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

Moutiers, Gilles; Guével, Eric Le; Villien, Luc; Terrier, François

doi:10.1039/a605249e

Cited by 18 publications

(10 citation statements)

References 55 publications

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“…(7). For example, the N values of 9.69, 8.62, and 5.88 have been estimated for 4‐methylphenoxide ion (p K a = 10.19) , 4‐bromophenoxide ion (p K a = 9.34) [35], and 4‐nitrophenoxide ion (p K a = 7.14) , respectively. As can be seen, these values agree well with those calculated through to the N versus correlation (Eq.…”

Section: Resultsmentioning

confidence: 99%

Nucleophilicities of Para-Substituted Phenoxide Ions in Water and Correlation Analysis

Gabsi

Boubaker

Goumont

2014

Int. J. Chem. Kinet.

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Second-order rate constants for the reactions of 2-aryl-4,6-dinitrobenzotriazole 1-oxides 1a-d with some 4-X-substituted phenoxide ions 2a-d (X = OCH 3 , H, Cl, and CN) have been measured in aqueous solution at 20 • C. The pK a values for the σ -complexation processes of a series of benzotriazole 1a-d measured in water have been used to determine their electrophilicity parameters E according to the correlation E = -3.20 -0.662 pK a (F. Terrier, S. Lakhdar, T. Boubaker, and R. Goumont, J Org Chem, 2005, 70, 6242-6253). For these reactions, plots of log k versus the electrophilicity parameters E of the benzotriazoles 1a-d were linear, allowing to derive the nucleophilicity parameters N and s for phenoxide ions as defined by the Mayr equation log k 1 (20 • C) = s (E + N) (H. Mayr, M. Patz. Angew Chem, Int Ed Engl 1994, 33, 938-957). The N values are found to cover a range of nucleophilicity from 6.85 to 10.22, going from 4-cyanophenoxide 2d for the least reactive ion to 4-methoxyphenoxide 2a for the most reactive nucleophile. Good linear correlations were found between the nucleophilicity parameters N of phenoxide ions 2a-d and the pK a values of their conjugate acids (N = -3.05 + 1.25 pK a ) and the σ p constants of the substituents X (N = 9.21 -2.51σ p ). C 2014 Wiley Periodicals, Inc. Int J Chem Kinet 46: 711-717, 2014

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Section: Resultsmentioning

confidence: 99%

Nucleophilicities of Para-Substituted Phenoxide Ions in Water and Correlation Analysis

Gabsi

Boubaker

Goumont

2014

Int. J. Chem. Kinet.

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“…Another important discovery in this regard was the fact that in many processes the reactivity of α‐nucleophiles such as oximates in aqueous solution is subject to a saturation effect 18−21. This behaviour prompted the suggestion that asynchronicity of nucleophile desolvation and bond formation in the related transition states is also a major factor governing the α‐effect 9,17−22…”

Section: Methodsmentioning

confidence: 99%

The α-Effect in SNAr Substitutions − Reaction between Oximate Nucleophiles and 2,4-Dinitrofluorobenzene in Aqueous Solution

et al. 2001

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The second-order rate constants (k 1 ArO , k 1 Ox ) for S N Ar substitutions of 2,4-dinitrofluorobenzene (DNFB) by a series of phenoxide and oximate nucleophiles have been measured in aqueous solution at 25°C, using both a potentiometric procedure involving the use of a fluoride ion selective electrode (FISE) and a classical spectrophotometric procedure. While the rate data for the phenoxide ions conform to a linear Brönsted plot with a slope (β Nu = 0.71) fitting the 0.5−0.7 range commonly found for S N Ar reactions, those for the various oximates studied do not define a meaningful linear plot. Interestingly, the observed variations in k 1 Ox reveal a tendency of the reactivity of oximates of pK a Ͼ 7.5−8 to level off rapidly, a situation reminiscent of that encountered in other Since Edwards and Pearson first drew attention to the enhanced reactivity of nucleophiles possessing a heteroatom with an unshared pair of electrons adjacent to the nucleophilic atom, numerous studies of the so-called α-effect phenomenon have been reported. [1Ϫ7] In the first review of this phenomenon, [3] it was noted that the rate constant ratio, k OOH Ϫ /k OH Ϫ , for nucleophilic attack on carbon substrates in different states of hybridization increased in the order sp 3 Ͻ sp 2 Ͻ sp, as represented by substrates of types ArCH 2 Br, ArC(O)OR, and ArCN, respectively. While the k α-Nu / k normal-Nu rate ratio subsequently became understood as referring to nucleophiles with the same pK a Ϫ i.e. as a positive deviation exhibited by an α-nucleophile from a Brönsted-type nucleophilicity plot Ϫ the importance of the hybridization of the carbon centre became firmly established as a factor determining the magnitude of the α-effect. [5Ϫ9] Systematic studies have also revealed the importance of the α-effect in nucleophilic reactions at sulfur and phosphorus centres with consequent biological and decontamination applications. [8Ϫ13] Several factors have been recognized as possible contributors to the α-effect phenomenon: ground-state destabilization of the α-nucleophile, transition-state stabilization, thermodynamic stability of products, and solvation differences of the nucleophiles. [5,8,14Ϫ16] Extensive discussions of these [ ‡] Visiting professor at the University of Versailles.[a] SIRCOB (ESA 3279 nucleophilic reactions of these species at carbonyl and phosphonyl centres. Our current finding reinforces the idea of a general oximate behaviour pattern originating from an especially strong need for partial desolvation before nucleophilic attack, i.e., asynchronicity or TS imbalance. A major consequence of the observed levelling off is that the extra reactivity reflecting the α character of oximate nucleophiles decreases significantly in magnitude on going from weakly basic oximates (k 1 Ox /k 1 ArO ഠ 100) to strongly basic ones (k 1 Ox /k 1 ArO ഠ 10). On the basis of the k 1 Ox /k 1 ArO ratio measured at low pK a , the α-effect associated with the S N Ar substitution of DNFB is of the same order as that measured for other reactions of oxim...

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“…Indeed, entropy–enthalpy compensation phenomenon observed in reactivation of phosphorylated butyrylcholinesterase suggested the displacement of water molecules and conformational change during the oxime binding process of reactivation [ 43 ]. Acidity and kinetic measurements of proton transfer in water and water-DMSO mixtures for oxime reactions also support the oxime desolvation hypothesis prior to nucleophilic attack on electrophilic phosphorus atoms [ 23 , 44 , 45 ].…”

Section: Resultsmentioning

confidence: 75%

Water structure changes in oxime-mediated reactivation process of phosphorylated human acetylcholinesterase

2018

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The role of water in oxime-mediated reactivation of phosphylated cholinesterases (ChEs) has been asked with recurrence. To investigate oximate water structure changes in this reaction, reactivation of paraoxon-inhibited human acetylcholinesterase (AChE) was performed by the oxime asoxime (HI-6) at different pH in the presence and absence of lyotropic salts: a neutral salt (NaCl), a strong chaotropic salt (LiSCN) and strong kosmotropic salts (ammonium sulphate and phosphate HPO42−). At the same time, molecular dynamic (MD) simulations of enzyme reactivation under the same conditions were performed over 100 ns. Reactivation kinetics showed that the low concentration of chaotropic salt up to 75 mM increased the percentage of reactivation of diethylphosphorylated AChE whereas kosmotropic salts lead only to a small decrease in reactivation. This indicates that water-breaker salt induces destructuration of water molecules that are electrostricted around oximate ions. Desolvation of oximate favors nucleophilic attack on the phosphorus atom. Effects observed at high salt concentrations (>100 mM) result either from salting-out of the enzyme by kosmotropic salts (phosphate and ammonium sulphate) or denaturing action of chaotropic LiSCN. MDs simulations of diethylphosphorylated hAChE complex with HI-6 over 100 ns were performed in the presence of 100 mM (NH4)2SO4 and 50 mM LiSCN. In the presence of LiSCN, it was found that protein and water have a higher mobility, i.e. water is less organized, compared with the ammonium sulphate system. LiSCN favors protein solvation (hydrophobic hydration) and breakage of elelectrostricted water molecules around of oximate ion. As a result, more free water molecules participated to reaction steps accompanying oxime-mediated dephosphorylation.

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

Cited by 18 publications

References 55 publications

Nucleophilicities of Para-Substituted Phenoxide Ions in Water and Correlation Analysis

Nucleophilicities of Para-Substituted Phenoxide Ions in Water and Correlation Analysis

The α-Effect in SNAr Substitutions − Reaction between Oximate Nucleophiles and 2,4-Dinitrofluorobenzene in Aqueous Solution

Water structure changes in oxime-mediated reactivation process of phosphorylated human acetylcholinesterase

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