Sulfides to sulfoxides or sulfones

Abstract This chapter principally concerns oxidations of organic substrates containing N, O, S, P, As and Sb. Oxidations of amines are covered first, including primary amines to nitriles or amides secondary amines to imines or other products tertiary amines to N-oxides or other prodncts (Section 5.1) and the oxidation of amides (5.2). Oxidation of ethers to esters or lactones follows (5.3), then of sulfides to sulfoxides or sulfones (5.4) and of phosphines, arsine and stibines to their oxides (5.5). A final section (5.6) concerns such miscellaneous oxidations not covered by other sections in the book. 

Peracids have been employed for the transformation of sulfides to sulfoxides or sulfones.377 Reactions with peracids proceed at higher rates in solvents which favour internal hydrogen bonding, e.g. benzene, chloroform or tetrachloro-methane, than in solvents which themselves hydrogen-bond to the peracid. 

In most instances, double and triple bonds remain unaffected by the oxidation of sulfides to sulfoxides or sulfones, even when hydrogen peroxide and peroxy acids are used [163, 264, 322, 324, 506, 521], Evidently, the affinity of sulfur to oxygen is higher than that of the carbon-carbon multiple bonds (equations 574 and 575). 

The oxidation of sulfides to sulfoxides or sulfones depend on the reaction conditions used. Hydrogen peroxide and ferf-butyl hydroperoxide have both been used as also has a variety of peroxycarboxylic acids. Since all of the peroxycarboxylic acids are stronger oxidants than hydrogen peroxide, reagents such as MCPBA have been used to oxidise sulfides to sulfoxides under very mild conditions. In the example shown in Eq. (38) the diastereomeric sulfoxides were obtained in good yield [46]. 

The oxidation of sulfides to sulfoxides or sulfones, as well as the reverse reduction strategy, is a well-known method for the design of a safety-catch protocol. As nucleophilic displacement is facilitated if electron-withdrawing groups are placed... 

Scheme 25. Possible sites for oxidation with ruthenium tetroxide (7) Oxidation of sulfide to sulfoxide or sulfone [140].

Jimenez et al. have developed tridentate cyclopentadienyl-silesquioxanate titanium complexes for the epoxidation of cyclic and linear alkenes with aqueous hydrogen peroxide under mild reaction conditions with excellent reactivity and selectivity. The authors extended the studies of these complexes as catalysts for the oxidation of sulfides to sulfoxides or sulfones under mild reaction conditions. The catalysts showed high chemoselec-tivity and proved to be very stable as no loss of activity or selectivity was observed after 14 cycles. 

Shaabani, A. and Rezayan, A.H. 2007. Silica sulfuric acid promoted selective oxidation of sulfides to sulfoxides or sulfones in the presence of aqueous HjOj. Catal. Commun. 8(7) 1112-1116. 

Data presentations should include the parent compound and all toxic transformation products. This is particularly important for oxidation of sulfide linkages to sulfoxides or sulfones. These products are often equally toxic to the parent with increased availability. Attention should also be given to oxidative desulfuration of phosphorothionate esters. 

Selective oxidation of sulfides to sulfoxides and sulfones was achieved over sodium periodate (NaI04) on silica (20%), under solvent-free conditions to afford either sulfoxides or sulfones as desired (Scheme 5) [49]. 

Sulfides are easy to oxidize and, depending on the type and quantity of oxidizing agent used, they can be cleanly oxidized either to sulfoxides or sulfones. 

Imidazole- and benzimidazole-2-thiols usually exist largely as the thione tautomers. The thiol (thione) group is susceptible to alkylation (especially in alkaline media), and can be oxidized to sulfide, disulfide and sulfonic acid. This oxidation can often be carried out quite selectively by careful choice of oxidizing agent. The sulfur function can be removed with nitric acid, iron(III) chloride, hydrogen peroxide or, most commonly, Raney nickel. Alkyl- and arylthio groups can be oxidized to sulfoxide or sulfone. 

In addition to the ozonolysis of alkenes and a few aromatic compounds [93, 104], ozone oxidizes other groups. Thus saturated hydrocarbons containing tertiary hydrogen atoms are converted into tertiary alcohols [105, 106], and some alkenes are transformed into epoxides [107] or a,p-unsat-urated ketones [108], Benzene rings are oxidized to carboxylic groups [109, ethers [110] and aldehyde acetals [111] to esters aldehydes to peroxy acids [772] sulfides to sulfoxides and sulfones [775] phosphines and phosphites to phosphine oxides and phosphates, respectively [775] and organomer-cury compounds to ketones or carboxylic acids [114]. 

Sodium perborate, NaBOa dlljO (mp 60 °C dec), is used for oxidations of primary aromatic amines to azo compounds [795] or nitro compounds [194] and of sulfides to sulfoxides and sulfones [794]. This reagent does not affect alcohols and only slightly affects alkenes [794]. 

The most important applications of peroxyacetic acid are the epoxi-dation [250, 251, 252, 254, 257, 258] and anti hydroxylation of double bonds [241, 252, the Dakin reaction of aldehydes [259, the Baeyer-Villiger reaction of ketones [148, 254, 258, 260, 261, 262] the oxidation of primary amines to nitroso [iJi] or nitrocompounds [253], of tertiary amines to amine oxides [i58, 263], of sulfides to sulfoxides and sulfones [264, 265], and of iodo compounds to iodoso or iodoxy compounds [266, 267] the degradation of alkynes [268] and diketones [269, 270, 271] to carboxylic acids and the oxidative opening of aromatic rings to aromatic dicarboxylic acids [256, 272, 271, 272,273, 274]. Occasionally, peroxyacetic acid is used for the dehydrogenation [275] and oxidation of aromatic compounds to quinones [249], of alcohols to ketones [276], of aldehyde acetals to carboxylic acids [277], and of lactams to imides [225,255]. The last two reactions are carried out in the presence of manganese salts. The oxidation of alcohols to ketones is catalyzed by chromium trioxide, and the role of peroxyacetic acid is to reoxidize the trivalent chromium [276]. 

The applications of ruthenium tetroxide range from the common types of oxidations, such as those of alkenes, alcohols, and aldehydes to carboxylic acids [701, 774, 939, 940] of secondary alcohols to ketones [701, 940, 941] of aldehydes to acids (in poor yields) [940] of aromatic hydrocarbons to quinones [942, 943] or acids [701, 774, 941] and of sulfides to sulfoxides and sulfones [942], to specific ones like the oxidation of acetylenes to vicinal dicarbonyl compounds [9JS], of ethers to esters [940], of cyclic imines to lactams [944], and of lactams to imides [940]. 

The oxidation of sulfides to sulfoxides and sulfones is achieved in a selective manner using MW irradiation under solvent-free conditions with desired selectivity to either sulfoxides or sulfones over sodium periodate (NaIO4) on silica (20%) (Scheme 8) A noteworthy feature of the protocol is its applicability to long chain fatty sulfides that are insoluble in most solvents and are consequently difficult to oxidize. Further, it circumvents the use of oxidants such as nitric acid, hydrogen peroxide, chromic acid, and peracids, which are conventionally used for the oxidation of sulfides to the corresponding sulfoxides and sulfones. 

In Summary The naming of thiols and sulfides is related to the system used for alcohols and ethers. Thiols are more volatile, more acidic, and more nucleophilic than alcohols. Thiols and sulfides can be oxidized, thiols to disulfides or sulfonic acids and sulfides to sulfoxides and sulfones. 

Oxidation of a sulfide to a sulfoxide or sulfone is accompanied by a decrease m shield ing of the H—C—S—C proton by about 0 3-0 5 ppm for each oxidation... 

Section 16 16 Oxidation of sulfides yields sulfoxides then sulfones Sodium metaper lodate IS specific for the oxidation of sulfides to sulfoxides and no fur ther Hydrogen peroxide or peroxy acids can yield sulfoxides (1 mole of oxidant per mole of sulfide) or sulfone (2 moles of oxidant per mole of sulfide)... 

One equivalent of a peroxy acid or of hydrogen peroxide converts sulfides to sulfoxides two equivalents gives the conesponding sulfone. 

The oxidation of sulfides to sulfoxides are occasionally found to be unsatisfactory, since the resulting sulfoxides are easily oxidized to sulfones. In order to avoid the further oxidation of sulfoxides into sulfones, several oxidizing agents have been selected. Recently, we found that BTMA Bt3 is the most effective and satisfactory oxidizing agent for this purpose. That is, the reaction of sulfides with a calculated amount of BTMA Br3 and aq. sodium hydroxide in dichloromethane at room temperature, or in 1,2-dichloroethane under reflux, gave sulfoxides in good yields (Fig. 28) (ref. 36). 

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