Photocyclization of 2-alkenylbiphenyls is a well-known route to dihydrophenanthrenes. A similar photocyclization of alkynylbiphenyls to phenanthrene was first reported by Lewis and co-workers in 1976, but this reaction has been little explored. We report here that this is a general photoreaction, which provides an efficient route to some phenanthrene derivatives. Photocyclization of the parent structure 2-ethynylbiphenyl is nearly quantitative; for derivatives, yields vary from good to moderate. Cyclization of 2,2'-diethynylbiphenyl yields pyrene. One unusual observation is that this photoreaction is highly solvent dependent, proceeding easily in acetonitrile but not cyclohexane. In principle, this photoreaction might proceed through either a strained cyclic allene or a carbene intermediate. The mechanism has been probed through the use of deuterium labeled substrates and through computations. Results of experimental and computational studies will be described.

Photocyclization of 2-ethynylbiphenyls: An efficient route to phenanthrenes

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