A Becke3LYP/6-31G* Study of the Cope Rearrangements of Substituted 1,5-Hexadienes Provides Computational Evidence for a Chameleonic Transition State

Abstract

B3LYP/6-31G* calculations have been performed on the chair Cope rearrangements of a wide variety of 1,5-hexadienes, substituted with cyano or with vinyl groups. In agreement with experimental data from the study of phenyl substituent effects, cyano and vinyl groups at C(2) and C(5) are found to provide cooperative lowering of the activation enthalpy, as are substituents at C(1), C(3), C(4), and C(6). In contrast, the stabilization of the transition structure by substituents at C(2) and C(4) or at C(1), C(3), and C(5) is predicted to be competitive, rather than cooperative. These findings are consistent with what Doering has termed a chameleonic transition state for the Cope rearrangement, one in which the relative importance of the cyclohexane-1,4-diyl and bis-allyl radical resonance contributors can be altered by substituents, depending on the carbons to which the substituents are attached. The computed bond lengths in the transition structures and the calculated and experimentally observed kinetic isotope effects are all consistent with a chameleonic transition state for the Cope rearrangement.