CF3 Oxonium Salts, O-(Trifluoromethyl)dibenzofuranium Salts:  In Situ Synthesis, Properties, and Application as a Real CF3 + Species Reagent

We report in situ synthesis of the first CF3 oxonium salts, thermally unstable O-(trifluoromethyl)dibenzofuranium salts, which furthermore have different counteranions (BF4 -, PF6 -, SbF6 -, and Sb2F11 -) and ring substituents (tert-butyl, F, and OCH3), by photochemical decomposition of the corresponding 2-(trifluoromethoxy)biphenylyl-2‘-diazonium salts at −90 to −100 °C. The yields markedly increased in the order of BF4 - < PF6 - < SbF6 - < Sb2F11 -. The CF3 oxonium salts were fully assigned by means of 1H and 19F NMR spectroscopy at low temperature. The CF3 salts decomposed to form CF4 and dibenzofurans. The half-life times at −60 °C of the 2-tert-butyl salts having different counteranions were 29 min for BF4 - salt 2d, 36 min for PF6 - salt 2c, 270 min for SbF6 - salt 2a, and 415 min for Sb2F11 - salt 2b. Those at −60 °C of the Sb2F11 - salts having different 2-substituents were 13 min for F salt 3b, 63 min for H (unsubstituted) salt 1b, and 415 min for tert-butyl salt 2b. Thus, the stability of the CF3 oxonium salts increased in the order of BF4 - < PF6 - < SbF6 - < Sb2F11 - and F < H < tert-butyl, which is in accord with the increasing orders of the non-nucleophilicity of counteranions and the electron-donating effect of ring subsitituents. 2-tert-Butyl-O-(trifluoromethyl)dibenzofuranium hexafluoroantimonate (2a) was thus chosen and successfully applied as a real CF3 + species source to the direct O- and N-trifluoromethylations of alcohols, phenols, amines, anilines, and pyridines under very mild conditions. The thermal decomposition method with a mixture of diazonium salt 17a and aryl- or alkylsulfonic acids, pyridine, or pyridines having an electron-withdrawing group also afforded CF3O or CF3N products. The trifluoromethylation mechanism is discussed and an SN2 mechanism containing the transient formation of free CF3 + is proposed. Thus, the present study has demonstrated that the exceedingly reactive CF3 + species can be generated much easier than the CH3 + species, contrary to the common sense that CF3 + is extremely difficult to generate in solution.