Nitro Promoters for Selectivity Control in the Core Hydrogenation of Toluidines: Controlling Adsorption on Catalyst Surfaces

Abstract

The diastereoselectivity in the hydrogenation of multi‐substituted aromatic ring systems was used as a sensitive tool to gain molecular‐level insight into the reaction pathways on the surface of ruthenium. Here, the hydrogenation of para‐, meta‐, and ortho‐toluidine was studied in detail. The chemoselectivity towards primary or secondary amines depends primarily on the choice of the metal; notably, nitro compounds such as NaNO2 are highly effective promoters for ruthenium catalysts, which significantly increase the chemoselectivity towards primary amines as well as catalytic activity. Lower activation energy and altered diastereoselectivity indicate that in the presence of nitro additives, the substrate molecules are adsorbed in a coordination mode favourable for the hydrogenation of the aromatic ring.

Enhanced Selectivity in the Hydrogenation of Anilines to Cyclo‐aliphatic Primary Amines over Lithium‐Modified Ru/CNT Catalysts

“…Ruthenium and rhodium were envisioned to be the most suitable metals for the hydrogenation of aromatic amines, based on the successful use of these two metals in related hydrogenation reactions . Carbon nanotubes (CNT) were chosen as the support, as they are generally inert materials, and specific modification, such as directed oxidation of surface groups, results in well‐defined and predictable functional groups . Moreover, the latter act as suitable anchoring sites for metal nanoparticles .…”

“…Note that for formation of the thermodynamically more stable trans product, the hydrogen atoms have to approach the aromatic ring from one as well as the opposite side. To allow for this, the imine formed as intermediate, or another partially hydrogenated intermediate, has to flip over, while being adsorbed on the catalyst surface, or desorb and re‐adsorb with the opposite face . The cis product, on the other hand, is formed when all hydrogen atoms approach the aromatic ring from the same side.…”

“…Consequently, all acidic groups should be removed by treatment of the parent catalysts with LiOH. In the hydrogenation of toluidines over Ru/C, however, the use of LiOH as an additive was reported to improve the selectivity only slightly compared to the reaction in the absence of LiOH . In that case, isopropanol had been used as solvent.…”

“…With most catalysts, the formation of the corresponding diamine predominates as side reaction in the hydrogenation of anilines. In a recent study, we investigated the effect of nitrate additives on the hydrogenation of toluidines . In the presence of nitrates, catalyst activity and the selectivity toward the primary amine were increased significantly.…”

Molecular Assembly Line: Stepwise Hydrogenation of Multifunctional Substrates over Catalyst Mixtures

“…Closer inspection of the time–concentration profiles reveals that also the rate of aniline hydrogenation increased in the presence of Pt/CNT compared that for the parent Ru/CNT catalyst. This is readily explained by the formation of hydrogen‐bonded aggregates between nitro groups and amine groups . This specific interaction enables the aromatic ring of aniline to adsorb at the metal surface in the presence of other coordinating groups leading to accelerated hydrogenation.…”