Pericyclic reactions
•
•
•
•
Electrocyclisation
Sigmatropic
Cycloadditions
Cheletropic reactions…
• Frontier orbitals
• Correlation diagrams (MOs, States)
• Aromaticity of the Transition State
1
Pericyclic reactions
A pericyclic reaction is a reaction wherein the
transition state of the molecule has a cyclic
geometry, and the reaction progresses in a
concerted fashion.
2
Electrocyclic reaction
ring closure of conjugated systems
An electrocyclic reaction is a pericyclic reaction where the net result
is one p bond being converted into one s bond. electrocyclic
reactions are photoinduced or thermal
p
p
p
s
p
p
TS
3
Woodward- Hoffmann rules
and symmetry conservation
• Concern pericyclic reactions.
• Tell about the mechanism passing through
the lowest activation barrier
• Does not tell anything about the
thermodynamic (reaction or reverse
reaction)
• Based on symmetry conservation
4
Why Woodward- Hoffmann rules
are important?
• Allows synthesis of compounds with
determined asymmetric carbons.
• First example of useful application of
theory
• Simplify sophisticated systems
• Sample the main methods of analysis
using theory
• Prevails over alternative explanations such
as steric effects.
5
Woodward- Hoffmann rules
Robert Burns Woodward 1917-1979
American, Nobel 1965
Roald Hoffmann 1937
American, Nobel 1981
6
Roald Hoffmann 1937
American, Nobel 1981
7
Conservation of Orbital
Symmetry
H C Longuet-Higgins E W Abrahamson
Hugh Christopher Longuet-Higgins
1923-2004
8
What symmetry is preserved?
A mirror
A C2 axis
W-H rules say that which symmetry element has to be preserved.
9
Up to now,
implicitly we have only considered the mirror symmetry
j = 3, AS
j = ± 2, AA
j = ± 2, SS
j ± 1, SA
j = ± 1, AS
j = 0, SS
10
Electrocyclic reaction
The orientation of CH3 depends on the symmetry
conservation: when the mirror symmetry is preserved
(here called disrotatory mode: rotation in opposite
senses) we obtain the following reactions with
asymmetric carbons
disrotatory
(2Z,4Z,6Z)-octatriene
disrotatory
The conrotation would give the opposite correspondance; the knowledge of the
mechanism allows you to make the compound with the desired configuration.
11
The conservation of the C2 axis
(here called conrotatory moderotation in the same sense) is not
observed by thermal cyclization
It is the mechanism for
photochemical process
12
Electrocyclic reaction
ring closure of conjugated systems
An electrocyclic reaction is a pericyclic reaction where
the net result is one p bond being converted into one s
bond. Electrocyclic reactions are photoinduced or
thermal
13
Sigmatropic reaction
Sigmatropic reaction is a pericyclic reaction wherein the net result is one s
bond changed to another s bond.
1
1’
3
2’
[2,3]
[3,3]
14
Sigmatropic reaction
Sigmatropic reaction is a pericyclic reaction wherein the net result is one s
bond changed to another s bond.
15
Cope Rearrangement
sigmatropic [3,3]
Cope Rearrangement
Oxy-Cope Rearrangement
1Arthur Cope1902-1958
1’
3
3’
1
1’
3
3’
16
Claisen rearrangement
sigmatropic [3,3]
Rainer Ludwig Claisen
(1851-1930) German
1
1
1’
1’
3’
3
3’
3
17
cycloaddition
A cycloaddition is a reaction, in which two
π bonds are lost and two σ bonds are
gained. The resulting reaction is a
cyclization reaction.
18
cycloaddition
This generates chiral compounds. Steric
hindrance does not systematically explain.
19
Cheletropic reaction
A Cheletropic reaction is a pericyclic reaction where the net result is the
conversion of a pi bond and a lone pair into a pair of sigma bonds; with both
new sigma bonds adding into the same atom.
.
20
Transition State Aromaticity
(Dewar and Zimmermann)
Does not explicit MOs (does not require any
calculation)
Based only on the signs of the overlaps
and on the count of the electrons involved.
21
Huckel annulene
The reaction goes through a ring
22
Huckel annulene
The reaction preserving a mirror symmetry goes through a ring
23
Hückel annulene
By convention all the AOs are oriented in the same direction
(+ above the plane – below): The overlaps are positive:
S>0
S>0
S>0
+
+
+
+
+
S>0
+
S>0
Reversing the sign of one AO still makes an
even number of positive overlaps:
This defines Hückel annulene
+
+
For real unsaturated compound,
this is always the existing situation
S>0
+
+
+
S<0 S<0
+
24
Aromaticity
according to Dewar
Michael J. S. Dewar (Michael James Steuart Dewar)
English
born in Ahmednagar, India in 1918
The Dewar-Chatt-Duncanson model is a model in
organometallic chemistry which explains the type of
chemical bonding between an alkene and a metal (pcomplex) in certain organometallic compounds. The
model is named after Michael J. S. Dewar, Joseph
Chatt and L. A. Duncanson .
25
Radical chain + C radical atom
comparing the chain with the ring: Aromaticity
First order term.
S
E=0
E=0
4/√(N-1) for the ring
2/√(N-1) for the chain
A
E=0
E=0
0 for the ring
2/√(N-1) for the chain
26
Radical chain + C radical atom
comparing the chain with the ring: Aromaticity
Aromaticity according to Dewar
S
A
When the SOMO is symmetric
The ring is more stable than the chain
The polyene is AROMATIC
N-1 is even
N = 4n +2
When the SOMO is antisymmetric
The ring is less stable than the chain
The polyene is ANTIAROMATIC
N-1 is odd
N = 4n
The SOMO is once upon twice S (n=2n-1) or A (2n+1)
27
Hückel-type annulene
-2 

2

2
j=3
j=±2
j=±1
j=0
Aromatic 4n+2 electrons; antiaromatic 4n electrons
28
August Ferdinand Möbius
German 1780-1868
was a descendant of Martin Luther
by way of his mother.
http://www.youtube.com/watch?v=JX3VmDgiFnY
29
Moebius rings: Aromaticity
One negative overlap.
A
S>0 A S<0
…
A
…
p orbital
binding through
opposite lobes
S
E=0
E=0
0 for the ring
2/√(N-1) for the chain
A
E=0
E=0
4/√(N-1) for the ring
2/√(N-1) for the chain
30
Möbius annulene
aromaticity rules are reversed
2
Aromatic 4n electrons; antiaromatic 4n+2 electrons
31
Aromatic systems have the largest HOMO-LUMO gap
(the most stable ground state and the least stable first
excited state)
Antiaromatic systems have half filled degenerate
non bonding levels (the smallest gap; the most stable
first excited state and the least stable ground state )
2
32
Aromaticity rules for are the
opposite for thermal and
photochemical reaction
D
hn
33
Electrocylic
34
Electrocyclic
C2 axis of symmetry
Mirror symmetry
35
Sigmatropic
Mirror symmetry
C2 axis of symmetry
36
Sigmatropic
37
Cycloadditions
Suprafacial and antarafacial attack
Suprafacial attack
Antarafacial attack
38
Bond formation
Supra
Supra
Antara
Supra
39
Cycloaddition
Supra-Supra
Hückel
Mirror symmetry
Supra-Antara
Möbius
C2 axis of symmetry
Antara-Antara
Hückel
Mirror symmetry
unlikely
40
Transition State Aromaticity
(Dewar and Zimmermann)
Does not explicit MOs
Based only on the sign of the overlaps
And on the electron count
41
For a thermal reaction (ground state)
4n (4) electrons one S<O
4n+2 (6) electrons all the S>O
For a photochemical reaction (excited state)
4n (4) electrons all the S>O
4n+2 (6) electrons one S<O
42
Bimolecular reactions: Favorable
interaction in Frontier Orbitals.
- this determines the conservation
of a symmetry operation (axis or plane)
Unimolecular reactions: Symmetry
conservation of the HOMO
- the HOMO accommodates the most
mobile electrons
- its amplitude is generally large at the
reaction sites
43
Electrocyclic: symmetry
conservation of the HOMO
The HOMO is U symmetric for C2 (antisymmetric for s)
44
The “conservation of the HOMO” requires calculating the HOMO
The “TS aromaticity” does not: it only look at the sing of overlaps and
the # of electrons involved.
45
Symmetry alternates for MOs
in a linear polyene
HOMO symmetry switches according to N
G: three nodes
U: two nodes
G: one node
U: no node
For a mirror symmetry U=S an G=A, for a C2 symmetry U=S and G=A
46
Photochemical reaction
6-e
Conrotatory
47
Electrocyclic
Molecules with symmetric HOMOs give disrotatory
ring-closure products.
Ring Closure With Symmetric HOMO
ground state 4n+2 electrons D
excited state 4n electrons
hn
Molecules with symmetric HOMOs have the top lobe of one orbital
in the same phase as the top lobe of the other orbital.
48
Electrocyclic
Molecules with antisymmetric HOMOs give
conrotatory ring-closure products.
Ring Closure With antiSymmetric HOMO
ground state 4n electrons D
excited state 4n+2 electrons hn
Molecules with antisymmetric HOMOs have the top lobe of one orbital in
the same phase as the bottom lobe of the other orbital.
49
(2E,4Z,6E)-Octatriene ring closure is
disrotatory, yielding cis-5,6-dimethyl-1,3cyclohexadiene
The HOMO of (2E,4Z,6E)-octatriene is symmetric because MOs of linear
conjugated pi systems alternate in symmetry starting with the lowestenergy MO being symmetric. (2E,4Z,6E)-Octatriene has six MOs (from six
atomic p orbitals overlapping), half of which (three) are filled in the ground
state. The third-lowest-energy orbital has to be the HOMO, and it has to be
50
symmetric rather than antisymmetric.
(2E,4Z,6Z)-Octatriene ring closure is
disrotatory, yielding trans-5,6-dimethyl-1,3cyclohexadiene.
The HOMO of (2E,4Z,6Z)-octatriene is symmetric because MOs of linear
conjugated pi systems alternate in symmetry starting with the lowest-energy
MO being symmetric. (2E,4Z,6Z)-Octatriene has six MOs (from six atomic p
orbitals overlapping), half of which (three) are filled in the ground state. The
third-lowest-energy orbital has to be the HOMO, and it has to be symmetric
51
rather than antisymmetric.
Photochemically induced (2E,4Z,6Z)-octatriene
ring closure is conrotatory, yielding cis-5,6dimethyl-1,3-cyclohexadiene.
The HOMO of (2E,4Z,6Z)-octatriene which has been excited by light is
antisymmetric because MOs of linear conjugated pi systems alternate in
symmetry starting with the lowest-energy MO being symmetric. (2E,4Z,6Z)Octatriene has six MOs (from six atomic p orbitals overlapping), half of which
(three) are filled in the ground state. The third-lowest-energy orbital has to be
the HOMO in the ground state, and the fourth-lowest-energy orbital has 52
to be
the HOMO of the excited state.
(2E,4Z)-Hexadiene undergoes conrotatory
ring closure to yield cis-3,4dimethylcyclobutene.
The HOMO of (2E,4Z)-hexadiene has to be antisymmetric because this
compound has to have four pi MOs, two of which are filled. The HOMO has to
be the second-lowest-energy orbital. Since the lowest-energy orbital has to be
symmetric, the HOMO has to be antisymmetric.
53
(2E,4E)-Hexadiene undergoes conrotatory
ring closure to yield trans-3,4dimethylcyclobutene.
The HOMO of (2E,4E)-hexadiene has to be antisymmetric because this
compound has to have four pi MOs, two of which are filled. The HOMO has to
be the second-lowest-energy orbital. Since the lowest-energy orbital has to be
54
symmetric, the HOMO has to be antisymmetric.
Suprafacial and Antarafacial Sigmatropic Rearrangements
A sigmatropic rearrangement in which a migrating group remains on the
same face of the pi system as it migrates is suprafacial. If the migrating
group moves from one face of the pi system to the opposite face, the
migration is antarafacial.
Suprafacial migrations normally occur when the HOMO of the p system is
symmetric, and antarafacial migrations normally occur when the HOMO of
the p system is antisymmetric and the migration transition state is a ring with
seven or more atoms in it
55
Migration of Hydrogen
Since hydrogen's s orbital has only one phase, the phase of the lobe of
the developing p orbital of the atom it migrates from and the lobe of the p
orbital of the atom it migrates to must have the same phase. Thus,
hydrogen is forced to migrate suprafacially in cases where there is an
odd number of electron pairs involved in the migration (symmetric
HOMO) and antarafacially in cases where there is an even number of
pairs of electrons involved in the migration (antisymmetric HOMO).
56
Migration of Carbon Using One Lobe
Suprafacial and antarafacial migration of carbon with carbon using the same lobe to
bond to its destination position that it uses to bond to its original position.
Caption
Notes
When carbon uses only one lobe to migrate in a sigmatropic rearrangement, it
must migrate suprafacially when an odd number of electron pairs are involved
in the migration (symmetric HOMO) and antarafacially when an even number
of electron pairs are involved in the migration (antisymmetric HOMO). 57
This
type of migration results in retention of configuration at the migrating carbon
Migration of Carbon Using Both Lobes
Suprafacial and antarafacial migration of carbon with carbon using the opposite lobe to
bond to its destination position from the one that it uses to bond to its original position.
When carbon uses both lobes to migrate in a sigmatropic rearrangement, it
must migrate antarafacially when an odd number of electron pairs are involved
in the migration (symmetric HOMO) and suprafacially when an even number of
58 of
electron pairs are involved in the migration (antisymmetric HOMO). This type
migration results in inversion of configuration at the migrating carbon.
Cycloaddition 4+2
Supra-supra
The Diels-Alder reaction represents
the prototype of cycloadditions.
Besides the Grignard reaction, it is
the most cited name reaction in
chemical literature.
The reaction principle was discovered in
1928 by Otto Diels and his student Kurt
Alder. Both were honored with the Nobel
Prize for Chemistry in 1950.
59
The Diels-Alder reaction
represents the prototype of
cycloadditions. Besides the
Grignard reaction, it is the most
cited name reaction in
chemical literature.
Otto Diels 1876-1954
Kurt Alder. 1902-1958
The reaction principle was discovered in 1928 by Otto
Diels and his student Kurt Alder. Both were honored with
the Nobel Prize for Chemistry in 1950.
60
Frontier orbital intractions:
Diels-Alder 6-e
Cycloaddition 4+2 Supra-supra
61
Cycloaddition 2+2 Supra-antara
Frontier MO analysis of
a [2 + 2] cycloaddition
reaction under thermal
and photochemical
conditions.
Under thermal conditions, this cycloaddition would have to be antarafacial,
which is impossible for a [2 + 2] cycloaddition (forms a four-membered ring).
Under photochemical conditions, this reaction allows suprafacial ring formation.
62
Cycloaddition 2+2
Supra-antara
63
Cycloaddition 2+2
Supra-antara
LUMO
Supra
HOMO
antara
HOMO
supra
LUMO
antara
64
The Alder rule
diene with D ligand, dienophyle with A ligand
The reverse Alder rule
D
D
A
D
A
A
LUMO
LUMO
HOMO
HOMO
65
The Alder rule
The reverse Alder rule
D
D
A
D
A
A
LUMO
D
A
LUMO
HOMO
HOMO
It is better to have substituent with opposite properties.
Each pair favors one Frontier orbital interaction
66
Regioselectivity
The Alder rule
D
D
A
D
A
or
**
A
D
Concerted reaction but not synchronous.
The atoms with the largest coefficients
bind first.
A
67
Electrocyclic
4n+2
disrot
4n
conrot
4n+2
conrot
4n
disrot
4n+2
supra
4n
antara
4n+2
antara
4n
Supra
4n+2
Suprasupra
4n
Supraantara
4n+2
Supraantara
4n
Supra68
supra
D
Electrocyclic
hn
Sigmatropic
D
Sigmatropic
hn
cycloaddition
D
cycloaddition
hn
Correlation of MOs
Dewar - Zimmermann
69
Electrocyclic 4-e
D C2 conservation hn s conservation
70
Electrocyclic 4-e
D C2 conservation hn s conservation
71
Electrocyclic 4-e
D C2 conservation
SA2A
S2S2
S2AS
S2A2
sp2s*
p12p3p4 S2AS
s2p*2
p12p22 A2A2
A
Barrier
s2pp*
p12p2p3 A2SA
S
s2p2
p12p22 A2S2
No barrier
cyclobutene
butadiene
Diagram of states
72
Electrocyclic 4-e
hn s conservation
p12p32 S2S2
S2A2
S2AS
s2p*2
p12p22 S2A2
S
No barrier
A
s2pp*
p12p2p3 A2SA
S
S2S2
s2p2
p12p22 S2A2
cyclobutene
butadiene
Diagram of states
73
Correlation of MOs
Dewar – Zimmermann
Electrocyclic 6 - e
74
Electrocyclic 6-e
D s conservation hn C2 conservation
75
Cycloaddition 2+2
D C2 conservation hn s conservation
76
Cycloaddition 4+2
Diels-Alder
Mirror symmetry
s supra-supra
Disrotatory rotation of
the terminal AOs from
the butadiene
C2 symmetry
Supra: ethene in plane
Antara: butadiene
Conrotatory rotation of
the terminal AOs from
the butadiene
77
Cycloaddition 4+2
D s conservation
Y4
A
A
S
p* A
A
Y3
Y2
S
A
S
p
Y1
S
S
A
S
78
Cycloaddition 4+2
hn C2 conservation
Y4
S
A
S
p* A
S
Y3
Y2
A
S
A
p
Y1
S
A
A
S
79
Cycloaddition 6-e
D s conservation
sS2sAp2sS*
S2AS2S
S2SA2A
Y12pY32s*
S2S2ASY1
S2S2A2
2p2Y
2 Y3
Y12p2Y22
Butadiene + ethene
A
Barrier
A
sS2sA2pp* S2A2SA
S
No barrier
sS2sA2p2 S2S2A2
2s+1s
Diagram of states
80