RESEARCH IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES

VI. SYNTHESIS OF 9-(β-HYDROXYETHYL)CARBAZOLE

V. P. Lopatinskii, E. E. Sirotkina, I. P. Zherebtsov

(Presented by Prof. L. P. Kulev, Doctor of Chemical Sciences)

Polymers and copolymers of 9-vinylcarbazole are used as heat-resistant high-frequency dielectrics and are obtained from 9-vinylcarbazole by conventional methods [1—6]. At present, 9-vinylcarbazole is produced by only one method, proposed by V. Reppe, which consists in the action of acetylene on carbazole under pressure, at high temperature and in the presence of alkaline catalysts [1—4, 7, 8]. Despite the considerable simplifications of this method introduced in recent years, which have made it possible to achieve good yields of 9-vinylcarbazole [9—14], the process is carried out under highly explosion-hazardous conditions, is sensitive to moisture, and is performed periodically.

Other methods, for example the vinylation of potassium carbazole with vinyl chloride [15, 8] and synthesis via 9-(β-chloroethyl)carbazole [16, 17], lead to low yields of 9-vinylcarbazole.

One of the methods for obtaining 9-vinylcarbazole is its synthesis via 9-(β-hydroxyethyl)carbazole according to the scheme

\[ \text{carbazole} \;\longrightarrow\; 9\text{-(}\beta\text{-hydroxyethyl)carbazole} \;\longrightarrow\; 9\text{-vinylcarbazole} \]

\[ \mathrm{NH} \;\longrightarrow\; \mathrm{NCH_2CH_2OH} \;\longrightarrow\; \mathrm{NCH{=}CH_2} \]

The most important stage of this process is the preparation of 9-(β-hydroxyethyl)carbazole, which has been accomplished in two ways: by the reaction of sodium or potassium carbazole with ethylene oxide in xylene for 5 hours at 50°C, with a yield of 92% of theory [18, 1, 19], and by the reaction of sodium carbazole with ethylene chlorohydrin on boiling in acetone [20] or methyl ethyl ketone [21] for 30 hours, with a yield of 40% of theory. 9-Vinylcarbazole was obtained from 9-(β-hydroxyethyl)carbazole by heating with NaOH in vacuo [22] and by dehydration with KOH [18, 19, 1], as well as by pyrolysis of 9-(β-acetoxyethyl)carbazole at 575—600°C in the absence of catalysts in a nitrogen atmosphere [23, 24], or at 540° in the presence of a copper–boron–aluminum catalyst and at a residual pressure of 3—20 mm, with a yield of 23% [21, 1, 19].

In studying the synthesis of 9-(β-hydroxyethyl)carbazole, we found that quantitative conversion of carbazole can be achieved if its interaction with ethylene oxide is carried out under conditions in which the intermediate formation of potassium or sodium carbazole is observed, i.e., in alkaline acetone or methyl ethyl ketone (Table 1). A method close to this was used for the synthesis of 9-(β-hydroxyethyl)carbazole from carbazole and ethylene chlorohydrin [20, 21], but it led to a low yield of the reaction product.

In the present work, we investigated the influence on the yield of 9-(β-hydroxyethyl)carbazole and on the rate of the process of the following factors: temperature, catalysts, solvents, and the ratio of reagents. Acetone, methyl ethyl ketone, benzene, dioxane, methyl and ethyl alcohols were tested as solvents. It was found that in aliphatic alcohols and dioxane hydroxyethylation does not occur; in benzene the yield of the reaction product is insignificant (2% of theory), while the best solvents are aliphatic ketones—acetone and methyl ethyl ketone. A study of the influence of temperature showed that quantitative conversion of carbazole to 9-(β-hydroxyethyl)carbazole is achieved at 45–50°C in 50 minutes. Lowering the temperature leads to an increase in the duration of the process and to a decrease in the yield of the reaction product; when the temperature is increased above 60°C, strong resinification is observed. Experiments showed that hydroxyethylation of carbazole can be successfully carried out using powdered caustic potash as catalyst; in the presence of caustic soda the reaction proceeds less vigorously, lithium hydroxide exhibits weak activity, and calcium hydroxide is completely inactive. In most experiments the catalyst was introduced in an equimolar ratio to carbazole. Further experiments showed that the amount of catalyst can be reduced to 0.25 mole per mole of carbazole, but in this case a high yield of 9-(β-hydroxyethyl)carbazole (85–90% of theory) can be obtained (at 50°C) only in 3.5 hours.

Table 1

As the investigation showed, the considerable excess of ethylene oxide that was introduced in the first experiments is not necessary. In opti-

under optimal conditions, quantitative conversion of carbazole into 9-(β-oxyethyl)carbazole can be achieved at a molar ratio of ethylene oxide to carbazole of 1.1 : 1; in this case the duration of the reaction is not increased.

The reaction product was identified as 9-(β-oxyethyl)carbazole by the melting point of the recrystallized substance, by the results of analyses for nitrogen content, by the content of hydroxyl groups according to Verley, and by the acetyl derivative known previously. Finally, 9-vinylcarbazole was obtained from 9-(β-oxyethyl)carbazole by dehydration with caustic potassium.

Experimental Part

Starting reagents. Carbazole was isolated from crude anthracene of the Kemerovo Coke-Chemical Plant by the formaldehyde method [25] and had m.p. 242–244°C. Ethylene oxide had a purity of 99%. The benzene used was dry, cryoscopic grade. The other reagents had the following grade: acetone—“ch.d.a.”; methyl ethyl ketone, caustic potassium, caustic sodium, dioxane, methanol, xylene—“pure” grades; petroleum ether—purified.

9-(β-Oxyethyl)carbazole. Into a round-bottom flask of heat-resistant glass with a capacity of 100 ml were introduced 33.4 g of carbazole, 11.1 g of powdered caustic potassium, 66.8 ml of methyl ethyl ketone, and 9.68 ml of ethylene oxide. The solvent and ethylene oxide were preliminarily cooled to 0°C. The closed flask was placed in a thermostat, where it was kept for 50 min at 50°C with periodic shaking of the reaction mass. After completion of the reaction (at this point all the carbazole passes into solution), the flask was cooled on ice, and the contents were poured into 200 ml of ice water. The precipitate that separated was filtered off, washed with water, and dried to constant weight in a vacuum drying cabinet. The crude product was purified by vacuum distillation. At 220°C and 4–5 mm Hg, 40 g of substance distilled over (yield 95% of theory), with m.p. 80–81°C. Recrystallization from a 1 : 1 mixture of benzene and petroleum ether gave a substance with m.p. 82–83°C (according to the literature, 9-(β-oxyethyl)carbazole has m.p. 81–82°C [18], 83–83.5°C [21]). 9-(β-Oxyethyl)carbazole crystallizes in the form of long white needles, is very readily soluble in aromatic hydrocarbons, acetone, methyl ethyl ketone, methyl and ethyl alcohols, dioxane, diethyl ether, and nitrobenzene; it is less soluble in hexane, heptane, octane, isooctane, and light gasoline. Results of analyses: calculated, %: N 6.63; found, %: N 6.33; 6.3; 6.35. Calculated, %: OH 8.05; found (by Verley), %: OH 8.1; 8.07; 7.93. On reaction with acetyl chloride, 9-(β-acetoxyethyl)carbazole was obtained, m.p. 75.5°C (from methanol) (according to the literature, 9-(β-acetoxyethyl)carbazole melts at 74–74.5°C [21]).

9-Vinylcarbazole. Obtained by dehydration of 5 g of 9-(β-oxyethyl)carbazole with 0.5 g of caustic potassium at 250–260°C in a yield of 75% of theory. The substance has m.p. 66°C and gives a picrate—red needles with m.p. 184°C. Results of analyses: calculated, % N: 7.25; found, %: 7.2; 7.2. Thermal polymerization of 9-vinylcarbazole in a block with 0.5% benzoyl peroxide at 100°C for 5 hours gives colorless poly-9-vinylcarbazole.

Conclusions

1. The influence of temperature, reagent ratios, catalysts, and solvents on the process of the interaction of carbazole with ethylene oxide has been investigated.

1. It has been shown that, under optimal conditions, quantitative conversion of carbazole to 9-(β-oxyethyl)carbazole can be achieved in 50 minutes instead of 5 hours by the previously known method.

2. The conversion of 9-(β-oxyethyl)carbazole into 9-vinylcarbazole has been carried out in high yield.

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