Mechanism of the Photooxidation of and Photoepoxidation with Cyclic α-Diketones

The photooxidation of phenanthrenequinone (PhQ) under oxygen afforded diphenic acid and its peroxy acid, while acenaphthenequinone (AcQ) predominantly yielded 1,8-naphthalenedicarboxylic anhydride. The photolysis of PhQ or AcQ in the presence of olefins afforded predominantly 1 : 1 cycloadducts and the photoepoxidation was a very minor one accompanying significant amount of C–C cleavage of olefins. These results show that the photoreaction of PhQ, or AcQ with olefins is very fast and competitive with the quenching and/or reaction with oxygen. The photoepoxidation of stilbene in the presence of dimethyl sulfoxide resulted in a high selectivity (>98%) for the trans-epoxide. The same was true for the epoxide formed in the coautoxidation of benzaldehyde and stilbene. These facts suggest that the photoepoxidation with cyclic α-diketones as a minor reaction proceeds chiefly by way of acylperoxyl radical, accompanying the epoxidation with peroxy acids produced concomitantly. Mechanisms of the photooxidation of and photoepoxidation with the cyclic diketones are discussed in comparison with the case of acyclic diketones.