Gilles Moutiers

Revisiting the reactivity of oximate α-nucleophiles with electrophilic phosphorus centers. Relevance to detoxification of sarin, soman and DFP under mild conditions

Following a potentiometric determination of the relevant pKa values of the (R1R2)C=NOH functionality, the second order rate constants (k(Ox)) for reaction of a large set of oximate bases with two model organophosphorus esters, i.e. bis-(4-nitrophenyl)phenylphosphonate (BNPPP) and bis-(4-nitrophenyl)methylphosphonate (BNPMP), and three toxic compounds, i.e., sarin (GB), soman (GD) and diisopropylphosphorofluoridate (DFP), in aqueous as well as a 30 : 70 (v/v) H2O-Me2SO mixture have been measured. The corresponding Brønsted-type nucleophilicity plots of log k(Ox)vs. pKa(Ox) reveal a clear tendency of the reactivity of the oximates to suffer a saturation effect with increasing basicity in aqueous solution. In the case of BNPMP and the three toxic esters, this behaviour is reflected in a levelling off at pKa approximately 9 but a more dramatic situation prevails in the BNPPP system where the attainment of maximum reactivity at pKa approximately 9 is followed by a clear decrease in rate at higher pKas. Interestingly, a number of data reported previously by different authors for the sarin, soman and DFP systems are found to conform rather well to the curvilinear Brønsted correlations built with our data. Based on this and previous results obtained for reactions at carbon centers, it can be concluded that the observed saturation effect is the reflection of an intrinsic property of the oximate functionality. An explanation of this behavior in terms of an especially strong requirement for desolvation of the oximates prior to nucleophilic attack which becomes more and more difficult with increasing basicity is suggested. This proposal is supported by the observed changes in pKa(Ox) and k(Ox) brought about by a transfer from H2O to a 30 : 70 H2O-Me2SO mixture. The implications of the saturation effect on the efficiency of oximates as nucleophilic catalysts for smooth decontamination are emphasized. Also discussed is the effect of basicity on the exalted (alpha-effect) reactivity of these bases.

Electrochemical Oxidation ofσ-Complex-Type Intermediates in Aromatic Nucleophilic Substitutions

A series of sigma-adducts (1H-...7H-) derived from the addition of 2-nitropropenide ion to various nitrobenzofuroxans and nitrobenzofurazans have been oxidized electrochemically. The results show that the rearomatization of the carbocyclic ring of these adducts as well as that of a few additional 4,6-dinitrobenzofuroxan adducts (8 H- a-c) is associated with much higher oxidation potentials than found for the same process in the dinitro- and trinitrobenzene series. Especially high Eo values are measured for the oxidation of the 2-nitropropenide 4,6-dinitro- and 4-nitro-6-trifluoromethylsulfonylbenzofuroxan adducts 1H- and 4H- in acetonitrile: E (1H-)= 1.15 V versus SCE; Eo(4H-)=1.33V versus SCE. These values fit well with the available evidence that the chemical oxidation of these adducts requires the use of very strong oxidizing agents to proceed efficiently. The mechanism for the oxidation process has been established. It is shown to involve transfer of two electrons and liberation of one proton per sigma-complex precursor with no evidence whatsoever for the intermediacy of radical anionic species.

Cu(II) extraction by supercritical fluid carbon dioxide from a room temperature ionic liquid using fluorinated β-diketones

Copper(II) can be extracted in supercritical CO2 from a room temperature ionic liquid using CO2-philic fluorinated β-diketonate ligands; thanks to the 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIMTf2N) ionic liquid properties, there is no need to add modifiers to the neat supercritical CO2 to reach high extraction efficiencies.

The levelling effect of solvational imbalances in the reactions of oximate α-nucleophiles with electrophilic phosphorus centers. Relevance to detoxification of organophosphorus esters

A study of the reactions of oximate alpha-nucleophiles with diisopropylphosphorofluoridate (DFP) and two model phosphonates, has revealed either a levelling-off in reactivity or a bell-shaped behaviour in accordance with a critical decoupling of desolvation and bond formation (solvational imbalances); the relevance of these results to detoxification is emphasized.

Rapid Marcus Curvature Due to Extremely Strong Solvational Imbalances inthe Deprotonation of a Trinitrobenzylic Carbon Acid by Oximate Bases

We report the first instance of the levelling of oximate reactivity in a deprotonation process, which is shown to have as origin the desolvation of the oximate ion prior to actual proton transfer, in accord with the large predominance of the work term (wr) far exceeding the intrinsic barrier term in the Marcus formulation.

The low intrinsic reactivity of picrylacetone: an index for the π-acceptor capability of a 2,4,6-trinitrophenyl structure

Rates of deprotonation of picrylacetone, i.e., 2,4,6-trinitrophenylacetone 4, by a variety of bases B (phenoxide and carboxylate ions, OH–) and of protonation of the resulting carbanion (C-4) by the conjugated acids BH+ have been measured in a 50%H2O–50%Me2SO (v/v) mixture at 25 °C. The intrinsic reactivity (in the Marcus sense) of 4, as determined from the Bronsted plot for phenoxide reactions (βArO= 0.38) is low: log k0ArO= 2.37. This value is more typical for the formation of a strongly resonancestabilized benzyl-type carbanion than for that of an enolate-type carbanion and it may be regarded as a good measure of the high π-acceptor capability of a conjugated 2,4,6-trinitrophenyl structure. Definitive evidence that the negative charge of C-4 is essentially delocalized through the picryl moiety comes from the observation that this carbanion undergoes instantaneous protonation at its p-nitro group at low pH (pH < 6). The pKa′ value associated with the ionization of the resulting nitronic acid (C-4H) is ≈5, which compares well with similar data previously reported in the literature.

Similar catalytic behaviour of oximate and phenoxide bases in theionization of bis(2,4-dinitrophenyl)methane in 50% water– 50%Me2SO. Revisiting the role of solvational imbalances indetermining the nucleophilic reactivity of α-effect oximate bases

Rates of proton abstraction from bis(2,4-dinitrophenyl)methane 1 by a series of oximate bases and of reprotonation of the related diphenylmethyl carbanion C-1 by the conjugate oxime acids have been measured in a 50∶50 (v/v) H2O–Me2SO mixture at 25 °C. The results reveal an essentially identical behaviour of oximate (Ox-) and phenoxide (ArO-) bases in these proton transfer reactions. The Bronsted lines for the two types of catalysts are nearly the same, showing no tendency to level off at high pKa and providing very similar values for the intrinsic reactivity of 1: log k0Ox = 0.9 ± 0.1; log k0ArO = 0.5 ± 0.1. A comparison with previous results obtained with carboxylate ions as well as primary and secondary amines indicates that solvational imbalances due to the catalysts are important in determining the intrinsic reactivity of 1. This conclusion implies that ArO- and Ox- bases undergo comparable solvation changes along the coordinate of the ionization reactions of 1, a result which is indirectly supported by the finding that a transfer from water to H2O–Me2SO mixtures rich in Me2SO induces similar variations in the acidity of oximes and phenols of similar pKaH2O. The identical behaviour of ArO- and Ox- bases in the ionization of 1 is in marked contrast with the situation observed in nucleophilic addition or substitution reactions. In agreement with their α-effect character, oximates are much more reactive than phenoxides in these processes but a typical feature is that the nucleophilic reactivity of Ox- species is subject to a very rapid levelling off at pKa ca. 8.5. Based on the information obtained in this work, this peculiar behaviour is re-examined and suggested to be a reflection of especially large solvational imbalances in the transition states for nucleophilic reactions.

THE NITROALKANE BEHAVIOUR OF (4-NITROPHENYL)NITROMETHANE : A KINETIC AND STRUCTURAL STUDY IN H2O-ME2SO MIXTURES

Rates of proton abstraction from (4-nitrophenyl)nitromethane (1) by a variety of bases B (phenoxide and carboxylate ions, primary amines) and of reprotonation of the resulting carbanion (C-1) by the conjugate acids BH have been measured in a 50∶50 (v/v) H2O–Me2SO mixture at 25 °C. In contrast with the situation at pH 4.2 where only one relaxation time corresponding to a simple equilibrium approach according to 1 + B ⇌ C-1 + BH is observed, the interconversion of 1 and C-1 proceeds in two steps in acidic media. Kinetic studies as well as data obtained by 1H and 13C NMR or UV–VIS spectroscopy indicate that the first step corresponds to the protonation of the exocyclic nitro group of C-1, giving rise to the corresponding nitronic acid C-1a,H. All rate and equilibrium parameters obtained for the aci-nitro behaviour of 1, including the log k0 values measuring its intrinsic reactivity towards the various types of bases studied, emphasize a nitroalkane behaviour rather than a 4-nitrotoluene behaviour of this carbon acid in 50∶50 (v/v) H2O–Me2SO. Although they clearly reveal some contribution of the nitrophenyl ring to the stabilization of C-1 in Me2SO, the NMR data remain consistent with a major role of the exocyclic nitro group in governing the ionization behaviour of 1 in this dipolar aprotic solvent.

The extremely low intrinsic reactivity of the P -(formylmethyl)triphenylphosphonium cation: an artefact due to strong covalent hydration of the carbonyl group

The title cation is found to exhibit the lowest Marcus intrinsic reactivity known so far for a carbon acid; this situation is shown to reflect a rate-limiting hydration/dehydration reaction of the aldehyde functionality.