Diverting o-Borylaryl Triflate Reagents from an Aryne Mechanism to a Tandem Cross-Coupling Catalysis Pathway to Form Unsym-metrical Carbazoles

Cross-coupling has been used for decades to construct key C–C and C–heteroatom bonds. Generally, it is used to construct a single bond in a single synthetic step. Our group and others have utilized o-borylaryl triflates as aryne precursors to build molecular complexity by forming two bonds in a single synthetic step. We hypothesized that these same o-borylaryl triflates precursors could be used as tandem cross-coupling reagents if we could control their reaction mechanism. By utilizing a hal-ide coupling partner, we were able to leverage the differences in mechanism and rate of oxidative addition for an aryl halide versus an aryl (pseudo)halide using ligand control. Carbazoles are key scaffolds found in natural products, pharmaceuticals, and materials and thus were the chosen target. Twenty-seven carbazoles were synthesized and multiple mechanistic studies reveal that the substituent on the coupling partner impacts the rate and yield of the reaction. We report findings that allow for strategic choices in the incorporation of substituents into coupling partners to maximize reaction efficiency in the generation of unsymmetrical carbazoles.