The Preparation Of Polysaccharide-based Heterogeneous Catalyst And Its Application In The Construction Of C-X Bond

The Ullmann coupling reaction is one of the most important strategies for constructing C-X bonds,which plays a vital role in the fields of pharmaceutical and agrochemical industries.However,the primary Ullmann coupling reaction was conducted under harsh conditions,such as high temperatures,strong bases,excess copper salts,which greatly hindered the application of the Ullmann reaction.Later,the introduction of ligands broke through these limitations and made the reaction conditions milder.However,the recovery of ligands remained challenging and need high catalytic cost.More researchers had begun to carry out research about heterogeneous catalysts.Among the heterogeneous catalyst support materials,Chitosan,as a cheap and available natural molecular polymer,contains functional group with high metal affinity.In this paper,we carried out the following investigations by using chitosan as a catalyst carrier in the Ullmann coupling reaction:In this paper,the amino groups in chitosan molecular structure were functionally modified to increase the active sites of chitosan anchoring metal.The model reaction catalyzed by pyridine-2-carboxaldehyde functionalized-chitosan@CuI exhibited an excellent yield（up to 99%）.The effects of bases,solvents and temperature on Ullmann reactions were tested in detail.Then the optimum conditions for this coupling reactions could be concluded:using Cs₂CO₃ as the base,DMSO as the solvent,110 ^oC,24 h.The extensive study about the substrate scope was conducted under the optimum conditions and confirmed this catalytic system had a wide range of substrate applicability.In addition,the reusability of catalyst was evaluated and the yield still achieved above 90%after being reused five times.The structure of catalyst was analyzed by TG,XRD,SEM,EDS and XPS.The outcomes indicated that the free electron pair on the N atom of functionalized-chitosan forms a stable coordination bond with copper,which resulted in the little copper leaching after several times of recycling.Although the catalyst had excellent catalytic performance and recyclability,the power complex catalyst still faced the question of weight loss in the recovery process.Therefore,by using the sol-gel properties of chitosan,we prepared chitosan beads by pH inversion.Chemical cross-linking is not only to introduce some chelating functional groups,but also increase the mechanical strength of the beads.The cross-linked chitosan beads supported copper complex were prepared with various copper salts and then used in Ullmann coupling reaction.By carrying out an optimization study,the best reaction conditions for coupling reaction were demonstrated as below:using Na₂CO₃ as base,water as solvent,100 ^oC,24 h.A series of characterizations of the catalysts showed that the beads formed a three-dimensional network structure after cross-linking and still maintained good stability at 200^oC.In last,the reusability of catalyst by simple filtration is evaluated.After the catalyst was reused five times,the copper content lost 33%and the weight lost 16%,when the catalyst was reused ten times,the product yield still achieved 65%.At present,the copper used in Ullmann coupling reaction was mainly+1 oxidation state of Cu,we found that chitosan supported the zero-valent copper still had a good catalytic effect in the aqueous phase.In addition,the appearance of zero-valent copper particle is controllable.Avoiding the disadvantage that+1 oxidation state of Cu is easily oxidized in the air.In this paper,the functional groups of chitosan can reduce Cu^III to Cu⁰ without using reducing agent,eg NaBH₄.Simultaneously,the SEM shown these copper particles were evenly dispersed to the surface of chitosan and the average diameter of copper particles were about 200-300 nm.The XPS indicated that the copper element forms a stable coordinate bond with the amino group on the chitosan.The catalyst was used in Ullmann coupling reaction in water under the optimum conditions,it was obvious that the catalyst exhibited excellent catalytic performance in substrate scope and still had the catalytic activity after reused five times.