1 Chemistry 4420 Dr. Y. Zhao Topic 8 Pericyclic Reactions

Chemistry 4420 Dr. Y. Zhao
Topic 8 Pericyclic Reactions
• Classes of Pericylcic Reactions
• Electrocyclic Reactions
• Cylcoaddition and Cycloreversion Reactions
• Sigmatropic Rearrangements
• Ene Reactions
1. Classes of Pericyclic Reactions
Pericyclic reactions refer to the reactions in which bonds
are formed and broken at the termini of one or more
conjugated π-systems. The electrons move around in a
circle, all bonds are made and broken simultaneously.
There is no intermediates intervene (concertedness).
Pericyclic reactions can be classified in four types:
(a) Electrocyclic reactions (ring opening or closing)
(b) Cylcoaddition and Cylcoreversion reactions
(c) Sigmatropic Rearrangement
(d) Ene Reactions
Examples of various pericylcic reactions
(a) Electrocyclic reactions
R
4π electron ring opening 6π electron ring closing
O
R
2π electron ring closing
O
R R
O
R R
(b) Cycloaddition and Cycloreversion reactions
Note that, there are two conventions to depict cycloaddition
reaction, both using [m + n]; however, the meaning of m and
n are different. In the old rule, m and n denote the number
of atoms in each components. Woodward-Hoffmann later
revised the rule by denoting m and n the number of
electrons in each component. So, be careful with the
different usages in the literature.
O O
Ph
H
[4 + 2] cycloaddition or Diels-Alder reaction [2 + 2] cycloaddition reaction
O COOEt
O
COOEt
Ph N
Ph N
Ph
COOEt
Ph
COOEt
[3 + 2] or 1,3-dipolar cycloaddition reaction
O
C
H
H
H
1
O
Ph

Chemistry 4420 Dr. Y. Zhao
Other cycloadditions including [8 +2], [4 + 3] and [6 + 4]
cycloadditions are also known. A special class
cycloadditions where one of the component is a single
atom are called cheletropic reactions. For example, [2 +1]
and retro-[4 + 1] cycloadditions.
H
Cl
Cl
C
Cl Cl
H
[2 + 1] cycloaddition or carbene insertion
Ph
Ph
O
Ph
Ph
Ph
Retr o-[4 + 1] cycload dition
Ph
Ph Ph
(c) Sigmatropic Rearrangements
Sigmatropic rearrangements involve the cleavage of a σbond
connecting the end of one fragment with the end of
another, with concerted formation of another σ-bond at the
other ends of the fragments. Like cycloaddition,
sigamtropic rearrangement also uses [m + n] to indicate
the atoms involved in each component.
O
Ph
O
[ 3,3] sigmatropic rearrangement
or Oxy-Cope rearrangement
R O
R O
[3,3] sigmatropic rearrangement
or Claisen rearrangement
OBn
H
H
H
H
[1,5] sigmatropic rearrangement
Ph
H
OBn
Why [3,3]?
O
1 2
bond to be broken 1' 3' Ph bond to be formed
2'
R
HO
H
3
Cl
Cl
H
[1,2] sigmatropic rearrangement
or 1,2-alkyl shift
H
H
H
CH3
H3C
[1,3] sigmatropic rearrangement
(d) Ene Reactions
H
H
H
O
S
Ph
O
S
Ph
[2,3] sigmatropic rearrangement
The ene reaction is always a 6-electron reaction. It shares
some characteristics with the [4 + 2] cycloaddition and the
[1,5] sigmatropic rearrangement.
H
H
Alder ene reaction
H
O
Se Ph
O
H
H
retro-hetero-ene reaction
O +
Ph
H O
O
Se Ph
Summary of Pericyclic Reactions
electrocyclic reaction
one π bond one σ bond
sigmatropic rearrangement
oneσ bond new σ bond
H
O
Ph
retro-ene reaction
electrocyclic reaction
two π bond two σ bond
H H
ene reaction
oneπ bond one σ bond
andoneσ bond migrate
+
H
O
O
2

Chemistry 4420 Dr. Y. Zhao
• Some Features of Pericyclic Reactions
(a) Pericyclic reactions are stereospecific
CH3 CH3 175
H
H
oC CH 3
H
H
CH3 CH3 H 175
H
CH3 oC Stereospecificity (stereospecific): A reaction is termed
stereospecific if starting materials differing only in their
configuration are converted into stereoisomeric products.
According to this definition, a stereospecific process is
necessarily stereoselective but not all stereoselective
processes are stereospecific. Stereospecificity may be total
(100%) or partial. The term is also applied to situations where
reaction can be performed with only one stereoisomer. For
example, the exclusive formation of trans-1,2-dibromocyclohexane
upon bromination of cyclohexene is a
stereospecific process, although the analogous reaction with
(E)-cyclohexene has not been performed.
(b) Pericyclic reactions are dependent on conditions
CH 3
CH3 H
H
CH 3
H
H
CH 3
2. Electrocylcic Reactions
hv
CH 3
H
H
CH 3
• Conrotatory and Distrotatory Processes
H 3C CH 3
H
H
CH 3 H
conrotatory (rotate in the same direction)
H
CH 3
H 3C CH 3
H
H
H 3C
CH 3
CH 3
H
H
H H
disrotatory (rotate in different directions)
CH 3
The terms were coined by Woodward and Hoffmann in 1965.
The Woodward-Hoffmann Woodward Hoffmann Rules
4n π e
4n + 2 π e
Ground state
(thermal)
Conrotatory
Disrotatory
Excited state
(photochemical)
Disrotatory
Conrotatory
• Explanations for the Woodward-Hoffmann Rules
(a) FMO Theory (Fukui)
(b) Aromatic Transition States (Dewar-Zimmerman)
(c) Conservation of Orbital Symmetry (Woodward-Hoffmann)
FMO Approach
A
B A
B
disrotatory
Ψ HOMO σ
Lobes with the same phase attract attract each each other other to form form a
new bond; lobes with opposite phases repel repel each other.
A
B A
B
conrotatory
Ψ HOMO σ
A
B
A
B
B
A
A
B
4n + 2 π
4n π
3

Chemistry 4420 Dr. Y. Zhao
The FMO approach is very useful and simple; however, it is
not satisfactory and exact theoretically as only the FMOs are
considered.
Aromatic Transition States (the Dewar-Zimmerman Dewar Zimmerman model)
Hückel transition state: the p orbitals around a ring have
zero or an even number of phase inversions.
Möbius transition state: the p orbitals around a ring have
an odd number of phase inversions.
In a Hückel system, 4n +2 electrons, aromatic; 4n
electrons, antiaromatic.
In a Möbius system, 4n electrons, aromatic; 4n + 2
electrons, antiaromatic.
Hence, the Woodward-Hoffmann rules can also be
phrased as: Reactions are allowed if they proceed by
aromatic transition states and are forbidden if they
proceed by antiaromatic transition states. states
How to apply the Dewar-Zimmerman model?
A B A B
A B A B
A
B
disrotatory
A
B
Zero phase inversion
Hückel topology
4 e, antiaromatic
Forbidden
A B A B
A B A B
A
B
conrotatory
A
B
One phase inversion
Möbius topology
4 e, aromatic
Allowed
(1) Draw all the p, s and sp 3
hybridized orbitals (orbital
phases can be assigned
arbitrarily).
(2) Connect all the orbitals that
interact in the starting materials
before the reaction begins.
(3) Allow the reaction proceed
to a postulated transition state.
(4) Connect the lobes that
begin to interact.
(5) Determine the aromatic or
antiaromatic transition state.
Note: the Dewar-Zimmerman is probably the easiest model used
to explain pericyclic reactions. For further reading, see. H. E.
Zimmerman, Acc. Chem. Res. 1971, 4, 272.
Conservation of Orbital Symmetry–Correlation Symmetry Correlation Diagrams
The correlation diagram approach was first propsed by Longuet-
Higgins and Abrahamson a few years after the original paer by
Woodward and Hoffmann. [JACS, 1965, 87, 2045]
The principle of conservation of orbital symmetry says each
orbital of the starting material must be converted to an orbital
with the same symmetry.
4

Chemistry 4420 Dr. Y. Zhao
Take the ring closing of butadiene under thermal conditions
as an example,
[2 + 2]
C2 Transition state for a conrotatory
reaction. Symmetry around the
mirror plan is not maintained, but
symmetry around the axis of
rotation is maintained.
Orbital symmetry
σ
Transition state for a disrotatory
reaction. Symmetry around the
mirror plan is maintained, but
symmetry around the axis of
rotation is not maintained.
σ 2 *
π 2 *
π 1
σ 1
Conrotatory interconversion
Thermally allowed
5
π 4 *
π 3 *
π 2
π 1

Chemistry 4420 Dr. Y. Zhao
Disrotatory interconversion
Thermally forbidden
Correlation diagram for the conrotatory pathway
Correlation diagram for the disrotatory pathway
6