RESEARCH IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES

II. Acetylation of 9-methylcarbazole

V. P. Lopatinskii, E. E. Sirotkina, M. M. Anosova

(Presented by Professor, Doctor of Chemical Sciences L. P. Kulev)

Diacetyl derivatives of carbazole are of interest for the synthesis of divinylcarbazoles, which, upon polymerization, are capable of yielding heat-resistant polymers with good dielectric properties [1]. Of greatest importance with respect to heat resistance may be those divinylcarbazoles whose vinyl groups are located in the benzene rings of the carbazole molecule. Of such divinylcarbazoles, only 9-ethyl- and 9-benzyl-3,6-divinylcarbazoles have been described in the literature [1]. Table 1 below gives information on the known diacetyl derivatives of carbazole.

Table 1

Diacetylcarbazoles

9-Alkyl-3,6-diacetylcarbazoles were obtained by two methods: either by alkylation of 3,6-diacetylcarbazole [2] or of its potassium salt [1], or by acetylation of 9-alkylcarbazoles with acetyl chloride in the presence of anhydrous aluminum chloride in carbon disulfide [3] or in dry benzene [5]. By the latter method only 9-ethyl-3,6-diacetylcarbazole was obtained, in low yield (10% of theory); the main product of the acetylation was 9-ethyl-3-acetylcarbazole [5].

The aim of the present work is to study the conditions under which a high yield of 9-methyl-3,6-diacetylcarbazole is obtained in the acetylation of 9-methylcarbazole in benzene. Acetyl chloride was used as the acetylating agent; its activity was increased

Table 2

Acetylation of 9-methylcarbazole

by adding small amounts of acetyl bromide (0.1 mole per mole of 9-methylcarbazole), as had previously been recommended for the acetylation of 9-acetylcarbazole [6]. In addition to benzene, chlorobenzene, nitrobenzene, and carbon tetrachloride were tested as solvents. However, in their presence the yields of diacetyl derivatives were lower and did not exceed 10—11% of theoretical. Therefore, subsequent experiments were carried out only in pure dry benzene. The effect on the yield of 9-methyl-3,6-diacetylcarbazole of temperature, the amount of aluminum chloride and solvent, and also the duration of acetylation was studied. The results of some experiments are given in Table 2.

The experimental results show that the yields of 9-methyl-3,6-diacetylcarbazole can be increased to 64—68% of theoretical if the acetylation is carried out at 0°C in the presence of a considerable excess of anhydrous aluminum chloride (3 moles per mole of 9-methylcarbazole). Under these conditions the reaction product is obtained in a purer form. A further increase in the amount of AlCl₃ practically does not change the yield of the diacetyl derivative (experiment 7). The experiments showed that, for successful performance of the reaction, an amount of benzene sufficient to dissolve the weighed portion of the starting 9-methylcarbazole may be used; increasing the amount of solvent has no effect on the yields of 9-methyl-3,6-diacetylcarbazole. It should be noted that the principal amount of 9-methylcarbazole reacts during the first half-hour, and that two hours of stirring is sufficient for completion of the reaction.

Control of the quality of recrystallized 9-methyl-3,6-diacetylcarbazole was simultaneously carried out by the ultraviolet absorption spectra of the substance in ethanol (Fig. 1¹).

Fig. 1. Ultraviolet spectrum of 9-methyl-3,6-diacetylcarbazole in ethanol.

The synthesis of the starting 9-methylcarbazole was carried out by us according to a modified Stevens and Tucker procedure [7], differing in that methylation of carbazole is performed with methyl iodide in acetone solution in the presence of powdered caustic soda. Under these conditions a quantitative yield of 9-methylcarbazole is readily obtained.

Experimental Part

Starting reagents. Carbazole was isolated from crude anthracene from the Kemerovo coke-chemical plant by the formaldehyde method [8] and had m.p. 244°C. Benzene used, acetyl chloride, acetyl bromide, and anhydrous aluminum chloride were of “pure” grade.

9-Methylcarbazole. Into a three-necked round-bottomed flask equipped with a dropping funnel, reflux condenser, and stirrer were introduced 10 g of carbazole, 50 ml of acetone, and 3.5 g of powdered NaOH (or KOH). The contents of the flask were heated to 35°. Then, with stirring, 5.6 ml of methyl iodide was gradually added to the flask over 15 minutes. The end of the dropping funnel containing the methyl iodide was to be immersed in acetone. After addition of the methyl iodide, the temperature was raised to the boiling point of the reaction mixture and heating was continued for 1.5 hours. The end of the reaction is determined by introducing a drop of the reaction mixture into a test tube with boiling water; when the reaction is complete, an oily drop of 9-methylcarbazole, melting below the boiling point of water, spreads over the surface of the water.

The reaction product was precipitated with water, filtered off, washed with water to a neutral reaction, and dried at 50–60°C. The dry substance was recrystallized from ethanol. Yield of recrystallized 9-methylcarbazole — 10.4 g (96% of theoretical), m.p. 87–88°C.

9-Methyl-3,6-diacetylcarbazole. 5 g of 9-methylcarbazole was dissolved in 38 ml of dry benzene, and to the solution were added 6.5 g of acetyl chloride and 0.34 g of acetyl bromide. The mixture was cooled in ice to 0°C, and then 11.1 g of anhydrous aluminum chloride (3 moles per mole of 9-methylcarbazole) was gradually introduced into it with stirring. The reaction mass changed its color from yellow-green (after addition of acetyl chloride) to reddish-brown, and after completion of the addition of AlCl₃ became dark green. Stirring at 0°C was continued for two hours. The rapid removal of HCl from the reaction vessel was facilitated by carrying out the reaction under strong draft. After completion of the acetylation, ice was added to the reaction mass; after stirring—

with which its color changed to greenish-gray. The precipitate that separated was filtered off, and the diacetyl derivative was extracted from it by heating with 150 ml of benzene to boiling. On cooling to room temperature, 5.0 g of 9-methyl-3,6-diacetylcarbazole was obtained, m.p. 195.5–196°C (yield 68.4% of theoretical). Recrystallization from benzene gave 4.1 g of substance, m.p. 197–198°C, separating in the form of white, lustrous, needle-like crystals.

Found, %: N 5.32. \( \mathrm{C}_{17}\mathrm{H}_{15}\mathrm{O}_{2}\mathrm{N} \). Calculated, %: N 5.28.

Conclusions

1. The influence of conditions on the yield of 9-methyl-3,6-diacetylcarbazole in the acetylation of 9-methylcarbazole with acetyl chloride in benzene has been investigated.

2. It has been shown that, under optimal conditions, 9-methyl-3,6-diacetylcarbazole can be obtained in a yield of 64–68% of theoretical.

3. A simple method has been developed for the synthesis of 9-methylcarbazole, making it possible to obtain it in quantitative yield.

Literature

1. S. Kawai, K. Yoneshige et al., J. Chem. Soc. Japan, Pure Chem. Sect., 73, 103, 1952; C. A. 47, 3605, 1953.
2. S. Plant, K. Rogers, S. Williams. J. Chem. Soc., 1935, 741.
3. N. Buu-Hoi, R. Royer, Rec. trav. chim., 66, 533, 1947; C. A. 42, 5014, 1948.
4. D. Mitchell, S. Plant. J. Chem. Soc., 1936, 1295.
5. N. Buu-Hoi, R. Royer. J. Org. Chem., 15, 123, 1950.
6. A. A. Berlin, ZhOKh, 14, 1096, 1944.
7. T. Stevens, S. Tucker, J. Chem. Soc., 123, 2140, 1923.
8. L. P. Kuz’ev, V. P. Lopatinskii, Proceedings of the Tomsk Polytechnic Institute, 102, 1957.

1. The spectral analyses were carried out by V. V. Konstantinova, to whom the authors express their gratitude. ↩