Jagdamba Singh(Pericyclic & Photochemistry)

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]* [[] + 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- O2 + R-C-R R-C-R I 0-0' OR I OR I O-O-H OR I ~ ejr-C- I 0-0' OR I (iv) ejr-Q- + OR I 0 II R-C-R ~ I ejr-C- 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) ~ 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 OR .0 cjl cjl 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: 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 (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) = - . 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

Jagdamba Singh(Pericyclic & Photochemistry)

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