Synthetic and mechanistic aspects of the photocyclization of 2-(β-arylvinyl)biphenyls into 9-aryl-9,10-dihydrophenanthrenes

Abstract

The synthesis is described of several 2-(β-arylvinyl)biphenyls (4c), some of which contain deuterium on C-2′–6′(14c). On irradiation they yield 9-aryl-9,10-dihydrophenanthrenes (6c) in which the isotope if present is found on C-5–8 and at the cis- or trans-position of C-10 (15c). The photoreaction consists of a stereoselective, probably concerted, conrotatory cyclization from the S₁ state of both cis- and trans-(4c) to a 9-aryl-8a,9-dihydrophenanthrene derivative (5c), followed by a thermal, suprafacial [1,5] sigmatropic hydrogen shift. The rate of the hydrogen shift is so fast that (5c) cannot be detected spectroscopically in the reaction mixture and equally is not oxidized by dissolved oxygen to a 9-arylphenanthrene (9c), which does arise in the presence of iodine as oxidant.