RESEARCH IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES

74. Study of the Acetylation of 9-Methylcarbazole Using the Method of Thin-Layer Chromatography

V. P. LOPATINSKII, E. E. SIROTKINA, V. L. IVASENKO, V. T. NOVIKOV

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

Acylation of 9-methylcarbazole by the Friedel–Crafts method, with the aim of obtaining 3-acetyl-9-methylcarbazole using acetyl chloride in the presence of aluminum chloride, can be carried out by two procedures that differ in the order in which the components of the reaction system are charged:

a) 9-methylcarbazole and acetyl chloride are dissolved in benzene or another solvent, and the catalyst is added to the solution;

b) 9-methylcarbazole is dissolved in benzene, the catalyst is introduced into the solution, and the resulting sparingly soluble complex of 9-methylcarbazole with aluminum chloride is treated with acetyl chloride.

When the acylation process is carried out by procedure “a,” the preparative yield of 3-acetyl-9-methylcarbazole is 30–40% (1); when procedure “b” is used, the preparative yield of the desired product is more than 80% (2); in both cases the principal by-product is 3,6-diacetyl-9-methylcarbazole.

We have established that, when the acylation process is carried out in a homogeneous system (solvent: methylene chloride; initial concentration of 9-methylcarbazole, 2%), the order in which the components are charged does not affect the preparative yield of 3-acetyl-9-methylcarbazole. It was therefore decided to investigate the influence of the appearance of a solid phase in the reaction system, and of the duration of the process, on the formation of the by-product (3,6-diacetyl-9-methylcarbazole), depending on the order in which the components are charged.

Thin-layer chromatography was used as the analytical method.

The results of studies of the influence of the heterogeneity of the reaction system and of the duration of the acylation process, depending on the order in which the components of the reaction system are charged, are presented in Fig. 1.

As can be seen from the data presented in Fig. 1, the amount of 3,6-diacetyl-9-methylcarbazole increases with increasing duration of the acylation process; moreover, when the acylation process is carried out according to procedure “a,” the rate of formation of 3,6-diacetyl-9-methylcarbazole is considerably greater. This fact can be explained by the circumstance that, when the process is carried out according to procedure “a,” there is a significant ...

—amount of acylium ion that can attack both 9-methylcarbazole and the sparingly soluble complex of 3-acetyl-9-methylcarbazole with aluminum chloride (solubility of the complex 0.5% in benzene), since introduction of an acetyl group into one of the benzene rings of the 9-alkylcarbazole molecule does not alter the activity of the second ring (3–6). When the acylation of 9-methylcarbazole is carried out by procedure “b,” the amount of attacking acylium ion in the reaction system is strictly equivalent to the 9-methylcarbazole present in solution, since 9-methylcarbazole enters the solution upon destruction of its complex with aluminum chloride under the action of the acetyl chloride entering the system.

Earlier, V. L. Ivasenko established, by means of IR spectroscopy, that the principal compounds present in the reaction system, in order of their complex-forming activity with aluminum chloride, may be arranged as follows: 3-acetyl-9-methylcarbazole > acetyl chloride > 9-methylcarbazole (7). As a result, the 3-acetyl-9-methylcarbazole formed is removed from the system in the form of a sparingly soluble complex with aluminum chloride.

Fig. 1. Dependence of the yield of acylation products on time, method, and amount of solvent

The appearance of a diacetyl derivative when acylation is carried out by method “b” can be explained by isomerization of the acyl group by an intermolecular mechanism, since it was found that, under the reaction conditions, 3-acetyl-9-methylcarbazole can undergo changes under the action of aluminum chloride, giving diacetyl derivatives.

Experimental Part

To construct the dependence of product yield on the duration of the acylation process, the reaction-interruption method was used. To carry out the acylation of 9-methylcarbazole, procedures known from the literature [8–9] were used. The ratio of reagents in the initial system (9-methylcarbazole : acetyl chloride : aluminum chloride : benzene) was 1 : 1.25 : 1.1 : 12; temperature, 10°C.

For identification of the products, the method of thin-layer chromatography was used. Separation was carried out on a fixed layer of activated silica gel, grade KSM, layer thickness 0.5 mm, with standard—

conditions (10–11). Several solvent systems were selected. The determined values are given in the table.

Table 1

\(R_f\) values for several carbazole compounds

REFERENCES

1. N. Buu-Hoi, R. Royer. J. Org. Chem., 15, 123, 1950.
2. V. P. Lopatinskii, E. E. Sirotkina. Izv. TPI, 126, 62, 1964.
3. Heterocyclic Compounds. IL, vol. 3, 1953.
4. N. Buu-Hoi. Rec. trav. chim., 66, 533, 1947.
5. Yu. S. Zalkind, M. E. Komarennko. ZhPKh, 12, 1134, 1939.
6. V. F. Borodkin. ZhPKh, 23, 759, 763, 1105, 1950; 21, 186, 1948.
7. V. L. Ivasenko. Dissertation work, TPI, 1968.
8. V. P. Lopatinskii, E. E. Sirotkina, M. M. Anosova. Izv. TPI, 111, 40, 1961.
9. S. Plant, K. Rogers, S. Williams. J. Chem. Soc., 741, 1935.
10. Stahl. Thin-Layer Chromatography. “Mir,” 1965.