Adsorption of a Chiral Amine on Alginate Gel Beads and Evaluation of its Efficiency as Heterogeneous Enantioselective Catalyst

International audienceAn organic catalyst derived from quinine can be adsorbed on alginate gels by following a simple protocol. The resulting solvogel beads are competent and highly stereoselective catalysts for an asymmetric Michael reaction.A representative Lewis base organic catalyst (9‐amino‐9‐deoxy epi‐quinine, QNA) can be adsorbed in high yields onto acidic alginate gels (AGs) using a very simple and straightforward protocol. The resulting solvogel beads (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. In these reactions, the carboxylic functions of the biopolymer act as both acidic co‐catalyst and non‐covalent anchoring site for the tertiary amine catalyst (as observed by IR spectroscopy). Use of heterocationic gels, derived from alkaline earth metal gels by proton exchange, provided materials with better mechanical properties and higher porosities, ultimately resulting in higher catalytic activities. This work represents the first utilization of alginates, abundant and renewable biopolymers, as gel supports/media for asymmetric organocatalytic processes

Blue Chemistry. Marine Polysaccharide Biopolymers in Asymmetric Catalysis: Challenges and Opportunities

Alginate, chitin (precursor of chitosan), and carrageenan are natural polysaccharides derived from marine sources and available in nearly unlimited amounts. In contrast with other natural polysaccharides (i.e. cellulose), their monomers bear functional groups (amine, carboxylate, sulfate). These functional groups can be used to anchor catalytic species, or even as catalytically active units. In this mini-review, the utilization of marine polysaccharides in asymmetric catalysis is discussed. Examples include: i) combinations with chiral catalysts, resulting in heterogeneous catalytic systems, and ii) utilization of the biopolymers as chirality inducing elements \u2013 serving as chiral ligands or organocatalysts. The reviewed works propose innovative and unconventional utilizations of these renewable materials, providing not only a useful alternative to oil-based polymers, but also unforeseen and fascinating opportunities in the field of asymmetric catalysis