Metolachlor posses two chiral elements, an asymmetric carbon stom and a hindered rotation chiral axis, it can exist in four stereoisomeric forms (αS,1’S- /αR,1’R- and αS,1’R-/αR,1’S-metolachlor (αSS-, αRR-, αSR- and αRS-metolachlor for short). Metolachlor was introduced to the market in 1976 containing a mixture of all four metolachlor stereoisomers and in 1982, Moser et al. found that S-metolachlor (with respect to the C chiral center, including αSS- and αRS-metolachlor) showed the highest herbicidal activity. Nowadays, metolacholr and S-metolachlor are still widely used as the main amide herbicides in many countries and regions. Fist of all, we established a semi-preparative separation method of metolachlor by HPLC-PAD, Rac-metolachlor was first separated on a Ultimate Amy-S column to get baseline separated of S-metolachlor and other two stereoisomers and then the S-metolachlor was separated on a Chiralcel OD-H column. The four separated stereoisomers had purity of ≥99% and their absolute configuration were characterized by electronic circular dichroism. The enantioseparation of metolachlor by UPC2-MS/MS with a chiral AMY1 column was also established for fast and high sensitivity detection. The enantioselective bioactivity toward weed (Echinochloa crusgalli) was assessed by measuring the growth rates after the treatments with metolachlor stereoisomers, S-metolachlor and racmate. The order of the bioactivity to weed was found to be: S&gt;αSS&gt;αRS&gt;rac&gt;αSR&gt;αRR. Investigations on the stereoselective degradation of metolachlor in rice field soil showed that highest-bioactive αSS stereoisomer display slower degradation rate than its enantiomers, the EF trended to increase little during 35 days of soil culture. Moreover, metolachlor was configurationally stable in the test soil matrices. This is the first time to study the differences of four stereoisomers, on the base of biological activity and environmrntal behavior, it is likely that the use of S-metolachlor instead of the racmate help to increase the bioactivity and will not increase the environmental pollution.

Chiral amide herbicide metolachlor: Enantioseparation, stereoselective bioactivity and environmental behavior

256th National Meeting (2018)

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