Photoinduced electron transfer reactions of chloranil with small-ring hydrocarbons. Contrasting reactivities of radical cation intermediates

Abstract

Products that result from the irradiation of chloranil with phenylcyclopropane and with 1,2-diphenylcyclobutene in various solvents have been identified. The cyclopropane gave products that derive from nucleophilic attack on the small-ring radical cation that is formed on electron transfer quenching of the quinone excited triplet state. Contrastingly, the cyclobutene ring remains intact following an analogous radical cation formation, but undergoes an addition in which (formally) the quinone is added across an allylic CH bond. The results are consistent with the formation of radical ion pairs (triplet excited complexes, TECs) consisting of quinone radical anions and cations associated with the small-ring derivatives. Intra-ion-pair proton transfer and radical (ion) coupling reactions account for observation of the various products. Different pathways for photolysis of quinone and small-ring hydrocarbons in more polar media involving formation of free-radical ions were also established. The quenching of quinone excited triplet states and the appearance and decay of TEC-type and other intermediates were followed by laser flash photolysis.