Remarkable Positive Effect of Silver Salts on Asymmetric Hydrogenation of Acyclic Imines with Cp*Ir Complexes Bearing Chiral N-Sulfonylated Diamine Ligands

A family of bifunctional Ir, Rh, and Ru complexes bearing chiral monosulfonylated diamine ligands has been evaluated for asymmetric hydrogenation of acyclic imines (1) to the corresponding amines (2). A chiral Ir complex, [Cp*IrCl{(S,S)-Tscydn}] (3a), combined with silver salts caused a marked improvement in the catalyst performance in terms of the activity and selectivity. The use of an excess amount of the silver salt, AgSbF6, increased enantioselectivity up to 72% ee in the asymmetric hydrogenation of N-(1-phenylethylidene)benzylamine (1a). A stoichiometric reaction of 3a with AgSbF6 in acetonitrile afforded an isolable cationic complex, [Cp*Ir(Tscydn)(CH3CN)]+SbF6- (4) which was fully characterized by NMR spectroscopy and X-ray crystallography. The resulting cationic complex 4 readily reacted with H-2 under ambient conditions in the presence of triethylamine to give a hydridoiridium complex, [Cp*IrH{(S,S)-Tscydn}] (5) and showed comparable catalytic behavior to that for the catalyst system generated in situ from 3a and AgSbF6. On the basis of the additive effect on the outcome of the hydrogenation as well as the C-13{H-1} NMR spectrum of a reaction mixture of imine 1a and AgSbF6, the mechanism of the imine hydrogenation including heterolytic bond cleavage of H-2 on a cationic complex to generate a hydrido intermediate and the following H- transfer to the imine substrates activated by the silver cation was proposed.