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

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.
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