Research in the Chemistry of Carbazole Derivatives

XII. Synthesis of Certain 3-Acetyl-9-alkylcarbazoles

V. P. Lopatinskii, E. E. Sirotkina

(Presented by the scientific seminar of the Organic Chemistry Section)

3-Acetyl-9-alkylcarbazoles are of interest for the synthesis of 3-vinyl-9-alkylcarbazoles, from whose polymers one may expect electrophysical properties as good as those of the polymer of 9-vinylcarbazole, which is used in electrical engineering [1]. In the literature only two monoacetyl derivatives of 9-alkylcarbazoles have been described: 3-acetyl-9-methylcarbazole, obtained in low yield by acetylating 9-methylcarbazole with acetyl chloride in carbon disulfide in the presence of anhydrous aluminum chloride for 18 hours at room temperature [2], and 3-acetyl-9-ethylcarbazole, which was obtained by acetylating 9-ethylcarbazole with acetyl chloride in carbon disulfide, likewise in insignificant yield [2], and in dry benzene—with a yield of 53% of theory in 5 hours [3]. The preparation of both products was accompanied by the formation of by-product 3,6-diacetyl derivatives of 9-alkylcarbazoles. The picrate of 3-acetyl-9-isoamylcarbazole has also been described [4].

The aim of the present work was to develop a method for the synthesis of 3-acetyl-9-alkylcarbazoles that would give high yields of these compounds and would not be accompanied by the formation of significant amounts of by-products. To this end, using the synthesis of 3-acetyl-9-ethylcarbazole as an example, a systematic study was carried out of the influence of the principal factors on the yield of the final product and on the rate of acetylation. As a result of these experiments (see Table 1), it was established that the optimum acetylation temperature is 15°C, the amount of anhydrous aluminum chloride should be 1 mole, and the amount of acetyl chloride—1.25 moles per mole of 9-ethylcarbazole. With acetyl bromide, the same results are obtained as with acetyl chloride. As solvent it is convenient to use dry cryoscopic benzene, the amount of which should be 6 moles per mole of 9-ethylcarbazole. Attempts to use other solvents (toluene, xylenes, carbon tetrachloride) led to poorer results. Under these conditions, the best yields of 3-acetyl-9-ethylcarbazole were obtained when acetylation was carried out for 4 hours.

The optimum conditions found, under which a stable yield of 3-acetyl-9-ethylcarbazole is achieved (73% of theory), were

were checked in several series of experiments on the synthesis of other 3-acetyl-9-alkylcarbazoles. The results of these experiments basically confirmed the correctness of the choice of the indicated optimal conditions. Only with respect to the acetylation temperature was it established that almost identical

Table 1

Results of some experiments on the synthesis of 3-acetyl-9-ethylcarbazole

results are obtained both at 15°C and at 10°C. The basic data on the 3-acetyl-9-alkylcarbazoles we obtained are given in Table 2.

A substantial influence on the results of acetylation is exerted by the order in which the reagents are charged. We changed the procedure used earlier [3]. In our experiments all the aluminum chloride was charged at once into the benzene solution of 9-alkylcarbazole, and then, with stirring, acetyl chloride was gradually introduced. Only by using this procedure under optimal conditions can high yields of 3-acetyl-9-alkylcarbazoles be obtained, reaching 50–73% of theoretical. The developed procedure also has the advantage that the experimental results are well reproducible, and the by-products of acetylation (3,6-diacetyl-9-alkylcarbazoles), which are inevitably obtained by the method of Buu-Hoï et al. [2, 3, 4], are not formed here. Not

The 9-alkylcarbazoles that have entered into the reaction can be separated by distillation and used in repeated syntheses.

Table 2

Experimental Part

The starting 9-alkylcarbazoles were prepared by the method previously developed by us for 9-methyl- and 9-ethylcarbazoles [5, 6]. The characterization of the 9-alkylcarbazoles used by us is given in Table 3.

Table 3

9-ethylcarbazoles

Acetyl chloride and anhydrous aluminum chloride were of “pure” grade; benzene (cryoscopic) was subjected to additional drying.

3-Acetyl-9-methylcarbazole. A charge of 9-methylcarbazole, 181.2 g (1 mole), was dissolved in 525 ml of benzene, and anhydrous AlCl₃ in an amount of 133 g (1 mole) was added to the solution in small portions with thorough stirring. To the reaction mixture, cooled to 10°C, 89.1 ml (1.25 moles) of acetyl chloride was gradually added with stirring. At a temperature of 15°C the mixture was kept for 4 hours with stirring. After completion of the reaction, to the reaction mass was add-

ice was added. The precipitated 3-acetyl-9-methylcarbazole was filtered off, washed with water to a neutral reaction, dried, distilled in vacuo, and recrystallized from methyl alcohol.

At 214–215°C (1–2 mm Hg), and after subsequent recrystallization from methanol, 107 g of 9-methyl-3-acetylcarbazole with a melting point of 101–102°C was obtained. From the benzene layer an additional 18 g of 3-acetyl-9-methylcarbazole was isolated. The total yield of 3-acetyl-9-methylcarbazole was 56.04% of theory.

Analyses: found, %: N 5.9; 5.94. C₁₅H₁₃ON. Calculated, %: N 6.28.

3-Acetyl-9-methylcarbazole forms picrate—orange needles with a melting point of 106–107°C.

All other 3-acetyl-9-alkylcarbazoles were prepared by the method described above.

3-Acetyl-9-ethylcarbazole. From 195.2 g (1 mole) of 9-ethylcarbazole, 133 g of aluminum chloride (1 mole), and 89.1 ml of acetyl chloride (1.25 moles) in 525 ml of benzene, 174.8 g (73.7% of theory) of 3-acetyl-9-ethylcarbazole was obtained, with a melting point of 114–115°C and a boiling point of 240–245°C (at 10–12 mm Hg). Analyses: found, %: N 5.85; 5.80. C₁₆H₁₅ON. Calculated, %: N 5.91.

3-Acetyl-9-ethylcarbazole forms picrate—orange needles with a melting point of 136°C.

3-Acetyl-9-n-propylcarbazole. The reaction used 209.2 g (1 mole) of 9-n-propylcarbazole, 89.1 ml of acetyl chloride (1.25 moles), 133 g of aluminum chloride, and 525 ml of benzene. After completion of the acetylation, when the complex was decomposed, the entire reaction product was in the benzene solution. The benzene layer was separated from the aqueous layer, washed with water to a neutral reaction, dried with calcium chloride, and, after removal of the benzene by distillation, the residue was fractionated in vacuo. At temperatures up to 215°C (3–4 mm Hg), 18 g of 9-n-propylcarbazole with a melting point of 48–49°C was distilled off. At 217–220°C (3–4 mm Hg), 3-acetyl-9-n-propylcarbazole was distilled off; after recrystallization from methanol it was obtained in an amount of 133.1 g (53% of theory).

3-Acetyl-9-n-propylcarbazole separates in the form of colorless needles with a melting point of 68–68.5°C. It forms picrate—orange needles with a melting point of 100–101°C. Analyses: found, %: N 5.32; 5.33. C₁₇H₁₇ON. Calculated, %: N 5.57. The isolation of the reaction products in subsequent experiments was carried out in the same way as described for 3-acetyl-9-n-propylcarbazole.

3-Acetyl-9-n-isopropylcarbazole. The reaction used 104.6 g of 9-isopropylcarbazole, 44.5 ml of acetyl chloride, and 66.5 g of anhydrous aluminum chloride in 260 ml of benzene. 62.8 g (50.0% of theory) of 3-acetyl-9-isopropylcarbazole was obtained, with a melting point of 81.0–81.5°C and a boiling point of 205–207°C (at 2–3 mm Hg). 3-Acetyl-9-isopropylcarbazole is a colorless crystalline substance and forms an orange picrate with a melting point of 116–117°C. Analyses: found, %: N 5.29; 5.41. C₁₇H₁₇ON. Calculated, %: N 5.57.

3-Acetyl-9-n-butylcarbazole. To a solution of 223 g of 9-n-butylcarbazole in 400 ml of benzene, 133.3 g of aluminum chloride and 89.1 ml of acetyl chloride were added. 160 g (60.3% of theory) of colorless crystalline needles of 3-acetyl-9-n-butylcarbazole were obtained, with a melting point of 57–58°C and a boiling point of 210–213°C (at 1–2 mm Hg). It forms picrate—orange needles with a melting point of 97–98°C.

Analyses: found, %: N 5.34; 5.29. C₁₈H₁₉ON. Calculated, %: N 5.28.

3-Acetyl-9-isoamylcarbazole. To a solution of 237 g of 9-isoamylcarbazole in 500 ml of benzene, 133.3 g of aluminum chloride was added.

and 89.1 ml of acetyl chloride. 188.3 g of colorless crystalline needles of 3-acetyl-9-isoamylcarbazole were obtained, with a melting point of 69–70°C (at 2–3 mm Hg). It forms orange picrate needles with a melting point of 97–98°C.

Analyses: found, %: N 5.00; 5.06. \( \mathrm{C}_{19}\mathrm{H}_{21}\mathrm{ON} \). Calculated, %: N 5.19.

Conclusions

1. Procedures have been developed for obtaining six 3-acetyl-9-alkylcarbazoles, four of which have not been described in the literature.

2. The influence of various factors on the synthesis reactions of 3-acetyl-9-alkylcarbazoles has been studied, and the optimal synthesis conditions have been established, under which 50–73% yields of 3-acetyl-9-alkylcarbazoles are achieved.

References

1. Monomers. Collection 2, Izdatinlit, 1953.
2. N. Buu-Hoï, R. Royer. Rec. trav. chim., 66, 533, 1947.
3. N. Buu-Hoï, R. Royer. J. Org. Chem., 15, 123, 1950.
4. N. Buu-Hoï, R. Royer. J. Org. Chem., 16, 1198, 1951.
5. V. P. Lopatinskii, E. E. Sirotkina, M. M. Anosova. Izvestiya Tomskogo politekhnicheskogo instituta, 111, 40, 1961.
6. V. P. Lopatinskii, N. E. Sirotkina, L. I. Masailova. Izvestiya Tomskogo politekhnicheskogo instituta, 112, 1962.
7. N. Gilman, S. Spatz. J. Org. Chem., 17, 860, 1952.
8. W. Reppe. Liebig’s Ann. L. Chem., 601, 128, 1956.
9. B. Levy. Monatsh. f. Chem., 33, 177, 1912.
10. N. Buu-Hoï, N. Hoan. J. Am. Ch. Soc., 73, 98, 1951.