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.
and catalysis. In general, aromatic ketones constitute key players in this type of catalysis as they are involved in a plethora of examples in literature. A prime example of these...
Covering: up to 2015Sesquiterpenoids are consistently attracting the interest of the scientific community due to their promising clinical profile as therapeutic agents. Cycloisomerization of enynes and dienes is a powerful tool in the hands of organic chemists to access them. In the last 20 years the field has witnessed remarkable advances, especially by revealing the capability of platinum and gold complexes to initiate such reactions. Nowadays, cycloisomerizations continue to enrich our knowledge with atom-economical routes and impressive cascades to reach more complex molecules. The current review covers the basic mechanistic aspects of metal catalysis in cycloisomerization reactions and their progress to the synthesis of selected complex sesquiterpenoids.
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