A Non-Cope among the Cope Rearrangements of 1,3,4,6-Tetraphenylhexa-1,5-dienes

Abstract

The set of 1,3,4,6-tetraphenylhexa-1,5-dienes ( 1 ) represents a perturbation of Cope's rearrangement by four radical-stabilizing phenyl groups all positioned to drive the transition region toward the homolytic−colligative end of the mechanistic spectrum. The appearance of ( Z )-isomers being suppressed thermodynamically by a steric interaction of +2.6 kcal mol - 1 per cis double bond, an equilibration that is stereochemically not of any Cope type, emerges as the predominant reaction. It is an interconversion of rac -( E, E )-1 and meso -( E, E )-1 (48:52; 77.3−115.3 °C) with the following values of the enthalpy, entropy, and volume of activation: Δ H ⧧ = 30.7 ± 0.2 kcal mol - 1, Δ S ⧧ = +2.1 ± 0.4 cal mol - 1 K - 1, and Δ V ⧧ = +13.5 ± 0.1 cm - 3 mol - 1, respectively. Structures have been established by X-ray crystallographic analysis; a possible relationship between dihedral angle and bond lengths in the styrene portions is proposed. The entropy of activation is incompatible with a chair or boat Cope rearrangement; the volume of activation is neither low enough for a pericyclic Cope (“concerted”) mechanism nor high enough for a homolytic−colligative mechanism involving full dissociation as the rate-determining step. Trapping and a crossover experiment give some but only partial support to the intermediacy of free radicals. At higher temperatures, however, electron spin resonance experiments demonstrate an equilibrium with kinetically free ( E, E )-1,3-diphenylallyl radicals. These observations are rationalized in terms of geometric reorganization within the confines of a ‘cage'. Resolution by chiral chromatography of rac -( E, E )-1 allows recognition of a fast racemization (40−65 °C), of which Δ H ⧧ (21.3 ± 0.1 kcal mol - 1 ), Δ S ⧧ (−13.2 ± 0.3 cal mol - 1 K - 1 ), and Δ V ⧧ (−7.4 ± 0.4 cm - 3 mol - 1 ) are consistent with a pericyclic Cope rearrangement. Enriched ( Z )-isomers undergo Cope rearrangements in accord with the known influence of axiality and the chair/boat alternative on the energy of the transition region.