FACTORS CONTROLLING THE SYNCHRONOUS VERSUS ASYNCHRONOUS MECHANISM OF THE COPE REARRANGEMENT

MC-SCF potential surfaces for the Cope rearrangement of 1,5-hexadiene have been modeled by using a valence bond (VB) scheme parametrized with effective Hamiltonian methods. It is demonstrated that the mechanistic preference for a synchronous mechanism with an aromatic transition state versus an asynchronous mechanism with a biradicaloid intermediate is controlled by two factors: (i) the stability of the long bond in the Dewar VB structure and (ii) the softness of the Coulomb interactions between the terminal methylenes of the allylic fragments. Thus, the mechanism may be strongly affected by substituents. © 1990, American Chemical Society. All rights reserved.