Cobalt-Catalyzed Regiodivergent Stereoselective Hydroboration of 1,3-Diynes To Access Boryl-Functionalized Enynes

We report a regiodivergent and stereoselective hydroboration of 1,3-diynes with pinacolborane (HBpin) in the presence of cobalt catalysts generated in situ from bench-stable Co(acac)(2) and bisphosphine ligands. These cobalt catalysts were activated by the reactions with HBpin. A range of unsymmetrical and symmetrical 1,3-diynes reacted smoothly with HBpin in the presence of Co(acac)(2)/xantphos to selectively afford enynylboronate products with boron addition to the internal carbon of the 1,3-diyne unit. However, these 1,3-diynes reacted in the presence of Co(acac)(2)/dppf to form enynylboronate products with boron addition to the external carbon of the 1,3-diyne unit. These hydroboration reactions show good functional group compatibility and can be readily scaled up to gram-scales without using a drybox. Deuterium-labeling experiments suggest a cis-addition of HBpin to the less-hindered triple bond of 1,3-diynes. Furthermore, the comparison of the regioselectivity between cobalt-catalyzed hydrosilylation and hydroboration reactions of the same 1,3-diyne substrate suggests that Co(acac)(2)/xantphos-catalyzed regioselective 1,3-diyne hydroboration proceeds through a cobalt-hydride intermediate while Co(acac)(2)/dppf-catalyzed 1,3-diyne hydroboration proceed through a cobalt boryl intermediate.