Exciplex and Radical Ion Intermediates in the Photochemical Reaction of α-Phenylcinnamonitrile with 2,5-Dimethyl-2,4-hexadiene

The photochemical reaction of α-phenylcinnamonitrile with 2,5-dimethyl-2,4-hexadiene has been investigated in several solvents. In nonpolar, nonhydroxylic solvents four isomeric [2 + 2] cycloadducts are formed. Cycloaddition is proposed to occur via a singlet exciplex intermediate. In acetonitrile solution a novel acyclic adduct is formed via an electron transfer-proton transfer-radical coupling mechanism. In alcohol solvents, reduced α-phenylcinnamonitrile, a solvent-incorporated adduct, and an oxidative coupling product of 2,5-dimethyl-2,4-hexadiene are formed. The two latter products are formed via nucleophilic trapping of the diene cation radical by solvent. The difference between electrochemical and photochemical oxidative addition of methanol to dienes and styrenes is discussed.