Preparation Of Immobilized Chiral Ligand Catalysts And Their Application In Asymmetric Transfer Hydrogenation

Chiral alcohols are important intermediates for the synthesis of various chiral drugs,perfume flavors and pesticides.Asymmetric transfer hydrogenation of ketone compounds is one of the important preparation routes;The chiral ligand and the transition metal are combined to prepare a small molecule catalyst,which is applied to the asymmetric transfer hydrogenation reaction.There are many reports on good catalytic activity and enantioselectivity.However,the prepared small molecule catalysts are generally expensive and are not easily separated from the product after the reaction is completed.At the same time,the catalyst can not be recycled,which will cause a lot of waste.The catalyst immobilization is an effective method for solving the separation,recovery and recycling of the catalyst,but the supported catalyst also faces the problems of low catalytic activity of the heterogeneous system and difficulties in achieving green catalysis in the aqueous solution.In this paper,we have prepared a series of complexes of immobilized chiral catalysts based on polyionic liquids.First,a chiral monomer and methacrylic acid are copolymerized to obtain a random copolymer with carboxylic acid groups in the side chain,which is crosslinked by a complexation between carboxylic acid ions and a polyionic liquid in an ethanol ammonia solution to prepare a solid.This complex is used in an asymmetric transfer hydrogenation reaction of a ketone compound or an acyclic nucleoside compound.At the same time,the effects of different anions and their contents of PIL on the catalytic activity and enantioselectivity of the catalyst were studied.The details are as follows:1.The chiral ligand V-TsDPEN and[RuCl₂?p-cymene?]₂ were combined to prepare a small molecule catalyst V-TsDPEN-Ru;then V-TsDPEN and V-TsDPEN-Ru were separately polymerized with methacrylic acid to prepare,two random copolymers with carboxylic acid groups in the side chain.The structures of the two random copolymers were characterized by ¹H NMR and FT-IR.2.We prepared a polyionic liquid PIL-1 by reacting hydroxyl terminated polyepichlorohydrin with methylimidazole,and prepared a polyionic liquid PIL-2 by a simple counter ion exchange.The random copolymer-1 and PIL-1?n=1/1?were dissolved in dimethyl sulfoxide solution,and they were crosslinked by the interaction between ions in an ethanol ammonia solution to obtain a composite Compound-1.The ratio of random copolymer-1 to PIL-1?n=1/2?was adjusted to prepare complex-2,and then complex-1 and complex-2 were mixed with[RuCl₂?p-cymene?]₂ respectively to prepare catalyst Cat-1,Cat-2.The random copolymer-2 was combined with PIL-1 and PIL-2?n=1/2?in the same manner to prepare Cat-3 and Cat-4,The catalysts were characterized by FT-IR,SEM,XPS and ICP-AES.3.To investigate the catalytic performance of the immobilized chiral ligand catalysts,we applied them to asymmetric transfer hydrogenation reactions:?1?Catalysts were used as reaction substrates,formic acid and triethylamine were used as hydrogen sources.The catalytic activity and enantioselectivity in water of the supported catalysts were investigated.The activity,the reaction can be completely converted,and at the same time,an ee value of over 94%is obtained in various ketone compounds.?2?The catalytic activity and enantioselectivity of the supported catalysts were studied using acyclic nucleoside compound 6-chloro-9-acetonyl hydrazine as the substrate for the reaction.It was found that the catalyst activity of the Cat-3 in water was the highest,and it could be completely reacted in 6 h.At the same time,the reaction temperature and reaction solvent were screened.Finally,100%conversion and78.3%ee value were obtained in ethanol solution at 25°C.?3?The recycling performance of Cat-3 was investigated.It was found that in the catalyst process of the asymmetric transfer hydrogenation of acetophenone and 6-chloro-9-acetonyl hydrazine,after five cycles of recycling,the catalytic activity and enantioselectivity were not significantly reduced.