The photochemical cycloaddition of furan and benzene

Abstract

Irradiation (254 nm) of furan–benzene mixtures results in the formation of the 2,5;1′,4′- and 2,3;1′,2′-photocycloadducts (3) and (4) respectively as the major chemical processes: 2,3;1′3′- and 2,5;1′,3′-cycloadducts are minor reaction products. Adduct (3) yields the isomer (4) by a reversible thermal or irreversible photochemical Cope rearrangement and both isomers undergo retroaddition on sensitised irradiation. Furan quenches the fluorescence of benzene but no exciplex emission is observed. A 1:1 photoadduct of thiophen and benzene is formed in trace amounts but only in the presence of proton donors.