Enantioselective catalysis for agrochemicals. The case histories of (S)-metolachlor, (R)-metalaxyl and clozylacon

The application of enantioselective catalytic methods for the technical preparation of chiral agrochemicals is illustrated for three active ingredients of the acylanilide type. The key step-for the technical synthesis of the herbicide (S)-metolachlor (trade name DUAL MAGNUM(R)) is the enantioselective hydrogenation of an imine intermediate using a novel iridium ferrocenyl-diphosphine catalyst that exhibited an unprecedented high activity (average turnover frequency up to 350,000 h(-1)) with an enantioselectivity of 80%. (R)-metalaryl and clozylacon were synthesized with > 95% and 99% enantiomeric purity, respectively. The enantioselective hydrogenation of the corresponding enamides with Ph or Ru/binap catalysts was the key step in both cases. Some conclusions regarding the development of the catalyst system are made and important prerequisites for the use of enantioselective catalysts for the production of agrochemicals are discussed.