Synthesis, Characterization, And Application Of Metal-Organic Framework-Based Asymmetric Catalysts

Metal-organic frameworks (MOFs), as a new class of porous materials, have been rapidly developed in the last20years in terms of either their initial synthesis or their later application. Research on MOFs has become a hot topic, mainly due to MOFs possessing some excellent properties, such as large specific surface area, high porosity, adjustable pore size, and chemical modifiability. The synthesized MOFs have been not only applied to gas adsorption, storage, and separation but also to catalysis widiely, especially in the field of heterogeneous asymmetric organocatalysis. Based on the known structural characteristics and chemical modification of MOFs, Michael addition catalyzed by the chiral organocatalyst functionalized MOFs will be a promising and challenging research topic. The main results are summarized as follows:1. First, MIL-101with high thermo-stability, large specific surface area, and high porosity was successfully synthesized. Then the chiral (1R,2R)-1,2-cyclohexanediamine,(1R,2R)-CHDN in abbreviation, and (1R,2R)-1,2-diphenylethylenediamine,(1R,2R)-DPEN in abbreviation, were successfully embedded into MIL-101via a post-grafting method. The chiral diamine functionalized MIL-101catalysts (CDMILs in abbreviation) were used in the asymmetric Michael addition of hydroxycoumarin and benzylideneacetone. The results show that MIL-101incorporated with (1R,2R)-DPEN, CDMIL-4, possesses the best performance in the asymmetric Michael addition, in which a high yield of94%and a high enantioselectivity of83%ee can be achieved under the optimized reaction conditions. The reaction scope was investigated with respect to Michael adducts under the applied conditions, and the reusability of chiral heterogeneous catalyst CDMIL-4and the scalability of the reaction over CDMIL-4were conducted as well. Chiral heterogeneous catalyst CDMIL-4shows a better catalytic performance in the asymmetric Michael reactions, compared with that using the corresponding homogeneous catalyst (1R,2R)-DPEN in all cases. Although the leaching issue of heterogeneous catalyst CDMIL-4is observed, the current study confirms that MOFs offer a great potential in the application to heterogeneous asymmetric organocatalysis.2. NH2-MIL-53with high thermo-stability, large specific surface area, and high porosity was used as a support to prepare NCS-MIL-53via a post-synthesis modification with thiophosgene. Then, NCS-MIL-53was further modified with either (1R,2R)-CHDN or (1R,2R)-DPEN to synthesize the chiral organocatalysts, the so-called SN-MIL-53catalysts, bearing thiourea-primary amine functional groups. SN-MIL-53catalysts were used in the asymmetric Michael addition reactions. The results show that the catalysts are not active in the most screened reactions, except for the asymmetric Michael addition of nitrostyrene and5,5-dimethyl-cyclohexane-1,3-dione, in which a yield of80%and an enantioselectivity of25%ee are obtained. The Michael adducts were obtained with lower enantiomeric excesses, compared to those using the corresponding homogeneous Takemoto’s organocatalyst in all the cases.