A systematic approach to understanding organic reactivity in ionic liquids: changes in cybotacticity-induced solvent heterogeneity as an important determinant In reaction outcomes of substitution processes

摘要

Described herein is the initial development towards a general framework for predicting changes in organic reactivity in ionic liquids through the mechanistic studies of several representative substitution processes with well-defined mechanisms.The temperature-dependent kinetic behaviour of the ethanolysis of 1-fluoro-2,4-dinitrobenzene as a model nucleophilic aromatic substitution reaction was examined in molecular and ionic solvents. Activation parameters obtained and relevant molecular dynamics simulations performed highlight the importance of cybotactic regions characterised by anion-π interaction.Activation parameters and free energy relationships for the Menschutkin reactions between pyridine and a series of substituted benzyl bromides and unsubstituted benzyl halides in both molecular and ionic liquid solvents were investigated. The Menschutkin reaction was found to be entropically favoured in ionic liquid regardless of the leaving group involved.By systematically replacing the aromatic starting materials with non-aromatic ones, the effect of delocalised electron density in either the electrophile or the nucleophile of the aforementioned Menschtkin reaction was evaluated. Cybotacticity associated with nitrogen centre in the nucleophiles and related changes were found to be responsible for changes in reactivity observed on changing solvent to the ionic liquid and are consistent with related molecular dynamics simulations of the starting materials.Kinetic analyses were also carried out for the reaction between 1-(1-chloroethyl)-4-methoxybenzene and pyridine, which proceeds through both unimolecular and bimolecular pathways simultaneously. The results are discussed in relation to the bimolecular Menschutkin reactions presented in the early chapters of this thesis as well as previous work on a unimolecular substitution process and the potential for solvent-controlled reactivity is discussed.The Menschutkin reaction between benzyl bromide and pyridine was investigated in ionic liquids composed of either a 1,3- dialkylimidazolium cation or a 1,2,3-trialkylimidazolium cation. The generality of electrostatic interactions and the impact of charge density were examined based on the activation parameters obtained from temperature-dependent kinetics analysis.A general “one-pot” method for obtaining relative rates of reaction in complex mixtures has been established through the construction of free energy relationships for various reactions under competitive conditions. The advantages of the technique, compared to existing methods in the literature, are discussed in terms of accuracy, efficiency and applicability.