Well-Defined Cationic Cobalt(I) Precatalyst for Olefin-Alkyne [2 + 2] Cycloaddition and Olefin-Diene Hydrovinylation Reactions: Experimental Evidence for Metallacycle Intermediates

The synthesis and characterization of the cationic cobalt(I) arene complex, [(dppf)Co(ν6-C7H8)][BArF4] (dppf = 1,1′-bis(diphenylphosphino)ferrocene; BArF4 = B[(3,5-(CF3)2)C6H3]4) from an air-stable cobalt precursor is described. Dissolution in benzene-d6 or tetrahydrofuran (THF) resulted in rapid arene substitution and generated [(dppf)Co(ν6-C6H6)][BArF4] or [(dppf)Co(THF)2][BArF4]. The latter compound was characterized by a combination of X-ray diffraction and magnetometry and established an S = 1 cobalt(I) derivative. The isolated bis(phosphine)cobalt complexes were evaluated as precatalysts for carbon-carbon bond-forming reactions. The [2 + 2] cycloaddition of internal alkynes and olefins was observed with cobalt precatalyst loadings of 0.25 mol % with high chemoselectivity. The catalytic method was compatible with Lewis basic functional groups, an advantage over in situ-generated catalysts that rely on excess trialkyl aluminum activators. The cationic bis(phosphine)cobalt arene complex was also an effective catalyst precursor for the hydrovinylation of isoprene with ethylene. In both C-C bond-forming reactions, the corresponding cobalt(0) complex, [(dppf)Co(COD)] (COD = 1,5-cyclooctadiene), was inactive, providing strong evidence of the role of cobalt(I) during catalysis. In both catalytic reactions, deuterium crossover experiments provide experimental evidence of the role of metallacyclic intermediates during turnover.