Rhodium(I)-Catalyzed Intramolecular Ene Reaction of Vinylidenecyclopropanes and Alkenes for the Formation of Bicyclo[5.1.0]octylenes

An efficient catalytic system for the intramolecular ene reaction of allene and alkene of diarylvinylidenecyclopropanes has been established. The reaction was achieved by using [RhCl(CO)(2)](2) as the catalyst in co-solvents of toluene and acetonitrile. MeCN was found to play a crucial role in controlling the reaction toward formation of bicyclo[5.1.0]octylene derivatives. An alternative system consisting of [RhCl(CO)(2)](2) and toluene in the absence of MeCN was found to give [2 + 2] cycloaddition adducts. The structures have been unambiguously determined by X-ray structural analysis. Deuterium labeling experiments were conducted to confirm the mechanism hypothesis.