Dithianylium salts

A very generally applicable method for converting a variety of different functional groups into their partially or fully fluorinated analogs is the oxidative fluorodesulfuration of thiocarbonyl compounds, dithiolanes, dithianes, and dithianylium salts. 

Scheme 2.72 Synthesis of liquid c stals by oxidative fluorodesulfuration of xanthogenates (aboi e) [162], and by the oxidative alkoxydifluorodesulfuration of dithianylium salts (beloiv) [163].

Mechanistically the only difference from the chemistry depicted in Scheme 2.68 is the formation of a dithioorthoester as the central sulfur-containing intermediate if this is generated from strongly electron-deficient perfluoroalkyl dithianylium salts (Scheme 2.73) it can be isolated at room temperature. For alkyl or aryl dithia-nylium salts with less fluorination the dithioorthoester is a labile intermediate which is only stable at low temperatures (up to ca. —50 °C). This intermediate is subsequently fluorodesulfurated to yield the corresponding a,a-difluoroether. 

Scheme 2.73 Synthesis of ,c -difluoroether-linked liquid crystals by oxidative alkoxydi-fluorodesulfuration of dithianylium salts. The intermediate dithioorthoester is only stable at temperatures below ca — 50 C [147a].

Dithianylium salts in combination with oxidative fluorodesulfuration chemistry are also useful reagents for synthesis of gem-difluoromethylene analogs of carboxylic acid derivatives other than esters. If the fluorodesulfuration is conducted in the presence of other O- or M-nucleophiles the corresponding a.a-difluoroalkyl compounds are obtained in reasonable to good yields (Scheme 2.74). 

Scheme 2.74 Fluorodesulfuration of dithianylium salts in the presence of different O- and N-nucleophiles enables convenient access to a variety of rz/z-difluoroalkylated products (R = n-CjHz or n-CsH,) [147b].

Oxidative Alkoxydifluorodesulfuration of Dithianylium Salts A.7.1 Dithianylium Triflate 27... 

Scheme A.11 Synthesis of a,a-difluoro ethers via dithianylium salts [16].

R-CF2OOR Dithianylium salt oxidative fluorodesulfliration in the presence of R -OOH 72... 

Treatment of propane-1,3-dithiol with trifluoroacetic anhydride and then with trifluoromethanesulfonic acid affords 2-trifluoromethyl-l,3-dithianylium triflate 43. This first a-perfluoroalkylcarbenium salt is thermally stable and acts as an electrophilic polyfluoroalkylating agent <03TL5995>. 

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