The Metolachlor Herbicide: An Exercise in Today's Stereochemistry

Metolachlor is one of the most widely used agents registered for the protection of many cultivated plants against weeds. Because of axial and central chirality, this molecule forms four stereoisomers, the investigation of which by 1H NMR and chromatography is described. It is shown that the isomers do not interconvert at room temperature. Non-enantioselective and enantioselective preparations on industrial scales, developed by the Syngenta Company, are given, including the composition of the catalyst used for an enantioselective hydrogenation step. The bioactivities of the four separated species are discussed. The mixture of the two stereoisomers with stronger biological effects, commonly designated (S)-metolachlor, has largely replaced the 1:1:1:1 mixture in practical agriculture. The load to the environment diminishes by using a lower dosage of (S)-metolachlor instead of a higher dosage of the 1:1:1:1 mixture. The present article exemplifies several general stereochemical aspects that are placed together in the conclusion of the article.