A two-step continuous flow synthesis of 4-nitropyridine

Abstract

4-Nitropyridine, a key intermediate in medicinal products, was successfully prepared from pyridine N-oxide in a two-step -approach. Pyridine N-oxide was nitrated with HNO3 and H2SO4 to give 4-nitropyridine N-oxide, followed by reaction with PCl3 to give the final product. The continuous flow methodology was used to minimise accumulation of the highly energetic and potentially explosive nitration product to enable the safe scale-up of 4-nitropyridine with no 2-nitropyridine by-product. By employing continuous extraction in the nitration step and applying the optimised conditions, a throughput of 0.716 kg 4-nitropyridine product per day from pyridine N-oxide with 83% yield and high selectivity in a continuous flow system was achieved.