Pd Metal Catalysts for Cross-Couplings and ... - ACS Publications

Review Cite This: Chem. Rev. 2018, 118, 2249−2295

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Pd Metal Catalysts for Cross-Couplings and Related Reactions in the 21st Century: A Critical Review Andrea Biffis,*,† Paolo Centomo,† Alessandro Del Zotto,‡ and Marco Zecca† †

Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, I-35131 Padova, Italy Dipartimento di Scienze Agroalimentari, Ambientali e Animali−Sezione di Chimica, Università di Udine, Via delle Scienze 206, I-33100 Udine, Italy

‡

ABSTRACT: Cross-couplings and related reactions are a class of highly efficient synthetic protocols that are generally promoted by molecular Pd species as catalysts. However, catalysts based on more or less highly dispersed Pd metal have been also employed for this purpose, and their use, which was largely limited to the Heck reaction until the turn of the century, has been extended in recent years to most reactions of this class. This review provides a critical overview on these recent applications of Pd metal catalysts. Particular attention is devoted to the discussion of the mechanistic pathways that have been proposed to explain the catalytic role of Pd metal. Furthermore, the most outstanding Pd metal based catalytic systems that have emerged are illustrated, together with the development of novel approaches to boost the reactivity of Pd metal. A section summarizing the current industrial applications of Pd metal catalyzed reactions of this kind concludes the review.

CONTENTS 1. Introduction and Historical Overview 2. Debate on the Reaction Mechanism(s) 3. Pd Metal Catalysts for the Suzuki-Miyaura Reaction 3.1. Mechanistic Studies 3.2. Outstanding Catalytic Systems 3.3. Innovative Pd Metal Based Catalytic Systems 3.3.1. Pd/Au Nanoalloys 3.3.2. Systems Based on Photophysical Activation 3.3.3. Asymmetric Suzuki Reaction with Pd Metal 4. Pd Metal Catalysts for the Sonogashira Reaction 4.1. Mechanistic Studies 4.2. Outstanding Catalytic Systems 5. Pd Metal Catalysts for Other C−C Cross-Coupling Reactions 6. Pd Metal Catalysts for the Heck Reaction 6.1. Mechanistic Studies 6.2. Outstanding Catalytic Systems 7. Pd Metal Catalysts for C−N Coupling Reactions 8. Pd Metal Catalysts for Direct C−H Arylation Reactions 9. Industrial Applications of Pd Metal Catalysts for Cross-Couplings and Related Reactions 10. Conclusions and Outlook Author Information Corresponding Author ORCID Notes Biographies Acknowledgments © 2018 American Chemical Society

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1. INTRODUCTION AND HISTORICAL OVERVIEW Cross-couplings and related reactions, such as the Heck reaction, the Buchwald-Hartwig reaction and the direct arylation reaction (Figure 1), currently rank among the most versatile and useful tools for carrying out organic syntheses both in academic laboratories and in industrial production plants. They represent a comparatively mature technology, whose foundations were laid by the work of scientists in the

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Figure 1. Cross-couplings and related reactions focused upon in this Review. Received: July 25, 2017 Published: February 20, 2018 2249

DOI: 10.1021/acs.chemrev.7b00443 Chem. Rev. 2018, 118, 2249−2295

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reported in refs 1−6 as well as to additional reviews devoted to a specific cross-coupling or related reaction, which will be referred to in the subsequent sections. Interestingly, the use of Pd metal as catalyst for these reactions is as old as the reactions themselves: first mention of the successful use of Pd black in the Heck reaction dates back to the early 1970’s,7−9 as well as the first application of supported Pd metal, with the pioneering work by Julia and coworkers.10,11 Incidentally, Julia managed to obtain stilbene from styrene and chlorobenzene in respectable yield (62%, with 75% selectivity based on the aromatic halide) over Pd on carbon, in methanol at 120 °C and under pressure (to preserve the liquid state of the solvent). This shows that the activation of aryl chlorides has been a challenge since the very beginning of Pd metal catalysis as applied to coupling reactions. In more recent times, extensive research has been performed on catalysts based on highly dispersed/nanosized Pd metal on a solid support.12−15 These materials were generally found to deliver good to excellent catalytic performances in these reactions and could often be recovered and recycled, which adds to their economical significance. The manufacture and application of nanoengineered, heterogeneous, or “nanoheterogeneous” (i.e., based on colloidally stable Pd nanoparticles/nanoclusters dispersed in solution) catalytic systems based on Pd metal for these reactions has been continuously developed up to the present day, producing catalysts which reach high activity, selectivity, and durability. Indeed, crosscoupling reactions have nowadays largely replaced hydrogenations as a handy tool for evaluating the potential of Pdbased catalysts of this kind.16 In spite of the enduring success of Pd metal catalysts for cross-couplings and related reactions, comparatively few systematic surveys have been carried out on the factors affecting the performance of these catalysts and on their exact mechanism of action (in particular their “homogeneous” or “heterogeneous” nature, see below). Indeed, the reaction mechanisms of these catalysts in cross-couplings and related reactions are still debated. There has been on the one hand a fairly general consensus on a mechanistic hypothesis based on leaching into solution of catalytically competent, molecular Pd species from Pd metal, underpinned by several critical reviews starting from our first contribution to this discussion in 2001.13,17−21 As will be apparent in the following, some more recent studies point instead toward a “heterogeneous” reaction mechanism taking place at the Pd metal surface and/or at defect/edge sites of Pd nanoparticles, at least for certain reactions and under certain conditions.22 Several reviews have treated Pd metal catalysts for these reactions in some detail, but most of them actually deal with (putatively) heterogeneous Pd metal based catalysts in general, and besides (supported) Pd metal, they mainly treat supported palladium(II) ions or complexes of several kinds.16,23−31 In the last ten years, only two review articles and a book chapter have specifically showcased Pd metal nanoparticle catalysts,15,32,33 but nanoparticles are by far not the only form of Pd metal that has been employed as catalyst for this rection class. Furthermore, none of these reviews critically discuss the reaction mechanism(s) and the notable efforts made in recent years to clarify it. Consequently, this review aims at filling this gap. It will provide in first instance a brief historical account on the mechanisms underlying catalysis of cross-coupling and related reactions by Pd metal and will then present and discuss for

1970’s, among them those who gave their names to these reactions. Their development and extensive application became tumultuous after the full realization of their synthetic usefulness and the discovery in the 1990’s of the dramatic activity enhancement exhibited by metal complexes with bulky, strongly σ-donating ligands. Eventually, the success and popularity of this research field led to the attribution of the Nobel Prize in Chemistry to R. F. Heck, E.-i. Negishi, and A. Suzuki in 2010.1 Generally, all these reactions need a transition metal catalyst to proceed at a synthetically useful rate. Although several metal centers are in principle capable of catalyzing the various steps of these reactions, there is no doubt that catalysts based on Pd dominate the scene to the point that, beside their synthetic application at the laboratory and industrial scale, these reactions have also become standard methods to evaluate the reactivity of Pd species as potential catalysts. Pd salts or complexes, preformed or generated in situ upon addition of a ligand, are commonly employed as sources of Pd for cross-couplings and related reactions. Usually, palladium(II) species are chosen as starting material because of their higher stability. Such compounds are then reduced in situ to palladium(0) species which enter the catalytic cycle. The reaction mechanisms are quite well understood and, especially in the case of Pd complexes, are considered to be organometallic in nature, taking place in the coordination sphere of the metal. Although the mechanisms for the various Pdcatalyzed cross-coupling reactions differ in some detail, they conform to the general catalytic cycle depicted in Figure 2. As

Figure 2. General catalytic cycle for cross-coupling reactions.

will be apparent in the following, several Pd-catalyzed reactions related to cross-couplings (e.g., Heck reactions, BuchwaldHartwig couplings, direct arylations) share with crosscouplings at least the initial steps of the catalytic cycle (i.e., the activation of the aryl halide by oxidative addition, which is incidentally the rate-determining step of the reaction in several instances) and are consequently influenced to a large extent by the same factors governing cross-couplings. An enormous number of review articles and monographs have been dedicated to these reactions, covering their synthetic applications, the reaction mechanisms, and the development of catalytic systems based on both homogeneous and supported “molecular” Pd species (ions, complexes). The interested reader is referred to the general bibliographies 2250


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Figure 3. Schematic picture of the different nature and location of potentially competent catalytic species in a cross-coupling reaction catalyzed by supported Pd metal.

contributions apart. (2) As it has been demonstrated already at the turn of the century by de Vries, Leadbeater, and others, low levels of Pd in solution (at subppm level) can be extremely active for the catalysis of these reactions (“homeopathic catalysis”).36−38 This is particularly true for reactions carried out with the most reactive substrates (i.e., organic bromides and iodides) under relatively harsh reaction conditions (especially high temperatures). To make an example, ca. 10 ppb of [Pd(OAc)2] in a N-methylpyrrolidone solution are enough to achieve in 4 h a 90% trans-stilbene yield in the ligand-free Heck reaction of PhBr (99% conversion) with styrene at 140 °C, using Ca(OH)2 as the base.38 Clearly, detecting such low levels of Pd in solution and establishing correlations between their formation and the onset of catalytic activity is not an easy task. (3) The determination of the homogeneous or heterogeneous nature of the catalytically competent species, which has been most often carried out by, for example, quickly filtering off a supported metal catalyst in the course of the reaction and determining whether the reaction continues in solution (“hot filtration test”), does not necessarily tell much about the true nature of the catalytically active species. For example, supported Pd metal may leach molecular Pd species, which, however, depending on the nature of the support and on the reaction conditions (particularly on the solvent) remain largely or even completely adsorbed on the support, where they exert their catalytic function.39,40 In this case, the hot filtration test will be negative in spite of the “molecular” nature of the catalyst. Conversely, leached molecular Pd species in solution can produce soluble Pd nanoclusters/colloids which may act as “quasi homogeneous” catalysts even after filtration, albeit with a “heterogeneous”, surface reaction mechanism (Figure 3). (4) As has been proposed by Schmidt,41 the nature of the catalytically competent species may be dynamic (i.e., the catalytically active Pd species can interconvert and consequently change in the course of the reaction from molecular Pd species to Pd metal). Given the difficulties listed above, which have emerged in the course of the last two decades of research on these mechanisms, the question may be asked whether it is important at all to identify the reaction mechanism(s) underlying these reactions with Pd metal catalysts. Indeed,

each reaction the role as catalytically competent species of leached Pd species, of exposed surface metal atoms, of defect/ edge sites and of surface PdO, highlighting also the influence of Pd metal morphology and of the support. The review will also provide a critical account on the use of Pd metal as catalyst for the title reactions: far from being a comprehensive listing of all Pd metal-based catalysts proposed to date, which would occupy a lot of journal space without being of much use to the scientific community, it will rather concentrate on systems displaying outstanding catalytic performance and on its rationalization, upon considering in particular the plethora of new findings that the new century has witnessed in this fundamental research field. The review will also present the most novel approaches to catalytic systems based on Pd metal for this scope and will conclude with a survey on the application of Pd metal catalysts in industrial processes involving cross-couplings and related reactions.

2. DEBATE ON THE REACTION MECHANISM(S) Mechanistic studies on cross-coupling reactions catalyzed by Pd metal have been carried out in most instances only in the last 17 years (i.e., from the turn of the century onward). The only exception in this regard is the Heck reaction, which was the first coupling reaction to be carried out with Pd metal catalysts and also the first to be investigated mechanistically already in the 1990’s.13,16,17,20,23 Detailed mechanistic investigation on these reactions with Pd metal catalysts is not a trivial issue. The main question that is generally to be answered is where the catalytic event takes place [i.e., at the surface of Pd metal, at defect sites (edges, vertexes) of Pd metal (nano)particles/colloids or on “molecular” Pd species released by Pd metal]. The analysis of these possibilities is further complicated by the following observations. (1) As it has been quite recently pointed out by Ananikov, the answer to the question “where does the catalytic event take place?” can be multiple [i.e., Pd metal (nano)particles and leached soluble Pd species can be both active for the reaction, albeit at different reaction rates and possibly with different reaction selectivities (“cocktail” catalysis)].34,35 In these cases, only careful mechanistic investigations focused on reaction kinetics and selectivities can tell the various 2251


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Figure 4. Correlation between reaction profile (left) and Pd leaching (right) in the course of a typical Heck reaction catalyzed by supported Pd metal (Pd/SiO2). Reproduced with permission from ref 48. Copyright 2000 Wiley-VCH.

reaction temperature is generally prescribed in such standard “hot filtration test”).45,46 On the other hand, Schmidt and Mametova, who studied the kinetics of the Heck coupling of iodobenzene and styrene in DMF at 80 °C, in the presence of NBu3 over a 5% Pd/C catalyst, showed that the kinetic plot was not altered by a hot filtration test removing the catalyst from the reaction mixture right after having reached the maximum apparent reaction rate.47 They also recorded that notable amounts of palladium were leached when Pd/C came in contact with components of the reaction mixture under reaction-like conditions and suggested in particular that leaching was due to the oxidative action of iodobenzene. These findings were later confirmed by thorough studies stemming from a few pioneering groups. In particular, Arai and co-workers48−50 observed that Pd metal catalysts such as a 10% Pd/C and a 1% Pd/SiO2 exhibited extensive leaching in the course of the Heck reaction of iodobenzene and methyl acrylate at 75 °C, using NEt3 as the base and Nmethylpyrrolidone as the solvent. They monitored the Pd concentration in solution throughout the reaction course and confirmed that maximum apparent reaction rate was attained at maximum Pd concentration (Figure 4). Moreover, they also noticed that palladium reprecipitated over the support at high conversion of iodobenzene, the extent of reprecipitation depending on factors including the temperature (at higher T, faster and more complete reprecipitation occurred), the nature of the support (more extensive reprecipitation was obtained on carbon rather than on silica), and in a more complicated and unpredictable way, the nature of the base. The nature of the solvent was also found to impact the location and speciation of Pd after reaction: for example, the addition of water as cosolvent was found to favor the formation of large colloidal palladium particles, a few hundred nanometers in size.50 These relatively large colloids did not reprecipitate over the support. They could be nonetheless recovered but proved catalytically inactive in recycling experiments. Such results were shortly thereafter confirmed by the group of Köhler,51 who investigated on the Heck reaction with aryl bromides at higher temperature (140 °C) using Pd/C as catalyst. They also observed Pd leaching and reprecipitation on the support, with extensive restructuring of the Pd nanoparticles, as well as a correlation between maximum apparent reaction rate and maximum palladium concentration in solution. However, whereas Arai interpreted his results in terms of release of Pd(0) species in solution, Köhler favored leaching of Pd(II) species caused by oxidative addition of the organic halide. This mechanistic hypothesis was formulated shortly before by our group, as we observed extensive leaching of molecular Pd species from two different supported Pd metal catalysts upon treatment with excess aryl bromides and even

there are very good reasons for further pursuing this task in the 21st century. First of all, this is a general problem that involves not only Pd metal catalysts but all kinds of Pd-based catalysts, including putatively homogeneous ones (Pd salts and complexes) that may well undergo decomposition in situ upon formation of catalytically competent Pd metal nanoclusters/colloids. Incidentally, this issue emerges also in several other metal-catalyzed reactions, and in fact numerous advanced methodologies and techniques have been proposed to investigate it, which have been very extensively reviewed in recent times;42,43 examples of the application of such techniques to the study of Pd metal catalysts in cross-couplings and related reactions will be highlighted in the subsequent sections. Further, it is quite obvious that rational catalyst optimization cannot leave apart a detailed knowledge of the reaction mechanism and of the actual catalytically active species: only after these have been identified will it be possible to devise means to further advance in the development of more efficient catalytic systems. As it will be apparent in the subsequent sections, examples of Pd metal catalysts exhibiting outstanding catalytic performances in these reactions have indeed been reported in the recent past, hence there is an acute need to rationalize these performances. Knowledge of the true catalytically active species and of their mechanism of action is important also for establishing intellectual property rights on new catalytic systems to be patented. Finally, there is a strong academic interest in evaluating the possibility for rather complex organometallic reactions to be carried out on Pd nanoparticles (and not in the coordination sphere of molecular Pd species) and more generally in determining the behavior of Pd metal nanoparticles under reaction conditions, since it is by now generally accepted that such nanoparticles can undergo extensive modifications even under mild conditions.44 As already mentioned in the Introduction, mechanistic investigations on these reactions with Pd metal catalysts were initially confined to the Heck reaction. Mizoroki, already in 1973, gathered the first evidence supporting the view that with palladium black as (pre)catalyst, the reaction was actually promoted by leached soluble palladium species; for example, he noticed that the reaction rate was independent of the amount of employed Pd black.9 During the 1990’s, contradictory reports on the release of Pd species into solution by Pd metal catalysts for the Heck reaction appeared in the literature. Augustine and O’Leary investigated the reaction between 4substituted aroyl chlorides and butylvinylether and observed no change in the composition of the reaction solution in 1,4dioxane after separation by filtration of the Pd/C catalyst at around 20% conversion (though it needs to be remarked that the reaction was carried out at 100 °C, whereas filtration was performed at room temperature; catalyst separation at the 2252


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chlorides (in the former case even at room temperature!);52 leaching was further promoted by the presence of coordinating bases (e.g., acetate anions) and produced catalytically competent Pd(II) species for the Heck reaction. Further support to this hypothesis was provided by Reetz and Westermann, who observed that Pd colloids stabilized by tetraoctylammonium bicarbonate reacted with stoichiometric amounts of PhI to quantitatively give Pd(II) species, and that these soluble Pd species underwent Heck reaction with styrene, in the presence of NaOAc to afford stilbene quantitatively.53 Actually, Reetz and Westermann favored a reaction mechanism taking place at the surface of the Pd colloids, and indeed such a mechanism cannot be completely excluded by their study, since it has been later shown that even in cases in which Pd nanoparticles are completely destroyed by reaction with excess aryl halide, carrying out cross-coupling or related reactions under the same reaction conditions may lead to the establishment of a “steady state” between Pd nanoparticle dissolution and reformation, preserving the presence of Pd nanoparticles throughout the catalytic event; examples of this behavior will be reported in the following sections. In the same years, though, studies providing evidence in favor of a surface reaction mechanism continued to appear. For example, working with PVP-stabilized Pd colloids the group of Bradley was able to establish a direct proportionality between the initial reaction rates of Heck reactions and the number of defective sites on the metal surface, rather than the total metal surface area or the total amount of palladium introduced, which was interpreted on the basis of a reaction taking plase at these defect sites.54 It must be however borne in mind that this finding is not a conclusive proof of the active catalytic role of defect sites on the Pd nanoparticles: such observation can be well explained also by assuming that Pd atoms located at defective sites (which are surely the most reactive on the Pd nanoparticle surface) are more prone to leaching and are more easily brought into solution, where they exert their catalytic action as molecular Pd complexes.13,18 Other cross-coupling reactions, especially the Suzuki reaction, have been the object of significant mechanistic investigations only in the 21st century. However, in spite of its later start, the debate on the mechanism of these reactions substantially mirrored the one on the mechanistically related Heck reaction, at least in the first years of the new century. For example, the group of Sowa reported on the activation of aryl chlorides for the Suzuki reaction at relatively mild temperature (80 °C, K2CO3 as the base) by ligandless Pd/C without further additives.55 They initially suggested a surface reaction mechanism taking place on Pd metal nanoparticles but did not collect any evidence for it; however, in a later publication devoted to the Suzuki coupling of an aryl bromide en route to the synthesis of a pharmaceutical intermediate, they demonstrated that under otherwise identical reaction conditions the catalyst released catalytically competent species in solution, in a very similar way to that reported with catalysts of the same kind for the Heck reaction.56 The group of El-Sayed investigated Suzuki couplings using as catalysts palladium colloids stabilized by poly vinylpyrrolidone (PVP) or by dendrimers.57,58 Similarly to LeBlond et al. for the Heck reaction, they were able to establish a direct proportionality between the initial reaction rates of these reactions and the number of defective sites on the metal surface, rather than the

total metal surface area or the total amount of palladium introduced. In later years, several mechanistic studies of this kind have appeared in the literature. In most instances, though, they have dealt with a single cross-coupling reactions, most notably Suzuki and Heck reactions, and only occasionally to other cross-couplings and related reactions. The most notable result of these studies is that subtle differences in the reaction mechanism are sometimes apparent, depending on both the peculiar reaction to be investigated and the reaction conditions; consequently, extreme care should be taken in transferring the conclusions of a mechanistic study of this kind to another reaction or even to other reaction conditions. Therefore, these investigations will be discussed in the sections devoted to the single reactions. In the remaining part of this section, we shall instead concentrate on those few studies which have nevertheless collected information on a more or less general mechanism underlying these reactions and that can therefore be taken as relevant to most or even all reactions that concern this review. In 2006−2007, the group of Rothenberg published the results of a thorough study on the reaction mechanism underlying Heck and cross-coupling reactions.59,60 They made use of a two-compartment membrane reactor, equipped with mesoporous alumina membrane able to retain Pd nanoclusters of size greater than about 5 nm (Figure 5). By subjecting

Figure 5. Concept of the two compartment reactor employed for the leaching studies of ref 59.

preformed metal nanoclusters of 14 ± 3 nm in size to the action of different reaction components on one side of the membrane, it was possible to detect in the other side of the membrane the formation of small Pd species that were capable of crossing the membrane. In this way, they were able to demonstrate the presence of two leaching mechanisms forming small Pd species in solution, namely oxidative addition of the aryl halide to sites at the nanocluster surface leading to organometallic palladium(II) complexes, and formation of palladium(0) atoms/very small clusters (undetectable by TEM) under nonoxidizing conditions. They also incidentally observed that the nature of the solvent influenced to a certain degree the extent of leaching with the two mechanisms. However, they did not assess the correlation of leaching by either mechanism with the onset of catalytic activity. This gap was filled in 2013 by a comprehensive study by Leyva-Pérez et al. on the nature of catalysts for cross-coupling reactions,61 which demonstrated that at a reaction temperature of 135 °C in N-methylpyrrolidone as reaction solvent, several Pd-based catalysts (palladium(II) complexes, palladium(II) salts, Pd nanoparticles, and Pd on carbon) all produce Pd nanoparticles in solution, from which low amounts of catalytically competent species are leached upon oxidative addition of the aryl halide. Most notably, upon addition of small amounts of hard bases 2253


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the development of catalytic systems featuring unusually high catalytic efficiencies.

such as water or cyclohexylamine, a sharp increase in catalytic activity is observed, which correlates with the more extensive release into solution of small Pd nanoclusters 3−4 atoms large. Thus, the results by Levya-Perez et al. confirm previous findings in that leaching of palladium(II) species by oxidative addition of the organic halide reagent is not the only mechanism available to Pd leaching and that palladium(0) species can also leach, particularly in the presence of hard Lewis bases. Furthermore, these leached species, and particularly the small Pd nanoclusters, are competent, highly active catalyst for Heck and cross-coupling reactions at high temperature. This incidentally explains the common observation that the additions of small amount of water promotes the catalytic performance of the systems and also results in the formation of large Pd colloids in solution rather than in reprecipitation of Pd metal on the support. Concluding this section, it can be quite safely stated that when cross-coupling and related reactions are run at temperatures higher than about 120 °C, there is a high probability that the reaction is actually carried out by soluble palladium species, which form predominantly upon oxidative addition of the aryl halide to defect sites on the Pd metal nanoparticles in the absence of nucleophiles or other species able to stabilize palladium(0), predominantly upon leaching of palladium(0) atoms or very small palladium(0) clusters (3−4 atoms) when nucleophilic species such as water, primary amines, etc. are present. Of course, the situation for reactions carried out at lower temperatures might be different, as well as for reactions with aryl chlorides, which are not activated under these conditions. Furthermore, under milder conditions, or with aryl chlorides as substrates, the efficiency of the catalytic process critically depends on the peculiar cross-coupling reaction under study. Suzuki reactions tend to be the most facile, whereas Heck and Sonogashira reactions are certainly more demanding. As will be apparent in the following sections, under these conditions there is a chance that a heterogeneous reaction mechanism can indeed be in operation.

3.1. Mechanistic Studies

A few interesting mechanistic studies on the Suzuki reaction have been conducted using macroscopic forms of Pd metal such as foil, wire, and sponge. First, it has been demonstrated that 0.25 mm thick Pd foil promotes the formation of 4-nitro1,1-biphenyl starting from 1-iodo-4-nitrobenzene and phenylboronic acid pinacol ester in DMF, using NEt(i-Pr)2 as the base.64 The Pd foil leached catalytically competent Pd species into solution during reaction, resulting in “pitting” on the surface of the foil visible by SEM. Furthermore, after reaction the foil was found by XPS analysis to be covered with a layer containing oxygen, nitrogen, and iodine beside palladium; on the basis of the XPS data, such a layer was proposed to contain PdO as well as products of oxidative addition of 1-iodo-4nitrobenzene to Pd. Incidentally, more recently researchers working on Pd nanoparticles supported on carbon nanotubes as catalysts for Suzuki reactions observed formation of quite the same kind of layer on their nanoparticles after Suzuki reaction of iodobenzene with phenylboronic acid in DMF at 60 °C using K2CO3 as the base.65 By adopting a sophisticated tailor-made microreactor which allowed local heating of a small area of the Pd foil without significant heating of the rest of the foil and of the bulk reaction solution, it was also demonstrated that Pd leaching was followed by redeposition of the metal, mainly on the cooler areas of the heated reactive section of the foil, thus providing a visual demonstration of the leachingreprecipitation mechanism (see section 2).64 Some years later, this topic was revisited, and the study was extended to Pd wire (0.25 mm diameter) and Pd sponge.66 The reaction between 1-bromo-4-nitrobenzene and 4-methylphenylboronic acid, run at 60 °C in ethanol/water mixtures in the presence of KOH, gave quantitatively the coupling product, and the reaction times were almost independent of the physical nature of the metal. Furthermore, the wide scope of the protocol was confirmed by combining different organic precursors. The results of standard tests to determine the nature of the catalytically competent species, such as the previously mentioned “hot filtration test”, the “three-phase test” (a catalytic trial run in which one of the substrates is bound to an insoluble solid support, so that only soluble Pd species will be able to reach it and effect the coupling),18,39,40 along with the “chelation test” [a catalytic trial run in the presence of an excess of a chelating ligand anchored on a solid support and able to sequester both molecular Pd(0) or Pd(II)], confirmed that the true catalyst is a soluble form of leached palladium. Selected SEM images of the surface of Pd foil are reported in Figure 6. As can be clearly seen, before its use in catalysis (Figure 6a), the untreated surface shows some small and

3. PD METAL CATALYSTS FOR THE SUZUKI-MIYAURA REACTION The Suzuki-Miyaura reaction (Scheme 1), hereafter termed for sake of brevity Suzuki reaction, is undoubtedly the most Scheme 1. Suzuki-Miyaura Reaction

intensively investigated C−C coupling reaction, particularly in the 21st century.62,63 It is also the cross-coupling reaction that has the most important technological significance, in view of the fact that it employs as organometallic reagents organoboron derivatives that are nontoxic, air- and moisture tolerant, and (by now) widely available in great variety and at a reasonable cost; about 1800 compounds (boronic acids, boronate esters, and MIDA boronates) are currently commercially available. Studies on Pd metal catalysts for cross-couplings in the 21st century have also mostly targeted the Suzuki reaction and have involved new investigations on the reaction mechanism and

Figure 6. SEM images of Pd foil (a) before use and (b) after 4 consecutive uses (the side of the square is 500 μm). 2254


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cuboctahedral NCs > octahedral NCs. For instance, the conversion at the end of the reaction (after ca. 30 h) was 94, 78, and 58%, respectively. Notably, with the less reactive octahedral NCs, the catalytic reaction started after ca. 5 h induction time. Interestingly, it was also observed that the reaction profile was the same when NC cubes of two very different sizes (10 and 20 nm) were employed in such a relative amount to make equivalent the total number of surface Pd atoms. This means that catalytic effectiveness is associated with the shape of the crystal rather than the numbers of the edge and corner sites. TEM analysis, corroborated by ICP measurements, revealed that after catalysis the morphology of all NCs was detectably modified following the release of metal atoms into the solution phase. It is worth noting that the most evident erosion (the highest Pd concentration in the filtrate) was observed for the most reactive cubic NCs. Thus, the leaching of palladium atoms from the {100} surface with respect to the {111} one is more conspicous. Under inert atmosphere (Ar), the amount of leached metal was comparatively much lower and similar for all three types of NCs. Therefore, the presence of molecular oxygen is crucial for having a high catalytic activity correlated with the shape of the nanocrystal. In this regard, the complete absence of coupling reaction within 1 day when all operations were conducted under argon atmosphere is quite remarkable. The role of O2 is the formation of PdO on the surface of the NC, as confirmed by XPS analysis. The oxidized palladium atoms are more easily released into the solution phase and, at the same time, can directly undergo transmetalation with the arylboronic acid. In fact, it was observed that both the base and the arylboronic acid act as primary leaching reagents, while the contribution of the aryl halide is less significant. With regard to cross-coupling and related reactions, XAS (Xray absorption spectroscopy) techniques are powerful and promising tools for the characterization of metal catalysts.71,72 Being element-specific, XAS allows the selective investigation of the Pd-containing species, getting rid of interferences coming from the environment (support and/or reaction mixture). In addition, XAS can be exploited under actual reaction conditions and can be successfully applied to both heterogeneous and homogeneous systems, making possible in situ studies, differently from laboratory techniques requiring ultrahigh vacuum such as electron microscopy and XPS. Furthermore, these techniques can be applied to concentrated samples in the transmission mode, whereas fluorescence experiments are suitable for highly diluted and nonhomogeneous systems. Moreover, differently from XRD and NMR, XAS also allows the investigation in solution of metal species with short-range order and fine nanoparticles. Finally, Quick EXAFS makes possible time-resolved investigations of the coupling process, by probing the reaction system on the subsecond scale. Consequently, it is not surprising that in recent years a small but growing number of XAS studies regarding these reactions has appeared in the literature, which will be discussed in this and in the subsequent sections. As to Suzuki cross-coupling, a recent investigation with operando XAS techniques by Brazier et al.,73 although not providing a direct insight on the mechanism, supports the established evidence for this reaction. The authors studied the reaction of 4-bromoanisole with 4-fluorophenylboronic acid (in water/ethanol 1:1 solution, 1 equiv. K2CO3), at 75 °C, in the presence of 4.5% Pd/Al2O3 and Pd/Encat 30 NP (a polyurea-encapsulated commercial form of Pd) as the catalysts,

randomly distributed cavities, generally not larger than 100 μm, which appear as dark spots. EDXS measurements indicates that within these cavities the metal is prevalently present as oxide, the rest of the smooth surface being almost pure palladium (Pd > 96%). After four consecutive uses in catalysis, the black spots disappeared (Figure 6b) and the almost complete absence of oxygen inside the residual cavities was confirmed by EDXS analysis. As this sample showed negligible catalytic activity, it is unequivocally demonstrated that the effective catalytic species in solution arises from Pd2+ ions released by amorphous PdO present on the foil surface. Analogously to the foil, the surface of Pd wire is spangled with several deep holes containing prevalently PdO, while the rest of the surface analyzes as almost pure palladium (Pd > 95%). Contrarily to the foil, the wire showed still catalytic efficiency after eight consecutive uses, although after the sixth run the product yield dropped down considerably. This may be reasonably ascribed to a higher PdO content of the wire with respect to the foil. Finally, it is worthy noting that the oxygen content on the surface increases on heating the Pd wire at 900 °C for 24 h on air, but the catalytic activity remained practically unchanged. This is due to the formation of crystalline PdO, since lower amounts of Pd2+ are leached into the solution phase from crystalline PdO under otherwise identical reaction conditions.67−69 To confirm the unique role played by amorphous PdO, the catalytic activity of a sample of Pd wire was completely suppressed by warming it at 100 °C under a stream of hydrogen, which reduces PdO to metallic palladium. Interesting mechanistic insights on the Suzuki reaction have also emerged from the study of tailored Pd nanostructures. For example, the studies of Holmes, McGlacken, and co-workers constitute a breakthrough for the knowledge of how crystalline palladium metal nanoparticles catalyze the Suzuki coupling.70 In particular, the origin of shape sensitivity and the role of molecular oxygen have been well-outlined. Cubic, cuboctahedral, and octahedral Pd nanocrystals (NCs) displaying {100}, {111}, and both (6 {100} and 8 {111}) surface facets, respectively, with comparable size (about 20 nm) were produced and supported on activated carbon (Figure 7). Employed as catalysts in the room temperature coupling between 4-bromoanisole and phenylboronic acid in 1:3 water/ ethanol, with K2CO3 as the base, the three types of NCs showed very different reactivity: the order was: cubic NCs >

Figure 7. Different types of Pd nanocrystals employed as catalysts for Suzuki coupling. Reproduced with permission from ref 70. Copyright 2014 Wiley-VCH. 2255


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by using a suitable plug flow reactor, specifically designed for these measurements. “Single pass experiments” were performed by determining the conversion of 4-bromoanisole versus time, flowing the reaction mixture through the catalytic bed of pristine and pretreated (0.05 mL min−1 50% aqueous ethanol, 80 °C, 18h) catalysts at a relatively high flow (0.05 mL min−1) to obtain a relatively low single pass conversion (99% selectivity) yield (%)/t (h) R

thermal heating

MW heatingb

H NO2 MeC(O) HC(O) PhC(O) Me HOCH2 MeO

98/30 95/10 97/15 98/15 93/15 96/30 92/30 76/40

95/0.5 96/0.5 93/0.5 95/0.5 91/0.5 92/0.5 90/0.5 80/1

a

Chloroarene (1 mmol), olefin (1.2 mmol), LDH−Pd0 (3 mol %), [NBu4]Br (10 mmol), and tri-n-butylamine (1.2 mmol), 130 °C. b MW 400 W, 130 °C. 2274


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release of the parent aryl halides was observed, it was argued that these fragments arose from the decomposition of ArPdX moieties. Accordingly, the XPS spectra of the materials clearly showed the presence of PdII after the treatment (and also after the reactions). Nevertheless, the XPS spectra of the fresh catalysts were not reported, hence it is not possible to know if PdII was already present before the treatment (or the reaction) and, in this case, if the PdII/Pd0 ratio changed. In a later extension of this study,80 Choudary and co-workers isolated the materials obtained from the reaction of LDH-Pd with chlorobenzene or 4-chloroanisole and characterized them again with XPS and TG-MS. Also in this case, XPS showed the presence of PdII after the treatment (but again no XPS data were provided for fresh LHD-Pd0) and TG-MS showed the release of organic fragments compatible with the nature of the chloroarene but not the chloroarene itself. More importantly, these isolated intermediate materials were used as reagents for a number of coupling reactions (Heck, Suzuki, Sonogashira, Stille), which afforded their typical products. This confirmed rather convincingly that the organic fragments were likely not coming from the aryl halide but from ArPdX moieties and that such moieties were able to act as “single site” catalytic species. However, the hypothesis that ArPdX moieties were formed on the metal surface and were never detached from there is not the only one compatible with the experimental data: they could have been formed or leached into the liquid phase as a part of a homogeneous catalytic cycle and later recaptured by the solid. In this case, it cannot be ruled out that they were readsorbed on the support surface rather than on the metal’s. As to TEM analysis, 79 the authors reported no difference in the morphologies and the sizes of the metal nanoparticles of the fresh catalyst and of the used catalyst. However, no micrographs were provided. The catalyst was recycled five times in the reaction of chloroanisole (Figure 23), and there is a clear, albeit modest, steady decrease of the yield from one catalytic run to the next. During the reaction of 4chloroanisole, the concentration of palladium in solution was found to pass through a maximum at ca. 25% conversion. This peak in palladium leaching corresponded to 4.5% of the total palladium amount. It then declined to less than 1% at the end of the reaction. In a control experiment, a peak of palladium leaching corresponding to 10% of the total metal amount was found also for the coupling of iodobenzene under the same conditions. Similar trends of the metal leaching during the reaction were also found by Pröckl et al.159 They showed that leaching and catalytic activity were strictly connected to each other when palladium(II) ion exchanged NaY zeolites were used as the precatalysts. Finally, the split test was negative: according to the reported data, the reaction in the liquid phase stopped after the separation of the catalyst. Taking into account that [NnBu4]Br melts at 103 °C and that the authors mention a “simple filtration”, filtration should have been carried out at a higher temperature, but no experimental details on the split test were given. Even neglecting the criticism by Schmidt and Kurokhtina148 to tests which imply the separation of different potentially active species, it is difficult to evaluate the reliability of this test. At any rate, the authors argued “that the Heck olefination of 4-chloroanisole occurs exclusively and with iodobenzene predominantly on the surface of the nanopalladium particle, presumably at defect sites”,79 in spite of the not negligible leaching of the metal. In concluding that the catalyst was actually a heterogeneous one, they attributed its remarkable ability to activate chloroarenes to the basic

(MW) heating conditions, the reactivity of the chloroarenes was leveled off, with the exception of (4-MeO-C6H4)-Cl which still required a longer time to afford a lower yield. The authors addressed the issue of the heterogeneity of the catalyst with the methods that were considered the most appropriate at the time: recycling tests (Figure 23), assessment

Figure 23. Recycling of LDH-Pd0 in the Heck reaction of 4chloranisole with styrene (a: MW heating; b: conventional heating; see footnotes to Table 6 for experimental conditions). Reproduced with permission from ref 79. Copyright 2002 American Chemical Society.

of leaching in solution during the reaction, split test (hot filtration tests, Figure 24), ex situ characterization of the catalyst after duty (XPS, TG-MS, and TEM). After the treatment with solutions in NBu4Br of either iodobenzene at 75 °C or 4-chloroanisole at 130 °C, organic fragments such as C6H5 or MeOC6H4 were observed, respectively, in the ex situ TG-MS analysis (a sort of TPD) of the precatalyst. As no

Figure 24. Conversion and palladium leaching during the Heck reaction of 4-chloranisole with styrene over LDH-Pd0 (a and c) see footnotes to Table 6 for experimental conditions) and split test (b). Reproduced from ref 79. Copyright 2002 American Chemical Society. 2275


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and n-butylacrlate as the coupling partners. The authors dwelled upon the heterogeneity of the catalyst, which was assayed with the mercury test, the hot filtration test, the threephase test, the solid-phase poisoning test (scavenging of soluble palladium species with insoluble solids bearing mercapto groups) and the recycling of the catalyst. From the results of the tests, they concluded that the catalyst was heterogeneous. However, the chemoselectivity of the Heck reactions reported by Jana et al. for their catalyst is close to that generally observed with soluble precatalysts. In fact, the catalyst was fully chemoselective toward the Heck reaction, with 100% stereoselectivity toward the E isomer of stilbenes and of n-butylcynnamates; in the case of styrene couplings, the formation of small amounts of 1,1-diphenylethylene was also reported. As argued above, this selectivity pattern is a strong clue that the reactions were not heterogeneous, in spite of the results of the heterogeneity tests mentioned above. The specific activity of palladium was good even with chlorobenzene, but in view of the high ArX/Pd initial molar ratio (2.67 × 104; see ref 168 for a comment on specific activities), this could be connected to a “homeopathic effect”,17−21 which strengthens the hypothesis that this catalyst was anyway working on a palladium release-redeposition basis. To the general criticism raised by Schmidt and Kurokhtina148 to tests of the kind used by Jana and co-workers, we can add some remarks as a specific comment to their paper. First of all, the simple retention of the “activity” upon recycling cannot be any longer considered as a reliable proof of heterogeneity. On the one hand, this argument can be impaired by the presence of a release-redeposition mechanism or by the homeopathic action of tiny amounts of palladium in solution. On the other hand, if the results of the recycling experiments are reported as final yields at the same, fixed reaction time for each run, the conclusion that the catalyst retained its activity based on this observation might be wrong, as pointed out above also for the Suzuki reaction. In fact, the same final yield could be always achieved in recycles even in the case of the progressive deactivation of the catalysts from one run to the next provided the fixed reaction time is long enough. This is especially true if the yield is close to quantitative, a common circumstance with aryl iodides, which are most often used in recycling experiments. This can be highly rewarding for the authors because it allows them to report of very high and unaltered yields in a number of consecutive runs (even a great one) but has little significance from the point of view of the assessment of the catalyst activity and stability. As a matter of fact, the sound assessment of the activity requires the measurement of reaction rates: this can be done reliably only collecting kinetic plots, which should be therefore used also in the evaluation of the catalyst stability upon recycling. As to mercury poisoning, it was carried out at room temperature before starting the reaction. Mercury was still present in the reaction mixture during the coupling, which was completely quenched. There are two circumstances that cast doubts on the relevance of these findings. In the first place, the palladium nanoparticles were supposed to be present inside the mesopores of MCM-41, which are pretty narrow (the reported average diameter is 2.03 nm). To form the alloy with them, mercury had therefore to be divided into particles smaller than 2 nm, and it seems unlikely that this could be accomplished simply by stirring the mixture. Second, in the case of leaching during the reaction, mercury could form the alloy also (or

nature of the support. The putative heterogeneous catalyst would benefit from increased electron density transferred from the support to the active metallic phase. However, it is our opinion that the pieces of evidence provided by Choudary and co-workers are not compelling proof of a heterogeneous reaction, especially in view of the current knowledge on the putative heterogeneous Heck reaction. In addition, the reported chemo- and regioselectivity are very much like those observed with soluble precatalysts. This is another hint that LDH-Pd0 is more likely than not the precursor of a soluble catalyst. In the past decade, interest into the activation of chlorides was not higher than in the past, at least as far as the use of supported palladium nanoparticles as the precatalyst is concerned. As already argued by Köhler et al. in 2006,16 the Heck reaction is now mainstream chemistry and most of the results concerning supported palladium are published to highlight new materials rather than to address specific issues of the reaction, either mechanistic or synthetic. In this sense, the Heck reaction came up as a fashionable alternative to hydrogenation as a model reaction, and standard substrates and conditions (most often “facile” ones) are generally employed. Only occasionally chloroarenes were included in the investigation.160−166 Only in one case the investigation was specifically focused on the activation of chlorobenzene.167 In 2008, Jana et al. published their study of the Heck reaction with palladium supported on MCM-41.160 The catalyst was prepared by incorporating Pd(OAc)2 into the support during its preparation by the sol−gel method. After calcination, the solid was treated with hydrazine to reduce the metal. A critical issue of this catalyst is its very low content of palladium, for which the authors reported 0.008% w/w, which corresponds to only 0.75 μmol g−1. Not surprisingly, the authors were unable to detect signals from palladium in the powder XRD spectrum or nanoparticles in the TEM microphotographs. Nonetheless, they attributed these findings to the good dispersion of the metal. The catalytic results are illustrated in Table 7.168 They were able to activate chlorobenzene too at 100 °C, a temperature which can be considered low for this substrate. The conversions were low and fair, respectively, with styrene Table 7. Heck Reactionsa with Pd/MCM-41160 (4-R-C6H4)−X + CH2CH−R′ → E-(4-R-C6H4)−CHCH−R′ (complete selectivity) X

R

R′

t (h)

conv. (%)

sel. (%)b

I Br Br Br Br Cl I Br Br Br Br Cl

H H NO2 MeC(O) MeO H Hc H NO2c MeC(O)c MeO H

Ph Ph Ph Ph Ph Ph OC(O)nBu OC(O)nBu OC(O)nBu OC(O)nBu OC(O)nBu OC(O)nBu

24 24 12 12 24 24 12 24 10 10 24 24

88 56 96 90 30 15 100 76 100 100 45 45

90 96 95 93 80 100 100 100 100 100 100 100

a

1 mmol ArX, 1.5 mmol alkene, 1.5 mmol NaOAc, 0.05 g cat (0.0375 μmol Pd), 8 mL DMF, 100 °C. b1,1-Diphenylethylene as the side product. c80 °C. 2276


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“yellowish”. Regretfully, no further data nor reference to published work were provided, and it cannot be ruled out that the amount of grafted bromoacetophenone was so small to make the three-phase test irrelevant. In fact, it was carried out as the competitive coupling with styrene of 4-nitrobromobenzene and of the bromide grafted in PBA-MCM-41. The former reacted to 95% conversion and the latter not at all under conditions otherwise similar to those described in the footnote to Table 7. However, without knowing the relative amount of the soluble and of the grafted aryl halide, it is difficult to assess the relevance of this test. Wan et al. reported in 2009 on the Heck and Ullmann reactions of chloroarenes in aqueous medium.167 Their precatalyst was palladium-supported by a mesoporous silica, which was prepared by the sol−gel method in the presence of a soluble phenolic resin. After a final step of carbonization of the phenolic resin, mesoporous black powders formed by silica and carbon were obtained (MSC, pore diameter 6.3 nm) and used as the support for palladium. The precatalyst, Pd/MSC, contained 0.47 mmol·g−1 of palladium, corresponding to 5.0% (w/w). For comparison, precatalysts supported by mesoporous silica (MS), mesoporous carbon (MC), and a mesoporous polymer (MP) were also employed. The latter two were obtained from the same phenolic resin employed for MSC. XPS analysis of the precatalysts showed that palladium was present in the zerovalent state in all of them. The metal nanoparticles in Pd/MS and Pd/MP were relatively large (8.1 and 6.4 nm from the Scherrer analysis of the XRD spectra) and relatively small in Pd/MC and Pd/MSC. For the latter precatalyst, the authors reported a nanoparticles diameter of 3 nm (smaller than the diameter of the mesopores) from the TEM analysis. However, no particle counting was presented, and this value is apparently the result of a not fully quantitative estimation. The metallic phase was also characterized by chemisorption, from which the specific surface area and size of palladium nanoparticles were obtained. The reported values of the nanoparticles diameter are in acceptable agreement with the picture arising from TEM and XRD, but to our surprise, the authors stated that H2 was used in the chemisorption experiments. This is generally not recommended because it is well-known that dihydrogen is not simply adsorbed on the surface of palladium, but it also dissolves in its crystal lattice.170 We therefore suspect that the amount of H2 taken up by the metal phase exceeded the capacity of the surface, and that these measurements of the surface area and of the nanoparticles size were over- and underestimated, respectively. These catalysts were employed in the Heck coupling of chlorobenzene with styrene in water at 100 °C (Table 8).

more likely) with the nanoparticles in solution. Even excluding the possibility of dissolution of Pd0 atoms in mercury and assuming that the nanoparticles are not active, this would lower the catalytic activity. In fact, the reaction of the nanoparticles with the halide would be quenched (i.e., a pathway which helps in keeping the concentration of molecular species in solution high), contrasting the nanoparticles growth. As a matter of fact, scavenging of palladium upon amalgamation of the nanoparticles could lead to the deactivation of the system even in the case of catalysis in solution, hence the observed quenching is not conclusive. In the hot filtration test, the authors simply checked for palladium in solution after separating the catalyst at the reaction temperature upon filtration at 30% conversion and at the end of the reaction. In both cases no metal was found in the liquid phase. However, no data of the residual activity in the recovered solution, after the split of the catalyst at partial conversion, were reported. The substrates and the conditions of the test were not declared, so we can assume that at least the amounts of the reactants, of the catalyst and of the solvent were the same reported in the experimental section. For Pd/ MCM-41, 0.05 g were typically used, corresponding to about 4 μg of palladium. The volume of the liquid phase can be estimated in about 10 mL (8 mL of solvent + the volume of the alkene and of the haloarene). As the consequence, the concentration of palladium in the case of total release would have been 0.4 ppm. With such a low maximum level even a slight perturbation could give a false negative test. Of course, finding no palladium at the end of the reaction is of no relevance with respect to the heterogeneity of the reaction. The authors also reported that the liquid phase remained colorless upon addition of the catalyst. According to them, this qualitatively supports the absence of leaching, but the simple addition of the catalyst to the solvent seems of little relevance; in any case, the assumption that a color change due to the presence of 0.4 ppm (at most) of palladium in solution can be appreciated by sight seems rather optimistic. The very low palladium concentration which can be envisioned in solution before it is scavenged makes troublesome also the interpretation of the results of the solid-phase poisoning tests, which showed no quenching of the reaction. On the one hand, it implies that the gradient of palladium concentration between the liquid and the solid phase of the scavenger was very small, if any. On the other hand, very little amounts of the scavenger were required for the experiments with relatively small S/Pd ratios (for instance the authors reported that only 4.3 mg of a commercial SH-SiO2 scavenger were enough to ensure a S/Pd ratio of 7/1 with 1 g of the catalyst), hence the area of the interface between the liquid phase and the scavenger was very low. Under conditions like these, it cannot be ruled out that the diffusion rate of the palladium species toward the mercapto group of the scavenger was not high enough for an effective competition with the catalytic reaction. In fact, it has already been argued by Richardson and Jones169 that the quenching ability in the Heck reaction exhibited by silica-supported thiols actually reflects the ratio of the rates of palladium scavenging and of the coupling reaction. For the three-phase test, the authors functionalized MCM41 with 3-aminopropyl-triethoxysilane and then condensed the amino groups with 4-bromoacetophenone to form the corresponding grafted imine. As the characterization of this material (PBA-MCM-41), the authors only reported that it was

Table 8. Catalytic Results in the Heck Reaction of Chlorobenzene and Styrenea with Pd/MSC167 catalyst

conv (%)b

sel (%)c

Pd/MSC Pd/MP Pd/MC Pd/MS

27 9 3 97%). After the fifth recycle, the Pd content lowered from the initial 2.38% (w/w) to 2.21% (w/w), corresponding to the loss of ca. 6% of the initial amount of Pd. The authors took into account the possibility of the redeposition of Pd and analyzed the exhausted catalyst with TEM (Figure 26c). Differently from the case of the fresh

The only precatalyst which showed some appreciable activity was Pd/MSC. There was no relationship between the apparent activity and the specific surface area or size of palladium nanoparticles. The yield in E-stilbene is low if compared with other supported catalytic systems in water at 100 °C and otherwise similar conditions (see for instance ref 161, which is commented below). In fact, the yield in Estilbene achieved by substituting chlorobenzene with bromobenzene increased only from 27% to 32%, which indicates that the performance of this catalyst was fair but not exceptional. The authors also reported that the change in the reaction yield observed in a hot filtration experiment and indicated as “100 °C) are generally required; and (iii) only aryl iodides and aryl bromides can be employed as substrates. Very few reports describe activation of aryl chlorides; the best example is provided by the group of Glorius, who achieved activation of aryl chlorides for the selective arylation of benzo[b]thiophenes at the C3 position. The reaction employs however up to 10 mol % Pd/C + CuI as the catalytic system and a reaction temperature of 150 °C.191 In this report, the authors performed several tests in order to demonstrate the heterogeneous nature of the catalyst (mercury poisoning, hot filtration test, three-phase test) which indeed supported their hypothesis. However, the fact that Pd leaching (1000 Ton/year). The catalyst employed for obtaining the key precursor 13 by a Suzuki coupling is Pd(PPh3)4 (Figure 38), and the catalysis is homogeneous. However, the interest for developing more economical and useful catalysts for the production of 12 has led to two patents203,204 describing the use of solid Pd-based catalysts (Pd/C and MS-Pd, MS = molecular sieves) (Figure 38). In the Pd/C catalyzed reaction, different from the other two, the

Compound 2, a key-precursor of 1, is obtained in 91% isolated yield (∼6 kg) at 78 °C after 5 h by a Suzuki coupling starting from a bromoaryl derivative and using about twice the amount of Na2CO3 (Figure 32). The catalyst of choice is a selected

Figure 32. Application of the Suzuki reaction at SmithKline Beecham Pharmaceuticals.

form of Pd/C (1.2 mol %) supplied by Johnson Matthey, and also the solvent plays a crucial role. In fact, the reaction occurs in high yield when it is performed in refluxing MeOH/H2O. On the contrary, no product is obtained in DME/H2O and, surprisingly, in EtOH/H2O as well. Notably, the metal amount in the isolated product is below 6 ppm. It should be stressed that the use of Pd(PPh3)4 as a homogeneous catalyst (Na2CO3, DME/H2O) resulted in quite a lower yield (64%) and a much higher metal contamination of the isolated product (40÷80 ppm). The preparation of kilograms of compound 3, a PDE4 inhibitor useful against asthma and chronic obstructive pulmonary disease (Figure 33), has been developed at Merck Research Laboratories and Merck Frosst Center for Therapeutic Research.201,202 The phenylquinoline intermediate 4 is synthesized by a Suzuki coupling starting from the commercially available boronic acid. The best conditions (yields of 4 up to 92%) involve the use of Pd/C (2.6 mol %),

Figure 33. Application of the Suzuki reaction at Merck. 2285


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Figure 34. Application of the Suzuki reaction at Eli Lilly.

Figure 35. Application of the Suzuki reaction at Solvias.

Figure 38. Application of the Suzuki reaction at BASF.

temperature (190 °C) starting from 2-ethylhexylacrylate and pbromoanisole (Figure 39).205 The catalyst is Pd/C (TON ∼ 6000, TOF ∼ 2000 h−1), and the reaction is run in NMP using Na2CO3 as the base. It has been found that a consistent amount of water (up to 15%) has a beneficial role on the reaction rate. Rilpivirine 16 (TMC 278, Edurant) is a potent nonnucleoside reversed transcriptase inhibitor used, in combination with other antiretroviral agents, for the treatment of HIV infection (approved by FDA in 2011). Its preparation on an industrial-scale (about 1 ton) has been developed by researchers at Johnson & Johnson. The key-intermediate 4-

Figure 36. Application of the Suzuki reaction at Eli Lilly (2).

precursor is 2-iodoaniline which leads to intermediate 14. In this manner, the necessary −NO2 reduction step is avoided. Coming to the Heck reaction, the industrial synthesis of 2ethylhexyl-p-methoxycinnamate 15 (Eusolex 2292, Uvinul MC80), which is the most common UV−B sunscreen, is accomplished by means of a simple Heck coupling at very high

Figure 37. Application of the Suzuki reaction at Eli Lilly (3). 2286


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Figure 39. Application of the Heck reaction to the preparation of 2ethylhexyl-p-methoxycinnamate. Figure 41. Application of the Sonogashira reaction at Medichem.

cyanovinyl-2,6-dimetilaniline 17 is prepared by reacting 4iodo-2,6-dimetilaniline with acrylonitrile in a 1:1.5 molar ratio in DMAc at 140 °C (Figure 40). The best catalyst is 10% Pd/

application field of Pd metal catalysts has been extended to mechanistically related but more complex coupling reactions, such as C−N couplings (Buchwald-Hartwig coupling) and direct C−H arylations. Applications in the Suzuki reaction have been particularly successful and have often resulted in the development of catalytic systems capable of activating aryl chlorides under relatively mild conditions. Furthermore, investigations have been extended to more sophisticated Pd metal based catalytic systems, such as nanoalloys and bimetallic nanoparticles capable of plasmonic activation of the catalytic event. The development of such tailor-made nanocatalysts has however not resulted up to now in a real breakthrough in terms of catalytic performance. To this end, much more important seems to be the proper choice and optimization of the support for the dispersed Pd metal phase. Investigations on the mechanism of these reactions when a Pd metal catalyst is employed were also pursued, and whereas studies on the Heck reaction have set a bit their pace in the last years, after the remarkable number of reports published at the beginning of the new century, research efforts have concentrated more on other cross-coupling reactions, most notably the Suzuki coupling. Such investigations have allowed for the highlighting of previously unknown or not so well-known features of the reaction, such as the increased reactivity displayed by defect sites as well as by certain facets of nanocrystalline Pd nanoparticles, the remarkable stability of Pd nanoparticles throughout the reaction under certain conditions, the oxidation state of leached Pd species (+2, but also 0), and the important role played by surface oxidation of Pd metal and by water as cosolvent. However, these studies have not yet resulted in conclusive proof that the coupling reactions can actually take place at the metal surface, with a “heterogeneous” reaction mechanism. Gathering this proof has proved to be a formidable task; in our opinion, in order to have a chance to get over this hurdle, it is mandatory to target on the one hand model reactions with less active substrates, most notably aryl chlorides, and reaction conditions for which it is known that molecular Pd species leached into solution are inefficient catalysts; up to now, all mechanistic studies that we are aware of have been conducted with aryl bromides or iodides. On the other hand, more sophisticated techniques (e.g., quick EXAFS), which allow for the characterization of the working catalyst during the catalytic event, as well as careful SADS kinetic studies evaluating the differential selectivity of the reaction, need to be more extensively applied.

Figure 40. Application of the Heck reaction at Johnson & Johnson.

C wet (0.5 mol %), and the base of choice is sodium acetate (1.2 equiv). In comparison with homogeneous catalysts (Pd2(dba)3/P(t-Bu)3 or Pd(OAc)2/P(o-tolyl)3), Pd/C gives a comparable yield but a much less residual metal contamination of 17. It should be noted that 0.1 mol % catalyst is enough to achieve complete conversion, even though 0.5 mol % is instead used in the plant (250 kg scale) to ensure robustness. Compound 17 is obtained in 81% yield as a 1:4 mixture of Z/E isomers (with C=O function promoted by Ru(II) and Os(II) complexes, the Pd-catalyzed cross-coupling reactions and, more recently, reactions catalyzed by Au(I) complexes. Marco Zecca graduated in Industrial Chemistry in 1986 at the University of Padova under the guidance of Prof. Benedetto Corain. He was awarded the “Silvio Bezzi” Gold Medal as the best graduate in Chemistry and Industrial Chemistry of his year. In 1990, he was appointed Assistant Professor by the University of Padova. Since 2002, he has been Associate Professor of Inorganic Chemistry. His scientific activity has been mainly focused on the catalytic applications of transition metals and of cross-linked organic polymers. He is currently working on polymer-supported metal catalysts for the direct synthesis of hydrogen peroxide and on polymer-based catalysts for the transformation of biomass-derived intermediates into valuable chemicals.

AUTHOR INFORMATION Corresponding Author

*E-mail andrea.biffi[email protected]. ORCID

ACKNOWLEDGMENTS Financial support from the University of Padova (PRAT 2012 to P.C. and P-DiSC 2016 to M.Z.) is gratefully acknowledged.

Andrea Biffis: 0000-0002-7762-8280 Paolo Centomo: 0000-0002-0124-9284 Notes

The authors declare no competing financial interest.

DEDICATION Dedicated to the memory of our mentor and friend, Benedetto Corain (1941−2014).

Biographies Andrea Biffis graduated in Chemistry (honors) at the University of Padova (with Prof. Benedetto Corain) and received his Ph.D. in 1998 from the University of Düsseldorf, Germany (with Prof. Günter Wulff). After a postdoctoral stay at the University of Essen, Germany (with Prof. Günter Schmid), he returned to Padova, where he was appointed assistant professor in 2001 and associate professor in 2011. Prof. Biffis has at present coauthored more than 90 ISI publications and 6 chapters in monographs. The research interests of Prof. Biffis deal mainly with the development of late transition metal catalysts (metal nanoparticles, metal complexes) for the synthesis of fine chemicals, as well as with strategies for catalyst recovery and recycling, employing biphasic systems or soluble crosslinked polymers (microgels) as catalyst support.

REFERENCES (1) Johansson Seechurn, C. C. C.; Kitching, M. O.; Colacot, T. J.; Snieckus, V. Palladium-Catalyzed Cross-Coupling: A Historical Contextual Perspective to the 2010 Nobel Prize. Angew. Chem., Int. Ed. 2012, 51, 5062−5085. (2) Metal Catalyzed Cross-Coupling Reactions and More, 3 Vol. Set; De Meijere, A., Bräse, S., Oestreich, M., Eds.; Wiley-VCH: Weinheim, 2014. (3) Cross-Coupling Reactions, Workbench ed.; Molander, G. A., Larhed, M., Wolfe, J., Eds.; Science of Synthesis Series; Thieme Chemistry: Stuttgart, 2013. (4) Palladium-Catalyzed Coupling Reactions: Practical Aspects and Future Developments; Molnár, Á ., Ed.; Wiley-VCH: Weinheim, 2013. (5) Li, H.; Johansson Seechurn, C. C. C.; Colacot, T. J. Development of Preformed Pd Catalysts for Cross-Coupling Reactions, Beyond the 2010 Nobel Prize. ACS Catal. 2012, 2, 1147−1164. (6) Cross Coupling Reactions in Organic Synthesis Themed Issue. Chem. Soc. Rev. 2011, 40, 4877−5208.10.1039/c1cs90039k

Paolo Centomo graduated in Chemistry in 2002 and obtained his Ph.D. in Chemistry in 2006 at the University of Padova under the supervision of Prof. Benedetto Corain. In the same year, he obtained a postdoc position (assegno di ricerca, extended in 2008 and 2009) at the Department of Chemical Sciences of the University of Padova. In 2010, Dr. Centomo was appointed assistant professor of the Deparment of Chemical Sciences of the University of Padova. The 2289


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