Carbon tetrachloride‐promoted stereo selective hydrolysis of methyl‐2‐chloropropionate by lipase

Abstract

Esters of S-2-chloropropionic acid are important intermediates for synthesis of the active isomers (generally R-isomers) of many commercially important herbicides. While these esters are commercially available, their high cost is preventing their use in the synthesis of isomeric herbicides. There are several advantages of isomeric herbicides over their racemic counterparts. Among them, raw material cost reduction for the same activity, formulation cost reduction and lower mammalian toxicity. Cambou and Klibanov' have resolved 2-chloropropionic acid isomers using an octyl ester and lipase enzyme from Candida cylindracea. Stereoselective ester synthesis and transesterification reactions involving 2-chloropropionic acid and lipase enzyme have also been reported.' These reports dealt primarily with preparative procedures. We undertook the current study with the hope. of developing a commercial process for a S-2-chloropropionic acid ester. For low production cost and wide applicability we were interested in a methyl ester. Also, to obtain reaction rates which are acceptable at economical levels of enzyme usage, we had to depend on the hydrolysis reaction for resolution rather than esterification or transesterification reactions. The present report deals with interesting observations made during the kinetic studies of lipase-catalyzed hydrolysis of methyl-2-chloropropionate '(MCP) when an organic solvent, carbon tetrachloride, was used as a diluent for the substrate. MATERIALS AND METHODSCandida cylindracea lipase (substantially free of protease and a-amylase activity) and porcine liver esterase were purchased from Sigma Chemical Co., reference ~-methyl-2-chloropropionate sample was obtained from Rhone-Poulenc. Racemic methyl-2-chloropropionate was purchased from Aldrich Chemical Co. Other chemicals were reagent grade. The kinetic experiments and enzyme activity measurements were done using automatic pH-stat equipment with recorder (Metrohm AG CH-9100). The re- action rates were determined by manually taking the slopes of titrant-versus-time curves. The total ester concentration in carbon tetrachloride was determined by end-point hydrolysis and titration using nonspecific porcine liver esterase as the catalyst. The isomeric purity was determined by optical rotations on a Perkin-Elmer model 241 polarimeter.The specific rotation, [a];' = -26.9' neat, [given in the Handbook of Chemistry and Physics (1983-1984)] was utilized to calculate the optical purity of undiluted MCP samples. The optical purity of MCP in CCI, solution was determined from a calibration curve constructed from L-MCP (Rhone-Poulenc) solutions prepared in CCl,. The slope of this line was 3.73" rotation/lM concentration L-MCP.Unless otherwise stated the kinetic experiments were performed using 1mM phosphate buffer containing 50mM NaCl. The pH-stat experiments were conducted at room temperature (21-23"). The practical scale resolution experiments were carried out in a cold room (4°C) using 20-or 40-mL portions of MCP. For these experiments, a calcium carbo...

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