【摘要】：In serving the humanity, the role of life and material sciences is undeniable. An arousing demand of enantiomerically pure compounds in the field of pharmaceutical, agrochemicals, cosmetics, fine chemicals and material sciences has made the asymmetric synthesis as an indispensible for new era. Splendid amount of research work from all over the globe is already in place to synthesize special kinds of chiral compounds by using asymmetric catalysts. At the same time, heterogenization of such asymmetric catalysts is of crucial importance to make the processes more facile, economic and environmentally benign. Owing to high surface area, tunable pore sizes and ease of functionalization made the mesoporous silica as a prime candidate for plenty of application including the field of asymmetric heterogeneous catalysis.
The present work is aimed at the development of single unit heterogeneous chiral catalyst system based on mesoporous silica for important asymmetric transformations. As cinchona alkaloids has been classified as a privileged class of chiral catalyst and known to induce chirality to plenty of achiral compounds including a-keto esters. Enantioselective hydrogenation of a-keto esters (Orito's reaction) using Pt/cinchona chiral catalyst system is regarded as the most studied reaction system. Initially (chapter 3), the research work was focused on the grafting of cinchona alkaloid over different kinds of mesoporous silica having different morphologies and nature i.e., SBA-15, A1-SBA-15, MCM-41 and MCF. Subsequently Pt deposition over thus cinchona functionalized silica created a chiral catalyst system for the enantioselective hydrogenation of ethyl pyruvate. The catalyst was found to provide enantioselective product with an enantiomeric excess (e.e.) ranging from≈35 to 50% depending upon the nature and the morphology of the silica support. The second part of this study (chapter 4) was concerned to the one pot synthesis of cinchona functionalized mesoporous silica. In that method, the main silica precursor is co-condensed with a cinchonidine molecule linked organosilane which is renovated by triethoxy silane moiety at its C11 position to yield cinchona functionalized silica. The subsequent deposition of Pt nanoparticles over functionalized silica provides a catalytic system for the enantioselective hydrogenation of a-activated ketone (Orito's reaction). Thus-developed catalyst system is found to be comparable in enantioselectivities with an enantiomeric excess (e.e) of 35.6% when compared to its rival synthesis route (grafting method) which resulted in e.e of 39.1%, with better cinchonidine incorporation efficiency into silica framework. While in the third part of the project (chapter 5), Cinchonidine was tethered directly without prior modification over carboxylate functionalized SBA-15 by the reaction of vinyl group in cinchonidine with -COOH group in functionalized SBA-15 through ester linkage. Then Pt nanoparticles were deposited over cinchonidine tethered SBA-15. The highest enantiomeric excess was achieved as 70.8% and the catalyst recyclability was authenticated even after 3rd reuse without significant loss in enantiomeric excess. Finally in chapter 6, trans-4-hydroxy-L-proline (another chiral organocatalyst) was grafted over acylchloride functionalized SBA-15 (SBA-R-COCl) through O-acylation reaction to yield proline grafted silica catalyst for asymmetric aldol reactions.