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)]­[BAr4F] (depe = 1,2-bis­(diethylphosphino)­ethane; BAr4F = 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.