Dynamic Kinetic Resolution of Secondary Alcohols Combining Enzyme-Catalyzed Transesterification and Zeolite-Catalyzed Racemization

摘要

Hydrophobic zeolite beta containing low concentrations of Zr or Al was found to be a good catalyst for the racemization of 1-phenylethanol.The formation of styrene as a side product could be minimized by reducing the metal concentration in the zeolite beta.Combined with an immobilized lipase from Candida antarctica,the dynamic kinetic resolution of 1-phenylethanol to the(R)-phenylethyl-ester can be achieved with high yield and selectivity.The reaction was best conducted in toluene as solvent at 60 deg C,with higher temperatures leading to a loss in the enantioselectivity of the formed ester.By using high-molecular-weight acyl-transfer reagents,such as vinyl butyrate or vinyl octanoate,a high enantiomeric excess of the product esters of 92 and 98%,respectively,could be achieved.This is attributed to a steric effect:the bulky ester is less able to enter the pore space of the zeolite catalyst where the active sites for racemization are localized.Close to 100% conversion of the alcohol was achieved within 2 h.If the more common acyl donor,isopropenyl acetate,was used,the enantiomeric excess(ee)of the formed ester was only 67 %,and the reaction was considerably slower.