Pericyclic reactions involve the concerted formation and breaking of bonds in a cyclic transition state. There are several types of pericyclic reactions including electrocyclization reactions, cycloaddition reactions, and sigmatropic rearrangements. Electrocyclization reactions involve the conversion of a linear conjugated polyene to a cyclic product in one step. The stereochemistry of the product depends on whether heat or light is used to promote the reaction. Cycloaddition reactions convert two linear conjugated polyenes to a cyclic product in one step, often with high stereochemical control. Sigmatropic rearrangements involve the concerted migration of atoms or groups of atoms. The Woodward-Hoffmann rules can be used to predict the stereochemistry

1. Pericyclic reactions are; “Any concerted reaction in which bonds
are formed or broken in a
cyclic transitions state”. (electrons move around in a circle).
i.e. there is a single transition state from start to finish, in contrast to
a stepwise reaction.
Pericyclic Reactions
Transition state
reaction co-ordinate
Energy
starting
material
product
Concerted reaction
Transition states
reaction co-ordinate
Energy
starting
material product
Multistep reaction
inter-
mediate inter-
mediate

2. 2
Properties of pericyclic reactions:
(a) Little, if any, solvent effect (b) No nucleophiles or
electrophiles involved.
(c ) Not generally catalysed by Lewis acids.
(d) Highly stereospecific. (e) Often photochemically promoted.
1) Electrocyclisation reactions – Linear conjugated
polyene converted into a cyclic product ( Pi-
bonds convert into sigma and new pi-bond)
in one step. The mechanism is not particularly
surprising, but the stereochemistry changes
depending
on whether heat or irradiation (typically UV-light) is
used to promote the reaction. e.g.

3. 3
Me
Me
heat Me
Me
Me
Me
irradiation (hυ) Me
Me
2) Cycloaddition reactions – Two linear conjugated polyenes
converted onto a cyclic product in one step. Again, the
stereochemistry of the reaction is remarkably reproducible. e.g.
(two cis or Z alkenes)
+ but no
irradiation
(hυ)

4. mple of a cycloadditions to give a 6-membered ring:
(one cis or Z alkene,
and a E,E-diene)
heat
Me
Me
Me
Me
O
O
O
O
O
O
One diastereo-
isomer is favoured.
(Diels-Alder reaction!)
3) Sigmatropic rearrangement reactions: This involves a
concerted migration of atoms or of groups of atoms. E.g.
migration of a σ-bond.

5. H H
This would be classified as a [1,2]-sigmatropic
rearrangement (or shift).
This would be classified as a [1,5]-sigmatropic
rearrangement (or shift).
The numbering refers to the number of atoms in the
transition state on either side of where bonds are made
or broken.
H
Me
H
Me
3) Sigmatropic rearrangement reactions: A high level of
stereochemical control is often observed.
This would be classified as a [3,3]-sigmatropic
rearrangement (or shift).
Me
Me
Me
Me
only, no:
Me
Me
observed

6. Woodward-Hoffmann theory for prediction of the stereochemistry
of pericyclic reactions: Cycloaddition reactions.
In cycloaddition reactions, the situation is
slightly different because a) two molecules are
used and b) electron flow takes place from the
highest occupied molecular orbital (HOMO) of
one molecule to the lowest unoccupied
molecular orbital (LUMO) of the other. The
stereochemistry therefore follows from the
wavefunction signs of the orbitals on each
molecule.
Consider the reaction of a butadiene with an
alkene (the Diels-Alder reaction):
The reaction is usually heat-promoted,
but sometimes it is carried out photochemically.

7. In this process the reagents approach each other in a ‘face to
face’ manner, i.e. so that the p- orbitals of the π-system can
combine with each other. The relevant orbitals are shown
below:
H H
HH
H H
HH
LUMO
HOMO
Alkene Butadiene
H H
HH
H H
H H
HH
H H
LUMO
HOMO
So the following combinations can be employed in the
suprafacial cycloaddition reaction:
H H
HH
H H
HH
LUMO
HOMO
Alkene
Butadiene
H H
HH
H H
H H
HH
H H
LUMO
HOMO

8. In both cases, the appropriate signs of the wavefunctions
on the orbitals are matched so that the reagents can
approach each other in a face to face manner and also
form bonds easily.
In practice, it is usually the combination of diene HOMO
with alkene LUMO which leads to the product, rather than
the diene LUMO and alkene HOMO. Electron-
withdrawing groups on the alkene lower its LUMO
energy and improve the matching to the diene HOMO. In
turn this increases the reaction rate. Hence, electron-
withdrawing groups on an alkene generally increase the
reaction rate, often very significantly. As might be
predicted, electron-donating groups on the diene also
improve the rate – by pushing its HOMO energy closer to
that of the alkene LUMO

9. More closely-matched orbitals give a greater
energetic benefit when combined. Hence the
closely related butadiene HOMO and alkene
LUMO represent the favoured combination. When
electron-withdrawing groups are present on the
alkene, the benefit is even greater because the
HOMO/LUMO levels are even closer. Lewis acids
speed it even further.
HH
Alkene with
e-withdrawing groups
Butadiene
H H
HH
H H
Energy
H H
HH
Alkene
O
OO
HOMO
HOMO
HOMO
LUMO
LUMO
Ψ 4
Ψ 3
Ψ 2
Ψ 1

10. The Diels-Alder reaction proceeds in a
suprafacial manner, i.e. the reagents add together
in a perfectly-matched face-to-face fashion.
H H
HH
H H
HH
LUMO
HOMO
H H
HH
H H
H H
HH
H H
LUMO
HOMO
[2+2] [4+4]
thermal
H H
HH
H H
HH
LUMO
HOMO
H H
HH
H H
H H
HH
H H
LUMO
HOMO
[2+2] [4+4]
photo-
chemical
(excited state)
(excited state)
antara-
facial
supra-
facial

11. Woodward-Hoffmann theory for prediction of the stereochemistry
of cycloaddition reactions: summary:
Ring size
4,8,12…
6,10,14…
No. electrons
4n
4n+2
Thermal
Antarafacial
Suprafacial
Photochemical
Suprafacial
Antarafacial
Note…the rules also work in reverse:
H
H
Me
Me
heat
+
Me
Me
antarafacial
Although you might also get competing radical reactions:
H
H
Me
Me
heat
+
Me
Me
H
Me
Me
. .

12. Sigmatropic Rearrangements
[3,3] Rearrangements; Claisen rearrang. etc.
Cope rearrangem ent
1 2
2
3
31
1 2
3
2
31R R
O xy-Cope rearrangem ent
1 2
2
3
31
1 2
3
2
31
taut
HO HO
1 2
3
2
31O
Claisen rearrangem ent
O
Allyl-vinyl ether or
Allyl aryl ether
O
σ-bond to
be broken
1 2
2
3
31
O
O
1 2
3
2
3
1
σ-bond to
be form ed
O
H
taut
HO
Suprafacial

13. [1,5] Rearrangement (H-shift)
H
σ-bond to
be broken
1
2
31
H
1
2
3
1 4
5
H
4
5
Suprafacial
H
H
No. of electrons
4n
4n + 2
Reactions in the ground state
(term al)
Antarafacial
Suprafacial
Reactions in exited state
(Photochem .)
Suprafacial
Antarafacial
[1,3] Rearrangement (H-shift)
SuprafacialHH
H
HH
hν
Antarafacial
Too strained TS#

14. Woodward-Hoffmann theory for prediction of the stereochemistry
of pericyclic reactions: Electrocyclisations.
The ‘Woodward-Hoffmann’ theory explains the
stereochemical outcome of pericyclic reactions by
considering the symmetry of the ‘frontier orbitals’
which are involved in the reaction. These are the
orbitals which actually contribute to the bond making
and breaking process. They are also the ‘outermost’
orbitals (of highest energy) in a structure, hence the
term ‘frontier’.
Electrocyclisations.
Consider the conversion of butadiene into
cyclobutene: The mechanism is quite simple, but the
stereochemistry of the product is directly related to (i)
the stereochemistry of the starting material and (ii)
whether heat or irradiation is employed to promote the
reaction.

15. irradiation
(hυ) or heat (∆)
irradiation (hυ)
Heat (∆)
E,E
E,E
cis
trans
E,Z
E,Z
Heat (∆)
irradiation (hυ)

16. Woodward-Hoffmann theory applied to cyclobutene formation.
Note: the view of the
butadiene is 'edge-on'
with the single bond
'further back'.
H H
H HH H
ψ 3
ψ 4
ψ 1
ψ 2
Energy
There are 4
electrons in this
bonding system.
Filling orbitals from
the lowest first soon
reveals the nature
of the outermost or
'frontier' orbital.
0 nodes
1 nodes
2 nodes
3 nodes
What is happening in the cyclisation is that p-orbitals (which
form the pi-bonds) are combining in order for a new sigma
bond to be formed between the ‘ends’ of the conjugated
system.

17. Symmetry Allowed React.
Woodward Hoffmann rules
Symmetry in reactants are preserved during pericyclic react.
Results can generally be predicted just by looking at Front Orbitals
(FMO; HOMO and LUMO) - Fukui
HOM O: Highest occupied M O
LUM O: Low est uoccupied M O

18. Molecular orbitals
1,3-butadiene
S: Symmetric
A: Antisym.
HOM O
Conrotatory
Both rotate sam e way
HOM O σ-bond
LUM O Π-bond
Stereospesific react.
H3C CH3
HH
E,E
H
CH3
H
CH3
Cis
H3C H
H3CH
E,Z
H
CH3
CH3
H
Trans

19. Molecular orbitals
hexatriene
HOM O
HOMO σ-bond
LUM O Π-bond
Disrotatory
Rotation opposite way
LUM O Π -bond
H3C CH3
E E
CH3 CH3
Cis
H3C
E Z
CH3
Trans
H3C CH3
No. of electrons
4n
4n + 2
Reactions in the ground state
Conrotatory
Disrotatory
Symmetry allowed react

20. Photochemical electrocyclic react.
hν
HO M O exitet state
(= LUM O ground state)
Disrotatory
No. of electrons
4n
4n + 2
Reactions in the ground state
(term al)
Conrotatory
Disrotatory
Reactions in exited state
(Photochem .)
Disrotatory
Conrotatory