Photodimerization of α-Phenylmaleimides

Abstract

Irradiation of a solution of N-methyl-a-phenylmaleimide gave in almost quantitative yields three (2+2) dimers of trans-head-to-head, trans-head-to-tail, and cis-head-to-head types and only one (4+2) dimer, cis, transoid, cis-1-phenyl-1, 2, 3, 4-tetrahydronaphthalene-1, 2:3, 4-bis(N-methylcarboximide). The photodimer distribution was solvent-dependent; the yield of the (4+2) dimer increased with the increase in the solvent polarity. The (4+2) dimer was found to be formed on the prolonged heating of the maleimide. N-(p-Tolyl)-α-phenylmaleimide photodimerized similarly with lower efficiency. Discussion is made on the formation of the (4+2) dimer which may involve the intermediate formed through a photochemically forbidden (4s+2s) process. The photochemically as well as thermally induced reversion of the cyclobutane isomers to the starting material was also carried out.