Photocyclization of 2-vinylbiphenyls: stereochemistry of the triplet state cyclization

Abstract

The photochemistry of five- to eight-membered ring containing 2-cycloalkenyl biphenyls was examined under direct and sensitized irradiation. From the singlet excited state only trans 9,10-cycloalkyl-9,10-dihydrophenanthrenes were obtained. From the triplet state the trans isomer was exclusively formed from 2-cyclopentenyl and 2-cyclohexenyl biphenyls while the cis isomer also appeared with 2-cycloheptenyl and 2-cyclooctenyl biphenyls. We propose that the cis isomers resulted from the cyclization of the perpendicular triplet of cycloheptenyl biphenyl and from the perpendicular and trans triplet of cyclooctenyl biphenyl. Whereas the photocyclization of 2-vinylbiphenyls was regarded as a singlet-state reaction, the quantum yields of reaction of 2-cycloalkenyl biphenyls, from both singlet and triplet states, were high (ΦS = 0.20–0.26; ΦT = 0.15–0.46).