Heterogeneous Asymmetric Knoevenagel-Phospha-Michael Concurrent Tandem Reaction Catalysed By Grafted Chiral Bases And Inherent Achiral Hydroxyls On Mesoporous Silica Surface

Concurrent tandem reactions, especially asymmetric concurrent tandem reactions, attract much attention in organic synthesis for their high-efficiency, high reaction rate and convenience in operation. How to control the influence between different catalytic sites is the most important problem in concurrent tandem reactions, which are composed of several single reactions. Here, we prepared several bi functional heterogeneous catalysts with inherent achiral hydroxyls of mesoporous silica (SBA-15materials) as acidic sites and immobilized chiral amines as basic sites. The bifunctional catalysts successfully catalyzed different concurrent tandem reactions and their catalytic efficiency was significantly improved by synergy between the basic sites and acidic sites. The results are as follows:1. The chiral basis center initially selected for immobilization in this work is (S)-(+)-Prollnol. To enhance the catalytic efficiency, the other heterogenous catalysts were designed. The immobilization of the chiral amines was achieved by the reactions between-OH/-NH2/-C=C-and-SH/-Br with (S)-(+)-Prollnol,(S)-(-)-1-Methyl-2-Pyrrolidinemethanol,(S)-(-)-2-Aminomethyl-1-ethylpyrrolidine, Quinine,9-amino (9-deoxy)-epihydroquinine as grafted amine moieties. The well-ordered channels were detected by XRD. Then we used TEM to observe the channels directly. The specific surface area and pore diameter were determined by N2adsorption-desorption. The density of acidic sites and the structure of the amines on the mesoporous silica surface were tested by solid NMR. The molar ratio of the acid and base sites was calculated from the result of29Si MAS NMR and elemental analysis. The molar ratio indicated that there are enough hydroxyl groups around basic sites.2. The heterogeneous catalysts were applied in tandem Knoevenagel-phospha-Michael reaction and their catalytic performance was observed. The result indicated that surface hydroxyl groups cooperating with the grafted amines promoted the Knoevenagel-phospha-Michael tandem reaction, and improved the enantioselectivity in a narrow scale. The catalytic activity of the heterogenous catalysts was improved with the strengthen of the basic centers, and the heterogeneous catalyst containing (S)-(-)-2-Aminomethyl-1-ethylpyrrolidine as basic sites showed best performance with yield up to>99%and ee value up to98%. Besides, quinolone group show a good catalytic activity in Knoevenagel reaction and it can promote the reaction by attracting substrate based on the principle of the dissolution in the similar material structure.3. We also applied the bifunctional catalysts in Aldol-Michael tandem reaction and Knoevenagel-Michael tandem reaction. An excellent asymmetric selectivity of>99%was achieved in Aldol-Michael tandem reaction.