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

• Published in: Applied catalysis. A, General General, 2016-05, Vol.517, p.176-186
• Main Authors: Franconetti, A., Domínguez-Rodríguez, P., Lara-García, D., Prado-Gotor, R., Cabrera-Escribano, F.
• Format: Article
• Language: English
• Subjects: Aldehydes; Aromatic aldehydes; Catalysis; Catalysts; Chitosan; Cyanoacrylates; Density functional theory; Derivatives; Heterogeneous organocatalysis; Hydrogels; Knoevenagel condensation; Stereoselectivity
• ISSN: 0926-860X; 1873-3875
• DOI: 10.1016/j.apcata.2016.03.012

[Display omitted] •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.