Synthesis, structure, and catalytic activity of rhodium complexes with new chiral binaphthyl-based NHC-ligands

Three new chiral NHC–rhodium complexes have been prepared from the reactions between [Rh(COD)Cl]2, NaOAc, KI, and dibenzimidazolium salts 3, 4 or 5, which are derived from (S)-2,2′-diamino-1,1′-binaphthyl. The steric and electronic effects of the ligand play an important role in the complex formation. For example, treatment of pyridine substituted dibenzimidazolium salts 3 or 4 with 0.5 equiv of [Rh(COD)Cl]2 in the presence of NaOAc and KI in CH3CN at 85 °C gives the chiral Rh(III) complexes 6 and 7, respectively. However, under similar reaction conditions, pyridine-N-oxide substituted dibenzimidazolium salt 5 affords a binuclear Rh(I) complex 8. All compounds have been characterized by various spectroscopic techniques, and elemental analyses. The solid-state structures of compounds 4–8 have been further confirmed by X-ray diffraction analyses. Rhodium complexes 6–8 show good catalytic activity for the asymmetric hydrosilylation of acetophenone with moderate ee values.

Graphical abstractThree new chiral NHC–rhodium complexes have been prepared and structurally characterized. They have good catalytic activity for the asymmetric hydrosilylation of acetophenone with moderate ee values.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Three new chiral NHC–rhodium complexes have been reported. ► The nature of the ligand play an important role in the complex formation. ► The rhodium complexes are active catalysts for the asymmetric hydrosilylation.