Sulfonium-ions, cleavage

The El mass spectra of thiols and thioethers also show a series of onium ions generated by a-cleavage of the molecular ion (Table 6.9). Sulfonium ions can easily be recognized from the isotopic pattern of sulfur (Fig. 6.9). The fragmentation patterns of thioethers will be discussed in greater detail later (Chap. 6.5.2 and 6.12.4). 

Example The oxidative addition of dimethyl disulfide (DMDS) transforms the double bond to its 1,2-bis-thiomethyl derivative (a). Induced by charge localization at either sulfur atom, the molecular ions of DMDS adducts are prone to a-cleavage at the former double bond position (b). This gives rise to sulfonium ions that are readily identified from the mass spectrum (Chap. 6.2.5). The method can be extended to dienes, trienes, and alkynes. [70,71] (For the mass spectral fragmentation of thioethers cf. Chap. 6.12.4). 

Aliphatic onium ions such as immonium, oxonium, and sulfonium ions have been introduced as even-electron ionic products of the a-cleavage occurring from molecular ions of amines, alcohols, and ethers or thiols and thioethers, respectively (Chap. 6.2.5). All these and analogous onium ions are capable of further fragmentation reactions, the majority of which are alkene losses [141] yielding fragments of high relevance for structure elucidation. 

The proposed mechanism of the oxidative cleavage of S-protecting groups by the chlorosilane/sulfoxide procedure is outlined in Scheme 8. 95 The first reaction is considered to be formation of the sulfonium cation 9 from diphenyl sulfoxide (7) and the oxygenophilic silyl compound 8. The formation of a sulfonium ion of this type is known and has been utilized for the reduction of sulfoxides. 97 Subsequent electrophilic attack of 9 on the sulfur atom of the S-protected cysteine residue leads to the formation of intermediate 10, whereby the nature of the silyl chloride employed should be the main factor that influences the electrophilicity of 9. The postulated intermediate 10 may then act as the electrophile and react with another S-protected cysteine residue to generate the disulfide 11 and the inert byproduct diphenyl sulfide (12). This final step is analogous to the reaction of a sulfenyl iodide as discussed in Section 6.1.1.2.1. 

The problem stipulates that a bridged sulfonium ion is an intermediate. Therefore, use the tt electrons of the double bond to attack one of the sulfur atoms of thiocyanogen and cleave the S—S bond in a manner analogous to cleavage of a Br—Br bond in the reaction of bromine with an alkene. 

The [Fe4S4(LS3)(SR )] cluster has been shown to engage in an electrophilic attack of the sulfonium ion, while causing reductive cleavage of the cofactor S-adenosylmethionine. This behavior is analogous to the enzymatic action of biotin synthase and other enzymes in the S-adenosylmethionine family see Iron-Sulfur Proteins). ... 

Cleavage of thioglycosides. Oxidation with the hypervalent iodine reagent facilitates replacement of the thio group. O-Glycosylation occurs via the sulfonium ions but not the oxonium ions. 

The reduction of benzo-fused bicyclic sulfonium ions with samarium diiodide also results in the cleavage of the —C bonds. However, as shown in Scheme 9, the regioselectivity here depends on... 

Intramolecular nucleophilic substitution reactions also are thought to compete with hydrolysis (i.e., 8 2 reaction with H2O) in the hydrolysis of several or-ganophosphorus esters. For example, the C-S cleavage of thiometen (R = H) and disulfoton (R = CH3) can occur by intramolecular nucleophilic substitution by the sulfur atom of the electron-withdrawing group, which results in the formation of the cyclic sulfonium ion which subsequently reacts with water to give 2-(ethylthio)ethanol (Wanner et al., 1989). 

S-nitro or S-nitrito sulfonium ions are similar intermediates in the oxidative cleavage of ethylenethioacetals with nitronium teirahuoroborate or sodium nitrate-trifluoroacetic acid [127cj. 

On the other hand, cleavage of the S-C bond is facile in nucleophilic substitutions involving sulfonium salts92 and the leaving ability of the dial-kylsulfide group compares with that of a bromide ion. As a result, there are several examples of type B substitutions in the literature. 

Nucleophilic substitution at the a-carbon atom does not occur in the case of the most studied and stable bicyclic disulfonium dications.96 Although the reaction of dication 34 with bromide ions formally leads to the S C bond cleavage, the reaction mechanism involves initial nucleophilic attack at the sulfonium atom by the bromide anion. The bromosulfonium salt intermediate... 

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