Rhodium and iridium NHC complexes from chiral NHC precursors: Synthesis, structure, transfer hydrogenation catalysis and method validation

The synthesis of Rh(III)Cp*–NHC and Ir(III)Cp*–NHC complexes from amino acid ligand precursors is reported. These complexes were tested as catalysts in TH with good conversions in 2 h and TON values greater than 100. [Display omitted] Heteroleptic [RhIIICp*(NHC)] and [IrIIICp*(NHC)] complexes bearing a N-amino acid functionality derived from imidazolium salts were synthesized and characterized by NMR, IR, HR-MS, and single-crystal X-ray diffraction. A discussion of the structure of the [RhCp*{κ2(C,O)–NHC}Cl] complex 3f as a pair of diastereomers is presented. Catalysts were assessed for the transfer hydrogenation (TH) of ketones in basic propan-2-ol, obtaining good yields for the reduction products. A chromatographic method applied for measurements of compounds in TH as analytes was validated. The Rh and Ir complexes described here offer an original approach in the development of catalysts based in classic NHC precursors.