Asymmetric Electroreduction Of Aromatic Ketones And Benzyl Chloride

Chirality is common in nature. Enantiomers with different configuration often exhibit different or even opposite properties. Chiral compounds with high optical purity are very important for the biological, chemical, pharmaceutical, agricultural and non-linear optical materials industry. Effective method for obtaining chiral compounds with high optical purity has always been a great challenge to the modern chemical workers. Asymmetric catalytic synthesis is the most economic method for obtaining chiral compounds with high optical purity, and also bears the most challenging.There are three main types of catalysts for asymmetric catalysis, such as metal complexes catalyst, biological catalyst and organic catalysts. Cinchona alkaloid is one of the organic catalysts. At the beginning of21st century, asymmetric organic catalytic reactions experienced rapid development especially in the reactions catalyzed by cinchona alkaloids, which have been recognized as a common method of organic synthesis in the fields of academic and industrial. The reactions can be completed in a cheap and environment-friendly reagent and solvent, moreover, the mass production in the chemical process could be realized. Asymmetric synthesis catalyzed by cinchona alkaloid has caused great concerns from chemists because of its remarkable achievement. Furthermore, as a kind of metal catalyst, cobalt Schiff base complexes are widely used in the field of catalytic reduction research of epoxides and halides.Electrochemical technology is a research dealing with a clean reagent, the electron, under mild and safe conditions. The required high energy intermediates can be generated only by controlling the electrode potential in the electrochemical reaction. Therefore, electrochemical method is an effective synthesis method and technique. In addition, the application of electrochemical technology for asymmetric synthesis is of great significance to the obtaining of chiral compounds with high optical pure.CO2, which is other than CO and phosgene, is an ideal Cl resources. It is a rich source, low cost, non-toxic, and recyclable. However, CO2is very stability, so far effective method of utilizing CO2into useful chemicals has been made much attentions. The electrochemical technology, which can activate and reduce CO2under ambient pressure, is an effective method for fixing CO2.The electron in the electrochemical is not chiral, so it is theoretically impossible to asymmetric transform small molecule like CO2into chiral compound only through simple electron transfer. However, if a chiral environment can be created via an external force, electrochemical asymmetric synthesis would be realized. One effective methods of synthesizing chiral compounds is the addition of inducer into the system. Chiral compound can be obtained with the achiral substrate, which is relatively cheap and abundant, in the presence of small amount of chiral inducer. There are great theoretical significance and potential application value in the electrochemical asymmetric synthesis. However, compared with organic asymmetric synthesis, there are relatively only a few reports on the electrochemical asymmetric synthesis, especially on the asymmetric electrocarboxylation. So far, only a few reports with its substrate containing chiral group on the asymmetric fixing of CO2into chiral compounds though electrochemical technique except for our group.Electrochemical technique affords a handy and mild way to obtain chiral aromatic alcohols, aromatic carboxylic acids, and carboxylic acids though asymmetric fixing CO2in the presence of inducer. The asymmetric electroreduction of prochiral acetophenone, carried out on green and environment-friendly electrode Ag, and the study of obtaining the chiral2-phenyl propionic acid from organic halide and CO2in the presence of the electrogenerated chiral [CoⅠ(salen)]-complex are particularly new all over the word. Therefore, there are great significance and good development space for the study.The main research contents of the thesis are as follows:(1) Alkaloid induced asymmetric electroreduction of acetophenoneThe asymmetric electroreduction of prochiral acetophenone, which was induced by cinchona alkaloid cinchonidine (CD) and carried out on Ag cathode in an undivided cell under atmospheric N2atmosphere condition, yields two main products the chiral phenylethanol and the dimerization yield pinacol with no optical rotation. The effect of various experiment conditions, such as the water in the solvent (MeCN/H2O), proton type in the solvent (MeCN/Proton), supporting electrolyte, cathode material, current density, and inducer type, on the yield and enantiomeric excesses (ee) of the product was studied. Phenylethanol in21.6%ee with3.6%yield and pinacol in83.2%yield with a5.5dl/meso ratio are obtained under the optimized conditions. In addition, the electrochemical behavior of acetophenone was studied by cyclic voltammetry. The possible reaction mechanism was proposed combined the cyclic voltammetric behavior of acetophenone with electrolytic results. This work achieves asymmetric electroreduction of prochiral acetophenone induced by cinchona alkaloids on green and environment-friendly Ag cathode for the first time, avoiding the use of toxic and harmful Hg electrode. It provides a new way for the synthesis of optically active product by electrochemical technique under green, environment-friendly, and mild condition.(2) Asymmetric electrocarboxylation of aromatic ketones with CO2induced by cinchona alkaloidsThe asymmetric electrocarboxylation of prochiral aromatic ketones, such as2-acetonaphthone,6-methoxy-2-acetyl naphthalene, and4-methoxy-l-acetonaphthone induced by cinchona alkaloids with the auxiliary inducer proton was studied for the first time in an undivided cell under atmospheric CO2atmosphere condition, yielding the corresponding optically active carboxylic products2-hydroxy-2-aryl propionic acids. Using2-acetyl naphthalene as the model substrate, the effect of various synthetic conditions, such as cathode material, current density, inducer, phenol/inducer ratio, proton source type, and inducer quantity, on the yield and enantiomeric excesses (ee) of the target carboxylation product was studied. In addition, electrocarboxylation of other prochiral aromatic ketones under the optimized reaction conditions of2-acetonaphthone was actualized. The studied aromatic ketones could be converted into optically active product2-hydroxy-2-aryl propionic acids in32.2%～-41.3%yield with48.1%～48.6%ee. In addition, the electrochemical behavior of the model substrate2-acetonaphthone was also investigated by cyclic voltammetry in the absence and presence of atmonpheric CO2. The possible reaction mechanism was proposed combined the cyclic voltammetric behavior of2-acetonaphthone with electrolytic results of aromatic ketones. There are several advantages, such as simple operation, short reaction time and mild conditions, in the reaction. The employing of cinchona alkaloids achieves the asymmetric electrocarboxylation of prochiral aromatic ketones avoiding the use of stoichiometric quantities of optically active chiral auxiliary reagent. This work provides a new way for asymmetric electrochemical fixing CO2into optically active aromatic carboxylic acid.(3) Asymmetric electrocarboxylation of1-phenylethyl chloride induced by electrogenerated chiral [CoⅠ(salen)]-complexesThe asymmetric electrocarboxylation of1-phenylethyl chloride induced by electrogenerated chiral [CoⅠsalen)]-complexes was studied for the first time under atmospheric CO2atmosphere condition in an undivided cell, yielding the corresponding optically active2-phenylpropionic acid. In order to improve the yield and ee of2-phenylpropionic acid, the effect of various synthetic conditions, such as electrolytic temperature, cathode material, current density, preinducer quantity, inducer type, electrolysis potential, substrate concentration, and charge passed, on the reaction was studied under galvanostatic and potentiostatic condition. Under the optimized condition, the optically active2-phenylpropionic acid can be obtained in83%ee with37%yield. In addition, in order to study the inducing mechanism of the reduction, cyclic voltammetry was carried out to investigated the electrochemical behavior of the preinducer Con-(R,R)(salen) in the absence and presence of1-phenylethyl chloride and atmonpheric CO2. This work opens up a new way for the asymmetry electrosynthesis of chiral aromatic carboxylic acid compound, extending the application of the chiral Con(salen) complexes in the asymmetry electrochemical fixing of CO2.