Resolution and Racemization of Sulphoxides

Resolution and Racemization of Sulphoxides.—Resolution of sulphoxides into enantiomers usually requires the presence of an acidic or basic grouping that can be used for diastereoisomeric salt formation with a chiral acid or base sulphoxides without such handles may be partially resolved (4.4— 14.5% optical purity) by stereospecific inclusion into jS-cyclodextrin.  

The mechanism of the racemization of sulphoxides by halide ions in acid solution, and the concurrent reduction to the sulphide, is established by rate studies to involve the conversion of the O-protonated sulphoxide into the halogenosulphonium ion in the rate-determining step. Chlorinolysis of a sulphoxide in AcOH gives the products obtained by chlorinolysis of the corresponding sulphide ArSR, where R is a group capable of independent carbonium ion stability, together with products of the Pummerer reaction e.g. PhCHO, PhSH)  

Corresponding C—S bond cleavage in aqueous HCl does not feature in the currently accepted mechanism for sulphoxide racemization, in which 

Quadricovalent sulphur intermediates should also be considered eligible to account for the racemization of sulphoxides  

the formation of (96) and reversal of the path by which it is formed results in racemization. Treatment of a methyl sulphoxide R SO Me with HCl in CH2CI2 gives RSCH2CI when the water formed in this reaction is removed by molecular sieves since this seems to be accounted for in terms of a quadricovalent intermediate RgSClg, the suggestion that sulphoxide racemization by HCl involves the same intermediate is supported.  

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