Sulfoxides to sulfones

Electrocatalytic formation of Me SO from DMSO was effected using cw-[Ru(0) (py)(bpy)2] VPt electrode/water-DMSO cf. mech. Ch. 1 [113], Oxidation of thian-threne-5-oxide by RuO /aq. Na(IO )/CCyO°C gave a mixture of the sulfone and thianthrene-5,5-dioxide with minor amounts of the disulfoxide and also thian-threne-5,5,10-trioxide comparisons were made between the behaviour of RuO, CrOjClj and [MnO ] for these reactions cf. mech. Ch. 1) [114]. 

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When the desired product is a sulfoxide sodium metaperiodate (NaI04) is an ideal reagent It oxidizes sulfides to sulfoxides m high yield but shows no tendency to oxidize sulfoxides to sulfones... 

Sulfoxides are readily reduced to sulfides however, analogous to the oxidation aldehydes, the oxidation of sulfoxides to sulfones is irreversible as illustrated by the drug sulindac in Figure 5.13. 

The reagent RuCl /aq. Na(IO )/CH3CN also oxidised sulfoxides to sulfones (Table 5.2) [435], and oxidations of P-sulfidecarboxylic acids to the corresponding... 

Oxidation of a snlfide to sulfoxide is known to be an electrophilic reaction, in contrast with nncleophUic oxidation of sulfoxide to sulfone. Since 2-nitrobenzenesulfinyl chloride/KOi oxidizes sulfides to sulfoxides selectively, intermediate 48 must be the actual active intermediate. Moreover, in the presence of l,4-diazabicyclo[2.2.2.]octane (DABCO), which is a radical capturing reagent, the oxidation of methyl phenyl sulfide to the sulfoxide was inhibited. In order to further detect the intermediate 48, pure 5,5-dimethyl-l-pyrroUne-l-oxide (DMPO) was used as a trapping reagent and spin adduct was obtained. The ESR spectrum of the DMPO spin adduct was obtained by the reaction of 02 with 2-nitrobenzenesulfinyl chloride (hyperfine coupling constants, aH = 10.0 G and aN = 12.8 G). 

Diphenylphosphinic chloride reacts with the superoxide anion radical (Oi ) in CH3CN under mild conditions to form the diphenylphosphinic peroxy radical intermediate 77, which shows strong oxidizing abilities in the epoxidation of alkenes, oxidation of sulfoxides to sulfones, desulfurization of thioamides to amides and oxidation of triphenylphos-phines to triphenylphosphine oxide in good to excellent yields (equation 105). ... 

With peroxides and peroxycarboxylic acids the oxidation of sulfides to sulfoxides proceeds much more rapidly than that of sulfoxides to sulfones. The oxidation occurs by electrophilic attack of the peroxide on sulfur and, as the nucleophilicity of the sulfur atom is reduced in the sulfoxide compared to that of the sulfide, the sulfoxides are normally easily isolable (generally 1 mol of oxidant is used). Over-oxidation of sulfoxides can also be avoided with specific reagents sodium metaperiodate has been usually used. Sulfoxides are also obtained exclusively by oxidation of sulfides with the couple Mn02/Me3SiCl in methanol [76]. 

Sulfones. The oxidant is useful for oxidation of sulfides and sulfoxides to sulfones in CH2Cl,-CH3COOH at - 10°. 

The hydridorhodium(III) species HRh(NH3)52+ and HRh(CN)4-(H20)2 are known to react with 02 to give hydroperoxide species, Rh—OOH (102, 103, 104) the sulfoxide oxidation via such an intermediate could be accommodated by several mechanisms akin to those discussed already for olefinic substrates (cf. Reactions 17 and 18). The oxidation also could occur via liberated hydrogen peroxide, which oxidizes sulfoxide to sulfone and can be formed by treatment of Rh—OOH species with acid (104). Such a mechanism would amount to a novel catalytic conversion of H2/02 mixtures to in situ H202. The part of the enzymatic cycle of P 450 that utilizes 02 and two electrons (from a hydride source) can be bypassed by using hydrogen peroxide (16, 18). 

Dioxirane species have been found to be effective oxidants for sulfides to sulfoxides and sulfoxides to sulfones.411 The oxidations both appear to be electrophilic in nature. 

Organic hydroperoxides are generally used for the preparation of sulfoxides from sulfides, - while sulfones can be obtained in neutral organic solvents in the presence of metal catalysts such as V, Mo and Ti oxides at 50-70 C. Two polymer-supported reagents which involve peroxy acid groups and bound hypervalent vanadium(V) and molybdenum(VI) compounds have been developed for facile oxidation of sulfoxides to sulfones. 

A number of halogen compounds are capable of oxidizing sulfoxides to sulfones, although synthetically useful procedures are rather limited. Chlorine and sulfuryl chloride oxidize sulfoxides to sulfones in aqueous solvents, but the oxidation is often accompanied by many side reactions as shown in equations... 

In shaip contrast to peroxy acid oxidation the oxidation of sulfoxides to sulfones with various transi tion metal salts proceeds much faster than that of sulfides to sulfoxides and consequently sulfoxides may be selectively oxidized to sulfones in the presence of sulfides. 

A number of other oxidants which produce sulfones from sulfoxides are known, such as N02BF4, ° oxygen with Ir or Rh catalysts, ozone,KHSOs (Oxone) and K2S20s. Oxone is a highly chemoselective oxidant for the conversion of sulfides to sulfones without affecting hydroxy or al-kenic groups (equation 37). Similarly flavin (14) oxidizes aryl methyl sulfoxides to sulfones fairly selectively. ... 

A similar effect is observed in the osmylation of allylamides bearing a bis-homoallylically located sulfoxide group94. In this case the asymmetric 1,5-induction of the stereogenic sulfur atom totally overwhelms the weak bias of the allylic chirality. The concomitant sulfoxide-to-sulfone transformation suggests sulfoxide involvement in the oxidation mechanism. In this example, as well as in the previous one, replacement of the sulfur-based directing group by a sulfone moiety leads to a drop in diastereoface differentiation. 

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