Iron-Catalyzed 5‑Endo-Dig Synthetic Approach to Indenes and Its Bidirectional Extension to Narrow Bandgap π‑Systems

The indene skeleton is a key structure in a variety of compounds, with applications in medicinal and materials science. Traditional syntheses often require harsh conditions or reactive intermediates due to the temporary disruption of the aromaticity of the developing indene ring. To circumvent this problem, we have investigated iron-catalyzed 5-endo-dig cyclization for the construction of the five-membered carbocycle component of indene, which does not interrupt the benzene ring’s aromaticity. A reduced iron reactive species generated by the reduction of FeCl₂ with metallic magnesium is a key reactive species that effectively cleaves the C–O bond in the starting material to generate a dormant radical-stabilized iron, which smoothly undergoes the 5-endo-dig closure of an indene ring. The synthetic conditions are so mild that we synthesized a high highest occupied molecular orbital (HOMO), narrow bandgap conjugated compound that has a HOMO level as high as −4.59 eV and an optical bandgap of 1.50 eV; hence, it is unstable even under air.