Synthesis of chiral N-heterocyclic carbene (NHC) ligand precursors and formation of ruthenium(II) complexes for transfer hydrogenation catalysts

Abstract

The S enantiomers of ester functionalized NHC ligand precursors, 1-[(2S)-1-ethoxy-3-methyl-1-oxobutan-2-yl]-3-ethyl-1H-imidazol-3-ium bromide (3a) and the methyl ester analog 3b, were synthesized from l-valine and fully characterized. The absolute stereochemistry of the chiral center was confirmed by single crystal X-ray diffraction (for 3a) and enantiopurity (of 3a and 3b) was determined by micellar electrokinetic chromatography. Deprotonation of this NHC ligand precursor (3a or 3b) and coordination to [RuCl2(p-cymene)]2 resulted in the isolation of a monodentate complex (with only NHC coordination) (4a or 4b) and a bidentate complex (5). In the bidentate complex, in addition to NHC coordination, a carboxylate donor group (from in situ hydrolysis of the ester) coordinates to Ru. All ruthenium complexes have been characterized by spectroscopic and analytical methods, plus single crystal X-ray diffraction for 4a and 5. The evidence shows that both Ru complexes are racemic, but with alternate synthetic methods perhaps racemization during complexation of 3a or 3b to metals could be avoided. The ruthenium complexes described herein serve as catalysts for ketone transfer hydrogenation in basic isopropanol.