Rhodium(III)-Catalyzed Oxidative Coupling of N-Phenylindole-3-carboxylic Acids with Alkenes and Alkynes via C4–H and C2–H/C2′–H Bond Cleavage

The rhodium(III)-catalyzed direct alkenylation of N-phenylindole-3-carboxylic acids with alkenes including acrylate ester, acrylamide, and acrylonitrile proceeds smoothly at the C4-position through regioselective C–H bond cleavage directed by the carboxyl group. In marked contrast, the indole substrates react with diarylacetylenes accompanied by cleavage of the C2–H and C2′–H bonds and decarboxylation to produce 5,6-diarylindolo[1,2-a]quinolone derivatives. DFT calculations have suggested that the smooth insertion of an alkene to a C4-rhodated six-membered metallacycle intermediate leads to the C4 alkenylated products, while the latter annulation at the C2- and C2′-positions is attributable to facile reductive elimination in the corresponding seven-membered metallacycles formed by the double C–H bond cleavage and alkyne insertion.