P,N,O type chiral imino- and aminophosphine ligands and their applications in Ru(II)-catalyzed asymmetric transfer hydrogen reactions

Chiral P,N,O type imino- (1a-d) and aminophosphine ligands (2a-d), substituted with methyl-, isopropyl-, phenyl- and benzyl groups, were synthesized and characterized by spectroscopic techniques such as NMR, FTIR and HRMS. The structure of the ligand 1c was also determined by single crystal X-ray diffraction analysis. The X-ray data revealed that compound 1c exhibited triclinic-P1 space group with C40H34NOP molecular formula. The catalytic performances of these imino- and aminophosphine ligands were tested in ruthenium catalyzed asymmetric transfer hydrogenation of aromatic ketones in 2-propanol. Ruthenium(II) complexes were generated in situ from Ru(cod)Cl-2, Ru(dmso)(4)Cl-2, Ru(PPh3)(3)Cl-2 and [Ru(p-cymene)Cl-2](2) precursors. According to the chromatographic analyses, isopropyl- substituted chiral aminophosphine ligand 2-((2-(diphenylphosphinyl)benzyl) amino)-3-methyl-1,1-diphenylbutan-1-ol (2b) and [Ru(cod)Cl-2] combination were found to be the best catalyst system, affording (R)-enriched 1-(4-bromophenyl)ethanol in 85% ee and 98% conversion.