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 a,�-unsaturated carbonyls. Reactions are readily catalyzed by 2.5�10 mol % of a simple N-heterocyclic carbene (NHC).  A variety of cyclic and acyclic unsaturated ketones and esters can serve as substrates.  Transformations deliver �-boryl carbonyls bearing tertiary as well as 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 of the �-boronate.

Efficient C-B bond formation promoted by <i>N</i>-heterocyclic carbenes: Synthesis of tertiary and quaternary B-substituted carbons through metal-free catalytic boron conjugate additions to cyclic and acyclic <i>α,β</i>,-unsaturated carbonyls

239th National Meeting (2010)

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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 α,β,-unsaturated carbonyls