Insecticidal activity of new fluorinated pyrethroids and their stability toward chemical oxidation and photoreaction

The insecticidal activity of several new fluorinated pyrethroids was measured using houseflies untreated and treated with piperonyl butoxide (PB). 10, 10‐Difluorophenothrin and 10, 10‐difluoroallethrin, which include fluorine atoms at a position most easily attacked by mixed function oxidases (mfo) in the acid moiety, showed smaller topical LD₅₀ values on the PB‐untreated flies than the parent insecticides but rather large values on the PB‐treated flies, suggesting that the introduction of fluorine prevents metabolic oxidation, although it has a negative effect on the interaction with a target site. Monofluorinated analogues of phenothrin, in which the structure of chrysanthemic acid was rearranged, still had moderate insecticidal activity, but flies were scarcely killed by 7 fluoroallethrin into which fluorine was introduced at a position most easily oxidized by mfo in the alcohol moiety. Competitive oxidation of the fluorinated derivatives and the parent compounds showed that fluorine at both the 10‐ and 7′‐positions stabilizes the alkenyl side‐chains of the pyrethroids against selenium dioxide and m‐chloroperoxybenzoic acid. Photochemical studies with 10, 10‐difluorophenothrin indicated that the photo‐instability of the cyclopropanecarboxylic part could not be improved by this modification.