Perturbation of Cope's rearrangement: 1,3,5-triphenylhexa-1,5-diene. Chameleonic or centauric transition region?

Two types of perturbations of Cope's rearrangement are distinguished by the ir occupancy of sets of four "active" and two "nodal" positions. A "chamele onic" model of a continuum of chair-like transition regions is defined as e xtending from two noninteracting allyl radicals at one extreme to cyclohexa -1,4-diyl diradical at the other. Perturbations are analyzed quantitatively in terms of obligatory corrections for conjugative interaction in the educ t, and a model of the transition region that specifies transference of stab ilization energies of the perturbing substituents on allyl radicals if occu pying active positions, and on secondary radicals if occupying nodal positi ons. When this model is applied to the chameleonic 2,5- (nodal) and 1,4- (a ctive) diphenylhexa-1,5-dienes, good agreement with empirical lowering of e nthalpies of activation per phenyl group of -8.7 and -4.4 kcal mol(-1), res pectively, is obtained, in a perturbation of mixed type, 1,3,5-triphenylhex a-1,5-diene (1,3-diphenyl-active: 5-phenyl-nodal), a novel question is addr essed: Will the stronger of the two types alone prevail (transition region remaining chameleonic), or will the stabilizing capacity of both be realize d (centauric domain)? The result is close to, but perhaps somewhat shy of, the full additivity expected of the centauric model.