(64b) Ionic Liquids in Continuous Flow Microreactors

Over the last years, ionic liquids (ILs) have attracted increasing attention due to their thermal stability, non-volatility and almost negligible vapor-pressure. Numerous reports on various applications since have been published. Most of them decribe the employment of ILs as alternatives to highly polar solvents for synthetic applications. However, the preparation and subsequent work-up of these compounds is tedious and time-consuming, mostly because purification cannot be achieved by simple distillation methods like for conventional organic solvents. Here we report on the development of an alternative approach for the facile synthesis of various novel imidazolium-based ILs that affords highly pure products without the necessity of subsequent purification steps. The highly exothermic quarternization reaction involving the attack of an alkylating agent (typically an alkyl bromide) on an imidazolium species frequently leads to the formation of undesired side products when the reaction is carried out in a standard batch reactor. Microreactors offer a high degree of control over such reactions because of their high surface-to-volume ratio and superior heat- and mass transport. 1-hexyl-3-methylimidazolium bromide was used as a model compound and prepared in a microreactor under continuous flow conditions in order to evaluate the ideal reaction conditions. Based on the afforded results, we prepared several novel ILs by reacting different imidazoles with alkyl and vinyl bromides. Within the scope of these studies, we focused on ILs containing differently substituted phenyl rings and characterized these compounds with resepct to further use for direct application or subsequent reaction sequences.