Polysaccharide Encapsulated Catalysts: Towards the Sustainable Production of Fine Chemicals

The present work contributes to broaden the use of alginate gels in asymmetric heterogeneous catalysis. In the first part of this study, the use of alginate gels as supports to prepare a heterogenized version of an amino Cinchona alkaloid for asymmetric Michael addition is described. The results demonstrated that the adsorption of a representative Lewis base organic catalyst (9-amino-9-deoxy epi-quinine, QNA) takes place with high yields onto acidic alginate gels (AGs) using a straightforward, robust and fully reproducible protocol. The resulting chiral gels (QNA@AGs) are active as heterogeneous catalysts in the addition of aldehydes to nitroalkenes, affording the corresponding adducts in good yields and moderate to excellent diastereo- and enantio-selectivities. The use of heterocationic gels, derived from alkaline earth metal gels by partial proton exchange, provided materials with better mechanical properties and higher porosities, ultimately resulting in higher catalytic activities. The alginate gels were also assessed as a possible way of transfering chirality from the support to a reaction outcome. The Friedel-Crafts alkylation of nitroalkenes with indoles was selected as a model reaction to evaluate the enantio-induction by metal Lewis alginate gels. The library of alginate gels tested is active in the benchmark reaction. The Cu and Ba- alginate gels afford good activity and the enantiomeric-induction is proved. Furthermore, these two metals allow 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 catalyst is proved using Cu-alginate gels, which can also be recycled without loss of stereochemical activity for at least five reaction cycles. This work represents the first utilization of alginates, abundant and renewable biopolymers, as gel supports/media for asymmetric organocatalytic processes and the first example of induction of enantioselectivity for a C-C bonding reaction with interest in the fine chemical industry.