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Cross [2 + 2] Cycloaddition of Bicyclic Alkenes with Alkynes Mediated by Cobalt Complexes: A Facile Synthesis of Cyclobutene Derivatives
journal contributionposted on 2001-11-30, 00:00 authored by Kuan Cheng Chao, Dinesh Kumar Rayabarapu, Chun-Chih Wang, Chien-Hong Cheng
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.