Reactivity of the Insecticide Fenitrothion Towards O and N Nucleophiles.

NATALIA M. ROUGIER; RAQUEL V. VICO; RITA H. DE ROSSI; ELBA I. BUJÁN

Berkeley, CA

Simposio; Ninth Tetrahedron Symposium; 2008

Elsevier

Nucleophiles can react with alkylarylphosphates through three pathways, i.e. SN2(P) with P?O bond fission, SN2(C) with aliphatic C?O bond fission and SNAr with aromatic C?O bond fission. In order to get a better understanding of the factors that govern the regioselectivity of the reaction we have studied the reaction of Fenitrothion (O,O-dimethyl O-(3-methyl-4-nitrophenyl)-phosphorothioate) (1), an anticholinesterase neurotoxic used as a broad spectrum insecticide, with a series of nucleophiles that includes oxygen, nitrogen, bidentated and alfa nucleophiles. The kinetics of the reaction of 1 with the choosen nucleophiles was studied in basic media in 2% 1,4-dioxane/water at 25°C at ionic strength I = 1 M (NaCl) and followed by UV-visible spectrophotometry. The pH of the reaction media was varied between 9.00 and 13.00 depending on the nucleophile. Product identification was made by UV-visible spectrophotometry, NMR spectroscopy (1H, 13C, 31P) and GC-MS. Depending on the nucleophile, different sites of nucleophilic attack were observed. In the reactions of 1 with HO? and HOO? quantitative formation of 3-methyl-4-nitrophenoxide (2) was found, indicating P?O bond fission via SN2(P) pathway. With NH2OH and BuNH2 formation of demethylfenitrothion (3) and 2 was observed, indicating competition between SN2(P) and SN2(C) pathways. No evidence of SNAr pathway was found in any of the reactions studied. As the pH increases, the SN2(P) pathway is favored. The SN2(C) pathway is the favored reaction with BuNH2.