Sulfenic acid oxidation

The reaction of thiirane 1-oxides with water or methanol is usually acid-catalyzed and gives /3-substituted sulfenic acids which dimerize to thiolsulfinates (54 Scheme 70) (72JA5786). If acetic acid is used a mixture of disulfide (55) and thiolsulfonate (56) is obtained. Treatment of thiirane 1,1-dioxides with hydroxide ion may involve attack on carbon as well as on sulfur as exemplified by 2-phenylthiirane 1,1-dioxide (Scheme 71). 

Section 15.13 Thiols are compounds of the type RSH. They are more acidic than alcohols and are readily deprotonated by reaction with aqueous base. Thiols can be oxidized to sulfenic acids (RSOH), sulfinic acids (RSO2H), and sulfonic acids (RSO3H). The redox relationship between thiols and disulfides is important in certain biochemical processes. 

The addition of sulfenic acids to olefins207 has been successfully applied in the synthesis of thietanoprostanoids, the thietane analogues of prostaglandin245. The general synthetic scheme is presented in equation 83207. The key step is the thermolysis of either erythro- or (7ireo-2-f-butylsulphinyl-3-vinyl-l-ol (209) to give the corresponding alkenesulfenic acids 210, which cyclize spontaneously to a mixture of stereoisomeric thietane oxides. 

The main result of the thermolysis of the three-membered ring sulfoxides and sulfones is the extrusion of the sulfur monoxide and the sulfur dioxide moieties (Section III.C. I)99 10 5. Only in the presence of a suitably disposed /J-hydrogen does the ordinary sulfoxide-sulfenic acid fragmentation take place in the thiirane oxide series (equation 9). 

It is noteworthy that, based on the sulfoxide- sulfenic acid rearrangement, the readily accessible 1,3-dithiolane systems (316) may be utilized (equation 116) as an efficient entry into the 1,4-dithiane series303, including the construction of carbocyclic fused systems304. The oxidation of the dithienes 318 to the corresponding sulfoxides (319 and 320) and sulfones is a simple, straightforward process. 

Jones and Lewton250 have also demonstrated the utility of the intramolecular addition of sulfenic acids to olefins as a stereospecific method for the synthesis of thiolan 1-oxides. 

Gilbert and coworkers18 were able to detect ESR spectra of sulfinyl radicals together with other sulfur-centered radicals during the oxidation of disulfides and thiols with a titanium(III)-peroxide couple reaction mechanisms involving sulfenic acids as intermediates have been discussed. 

The most common procedure previously employed to effect the elimination of thiols from thioacetals has been heating in the presence of a protic acid. For example, propionaldehyde diethyl thioacetal is converted to 1-ethylthio-l-propene on heating at 175° in the presence of phosphoric acid. The relatively high temperature and acidic conditions of such procedures are, however, distinct disadvantages of this method. Another approach consists of oxidation of a thioacetal to the mono S-oxide and thermal elimination of a sulfenic acid at 140-150°. ... 

In the presence of a suitably disposed /i-hydrogen—as in alkyl-substituted thiirane oxides such as 16c—an alternative, more facile pathway for thermal fragmentation is available . In such cases the thiirene oxides are thermally rearranged to the allylic sulfenic acid, 37, similarly to the thermolysis of larger cyclic and acyclic sulfoxides (see equation 9). In sharp contrast to this type of thiirane oxide, mono- and cis-disubstituted ones have no available hydrogen for abstraction and afford on thermolysis only olefins and sulfur monoxide . However, rapid thermolysis of thiirane oxides of type 16c at high temperatures (200-340 °C), rather than at room temperature or lower, afforded mixtures of cis- and trans-olefins with the concomitant extrusion of sulfur monoxide . The rationale proposed for all these observations is that thiirane oxides may thermally... 

There are several reactions in which the sulfoxy oxygen exhibits its nucleophilicity, the most noticeable being the thermal rearrangement of thiirane oxides (in the presence of a suitable disposed 8-hydrogen) to allylic sulfenic acids (see equation 9 in Section III.C.l). 

The thermolysis of acyclic- and/or six- and larger ring sulfoxides to yield olefins and sulfenic acids is well documented . The formation of allylic sulfenic acids and thiosulfinates in the thermolysis of thiirane oxides containing hydrogen on the a-carbon of the ring substituent (which is syn to the S—O bond) has been discussed previously in terms of /i-elimination of hydrogen, which is facilitated by relief of strain in the three-membered ring (Section llI.C.l). 

More recently, several similar thermal rearrangements of other cyclic sulfoxides involving reversible intramolecular cycloelimination of sulfenic acid have also been reported . A detailed study of this rearrangement which, in certain cases, is accompanied by ring expansion or ring contraction has been performed by Jones and coworkers. These authors found that at 140 °C in xylene, conditions under which acyclic sulfoxides readily decompose , thian 1-oxide (180a) was inert after 6 days. 

During the chain oxidation of hydrocarbons, sulfides and disulfides terminate chains by reacting with peroxyl radicals [40,42,44], which, as opposed to phenols, are weak inhibitors (see Table 17.6). The mechanism and stoichiometry of the termination reaction by sulfides remain yet unclear. Since sulfenic acid is an efficient scavenger of free radicals, the oxidation of tetralin in the presence of dialkylsulfoxide occurs only if the initiation rate v > vimin is proportional to the concentration of sulfoxide [5], so that the rate of oxidation is... 

The resulting products, such as sulfenic acid or sulfur dioxide, are reactive and induce an acid-catalyzed breakdown of hydroperoxides. The important role of intermediate molecular sulfur has been reported [68-72]. Zinc (or other metal) forms a precipitate composed of ZnO and ZnS04. The decomposition of ROOH by dialkyl thiophosphates is an autocata-lytic process. The interaction of ROOH with zinc dialkyl thiophosphate gives rise to free radicals, due to which this reaction accelerates oxidation of hydrocarbons, excites CL during oxidation of ethylbenzene, and intensifies the consumption of acceptors, e.g., stable nitroxyl radicals [68], The induction period is often absent because of the rapid formation of intermediates, and the kinetics of decomposition is described by a simple bimolecular kinetic equation... 

Peroxynitrite easily oxidizes nonprotein and protein thiyl groups. In 1991, Radi et al. [102] have shown that peroxynitrite efficiently oxidizes cysteine to its disulfide form and bovine serum albumin (BSA) to some derivative of sulfenic acid supposedly via the decomposition to nitric dioxide and hydroxyl radicals. Pryor et al. [124] suggested that the oxidation of methionine and its analog 2-keto-4-thiomethylbutanic acid occurred by two competing mechanisms, namely, the second-order reaction of sulfide formation and the one-electron... 

The attack by the thiolate anion on the N-oxide oxygen of 62 produces the intermediate sulfenic acid derivative 65, which, in the presence of thiols, further reacts with the thiolate anion, to give the oxime 66, which has been isolated among the reaction products. By contrast, spontaneous loss of the halide anion from 65 affords the ni-troso intermediate 67 that, by losing NO and the thiyl radical directly, or through 68, produces the a-nitrosoolefm 69. By a Michael type reaction with water this last product immediately yields the final oxime 70, which has been isolated among the reaction products. 

A range of other sulfur(IV) and (VI) derivatives formally obtained by the oxidation of thioglycosides have been prepared, but they are apparently not employed to date in O-glycosylation reactions. These include glycosyl sulfenamides and sulfonamides, as well as sulfinates and sulfonates [289]. S-Glycosyl sulfenic acids have been prepared as transients by the syn-elimination of S-(2-cyanoethyl) glycosyl sulfoxides [311]. 

Disulfides can be reduced to two thiols (Fig. 5.14). The best example is the reduction of oxidized glutathione (GSSG) back to the reduced form (GSH) (Fig. 5.14), which is mediated by glutathione reductase. In addition, exchange can occur with other thiols mediated by protein disulfide isomerase. In principle, sulfenic acids can probably also be reduced back to thiols, but because of the reactivity of the sulfenic acid, this is not generally observed. 

Because anhydrides of sulfenic acids have the thiolsulfinate structure they can also be formed from disulfides by oxidation. Treatment of a disulfide with one mole of peracid, for example, gives the corresponding thiolsulfinate (25a) (Small et al., 1947 Block and O Connor, 1974b). Murray and Jindal (1972)... 

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