C(sp³)-C(sp²) Reductive Elimination versus β-Hydride Elimination from Cobalt(III) Intermediates in Catalytic C-H Functionalization

The cationic bis(phosphine) cobalt(I) arene complex, [(depe)Co(η⁶-C₇H₈)][BAr₄^F] (depe = 1,2-bis(diethylphosphino)ethane; BAr₄^F = B[(3,5-(CF₃)₂)C₆H₃]₄), was investigated as a precatalyst for three-component coupling of arenes, alkenes, and alkynes. Although a sterically attenuated catalyst was targeted with the goal of accelerating C-H functionalization, hydrovinylation derived from the alkene and alkyne was favored over three-component coupling. Remarkably, no catalytic hydrovinylation was observed in the absence of arene, demonstrating the role of C-H activation in the hydrovinylation process and ruling out a mechanism that involves β-H elimination from a cobalt(III) metallacycle. Deuterium labeling supported β-H elimination from a putative cobalt(III)-alkyl,aryl intermediate that forms after C-H activation by a cobalt(III) metallacycle. By varying the arene and alkene coupling partners, factors affecting the selectivity of C(sp³)-C(sp²) reductive elimination versus β-H elimination from the cobalt(III)-alkyl,aryl intermediate were determined.