Photocyclization of a conformationally constrained 2-vinylbiphenyl

The synthesis and photochemical behavior of a rigid analog of syn-2-vinylbiphenyl are reported. This analog undergoes quantitative conversion to a tetrahydrobenzanthracene derivative upon irradiation in fluid solution. Product formation is proposed to occur via photocyclization to yield an unstable intermediate followed by an intramolecular thermal hydrogen shift. The intermediate can be observed upon irradiation at low temperature either in a rigid glass or in liquid propane solution. The temperature dependence of its decay is indicative of a tunneling mechanism for the hydrogen shift process.