RESEARCH IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES

V. SYNTHESIS OF 9-ETHYL-3,6-DIACETYLCARBAZOLE

V. P. Lopatinskii, E. E. Sirotkina, L. I. Misailova

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

Only a few 9-alkyl-3,6-diacetylcarbazoles have been described in the literature [1–4]; these are of interest for the synthesis of 9-alkyl-3,6-divinylcarbazoles, from which heat-resistant polymers can be obtained [1]. A summary of the known 9-alkyl-3,6-diacetylcarbazoles was given in our previous communication [5]. Their synthesis was carried out 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 dry benzene [4]. By the latter method, 9-ethyl-3,6-diacetylcarbazole was obtained; however, its yield did not exceed 10% of theoretical [4].

The purpose of the present work was to study the conditions under which a high yield of 9-ethyl-3,6-diacetylcarbazole is achieved in the acetylation of 9-ethylcarbazole in benzene. Acetyl chloride was used as the acetylating agent, its activity being increased by the addition of small amounts of acetyl bromide (0.1 mole per mole of 9-ethylcarbazole), as had previously been recommended in the acetylation of 9-acetylcarbazole [6]. The effect on the yield of 9-ethyl-3,6-diacetylcarbazole of temperature, the amount of aluminum chloride, acetyl chloride, and solvent was investigated.

The results of some experiments are given in Table 1.

The results of the experiments show that the yield of 9-ethyl-3,6-diacetylcarbazole can be increased to 81.1% of theoretical if acetylation is carried out at 0°C in the presence of a considerable excess of anhydrous aluminum chloride (3 moles per mole of 9-ethylcarbazole). A further increase in the amount of AlCl₃ and an increase in temperature lead to a decrease in the yield of the reaction product, as well as to deterioration in the purity of the product.

The experiments showed that, for the successful accomplishment of the reaction, it is possible to use such an amount of benzene as is sufficient to dissolve the suspension of the initial 9-ethylcarbazole. The main amount of 9-ethylcarbazole reacts during the first half-hour, but the maximum yield was obtained after two hours of stirring.

The synthesis of the starting 9-ethylcarbazole was carried out by a modified procedure of Stevens and Tucker [7], which differed in that carbazole was alky-

Table 1

*) in moles per mole of 9-ethylcarbazole

...was ethylated with ethyl iodide or ethyl bromide in acetone solution in the presence of powdered NaOH or KOH. Under these conditions, a quantitative yield of 9-ethylcarbazole was readily achieved.

Experimental Part

Starting reagents. Carbazole was isolated from crude anthracene from the Kemerovo coke-chemical plant with the aid of paraformaldehyde [8] and had a melting point of 244°C. Dry cryoscopic benzene was used. Acetyl chloride, acetyl bromide, ethyl bromide, and anhydrous aluminum chloride were of “pure” grade.

9-Ethylcarbazole. Into a three-necked round-bottom 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 the stirrer operating, 5.6 ml of ethyl bromide was gradually added to the flask over 15 minutes. After addition of the ethyl bromide, the temperature was raised to the boiling point of the reaction mass and heating was continued for 1.5–2 hours. The end of the reaction was determined by introducing a drop of the reaction mass into a test tube with boiling water: upon completion of the reaction, an oily drop of 9-ethylcarbazole, melting below the boiling point of water, spread over the surface of the water. Pro-

the reaction product was precipitated with water, filtered off, washed with water to neutral reaction, and dried at 40–50°C. The dry substance was recrystallized from ethanol. The yield of recrystallized 9-ethylcarbazole was 11.0 g (94.3% of theory), m.p. 68–69°C (according to the literature, 9-ethylcarbazole melts at 67–68°C [9], 71°C [10]).

9-Ethyl-3,6-diacetylcarbazole. 5 g of 9-ethylcarbazole was dissolved in 34.5 ml of dry benzene, and 5.03 g of acetyl chloride and 0.3 g of acetyl bromide were added to the solution. The mixture was cooled on ice, and then, with stirring, 10.4 g of anhydrous aluminum chloride (3 moles per mole of 9-ethylcarbazole) was gradually introduced into it. Stirring at 0°C was continued for two hours. After completion of the acetylation, the reaction mass was poured onto ice; the precipitate that separated was filtered off and recrystallized from benzene. The product yield was 5.8 g (81.1% of theory), with m.p. 181°C. Repeated recrystallization raises the melting point of the substance, which separates as small white crystals, to 182–182.5°C (according to the literature, 9-ethyl-3,6-diacetylcarbazole melts at 182°C [3, 4], 183.5°C [1]). Found, %: N 5.07; 5.09. \(\mathrm{C}_{18}\mathrm{H}_{17}\mathrm{O}_{2}\mathrm{N}\). Calculated, %: N 5.01.

Conclusions

1. The influence of conditions on the yield of 9-ethyl-3,6-diacetylcarbazole in the acetylation of 9-ethylcarbazole with acetyl chloride in benzene has been studied.

2. It has been shown that, under optimal conditions, 9-ethyl-3,6-diacetylcarbazole can be obtained in a yield of 81.1% of theory.

3. A procedure has been developed for the synthesis of 9-ethylcarbazole that makes it possible to obtain it in quantitative yield.

References

1. S. Kawaiaa, J. Chem. Soc. Japan, Pure Chem. Sect., 73, 103, 1952; C. A. 47, 306, 1953.

2. S. Plant, K. Rogers, S. Williams, J. Chem. Soc., 741, 1935.

3. N. Buu-Hoi, R. Royer, Rec. trav. chim., 66, 533, 1947; C. A. 42, 5014, 1948.

4. N. Buu-Hoi, R. Royer, J. Org. Chem., 15, 123, 1950.

5. V. P. Lopatinskii, E. E. Sirotkina, M. M. Anosova. Izvestiya Tomskogo politekhnicheskogo instituta, 111, 1961.

6. A. A. Berlin. ZhOKh, 14, 1096, 1944.

7. T. Stevens, S. Tucker, J. Chem. Soc., 123, 2140, 1923.

8. V. P. Lopatinskii, E. E. Sirotkina. Izvestiya Tomskogo politekhnicheskogo instituta, 111, 1961.

9. C. Graebe, Ann., 202, 23, 1880.

10. W. Reppe, Lieb. Ann., 601, 128, 1956.