Thiosulfonium salts

Dealkylation of labile disulfonium dications 75 derived from 2,2 -bis(alkylthio)-biphenyls gives rise to thiosulfonium salt 76. This reaction can also be classified as nucleophilic substitution at the a-carbon atom. The intermediate dication 75 is highly reactive, but can be detected spectroscopically (Scheme 29).93... 

The propensity of S-S dications to undergo dealkylation was found to decrease in the order of methyl > ethyl > benzyl. This order of reactivity parallels the increase in the stability of the corresponding carbocations.94 Dealkylation of dication 77 affords thiosulfonium salt 78 in quantitative yield.95 Kinetic studies suggest SN1 mechanism of dealkylation. In addition, reaction of sulfoxide 79 with a substituent chiral at the a-carbon results in racemic amide 80 after hydrolysis. 

The high reactivity toward sulfur nucleophiles is not the only noteworthy aspect of chemisty of thiosulfonium salts. Certain representatives also undergo interesting rearrangements that will now be discussed. 

An alternative route from alkenes to 2-azasulfides reported by the groups of Caserio and Trost involves addition of a thiosulfonium salt, e.g. dimethyl(methylthio)sulfonium tetrafluoroborate (MeSS-Me2+ BF4-), followed by treatment of the resultant thiosulfenylated adduct with an amine or other nitrogen nucleophiles (Schemes 2320 and 24).35 Trost reports that the addition of the thiosulfonium salt can be followed by addition of an oxygen nucleophile, such as acetate, or a carbon nucleophile, such as cyanide, effecting oxosulfenylation and cyanosulfenylation, respectively (Scheme 25).36... 

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