Sulphenes mechanism

From a study of the hydrolysis of sultone 4 in D20-0D solution, Kaiser and his coworkers showed that carbanion formation occurs rapidly and reversibly from the sultone in basic solution136. They were able to eliminate the concerted mechanism (equation 99) as a major reaction pathway and concluded that a carbanion-sulphene mechanism (equation 100) does not provide an important pathway for the hydrolysis of five-membered sultones. 

Jones and coworkers200 found that a variety of sulphenic acids may be generated by thermolysis of the readily available /J-cyanosulphoxides (equation 81) and observed their highly regiospecific addition also to non-conjugated alkynes (Table 12). As expected for a pericyclic mechanism, the reaction afforded the product of a stereospecific cis-addition. However, the regioselectivity of the addition suggests that the partial carbon-sulphur bond in the transition state 148 is polarized in such a way that the carbon atom has some cationic character (equation 82). 

The cystine link also has a profound effect upon the mechanical properties of the fibre. The disulphide bond is covalent and not very sensitive to pH, but there are a number of reagents which can break it down. Water can bring about hydrolysis, especially when in the form of steam with the formation of sulphenic acid groups ... 

Addition of in situ-generated sulphenes (see Section I.A.2) to a,/ -unsaturated carbonyl compounds to give y-unsaturated <5-sultones is described in an early paper by Gandini and coworkers (equation 45)64. Their study compares the behaviour of sulphenes towards a,/ -unsaturated carbonyl compounds to that of ketenes and, by analogy, two possible mechanisms for the formation of <5-sultones via / -sultones are outlined in equation 46. 

On the other hand, for the five-membered sultone, although the benzylic proton is labile and the phenoxide ion is a good leaving group, the ElcB mechanism is suppressed because the transition state 45 for this reaction would lead to the high-energy perpendicular sulphene. It seems reasonable to conclude therefore that the difference in reactivity of sultones and phenyl benzenesulphonate, which does not possess a potential carbanion centre, reflect differences in rate of attack at sulphonyl sulphur in cyclic and acyclic esters. 

Sulphenes, generated by reaction with tertiary amines (see Sections II.A.2 and II.B.2), react with SchifFs bases to give /7-sultams258,259. In all cases so far, Z-sultams predominate in the product mixture so that Hirotoka and Kobayashi have suggested a concerted [tc2s + 7t2s] mechanism for the reaction (equation 205)259. 

Kinetic studies in the benzothiazole series include examination of the base-catalysed reaction between 2-fluoro-6-nitrobenzothiazole and aliphatic amines, " investigation of the mechanism of formation of sulphenic acid amides... 

The proposed mechanism of thermolysis (Scheme 4) implies that rotation of the AB quinolizidine ring at the sulphenic-acid stage leads to the original sulphides (15) or (18) or to new products (21) or (22). 

The most common route to sulphenes, the elimination of hydrogen halide from a sulphonyl halide, has been the subject of a recent mechanistic and kinetic study which led to the conclusion that this reaction most probably proceeds by an El mechanism. Tsuge and Iwanami used this reaction for the generation of benzoylsulphene (130) from benzoylmethane-sulphonyl chloride. The reaction of (130) with a series of anils (131) was found to be somewhat dependent on the presence or absence of triethylamine and very dependent on the nature of R in (131). For R = alkyl, the (4 -I- 2) cycloadduct (132) is the predominant or only product, whereas for R = aryl the (2 + 2) cycloadduct (133) is the reaction product. Similar results were obtained for the reaction of (130) with carbodi-imides. The reaction of (130) with aryl cinnamylideneamines gave Diels-Alder adducts (134). ... 

King and Lee have provided evidence that the normal elimination route for sulphene formation involves an 2 mechanism with direct and initial attack on the a-proton by the base, not initial substitution for the chloride. Using the axial and equatorial isomers of trans-l-chXoTO-sulphonyldecalin with triethylamine as the base, it was found that the axial isomer formed sulphene 71 times as fast as did the equatorial isomer. By comparison, it was found that the equatorial isomer hydrolysed 14 times as fast as did the axial isomer, and this reaction presumably involved attack at sulphur. 

Apart from one citation, all the papers collected here to illustrate the reactions of vinyl sulphoxides deal with addition reactions. " As would be expected from the nature of the sulphoxide functional group, there are several novel features in these addition reactions compared with those of other vinyl compounds. Pyrolysis of alkyl styryl sulphoxides gives benzothiophens. The proposed mechanism, which involves formation and homolysis of a sulphenic acid followed by cyclization, (54) (55), is supported by the formation of benzo-... 

Elimination reactions of aryl arylmethanesulphonates leading to phenyl-sulphenes have been shown to involve E ch characteristics where the leaving group is a phenol with pJlTa less than 6, otherwise a concerted E2 mechanism applies. 

Dichlorosulphine is formed in good yield by the thermal decomposition of p-anisyl trichloromethanesulphenate in acetonitrile or chloroform. The reaction is very sensitive to solvent polarity, and, in non-polar solvents, rearrangement of the sulphenate to the sulphoxide occurs by a polar mechanism. Since the trichloromethanesulphenate anion is known to give dichlorosulphine, an intermediate ion-pair is postulated, i.e. 

A detailed study of the mechanism of the allyl sulphoxide-sulphenate rearrangement and its uses in synthesis has been reported. Rates of steroidal allyl sulphoxide rearrangements are influenced by chirality at sulphur, and configurational aspects established for compounds of this... 

Omeprazole becomes completely ionised and thus trapped in secretory channels of the parietal cells due to an acid-catalysed rearrangement which converts it into a quaternary amine. The mechanism of the structural rearrangement is not known exactly. It seems likely that it is the sulphenic acid (Fig. 12.29) intermediate that reacts with the H K ATPase and rather than the sulphenamide (Fig. 12.29), which is a more stable end product of the rearrangement. ... 

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