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

摘要

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.