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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 .
With oxidative reagents of a nucleophilic nature such as sodium and potassium permanganate, sulfoxides react faster than sulfides and a chemoselective oxidation of a sulfoxide in preference to that of a sulfide is available [77-79].
The following experimental procedures illustrate the use of these reagents for the synthesis of the monosulfoxide (2) and monosulfone (3) derived from the useful 1,3-dithiane (1) of Corey and Seebach (they can and have been used in a similar fashion).
2.6.1 Oxidation of sulfides
Rl/KR2 ------> R'/?XR2 ------*
To a cooled solution of dithiane (1) (2.00 g, 16.7 mmol) in methanol (125 ml) was added an aqueous solution (35 ml) of sodium periodate (3.68 g,
17.5 mmol) at such a rate (approximately 30 min) to maintain the temperature at 20°C. Stirring and cooling were continued for an additional 30 min. The reaction mixture was then filtered to remove sodium iodate, and the resulting solution taken to near dryness on a rotary evaporator. Extraction of the solids with chloroform produced a solution which was dried over sodium sulfate, and filtered. Evaporation left the crystalline sulfoxide (2) (2.13g, 94%), m.p. 86-87°C.
From  with permission.
To compound (2) (408 mg, 3 mmol) in water (30 ml) containing magnesium sulfate (1 g) as a buffer was slowly added a solution of potassium permanganate (316mg, 2mmol) in water (20ml). Discoloration of the potassium permanganate occurred instantly. After 1.5 h, excess sodium metabisulfite was added and the resulting clear, colourless solution was extracted with chloroform and the extracts dried over sodium sulfate. Evaporation of chloroform afforded (3) (412mg, 91%), m.p. 138-139°C.
From  with permission.
See also  for a similar result with 1,3-dithietane-l-oxide.
The 1,3-dithiane-l,3-dioxides can also be prepared by sodium periodate oxidation of the 1,3-dithiane, and the irans isomer was thus obtained in a 58% recrystallized yield . Chiral molecules of this type (C2 symmetry) are particularly suitable for use as chiral acyl anion equivalents [11,82].
The use of dimethyldioxirane in the oxidation reactions of sulfides deserves to be mentioned [83, 84] as this mild neutral oxygen transfer reagent allows, for instance, highly reactive compounds such as a-oxosulfones to be obtained. Although the oxidation can usually be controlled at the sulfoxide level by using a stoichiometric amount of the
2 6 SULFOXIDES
oxirane, in this case the a-oxosulfoxide could not be obtained. The authors suggest disproportionation into the thiolester and the oxosulfone.
S At1 S
AT MeCOMe/CH2Cl2 ^ 'q
-30°C. 2-4 h
A selective photo-oxidation of sulfides to sulfoxides with tetranitromethane as oxidizing agent is also of interest .
hvcT (Pyrex) SN
R1 R CHjCla. 6-8 h
78-97% (11 ex.)
Chemoselectivity, high yields and compatibility of reactive functional groups with the reaction conditions are noteworthy features.
2.6.2 Asymmetric synthesis of sulfoxides
Enantiomerically pure sulfoxides appear to be extremely valuable auxiliaries in asymmetric synthesis. Many reviews deal with their synthesis and their uses to control the formation of stereogenic centres mainly in carbon-carbon bond formation [86-95].
A quite general method of access to optically pure sulfoxides is due to Andersen [96-98]: a menthyl sulfinate ester is reacted with a Grignard reagent. Both enantiomers of menthyl p-toluenesulfinate are commercially available. A large-scale preparation of (-)-menthyl (S)-p-toluenesulfinate as well as that of (fl)-(+)-methyl p-tolyl sulfoxide is described  by Solladie et al. Other related approaches are presented and discussed in .
(lR,2S,5R)-( —)-Menthyl (S)-p-toluenesulfinate
The powdered sodium salt of p-toluenesulfinic acid (50 g, 0.281 mol, carefully dried overnight by azeotropic distillation in toluene) was added in small portions to a solution of thionyl chloride (100 ml, 1.4 mol) in benzene (500 ml) at 0°C. Then the mixture was allowed to reach room temperature and the solution concentrated to a quarter of its original volume by distilling benzene and the excess uf thionyl chloride. The residue was diluted with anhydrous ether (500 ml, formation of a white precipitate of sodium chloride) and cooled at 0°C. A solution of (-)-menthol (48.3 g, 0.309 mol) in pyridine (50 ml) was then added dropwise. After the addition, the mixture was stirred for lh at room temperature and hydrolysed with water (200 ml). The organic layer was washed with 10% hydrochloric acid (200ml) and saturated brine (100ml). After drying the solution with sodium sulfate, the solvent was evaporated. The residue was dissolved in acetone (200 ml) and treated with concentrated hydrochloric acid (5 drops). The solution was allowed to crystallize at -20°C. After filtration of the first crop of crystals, the mother liquor was concentrated, concentrated hydrochloric acid (1 drop) was added and the solution allowed once again to crystallize at -20°C. This operation was repeated four times. Finally the product was recrystallized once from hot acetone, giving white crystals (66g, 80%). m.p. 110°C, [a]D21 = -202° (ñ 2.0 in acetone).