Thermally Robust and Highly Active PCP Cobalt(II) Catalyst for Linear-Selective Hydroboration of Terminal and Internal Olefins

We report the design and synthesis of cobalt complexes of L2X-type ligands for olefin hydroboration guided by the consideration of catalyst stability. Among the series of PCP-ligated Co­(II) and Co­(III) pincer complexes, the complex with iPr phosphino substituents exhibits broad scope, high functional group tolerance, and high catalytic activity at ppm catalyst loadings (17 ppm, 0.001 mol %) in the hydroboration of terminal olefins with pinacolborane (HBpin), furnishing anti-Markovnikov addition products. We also show that linear-selective hydroboration occurs with internal acyclic olefins at elevated temperatures, including those conjugated with the arene or carbonyl groups that are difficult to be achieved by previous catalysts. Such a catalytic property makes this cobalt catalyst suitable for terminally selective formal borylation of arylalkane through the dehydrogenation–hydroboration sequence. Experimental mechanistic data provide evidence for the involvement of a Co­(II) hydride intermediate. Deuterium-labeling experiments show that the formation of terminal alkylboronate ester from internal olefins likely proceeds via the initial generation of secondary alkyl species, followed by chain-walking to form primary alkyl species, which then reacts with HBpin to produce the linear product and regenerate the hydride species.