Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 2 / Section I / 35 / c / ii / Solution

Sulfuryl chloride converts the aryl thiol into an electrophilic sulfur reagent, conveniently represented as $\mathrm{ArSCl}$ after chlorination and dehydrogenation. Triethylamine traps the generated acid. The pyrrole's two $\alpha$ positions are already occupied; its less deactivated remaining carbon, C4, attacks electrophilic sulfur, displacing chloride. Loss of the C4 proton restores the aromatic ring and gives the C4 arylthio-substituted pyrrole.

The net new bond is C(pyrrole)–S(aryl). Sulfur is retained from the thiol and becomes a thioether; the transformation is not sulfonylation by transfer of sulfuryl chloride's SO${}_2$ group. Possible disulfide formation during thiol chlorination does not change the electrophilic-sulfenylation logic of the final ring substitution.