Chiral sulphur compounds

The most important class of chiral sulphur compounds is the sulphoxides. As a result of the presence of the lone pair, the stereochemistry at sulphur is pyramidal and molecules of the general type (38) and (39) are opposite enantiomers. The sulphoxides are of considerable importance in asymmetric synthesis (see chapter 5) and, as for the phosphines, are generally obtained in enantiomerically pure form by resolution. 

If the lone pair in (38) or (39) is replaced by a double bond to oxygen the resulting sulphone is obviously achiral but if it is instead replaced by a double bond to nitrogen, we get another type of chiral sulphur compound, the sulphoximines (40) which have also been used for asymmetric synthesis (see section 5.1.3). The sulphonium salts (41) are also potentially chiral. A review of the chiral compounds of sulphur is available,... 

Chiral sulphoxides are the most important group of compounds among a vast number of various types of chiral organosulphur compounds. In the first period of the development of sulphur stereochemistry, optically active sulphoxides were mainly used as model compounds in stereochemical studies2 5 6. At present, chiral sulphoxides play an important role in asymmetric synthesis, especially in an asymmetric C—C bond formation257. Therefore, much effort has been devoted to elaboration of convenient methods for their synthesis. Until now, optically active sulphoxides have been obtained in the following ways optical resolution, asymmetric synthesis, kinetic resolution and stereospecific synthesis. These methods are briefly discussed below. 

The chirality exhibited by the pyramidal sulphur compounds (e.g. sulphonium salts, sulphoxides and sulphinic esters) should be noted, but cannot be considered within the scope of this book. 

A text on aspects of asymmetry in carbohydrates has appeared, which includes reviews on the composition of reducing sugars in solution, asymmetric reactions of carbohydrates containing carbonyl groups, and prochirality and pseudoasymmetry in carbohydrate biochemistry, besides articles on nitro-sugar stereochemistry, chiral sulphur derivatives, and carbohydrate crown-ether compounds. A general survey of organic chemistry includes an extensive account of monosaccharide chemistry. ... 

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... 

Compounds containing a chiral sulphur atom-pages Z7-Z8. 

Lyotropic polymeric LC, formed by dissolving two aromatic polyamides in concentrated sulphuric acid, have been studied using variable-director 13C NMR experiments.324 The experimental line shapes at different angles w.r.t the external field were used to extract macromolecular order and dynamic in these ordered fluids. An interesting application of lyotropic LC is for the chiral discrimination of R- and S-enantiomers, and has recently been demonstrated by Courtieu and co-workers.325 The idea was to include a chiral compound 1-deutero-l-phenylethanol in a chiral cage (e.g., /1-cyclodextrin) which was dissolved and oriented by the nematic mean field in a cromolyn-water system. Proton-decoupled 2H NMR spectrum clearly showed the quad-rupolar splittings of the R- and S-enantiomers. The technique is applicable to water-soluble solutes. 

Optical activity was first observed with organic compounds having one or more chiral carbon atoms (or centres) (i.e. a carbon substituted with four different groups). In the structures (1) to (17) the chiral carbons are specified with an asterisk. Subsequently compounds having chiral centres at suitably substituted heteroatoms (e.g. silicon, germanium, nitrogen, phosphorus, arsenic, sulphur, etc.) were also synthesised. Molecular dissymmetry, and hence chirality, also... 

All compounds with more than one chiral centre yield diastereomeric mixtures. All heteroalkylated species have a chiral centre at the sulphur. In the compounds 6 and 2 the carbon 5 is chiral, too. After phosphorylation there is one more centre at the phosphorus. So the appearance of two diastereomeres b 1 and 6 and four b 2 respectively is expected. This expectation is verified by NMR-results. In l3C-NMR as well as 31P-NMR spectra one can see multiple resonances corresponding to almost all nuclei of the compounds. The 31P-spectrum of 1 and the l3C-spectrum of 6 are the most significant for tbs phenomenon. 

Cyclic sulphates provide a useful alternative to epoxides now that it is viable to produce a chiral diol from an alkene. These cyclic compounds are prepared by reaction of the diol with thionyl chloride, followed by ruthenium catalyzed oxidation of the sulphur (Scheme 13) [109]. This oxidation has an... 

Kobayashi obtained enantiopure phthalocyanines by introducing a helical type chiral element, or a chain containing an asymmetric carbon. The use of two or four chiral binaphthyl substituents [(P) or (5)] or [(M) or (/ )], attached to the phthalocyanine via oxo or sulphur bridges, leads to compounds of the type BNpPcM and TNpPcM with M = H2, Zn and Co (Figure 6.32). 

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. 

---