Research On The Synthesis Of Chiral α-phenylethanol By Asymmetric Transfer Hydrogenation

Chiral a-phenylethanol as the precursor and intermediate in the synthesis of many complex chiral compounds can be widely applied in medicine, spices and pesticides due to its simple structure. Asymmetric Transfer Hydrogenation of acetophenone is an effective method to obtain the enantiomerically pure chiral a-phenylethanol.In this paper, three kinds of supported catalysts loading respectively with chitosan-Pd (CS-Pd), chitosan-Ru (CS-Ru) and TsDPEN-Ru were evaluated in asymmetric transfer hydrogenation of acetophenone. Mesoporous silica microspheres as support were synthesized by polymerization-induced colloid aggregation. The influence of support structure on performance was investigated through adjusting the proportion of urea and formaldehyde and reaction conditions of asymmetric transfer hydrogenation have been optimized.The characterization results of SiO2-CS-Pd showed that the structure of the microspheres broke after impregnation and support with large pore size and low surface area leaded to more agglomeration of palladium. The conversion of acetophenone achieved 27.4% and enantiomer excess (ee) value of R-a-phenylethanol reached 34.4% at 16h,40℃ and 2.0mL of acetophenone.The characterization results of SiO2-CS-Ru presented the catalyst still kept the microspheres with mesoporous and ruthenium only agglomerated a little on support. The catalyst with lower pore size made better activity of asymmetric transfer hydrogenation. Under 20h,40℃ and 0.2mL of acetophenone, the performance showed acetophenone conversion of 53.1% and R-a-phenylethanol ee value of 45.6%.The characterization results of SiO2-TsDPEN-Ru indicated that ruthenium and chiral diamine ligands were loaded on support through hydrogen bonds. After impregnation, the pore size and pore volume increased and surface area decreased. Performance results display that the addition of chiral sulfonamide ligand enhanced the activity and optical selectivity for asymmetric transfer hydrogenation of acetophenone. The acetophenone conversion and R-a-phenylethanol ee value reached up to 64.1% and 93.4%. R-a-phenylethanol ee value still maintained above 80% after recycling catalyst four times.