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

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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: ΔH⧧ = 30.7 ± 0.2 kcal mol-1, ΔS⧧ = +2.1 ± 0.4 cal mol-1 K-1, and ΔV⧧ = +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 ΔH⧧ (21.3 ± 0.1 kcal mol-1), ΔS⧧ (−13.2 ± 0.3 cal mol-1 K-1), and ΔV⧧ (−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.

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