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Phenyl and cyano substituent effects on the activation free energy of thermoneutral Cope rearrangements of 1,5-dienes are analyzed in terms of the activation free energies for formation of the two non-concerted mechanistic alternatives, cyclohexane-1,4-diyl and two allylic radicals. The experimental data, adjusted upwards in the case of 2-phenyl-1,5- hexadiene, is reasonably correlated utilizing the harmonic mean values of the activation free energies and not some simple average of them or either one independently depending on which is lower. The latter two approaches appear to represent Centauric and Chameleonic models, respectively, as proposed by Doering. The harmonic mean is derived from consideration of the saddle point surface represented by a More O'Ferrall-Jencks diagram which appears to characterize the transition state structure and energetics of the Cope rearrangement. The conclusion from the correlation is that substitution of radical stabilizing groups alters the nature and stability of the transition state in accord with the position of substitution, and further substitution has an even greater effect if the position of substitution reinforces the change induced by the original substitution. If the position of substitution is opposed to the original substitution, the increase in stability of the transition state is attenuated.

Substituent effects on the Cope rearrangement: neither centaurs nor chameleons can characterize them