Radical Sulfinyl

Sulfoxides. Sulfoxides, R — SO—R, are named by placing the names of the radicals in alphabetical order before the word sulfoxide. Alternatively, the less senior radical is named followed by sulfinyl- and concluded by the name of the senior group. For example, CH3CH2—SO—CH2CH2CH3 is named either ethyl propyl sulfoxide or l-(ethylsulfinyl)propane. 

TABLE 9. Bond energies in sulfoxides and sulfones, and enthalpies of formation of sulfinyl and sulfonyl radicals... 

The effect of sulfur-containing substituents on free radicals appears to be a topic of current interest, so we will discuss it briefly insofar as it concerns sulfinyl and sulfonyl groups. Our discussion will refer mainly to a few recent papers, which usually contain copious references to earlier work see also Block247. 

The absolute stereochemistry at the sulfoxide sulfur atom in some /J-phenylsulfinyl radicals (prepared in situ by treating 2-bromo-3-phenylsulfinylbutanes with tributylstan-nane) controls the stereochemistry (i.e., cis vs. trans) of the olefinic products which are formed104. Implicit in this result is that loss of the sulfinyl group occurs more rapidly than rotation about C-2-C-3 of the intermediate radical105. 

The total yield of OH radicals is 6.0 and hence the yield of sulfinic acid of G = 2.7 indicates that 45% of the OH radical produced in the radiolysis attack the sulfinyl group of MTMSO. The authors48 said that the residual 55% attack the sulfide S—C bond, but this claim ignores completely other routes, such as hydrogen abstraction, which was found for other sulfoxides45. 

This review is concerned with the formation of cation radicals and anion radicals from sulfoxides and sulfones. First the clear-cut evidence for this formation is summarized (ESR spectroscopy, pulse radiolysis in particular) followed by a discussion of the mechanisms of reactions with chemical oxidants and reductants in which such intermediates are proposed. In this section, the reactions of a-sulfonyl and oc-sulfinyl carbanions in which the electron transfer process has been proposed are also dealt with. The last section describes photochemical reactions involving anion and cation radicals of sulfoxides and sulfones. The electrochemistry of this class of compounds is covered in the chapter written by Simonet1 and is not discussed here some electrochemical data will however be used during the discussion of mechanisms (some reduction potential values are given in Table 1). 

TABLE 1. ESR data and bond dissociation energies of some sulfinyl radicals... 

Sulfinyl radicals studied by ESR spectroscopy have been generated photolytically either from the appropriate sulfoxide or from the corresponding chlorides, namely10,12"14,16... 

However, in most cases photolysis of XS(0)Y alone produced much weaker signals from the radical XSO than when mixtures of the compounds with peroxides were irradiated. A mechanism has been proposed which involves hydrogen abstraction to form species 4, the fragmentation of which gives the sulfinyl radical, namely13,14,16... 

Gilbert and coworkers18 were able to detect ESR spectra of sulfinyl radicals together with other sulfur-centered radicals during the oxidation of disulfides and thiols with a titanium(III)-peroxide couple reaction mechanisms involving sulfenic acids as intermediates have been discussed. 

Scheme 2 summarizes the mechanism for the formation of thiosulfonate from sulfinyl radicals it is shown that the sulfinyl radicals combine to give both yic-disulfoxides and O, S-sulfenyl sulfinates, although they may rearrange to thiosulfonates either via a free radical route or via a concerted mechanism. The reader is referred to the recent review of Freeman22, who has collected and discussed the vast amount of information published on yic-disulfoxides and O, S-sulfenyl sulfinates. 

Early investigations have indicated that sulfinyl radicals apparently do not add, at least in the usual way, to olefmic double bonds24. However, some recent results by lino and Matsuda25 obtained by studying the thermal decomposition of benzhydryl p-tolyl and benzhydryl methyl sulfoxides in the presence of cis-/2-deuteriostyrene lead one to believe that sulfinyl radicals add reversibly to CH2 =CHPh. The molar ratio of trans to cis /3-deuteriostyrene that they observed at nearly 50% conversion was explained by addition-elimination reaction of sulfinyl radicals. 

It seems to the author that another possible pathway can coexist with the above-mentioned reversible addition of sulfinyl radicals to olefinic double bonds, that is, the reversible addition by the sulfinyl oxygen, since a substantial amount of the unpaired electron spin-density resides on the oxygen atom. 

Norman and coworkers21 23 used ESR spectroscopy to study in detail the reaction of several organic compounds containing the sulfinyl moiety with the Ti(III)-H202 system in particular, dimethyl sulfoxide yields the methyl and methanesulfonyl radicals as shown in reactions 1-321. 

Correa and Waters50 also proposed a mechanistic scheme where the key step of the overall reaction involves the recombination of sulfonyl radicals to form an intermediate with an O—S bond, the decomposition of which yields a sulfinyl radical (ArSO ) and an oxygen-centered radical ArS020 Later this suggestion was further strengthened on the basis of ESR studies51 and thanks to the elucidation of the electronic structure of sulfonyl radicals. However, it seems to the author that the available literature data point to an overall mechanism for equation 17 more complex that the one suggested50 (cf., for... 

Sulfinyl radicals have also been invoked in the synthetically useful reaction of alkane-and arenesulfmyl chlorides with activated zero-valent metals (Ag, Cu, Zn) to give the corresponding symmetrical thiosulfonates ... 

On the other hand, it seems to be generally agreed that combination of two sulfinyl radicals leads to thiosulfonate (RSO2SR) as the final product the usual suggestion has been that it takes place via an initial head-to-tail combination of the radicals to give a O, S-sulfenyl sulfinate (5) which then rearranges to the thiosulfonate. 

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