The effects of an ionic liquid on unimolecular substitution processes: the importance of the extent of transition state solvation

Abstract

The reaction of bromodiphenylmethane and 3-chloropyridine, which proceeds concurrently through both unimolecular and bimolecular mechanisms, was examined in mixtures of acetonitrile and an ionic liquid. As predicted, the bimolecular rate constant (k2) gradually increased as the amount of ionic liquid in the reaction mixture increased, as a result of a minor enthalpic cost offset by a more significant entropic benefit. Addition of an ionic liquid had a substantial effect on the unimolecular rate constant (k1) of the reaction, with at least a 5-fold rate enhancement relative to acetonitrile, which was found to be due to a significant decrease in the enthalpy of activation, partially offset by the associated decrease in the entropy of activation. This is in contrast to the effects seen previously for aliphatic carbocation formation, where the entropic cost dominated reaction outcome. This change is attributed to a lessened ionic liquid-transition state interaction, as the incipient charges in the transition state were delocalized across the neighbouring π systems. By varying the mole fraction of ionic liquid in the reaction mixture the ratio between k1 and k2 could be altered, highlighting the potential to use ionic liquids to control which pathway a reaction proceeds through.

A mild and efficient procedure for alkenols oxyselenocyclization by using ionic liquids

“…In contrast to the stabilization of the cyclic ether product caused by the NTf 2 acceptance of the proton, the protonated cyclic ether form in unsupported reaction is energetically very unfavorable in its transformation to the final product by later deprotonation. This behavior has been observed for other reported reactions in ILs, where the IL effect is correlated with the hydrogen bond accepting ability of the IL anion, as well as the transition state IL solvation …”

Investigating Variation of the Pnicogen Nucleophilic Heteroatom on Ionic Liquid Solvent Effects in Bimolecular Nucleophilic Substitution Processes

“…χ 2 >0.2 is unusual and resembles trends observed for unimolecular substitution processes ,,. These observations could intimate that there are similar interactions occurring in the triphenylantimony 1 d system that are responsible for the observed effects on the rate constant as have been noted in the unimolecular substitution processes (those between the ionic liquid and the transition state), once again this shall be discussed subsequently when further data is presented.…”

Ionic Liquids as Solvents for SN2 Processes. Demonstration of the Complex Interplay of Interactions Resulting in the Observed Solvent Effects

“…The former argument is considered more likely given the charge development in the transition state ( Figure 2) and the potential for either the cation or the anion of the ionic liquid to interact with that charge development. [29] For the reactions studied here these results demonstrate that such transition state-ionic liquid interactions must be taken into account when considering solvent effects in these systems before using ionic liquids to promote the rate of reaction in a synthetic scenario. Significantly, the dominance of these interactions (which have been demonstrated by the difference in effects of the salt in the bromide 2 d and chloride 6 d cases) is novel for an S N 2 process with a neutral nucleophile, though some previous studies have suggested similar interactions result in rate constant enhancement.…”

“…[21] Rather, such activation parameter changes have been seen before for S N 1 processes where a greater degree of charge development in the transition state would be expected. [29] For the reactions studied here these results demonstrate that such transition state-ionic liquid interactions must be taken into account when considering solvent effects in these systems before using ionic liquids to promote the rate of reaction in a synthetic scenario.…”