Gold catalysts for the synthesis of aromatic azocompounds from nitroaromatics in one step

Abstract

One step selective hydrogenation of nitroaromatics to obtain symmetric azocompounds with high yields has been performed with a gold supported on cerium oxide catalysts. Au/TiO 2 and Au/CeO 2 catalysts direct the reaction by two different pathways and with different selectivities. In situ FTIR studies reveal that the surface concentration of the intermediate nitrosobenzene is decisive in directing the reaction trough the different reaction pathways. In this way, while on Au/TiO 2 a fast hydrogenation of the nitrosobenzene intermediate leads to a low surface concentration of the nitrosocompound, on Au/CeO 2 nitrosobenzene is more stabilized on the catalyst surface leading to a lower hydrogenation and a higher coupling rate, resulting in high selectivities to azobenzene. On Au/CeO 2 the relative weak adsorption of the azo with respect to the azoxycompound on the catalyst surface, avoids the consecutive hydrogenation of azocompounds to the corresponding anilines until all the azoxy has been consumed. Asymmetric azobenzenes have also been obtained with very high yields on TiO 2 , through the Mills reaction.

Catalytic Materials Based on Surface Coating with Poly(ethyleneimine)‐Stabilized Gold Nanoparticles

“…Thus, Corma et al. have demonstrated that the reaction proceeds through the direct route if it is catalyzed by Au‐TiO 2 , and Noschese et al. showed that the condensation route seems to prevail if the AuNPs are supported onto a nanoporous polymeric matrix .…”

Highly efficient and selective reduction of nitroarenes into anilines catalyzed by gold nanoparticles incarcerated in a nanoporous polymer matrix: Role of the polymeric support and insight into the reaction mechanism

“…The course of the reaction depends on the experimental conditions, namely the metal catalyst, the temperature, the reductant and the nitroaromatic/reductant molar ratio. In two elegant papers Corma et al demonstrated that NB reduction catalyzed by Au-TiO 2 in the presence of H 2 proceeds through the direct route involving the phenylhydroxylamine intermediate (Scheme 1b) [6b,36]. The FTIR monitoring of the reaction showed that at low coverage degree of the AuNPs surface, nitrosobenzene is rapidly transformed into phenylhydroxylamine and then into AN; the latter reaction is commonly considered the rate determining step.…”

Gold‐Catalyzed Direct Hydrogenative Coupling of Nitroarenes To Synthesize Aromatic Azo Compounds

“…[12] Thanks to the discovery that supported gold nanoparticles (NPs) are capable of uniquely facilitating the direct nitronitroso-hydroxylamine pathway, it is now possible for targetspecific synthesis of substituted anilines and related derivatives by using catalytic nitro-group hydrogenation in a highly regio-and chemoselective manner. [15] In view of the likely intermediacy of the azo compound in the selective hydrogenation of nitroarenes, and considering that the selectivity of the nitro reduction might be tuned in favor of azo products by rational regulation of the synergy between the metal and support in the gold catalyst system, we were motivated to explore gold deposited on functional support with tailored surface redox and acid-base properties as a catalyst for the direct hydrogenative coupling of nitro compounds. [14] One major limitation of this procedure, however, is that it is potentially dangerous when applied to large-scale commercial applications.…”