Development of Safe One-Pot Synthesis of <i>N</i>-1- and <i>C</i>-2-Substituted Benzimidazole via Reductive Cyclization of <i>o</i>-Nitroarylamine Using Na<sub>2</sub>S<sub>2</sub>O<sub>4</sub>

We report that the reductive cyclization of <i>o</i>-nitroarylamine with aldehyde using sodium dithionite (Na<sub>2</sub>S<sub>2</sub>O<sub>4</sub>) could be accelerated by addition of H<sub>2</sub>O, which made it possible to control the heat release of the reaction by semibatch-type operation. Safety evaluation was performed using DSC, ARSST, in situ IR analysis, and Multimax.