Perturbation of the degenerate, concerted cope rearrangement by two phenyl groups in "active" positions of (E)-1,4-diphenylhexa-1,5-diene. Acceleration by high pressure as criterion of cyclic transition states

Article abstract of DOI:10.1021/ja00089a018

Previous examinations of radical-stabilizing substituents in the two distinct types of position in the hypothetical "aromatic" transition state of the thermal Cope rearrangement, designated "a" or active and "n" or nodal after the allyl radical have concentrated on their effect in the "n" positions. In order to provide a quantitatively reliable reference for the "a" position, the activation parameters of the degenerate rearrangement of (6-¹³C)-1,4-diphenylhexa-1,5-diene have been evaluated: E_a = 30.8 ± 0.4 kcal/mol; log A = 10.14 ± 0.2. The soundly energetically-based proposition that these observations relate to a concerted mechanism is strongly supported by the observation of a 3.0-fold increase in rate of approach to equilibrium on increasing the pressure from 1 bar to 6000 bar (162°C; benzene-d₆). This rearrangement like that of cis-1,2-divinylcyclobutane and rac- and meso-3,4-diphenylhexa-1,5-diene, has a negative volume of activation. In contrast, trans-1,2-divinylcyclobutane, which does not rearrange by a cyclic transition state and gives cycloocta-1,5-diene, 4-vinylcyclohexene, and butadiene as products, has a positive volume of activation. To place the possibility of reaction by the homolytic/colligative (dissociative/recombinative) mechanism on a "quantitative" base, a further sighting on the heat of formation of the cinnamyl radical is provided by activation parameters for thermal syn-anti equilibration between (E)- and (Z)-1,1′-bi-3-phenylcyclohex-2-enylidene: E_a = 35.8 ± 0.2 kcal/mol; log A = 12.7 ± 0.1. After correction for conjugative interaction between phenyl and the double bond in the educts and without regard for any proposed structure for the transition state, the two phenyl groups in "a" positions appear to have lowered the enthalpy of activation by 7.7 kcal/mol relative to the paradigm, hexa-1,5-diene, whereas the two phenyl groups in the "n" positions of 3,5-diphenylhexa-1,5-diene have lowered the enthalpy of activation by 17.0 kcal/mol.