Stereodynamics and Asymmetric Hydrosilylation with Chiral Rhodium Complexes Containing a Monodentate N-Heterocyclic Carbene

Imidazolinium salts derived from (1S,2S)-diphenylethylenediamine were used to prepare chiral N-heterocyclic carbene (NHC) complexes of rhodium of the form (cod)Rh(NHC)Cl via silver transmetalation. The neutral complexes were readily converted to cationic amine analogues [(cod)Rh(NHC)(L)]SbF₆, where L = isoquinoline and 3,5-lutidine. The NHC ligands contain unsymmetrically substituted wingtips that give rise to chiral axes; the conformations of the substituents, as well as other ligands, are influenced by the fixed chirality in the backbone of the ligand. The stereodynamics of both the neutral and cationic complexes were studied, and it was observed that strong conformational preferences exist both in solution and in the solid state. The interplay of the various chiral elements provides the environment that eventually controls the enantioselectivity in the catalytically active species. These complexes are active catalysts for the hydrosilylation of acetophenone with diphenylsilane and furnished (S)-1-phenylethanol in up to 58% enantiomeric excess.