Facile Assembly of Carbazolyl-Derived Phosphine Ligands and Their Applications in Palladium-Catalyzed Sterically Hindered Arylation Processes

Phosphine ligands embodying a carbazolyl motif have been found to be successful in many palladium-catalyzed biaryl syntheses and direct C–H bond arylation processes. Here, a practical scaled-up synthesis of a series of carbazolyl-derived phosphine ligands, the PhenCarPhos series, is described. The original protocol for accessing the target ligand skeleton via aromatic C–N bond formation is limited by the use of a substoichiometric amount of copper salt and diamine catalysts, which both add cost and generate purification problems (significant amounts of side products and copper residues). In order to develop a more attractive and scalable synthetic pathway, a simple nucleophilic substitution method was attempted involving simple heating of 1-bromo-2-fluorobenzene, a carbazole derivative, and KOH in DMF without inert atmosphere protection. This route enables the large-scale synthesis of the desired ligand skeletons and minimizes the association of inseparable reduction side products. Particular examples of the use of these ligands in Pd-catalyzed sterically hindered arylation processes are also shown.