Studies On Cellulose-Supported Palladium Catalyzed Alkyne Functionalization

With the depletion of fossil resources,energy crisis,environmental pollution and other issues becoming more and more prominent in today's era,the recovery and reuse of resources has become the main trend of development.Eco-friendly polymers,such as cellulose,starch,chitosan,alginate and gelatin,have attracted extensive research interest due to their wide application,low cost,non-toxicity and biodegradability.Among these,cellulose,as the most abundant renewable resource in the world,has been widely used in industrial production and development of new functional materials.Transition metal catalysis can not only effectively improve the efficiency of chemical reactions,but also meet the requirements of contemporary sustainable development.Palladium nanoparticles have garnered maximum attention due to their versatile catalytic activity for many organic reactions including cross-coupling,hydrogenation,cyclization and oxidation.In recent decades,palladium-catalyzed cyclization of alkynes has become an important method for the construction of heterocyclic compounds.Generally,the homogeneous palladium-catalyzed reaction has amazing reactivity,which is attributed to the efficient conversion of the reaction through the catalytic active site at the molecular level.However,there are also disadvantages such as difficult product separation,difficult catalyst recycling,and so on.Cellulose-supported heterogeneous catalysts effectively solve these problems due to their advantages such as high catalytic efficiency,cheap raw materials,easy separation from reaction products,and recycling.Therefore,it is of practical theoretical significance and application value to use cellulose-supported metal palladium to construct heterogeneous catalysts and then apply them to catalyze alkyne cyclization reactions.This thesis is mainly concerned with the cellulose-supported heterogeneous palladium catalyst system to catalyze the functionalization of alkynes for efficient preparation of heterocyclic compounds.Successfully designed and synthesized of a green,recyclable heterogeneous cellulosic Pd(salen)-type catalyst.Next,this catalyst was used in alkyne functionalization for synthesis naphthoquinofuran derivatives.Finally,the performance of traditional homogeneous catalysts and the cellulose Pd(salen)-type catalyst was compared in the synthesis of Benzo[4.5]imidazo[1,2-a]pyridines.Catering to the current atomic economy,step economy,environmental protection and sustainable green chemistry concepts.The details are as follows:(1)Design and characterization of cellulosic Pd(salen)-type catalyst:A green recyclable cellulose-supported Pd(salen)-type catalyst was synthesized using commercially available microcrystalline cellulose as raw material through sequential three steps:chlorination with thionyl chloride,modification by ethylenediamine,and the formation of Schiff base with salicylaldehyde to immobilize palladium chloride through multiple binding sites.This novel heterogeneous cellulosic Pd(salen)-type catalytic complex was fully characterized by FT-IR,SEM,TEM,XPS,ICP-AES and TG.The traditional cross-coupling chemistry,such as Suzuki,Heck,Sonogashira,Buchwald-Hartwig amination and etherification,was then investigated in the presence of the above cellulose-palladium nanoparticle.Studies have shown that the synthesized catalyst shows high activity and efficiency for all types of transformations,providing the corresponding carbon-carbon or carbon-heteroatom coupling products in a general and mild manner.Furthermore,the catalyst demonstrates high to excellent yields and is easily recycled by simple filtration for upto twelve cycles without any significant loss of catalytic activity.(2)Cellulose-supported palladium catalyzes oxidative cyclization of 2-hydroxynaphtho-quinone with internal alkynes:One-pot synthesis of naphthoquinone fused 2-disubstituted furan derivatives in the presence of cellulosic Pd(salen)-type catalyst with 2-hydroxy-naphthoquinone and an internal alkyne as substrates through C-H/C-O bond activation.The success of the reaction depends heavily on careful optimization of the conditions.The tolerance of the reaction was investigated under optimized conditions and found that a broad substrate scope with good yields and excellent regioselectivity for aryl,alkyl-substituted alkynes.27 naphthoquinofuran derivatives were successfully synthesized in yields of 38%to 78%.In the whole reaction path,the reaction exhibits high atom economy and step efficiency Unlike the traditional homogeneous catalytic reaction,the use of heterogeneous catalyst greatly reduces the cost of the reaction(3)Based on C-X bond initiation/C-H bond activation,Palladium-catalyzed annulation reaction of benzimidazoles and alkynyl bromides with internal alkynes:In this part,aryl-fused benzimidazole frame was successfully construct with three components of benzimidazole,alkynyl bromide and internal alkyne as substrates through nucleophilic addition reaction,C-X bond initiation and C-H bond cyclization.In all cases,the procedure This approach can be performed by first performing a nucleophilic addition reaction to provide(Z)-N-(1-bromo-1-en-2-yl)benzimidazole.Sequentially,these adducts containing alkenyl bromide can undergo Pd-catalyzed intermolecular C-H annulation in the presence of internal alkynes(83%).The corresponding target cyclization product can also be obtained through a one-pot twostep operation(58%).And this synthetic method exhibits a broad substrate scope with good to high yields and excellent stereo-and regioselectivity.Due to the three-component reaction substrate,the product diversity of the reaction is greatly improved.In addition,the efficiency of cellulosic Pd(salen)-type catalyst with traditional homogeneous catalysts were compared,and uses experimental data to better illustrate the characteristics of high efficiency and easy separation of heterogeneous catalyst systems.The paper has studied the optical properties of the synthesized products and found that these compounds generally exhibit blue or green fluorescences(454-503 nm)in CH2Cl2,and the fluorescence quantum yields remained in the range of 0.19-0.89.