title: Dynamic kinetic resolution of secondary alcohols combining enzyme-catalyzed transesterification and zeolite-catalyzed racemization.
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.
authors.affiliation: Technische Universität München, Department of Chemistry, Lichtenbergstrasse 4, 85748 Garching, Germany.
authors.forename: Yongzhong
authors.initials: Y
authors.lastname: Zhu
authors.forename: Kam-Loon
authors.initials: KL
authors.lastname: Fow
authors.forename: Gaik-Khuan
authors.initials: GK
authors.lastname: Chuah
authors.forename: Stephan
authors.initials: S
authors.lastname: Jaenicke
mesh_terms: D000438:Alcohols / Q000737:chemistry; D001592:Benzyl Alcohols / Q000737:chemistry*; D002087:Butyrates / Q000737:chemistry; D002384:Catalysis; D004800:Enzymes, Immobilized / Q000737:chemistry*; D004951:Esterification; D005656:Fungal Proteins; D007700:Kinetics; D008049:Lipase / Q000737:chemistry*; D015394:Molecular Structure; D012997:Solvents; D013237:Stereoisomerism; D013696:Temperature; D014753:Vinyl Compounds / Q000138:chemical synthesis / Q000737:chemistry; D017641:Zeolites / Q000737:chemistry*
chemical_list: D000438:Alcohols; D001592:Benzyl Alcohols; D002087:Butyrates; D004800:Enzymes, Immobilized; D005656:Fungal Proteins; D012997:Solvents; D014753:Vinyl Compounds; C095498:vinyl butyrate; D017641:Zeolites; C002017:methylphenyl carbinol; D008049:Lipase; C431349:lipase B, Candida antarctica; C011566:vinyl acetate