Ruthenium(II)-assisted asymmetric hydrogen transfer reduction of acetophenone using chiral tridentate phosphorus-containing ligands derived from (1R, 2R)-1,2-diaminocyclohexane

Transfer hydrogenation reduction of ketone catalyzed by Ru(II) and Rh(I) complexes with ligands derived from (1R,2R)-cyclohexane-1,2-diamine

“…The low yields in these reactions suggest, either steric crowding around the metal center in the transition state, or more likely the tridentate ligands with additional heteroatom on these substituents binds to ruthenium leading to a catalytically inactive species (Figure 1). Ligands 4, 5 with ortho diphenylphosphine substituent reported previously (Scheme 2), 22 . This is in keeping with Lemaire's observation, where the ligand displaces only one of the COD ligands, and since the chiral center is far from the metal, it is unable to induce enantioselectivity in the formation of the alcohol.…”

“…25 Ligands 6, 7 reported previously gave similar results, a racemic mixture in 80-90% yield, suggesting that the ligands must be binding to rhodium in a bidentate fashion (Scheme 2). 22 and monourea derivatives 25 were prepared by previously reported methods. …”

“…21 Recently we reported the use of chiral [NNP] ligands derived from cyclohexane-1,2-diamine in the enantioselective reduction of ketones. 22 These phosphorous containing ligands gave high enantioselectivities (80-99%) and low yields (20%) in the hydrogen transfer reduction of ketone. Continuing on the same lines, our objective was to synthesize new chiral ligands maintaining the cyclohexane 1,2-diamine as a backbone and test them in the transfer hydrogenation of ketone with metals such as Ru(II) and Rh (I).…”

Phosphines and Related Tervalent Phosphorus Systems

“…Imine formation from the commercially available o-(diphenylphosphino)benzaldehyde continues to be exploited. Among new phosphinoarylimines reported are those from 2-aminomethylpyridine, 379 1-phenylazo-2-naphthylamine, 380 chiral sulfinamides, 381,382 chiral monosulfonamido derivatives of trans-1,2-diaminocyclohexane, 383 and the primary amines H 2 N(CH 2 ) n SePh (n¼3,4). 384 Imine formation with o-diphenylphosphinobenzaldehyde has also been used for the surface functionalisation of dendritic primary alkylamines, giving dendrimeric P,N ligands.…”

Chiral Tridentate Ligands in Transition Metal-Catalyzed Asymmetric Hydrogenation

“…Based on Noyori's catalysts, the chiral diamine moiety was employed in the design PNN ligands. In 2004, chiral diamine-based PNN-L1 88 was synthesized by Somanathan and co-workers and was employed in Ru-catalyzed ATH of ketones. Although high enantioselectivity was achieved for acetophenone, only 15−20% yield was observed.…”