Photochemistry of 2-halogeno-1,1-diphenylethylenes. The photo-Fritsch–Buttenberg–Wiechell rearrangement

Abstract

Competing ionic and radical photobehaviour has been observed for 2-halogeno-1,1-diphenylethylenes. Irradiation of 2-fluoro-1,1-diphenylethylene in cyclohexane or ether afforded cis- and trans-fluorostilbene. Irradiation of the chloro, bromo, or iodo analogues gave a complex mixture of up to eight products, the three major products being diphenylacetylene, 1,1-diphenylethylene, and 1,1,4,4-tetraphenylbutadiene. The product distribution proved to depend on the halogen and on the solvent. Benzophenone or xanthone sensitised irradiation of 2-fluoro-1,1-diphenylethylene in cyclohexane gave 1-cyclohexyl-1-fluoro-2,2-diphenylethane, while irradiation of 2-chloro 1,1-diphenylethylene gave 2-cyclohexyl-1,1-diphenylethylene.