Metal triflimidates: better than metal triflates as catalysts in organic synthesis--the effect of a highly delocalized counteranion.

The continuously increasing need for novel and selective methods in organic synthesis to aid drug discovery and to address environmental concerns is a constant source of stimulation to develop novel and more efficient reaction systems. This has often resulted in a focus on transition metals, ligands, and additives, with much less attention paid to the counterion(s) of the metal cation. Recently, metal salts with one or more triflimidate counterion(s) have appeared as a unique class of catalysts that display outstanding σ- and π-Lewis acid character. The highly delocalized nature of the triflimidate counterion, combined with its high steric hindrance results in virtually no nucleophilic behavior and an extremely high positive charge density on the metal cation, thus enhancing its Lewis acid character. Consequently, these metal triflimidates often outperform their metal halide or triflate analogues. This Review describes general methods for the preparation of metal triflimidate salts and their use as catalysts.