Cobalt-Catalyzed Cyclization/Hydroboration of 1,6-Diynes with Pinacolborane

Herein, we report a protocol for cyclization/hydroboration of 1,6-diynes with pinacolborane using a cobalt catalyst generated in situ from a Co­(II)–phenanthroline complex, tetrabutylammonium fluoride, and pinacolborane. This protocol, which features good functional group tolerance, a broad substrate scope, and excellent stereoselectivity, enables the synthesis of useful cyclic 1,3-dienylboron compounds from readily accessible feedstocks. The proposed mechanism involves low-valent cobalt-promoted cyclometalation and subsequent hydroboration, as indicated by control experiments. Our findings demonstrate that combining hydroboration with C–C bond formation is an efficient way to synthesize structurally diverse organoboranes.