Bismuth nitrate-promoted disproportionative condensation of indoles with cyclohexanone: a new-type azafulvenium reactivity of indole

The indole nucleus is a privileged heterocyclic scaffold, which is present in several natural and synthetic compounds. New synthetic strategies to access functionalized indoles have therefore attracted the attention of both synthetic and medicinal chemists over the decades. In this study, the bismuth nitrate-promoted disproportionative condensation of indoles with cyclohexanone in one pot, to yield C3-cyclohexyl substituted indoles and 1,3-di(1H-indol-3-yl)benzene derivatives is reported for the first time. Using 3-methylindole with cyclopentanone, cyclohexanone, and cycloheptanone results in new and different synthetic pathways. The plausible reaction mechanisms are presented and supported with DFT (M06-2X/6-31+G(d,p)) calculations.