Synthesis of Unsymmetrical Carbazoles: A Mechanistic Cautionary Tale When Using ortho -Borylaryl Triflates in Catalysis

Carbazoles are desirable products with ample usage in the fields of pharmaceutical and materials chemistry. Usual methods for their synthesis often focus on the formation of a single C–C or C–N bond from complex diphenylamine- or biphenyl-derived substrates—i.e., multiple reaction steps are necessary for obtaining the desired carbazole product. Furthermore, regioselectivity issues arise when unsymmetrically substituted phenyl rings are involved in the reaction. We report the use of o-borylaryl triflates in a single-step palladium-catalyzed annulation reaction to yield carbazoles. Twenty-six carbazoles were obtained in up to 99% yield. In addition, this report includes an unexplored tandem cross-coupling reactivity mode for o-borylaryl triflates, which is in contrast to previous reports that invoke them as aryne precursors. Utilizing Hammett parameters as a rationale, we developed a predictive approach to optimize the placement of substituents on the starting materials to maximize yields.