Trifluoromethyldibenzothiophenium triflate

The reaction of various nucleophiles with A-trifluoromethyldibenzothiophenium triflate constitutes an excellent general method for the electrophilic perfluoroalkylation of nucleophilic substrates. This forms the subject matter of Section 3.10.5. 

Reaction of enamines with perfluoroalkyl iodides gives a-perfluoroalkyl ketones via a free radical mechanism . The reaction is rapid and uncatalysed for many enamines in the absence of oxygen only when the double bond of the enamine is not sufficiently electron rich is irradiation with ultraviolet light necessary . The mechanism for unassisted trifluoromethylation involves the formation of a charge transfer complex as a result of electron donation by the nitrogen, followed by the addition of trifluoromethyl radicals to the double bond (Scheme 17). Difluorodihalomethanes (Scheme 18), trifluoromethyldibenzothiophenium triflate and trifluoromethyl metal reagents have also been used. 

Recently, simple copper-catalyzed trifluoromethylation of aryl boronic acids under mild conditions was developed. Using (trifluoromethyl)trimethylsilane (McsSiCFs) [93], trifluoromethyldibenzothiophenium triflate [94], or Togni s reagent [95], 2-trifluoromethylbenzo[fc]thiophene 173 was prepared in 45-73 % yields. 

The 3 -trifluoromethyldibenzothiophenium salts are stable solids. Thermolysis leads to dibenzothiophene and trifluoromethyl triflate. Alkaline hydrolysis leads to the 3 -oxide (Equation 111). 

Scheme 2.154 Synthesis of trifluoromethyldibenzothiophenium and -selenophenium triflates (X = S, Se) and their subsequent nitration [28b]. On the extreme left and the extreme right, the reactive sulfonium intermediates for both synthesis routes are depicted undergoing ring-closure by intramolecular electrophilic substitution.

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