PERTURBATION OF THE DEGENERATE, CONCERTED COPE REARRANGEMENT BY 2 PHENYL GROUPS IN ACTIVE POSITIONS OF (E)-1,4-DIPHENYLHEXA-1,5-DIENE - ACCELERATION BY HIGH-PRESSURE AS CRITERION OF CYCLIC TRANSITION-STATES

Previous examinations of radical-stabilizing substituents in the two d istinct types of position in the hypothetical ''aromatic'' transition state of the thermal Cope rearrangement, designated ''a'' or active an d ''n'' or nodal after the allyl radical, have concentrated on their e ffect in the ''n'' positions. In order to provide a quantitatively rel iable reference for the ''a'' position, the activation parameters of t he degenerate rearrangement of (6-C-13)-1,4-diphenylhexa-1,5-diene hav e been evaluated: E(a) = 30.8 +/- 0.4 kcal/mol; log A = 10.14 +/- 0.2. The soundly energetically-based proposition that these observations r elate to a concerted mechanism is strongly supported by the observatio n of a 3.0-fold increase in rate of approach to equilibrium on increas ing the pressure from 1 bar to 6000 bar (162 degrees C; benzene-d(6)). This rearrangement, like that of cis-1,2-divinylcyclobutane and rac- and meso-3,4-diphenylhexa-1,5-diene, has a negative volume of activati on. In contrast, trans-1,2-divinylcyclobutane, which does not rearrang e by a cyclic transition state and gives cycloocta-1,5-diene, 4-vinylc yclohexene and butadiene as products, has a positive volume of activat ion. 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 p rovided by activation parameters for thermal syn-anti equilibration be tween (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 conjugativ e interaction between phenyl and the double bond in the educts and wit hout regard for any proposed structure for the transition state, the t wo phenyl groups in ''a'' positions appear to have lowered the enthalp y of activation by 7.7 kcal/mol relative to the paradigm, hexa-1,5-die ne, whereas the two phenyl groups in the ''n'' positions of 3,5-diphen ylhexa-1,5-diene have lowered the enthalpy of activation by 17.0 kcal/ mol.