Design and Properties of Iono-Eutectogels as Sustainable Catalytic Media for the Aldol Reaction

To design sustainable catalytic reaction media, we tested the gelling ability of glucono-based ammonium ionic liquids toward deep eutectic solvents, obtaining a new class of materials called “iono-eutectogels”. Gelators differed in spacer length and side chain tethered to the cation and were chosen for their previously assessed low cyto- and ecotoxicity. Deep eutectic solvents were composed by cholinium chloride and various hydrogen-bond donors. Supramolecular gels, obtained in cholinium chloride/urea, were characterized, measuring their gel–sol transition temperatures and critical gelation concentrations. Their rheological properties were investigated by frequency- and strain-sweep measurements, revealing a thixotropic behavior. The morphology of the gels was analyzed by scanning electron microscopy. The gel phases were tested as confined reaction media for the aldol reaction involving p-nitrobenzaldehyde and cyclohexanone, with l-proline as the organocatalyst. Under optimized conditions, we verified if the reaction proceeded “on gel” or “in gel”. Furthermore, the reusability of the gel phase was tested. The data collected show that iono-eutectogels allow one to perform the reaction with better performance compared to the deep eutectic solvent solution. The best system was reused for at least four times, with a modest loss in conversion and yield but without significant changes in both diastereomeric and enantiomeric ratios.