- Reactivity of the insecticide fenitrothion toward O and N nucleophiles
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The reactivity of Fenitrothion (1) toward several O- and N-based nucleophiles, including ambident and α-nucleophiles, was investigated in basic media at 25 °C in water containing 2% 1,4-dioxane. In the reactions with HO- and HOO- quantitative formation of 3-methyl-4-nitrophenoxide (2) was observed indicating a SN2(P) pathway. In the reactions with NH2OH, NH2O-, and BuNH2, demethylfenitrothion (4) was formed along with 2, indicating competition between the SN2(P) and SN2(C) pathways; no evidence of a SNAr pathway was observed in any case. The observed rate constants were dissected into the values corresponding to the SN2(P) and SN2(C) pathways. The yield of 4 depends on the nucleophile and on the pH of the reaction, being the main product in the case of BuNH2. With HOO-, NH2OH, and NH 2O- a significant α-effect was observed, confirming the participation of the nucleophile in the rate-limiting step of the reaction.
- Rougier, Natalia M.,Vico, Raquel V.,De Rossi, Rita H.,Bujan, Elba I.
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- Effect of cyclodextrin on the hydrolysis of the pesticide fenitrothion [O,O-dimethyl O-(3-methyl-4-nitrophenyl)phosphorothioate]
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The hydrolysis of fenitrothion was studied at HO- concentrations between 0.099 and 0.999 M. The second-order rate constant is 2.0 × 10-3 M-1 s-1. The reaction is inhibited by β-cyclodextrin at HO- con
- Vico, Raquel V.,Bujan, Elba I.,De Rossi, Rita H.
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- Accelerating the initial rate of hydrolysis of methyl parathion with laser excitation using monolayer protected 10 nm Au nanoparticles capped with a Cu(bpy) catalyst
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Using a low power green laser, we have demonstrated a rate acceleration of ~2-fold for the hydrolysis of methyl parathion by irradiating the plasmon absorption band of Au nanoparticles capped with a Cu(bpy) catalyst.
- Trammell, Scott A.,Nita, Rafaela,Moore, Martin,Zabetakis, Dan,Chang, Eddie,Knight, D. Andrew
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supporting information; scheme or table
p. 4121 - 4123
(2012/06/18)
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- Study on the transesterification of methyl aryl phosphorothioates in methanol promoted by Cd(II), Mn(II), and a synthetic Pd(II) complex
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Methanol solutions containing Cd(II), Mn(II), and a palladacycle, (dimethanol bis(N,N-dimethylbenzylamine-2C,N)palladium(II) (3), are shown to promote the methanolytic transesterification of O-methyl O-4-nitrophenyl phosphorothioate (2b) at 25 °C with impressive rate accelerations of 10 6-1011 over the background methoxide promoted reaction. A detailed mechanistic investigation of the methanolytic cleavage of 2a-d having various leaving group aryl substitutions, and particularly the 4-nitrophenyl derivative (2b), catalyzed by Pd-complex 3 is presented. Plots of k obs versus palladacycle [3] demonstrate strong saturation binding to form 2b:3. Numerical fits of the kinetic data to a universal binding equation provide binding constants, Kb, and first order catalytic rate constants for the methanolysis reaction of the 2b:3 complex (kcat) which, when corrected for buffer effects, give corrected (kcat corr) rate constants. A sigmoidal shaped plot of log(k catcorr) versus sspH (in methanol) for the cleavage of 2b displays a broad sspH independent region from 5.6 ≥ sspH ≥ ~10 with a k minimum = (1.45 ± 0.24) × 10-2 s-1 and a [lyoxide] dependent wing plateauing above a kinetically determined sspKa of 12.71 ± 0.17 to give a k maximum = 7.1 ± 1.7 s-1. Bronsted plots were constructed for reaction of 2a-d at sspH 8.7 and 14.1, corresponding to reaction in the midpoints of the low and high s spH plateaus. The Bronsted coefficients (βLG) are computed as -0.01 ± 0.03 and -0.86 ± 0.004 at low and high sspH, respectively. In the low sspH plateau, and under conditions of saturating 3, a solvent deuterium kinetic isotope effect of kH/kD = 1.17 ± 0.08 is observed; activation parameters (ΔHPd? = 14.0 ± 0.6 kcal/mol and ΔSPd?= -20 ± 2 cal/mol?K) were obtained for the 3-catalyzed cleavage reaction of 2b. Possible mechanisms are discussed for the reactions catalyzed by 3 at low and high sspH. This catalytic system is shown to promote the methanolytic cleavage of O,O-dimethyl phosphorothioate in CD3OD, producing (CD3O)2P=O(S-) with a half time for reaction of 34 min.
- Edwards, David R.,Neverov, Alexei A.,Brown, R. Stan
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scheme or table
p. 1786 - 1797
(2011/04/23)
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- The mechanism of the hydrolysis of μ-monothiopyrophosphate
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The hydrolysis of μ-monothiopyrophosphate (MTP) is described as a function of pH, temperature, and solvent composition. The hydrolysis of its precursor, tetramethyl μ-monothiopyrophosphate (Me4MTP), is also examined. The profile of log kobs versus pH for the hydrolysis of MTP exhibits two breaks corresponding to pKa values of 4.73 and 8.49, which are assigned to the third and fourth dissociation constants of MTP, respectively. The rate constants for hydrolysis of the tetraanion and trianion of MTP at 25°C are 0.0032 and 1.39 min-1, and at 5°C the trianion and dianion undergo hydrolysis with rate constants of 0.103 and 1.93 min-1, respectively. The enthalpy of activation at pH 7.0 is 19.7 kcal-mol-1, and the entropy of activation is 0.2 eu; the latter observation indicates that the mechanism involves a dissociative transition state. In contrast, Me4MTP undergoes hydrolysis with kobs = 1.21 min-1 at 25.3°C, an enthalpy of activation of 13.0 kcal-mol-1, and an entropy of activation of -22.7 eu. These values suggest that the hydrolytic mechanism for Me4MTP involves an associative transition state These mechanisms are strongly supported by comparisons of the hydrolytic rates for MTP and Me4MTP with those for pyrophosphate (PPi) and tetraalkyl pyrophosphate. The rate ratios for MTP/PPi are 5.8 × 107 for the dianions, 1.7 × 107 for the trianions, and between 0.6 × 107 and 2.4 × 107 for the tetraanions. The large rate enhancements induced by the replacement of oxygen by sulfur are attributed to the weakness of the P-S bond relative to the P-O bond. This weakness will be expressed in a dissociative transition state, which involves a high degree of bond cleavage between P and the leaving group. In contrast, the rate ratio for Me4MTP/Me4PPi (tetramethyl pyrophosphate) is 48, which is indicative of little cleavage of the bond between P and the leaving group in the transition state, that is, an associative transition state.
- Halkides, Christopher J.,Frey, Perry A.
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p. 9843 - 9848
(2007/10/02)
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- KINETICS OF THE REACTION OF METAPHOS WITH ETHYLENEDIAMINE IN MICELLAR SOLUTIONS
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The kinetics and mechanism of the reaction of O,O-dimethyl O-4-nitrophenyl phosphorothioate (metaphos) with ethylenediamine in the presence of the cationic surfactant cetyltrimethylammonium bromide have been studied by a spectrophotometric method.The reaction takes place along two paths in an aqueous medium, viz., as alkylation of the amine and as nucleophilic substitution at the phosphorus atom.In the absence of micelles, alkylation of ethylenediamine mainly takes place.In a micellar solution the alkylation process is inhibited, and the reactions at the phosphorus atom are catalyzed and become predominant.This results in a sharp change in the ratio between the products of the parallel reactions in the presence of the surfactant micelles.
- Fedorov, S. B.,Bel'skii, V. E.,Kudryavtseva, L. A.,Ivanov, B. E.
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p. 1086 - 1089
(2007/10/02)
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