Functions incorporating a halogen and a chalcogen

The most frequently reported compounds containing a tetracoordinated carbon bearing a halogen and a chalcogen are α-haloethers, -sulfides, -sulfones, -sulfoxides and -selenides, and α-haloalkyl esters. Although many of these compounds have limited stability, they are useful synthetic intermediates and because of this there has been a high level of interest in their preparation and use since 1995. This is particularly true of α-fluoroethers and sulfides, which have been prepared using not only the extensive range of chemical fluorinating agents now available, but increasingly electrochemical methods. More generally, the commercial importance of perfluorinated and partially fluorinated ethers, and the fact that glycosyl halides continue to be important intermediates in synthetic carbohydrate chemistry, contributes to the frequency with which this functional group is encountered in the literature. The preparation of α-halosulfoxides and sulfones almost invariably begins with a sulfide and involves an oxidation"chlorination sequence or the reverse. Irrespective of the actual sequence used, the reagent of choice for the oxidation step is currently MCPBA. Although both approaches continue to be used for the α-fluoro compounds, the preparation of the α-chloro analogs almost always involves chlorination with NCS followed by MCPBA oxidation, frequently as a one-pot procedure. The facility with which α-carbanions can be produced from these α-halosulfoxides and sulfones contributes to their synthetic importance. The methods used to prepare α-haloselenides mirror those used for the corresponding compounds involving the lighter chalcogens. α-Halotellurides and related compounds are quite unstable and little new work has been reported in this area.