Sulphoxides sulphide oxidation

II. SYNTHESIS OF ACHIRAL AND RACEMIC SULPHOXIDES A. Oxidation of Sulphides... 

Alumina supported sodium metaperiodate, which can be prepared by soaking the inorganic support with a hot solution of sodium metaperiodate, was also found to be a very convenient reagent for the selective and clean oxidation of sulphides to sulphoxides . The oxidation reaction may be simply carried out by vigorous stirring of this solid oxidant with the sulphide solution at room temperature. As may be expected for such a procedure, solvent plays an important role in this oxidation and ethanol (95%) was found to be... 

In 1960, Montanari and Balenovic and their coworkers described independently the first asymmetric oxidation of sulfides with optically active peracids. However, the sulphoxides were formed in this asymmetric reaction (equation 130) with low optical purities, generally not higher than 10%. The extensive studies of Montanari and his group on peracid oxidation indicated that the chirality of the predominantly formed sulphoxide enantiomer depends on the absolute configuration of the peracid used. According to Montanari the stereoselectivity of the sulphide oxidation is determined by the balance between one transition state (a) and a more hindered transition state (b) in which the groups and at sulphur face the moderately and least hindered regions of the peracid,... 

As discussed with reference to co-oxidation with hydrocarbons, sulphide oxidation chemistry is complicated by the further reactions of sulphoxide products. The autoxidation of sulphides in the absence of hydrocarbons is a free radical process [149] leading, in the first instance,... 

Stereoselective formation of aromatic sulphoxides by oxidation of sulphides and sulphoxides in the presence of bovine serum albumin. J Chem Soc Chem Commun 1052-1053... 

Side-chain chlorination to 2,3-dichloromethylpyridine with JV-chloro-succinimide followed by ring-closure with sodium sulphide, oxidation to the sulphoxide with phenyliodonium chloride, and dehydration on AI2O3 according to the method of Cava gave (540). Curiously, attempts to prepare the benzo-analogue (541) by the same method were unsuccessful. However, the transient existence of (541) could be demonstrated by trapping with A -phenylmaleimide. ... 

Sulphide and sulphoxide complexes have been extensively studied since cis-RuC12(DMSO)4 (DMSO, dimethyl sulphoxide, (Me)2SO) was found to have anti-tumour properties and to be a precessor for radiosensitizing agents. Such complexes can act as catalysts for the oxidation of sulphides with molecular oxygen. 

RuCl3(DMSO)3 reacts with sulphides to form mixed sulphide/sulphoxide complexes that are catalysts for oxidation of thioethers to sulphoxides [114a] ... 

Popova and colleagues47 carried out TLC of oxidation products of 4,4 -dinitrodiphenyl sulphide (the sulphoxide and sulphone) on silica gel + a fluorescent indicator, using hexane-acetone-benzene-methanol(60 36 10 l) as solvent mixture. Morris130 performed GLC and TLC of dimethyl sulphoxide. For the latter, he applied a 6% solution of the sample in methanol to silica gel and developed with methanol-ammonia solution(200 3), visualizing with 2% aqueous Co11 thiocyanate-methanol(2 1). HPLC separations of chiral mixtures of sulphoxides have been carried out. Thus Pirkle and coworkers131-132 reported separations of alkyl 2,4-dinitrophenyl sulphoxides and some others on a silica-gel (Porosil)-bonded chiral fluoroalcoholic stationary phase, with the structure ... 

The oldest and generally applied sulphoxide synthesis consists of the oxidation of sulphides to sulphoxides. This reaction was reported for the first time by Marcker9 as early as 1865. He found that treatment of dibenzyl sulphide with nitric acid afforded the corresponding dibenzyl sulphoxide in a high yield. Since that time the oxidation of... 

The simplest procedure for oxidation of sulphides to sulphoxides used till now involves the oxidation of sulphides with hydrogen peroxide alone or in the presence of various catalysts (equation 1). 

TABLE 1. Oxidation of sulphides to sulphoxides, RlR2SO, with hydrogen peroxide... 

A highly selective and rapid oxidation of sulphides to sulphoxides occurs when hydrogen peroxide/selenium dioxide system is used32. The reaction takes place immediately upon addition of a solution of hydrogen peroxide and selenium dioxide to a solution of a sulphide in methanol at room temperature. Yields of sulphoxides (Table 2) are in the range between 80 and 95%. It is most probable that perseleninic acid 19 is the true oxidizing agent. 

A rapid and clean oxidation of sulphides to sulphoxides can also be carried out using the titanium(III) trichloride/hydrogen peroxide reagent35. On a milimole scale, the oxidation takes place in a time shorter than 20 min upon addition of a solution of hydrogen peroxide to a solution of the sulphide and titanium(III) trichloride in methanol at room temperature. It was suggested that the formation of a sulphoxide in this reaction resulted from a direct coupling of the hydroxy radical with cation radical 20 formed at the sulphur atom of the sulphide (equation 6). 

Horner and Jurgens39 reported that benzoyl peroxides 21 in the presence of sulphides decompose to give sulphoxides and a-acyloxysulphides 22 (equation 8). The latter compounds are undoubtedly formed as a result of the Pummerer reaction. The oxidation reaction leading to sulphoxides has been shown to be an ionic process40. However, till now it has not found wider synthetic applications. Ganem and coworkers41 showed that 2-hydroperoxyhexafluoro-2-propanol 23 formed in situ from hexafluoroacetone and... 

Based on the kinetic studies, a mechanism for this oxidation was proposed45 which involves a nucleophilic attack by the sulphide on a cyclic hydrogen-bonded form of the peracid (equation 9). Since oxidation using peracids occurs under very mild conditions, it can be successfully applied to the preparation of base sensitive sulphoxides. Thus, di(a-bromobenzyl) sulphoxide 25, which is very labile in the presence of a base, was obtained by careful oxidation of a-di(a-bromobenzyl) sulphide by means of m-chloroperbenzoic acid (MCPBA)46 (equation 10). 

Oxidation of a thiiraneradialene with equimolar amounts of MCPBA in CH2C12 at about 0°C gave the corresponding thiiraneradialene S-oxide 26 in a quantitative yield47 (equation 11). 5-Membered heterocyclic sulphoxides such as 1,3-benzoxathiolane sulphoxide 27, 1,3-benzdithiolane sulphoxide 28 and 1,3-dithiolane sulphoxide 29 were readily obtained from their sulphide precursors by oxidation with MCPBA in dich-loromethane solution48. 

The use of optically active peracids for asymmetric oxidation of sulphides will be discussed in Section III dealing with the synthesis of optically active sulphoxides. 

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