Dynamic kinetic resolution of secondary alcohols combining enzyme-catalyzed iransesterification 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 1phenylethanol 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-molecularweight 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 2h. 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.