Immobilization Of Poly(L-leucine)s And Chiral Iminium Salts On Inorganic Materials And Application In Asymmetric Epoxidation Of α,β-unsaturated Ketones

This dissertation includes three parts.1. Immobilization of poly(L-leucine) on mesoporous molecular sieve MCM-41 and asymmetric epoxidation of α,β-unsaturated ketones with the immobilized poly(L-leucine) catalysts.Poly(L-leucine)s were immobilized onto mesoporous molecular sieve MCM-41 through physical absorption or covalently bonding. (1) Poly(L-leucine)s with various chain lengths were prepared by the n-BuNH₂ initiated polymerization of N-carboxyanhydrides of L-leuine (LeuNCA) in THF and immobilized on the MCM-41 by absorption. (2) Amino-functionalized mesoporous molecular sieve (MCM-AP) was prepared by reaction of (3-aminopropropyl)triethoxysilane (APTES) with MCM-41. Utilizing the MCM-AP as initiator, LeuNCA was polymerized in THF at room temperature for 72 h under a nitrogen atmosphere to provide the MCM-41-immobilized poly(L-leucine) catalyst, MCM-AP-PLL, with different degrees of polymerization (n). (3) Utilizing APTES as initiator, LeuNCA was polymerized in THF and subsequently covalently grafted on MCM-41 to obtain another catalyst AP-PLL-MCM. These catalysts were characterized by FT-IR, elemental analysis, 13C CP/MAS NMR, TG-SDTA and XRD.The Julia'-Colonna asymmetric epoxidation of benzalacetophenone was used as the model reaction to evaluate the performance of the immobilized poly(L-leucine). Under optimal condition, these immobilized catalysts, especially the covalently grafted ones, showed excellent catalytic activities and enantioselectivities. At average chain length n = 30, the MCM-AP-PLL catalyst exhibited the highest activity and enantioselectivity (up to 96.5%ee with 89.7% yield). The AP-PLL-MCM catalyst with n = 40 gave desired epoxide with 92.1%ee and 90% yield. MCM-adsorbed catalyst with n = 45 displayed 88.3%ee and 93.2% yield in the epoxidation under otherwiseidentical conditions. The effect of chain length is significant on catalytic activity and enantioselectivity, showing that it is necessary to use poly(L-leucine) with n ≥ 30 to achieve good results. The MCM41-grafted Poly(L-leucine) catalysts could be very easily separated from the reaction mixture by filtration, which made recycling of catalyst very simple. The recovered MCM-AP-PLL catalyst retained catalytic properties at least for six runs (92%ee).2. Silica-grafted Poly(L-leucine) catalysts and their application in the asymmetric epoxidation of α,β-unsaturated ketones.Base on previous works of our group, we improved the immobilization method solving the problems of repeatability in preparing the immobilized catalysts and decrease of enantioselectivity in catalyst recycling. The silica-grafted AP-PLL-Silica gel was prepared similar to the MCM-41 analog. Using APTES as initiator, LeuNCA was polymerized in THF and then bound to silica gel via the covalent Si-O bond, which was characterized by FT-IR, elemental analysis, and ¹³C CP/MAS.With poly(L-leucine) n = 30, this grafted catalyst showed excellent enantioselectivity (97.0%ee with 92.4% yield) in the model reaction of epoxidation of benzalacetophenone, significantly higher than the 87.3%ee of unsupported polyamino acid, 79.2%ee of silica-adsorbed catalysts and 92%ee of APTES, respectively. The effects of chain length of poly(L-leucine) on catalytic activity and enantioselectivity were also investigated. The average chain length affected both catalytic activity and enantioselectivity significantly. When chain length greater than 30 (n ≥ 30) is necessary to achieve acceptable enantioselectivity.The recycling and reuse of AP-PLL-Silica gel catalysts were further investigated. The catalyst was easy to recover and had a very low loss rate of about 2% per recycle. It is noteworthy that recycling did not decrease the catalytic performance of immobilized catalyst. After six times of recycling the enantioselectivity of the epoxidation of benzalacetophenone is still up to 95%ee.3. Exploration of synthesis and immobilization chiral iminium salts catalyst.(1) A racemic binaphthyl-based chiral iminium with grafting groups was designed and synthesized through seven steps.(3) Two kinds of...