Thio-Induced Organophosphate Breakdown Promoted by Methimazole: an Experimental and Theoretical Study

Investigating the reactivity of small nucleophilic scaffolds is a strategic approach for the design of new catalysts aiming at effective detoxification processes of organophosphorus compounds. The drug methimazole (MMZ) is an interesting candidate featuring two non-equivalent nucleophilic centers. Herein, phosphoryl transfer reactions mediated by MMZ were assessed by means of spectrophotometric kinetic studies, mass spectrometry (MS) analyses, and density functional theory (DFT) calculations using the multi-electrophilic compound O,O-diethyl 2,4-dinitrophenyl phosphate (DEDNPP). MMZ anion acts primarily as an S-nucleophile, exhibiting a nucleophilic activity comparable to that of certain oximes featuring alpha-effect. Selective nucleophilic aromatic substitution was observed, consistent with the DFT prediction of a low energy barrier. Overall, the results bring important advances regarding the mechanistic understanding of nucleophilic dephosphorylation reactions, which comprises a strategic tool for neutralizing toxic organophosphates, hence promoting chemical security.

Theoretical investigation of nucleophilic substitution reaction of phenyl carbonyl isothiocyanates with pyridines in gas and polar aprotic solvent

This study focuses on the mutual interaction of substituents in the nucleophile and substrate - cross interaction constant, ρXY, in the uncatalyzed aminolysis by substituting pyridine with phenyl carbonyl isothiocyanate. The mechanism was found to be a stepwise process with a rate-limiting breakdown of the -NCS leaving group. This stepwise reaction mechanism considers the cross-interaction constant (CIC) with rate-limiting breakdown of tetrahedral intermediate in gas and solvent phases. The corresponding Hammett coefficients are related to the substituents associated with (1) the nucleophiles (X), ρX (-1.93 to -6.54 for the gas phase and 10.5 to 18.9 in the solvent model), and with (2) the substituents associated with the phenyl ring of the substrate (Y), ρY (0.41-3.48 for the gas phase and 1.83 to -10.70 for the solvent model). It also includes the Brønsted coefficient with X, βX (0.11-1.52 for the gas phase and -2.57 to 3.96 for the solvent model), and CIC values, ρXY (0.69 for the gas phase and 0.87 for the solvent model). In this work, the NBO analysis, reaction potential, reaction electronic flux (REF), dual descriptor, and the structure-energy relationships were considered in interpreting the mechanistic criteria.

Solvent and solvation effects on reactivities and mechanisms of phospho group transfers from phosphate and phosphinate esters to nucleophiles

Organophosphorus esters fulfil many industrial, agricultural, and household roles. Nature has deployed phosphates and their related anhydrides as energy carriers and reservoirs, as constituents of genetic materials in the form of DNA and RNA, and as intermediates in key biochemical conversions. The transfer of the phosphoryl (PO₃) group is thus a ubiquitous biological process that is involved in a variety of transformations at the cellular level such as bioenergy and signals transductions. Significant attention has been paid in the last seven decades to understanding the mechanisms of uncatalyzed (solution) chemistry of the phospho group transfer because of the notion that enzymes convert the dissociative transition state structures in the uncatalyzed reactions into associative ones in the biological processes. In this regard, it has also been proposed that the rate enhancements enacted by enzymes result from the desolvation of the ground state in the hydrophobic active site environments, although theoretical calculations seem to disagree with this position. As a result, some attention has been paid to the study of the effects of solvent change, from water to less polar solvents, in uncatalyzed phospho transfer reactions. Such changes have consequences on the stabilities of the ground and the transition states of reactions which affect reactivities and, sometimes, the mechanisms of reactions. This review seeks to collate and evaluate what is known about solvent effects in this domain, especially their effects on rates of reactions of different classes of organophosphorus esters. The outcome of this exercise shows that a systematized study of solvent effects needs to be undertaken to fully understand the physical organic chemistry of the transfer of phosphates and related molecules from aqueous to substantially hydrophobic environments, since significant knowledge gaps exist.

Nucleophilic Neutralization of Organophosphates: Lack of Selectivity or Plenty of Versatility?

Neutralization of organophosphates is an issue of public health and safety, involving agrochemicals and chemical warfare. A promising approach is the nucleophilic neutralization, scope of this review, which focuses on the molecular nucleophiles: hydroxide, imidazole derivatives, alpha nucleophiles, amines and other nucleophiles. A reactivity mapping is given correlating the pathways and reaction efficiency with structural dependence of the nucleophile (basicity) and the organophosphate (electrophilic centers, P=O/P=S shift, leaving and non-leaving group). Reactions extremely unfavorable (>20 years) can be reduced to seconds with various nucleophiles, some which are catalytic. Although there is no universal nucleophile, a lack of selectivity in some cases accounts for plenty of versatility in other reactions. The ideal neutralization requires a solid mechanistic understanding, together with balancing factors such as milder conditions, fast process, selectivity and less toxic products.

Mechanistic study of carboxylic acid and phosphate ester cleavage by oximate metal complexes surpassing the limiting reactivity of highly basic free oximate anions

Two tridentate and one tetradentate new ligands containing the terminal oxime group separated from secondary amino and pyridine groups as additional binding sites by two or three methylene groups have been prepared. Their acid-base properties, as well as the composition and stability of their complexes with Zn(ii) and Cd(ii) ions, were determined by potentiometric and spectrophotometric titrations. The X-ray structure of a Cd(ii) complex of a related tridentate oxime ligand previously studied in solution was determined. All oximate complexes show high reactivity in the cleavage of aryl acetates, paraoxon, parathion and 4-nitrophenyl diphenyl phosphate, with rate constants significantly surpassing the limiting rate constants observed for highly basic free oximate anions. The second-order rate constants for individual oximate complexes in solution are assigned to each ligand, metal cation and substrate. The results of the cleavage of 4-substituted phenyl acetates were analyzed in terms of Brønsted correlations with the leaving group pKa, which demonstrated a change in the rate determining step from the nucleophilic attack to the leaving group departure upon an increase in the leaving group basicity. The zero slope of the Brønsted correlation for the nucleophilic attack indicates transition state stabilization through electrophilic assistance by the metal ion. This interpretation is supported by metal selectivity in the relative efficiency of the cleavage of paraoxon and parathion. The existence of the alpha-effect in ester cleavage by coordinated oximates is confirmed by an analysis of the Brønsted correlations with the nucleophile basicity for metal bound oximate and alkoxo or hydroxo nucleophiles. The very high reactivity of the oximate complexes of the new ligands is attributed to transition state stabilization and to the removal of the solvational imbalance of oximate anions that impedes the expected increase in the reactivity of highly basic free anions.

Experimentally realized mechanochemistry distinct from force-accelerated scission of loaded bonds

Stretching polymer chains accelerates dissociation of a variety of internal covalent bonds, to an extent that correlates well with the force experienced by the scissile bond. Recent theory has also predicted scenarios in which applied force accelerates dissociation of unloaded bonds and kinetically strengthens strained bonds. We report here unambiguous experimental validation of this hypothesis: Detailed kinetic measurements demonstrate that stretching phosphotriesters accelerates dissociation of the unloaded phosphorus-oxygen bond orthogonal to the pulling axis, whereas stretching organosiloxanes inhibits dissociation of the aligned loaded silicon-oxygen bonds. Qualitatively, the outcome is determined by phosphoester elongation and siloxane contraction along the pulling axis in the respective rate-determining transition states. Quantitatively, the results agree with a simple mechanochemical kinetics model.

Investigating the breakdown of the nerve agent simulant methyl paraoxon and chemical warfare agents GB and VX using nitrogen containing bases

A range of nitrogen containing bases was tested for the hydrolysis of a nerve agent simulant, methyl paraoxon (MP), and the chemical warfare agents, GB and VX.

Reactivity and selectivity of the reaction of O,O-diethyl 2,4-dinitrophenyl phosphate and thionophosphate with thiols of low molecular weight

A kinetics control product by sulfhydryl attack of thiols was observed in the reactions of both triesters with Cys and Hcys, followed by an intramolecular amine attack leading to a thermodynamic control product.

Reaction mechanisms in ionic liquids: the kinetics and mechanism of the reaction of O,O-diethyl (2,4-dinitrophenyl) phosphate triester with secondary alicyclic amines

In the title reaction, the ionic liquids used stabilized the zwitterionic pentacoordinate intermediate (P±), leading to a change in the mechanism from concerted to stepwise.

From α-nucleophiles to functionalized aggregates: exploring the reactivity of hydroxamate ion towards esterolytic reactions in micelles

Hydroxamate ions as α-nucleophiles for esterolytic reactions in water and micelles.

Ionic liquids: anion effect on the reaction of O,O-diethyl O-(2,4-dinitrophenyl) phosphate triester with piperidine

The ionic liquids can be considered as designer solvents in the titled reaction because by an appropriate choice of the anion it is possible to steer the selectivity of this reaction.