Trifluoromethanethiolate ion. Part 2. Nucleophilic substitution in pentafluoropyridine. Synthesis and characteristics of trifluoromethylthio and trifluoromethylsulphonyl derivatives

Abstract

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.