3- Indolyl sulfides

Desulfurization of 3-indolyl sulfides. Nonreductive removal of thio groups uses CF3COOH to protonate the 3-indolyl sulfides, thus allowing thiophilic attack by a thiol reagent (e.g., o-mercaptobenzoic acid). The corresponding 3-unsubstituted indoles are produced. 

Indolyl sulfides 1 are easily prepared by sulfenylation of indoles with sulfenyl ehlorides. The mechanism of the sulfenylation is well known and follows the usual SfiAr-type substitution pathway depicted in Scheme 2.1. 

Novel heterocycles. Some novel heterocycles have been prepared by de-oxygenation of indolyl o-nitrophenyl sulfides, prepared by reaction of indoles with 2-nitrobenzenesulfenyl chloride. The reaction involves intermediate spirocycUc compounds. 

One drawback to the use of sulfenyl chlorides for the sulfenylation of indoles is their exeeptional reactivity. If the 2-position of the indole is unoccupied, the slightest excess of reagent leads to a seeond sulfenylation, and a full seeond equivalent leads to excellent yields of 2,3-indolyl bis-sulfides 2 (Seheme 2.2). 

If quatemization of 6 did oeeur, the rearrangement would also proceed in the presence of alkylating agents like methyl iodide, benzyl bromide or methyl trifluoro-methanesulfonate. Alkyl halides would lead to A-alkylated derivatives 15, which after rearrangement in the medium and aqueous workup would yield the 2,3-indolyl bis-sulfide 7 (Seheme 2.10). 

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