Fluorodesulfurization Reactions

Fluorine/iodine mixtures, which can be regarded as sources of both iodonium and fluoride ion, have been used to prepare glycosyl fluorides from the corresponding thioglycosides and gem-difluorides from some diaryl-1,3-dithio-lanes (Fig. 68) [161,162]. Activation of the carbon-sulfur bond towards nucleophilic attack by fluoride ion is achieved by complexation of the thiophilic iodonium species with the sulfur atoms. 

Carbon-sulfur double bonds have been transformed to CF2 groups in the preparation of difluoroformals from thionocarbonates (Fig. 69) [163,164]. 

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Hypervalent iodine compounds, such as l-(difluoroiodo)-4-methylbcnzcnc or l-(difluoroiodo)-4-methoxybenzene, can serve as oxidant and simultaneously as the source of fluoride ions. They are used for fluorodesulfurization reactions under mild conditions.229 230 The reagents arc crystalline and soluble in organic solvents. 

Scheme 2.69 Examples of the versatility of the different varieties of fluorodesulfuration reactions [145, 154, 155, 157].

Trifluoromethyl hypofluorite reacts with various sulfides in trichlorofluoromethane at — 78 C forming difluoro- 624 and tctrafluorosulfancs 7,25 while fluorodesulfuration is observed in the reaction of amino acids, e.g. formation of 8.26 In a similar manner, trivalent phosphorus compounds react with trifluoromethyl hypofluorite to give difluoro-substituted derivatives 9.27... 

O Hagan and coworkers recently found that fluorodesulfuration is not only a versatile tool for the synthetic organic chemist but also - in a non-oxidative variant - so far the only known pathway in nature for enzymatic incorporation of inorganic fluoride ions into secondary metabolites [166] (Scheme 2.75). As the key step of this enzymatic reaction sequence a trialkylsulfonium ion (SAM) reacts with inorganic fluoride in a nucleophilic replacement with methionine as the leaving group. 

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