond breaking
)
Ph
Less stable 1, 3-diradical
1
~Ph
Ph
(2)
(Not observed)
Scheme 11
Formation of only one product (1) shows that there is retention ofconfiguration at carbon-l.
Thus cis-trans isomerisation of the bond C1-C2 must be slow in comparison to formation of
cyclopropane derivative. Similarly, formation of product (1) and not (2) shows that less stable
radical participates in the three membered ring opening which gives more stable 1, 3-diradical.
The di-n-methane rearrangement has been studied in a sufficient number of cases to
develope some of the patterns regarding the substituent effects. When the central sps carbon
(Cs-earbon) is unsubstituted, the di-n-methane rearrangement mechanism becomes less
favourable by mechanism given in Schemes 10 and 11. In such cases, the route is 1, 2-shift and
closure to a three membered ring. The case of 1, 5-diphenyl 2, 4-dideutero-1, 4-pentadiene is
illustrative (Scheme 12).
hv
DiY
Ph
Ph
1, 2-Diradical
11,2, H-shift
Scheme 12
(allyl radical)
$
The driving force for 1, 2.migration may be the fact that a more stable allylic radical
results.
The resistance of the unsubstituted system at carbon·3 to the di·n·methane
rearrangement probably occurs of the second step of the rearrangement which leads the
formation of primary alkyl radical which is energetically unfavourable.
Second
step
First step
~
Primary-1, 4-diradical
Primary 1, 3-diradical
Thus substituents on the central carbon and terminal vinylic carbons are essential for
the occurrence of the di-;r-methane rearrangements.
Monocyclic dienes also give this rearrangement. One of the most common example of
monocyclic 1, 4·diene is compound (1).
hv
..
<\1'
i'"
\
-·f
I
-'
A variant of the di-n-methane rearrangement involves an aryl-vinyl interaction as in
the conversion of the propene derivatives (3-phenyl propenes, I-phenyl propenes, and 1, 3­
diphenylpropenes) into cyclopropanes. In such cases rearrangement occurs via 1, 2-shift
(Scheme 13).
Vi
CH2"'-CH
~
hv
~
II
CH-C6H s
1,3-Diphenylpropene
~C
V
1,3-Diradical
6H s
f-(- - - - ­
1, 2-Diphenylcyclopropane
Scheme 13
Examples of this category are as follows:
(i)
Q Q:
hv
~
Ph
Ph
Ph
l,2-Shift
of Ph
VPh (fPh
l
Ph
Ph
Ph
3-Phenylpropene
derivative
(ii)
Q Q:
hv
Ph
Ph
Ph
1,2-Shift
Ph
)
~fl
Ph
3·Phenylpropene
derivative
1
6
~h
+
I
"'Ph
Ph
Ph
Phenyl substituted propenes gives di-n-methane rearrangement by singlet s&ate.
Exceptions of this generalisation are found in the systems such as 5, 5-diphenyl-l,
3-cyclohexadiene for which direct irradiation causes electrocyclic ring opening, and triplet
sensitisation leads to a di·n-methane rearrangement.
r1"t1
Ph
~Ph
hv
direct
1
80%
hv Sens
1,2-Shift
.
)
(J--4
YPh
~Ph
Ph
Ph
I-Phenyl-3-vinyl-3-methylcyclohexene gives di-1t-methane rearrangement in the presence
of sensitisers.
~h
hv
Sens
G:h
1, 2-Shift of
)
vinyl group
In the above case vinyl group migrates because migrating power of vinyl group is more
than the phenyl and alkyl groups.
,
:
3-Phenyl propene where position-I is not substituted behaves as 1, 4-diene. In such
cases phenyl ring is part of the 1, 4-diene system.
Ph
Ph
Ph
~
oY.
hv
1
Ph
Ph
Ph
Ph
+--
...
.
~
3, 3, 3·Triphenylpropene
Ph
Ph
+-­
~
,
Ph
cFl
In such cases regioselectivity operates in such a way as to regenerate the aromatic
system in the three membered ring opening process.
Unlike linear and monocyclic 1, 4-dienes, polycyclic dienes undergo the di-1t-methane
rearrangement from the T 1 state. Barrelene was one of the first examples. A variety ofbarrelene
derivatives have been photolysed in the presence of sensitiser.
4
4
2
2
hv
Sens
2
~
1
4
,CD,
3
7
4
1
-
2
8
7
:~::::;=::::::=:::::;:
Variety of aromatic compounds, aryl esters, aryl ethers, anilides, etc" undergo photochemical
rearrangements. In these rearrangements acyl and alkyl groups migrate to ortho and para
positions on .photolysis.
Photo-Fries Rearrangement
The most extensively investigated photo rearrangement is Photo-Fries rearrangement
o
II
O-C-R
6
~
Phenol ester
@-LR +~+~
OR 0
Qrtho acyl phenol
C=O
I
R
p-Acyl phenol
This rearrangement is an intramolecular rearrangement. In this rearrangement substrate
undergoes dissociation into phenoxy and acyl radicals which combine within the solvent cage to
give intermediates which on aromatisation (or enolisation) give the product. The phenol is
produced by phenoxy radicals which escape from the solvent cage.
0
II
hv
Ph-Q-C-R
~
[Ph-olJ'
~
&~R
II
~
0
0
+
R
Q
C-R
• 0II
]
PhOQ-R
Solvent cage
Excited state
0
[
1
0
0
+
"
0
1
1
1
@-II
o
OR
OR
OR 0
C-R
r9J
C=O
I
R
©
"
\.""­
rrangemel')t$
,r
!
When this reaction is carried out in gaseous phase, only phenol is obtained. This confirms
the formation of solvent cage.
Similar rearrangements are also obtained from the following compounds:
6S0~
OH
hv
~
@-SO~
OH
+
0
NH-C-R
©
$
OH
+
©
SOsR
II
NH 2
hv
~
~COR
+
...
-1
259
¢'
NH 2
+
©
COR
O-CH 2-CH=CH 2
©
~
©OH
OH
OH
CH,.--CH=CH,
+
~
+
CH 2-CH=CH 2
©
..
..r
i
--,
...--­
• i
""',
9
CHAPTER
Uebon and Photo,
.....
l~
Photo reduction of carbonyl compounds is one of the most thoroughly investigated photochemical
processes. In fact, one can carry out the reduction of benzophenone in benzene in a pyrex tube
by exposing it to sunlight for few days. The reduction takes place in the presence ofbenzhydrol.
Benzpinacol
The first step is excitation of the benzophenone to the l[n, n*] state which intersystem
crosses to the 3[n, n*] state. Hydrogen abstraction from alcohol gives two diphenyl hydroxy
methyl radicals which combine to form benzpinacol.
ISC
hv
n
)
)
~1t*
Ketones undergo photo reduction in the presence of a variety of hydrogen atom donors
other than secondary alcohols. The effect of substituents in the aryl ring can be quite dramatic
because of the changes in the electronic configuration of the low-lying triplet state. Hydrogen
atom abstraction is much more efficient for n, n* than n, n* for ketones.
Excited ketones can be reduced by tertiary amines. The key reaction in this case is
electron transfer from the amine to the ketone producing ketyl radical and the cation radical
from the amine.
262
I
..
~
--~---------------------------------------
..
,
OH
[ CH:r-~-CHa +
hv313 nm
.
Hydrogen abstractIOn
)
OH
0-0 0-rOH
+
. ]
1
CH 3
1
0
1
+ CHa-C-CHa
I
H
CH a
I::::::::::::
it:
Many excited aromatic hydrocarbons like benzene, biphenyl, naphthalene, methoxy derivative
of naphthalene and anthracene react with amine to give reduction products and aminated
products. The reaction is initiated by the transfer of an electron from nitrogen lone pair orbital
to the half filled highest bonding molecular orbital of the excited aromatic compound.
••
ArH* + ~N ~
•
+
[ArHr + R3~
[oor L Q:2. ~ Q;)
H
H
H
H
H
®
H
H
H
Excited aromatics are also reduced by transfer of hydride ion by metal hydride or by
transfer of hydrogen atom by metal hydride.
CN
i
L
CN
~
~
. (i) hv )
(u) NaBH 4
~H
~
H
H
l
r
6o~ 60J
Reaction takes place as follows:
CN
CN
Incorporation of oxygen in the presence of UV light is known as photochemical oxidation.
There are two mechanisms by which oxygen is incorporated in photochemical oxidation
reactions.
The first is the Backstrom mechanism which is also known as photosensitisation oxidation
mechanism. Its main characteristic is abstraction of hydrogen by the sensitiser (Sens) in its
excited triplet state followed by addition of oxygen to the newly created radical. This type of
photooxygenation is known as type I photooxygenation.
it
Sens + hv ~ Sens
it
Sens + A - H
A+0 2
A-O-O + AH
~ Sens - H + A
~ A-O-O
~
A-O-O-H + A
A-D-O + Sens-H ~ A~O-O-H+ Sens
Examples of photosensitised reactions that take place by this mechanism are found
among the oxidation of secondary alcohols to hydroxy hydroperoxides which in aqueous medium
decompose to form ketones and hydrogen peroxide.
OH
H
I
I
R-C-R' +
R-C-R'
I
I
H 20
~
O-O-H
OH
Mechanisms:
Hydroxy hydroperoxide
OH
o
(i)
II
cj)-~
Sens
hv
~
I
o
R-e-R
cj)-H
---~)
I
~
OH
I
cj)-~:--4>
OH
I
+ R-9-R
OR
I
(ii)
R-y-R
°2
~
OR
I
R-C-R
I
R
OR
I
R-C-R
)
I
OR
I
(iii)
ejr-Q-<I>
O2
+ R-C-R
R-C-R
I
0-0'
OR
I
OR
I
O-O-H
OR
I
~
ejr-C-<I>
I
0-0'
OR
I
(iv)
ejr-Q-<I>
+
OR
I
0
II
R-C-R
~
I
ejr-C-<I>
OR
I
+ R-C-R
0-0'
I
O-O-R
The second mechanism known as photosensitised oxygen transfer involves the direct
combination of the substrate with oxygen. There are two proposals regarding the state of
oxygen involved. This type of photooxygenation is known as type II photooxygenation.
Schenck's favours an oxygen-transfer step in which the triplet sensitiser forms an adduct
with triplet oxygen.
Sense
hv
~
3Sens + O2 ~
3S ens
Sens-O-O
A + Sens-O-O ~ A0 2 + Sens
Foote favours the idea of singlet oxygen being the sole agent in the transfer step:
hv
Sens
O2 (Triplet) + Sens* (Triplet)
A + O2 (Singlet)
~
3S ens *
O2 (Singlet) + Sens
A0 2
Alkenes, dienes and polyenes are attacked by singlet oxygen. The singlet oxygen may be
generated by thermal methods (e.g., by the reaction of hydrogen peroxide with sodium
hypochlorite), by excitation of ground state oxygen (triplet oxygen) in a microwave discharge
or by the use of visible radiation and photochemical sensitiser such as methylene blue, Rose
Bengal, chlorophyll, riboflavin, fluorescein or halo fluorescein.
Acyclic or cyclic conjugated dienes gives 1, 4-cycloaddition reaction with singlet oxygen
to form six-membered cyclic adduct, i.e., cyclic peroxide.
'(orf
0
0
+ II
0
°2 )
hV,Sens
hv, Sens
)
'(6
®
2
00
~
hv, Sens
° )
2
hV,Sens
O
2
)
~
0-0
~
The formation of hydroperoxides in photosensitised oxygen-transfer reactions that follow
the Schenck type of mechanism occurs only when hydrogen is present on allylic carbon. The
reaction has, moreover, some definite steric and electronic requirements. Oxygen always
becomes attached to one of the double bonded carbons, which then shifts to the allylic position.
The reaction is like ene reaction.
IC~
II
0
29)
II
UO
0
?€CH,-H1
hv,Sens
O2 )
hv,Sens
~~
0
~~
~
H~
IC
I
hv )
Sens
2C
C'
sC
'"
II
o-0-O-H
)
/O-O-H
(CO-O-H
(O-O-H
~.
Na2S0a
Na2S0a
)
)
N.,SO,
)
oOH
(COH
(OH
..
The above reactions do not proceed through free radicals. The oxidation with singlet
oxygen occurs by concerted mechanism like the ene reaction.
1
~
Sens
J
\
Some substituted alkenes behave differently with singlet oxygen and form a dioxetane
in a cycloaddition reaction. Some dioxetanes are stable but others readily decompose to two
carbonyl compounds. Electron rich alkenes undergo this reaction and reaction is stereospecific.
l
Photo Reduction and Photo Oxidation
W
hv, Sens)
°2
©QsO~
a
12+21
261
©CCH2-CHO
CHO
Cycloaddition
(OEt
OEt
)(OEt
°2
)
hv, Sens)
O2
OEt
EtO'r----( OEt
hv, Sens
0-0
EtO 'r---f~OEt
~
0-0
The oxidative cyclisation of conjugated trienes to form aromatic system is one of the
more extensively studied photochemical oxidations. The conversion of cis-stilbene to
phenanthrene is a main example of such a ring closure. The reaction takes place in the presence
of hydrogen acceptors (0 2 , 12 , FeC1 3).
f'
•
"
10
CHAPTER
mistry of Alkenes,
~omatic Compo.....,
::::::::::=:::::::::::::::::::::::::::
Electronic absorption spectra of simple alkenes consists of an intense broad band from 140 nm
to 190 nm (for ethylene) with absorption threshold of 200 nm (for ethylene) to 240 nm (for 2,
3-dimethyl-2-butene). The diffuse bands in the spectrum of ethylene are attributed to a 1t ~ 1t*
transition. The absorption at 174 nm for ethylene is the first singlet Rydberg transition. This
Rydberg transition is due to the 2p1t ~ 38 excitation.
The excited states of alkenes are complex because different electronic states have different
electronic configuration. Although the nature of most electronic states of alkenes is not known
in detail, some are well understood. Alkenes have two low-lying excited singlet states: the
1 [2pn, 38] Rydberg state and the 1[1t, 1t*] valence state. Calculation indicates that there is apparently
an additional state, i.e., the (2p1t, 3p) Rydberg state. The lowest triplet state is practically pure
3(1t, 1t*) and T2 is essentially a pure Rydberg 3 [2p1t, 38] state (Fig. 10.1)
53 (It, 3p)
5, (It, 3s)
T1 (It, It')
Fig. 10.1 Different electronic states of ethylene
Tlie singlet-triplet splitting (S-T splitting) is generally large for alkenes (ethylene 70
kcal/mole-1). As a result, ISC is slow and inefficient. Therefore, direct irradiation of alkt!'n.es
induces singlet excited state reactions, and sensitisation is required for triplet state reactions.
Excitation of a planar alkene like ethylene results initially in the formation of planar
excited state molecule according to the Frank-Condon principle. The initially formed planar
269
I'
270
Photochemistry and PericycUc Reactions
excited state n, n* species, wherever as a singlet or a triplet relaxes by rotation of the terminal
group-methylene in the case of ethylene-through 90 0 around the central bond to give the
lowest energy conformation possible. Thus the molecule can rotate from the planar
configuration, produced by the Frank-Condon excitation, to reach an energy minimum. This
energy minimum is arrived at by rotation about the central bond so that the methylene groups
are at right angles to each other so relieving the unfavourable interactions between the singly­
filled orbitals on the carbon atoms. (Fig. 10.2)
hv
Rotation
.~
~
",,0 \) /
C--C
/0/\2)
Frank-Condon planar state
Fig. 10.2
Relaxed twisted state
Formation of relaxed twisted state (orthogonal state)
Spectroscopic evidence has been obtained which shows that there is additional distortion
of the methylene groups from planarity as the carbon atoms tend towards tetrahedral (sp 3)
hybridisation. In this energy minimum there is essentially no n (pi) bond, the electronic
interaction is at a minimum and the terminal carbon atoms are orthogonal.
Several modes of reactions can be predicted from the excited state olefin from this
orthogonal model. These are cis-trans isomerisation by rotation about central bond, hydrogen
abstraction by the half-filled orbitals on carbon, addition to other alkenes and rearrangement.
The most facile photochemical process of alkenes is cis-trans isomerisation.
The most commonly observed photochemical reaction of alkenes in solution in cis-trans
isomerisation which can occur by both direct and sensitised irradiation. This isomerisation
reaction is usually associated with n, n* excited states.
10.2.1 CIs-Trans Isomerisation of Alkenes by Direct Irradiation
Direct irradiation for the simplest alkenes (whose "'max is below 200 nm) is difficult to achieve
as a result of the high energy absorptions. However, with more substituted alkenes the UV
absorptions are pushed above 200 nm and so direct excitation can be more readily achieved.
The most common substrate for isomerisation is the stilbenes. The irradiation of either
cis- or trans- stilbene at 313 nm results in the formation of 93% cis- and 7% trans- stilbenes.
No matter how long, within reason, the irradiation is continued the ratio of products does not
alter and such a state is referred to as a photostationary state. The composition of the
photostationary state is determined by the excitation co-efficient of the two isomers at the
exciting wavelength. For most alkenes (Emax) trans is greater than (E max ) cis. Therefore, in an
equilibrium mixture more molecules of trans than cis isomer will reach the excited singlet
state, followed by rapid relaxation to So' At photostationary state, the rate of cis ~ traits and
trans ~ cis isomerisation becomes equal; no change in the composition of the reaction mixture
occurs upon further irradiation.
The energy of a n ~ n* excited species is a function of the angle twist about the carbon­
carbon C1 (sigma) bond, as mentioned earlier, and trans-to cis- isomerisation is believed to be
affected by distortion of the trans- excited state initially produced, to an excited state common
to both cis- and trans isomers. This excited state is termed as a phantom state.
The mechanism by which the cis-trans isomerisation occurs, in terms of the sample
model involves the excitation of an electron to a planar excited state which subsequently relaxes
to the twisted state. A simple representation is given in the figure 10.3. A small activation
energy of 2 kcaVmole is required to twist the planar trans singlet to the twisted singlet state.
Decay from this twisted state can give either cis or trans stilbene.
H
")k--Ph
H
::.k--Ph
Ph
/
!
::.k--Ph
,
Ph
/
Fig. 10.3
However, as mentioned earlier the trans fOo11 absorbs more light at the exciting
wavelength (c max trans = 16300) than does the cis form (E max cis =2880) and consequently the
trans isomer is converted almost completely into the cis.
The synthetic utility value of trans-cis isomerisation lies in the fact that the more stable
alkene can be readily converted into the less stable alkene. Some examples of the result of
direct irradiation of alkenes are shown below:
.
hv
<:
....
H3C~
_ /H
/C-C"",
H
COOH
CsHs-CH 2-CH 2 " ,
/
H
C=C
H/
Cl",
hv
"'CHs
/CH 2 Cl
C=C
H/
hv
hv
H3C~
_ /COOH
/C-C"",
H
H
CsHs-CHc CH2 " ,
/CH 3
/C=C",
H
H
Cl",
H/
"'Cl
hv
/Cl
C=C
"'CH 2 Cl
\
~
o
0
10.2.2 Sensitised Cis-Trans Isomerisation
Sensitised cis-trans isomerisation of an alkene can be brought about by the use of a triplet
sensitiser such as ketone. The sensitised isomerisation of stilbene is a typical example. The
composition of the photostationary state of the sensitised isomerisation varies with the energy
of the sensitiser employed. In this process if the donor species fulfils the conditions for effective
sensitisation and its triplet energy is greater than that of either of the geometrical isomers,
then triplet energy transfer to both cis and trans isomers and isomerisation takes place. The
initially formed cis and trans triplet excited species undergo distortion to a common phantom
triplet which, on collapsing to the ground state (So) affords a mixture of isomers. Because the
sensitiser can excite both isomers, the proportion of cis-trans isomer in the photostationary
state from a sensitised reaction is lower than that obtained from direct irradiation. This is in
contrast to the unsensitised isomerisation. Sensitisers of high energy give photostationary
state with approximately the same composition of isomers (55% cis), while direct photolysis
results in a higher proportion of cis isomer (93%) in case of stilbene. As the sensitiser energy
is reduced, anomalous results are observed.
Sens (So)
.',
l
,.
R
F
+ Sens (So)
R
R
'"
R/
R
R
Relaxed triplet
The composition of photostationary state depends very much on the triplet energy
sensitiser. With sensitiser having triplet energy above 60 kcallmole, cis-trans ratio is slightly
more than one but a range of sensitiser having triplet energy of 52 to 58 kcallmole offered
much higher cis-trans ratio in the photostationary state. The higher cis-trans ratio in this
region results from the fact that energy required for excitation of trans isomer is less than that
for excitation of cis isomer.
The sensitiser having triplet energies in the range of 52 to 58 kcallmole, selectively
excites the trans isomer. Since the.rate of trans-cis conversion is increased, the composition of
the photostationary state is then enriched in cis isomer.
Sometimes, photochemical cis-trans isomerisation may also take place in the presence
of halogens. It appears that under these conditions there is a photochemical product of halogen
atoms which adds to the alkene yielding a radical. Elimination of halogen from this radical
yields a constant ratio of cis and trans isomers.
hv
.
•
--~)X+X
R
+
R
X'c-C/
X
H/
+
""H
R~./H
XH/
c
""R
1
R",,'
.
H
/R
/c~c""
+
H
R""
H
/H
/c=c""
R
Rea.ctions competing with cis-trans isqIlleriflati.Qllflt iI;lcreased concentrations of the alkene(s)
are photocycloaddition reactions. Ali ll;1kene til, its S~"or T 1 state reacts with either the same
,"
,"
.;:
• . '"
.
"
.::;.'
I.",
.\,_.
.;'
,_.
,'.'
(photodimerisation) or a different (photocycloaddition) alkene in the ground state to produce a
cyclobutane derivative.
Although the dimerisation of simple alkenes (whose Amax is equal or less than 200 nm)
is technically difficult, increase alkyl substitution of ethene causes a bathochromic shift of
the absorption band, so that direct irradiation (A abs = 224 nm) of neat 2, 3-dimethyl-2-butene
gives the corresponding dimers.
hv
The dimerisation reaction is stereospecific and this is demonstrated by the irradiation
of cis-2-butene which yields all cis dimer and the cis-anti-cis dimer while the trans-2-butene
affords the cis-anti-cis dimer and the trans-anti-trans dimer (Scheme 1).
2(
hv
214nm
+
"~I
Cis-2-butene
1
)=( )=l
)=J/
~.
Cis-anti-cis dimer
All cis dimer
57%
hv
214nm
43%
+
,."",",
Trans-2-butene
Cis-anti-cis dimer
Trans-anti-trans dimer
39%
60%
.U;
Scheme 1
Cyclohexene also dimerises, yielding a mixture of dimeric products while norbornene
affords two dimers (A) and (B) in a ratio of 1 : > 10 (Scheme 2)
hv
(A)
...
Scheme 2
Photosensitised dimerisation (triplet state dimerisation) occurs easily with cycloalkenes,
particularly with small ring alkenes because they cannot undergo cis-trans isomerisation.
Dimerisation appears to favour the formation of cis-trans-cis arrangement of rings in
the dimer. Example of dimerisation of cyclopropenes, cyclobutenes, cyclopentenes and
cyclohexenes have been reported.
The dimerisation of alkenes and cycloalkenes is best sensitised by aliphatic ketones.
Oxitane are formed from aromatic ketones.
hv
Acetone
H
H
H
H
hv
Acetone
H
H
+
H
+
H
H
42%
H
27%
Norbornene can also be made to dimerise on sensitisation and that is preferred for the
formation of the exo-trans-endo dimer.
2~
hv
Acetone )
Four dimers that differ in regio and stereochemistry are formed by photodimerisation
of indene. The ratio of dimers depends on the irradiation conditions. The triplet sensitised
dimerisation, as a step wise bond formation process, generally produces the thermodynamically
more stable exo products from the more stable 1, 4·biradical intermediate (head-to-head).
hv
Acetone
H
H
(1)
+
H
H
H
H
+
+
H
H
(4)
H
H
(3)
H
H
(2)
10.3.1
Intramolecular Dimerisation
The photochemistry of nonconjugated dienes with isolated double bonds resembles with simple
alkenes. Such type of alkenes give intramolecular I 2 + 2 I cycloaddition reactions to give
bicyclic products. The products of sensitised intramolecular cycloadditions depend on the
number of -CH2- groups between the two double bonds. If the number of intervening groups
is odd, e.g., 1, 4-pentadiene or 1, 6-pentadiene, the major product results from a normal I 2 + 2 I
cycloaddition.
r
CH =CH 2
rCH-CH2
hv
(CH 2 )n
.
Sens
"-CH=CH2
(CHJn
I I
"-CH-CH 2
n = 1, 3, 5 etc.
Normal 12 + 21
adduct
If the number n is even e.g., 1, 5-hexadiene, the major product is formed via a "cross",
i.e., x [2 + 2] cycloaddition to form two crossed 0' (sigma) bonds.
rCH-CH2
(C~X
hv
Sens
CH-CH2
x [2 + 2] Adduct
n= 2,4,6, ...
This feature is even more prominent in cyclic dienes, e.g., n =m
cycloadduct is the only product.
in which x [2 + 2]
1
2
0:
6
hv
Acetone
)
G)
5
Biradical
~
.00
=2 (1, 5-cyclooctadiene)
,.
­
-,
<X>
If n = 1 and m = 3 or vice-versa (i.e., 1, 4-cyclooctadiene), the normal
is formed.
o
hv
,
I 2 + 2 I cycloadduct
)
Acetone
The photoreaction of 1, 5-hexadiene from the T1 state follows the rule of five which
states that if triplet cyclisation can lead to rings of differ~nt sizes, the one formeq by 1, 5 addition
is preferred kinetically. If there are several possibilities for 1, 5 addition leading to different
biradicals, the most stable one ·is formed.
i
hv
Acetone)
Minor product
(Behaves as
1,6-diene)
~~~,
hv )
Acetone
to rule five
(Major product)
(Behaves as
1,5-diene)
Intramolecular I 2 + 2 I cycloadditions can produce cage structures which are molecules
of theoretical interest with respect to strain and bond angles. Norbornadiene (A) is a typical
example of such reaction. The UV spectrum of the compound exhibits absorptions at 205 nm,
214 nm and 220 nm and a shoulder at 230 nm in ethanol. Such a spectrum is not expected for
a non-conjugated diene. Molecular model also indicates that bonding is feasible between C-2
and C-6 and between C-3 and C-5 without too much strain. Due to these reasons compound
(A) undergoes I 2 + 2 I cycloaddition by direct irradiation and in the presence of sensitiser.
2~'
3
hv
253.7 nrn
5
In the presence of sensitiser product formation takes place by triplet state.
hv
)
~
Spin
inversion)
~
~
Quadricyclone
The I 2 + 2 I cycloaddition reaction of norbornadiene in the presence of sensitiser is a
reversible reaction. The introduction of groups other than -CH2- at C-7 position does not
change the relationship between isolated double bonds. In such type of compounds the reaction
is not reversible. The reaction is brought about by direct irradiation or sensitised irradiation.
a
~COOMe
COOMe
COOMe
COOMe
:-\__--=__ COOMe
COOMe
Conjugated dienes can give the following type of photochemical reactions:
(i) Cis-trans isomerisation
(ii) Sigmatropic shift
(iii) Disrotatory electrocyclic ring closure
(iv) Intramolecular x [2 + 2] cycloaddition
The above photochemical reactions of conjugated dienes depend to a large extend on the
excited state population, and on the phase in which the reaction is performed. Singlet excitation
generally leads to intramolecular process (i.e., disrotatory electrocyclic ring closure and
sigmatropic rearrangement) whereas dimerisation and addition reactions are more common
from the triplet excited state. Reaction in the gas phase at low pressure often leads to greater
fragmentation than in solution.
The solution phase photolysis of butadiene leads to cyclobutene and bicyclobutane,
whereas in the vapour phase (gas phase) I-butyne, methylallene, acetylene, ethylene, methane,
hydrogen and polymeric materials are produced.
D
hv
)
Cyclohexane
1
+
10
<1>
1
Vapour phase
10.4.1 Photochemistry of Conjugated Dienes In Solution
Intramolecular x{2 + 2J Cycloaddition Reaction
1, 3-Butadiene exists in solution as a rapidly equilibrating mixture of 8-transoid (95%) and
8-cisoid (5%) conformers.
Irradiation of butadiene promotes an electron from HOMO to LUMO ('112 ~ '113*) which
results in the increased bonding between C-2 and C-3 at the expense of C-1 and C-2 and C-3
and C-4. Thus the lower excited states of 8-trans and 8-cis butadiene should exhibit still
larger energy barriers to rotation about the C2-C 3 bond because of its double bond character.
Thus conformational character of butadienes are retained in the excited states.
hv
~
S-trans
.~.
Trans
hv
~
S-cis
Cis
The exact energies of the 8 1 states are not known, but cisoid 8 1 probably lies below
transoid 8 1 , There is large energy gap between 8 1 and T 1 • Also inter system crossing does not
take place in this case. The energy gap between 8 1 and T1 accounts for the fact that inter
system crossing does not occur. Direct irradiation of 1, 3-butadiene in solution thus gives rise
to chemistry only from the 8 1 state.
The energy of the excited states of butadienes are shown in the Fig. 10.4.
81
107-124 kcaVmole
T1
T1
53.3 kcal/mole
80
-
.......­
....
....
95%
5%
Fig. 10.4
Irradiation of butadiene in cyclohexane produces cyclobutene from an electrocyclic
reaction of the S-cis isomer, and bicyclo [1, 1, OJ-butane, the product of an intramolecular x
[2 + 2] cycloaddition of the S-trans isomer, in a ratio of 6 : 1.
>
l
Electrocyclic
ring closure
(converted)
1
Non-concerted
process
~.
D
Biradical
1
~
Bicyclobutane formation is particularly important in systems that contain a rigid-S­
trans diene geometry.
~hv
+
)
x [2 2]
.
Cycloaddition
J:j) ~ Jd)
.
•
•
However x [2 + 2] cycloaddition also occurs with cis fused dienes particularly in case of
cyclopentadiene.
o
hv
---------------------~----~====~=======~--
If a concentrated solution of 1, 3-butadiene is irradiated, some dimer formation can be
detected (maximum yield 10%). We expect dimer formation to be concentration dependent,
since it involves the bimolecular reaction of an excited butadiene with a ground state butadiene.
The ring closure reactions (i.e., formation of cyclobutene and bicyclobutane), on the other hand,
are unimolecular. Thus dimerisation can only be possible on high concentration of 1,
3-butadiene. Dimerisation gives four different dimers:
1 [CH 2=CH-CH=CH 2]
+ CH2=CH-CH=CH z
Excited state, 8 1
Ground state, 8 0
1
H
cC Y=
+
--
-
H
"
H
(II)
(I)
/
30%
If 0
+
(IV)
8%
(III)
50%
When batadiene is treated in the presence of sensitiser, the product formation takes
place by triplet state.
The chemistry of the T1 state of 1, 3-butadiene is quite different from that of the 8 1
state. A mixture of dimers is formed in very good yieli:l (==< 75%) and neither cyclobutene nor
bicyclobutane can be detected in the sensitised reaction (triplet reaction). The dimer mixture
obtained from triplet also differ significantly from that given by singlet. The dimer composition
in triplet state depends upon the energy of the sensitiser used to populate T1, The results for
the two sensitiser, acetophenone and benzil are given below:
Sensitiser
E T (heal/mole)
%
etet
II
C6~~5COCH3
C6H5COCOC 6H 5
74
57
Composition of dimers
H
82
14
4
49
8
43
The origin of this remarkable change in product composition lies in the relative energies
of the sensitiser. Acetophenone, with E T = 74 kcal/mole, is sufficiently energic to transfer
energy to either s-cis or s-trans butadiene. Since the s-tarns form predominates by a large
margin, and since energy transfer occurs at nearly every collision, the dimer composition from
the acetophenone sensitised experiment must represent primarily the reaction of s-trans- T 1 •
Benzil (E T = 54 kcal/mole), on the other hand, is energetic enough to transfer energy to s-cis
butadiene but not s-trans butadiene. Thus the dimer composition from the benzil-sensitised
experiment must represent primarily the reaction of s-cis- T 1 • If a sensitiser with a triplet
energy of less than 54 kcal/mole were used, no dimer formation would be observed.
Thus when acetophenone is the sensitiser, s-trans-T1 is the reacting species which reacts
with s-trans ground state to give dimer.
Cis and trans
In benzil sensitised reaction, s-cis-T1 adds to s-trans ground state butadiene.
Cis and trans
Benzene and substituted benzene undergo valence isomerisation by irradiation. Selective
excitation into 8 1 gives preferentially meta and ortho product while excitation into 8 2 'gives
para bonded products by valence isomerisation. These processes are described by biradical
intermediate for case of visualisation.
o
l
--1
l_hv
82
81
lor
1
o
lor
1
+-+
G):
<-->
0:
·U
I
u.
Irradiation of liquid benzene under nitrogen at 254 nm causes excitation to 8 1 state and
the products, benzvalene and fulvene are formed via 1, 3-biradical.
.
9.
o
hv
)
U
8 1 Biradical
(A)
11, 2-H shift
Hb
H
(Bondb'oakmg
Fulvene
The formation of fulvene can now be considered to arise by reaction of the biradical (A)
by a 1, 2-hydrogen migration and bond breaking.
Formation of benzvalene can take place as follows:
V
1, 3·Bonding
(A)
Benzvalene
Dewar benzene is formed via 8 2 state upon short wavelength irradiation (205 nm) .
.
oI
hv
.
[QJ]
Dewar benzene
It has been confirmed that Dewar benzene is converted into prismane either by concerted
path or by formation of a biradical.
~
~
~
12+21
Cycloaddition
~
.~.~ fu,m,~
.!)
All these strained intermediates are thermally labile and ultimately isomerises into
benzenoid compounds.
Monocyclic aromatic compounds undergo remarkable photochemical rearrangements.
For example, o-xylene on irradiation gives mixture.of 0, m and p-xylenes.
t,
AI:'
~
hv
1,2.Alkyl sIJft
",:I'
;?
v
L~-
CH,
i
J
"v
1
Conversion of o-xylene into m-xylene and m-xylene into p-xylene is due to 1, 2-alkyl
group shift. Similarly, conversion of a-xylene into p-xylene and vice-versa is due to the 1,
3-alkyl group shift.
1, 2-Alkyl group shift takes place by benzvalene as well as prismane intermediates whereas
1, 3-alkyl group shift takes place only by prismane intermediate.
10.5.1 1,2-Alkyl Shift
1, 2-Alkyl group shift (rearrangement of the atoms in the benzene ring) takes place via formation
of benzvalene and prismane as reaction intermediate.
(A) Mechanism of 1, 2-shift via benzvalene intermediate
Bond formation between
)
:6r
Benztalene
Reorganisation of
carbons of the ring
by (i) bond breaking
between C1 - C2 and
Ca - C4 and (ii) bond
formation between
C1 - Cg and C2 - C4
(B) Mechanism of 1, 2-alkyl group shift via prismane intermediate
R
dr
R
~R
R
12 + 21 Cycloaddition
)
6~
5~
Prismane
Dewar benzene
Reorganisation of the
ring by bond breaking
between C 1 - C2 and
Cg - C4 and bond
formation between
C2 - Ca and C1 - C4
R
6
R~R
1
2
.
5
4
a
Dewar benzene
,~.-irR
5~JL
Open book structure
(Dewar benzene)
10.5.2 Mechanism of 1, 3-Alkyl Group Shift
1, 3-Alkyl group shift takes place only via formation of prismane as reaction intermediate.
6
5~R
40-­
a
2
R
6
---7
R
:0'r
~R
12 + 21 Cycloaddition
reaction
==
)
~RR
4
Dewar benzene
2
Reorganisation of the
carbons of the ring by
(i) bond breaking
between Ca - C4 and
C5 - C6 and
(i~) bond formation
between Ca - C6
and C4 - C5
R
¢
~
R
R
,~R
r­
i­
~R
Open book structure
The photo isomerisation of 1,3,5-trimethylbenzene to 1,2,4-trimethylbenzene is also due
to 1,2-alkyl group shift. This has been confIrmed by isotopic labelling experiments.
HaC
£·=C
hv
---7
CHa
14
Similarly, 1,3,5 tri-t-butylbenzene and 1,2,4-tri-t-butylbenzene are interconvertible to
each other by 1, 2-alkyl group shift. In this case interconversion takes place via benzvalene as
reaction intermediate.
l\
l\
~
I
j
.1
"
hv
~
I
I
...~
hvllhv
~
1
".
_"_%'1'*"""".';°""7'1'1"1"*";""':'
Benzene and its derivatives in the 8 1 state undergo cycloaddition reactions with various 1t(Pi)
systems, specially alkenes, alkynes and dienes. The T 1 state of aromatic compounds does not
give cycloaddition reactjl)ns because the T1 state of aromatic compounds is quenched by the
transfer of excitation energy to the alkene. Under certain structural and electronic
circumstances a reverse model, i.e., 8 1 alkene + 8 0 arene prevails. Three different modes of
cycloaddition reactions have been observed: 1, 2 or ortho, 1, 3 or meta and 1, 4 or para. The
1, 4-addition is the least efficient cycloaddition reaction. These reactions occur either
bimolecularly or intramolecularly. Thus, there are at least four broad mechanistic pathways
possible for these reactions. These are as follows:
(i) Interaction of excited aromatic with ground state alkene.
(ii) Interaction of excited alkene with ground state aromatic.
(iii) Formation ofbiradical intermediate from excited aromatic and reaction of the biradical
with alkene to give product.
(iv) Involvement of dipolar entities such as
(aromatic
-+
olefin ~) or
(aromatic ~
olefin -+), obtained either by excitation of charge-transfer complexes
pre-existing in the ground state or by electron transfer in an exciplex.
10.6.1 1,2-Cycloaddition Reactions
Electron poor alkenes (alkenes having electron withdrawing groups) give 1, 2-addition reaction
with arenes in addition to some 1,4 products. The para product is the secondary product. The
formation of the ortho product is generally favoured by donor-acceptor interaction. The more
polar the interactions (charge-transfer) in the exciplex, the longer the proportion of ortho and
para cycloadditions.
o
~8
---------
1
x y
--~)
[81-X=y]*
Exciplex
.
~
O=x
I
Y
::::::-..
The 1,2-cycloaddition reactions are concerted reactions. The photocycloaddition of cis
and trans-2-butene to benzene in the liquid phase gives the 1,2-adduct stereospecifically showing
that the reaction is a concerted reaction. This also confirms that the product formation is
taking place via formation of exciplex.
o
o
+
hv
o=cCN
eN
hv
Thus benzene and its derivatives generally form charge transfer complexes (exciplexes)
with dienophiles, and upon irradiation, give ortho-cycloaddition adducts. The adduct is usually
with exo orientation.
o
+
CH z
II
hv
~
CH-CN
CN
,
1
Irradiation of benzene in the presence of maleic anhydride yields an exo 1, 2-cycloadduct.
The product formation takes place via 8 1 state of a charge transfer complex. Exo-1, 2-adduct is
a result of a photochemical I 2 + 2 I ortho-cycloaddition reaction followed by a thermal I 4 + 2 I
cycloaddition.
0
---'
0
+
H 0
<>
hv
0
H 0
o
+~
H
H
0
~\
':
5
The primary products from irradiation of alkynes, specially those with electron­
withdrawing groups, with benzene are the corresponding ortho cycloaddition products. These
adducts are often unstable at ambient temperature.
These unstable adducts undergo ring opening reaction (electrocyclic ring opening) to
give substituted cyclooctatetraene.
COOMe
;\
j
.'
0
I
C
+
III
C
I
COOMe
~
~COOMe
(Unstable)
COOMe
GCOOMe
-----+
-.
COOMe
This type of cycloaddition is more efficient by an intramolecular process. This reaction
is possible when alkyne moiety is linked to the arene by at least a two carbon chain.
CH2
.___CH2
\
Y)lli
tV
C
I
COOMe
hv
C=:?H'
(J=lcooMe
10.6.2 1,3-Photoaddltion of Benzene
This remarkable process leads to tricyclic system in which an olefinic double bond has added·
across the meta-positions of a benzene ring.
The hypothetical presentation for 1,3 addition can be shown as follows:
CH2
~ R+~H2
o_
~
254nm
(J)
(gas or liquid)
+
R
<}-R+~H,
CH2
cp
R
l<f~~]::]
~
t
( }-R
(ER
lhv
l~~~::]::]
0+(:
hv
~
(Q-R
R
The reaction seems to be restricted to double bonds bearing only alkyl substituents such
as 2-butene, norbornene, allene and cyclobutene. It occurs with 254 nm light in both gas phase
and liquid phase. It is stereospecific and it involves singlet excited benzene. It is known that
neither fulvene nor benzvalene is a precursor. Product formation takes place via formation of
prefulvene biradical.
In 1,3-cycloaddition reaction, endo product is the major product.
o~ct
o
H
10.6.3 1,4-Photoaddltlon of Benzenes
In this reaction olefinic double bond has added across the para-positions of a benzene ring.
R
(~
R
The reaction is concerted and therefore stereospecific and it involves singlet excited
state benzene. The reaction is similar to Diels-Alder reaction because benzene behaves as
diene in this case.
l,4-Adduct
Butadiene also gives [4 + 4] cycloaddition reaction with benzene and reaction is
stereospecific.
v
~
~l
\'
;
Q)
J&
0+/
c..
~T
Cis
hv )
12 +21
rfi
hv
Dimerisation
)
QXD
The photoaddition of olefinic systems is not restricted to benzene and its simple
homologues. Analogous process occurs with condensed aromatics such as naphthalene,
phenanthrene or anthracene. In the case of anthracene, the cycloaddition is [4 + 2] while in
case of phenanthrene and naphthalene it is [2 + 2].
hv
----+
Ph
Ph
I
"II
hv
[2 + 2])
~hPh
(X)
hv
[2+ 2])
Ph
_::::::::;; ::;::;::.::::::.:::: .::::::::.:::;: ::::: ::::::::::::::::::
...
..
..
\
Polynuclear aromatic hydrocarbons such as naphthalene and anthracene form transannular
peroxides when excited in the presence ofoxygen, and the point of attack is substituent dependent.
A
B
A
B
A
B
A
B
~~~+
~
02~
~H. hv ~H,
~ o;~
hv
----+
OCHa
OCHa
Mechanistic studies indicate that the triplet excited state of the hydrocarbon excites
oxygen by energy transfer, into its 18g state, and the stnglet oxygen so produced adds thermally
to the ground state hydrocarbon. Thus reaction is [4 + 2] cycloaddition reaction.
:::.::::: JidS:::
J::.::J:.:J::::::::::::::~:I:::.
Light-induced substitution in the aromatic compounds can occur either on a ring carbon atom
or on an atom of a substituent. These substitution reactions can proceed by heterolytic or
radical mechanism. The most· common reactions are the heterolytic substitutions which can
be classified into two categories:
(i) Type A: In these reactions the orientation rules are different from those in the ground
state.
(ii) Type B: In these reactions the orientatio.Q. rules are the same as in the ground state
but the process is accelerated by light absorption.
All these reactions seem to be functions of the lowest singlet excited state, and many
proceed with high quantum yield.
,
J
!
10.8.1 Type A Reaction.
(i) Photonucleophilic Substitution: Nitro group is an activating group and ortho and para
directing group for nucleophilic substitution reactions in the ground state. In the presence of
light (i.e., in the excited state) the nitro group is m-directing group.
$'
.
NH 2
(Ground state reactIon)
rOIO CHs
I
rOI NHCH s
hV)
I
CH aNH 2
N0 2
N02
(Reaction in excited state)
cf~H
N02
N02
In many cases the cyano group behaves similarly, ::d analogous reactions are found
with substituted naphthalenes.
..
(ii) Photoelectrophilic Substitution: On irradiation in the presence of CF 3coon,
toluene gives mainly m-deuterio toluene instead of o-and p.derivatives.
eHs
CF,~~Or! . b-
D
Anisole also gives m-derivative whereas nitrobenzene gives p-derivative.
OCH s
hv
)
CFsCOOD
hv
)
CFaCOOD
~D
~'
n
Notice that the observed orientation differs from that expected in the ground state; the
nitro group activates the p-position and the methoxy group activates the m-position.
Orientation rules for nucleophilic and electrophilic substitution reactions may be obtained
from simple considerations of the changes in electron density on making the transition
8 0 ~ 8 1 , For simplicity consider benzyl carbocation as model for benzene substituted with
electron-withdrawing group and benzyl carbanion with electron-donating group.
The seven 1t molecular orbitals of benzyl system are related to those of benzene itself
and take the form of Fig. 10.1; where the number are the electron densities at the carbon
atoms. The
is the non-bonding molecular orbital, and the pairs
and
are no
longer degenerate.
"'4
"'2' "'3
"'4' "'5
0'214~25 O.14~O.'4 °tl
0.25 .25 ~
'V2
'V3
o
0
0.14
'V4
0
o~
'Vs
'V6
Fig. 10.6 The 1t molecular orbitals of benzyl systems in schematic form
Consider fIrst the ground state benzyl cation. The six electrons are accommodated in
and "'3' The HOMO in this case is "'3' Hence the 8 0 ~ 8 1 state corresponds to "'3 ~ "'4
transition. In
the electron density in the a-position is zero and in
it is 0.57. Thus the
transition induces a very large increase in the charge density at the a-position. Similarly, one
fInds that excitation leads to the o-and particularly the m-position becoming more positive
(electron defIcient) and the p-position becoming somewhat more negative (electron rich). The
results are reverse in case of benzyl carbanion (Table 10.1).
"'1'
"'2
"'3
"'4
Table 10.1 Change in electron densities at different positions in benzyl
carbocation and benzyl carbanion on excitation
Benzyl carbocation
Benzyl carbanion
a
,.
+
+
\:
+
o
+
m
1
+
+
p
t
If one assumes that the rate of photosubstitution is correlated with the charge density
at the point of attack, there is an immediate explanation of the observed result-an electron
withdrawing substituent decreases electron density at m-position hence it is m-directing group
for nucleophilic substitution reaction. In valence bond terms this can be explained as follows:
e
e
O'N~
e
e
O'N~
O'N/ O
~
6nu
~
~u
Mechanism of photonucleophilic substitution
~
&NU
-­
!
•
I
J
.,
Similarly electron donating group increases electron density at m-position therefore,
such groups are m-director for electrophilic substitution reactions.
hv
~
Mechanism of photoelectrophilic substitution
10.8.2 Type B Substftutions
These reactions take place on irradiation but have orientation rules characteristic of the ground
state. In nucleophilic substitution reactions, nucleophile is always neutral. The detailed
mechanism of Type B reactions is not yet clear.
"
+
,
1
10.8.3 Radical Substitutions
Haloaromatic compounds on irradiation undergo homolysis in ground state to produce radicals
which then give rise to products by well-established thermal pathway.
hv
•
Ar-X ~ Ar+X
However, when chlorobenzene is excited in the liquid phase, the Ph and ci radicals
seem to recombine within the solvent cage generating transient 1t-chlorobenzene, an isomer of
chlorobenzene in which the chlorine atom forms a 1t complex with phenyl radical.
C,H,.--cl
~
[C,H, + ci]
Solvent cage
~
G-
ci
1t-Complex
Halogen atom of 1t complex is more selective than halogen free radical.
The light-induced homolysis of aryl-halogen bonds has important synthetic implications.
Irradiation of iodinated aromatics in benzene often gives the phenylated aromatics in high yield.
@-@-I
90% Yield
OH
hv (254 nm»
H,C,*C,H.
C6H5H
C6H5
75% Yield
Intermolecular reactions of this type can be used for the preparation of polynuclear
aromatic compounds.
hv
-----+
Fluoranthene
22% yield
Phenanthrene
90% yield
Similar reaction is also given by iodoacetylenes.
hv
C6H s-C == C-I - - + C6 H s-C == C-C 6H s
CeRe
75% Yield
t:::=::;:::::::::::::,:::;:~::;::
Diazoalkanes exhibit a weak absorption band in the visible region (400-500 nm) which is
attributed to an n ---+ n* transition.
Ell
e
eEll
••
Ell
••
e
R2C=N=N ~ R2C-N=N ~ R2C-N=N
The corresponding excited states fragment readily to give molecular nitrogen and a
carbene and this is a widely used method for the generation of these divalent carbon species.
Direct irradiation leads initially to a singlet state of the diazo compound and hence to a singlet
carbene, although collisional deactivation gives triple.t carbene before further reaction occurs.
It may be important for those carbenes whose triplet state is lower in energy than the singlet
state, especially at high pressure in the vapour phase or in solution. Triplet sensitised
decomposition of diazoalkanes gives rise directly to triplet carbene by way of the triplet excited
state. These processes provide a valuable route to carbene and are employed for intramolecular
reaction as in the ring contraction of cyclic diazoketones and in ti-lactam formation from
a-diazoamides.
CaH6',
CN2
CaH/
5
hv )
288nm
/~~
2
Cyclic diazoketone
~
CaH6',tt
CH/C + N2
6
I
-~
5
~o
..
,
-----+
~c=o
I
I
>­
~COOR
hv )
Pyrex
~N~O
::::::::::::=:::::::::::::::::;::::::7::]::::::
Azides are similar to diazoalkanes. It also gives molecular nitrogen and nitrene in excited
state.
Aliphatic azides give aliphatic nitrenes. Aliphatic nitrenes normally give imine by a
hydrogen shift.
CH a-CH 2-CH2-N a
hv
---7
..
CH a-CH 2-CH 2-N
Hydrogen shift
) CH a-CH 2-CH=NH
Imine
If a-carbon has no hydrogen, then in that case alkyl shift takes place.
CH a-CH2-CH 2-C=N-CH a
I
CRa
N·methyl imine
Intermolecular hydrogen abstraction from solvent can also occur to give an amine.
..
Ether
CH a-CH 2-CH 2-N -----+ CH a-CH 2-CH 2-NR 2
Singlet state nitrene gives addition reaction with olefines like carbenes to give aziridine.
o
+ NaCOOC2H 5
1
hv )
254nm
(pN-COOC 2H 5
Aziri.dine
derivative
NCOOC 2R 5
Singlet state vinyl or aryl nitrp.nes undergo intramolecular cycloaddition to form a
l-azirine. In the aromatic systems this can lead to ring-expansion with eventual formation of
a 6H-azepine when a nuc1eophile is present. In the absence of nuc1eophile an azo compou:qp.
may be formed.
hv
~
[cY]
~
Vinyl nitrene
hv
~
~
Aryl nitrene
~
l-Azirine
~
d1
(C 2H 5)2 NH
In the absence of nucleophile, aryl nitrene undergoes coupling reaction to give azo
compound.
[¢~Ha] ~
CHaO*N=N*OCHa
...
,.
299
Photochemistry
·c Photochemistry, Plenum, New York
'Proach, VCH, Weinhim, Germany,
"
.
_
...,.
I
..~
11
CHAPTER
on Reactio
"Rat Least
~:::::::::::::;;::::::::;;:::::::;::::;/:<::'::::::::::;:::::~:::;:':::::,:::
Substitution reaction at sp3 hybrid carbon takes place when carbon has either leaving group
or has at least one hydrogen. Replacement of hydrogen is possible when neighbouring group
activates the carbon. Ionic attack at unactivated C-H bonds is uncommon. Free radical on
the other hand, can often be obtained with enough energy to break unactivated C-H bonds.
Intermolecular reactions of this type (free radical halogenation, sulphonation, nitration etc.)
are of limited value because the reagents are unselective and mixture of products generally
results. However, it has been found that in many molecules intramolecular free radical attack
at sp3 hybrid carbon having hydrogen can become quite specific when molecules meet certain
structural and geometrical requirements. The reaction leads to the introduction of functional
group at this carbon and a number of reactions of this type have gained of synthetic importance.
The key step in these reactions is an intramolecular abstraction of hydrogen from
8-carbon (carbon-5) resulting in the transfer of a hydrogen atom from 8-carbon to the attacking
free radical in the same molecule. Transfer of hydrogen atom from 8-carbon leads to the
formation of six-membered transition state which is very common in photochemistry.
The reaction can be represented as follows (equation 1):
. ex
.
-y
X-H
.
U~
~
O-H
...(1)
Z=Yor some
different group
In the above reaction X in most of the cases is either oxygen or nitrogen.
The most common reaction of this class is Barton reaction (when X is oxygen) and
Hoffmann-Loeffler-Freytag reaction (when X is nitrogen).
;::::::::::::':::=:;:::;;:;:::::::::::::::~:::::::::::::::::::::::::
It has been known for many years that vapour phase photolysis or pyrolysis of organic nitrites
(X = 0, Y = NO) gives alkoxy radicals and nitrogen monoxide. It has been found that when the
301
PhotochemiStry and Pericyclic R
structure of the molecule is such as to bring the
-t-O-N=O group and a -t-H bond
I
I
into close proximity, or potentially close proximity (generally 1, 5-positions), the alkoxyl radicals
produced by photolysis of the nitrites in solution have sufficient energy to bring about selective
intramolecular hydrogen abstraction according to the equation (1) to give carbon radical. This
carbon radical captures nitrogen monoxide to give nitrosoalcohol. This nitrosoalcohol may be
isolated as dimer or may rearrange into oxime. The nitroso and oximo compounds produced
may further be transformed into other functional derivatives such as carbonyl compounds,
cyano compounds and amino compounds. The photolytic conversion of organic nitrites into
nitrosoalcohols is known as Barton reaction. The complete reaction is as follows (Scheme 1):
R
I~.
H-G-:--H
1\.-1
CH 2
0
1
CH 2
+ NO
-...::........CH~
,
Alkoxy radical
1
R
1
H-C·
O-H
1
I
CH 2
CH 2
+ NO
~CH-2
1
R
1
Tautomerisation
H-C-N=O
1
(
O-H
1
CH 2
CH 2
~CH-2
Nitrosoalcohol
1
Dimerisation
H?
V
e
e
$
$~
o 0
R
I I
yN=NyR?H
Dimer
Scheme 1 Formation of nitrosoalcohol
Thus Barton reaction is an example of the remote functionalisation of an unreactive
saturated carbon atom. The reaction starts from the 8 1 (n rt*) state (A = 31(}-390 nm) with t h e ' ( ,
cleavage of the R-O-NO bond (Ediss = 150 - 190 kJ) to produce nitric oxide and an activated
alkoxy radical (Scheme 1). This is followed by abstraction of hydrogen via a six-membered
cyclic transition state and the formation of hydroxyl group and a carbon radical that combines
with nitric oxide (Scheme 1a). The nitroso compound dimerises or preferentially isomerises to
the oxim~.
The reaction is effected by irradiation under nitrogen of a solution of the substrate in a
suitable non-hydroxylic solvent with light from a high pressure mercury arc lamp. A pyrex
filter is usually employed to limit the radiation of wavelength greater thaa 300 nm.
Quantum yield of the reaction is 0.76. This confirmed that the reaction is a photochemical
reaction and not a free radical chain reaction. In free radical chain reactions quantum yield is
always more than one. Photolysis of nitrite ester is reversible process which has been confirmed
by the use of N15 nitrite ester. Photolysis of nitrite ester gives an alkoxyl radical and nitrogen
monoxide in reversible step which are completely dissociated from each other. The alkoxyl
radical rearranges rapidly, by abstraction of hydrogen, to a carbon radical. This carbon radical
can be captured by radical trapping reagents to give transfer products. The normal fate of the
carbon radical in the absence of trapping reagents is to react relatively slowly with nitrogen
monoxide to give nitrosoalcohol.
Six-membered
transition state
Scheme 18
There is ample evidence to support the view that hydrogen transfer step takes place
through a six-membered transition state. In practice reaction occurs at most exclusively by
abstraction of a hydrogen atom from the a-carbon. Photolysis of l-octyl nitrite in benzene gave
only dimer of 4-nitroso-1-octanol.
~O-N=O
... (2)
Dimer
Similarly 4-phenyl-1-butylnitrite and 5-phenyl-1-pentyl nitrite are readily converted
into dimer of 4-nitroso-4-phenylbutanol and 4-nitroso-5-phenylpentanol respectively.
Ph-CH -CH -CH -CH -O-N
2
2
2
2
I=9
hv
-----7
C 6H 6
Ph -iH - CH2 - CH 2 - CH20H]
[
NO·
-----7
Dimer
... (3)
[Ph - CH 2
-
t:-
CH 2
-
CH 2
CH 2
-
-
OH] ------> Dime'
...(4)
Structure of the 4-nitroso-4-phenylbutanol and 4-nitroso-5-phenylpentanol can be
rationalised by assuming a six-membered cyclic transition state in the hydrogen transfer step.
In case of 5-phenyl-l-pentylnitrite, none of the products is formed by abstraction of hydrogen
from benzylic carbon which would require a seven membered transition state. In the same
way, photolysis of 3-phenyl propyl nitrite (C6H5-CH2-CH2-CH2-0-NO) did not yield
any product corresponding to abstraction of benzylic hydrogen atom through five-membered
cyclic transition state, even though abstraction of benzylic hydrogen should be highly favourable
thermodynamically.
Intramolecular hydrogen abstraction by alkoxyl radicals is always accompanied to a
greater or lesser extent by disproportionation, radical decomposition and intermolecular
reactions. In case of alkoxyl radicals derived from tertiary nitrites the reaction follows the
normal course if o-carbon is secondary or tertiary. But if o-carbon is primary then Barton
reaction is superseded by alkoxyl decomposition.
CHa
I
CH a
I
Ii
CHa-C
CH2-CH2-CH2-CH a
I
r
O-N=O
CHa
I
Ii
CHa-C
CH 2-CH 2-CH a
I
r
O-N=O
CH a-C-CH2-CH2-CH-CHa
I
I
OH
NO
CHa
I
... (5)
•
CHa-C~~2-CH2-CHa+NO
~J
Disproportionation reactions also occur in primary and secondary nitrites having no
hydrogen on o-carbon.
CH -CH-CH -O-N=O ~ CHa-CH-CH-O + NO
a
I
CHa
2
I
I
CHa H
1
\
.
(
dl-Bornyl and dl-isobornyl also undergo disproportionation rather than Barton reaction.
c1r
dJ--
0NO
ONO
dJHO
+
~H
1
+
+
dJHO
The Barton reaction is potentially useful in the synthesis of natural products, such as
hormones and alkaloids. The photolysis of the ll-nitrite of corticosterone acetate has been
utilised on an industrial scale in the partial synthesis of aldosterone-21-acetate.
OAc
o
The key step in the synthesis of the alkaloid perhydrohistorionicotoxin is the photolysis
of the nitrite of a spirocyclic alcohol having three different y-hydrogen atoms. The transition
state of optimal geometry is formed between both rings of the spirocyclic molecule.
hv
When the double bond of an alkenyl nitrite is in close proximity to the alkoxy radical
formed by irradiation, the sole product is a result of its addition to the C=C bond with the
formation of a cyclic ether.
~
HON~
hv
Some other examples are as follows:
hv
~
(i)
AcO
AcO
1
ONO
hv
(it)
~
AcO
AcO
HaN
R
R
hv
~
(iii)
:: :1:=:::::::;::::::
This reaction is given by N-haloamines having hydrogen on o-carbon. The reaction is affected
by warming a solution of the halogenated amine in strong acid (concentrated sulphuric acid or
concentrated CFgCOOH) or by irradiation of the acid solution with ultraviolet light. The product
of the reaction is the 0-halogenated amine. This product is not generally isolated in this reaction.
This product on basification gives pyrrolidine and its derivatives.
x
H
I
I
(J-R
hv
o
X N- R
or~
e
Q-R
OH
~
... (1)
o-Halogenated
Pyrrolidine
amine
Both N-chloro and N-bromo amines are used as starting material but the N-chloroamines
give better result. N-chloroamines are obtained from primary and secondary amines by the
action of sodium hypochlorites or N-chlorosuccinamide (NCB).
NaOClur
--~)
NCS
R-NHCl
...(2)
R'NH NaOCl or) R'N_Cl
R/
NCS
R/
The first example of this reaction was reported by A.W. Hoffmann equation 4.
~
I
Cone. H2S0 4, 140°C
("'\
l.N~Br
)
NaOH
... (3)
)
... (4)
I
Br
H
o-Coneeeine
Latter, further examples of this reaction were reported by Loeffler. One of the reactions
reported by Loeffler is
lOJ-9
H2~04) ~
NaOH)
N H/ .......... CH
a
I
N
... (5)
CH 3
When haloamines are the derivatives of primary amines, reaction takes place in the
presence of Fe (II) as initiator.
CHa-CH2-CH2-CH2-CH2-CH2-CH2-NHCl
l
cone.
Cl
I
H2S04j~
Fe(ll)
CHaCH2CH2-CH-CH2-CH2-CH2-NH2
NaOH
)
~
N
I
H
With N-halocycloalkylamines cyclisation leads to bridged-ring structures, but in these
cases products may not be exclusively pyrrolidine derivatives. In some cases six-membered
heterocyclic ring is formed instead of five membered heterocyclic ring. N-Bromo-N-methyl
cycloheptylamine gives tropane. In this case pyrrolidine ring is formed.
(I)
Cone. H2S0 4I ~
(iz)
NaOH
)
Similarly N-chloro-4-ethyl piperidine gives a mixture of the azabicycloheptane and
quinuclidine
o
NaOH
CP+
)
N
I
Quinuclidine
abnormal product
(six-membered
heterocyclic ring)
Azabicycloheptane
(normal product)
H
N-chloro-N-methylcyclooctylamine yields only N-methylgranatamine in which
six-membered heterocyclic ring is formed.
C1
(J
0
)
IN-CH
I
NaOH
~
)~
N-Methyl­
granatamine
N-CH g
I
g
H
CI
The reason for the formation of six-membered heterocyclic ring instead of five-membered
heterocyclic ring is not clearly understood.
The reaction is believed to proceed by a free radical chain process (Scheme 2).
CI
H
O-R
I
.,<]1/
O~-R
-cr
CWorammonium ion
CI
III
hv
~
I
1
R'-N-R
Chloramine
<
1
E1J/H
•
N-H
U"R
Carbon radical
Base
R
~. H
0
I
-
LJ
R
Scheme 2 Mechanism of the reaction
ii
I
In acidic medium, the reacting species is chlorammonium ions. This dissociates
photochemically to form the ammonium radical. This ammonium radical abstracts a suitably
situated hydrogen atom on the 8-carbon to give the carbon radical. This carbon radical abstracts
a chlorine atom from another molecule of chloramine, thus propogating the chain and at the
same time forming the 8-chloroamine from which the cyclic amine is subsequently obtained
(Scheme 2).
The free radical nature of the reaction is supported by the fact that reaction does not
proceed in the dark at 25°C. The reaction is initiated by heat and Fe(II) and inhibited by
oxygen and p-benzoquinone. These results also support the free radical nature of the reaction.
The abstraction of hydrogen from 8-carbon is supported by the fact that in a number of cases
8-chloroamines are isolated from the reaction mixture.
N-Haloamides also give this reaction. In this case N-halogen bond undergoes facile
rupture upon photolysis. Barton has made use of this reaction to develop a useful method for
the synthesis of y-lactones. In case of amides, the starting material is N-iodoamides prepared
either with lead tetraacetate-iodine or by tertiary butyl hypochlorite-iodine mixture
(Scheme 3).
0
RUH~
~
Pb(OAc)iI 2
)
~
R'yI
)
=
~RUH
II
R'-C-NHI
?H
~NH
RU~
Base)
°
"
R'-C-NHI
)
or 12
R----r-9
0
-I"
0
L~J
Tautomerisation
R ' UHI
R'(tH~
H 20)
~NH
y-Lactone
Scheme 3 Preparation of y-Iactone from N-iodoamide having hydrogen on a-carbon
....
"
The photolysis of hypohalites needs little discussion, since it proceeds in a similar fashion to
that of nitrite esters but reaction is free radical reaction and not a concerted reaction. This
reaction is a free radical reaction as in the case of Hoffmann-Loeffler-Freytag reaction. This
reaction is given by hypohalites which have at least one hydrogen on D-carbon. The initial
products of the reaction are 1, 4-dichlorohydrins formed again by preferential abstraction of
hydrogen present on D-carbon atom of the molecule. The 1, 4-dichlorohydrins of this reaction
are easily converted into tetrahydrofurans by treatment with alkali. The reaction proceeds
through a six-membered cyclic transition state as in the photolysis of nitrite esters (Scheme 4).
,
I
I
I
RU-Cl
~
-cr
~
III
R'OCl
Six-membered cyclic TS
(
NaOH
RDCl
O-H
Furan
derivative
I
1
R'OCl
(
Carbon free
radical
Scheme 4
I
i
1
I'
;1
~
.
,
"
'1,..
. I..
.~
12
I
CHAPTER
•
mistry in Natu
;::::::::::::::::
Natural products such as santonin, a molecule which offers the possibility of numerous
structural and stereochemical variations, have provided excellent models for experimental
and theoretical chemists to conduct studies aimed at clarifying the chemistry of excited-state
molecules.
Natural products having cross conjugated ketone system (terpenoids and steroids)
undergo rearrangement (see rearrangements of dienone) by diradical intermediate. This
rearrangement takes place via an n, n* triplet state. This photo rearrangement shows strong
solvent dependence.
General Photoisomerisation
hv
~
Neutral
( me dium
Lumiketone
Lumiketone is obtained from (D) when medium is neutral, i.e., in the presence of dioxane
(D) gives two isomeric ketones (E) and (F) in acidic medium (45% CH 3 COOH).
312
-
R,~ ~<)R,~,
O~
O~
(Dj)
(D:J
1
1
Rj
HO
Rj
~
YLJ
R
j
(D)
1
Enrui>;ati"n
Off
o$J
CH OH
o~
R
R
j
j
Hydroxy ketone
Hydroxy ketone
(E)
(F)
Stability of intermediate cation D 1 and D2 detl;lrmines the reaction path in acidic medium.
When R 1 is +1 group and R 2 = H, then major product is (F) which is formed from D • Similarly
2
ifR2 is +1 group and R 1 = H, then major product is (E) which is formed from D .
1
Photoisomerisation of Santonin in Neutral Medium
Santonin in the presence of dioxane gives Luminosantonin
o~
o
Santonin
hv
Dioxane
)
o==d:-J
To
Vo
Lumino santonin
,
Photoisomerisation in Acidic Medium
o~
a
)
(it) 45% CHaCOOH
a
a
Hydroazulenone
Conversion of santonin into hydroazulenone derivative takes place by formation of
intermediate (D 2). A methyl substituent effect is observed in this case.
Steroid (1) also gives the above result in the presence of acidic medium.
OAc
D2
45% AcOH
Intermediate
Reflux
o~
)
a
(2)
Bicyclo [3, 1, 0] hexane-2-one (1) give rise to a large number of photo chemical transformation.
(i) The primary process leads to the breaking of bond between C 1 and C 5 followed by C1
and C2 • These bond cleavage give diene-ketone without skeletal rearrangement.
$R
a
3
4
1
6
5
R'
qR
a
hv
)
Bond cleavage
between C1-C 5
(due to ring strain)
(1)
~
~
::::,..1
R'
R'
(A)
(2)
(ii) Breaking of bond between C 5 -C S leads to the formation of diene-ketene with
rearrangement.
a
~~~
(3)
~
C:C<.?R
O
::::,..
I ... R
(4)
Diene ketene
­
l
~.
I­
I
(iii) Breaking of bond between C1 -C 5 leads to the formation of cyclopropanone derivative.
o
o
~R~
6<:
R
(5)
(6)
This cyclopropanone derivative undergoes thermal rearrangement as follows:
0)
t:iR
: : ,. .
R
---4
~
Rearrangement
)
a~
1
CcR
1
e
OR
0
00::
o
R
(C)
1
..
CR
g:
Rearrangement
)
~R
OR
~
¢l
: : ,. .
R
R
(B)
1. 1, 5 bond cleavage should be favoured by the .presence of substituents on C-l and C-5
and 5, 6 bond cleavage should be favoured by substitution on C-6.
2. Phenolic products (B) and (C) are formed by cyclopropanone intermediate (6).
Photochemistry of Umbellulone
Umbellulone is a tripenoid. On irradiation with VV light in the presence of CR 30R at room
temperature it gives two isomeric compounds (2) and (3).
0
0
if
hv
CHaOH
)
~II~
~
~
iJif
(3)
Umbellulone
::
0
(2)
Photochemistry of Lumisantonin
Lumisantonin on irradiation gives ketene (1) as a primary photo product which then converted
into photosantonic acid.
'H __ H
O~'
- ,-­
1
~
.;
~
O-c
o"
Lumisantonin
Photosantonic ester
(1)
As mentioned earlier that ~, y-unsaturation promotes a-cleavage of carbonyl compound because
of allylic stabilisation of the radical produced. Two modes of overall reaction are observed.
The first mode involves a 1, 3-acyl shift in the formation of an isomeric ~, y-unsaturated
ketone.
The second mode of reaction involves ring closure to a cycloporpyl ketone.
Example
OAc
OAc
hv/Terbutyl alcohol
.
Ioser to
)
nngc
cyclopropyl ketone
o
o
OAe
OAc
hv/Acetone)
ring closer
to ketone
o
.,:i_"h0<WWMll<7rttti,;'..'lMlrlr-j"mr:71tmt
]1;'1"'" . " "
As mentioned earlier, photochemical reaction of aliphatic a, ~ epoxy ketones give 1, 3 diketones
and a-hydroxy ketones by migration of alkyl and hydrogen respectively.
[
R 1,,-
0
/
\
C-CH-C-R
R/
II
3
hv )
n --+ It*
/0~0
R1,,/C-CH-C-R
R2
I
3
0
0
II
~6
0
II
R-C-CH-C-R
2
I
3
R1
~
1
o·
®~~R
R/
9)
~
3
2
1, 3-Diketone
Examples
~o
~o
~
1, 3-Diketone
ofl)
YO
~
0
0
0
1, 3-Diketone
oW
c;P
~
0
0
1,3-Diketone
lo
H
om
0
~
J
"
s
H
:::;:::::~::::::==::=::
Since KAUTSKY discovered the reactivity of singlet oxygen, dye-sensitised photooxygentation
cf organic compounds has been extensively investigated from synthetic and mechanistic
viewpoints.
Molecular oxygen has a unique electronic configuration in that the electronic character
of ground state oxygen is triplet
O2 = 0 (18)2 0*(18)2 0 (28)2 0*(28)2 0 (2pX)2 n (2py)2
=n (2pZ)2 n*(2py) = n*(2pz)
j
It has two unpaired electrons one in n*(2py) and other in n*(2pz), hence it has triplet
multiplicity =28 + 1 =(2 x 1 + 1) =3
On the other hand, two kinds of singlet oxygen can be generated photochemically. These
are l!!.g and lr,+g singlets.
Electronic configuration of l!!.g singlet oxygen
cr (18)2 cr*(18)2 cr (28)2 cr*(28)2 cr (2px)2 n (2py)2 = n (2py)2 n*(2py)2 n*(2pz)O
J,
"acant orbital
Electronic configuration of l~/ singlet oxygen
cr (18)2 cr*(18)2 cr (28)2 cr*(28)2 cr (2pX)2 cr (2pX)2 n (2py)2 = n (2pZ)2 n *(2py)i n*(2py) J,
.~
Jj.
Half
Half
filled filled orbital
orbital
Opposite
spins
The l!!.g singlet oxygen is mostly responsible for usual photooxygenation reaction.
Photooxygenation Reaction
Ci80id diene8: Singlet oxygen gives additions reactions with ci80id dienes and reaction proceeds
in a concerted manner by formation of six-membered TS.
Thus dienes give 1, 4-cycloaddition reaction with singlet oxygen in concerted manner
with monoalkenes. The reaction is more complex and follows the general scheme shown below:
~
Sens
C-OH
I
C
II
C
The reaction is identical to ene reaction. The obtained peroxide on reduction with Na 2SO a
gives alcohol.
Some examples of photooxygenation of naturally occurring dienes are given below:
+
1
0
----4
HO
HO
C9H17
C9H 17
1
0
----4
AcO
AcO
«0
Ph
;::,...
at a
Ph
1
0
~
~
----4
~
;::,...
. Ph
Ph
~C
Ph
­
W-Ph
0
Addition with Mono-oleflns
Various examples of the photooxygenation of mono-olefin are given below:
/qa
H
Me
t
c
,",
i~_
I,
Allyli(' !,~~itions
having hydrogen
(i) 102
)
(il) Na 2 SO S
H Me H2=e
2
I
+
H2=e
OH
OH
(a)
+
(b)
OH
(c)
I
l
~CH'OH
~
Q3J/
f
1•
carbon
1
Allylic carbon
having hydrogen
~,y-Unsaturated
l
~
i
ketones also give photooxygenation in a concerted manner with singlet
oxygen.
I
oJ?O
Allylic carbon
Carotenoids and Related Terpenoids
Photooxygenation of carotenoids and related terpenoids afford ketene derivative in addition to
normal oxygenation products. This unique ketene formation is attributed to the non-planarity
of the C5 • C6 and C7• Cs double bonds, resulting from steric crowding of the methyl groups at
C-l and C-5. This unique photooxidation has been applied to the synthesis of naturally occurring
ketenic carotenoids.
' . )
1,4­Addlhon
rt~~~
-
-X~~H
..
..
I
,­
I
I
rt"0-O-H
O.jhv
XC~C
)
1
AY
OH
N.,SO,
rt"0R
XC"C
hY
OR
::::::=:::;:::::::::::::
The main reactions given by saturated ketones are a.-cleavage and hydrogen abstraction at
8-position.
hv
)
~CO/EtOH
Hydrogen abstraction
....
R R
"
j
1
13
CHAPTER
t
J1emistry in Na
lied Photoche
Earlier Chapters of this book were related with the fundamental process of photochemistry.
In this Chapter, we will study some of the ways in which photochemistry has an impact on our
lives. Natural photochemical phenomena have contributed to the evolution of life and have
allowed its continued existence on the Earth.
::::::;:::::.•.::•.:::::::::r=
Before discussing the photochemical reactions in the atmosphere, let us discuss atmospheric
structure because photochemical reactions depend on atmospheric structure.
The atmosphere may be broadly divided into four regions as shown in Table 13.1.
Table 13.1
0-11
11-50
15 to -56
-56 to-2
N2• 02' CO 2, H 2O
2.
Troposphere
Stratosphere
3.
Mesosphere
50-85
-2 to -92
°2,NO
4.
Thermosphere
85-500
1.
-~2
to 1200
°3
+
+
Ell
+ Ell
°2,0,NO
Oxygen plays an important role in the troposphere whereas ozone plays a key role in
the stratosphere.
Oxygen and Ozone Chemistry
The chemical activity of oxygen plays an important role in the lower atmosphere (troposphere).
The stable forms of almost all elements are oxides. The atmosphere contains gases such as
carbon dioxide and sulphur dioxide.
In the upper atmosphere the species of oxygen are 02' 0, 0·, O+, 0. 2 and 03' UV light
causes photochemical dissociation, ionisation etc.
02+hv
240-260
~
run
0+0
...(1)
308
~
nm
322
... (2)
°+
hv ~ 0' + e
02 + hv ~ 02++ e
(3)
(4)
Ozone is the important species in the stratosphere, acting as a protective radiation shield
for living organism on earth. The maximum ozone concentration is around 10 ppm in the
stratosphere at an altitude of 25-30 krri.
Ozone is formed by a photochemical reaction, followed by a three body reaction. The
photochemical reactions taking place in this region are
°°
02 + hv (242 nm) ~
+
+ 02 + M (N 2 or 02) ~ 03 + M + Heat
...(5)
The third body (M) absorbs the excess energy liberated by the above reaction and thereby
stabilizes the 03 molecule.
Ozone strongly absorbs ultraviolet radiations in the region 220-330 nm and thereby
protects life on earth from severe radiation damage. Only small fraction of the ultraviolet
light reaches the lower atmosphere and the Earth.
The mechanism of ozone removal is not well known. It is believed that ozone is eliminated
by reaction with atomic oxygen, reactive hydroxyl radicals and mainly by nitric oxide.
°
°
03 +
~ 02 + 02 + Heat
03+HO' ~ 02+0'-O-H
O'-H ~ HO'+0 2
3.
03+NO ~ N0 2 +0 2
N0 2 +O ~ NO+0 2
It may be noted that nitric oxide is produced in the stratosphere, below 30 km, by the
reaction of nitrous oxide with excited oxygen atoms and above 30 km by ionising radiation on
nitrogen
N20+O' ~ 2NO
...(6)
N 2 +hv ~ N+N
...(7)
...(8)
02+N ~ NO+O
1.
2.
Nitrogen Oxides
The nitrogen oxides in the atmosphere are nitrous oxide (N20), nitric oxide (NO) and nitrogen
dioxide (N0 2).
Nitrous oxide originates from microbiological processes and occurs in unpolluted air at
a level of about 0.25 ppm. In the lower atmosphere it has practically no influence on chemical
reactions. At higher altitudes it helps deplete the ozone layer.
N 20 + hv ~
°
N2 +
°
(9)
N2 0 +
~ NO + NO
(10)
NO + 03 ~ N0 2 + 02
(11)
Nitric oxide and nitrogen dioxide are important constituents of polluted air. These oxides,
collectively designated as NO x ' enter the atmosphere mainly from anthropogenic (man-made)
sources, i.e., combustion of fossil fuels in both stationary and mobile sources. The 'annual
global input of NO x is of the order of 86 million tonnes.
,
I
-I
i
i
The photodissociation of N0 2 can yield a series of inorganic reactions:
N0 2 + hv
N0 2 +hv
° + °2+ M
NO+O a
<a98nm
)
NO+O
)
N0 2
>430nm
)
)
t
°a+ M
N0 2 +0 2
)
NO a + 02
N0 2 + °a
)
0+N02 +M
N0 3 +M
)
N0 2 +N03
N 20 5
)
2N0 2
NO + N0 3
)
O+NO+M
N0 2 +M
Nitrogen dioxide finally ends up as HNO a
3N0 2 + HOH
) 2HNOa + NO
) 2HNO a
N 20 5 + HOIl
In the stratosphere, N0 2 also reacts with the hydroxyl radical to form HNO a.
) HN03
HO + N0 2
Apart from NO x ' chlorine also plays an effective role in removing ozone in the
stratosphere.
CI +03 ~
~
l
...(12)
CI-O + 02
CI-O+O ~ CI + 02
CI-0+N02
I
...(13)
°
°
CI-o-N~
...(14)
Chlorine nitrate
The Sources ofCI· (Chlorofluorocarbons)
1. Chlorofluorocarbons are used as refrigerant aerosol spray and propellants. They are
inset in the troposphere but slowly diffuse into the stratosphere where they are subjected to
UV radiation at about 200 nm generating chlorine free radical. The chlorine free radical
immediately reacts with ozone.
hv
CFCla
CI + 03
CIO + 02
CFCl 2 + CI
...(15)
~
CIO + 02
...(16)
~
CI+0 2
...(17)
~
In a cyclic reaction each CIO can initiate a series of chemical reactions which leads to
the destruction of upto 100,000 molecules of ozone without being destroyed itself in the process.
2. Volcanoes inject chlorine gas and HCI directly into the stratosphere. CI2 on exposure
to UV light (300 - 400 nm) forms ci'while HCI reacts with HO to give CIO
CI + 0a -----t
CIO +
6
~
CIO + 02
...(18)
CI + 02
...(19)
I
Cl +CH 4
~
CIO+N0 2 ~
C~ + HCl
... (20)
°
°
CI-O-N~
...(21)
By reaction with CH 4 , Cl is trapped as HCl which is rained out in the troposphere.
Organic Compounds
Hydrocarbons and other organic compounds in the atmosphere .are susceptible to oxidation
through a series of steps of chemical and photochemical reactions. As a result many noxious
secondary pollutant products and intermediates are produced, which belong to the category of
photochemical smog.
CH 4 + 02 ~ CR a + HO
(22)
~ CR a + H 20
(23)
CRa + 02 + M ~ CHaCOO + M
(24)
Organic compounds in the atmosphere readily enter into reactions with 0a' N0 2 and
CH 4 +
free radicals such as HOO,
HO
OH etc.
Natural sources, particularly trees, emit large quantity of hydrocarbons in the
atmosphere. Methane is the major naturally occurring hydrocarbon emitted into the
atmosphere. It is produced in considerable quantities by bacteria in the anaerobic decomposition
of organic matter in water, sediments and soil.
Bacteria
--~)
CO 2 + CH 4
...(25)
Domestic animal contribute about 85 million tones of CH 4 to the atmosphere each year,
CH 4 has a mean residence time of about 3-7 years in the atmosphere.
It has been estimated that anthropogenic sources contribute about 15% of the
hydrocarbons emitted to the atmosphere every year. Automobiles are the major source in this
respect. About 20 different hydrocarbons were identified in the atmosphere. Among them are
methane, ethane, ethene, ethyne, propane, n-butane, isopentane, toluene, n-pentane, m-xylene
and isobutane.
Hydrocarbons are removed from the atmosphere by several chemical and photochemical
reactions. They are oxidised through a series of steps. The end products are CO 2 , acids and
aldehydes which are washed by rain.
Hydrocarbons do not react readily with sunlight, but they are reactive towards other
substances produced photochemically. An important characteristic of atmosphere which is
loaded with large quantities of automobile exhausts, trapped by an inversion layer and at the
same time exposed to intense sunlight, is the formation of photochemical oxidant in the
atmosphere. This gives rise to the phenomenon of Photochemical smog. Photochemical' smog
is an oxidising smog having high concentration of oxidant (i.e., 0a and N0 2).
The probable mechanism of smog-forming reactions is illustrated below (Scheme 1):
Reactive hydrocarbons (having )C = C() .
1° .
3
R-CH 2
°2
~
R-CH 2-O-O
!
lNO
RCHi)+N0 2
H-O-O.J02
~
NO
+
RCHO
(Stable aldehyde)
Scheme 1 Smog formation reactions
R-CHO
1e)H
o
II
R-Q
•
0
II
,f-O
R-G-O-O-N '::i
o
Peroxy acyl nitrate (PAN)
Scheme 2 PAN formation reactions
The concentration level of 0 3 and N0 2 is so high that 03 can be virtually smelled.
Moreover, a brown haze is found in the atmosphere due to the heavy load of particles. The
various consequences that have been recognised so far as a result of these pollutants include
damage to materials such as rubber, damage to vegetation, reduced visibility and a tremendous
increase in respiratory disorders. What might be seen as an immediate effect is eye irritation
which is caused by substances such as formaldehyde, acrolein and PAN. PAN is an irritant to
the respiratory system and eyes and has also been found to be highly toxic to plants, that is, it
is phototoxic.
I
.
,IL'
::::::=!;::=::::::,:::::=::::=:::~:::
One of the most important areas of research in organic photochemistry is that which concerns
the chemistry of vision. The eye is an extraordinary sensitive instrument. Its wavelength
response is restricted to 400 to 800 nm, but its degree of sensitivity is such that a fully dark­
adopted eye can clearly detect object in light so dim as to correspond to a light input over the
retina of only about 10,000 quanta per second.
The gross anatomy of the vertebrate eye is the system of lens and retina. The retina is
made up of two kinds of light sensitive cells known as rods and cones. The rods and cones
present in the retina act as the receptors of the eye.
Rods are located primarily at the periphery of the retina. The rods have been found to
have high sensitivity and thus can function at low light intensities. Thus rods are responsible
for vision in dim light. Rods, however, are colour-blind and "see" only in shades of grey. Cones
are found mainly in the centre of the retina and are responsible for vision in bright light.
Cones also possess the pigments that are responsible for colour vision.
Some animals do not possess both rods and cones. The retina of pigeons contain only
cones. Thus, while pigeons have colour vision, they see only in the bright light of the day. The
retina of owls, on the other hand, have only rods; owls see very well in dim light but are colour
blind.
The chemical changes that occur in rods are much better understood than those in
cones. For this reason we shall concern ourselves here with rod vision fIrst.
When light strikes rod cells, it is absorbed by a compound called rhodopsin. This
initiates a series of chemical events that ultimately results in the transmission of a nerve
impulse to the brain.
Rhodopsin was discovered in 1877 by the German Physiologist Franz Boll. He noticed
that the initial red-purple colour of a pigment in the retina of frogs was ''bleached'' by the
action of light. The bleaching process led fIrst to a yellow retina and then to a colourless one.
The rhodopsin is also known as "visual purple" or Sehpurpur because of its colour shade.
Rhodopsin has absorption maximum'" 500 nm. Hum.an rhodopsin has a molecular weight of
41,000 and has 348 amino residues. It is found in membranes of the rod disks and makes up
90% of the total protein in these membranes. Rhodopsin functions when vision takes place
under low light intensity, for example, at night time. It cannot distinguish different colours,
since it has only one pigment whose Amax is 498 nm.
The chromophore of rhodopsin is a polyunsaturated aldehyde, ll-cis retinal. Rhodopsin
is the product of the reaction of this aldehyde and a protein called opsin (Fig. 13.1).
C=N-Opsin
I
H
"
ll-Cis retinal
Rhodopsin
'
The reaction is between the aldehyde group of ll-cis retinal and an a-amino group of a
amino group of the chain of the protein and involves the loss of a molecule of water. Other
secondary interactions involving-SH group of the protein probably also hold the cis retinal in
place. The site on the chain of the protein is one on which cis retinal fits precisely (Fig. 13.1).
Ir
Rhodopsin
Fig. 13.1 The formation of rhodopsin from 11-cis-retinal and opsin
The conjugated polyunsaturated chain of ll-cis retinal gives rhodopsin the ability to
absorb light over a broad region of the visible spectrum. Fig. 13.2 shows the absorption curve
of rhodopsin in the visible region and compares it with the sensitivity curve for human rod
vision. The fact that these two curves coincide provides strong evidence that rhodopsin is the
light-sensitive material in rod vision (Fig. 13.2).
1
100 r - - - ' 3 l e ! l r - - - r - - - - ,
~
~
.~ 50 f---:H---t--\\--I----~
Q)
Ul
"'iii
~
Ul
:>
500
600
700
Wavelength (nmj
Fig. 13.2 A comparison of the visible absorption spectrum of rhodopsin
and the sensitivity curve for rod vision
Irradiation of rhodopsin leads to a series of conformational changes which can be noticed
by the appearance and disappearance of various intermediates of different colours. The process
may be described by the Fig. 13.3.
When rhodopsin absorbs a photon oflight, the ll-cis-retinal chromophore isomerises to
the all trans form. The first photoproduct is an intermediate called bathorhodopsin, a compound
that has about 150 kJ/mole more energy than rhodopsin. Bathorhodopsin then, through a
series of steps, becomes metarhodopsin (also all trans). The high energy of all-trans chromophore
of metarhodopsin II cannot fit well with the protein; it is released. The free trans-isomer (i.e.,
all-trans-retinal) is then enzymatically converted to cis-form, which instantaneously recombines
I
----..
with the opsin. These molecular changes produce electrical potential which is transmitted to
brain within 25 million of a second after the flash of the colour (Fig. 13.4).
Rhodopsin
(red; Amax = 498 nm)
~
Bathorhodopsin
(red; Amax = 543 nm)
Opsin
Lumirhodopsin
(orange-red; Amax = 497 nm)
ll-cis retinal
Metarhodopsin I
(orange; Amax = 478 nm)
· + a 11 -trans-ret'Ina1 <Thermal
Met ar h 0 dopSIn
'II
E(--Opsm
(colourless; "'max = 378 nm)
(yellow; "'max = 380 nm)
Retinol
Fig. 13.3 The vision cycle of retinal opsin
(11.c;')
11 12
Bathorhodopsin
l}
1.
Several
steps
!
I~
I~ ,
Enzymatic cascade
and nerve impulse
Fig. 13.4 The important chemical steps of the visual process. Absorption of a photon of light by the
11-cis-retinal portion of rhodopsin generates a nerve impulse as a result of an isomerization
that leads, through a series of steps, to metarhodopsin II. Then hydrolysis of metarhodopsin
\I produces all-frans-retinal and opsin. This illustration greatly oversimplifies the shape of
rhodopsin; the retinal portion is actually embedded in the center of a very complex protein
structure
The first step to form bathorhodopsin occurs on a time-scale of tens of picoseconds and
each subsequent step is 102 - 10 3 times slower than its predecessor.
Rhodopsin has an absorption maximum at 498 nm. This gives rhodopsin its red-purple
colour. Together, all-trans-retinal and opsin (i.e., mBta rhodopsin II) have an absorption
maximum at 387 nm and thus, are yellow. The light-initiated transformation of rhodopsin to
all-trans-retinal and opsin corresponds to the initial bleaching that Boll observed in the retina
of frogs. Further bleaching to a colourless form occurs when all-trans-retinal is reduced
enzymatically to all-trans-vitamin-A. This reduction converts the aldehyde group of retinal to
the primary alcohol function of vitamin A (Fig. 13.5).
CHO
[H] Enzyme
)
All-trans-vitamin A
All-trans-retinal
Fig. 13.5 Formation of vitamin A
..
ir
1\
Thus precursor of ll-cis-retinal is vitamin A. This is the reason why vitamin A is good
for vision.
We now turn to a brief consideration of how the photochemical changes described so far
are converted to an electrical impulse that stimulates the brain. There is evidence that a
single quantum of radiation can stimulate a retinal rod. The absorption of one quantum does
not, however, result in vision, and several quanta (between two and six) must reach the same
rod within a relatively short period. Even, so, the process is remarkably efficient, and the
energy of the ultimate reaction generally exceeds that absorbed by the visual pigment. The
absorption of light appears to initiate a chain reaction that derives its energy from metabolism,
and visual excitation is a result of 'amplification' of the light signal received at the retina. The
photoreceptor is the biological equivalent of a photomultiplier, which converts photons to an
electrical signal with high gain and low noise. Both photoreceptor and photomultiplier achieve
high gain in a cascade of amplifying stages. Visual pigments are integral membrane proteins
that reside in the photoreceptors outer segment. Photoisomerisation of retinal triggers a series
of conformational changes in the attached protein that create or unveil an enzyme site. A
cascade of enzymatic reactions follows, which ultimately produces a neural signal.
Rhodopsin functions when vision takes place under low light intensity, for example, at
night time. It cannot distinguish different colours, since it has only one photopigment.
Colour vision is associated with the cones rather than with the rods. The photosensitive
pigment in the cones is called iodopsin. The chromophore of iodopsin is also ll-cis-retinal.
Rods contain only one colour pigment whereas cones contain three different colour pigments.
Thus three types of iodopsin are present in cones. The three type of iodopsin, which contain
pigments that absorb light at a maximum of 440 nm (blue), 535 nm (green) and 565 nm (red),
are responsible for colour vision. Cone cells are much less sensitive to light than rod cells, so in
dim lighting all objects appear in shades of grey. Thus the spectral response of the eye shifts
towards the red in going from dim to bright light, as expected.
Vertebrates appear to perceive colour through the operation of a three colour system.
Three different cone pigments seem to be implicated, with absorptions in the blue, the green
and the red wavelength regions.
~:::::,;::::::.::::.:::::;:;:::::=:.=:;:::::::::::
"'"
Silver halides are used as photosensitive materials for the most common form of photographic
process, both for monochrome and for colour photography. A light sensitive emulsion is formed
by coating microcrystalline grains of silver halide ~'..:spended in gelatin onto a suitable support,
for example, film, glass plate, paper, etc. It has been seen that formation of metallic silver
occurs which causes darkening of the emulsion ifthere happens to be a prolonged exposure to
light. This phenomenon is known as 'the print-out effect'. However, it has been found that 'a
latent image' is produced in the silver halide grains by shorter exposures. For converting a
latent image so produced into a visible silver deposit, a developer, which is a suitable reducing
agent, is used. In actual, all developers bring about the thermodymamic reduction of silver
haiide.
'rho formation of the latent image increases the rate of the reduction of silver halidelt to
metallic silver, rather than changing the ultimate reducibility of the emulsion itself. Unexposed
emulsion, when developed, leads the formation of 'fog' (darkening). Thus we can conclude that
the difference in the reduction rate for the exposed and unexposed areas governs their
discrimination during development. In normal course, after the development of the exposed
and unexposed areas, these areas are fIxed, that is they are made permanent, by dissolving
the unexposed grains of silver halides in sodium thiosulphate solution, immediately after
development.
AgBr ~ Ai~Br
o
OR
¢
¢
+ 2AgBr + 2011
e
+ 2Ag + 2Br + 2R 20
... (26)
o
OR
We will study the photographic sensitiser in Section 13.4. Photographic sensitiser is
used in colour photography. Thus colour photography is an important field where dyes have
proven to be of great importance in producing images. The difference in the use of dyes is that
here a larger quantity of the dye is used and this dye remains uncorporated till the fInal stage.
In order to obtain a complete range of visible hues, a three colour 'substractive' process is
used. The process involves the use of three layers of emulsion, each layer being spectrally
sensitised to a different spectral region (blue, green and red) and possessing integral filter
dyes, if required. The development process is arranged to form colours in each layer in inverse
intensity to the amount of the kind of light striking it to which it is sensitised. The developed
colours are complementary to the colours of the sensitising light. To make this point clear, it
will be well to discuss the basic structure of ordinary colour films and the colour changes
which occur on exposure, as shown in Fig. 13.6.
Blue-sensitive
Green-sensitive
Red-sensitive
Expose to
yellow
develop
Unexposed
develop
)
Yellow
Magenta
Cyan
~-\"~
Olll
o<;'e t.
ect.
0
e~el.
0.0
Yellow
Clear
Clear
Clear
Magenta
Cyan
Fig. 13.6 Schematic repregentation of the layer structure of colour film and the colour changes that
occur on development. The actual film also contains a filter (yellow) below the-blue-sensitive
layer to remove the blue light passing through this layer (blue), an anitihalation layer below
the red to prevent scattering of the light back through the emulsion, and a film base such
as cellulose acetate or polyethylene glycerol-terephthalate which supports the emulsion
If unexposed film is developed intense yellow, magenta and cyan (blue-green) colour
are formed in the respective layers. If white light then shines on the film, the yellow layer
I
­
substracts out the blue, the magenta substracts out the green and the cyan substracts out the
red. The appearance of the film is black as corresponds to no exposure to light. On the other
hand, if the film is exposed to strong blue light, no yellow results in the developed film and the
red and green are substracted from white light by the magenta and cyan layers so that on
viewing the transparency against white light only blue is transmitted, as befits the colour of
the original light. Similarly, exposure to strong yellow light (containing no blue) results in
formation of a yellow layer with no dye in the magenta or cyan layers. This is because the
green and red-sensitive emulsions are both made to be sensitive to yellow, while a blue-sensitive
emulsion is insensitive to yellow.
-
_.
Blue-sensitive AgBr
Green-sensitive AgBr
Red-sensitive AgBr
1
Green light
Blue-sensitive AgBr
Green-sensitive Ag*Br
Red-sensitive AgBr
1
Develop
AgBr
Ag
AgBr
1
White light
Ag*Br
Yellow
Ag
Clear
Ag*Br
Cyan
I Colour developer
-!-
Colour former
Ag + Yellow
AgBr + Yellow
Ag
AgBr
Ag+ Cyan
AgBr+ Cyan
Fig. 13.7 Schematic change in the development
of colour film exposed to green light
The chemistry involved in the production of the dye in colour films is highly ingenious
and is achieved through the general steps shown in Fig. 13.7, which for purpose of illustration
are carried through for an initial exposure to green light. The exposure activates only the
green-sensitive emulsion, and development with an ordinary developer such as metal­
hydroguinone produces silver metal only in the green-sensitive layer. The film now has the
visual appearance of a milky negative. The developer is then washed out and the film is exposed
to a strong white light which activates all the silver bromide remaining unreduced in the first
step.
The activated silver bromide so formed is now reduced with a colour developer, usually
N, N-diethyl-p-phenylenediamine. in the presence of a colour former. Production of dye occurs
in the direct proportion to the amount of activated silver bromide present in close conformity
with the following equation:
...(27)
Dye + 4Ag + 4Br
A different colour former is used in each layer of the emulsion and, althrough the complex
colour picture is formed in this step, the film is coal black because of the metallic silver produced
at the same time. The silver is then oxidised to silver bromide with dichromate solution
containing bromide ion and removed with thiosulphate solution. The final image thus contains
no silver.
The colour forming reactions are critical to the success of the overall process and, of
necessity, involve some degree of compromise between requirements for yield, reproducibility,
rate, suitability of colour, and light-fastness. In the reaction of the type show in equation
given below the colour former is a methylene compound R 2CH2 , where R groups have sufficient
e
electron-attracting character to have some degree of formation of R 2CH in the alkaline medium
used fer colour development.
H,C"
~
MgBi
C,H".
~ rffi<:
H5C2/N~NH2(_2Ag)C2H5/~~NH
- HBr
Br
<n
c,H"
~
)C 2H 5/ N -..Q;-NH-CHR2 ···(28)
(II)
The colour developer is oxidised by the activated silver bromide produced in the third
step of the Equation (28) to a quinonimmonium salt (1). This substance readily undergoes a
Micheal type conjugate addition with the anion R 2CH to give a N'-substituted, N, N-diethyl
phenylenediamine (II).
The product (II) is a photographic developer and is oxidised by activated silver bromide
to a n~w quinonimmonium salt (III) (Equation 29).
-
~:..
t
AgBr*
)
-Ag*,-HBr
,..(29)
The substance (III) has acidic hydrogen at the -CHR2 , and on loss of this hydrogen, a
neutral conjugated imine (IV) results which is the actual dye. Colour is expected for these
molecules because the R groups are electron-attracting, and the dimethylamino group at the
other end of the conjugated system is electron-donating. Proper colour of each layer of the film
depends on the nature of R 2CH2, Illustrative colour formers that give yellow, magenta and
cyan dyes with N, N-diethyl-p.phenylenediamine are shown below:
o
0
II
II
CSH 5-NH-C-CH2-C-CH s
Acetoacetanilide
(Yellow)
I-Phenyl-3-methyl-5·pyrazolone
(Magenta)
OH
@-CONH,
Salicylamide
(Cyan)
In some photographic processes (for example, Agfa-colour, Ektachrome, etc.) insoluble
compounds are used as colour formers and are past of the original emulsion while in other
processes (as in kodachrome) soluble colour formers are diffused into the layer of the emulsion
during the development process.
::::":::::::::':::::::::::::::,;;::::::
Light absorbing compounds can be classified into following categories:
(A) Photosensltlsers
As mentioned earlier that intersystem crossing (ISC) is very important phenomenon in
photochemistry, For many molecules ISC (8 1 ~ T 1) is not very efficient.
It is possible to excite such type of molecules (known as acceptor) by other molecule
(known as donor). Donor molecule produces T 1 in high yield by ISC and than this molecule
transfer its excitation energy to acceptor.
Excited state 8 1 or T 1 of donor molecule transfer its large amount of energy to the
acceptor molecule and in this process acceptor (A) species promoted to excited state and donor
(D) species return to the ground state.
hv
,
~
!
(D) ~ D*
D*+A ~ D+A*
Although photons are absorbed by D, it is A which becomes excited. This phenomenon is
known as photosensitisation and D is referred to as photosensitiser.
The most useful commercial sensitiser is photographic sensitisor.
Photographic Sensitisers
A simple emulsion of silver bromide in gelatin is mainly sensitive to blue, violet or ultra violet
light. As a result, a photograph made with such an emulsion represents blue and violet subjects
as white and all other subject colours as gray or black. That organic dyes can act as
photosensitisers for silver bromide emulsion was first discovered in 1873 and, it is possible to
make photographic emulsion which are sensitive over the whole region of the visible spectrum
as well as into the infrared. Since the dyes themselves absorb light of the wavelength to which
they sensitise silver bromide, it appears that light activated dye molecule can transfer their
energy to silver bromide and thus produce activated silver bromide capable of being reduced
to suitable developing agents.
Dye ~ Dye*
Dye*+AgBr
~
*
AgBr+Dye
.¢ . ¢
OH
20H +
I
fi
0
+ 2AgBr
~
OH
e
+ 2Ag + 2Br + 2H2 O
0
Developer
The most useful sensitising dyes are called cyanines and have the fallowing basic
structural unit:
tr-b=~-
-~-~? =
,
n
This type of system is excepted to be strongly light-absorbing by virtue of having electron­
attracting and electron donating groups at the opposite ends of a conjugated system. Examples
of such'cyanine dyes are:
[~
~]
~~~CH/CH~CHA~~
CR a
Ie
C2H5
Pinacyanol (Violet-blue)
~,
if
Krytocyanine (Purple-black)
Pinacyanol sensitises silver bromide emulsion is effective to red light and kryptocyanine
is effective for both red and infra red radiation.
(B) Ultraviolet Screening Agents
The destructive action of ultraviolet radiation on dyes, plastics, fabrics, etc. is well known and
there has been considerable research directed towards development ofsuitable protective agents
which would act as preferential absorbers of ultraviolet light. Such substances are often
called ultraviolet screening agents. A sunburn preventive agent is ultraviolet screening agent
which filter out the burning but not the tanning component of sunlight. Glyceryl p­
aminobenzoate is sunburn preventive agent which is non-toxic, nonstaining, nonvolatile and
able to filter out the burning component of sunlight.
Many ultraviolet screening applications require a substance which is not itself coloured,
is highly stable to light, and is an effective absorber. It is particularly to have a sharp drop in
absorption coefficient at the edge of the visible coloration. These requirements are well met is
many applications of o-hydroxy phenyl ketones, o-hydroxybenzophenones being particularly
useful. 2-(2-Hydroxy phenyD-benzotriazoles also possess excellent properties as screening
agents.
CH30~ ~OCR3
°cO
II
OR 0
OR
2, 2'-Dihydroxy·4, 4'-dimethoxybenzophenone
(Uvinol)
2-(2-Hydroxy-4-methylphenyl benzo [d] triazole
(Tinuvin-P)
(C) Optical Bleach
A useful way of improving the appearance of white fabrics particularly cotton, which tend to
yellow with continued washings, is to introduce on the fabric is a colourless substance which
fluoresces blue with the ultraviolet light. Such type of substances are known as optical bleaGP
or fluorescent whitening agents.
.
The principle behind the operation of an optical bleach is that the substance should
absorb is the ultraviolet and radiate in the visible region, so that the washed white textile
,
III
I
I
I I.'
apparently reflects more light than was incident upon it. A related large-scale application is
in the optical bleaching of paper.
A substance that is to be suitable as an optical bleach must satisfy following
requirements:
(i) It must not absorb at all in the visible region, since this would lead to coloration of
the fabric.
(ii) It must absorb strongly in the near-ultraviolet region, where there is still some in­
tensity available from natural or artificial light sources.
(iii) The fluorescence must lie in the short-wavelength end of the visible region, as other­
wise the fluorescence would give an apparent undesirable yellowing to white fabric.
(iv) The fluorescent substance must be photochemically stable, and it must not sensitise
degration or oxidation of the fibre material, and
(v) The substance must be soluble or dispersible in the aqueous detergent solution, but
must be sufficiently strongly adsorbed by the textile fibres for an appreciable concen­
tration to build up during washing and to remain after rinsing.
Over two hundred chemically different optical brightners are commercially available
and the choice for a given detergent depends on the type of textile and washing conditions for
which the detergent is intended. In many cases several brightners may be added to the d~tergent
to give a wide spectrum of activity. Almost all brightners used for cotton are derivatives ofbis­
triazyl-4, 4'-diamino stilbene-2, 2'-disulphonic acid (Fig. 13.8). The trans isomer of stilbene
derivative of the above compound gives better result.
I
NH
©
R=
I
N
/
I
"­
I
N
N
"­
/
/
"­
'CH2 CH3
I.
CH 2 CH 2
CH 2 CH 2
CH 2
CH 2 OH
CH 2 CH 2
OH
OH
OH
I
I
I
I
I
I
I
I
OH
X=
H
H
H
H
S03 Na
Code
TA
DM
DMEA
DDEA
DMDDEA
Fig. 13.8 Bis-triazinyJ-4, 4'-diamino-trans-stillbene 2, 2'-disulphonic acid
.
derivatives with their code names
339
Cotton fibres are hydrophilic fibres. Brightener of cotton fibres are also hydrophilic in
character. In most ofthe cases brightener adsorbs on the surface ofthe cotton fibres by hydrogen
bonding. Derivatives of 4, 4'-diamino-2, 2'-stilbene sulphonic acids are also used for cotton
fibres (Fig. 13.9).
Fig. 13.9 Bis 4, 4'-(phenylureido)-2, 2'-stilbene disulphonic acid (Blankophor-B)
.
'
Brightners suitable for nylons, polyesters and acetates are the derivatives of 1, 2-diben­
zoxazolylethylenes (Fig. 13.10) 2, 5-dibenzoxazoylthiophenes (Fig. 13.11) and styrlnapthoxazoles
(Fig. 13.12).
I
©r~ CH = CH-<~=©
1'­
R
R =Alkyl group
R
Fig. 13.10
Flg. 13.11
C6l.N
X
TQIO}-CH =CH-@
x= Cl, H
Fig. 13.12'
Nylon, polyester and acetates are hydrophobic fibres. Brightening of these fibres takes
place possibly involving the penetration ofthe molecules into canals between the fibre molecules.
We have already seen, photography is an irreversible photochemical process which involves
the production of an essentially permanent optical effect. Photochromism can be referred to as
a reversible light induced colour change. The absorption spectrum in photochromic system is
drastically altered in the presence ofirradiation but when this source ofirradiation is removed,
original state is once again achieved by the photochromic system. The reversal can alsOflOccur
by the introduction of a light of a different wavelength in some cases. Although, sometimes
changes in colour for example, red to green, might occur, the most common effect that can be
seen visibly in the appearance of colour in a previously colourless material.
I~
---------~---------=============----------
A major shortcoming that is exhibited by substances having photochromic properties is
the rapid fatigue in most of the known photochromic substances. Most of the photochromic
systems exhibit reversal only a limited number of times. The best prospect of good fatigue
characteristics is offered by photochromism based on isomerisation, since, with alternative
systems that involve bond cleavage, a very small lack of reversibility soon leads to chemical
decomposition is side reactions.
Isomerisation, dissociation and charge-transfer or redox reactions are the main chemical
reactions that are responsible for the photochemical behaviour of certain substances. Although
numerous substances are known to exhibit photochromic properties, one example is sufficient
to explain the preseTJ.ce of photochromiom. It has been seen that a large number of aromatic
nitro compounds show photochromic isomerisation. The process actually involves the
photoisomerisation of the colourless nitro form to the coloured aci.:form (Fig. 13.13).
02N
--r\/R
'=< CH.,R'
N0 2
Nitro·form
Light
I
J
Dark
02N-Q=C(~,
K-OH
~,
o
Aci-forrn
R = H, CHao CsHo etc.
R' =Electron-withdrawing group
Fig. 13.13 Photochromic isomerisation
Irradiation ofchromium hexacarbonyl, Cr(CO)6' in a plastic matrix leads to the formation
of a deep yellow colour as a result of photodisseciation. In the plastic, CO calUlot escape, and
-recombination occurs about four hours at room temperature.
Cr(CO)6 '"
Light"
Dark
Cr(CO)5 + CO
Both organic and inorganic charge transfer complexes or redox photochemical systems
are known. A typical reversible photochemical redox reaction occurs in a mixture of mercurous
iodide and silver iodide.
Hg 212 + 2AgI '"
Green
Yellow
Light,
Dark
2HgI2 + 2Ag
Red
"
Black
Heterolytic bond dissociation is responsible for photochromism exhibited by a large
number of spiropyran derivatives (Fig. 13.14).
'
Some derviatives of the class of compounds known as fulgides show a particularly
interesting type of photochromism. The forward photochromic reaction that is induced by
,i -' I
I,',
,"
,
light of wavelength A/may be reversed by light of a different wavelength Ar. (Fig. 13.15). Such
a system is widely used at present.
~NO,
CH s
Colourless
Light
JfDark
~NO'
CH s
Blue
Fig. 13.14
R
0
©t»
o
Colourless
,I
(I)
Fig. 13.15
: I
The most common of all applied photochemical processes is photography which involves the
making of a more or less permanent record of light and shade. In the series of photoimaging
techniques is photography which involves the capture and replication of image formation by
the use of photons. Office copying and the preparation of various kinds of printing plates are
some ofthe other large scale applications of photoimaging. If the properties (e.g., the soiubility)
of material used to protect some underlying medium can be modified by the image, then the
image can be transferred to the formely protected surface by means of subsequent treatment.
Photoresists is the name of given to such materials and these have found extensive uses
in the production of printing plates, integrated circuits and printed circuit boards for the
electronics industry. They may also be utilised for the manufacture of small components such
as electric razor foils, camera shutter blades, etc.
Three stages that are involved in photoimaging are capture, rendition and reli}dout. For
a typical imaging system these stages can be clearly shown by the Fig. 13.16. Among the three
stages, it is the image capture which is the photochemical process. The next stage, that is,
image rendition consists of a series of thermal reactions. The last stage being image readout
, I,
342
involves the development of the rendered image into a form that differentiates it physically
from the unexposed background. While ordinary photography involves optical changes,
solubility changes can lead to the generation of three dimensional relief patterns for printed
circuits, printing plates or three dimensional topographic maps. Production of lithographic
printing plates can be brought about by wettability changes. Enhanced tackiness can be used
to pick up pigments to yield pigmenttoned images in printing.
hv
I-Image capture
hv
~
--+
Photosensitive medium
~~Support
II-Image rendition
!
~olymerization in
lmages area
III-Image readout
(a)
Solubility
(Washout)
Photoresists
printing plates
(b)
Tackiness
(Pigment tone)
Proofs
line films
Fig. 13.16 Typical elements in photoimaging
I1~:mtIIt:~::'::~::;:~;::::::::::::::::'::;:i:::::i:l;:::::::::
Photopolymerization can be classified into two categories depending on whether each increase
in relative molecular mass requires its own photochemical activation step, or whether many
(thermal) polymerization steps follow the absorption of a photon. Photocrosslinking involves
the formation of crosslinks between pre-existing polymer chains and it falls into the first
category while the photoinitiated polymerization falls into the second.
Photopolymerisation: Imaging
There are numerous uses of photoresists. The manufacture of electronic integrated circuits is
one such important application where resists are used to define the doped regions on the
silicon substrate that will form resistors, capacitors, diodes and transistors of the finished
circuit, together with the metal conductors joining the components and the insulating and
passivating layers. The production of a complex circuit may involve several tens of successive
stages of imaging, followed by eteching, doping or other processing. An accuracy as good as a
few hundred nanometers is necessary at each stage. The required accuracy can be attained by
means of photograp}llc methods.
Now, since even more components are packed into a small space (very large scale
integration), (VLSI) shorter wavelength X-rays or electrons or ion beams are being used instead
of ultraviolet radiation. Photopolymer systems form the basis of photoresists. Refined versions
of photoresists are used for the manufacture of printing plates which are in turn used in the
semiconductor industry. Photocrosslinking forms the basis of a photoresist system due to the
fact that cross linked polymers are generally insoluble in any solvent. The material that will
be left behind after development by a solvent is that in the exposed regions, therefore the
resist will be negative working. When during development, the areas exposed to light in the
resist are removed, then those resists are known as positive working. Here the protective
coating where the light was obscured by the transferred pattern is left behind. Thus, negative
working resists leave the coating which has been exposed to light.
Now let us study one of the important photocrosslinking systems which involves the
photolytic cycloaddition of pendant cinnamate groups incorporated in polymer chains.
\
,,
... CH2 --CH
I
"
CH 2
0
I
I
0
I
t
... CH 2 --CH--CH 2 -CH
CH
I
C6H 5
C=O
C6H 5
C=O
CH
CH
CH
CH
CH
CH
CH
CH
C=O
C6H 5
C=O
C6H 5
I
II
I
I
I
I
II
II
I
0
I
I
II
I
hv
IIIIICH
~
I
IIII1CH
I
I
C=O
I
I
CH2
CH
CH2
I
I
C6H 5
0
I
... CH
I
I
0
0
C=O
I
C=O
C6H 5
I
I
CH
CH--CH
I
II
I
CH
CH--CH
I
C6H 5
I
I
C6H 5
C=O
I
O.
0
I
I
... CH--CH 2--CH--CH 2 •••
•••
Polyvinyl cinnamate
Fig. 13.17
The above reaction shows the formation of a crosslinked structure which is formed by
the reaction ofcinnamyl groups on irradiation ofpolyvinyl cinnamate. The cross linked structure
so formed has been found to be insoluble in solvents. Both the singlet and the triplet excited
states of the cinnamate chromophore can lead to dimerization. Dimerization of the ester can
only take place in the trans-isomer. However, a photoisomerization reaction brings about the
conversion of the cis-isomer into the trans-isomer. It has been discovered that radiations of
wavelength shorter than about 320 nm can be absorbed by the cinnamates. Thus, in order to
increase the sensitivity to near-uv and visible light by a factor of several hundred, triplet
sensitizers such as 4, 4' bis-(dimethylamino) benzophenone (Michler's ketone) are used.
Photopolymerisation: Curing
.~
Due to the availability ofsatisfactory low temperature thermal initiators for the bulk production
of thermoplastic linear polymers, photoinitiation of polymerisation is of little importance.
Photopolymerisation is rather concerned with in situ polymerisation of relatively thin films of
materials. In addition to the various uses of photolymerisation in imaging, it has proven to be
extremely valuable is numerous applications, for example, in the drying of decorative and
protective coatings on a variety of substrates and in the rapid and easily controllable hardening
. :
,
"
,
~
\
,
I,
,
.\
~
_ , " 'l'k.il'~·.I- J'.:=.:.L
I'"
,,""
'Lllllt.I~~.
i'
i
I
of resins in dentistry. Photochemical methods of curing, that is, drying or hardening processes
have been found to be more advantageous as compared to other methods. For example, large
quantities of solvents are used in many conventional coating techniques in order to reduce the
viscosity of the primary material so as to facilitate the coating operation. These solvents find
no use is the final cured coating. There may also be a necessity to bring about the heating of
the entire substrate which is energy inefficient. In contrast, we find that photoinitiation of the
cure works directly on the coating, leaving the substrate unaffected.
Heating operations are usually not required and the cure can be brought to completion
is a fraction of second if high intensities are used.
Photodegradatlon and Photostabllisatlon
Now let us briefly study two topics that deal with the durability of polymers in outdoor
environment. On being exposed to visible or ultraviolet radiation, especially in the presence of
atmospheric oxygen, most of the organic polymers undergo photodegradation which involves a
chemical change in the polymer. This brings about a deterioration in the mechanical properties
of the bulk polymer. In some contexts durability is a necessary factor, for example, in the
building or automotive industries. Photostabilization is desirable in order to extend the usual
lifetime of the material. On the other hand, the persistence of agricultural plastics and plastic
packaging materials after disposal has gained environmental concern. Therefore, there is a
deliberate effort to make the polymers light sensitive. In order to make articles such as plastic
cups short lived, biodegradable plastics are used. Exposure to light reduces theRe articles to a
fine powder, thus naturally disposing off abandoned waste.
The normal degrdation process involves a light-initiated autooxidation process. Thus,
in order to bring about photostabilisation of polymers, the aim should be to reduce the rates of
initiation or propagation of the chains, or to increase the rate of their termination. A decrease
in the rate of initiation can be brought about by a reduction of residual impurities in the
polymer while protection from oxygen would decrease the rate of propagation. However, these
two methods have been found to be unpractical.
The rate of initiation can also be reduced by the prevention of the absorption of light.
Photodegradation can be confined to the polymer surface by the use of highly, absorbent
materials such as carbon-black. Similarly, reflecting substances such as the white oxides of
zinc and titanium can also be used. In order to prevent the relatively long-lived triplets of
carbonyl compounds from participating in the- secondary photoinitiation steps, quenches may
be used. Retardation of photodegradation can also be brought about by the use of radical
scavengers such as phenols, hydroquinones and thiols which interfere with the propagation
steps. Ortho hydroxybenzophenones have proven to be very useful as stabilizers since they
operate both by screening as well as by quenching. Moreover, the hydroxybenzophenones
prevent the acceleration of autoxidation by reacting chemically with hydroperoxides.
In order to obtain environmentally desirable qualities, the polymer is rendered
photodegradable by a deliberate incorporation of photoactive groups into the polymer. For
example, light sensitive polymers are obtained by the copolymerization ofketop.ic species with
hydrocarbons. Photodegradation of the resulting material involves Norrish Type II scission of
the polymer chain rather than a radical mediated photooxidation.
i
r
U
I
-CH 2-CH-CH
-CH2­
I
2
hv
~
COR
Fig. 13.18
I
I,'
Ii:,,"
,,"
9,
"II
'
Ideally, disposable agricultural film or packaging should undergo a sharp and controllable
degradation initiated by exposure to ultraviolet radiation.
14
CHAPTER
and Their
Problem 1: Give product with its stereochemistry in the reactions given below.
(a)
n
n
~
)
(A)
)
(B)
HaC H CHaH
(2E,4Z)
(b)
hv
HaC H CHaH
(2E,4Z)
(c)
HaC
D
hv
H H
:>
(C)
)
(D)
CHa
(2E, 4Z, 6E)
(d)
HaC
D
~
H H
CHa
(2E,4Z,6E)
CH
(e)
(H~
~ H)
hv
a
)
(E)
CH a
Solution: The above problems are based on electrocyclic reaction.
(a) The compound is 1, 3-butadiene derivative (4n component). Its HOMO under thermal
condition is \jI2 which has C z symmetry. Hence conrotatory motion is possible in this
case.
346
H,¥_~H,
)
Conrotatory motion
H
H
(A)
Cis-geometry
(b)
The compound has 4n electrons, its HOMO under photochemical process is 'V3 having
m symmetry. Therefore disrotatory motion is possible.
hv
)
Disrotatory
Trans-geometry
(B)
Conrotatory
motion
(c)
~H3t
)
hv
H
CH 3
(C)
",~HOMO
C2-Symmetry,
(4n + 2) system
(d)
)
Disrotatory motion
"'3
(4n + 2) 7t System,
is HOMO in thermal
condition, m-Symmetry
Disrotatory
motion
(e)
)
hv
~
~
H
H
(E)
47t System, ",;is HOMO,
m-Symmetry
Problem 2: Give product with its stereochemistry in the reactions given below:
r:==i
(a) H3
H
(c)
H3
)
(A)
)
(B)
)
(C)
H
t==l
H3C
hv
CH3
Solution: These problems are based on electrocyclic reaction.
Conrotatory
motion
.H 3 9/:=~--~\SH~,
(a)~)
H
)
H3C
H
~
HH3C H
(2Z, 4E), 2, 4-Hexadiene
(A)
(i) 41t·System.
(ii) '1'2 is HOMO in thermal
condition
(iii) It has C2 -Symmetry in
(J
bond, hence product too
should have C 2-Symmetry
H3q~1;I
Conrotatory
motion )
(b)~
H
H3C
(2E, 4E)-2, 4-Hexadiene
(B)
CH3
hv
(c)
~
HH
CH 3
)
Disrotatory
motion
~
H4cH~HoC~H
3
(i) 4n-System.
(ii) 'fI3 is HOMO in photo­
III
chemical condition.
(iii) It has m-symmetry in (J bond
hence m-symmetry in n
bond should also be present
in the product. Therefore
motion is disrotatory.
3
H3C
~
HH
CH3
(2E, 4E) 2, 4-Hexadiene
(C)
Problem 3: Predict the product of the given reactions.
I,
L' ,
I'
f"
:
1
"
(a)
hv
0-0
II
~
HH
)
(A)
D
(b)
(
r:?
hv
h
D
)
(B)
H
(C)H
H
D
D
)
(C)
H
(D)
(e)
2
hv
(E)
)
(f)H~
(g)
n
O~O~O
)
hv
)
(F)
(G)
Solution: These problems are based on electrocyclic reaction.
(a)O-O "'"
)
Y-l
H
H
(i) 4n-System.
(ii) Its HOMO is 'Jf3* in photochemical cOlldition which has m-symmetry.
(iii) Motion is disrotatory.
Thus in the product both hydrogens are in the same plane.
hv
)
H
H
(A)
(b) The given diene is trans-diene. This undergoes photoisomerisation into cis-diene
because cycloaddition reaction is given by cis-diene and not by trans-diene.
Photoisomerisation
H D
¢
)
hv
: : ,. D
H
III
q
D
~
H1}D
motion
D
(B)
~
D
(c)
H
7
D1}H
H
H
¢
D
: : ,. D
)
7
H
III
~-i
D
(
Conrotatory
motion
H
D
H
D
H
(C)
CHa
(~
)(FY
\
HaC
(e)
CH a H
~CHa
hv
~
H
CH a
HaC
~
'\ CHa
+
HaC
CH a
HaC
CH a
(4n + 2) n-System
(Racemic mixture)
Photochemical
conrotatory motion
(D)
hv
)
(E)
(4n + 2)n-System in open
chain product hence ring
opening is conrotatory
H
~ ~
(f)
H
H
Il-c
C H
HOMO
"'2
III
C
1
~'
~
C
C
This reaction can be represented as follows:
H
9=>
)
H
(F)
hv
(g)
41t-System, excited HOMO
is
which has m-symmetry,
motion is disrotatory.
"'3
)
~
HO
(G)
Problem 4: Give product and stereochemistry of the product in each case.
-83°C
--~)
(B)
"'I'
FeCl a
--~)
(C)
--~)
(D)
Solution: These problems are based on electrocyclic reaction.
-830C
_
_~)
H3C~~CH 3
~
~
H
H3 C H CHs
(A)
System is conjugated (+ve charge, 1t conjugation) having (4n) 1t electrons. HOMO in
ground state is 'Jf2 which has C2 symmetry. Motion is conrotatory.
~
H3C g
CH
3
/?I
(b)
'H
~
CH3 H
l.......
-83°C
)
HSC:ht­
~
CH
" =
a
H ~ § H
Hi~ CH s
CH
~
(B)
System is conjugated having (4n) 1t electrons. Its ground state HOMO is
has C2 symmetry. Motion is conrotatory.
MesSi
OFeCl s
C6j
FoCI,)
HH
(C)
47t Electrons
o
~
ConrotatQl'y
motion
H H
i
OH
q:p
~(_H_(l>_MeCiP
H H
H H
(E)
(D)
Problem 5: Give product and stereochemistry of the product in each case.
(a)
BuLi
)
(1)
(i) ,\
(ii) HEll)
(2)
'Jf2
which
353
(b)
I
(l
BuLi
Ph~Ph
--.,)
(3)
(4)
)
(5)
)
Solution: These problems are based· on electrocyclic reaction.
Ph
PhXH
N N
(a)
A
BuLi
)
Jl~
NV
J12
Ph~~1
Ph H H Ph
Ph
Disrotatory
e 1ectrocycrIC
ring closure
)
61t Electrons
(1)
Ph
A Ne
N
Ph
A NH
H$
N
)
PhKph
PhKph
(2)
Allylic hydrogen is acidic
(b)
Ph
rl
~
rP/)
BuLi
)
Ph
Ph
Disrotatory
· )
e1ectrocyc1IC
ring opening
~nl
Ph
Its open system will have 61t electrons
(3)
He)
Q Q
+
Ph
Ph Ph
Ph
00
~
Problem 6: State whether these reactions proceed by t~e conrotatory or the disrotatory mode.
(a)
R
T'~
<~
)
----
:,
:'
H
(b)
C
HOAc!Ll
H
R
R
R
~
:
TsO
R
:
HOAc!Ll
)
<=
~-$
' ..... ,.....
R
Pbotoch~mlstrYlilnd Pericyclic Re~tions
354
Solution: These problems are based on electrocyclic ring opening reaction
(a)
~
H
)
H
Its open chain system will have
2lt electrons. This system will
have (4n + 2) It electrons. Its
HOMO will be 'VI having mirror
plane symmetry. Motion is
disrotatory.
OTs
~
H
H~H
)
R
R
I
H
HyyH
R
(b)~
)
R
RyyR
H
R
R
Thus
H
This has (4n + 2)7t conjugated
electrons. Its"HOMO will be 'VI which
has mirror plane symmetry. Motion
will be disrotatory.
OTs
~
RJ
R~R
H
\..R
H
1
R~R
H
,
.
~
'
"
,
H
Problems and Their Solutions
-,
,
355
Problem 7: Draw the structure of the chiefproduct of each of the following thermal reactions.
>
(b)
D
>
HaC HCH 3 H
Cis/trans
(c)
~;,
)
CH a
Solution: These problems are based on electrocyclic reaction.
(a)
)
This system has 41t electrons. Its
HOMO in thermal condition is 'lf2
which has C 2 symmetry. Thus
motion will be conrotatory.
HaC~
H
(b)
Conrotatory
motion )
­
H
a
)
CHa
((
~
CHa CH a
CH 3
System has
(4n + 2) 1t electrons
Its HOMO in thermal condition is 'V3 which has mirror plan symmetry. Therefore
motion will be disrotatory motion. Thus
o:CH,
)
""CH a
.
Conrotation
ring opening
~
Chehttropic
,: ~imihation
OfS~2
)
)
~
HaC H CHaH
Trans/cis or
(2E,4Z)
358
Problem 8: Complete the following reactions and give their mechanisms.
0
(a)
(b)
II
0
0
0
~
hv
+ C=G-COOEt
°
(c)
hv
+ CsHs-C-CsH s
0N
)
I
COOEt
hv
+ EtOOC-C == G-COOEt
~
I
H
Solution: These problems are based on
12 + 21
cycloaddition reaction
~Ph
0
hv
c.Y
o
)
12+21
Cycloaddition
(b)
O~ ~-COOEt
o "-.-C-COOEt
hv
N ............. C-COOEt
I
>
12+21
COOEt
dCOOEt
o
Cycloaddition
~C-COOEt
(c)O~ + III
Oxetane
derivative
hv
)
12+21
Cycloaddition
COOEt
&COOEt
N
I
H
H
~lec~rocy~lic
1
nngopenmg
COOEt
[fCOOEt
N
I
H
Problem 9: Complete the following reactions:
(a)
~0'f0
~
/OC 2H s
+ CH2 =C"
OC 2H s
hv
>
Pn:lbfems .anJIt Tlheir Solutions
(b)
(c)
"
)
0
))
~OH
(d)
hv
/
C6H 5-C == N + /C = C"
hv
+ CH 2=CH-COOCH a
)
hv
0
)
QOH+ 0
hv
)
12 + 21
Solution: These reactions are based on
cycloaddition reaction.
hv
12 + 21
©go
)
Cycloaddition
EtO
Ph V
(b)
I¥"'II
hv
III----A
I2 + 2 I Cycloaddition
C + C
)
N
OEt
\~
Ph
0
(C)Q
+
OH
CH-COOCH a
II
CH 2
hv
I2 + 2 I Cycloaddition
1
0
0
~COOCHa
"+
l:P-COOCHa
OH
OH
1
°nCOOCH,
OH
o
0QCOOCH'
0
Ell
WCOOCH,
OH
Ell
1
1
Ell
0
1
QCOOCH,
0
357
358
Photochemistry and Pericyclic Re,actii9:~a''i'~~:f~·~.r
o
(d)Q
+
OH
0
hv
)
I2 + 2 I Cycloaddition
1
00>
o
hv
)
I 2 + 2 I Cycloaddition
(oxetane formation)
Problem 10: How these conversions can be achieved? In these conversions atleast one step
should be a photochemical reaction.
OH~Y
®
,,
)
'
!
HCHa
a-Bourbonene
,,\\,,\\(CHJ6-COOCHa
(CHJ~-CH3
o
'.
, ..
Problemsaod Their Solutions
Solution:
hv
)
I2 + 21 Cycloaddition
($
H CH3
1
(i) CHaMgBr
(ii) HOHlH'"
CHVY
H
@
3
1?
;;;
(
=
=
H CH 3
1~
Allylic rearrangement
o
F<CHJ.-CH,
+
Cl
l
hV
12 + 21 .Cycloaddition
\\\\\\\ (CHJ6COOCHa
(CHJ4-CH 3
o
359
Problem 11: How these conversions can be achieved? In these conversions atleast one step
should be a photochemical reaction.
o
OH
i> ~ M
Y
(a)(Cl +
Cl
AcO
o
(b)~
(-)
~-Boubonene
Solution:
o
Cl
(a) (
+
Cl
hOi>
hv
)
12 + 21 Cycloaddition
Cl~~
1
Cl
2 3
5
4
O-Ac
O!
1
(i)
(ii) H
r::X;S:HJ
(b)~
hv
~
)
12 + 21 Cycloaddition
Problem 12: Complete the following reactions:
(a)G
)
0
Cl
(b)
O
(c)
O~
I
II
CH 2 + CH3-CH-C-Cl
Cl
0
I
II
+ CH -CH-C-Cl
EtaN
3
A
)
o
(d)
CO
~
I
II
+ C12-CH-C-Cl
•
Problems and Their Solutions
12 + 21
Solution: Ketene gives
is thermal reaction.
cycloaddition reaction in the absence ofUV light. The reaction
CH3
(a)
~
,,/CH3
C
+
1/
C=O
CH-C=C=O
I
3
Cl
Ketene
12 + 21
)
Cycloaddition
Cl
I
(e)
0
II
CH 3-CH-C-Cl
CH-C=C=O
I
3
Cl
12+21
)
Cycloaddition reaction
o
(d)
Cl
Cl
I
I
"CH-C-Cl
Cl/
CH=C=O
r'
H ,,/--,1
C
II
361
C=O
)
CO=C
CI
362
Photochemistry and Peticyctlc Reaction$
Problem 13: Complete the following reactions:
(al~
hv
(b)
)
hv
)
hv
)
CuCl
(dlJJ
hv
)
12 + 21
Solution: These problems are based on
(al~
hv
12 + 21 Cycloaddition
>
hv
(b)
cycloaddition reaction.
12 + 21 Cycloaddition
>
reaction
OH
I
CH
"
. CH 2=CH ..........
(c)
H 3C-C-CH 3
hv
)
12 + 21 Cycloaddition reaction
CHz=CH-CH{
(dlee
o
hv
)
12 + 2 1 Cycloaddition reaction
Problem 14: Complete the following reactions and give product in each case.
ClaCCOOAg
>
)
CHa O-SiMea
I
I
/CH a
C",
CHa
(b) CHa-C-C
I
Cl
.'
Solution: These problems are based on
AgCI0 4
14 + 21
)
cycloaddition reaction
21
14+
Cycloaddition reaction
~CH,
D
)
10
1
~CH,
AgCI0 4
(b)
)
j
~
Ha
CHa
HaC
o
CH a
~(----
~Ha
e CH
HaC
1, 2-Shift
a
O-S~ea (
CHa
O~
14 + 2\
CYcl~addition
reactIOn
364
Problem 15: Give product in the given reactions:
(alV 5
ffi
o
H~_20~)
NH,_C_l_ _
+
(bl~
)
o
Solution:
@
(a)
~
I
1
+
NH3Cl
14 + 21
Cycloaddition reaction
--.:...----~)
o
l-H'o
~
H
(bl~
Intramolecular
14 + 21 Cycloaddition
)
Problem 16: Complete the given reaction and give its name (if any).
o
)
)
(ii) HCHO
Solution:
o
ffi
+ CH 2 =CH-PPh3
--~)
Me2NLi
)
The first step is Diels-Alder addition, (14 + 21 cycloaddition) and second step is Wittig
reaction.
i~'
~_ ..; -#
• to'
Problems
and
Their Solutions
365
Problem 17: Complete the following reactions and give their name (if any):
o
.1.:
•. 1
(ai~
160°C
O°C
)
)
(bil(C)
Solution: The above reactions are intramolecular Diels-Alder reactions.
(a)~
(bi
'fa
O°C
)
Intramolecular
14 + 21 Cycloaddition
160°C
)
Intramolecular
14 + 2 I Cycloaddition
Problem 18: Complete the following reactions:
o
Ph
o
Q
(biC¢SO' +
o
Ph
o
(0
Ph
(c)
Ph~O + Ph-C",C-Ph ~
Ph~
Ph
Solution:
0
@COOCH,
(a)
if~o
~
nCOOCH3
)
Chelotropic
elimination
If
~
Q
0
0
II I
O~
)
14 + 21
Cycloaddition
0
H 3C-O-C
0
0
Ph
Ph
(b)
~O
~ I ~
Ph
~
Chelotropic
elimination
)
c<
~
:::,....
Ph
¢o
0
Diels-Alder
reaction
Ph 0
)
$'
~
h
Ph 0
Ph
(c)
Ph~C=O
~
_ _ __ _
Ph
~) p~,rPh
Ph
Chelotropic
elimination of CO
Ph
Ph
PhyYPh
Ph-C", G-Ph
~
Diels-Alder addition)
Ph~Ph
Ph
Ph
Problem 19: Draw the structure of the chief product of each of the following thermal reactions:
COOEt
CH2
I
(a)CO + C
III
C
I
II
I
)
CH
)
COOMe
COOEt
14 + 21
o
Solution: These two reactions are
COOEt
(a)Co
~
I
+ C
)
cycloaddition reactions.
~COOEt
III
I
)
COOEt
C
O~COOEt
V-COOEt
COOEt
(b)
~~
+ CH 2
)
II
CH-COOMe
d~)
,.) + CH-COOMe __
~
II
CH a
6COOMe
CH 2
Problem 20: Draw the structure of the chiefproduct of each of the following thermal reactions:
(a) (
°II
+~
N-C,
-500C
+ II
/N-Ph -~)
N-C
(b) (
)
II
o
+
I(
CHO
1100C
--~)
(d) (
~
0
+
I(CHO
Problems and Their Solutions
Solution: These problems are based on
14 + 21
cycloaddition reaction.
.J
... ~
..
)
o
I
I
N_I~
-.<'
,
I
N­
(N-Ph
I
o
CHO
+ I(
0
(d)
+
(::,...
(CHO
I
r10 y
--.,.) V
t.
CHO
Problem 21: Give name and mechanism of the following reactions:
&to
~
(a)
(b)
~
QNH
~
I N"H
) OCO
~ ~ ~
t.
-..:::::
I~
I
ex
HgO
)
~
20·C
[NH]
+ II
NH
H
+ [W
NH ]
~
) H2 + N2
N
) III
N
+H2
Solution: These two reactions are retro Diels-Alder reaction.
t.
~H
(b)0r-oNH
t.
)
)
~
~
r7Y
~
+ [NH=NH]
+ [NH]
II
NH
Problem 22: Give name and mechanism of the following conversion:
08+
COOMe
I
III
C
I
C
COOMe
t.
-~)
0>
I
~
+
cQ(COOMe
0
COOMe
367
368
Phofocl1emislry and Pericyclic
Reaettons
Solution: First step is Diels-Alder addition and second step is retro Diels-Alder reaction.
~
~COOM
~
'p
::::,. .
C-COOMe
"'--III
~ COOM
)
C-COOMe
Retr~
1
Diels-Alder
reactIon
~COOMe
+
~COOMe
Problem 23: Complete the given reaction and give name of the reaction at each step (where it
is possible).
C-COOEt
+
!
I
I
;
1
III
--~)
(A)
C-COOEt
(B)
)
One mole
(C) + (D)
-~)
Solution:
C-COOEt
C
: 0 +
III
GJC
Diels-Alder
addition
C-COOEt
COOEt
101
)
COOEt
(A)
H~Pd
One mole
partial
reduction
1
+ OCCCOOEt
.0
~COOEt
(
COOEt. Retro Diels-Alder
addition
~COOEt
(B)
(C)
Problem 24: Complete the given reaction:
CH-COOMe
II
L1
)
(A)
)
(B)
CH-COOMe· ;:
1
6500C
('COCH 3
~
H 3 COOC
+
CH 3
o
I' '
\
"
Solution:
.'
CH-COOMe
II
)
CH-COOMe
Diels-Alder
addition
'f
'I
~COOMe
COOMe
(A)
:)NH
(i)
1
(ii) CH 3-Br (SN R
O
:;:....
+
MeOOC
J:
Retro Diels-Alder
addition
COOMe
I .."
~
n
)
COOMe
(
CH3
o
II
CH2 =CH-C-OR
)
Solution: Reaction is Diels-Alder addition. Product of the reaction will be bicyclic compound.
Hence two products will be fOrnied, one is exo and other will be endo. Endo product will be
major product because of the secondary interaction in the molecule.
0 +II
~H + ~COOR
.1
CH 2
)
CH-COOR
H
COOR
Endo
Exo
Problem 26: Complete the following sequence of reactions:
0
0+
II
CH-C-CH
II
II
.1
)
(A)
hv
(B)
)
CH-C-CH
II
0
Solution:
0
II
0
CH-C-CH
+ II
II
CH-C-CH
II
0
.1
)
Diels-Alder
addition
~o
(A)
hv
)
12 + 2/
Cycloaddition
~O
0
(B)
Problem 27: Indicate how the given conversions take place?
O
(a) :
~
~CHO·
+ CH=C-CHO
+
CH s
CHs
(blQ
0
)
C-COOEt
+ III
C-COOEt
~COOEt
)
yCOOEt
CH s
CHs
Solution:
(a)O
CH s
(blQ
Retro
Diels-Alder
addition)
CH
III
+ C-CHO
)
I '"
V
CHO
,....
Diels-Alder
addition
$(
CHs
C-COOEt
III
~
+ C-COOEt ---~) I
COOEt
14+2/
Cycloaddition
CH s
COOEt
Retro
Diels-Alder
reaction)
CH2
+ C"H2
CH s
~COOEt
+ S
~COOEt
CH s
CHs
Problem 28: Predict major product in the given thermal reactions:
NR2
(al'
HsC,!
(b)
.
(c)
+
-...:::::
EtO'!
--::::.
/(COOEt
+
+
~
)
(COOEt
Il. COOEt
~
~
)
)
Solution: The above reactions are Diels-Alder addition. Both diene and the dienophile are
unsymmetrically substituted. Generally there is a preference for the ortho andpara orientations
. as follows:
)
Favoured
Ortho like orientation
,I"
11
i
ERG
+
U
Il EWG
EWG
Para like orientation
Thus
NR2
NR2
&COOEI
+
Major
Q
u
EtO
A ~
COOEt
Minor
1
+
.
EtOuCOOEt
1
COOEt
Major
HaC
(c)
'C~
+
Il COOEt
A
Minor
HaCuCOOEt HaC
~
I
+
U
Minor
product
1
COOEt
Major
product
Problem 29: Complete the following reactions:
o
II
CHz=CH-C-CHa
Zn·Ar • (A)
- - - - -...
~ (B)
A
Ph
(blCJ9>
+
o
A
(A)
Ph /
COOMe
HaCWCHa
(c)
~()
----+) (A)
--~)
(B)
o
'L~,' :
\"
I,
Solution:
.,.:,
CH 3
(a)
¢CI C~
:::,..
Q,
Br
CH 2..Br
2
Z_n_-A_g_/!'J._~)
Dehalogenati<;m.and
ring formation
~
:
)
Electrocyclic
ring opening
CH 3
CH 3
(A)
. .
"1
!'J.
• J;
II
CH:r-CH-0-CH3
0
Dienophile
.
'.
Ph
+0
I4 + 2 I Cycloaddition
)
c40
Ph
(A)
', .. ;. :. ".' ,.'CHil' ,';:.
COOMe
(C)CH3VCH3
1
'
o
, OMe
'.,
"OMe
.f CH=C/
2
)
Meooc~?!'
"
. C=O'OMe
H 3C .'
',' c:.,OMe
(A)
o
III
1CO,~MeO
I
?i
Meo-c
n
H 3C
.' ,~.,
CH 3
I.
0
CH 3
MeO----'C~H
H 3C
OMe
(B)
Problem 30: Give mechanism for the following re,actions:
)
HO
'r· " ,
)LA OMe
!
)
...\.
(c)
::::l
'--.f'N
O~
)
Pr
Solution:
(Electrocyclic
ring opening)
:; , .
6, Electrocyclic ring opeJ;ling "' '
')
"
'i"
". i _ -:"
., ,; ,.: ~'l . l
" ~
..
•.
.
" - l
- ~ ;
1'1Cycloaddition
4+21
reaction
" ,intr9: ffio1 l:!cular
~. Intramolecular
Diels-Alder addition
(c)~
O~ Pr
Problem 31: Give product in the following reactions:
(a) CHa-CHO
+ CsHs-NHOH + CsH s-CH=CH2
(b) C~=CH-CH=CH2+ C~N2
H'\.
(c)
H6
1Il
/CeH 6
C/C~'\.S!.
CHFCH-eH 20H
SOOC
U
a
(d) CH-COOMe
CH2N2
II
<C2Ha)20
BOoc)
)
~
~
CH-COOMe
Solution: These problems are based on 1, 3-dipolar addition reaction.
(a) CaHr;-NHOH
+ CHa-CHO
1-
HOH
[
("rCH=CH2]
N......N
Tautomerisation
- - - -..~
~
I
II
NH-N
CH=CH 2
Ph
I
80°C
)
C H _CH/N.......O
6
I
5
I
CH 2-CH-CH 20H
Problem 32: Give product in the following reactions:
COOMe
I
(a)
C6 H5N3 + C
(b) CH 2N 2
)
III
+ CH==CH
!J.
~
C
I
COOMe
o
(cJ C,H,N, +
¢o
o
Solution: These reactions are based on 1, 3-dipolar addition.
Ell
e
(a) CaH5N3==C6H5-N=N=N
~
N (N-CH
e ~N . . . . . . . . $
i\?
6
e
Ell"
C6H 5-N-N=N
/C 6H 5
Jj-N,
5
)
N
,ijC-COOMe
"'C:?'
1
I
MeO-C-C==C-COOMe
--~)
N=N
V
COOMe
Tautomerisation
<:
N-NH
V
Ell
••
e
(c) C6H5-Na==C6Hs-N-N=N
orN~
~
~ I
Ell
e
~(-~) C6H s-N=N=N
I
o
+
~
NEIl
I
C6 H 5
Problem 33: Give product(s) in the following reactions. Give mechanism for the reaction (b):
)
e
(b)@-CHO
CH 3-NHOH/OH
--=--------~)
(A)
CH 2=CH2
---~)
(B)
I
I
-
(~
(b)
C6H 5-C-H
)
c.
CHa-NH-OH
(A)
Base
(.
.r-<Ca!J;5
)
z..O"N-CHs
(B)
----~-
(c)
C6H5-~-~
CH= CH 2
I
0-(CH 2)a-CHa
en
Problem 34: Give product(s) in the following reactions:
(a) C6H 5 CHN(CH a)0 +
(b)
0
1
e
.... 0
+ CH 2=CH-CH a
t.)
t.)
t.
(c) C6H 5 CHN(CH a)0 + CH 2=CH-C00C 2H 5 ~
Solution: These problems are based on 1,3-dipolar addition reaction.
(b)
0 al . .
1
e
O
~(--+)
Oal
CH:r-CH-CHa
+
.)
e
.... 0
(Major product)
e/CHa
(c) C6H5CHN(CH~0=C6H5-CH=N "'-0
e
(
(Minor product)
Problem 35: Give product in each of the following reactions:
8
)
)
)
~l·
~~N
$
N02
(b)
C6H5-N=N=N
~(-~) C6H5-~-N=~
C====C
I
I
2
COOMeCOOMe
(c)
CH 2N2==CH 2=N=N
~(-~) c~
CH 2 =C",,-O]
Problem 37: Complete the following reactions and give product in each step:
1 mole
-~)
Ozone
C6H s-N=C=O
-­
(
n
(d)!y
C6H s-CH 2NHOH
ti)
)
(A)
(A)
(B)
NaBH 4
CsHn-C=CH
----~)
ZnlCHaCOOH
----~)
) (C)
(B)
(B)
S-CH 2-CHO
Solution:
(a)
,V
HC
3
I
h
0CH 3
LilNHa (l)
)
)
Birch
reduction
(1, 3-Dipolar
addition)
1
·\
•
~°
I
H3 C
OMe
NOMe
I
1 CHOb
H3 C
CH 20H
(C) •
(b)
1
°
A.,··CH 3
~EIl
(A)
e
C=N-O
()
(c)
Intra­
molecular
1, 3-dipolar
()
I
I
r~rCH2-CH=CH2 ~(_ _~) EIl~CH2-CH=CH2_a_dd_it_io_n~)
n
(d)y
)Q
:,
C6H l lCH 2NHOH
S-CH -CH=N-O
S-CH2-CHO
I
2
CH2
I
CsH s
I
1
Intramolecular
1, 3-dipolar addition
CH2-C sH s
ZnlCHaCOOH
(
Eo
(A)
Problem 38: What product(s) will be obtained if diphenylnitrilimine is heated with
(i) cis-stilbene
(ii) trans-stilbene
Give stereochemistry of the product in both the cases.
Solution:
Ell
8
CsHs-C=N-N-CsH s ·
".,
"
Ell
!
8
CsHs-C=N-N-CsH s
Cis-stilbene
H
H
381
Problem 39: Complete the following reactions and give product in each step:
"
a
I
EtaN
(a) CH 3-CH 2-C-CH-CH 3
(A)
)
I
EtOOC-C =C-COOEt
) (B)
HOH
li
CI
CH:r-CH-COOEt
)
(A)
--"------~)
ZnJli
--~)
(A)
li
--~)
(B)
C6 H 5-CH=CH-C 6 H 5
------~)
(B)
(C)
Solution:
a
OH
II
EtaN/HOH
---~)
(a) CH a-CH 2-C-CH-CH 3
I
I
CH 3-CH=G-CH-CH 3
1
I
CI
Cl
e
a
I
ffi
CH 3-CH= C-CH-CH a
1,3-Dipolar species (A)
,tt
I", •
ffi
CH 3-CH=C-CH-CH
~
J
CH 3-CH=C-CH-CH 3
a
~~
1, 3-Dipolar addition
----~)
EtOOC-C-G-COOEt
I
c/
)C=C"
COOEt
COOEt
(B)
CsH s
ffi
)
1
N
e
H-C"" 'C-H
CsH{
\CsH s
(A)
CH 2=CH-COOEt
1
1,3-DiPolar
addition
CsH s
I
H ........C/N,C/H
CsH( I
I.. . . . CsH s
CH 2-CH-COOEt
(B)
CSH 5-CH=y-yH-C sH 5
O~CH...CH-CsH5
I
CSH 5
(C)
Problem 40: Complete the following reactions:
(a)
(c)
OIl!
~)
o
0
!IV'
(b)
hv
)
HXH.
hv
H
(d)
)
hv
).
Solution: These problems are based on sigmatropic rearrangement
(lIl
(a)
cQ£rY:'
1
3
2
~
0
A5
(b)~i7
Intramolecular
[1,5] Shift
hv
---~)
[1,7] Shift
H~
)
)
14+ 21
Cycloaddition
reaction
OJ
~
~
hv
­
H'
H
hv
)
[1, 7] Shift
Provitamin D2
)
[1,7] Shift
H
(d)
'CJ:>=o
Problem 41: Complete the following reactions, give their name and mechanism:
(a)
Q)
(b)
t:.);
v-v
t:.) ; (c)
)
Solution: These reactions are [3, 3] sigmatropic rearrangement.
~3
dif:'\~
(b)~
(a)~2
1
(c)
)
H
Problem 42: Complete the following sequence of reaction and give structures of (A), (B) and (C):
H sC-C-CH2-CHO +
II
Solution:
CH 2
CH s"'/C=CH-CH 2 0H
CH s
Helt:.
t:.
~
) (A)
(B)
t:.
~
(C)
Nucleophilic addition reaction of
carbonyl compound (Hemiacetal formation)
)
t:. I He,
1
Dehydration
of alcohol
s~o==
~O
3~l
~
[3, 3] Sigmatropic
rearrangement
(Claisen
rearrangement)
(
2
(B)
1
f3, 3] Sigmatropic rearrangement
(Cope rearrangement)
~CHO
""";l......
Citral
(C)
(A)
384
. Photoco,emistry and pericycllc Reactions
Problem 43: Complete the following sequence ofreaction and give structures of (1\), (B) and (C):
OH
~
Solution:
KH )
e
(A)
,1
--~)
Hal
(B)
e
6>
~
)
2
~
[3,3]
Cope rearrangement
3:?'
KH
)
1
,1
(B)
(A)
OH
al
OK
al
OK
OH
(C)
)
0
Tautomerisation
Hal
~
)
(C)
Problem 44: Give mechanism of the following conversion and comment on the reaction:
(i) Hal
(ii),1
(iii)
)
Hal
~
~N~
I
H
Solution:
)
Enamine formation
1
Tautomerisation
[~J
1
13, 31 Sigmatropic
rearrangement
<
H
~N+./
C4b
Vl.N~
H
H
H
~
I NH2
I;)
~
I
385
Problem 45: Give mechanism of the given reaction:
(i)RLi
..;r
U ...
•
(ii) HOHIH$
R
Solution:
RLi
CrOaIH$
(Ester formation)
)
)
1, 2-Addition
reaction
o
II
I
+ Cr-OH
~(---
OH
Problem 46: Identify (A), (B) and (C) in the given reaction sequence:
Ph-S-CI
Py
t:..
R-CH=CH-CH 2-OH --Py-----.+ (A) ---------+ (B) ---------+ (C)
Solution:
R~OH+Ph-S-Cl
Py )
SN
R~O/S-Ph
R~Q/§'Ph
Py )
(\
(B)
H
(A)
III
Ph
[2,3]
Sigmatropic shift
Ph....... S~O
R~
(
Problem 47: Complete the following reactions and give their names:
(a)
·"
-:
(c)
'0
X
0
I
t:..
)
BuLi
)
(b)
(A)
t:..
)
(B)
6\
~
2
R
(C)
O~
I
.S
t:..
)
3
1
2
1
Solution:
1
U
0
(a)
I
2
2~
°
[3, 3] Sigmatropic
rearrangement
(Claisen rearrangement)
h
~
~
I
1
I3 ~
)
3
r
l
~
:
1
I
[3. 31 Shill
I
W
)
'¥
Allylic
(C)~b(
;(
Tautomerisation
(Aromatisation)
OI
[5,5] Shift
BuU.
OH
)
[2,3)
Shift
~
l
r
)~
tJ.
6H
(B)
(A)
Problem 48: Which ofthe following known sigmatropic rearrangements would proceed readily
and which slowly? Explain your answer:
A.
<alQ.
(b)
D
Solution:
(a)~214-1
Q
H
tJ.
---)~
CN
~
~
COOEt
tJ.)
0.CN
~COOEt
HtiH.
I
I
- D
HlD
1
The above reaction is thermal reaction. It is [1, 5] sigmatropic rearrangement which
is thermally allowed. Therefore reaction will proceed readily.
2
(b)
3-:ftll
CN
~COOEt
2
.
)
CN
~ COOEt
,&
The reaction is thermal reaction. It is [3, 3] sigmatropic rearrangement or Cope
rearrangement which is thermally allowed reaction. Therefore reaction will proceed
readily.
',',j
~,
381
Problem 49: Classify each of the following sigmatropic rearrange~nts as (i, j or m, n) and
indicate which will be allowed photo chemically and which will be *hermally allowed:
HaC
(a)~
0]
(b)
(c) (yCH=CH-CH-:--C,H2 _ _~
)
~OD
a
~
",
H
)
Y
CH a
CH CH
- II
0
CH
I
.', -~.
CH2D
~)qJ
(d)
H
Solution:
~
[3, 3] Sigtnatropic'rearrangement;)
thermally allowed; Supra-Supra
(a)~~
1
0
3
(b)
~
~
1
~H
1
a
Q7'
et?O-D-Y:" "
a
oJ
etr
5
[1, 7} Antarafacial
::r a CH=CH-C,H==,
• ,C"H,'2
(c)
~
2
1
1
1
(d)
2
[3,3], Supra-Supra
,.
2 h
1
3
\
)
h
::,...
3
~
H
Problem 50: Complete the following reactions and give their mechanism:
CHa!l
Ii
·(b)~
'
)
",~,c'. ~
'";., . ..
.
1\
(c) CH 2=CH-O-CH2---CH-CH 2 ~
Solution:
.'
CH-CH=CH-CH2D
)
~
(a):~H
3
2 1
CHa
I
I
r
1
[1,5]
)
(C) 311
It
CHO
11
~2
2(,'0
)
[3, 3] Sigmatropic
rearrangement
3
U
l
Problem 51: Complete the following reactions and give their mechanism: .
(a)(j
a
)
)
Jo~
(e)
C~ CH,-CH=CH:
•
Solution:
)
[3,3] Shift
)
[3,3]
SilJmatropic
rearrangement
(Jl[~H-CH=CH,
~ C Hs
1
6
Tautomerisation
1
10 CH z ·
~CH2
(C)Cl~2q
I
~
CH z
3
)
[3,3]
Sigmatropic
rearrangement
1
Tautomerisation
I
II)'
~ f::./,,
~I
..,
I
.
Problem 52: Complete the following reactions and give their mechanism:
O-CH2-CH=CH2
(a)
O-CH 2-CH=CH 2
¢
I
":
A
~
HaC,&CH
":
a
)
I
(b)
A
)
h
NHCOC 2H s
C6H s
I
(el
Cl'¢ CH-CH-CH,
)
CH a
Claisen )
rearrangement
~fH
I~
H
¢-O:CH'--CH
Tautomerisation
C!!t CH
) 0
II
II
CH 2
~CH
(b)
2
HaC~~
CHal1 3
I 3 CH2
Claisen rearrangement
.
)
~
1
[3, 3] Sigmatropic
rearrangement
OH
y
CHa*CHa
CH 2-CH=CH2
\,
,
Tautomerisation
(
h
C2H s-C-NH
COC 2H s
lOlCH
I
II
CH2
[3~31
•.
Sigmatropic
rearrangement
Cl~~H,-cH=CH--C.H'
Y- _
CsH '
l.. ·..~tiD.
OH
Cl
V'-'::
,-C~2-CH=CH-C6H5
I '
h-
CH s
OH
y
HsCW CH s
Tautomerisation
~(-----
'~
H,CY",CH.
'",'R"
CH2-CH=CH-C6H 5
C~-CH=CH-C6H5
Prohlem 53: Complete the followlTig reactions:
(a)
CH,-CH~CH...
o
¢o
hv
)
--~);
o
HO"
(C)Gl) +
Be/'
~
OH
)
(A)
HOH)
(B)
~-
I
Solution: The above problems are based on ene reaction.
o
(a)~i)/
)
(b)
hv
O2
w
o
o
Singlet oxygen
----).)
)
OH
c?/OH
. )[a6~H]
le=O
arfJ
1
-----.+-HOH)
I
1
2
. 3
1
[2, 3] Sigmatropic
rearrangement
OJ?
+HOH
(
OH
(B)
Problem 54: Giue mechanism and name ofgiven r.eactions:
(a) HC
g 1
CHa
(i) A
+ Se02
)
(ii) H2 O
CRg
0
II
r
yS'S
(b)
.
I
to
)
~=o
Zwiebelane
'L,
(8 +
(c):::"..
~
S
to
)
HgC1CR20R
CR a
OJ?
O-Se---O-H
(A)
392
Solution:
(a) H C~CH3
I
3
CH 3
O=Se=O H
H,c~H,
+Se0 2 ~
Ene
,
)
reaction
L1
CH 3
[2, 3] Sigmatropic
rearrangement
)
o
o
~\f: S~~~i') ~
II
(b)
)
:
-
~ ~s~o
12 +21
Cycloaddition
-
iii
rearrangement
Thioaldehyde
L1
14+21
)
CS~
I S
Cycloaddition
Problem 55: Give mechanism and name of the given reactions:
(a)C
H
. 450·C
)
¢
H
230·C
CH s
)
11
V(1)
(
COOEt
+11 I
~COOEt
(2)
Solution:
(a)~
~
.­
.
Ene
. )
reactIon
450·C
H
¢
H
j
1­
CH a.......... CH
CH2-COOEt
II
230·C
I
CH
CH 2
'CH{
--~)
(1)
and
CHa.. . . .
230·C
C
CH
II
I
Ha
CH
CH-COOEt
'CH/
2
)
(2)
Problem 56: Give mechanism of the given reactions:
(a)
• >
~ + Nr"COOEt
~OOEt
Me
(blpCOOEt
(C)o
l\
• > ffCOOEt
~
+
)
o-~OOEt
hv
)
+1(
CR a
+
CR;;
Solution: These problems are based on chelotropic addition reaction.
~
(a)
COO~
N=N=CH-COOEt--~~)[CH-COOEt+N:J-_-..::::_~-~>~
)
(b)
COOEt
COOEt
1
Intramolecular
chelotropic addition
H
PCOOEt
Me
>€
(c)
Ell
N=N=CH-COOEt ~(-~)
~
Ell
e
N=~ CH-COOEt --~) N 2 + CH-COOEt
0
1
Electrocyclic
ring opening
o-COOEt
Chelotropic
addition
<
hV)~
~
..
Q><Q
H3 C
+
CH 3
Q;<Q
H3C
CR 3
Problem 57: Explain the fonnatwn ofproducts in these reactions:
(a)
lYJ
O~~
£@H
MeAlCl2
)
0
~
o 0
II II
",
'H
o
CH
II 2
(b) CHao-c-e-H + e-CH a
.
I
CH a
Ti(OR)4)
Meo~
OH
"
i."
H
{cl~O
~OH
~
(d)C)=O +
(el6 +
0
~
o
)
'. o?
)
.
COOEt
COOEt
Solution:
(al~
CHaAJ.C~
)
Ene
reaction
)
(c)~
Ene
reaction
)
~O
if
(dle
(el
16+41
Cycloaddition
rea:tion
16+41
Cycloaddition
d~
~
~OH
~
~':,"".
)
(
)=0<) -~
o
rh
vy
. COOEt
COOEt
Problem 58: Complete ihe following reactions and give their mechanism:
o
\I
"
"
(a).yC~
hv
313 n.m
)
(b) CHa-CHO
hv
)
25n.m .
,
Solution:
o
(a)
A
hv
)
Ha
l
r
CHa-C- + CO
-+
+
313nm
a.·Cleavage
I
.~
f
,
I
CH a
CH a CH a
Dimerisation
)
I
I
I
I
CHa-C-C-CH a
CH a CH a
Elimination
)
hv
)
254nm
[CIj,,,
~+iI] ~
[CiI,+H"+COj
vv
~
CH.+CO
Problem 59: Give mechanism for the following reactions:
o CH a
II
I
hv
1
313nm
Solution
CHa-----:-C-C-CHa
(a)
CH
a
)
o
II
(b)
hv
C6Hs-CH2-C-CH2-C6Hs ~ C6Hs-CH2-CH2-C6Hs + CO
o
CH a
(c) ( (
.
CH a
hv
--~)
I
OHC-(CH2)a-C=CH-CH a
CH a
Solution:
o
(a)
I
CHa
I
CHa-C-C-CHa
r~H,
hv
)
a-Cleavage
o
CH a
I
I
CH a-<; + CH a
-?­
CHa
Weaker bond due to
sterle hindrance
,I
Disproportionation
reaction
0
II
/CH a
) CHa-C-H + CH2=C""
CH a
a-Cleavage
)
hv
o
(c)
cX
ql
0
CHa
hv
)
a-Cleavage
CHa
H
CHa
~
HCHa
I
'
O~C~
+
~;;C~
Problem 60: Complete the following reactions and give their mechanism:
(a)
o~
(c)5!0
hv
(b)
hv
Vapour phase>
.k../.O
v---Y
(dlrJr°
)
hV)
Vapour phase
hv
)
Solution:
(alo~
o~
hv
)
a-Cleavage
Removal of CO
)
.~
(1)
(i)
ail
~:
..
Js ~>~
Diradical
.
.;fs
Formation
of
new
diradical
)
(3." ]
)
2)
I
(b)
(5y
~.
5
o
(d)
)
a-Cleavage
Cyclisation
)
(/i---'6
4
(c)
l
Removal of CO
hv
---~)
~O
y
hV),}O o~O
.-Cl.a~g. ~
Deca-elboOnYlation)
~ro
(.;<
X):
~O _~hv_~) H~ ~O _~) ~ ~~
ltJ
l.tJ
ltJ.. . .
a-Cleavage
Problem 61: Give mechanism for the given conversions:
o
(a)O¢{)
hv
o
OH
~
Solution:
o
o
(a)O<¢<O
hv
)
a-Cleavage
[o--~ ~-O1
o
2CO +
0
1
(X)
o
d;p1
r~latiOO l
o
)
6
(b)~
,
~.
1
2CHg-CH2-D-CH2-CHg
Hydrogen abstraction
OH
~
.
+ [2 CH 3-CH-O-CH 2-CH 3]
1
Dirnerisation
CH 3-CH---0-CH 2-CH 3
I
CH 3-CH-O-CH 2-CH 3
Problem 62: Complete the following reactions and give their mechanisms:
o
OH
II
I
+ CH 3-CH-CH 3
(a) C6H 5-C-C6 H 5
hv
)
OH
(b)
r()f
CHO
I
CHg-CH-CHg
~COOH
)
hv
Solution: These two reactions are intermolecular photo reduction in which isopropyl alcohol
works as hydrogen donor.
. ­
(a)
..
,.
o
II
CSH 5-C-C sH 5
hv
~
8 1 (n
ISC
~ n*) ~
T 1(n
~ n*)
r
.
OH
I
(ii) CHa-C'
I
OH
I
I
•
I
I
I
I
CHa-C-C-CH a
-~)
+ CHa-C'
CHa
CHa
,
OH OH
CHa CH a
,
1
1
6
o
I
II
C-H
hv
(b) ( (
I
~
C-OH
r?\TC-H
~.
C-OH
)
II
II
o
o
Hydrogen transfer
l
CH 3,
. /CH-OH
CH 3
OH
OH
I
I
OH
C-H
I
©C .
+ CHa-y-CHa
C-OH
-~)
II
o
1
III
OH
OH
I
I
CH
CH
°
©CC-OH HO-C)QJ
L
II
o
II
0
Ester
formation
0)
0
OH
I
I
CH-e-CHa
© C COOH CHa
Problem 63: Provide a mechanistic rationalisation for each of the following reactions:
o
( ) Phx.Jl ~Ph
h'ol,
a Ph~Ph
>
)
I
o
(b)Ph~Ph
,
Ph
Ph
Ph7Ph
hv
)
Ph/U":'-Ph
.
I
o
II
Jl/~CHg
hV)
LJ
CH g
CH~gCH',
-.
CHg
hv
)
,,)
,
'-".
o
Solution:
o
0:' .
"',
Ph,YL./Ph
hv
(a)Ph~Ph,
Ph,.0l •./Ph
)
P h ' , ' Ph
Ph '>r------,<' Ph
Ph, l-J 'Ph
Cyclisation
)
.
Ph
Ph
.,;~
Ph~ '.
(ii) Ph,
)
-'0
(b)Ph~Ph
Ph/U"Ph
C) Ph>,.
l
Ph
V
hv
)
a-Clea.vage
.r<Ph
Ph
Decarbonylation
Ph~Ph
a-Cleavage
(i) PhD'<Ph
-,
II
' .
Cyclisation
)
"j..
~:
-.
..
",
I
-co
)
Ph
Ph
Ph>LfPh
(c) The reaction is 1,3-acyl shift
n~CHa
~CHa
hv
~ n~CHa
~CH
'CR:
~
"fi \.. . CHa
o
o
~. r-Unsaturated ketone and
t
o
0
1
Allyl free radical
Ring closure
hence it will give a-cleavage
Y"<CH,
CRa
0
(<l) The reaction is 1,3-acyl shift
0&
CRa
~,
•
&
2
1
eRa
CR a
6
7
9
'Y.Unsaturated ketone
hence a-cleavage
oS
CHa
1
Ring
closure
eRa
o
Problem 64: Provide a mechanistic rationali.8ation for each of the following reactions:
CHa
I
C=O
+
hv
o
(c)
0
hv
Cyclohexane
~
+
(II)
...
t:~··
~
I'
..... ~
'II'.
Cf~
Solution:
(a) The substrate has hydrogen on y carbon hence intramolecular hydrogen abstraction
is an important process in this reaction.
b)~H
II)
3
4
(Is/R
CH g-C-C-CH2-C"
2
II
R
CH 2
11,
5·Hydrogen shift
OH
I
./R
CH 3-C-C-CH 2-C,
• II
'R
i CH2
<
)
Allyl free radical
1
Cyelisation
Cyclisation
OH
I
HaC-CLJcR
Tautomerisation
)
R
I, 5-Hydrogen
Allyl free radical
<
)
shift
)
1
CH g
.
(
Tautomerisation
rAOH
R~H
R
(c) Cyclopentenone abstracts hydrogen from hydrogen abstractor i.e., cyclohexane.
6
hv
)
.6
o
OR
)
.6
+
~T'UtD;non..tion>
OH
.
(i) [ ]
·0
+
o
(ii)O
;
t
&0..
·0 -~.
+
J
•
(II)
Problem 65: Provide a mechanistic rationalisation for each ,of the following reactions:
(a)
Ph
hv
)
.
Ph
~
o
09"
o
(c)~
.
1
~Ph
hv
(b)·
.
HOlt<}>
.
9 1\'
~
)
Ph
o
~
hv
Solution:
)
. . ..
.,.
(a) This compound gives Norrish Type-II reaction in which intramolecular hydrogen
abstractiQn by the carbonyl oxygen .from y carbon takes place
.r·,
1Y
°
hv
)
J~
Ph '.;
Hydrogen abstraction
from y-carbon
1
ex
HO~
.Phl\(\
OH
4<-'....;Cy:--'b_li_Sll_ti_on_'_·
0
I~·
p
Pli"'oc
ex
(b) The given reaction is an example of Norrish TypeH reaction
~Ph
hv
)
H~
9
'Y. Hydrogen abstraction
by carbonyl oxygen
Ph
.
o
~Ph
.~
OH '"
(
Tautomerisation
~Ph
)
(
~-Cleavage
·C-Ph
I
OH
. (c) Photolysis ofacetylcyclopropane leads to breaking ofthe cyclopropane ring (~-cleavage)
followed by hydrogen abstraction from the y-carbon,
~
0
H
0
r-!i ~"~CHz-CH-CH-C-CH3
hv
•
I
• I
L?<i
y
~
y
o
Hydrogen
II
abstraction Bond form,ation
,between ~ arid yCHz=CII-CHz-C-CH3
from
(Minor)
y-carb~n.
o
a
II
"Bond formation
CH
-CH=CH-C-CH
3
3
between a. ana y
(Major)
Problem 66: Give step-wise product formation in the given reaction:
6~ o
+ CH3~H2~H2-CH=CH2 + CH3~H=CH2 + CH2=CH 2
",I,
-
•
.
.
+ CH2 ' CH~H2~H2~H2~HO
Solution: Cyclic ketone' undergo~s",a-cl~av.ag~to give. biradicaL The biradical after
decarbonylation may recombine, abStract a: hydrogen or'undergQes secondary fission reactions
to yield hydrocarbons. Abstraction of hydrogen in biradical ta,kes place from a-carbon.
6
0
1,.
,.6.
-
hV)
Decarbonylation)
-co
·u·
+
co
.Diradi:eal
(i) Recombination
(ii) Abstraction of hydrogen atom froin a-clirbon
.
.
C~-CHz:"':""CHz-CHz-CHz
a
1
CH2=CH~CHz-CHz-CH3
(iii) Secondary fission reactions
CHz-GHz+CHz-CHz-CHz
1
. I
I
)9. ~
\
)
(iV)lA
H
Problem 67: Provide a mechanistic rationalisation for each of the following reactions:
<O)Q
6<
,=
h,)
D
hv
(b)Ph~Ph
)
~
,
Ph~D
~
Ph
0.003
(I
hV)
~
256nm
Ph
1
Triplet
sensitiser
I
Ph
80%
I
(A)
Lp-Ph
Ph
,=
0.20
(B)
Solution: These problems are based on Di-7t methane rearrangement.
Formation of new bond
between C-2 and C·4
(a) 2 Q 4
1
5
6
1°
°5
6
5~1611
4lA<
2
~
=
1
2 3
cp =0.003
D
(b)
2Q34
)
I
/\4
4
(
Formation of new
bond between C3-Cl
6 5
D
~
5
Formation of
bond between
D
~CHhQ)
~
a'
D
Q--\;O
.
..
2
1°
F\I~
~
~
Bond breaking
lbetween C3-C4 and
Bond formation
between C4-CS
~
6
4
5
Bond formation
between CZ-C 4
I
Bond breaking
between Ca-C4 and
bond formation
between C4-C 5
1
I
(c) In singlet state
~
~ ~.
Ph
~Ph
Ph -----+
~Ph
10
~
Ph
~Ph
Ph
Ph
(A) 80%
Triplet sensitiser
~ Ph _
hv
~Ph
)
I
Formation of new
oJ, bond between Cz-C 4
Breaking of
Cs-C4 bond
and formation
OfCI-C3 bond
Problem 68: Provide mechanistic rationalisation for each of the following reactions:
(a)
A
hV.) CL:)
(c)~
hv
(b>Op
hv
)
)
Solution: These problems are based on di-1t methane rearrangement.
(a)
A
~
8
2
1
3
~
6
4
5
Bond formation
between C-2, C-4
83
)
2
~
I"
6
4
5
Bond breaking
between C3-C 4
and bond formation
between C-4, C-5
j
.....
3
2
8CL:)4
.7
6
5
E
Bond formation
between C-1, C-3
6
.
2mt
7
1·
6
5
3
2
(b)
:
OJ?
4
4
I
Q:;t.J
f6 ,
:::,..
1
3
:::,..
5
0,
1
3.
05
6
7
7
Bond breaking
'betwe~nC3-C4 and
bond formation
between C.-C s
(
(c)
0
hv )
2~7
1
Bond form,a"tion,
betweenC r',,,,C 3
Bond 'formation
"between C2=C4 ,.'~
,
"
"
)
2
~
3~:
~"7
i
5
,
'5
~:t~e:~e~~in an~
,1, bond'formatiol1
. 9,,\i
;,." .
'" ' ,', "
between C4-.C S
.'
,
6"'
~7
1
(
Bond'formation ,,',
between C1-C 3
5
.".",
2~7
,1
5
,',"
Problem 69: Give mechanism and product of thereactitm:
;
~.~?
Solution: Compound can be treated as 1,4 pentadiene'derivative. Hence reaction: will be di-1t
methane rearrangement.
10~98
7
3
2
1
4
6
I
5
9
9
1O~
7
hv
)
3
2_
1-
6
4
5
Bond formation
between Cr-C4
1O~
7
)
3
2
1-
4
6
-5
'~
-l.
i.'
I
"']
Problems arid Theii' Solutions
~
LJLU4
10
7
.
6
(
3
Bond formation
between C1-C 3
.
9~
10~98
7
2
LJLLJ4
5
10
7
6·
I
409
l'
5
.3
4
6
5
Problem 70: Complete the following reactions:
o
II
.
(a) C6 H s-C-C 6H s
(b)
~
. CH 3 "",
+
hv
/C=CH 2
)
CH3
hv
+ Acetone
)
hv
Solution: These problems are based on
hv
--~)
1
)
12 + 21
[n, 1t*]
(c)~
Benzophenone
(e)~
Benzophenone
hv
hv
)
)
photocycloaddition and Paterno-Biichi reaction.
)
3
[n, 1t*]
+
(More stable)
1
(i) Spin inversion
(il) Recombination
Ph . . . . O-CH
I I ;,-CH 3
....... G--C/
Ph/"
~CH3
(Major product)
+
(Minor product)
j
(b)
Acetone
hv
hv
~ 1 [Acetonel ~ 3 [Acetonel
I
3
+. [Acetone]
(c)
Benzophenone
hv
~~~)
1
[Benzophenone]
~-~)
3
[Benzophenone]
1~
~:~
CH
6 5
(d)
C6 H 5
(e)
~
( _
>~-O
...-Ph
C""'-
hv
+ ;111
Ph
)
'~C"""""""Ph
~
Ph
~
0
Ph
O=C-Ph
__~Ph
I
)
)C=C-.-Ph
Ph
Ph
Ph
(i)~~C<
"---.~ Ph
)
~
hV)
~
12+21
Cycloaddition
Problem 71: Provide mechanistic rationalisation for each of the following reactions:
R
<al¢
R
hv )
Q
(A)
6'R
R
R
+
R
(B)
l
"I
I
I
I
I
I
R
R
(b)~
hv
~R
¢
)
i
R
Solution: These two reactions are examples of isomerisation of disubstituted benzenes:
:¢:
R
<oj
Cy~~:d~tion
R*R
h'.
R~2
:~
•
R
6
Prismane
1
R
Q
R~f)JL
4 1
-
(
R
5
6
Open-book structure
R
(A)
Formation of B
R
R
¢
6S:J1
2
1
5 --- ~-- ,
12+21
Cycloaddition
•
R
R
Prismane
1
R
Q-R
(
R~
R
Open-book structure
R
R
hv
R
)
d:l
R
R
~.
I
CflR
t
¢~( R~R
R
412
Problem 72: Provide mechanistic rationalisation for each of the following reactions:
Ph
(a)
~
Ph
Ph
Ph~Ph
hv
Ph
Ph
(b)~
hv
)
Ph~Ph
Ph
)
o
+
+.
(5'
(B)
(A)
(C)
R
hV)
~R
R
1,2,4,5-Tetra
alkyl benzene
R
1,2, 3, 5-Tetra
alkyl benzene
Solution:
hv
)
12 + 21 Cycloaddition
Ph¥Ph
Ph~Ph
Ph
1
Ph
Ph*Ph
(
IhPh
Ph
Ph
(b) Formation of A
hv
)
12 + 21 Cycloaddition
Nor-bornadiene
~
~
Quadricyclin
I ­
,I
.;....
...:
Formation of B
o
)
Retro Diels-Alder
reaction
+ CH=CH
B
Formation of C
CH 2
hv
6
-
)
1
H2 C CH
()
R
R
J:XR
1
R
R
R
R~:
(
R
R
Problem 73: Give mechanism of the given reactions:
o
hv
H3CO~
.. I
I
).
Rtf
R'
R
o
(b)
.
H CO 0
~
A0
H3CO~
n3'-'~
RaOC,
HOH H C-O-C
U~D-J~
LlJ] ~ a
w
0
).l
LU
~
~
Solution:
hv
(a)
)
a-Cleavage
1
HaCO 0
Bond formation between 3-3 and
bond breaking between I-I
2
Cope rearrangement
~<
__
~-1'--""(1
R'
R"
R
rl$R
R'
o
_
H3CO~2
R"~
R'
R
o
hv
)
IH,'C:o
hu
)
a-Cleavage
o
OC\:::63
I
<
(B)
Cope
rearrangement
hvlHOH
~
o
CHaO
H co'-H
H
0
I
-~)
~
H3C-o-llJ]
Jl'
(C)
,\
.i
t.
,­
Problem 74: Give mechanism of the given reaction:
hv
)
Solution:
0
(
+
hv
0=(
)
I2 + 21 Cycloaddition
lhv
,·Ct
4
3
4CJC(
5
67
(
4 ~
5
8
7
1
.6t
2
~.
5 6
2
3~
(
4""",- •
6
5
6 7
7
8
Problem 75: Two sequential pericyclic reactions are involved in the 3,4 dimethyl furan synthesis.
Identify them, and propose a mechanism for the transformation:
+
)
Solution:
14+21
Cycloaddition
1}(
Ph
)
CH
3
CH 3
1
Retro Diels-Alder
reaction
H3C
:Co
+ PhCN
H3 C
Problem 76: The following rearrangement of N-allyl-N, N-dimethylanilinium ion has been
observed. Propose a mechanism for the reaction.
)
Solution:
.
(3,3) Sigmatropic shift
--------'.~)
.
A
.,
~ ~
...
Ell.......CHa
::;.--N'CH
d
~
:::::.....
H
a
CH 2
I
I
II
t
t
CH
1
CH 2
1
Aromatisation
o:
: : :. . . I
I
Ell
NH(CH a)2
'.
CH 2-CH=CH 2
Problem 77:Provide a mechanistic rationalisation for each of the following reactions:
."
,
!
•
t
1
1
(i) hv, Pentane
l
(ii) CHaOH
~
(b)
.
CH
CH 2 COOCH a +
a)C=CH-CH=CH-CHO
CH a
o
0
OCH a
c=(C. . . CHa
CH~~H
)
. . . . CH a
-,
"
ex'
CH
C- a
"CH
a
Solution:
o
X/CHa
(a)L]'CHa
hv, Pentane
)
a-Cleavage
1
Hydrogen transfer
CHa
)C=.CH-CH=CH-CHO
.
CH a
. .'
j
.
&
o
0
&
C Ha
~
CHa Rearrange~~
CH3 ---~).
CH3~
hv
CHgOH
Norrish Type I
reaction
A
------~)
A
CH OH
3
CH=C=O~
L(
•
0
C
CH3 0H
CH COOCH
2
~
)
. . . . . CH a
'CH3
1C~OH
Problem 78: Explain the following observations:
o
o
I~'l~)
)l/OMe
Ll
Cl.J
)
(A)
Solution:
Formation of (A)
HeC~0
2
[3,3]
Sigmatropic
rearrangement
3
3
1
~,- etY
o
3
)
4
5
2
3
I
1
7
5
6
(A)
Formation of (B)
..
o
;'l)
o
hv
)
a-cleavage
~
~
~'CI ~I
III
o
HCO......-ll----l3
;'Cl ~
o
o
<
~
HCO.........rrJ
3
OCH3
3
-. . . -- .. -
..
,
I
-.".
418PhotochemiStry"and PericyClic Reactioi1s,
I
Problem 79: Give mechanism of the following concerted reactions:
I
)
(b)~
~
Solution:
\
)
l
H
H 3 C"",-
It 31 >
Sigrnatropic
shift
Electro­
cyclic
closure
H;:"'C=lrr
~
"",C=C=C
)
H
cP
H""
H
(b)
~
~
Disrotatory
closure
,;;7
"":
---h-v~-~)::::""
controtoryl
Closure hv
~
H
H
Problem 80: Prdiet the product(s) and give suitable mechanistic scheme for the following
photochemical transformation:
o
II
hv
(a) CH g-C-CH 2-CH Z-CH g ~
hv
(b) CH 2=CH-CH=CH 2
l
hv
Sens
cProbtsms and Their Solutions
XX
D
(cl
Ph
Ph Ph
D
hv
)
(d)
c=:C>=o
419
hV)
Ph
CH g
~CHa
(nil)
hv
)
Ph
;1
(hlo?=0
(i)CX
Solution:
Ph
o
hV)
CHa
CH a
II
p
a
b
~
hv
Y
CHa+C+CH 2-CH 2-CH 3
(i) Breaking of bond 'b'
2CHaCO'
Norrish Type I
.
CHaCO + CHa-CH 2-CH2
II II
At above 100°C temp.
CHaCO'
)
CHa-C-C-CHa
o
\i
)
o
a
(a)
hv
~
0
.
CH a + CO
2CH a ~ CHa-CHa
(ii) Breaking of bond 'a'
~..
(
~-k\
CH a-CH 2-CH2-C ~ CH g-CH2-CH2- C
o
I
a
P
Y
(iii) CH 3-C-CH 2-CH2-CH a
.­
..
Norrish Type II
hv)
CH?
CH-C/ ~"""""CH
a ~I
I 2
~
. . . . CH 2
~H'
y-Hydrogen abstraction
l
~I
"
I
I
r
Ring closing
(
[J
hv
)
Through singlet
state
Cyclobutene
+<1>
Bicyclo [1, 1, 0]
butane
nJrn
~
~~.
11,
D
(d)
D
H
~:x:1~
0:)=0
.
~
2 Shift of H
H
~V~
hv
)
~O
1
Hydrogen
(e) R-CH2--CH~H2-C-NH2
CD
tra~sfer
0/=0 +
o
II
~CO)
c::V
CHO
12 , Pb(OAc).
hv
)
(N-Iodoamide)
!hV
P
a
C¥2--C~2
R-eH'Y
C=O +
~.k
r
.
/CH2-C~2
Abstraction
ofy-hydrogen
)
R-CH
/
I·
C=O
/CH2-C~2
R-CH
C=O
)
I
NH 2
I
NH 2
I
(y-Iodoamide)
(g)
jr
CH2-CH2
I
R-CH
I
I
I
OH
l-HI
,
R~O
H 2O
<
R~NH
y-Lactone
&
I:
(f) (i)
hv
3
)
H'C~"CH'
CH3
CH
CH
(h)
'11]
Dewar structure
Prismane structure
H 3C
&
1
CH,~CH,
(
CH3
Open book structure
Meta isomer
CH3
(ii)
"
I
C=NH
&CH,
hv
~l
¢'
CH 3
Para isomer
)
H,ClJJ
)
H 3C
CH3
HC
CH 3
(
CH'JJ
~
CH3
-
3
~
nJl
H 3C
r
I
422
HaC'r----( CH 2 C6 Hs
~
Dimerization
Ph-CH2
(g)
H
>
<CH a
C=C H
HaC
hv
CH 2 C6 Hs
Isomerization
Ph-CH2
H
Ph-CH 2
+
>
>
C=C
<CH a
H
C'tS
C=C
H
<H
CH a
Trans.
Ph
Ph
(hlO:>=0
hv
)
Norrish Type I
I
-.: :
c¢
C=O
b-
••
Ph
Ph
t-
\
co
~Ph
(
~Ph
hv
)
Norrish Type I
a
1
-co
2Ha
CHa
)
Hydrogen transfer
q~H,
+
+
CHa
Problem 81: Write the products formed in the following photochemical transformations:
o
0
(a)~
(c)~
0
,­
."
II
hv
)
(b)
hv
)
Pyrex
(~
OMe
I
hv
Ph-e-CH-Ph
)
0
cP
Q
•
hv
)
366_
~,".
'l OMe
+
COOEt
(f)O
hv
)
hv
+
COOEt
Solution:
o
, II
o
,
a-Cleavage
(a) -C-C+C-
I
I II
I
I
I
-C-C' + 'C-
)
I
)
III
Norrish Type I
(i) At temp. above lOO°C decarbonylation of free radical takes place with the forma­
tion of tertiary free radical and carbon monoxide.
o
I II
-C-C'
I
I
2-C'
I
--~)
-C'+CO
I
I I
I I
-C-C­
--~)
I
(A)
o
I II
(ii) 2 -C-C'
I
o
--~)
0
I II II I
-C-C-C-C­
I
I
(B)
(iii) By hydrogen abstraction
o
I II
I
-C-C·+·CI
I
I
I
-G-H
+ "­/C=CH z
)
I
(E)
o
OMe
I
II
(C)
(D)
I
I
-C, +'CI
I
o
I
-C-CHO
+ "­/C=CH z
)
II
hv
(b) (I) Ph-t+CH-Ph
(t)
o
o
II
2Ph-C'
--~)
OMe
I
2Ph-CH·
II
0
II
Ph-C-C-Ph
OMeOMe
--~)
I
OMe
I
Ph-C' + QH-Ph
)
Norrish Type I
.
(D)
I
Ph-CH-CH-Ph
(ii) At temp. above lOODC
o
II
Ph-C·
~
Ph + CO
2Pn
~
Ph-Ph
•
OMe
OMe
I
o
I
Ph-CH-Ph
~
Ph +·CH-Ph
(II)
o
OMe
Ph-C-CH-PH
" I
2Pn
)
I
II
2Ph-CH-Q
OMe
I
Pli.+"
·C-CH-PH
hv
---~)
Norrish type I
Ph-Ph
OMeO 0
OMeO
(III)
1
~
OMe
Ph-CH-C-C-CH-Ph
I "" I
Also undergoes Norrish Type II by involving y-hydrogen abstraction.
Ph-C---c..CH-Ph
I
OH
1>
~(
·C-H
I
H
(I, 4-Biradical)
Ring closure
1
OH
I
Ph-C--CH-Ph
I
I
H2C
HCHO + Ph-C=CH-Ph ~ Ph-C-CH 2-Ph
I
0
II
OH
0
(c) (i) Breaking of bond 1
~
~
o
BYbreokingof7 "1
-hv
Norrish type I )
.
Pyrex
•
0
Qq
(A)
(Ketene)
0
(B)
0
(Unsaturated aldehyde)
:'1 ,
(ii) Breaking of bond 2
~
o
Norrish Type I
~
)
~
1
(iii)
OR
G:::J
(iV)~
o
Q
0
(d)
<I>
~
II
3
o
hv
c:p
hv
Intranlolecular )
hydrogen abstraction
from y-carbon
(Norrish Type-ii)
OR
OR
0
OR
6<: + Cc~ + ~
)
1.0
366 nnl
<I>
OR
.0
cjl
cjl
<I>
0
(e)
,
0 +l
(f)O
0
0
hv
)
Photocyclo
addition
reaction
OMe
(p-OMe +
COOEt
+
hv
'II
(2 + 2) Cycloaddition
COOEt
"
Q
)
~OMe+
~COOEt
~
COOEt
1
Electrocyclic ring opening
COOEt
~ /- COOEt
Cis-isonler
(
C
~
(COOEt
Trans-isonler
COOEt
S-oMe
Problem 82: The given compound (AJ is extremely stable is spite ofthe fact that its rearrangement
to toluene is energetically favoured. Explain why the rearrangement is so slow?
C)=CH z
--~)
o - C Ha
(A)
Solution:
l
~CHz
(~
\A)
During the rearrangement 1, 3-sigmatropic shift of hydrogen is involved. The [1, 3]
migration is allowed to take place only antarafacially but such a transition state would be
extremely strain and geometrically not fissiable. Due to this reason process is slow.
Problem 83: Vinyl substituted cyclopropanes undergo thermal rearrangement to yield
cyclopentenes. Propose a mechanism for the reaction and identify the pericyclic process involved.
)
o
Solution: This is degenerate Cope rearrangement. This rearrangement is given by cis-vinyl
cyclopropanes. This rearrangement is not given by trans vinyl cyclopropanes. The mechanism
of the transformation is as follows:
lC/;5
~
Z
)
Problem 84: What is difference between sigmatropic migration of hydrogen and alky or aryl
groups?
Solution. Hydrogen atom has its electron is a 1s orbital which has only one lobe. A carbone
free radical (for imaginary transition state) has its odd electron in a p-orbital which has two
lopes of opposite sign.
.
1, 5 Sigmatropic rearrangement
R-CHz-CH=CH-CH = CHz
R"
1
CH z-CH=CH-CH=CH 2
In thermal condition 1, 3-pentadiene free radical has m-symmetry.
During thermal suprafacial [1,5] process symmetry can be conserved
only when the migrating carbon moves in a manner that the lobe which was
originally attached to the n-system remains attached in TS.
Thus a suprafacial [1, 5] thrmal rearrangement proceeds with retension
of configuration at the migratory carbon.
_1
.
~
e"~
(;(i7J
A
l
r
I
I
Problem 85: Predict the product of each of the following reactions and also give name of the
reactions:
380°C
( ..
n
)
)~!J
!
100°C
)
CH 3
o
MeO
KH, Dioxane
(v)
~
KHlTHF
(iv)
)
(iiO'?-OH
)
30'C
~)
Solution: These problems are based on [3, 3] sigmatropic shift and reactions are Cope
rearrangement.
~; Cope
rearrangement
(iil~
CH 3
~
~
)
Cope
rearrangement
CH 3
0
(iiil~OH
c<JJ
)
(ilCCY
0
KH
)
Anionic oxy-Cope
rearrangement
~.
2
11 2
)
0
3
~
Tautomerisation
<
o
(iv) MeO
KH
)
I
1
Anionic oxy-Cope
rearrangement
l
OMe
'
..
MeO
Tautomerisation
1
(
n~
3
(V)2
o
~, Cope
rearrangement
3
2
)
Problem 86: Complete the following reactions:
(i)~
hv
+ CH,==CH,
)
o
o
(ii)
Qo
hv
+ CSH 5-C==C-C sH 5
)
o
OCOMe
JI
II-U
o
(iii)U
C
hv
Cyclohexane
)
.11
'f
Solution: These problems are based on
(oQ==J
hv
+ CH 2=CH 2
12 + 21
cycloaddition reactions.
~
~
0
0
Ph
I
0
(ii)~D¢O
Ph
(iii)
hv
N°
~
0
0
OCOMe
~
II
hv
~
)
C
0
0
~
o
C):\~
CH
II
(iv)~r-CH3
2
~~CH2
CH g
""CH
hv
(Intramolecular
12 + 21 addition)
~
2
Problem 87: Complete the following given reactions:
CH 2
(i)
$
+ OHG-C==G-COOMe
Ell
L1
HOHIH
> DCC)
~ ---~7
OTs
COOC2H 5
(ii)
cr
I
I
CH-CH2-CH2-CH=CH2
300.C
~
Solution: These problems are based on ene reaction.
CH2-CH-C=C-COOMe
CH 2
:=:-\
H + CH-C==e-COOMe
(~
(i)
I
OH
_6~)
e
HOHIH
)
o
o
CH 2-CH-C=C-COOH
I
CH2-CH-C=C
1/
OH
DCC
)
O-C=O
o
o
)
~
(iii) CH 3-C=CH 2 + CH
0I :J'"I
1
CH 2
C
H
COOMe
A
y
3
AC20/BFa
------+)
~-CH2-CH2-0Ac
CH2
Problem 88: Identify (A) and (B) in the given reaction sequence:
CHs-eH=CH-CH20H + CCIs-e=N
K2CO a
11
11
) (A)
~
(B)
..
1,
Solution: First step of the reaction is addition reaction between alcohol and cyanide to give
an imine. The second step is [3, 3] sigmatropic rearrangement.
CH3-{;H=CH-{;H 2-O-H + N-C-CCI3
1
K2 CO a ltJ.
CH3-{;H=CH-{;H2-O-C = NH
I
CCl 3
(A)
III
H3C~:
'
01
H 3N
---~)
CH 3-CH-CH=CH 2
I
2
NH-C-CCI3
CCl3
o
II
(B)
Problem 89: Give mechanism of the given reaction:
LDA
)
20°C
Solution: Mechanism of the reaction is as follows:
[2,3] Shift
)
Problem 90: Complete the given reaction and give its mechanism:
t
BOK )
25°C
y
Solution:
HS C2
[2,3)
OCOCeHs
~
Base)
Shift
)
81 V
CH IH-CH 3
12
2
3
Hce
1
COOC 2 Hs
Sulphur ylide
Problem 91: Complete the given reactions:
Ar
1
(i)
in situ
HAH
+ CH 2=CH-CI
MeOOC
COOMe
)
o
(ii)~
CI
(iii)
Q
+0
o
/
+ CH 2=C""
L1 (in
situ)
08i(CH 3h
L1
)
CH(OCH 3)2
Solution: These problems are based on cycloaddition reactions
Ar
I
(i)
H~H
MeOOC
A .
)
COOMe
I
Ar
(
CH 2=CH-CI
1, 3-Dipolar addition
I
H
MeOOC'C/N . . . . C/
H . . . . 6l
e 'COOMe
1, 3-Dipolar species
I
r
i
°II
Base
)
CH -CH............ C"--CH-CH
s e~~~
s
1
R)
SN2
e
o
I
6l
CHs-CH=C-CH-CH s ~(--
1
C
CHs-CH<1 "--CH-CHs
o
e
II
EB
CHs-CH-C-CH-CHs
~CH2
)
~.I /OSi(CHsh
c". CH(OCHs)2
Problem 92: Provide a mechanism for each of the following transformations:
Ph
-\
(i)
Ph~ 0 + Ph-C=C-Ph
Ph
~
Ph
~
Ph
)
P h + r Ph
Ph~Ph
Ph
I
I
I'
i.
J
j
j
434
I
II
NyN
CHaO"
CH 0/
a
l
COOMe
o
+
I
1
COOMe
COOMe
N~N
rI1
N~Ph
LJ
COOMe
(iv)
)
+ Ph-C=CH 2
(m''') I~N
II
NyN
II
N7COCHa
II
_ _~)
C=CH-C-CH
N a
a
OMe
COOMe
COOMe
Solution:
Ph
Ph
(i) P h * 0
Ph ~
+ Ph-C==C-Ph
14+21
cycloaddition
Ph~Ph
)
.Ph~Ph
Ph
Ph
1
Retro Diels­
Alder reaction
Ph
Ph:¢cPh
I
Ph
fi
+ CO
Ph
Ph
(") ¢N
CH a
LL
II
[4,2]
Cycloaddition
.
)
::::"...N
CH a
1
Retro Diels­
Alder reaction
CH a
~~.
Y'NMe 2
CH a
+ N==N+MeOH
n
[4 + 2] cyc1oaddition
reaction
~N
(iii)
~
~
y
+
)
Ph-C=CH 2
I
U
COOMe
1
Retro Diers­
Alder reaction
~
+N=N+OH
N'f'Ph
COOMe
COOMe
Diels·Alder
reaction
)
(iu)
~~IC~CHs
N N
OCHg
CH g
1
Retro Diels­
Alder reaction
COOMe
g
N?=COCH
II
+N=N+CHgOH
N.o OCHg
COOMe
Problem 93: Give possible mechanism for the given c<!nuersion:
Basel~
)
Gl.CH=CH 2
I
CH3
Solution: First step of the reaction is formation of sulphur ylide which undergoes [2, 3]
sigmatropic rearrangement. Reaction takes place as follows:
Base
)
Gl.CH=CH
I
CHs
2
~CH=CH2
I
CH 2
e
Sulphur ylide
[2,3] Shift
)
Problem 94: Complete the given reactions and give their mechanisms:
(alQ
(bl~
hv
)
hv
(c)~
VLAJl)
)
Sens
~j
hv
Solution:
(a) This is an example of Di.1[ methane rearrangement which is given by 3-substituted
1,4-diene.
:Q:
hv
'Q'
)
•
•
Q.
Bond formation
)
between C2-C 4
1
.
cA xry
=
Formation of
new bond between
Ca-C 1
)
Bond
brooking
between
Ca-C 4 and
p.ew bond formation
between C4-C 6
9
(b) This is an example of Di-1! methane rearrangement.
hv
Sens
)
~.
~.
~.
Formation of new bond
between C:r-C4
)
Bond breaking
between C2-C 3
and new bond
formation between
Ca-C s
I
-I
4
Formation of
new bond between
C-2 and C-4 carbon
(c)
)
Breaking of C2-C3 bond
and formation of new
bond between C-3 and C-5
1
Problem 95: Complete the following reaction:
(a)~
hv
---,--~)
T\Sens
(b)~
?
Solution:
(a) In presence of sensitiser this compound give Di-n-methane rearrangement by triplet
state.
i
~
7
::::,...
1
7 6
2
hv
1
Tl Sens
)
4
~
7
I
::::,...
76
·2
-;?
)
1
~
1
•
Formation of new bond
between C-2 and C-4
~
I7
3
6
•1
2
B".king of
C2-C 3 bond
cruz
3
:::-....
7
4
-4
6
1
-
cJP:)'
~ I7
6
4
Formation of new bond
between C-3 and C-5
(
1
~'
~ I
7
6
1
(b) In the absence of sensitizer this compound give bicyclic compound by singlet state.
~,
h',~,
~~5
~
Sj
165
,
~I
I
~
In absence of sensitized first 2 + 2 cycloaddition takes place followed by 3-bond break'§.
Although this compound have 1,4-diene system but in absence of sensitiser this does not give
di-n-methane rearrangement. Instead of di-n-methane rearrangement this give 2 + 2
cycloaddition (c.f. - Prob. 95 - a).
·.- .;"
J #
,
l
l
Problem 96: Give the product of following reactions:
~CN
~,.,.
(a)
H C
?
(b)
o
Me~Me
y
COOCHa
a
(c)
hv
--~)
Me~
y "
~
hv
?
--~)
OAc
Me
o
hv
?
---~)
(d)
C6 H n NH 2
y ""
Me~
hv
)
Solution:
(fCN
O·
O·
hv
CN
I I
)
I
)
COOCHa
HaC
"
1"
CH2gShift
groupof
O·
«CN
OH
~
~CN
I
H
(
CH a
COOCHa
CH 3
COOCHa
(b)
The migration of an acetoxyl group to an adjacent carbon is a typical example of
neighbouring group participation and is a characteristically polar process.
oe
o
;:;:.
Me'¢fh-
Me
Me
Me
Me
I V OAc
hV)
e
o
.
--+)
Me~$
~or
H
Me
1, 4, 6-Trimethyl-6-acetoxy
2, 4-cyclohexadienone
1
'¢c
OH
Me
I
A
Me 0
II'
O-C-CH a
Me
I
439
(c)
Irradiation of 2,4,6-trimethyl-6-allyl-2,4-cyclohexadienone in ether containing
cyclohexoyl amine gives the expected a, ~-unsaturated amide.
2
3
o
.
C H NH
6
(d)
~5
79
~4 ~6
1 2
11
A'
8
When the above compound is irradiated in presence of mixture of ether and water
there is no reaction.
o
~
hv
No reaction
)
Problem 97: Give the product of the following reactions:
(aJ
o~
hv
)
(bJ~
?
O,;?
(C)~O
hv
)
MeOH
hv
Ph
(d)~O
?
?
)
hv
H2O
)
?
Solution:
(a)
~y
hv
:;(~H
a
)
y-Hydrogen
abstraction by oxygen
of carbonyl group
~
Breaking of
~-bond
)
This is an example of Norish Type II reaction.
(b) H
fi
~
H 0"""
OH
hv
-_----=-:-'---~)
a
Ph
y-Hydrogen
abstraction·
by oxygen of
carbonyl group
~
Breaking of
C
/'
0H
'Ph
~-bond
)
~Ph
II
Tautomerisation
o
~Ph
This is also an example of Norish type II reaction.
(c)
(d)
N
·0
~
J
~.AO
~
~)A-J. .
~O
MeOH
a-Cleavage
hV)
MeOH
hv
MH
OMe
~
)
MeOH
a-Cleavage
0
Solvent adduct
j"",,~.
H~O
~
U'CHO
Problem 98: Complete the following reactions:
0
&
I
(a)
R'
R
hv
--.........-------» ?
fi
(R
.
Anhydrous ether
=OAc, R' =Me)
'.
hv
?
Aq. ether
----=----~)
hv
~OAC
?
(c)
Y
OAc
CH3
o
Me~Me
hv
U . . OAc
(d)
---~)
?
~Me
(e)y
hv
--~)
OAc
?
CH3
Solution:
\0\\
);\\
01:\\\ 0
\Cfl?) R'
o
er
I
(a)
fi
-0"
\~\\}."
R'
R
hv
)
( a-Cleavage
)
(
\ I
~
R
Ketene
(R = OAc, R' = Me)
1
hv (Anhydrous ether)
o
&D
I
•
R'
R
fi
R-H )
( R'=Me
OR
erMe
~I"':
o
Jl
_ .:
_R'
R'NH/..........,..,~~
R
O~ . . . . OH
R'
C'c~
2
.. R
fi
79%
(R =OAc, R' =Me)
I
o
)1
hv
(b)
6, 6-Dimethyl 2, 4
cyclohexadiene
Aq. ether
(a. Cleavage)
Y
.~Me
hu
HOH)
LJ"Me
J(r<0Ac
V
)
OAc
CR g
RVC
OR
Me
: :,. . I Me
o
o
~OAc
(c)
)
o
II
~~¥.OAC
)
OAc
CR 3
4-Methyl 6, 6-diacetly-2,
4-cyclohexadiene
L­
o
o
Me~"rC'Me
Me~Me
l)"OAc
(d)
~"OAc
a-Cleavage
2,6-Dimethyl­
6-acetoxy-2,4­
cyclohexadiene
o
~Me
y
(e)
hv
OAc
V
H 2 0- E~O
a.Cleavage
4,6-Dimethyl­
6-acetoxy-2,4
cyclohexadiene
)
OAc
~
r-9
35M
~~
4 1
6
e
2
---~
'-,.
""
~
I
(Expected product
but it is unstable)
1
6
~
""
3
COOR
4
5
III
""
OA
&
O~
.1
J,'r.r<. Me.
)
c
:::,....
I
Me
OAc
o \
G~l
3 4 5 6 OAc
"" "" Me
2
II
a·~·Unsaturateacid
(Unexpected product)
OAc
Me , , < - - - - - - - - - - - - - - - -__
Repulsion is minimised between acetyl group
(at CJ and methyl group (at C 4 or CJ when
geometry of the compound is reversed
Problem 99: Give the product of following reactions:
COOCH 3
I
(a)(
C
+
6.
III
?
)
C
I
COOCH 3
COOCH 3
(b)
I
XJ
C
III
+
C
I
COOCH 3
(c)
0
hv
)
COOCH 3
(d)
O
:::-.. .
I
I
c
hv )
+ III
® _~) (y)
C
I
COOCH 3
(e)
V
Solution:
(a) This is
CN
NC
I 2 + 4 I cycloaddition reaction
COOCH 3
~
~
(b)
This is also I 2 + 4
ethylene molecule.
I
Y!
+
6.
)
C
I
COOCH3
I cycloaddition reaction followed by aromatization by removal of
CH 3
I
C
+
III
C
I
CH 3
~
I
I'
...
~
R)3;(COOMe
R
. i
COOMe
®
(c) This is example of 1, 3-photoaddition of benzene to form tricyclic system. This reac­
tion is restricted to double bond bearing only alkyl substituent such as but-2-ene,
nonbornene, alkene and cyclobutene. A biradical "Prefulvene" is precursor of this
reaction.
o
COOMe
I
(d)
~ I
O
c
+
hv
m
)
O=(cooMe
oc
COOMe
~II
COOMe
I
-
COOMe
COOMe
The above sequence of reactions are I 2 + 21 photocycloaddition reaction followed by
ring expansion.
(e) This is special type of reaction in which 14 + 41 photocyclo addition reaction takes place.
o +X
~
NC~
NC~
hv
Problem 100: Complete the following reactions:
(al~
hv
~
Photocyclo-additiOl?
)
Photosensitiser
?
(b)
07
"
(C)lf
hv
(~¢
)
0
(el
qo
'"
hv
)
hv )
NC~
?
Ph
0
..
=
I
C
+
III
C
hv
(Pyrex)
)
?
I
Ph
?
"­
1
j
- -
...... .----------------------------------­
Solution:
(a) Iftriplete excitation energy of ketone-acetone (sensitizer) is greater than triplet exci­
tation of alkene, dimerisation takes place.
hv
)
CHaCOCH a
However, when ketone (sensitiser = benzopheonone) having less triplet energy than
alkene, oxetane formation takes place.
(i)~
hv
(ii)
(blCl?
H
~
)
hv
~
[2+2]
cycloaddition)
l
~
~
)
CH 3
This is example of ene reaction
(C)~
h
1
hv
)
[2 + 2] Photocycloaddition
~I
~
+
Di·Jt-Methane
v rearrangement
--7-)
a(y<
(a)~~c:R
' \ . (b)
(d) In this example two types of [2
6'
----+
~
+ 2] photocycloaddition reaction takes place.
C==C and C=C and formed· CD.
(ii) [2 + 2] photocycloaddition reaction between C-C and C=O, followed by ring
opening reaction to form compound @ in 60% yield.
0
Ph
0
0
Ph
I
C
hv )
)
+
I I ,.
I
I
I I
+ III
(Pyrex)
C
Ph
I
0
0
0
0
Ph
@60%
40%
(i) [2 + 2] photocycloaddition reaction between
¢
¢o:Ph tJPh
Ph
Q
CD
,
,r,
-,'
Ii
(e) When carvone is irradiated to photolysis [2 + 2] photocycloaddition reaction takes
qo
place.
[2 + 2] Photo )
cycloaddition
=
hv
)
Br°
Problem 101: Consider the following two isomeric compounds (AJ and (B) whose structures
are given below:
Q=CR,
H H
(A)
(B)
Compound (AJ on heating readily converts into toluene but (BJ does not. Explain this
observation.
Solution: (A) gives [1, 5J sigmatropic rearrangement which is thermally allowed. Therefore
reaction will proceed readily. Compound (B) can only be converted into toluene if reaction is
[1, 3] sigmatropic rearrangement. [1, 3] Sigmatropic rearrangement is thermally allowed but
geometrically not possible.
Hence reaction does not take place.
Problem 102: Complete the given reaction and give its name:
HO
HC~
H:C~
....
HO
Solution: The reaction is [3,3] sigmatropic rearrangement i.e., Cope rearrangement.
o
Tautomerisation
)
II
CHa-C)
HaC-C
II
o
Substrate of Cope rearrangement is derivative of allyl alcohol, therefore this Cope
rearrangement is known as Oxy-Cope rearrangement.
Problem 103: Complete the following reactions:
(a)
HaC"
/C=CH-CH2-O-CH=CH 2
HaC
Heat
)
(b)c6
0-CH=CH 2
,
'I
j
Solution:
(a) The substrate is allyl vinyl ether therefore reaction will be Claisen rearrangement.
[
)
3,3-Dimethyl­
4-pentenal
(b)
The substrate is allyl vinyl ether therefore, the reaction is aliphatic Claisen
rearrangement.
cp
)
CHz-CHO
Problem 104: What starting material would give the following compound in an aliphatic
Claisen rearrangement:
yCH'
CH-CHO
I
CH a
Solution: Deduce the starting material by drawing the curved arrows for the reverse of
Claisen rearrangement:
if
Problem 105: Complete the following reaction:
~
---.+)
(A)
~
--~)
(B) + (C)
Solution: The first step is Diels-Alder addition reaction and second step is retro Diels-Alder
addition.
447
COOCH 3
I
+ C
III
I
COOCH 3
1
Diels:Alder
reactIOn
MeOOC
MeOOC
MeOOC
MeOOC
(A)
Retro
Diels:Alder
reactIOn
1
y
MeOOCn
I
MeOOC
+
CH 3
.h-
(B)
CH
II
CH 2
(C)
The driving force for the retro Diels-Alder reaction is formation of a product that is
stabilised by aromaticity.
Problem 106: Identify (AJ and (BJ in the given reaction sequence:
J.
0
to.
) (A)
MeOOC--C '" C-COOMe
to.
(B)
)
to.
MeoocX)
)
MeOOC
.h­
(C)
+0
(D)
Solution:
H
Disrotatory
)
e1ectrocycIic
reaction
ep
H
(C) + (D)
Retro Diels­
Alder reaction
<
1
MeOOC-C =< C-COOMe
to.
(A)
Meooc~
_
Meooc~
MeOOC
(B)
Diels:Alder
reactIOn
MeOOC
~
.I
I
Problem 107: Give the stereochemical structure of the products in the following reactions:
o
r.t.
(A)
)
150°C
~o
)
(B)
o
(C)
----~)
Solution:
o
o
ceR
Diels-Alder
)
reaction
o
«0
(A)
o
xxi
Retro Diels­
Alder reaction
)
150°C
o
(B)
o
~o
o
o
o
(C)
Problem 108: Propose a mechanism for following transformation:
o
et
~ 0
-G-COOMe
+
III
---~)
C-COOMe
COOMe
(XI COOMe + CO
~
Solution:
®
0:7
~
o
+
C-COOMe
II
-"-COOMe
)
~
CO
COOMe
I
0
Retro DielsAlder reaction
)
COOMe
2
ex
~
I
COOM
e
COOMe
+ CO 2
Problem 109: What do the pericyclic selection rules have to say about the position ofequilibrium
in each of the following reactions ? Which side of each equilibrium is favoured and why:
OH
0
(a)~~)~
Solutions
(b)
~
~
(ely
o
COOMe
~)
~COOMe + CH
V
z
449
CH z
9
o
Solution:
(a) The right side of equation is favoured at equilibrium because a C=O double bond is
formed at the expense of C=C double bond. C=O Double bond is stronger than
C=C double bond.
(b) The right side of the equation is favoured at equilibrium because one product is
aromatic and therefore highly stable.
(c) The left side of the equation is favoured at equilibrium because three double bonds
are in conjugation to each other.
Problem 110: Complete the following reactions:
hv
)
(U)~O
hv
)
Solution:
(L) Compound has hydrogen on y-carbon and a-carbon is highly substituted hence it will
give cyclobutane derivative as follows:
o
o
hv
)
y-hydrogen
abstraction
l,4-diradical
1
o
"
(iz) Compound has y-hydrogen and a-carbon has highly substituted. Therefore, compound
will form l,4-diradical which will undergo cyclisation.
~OH
hv )
1
~
OR
Problem 111: Complete the following reactions:
(i)
o
Me~o
~COOEt
I
~
9'
Me
~
(ii)
Me
Ts
~N<Et
Et
Mexfx,
hv )
~
Solution:
(z) Compound has no hydrogen on y-position. It has hydrogen on ~ as well as 0 positions.
Therefore, hydrogen abstraction will taken place preferabily from 0 positions to give
1,5-diradical which on cyclisation will give five membered ring compound.
o
Me~'('COOEI
MeN
Ts
lhV
Me~o
•
I·
~~COOEt
9'
Me
~
Ts
1
COOEt
Me
Me
-Ts
(it)
Me
N<Et
Et
Me
OH
Me
Me
1
OH
~~-N<
Me
E
t
Et
Me
Problem 112: Complete the following reactions:
o
OCH 3
Ph
(t)
hv )
I
hv )
(ii):
((
C-Ph
II
°
Solution:
(i) Compound has hydrogen on B carbon .hence hydrogen abstraction will takes place
from this carbon.
Ph
OH
Ph
hv )
1
OH
Ph
(iz)
rf(0-CH3
hv
~.
~
0-6Hz
I
-::?
~
0yPh
.
Ph
OH
o
1
o
W
HO
. Ph
Problem 113: Complete the following reactions:
Solution:
Compound has hydrogen on E -position which is allylic in nature. Hence, hydrogen
abstraction will takes place from E -position.
Problem 114: Complete the following reactions and give stereochemistry of the product
(ifpossible) :
(z)
OMe
+
(iz)
I(
hv )
benzene
,..--,/COOMe
+
UJ
hv
)
Solution: Enone gives 12 + 21 cycloaddition reaction with alkenes. cis-Fused 4/5, 4/6 and
SiS systems are common and more stable than their trans isomers.
(0 Alkene is electron rich hence adduct will be head to tail.
o
hv
+
)
OMe
Major product
+
OMe
Minor product
(iO Alkene is electron-deficient hence adduct will be head to head.
o
OR
MeOOC~
+
U
COOMe
)
R
Major product
Problem 115: Complete the following reaction and give stereochemistry of the product:
o
Q+II
hv
)
(A)
As major product
1
(i) MeLi
(ii) H 20 I HEll
(B)
1
t.
) (C)
Solution: The first step is 1 2 + 2 1 cycloaddition between enone and alkene.
o
OH
cp
Q+II ".
CH 3
l
<i) MeLi
(ii) H 20 IH$
b .' H~~-,-h
l
o!!-/··..· . ·.
CH2 H
.-ll.. .·
H 3c
"'"
Ho~...·.....
-H 20
CH 3
Problem 116: Give mechanism and type ofphotochemical reaction of the following:
(i)
((
+
1C;;Me,
cPy
CHO
(i) hv
)
(ii) H$
°
o
100% yield
o
(ii)
~o
0
AA
Solution:
(i) Reaction is
hv
~O
12 + 21
((
)
+
cycloaddition reaction.
1C;;Me,
hv
o
d"-CHO
y
o
(
)
(i£)
0
0
~
hv
~
10
~o
od;H
(
~O
OH
0
)
1
Problem 117: Give mechanism of the following reactions:
0
(£)
Ph~
0
hv )
0
Ph:£)
+
Ph
""""Ph
Ph
Ph~
~111J.j
Ph
."'~'" Ph
Ph
0
hv )
0
(it)
Solution: The substrate is enone, therefore reaction is cyclic enone rearrangement.
(£)
hv )
...
1
Ph
~(-~) A
PhA
Ph~
Ph~
Ph
Ph
Ph
~
Ph~
Ph
PhA
Ph
•
Ph
~
(
Ph
Ph~H
Ph
•
Ph
6
o
hv
)
i
...
I
6
o
(
1
o
o
Problem 118: Complete the following reaction:
hvlCyclohexane
(1)
)
Probtems and Their Solutions
457
Solution:
(1) Substrate is cross-conjugated enone and reaction is taking place in presence of non­
polar solvent. Thus product formation will takes place by diradical formation:
.
0
Q
•,.
0
hv
cyclohexane
0
Q
)
"Q1
.
0
0
~
Q"
(
1
Q"
(
0
0
CJx
cA
-
(2)
0
0
Q
)
0
hv
)
H:i0lDioxane
0
Q
)
1.
0
0
Crx
oQ
(
QO
(
III
0
cA
0
hv
)
6c
0
(
)
o~
1
a Q1
1
ex 0­
0
R
~
e
e
e
0
)
'
Ell
.~
e
0
OH
)
Problem 119: Give mechanism of the following transformation:
o
n
X
hv
)
MeOHlHCI
CCls
Solution: Compound is cross-conjugated enonone. Hence, it will give cross-conjugated enone
rearrangement. The reaction is taking place in presence of protic solvent hence intermediate
will be zwitter ion.
nX
.
CCI3
Q ~(~)
6
o
hv
HCYMeOH
)
CCla
~
o
e
~~~e
(
r:!-Me
~i
MeOH
CCh
6
Q'
(
CCh
CCh
1
~OCH3
0
)
~~CH3
CCh
CCh
~
I
Problem 120: Complete the following reaction:
i
~1
)I
0
hv
#
)
Ph
Solution: Substrate is /3, y-unsaturated ketone and reaction is carried out is presence of
sensitiser, therefore, reaction is oxa-di-n-methane rearrangement
:n
.>'
hv
3Sens
I I
i
!
I
-i!
Ph
0
Ph
p::X
•
Et
f?
9
1
Et
+
0
E~>-<
•
~
~
i
)
pj>-<o
E'j>-<
~
+
1
0
Ph
0
15
CHAPTER
of Excited States and Inve~tiB
Reaction Mechanism
Photochemical reactions take place by absorption of uv light or visible light by the substrate
molecule. Ground state molecule converts into an excited by the absorption of energy. The
excited species can give rise to high energy products, such as radicals, biradicals or strained
ring compounds which are not readily formed from the ground state.
For photochemical reaction to be readily observable, the rate constant for the initial
photochemical step (primary process) involving the excited state must be high (10 6 - 109 /sec).
This is because the excited state is short-lived, decaying to the ground state very rapidly, an
an efficient photochemical reaction must compete successfully with these very rapid
photophysical processes. The decay processes may be radiative (fluorescence from 8 1 and
phosphorescence from T 1) or non-radiative (internal conversion and intersystem crossing). It
may be that there are many photochemical reactions as yet unobserved because their quantum
yield are very low as a result of such competition. The activation energy for excited state
reactions are generally small, often less than 7 kcallmol, and this is a corollary of the fact that
only fast photochemical reactions are· detectable.
Both excited states (singlet and triplet state) give chemical reaction and energy transfer.
The energy transfer process is a bimolecular process, often referred to as quenching, in which
the excited state (singlet or triplet) is converted to ground state in a non-radiative manner
whilst the second molecule (the quencher, Q) is excited to a higher energy state. All the processe
can be shown as follows (Scheme 1).
Chemjcal
reaction
Chemjcal
reaction
Scheme 1
460
."I'
Quenching of Excited States and Investigation of Reaction Mechanism
461
In the chemical reaction the process may be a simple concerted formation of products
from the (Sl or T 1) or it may occur through one or more intermediates which may also be able
to revert to ground state starting material.
/So
Intermediate ~
Product
The lifetime of excited states is very important for photochemical reactions. The lifetime
of these excited states affects the rate of the photochemical reaction directly because this is
the time which is provided for the photochemical transformation. The lifetime of an excited
state can in principle be determine by the use of the suitable quencher.
Rate of photochemical reactions can be measured by measuring the quantum yield as a
function of quencher concentration. Thus quenching process and quencher is very important
for the determination of the rate of the photochemical reactions. Therefore. before discussing
the lifetime and rate of the reaction, let us first discuss about the quencher.
A substance which accelerates the decay of an excited state (Sl or T 1) to the ground state or to
a lower excited state is described as a quencher and is said to that excited state. The quenching
process can be represented as:
--_Q-=---~) 1\
... (1)
where A' is the ground state or another excited state of A and Q is the quencher.
The quenching process may occur by many different mechanisms and is induced by
many different substances. Of these substances oxygen is the most ubiquitous and one of the
most efficient quencher.
Quenching process is a bimolecular process. This-bimolecular process is an intermolecular
process. The bimolecular entity in which the quenching occurs can be either an encounter
complex or an excimer/exciplex. The encounter complex is represented as (A* ------ Q). The
distance between A* and Q in this species is of the order of 7A or more and they have random
relative orientations. In this species, there is some significant overlap of the orbitals of the
components.
Excimers is represented by (AA)* and exciplexes by (AQ)*. These two have definite
geometries. In aroma.tic excimers, the components are organised into parallel planes.
Quenching can occur either by the formation of encounter complex, excimer or exciplex.
Bimolecular quenching depends on the reversible formation of an encounter complex or exciplex
which subsequently relaxes to the ground state entities by various modes. Scheme 2 shows
relaxation modes open to exciplexes.
-+-<:::"
",
....................
""'~""
E
-+-<:~
lA*
,"
~::>
/,--t--"",
...............
"
"
",
""'---tt----",/"
LUMO
~~:::>-rr- HOMO
A
1 (AA)*
Fig. 15.1
Thus excimers exist only in the excited state and hence dissociate very easily. The
dissociation of excimer is shown below:
Excimers exist only in the excited state, therefore, their existence cannot be detected in
the ground state. Excimers are non-polar species and have zero dipole moment.
Excimer formation is observed quite frequently with aromatic hydrocarbons. Excimer
stability is particularly large for pyrene, where the enthalpy dissociation is ~H = 10 kcallmole.
The excimers of aromatic molecules adopt a sandwich structure, and at room temperature, the
constituents can rotate relative to each other. The interplanar separation in excimer is 300­
350 pm.
15.2.2 Exclplex
Formation of exciplexes take places is solution of mixed solutes. The term exciplex refers to an
excited complex with a definite stoichiometry (usually 1 : 1). In most of the cases, an exciplex
is formed between an excited species (say M*) and one different molecule in the ground state
(say Q).
M
_---:.:h..:....v-~)
M* -_Q-=---~) (MQ)*
Exciplex
Contrary to the situation for excimers, one component of the exciplex acts predominadtly
as donor (D); the other acts as the acceptor (A). On the basis of quantum mechanical calculation,
it has been found that exciplex corresponds to a contact ion pair. Experimentally, the charge
464
Photochemistry and Pericyclic Reactions
transfer character is revealed by the high polarity, with exciplexes from aromatic hydrocarbons
and aromatic tertiary amines having dipole moments> 10 D.
From a simple molecular treatment it follows that the electron transfer leading to exciplex
formation can occur either from an excited donor to an acceptor or from a donor to an excited
acceptor. In both cases, the singly occupied orbitals of the resulting exciplex corresponds to
the HOMO of the donor and LUMO of the acceptor (Fig. 15.2).
(a)
QlLUMO-+
\
E
)
QlROMO+ -#­
A*+D
­
(b)
+
QlROMO-#­
A+D*
8
(A
........."
.
:\......................
\
*~
-+-­
--+
E).. IL""O
\
..........,
.
.rf.)
_
.
-#--#­
A+D+hv
\
\
:\
\
.
~
-
-#­
-#A+D+ hv
Fig. 15.2 Exciplex formation by charge transfer (a) from the donor to the excited acceptor and
(b) from the excited donor to the acceptor. Exciplex emission is indicated by a broken arrow
Detection of exciplex and excimer is difficult but their formation take place in many
photochemical processes, e.g.
(i) Photodimerisation of polyarenes
hv
----'-'-'----~)
A * __A_~) (AA)*
Excimer
1
Product
Dimer
,
I
~, :!\
I'"
"
''"
','
(ii) Photoaddition of ketone to electron deficient olefines
o
A
hv
)
O-__(CNJ*
[A-­ R
[JJ
Exciplex
1
n
CN
R
(iii) Triplet-triplet annihilation
3A* + 3A* ~ l(AA)* ~ lA
+ lA*
(iv) Exciplexes are also involved in many quenching processes (See Sec. 15.3).
=:;:::::::':::::::
There are many quenching processes which are given in Scheme 3.
r-------'c--~ Products
A*
\
A
(photochemical quenching)
r--=--~
Self-quenching
Photophysical
quenching
'----Q-'"""'"iI' Impurity quenching .
Electron tr.ansfer
quenching
Heavy
atom
quenching
Scheme 3: Quenchlng:processes
Energy
transfer
In this process singlet converts into triplet and hence fluorescence phenomenon is
quenched by quencher Q.
.
15.3.3 Quenching by Molecular Oxygen (Triplet State) and Paramagnetic Species
The quenching of the excited state of many organic molecules is quenched by triplet oxygen.
This phenomenon is diffusion-controlled.
lAo +30 2 -----+3A* +302
This phenomenon occurs as follows:
Ie
)
)
Highly excited
triplet exciplex
Lowest excited
triplet
1
In this process, lAo is converted into 3A* by oxygen. Thus phenomenon given by singlet
state of A is quenched by oxygen.
:
:::::::::::::::::::::~::::::;::::1::::=
Electronic energy transfer phenomenon is the most important in photochemistry. In this
phenomenon an excited donor molecule D* collapses to its ground state with the simultaneous
transfer of its electronic excitation energy to an acceptor molecule A. Molecule A is thereby
promoted to an excited ,state.
•
D* + A -----+ D + A*
It should be noted that the acceptor can itself be an excited state as in triplet-triplet
annihilation.
3A* + 3A* -----+ A + lA*
(D) A*
(D*) (A)
The energy transfer can be formulated as:
D*+ A~D···A)*-----+D+A*
The process is a bimolecular:process which takes place by the format1.on of an
intermediate. This intermediate may be''an exciplex··or a collision complex. The· energy transfer
process is governed by Wigner's rule. According to this rule, an excited tripleistate produces
another triplet and a singlet excited state produces another singlet.
::.
...(i)
...(ii)
The singlet-singlet energy transfer [equation (ii)] is rare and takes place only under
special conditions. Many of the examples of this phenomenon involves the use of biacetyl as
quencher. Biacetyl quenches the fluorescence shown by aromatic hydrocarbons.
The triplet-triplet energy transfer [equation (i)] is very common in photochemistry.
Photosensltlsed Processes
Hoften happens that the Sl and T 1 states of the molecule have different photochemical reactions.
(Scheme 4). For example, Sl-butadiene gives cyclisation reaction to form cyclobutene but T 1­
butadiene undergoes dimerisation.
----+)
U
Similarly p, y-unsaturated ketones undergo 1,3-acyl migration from 8 1 state but 1,2­
migration from the T 1 state.
A __h_v_~) 1A* __I_SC_~) 3A* ..
1
.
~sen~J=t
~tion
1
Product (P~
A + 3S*
A+Q
or
A+ 3Q*
SCheme 4
Selection between the products P 1 and P 2 may be obtained by quenching or sensitisation
of 3A*- control over the nature of the products can be obtained for such systems by sensitisation
and/or quenching techniques. Thus in Scheme 4, the formation of product P 2 may be suppressed
by quenching 3A* with a triplet quencher Q whose triplet energy is less than that of 3A*,
Conversely, the product P 2 may be obtained free from P 1 by specially producing 3A* by triplet
transfer from a photosensitiser 3S* of higher triplet energy, thereby passing 1A*. Thus, the
process of sensitisation allows to populate one excited state (generally T 1) which is otherwise
very difficult by selective radiation. The quenching process is reverse to sensitisation. In
quenching process, excited species (generally T 1) is removed by ground state molecule (i.e.
quencher). Hence reaction does not take place by the excited state (T 1)' Thus, sensitisation
and quenching processes are frequently used in photochemistry. The most common triplet
sensitisers are aldehydes and kelones which are used in a wide variety of photoreackons.
The most generally useful triplet state quenchers are conjugated dienes which· have
relatively low triplet state energy (53--59 kcallmole),
Let us take the examples of some photochemical reactions which demonstrate the use
of quencher.
Anthracene on irradiation with light undergoes dimerisatioh reaction. When this reaction
is carried out in the presence of molecular oxygen, dimer formation does not take place. This
result confirms that dimerisation takes place by T 1 state and not by 8 1 state of the anthracene.
Dimerisation
A*
ISC)
_ _h;.;..V_--+) 1
Adduct
.
Anthracene dimer
Quenching by Oxygen
'[(OJr
1
+
0,
Adduct of oxygen and anthracene
3A
Y}
*
Dimer
Similarly, cyclopentenone dimerises by irradiation with UV light. If the same reaction
is carried out in the presence of 1,3-cyclopentadiene, no dimer is formed because 1,3-pentadiene
inhibits the reaction. In this reaction, it behaves as a quencher. This result also shows that
dimerisation takes place by T 1 state ofthe ketone.
o
(
Dimer
hv
6
hv
------'-7-~)
OJ
No reaction
A great deal of effort has been directed towards the elucidation of the mechanisms of organic
photochemical reactions in recent years. General methods of trackling the problems involved
in the investigation of reaction mechanisms have been developed. Investigation of reaction
mechanism for a particular reaction requires identification of excited state intermediates and
other short-or long-lived species on the reaction path, and determination of the rate constants
for various chemical and physical steps in the sequence. The whole process is divided into
quantitative and qualitative methods. Qualitative methods include methods for identification
of reaction products, intermediates and excited states. Quantitative methods include methods
for the determination of quantum efficiencies and rate constants.
15.6.1 Identification of Products
The most fundamental basis for mechanistic speculation is the identification of the reaction
products and byproduct(s) (if any). Identification of the chemical structure of the products is
achieved by standard analytical, spectroscopic, chemical and degradation techniques. For
example, when compound (A) is irradiated with. light in the presence of methanol, it give
compound (B).
mOOCH,
a
H
(A)
H
(B)
Identification of product (B) shows that product formation takes place by forwation of
ketene as one of the reaction intermediates.
Similarly when compound (C) is irriated with UV light it gives the product (D)
o
o
d)
hv
p)
)
(C)
(D)
Isolation of product (D) shows that this reaction is rearrangement reaction.
15.6.2 Intermediatea
In photochemical reactions a large amount of energy is supplied in a specific (electronic) manner.
This can affect conversiV'iS to high energy systems. These high energy systems (i.e., reactive
intermediates) may be sta\jl~ under the conditions of reaction, but someone thermally unstable
or chemically active. DetectiLll and identification of such intermediates is important in the
elucidation of the overall reacli-on pathway leading to the isolated products. The methods
employed depend to a large eX\l:lnt on the lifetime of the reactive intermediate under
investigation.
(i) Identification by spectrophotOl-eter
Long-lived species (1: > 10 1 to 102 S) can l)e identified by either IR or UV spectrometer. For
example, 2-pentanone on irradiation with TN light gives ethene and acetone. Formation of
acetone takes place by formation of enol. FOhnation of enol as reactive intermediate can be
confirmed by performing the reaction in a cell an infrared spectrophotometer.
'*'
o
~
hv
OH
,
)
I
CH 2 =CH 2 + ~li2=C-CHa
= 3630 cm- 1
tV2-= 3.3 min at 27°C.
vQ-"!
Similarly in photoreduction of benzophenone by isopr~a'ol a coloured compound is
formed. Formation of this coloured compound has been confir~d "hen reduction is carried
out in the cell of an ultraviolet spectrometer.
@-bf=vC~~,
~CH a
Coloured compound
(iii) Flash photolysis
Flash photolysis is used for the characterisation of short-lived or very short-lived intermediates
(with a lifetime less than 10-11 10-12 sec). This is one of the most powerful tools for studying
quantitatively and qualitatively the excited states and intermediates in photochemical
processes. In principle, flash photolysis involves the generation of a high concentration of a
short-lived intermediate using a very high intensity pulse of radiation of a very short duration.
At ash.ort time interval after generating the pulse, the system is analysed by observing its
emission or absorption characteristics.
A basic flash system in diagrammatic form is shown in Fig. 15.3. The conventional flash
sourl~es are based on gas discharge lamps (flash durations less than one ns). Detection
tecrmiques vary according to the nature of the system and the information required. The
em;Lssion spectrum of an intermediate can be photographed using a spectrograph, or the visible
ab f30rption spectrum can be similarly recorded if an analytical beam passing through the
reaction cell is triggered to flash at a predetermined time interval after the initial flash.
AJternatively" the process can be followed kinetically by monitoring the absorption or emission
at a particular wavelength and coupling to an oscilloscope with a time based sweep. These
methods give the nature of the species generated and its lifetime. This method is very useful
for th/,3 identification of short lived free radicals particularly in the photo reduction reactions
of aromatic ketones.
,------1
Trigger
1-------1: :1------,
Photolysis flash
I
..
Reaction vessel
/
MgO coated reflector
(or photomultiplier)
Fig. 15.3 Basic flash photolysis'arrangement
(iv) EPR and CIDNP
Photochemical reactions take place via formation of free radicals as reactive intermediates.
The most useful method for detecting and characterising unstable radical intermediates is
electron paramagnetic resonance spectroscopy. EPR opectroscopy is a highly specific tool for
detecting radical species because only molecules with unpaired electrons give rise to EPR
spectra..
Another techniqUl:l that is highly specific for radical identification is known as CIDNP
(chemically induced dynamic nuclear polarisation). The instrumentation required for such
studies is a normal N11·R_~~ectrometer. CJQNPi$..abserved as a strong perturbation of'the
intensity of NMR signals in products formed in certain types of free radical reactions. The
effect is investigated by irradi!'.ting the sample in the sample tube of an NMR spectrometer
. and the in~'::j,J.sity of the signals for the protons attached to the atoms forming the new bond in
the product is observed. The signal may be of higher or lower intensity than normal, and may
Thus, CIDNP studies show that formation of product (F) takes place by formation of
C6H 5 - QH - CH a free radical. Thus mechanism of the reaction is as follows:
o
1/
. C6H 5-C-C 6H 5
1.
hv
o
I
C6H 5-Q--C 6H 5
1
C6H5-CH z-CH a
OH
[
CoHs-+
+
C6 H 5
1
OH
I
C6H 5-C -
I
CH-C 6H 5
I
C6H 5 CH a
(v) Trapping of reaction intermediates
Sometimes an intermediate may be detected by adding to the reaction a 'trapping agent'. The
trapping agent is added so that it combines with the intermediate to form the product that
cannot be accounted for otherwise.
(i) Transient alkyl free radicals are trapped by nitroso compounds. Nitroso compounds
react with alkyl free radicals to form long-lived nitroxides. These nitroxides are stable.
.
R + R'-NO
~
R-N-R
I
Q
The most suitable reagent for trapping of free radical is 2,2,4,4-tetramethyl piperidine
N-oxide. For example, irradiation of 1,3-diphenyl acetone gives 1,2·diphenyl ethane and CO.
o
II
C6H5-CHz-C-CHz-C6H5
1
hv
Formation of enol form in the above case can be confirmed by trapping the enol form by
maleic anhydride.
o
II
~C-Ph
~CH3
OH
~,==h=v=~, ~Ph
U
CH2
Diene
o
¢o
o
Ph
OHO
~
o
Adduct
Isolation of adduct confirmed that enolisation is a more dominant process than photo
reduction.
_:::::::=::::::;:::=':::::::::===:::=::::::
In non-kinetic studies, suitable isotopes of carbon (13C), hydrogen (D), nitrogen (15N), oxygen
(180), and radioactive isotopes of carbon (14C), hydrogen ell), sulphur ( 5 8), iodine (1 281 and
131I) and bromine (82Br) have been introduced in strategical positions of reacting molecules to
have an insight into reaction mechanism by noting which parts of reactant form which parts
of the products. For example, 17-ketosteroids on irradiation in aqueous solution give a
reasonable yield of ring-opened carboxylic acid.
o
{f>
hv
)
The mechanism may involve a direct addition of water or may involve a ketene as
intermediate and the use of a deuterium labelled reactant distinguishes between the two
pathways.
(i)
0
{i)<~
{~ ~
OH
H 20
)
1
{1J<:~
C-OH
Pattern not observed
0
(ii)
ct)<~
0
)
{1Y~
1
ri)=c=o
Ketene
lH~
{:t:t
0
D II
C-OH
Observed pattern
This isotopic labelling experiment shows that product formation is taking place v
formation of a ketene as reactive intermediate.
In photochemical reactions product formation takes place either by lowest excited singlet st
or by the lowest excited triplet state. In some cases, product formation takes place by b
singlet and triplet states.
hv
S
hv
S
hv
S
)
lS*
)
lS*
ISC
lS*
ISC
)
)
~
Product
)
3S*
)
3S*
)
Product
/
Product
Lowest singlet
01:
triplet state of the reactant is characterised by their spectroscopic
data.
The longest wavelength band normally seen in the ultraviolet absorption spectrum arises
from singlet ~ singlet absorption and corresponds to the lowest energy-electronic transition.
The nature of this transition is usually apparent from the magnitude of the extinction coefficient,
the effect of solvent polarity on the position of the maximum and the direction of polarisation
of the absorption band. For n ~ n* transitions E max is usually small (10 1 to 10 2 mol-1 cm-1), the
band shows a large blue shift in a more polar solvent, and the transition is polarised in a
direction perpendicular to the molecular plane. For n ~ n* transitions, E max is large (10 3 to
10 4 mol-1cm-1), the band shows a small red shift in a more polar solvent and the direction of
polarisation is parallel to the molecular plane. For intramolecular charge-transfer transitions,
which can be regarded as extreme cases of n ~ n* transitions, E max is very large (10 4 to 105 l
mol- 1cm- 1) and the band shows a pronounced red shift as the solvent polarity increases.
It is not normally possible to observe singlet-triplet absorption spectra by conventional
techniques because of the very low extinction coefficient associated with these transitions.
The intensity of So ~ T 1 (n ~ n*) absorption can be increased by the use of high pressures of
added oxygen, and triplet (n ~ n*) energies have been determined for conjugated dienes using
this method.
Lowest energy excited states of a molecule may be n ~ n* or n ~ n*. These states are always
not directly responsible for photochemical reactions. Higher energy or non-emitting states
may also be involved in photochemical reactions. The re'acting states may be either singlet or
triplet. Sensitisation and quenching methods are generally most useful for characterisation of
reacting states.
Most quantitative studies have involved triplet sensitisers or triplet quenchers, partly
because the results of such studies are more easily interpreted than those involving singlet
sensitisers and quenchers. Triplet sensitisers must be chosen carefully for the purpose. The
sensitiser should fulfil the following properties:
(i) The sensitiser must absorb radiation strongly of the wavelength employed so that
absorption by the substrate is minimised.
(ii) The sensitiser must have a high quantum of efficiency for intersystem crossing, and its
triplet state so formed must be long-lived so that transfer of its energy to another molecule
can be efficient.
The most widely used compounds which are good triplet sensitisers are aromatic ketones
such as acet.ophenone and benzophenone.
The qualitative use of triplet sensitiser is of value as follows:
If triplet sensitisation leads to products which are different from those obtained on
direct irradiation, then the reactive state in the direct irradiation is not the same as the triplet
state obtained by sensitisation. If triplet sensitisation produces the same products as direct
irradiation, then it is proved that reaction can occur through the triplet state.
Quenching techniques can in a similar manner yield information about the multiplicity
ofthe excited state in a photochemical reaction. The most generally useful triplet state quinchers
are conjugated dienes, which have relatively low triple state energies (200-250 kJ mol-I) and
lowest singlet states of much higher energy (> 400 kJ mol-I) so that singll3t quenching is of less
importance. If photochemical reaction of a compound can be quenched efficiently by a known
triplet quencher, the there is strong evidence for a reactive triplet excited state.
:::::::~::::l=:::::::::=:::::
Mter identification of products and reaction intermediates, we need qunatitative data for the
reaction pathway. Quantitative data give more complete description of a reaction mechanism.
The most important quantitative data are rate of the reaction, quantum yield qf the reaction,
and lifetime of reaction intermediates. Since kinetic analysis provides a powerful tool for
unravelling the complexities of many photochemical phenomenon, certain quantitative relations
involving experimental quantities, such as quantum yield and lifetime will now be established
to show how the key rate constant may be extracted from these data.
15.10.1 Quantum Yield
On the absorption of radiation, the ground state molecules are excited. Once the excited state
of the re"actants has been formed, either by direct or sensitised energy transfer, the stage is set
for a photochemical reaction. There are still, however, competitive processes that can occur
and result in the return of unreacted starting material. The excited state can decay to the
ground state by emission of light, a radiative transition. The rate of emission is very fast
(k = 10 5 - 109/sec) for radiative transitions between electronic states of the same multiplicity,
and somewhat slower (k = 10 3 - 105/sec) between states of different multiplicity. The two
process are known as fluorescence and phosphorescence, respectively. If energy is emitted as
light, the reactant is no longer excited, of course, and a photochemical reaction will not occu,r.
Excited state can also be quenched. Qmmching is the same physical process as
sensitisation, but the word quenched is used when a photoexcited state of the reactant is
deactivated by transferring its energy to another molecule in solution. This substance is called
quencher.
Finally, non-radiative decay can occur. This name is given to the process by which the
energy of the excited state is transferred to the surrounding molecules as vibrational energy
without light emission.
Because of the existence of these competing processes, not every molecule that is excited
undergoes a photochemical reaction. The fraction of molecules that react, relative to those
that are excited is called the quantum yield (<1». The quantum yield is a measure of the energy
of the absorption of light in producing reaction product. A quantum yield of unity mea:t:J.s that
each molecule excited (which equals the number of quanta of light absorbed) forms pr2duct. If
the quantum yield is 0.01, then only lout of every 100 molecules that are excited undergoes
photochemical reaction. The quantum yield can vary widely, depending on the structure of the
reactants and the reaction conditions. Quantum yield can be greater than one in chain reactions'
in which a single photoexcitation initiates a repeating series of reactions leading to many
molecules of product per initiation step.
The quantum yield can be related to the rate constants in a particular system but
normally it is not a useful, direct measure of the reactivity of an excited state, that is, of the
rate constant for the primary chemical step from the excited state.
The quantum yield (<I» for the formation of a particular product is given by:
<I>
Amount of product formed (in moles or molecules)
= Amount of radiation absorbed (in Einstein or photons)
Rate of the formation of product
Rate (i.e., intensity) of the absorption of radiation
=---------,------:--,------='------,-----,---­
d[P] 1
dt Ia
=--.­
The quantum yield can be measured by measuring (i) number of moles reacting and
(ii) number of Einstein absorbed.
Let us consider a simple photochemical reaction (Scheme 5)
A+hv
A*
A*
~
kd)
~
A
A*
Rate = Ia
Rate = kd[A*]
Product (B) Rate = kr[A*]
Scheme 5
In the above reaction:
Ia = Rate of formation of A *
kd = Rate of destruction of A*
kr Rate of formation of product
In the above reaction A is the substrate, A* is the excited state of A and B is the product.
The rate of formation of the product is given by the equation (1).
=
d[B] = kr[A*]
dt
... (1)
To replace the concentration of A * which is not directly measurable, we make use of the
steady-state approximation. The steady-state approximation is applicable to excited states
(S1 and T1) and it follows that the rate of the formation and destruction of S1 (or T1) are equal.
The rate of the formation of singlet state is always directly proportional to the intensity of light
absorbed by the substrate A. Thus,
rate of formation of
A* = Ia
A* = kd[A*] + kr[A*]
rate of destruction of
Thus, according to steady-state
Ia = kd[A*] + kr[A *]
. .ooPericYclic Reactions
Therefore
Ia
[A*]
= kd + kr
... (3)
On substituting the value of [A*] from equation (3) into (1), we will get equation (4)
d[B]
dt
= kr. _I~
... (4)
kd + kr
Thus, quantum yield for the formation of product B is
<PB
. d[B] 1
dt'Ia
=
kr
...(5)
kd+kr
The quantum yield measurement provides a value for kr only if the value of kd + kr is
known.
=---
15.10.2 Experimental Determination of the Quantum Yield
In general organic photochemistry follows two main lines of study:
(i) The elucidation of the mechanism of the reaction in question, and
(ii) The use of the reaction for synthetic purposes.
These two types of studies involve different types of equipments. Mechanistic
photochemist often requires elaborate equipment involving the use of a monochromatic source
of light, a filter train, a beam collimator and means of measuring both the incident light and
the light absorbed by the reactants. The ray diagram of the apparatus is given in Fig. 15.5.
Thermostat
Detector
Reaction chamber
Light source
Fig. 15.5 Measurement of quantum yield
Light source: The light source to be used for a given process is determined by the
absorption spectrum of the compound under study. The light sources are:
(i) High intensity tungsten lamp
(ii) Xenon arc lamp
(iii) Mercury lamps.
QJerilOhlngofExc~ted States and Investigation
of Reaction Mechanism
483 .
Among these the most common lamps are mercury lamps. Mercury lamp may be any
one of these:
(i) Low pressure of resonance lamp
(ii) Medium pressure lamps
(iii) High pressure lamps.
Monochromator: In all photochemical experiments, it is desirable to work with light
of a single wavelength (monochromatic light), as far as possible.
All the light sources emit light of various wavelengths, which are mixed together.
Therefore, it becomes necessary to use a monochromator. Monochromator works as a filter for
radiation source. When light from the source passed through the lens and allowed to pass
through filter, it will absorb all of undesired wavelengths and transmit light of definite
wavelength which is needed for the reaction.
Monochromatic light in fact can be achieved by either a monochromatic source (a mercury
vapour lamp and diffraction grating) or by a system of filters. The simplest filter is glass
sleeve (quartz, vycor, corex or pyrex). Other narrow band-pass glass filters are also available.
The alternative to glass filters is the solution filters. Solution filters are solutions of
Na 2W0 4, SnC1 2 in HCl, 2M Na 3V04 , etc.
Reactors: Reaction vessel for the quantitative work required special design. For this
purpose, an apparatus composed of an immersion well apparatus surrounded by a merry-go­
round or carousel assembly which can take a number of sample tubes is used. These two
apparatus are used either for carrying out preliminary studies on a variety of compounds or
for the determination of the quantum yield of a given reaction. The most important essential
feature for the use of a merry-go-round assembly is to ensure that the light incident upon
the sample or on the actinometer is completely absorbed. This usually means that solutions
should have an optical density greater than five. Complete absorption of incident light can
be achieved by using combinations of filter solutions in three stage filter cell. Figure 15.6,
for quantum yield determinations, the light incident on the face of the sample cell can be
determined by the use of beam splitter to deflect about 10% of the light to an actinometer
cell or photomultiplier (Fig. 15.6).
r
Refl~~~~~~~~~]IDj§:IJD
Beam
Sample
splitter cell
/'-Hg super high
pressure lamp
1
Actinometer
ll
Filter train:
:
::
r - - I Actinometer
~cell
Fig. 15.6 Apparatus using focused light beam and filter train
Actinometry: The accurate measurement of the quantum yield depends mainly on the
accuracy of the actinometer employed, although analytical determination of the product (or
reactant consumed) is also important. Actinometer measures radiation intensity (i.e., 10 , I and
10 - I = Ia). In fact, radiation intensity can be measured by radiometric methods (Thermopiles,
thermistors or bolometers) or by photoelectric methods (phototubes or photovoltaic cells). These
are known as physical actinometers. In fact, intensity of radiation for quantum yield
determination is measured by chemical actinometer. The process is known as actinometry. A
chemical actinometer gives accurate value of 10 , I and thus Ia. A chemical actinometer should
have the following features: thermal stability, availability, reproductibility, uniform response
over a large wavelength range, and the case of the analysis of chemical change. The normal
range for solution phase photochemistry is from 250-450 nm chemical actinometer is best
suited to solution photochemistry. In the 250-450 nm range, the ferrioxalate actinometer
gives very satisfactory result. In this actinometer, 6 x 10-3 M aqueous solution of potassium
ferrioxalate is used. When this solution is irradiated by light it produces Fe++ (as ferrous
oxalate which does not absorb the light) and CO 2,
hv+[Fe(C20 4 )a]a- ~ [Fe(C20 4 )]2- +C20/"
C20/ + [Fe(C 2 0 4 h]g- ~ C20~- + [Fe(C20 4 hf 2­
.
[Fe(C20 4 )g]2- ~ [Fe(C20 4 )]2- + 2C02
These equations show that only one photon is absorbed by [Fe(C20~:J3-- which produces
oxa,lyl radical anion. This radical anion, in dark, reacts with another molecule of
[Fet~P">:J,3--. Thus, the observed quantum yield is twice the quantum yield of the primary
pho~~che~ical process.
~~e IRctinometer is monitored by measuring the concentration of Fe 2+ formed by
corhp~e,,~iQn with 1, 10 phenanthroline which produced a red complex analysed
spectrophotometricallyat 510 nm. The formation of Fe2+ is almost constant at short wavelengths
(ljlFe 2+ = 1.24 at 254 - 358 nm).
Many other systems can be used as actinometer, an often it is convenient to a reaction
whose quantum yield has been determined with reference to ferrioxaJ:ce. The Norrisch type­
II elimination of valerophenone is one such system. Valerophenone on irradiation at 313
produces acetophenone with quantum yield of 0.33.
o
Ph~
o
hv
-..:.:..-~)
313
II
C6H s-C-CHg + CHg-CH=CH2
<1>=0.33
2-Hexanone on irradiation at 313 produces acetone with ljl = 0.22 . A
o
~
hv
o
II
-----+) CHg-C-CHa + CHg-CH = CH 2
313
$=0.22
The new attractive system is Aberchrome. This system is operative in the 310-370 nm
and 436-545 nm ranges. This compound on irradiation at 310-370 nm converts into its isomer.
0
0
0
Aberchrome
1
31 0-370
4
I
0
0
In recent years, electronic means of measuring incident and transmitted light have
become popular. Nowadays automatic and semi-automatic electronic systems have advantages
in simplicity of use.
15.10.3 Procedure for Determining Quantum Yield of a Photochemical Reaction
The monochromatic light is first passed through the empty reaction chamber from the light
source and the intensity of light (I~ is measured by the detector. Now the reactants are taken"
in the reaction chamber and the same light is again passed through the chamber. The reactants
absorb radiation and a photochemical reaction is induced in the reaction chamber. The intensity
(I) of the transmitted light is again measured by the detector. The difference in the two
intensities gives the intensity of absorbed radiation (Ia =10 - I). The rate of this photochemical
reaction is measured by a suitable technique. By kl\owing the rate of reaction, wavelength of
light and intensity of absorbed radiation, the quantum yield of the process can be known by
the following equation:
<l>
= - - -Rate
- -of-reaction
----­
Intensity of absorbed radiation
= d[P].
dt
1
Ia
mmr_"wP'IP'tCi!r:'!iTti'!"Wr","'mt·:'m'f"",m",sm
All photophysical and photochemical processes go through excited states. These excited states
are excited reaction intermediates. These excited intermediate states may be either singlet or
486.
Photochemistry and Pericyclic Reactions
triplet in nature. The lifetime of these excited states affects the rate of photochemical reaction
directly because this is the time which is provided for the photochemical transformation. Mostly
photoexcited species undergo chemical change but there are equal chances of photophysical
deactivation also. Hence, the number of excited molecules and the number of molecules
converted into product is not the same. Thus, it is very important to make a relationship
between rate constant of photochemical reaction and lifetime of excited state of the reactant.
Consider the following photochemical reaction:
A+hv ---) A*
Rate
= Ia
A*
---} Product
Rate
= kr[A*]
~A
Rate
= kd[A*]
A*
For this reaction
Ia
kr = kr +kd = Ia . T
... (6)
The measured lifetime of an excited state can provide an estimate of the rate constant
for reaction in the excited state. The lifetime is defined by the expression:
T = (kr + kd)-1
... (7)
where
kr = Rate constant for the photochemical reaction
kd = Sum of rate constants for deactivation of A*.
_1I!tffJ~""~'ti5"
,.,e",!r'.rtS,FIIi
"l''If
The lifetime of an excited state which undergoes measurable physical or chemical change
(such as luminescence or product formation or both) can in principle be determined by the use
of a suitable quencher which interacts with the excited state in an energy transfer process
whose rate constant is known.
The fact one molecule can quench the excited state of another molecule by energy transfer
enables us to calculate the lifetime of the excited state in the absence of quencher. As an
example, we will consider a molecule that is excited to 81' undergoes intersystem crossing to
T 1 and from T 1 either gives products or is quenched. The individual steps and their rates are
given in Scheme 6. In Scheme 6, it is assumed that no emission (fluorescence from 8 1 and
phosphorescence from T 1) take place from the singlet state or the triplet state.
A + hv ---)
lAO
kd
lA*
k lSC
3A*
)
ISC
~
3A*+Q ~
)
lA*
Rate = Ia
A
3A*
= kd[lA*]
Rate =k,sc [IA*]
P(product)
Rate
= kr[3A*]
A + 3Q*
Rate
=kq[3A*][Q]
Rate
· 9 of Excited States and h'JV
Here,
Ia
kd
=Rate of absorption = Rate of formation of lA*
= Sum of the rate constants for deactivation of lA* except for
transfer to 3A*
k rsc = Rate constant for intersystem crossing
kr = Rate constant for the formation of product
kq = Rate constant for quenching of 3A* by quencher Q
Scheme 6
In the above photochemical reaction, A is the substrate, Q is quencher and P is the
product.
The rate of the formation of the product is given by equation (8)
...(8)
.
I
To replace the concentration of 3A* (and in process also for lA*) which is not directly
determinable, use is made of the steady-state approximation by applying it to the unstable
intermediate 3A* (and also lA*).
According to steady-state approximation for lA*
Ia = kd[lA*] + k rsc [lA*]
... (9)
= [lA*] (kd + k rsc)
[lA*]
Similarly for 3A*
k rsc [lA*]
Ia
= kd + k
... (10)
rsc
= kr[3A*] + kq [3A*]
= [3A*] (kr + kq[Q])
1
[Q]
...(11)
*
3A* _ krsd A ]
] - (kr+kq[Q])
[
...(12)
On substituting the value of [lA*] in equation (12) from equation (10) equation (13) is
obtained:
*
[3A]
krsc
Ia
= (kr+kq[Q])'(kd+krsc )
... (13)
Substituting the value of [3A*] in equation (8) from equation (13) gives equation (14).
d[P]
dt
= kr.~-c-.
Ia
(kr + kq[Q]) (kd + krsc )
... (14)
The quantum yield of the product (<I>product) is equal to the ratio of the rate of the fo:tmation
of the product to the rate of the light absorption, i.e.,
d[P]
<l>product
=
1
&' Ia
...(15)
Substituting value of
of the product
d~~]
from equation (14) in equation (15) gives the quantum yield
(<Pproduct).
kr ·kISC
A+hv
~
IA*
Rate = 1a
IA*
~
A
Rate = kd [lA*]
ISC ) 3A*
IA*
3A*
Rate = kISdIA*]
Rate = kr[3A*]
P
~
Scheme 7
The rate of the formation of product is given by equation (17)
d[P]
...(17)
= kr[3A*]
dt
Concentration of 3A* and IA* can be obtained by applying the steady-state approximation
as follows:
1a = kd[IA*] + k ISC [lA*]
-
= [lA*] (kd + k ISC )
[IA*] =
1a
(kd +kISC )
...(18)
Similarly
k ISC [IA*]
[3A*]
= kr [3A*]
= k1src eA*]
... (19)
Substituting the value of [IA*] in equation (19) from equation (18) gives equation (20).
[3A*] =
~kSC
r (
kd 1a
+ kISc)
... (20)
On substituting the value of [3A*] in equation (17) from equation (20), we get equation
(21).
d[P]
dt
= kr. ~sc .
kr
1a
(kd + kISc)
....(21)
The quantum yield of the product (<P~roduct ) in the absence of quencher can be obtained
as follows:
o
<Pproduct
== d[P].~
dt Ia
On substituting the value of
",0
"'product
d~~]
...(22)
in equation (22) from equation (21), we get equation (23)
kISC
== (kd + k
ISC
)
... (23)
On dividing equation (23) by equation (16) equation (24) is obtained
<P~roduct
<Pproduct
(kr + kq[Q])(kd + kISC )
==
(kr +kq[Q])
kr
== 1 + kq[Q]
kr
...(24)
If the product formation is all that 3A* does in the absence of quencher, then
1
== 't
kr
-
...(25)
1
where't is the actual lifetime of 3A*- the time required for 3A* to decrease to - of its initial
e
value. Substituting (25) into (24), we obtain the Stern.Vol!Der equation:
<P~roduct
'"
== 1 + kq[Q] . 't
... (26)
"'product
A plot of <P~roduct I <Pproduct versus quencher concentration Fig. 15.7 should give a straight
line with slope kq.'t (Fig. 15.7). If kq is assumed to be diffusion-controlled, then from the Debye
equation, we can obtain kq from equation (27).
8RT
kq == k diffusion == 3000"
where
1'] == Solvent viscosity in poise
Now actual lifetime of 3A*, 't is readily obtained.
... (27)
490
Photochemistry and Pericyclic Reactions
...,
'-'
::l
"0
B
~'-'""
i F-JI~:~:~:t~~~~~_k~:
.3i....5
o
/
""
~ ... ,~
--------­
- - - -..~ Concentration of Q
<P~roduct
•
FIg. 15.7 Plot of - - -
.
versus quencher concentration.
Q>product
, ';"3". ' O f i . '
i~~I~fr~~~f_i
• •~",v'.i.
Stern-Volmer equation can also be obtained for the photochemical reaction which takes place
by singlet state. Consider a molecule that is excited to S1' The Sl either gives products or is
quenched. The individual steps and their rates are given in Scheme 8. In Scheme 8, it is
assume that no emission takes place from the singlet state.
A+hv
~
1A*
1A*
~
A
lA*
~
P
1A* + Q
~
A + 1Q*
=Ia
Rate =kd [1A*]
Rate = kr [lA*]
Rate =kq [1A*][Q]
Rate
Scheme 8
In the above photochemical reaction, A is a substrate, Q is quencher and P is product.
The rate of the formation of product is given by equation (28).
d[P]
dt
= kr[l A*]
To replace the concentration of 1A* which is not directly determinable, use is made
the steady-state approximation by applying it to unstable 1A*.
0
Quenching of Excited States and In\feStlgafion.of·Reaction Mechanism
491
According to steady-state approximation
Ia = kd [lA*] + kr[lA*] + kq [lA*] [Q]
= [IA*] (kd + kr + kq [Q])
1 * _
Ia
[ A] - (kd + kr + kq[Q])
... (29)
Substituting the value of [IA*] in equation (28) from (29) gives equation (30).
= kr'
Ia
(kd + kr + kq[Q])
The quantum yield of the product
d[P]
<!>product
(<!>product)
... (30)
of the reaction is given by equation (31)
1
= &' Ia
... (31)
On substituting the value of d[P] in equation (31) from (30), we get equation (32)
dt
<!>product
=
=
kr ·Ia
1
(kd + kr + kq[Q]) Ia
kr
--kd + kr + kq[Q]
... (32)
In the absence of quencher, the individual steps and their rates are as follows (Scheme 9).
In Scheme 9, it is assumed that no emission takes place from the singlet state.
A+hv
~
lA*
Rate = Ia
~A
Rate;= kd [lA*]
~P
Rate = kr [lA*]
Scheme 9
The rate of the product formation is given by equation (33).
...(33)
C0~centration of
lA* can be obtained by the applying steady-state approximation as
follows:
Ia = kd[lA*] + kr[lA*]
= [IA*] (kd + kr)
*
Ia
[1 A ] = -k-d-+-k-r
... (34)
A plot of l1>~roduct
versus quencher concentration (Fig. 15.8) should give a straight
line with slope kq.'t (Fig. 15.8). If kq is assume to be diffusion-controlled then from the Debye
equation. We can obtain kq from equation (42).
ll1>product
8RT
kq = k diffusion = 3000r\
Now actual lifetime of lA*,
't
... (42)
is readily obtained.
~o product
~product
- - - - . . . . Concentration of Q
Fig. 15.8 Plot of
l1>~roduct
versus quencher concentration
l1>product
lUi
_In.~t'_rt:r'''·''j=,
. . .t'"t;o;1t1E1rtrnr.
Photochemical processes are very fast. Thus special techniques are required to obtain rate
measurements. One method is flash photolysis. The excitation is effected by a short pulse of
light in an apparatus designed to monitor very fast spectroscopic changes. The rate
characteristics of the reactions following radiation can be determined from these spectroscopic
changes.
Another useful techniques for measuring the rates ofcertain reactions involves measuring
the quantum yield as a function of quencher concentration. A plot ofthe inverse of the quantum
yield versus quencher concentration is then made (Stern- Volmer plot). Because the quantum
yield indicates the fraction of excited molecules that go on to the product, it is a function of
rates of the processes that result in other fates for the excited molecule. These processes are
described by the rate constants kq (quenching) and kn (other non-productive decay (like kd,
kIse ' etc.) to ground state.
494
Photochemistry and Pericyclic Reactions
The photochemical reactions may be either unimolecular or bimolecular. In unimolecular
reactions, product formation takes place either by singlet state or by triplet state. In bimolecular
reactions product formation takes place between excited state of one molecule and ground
state of another molecule (Scheme 10).
Unimolecular photochemical reaction
A' ~ Product
Rate = k[A*]
Bimolecular photochemical reaction
A' + B
Rate
~
Product
= k[A* ] [B]
Scheme 10
15.14.1 Rate of Unimolecular Photochemical Reactions from Singlet Excited State
The rate of the unimolecular reaction can be measured by measuring the quantum yield of the
reaction in the presence of the quencher. The mechanism of unimolecular reaction in the
presence of a quencher is given in Scheme 11. In this scheme, it is assumed that no emission
takes place from the singlet state.
A+ hv
~
=10
Rate =kd [IA*]
Rate = kr [lAO]
lA'
lA'
IC
kd
lA'
~
Rate
)A
,r
(P) Product
Rate = kq [lA' ][Q]
Scheme 11
The quantum yield of the above reaction is given in equation (32) (on page 491)
<j>
1
=
kr
kd + kr + kq[~]
kd+kr+kq[Q]
= ------­
kr
<j>
= 1+ kd + kq[Q]
kr
A plot of
slope kq/kr.
1
~
... (32)
kr
...(43)
versus quencher concentration [Q], (Fig. 15.9) then gives a line with the
Quenching of Excited States and InVestigation of Reaction Mechanism
495
1
I
I
I
<\>
:
I
I
Tan a
kq
= -k
= Slope
r
I
I
I
I
ke
Intercept = a = 1 + -,;;:
}
- - - - . . Concentration of Q
1
Fig. 15.9 A plot of
~
versus quencher concentration Q
It is usually possible to assume that quenching is diffusion-controlled, permitting
assignment of a value to kq by Debye equation.
8RT
= 3000rj
kq
where II is solvent viscosity in poise.
The rate of photochemical reaction, kr, for the excited intermediate (i.e., the rate of the
reaction) can then be calculated from equation (44).
kq
Slope
= kr
kq
8RT
kr=--=
---­
slope 3000rj x (Slope)
... (44)
15.14.2 Rate of Unimolecular Photochemical Reactions from the Triplet State
The unimolecular photochemical reaction from triplet state is given in Scheme 12. This
unimolecular reaction is carried out in the presence of a quencher Q. In this scheme, it is
assumed that no emission takes place from the singlet and the triplet
A+hv
~
kd
IA*
)
Rate = Ia
A
Rate
[lA*]
= kISC [IA1
Rate = kr [3A1
Rate = kq [3A* ][Q]
Rate
~ (P) Product
= kd
Scheme 12
The quantum yield of the above reaction is given in equation (16) (on page 488)
kr ·kIse
</l = kr +kq[Q](kd +kIse )
... (16)
We know that
...(45)
On substituting the value of
equation (46).
kIse
in equation (16) from equation (45), we get
(kd +kIse )
kr
</l
.,,'.
On convert </l in
1
~
~I
... (46)
from equation (46), equation (47) is obtained
1
•
= kr + kq[Q] . ~se
</l
=
kr+kq[Q] 1
kr
</lIse
=
[1 + kkr[Q]J_l­
...(47)
q
~se
1
1 kq[Q]
=--+--.-­
</lIse </lIse
kr
A plot of
1
~
... (48)
versus quencher concentration (Fig. 15.10) [Q], then gives a straight line
.h
WIt
slope -1- .kq
­
~se kr
1
q,
a
1
kq
Tana=Slope=-' - '
q,ISC kr
---------------------------­
} In<ereopt =
.,~
---~.~
.Fig. 15.10
Concentration of Q
1
A plot of -q, versus
a
f RE~acl:ion Mechanism
8lope (8)
Intercept (a)
1
kq
= --.
­
l!>Ise kr
from the plot
1
= A.~~
(from the plot)
'VIse
From equation (49) and equation (50), we get the value of kr as follows:
497
...
(49)
... (50)
1 kq
8lope (8) = - . ­
<i>Ise kr
1 kq
8=-'­
a kr
8
kr = -·kq
a
... (51)
It is usually possible to assume that quenching is diffusion-controlled, permitting
assignment of a value of kq from Debye equation (52).
8RT
kq
= 3000r)
... (52)
The rate of photochemical, reaction, kr, for the excited intermediate (i.e., rate of the
reaction) can then be calculated from equation (53)
kr
8
= -·kq
a
- 8- .8RT
-­
...(53)
- a 3000"
In this equation, the value of S and a is obtained from the plot. The value of RT and lJ is
known. Hence, the value of kr will be obtained.
.
15.14.3 Rate of Bimolecular Reaction from Triplet State
The mechanism and rate of different steps is given in Scheme 13. The reaction takes place by
formation of triplet state. This bimolecular reaction is carried out in the presence of quencher
Q. In this scheme, it is assumed, that no emission takes place from the singlet and the triplet.
A+hv
IA*
IA*
~
k
IA*
Rate
= Ia
= kd [IA*]
ISC
) [A]
Rate
k.Isc
) 3A*
Rate
3A* + B ~ (P) Product
3A*+Q ~ A+3Q*
= kIse [IA*]
Rate = kr [3A*][B]
Rate = kq [3A*][Q]
Scheme 13
Rate of the formation of product is given by equation (54)
d[P] = kr[3A * ] [B]
dt
...(54)
According to steady-state approximation
Ia = kd[IA*] + k rsc [IA* J
= [lA*](kd + k rsc)
Ia
*
PA ] =
...(55)
(kd+krsc )
Similarly
kIScl IA*]
=kr[3A* ][B] + kq
[3A* ][Q]
= [3A*] (kr[B] + kq[Q])
l
3A* _ krsd A*]
[
] - kr[B] + kq[Q]
...(56)
On substituting the value of FA*] in equation (56) from equation (55), we get
equation (57).
Ia
[3A*]
= ~(k-d-+-krsc)
krsc
(kr[B]+kq[Q])
... (57)
Substituting the value of [3A*] in equation (54) from equation (57) gives equation (58)
d[P]
-= kr[3A* ][B]
dt
Ia
krsc
= kr[B] . (kd + krsc ) . kr[B] + kq[Q]
= _ _k_r~[B---"],---'k2IS~C,,---·_Ia_
(kd + kISC )(kr[B] + kq[Q])
The quantum yield of the product
_ d[P]
G>product -
On substituting value of
(G>Product)
...(58)
of the reaction is:
1
&' Ia
...(59)
~JP] in equation (59) from equation (58), we get equation (60).
dt
kr[B]· krsc ·Ia
lJlproduct
= (kd + kISC )(kr[B] + kq[Q])· Ia
kr[B]· hrsc
= ------=--=----=--(kd + krsc )(kr[B] + kq[Q])
...(60)
Quenching of E
d Investigation of Reaction Mechanism
499 .
In the absence of the quencher, the individual steps of the reaction and the!r rates are
as following (Scheme 14):
A + hv
-----7
IA*
-----7
A
-----7
3A*
= Ia
Rate = kd [IA*]
Rate = krsc [IAj
Rate = kr [3A* ][B]
Rate
3A* + B -----7 P
Scheme 14
Concentration of 3A* and IA* can be obtained by applying the steady-state approximation
as follows:
Ia = kd [IA*] + k rsc [IA*]
= [IAj
[IA*]
=
(kd + kISC)
Ia
kd +krsc
... (61)
Similarly
kISdIA*] = kr [3A*] [B]
I
[3A*] = kISd A*]
kr[B]
... (62 )
On substituting the value of [3A*] in equation (62) from eqution (61), we get equation
(63).
[3A*] = kISC .
Ia
kr[B] (kd + kISC )
... (63)
Thus, rate of product formation is as follows:
d[P]
-= kr[3Aj[B]
dt
... (54)
On substituting the value of [3A*] in equation (54) from equation (63)-equation (64) is
obtained
d[P]
~
dt
*
= kr[B] . [3A ]
=kr[B]
kISC ·Ia
. kr[B](kd + k
rsc
)
...(64)
The quantum yield (<P~roduct) of the product in the absence of quencher is:
d[P]
D
<Pproduct
=
1
ili' Ia
...(65)
Substituting the value of d[P] from equation (64) in equation (65) gives equation (66).
dt
<Il~roduct =
kr[B] . kISC
la
kr[B](kISC + kd) la
... (66)
- kd +kISC
Dividing equation (66) by equation (60) gives equation (67).
<Il~roduct
=__
(kd +k )
kr[B]· kISC
(kd + kISC )(kr[B] + kq[Q])
ISC
----'-_-----'="-0--_
<Ilproduct
=
kr[B] + kq[Q]
kr[B]
= 1+ kq[Q]
... (67)
kr[B]
At constant [B], a plot of
<Il~roduct
versus quencher concentration [Q], (Fig. 15.11) is
<Ilproduct
linear and the slope is equal to
k~[~]'
kq
Slope = S = kr[B]
- - - - ­ Concentration of Q
Fig. 15.11 A plot of <Ilgroduct l<pprodUct versus Q at constant [B]
kq
Slope (S) = kr[B]
... (68)
kq
kr
= S[B]
It is usually possible to assume that quenching is diffusion-controlled, permitting
assignment of a value of kq from Debye equation.
8RT
kq = 3000rt
Thus
8RT
kr
...(69)
= 300011(S)[B]
The value of kr can be calculated from equation (69) because value of R, T, ll, Sand [B]
is known to us.
"'7111'"'1 ''1'1'1'1''11111'''
1
The establishment of a photochemical mechanism can be aided by the study of the effects on
the reaction of controlled change in the intensity of radiation employed on the system.
The effect of intensity can be used to determine the number of quanta of radiation
required to affect a particular chemical change. Under normal circumstances, where one
molecule of product arises from only one singly excited state of the reactant, the rate of the
formation of the product is directly proportional to the intensity of absorbed radiation. Such
reactions are primary photochemical reactions. Primary photochemical reactions thus are
monophotonic photochemical reactions. In monophotonic photochemical reactions, each absorbed
quanta excites one molecule of the reactant which then reacts to give the products. Thus
Rate ala.
But in case of secondary photochemical reactions, the proportionality depends on the
following:
(i) If the reactive excited state is doubly excited (which requires two photons for its
formation), then the rate of the formation of"product is proportional to the square of
intensity.
Rate a la 2
Such reactions are known as biphotonic photochemical reactions. In biphotonic
photochemical reactions, either two quanta are simultaneously absorbed by one molecule or
more commonly reaction occurs by the formation of two excited molecules. Reactions of this
category have not been widely reported, but they do occur when a high concentration of first
excited state can be generated.
(ii) In this case, the rate of the formation of product is proportional to the square of the
intensity of radiation.
Rate a .jk;,
This situation arises when two singly excited states are involved in the formation of one
molecule. of the product. This is not usually the direct interaction of two excited states, but
rather of two intermediates formed from separate excited states.
--
- - -
-
--
- -
--~ -~------------------
502 .
Let us take the example of the formation of benzpinacol.
1
hv
OH
I
OH
I
I
I
CSH 5-C-C-C sH 5
C 6H 5 C SH 5
The formation of product can be explained by two possible mechanisms.
First mechanism
(i)
, I',
(ii)
(iii)
1
In this mechanism is
Rate a; la2
second mechanism 0
(i)
II
C6H s-C-C6H s
0
II
OH
(ii)
I
CHa-Q-CH a + C6 Hs-C-C 6 Hs
1
(iii)
According to this mechanism
Rate ex la
Experimentally it has been found that rate of the reaction is directly proportional of la.
Thus, second mechanism is correct.
Index'
A
a-cleavage 198
Absorbance 165
Actinometry 484
Aliphatic azides 297
Aliphatic Claisen rearrangement 132
Aliphatic nitrenes 297
Alkenes 269, 273
Alkenyl chain 111
1, 3-alkyl group shift 286
Allyl carbanion 26
Allyl carbocation 33
Allyl free radical 116
Allyl vinyl ether 113
Allylic system 10
4-aminoalkenes 129
Anions 90
Anisole 293
Antara 58
Antarafacial process 104, 117
Anthracene 263
Anionic oxy-cope rearrangement 129
Antibonding molecular orbitals 15
Aromatic compounds 257
Aromatic hydrocarbons 263
Autoxidation 344
Aza cope rearrangement 129
Aza-Claisen rearrangement 134
Aza-di-7t-methane 250
A~iti<:s 297
Azo compound 297
B
Backstrom mechanism 264
Barbaralene 146
Barbaralone 146
Barton reaction 301-02
Beer's law 164
Benzene 282
Benzophenone 180, 262
Benzvalene intermediate 285
Benzyl carbanion 294
l3-cleavage 203
l3-hydrogen abstraction 213
Bicyclic compounds 78
Bicyclo [3, 1, 0] hexane-2-one 314
Bimolecular reaction 216, 497
Biphenyl 263
Bonding molecular orbitals 15
1, 3-butadiene 4, 281
Bridged homotropilidenes 146
13, y-unsaturated ketones 243
Bullvalene 146
c
C2 -symmetry 7,21
Carbon-carbon sigma bond 14
Carbonyl compounds 128, 262
Carotenoids 320
Cations 90
Cation radical 262
Chlorofluorocarbons 324
Chlorophyll 265
CIDNP 473
cis-I, 2-divinylcyclopropane 144
cis-3, 4-dimethylcyclobutene 24
cis-retinal 328
cis-stilbene 267
Claisen-cope rearrangement 132
505
506
. - II
'r '
Photochemistry and PericyclicReaetions
Colour films 332
Colour photography 332
Component 46
Cones 327
Conjugated dienes 278
Conjugated polyenes 8
Conrotatory motion 18, 52
Conrotatory ring opening 19
Conservation 3
Conservation of orbital symmetry 3, 14
Constructive overlap 15
Cope rearrangement 125, 128, 143, 147
Curing 343
Cyanine dyes 336
Cyclic ketones 190
Cyclo-octatetraene 146
Cycloaddition reactions 1, 58, 61, 79
[2 + 2] cycloaddition 224
[6 + 4] cycloaddition 91
[8 + 2] cycloadditions 92
1, 2-cycloaddition reactions 287
1, 3-cyclohe~adiene 34
Cycloaddition(s) 61, 104
Cyclobutanones 198
Cyclododecanone 211
Cyclohexanone 229
Cyclopentanones 196, 229
Cycloporpyl ketone 316
Cyclopropanone 315
Cycloreversion 87
Cycloreversion or retrograde cycloaddition 105
o
Debye equation 493
Decarbonylation 190
2, 4, 6, 8-decatetracne 36
Degenerate cope rearrangement 143
Dehydrobullvalcne 146
Delocalised orbitals 15
Destructive overlap 15
Diimide 156
Di-n-methane 251
Di-1t-methane type rearrangement 233
Diazo compounds 296
Diazoalkanes 296, 297
Diazoketones 296
Diels-Alder 63
Diels-Alder addition 216
Diels-Alder reaction 72
Diene 63
Dienones 235
Dienophile 63, 64
2, 6-dimethyl-cyclohexanone 195
1, 3-diphenylacetone 186
Dioxetanes 26'6
1, 3-dipolar addition 101, 105
1, 3-dipolar molecule 99
Dipolarophile 101
1, 3-dipolar species 99
1, 3-dipole 105
1, 3-dipole addition 104, 105
1, 3-photoaddition 289
1, 4-photoadditions 290
Disrotatory 34
Disrotatory motion 19, 52
Disrotatory process 18
Disrotatory ring-closure 21
Disrotatory ring-opening 19
dl-bornyl 305
dl-isobornyl 305
Donor molecule 178
E
Einstein's law 182
Electrocyclic 17
Electrocyclic reaction 1, 52
Electrocyclic ring-opening 29
Electronic energy 160
Electronic excitation 167
Electron-transfer quenching 466
Endo and exo stereochemistry 70
Endo orientation 78
Ene reaction 153, 158
Energy cascade 175
Enophile 153
Epoxy ketone 204, 316
EPR 473
Excimer(s) 226, 462
Exciplex 221, 226, 463
Excited state 13, 479
F
Fenchone 202
Flash photolysis 473
Index
Fluorescein 265
Fluorescence 176
Fluxional molecules 146, 147, 150
FMO method 60, 115
Frank-Condon principle 167
Frontier orbital 76
Frontier-orbital approach 52
G
Germacrone 150
Grotthurs-Drapper law 182
Group transfer reactions 2, 153
H
Halo fluorescein 265
Heavy atom quenching 466
2, 4, 6-heptatricnyl 11
(2E, 4Z)-2, 4-hexadiene 24
1, 3, 5-hexatriene 34
Hoffmann-Loeffler-Freytag 301, 306, 310
HOMO 13, 15
HOMO-LUMO transition 169
Huckel 85
Huckel-Mobius 50
Hydrazine 156
Hypochlorite 265
Hypohalitcs 309
Imaging 342
Intermolecular cycloaddition 297
Intermolecular photo reduction 216
Internal conversions 176
Intersystem crossing 174, 177
Intramolecular 103
Intramolecular Diels-Alder reactions 73
Intramolecular dimerisation 276
Inversion of configuration 122
Ireland-Claisen rearrangemen~ 134
Isomerisation 312, 340
T,:::;topic labelling 477
J
Jablonski diagram 175
507
K
Ketyl radical 262
-Kimel-Cope rearrangement 134
L
Lambert's law 164
Lifetime 485, 490
Low temperature photolysis 472
Lumiketone 312
Lumiketone rearrangement 230
Luminescene 174
Luminosantonin 313
Lumisantonin 316
LUMO 13, 15
M
m-symmetry 6, 21
Mechanism of Diels-Alder reaction 65
Mesosphere 322
Metalla-ene reaction 153
Methylene blue 265
2-methyl-3, 5-diphenylisoxazoliding 202
Microscopic reversibility 37, 38
Migrating group 112
Mobius 85
Modified 143
Modified cope rearrangement 143
Molar absorptivity 165
Molecular orbitals 15
Morse curve 161, 167, 181
N
N-chlorosuccinamide (NCS) 307
N-diethyl-p-phenylenediamine 334
N·haloamides 309
N-halocycloalkylamines 307
Naphthalene 263, 291
Nazarov cyclisation 27
Neural signal 331
Nitrene 298
Nitrite esters 309
Nitrobenzene 293
Nitrogen oxides 323
Nitrosoalcohol 303
-
• c ?i4Ji#4>
4-nitroso-4-phenylbutanol 303
4-nitroso-5-phenylpentanol 303
Nitrous oxide (N20) 323
N, N-diethyl-p-phenylenediamine 334
Nonbonding molecular orbitals 15
Norrish type I and II cleavage 226
o
1, 3, 5, 7-octatetraene 26
Opsin 328
Optical bleach 337
Orbital correlation diagram 52
Organic cations 147
Organometallic compounds 147
Orientation effects 72
Ortho hydroxybenzophenones 344
Oxacarbene 200
Oxa-Di-7t-methane rearrangement 259
Oxy-cope rearrangement 128, 129
Ozone chemistry 322
p
p-benzoquinone 309
PAN formation reactions 326
Paterno-Biichi reaction 218, 223, 226
2, 4-pentadienyl 11
Perturbation molecular orbital (PMO) theory 4,
50
Perturbation theory 4
Phenanthrene 267, 291
Phenoxy radicals 258
Phosphorescence 174
Photo chemical reactions 84
Photo induced reaction 77
Photo rearrangements 229
Photo reduction 262
Photo-Fries rearrangement 258
Photo-induced cyclisation 24, 27, 31
Photo-induced ring opening 29, 30, 35
Photoaddition of alkenes 287
Photochemical dissociation 322
Photochemically allowed reaction 52
Photochromic isomerisation 340
Photocycloaddition 298
Photoelectrophilic substitution 293
Photoenolisation 215
Photographic sensitisers 336
Photolysis 192, 257, 309
Photonucleophilic substitution 293
Photooxidation 344
Photooxygentation 267, 317
Photopolymerisation: Imaging 342
Photoracemisation 208
Photoreduction 267
Photosensitisation 177, 179
Photosensitised oxygen transfer 265
Photosensitiser 177, 179
Photostabilisation 344
Photostationary 270
Phototoxic 326
Polyenes 265
Potassium azodicarboxylate 156
Prefulvene biradical 290
Prismane intermediate 285
2-pyrazoline 102
Pyrex 166
Q
Quantum yield 182, 480
Quartz 166
Quencher 461
Quencher(s) 187, 461
Quenching 180
Quenching by oxygen 469
Quenching mechanism 465
R
Radical substitution 295
Rate constants 490
Reaction intermediates 475
Retro-Diels-Alder reaction 87, 89
Retrocycloaddition 87
Rhodopsin 327, 328
Riboflavin 265
Ring opening reactions 18
Ring-closing reactions 20
Rods 327
Rose Bengal 265
Rotational energy 160, 161
s
S-(+)-a-phellandrene 150
s-cis conformation 66
Santonin 313
Schrodinger wave equation 162
Selection rules 45, 124, 142
Semibullvalene 146
1, 2-shift 285
[1, 3] shift 115
[1, 5] shifts 115
Sigmatropic rearrangement 1, 111
Sigmatropic shifts 147
[1, 3] sigmatropic rearrangement 115, 141
[1, 5] sigmatropic rearrangement 118, 142
[2, 3] sigmatropic rearrangement 114, 134
[3, 3] sigmatropic rearrangement 112, 130
[4, 5] sigmatropic rearrangement 137
[5, 5], sigmatropic rearrangement 136
[9, 9] sigmatropic rearrangement 137
[1, 3] sigmatropic shifts 117
[1, n]-sigmatropic shift 147
[m, n]-sigmatropic shift (m *" 1) 147
Singlet carbenes 96, 296
Singlet oxygen 265, 318
Singlet state 172
Singlet-triplet 174
Smog formation reactions 326
Solvent cage 258
Stereochemical 66
Stereochemistry 62, 67, 68, 100, 119, 128, 134
Stereochemistry of cope rearrangement 128
Stereospecific 21, 157
Stern-Volmer plot 493
Steroids 312
Stilbenes 270
Stratosphere 322
Substituted benzene 282
Sulphur dioxide 322
Supra 58
Suprafacial 58
Suprafacial migration 119
Suprafacial process 104, 116
Symmetry allowed reaction 15, 52
Symmetry-forbidden reaction 15, 52
syn stereochemistry 67
(4n + 2)n systems 31
4nn systems 24
T
Tautomerism 147
Terpenoids 312, 320
2, 2, 4,4-tetramethylpiperidine-1-oxyl 186
Thermal chemistry 160
Thermal energy 160
Thermal induced reaction 76
Thermal reactions 84
Thermal ring-opening 30, 34
Thermal-induced cyclisation 24, 26, 31, 32, 34
Thermal-induced ring opening 29
Thermally allowed reaction 52
Thermosphere 322
Translational energy 160
Triazoline 102
Triplet carbene 296
Triplet oxygen 265
Triplet state 173
Triplet-singlet 174
Troposphere 322
u
Ultraviolet screening agents 337
Umbellulone 315
Unimolecular photochemical reactions 494
Unsaturated ketone 316
v
Valence tautomerism 147
Vibrational cascade 175
Vibrational energy 160, 161
Vision 327
Vitamin A 331
w
Wittig rearrangement 135
Woodward-Hoffmann rule 46, 52, 84, 137, 156
z
Zimmerman 239
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