Solvent-free, [Et3NH][HSO4] catalyzed facile synthesis of hydrazone derivatives

Abstract

In the present study, a library of hydrazone analogues 2(a–j) and 4(a–e) were synthesized, which were typically accessed via a solvent-free facile nucleophilic addition between hydrazine hydrate and appropriately substituted aromatic aldehydes 1(a–j) and 3-formylchromones 3(a–e). The molecular structure of compound (2f) was well supported by single crystal X-ray crystallographic analysis and also verified by DFT calculations. This new synthetic, eco-friendly, sustainable protocol resulted in a remarkable improvement in the synthetic efficiency (90–98% yield), high purity, using [Et₃NH][HSO₄] as a catalyst and an environmentally benign solvent eliminating the need for a volatile organic solvent and additional catalyst. This ionic liquid is air and water stable and easy to prepare from cheap amine and acid. The present methodology is a green protocol offering several advantages such as, excellent yield of products, minimizing production of chemical wastes, shorter reaction profile, mild reaction conditions, simple operational procedure, easy preparation of catalyst and its recyclability up to five cycles without any appreciable loss in catalytic activity. The optimization conditions carried out in the present study revealed that 20 mol% of ionic liquid catalyst under solvent-free condition at 120 °C are the best conditions for the synthesis of hydrazone derivatives in excellent yields.