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

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 )‐phenylethylester can be achieved with high yield and selectivity. The reaction was best conducted in toluene as solvent at 60 °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.