Rhodium‐Catalyzed C6/C7 Annulation of Indoles With Internal Alkynes: Synthesis and Fluorescent Properties of Benzo[ g ]indoles

A novel rhodium‐catalyzed C6/C7 annulation of N ‐acetyl‐indoles with alkynes is reported for the construction of benzo[ g ]indoles. The synthetic protocol uses the readily available monophenylcyclopentadienyl rhodium complex [Cp ^Ph RhCl ₂ ] ₂ as a catalyst and Cu(OAc) ₂ as an oxidant. The reaction proceeds smoothly via double C–H activation under chelation assistance by the acetyl group. It tolerates various functional groups in both indole and diarylacetylene coupling partners, such as fluoro, chloro, methyl, and methoxy. One‐pot synthesis of benzo[ g ]indoles from commercially available acetanilides can be achieved by increasing alkyne and catalyst loading. The obtained N ‐acetyl‐benzo[ g ]indoles emit fluorescence in the violet‐blue region (420–465 nm) with quantum yields of up to 33%. Deacetylation of the nitrogen atom results in an increase of the emission quantum yield to 46%.