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

摘要

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

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