Trifluoromethanethiolate ion. Part 2. Nucleophilic substitution in pentafluoropyridine. Synthesis and characteristics of trifluoromethylthio and trifluoromethylsulphonyl derivatives
The CF3S– anion, generated from thiocarbonyl difluoride or its trimer and caesium fluoride, reacted with pentafluoropyridine (1) at –15 °C to give a high yield of 2,3,5,6-tetrafluoro-4-trifluoromethylthiopyridine (2). When the trimer was used as a precursor of the CF3S– anion, compound (2) reacted further at 20 °C to give a mixture of mono-(2), bis-(3) and (4), and tris-(trifluoromethylthio)(5) substituted fluoropyridines. At 100–110 °C, 2,4,6-trifluoro-3,5-bis(trifluoromethylthio) pyridine (4) was obtained as the only product. On oxidation with CrO3 in conc. sulphuric acid compound (2) gave 2,3,5,6-tetrafluoro-4-trifluoromethylsulphonylpyridine (12), and the oxidation of compound (4) led to a mixture of 2,4,6-trifluoro-3,5-bis(trifluoromethylsulphonyl)pyridine (13) and two isomeric fluoro-3,5-bis(trifluoromethylsulphonyl)pyridinediols (14a) and (14b). The sulphone (12) reacted readily with monomeric CF2S to yield a mixture of compounds (3), (5), and 2,4,5,6-tetrafluoro-3-trifluoromethylthiopyridine (15). The reaction of compound (2) with potassium thiophenolate and potassium phenolate also led to a substitution of fluorines and the CF3S group with the PhS or PhO substituents to give compounds (7)–(11). The sulphone (13) readily reacts with methanol to yield 2,4,6-trimethoxy3,5-bis(trifluoromethylsulphonyl)pyridine (18). The structures of the new pyridine derivatives (2)–(5), (7)–(15), and (18) have been elucidated from the m.s., 19F n.m.r., and 13C n.m.r. spectra. The X-ray molecular structure of derivative (18) has been obtained.