Sequential Nitration/Hydrogenation Protocol for the Synthesis of Triaminophloroglucinol: Safe Generation and Use of an Explosive Intermediate under Continuous-Flow Conditions

Abstract

A continuous-flow process for the synthesis of triaminophloroglucinol has been developed. The synthetic procedure is based on a sequential nitration/reduction protocol which uses phloroglucinol as an inexpensive substrate. During the initial exothermic nitration step employing a combination of ammonium nitrate and sulfuric acid, the temperature was controlled through the enhanced heat transfer derived from the high surface-to-volume ratio of the utilized capillary tubing. Clogging of the tubing due to precipitation of trinitrophloroglucinol (TNPG) was avoided by immersing the tubular reactor in an ultrasound bath during the process. The nitration mixture was diluted with water and immediately subjected to catalytic hydrogenation of the nitro groups using a commercially available continuous-flow reactor and PtO2 as heterogeneous catalyst, thus avoiding the isolation of the highly unstable and explosive TNPG intermediate.