RESEARCH IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES

XXVIII. Reduction of Certain 3-Acetyl-9-alkylcarbazoles with Aluminum Isopropoxide

V. P. Lopatinskii, E. E. Sirotkina

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

In a previous communication [1] we showed that, upon reduction of 3-acetyl-9-methyl- and 3-acetyl-9-ethylcarbazoles with aluminum isopropoxide in isopropyl alcohol, the isopropyl ethers of 3-(α-hydroxyethyl)-9-alkylcarbazoles are formed. When isopropyl alcohol is replaced by aromatic hydrocarbons, the final products of the reduction are the corresponding 3-vinyl-9-alkylcarbazoles. In the present work the series of starting 3-acetyl derivatives of 9-alkylcarbazoles was expanded, and it was shown that increasing the alkyl radical in the 9-position of the 3-acetylcarbazole molecule has little effect on the ability of the acetyl derivatives to undergo reduction with aluminum isopropoxide both in isopropyl alcohol and in aromatic hydrocarbons. The final products of reduction in isopropyl alcohol are the isopropyl ethers of the expected carbinols (yield 50–70% of theoretical), and in aromatic hydrocarbons they are the 3-vinyl derivatives of 9-alkylcarbazoles, isolated in yields of 46–70% of theoretical. In benzene and toluene the formation of vinyl derivatives proceeds at a low rate and with low yields. The starting 9-alkylcarbazoles and 3-acetyl-9-alkylcarbazoles were obtained by methods developed by us earlier (2–4).

Experimental Part

Reduction with Aluminum Isopropoxide in Isopropyl Alcohol

The reduction is carried out in a round-bottom flask into which aluminum isopropoxide, ketone, and isopropyl alcohol are placed. The flask is connected to a dephlegmator, the upper part of which is connected to a straight condenser. The flask is heated on a water bath so that distillation proceeds at a rate of 5–7 drops per minute. When the test for acetone is negative, the reduction is stopped. The contents of the flask are cooled and hydrolyzed with a 15–20% aqueous alkali solution; the reaction product, which separates as an oil, is extracted with benzene or ether, the organic layer is separated, dried over calcium chloride, and, after removal of the solvent, the residue is distilled in vacuum. The reduction is completed in 7–8 hours with an excess of isopropoxide

aluminum 200% and for 2–2.5 hours with an excess of 800%. Isopropyl alcohol must be taken in an amount of 25–30 moles per mole of the acetylene derivative. The ethers are thermally stable and do not change upon prolonged heating to 250°C. When heated above 350°C, the ethers decompose with formation of isopropyl alcohol and a polymer of 3-vinyl-9-alkylcarbazole. When these ethers are heated to 90–140°C in the presence of activated alumina or potassium bisulfate, cleavage of the ethers occurs with formation of the corresponding monomer (or polymer) and isopropyl alcohol, identified by its boiling point and refractive index. Table 1 gives the principal data characterizing the ethers obtained by us.

Table 1

Reduction with aluminum isopropylate in aromatic hydrocarbons

The reduction was carried out in a round-bottom flask equipped with a straight condenser. The flask, containing the ketone, aluminum isopropylate, and solvent, was heated in an oil bath to 130°C; at the same time the bulk of the acetone was distilled off. Toward the end of the reaction the bath temperature was raised to 140–150°C. When the test for acetone in the distillate was negative, the contents of the flask were cooled and slowly treated with a cold 15–20% aqueous solution of caustic soda. The separated xylene layer was separated from the aqueous layer and dried with solid caustic potash (or caustic soda). It is best to carry out the reaction in p-xylene, since the bulk of it was readily separated by freezing out and filtration. The filtrate, containing the monomer and a small amount of xylene, was distilled in vacuum. At first, under a slight vacuum, xylene was distilled off and then the monomer; in the residue there remained a transparent yellow thermopolymer of 3-vinyl-9-alkylcarbazole. The monomers were analyzed for unsaturation by the Martin method [5]. The characteristics of the monomers obtained by us are given in Table 2. The best conditions for direct conversion of 3-acetyl-9-alkylcarbazoles into the corresponding vinyl derivatives are a temperature of 140–150°C, an excess of aluminum isopropylate of 200–250%, and an amount of solvent of 10–12 moles per mole

acetyl derivative. Under these conditions the reduction is completed in 1.5–2.5 hours, with yields of vinyl derivatives of 46–70%.

Table 2

3-Vinyl-9-alkylcarbazoles

Conclusions

1. It has been shown that reduction of 3-acetyl-9-alkylcarbazoles with aluminum isopropylate in isopropyl alcohol leads to the formation of isopropyl ethers of 3-(α-hydroxyethyl)-9-alkylcarbazoles in yields of 50–70% of theory.

2. It has been established that, when isopropyl alcohol is replaced by xylene, reduction of 3-acetyl-9-alkylcarbazoles proceeds with yields of 46–70% of theory.

3. Six isopropyl ethers of 3-(α-hydroxyethyl)-9-alkylcarbazoles and five 3-vinyl-9-alkylcarbazoles have been obtained for the first time.

References

1. V. P. Lopatinskii, E. E. Sirotkina, Izvestiya Tomskogo politekhnicheskogo instituta, 120, 1964.

2. V. P. Lopatinskii, E. E. Sirotkina, M. M. Anosova, Izvestiya Tomskogo politekhnicheskogo instituta, 111, 40, 1961.

3. V. P. Lopatinskii, E. E. Sirotkina, L. I. Misailova, Izvestiya Tomskogo politekhnicheskogo instituta, 112, 39, 1963.

4. V. P. Lopatinskii, E. E. Sirotkina, Author’s Certificate No. 158884 (1963); Bulletin of Inventions, No. 23, 1963.

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