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

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