Gold Nanoparticle Catalyzed Selective Reduction For Clean Synthesis Of Fine Chemicals

The catalytic potential of gold (Au) has long been ignored owing to its high chemical inertness, although it has been used for coinage, jewelry, and other arts for thousand years. However, since Haruta’s discovery in the late1980s that well-dispersed gold catalysts can exihibit ultrahigh catalytic activity in the low temperature CO oxidation, intensive and extensive research efforts have been devoted to the subject of Au catalysis. Over the last decase, it has been well established that besides unique catalytic activity for anumber of gas phase reactions including CO oxidation, epoxidation of propylene, and wate-gas-shift, supported gold catalysts also exhibit unique performance in a braod range of liquid phase organic transformations, especially for selective reduction reactions. Against this background, we have studied gold catalysts for selective hydrogenation and reductive amination in liquid fine chemical synthesis. The main conclusions are described as follows:1. Gold catalysts for the chemoselective hydrogenation of carbonyl compounds in neat waterChemoselective hydrogenation of α,β-unsaturated carbonyl compounds (unsaturated aldehydes or ketones, UALs or UKEs) to the corresponding allylic alcohols is an important step in the industrial synthesis of fine chemicals, particular of pharmaceuticals, perfumes and cosmetics. Although extensively studied, selective allylic alcohol synthesis facilitated by catalytic hydrogenation rather than traditional stoichiometric reduction remains a challenging issue. Conventional hydrogenation catalysts based on supported Pt, Pd, Ru, and Rh produce mainly saturated aldehydes. Although great efforts have been made to overcome this problem over the past decades, catalyst preparation often remains tedious and elusive, and the amount of the modifying agent must be precisely controlled. In addition, most publications are available that address the use of supported metal NPs for producing allylic alcohols from carbonyl compounds hydrogenation in organic solvents. Alternative clean and efficient catalyst that would enable the preferential hydrogenation of the C=O group versus C=C is highly desirable.We demonstrate that gold supported on metal oxides can catalyze the reduction of UALs and UKEs to the corresponding allylic alcohols in high yields in aqueous media. Mesostructured ceria with high surface area has been prepared via template-assisted precipitation method. The catalytic properties of the Au/CeO2catalysts with different surface areas in various solvents were investigated using chemoselective hydrogenation of crotonaldehyde (CAL) as a model reaction. Comparing the results of Au/CeO2with those corresponding to Au/CeO2indicated that an increased surface area of ceria support (150m2·g-1) is favorable for obtaining a gold catalyst with enhanced catalytic activity and selectivity, when neat water was applied as the solvent, the specific rate of the initial activity is dramatically boosted to an unprecedented value of226.4μmol-gAu-1·S-1, which is almost3times that of the isopropanol system and nearly one order of magnitude higher than that for cyclohexane, with an excellent selectivity up to ca.90%can be achieved for CROL production. To rationalize the beneficial effect achieved by using H2O as a solvent, an intermolecular competitive hydrogenation of benzaldehyde and styrene using the Au/CeO2catalyst in different media under similar reaction conditions has been investigated. It is revealed that the preferential reduction of benzaldehyde proceeds much more rapidly in water, in sharp contrast to that occurred in organic solvents. Therefore, the intrinsic higher rate for the Au-catalyzed aldehyde reduction in water is responsible for the high chemoselectivity observed. Moreover, this new Au/CeO2catalytic system has also been suitable for the environmentally clean reduction of a range of unsaturated ketones in water. 2. Gold-catalyzed chemoselective reduction of citral using CO and H2O as the hydrogen sourceSupported gold nanoparticles have recently emerged as active and selective catalysts for a broad array of organic transformations including chemoselective reduction of nitro or unsaturated carbonyl compounds by molecular hydrogen. One critical limitation associated with the current Au-catalyzed hydrogenation process, however, is the unfavorably low hydrogen-delivery capacity compared to the conventional hydrogenation metals. Very recently, our group have developed a highly effective heterogeneous gold-catalyzed, CO/H2O-mediated reduction approach for deoxygenation epoxides, chemoselective reduction unsaturated carbonyl compounds and nitro compounds under very mild conditions. The unique activity of gold catalyst enables us to reasonably conclude that the reaction does not proceed through the seemingly simple reduction of unsaturated carbonyl compounds with H2in situ generated from the gold-catalyzed water-gas shift reaction (CO+H2Oâ†'CO2+H2, generally ignited above100℃), and then leads to the present study that seeks to elucidate the mechanism of this transformation.Chemoselective reduction of citral using CO and H2O as the hydrogen source was performed over four different metal oxide-supported Au catalysts with similar Au particle sizes (Au/CeO2, Au/TiO2, Au/Fe2O3, and Au/Al2O3). The activity of the supported gold catalysts for citral reduction strongly depends on the nature of the oxide support, ranking in the order Au/CeO2>AU/TiO2> Au/Fe2O3> Au/Al2O3. From the experimentally proven correlation between WGS reaction and citral hydrogenation, the intrinsic higher rate for the Au/CeO2-catalyzed WGS reaction is responsible for the genesis of catalytically active sites for aldehyde reduction.On this basis, a series of CeO2-supported gold nanocatalysts deposited with varying gold size were further studied in relation to their performance in the reduction of citral using CO and H2O as the hydrogen source. The size of gold nanoparticle was found decrease with the decrease of gold loading. The results of the activity tests of gold supported on ceria with0.5～8wt%gold loading and catalyst characterization techniques (XRD, TEM, XPS, ICP and XANEs) suggest that selectivity toward the formation of unsaturated alcohol increased with the decrease in the gold particle sizes. In accordance with in situ FTIR experiments for CO/H2O adsorbed, we propose that heterolytic cleavage of H2yield a Hδ+/Hδ-pair at metal/suooprt interface on which higher dispersion of gold is favored, so that the hydrogenation of the polar C=O group increases over that of the non-polar C=C group.3. Gold-catalyzed reductive N-alkylation of nitriles with aminesThe amine is one of the most important partial structures of biologically active compounds and functional materials. Mono-and polyamines are produced by catalytic hydrogenation of the corresponding nitriles, or reduction of nitro compounds. The most commonly used method for synthesis of higher amines 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 nitriles. Nevertheless, despite tremendous efforts in the last two decades, only two examples of heterogeneous catalyst systems for the reductive coupling of nitriles and amines have appeared, and these systems have often suffered from problems such as an excess of nitrile, narrow applicability to a limited number of amines, and the use of organic solvents. From the environmental and atom economical point of view, the development of new practical and efficient one-pot process that can allow the direct synthesis of higher amines under green conditions still remains a major challenge.The catalytic activity and selectivity for the solvent-free reaction of equimolar amounts of benzonitrile and aniline to give N-phenylbenzylamine (NPB) were compared by using various catalysts. Among the Au NP catalysts tested, Au/TiO2exhibited the highest activity toward this alkylation to afford NPB in99%yield with a trace of the semi-hydrogenated product N-benzylidenebenzenamine. Structurally diverse nitriles, including aromatic, aliphatic ones react with various amines to give the desired products in good to excellent yields. Furthermore, various structurally diverse nitroarenes, regardless of the presence of electron-withdrawing or donating functional groups, also could be mono-alkylated with benzonitrile to give the corresponding secondary amines in excellent yields. By monitoring the reaction, a plausible mechanism wqas proposed. Motivated by the suggested mechanism, the applicability of this protocol id further extended to the synthesis of tertiary amines through direct amination of various amines using2equiv of nitriles.