Vanadium-Catalyzed Stereo- and Regioselective Hydroboration of Alkynes to Vinyl Boronates

Molecular complexes of vanadium catalyze cis-selective anti-Markovnikov hydroboration of alkynes to generate vinyl boronate esters with appreciable turnover numbers of up to 4000 at room temperature. This represents the first example of the use of vanadium in homogeneous catalytic hydroboration of alkynes. The method is tolerant to various functional groups, including C═C double bonds. Accordingly, 1-hexen-5-yne can be quantitatively and selectively reduced at the triple bond, leaving the double bond unaffected. Preliminary computational analysis of the catalytic cycle reveals both two-state reactivity and previously unknown complexity associated with the redox-active ligand. Specifically, it was found that the ligand can shuttle up to two electrons back-and-forth to and from the metal, which thus adapts three different oxidation states on the catalytic reaction coordinate.