Construction of Carbazoles via Pd-Catalyzed Oxidative Coupling of Alkynes

Significance

The previously reported reaction of benzoic acids with alkynes catalyzed by iridium or rhodium to yield naphthalenes or isocoumarins, respectively, guided the present research: The ­oxidative coupling of heterocycles or heterocyclic carboxylic acids with alkynes. In the case of indoles, the reaction proceeds under palladium catalysis to yield tetrasubstituted N-protected carbazoles in generally good yields. Lower yields were obtained when R^² = alkyl or EWG aryl substituents. Thorough optimization and substrate scope studies were carried out. Other hetero­cyclic systems besides indole-3-carboxylic acid were shown to undergo successful coupling reactions. Generally poor yields were observed for R^¹ = H rather than R^¹ = CO₂H.