Chemistry

D. N. Kravtsov

PHENYLMERCURY DERIVATIVES OF SUBSTITUTED
2-PHENYLSULFONYLAMINOPYRIDINES
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(Presented by Academician A. N. Nesmeyanov on 29 X 1964)

It was found earlier that arylmercury derivatives of nitrosophenols are tautomeric in solution (¹). In light of this, it seemed of interest to investigate the structure in solution of \( \mathrm{C_6H_5Hg} \)-derivatives of tautomeric 2-arylsulfonylaminopyridines (²–⁵). Only one paper discusses possible structures for organometallic compounds of this type, but it gives no experimental evidence in favor of one structure or another (⁶).

Table 1

Phenylmercury derivatives were obtained by the action of \( \mathrm{C_6H_5HgOH} \) on arylsulfonylaminopyridines (Table 1). The \( \mathrm{C_6H_5Hg} \) group in them is bonded to one of the nitrogen atoms, which is confirmed by their cleavage into phenylmercury halide and the K derivative of arylsulfonylaminopyridine under the action of KI and KBr, as well as by the absence of the ability to form a K salt under the action of KOH. Consequently, a priori, either an aminopyridine or a pyridonimine structure is possible for them:

\[ \begin{array}{cc} \text{[pyridine structure: } N\bigl(SO_2C_6H_5\bigr)\bigl(HgC_6H_5\bigr)\text{]} & \text{[pyridonimine structure: } {=}NSO_2C_6H_5,\; NHgC_6H_5\text{]} \end{array} \]

Determination of the structure of the \( \mathrm{C_6H_5Hg} \)-derivatives in solution was carried out on the basis of absorption spectra, using as a criterion the characteristic maximum of the imino form in the region of 325 mµ (², ⁵). Preliminarily, using the example of the \( \mathrm{C_6H_5Hg} \)-derivative of N-phenylsulfonyl-\(\alpha\)-naphthylami-

It was found that replacement of hydrogen by a \( \mathrm{C_6H_5Hg} \) group does not change the shape of the spectral curve in the region \(260\text{–}340\ \mathrm{m\mu}\) and only slightly affects the position of the absorption maximum and its intensity. Thus, in dioxane, for the free compound and the \( \mathrm{C_6H_5Hg} \) derivative, respectively:
\(\lambda_{\max}=289\ \mathrm{m\mu},\ \varepsilon_{\max}=7400\) and \(\lambda_{\max}=292\ \mathrm{m\mu},\ \varepsilon_{\max}=8100\). It was also found that, when the structure is fixed, the shape of the spectral curve of the \( \mathrm{C_6H_5Hg} \) derivative does not depend on the nature of the solvent.

Fig. 1. Absorption spectra in acetonitrile: 1 — 1-methyl-2-phenylsulfonyliminopyridine. 2 — \(2\text{-}\mathrm{C_5H_4NH(R)SO_2C_6H_5}\), \(R=\mathrm{CH_3}\), 3 — \(R=\mathrm{Na}\), 4 — \(R=\mathrm{C_6H_5Hg}\)

Fig. 2. Absorption spectra in \( \mathrm{C_4H_8O_2} \): 1 — 1-methyl-2-phenylsulfonyliminopyridine. 2 — \(2\text{-}\mathrm{C_5H_4NN(R)SO_2C_6H_5}\), \(R=\mathrm{H}\). 3 — \(R=\mathrm{C_6H_5Hg}\); 4 — \(2\text{-}\mathrm{C_5H_4NN(R)SO_2C_6H_5}\), \(R=\mathrm{C_6H_5Hg}\), in \( \mathrm{CH_3CN}\); 5 — the same in \( \mathrm{HCON(CH_3)_2}\); 6 — the same in \( \mathrm{C_5H_5N}\)

Comparison of the spectrum of the \( \mathrm{C_6H_5Hg} \) derivative of 2-phenylsulfonylaminopyridine in such a strongly polar solvent as acetonitrile with the spectra of the isomeric methyl derivatives and the Na salt (Fig. 1) shows that in this case it cannot be assigned the structure of either of the limiting isomeric forms or an ionic structure. The presence in the spectrum of the \( \mathrm{C_6H_5Hg} \) derivative of absorption in the region of \(325\ \mathrm{m\mu}\), characteristic of the pyridonimine form, whose intensity is greatly reduced and does not depend on concentration or on addition of \( \mathrm{C_6H_5HgOH} \) to the solution (which rules out the possibility of hydrolysis), should be interpreted as the result of the presence in solution of two forms: the aminopyridine and pyridonimine forms. From Fig. 2 it is seen that \(\varepsilon\) of the maximum in the region of \(325\ \mathrm{m\mu}\) depends substantially on the nature of the solvent, and in weakly polar solvents of the dioxane type it disappears altogether. Obviously, as for the free compound, this indicates mutual interconversion of the two forms as a result of tautomeric equilibrium:

\[ \begin{aligned} &\text{[[structural formula: aminopyridine form with } \mathrm{SO_2C_6H_5} \text{ and } \mathrm{HgC_6H_5}\text{ substituents]]} \rightleftharpoons \text{[[structural formula: pyridonimine form with } \mathrm{=NSO_2C_6H_5} \text{ and } \mathrm{NHgC_6H_5}\text{]]} \end{aligned} \]

Thus, for example, in dioxane, in contrast to acetonitrile, the phenylmercury derivative exists practically entirely in the aminopyridine form. At the same time, its maximum is characterized by a bathochromic shift of \(20\ \mathrm{m\mu}\) compared with the maximum of the aminopyridine form of the free compound and by a 23% decrease in intensity. This effect is probably caused

with formation of a coordination bond of the \(C_6H_5Hg\)-group with the heterocyclic nitrogen atom:

\[ \begin{array}{c} \text{pyridine ring bearing } N\!\left(SO_2C_6H_5\right)\!\left(HgC_6H_5\right) \\ \text{and a dotted coordination bond from the ring } N \cdots Hg \end{array} \]

analogous to that existing in the \(C_6H_5Hg\)-derivative of 8-oxyquinoline (7). Examination of Stuart–Briegleb models shows that, upon rotation of the

\[ -\!N \begin{matrix} \diagup SO_2C_6H_5\\[-2pt] \diagdown HgC_6H_5 \end{matrix} \]

group about the \(C—N\) bond, overlap of the van der Waals radii of the mercury atom and the heterocyclic nitrogen is possible, whereby favorable conditions are created for the formation of a four-membered chelate ring.

Proceeding from the content of the pyridonimine form in various solvents at \(20^\circ\) (found by comparing \(\varepsilon_{\max}\) of the pyridonimine form with \(\varepsilon_{\max}\) of the methylated standard), it may be concluded that for 2-phenylsulfonylaminopyridine the pyridonimine form is more stable than for its \(C_6H_5Hg\)-derivative, especially in low-polar solvents (Table 2). According to the data for solutions in aliphatic nitriles, with decrease of the dielectric constant in a series of solvents of one chemical type, the content of the pyridonimine form falls more rapidly for the \(C_6H_5Hg\)-derivative than for phenylsulfonylaminopyridine.

Fig. 3. Dependence of \(pK_T\) in pyridine on \(\sigma_n\) for 2-arylsulfonylaminopyridines (B) and their phenylmercury derivatives (A)

Table 2

Determination of the constants of tautomeric equilibrium in pyridine for para-substituted 2-phenylsulfonylaminopyridines and their phenylmercury derivatives shows that in both cases (Fig. 3) a linear dependence of \(pK_T\) on the Hammett constant \(\sigma_n\) for the corresponding substituent is observed (by the tautomeric-equilibrium constant is meant the ratio of the concentrations in solution of the pyridonimine and aminopyridine forms for the given compound).

On the basis of the parallelism of the corresponding straight lines, it may be asserted that polar factors influence in the same way the position of the tautomeric equilibrium of 2-arylsulfonylaminopyridines and their \(C_6H_5Hg\)-derivatives.

Thus, the results obtained show that even when like-named atoms are present at the ends of the tautomeric system, the position of equilibrium is not the same for migrating groups of different nature and is determined not only by the relative stability of the two electromeric forms of the common anion and by the relative stability of the covalent bonds formed by the departing cation with each of them (⁸). Solvation and chelation effects may also play an important role. At the same time, the effect on the position of tautomeric equilibrium of polar effects that influence only the relative stability of the electromeric forms of the common anion apparently does not depend on the nature of the migrating group.

In conclusion, I take this opportunity to express my deep gratitude to Academician A. N. Nesmeyanov for his attention to the work.

Institute of Organoelement Compounds
Academy of Sciences of the USSR

Received
29 X 1964

CITED LITERATURE

¹ A. N. Nesmeyanov, D. N. Kravtsov, DAN, 135, 331 (1960). ² Yu. N. Sheinker, DAN, 113, 1080 (1957). ³ Yu. N. Sheinker, E. M. Peresleni et al., ZhFKh, 33, 2090 (1959). ⁴ Yu. N. Sheinker, E. M. Peresleni, ZhFKh, 35, 2623 (1961). ⁵ T. A. Mastrukova, Yu. W. Sheinker et al., Tetrahedron, 19, 357 (1963). ⁶ Yu. D. Gnesin, Uch. zap. Pyatigorsk. gos. farm. inst., 2, 271 (1957). ⁷ G. Faraglia, L. Roncucci, R. Barbieri, Gazz. chim. ital., 93, 1413 (1963). ⁸ D. V. Beker, Tautomerism, Moscow, 1937, p. 35.