Site- and Enantioselective Formation of Allene-Bearing Tertiary or Quaternary Carbon Stereogenic Centers through NHC–Cu-Catalyzed Allylic Substitution

Abstract

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_N2′ mode of addition. Allenyl-containing products are generated in up to 95% yield, >98% S_N2′ 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