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(η6-C7H8)]­[BArF 4] (dppf = 1,1′-bis­(diphenylphosphino)­ferrocene; BArF 4 = B­[(3,5-(CF3)2)­C6H3]4) from an air-stable cobalt precursor is described. Dissolution in benzene-d 6 or tetrahydrofuran (THF) resulted in rapid arene substitution and generated [(dppf)­Co­(η6-C6H6)]­[BArF 4] or [(dppf)­Co­(THF)2]­[BArF 4]. 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.