Abstract
Gold nanoparticles have been demonstrated to be efficient catalysts for a wide range of reactions in the past decades, such as oxidation and hydrogenation. In recent research, gold nanoparticle catalysts have been utilized in carbon-carbon coupling reactions. These coupling reactions have been established as convenient and general approaches toward biaryl or propargylamines, which are biologically active compounds, natural products, and pharmaceutical organic compounds. This review aims to highlight the current achievements in the field of gold nanoparticle-catalyzed coupling reactions, including Ullmann homocoupling of halides, oxidative homocoupling of organoboronates, Suzuki cross-coupling of phenylboronic acid and halides, Sonogashira cross-coupling of iodobenzene and phenylacetylene, and A3-coupling reaction of phenylacetylene, amines, and aryl or alkyl aldehydes. The catalytic mechanisms of these carbon-carbon coupling reactions are discussed. Finally, we provide our perspectives on some future work on gold nanocatalysis in coupling reactions.
1 Introduction
The field of metal nanoparticle catalysis represents a burgeoning area with increasing application in chemical synthesis. The small metal nanoparticles (e.g., 1–10 nm) exhibit extraordinary catalytic activity, sometimes better than the corresponding metal complexes. The high activity of nanocatalysts is attributed to several important factors, including the high surface-to-volume ratio, surface geometric effect (e.g., surface atom arrangement and low-coordinated atoms), the electronic effect, as well as the quantum size effect. Metal nanoparticles suspended in solution are often used as effective heterogeneous catalysts due to the advantages of simplified isolation of product and facile recovery and excellent recyclability [1, 2], which renders metal nanocatalysts environmentally friendly. Among the precious metal nanoparticles, ligand-protected (e.g., by thiolate and phosphine) or oxide-supported gold nanoparticle catalysts have become a hot research topic in recent years.
In general, there are two main methods to synthesize oxide-supported Au nanoparticle catalysts. In the traditional method, the catalysts are typically prepared by reduction of Au(III) salts (such as HAuCl4 or NaAuCl4) by NaBH4 in the presence of oxide supports in water solution. Although it is expected that the ionic gold (Au3+) would be completely reduced to metallic gold particles (Au0 in the metal core and Auδ+ on the surface of nanoparticles), recent experiments have shown that Au nanoparticles/oxides contain residual cationic gold species in the oxidation state of either +1 or +3 [3–7], which are adsorbed on the surface of the Au particles. These minor cationic species (Au+ and Au3+) on the Au nanoparticles, even less than 1 ppm, may act as soft Lewis acids or participate in some catalytic reactions and have been proposed on many occasions to be in part responsible for the catalytic activity; some examples will be presented in this review. Thus, it is quite challenging to elucidate the exact catalytic active species in gold nanoparticle catalysts because three gold states (i.e., 0, +1, and +3) possibly coexist in the catalysts prepared by the traditional method and many organic reactions can be catalyzed by both Au(I) or Au(III) complexes and by gold nanoparticles (containing primary Au0 and minor Auδ+) [8–10]. Another apparent disadvantage of the traditional preparation method is the broad size distribution of gold particles on the support; for example, the size of gold nanoparticles is typically distributed from ∼10 to ∼30 nm, and sometimes it may be larger than 50 nm. The catalytic performance (e.g., conversion and selectivity) of nanocatalysts is drastically influenced by the size of metal nanoparticles (examples to be presented in this review).
The second preparation method of nanocatalysts is to first prepare ligand-protected gold nanoparticles in organic or water solutions and then deposit such colloidal particles onto oxide supports. This method has been widely employed especially in recent years. Typical organic ligands include thiolate, phosphine, and amine. The ligand-protected nanoparticles are particularly robust under ambient or thermal conditions (e.g., <200°C). The core gold atoms of the gold nanoparticles are zero-valent (Au0), but the surface atoms appear to be partially positive (Auδ+) due to charge transfer from surface gold atoms to ligands. However, if ligand-off catalysts are preferred, the oxide-supported gold nanoparticles can be thermally calcined at a moderate temperature higher than the ligand desorption temperature (e.g., 300°C, several hours in vacuum); this treatment completely removes the protecting ligands on the gold nanoparticles. It is important to preserve the size of gold nanoparticles during ligand removal. The ligand-off particles would not aggregate if they interact sufficiently with the oxide support; the particle-support interactions result in fixation of particles on support surfaces [11–14]. Compared to the catalysts prepared by the traditional method, the advantage of the “ligand-off” gold nanoparticles is that they contain only metallic gold (Au0 and a minor amount of Auδ+ species, which tend to form when the nanoparticles are exposed to air or O2), but no Au+ or Au3+ species are present on the surfaces of such Au nanoparticles. For example, the protecting ligands of the thiolate-capped Au144(SCH2CH2Ph)60 nanoclusters (NCs) can be easily removed via thermal treatment at 300°C in air (ligand desorption temperature, ∼200°C) without sintering the Au144 particles because the ligand-off Au144 particles can be effectively stabilized on a heterostructured mesoporous support (e.g., CuO-mSiO2) [12]. It is worth noting that ligand-off gold nanoparticles might have a structure different from the original structure of ligand-protected particles.
Since the early work by Haruta et al. [15], gold nanoparticles (size ranging from 1 to 100 nm) have been demonstrated to be very efficient catalysts in a wide range of chemical reactions. The gold nanoparticles have been widely applied in CO oxidation to CO2 and selective oxidation reactions [16–20] (e.g., styrene oxidation to styrene epoxide and benaldehyde, alcohol oxidation to ketone, aldehyde, and even acid), selective hydrogenation [21, 22] (e.g., CO2 reduction to CO, nitrophenol to aminophenol, and chemoselective hydrogenation of α,β-unsaturated ketones to α,β-unsaturated alcohols). These catalytic processes have been thoroughly reviewed in the past years [22–24]. In this review, we focus on the application of small Au nanoparticles (especially the 1- to 20-nm size range) in catalyzing carbon-carbon coupling reactions (Scheme 1), including Ullmann homocoupling of halides, oxidative homocoupling of organoboronates, Suzuki cross-coupling of phenylboronic acid and halides, Sonogashira cross-coupling of iodobenzene and phenylacetylene, and A3-coupling reaction of phenylacetylene, amines, and aryl or alkyl aldehydes. These carbon-carbon bond formation processes are one of the most important tools in modern catalytic organic synthesis. The formed bonds are often found in natural products such as alkaloids as well as in numerous biologically active parts of pharmaceutical and agrochemical specialities. Moreover, mechanistic aspects of these carbon-carbon coupling reactions are discussed in this review.
The various carbon-carbon coupling reactions catalyzed by gold nanoparticles covered in this review (where X=I, Br, and Cl).
2 Ph-B(OH)2, Csp2-I, and Csp-H bond activation by gold nanoparticles: theoretical calculations
Ph-B(OH)2 bond activation is the key step in oxidative homocoupling reaction of organoboronates and in Suzuki cross-coupling reaction of phenylboronic acid and halides. However, the activation of the Csp2-X bond (hereafter, X=I, Br, and Cl) of aryl halides is the key point in Ullmann homocoupling, Suzuki cross-coupling, and Sonogashira cross-coupling reactions. The Csp2-I bond of iodobenzene is chosen as a model for theoretical calculations, as it is easier to be activated by nanogold than Csp2-Br and Csp2-Cl bonds of aryl halides. Meanwhile, the activation of the terminal Csp-H bond of phenylacetylene is critical in Sonogashira cross-coupling reaction of phenylacetylene and iodobenzene and A3-coupling reaction of phenylacetylene, amines, and alkyl or aryl aldehydes. To understand the details on the activation mechanism of the Ph-B(OH)2 bond, Csp2-I bond of iodobenzene, and Csp-H bond of phenylacetylene by gold nanoparticles, density functional theory calculations are helpful (vide infra).
Boronat and Corma used naked or partially oxidized Au38 nanoparticles as quasi-molecular catalysts to model the oxidative homocoupling reaction of the organoboronate catalysis process [25]. The geometries and energy profiles of the various states have been considered (Figure 1). The adsorption energy of phenylboronate
![Figure 1 Calculated energy profiles for (A) dissociation of anion and (B) coupling of two phenyl fragments yielding biphenyl on a naked Au38 nanoparticle (black line) and on a partially oxidized Au38 nanoparticle (i.e., carrying two oxygen atoms) (gray line). The optimized distances are given in angstroms. From [25] with permission.](/document/doi/10.1515/ntrev-2013-0020/asset/graphic/ntrev-2013-0020_fig1.jpg)
Calculated energy profiles for (A) dissociation of
The same Au38 nanoparticle was also used as an idealized model for 1-nm gold nanoparticles. The adsorption and possible dissociation of iodobenzene were calculated [26]. The calculation results suggested that iodobenzene interacts strongly with the surface atoms of Au nanoparticles, forming an adsorption complex in which the I atom is directly bonded to a Au atom at a distance of 2.772 Å. There is a net transfer of electron density from the I atom (hence, I bears a net +0.314e charge) to the Au nanoparticle (hence, the particle bears -0.259e charge). In the transition state, the C-I bond length increases to 2.475 Å, and the I atom and the phenyl fragment each interact with a different Au atom, with optimized Au-I and Au-C distances of 2.781 and 2.208 Å, respectively. The calculated activation energy is 11.3 kcal mol-1, which indicates that the rupture of the C-I bond on gold particles is a feasible process.
In further work, four catalyst models [i.e., Au(111) facet, Au38, Au38O2, and Au/CeO2] have been employed to study the mechanism and the active sites of the gold nanoparticle-catalyzed Sonogashira cross-coupling reaction of iodobenzene and phenylacetylene, and the competitive homocoupling reactions (Figure 2) [27]. The Au(111) facet – a model for large gold particles (diameter >5 nm) – contains high-coordinated Au0 atoms, and the cuboctahedral Au38 particle – a model for 1-nm small gold particles – contains low-coordinated Au0 atoms. Low-coordinated metallic Au0 and cationic Auδ+ sites were included in the oxidized particle model (Au38O2); in the latter, one Au atom is directly bonded to two O atoms, adopting an oxide-like structure, and four Au atoms are directly bonded to one O atom. The Au/CeO2 model contains neutral Au0 atoms on top of the rod and cationic Auδ+ sites at the metal-support interface. Mechanistic studies showed that iodobenzene dissociation occurs on low-coordinated Au0 atoms present in small gold particles (e.g., Au38 and Au38O2), whereas phenylacetylene is preferentially adsorbed and activated on Auδ+ species existing at the metal-support interface. When this occurs, the activation energy of the rate-determining step for the Sonogashira cross-coupling reaction, which has been found experimentally to be bimolecular coupling, is minimized. The product distribution obtained with Au/CeO2 catalyst containing different ratios of Au0/Auδ+ sites confirms the positive role played by cationic gold in the Sonogashira cross-coupling reaction. Importantly, only metallic Au0 atoms present in gold particles are required to perform the homocoupling of iodobenzene, which explains the product selectivity of the Sonogashira cross-coupling reaction.
![Figure 2 Optimized structures of the gold catalyst models used in the theoretical study: (A) Au(111) facet containing high-coordinated neutral Au0 atoms, (B) Au38 cuboctahedral particle containing low-coordinated neutral Au0 atoms, (C) partially oxidized Au38 particles (Au38O2) containing low-coordinated metallic Au0 and cationic Auδ+ sites, and (D) Au/CeO2 model containing neutral Au0 atoms on top of the rod and cationic Auδ+ sites at the metal-support interface. Au, yellow; O, red; Ce, blue. From [27] with permission.](/document/doi/10.1515/ntrev-2013-0020/asset/graphic/ntrev-2013-0020_fig2.jpg)
Optimized structures of the gold catalyst models used in the theoretical study: (A) Au(111) facet containing high-coordinated neutral Au0 atoms, (B) Au38 cuboctahedral particle containing low-coordinated neutral Au0 atoms, (C) partially oxidized Au38 particles (Au38O2) containing low-coordinated metallic Au0 and cationic Auδ+ sites, and (D) Au/CeO2 model containing neutral Au0 atoms on top of the rod and cationic Auδ+ sites at the metal-support interface. Au, yellow; O, red; Ce, blue. From [27] with permission.
3 Reactivity and catalytic properties of gold nanoparticles
3.1 Ullmann homocoupling reaction of aryl halides
During the first 70 years of the 20th century, copper was almost the only metal usable for aryl-aryl bond formation, initially in the form of copper bulk metal for reductive symmetric coupling of aryl halides (so-called Ullmann homocoupling reaction) [28, 29]. Recently, gold nanoparticles were applied to Ullmann homocoupling reaction, as the Csp2-I bond of iodides can be activated to the C-Au-I intermediate by gold nanoparticles.
Karimi and Esfahani recently found out that gold nanoparticles with average size of 3–15 nm can be used in Ullmann homocoupling reaction [30]. The gold nanoparticles were supported on the mesochannels of the bifunctional periodic mesoporous organosilicas (PMOs) [30]. The initial application of the Au@PMOs catalyst was investigated in the C-O cross-coupling reaction of phenol and iodobenzene (Scheme 2). However, it was interesting that the product of the test was the Ullmann homocoupling product (i.e., biphenyl) instead of the C-O cross-coupling product (i.e., biphenyl ether) (Scheme 2). The coupling reaction is applicable to aryl iodides by using 1 mol% of catalyst and 3 equiv. of K3PO4 as base and N-methylpyrrolidone as solvent at reaction temperature of 100°C (for 16 h). The yields of the symmetric biaryls were 80–95%; note that the conversion of bromobenzene was below 5%. The recovery of the Au@PMOs catalyst can be successfully achieved in five successive reaction runs.
Ullmann homocoupling reaction catalyzed by Au@PMOs.
Monopoli and coworkers reported that the ∼1-nm gold nanoparticles, which possess large surface areas, showed good catalytic activity in Ullmann homocoupling reaction of aryl iodides [31]. The reaction was carried out in two different sets of conditions: (i) H2O/TBAOH/glucose (TBAOH=tetrabutylammonium hydroxide) and (ii) molten TBAA/glucose (TBAA=tetrabutylammonium acetate), both at 90°C (Scheme 3). The ionic liquids (ILs) TBAA and TBAOH play the dual role of surfactant and base in the coupling reaction. This reaction can also be applied to aryl iodides, and the yield of the symmetric biaryls ranges from 40% to 98%; note that the bromobenzene is unreactive. The catalytic activity of the larger gold nanoparticles (e.g., ∼20 nm) was slightly lower than that of the smaller ones due to the reduced surface area.
Ullmann homocoupling reaction catalyzed by Au nanoparticles in ionic liquids.
Dhital and coworkers described the unique catalytic activity of bimetallic Au/Pd alloy nanoparticles for Ullmann homocoupling of chloroarenes in aqueous media at low temperature (27–45°C) [32]. It is very interesting that there is no reaction when using the monometallic Au:PVP, Pd:PVP [PVP=poly(N-vinyl-2-pyrrolidone)], or their mixture as catalysts. But surprisingly the Au0.5Pd0.5:PVP (Au:Pd=50:50) catalyst exhibited very high catalytic activity. In terms of the Au/Pd ratio, the Au0.5Pd0.5:PVP catalyst was found to be much better than corresponding Au0.8Pd0.2:PVP and Au0.2Pd0.8:PVP catalysts. Further, quantum chemical calculations were performed using
![Scheme 4 Possible pathways for Ullmann homocoupling of chloroarenes catalyzed by bimetallic Au0.5Pd0.5:PVP catalyst. From ref. [32] with permission.](/document/doi/10.1515/ntrev-2013-0020/asset/graphic/ntrev-2013-0020_scheme4.jpg)
Possible pathways for Ullmann homocoupling of chloroarenes catalyzed by bimetallic Au0.5Pd0.5:PVP catalyst. From ref. [32] with permission.
Recently, Li et al. explored the catalytic application of atomically precise thiolated-protected Au25(SR)18 (R=CH2CH2Ph, the core size is ∼1.3 nm) nanoparticles for the Ullmann-type homocoupling reaction of aryl iodides and obtained high catalytic activity [34]. The Au25(SR)18/oxide catalyst was prepared by impregnation of oxide powders in a dichloromethane solution of Au25(SR)18 (loading of Au25(SR)18: ∼1 wt%). The catalyst gave rise to 99.8% conversion of iodobenzene under N2 atmosphere at 130°C using N,N-dimethylformamide (DMF) as solvent and K2CO3 as base. The Au25(SR)18-catalyzed homocoupling reaction was tested with a range of reaction substrates with various substituents; the electron-rich substrates gave much higher conversion of iodides than electron-deficient ones in this reaction (Table 1). The support effect was also investigated, but no distinct effect of the oxide supports was observed (e.g., CeO2, SiO2, TiO2, and Al2O3). Structure-property correlation was further achieved based on the crystal of the Au25(SR)18 nanoparticle. The structure of Au25(SR)18 is composed of an icosahedral Au13 kernel and six -SR-Au-SR-Au-SR- staple-like protecting semi-rings [35]. The positively charged surface gold atoms (Auδ+) in Au25(SR)18 nanoparticles are rationalized to be the active sites for activating iodobenzene, whereas the electron-rich, redox-active Au13 kernel [35] takes part in the electron transfer process in the catalytic reaction.
Homocoupling of aryl iodides using Au25/CeO2 catalyst. (A) Crystalline structure of Au25(SR)18 (ball-stick model); (B) space-filling model and the triangular catalytic active sites. From ref. [34] with permission.
![Table 1 Homocoupling of aryl iodides using Au25/CeO2 catalyst. (A) Crystalline structure of Au25(SR)18 (ball-stick model); (B) space-filling model and the triangular catalytic active sites. From ref. [34] with permission.](/document/doi/10.1515/ntrev-2013-0020/asset/graphic/ntrev-2013-0020_fx1.jpg)
3.2 Oxidative homocoupling reaction of organoboronates
Gold nanoparticles are also an excellent catalyst for the oxidative homocoupling reaction of arylboronic acids, which is recognized as one of the convenient methods for such reactions, especially in the preparation of symmetrical biaryls like the Ullmann homocoupling reaction [17]. The gold particle-catalyzed aerobic oxidative homocoupling of phenylboronic acid usually gives rise to two main products: biphenyl and phenol (Scheme 5) [17].
Oxidative homocoupling of phenylboronic acid to yield biphenyl and phenol.
Tsukuda’s group first reported gold nanoparticle-catalyzed oxidative homocoupling reaction of phenylboronic acid in 2004 [36]. The reaction was catalyzed by sub-2-nm Au:PVP particles in water under aerobic conditions. Substituent effects (i.e., steric and electronic effects) of reactants and size effects of gold particles have been investigated in the coupling reaction. 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 1.3 to 9.5 nm). 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
![Scheme 6 Proposed scheme of oxidative homocoupling of phenylboronic acid catalyzed by Au:PVP NCs. From ref. [36] with permission.](/document/doi/10.1515/ntrev-2013-0020/asset/graphic/ntrev-2013-0020_scheme6.jpg)
Proposed scheme of oxidative homocoupling of phenylboronic acid catalyzed by Au:PVP NCs. From ref. [36] with permission.
Oxidative homocoupling of potassium aryltrifluoroborate to yield biphenyl and phenol.
Dhital et al. prepared Au:chitosan particles (∼2.3 nm) protected by chitosan (β-1,4-linked poly(d-glucosamine) and investigated the catalytic performance for oxidative homocoupling of arylboronic acids under acidic conditions (pH 4.57) [38]. 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 [36]; the difference is not clear. In terms of reaction mechanism, the homocoupling reaction proceeded via an anionic species
Chaicharoenwimolkul and coworkers have investigated the catalytic activity of gold particles capped with ligands containing ferrocene moieties [40]. 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. However, the selectivity for product I (Scheme 8) catalyzed by gold particles with ferrocene moiety is near 100%, which is much better than that catalyzed by Au(III) salt (34%) in the demetalation process of ferrocenylboronic acid (Scheme 8).
Demetalation reaction of ferrocenylboronic acid catalyzed by Au(III) and Au NCs stabilized by ligands containing ferrocene moieties.
In 2012, Wang and coworkers reported that ultrasmall gold nanoparticles (1–4 nm) supported on Mg-Al mixed oxides (Au/MAO), and the catalyst showed high catalytic performance and good recyclability in aerobic oxidative homocoupling of phenylboronic acid under base-free conditions (Scheme 9) [41]. The reaction was run at 100°C for 12 h under 1.5 MPa of O2 atmosphere. They found that the high conversion of phenylboronic acid and high selectivity for biphenyl in the Au/MAO catalyst was related with three factors, including oxygen, hydroxyls on MAO, and small amounts of H2O. 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.
![Scheme 9 The proposed mechanism for aerobic homocoupling of phenylboronic acid over a Au/MAO catalyst. Redrawn from [41].](/document/doi/10.1515/ntrev-2013-0020/asset/graphic/ntrev-2013-0020_scheme9.jpg)
The proposed mechanism for aerobic homocoupling of phenylboronic acid over a Au/MAO catalyst. Redrawn from [41].
Zheng and coworkers applied the polystyrene-polyamidoamine (PS-PAMAM)-supported gold nanoparticle catalyst for the oxidative homocoupling of phenylboronic acids [42]. The reaction was carried out in water and air using K2CO3 as base. It is very interesting that steric factors also played a major role in the product selectivity (Table 2). The unsubstituted and meta/para-substituted (i.e., R=Me and OMe) phenylboronic acids almost exclusively afforded biphenyl products (Table 2, entries 1–3, 5, and 6), whereas the ortho-substituted ones afforded phenols (Table 2, entries 4 and 7). In addition, the PS-PAMAM-Au particle catalyst also had good catalytic activities in the aerobic oxidation of benzyl alcohols to ketone.
Homocoupling of various arylboronic acids catalyzed by PS-PAMAM-Au NCs (data from [42]).
 | |||
|---|---|---|---|
| Entry | R | Yields (%) | |
| Biphenyl | Phenol | ||
| 1 | H | 99 | Trace |
| 2 | 4-Me | 98 | Trace |
| 3 | 3-Me | 96 | Trace |
| 4 | 2-Me | Trace | 95 |
| 5 | 4-OMe | 98 | Trace |
| 6 | 3-OMe | 94 | Trace |
| 7 | 2-OMe | Trace | 95 |
In heterogeneous catalysts (i.e., metal particles loaded on a support), the effect of the support (crystallite size, degree of hydroxylation, and crystallinity) often plays an important role in the metal nanoparticle-catalyzed carbon-carbon coupling reaction. 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 [43]. The supported palladium nanoparticle catalyst showed a selectivity of about 75% toward biphenyl, whereas ∼100% selectivity was obtained when using oxide-supported gold nanoparticles (i.e., CeO2, TiO2, and ZrO2) as the catalyst except in the case of SiO2 as support. 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 [43].
3.3 Suzuki cross-coupling reaction
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 [44]. 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.
In early work, it was reported that gold supported on nanocrystalline ceria particles (Au/CeO2) did not catalyze the Suzuki cross-coupling between phenylboronic acid and p-iodobenzophenone (Scheme 10) [45]; instead, all the phenylboronic acid was transformed into biphenyl (oxidative homocoupling product) as discussed in section 3.2. The catalytic result implied that Au/CeO2 only promotes the quantitative (∼100%) oxidative homocoupling of phenylboronic acids to the corresponding symmetric biphenyl under the typical reaction conditions for this Suzuki cross-coupling (5 mol% Au/CeO2, 55°C, K2CO3, toluene as solvent). The authors deemed that the Au3+ species, instead of Au+ and Au0, catalyzed this coupling reaction, and Ce4+ species in the support (CeO2) played the oxidant role in the reaction (Scheme 10, lower panel), as the homocoupling reaction proceeded with the same activity even in the absence of oxygen (note that the necessity of O2 in the homocoupling is still under debate). Intriguingly, the cross-coupling reaction was found to require oxygen and water to occur. The whole procedure was proposed as the following: The OH groups on the surface of the CeO2 interact with boronic acid to form boric acid, while two phenyl groups interact with Au3+ to form a Csp2-Au bond. Thus, homocoupling occurs and Au3+ is reduced to Au+, and then the Au+ was reoxidized to Au3+ by Ce4+ species (Scheme 10, lower panel).
![Scheme 10 Catalytic activity of Au/CeO2 for the oxidative homocoupling of phenylboronic acid versus its cross-coupling with p-iodobenzophenone and mechanistic rationalization involving Au3+ species. Redrawn from [45].](/document/doi/10.1515/ntrev-2013-0020/asset/graphic/ntrev-2013-0020_scheme10.jpg)
Catalytic activity of Au/CeO2 for the oxidative homocoupling of phenylboronic acid versus its cross-coupling with p-iodobenzophenone and mechanistic rationalization involving Au3+ species. Redrawn from [45].
However, in 2009, Han et al. first reported the application of poly(2-aminothiophenol) (PATP) polymer-stabilized gold nanoparticles for the Suzuki cross-coupling reaction of aryl halides with arylboronic acids using NaOH as base in water and air [46]. The results showed that the catalytic activity of gold nanoparticles decreased with increasing sizes of gold nanoparticles from 1.0 to 2.0 and 5.0 nm. 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 11). 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 [47]. The catalytic activity was influenced by the size of gold particles: ∼7.5-nm Au particles are less catalytically active than the ∼3-nm Au particles.
Suzuki cross-coupling reaction of arylboronic acids and aryl halides catalyzed by Au/PATP.
Selvam and Chi have synthesized luminescent gold nanoparticles with average size of 1.7–2.0 nm, which was achieved by thermal decomposition of gold organometallic precursor CH3AuPPh3 in the presence of thiol surfactants. The luminescent gold nanoparticles were also investigated in the Suzuki cross-coupling reaction of phenylboronic acid and iodobenzene, and a 68% yield was obtained [48].
Bimetallic metal nanoparticles are excellent catalysts in organic catalytic reactions, which sometimes perform even better than the corresponding monometallic particles. The Suzuki cross-coupling reaction catalyzed by bimetallic Pd/Au nanoparticles was first reported by Zheng and coworkers [49]. The Pd and Pd/Au alloy nanoparticles were prepared by controlling the pH value, the amounts of precursors and the contact time between the metal ions (Pd2+ and/or Au3+) and the SBA-15 supported G4-PAMAM dendrimers [G represents the generation, PAMAM=poly(amido-amine); Figure 3]. The transmission electron microscopy (TEM) images indicate that the nanoparticles were nearly monodispersed and stabilized in channels of SBA-15 planted by dendrimers (Figure 3). The as-prepared Pd and Pd/Au particle catalysts were then tested in Suzuki cross-coupling reactions under microwave irradiation conditions. The reactions were carried out in a mixture of water and ethanol (3:2) at 100°C using K3PO4 as base. The catalytic results showed that the Pd nanoparticles have poor performance in the cross-coupling of arylboronic acids with aryl bromides. In contrast, the Pd/Au bimetallic nanoparticles exhibited superior catalytic activity under the same conditions. In addition, these bimetallic nanoparticles can also be applied to the cross-coupling between aryl chloride and arylboronic acids with moderate yields. The Pd/Au bimetallic nanoparticle catalyst could be recovered and reused.
![Figure 3 Procedure for the preparation of Pd and Pd/Au nanoparticle catalysts (top panel). Scheme of Suzuki cross-coupling reaction catalyzed by Pd or Pd/Au nanoparticles (bottom panel). Redrawn from [49].](/document/doi/10.1515/ntrev-2013-0020/asset/graphic/ntrev-2013-0020_fig3.jpg)
Procedure for the preparation of Pd and Pd/Au nanoparticle catalysts (top panel). Scheme of Suzuki cross-coupling reaction catalyzed by Pd or Pd/Au nanoparticles (bottom panel). Redrawn from [49].
Nanoflower-like Pd/Au nanoparticles with a Au core and Pd petals (Figure 4) were formed by reduction of Pd(II) by hydroquinone in the presence of gold nanoparticles and polyvinylpyrrolidone [50]. The bimetallic Pd/Au nanoparticles were characterized by high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) (Figure 4). The catalytic activity of as-synthesized bimetallic nanoparticle catalyst was probed and compared against the activity of Pd nanocubes and thin-shelled Au/Pd core-shell nanoparticles in the Suzuki cross-coupling reaction of iodobenzene and phenylboronic acid. 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. In another report, flower-like bimetallic Au/Pd particles were also tested for the Suzuki cross-coupling reaction of iodobenzene and phenylboronic acid [51]; above 98% yield of biphenyl was achieved.
![Figure 4 (A) HAADF-STEM image of nanoflowers consisting of a Au core and Pd petals. (B–D) Elemental maps obtained with EDX spectroscopy showing the distribution of Au (B) and Pd (C) in the nanoflowers, and an overlain image (D). From [50] with permission.](/document/doi/10.1515/ntrev-2013-0020/asset/graphic/ntrev-2013-0020_fig4.jpg)
(A) HAADF-STEM image of nanoflowers consisting of a Au core and Pd petals. (B–D) Elemental maps obtained with EDX spectroscopy showing the distribution of Au (B) and Pd (C) in the nanoflowers, and an overlain image (D). From [50] with permission.
Biodeposited Pd/Au bimetallic nanoparticles (denoted as bio-Pd/Au) were synthesized by co-precipitation of Pd and Au salts using bacterial cells as reducing and stabilizing agents [52]. The bio-Pd/Au catalyst was examined in Suzuki cross-coupling reaction of aryl iodide and phenylboronic acid (Figure 5); the reaction was run at 70°C in a mixture of EtOH/H2O (2:1) using K2CO3 as a base. The authors pointed out that the bio-Pd/Au catalyst needs improvement for long-term stability [52].
![Figure 5 Bio-Pd/Au bimetallic nanoparticles for the Suzuki cross-coupling reaction. From [52] with permission.](/document/doi/10.1515/ntrev-2013-0020/asset/graphic/ntrev-2013-0020_fig5.jpg)
Bio-Pd/Au bimetallic nanoparticles for the Suzuki cross-coupling reaction. From [52] with permission.
Trimetallic Au-Ag-Pd nanoparticles with Au-core/Ag-interlayer/Pd-shell structure capped by cetyltrimethylammonium bromide (CTAB) were formed by temperature-controlled self-organization [53]. The catalytic activity of Au-Ag-Pd:CTAB catalyst was tested in Suzuki cross-coupling reaction of a scope of arylboronic acids and aryl iodides (Scheme 12). The optimized reaction was done in the mixture of DMF/H2O and using sodium acetate (NaOAc) as base at 100°C. The trimetallic nanoparticles showed a much higher catalytic activity than monometallic Pd nanoparticles, which was due to the special core/shell structure of the Au-Ag-Pd particles. The Au-Ag-Pd trimetallic particle catalyst can also be recycled and reused.
Suzuki cross-coupling reaction of arylboronic acids and aryl halides catalyzed by Au-Ag-Pd:CTAB catalyst.
3.4 Sonogashira cross-coupling reaction of phenylacetylene and iodides
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 [54]. 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 Csp2-I bond of iodobenzene and the Csp-H bond of phenylacetylene on one gold atom or the neighbor gold atoms [28]. 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) [55].
Coupling reactions of iodobenzene and phenylacetylene to yield the desired Sonogashira cross-coupling product DPA and two homocoupling side products DPDA and BP (Ullmann homocoupling product).
Kyriakou and coworkers examined the catalytic activity of gold nanoparticles supported on silica in Sonogashira cross-coupling reaction of iodobenzene and phenylacetylene [56]. Three different sizes (A: 23 nm, B: 12 nm, and C: 2.8 nm) of the silica-supported Au nanoparticles were chosen for testing the size effect; the reaction was run at 145°C in DMF for 160 h (other conditions: 0.5 mmol iodobenzene and phenylacetylene; 60 mg catalyst; 0.3 mmol base). Catalysts A and B behaved similarly with ∼100% conversion of iodobenzene after 160 h. However, catalyst C, which contained the smallest Au particles, gave a much lower yield for the cross-coupling product and ∼70% conversion of iodobenzene, as the coupling reaction had already ceased at 140 h (deactivated catalyst), which was supported by the X-ray photoelectron spectroscopy and inductively coupled plasma mass spectrometry data. 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 TiO2-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.e., BaO, CeO2, Al2O3, La2O3, TiO2, and SiO2) for the 20-nm gold catalyst in the above coupling system [57]. The catalytic results showed that Au0 nanoparticles supported on SiO2, γ-Al2O3, and BaO were active but relatively unselective; however, as with lanthana, ceria-supported Au0 nanoparticles showed high selectivity. It indicated that the catalytically active sites were associated with the metallic gold particles, and the author deemed that the support behavior cannot be accounted for in terms of redox, acid/base, or strong metal-support interaction effects; it may instead be tentatively ascribed to metal-to-support hydrogen spillover. They also investigated the same coupling on Au(111) [58]. Chitosan as an efficient porous support for the 1.6 nm gold particles was also examined in the Sonogashira cross-coupling reaction [59].
De Souza and coworkers reported Sonogashira cross-coupling reaction catalyzed by supported gold catalysts under microwave irradiation, which is widely applied in organic synthesis [60]. 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. The Au/SiO2 catalyst gave the best result for both aryl and alkyl acetylenes among the three different oxide-supported gold catalysts (i.e., Ce2O3, Nb2O5, and SiO2).
Venkatesan and Santhanalakshmi synthesized CTAB-capped Au/Ag/Pd trimetallic nanoparticle, which has been used as catalyst for Suzuki cross-coupling reaction [53]. Further, they expanded the application of the trimetallic nanoparticles (4.2±0.5 nm) in the Sonogashira cross-coupling reaction of arylacetylene and halides [61]. The reaction was carried out at 120°C using K2CO3 as base and DMF-H2O as solvent. When moving on to aryl halides, the reactivity of I≈Br>>Cl was observed (yield of biphenylacetylene: 99% for iodobenzene, 95% for bromobenzene, and 30% for chlorobenzene). The yields with other substrates ranged from 70% to 96%. 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.
3.5 A3-coupling reaction of phenylacetylene, aldehdye, and piperidine
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 [62]. Among MCRs, the A3-coupling reaction of aldehydes, amine, and terminal alkynes is one of the best examples, where propargylamine is obtained as the major product [63]. Gold nanoparticles or nanoparticles could be applied to this A3-coupling, as the Csp-H bond of terminal alkynes can be activated by gold particles (including Au0, Auδ+, Au+, and Au3+ species), which is the key step during the A3-coupling processes [64].
Kidwai and coworkers first reported the application of ∼20-nm gold nanoparticles for the A3-coupling reaction of aldehyde, amine, and alkyne under optimized conditions (1.0 equiv. of benzaldehyde, 1.0 equiv. of piperidine, 1.5 equiv. of phenylacetylene, 10 mol% Au-NPs; MeOH; 80°C; N2) [65]. The size effects of the gold particles were investigated; it was found that the catalytic activity of gold nanoparticles decreased with increasing size due to less surface area available for reactant adsorption/activation. Although the Au catalyst was demonstrated to be recycled and reused, the yield dropped drastically from 92% (first cycle) to 63% (fifth cycle) with the reaction time being significantly prolonged from 5 to 18 h in order to attain 63% yield. A range of aromatic aldehydes and amine were applied in this A3-coupling reaction, and aryl aldehydes possessing electron-withdrawing groups were found to exhibit better reactivity than those with electron-donating groups. The isolated yields of the propargylamine were from 67% to 96%, and the conversions of aldehydes were from 73% to 97%. A tentative two-step mechanism was proposed for the A3-coupling reaction (Figure 6). During the first step, which is reversible, the aldehyde reacts with amine to yield an iminium ion (C=N+R1R2) intermediate. In the second step, the iminium ion intermediate couples with the activated phenylacetylene (deprotonated by gold catalyst), which is the key step in the A3-coupling reaction. Mesoporous carbon nitride (MCN) with built-in groups of -NH2 and -NH can also be used as stabilizer for gold nanoparticles (<7 nm) [66]. The gold particles supported/encapsulated by MCN exhibited a moderate catalytic activity with 47–69% yields of propargylamines. The commercial Nano Active™ Magnesium Oxide Plus [NAP-MgO, was purchased from Nano Scale Materials, Inc. (Manhattan, USA)] was also used as support for gold nanoparticle catalyst in the A3-coupling reaction [67]. The nanogold catalyst (10–12 nm) was synthesized via stabilization of
![Figure 6 Proposed mechanism of Au nanoparticle-catalyzed A3-coupling of aldehyde, amine, and alkyne. Redrawn from [65].](/document/doi/10.1515/ntrev-2013-0020/asset/graphic/ntrev-2013-0020_fig6.jpg)
Proposed mechanism of Au nanoparticle-catalyzed A3-coupling of aldehyde, amine, and alkyne. Redrawn from [65].
![Scheme 14 Preparation of NAP-Mg-Au(0) catalyst. Redrawn from [67].](/document/doi/10.1515/ntrev-2013-0020/asset/graphic/ntrev-2013-0020_scheme14.jpg)
Preparation of NAP-Mg-Au(0) catalyst. Redrawn from [67].
Karimi and coworkers reported gold nanoparticle-catalyzed A3-coupling reaction of aldehydes, secondary amines, and terminal alkynes, which was conducted at 60°C using chloroform as solvent and Au@PMO-IL as catalyst (PMO-IL=PMO containing ionic liquid) [68]. An 88% isolated yield was obtained when using 4-methylbenzaldehyde, piperidine, and phenylacetylene as reactants, and the substrate scope of the methodology was established by employing a range of functionalized aromatic and aliphatic aldehydes (Scheme 15). Generally, the yields of all the tested catalytic reactions were from 75% to 88%, regardless of the steric effects. The TEM image showed that the Au@PMO-IL catalyst was kept intact after the third reaction cycle.
A3-coupling reaction of a combination of aldehydes, secondary amines, and terminal alkynes catalyzed by Au@PMO-IL.
In general, the A3-MCR is performed as a “one-pot” process. However, Abahmane and coworkers found that aldimine formation from aldehydes and secondary amines catalyzed by montmorillonite K10 (MM K-10) beforehand can benefit the A3-coupling reaction [69], as the A3-coupling reaction was investigated as a two-step flow process shown in Figures 6 and 7D. In order to investigate the performance of the process, three different reaction regimes A–C were set up (Figure 7): A) the catalytic potentials of PBCR1 (PBCR, packed-bed capillary reactors) and PBCR2 for the three-component reaction were investigated independently of one another; B) solutions of all three building blocks were mixed and fed sequentially through PBCR1 and PBCR2 at different temperatures; C) the aldehyde and amine building block solutions were mixed and fed through PBCR1, and then the alkyne solution was added to the intermediate product stream before implementation of PBCR2 (PBCR1 was filled with MM K-10, and PBCR2 was filled with Au-NP@Al2O3) (Figure 7). Regime C gave the best reaction performance in those reaction regimes, as the MM K-10 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).
![Figure 7 (A–C) Flow chemistry setup schemes for the different reaction regimes, P1 and P2: pumps; PBCR1 (MM K-10) and PBCR2 (Au-NP@Al2O3); BPR, back-pressure regulator. (D) Scheme of the A3-coupling reaction and the combination of the various aldehydes and amines used in the reaction regime C. Redrawn from [69].](/document/doi/10.1515/ntrev-2013-0020/asset/graphic/ntrev-2013-0020_fig7.jpg)
(A–C) Flow chemistry setup schemes for the different reaction regimes, P1 and P2: pumps; PBCR1 (MM K-10) and PBCR2 (Au-NP@Al2O3); BPR, back-pressure regulator. (D) Scheme of the A3-coupling reaction and the combination of the various aldehydes and amines used in the reaction regime C. Redrawn from [69].
Recently, González-Béjar and coworkers reported the photocatalysis of gold nanoparticles on ZnO in the presence of aldehydes, amines, and phenylacetylene, which led to rapid and selective formation of propargylamines with good yields (50–95%) at room temperature [70]. The surface plasmon band of supported Au particles allows for selective excitation of Au nanoparticles at 530 nm over the organic substrates and the bare supports as the latter two have no absorption at this wavelength. The AuNP@ZnO photocatalyst performed much better than Au particles supported on Al2O3 and TiO2 (95% yield for ZnO vs. 27% for Al2O3 and 20% for TiO2); the reaction was carried out in dry acetonitrile for 2 h under N2. 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.
![Figure 8 Proposed mechanism for the plasmon-mediated A3-coupling of isovaleraldehyde, piperidine, and phenylacetylene catalyzed by AuNP@ZnO, which was irradiated at 532 nm. Redrawn from [70].](/document/doi/10.1515/ntrev-2013-0020/asset/graphic/ntrev-2013-0020_fig8.jpg)
Proposed mechanism for the plasmon-mediated A3-coupling of isovaleraldehyde, piperidine, and phenylacetylene catalyzed by AuNP@ZnO, which was irradiated at 532 nm. Redrawn from [70].
4 Conclusions
Over the past decade, significant research has demonstrated the extraordinary catalytic activity of gold nanoparticles in the carbon-carbon coupling reactions (such as Ullmann homocoupling of halides, oxidative homocoupling of organoboronates, Suzuki cross-coupling of phenylboronic acid and halides, Sonogashira cross-coupling of iodobenzene and phenylacetylene, and A3-coupling reaction of phenylacetylene, amines, and aryl or alkyl aldehydes), in addition to the selective oxidation and hydrogenation reactions. Among the gold nanoparticles, the size from 1 to 20 nm is more efficient in the carbon-carbon coupling reactions. In general, the smaller nanoparticles exhibit higher catalytic activity in the carbon-carbon coupling reactions due to their larger surface area than that of larger nanoparticles. With respect to the catalytic supports, the crystalline CeO2 support with Ce3+ and Ce4+ species often performs the best among the oxide supports (e.g., TiO2, SiO2, Al2O3, Fe2O3, MgO, BaO). The detailed mechanisms involved in CeO2-supported gold nanocatalysts merit future studies. Among the carbon-carbon coupling reactions, research on the gold nanoparticle-catalyzed Sonogashira cross-coupling reaction has started to attract research interest in the recent couple of years; so far it is only focused on iodobenzene, and future work should devise strategies to activate bromobenzene and even chlorobenzene.
Research on gold-based bimetallic or multimetallic (e.g., alloying with copper, palladium, platinum, silver) nanoparticles is particularly promising [33, 71]. The unique alloy catalysts can achieve a large variety of catalytic reactions with high activity and selectivity, and such catalysts can be used for multistep catalysis due to their unique multisite and electronic effect between metal components.
Expansion of supports is also another important direction, such as high surface area solid hybrid materials (e.g., heterostructured transition-metal oxide-mesoporous silica supports [12] and metal-organic frameworks [72]). The hybrid materials can largely enhance the stability of the metal particles during the catalytic reactions, which is very important for the durability and recyclability of nanocatalysts, and the steric effect of the pores or the channels of the hybrid materials may also improve the selectivity during the catalytic process. Optimization of gold nanoparticle catalyst and experimental conditions is further needed to enhance the catalytic activity in the carbon-carbon coupling reactions, which can be applied in industrial catalysis in the future.
Understanding the carbon-carbon coupling catalytic behavior and mechanism, the correlation between catalytic performance and the structure and intrinsic properties of nanocatalysts should be further pursued in future work. In this regard, the atomically precise gold nanoparticle catalysts, such as thiolate-capped gold nanoparticles or nanoclusters [73], are particularly attractive, as these nanoclusters are well defined to the atomic level and they are particularly robust and exhibit excellent catalytic performance in some catalytic organic reactions [74]. The atomically precise gold nanocluster catalysts eliminate the size polydispersity issue in conventional nanocatalysts; hence, such nanocluster catalysts offer some exciting opportunities in investigating the fundamental aspects of carbon-carbon coupling and other catalytic processes. In addition, the crystallographically characterized structures [73] of gold nanoclusters permit precise correlation between structure and catalytic activity as well as employing them as new model catalysts for theoretical calculations to attain deeper insight into reactant adsorption and activation details [75–77].
Finally, we expect that gold nanoparticle catalysts can be further expanded to carbon-heteroatom coupling reactions to form C-O and C-N bonds. The carbon-heteroatom coupling reactions are readily catalyzed by homogeneous gold(I) and some gold(III) complexes [78, 79]; it would be highly desirable to utilize nanogold for such reactions. The gold nanoparticle catalysts may also be expanded to another type of new carbon-heteroatom coupling reactions that are much more difficult to achieve compared to C-O and C-N systems, such as carbon-sulfur, carbon-phosphorous, carbon-silica, and carbon-boron coupling [80]. These expansions may offer new exciting possibilities for gold nanoparticle catalysts.
About the authors
Gao Li received his BS in Chemistry from Hunan Normal University (Changsha, China) in 2004, and his PhD in Applied Chemistry from Shanghai Jiao Tong University (Shanghai, China) in 2011. He is currently a postdoctoral research associate at Carnegie Mellon University (PA, USA). His current research interests focus on the preparation and catalytic applications of gold nanoparticles.
Rongchao Jin is an associate professor of chemistry at Carnegie Mellon University. He received his BS in Chemical Physics from the University of Science and Technology of China (Hefei, China) in 1995, his MS in Physical Chemistry/Catalysis from Dalian Institute of Chemical Physics (Dalian, China) in 1998, and his PhD in Chemistry from Northwestern University (IL, USA) in 2003. After 3 years of postdoctoral research at the University of Chicago (IL, USA), he joined the chemistry faculty of Carnegie Mellon University (PA, USA) in 2006. His current research interests focus on atomically precise noble metal nanoparticles and their applications in catalysis, optics, and sensing.
The authors are grateful for support by the U.S. Department of Energy Office of Basic Energy Sciences (grant DE-FG02-12ER16354).
References
[1] Astruc D, Lu F, Aranzaes JR. Nanoparticles as recyclable catalysts: the frontier between homogeneous and heterogeneous catalysis. Angew. Chem., Int. Ed. 2005, 44, 7852–7872.Search in Google Scholar
[2] Jin R. The impacts of nanotechnology on catalysis by precious metal nanoparticles. Nanotechnol. Rev. 2012, 1, 31–56.10.1515/ntrev-2011-0003Search in Google Scholar
[3] Goguet A, Ace M, Saih Y, Sa J, Kavanagh J, Hardacre C. Remarkable stability of ionic gold supported on sulfated lanthanum oxide. Chem. Commun. 2009, 32, 4889–4891.Search in Google Scholar
[4] Ono LK, Cuenya BR. Formation and thermal stability of Au2O3 on gold nanoparticles: size and support effects. J. Phys. Chem. C 2008, 112, 4676–4686.10.1021/jp711277uSearch in Google Scholar
[5] Fu L, Wu NQ, Yang JH, Qu F, Johnson DL, Kung MC, Kung HH, Dravid VP. Direct evidence of oxidized gold on supported gold catalysts. J. Phys. Chem. B 2005, 109, 3704–3706.10.1021/jp045117eSearch in Google Scholar PubMed
[6] Concepcion P, Carrettin S, Corma A. Stabilization of cationic gold species on Au/CeO2 catalysts under working conditions. Appl. Catal., A 2006, 307, 42–45.10.1016/j.apcata.2006.03.004Search in Google Scholar
[7] Klimev H, Fajerwerg K, Chakarova K, Delannoy L, Louis C, Hadjiivanov K. Oxidation of gold metal particles supported on TiO2: an FTIR study by means of low-temperature CO adsorption. J. Mater. Sci. 2007, 42, 3299–3306.Search in Google Scholar
[8] Stratakis M, Garcia H. Catalysis by supported gold nanoparticles: beyond aerobic oxidative processes. Chem. Rev. 2012, 112, 4469–4506.Search in Google Scholar
[9] Wittstock A, Wichmann A, Baumer M. Nanoporous gold as a platform for a building block catalyst. ACS Catal. 2012, 2, 2199–2215.Search in Google Scholar
[10] Takei T, Akita T, Nakamura I, Fujitani T, Okumura M, Okazaki K, Huang JH, Ishida T, Haruta M. Heterogeneous catalysis by gold. Adv. Catal. 2012, 55, 1–126.Search in Google Scholar
[11] Liu Y, Tsunoyama H, Akita T, Tsukuda T. Efficient and selective epoxidation of styrene with TBHP catalyzed by Au25 clusters on hydroxyapatite. Chem. Commun. 2010, 46, 550–552.Search in Google Scholar
[12] Ma G, Binder A, Chi M, Liu C, Jin R, Jiang D, Fan J, Dai S. Stabilizing gold clusters by heterostructured transition-metal oxide-mesoporous silica supports for enhanced catalytic activities for CO oxidation. Chem. Commun. 2012, 48, 11413–11415.Search in Google Scholar
[13] Menard LD, Xu F, Nuzzo RG, Yang JC. Preparation of TiO2-supported Au nanoparticle catalysts from a Au-13 cluster precursor: ligand removal using ozone exposure versus a rapid thermal treatment. J. Catal. 2006, 243, 64–73.Search in Google Scholar
[14] Liu Y, Tsunoyama H, Akita T, Xie S, Tsukuda T. Aerobic oxidation of cyclohexane catalyzed by size-controlled Au clusters on hydroxyapatite: size effect in the sub-2 nm regime. ACS Catal. 2011, 1, 2–6.Search in Google Scholar
[15] Haruta M, Yamada N, Kobayashi T, Iijima S. Gold catalysts prepared by coprecipitation for low-temperature oxidation of hydrogen and of carbon-monoxide. J. Catal. 1989, 115, 301–309.Search in Google Scholar
[16] Haruta M. Catalysis of gold nanoparticles deposited on metal oxides. CATTECH 2002, 6, 102–115.10.1023/A:1020181423055Search in Google Scholar
[17] Tsukuda T, Tsunoyama H, Sakurai H. Aerobic oxidations catalyzed by colloidal nanogold. Chem. Asian J. 2011, 6, 736–748.Search in Google Scholar
[18] Zhu Y, Qian H, Zhu M, Jin R. Thiolate-protected Aun nanoclusters as catalysts for selective oxidation and hydrogenation processes. Adv. Mater. 2010, 22, 1915–1920.Search in Google Scholar
[19] Li G, Qian H, Jin R. Gold nanocluster-catalyzed selective oxidation of sulfide to sulfoxide. Nanoscale 2012, 4, 6714–6717.10.1039/c2nr32171hSearch in Google Scholar PubMed
[20] Della PC, Falletta E, Rossi M. Update on selective oxidation using gold. Chem. Soc. Rev. 2012, 41, 350–369.Search in Google Scholar
[21] Zhu Y, Qian H, Drake BA, Jin R. Atomically precise Au25(SR)18 nanoparticles as catalysts for selective hydrogenation of α, β-unsaturated ketones and aldehydes. Angew. Chem., Int. Ed. 2010, 49, 1295–1298.Search in Google Scholar
[22] Hashmi ASK, Hutchings GJ. Gold catalysis. Angew. Chem., Int. Ed. 2006, 45, 7896–7936.Search in Google Scholar
[23] Daniel MC, Astruc D. Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology. Chem. Rev. 2004, 104, 293–246.Search in Google Scholar
[24] Corma A, Garcia H. Supported gold nanoparticles as catalysts for organic reactions. Chem. Soc. Rev. 2008, 37, 2096–2136.Search in Google Scholar
[25] Boronat M, Corma A. Molecular approaches to catalysis: naked gold nanoparticles as quasi-molecular catalysts for green processes. J. Catal. 2011, 284, 138–147.Search in Google Scholar
[26] Corma A, Juárez R, Boronat M, Sánchez F, Iglesiasc M, García H. Gold catalyzes the Sonogashira coupling reaction without the requirement of palladium impurities. Chem. Commun. 2011, 47, 1446–1448.Search in Google Scholar
[27] Boronat M, Combita D, Concepción P, Corma A, García H, Juárez R, Laursen S, López-Castro JD. Making C-C bonds with gold: identification of selective gold sites for homo- and cross-coupling reactions between iodobenzene and alkynes. J. Phys. Chem. C 2012, 116, 24855–24867.10.1021/jp3071585Search in Google Scholar
[28] Hassan J, Sévignon M, Gozzi C, Schulz E, Lemaire M. Aryl-aryl bond formation one century after the discovery of the Ullmann reaction. Chem. Rev. 2002, 102, 1359–1469.Search in Google Scholar
[29] Monnier F, Taillefer M. Catalytic C-C, C-N, and C-O Ullmann-type coupling reactions. Angew. Chem., Int. Ed. 2008, 47, 3096–3099.Search in Google Scholar
[30] Karimi B, Esfahani FK. Unexpected golden Ullmann reaction catalyzed by Au nanoparticles supported on periodic mesoporousorganosilica (PMOs). Chem. Commun. 2011, 47, 10452–10454.Search in Google Scholar
[31] Monopoli A, Cotugno P, Palazzo G, Ditaranto N, Mariano B, Cioffi N, Ciminale F, Nacci A. Ullmann homocoupling catalysed by gold nanoparticles in water and ionic liquid. Adv. Synth. Catal. 2012, 354, 2777–2788.Search in Google Scholar
[32] Dhital RN, Kamonsatikul C, Somsook E, Bobuatong K, Ehara M, Karanjit S, Sakurai H. Low-temperature carbon-chlorine bond activation by bimetallic gold/palladium alloy nanoclusters: an application to ullmann coupling. J. Am. Chem. Soc. 2012, 134, 20250–20253.Search in Google Scholar
[33] Gao F, Goodman DW. Pd-Au bimetallic catalysts: understanding alloy effects from planar models and (supported) nanoparticles. Chem. Soc. Rev. 2012, 41, 8009–8020.Search in Google Scholar
[34] Li G, Liu C, Lei Y, Jin R. Au25 nanocluster-catalyzed Ullmann-type homocoupling reaction of aryl iodides. Chem. Commun. 2012, 48, 12005–12007.10.7312/li--16274-049Search in Google Scholar
[35] Zhu M, Aikens CM, Hendrich MP, Gupta R, Qian H, Schatz GC, Jin R. Reversible switching of magnetism in thiolate-protected Au25 superatoms. J. Am. Chem. Soc. 2009, 131, 2490–2492.Search in Google Scholar
[36] Tsunoyama H, Sakurai H, Ichikuni N, Negishi Y, Tsukuda T. Colloidal gold nanoparticles as catalyst for carbon-carbon bond formation: application to aerobic homocoupling of phenylboronic acid in water. Langmuir 2004, 20, 11293–11296.10.1021/la0478189Search in Google Scholar PubMed
[37] Sakurai H, Tsunoyama H, Tsukuda T. Oxidative homo-coupling of potassium aryltrifluoroborates catalyzed by gold nanocluster under aerobic conditions. J. Organomet. Chem. 2007, 692, 368–374.Search in Google Scholar
[38] Dhital RN, Murugadoss A, Sakurai H. Dual roles of polyhydroxy matrices in the homocoupling of arylboronic acids catalyzed by gold nanoclusters under acidic conditions. Chem. Asian J. 2012, 7, 55–59.Search in Google Scholar
[39] Sophiphun O, Wittayakun J, Dhital RN, Haesuwannakij S, Murugadoss A, Sakurai H. Gold/palladium bimetallic alloy nanoclusters stabilized by chitosan as highly efficient and selective catalysts for homocoupling of arylboronic acid. Aust. J. Chem. 2012, 65, 1238–1243.Search in Google Scholar
[40] Chaicharoenwimolkul L, Munmai A, Chairam S, Tewasekson U, Sapudom S, Lakliang Y, Somsook E. Effect of stabilizing ligands bearing ferrocene moieties on the gold nanoparticle-catalyzed reactions of arylboronic acids. Tetrahedron Lett. 2008, 49, 7299–7302.Search in Google Scholar
[41] Wang L, Zhang W, Su DS, Meng X, Xiao F. Supported Au nanoparticles as efficient catalysts for aerobic homocoupling of phenylboronic acid. Chem. Commun. 2012, 48, 5476–5478.10.1097/01.ccm.0000424304.17705.e9Search in Google Scholar
[42] Zheng J, Lin S, Zhu X, Jiang B, Yang Z, Pan Z. Reductant-directed formation of PS-PAMAM-supported gold nanoparticles for use as highly active and recyclable catalysts for the aerobic oxidation of alcohols and the homo-coupling of phenylboronic acids. Chem. Commun. 2012, 48, 6235–6237.Search in Google Scholar
[43] Willis NG, Guzman J. Influence of the support during homocoupling of phenylboronic acid catalyzed by supported gold. Appl. Catal. A 2008, 339, 68–75.10.1016/j.apcata.2008.01.019Search in Google Scholar
[44] McGlacken GP, Bateman LM. Recent advances in aryl-aryl bond formation by direct arylation. Chem. Soc. Rev. 2009, 38, 2447–2464.Search in Google Scholar
[45] Carrettin S, Guzman J, Corma A. Supported gold catalyzes the homo-coupling of phenylboronic acid with high conversion and selectivity. Angew. Chem., Int. Ed. 2005, 44, 2242–2245.Search in Google Scholar
[46] Han J, Liu Y, Guo R. Facile synthesis of highly stable gold nanoparticles and their unexpected excellent catalytic activity for Suzuki-Miyaura cross-coupling reaction in water. J. Am. Chem. Soc. 2009, 131, 2060–2061.Search in Google Scholar
[47] Li Y, Fan X, Qi J, Ji J, Wang S, Zhang G, Zhang F. Gold nanoparticles-graphene hybrids as active catalysts for Suzuki reaction. Mater. Res. Bull. 2010, 45, 1413–1418.Search in Google Scholar
[48] Selvam T, Chi K. Synthesis of hydrophobic gold nanoclusters: growth mechanism study, luminescence property and catalytic application. J. Nanopart. Res. 2011, 13, 1769–1780.Search in Google Scholar
[49] Zheng Z, Li H, Liu T, Cao R. Monodisperse noble metal nanoparticles stabilized in SBA-15: synthesis, characterization and application in microwave-assisted Suzuki-Miyaura coupling reaction. J. Catal. 2010, 270, 268–274.Search in Google Scholar
[50] Xu J, Wilson AR, Rathmell AR, Howe J, Chi M, Wiley BJ. Synthesis and catalytic properties of Au-Pd nanoflowers. ACS Nano 2011, 5, 6119–6127.10.1021/nn201161mSearch in Google Scholar PubMed
[51] Han J, Zhou Z, Yin Y, Luo X, Li J, Zhang H, Yang B. One-pot, seedless synthesis of flowerlike Au-Pd bimetallic nanoparticles with core-shell-like structure via sodium citrate coreduction of metal ions. CrystEngComm 2012, 14, 7036–7042.10.1039/c2ce25824bSearch in Google Scholar
[52] Heugebaerta TSA, De Corteb S, Sabbea T, Hennebelb T, Verstraeteb W, Boonb N, Stevens CV. Biodeposited Pd/Au bimetallic nanoparticles as novel Suzuki catalysts. Tetrahedron Lett. 2012, 53, 1410–1412.Search in Google Scholar
[53] Venkatesan P, Santhanalakshmi J. Synthesis, characterization and catalytic activity of trimetallic nanoparticles in the Suzuki C-C coupling reaction. J. Mol. Catal. A: Chem. 2010, 326, 99–106.Search in Google Scholar
[54] Chinchilla R, Najera C. The Sonogashira reaction: a booming methodology in synthetic organic chemistry. Chem. Rev. 2007, 107, 874–922.Search in Google Scholar
[55] Garcia P, Malacria M, Aubert C, Gandon V, Fensterbank L. Gold-catalyzed cross-couplings: new opportunities for C-C bond formation. ChemCatChem 2010, 2, 493–497.10.1002/cctc.200900319Search in Google Scholar
[56] Kyriakou G, Beaumont SK, Humphrey SM, Antonetti C, Lambert RM. Sonogashira coupling catalyzed by gold nanoparticles: does homogeneous or heterogeneous catalysis dominate? ChemCatChem 2010, 2, 1444–1449.10.1002/cctc.201000154Search in Google Scholar
[57] Beaumont SK, Kyriakou G, Lambert RM. Identity of the active site in gold nanoparticle-catalyzed Sonogashira coupling of phenylacetylene and iodobenzene. J. Am. Chem. Soc. 2010, 132, 12246–12248.Search in Google Scholar
[58] Kanuru VK, Kyriakou G, Beaumont SK, Papageorgiou AC, Watson DJ, Lambert RM. Sonogashira coupling on an extended gold surface in vacuo: reaction of phenylacetylene with iodobenzene on Au(111). J. Am. Chem. Soc. 2010, 132, 8081–8086.Search in Google Scholar
[59] Primo A, Quignard F. Chitosan as efficient porous support for dispersion of highly active gold nanoparticles: design of hybrid catalyst for carbon-carbon bond formation. Chem. Commun. 2010, 46, 5593–5595.Search in Google Scholar
[60] de Souza ROMA, Bittar MS, Mendes LVP, da Silva CMF, da Silva VT, Antunes OAC. Copper-free Sonogashira reaction using gold nanoparticles supported on Ce2O3, Nb2O5, and SiO2 under microwave irradiation. Synlett 2008, 12, 1777–1780.10.1055/s-2008-1078565Search in Google Scholar
[61] Venkatesan P, Santhanalakshmi J. Designed synthesis of Au/Ag/Pd trimetallic nanoparticle-based catalysts for Sonogashira coupling reactions. Langmuir 2010, 26, 12225–12229.10.1021/la101088dSearch in Google Scholar PubMed
[62] Syamala M. Recent progress in three-component reactions. An update. Org. Prep. Proced. Int. 2009, 41, 1–68.10.1080/00304940802711218Search in Google Scholar
[63] Peshkov VA, Pereshivko OP, Van der Eycken EV. A walk around the A3-coupling. Chem. Soc. Rev. 2012, 41, 3790–3807.Search in Google Scholar
[64] Zhang X, Corma A. Supported gold(III) catalysts for highly efficient three-component coupling reactions. Angew. Chem., Int. Ed. 2008, 47, 4358–4361.Search in Google Scholar
[65] Kidwai M, Bansal V, Kumar A, Mozumdar S. The first Au-nanoparticles catalyzed green synthesis of propargylamines via a three-component coupling reaction of aldehyde, alkyne and amine. Green Chem. 2007, 9, 742–745.Search in Google Scholar
[66] Datta KKR, Reddy BVS, Ariga K, Vinu A. Gold nanoparticles embedded in a mesoporous carbon nitride stabilizer for highly efficient three-component coupling reaction. Angew. Chem., Int. Ed. 2010, 49, 5961–5965.Search in Google Scholar
[67] Layek K, Chakravarti R, Kantam ML, Maheswaran H, Vinu A. Nanocrystalline magnesium oxide stabilized gold nanoparticles: an advanced nanotechnology based recyclable heterogeneous catalyst platform for the one-pot synthesis of propargylamines. Green Chem. 2011, 13, 2878–2887.Search in Google Scholar
[68] Karimi B, Gholinejad M, Khorasani M. Highly efficient three-component coupling reaction catalyzed by gold nanoparticles supported on periodic mesoporous organosilica with ionic liquid framework. Chem. Commun. 2012, 48, 8961–8963.Search in Google Scholar
[69] Abahmane L, Köhler JM, Groß GA. Gold-nanoparticle-catalyzed synthesis of propargylamines: the traditional A3-multicomponent reaction performed as a two-step flow process. Chem. Eur. J. 2011, 17, 3005–3010.Search in Google Scholar
[70] González-Béjar M, Peters K, Hallett-Tapley GL, Grenier M, Scaiano JC. Rapid one-pot propargylamine synthesis by plasmon mediated catalysis with gold nanoparticles on ZnO under ambient conditions. Chem. Commun. 2013, 49, 1732–1734.Search in Google Scholar
[71] Ferrando R, Jellinek J, Johnston RL. Nanoalloys: from theory to applications of alloy clusters and nanoparticles. Chem. Rev. 2008, 108, 845–910.Search in Google Scholar
[72] Vilhelmsen LB, Walton KS, Sholl DS. Structure and mobility of metal clusters in MOFs: Au, Pd, and AuPd clusters in MOF-74. J. Am. Chem. Soc. 2102, 134, 12807–12816.Search in Google Scholar
[73] Qian H, Zhu M, Wu Z, Jin R. Quantum sized gold nanoclusters with atomic precision. Acc. Chem. Res. 2012, 45, 1470–1479.Search in Google Scholar
[74] Li G, Jin R. Atomically precise gold nanoclusters as new model catalysts. Acc. Chem. Res., doi: 10.1021/ar300213z.10.1021/ar300213zSearch in Google Scholar PubMed
[75] Kauffman DR, Alfonso D, Matranga C, Qian H, Jin R. Experimental and computational investigation of Au25 clusters and CO2: a unique interaction and enhanced electrocatalytic activity. J. Am. Chem. Soc. 2012, 134, 10237–10243.Search in Google Scholar
[76] Pei Y, Shao N, Gao Y, Zeng XC. Investigating active site of gold nanoparticle Au55(PPh3)12Cl6 in selective oxidation. ACS Nano 2010, 4, 2009–2020.10.1021/nn100184mSearch in Google Scholar PubMed
[77] Zhu Y, Wu Z, Gayathri GC, Qian H, Gil RR, Jin R. Exploring stereoselectivity of Au25 nanoparticle catalyst for hydrogenation of cyclic ketone. J. Catal. 2010, 271, 155–160.Search in Google Scholar
[78] Aguilar D, Contel M, Navarro R, Soler T, Urriolabeitia, EP. Gold(III) iminophosphorane complexes as catalysts in C-C and C-O bond formations. J. Organomet. Chem. 2009, 694, 486–493.Search in Google Scholar
[79] Ciobanu M, Cojocaru B, Teodorescu C, Vasiliu F, Coman SM, Leitner W, Parvulescu VI. Heterogeneous amination of bromobenzene over titania-supported gold catalysts. J. Catal. 2012, 296, 43–54.Search in Google Scholar
[80] Corma A, Leyva-Pérez A, Sabater MJ. Gold-catalyzed carbon-heteroatom bond-forming reactions. Chem. Rev. 2011, 111, 1657–1712.Search in Google Scholar
©2013 by Walter de Gruyter Berlin Boston
This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
