Synergistic Effects of Brønsted Acid Site and Lewis Acid Site of Nb2O5 on the Direct Amination of Alcohols

Abstract Direct amination of alcohols is an attractive synthetic method of secondary amines as water is generated as the sole byproduct. Solid acid catalysts have been developed with the advantages of their high stability and easily tunable acid nature. Herein, we report the synergistic effects of the Brønsted and Lewis acid sites of Nb2O5 on the amination of alcohols. Calcination of Nb2O5 altered the ratio of Brønsted acid sites to Lewis acid sites along with the removal of water, promoting the direct amination of alcohols. Nb2O5 calcined at 573 K (Nb2O5-573) acted as a reusable heterogeneous catalyst for the direct amination of alcohols to give the corresponding secondary amines in high yields. The formation rate per acid site of Nb2O5-573 was 21.7 h–1, which is higher than those of the previously reported solid acid catalysts (0.4–8.5 h–1). A variety of alcohols and aromatic amines were applicable with this catalytic system to afford the corresponding secondary amines in moderate to high yields. Catalyst characterizations and control experiments using chiral alcohols demonstrated that the direct amination of alcohols over Nb2O5 mainly proceeded through an SN2-type nucleophilic substitution mechanism, whereas an SN1-type nucleophilic substitution mechanism has been proposed for the previously reported solid acid catalysts. Both Brønsted and Lewis acid sites interact with the hydroxy group of alcohols to enhance the leaving ability of the hydroxy group, resulting in the improvement of nucleophilic substitution by amines.