A Heterogeneous Niobium(V) Oxide Catalyst for the Direct Amidation of Esters

This study reports the first example of a heterogeneous catalytic system for the direct amidation of various esters with amines. Of 25 types of catalyst, Nb 2 O 5 shows the highest activity in the amidation of methyl benzoate with aniline. Nb 2 O 5 gives high yields in the amidation of various esters and amines under solvent‐free conditions, is reusable, and shows higher turnover numbers than previously reported homogeneous catalysts such as La(OTf) 3 . IR spectroscopic studies of ethyl acetate adsorbed on the catalysts show a strong acid–base interaction between the Nb 5+ Lewis acid site and carbonyl oxygen, which can result in high reactivity of the ester with a nucleophile (amine) and, thus, high activity of Nb 2 O 5 . Kinetic results show that the activity of Nb 2 O 5 does not markedly decrease with increasing aniline concentration, in contrast to reference catalysts TiO 2 and La(OTf) 3 . The relatively low negative impact of basic molecules on the Lewis acid catalysis of Nb 2 O 5 also enables its high activity.