New Strategies For Green Catalytic Systems Based On Heterogeneous Hydrogen Transfer Processes

One of great challenges in the modern sustainable chemistry is the development of new green catalytic technologies that can afford resource-saving, environmentally benign, mild and atom-economic synthesis of commodity and fine chemicals. In the specific field of controlled reduction, the conventional technologies using hazardous and stoichiometric reducing agents such as such as sulfide (H2S or NaSH), or Fe/HCl, produce a large number of byproducts and has been falling into disuse. Catalytic hydrogenation is a powerful clean technology that has attracted considerable recent interest. Classical hydrogenation metal catalysts (Pt, Pd, Ru, Rh etc.) can hydrogenate many different functional groups with high activity. However, when dealing with multifunctional molecules, the problem often arising is that the classical hydrogenation catalysts reduce sensitive functions simultaneously, leading to an undesired selectivity. In contrast, gold can be especially sensitive to discriminate different functional groups in poly-substituted compounds, offering an appealing alternative for manufacturing complex chemicals and fine chemicals. Due to unfavorably low hydrogen-delivery rates, however, the activity of the gold-catalyzed hydrogenation process was too low for practical applications.Catalytic transfer hydrogenation (CTH) employing hydrogen donors is a powerful tool for achieving controlled reduction of many types of organic compounds under mild conditions. In comparison with catalytic reduction using molecular hydrogen (H2), CTH using hydrogen donors has real and potential advantages. H2, a gas of low molecular weight and therefore high diffusibility, is easily ignited; the use of hydrogen donors obviates these difficulties, no pressure vessels are needed, and simple stirring of solutions is usually all that is required. Potentially, CTH methods could afford enhanced selectivity in reduction. With a catalyst and H2, changes of catalyst, solvent, and temperature are possible variations in reaction conditions but, with hydrogen donors, a new dimension is opened up because the choice of hydrogen donor can affect the reaction through its competitive adsorption onto the catalyst surface. Meanwhile, CTH is a very useful reaction in organic synthesis. In particular, the substrate is activated by catalytic dehydrogenation. This activation is followed by a bond construction step. These steps proceed under ’one-pot’ conditions with a single catalyst or at most two catalysts acting together. The advantages of dehydrogenative activations of organic molecules of the various reactions are clear. They often lead to lower waste processes and involve lower toxicity starting materials. They are still somewhat high-barrier processes, however, and often require higher temperatures than classical reactions. The challenge therefore is to define catalysts and conditions that lead to the same transformations under milder conditions and with efficient induction. In doing so, more difficult reactions may become possible. Against this background, our research focused on design of green heterogeneous catalytic systems for hydrogen transfer reactions. The main conclusions are described as follows:1. Supported gold nanoclusters-catalyzed, formate-mediated reductionDuring the last decade, the increasing awareness of environmental concerns has stimulated the development of metal-catalyzed reactions in aqueous media, since water represents the most benign, abundant and inexpensive solvent known. In addition, the use of water as the reaction medium has reportedly led to unique reactivity and selectivity that can not be obtained in conventional organic solvents. In this context, the selective reduction of aldehydes into the corresponding alcohols in aqueous media is a fundamental and useful reaction. We report for the first time a highly efficient aqueous formate-mediated of aldehydes catalyzed by supported gold nanoclusters based on CTH methodology. Our results show that the reaction is general and can proceed at temperatures as low as25℃without inert atmosphere. Moreover, we discover that mesoporous CeO2featured with extraordinary surface redox capabilities are far superior to conventional oxides (for example, HO2, Al2O3or Fe2O3etc.) as supports for Au nanoclusters. This provides the first experimental evidence for the underlying surface redox nature of an oxide support in the design and development of new gold nanocatalysts with improved hydrogenation activity.By using HCOONH4as the reductant instead of the hydrogen gas, the combination of Au nanoparticles and titania support can be uniquely efficient for liquid phase organic synthesis, particularly for the chemoselective and regioselective transfer hydrogenation of aromatic nitro compounds at ambient conditions. Of significant practical importance is that the Au/TiO2catalyst tolerates a wide variety of synthetically useful functional groups including halogens, ketones, esters, heterocyclic ring and olefins, thus opening up a new avenue for efficient and sustainable production of amine compounds. Moreover, this gold-based TH system could also accomplish the direct conversion of aromatic nitro compounds bearing different functional groups to the corresponding N-formanilides chemoselectively. Thus, the present gold-catalyzed reduction constitutes a breakthrough in chemo-and regioselective reduction on heterogeneous catalysts, especially as many of the substrates used are highly relevant for fine-chemicals production.Stereoselective semihydrogenation of carbon-carbon triple bonds is a highly desired tool for synthetic organic chemistry, and many of the products obtained through this reaction are useful in the synthesis of natural products, such as biologically active compounds. The semihydrogenation of alkynes is traditionally performed with Lindlar catalyst and dihydrogen. This system reduces alkynes to Z alkenes but requires an elaborate experimental setup and strict monitoring of the hydrogen uptake to prevent over-reduction to the alkane. Apart from these practical inconveniences, partial isomerization of the Z alkene to the E alkene, a double-bond shift, and problems with reproducibility are typical for this reaction. We showed in this section that titania supported Au (Au/TiO2) is an excellent catalyst for the semihydrogenation of alkynes under hydrogen-transfer conditions. The over-reduction to alkanes is fully inhibited when HCOOH/NEt3is used as hydrogen donor, providing a very high stereoselectivity in a simple procedure.2. Supported gold nanoclusters-catalyzed, CO/H2O-mediated reductionIn view of the excellent performance of supported gold for low temperature water-gas shift (WGS) reaction (CO+H2O=CO2+H2) to produce high purity H2for a number of technological applications, it is conceivable that a combination of the direct hydrogenation and WGS reaction over supported gold may provide an attractive alternative for more efficient green reductions. From our continuing studies on supported gold NPs catalysis, we have discovered a highly effective heteroge-neous gold-catalyzed, CO/H2O-mediated approach for chemoselective reduction of nitro compounds under very mild conditions. In particular, by using very small gold NPs (ca.～1.9nm) supported on TiO2, the clean reduction of a wide range of nitro compounds, including not only substituted nitroarenes but also notoriously difficult nitroalkanes, to the corresponding amines was achieved without any additive under atmospheric CO at room temperature. Under such mild condition, Au/TiO2-VS showed an unprecedented turnover frequency (TOF) of about2orders of magnitude higher than the most active homogeneous [Rh(CO)2(acac)] complex, whereas other supported platinum-group metal catalysts tested were completely inactive. The unique ambient activity of gold catalyst enables us to reasonably conclude that the reaction does not proceed through the seemingly simple reduction of the nitro groups with H2in situ generated from the gold-catalyzed WGS reaction. By monitoring the reaction with in situ diffuse reflectance FT-IR (DRIFTS) spectroscopy, we observed the presence of an unusual reactive intermediate carboxyl species, which plays a central role in the CO/H2O-mediated reduction processes.Encouraged by these results, we were able to extend this exceedingly efficient system to the reduction of α,β-unsaturated aldehydes. We have demonstrated that gold supported on ceria (Au/CeO2) is extremely high chemoselective for the reduction of a series of α,β-unsaturated aldehydes to the corresponding high-valued unsaturated alcohols in the presence of CO and H2O. It is of interest to note when molecular hydrogen was applied instead of CO/H2O, the activity and selectivity is much lower for reduction of crotonaldehyde at1h with Au/CeO2.3. Efficient and clean gold-catalyzed one-pot selective N-alkylation of amines/urea with alcoholsWith a production on million-ton scale per year, amines represent an important class of compounds in bulk chemicals and pharmaceutical raw materials. Conventional routes to this class of nitrogen-containing compounds have relied heavily upon the amination of aryl halides and reductive amination of carbonyl compounds. Despite of their utility, these procedures can be problematic due to the instability of carbonyl reagents, the toxic nature of alkyl halides and/or the concomitant formation of large quantities of undesired waste. The search for new facile and cost-effective procedures that avoid the use of hazardous and expensive starting materials has attracted substantial interest. An environmentally friendly alternative is the direct N-alkylation of amines with alcohol via hydrogen autotransfer, also known as hydrogen-borrowing process. This domino reaction sequence is based on the in situ dehydrogenation of alcohols to give the corresponding aldehydes or ketones. Subsequent imine formation followed by reduction with the initially produced hydrogen leads to the amine. The advantages of this type of amination are the availability of alcohols and the high atom efficiency of the reaction sequence, which forms water as the only by-product. Moreover, no additional hydrogen source is required. The excellent performance of gold catalysts for hydride transfer has led us to investigate the possibilities offered by gold catalysts for environmentally clean N-alkylation of amines using alcohols as alkylating agents. In this context, we have found that the direct synthesis of a wide variety of higher amines can be realized by a novel gold-catalyzed approach based on the direct N-alkylation of amines/urea with an equimolar amount of the alcohols under mild conditions.4. Titania-supported irdium subnanoclusters as an efficient heterogeneous catalyst for direct synthesis of quinolines from nitroarenes and aliphatic alcohols The search for facile, clean and sustainable catalytic procedures towards the synthesis of complex molecules from convenient starting materials is one of the central priorities in chemistry. An attractive way to address these criteria is the development of one-pot tandem catalysis that allows a rapid increase in molecular complexity in a highly concise fashion. We reported in this chapter a versatile multitask catalyst system consisting of subnano-sized iridium clusters deposited on titania (Ir/TiO2-NCs) can successfully promote the direct tandem synthesis of diverse quinoline derivatives from simple and readily available nitroarenes and aliphatic alcohols under mild and additive-free conditions. Notably, the process tolerates the presence of various reactive functional groups and is very selective to highly valuable quinolines. Based on the result of a series of carefully designed control experiments, it was finally established that the initial steps of the transformation are the dehydrogenation of alcohols and transfer hydrogenation of nitrobenzene, generating aminoarenes and aldehydes. Subsequently, an acid-catalyzed Michael addition of aniline to the self-aldol condensation product might occur, followed by ring closure with dehydration to form dihydroquinoline, which would be finally dehydrogenated to afford the desired quinoline products.