Research in the Chemistry of Carbazole Derivatives

40. Interaction of 3-chlorocarbazole with vinyl acetate and some chemical transformations of 3-chloro-9-(α-acetoxyethyl)carbazole

V. P. Lopatinskii, Yu. P. Shekhirev, V. M. Sutyagin,
E. A. Danilova

(Recommended by the scientific-methodological seminar of the Faculty of Chemical Technology)

Previously we showed [1] that carbazole adds to vinyl acetate at 10–20°C below zero in acetone in the presence of caustic potash, giving 9-(α-acetoxyethyl)carbazole. The latter is chemically very active and can be used in a number of syntheses. In this connection, it seemed of interest to us to carry out the addition reaction of carbazole derivatives to vinyl acetate. The present communication describes the results of studies on the interaction of 3-chlorocarbazole with vinyl acetate.

Our investigations showed that 3-chlorocarbazole, similarly to carbazole itself, readily enters into reaction with vinyl acetate in acetone in the presence of solid caustic potash; moreover, in this case as well the reaction proceeds simultaneously in two directions: with formation of 3-chloro-9-(α-acetoxyethyl)carbazole and 3-chloro-9-acetylcarbazole according to the scheme:

\[ \mathrm{3\text{-}chlorocarbazole} + \mathrm{CH_2{=}CHOC(O)CH_3} \longrightarrow \begin{cases} \mathrm{3\text{-}chloro\text{-}9\text{-}(\alpha\text{-}acetoxyethyl)carbazole},\\ \mathrm{3\text{-}chloro\text{-}9\text{-}acetylcarbazole} + \mathrm{CH_3CHO}. \end{cases} \]

Since the content of these two substances in the reaction products depends strongly on the conditions under which the process is carried out, in order to achieve high yields of the addition product of interest to us it was necessary to study the influence of various factors on this reaction. For this purpose we developed a method for the quantitative determination of 3-chloro-9-(α-acetoxyethyl)carbazole and 3-chloro-9-acetylcarbazole in their joint presence. The method was based on the different stability of these products toward hydrolysis. According to our observations, 3-chloro-9-(α-acetoxyethyl)carbazole is quantitatively hydrolyzed upon heating with water in acetone solution, whereas 3-chloro-9-acetylcarbazole is stable under these conditions. Hydrolysis with a 0.1 N solution of caustic potash in 50% methanol leads to quantitative saponification of both compou-

compounds. Therefore, by hydrolyzing part of the reaction products with water, we determined the conversion of 3-chlorocarbazole to 3-chloro-9-(α-acetoxyethyl)carbazole—($a$); upon hydrolysis of another part of the reaction products with an aqueous-methanolic alkali solution, the total conversion of 3-chlorocarbazole was determined—($b$). From the difference between these values we determined the conversion of 3-chlorocarbazole to 3-chloro-9-acetylcarbazole—($c$).

Using this method, we studied the influence of temperature and the amount of catalyst on the reaction of 3-chlorocarbazole with vinyl acetate. The results obtained are given in Table 1.

Table 1

Effect of temperature and the amount of caustic potash on the reaction of 3-chlorocarbazole with vinyl acetate

*) The designation of these quantities is indicated in the text.

Note. All experiments were carried out with an amount of vinyl acetate of 1.53 mol per mol of 3-chlorocarbazole. For carrying out the reaction, 3-chlorocarbazole with m.p. 192–193°C was used.

The data presented show that, in the absence of a catalyst, 3-chlorocarbazole does not react with vinyl acetate in the temperature range (−10)–30°C; in the presence of small amounts of caustic potash, however, the reaction proceeds simultaneously in the two directions indicated above. At the same time, the rates of the addition reaction and of the acetylation reaction depend differently on the amount of catalyst. If the acetylation reaction proceeds at the maximum possible rate for it at the given temperature already with 1% catalyst, then achieving a high rate of the addition reaction requires 3–5% catalyst.

As can be seen from Table 1, the temperature conditions of the process exert an especially strong influence on the reaction of 3-chlorocarbazole with vinyl acetate. As the temperature is raised, the content of the addition product sharply decreases and, at the same time, the content of the acetyl derivative increases. In the temperature range 25–50°C, the overall conversion of 3-chlorocarbazole also decreases, which apparently is explained by partial decomposition, under these conditions, of the 3-chloro-9-(α-acetoxyethyl)carbazole being formed. To obtain high yields of the addition product, the reaction temperature must be maintained at a level 10–20°C below zero. Subsequently we showed that the quan-

the amount of vinyl acetate introduced into the reaction has little effect on the formation of the two possible reaction products; however, the overall conversion of 3-chlorocarbazole depends on it. A sufficient amount of vinyl acetate to achieve complete conversion of 3-chlorocarbazole is 1.3–1.5 moles per mole of 3-chlorocarbazole. With smaller amounts, part of the 3-chlorocarbazole remains unreacted and makes it difficult to isolate and purify the reaction products. Special experiments have established that, for carrying out the reaction, crude 3-chlorocarbazole with m.p. 186–187°C, obtained as a result of chlorination of carbazole with sulfuryl chloride, may be used without any additional purification.

The investigations carried out made it possible to synthesize 3-chloro-9-(α-acetoxyethyl)carbazole in good yields, up to 77%, and to study some of its chemical properties.

Analogously to the 9-(α-acetoxyethyl)carbazole previously obtained by us, 3-chloro-9-(α-acetoxyethyl)carbazole is readily hydrolyzed by water to 3-chlorocarbazole, acetaldehyde, and acetic acid. When heated with methanol it forms 3-chloro-9-(α-methoxyethyl)carbazole in a yield of 70% and higher.

On distillation under vacuum, 3-chloro-9-(α-acetoxyethyl)carbazole readily decomposes with liberation of acetic acid and formation of 3-chloro-9-vinylcarbazole. In contrast to 9-(α-acetoxyethyl)carbazole, the yield of the vinyl derivative in this case is high and averages 80–90%. After one crystallization from methanol, the 3-chloro-9-vinylcarbazole obtained in this way contains 92–99% of the principal substance; the yield of such a product is 60–75% of theory. The resulting 3-chloro-9-vinylcarbazole polymerizes in bulk at 120–140°C without an initiator, or at 80–90°C in the presence of benzoyl peroxide. The properties of its polymers are currently being studied by us. The physical constants and analytical results for the compounds synthesized by us are given in Table 2.

Table 2

Experimental Part

The 3-chlorocarbazole used in this work was synthesized by us by chlorination of carbazole with sulfuryl chloride [2] and had m.p. 186–187°C; after two crystallizations from alcohol it melted at 192–193°C.

Before the experiments, vinyl acetate was distilled and had constants coinciding with those in the literature.

3-Chloro-9-(α-acetoxyethyl)carbazole (I). Into a three-necked round-bottom flask are successively charged 10 g of 3-chlorocarbazole, 0.5 g of finely ground caustic potash, and 50 ml of acetone. The flask is placed in a cryostat at a temperature of (−19°)C, and, when the indicated temperature is established in the reaction mass, 7.2 ml of vinyl acetate is added dropwise to the flask, after which the mixture is stirred at the specified temperature for another 30 min. Upon completion of the reaction, the resulting solution is filtered off from the catalyst, and acetone is distilled off from the filtrate. The remaining oil is dissolved in 50 ml of heptane; upon cooling, 9.7 g of I crystallizes from the solution, m.p. 82–83°C (yield 67.8%). For further purification it is crystallized from the same solvent.

3-Chloro-9-(α-methoxyethyl)carbazole (II). To 4 g of I, 40 ml of methanol is added, and the mixture is boiled on a water bath with a reflux condenser for 10 min. Upon cooling, 2.6 g of II crystallizes from the solution, m.p. 75–76°C (yield 72.2%). Further purification of II is carried out by crystallization from methanol.

3-Chloro-9-vinylcarbazole (III). 12.05 g of I is placed in a Claisen flask equipped with a spider adapter with two receivers and heated under vacuum (5–10 mm Hg). On heating, the product first melts, and then acetic acid distills off, condensing in one of the receivers. Upon further heating at 220–236°C (10) mm Hg, III distills off as a colorless liquid, which rapidly crystallizes into a white solid, m.p. 57–58°C. Yield 8.6 g, 90% of theoretical. After two crystallizations from methanol, III melts at 68–69°C; literature data: m.p. 69°C [3].

Conclusions

1. It has been shown that, in the reaction of vinyl acetate with 3-chlorocarbazole in acetone in the presence of caustic potash, 3-chloro-9-(α-acetoxyethyl)carbazole and 3-chloro-9-acetylcarbazole are formed simultaneously.

2. The influence of the conditions for carrying out the process on this reaction has been studied, and it has been shown that the temperature conditions of the process are decisive in directing the reaction one way or the other.

3. The synthesis of 3-chloro-9-(α-acetoxyethyl)carbazole has been carried out for the first time, and some of its chemical properties have been studied.

4. By thermal decomposition of 3-chloro-9-(α-acetoxyethyl)carbazole, 3-chloro-9-vinylcarbazole has been synthesized in good yield.

References

1. V. P. Lopatinskii, Yu. P. Shekhirev. Izv. TPI, 136, 1964.
2. G. Mazzara, E. Lamberti-Zanardi. Gazz. Chim. Ital., 26, II, 236, 1896.
3. Chemistry of Acetylene. Collection of articles. Izdatinlit. Moscow, 1954.