RESEARCH IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES 75. PHOTOCHEMICAL SYNTHESIS OF 1-ACETYLCARBAZOLE

G. N. Ivanov, V. Ya. Tolmacheva, V. P. Lopatinskii

(Presented by the scientific-methodological seminar of the organic departments and laboratories of the CTF)

This communication presents the results of a study of the reactivity of 9-acetylcarbazole in the excited state under the use of UV radiation. Although reactions of ketone groups in the excited state have previously been studied for a large number of compounds [1, 2, 3], in the literature we have not encountered examples of photochemical reactions of acetyl derivatives of carbazole. The use of readily available 9-acetylcarbazole as the starting material was also of interest because the direct bond of the ketone group to the nitrogen atom made it possible to expect the manifestation of a specific reactivity of the carbonyl oxygen toward reduction, addition, cyclization, and other reactions [1, 2, 3].

As the source of UV radiation, the full spectrum of a PRK-7 mercury lamp was used, operating at an electrical power of 1 kW. Solutions of 9-acetylcarbazole in aliphatic alcohols were irradiated; of these, isopropyl alcohol proved to be the best. The temperatures in the experiments were 35–45°C; the duration of irradiation depended on the concentration of the starting substance and the volume of the reaction mixture. The results of several series of experiments made it possible to establish the optimum index—0.01 g/cm³ × 0.02 h, at which the highest yield of the transformation product, 1-acetylcarbazole (25–28%), was achieved. For example, with a reaction-mixture volume of 1000 cm³ and a 10 g charge of 9-acetylcarbazole, the required irradiation time is 20 h. Increasing the irradiation time and the concentration of the starting substance increased the yield of 1-acetylcarbazole only insignificantly.

In the reaction products, the main mass (40%) consisted of the starting 9-acetylcarbazole. The residue after isolation of the acetyl derivatives was a resinous mass, the composition of which has not yet been studied. A diagram of the reaction vessel is shown in Fig. 1.

Experimental Part

Reagents. 9-Acetylcarbazole, recrystallized from ethanol, had m.p. 74°C. Isopropyl alcohol was of “chda” grade.

1-Acetylcarbazole. 10–12 g of 9-acetylcarbazole was dissolved in 1000–1200 cm³ of isopropyl alcohol, and the solution was poured into the reactor. After

connection of a reflux condenser to the reactor, the reaction mixture was purged with a stream of helium (or nitrogen purified of oxygen) in order to remove oxygen from the space above the reactor. From the moment the mercury lamp was switched on, the inert-gas flow was reduced and kept constant at the level required to ensure the necessary circulation of the solution.

Fig. 1. Diagram of the reaction vessel:
1 — reaction flask, 2 — thermometer, 3 — inert-gas supply tube, 4 — bubbler, 5 — drain tube, 6 — mercury lamp, 7 — quartz jacket, 8 — choke

After 20–24 hours of irradiation, the reaction mixture was drained from the reactor, most of the isopropyl alcohol was distilled off, and, upon crystallization of the residue, yellowish crystals with m.p. 129–130°C were isolated (yield 25–28%, calculated on 1-acetylcarbazole). Upon recrystallization from methanol the m.p. rose to 132–133°C (lit. m.p. 133–134° for 1-acetylcarbazole [4]). Found: acetyl groups by oximation, 12.9%. Calculated (for monoacetylcarbazole), 13.3%. The product gives IR spectra and chromatograms that do not coincide with the spectra and chromatograms of authentic samples of 2-, 3-, and 9-acetylcarbazoles.

REFERENCES

1. J. Calvert, J. Pitts. Photochemistry. IL, Moscow, 1968.
2. A. Schönberg. Preparative Organic Photochemistry. IL, Moscow, 1963.
3. N. Turro. Molecular Photochemistry. Mir, 1967.
4. H. F. R. Manske, M. Kulka. Can. J. Res. B. 28, 443–50, 1950; C. A. 45, 2467, 1951.