Chemistry

Yu. I. Naumov, V. A. Izmail’skii

On the Microstructure and Electron-Acceptor Properties of the Sulfofluoride Group

(Presented by Academician M. I. Kabachnik, January 30, 1965)

The study of the UV spectra of benzenesulfonyl fluoride derivatives is of interest both from the standpoint of evaluating the degree of electron-acceptor character of the SO₂F group and from the standpoint of studying the microstructure of the SO₂Y group \((Y = F, Cl, Me, OH, NH_2\), etc.). The literature data are contradictory, and there is as yet no complete clarity regarding the nature of the \(S \ldots O\) bond. According to \((^1)\), in the SO₂ group there is not a double bond \(S = O\), but only a mesomeric shift of the coordination bond toward the double bond \(\overset{+}{S}—\overset{\cdot\cdot}{O}:\overset{\ominus}{\cdot\cdot} \leftrightarrow S=O:\), with participation of the vacant \(3d\)-orbitals of sulfur. The double bond \(S=O\), by its character, should differ from a \(\pi\)-bond. Against an \(S=O\) bond it was argued that there is no spectral band of the \(n \to \pi^*\) transition type for the SO group and no addition reactions. However, recently a 1,4-addition reaction to \(C=C—S=O\) has been described \((^2)\).

It could be supposed that an increase of the positive charge on the S atom as a result of the inductive influence of the F atom \((\delta^+ S \to F^{\delta-})\) would strengthen the electron-acceptor properties of the SO₂F group in comparison with SO₂Me \((S \leftarrow CH_3)\) and SO₂NH₂, and would shift the bond \(S \to O\) toward \(S=O\). In favor of this assumption is the shortening of the \(S \ldots O\) bond in SO₂F₂ to 1.37 Å, instead of 1.50 Å in \(SO_2O_2^{--}\) \((^1)\).

Table 1

* Taken from \((^6)\), except for the value for \(SO_3^{\ominus}\) \((^7)\) instead of SO₃H.
* \(\varepsilon_{\max}\) 18,900 and 26,100, respectively, at \(C = 10^{-4}\) mol/l.
** Calculated from the curves of Fig. 4.

The study of the UV spectra of derivatives of the series \(A — \Phi — SO_2F\) \((\Phi =\) cyclohexylidene ring or \(C_6H_5—\), \(A = NMe_2, NH_2, OMe, Br, Cl, Me, H)\) showed that the sulfofluoride group is a weaker acceptor than groups of type B with a double \(\pi\)-bond, COOH, COCH₃, NO₂. However, the spectral curves of benzenesulfonyl fluorides contain K-bands \((\pi \to \pi^*\) transitions), typical of compounds of the structure \(n\)-B—\(\Phi\)—A \((^{3,4})\) (Tables 1 and 2, Figs. 1 and 2). The positions of \(\lambda_{\max}\) and \(\varepsilon_{\max}\) of the K-band and the similar character of the bands clearly confirm the presence of conjugation of the SO₂F group with the benzene nucleus.

Our conclusions on the presence of conjugation in the system \(A\Phi SO_2F\), where \(A = NH_2\) and other electron-donor groups (Fig. 2), confirm the conclusions \((^8)\) on the absence in solutions of the bipolar form of sulfanilic and \(N,N\)-dimethylsulfanilic acids. According to our data, the K-bands of the system \(A-\Phi-SO_2F\) are bathochromically shifted K-bands of the chromophores \(A-C_6H_5\) or \(C_6H_5SO_2Y\), which in turn are genetically connected with the K-band of benzene \((\lambda_{\max} 203.5,\ \varepsilon 7000)\).

Recently M. A. Mostoslavskii \((^9)\) pointed out a linear correlation of \(\lambda_{\max}\) for two chromophores of similar microstructure upon introduction into them of identical substituents. We applied this rule in order to determine to what extent the groups \(SO_2F\), \(SO_2Y\) are related in their action to groups containing a \(\pi\)-bond (\(COOH\), \(COMe\), \(NO_2\)). Comparison of \(\lambda_{\max}\) for aniline and dimethylaniline derivatives (Fig. 3, \(a\)) showed the presence of a linear correlation. Small deviations from the straight line for the groups \(SO_2F\), \(SO_2NH_2\) may be regarded as an indication of some difference in the mechanism of their interaction with the \(\pi\)-electrons of the benzene nucleus from that of groups of type B with a \(\pi\)-bond.

Fig. 1. Influence of electron-acceptor groups on the spectrum in compounds of the type \(A-\)⌬\(-B\):
\(1\)—\(Me_2N\Phi COMe\); \(2\)—\(H_2N\Phi COMe\); \(3\)—\(Me_2N\Phi COOH\); \(4\)—\(Me_2N\Phi SO_2F\); \(5\)—\(H_2N\Phi COOH\); \(6\)—\(H_2N\Phi SO_2F\)

On the other hand, comparison of \(\lambda_{\max}\) for benzenesulfofluorides with \(\lambda_{\max}\) for benzenesulfamides and methyl phenyl sulfones shows a linear correlation (Table 2, Fig. 3, \(b\) and \(c\)). This indicates a relationship in the electronic structure of their chromophoric systems, whereas for the series of benzenesulfofluorides and \(p\)-substituted benzoic acids (Fig. 3, \(d\)) a linear correlation is not always observed.

Fig. 2. Absorption spectra of \(A-\Phi-SO_2F\);
\(A = 1\)—\(NM_{12}\); \(2\)—\(NH_2\); \(3\)—\(MeO\); \(4\)—\(Br\); \(5\)—\(Cl\); \(6\)—\(Me\); \(7\)—\(H\)

We see the reason for this in the fact that in \(SO_2F\) and \(SO_2Y\), even in the presence of the \(S=O\) bond, we do not have conditions for coplanarity and normal conjugation. The double bond \(S=O\) does not have the character of a \(\pi\)-bond; the groups \(SO_2Y\) have a special tetrahedral structure \((^1,\,^{10})\), which prevents coplanarity of the atoms connected with \(S\) and the benzene nucleus. As a consequence of this, conjugation and displacement of the electron upon excitation by light occur without participation of the oxygen electrons of the \(SO\) group, with partial expansion of the shell up to 12 (Fig. 1, \(c\)) (and up to 14 electrons when two \(S=O\) groups are retained (Fig. 1, \(d\))). Conjugation may occur, however, even when theileswi

of the absence of a double bond in the SO group (II):

Investigation of the correlation of \(\Delta \lambda_{\max}^*\) of the K-bands of the \(\mathrm{H_2N\Phi B}\) and \(\mathrm{Me_2N\Phi B}\) series with the Hammett constants \(\sigma^-\) and their mesomeric components \(\sigma_R\) \((^{5,11})\) by the method \((^{12})\) (Table 1, Fig. 4) makes it possible to estimate for \(\mathrm{SO_2F}\) the constants \(\sigma \sim 0.64\) (which is close to \(\sigma^-\), calculated for \(\mathrm{SO_2Me}\) \((^{13,14})\)) and \(\sigma_R\) 0.46.

Table 2

\(\lambda_{\max}\) of the absorption bands of substituted benzenesulfofluorides
\(\mathrm{A{-}\Phi{-}SO_2F}\) (Fig. 2) in dichloroethane

\(C = 10^{-2}\) mol/l

* There is also \(\lambda\) 223, \(\varepsilon\) 8500, analogous to the \(x'\)-band \((^3)\).

Fig. 3. Correlation between \(\lambda_{\max}\) of two compounds of structure \(\mathrm{A{-}\Phi{-}B}\) \((\Phi =\) phenylene\()\):
a: comparison of \(\lambda_{\max}\) \(I\)—\(\mathrm{Me_2N\Phi B}\) and \(II\)—\(\mathrm{H_2N\Phi B}\); \(\mathrm{B} = 1\)—H, 2—\(\mathrm{SO_3H}\), 3—\(\mathrm{SO_2NH_2}\), 4—\(\mathrm{SO_2F}\), 5—COOH, 6—COMe, 7—\(\mathrm{NO_2}\);
b: comparison of \(\lambda_{\max}\) \(III\)—\(\mathrm{A{-}\Phi{-}SO_2F}\) and \(IV\)—\(\mathrm{A{-}\Phi{-}SO_2NH_2}\), \(\mathrm{A} = 1\)—H, 2—Me, 3—Cl, 4—Br, 5—MeO, 6—\(\mathrm{NH_2}\), 7—\(\mathrm{NMe_2}\);
c: comparison of \(\lambda_{\max}\) \(III\) and \(V\), \(\mathrm{A{-}\Phi{-}SO_2Me}\); \(\mathrm{A}\) = the same;
d: comparison of \(\lambda_{\max}\) \(III\) and \(VI\), \(\mathrm{A{-}\Phi{-}COOH}\); \(\mathrm{A}\) = the same.

The data obtained make it possible to conclude that conjugation of the \(\mathrm{SO_2F}\), \(\mathrm{SO_2Y}\) groups occurs with participation of vacant sulfur \(3d\)-orbitals.

* The values of the shift of the K-band in Fig. 4 were calculated by us relative to the corresponding bands of aniline \(\lambda_{\max}\) 230 mµ and dimethylaniline \(\lambda_{\max}\) 250 mµ.

(\(\mathrm{I_B}\) or \(\mathrm{II_B}\)) through the formation of a \(\mathrm{C}\cdots \mathrm{S}\) bond. The electrons of the O atom are not drawn into the chromophore system. The conjugated chromophore system terminates at the sulfur atom. In support of this conclusion one may cite the observation that the \(\Delta\lambda\) effects of the \(\mathrm{SO_2F}\) and \(\mathrm{SO_2Me}\) groups practically coincide (\(n\)-\(\mathrm{H_2N\Phi SO_2F}\) \(\lambda_{\max}\) 270, \(n\)-\(\mathrm{H_2N\Phi SO_2CH_3}\) \(\lambda_{\max}\) 269 \(^{10}\)), despite the opposite direction of the polarizing action in the \(\mathrm{S}\to\mathrm{F}\) and \(\mathrm{S}\to\mathrm{CH_3}\) bonds. The closeness of the effects of \(\mathrm{SO_2F}\) and \(\mathrm{SO_2Me}\) is also confirmed by the closeness of the \(\sigma\) found by us for \(n\)-\(\mathrm{SO_2F}\), 0.64, to that calculated for \(\mathrm{SO_2Me}\), 0.65–0.68 \(^{13,14}\).

Fig. 4. Correlation of shifts of maxima (\(\Delta\lambda\)) with \(\sigma\), \(\sigma_R\) for \(\mathrm{H_2N—\Phi—B}\); \(\mathrm{Me_2N—\Phi—B}\): 1 — \(\mathrm{SO_3H}\); 2 — \(\mathrm{SO_2NH_2}\); 3 — \(\mathrm{COOH}\); 4 — \(\mathrm{COMe}\); 5 — \(\mathrm{NO_2}\).
\(a\) — \(\mathrm{H_2N—\Phi—B}\), \(b\) — \(\mathrm{Me_2N—\Phi—B}\).

\(n\)-\(\mathrm{Me_2N}\)-benzenesulfonyl fluoride was obtained by the action of an excess of \(\mathrm{CH_3I}\) on \(n\)-\(\mathrm{H_2NC_6H_4SO_2F}\), with decomposition of the quaternary salt. From benzene: leaflets, m.p. \(110.5–111^\circ\).

\[ \begin{aligned} &\text{Found, \%: } && \mathrm{N}\ 6.82;\ 6.95;\quad \mathrm{F}\ 9.15;\ 9.12\\ &\mathrm{C_8H_{10}FNO_2S}.\ \text{Calculated, \%: } && \mathrm{N}\ 6.89;\quad \mathrm{F}\ 9.23 \end{aligned} \]

Laboratory of Dyes and Problems of Color
at the Moscow Pedagogical Institute
named after V. I. Lenin

Received
30 I 1965

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