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

Abstract

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.

Mechanism of the Forbidden [3s,5s]-Sigmatropic Shift: Orbital Symmetry Influences Stepwise Mechanisms Involving Diradical Intermediates

“…[6,7] For example, the energy of cyclohexan-1,4-diyl, 2, a potential intermediate in the Cope rearrangement of 1,5-hexadiene, 1, has been estimated from twice the CH bond dissociation energy of cyclohexane (Scheme 1). [8] This bond dissociation energy is Scheme 1. Several [3,3]-sigmatropic shifts (Cope rearrangements) and a potential diradical intermediate, 2.…”

The Niecke Biradicals and Their Congeners – The Journey from Stable Biradicaloids to Their Utilization for the Design of Nonlinear Optical Properties

“…Thus it can be predicted that by suitable substitution a biradical transition state should come to the fore, when compared with the concerted structural alternative. This analysis, first pursued by Doering et al, is now well established by experimental, as well as theoretical contributions at MCSCF, and DFT level . Overall the Doering view of a biradical in competition with a biradicaloid requires a structural change enforced by radical stabilizing substituents, which exert energy effects (on the activation barrier of the reaction) being equal or even larger than the energetic difference between a “full” biradical structure and the electron delocalized structure.…”

The Cycloaddition-Cycloelimination Pathway to Homotropilidenes − Synthesis and Properties of Homotropilidenes

“…Unfortunately, our kinetic data do not allow us to conclude whether the individual Cope rearrangement occurs through a concerted mechanism or a nonconcerted one. According to an article by Doering et al7,8 and calculations (Becke3LYP/6−31G* level) by Borden and Houk52,53 on 1,5‐hexadienes bearing cyano and vinyl groups in different positions, these systems are specified as the so‐called “chameleonic” transition states for the Cope rearrangement. The relative importance of the cyclohexane‐1,4‐diyl ( 4 ) and bis(allyl) radical ( 3 ) resonance contributors can be altered by substituents, depending on the carbon atoms to which the substituents are attached.…”

“…This [3,3]‐sigmatropic rearrangement, by Woodward−Hoffmann nomenclature,2,3 not only turned out to be of tremendous synthetic value,4−6 but its mechanism has been the topic of heated discussion for nearly 50 years. In two remarkable contributions, Doering quite recently offered a critical discussion of all mechanistic aspects 7,8. Houk summarized all relevant articles to show how quantum mechanical calculations have been used to analyse and predict the rates and mechanisms of this and other pericyclic reactions 9,10…”