Acceptorless dehydrogenative synthesis of imines from alcohols and amines using niobium(v) oxide supported nickel as heterogeneous catalyst

Abstract

Imines are essential compounds due to their wide range of reactivity, which has led to their widespread use in organic and pharmaceutical synthesis due to their structural similarities with biologically active N-heterocyclic compounds. The conventional synthesis methods of imines suffered from numerous important drawbacks, including low atom efficiency, high temperature, time-consuming, homogeneous and high catalyst loading, and challenges in separation from reaction systems. In this study, easily recoverable and reusable Ni/Nb2O5 was used as a heterogeneous catalyst for the effective oxidative coupling of benzyl alcohol with amines to imines. Catalytic sites of Ni0 help in oxidation of alcohol to aldehyde with the loss of hydrogen, followed by the cleave C=O bond with activating carbonyl group of the aldehyde by highly active water tolerant Lewis’s acid site, Nb5+ on niobium oxide and gives coupling reaction with formation of new C=N bond. The catalysts were prepared by impregnation method, whereas, the reduction of Ni2+ was carried out by NaBH4 in the presence of ethylene glycol as a capping agent to maintain Ni0 particle size. Herein 3, 5, and 10 wt.% Ni0 were incorporated on Nb2O5 and successfully used in the reaction of benzyl alcohol with three different amines; aniline, 2-chloroaniline, and benzylamine to corresponding imines with high yields which were identified by FT-IR, GC-MS, 1H-NMR, and 13C-NMR spectroscopy. The catalyst was separated easily with centrifugation and reused several times without a significant decrease in its catalytic activity. In conclusion, we have demonstrated a simple, versatile and sustainable method of imines synthesis from less reactive and easily available alcohol instead of aldehyde using Nb2O5 supported Ni as an effective and reusable heterogeneous catalyst. Keywords: Imine, benzyl alcohol, amine, heterogeneous, nickel, niobium oxide.