Gold nanoparticle (AuNPs) and gold nanopore (AuNPore) catalysts in organic synthesis.

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Cite this: DOI: 10.1039/c3ob42207k

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Gold nanoparticle (AuNPs) and gold nanopore (AuNPore) catalysts in organic synthesis Balaram S. Takale,*a Ming Bao*a and Yoshinori Yamamoto*a,b Organic synthesis using gold has gained tremendous attention in last few years, especially heterogeneous gold catalysis based on gold nanoparticles has made its place in almost all organic reactions, because of

Received 8th November 2013, Accepted 17th December 2013

the robust and green nature of gold catalysts. In this context, gold nanopore (AuNPore) with a 3D metal

DOI: 10.1039/c3ob42207k

framework is giving a new dimension to heterogeneous gold catalysts. Interestingly, AuNPore chemistry is proving better than gold nanoparticles based chemistry. In this review, along with recent advances, major

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discoveries in heterogeneous gold catalysis are discussed.

1 Introduction For both industrial and academic research the development of efficient methods which minimize environmental problems like toxic waste, cost, and energy, is a great challenge for

a

State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116023, China. E-mail: [email protected], [email protected], [email protected] b WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai 980-8577, Japan

Dr Balaram S. Takale (MumbaiIndia) received his MSc in Organic Chemistry at the Institute of Science-University of Mumbai in 2008. In September 2012 he received his PhD in Chemistry from the Institute of Chemical Technology-University of Mumbai on the development of new chemical methods for the synthesis of bioactive molecules. Currently he is performing postdoctoral research in the group of Balaram S. Takale Prof. Yoshinori Yamamoto and Prof. Ming Bao (Dalian University of Technology, China) on the applications of nanosized gold in organic synthesis.

This journal is © The Royal Society of Chemistry 2014

chemists. In recent years green chemistry has attracted tremendous attention from the synthetic community. Traditional methods using stoichiometric reagents have been challenged by catalytic methods and transition metal catalysts play an important role for this purpose. Though homogeneous catalysts have proved efficient to overcome some of the above problems, their use is accompanied with a problem of residual metal impurities left in the final chemical products. This is especially a serious problem in pharmaceutical products, which are required in high purity. In this context heterogeneous catalysts are considered to minimize contamination of metal residues in the products. However, much work using

Prof. Ming Bao studied Chemistry at Northeast Normal University, China, and obtained his MSc degree at the same university in 1989. After gaining research experience at Northeast Normal University for eight years, he transferred to the group of Prof. Yoshinori Yamamoto at Tohoku University, Japan, where he received his PhD in 2001. From 2001 to 2004 he conducted postdoctoral research at the Ming Bao National Institute of Advanced Industrial Science and Technology (AIST), Japan, with Dr Shigeru Shimada and Dr Masao Shimizu (JSPS fellow 2002–2004). In 2005 he began his independent career at Dalian University of Technology. His current research interests focus on the development of novel synthetic methodologies using transition-metal catalysts.

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Review

heterogeneous catalysis has been carried out on gas phase reactions and not so much attention has been given to liquid phase reactions. Gold is considered as one of the least reactive metals. Because of the noble properties of this metal its chemistry has not been well developed until recently, however, the first major breakthrough for gold nanoparticles in organic synthesis was performed by Bond and co-workers for selective reduction of 1,3-butadiene to butane.1–3 In last few years, reports on gold-based catalysts have increased significantly, because various organic transformations can be performed under mild conditions with high regio- and chemo-selectivities. Bulk gold metal is considered inactive, however, gold in the form of nanometer size particles or nanoporous gold framework possesses a higher surface to volume ratio and this leads to high chemical reactivity (Fig. 1). Until now, the majority of work on heterogeneous gold catalysis has been focused on gold nanoparticles, primarily because of their easy and well-known preparation methods together with interesting synergistic effect of support on a variety of reactions. Additionally, gold nanoparticle size or their distribution on a support can be controlled by the preparation method, and these catalysts often show high chemoand regio-selectivity compared to other transition metals. However, agglomeration of nanoparticles could not be overlooked when considering AuNPs. The active species in the reactions catalysed by supported gold nanoparticles is considered to be a cationic gold species such as Au(I) or Au(III), however, it is difficult to understand the precise mechanism involved in these reactions because the support also plays an important role in the catalysis. The recent advances in nanotechnology have expedited the research on unsupported nanostructured gold. In this context, nanoporous gold has gained increasing attention owing to its great potential in

Yoshinori Yamamoto received MSc and PhD degrees from Osaka University, and became a full professor at Tohoku University in 1986. He was awarded the Chemical Society of Japan Award (1996), the Humboldt Research Award from Germany (2002), Purple Ribbon Medal from The Cabinet (2006), A. C. Cope Scholar Award from the ACS, USA (2007), and Centenary Prize from the RSC, UK Yoshinori Yamamoto (2009). He was the Regional Editor of Tetrahedron Letters (1995–2012). After formal retirement from Tohoku University in 2012, he is now Professor at the state key laboratory of fine chemicals, Dalian University of Technology (DLUT) in China. Presently, he is interested in the interdisciplinary research between organic synthesis and materials science.

Org. Biomol. Chem.

Organic & Biomolecular Chemistry

Fig. 1

Reactivity comparison of gold catalysts.

catalysis. This nanostructured framework has a high surface to volume ratio, pore size 2–5 nm, and ligament size ∼30 nm. It has been established that gold atoms present on the stepped surface of the catalyst are really active species in nanoporous gold catalysts, however, mass transport phenomenon in this case cannot be excluded. Although much remains uncertain about the activity of these catalysts, their easy recyclability, reuse and environmentally friendly character make it a promising catalyst in the near future. Many reviews describing gold catalysis in organic synthesis have appeared recently.4–6 In 2007, Yamamoto7 wrote a perspective on the application of Lewis acids for selective organic transformations, which describes homogeneous gold catalysts in a variety of organic transformations and his work on metal nanopores is also reviewed recently.8 The purpose of this review is to present readers with detailed information on nanosized gold catalysts for organic transformation. In this review, we tried to highlight major discoveries in AuNPs that happened before 2012, and recent results on AuNPs, along with our original research results based on AuNPore. The two topics AuNPs and AuNPore are covered in different sections. Finally, it should be mentioned that in this review we do not mention metal nanoclusters, which belongs in the category of MNPs, but consists of a few to several tens of atoms. Therefore its size is much smaller than the MNPs mentioned in this review. A good review on nanoclusters for organic synthesis has appeared quite recently.9

2 Gold nanoparticles in organic synthesis 2.1

Hydrogenation reactions

2.1.1 Hydrogenation of unsaturated carbonyls. Unlike other metals, such as Pd, gold requires high temperature for


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Fig. 2

Review

Dissociation of H2 on supported Au nanoparticles.

Scheme 1 catalysts.

Reduction

of

α,β-unsaturated

aldehydes

using

metal

hydrogenation reactions, hence its catalytic properties have been neglected for hydrogenations. Bone and Wheeler reported in 1906 the first catalytic use of gold for hydrogen activation. Various experiments were performed at 250 and 600 °C. Though analytical techniques were poor in the early 20th century, they managed to measure the change in hydrogen and oxygen pressure in an absorption apparatus by heating the gases to 600 °C on a gold metal gauze. A microscopic examination depicted that there was no formation of metal hydride even at such elevated temperatures; a slight decrease of hydrogen pressure was observed, but it was merely due to superficial occlusion or condensation of H2.10 With the help of neutron scattering experiments it has been found that dihydrogen undergoes heterotopic or heterolytic dissociation. The cleavage is heterolytic, and these two hydrogen atoms have dissymmetric locations; one bonded to AuNPs and the other to O-atoms of ceria support. H+ attaches to oxide supports, and H− bonds to the Au surface (Fig. 2). Dissociation of hydrogen in hydrogenation reactions is believed to be a rate determining step.11 Regarding reduction of unsaturated bonds, the most studied reaction is the selective reduction of a CvO bond in the presence of other unsaturated bonds. Chemoselective reduction of CvO of enals was examined with the help of stoichiometric inorganic reagents, such as NaBH4; however, generation of a large amount of solid inorganic waste urged researchers to develop efficient methods with minimum waste generation and with high selectivity. In this context, many transition metal catalysts and H2 gas as reducing agents have been investigated.12 Nevertheless, chemoselectivity has been a problem associated with those catalysts (Scheme 1). This is because hydrogenation of a CvC bond is favoured over a CvO bond by nearly 35 kJ mol−1, hence there have been various modifications in catalysts to achieve maximum selectivity for CvO reduction. It is also been proposed that selectivity for CvO reduction can be improved by increasing four electron repulsive interaction with metal, especially using a metal with an extended d orbital.13 The radial expansion of the d orbital or broadening of the d-band width increases the four electrons repulsion and hence there is less chance of adsorption through η4 geometry, which in turn leads to CvO adsorption. For example, Os and Ir have high d-band width


hence these metals favour CvO reduction. In this context, gold shows complete d-bands and has no unpaired d electrons, hence it is expected that it shows high selectivity for CvO reduction. The very first application of gold for the reduction of unsaturated carbonyls was described by Shibata et al. in 1988. They used Au–Zr alloy to reduce crotonaldehyde to 2-butenal with 60% chemoselectivity, however, with low conversion (16%).14 Unfortunately even a decade after this discovery, no more work appeared in this field until Hutchings and Bailie reconsidered reduction of crotonaldehyde using supported Au catalysts prepared by co-precipitation. ZnO and ZrO2 supported gold catalysts, along with impact of thiophene doping in these catalysts, were studied. Reactions generally were performed at 250 °C with atmospheric pressure of H2, and Au/ZnO was more selective to CvO bond hydrogenation than Au/ZrO2, however, sulphur modified Au/ZnO or Au/ZrO2 gave much higher selectivity for the formation of crotyl alcohol as compared to undoped catalysts.15 Following Hutchings’ work, reduction of unsaturated carbonyls was extensively studied; for example, crotonaldehyde, acrolein and cinnamaldehyde were studied frequently due to the importance of their respective alcohols in fragrance and perfumery. In this part we will combine some old as well as new discoveries related to these three moieties. Claus and coworkers reported partial hydrogenation of acrolein using supported gold nanoparticles with different structural properties. Unsupported gold powder exhibited high activity and resulted in 93% selectivity for propionaldehyde and 7% selectivity for allyl alcohol. Hence, the main attention was focussed on the preparation of supported gold nanoparticles on different metal oxides at different pH to increase the selectivity towards allyl alcohol. When they used Au/ZrO2-DP22 ( prepared at pH 5) at 593 K, maximum selectivity to allyl alcohol (44%) was achieved with low conversion (18%).16 Further, they prepared monometallic gold nanoparticles and bimetallic Au-In nanoparticles supported on ZnO. Interestingly, a synergistic effect was observed for reduction of acrolein, and the bimetallic catalyst Au-In/ZnO showed improved selectivity (63%) for allyl alcohol as compared to Au/ZnO.17 In an attempt to clarify the effect of particle size and support on selectivity in the hydrogenation of α,β-unsaturated aldehydes Zanella et al. prepared supported gold nanoparticles (Au/TiO2) by deposition-precipitation using urea to get higher gold loading. The results obtained with this method were compared with Au/TiO2 prepared via Haruta’s method18 by studying the hydrogenation of crotonaldehyde.19 Yang et al. described the chemoselective reduction of crotonaldehyde. Accordingly, they prepared mesoporous γ-aminopropyltrimethoxysilane (APTMS) supported bimetallic Au-In nanoparticles (Au-In/APTMS-SBA-15), and compared its activity with monometallic nanoparticles. Interestingly, maximum selectivity for crotyl alcohol (75%) was achieved with excellent conversion (94%) when crotonaldehyde was heated at 120 °C with 20 bar H2.20

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Review

Scheme 2


Scheme 3

Intermediate involved in the reduction of nitroarenes.

Scheme 4

Reduction of nitroarenes using AuNPs/H2.

Reduction of α,β-unsaturated aldehydes using AuNPs.

Chen and co-workers reported liquid phase hydrogenation of crotonaldehyde using Au/Mg2AlO hydrotalcite catalysts. Further, promoters such as Fe, Mo or W were co-deposited with Au on support to increase selectivity for crotyl alcohol and maximum conversion (51.5%) was accomplished with high selectivity (65.8%) for crotyl alcohol using Fe as promoter.21 Galvagno and his group did a comparative study on hydrogenation of α,β-unsaturated carbonyls using different Au supported nanoparticles. Amongst all catalysts studied Au/FeOOH has showed highest activity (conversion, 50%) and selectivity (91%) for unsaturated alcohols.22 In contrast, Shi et al. reported an unusual property of immobilized-PVA-stabilized gold nanoparticles on silica for selective hydrogenation of the CvC bond in cinnamaldehyde.23 The results of the reduction of enals are summarized in Scheme 2. 2.1.2 Reduction of nitro group. Aromatic amines are industrially important fine chemicals with various applications, especially in dyes and pigments. To this end, reduction of nitro-aromatics is realized as a common way to produce anilines. A large number of methods has been developed, and the main intermediates involved in the reduction are azo, azoxy, nitroso, hydroxyl amine compounds, etc., which makes reduction of nitro to amine more complicated, as many by-products can be formed (Scheme 3). There have been many attempts to achieve selective reduction of nitro to amine.24 In this context, Qiu25 and Corma26 independently developed gold catalyzed hydrogenation of nitroarenes. In both cases, high conversion (∼100%) and high selectivity (>90%) were achieved. In the case of Corma’s work, the presence of other reducible groups did not show any significant influence on the yield of the expected product (aniline) or on the conversion of the reactant. Additionally, those functional groups could tolerate the conditions used for the reaction and did not undergo reduction, for example, 3-nitrostyrene underwent

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selective reduction of the nitro group to give 3-aminostyrene (Scheme 4). Interestingly, 1-nitro-1-cyclohexene underwent reduction of both nitro and olefin groups to give cyclohexanone oxime in a high yield. To find out the rate determining step kinetic and isotopic studies were done on this reaction, and it was found that dissociation of H2 on gold was the rate determining step. Further, to improve the rate, bimetallic Au–Pt/TiO2 catalyst was prepared and surprisingly this catalyst increased the rate of H2 dissociation, leading to higher catalytic activity and retaining the high selectivity. In addition, reduction of 3-nitrostyrene could be completed in 30 min for which 9 h was needed by Au/TiO2 catalyst.27 2.1.3 Transfer hydrogenation using gold. Molecular hydrogen has been associated with handling problems attributed to its flammable and explosive nature. Transfer hydrogenation in contrast has the advantage of easy handling, recovery, and recycling of the catalysts, and in addition its use leads to minimized waste. Five main hydrogen reservoirs are generally used for transfer hydrogenations using various metals (Fig. 3). Recently, Fan and co-workers28 used HCOOK as a hydrogen source for hydrogenation of aldehydes. In their study, supported gold catalysts proved more efficient than others, among which gold nanoparticles supported on mesoporous CeO2 showed the highest yield (97%) of desired product.


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Fig. 3

Transfer hydrogenating reagents.

Scheme 5

Scheme 6

Review

Scheme 7

Selective reduction of –NO2 in the presence of CvO.

Scheme 8

Reduction of –NO2 via WGS on AuNPs.

Scheme 9 complex.

Reduction of nitroarenes using hydrazine/ammonia-borane

Reduction of aldehydes using HCOOK/AuNPs.

Reduction of carbonyls using isopropanol/AuNPs.

Interestingly, the reaction could be carried out in water at ambient temperature and even in the presence of air. Under the reaction conditions, selective reduction of aldehyde in the presence of ketone was achieved; only benzaldehyde was reduced in the presence of acetophenone. A proposed mechanism is shown in Scheme 5. Decomposition of formate in the presence of water on catalyst led to bicarbonate and H2. The formed hydrogen species transfers to the gold surface by reverse spill over to form a Au–H species, which is consumed by aldehyde (Scheme 5). In 2008 Cao and co-workers29 reported a Au/TiO2 catalyzed transfer hydrogenation of ketones using isopropanol as hydrogen source. Accordingly, 0.8 mol% catalyst was used along with 0.3 equiv. KOH at 82 °C. After comparing different catalysts, Au/TiO2 was found to be more active than others, giving 99% conversion with 100% selectivity. A variety of aldehydes along with ketones was studied and all of them could be converted to the corresponding alcohols with high conversion rates (Scheme 6). Interestingly, p-nitroacetophenone underwent selective reduction of the nitro group leaving the ketone untouched (Scheme 7). Deng and co-workers30 reported supported Au catalysts in combination with a low temperature water gas shift (WGS) reaction for the reduction of nitroarenes. A series of Au catalysts with different loading was prepared by a co-precipitation method. These catalysts (Au/Fe(OH)x and Au/Fe2O3) were used


in the reduction of nitroarenes with 15 bar CO pressure at 100–120 °C. A >99% conversion and 99.6% selectivity could be achieved using 1.5% Au/Fe(OH)x. Further, they did not find evidence for H2 gas evolution in these reactions and it was proposed that reduction was merely due to Au–H species being generated in situ via a WGS reaction. In the same year, Cao and co-workers31 reported the gold catalyzed reduction of nitrobenzenes using CO and water as reducing agents. Gold nanoparticles, with a diameter of about 1.9 nm supported on TiO2 (Au/TiO2), showed highest activity. Interestingly, the reaction proceeded with high efficiency at 1 atmospheric pressure of CO and this system could be applied even to non-activated nitro compounds. A typical reaction was carried out at 100 °C with 0.01 mol% Au loading. High conversion (up to 99%) with high selectivity (>99%) was observed with almost all substrates studied (Scheme 8). Gkizis et al.32 disclosed activation of hydrazine hydrate by gold nanoparticles for the reduction of nitroarenes. Reaction was carried out using 99% conversion. Various substituted nitroarenes were treated with this reduction system and the reduced products, anilines, could be obtained in good to excellent yields (Scheme 10).

2.2

Scheme 12

Cycloaddition of allenynes using AuNPs.

Scheme 13

Cyclization of arylpropargyl ethers.

Alkyne activation

Alkyne activation is an important area of synthetic organic chemistry. Gold salts, being Lewis acids, have a soft carbophilic nature, which leads them to activate π-bonds for further nucleophilic addition. However, heterogeneous catalysts in this case do not behave a similar way to homogeneous catalysts;35 therefore the research on alkyne activation by AuNPs is still underdeveloped. Nevertheless their use in this case will be greener and more economical; hence alkyne activation by AuNPs is field attracting much interest in synthetic chemistry.36,37 In 2007 Garcia and co-workers38 reported a benzannulation reaction of o-alkynylbenzaldehyde using heterogeneous catalysts such as AuNPs. A variety of supports like Fe2O3, CeO2, TiO2, and C was used and compared with Yamamoto’s39 homogeneous gold (AuCl3) catalyzed benzannulation reaction. In this context, they observed excellent conversion with nearly 90% selectivity for o-phenylnaphthylketone, and other cycloadducts formed could not be characterized due to being formed in only trace amounts. It is noteworthy that particle size should be in the range of a few nanometers, and the overall turnover number measured for supported gold catalysts was 4–7 times higher than that of AuCl3 (Scheme 11). Similarly, Oh and co-workers40 reported an intramolecular version of benzannulation. The [3 + 2] cycloaddition of

Scheme 11

Benzannulation of o-alkynylbenzaldehyde using AuNPs.

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allenynes proceeded with the help of monodispersed spherical gold nanoparticles prepared from AuCl3 (Scheme 12). In 2011, Stratakis and group41 reported cycloisomerization or oxidative dimerization of propargyl ethers using gold supported on TiO2. Treatment of phenylpropargyl ether with Au/ TiO2 in dichloroethane at 70 °C resulted in 82% yield of two products in which the cyclized product was the major product (83%) and the dimerized product was obtained in low yield (17%). The proposed mechanism involves formation of an auric-monocyclic intermediate which can either undergo Au(III) elimination to give a cyclized product or it can add to another molecule of propargyl ether to give a diorganogold intermediate which led to a dimerized product by elimination of Au(I) species. The Au(I) species was reoxidized to a Au(III) species under the aerobic conditions (Scheme 13). Buceta et al.42 reported cycloisomerization of alkynylanilines to indoles using gold nanoparticles supported on carbon. Cycloisomerization of 2-( p-tolylethynyl)aniline in toluene at 90 °C was studied and AuNPs supported on different supports is studied for this reaction; Au/TiO2 gives a low yield (13%), Au/CeO2 afforded a moderated yield (51%) and Au/C, prepared via a sol-immobilization method, gave 11% yield of the desired product. However, when Au/C was prepared by incipient wetness with aqua regia using Hutchings’ procedure,43 the


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Scheme 14

Cyclization of 2-( p-tolylethynyl)aniline using AuNPs.

Scheme 15

Cyclization of ω-alkynylfurans to phenols using Au/CeO2.

Review

Scheme 17

Scheme 16

Cyclotrimerization of alkyne using AuNPs.

desired product could be obtained in 96% yield after 16 h (Scheme 14). Hashmi and co-workers44 showed comparison of homogeneous and heterogeneous gold catalysts for cyclization of ω-alkynylfurans to phenols. The Au/CeO2 catalyst showed high activity with nearly 100% yield of phenol in 20 h, however, use of CDCl3 as solvent led to leaching of gold. Hence, to maintain the heterogeneity of the reaction, CD3CN was chosen as the solvent to give the desired product in 100% yield (Scheme 15). Yet, leaching analysis showed 25 ppm of gold in solution. However, the turnover number (TON) was similar to that of homogeneous gold catalyzed reactions.45 Similar to homogeneous gold catalysts where Au(III) is the active catalytic species, the activity of AuNPs is considered to be due to Au(III) on the surface. Corma’s group46 disclosed cyclotrimerization of electrondeficient alkynes. When they heated ethyl propiolate with a catalytic amount of Au–TiO2 in dichlorobenzene in air, cylcotrimerization proceeded selectively in good yields (Scheme 16). However, use of other supported AuNPs like Au–CeO2, Au–FeO2, Au–Al2O3, and Au–C failed to give any product under those conditions, although a very small amount of product was observed when using Au–ZnO. It was thought that the higher activity of Au–TiO2 compared to Au–CeO2 could be due to the prominent metallic character of gold in Au–TiO2, and it was clarified by using of phenylethanol as reductant for supported AuNPs; interestingly, this resulted in increased activity for the cyclotrimerization reaction. For example, a fully reduced sample of Au–CeO2 with phenylethanol resulted in increased performance in the cyclotrimerization, which could be due to more metallic character of gold in the reaction.


Scope for the cyclotrimerization and oxidative alkynylation.

The reaction was then performed on various substrates. In addition to the expected [2 + 2 + 2] cycloaddition products, an oxidative arene alkynylation product was obtained in the case of terminal alkyne (R1 = H) when 5 mol% of Au–C was used in combination with 1,3,5-trimethoxybenzene (Scheme 17). In 2005, Caporusso and co-workers47 reported acetone solvated gold nanoparticles for hydrosilylation. They studied hydrosilylation of 1-hexyne with hydrosilane at 70 °C in the presence of Au–C. With these conditions, only the trans isomer, β (E) was formed predominantly, however polar silanes such as (EtO)3SiH showed low conversion and Cl2MeSiH resulted in no reaction. Interestingly, in comparison with Pt nanoparticles, AuNPs possessed almost 100% selectivity towards the formation of β (E) isomers. Further, they extended this hydrosilylation by screening nanoparticles deposited on supports such as, Al2O3, Fe3O4, CeO2, TiO2, and ZrO2, and excellent selectivity with totally perfect conversion was observed in each case (Scheme 18).48 Corma and co-workers49 reported that 5 mol% Au–CeO2 with average gold particle size of 4 nm was active for selective hydrosilylation of alkynes and other functional groups such as olefin, aldehyde, ketones and imines. It is well known that Au catalysts possess three different oxidation states, Au(0), Au(I) and Au(III). The reactions with homogeneous Au(I), Au(III) catalysts, and colloidal Au(0) particles were carried out independently, and it was found that Au(III) catalysts possessed higher activity compared to Au(0) or Au(I). Interestingly, Au–CeO2 held very high activity for hydrosilylation of terminal alkynes, which suggested that Au–CeO2 selectively catalyzed hydrosilylation owing to the presence of stabilized Au(III) on the surface. In continuation of their work on supported gold nanoparticles, Stratakis and co-workers50 reported Au–TiO2 assisted hydrosilylation of alkynes. Treatment of terminal alkynes and hydrosilane with 0.5–1 mol% Au–TiO2 in dichloroethane under inert atmosphere resulted in completion of the reaction in 2–4 h. Previous methods have been applied only on hexyne and phenylacetylene, but this method can be applied to a variety of substrates. cis-Hydrosilylation proceeded predominantly, though in some cases disilylation adducts and 90% (Scheme 20). The proposed mechanism involves formation of a silyl–gold species which undergoes addition to alkyne to produce a vinyl–gold species. Further, the vinyl–gold species loses H2 and regenerates the gold catalyst to give the cyclized product (Scheme 21). Very recently, they extended this method to tethered silanes and respective products could be formed in moderate to good yields (Scheme 22).53


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Scheme 23 Cross-dehydrogenative monohydrosilanes.

Scheme 24

Review

coupling

of

various

Scheme 25 Au/TiO2.

Synthesis of quinolines from propargylamines using

Scheme 26 coupling.

Synthesis

Cycloisomerization of 1,6-enynes using Au/TiO2.

Mizuno and co-workers54 studied a cross dehydrogenative coupling of terminal alkynes with monohydrosilanes using gold supported on a cryptomelane-type manganese oxidebased octahedral molecular sieve (OMS-2) under aerobic conditions. A variety of monohydrosilanes as well as terminal alkynes could be used for this transformation and the coupling products could be obtained in good to excellent yield (Scheme 23). A leaching study proved that the reaction is truly heterogeneous. In 2012, the Stratakis group55 utilized gold nanoparticles supported on TiO2 for the first time in the cycloisomerization of 1,6-enynes. It is believed that the reaction proceeds via 5-exo-cyclization to form a gold-cyclopropyl carbene, which further undergoes cleavage. Various internal and terminal alkynes were studied, and 5-exo cyclized products were observed in most cases, however, minor amounts of 6-endo cyclized products were also observed in certain cases. The reported56 homogeneous gold(I) catalyzed double cleavage products were not observed (Scheme 24). Litinas and co-workers57 reported the synthesis of quinoline and pyridocoumarins from propargyl amines by gold nanoparticles supported on TiO2. Consequently, treatment of 6-propargylaminocoumarin in 1,2-dichloroethane with Au–TiO2 at 70 °C for 48 h resulted in excellent yield of the desired product. The reaction proceeded via 6-endo-dig cyclization and 1,3-H shift. Use of water as the solvent resulted in lower yields of the product. Additionally the reaction could


of

propargylamines

via

three-component

be carried out on other supported gold nanoparticles, for example, Au–Al2O3 was also efficient for this reaction. However, treatment of N-propargylanilines with Au–TiO2 for 3 days resulted in moderate yields of the desired products along with dimerized products in significant amounts (Scheme 25). Synthesis of propargyl amines via a three-component coupling is well known by homo- and heterogeneous catalysts.58 In 2007, Kidwai and co-workers59 reported that unsupported Au-nanoparticles mediated a three-component coupling for the synthesis of propargyl amines (Scheme 26). The reaction proved efficient and green with >90% yield, however, the high catalyst loading (5–50%) required for this reaction made further development of AuNPs mediated synthesis of propargyl amines necessary. In this context, Zhang and Corma60 used supported gold nanoparticles, having supports such as SiO2, TiO2, C, Fe2O3, ZrO2, and CeO2, and they found that Au/CeO2 and Au/ZrO2 were very efficient, and could give the desired products in >99% yields. Additionally, a variety of substrates was studied, and all could be transformed to respective products in good to excellent yields (Scheme 26). Surprisingly, substituted indoles could be synthesized via coupling of N-protected ethynylaniline (instead of

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Review


Scheme 27


Synthesis of indoles via three-component coupling.

Scheme 29

Scheme 28 Proposed mechanism for three-component coupling using AuNPs.

phenylacetylene) and aldehydes (Scheme 27). After this, a variety of AuNPs mediated reactions was reported for the three-component coupling of aldehydes, amines and alkynes. For example, Ying and co-workers61 reported nanocomposites, such as PbS-Au, CdS-Au, and CdSe, for the synthesis of propargylamines. Further, Datta et al.62 also reported the three-component coupling via gold nanoparticles embedded in mesoporous carbon nitride stabilizer. These highly dispersed Au-nanoparticles proved to be very active, efficient and easily recyclable catalysts. Anand et al.63 tested the three-component coupling using highly recyclable gold catalyst Au-nanoparticles supported and immobilized on lipoic acid functionalized SBA-15 (SBA-LAG). Interestingly, the reaction could be carried out in solvent free conditions to give the resultant products in good to excellent yields. Before this work, Karimi et al.64 demonstrated the use of gold nanoparticles supported on periodic mesoporous organosilica with imidazolium ionic liquid framework (Au/PMO-IL). The reaction could be performed in various solvents and chloroform proved best in that case. Various substrates were studied in this coupling reaction, and the respective products could be obtained in moderate to good yields. The general mechanism of this reaction is proposed in Scheme 28. Insertion of Au in the Csp–H bond of the alkyne leads to a gold-acetylide hydride species, which attacks nucleophilically the iminium ion, formed from an aldehyde and amine, to give the corresponding propargyl amine. 2.3

Coupling reactions

C–C coupling reactions are very important transformations as they can give direct access to substituted arenes, but this area has been mostly covered by Pd-catalysts because of their general applicability, mild conditions, wide substrate scope, and high chemo- and regio-selectivity. In this context gold seems to be very less active. However, in 2004 Tsukuda and co-workers65 reported colloidal gold nanoparticles for the aerobic homocoupling of phenylboronic acid in water. Accordingly, they used poly(N-vinyl-

Org. Biomol. Chem.

Homocoupling of arylboronic acids using AuNPs.

2-pyrrolidone) (PVP) stabilized AuNPs along with K2CO3 in the presence of water and air. It was interesting to find that the use of deaerated water resulted in no reaction which revealed that O2 was necessary for the reaction (Scheme 29). An attempt to achieve Suzuki–Miyaura coupling between aryl halides and aryl boronic acids using gold nanoparticles supported on ceria failed,66 instead, homocoupling of aryl boronic acids was observed. In this case of homocoupling by AuNPs, Corma and co-workers studied the reaction of p-iodobenzophenone and phenylboronic acid using Au/CeO2. To their delight, all phenyl boronic acids were transformed into biphenyls and only a very small amount (99%. Han et al.92 developed a new method for the synthesis of gold nanoparticles supported on poly(o-phenylenediamine) (PoPD). The size of nanoparticles can be easily tuned from 3–15 nm by changing concentration of metal salt. The PoPD was used as both stabilizer and reductant in generation of gold particles from HAuCl4. Further, oxidation of benzyl alcohol could be carried out at room temperature, however the alcohol underwent complete oxidation to give benzoic acid as the exclusive product. Tsukuda and co-workers93 immobilized Au nanoclusters (Au11:TPP), with size nearly 1 nm, in mesoporous silica (SBA-15) with the help of organic solvents. The Au clusters could be deposited uniformly on silica by optimization of solvent mediated interactions; further, aggregation of Au-clusters was minimized by removing protecting ligands through calcination. When these supported Au-nanoclusters along with H2O2 were used for the oxidation of benzyl alcohol in a microwave oven heating at 60 °C, an overall conversion of 100% could be achieved with 91% selectivity for benzoic acid. Choudhary et al.94 studied liquid phase oxidation of benzyl alcohol to benzaldehyde using gold nanoparticles supported


Review

on metal oxides in combination with tert-butylhydorperoxide. In this context, they prepared a variety of metal supports such as TiO2, MnO2, Fe2O3, CoOx, NiO, CuO, ZnO, ZrO2 etc. However, Au/TiO2 prepared via a homogeneous deposition (HDP) method was found to be more active than others, although Au/ZrO2 prepared via HDP also showed good activity for example, Au/ZrO2 exhibited higher TOF (206.1) than Au/ TiO2 (108). The combining effect of two nanosized metals on catalysis is gaining increasing importance, since an added metal can significantly alter the geometrical and electronic properties of nanoparticles, resulting in selective organic reactions. In this respect, Prati and co-workers95 developed Au–Pd and Au–Pt single phase catalysts and used these for the oxidation of benzyl alcohol. Only use of Pt or Pd, without Au, showed poor conversion, however positive synergistic effect could be observed when Au was added to Pd and conversion could be increased up to 96% with 94% selectivity towards benzaldehyde. In fact, it was found that Au–Pd was not only more active than monometallic Pd or Pt, but also than Au–Pt. 2.4.4 Oxidation of alkenes. In 2005, Hughes et al.96 utilized supported gold nanoparticles for the oxidation of cyclohexene in polar solvents. By using 1% Au/C and oxygen, cyclohexene could be converted fully to only CO2, formic acid and oxalic acid; however no C6 products were observed. Interestingly, when the reaction was carried out in solvent free conditions, cyclohexenone and cyclohexanol could be observed. Besides, the study using a wide range of apolar solvents revealed that the solvent played an important role in this transformation. 1,2,3,5-Tetramethylbenzene was found to be the best solvent and the highest selectivity (50%) toward the formation of cyclohexene oxide was achieved using 1% Au/C in this solvent (Scheme 36). Reaction could only be initiated in the presence of an initiator such as tert-butyl hydroperoxide or H2O2; moreover, the reaction could not proceed either in the absence of the catalyst or in the absence of an initiator, suggesting the importance of both gold and initiator. Further, styrene and stilbene also underwent epoxidation and the selectivities for the formation of epoxides were greater than 90%. Additionally, Bi-modified Au/C was tested for oxidation and the selectivity for C6 product was markedly improved even with low catalyst loading (0.5%) (Scheme 36). Caps and co-workers97 disclosed gold catalyzed aerobic epoxidation of stilbene in methylcyclohexane, in which they

Scheme 36

Epoxidation of cyclohexene using AuNPs.

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2.5

Scheme 37

Epoxidation of stilbene using Au/SiO2-Aerosil 972.

Scheme 38

Epoxidation of stilbene using Au/In2O3.

used hydrophobic silica, such as hydrophobic Aerosil treated with silane or siloxane, as a support for gold nanoparticles prepared from AuPPh3Cl complex. These catalysts were applied for the epoxidation of trans-stilbene in methylcyclohexane as solvent at 80 °C. Au/SiO2-Aerosil 972 showed maximum selectivity (70%) with overall conversion of 64% after 24 h, and extending the reaction time up to 72 h increased conversion (99%) with improved selectivity (79%). Further they found the experimental evidence for the formation of 1-methylcyclohexyl hydroperoxide (MCOOH) in the gold catalyzed co-oxidation of methylcyclohexane and trans-stilbene, with the help of GC-MS in selective ion monitoring.98 Consequently, MCOOH was found to accumulate during the co-oxidation process which suggested that the decomposition of hydroperoxide was the rate-limiting step in the current process (Scheme 37). Mishra et al.99 prepared gold nanoparticles supported on indium oxide (Au/In2O3) and tested this catalyst for the aerobic epoxidation of trans-stilbene at low temperature. In addition, a comparative study was done by using other supported gold catalysts such as Au/α-Ga2O3 and Au/β-Ga2O3, and Au/Al4Ga2O9. Interestingly, Au/In2O3 was found to be the best, and 78% of stilbene could be converted with almost 68% selectivity for epoxide (Scheme 38). Moderate to good yields of the product were observed by the use of other gold catalysts, and the mechanism of the reaction is similar to that predicted by Caps and co-workers.97

Org. Biomol. Chem.

Miscellaneous reactions

Kaneda and co-workers100 reported the cyclocarbonylation of o-aminophenol using Au-nanoparticles supported on hydrotalcite [Mg6Al2–CO3(OH)16]. The reaction was performed using 50 atm CO/O2 (48/2), and the cyclocarbonylation could not proceed in the absence of O2. Highest yield of 99% was observed in the case of 2-aminophenol after using Au/HT as a catalyst (Scheme 39). Other supported catalysts, such as Au/ Al2O3 (97% yield), Au/CeO2 (92% yield), Au/TiO2 (82% yield), and Au/SiO2 (62% yield), were found to be equal or less competitive. In all other substrates studied, the yields were quite high (>95%). Further, the same group has used hydroxyapatite supported AuNPs for the deoxygenation of amides to amines, sulfoxide to thioether and pyridine N-oxide to pyridine.101 In all the cases, excellent yields of the desired products were obtained with high selectivity, additionally, the catalyst could be reused without loss in activity. Other supported AuNPs studied for these reactions also led to good yields of the desired products, but Au/HAP proved to be best (Scheme 40).

Scheme 39

Cyclocarbonylation of o-aminophenol using Au/HT.

Scheme 40

Deoxygenation reactions using Au/HAP catalyst.


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Scheme 41

Review

Scheme 42

Desulfurization of 2-MBI and 6-TG using NaBH4/AuNPs.

Scheme 43

Synthesis of quinaldines from nitroarenes and alcohol.

Scheme 44

Reductive coupling of nitroarenes and alcohol.

Synthesis of silyl isocyanate using AuNPs.

Mizuno and co-workers102 presented a novel AuNPs promoted dehydrogenative coupling of hydrosilane and isocyanic acid (generated in situ from urea), which resulted in silyl isocyanates. To realize this, they used various metals (Au, Pd, Rh, Ag, Pt, Ru, and Cu) supported on Al2O3, additionally, other supported gold catalysts (Au/CeO2, Au/SiO2, Au/TiO2, etc.) were also prepared. Surprisingly, when they tested all these catalysts for the coupling reaction, only Au/Al2O3 was found to be the best catalyst, giving 91% yield of dimethylphenylsilyl isocyanate in 3 h, and Pd/Al2O3 or Au/SiO2 could only give low yield (∼50%). In the case of substrate study, all silanes could result in good to excellent yields of the desired products, except alkoxy silanes which resulted in low yields of respective products. The mechanism proposed involves in situ formation of isocyanic acid from urea, which further combines with Au-activated silane to produce the product by releasing hydrogen gas (Scheme 41). Recently, Zhang and co-workers103 presented a mild method for desulfurization of mercaptobenzimidazole (2-MBI) and thioguanine (6-TG) on colloidal gold nanoparticles using NaBH4 in water. AuNPs of different size were prepared and MBI or TG were adsorbed on 10, 15, 50 nm diameter gold nanoparticles. The rate of desulfurization depended on the particle size, and an increase in the particle size resulted in decrease in the rate of desulfurization. Additionally, via an isotopic labelling experiment, they proposed that a desulfurization process would proceed via hydrogen uptake on AuNPs rather than hydride attack from BH4−. Experimental evidence proved that AuNPs captured and concentrated 2-MBI or 6-TG, and activated them along with NaBH4 for desulfurization, and further scavenged cleaved sulphur by forming Au–S bond. Interestingly, AuNPs could behave as both catalyst and reactant in this case, because an increase in the sulphide concentration on Au surface drastically reduced the rate of desulfurization (Scheme 42). Swaminathan and Selvam104 presented a direct route to quinaldines from nitroarenes using Au-nanoparticles supported on TiO2. This redox reaction was carried out under mild conditions using UV light. Additionally, this method could be performed without oxidant or reductants (Scheme 43).


In another case, Cui et al.105 demonstrated the reductive coupling of nitroarenes and alcohols using Au nanoparticles supported on Ag–Mo nanorod (Au/Ag–Mo-NR) in glycerol as solvent. Gold particles of 5–8 nm size were well dispersed and gold loading was found to be 0.3 wt% and glycerol provided necessary hydrogen for reduction of nitroarenes, which was confirmed by LC-MS analysis. Interestingly, selectivity depended on solvent and temperature; use of toluene at 120 °C resulted in imine as the product, while xylene as solvent at 150 °C resulted in an amine product (Scheme 44).

3

Gold nanopore in organic synthesis

Despite the above applications of AuNPs in organic synthesis, selection of suitable oxide supports, size of particles, and

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deposition techniques play a major role for their activity.106–109 In addition, a mechanistic pathway involved in the gold nanoparticle mediated reactions is still under debate. The main reason for such controversy seems to be ascribed to a composite system required in supported gold nanoparticles. In contrast to supported gold nanoparticles, unsupported nanoporous gold (AuNPore), which possesses a 3-dimensional bicontinuous open pore framework having high surface to volume ratio, has opened a door to help clarify the mechanism in heterogeneous gold catalysis. The reactions using AuNPore take place in the absence of supports, and agglomeration observed in AuNPs is excluded completely. In addition, easy recovery and reuse makes it further an attractive catalyst in organic synthesis. Nanoporous-gold is generally prepared by dealloying of the less noble metal from Au–M alloy (M = Ag, Cu, Al etc.) with different techniques.110–114 As expected, AuNPore was used for the first time in the wellknown industrially important reaction such as CO oxidation. In 2006, Zielasek et al.115 and Xu et al.116,117 independently applied nanoporous gold (AuNPore) for CO oxidation with molecular oxygen at temperature as low as −20 °C, however it was later found that residual Ag in AuNPore left while leaching of alloy (Au–Ag) played a crucial role for O2 activation.118 In particular, most of the residual Ag segregates to the surface, resulting in surface concentration of up to 10 atom%. Hence, activity of thin monolithic gold catalyst (AuNpore) has been under debate in past few years.119 Regardless of residual Ag, the mass transport phenomenon is also an important entity for catalytic activity. AuNPore possesses uniform pore size distribution throughout a 3-dimensional open framework which provides an efficient mass transfer. Bäumer and co-workers120 further made various modifications in the fabrication of this nanoporous catalyst and proved that CO oxidation was slower in the case of 200 micron-thick-AuNPore disk. Further, activation of oxygen could be controlled by modifying the AuNPore surface. For example use of metal oxides such as TiO2 and PrO2 on the AuNPore surface led to increase reactivity of CO oxidation.121 Recently Fujita et al.122 demonstrated an atomic origin of AuNPore with the help of TEM techniques; residual Ag plays an important role in suppressing (111) faceting and to stabilize steps and kinks which further helps in the preservation of low co-ordinated gold. Metal catalysts, including MNPs and MNPore, are very well known for use in gas phase reactions, so it must be worthwhile to focus on the use of MNPore in liquid phase organic chemistry. With interesting results in AuNPorecatalyzed gas phase reactions, Yamamoto and co-workers focused on liquid phase reactions using AuNPore. 3.1

AuNPore catalysed oxidation reactions

In 2010, Yamamoto and co-workers123 reported the first use of AuNPore for the conversion of silanes to silanols in acetone at ambient temperatures. Silanols are handy building blocks for silicon-based polymer synthesis, which makes the development of convenient and efficient methods for silanol synthesis

Org. Biomol. Chem.


Scheme 45

Oxidation of silanes to silanols using AuNPore.

important. Accordingly, the reaction of PhMe2SiH with water in the presence of 1 mol% AuNPore in acetone resulted in the corresponding silanol, PhMe2SiOH, in 100% yield and H2 was a sole co-product of this reaction. A wide variety of silanes was oxidized to the corresponding silanols in good to excellent yields. The nanoporous gold can be recovered simply by picking up with tweezers without performing tedious work up procedures. Further, it can be used for many cycles without decrease in activity (Scheme 45). As described in the previous section, the oxidation of alcohols is an important transformation in organic synthesis.124 Further, the use of heterogeneous catalysts with naturally available molecular oxygen is highly appreciated. Many heterogeneous systems, in particular, supported gold nanoparticles have been used for such transformation, however AuNPore possessing easy recovery and reuse characteristics will supersede AuNPs. Hence, Yamamoto and co-workers125 performed the oxidation of 1-phenylbutanol with AuNPore in the presence of molecular oxygen. To their delight, the corresponding oxidized product could be isolated in >95% even after 5th use. The scope of this method was further studied on different substrates; in addition to aryl, heteroaryl and aliphatic secondary alcohols, some primary alcohols were also oxidized to the corresponding carbonyl compounds (Scheme 46). This reaction was also studied for continuous flow reactor. A stainless steel tube column was used for this purpose, and a methanolic solution of substrates and oxygen gas was introduced independently with flow rates 3 mL h−1 and 30 mL h−1, respectively. The efficiency of the flow system was higher than that of the batch system. As primary alcohols can be oxidized to aldehydes in the above reaction, a major question can arise about the oxidation of methanol which is used as a solvent. Possibly, this turned


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formed as an intermediate in the current reaction, however more study is needed to confirm this path.


3.2

Scheme 46

Oxidation of alcohols using AuNPore/O2.

Scheme 47

Oxidative formylation of amines using AuNPore/O2.

the authors to perform the oxidation reaction of methanol in the presence of amines to give different formamides. The debate of activation of molecular O2 by residual silver in AuNPore was further proved by carrying out the reactions with AuNPore containing different of amount of residual silver. AuNPore was fabricated according to the reported procedures.126,127 From all AuNPore catalysts studied, AuNPore containing Au97Ag3 was found to be the best catalyst, and besides it could be used many times without compromise in the yield of the desired products (Scheme 47). This study shows that residual silver plays an important role in the activation of molecular oxygen. A mechanistic pathway was postulated by doing some control experiments, in which the reaction of methyl formate with amine gave a good yield of formamide. Hence it can be considered that methyl formate was


AuNPore catalysed reduction reactions

AuNPore has been successfully applied for oxidation reactions, but its application for reductions has been ignored because of the high energy required to dissociate H2 from the gold surface. Surprisingly, oxidation of silanols in water results in H2 as a co-product,123 hence use of this hydrogen rather than hazardous H2 gas could be useful and economical. Until now two reduction protocols have been developed using AuNPore/ silane, and this section gives an outline of those protocols. 3.3.1 Semihydrogenation of alkynes. Many efficient homogeneous catalytic systems have been known for the semihydrogenation of alkynes, and heterogeneous catalysts based on Pd and Ni metals with H2 have been applied, too, for the reduction of alkynes to cis-alkenes. However over-reduction and stereo-isomerization with those catalysts cannot be avoided. Recently, the gas phase hydrogenation of alkynes with AuNPs/H2 was reported, though the selectivity was higher and the activity was poorer compared to PdNPs/H2. Jin, Yamamoto and co-workers128 reported the reduction of alkynes with a AuNPore/silane/water system as hydrogen source. In the reduction of phenylacetylene, solvents such as CH2Cl2, tetrahydrofuran, toluene, and acetonitrile gave poor yields of the desired product (styrene), while dimethylformamide (DMF) was proved to be the best solvent, resulting in 96% yield of the product (Scheme 48). DMF may undergo degradation to Me2NH (or Me3N) due to the conditions used in the present study. This inspired the study of the effect of different amine additives on semihydrogenation of alkynes. Interestingly, 1 equiv. of pyridine as an amine additive improved the yield to 98% (Scheme 49).

Scheme 48

Reduction of an alkyne using DMF as solvent on AuNPore.

Scheme 49

Effect of amine additives on reduction of an alkyne.

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Scheme 52 AuNPore.

Scheme 50

Substrate scope for reduction of alkynes to alkenes.

of which toluene proved the best solvent for the current reduction. Various substituted quinolines were treated with 2 mol% AuNPore, 1.25 equiv. PhMe2SiH, 1.5 mmol H2O in toluene to give the corresponding tetrahydroquinolines in good to excellent yields (Scheme 52).129

3.3

Scheme 51

Proposed mechanism of reduction of alkynes to alkenes.

Various substituted alkynes, both terminal and internal alkynes, were subjected to reduction via two methods; (1) using DMF as a solvent at 35–55 °C and using acetonitrile as a solvent, (2) using pyridine as a base at 80 °C. In both cases, good to excellent yields of the desired products with nearly 100% stereoselectivity were obtained, although in some cases over-reduced products were also formed (Scheme 50). Various control experiments, including deuterium labelling, supported the postulated mechanism in Scheme 51. Amine additives used for the reduction of alkynes opened the path for the reduction of quinolines. Interestingly, when the reduction of alkynes was performed using quinoline as an amine additive, 1,2,3,4-tetrahydroquinoline was isolated as a by-product in 30% yield. This spurred the interest in optimizing the conditions for selective hydrogenation of quinoline. The reaction was performed in solvents such as dimethylformamide, acetonitrile, tetrahydrofuran, ethanol, and toluene, out

Org. Biomol. Chem.

Reduction of quinolines to tetrahydroquinolines using

Hydrosilylation of alkynes using AuNPore

Vinyl silanes are important intermediates in the synthesis of organosilicon compounds, hence regio- and stereoselectivity is highly important in the hydrosilylation of alkynes. Until now, many homo- and heterogeneous catalysts have been reported for the hydrosilylation of alkynes. Typically, transition metal catalysts result in cis-hydrosilylation and Lewis acid catalysts result in trans-hydrosilylation of alkynes. In addition to β-silylation, α-silylation often occurs in equal extent (Scheme 53).130–132 Recently, the same group tried133 AuNPore, fabricated by leaching of Au20Al80 alloy in 20% aqueous solution of NaOH (resulting in AuNPore with composition Au96Al4) for hydrosilylation of alkynes. Interestingly, this AuNPore was found to be more active than the other AuNPore (Au98Ag2) prepared from Au30Ag70 alloy. For example, the hydrosilylation of phenylacetylene with triethylsilane with AuNPore, composed of Au96Al4, resulted in 74% yield of the desired product. Further optimization of conditions improved the yield up to 98% (Scheme 54). Change in the alkyne did not exert significant effect on the yield, however, change in silane resulted in decrease in the yield, and even needed more catalyst loading.

Scheme 53

Catalysts based selectivity for hydrosilylation of alkynes.


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Scheme 54

3.4

Review

Hydrosilylation of alkynes using AuNPore.

Diboration of alkynes

Jin, Yamamoto and co-workers134 described the novel catalytic property of nanoporous gold for diboration of alkynes without using any additives. The catalytic activity of PdNPs135 had already studied for this reaction; hence it was thought that the activity of nanoporous Pt (PtNPore) would be interesting in this chemistry. To realise this, PtNPore was prepared from Pt–Cu alloy and studied for diboration. Interestingly, when they treated phenylacetylene with bis( pinacolato)diboron in the presence of PtNPore in toluene as solvent at 100 °C, the respective cis-diborane adduct was obtained exclusively in 99% yield. However, leaching of Pt atoms from PtNPore was observed, suggesting that reaction proceeded through homogeneous Pt species. Surprisingly, when the reaction was carried out using AuNPore, the respective product could be obtained in 96% yield. Additionally, no Au was leached into the solution, which showed heterogeneity of the AuNPore catalyzed reaction. Other catalysts such as AuCl, Au–Ag alloy, PdNPore, CuNPore, and AgNPore showed no reaction, confirming the necessity of both gold and its nanoporous structure (Scheme 55). After studying different solvents, use of toluene resulted in a high chemical yield. Besides, the catalyst can be reused several times without significant loss in activity.

Scheme 56

Substrate scope for diboration of alkynes using AuNPore.

Scheme 57 AuNPore.

Mechanistic pathway for diboration of alkynes using

Further, various terminal and internal alkynes were subjected to diboration. In most cases, the cis-adducts were formed predominantly, while in some cases the formation of small amounts of the trans-adduct was observed (Scheme 56). A possible mechanism for the cleavage of the B–B bond on AuNPore surface is shown in Scheme 57. The B–B bond cleaved to form a Au–Bpin species takes place first, and then this species reacts with an alkyne attached to the gold surface, and finally the product can be formed via three different paths (Scheme 57). 3.4

Scheme 55

Catalyst screening for diboration of phenylacetylene.


Benzannulation using AuNPore

Aromatic compounds are core chemicals possessing a variety of applications in pharmaceuticals, optoelectronics, polymers etc.136 Many methods have been developed to achieve their selective synthesis, however, most of them have failed to accomplish high regioselectivity regarding the substituent

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Scheme 58


Benzannulation of o-alkynylbenzaldehyde using AuNPore.

position on aromatic ring. A benzannulation strategy has long been used for the construction of substituted aromatic compounds.137 To continue the research on benzannulation, Yamamoto and co-workers138 treated o-alkynylbenzaldehyde and phenylacetylene with AuNPore (20 mol%) in o-dichlorobenzene at 150 °C, and the respective annulated product could be obtained in moderate yield. Interestingly, the nanoporous structure was very important for the benzannulation, and as the size of gold nanopores increased the yield decreased significantly, and the reaction with AuNPore of 100 nm pore size could not proceed at all. A plausible mechanism is shown in Scheme 58. A benzopyrylium intermediate similar to Lewis acid catalyzed benzannulation is also involved in this case (Scheme 58).

4 Conclusions and future prospect In summary, we have presented an outlook on heterogeneous gold catalysts in organic synthesis. Many gas phase reactions have been developed using heterogeneous gold catalysts, hence the major purpose of this review was to focus on liquid phase reaction carried out using heterogeneous gold catalysts. For many years gold has been treated as a noble or inert metal with no role in catalysis, and this misunderstanding has continued until recent years. Gold nanoparticles as heterogeneous catalysts are overcoming various drawbacks faced by homogeneous gold catalysts, especially, they are not sensitive to air or moisture, the reactions can be performed using mild conditions, and high selectivity can be achieved even in complex reactions. The initial section of this review discusses very simple reactions, for example, reduction of carbonyl in

Org. Biomol. Chem.

the presence of CvC which has been studied extensively. AuNPs show promising selectivity towards CvO reduction, 100% selectivity with high conversion is yet to be achieved. Moreover, alkyne activation via gold catalysts is a dominant area; various papers have appeared recently based on alkyne activation. Homogeneous gold catalysis is important primarily due to the soft carbophilic nature of gold which activates alkyne for further reactions. However, heterogeneous gold catalysis seems to receive little attention in alkyne activation owing to a debate of active gold species in AuNPs. In contrast, oxidation reactions using AuNPs are proving to be evergreen reactions with current work on these reactions proceeding in the same rate as before. For example oxidation of toluene to benzyl benzoate using bimetallic Au–Pd supported metal particles proved very efficient, selective with high turnover numbers, which shows its industrial importance. In this context, the research on nanoporous gold (AuNPore) has only recently started; low temperature gas phase oxidation of CO was achieved effectively using AuNPore. Interesting liquid phase reactions have also appeared using this catalyst. In some cases, it is proving to be better than AuNPs due to its easy recovery and reuse. Further it does not need any support, which eliminates particle agglomeration as appeared in the case of AuNPs. The role of residual impurities in the case of AuNPore and support effect or additional metal effect in the case of AuNPs are interesting and promising for discovering “Etwas-Neues” in this research field. For example, the use of multi-metallic alloy nanoparticles to realize synergistic effects might open a door to new activity, selectivity, reactions, and applications. Along with the classical reactions, complex or asymmetric reactions are also manipulated using gold catalysts and those will present a significant area of future research. Additionally, new discoveries using gold catalysts are appearing very fast and it will be worthwhile to explore and make them an industrially important protocols. In conclusion, “the Golden era in the use of heterogeneous gold catalysts in chemistry has already started, and the burst will appear very soon”.

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