Iron‐Promoted Domino Annulation of α‐Enolic Dithioesters with Ninhydrin under Solvent‐Free Conditions: Chemoselective Direct Access to Indeno[1,2‐ b ]thiophenes

An efficient, straightforward, and highly chemoselective strategy has been devised for the synthesis of a broad range of indeno[1,2‐ b ]thiophenes through the annulation of α‐enolic dithioesters with ninhydrin under solvent‐free conditions. The advantages of this nucleophilic domino substitution/cyclization sequence are highlighted by the use of inexpensive and readily available FeCl 3 · 6H 2 O as the catalyst, the operational simplicity, a short reaction time, its functional‐group tolerance, and the concomitant formation of the C–C and C–S bonds to give a thiophene ring in a single operation. The method is economic with regard to the number of steps as well as to being a one‐pot carbon‐efficient process that is devoid of toxic reagents and solvents. Importantly, one of the newly synthesized indeno[1,2‐ b ]thiophene compounds exhibited a high selectivity and sensitivity towards the Fe 3+ ion over other metal ions.