Support Synergy and Interface Synergy in Promoting the Proline Immobilized in Mesoporous Silica Catalyzed Direct Asymmetric Aldol Reaction
|School||Beijing University of Chemical Technology|
|Course||Chemical Engineering and Technology|
|Keywords||proline support synergy interface synergy direct asymmetric Aldol reaction|
Proline and its derivatives are one class of the most important small organic-catalysts. How to achieve their highly efficient catalysis in direct asymmetric Aldol reactions becomes a hot spot in the field of asymmetric reaction. In recent years, heterogeneous proline catalyst has been followed with enormous interest in the catalytic to direct asymmetric Aldol reactions. However, the catalysis of heterogeneous proline in asymmetric Aldol reaction has generally low efficiency and/or enantioselectivity.We prepared a heterogeneous catalyst that proline immobilized on mesoporous silica with with periodically alternating hydrophobic and hydrophilic blocks. The catalysis of heterogeneous catalyst to the asymmetric Aldol reaction were studied, for comparison, the heterogeneous catalysts with hydrophobic surface and hydrophilic surface were prepared. The effect of the surface properties of heterogeneous catalysts to the catalytic properties was studied. The ordered structure of mesoporous materials were characterized by the measures of XRD, N2 adsorption-desorption, and so on. The immobilized of proline on mesoporous materials were characterized by the measures of elemental analysis and FT-IR, et al. The surface properties of the different catalysts were characterized by the measures of Si NMR and dynamic moisture sorption. The Aldol reaction of p-nitrobenzaldehyde and cyclohexanone without organic solvents was performed to research the catalytic ability of the heterogeneous catalysts. Our studies have shown that:1. The surface properties of the heterogeneous catalysts affect the catalytic activity. The pore walls of mesoporous heterogeneous catalysts promote the organic reactants near the active sites of catalysts, thereby effectively improving the conversion rate of the Aldol reaction. The heterogeneous catalysts with different surface have different adsorption capacity to reaction substrate. The more hydrophobic of the pore walls, the improvment of catalyst to the reaction rate is more evident. The periodically alternating hydrophobic and hydrophilic catalyst can effectively catalyze the direct asymmetric Aldol reaction in solvent-free condition which can not be catalyzed by homogeneous proline.2 When the reaction progressed in aqueous media, the pore walls of the catalysts enriched the reaction substrates into the channels, the reaction was much faster than the reaction progressed in solvent-free condition. The effective oil-water interface in the channels of catalysts can further improved the catalytic activity. Compared to the concave continuous oil-water interfaces in the channels of hydrophobic catalyst and the invaid oil-water interface in the channels of hydrophilic catalyst, the convex discontinuous oil-water interfaces in the channels of the periodically alternating hydrophobic and hydrophilic catalyst have more powerful improve to the reaction rate.3 Compared to the homogeneous catalysis of proline and its derivatives, the catalysis of heterogeneous catalysts had an configuration inversion of the major product for the asymmetric Aldol reaction due to the confinement effect. The active sites presence in different positions in the channels of three different heterogeneous catalysts, but the pore walls of three class of heterogeneous catalysts all hindered the aldehyde re attack on anti-enamine, leading to the favored production of inverse isomers. The catalyst with periodically alternating hydrophobic and hydrophilic surface has more powerful confinement effect than the catalysts with hydrophobic and hydrophilic surface, leading to higher enantioselectivity.4 The oil-water interface in the channels of catalysts enhance the enantiomer excess of the reaction. When there was an effective oil-water interface in the channels of catalysts, the si attack on syn-enamine is supposed to be further favored due to the hydrogen bonds of the free OH group of the interfacial water with the reactants and transition state, giving good yield and high enantioselectivity. The properties of the surfaces of catalysts affect the shape of the oil-water interface. In the channels of hydrophobic catalyst forming a concave continuous oil-water interface, and in the channels with with periodically alternating hydrophobic and hydrophilic surface forming a convex intermittedt oil-water interface. The continuous concave interface lowers the stability of the transition state for (2R, 1’S) isomer due to the interaction of phenyl ring with water, leading to the decrease of the enantioselectivity.