Diphenyl ether herbicides

The 0- and -monochloro- and 2,4- and 3,4-dichlorobenzotrichlorides are iatermediates ia the manufacture of the corresponding chlotinated benzoic acids and benzoyl chlorides. Fluotination of the chlotinated benzotrichlorides produces the chlotinated benzotrifluorides, iatermediates ia the manufacture of dinitroaniline and diphenyl ether herbicides (76). 

Diphenyl ethers are both systemic and contact herbicides and are used for the selective control of annual broad-leaved weeds and grasses in a variety of crops (such as soybeans, maize, rice, wheat, barley, peanuts, cotton, onions and ornamental trees) under different application scenarios. This class of herbicides contains a diphenyl ether moiety as the core substructure. Acifluorfen, bifnox, chlomethoxyfen, chlorni-trofen, fluoroglycofen-ethyl and fomesafen, etc., are representative compounds of the diphenyl ether herbicides (Figure 1). 

The diphenyl ether herbicides are nonvolatile compounds, generally very lipophilic and insoluble in water. Solubility in water and octanol-water partition coefficients (logXow) of the various diphenyl ether herbicides range from 120mgL (acifluorfen) to 0.16 mg (oxyfluorfen) and from 2.9 (fomesafen) to 5.4 (acifluorfen), respectively. Diphenyl ether herbicides are stable in an acidic or alkaline condition, but some compounds are gradually degraded under the sunlight. ... 

Because of the limited root uptake and slow rate of systemic translocation, the diphenyl ether herbicide residues detected in the aerial plant portion are low. 

In Japan, bifenox is the only registered diphenyl ether herbicide. The tolerance and/or maximum residue limits (MRLs) are established at 0.1 mg kg for cereals such as rice grain, barley and wheat, and 0.05mgkg for potatoes (Ministry of Health, Labour and Welfare, Japan). Ibe California Department of Food and Agriculture (CDFA) established the minimum detectable quantity of diphenyl ether herbicides at 0.1 mgkg for bifenox, nitrofen and oxyfluorfen. ... 

Extraction and cleanup of diphenyl ether herbicide metabolites in plants... 

To determine the diphenyl ether herbicides in the samples, GC/ECD or GC/NPD is used in general. GC/ECD is preferred to GC/NPD owing to its higher sensitivity. An aliquot of GC-ready sample solution is injected into the gas chromatograph under the conditions specified below. In addition, multi- and confirmatory analysis of residues is carried out using gas chromatography/mass spectrometry (GC/MS) in the selected-ion monitoring (SIM) mode. 

Okumura et al reported State regulatory programs for pesticide residues in food crops analyzed by the CDFA. In the multiresidue analysis of several organochlorine pesticides including diphenyl ether herbicides, bifenox, nitrofen and oxyfluorfen, HPLC has also been used. 

Quantitative analysis is performed by the calibration technique. A new calibration curve with a standard solution of each diphenyl ether herbicides is constructed, plotting the peak area against the amount of standard solution injected. Each diphenyl ether herbicide in the sample is measured by using the peak area for each standard. Before each set of measurements, the GC and HPLC system is checked by injecting more than one standard solution containing ca 0.01-2 mg L of each compound. 

The residual amount (R, mgkg ) of diphenyl ether herbicides in the sample is calculated by the following equation ... 

Wi = amount of diphenyl ether herbicide for Vj read from the calibration curve (ng)... 

Diphenyl ether herbicides are generally extracted from 10 to 50 g of air-dried soil with an organic solvent such as acetone, methanol and benzene by sonication, mechanical shaking or Soxhlet extraction. If necessary, the extracts are then cleaned by column chromatography or SPE. The extract is evaporated completely to dryness and the residue is dissolved in an appropriate volume of the solvent for GC analysis. The reduced amine metabolites are extracted under alkaline conditions. 

Analytical method for the metabolites of diphenyl ether herbicides in soil... 

Under flooded soil conditions, the diphenyl ether herbicides are substantially transformed into the amino derivatives, which are incorporated tightly into the soil particles. An analytical method for these amino derivatives in soil has been developed. 

Environmental pollution caused by pesticides has become a serious problem. Especially during and/or after pesticide application to crops, the pesticides are released into sensitive environmental areas, and also into ground and surface water, and could be harmful or dangerous to humans and other species. Therefore, very low concentrations of diphenyl ether herbicides in environmental waters must be monitored. 

The concentration of chlornitrofen in river water released from flooded paddy fields 30-60days after application was detected in the range 0.039-1.25 agL . Eurther, it was reported that the DTso of diphenyl ether herbicides in groundwater, river water and seawater were 17-84, 14-140 and 10-88 days for chlornitrofen, and 18-131,4-206 and 6-23 days forbifenox, respectively. Diphenyl ether herbicides in... 

Water samples of 500-1000 mL are extracted and purified simultaneously through an SPE cartridge such as Carbograph-1, Cig and RP-18, usually followed either by HPLC with ultraviolet (UV) or photoconductivity detection or by GC/ECD. The acidic-type diphenyl ether herbicides are derivatized with diazomethane and various kinds of chloroformates and determined by GC and HPLC. 

In recent years, the extraction of diphenyl ether herbicides from water samples such as river water, groundwater and drinking water by SPE has increased in popularity. 

Ruzo, L.O., Lee, J.K., and Zabik, M.J. Solution phase photodecomposition of several substituted diphenyl ether herbicides, J. Agric. Food Chem., 28(6) 1289-1292, 1980. 

Trebst, A. (1979). Inhibition of photosynthetic electron flow by phenol and diphenyl ether herbicides in control and trypsin-treated chloroplasts. Z. Naturforsch. Sect. C Biosci., 34 986-991. 

Yamagishi, T., Miyazaki, T., Akiyama, K., Morita, M., 1981. Polychlorinated dibenzo-/ -dioxins and dibenzofurans in commercial diphenyl ether herbicides, and in freshwater fish collected from the application area. Chemosphere 10, 1137-1144. 

Diphenyl ether herbicides. The diphenyl ether class of herbicides belongs to a broader area of herbicides known as protoporphyrinogen oxidase (Protox) herbicides [20,21], Diphenyl ether herbicides were initially introduced over four decades ago in the 1960s with the discovery of nitrofen by Rohm and Haas (now Dow AgroSciences) [22] and bifenox by Mobil [23], These early diphenyl ether herbicides did not attain a significant role in the control of weeds in commercial crops until the 1970s, with the introduction of the trifluoromethyl compounds oxyfluorfen [24] and acifluorfen-sodium [25], Replacement of the chlorine with a trifluoromethyl group resulted in a dramatic increase in the herbicidal properties of these compounds, in terms of both potency and the spectrum of weeds controlled. 

We have also noticed renewed interest in the use and synthesis in agrochemicals containing the pentafluorosulfanyl group, first in the early 1990s, mostly from work done at Zeneca in the UK. In those early patents, the -CF3 group was replaced with - SF5 in known diphenyl ether herbicides and in insecticides such... 

M. Matringe, J.M. Camadro, R. Labbe, R. Scalla, Protoporphyrinogen oxidase as a molecular target for diphenyl ether herbicides, Biochem. J. 260 (1989) 231-235. 

Many herbicides are nitro-PCDEs such as nitrofen and DNP [28,29]. Different diphenyl ether herbicides are presented in Table 3. The use of nitrofen, 2,4-di-chloro-4 -nitro-diphenyl ether, was restricted to crops without detectable residues at harvest in the 1980s, since it has caused neonatal deaths in rats [30]. A aminoderivative of PCDEs, Aminofen, is an intermediate in the chemical industry and has been used in the manufacture of herbicides Illoxan and Diclofop-methyl [4]. Amino-PCDEs are also impurities of Eulan WA New that contains... 

The diphenyl ether herbicides are active only in the presence of light and canse chlorosis of leaf tissue. They inhibit the Hill reaction in photosynthesis and photophosphorylation. However, the primary mode of action probably involves the photosynthetic rednction to form radicals, which initiate destructive reactions in lipid membranes leading to cell leakage. 

Table 5.17 Commercial products containing diphenyl ether herbicides.

Diphenyl-ether herbicides such as aclonifen, bifenox and lactofen are relatively new herbicides, used for weed control in the growth of seeded legumes, such as soybeans. MS and MS-MS of five neutral diphenyl-ethers herbicides and three acid metabolites was reported [64]. Negative-ion ESI is preferred for the acidic compounds [47, 64] and either negative-ion APCI [64] or positive-ion ESI [47] is used for the neutral ones. Stmcture informative fragmentation was observed in negative-ion MS-MS [64]. 

A. Lagana, G. Fago, L. Fasciani, A. Marino, M. Mosso, Determination of diphenyl-ether herbicides and metabolites in natural waters using LC with DAD and MS-MS detection. Anal. Chim. Acta, 414 (2000) 79. 

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