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

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.