Sulfene, reaction intermediate

The reaction of 16a on heating with methanol to give the sulfenic acid intermediate 142 and the sulfmate 143 (which was further transformed into the disulfide 144) was interpreted in terms of the mechanism shown in equation 58161. 

Winterfeldt has shown that the reaction of dimethyl sulfoxide with DMAD gives tetramethyl furantetracarboxylate (345), and it was suggested that this reaction may proceed through the intermediates 342, 343, and.344 (Scheme 52). The reaction of /-butylsulfenic acid with methyl propiolate, however, gives /3v -bis(/ra/is-carbomethoxy)divinyl sulfoxide (348) (Scheme 53). The penicillin S-oxide (349) is known to react with DMAD to give the adduct 352, and this reaction has been assumed to proceed through the sulfenic acid intermediate 350 (Scheme 54). ... 

As a consequence of the conformational mobility of the thiepane 1-oxide ring (115) it was possible to form the necessary planar five-membered cyclic transition state for a thermal E elimination reaction (equation 19). The acyclic sulfenic acid intermediate was not isolated but rearranged to c/s-2-methylthiane 1-oxide by an intramolecular cyclization mechanism (75TL2235). 

The oxidation of thiols follows a completely different course as compared with the oxidation of alcohols, because the capacity of the sulfur atom to form hypervalent compounds allows it to become the site of oxidation. Thiols are readily oxidised to disulfides by mild oxidants such as atmospheric oxygen, halogens or iron(III) salts (Scheme 6). This type of reaction is unique to thiols and is not undergone by alcohols, it is a consequence of the lower bond strength of the S-H as compared with the O-H bond, so that thiols are oxidised at the weaker S-H bonds, whereas alcohols are preferentially oxidised at the weaker C-H bonds (Scheme 7). The mechanism of oxidation of thiols may be either radical or polar or both (Scheme 6). The polar mechanism probably involves transient sulfenic acid intermediates like (7) and (8). In contrast, thiols react with more powerful oxidants, like potassium permanganate, concentrated nitric acid or hydrogen peroxide, to yield the corresponding sulfonic acids (10). This oxidation probably proceeds via the relatively unstable sulfenic (7) and sulfinic acids (9), which are too susceptible to further oxidation to be isolated (Scheme 8). 

Hori et al. reported the ring expansion of 3-acetylbenzothiazoline 1-oxide (75) in refluxing acetic anhydride118-120 to give 4-acetylbenzothiazines (76) via a sulfenic anhydride intermediate this reaction is nonstereospecific, supporting the proposed nonconcerted mechanism. 

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... 

Ozonolysis causes usually in other furan containing natural compounds the loss of C H 0 unit from the molecule and a carboxylic group is formed. It can be assumed that in the NTBN molecule the positively charged nitrogen atom withdraws the electrons from the furan ring so the C4 - Cll bond bears some olefinic character. This bond could be attacked by ozone molecule in the subsequent reaction step and quinolizidone derivatives are formed. X-ray analysis of NTBN bromohydride shows that C4 - Cll bond is really shortened. In the next reaction step lactam rings could be cleaved by acid in ethanol and corresponding esters are formed. The C-15 compounds are presumably formed via sulfenic acid intermediate. 

On the basis of these findings, the reaction of acyl imines with methanesulfony 1 chloride-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 exdemely high, therefore the carbanion, formed on reaction of triethylamme with methanesulfonyl chloride, should undergo nucleophilic attack at C-4 of the hetero-1,3-diene faster than sulfene formabon by chloride elimination. 

The reaction of enamines with ketene (146) and sulfene (147) are presumed to proceed by a two-step process involving an iminium intermediate such as 99. In fact, reaction with all electrophilic olefins such as acrylonitrile and related reagents could be thought of as going through an iminium intermediate similar to 99. Another example is given by addition to an enamine... 

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. 

The mechanism of this reaction is not known. However, some evidence" suggests the mechanism (equation 39) with the zwitterion 101 as a key intermediate. This is in accord with the known favored attack of nucleophiles at the sulfur atom of sulfenes . 

The formation of cyclic sulfinic esters (sultines) from vinyl sulfenes is known , and the trapping of the expected intermediate vinyl sulfene in the thermolysis of thiete dioxide (6fc and 194) has been convincingly achieved . Specifically, thermolysis of thiete dioxide 6b in the presence of norbornenes gave cycloadducts of the Diels-Alder type (i.e. 252b), resulting from the trapping of the vinyl sulfene formed. The accumulated evidence thus supports the proposed mechanism for these thermolytic reactions. 

The synthetic utility of the remarkably facile and efficient [2,3]-sigmatropic rearrangement of propargylic sulfenates has been further demonstrated in a variety of preparations and interesting reactions of allenyl sulfoxides , including the preparation of vinylallenes " which are useful intermediates in organic synthesis in general and natural polyenes, such as Vitamins A and D, in particular Two typical examples, taken... 

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