Graphite cathodic reduction

Shigehara K, Anson EC. 1982. Electrocatal3dic activity of three iron porphyrins in the reduction of dioxygen and hydrogen peroxide at graphite cathodes. J Phys Chem 86 2776. 

Miller and his co-workers60) reported surprisingly high optical yields, close to 50 %, in the reduction of 2-acetylpyridine in the presence of strychnine. They also prepared chemically modified electrodes with optically active amino acids and attempted asymmetric induction in both reduction and oxidation61 . The best optical yield, only 14.5 %, seemed to be obtained in the reduction of 4-acetyl-pyridine on a graphite cathode modified with (S)-phenylalanine methyl ester. 

The formation of TAA-metals by cathodic reduction of TAA+ cations at several solid electrodes has recently been reported n. These coloured materials were observed at Hg, Pb, Sn, Bi and Sb cathodes, while Pt and Cr were unreactive. They all act as reducing agents and seem to incorporate both TAA+ and metal atoms (from the cathode) into their structure. They behave similarly and are probably related to the compounds resulting from the reduction of TAA+ at graphite cathodes 2). The best known and most extensively studied TAA-metals are those generated at mercury cathodes. They are also the likely catalysts for the organic electroreductions described below. Because TAA-mercury may be pervasively involved in the preparative reductions which are the topic of this review, the next few paragraphs provide information about their composition and evidence for their involvement as catalysts. 

In Section 2 we showed that the properties of amorphous carbon vary over a wide range. Graphite-like thin films are similar to other carbonaceous materials (glassy carbon, and the like) in their electrode behavior. Redox reactions proceed in a quasi-irreversible regime on these films. In particular, the cathodic reduction of nitrate previously studied on crystalline diamond electrodes (see Section 6.4.2) was performed with amorphous carbon films prepared by UHV laser deposition and comprising both sp2- and sp3-carbon [152], Reduction current density as high as 2 mA cur 2 was reached in neutral or alkaline solutions. The use of such electrodes in microgravimetry is discussed [153]. 

Kariv-Miller and coworkers have developed indirect electroreductive cyclizations with the dimethyl-pyrrolidinium ion (DMP") as a mediator. Preparative electrolysis of 6-hepten-2-one (9) at a graphite cathode afforded cu-dimethylcyclopentanol (10) in 90% yield (equation 5). The reduction is believed to occur via the ketyl radical anion, which cyclizes onto the alkenic bond. In the absence of DMP simple reduction to 6-hepten-2-ol takes place.Very recently it was shown that instead of DMP several aromatic hydrocarbons can be used as mediators to initiate the cyclization reaction. The carbonyl group can also be cyclized onto an alkynic bond and even an aromatic ring. - ... 

The electrochemical reduction of formaldehyde to the corresponding pinacol, dihy-droxyethane, has been closely examined as a possible technical scale process. Yields are very dependent on the choice of reaction conditions. Best results are obtained with a graphite cathode and sodium formate as electrolyte at 57°C [20]. The reduction of acetone to pinacol has also been examined from a technical point of view. Moderate yields of pinacol are obtained at a lead cathode in acid solution together with isopropanol and propane. The propane arises by hydrolysis of lead alkyl intermediates and under some conditions tetraisopropyllead is formed [21]. A pilot plant scale production of acetone... 

Attempts to achieve optical induction during the reduction of aromatic ketones to the secondary alcohol with the help of a single layer of chiral catalyst covalently attached to a graphite cathode have been much less successful. Reduction of 4-acetylpyridine at a graphite surface coated with (S)-phenylalanine methyl ester afforded an enantiomeric excess of the (S)-pyridinylethanol. In other experiments, ethyl glyoxylate afforded ethyl (—)-mandelate with 9.7% enantiomeric excess [101]. Results using such modified surfaces have, however, proved highly irreproducible [102]. 

Derivation (1) Reduction of the fluoride with calcium, (2) electrolysis of the chloride with sodium chloride or potassium chloride in an iron pot that serves as an anode and graphite cathode. 

The AGo values can be transformed into the corresponding standard electrochemical potentials, (Eq. 4.1). For instance, thc conversion of PTFE (AGr= —365kJ/mol CF2) into polyyne and HI (reaction (4.11b) in a hypothetical cell with standard hydrogen electrode) would have AGo = —71kJ/mol. The corresponding standard redox potential PTFE/polyyne is Eq = 0.74 V, which is just 0.36 V smaller than the standard potential of PTFE/graphite (Eq = 1V) [3]. Apparently, in terms of the reaction thermodynamics, the electrochemical carbyne should be easily accessible via cathodic reduction of PTFE. Analogously, the oxidation of acetylene (AGr = —209.9 kJ/mol) to polyyne (Eq. 4.11a) corresponds to AGo = 2.1kJ/mol, Ea = -0.02 V. 

The largest group of elements comprises those isolated from solution in the elemental form as a result of reduction, usually electrochemical. In acid solution, the electrolytic deposition of metal on a solid cathode is limited to noble and semi-noble metals. Trace analysis of copper and its compounds may serve as an example [100]. An anodic dissolution technique may be applied for the isolation of macroscopic amounts of copper. A sample in the form of a bar, plate, or wire is the anode in the electrolytic system. When current is passed through the electrolyte (nitric acid + persulphate), Cu is deposited on the graphite cathode, while most trace elements accumulate in the solution. In the trace analysis of platinum, the matrix has been also separated on a cathode [101]. 

The dimer, 10,10 -bisvindoline, is the major product (60% yield) obtained when vindoline is oxidized electrochemically with the addition of trifluoro-acetic acid to the anodic compartment using a platinum anode and a graphite cathode, followed by controlled potential cathodic reduction. Two minor products, so far unidentified, were also obtained (205). 

Preparation of binary-alkali metal graphites by cathodic reduction of graphite in molten salt electrolytes is possible in principle but has no advantage over conventional vapor-phase intercalation, particularly because of difficulties related to the separation of the product from the melt. Cathodic intercalation of alkali cations from solid or organic polymer electrolyte is also possible . 

Cathodic reduction of graphite in dipolar aprotic solutions of alkali and [NRJ salts (e.g., in ethers, esters, amides) usually yields solvated cation intercalation compounds " however, at high cation density the solvent may be squeezed out . The corresponding phosphonium and sulfonium graphite intercalation compounds are unstable their formation is accompanied by reductive decomposition of these cations . 

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