The effects of ionic liquids on the ethanolysis of a chloroacenaphthene. Evaluation of the effectiveness of nucleofugality data to predict reaction outcome

The reaction of a chlorobenzene in mixtures containing ethanol and eight different ionic liquids was investigated in order to understand the effects of varying proportions and constituent ions of an ionic liquid on the rate constant of the process. The results were found to be generally consistent with previously studied reactions of the same type, with small proportions of an ionic liquid resulting in a rate constant increase compared to ethanol and large proportions causing a rate constant decrease. Temperature dependent kinetic studies were used to interpret the changes in reaction outcome, particularly noting an entropic cost on moving to high proportions of ionic liquid, consistent with organisation of solvent around the transition state. While attempts to use empirical solvent parameters to correlate outcome with the ionic liquid used were unsuccessful, use of recently acquired nucleofugality data for chloride and estimations for the electrofuge allowed for excellent prediction of the effects of ionic liquids, with rate constants quantitatively predicted in systems containing both different proportions of ionic liquid (mean absolute error (MAE) log(k₁) = 0.11) and different ionic liquids (MAE log(k₁) = 0.33). Importantly, this demonstrates the ready application of these quantitative reactivity parameters.

Regulating the Acidity of SO3 H-Functionalized Ionic Liquids: Hydrogen Bonding or Electrostatic Potential?

The SO₃ H-functionalized ionic liquids (ILs) with high acidity are important catalysts for acid-catalyzed reactions. However, the acidity of these ILs have been found to decrease due to intramolecular hydrogen bonds (H-bonds). In this work, a series of anionic SO₃ H-functionalized ILs were designed and the factors resulting in weak acidity were investigated, including H-bonds strength and electrostatic potential on the leaving proton (ESPLP). Using catalytic experiment, atoms in molecules topology analysis and electrostatic potential calculation, it was found that the acidity of ILs was correlated with the value of ESPLP rather than the H-bond strength. Meanwhile, there were several ways to increase the acidity of anionic SO₃ H-functionalized ILs, such as enhancing the electron-withdrawing ability of cation, introducing H-bond acceptor sites on cation or separating the intramolecular H-bonds through substitution position. These strategies made the conversion of TBA in Friedel-Crafts alkylation increase from 19 % to 84 %.

Ionic Liquids as Solvents for SN 2 Processes. Demonstration of the Complex Interplay of Interactions Resulting in the Observed Solvent Effects

Bimolecular nucleophilic substitution reactions between triphenylphosphine and benzylic electrophiles have been examined in an ionic liquid to probe interactions with species along the reaction coordinate. Trends in the rate constant were found on both varying the leaving group and the electronic nature of the aromatic ring. In all the cases considered, interactions between the components of the ionic liquid and the transition state were shown to be more significant in determining reaction outcome than previously observed for this class of reaction. This demonstrates the importance of considering interactions of the ionic liquid components with all species along the reaction coordinate when investigating the origin of ionic liquid solvent effects, along with how such effects might be exploited.

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

A series of nucleophiles containing Group 15 nucleophilic heteroatoms has been used to expand and develop the current understanding of ionic liquid solvent effects on bimolecular nucleophilic substitution processes. It was found that when using arsenic-, antimony- and bismuth-based nucleophiles, rate constant enhancement was observed for all solvent compositions containing ionic liquids. This rate constant enhancement was driven by ionic liquid/transition state interactions, which contrasts with previous studies on earlier Group 15 nucleophiles. This study provides a holistic understanding and augments the predictive framework for the effects of ionic liquids on bimolecular nucleophilic substitution processes, with the potential for these periodic trends to be broadly applied.

Controlling the outcome of SN2 reactions in ionic liquids: from rational data set design to predictive linear regression models

Rate constants for a bimolecular nucleophilic substitution (SN2) process in a range of ionic liquids are correlated with calculated parameters associated with the charge localisation on the cation of the ionic liquid (including the molecular electrostatic potential). Simple linear regression models proved effective, though the interdependency of the descriptors needs to be taken into account when considering generality. A series of ionic liquids were then prepared and evaluated as solvents for the same process; this data set was rationally chosen to incorporate homologous series (to evaluate systematic variation) and functionalities not available in the original data set. These new data were used to evaluate and refine the original models, which were expanded to include simple artificial neural networks. Along with showing the importance of an appropriate data set and the perils of overfitting, the work demonstrates that such models can be used to reliably predict ionic liquid solvent effects on an organic process, within the limits of the data set.

Understanding the effects of solvate ionic liquids as solvents on substitution processes

The effects of solvate ionic liquids as solvents have been considered for two substitution processes where the solvent effects of typical ionic liquids have been extensively investigated previously; the bimolecular nucleophilic substitution (SN2) reaction between pyridine and benzyl bromide and the nucleophilic aromatic substitution (SNAr) reaction between ethanol and 1-fluoro-2,4-dinitrobenzene. It was found that use of solvate ionic liquids gave rise to similar trends in the activation parameters for both substitution processes as typical ionic liquids, implying the microscopic interactions responsible for the effects were the same. However, different effects on the rate constants compared to typical ionic liquids were observed due to the changes in the balance of enthalpic and entropic contributions to the observed rate constants. From these data it is clear that the reaction outcome for both of these substitution reactions fall within the 'predictive framework' established in previous studies with a cautionary tale or two of their own to add to the general knowledge of ionic liquid solvent effects for these processes, particularly with respect to potential reactivity of the solvate ionic liquids themselves.

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

Nucleophilic aromatic substitution (S_N Ar) reactions of 1-fluoro-4-nitrobenzene using similar nitrogen and sulfur nucleophiles were studied through extensive kinetic analysis in mixtures containing ionic liquids. The interactions of the ionic liquid components with the starting materials and transition state for each process were investigated in an attempt to construct a broad predictive framework for how ionic liquids affect reaction outcome. It was found that, based on the activation parameters, the microscopic interactions and thus the ionic liquid solvent effect were different for each of the nucleophiles considered. The results from this study suggest that it may be possible to rationally select a given ionic liquid mixture to selectively control reaction outcome of an S_N Ar reaction where multiple nucleophiles are present.

Understanding the effects of ionic liquids on a unimolecular substitution process: correlating solvent parameters with reaction outcome

The polarisability of an ionic liquid is key in determining the rate constant of a unimolecular substitution process.

Ion-Reagent Interactions Contributing to Ionic Liquid Solvent Effects on a Condensation Reaction

Molecular dynamics simulations of solutions of hexan-1-amine or 4-methoxybenzaldehyde in acetonitrile, an ionic liquid/acetonitrile mixture (χIL =0.2), and a number of different (neat) ionic liquids were performed, to further understand the solvent effects on the condensation reaction of these species. This work indicates that, in the presence of an ionic liquid, the amine group of hexan-1-amine is exclusively solvated by the components of the ionic liquid, and not by acetonitrile, and that the anion interacts with the aldehyde group of 4-methoxybenzaldehyde. These interactions showed little dependence on the proportion of the ionic liquid present. When varying the cation of the ionic liquid there were changes in the cation-amine interaction, and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ([Bm2 im][N(CF3 SO2 )2 ]) was found to order more than expected about the amine. This ordering is likely the origin of the large rate constant values determined in [Bm2 im][N(CF3 SO2 )2 ] for this condensation reaction and explains an anomaly seen previously. When changing the anion, changes were seen in the interactions between both the cation and anion with hexan-1-amine, and the anion with 4-methoxybenzaldehyde. The differing magnitude of these interactions likely causes subtle changes in the activation parameters for this condensation reaction, and provides an explanation for the anomalous rate constant values previously determined when varying the anion.

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

Variety of ionic liquids with different anions used as solvents for a nucleophilic aromatic substitution reaction.