Cross [2 + 2] cycloaddition of bicyclic alkenes with alkynes mediated by cobalt complexes: a facile synthesis of cyclobutene derivatives.

Bicyclic alkenes 1a-e and 5 undergo [2 + 2] cycloaddition with a variety of alkynes PhC(triple bond)CPh, (TMS)C(triple bond)CH, HC(triple bond)C(CH(3))(2)OH, (TMS)C(triple bond)CCO(2)Et, PhC(triple bond)CCH(3), C(2)H(5)C(triple bond)CC(2)H(5), CH(3)C(triple bond)CC(3)H(7), and CH(3)C(triple bond)CC(2)H(5) in the presence of Co(PPh(3))(2)I(2), PPh(3), and Zn powder in toluene to afford the corresponding exo-cyclobutene derivatives 3a-m, 6, and 8a-g in fair to excellent yields. The yield of this cycloaddition is highly sensitive to the cobalt catalyst, solvent, ligand, and temperature used. A mechanism involving a metallacyclopentene intermediate is proposed to account for this cobalt-catalyzed cyclization.