Chiral Bicomponent Melamine/Amino Acid Eutectogels: Efficient Catalysts for the Aldol Reaction

Supramolecular gels, characterized by reversibility, stimuli responsiveness, and high surface area, represent a promising class of materials in catalysis. In this work, we explored the possibility to obtain supramolecular melamine/amino acid-based eutectogels, and we tested them as heterogeneous catalysts for the aldol reaction between p-nitrobenzaldehyde and cyclohexanone. Gels were characterized by determining the critical gelation concentration and gel-sol transition temperature. Mechanical features of soft materials were assessed performing strain and frequency sweep measurements, and their self-sustaining properties were also investigated. Morphology of gel phases was analyzed by scanning electron microscopy. Resonance light scattering investigation was carried out to study gelation process and to investigate the size of aggregates featuring gel phases. Fluorescent behavior in gel phase and in solution was also studied, suggesting that gel formation was driven by an aggregation enhanced emission (AEE) process. Performing the aldol reaction, the experimental conditions were optimized in terms of reaction time, temperature and amount of catalyst. Under the best experimental conditions, we obtained 73% of yield and 76-24 anti/syn diasteroisomeric ratio with 75% of enantiomeric excess_anti. Products were easily separated with simple extractions with a green solvent (ethyl acetate), and gel phase was recycled at least five times without loss in stereoselectivity.