Research in the Chemistry of Carbazole Derivatives

XXVI. Synthesis of 3-Vinyl Derivatives of 6-Chloro- and 6-Nitro-9-ethylcarbazoles

V. P. Lopatinskii, E. E. Sirotkina, S. D. Pukhalskaya

(Presented by the scientific seminar of the Faculty of Chemical Technology)

Our previous studies established that, in the reduction of 3-acetyl derivatives of 9-alkylcarbazoles with aluminum isopropoxide in isopropyl alcohol, the final products are the corresponding isopropyl ethers, instead of the expected carbinols [1]. In order to avoid ether formation, reduction with aluminum isopropoxide was carried out in aromatic hydrocarbons; however, in this case as well the final products were not carbinols, but the corresponding 3-vinyl derivatives of 9-alkylcarbazoles [2]. It is known that when electrophilic substituents are introduced into the aromatic nucleus, the mobility of the atoms at the α-carbon of an aliphatic radical is considerably reduced [3]. It was therefore of interest to examine the behavior of 6-chloro- and 6-nitro-3-acetyl-9-alkylcarbazoles upon reduction with aluminum isopropoxide both in isopropyl alcohol and in aromatic hydrocarbons. For this purpose we studied the reduction of 3-acetyl-6-chloro- and 3-acetyl-6-nitro-9-ethylcarbazoles. It turned out that these acetyl derivatives are not reduced at all by aluminum isopropoxide in isopropyl alcohol, benzene, or toluene, while in xylene the reduction proceeds at approximately the same rate as for 3-acetyl-9-alkylcarbazoles.

3-Acetyl-6-chloro-9-ethylcarbazole was synthesized by the method we developed [4]. By this method, 3-acetyl-6-nitro-9-ethylcarbazole was obtained in low yield; therefore, it was also synthesized by nitration of 3-acetyl-9-ethylcarbazole with a nitrating mixture consisting of nitric and acetic acids.

Experimental Part

3-Acetyl-6-chloro-9-ethylcarbazole. A charge of 23 g (0.1 mole) of 3-chloro-9-ethylcarbazole (m.p. 71°C) was dissolved in 55 ml of benzene, and 13.3 g (0.1 mole) of anhydrous aluminum chloride was added to the solution with thorough stirring. To the reaction mixture, cooled to 10°C, 8.9 ml (0.125 mole) of acetyl chloride was gradually added with stirring. At a temperature of 10–15°C the mixture was maintained for 3–3.5 hours with stirring. After completion of the reaction, ice was added to the reaction mass. The precipitated 3-acetyl-6-chloro-

9-Ethylcarbazole was filtered off, washed with water to a neutral reaction, and dried. The crude product (weight 25 g, i.e., 67.5% of theoretical), recrystallized from ethyl alcohol, was obtained in the form of silky colorless needles with m.p. 139–140°C. According to literature data, m.p. 138°C [5,6].

Analytical results. Found, %: N 5.4; 5.25; C₁₆H₁₄ON; Calculated, %: N 5.15.

3-Acetyl-6-nitro-9-ethylcarbazole

a) Prepared by the method described above from 6.3 g (0.025 mole) of 3-nitro-9-ethylcarbazole, 3.3 g (0.025 mole) of anhydrous aluminum chloride, and 2.2 ml of acetyl chloride in 15 ml of benzene over 4 hours, with a yield of 2 g (27.3% of theoretical). M.p. 232°C after two recrystallizations from xylene. Analytical results. Found, %: C 67.5; 67.6. H 4.77; 4.75. N 9.55; 9.50. C₁₆H₁₄O₃N₂; Calculated, %: C 68.08; H 4.96; N 9.93. It is not described in the literature.

b) Prepared by nitration of 3-acetyl-9-ethylcarbazole. A weighed portion of 23.7 g (0.1 mole) of 3-acetyl-9-ethylcarbazole was dissolved in 160 ml of glacial acetic acid, cooled to 10°C, and nitrated with a nitrating mixture consisting of 12.5 g of nitric acid (sp. gr. 1.4) and 125 g of glacial acetic acid. The nitrating mixture was added slowly over the course of an hour, after which stirring was continued for another 2 hours at 10°C. At the end of the reaction, the precipitated solid was filtered off, washed with water until the wash waters were neutral, and dried. The reaction product was purified by recrystallization from xylene. Pure 3-acetyl-6-nitro-9-ethylcarbazole—orange-yellow needles with m.p. 232°C, yield 23 g (81.5% of theoretical). The product is identical to the 3-acetyl-6-nitro-9-ethylcarbazole obtained by method a.

3-Vinyl-6-chloro-9-ethylcarbazole

Into a 200-ml flask were charged 5.43 g (0.02 mole) of 3-acetyl-6-chloro-9-ethylcarbazole, 4.08 g (0.02 mole) of aluminum isopropylate, and 35 ml of p-xylene. The flask was connected to a straight condenser and heated in an oil bath. Acetone was distilled off slowly at 130–135°C; toward the end of the reaction the temperature was raised to 140–150°C and the residue of acetone was distilled off in vacuum (residual pressure 200 mm Hg). When the test for acetone was negative, which required 2–2.5 hours, the reaction was terminated, and the contents of the flask were cooled and slowly treated with a cold 20% aqueous solution of caustic soda (200 ml). The xylene layer was separated and dried with solid caustic potash. Under slight vacuum, first xylene was distilled off, and then the reaction product. At 193–195°C (4–5 mm Hg), 3.8 g distilled over (69.5% of theoretical).

Analysis of the oil for unsaturation by Martin’s method [7] showed an unsaturated-content value of 97.09%. On standing in the cold, the oil solidified. Recrystallization from methanol gave colorless needles with m.p. 70–71°C.

Analytical results. Found, %: C 75.45; 75.24. H 5.63; 5.66. N 5.52; 5.5. C₁₆H₁₄ClN.

Calculated, %: C 75.08; H 5.48. N 5.47. Unsaturation by Martin’s method—99.2; 99.5%.

3-Vinyl-6-nitro-9-ethylcarbazole

Prepared by reduction of 5.64 g (0.02 mole) of 3-acetyl-6-nitro-9-ethylcarbazole with 4.08 g of aluminum isopropylate in 75 ml of p-xylene. The reaction was carried out as described above. The reduction was completed in 60—

80 min. After decomposition of the reduction products with a saturated solution of ammonium chloride or caustic soda, the xylene layer that separated was removed from the aqueous layer, dried with calcium chloride, filtered, and left to crystallize in the cold. There crystallized 3 g (56.3% of theory) of a yellow crystalline substance with m.p. 138–145°C. After two recrystallizations successively from benzene and ethanol, bright-yellow needles with m.p. 148.5–149.5°C were obtained. Results of analyses. Found, %: C 71.77; 71.79. H 5.4; 5.0. N 10.3; 10.1. $C_{16}H_{14}O_{2}N$. Calculated, %: C 72.18; H 5.26; N 10.52. Content of impurities by Martin—98.9; 99.0%.

Conclusion

For the first time, 3-vinyl derivatives of 6-chloro- and 6-nitro-9-ethylcarbazoles have been obtained by reduction of the corresponding 3-acetyl derivatives with aluminum isopropylate in xylene.

LITERATURE

1. V. P. Lopatinskii, E. E. Sirotkina, M. M. Anosova. Synthesis and certain transformations of acetyl derivatives of 9-alkylcarbazoles, Materials of the X Scientific Conference of the Kirghiz State University, Frunze, 64, 1962.

2. V. P. Lopatinskii, E. E. Sirotkina. XXV, Reduction of 3-acetyl-9-alkylcarbazoles with aluminum isopropylate and synthesis of 3-vinyl-alkylcarbazoles. Izvestiya TPI, 136, 1964 (in press).

3. S. N. Ushakov, P. A. Matuzov. ZhOKh, 14, issue 1–2, 1944.

4. V. P. Lopatinskii, E. E. Sirotkina. XII, Synthesis of certain 3-acetyl-9-alkylcarbazoles, Izvestiya TPI, 126, 1964 (in press).

5. N Buu-Hoi, Rec. trav. chim., 66, 533, 1947; C. A., 5014, 1948.

6. N. Buu-Hoi, J. Org. Chem., 15, 123, 1950.

7. R. Martin, Anal. Chem., 21, 921, 1949.