Electrocyclic ring opening of 1α,4α- and 1α,4β-bicyclo[2.2.0]hexa-2,5-dienes (cis and trans Dewar benzenes): MNDO (modified neglect of diatomic overlap) semiempirical molecular orbital calculations

Abstract

trans-Dewar benzene is calculated by the semiempirical SCF–MO method MNDO to have a barrier of 21 kJ mol^–1 towards the allowed conrotatory electrocyclic ring opening to benzene, suggesting that the species may have a transient existence at low temperatures, while the forbidden disortatory opening of cis-Dewar benzene is correctly predicted to involve passage over a much higher barrier via a transition state of C_2v symmetry.