Catalysis by gold nanoparticles: carbon-carbon ...

DOI 10.1515/ntrev-2013-0020 Nanotechnol Rev 2013; 2(5): 529–545

Review Gao Li and Rongchao Jin*

Catalysis by gold nanoparticles: carbon-carbon coupling reactions 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. Keywords: carbon-carbon coupling; gold; heterogeneous catalysis; nanocatalysis; nanoparticles. *Corresponding author: Rongchao Jin, Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA, e-mail: [email protected] Gao Li: Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA

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, ligandprotected (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 selecti vity) of nanocatalysts is drastically influenced by the size

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530 G. Li and R. Jin: Nanogold catalysts for carbon-carbon coupling reactions 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 colloi dal 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., 30 kcal mol-1), which indicates that positively charged gold atoms should intervene at this stage to lower this barrier. Thus, calculations based on the partially oxidized Au38 nanoparticles (i.e., carrying two O atoms) give 6.5 kcal mol-1 as the activation energy, which makes the homocoupling feasible. The calculation results indicate that the oxygen atom is one of the intriguing facts of this

C-C bond formation during the oxidative homocoupling reaction of organoboronates. 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

A

B

0

E (Kcal/mol) COX

Au+ Ph-B(OH)3-

1.58

1.64 2.45

2.31

E (Kcal/mol)

TSCCAuB

4.68

TSCCAuA

30

-10

2.26 2.17

2.16

2.08

2.07

2.28

20 2.16

-20 10 -30

CAu 1.66

-40

1.54

3.82

1.99 2.46

1.48

CAu

4.34

0

2.20 2.62

2.32

2.23

RAu

TSdisAu

2.08

2.07

TSCCOX

2.14

TSCCAuB

2.22 2.17

2.08

TSCCAuA

-10

TSdisAu

ROX

− , anion and (B) coupling of two phenyl fragments yielding biphenyl on a Figure 1 Calculated energy profiles for (A) dissociation of Au20 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.


532 G. Li and R. Jin: Nanogold catalysts for carbon-carbon coupling reactions

A

B

Au0

Au0

3 R eactivity and catalytic properties of gold nanoparticles 3.1 U llmann homocoupling reaction of aryl halides

C Auδ+

Au0

D Au0

Auδ+

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 lowcoordinated 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.

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 phenyl acetylene 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 crosscoupling reaction.

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. 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

Scheme 2 Ullmann homocoupling reaction catalyzed by Au@PMOs.


G. Li and R. Jin: Nanogold catalysts for carbon-carbon coupling reactions 533

R

I

Au NPs, 90°C (i) H2O/TBAOH/glucose or (ii)TBAA/glucose

R

R = H, F, Br, Me, CF3, OMe and 1-naphthyl iodide

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.

R

Yield: 40-98%

Scheme 3 Ullmann homocoupling reaction catalyzed by Au nanoparticles in ionic liquids.

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. 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 per− formed using Au20 , Au10 Pd10− , and Au16 Pd4− as models for the Au:PVP, Au0.5Pd0.5:PVP, and Au0.8Pd0.2:PVP catalysts, respectively. The results indicated that involvement of Au as a nearest heteroatom is crucial to initiate the catalytic power, and more importantly, the Au% composition at

A

D

Me2NH, CO2 KCI, HCI

H CI H

Ar

Ar-CI (1)

Au/Pd

Ar-Ar (2) HCl

50% in the bimetallic catalysts gives rise to the optimum catalytic activity. The authors argued that alloy effects [33] such as “ligand effect” and “ensemble effect” enhance the catalytic performance in this Au/Pd bimetallic system (Scheme 4). 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

CI

DMF, KOH

Au/Pd

Ar CI

Ar-CI (1)

G 1C

CI Ar

Ar-H Me2NH, CO2 KCI C

Ar CI

CI

Ar

Au Pd Pd

1A

DMF, KOH B

Ar

Me2NH, CO2 KCI

Au/Pd Ar-CI (1)

Au/Pd

E

Au/Pd 2E DMF, KOH H

Ar

CI

Ar-Ar (2)

2G

1B Ar-H

Scheme 4 Possible pathways for Ullmann homocoupling of chloroarenes catalyzed by bimetallic Au0.5Pd0.5:PVP catalyst. From ref. [32] with permission.


534 G. Li and R. Jin: Nanogold catalysts for carbon-carbon coupling reactions (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 electrondeficient 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 electronrich, redox-active Au13 kernel [35] takes part in the electron transfer process in the catalytic reaction.

3.2 O xidative 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]. Tsukuda’s group first reported gold nanoparticlecatalyzed 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 B(OH)2

Au NCs

+

Base Biphenyl

OH Phenol

Scheme 5 Oxidative homocoupling of phenylboronic acid to yield biphenyl and phenol.

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 surfaceadsorbed [ArB(OH) 3- ] 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) [37]. 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 AuCl4- even under acidic conditions, which is similar to the afore-discussed mechanism (Scheme 6). The Au/chitosan catalyst can also be recycled. In a subsequent work,

Oxidation O2

Reductive elimination Ph-Ph

Transmetalation Ph-B(OH)3-

2OH-, [(OH)2BO]2

Transmetalation Ph-B(OH)3-

Scheme 6 Proposed scheme of oxidative homocoupling of phenylboronic acid catalyzed by Au:PVP NCs. From ref. [36] with permission.

R

BF3K

Au:PVP pH = 6.68 buffe, air R

R

+

R

OH

R= Me, OMe, F, CF3, Br

Scheme 7 Oxidative homocoupling of potassium aryltrifluoroborate to yield biphenyl and phenol.



Fe

xid ati on 3: O Ste p

ion lat

+

K2CO3, H2O, in air

ta me ns

Au Ncs: ligand

HO2 + 1/2 O2

Tra

Fe

O

2 Ph-B(OH)2 + 2 OH-

2 OH-

1:

B(OH)2

Aun

p Ste

Sophiphun et al. synthesized bimetallic Au/Pd:chitosan catalysts with various Au/Pd ratios (0.91:0.09 to 0.72:0.28), which was applied in the same homocoupling reaction [39]. 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). 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. 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

2 B(OH)3

Ph Aun2-

Step 2: Coupling

Aun2-

Ph

Ph-Ph

Scheme 9 The proposed mechanism for aerobic homocoupling of phenylboronic acid over a Au/MAO catalyst. Redrawn from [41].

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. 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].

O

I

II

Scheme 8 Demetalation reaction of ferrocenylboronic acid catalyzed by Au(III) and Au NCs stabilized by ligands containing ferrocene moieties.

3.3 Suzuki cross-coupling reaction Suzuki cross-coupling reaction, also named as SuzukiMiyaura reaction, is one type of carbon-carbon coupling


536 G. Li and R. Jin: Nanogold catalysts for carbon-carbon coupling reactions Table 2 Homocoupling of various arylboronic acids catalyzed by PS-PAMAM-Au NCs (data from [42]).

B(OH)2

R

+

PS-PAMAM-Au NCs K2CO3, H2O, in air

R

R Biphenyl

Entry

1 2 3 4 5 6 7

R

H 4-Me 3-Me 2-Me 4-OMe 3-OMe 2-OMe

I

+ R

OH

B(OH)2

O

Phenol

×

Homo-coupling O Cross-coupling

Yields (%) Biphenyl

Phenol

99 98 96 Trace 98 94 Trace

Trace Trace Trace 95 Trace Trace 95

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). However, in 2009, Han et al. first reported the application of poly(2-aminothiophenol) (PATP) polymer-stabilized gold nanoparticles for the Suzuki cross-coupling

H2O

H2

Ce3+

Au3+

Ce4+

Au+

PhB(OH)2 K2CO3 Biphenyl B(OH)3/K2CO3

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].

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 crosscoupling 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. 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 organo metallic 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

R

X + R′

B(OH)2

Au:PATP NaOH, H2O, 80°C

R

R′

R = H, COOH, OMe, CHO; R′ = H, NO2, Cl, OMe; X= Cl, Br, I

Scheme 11 Suzuki cross-coupling reaction of arylboronic acids and aryl halides catalyzed by Au/PATP.



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].

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

B

A

10 nm

Au

10 nm

D

C

Pd

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. 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 highangle 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

10 nm

Pd Au

10 nm

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.


538 G. Li and R. Jin: Nanogold catalysts for carbon-carbon coupling reactions 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 crosscoupling 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. 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]. 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.

R

X + R′

B(OH)2

Trimetallic NCs NaOAc, DMF/H2O

R

R′

R = H, COMe, OMe, Me, NO2; R′ = H, Me, OMe; X = Cl, Br, I

Scheme 12 Suzuki cross-coupling reaction of arylboronic acids and aryl halides catalyzed by Au-Ag-Pd:CTAB catalyst.

3.4 S onogashira 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 goldcatalyzed 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]. 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 Homo-coupling DPDA + Au NCs Cat., Base, Solvent

I

Sonogashira Cross-coupling DPA Ullmann

Homo-coupling BP

Figure 5 Bio-Pd/Au bimetallic nanoparticles for the Suzuki crosscoupling reaction. From [52] with permission.

Scheme 13 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).



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 crosscoupling 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 CTABcapped 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 A 3-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 builtin groups of -NH2 and -NH can also be used as stabilizer for gold nanoparticles (