Sulfenates, formation

Alkanesulfonyl halides are not the only alkanesulfonyl derivatives that can undergo substitution by an elimination-addition mechanism. A number of aryl esters of phenylmethanesulfonic acid, PhCH2SOzOAr, undergo alkaline hydrolysis and aminolysis by such a pathway, and study of these reactions has been particularly valuable in providing insight into the detailed mechanism for sulfene formation (Williams et al., 1974 King and Beatson, 1975 Davy et al., 1977). 

Davy et al. (1977) conclude that sulfene formation from the esters of weakly acidic phenols clearly involves a reversible ElcB mechanism, (203) k2 < /c j[H20], i.e. step k2 is rate-determining. Clearly, too, for the esters of... 

That there is also considerable carbanion character on the carbon adjacent to the sulfonyl group in the rate-determining transition state for sulfene formation from phenylmethanesulfonyl chlorides is shown (King and Lee, 1969) by the fact that p for sulfene formation from ArCH2S02Cl and pyridine is +2.3, with the rates being better correlated by a - rather than o-values. 

On the basis of these findings, the reaction of acyl imines with methanesulfonyl chlonde-triethylamine is not expected to proceed via a sulfene intermediate as previously proposed [99]. Again, a carbanion intermediate accounts nicely for the experimental facts. The electrophihcity of the hetero-l,3-diene is extremely high, therefore the carbanion, formed on reaction of triethylamine with methanesulfonyl chloride, should undergo nucleophilic attack at C-4 of the hetero- 1,3-diene faster than sulfene formation by chloride elimination. 

Evidence for sulfene formation is obtained by carrying out the reaction in the presence of suitable trapping agents, namely substrates which undergo nucleophilic additions, like alcohols and amines (Scheme 66). 

Brase and coworkers used a similar concept to prepare sulfonic esters via polymer-bound triazenes [67]. The methodology was subsequently expanded to phosphoric and phosphinic acids with good results [68]. For the alkylation of sodium sulfonates, this method has the advantage that sulfene formation is... 

B. Sulfenes from RR XS02Lg Thiofugal Sulfene Formation. 712... 

A number of methods including product analysis, isotopic labelling, kinetics and kinetic isotope effects, and stereochemical studies have been used to show the presence of sulfenes. The case for the existence of sulfenes has been summarized before1,3 and, except for the next section (IV.A.l), which is necessarily complex, the present account will note, as briefly as possible, the evidence for each method of sulfene formation, the reasons for concluding that sulfenes are present and the extent to which non-sulfene reactions also appear with this method. 

Sulfene formation is not restricted to the simple alkanesulfonyl chlorides rather it is the common, though not invariable, route when a compound of the general formula RR CHS02Lg, which can reasonably eliminate HLg, is treated with a basic reagent. The nature of the base, of the leaving group, Lg, and of the alkyl portion, can all determine... 

Though benzenesulfonyl chloride itself cannot form a sulfene by elimination of HC1, various substituted arenesulfonyl chlorides have this potential. The very first discussion of sulfenes in the literature54 concerned the possibility of sulfene formation from a substituted 4-hydroxybenzenesulfonyl chloride (10) as in equation 10. Subsequent studies clarified the nature of the product (which was polymeric)55,56 and of the yellow color ascribed by the early workers to the sulfoquinone intermediate relatively recently a good kinetic case has been presented57 for the reaction of 10 (X = Me) according to equation 10. 

The preceding section dealt primarily with the evidence that demonstrated sulfenes to be intermediates in these processes and also, where the evidence warranted it, with the mechanism of the sulfene formation. A continual point of concern is whether or not the reaction is really proceeding by way of the sulfene or not. In this section we shall attempt to find what circumstances of substrate, base and (where different) nucleophile tend to lead to the elimination-addition (sulfene) reaction, and which to some other process, notably the direct displacement reaction. Sometimes, indeed, it takes only a rather small change in conditions to alter the mechanism completely. For example, it has been noted above that azide ion reacts with a-disulfones or sulfonyl chlorides by the direct displacement49. This is also seen in the reaction of methanesulfonyl chloride with aqueous sodium azide when the... 

The Truce-Campbell picture of competing displacement and elimination, which has been taken up by others1,61,62, is, in our view, incorrect. There are indeed other instances in which sulfene formation competes with direct displacement (see below), but it is our conclusion that the reaction of simple alkanesulfonyl chlorides with alcohols in the presence of triethylamine in benzene (or other aprotic organic solvents) is not one of them. Since the real picture is already complex, we feel that we had best get rid of any complications that are merely errors. 

Sulfonyl fluorides have not been much used as sources of sulfenes, presumably because of their low reactivity relative to, say, sulfonyl chlorides. Phenylmethanesulfonyl fluoride does not react with triethylamine under mild conditions, but does react with p-toluidine in methanol at 45 °C after a few days the deuterium exchange pattern was consistent with sulfene formation by an (ElcB)rev mechanism93. The mechanism of the reaction of phenylmethanesulfonyl fluoride evidently changes with conditions, since Williams and coworkers had observed that its reaction with aqueous hydroxide showed rate-controlling proton transfer, as in either an E2 or (ElcB)irr process46. Sulfonyl fluorides have been proposed as sulfene sources from such fluorinated sulfonyl fluorides as 1,2,2,2-tetrafluoro-ethanesulfonyl fluoride by Knunyants and Sokol skii and coworkers94. 

One further form of competition with sulfene formation is the substitution of the S02Lg group, particularly the halosulfonyl function, by the nucleophile. This is perhaps best illustrated by the reaction of bromide ion with phenylmethanesulfonyl bromide in an SN2-on-carbon process to give benzyl bromide and sulfur dioxide95. This, and the other competing modes of reaction of phenylmethanesulfonyl bromide and chloride summarized in Scheme 2, serve to emphasize that sulfene formation is by no means the... 

Block and Aslam98 have reported sulfene formation as in reaction 14. It proceeds readily at room temperature in two hours, and has certain advantages in comparison with the reaction of methanesulfonyl chloride with triethylamine. These include formation of the sulfene adduct with cyclopentadiene, bromomethanesulfonyl bromide with bromine,... 

Not long after the appearance of our original report on carbofugal formation of sulfenes, Dykman105 rationalized the overall transformation shown on the first line in equation 18 by including the reaction in the second line. The closely related sulfene formation,... 

In 1916 Staudinger and Pfenninger107, prompted by the ketene-sulfene analogy, reported that diphenyldiazomethane and sulfur dioxide gave products appropriate to the intermediate formation of diphenylsulfene as in equation 19. Sulfene formation in this way... 

The mechanism by which the sulfene is formed is not known, though the intermediacy of either 23 or 24 (or both) seems reasonable. Sulfene formation from 23 would be a carbofugal process like that from an a-haloalkanesulfinate anion already discussed in Section IV.C. 

In contrast to the above thermal elimination processes, in which there is good reason to invoke sulfene formation, the chlorination of 26 is perhaps more appropriately labelled a... 

The reaction of methanesulfonyl chloride with cyclohexanone enamine in the presence of a proton acceptor produced a four-membered cyclic aminosulfone (equation 64). Stork and Borowitz36 suggested sulfene formation as the first step of this reaction. 

An interesting olefination of electron-deficient ketones by alkanesulfonyl halides using KF probably involves sulfene formation. The removal of the fluorenylmethoxy-carbonyl (Fmoc) group from amino acids and peptides with KF/18-crown-6 does not affect methyl, ethyl, f-butyl, benzyl, and p-methoxybenzyl groups. Actually tran-scarbamoylation (into Boc and Cbz groups) can be accomplished in one flask, by presenting the proper electrophiles in the reaction mixture. ... 

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