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.

Abstract

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 CuCl(2)•2H(2)O and used in situ, and typically proceed to completion within 15-20 min. 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.