Herbicides pyridine derivatives

Pyridine is a polar, stable, relatively unreactive liquid (bp 115°C) with a characteristic strong penetrating odor that is unpleasant to most people. It is miscible with both water and organic solvents. Pyridine was first isolated, like pyrrole, from bone pyrolysates. Its name is derived from the Greek for fire (pyr) and the suffix idine used to designate aromatic bases. Pyridine is used as a solvent, in addition to many other uses including products such as pharmaceuticals, vitamins, food flavorings, paints, dyes, rubber products, adhesives, insecticides, and herbicides. Pyridine can also be formed from the breakdown of many natural materials in the environment. 

Pyridine derivatives with several halogen atom substituents show herbicidal activity. The herbicidal action of 2,3,4-trichloro-4-pyridinol (pyrichlor, 1) and of... 

Pyridine derivatives are precursors of many chemical products of medicinal, agricultural, and industrial importance. Pyridine itself is used not only as a solvent, but also as a starting material for pharmaceuticals, herbicides, insecticides, and fungicides. a-Picoline is a precursor for 2-vinylpyridine which is used in the production of an adhesive for textile tire cord. The major use of y-picoline is in the production of isonicotinic hydrazide, an antituberculosis agent. / -Picoline and 2-methyl-5-ethylpyridine are important intermediates in the production of two members of the vitamin B family, nicotinamide, and nicotinic acid (also known as vitamin B3). All this shows there is a substantial need for the production of picolines. The following text describes a novel route to a-picoline which might in the future lead to new processes. 

Other pyridine derivatives with biological activity are the herbicides paraquat and diquat (see p 295). 

The aromatic ring component illustrates the relative contributions of enzyme inhibition and physicochemical properties to herbicidal activity. The benzene imidazolinones are approximately ten-fold more potent than the corresponding pyridine derivatives as enzyme inhibitors but are less potent as herbicides. 

Many synthetic pyridine derivatives are important as therapeutic agents, for example Isoniazide is a major antituberculosis agent, Sulphapyridine is one of the sulfonamide antibacterials, Prialdoxime is an antidote for poisoning by organophosphates, and Lacidipine is one of several antihypertensive 1,4-dihy-dropyridines. Some herbicides (Paraquat) and fungicides (Davicil) are also pyridine derivatives. 

Pyridine and derivatives of pyridine occur widely in nature as components of alkaloids, vitamins, and coenzymes. These compounds are of continuing interest to theoretical physical, organic, and biochemistry and to industrial chemistry. Pyridine and derivatives have many uses, e.g., herbicides and pesticides, pharmaceuticals, feed supplements, solvents and reagents, and chemicals for the polymer and textile industries. 

One-third of all pyridine electrochemical citations deal with the electrolysis of quaternary salts of pyridines two out of five cathodic reports are concerned with them. Moreover, the products of reduction and the salts themselves are commercially valuable. A whole class of biochemical transformations depends on the reactivity of pyridinium ions. Agricultural products are also derived from these salts, and the value of bipyridiniiim herbicides is directly linked to their redox chemistry. 

The herbicides commonly used as arboricides include members of the triazines, ureas, and uracils, for grass and weed control in nursery and Christmas tree plantings. Other compounds include phosphonate and phosphate derivatives, pyridine-based organic acids of high activity, and organic arsenicals. Such a wide... 

Use of pyridines and their benzo derivatives in agriculture is a consequence of their considerable bioactivity in herbicide, insecticide, and fungicide applications. The reader should refer to CHEC(1984) 1984CHEC(2)511> and CHEC-II(1996) <1996CHEC-II(5)245>, as many of the compounds discussed therein remain of great commercial importance. Interest continues in developing new products containing pyridine-based compounds, some of which are mentioned below. 

Various chemical classes of auxin herbicides with different weed spectra and types of selectivity have been synthesized and commercially introduced over the years. They include phenoxycarboxylic acids, benzoic acids, pyridine carboxylic acids, aromatic carboxymethyl derivatives and a relatively new category, the quinolinecarboxylic acids (Figure 1). These compounds basically act as synthetic mimics of indole-3-acetic acid (lAA), which is the principal natural auxin in higher plants [2,3,5,7]. 

The Pesticide Manual describes 5 pyridine carboxylic acid derivatives, 3 benzoic acids, and 10 aryloxyalkanoic acids in current use as herbicides. 2,4,5-T seems to be banned worldwide and is not included in the current issue (Tomlin, 2000) but is described in earlier issues (e.g., Worthing, 1979). It was first registered in 1948 by Amchem Products, Inc., Pennsylvania, and the Dow Chemical Company, and was mainly used to control shrubs and trees, for example, in forests and along railways. Although the content of dioxin was eventually controlled, with a limit of <0.05 mg/kg in the preparation, it was banned soon after the termination of its use in Vietnam. Other products with 2,4,5-trichlorophenol were also banned or restricted after the Seveso accident (see Hay, 1978a, 1978b). 2,4-D is also produced from a chlo-rophenol and may contain dioxins, but of the much less toxic congeners. It... 

Of the N-alkylcarbamates only the aryl esters of N-alkylcarbamic acid possess herbicidal activity. The herbicidal activity of N-alkylcarbamic acid alkylesters and of the heterocyclic derivatives of N-alkylcarbamic add is mentioned only in the patents literature, but these latter derivatives have not found agricultural application. Examples are the N-dimethylcarbamic acid enol esters proposed by Whetstone and Kuddema (1959) and the N-methylcarbamic add pyridine esters proposed by Johnston (1964). 

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. 

Activity investigations of substances derived from 2,2-bipyridyl prove that the pyridine rings of the molecule must lie in the same plane for a suitable herbicidal effect to be exerted. 

Key products derived from pyridine (1) are the non-selective herbicides paraquat (5) and diquat (6) and the fungicide zinc pyrithione (7) used in a well-known fluh -dandruff shampoo. y5-Picoline (3) is a key precursor to vitamin B3 and both niacinamide (8) and niacin (9) are produced on the large scale (i.e. several thousand tons per year). The insecticide chloropyrifos (10) can be prepared from 6-picoline. 

The Pesticide Manual describes 5 pyridine carboxylic acid derivatives, 3 benzoic acids, and 10 aryloxyalkanoic acids in current use as herbicides. 

Phenoxypyridinecarbonamides are surprisingly flexible, when the pyridine ring is substituted (review in Refs. [27, 34]). The first active pattern consisted of nicotinamides with a 2-phenoxy substitution (3, Fig. 4.1.4). For the latter, the m-position (R ) was important with 3-CF3 and 3-Cl being most active, while double substitution led to a decrease of activity. While small substituents R such as H or CH3 gave good herbicidal activity, Br or Cl were weaker. In the amide part, N-phenyl and N-benzyl derivatives showed comparable activity ethylene as a spacer... 

There has been an increasing interest in the synthesis and properties of heterocycles bearing a trifluoromethyl substituent a selection of these is given in the Table. 2,6-Bis(trifluoroinethyl)pyridin-4-ol is readily prepared by the reaction of l,l,l,7,7,7-hexafluoroheptane-2,4,6-trione with aqueous anunonia at 125—130 °C its sodium salt reacts with halogens to give herbicidal 3,5-dihalogeno-derivatives. ... 

Pyridine has long been used as a solvent in the production of rubber chemicals, textile water-repellant agents, and in the synthesis of drugs. The derivatives 2-benzylpyridine and 2-aminopyridine are used in the preparation of antihistamines. Another market for pyridine is in the manufacture of the nonpersistent herbicides diquat and paraquat. 

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