Enzymatic hydrolytic resolution of (R,S)-α-chlorophenyl acetates in biphasic media

A thermally stable carboxylesterase (SNSM-87) from Klebsiella oxytoca is explored as an enantioselective biocatalyst for the hydrolytic resolution of (R,S)-α-chlorophenyl acetates in biphasic media, where the ethyl ester possessing the highest enantioselectivity (E^* = 95) is selected as the best substrate and rationalized from the linear free energy relationships in terms of the logarithms of kinetic constants and enantiomeric ratio varied with the inductive parameter of leaving alcohol. An expanded Michaelis-Menten mechanism for the rate-limiting acylation step is adopted for the kinetic analysis. Two-stage Brønsted slopes of 58.0 and 2.28 for the fast-reacting (R)-esters, as well as only one-stage slope of 4.15 for the slow-reacting (S)-esters, were found and elucidated from the change of rate-limiting step. The replacement of α-chloro substituent to α-methyl or α-hydroxy moiety indicates that the α-substituent has profound effects on varying the enzyme activity, enantioselectivity, and optical-preference for SNSM-87 and lipases of pCPL, Lipase MY, NOVO 435 and Lipase PS.