Syntheses of Agrochemicals with Phosgene and Derivatives

The carbamate and urea derivatives used as pesticides are commonly prepared with phosgene. The substituted ureas are mainly used as herbicides, while carbamates are used as insecticides and acaricides. It is estimated that 93% of all carbamates and 92% of all urea-based herbicides on the U.S. market are commercially synthesized with phosgene [206]. Of the substituted ureas, Monuron and Diuron are the most important pesticides in volume terms, and of the carbamates. Car-baryl (Sevin) is the single most important chemical. Eptam (S-ethyl-N,N-dipropyl 

These products are made in a two-stage process by reacting phosgene with an alcohol, thiol, or amine to give a chloroformate, thiocUoroformate, carbamoyl chloride, or isocyanate, which is then combined with an amine to give carbamates (X = O), thiocarbamates (X = S), or ureas (X = NH). 

Monoisocyanates prepared by aminolysis of phosgene are predominantly used as intermediates in the manufacture of agricultural products such as herbicides and insecticides [207, 208]. The commercially used aliphatic isocyanates include methyl, propyl, isopropyl, butyl, isobutyl, octadecyl, and cydohexyl isocyanate. The aromatic isocyanates used as building blocks for agricultural chemicals include phenyl isocyanate and halogen-substituted phenyl isocyanates. 

The accident involving methyl isocyanate (MIC) in Bhopal, India (1984) has 

Selected important carbamate, thiocarhamate, and urea herbicides and their chemical structures are presented below. 

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Syntheses of Agrochemicals with Phosgene and Derivatives 565 Tab. 5.3. Pesticides suggestive of alkyl or aryl isoqranate precursors. 

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