Study in the Chemistry of Carbazole Derivatives
Abstract Generated abstract
This study examines the photochemical oxidation of carbazole and several N-substituted and acetyl-substituted carbazole derivatives by atmospheric oxygen in polar media containing small amounts of sulfuric acid. Irradiation experiments, direct photolysis in the absence of oxygen, spectrophotometric kinetic measurements, and IR, UV, luminescence, EPR, and chemical analyses are used to propose that oxidation is preceded by photochemical cleavage to carbazole or 3-acetylcarbazole, followed by sensitization of oxygen and subsequent dark oxidation. Two main oxidation products are distinguished, depending on whether the starting compound lacks or contains acetyl substitution in the ring, and hydroperoxide intermediates are detected only for some acetylated derivatives. Product yields are shown to depend on acid concentration and initial substrate concentration, with N-polyvinylcarbazole following the general behavior of N-derivatives of carbazole.
Full Text
Research in the Chemistry of Carbazole Derivatives
98. Photochemical oxidation of carbazole and its derivatives by atmospheric oxygen
G. N. Ivanov, V. Ya. Tolmacheva, V. P. Lopatinskii,
O. V. Rotar’, T. G. Vedernikova, G. S. Yakusheva,
G. M. Samoilenko
(Presented by the scientific-methodological seminar of the organic departments and laboratories of TPI)
The objects of photochemical oxidation were: carbazole (I), \(N\)-methylcarbazole (II), \(N\)-acetylcarbazole (III), \(N\)-polyvinylcarbazole (IV), 3-acetylcarbazole (V), \(N\)-methyl-3-acetylcarbazole (VI), 3,6-diacetylcarbazole (VII), \(N\)-methyl-3,6-diacetylcarbazole (VIII). It has been established that the reaction proceeds effectively in polar solvents with the addition of small amounts of dilute sulfuric acid.
On the basis of the investigations carried out[^1], it has been shown that, upon oxidation of the indicated substances, two products are obtained, one of which is formed upon oxidation of I, II, III, and IV (product A), and the other upon oxidation of V, VI, VII, VIII (product B). The following structural formulas are proposed for them:
\[ \begin{array}{c@{\qquad}c} \text{A: [[structural formula of an oxidized carbazole derivative with } N,\ OH,\ \text{and } {=}O \text{ shown]]} & \text{B: [[structural formula of an acetyl-substituted oxidized carbazole derivative with } O{=}C{-}CH_3,\ N,\ OH,\ \text{and } {=}O \text{ shown]]} \end{array} \]
The results obtained in photooxidation made it possible to assume that the oxidation reaction of the named compounds is preceded by a process of photochemical cleavage with the formation of intermediate carbazole (for A) and 3-acetylcarbazole (for B), which subsequently act as the oxidized objects. This hypothesis was confirmed by direct photolysis of compounds II, III, IV, VI, VII, VIII in the absence of oxygen. The established fact is in good agreement with
with ideas about the acid–base properties of heterocycles in the excited state [1, 2]. In the cases examined, carbazole in its lower excited state of the \(T_{\pi-\pi^*}\)-type [3] can apparently act only as a sensitizer of the oxidative process. Deactivation of the excited carbazole is directed toward the formation of active singlet oxygen, whose action is reduced to electrophilic attack on the carbazole fragment in the ground state.
Thus, the overall oxidation reaction may be represented by the following stages:
1) photolysis of a carbazole derivative or of 3-acetylcarbazole,
2) sensitization of atmospheric oxygen,
3) dark oxidation reactions.
\[ \begin{aligned} &\text{carbazole derivative }(X)_n\text{–}N\!-\!R \ \xrightarrow[\mathrm{H^+,\ H_2O}]{h\nu}\ \text{carbazole }(X)\text{–}N\!-\!H\ (K) + R^+ + (X) \end{aligned} \tag{1} \]
\[ K(S_0)\ \xrightarrow{h\nu}\ K(S_1)\ \longrightarrow\ K(T)\ \xrightarrow{+\,O_2^{T}}\ K(S_0)+O_2^{S_1} \tag{2} \]
\[ \begin{aligned} &\text{carbazole }(X)\text{–}N\!-\!H + O_2^{S_1} \xrightarrow[\mathrm{H^+,\ H_2O}]{} \text{hydroperoxide }(IX)\text{ with }(X)\text{ and }N\!-\!H,\ -O\!-\!OH \\ &\xrightarrow[\mathrm{O_2}]{\mathrm{H^+,\ H_2O}} \text{oxidation product }(X)\text{ containing }N\!-\!H,\ \mathrm{OH},\ \mathrm{C{=}O}. \end{aligned} \]
The formation of the hydroperoxide compound (IX) in appreciable amounts was recorded only in cases where compounds VI–VIII were subjected to oxidation. In the remaining cases the hydroperoxide could not be detected, which is apparently due to the different stability of IX with an acetyl substituent in the ring and of \(N\)-derivatives of carbazole.
To reveal the quantitative regularities of the processes taking place, kinetic determinations were made of the yield of oxidation products as a function of time, the initial concentration of the compounds being oxidized, and the amounts of acids added. The course of the reactions was monitored by spectrophotometric methods. The experimental data are summarized in the table. As can be seen from the table, the yield of oxidation products increases markedly with increasing acid concentration in the series of compounds I–III. The same dependence is observed
with a decrease in the initial concentration of all the compounds studied. Oxidation of the polymer follows the general pattern of oxidation of \(N\)-derivatives of carbazole.
For compounds V–VII, a regular change is observed in the yield of oxidation products with increasing acid concentration. This is apparently explained by the fact that in these cases an additional protonation process at the carbonyl oxygen in the acetyl radicals is superimposed.
Table
Dependence of the yield of oxidation products on the conditions of their preparation
| No. | Starting compound | Initial acid concentration, wt. % | 0.25 | 0.5 | 1.0 | 2.0 | 6.0 |
|---|---|---|---|---|---|---|---|
| yield, % | yield, % | yield, % | yield, % | yield, % | |||
| I | \(N\)-acetylcarbazole | 0 | 43 | 41 | 37 | 24 | 16 |
| I | \(N\)-acetylcarbazole | 1.0 | 68 | 67 | 62 | 59 | 34 |
| I | \(N\)-acetylcarbazole | 2.6 | 85 | 82 | 80 | 80 | 50 |
| I | \(N\)-acetylcarbazole | 3.0 | 88 | 87 | 86 | 83 | 53 |
| I | \(N\)-acetylcarbazole | 5.2 | 93 | 90 | 88 | 84 | 62 |
| I | \(N\)-acetylcarbazole | 7.8 | 95 | 91 | 89 | 85 | 68 |
| II | Carbazole | 0 | 42 | 32 | 24 | 16 | 10 |
| II | Carbazole | 2.6 | 85 | 80 | 77 | 70 | 40 |
| II | Carbazole | 5.2 | 89 | 83 | 81 | 76 | 43 |
| II | Carbazole | 7.8 | 92 | 86 | 83 | 78 | 45 |
| III | \(N\)-methylcarbazole | 0 | 36 | 24 | 12 | 10 | 4 |
| III | \(N\)-methylcarbazole | 2.6 | 68 | 58 | 50 | 32 | 11 |
| III | \(N\)-methylcarbazole | 5.2 | 72 | 66 | 62 | 46 | 15 |
| III | \(N\)-methylcarbazole | 7.8 | 74 | 70 | 66 | 50 | 18 |
| V | 3-acetylcarbazole | 0 | 81 | 60 | 42 | 36 | 15 |
| V | 3-acetylcarbazole | 1.0 | 48 | 42 | 33 | 23 | 6 |
| V | 3-acetylcarbazole | 2.6 | 48.5 | 47 | 30 | 7 | |
| V | 3-acetylcarbazole | 5.2 | 51 | 50 | 33 | 8 | |
| V | 3-acetylcarbazole | 7.8 | 57 | 53 | 45 | 36 | 11 |
| No. | Compound | ||||||
|---|---|---|---|---|---|---|---|
| VI | N-methyl-3-acetylcarbazole | 0 | 34 | 31 | 21 | 10 | 8 |
| VI | N-methyl-3-acetylcarbazole | 0.005 | 27 | 17 | 9 | 7 | 3 |
| VI | N-methyl-3-acetylcarbazole | 1.0 | 36 | 29 | 19 | 11 | 7 |
| VI | N-methyl-3-acetylcarbazole | 2.6 | 43 | 36 | 25 | 14 | 8 |
| VI | N-methyl-3-acetylcarbazole | 5.2 | 46 | 40 | 29 | 17 | 10 |
| VI | N-methyl-3-acetylcarbazole | 7.8 | 48 | 42 | 34 | 19 | 14 |
| VII | N-methyl-3,6-diacetylcarbazole | 0 | 51 | 32 | 21 | 10 | 6 |
| VII | N-methyl-3,6-diacetylcarbazole | 0.5 | 5 | 6 | 7 | 4 | 3 |
| VII | N-methyl-3,6-diacetylcarbazole | 1.0 | 8 | 7 | 8 | 4 | 3 |
| VII | N-methyl-3,6-diacetylcarbazole | 2.6 | 11 | 8 | 9 | 5 | 5 |
| VII | N-methyl-3,6-diacetylcarbazole | 5.2 | 17 | 11 | 10 | 6 | 5 |
| VII | N-methyl-3,6-diacetylcarbazole | 7.8 | 18 | 14 | 11 | 7 | 6 |
| VIII | 3,6-diacetylcarbazole | 0 | 22 | 15 | 10 | 6 | 4 |
| VIII | 3,6-diacetylcarbazole | 2.6 | 7 | 7 | 6 | 4 | 3 |
| VIII | 3,6-diacetylcarbazole | 5.2 | 4 | 4 | 3 | 2 | 3 |
| VIII | 3,6-diacetylcarbazole | 7.8 | 4 | 4 | 3 | 3 | 3 |
| VIII | 3,6-diacetylcarbazole | 10.0 | 5 | 5 | 4 | 4 | 4 |
| IV | N-polyvinylcarbazole* | 0 | 24 | 17 | 15 | 12 | 6 |
| IV | N-polyvinylcarbazole* | 0.48 | 80 | 66 | 63 | 59 | 33 |
* In benzene solution.
Experimental Part
An alcoholic solution of carbazole or its derivative with additions of sulfuric acid, specific gravity 1.3, was irradiated in the reactor with the full spectrum of a DKsSh-1000 xenon lamp placed inside the reactor, at a temperature of 40°C. Oxygen was supplied to the reaction volume by intensive bubbling of air through the solution with simultaneous stirring by a mechanical stirrer. The process was thermostated by supplying hot water to the outer jacket of the reactor and by air cooling of the lamp. After irradiation, the mixture of products was concentrated by distilling off part of the alcohol, and the resulting solution was treated with a 10-fold volume of 40% alkali solution, which made it possible to transfer the oxidation products into the aqueous phase in the form of salts. After further treatment
upon acidification of the solution, oxidation products were isolated—an amorphous brown powder, thermally unstable (decomposes at \(80^\circ\mathrm{C}\)).
The IR spectrum (\(\mathrm{KBr}\)) showed the presence of the following chromophoric groups: 1715, \(1680\ \mathrm{cm}^{-1}\) (\(\mathrm{C{=}O}\)); 3300–\(3200\ \mathrm{cm}^{-1}\) (\(\mathrm{OH}\)); \(1640\ \mathrm{cm}^{-1}\) (\(={N}-\)); \(1610\ \mathrm{cm}^{-1}\) (\(\mathrm{C{=}C}\) of the benzene ring); \(750\ \mathrm{cm}^{-1}\) (\(\delta_{\mathrm{CH}}\)—for product A); \(850\ \mathrm{cm}^{-1}\) (\(\delta_{\mathrm{H}}\)—for product B); \(1250\ \mathrm{cm}^{-1}\) (\(\delta_{\mathrm{OH}}\)—phenolic). The \(3400\ \mathrm{cm}^{-1}\) band (\(\mathrm{NH}\) group of carbazoles unsubstituted in the \(N\)-position) was absent in all oxidation products.
To confirm the structures of the compounds obtained, UV spectra, luminescence spectra, and an EPR spectrum were also recorded. Chemical methods established the presence of carbonyl, the absence of an \(\mathrm{NH}\) group, and the ability of the substances to be reduced by atomic hydrogen.
References
- J. Calvert, J. Pitts. Photochemistry. Moscow, 1968.
- Molecular Photonics. Leningrad, “Nauka,” 1970.
- R. N. Nurmukhametov, A. K. Piskunov, D. N. Shigorin. ZhOKh, 1969, 43, 1177.