The Cyclohexadienyl Radical in the Thermal Syn−Anti Isomerization of Two Crossed Pentaenes of the Type of Bis-Homofulvalene

Abstract

To augment the elucidation of the dependence of enthalpy of stabilization on configuration among the pentadienyl radicals, a designer pentaene of the (Z,Z) type, (R,R)-5,5‘,6,6‘,7,7‘,8,8‘-octahydro-4a,4a‘-dimethyl-2,2‘-bi-4a(H)-naphthylidene, has been prepared and subjected to a kinetic study of cis, trans isomerization about its central double bond. The resulting activation parameters and those of a more complicated example, 3,3‘-bicholesta-1,4-dienylidene, are essentially identical, their mean values being ΔH ⧧ = 36.8 kcal/mol and ΔS ⧧ = +1.8 eu. This enthalpy of activation is significantly higher than those of pentaenes of the (E,E) and (E,Z) configuration. Conjugative interaction in coplanar cross-conjugated systems of the bis-homofulvalene type not having been experimentally evaluated or estimated by theoretical calculation, an enthalpy of stabilization for the cyclohexadienyl radical is not definitively to be extracted at the present time. If conjugative interactions are to be equated to four Kistiakowsky butadiene units and the difference in steric energies between educt and 90°-twisted state are to be estimated by the MMEVBH program, an enthalpy of stabilization of the cyclohexadienyl radical of ∼20 kcal/mol results.