Synthesis of Ruthenium N-Heterocyclic Carbene (NHC) Complexes as Catalysts for Transfer Hydrogenation of Carbonyl Compounds

Abstract

N-heterocyclic carbene (NHC) based ruthenium complexes were studied as transfer hydrogenation catalysts of carbonyl compds. The influence of bis vs. mono NHC ligands and the role of pendant ether groups were investigated. Ruthenium complexes contg. a monodentate NHC ligand with a tethered di-Et ether N-substituent, ImEt,CH2CH2OEtRu(Cl)2(arene), (arene = benzene (1 ), p-cymene (2 ), hexamethylbenzene (3 )) and a ruthenium complex with a bis-NHC ligand, [RuCl(methylenebis(N-ethylIm)2)(p-cymene)]PF6 (4 ) were synthesized (Im = Imadazole). X- ray crystal structures of complexes 1 , 2 , and 4 were obtained. These complexes are active transfer hydrogenation catalysts for the redn. of acetophenone in basic isopropanol with complex 1 being the most active. It was found that electron deficient ketones were the most easily reduced. Synthesis of ruthenium complexes contg. chiral NHC ligands as catalysts for enantioselective transfer hydrogenation is in progress. Organometallic catalysts for water oxidn. are also being investigated. [on SciFinder(R)]