Enantioselective Friedel-Crafts reaction catalysed by alginate aerogels

Alginates are polysaccharides derived from brown algae, available in nearly unlimited amounts at very low prices. In the presence of some divalent metals, these renewable biopolymers can readily form hydrogels, solvogels and aerogels, characterized by high surface areas, good mechanical properties, tolerance to different media, and easy manageability. For these reasons, alginates are nowadays being thoroughly studied in heterogeneous catalysis; several applications in supported metal catalysis and as heterogeneous Brønsted acids have emerged. However, none of these studies has given an answer to the following intriguing question: can we use the intrinsic chirality of alginates to induce enantioselectivity in a chemical reaction? In order to answer this question a representative reaction, the Friedel-Crafts alkylation of nitroalkenes with indoles, was tested in this master thesis. This study, which involved a large screening of a variety of alginate gels under different reaction conditions, showed that Cu2+ and Ba2+ are the best cross-linking metals to promote the Friedel-Crafts reaction. Indeed, good activity with moderate enantiomeric excesses were obtained under the optimized reaction conditions. Furthermore, these two metals allowed the access to both enantiomers of the products, an important aspect given that only one enantiomeric form of alginates is available. Finally, the heterogeneous nature of the catalysis by one of the two gels was proved, and a good recyclability was demonstrated, by showing that the same catalyst can be used at least five times with similar results.