Synthesis 2015; 47(14): 2017-2031
DOI: 10.1055/s-0034-1380868
feature
© Georg Thieme Verlag Stuttgart · New York

Catalysis Inside Dendrimers

Didier Astruc*, Dong Wang, Christophe Deraedt, Liyuang Liang, Roberto Ciganda, Jaime Ruiz
  • ISM, UMR CNRS N°5255, Université Bordeaux, 351 Cours de la Libération, 33405 Talence Cedex, France   Email: d.astruc@ism.u-bordeaux1.fr
Further Information

Publication History

Received: 17 March 2015

Accepted after revision: 02 May 2015

Publication Date:
22 June 2015 (eFirst)

 

Abstract

Modern methods that involve intradendritic catalysis are introduced in this feature article. Supramolecular principles, therefore, derive from the concept of the unimolecular dendritic micelle introduced by Newkome. When the micellar effect is combined with intradendritic ligand acceleration, copper(I) catalysts or palladium nanoparticles (PdNPs) are required at the ppm level for efficient reactions. Applications range from organic catalysis to catalysis by metal complexes of intradendritic ligands and dendrimer-stabilized nanoparticles (NPs) that are located at the dendrimer core or between the dendrimer tethers. Bimetallic nanoparticle-cored dendrimers including superparamagnetic iron oxide nanoparticles (SPIONs) are especially promising due to very facile magnetic separation.

1 Introduction

2 Supramolecular and Organic Catalysis inside Dendrimers

3 Micellar Dendritic Catalysis: Olefin Metathesis and Click ­Reactions

3.1 Olefin Metathesis

3.2 Click Chemistry

4 Combined Micellar Dendritic Effect and Intradendritic Ligand ­Acceleration

5 Efficient Click Catalysis at the Dendrimer Core

6 Dendrimer-Encapsulated and Dendrimer-Stabilized Nanoparticle Catalysts

7 Catalysis by Nanoparticle-Cored Dendrimers

8 Concluding Remarks


#

Biographical Sketches

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Didier Astruc studied in Rennes including two theses passed with R. Dabard before postdoctoral studies at MIT with R. R. Schrock. He became Professor of Chemistry in Bordeaux in 1983 and he took sabbatical leave at UC Berkeley with Peter Vollhardt in 1990–1991. He is a Member of the Institut Universtaire de France and of several Academies. His interests are in inorganic and organometallic chemistry and nanosciences, including applications to catalysis, sensors, and nanomedicine.

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Dong Wang studied organic chemistry with B. Chen in Lanzhou where he received a doctoral degree. He passed a second Ph.D. with Prof. Didier Astruc in 2014 on magnetic and dendritic nanocatalysis and is presently undertaking postdoctoral studies with D. Song in Toronto.

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Christophe Deraedt passed his Ph.D. with Didier Astruc on catalysis in dendritic nanoreactors in 2014 and is now doing postdoctoral studies in Tokyo with M. Fujita on nanoreceptors.

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Liyuan Liang passed her Ph.D. with Didier Astruc in 2011 on dendritic nanoreactors in catalysis and molecular recognition. She is now an Associate Professor in Chong­qing. Her interests are in organic/ inorganic hybrid nanomaterials.

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Roberto Ciganda obtained his Ph.D. with M. A. Garralda Hualde in 2013 at the University of San Sebastian before working there with R. Hernandez. He is now undertaking postdoctoral studies with ­Didier Astruc on catalysis with metallodendrimers and nanoparticles.

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Jaime Ruiz studied with E. Roman in Santiago de Chile, obtained his Ph.D. with Didier Astruc in organometallic chemistry in 1989, and undertook postdoctoral studies with A. H. Cowley at the University of Texas at Austin. After his Habilitation, he became an Engineer. His interests are in inorganic electrochemistry, nanoparticles, and metallodendrimers.

1

Introduction

Catalysis science is rapidly developing at the interface between homogeneous and heterogeneous processes with combined implications for molecular chemistry and sophisticated supports.[1] [2] [3] The main modern nanotechnology approaches include the use of surface functionalization,[4] nanoparticles,[5–13] micelles and reversed micelles,[14] [15] polymers,[16] [17] dendrimers,[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] and various other nanoreactors,[28] [29] [30] [31] sol-gel,[32] [33] [34] [35] [36] [37] organocatalysts,[38] [39] [40] ionic liquids,[41] [42] [43] and atomic layer deposition.[44] [45] [46] In this feature article, we have selected to highlight the major progress and developments in catalysis inside dendrimers, essentially with our recent work and that from other research groups.

Dendrimers have the advantage over polymers and some other supports that they are well-defined and offer precise locations for catalytic sites, i.e. at the dendron focal point, at the dendrimer core, on the branches, at branch intersections, and at the branch termini,[47] and molecular engineering allows one to control and optimize the catalytic efficiency of a given site.[48] Dendrimers belong to mesoscale structures bridging homogeneous and heterogeneous catalysis.[49] A number of recent reviews have already covered dendrimer catalysis overall.[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] Following the pioneering work of Brunner on ‘Dendrizymes’, dendritic catalysis was also considered for asymmetric synthesis in the early 2000s and it has been reviewed.[13,50–57]

Early examples of intradendritic catalysis were reported by the Suslick group with manganese–porphyrin-centered dendrimers providing enhanced catalytic selectivity,[58] the van Leuwen group with 1,1′-bis(diphenylphosphino)ferrocene-centered dendrimers for palladium catalysis[19] and the Crooks group with catalysis by PAMAM [poly(amidoamine)] dendrimer-encapsulated palladium nanoparticles.[5] [59]


# 2

Supramolecular and Organic Catalysis inside Dendrimers

In organocatalysis, the Fréchet group reported 4-(dialkylamino)pyridine-containing dendronized polymers that catalyzed acylation reactions using sterically demanding tertiary alcohol substrates. In these catalysts, the nanoenvironment played the dominant role in determining the catalytic activity. Thus intradendritic catalysis somewhat resembles enzymatic catalysis whereby the dendritic interior plays the essential role of a second coordination sphere for the catalytic metal center located inside the dendrimer, eventually providing a favorable gradient as postulated in Fréchet’s reaction.[53] [60] Dendritic organocatalysis has been reviewed until 2010.[61] The Naota group have designed dendritic organocatalysts featuring association complexes in dendritic cores consisting of synthetic flavin with diaminopyridine and melamine receptors for aerobic hydrogenation of styrene at ambient conditions.[62] The Kaneda group reported catalysis in the internal nanocavity of polypropylenimine (PPI) dendrimers functionalized with C16 alkyl chains acting as efficient tertiary amine catalysts for the intramolecular Michael reaction. The substrate was accommodated in a reactive conformation within a sterically confined nanocavity consisting of regularly arranged tertiary amino groups of the PPI dendrimers.[63] In a 2014 account,[57] the Fan group reviewed their original work involving several series of chiral dendritic phosphorus ligands, including diphosphines, monodentate phosphoramidites, and P,N-ligands attached to the core or the focal point of Fréchet-type dendrons and their use in the asymmetric hydrogenation of prochiral olefins, ketones, and imine-type substrates using ruthenium, rhodium, or iridium complexes.[64] [65] [66] [67] [68]


# 3

Micellar Dendritic Catalysis: Olefin Meta­thesis and Click Reactions

3.1

Olefin Metathesis

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Equation 1 Synthesis of the amphiphilic dendrimer 1

Dendrimers that are constructed with a hydrophilic periphery and a hydrophobic interior are unimolecular micelles. The Newkome group proposed in their original 1985 paper[69] and showed that dendrimers are unimolecular micelles,[70] a concept that was exploited towards molecular recognition[70] [71] and drug delivery,[72] but not in catalysis. Olefin metathesis reactions[73,74] in water are catalyzed mostly by Grubbs-type ruthenium benzylidene complexes,[75] [76] [77] but the ruthenium–methylidene intermediate complexes that are formed during the reactions are reactive in side reactions. This drawback limited turnover numbers, even with some water-soluble functionalized ruthenium benzylidene catalysts. Indeed, several commercial ruthenium–benzylidene catalysts, including Grubbs’ 2nd generation catalyst, are stable in suspension in water, but only in the absence of a terminal olefin due to the reactivity of the ruthenium–methylidene intermediate during olefin meta­thesis.[78] As in several ‘compatible’ organic solvents, olefin metathesis reactions in water require the catalyst to be present in an amount of the order of 5 mol% versus the substrate because of side reactions of the ruthenium–­methylidene intermediate. In the presence of an amphiphilic dendrimer 1 (Equation 1), however, it was found that excellent yields of ring-closing metathesis for a variety of terminal olefins could be obtained using 0.07 mol% 2nd generation Grubbs catalyst 2 at ambient temperature (Table [1]).[78]

This was tentatively taken into account by encapsulation of the catalytic intermediate inside the dendrimer that protects it from side reactions. The dendrimer is stable under the reaction conditions and is recycled more than ten times without yield decrease. In 2014, the Klein Gebbink group reported ruthenium–benzylidene catalysts of ring-closing metathesis reactions that were covalently loaded onto dendrimers for recovery and reuse.[79]

Table 1 Ring-Closing Metathesis Reactions Using Grubbs’ 2nd Generation Catalyst 2 and Dendrimer 1 (H2O, Air, 25 °C, 24 h)

Entry

Substrate

Product

Catalyst 2 (mol%)a

Conversion (%)

1 (0 mol%)

1 (0.083 mol%)b

1

0.1

 0

86c

2

0.1

 0

90d

0.06

 0

66d

0.04

 0

62d

3

0.1

 6c

89c

4

0.1

 0

90d

5

2

27d

97d

6

2

30d

99d

a Ru catalyst 2 (mol%) vs. substrate; e.g. 2 (4 mg) dispersed in H2O (47 mg), which corresponds to 2 (0.1 mol%).

b 1 (28 mg, 0.083 mol%).

c The mixture was filtered to remove Ru catalyst or resulting residual species, then extracted with Et2O and the solvent removed. The catalyst-free extract was analyzed by 1H NMR in CDCl3.

d The mixture was filtered to remove Ru catalyst or resulting residual species, then extracted with Et2O. The catalyst-free extract was analyzed by GC (injection of the extracted Et2O solution).

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Scheme 1 CuAAC reaction between benzyl azide and phenylacetylene catalyzed by copper(I) complex 3 in the presence of small amounts of the dendrimer 1

# 3.2

Click Chemistry

The dendrimer 1, which was used for micellar dendritic catalysis of olefin metathesis reactions, was also efficient for copper(I)-catalyzed alkyne azide cycloaddition (CuAAC) click reactions[80] [81] [82] [83] using the catalyst [Cu(hexabenzyltren)]Br 3.[84] Indeed the copper(I) complex of the hexamethyltren ligand N(CH2CH2NMe2)3 (Me6tren) was shown by the Matyjaszewski group to be an efficient catalyst for click reactions.[85] These authors compared various polyamine ligands and showed that the Me6tren ligand provided a 50-fold increase of the click rate constant compared to copper(I) bromide.[86] The benzyl analogue 3 is soluble, which allows easy recycling,[84] and it has been utilized for otherwise difficult click reactions, such as those involving gold nanoparticles.[87] [88] Taking advantage of the molecular-micelle effect of dendrimer 1, click reactions are catalyzed using only 0.1 mol% of copper(I) catalyst 3 (Scheme [1,] Tables 1–3). Remarkably, this hydrophobic solid catalyst 3 was localized inside the dendrimer in deuterium oxide by 600 MHz 1H NMR, bringing an enlightening support for the role of 1 as a micellar nanoreactor.[89]

Table 2 CuAAC Reactions between Benzyl Azide and Phenylacetylene (Scheme [1])a, [89]

Entry

Catalyst 3 (mol%)

Dendrimer 1 (mol%)

Yieldb (%)

1

0.1

0

 2

2

0.1

1

91c

3

0.2

1

92

4

0.5

1

98

a Reaction conditions: BnN3 (0.1 mmol), phenylacetylene (0.105 mmol), H2O (2 mL), 25 °C, 3 h.

b Isolated yield.

c This reaction was repeated 10 times with the same recycled dendrimer 1, and the tenth reaction the yield remained 91%.

Table 3 CuAAC Reactions between Various Azides and Alkynes Using Catalyst 3 (0.1 mol%) in the Presence of Catalytic Dendrimer 1 (Scheme [1])a, [89]

Entry

Azide

Alkyne

Yieldb (%)

Conversionc (%)

1

89

 97

2

93

100

3

93

100

4

90

 97

5

95

100

6

91

 98

7

90

 97

8

96

100

a Reaction conditions: azide (0.1 mmol), alkyne (0.105 mmol), dendrimer 1 (1 mol%), 3 (0.1 mol%), H2O (2 mL), 25 °C, 3 h.

b Isolated yield.

c 1H NMR conversion.

The catalyst 3 was also used for the ‘autocatalytic’ synthesis of the nanoreactor 1. With only 8 mol% of 3 per branch, the ‘click’ reaction between the nona-azide core and the tris-TEGylated alkynyl dendron is complete in ten hours at 30 °C in the presence of only 1 mol% of 1 per branch (Equation 2), leading to 1 in 81% isolated yield. If 1 is not introduced at the beginning of the reaction, the yield under these conditions is only 39% in 20 hours, showing the strong autocatalytic effect of 1 on its own formation.[89]


#
# 4

Combined Micellar Dendritic Effect and ­Intradendritic Ligand Acceleration

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Equation 2

Amphiphilic dendrimers such as 1 are efficient unimolecular micelles for intradendritic catalysis, but if these dendrimers contain ligands in their interior, it is also possible to accelerate the intradendritic reactions. This is exemplified by the CuAAC click reaction; we know from the work of the Matyjaszewski group that nitrogen ligands accelerate the reactions.[85] Indeed, the original Sharpless–Fokin catalyst consisting of copper(II) sulfate pentahydrate and sodium ascorbate (NaAsc) remains, at present, the most frequently used catalyst for this reaction, but it is too slow at room temperature for efficient catalysis. Thus many copper(I) complexes containing such nitrogen ligands have been reported to accelerate the Sharpless–Fokin reaction.[81] [82] This is also the case with the 1,2,3-triazole ligands resulting from CuAAC click reactions between terminal alkynes and azides.[89] Indeed, when this reaction is conducted in the presence of the dendrimer 1, it benefits from both the micellar dendritic effect and intradendritic ligand acceleration [Scheme [2] (a)]. The coordination of copper(II) to the triazole rings of 1 has been shown by 1H NMR spectroscopy. After adding copper(II) sulfate pentahydrate to a deuterium oxide solution of 1 (1 equiv per triazole), the NMR signal of the triazole hydrogen atom of 1 at δ = 7.90 disappears to give a very broad signal due to the paramagnetic copper(II) species. When sodium ascorbate is added to reduce copper(II) to copper(I), the NMR signal of the triazole proton of 1 reappears but is shifted (δ = 8.08 instead of 7.90 when 1 alone is present) showing the coordination of all the triazole rings to copper(I) [Scheme [2] (b)]. The combined micellar/intradendritic acceleration is so dramatic that the reaction is quantitative with only 4 ppm of copper(I) in water at 30 °C for 24 hours and reaches 50% yield using 1 ppm of copper(I).[89]

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Scheme 2 (a) Dendrimer 1 as nanoreactor/ligand for CuAAC catalysis. (b) Comparison of the NMR signals of the triazole proton of 1 alone (δ = 7.90), with copper(II) (very broad due to the paramagnetism) and copper(I) (shift to δ = 8.08) showing the coordination of the intradendritic triazoles of 1 to the copper ions.
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Scheme 3 CuAAC reactions with 1-CuI as a catalyst for various applications

Each of the micellar and intradendritic ligand acceleration effects were demonstrated separately upon comparison with a related dendrimer that did not contain triazole ligands. This remarkably low catalytic amount compatible with industrial production compares with the large amount of catalyst used for CuAAC reactions with the Sharpless–Fokin catalyst alone. In many reported cases, the Sharpless–Fokin catalyst is even used in stoichiometric or even higher amounts.[90] [91] [92] [93] [94] In order to evaluate the scope and the applicability of the dendritic catalyst 1 with copper(II) sulfate pentahydrate and sodium ascorbate, the CuAAC reaction with 1-CuI was examined with hydrophobic biomolecules with medicinal, targeting, and labeling interests. 1-Ethynylcyclohexanol (4) was selected because of its simple structure and it is an active metabolite of the (old) central nervous system depressant drug ethinamate. 7-(Propargyl­oxy)coumarin (5) and 3-(d-biotinylamido)prop-1-yne (6), known for its role as a vitamin and co-enzyme in the synthesis of fatty acids, were also tested (Scheme [3]).[89]


# 5

Efficient Click Catalysis at the Dendrimer Core

Metallodendritic catalysts were constructed around the click reaction catalyst [Cu(hexabenzyltren)]Br 3, upon functionalization with dendrons in the para positions of the benzyl substituents (Scheme [4]).

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Scheme 4 Syntheses of the tren ligands G1 and G2 and structures of their CuBr complexes 7 (G1) and 8 (G2)

These dendritic copper(I) catalysts were found to be efficient for CuAAC reactions. Interestingly the larger metallodendritic catalyst 8 reacts faster than the parent catalyst [Cu(hexabenzyltren)]Br 3 and smaller metallodendrimer 7 for the selective cycloaddition between phenylacetylene and benzyl azide at 22 °C in toluene using with 0.1% catalyst; this behavior is reminiscent of that of enzymes.[95] These catalytic reactions were conducted in toluene, because the dendritic catalysts and substrates are hydrophobic, but with the water-soluble metallodendritic catalyst 9 (Figure [1]), excellent catalytic results were obtained with hydrophobic substrates in water. Thus again, the unimolecular micelle effect of the catalyst is operating favorably here, which also confirms the monometallic CuAAC mechanism in this case.[84]

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Figure 1 Structure of the water-soluble dendritic copper(I) catalyst 9

# 6

Dendrimer-Encapsulated and Dendrimer-Stabilized Nanoparticle Catalysts

The stabilization of noble-transition-metal nanoparticles (NPs) by a variety of macromolecules has long been applied to catalysis.[13] [16] [96] [97] With PAMAM dendrimers, the seminal work by the Crooks group paved the way for catalysis using dendrimer-encapsulated nanoparticles.[5,25,98] The PAMAM dendrimers were shown to behave as generation-dependent nanofilters.[5] Later, the dendrimers assembled by click chemistry containing the intradendritic 1,2,3-triazole ligands were shown to be valuable ligands for transition-metal cations that could be recognized using electrochemistry of triazolylferrocenyl termini.[91] [92] These intradendritically coordinated transition-metal cations were reduced using various reductants to zero-valent metal atoms coalescing to dendrimer-stabilized nanoparticles[93] that were shown to be active catalysts for a variety of reactions. With palladium, complexation of triazolylferrocenyl dendrimers was achieved using palladium(II) acetate. Then reduction was conducted using methanol leading to dendrimer-encapsulated nanoparticles for the second and third dendrimer generations. Transmission electron microscopy (TEM) showed that the nanoparticles contained a number of atoms matching the number of intradendritic triazole ligands. These palladium nanoparticles (PdNPs) were more active in toluene for olefin hydrogenation as they were smaller, which was consistent with a classic trend.[92] [99] On the other hand, for Suzuki–Miyaura C–C cross coupling with iodoarenes at room temperature, catalytic efficiency was independent of the size of the palladium nanoparticles and method of dendrimer generation, and even of the palladium nanoparticle concentration down to the ppm concentration.[99] [100] [101] These data suggested a leaching mechanism with extremely efficient palladium atoms or small clusters of zero-valent palladium fragments in solution. Later, experiments using potassium tetrachloropalladate(II) as the palladium(II) source were conducted in aqueous media (EtOH–H2O, 1:1) with water-soluble dendrimers such as 1 acting as micellar nanoreactors. For Suzuki–Miyaura reactions the amount of palladium atoms was also down to the ppm range using iodoarenes at room temperature[102] or bromoarenes at 80 °C (Equation 3).[103] [104] [105] [106]

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Equation 3
Zoom Image
Scheme 5 Palladium nanoparticles stabilized by the dendrimer 1 [105]

The core size of the palladium nanoparticles ranged from 1.4 to 2.8 nm, indicating that the atoms were provided by several dendrimers that stabilized a single palladium nanoparticle (Scheme [5]). The palladium nanoparticles stabilized by the dendrimer 1 were also efficient using this solvent for copper-free Sonogashira coupling reactions in the presence of triethylamine between iodobenzene and various terminal alkynes at 80 °C in good yields with 100 ppm palladium and for Heck reactions between iodobenzene and styrene or methyl acrylate at reflux in the presence of potassium hydroxide.[105]

These palladium nanoparticles (0.2% mol Pd) were also very efficient for the sodium borohydride reduction of 4-nitrophenol to 4-aminophenol in water at 20 °C with a rate constant k app = 0.004 s–1; TON values up to 22500 were obtained, which was higher than other reported values (Equation 4).[105]

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Equation 4

Dendrimers such as 1 and related dendrimers also stabilize other transition-metal nanoparticles such as platinum (PtNPs)[107] and gold nanoparticles (AuNPs)[108] [109] [110] [111] [112] [113] [114] according to the same principle using intradendritic triazole coordination followed by reduction of the cations to zero-valent atoms. Catalysis of 4-nitrophenol reduction by sodium borohydride was also conducted using gold nanoparticles stabilized by 1 and other related water-soluble dendrimers in water. The weak coordination of triazole groups provides a key stabilization factor, but does not inhibit catalysis.[112–114] Comparison between such triazole structures showed that this ligand indeed provided an ideal compromise between stabilization and gold nanoparticle catalytic activity.[114] [115]


# 7

Catalysis by Nanoparticle-Cored Dendrimers

Nanoparticle-cored dendrimers and related structures appeared at the beginning of the 2000s,[116] [117] [118] and their catalytic functions were examined. Such thiolate-stabilized palladium nanoparticles with 2-nm cores were shown to be active catalysts for Suzuki reactions of various haloarenes at 20 °C. This excellent activity indicated that, contrary to common belief, the bulk and coordination of the thiolate ligands were not an obstacle to efficient catalysis.[119] The reduction of 4-nitrophenol has also been examined using various gold nanoparticle centered nanostructures,[112–114] and it was shown that catalysis worked better as the stabilizing organic framework was less bulky.[114] At this time the dendritic and dendronic frameworks were counter-productive compared to simple, nonbulky ligands. These results confirmed that ligand displacement by substrates in the gold nanoparticle surfaces is the dominant factor of the mechanism involving restructuring the surface. This was in accord with the mechanism proposed by Ballauff and others following the Langmuir–Hinshelwood kinetic model that involves adsorption of both reactants on the surface of the gold nanoparticles.[8] [120] [121] Here the dendritic bulk is useful, insofar as it allows discrimination between efficient catalysis in the presence of surface bulk favoring a leaching mechanism in the case of the Suzuki–Miyaura reaction and bulk-inhibited catalysis with nanoparticle surface restructuration in the case of the 4-nitrophenol reduction by sodium borohydride.

Another type of nanoparticle-cored molecular structure that has appeared very useful in catalysis is that involving magnetic nanomaterials, in particular superparamagnetic iron oxide nanoparticles (SPIONs) for magnetic catalyst recovery.[10] [11] , [122] [123] [124] [125] [126] [127] Some of these nanostructures have involved dendritic or dendronic stabilization in order to improve catalyst loading and recovery by steric protection.[128] [129] [130] [131] Therefore two strategies may be used. The first involves covalent binding of dendrons containing triazole or related ligands to a silica-coated iron oxide core (Figure [2]),[128] whereas the second consists of impregnating dendritically preformed nanoparticles on a magnetic support such as maghemite γ-Fe2O3@SiO2 nanoparticles (Figure [3]) by sonification.[129] Using the former method, Suzuki, Sonogashira, and Heck reactions were conducted, the catalyst was recovered and reused many times and a series of pharmacologically relevant or natural products were successfully synthesized.[128]

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Figure 2 Superparamagnetic iron oxide nanoparticles cored dendrimer[128]

Using the second method (Figure [3]), Suzuki–Miyaura reactions between bromobenzene and phenylboronic acid were carried out, and the catalyst was used at least five times without much loss of activity, the yields decreasing from 99% yield (1st run) to 91% (5th run). Inductively coupled plasma optical emission spectrometry (ICP-OES) showed that only 0.3% of palladium that composed the catalyst was lost after the first run. Copper-free Sonogashira and Heck reactions of iodobenzene also proceeded quantitatively with full catalyst recovery.[129] This dendritic nanotechnology also allowed efficient and selective oxidation of benzylic alcohol to benzaldehyde with only 0.09–0.20 mol% of palladium from this catalyst in the presence of potassium hydroxide as base in water, at 60 °C upon bubbling oxygen for five minutes.[129] In parallel, the Bronstein group also designed magnetically recoverable catalysts based on polyphenylenepyridyl dendrons and dendrimers.[129] [130]

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Figure 3 Impregnation of superparamagnetic iron oxide nanoparticles with dendrimer-encapsulated nanoparticles[129]

# 8

Concluding Remarks

The perspective of the useful inclusion of substrates inside dendrimers appeared at the very beginning of dendrimer chemistry in Newkome’s publication in 1985 and was later extensively applied to a variety of functions including catalysis. In this feature article, we have reviewed the recent concepts of intradendritic catalysis that have enriched dendrimer catalysis. In general, the principles that are involved are relevant to supramolecular catalysis, i.e. the interior of the dendrimer is the subject of confinement generating weak interactions between the dendritic core and tethers and the guest substrates. A most useful application of the dendrimer topology is that of dendrimer micelle with hydrophilic tethers and hydrophobic interior that solubilizes a large variety of organic molecules including ligands and catalysts. This strategy was first applied by our research group using the easily synthesized dendrimer 1 with olefin metathesis in water for which the reactive Ru=CH2 intermediate is seemingly well protected from side reactions inside the dendrimer. In the next example, the CuAAC click reaction, encapsulation of the catalyst ­[Cu(hexabenzyltren)]Br 3 in dendrimer 1 was demonstrated by 600 MHz NMR (D2O). In both example, the catalytic improvements in the presence of the dendrimer 1 are dramatic, and this dendrimer could be recycled more than ten times. Combining the benefits of intradendritic ligand acceleration and micellar effects led to impressive catalytic efficiency of a copper catalyst at the ppm level. Click catalysis by the dendrimer core is reminiscent of enzyme catalysis, because the dendrimer accelerates catalysis compares to the non-dendritic model. Finally promising applications of very efficient catalysis by nanoparticle-cored dendrimers are those involving nanoparticles including bimetallic nanoparticles involving superparamagnetic iron oxide nanoparticles. In nanoparticle catalysis, the modulation of the dendrimer generation also allows an ideal control of the size of the encapsulated or stabilized nanoparticles and provides precious mechanistic information.


#
#

Acknowledgement

Financial support from the University of Bordeaux, the Centre National de la Recherche Scientifique (CNRS), the Ministère de l’Enseignement et de la Recherche (MER, Ph.D. grant to CD), the China Scholarship Council (CSC) of the People’s Republic of China (Ph.D. grant to D.W.), the Agence Nationale de la Recherche (ANR) and L’Oréal is gratefully acknowledged.

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Equation 1 Synthesis of the amphiphilic dendrimer 1
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Scheme 1 CuAAC reaction between benzyl azide and phenylacetylene catalyzed by copper(I) complex 3 in the presence of small amounts of the dendrimer 1
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Equation 2
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Scheme 2 (a) Dendrimer 1 as nanoreactor/ligand for CuAAC catalysis. (b) Comparison of the NMR signals of the triazole proton of 1 alone (δ = 7.90), with copper(II) (very broad due to the paramagnetism) and copper(I) (shift to δ = 8.08) showing the coordination of the intradendritic triazoles of 1 to the copper ions.
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Scheme 3 CuAAC reactions with 1-CuI as a catalyst for various applications
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Scheme 4 Syntheses of the tren ligands G1 and G2 and structures of their CuBr complexes 7 (G1) and 8 (G2)
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Figure 1 Structure of the water-soluble dendritic copper(I) catalyst 9
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Equation 3
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Scheme 5 Palladium nanoparticles stabilized by the dendrimer 1 [105]
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Equation 4
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Figure 2 Superparamagnetic iron oxide nanoparticles cored dendrimer[128]
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Figure 3 Impregnation of superparamagnetic iron oxide nanoparticles with dendrimer-encapsulated nanoparticles[129]