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 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.

Utility of chemical indices for transition structures of pericyclic reactions: Case study of the cope rearrangement

“…Therefore, it is interesting to perform the modified B2LYP calculations. The Δ H ‡ values (20.5–22.3 kcal/mol) by RB2′LYP ( C 1 = 0.25–0.3) are close to the observed one (21.3 ± 0.1 kcal/mol) 38. The results of the UB2′LYP ( C 1 = 0.3–0.4) calculations converged on the unrestricted solutions.…”

“…The strength of two σ bonds between two allyl fragments is so strong even at the UB2′LYP ( C 1 = 0.3–0.4) level that the Cope rearrangement would not actually proceed by σ bond cleavage to afford free 1,3‐diphenylallyl radicals. Despite the fact that the heat of the formation of radical pairs is smaller than that of the 1,3,4,6‐tetraphenyl CTS, trapping and crossover experiments gave some, but only partial, support to the intermediacy of free radicals 38.…”

The Effect of Pressure on Organic Reactions

“…The utility of high pressure in the elucidation of reaction mechanisms can be also demonstrated by the example of the racemization and diastereomerization in 1,3,4,6-tetraphenyl-1,5-hexadiene (Scheme 21) indicating that a pericyclic Cope rearrangement competes here with a dissociative process involving free-radical intermediates [99]. Optically active tetraphenylhexadiene undergoes a facile racemization at temperatures just above room temperature.…”

The Cope rearrangement—the first born of a great family

“…The interallyl distance R ranges in substituted cases between 1.5 and 2.6 Å, suggesting a gradual switch between biradical and aromatic character. Radical‐stabilizing phenyl or cyano substitutents at ‘active positions’, e.g., 1b and 1c , favor the bisallyl radicals45 and are predicted to lengthen R . Large R value of 2.6 Å for 1,3,4,6‐tetracyano‐1,5‐hexadiene 1b had been computed at B3LYP/6–31G(d), indicating a bisallyl contribution in the TS 46.…”