Radical reactions of sulfur compounds

The reactions of sulphonyl halides with olefins have been reviewed, with other processes of producing free sulphonyl radicals.
Reactions of polynuclear aromatic hydrocarbons and quinones with free radicals have also been presented.
The decomposition of p-toluene sulphonyl iodide was investigated. The sulphonyl iodide decomposed very slowly when refluxed in carbon tetrachloride for eight hours but in the presence of metals, e.g. silver and copper powders, the decomposition was achieved in less than three hours. The products of this decomposition were p-toluenesulphonic anhydride and p-tolyl-p-toluenethiosulphonate. Copper salts of p-toluenesulphonic acid seem to be formed. The expected dimer, p-tolyldisulphone could not be isolated.
Free radicals were used instead of metals to decide whether the sulphonic anhydride was formed by a homolytic or heterolytic reaction. It was expected that the free radicals would combine with the iodine atoms yielding sulphonyl radicals:
TolSO2I + R. &longrightarrow; TolSO2&bullet; + R-I
The reaction mixtures were complex but the formation of p-toluenesulphonic anhydride could still be detected. The dimer, p-tolyldisulphone was also isolated but only in 3-18% yield. Azobis-isobutyronitrile and azobis(1-cyano-l-cyclohexyl) were used as sources of free radicals. The other products of reaction were of the type R-I, R-R and R'-SO2Tol (R = 2-cyano-2-propyl and 1-cyano-l-cyclohexyl; R' = 2-cyano-2-propyl).
Decomposition of the sulphonyl iodide under u.v. light gave also p-toluenesulphonic anhydride.
The mechanism proposed for the decomposition of the sulphonyl iodide was:
TolSO2I + { M
R&bullet;
 &longrightarrow; { TolSO2&bullet; + IM
TolSO2&bullet; + IR
TolSO2I      &longrightarrow;&longleftarrow;TolSO2&bullet; + I&bullet;
I&bullet; + { M
R&bullet;
 &longrightarrow; { IM
IR
2 TolSO2&bullet;      &longrightarrow;     TolSO2-O&bullet; + TolSO&bullet;
2 TolSO2&bullet;      &longrightarrow;     TolSO2SO2Tol
2 TolSO2&bullet;      &longrightarrow;   [TolSO-STol]
         &downarrow;
        O

  &longrightarrow;        O
       ||
Tol S-S-Tol&bullet;
       &downarrow;
      O



TolSO2&bullet;  + TolSO2-O&bullet;   &longrightarrow;   (TolSO2)2O
TolSO2&bullet;  + R&bullet;   &longrightarrow;   TolSO2-R
R&bullet; = Free radical; M = Metal
p-Toluenesulphonyl iodide reacts with quinones in the presence of copper "bronze. Attack by the sulphonyl group concerned exclusively the oxygen atoms of quinones. Di-ptoluenesulphonates of 1,4-dihydroxybenzenes were obtained. p-Toluenesulphonic anhydride did not react with quinones. The sulphonyl iodide attacked hydroquinone at the nucleus to give a sulphone and not a sulphonate, i.e.
The mechanism proposed for the reaction of p-toluene-sulphonyl iodide with quinones was as follows:
TolSO2I + Cu &longrightarrow; TolSO2• + CuI
TolSO2&bullet; + Q &longrightarrow; TolSO2-Q&bullet;
TolSO2-Q&bullet; + TolS02 &longrightarrow; TolSO2-Q-SO2Td
Quinones of higher redox potential were more reactive towards tolylsulphonyl radicals than quinones of low potential. Shielding of the quinone oxygen atoms by methyl or chlorine groups also reduced the reactivity.
Further indications of the free radical nature of the reactions of p-toluene sulphonyl iodide were obtained by reactions with polynuclear aromatic hydrocarbons. In the presence of copper bronze the sulphonyl iodide reacted with anthracene to give 9,10-di-p-tolylsulphonyl-9,10-dihydroanthracene .
In the absence of metal, 9-p-tolylsulphonylanthracene was also formed. 2, 3-Benzanthracene gave 9,10-di-p-tolylsulphonyl-9 , 10-dihydro-2 , 3-benzanthracene . 1 , 2-Benzanthracene, phenanthrene did not react.
Evidence for free radical reactions were obtained with 9,10-di-methylanthracene and 9-methylanthracene. Substitution products of the methyl group were obtained in both cases, which indicated the presence of species able to abstract hydrogen from the side chain.
The reaction with cumene gave no bicumyl but p
p-tolyl-sulphonyloxy radicals, TolSO2-0&bullet;, have been produced.
The mechanism proposed to the reaction of p-toluenesulphonyl iodide with anthracene was as follows:
9-Methyl- and 9,10-dimethylanthracene reacted as follows:
The effect of light and inhibitors (hydroquinone) on the addition of p-toluenesulphonyl iodide to acrylonitrile was studied. The reaction was accelerated by light and retarded by hydroquinone and darkness.
The production of sulphinyl radicals by reaction of sulphinyl chlorides, Ar•SO•Cl with copper bronze was investigated.
No evidence for the formation of sulphinyl radicals was obtained since no reaction occurred with olefins, p-benzoquinone or anthracene. Sulphinyl radicals produced by decomposition of 2,4-dinitrobenzenesulphenyl nitrate failed to react with p-benzoquinone.