A STUDY IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES

63. SYNTHESIS OF 3-ACETYL-6,9-DIETHYLCARBAZOLE

E. E. Sirotkina, V. P. Lopatinskii, R. F. Chazova

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

3-Acetyl derivatives of carbazole are of interest as intermediates for the preparation of vinyl monomers, physiologically active substances, and a number of other valuable products. In connection with this, a method was developed for the synthesis of 3-acetyl-6,9-diethylcarbazole.

The starting 6,9-diethylcarbazole has been described in the literature and was obtained by reduction of 6-acetyl-9-ethylcarbazole by the Clemmensen method; however, the duration of the reaction in this case was 7 days [1]. Experiments on the reduction of 6-acetyl-9-ethylcarbazole with metallic sodium in alcohol led to low yields (not more than 5%) of 6,9-diethylcarbazole. In the present work, for the reduction of 6-acetyl-9-ethylcarbazole, the Kizhner–Wolff method was used, consisting in the reductive decomposition of hydrazones and semicarbazones in the presence of catalysts (caustic potassium, aluminum alcoholate in a mixture with platinum black) upon heating. The reduction of 6-acetyl-9-ethylcarbazole was carried out both with aqueous solutions of hydrazine and with hydrazine sulfate upon heating in a diethylene glycol solution in the presence of caustic potassium. The results of these studies are presented in Tables 1, 2, and 3.

Table 1

Reduction of 6-acetyl-9-ethylcarbazole with a 90% aqueous solution of hydrazine hydrate

From the data in Tables 1 and 2 it is evident that aqueous solutions of hydrazine can be successfully used for the reduction; however, in the case of dilute 58% solutions, high yields of 6,9-diethylcarbazole are achieved with a large excess of hydrazine and with a longer reaction time than in reduction with a 90% solution. The data in Table 3 show that reduction of 6-acetyl-9-ethylcarbazole with hydrazine sulfate leads to low yields of 6,9-diethylcarbazole; in this case, small amounts of hydrazine can be isolated, while the reaction products mainly become resinified. Attempts to obtain 6,9-diethylcarba-

Table 2

Reduction of 6-acetyl-9-ethylcarbazole with a 58% aqueous solution of hydrazine hydrate

Table 3

Reduction of 6-acetyl-9-ethylcarbazole with hydrazine sulfate

decomposition of the hydrazine of 6-acetyl-9-ethylcarbazole on heating in the presence of caustic potash did not lead to positive results.

Acetylation of 6,9-diethylcarbazole with acetyl chloride in the presence of aluminum chloride in benzene solution gave 3-acetyl-6,9-diethylcarbazole in 60% yield by the previously developed method [2].

Experimental Part

6,9-Diethylcarbazole. A distillation flask containing 17.5 g (0.074 mol) of 3-acetyl-9-ethylcarbazole (m.p. 114°C), 50 g of 58% hydrazine, 150 ml of diethylene glycol, and 22.8 g (0.4 mol) of caustic potash is placed in an oil bath, a straight condenser is attached, and the mixture is heated to 140–150°C; water distills off. The flask is kept at this temperature for one hour, after which the temperature is raised to 165°C and maintained until the yellow coloration in the flask disappears. After completion of the reaction, the contents of the flask are cooled, water is added, and the reaction product is extracted with benzene. The benzene extract is washed successively with an aqueous solution of hydrochloric acid and with water, then dried over calcium chloride. After removal of the benzene, the residue is distilled in vacuo. At 205–210°C and 3–5 mm Hg, 15.7 g (95%) of an oil distilled over, which solidified on standing. Recrystallization from ethyl alcohol gave 12.5 g of colorless crystalline needles with m.p. 44°C. The melting point of 6,9-diethylcarbazole, according to literature data, is 45°C [1]. This procedure was used in studying the influence of various factors on the reduction process.

3-Acetyl-6,9-diethylcarbazole. To a solution of 7 g (0.031 mol) of 6,9-diethylcarbazole in 16.6 ml of cryoscopic benzene, 4.12 g (0.031 mol) of aluminum chloride is added at room temperature; a solid complex precipitates. To the complex cooled to 5°C, with constant-

with stirring, 2.9 g (0.039 mole) of bromoacetyl (or chloroacetyl) is added dropwise.

The reaction mixture is kept at this temperature for 4.5 hours. After completion of the reaction, crushed ice is added to the reaction mixture, and the precipitated product is extracted with benzene. The benzene extract is washed with water, dried over calcium chloride, and, after distilling off the benzene from the residue by vacuum distillation at 170–175°C and 5 mm Hg, 4 g (57%) of an oil is obtained, which solidifies on standing. Recrystallization from methanol gives colorless crystalline needles with m.p. 69–70°C. Not described in the literature.

Found, %: N 5.6; 5.7. $\mathrm{C}_{18}\mathrm{H}_{19}\mathrm{NO}$.

Calculated, %: N 5.28.

On reaction with 2,4-dinitrophenylhydrazine, a hydrazone was obtained in the form of red crystals with m.p. 294–295°C.

Conclusions

1. A method has been developed for the synthesis of 6,9-diethylcarbazole by reduction of 6-acetyl-9-ethylcarbazole with aqueous hydrazine solutions in the presence of caustic potash.

2. 3-Acetyl-6,9-diethylcarbazole has been synthesized for the first time.

LITERATURE

1. N. Bull-Hoi, R. Royer, J. Org. Chem., 15, 123, 1950.
2. V. P. Lopatinskii, E. E. Sirotkina. Izv. TPI, 126, 62, 1964.