This book describes the properties of gold, the methods for preparing gold catalysts and ways to characterise and use them effectively in reactions. The reaction mechanisms and reasons for the high activities are discussed and the applications for gold catalysis considered.
Catalysis by Gold Geoffrey C. Manuel P. The safety and environmental issues posed by HgCl2 prevent a return to the former catalyst but Hutchings and co-workers have proven that gold supported on carbon can be a viable alternative. Our project will inform the development of such catalysts but will also provide an important new window on gold catalysis. Whilst catalysis by gold nanoparticles is an established phenomenon particularly in relation to selective oxidation processes, the key points of the active site and the reaction mechanism remain very controversial despite extensive work.
Different researchers have separately emphasised the importance of different aspects of gold catalysts for example: the size of the gold particles; the interface between the gold and the support and the oxidation state of the gold. They recommend that more diverse systems be investigated at a fundamental level if further progress is to be made in understanding the nature of the active site in gold catalysis and thus provide direction to the development of better gold catalysts.
A powerful combination of surface spectroscopy, surface microscopy and surface kinetic methods applied in conjunction with computational chemistry will be focussed on identifying and characterising the active states of gold in ideal models of the hydrochlorination catalyst.
The significance of the oxidation state of gold in heterogeneous catalysis. Davies, Professor PR.
The reaction was catalyzed by subnm Au:PVP particles in water under aerobic conditions. Substituent effects i. The selectivity was influenced by the steric effect of the reactants, and catalytic activity based on conversion of the phenylboronic acid was found to be dependent on the size of gold particles; note that no apparent tendency was found on the electronic effect of the reactants. The smallest gold particles Au:PVP gave the highest activity average diameter ranging from 1.
The Au:PVP catalyst is recyclable and reusable. The mechanism of the aerobic homocoupling was proposed on the basis of the well-established mechanism for the Pd II -based complex catalysts Scheme 6. In this mechanism, oxygen plays a key role in the activation of phenylboronic acid to surface-adsorbed anion. The Au:PVP particle catalyst was further developed to catalyze the oxidative homocoupling reaction of potassium aryltrifluoroborate in water under air atmosphere Scheme 7 .
Dhital et al. They found that the Au:chitosan catalyst gave higher conversion and better selectivity for biaryl than the Au:PVP catalyst, which was attributed to the higher stability of the Au:chitosan catalyst under acidic conditions. The Au:chitosan catalyzed coupling reaction showed the substituent effect of substrates: Those with electron-donating groups performed better than the ones with electron-withdrawing groups, whereas in the Au:PVP case no substituent effect was observed ; the difference is not clear. In terms of reaction mechanism, the homocoupling reaction proceeded via an anionic species even under acidic conditions, which is similar to the afore-discussed mechanism Scheme 6.
In a subsequent work, Sophiphun et al. Chaicharoenwimolkul and coworkers have investigated the catalytic activity of gold particles capped with ligands containing ferrocene moieties . Two substrates, phenylboronic acid and ferrocenylboronic acid, were chosen for the reactions catalyzed by gold particles to test the effect of capping ligands. In the case of oxidative homocoupling of phenylboronic acid, it was found that the selectivity for the biphenyl catalyzed by gold particles is lower than that catalyzed by Au III salt.
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As shown in the proposed mechanism Scheme 9 , which is totally different from that in Scheme 8 , the procedure of the coupling reaction of phenylboronic acid followed a different order: transmetalation, coupling also called reductive elimination , and oxidation as the last step; the last step is suggested as the first step in the mechanism shown in Scheme 8. Overall, the mechanism of homocoupling reaction of phenylboronic acid needs more detailed investigation in future work.
Redrawn from . Zheng and coworkers applied the polystyrene-polyamidoamine PS-PAMAM -supported gold nanoparticle catalyst for the oxidative homocoupling of phenylboronic acids . The reaction was carried out in water and air using K 2 CO 3 as base. It is very interesting that steric factors also played a major role in the product selectivity Table 2. In heterogeneous catalysts i. It was reported by Willis and Guzman that the supported gold nanoparticle catalyst average diameter, 10 nm was more active than the supported palladium nanocatalyst average diameter, 15 nm under the same reaction conditions in the oxidative homocoupling of phenylboronic acid .
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Meanwhile, the activity of the supported gold nanocatalysts increases as the size of the crystallite support particles becomes smaller because the initial surface coverage of OH groups in the supports increases . Suzuki cross-coupling reaction, also named as Suzuki-Miyaura reaction, is one type of carbon-carbon coupling of aryl halides with arylboronic acids to generate asymmetric biaryls .
In general, the catalyst for Suzuki cross-coupling reaction is almost dominated by palladium complexes. In recent research, gold nanoparticles for this cross-coupling application have aroused considerable attention.
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Intriguingly, the cross-coupling reaction was found to require oxygen and water to occur. Redrawn from . However, in , Han et al. The results showed that the catalytic activity of gold nanoparticles decreased with increasing sizes of gold nanoparticles from 1.
Catalysis by Gold
A thinner polymer stabilizing layer on gold nanoparticles was also found to be crucial for improving the catalytic activity of nanoparticles. The application of gold nanoparticles was tested with a variety of substituents Scheme Steric effect was observed in the cross-coupling reaction.
The gold nanoparticle catalyst showed very good recyclability. Moreover, gold nanoparticles supported on graphene were also examined in this cross-coupling reaction . Selvam and Chi have synthesized luminescent gold nanoparticles with average size of 1. Bimetallic metal nanoparticles are excellent catalysts in organic catalytic reactions, which sometimes perform even better than the corresponding monometallic particles. The transmission electron microscopy TEM images indicate that the nanoparticles were nearly monodispersed and stabilized in channels of SBA planted by dendrimers Figure 3.
The catalytic results showed that the Pd nanoparticles have poor performance in the cross-coupling of arylboronic acids with aryl bromides.
Heterogeneous Catalysis by Gold-based Bimetallic Catalysts
In addition, these bimetallic nanoparticles can also be applied to the cross-coupling between aryl chloride and arylboronic acids with moderate yields. Redrawn from . The catalytic results implied that there was no effect of the surface structure or subsurface composition of the particles in this cross-coupling reaction, and it is unfortunate that the reaction was instead primarily catalyzed by molecular Pd species that leached from the nanostructures.
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The significance of the oxidation state of gold in heterogeneous catalysis
The Au-Ag-Pd trimetallic particle catalyst can also be recycled and reused. Sonogashira cross-coupling of halides with terminal alkylene is one of the important carbon-carbon cross-coupling reactions, which is usually catalyzed by palladium complexes and Pd nanoparticles . Recently, gold nanoparticle catalyst was also found to be capable of catalyzing the Sonogashira cross-coupling between iodobenzene and phenylacetylene, as gold particles can activate the C sp2 -I bond of iodobenzene and the C sp -H bond of phenylacetylene on one gold atom or the neighbor gold atoms .
Three products would be obtained during the gold-catalyzed Sonogashira cross-coupling process of phenylacetylene and iodobenzene Scheme 13 : the target cross-coupling product, diphenylacetylene DPA , and two side products, biphenyl BP and diphenyldiacetylene DPDA . Kyriakou and coworkers examined the catalytic activity of gold nanoparticles supported on silica in Sonogashira cross-coupling reaction of iodobenzene and phenylacetylene .
Three different sizes A : 23 nm, B : 12 nm, and C : 2. That the larger gold particles performed better may be due to adsorption geometries and steric effects of the particles. The effects of metal-support interactions were found to be not of primary significance in this coupling reaction, as it was compared with the TiO 2 -supported gold particle catalyst. The time-dependent behavior of selectivity and yield was also examined; the conversion of iodobenzene increased and the selectivity for cross-coupling product DPA decreased at longer reaction times.
Further, the same group examined more supports i. They also investigated the same coupling on Au . Chitosan as an efficient porous support for the 1. De Souza and coworkers reported Sonogashira cross-coupling reaction catalyzed by supported gold catalysts under microwave irradiation, which is widely applied in organic synthesis . Good yields of the target cross-coupling products were obtained with short reaction times with a range of functionalized aryl and alkyl acetylenes when DMF was used as solvent.
Further, they expanded the application of the trimetallic nanoparticles 4.
The trimetallic nanoparticle catalyst obtained better results than monometallic and bimetallic catalysts, which is probably due to the delicate electronic effect between elements in the particles. In the past few years, an increasing number of multicomponent reactions MCRs have been developed for the synthesis of diverse organic molecules through a combination of three or more starting materials in a one-pot operation . Among MCRs, the A 3 -coupling reaction of aldehydes, amine, and terminal alkynes is one of the best examples, where propargylamine is obtained as the major product .
A range of aromatic aldehydes and amine were applied in this A 3 -coupling reaction, and aryl aldehydes possessing electron-withdrawing groups were found to exhibit better reactivity than those with electron-donating groups. A tentative two-step mechanism was proposed for the A 3 -coupling reaction Figure 6. In the second step, the iminium ion intermediate couples with the activated phenylacetylene deprotonated by gold catalyst , which is the key step in the A 3 -coupling reaction. Manhattan, USA ] was also used as support for gold nanoparticle catalyst in the A 3 -coupling reaction .
The nanogold catalyst 10—12 nm was synthesized via stabilization of ion on nanocrystalline magnesium oxide support followed by NaBH 4 reduction Scheme The NAP-Mg-Au 0 catalyst gave rise to high catalytic activity turnover number up to in A 3 -coupling reaction of a series of aryl aldehydes, amines, and alkynes. Proposed mechanism of Au nanoparticle-catalyzed A 3 -coupling of aldehyde, amine, and alkyne.
Redrawn from . However, Abahmane and coworkers found that aldimine formation from aldehydes and secondary amines catalyzed by montmorillonite K10 MM K beforehand can benefit the A 3 -coupling reaction , as the A 3 -coupling reaction was investigated as a two-step flow process shown in Figures 6 and 7D. Regime C gave the best reaction performance in those reaction regimes, as the MM K can promote the first step to form iminium ion intermediate Figure 7D. Regime C was established including a range of functionalized aromatic and aliphatic aldehydes and cyclic and acyclic aliphatic amines.
The catalytic results suggested that the use of aliphatic aldehydes as well as acyclic aliphatic amines in the flow reaction system can also lead to the desired propargylamines, and an excess of phenylacetylene is not necessary in the flow reaction process as opposed to the one-pot process. D Scheme of the A 3 -coupling reaction and the combination of the various aldehydes and amines used in the reaction regime C. Redrawn from . The surface plasmon band of supported Au particles allows for selective excitation of Au nanoparticles at nm over the organic substrates and the bare supports as the latter two have no absorption at this wavelength.
ZnO as a most basic semiconductor plays a key role in assisting the loss of acidic hydrogen on the terminal alkyne of phenylacetylene Figure 8. A range of functionalized aromatic and aliphatic aldehydes as well as amines have been examined in this photocatalytic system. Proposed mechanism for the plasmon-mediated A 3 -coupling of isovaleraldehyde, piperidine, and phenylacetylene catalyzed by AuNP ZnO, which was irradiated at nm.