Synthesis of 2-Sulfenylindoles via Palladium-Catalyzed Thiolative Annulation of Ynamides

The Pd(0)-catalyzed heteroannulation of ynamides 1 in the presence of dialkylsulfides 4 acting as nucleophiles delivers 2-sulfenylindoles 5 in up to 85% yield. The underlying reaction cascade is proposed to involve formation of a σ,π-chelated palladium species A being transformed via the addition of dialkylsulfide to the ynamide triple bond into palladacycle B displaying a sulfonium moiety. Subsequent dealkylation of the latter by in situ-liberated iodide provides 2-alkylthioindoles 5. 2-Sulfenylindoles 5 are readily de-N-tosylated with TBAF in THF to afford the parent 2-sulfenyl-1H-indoles.