Preparation Of Hollow Polymer Microspheres/Microbowls-Supported Chiral Organocatalyst And Their Application In Asymmetric Catalysis

In homogeneous catalysis,the chiral organocatalyst?5-20 mol%?is difficult to be recovered from reaction mixture and reused for next reaction,which results in the high operating costs of asymmetric reaction and difficulty in large-scale synthesis.The heterogenization of organocatalyst is an important strategy to achieve the reusability and actualize the large-scale synthesis of catalyst.However,the factors,such as micro environmental diversity of catalyst support and embedded active catalytic sites,result in the challenge of mass transfer limitation and the decrease in stereoselectivity.Therefore,the construction of heterogeneous organocatalyst with small scale and regular morphology,large specific surface area,controlled skeleton structure,high catalytic performance and reusability is an important and key strategy to solve the large-scale asymmetric synthesis.In order to improve the catalytice of heterogeneous organocatalysts,microspheres and microbowls-supported organocatalysts with hollow structure were designed by using amphipathic cross-linked polymers with good compatibility,and their catalytic performances were investigated in heterogeneous asymmetric aldol reaction and Michael addition reaction.First,hollow microspheres were prepared by emulsion polymerization using cross-linker DVB / emulsifier SDS,cross-linker DVB / emulsifier PVA and cross-linker EGDMA / emulsifier PVA,respectively.On the basis of their morphology,stability of preparation system and catalytic performances,and the route of cross-linker EGDMA / emulsifier PVA was chosen for both microspheres and microbowls.Then,we screened the amount of monomer and the type and ratio of solvent.And their morphology,structure and composition were characterized by FT-IR,elemental analysis,SEM,TEM,N₂ Adsorption-desorption and thermogravimetric analysis.The hollow microspheres supported-PSQDNH₂ catalysts for asymmetric aldol reaction,and achieved good yields?63-87%?and stereoselectivity?95% ee,90/10 anti/syn?.The hollow microspheres-loaded J?rgensen-Hayashi catalysts for asymmetric Michael reaction,and achieved good yields?59-82%?and excellent stereoselectivity?95-99% ee,90/10-96/4 anti/syn?.After completion of the reaction,the catalyst was recovered by centrifugation and dried.After 5 cycles,the yield decreased slightly and the stereoselectivity was almost maintained.Second,hollow microbowls were prepared by controlling the thickness of the hollow shell,and their catalytic performances comparable to that of homogeneous catalysis was obtained in the asymmetric Michael addition reaction.Adjusting the reaction conditions to control the shape of the bowls,we prepared a series of loaded J?rgensen-Hayashi functional hollow polymer microbowls.And their morphology,structure and composition were characterized by FT-IR,elemental analysis,SEM,TEM,N2 Adsorption-desorption and thermogravimetric analysis.In the heterogeneous asymmetric Michael addition reaction of nitroene with n-propyl,excellent yield?86-98%?and stereoselectivity?96-99% ee,81/19-92/8 anti/syn?were obtained.The catalyst was recycled three times with a slight decrease in yield,unchaged enantiomeric excesses and diastereomeric excess.Thirdly,we studied the formation mechanism of hollow microbowls in this paper.The effects of different preparation conditions on the morphology were compared;SEM was used to monitor the preparation process and the morphological changes of the process;we compared the shell thickness of the hollow microspheres and the hollow microbowls;the effects of solvent evaporation and centrifugal force on the formation of hollow microbowls were investigated.The formation mechanism of hollow micro-bowl was studied and elucidated: due to the sequential addition of DMSO and H₂O in the mixed solvent,the emulsion polymerization system was affected.So that the thickness of the shell in the core-shell precursor was not the same,and the thickness of the shell-like core shell is smaller?<40 nm?.When the core was removed,the support strength became small,resulting in shell deformation under the centrifugal force and solvent evaporation.Ultimately,the formation of hollow microbowls was complete.