Palladium-Catalyzed Direct One-Pot Synthesis of Carbazoles

Significance

An efficient catalytic system for the one-pot synthesis of carbazoles by N-arylation and oxidative coupling is reported. The N-arylation was carried out under standard conditions using triflates as leaving groups, since the use of halides effectively poisons the palladium catalyst in subsequent oxidative cyclization step. To effect this step, acetic acid was added as a co-solvent, because the common N-arylation solvents such as toluene alone were shown to be ineffective. Oxygen atmosphere (1 atm) was used, although air was shown to effective as well affording similar yields, albeit under longer reaction times (R^¹ = Me, R^² = H with O₂: 9 h, 69% yield; with air: 17 h, 67% yield). The substrate scope was well studied and the reaction was shown to tolerate both EWGs and EDGs, although poor yields with ortho substituents were observed and the reaction failed completely when R^¹ = R^² = CO₂Me (however, the N-arylation product was obtained in 89% yield).