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⋮CPh, (TMS)C⋮CH, HC⋮C(CH₃)₂OH, (TMS)C⋮CCO₂Et, PhC⋮CCH₃, C₂H₅C⋮CC₂H₅, CH₃C⋮CC₃H₇, and CH₃C⋮CC₂H₅ in the presence of Co(PPh₃)₂I₂, PPh₃, 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.