IBzH (IBenzhydryl): Sterically-Flexible -Aliphatic -Heterocyclic Carbenes (NHCs) for Iron-Catalyzed C(sp)-C(sp) Cross-Coupling of Unactivated Haloalkanes.

Iron-catalyzed cross-coupling has emerged as a pivotal concept for the synthesis of valuable products across various facets of chemical research, including pharmaceuticals, organic materials and biological probes. In this respect, the use of -heterocyclic carbenes (NHCs) as ancillary ligands to iron has been particularly powerful. However, the major limitation is that the successful iron-catalytic systems have been almost exclusively limited to -aryl--heterocyclic carbenes, which significantly restricts future developments of this commanding catalysis platform. Herein, we report IBzH (IBenzhydryl), a class of -heterocyclic carbenes that are based on benzhydryl substitution of the imidazole ring. We demonstrate that this -alkyl yet sterically-flexible ligand class promote the challenging C(sp)-C(sp) iron-catalyzed cross-coupling of unactivated haloalkanes, superseding the performance of other NHC ligands. Alkyl-alkyl cross-coupling is also described. Large scale synthesis and the evaluation of steric and electronic properties is presented. Considering the major advantages of sterically-flexible -heterocyclic carbenes, we anticipate that this class of -alkyl NHC ligands will find broad application.