RESEARCH IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES

89. PHOTOCHEMICAL TRANSFORMATIONS OF 9-ACETYLCARBAZOLE

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

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

It is known that, in the excited state, molecules of organic compounds have a different distribution of electron density than in the ground state \([1, 2]\). One of the principal indicators reflecting changes in electronic states in molecules is the change in their acidity upon excitation relative to the ground state. The change in pK upon excitation of organic weak acids leads to a regular increase in acidity by several orders of magnitude, which is manifested in an increase in the mobility of protons or groups equivalent to them.

Earlier we showed \([3]\) that upon excitation of 9-acetylcarbazole in alcoholic media, the formation of isomers of 1-acetylcarbazole and 3-acetylcarbazole is observed. The mechanism of isomerization in this case can be explained by the existing concepts of excited-state photochemistry for reactions of the heterolytic type \([1]\). The sharp increase in the acidity of the starting compound leads to dissociation of the acetyl fragment in the primary act, followed by its incorporation by an ionic mechanism into the aromatic nucleus.

\[ \left[ \begin{matrix} \mathrm{N}\\[-2pt] |\\[-2pt] \mathrm{C(=O)CH_3} \end{matrix} \right]^{S_0} \xrightarrow{h\nu} \left[ \begin{matrix} \mathrm{N}\\[-2pt] |\\[-2pt] \mathrm{C(=O)CH_3} \end{matrix} \right]^{S_1(T)_3} \longrightarrow \left[ \begin{matrix} \mathrm{N}\\[-2pt] | \end{matrix} \right]^{S_0} + \left[ \mathrm{C(=O)CH_3} \right] \longrightarrow \]

\[ \frac{[\mathrm{H}^+][\mathrm{H_2O}]}{\phantom{.}} \quad \text{isomers of 1- and 3-acetylcarbazole} \]

\[ \left[ \begin{matrix} \mathrm{N}\\[-2pt] | \end{matrix} \right] = \text{carbazole nucleus} \]

As is evident from this scheme, the occurrence of dark ionic reactions must inevitably be accompanied by the appearance in the reaction medium of unsubstituted carbazole as the most probable by-product. Indeed, in the experiments carried out, a simultaneous increase was observed in the yield of the isomers and in appreciable amounts of carbazole, which reached an average value of 8%.

Such phenomena are not observed if the irradiation of 9-acetylcarbazo-

be carried out in the presence of atmospheric oxygen. In this variant (depending on the conditions), the yield of isomers and carbazole decreases sharply. In the final reaction products, in addition to isomers and carbazole, oxidation products intensely colored red-brown were isolated; their yield reached a maximum value of 24% by weight of the initial 9-acetylcarbazole at an initial concentration of the latter of 0.1–4.0 g/liter.

In view of the foregoing, it seemed of interest to investigate the structure of the oxidation products obtained. Clarification of this circumstance could provide a key to understanding the mechanism of the photochemical destruction of carbazole derivatives under the action of atmospheric oxygen, which is of practical importance in the use of photosensitive materials based on carbazole, as well as in assessing the stability of certain substituted carbazoles.

The most characteristic chemical feature of the oxidation products is their ability to form, with alkali-metal ions, salts soluble in aqueous media, which we used in isolating them.

Chromatographic analysis showed that these products consist of two individual compounds. By chemical reactions, the presence of ketone oxygen was detected in their composition, which is not identical with the oxygen in the acetyl radical: the substance is not saponified, is readily reduced by atomic hydrogen, and, upon isolation of the reduced product, the latter is rapidly oxidized by air back to the starting substance.

Spectrophotometric analysis of functional groups on an IKS-14 showed that the product has a carbazole skeleton; its composition includes carbonyl oxygen, an imine structure, and a hydroxyl group; C-substitution is observed in one benzene ring (although the position of attachment is unclear); hydrogen at the nitrogen atom is absent, as are peroxide and hydroperoxide groups. The presence of carbonyl oxygen not of acetyl origin is proven by the synthesis of an identical oxidation product from unsubstituted carbazole.

The formal assumption concerning the formation of carbazolecarboxylic acids was rejected on the basis of material-balance considerations, since the missing (at least one) carbon units required for their formation could be supplied only by the carbon atoms of the alcohol. However, irradiation of dry 9-acetylcarbazole and carbazole, slightly moistened with an aqueous solution of hydrochloric acid, leads to the formation of the same oxidation products, which indicates the inertness of the alcoholic medium in the oxidation process of 9-acetylcarbazole. The oxidation product is a thermally unstable compound. On heating, the substance gives off gaseous products containing no oxygen and sinters into a black mass, which no longer has the properties of an acid and loses the ability to dissolve in alcohols, benzene, and chloroform.

At room temperature the substance gives an EPR signal ($10^{19}$ spins/gram), which does not disappear after thermal treatment up to 200°C.

On the basis of literature data [4, 5, 6] and the properties of our product, we propose the following structures for the oxidation products:

Experimental Part

The oxidation reaction was carried out in isopropyl alcohol at the above-indicated concentrations of 9-acetylcarbazole.

The full spectrum of a DKS-1000 xenon lamp, operating at an electrical power of 300 watts, was used as the radiation source. In the experiments the temperature was maintained within the range 40–60°C. The reactor apparatus was arranged similarly to that described in [3]. Oxygen was supplied to the reaction volume by intensive bubbling of air through the solution, with simultaneous stirring by a mechanical stirrer; the course of the oxidation reaction is accompanied by coloration of the solution to a reddish-brown color.

Analytical monitoring of the end of the reaction was carried out by withdrawing a 1 cm³ sample and then spectrophotometrically determining the concentration of the oxidation products on an FEK-57 instrument until the concentration of the latter did not change for one hour. Usually this did not exceed 3–4 hours. The working light filter used in measuring the concentration of the oxidation products was a filter with a maximum transmittance at 420 nm.

For separation of the oxidation products, the following scheme was adopted. From the reaction mixture, 90% of the solvent was distilled off under vacuum from a water-jet pump. Then the remaining concentrated solution was treated with a 10-fold volume of 40% alkali, which made it possible to transfer the oxidation products into the aqueous phase in the form of salts, while the remaining 9-acetylcarbazole, isomers, and carbazole precipitated. The dark-brown mother liquor was filtered off from the precipitate and treated with a 20% solution of hydrochloric acid until a weakly acidic reaction was obtained. In this process the oxidation products precipitated. The precipitate was filtered off, washed with distilled water to a neutral reaction, and dried at a temperature of 25–35°C to constant weight.

The starting substances for the synthesis and the oxidation products were characterized by the following parameters.

9-Acetylcarbazole, recrystallized from ethanol, had m.p. 74°C and contained no carbazole impurities (according to the chromatogram).

Isopropyl alcohol was of “chemically pure for analysis” grade.

The oxidation products were a dark-brown powder, thermally unstable and without a definite melting point (the onset of melting with simultaneous decomposition at approximately 80°C was determined by the thermogravimetric method on a derivatograph).

The product dissolves well in aqueous solutions of alkalis, soda, alcohol, acetone, chloroform, and acetic acid; very poorly in benzene and carbon tetrachloride.

Conclusions

It has been shown that, upon irradiation of 9-acetylcarbazole in the presence of atmospheric oxygen, isomers of 9-acetylcarbazole and oxidation products with a quinone-imine structure are formed simultaneously.

References

1. Kuz’min M. G., Ivanov V. L. Molecular Photonics, Leningrad, “Nauka,” 1970.
2. Calvert J., Pitts J. Photochemistry. Moscow, “Mir,” 1968.
3. Ivanov G. N., Tolmacheva V. Ya., Lopatinskii V. P. Author’s Certificate No. 332085, “Method for obtaining 1-acetylcarbazole,” 1971.
4. Iwaoka T. et al. Bull. Chem. Soc. Jap., 44, 341, 1971.
5. Luckhurst G. R. J. Molecular Physics, 12, 493, 1967.
6. Neugebauer F. A. et al. Chem. Ber. 105, No. 8, 1972.