The effect of varying the anion of an ionic liquid on the solvent effects on a nucleophilic aromatic substitution reaction

Abstract

A variety of ionic liquids, each containing the same cation but a different anion, were examined as solvents for a nucleophilic aromatic substitution reaction. Varying the proportion of ionic liquid was found to increase the rate constant as the mole fraction of ionic liquid increased demonstrating that the reaction outcome could be controlled through varying the ionic liquid. The solvent effects were correlated with the hydrogen bond accepting ability (β) of the ionic liquid anion allowing for qualitative prediction of the effect of changing this component of the solute. To determine the microscopic origins of the solvent effects, activation parameters were determined through temperature-dependent kinetic analyses and shown to be consistent with previous studies. With the knowledge of the microscopic interactions in solution, an ionic liquid was rationally chosen to maximise rate enhancement demonstrating that an ionic solvent can be selected to control reaction outcome for this reaction type.

The Dependence of Ionic Liquid Solvent Effects on the Nucleophilic Heteroatom in SNAr Reactions. Highlighting the Potential for Control of Selectivity

“…Such an effect is consistent with the changes observed here. For the S N Ar reaction shown in Scheme involving an oxygen nucleophile, at low proportions of added salt, starting material‐ionic liquid interactions dominate, while at high proportions of ionic liquid, interactions with the transition state dominate ,. As such, the interactions implicit from the activation parameters (and the resulting rate constants) for an S N Ar process involving a nitrogen nucleophile are consistent with a combination of the previously observed ionic liquid effects.…”

“…This trend in the mole fraction dependence of the rate constant is markedly different from the case for the reaction shown in Scheme where an oxygen rather than nitrogen nucleophile was used, where the bimolecular rate constant continued to increase as the amount of ionic liquid in the reaction mixture increased –. Instead, the observed trend is more akin to what has been reported for bimolecular processes involving nitrogen nucleophiles, suggesting that the dominant interaction(s) driving the ionic liquid solvent effects in these systems may be the same.…”

“…For the previously studied S N Ar reaction involving ethanol as the nucleophile (Scheme ) the same trend in the activation parameters (an increase) was observed on addition of small amounts of ionic liquid in the reaction mixture ( χ 6 <0.5) –. Those changes in activation parameters were also attributed to ionic liquid‐starting material interactions, particularly the anion of the ionic liquid interacting with the electrophile 3 .…”

“…In an attempt to further understand the ionic liquid‐starting material interactions observed in the S N Ar reaction discussed above, the mole fraction dependence of the rate constant and temperature dependent kinetic studies conducted above were carried out with an ionic liquid containing a different cation; tetraoctylammonium bis (trifluoromethanesulfonyl)imide 7 ). The [TOA] + cation has a particularly hindered charge centre and this ionic liquid has been used previously to observe the effects of minimising potential interactions with the cation of an ionic liquid ,–,. The hindered nature of the charged centre of [TOA] + also means that the anion of the ionic liquid would be expected to associate less with the cation and thus be more available to interact with species along the reaction coordinate.…”

“…Previous studies have investigated the S N Ar reaction between ethanol and 1‐fluoro‐2,4‐dinitrobenzene 1 (Scheme ) in mixtures containing one of a range of different ionic liquids . In those studies, the key interactions responsible for rate constant enhancement were shown to vary with the proportion of the ionic liquid added,, although at all solvent compositions containing an ionic liquid an increase in the rate constant was observed compared to the ethanol case.…”