Diiminium Nucleophile Adducts are Stable and Convenient Lewis Superacids

Abstract

Strong Lewis acids are essential tools for a manifold of chemical procedures that aim to react weakly basic centres but their scalable deployment is severely limited by their costs and safety concerns. We report that dicationic relatives of guanidinium can be conveniently synthesised in a two-step one-pot procedure from tetramethylurea. Triflic anhydride is used to generate an isouronium intermediate. Substitution of the bound triflate with pyridines yield the dicationic tetramethyldiiminium bis(triflate) nucleophile adducts (TMDINu). Their proposed diiminium character is demonstrated by substituting pyridine from the corresponding adduct with other nucleophiles. The observation of a chelation effect in the 2,2’-bipyridine adducts supports Lewis acidic character of the diiminium -system and flexibility towards accepting another bond. High fluoride, hydride, and oxide affinities are demonstrated, leading to their classification as soft and hard Lewis superacids. An example reaction is reported which shows that the tetramethyldiiminium bis(triflate) pyridine complex (TMDIPy) is more effective than conventional reagents in the activation of electron-poor amines for amide couplings.