Efficient C-B bond formation promoted by N-heterocyclic carbenes: synthesis of tertiary and quaternary B-substituted carbons through metal-free catalytic boron conjugate additions to cyclic and acyclic alpha,beta-unsaturated carbonyls

Metal-free nucleophilic activation of a B-B bond has been exploited in the development of a highly efficient method for conjugate additions of commercially available bis(pinacolato)diboron to cyclic or acyclic alpha,beta-unsaturated carbonyls. The reactions are readily catalyzed by a simple N-heterocyclic carbene (NHC) present at 2.5-10 mol %. A variety of cyclic and acyclic unsaturated ketones and esters can serve as substrates. The transformations deliver beta-boryl carbonyls bearing tertiary or quaternary B-substituted carbons in up to >98% yield. Preliminary studies indicate that although related Cu-NHC-catalyzed reactions are equally efficient, the metal-free variant is more functional-group-tolerant; in contrast to the Cu-catalyzed reactions, the metal-free processes proceed readily in the presence of a terminal alkyne and do not promote concomitant diboration of an aldehyde. Representative functionalization of the resulting boron enolates demonstrates the strong influence of the Lewis acidic B atom of the beta-boronate.