Electrocarboxylation Of α, β-unsaturated Ketons

As the development of the global economy and the increased need of the energy sources,the content of carbon dioxide(CO₂)in atmosphere is increasing and it is the primary component of greenhouse gases.Converting it to chemical products can both reduce the environmental pollution by controlling the emission of greenhouse gases and utilize the cheap and abundant C1 resource.This is a very important thing in the whole world.γ-keto acids are useful intermediate for electrosynthesis ofγ-lactones cyclopetenones,butenondes,1,3-cyclo-pentanediones and heterocyclic compounds. Generally the organic methods to synthesize it involved heatingβ-keto amine in KCN or NaCN solution and the reaction of ethyl acetoacetate with haloid alkane catalyzed by sodium alcoholate.However,traditional methods usually have multi-stage reactions and can not be applicable to industry due to its complexity and serious pollution of environment.Electrochemical synthesis get rid of the disadvantages above,and have the higher yield and good selectivity.There are several reports of reductive carboxylation ofα,β-unsaturated ketons,whereγ-keto acids were produced.However, the yields were not satisfactory in the earlier works because of the formation of dimeric and polymeric products,or it was not clean and polluted the environment.We would like to report the effective and environment protected transformation of aromaticα,β-unsaturated ketons intoγ-keto acids by electrochemical carboxylation with CO₂ in acetonitrile under the controlled potential conditions.The experiment of the paper is divided into four main parts.In the first part,the quantitative analysis ofα-phenyllevulinic acid andα-phenyl-β-benzoylpropionic acid using HPLC was discussed.The floating solvent was the mixture of acetonitrile and water which containing 2%acetic acid.Phenyl methyl ether was the inner standard substance.The quantitative analysis which is exact and sensitive was carded out on Supelcosill C-18-DB(5μm,150mm×4 mmI.D.)and the UV wavelength was 254 nm.In the second part,electrocarboxylation of benzalacetone was studied which include the electrochemical behavior and electrosynthsis ofα-phenyllevulinic acid from benzalacetone and carbon dioxide.The electrochemical behavior of benzalacetone in acetonitrile containing 0.1mol·L ^-1TEABF₄ was studied in detail The influence of temperature,scan rate and concentration on benzalacetone reduction is investigated by cyclic voltammetry.The results show that the reduction of benzalacetone is an irreversible process by diffusion control.The transfer coefficient and diffusion coefficient of benzalacetone was figured out by cyclic voltammetry and chronocoulometry.The electrosynthesis ofα-phenyllevulinic acid from CO2 and benzalacetone were carried out under controlled potential.The effects of solvent,supporting electrolytes,electrodes,electrolysis potential,benzalacetone concentrations and temperature on the yield ofα-phenyllevulinic acid have been examined to optimize the synthetic conditions.After optimizing the synthetic parameters,the yield ofα-phenyllevulinic acid reached 68.86%on Stainless steel-Mg couple electrodes under a controlled potential -1.6 V until 2 F·mol^-1of charge passed through the cell when 0.1 mol L^-1benzalacetone was applied at 0℃in MeCN solution containing 0.1 mol L^-1 TEABF₄ as supporting electrolytes.In the third part,the electrochemical behavior of benzalacetophenone was discussed and the optimized conditions for the preparation ofα-phenyl-β-benzoyl propionic acid from benzalacetophenone by electrochemical method were studied. Cyclic voltammetry and Chronocoulometry were used to study the electrochemical reduction behavior of benzalacetophenone on GC electrode.The results showed that the reduction of benzalacetophenone on GC electrode was a diffusion-controlled irreversible process.The diffusion coefficient D of benzalacetophenone in 0.1 mol L^-1-TEABF₄-MeCN solution was 2.597×10^-5cm².s^-1,while the transfer coefficient was 0.29～0.38.Stainless steel as working electrode,SCE as reference electrode and Mg as auxiliary electrode,α-phenyl-β-benzoylpropionic acid was prepared by potentialstatic electro-reduction of benzalacetophenone.The optimized condition was: 0.1 mol L^-1-TEABF₄-MeCN solution,0.05 mol L^-1benzalacetophenone was added, the electrolysis potential was -1.75V.The yield was 88%with the theoretical electric quantity.The fourth part studied the effects of the nature and the position of substituted groups on the characteristics of the electroreduction ofα,β-unsaturated ketons by electrochemical methods such as cyclic voltammery,differential pulse voltammetry. In addition,the mechanism of the electrocarboxylation process was proposed.It involves a one-electron reduction and a chemical reaction followed by another one-electron reduction and a chemical reaction.The experimental results showed that the different substituted group on the same location of theα,β-unsaturated ketons and the same substituted group on the different location of theα,β-unsaturated ketons were greatly affected by the electronic domino effect and space resistance of the substituted group.The electrochemical behavial of benzalacetic acid, 4-methoxychalcone,and 4'-methoxychalcone were studied.The reduction potential of 4-methoxychalcone was the most positive,while the reduction potential of benzalacetic acid was the most negative.Under the optimized condition,several unsaturated ketones were electrochemically reduced in the presence of CO₂ to the corresponding monocarboxylic acids in satisfactory yield.The three points listed below reflect the innovation of this dissertation.1.γ-keto acids were produced from CO₂ andα,β-unsaturated ketons directly in an undivided cell equipped with a Mg sacrificial anode at the controlled potential conditions,which avoided the use of toxic substances compared with the traditional organic methods as well as utilized the greenhouse gas as material.2.The stainless steel electrode which was used as the cathode is more eco-friendly than mercury cathode used in the electrochemical reduction ofα,β-unsaturated ketons.The one-compartment electrochemical cell is easier to be prepared than two-compartment electrochemical cell and can be controlled well.3.The electrochemical behavior of benzalacetone and benzalacetophenone in MeCN is first studied in detail by cyclic voltammetry and differential pulse voltammetry. The influence of sweep rate,concentration and temperature shows that the reaction was irreversible and diffusion controlled.4.Effects of the nature and the position of substituted groups on the characteristics of the electroreduction ofα,β-unsaturated ketons are first studied and offer academic gist for studying the unsaturated ketons farther.So this paper is significant in the basic research and practical applications.