Ionic Liquid and Deep Eutectic Solvent Silica Microcapsules and their Application as Heterogeneous Organocatalysts in Various Chemical Transformations

In recent times, ionic liquids (ILs) and deep eutectic solvents (DESs) have gained considerable attention as alternatives to traditional organic solvents used in synthesis owing to their unique properties, which include high thermal and chemical stability, low vapor pressure and high versatility. Moreover, several investigations demonstrated the effectiveness of ILs and DESs as organocatalysts in diverse catalytic transformation. Unfortunately, most of the ILs and DESs exhibit high viscosities, which both complicate and limit their use. Therefore, numerous methods were developed for the immobilization of ILs and DESs on solid supports.

In this research, ILs and DESs were microencapsulated via interfacial polymerization using oil in oil (O/O) emulsions as templates for the formic acid-mediated non-aqueous sol-gel technique. The immobilization step provided easily handleable and dispersible solids. Thus, eliminating the negative effects of the high viscosity of ILs and DESs. Then, the encapsulated compounds were applied as catalytic microreactors in different organic reactions. For instance, the Bronsted acidic choline chloride: L-tartaric acid DES was employed in the acid-catalyzed synthesis of pyrroles from acetonyl acetone and various aniline derivatives. In addition, microencapsulated proline-based ILs and DESs were used as catalysts for the thiol Michael addition reaction and the asymmetric aldol condensation of acetone and cyclohexanone with different benzaldehyde derivatives. In general, high yields were obtained among all of the aforementioned reactions, which were conducted at room temperatures. In the case of the asymmetric aldol condensation, moderate enantioselectivities were acquired with the microencapsulated proline: L-lactic acid (1:2) DES. Furthermore, all of the encapsulated organocatalysts were successfully recycled post-reaction by simple centrifugation and were used for four additional runs without a significant decrease in the catalytic activity.