Enhanced Heterogeneous Asymmetric Catalysis Via The Cooperation Between The Immobilized Chiral Amine And Achiral Silanols Of Mesoporous Support

The acid-base cooperation in heterogeneous organocatalysis utilizing the surface silanols of solid supports as acid sites was expanded to the asymmetric catalysis in this work, employing mesoporous silica to provide acid sites while the immobilized chiral pyrrolidine derivatives to afford base sites.The results of XRD, nitrogen sorption analysis and SEM&TEM of the supports demonstrated well-ordered 2D hexagonal mesostructures with p6mm space group, and the pore diameter was 4-6 nm. Al NMR spectra showed that all the aluminum-containing samples were of framework aluminum species in tetrahedral coordination. The final Si/Al atomic ratios in samples were dertermined by XRF. The quantity, ratios and distribution of acid/base sites were analyzed by 13C CP/MAS NMR,29Si CP/MAS NMR and element analysis. Proper acid-base distribution model was then proposed.The catalytic ability of prolinol grafted SBA-SH was investigated using the aldol condensation of 4-nitrobenzadlehyde and acetone as model reaction. The catalysis displayed excellent acid-base cooperative effects in promoting catalytic activity while preserving the enantioselectivity. By deriving the structure of basic center as (2S,4R)-N-(pyridin-3-yl)-4-(4-vinylbenzyloxy)pyrrolidine -2-carboxamide (Pro-Py) and (2S)-N-(quinine-9-yl) pyrrolidine-2-carboxamide (Pro-QN), we finally achieved the achiral silanols catalyzed acid-base cooperative catalysis, promoting both the activity and enantioselectivity. By controlling the density and acid strength of silanol and acid-base sites ratios, we found that enough silanols, proper acid strength around the basic center and optimal acid-base sites ratios were pivotal to the acid-base cooperativity. Silanols also had positive effect to concentrate substrates on the surface by hydrogen bonding so that the reaction proceeded smoothly.Based on the catalysis results, surface silanol was proposed to acted as acid site in the heterogeneous acid-base cooperative catalysis via two different transition states, namely the direct electrophilic substrate activation by the surface silanol and the indirect silanol activation through transfer the silanol proton to the y site of chiral center. These two transition states resulted in enantiodifferentiation due to whether the chiral center contributed to the transition state or not.