Nanocatalysis Synthesis and Applications (Polshettiwar/Nanocatalysis) || Nanocatalysts for the Heck...

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NANOCATALYSTS FOR THEHECK COUPLING REACTIONS

T. Asefa, A. V. Biradar, S. Das, K. K. Sharma, and R. Silva

INTRODUCTION

Thefields of homogeneous and heterogeneous catalysis havemade considerable progressin the past few decades, enabling the synthesis of a diverse range of useful and uniqueorganic compounds.1 In particular, the catalytic Heck coupling reaction (sometimesalso called the Mizoroki–Heck reaction), which was named after the pioneering andindependent works of TsutomuMizoroki2 and Richard F. Heck,3 has become among themost indispensable catalytic reactions today because it allows the synthesis of new C Cbonds between various aryl or vinyl halides and activated alkenes. Since its inception inthe early 1970s, theHeck reaction hasmade great strides and has nowunarguably becomeamong the most powerful synthetic tools available for organic and inorganic chemistsalike as well as for many commercial processes today. The Nobel Prize awarded toRichard Heck in 2010 is also clearly a good testament of the importance of the reaction.

The classical Heck reaction involves C C bond formation between two sp2

hybridized carbon centers, one of which is an �-olefin and the second is an organiccompound containing a leaving group composed of bromo-, iodo-, or pseudohalide moi-ety(e.g., tosylate, mesitylate, triflate, and diazonium saltsdiazonium salt). Furthermore,in conjunction with other organic reactions such as carbonylation, esterification, andother types cross-coupling and intermolecular reactions, the Heck reaction can serveas the key step in the synthesis of various types of pharmaceuticals, agrochemicals,

Nanocatalysis: Synthesis and Applications, First Edition. Edited by Vivek Polshettiwar and Tewodros Asefa.© 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.

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12 NANOCATALYSTS FOR THE HECK COUPLING REACTIONS

R – X + R′Pd0

Base- HX

RR′

Scheme 2.1. General schematic depiction of the Heck reaction between an unsaturated

halide or triflate and an alkene that contains at least one hydrogen atom and is electron

deficient (R = aryl, benzyl, or vinyl; and R′ = aryl, acrylate ester, etc).

and natural products.4 In addition, the Heck reaction enables the functionalization ofvarious molecules with different substituents, which otherwise take multiple steps toproduce. Thus, an impressive list of synthetically useful transformations is possible viathe Heck reaction, allowing the reaction to get a place in the repertoire of syntheticorganic chemists. Nevertheless, intensive research efforts have been continuing becausesimpler, “greener,” and more efficient catalytic systems beyond the traditionally usedorganopalladium-based catalysts and phosphine ligands are needed for many existingand other possible new Heck reactions.5

Transition metals, particularly organopalladium complexes, still remain at the fore-front of catalysts used for theHeck reaction (Scheme 2.1). The Pd(II) derivatives are oftenused in the presence of PPh3 and excess base to catalyze the reaction as shown in Scheme2.1.6 It is noteworthy that the same catalytic systems are now known to catalyze othertypes of C C bond-forming reactions such as acylation, methylation, and carboxyalky-lation,7 the Sonogashira reaction,8 the Suzuki reaction,9,10 and the Stille reaction aswell.11, 12 Not surprisingly, these C C bond forming reactions are also widely utilizedas key steps for the syntheses of many organic chemicals, natural products, and a varietyof industrial products,13–16 as can be seen in the other chapters of this book.

It is worth noting that phosphines are generally used as electron-donating ligands toeffectively drive the Heck reaction. However, phosphines are toxic, moisture-sensitive,and costly; hence, they make large-scale industrial production of various coupling prod-ucts through the Heck reaction relatively expensive or less viable. Thus, research inphosphine-free catalytic systems for the Heck reaction has received greater attentionand is one of the research areas that has been widely pursued in recent years as well.17

HETEROGENEOUS CATALYSTS FOR THE HECK REACTION

Generally, the Heck reaction can be catalyzed by two different types of catalytic systems,namely homogeneous catalysts or heterogeneous catalysts, which include nanocatalysts.As the interest for “greener” chemical process with minimum potential effects in theenvironment is growing, research efforts toward the development of heterogeneouscatalysts for various reactions, including the Heck reaction, have surged. The latter, inturn, has led to, among other things, a greater effort in the development of new syntheticstrategies for stabilizing Pd-based homogeneous catalysts on solid support materials(also known as “heterogenization of homogeneous catalysts”) to produce heterogeneouscatalysts that are effective, easily recyclable, and help in reducing the costs associatedwith catalysts/catalysis.

SYNTHETIC METHODS TO CATALYSTS FOR THE HECK COUPLING 13

Owing to their intrinsic properties that are conducive for catalysis, nanomaterialshave also been in the spotlight for the development of efficient heterogeneous catalysts.The rapid pace of research in the fields of nanoscience and nanotechnology overthe last two decades has only strengthened these efforts and opened up many newresearch directions toward novel and efficient heterogeneous nanocatalysts for the Heckreaction. Heterogeneous nanocatalysts for the Heck reaction are appealing becausenanomaterials inherently have effective catalytic-active surfaces/features as well asinteresting physical attributes that make their separation from reaction mixtures easier.This, in turn, makes it possible to recycle “expensive” catalysts or create “greener”catalytic processes. Furthermore, depending on the size and shape of the nanoparticles,many nanocatalysts can lead to different (or the desired) products in high yields. Thesetopics as well as the advances made in nanocatalysts for the Heck reaction are discussedin more detail in many of the sections to follow.

SYNTHETIC METHODS TO CATALYSTS FOR THEHECK COUPLING

The general synthetic routes to catalysts for the Heck coupling reaction have recentlybeen reviewed by Beletskaya et al., Polshettiwar et al., and Whitcombe et al.18–20 In thischapter, we have briefly described some of the synthetic methods employed to make Pdcatalysts (homogeneous and heterogeneous) without unduly repeating the topics coveredin these aforementioned reports as much as possible.18–20 We have also described theadvantages and disadvantages of the homogeneous and heterogeneous catalysts used forthe Heck reaction. Most of the detailed discussion in the chapter, however, focuses onnanocatalysts for the Heck reaction. Since some of the nanocatalysts reported for theHeck reaction were also shown to successfully catalyze other C C bond coupling andhydrogenation reactions, some mention of the latter is unavoidable or is made along theway.

Homogeneous Catalysts for the Heck Coupling Reaction

As palladium can form strong as well as weak metal–ligand complexes with a widevariety of ligands containing donor atoms such as P, N, and O,21,22 it can be expected toform a diverse range of organopalladium complexes (or homogeneous catalysts) for theHeck reaction.23 Thus, homogeneous palladiumcatalysts are relatively easy to synthesizeas they typically involve the complexation of Pd(II) salts such as Pd(II) chloride or acetatewith organic ligands containing phosphines, phosphites, carbenes, thioethers, or pincerfunctional groups.21,22 ,24–29 The resulting Pd complexes are appealing as homogeneouscatalysts not only because they are relatively easy to synthesize and handle, but alsobecause many of them give reproducible catalytic products in reactions that are easyto run under ordinary reaction conditions. Moreover, the functional groups in thesehomogeneous catalysts often tolerate diverse reaction conditions.

Despite their appeal, homogeneous Pd-based catalysts for the Heck reaction doalso have some drawbacks. First, substrates having hydrogen atoms on their �-carbonscannot undergo the Heck reaction with homogeneous Pd-based catalysts because their


corresponding organopalladium derivatives tend to undergo rapid �-hydride eliminationto give olefins, instead of the desired C C coupling products. Second, aryl chlorides,which are relatively inexpensive and easy to obtain from commercial sources, are notgood substrates for many homogeneous Pd catalysts because these substrates react veryslowly in the Heck reaction involving many homogeneous catalytic systems. Third,homogeneous organopalladium catalysts require costly and relatively lengthy multistepprocedures to separate from the reaction mixtures, while at the same time they havegreater chances to contaminate the reaction products with heavy metals (or Pd species).The latter is of major concern particularly for pharmaceutical products, where theacceptable content of residual heavy metals in reaction intermediates or final productsis extremely low.

Heterogeneous Catalysis for the Heck Coupling Reaction

The heterogenization of homogeneous catalysts has been among the most widely usedmethods to produce heterogeneous catalysts for the Heck coupling reaction. With thismethod some of the common problems associated with homogeneous catalysts canalso be overcome. By using this method, a number of heterogeneous Pd catalysts havealready been successfully developed for the Heck coupling reactions of various sub-strates, including aryl bromides and aryl chlorides—reactants that are relatively difficultto activate and sluggish to undergo the Heck reaction.30,31 Some of these catalysts workunder phosphine-free conditions as well, that is, the more appealing type of Heck reac-tion. In cases where phosphines are included, the problems associated with phosphinescan be partly overcome by using unconventional solvents.32, 33 In the absence of stabi-lizing agents such as phosphines, other problems arise though. This includes the quickconversion of the Pd-based catalysts into the catalytically inactive palladium black; thisin turn decreases the shelf-life and reusability of the catalysts. This problem can be sig-nificantly minimized by using good inorganic support materials or tetra-alkylammoniumsalts.34 Finally, there are also examples of efficient catalysts for the Heck reaction basedon colloidal Pd particles, which are sometimes referred to as “quasihomogeneous” or“quasiheterogeneous” catalysts.35, 36 Their supported forms are also commonly used asheterogeneous catalysts for the Heck reaction.

Silica remains among the most commonly used support material for heteroge-nization of homogeneous catalysts, including Pd complexes or nanoparticles, to makevarious heterogeneous catalysts for the Heck reaction.37–39 In addition, other metaloxides such as Al2O3, TiO2, ZnO, ZrO2, and MgO,40–42 and materials such as hydro-talcite43 and zeolites44 have also been successfully utilized as support materials foranchoring various palladium-based catalytic species for the Heck reaction. Moreover,organic-functionalized materials such as mesoporous organosilicas are shown to be veryinteresting support materials for making heterogeneous catalysts because their organicgroups can serve as good anchor points to various metallic species such as organopal-ladium complexes and Pd nanoparticles (Pd-NPs). Among organic functional groups inmesoporous organosilica support materials, those with mercapto,45 amine,46 pyridine,47

and imidazole groups are particularly the most suitable to support catalytic-active pal-ladium species.48

SYNTHETIC METHODS TO CATALYSTS FOR THE HECK COUPLING 15

Carbon-based systems have traditionally been used as support materials for Pd andother metallic catalysts.49 In fact, activated carbon-supported palladium (often denotedas Pd/C) is the most widely used catalytic system for the Heck coupling reaction inmany commercial processes today.50

Supported homogeneous and nanoparticle catalysts (or heterogeneous catalysts) areadvantageous compared with their homogeneous or “quasihomogeneous” counterpartsbecause of the ease of separation and recovery of the former from catalytic reactionmixtures, and thus their reusability as catalysts for multiple catalytic reaction cycles.Furthermore, the need for use of expensive ligands and inert atmosphere, which are oftenrequired in homogeneous catalytic systems, can sometimes be avoided in heterogeneouscatalytic systems.

However, heterogeneous catalysts do have some disadvantages of their own too,compared with their homogeneous counterparts. The first and most common one is theycatalyze organic reactions more slowly than their soluble homogeneous counterpartsdo. This is mainly because the reactants have limited interaction with the supportedcatalytic groups (e.g., Pd nanoparticles). In the case of nanoparticle catalysts, it isbecause the reactants see only the atoms on surfaces of the nanoparticles, with no chanceof interacting with those in the middle of the particles. Nonetheless, since nanoparticleshave high surface areas and large numbers of surface atoms, this problem is not a majorone. The second disadvantage of heterogeneous catalytic systems is the decrease incatalytic activity of the catalysts due to leaching of the active catalytic species intothe reaction media. Finally, aggregation/sintering of the nanoparticles or their possibleleaching into catalytically deactivated systems such as Pd black are common problemsfor many heterogeneous catalysts.

Some Commercial Applications of the Heck Reaction

The usefulness of the Heck reaction has been demonstrated by its effectiveness inenabling the synthesis of many interesting commodity chemicals, drug intermediates,and drug molecules (Figure 2.1).51–53 One of the first reported examples of the indus-trial use of the Heck reaction was demonstrated by Ciba–Geigy with the productionof ProsulfuronTM, a highly active herbicide.54 The Heck reaction was also employedin the synthesis of anti-inflammatory drug NaproxenTM, which was developed andcommercialized by Albemarle. The synthesis of this drug involves the Heck reactionbetween 2-bromo-6-methoxynaphthalene and ethylene, followed by carbonylation.55–57

The Heck reaction is also utilized in the synthesis of 2-ethylhexyl-p-methoxycinnamate,one of the most common agents used for ultraviolet (UV) B sunscreens.58 The synthe-sis of this compound involves Pd/C-catalyzed Heck reaction between p-bromoanisoleand 2-ethylhexyl acrylate. Other notable commercial products that are made possi-ble by the Heck reaction include the asthma drug Singulair (Merck)59,60 and thepotent drug and partial antagonist for 5-HT1D-like receptors Eletriptan (Pfizer).61, 62

Besides pharmaceutically relevant compounds, other industrially important compoundssuch as 2- and 4-vinyltoluenes, which serve as comonomers for styrene polymersfor some household and other consumer products, are produced by using the Heckreaction.63


COOH

H CH3

OH3C

Naproxen

N

N

N N N S

H3C

OH3C

O CH2

CH2

F

F

F

H H

Prosulfuron

OH

s

o-O

Cl

Singular

O

O

CH3

CH3

OH3C

2 Ethylhexy-p-methoxy cinnamate

Figure 2.1. Important drugs that involve the Heck coupling reaction during their synthesis.

Mechanisms of the Heck Coupling Reaction

The mechanism of the Heck reaction was proposed first by Dieck and Heck in 1974for homogenous catalytic systems,64 which is still widely accepted only with slightmodifications. The mechanism for the Heck reaction between an aryl or vinyl halide(RX, where X = Br, I) and an acrylate or olefin catalyzed by the homogeneous catalystPd(OAc)2 in the presence of ligands (or PdCl2L2, where L is a ligand such as PPh3or P(o-tolyl)3) is depicted in Scheme 2.2. During the course of the catalytic reaction,

Pd0

R1–X′

R1–Pd–X

Oxidativeaddition

Reductiveelimination

R1–Pd–R2

Transmetalation

R2–MM–X

R1–R2

M = B, Sn, Si, Zn, Mg

Scheme 2.2. General mechanistic schemes of the Heck coupling reaction.

NANOPARTICLES FOR THE HECK COUPLING REACTION 17

palladium undergoes changes in its oxidation state between Pd(0) and Pd(II) while thereaction proceeds through the following four major steps:

1. Oxidative addition of the aryl halide into the Pd(0) complex to form �-aryl-palladium(II) halide.

2. Oxidative addition into �-aryl-palladium(II) halide (i.e., trans-ArPdXL2); thetrans-ArPdXL2 first coordinates the alkene after leaving one of the phosphineligands and then undergoes a syn insertion with the alkene to form a �-alkyl-palladium(II) halide.

3. �-Hydride elimination and dissociation gives hydridopalladium(II) halide thatis coordinated to the arylated alkene.

4. Finally, regeneration of the active Pd(0) complex by reductive elimination ofhydridopalladium(II) species.65–67

Despite this well-accepted general mechanism, the Heck catalytic reaction caninvolve different types of Pd(0) and Pd(II) intermediates, whose structure and reac-tivity vary depending on the experimental conditions, the catalytic precursors (e.g.,whether they are Pd(0) complexes, Pd(OAc)2, or palladacycles), the types of ligands(e.g., mono- or bisphosphines, carbenes, and bulky monophosphines), and any possi-ble additives (e.g., halides and acetates) used in the reaction.65 Furthermore, althoughthere is agreement on the general mechanism of the Heck reaction, there is also stillsome ambiguity about the mechanism of the Heck reactions catalyzed by heteroge-neous catalysts. Nonetheless, mechanistic studies of the Heck reactions catalyzed bysupported Pd-based heterogeneous catalysts have led to the conclusion that (1) solubleand catalytic-active palladium species form by leaching from the supported-Pd catalystsand (2) these soluble palladium species then involve in the reactions, and their catalyticactivities and readsorption on the solid material play major roles in the overall rates ofthe catalytic reaction.68 Nevertheless, it is widely accepted that most of the importantsteps of the catalytic cycle of the Heck reaction catalyzed by heterogeneous catalystsremain the same as the coordination sphere model proposed for the catalytic reactionscatalyzed by homogeneous catalysts, as shown in Scheme 2.3.69

NANOPARTICLES FOR THE HECK COUPLING REACTION

In this section, the Heck reaction catalyzed by transition metal nanoparticles, mainlyPd-NPs, is discussed.70–74 Discussion on other metal nanoparticles such as Ru,75 Ni,76

and trimetallic Au–Ag–Pd77, which have also been shown to catalyze the Heck reaction,is also included, though not in much detail.

Pd-NPs-Catalyzed Heck Reaction

The use of nanoparticles as catalysts for the Heck reaction was first reported byBeller et al. in 1996 using colloidal Pd-NPs stabilized by tetraoctylammonium


Pd0

Oxidative

addition

PPh3

PPh3

Br

PdII

PPh3

PPh3

Br

PdIIPPh3

Ph3PBr

PdII Br

PPh3

PPh3

H

H

Pd0 Br

Ph3P

PPh3H

PdII

PPh3

PPh3

BrH

Pi complex

Insertion

Pd intermediate

Elimination

Reductive

elimination

HBr/baseBase

Scheme 2.3. Mechanism of Pd-catalyzed Heck coupling reaction.

bromide (TOA).78 These nanoparticles were prepared by reduction of PdCl2 in the pres-ence of ammonium boronate. The catalytic reaction involved the arylation of styrene orbutyl acrylate by activated aryl bromides at relatively low catalyst loading (0.05 mol%)of the Pd-NPs. The nanoparticles showed good catalytic activity for this reaction; how-ever, they showed only limited catalytic activity when the less activated aryl chlorideswere used as a substrate (Scheme 2.4).78

The major advantages of metal nanoparticle catalysts, especially compared to theirbulk counterparts, include their unusual or high catalytic activity, and in some case their

R

Br + orCO2Bu

CO2Bu

R = 4-CHO, 4-F, H, 4-CH3CO, 4-OAc

TOA

TOATOA

TOATOA

TOA

R

Ror

Scheme 2.4. The first reported Pd-NPs-catalyzed Heck reaction by Beller and coworkers.78

(Adapted with permission from Ref. 78.)


excellent selectivity toward the desired products. These unusual catalytic propertiesof nanoparticles, especially compared to their bulk counterparts, are mainly due tothe very high surface area-to-volume ratios and the unique orientation of the surfaceatoms/surface-faceted structures of nanoparticles. Consequently, the catalytic propertiesof nanoparticles are highly dependent on their sizes and shapes.

Despite many of their advantages, nanoparticle catalysts have some inherentdrawbacks as well. Among these, leaching of the metallic species from nanoparticles isa common problem to many nanoparticle catalysts and nanoparticle-catalyzed reactions.To investigate leaching of palladium from Pd-NPs during the Heck reaction, Tathagaret al. evised a two-chamber reactor separated by a membrane, which was permeableto small Pd species, but not to the nanoclusters (Figure 2.2).79 By using this setup,they showed that the reaction in the second chamber was catalyzed; this indicates thatlow-molecular-weight palladium species must be leached from the nanoparticles andthen traveled from one chamber to the other and catalyze the reaction in the latter.The second common problem for nanoparticle catalysts (especially those composed ofmetals such as Pd and Au, whose surface energy is very high) is their inherent tendencyto aggregate/sinter, especially during high-temperature catalytic reactions. This, in turn,results in quick loss in the catalytic activities of the nanoparticles.

Membranewith 5 nm pores

Reaction mixturewithout catalyst

Side B

Leached Pd species diffusedthrough the membrane

Pd clusters (15 nm)cannot pass through the

membrane

Side A

Figure 2.2. A two-chamber reactor separated by a membrane, impermeable to Pd-NPs but

permeable to soluble low-molecular-weight Pd species. By using this reactor, it is shown that

low-molecular-weight palladium species leached off the nanoparticles and traveled from one

chamber (Side A) to the other (Side B), and catalyzed the reaction mixture in Side B. The

result here is also consistent with the hypothesis in Section “Mechanisms of the Heck Coupling

Reaction” that the Heck reaction by heterogeneous catalysts can be catalyzed by small leached

out Pd species.79 (Adapted with permission from Ref. 79.) (See color insert.)


Metal cationsin solution

Metal atomsin solution

Stable nanoparticlesin suspension

StabilizationReduction

Figure 2.3. Schematic representation of the synthesis of metallic nanoparticles in solution

for catalysis. The nanoparticles are typically synthesized by reduction of their corresponding

metal salts in solutions in the presence of organic stabilizers.80 (Adapted with permission from

Ref. 80.)

The simplest strategy to synthesize metal nanoparticles for the Heck coupling andother catalytic reactions is through the reduction of a metal salt with a reducing agentin the presence of a stabilizer (Figure 2.3).80 A stabilizer is necessary because nakednanoparticles are thermodynamically and kinetically unstable and tend to aggregateinto bulk metal that is inactive in catalysis. The stabilizers typically come in the formof organic ligands,81 dendrimers,82,83 or polymers,84 and contain donor atoms that arecapable of binding palladium.

Organic Ligand- and Polymer-Stabilized Pd-NPs for the Heck Reaction.As mentioned previously, many metals, such as Pd, have a tendency to aggregatedue to their high surface energies. For this reason, metallic nanoparticles requireligands or passivating agents around their surfaces to remain stable in solutions aswell as to be utilized as stable colloidal nanocatalysts. Various organic and polymericgroups have been successfully used as stabilizing agents for making Pd-NPs for theHeck reaction. For instance, palladium nanoparticles were stabilized with 1,5-bis(4,4′-bis(perfluorooctyl)phenyl)-1,4-pentadien-3-one. The resulting Pd-NPs were success-fully used as efficient and recoverable catalysts for the Heck reaction between ethylacrylate and ethyl cinnamate with iodobenzene.85 In another example, Pd-NPs stabi-lized by a fluorinated ligand, which rendered the nanoparticles dispersibility in fluorousmedium, were synthesized by reducing Pd(II) dichloride in methanol. In this case,besides being a solvent for the reaction, methanol served as a mild reducing agent forPd(II) ions. The resulting Pd(0) nanoparticles catalyzed the Heck reaction in the pres-ence of triethylamine as a base and acetonitrile perfluorooctyl bromide as a solvent athigher temperature (140 ◦C).85

Polymers have also been widely used as stabilizers for the synthesis of metallicnanoparticles, including Pd-NPs for the Heck reaction. Polyvinylpyrrolidone (PVP)is commonly used for making Pd-NPs because it strongly binds and stabilizes Pd-NPs due to its donor atoms, (i.e., the nitrogen atoms in it). In one example, PVP-stabilized Pd-NPs were used as catalysts to study the structure–catalytic activities of theHeck coupling reaction between p-bromobenzaldehyde and butyl acrylate.86 The resultsindicated the presence of a quantitative relationship between the initial reaction rates and


Defect sites

Terrace sites

2.5(b)

(a)

2.0

1.5

1.0

0.51.5 2.0 2.5 3.0

Pd diameter (nm)3.5 4.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Surface PdDefect Pd

No

rmal

ized

init

ial r

eact

ion

rat

e

No

rmal

ized

init

ial r

eact

ion

rat

e

Figure 2.4. The Heck reaction depends on the structures of the Pd-NPs used as catalysts for

the reaction. (a) Structure of a Pd-NP with more low-coordinated Pd surface atoms at defect

sites that are highly catalytically active, and with Pd terrace atoms that are less catalytically

active. (b) Initial normalized reaction rates versus size of the nanoparticles in the Heck coupling

reaction.86 (Adapted with permission from Ref. 86.)

the number of low-coordinated Pd surface atoms at “defect” sites on the nanoparticles,which were calculated using statistical analysis for face-centered cubic cuboctahedronof Pd-NPs (Figure 2.4).86 While the initial reaction rates did not vary with the numberof defect atoms, high catalytic activity was, however, exhibited by Pd sites possessinglow metal–metal coordination numbers. This clearly corroborates that the structure ofthe nanoparticles is vital for the catalytic activity of particles (Figure 2.4). This, in turn,implies that with the right synthetic strategy, nanomaterials with high catalytic activitiesfor the Heck coupling or other reactions can be systematically produced. For example,by synthesizing Pd nanocatalysts with approximately 2.8 nm particle size and using them


as catalysts in the Heck reaction, turnover number (TON) as high as 100,000, productyields as high as 99%, and turnover frequencies of over 80,000/h were obtained.86

A notable synthetic method for PVP-stabilized Pd-NPs involves a two-step proce-dure consisting of the dissolution of H2PdCl4, followed by the reduction of Pd(II) inthe presence of PVP with a reductant such as pyrogallol.87 This method can produce19.8 nm-diameter PVP-stabilized Pd nanoparticles that are effective for the catalyticHeck reaction between n-butyl acrylate and bromobenzene.87 In a different syntheticapproach, Evangelisti et al. employed gas-phase metal vapor synthetic technique tosynthesize 1.0–3.5 nm-diameter PVP-stabilized Pd-NPs.88 The resulting nanoparticleswere shown to catalyze the Heck reaction between aryl halides and n-butyl acrylate(acrylate/ArX = 2) at 75–125 ◦C under argon atmosphere, with N(n-Pr)3 as base inN-methyl-2-pyrrolidone as organic solvent. Durap et al. synthesized 4.5 nm-diameterPd-PVP nanoparticles by reducing Pd(II) (or K2PdCl4) in methanolic solution in thepresence of PVP by using NaBH4 as well as the methanol itself as reducing agents underrefluxing conditions.89 The resulting Pd-PVP nanoparticles were employed as catalystsfor the Heck coupling between styrene and brominated aromatic compounds (ArBr)including bromobenzene, bromoanisole, and 4-bromotoluene with styrene/ArBr ratio of1.5 : 1 in dimethylformamide (DMF) and K2CO3 at 120 ◦C under inert atmosphere. Thereactions produced the corresponding E-stilbene products in yields ranging from 34 toca. 100% in 45 min.

The Heck reaction often requires higher temperatures and longer reaction timesto lead to reasonable yields. However, by applying microwave chemistry, in whichmicrowaves are used as an alternative source of heating, a significant decrease in thereaction times for the Heck reaction can be achieved.90 For instance, 3-6 nm-diameterPVP-stabilized Pd(0) nanoparticles were shown to catalyze the Heck reaction betweeniodobenzene and different alkenes under microwave heating, affording moderate togood yields of 62–99% and good selectivity to the E-isomers in very short reactiontimes (e.g., 12 min).90 Generally, the products obtained from the Heck reaction withmicrowave heating are superior to those obtained with conventional heating, in terms ofreaction yields as well as product selectivity, "with the reactions requiring as short as12 min to reach completion in the former.

Dendron- and Dendrimer-Stabilized Nanoparticles for the Heck Reac-tion. Dendrons and Dendrimers are a family of highly branched macromolecules.Dendrons are partial dendrimers, or conversely, dendrimers can be formed by chemi-cally linking together a few dendrons. Their structural and chemical properties can becontrolled by modifying their cores, the type and number of branches used to makethem, and their terminal groups.91,92 Dendrons and Dendrimers with different degreesof branching, often represented by the so-called generations, or G, can be made bymultistep synthesis. They can also be obtained from commercial sources nowadays.

Dendrons and Dendrimers can be used as stabilizing agents for many types of metal-lic nanoparticles, including Pd-NPs.93 Depending on the degree of branching, differentdendrimer-encapsulated stable metal nanoparticles can be synthesized by following thesynthetic procedures depicted in Figure 2.5 for polyamidoamine (PAMAM) dendrimers.In typical synthesis, first, metal ions are trapped within the dendritic cores. The metal


Metal ions

Complexation

ProductReactant

Reduction

NaBH4

M0 nanoparticles

Figure 2.5. Synthetic strategy for making metallic nanoparticles stabilized in PAMAM den-

drimers for catalysis. First, metal ions are complexed with the tertiary amine groups of PAMAM.

The metal ions are then reduced into metal(0) using reducing agents such as NaBH4 to produce

dendrimer-encapsulated metal(0) nanoparticles within the dendrimer core.80 (Adapted with

permission from Ref. 80.)

ions are then reduced using a suitable reducing agent to produce metal nanoparticlesentrapped within the dendrimers. The dendrimer host prevents the nanoparticles fromaggregation and precipitation as bulk metal, while at the same time, it lets reactants andproducts diffuse into and out of where the metal nanoparticle catalytic sites reside.

Many dendrimer-encapsulated nanoparticles are proven to be good catalysts for anumber of organic reactions. For instance, PAMAM dendrimer-encapsulated Pd-NPshave been used as efficient catalysts for the Heck reaction between aryl halides andacrylic acid, giving good yields of trans-cinnamic acid under phosphine-free condi-tions.94 Similarly, dendrons such as G-3 dendrons with Pd-NP cores have also been usedto efficiently catalyze the Heck reaction.95 These nanoparticles have approximately 300Pd atoms and an average diameter of 2.0 nm, with each Pd-NP being attached with14 G-3 dendrons. They catalyze the Heck reaction between iodobenzene and styrene,yielding the coupling product with a high TON of 25,600.

Besides PAMAM, dendrimers with other ligands or donor atoms have also beensuccessfully used to encapsulate metal nanoparticles for catalysis. For instance, Yeungand Crook have synthesized Pd-NPs (2–3 nm in diameter) within covalently functional-ized poly(propylene imine) dendrimers.96 The resulting material is shown to be effectivecatalyst for the Heck reaction between n-butyl acrylate and aryl halides, yielding approx-imately 100% selectivity toward n-butyl-trans-cinnamate product at 90 ◦C.

Ionic Liquid-Stabilized Pd-NPs for the Heck Reaction (or Pd-NP-Catalyzed Heck Reactions in Ionic Liquid Media). Ionic liquids can also beused as stabilizing agents for metallic nanoparticles for the Heck reaction. Cassol andcoworkers synthesized Pd-NPs (0.001 mol%) stabilized in imidazolium ionic liquids.97

Although the material showed significant Pd leaching, whose amount varied during thecourse of the reaction, it successfully catalyzed the Heck reaction between butyl acrylateand aryl iodides. Recently, a basic and an ester-functionalized imidazolium-based ionicliquid or 3-methyl-1-(ethoxycarbonylmethyl)imidazolium hydroxide was synthesizedand successfully utilized to stabilize 5 nm-diameter Pd-NPs. The resulting Pd-NPs wereused as catalysts for the Heck reaction between aryl halides (aryl iodide, aryl bromide,


HOHO

NHR

NHR

N+

N+

N+

N+

N+

CH2OH

CH2OH

PdBr42–

PdBr42–

PdBr42–

PdBr42–

OO O

OO

n

Pd0

Figure 2.6. Chitosan-supported Pd-NP for the Heck coupling reaction.100 (Adapted with per-

mission from Ref. 100.)

and aryl chloride substrates) and olefins at relatively low temperature of 80 ◦C.98 Caloet al. reported that when Pd(OAc)2 and tetrabutylammonium acetate dissolved in tetra-butylammonium bromide (an ionic liquid), they formed Pd-NPs in situ that efficientlycatalyzed the stereospecific reaction of cinnamates with aryl halides to give �-aryl-substituted cinnamic esters.99 Calo et al. also showed that Pd-NPs supported on chitosan(Figure 2.6) could serve as very efficient heterogeneous catalysts for the Heck reactionbetween aryl bromides or activated aryl chlorides and butyl acrylate in ionic liquid (ortetrabutylammonium).100

Nanoparticles Composed of Metals Other than Pd for the HeckCoupling Reaction

Although the Heck reaction has been traditionally conducted using Pd-NPs, other transi-tionmetal nanoparticles such as Ru,101 Ni,102 and trimetallic Au–Ag–Pd103 nanoparticleshave also been shown to catalyze the reaction. This is mainly because many of thesemetals are also capable of inserting into the aryl halide bonds of the substrates during theHeck reaction. For example, certain Ru species were shown to readily activate an arylhalide bond of haloarenes by inserting Ru into the aryl halide bond and giving rise to anaryl–Ru–halo moiety.104 Upon addition of an olefin (styrene) and a base (KOAc) into thesolution containing the aryl–Ru–halo groups, an olefinated adduct (trans-stilbene) wasobtained quantitatively (98%) at 150 ◦C in 6 h.104 This experiment was actually whatalso led to one of the first investigations of the catalytic Heck-type reactions involvingmetals other than Pd. In this study, colloidal Ru nanoparticles, which were proposed tobe generated in situ within the catalytic medium containing [RuCl2(p-cymene)]2 andNaOAc, were proposed to implicated as the active catalyst for the reaction.101 To furtherprove this, Ru nanoparticles were separately prepared via a hydride reduction procedure

CORE–SHELL NANOPARTICLES FOR THE HECK COUPLING REACTION 25

using Ru(II) and dodecylamine as a stabilizing agent. The resulting Ru(0) nanoparticleswere also found to catalyze the Heck reaction in a very similar manner.

Recently, Martınez et al. showed that highly dispersed Ni nanoparticles withinsilica and carbon aerogels could serve as active and recoverable catalysts for theMizoroki–Heck reaction.105,106 The Ni nanoparticles in silica aerogels were preparedin multisteps. First, the sol–gel process was used to synthesize silica gel. The sil-ica gel was then grafted with N-(aminoethyl)aminopropyl-trimethoxysilane (or [N-((aminoethyl)aminoethyl)aminopropyl]trimethoxysilane). By using the supported aminegroups, metal (Ni(II)) ions were anchored onto the silica gel. The Ni(II)/silica gel wasthen dried with supercritical CO2 to afford silica aerogels, and the Ni(II) ions were sub-sequently reduced into Ni(0) by heating the sample in a stream of hydrogen at 550 ◦C(heating rate 10 ◦C min−1), followed by keeping the sample at 550 ◦C for 2 h. Inanother example, Zhang et al. reported a hydrothermal catalytic Heck reaction using Ninanoparticles, which were prepared by direct reduction of NiCl2 with NaBH4 in aqueousmedium under sonication in the absence of surfactants.102 The resulting Ni nanoparticlescatalyzed the Heck coupling reaction between aryl iodides or bromides and a variety ofalkenes in the absence of phosphine ligands, giving the corresponding coupling productswith moderate to good yields.

Mixed metallic or alloy-like nanoparticles composed of two or more than two dif-ferent metals have also been effectively used as catalysts for the Heck reaction.107–109 Infact, bimetallic and trimetallic nanoparticles often show higher catalytic activities thanthe individualmaterial or their physicalmixture.107 Tsai et al. synthesizedwell-dispersedalloy-like Au–Ag–Pd-nanoparticles with an average diameter of 4.4 ± 1.5 nm by laserirradiation of a solution containing gold, silver, and palladium colloidal nanoparticles.108

The synthesis of the nanoparticles was achieved by taking advantage of the meltingbehavior of themetals under laser irradiation, which led to the formation of the trimetallicalloy nanoparticles. When these trimetallic nanoparticles were tested as catalysts for theHeck reaction between 2-bromo-6-methoxynaphthalene and 3-buten-2-ol, they exhib-ited high catalytic activity, giving the nonsteroidal anti-inflammatory drug Nabumetone(4-(6-methoxy-2-naphthalenyl)-2-butanone) as a product, with reactant conversion andproduct yield of more than or equal to 90%.

CORE–SHELL NANOPARTICLES FOR THE HECKCOUPLING REACTION

As discussed earlier, one of the reasons for nanosized metal particles to exhibit efficientcatalytic activity for various chemical transformations is their large surface area-to-volume ratios. This very property of high surface area-to-volume ratio, however, alsorenders nanoparticles high surface energy and greater tendency to aggregate/sinter. Thelatter, in turn, results in loss of catalytic activities in the nanoparticles in various reac-tions, including the Heck coupling reaction. An innovative and promising strategy toovercome this common problem, that is, to eliminate or reduce the tendency of metallicnanoparticles to undergo aggregation/sintering, includes their synthesis with core–shellstructures.110–113 The cores of the core–shell structures will consist of the metal nanopar-ticles or supported metal nanoparticles. The shells will be porous materials that can hold


APTS

NaOH

TEOS /NH4OH

Silicananosphere

Pd-NPs

NH

2

NH 2

NH2

NH22HN

NH2 NH2

NH2N

H2

NH

2

NH

2

NH2

Scheme 2.5. Schematic representation of the synthesis of SiO2/Pd-NPs/porous SiO2 core–

shell–shell nanospheres for the Heck coupling and hydrogenation reactions.114 APTS = 3-

aminopropyltrimethoxysilane and TEOS = tetraethoxysilane. (Adapted with permission from

Ref. 114.)

the nanoparticles tightly and protect them from aggregation/sintering, while at the sametime, allow reactants and reagents to diffuse in and out of the cores of the materials,where the metallic nanoparticles are located.110–113 Such core–shell nanostructures cantypically be generated by using two sequential synthetic steps involving the prepara-tion of metal nanoparticle (or metal oxide/metal nanoparticle) cores, followed by thedeposition of the shell materials around the cores with an appropriate synthetic method.Recently, our research group reported a new synthetic method to such core–shell–shellnanocatalysts consisting of silica nanosphere (SiO2) cores decorated with Pd-NPs thatare further encapsulated by a nanoporous silica shell (porous SiO2) for catalysis (Scheme2.5 and Figure 2.7).114 The resulting nanoporous silica shells permitted reactants to reachthe Pd-NPs, while protecting the Pd-NPs from aggregation/sintering. These core–shell–shell nanospheres, labeled as SiO2/Pd–NPs/porous SiO2 core–shell–shell nanospheres,were proven to be versatile catalysts for the Heck coupling and hydrogenation reac-tions. Moreover, the nanospheres remained stable, showed negligible Pd leaching andaggregation, and were recycled multiple times without losing their catalytic activity.

Core–shell nanostructures composed of polymers were also used for the synthesisof heterogeneous catalysts for the Heck reaction. For example, Zheng and Zhang syn-thesized core–shell microspheres composed of pH-sensitive block copolymers such aspoly[styrene-co-2-(acetoacetoxy)ethyl methacrylate-co-methyl acrylic acid] (or PS-co-PAEMA-co-PMAA) to stabilize Pd-NPs for the Heck reaction.115 These pH-responsivepolymeric microspheres were synthesized using a one-step soap-free emulsion polymer-ization method in water (Figure 2.8).115 The polymer microspheres were then function-alized with 3 nm Pd-NPs, and the resulting materials were used to catalyze the Suzukiand Heck and coupling reactions in aqueous solutions.6 Furthermore, the pH-sensitiveor pH-responsive PMAA segment helped the microspheres to have different solubility atdifferent pHs, which in turn helped them to be easily recyclable after serving as catalystin catalytic reactions. This means, by simply changing the pH of the reaction mixture,the polymer microspheres could be dispersed or precipitated in the solution.

MESOPOROUS MATERIALS FOR THE HECK COUPLING REACTIONS 27

(i)

(ii)

(iii)

(i)

(iii)

100 nm

100 nm

100 nm

100 nm 100 nm

100 nm

100 nm

100 nm

100 nm

(v)

(a) (b)

(ii)

(iv)

(vi)

Figure 2.7. (a) Transmission electron microscopy (TEM) images of (i) SiO2/Pd-NP core–shell

nanospheres with 20 nm Pd-NP chemisorbed on aminopropyl-modified silica nanospheres

and the corresponding (ii) SiO2/Pd-NP/SiO2 core–shell–shell nanospheres, and (iii) SiO2/Pd-

NP/porous SiO2 core–shell–shell nanospheres etched for 120 min. (b) TEM images of SiO2/Pd-

NP/SiO2 core–shell–shell nanospheres (i) and the corresponding SiO2/Pd-NP/porous SiO2 core–

shell–shell nanospheres after etching for (ii) 50, (iii) 60, (iv) 70, (v) 80, and (v) 100 min. Scale

bars = 100 nm in all images. (Adapted with permission from Ref. 114.)

MESOPOROUS MATERIALS FOR THE HECKCOUPLING REACTIONS

As mentioned previously, various palladium-containing heterogeneous catalysts for theHeck reaction can be synthesized by supporting homogeneous palladium species orPd-NPs onto solid support materials. For the resulting heterogeneous catalysts to workeffectively, however, the right supportmaterial, which has high surface area, large enoughpore dimensions for mass transport of reactants/products, and higher loading capacity ofaccessible organopalladium species/Pd-NP groups, needs to be chosen or designed and


COOH

COOH: pH-responsive PMAA

: coordinative PAEMA

COOH

COOH

COOH

-COOH

COOH

L

L

L: PS

Figure 2.8. Schematic structure of a core–shell microsphere composed of poly[styrene-co-

2-(acetoacetoxy)ethyl methacrylate-co-methyl acrylic acid] (or PS-co-PAEMA-co-PMAA), which

was successfully used as a support material for Pd-NPs. The resulting microspheres were used as

efficient and reusable heterogeneous catalysts for the Suzuki and Heck reactions in water.115


synthesized. Using the right support material can result in heterogeneous catalysts thatnot only possess high catalytic efficiency but also easily become industrially attractiveby reducing the costs associated with catalysis.

Owing to their very high surface areas (typically 1000m2/g), tunable nanoscale poredimensions and easily modifiable surfaces to anchor organic groups, organometalliccomplexes, or metallic nanoparticles, mesoporous materials stand out as excellent sup-port/host materials for making heterogeneous catalysts for various reactions, includingtheHeck reaction.Mesoporousmaterials are synthesized by amethod called supramolec-ular self-assembly, which involves the self-assembly of silica precursors with surfactantmicelle templates.116,117 In this section, we discuss some of the development in thesynthesis of mesoporous material-based heterogeneous catalysts for the Heck reactions.

Mesoporous materials with compositions including organic, inorganic, carbon, orhybrid organic–inorganic materials have been synthesized by various methods. Manyof them have also already been successfully utilized as host materials for nanoparticlesor organometallic precursors. Furthermore, many of the resulting materials have beenshown to serve as heterogeneous catalysts for various Heck coupling reactions.118–120

Mesoporous material-based heterogeneous catalysts for the Heck reaction aremost commonly synthesized by immobilizing organopalladium complexes or palla-dium nanoparticles within the pores of organic-functionalized mesoporous materi-als. For instance, Huang et al. synthesized SBA-15-type mesoporous silica catalystscontaining Pd(II) species by self-assembling diphenylphosphine-containing organosi-lane (2-(diphenylphosphino)ethyltriethoxysilane), tetralkoxysilane, and Pluronic 123 assurfactant templating agents (Figure 2.9).121 After the Pluronic 123 templates were


Pd-1P

SiO2 PPh2 Pd(II)

Low

P c

onte

nt

Pd-2P

Pd(II) + PPh2-SBA-15Pd

PhPh

P

Si

PdPhPh

P

Si

PhPh

P

Si

Mid

P c

on

ten

t

Hig

h P

con

ten

tFigure 2.9. Immobilization of mono- or biscoordinated Pd organometallic complexes within

diphenylphosphine-functionalized SBA-15-type mesoporous silica for the Heck reaction.121


extracted from the resulting material, Pd(II) ions were anchored onto its diphenylphos-phine ligands by stirring the material with PdCl2(COD) in toluene at 303 K for 24 h.Different ratios of PdCl2(COD) and diphenylphosphine-functionalized mesoporous sil-ica were mixed and stirred in order to produce mesoporous materials with 1 : 1 and1 : 2 ratios of Pd(II) : diphenylphosphine ligands or samples labeled as Pd-1P or Pd-2P, respectively (Figure 2.9). The catalytic activities of the SBA-15-supported Pd(II)were then tested in the Heck reaction. Compared with catalyst Pd-1P, catalyst Pd-2Pshowed higher catalytic activity and selectivity for the Heck reaction at 358 K, givingreactant conversion of 90% with more than 99% selectivity toward the desired Heckcoupling product in 6 h. To check the recyclability of the catalysts—one of the mostimportant features sought after heterogeneous catalysts—the Pd-phosphine-containingSBA-15 materials were recovered and reused as catalysts in multiple reaction cycles.The catalysts showed no leaching of palladium and were proven to be recyclable withbarely any loss of catalytic activity.121

Owing to their relatively bigger pore dimensions and high surface area, SBA-type mesoporous silica materials in particular can be functionalized with relatively“bulky” groups, including ionic liquids. SBA-15-supported ionic liquid, which in turnwas used to stabilize palladium complexes for the Heck reaction, was reported byJung et al. (Figure 2.10).122 The resulting material was found to be highly stable andeasily recyclable catalyst with minimum loss (�0.1 ppm) of palladium during the Heckreaction. Such mesoporous silica-supported ionic liquids can also potentially providean alternative and stable ionic liquid reaction medium for many catalytic reactions,including the Heck reaction, compared with the more costly and highly viscous ionicliquids, whose properties have prevented their commercialization and wide range of usesas solvents for various organic reactions.

Periodic mesoporous organosilicas (PMOs), which are a family of mesoporousmaterials with organic–inorganic hybrid frameworks, have also been used to immobilize


Me

N a b

1

N

Me

Si(OEt)3

Cl–N

+

N

c, d

Me

Cl–N

+

N

SiEtOOO

Me

Cl–N

+

N

SiEtO

SBA-15 support2

SBA-15 support3

OO

Me

Cl–N

NBu

PF6–N

Me

=

= Pd(OAc)2

+

N

Si

OO

Me

Cl–N+

N

SiEtOOO

EtO

+

Figure 2.10. Synthetic scheme for the preparation of SBA-15/ionic liquid-supported Pd(OAc)2

catalyst (reagents and conditions): (a) (3-chloropropyl)triethoxysilane, 100 ◦C, 24 h; (b) SBA-15,

toluene, reflux; (c) 1-butyl-3-methylimidazolium hexafluorophosphate, Pd(OAc)2, tetrahydro-

furan (THF), room temperature, 1 h; (d) removal of THF.122 (Adapted with permission from

Ref. 122.)

organometallic palladium complexes.123,124 More importantly, PMOs can produce rathermore stable heterogeneous catalysts for the Heck reaction because their bridging organicgroups within their mesoporous channel walls can provide extra hydrothermal stabilityto the material as well as to the supported Pd catalytic groups. Moreover, the uniformdistribution of the organic groups in PMOs125 allows uniform distribution of the Pdcatalytic groups, without compromising the surface area and the pore volume of themesoporous structure.

For instance, phenylene-bridged PMO containing diphenylphosphine ligands wasprepared via supramolecular self-assembly of bissilylated bridging phenylene precur-sors with triblock co-polymers as templates. After removal of the polymer templates,diphenylphosphine-containing PMO was obtained. The PMO material was then stirredwith a palladium precursor, PdCl2(PPh3)2, in toluene producing Pd(II)/PMO catalyst(Figure 2.11).126 The presence of Pd(II) within the mesoporous catalysts was determinedby inductively coupled plasma optical emission spectrometer. The inclusion of organicmoieties and other surface parameters of the material was determined by solid-statenuclear magnetic resonance spectroscopy, nitrogen gas adsorption, X-ray diffraction(XRD), and TEM. The resulting Pd(II)-containing PMO was found to possess highlyordered mesoporous structure consisting of Pd(II) complexes that did not affect theoverall integrity of the material.126 More importantly, the phenylene moieties in theframework of the resulting mesoporous material (Pd-PPh2–PMO-Ph catalyst) renderedhydrothermal stability as well as favorable environment for hydrophobic reactants toundergo more efficient catalytic transformation during the Heck coupling reaction.

Besides mesoporous silicas and PMOs, other mesoporous materials have beenutilized as support materials for making heterogeneous catalysts for the Heck reaction.


Si

(A)

(B)

(C)

(EO)20(PO)10(EO)20

Si

Si SiRORO OR

OR

OR

ORO

Si PPh2

PdCl2(PPh3)2

PPh3

Pd

Cl

Cl

O

O

O

O

OO O

O

O

Si PPh2

OR

OR

OR

Figure 2.11. Schematic depiction for the synthesis of Pd(II)-containing PMO catalysts (Pd-

PPh2–PMO-Ph) for the Heck reaction.126 (Adapted with permission from Ref. 126.) (See color

insert.)

This includes mesoporous carbons. Although carbon materials such as activated carbonhave long been used for making a variety of Pd-based heterogeneous catalysts, theirmicroporous structure, low surface area, and broad pore size distribution have madethem to have limited application in catalysis. But the recent successes in the synthesis ofhigh-surface-area mesoporous carbons with narrow pore size distribution and high porevolume have opened up new opportunity for more effective carbon-supported Pd(II)ions and Pd-NP catalysts for the Heck reaction.

For instance, Hu et al. reported a simple synthetic method for the preparationof mesoporous carbon-supported Pd-NPs and investigated their use as a heterogeneouscatalyst for the Heck reaction.127 To synthesize the material, the authors first mademeso-porous silica and then immobilized it with Pd(NO3)2 and sucrose. After carbonization ofthe sucrose at 900 ◦C, the authors then removed the silica host from the resulting meso-porous silica/carbon composite material by using dilute hydrofluoric acid (HF) solution(Caution: HF is toxic and poisoness) to produce the mesoporous carbon-supported Pd-NPs (Figure 2.12).127 The wt% and the size of the Pd-NPs formedwithin themesoporouscarbon were controlled by changing the relative amount of Pd(NO3)2 used during thesynthesis of the parent material. Of the different materials synthesized, the material with

TEOS

Sucrose

Sol–gel Carbonization

Palladium particle

Template

removal

Palladium precursor

Figure 2.12. Synthetic scheme for mesoporous carbon-supported Pd-NPs for the Heck reac-

tion.127 (Adapted with permission from Ref. 127.)


5.0 wt% Pdwas found to give the highest catalytic activity for the Heck reaction betweeniodobenzene and styrene, even when compared with commercially available Pd-basedcatalysts with a higher Pd loading (e.g., 10% Pd on activated carbon from Aldrich and20% Pd on carbon from E-Tek Inc.).

Other mesoporous materials such as those composed of mixed metal oxides andthose possessingmagnetic properties have been employed for the stabilization of Pd-NPsfor the Heck reaction. Besides stabilizing the catalytic groups, such materials allow easeof separation of the catalysts by using an external magnetic field. Gao et al. developed asynthetic method to high-surface-area superparamagnetic mesoporous NiFe2O4 for useas magnetically separable support material for catalytic groups.128,129 The material wassynthesized by controlled thermal decomposition of Ni and Fe oxalate precursors witha template-free synthetic approach. Pd-NPs were then synthesized within the pores ofthe resulting porous material with a two-step procedure, consisting of wet impregnationof aqueous ammonium tetrachloropalladate(II) (or (NH4)2PdCl4) into the pores of thematerial at room temperature, followedby reduction of the resultingmaterial in 5%H2/Arat 200 ◦C to yield Pd(0) nanoparticles.129 This procedure produced a high-surface-areamagnetic mesoporous NiFe2O4 catalyst with uniformly dispersed Pd(0) nanoparticlesthat showed high catalytic activities in C C bond-forming Heck and Suzuki couplingreactions. After the catalytic reactions, the solid catalyst was easily separated for reuseusing a magnetic field.

Interestingly, in some cases, besides the metallic or Pd-NPs, the support materialsthemselves have been implicated as the catalytic active sites during some C C and Heckcoupling reactions. For instance, basic metal oxide support materials were shown to playa synergistic role and accelerate the catalytic activities of Pd-NPs during the Heck reac-tion.130–132 From CO2 thermal-programmed desorption experiments, it was determinedthat the Brønsted basic sites of such mesoporous metal oxide support materials couldcooperatively participate with the Pd(0) nanoparticles in the oxidative addition step ofthe reactants. This process helped with the formation of Ar-Pd(II)-X species on the sur-face of the materials, as shown in Scheme 2.6, which led to increased catalytic activitiesof the materials.

Mesoporous materials composed of organic polymers, dubbed mesoporous organicpolymers (MOPs), were also employed for entrapment and stabilization of active metal

OH

OH

Ar-X

δ+

δ+

δ−

δ−

OH

OHe

e

OH

Oxidative addition

Pd

Pd

OH

OHX

X

II

II

Ar

ArOH

OH

OH Pd

Pd

Scheme 2.6. Mesoporous silica-stabilized Pd(0) nanoparticles, which served as cooperative

catalysts for C C and Heck coupling reactions due to the involvement of the support material

in the catalytic process.129 (Adapted with permission from Ref. 129.)


Br

BrPd

Br

Br

PdN

N

R

R

polymer encapsulated Pd nanocluster

R

X = Br, Cl

X

B(OH)2

R

N

N

R

RBr

BrPd

N

N

R

R

N

N

Rn

R

Figure 2.13. An example of a functionalized mesoporous organic polymer (MOP) catalysts,

polytriallylamine (MPTAT-1), synthesized via organic–organic radical polymerization of 2,4,6-

triallyloxy-1,3,5-triazine in aqueous medium in the presence of an anionic surfactant (sodium

dodecyl sulfate) as a template. The template-free MPTAT-1 acts as an excellent support for

immobilizing Pd(II) at its surface and the resulting material show very good catalytic activity

in several C C cross-coupling reactions including the Mizoroki–Heck reaction.135

centers for the Heck reaction. MOPs, which are typically synthesized by soft-templatingsynthetic approach, possess organic functionalities intact within their porous networkthat can anchor metallic species (catalytic groups). By using a soft-templating syntheticmethod, Chandra and Bhaumik synthesized two MOPs, denoted as mesoporous poly-triallylamine MPTA-1 and MPTA-2 by using sodium lauryl sulfate and a mixture ofsodium lauryl sulfate and Brij 35, respectively, as structure-directing agents (SDAs).The resulting MOPs contained nitrogen donor sites, which were used to stabilize Pd cat-alytic groups.118 N2 adsorption/desorption isotherms of theMOPs gave type IV isotherm,which is characteristic of mesoporous materials, but with different surface areas, that is,134 m2/g for MPTA-1 and 70 m2/g for MPTA-2 (Figure 2.13). Pd-NPs were synthesizedwithin the mesoporous polymer network of MPTA-1 by refluxing the material withPd(II) acetate in methanol, where methanol was used both as a solvent and as a mildreducing agent for the Pd(II) ions.119 The formation of Pd(0) from Pd(II) was confirmedby XRD, TEM, and UV-visible spectroscopy. Furthermore, the Pd-MPTA-1 was shownto efficiently catalyze the Heck coupling reaction between a variety of aryl halides andalkene in an aqueous environment at 100 ◦C. In addition, the Pd(0) containing MOP wasrecovered via centrifugation and recycled successfully multiple times without losing itscatalytic activity. Moreover, a hot filtration test was performed after the reaction yielded67.0% product with the solid catalyst, and the result indicated that there was no leachingof Pd nor homogeneous catalysis present in the system.119


POLYMERIC-BASED NANOCATALYSTS FOR THE HECKCOUPLING REACTIONS

In the advent of the development of various synthetic methods to micro- and nanoscalepolymers, research in exploiting these systems as support materials for various nanocat-alysts has also continued. By supporting active catalysts such as Pd complexes andPd-NPs on nanostructured polymers, efficient nanocatalysts for the Heck reactions canbe synthesized. Most of the resulting systems are not only effective as catalysts but alsopossess many other advantages. One of the advantages of polymer-supported hetero-geneous catalysts is that they can act as “pseudohomogeneous” catalysts due to theirfavored interaction with many solvents (Figure 2.13).133,134 Normally, the “pseudoho-mogeneous” condition is reached in solvents where the polymer system presents elevatedhydrodynamic radii.135 In addition, some polymer-supported nanocatalysts can easilybe separated from reaction mixtures simply by changing the pH of the reaction mixtures,making catalytic workup procedures easier, as discussed above.

For example, Alacid and Najera reported the synthesis of polystyrene microsphere-supported palladacycle from Kaiser oxime resin (Figure 2.14).136 The resulting materialwas used as a catalyst (or precatalyst) for the Heck catalytic reaction.136 Furthermore,they investigated themechanismof the catalytic reaction of the palladacycles usingHg(0)poisoning experiment, which involved addition of Hg(0) into the reaction mixture. Uponaddition of Hg(0), the reaction stopped, indicating the formation of amalgam betweenPd and Hg(0). Most importantly, this result suggested that Pd species was present in thesolution and the palladacycles were responsible for the catalytic Heck reaction.

In another example, Pd-NPs, which were prepared within poly(methylmethacrylate)(PMMA) microspheres from the reduction between PdCl2 and formaldehyde, wereshown to catalyze the Heck reaction.137 These PMMA-encapsulated Pd-NPs, whichpossessed 0.79wt%Pd, catalyzed the reaction between bromobenzene and styrene underligand-free conditions in air for four consecutive cycles, resulting in the correspondingstilbene product with 89%, 88%, 85%, and 83% isolated yields, respectively. After thereactions, only negligible amount of palladium was detected in the reaction mixtures.

Polymer systems with core–shell nanostructures have also been employed as cat-alyst support for the Heck reaction. Zhang et al. reported the synthesis of core–shellmicrospheres composed of polystyrene (PS) cores and poly(methylacrylic acid) shells,whose carboxylic acid moieties were converted into iminodiacetic acid (IDA) groups,as depicted in Figure 2.15.138 The IDA groups were then used to chelate Pd2+ ions andproduce a heterogeneous Pd catalyst, labeled as PS-co-PMAA-IDA-Pd. The resultingmaterial was successfully used as a catalyst for the Heck reaction in aqueous medium.

OH

NO2

ClPd N

PS

Figure 2.14. Palladated Kaiser oxime resin, which can serve as a precatalyst for Mizoroki–Heck

reactions.136 (Adapted with permission from Ref. 136.)

POLYMERIC-BASED NANOCATALYSTS FOR THE HECK COUPLING REACTIONS 35

COOH

COOH

COOH

HOOC

PS

PS-co-PMAA

COOH

COOHO

O

N

N

O2C

CO2–

CO2–

Pd2+

Pd2+

PdCl2

O2C

PS

PS-co-PMAA-IDA-PD

–

–

COOH

COOHO

O

N

N

O2C

CO2–

CO2–

O2C

PS

PS-co-PMAA-IDA

–

–

COOH

O

NO

O

SOO

O O–

COOHHN

IDA

pH=9.5

sulf-NHSEDC

Cl–Cl–

N

NN

N

+NH+NH

C

C

NH

HOO

SO

O

O

OO O–

Figure 2.15. Schematic representation of the synthesis of IDA-functionalized polystyrene-

poly(methylacrylic acid)-IDA-palladium (PS-co-PMAA-IDA-Pd) core–shell-type polymer nano-

catalysts with supported Pd(II) ions for the Heck coupling reaction.138 (Adapted with permission

from Ref. 138.)

Interestingly, the material was found to be highly dispersible in the reaction medium,almost like a homogeneous catalyst. At the same time, owing to its pH-responsive prop-erties as a result of its PMAA functional group, the material could be separated as typicalheterogeneous catalysts, simply by adjusting the pH of the reaction medium and makingthe polymer to settle down via hydrogen bonding.

Besides core–shell geometry, polymers with other architectures were used as sup-port material for making heterogeneous catalysts for the Heck reaction.119,139, 140 Forinstance, as discussed above, mesoporous polymers such as MPTA-1139 can be usedto support Pd(II) catalytic species by mixing the MPTA-1 with Pd(OAc)2 in methanol.Methanol in this case was used as both a solvent and a reducing agent for Pd(II) ions.The resulting mesoporous polymer-supported Pd(II) material was found to efficientlycatalyze the Heck coupling reaction between alkenes and a variety of aryl and heteroaryliodides, bromides, and chlorides in aqueous medium. Furthermore, the catalyst wasproven to be highly stable, showing no loss of catalytic activity in subsequent reactioncycles.140 Another notable example of polymeric systems for the Heck reaction includespolymers possessing hollow nanostructures that were synthesized by using supercriticalCO2/ethanol; these polymers were successfully used to encapsulate metal nanoparticles,as shown in Figure 2.16, and subsequently to catalyze the Heck reaction.140

Polymers such as benzazepines in polyethylene glycol were shown to be excellentsupport materials for metal complexes for the Heck reaction.141 He and Cai synthesizedpolymer-supportedmacrocyclic Schiff base palladiummaterial with 0.5mol%Pd, whichshowed catalytic activity for the Heck reaction between aryl iodides or bromides andethyl acrylate or styrene inDMF.142 Thematerial exhibited high catalytic activity, gettingthe reaction almost into completion within 1–4 h and giving approximately 100% reac-tant conversion and approximately 100% isolated yield of the corresponding coupling


(a) (b) (c)

(d) (e) (f)

1 μm100 nm

100 nm100 nmEnergy (keV)0

0

200

400

Cou

nts

600C

NO

PdCu

Cu

3 K 6 K 9 K 12 K

5 nm

Figure 2.16. (a) Scanning electron micrograph image, (b) TEM image, (c) high-resolution

TEM image, and (d) energy dispersive spectrum profile of Pd-NPs supported within hollow

polystyrene spheres, which were prepared using supercritical fluid. (e) TEM image of Pd-NPs

supported within hollow polystyrene spheres prepared using ethanol as a solvent and (f) TEM

image of Pd-NPs supported within hollow polystyrene spheres prepared in water.140 (Adapted

with permission from Ref. 140.)

products with a very high selectivity. In addition, the polymer-supported macrocyclicSchiff base palladium catalyst was easily recoverable and reusable multiple times withonly a minor loss of catalytic activity, even after being used multiple times.

Another example of a polymer-supported metal complex catalyst for the Heckreaction was reported by Ochiai et al.143 First, a prepolymer bearing azine functionalgroups was synthesized by starting with poly(methyl vinyl ketone). Its hydrazine groupswere then chemically converted into hydrazone groups. The hydrazone moieties weresubsequently modified with aromatic aldehydes to produce azine groups, which wereused to support Pd(II) complexes. The resulting polymer supported-Pd(II) material wasfound to be a highly active heterogeneous catalyst for the Heck reaction, showing highcatalytic turnover frequency and good recyclable properties. Other polymer-supportedPd complexes affording very high turnover frequencieswere also reported recently.144,145

For example, a poly(styrene-divinylbenzene)-supported Schiff base with Pd complexeswas shown to give high turnover frequencies reaching up to 28,000 h−1, along with veryhigh selectivities with values more than 90%.146

Besides Pd(II) complexes and Pd-NPs, PdO nanoparticles can be stabilized withpolymers.145 The resulting materials can be used as heterogeneous nanocatalysts forthe Heck reaction. For instance, monodisperse PdO nanoparticles with average size of

POLYMERIC-BASED NANOCATALYSTS FOR THE HECK COUPLING REACTIONS 37

10 20

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Figure 2.17. (a) XRD patterns, (b) JCPDS data (#41-1107), (c) TEM micrograph (scale bar =20 nm), and (d) size distribution of polystyrene-supported-PdO nanoparticle synthesized by

thermal decomposition Pd(OAc)2 within linear polystyrene.145 (Adapted with permission from

Ref. 146.)

2.5 nm in diameter were synthesized within linear polystyrene by thermal decompositionof Pd(OAc)2 (Figure 2.17).147 The material was then successfully used as a catalyst forthe Mizoroki–Heck reactions in aqueous solution. The polymer served as a protectiveshell, inhibiting any possible growth of the PdO nanoparticles and helping the materialretain its catalytic activity.

The rational design of polymers, for example, by tuning their cross-linking density,was employed for the development of effective polymer-supported Pd nanocatalystsfor the Heck reaction. This approach can result in catalysts having tunable solubilityin different reaction media and useful properties for catalysis. For instance, when thepolymer has low degree of cross-linking and thus higher solubility, it can catalyze thereaction almost like a homogeneous catalyst. On the other hand, when the cross-linkingdensity is higher and the polymer’s solubility is lower, the material behaves as a colloidalor heterogeneous catalyst. These properties of polymers can be exemplified by the work


cat

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Figure 2.18. Schematic representation of nanofiltration of phosphinated polymer-supported

Pd catalyst after being used in the Heck, Sonogashira, and Suzuki coupling reactions.148


reported by Leyva et al. on polystyrene-supported catalysts containing SPhos phosphineligands and Pd-NPs, which were found to be extremely active ligands in the aminationand Suzuki coupling of sterically hindered aryl chlorides.147 While the non-cross-linkedand soluble linear polystyrene with polyethylene glycol-anchored phosphine Pd(II)enabled the catalyst to behave as a homogeneous catalysts, the cross-linked counterparthelped the material exhibit the typical properties of a heterogeneous catalyst.

Some Pd-based catalysts supported onto soluble polymers can serve as homoge-nous catalyst while they can also be easily recovered as heterogeneous catalyst withseparation techniques such as nanofiltration. For example, a highly soluble phosphi-nated polymer containing Pd(II) ions was prepared by anionic polymerization of 4-methylstyrene, followed by phosphination of the polymer and reaction with Pd(II) ions,and the resulting material was used as a catalyst for the Heck, Sonogashira, and Suzukicoupling reactions.148 The material catalyzed the Heck reaction between n-butyl acry-late and 4-bromoacetophenone (or 1-bromo-4-methoxybenzene or bromobenzene) inN-methylpyrrolidone, resulting in the corresponding products with yields ranging from 80to 87% in 16 h.More importantly, the catalystwas almost completely recovered (99.95%)after the reactions, simply by filtering the reactionmixtureswith poly(dimethylsiloxane)-casted porous poly(acrylonitrile) membrane (Figure 2.18).148

Polymers with surface-grafted ionic liquid groups that can support catalytic-activePd species can be used as catalyst for theHeck reaction under solvent-free conditions. Forinstance, a copolymer prepared by grafting 1-aminoethyl-3-vinylimidazolium bromideonto cross-linked polydivinylbenzene was further functionalized with ionic groups tosupport Pd(II) ions.149 The Pd(II) ions were subsequently reduced into metallic Pd(0)with sodium borohydride (Scheme 2.7). The resulting polymer/ionic liquid-supportedPd-NPs catalyzed the Heck reaction between iodobenzene andmethyl acrylate at 120 ◦C,affording high conversion (98%) in 1 h. In addition, owing to its insolubility and stability,the catalyst could be easily recovered from the reaction mixture and reused multipletimes.

CARBON NANOMATERIAL-SUPPORTED HECK COUPLING REACTIONS 39

Scheme 2.7. Example of synthesis of the cross-linked polymer-supported ionic liquid that

were used for supporting Pd species for solvent-free Heck reactions.149 (Adapted with permis-

sion from Ref. 149).

CARBON NANOMATERIAL-SUPPORTED HECKCOUPLING REACTIONS

Intensive research in recent decades has already led to our understanding of the manyfascinating properties of various carbon nanomaterials and their potential applications,including their use as stable and robust support materials for various catalytic groups.Carbon-based materials can serve as versatile support materials for heterogeneous cat-alysts because they do not render much restriction under what reaction conditions theycan work (e.g., they are stable at high temperatures and under high pressures, conditionsthat are far too common in catalysis). Different carbon materials such as highly orga-nized pyrolytic carbon (HOPG), amorphous carbon nanomaterials, carbon nanotubes(CNT), fullerenes, graphene, and graphene oxide have actually been already shown tobe useful as support materials for metal nanoparticles, including Pd, for the Heck reac-tion.150–153 As discussed above in Section “MesoporousMaterials for the Heck CouplingReactions,” carbon materials with mesoporous structures have also been investigated assupport materials for catalysis.127

To synthesize carbon-supported nanocatalysts, different wet and gas-phase syn-thetic methods have been developed. For example, Pd-NPs were successfully depositedonHOPGby using atomic beam deposition.150 The resultingmaterial was shown to serveas an active heterogeneous nanocatalyst for the Heck reaction at 373 K in a 2 : 1 mixtureof toluene andDMF, affording 35% of isolated yield of the C C coupling alkene productafter 12 h. Although the nanocatalyst had extremely low content of Pd, that is, approx-imately 0.38 �m equivalent, it showed relatively good catalytic activity. Similiarly,graphene was successfully used as a support material for Pd-NPs, and the resulting mate-rial was shown to have excellent catalytic activity for C C cross-coupling reactions.151

A typical synthetic strategy to synthesize graphene-supported Pd-NPs starts withpreparation of graphene oxide—a material that is relatively easy to synthesize—andthen deposition of Pd(II) ions on it.151 Various wet and physical deposition methods canthen be used to reduce reduce the Pd(II) (and even the graphene oxide) in the resultingmaterial to produce graphene oxide- (or graphene-) supported Pd-NPs, respectively.The latter forms because the graphene oxide itself can be reduced into graphene by thereductant used for reducing the Pd(II) ions. In another synthetic approach, the grapheneitself is used as a reducing agent for the metal ions (in addition to functioning as asupport material for the metal nanoparticles).

For example, Xiang et al. demonstrated a rapid preparation method for noblemetal nanocrystals on reduced graphene via a coreduction method, which involved the


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Figure 2.19. TEM images and particle size distributions of Pd-NPs on reduced graphene

oxide (Pd/PRGO catalyst) synthesized by direct laser irradiation and without using external

reducing agent and surface-capping ligands. The samples in A, B, and C were prepared in

aqueous, 50% ethanol−water, and 50% methanol−water solutions, respectively.155 (Adapted

with permission from Ref. 155.)

REFERENCES 41

reduction of Pd(II) ions into Pd(0) and the reduction of graphene oxide into graphene.152

The process led to the formation of Pd(0)/graphene composite nanomaterial, whichshowed high catalytic activity for the Heck reaction.152 Chen et al. demonstrated thatwell-dispersed Pd-NPs on the surface of graphene oxide could be directly generatedfrom a reaction between PdCl4− and graphene oxide.153 Rumi et al. showed that Pd(II)-exchanged graphite oxide (GO) could be applied as a precatalyst for the synthesis ofair-stable GO-supported Pd nanoparticles for the Suzuki–Miyaura, Mizoroki–Heck, andSonogashira reactions.154

Laser irradiation was also used to prepare Pd-NPs on partially reduced grapheneoxide for the Heck reaction (Figure 2.19).155 The typical synthesis involved irradia-tion of solutions containing graphene oxide and Pd(NO3)2 with a pulsed laser oper-ating at 532 nm wavelength, which led to the reduction of Pd(II) ions without theuse of any external reducing agent and in the absence of surface-capping ligands. Thesize distribution of the palladium nanoparticles depended on the type of solvent usedthough, as shown in Figure 2.19. By correlating the catalytic activity with other resultsobtained from X-ray photoelectron spectroscopy and XRD, the authors claimed thatthe defect sites generated on the partially reduced graphene oxide, due to the pho-tochemical processes, might be responsible for the very high catalytic activities ofmaterials.155

Finally, CNTs, which possess many interesting properties for nanoelectronic andnanosensor applications, are also successfully utilized as support materials for catalystsfor the Heck reaction. Sokolov et al. developed a method for the synthesis of Pd/CNTby the direct reaction of a zerovalent palladium complex on CNT, which effectivelycatalyzed the Heck reaction.156 The Pd-NPs were grown on the surface of the CNT fromPd complex containing an easily removable ligand, dibenzylideneacetone. The resultingmaterial was used as a catalyst for three different C C bond-forming cross-couplingreactions such as the Suzuki, Heck, and Sonogashira reactions (e.g., the Heck reactionbetween 5-iodosalicylic acid and acrylic acid in aqueous medium. The reaction wascarried at 100 ◦C affording 97–99% yield in 15 min. Furthermore, the catalyst waseasily regenerated and reused multiple times.

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