Photochemical reactions between dibromomaleic anhydride and benzo[b]thiophene.

On the irradiation of dibromomaleic anhydride in the presence of benzo[b]thiophene, both photosubstitution and photocycloaddition are simultaneously induced. The ratio of the substituted product to the cycloadduct increased with solvent polarity, and the same Stern-Volmer plots were obtained with both of the reactions. Kinetic parameters, obtained under various concentrations of benzo[b]thiophene, indicate that the reactions proceed via a common triplet exciplex. The activation energy for the photosubstitution is higher than that for the cycloaddition by 1–2 kcal/mol, where the former reaction is considered to proceed via a more polar transition state than the latter.