Large yet Flexible N‐Heterocyclic Carbene Ligands for Palladium Catalysis

Abstract A straightforward and scalable eight‐step synthesis of new N‐heterocyclic carbenes (NHCs) has been developed from inexpensive and readily available 2‐nitro‐ m ‐xylene. This process allows for the preparation of a novel class of NHCs coined ITent (“Tent” for “tentacular”) of which the well‐known IMes ( N , N ′‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene), IPr ( N , N ′‐bis(2,6‐di(2‐propyl)phenyl)imidazol‐2‐ylidene) and IPent ( N , N ′‐bis(2,6‐di(3‐pentyl)phenyl)imidazol‐2‐ylidene) NHCs are the simplest and already known congeners. The synthetic route was successfully used for the preparation of three members of the ITent family: IPent ( N , N ′‐bis(2,6‐di(3‐pentyl)phenyl)imidazol‐2‐ylidene), IHept ( N , N ′‐bis(2,6‐di(4‐heptyl)phenyl)imidazol‐2‐ylidene) and INon ( N , N ′‐bis(2,6‐di(5‐nonyl)phenyl)imidazol‐2‐ylidene). The electronic and steric properties of each NHC were studied through the preparation of both nickel and palladium complexes. Finally the effect of these new ITent ligands in Pd‐catalyzed Suzuki–Miyaura and Buchwald–Hartwig cross‐couplings was investigated.