Perturbation of Cope's Rearrangement:  1,3,5-Triphenylhexa-l,5-diene. Chameleonic or Centauric Transition Region?

• Published in: Journal of the American Chemical Society 1999-11, Vol.121 (43), p.10112-10118
• Main Authors: Doering, W. v. E, Wang, Yonghui
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja9908568

Two types of perturbations of Cope's rearrangement are distinguished by their occupancy of sets of four “active” and two “nodal” positions. A “chameleonic” model of a continuum of chair-like transition regions is defined as extending 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 educt, and a model of the transition region that specifies transference of stabilization energies of the perturbing substituents on allyl radicals if occupying active positions, and on secondary radicals if occupying nodal positions. When this model is applied to the chameleonic 2,5- (nodal) and 1,4- (active) diphenylhexa-l,5-dienes, good agreement with empirical lowering of enthalpies of activation per phenyl group of −8.7 and −4.4 kcal mol-1, respectively, is obtained. In a perturbation of mixed type, 1,3,5-triphenylhexa-1,5-diene (1,3-diphenyl-active; 5-phenyl-nodal), a novel question is addressed:  Will the stronger of the two types alone prevail (transition region remaining chameleonic), or will the stabilizing capacity of both be realized (centauric domain)? The result is close to, but perhaps somewhat shy of, the full additivity expected of the centauric model.