In 2009, we disclosed the first catalytic metal-free boron conjugate additions (BCAs) to a,�-unsaturated ketones and esters. The reactions are catalyzed by a small, achiral NHC, delivering �-boryl carbonyls that are versatile building-blocks in the synthesis of biologically active molecules. Subsequent mechanistic investigations including NMR experiments, computational studies as well as crystallographic data indicate NHC may serve as a Lewis base to activate the diboron reagent. Here, we report the enantioselective version of the NHC-catalyzed BCA process. These C-B bond forming reactions are promoted by 2.5-7.5 mol % of a readily accessible C1-symmetric chiral imidazolinium salt (NHC precursor) and a common organic base (DBU) in the presence of THF and methanol. a,�-Unsaturated acyclic ketones, esters, Weinreb amides and even aldehydes are suitable substrates (60-94% yield and 85.5:14.5 to 98:2 er). Cyclic �-boryl ketones can also be obtained efficiently but with diminished enantioselectivity (71:29 to 92:8 er). The utility of the current approach is demonstrated through comparison with the alternative Cu-catalyzed variant; in cases involving a polyfunctional substrate (such as aldehyde, alkyne, phenol or allene groups), NHC-catalyzed BCA reaction affords a higher chemoselectivy than the more established Cu-catalyzed method.

NHC-catalyzed enantioselective boron conjugate additions to <i>a</i>,<i>�</i>-unsaturated ketones, esters, Weinreb amides, and aldehydes

248th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.

NHC-catalyzed enantioselective boron conjugate additions to a,�-unsaturated ketones, esters, Weinreb amides, and aldehydes