Cryptocope rearrangement of 1,3-dicyano-5-phenyl-4,4-d2-hexa-2,5-diene. Chameleonic or centauric?

Article abstract of DOI:10.1021/ja992137z

The 'centauric' model for evaluation of the effect of radical- stabilizing perturbations on the Cope rearrangement conjectures independent action of substituents that make conflicting electronic demands on the two halves of the transition region. The present test of this conjecture compares 1,3-dicyano-[5-protio]-hexa-1,5-diene (1(H)) and 2-phenylhexa-1,5-diene, with 1,3-dicyano-5-phenylhexa-1,5-diene (1(Ph)). Thermochemical information required for a proper comparison includes new data of the van't Hoff type on conjugative interaction of cyano with the carbon-carbon double bond, reevaluation of the radical-stabilizing potential of the cyano group on secondary and allyl radicals, comparison with the (reevaluated) stabilizing effect of cyano in 'nodal' positions of the Cope transition region, and determination of the enthalpy and entropy of activation of the cryptoCope rearrangement of otherwise Cope-incompetent, thermodynamically more stable hexa-2,5-dienes related by prototropy to the Cope-competent hexa-1,5-dienes above. The 'chameleonic' model is concluded to be unsatisfactory, while the 'centauric' is in better, if not complete, accord with experiment.