Chemistry

V. N. SETKINA, A. G. GINZBURG, E. I. FEDIN,
Corresponding Member of the Academy of Sciences of the USSR D. N. KURSANOV

ON THE ISOTOPIC EXCHANGE OF HYDROGEN IN HEXASUBSTITUTED BENZENES

Hydrogen atoms of aliphatic C—H bonds in molecules of alkylbenzenes do not enter into the reaction of isotopic hydrogen exchange in acidic media even under very severe conditions (in anhydrous DF in the presence of BF₃) (¹, ²). In accordance with this, we found that both hexamethylbenzene (I) and hexaethylbenzene (II) practically do not enter into the reaction of isotopic hydrogen exchange with deuterosulfuric acid.

However, hexasubstituted benzene derivatives containing, as substituents, three cycloalkane rings fused with the benzene ring are capable of entering into the reaction of isotopic hydrogen exchange with acids. We found that the hydrocarbons tricyclotetramethylbenzene (III) and tricyclotrimethylbenzene (IV) enter into the reaction of isotopic exchange both with deuterosulfuric acid and with deuterium chloride (in the latter case in the presence of anhydrous FeCl₃). The results of experiments on isotopic exchange with D₂SO₄ in nitrobenzene medium are given in Table 1.

(III)  (IV)

Table 1

Having compared the proton resonance (p.r.) spectra of the initial hydrocarbons and of those isolated after exchange, we established that the exchange reaction proceeds in the β-position. In molecules III and IV there are two types of hydrogen atoms: α- and β-atoms with respect to the benzene ring. The p.r. spectrum of III consists of two peaks of equal intensity with shifts \(\tau_\alpha = 7.4\) and

Fig. 1. P.r. spectra of initial III (a), initial IV (b)

Fig. 2. P.r. spectra of III isolated after exchange (a) (deuterium content 30.4 at.%), and of IV (b) (deuterium content 21.0 at.%)

\(\tau_\beta = 8.2\) (Fig. 1a). The assignment of the peaks in the p.r. spectrum of III was made on the assumption that the inductive effect of the benzene ring should lead to different shielding of the α- and β-protons by the electrons of the C—H bonds. Compared with the α-protons, the β-protons are shielded to a lesser extent; therefore the β-proton signal is located in the stronger field, and the α-proton signal in the weaker field. An additional check of this assignment was also made from the p.r. spectrum of IV, where the signals from the α- and β-protons are unambiguously determined by their different integral intensities and by spin–spin interaction. At a frequency of 40 MHz the spin–spin splitting was poorly resolved, but the spectrum at 60 MHz (Fig. 1b) clearly demonstrated the presence of a triplet signal in the weak field (\(\tau_\alpha = 7.4\)) and a quintuplet signal of half the integral intensity in the strong field (\(\tau_\beta = 8.05\)), which completely confirms the assignment made earlier.* In the p.r. spectra of the hydrocarbons isolated after exchange, the signal of the β-protons decreases, whereas the signal of the α-protons remains unchanged (Fig. 2). Measurement of the ratio of inte-

* We express our gratitude to P. V. Petrovskii and V. I. Sheichenko for the p.r. spectra.

...of the integral intensities of the signals in the NMR spectra shows good agreement with the ratio calculated theoretically from the isotope-analysis data (Table 2).

Table 2

To determine the relative reactivity of III and IV, the kinetics of the isotopic exchange of hydrogen was studied. As is seen from the data in Table 1, equilibrium of the exchange reaction for III is reached at \(30.2^\circ\) in 48 h. For IV the exchange proceeds at a lower rate. The first-order rate constants of isotopic exchange, calculated from the equation, are given in Table 3.

Table 3

Kinetics of isotopic exchange of III (at the ratio hydrocarbon : \( \mathrm{D_2SO_4} : \mathrm{C_2H_5NO_2} = 1:7.4:93.1\) and temperature \(25^\circ\))

Table 4

Kinetics of isotopic exchange of IV (at the ratio hydrocarbon : \( \mathrm{D_2SO_4} : \mathrm{C_6H_5NO_2} = 1:7.4:78.8\) and temperature \(25^\circ\))

The data obtained make it possible to put forward an assumption concerning the mechanism of the reactions found. Apparently, the reaction begins with oxidation of the \(\alpha\)-carbon atom of the side ring by deuterosulfuric acid, with formation of carbonium ions (V and VI), which are in equilibrium with the corresponding olefins:

\[ \text{(V): carbonium-ion structural formula} \rightleftharpoons \text{corresponding olefin structural formula} \]

\[ \text{(VI): carbonium-ion structural formula} \rightleftharpoons \text{corresponding olefin structural formula} \]

Further isotopic exchange occurs through reversible addition and elimination of a proton (deuteron). The proton (deuteron) is directed to the \(\beta\)-position, since the carbonium center in the \(\alpha\)-position is stabilized more strongly through conjugation with the benzene ring than is the carbonium center in the \(\beta\)-position. Work to elucidate the mechanism of the reactions found will be continued by us.

Experimental Part

Tricyclotetramethylbenzene (dodecahydrotriphenylene, III) was obtained by the method of S. V. Svetozarskii (³) and purified by repeated recrystallization from benzene, sublimation, and zone melting. For the experiments a preparation with m.p. 232–233°, whose UV spectrum coincides with the literature data, was used.

Tricyclotromethylbenzene (trindane, IV) was obtained by the method of R. Mayer (⁴) and purified by recrystallization from methanol and by zone melting. For the experiments a gas-chromatographically pure preparation with m.p. 96–97° was used.

Hexamethylbenzene (I) was obtained by passing a mixture of phenol and methanol vapors, with a large excess of the latter (molar ratio 1 : 15), over alumina at 380–410° (⁵). For the experiments a gas-chromatographically pure preparation, purified by recrystallization from absolute ethanol and by zone melting, m.p. 164–165°, was used.

Hexaethylbenzene (II) was obtained by ethylation of benzene with ethyl chloride by the Friedel—Crafts method (⁶). The preparation was purified by recrystallization from absolute ethanol. The gas-chromatographically pure preparation had m.p. 129–130°.

Experimental procedure. A weighed portion of substance (0.5–0.9 g) is dissolved in 30–35 ml of nitrobenzene purified and distilled on a column; the required amount of 100% D₂SO₄ is added, the mixture is stirred, and it is placed in a thermostat. In the case of III and IV, the color of the nitrobenzene solution changes to dark red; in the case of I and II, no change in coloration is observed. To quench the reaction, the reaction mixture is poured into 100 ml of a 10% NaHCO₃ solution. The nitrobenzene layer is washed repeatedly with water and dried over MgSO₄. The nitrobenzene is distilled off under vacuum in a nitrogen atmosphere; the remaining hydrocarbon is sublimed or recrystallized. In experiments with IV, the residue after removal of nitrobenzene is chromatographed on an Al₂O₃ column, using n-hexane as eluent. With this treatment it is possible to isolate 50–70% of the initial charge taken into the reaction.

The NMR spectra were recorded on a YaMR-TsLA-5535 spectrometer; verification of spin–spin splitting in IV was carried out on an INM-C-60 spectrometer. CCl₄ was used as solvent, and tetramethylsilane (TMS) as internal standard.

Institute of Organoelement Compounds
Academy of Sciences of the USSR

Received
27 V 1964

CITED LITERATURE

¹ A. I. Shatenshtein, Isotopic Exchange and Substitution of Hydrogen in Organic Compounds, Moscow, 1960, p. 224.
² S. Olsson, L. Melander, Acta chem. scand., 8, No. 3, 523 (1954).
³ S. V. Svetozarskii, E. N. Zil’berman, T. A. Razuvayev, ZhOKh, 29, No. 5, 1454 (1959).
⁴ R. Mayer, Ber., 89, No. 6, 1443 (1956).
⁵ P. S. Landis, W. O. Haag, J. Org. Chem., 28, No. 2, 585 (1963).
⁶ E. Wertyporoch, Ann., 500, No. 2, 287 (1933).