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

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 which 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.