Bipyridylium salts

Although both are quaternary salts of bipyridyls, diquat is a 2,2 -bipyridyl whereas paraquat is a 4,4 -bipyridyl. 

Given their relatively high costs relative to the herbicides discussed so far (this is due to the high cost of pyridine), the rapid expansion in their use is remarkable. Clearly it relates to their novel applications. Their mode of action involves reduction of the cation to a free radical during interference with photosynthesis. Re-oxidation back to the original cation by molecular oxygen then takes place, accompanied by production of hydrogen peroxide which is extremely toxic to plant cells. 

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A Microcalorimetric Investigation of the Interactions Between Clay Minerals and Bipyridylium Salts, M. H. B. Hayes, M. E. Pick, M. Stacey, and B. A. Toms, Proc. 1972 Int. Clay Conf., (1973) 675-682. 

Bipyridylium compounds are reduced similarly with one electron per nucleus,122-124 the semiquinone (121) having a considerable stability. An attempt has been made to correlate the electrochemical properties of quaternary bipyridylium salts with their herbicidal activity.125... 

Whereas cation radicals are formed from neutral molecules by one electron oxidation processes, bipyridylium salts readily give rise to stable cation radicals by one electron reduction processes. Several types of bipyridylium salts have been shown to have interesting herbicidal properties (Akhavein and Linscott, 1968) and the most important of these are exemplified in Scheme 3 by paraquat dichloride (PQ2+) (l,T-dimethyl-4,4 -bipyridylium dichloride) and diquat dibromide (PQ2+ 6,7-dihydrodipyrido[l,2- 2,l,-c]-pyrazine-di-ium dibromide). Herbicidal activity appears to depend, in part, on the ease of (reversible) one electron reduction of PQ2+ and... 

DQ2+ to form stable but air sensitive cation radicals PQ + and DQ + respectively (Dodge, 1971). One electron reduction of bipyridylium salts may be achieved elcctrochemically (Elofson and Edsberg,... 

Data on herbicides are presented and reviewed, which allows the distinction between two different modes of bleaching. The first mode is caused by inhibited carotene biosynthesis exhibited by particular phenylpyridazinones, substituted phenylfuranones or amitrole. Decrease of carotenes leads to subsequent photodestruction of chlorophyll, peroxidation of other membrane components, and decay of electron transport activity. The second mode, represented by p-nitrodl-phenylethers, is associated with peroxidation of membrane-bound polyunsaturated fatty acids concurrently with the breakdown of carotenes, chlorophylls, and decay of photosynthetic electron transport. Short-chain hydrocarbon gases are reliable markers. The action of peroxidizing diphenylethers appears to be related to that of bipyridylium salts, although no light-induced oxygen uptake can be measured. 

The pyridine ring occurs in the well-known and widely distributed compotmd homarine (Polychronopoulos et al, 2001), and in an unusual bipyridylium salt that has been isolated from Callista chione from the Aegean Sea and previously known as a synthetic compound (Vagias et al, 2000). 

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