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Construction of Carbazoles via Pd-Catalyzed Oxidative Coupling of Alkynes
M. Yamashita, H. Horiguchi, K. Hirano, T. Satoh*, M. Miura*
Osaka University, Japan
20 November 2009 (online)
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 R² = 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 R¹ = H rather than R¹ = CO2H.