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.