Chemical constitution and activity of bipyridylium herbicides. Part VI. Diquaternary salts of 4,4′‐iminodipyridine and 4,4′‐thiodipyridine

Abstract

19In work reported in earlier papers of this series (1-5) much evidence has bc:r:n obtained to support the view that thc mode of action of the bipyridylium herbicides, paraquat (I) and diquat ([I), is connected with their ability to be reduced in aqueous solution to stable radical cations by a onr: electron transfer which is reversed by air. The effect on herbicidal activity of the introduction between the pyridine rings of paraquat of a group which is capable of acting as an electron transfer bridge has so far betm studied only with diquaternary salts of trans-I ,2-di-(4-pyridyl)ethylene (5). We now report our results with diquaternary salts of 4,4'-iminodipyridine and 4,4'-thiodipyridin (s .Unlike 2,2'-iminodipyridine, from which only the dcrivative (111) was isolated from reaction with excess methyl iodide (6), 4,4'-iminodipyridine afforded the diquaternary salt (IV; X = NH) with this reagent. The salt (IV; X = NH) in aqueous solution is in reversible equilibrium, dependent o n pH, with another species, which is considered t o be the monoquaternary derivative (V) analogous to 111. At pH values lower than about 0.8, the predominant form is (IV; X = NH). A t pH 11.3, the UV maxima are due to the more conjugated system (V) formed by lobs of hydrogen iodide from IV (X = NH). The long wavelength absorption maxima of V is similar to that reported (6) for the analogous derivative (111) obtained from 2,2'-iminodipyridine. In the 2,2'-iminodipyridine series, compound 111 rcverts to the diquaternary salt (VI) only in strong acid solution (6). This dilference in the east: with which hydrogen iodide is lost from VI and I\-(X = NH) presumably reflects the difference in the proximity of the two positively charged nitrogens in the two series.Aqueous solutions of diquat, paraquat and several related bipyridylium salts on treatment with zinc dust give intense green or violet solutions due t o the corresponding radical cations (7). When IV (X = NH) was treated with zinc powder in aqueous solution, there was n o apparent change and this was confirmed by NMK and UV spectroscopy. On polarographic examination there was n o evidence of reduction above -1.2 volts. It is clear that the diquaternary salt (IV; X NH) is not reduced t o a radical cation a t potentials attainable in aqueous solution. In accord with the theory of the mode o f action of bipyri-dylium herbicides, the salt (IV; X = NH) was inactive against six plant species when tested as a post-emergent herbicide at 8 Ibs/acre.

Chemical constitution and activity of bipyridinium herbicides. Part XIV. Reduction potential and herbicidal activity of 4,4′‐(1,3,4‐thiadiazole‐2,5‐diyl)‐ and 4,4′‐(1,3,4‐oxadiazole‐2,5‐diyl)bis(1‐methylpyridinium) diiodides

“…The stability of the radical cations is due to the delocalization of the odd electron over both pyridine rings which are essentially coplanar. The introduction between the pyridine rings of diquat or paraquat of an atom or group which is capable of acting as an electron transfer bridge usually results in loss of herbicidal activity since diquaternary salts of this type give, inter alia, unstable radical cations on reduction (3)(4)(5)(6)(7)(8)(9). Recently, however, it was found that the introduction of the planar 1,3,5-triazine ring system between the pyridine rings of paraquat gave the salt (IV) which showed very good herbicidal activity (10).…”