Photorearrangement of vinylidenecyclopropanes to 1,2,3-butatriene derivatives

[GRAPHICS] Photoirradiation of benzene solutions containing 1-diarylvinylidene 2,2,3,3 -tetramethylcyclopropanes (2a-d) afforded rearranged products 1,2,3-butatri enes (3a-d) in good to high yields. Photorearrangement from 2,2,3-trimethyl and 2,2- and 2,3-dimethyl derivatives 2e-g also proceeded, but the rates o f the rearrangement were lower than those of 2a-d, A singlet mechanism is p roposed far this photorearrangement, where alkyl migration occurs from 1,3- biradical intermediates generated via the homolysis of the C1-C2 bond. Gene ration of diarylvinylidene carbenes from 1,3-biradicals might be competitiv e with the formation of 3.