Competing pathways in the reaction of the pesticide fenitrothion [O,O-dimethyl O-(3-methyl-4-nitrophenyl) phosphorothioate] with some nitrogen nucleophiles in aqueous solution

Abstract

The reaction of the title compound, 1, with the nitrogen-based nucleophiles n-butylamine, ethanolamine, and glycyl ethyl ester, has been investigated in aqueous solution at room temperature. In all cases, attack at the phosphorus center of the substrate by the nucleophile is in competition with O-Ar bond cleavage, the latter being an SNAr reaction. Product analyses show that the % SNAr product is independent of the concentration of glycyl ethyl ester but increases with n-butylamine and ethanolamine concentrations, indicating that the reactions involving n-butylamine and ethanolamine are base catalyzed while the reaction of glycyl ethyl ester is not. The preponderance of SNAr products at high nucleophile concentrations argues for the rate-limiting decomposition of the Meisenheimer-type intermediates (PH) formed in the reactions of n-butylamine and ethanolamine. Formation of PH is rate determining in the reaction involving glycyl ethyl ester. Site preferences observed in the reaction of 1 with oxygen and nitrogen nucleophiles are discussed in terms of (i) the Hard Soft Acid Base (HSAB) theory, (ii) stabilization of the transition state for the cleavage of alkyl-O bond by hydrogen bonding, (iii) competitive advantage of nitrogen nucleophiles over their oxygen counterparts in their attack at aromatic carbon centers, and (iv) the relative ease of formation of σ-complex intermediates in S NAr reactions by amine nucleophiles compared to their oxygen counterparts.