Asymmetric Hydrogenation of N-Arylimines Catalyzed by the Xyl-Skewphos/DPEN-Ruthenium(II) Complex

The new catalyst system of RuBr2[(S,S)-xyl-skewphos][(S,S)-dpen] and potassium tert-butoxide shows high reactivity and enantioselectivity in the hydrogenation of N-arylimines [Xyl-Skewphos=2,4-bis(di-3,5-xylylphosphino)pentane, DPEN=1,2-diphenylethylenediamine]. A range of imines derived from aromatic and heteroaromatic ketones as well as some ferrocenyl and vinylic imines are hydrogenated with a turnover number as high as 18,000 under 10-50 atm of hydrogen to afford the amine products in up to 99% enantiomeric excess. The reaction is applied to the synthesis of a chiral N-arylamine, which is the synthetic intermediate of a biologically active compound. The mode of enantioselection is interpreted by using transition-state models based on the X-ray structure of the Xyl-Skewphos/DPEN-RuBr2 complex.