Ji Ni

Gold Supported on Hydroxyapatite as a Versatile Multifunctional Catalyst for the Direct Tandem Synthesis of Imines and Oximes

Two birds with one auric stone: The title system acts as a highly efficient heterogeneous catalyst for the one-pot tandem synthesis of imines or oximes from alcohols and the corresponding amines under mild conditions (see scheme; HAP = hydroxyapatite).

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

Efficient and clean gold-catalyzed one-pot selective N-alkylation of amines with alcohols.

The N-alkylation of primary amines to secondary amines is of fundamental importance in organic synthesis, granting access to valuable N-alkylamines that are widely used as pharmacophores in numerous biologically active compounds, dyes, agrochemicals and functionalized materials. The most commonly used method for N-alkylation is the coupling of amines with alkyl halides in the presence of stoichiometric amounts of bases. This procedure, however, can be problematic due to overalkylation, the toxic nature of many alkyl halides, as well as the concomitant formation of large quantities of undesired waste. An alternative environmentally-benign approach is the N-alkylation of amines with readily available alcohols via hydrogen autotransfer, also known as hydrogen-borrowing process. The application of alcohols as alkylating agents has the benefit of high atom efficiency and the formation of water as the only side product. Until now, reports are largely focused on Ruor Irbased homogeneous systems, which are not practically useful because of the problem of reusability and/or the indispensable use of large amounts of additives or co-catalysts. Although there are several reports on the cross-coupling of amines and alcohols using heterogeneous catalysts, most of them suffer from harsh reaction conditions, low turnover numbers (TONs) and frequencies (TOFs), limited substrate scope, and/or the use of excess alcohols/amines to achieve high yields. Catalysts based on supported gold nanoparticles (NPs) have shown their efficiency in a broad range of transformations. We and others have recently reported that supported gold NPs acted as robust catalysts for the transfer hydrogenation (TH) of carbonyl compounds with 2-propanol and oxidant-free dehydrogenation of alcohols under mild conditions. The excellent performance of gold catalysts for hydride transfer has led us to investigate the possibilities offered by Au catalysts for one-pot N-alkylation of amines using alcohols as alkylating agents. Herein, we present a simple and versatile gold-catalyzed system for highly selective N-alkylation of amines under mild conditions. The catalytic system described here is one promising candidate for the direct coupling of amines and alcohols because of the following advantages: i) extremely high atom-economical system achieved by the employment of equimolar amounts of starting materials with water as the sole by-product; ii) high catalytic activity and selectivity; iii) without any additives and/or cocatalysts, and iv) the reaction can proceed smoothly under a 100 mmol scale solvent-free conditions. To the best of our knowledge, this new heterogeneous Aumediated catalysis is greener than any of the known alternatives and involves an easy work up. Initially, the catalytic activity and selectivity for the additive-free reaction of equimolar amounts of aniline (1 a) and benzyl alcohol (2 a) to give N-phenylbenzylamine (3 a) were compared by using various catalysts (Table 1). The titania supported gold catalysts such as Au/TiO2-WGC (provided by the World Gold Council) and Au/TiO2-VS (very small Au NPs ca. 1.8 nm, see Supporting Information) showed high activities for the transformation (entries 1–3); in particular, the reaction with Au/TiO2-VS gave 3 a in 92 % yield with high selectivity (entry 1). In this case, the average TOF and the TON reached up to 13.1 h 1 and 184, respectively. There are very few catalysts that are effective under such benign conditions, the most prominent of which is a homogeneous [Cp*IrCl2]2 complex in the presence of a large excess of NaHCO3 (TOF: 5.5 h , TON: 94). In the absence of the catalyst, the desired secondary amine 3 a was not produced. No formation of 3 a was observed in the presence of just TiO2 (entry 12). Under the conditions described in Table 1, Au deposited on other supports as well as Pd, Ru catalysts supported on TiO2 were not effective (entries 4– [a] L. He, X.-B. Lou, J. Ni, Dr. Y.-M. Liu, Prof. Dr. Y. Cao, Prof. Dr. H.-Y. He, Prof. K.-N. Fan Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry Fudan University, Shanghai 200433 (P. R. China) Fax: (+86)-21 65643774 E-mail : yongcao@fudan.edu.cn Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201001848.

Gold Supported on Nanocrystalline β‐Ga2O3 as a Versatile Bifunctional Catalyst for Facile Oxidative Transformation of Alcohols, Aldehydes, and Acetals into Esters

Esterification is one of the most fundamentally important reactions in organic synthesis. Although a number of methods have been developed, the search for new, facile, cost-effective, and environmentally friendly procedures that avoid the use of large excess of reagents and expensive activators has attracted substantial interest. An attractive alternative is the direct catalytic transformation of alcohols or aldehydes to esters, without the use of the corresponding acid or acid derivative. In particular, the direct oxidative conversion of alcohols or aldehydes under mild conditions is an attractive goal. As opposed to the traditional esterification method, in which a two-step synthetic procedure first involving the synthesis of carboxylic acids or activated carboxylic acid derivatives, such as acid anhydrides or chloACHTUNGTRENNUNGrides, is required, the single-step nature of the oxidative esterification procedure has economic and environmental benefits in the synthesis of esters. However, relevant reports are limited to a few nonselective heterogeneous reactions or homogeneous systems, which utilize stoichiometric amount of oxygen donors and long reaction times. In general, heterogeneous systems capable of catalyzing oxidative esterification of alcohols or aldehydes using molecular oxygen (O2) as oxidant are relatively scarce. One notable exception is the ethylene glycol to methyl glycolate (MGC) process based on a specific Au-based catalyst system with proprietary formulations recently developed by the company Nippon Shokubai; this result represents a milestone towards greener commercial process for clean and efficient production of carboxylic esters. Solid catalysts based on supported gold nanoparticles have attracted tremendous recent attention owing to their unique catalytic properties for a broad spectrum of organic transformations, especially for aerobic oxidation of alcohols under mild conditions. Over the last few years, our group, Hutchings et al., Baiker et al., and Corma et al. reported that gold nanoclusters deposited on TiO2, CeO2, and Cu-Mg-Al or Ga-Al mixed-metal oxides are highly effective for aerobic alcohol oxidation under solventfree conditions. One critical issue associated with the goldcatalyzed primary-alcohol oxidation process is the selectivity toward aldehydes. Most recently, several studies have revealed that, depending on the substrate or acidic nature of the supports, the yielding of esters may severely reduce the selectivity toward target products of aldehyde, with hemiacetal being identified as the key intermediate for ester formation. Taking into account that inorganic oxides contain some appropriate acid sites that are able to facilitate the hemiacetal formation, it appeared to us that a new concept of catalyst could be brought forward if the gold nanoparticles in the presence of these acidic sites can cooperatively work together by introducing a solid bifunctional catalyst that facilitates the generation and consecutive oxidation of the intermediate hemiacetals to the corresponding esters. To explore this possibility, we chose a solid catalyst formed by gold supported on nanocrystalline b-Ga2O3 (denoted as Au/b-Ga2O3). The reason for this choice is that gallium oxide has emerged as an exceptional catalytic or supporting material that is highly efficient for a wide range of acid-catalyzed reactions; it is also known that significantly increased surface Lewis acidity can be achieved on nanocrystalline b-Ga2O3. [16] The nanocrystalline b-Ga2O3 support was prepared by an alcoholic gel-precipitation method. The X-ray diffraction (XRD) pattern of the as synthesized nanocrystalline support shows well-defined diffraction features characteristic of bGa2O3 (Figure S1 in the Supporting Information). Transmission electron microscopy (TEM) shows that the support is highly porous in nature; it consists of interconnected parti[a] F.-Z. Su, J. Ni, H. Sun, Prof. Dr. Y. Cao, Prof. Dr. H.-Y. He, Prof. K.-N. Fan Department of Chemistry & Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials Fudan University, Shanghai 200433 (P. R. China) Fax: (+86)21-6564-2978 E-mail : yongcao@fudan.edu.cn Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200800982.

A green and efficient oxidation of alcohols by supported gold catalysts using aqueous H2O2 under organic solvent-free conditions

The use of supported gold nanoparticles as an efficient, green and reusable catalyst for the oxidation of various alcohols to the corresponding carbonyl compounds using aqueous hydrogen peroxide as an environmentally benign oxidant is presented. The reaction proceeds with good to excellent yields in particular for nonactivated alcohols under base-free conditions.

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,