Phenylureas methyl-diuron

Pesticides containing methyl or other alkyl substituents maybe linked to N or 0 (i.e., N- or O-alkyl substitution). An N- or O-dealkylation catalyzed by microorganisms frequently results in loss of the pesticide activity. Phenylurea (see Chap. 1) becomes less active when microorganisms AT-demethylate the molecules (e. g., the conversion of Diuron to the normethyl derivative, Fig. 7). The subsequent removal of the second AT-methyl group renders the molecule fully nontoxic [169]. On the other hand, the microbial O-demethylation of Chloroneb creates the non-toxic product 2,5-dichloro-4-methoxyphenol (Fig. 7). 

Due to the thermal lability of the urea group, phenylureas are not amenable to GC-MS. They are frequently analysed by LC-MS. The general stmcture is shown below. The phenyl ring is substituted with halogen(s), methoxy, methyl, trifluoromethyl, or 2-propyl substitution. The Rj side chains are methyl groups for most phenylureas, while the Rj side chain can be methyl like in diuron, methoxy like in linuron, butyl like in neburon, or a proton like in monomethylmetoxuron. 

The triazines, such as simazine (4.62), are the most used of all soil-applied herbicides small changes in the substituents give excellent control over selectivity and persistence. These substances [and the phenylureas, such as diuron (4.63), which are also soil-types] act by interfering with the Hill reaction in photosynthesis (see Section 4.6). Both of these parent substances, simazine and diuron, have undergone extensive modification for particular purposes, but each is still extensively used. A newcomer, chlorsulfuron (6.73), combines molecular features from both parents, but is notable for the greater dilution at which it acts, and also for the speed (all cell division is halted within 1 hour). This substance, l-(2-chlorophenylsulfonyl)-3-(4-methoxy-6-methyl-l, 3,5-tri-azin-2-yl)urea, is taken up by wheat, oats, and barley plants which rapidly inactivate it. It is harmless to Man (Campion and Tichon, 1981). 

Phenoxy acids (2,4-dichlorophenoxy acetic acid, MCPA, and 2,4,5-trichlorophenoxy acetic acid) and similar acid herbicides (benzaton, dicamba, dichlorprop, and mecoprop) sulfonurea herbicides (chlorsulforon, metsulforon methyl, tribenuron methyl, and thifensulfuron methyl) triazine herbicides (methoxy-s-triazines, chloro-s-triazines, alkylthio-s-triazines) phenylurea herbicides (fenuron, monolinuron, and diuron), metabolites of the fungicide thiophanate-methyl (carbenazim and 2-aminobenzimidazole)... 

---