Electrocatalytic Carboxylation Of CO₂ And Organic Compounds

As entering into the 21 century, the international community has focused on the sustainable development referring to the ecological environment, natural resources, economy and so on. With the establishment of many policies concerning environmental protection, the chemistry industry has concentrated on how to eliminate or reduce the waste from the source, which poses new demands and challenges for the chemisty. The environmental economy has becoming one of primary drive for technology innovation. Consequently, the most importance for the synthesis chemistry is not what to prepare, but how to prepare. At present, synthesis chemistry is developing towards the green chemistry.Electrochemisty provides a simple, convenient synthetic technique. Electron is used as reagent in electroorganic synthesis, avoiding the use of other reducing agent and oxidant. The reactant could be reduced or oxidized on the electrode by getting or losing electrons. In electrochemistry processes, the electrons are consumed stoichiometrically with respect to the substrate, Unlike classical organic synthesis, a certain amount of oxidant or reducing reagent is added before reaction, so it is the electrons that are used as clean, controlled, and nonpolluting redox reagents. This feature is often considerd as being environmentally favorable. So compared with classical organic synthesis, electrochemistry synthesis is greener.Carbon dioxide is the largest contributor to the green house effect, which may increase the earth average temperature to such a value that may cause catastrophic events. Therefore, great efforts have been placed toward the utilization of CO2, which can control the emission of CO2, reduce the environmental pollution and convert CO2 into the commodities. However, since CO2 shows a very low reaction activity, its activation could be realized only under severe reaction conditions via the conventional chemical methods. In contrast to thermo-chemical reactions, CO2 can be readily activated through an electrochemical reaction at mild conditions. An electrochemical method has become one of efficient routes for the utilization of CO2. Ionic liquids are non-volatile novel green solvents which combining ionic conductivity and good solvating properties for organic compounds. In fact, such a non-volatile systems, used instead of conventional organic solvent/supporting electrolyte system and maximally facilitating products isolation procedure, seem to be an ideal media for electrochemistry.Organic carbamates and dimethyl carbonate(DMC) are an important class of compounds whose versatility allows their application in several fields of the chemical and pharmaceutical industry. Dimethyl carbonate can be used not only as a carboxylation and methylation agent in organic synthesis, but also as a high energy battery electrolyte and environment-friendly gasoline additives for mobile. Therefore, dimethyl carbonate has been highly regarded as a novel green chemical product in the world. The conventional synthetic methods for the carbamates and DMC have several drawbacks:the use of hazardous reagents, expensive and complicated catalysts, high pressure and high temperature, and low yields. Consequently, alternative methods of the synthesis are highly desirable. On the other hand, organic halides are widely used in insecticide. They represent a class of toxic, carcinogenic and nonbiodegradable organic pollutants. Considering the environmental protection, it is very important to convert the organic halides into the useful chemical products.Under mild condition, electrochemical activation of CO2 to the useful chemical products is a simple procedure. Specially, application of ionic liquid in fixation of CO2 is of great practical significance. The details are given as follows:(1) Electrochemical activation of CO2 for the synthesis of ethyl phenylcarbamate under mild conditionsA novel electrochemical procedure for the synthesis of ethyl phenylcarbamate from aniline and carbon dioxide was developed via the selective cathodic reduction of carbon dioxide in CO2-saturated DMF solution containing 0.1 mol L-1 Et4NBr at room temperature, followed by the addition of EtI as an alkylating agent. The synthesis was carried out under mild[p(CO2)=0.1 MPa, t=20℃] and safe conditions. Influences of the nature of the electrodes, the current densities, the passed charges during electrolysis, temperature, and supporting electrolytes on the yield of ethyl phenylcarbamate were studied to optimize the electrolytic conditions.(2) Activation of carbon dioxide by electrocatalysis for synthesis of carbamatesCO2 was electrochemically activated by the Ni(bpy)3Cl2 catalyst and reacted with amines and iodoethane to give carbamates (15.8%-82.2%) under mild conditions [p(CO2)= 0.1 MPa,20℃]. The CO2 activation mechanism and the synthesis reaction mechanism were proposed. The electroreductive potential of CO2 was moved from-2.1 V to-1.6 V owing to the Ni(bpy)3Cl2 catalyst in DMF solution, indicating that the reaction condition was effectively improved. The catalyst showed a good effect on the CO2 activation. The effects of the catalyst, the electrolytic potentials, the nature of the working electrodes, the temperature, the supporting electrolytes, and the passed charge amount per mole of aniline supplied to the electrode on the yield of ethyl phenylcarbamate were studied.(3) Electrosynthesis of dimethyl carbonate from CO2 in mild conditionElectrosynthesis of dimethyl carbonate from methanol was studied at room temperature in the presence of atmospheric pressure of CO2. The only product obtained was dimethyl carbonate in a CH3CN-Et4NBr (tetraethylammonium bromide) solution previously electrolyzed under galvanostatic control. Influences of the nature of working electrodes, current densities, the passed charges on electrolysis, supporting electrolytes and different procedures of adding methanol were studied to optimize the electrolytic conditions, The maximal yield is 74% on Cu/C electrodes under a constant current of 17 mA cm-2 until 2 F mol-1 of charge has passed through the cell. With respect to the methods so far reported, the yield of dimethyl carbonate in this paper is highest.(4) Electrocatalytic carboxylation of aliphatic halides at silver cathode in acetonitrileA simple and efficient electrocarboxylation reaction of aliphatic halides has been developed using silver as cathode, magnesium as anode, and CH3CN saturated CO2 as solvent in an undivided cell. The influence of some key factors (such as the nature of electrode materials, supporting electrolytes, and temperature) on this reaction was investigated. Under the optimized condition, the corresponding carboxylic acids were obtained in moderate to good yields(22%-89%). The electrochemical behaviour was studied at different electrodes (Ag, Cu, Ni and Ti) by cyclic voltammetry, which showed significant electrocatalytic effect of the silver electrode towards the reductive carboxylation of aliphatic halides. (5) Electrocatalytic carboxylation of benzyl chloride at silver cathode in ionic liquid BMIMBF4The feasibility of electrocarboxylation of benzyl chloride has been investigated at silver cathode in CO2-saturated room-temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) solution for the first time. The electrochemical behavior was studied at different electrodes by cyclic voltammetry, which showed significant electrocatalytic effect of the silver electrode on the reduction of benzyl chloride. The highest yield of 45% of phenylacetic acid was obtained under optimal conditions. The recovered ionic liquid was reused for four times with gradual decrease in the yield of phenylacetic acid.(6) Electrocatalytic dimerisation of benzyl bromides and phenyl bromide at silver cathode in ionic liquid BMIMBF4A simple and eco-friendly electrochemical route was developed by using silver as the cathode, magnesium as the anode and ionic liquid BMIMBF4 as solvent for the electrochemical dimerisation of aromatic bromides. The electrochemical behaviour was studied at different electrodes (Ag, Cu, Ni and Ti) by cyclic voltammetry, which shows significant electrocatalytic effect of the silver electrode towards the reductive dimerisation of aromatic bromides. Biaryls were obtained in moderate to good yield (12%-68%). A recycling study confirmed that the solvent can be reused multiple times without activity loss.