Journal
ACS CATALYSIS
Volume 4, Issue 10, Pages 3504-3511Publisher
AMER CHEMICAL SOC
DOI: 10.1021/cs500379u
Keywords
gold nanoparticles; heterogeneous catalysis; nitro compounds; selective reduction; kinetic isotope effects; Hammett kinetics
Categories
Funding
- European Union
- Greek Ministry of Education [ERC-09, MESOPOROUS-NPs, ARISTEIA-2691]
- Aristotle University of Thessaloniki Research Committee [KA 89309]
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Herein, we show that mesoporous titania-supported gold nanoparticle assemblies (Au/MTA) catalyze the activation of NaBH4 and 1,1,3,3-tetramethyl disiloxane (TMDS) compounds, which act as transfer hydrogenation agents for the reduction of nitroarenes to the corresponding anilines in moderate to high yields. On the other hand, nitroalkanes are reduced to the corresponding diazo and hydrazo compounds under the studied conditions. The substantial measured primary kinetic isotope effects found here suggested that B-H bond cleavage occurs in a rate-determining step and [Au]-H active hybrids are formed, which are responsible for the reduction of nitroarenes to the corresponding amines. Formal Hammett-type kinetic analysis of a range of para-X-substituted nitroarenes lends support to this hypothesis. Nitro compounds substituted with electron-withdrawing groups were reduced faster than the corresponding compounds with electron-donating groups. The presence of water enhanced the catalytic activity of Au/MTA in aprotic solvents. Nuclear magnetic resonance studies support the formation of the corresponding hydroxylamines as the only intermediate products. On the basis of the high observed chemoselectivities and the fast and clean reaction processes, these catalytic systems, i.e., Au/MTA-NaBH4 and Au/MTA-TMDS, show promise for the efficient synthesis of aromatic amines at industrial levels.
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