Sulfur electrochemical reduction

Electrowinning from Aqueous Solutions. Electrowinriing is the recovery of a metal by electrochemical reduction of one of its compounds dissolved in a suitable electrolyte. Various types of solutions can be used, but sulfuric acid and sulfate solutions are preferred because these are less corrosive than others and the reagents are fairly cheap. From an electrochemical viewpoint, the high mobiUty of the hydrogen ion leads to high conductivity and low ohmic losses, and the sulfate ion is electrochemicaHy inert under normal conditions. 

Deuteration by Electrochemical Reduction of a Steroidal Ketone in the Presence of Deuterium Oxide-10% Deuterio-sulfuric Acid... 

Deuteration by electrochemical reduction of a steroidal ketone in the presence of deuterium oxide-10% deuterio-sulfuric acid, 168... 

The picolinamide is prepared in 95% yield from picolinic acid/DCC and an amino acid and is hydrolyzed in 75% yield by aqueous Cu(OAc)2 or by electrochemical reduction (sulfuric acid, MeOH, 20°, 20-94% yield)."... 

The electrochemical reduction and oxidation of sulfur and of polysulfide dianions at inert electrodes has been studied in aprotic solvents and in liquid ammonia. In the latter case, sulfur-nitrogen compounds are involved and these systems [90] will not be discussed here. 

The red tetrasulfide radical anion 84 has been proposed as a constituent of sulfur-doped alkali hahdes, of alkah polysulfide solutions in DMF [84, 86], HMPA [89] and acetone [136] and as a product of the electrochemical reduction of 8s in DM80 or DMF [12]. However, in all these cases no convincing proof for the molecular composition of the species observed by either E8R, Raman, infrared or UV-Vis spectroscopy has been provided. The problem is that the red species is formed only in sulfur-rich solutions where long-chain polysulfide dianions are present also and these are of orange to red color, too (for a description of this dilemma, see [89]). Furthermore, the presence of the orange radical anion 8e (see below) cannot be excluded in such systems. 

In the same year, Evans and coworkers reported the electrochemical reduction of protons to H2 catalyzed by the sulfur-bridged dinuclear iron complex 25 as a hydrogenase mimic in which acetic acid was used as a proton source [201]. The proposed mechanism for this reaction is shown in Scheme 60. The reduction of 25 readily affords 25 via a one electron reduction product 25. Protonation... 

The stereochemistry of electrochemical reduction of acetylenes is highly dependent upon the experimental conditions under which the electrolysis is carried out. Campbell and Young found many years ago that reduction of acetylenes in alcoholic sulfuric acid at a spongy nickel cathode produces cis-olefins in good yields 126>. It is very likely that this reduction involves a mechanism akin to catalytic hydrogenation, since the reduction does not take place at all at cathode substances, such as mercury, which are known to be poor hydrogenation catalysts. The reduction also probably involves the adsorbed acetylene as an intermediate, since olefins are not reduced at all under these conditions and since hydrogen evolution does not occur at the cathode until reduction of the acetylene is complete. Acetylenes may also be reduced to cis olefins in acidic media at a silver-palladium alloy cathode, 27>. 

The 14 re-electron [S Ns]"1" cation is formed by the reaction of S4N4 with the [SN]+ cation.42 The planar, 10-membered ring usually has an azulene shape (19), with alternating sulfur and nitrogen atoms. Electrochemical reduction of S5N5+ salts in acetonitrile produces the polymer (SN)X. 

Lastly, the electrochemical reduction of 3-imidazolin-5-thiones in an AN solution showed a rather unexpected migration of the aryl group from nitrogen to sulfur [196] occurring under electron transfer, although the mechanism was not strictly proven by the authors (Scheme 40). 

Reduction of dibenzothiophene with sodium in liquid ammonia has been shown to be sensitive to the experimental methods employed however, the major product is usually 1,4-dihydrodibenzothiophene. 27 -28i The electrochemical reduction of dibenzothiophene in ethylene-diamine-lithium chloride solution has been shown to proceed via stepwise reduction of the aromatic nucleus followed by sulfur elimination. In contrast to the reduction of dibenzothiophene with sodium in liquid ammonia, lithium in ethylenediamine, or calcium hexamine in ether, electrolytic reduction produced no detectable thiophenol intermediates. Reduction of dibenzothiophene with calcium hexamine furnished o-cyclohexylthiophenol as the major product (77%). Polaro-graphic reduction of dibenzothiophene 5,5-dioxide has shown a four-electron transfer to occur corresponding to reduction of the sulfone group and a further site. ... 

The electrochemical behavior of sulfur, sulfide (H8 , S ) and polysulfide ions in water is much less documented than for nonaqueous solvents. Experimental studies are less numerous and do not include a systematic study versus the stoichiometry n of polysulfides M28 . The conclusions of these investigations are often speculative, since the experimental curves do not display strong evidence for chemical species involved in the proposed mechanisms. Moreover, the very low solubility of sulfur in water does not allow the study of its electrochemical reduction in water. 

Cleavage of a Mo L bond. The electrochemical reduction of the acetonitrile complex ds-19 + is an overall two-electron process leading to the cleavage of the Mo—NCMe bond. The CV of cis-[Mo2(cp )2(At-SR)2(CO)3(MeCN)]2+ appeared to be strongly dependent upon the nature of the sulfur substituents and of the... 

Sulfur forms a series of homoatomic dianions catena-S (x = 2-8), which, without exception, have unbranched chain structures in the solid state.The electrochemical reduction of cyclo-Sg in aprotic solvents occurs via an initial two-electron process to produce catenaS P In solution, catena- and other long-chain polysulfides, e.g. catena- and catenaSi, dissociate via an entropy-driven process to give radical anions S (x = 2-4), including the ubiquitous trisulfur radical anion (x = 3). This intensely blue species is the chromophore in the mineral lapis lazuli, which is used in the manufacture of jewellery. 

Perfluorobenzoic acid on electrochemical reduction affords a mixture of 2.3,5.6-tetrafluoroben-zoic acid, 2,3,5,6-tetrafluorobenzyl alcohol, and perfluorobenzyl alcohol, depending on the applied potential.119 12(1 It is noteworthy that the latter is not reduced to the tetrafluorobenzyl alcohol under the same conditions the result allows supposition that the p-hydroxymethyl group is more deactivating than activating.119120 Perfluorobenzoic acid is product-seleclively reduced to a mixture of 2,3,5,6-tetrafluorobenzyl alcohol (17) and 2,3,5,6-tetrafluorobenz-aldehyde (16) at a lead cathode in dilute sulfuric acid 123 these conditions were determined... 

It is pertinent to note that the chemistry of the 1,2-diseleno ligands is less extensive than that of the analogous sulfur chelates. However, it is interesting to note that chemical or electrochemical reductive coupling of CSe2 has provided 1,2-diseleno anions which can react with a variety of metal salts to give the corresponding square planar bis chelates (Scheme 5).47 48... 

The sensor is based on high affinity of gold to mercury and on chemo-resistive properties of ultrathin gold layers adsorption of mercury leads to increase in the surface resistance [1,2]. However, this effect is not selective similar changes are caused by adsorption of water vapor and sulfuric compounds. The use of monomolecular layer of alkylthiols as a filter excludes this interference completely [3]. Sensor calibration is performed by thermoinjection of nanogram-amount of mercury quantitatively deposited by electrochemical reduction [4],... 

Recently, the electrochemical reduction of sulfur dyes was studied. For continuous processes a mediator is not necessary. In discontinuous application it is possible to reuse dyebaths [72],... 

In an electrochemical reduction in acetonitrile with a sacrificial sulfur/graphite electrode for generation of the anions S 2, the allene 62 afforded a trithiolane 63 (77%) along with an enethiol 64 (8%) (Equation 22) <1996MI1987>. However, allenes with different structure gave different products or remained unchanged. 

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