Reaction characteristics of dynamic kinetic resolution over various racemization catalysts

Abstract

Among the techniques that can give the optically pure compounds, DKR have recently emerged due to the efficiency of the substrates usage. In DKR, the homogeneous transition metal complexes have been widely used as a racemization catalyst, but they have some problems of high cost and catalyst recovery. Therefore, we performed the reaction studies over three types of heterogeneous catalysts. First, we performed DKR over H-beta zeolite as a racemization catalyst. This reaction was run in two liquid phases: zeolite in an aqueous phase and enzyme in an organic phase. The reaction characteristics of DKR of DMBA were investigated by changing the reaction variables such as the amount of the zeolite and the enzyme, the reaction temperature, and the organic solvent. The relative amount of zeolite and enzyme had great influence on the product distribution. While the generation of 4-methylstyrene was increased at higher temperatures, the product yield decreased with the enzyme deactivation. The reaction conversion and yield also depended on the polarity of the solvents. The reaction efficiency decreased when the highly polar solvent was used. When this system was applied to other secondary alcohols, the reaction conversion, yield and selectivity were dependent on the nature of the functional groups in each substrate. Secondly, immobilized homogeneous Ru complexes were used as the racemization catalyst in DKR of 1-phenylethanol. And the reaction characteristics were also investigated by similar ways. Immobilized catalysts showed rates similar to those of homogeneous catalysts in the conventional reaction conditions and the immobilized Shvo’s catalyst showed no leaching during reaction. The yield to the ester of immobilized catalysts was found to be dependent on the solvent and alkali additive. When DKR of 1-phenylethanol was performed in n-octane, the reaction proceeded well without an alkali additive over Ru-p-cymene, which is a new observation. The main factors influencing the DKR over immobilized Ru complexes were the solvent, the support, the alkali additive and the catalyst type. Finally, heteropoly acids (HPAs), which are one of the useful solid acids, were investigated on its applicability as a racemization catalyst in DKR of 1-phenylethanol. Phosphotungstic acid supported on alumina (PTA/alumina) prepared by using ethanol with heat-treatment at 100°C, showed the best performance in DKR of 1-phenylethanol. Even though the rate of the reaction used PTA/alumina was slower than the immobilized Shvo’s catalyst reaction, the PTA/alumina showed similar selectivity and yield with H-beta zeolite and the immobilized Shvo’s catalyst. Moreover, the reaction efficiency was also influenced by other reaction variables such as solvent, temperature, and kind of acyl donor. All the catalysts and enzymes used in this research were reused in order to demonstrate their durability. They showed stable activity during reuse test.