4,4 -bipyridinium diquatemary salts

Bipyridinium diquatemary salts (1.0 mmol) and acetylenedicarboxylate (1.0 mmol) were dissolved into 1 mL of acetone and adsorbed on KF-Merck Alumina 70-230 mesh. The solvent was then evaporated and the mixture was irradiated at atmospheric pressure in a focused microwave reactor Prolabo MX350 with measurement and control of power and temperature by infrared detection for the time and at power indicated in Table 1. All derivatives were characterized by IR, XH and 13C NMR spectroscopy, and elemental analyses. 

Keywords 4,4 -bipyridinium diquatemary salts, acetylenedicarboxylate, 1,3-di-polar cycloaddition, microwave irradiation, 7,7 -bis-indolizine... 

Cycloaddition of 4,4 -bipyridinium ylides 218, generated in situ from 4,4 -bipyridine and substituted phenacyl bromides 216 via 4,4 -bipyridinium diquatemary salts 217, with activated alkynes has been achieved under solvent-free conditions and in the presence of basic catalyst KF on alumina within 7-10 min by MWI to give 7,7 -feir-indolizines 220 in 81-93% yields via intermediate 219 (Scheme 46). On heating in benzene (or in benzene and A-methylpyrrolidone (NMP)), the reaction times were longer (3 h) and the yields were lower (50-59%) (00SL1013). 

Some bipyridinium salts are remarkable herbicides. They rapidly desiccate all green plant tissue with which they come into contact, and they are inactivated by adsorption on to clay minerals in the soil. This potent herbicidal activity is found only in quaternary salts, e.g. diquat (254) and paraquat (255), with redox potentials for the first reduction step between -300 and -500 mV (equations 158 and 159) (B-80MI20504). The first reduction step, which is involved in herbicidal activity, involves a completely reversible, pH independent, one-electron transfer to yield the resonance stabilized radicals (256) and (257). The second reduction step, (256 -> 258) and (257 -> 259), is pH dependent and the p-quinoid species formed are good reducing agents that may readily be oxidized to diquatemary salts. 

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