Monodentate Non-<i>C</i>₂-symmetric Chiral <i>N</i>-Heterocyclic Carbene Complexes for Enantioselective Synthesis. Cu-Catalyzed Conjugate Additions of Aryl- and Alkenylsilylfluorides to Cyclic Enones

A new class of enantioselective conjugate addition (ECA) reactions that involve aryl- or alkenylsilyl fluoride reagents and are catalyzed by chiral non-C2-symmetric Cu-based N-heterocyclic carbene (NHC) complexes are disclosed. Transformations have been designed based on the principle that a catalytically active chiral NHC−Cu−aryl or NHC−Cu−alkenyl complex can be accessed from reaction of a Cu−halide precursor with in situ-generated aryl- or alkenyltetrafluorosilicate. Reactions proceed in the presence of 1.5 equiv of the aryl- or alkenylsilane reagents and 1.5 equiv of tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF). Desired products are isolated in 63−97% yield and 73.5:26.5−98.5:1.5 enantiomeric ratio (47%−97% ee). A major focus of the present studies is the design, evaluation, and development of new chiral imidazolinium salts and their derived NHC−Cu complexes as catalysts that promote reactions of various carbosilanes to a range of electrophilic substrates. Toward this end, nearly 20 new chiral monodentate imidazolinium salts, most of which are non-C2-symmetric, have been prepared and fully characterized and their ability to serve as catalysts in the ECA reactions has been investigated.