Asymmetric Epoxidation Of Unfunctionalized Olefins Catalyzed By Recyclable Chiral Mn(Ⅲ) Salen Catalysts

Salen is a common chelating ligand, in which two N atoms and O atoms were on the same plane, and it can coordinate with many kinds of transit metal. As a result of the existence of unoccupied orbital in metal salen complexes axially, which can coordinate with many functionalized groups, metal salen complexes are widely applied in catalysis field. With the increased demand of chiral compounds, enantioselective metal salen complexes are used in chiral catalysis area, and because of their high activity and enatioselectivity, they have become privileged chiral catalysts. Among metal salen complexes, Mn(Ⅲ) salen complex proves to be an important complex, which can catalyzed many reactions, such asymmetric epoxidation of olefins, oxidation kinetics resolution of racemic secondary alcohols and asymmetric oxidation of sulfide. However, due to the difficulty in product separation and catalyst recycling for homogeneous Mn(Ⅲ) salen complex, heterogeneous Mn(Ⅲ) salen catalysts have attracted much attention in last decade.First, it was investigated that the effect of fine tuning pore size for the catalytic performance of unsymmetric Mn(Ⅲ) salen heterogeneous catalysts, and an unsymmetric Mn(Ⅲ) salen complex was immobilized on a series of MCM-41and MCM-48with fine-tuning pore size. These heterogeneous catalysts were characterized by FT-IR, DR UV-vis, N2sorption and XRD, and these characterizations indicated that the active specie was grafted successfully and the structures of supports were kept well. The dosage of3-Mercaptopropyltrimethoxysilane was studied in epoxidation of olefins, which could influence the catalytic performance of heterogeneous catalysts. The optimum organosilane was determined, and on the basis of this, the effect of fine-tuning pore size for catalytic performance of catalysts was investigated. It was found that the heterogeneous catalysts immobilized on mesoporous silica with larger pore size showed higher catalytic activity and enantioselectivity.Second, it was studied that the comparison of catalytic performace of homogeneous and heterogeneous Mn(Ⅲ) salen catalysts using different oxidants, and two kinds of unsymmetric Mn(Ⅲ) salen complexes were grafted on mesoporous silica MCM-41, MCM-48and SBA-15. Several characterizations were employed and they indicated that Mn(Ⅲ) salen complexes were immobilized on the support successfully and the mesoporous structures of supports were retained very well. Two oxidants, m-CPBA and NaCIO, were applied in asymmetric epoxidation of unfunctionalized olefins. As the heterogeneous catalysts Mn(Ⅲ) salen catalysts were chosen, the conversions obtained by two oxidants showed remarkable different (NaCIO obtained great higher conversions), while the conversions obtained by two oxidants presented no obvious difference using homogeneous catalysts. In other words, the heterogenization of Mn(Ⅲ) salen complexes gave different influence for two oxidation system of m-CPBA and NaCIO. In order to research this phenomenon, UV-vis spectra and reaction kinetics were employed to study two oxidation system mentioned above, and it was found that as a result of the mildness of NaClO, no obvious accumulation of intermediate [Mn(Ⅴ)=O salen]+was observed. This was the reason that NaClO gave higher conversions in heterogeneous condition. In addition, compared with the heterogeneous catalysts immobilized MCM-41, the heterogeneous catalysts immobilized on MCM-48and SBA-15showed better catalytic activity and enantioselectivity, and this was due to3D pore structure of MCM-48and larger pore size of SBA-15, which improved molecular diffusion.Third, in this paper, pyrolytic waste tire char (PWTC) was chosen as support, and without any organosilane and oxidation process, Mn(Ⅲ) salen complexes were immobilized on PWTC directly. The prepared heterogeneous Mn(Ⅲ) salen catalysts obtained good catalytic results in asymmetric epoxidation of olefins, and especially in the epoxidation of1-phenylcyclohexene, they obtained higher ee values than their homogeneous counterparts. Moreover, the heterogeneous catalyst could be recycled three times.Fourth, with the employment of1,2-ethanedithiol,1,3-propanedithiol and1,6-hexanedithiol, a series of cyclic sulfide Mn(Ⅲ) salen complexes were synthesized. Though the characterizations of NMR, MS and FT-IR, it was confirmed that the cyclic sulfide structure was synthesized successfully. In asymmetric epoxidation of olefins, the oxidation system of NaClO/PPNO was chosen, and dichloromethane was employed as solvent. These cyclic sulfide Mn(Ⅲ) salen complexes presented good catalytic performance in asymmetric epoxidation of unfunctionalized olefins. With the addition of hexane, these Mn(Ⅲ) salen complexes were separated easily and they could be recycled six times.