A Non-Cope among the Cope Rearrangements of 1,3,4,6-Tetraphenylhexa-1,5-dienes.
In: Journal of the American Chemical Society (J.Am.Chem.Soc.), Jg. 122 (2000) ; Nr. 2, S. 193-203
Zeitschriftenaufsatz / Fach: Chemie
The set of 1,3,4,6-tetraphenylhexa-1,5-dienes (1) represents a perturbation of Cope's rearrangement by four radical-stabilizing Ph 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 stereochem. 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 Deg) with the following values of the enthalpy, entropy, and vol. of activation: DH.thermod. = 30.7 +- 0.2 kcal mol-1, DS.thermod. = +2.1 +- 0.4 cal mol-1 K-1, and DV.thermod. = +13.5 +- 0.1 cm-3 mol-1, resp. Structures have been established by X-ray crystallog. anal.; 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 vol. of activation is neither low enough for a pericyclic Cope (\"concerted\") mechanism nor high enough for a homolytic-colligative mechanism involving full dissocn. as the rate-detg. step. Trapping and a crossover expt. give some but only partial support to the intermediacy of free radicals. At higher temps., however, ESR expts. demonstrate an equil. with kinetically free (E,E)-1,3-diphenylallyl radicals. These observations are rationalized in terms of geometric reorganization within the confines of a 'cage'. Resoln. by chiral chromatog. of rac-(E,E)-1 allows recognition of a fast racemization (40-65 Deg), of which DH.thermod. (21.3 +- 0.1 kcal mol-1), DS.thermod. (-13.2 +- 0.3 cal mol-1 K-1), and DV.thermod. (-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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