A comparison of catalytic activity for imine hydrogenation using Ru ditertiary phosphine complexes, including chiral systems

Abstract

A family of ruthenium ditertiary phosphine complexes was investigated for catalytic activity toward imine hydrogenation. The diphosphines (PP) used include chiral (chiraphos, diop, binap) and achiral (dppe, dppb) systems (chiraphos=Ph 2PCH(Me)CH(Me)PPh 2; diop=Ph 2PCH 2{ CHOCMe 2O C}HCH 2PPh 2; binap=2,2′- bis(diphenylphosphino)-1,1′-binaphthyl; Ph 2P(CH 2),:PPh 2 ( n=2, dppe; n=4, dppb)). Activity was observed in MeOH at low catalyst concentrations (0.77 mM Ru), under moderate conditions (room temperature (r.t.), 1000 psi H 2). The air-stable Ru 2Cl 5(PP) 2 complexes were more active than the commonly used dimeric Ru 2(II,II) systems to which they give rise in situ. Asymmetric induction in the prochiral ketimine PhCH 2NC(Me)Ph was consistent, though modest, within the diop and binap series of neutral complexes, implying a common catalytic intermediate within each series. A maximum e.e. of 27% was found using Ru 2Cl 5(chiraphos) 2 as catalyst; this represents a non-optimized figure for asymmetric induction, as no other prochiral ketimines were screened.