Direct Ortho Arylation of 9-(Pyridin-2-yl)-9H-carbazoles Bearing a Removable Directing Group via Palladium(II)-Catalyzed C–H Bond Activation

A one-pot synthesis of ortho-arylated 9-(pyridin-2-yl)-9H-carbazoles via C–H bond activation is presented. Silver nitrate and tert-butyl alcohol were found to be the best oxidant and solvent for the process, respectively. The product yields are from modest to excellent, and the reaction showed sufficient functional group tolerance. p-Benzoquinone served as an important ligand for the transmetalation and reductive elimination steps in the catalytic process. The key intermediate of the reaction, a 9-(pyridin-2-yl)-9H-carbazole palladacycle, was isolated, and its structure was unequivocally confirmed by X-ray crystallography. No kinetic isotope effect (kH/kD = 1.00 ± 0.17) for the C–H bond activation step was observed. In addition, a Hammett experiment gave a negative ρ value, −2.16 ± 0.02. The directing group, pyridinyl, was demonstrated to be a removable functional group. Finally, a rational catalytic mechanism is presented on the basis of all experimental evidence.