Effective N-Heterocyclic Carbenes with Indole Moiety Applied in the Pd-Catalyzed Synthesis of Sterically Hindered Biaryls

Indolyl-based benzimidazolium salts have been designed and prepared using three easy steps, namely indole alkylation, iodine-catalyzed C-N bond coupling, and substitution. These salts have high catalytic performance in Pd-catalyzed sterically hindered biaryl synthesis. The variability of the salt scaffold can be modified by the substitution of indole and alkyl halides. The Suzuki-Miyaura cross-coupling (SMC) of aryl chlorides was performed with satisfactory functional group tolerance and biaryls were obtained in 42-87% yield at 90 degrees C by using 0.5 mol% of Pd(OAc)(2)/1 i as the catalyst. Our results highlight the importance of maintaining a balance between the length of the carbon chain of substituents on the indole moiety and the steric congestion of substituents on the indole moiety for catalytic activity; this can be beneficial in difficult coupling processes. In addition, an asymmetric SMC reaction of aryl chloride under optimal reaction conditions was achieved with an 8% ee preliminarily.