Bulky α-diimine palladium complexes: highly efficient for direct C-H bond arylation of heteroarenes under aerobic conditions.

Through the strategy to enhance the bulkiness on both the backbone and the N-aryl moieties, we designed and synthesized a type of bulky α-diimine palladium complex (i.e., {[Ar-N[double bond, length as m-dash]C(R)-C(R)[double bond, length as m-dash]N-Ar]PdCl2, (Ar = 2-benzhydryl-4,6-dimethylphenyl)}, C1, R = H; C2, R = An; C3, R = Ph). The structures of these palladium complexes were well characterized, while C1 and C3 were further characterized by X-ray diffraction. The catalytic performances of the precatalysts were screened for direct C-H bond arylation of heteroarenes. The bidentate N,N-palladium complex C3 with both a backbone and N-aryl bulkiness was found to be a highly efficient precatalyst under aerobic conditions. With a low palladium loading of 0.5-0.1 mol%, a variety of heteroarenes with challenging bulky steric aryl bromides as well as heteroaryl bromides are all applicable for this cross-coupling reaction.