Abstract
Asymmetric metal and organocatalysis typically relies on low-molecular-weight chiral compounds as enantioinducing elements. However, evoking biocatalysis, examples of inherently chiral natural (e.g., ds-DNA) and synthetic (e.g., chiral organic frameworks) macromolecules as enantioselective catalysts are rapidly emerging. In this context, we report barium alginate gel beads as heterogeneous and reusable catalysts for the benchmark addition of indoles to nitroalkenes, affording the corresponding adducts in moderate to good yields and enantioselectivities (up to 93% ee). This rare example of high enantioinduction offered by a polysaccharide in catalysis was realized thanks to the versatility of alginates, a family of biopolymers derived from brown algae. In the presence of multivalent metals, including abundant alkaline earth metal ones, alginates form gels featuring appealing properties for heterogeneous catalysis, such as high surface area and stability in different media.