Development of Ferrocene-Based Planar Chiral Imidazopyridinium Salts for Catalysis

Imidazol-2-ylidenes and their N,N′-diaryl derivatives constitute an important class of N-heterocyclic carbenes (NHCs) which possess a unique level of versatility in both organocatalysis and transition-metal catalysis. However, there remain few examples of carbenes of this type that are chiral. Here we describe the development of a family of closely related imidazopyridine-based NHCs that derive their asymmetry purely from planar chirality through the incorporation of the classic metallocene framework. An NMR analysis of ⁷⁷Se complexes of this family is used to assess their degrees of σ donicity and π acidity in comparison to a larger family of NHCs. Percent buried volume (%V_bur) calculations were performed to measure the steric bulk of this distinct class of NHCs. Additionally, these NHCs were employed in various enantioselective transformations to survey their use as organocatalysts as well as ligands for transition-metal-catalyzed processes.