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: 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 a...