The pharmacophore of the neonicotinoid insecticide imidacloprid, nitroiminoimidazolidine, was modified to heterocycles such as thiazolidine, pyrrolidine, dihydroimidazole, dihydrothiazole, and pyridone conjugated to nitroimine (=NNO2) or nitromethylene (=CHNO2). Their 6-chloro-3-pyridylmethyl or 5-chloro-3-thiazolylmethyl derivatives were examined for insecticidal activity against the American cockroach by injection and for neuroblocking activity using the cockroach ganglion. Most of the compounds having the neonicotinoidal pharmacophore exhibited insecticidal activity at the nanomolar level, which was enhanced in the presence of synergists, and high neuroblocking activity at the micromolar level. Quantitative analysis for the compounds showed that the neuroblocking potency is proportional both to the Mulliken charge on the nitro oxygen atom and to the partition coefficient log P value. The equation for the insecticidal versus neuroblocking potencies indicated that both potencies are related proportionally with each other when the other factors are the same.
Five new neonicotinoid compounds bearing a 5-chloro-6-fluoronicotinyl group were prepared and their insecticidal activity against brown rice planthopper, green peach aphid and common cutworm under laboratory conditions was compared with 1-(6-chloro-5-fluoronicotinyl)-2-nitroiminoimidazolidine. 5-Fluoroimidacloprid controlled the two hemipteras at 0.2 ppm at the same dose level of imidacloprid, while the positional isomers showed far lower activity. None of the tested compounds controlled the lepidoptera at 200 ppm.
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