| Abstract
| - Bicyclic alkenes 1a−e and 5 undergo [2 + 2] cycloaddition with a variety of alkynes PhC⋮CPh,(TMS)C⋮CH, HC⋮C(CH3)2OH, (TMS)C⋮CCO2Et, PhC⋮CCH3, C2H5C⋮CC2H5, CH3C⋮CC3H7, andCH3C⋮CC2H5 in the presence of Co(PPh3)2I2, PPh3, and Zn powder in toluene to afford thecorresponding exo-cyclobutene derivatives 3a−m, 6, and 8a−g in fair to excellent yields. The yieldof this cycloaddition is highly sensitive to the cobalt catalyst, solvent, ligand, and temperatureused. A mechanism involving a metallacyclopentene intermediate is proposed to account for thiscobalt-catalyzed cyclization.
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