Thiolate ions oxidation reactions

The island mechanism assumes that adsorption takes place in wet conditions and H2O adsorbed on the carbon surface, is able to create water clusters or small water islands, where MM and oxygen first are dissolved in molecular form and later, depending on the pH, MM can dissociate to thiolate ion. Oxidation occurs due to the surface reaction between adsorbed thiolate ions and dissodatively adsorbed oxygen in water "islands" and/or between adsorbed MM and oxygen on a dry part of the carbon surfece. The product of reaction, DMDS, is adsorbed in a molecular form on the carbon surface (Fig. 35). 

The reaction mechanism probably simply involves one-equivalent oxidation of thiolate ion. 

The conversion (19) of thiols to disulphides coupled with reduction of flavin (vitamin B2 family) is a topic of import in connection with coenzyme reactivity in flavoenzymes. Since flavin oxidation of thiols involves nucleophilic attack of thiolate ion in the rate-determining step (Loechler and Hollocher, 1975 Yokoe and Bruice, 1975), this biologically important reaction would be markedly affected by hydrophobic environments. 

The [Fen(CN)5NO]3 ion can be oxidized back to NP in the presence of oxygen (see the reactions of NP with thiolates below) (57,60). Detailed kinetic studies on this important bioinorganic oxidation reaction are not available. In a more general context, the mechanism of the autoxidation reactions of NO complexes awaits a systematic study, which could in principle be afforded with the known (MX5(NO + ) series (M = Fe, Ru or Os X = amines, polypyridines, etc.), if appropriate reductants were used to generate the NO complexes. 

The (oxidizing) a-carboxyalkyl radicals do not react readily with thiols (Table 6.4). They are, however, rapidly reduced by thiolate ions [reaction (20)]. The reactions of thiols with DNA radicals play a very important role in the chemical repair of DNA radicals in cells (Chaps 12.10 and 12.11). The reversibility of the H-donation of thiols, that is, H-abstraction by thiyl radicals, is discussed in Chapter 7.4. 

Another common type of reaction involving sulfur compounds is the oxidation of thiols or thiolates to disulfides. This process is found to be very sensitive to the presence of metal ions. The metal can act as the primary oxidant, or dioxygen may be involved in a reaction with a co-ordinated thiol (Fig. 9-10). Very often, oxidation reactions involving metal ions and thiols are catalytic in the metal. 

Oxidation of DMDS by Q-> Oxidation of DMDS in alkaline medium is preceded by a hydrolysis reaction which involves nucleophilic displacement of sulfur from sulfur by hydroxide ion to yield a thiolate ion and a sulfonate ion (41) ... 

Thiolate ions have been reported to undergo reactions with aliphatic a-halonitro compounds5114,115 to yield the substitution products a-nitrosulphides or disulphides by oxidative dimerization. The substitution reaction is favoured by weakly nucleophilic thiolate ions and proceeds by an S l mechanism. On the other hand, strongly nucleophilic thiolate ions favour the redox reaction by an ionic abstraction (X-philic) mechanism. 

Our most recent report (3) on reactions of pyronin cation showed a correlation between the rates of reactions of a variety of nucleophiles and the one-electron oxidation potentials of the nucleophiles in aqueous solution. The correlation included anionic, neutral, and a-effect nucleophiles only thiolate ion nucleophiles showed significant deviations from the correlation. 

In the first step of the mechanism for the FAD-catalyzed oxidation of dihydrolipoate to lipoate, the thiolate ion attacks the C-4a position of the flavin ring. This reaction is general-acid catalyzed—as the thiolate ion attacks the ring, a proton is donated to the N-5 nitrogen. A second nucleophilic attack by a thiolate ion, this time on the sulfur that is covalently attached to the coenzyme, generates the oxidized product and FADH2. Section 25.4 discusses where this FAD-catalyzed reaction fits into metabolism. 

A well-known example of a complex catalytic reaction that takes place on the surface of carbon is the oxidation of hydrogen sulfide [329,330], When water is present on the carbon surface and the surface has the basic pH required for dissociation of H2S, oxidation of the HS ions by active oxygen occurs either to elemental sulfur or sulfuric acid. The latter is formed when the reaction takes place in very small pores, where only sulfur radicals very susceptible for further oxidation to SO3 are formed. Catalytic oxidation also occurs in the case of methyl mercaptan adsorption [331], where on basic carbon, thiolate ions formed as a result of dissociation are further oxidized to dimethyldisulfide strongly adsorbed in the pore system. In the case of desulfurization, inorganic constituents of carbon such as iron and calcium also play a crucial role. Those elements, present even in small amounts, contribute significantly to the oxidation reactions as catalysts [332,333],... 

In perchloryl fluoride (FCIO3) the fluorine is a soft acid. Formation of the C-F bond is observed when it is treated with carbanions. Thiolates are oxidized to disulfides by this reagent (92, 93). This latter reaction presumably involves sulfenyl fluoride intermediates. In contrast, alkoxide ions effect displacement at chlorine to afford perchlorate esters or further transformation products. 

The products of the reactions of thiols or thiolate ions with alkylene oxides may correspond to addition across the C—O bond, or reactions of the isomeric aldehyde or ketone if the temperature is sufficiently high. Simple addition is observed in reactions such as... 

The hypothesis that the amine-catalysed oxidation of thiols is a particular case of the more general reaction of oxidation by molecular oxygen of thiolate ions is confirmed by the finding that arene-thiols, which are more acidic and hence more dissociated, are oxidized faster than arylalkane-and alkane-thiols in the presence of amines . 

A second thiolate ion attacks the sulfur that is covalently attached to the coenzyme, forming the oxidized product and FADH2. This is a general-base catalyzed reaction a base removes a proton from the sulfur to make it a better nucleophile. 

Sodium mercaptides are prepared from the mercaptans and aqueous or alcoholic solutions of sodium hydroxide or alcoholic sodium eth-oxide. The sodium mercaptide reacts with halides, chlorohydrins, esters of sulfonic acid, or alkyl sulfonates [6] to give sulfides in yields of 70% or more. A recent report describes a general procedure for synthesizing aryl thioesters by a nucleophilic displacement of aryl halide with thiolate ion in amide solvents. No copper catalysis is necessary as in an Ullmann-type reaction. 

Nucleophilic substitution by S and Sg (generated electrochemically) at nitrohaloaromatics have been studied, and the mechanism suggested involves Sl and Sg as the active species, and not radical anions. The reaction of sulfur with thiolate ions RS in dimethylacetamide has been studied electrochemically. The reaction leads to the formation of RS, which is then oxidized to RSSR and polysulfide ions. Rate constants for the inter-molecular thiolate/disulfide exchanges, shown in Eq. (33), are much faster ... 

Thiols, the sulfur analogs of alcohols, are usually prepared by Sjv 2 reaction of an alkyl halide with thiourea. Mild oxidation of a thiol yields a disulfide, and mild reduction of a disulfide gives back the thiol. Sulfides, the sulfur analogs of ethers, are prepared by an Sk2 reaction between a thiolate anion and a primary or secondary alkyl halide. Sulfides are much more nucleophilic than ethers and can be oxidized to sulfoxides and to sulfones. Sulfides can also be alkylated by reaction with a primary alkyl halide to yield sulfonium ions. 

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