Geometrical Isomerism of the Exocyclic C=C Bonds of Heptafulvene Derivatives. Steric and Electronic Effects on the Rotational Barrier

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Abstract
The rotational barriers of the exocyclic double bonds of a series of methyl 8-cyanoheptafulvene-8-carboxylates were measured to evaluate the steric effect and the contribution of the polarized structures. An incorporation of five-membered heterocycles lowered the activation energies sufficiently to allow the study of steric and electronic factors. Thus, the Type I compounds, 8-[cyano(methoxycarbonyl)methylidene]-2-methyl-8H-cyclohepta[b]furan and its sulfur and nitrogen analogues, which have a different extent of steric effect, had rates of rotation in the order of N > S > O. On the other hand, the Type II derivatives, 6-(1-cyano-1-methoxycarbonylmethylidene)-2-methylcyclohepta[b]furan and its sulfur and nitrogen analogues, for which the steric factor is unimportant, showed an order of N > O > S. This was also the case for the Type III derivatives, methyl 8-cyano-3-methoxyheptafulvene-8-carboxylate, methyl 8-cyano-3-(methylthio)heptafulvene-8-carboxylate, and methyl 8-cyano-3-(dimethylamino)heptafulvene-8-carboxylate. The unusually low values of ΔH‡ for the nitrogen analogues indicated that the ground state structures were destabilized by the steric repulsions.