Preparation, Characterizations And Catalysis Of Heterogeneous Chiral Catalysts

During the past decades, the synthesis and application of chiral catalysts has been makeremarkable achievements. The homogeneous chiral catalytic reaction always occupies theleading status in the field of chiral catalysis. Thousands of chiral ligands and catalysts havebeen used in the asymmetric catalytic reaction referring to the forming of C-H, C-C, C-O andC-N bonds, and exhibit high catalytic activity and enantioselectivity. A large number of chiralcatalysts have been reported, however, only a few of those can be applied in large-scaleindustrial production. Because there are still some disadvantages in the homogeneouscatalysis system, such as difficult to separate the catalyst and the product, difficult to purifythe product, difficult to recycle the high cost of catalysts etc. Thus, heterogenization ofhomogeneous chiral catalysts will be an effective approach to solve the above problems.Homogeneous catalysts is linked with specific supporters through a sort of methods toseparate catalysts from the reaction system and catalysts can be recoveried and recycled,which is known as the immobilization of chiral catalysts. Based on the strategy ofimmobilization of chiral catalysts, a pyrene-tagged cinchona quaternary ammonium salt,chiral phase transfer catalyst （PTC-1） was synthesized and firstly immobilized onmulti-walled carbon nanotubes （MWCNTs） forming a novel heterogeneous catalyst（PTC-1/MWCNTs） and PTC-1/MWCNTs has been used in asymmetric alkylation of RX toN-（diphenylmethyene）glycine tert-butyl ester. UV-Vis spectra show different organic solventsaffect PTC-1adsorbing onto and desorbing from MWCNTs.5.3mg PTC-1can be loaded on10mg of MWCNTs in toluene, while0.02025mg PTC-1can be desorbed from12.7mgPTC-1/MWCNTs in CHCl₃. PTC-1/MWCNTs exhibit high catalytic activity andenantioselectivity in the asymmetric alkylation of different halohydrocarbons （RX） toN-（diphenylmethyene）glycine tert-butyl ester Moreover, PTC-1/MWCNTs catalysts can berecovered and recyled, showing PTC-1/MWCNTs is effective in asymmetric alkylation ofmultiple halohydrocarbon.Derivatized homogeneous catalysts form solid materials through one-pot synthesis toseparate catalysts from the reaction system and catalysts can be recoveried and recycled aswell. Based on the strategy of one pot synthesis, the alkyne-derivatized1,1′-binaphthol ligandand azides form organic polymer （Polymeralkyne-azide） containing chiral1,1′-binaphthol ligand through click reaction in one pot. Polymeralkyne-azidedisplays poor solubility in sixcommon organic solvents （EtOH, CH₂Cl₂, CHCl₃, THF, MeOH and EtOAc）, showing thatchiral1,1′-binaphthol ligands has been immobilized in organic polymer. A newheterogeneous chiral catalyst （Polymeralkyne-azide Ti） can be obtained through the coordinationof Polymeralkyne-azidewith Ti（OⁱPr）4. Comparing to supported catalysts, Polymeralkyne-azide Ticontains a large number of catalytic active centers which are uniformly distributed in theframework of the catalyst. Asymmetric addition reaction of diethylzinc with benzaldehydewas chose as a model reaction to investigate the effect of the proportion between Polymeralkyne-azideand Ti（OⁱPr）4on the chiral induction in details. When the molar ratio of Polymeralkyne-azideand Ti（OⁱPr）4is1:6, the enantiomeric excess can reach as high as65%. Besides,Polymeralkyne-azide Ti can also be used in the asymmetric addition reaction of diethylzinc toother aromatic aldehydes, the chiral products with moderate stereoselectivity have beenobtained. Polymeralkyne-azide Ti can be separated from the reaction system throughcentrifugation resulting in quick and easy experiment operations.Here, two new heterogeneous chiral catalysts were prepared using different methods andsuccessfully used in the corresponding asymmetric catalytic reactions, which is a helpfulattempt for heterogenization of homogeneous chiral catalyst as well as provides a novel ideaand method for design and synthesis of heterogeneous chiral catalysts.