Acceleration of nucleophilic attack on an organophosphorothioate neurotoxin, fenitrothion, by reactive counterion cationic micelles. Regioselectivity as a probe of substrate orientation within the micelle

31P NMR and UV-vis spectrometric evidence has revealed an unexpected regioselectivity in the reaction of fenitrothion, 1, an organophosphorus pesticide, with the cetyltrimethylammonium (CTA) surfactants CTAOH and CTAMINA, that incorporate the reactive counterions OH- and MINA- (the anti-pyruvaldehyde 1-oximate anion). While both micellar solutions accelerate decomposition of 1 compared to aqueous OH- alone, CTAMINA produced the largest rate enhancement (ca. 10(5)) at a pH (8.39) appropriate for environmental applications. In the absence of surfactant, reaction proceeds solely via the SN2(P) pathway. In the presence of surfactant but below the critical micelle concentration (cmc), a competitive SN2(C) pathway was observed in addition to SN2(P). Above the cmc, however, the CTAOH reaction again proceeded solely via the SN2(P) pathway while both pathways were operative with CTAMINA. The changes in reactivity and mechanistic pathway are discussed in terms of premicellar and micellar influences on rates and regioselectivity. A proposal that would account for the observed regioselectivity in the micellar system is that the aromatic ring and aliphatic side-chains of 1 are oriented toward the micellar interior, while the P=S moiety faces the aqueous pseudophase.     

Degradation of the pesticide fenitrothion as mediated by cationic surfactants and alpha-nucleophilic reagents

The reaction of fenitrothion with a series of alpha-nucleophile oximates having pK(a) values in the range of 7.7-11.8 was studied both in the absence and presence of cetyltrimethylammonium (CTA(+)) surfactants. Reaction with CTA-oximates was found to proceed through two pathways: S(N)2(P) and S(N)2(C); an S(N)Ar pathway was not observed. Accordingly, the observed rate constants were dissected into the two corresponding S(N)2(P) and S(N)2(C) pathways. Use of the pseudophase ion exchange (PPIE) model for micellar catalysis in the CTA(+) system allowed evaluation of micellar second-order rate constant (k(2m)) parameters and binding constants, (K(S)). K(S) values for CTA-oximates were found to vary with the counterion, and the rate enhancement depended on a combination of K(S) and k(2m) values. k(2m)/k(2w) values ranged from 0.0025 to 0.64, suggesting that a concentration effect is mainly responsible for the rate enhancement. In the absence of surfactant, an alpha-effect (i.e., k(alpha)/k(normal)) varying from 8 to 450 was observed for the oximate reaction, decreasing with increasing pK(a). It is proposed that differential solvation (transition-state imbalance) is a cause of the alpha-effect in this system.