Synthesis Of New Immobilized Chiral Catalysts And Studies On Their Catalysis For Aldol Reactions

With the advances made in immobilized chiral catalysts in recent years, a number of high-performance and recyclable catalysts have been developed for asymmetric catalysis in organic solvent as well as aqueous solution, such as polymer-supported chiral organocatalysts and silica-supported chiral organocatalysts.Well-defined polymers which immobilized chiral organocatalysts assembled or aggregated in solvents/solutions to create a favorable, isolated and stable nanostructure around the catalytic sites, however, it is very difficult to prepare due to multistep synthesis. As for silica-supported chiral organocatalysts, have the advantages of easy recycling, but most of them are single structure with supported small molecules, which result in low catalyst loadings. To design and synthesis novel immobilized chiral organocatalysts, two silica-supported chiral organocatalysts are presented in this dissertation. First, the derivative of proline hydrochloride salt is polymerized onto surface of silica nanoparticles by emulsion polymerization and we investigate the catalytic capability of the catalyst for aldol reaction in the organic phase. In addition, hairy silica nanoparticles grafted poly(O-acryloyl-trans-4-hydroxy-L-proline-co-NIPAM) chains are obtained from surface of silica nanoparticles by surface-initiated reversible addition fragmentation active radical polymerization. And hairy silica nanoparticles have been used in direct asymmetric Aldol reaction to investigate the catalytic activity and recyclability of catalyst in organic solvent and aqueous solution. The details are summarized below:(1) Based on emulsion polymerization, the derivative of proline hydrochloride salt is polymerized onto surface of silica nanoparticles to obtain Organic-inorganic hybrid nanoparticles. The results show that the loadings of catalytic unit on the surface of silica nanoparticles are about 2.38 mmol / g. The hybrid nanoparticles give good catalytic activity, diastereoselectivity and enantioselectivity(50% yield, anti/syn 72/28 and e.e. 89%) in optimal reaction conditions(DMF / H2O(95/ 5), 10 mol % catalyst loading, r.t.) for asymmetric Aldol reaction. In addition, recovery and recycling of this hybrid nanoparticles in organic solvent is also investigated. The results suggest that the catalyst activity is being diminished after three cycles.(2) Based on “grafted from” method, RAFT chain transfer agents are attached on the surface of silica nanoparticles and hairy silica nanoparticles grafted poly(O-acryloyl-trans-4-hydroxy-L-proline-co- NIPAM) chains were obtained from surface of silica nanoparticles by surface-initiated reversible addition fragmentation active radical polymerization. The results show that the loadings of catalytic unit on the surface of hairy silica nanoparticles are about 0.151 mmol / g. The experiment results show that hairy silica nanoparticles give good catalytic activity(97 % yield), bad diastereoselectivity and enantioselectivity(anti/syn 43 /57 and e.e. 36%) in optimal reaction conditions(DMF / H2O(95:5), 10 mol % catalyst loading, r.t.). In addition, recovery and recycling of the hairy silica nanoparticles in organic solvent is also investigated. The results suggest that the catalyst activity is being diminished after three cycles. On the other hand, the hairy silica nanoparticles(10 mol % catalyst loading) promoted aldol reaction between cyclohexanone and p-nitrobenzaldehyde in aqueous solution. However, the aldol reaction failed to achieve enantioselectivity in this relatively hydrophobic field.