Sulfoxide, electrochemical reduction

I reasoned that it may be possible to change the pathway for breakdown of radical ion intermediates such as XVII by either changing the acetoxy leaving group in II to one which would leave more readily (e.g., halide, SCN or pyridinium) or by changing the electronic character of the sulfur atom (e.g., by oxidation to the sulfoxide). The use of the sulfoxide of II as the electrochemical reduction substrate proved to be most successful, eliminating both of the unwanted pathways B and C in Scheme 10. Work at CSU on the other major practical objectives (increasing... 

Transient quantities of 02 in aqueous solution can be generated by pulse radiolysis of 02 and by photolysis of H202 in aqueous media. In aprotic media stable solutions of OJ can be prepared by electrochemical reduction of molecular oxygen and by the base-induced decomposition of H202. Superoxide species can also be made from basic dioxygen-saturated solutions of aniline in dimethyl sulfoxide 34... 

Reaction of l-chloro-4,4-bis(chloromethyl)pentane with magnesium in diethyl ether, followed by quenching with carbon dioxide, gave 4-(l-methylcyclopropyl)butanoic acid in 68% yield. Cyclopropane derivatives with electron-withdrawing substituents 5 were prepared by elec-troreductive dechlorination of 2,4-dichlorobutanoic acid derivatives in dimethyl sulfoxide solution in the presence of tetraethylammonium 4-toluenesulfonate at ambient temperature (yields 51 -90%).The starting materials for compounds 5 can easily be obtained by copper (I)-catalyzed photochemical addition of dichloromethane to electron-deficient alkenes. Electrochemical reductive 1,3-debromination has also been achieved however, it is of little synthetic value (experimental details are described in ref 16, with yields ranging from 39 to 94%). meso- and dimethyl sulfoxide gave equal amounts of cis- and transA, 2-dimethylpropane. ... 

Electrochemical reduction of CO2 in nonaqueous solutions is significant from the following viewpoints Firstly, hydrogen evolution reaction can be suppressed. Secondly, the concentration of water as a reagent can be accurately regulated and the reaction mechanism may be more easily studied. Thirdly, the solubility of CO2 in organic solvents is much liigher than in water. Various metal electrodes have been tested for CO2 reduction in some nonaqueous solvents, such as propylene carbonate (PC), acetonitrile (AN), DMF, and dimethyl sulfoxide (DMSO), as tabulated in Table 5. Methanol is also used for CO2 reduction, and mentioned in the next Section. 

Reaction of adsorbed inhibitors. In some cases, the adsorbed corrosion inhibitor may react, usually by electrochemical reduction, to form a product that may also be inhibitive. Inhibition due to the added substance has been termed primary inhibition and that due to the reaction product, secondary inhibition. In such cases, the inhibitive efficiency may increase or decrease with time according to whether the secondary inhibition is more or less effective than the primary inhibition. Sulfoxides, for example, can be reduced to sulfides, which are more efficient inhibitors. 

This review is concerned with the formation of cation radicals and anion radicals from sulfoxides and sulfones. First the clear-cut evidence for this formation is summarized (ESR spectroscopy, pulse radiolysis in particular) followed by a discussion of the mechanisms of reactions with chemical oxidants and reductants in which such intermediates are proposed. In this section, the reactions of a-sulfonyl and oc-sulfinyl carbanions in which the electron transfer process has been proposed are also dealt with. The last section describes photochemical reactions involving anion and cation radicals of sulfoxides and sulfones. The electrochemistry of this class of compounds is covered in the chapter written by Simonet1 and is not discussed here some electrochemical data will however be used during the discussion of mechanisms (some reduction potential values are given in Table 1). 

Cation and anion radicals form from sulfoxides or sulfones by either chemical or electrochemical oxidation and reduction, respectively. The ability of compounds to accept... 

Oxidation of thiophene with Fenton-like reagents produces 2-hydroxythiophene of which the 2(570 One isomer is the most stable (Eq. 1) <96JCR(S)242>. In contrast, methyltrioxorhenium (Vn) catalyzed hydrogen peroxide oxidation of thiophene and its derivatives forms first the sulfoxide and ultimately the sulfone derivatives <96107211>. Anodic oxidation of aminated dibenzothiophene produces stable radical cation salts <96BSF597>. Reduction of dihalothiophene at carbon cathodes produces the first example of an electrochemical halogen dance reaction (Eq. 2) <96JOC8074>. 

Electrochemically, the reduction of activated sulfoxides (unsaturated or aromatic) in protic media exhibits a specific step leading to corresponding sulfides. However, in acidic solutions of sulfoxides a hydrogen reduction wave can be seen [223, 224] because of the reduction of their pro-tonated form. On the contrary, in the absence of electrophiles (in aprotic DMF), the reduction of aromatic sulfoxides was reported to afford [225] a complex between the reduced form and the substrate (the process therefore would be analogous to... 

The ideal solvent for electrochemical studies should satisfy a number of requirements. In addition to the properties required for any good solvent for organic chemistry, such as a high solvating power and a low reactivity towards common intermediates, solvents for electrochemical use should be difficult to oxidise or reduce in the potential range of interest. Traditionally, the recommended potential limits are +3 V (versus the SCE) for oxidations and —3 V for reductions. Also, the solvent should have a dielectric constant higher than about 10 in order to ensure that the supporting electrolyte is well dissociated. Commonly used solvents are acetonitrile (MeCN) and dichloromethane for oxidations, and MeCN, N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) for reductions. 

As can be seen by inspection of Table 1, the anodic and cathodic limit of a particular SSE depends on an electrochemical process involving either solvent or supporting electrolyte. In for example acetonitrile the anodic limit is dependent on the nature of the anion and the cathodic limit on the nature of the cation, whereas in dimethyl sulfoxide (see Table 2) the anodic limit is due to oxidation of solvent in cases where an oxidation-resistant anion is present, and the cathodic limit is dependent on a process involving reduction of the cation. Thus, one can order anions in a series of increasing resistance towards anodic oxidation ... 

DMF has been widely used as an electrochemical solvent, especially for the reduction of aromatic hydrocarbons.88 The polarography of a number of metal ions in DMF also has been reviewed.89 In general, die voltage range attained in reductions is comparable to acetonitrile and dimethyl sulfoxide, but DMF is less suitable for the study of oxidations. It has been suggested that the cyclic amide, iV-methylpyrrolidone, may have most of the favorable properties of DMF, but with less tendency to hydrolyze.90,91 However, it is less available and more expensive. 

Dimethyl sulfoxide is an important solvent in nonaqueous electrochemistry due to its high polarity (dielectric constant of 47), its high donor number (29.8), and a relatively wide electrochemical window. The limiting cathodic voltages in which this solvent can be used depend on the cation used (as expected from the discussion on the cation effects on the reduction processes of the above nonaqueous solvents). Using salts of alkali metals (Li, Na, K), the cathodic limit obtained was around -1.8 — -2 V versus SCE [49], whereas with tetrabutyl ammonium, the cathodic limit was as low as -2.7 — -3 V versus SCE [49], There is evidence that in the presence of Na ions, DMSO reduction produces CH4 and H2 on plati-... 

Redox processes involving 178 have also been studied.Anodic oxidation of thianthrene has been eifected in a wide variety of solvents. Use of trifluoracetic acid gives stable solutions of 178 and, if perchloric acid is included, the solid perchlorate salt may be isolated on evaporation of the solvent after electrolysis. Dichloromethane at low temperatures has been used and, at the opposite extreme, fused aluminum chloride-sodium chloride mixtures. " Propylene carbonate permits the ready formation of 178, whereas the inclusion of water in solvent mixtures gives an electrochemical means of sulfoxidizing thianthrene. Reversible oxidation of 178 to thianthrenium dication may be brought about in customary solvents such as nitriles, nitro compounds, and dichloromethane if the solvent is treated with neutral alumina immediately before voltammetry addition of trifluoracetic anhydride to trifluoracetic acid equally ensures a water-free medium. The availability of anhydrous solvent systems which permit the reversible oxidation and reduction of 178 has enabled the determination of the equilibrium constants for the disproportionation of the radical and for its equilibria with other aromatic materials. ... 

Electrochemical formation of Ti2+ allowing the reduction of hydroxylamines, sulfoxides, nitroso [60] ... 

The [Co(trans-diammac)] -mediated DMSO reductase electrochemistry in Figure 5.20D was scaled up by using a large surface area reticulous vitreous carbon working electrode to enable the bulk catalytic reduction of racemic mixtures of chiral sulfoxides methyl p-io y sulfoxide, methyl phenyl sulfoxide and phenyl vinyl sulfoxide. Extraction of the electrochemical solution with chloroform followed by chiral HPLC revealed that R. capsulatus DMSO reductase is able to kinetically discriminate between the S- and i -sulfoxides the 5-enantiomer being reduced preferably (Figure 5.21). 

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