A Bicyclic N‑Heterocyclic Carbene as a Bulky but Accessible Ligand: Application to the Copper-Catalyzed Borylations of Aryl Halides

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.