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

Thecationic bis(phosphine) cobalt(I) arene complex, [(depe)Co(& eta;(6)-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-componentcoupling of arenes, alkenes, and alkynes. Although a sterically attenuatedcatalyst was targeted with the goal of accelerating C-H functionalization,hydrovinylation derived from the alkene and alkyne was favored overthree-component coupling. Remarkably, no catalytic hydrovinylationwas observed in the absence of arene, demonstrating the role of C-Hactivation in the hydrovinylation process and ruling out a mechanismthat involves & beta;-H elimination from a cobalt(III) metallacycle.Deuterium labeling supported & beta;-H elimination from a putativecobalt(III)-alkyl,aryl intermediate that forms after C-Hactivation by a cobalt(III) metallacycle. By varying the arene andalkene coupling partners, factors affecting the selectivity of C(sp(3))-C(sp(2)) reductive elimination versus & beta;-Helimination from the cobalt(III)-alkyl,aryl intermediate weredetermined.