Reduction of Nitroarenes into Aryl Amines and N-Aryl hydroxylamines via Activation of NaBH(4) and Ammonia-Borane Complexes by Ag/TiO(2) Catalyst

In this study, we report the fabrication of mesoporous assemblies of silver and TiO(2) nanoparticles (Ag/MTA) and demonstrate their catalytic efficiency for the selective reduction of nitroarenes. The Ag/TiO(2) assemblies, which show large surface areas (119–128 m(2)·g(−1)) and narrow-sized mesopores (ca. 7.1–7.4 nm), perform as highly active catalysts for the reduction of nitroarenes, giving the corresponding aryl amines and N-aryl hydroxylamines with NaBH(4) and ammonia-borane (NH(3)BH(3)), respectively, in moderate to high yields, even in large scale reactions (up to 5 mmol). Kinetic studies indicate that nitroarenes substituted with electron-withdrawing groups reduced faster than those with electron-donating groups. The measured positive ρ values from the formal Hammett-type kinetic analysis of X-substituted nitroarenes are consistent with the proposed mechanism that include the formation of possible [Ag]-H hybrid species, which are responsible for the reduction process. Because of the high observed chemo selectivities and the clean reaction processes, the present catalytic systems, i.e., Ag/MTA-NaBH(4) and Ag/MTA-NH(3)BH(3), show promise for the efficient synthesis of aryl amines and N-aryl hydroxylamines at industrial levels.