Sulphoxides halogenation

Chiral alcohols have also been used in an asymmetric synthesis of sulphoxides based on halogenation of sulphides. Johnson and coworkers have found319 that the reaction of benzyl p-tolyl sulphide with JV-chlorobenzotriazole (NCBT) followed by addition of (—) menthol and silver tetrafluoroborate afforded diastereoisomeric menthoxysulphonium salts 267 which, upon recrystallization and hydrolysis, gave benzyl p-tolyl sulphoxide with 87% optical purity (equation 145). More recently, Oae and coworkers reported320 that optically active diaryl sulphoxides (e.e. up to 20%) were formed either by hydrolysis or thermolysis of the corresponding diaryl menthoxysulphonium salts prepared in situ from diaryl sulphides using ( —) menthol and t-butyl hypochlorite. 

The stereochemistry, kinetics and mechanism of a-halogenation of sulphoxides have been widely investigated587,588 and exhaustively reviewed257,589. Therefore, they will not be discussed here. 

Halogenation of diphenyl or methyl phenyl sulphoxides by Cl2 or Br2 affords mainly para-halogeno derivatives, whereas the meta-isomers are formed in low percentages or not at all606,607. In contrast, nitration in concentrated sulphuric acid leads to metasubstitution whose extent increases with acidity of the medium (up to 100%)6°8. 

Hypobromite has also been used as a sulphoxide oxidant. In this case, as with hypochlorite, multi-halogenated species are usually found. Thus, treatment of ethyl methyl sulphone with a basic aqueous hypobromite solution at 45 °C yielded the product shown in equation (37) in high yield97. In the presence of a-carboxylate groups, C—C bond cleavage occurs96 (equation 38). 

McLafferty rearrangement 133, 163 Meisenheimer complexes 699, 702 Metal-chelated intermediates 838 Metal-halogen exchange 781, 784 Methionine, oxidation of 852-855 Methionine sulphone 853 Methionine sulphoxide 851-869 reduction of 1063 residues of... 

Oxidative chlorination of diaryl sulphoxides with chlorine is generally much more useful than for alkyl-containing substrates. The sulphone is obtained cleanly, with no (or little) halogenation . The rate of reaction is first-order with respect to both the sulphoxide and chlorine and has an order of minus one with respect to chloride ion. This is consistent with the mechanism given in equation (29). 

As with chlorine-containing oxidants, JV-bromo species have been used to oxidize sulphoxides to sulphones (with no bromine incorporation) through the initial formation of a bromosulphonium ion, by nucleophilic attack of the sulphoxide sulphur atom on the electrophilic halogen atom. Such reactions involve JV-bromosuccinimide ° bromamine-T, iV-bromoacetamide ° and iV-bromobenzenesulphonamide. All reported studies were of a kinetic nature and yields were not quoted. In acid solution all oxidations occurred at or around room temperature with the nucleophilic attack on the electrophilic bromine atom being the rate-limiting step. In alkaline solution a catalyst such as osmium tetroxide is required for the reaction to proceed . ... 

Diffusion control and pre-association in nitrosation, nitration, and halogenation, 16, 1 Dimethyl sulphoxide, physical organic chemistry of reactions, in, 14, 133 Diolefin crystals, photodimerization and photopolymerization of, 30, 117 Dipolar aptotic and protic solvents, rates of bimolecular substitution reactions in,... 

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