Electrochemical Reduction Of Aromatic Ketones

Ecological environment has been severely destroyed by human's activies and industrial revolution, and caused many ecological and environmental problems, eg. air pollution, water pollution, soil pollution, green house and so on. Moreover, many ecological and environmental problems were caused by chemicals. To solve these problems and protect ecological environment, the green chemistry has become more and more promising. In recent decades, an electrochemical methodology based on the use of an undivided cell equipped with a sacrificial anode (Mg, Al) has been a very convenient,"green" and cheap way in electrochemical synthesis.CO2is recognized to be a naturally abundant, cheap, recyclable, nontoxic building block in organic synthesis. However, the conversion and utilization of carbon dioxide have been challenging for chemists from the ecological and economic points of view due to its high chemical stability. In recent years electrosynthesis has been recognized as a convenient, low cost and ecofriendly method for CO2fixation. The electrochemical activation of aromatic ketones is also one of the most important reactions in organic electrochemistry. Therefore, a-hydroxy acids can be prepared in a much more environmental friendly way via the electrocarboxylation of the corresponding aromatic ketones with sacrificial anodes. α-Hydroxy acids are an important class of compounds, used to cure certain skin diseases, or used as pharmaceutical/fine chemical intermediates in the production of certain anti-inflammatory drugs. Form an applicative point of view, one of the most active current areas of chemical research focuses on how to synthesize chiral compounds, because different biological activities. Drawing inspiration from generic electrocarboxylation, there is also a great need to synthesize chiral carboxylic acids in organic electrochemistry.Room-temperature ionic liquids have received considerable attention in green chemical applications as attactive alternatives to molecular solvents because they exhibit many desirable properties, such as negligible volatility, high thermal and chemical stability, low toxicity, non-flammability and the ability to dissolve large amounts of organic and inorganic compounds. For electrochemical applications, room-temperature ionic liquids have additional advantages of high conductivity and electrochemical stability.The mechanism of electrochemical reduction of acetophenone in1-butyl-3-methylimidazolium tetrafluroborate ([Bmim][BF4]) under nitrogen (N2) and carbon dioxide (CO2) atmospheres have been investigated using transient voltammetry, steady-state voltammetry, bulk electrolysis and numerical simulation. To study the mechanism incorporating protonation and hydrogen bonding interaction of the benzophenone dianion with water, reduction of benzophenone in ionic liquids has been investigated in five different ionic liquids, using transient cyclic voltammetry, near steady state voltammetry and numerical simulation. To obtain the reversible potentials of electrode processes in ionic liquids devoid of significant modification by adventitious water, voltammetric reduction of benzophenone and1,4-benzoquinone microparticles adhered to a glassy carbon electrode placed in contact with the "wet" ionic liquid,1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpyrd][NTf2])(water content-0.13M) have been investigated under bench top conditions. The results are rationalized in terms of the method providing high localized analyte concentrations that significantly exceed the water concentration near to the electrode surface.The details are given as follows:(1) Influences of the operative parameters and the nature of the substrate on the electrocarboxylation of benzophenonesThe electrocarboxylation of a series of benzophenones under galvanostatic conditions has been carried out in aprotic solvents using an undivided bulk electrolysis cell equipped with a Mg sacrificial anode. Systematic studies have been carried out in order to establish the qualitative relationships between the yield of carboxylation reaction and the operational and intrinsic parameters for the electrocarboxylation of benzophenones. For the diaryl ketones chosen for these studies, the yields of the target benzilic acids have been found to be strongly dependent on different parameters such as solvents, supporting electrolytes, cathode materials, current density, temperature and the nature of the substrates. (2) Alkaloid induced asymmetric electrocarboxylation of4-methylpropiophen-oneThe alkaloid-induced electrocarboxylation of4-methylpropiophenone is examined in mild conditions. Comparative studies with several inductors indicate that the efficient enantiodiscrimination of the electrocarboxylation depends on the nucleophilic quinuclidine nitrogen atom and the OH group of the inductors. And the mechanism of the alkaloid-induced electrocarboxylation of4-methylpropiophenone has been investigated via a reaction mechanism incorporating protonation interaction of the4-methylpropiophenone anion radical with chiral proton from alkaloid and general electrocarboxylation with CO2.(3) A unique proton coupled electron transfer pathway for electrochemical reduction of acetophenone in ionic liquid [Bmim][BF4] under a carbon dioxide atmosphereThe mechanism of electrochemical reduction of acetophenone in1-butyl-3-methylimidazolium tetrafluroborate ([Bmim][BF4]) under nitrogen (N2) and carbon dioxide (CO2) atmospheres have been investigated using transient voltammetry, steady-state voltammetry, bulk electrolysis and numerical simulation. Under a N2atmosphere, acetophenone undergoes a one-electron reduction to the anion radical followed by a rapid dimerization reaction with a rate constant of5×105M-1s-1leading to the formation of2,3-diphenyl-butane-2,3-diol. In contrast, under a CO2atmosphere, the electrochemical reduction of acetophenone is an overall two-electron transfer chemically irreversible process with the final electrolysis product being1-phenylethanol, instead of the anticipated2-hydroxy-2-phenylpropionic acid resulting from an electrocarboxylation reaction. A proton coupled electron transfer pathway leading to the formation of1-phenylethanol requires the presence of a sufficiently strong proton donor which is not available in neat [Bmim][BF4]. However, the presence of CO2enhances the C-2hydrogen donating ability of [Bmim]+due to strong complex formation between the deprotonated form of [Bmim]+, N-heterocyclic carbine, and CO2, resulting in a thermodynamically favorable proton coupled electron transfer pathway.(4) Remarkable sensitivity of the electrochemical reduction of benzophenone to proton availability in ionic liquidsReduction of benzophenone has been investigated in five different ionic liquids, using transient cyclic voltammetry, near steady state voltammetry and numerical simulation. Two reversible and well-resolved one-electron reduction processes are obtained in dry (≤20ppm water)1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpyrd][NTf2]) and1-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide ([Bmpipd][NTf2]), which do not contain any readily available proton source. Upon addition of water, the second process becomes chemically irreversible and shifts in the positive potential direction by approximately600mV and the two reduction processes merge into a single two-electron proton coupled process when about0.6M H2O is present. This large dependence of potential on water content, which is not observed in molecular solvents (electrolyte), is explained via a reaction mechanism incorporating protonation and hydrogen bonding interaction of the benzophenone dianion with as many as seven water molecules. In the three imidazolium-based ionic liquids used in this study, the first benzophenone reduction process is again reversible, while the second reduction process is now chemically irreversible due to the availability of C2-H imidazolium protons in these ionic liquids. The reversible potentials for benzophenone reduction are remarkably independent on the identity of the ionic liquids implying either weak interaction with the ionic liquids or of relatively insignificant differences in levels of ion pairing. Thus, the magnitude of the separation of potentials of the reversible first and irreversible second reduction processes mainly reflect the proton availability from either the ionic liquid itself or adventitious water. Consequently, voltammetric reduction of benzophenone provides a sensitive tool for the determination of proton availability in ionic liquids.(5) Voltammetric studies in "wet"1-butyl-l-methylpyrrolidinium bis(trifluoro-methylsulfonyl)imide ionic liquid using electrodes with adhered microparticlesVoltammetric reduction of benzophenone and1,4-benzoquinone microparticles adhered to a glassy carbon electrode placed in contact with the "wet" ionic liquid,1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpyrd][NTf2])(water content～0.13M) have been investigated under bench top conditions. For both compounds, two well-defined reversible reduction processes were observed. Significantly, all voltammetric characteristics, except peak current magnitudes, closely resemble derived from the voltammograms obtained in a dry box with carefully dried [Bmpyrd][NTf2](water content～1mM) containing10mM of benzophenone or1,4-benzoquinone. In contrast, in "wet"[Bmpyrd][NTf2] solutions containing10mM or100mM of dissolved benzophenone, the second reduction process is chemically irreversible since the electrogenerated benzophenone dianion, a strong base, rapidly reacts with water to form benzhydrol, which is electroinactive in the potential region of interest. Although both reduction processes for1,4-benzoquinone still remain reversible in "wet"[Bmpyrd][NTf2], the reversible potential for the second process shifts to more positive values, relative to the value in "dry" conditions and the water effect remains significant under conditions of cyclic voltammetry even when a fast scan rate of1000V s-1is applied. Data suggest that the reversible potentials of electrode processes in ionic liquids devoid of significant modification by adventitious water can be obtained conveniently under bench top conditions using the voltammetry of adhered microparticles, without the need for extensive drying and use of experimentally more cumbersome dry box conditions. The results are rationalized in terms of the method providing high localized analyte concentrations that significantly exceed the water concentration near to the electrode surface.