Photocyclization between Quinones and Allenes via Photoinduced Electron Transfer

Article abstract of DOI:10.1021/jo00272a045

Photochemical reactions of halo-1,4-naphthoquinones with 1,1-diphenylallenes afforded spiropyran adducts derived from <2 + 4> cycloaddition between carbonyl group of quinone and allene in good yield.In the photoreactions of 2,3-dichloro-1,4-naphthoquinone with monophenylallenes, <2 + 2> cycloadducts were obtained in addition to spiropyran derivatives.Considering the relationship between formation of spiropyran adducts and the free energy changes (ΔG) together with substituent effects, solvent effects, and CIDNP (chemically induced dynamic nuclear polarization), we propose an electron-transfer mechanism.That is, radical ion pair formation from excited triplet quinone and allene is followed by conversion to a biradical.Subsequent bond formation between the ketyl radical in quinone moiety and ortho position of the phenyl substituents on the allene skeleton leads to the spiropyran adduct after subsequent 1,5-hydrogen shift.