Heterocyclic nitrogen herbicides

Other Heterocyclic Nitrogen Derivative Herbicides. The herbicides in this group are heterocycHc nitrogen derivatives that do not readily fall into one of the previously discussed groups. They have a wide range of uses and properties. Most of these herbicides are used for selective, pre-and/or post-emergence weed control. Amitrole is used for post-emergence, nonselective weed control in non-croplands and also as an aquatic herbicide (2,296). 

Fig. 20.6 shows the trend in consumption for the three most important types of herbicides. Carboxylic acids have been the standby for many years and were the first type of herbicide. They were replaced as number one by the heterocyclic nitrogen compounds in the 1970s. Carboxylic acids are making a comeback in the 1990s. A close third are the amide herbicides. The present percentage of consumption for herbicides is given in Table 20.7. 

As sulfones are known to be readily displaced from electron deficient nitrogen-containing heterocycles, Bessard noted significant rate enhancements as well as improved yields in the displacement of the chloride on pyrimidine 49 by alcohols, through the use of sodium methylsulfinate as a catalyst <00T4739>. The production of trialkoxypyrimidines 51 as potential herbicides, presumably formed from the displacement of methysulfinate from intermediate 50 by the various alcohols, required a less than a stoichiometric amount of sodium methylsulfinate (typically 0.10-0.25 equivalents). 

Nitrogen derivative herbicides, heterocyclic, 13 323-324 Nitrogen dioxide (N02). See also Nitrogen oxides (NO )... 

The most widely used herbicides today are triazine compounds (three nitrogens in the heterocyclic aromatic ring). Atrazine is used especially on com but also on pineapple and sugarcane. It is synthesized by reacting cyanuryl chloride successively with one equivalent of ethylamine and one equivalent of isopropylamine. Cyanuryl chloride is made in one step from... 

Adrian N. R. and Suflita J. M. (1990) Reductive dehalogenation of a nitrogen heterocyclic herbicide in anoxic aquifer slurries. Appl. Environ. Microb. 56(1), 292-294. 

Based on available herbicide data for related Cyanamid pyridazines and patented compounds, the following structural modifications were proposed 1) substitution of the chlorine at the 3-position 2) replacement or derivatization of the 4-methyl group 3) introduction of substituents at the 5-position 4) alternate substitution in the 6-phenyl ring as well as reduction to the corresponding cyclohexyl derivatives and replacement of the phenyl by heterocycles and 5) oxidation and quaternization of the nitrogens at positions 1 and 2. 

Diary] and triaryl or naphthyl carbamates exhibit low herbicidal activity. The substitution of the aryl radical for a heterocyclic radical gives heterocyclic alkyl and dialkyl ureas, of which many examples have been prepared in recent years. The herbicidal activity of urea derivatives containing a heterocyclic radical, such as benzthiazole, thiazole, thiadiazole, oxadiazole and pyridine, is favourable if one or two methyl groups are substituted at the N -nitrogen. The carrier of total or selective action in these derivatives is presumably the heterocyclic part of the molecule. More recently several new groups of compounds have become known, mainly in the patent literature, for which the structure-activity on relationships are still to be elucidated. 

Chloroacetanilides and nitrogen heterocyclic herbicides Soil (5 g) PFE Pretreatment with 31.6% of water. 32 ml acetone, 1500 psi, 100°C Dried over Na2S04, filtered, evaporated to 1 ml, and adjusted to 5 ml with MTBE 93 to 103 1 to 7 GC-MS 2000 196... 

Continuous study of oxyacetamide chemistry shifted research from the paddy herbicide to an upland herbicide with increasing water solubility that is suitable for such upland use. To this end, benzanellated analogues such as the benzothia-zole moiety of mefenacet were changed to simple five-membered heterocycles that contain at least one nitrogen atom to increase water solubility, and sulfur or oxygen atom to decrease lipophilicity, for instance thiazoles, thiadiazoles, oxazoles and oxadiazoles (Fig. 8.2). Consequently, many patent applications of the new class of heteroxyacetamide herbicides were disclosed [17, 18]. 

Nitrogen heterocycles are a class of naturally occurring compounds essential to life since their structural units are present in many natural products such as vitamins, hormones, antibiotics, and alkaloids and also in many pharmaceuticals and herbicides among other synthetic compoxmds, exhibiting a broad range of biological activities [83]. 

---