RESEARCH IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES

XXV. REDUCTION OF CERTAIN 3-ACETYL-9-ALKYLCARBAZOLES WITH ALUMINUM ISOPROPOXIDE AND SYNTHESIS OF 3-VINYL-9-METHYL- AND 3-VINYL-9-ETHYLCARBAZOLES

V. P. Lopatinskii, E. E. Sirotkina

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

In the course of investigating the synthesis and properties of 3-acetyl-9-alkylcarbazoles, we attempted to reduce their carbonyl groups to alcohols for subsequent dehydration, which was to lead us to the corresponding 3-vinyl-9-alkylcarbazoles. For the reduction we chose the Meerwein–Ponndorf method [1], which, as a rule, gives a minimal amount of by-products.

Testing this method showed, however, that under the usual reduction conditions (aluminum isopropoxide in isopropyl alcohol) it is not carbinols that are formed, but their isopropyl ethers, in yields of 56–85% of theory. When these ethers were heated to 90–140°C in the presence of active alumina or potassium bisulfate in xylene, or without it, cleavage of the ethers occurred, and the solid residue was usually a polymer of the corresponding 3-vinyl-9-alkylcarbazole. Only in some experiments was it possible in this way to obtain a small amount of monomers. At the same time, an equimolecular amount of isopropyl alcohol was evolved, identified by its boiling point and refractive index. The ethers were unchanged upon prolonged heating with caustic potash to 250°C.

To avoid the formation of isopropyl ethers, the reduction of 3-acetyl-9-alkylcarbazoles with aluminum isopropoxide was carried out in other solvents, of which aromatic hydrocarbons proved the most suitable, especially xylene. The choice of xylene is associated with the need to maintain the temperature within 130–140°C, at which the reaction is completed in 2–2.5 hours with an excess of aluminum isopropoxide of 200–250%. In benzene and toluene the reaction proceeds much more slowly.

In these experiments it was found that the final products of the reduction are monomeric 3-vinyl-9-alkylcarbazoles, which partially polymerize under the reaction conditions. Thus, the reduction of 3-acetyl-9-alkylcarbazoles with aluminum isopropoxide in aromatic hydrocarbons is accompanied by dehydration of the carbinols, which is a rather rare case [1]. The best yields of monomers were obtained in experiments with an excess of aluminum isopropoxide of 200–250% and with the amount of xylene 12–15 moles per mole of 3-acetyl-9-alkylcarbazole. A further increase in the amount of aluminum isopropoxide has no substantial effect on the yield of monomers. Polymers-

The 3-vinyl-9-alkylcarbazoles formed during the reaction are transparent, yellowish, amorphous substances with softening temperatures of 150–190°C.

Below, in Table 1, are the principal data characterizing some of the products obtained by us.

Table 1

At present we have studied 3-vinyl-9-ethylcarbazole better than the others; it polymerizes very readily on heating, as well as in the presence of ordinary initiators of radical polymerization. In bulk polymerization in the presence of benzoyl peroxide or dinitrile of azodiisobutyric acid, it forms a colorless transparent polymer with a softening temperature of 200–220°C. Suspension polymerization in the presence of potassium persulfate gives a polymer with a softening temperature of 210–220°C. The electrophysical properties of these polymers are being studied.

3-Vinyl-9-methylcarbazole is also capable of readily polymerizing by the usual methods. More detailed results of investigations of the polymerization processes of both monomers will be presented in a subsequent communication.

Experimental Part

Starting reagents

3-Acetyl-9-methylcarbazole with m.p. 101–102°C and 3-acetyl-9-ethylcarbazole with m.p. 114–115°C were prepared by the procedure developed by us [2]. Isopropyl alcohol, grade “purified,” was freed from acetone. Aluminum isopropylate was prepared by the known procedure [1].

Isopropyl ether of 3-(α-hydroxyethyl)-9-methylcarbazole

A 300-ml round-bottom flask equipped with a dephlegmator is charged with 22.3 g (0.1 mole) of 3-acetyl-9-methylcarbazole, 20.4 g (0.1 mole) of aluminum isopropylate, and 150 ml of absolute isopropyl alcohol. The mixture is heated on a water bath. Acetone is slowly distilled off as it forms. The temperature of the vapors being distilled is maintained within 60–70°C. After acetone ceases to be detected in the distillate (after 6–7 hours), most of the isopropyl alcohol is distilled off in vacuo. The residue is cooled to room temperature and slowly treated with a cold 15% aqueous solution of NaOH (or NH₄Cl). The oily layer is separated, washed with water, the isopropyl alcohol is distilled off from it, and the residue is distilled in vacuo. At 180–181°C (2–3 mm Hg), a colorless oil distilled over (15 g, 56.18% of theory), which solidifies on standing. Recrystallization from methanol gave colorless

crystalline substance with m.p. 52°C. Analyses: found, %: C 81.00; 81.04. H 8.00; 7.96. N 5.42; 5.5. C₁₈H₂₁NO. Calculated, %: C 80.9; H 7.86; N 5.24.

By heating with KHSO₄ at 140°C, a liquid was obtained from the ether with b.p. 80–81°C and \(n_D^{20}\) 1.3773, identified as isopropyl alcohol, and a resinous solid residue, the analyses of which show that it is a polymer of 3-vinyl-9-methylcarbazole. Found, %: C 86.7; H 6.42; N 6.62. \((\mathrm{C}_{15}\mathrm{H}_{13}\mathrm{N})_n\). Calculated, %: C 86.87; H 6.27; N 6.76.

Isopropyl ether of 3-(α-hydroxyethyl)-9-ethylcarbazole. Obtained by the above-described procedure from 6 g of 3-acetyl-9-ethylcarbazole, 5 g of aluminum isopropylate in 60 ml of absolute isopropyl alcohol. There was isolated 6.0 g of a colorless crystalline substance (85.7% of theoretical), m.p. 88–89°C (from methanol) and b.p. 215–218°C (2–3 mm Hg). Analyses: found, %: C 81.02; H 8.28; N 4.86. C₁₉H₂₃NO. Calculated, %: C 81.1; H 8.18; N 4.97. Forms a picrate—red needles with m.p. 109–110°C.

3-Vinyl-9-methylcarbazole

Into a 300-ml flask are charged 22.3 g (0.1 mole) of 3-acetyl-9-methylcarbazole, 20.4 g (0.1 mole) of aluminum isopropylate, and 150 ml of dry xylene. The flask is connected to a straight condenser and the reaction mixture is heated on an oil bath. The process of distilling off acetone is carried out slowly at a temperature of 130°C; toward the end of the reaction the temperature is raised to 140°C and the remainder of the acetone is distilled off in vacuo. When the distillate gives a negative test for acetone, which requires 2–2.5 hours, the reaction is stopped, the contents of the flask are cooled, and slowly treated with cold 20% aqueous caustic soda solution (200 ml). The xylene layer is separated and dried with solid caustic potash. First the xylene and then the reaction product are distilled off in vacuo. At 192–194°C (1–5 mm Hg), 10.0 g (50.0% of theoretical) of a colorless oil distills, which solidifies on standing. After recrystallization from methanol, colorless needles are obtained with m.p. 70–71°C. Analyses: found, %: C 86.94; 86.58. H 6.54; 6.53. N 6.75; 6.79. C₁₅H₁₃N. Calculated, %: C 86.87; H 6.27; N 6.76.

3-Vinyl-9-ethylcarbazole

Obtained by the above-described procedure from 71.1 g (0.3 mole) of 3-acetyl-9-ethylcarbazole, 61.2 g (0.3 mole) of aluminum isopropylate in 300 ml of xylene in 2 hours. At 180–183°C (2–3 mm Hg), 34.8 g (52.8% of theoretical) of a colorless oil was distilled, which solidified on standing. After recrystallization from methanol, colorless needles are obtained with m.p. 66–67°C. Analyses: found, %: C 86.83; 87.01. H 7.1; 6.88. N 6.24; 6.21. C₁₆H₁₅N. Calculated, %: C 86.89; H 6.79; N 6.32.

Conclusions

1. The reduction of 3-acetyl-9-methyl- and 3-acetyl-9-ethylcarbazoles with aluminum isopropylate in isopropyl alcohol and xylene has been studied.

2. It has been shown that reduction in isopropyl alcohol gives the isopropyl ethers of 3-(α-hydroxyethyl)-9-methyl- and 3-(α-hydroxyethyl)-9-ethylcarbazoles.

3. A method has been developed for the one-stage conversion of 3-acetyl-9-methyl- and 3-acetyl-9-ethylcarbazoles into 3-vinyl-9-methyl- and 3-vinyl-9-ethylcarbazoles by reduction of the acetyl derivatives with aluminum isopropylate in xylene.

1. It has been established that 3-vinyl-9-methyl- and 3-vinyl-9-ethylcarbazole are readily polymerized by conventional methods.

REFERENCES

1. Organic Reactions, Collection 2, Izdatinlit, 1950.
2. V. P. Lopatinskii, E. E. Sirotkina. Bulletin of the Tomsk Polytechnic Institute, 126, 1964.