Lu-Cun Wang

Efficient and Selective Room-Temperature Gold-Catalyzed Reduction of Nitro Compounds with CO and H2O as the Hydrogen Source†

The selective reduction of nitro compounds to the corresponding amines is one of the most important transformations in synthetic organic chemistry. Although a number of methods have been developed, the search for new facile, chemoselective, cost-effective, and environmentally friendly procedures that avoid the use of expensive and hazardous stoichiometric reducing agents in large excess has attracted substantial interest. An attractive alternative is the catalytic reduction of nitro compounds with cheap and readily available CO and H2O as the hydrogen source. In particular, the specific reduction of a nitro group under mild conditions in the presence of other functionalities is desirable. As opposed to commonly used catalytic hydrogenation, which involves H2 as the reductant, [3] the use of CO and H2O as the hydrogen source leads to remarkable chemoselectivity and is of great industrial potential, especially when an efficient and reusable catalytic system can be employed. However, relevant studies have largely focused on various rutheniumor rhodium-based homogeneous systems, which are not practically useful because of their low turnover numbers (TONs) and turnover frequencies (TOFs), and the requirement of organic and/or inorganic bases in large excess as cocatalysts. Despite tremendous efforts in the last two decades, few examples of heterogeneous catalyst systems for the reduction of a nitro compounds with CO/H2O as the reductant have appeared, and these systems have often suffered from low efficiency as well as limited substrate scope and catalyst reusability. Supported gold nanoparticles have emerged as active and extremely selective catalysts for a broad array of organic reactions owing to their unique catalytic properties under mild conditions. Whereas the potential of gold-catalyzed selective oxidation reactions for atom-economical and sustainable organic synthesis is widely recognized, the possibilities offered by catalytic reduction with supported gold nanoparticles have remained largely unexplored. Recently, Corma and Serna reported that the chemoselective reduction of a nitro group in the presence of other reducible functionalities is possible with supported gold nanoparticles. One critical limitation associated with the current gold-catalyzed processes for the reduction of nitro compounds, however, is that the hydrogen-delivery rate is too low for practical applications. 11] Herein, we describe a highly effective gold-catalyzed, CO/H2O-mediated reduction that circumvents inconvenient H2 activation to enable the rapid, efficient, and chemoselective reduction of a wide range of organic nitro compounds under mild conditions. The reaction is general and proceeds efficiently under an atmosphere of CO at room temperature. To the best of our knowledge, this gold-based catalytic system is the most efficient, simple, and environmentally friendly catalytic system for the selective reduction of nitro compounds that has been developed to date. Initially, nitrobenzene was used as a model substrate in investigations of the catalytic activity of different solid catalysts under a CO atmosphere at room temperature. The Pt, Pd, and Ru catalysts tested were not active for this reaction. Of the various gold catalysts tested, very small Au nanoparticles (with a diameter of about 1.9 nm) supported on TiO2 showed the highest activity (this catalyst system is denoted as Au/TiO2-VS; see details in the Supporting Information). As observed for other gold-catalyzed processes, both the nature of the support and the particle size had a strong influence on the activity of the Au nanoparticles. Thus, at 25 8C under 1 atm of CO, aniline was produced exclusively with an average TOF in the range of 0.9–33 h 1 (Table 1, entries 1–4). No trace of azo or azoxy compounds, byproducts frequently formed under homogeneous CO/H2O catalysis, was observed. Of particular note is that the reaction proceeded efficiently at a pressure of only 1 atm, which enables the use of common glass reactors. There are very few catalysts that are effective under such mild conditions, the most active of which is a homogeneous [Rh(CO)2(acac)] complex in the presence of a large excess of NaOH. However, the TOF of Au/TiO2-VS is 97 times greater than that of [Rh(CO)2(acac)] under base-free reaction conditions (Table 1, entry 9). The high activity of Au/TiO2-VS under ambient conditions significantly improves the economical and environmental impact of this gold-catalyzed reduction process. Next, the reaction conditions were optimized for the reduction of nitrobenzene through variation of the pressure and solvent. First, the effect of the pressure of CO (PCO) was investigated. The reaction rate increased dramatically as PCO was raised from 1 to 5 atm (Table 1, entries 1, 10, and 11) but leveled off at 5–15 atm (Table 1, entries 12 and 13). These [*] L. He, Dr. L. C. Wang, H. Sun, J. Ni, Prof. Y. Cao, Prof. H. Y. He, Prof. K. N. Fan Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials Department of Chemistry, Fudan University Shanghai 200433 (China) Fax: (+ 86)21-6564-3774 E-mail: yongcao@fudan.edu.cn

Aqueous Room‐Temperature Gold‐Catalyzed Chemoselective Transfer Hydrogenation of Aldehydes

Reduction of aldehydes to the corresponding alcohols is one of the most fundamental and useful reactions that are important in the pharmaceutical and chemical industry. Among the various available reduction protocols for the synthesis of valuable hydroxy compounds, catalytic transfer hydrogenation (TH) has emerged as the most viable, mainly due to the non-involvement of highly flammable and explosive molecular hydrogen or highly expensive metal hydride donors. Moreover, TH processes generally involve easy handling and recovery of products, recycling of catalyst and minimization of undesired toxic wastes. One of the recent highlights in this field is the use of cheap and easily accessible formic acid or its salts as possible in situ hydrogen sources, which has attracted special attention owing to their ease of hydrogen donation as compared to most other transfer reduction agents. In this context, TH of aldehydes in the presence of formate salts is an area of growing interest. Although many transition-metal-based TH catalysts have been developed, c] the focus has been largely on homogeneous rather than heterogeneous catalysis. To the best of our knowledge, few heterogeneous catalysts have been reported that enable fast, chemoselective, and productive TH of aldehydes with inexpensive, eco-friendly formates, and that can tolerate the presence of synthetically useful functional groups. Supported gold nanoclusters have attracted increased interest in the past few years as new generation of advanced catalysts for a number of organic transformations including chemoselective reduction of unsaturated carbonyl or nitro compounds by molecular hydrogen. One critical limitation associated with the present Au-catalyzed hydrogenation process, however, is the unfavorably low hydrogen-delivery rates compared to other noble metals. Very recently, we have described the use of a facile gold-catalyzed, 2-propanol-mediated TH strategy that bypasses the inconvenient H2 activation thus enabling fast and chemoselective reduction of a range of aromatic ketone and nitro groups. From an environmental and an economic viewpoint, efficient functional transformations under milder conditions would be most desirable. Herein, we report a highly efficient, aqueous, room temperature, formate-mediated TH of aldehydes catalyzed by supported gold nanoclusters. Our results have shown that the reaction is general and can proceed at temperatures as low as 25 8C. Moreover, an inert atmosphere is not required. Initially, various heterogeneous catalysts were applied to the transformation of benzaldehyde to benzyl alcohol in aqueous HCOOK without inert gas protection at 80 8C (Table 1). The benefit of using redox CeO2 as a support,

On the Role of Residual Ag in Nanoporous Au Catalysts for CO Oxidation: A Combined Microreactor and TAP Reactor Study

The influence of residual Ag on the formation of stable adsorbed active oxygen species (oxygen storage capacity, OSC) and its correlation with the activity and stability in the CO oxidation reaction on nanoporous gold (NPG) was investigated by a combination of kinetic and temporal analyses of products (TAP) reactor measurements, comparing four different NPG samples with different Ag contents. The data demonstrated that oxygen can be activated and stored on the NPG catalysts at room temperature. Clearly the surface Ag content influenced both the OSC and activity, with a more qualitative correlation for the fresh catalysts and an approximately linear relationship for the stable catalysts after 1000 min on stream. A strictly linear correlation between catalytic activity and OSC, both before and after the reaction, indicated that the active oxygen detected by the TAP reactor measurements also represents the active oxygen species for the reaction at atmospheric pressure. Consequences for the mechanistic understanding of the CO oxidation reaction on NPG catalysts are discussed.

Catalytic activity of nanostructured Au: Scale effects versus bimetallic/bifunctional effects in low-temperature CO oxidation on nanoporous Au

Summary The catalytic properties of nanostructured Au and their physical origin were investigated by using the low-temperature CO oxidation as a test reaction. In order to distinguish between structural effects (structure–activity correlations) and bimetallic/bifunctional effects, unsupported nanoporous gold (NPG) samples prepared from different Au alloys (AuAg, AuCu) by selective leaching of a less noble metal (Ag, Cu) were employed, whose structure (surface area, ligament size) as well as their residual amount of the second metal were systematically varied by applying different potentials for dealloying. The structural and chemical properties before and after 1000 min reaction were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The catalytic behavior was evaluated by kinetic measurements in a conventional microreactor and by dynamic measurements in a temporal analysis of products (TAP) reactor. The data reveal a clear influence of the surface contents of residual Ag and Cu species on both O2 activation and catalytic activity, while correlations between activity and structural parameters such as surface area or ligament/crystallite size are less evident. Consequences for the mechanistic understanding and the role of the nanostructure in these NPG catalysts are discussed.

Reactive removal of surface oxygen by H2, CO and CO/H2 on a Au/CeO2 catalyst and its relevance to the preferential CO oxidation (PROX) and reverse water gas shift (RWGS) reaction

Aiming at further insight into the mechanism of the preferential CO oxidation (PROX) and the (reverse) water gas shift (R)WGS reaction over Au/CeO2 catalysts, we investigated the removal of stable, active surface oxygen from a Au/CeO2-supported catalyst by H2 by quantitative temporal analysis of products (TAP) measurements over a wide range of temperatures (30–300 °C) and compared it with the removal of active oxygen by reaction with CO and a CO–H2 mixture. It is demonstrated that a surface-oxidized Au/CeO2 catalyst can be reduced by H2 pulses only at temperatures higher than 80 °C, whereas significant reduction of the catalyst surface by CO or CO/H2 pulses is possible already at 30 °C. Even at 300 °C, removal of surface oxygen by H2 or CO pulses is possible only in the presence of Au nanoparticles, underlining that these processes are Au assisted. At all temperatures investigated, the amount of H2 necessary to remove the available active surface oxygen is much higher than that of CO. Hence, over the whole range of temperatures the efficiency of H2 to surface reduce a Au/CeO2 catalyst is much lower than that of CO or of a CO–H2 mixture. On the other hand, it is significantly higher than that for active oxygen deposition from CO2, indicating that under steady-state reaction conditions during the RWGS reaction surface lattice oxygen vacancies are present on the surface. Furthermore, the influence of adsorbates resulting from H2, such as hydroxyl groups or water, on the oxygen storage capacity (OSC) of Au/CeO2 catalysts and on the active oxygen deposition from CO2 was elucidated. Implications of these results for the mechanistic understanding of the PROX and the (R)WGS reaction in H2-rich gases are discussed.