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-alpha] 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.