Chitosan as a Sustainable Organocatalyst: A Concise Overview

Abstract

Increased demand for more sustainable materials and chemical processes has tremendously advanced the use of polysaccharides, which are natural biopolymers, in domains such as adsorption, catalysis, and as an alternative chemical feedstock. Among these biopolymers, the use of chitosan, which is obtained by deacetylation of natural chitin, is on the increase due to the presence of amino groups on the polymer backbone that makes it a natural cationic polymer. The ability of chitosan-based materials to form open-network, macroporous, high-surface-area hydrogels with accessible basic surface sites has enabled their use not only as macrochelating ligands for active metal catalysts and as a support to disperse nanosized particles, but also as a direct organocatalyst. This review provides a concise overview of the use of native and modified chitosan, possessing different textural properties and chemical properties, as organocatalysts. Organocatalysis with chitosan is primarily focused on carbon-carbon bond-forming reactions, multicomponent heterocycle formation reactions, biodiesel production, and carbon dioxide fixation through [3+2] cycloaddition. Furthermore, the chiral, helical organization of the chitosan skeleton lends itself to use in enantioselective catalysis. Chitosan derivatives generally display reactivity similar to homogeneous bases, ionic liquids, and organic and inorganic salts. However, the introduction of cooperative acid-base interactions at active sites substantially enhances reactivity. These functional biopolymers can also be easily recovered and reused several times under solvent-free conditions. These accomplishments highlight the important role that natural biopolymers play in furthering more sustainable chemistry.

Preparation and Chemical/Microstructural Characterization of Azacrown Ether-Crosslinked Chitosan Films

“…We also hypothesize that the crosslinking chemistry involves the hydroamination of the alkene arms of DAC (Scheme 1). The formal addition of the N–H bond of the amine across the alkene C=C double bond is said to yield alkylated amines in a direct, 100% atom-economic process [24,25,26,27,28]. …”

Valorization of Waste: Sustainable Organocatalysts from Renewable Resources

“…Hydrogels are polymeric materials, the hydrophilic structure of which makes them able to hold large amounts of water in their three‐dimensional networks. Chitosan‐based hydrogels can be formed by exploiting two distinct hydrogel engineering approaches . Reversible physical hydrogels are generated if chitosan polymer chains are physically associated through electrostatic, hydrophobic, and hydrogen‐bonding forces that lead to the gelation of a chitosan solution.…”

“…Chitosan has been exploited as an effective support for transition‐metal catalysts and reactive nanoparticles, but, more recently, the application of hydrogels and especially aerogels of native and modified chitosan in organocatalysis has emerged as a promising area. In this respect, examples of the use of these materials, either as direct organocatalysts or supported organocatalysts, are flourishing …”

Self‐Organized Porous Titanium‐Chitosan Hybrid Materials with Tunable Functions

“…The incorporation of Ti introduces new acidic sites within the CS framework while avoiding the consumption of active primary amine sites. In addition, the coexistence of the NH 2 groups on CS and TiO 2 Lewis acid sites were also shown to have a synergic catalytic behavior in the monoglyceride synthesis . By fine tuning of the synthesis conditions, the Ti sites can be either grafted or inter‐grafted (i. e. crosslinking) to the CS backbone.…”