First examples of stable polyfluorinated bis- and tris-(alkoxy)methyl cations

Stable polyfluorinated bis- and tris-(alkoxy)methyl cations were prepared by the reaction of the corresponding difluoroformals (R_fO)₂CF₂ (R_f = -CH₂CF₃, -CH(CF₃)₂, -CH₂CF₂Cl) with an excess of SbF₅. Although the cation (CF₃CH₂O)₂CF⁺ (1a) is stable at ambient temperature, the chlorinated analog (ClCF₂CH₂O)₂CF⁺ (3a) can be generated only at low temperature in SO₂ClF solvent and rapidly decomposes at ambient temperature. Although the salt (CF₃)₂CHO]₂CF⁺Sb_nF_5n+1⁻ (2a) is slightly more stable than the salt of cation 3a, at ambient temperature it undergoes rapid disproportionation with formation of equal amounts of (CF₃)₂CHO]₃C⁺Sb_nF_5n+1⁻ (2b) and CF₃OCH(CF₃)₂ (2c). Stable solid salt 2b (n = 2) was isolated and fully characterized by ¹H, ¹⁹F and ¹³C NMR spectroscopy and its structure was confirmed by single crystal X-ray diffraction.