Reactions of Sulphoxides

Reactions of Sulphoxides.—The general features of the reactions of sulphoxides can be illustrated by reference to the simplest structural types, without regard for stereochemical details. A minority of the new work published within the period under review falls within this category when the well-defined uses of dimethyl sulphoxide as a reagent are considered separately (p. 50), but interpretation of the reactions of sulphinyl compounds with chirality at sulphur fully in view is the outstanding feature of recent work. 

The methylsulphinyl substituent is pam-directing in the electrophilic bromination of Ph-SO Me, while the appearance of p-bromophenyl 

Koulkes-Pujo, L. Gilles, B. Lesigne and J. Sutton, Chem. Comm., 1971, 749. Gollnick and H. U. Stracke, Tetrahedron Letters, 1971, 203, 207. 

Otsuji, Y. Tsujii, A. Toshida, and E. Imoto, Bull. Chem. Soc. Japan, 1971, 44, 219. F. Tavares, R. E. Estep, and M. Blezard, Tetrahedron Letters, 1970, 2373. 

The synthesis of ketones from j9-keto-sulphoxides shows the comparability of these compounds with j8-dicarbonyl compounds. Other synthetic uses include the formation of a-chloro-oximes RC(Cl)=NOH on nitrosation 

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A detailed discussion of the different acidities of the diastereotopic a-methylene protons in sulphoxides, as well as of the stereochemistry of reactions of sulphoxide a-carbanions with electrophilic reagents is beyond the scope of this chapter. A recent review by Wolfe pertinent to these problems is available392. 

Reaction of sulphoxides with disulphides 493 in the presence of BuLi or NaH yields mono-, and disulphenylated products (equation 297). The formation of monosulpheny-lated sulphoxide as the main reaction product (55%) takes place only when Et2S2, sulphoxide and BuLi are used in a 1 3 3 ratio556. 

A mechanistic study6 5 of the reaction of sulphoxides with the sodium salt of 1 -methyl-1 -phenylethyl hydroperoxide in benzene-alcohol solutions at room temperature showed that the reaction is first order with respect to sulphoxide and the hydroperoxide salt. The rate was also shown to be inversely proportional to the alcohol concentration in the solvent and this dependence was shown to increase with increasing pH. Also, more sterically crowded alcohols caused a decreased rate of reaction. 

High yields of sulphoximines are also obtained from the reaction of sulphoxides with chloramine-T in the presence of copper(II) salts or copper metal (equation 68)180,184. 

The anodic oxidation reaction of sulphoxides was not much studied, and just a few reports are available so far. The conversion into the corresponding sulphones of some phenyl alkyl and diaryl sulphoxides (oxidation potential for 86 + 2.07 V vs. SCE in acetonitrile/NaC104 electrolyte, Pt anode) has been reported. Similarly, diphenyl suiphoxide was long known to be transformed in a quantitative yield into the sulphone (Pt anode, solvent glacial acetic acid). Additional examples of the oxidation of a suiphoxide function attached to aryl groups are available . 

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,... 

Sulphur Compounds.—New light has been thrown on the mechanism of the Pummerer reaction of sulphoxides (468a) by the use of conformationally defined steroidal sulphoxides. °° The reaction, in warm acetic anhydride, involves both a reduction at sulphur and an oxidation at the a-carbon atom. The possible reaction routes are outlined in Scheme 15. 

Reviews cover the topics alkynethiolates in synthesis " oxygen-exchange reactions of sulphoxides sulphonyldiazomethanes C—S bond cleavage acyl isothiocyanates radical reactions of sulphur compounds addition of sulphenyl halides to olefins " sulphenamidcs mercaptoethylation of amines sulphur as a chiral centre " stereochemistry of S and S " compounds " reductive cleavage of sulphides, synthetic uses of alkene- and alkyne-thiolates, and thio-Claisen rearrangements nucleophilic displacements at sulphur in disulphides aromatic... 

Allenmark s work on anchimerically assisted reactions of sulphoxides" continues with a study of the rates of reduction of the syn- and anti-forms of e fo-cis-3-benzenesulphinylbicyclo[2,2,l]heptane-2-carboxylic acids. The syn-was more reactive than the an/i-isomer by a factor of more than 3.2 x 10 . The relative ease of formation of the acyloxysulphonium ion resulting from nucleophilic attack by the carboxy-group on sulphur accounts for this result, and also for the relative racemization rates of jS-carboxy-substituted sulphoxides. Pummerer reaction of one enantiomer of a benzyl o-carboxyphenyl sulphoxide in the presence of DCCI leads to 2-phenyl-3,l-benzoxathian-4-one (47) in an optically active form, indicating the transfer of chirality from S to C. ... 

Properties and Reactions of Sulphoxides.—Simple reactions resulting in the reduction of sulphoxides to sulphides, reported recently, concern SnCU-HCl, NaHSOj (SO2 is ineffective), RCS2H, and MeCOCl. During the reaction of butyl methyl sulphoxide with L-cysteine, ( -)-2 ner-captopropionic acid, or other optically active thiols, the (-HR)-sulphoxide accumulates, indicating that the (S)-enantiomer is reduced faster by the thiol. ... 

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