Synthesis of Methyl 8-Cyanoheptafulvene-8-carboxylate Derivatives

Heptafulvenes, fundamental members of non-alternant conjugated compounds, are known to be stabilized by introduction of electron-withdrawing substituents on the C-8 position, e.g., 8,8-dicyanoheptafulvene (1).i' The bond lengths of the exocyclic C=C of the heptafulvenes have frequently been discussed as a measure of the contribution of a dipolar form.') For example, the exocyclic C=C bond length of 1 was 1.42 A, which has a considerable single bond character.i) On the other hand, that of 3,4-diacetoxy-8,8dimethylheptafulvene (2),2) carrying no electron-withdrawing substituents to assist the polarized 6rr-contribution, was 1.343 A, a typical length for the normal C=C bonds. These were explained in terms of the different dipolar contribution of molecules.