Three cycles in the dioxomolybdenum-catalyzed reduction of nitrobenzenes to anilines with pinacol. A computational study towards the valorization of biomass subproducts.

In this work, we use density functional theory to unravel the mechanism of the nitrobenzene to aniline reduction, catalyzed by dioxomolybdenum (VI) dichloride. The use of pinacol as an oxoaccepting reagent and the production of only acetone and water as byproducts, signals a novel and environmentally friendly way to add value to the oxygen-rich biomass-derived polyols. The reaction proceeds through three consecutive cycles, each one responsible for one of the three reductive steps needed to yield nitroaniline from nitrobenzene, with nitrosobenzene and benzylnitrene as intermediates. Each cycle regenerates the Mo(VI) catalyst and releases two acetone molecules. The mechanism involves singlet/triplet state crossings, a feature that has been found to be key in related polyoxomolibdate catalyzed processes. The role of the Mo-coordinated water, product of the reduction of pinacol, as the provider of the mysterious protons needed to reduce the nitro group, was revealed. The disclosure of this challenging mechanism and its rate limiting step can contribute to the design of more effective Mo(VI) catalysts.