## 4 Chitosan as Catalytic Support
### Catalytic Metal-Chitosan Composites
The resulting mixture was crosslinked with GA (designated as MCS-GT NPs). Resulting MCS-GT NPs (with sizes of about 20 nm) were treated with cobalt acetate solution (MCS-GT@Co(II) NPs). Figure 23: Stepwise synthesis of MCS–GT@Co(II) magnetic NPs. Reproduced with permission from authors of [137], Wiley, 2018. Figure 22: Scheme of QCS@Ag–TiO\({}_{2}\) synthesis. Reproduced with permission from authors of [148], Elsevier, 2020. MCS-GT@Co(II) NPs were tested in one-pot three- and four-component condensation reactions to synthesize 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles of biological and pharmaceutical importance. The MCS-GT@Co(II) catalyst allowed high yields (in most cases 90-99%) and purity of products, short reaction time, atom economy, easy and environmentally benign work-up procedure, clean and mild reaction conditions, use of various substituted aldehydes and amines, easy separation of catalyst by applying an external magnet, recyclability and reusability of the catalyst and no use of column chromatography for the purification of products [137]. Photolatent heterogeneous Cu(II) precatalyst supported by CS powder (CuBP-CS) was developed and tested in copper-catalyzed azide-alkyne cycloaddition [138]. The benzophenone (BP) photosensitizer was employed in catalytic amount, i.e., 10 mol% with respect to the Cu(II) ions, to effectively promote the Cu(II) to Cu(I) photoreduction process (Figure 24). This allowed for the effective synthesis of a range of triazoles, in particular those bearing sugar moieties, under light illumination and sodium ascorbate-free conditions. The catalytic activity of CuBP-CS was shown to be effectively switched on when illuminated with a low intensity UVA-emitting lamp. The catalyst could be recovered and reused several times with no appreciable decrease in catalytic activity. As a practical application of the procedure, the anticonvulsant drug rufinamide was isolated at 82% yield on a 10 mmol-scale synthesis [138]. CS-supported copper catalysts were synthesized [139] using a simple procedure: different Cu(II) and Cu(I) salts or Cu powders were added to a CS suspension. Series of CS@copper catalysts were obtained and tested in the synthesis of alkyl/aryl-mixed phosphates by conversion of P-N bonds into P-O bonds through a nucleophilic substitution reaction under aerobic conditions. The deaminated esterification was conducted underrelatively mild conditions, generating nitrogen gas as a byproduct. The developed methodology enabled the preparation of mixed phosphates in moderate-to-excellent yields (up to 94%). However, the decreased copper content of the recovered catalyst indicated that active species were leaching during the reaction or workup procedure and might have been responsible for the loss of activity. CS-supported CuSO4(CTS-CuSO4) and CS-supported Cu NP/Cu2O microcrystal (CuNP/Cu2 OMCs)-heterogeneous catalysts were developed [140] using CS powder without any additional shaping or crosslinking. Different crystal structures of Cu2 OMCs were obtained in combination with Cu NPs through the chemical reduction of CuSO4 using ascorbic acid as a reducing agent [140]. These composites were applied for the synthesis of bis(indoly)methanes under mild solvent-free conditions (80 \({}^{\circ}\)C). Depending on the aldehyde, in most cases, an yield of the target products from 15% up to 100% was achieved using the catalyst with the highest CuSO4 content (78.2 wt.% CTS-CuSO4). In was also found that CTS-CuNP/Cu2 OMC catalyst was an effective alternative to CTS-CuSO4 in the synthesis of bis(indoly)methanes  
Figure 24: Synthesis of CuBP-CS. Reproduced with permission from authors of [138], [PERSON], 2018.  
when aliphatic aldehydes were used. Reusability of the obtained catalysts was up to five catalysis/recovery cycles. Cu-containing, CS-based catalysts can be also used for the oxidation of sulfides into sulfoxides, which are versatile synthetic intermediates [145; 146].