Abstract:
The inverse electron demand Diels-Alder reaction has recently attracted increasing research interest as it allows the stereospecific synthesis of useful compounds. Ionic liquids have recently been proposed as an alternative class of solvents to traditional organic solvents, due to their beneficial features that include low vapour pressure, low volatility, and high thermal stability. Ionic liquids display interesting structural features such as an array of alternating cations and anions as well as the formation of well-defined polar and nonpolar domains.
The aim of this project was to investigate the effect of ionic liquid nanostructure on the reaction rate of the inverse electron demand Diels-Alder reaction. Ionic liquids with long alkyl chains can display well-defined polar and nonpolar domains within the nanostructure. The reaction between acridizinium bromide and cyclopentadiene was chosen as the model reaction. The reaction was studied in acetonitrile as well as [OTf], [NTf2] and [N(CN)2]-based imidazolium ionic liquids of different alkyl chain lengths and analysed by UV-VIS spectroscopy to determine second-order rate constants. The C4 variants of these ionic liquids showed similar reaction rates to acetonitrile. The [OTf] and [NTf2]-based C10 variants significantly increased the rate of reaction compared to C4 variants and acetonitrile, suggesting that longer alkyl chain lengths led to increased rate of reaction. This was attributed to the longer alkyl chains providing a more well-defined nonpolar environment, which led to an increase in reactivity. It was also suggested the increased polarity of [OTf]− brought upon stronger cation-anion interactions, increasing hydrophobic interactions between the reactants which would accelerate the reaction due to the smaller volumes of the transition state formed compared to the isolated reactants.
The [N(CN)2]-based ionic liquids displayed inconsistent kinetic data based on the pseudo-first order kinetic model, and it was initially hypothesized that the silver content from the synthesis was a source of contamination. ICP-MS results revealed that the silver content was minimal and ruled out as a source of contamination. The hypothesis of the high viscosity providing inconsistent kinetic results observed with [N(CN)2]-based ionic liquids remain an area to be explored beyond the scope of this project. Overall, this project provides evidence of the ionic liquid amphiphilic nanostructure influencing the relative stability of the reactants and transition state in the model reaction studied.