Functionalized Silica Matrices and Palladium: A Versatile

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Functionalized Silica Matrices and Palladium: A Versatile Heterogeneous Catalyst for Suzuki, Heck, and Sonogashira Reactions Pitchaimani Veerakumar,*,†,§ Pounraj Thanasekaran,*,‡ Kuang-Lieh Lu,‡ Shang-Bin Liu,†,§ and Seenivasan Rajagopal*,∥ †

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan § Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan ∥ School of Chemistry, Madurai Kamaraj University, Madurai-625021, India ‡

ABSTRACT: Remarkable developments have been accomplished in silica-supported palladium nanoparticles (PdNPs)-mediated organic transformations for the generation of Suzuki (C−C), Heck (CC), and Sonogashira (CC) coupling reactions in academic as well as industrial communities. Various synthetic methods were adopted to prepare highly dispersed PdNPs encapsulated within various forms of silica supports. The type of reaction examined size and shape, stability, and recycling ability of silica-supported PdNPs, and the influence of different reaction parameters on carbon−carbon bond-forming reactions are discussed. In these reactions, the silica-supported PdNPs exhibited superior performances compared to their unsupported colloidal metal nanoparticles (MNPs), revealing the advantages of designing nanocatalysts. Recent progress in the synthesis, catalytic results, stability, and recycling possibilities of silica-supported PdNPs are discussed, along with the prospective outlook of relevant research fields. KEYWORDS: Heterogeneous catalysis, Palladium, Silica supports, Suzuki reaction, Heck reaction, Sonogashira reaction

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in solvents.28 Among nanostructured supports, silica-supported PdNPs have drawn considerable attention.29 Since the stability of inorganic supports is important under oxidizing conditions, the majority of heterogeneous catalysts come from silica supports. Silica-supported catalysts are advantageous over zeoliteencapsulated catalysts, colloidal nanoparticles, and ionic liquidsupported metal catalysts (Scheme 1). Amines,30,31 imines,32 phosphine,33 thiols,34 and sulfonic acids35 could be anchored to the surface of silica and then interacted with PdNPs. These materials result in enhancing stability and dispersity in solvents that could allow their practical use in catalytic reactions. Silica matrices including functionalized silica gel,36 silica yolk− shell,37 nanoporous silica,38 SBA-15,39 SBA-16,40 MCM-41,41 MCM-48,42 SiO2 aerogels,43 silica wet gel,44 fluorous silica gel,45 and organosilicas46 have been used to support PdNPs on their surfaces. These materials are utilized in Suzuki−Miyaura, Heck, and Sonogashira cross-coupling reactions because they possess excellent reactivities under sustainable conditions and can be readily recovered and reused by simple filtration after completion of the reaction. The straightforward workup process and simple

INTRODUCTION Palladium nanoparticles (PdNPs) have become one of the most interesting catalysts because of their size- and shape-dependence as well as efficient catalytic activities in catalytic reactions.1−3 Due to the high surface energy of palladium, they can easily aggregate to form Pd-black.4 Despite their great contributions, low stability and inconvenient recovery make their use limited.5 PdNPs are preferred as heterogeneous catalysts over homogeneous catalysts because they can be easily separated from the reaction mixture by filtration or centrifugation and reused several times.6 To overcome the degree of aggregation or atom/ion leaching,7 polymers,8 dendrimers,9 ionic liquids,10 surfactants,11 and others12 are employed as stabilizing or capping agents. Thus, organic functionality is a fundamental parameter that could influence their catalytic performance. Encapsulation or immobilization of active nanometals on carbon-based materials,13 metal−organic frameworks,14 alumina,15 polymers,16 zeolites,17 clays,18 and inorganic matrices19,20 have emanated as effective strategies that could enhance their recovery and reduce their tendency to undergo agglomeration.21,22 Usually, immobilization is accomplished using covalent anchoring,23 polymerizations,24 encapsulations, 25 sol−gel condensation,26 or the Stober method.27 For versatile catalytic applications, PdNPs on supports can be protected with organic molecules to enhance their dispersity © 2017 American Chemical Society

Received: March 27, 2017 Revised: June 9, 2017 Published: June 16, 2017 6357

DOI: 10.1021/acssuschemeng.7b00921 ACS Sustainable Chem. Eng. 2017, 5, 6357−6376

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ACS Sustainable Chemistry & Engineering Scheme 1. Advantages of Using SiO2 as Support for Preparation of Heterogeneous MNP-Supported Catalysts

Scheme 2. First Example of Pd and Pd/Ni NPs-Catalyzed Suzuki Coupling

handling of catalysts are extremely practical applications for industrial-scale catalysts.47 Encouraged by catalytic applications of PdNPs, more details on C−C coupling reactions of PdNPs can be found in several reviews.1,2,16,48−50 However, no detailed coverages for the use of heterogeneous PdNPs immobilized on various forms of SiO2 supports have been reported. Thus, silica-supported PdNPscatalyzed cross-coupling reactions are very important subjects, and reviewing these would be timely. This review covers the recent progress on syntheses, morphological features, and catalytic performances of various SiO2 matrices-decorated PdNPs with respect to Suzuki, Heck, and Sonogashira coupling reactions, and their future prospects are discussed. Solid-Supported Palladium Nanocatalysts: Coupling Reactions. Silica-supported PdNPs play a significant role in heterogeneous catalysis because of low cost, accessibility, high stability, and large surface area with excellent porosity. They provide the advantages of reaction pathways under mild reaction conditions and comprise environmentally friendly processes toward sustainable chemistry.51,52 The C−C coupling reactions, including Heck, Suzuki, Negishi, Corriu−Kumada, Sonogashira, Stille, Hiyama, Tsuji−Trost, and Ullmann reactions, have highlighted the practical applications of PdNPs in chemical industries. The types of Pd catalysts that have been applied in Suzuki−Miyaura, Heck, and Sonogashira reactions and the desired products obtained are most important in the field of pharmaceutical industries. Thus, silica-supported PdNPs-catalyzed cross-coupling reactions are well known and very important subjects, thus addressing their use is a timely topic. Recent Progress in Suzuki−Miyaura Cross-oupling Reactions. Palladium-catalyzed carbon−carbon bond-forming reactions developed by Suzuki have made a significant impact in the field of organic chemistry.53 It is a versatile route for the construction of biaryls, which are partial structures in pharmaceuticals54 by the reaction of aryl halides with PhB(OH)2 using heterogeneous PdNPs under milder conditions. Marck and co-workers reported the first example of the Pd/C-catalyzed Suzuki reaction.55 Reetz and co-workers56 first demonstrated the use of Pd and Pd/Ni NPs for the Suzuki coupling of aryl bromides and chlorides with Ph(OH)2 using 2 mol % Pd catalysts (Scheme 2). The Pd/Ni NPs had more activity for the transformation of aryl chlorides. In 2004, PdNPs (2−5 nm) entrapped in SiO2 gel (SiO2/TEG/ Pd, 1) were prepared by heating a mixture of [Pd(PPh3)4],

tetra(ethylene glycol), and tetramethoxysilane via a sol−gel process followed by treatment with water (Scheme 3).57 Aryl iodides and bromides were coupled with Ph(OH)2 using a 0.75 mol % Pd catalyst and K3PO4 in toluene at 110 °C to give products in 86−100% yields, although aryl chlorides were less efficient ( Pd@pSiO2 before thermal treatment (72%) > Pd@SiO2 core− shell NPs (40%) > free-standing PdNPs (35%). The superior catalytic activity of 3 was attributed to the completely exposed surface of Pd cores, rapid diffusion of the reactants through SiO2 layers, and clean metal surface aided by high temperature. Under identical conditions, bromo- and chloro-benzenes gave products in 61−100% yields. These catalysts exhibited a high turnover frequency of 78,000 h−1, as well as being recycled 10 times, indicating the heterogenesity of 3. Because of a large surface area and pore volume, as well as adjustable pore size of the mesocellular foam,60 the Bäckvall group developed the Pd0-AmP-MCF (4) catalyst using aminopropyl-functionalized siliceous foam with PdNPs.61 The Suzuki reaction gave a 4-fold decrease in yield under conventional heating instead of MW irradiation,62 indicating the unavailability of efficient heat transfer to the PdNPs. This catalyst was active in the coupling of aryl iodides and bromides (longer reaction times with higher catalyst loading) with Ph(OH)2 under optimized conditions to give products in 97−99% and 40−99% yields, respectively. This catalyst showed good performance in the coupling of heteroarylhalides with heteroarylboronic acids under MW irradiation.63 This catalyst could be recycled three times with a minimal Pd leaching and aggregation. 6358


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ACS Sustainable Chemistry & Engineering Scheme 3. Preparation of Recyclable Catalyst (1)

TEM images showed that these catalysts were spherical in shape with diameters of 30 and 28 nm, respectively. Higher conversions were obtained when these catalysts were tested in the Suzuki reaction of boronic acids with m-bromoanisole or o-bromopyridine using DMF−H2O and K2CO3 at 70 °C. A SiO2-acac-PdNPs catalyst (11) was prepared by the reaction of acetylacetone-modified silica with Pd(OAc)2 in the presence of CH3CN (Scheme 8).69 TEM images indicated the formation of PdNPs (6−12 nm). The Suzuki coupling of aryl iodides and bromides with Ph(OH)2 was rapid using a 0.04 mol % catalyst, NaHCO3, in refluxing H2O to give products in 91−94% yields. However, aryl chlorides reacted slowly to give 84−88% yields. In chemoselectivity reaction, Br is a better leaving group compared to Cl, giving products in 85−91% yields. This catalyst was stable in air and water and could be reused six times. Starting from the reaction of Schiff-base-functionalized silica with Pd(OAc)2 in MeOH, Mahjoub and co-workers41 prepared a MCM(Pd)-41 catalyst (12). HR-TEM images of 12 showed the distribution of PdNPs in pore channels. Excellent yields (80−95%) were obtained when aryl iodides or bromides reacted with boronic acids under solvent-free conditions using a 0.01 g Pd catalyst and K2CO3 at 100 °C. This catalyst could be recycled 10 times. The Sarkar group70 prepared a SBA-15-Pd catalyst (13) by treating mercaptopropylated SBA-15 with (CH3CN)2PdCl2 in CHCl3. Using a 0.1 mol % Pd catalyst, the coupling of aryl iodides and bromides with Ph(OH)2 in the presence of K2CO3 in aqueous ethanol at 90 °C gave biaryls in 90−97% and 87−94% yields, respectively. However, the coupling of aryl chlorides gave 51−54% yields. This catalyst could be reused three times. Using a modified Aerosil-380, SBA-15, plugged SBA-15, and m-MCF with thiol ligands, SiO2−SH·Pd (14), SBA-15SH·Pd (15), plugged SBA-15SH•Pd (16), and m-MCF-SH·Pd (17) catalysts with the same-sized PdNPs (2 nm) were prepared.34 Catalyst 16 was active in the Suzuki reaction of Ph(OH)2 with p-iodoanisole to yield 80% conversion under optimized condition (ethanol, Na3PO4·12H2O, 0.12 mol % Pd catalyst, 60 °C), while bromo- and chloro-benzenes in DMF gave products in 80% and 10% yields, respectively. On the basis of structural characteristics, catalyst 16 showed the best BET surface area, pore volume, and recyclability compared to other supports. A Pd@imine-SiO2 (3.26 nm) catalyst (18) was synthesized by treating imine-functionalized SiO2 with Pd(OAc)2 in acetone (Scheme 9).71 With optimized conditions (0.463 mol % Pd catalyst, iPrOH/ H2O, Na2CO3, RT), catalyst 18 showed 90−100% yields for the

Scheme 4. Proposed Procedure for Formation of Encapsulated Pd/SiO2 Nanobeads (2)

Using different Pd sources and silyl-functionalized KIT-6 with different pore-sized PdNPs (5−12 nm), KIT-6(X)-SH (5a−d) (X = 60, 80, 100, and 130 °C) was prepared under hydrothermal condition.64 All catalysts underwent the Suzuki−Miyaura reaction between p-bromoacetophenone and pinacol ester of Ph(OH)2 using a 1.0 mol % Pd catalyst in DMF/H2O at 80 °C to give 90−100% yields. These catalysts were reused 6−8 times, which is higher than SBA-15-based materials. However, the activity was decreased due to the formation of large PdNPs and Pd leaching. A Pd(II)−SBA-16 (6) catalyst was prepared by the reaction of functionalized SBA-16 with PdCl2 (Scheme 6).65 This catalyst showed a type-IV isotherm with an H2 hysteresis loop, indicating the presence of a mesoporous cage-like structure. Under optimized conditions (0.5 mol % Pd catalyst, K2CO3, EtOH/H2O, 80 °C), aryl iodides and bromides couple with arylboronic derivatives to give the desired products in 94−100% and 81−95% yields, respectively, but, aryl chlorides did not couple. This catalyst could be reused five times. Song et al.66 prepared a nanoreactor (7) that was composed of hollow spheres of silica with PdNPs located inside the pores. HRTEM images showed the distribution of PdNPs (2−10 nm) on SiO2. This catalyst showed 99.5% activity in the coupling of aryl iodides with Ph(OH)2 using a 10 mg Pd catalyst and K2CO3 in refluxing ethanol. However, bulky groups on the Ph(OH)2 failed to couple. A commercial Pd/C catalyst (Pd/C, 8)66 performed well in the Suzuki reaction (90% yield) without shape selectivity. However, the shape selectivity of 1-naphthaleneboronic acid using (7) was obtained by the collective diffusion barriers from reactants. A PNP-SSS catalyst (9) was synthesized by immobilizing palladium nanoparticles (PNP) on a silica-starch substrate (Scheme 7).67 By selecting appropriate boronic acids and diarylhalides under optimized conditions (0.08 g of Pd catalyst, water, NaOH), a wide range of p-teraryls could be synthesized in excellent yields. This catalyst could be reused six times without Pd leaching. The fabrication of SBA-15−Pd-X (X = 1%, 2%, 5%, 10%, 20%, and 50% Pd, 10a−f) and MSU-2−Pd-X (X = 1%, 2%, 5%, 10%, 15%, and 20% Pd, 10a−f) by functionalizing SBA-15 or MSU-2 through the reduction of [PdCl2(cod)] was reported.68

Scheme 5. Synthetic Procedure of Pd@pSiO2 Yolk−Shell Nanocatalysts (3)

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ACS Sustainable Chemistry & Engineering Scheme 6. Preparation of Catalyst Pd(II)−SBA-16 (6)

Scheme 7. Synthetic Route for Preparation of PNP-SSS Catalyst (9)

Scheme 8. Synthetic Route for Preparation of Catalyst (11)

Scheme 9. Preparation of Supported Schiff-Base Pd(II) Complex (18)

of fungicide Boscalid, which was obtained by coupling of o-nitroaniline with p-Cl-Ph(OH)2. Working with a higher catalyst loading allowed the catalyst to be efficiently recycled. Zhao et al.3 prepared Pd-MSNSs-T (T = 10, 30, 50, and 70 °C, 21a−d) through amino-functionalized silica with K2PdCl6 in the presence of NaBH4. TEM images of 21c showed that PdNPs were 1.3−5 nm in size. Among these catalysts, the immobilized PdNPs in 21b retained their locations in channels without aggregation. The Suzuki reaction proceeded with 83−96% conversion when aryl bromides and iodides were coupled with Ph(OH)2 using a 0.075−0.75 mol % Pd catalyst and K2CO3 in MeOH. This catalyst can be reused six times without apparent deactivation. A phosphine-free Pdnp-nSTDP (22) catalyst was prepared based on PdNPs immobilized on a nanosilica polymer (Scheme 10).23 TEM images indicated that a dendritic polymer would be a good host for 3.1 nm-sized PdNPs. Under optimized conditions (0.006 mol % Pd catalyst, K2CO3, DMF/H2O), aryl iodides and bromides were coupled with Ph(OH)2 to give the desired products in 90−96% yields at RT or 91−96% yields under MW. By increasing the temperature and reaction time, the coupling of aryl chlorides gave biaryls in high

Suzuki reactions of 4-aryl bromides with Ph(OH)2 derivatives. However, aryl chloride did not couple. This catalyst could be reused eight times without Pd leaching. Paul and co-workers72 developed Pd(0)-EDA/SC-1−3 (19a), Pd(0)-EDA/SC-2 (19b), and Pd(0)-EDA/SC-3 (19c) based on immobilization of PdNPs on ethylenediamine-functionalized silica cellulose. SEM and TEM images confirmed uniform distribution of PdNPs on EDA/SC. Among these catalysts, catalyst 19b was active in the Suzuki reaction of aryl bromides with arylboronic acids under optimized conditions (K2CO3, TBAB, 2.5 mol % Pd catalyst, water, MW) to give 75−92% yields. Under identical conditions, thermal coupling gave a similar % product yield. This catalyst was reused five times without loss of activity. A Pd−SO3H/SiO2 catalyst (20) was prepared by the reaction of Pd(OAc)2 with silica-supported phenylsulfonicacids.35 The 0.25 wt % Pd catalyst (20) and 0.5 wt % Pd catalyst (20) onto silica were also prepared for comparative studies. TEM images showed that spherical PdNPs with sizes of 10−16 nm and 37−45 nm for 0.25% 20 and 0.5% 20, respectively, were distributed onto silica. The coupling of anilines with boronic acids was performed to give 52−84% yields using 50 mol % H+ and 3.7 mol % Pd. This protocol was applied to the preparation 6360


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ACS Sustainable Chemistry & Engineering Scheme 10. Synthesis of Pdnp-nSTDP Catalyst (22)

was required, giving 20−76% yields. This protocol was extended to the Suzuki cross-coupling of 7-bromo-2,3-diphenylpyrido[2,3-b] pyrazine, which are structures found in anticancer agents and antibiotics, with arylboronic acids using K2CO3 in THF under reflux to give the desired products (79−84%). A hot filtration test proved the heterogenesity of 26. Organic-functionalized NH-SBA-15 reacted with PdCl2 via the coordination bonds followed by reduction with formalin to obtain Pd-SBA-15 (27a).77 Three more samples were obtained using 0.05 g PdCl2 for 27b, 0.2 g PdCl2 for 27c, and SBA-15 for 27d to compare the reactivity. TEM images confirmed the encapsulation of PdNPs (3−6 nm) in the silica pores. These catalysts were active in the Suzuki reaction of aryl bromides with Ph(OH)2 to give their biaryls. Among others, catalyst 27d showed the lowest activity with a high degree of Pd leaching, indicating the importance of amino groups in preventing agglomeration and leaching of PdNPs. Catalyst 27a showed the highest activity with moderate Pd leaching, as it contained an optimal N:Pd molar ratio. Under optimized conditions (K2CO3, 0.05 mol % Pd catalyst, water, 80 °C), the Suzuki reaction of bromobenzenes with Ph(OH)2 gave 90−99% yields of biaryls. This catalyst could be reused six times with a minor aggregation and Pd or N leaching. Polymer-encapsulated silica-supported Pd(0) catalysts (28a and 28b) were prepared by the reaction of amine-modified silicas with Pd(OAc)2, followed by reduction with NaBH4.78 TEM images showed that PdNPs (100 nm) were surrounded by polymer coatings. Under optimized conditions (MeOH:H2O:DME, DIPEA, 200 mg of Pd catalyst, 1.5 g of sand, 120 °C), these catalysts showed excellent reactivity (>95%) with deactivated aryl iodides and a lower conversion with activated aryl iodides (56−86% for 28a; 19−43% for 28b). Both catalysts were used for >50 h of consecutive operation with minimal Pd leaching. The Das group32 has developed a Pd@imine−SiO2 catalyst (29) by immobilizing Pd(OAc)2 onto silica through Schiff-base condensation between APTES-functionalized silica and acetamide. SEM studies suggested that the presence of Pd caused a decrease in silica size. This catalyst was active in the Suzuki coupling of aryl bromide and iodides with Ph(OH)2 using a 0.08 mol % Pd catalyst and K2CO3 in iPrOH/H2O at 60 °C to yield products in 84−99% and 96−98%, respectively, but the coupling of aryl chlorides was poor. This catalyst can be reused six runs without Pd leaching.

yields. The efficiency of 22 was examined for the synthesis of starand banana-shaped molecules, and the yields were 20−91% at RT and 30−94% under MW irradiation. This catalyst could be recycled seven times without metal leaching. A nanocatalyst, Nano-Pd/SiO2 (23a), prepared from PdNPs on SiO2 via a chemical vapor deposition technique was reported.73 SEM and AFM images indicated that PdNPs (30 nm) were uniformly distributed on the surface of SiO2. A 5% Pd/SiO2 catalyst (23b) was also prepared for comparing the reactivity purpose. The Suzuki reaction of Ph(OH)2 with p-iodophenol in H2O at RT using K2CO3 showed that catalyst 23b was less reactive (85% yield) compared to that of 23a, 95% yield, due to aggregation of PdNPs. Under optimized conditions (0.3 mol % Pd catalyst, K2CO3, H2O), the desired products were obtained in 82−95% yields at RT from the reaction of p-iodoand bromo-phenols with Ph(OH)2. Halophenols showed a high selectivity, which was attributed to H-bonding between the OH groups of the silica and halophenols. This catalyst could be recycled four times without Pd leaching. The reaction of PdCl2 with SiO2-diethanolamine-1,4-diazabicyclo[2.2.2]octane followed by reduction with hydrazine afforded DEA-DABCO-Pd(0) (24).74 TEM images of 24 showed that the size of the PdNPs was 10−20 nm. The Suzuki reaction of o- and m-nitroaryl halides gave trace amounts of product, while p-aryl bromides afforded coupling products in 92−99% yields under optimized conditions (50% EtOH, K2CO3). However, para-substituted aryl chlorides were less reactive to give products in 77−89% yields, when K3PO4, 0.08 mol % Pd catalyst, and DMF were used at 130 °C. This catalyst was reused five times without significant loss of activity. A silica−terpy−Pd(II) nanocatalyst (25) was prepared by the reaction of SiO2−terpy with Pd(OAc)2 at RT.75 This catalyst was active in the Suzuki coupling of aryl iodides with arylboronic acids using a 1.0 mol % Pd catalyst and K2CO3 in water at 90 °C to give 86−91% yields. The coupling of chloro- and bromobenzenes failed to couple. This catalyst could be reused four times without Pd leaching. A silica-tethered Pd−DABCO catalyst, Pd−DABCO@SiO2 (26),76 was prepared by the reaction of DABCO-functionalized silica with Pd(OAc)2. When the optimized protocol (K2CO3, 1.0 mol % Pd catalyst, air, 80 °C) was applied to reactions of aryl bromides with arylboronic acid, the coupling products were formed in 70−96% yields. For aryl chlorides, a much longer time 6361


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ACS Sustainable Chemistry & Engineering

Scheme 11. Schematic Illustration Showing Synthesis of Shell-in-Shell @Pd/meso-TiO2/Pd@meso-SiO2 Nanocatalyst (30a)

Pd(OAc)2 through an in situ polymerization method.82 Under optimized conditions (0.14g of Pd catalyst, KOH, water, 40 or 90 °C, air), the coupling of aryl iodides with Ph(OH)2 gave 85−97% yields, while bromo- and chloro-benzenes yielded 90−95% at 40 or 90 °C. This catalyst could be reused nine times with a low level of Pd leaching. Hollow Pd spheres (35) were prepared by heating mercaptopropylsilyl-functionalized silica with Pd(acac)2 at 250 °C followed by treatment with HF.83 TEM images showed that the shell was composed of 10 nm-sized PdNPs. Suzuki coupling between Ph(OH)2 and C6H5I was conducted using 3 mol % Pd catalyst, and K3PO4 in EtOH at 78 °C to produce the desired biaryls in 96−99% yields. However, aryl bromides required 5 times more Pd catalyst to achieve a similar yield, while aryl cholorides did not couple. This catalyst could be reused seven times without Pd leaching. Two effective catalysts, SBA-15-SH·Pd (36a) and SBA-15NH2·Pd, (36b), were prepared by the reaction of mercaptopropyl- or amine-modified SBA-15 with Pd(OAc)2.84 Catalyst 36a worked well in the Suzuki coupling of aryl chloride with arylboronic acid using a 2 mol % Pd catalyst and K2CO3 to give product in 96% yield in H2O. Bromobenzenes gave 82−97% yields in H2O or DMF/H2O. It could be reused four times with a low extent of Pd leaching. The Sangtrirutnugul group prepared a T10-Pd catalyst (37) from a reaction between Pd(COD)Cl2 and pyridine−triazolefunctionalized decameric silsesquioxane.85 XPS and XRD analyses of 37 confirmed the presence of Pd. Under optimized conditions (1.4 mol % Pd catalyst, K2CO3, EtOH:H2O, and 60 °C), coupling products derived from bromoarenes and Ph(OH)2 were obtained in 65−98% yields. This catalyst could be recycled five times with no detectable deactivation. The Cai group86 developed a MCM-41−2N−Pd(II) catalyst (38) by heating 3-(2-aminoethylamino)propyl-functionalized MCM-41 and Pd(OAc)2. XPS studies confirmed the presence of Pd in this catalyst. This catalyst was active in the coupling of p-aryl bromides with Ph(OH)2 under optimized conditions (0.2 mol % Pd catalyst, K2CO3, xylene, 90 °C) to produce their desired products in 91−97% yields, while o- or heteroaryl bromides afforded 83−89% yields. This catalyst could be recycled 10 times without Pd leaching. Strawberry-like nanomaterials, NH2−SiO2@Pd (5.1 nm, 4.47 wt % Pd catalyst (39a); 6.0 nm, 5.95 wt % Pd catalyst (39b)) and PPh2-SiO2@Pd (39c), were prepared by the reaction of APTES- or 2-(diphenylphosphino)ethyltriethoxysilane-functionalized silica with Pd(II), respectively.87 TEM images showed that 3.6 nm-sized PdNPs were dispersed on the silica surface. The Suzuki coupling reaction between Ph(OH)2 and aryl iodides catalyzed by 39a and 39b using 0.1 mol % Pd catalyst and K2CO3 in DMF−H2O at 100 °C produced biphenyls with a 94−96% conversion. A SiO2-pA-Cyan-Cys-Pd (40) catalyst based on a propylamine−cyanuric−cysteine Pd complex immobilization on silica was prepared.88 TEM images showed the size distribution of PdNPs (15−30 nm) on the surface of silica. This catalyst was

The Zhang group prepared a shell-in-shell structured nanocatalyst, @Pd/meso-TiO2/Pd@meso-SiO2 (30a), as shown in Scheme 11.54 EDX confirmed the presence of PdNPs (∼5 nm) in 30a. Contrasting catalysts, @meso-TiO2/Pd@meso-SiO2 (30b), @mesoTiO2/Pd@SiO2 (30c), @Pd/meso-TiO2/Pd (30d), and @mesoTiO2/Pd (30e), were also prepared to compare catalytic performance. When catalyst 30a was explored in the Suzuki coupling of iodobenzenes with Ph(OH)2 under optimized conditions (80 °C, EtOH, Cs2CO3, 10 or 25 mg of Pd catalyst), it showed a 53−100% conversion and TOF of 8220−15,546 h−1, which were higher than (30b−e). The mesopores in the SiO2 shell allowed the reactants to selectively diffuse into the nanoreactors, thus achieving catalytic selectivity. This catalyst showed outstanding performance in this reaction. Cao and Song37 fabricated a NC/Pd@mSiO2 nanoreactor (31), as shown in Scheme 12. TEM images showed that Scheme 12. Synthetic Strategy for NC/Pd@mSiO2 Yolk− Shell Nanoreactor (31)

5 nm-sized PdNPs were uniformly distributed on the surface of PDA spheres. This nanoreactor was active in the Suzuki coupling of bromoand iodo-benzenes with Ph(OH)2 in the presence of K2CO3 and 10 mg of Pd catalyst in ethanol/H2O at 80 °C. The conversion of bromo- and iodo-benzenes reached 99%, but chlorobenzene gave only 26%. This nanoreactor was reused five times without Pd leaching. A functionalized KCC-1-NH2 composite was reacted with PdCl2 followed by hydrogen reduction to afford a KCC-1-NH2/Pd nanocatalyst (32).79 TEM images of 32 demonstrated the loading of PdNPs (1−5 nm) into fibers of KCC-1. Under optimized conditions (K3PO4, EtOH/H2O, 100 °C, 0.5 mol % Pd catalyst), this catalyst catalyzed the coupling of arylhalides with arylboronic acids to produce the corresponding biaryls in 77−97% yields. The catalyst could be reused seven times without Pd leaching. Nanocatalyst Pd@SiO2 (33) was prepared by heating a silylated Pd complex with silica gel in toluene at 100 °C.80 This catalyst showed 80−94% yields in the coupling of heteroaryl chlorides with arylboronic acids using a 0.5 mol % Pd catalyst at 60 °C in water. These reactions gave surprising results since chlorothiophenes in the coupling reaction are challenging due to the strong affinity for Pd.81 TBAB was used as a phase transfer to enhance the reactivity in water. The catalyst could be reused six times with minor Pd leaching. As silica KIT-6 possesses large tunable pores with thick pore walls, high stability, and high surface area with large pore volume, nanocomposite Pd-PHEMA/KIT-6 (34) was fabricated by the reaction of poly(2-hydroxyethyl methacrylate)/KIT-6 with 6362


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ACS Sustainable Chemistry & Engineering Scheme 13. Synthesis of Silica-Supported PdNPs (41)

Scheme 14. Preparation of Ionic Liquid-Supported Pd Catalyst (42)

Scheme 15. Synthesis Route to Composite Nanoreactor (45)

43b3Pd showed poor activity. Long alkylamine chains containing catalysts 43g3Pd and 43h3Pd gave promising results in terms of activity and recyclability. These catalysts could be recycled four to six times with Pd leaching. The Sarkar group prepared a SBA-16-supported Pd complex (44) by treating chloropropylated SBA-16 with PdCl2 in THF at 50 °C.92 TEM images of 44 indicated a well-ordered mesopore structure. The Suzuki reactions of aryl iodides and bromides proceeded in coupling with arylboronic acids under optimized conditions (0.01 mol % Pd catalyst, K2CO3, EtOH/H2O, 90 °C) to give products in 88−94% yields. This catalyst provided 53−73% yields with chloroarenes. Generally, this catalyst is more active than an SBA-16-supported Pd−guanidine catalyst.93,94 TEM images of reused 44 confirmed its heterogeneous pathway. Fabrication of Pd@meso-SiO2 (45) was achieved by coating a thin layer of silica on Pd/C using TEOS and CTAB followed by calcination treatment (Scheme 15).66 TEM studies of 45 showed the distribution of PdNPs (5 nm) on the surface of silica. Under optimized conditions (K2CO3, 10 mg of Pd catalyst, ethanol, 80 °C), this nanocomposite showed a 52−99.5% conversion in the coupling of aryl iodides with Ph(OH)2 derivatives, while C6H5Br afforded only a 32% conversion. This catalyst could be reused four times without deactivation. Although catalysts 45 and Pd@C composites (8)66 had the same initial 4.0 wt % Pd loading, Pd leaching from 8 was 8 times higher than that of 45, showing its performance in a stable manner. A series of SiliaCatPd0-Hydrogel catalysts, SiliaCatPd0-1 (46a) (0.03 mmol/g Pd loading), SiliaCatPd0-2 (46b) (0.112 mmol/g Pd loading), SiliaCatPd0-3 (46c) (0.148 mmol/g Pd loading), and SiliaCatPd0-4 (46d) (0.163 mmol/g Pd loading), were prepared by the hydrolysis and condensation of MeSi(OEt)3 and Si(OEt)4 and, consequently, doped with K2PdCl4 followed by hydrogenation.95 An SEM image of 46a revealed the size of organosilica (60−125 μm). Catalyst 46a promoted 95−100% yields in the Suzuki coupling of aryl and heteroaryl iodides and bromides

efficient for the Suzuki reaction of aryl iodides or bromides to give 87−95% yields using 0.5 mol % Pd catalysts, K2CO3, and H2O at 100 °C. However, this catalyst provided 52−61% yields for chloroarenes. This catalyst could be recycled five times without losing its activity. Sarkar et al.89 prepared a silica-supported polyethylene glycolencapsulated Pd catalyst (41) by treating Fischer carbine acyl metal salt with a mixture of K2PdCl4 and functionalized silica in water (Scheme 13). TEM images of 41 confirmed that spherical PdNPs (12−14 nm) were dispersed across the silica. It catalyzed the Suzuki coupling of Ph(OH)2 with aryl bromides and iodides using K2CO3 and a 1 mol % Pd catalyst in DMF at 100 °C to generate products in 88−95% yields. The catalyst can be reused four times with no Pd leaching. The Jin group90 prepared an ionic liquid-supported Pd catalyst (42) by immobilization of triethoxysilylated ionic liquid on silica followed by reaction with Pd(OAc)2 (Scheme 14). Under optimized conditions (0.1 mol % Pd catalyst, Na2CO3, DMF:H2O, 65 °C), aryl bromides, and iodides underwent coupling with Ph(OH)2 to give their desired products in 83−100% yields, but 52−66% yields were observed with aryl chlorides. This catalyst could be reused six times without Pd leaching. Organically modified silicas with different concentrations were reacted with Pd(OAc)2 at 100 °C to give 43a−h; ([NBu3]+ modified silica 43a0.5Pd, 43a1Pd, 43a3Pd; N-methyl-imidazolium-modified silica 43b0.5Pd, 43b1Pd, 43b3Pd; NH2-modified silica 43c0.5Pd, 43c1Pd, 43c3Pd; NHMe-modified silica 43d3Pd; NEt3-modified silica 43e3Pd; NHPh-modified silica 43f3Pd; NHCH2CH2NH2-modified silica 43g3Pd; NH(CH2)2NH(CH2)2NH2-modified silica 43h3Pd.91 TEM results indicated that particle size was 5 nm. The Suzuki reaction of Ph(OH)2 with p-bromoanisole catalyzed by 43c0.5Pd using K3PO4 in toluene at 110 °C afforded biphenyl in 90% yield, while substituted bromoarenes coupled with Ph(OH)2 using 43c3Pd−43h3Pd give products in 75−98% yields. However, catalysts 43a3Pd and 6363


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Table 1. Catalytic Reactions Using Silica-Supported PdNPs and Their Catalytic Performances in Suzuki Cross-Coupling Reactionsa

a

NR = Not reported.

We compiled reports on some silica-based Pd catalysts for Suzuki coupling reaction in Table 1. Recent Progress in Heck Cross-Coupling Reactions. The Heck reaction is a fundamental synthetic transformation that has been used for the synthesis of alkenes from the coupling of alkenes with aryl halides in the presence of Pd catalysts. Such phosphine/ligand-free Pd catalysts are regarded as ecofriendly methods for use in the Heck reaction,39,106−108 which is ubiquitous in pharmaceutical industries. Moreover, silica materials are one of the most solid supports for PdNPs in terms of applications to Heck reactions.109 This section focuses on the development of silicasupported Pd catalysts for use in Heck reactions.

with Ph(OH)2 under optimized conditions (K2CO3, 0.1−0.5% mol Pd catalyst, MeOH or EtOH). The catalyst could be reused seven times with minimal leaching of Pd and Si. A hot filtration test showed its heterogeneous manner. A phosphine-free Pd/FSG nanocatalyst (47) was prepared by the reaction of Pd(OAc)2 and fluorous silica in perfluorooctane.96 The size of PdNPs was 2.9 nm. When this catalyst was tested in the Suzuki reaction of Ph(OH)2 with aryl bromides using K2CO3, and 0.1 mol % Pd catalyst in MeOH/H2O, a 85−95% yield was obtained, while aryl iodides gave products in 95−98% yield. Aryl chlorides gave only 7−21% yields. This catalyst could be reused five times with a low Pd leaching. 6364


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ACS Sustainable Chemistry & Engineering Beller et al.110 first demonstrated the use of PdNPs (61) in the Heck reaction using Pd colloids derived from tetraoctyltriethylboronate and [PdCl2]. This catalyst (0.05 mol % Pd catalyst) was active in the arylation of styrene or butyl acrylate by activated aryl bromides but showed moderate or little activity for deactivated aryl bromides and aryl chlorides, respectively (Scheme 16).

The channels were less ordered in 65 compared with MCM-41, indicating amorphization of MCM-41. This catalyst was active for the Heck reaction of aryl halides with styrene or n-butyl acrylate using 0.05g of Pd catalyst, NaOAc, and DMF at 80−100 °C. Iodobenzene gave 88% and 100% conversion with styrene and n-butyl acrylate, respectively. C6H5Br displayed 56% and 76% conversions, and chloroarene gave 15% and 45% conversions. Note that only a few heterogeneous PdNP catalysts are capable of activating C6H5Cl in the Heck reaction.116 This catalyst could be reused four times with high selectivity. The PdNPs on silica-bonded N-propylmorpholine (PNPSBNPM, 66) were produced by reacting SBNPM with Pd(OAc)2 in EtOH.117 TEM images showed the location of spherical PdNPs (7 nm) onto supports. A 90% yield was achieved in the reaction of aryl iodide with styrene under optimized condition (2.4 mol % Pd catalyst, Na2CO3, DMF, 120 °C), but an increase in reaction time was necessary to obtain 75% and 55% yields, respectively, for the reaction of bromo- and chloroarenes. This protocol is a one-pot route for the preparation of (E)-1,4distyrylbenzenes in 78−90% yields from the reaction of p-substituted styrenes with aryl dihalides. High yields with a minor Pd leaching were found in a six-run recycling experiment. Kalbasi et al.118 prepared a composite Pd-PVP/KIT-6 (67) by heating a mixture of PVP/KIT-6 and Pd(OAc)2 in aqueous acidic solution followed by reduction hydrazine hydrate. TEM images of 67 showed the location of PdNPs (4.5 nm) inside the channels. Under optimized conditions (0.14g Pd catalyst, K2CO3, MeOH/H2O, 60 °C), although C6H5I coupled with a high efficiency (97% yield), 12 h of reaction time was needed for deactivated iodobenzenes to give 40−98% yields. However, chloro- and bromo-benzenes afforded the coupling products in 90−98% yields, but significantly longer reaction times were needed, compared to that of C6H5I. This catalyst can be reused eight times with a lower Pd leaching. Compared to Pd-SBA-15 (27a),77 this catalyst displayed more recyclability as 3D porous networks of KIT-6 permitted a faster diffusion of reactants and hence avoided pore blockage, provided more adsorption sites, and prevented leaching of PVP and PdNPs into the solution. Bazgir75 investigated the activity of catalyst 25 in the Heck reaction of aryl halides with olefins under optimized conditions (K2CO3, 0.5−1.2 mol % Pd catalyst, N-methyl-2-pyrrolidone, 100 °C). Yields of 49−91% were obtained in coupling of arylhalides with styrene and aliphatic alkenes. Aryl chlorides also coupled to afford 65−92% yields. Under optimized conditions (0.5 mol % Pd catalyst, solvent-free, 130 °C), catalyst Pd(0)/ SDPP (58) performed the Heck reactions of iodo- and bromobenzenes to give the desired products in 60−99% yields.103 For coupling of deactivated aryl bromides with n-butyl acrylate, an n-Bu4NBr additive was applied, resulting in 52−89% yields of products. The Heck reaction of sterically hindered 1-iodonaphthalene afforded the desired products in 78−90% yields. This catalyst could be reused six times with a low level of Pd leaching.

Scheme 16. PdNPs (61) Catalyzed Heck Coupling Reactions

The Shi group prepared a Pd−SBA catalyst (62) by reacting trimethoxysilane-functionalized SBA-15 with Pd(OAc)2 in THF.111 TEM images permitted the morphology and distribution of PdNPs on the surface of SBA-15. Catalyst 62 performed well in Heck reactions of aryl halides with styrene (88−99% yield) and methyl acrylate (91−99% yield) using Et3N and a 0.02 mol % Pd catalyst in air at 120−170 °C. This catalyst could be reused over five times without Pd leaching. Moreover, this catalyst performed well compared to other heterogeneous Pd catalysts112 owing to high dispersion of Pd on the surface and pores of host. The PdNP/Sα−, Sβ−, and Sγ−CD catalysts (63a−c) were prepared by refluxing a mixture of silica chloride and α−, β−, and γ−CDs followed by treatment with Pd(OAc)2.113 The particle sizes for 63b, 63c, and 63a were 3, 5, and 10 nm, respectively, while the size of silica was in μm. Among them, catalyst 63b exhibited an optimal activity for Heck reaction of arylhalides with stilbenes to give trans products (61−99%) with good selectivity using K2CO3 and a 0.05 mg Pd catalyst in H2O reflux. This catalyst could be reused five times with a low extent of Pd leaching. Semi-heterogeneous PdNPs stabilized on the surface of (tris(hydroxymethyl)aminomethane)-functionalized SiO2, SiO2Tris-PdNPs (64), was reported (Scheme 17).114 A TEM image of 64 showed that PdNPs had diameters of 11 nm. The efficiency of 64 was studied for Heck reactions of aryl bromo-/iodo or chloro derivatives and olefins under optimized conditions (0.0002 mmol Pd catalyst, NaHCO3, DMF, 140 °C). Depending on the nature of the substituents in arylhalides and olefins used, the yield of the coupling product varied from 48% to 99%. In this reaction, methyl acrylate was used to suppress double arylation and enabled the reaction faster than that of styrene. Catalysts were recycled seven times without any loss of activity. Hot filtration and poisoning tests showed its homogeneous and heterogeneous pathways. A Pd(0)-MCM-41 (65) nanocatalyst was prepared by incorporation of Pd(II) into silica followed by reduction.115 Scheme 17. Synthetic Route for Preparation of Catalyst 64

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ACS Sustainable Chemistry & Engineering Scheme 18. Schematic Representation of Synthesis of Nanocatalyst 69

Scheme 19. Synthesis of Catalytic Material 70

Scheme 20. Schematic Illustration for Preparation of Catalyst 71

the corresponding trans-stilbenes in 80−98% yields, while aryl chlorides gave 85−90% yields. This material could be reused five times without Pd leaching. This protocol resulted in higher conversions and yields compared to others. A SiO2/PdNP/porous-SiO2 nanosphere (69) was obtained with a uniform nanoporous shell using an optimal etching time of 80 min (Scheme 18).121 TEM images showed that 5 and 20 nm-sized PdNPs were anchored to silica. It was efficient in the coupling of aryl iodides with styrene to form trans-stilbene in 100% yield using 10 mg of Pd catalyst and Et3N as a base in DMSO at 120 °C. The coupling of bromo- and chloro-benzenes was not reported. The advantages of using catalyst 69 in Heck reactions are (i) large number of PdNPs are incorporated in supports, thus conferring high activities, (ii) nanoporous SiO2 around PdNPs permitted the reactants to reach the surface of PdNPs and hindered PdNPs aggregation, and (iii) they are easily dispersed in solutions, thus recovered by simple centrifugation. Nanostructured Pd containing modified silica (70) was prepared by hydrolysis−polycondensation of TEOS and N,Ndimethyl-pyridin-4-yl-(3-triethoxysilyl-propyl)-ammoniumiodide followed by treatment with Pd(OAc)2 (Scheme 19).122 TEM images of 70 revealed the nanostructured morphology with a high degree of regularity. This catalyst was used in the Heck reaction of aryl iodides and bromides with methyl acrylate and styrene to give products in 70−90% yields under optimized conditions (Et3N, 0.1 mmol Pd catalyst, refluxed CH3CN). However, aryl chlorides did not couple. It could be recycled five times with a low level of Pd leaching. A Pd-supported catalyst (71) was prepared by anchoring a phosphine ligand on Ph-SBA-15 followed by treatment with

The Pdnp-nSTDP catalyst (22) was used in the Heck reaction of aryl iodides and bromides with styrenes to give the desired products in 90−95% yields under optimized conditions (K2CO3, 0.01 mol % Pd catalyst, DMF/H2O, 85 °C).23 Aryl chlorides gave 88−92% yields only at longer reaction times. Under MW irradiation, the desired products were formed in 87−96% yields in 8−20 min using the same reaction conditions. Finally, 75−88% yields of star-shaped products were obtained by the Heck reaction of 1,3,5-tribromobenzene with styrenes under MW irradiation. Catalyst MCM-Py-Pd-48 (59) catalyzed the Heck reaction of C6H5I with acrylates, isopropylacrylamide, and styrene in the presence of Na2CO3 in aqueous DMA with a 0.13 mol % Pd catalyst at 130 °C to yield the desired products in 87−96% yields.104 The coupling of aryl bromides with isopropylacrylamide and styrene afforded the desired products in 82−95% yields. This catalyst had a better activity than a hybrid silicasupported Pd complex.119 It could be reused five times with loss of Pd. Two conditions were suitable for the Heck coupling of iodoarenes but not for bromoarenes using the catalyst SiO2@ PdNP (53).99 The coupling of C6H5I with olefins using a 10 mmol−1 Pd catalyst and CTAB in water afforded the desired products in 72−90% yields. The coupling of p-bromo-o-iodoanisole yielded an (E)-ethyl 3-(4-bromo-2-methoxyphenyl) acrylate product, while p-bromotoluene failed to react, showing the selectivity. Amine-functionalized SBA-15-anchored Pd(II), SBA-15/ CCMet/Pd(II) (68), was prepared via immobilization of PdNPs on SBA-15 followed by treatment with PdCl2.120 TEM images of 68 displayed a pore diameter size of 7.58 nm. After optimizing the conditions (1.0 mol % Pd catalyst, Et3N, DMF), the coupling of aryl iodides or bromides with styrenes afforded 6366


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ACS Sustainable Chemistry & Engineering Scheme 21. Preparation of Chemically Modified Mesoporous Silica Gel and Immobilized Pd Catalyst (74)

Pd(II) (Scheme 20).123 TEM images of 71 indicated the presence of well-ordered hexagonal pore channels. This catalyst was employed in the Heck reaction of p-NO2− C6H4Br and methyl acrylate under optimized conditions (3.0 mol % Pd catalyst, nBu4NOAc, supercritical CO2) to give the desired product in 98% yield. This catalyst could be reused several times without loss of activity. Banerjee et al.101 reported on Pd-MCM-48 catalyst (55) for Heck reactions of aryl bromides and iodides with butyl acrylate, acrylonitrile, styrene, and 1-methoxy-4-vinylbenzene using a 0.6 wt % Pd catalyst and NaOAc in DMF at 100 °C to afford the desired products in 85−98% yields under ligand-free and aerobic conditions. Because of its large surface area, higher metal dispersion, and interpenetrating network of 3D pore sizes, this catalyst showed excellent activity. However, chloroarenes were not active. This catalyst could be reused four times without significant loss of activity. A poly(N-vinylpyrrolidone)-grafted silica Pd complex (72) was active in Heck reactions using DMF, K2CO3, and a 0.5 mmol % Pd catalyst at 120 °C.128 Aryl iodides reacted with n-butyl acrylate or styrene to produce the desired products in 93−95% yields, while bromobenzenes gave 80−95% yields. Chloroarenes were active only at high temperature or using a TBAB additive to give products in 40−90% yields. This catalyst could be recycled seven times without loss of activity. The heterogeneity of 72 was confirmed by a hot filtration test. Functionalized SiO2−Pd nanocomposites, 39a and 39b, performed well in the Heck coupling of C6H5I with styrene in the presence of a 0.1 mol % Pd catalyst using 1,3,5-trimethoxybenzene at 100 °C to afford 76% and 71% conversions for 39a and 39b, respectively.87 When the same reaction continued for a longer reaction time, an excellent yield was obtained. These catalysts could be reused eight times without loss of activity. SiO2-acac-PdNPs (73) was active in a Heck reaction of aryl iodides and bromides with acrylates to produce the desired products in 72−93% yields using NMP, NaHCO3, and a 2 × 10−4 mmol Pd catalyst at 140 °C.124 Chloroarenes gave products in 55−80% yields. This catalyst could be recycled nine times with a low extent of Pd leaching. The nanocatalyst Pd-MCM-41 (12) catalyzed the Heck coupling of aryl iodides with styrene and n-butyl acrylate under optimized conditions (solvent-free, 0.005g of Pd catalyst, n-Pr3N, 130 °C) to give 82−95% yields.41 Aryl bromides afforded 80−92% yields, but a longer reaction time was needed. Clark et al.106 prepared PdNPs entrapped in silica gel (74) by modifying the silica using aminopropyl(triethoxy)silane and 2-pyridinecarbaldehyde followed by complexation with Pd(II) (Scheme 21). This catalyst exhibited a pore diameter size of 6−80 nm. This catalyst was applied to the Heck reaction of methacrylate with C6H5I or p-iodophenol in the presence of 0.2 g of Pd catalyst and Et3N in CH3CN to afford 82% and 31% conversion, respectively. Allyl alcohol showed 21% and 15% conversion for C6H5I and 4-iodophenol, respectively, while but-3-en-2-ol gave 31−41% conversion along with side products. Thus, the activity and selectivity was poor, owing to the formation of triethylamine hydroiodide, which was likely to become adsorbed

on surface of the catalyst and block active sites. The catalyst could be reused five times without Pd leaching. A palladium-doped SiO2 nanoparticles (Pd/SiO2 NPs, 54) nanocatalyst was active in the Heck reaction of iodo- and bromoarenes with olefins to give products in 89−97% yields under optimized conditions (Et3N, dodecane, 0.75 wt % Pd catalyst, DMF, MW).100 Chloroarenes failed to react. An SBA-16-supported 1,2-diaminocyclohexane Pd complex (44) catalyzed the Heck reactions of aryl iodides with butyl acrylate, isopropylacrylamide, and styrene to give products in 91−97% yields under optimized conditions (500 mol Pd catalyst, Na2CO3, DMA/H2O, 125 °C).92 Aryl bromides promoted the coupling to give the desired products in 89−96% yields, while aryl chlorides gave 48−71% yields. The catalyst was reused five times with a minimal Pd leaching. Polymer-encapsulated silica-supported PdNPs (28a and 28b) were active for Heck reactions of C6H5I with olefins to give products in 30−95% yields using NMP and N-methyldicyclohexylamine at 140 °C.78 In this reaction, catalyst 28a was more active than 28b and could be recycled without leaching. We compile reports on the use of some silica-based Pd catalysts for Heck coupling reaction in Table 2. Recent Progess in Sonogashira Cross-Coupling Reactions. The Pd-catalyzed Csp2−Csp cross-coupling reaction developed by Sonogashira has had a significant impact in the field of pharmaceutical synthesis.131−135 It is a versatile route for the preparation of arylacetylenes by the coupling of alkynes with aryl or alkenylhalides (or triflates) with or without Cu(I) co-catalyst using PdNPs and a base in solvents. Although Cu(I) facilitates the reaction by the in situ generation of Cu acetylide, it causes the induction of so-called Glaser-type oxidative homocoupling of alkyne to afford a diyne.136 In order to suppress the formation of this byproduct, several procedures were developed. Such reactions are known as copper-free Sonogashira reactions. Amine-free, ligand-free, and solvent-free conditions have been performed since the Cu-free variant requires the use of amine, which can be detrimental to the environmental and economic advantages of this methodology. Aryl iodides, bromides, and triflates are most used coupling partners, but the coupling of aryl chlorides remains as a challenging task. Becasue of the benefits, chemists have attempted to capture some achievements in moving to a greener pharmaceutical industry.137 Nanostructured Pd(0) xerogels (46a−46d) showed 99−100% product yields in the coupling of phenylacetylene and iodobenzenes using trimethylamine or K2CO3 as a base and 0.1 mol % SiliaCat Pd0 in EtOH.95 The sol−gel encapsulation of Pd prevented the use of deaerated conditions and avoided the formation of homocoupled acetylenes. Trademarked SiliaCatPd0Hydrogel is now commercially available due to its excellent stability and versatility of use among Pd heterogeneous catalysts.46,135 A Pd-LHMS (82) catalyst was obtained by the reaction of Pd(II) with 2D-hexagonal organosilica (Scheme 22).136 HR-TEM images of 82 showed the hexagonal arrangement of the pore channels array with pore dimensions of 2.6 nm. This catalyst showed 72−90% yields in the Sonogashira coupling of aryl iodides and bromides with phenylacetylene using 6367


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Table 2. Catalytic Reactions Using Silica-Supported PdNPs and Their Catalytic Performances in Heck Cross-Coupling Reactionsa

a

NR = Not reported.

Under optimized conditions (0.05 mol % Pd catalyst, K2CO3, ethylene glycol, 120−130 °C), catalyst Pd/NH2−SiO2 (49) underwent a Sonogashira reaction of iodobenzenes with phenylacetylene

0.03 g of Pd catalyst and hexamine as a base in the absence of a co-catalyst in water or DMF at 120 °C. This catalyst could be reused four times without Pd leaching. 6368


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ACS Sustainable Chemistry & Engineering Scheme 22. Synthetic Pathway for Formation of Pd-LHMS (82)

Scheme 23. Synthesis of Poly(4-vinylpyridine)-Grafted Silica-Pd(0) (90)

to give products in 85−98% yields.31 However, C6H5Br was less efficient owing to its electronic effect. Neither copper nor other ligands were used nor byproducts observed compared to other nanocatalysts.138,139 The catalyst was reused three times with a minimal Pd leaching. A nanostructured Pd(0) xerogel (46a) was active in the coupling of aryl iodide with phenylacetylene using K2CO3 and over a 0.1 mol % Pd catalyst in refluxing MeOH or EtOH to afford products in 100% yields.95,140 Under MW irradiation, iodoarenes were coupled with phenylacetylene using a 0.1 mol % Pd catalyst in MeOH to afford 88−100% yields, while bromobenzenes gave 4−54% yields due to the stability of the C−Br bond. However, bromoarenes afforded the desired products in 18−100% yields using a 0.5 mol % Pd catalyst under MW irradiation. The catalyst could be reused five times with a low level of Pd leaching. An amine-modified SBA-16-supported Pd complex (44) was active in the Sonogashira reaction of aryl iodides and bromides to give products in 90−96% using piperidine and a 200 mol Pd catalyst under copper- and solvent-free conditions at 80 °C.92 Chloroarenes afforded 68−72% product yields. This catalyst was reused without loss of activity, showing better activity than a Pd(II)-Schiff-base supported on a multiwalled CNT.141 Nanocatalyst SiO2−PEG-PdNP (41) was used in Sonogashira reactions of iodoaryls with alkynes to afford 78−95% yields using K2CO3, 1.0 mol % Pd catalyst, and DMF at 110 °C under amine-, Cu- and phosphine-free conditions.89 Aryl bromides did not couple. This catalyst could be recycled four times without decreasing activity. MCM-41- and SBA-15-supported-Pd catalysts (83) and (84) were explored in the MW-assisted Sonogashira reaction of phenylacetylene with C6H5I to give 13% yield using 50 mg of Pd catalyst and K2CO3 under solvent-free conditions.142,143 However, the use of KF/Al2O3 enhanced the yield to 72%, while Pd−Y (85), Pd-VSB-5 (86), and Pd-SBA-15 (84) gave 9−70% yields. Among liquid bases, DBU gave 93% yield.

A deprotonation mechanistic pathway was proposed for this reaction. Organopalladium(II)-functionalized SBA-16 (Pd(II)-PMOSBA-16, 87) was obtained by co-condensation of Pd[PPh2(CH2)2Si(OCH2CH3)3]2Cl2 with TEOS.144 SEM images of 87 showed its length (2 mm) and diameter (500 nm). Under optimized conditions (CuI, n-decane, H2O, 90 °C, 0.6 mol % Pd catalyst), aryl iodides coupled with alkynes to give 86−98% yields, but aryl bromides afforded 85−96% yields if 0.9 mol % Pd catalyst was used. The efficiency of this catalyst was comparable to that for Pd(PPh3)2Cl2 (88) but faster than that for Pd(II)-SBA-15 (89) under identical conditions. The ordered and interconnected mesopores of 87 facilitated the transport of reactants and products without pore blockage. Catalysts 87 and 89 could be reused five times without Pd leaching, indicating inhibition of leaching by Pd(II) embedded in silica. Farjadian and Tamami145 reported on the preparation of PdNPs supported on poly(4-vinylpyridine)-grafted silica (90) (Scheme 23). SEM and TEM images of 90 showed the dispersion of PdNPs (30−50 nm) throughout the catalyst surface. This catalyst underwent the coupling of iodo- and bromoarenes with phenylacetylene to give products in 40−93% yields under optimized conditions (NMP, K2CO3, 0.3 mol % Pd catalyst, 120 °C). Iodoarenes reacted faster than bromoarenes, while aryl chlorides needed TBAB to promote the reaction and gave 65−85% yields. This catalyst could be reused six times with neglibible Pd leaching. For the activation of aryl chlorides, earlier findings showed the requirement of high catalyst loadings, elevated reaction temperatures, and co-catalyst CuI.145,146 However, the Pd@SiO2 catalyst (33) was capable of coupling o-, m-, and p-chloropyridine or thiophene derivatives with phenylacetylenes under optimized conditions (0.5 mol % Pd catalyst, piperidine, TBAB, H2O) to afford the corresponding products in 82−96% yields.80 This protocol was the first report on the heterogeneous Sonogashira reaction of heteroaryl chlorides in water. 6369


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Table 3. Catalytic Reactions Using Silica-Supported PdNPs and Their Catalytic Performances in Sonogashira Cross-Coupling Reactionsa

a

NR = Not reported.

transformations in Suzuki, Heck, and Sonogashira reactions. This class of Pd catalysts are efficient candidates for industrially important catalytic reactions. Various chemical and physical synthetic strategies were adopted to prepare these nanocatalysts with well-controlled sizes and shapes. As demonstrated throughout this perspective, significant efforts were made to employ these catalysts to overcome the problems regarding stability, separation, and recovery and the possibility of using a wide range of ligands instead of homogeneous catalysts. Thanks to the efforts of researchers in related fields, a number of studies have been conducted on the synthesis and applications of silica-supported PdNPs to various organic reactions. Different strategies have been adopted for increasing the reactivity and stability of these catalysts using various forms of SiO2 as size-controlling supports, which provide cavities for confining PdNPs in a controlled manner. Among them, mesoporous silicates possess a larger surface area, uniform pore structure, inert environment for the immobilization of PdNPs and thus become excellent candidates for exhibiting good catalytic activity in C−C bond formation. The use of water as a solvent is of importance for developing greener manufacturing protocols. Hence, silica is modified with several ligands to include suitable catalytic sites for reactions in aqueous media. The coupling reactions under solvent-free conditions applying thermal and MW methods revealed excellent activity through several cycles. However, several drawbacks to the current heterogeneous catalysts are still continuing in coupling reactions. Although Pd catalysts supported over modified silicas via N-, S-, and/or P-donors have been extensively studied, the search for newly emerging siliceous materials with specific properties and modifying the “donor substituents” appear desirable for improving catalytic efficacy. The performance of Pd nanocatalysts strongly depends on the particle size, shape, composition, and interaction with these supports. However, the relationship between such factors and relativities is less understood and needs to be studied in more detail. Only a few heterogeneous Pd catalysts are capable of activating C6H5Cl, but a large difference exists between their activity in homogeneous and heterogeneous phases. The mechanism of coupling reactions using heterogeneous catalysts is still unclear. Most of the Pd-catalyzed reactions are performed under inert atmospheres because they are sensitive to oxygen or moisture. Some catalysts require higher reaction temperatures and higher catalyst loadings and have limitations in stereoselective reactions. Several PdNPs immobilized on supports suffer from Pd leaching and aggregation/ agglomeration during reactions, resulting in limited recyclability.

A pH-responsive silica-supported PdNPs catalyst (Pd-5C-1N, 91) was prepared and its application explored for the Sonogashira reaction.22 A TEM image of 91 showed the location of PdNPs (size = 0.4 nm) in the pores of microspheres (200−300 nm). Under optimized conditions (Et3N, H2O-ether, 1.0 mol % Pd catalyst), 73−99% and 81−86% yields, respectively, were achieved in the coupling of aryl and heteroaryl iodides with alkynes, while aryl bromides gave 33−46% yields. The recycling ability of 91 was examined by changing the pH of the reaction mixture at different ranges. Aryl chlorides gave 81−91% yields in comparison with other heterogeneous catalysts for copper-free reactions.147 The use of K2CO3 and refluxing water had no effect on the grafted starch during the reaction. This catalyst could be reused five times with negligible Pd leaching. An MCM-48-supported 2-pyridinylmethanimine (Py) Pd-catalyst (59) promoted the coupling of aryl iodides and bromides with phenylacetylene under optimized condition (piperidine, 0.013 mol % Pd catalyst (for aryl iodides), 0.025 mol % Pd catalyst (for aryl bromide), solvent-free, 80 °C) to give products in 93−96% and 91−94% yields, respectively.104 It could be reused five times with low levels of Pd leaching. This catalyst showed a better performance than di(2-pyridyl)-methylamine hybrid silica-PdCl2.119 Easy synthetic procedures, long shelf life, stability, and compatibility make this catalyst an ideal system. A mercaptopropylated SBA-15-supported PdNPs catalyst (13) was efficient in the Sonogashira reaction of iodo- and bromobenzenes with alkynes to give desired products in 91−98% and 88−96% yields, respectively, using a 0.1 mol % Pd catalyst and piperidine under solvent-free conditions.70 This catalyst could be reused six times without loss of activity. This catalyst promoted the reaction, which was 12 times faster than a Pd(II)Schiff-base complex supported on multiwalled CNT under copper-free conditions.141 Under optimized conditions (CH3CN, Et3N, 0.1 mmol Pd catalyst), silica PdNPs (92) catalyzed the Sonogashira coupling of heterocyclic- and steric-substituted aryl iodides with alkynes to give products in 65−82% yields.131 Although the immobilized pyridine located on the surface of silica in 92 caused low Pd leaching, the anchoring of Pd species minimized the deterioration, thus allowing this catalyst to be recycled. We compile reports on the use of some silica-based Pd catalysts for the Sonogashira coupling reaction in Table 3.

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CONCLUSIONS AND FUTURE PERSPECTIVES The aim of this perspective is to provide readers with important developments of silica-supported Pd catalysts for organic 6370


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ACS Sustainable Chemistry & Engineering These problems might be overcome for large-scale applications of heterogeneous systems. Because of excellent stability and versatility of Pd heterogeneous catalysts, only a few Pd catalysts, trademarked SiliaCatPd0Hydrogel, are commercially available. Hence, there is still considerable room for developments in this area. New organic processes employing these catalysts can be designed in order to exploit the multifaceted catalytic chemistry. Their performance could be altered by examining inorganic and organic supports so that the confinement of PdNPs within the porosity of supports would enhance the stability and reactivity of these catalysts. To achieve a high catalytic activity while minimizing the cost of catalysts and contamination of the products, researchers should develop high-TON Pd catalysts that leave a low level of Pd contents. Furthermore, stability, durability, and cost are all issues that need to be addressed, if such systems are to enjoy widespread usage in industries for commercial purposes.

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Dr. Pounraj Thanasekaran received his Ph.D. in 1998 under the supervision of Prof. S. Rajagopal from Madurai Kamaraj University, India. From 1999−2001, he received a Research Associateship from the Council of Scientific and Industrial Research, India. He worked as a Postdoctoral Fellow at the Institute of Chemistry, Academia Sinica, Taiwan, in Prof. Kuang-Lieh Lu’s group (2001−2006) then in Prof. David M. Stanbury’s group, Auburn University, Alabama, USA (2006−2007). He served as a Lecturer in the School of Chemistry, Madurai Kamaraj University (2007−2008) before moving to Prof. LanChang Liang’s laboratory, National Sun-Yat Sen University, Taiwan, as a Postdoctoral Fellow in 2008−2009. He worked again as a Postdoctoral Fellow with Prof. Kuang-Lieh Lu until 2014. Since 2014, he has been working as a Postdoctoral Fellow with Dr. Hsien-Ming Lee. His current research focuses on the development of upconversion nanoparticlemediated NIR photoreleasing and biomedical imaging studies, cellular drug delivery systems using liposomes, and nanoparticles. His research interests also include synthesis, photophysical properties, and applications of supramolecular functional materials.

AUTHOR INFORMATION

Corresponding Authors

*P. Veerakumar. E-mail: [email protected]. *P. Thanasekaran. E-mail: [email protected]. *S. Rajagopal. E-mail: [email protected]. ORCID

Pitchaimani Veerakumar: 0000-0002-6899-9856 Kuang-Lieh Lu: 0000-0002-5529-7126 Notes

The authors declare no competing financial interest. Biographies

Dr. Pitchaimani Veerakumar obtained his M.Sc. (2004), M.Phil., (2007), and Ph.D. (2012) at the School of Chemistry, Madurai Kamaraj University, Madurai, India. After his doctoral research (2008−2012), he joined as a Postdoctoral research fellow (2012−2016) the Atomic and Molecular Sciences, Academia Sinica (Prof. Shang-Bin Liu), Taiwan. Presently, he is jointly appointed as a Postdoctoral research fellow in the Department of Chemistry, National Taiwan University (Prof. KingChuen Lin), Taiwan. His research interests include the synthesis and characterization of mesoporous porous carbon/silica and their catalytic developments, preparation of high surface area porous carbons from biowastes for sustainable energy applications, and synthesis and modification of novel porous carbon materials for applications as electrocatalysts in supercapacitors and electrochemical sensors.

Prof. Kuang-Lieh Lu obtained his Ph.D. in 1989 from the National Taiwan University. He is currently a Research Fellow in the Institute of Chemistry at Academia Sinica and an Adjunct Professor at National Taiwan Normal University and National Central University. Current research interests include metallacycles, metal−organic materials, supramolecular chemistry, and green chemistry. In the past few years, his research effort has been devoted primarily to the design of very efficient synthetic self-assembly strategies and to the understanding of the simplicity-to-complexity processes that occur in supramolecular systems. 6371


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(2) Fihri, A.; Bouhrara, M.; Nekoueishahraki, B.; Basset, J.-M.; Polshettiwar, V. Nanocatalysts for Suzuki cross-coupling reactions. Chem. Soc. Rev. 2011, 40, 5181−5203. (3) Shen, D.; Chen, L.; Yang, J.; Zhang, R.; Wei, Y.; Li, X.; Li, W.; Sun, Z.; Zhu, H.; Abdullah, A. M.; Al-Enizi, A.; Elzatahry, A. A.; Zhang, F.; Zhao, D. Ultradispersed palladium nanoparticles in three-dimensional dendritic mesoporous silica nanospheres: toward active and stable heterogeneous catalysts. ACS Appl. Mater. Interfaces 2015, 7, 17450− 17459. (4) Iwasawa, T.; Tokunaga, M.; Obora, Y.; Tsuji, Y. Homogeneous palladium catalyst suppressing Pd black formation in air oxidation of alcohols. J. Am. Chem. Soc. 2004, 126, 6554−6555. (5) Perez-Lorenzo, M. Palladium nanoparticles as efficient catalysts for Suzuki cross-coupling reactions. J. Phys. Chem. Lett. 2012, 3, 167−174. (6) Chen, Y.-H.; Hung, H.-H.; Huang, M. H. Seed-mediated synthesis of palladium nanorods and branched nanocrystals and their use as recyclable Suzuki coupling reaction catalyst. J. Am. Chem. Soc. 2009, 131, 9114−9121. (7) Ji, Y.; Jain, S.; Davis, R. J. Investigation of Pd leaching from supported Pd catalysts during the Heck reaction. J. Phys. Chem. B 2005, 109, 17232−17238. (8) Li, Y.; Hong, X. M.; Collard, D. M.; El-Sayed, M. A. Suzuki crosscoupling reactions catalyzed by palladium nanoparticles in aqueous solution. Org. Lett. 2000, 2, 2385−2388. (9) Biradar, A. V.; Biradar, A. A.; Asefa, T. Silica dendrimer coreshell microspheres with encapsulated ultrasmall palladium nanoparticles: Efficient and easily recyclable heterogeneous nanocatalysts. Langmuir 2011, 27, 14408−14418. (10) Cassol, C. C.; Umpierre, A. P.; Machado, G.; Wolke, S. I.; Dupont, J. The role of Pd nanoparticles in ionic liquid in the Heck reaction. J. Am. Chem. Soc. 2005, 127, 3298−3299. (11) Souza, B. S.; Leopoldino, E. C.; Tondo, D. W.; Dupont, J.; Nome, F. Imidazolium-based zwitterionic surfactant: A new amphiphilic Pd nanoparticle stabilizing agent. Langmuir 2012, 28, 833−840. (12) Briggs, B. D.; Pekarek, R. T.; Knecht, M. R. Examination of transmetalation pathways and effects in aqueous Suzuki coupling using biomimetic Pd nanocatalysts. J. Phys. Chem. C 2014, 118, 18543−18553. (13) Veerakumar, P.; Madhu, R.; Chen, S.-M.; Veeramani, V.; Hung, C.-T.; Tang, P.-H.; Wang, C.-B.; Liu, S.-B. Highly stable and active palladium nanoparticles supported on porous carbon for practical catalytic applications. J. Mater. Chem. A 2014, 2, 16015−16022. (14) Dhakshinamoorthy, A.; Asiri, A. M.; Garcia, H. Metal−organic frameworks catalyzed C−C and C−Heteroatom coupling reactions. Chem. Soc. Rev. 2015, 44, 1922−1947. (15) Gniewek, A.; Ziolkowski, J. J.; Trzeciak, A. M.; Zawadzki, M.; Grabowska, H.; Wrzyszcz, J. Palladium nanoparticles supported on alumina-based oxides as heterogeneous catalysts of the Suzuki−Miyaura reaction. J. Catal. 2008, 254, 121−130. (16) Yin, L.; Liebscher, J. Carbon−Carbon coupling reactions catalyzed by heterogeneous palladium catalysts. Chem. Rev. 2007, 107, 133−173. (17) Xue, S.; Jiang, H.; Zhong, Z.; Low, Z.-X.; Chen, R.; Xing, W. Palladium nanoparticles supported on a two-dimensional layered zeolitic imidazolate framework-l as an efficient size-selective catalyst. Microporous Mesoporous Mater. 2016, 221, 220−227. (18) Li, P.; Huang, P.-P.; Wei, F.-F.; Sun, Y.-B.; Cao, C.-Y.; Song, W.-G. Monodispersed Pd clusters generated in situ by their own reductive support for high activity and stability in cross-coupling reactions. J. Mater. Chem. A 2014, 2, 12739−12745. (19) Sharma, R. K.; Sharma, S.; Dutta, S.; Zboril, R.; Gawande, M. B. Silica-nanosphere-based organic−inorganic hybrid nanomaterials: Synthesis, functionalization and applications in catalysis. Green Chem. 2015, 17, 3207−3230. (20) Ncube, P.; Hlabathe, T.; Meijboom, R. Palladium nanoparticles supported on mesoporous silica as efficient and recyclable heterogenous nanocatalysts for the Suzuki C−C coupling reaction. J. Cluster Sci. 2015, 26, 1873−1888. (21) Wang, Y.; Liu, J.; Wang, P.; Werth, C. J.; Strathmann, T. J. Palladium nanoparticles encapsulated in core−shell silica: A structured

Prof. Shang-Bin Liu obtained his Ph.D. (1985) at the College of William and Mary, Virginia, USA. He then worked at the Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan, and jointly at the National Taiwan Normal University. His research interests include developments of solid-state NMR techniques for porosity, acidity, and basicity characterization of porous catalytic and adsorptive materials as well as synthesis, modification, and characterization of novel porous carbon materials for applications as catalyst supports in fuel cells and/or as adsorbents in fuel storage and CO2 capture.

Prof. Seenivasan Rajagopal received his Ph.D. in 1984 from Madurai Kamaraj University under the tutelage of Prof. C. Srinivasan. In 1985, he was selected as an UNESCO fellow for postdoctoral research with Prof. Shigeo Tazuke at the Tokyo Institute of Technology, Tokyo, Japan. After over 40 years of teaching and research, at present, he is serving as an UGC-BSR Faculty Fellow, Department of Physical Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai. He has authored more than 100 papers with international repute. His current research is primarily focused on synthesis, characterization, and photophysical studies of some novel Ru, Os, and Re organometallic complexes. His research also targets the development of synthesis of novel metal nanocatalysts for organic transformations and luminescent materials for protein-binding studies and biosensors.

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ACKNOWLEDGMENTS The authors thank the Ministry of Science and Technology (MOST) and Academia Sinica Taiwan for financial supports. S.R. thanks the University Grants Commission-Basic Scientific Research, New Delhi, for financial support.

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