Rationalising the effects of ionic liquids on a nucleophilic aromatic substitution reaction

Abstract

The nucleophilic aromatic substitution reaction between 1-fluoro-2,4-dinitrobenzene and ethanol was examined in a series of ionic liquids across a range of mole fractions. Temperature-dependent kinetic analyses were undertaken to determine the activation parameters for this reaction at the highest mole fraction. As the mole fraction of ionic liquid was increased, the rate constant of the reaction also increased, however the microscopic origin of the rate enhancement was shown to be different between different ionic liquids and also between different solvent compositions. These results indicate a balance between microscopic interactions that result in the observed solvent effects and a qualitative method for analysing such interactions is introduced.

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.…”

“…Importantly, these data do not suggest a lack of interaction between the cation of [Bmim][N(SO 2 CF 3 ) 2 ] 6 and the starting materials of the process. Whilst interactions between the cation and the nucleophile are not implicit from the activation parameter data (which in any case are based on small rate constant changes), understanding gained from previous examples,,, along with the reduced rate constant enhancement observed relative to the imidazolium salt 6 case, does suggest interactions between the nucleophile and the cation of the salt occur.…”

“…Whether or not the magnitude of each of the possible interactions has changed is unclear based on these data alone . When the same salt was used in the related aromatic substitution involving an oxygen nucleophile (Scheme ),, there was a decrease in both activation parameters relative to the molecular solvent case indicating that the dominant interaction was of the ionic liquid 7 with the transition state of the process; changing the nature of the anion of the salt also implied that it was involved in these interactions . The difference in this case may be due to the nature of the transition state, though this seems less likely than potential additional anion‐starting material interactions possible between the amine 4 a and the anion.…”

“…Overall, the work in this section shows that for S N Ar reactions containing a nitrogen nucleophile, ionic liquid‐starting material interactions are responsible for the changes in reaction outcome across all proportions of the salt 7 considered ( cf . ionic liquid‐transition state interactions at high salt concentrations in the case of oxygen nucleophiles) ,. Whilst specific sites of interactions are difficult to isolate, those involving both the cation and anion are implied.…”

“…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.…”