Magnesium-catalyzed hydroboration of esters: evidence for a new zwitterionic mechanism

A magnesium-catalyzed ester hydroboration reaction rapidly and efficiently (<0.5 mol% catalyst) provides alkoxy borane products via ester cleavage. Oxidized functional groups, such as cyano-, nitro-, cyclopropyl- and conjugated olefins, are unaffected by the ester reduction. Moreover, metal-catalyzed hydroboration reactions have been previously suggested to involve second-order interactions of hydroborane reagents and M–X (X = R and OR) for B–X bond formation. Catalytic kinetic studies rule out this traditional σ-bond metathesis mechanism for B–O bond formation, and instead a pathway involving a zwitterionic alkoxyborate is proposed.