Synthesis of Electron-Rich CNN-Pincer Complexes, with N-Heterocyclic Carbene and (S)-Proline Moieties and Application to Asymmetric Hydrogenation

New chiral CNN-pincer-type gold, palladium, and rhodium complexes containing N-heterocyclic carbene substituent and (S)-N-tert-butyl-methylpyrrolidine-2-carboxamide as chiral auxiliary have been synthesized and studied for asymmetric hydrogenation. The complexes were prepared by the silver carbene transfer route from the respective silver complex. The reaction with [RhCl(cod)](2) (cod = cycloocta-1,5-diene), PdCl2(CH3CN)(2), or K[AuCl4] affords the corresponding cationic [Rh(cod)-(ligand)]Cl, [PdCl(ligand)]Cl,and [AuCl(ligand)]Cl, complexes in which the ligand functions effectively in a CNN coordination mode. The complexes catalyze the enantiosclective hydrogenation of prochiral alkenes. Enantioselectivity is very sensitive to the NHC N-substituent, resulting in a useful switch in the predominant enantiomer.