Chapter 30 Orbitals and Organic Chemistry: Pericyclic Reaction

Polar mechanismRadical mechanismConcerted process

Electrocyclic reactionCycloadditionSigmatropic rearrangement

1,3,5-hexatriene

Woodward-Hoffmann rules

A pericyclic reaction can take place only if the symmetries of the reactant MOs are the same as the symmetries of the product MOs.In other words, the lobes reactant MOs must be of the correct algebraic sign for bonding to occur in the transition state leading to product.

If the symmetries of both reactant and product orbitals match up, or “correlate”, the reaction is said to be symmetry-allowed.

If the symmetries of reactant and product orbitals don’t correlate, the reaction is symmetry-disallowed.

Kenichi Fukui

Frontier orbitalsHighest occupied molecular orbital (HOMO)Lowest unoccupied molecular orbital (LUMO)

An electrocyclic reaction is a pericyclic process that involves the cyclization of a conjugated polyene. One bond is broken, the other bonds change position, a new bond is formed, and a cyclic compound results.

heat

a conjugated triene a cyclohexadiene

heat

(2E,4Z ,6E)-Octatriene cis-5,6-Dimethyl-1,3-cyclohexadiene

CH3

CH3

HH

CH3

CH3

H

H

CH3

CH3

H

H

(2E,4Z ,6Z )-Octatriene

heat

CH3

H

H

CH3

trans-5,6-Dimethyl-1,3-cyclohexadiene

h

Stereochemistry The most striking feature of electrocyclic reactions

Outermost sign

Polyenes with an even( 偶数 )number of electron pairs undergo thermal electrocyclic reactions in a conrotatory sense, Whereas polyenes with an odd number of electron pairs undergo the same reactions in a disrotatory sense.