Dynamic kinetic resolution of secondary alcohols combining enzyme-catalyzed transesterification and zeolite-catalyzed racemization |
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Authors: | Zhu Yongzhong Fow Kam-Loon Chuah Gaik-Khuan Jaenicke Stephan |
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Institution: | 1. Technische Universit?t München, Department of Chemistry, Lichtenbergstrasse 4, 85748 Garching, Germany;2. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Kent Ridge, Singapore 119543, Singapore, Fax: (+65)?6779‐1691 |
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Abstract: | 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 degrees 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. |
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Keywords: | heterogeneous catalysis kinetic resolution racemization zeolites zirconium |
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