The Synthesis And Asymmetric Catalysis Research Of Solvent-regulated Phase Transfer Schiff Base Complexe

Chiral Schiff base complexes are excellent catalysts in the fields of asymmetric catalysis.However,the homogeneous complexes are not easily recovered for reuse or recycle and the heterogeneous catalysts often suffer from decrease of catalytic efficiency.To address the issue, attempts were made to develop novel kinds of chiral Schiff base complexes that can be used as highly effective solvent-regulated phase transfer catalyst in the corresponding catalytic reaction.The chiral Schiff base complexes with the attractive feature of tunable miscibility could be miscible with reaction medium and immiscible with extractant.So they acted as homogeneous catalyst in corresponding reaction system,leading to excellent catalytic activity.After completion of the reaction,the catalysts could be facilely separated from the reaction system by addition of another solvent that is immiscible with the catalyst,therefore,the catalysts could be precipitated from the system.The integration of reaction and separation made the catalysts perform as "one-phase catalysis and two-phase separation".A metal-organic assembly with quasi-salen Mn(â…¢) active structural units was synthesized via a process involving in situ self-assembly of linked 4,4'-methylenebis(6-(sec-butylimino)methyl) phenol) ligand with manganese ion.The synthesized quasi-salen Mn(â…¢) assembly showed typical properties of solvent-regulated phase transfer catalyst and high catalytic efficiency bearing comparison with homogeneous salen Mn(â…¢) catalyst in the epoxidation of unfunctionalized alkenes.Furthermore,it could be easily separated by simple phase separation techniques,and could be recovered easily by the addition of hexane and subsequently reused at least 8 times without loss of activity.Density functional theory (DFT) at the B3LYP level in Gaussian 03 programs was employed to evaluate the optimized geometries of the synthesized quasi-salen Mn(â…¢) assembly and confirmed the structural stability of the complex.A novel polymeric chiral salen Mn(â…¢) complex with chiral diamine bridging was synthesized.With larger molecular weight,it acted as a solvent-regulated phase transfer catalyst in the asymmetric epoxidation of styrene under the given reaction conditions.Compared with the homogeneous monomeric chiral salen Mn(â…¢) complex,it showed the similar yield and enantioselectivity of the epoxide.Under optimal reaction conditions,the yield and the enantioselectivity of the epoxide were as high as 98%and 47%ee,respectively.Moreover,the polymeric chiral salen Mn(â…¢) complex could be conveniently recovered from the products by control of the solvent and be reused at least 3 times without losses of both activity and enantioselectivity.A novel polymeric ionic liquid(IL)-functionalized chiral salen Mn(â…¢) complex was successful synthesised by covalently-polymerization between amino(-NH₂) group of 1,3-dipropylamineimidazolium bromide with chloromethyl(-CH₂Cl) group at two sides of 5,5' positions in the salen ligand.The synthesized polymeric complex was totally miscible with dichloromethane but insoluble in non-polar organic solvents,thereby, could act as a "solvent-regulated phase transfer catalyst" in the enantioselective epoxidation of styrene.It showed comparable activity and enantioselectivity as a catalyst in asymmetric epoxidation of styrene relative to the monomeric chiral salen Mn(â…¢) complex and readily recovered from reaction system by simple precipitation.Moreover,with the catalytic site isolation of the rigid polymer framework,the polymeric chiral complex could be used at least 10 times without significant loss of catalytic activity and enantioselectivity.UV-vis spectra was used to monitor online the catalytic process and strongly supported the positive influence of the polarity of the IL on the stabilization of the transition state.Different strategies have been employed to attach the IL onto the chiral salen Mn(â…¢) complex to synthesize a novel series of IL-functionalized chiral salen Mn(â…¢) complex.The complexes with intriguing peculiarity of solubility are miscible with dichloromethane and insoluble in n-hexane.It therefore will work under homogeneous conditions during the reaction stage but will be precipitated at the end of the reaction by simple addition of n-hexane resulting in facile separation and reuse.The catalytic performances strongly depended on the attachment positions of chiral salen Mn(â…¢) complex with the IL.The complex prepared by the way of covalent linkage of the amino group in the IL with salen ligand at one side of 5 position showed the best catalytic performances in the epoxidation reaction and can be easily recovered for reuse at least 10 times.A novel hydrophilic IL of 1-propylamine-3-methylimidazolium tetrafluoroborate was covalently bonded onto chiral Schiff VO(â…£) complex successfully.It was found that the IL-functionalized chiral Schiff VO(â…£) complexes showed high activity and comparable enantioselectivity in asymmetric oxidation of the aryl methyl sulfides using hydrogen peroxide(30%) oxidant for its special 'hydrophilicity' and the retention of homogenous reaction system.Moreover,it could be readily separated by the addition of hexane and thereby subsequently reused at least 6 times without loss of activity.Different substitutes have been employed at 3 position of the Schiff base ligand in order to investigate the effect of electronic properties on the catalytic performances of the complexes.As expected,the electronic properties of the ligand have a significant effect on the catalytic properties.