| The one-pot synthesis of optically active complex molecules is of greatinterest in fine chemical industry. The bifunctional catalysts with coexistingacidic and basic functions are of great interest owing to their potential novelfunctions and a wide range of applications to various catalyses. To ourknowledge, however, no acid-base bifunctional heterogeneous asymmetricone-pot catalysis has been reported, and the asymmetric one-pot catalysisusing the surface achiral sites as one catalytic centre of the bifunctionalcatalysts is especially challenging. Here, we report the one-pot asymmetriccatalysis of the bifunctional heterogeneous catalysts with inherent achiralhydroxyls of mesoporous silica (SBA-15materials) as acidic sites andimmobilized chiral amines as basic sites. Our studies have shown that:1. The chiral amine initially selected for immobilization in this work is(S)-diphenylprolinol silyl ether, an efficient homogeneous catalyst for Michael reactions. To further enhance the catalytic efficiency and get a simplerstructure, the other heterogeneous catalysts were designed. The results ofPXRD, nitrogen adsorption/desorption isotherms of the supports imagesrevealed that the support SBA-15-SH had the well-ordered hexagonallysymmetric channel structure. The transmission electron microscopy (TEM)demonstrated that the the well-ordered hexagonally symmetric channelstructure of heterogeneous catalysts had not been destroyed. To achieve aquantitative determination of the silanol groups on the surface, the29Si MASNMR spectra were recorded. The molar ratio of the acid and base sites wasobtained by comparing the silanol density and the chiral base loadings whichdetermined by elemental analysis, It can be seen from the data that the molarratio of acid to base sites on heterogeneous catalysts are all around20:1.2. The catalysts have been found to afford enhanced activity andexcellent enantiomeric excess in the asymmetric Henry-Michael one-potreaction with the synergetic catalysis of surface-inherent acids andimmobilized bases in the Henry reaction and the acid-promoted asymmetriccatalysis of immobilized chiral bases in the Michael reaction. Besides,comparing the catalysts between before and after shielding surface hydroxylgroups, the synergy between hydroxyl groups and chiral base has been proved.Lowering the hinderance between the acidic and basic sites would enhance thecatalytic activity of the Henry reaction. The synergetic effect among oneacidic site and two basic sites could promote the catalytic activity of the asymmetric one-pot reaction. The heterogeneous catalyst which contained(1-methyl-1H-imidazol)pyrrolidine-2-carbothioate as basic sites revealed thebest conversion and e.e., both up to99%.3. The heterogeneous catalysts were also used to catalyze the asymmetricMichael-Aldol Tandem reaction and the Knoevenagel-Michael Tandemreaction, respectively. The enantiomeric excess of the asymmetricMichael-Aldol Tandem reaction could be up to68%, and the conversion ofKnoevenagel-Michael Tandem reaction is up to85%. The results alsoinvestigated that the hydroxyl groups could promote the reaction as acid sites. |
