Chitosan-based catalysts reaction

Preparation of the chitosan based catalysts for several hydrogenation reaction in the liquid phase... 

The goal of this work was the investigation of alternative methods of Pd/chitosan based catalyst preparation for several reactions of hydrogenation in the liquid phase. The work focused on the following directions ... 

Preparation of chitosan based catalysts for several reactions of liquid phase hydrogenation... 

Different chitosan-based catalysts are compared in Table 3. The reactions were performed at 70°C due to the thermal stability of the catalyst. The first ones focus on the influence of the drying procedure in comparison with an uncatalyzed reaction and a known heterogeneous catalyst (Table 3, entries 1-5). Lyophilized chitosan (Cl) does not display any activity when the aerogel is as efficient as the functionalised silica (C3). This result illustrates the accessibility to the amino groups of chitosan in its aerogel form. 

With the aim of avoiding the use of phosphorus-based ligands, we had previously developed catalysts based on silica supported bidentate iminopyridine ligand systems. These coordinate Pd well, and the catalysts are active and stable in both the Heck (20,22) and Suzuki (21,22) reactions. Since the catalysts are based on the functionalisation of an aminopropyl groups attached to the silica, these are ideal candidates to translate to a chitosan-based support, and indeed this was achieved in a straightforward manner (Figure 2). 

Sodium borohydride reduces disulfides to thiols, which can then be used to reduce nitro groups. Based on the redox properties of 1,2-dithiolane, lipoamide (17) was used for the selective reduction of mono-substituted nitrobenzenes to the corresponding anilines. Lipoamide (17) can also be immobilized on hydrophilic polymers such as polyvinylamine, polyethyleneimine and chitosan. These polymeric reducing catalysts can be recycled and are easy to separate from the reaction mixture. The system has been used to reduce nitroarenes to anilines. ... 

The enzymatic treatment of chitosan in the presence of tyrosinase and phenol derivatives produced new materials based on chitosan.91 During the reaction, unstable o-quinones were formed, followed by the reaction with the amino group of chitosan to give the modified chitosan. The tyrosinase-catalyzed modification of chitosan with phenols dramatically altered rheological and surface properties of chitosan. The modification with chlorogenic acid onto chitosan conferred the water solubility of chitosan under basic conditions.92 A new water-resistant adhesive was developed by the tyrosinase-catalyzed reaction of 3,4-dihydroxyphenethylamine and chitosan.93 Poly(4-hydroxystyrene) was modified with aniline by using tyrosinase catalyst.94 The incorporated ratio of aniline into the polymer was very low (1.3%). 

Palladium nanoparticles supported on chitosan (poly-[/3(l )-2-amino-2-deoxy-D-glucan]) are very efficient heterogeneous catalysts in the reaction of aryl bromides or activated aryl chlorides in tetrabutylammonium bromide as solvent and tetrabutylammo-nium acetate as base (reaction time 15 min, in quantitative yield) [47]. The catalyst was... 

Chitosan has been explored as a catalyst support for two different metals, palladium and nickel. In both cases, stable catalysts have been produced, which display good activity in chosen reactions. Generally, activity appears to be similar to existing silica-based analogues, which should allow more extensive replacement of silica systems with renewable supports. Initial attempts to form films of catalyst appear to be very promising, and further work is planned in this direction. 

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