Nucleophilic Reactions at Cationic Centers in Ionic Liquids and Molecular Solvents

Abstract

We report a quantitative comparison of the rates of nucleophilic reactions in ionic liquids and molecular solvents taking place at a cationic center. Cationic sulfonium electrophiles were reacted with three amine nucleophiles (n-butylamine, di-n-butylamine, and tri-n-butylamine) in several molecular solvents (toluene, dichloromethane, tetrahydrofuran, acetonitrile, and methanol) and ionic liquids {1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, and 1-butyl-3-methylimidazolium trifluoromethanesulfonate}. The solvent effects on these reactions are examined using a linear solvation energy relationship based on the Kamlet−Taft solvent scales (α, β, and π*). These correlations reveal that hydrogen-bonding interactions provide the dominant effects in determining the rate of reaction. In particular, hydrogen bonds donated by the sulfonium electrophile to the solvent are the most important controlling factor on the rate of nucleophilic reaction.

Ionic Liquids Entrapped in Reverse Micelles as Nanoreactors for Bimolecular Nucleophilic Substitution Reaction. Effect of the Confinement on the Chloride Ion Availability

“…As a chemical probe, we use the nucleophilic substitution of Cl − on dimethyl-4-nitrophenylsulfonium (S + in Scheme 2). 43,44 Our results point that Cl − nucleophilicity in toluene/BHDC/ bmimCl RMs is lower than in neat IL, as a consequence of ion-pairs formation between Cl − and the cationic head group of the surfactant. In the case of toluene/BHDC/bmimBF 4 , the reactivity is mainly governed by an ion-exchange process between Cl − and BF 4 − , in such a way that the composition of the entrapped IL hardly depends on the surfactant concentration.…”

Charge Screening in the SN2 Reaction of Charged Electrophiles and Charged Nucleophiles: An Ionic Liquid Effect

“…This is most likely due to these solvents being able to form strong hydrogen bonds with the nucleophile, thus greatly reducing reactivity. 5 In fact, the sulfonium chloride salt could be isolated as a stable species and dissolved in D 2 O for NMR analysis without any decomposition products detectable after 1 week. Hence, it is only in ionic liquids that pseudo-first order kinetic behavior for this nucleophilic substitution reaction is seen.…”

“…Similarly, the reactions of neutral nucleophiles with ionic electrophiles showed no special ionic liquid effect. 5 We report here the reaction of chloride ions with an ionic electrophile, which provides the first demonstration of just such an "ionic liquid effect" on a chemical reaction.…”

“…In spite of efforts to detect such a phenomenon, detailed quantitative investigations on the effects of ionic liquids on reactions of solutes have repeatedly demonstrated that the results can be correlated with, and hence rationalized in terms of, polarity behaviours originally derived to describe molecular solvents. 4,5 In particular, we have previously reported quantitative kinetic studies of nucleophilic substitutions by halides in reactions with neutral nucleophiles in a range of ionic liquids, showing that both cation and anion variation can change the nucleophilicity of the halide ion. 4 By comparing the kinetic data obtained against empirical solvent polarity measurements, 4 it became apparent that this could be understood by the application of the classical Hughes-Ingold predictions 4 of solvent polarity on reaction rates combined with an analysis of hydrogen-bonding effects, with the ionic liquid acting in the same way as polar molecular solvents.…”

Esterification in Ionic Liquids: The Influence of Solvent Basicity

“…23,24 Recently, LSER methods have also been applied to ILs in the investigation of chromatography stationary phases, 25 molar volume 26 and gas solubilities. 27 The general form of an LSER is shown in eq 1, where XYZ represents some solvent dependant property (herein, the natural logarithm of the rate constant).…”