Synthesis Of Novel Water-soluble Chiral Ligands And Its Ru Complex Catalyzed Asymmetric Transfer Hydrogenation Of Acetophenone

In this paper, based on the concept of thermoregulated phase-transfer catalysis (TRPTC),water-soluble chiral ligands(â… andâ…¡) containing poly(ethylene glycol) chain have beendesigned and synthesized, of which the structure is shown below. In order to explore a newapproach for recycling of the asymmetric catalyst, they were initially applied in theasymmetric transfer hydrogenation of acetophenone in aqueous/organic biphasic system.â… â…¡Using MeOPEG₇₅₀ (CH₃(OCH₂CH₂)_nOH, n=17) as raw material, through three stepsbenzenesulfonyl chloride modified with MPEG was synthesized. Chiral ligandsâ… wereprepared via benzenesulfonyl chloride modified with MPEG reacting with L(D)-valinol,(+)-pseudophedrine gradually; chiral ligandsâ…¡were obtained by the reaction ofbenzenesulfonyl chloride modified with MPEG with chiral 1,2-diphenylethylenediamine. Thestrucuures of these ligands have been characterized through NMR technique, and the resultsare similar to those reported for other ligands. Both chiral ligands have function oftemperature-dependent water solubility-cloud point.in this study, Ru complex catalyst formed in situ by [RuCl₂(p-cymene)]₂ with chiral ligandsâ… andâ…¡have been used in aqueous/cyclohexane biphasic asymmetric transfer hydrogenationof acetophenone. With sodium formate as the hydride source, the effects of temperature,reaction time, the concentration of the formate etc. have been tested and discussed. Theactivity of chiral ligandâ… Ru complex catalyst was low, there was nearby no conversion.chiral ligand (R,R)-â…¡Ru complex catalyst gave a conversion of 73% and an ee value of93.9%(R) under the conditions of S/C=100(molar ratio), 20℃, 8h, 5 equiv of HCOONa.chiral ligand (S,S)-â…¡Ru complex catalyst afforded 93.9%ee(S) and a qualitive conversionunder the conditions of S/C=100(molar ratio), 40℃, 6h, 1 equiv of HCOONa. In a word,chiral ligandsâ…¡have enantioselectivity, and it is worth further studying.The recyclability of Ru complex catalyst of chiral ligand (R,R)-â…¡was studied initially. Theconversion decreased sharply, but almost no loss in enantioselectivity.