Site- and enantioselective formation of allene-bearing tertiary or quaternary carbon stereogenic centers through NHC-Cu-catalyzed allylic substitution
Jung, B.; Hoveyda, A.H.
Journal of the American Chemical Society 134(3): 1490-1493
ISSN/ISBN: 1520-5126 PMID: 22214185 DOI: 10.1021/ja211269w
Catalytic enantioselective allylic substitutions that result in addition of an allenyl group (<2% propargyl addition) and formation of tertiary or quaternary C-C bonds are described. Commercially available allenylboronic acid pinacol ester is used. Reactions are promoted by a 5.0-10 mol % loading of sulfonate-bearing chiral bidentate N-heterocyclic carbene (NHC) complexes of copper, which exhibit the unique ability to furnish chiral products arising from the S(N)2' mode of addition. Allenyl-containing products are generated in up to 95% yield, >98% S(N)2' selectivity, and 99:1 enantiomeric ratio (er). Site-selective NHC-Cu-catalyzed hydroboration of enantiomerically enriched allenes and conversion to the corresponding β-vinyl ketones demonstrates the method's utility.