Organosilane insecticides. Part I: Biological and physical effects of isosteric replacement of silicon for carbon in etofenprox and MTI-800

Substitution of silicon for carbon was explored in etofenprox and MTI-800, primarily to establish the suitability of silicon for pesticide construction and to probe steric effects. It was shown that silicon is an effective building block for insecticides with subtle qualitative and quantitative differences relative to the carbon analogs. The silanes were slightly less toxic to insects than the carbon analogs (relative potency of 0.2-0.6) but the silane analog of MTI-800 was considerably less toxic to fish (0% mortality at ≥50 mg liter^?1) than MTI-800 itself (LC₅₀ = 3 mg liter^?1). The mode of action of both carbon and silicon compounds was similar in the intact, electrode-implanted cockroach and involved repetitive firing of a sensory nerve; potency measurements were also made using an in-vitro nerve assay. The possible metabolic and physicochemical contributions to the observed insecticide potencies were explored using synergism studies, physical chemistry measurements and quantum mechanical calculations.