Neutral and Cationic Iridium(I) Complexes Bearing Chiral Phenanthroline-Derived Benzimidazolylidenes: Synthetic, Structural, and Catalytic Studies

A series of neutral and cationic iridium(I) complexes bearing chiral and achiral phenanthroline-derived benzimidazolylidene ligands were synthesized and characterized. Spectroscopic and crystallographic data indicate that the principal plane of the NHC ligand in the achiral complexes is positioned orthogonally with respect to the Ir-Cl or Ir-P bonds, while the planes of NHC ligands in the corresponding chiral complexes are twisted by a torsion angle that depends on the size of the 2,9-substituents. All of the new complexes showed varying degrees of catalytic activity and enantioselectivity toward hydrogenation of acetamidoacrylates, with the best results being achieved using 2,9-diphenyl-substituted (S,S)-20a, which afforded (-)-(R)-methyl 2-acetamidopropanoate (12a) in 97% yield and 81 % ee.