Asymmetric Hydrogenation Of Acetophenone On Supported Chiral Ruthenium Complexes

Enantioselective hydrogenation of prochiral ketones to optically active secondary alcohols is among the most fundamental subjects in modern synthetic chemistry.An important way of producing chiral secondary alcohols is the asymmetric hydrogenation of prochiral ketones.The chiral ruthenium catalyst in the presence of an inorganic base and isopropanol,has been proved very efficient in the enantioselective hydrogenation of prochiral ketones.However,the difficulties in separation and recovery of the catalysts limit their reuse.Therefore,there has been an increasing demand for the preparation of heterogeneous catalysts,in which immobilization of homogeneous catalysts on solids especially on nanostructured solids is the most important strategy to realize this goal.Upon immobilization of chiral complexes onto nanostructured solid supports with well-controlled surfaces and pores,the solid supports may act as a nanoreactor and hinder or even block some of the reaction channels.In recent years,heterogeneous chiral catalysts supported on inorganic earrier is favored by researchers.Inorganic carrier has many advantages,such as easy synthesis,raw materials can be cheap industrial products,hydrothermal stability and high thermal stability.Asymmetric catalysts supported on inorganic carriers,have good hydrothermal stability and thermal stability,insoluble in organic solvents.The rigid structures of inorganic carrier,prevent the active component from polymerization effectively,and can be recycled use many times.Recent studies have shown that,heterogeneous catalysts supported on inorganic carrier possess the same catalytic effect to the corresponding homogeneous catalysts.To expand the new routes of heterogeneous asymmetric catalyst synthesis,open environment friendly new way of green chemical synthesis of chiral drugs,in this article,heterogeneous catalysts supported on inorganic carrier and their catalytic acetophenone asymmetric hydrogenation performance are investigates,and the factors affecting catalytic properties are discussed.The specific research results are as follows:(1)Chiral Ru complexes,(1S,2S)-DPEN-Ru(TPP)2,are encapsulated in the nanocavities of zeolite Y possessing various alkali metal cations(Li+,Na+5 K+,and Cs+)exchanged by the flexible ligand method.The influence of the alkali metal cations on the electronic transitions,redox and catalytic abilities of the guest(1S,2S)-DPEN-Ru(TPP)2 complexe in the restricted void space are investigated.Ru complexes encapsulated in Li+exchanged zeolite Y exhibit higher activity and the enantioselectivity than that in Na+,K+,or Cs+exchanged zeolite Y.The results of characterization,catalytic reaction and DFT calculation suggest that the higher activity and the enantioselectivity of Ru complexes encapsulated in Li+exchanged zeolite Y are mainly resulted from its higher encapsulation amount of Ru complex and large available space and relative weak interaction with acetophenone/DPEN.(2)The chiral ruthenium complexes,(1S,2S)-DPEN-Ru-BINAP,are encapsulated in the nanocavities of zeolite Y possessing various alkali metal cations(Li+,Na+,K+,and Cs+)exchanged by the flexible ligand method.The influence of the alkali metal cations on the electronic transitions,redox and catalytic abilities of the guest(1S,2S)-DPEN-Ru-BINAP complexe in the restricted void space are investigated.Ru complexes encapsulated in zeolite Y exchanged by K+and Cs+exhibit higher activity and the enantioselectivity.(1S,2S)-DPEN-Ru-BINAP/CsY possess the highest stability and Ru content.The influence factor of catalytic and redox properties of(1S,2S)-DPEN-Ru-BINAP/MY are the comprehensive results of its higher encapsulation amount of Ru complex,electronic effects,the role of space limitations and diffusion effects.(3)The cage-like mesoporous silica SBA-16 is highly ordered porous silica with large mesopores arranged in body centered cubic Im3m symmetry.Its large cages can accommodate metal complexes with large molecular size,whereas the smaller pore entrances may prevent leaching of the metal complex confined in the mesoporous cages.SB A-16 has been synthesized and introduced in asymmetric catalysis system as a support.Additionally,the existence of plentiful hydroxyl groups in SBA-16 provides the possibility of tailoring the pore entrance size by a simple silylation reaction.The chiral complexes(1.S,2S)-DPEN-RuCl2(TPP)2 are successfully encapsulated in the mesoporous cage of SBA-16.The prepared heterogeneous catalysts whose templates were extracted by pyridine-ethanol exhibit high catalytic activity and excellent enantioselectivity in asymmetric hydrogenation of acetophenone.The catalysts are stable highly and can be recycled for several times without significant loss of activity and enantioselectivity.(4)Al-SBA-16 with the body centered cubic Im3m symmetry,has been synthesized and introduced in asymmetric catalysis system as a support.The chiral complexes(1S,2S)-DPEN-RuCl2(TPP)2 are successfully encapsulated in the mesoporous cage of Al-SBA-16,and proved by powder XRD,N2 adsorption,FT-IR,UV-vis.The prepared heterogeneous catalysts exhibit high catalytic activity and excellent enantioselectivity in asymmetric hydrogenation of acetophenone which were over comparable with that obtained by its homogeneous counterpart.The ee value is more than 70%,and reactant conversion rate reaches 100%.(5)The catalytic properties of supported catalysts of[RuCl2(mTPPTS)]2 obtained from hydrotalcite-like compounds have been assessed in the enantioselective hydrogenation of acetophenone.The heterogenious catalysts are characted by XRD,IR,UV-vis,N2 and SEM spectroscopy.The results show that the prepared heterogeneous catalyst exhibite better catalytic effect than that of its corresponding homogeneous catalyst,and the ee value is of more than 70%which is higher(10%)than that of homogeneous catalyst.