STUDIES IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES

61. Interaction of 3-Methylcarbazole with Vinyl Acetate and Some Chemical Transformations of 3-Methyl-9-(1-acetoxy)ethylcarbazole

V. P. Lopatinskii, Yu. P. Shekhirev, V. M. Sutyagin, V. P. Chernitsyna

(Presented by the scientific-methodological seminar of the Chemical-Technological Faculty)

Earlier we showed [1] that carbazole adds to vinyl acetate at \(-(10–20)^\circ\mathrm{C}\) in acetone in the presence of caustic potash, forming 9-(1-acetoxy)ethylcarbazole. The latter is chemically very active and can be used in a number of syntheses, for example, to obtain 9-vinylcarbazole [2].

Later we investigated the interaction of 3-chlorocarbazole with vinyl acetate in an alkaline medium [3]. The present communication describes the results of studies on the interaction of 3-methylcarbazole with vinyl acetate.

Our investigations showed that 3-methylcarbazole, analogously to 3-chlorocarbazole and carbazole, reacts with vinyl acetate in acetone in the presence of solid caustic potash; moreover, in this case the reaction proceeds simultaneously in two directions: with the formation of 3-methyl-9-(1-acetoxy)ethylcarbazole and 3-methyl-9-acetylcarbazole.

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.

Earlier we developed a method for the quantitative determination of 3-chloro-9-(1-acetoxy)ethylcarbazole and 3-chloro-9-acetylcarbazole when present together [3]. This method was based on the different stability of these products toward hydrolysis.

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

The results show that 3-methylcarbazole, in the absence of a catalyst, does not react with vinyl acetate in the temperature range \((-10) — (+20)^\circ\mathrm{C}\); in the presence of small amounts of caustic potash, however, the reaction proceeds simultaneously in the two directions indicated above. In this case, the rates of the addition reaction and the acetylation reaction depend differently on the amount of catalyst.

As can be seen from Table 1, the temperature conditions of the process have a particularly strong effect on the reaction of 3-methylcarbazole 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 \(0–20^\circ\mathrm{C}\)

the total conversion of 3-methylcarbazole also decreases, which, apparently, is explained by partial decomposition of the 3-methyl-9-(1-acetoxy)-ethylcarbazole formed under these conditions. To obtain high yields of the addition product, the reaction temperature must be maintained at 20–15°C below zero.

Table 1

Effect of temperature and the amount of potassium hydroxide on the reaction of 3-methylcarbazole with vinyl acetate

Note: a—conversion of 3-methylcarbazole to 3-methyl-9-(1-acetoxy)ethylcarbazole;
b—overall conversion of 9-methylcarbazole;
c—conversion of 3-methylcarbazole to 3-methyl-9-acetylcarbazole.

Amount of vinyl acetate—1.53 moles per mole of 3-methylcarbazole.

The studies carried out made it possible to synthesize 3-methyl-9-(1-acetoxy)-ethylcarbazole in good yields and to study some of its chemical transformations.

Upon distillation in vacuum, 3-methyl-9-(1-acetoxy)-ethylcarbazole decomposes with liberation of acetic acid and formation of 3-methyl-9-vinylcarbazole. In contrast to 3-chloro-9-(1-acetoxy)-ethylcarbazole, the yield of the vinyl derivative in this case is lower and averages 70% of theoretical.

Experimental Part

3-Methylcarbazole was prepared by the method of Borsche [4] and had m.p. 202–203°C.

Vinyl acetate was used freshly distilled and had constants coinciding with those reported in the literature.

3-Methyl-9-(1-acetoxy)ethylcarbazole. (I)

Into a three-necked round-bottom flask fitted with a stirrer and thermometer, 5 g of 3-methylcarbazole, 50 ml of acetone, and 4 ml of vinyl acetate are introduced successively. The flask is placed in a cryostat, cooled to −(18–20°), and 1 g of finely powdered potassium hydroxide is added, and then

the mixture is kept at the same temperature for 20–25 min. After completion of the reaction the catalyst is filtered off, acetone is distilled off from the filtrate, and the remaining oil is dissolved in 10 ml of hexane. On cooling, 4.4 g of (I) with m.p. 81–82°C (60%) is obtained from the solution. After a second recrystallization from hexane the product has m.p. 84–85°C.

Found, %: N 5.24; C₁₇H₁₇NO₂

Calculated, %: N 5.24

By an analogous procedure at 50°C, 3-methyl-9-acetylcarbazole was obtained and isolated, m.p. 74–75°C.
Found, %: N 6.45; C₁₅H₁₃NO
Calculated, %: N 6.28

3-Methyl-9-vinylcarbazole (II)

Into a three-necked round-bottom flask equipped with a thermometer and stirrer are successively charged 20 g of 3-methylcarbazole, 100 ml of acetone, and 16 ml of vinyl acetate. The flask is placed in a cryostat, cooled to −(18–20)°C, and 2 g of caustic potash is added in small portions, maintaining the temperature at the specified level. The reaction mixture is then kept at the same temperature for 20–25 min. After completion of the reaction the catalyst is filtered off, acetone is distilled off from the filtrate, the residue is distilled in vacuum, and the fraction boiling in the range 165–175°C (2–5 mm Hg) is collected. Substance (II) distills as a colorless transparent liquid, which does not crystallize on prolonged standing, with \(n_D^{20}\) 1.6720 and a yield of 17.0 g (74.3%).

On crystallization from ethanol or methanol, a mixture of 3-methyl-9-vinylcarbazole and 3-methylcarbazole is formed, which is difficult to separate into individual components. By repeated crystallization, pure 3-methyl-9-vinylcarbazole was isolated in the form of colorless needles with m.p. 32°C.

The structure of the product was established by elemental analysis, infrared spectrum (1800, 1600, 1440 cm⁻¹), and determination of double bonds by the method of addition of mercuric acetate [5] and by the method of hydrolytic oxidation [6].

Found, %: N 6.74; C₁₅H₁₃N.

Calculated, %: N 6.76

The content of the basic substance in the product is 99.9%. 3-Methyl-9-vinylcarbazole polymerizes well at 120°C in the presence of azobisisobutyronitrile, forming a transparent polymer colored light yellow.

Conclusions

1. The influence of conditions on the reaction of vinyl acetate with 3-methylcarbazole in acetone in the presence of caustic potash was investigated, and it was shown that 3-methyl-9-(1-acetoxy)ethylcarbazole and 3-methyl-9-acetylcarbazole are formed simultaneously.

2. The synthesis of 3-methyl-9-(1-acetoxy)ethylcarbazole was carried out for the first time.

3. By thermal decomposition of 3-methyl-9-(1-acetoxy)ethylcarbazole, the synthesis of 3-methyl-9-vinylcarbazole was carried out for the first time.

References

1. V. P. Lopatinskii, Yu. P. Shekhirev. Izv. TPI, 1965, 136, 115.
2. V. P. Lopatinskii, Yu. P. Shekhirev, V. M. Sutyagin, A. P. Bychkova. Methods for Obtaining Chemical Reagents and Preparations, USSR Academy of Sciences (in press).
3. V. P. Lopatinskii, Yu. P. Shekhirev, V. M. Sutyagin, E. A. Danilova. Izv. TPI (in press).
4. W. Borsche, A. Witte, W. Bohte. Ann., 1908, 359, 74.
5. R. W. Martin. Analyt. Chem., 1949, 21, 921.
6. V. P. Lopatinskii, I. P. Zherebtsov, L. P. Poltorakova. Plastics (in press).