Abstract
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.
Highlights
Ionic liquid solvents affect the reaction outcome of organic reactions differently to the more commonly-used molecular solvents.[1,2,3,4,5,6] Ionic liquids are composed of ions,[7, 8] leading electrostatic interactions to dominate, often affording favourable effects on reaction rates and increased selectivity in the formation of products
The rate constant data for the reaction shown in Scheme 1 at 295 K in mixtures containing the ionic liquids having the cations shown in Figure 1 are summarised in Table 1; these cations have been organised by structure to allow the effect of changes to structural features to be readily seen
It should be noted that, in all cases, the anion is bis(trifluoromethanesulfonyl)imide ([N(SO2CF3)2]-); the solvent effects on the reaction in Scheme 1 are known for a wide range of ionic liquids based on this anion, at least in part because it is it is relatively non-coordinating and the cation is relatively 'available', maximising the key interaction responsible for rate constant enhancement
Summary
Ionic liquid solvents affect the reaction outcome of organic reactions differently to the more commonly-used molecular solvents.[1,2,3,4,5,6] Ionic liquids are composed of ions,[7, 8] leading electrostatic interactions to dominate, often affording favourable effects on reaction rates and increased selectivity in the formation of products. Systematic analysis of a variety of ionic liquids on a range of different reactions (examples include: alkyl substitution,[10,11,12,13,14,15,16,17,18,19,20] aromatic substitution,[21,22,23,24,25,26,27,28,29,30] condensation[31,32,33] and pericyclic[34,35,36,37,38,39,40] processes) has provided a greater understanding of the effects a.Department of Chemical and Environmental Engineering, University of Nottingham, NG7 2RD Nottingham, United Kingdom b.School of Chemistry, University of New South Wales, UNSW Sydney, 2052, Australia c.
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