A Labile arid Catalytically Active Imidazol-2-yl Fragment System

摘要

N-heterocyclic carbenes (NHCs) and their complexes are excellent catalysts for a broad array of organic transformations, where the NHC ligands impart useful electronic and steric properties to metal centers. In these systems, with commonly used ancilliary NHC ligands that are substituted at nitrogen atom(s) by alkyl, aryl, or other groups, all catalytic transformations take place at the metal center, which is stabilized and/or activated by the NHC ligand. However, transformations that may possibly involve both the metal center and at one ring nitrogen of the NHC ligand are much less common, and are limited to protic NHC complexes or their conjugated bases. Thus, the N—H function of a protic NHC complex (A or D; Scheme 1) could behave as Br0nsted acid, whereas the basic nitrogen atom in the imidazol-2-yl complex (B or C) may behave as a Br0nsted base. Moreover, reactivity of A with a base could lead to B, a transient species with both a vacant metal coordination site and a basic nitrogen atom, which could bind (C) and activate a substrate (C to D). These reactivity patterns might also be compatible with protic NHC complexes derived from other NHC ligands.