Zn-modified Mesoporous BMMs Immobilized Bipyridine-proline And Its Catalytic Asymmetric Aldol Reaction

Traditional catalytic reactions can be divided into homogeneous catalytic reactions and heterogeneous catalytic reactions.The homogeneous catalytic reaction are limited due to difficulty in purification of the product and recovery of the catalyst.From the point of view of practical application and environmental friendliness,it is particularly important to achieve the conversion of homogeneous catalysts to heterogeneous catalysts.Some reserachers have successfully prepared different types of heterogeneous catalysts by immobilizing organic catalyst on different organic supports?polystyrene,polyethyleneglycol and cyclodextrines?or inorganic supports?zeolithes,mesoporous silica and iron oxide nanoparticles?.Particularly,mesoporous silica materials are often selected as carriers for immobilized organic catalysts due to their high specific surface area,large pore volume,adjustable pore size,excellent thermal stability,mechanical stability,and chemical inertness.Based on this,the heterogeneous catalyst system with mesoporous silica as a carrier has entered a stage of rapid development.The bimodal mesoporous SiO₂?BMMs?were selected as the carrier to immobilize the bipyridine-proline due to a high surface area?>700 m²/g?,large pore volume?up to 3.5 cm³/g?,modifiable surface and controllable bimodal mesopores.Generally,most catalysts with proline or proline derivatives as active centers are used to catalyze asymmetric aldol reaction,asymmetric michael addition reaction and asymmetric mannich reaction,especially the direct asymmetric aldol reaction of cyclohexanone and 4-nitrobenzaldehyde.In our group,extensive studies have been carried out on preparation of the heterogeneous catalyst based on the mesoporous silica as well as catalytic application in the direct asymmetric aldol reaction,in which,the organic catalyst bipyridyl-proline derivative Z was representative.The members of our group before and after immobilized the bipyridyl-proline derivative Z onto the surface of double model mesoporous SiO₂?BMMs?and Zn-modified BMMs to prepare reusable heterogeneous catalysts ZBMMs,ZZnBMMs for the direct intermolecular asymmetric Aldol reaction of 4-nitrobenzaldehyde and cyclohexanone.Based on the preliminary work of our group,the specific research contents are as follows:?1?The grafted-Zn stability and the immobilized-Z leaching of obtained ZZnBMMs during washing and drying of prepared procedure were further investigated under various solvents,such as dimethyl sulfoxide?DMSO?,petroleum ether,anhydrous methanol?CH₃OH?,and dichloromethane?CH₂Cl₂?.The interactions between immobilized-Z and grafted-Zn in the mesoporous channels were elucidated by means of the powder X-ray diffraction?XRD?patterns,the scanning electron microscopy?SEM?,N₂ adsorption and desorption isotherms?N₂-sorption?,the UV-visible diffuse reflectance spectroscopy?UV-vis DRS?and the Fourier transform infrared spectroscopy?FT-IR?spectra.In particular,the fractal dimension of the sample during the above process was obtained by small angle X-ray scattering?SAXS?technique.The results showed that the solid catalyst ZZnBMMs has high stability in petroleum ether system at 20^oC.Finally,under the stable conditions,the catalytic performance of the solid catalyst ZZnBMMs in the asymmetric Aldol reaction between cyclohexanone and 4-nitrobenzaldehyde was preliminarily evaluated.?2?A series of heterogeneous catalysts Z-grafted ZnBMMs?abbreviated as ZZnBMMs-n?n represents a molar ratio of Z/Zn,corresponding to 5,20,50,70,and100,respectively?were synthesized by by post-grafting and coordination methods.This series of catalysts were successfully applied to the asymmetric Aldol reaction of cyclohexanone with bulkier aldehyde derivatives?2-naphthaldehyde,9-anthracenecarboxaldehyde as well as 1-pyrenecarboxaldehyde?.Based on the investigation of the catalytic performance of the solid catalyst,the influences between the molecular diameter of the three types of aromatic aldehydes and pore structures of the carrier?BMMs?on the catalytic activities and stability was further studied.The catalytic performance showed that 2-naphthaldehyde had higher reactivity than the aromatic aldehydes 9-anthracenecarboxaldehyde and 1-pyrenecarboxaldehyde??yield value 45.11%;dr value 82:18;ee value 52%?.?3?Furthermore,the reusability of the solid catalyst ZZnBMMs-100 in the asymmetric Aldol reaction between cyclohexanone with 2-naphthaldehyde was preliminarily evaluated.The solid catalyst ZZnBMMs-100 could be reused three times,while the diastereoselectivities?dr?did not decrease significantly?82/18-77/23?,and the enantioselectivity?ee?decreased slightly?52%-39%?.