RhCl3·3H2O-Catalyzed Regioselective C(sp2)–H Alkoxycarbonylation: Efficient Synthesis of Indole- and Pyrrole-2-carboxylic Acid Esters

The C2-selective C–H alkoxycarbonylation of indoles with alcohols and CO catalyzed by RhCl3·3H2O is disclosed that offers convenient access to diverse indole-2-carboxylic esters. The rhodium-based catalysts outperformed all other precious-metal catalysts investigated. In addition, this protocal was found applicable to the synthesis of pyrrole-2-carboxylic esters, and allowed the C–H alkoxycarbonylation in an intramolecular fashion. Preliminary mechanistic studies indicate that C–H cleavage is not likely involved in the rate-determining step, and a five-membered rhodacycle might be an intermediate involved in the reaction.