Formation of Vinyl- Vinylhalide- or Acyl-Substituted Quaternary Carbon Stereogenic Centers through NHC–Cu-Catalyzed Enantioselective Conjugate Additions of Si-Containing Vinylaluminums to β -Substituted Cyclic Enones

摘要

A catalytic method for enantioselective conjugate addition (ECA) of Si-containing vinylaluminum reagents to β-substituted cyclopentenones and cyclohexenones is described. Reactions are promoted by 1.0–5.0 mol % of a bidentate NHC–Cu complex, which is prepared from air stable CuCl2•2H2O and used in situ, and typically proceed to completion within 15–20 minutes. The requisite vinylmetals are generated efficiently by a site-selective hydroalumination of an alkyne with dibal–H. The desired products, containing a quaternary carbon stereogenic center, are obtained in 48–95% yield after purification and in 89:11 to >98:2 enantiomer ratio (er). The vinylsilane moiety within the products can be functionalized to afford acyl, vinyliodide or desilylated alkenes in 67% to >98% yield and with >90% retention of the alkene's stereochemical identity. The utility of the catalytic process is illustrated in the context of a concise enantioselective synthesis of riccardiphenol B.