Large yet Flexible N-Heterocyclic Carbene Ligands for Palladium Catalysis

A straightforward and scalable eight-step synthesis of new N-het-
ACHTUNGTRENUNGeroACHTUNGTRENUNGcyclic 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-tri-
methylphenyl)imidazol-2-ylidene), IPr (N,N’-bis(2,6-i(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.