A nickel half-sandwich complex supported by our original NHC ligand was developed as a robust precatalyst for Suzuki–Miyaura cross-coupling. The addition of PPh3 was a crucial element in the suppression of side reactions and in accelerating the cross-coupling reaction. By employing the optimal conditions, aryl–aryl, heteroaryl–aryl, and heteroaryl–heteroaryl couplings were achieved.

•Our original NHC–Ni complex aptly catalyzes a tert-butylation of aryl bromides.•n-BuLi works as a reductant of Ni(II) salts to generate the active species.•NMP is an effective co-solvent to yield the tert-butylated products.•Hindered substrates are also applicable to the coupling reaction we established.In this study, nickel-catalyzed coupling reactions between arylhalides and tert-alkyl Grignard reagents were developed. Our original bicyclic NHC ligands reduced the formation of isomerized products, and we found that NMP as a co-solvent suppressed the reduction process. Under the optimal conditions we developed, the catalyst loading was lowered to 0.5 mol %, and catalyst loading using ortho-substituted aryl bromides was also applicable at the level of 2.0 mol %.

Imidazol-2-ylidene and imidazolin-2-ylidene are frequently employed as the core structures of N-heterocyclic carbene (NHC) ligands, because these have two nitrogen sites adjacent to carbenic carbons. The electronic and steric effects of N-substituents to a bound metal have been aptly studied, and bulky substituents have commonly been introduced on nitrogen(s). However, too much bulkiness can sometimes result in an ineffectiveness of the hindered substrates. In this study, we thoroughly compared our original bicyclic NHC, which has bulky substituents on its noncarbenic carbons, with ubiquitous NHC ligands to probe its steric properties. A well-defined bicyclic NHC–CuCl complex was successfully synthesized and applied to the copper-catalyzed borylations of aryl halides at ambient temperature. A bicyclic NHC–CuCl proved to be a better catalyst than commonly used NHC ligand–Cu complexes, particularly for sterically hindered substrates, which suggested that the bicyclic NHC ligand offered a bulky but accessible environment to the bound copper.