Oxidative fluorodesulfuration

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. 

Tire principal method was initially discovered in the 1970s [141, 142] since the beginning of the 1990s it has been systematically developed into a valuable tool for fluoroorganic synthesis [143-147]. 

The general concept is that sulfur is introduced into the organic substrate as a direct synthetic precursor of fluorine. Tlte sulfur compound is then treated with a thiophilic, soft electrophilic oxidant, for example electrophilic halogenation agents (NBS, NIS, DBH, Br2, SO2CI2 [148], Fj [149], IF5 [150], BrFj [151], 4-MePhIF2 [152], or nitrosyl cations (NO BF., ) [146] in the presence of a fluoride source (50% or 70% HF-pyridine [143], HF-melamine [144], NEt3 3HF) [147]. 

The chemical oxidant can also be replaced by electrochemical oxidation [153, 

The sulfur species is thus activated by S-halogenation into a nucleofugic leaving group, which is substituted by fluoride. The fluorodesulfuration of thiocarbonyl compounds is supposed to follow a similar principal pathway. 

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Oxidative Fluorodesulfurization of Aryl Sulfides General Procedure ... 

Unsaturated Trifluoromethyl Compounds by Oxidative Fluorodesulfurization of Unsaturated... 

Oxidative fluorodesulfurization can also be achieved by the action of nitrosonium tetrafluo-roborate, as oxidant, and hydrogen fluoridc/pyridine, as a source of fluoride ions, on aryl sulfides.249 The starting compounds are easily prepared from ketones or aldehydes and ben-zenethio) using boron trifluoride monohydrate as catalyst, and subsequent reduction with triethylsilane.249 250... 

Scheme 2.68 Proposed mechanism of oxidative fluorodesulfuration of dithiolanes [142] (X stands for electrophilic bromine Br+ , iodine 1+ , or the nitrosyl cation, NO+).

Scheme 2.70 Syntheses of glycosyl fluorides from thioglycosides by oxidative fluorodesulfuration [145, 152].

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].

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). 

Generation of a Trifluoromethoxy Croup by Oxidative Fluorodesulfuration of a Xanthogenate... 

R-OCSSMe oxidative fluorodesulfuration R-OH 2-perfluoroalkyl-1,3-dithianylium triflate. 68, 70... 

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