Formation of benzo[c]thiophen-1-aminium iodide by the reaction of o-alkynylbenzothioamide with iodine

Reaction of o-alkynylbenzothioamide with molecular iodine was investigated. Unique benzo[c]thiophen-1-aminium iodide was obtained when the tertiary thioamide was used as a reactant. The reaction proceeded efficiently in less polar solvent, whereas the corresponding product was also obtained in a polar solvent. Single crystal X-ray analysis revealed that the benzo[c]thiophen-1-aminium structure possesses a resonance structure in N–C–S bond. Its stabilization shows a clear difference in the result from the reaction of amide analogue with iodine to give no stable compound.