A Becke3LYP/6-31G*study of the cope rearrangements of substituted 1,5-hexadienes provides computational evidence for a chameleonic transition state

B3LYP/6-31G* calculations have been performed on the chair Cope rearrangeme nts of a wide variety of 1,5-hexadienes, substituted with cyano or with vin yl groups. In agreement with experimental data from the study of phenyl sub stituent effects, cyano and vinyl groups at C(2) and C(5) are found to prov ide 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 transiti on 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 c onsistent with what Doering has termed a chameleonic transition state for t he Cope rearrangement, one in which the relative importance of the cyclohex ane-l,4-diyl and bis-allyl radical resonance contributors can be altered by substituents, depending on the carbons to which the substituents are attac hed. The computed bond lengths in the transition structures and the calcula ted and experimentally observed kinetic isotope effects are all consistent with a chameleonic transition state for the Cope rearrangement.