Pyrethroid photochemistry: mechanistic aspects in reactions of the (dihalogenovinyl)cyclopropanecarboxylate substituent

Abstract

The reaction quantum yields of six pyrethroid insecticides in methanol at 300 nm vary 3–10-fold between compounds with a cyclopentenone chromophore (allethrin) as compared with the phenoxybenzyl group (e.g., permethrin, decamethrin) in the alcohol moiety. Chlorine confers greater photostability than bromine in pyrethroids with dihalogenovinyl substituents in the acid moiety. Pathways for photodecomposition of methyl [1R,cis]-2,2dimethyl-3-(2,2-dibromovinyl)cyclopanecarboxylate in methanol at 250 or 300 nm involve cis-trans isomerization, reductive debromination, and conversion of the dibromovinyl group into a bromomethoxy-epoxide derivative. Isomerization, but not debromination, involves a triplet excited state.