Tetrahedron: Asymmetry 21 (2010) 1041–1054
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Tetrahedron: Asymmetry journal homepage: www.elsevier.com/locate/tetasy
Tetrahedron: Asymmetry Report Number 126
Palladium catalysis using dendrimers: molecular catalysts versus nanoparticles Didier Astruc ISM, UMR CNRS No. 5255, Université Bordeaux 1, 3305 Talence Cedex, France
a r t i c l e
i n f o
Article history: Received 10 March 2010 Accepted 19 April 2010 Available online 22 June 2010 Dedicated to our distinguished colleague Professor Henri Kagan at the occasion of his 80th birthday
a b s t r a c t Dendritic Pd catalysts, dendrimer-stabilized Pd nanoparticle (PdNP) catalysts, and their comparison and combined use for carbon–carbon coupling reactions are discussed with emphasis on the research carried out in the author’s laboratory during the last decade. Multinuclear star-shaped catalysts rather than dendritic catalysts can reach the efficiency of the best monometallic catalysts, whereas PdNPs stabilized by dendrimers can react with turnover numbers close to 106 and bring useful mechanistic indications. In both areas, leaching issues are examined. Finally, results of the literature in asymmetric Pd catalysis by chiral dendrimers and Pd nanoparticles stabilized by chiral ligands are also reviewed, revealing the importance of the dendritic and molecular ligand design and the role of leaching Pd atoms. Ó 2010 Elsevier Ltd. All rights reserved.
Contents 1. 2. 3. 4. 5. 6. 7. 8.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dendritic palladium molecular catalysis and Brunner’s concept of dendrizyme . . . . . . . . . . . . . . . . . Palladium nanoparticle (PdNP) catalysts in carbon–carbon coupling reactions . . . . . . . . . . . . . . . . . . Polymer- and PAMAM dendrimer-stabilized palladium nanoparticle pre-catalysts . . . . . . . . . . . . . . Highly efficient ‘click’-dendrimer-encapsulated and -stabilized Pd nanoparticle pre-catalysts . . . . . ‘Homeopathic’ catalysis of Suzuki–Miyaura C–C coupling by ‘click’ ferrocenyl dendrimer-stabilized evidence for a leaching mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Asymmetric catalysis: metallodendritic catalysts vs. nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Palladium catalysts are the most frequently used catalysts in synthesis.1 As with other catalysts, the most important problems are the cost related to the catalyst efficiency including turnover number of the catalyst (TON), turnover frequency (TOF), and removal of the catalyst from the reaction mixtures for both economic (catalyst recycling) and ecological reasons (prevent pollution of the reaction product by the catalyst).2–4 Chemoselectivity, regioselectivity, stereoselectivity, enantioselectivity, and diastereoselectivity, optimized with homogeneous catalysts, are the other key issues.5 Whereas research on molecular complexes in homogeneous catalysis1–5 remains a major source of mechanistic understanding6 of the key structural parameters (a concept pioneered in asymmetric catalysis by Henri Kagan with rhodium complexes7,8), the comE-mail address:
[email protected] 0957-4166/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetasy.2010.04.062
................................ ................................ ................................ ................................ ................................ PdNPs under ambient conditions and ................................ ................................ ................................ ................................ ................................
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munity involved with heterogeneous catalysis has introduced significant progress with the controlled design of metal nanoparticles and single-site catalysts on surfaces.9 The community of macromolecular chemists has proposed perfectly defined supports with metallodendritic catalysts. A variety of organic transformations are involved, from carbon–carbon coupling reactions and allylic substitution to oxidation reactions.10–18 Here we wish to focus on the comparison and interplay between palladium nanoparticles and dendrimers in palladium catalysis. Dendritic supports (Fig. 1) have appeared during the last two decades as an attractive means of combining the possibilities of catalyst recovery and recycling with the advantage of well-defined molecular catalysts.19–34 As this problem of the catalyst support is crucial, we will also compare (and also sometimes combine) dendritic supports and inorganic supports, the supported catalyst or pre-catalysts being a molecular palladium complex or a palladium nanoparticle.
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POLYMER SUPPORT Figure 1. Various possibilities of location of catalysts in dendrimers (circles).
O
O
O
O
O P
O
O
P
P
O
P
O
O
O
O
O O
O
O
Figure 2. Brunner’s dendrizyme model ligands in 1993.20
2. Dendritic palladium molecular catalysis and Brunner’s concept of dendrizyme Seminal results were obtained at Shell by van Leeuwen who patented the catalysis of CO/alkene polymerization in 1991– 1992. This study involved the comparison between mononuclear and star-shaped hexaphosphine-palladium catalysts. The starshaped catalyst gave 3% fouling whereas the mono-palladium catalyst gave 50% fouling, which was already a positive dendritic effect.19,26 In 1993, Brunner reported his first ‘dendrizymes’, that is, dendritic transition-metal catalysts in which the ligands were built up by branching dendronic units and optically active groups. It was expected that the enantioselective reaction would take place in the same way as in the pocket of an enzyme (Fig. 2).20 This concept was important because during the following years the field of dendrimer catalysis became established.21–28 Indeed, in 1994, the groups of van Leeuwen and van Koten reported dendritic diaminoaryl-Ni(II) catalysts for the Kharash addition of CCl4 reaction to metacrylate.21 A subsequent important step appeared when metallodendrimer recycling was reported by Reetz in 1997 with poly(propyleneimine) (PPI) dendrimers terminated by N(CH2PR2)2 groups with Pd complexes catalyzing the Heck reaction between bromobenzene and styrene.35 More than 98% of the catalyst with
16 peripheral Pd groups was recovered by precipitation, and the recovered catalyst (with an uncertain structure) displayed comparable activity and selectivity. This catalyst had a TON three times higher than the monometallic catalyst, showing a positive dendritic effect.35 We now know, however, that PdNPs form in the Heck reactions of bromobenzene, because they require high temperatures, leading to catalytically active PdNPs.36–38 Thus, we believe that the higher reactivity of the metallodendrimer in this case was due to the fact that such PdNPs formed were stabilized by the dendrimer (vide infra) which could not occur when the monometallic catalyst was used in the absence of dendrimer (Pd black formation). An important technological improvement with membrane nanofiltration was pioneered by the groups of Kragl and Reetz who reported in 1999 the retention of Meijer’s diaminopropyl-type dendrimers modified with palladium phosphine termini.39 These groups used the dendritic catalysts for the allylic substitution in a continuously operating chemical membrane reactor. Retention rates were higher than 99.9% resulting in a sixfold increase of the total turnover number for the dendritic Pd catalyst of generation 3 bearing 16 diphosphine-Pd groups at the periphery.39 Location of an efficient catalytic center is a challenging goal, because it also recalls the function of enzymes. Two such early
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OCH3 H3CO
O H3CO C
H3CO C O H3CO H3CO
C
C
O
O O N H
O
C O
C OCH3
OCH3 C O
N
NH
C
C
HN
N
N
O
O N
N
O
C
N
O O C
C
H3CO
O O
H3CO C O
C
C OCH3
N
N
C
C
C N
P
C
P
P
N C
OCH3
O O
OCH3 C
HN
O
Si
Si
Si
Si
OCH3
Si Si
Si
Si Si
Si
Si
C
Si
Si
Si
O
N
O
H3CO
O
HN
NH
N
O
O
N H
NH
C
OCH3
C
Si
Si
Si Si Si
Si Si
Si
Si Si
Si Si P Fe
Si Si
P Si Si
Si
Si Si
Si
Si
Si Si
Si
Si
Si
Si Si
Si
Si
Si
Si
Si
Si
Si
Si Si
Si
Figure 3. DuBois’ trisphosphine-cored dendronic ligand for Pd catalysis of cathodic CO2 reduction to CO (top) and van Leeuwen’s dendritic ferrocenyldiphosphine for Pd catalysis of allylic alkylation (bottom).40
examples for palladium catalysis are shown in Figure 3: Dubois’ early paper in 1994 reports on cathodic CO2 reduction to CO with a Pd-triphosphine terminated by a PAMAM-type dendron with 16 termini and van Leeuwen’s ferrocenyldiphosphine-cored carbosilane-centered dendrimer for which the palladium complex catalyzed allylic substitution.40 On the other hand, a remarkable example of a palladium catalyst located at the dendrimer periphery (the most frequent situation) was provided by Pugni and Togni’s chiral ferrocenyl phosphines (Fig. 4, see §7 for asymmetric palladadendritic catalysis).41
There is a large body of literature on palladodendrimer-catalyzed reactions during the last decade whereby the molecular palladium complex is covalently or supramolecularly attached to the dendrimer (including silica- or polymer-supported dendrons). A noteworthy improvement was the use of membrane reactors for ultrafiltration and re-use of the palladadendritic catalysts.12 This whole area has been recently reviewed in excellent and comprehensive reports by the groups of Newkome32,42 and de Jesus33,34,43 with catalyzed reactions including alkene hydrogenation, hydrovinylation, polymerization, and copolymerization, carbon–carbon coupling (Stille, Suzuki–Miyaura, Sonogashira), allylic substitution,
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PCy2 PPh2
PCy2 PPh2
Fe Fe
Fe Si
PCy2 PPh2
Fe
Fe
Si
NH Si
NH
NH
O O
O
PCy2 Fe PPh2 O
NH
Si
NH Fe
Si
NH
O
Si
P
NH
PCy2 PPh2 Fe
Si
NH
O
Fe
O
N N
Si NH
P
P O O
O O
O N
N
O
NH Fe
NH
O
O O
P
Ph 2P Cy2P
PCy2 PPh2
Si
Si
Ph2 P Cy2P
PCy2 PPh2
PCy2 PPh2
O O
O O
NH Si Ph2P Cy2 P
NH O O
Si
O O
Fe NH
Fe
NH
NH
PPh2 PCy2 Si
Si
Fe Si
Fe
Si Ph2P
Ph2P Cy2P
Fe Ph2P Cy2P
Fe
PCy2
Ph2P Cy2P
Figure 4. Togni’s chiral ferrocenyl dendrimer for enantioselective hydrogenation.41
AcO
P P P N
P N
P P N
N
P N P
N
P N P
P P N
P P N
N
N
N
N
N N
P N P
N P P
P N P
N N
N
P P N
N N N N P P
N P P
P N P
Pd(OAc)2
N P P N P P
P = PCy or t-Bu2
OAc AcO
OAc OAc Pd AcO Pd OAc P P P AcO Pd Pd P P N P N OAc AcO P P N N Pd P OAc AcO Pd P N N N P P N OAc AcO N N P Pd P N N Pd N N P OAc AcO P N N P OAc P N AcO N N N Pd Pd P N N P AcO OAc N N P N N P AcO Pd P OAc P Pd N AcO P N P OAc N N P P Pd AcO Pd OAc P P P P Pd Pd AcO OAc OAc AcO OAc AcO
Equation 1. Pd-catalyst loading at the periphery of (diphosphinomethyl)amino-terminated PPI dendrimers.
aldol-type condensation with isocyanoacetates, and Michael addition. In the beginning of the 2000s, our group has been interested in searching the efficiency and recovery/re-use of palladadendrimercatalyzed carbon–carbon coupling reactions of halogenoaromatics. Therefore, we used classic Pd-diphosphine-terminated PPI dendrimers for the Sonogashira and Suzuki–Miyaura reactions (Equation 1). Related palladadendrimers had been previously used by Reetz with phenyl substituents on the phosphorus ligands for his seminal studies of the Heck reaction35 (vide supra) and the analogous
RuCl2(benzylidene)-loaded dendrimers had been shown by our group to catalyze ring-opening metathesis polymerization of norbornene44 with mechanistically significant dendritic effects.45 The mononuclear complex being a very efficient copper-free catalyst for the Sonogashira coupling of alkynes with iodo-, bromo-, and chloroarenes at room temperature,46 we investigated the dendritic analogues of generation 1 (G1, 4 Pd), 2 (G2, 8 Pd), and 3 (G3, 16 Pd).47–50 A negative dendritic effect was disclosed upon increasing the generations, especially between G2 and G3. Only traces of cross-coupling products of aryl halides were found. The G2 and G3 cyclohexyl dendrimers could be recycled by precipitation with
D. Astruc / Tetrahedron: Asymmetry 21 (2010) 1041–1054
pentane and re-used at least six times without loss of activity, the integrity of the catalyst being shown by NMR spectroscopy. The same palladadendrimers were studied for the Suzuki–Miyaura coupling of electron-poor and electron-rich and hindered chloroarenes. The dendritic complexes were recovered by precipitation with pentane and re-used for three cycles (contrary to the mononuclear complex), but with loss of activity in terms of both conversion and TOF. Again, G1 gave yields comparable to those obtained with the mononuclear complex, but a clear negative dendritic effect was found upon generation increase.51 A polymer-supported version of these catalysts was patented.52 The negative dendritic effect found upon increasing the dendrimer generation in both the Sonagashira and Suzuki–Miyaura reactions had also been encountered in the metathesis catalysis of the Ru-benzylidene dendrimers. It is usually not emphasized in the literature, but it illustrates how the increased bulk at the dendrimer periphery can kinetically limit the substrate approach to the catalytic centers and subsequently the efficiency of catalysts loaded on the dendrimer branch termini. In the mid 1990s, we had anticipated this problem upon studying the redox catalysis of nitrate and nitrite electroreduction to ammonia in water catalyzed by star-shaped water-soluble electron-reservoir FeI complexes53 for which the kinetics was very sensitive to bulk around the metal center indicating a significant inner-sphere component in the electron-transfer rate-limiting step.54 Therefore, we had proposed that the recoverable multinuclear star-shaped catalysts should be more appropriate than metallodendrimers insofar as less bulk would mar the rate-limiting substrate approach to catalytic centers.55 More recently, we designed star-shaped Pd-alkylphosphine catalysts (Fig. 5) and probed their efficiency in the Suzuki–Miyaura reactions between electronrich, highly hindered chloroarenes and hindered aromatic boronic acids. We found that, remarkably, they were as efficient as the mononuclear Buchwald catalysts (3% catalyst, more than 90% yields at 70 °C). These star-shaped catalysts could be recovered and re-used 4 times, unlike the monomeric Buchwald catalysts, showing the advantage of the star-shaped frame.56 The van Koten group also showed the activity of molecular catalysts assembled on a rigid star-shaped species in a membrane reactor with continuous recycling.11 A common feature of all palladadendrimer-catalyzed reactions is the multiple re-use of the catalyst owing to their large size, whereas mononuclear catalysts are not recovered. The number of re-use is always small, however, which means that the catalysts
PCy2
PCy2
PCy2 PCy2
Cy2P
Cy2P
Figure 5. Star-shaped hexaphosphine ligand for very efficient, recoverable, and reusable Pd catalyst of Suzuki–Miyaura coupling between electron-rich, hindered aromatics and hindered arylboronic acids.
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are progressively de-activated upon leaching the catalytic metal from the dendritic frame. With simple chloroarenes, the amount of catalyst can be decreased down to 50 ppm, but not with the electron-rich or hindered substrates. De Vries has shown that molecular catalysts such as Pd(Ac)2 form Pd nanoparticles that are themselves catalytically active in Heck reactions at 130 °C.36 Thus, the nature of the real catalyst must be questioned in high temperature Pd-catalyzed reactions in which the molecular catalyst is progressively de-activated. Could it be that sophisticated molecular catalysts such as palladadendrimers that are used at 80 °C for many hours are just Pd nanoparticle precursors? In order to address this key point, let us now examine Pd catalysis by true Pd nanoparticles. 3. Palladium nanoparticle (PdNP) catalysts in carbon–carbon coupling reactions The formation of C–C bonds is one of the most important reactions of palladium catalysis in organic synthesis. They involve aromatic halides in Heck, Suzuki–Miyaura, Sonogashira, Stille, Corriu– Kumada, and Nigishi reactions. Many homogeneous and heterogeneous sources of palladium catalysts are efficient, sometimes in very low amounts down to the ppm. Most of these sources involve palladium nanoparticles (PdNPs) or heterobimetallic nanoparticles of 1–10 nm size (Fig. 6).36,57–60 The area was pioneered by Beller et al.61 and Reetz et al.62 in 1996. Mechanistic issues in the most important Heck reaction have been reviewed by Biffis et al.,63 Jones et al.,18 and de Vries et al.36,59 Aryl iodides and activated aryl bromides are easily coupled. Even aryl chlorides can couple, but much less selectively and in lower yields at temperatures of the order of 150 °C or higher. The source of PdNPs can be ligand free, Pd complexes (palladacycles, pincers, and others), solid supports (mesoporous silica, metal oxides, zeolites, hydroxyapatite, activated carbons, and organic polymers). In most cases, it is probable that leaching of Pd0 from the PdNPs (even temporarily and in very small amounts) into the solution provides the actual catalysts that are ligandless soluble mono- or biatomic palladium species.36,57–60 4. Polymer- and PAMAM dendrimer-stabilized palladium nanoparticle pre-catalysts Polymers containing heteroatoms have been shown to stabilize catalytically active PdNPs. They include polyvinylpyrrolidone, poly(2,5-dimethylphenylene oxide), polyurea, polyacrylonitrile, polyacrylic acid, multilayer polyelectrolyte films, polysilane shellcross-linked micelles, polysiloxane, polysaccharides, copolymers, conjugated polypyrrole, poly(vinylpyridine, poly(N,N-dialkylcarbodiimide, polyethylene glycol, chitosan, hyperbranched aromatic polyamides (aramids), and water-soluble polymers. Classic surfactants such as sodium dodecylsulfate are also efficient.60 In the 1970s, Sinfelt has extended this principle to catalysis with bimetallic Au-PdNPs (Fig. 5),64 a technique also elegantly developed by Toshima with core-shell Au-PdNP catalysts, essentially for selective olefin hydrogenations.65 Dendrimers are perfectly defined macromolecules with specific topologies,66–69 therefore the extension of PdNP stabilization by dendrimers by either PdNP encapsulation70,71 or peripheric stabilization72 was of great interest. Crooks, who pioneered the area, has remarkably and extensively developed it with the commercial polyamidoamine (PAMAM) dendrimers serving as generationdependent nanofilters for encapsulated PdNPs. The catalytic reactions were carried out in organic solvents, water, supercritical CO2 (sc CO2), or fluorous/organic biphasic solvents, exploiting the large flexibility of dendrimer solubilization provided by the nature of the branch termini.73–84 A comprehensive review of the subject is
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Figure 6. Various types of heterobimetallic transition-metal nanoparticles. Reprinted with permission from the American Chemical Society (Ref. 65, Toshima’s group).
available.75 In brief, the PdNP-catalyzed reactions studied were mostly selective hydrogenation,75,85,86 nitroarene reduction75,87,88 (including with PdAuNPs88,89), and carbon–carbon bond formations75,89–92 (Heck and Suzuki–Miyaura) using PAMAM and, to a lesser extent, PIP dendrimers.75,89–93 These dendrimer-stabilized PdNPs were shown to be less active in Suzuki–Miyaura coupling than poly(vinylpyrrolidone) polymers, however.91,92 Another promising area is that of dendrimer-encapsulated nanoparticles in heterogeneous catalysis pioneered by Chandler. PdNPs supported on mica or highly oriented pyrolytic graphites were calcinated at 630 °C forming large aggregates resulting from considerable PdNP size increase, although incorporation into sol–gel matrixes minimized this PdNP growth. The technique was also extended to other catalytically useful metals (mostly Pt and Au) and to bimetallic nanoparticles including PdAuNPs (Scheme 1).94–96 5. Highly efficient ‘click’-dendrimer-encapsulated and stabilized Pd nanoparticle pre-catalysts The commercial PAMAM and PIP dendrimers have allowed the development of useful concepts in Pd catalysis owing to their easy access, which often resulted in improved selectivity in hydrogenation of unsaturated substrates. These dendrimers had not been specially designed for catalysis, however. In 1993, we had reported the synthesis of arene-cored dendrimers by CpFe+-induced nonaallylation of mesitylene.97 Generation growth with 1?3 connectivity42,98 was carried out in a divergent way using selective hydrosilylation with HSiMe2CH2Cl followed by nucleophilic substitution by a phenolate triallyl dendron-producing dendrimers terminated by 3n+2 allyl branches (n = 0–7).99–101 Variations of the divergent growth included cross metathesis with an acrylate-functionalized dendron at the focal point102 or ‘click’ chemistry between a propargyl-modified dendron at the focal point and dendritic cores terminated by azido groups.103 The 1,2,3-triazolyl dendrimers obtained by ‘click’ chemistry were ideal ligands for PdII 104 and AuIII.105 When these click dendrimers were terminated by 1,2,3-
triazolyl ligands connected to ferrocenyl (Fc) groups, the Fc groups were used as redox sensors for the recognition and titration of PdII ions. This electrochemical recognition was straightforward using cyclic voltammetry, because all the peripheral redox Fc groups appeared equivalent in a single wave that is both chemically and electrochemically reversible.106 Various transition-metal cations and oxo-anions (including ATP2 ) could be sensed with selectivity, as monitored by the shift of the redox potential of the Fc wave. This titration technique was simple and useful in order to determine the number of PdII cations encapsulated in the ‘click’ dendrimers of various generations (Fig. 7).107 There are various known modes of coordination of PdII with triazole ligands, and the monohapto mode was confirmed by X-ray crystal structure determination.108 Reduction of the G1 (27 Fc) and G2 (81 Fc) dendritic-PdII complexes using NaBH4 or methanol provided PdNPs for which the sizes, determined by TEM, corresponded to the theoretical number of Pd atoms according to the one-to-one stoichiometry determined by electrochemical titration of the PdII precursors. This result was indicative of intra-dendritic PdNP formation and encapsulation. On the other hand, the PdNPs formed from the G0 dendrimer (9 Fc) were large. This small dendrimer cannot encapsulate PdNPs, but stabilization is still occurring by locating dendrimer around the PdNPs (Fig. 8). The fact that these G0-‘click’-dendrimer-stabilized PdNPs are large confirms that the PdNP size is independent of the dendrimer size when the latter is too small. Thus the smallest PdNPs are those formed from the G1 dendrimer containing 27 Fc groups, 36 triazolyl rings, and encapsulating PdNPs that contain 36 Pd atoms (Scheme 2). Selective hydrogenation of dienes to monoenes was achieved readily under ambient conditions for small dienes,109 but large steroidal dienes remained unreacted, in accordance with their lack of ability to reach the PdNP surface. All the rates (TOFs) and TONs of hydrogenation were all the larges as the PdNPs were smaller, as expected from the previous results with polymer-stabilized PdNPs65 according to a mechanism that involves mechanistic steps of the hydrogenation on the PdNP surface.109
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Scheme 1. Schematic diagram for the preparation of dendrimer-encapsulated bimetallic NPs. Reprinted with permission from Elsevier (Ref. 89a, Rhee’s group).
6. ‘Homeopathic’ catalysis of Suzuki–Miyaura C–C coupling by ‘click’ ferrocenyl dendrimer-stabilized PdNPs under ambient conditions and evidence for a leaching mechanism Whereas hydrogenation catalysis proceeds at the PdNP surface, as shown above, and therefore depends on the PdNP size, the catalysis of Suzuki–Miyaura C–C coupling110 between PhI and PhB(OH)2 was carried out at room temperature and does not depend on the PdNP size and whether its stabilization is intra- or interdendritic. This shows that the dendrimer is not involved in the rate-limiting step of the reaction. The dendrimer-stabilized PdNPs work identically whatever their size, and the TONs increase upon decreasing the amount of catalyst from 1% down to 1 ppm or upon dilution of the reaction solution. Thus, the efficiency of the catalyst is remarkable at homeopathic amount (54% yield at 25 °C with 1 ppm equivalent of Pd atom, i.e., TON = 540 000) and a quantitative yield is not even reached (75% yield) with 1% equivalent Pd atom.112 The ‘homeopathic’ catalysis was already observed for the Heck reaction at 150 °C and was rationalized by de Vries according to a leaching mechanism involving detachment of Pd atoms from the PdNP subsequent to oxidative addition of the organic halide PhI on the PdNP surface.36,59 This mechanism is established for high temperature reactions due to decomposition of the Pd catalyst to naked PdNPs, but it is less expected for a room temperature reaction. The ease of the room temperature reaction must be due, however, to the lack of ligation onto the dendrimer-stabilized PdNPs that therefore can easily undergo oxidative addition of PhI at their surface, which provokes the leaching of Pd atoms. These isolated Pd atoms are apparently extraordinarily reactive in solution, because they do not bear ligands other than the very weakly coordinating solvent molecules. The limit in their efficiency lies in that they are trapped by their mother NP if the solution is moderately concentrated. This trapping mechanism that inhibits
catalysis is all the less efficient as the catalyst is more diluted in the solution. Therefore it is not efficient under extremely diluted solutions, whereas it strongly inhibits catalysis at relatively high concentrations. It is likely that this concept can be extended to other PdNP-catalyzed C–C bond formation reactions (Scheme 3).111 Analogous ‘click’-dendrimer-stabilized PdNPs with other termini including sulfonate providing solubility in water were also active in aqueous medium for hydrogenation and Suzuki–Miyaura coupling reaction with high TOF and TON numbers,112 as were also related ‘click’-polymer-stabilized PdNPs.113 The G1-dendrimer-encapsulated PdNPs can be extracted by hexanethiol to yield PdNP-cored hexanethiol stars that also catalyze the Suzuki–Miyaura reaction, under ambient condition, between phenylboronic acid and iodobenzene, but not bromobenzene contrary to the G1-dendrimer-encapsulated PdNPs. PdNP-cored decanethiolate species were formerly found to be air and water stable and good catalysts for the latter Suzuki–Miyaura reaction. Thus, the thiolate ligands are not a poison for this catalysis, but the PdNPs are not as free in the presence of the alkylthiolate ligands as in the dendrimer-stabilized PdNPs that are extremely active catalysts (Equation 2).114 7. Asymmetric catalysis: metallodendritic catalysts vs. nanoparticles Since Brunner’s crucial dendrizyme concept 15 years ago, a number of reports have appeared on asymmetric catalysis by dendritic chiral transition-metal complexes. Stereogenic centers were located on the dendrimer branches, at the dendrimer core (or dendron focal point) or at the periphery.27 Peerling and Meijer reported the first catalytic studies with relatively high generation dendrimers, and showed a negative dendritic effect on the ee upon increasing generation.115 A special dendritic cooperative effect in asymmetric catalysis was demonstrated by Breinbauer and Jacob-
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Figure 7. Cyclic voltammetry: recognition of both oxo-anions and transition-metal cationic acetonitrile complexes by a second generation ‘click’ ferrocenyl dendrimer.
sen whereby the proximity of the metals in the low generation dendrimer with four metal centers was ideal to favorably influence the reaction (thus the dendritic effect was positive from the monometallic catalyst to the first generation dendrimer, but generation increase also produced a negative dendritic effect on the ee).116 By the turn of the century, about 30 publications had appeared on asymmetric catalysis using dendritic transition-metal catalysts, and all these early works have been reviewed in detail in 2001.26,27 The first report with Pd, however, was not before 2001, when Togni used Pugin’s Josiphos, a chiral ferrocenyl diphosphine, that was condensed via an amino arm onto cores terminated with carbonyl chloride termini (see the example in Fig. 3). These two authors had reported their first paper on a series of such chiral ferrocenyl dendrimers in 1998 together with Rh-catalyzed asymmetric hydrogenation of dimethyl itaconate in methanol with ees higher than 98%, nearly like Josiphos itself (99%).41 Subsequently, Togni reported Pd-catalyzed allylic substitution with 89–91% ee (compare Josiphos 93%). Negative dendritic effects were found upon increasing the dendrimer generation, resulting in slower catalytic reactions and lower selectivities.117 In 2003, Gade’s group reported asymmetric catalysis of allylic amination of 1,3-diphenyl-1-acetoxypropene with morpholine or sodium dimethyl malonate in DMSO at 45 °C118 using PyrphosPd-functionalized PPI and PAMAM dendrimers.119 Very interestingly, in this work a strongly positive dendritic effect was disclosed with up to 64 Pd sites. For instance, the ee for the Pd catalyst in-
creased from 9% for the mononuclear reference [(Boc-Pyrphos)PdCl2] to 69% for the Pd64-PAMAM-dendrimer (Scheme 4).118 The same dendritic ligands had produced a negative dendritic effect in cationic Rh complexes for asymmetric hydrogenation catalysis.118 In order to rationalize the positive dendritic effect disclosed in the Pd-catalyzed allylic amination, Gade et al. suggested that the control of the stereoselectivity of this catalytic reaction by the phenyl substituents in the C-2 PPh2-diphosphines was modified upon increasing the conformational flexibility.27,117–120 Daran and Manoury’s group designed new chiral ferrocenyl P,O ligands that were active in asymmetric Pd catalysis of allylic substitution of 1,3-diphenylprop-2-enylacetate, showing a strong influence of both planar and central stereogenicities.121 These authors, in collaboration with Caminade and Majoral, linked related chiral ferrocenyl phosphine-thioether ligands to the periphery of phosphorus dendrimers that were efficient for the palladium-catalyzed asymmetric allylic substitution reaction (ee up to 93%).122 The Toulouse group also reported the synthesis of a third generation phosphorus-containing dendrimer possessing 24 chiral iminophosphine end groups derived from (2S)-2-amino-1-(diphenylphosphinyl)-3methylbutane and its use in asymmetric allylic alkylation of rac(E)-diphenyl-2-propenyl acetate and pivalate with ee from 90% to 95%. The dendritic catalyst could be recovered and reused at least twice, with almost the same efficiency.123,124 The Barcelona group of Rossel, Seco, and Rodriguez recently designed carbosilane dendrimers peripherically functionalized with
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Figure 8. Pd nanoparticle surrounded and stabilized by several small G0 nonaferrocenyl dendrimers.
the first P-stereogenic mono- and diphosphine ligands for their use in asymmetric Pd-, Ru-, and Rh-catalyzed reactions.125–128 In asymmetric Pd-catalyzed hydrovinylation of styrene, the best results were obtained with the first generation dendrimer containing the 2-biphenyllyl-substituted phosphine yielding 79% ee toward the (S) enantiomer in CH2Cl2.127 Interestingly, this asymmetric Pd-catalyzed reaction could also be carried out in super-critical CO2 medium in which the selectivity and ee (81% ee) were comparable with those obtained in conventional solvents (see Scheme 5).128 Mechanistic issues are essential for the optimization of dendritic Pd catalysts. Therefore, the possibility of Pd leaching from the dendritic ligand should be envisaged. Only reactions carried out under very mild temperature conditions close to room temperature can diminish the risk of leaching, also depending on the strength of the Pd-ligand bonds. At high temperature and prolonged reaction times, Pd leaching should lead to the formation of PdNPs that would be stabilized by dendrimer either by encapsulation of the PdNPs or by peripheral stabilization if the dendrimer is small, as already shown in the previous section. Catalysis by PdNPs may also occur, but asymmetric catalysis would then be strongly perturbed. In many cases, however, asymmetric catalysis has also been demonstrated with PdNPs (vide infra). In this subfield of asymmetric catalysis, both catalysis processes by the nanoparticle surface and by leached atoms can be envisaged, because
asymmetric induction and catalysis could occur by either mechanism. Indeed, since Tamara and Fujihara reported the first asymmetric reaction catalyzed by nanoparticles in 2003,129 the field of asymmetric catalysis by nanoparticles has successfully expanded to a large variety of nanoparticle types, and has been the subject of an excellent recent review by Roy and Perricas.130 In their early report, Tamara and Fujihara synthesized chiral BINAP-stabilized gold and palladium nanoparticles and studied PdNP-catalyzed styrene hydrosilylation with trichlorosilane. The PdNPs were obtained by reduction of K2PdCl4 by NaBH4 in the presence of the BINAP ligand. In the presence of (S)-BINAP-PdNP (2.0 ± 0.5 nm), 1-phenyl-1-trichlorosilylethane was obtained in 81%, and subsequent oxidation with H2O2 in the presence of KF, a reaction known to proceed with retention of configuration at the carbon center, gave (S)-1-phenylethanol in 75% ee. When the reaction was carried out at 0 °C, the ee was even raised to 95%. (R)-1-Phenylethanol was obtained when the reaction was carried out with (R)-BINAP-Pd for the asymmetric hydrosilylation of styrene. Thus, the chiral BINAP-PdNP acted as both a promoter and an asymmetric induction reagent.129 This asymmetric catalysis is all the more remarkable as mononuclear Pd-BINAP complexes do not catalyze this reaction. The Fujihara group has more recently reported asymmetric catalysis of Suzuki–Miyaura reactions also using (S)-BINAP-PdNPs, and the coupled reaction product had higher enantiomeric purity than that ob-
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Scheme 2. Synthesis of ‘click’-ferrocenyl dendrimer-encapsulated PdNPs.
tained using the monometallic Pd-(S)-BINAP complex as catalyst.131 In 2004, the groups of Chaudret, Gomez, Philippot, and Claver showed that PdNPs stabilized by a chiral xylofuranide catalyze enantioselective allylic alkylation of rac-3-acetoxy-1,3-diphenyl1-propene with dimethylmalonate with 97% ee. Again, with the mono-palladium molecular catalyst, full conversion but no kinetic resolution was observed, because the (R) substrate reacted 12–20 times faster with the PdNPs than with the molecular catalyst.132 Strong arguments in favor of the colloidal-catalysis mechanism were provided, but the groups of Gomez, Dieguez, and van Leeuwen later reached a different conclusion after asymmetric allylic alkylation and Heck reactions carried out using PdNPs stabilized by sugar-based phosphite oxazolines. Indeed, TEM, continuous flow membrane reactor, kinetic, and poisoning experiments carried out by these researchers showed that leaching molecular spe-
cies containing monometallic species could account for catalysis.133 The groups of Gómez, Muller, Claver Chaudret, and Philippot have also reported asymmetric catalysis of the same allylic alkylation with PdNPs stabilized by other chiral ligands derived from carbohydrates.134 Recently, the Lambert group reported asymmetric catalysis of C@C bond hydrogenation. Enantiomeric turnover numbers up to about 100 product molecules per ligand molecule were found by PdNP chiral sulfide ligands containing a methenepyrrolidine substituent that played a key role in the asymmetric induction. Very large variations in asymmetric induction among six ligands were observed, enantiomeric excess increasing with increasing size of the alkyl group in the sulfide. It was suggested that these enantioselective reactions most likely involve initial formation of an enamine or iminium species, because an analogous tertiary amine led to racemic products.135
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ArX
8. Concluding remarks
Ar
PdNP
PdNP
X
The precision of the dendrimer structures, the specificity of their topology, and the variety of dendrimer generations provides a unique means to improve catalyst supports. Seminal studies by van Leeuwen and Brunner’s dendrizyme concept opened the field of dendrimer catalysis, then Reetz introduced recycling, and the groups of Kragl and Vogt pioneered the improvement of continuous membrane reactors. With peripheral catalyst loading, the multiple sites provide an exceptional density of catalysts, but steric constraints limit the access to the catalytic centers as shown by several of our examples. The design of star-shaped catalysts or first generation ‘dendrimers’, however, provided catalysts that were as efficient as mononuclear catalysts and could be recovered and reused, contrary to mononuclear catalysts, as shown by van Koten’s group and by our group. The Pd-catalyzed Suzuki–Miyaura reactions of electron-rich and hindered arenes represented good examples of this advantage. Although there are considerable literature reports of dendritic examples in palladium catalysis, there is also a striking paucity
oxiative addition on PdNP surface detachment of Pdn (A r)(X) from PdNP
Pd0n species quenching by PdNP Ar Pd n n = 1 or 2
Pd 0n
X
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oxidative addition transmetallation on Pd 0 n species reductive elimination
XB(OH) 2
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Ar Pdn Ar'
Scheme 3. Leaching mechanism in the ‘homeopathic’ catalysis of Suzuki–Miyaura C–C coupling at ambient temperature between PhI and PhB(OH)2 by ‘click’ ferrocenyl dendrimer-stabilized PdNPs.36
HS
S S S S S
+ No PdNP size change upon extraction
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PhX X = I or Br PhB(OH) 2 25ºC Ph-Ph
Suzuki cat: no PdNP size dependence less active than dendrimer-PdNP yield quantitative, air § water stable, recyclable Pd black slowly forms but only if PhX reacts (X = I , not Br)
Suzuki cat: no PdNP size dependence homeopathic down to 1 ppm Pd but yield never quantitative due to quenching active Pd atoms by PdNP Pd black slowly forms
Equation 2. Extraction of ‘click’-dendrimer-encapsulated PdNPs from the dendrimer with hexathiol leading to hexanethiolate-PdNPs.
O
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Scheme 4. Asymmetric palladadendritic catalysis of allylic amination with a dramatic positive dendritic effect on the ee, specifically with the Pd64 PAMAM dendrimer.
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Scheme 5. Asymmetric Pd-catalyzed hydrovinylation by chiral palladendritic complexes in CH2Cl2 or super-critical CO2. Model ligands are PPh(Me)(R) and Me3SiCH2PPhR (R = 2-biphenylyl, 9-phenantrenyl). A lower generation dendritic (tetraphosphine) ligand Si{CH2CH2SiMe2CH2PPh(R)}4 is also used. The cationic catalysts are obtained by reactions in situ between the chloro-Pd pre-catalysts and NaBAr4 {Ar = 3,5-C6H2(CF3)2} for chloride abstraction. The palladadendritic complex shown provided the best results in asymmetric catalysis (see text).127
of mechanistic studies and leaching catalytically active palladium species loaded in dendrimers. Usually, recovery and re-use are demonstrated in the reports, but it is limited to about five recycles. It is likely that leaching provides PdNPs that can also themselves be catalytically active. There is also a large body of literature on heterogeneous supported PdNP catalysts, but it is now recognized that most if not all these catalysts are in fact pre-catalysts that generates extremely active ligandless Pd species in solution. Homogeneous PdNP catalysis pioneered by the Beller–Herrmann and Reetz groups is very intriguing because the TONs are considerably higher than those obtained with molecular catalysts for olefin hydrogenation and Suzuki–Miyaura reaction. The most difficult C–C coupling reaction involving non-activated aryl chloride require the most reactive molecular catalysts, however, although all catalysts decompose at high temperature to PdNPs that are also active in Heck coupling of aryl chlorides. The fields of molecular and nanoparticle Pd catalysts are now intimately connected, since we know, after the mechanistic studies by de Vries, that even simple catalysts such as Pd(OAc)2 decompose to catalytically PdNPs at 130 °C for Heck reactions. The remarkable seminal studies by Crooks with commercial PAMAM dendrimers have demonstrated the potential of dendrimerencapsulated PdNPs acting as generation-dependent catalysts with the dendrimer peripheries serving as nanofilters in various media. Our contribution focused on the design of very active ‘click’dendrimer-encapsulated and ‘click’-dendrimer-stabilized PdNP catalysts showing the crucial role of the intradendritic triazole ligands in these nanoreactors that worked in organic as well as aqueous solvents. This approach not only provided very active PdNP catalysts for C–C coupling reactions under ambient condi-
tions, but also allowed to obtain key information on the leaching mechanism occurring in PdNP catalysis. Related leaching mechanisms may occur in other dendrimer-Pd-catalyzed reactions. For instance, de Jesus recently suggested that the Stille reaction catalyzed by PAMAM-dendrimer-encapsulated PdNPs occurred via Pd species that were leached from the PdNP,136 and in asymmetric catalysis, the intimate mechanism of allylic alkylation (surface vs leaching Pd atoms) is debated (vide supra). Design of dendrimers for catalytic performances is promising, as also demonstrated with other transition-metal catalysts by the groups of Yamamoto and Nishihara137 with the very interesting phenyldiazomethane dendrimers designed by the Keio group.138,139 Molecular Pd catalysis and PdNP catalysis are so intimately connected that it is sometimes difficult to decide which one, the Pd complex, the Pd atom, or the PdNP, is the true catalytic active species, but continuous research efforts are improving this knowledge and take advantage of each kinds of catalysis type for appropriate purposes. Finally, the mutual contribution of asymmetric catalysis and dendrimer/nanoparticle chemistry highlights the importance of avoiding leaching Pd centers to PdNPs from these sophisticated molecular catalysts. This can be achieved for low temperature reactions. Strongly positive dendritic effects are rare in asymmetric catalysis, and the remarkable example of Gade’s results29,118–120 illustrated here shows that careful search can lead to very substantial benefits in terms of ee. This property must continuously be searched and combined with robustness for high recyclability of these catalysts in the future. On the other hand, asymmetric nanoparticle catalysis is successfully developing with promising results and challenging mechanistic issues. Acknowledgments The author thanks the colleagues and students cited in the references for their contribution, and in particular Drs. Abdou K. Diallo, Denise Méry, Cátia Ornelas, Jaime Ruiz (Bordeaux), and Lionel Salmon (Toulouse) for their enthusiasm, many ideas, and fine work in the catalysis field. Financial support from the Institut Universitaire de France (IUF), the Ministère de la Recherche et de la Technologie (MRT), the Centre National de la Recherche Scientifique (CNRS), the Université Bordeaux 1, and the Agence Nationale de la Recherche (ANR) is gratefully acknowledged. References 1. Tsuji, J. Modern Palladium Catalysis. Palladium Reagents and Catalysts: New Perspectives for the 21st Century; Wiley: Chichester, 2004. 2. Catalyst Separation, Recovery and Recycling; Cole-Hamilton, D., Toose, R. P., Eds.; Springer: Heidelberg, 2006. 3. Gladysz, J. A. Chem. Rev. 2002, 102, 3215–3216. 4. Recoverable and Recyclable Catalysts; Benaglia, M., Ed.; Wiley: Chichester, 2009. 5. Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Angew. Chem., Int. Ed. 2005, 44, 4482– 4489. 6. Amatore, C.; Jutand, A. Acc. Chem. Res. 2000, 33, 314–321. 7. Kagan, H.; Dang, T. P. Chem. Commun. 1971, 481–482. 8. Kagan, H.; Dang, T. P. J. Am. Chem. Soc. 1972, 94, 6429–6433. 9. Modern Surface Organometallic Chemistry; Basset, J.-M., Psaro, R., Roberto, D., Ugo, R., Eds.; Wiley-VCH: Weinheim, 2009. 10. de Groot, D.; de Waal, B. F. M.; Reek, J. N. H.; Schenning, A. P. H. J.; Kamer, P. C. J.; Meijer, E. W.; van Leeuwen, P. W. N. M. J. Am. Chem. Soc. 2001, 123, 8453– 8458. 11. Dijkstra, H. P.; Kruihof, C. A.; Ronde, N.; van de Coevering, R.; Ramon, D. J.; Vogt, D.; van Klink, G. M. P.; van Koten, G. J. Org. Chem. 2003, 68, 675–685. 12. Hovestad, N. J.; Eggeling, E. B.; Heidbüchel, H. J.; Jastrzebski, J. T. B. H.; Kragl, U.; Keim, W.; Vogt, D.; van Koten, G. Angew. Chem., Int. Ed. 1999, 38, 1655– 1657. 2003, 345, 364–369. 13. Metal-Catalyzed Cross Coupling Reactions; Diederich, F., Stang, P., Eds.; WileyVCH: Weinheim, 1998. 14. Beletskaya, I. P. Chem. Rev. 2000, 100, 3009–3066. 15. Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 4176–4211.
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