REARRANGEMENT OF N-ACETYLDIPHENYLAMINE

E. E. SIROTKINA, R. M. KOGAN

(Presented by the scientific-methodological seminar of the KhTF)

The Fries–Rosenmund rearrangement, as a convenient preparative method for the synthesis of fatty-aromatic ketones, has not given positive results when applied to diarylamine derivatives. The ortho derivatives of N-substituted diarylamines that are formed either cyclize into the corresponding acridines [1], or, in the case of N-acetyldiphenylamine, the rearrangement could not be carried out even in the presence of such active catalysts as aluminum chloride and polyphosphoric acid [2, 3].

We have investigated the rearrangement of N-acetyldiphenylamine in the presence of equimolecular amounts of aluminum chloride and hydrogen chloride in the medium of several organic solvents, at different temperatures and reaction times. In all the cases studied, an intermolecular rearrangement is observed, with formation of diacetyldiphenylamine and diphenylamine. Among the solvents investigated (nitrobenzene, nitromethane, chlorobenzene, methylene chloride), the last proved to be the best; the highest yield of diacetyldiphenylamine (30% of theoretical) is achieved in 30 minutes of reaction time at a temperature of 40–41°C. With increasing reaction time, resinification of the products increases.

To determine the donor activity of the heteroatoms of the acetyl derivatives of diphenylamine, the IR spectra of complexes of N-acetyldiphenylamine and diacetyldiphenylamine with aluminum chloride were investigated. In the spectra of the complexes, new absorption bands appear in the regions 1400–1700 and 2900–3100 cm\(^{-1}\), indicating that both oxygen centers participate in complex formation. When hydrogen chloride is added to the system, the intensity of the absorption band of the bonded C—O group (1572 cm\(^{-1}\)) increases and the intensity of the absorption band of the tertiary amino group (1200–1400 cm\(^{-1}\)) decreases, which agrees well with the ability of diphenylamine derivatives to form compounds of the ammonium-salt type with strong acids. Salt formation at the nitrogen atom apparently leads to weakening of the C—N bond, as a result of which rearrangement of the acetyl group from the N-position into the aromatic nucleus occurs. In the absence of such a salt, rearrangement is not observed [2–3].

Experimental Part

Into a round-bottom flask equipped with a stirrer, dropping funnel, reflux condenser, and connected to an apparatus for generating gaseous hydrogen chloride, were placed 5.025 g (0.025 M) of N-acetyldiphenylamine \((T_{\mathrm{m}} = 101^\circ \mathrm{C})\), 20 ml of methylene chloride, and gaseous hydrogen chloride was passed through for 20 min. A hydrochloride salt of N-acetyldiphenylamine precipitated. After addition of the catalytic suspension (3.32 g of aluminum chloride in 10 ml of solvent), the reaction mixture was kept for 30 min at a temperature of \(40—41^\circ \mathrm{C}\) and then poured into water with ice. The precipitated crystals were filtered off, washed with water to neutral reaction, and dried in air.

The reaction products were separated by preparative thin-layer chromatography [4] on a 1-cm-thick layer of silica gel (KSK) in the solvent system isopropyl alcohol—benzene (1:6). 1.87 g of diacetyldiphenylamine was isolated (30% of theory). Analyses: \(T_{\mathrm{m}} = 87—88^\circ \mathrm{C}\) (one recrystallization from alcohol), molecular weight 250 (calculated 253), \(\mathrm{C}_{16}\mathrm{H}_{15}\mathrm{NO}_{2}\), N % 5.4 (calculated 5.53), total content of acetyl groups (by hydrolytic cleavage [5]), 33.0% (calculated 33.9), amount of C-acetyl groups (by oximation [5]) 16.4% (calculated 16.9). The IR spectra (IKS-14) of diacetyldiphenylamine indicate the presence of \(\mathrm{C—O}\) \((1674\ \mathrm{cm}^{-1})\), \(\mathrm{C—N}<\) \((1300\ \mathrm{cm}^{-1})\), and a 1,2-substituted benzene ring \((1038—1080\ \mathrm{cm}^{-1},\ 692\ \mathrm{cm}^{-1})\); consequently, one acetyl group is located at the nitrogen atom, the other in the ortho position of the aromatic nucleus.

References

1. A. E. Porai-Koshits, A. A. Kharkharov. Izvestiya AN SSSR, OKhN, 1086, 1944.
2. B. I. Ardashev, V. I. Minkin. Uspekhi khimii, 28, 1186, 1959.
3. J. M. Birchall, D. H. Thorpe, J. Chem. Soc. 22, 2898 (1968).
4. E. Stahl. Thin-Layer Chromatography. Moscow, “Mir,” 1965.
5. Houben-Weyl. Methods of Organic Chemistry. Moscow, 1963.