Fluorocarbon derivatives of nitrogen. Part 14. Studies on some (CF3)2 no-substituted fluoroaromatics; thermal rearrangement of 4-[bis(trifluoromethyl)amino-oxy]tetrafluoropyridine

The sodium salt (CF₃)₂ NO⁻Na⁺ (I) from (CF₃)₂NOH + NaH in Et₂O], is an alternative bis(trifluoromethyl)amino-oxylating agent to the adduct (CF₃)₂NOH.CsF (III). With pentafluoropyridine it affords 4-X.C₅F₄N (II) + 2,4-X₂.C₅F₃N (IV), X = (CF₃)₂NO]. It has been used to obtain a number of new bis(trifluoromethyl)amino-oxy-compounds; $\text{i.e.}$ the following conversions have been effected: perfluoro-(4-isopropylpyridine)→ 2-X.C₅F₃N.CF(CF₃)₂-4 (V) + 2,6-X₂.C₅F₂N.CF(CF₃)₂-4 (VI); 3-chlorotetrafluoropyridine → 4-X.C₅F₃N.Cl-3 (VII) and 2-X.C₅F₃N.Cl-5 (VIII) (not separated) + 2,4-X₂.C₅F₂N.Cl-5 (IX), 3,5-dichlorotrifluoropyridine → 2- (XI) and 4-X.C₅F₂N.Cl₂-3,5 (X) (not separated) + 2,4-X₂.C₅FN.Cl₂-3,5 (XII); and perfluorotoluene → 4-X.C₆F₄.CF₃-1 (XIII). Hexafluorobenzene resisted attack by (CF₃)₂NONa under the conditions used with these aromatic substrates ($\text{ca}$ 20 °C). Static pyrolysis (125 °C) of 4-bis(trifluoromethyl)amino-oxy]tetrafluoropyridine (II) gave a mixture of 6-bis(trifluoromethyl)amino]tetrafluoro-4-azacyclohexa-2, 4-dienone (XV) and 4-bis(trifluoromethyl)amino]tetrafluoro-4-azacyclohexa-2,5-dienone (XVI). The ¹³C chemical shifts, assigned by analysis of ¹⁹F-coupled and ¹⁹F broad-band decoupled ¹³C n.m.r. spectra, are in accord with a +M effect similar to that of fluorine for a (CF₃)₂NO- substituent in the 2- and 4- positions of a polyfluoropyridine and a slightly smaller -I effect; the steric effect of (CF₃)₂NO on the shifts is less than that of chlorine. In contrast, a ring carbon carrying a (CF₃)₂CF- substituent is markedly shielded compared with one carrying fluorine, presumably by a steric effect.