Native and modified chitosan-based hydrogels as green heterogeneous organocatalysts for imine-mediated Knoevenagel condensation

• Chitosan hydrogel beads and ureidyl-chitosan derivative hydrogel disks were efficient green organocatalysts for Knoevenagel condensation involving aromatic and heteroaromatic aldehydes.
• Imine intermediate formation was demonstrated by solid state 13C CP MAS NMR and coupling constants 3JC,H measuring by CLIP-HSQMCB experiment was used to analyze E, Z cyanoacrylate isomer mixtures.
• Kinetic parameters for both beads and disks catalysis were determined by using a model reaction.
• DFT calculations were performed to rationalise the observed E/Z stereoselectivities as well as to evaluate the role of ureidyl moiety on interaction with aldehydes and imine intermediate formation.

A variety of methylenemalononitriles and ethyl cyanoacrylates derived from both aromatic and heteroaromatic aldehydes were synthesized by Knoevenagel condensation catalysed with native and modified chitosan-based heterogeneous catalysts. The efficiency of our hydrogel organocatalysts, chitosan hydrogel beads and ureidyl-chitosan derivative hydrogel disks, was evaluated as function of pH, temperature and catalyst concentration by considering reaction rates, conversions, E/Z stereoselectivities, and kinetic studies of a model reaction between 4-nitrobenzaldehyde and ethyl cyanoacetate. An unprecedented study by solid state 13C CP MAS NMR of the employed catalyst when reaction was quenched after a 50% of conversion, has demonstrated that an imine-chitosan intermediate is formed during this process. Analysis of E/Z ethyl cyanoacrylate isomer mixtures for determining the corresponding stereoselectivity was carried out by NMR measuring carbon-proton coupling constants (3JC,H) using a novel CLIP-HSQMCB experiment. Additionally, DFT calculations let us rationalise the observed E/Z stereoselectivities as well as to evaluate the role of ureidyl moiety on interaction with aldehydes and imine intermediate formation with chitosan derivative.

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