Triazine herbicides metribuzin

Investigations on the nature of a colored impurity that occurs during the manufacture of the triazine herbicide metribuzin led to an unusual synthesis of 2,5-dihydrazino-l,3,4-thiadiazole (156). Addition of HjOj to the dihydrazide (155) in aqueous hydrochloric acid, and storage overnight followed by a complex workup, gave (156) in a crude yield of 28% <90JHC1053>. 

Scholz, K. (1982). Open Ring Structure of the Triazine Herbicide Metribuzin [SENCOR] in the Paper Published by Prestel et al. Miles Ag. Div. Report Number 86226.2... 

The asymmetrical-triazine herbicide metribuzin was found by Arvik et al. (1973) to prevent growth of five common soil algae in liquid culture when used at a concentration of 0.5 ppm (Chlorella vulgaris and Chlamydomonas) and one ppm (Chlorococcum, Anabaena and Schizo-thrix calcicola),... 

On the other hand, the broad weed control spectrum of chloramben, which was introduced in 1959, brought with it widespread acceptance. As an over-the-row band for control of both broadleaf and grass weeds, cost was minimized. With good crop tolerance, chloramben dominated the early soybean herbicide market. During 1972, 85% of the product was applied as the granular formulation (Table 4.2). However, as application practices changed for the soybean crop, banded applications became an inconvenience. Herbicides such as metribuzin (a triazine herbicide), linuron, and bentazon, which could be applied as an affordable broadcast treatment, soon became products of choice, and the marketing of chloramben was eventually discontinued in 1990. 

The triazine herbicides can be divided into four different structural classes chlorotriazines, methylthiotriazines, methoxytriazines, and atypical or asymmetrical triazines. The chlorotriazine group includes atrazine, simazine, pro-pazine, terbuthylazine, and cyanazine. The methylthiotriazine group includes ametryn, prometryn, and terbutryn. The methoxytriazine group will include prometon and secbumeton. Hexazinone and metribuzin were chosen to represent the atypical triazine group. The plant metabolism of the most researched member of each triazine group will be discussed in detail to cover all major biological and chemical transformations reported in the literature. 

Weed resistance to the triazine herbicides was first identified in the late 1960s, with a biotype of common groundsel that was resistant to simazine (Ryan, 1970). Since then, resistance to triazine herbicides has been reported in many weed species (Holt and LeBaron, 1990 LeBaron and McFarland, 1990 Gronwald, 1994). Most cases of triazine resistance have been reported in the US, Canada, and Europe, where triazine herbicides have been used extensively in corn monocultures (LeBaron and McFarland, 1990 Stephenson et al., 1990 LeBaron, 1991). Most of the. v-triazinc-resistant weed species have been selected against atrazine and usually show a high level of cross-resistance to other. v-triazine herbicides. In most cases, these weeds also show a low level of resistance to as-triazinones (e.g., metribuzin). Triazine-resistant weeds are often less vigorous than nonresistant weeds, which facilitates their management. 

Tierney and Talbert (1995) found that the best treatments for control of both ALS-resistant and susceptible common cocklebur were triazine herbicides (i.e., atrazine, cyanazine, and metribuzin). Tonks and Westra (1997) reported that mixtures including ALS inhibitors gave no better control of ALS-resistant kochia from Colorado and Kansas than the nonsulfonylurea herbicides alone. 

The pKa of an organic base is the pH in an aqueous system, where half of the compound is present in the neutral form and half in the protonated form. Substitutions in the 2 position greatly influence the basicity of the -triazines (Weber, 1967). The chloro-v-triazincs are very weak bases with pKa values between 1.6 and 1.9. The mcthoxy-v-triazines and mcthylthio-v-triazincs have pKa values between 4.0 and 4.8, and the hydroxy-v-triazincs have pKa values greater than 5.0. In soil solutions (pH 4.5 to 8.0), the chioro-. -triazines are overwhelmingly present in their neutral forms. The methoxy-5-triazines, mcthylthio-v-triazines, and hydroxy-y-triazmes, however, are present as neutral species in neutral and alkaline soil solutions, but may be present as both neutral and protonated species in acidic soil solutions. This chapter also includes data on metribuzin, which is an asymmetrical triazine herbicide having somewhat different soil properties. 

Smith, A.E. and A. Walker (1989). Prediction of the persistence of the triazine herbicides atrazine, cyanazine, and metribuzin in Regina heavy clay. Can. J. Soil Sci., 69 587-595. 

Several processes may play a role in the environmental dissipation of -triazine herbicides. Dissipation processes can include microbial or chemical degradation in soil metabolism or conjugation in plants photodegradation in air, water, and on soil and plant surfaces and volatilization and transport mechanisms. This chapter will address photolytic degradation and abiotic hydrolysis of the currently used triazine herbicides, the triazinone herbicides (metribuzin and metamitron), and the triazinedione herbicide hexazinone. 

Under the pH and temperature conditions of natural waters, the various 2-chloro-, 2-methylthio-, and 2-methoxy-.v-triazine herbicides are generally considered to be stable in solution between pH 5 and pH 9, stable in neutral, weakly acidic and weakly alkaline media, and stable to hydrolysis at 20°C in neutral, weakly acidic, and weakly alkaline media (Pesticide Manual, 1997). Metribuzin, metamitron, and hexazinone are also considered to be stable under these conditions (Pesticide Manual, 1997). Such hydrolytic stability of the triazine herbicides is supported by hydrolysis studies (Rhodes, 1980 Widmer et al., 1993 Noblet et al, 1996 Hequet et al, 1997) that indicate either very slow rates of hydrolysis or no measurable hydrolysis (Table 23.3) under these conditions. 

Although hydrolytically stable under environmental conditions, the hydrolysis of the triazine herbicides (including metribuzin) in aqueous solution is temperature and pH dependent (Pesticide Manual, 1997). Hydrolysis of atrazine to hydroxyatrazine increased with increasing temperature (Plust et al, 1981 Chan et al, 1992 Hequet et al, 1997) and decreased as pH values near 1 or 13 were changed toward values of 7 or neutrality (Armstrong et al, 1967 Gamble... 

Worldwide there have been a total of 20 commercialized triazine herbicides. Of the 20 triazines, 7 are currently registered for land use within the United States ametryn, atrazine, metribuzin, prometryn, simazine, terbutryn, and prometon. For purposes of this discussion, only dietary estimates for the 5 most widely used domestic triazines are presented since the USEPA revoked cyanazine tolerances in 2004, prometon is not used for food crops and terbutryn has very limited use. Additionally, propazine was used under USEPA Section 18 registrations in the 1990s, and in 2007 was registered for weed control in sorghum (USEPA, 2007). 

Triazine herbicides are particularly well suited for conservation tillage because they provide foliar and residual control of a broad spectrum of weeds. Atrazine, simazine, and metribuzin are used in com, atrazine and propazine in sorghum, metribuzin in soybean, and simazine reduces tillage required for weed control in many perennial and tree crops. Atrazine is also used extensively in chemical fallow cropping systems in rotations involving corn, sorghum, and wheat. Cyanazine was also used extensively in corn and cotton until 2002. 

Like most other classes of pesticides and herbicides, the degree of toxicity of triazines has been found to vary with compounds. Thus, the toxicity may not be solely attributed to the triazine ring. For example, while substances such as atrazine, metribuzin, and cyanazine are moderately toxic by all routes of exposure, compounds such as prometryn and propazine have a very low order of toxicity. However, no cases of human poisonings have been reported. A few selected triazine herbicides are discussed individually in the following sections. 

Cotterill [100] studied the effect of ammonium nitrate fertilizer on the electron capture or nitrogen specific gas chromatographic determination of Triazine plus other types of herbicide (Atrazine(2-chloro-4-ethylamino-6-isopropylamino, 1,3,5 triazine), Simazine (2-chloro-4.6 bis ethyl amino 1,3,5 triazine), Linuron (3,4,-chlorophenyl-l-methoxy-l-methyl urea), Metribuzin, Triallate and Phorate) residues in soil. 

The isomeric l,2,4-triazin-5-ones also have herbicidal activity, and of these the 4-amino compounds stand out. Metribuzin (3) and metamitron (4) (68SAP6804409) possess excellent selectivity, the former being manufactured on a very large scale. Metribuzin can be synthesized from 3,3-dimethyl-2-oxobutyric acid (Scheme 3). 

Yerkes et al. (1996) reported that although chlorophyll fluorescence measurements and C02 assimilation in resistant jimsonweed leaves were affected within 1 day of atrazine application, they returned to normal (at rates and levels equivalent to those in untreated leaves) within 5 days. Atrazine-resistant jimsonweed was cross-resistant to simazine, but was susceptible to prometryn, metribuzin, terbacil, and other herbicides. Chlorophyll fluorescence was unaffected in triazine-resistant pigweed, which showed cross-resistance to some triazines, moderate resistance to metribuzin and terbacil, and negative cross-resistance to bentazon and pyridate. 

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