N-heterocyclic carbene-based ruthenium-catalyzed direct amidation of aldehydes with amines

The direct amide synthesis from alcohols and amines is a highly environmentally-benign and atom economic process. In situ generated NHC-based Ru halide catalytic systems are active for the amidation of alcohols with amines. However, these systems show poor activities in the amidation of aldehydes with amines, though aldehydes are the proposed reaction intermediates formed by dehydrogenation of alcohols; imines are obtained as major byproducts in this case. In this study, an improved method was developed by incorporating the idea of using a hemiaminal intermediate as the precatalyst activator, for the direct amide synthesis from aldehydes and amines using a commercially available Ru complex [Ru(p-cymene)Cl-2](2), an N-heterocyclic carbene ligand, a base, and pyridine. Using this method, various amides were synthesized from several aldehydes and amines.