RESEARCH IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES

56. Application of the molecular-orbital method to the study of the reactivity of 9-substituted carbazoles

V. P. Lopatinskii, V. A. Ponomareva

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

An attempt to evaluate the reactivity of carbazole by the simple molecular-orbital method was first made in 1947 [1]. A recent paper by Schmid [3] is also known, concerning the calculation of reactivity indices in carbazole by the simple LCAO MO method; however, unlike Coulson and Longuet-Higgins [1], these calculations do not take into account the correction for the inductive effect of the heteroatom, as a result of which the author obtains incorrect values of the π-electron densities, which do not agree with experimental data. Calculations of frontier electron densities [2] also do not agree with experimental data. Apart from information on carbazole, the literature contains no calculations of reactivity indices for its substituted derivatives. Meanwhile, they constitute no less interesting an object of investigation.

In the present communication, the simple LCAO MO method is extended to 9-substituted carbazoles.

In order to choose the most appropriate parameter $\alpha_N$, carbazole was calculated at the following values of the Coulomb integral: $1\beta$; $1.5\beta$; $1.6\beta$; $1.8\beta$ and $2\beta$, without taking into account the inductive effect and with it taken into account; in the latter case the Coulomb integrals on the carbons adjacent to nitrogen were taken as from 0.1 to 0.2 of the value of the parameter on nitrogen.

Without taking the inductive effect into account, at any values of the parameter $\alpha_N$ on nitrogen, the following series of activities of the carbon atoms in the benzene rings of carbazole was obtained: $1 > 3 > 2 > 4$ (in electrophilic substitution reactions). Taking the induction parameter into account (with the exception of the case $\alpha_N = 1$, $\alpha_C = 0.1$, when the activities of positions 1 and 3 coincide), the activity series obtained is $(3 > 1 > 2 > 4)$, which agrees with experimental data. For all calculated compounds, the parameter chosen for carbazole nitrogen was $\alpha_N = 1.6$, and the parameter for the carbons adjacent to nitrogen was $\alpha = 0.16$. All the remaining parameters were taken from the literature [4]. In calculating methyl-substituted carbazole, the conjugation model for the methyl group [4] was used.

The results of the calculations are given in Table 1. The numbering of the carbon and nitrogen atoms in the molecules of compounds of the carbazole series is the generally accepted one (the numeral 14 denotes the first atom of the substituent in the 9-position). The table omits the values of the reactivity indices for positions 5, 6, 7, and 8, since they are identical to the corres—

Table 1

Reactivity indices of carbazole and its 9-alkyl-substituted derivatives

Continuation of the table

…corresponding indices at positions 4, 3, 2, and 1, which confirms the experimentally observed symmetry of the benzene rings in molecules of carbazole and its 9-substituted derivatives. In substances 3–10, the bond orders in the benzene rings of the 9-substituted molecules are numerically the same as the orders of the corresponding bonds in the carbazole molecule and therefore are not included in the table. The secular determinant was solved on an M-20 computer.

The data in the table show that electron-acceptor groups in the 9-position noticeably change the character of the distribution of $\pi$-electron density in the benzene rings of 9-substituted derivatives. The electronic structure of these compounds differs less from the electronic structure of benzene than does the structure of carbazole, which is especially pronounced in the case of a strong acceptor, the nitro group. It is characteristic that all acceptor groups attached to nitrogen have almost no effect on the $\pi$-electron densities at the 2, 4, and 7 carbon atoms. At the same time, the electron density at the 1 and 3 carbon atoms is substantially decreased. The order of the 9—10 (N—C) bond in the presence of acceptors in the 9-position sometimes decreases very significantly (acetyl, benzoyl, carboxyl), whereas the decrease in the order of the diphenyl bond (11—12) is weaker. Thus, the p-electrons of nitrogen play the principal role in transmitting electronic effects in carbazole and its 9-substituted derivatives. Taking these facts into account, one may conclude that when electron-acceptor substituents are introduced at nitrogen, the overall reactivity of the benzene rings in electrophilic substitution reactions decreases, especially at positions 3, 6, 1, and 8.

These regularities agree with known experimental facts. The increased $\pi$-electron densities at the 3, 6, 1, and 8 positions of the carbazole molecule promote the preferential (and in a number of cases the exclusive) direction of electrophilic reagents to these positions (for example, in halogenation, nitration, sulfonation, and acylation) [5]. Good agreement with the results of calculations of $\pi$-electron charges in carbazole is given by the determination of the relative rates of nitration of carbazole in acetic anhydride, which for positions 3, 1, and 2 are expressed by the values 77600, 32100, and 1100, respectively [6]. The same order of substitution is retained in the chlorination of carbazole, for example, in acetic acid, where among the chlorination products predominantly 3-chlorocarbazole was found (91%) and a small amount of 1-chlorocarbazole (9%), while no 2- or 4-substituted derivatives were detected [7].

A satisfactory correlation between calculated and experimental data is also observed when comparing the reactivity of carbazole and its 9-substituted derivatives.

The conclusion given above concerning the decrease in the activity of the benzene rings in electrophilic substitution reactions upon replacement of the hydrogen at nitrogen by electron-acceptor groups has a number of experimental confirmations. For example, iodination of carbazole proceeds more readily than that of 9-acetyl- and 9-benzoylcarbazoles [8], and bromination also proceeds more readily [9].

The opposite picture is observed when electron-donor substituents are introduced at nitrogen.

In contrast to acceptor groups, donor groups such as methyl and amino groups increase the $\pi$-electron charges on the carbon atoms in the benzene rings, especially at the 3, 6, 1, and 8 positions. This circumstance is also confirmed by the greater activity, for example, of 9-alkylcarbazoles in comparison with carbazole in electrophilic substitution reactions [5, 7].

Conclusions

1. The molecular orbital method was used to calculate the reactivity indices of carbazole and its 9-substituted derivatives.

2. A correlation was carried out between the calculated data and the results of the experimental study of the reactivity of carbazole and its 9-substituted derivatives, and their satisfactory agreement was shown.

References

1. H. Longet-Higgins, C. Coulson, Trans. F. S., 43, 87, 1947.
2. K. Fukui et al., J. Chem. Phys., 22, 1433, 1954.
3. R. Schmid, Helv. chim. acta., 45, 1982, 1962.
4. E. Streitwieser. Molecular Orbital Theory for Organic Chemists. Mir, 1965.
5. Heterocyclic Compounds. Izdatinlit, 1954.
6. M. Dewar, D. Urch. J. Ch. Soc., 3079, 1958.
7. P. de la Mare, O. Dusouqui E. Johnson. J. Chem. Soc. (B), 521, 1966.
8. S. Tucker. J. Chem. Soc., 546, 1926.
9. G. Mazzara, Zeonardi, Gazz chim. ital., 22, II, 573, 1892.