Sulfur-stabilized cations

Pummerer rearrangement

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Initially chlorination on the nucleophilic sulfur produces a chlorosulfonium salt. Deprotonation next to the positively charged sulfur leads to elimination of chloride producing a new sulfonium salt. Nucleophilic addition of chloride to the double bond completes the formation of the alpha-chloro sulfide.

The overall result is oxidation of the position next to sulfur and the use of (trifluoro)acetic anhydride on sulfoxides produces O,S-acetals by a similar mechanism.

S. K. Bur and A. Padwa, Chem. Rev., 2004, 104, 2401–2432.