Ammonium niobium oxalate (ANO) as an efficient catalyst in the Paal–Knorr synthesis of N-substituted pyrroles

Abstract

Ammonium niobium oxalate (ANO) has been proven to be an outstanding cheap, low-toxicity and readily available Lewis acid catalyst for carbonyl activation reactions. Herein, we describe the use of ANO as a catalyst in the Paal–Knorr pyrrole synthesis. The reactions were conducted under mild conditions, not requiring heating or special apparatus, allowing the synthesis of the desired products in up to 99% yield in just 30 min. Additionally, ANO could be reusable up to seven times, without remarkable loss of efficiency. The batch protocol was successfully translated to a continuous-flow environment, with 96% conversion of the starting dione and aniline to the desired pyrrole in 2 min of residence time (a STY of 2.5 g h⁻¹). The main protocol's features meet important green chemistry bases, making it a sustainable method to prepare N-substituted pyrrole derivatives.