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Organic-Inorganic Hybridizing And Grafting Of Sharpless Catalyst And Mn(salen) Catalysts, And Heterogeneously Asymmetric Epoxidation Reaction

Posted on:2003-02-22Degree:DoctorType:Dissertation
Country:ChinaCandidate:S XiangFull Text:PDF
GTID:1101360122975115Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
Enantioselective epoxidation of olefins is of great importance for the synthesis of a variety of enantiomerically enriched chiral intermediates in fine chemical production such as pharmaceutics, agrochemistry and flavors. There are two general systems for the enantioselective epoxidation of olefins: Sharpless catalytic system and Jacobsen system, effective for the epoxidation of allylic alcohols and nonfunctional alkenes, respectively. In this thesis, two catalytic systems were heterogenized on the surface of inorganic supports, and the activity and enantioselectivity of the heterogeneous catalysts were tested for the enantioselective epoxidation of olefins.For Sharpless catalytic system, the tartaric derivative was grafted on the surface of inorganic supports by organic-inorganic hybrid method. The chiral tartaric derivative was grafted onto inorganic supports in several steps through hydroxyl-protected tartaric acid dichloride and 3-aminopropyltriethoxysilane, then via selective cleavage of the isopropylidene acetal. After the protecting group was removed through selective hydrolysis, the desired organic-inorganic hybrid chiral materials were obtained. The heterogeneous catalytic system was simply synthesized by combination and complexation of titanium tetraisopropoxide with the hybrid chiral material. The as-synthesized catalysts were tested for the enantioselective epoxidation of allyl alcohol. The hybrid chiral catalysts show the enantioselectivity up to 80% ee, which is as good as that obtained from homogeneous Sharpless reaction. It indicates that the activity and enantioselectivity may result from the monomeric titanium species complexed with the hybrid chiral material. The significance of the heterogeneous system is that the product can be easily separatedfrom the catalyst by simple filtration after reaction. This is the first example of the application of the hybrid chiral materials to the heterogeneous asymmetric epoxidation of allyl alcohol. For the chiral hybrids, the environment surrounding the immobilized species is analogous to that of chiral complex.For Jacobsen catalytic system, the chiral Mn(salen) complexes were anchored on the surface of inorganic supports by a new strategy with the attention to synthesize the heterogeneous chiral catalyst for the asymmetric epoxidation of nonfunctional olefins. Inorganic supports were modified with phenol through the reaction between the silylating agent and the surface hydroxyl group. Then the Mn(salen) complexes were successfully anchored on the surface of inorganic supports through the axial complexation of the manganese by oxygen atoms of the phenoxyl group. The enantioselectivity of the anchored catalysts is obviously higher than that of the corresponding homogeneous catalysts because of the bulky axial group and the surface effect of the inorganic support.The stability of the anchored catalysts was investigated. It indicates that the anchored catalysts are quite stable and can be recycled under the epoxidation reaction conditions without detriment to the activity and enantioselectivity.The change of the anchored catalysts was investigated by UV-Vis and ESR spectroscopies before and after the catalytic reaction. Combined with the literature results, we suggest that the isolated mononuclear Mn species is the active site for the enantioselective epoxidation of olefins under the experimental conditions.
Keywords/Search Tags:Sharpless catalyst, Mn(salen), Organic-Inorganic hybrid, asymmetric epoxidation
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