Type II photoreaction of 1a,7a-dihydro-1H-cyclopropa [b] naphthalene-2,7-diones: photochemical generation of type II biradicals from cyclobutanols

Abstract

Irradiation of the title cyclopropa [b] naphthalene-2,7-diones (5a–d) in benzene or benzene–Bu^tOH (1 : 1) gave the cyclobutanols (6a–d) and the unsaturated keto alcohols (7a–c). The former are Norrish type II cyclisation products, while the latter may be formed by reverse disproportionation of the type II biradical (9). Irradiation of the 1a-methylcyclopropa [b] naphthalene-2,7-diones (5e–h) gave the bicyclic diketones (16e–h)via hydroxymethylcyclopropyl rearrangement of the type II biradicals (9), in addition to the corresponding cyclobutanols (6e–g) and keto alcohols (7e–g). Photoexcitation of the cyclobutanols (6a–g) exclusively led to the cleavage of the cyclobutane ring, giving rise to type II biradicals (9) whose fates were solvent-dependent. The relative decay rates of the type II biradical (9), by disproportionation, cyclisation, reverse disproportionation, and hydroxymethylcyclopropyl rearrangement, were determined on the basis of the product distribution in separate photolyses of compounds (5) and (6) in benzene.