Efficient click-polymer-stabilized palladium nanoparticle catalysts for Suzuki-Miyaura reactions of bromoarenes and reduction of 4-nitrophenol in aqueous solvents

Article abstract of DOI:10.1002/adsc.201300633

Palladium nanoparticles (size=1.6 ± 0.3 nm) stabilized by a polyethylene glycol containing triazolyl rings, synthesized by click reaction in water, are efficient catalysts for the Suzuki-Miyaura coupling of bromoarenes in aqueous ethanol and the borohydride reduction of 4-nitrophenol in water. Turnover numbers (TONs) reach 99,000 and turnover frequencies (TOFs) are up to 198,000 h^-1 for the C-C cross coupling reactions in water/ethanol.

Full text of DOI:10.1002/adsc.201300633

FULL PAPERS
DOI: 10.1002/adsc.201300633
Efficient Click-Polymer-Stabilized Palladium Nanoparticle
Catalysts for Suzuki–Miyaura Reactions of Bromoarenes and
Reduction of 4-Nitrophenol in Aqueous Solvents
a
b
a
a,
Christophe Deraedt, Lionel Salmon, Jaime Ruiz, and Didier Astruc *
a
ISM, UMR CNRS 5255, Univ. Bordeaux, 351 Cours de la Libꢀration, 33405 Talence Cedex, France
E-mail: d.astruc@ism.u-bordeaux1.fr
LCC, CNRS & University of Toulouse, 205 Route de Narbonne, 31077 Toulouse Cedex, France
b
Received: July 19, 2013; Revised: September 10, 2013; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300633.
À1
Abstract: Palladium nanoparticles (size=1.6Æ over frequencies (TOFs) are up to 198,000 h for
0
.3 nm) stabilized by a polyethylene glycol contain- the CÀC cross coupling reactions in water/ethanol.
ing triazolyl rings, synthesized by click reaction in
water, are efficient catalysts for the Suzuki–Miyaura
coupling of bromoarenes in aqueous ethanol and the Keywords: bromoarenes; catalysis; click chemistry;
borohydride reduction of 4-nitrophenol in water. green chemistry; 4-nitrophenol; palladium nanoparti-
Turnover numbers (TONs) reach 99,000 and turn- cles; polymers; Suzuki–Miyaura reaction
Introduction
temperature, with an amount of catalyst of 0.8–
[10c]
1
mol% Pd, the PdNPs being supported on TiO2.
Palladium is the most frequently used transition metal Beletskayaꢁs group proposed the term homeopathic
[1]
in catalysis, especially in the cross-coupling reactions Pd catalysis for Heck reactions using PdNP precata-
[
2]
[2a]
for carbon-carbon bond formation, among which lysts. De Vries used the “homeopathic” concept to
[
3]
the Suzuki–Miyaura reaction is the most useful one. investigate the mechanism of high-temperature Heck
Metal nanoparticles (NPs) have appeared as a very reactions using low PdNP loading.
[
10a]
Hongꢁs group
promising solution towards efficient catalysis under also used PdNPs stabilized by a bidendate ionic
mild, environmentally benign conditions because of ligand in 0.001 mol% Pd for the Suzuki–Miyaura cou-
their large surface-to-volume ratio and specific sur- pling of activated aryl bromides, the reactions being
[
4]
[10d]
face reactivities. The advantage of the NPs, com- carried out in only water at 1208C for a few hours.
pared to organometallic catalysts, is that one does not Here we wish to examine the stabilization and cata-
need to use large amounts of catalyst (generally less lytic efficiency of PdNPs produced upon NaBH re-
4
[
2a,b,4,5]
than 1 mol%),
and ligands (that are often ex- duction of the water-soluble Pd(II) precursor K PdCl4
2
pensive and sometimes toxic) are not required, which in the presence specific ethylene glycol oligomer 3
is also important for the treatment of the reactions, li- and polymer 5 formed by “click” reactions, i.e., con-
gands often being difficult to separate from the final taining 1,2,3-triazolyl (trz) linkages. We are also inter-
products.
PdNPs with a dimension of less than 10 nm have known
exhibited high catalytic activities toward different dendrimer
ested in comparing these properties with those of the
water-soluble arene-centered “click”
C H -1,3,5[C{(CH ) SiMe CH -trz-
AHCTUNGTRENNUNG
6
3
2
3
2
2
[
2a,b,4,5]
types of reactions.
They are often stabilized by CH OCH C H -2,3,4-{(OCH CH ) OCH } } ] , 6, that
2 2 6 2 2 2 3 3 3 3 3
[6]
polymers (PVP, PS, PEG, copolymers), dendrimers,
contains 9 trz linkages and 27 TEG termini (TEG=
cyclodextrins (CDs), inorganic materials, or ionic triethylene glycol). Triazolyl rings that were intro-
[
7]
[8]
[8c,9]
[10]
liquids.
For instance, among recent works,
duced by “click” functionalization in the oligomer,
Nomuraꢁ s group conducted quantitative Suzuki– polymer and dendrimer with TEG termini provide
Miyaura reactions with various substrates in water at the stabilization of the PdNPs after reduction of
8
08C with PdNPs (1.5 mol% Pd) stabilized by a PS Pd(II) by NaBH . The PdNPs stabilized by 6 present
4
[10b]
polymer.
Reddyꢁs group carried out Suzuki– a very good activity in the Suzuki–Miyaura reaction
[10e,f]
Miyaura reactions with various bromoarenes at room of various bromoarenes.
Bromoarenes are indeed
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Christophe Deraedt et al.
FULL PAPERS
very important aromatic substrates, because they are
often cheaper than their chloroarene analogues, and
therefore they are now subjected to further study.
The catalytic activity of PdNPs stabilized by 3 and
5
in the Suzuki–Miyaura coupling of bromoarenes in
an aqueous solvent, H O:ethanol=1:1 and the reduc-
2
tion of 4-nitrophenol to 4-aminophenol by sodium
borohydride in water have now been examined. 4-
Aminophenol is a potential industrial intermediate in
the manufacture of many analgesic and antipyretic
drugs, anticorrosion lubricants, and hair drying agents.
Therefore efficient catalysts for the borohydride re-
duction of 4-nitrophenol to 4-aminophenol are called
for.
Scheme 1. Synthesis of the PEG-bis-triazolyl oligomer 3 by
“
click” reaction between tetraethylene glycol bisazide 1 and
,5,8,11-tetraoxatetradec-13-yne 2.
2
“Click” chemistry was also used for the synthesis of
Results and Discussion
the polymer triazolyl-PEG 5. The reaction between
equimolar amounts of the commercial products tetra-
ethylene glycol bisazide 1 and 4,7,10,13,16-pentaoxa-
nonadeca-1,18-diyne 4 in DMF and in the presence of
Synthesis of the PEG-Bis-triazolyl Oligomer 3 and
Triazolyl-PEG Polymer 5
5
mol% CuBr as catalyst leads to the polymer triazol-
New PEG oligomers and polymers have been de- yl-PEG 5 in 80% yield (Scheme 2). The final polymer
1
13
signed with 1,2,3-triazole rings in their branches in has been characterized by H NMR, C NMR, IR,
order to provide the mild stabilization of PdNPs by MALDI TOF mass spectroscopy, size extrusion chro-
the synergistic combination of triazoles that bind the matography (SEC) and dynamic light scattering
PdNP surface and PEG that contribute by their steric (DLS, see the Supporting Information).
effect. Click chemistry offers a simple and practical
The SEC analysis provides the polydispersity of the
possibility for the synthesis of such oligomers and polymer, PDI=1.3, for this uncontrolled polymeri-
4
À1
polymers. The PEG-bis-triazolyl-PEG 3 was synthe- zation. The molecular weight, 5ꢃ10 gmol , is deter-
sized upon clicking tetraethylene glycol bisazide mined by SEC using polystyrene as a reference, which
1
with 2,5,8,11-tetraoxatetradec-13-yne 2 in H O/THF corresponds to 97 dimeric units, i.e., 194 triazolyl
2
(
50/50) with 4 mol% of CuSO ·5H O as catalyst and rings. The mass spectrum shows the presence of small
4 2
1
0 mol% sodium ascorbate as reductant of Cu(II) to oligomers with 18 triazolyl rings (molecular weight=
À1
Cu(I). This reaction leads to 3 in 90% yield 4626 gmol ), but polymers with higher molecular
(
Scheme 1).
weights cannot be observed due to saturation of the
1
The compound 3 has been characterized by detector with the small oligomers. In H NMR, the
1
13
H NMR, C NMR, heterogeneous single quantum peak at 2.4 ppm corresponding to the alkyne proton
coherence (HSQC), electrospray ionization (ESI) of the starting material is not observed, which allows
mass spectroscopy, and elemental analysis (see the the conclusion that all the monomeric dialkyne has
Supporting Information).
reacted with the monomeric diazide, but does not
Scheme 2. Synthesis of the triazolyl-PEG polymer 5 by “click” reaction between tetraethylene glycol bisazide 1 and
4,7,10,13,16-pentaoxanonadeca-1,18-diyne 4.
2
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Efficient Click-Polymer-Stabilized Palladium Nanoparticle Catalysts for Suzuki–Miyaura Reactions
permit any conclusion concerning the number of
units.
TEM analyses (Figure 2) have been conducted on
the PdNPs, showing a diameter of 2.0Æ0.5 nm upon
The polymer 5 and the oligomer 3 are both soluble stabilization by 3 and 1.6Æ0.3 nm upon stabilization
in water as expected and desired for the synthesis of by 5. The bright-grey colour around PdNPs in the
PdNPs in water.
TEM pictures shows the stabilizing oligomer or poly-
mer that appears to surround several PdNPs.
The DLS experiments show that the hydrodynamic
diameter of the polymer in the absence of PdNPs is in
the range 45–55 nm, which corresponds to an agglom-
PdNP Stabilization
The palladium-polymer complex is prepared in water erate of 5 in water. When the PdNPs are stabilized by
upon mixing aqueous solutions of the polymer and of 5, the hydrodynamic diameter remains unchanged.
K PdCl at 208C. In previous works with dendrimers There is an excess of polymer vs. the PdNPs in the
2
4
[
10e]
containing triazolyl TEG (6)
and sulfonated ter- synthesis, and it appears in the TEM images that an
[6c]
mini, the stoichiometry between the triazolyl group agglomeration of polymers surrounds the PdNPs. A
and the Pd(II) complex was 1:1, and this stoichiome- parallel experiment has been carried out with the
try was explained by the dendritic protection of the same conditions of PdNP synthesis but with the re-
interior triazole ligands. In the case of the polymer, placement of the new polymer 5 by commercial PEG
the best conditions for the stabilization of PdNPs are 2000, and in that case the PdNPs agglomerated sever-
a stoichiometry between triazolyl group and Pd(II) al hours after the Pd(II) reduction to Pd(0). Concern-
around 10:1. In aqueous K PdCl , a solution of the ing the PdNPs stabilized by the polymer 5, no trace of
2
4
polymer (or oligomer) is added in this stoichiometry. Pd aggregation is observed several days after the syn-
After five minutes of stirring of Pd(II)/triazole reac- thesis. This comparison shows the essential role of the
tion, a solution of sodium borohydride is rapidly triazole ligands in the PdNP stabilization by 5
added in order to form PdNPs upon reduction of (Scheme 3).
Pd(II) to Pd(0) (see the Experimental Section). The
On the other hand, when PdNPs are stabilized by
light-yellow solution of Pd(II) instantaneously the oligomer 3, some black Pd formation is observed
became orange/brown (see Figure 1), which indicated
the quick formation of PdNPs. A fast reduction of
Pd(II) to PdNPs is preferred to the dropwise addition
leading to a slower reduction, because a high reduc-
tion rate results in smaller PdNPs than a slow reduc-
tion rate. It is well known that the catalytic activity is
improved by reducing the PdNP size within the range
[14e]
of a few nm.
Scheme 3. Principle of PdNP stabilization. Pd(II) is added to
the polymer triazolyl-PEG 5 and is coordinated by nitrogen
atoms of the triazolyl ring. Pd(II) is then reduced to Pd(0)
forming PdNPs that are stabilized by the overall polymer
frameworks including crucial triazole-PdNP interactions.
Figure 1. During the PdNP synthesis, the reduction of Pd(II)
to PdNPs by NaBH4 is visible by the change of a light
(
(
yellow) solution (left), to a darker (orange/brown) solution
right).
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Christophe Deraedt et al.
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Figure 2. TEM pictures of the PdNPs. (a) TEM analysis of PdNPs stabilized by 5; (b) zoom on several PdNPs; (c) distribu-
tion of 1100 PdNPs; average diameter value: 1.6Æ0.3 nm. (d) TEM analysis of PdNPs stabilized by 3; (e) distribution of
1250 PdNPs: average diameter value: 2.0Æ0.5 nm.
after 2–3 days, indicating the decisive role of the bulk
of the molecular framework of the polymer 5.
Catalytic Studies
In order to check the catalytic activity of PdNPs, the
Suzuki–Miyaura coupling has been investigated. The
Suzuki–Miyaura cross coupling reactions of various
bromoarenes [Eq. (1)] have been carried out in the
environmentally the friendly mixture water:ethanol
(
1:1) in the presence of K PO , with as little palladi- also works very well with iodoarenes, because they
3 4
[11]
um as possible. All the results are summarized in are more easily activated, as it is well known.
Table 1. The most remarkable result is that obtained The Suzuki–Miyaura reaction works very well with
in the Miyaura-Suzuki reaction of 4-bromoacetophe- activated (electron-withdrawing substituents) as well
none and phenylboronic acid that is quantitative with as deactivated (electron-donating substituents) bro-
1
ppm of catalyst during 16 h at 808C. Under these or moarenes with 0.001 mol% of Pd (i.e., 10 ppm). What
3
analogous conditions, the TONs vary between 10 and is very remarkable is the TON up to 99,000 for the
6
10 with various bromoarenes. Of course, the catalysis reaction with the 4-bromoacetophenone (entry 13),
4
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Efficient Click-Polymer-Stabilized Palladium Nanoparticle Catalysts for Suzuki–Miyaura Reactions
Table 1. Suzuki–Miyaura of various bromoarenes catalyzed
by PdNPs stabilized by 5.
Suzuki–Miyaura Reaction between 4-Bromoaceto-
[a]
phenone and Phenylboronic Acid in EtOH:H O=1:1
2
[c]
[Eq. (2)]
Entry
R
Time Pd
Yield
TON; TOF
À1
[
h] [mol%] [%]
A
H
U
G
R
N
U
G
A[ h ]
H
U
G
R
N
U
G
1
2
3
4
5
4-OMe
4-OMe
4-OMe
4-OMe
H
12
12
20
20
23
23
20
24
2.5
24
14
18
0.1
0.01
99
99
98
80
99
99
99
72
99
90
96
98
99
990; 82.5
9900; 825
0.002
0.001
0.002
0.001
0.002
0.001
0.001
0.01
49,000; 2450
80,000; 4000
49,500; 2152
99,000; 4304
49,500; 2475
72,000; 3000
99,000; 39,600
9000; 375
[
[
a]
a]
6
7
8
9
1
1
1
1
H
4-Me
4-Me
4-NO2
4-CHO
4-Br
0
1
2
3
Kinetic Study of the Reaction
0.002
0.001
0.001
48,000; 3429
98000; 5444
99,000; 198,000
4-Br
4-COMe 0.5
The kinetic study (Figure 3) shows that the reaction
between 4-bromoacetophenone and phenylboronic
acid is complete in 30 min and that after 10 min the
isolated yield is already 72%.
[
b]
[
[
[
a]
All the reactions have been carried out in 10 mL of H O/
EtOH (1/1) with 1 mmol of bromoarene, 1.5 mmol of
2
phenylboronic acid, 2 mmol of K PO at 808C.
3
4
b]
The reaction has been also been conducted on a multi-
gram scale (n=15 mmol), and the yield remained the
same.
Isolated yields.
c]
and that the reaction is finished in 30 min, which cor-
À1
responds to a TOF=198,000 h that was never ob-
tained before this study with bromoarenes. The reac-
tions with deactivated arenes such as 1,4-bromoani-
sole (entry 4) or 1,4-bromotoluene (entry 8) lead to
a TON of 80,000 and 72,000, respectively, in a relative-
ly short time too. The reaction with activated bro-
moarenes such as 1,4-bromonitrobenzene or 1,4-di-
bromobenzene leads to a quantitative yield for both
substrates in a relatively short time (2 h and 14 h, re-
spectively). These results are expected because the
oxidative addition on the Pd species is faster when Figure 3. Kinetic study of the Suzuki–Miyaura reaction be-
bromoarenes are functionalized with electron-with- tween 4-bromoacetophenone and phenylboronic acid. The
drawing groups that weaken the carbon-halogen reaction reaches a quantitative yield in 30 min.
bond.
These results show the excellent catalytic activity of
PdNPs stabilized by the polymer 5 due to ideal stabi- Concentration Study
lization for catalysis by the triazolyl groups (as with
[
10e]
dendrimer 6 containing triazolyl TEG termini ). A study of the influence of the concentration has
Catalysis with PdNPs stabilized by the oligomer 3 is been conducted in order to optimize the reaction con-
also efficient down to 0.002 mol% of Pd (quite the ditions (Table 2). It shows that a total volume of the
same as PdNPs stabilized by the polymer 5), but then solvent mixture in the range 5–10 mL is necessary for
with less than 0.002 mol% the activity is almost non- optimized reactions.
existent due to the lack of sufficient stabilization.
The results show that the reaction with 10 ppm Pd
We attempted to conduct Suzuki–Miyaura reactions (PdNPs) is nearly quantitative with several bromoar-
with PdNPs stabilized by the commercial PEG 2000, enes; therefore the minimal quantity of catalyst nec-
under the same conditions as those indicated in essary for the reaction with the most reactive sub-
Table 1 (entry 3, for example), and no CÀC coupling strate, 4-bomoacetophenone, has been investigated
was observed.
(Table 3).
The series of TONs data obtained here in Suzuki–
Miyaura reactions with various bromoarenes are very
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Table 2. Influence of the volume of the solvent mixture
Table 4. Comparison of literature results obtained with
PdNP catalysts in aqueous medium for the Miyaura–Suzuki
reactions of bromoarenes.
EtOH/H O: 1/1 on the yield of the Suzuki–Miyaura reac-
2
tions between 4-bromoacetophenone and phenylboronic
[a]
acid.
[ref.]
R
Catalyst
Temp. TON TOF
À1
Total volume [mL]
Yield of the reaction [%]
[8C]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[h ]
[
12a]
0
2
5
1
2
4
.2
48
90
99
99
60
50
4-COMe
PSSA-co-MA-Pd(0) 100
99
38
1980
456
1680
49
119
12
38
10
39
14050
970
[
12b]
4-OMe[
Pd-SDS
Pd-PVP (MTPs)
Pd-PEG
100
100
25
12c]
4-OMe
1680
98
950
144
340
50
156
4681
970
[12d]
0
0
0
4-COMe[
12e]
4-COMe
Pd-1/FSG
Fe O -Pd
100
50
[
[
12f]
4-OMe
3
4
12g]
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
-OMe
-OMe
pEVPBr-Pd
Pd-PS
90
[
a]
The reactions have been carried with 0.001 mol% PdNPs
[12h]
[12i]
100
100
60
at 808C during 1 h.
-COMe
HAP-Pd(0)
PdCl (py) @SHS
[
12j]
-OMe
2
2
[10d]
-COMe
Pd/IL
Pd-MEPI
Pd-salt
120
100
90
[
12k]
Table 3. Influence of the PdNPs loading on the TONs and
TOFs of the Suzuki–Miyaura reactions between 4-bromo-
-OMe
24250 8083
[10c]
-COMe
4250
300
92
310
85000 2125
990
230
66
115
475
96
386
45
19600 2800
1062
30
8
[a]
[
12l]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
acetophenone and phenylboronic acid.
-OMe
Pd@PNIPAM
Pdx- ([PW O ] )
90
80
[4c]
7À
11 39 y
-COMe
À1
Entry Time [h] Pd [mol%] Yield [%] TON; TOF [h ]
[12m]
-OMe
Pd-block-co-poly
Pd-G3-p3
90
80
31
[12n]
-COMe[
1
1
1
1
3
4
5
6
0.5
2
16
24
0.001
99
99
99
–
99,000; 198,000
198,000; 99,000
990,000; 61,875
–
12o]
12p]
10a]
-COMe
Pd@CNPCs
Pd-PPy/PS
50
80
3960
77
66
29
95
7
0.0005
0.0001
0.00005
[
-COMe[
-COMe
PS-PdONPs
Pd-TiO2
80
80
[
10b]
-Me
[
a]
[12q]
All the reactions have been carried out in 10 mL of H O/
EtOH (1/1) with 1 mmol of 4-bromoacetophenone,
.5 mmol of phenylboronic acid and 2 mmol of K PO at
4-OMe
Pd@PMO-IL
Pd-XH-15-SBA
Pd -G0
75
90
80
2
[
12r]
4-NH2
[
12s]
2+
1
4-OMe
99
12
3
4
[
12t]
8
08C.
4-Me
-OMe
Pd(0)/Al O -ZrO
60
2
3
2
[
12u]
4
Pd
A
H
U
G
R
N
N
(OAc) /L
100
2
good in comparison with literature results reported in
Table 4, and the TOFs obtained here are superior to
those obtained with PdNPs stabilized by dendrimer 6
Table 5. Comparison between the properties of the PdNPs
stabilized by dendrimer 6 properties and those of the PdNPs
stabilized by 5.
(
Table 5).
In Table 4, the literature data found for Miyaura– Support:
Suzuki cross-coupling reactions of bromoarenes in Properties
Dendrimer 6
Polymer 5
aqueous solutions are summarized, indicating that the
PdNP catalysts reported here are among the very best
ones reported so far.
Interestingly, the PdNPs stabilized by polymer 5 Storage
provide lower TONs, but better TOFs than those with
dendrimer 6. The better TONs obtained with the den- Air stable
Size PdNP
TON (Suzuki–Miyaura)
TOF (Suzuki–Miyaura)
1.4Æ0.7 nm
1.6Æ0.3 nm
6
5
2.7ꢃ10
9.9ꢃ10
4
À1
5
À1
4.5ꢃ10 h
1.98ꢃ10 h
Several
months
yes
Several
weeks
yes
Easy synthesis of the sup-
port
Synthesis of the support
yes
yes
drimer 6 than with the polymer 5 are taken into ac-
count by a better protection by encapsulation of the
[
10e]
8 steps
1 step
PdNPs in the dendrimer,
whereas the better TOFs
obtained with the polymer result from freer access of
the substrates to the PdNP surface. See the compara-
tive findings between PdNP stabilized by the dendri- Reduction of 4-Nitrophenol to 4-Aminophenol
mer 6 and PdNP stabilized by 5 (Table 5).
The advantage of this system in comparison with The high efficiency of PdNPs described above in the
dendrimer 6 is that the polymer needs only one Suzuki–Miyaura-catalyzed CÀC coupling and the fact
simple synthetic step, and that PdNPs stabilized by that the rate of this catalysis clearly depends on the
the polymer 5 are almost as active for the Suzuki– size of the nanoparticles led to the idea that such cat-
Miyaura coupling as the dendrimer 6 but with the par- alysts might also be efficient for nitrophenol reduc-
ticular feature of a faster reaction (higher TOFs).
tion. This reaction is very fast and simple, with the ad-
vantage of the possibility of monitoring by UV-vis.
6
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Efficient Click-Polymer-Stabilized Palladium Nanoparticle Catalysts for Suzuki–Miyaura Reactions
spectroscopy. Indeed, a typical peak at 400 nm is di-
rectly linked to 4-nitrophenol (corresponding to 4-ni-
trophenolate that instantaneously appears in the pres-
ence of NaBH ) and at 300 nm to 4-aminophenol. The
4
disappearance of the yellow colour in the solution
shows the reaction progress. The reduction of 4-nitro-
phenol has been carried out in the presence of an
excess of NaBH4 (100 equiv.) and 0.2 mol% of
PdNPs/5 in water in the presence of catalytic amounts
of Pd from the above PdNPs.
The reaction is completed in 240 seconds, which
À3 À1
corresponds to a Kapp value of 6.0ꢃ10
s
(Figure 4).
When the reduction of 4-nitrophenol is carried out in
a concentration that is increased four times, the K
value is nearly ten times higher (Kapp =5.5ꢃ10 s ),
app
À2 À1
and the reaction is completed in less than 80 seconds.
The reaction rate constant Kapp is calculated using the
rate equation Àln
ACHTNUGNTERNUN(G C /C )=Kapp·t (the catalyst exhibits
a pseudo-first order kinetics).
t 0
The results obtained with the catalyst 5 are among
the best results ever recorded (here only Pd catalysts
have been compared in Table 6, but Au catalysts are
less impressive too) in terms of high TOF values, low
amounts of catalyst and similar to the literature in
Figure 4. Kinetic study of the 4-nitrophenol reduction by
NaBH using UV-vis. spectroscopy at 400 nm (top) and plot
4
of Àln
ACHTUNGTNERNGNU( C /C ) vs. time (s) for its disappearance (bottom).
0 t
term of rate constant Kapp
.
mise for an excellent activity in the Suzuki–Miyaura
Concluding Remarks
reaction of bromoarenes in EtOH:H O=1:1 with
a very low amount of catalyst confirming results with
2
The polymer 5 has been synthesized by a “click” reac- the triazolyl-TEG dendrimer 6. The use of only
tion between commercial PEG diazide and PEG 1 ppm Pd leads to a quasi-quantitative reaction with
diyne and shown to be a very good stabilizer of 4-bromoacetophenone (entry 15, Table 3 TON:
PdNPs of 1.6 nm diameter in comparison with 990,000), and by increasing the amount of catalyst,
PEG 2000 that leads to PdNP aggregation. The com- this reaction is complete in 30 min. The results ob-
bination of triazolyl rings and PEG offers a compro- tained in the reduction of 4-nitrophenol are also ex-
Table 6. Comparison of literature results obtained with PdNP catalysts for the reduction of 4-nitrophenol in water.
À1
À1
Catalyst/support
Pd [mol%]
Size Pd [nm]
NaBH [equiv.]
Kapp [s ]
TOF [h ]
4
[
14a]
À3
CNT/PiHP
Fe O4
3
4
10
77
0.36
2.1
2.6
100
0.56
0.2
0.2
2.7
80
5ꢃ10
300
300
13
819
139
326
6
1 068
7 500
22 500
[14b]
À2
À2
16.9Æ1.3
139
excess
100
100
11
1000
83
100
100
3.3ꢃ10
6.6ꢃ10
[14c]
PEDOT-PSS
<9
[14d]
À3
SPB
2.4Æ0.5
3.8Æ0.6
2.0
~8
3–5
4.41ꢃ10
[
14e]
À3
Microgels
1.5ꢃ10
[
14f]
À2
PPy/TiO2
1.22ꢃ10
[
14g]
À2
À3
SBA-15
CeO2
1.2ꢃ10
[
14h]
8ꢃ10
6ꢃ10
À3
5
5
1.6
1.6
À2
5.5x 10
Adv. Synth. Catal. 0000, 000, 0 – 0
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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ÞÞ
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Christophe Deraedt et al.
FULL PAPERS
cellent. Macromolecules such as dendrimer 6 or poly- Synthesis of the PEG-bis-triazolyl Oligomer 3
mer 5 are outstanding for a compromise between sta-
bilization and catalytic activity of PdNPs, whereas
small molecules such as the oligomer 3 are not suffi-
ciently efficient stabilizers to provide active PdNPs.
Bisazide 1 (0.122 g, 0.5 mmol) and 2 (0.202 g, 1 mmol,
2
equiv.) were dissolved in 2 mL THF in a Schlenk tube.
CuSO ·5H O was added (0.0048 g, 0.02 mmol, 0.04 equiv.)
4
2
after dissolving in 1 mL of water, followed by the dropwise
This is due to the fact that PdNPs can take shelter in addition of a freshly prepared solution of sodium ascorbate
the holes of the dendrimer or can be enveloped by (0.0124 g, 0.1 mmol, 0.1 equiv.) in order to set a 1:1 THF:wa-
several polymer molecules 5. The comparison be- ter ratio. The reaction mixture was stirred for 2 days at
2
58C under N . After removing THF under vacuum, CH Cl
2 2 2
tween the triazole-containing dendrimer 6 and poly-
mer 5 interestingly shows that the polymer provides
higher TOF values because of freer access than the
dendrimer, whereas the higher protection in the den-
drimer interior offers higher TONs than with the
polymer due to the better PdNP protection.
(
100 mL) and an aqueous ammonia solution (2.0M, 50 mL)
were successively added. The mixture was allowed to stir for
0 min in order to remove all the Cu(II) trapped inside the
trimer as [Cu(NH ) (H O) ] [SO ]. The organic phase was
1
A
H
U
G
R
N
U
G
ACHTUNGTRENNUNG
3
2
2
2
4
washed twice with water. The organic phase was dried with
sodium sulfate, and the solvent is removed under vacuum to
afford 3; yield: 0.290 g (90%). The product was fully charac-
The extremely low amount of Pd catalyst and the
1
13
biocompatibility of the PEG polymers opens an terized by H NMR, C NMR, HSQC, ESI, EA (see the
access to biological applications without the need for Supporting Information).
treatment of the product in order to remove the
metal. The multi-gram scale reaction reported here is
also very useful for potential industrial applications of
the Miyaura–Suzuki reactions of bromoarenes, espe-
cially given their modest cost among the halogenoar-
enes.
Synthesis of Triazolyl-PEG Polymer 5
[13]
[13]
Bisazide 1
(0.244 g, 1 mmol) and 4
(0.270 g, 1 mmol,
1
equiv.) were dissolved in 1 mL DMF in a Schlenk tube.
CuBr was added in the vessel (0.0102 g, 0.05 equiv.). The re-
action mixture was stirred for 2 days at 408C under N . The
2
solution was then added to 200 mL of Et O in order to pre-
2
cipitate the polymer formed. The precipitate was filtered,
and CH Cl (100 mL) and an aqueous ammonia solution
Experimental Section
2
2
(
2.0M, 50 mL) were successively added. The mixture was al-
lowed to stir for 10 min in order to remove all the Cu(II)
trapped inside the polymer as [Cu(NH ) (H O) ] [SO ]. The
General Data
A
H
U
G
R
N
N
ACHTUNGTRENNUNG
3
2
2
2
4
All the solvents (THF, EtOH, DMF, CH Cl , Et O) and
chemicals were used as received. H and C NMR spectra
were recorded at 258C with a Bruker AC 200, 300 or 400
2
13
2
2
organic phase was washed twice with water, then this opera-
tion was repeated once more to ensure the complete remov-
al of copper ions. The organic phase was dried with sodium
sulfate, and the solvent was removed under vacuum to
afford 5; yield: 0.410 g (80%). The product has been fully
1
(
200, 300 or 400 MHz) spectrometer. All the chemical shifts
are reported in parts per million (d, ppm) with reference to
1
Me Si (TMS) for the H NMR spectra. The infrared (IR)
1
13
4
characterized by H NMR, C NMR, IR, MALDI TOF,
spectra were recorded on an ATI Mattson Genesis series
FT-IR spectrophotometer. Size exclusion chromatography
DLS, SEC (see the Supporting Informatoin).
(
SEC) of the polymer 5 was performed using a JASCO
HPLC pump type 880-PU, TOSOHAAS TSK gel columns
G4000, G3000, G2000 with pore sizes of 20,75, and 200 ꢄ
Preparation of the PdNPs for Catalysis
(
À2
a): 1.62ꢃ10 mmol of polymer triazolyl PEG 5 (dimer
respectively, connected in series), a Varian (series RI-3) re-
fractive index detector, with DMF as the mobile phase and
calibrated with polystyrene standard. MALDI-TOF mass
spectra were recorded with a PerSeptive Biosystems Voyag-
er Elite (Framingham, MA) time-of-flight mass spectrome-
ter. This instrument is equipped with a nitrogen laser
À1
MW=514 g.mol , 0.8368 mg) were dissolved in 0.3 mL of
water in a Schlenk flask, and an orange solution of K PdCl
2
4
À5
(
3.2ꢃ10 mmol in 0.11 mL water) was added to the solution
of 5. Then 2.8 mL of water were added, and the solution
was stirred for 5 min at 208C. 0.1 mL of an aqueous solution
À4
containing 3.2ꢃ10 mmol of NaBH was added dropwise,
4
(
337 nm), a delayed extraction, and a reflector. It was oper-
provoking the formation of a brown/black colour corre-
sponding to the reduction of Pd(II) to Pd(0) and PdNP for-
mation [Figure S13 in the Supporting Inromation shows the
ated at an accelerating potential of 20 kV in both linear and
reflection modes. The mass spectra shown represent an aver-
age over 256 consecutive laser shots (3 Hz repetition rate).
Peptides were used to calibrate the mass scale using the two
points calibration software 3.07.1 from PerSeptive Biosys-
tems. Mentioned m/z values correspond to monoisotopic
masses. Elemental analysis (EA) results were obtained with
a Thermo Flash 2000 EA; the sample is introduced in a tin
container for CHS analysis.
full reduction of Pd(II) in Pd(0)]. The use of NaBH is es-
4
sential in order to obtain small, active PdNPs for efficient
catalysis. When PdNPs are formed in situ (without NaBH4)
during the Suzuki–Miyaura reaction, the catalytic activity of
the PdNPs is much lower. The same reaction as entry 13 was
carried out with PdNPs formed in situ, and 22 h were
needed for a quantitative reaction instead of 2 h with PdNPs
reduced by NaBH .
4
À3
b): 1.3ꢃ10 mmol of oligomer PEG
48 g.mol , 0.8368 mg) were dissolved in 0.3 mL of water in
3
(MW=
À1
6
a Schlenk flask, and an orange solution of K PdCl (3.2ꢃ
2
4
8
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Efficient Click-Polymer-Stabilized Palladium Nanoparticle Catalysts for Suzuki–Miyaura Reactions
À5
10
mmol in 0.11 mL water) was added to the solution of 5. Purification by Flash Chromatography of the
Then 2.8 mL of water were added, and the solution was
stirred for 5 min at 208C. 0.1 mL of an aqueous solution
containing 3.2ꢃ10 mmol of NaBH was added dropwise,
provoking the formation of a brown/black colour that corre-
sponds to the reduction of Pd(II) to Pd(0) and PdNP forma-
tion.
Coupling Products of the Suzuki–Miyaura Reactions
À4
Biphenyl (white powder): reaction between bromobenzene
and phenylboronic acid was followed by simple flash chro-
matography with petroleum ether as mobile phase and silica
as stationary phase; yield: 152 mg (99%) when 1 mmol of
bromoarene is used.
4
The stoichiometry, 10 triazolyl groups per Pd(II) is neces-
,
4
-Methylbiphenyl (colourless crystals): reaction between
sary in order to avoid PdNPs aggregation. When the stoi-
chiometries 1:1, 2:1, and 5:1 are used, PdNPs precipitate
after several hours due to aggregation.
4
-bromotoluene and phenylboronic acid was followed by
simple flash chromatography with petroleum ether as
mobile phase and silica as stationary phase; yield: 166 mg
(
99%) when 1 mmol of bromoarene is used.
-Methoxybiphenyl (white powder): reaction between
4
General Procedure for the Suzuki–Miyaura Catalysis
bromoanisole and phenylboronic acid was followed by flash
chromatography with petroleum ether as mobile phase and
silica as stationary phase at the beginning and then 95% pe-
troleum ether/5% diethyl ether; yield: 183 mg (99%) when
In a Schlenk flask containing tribasic potassium phosphate
(
(
2 equiv.), phenylboronic acid (1.5 equiv.), the aryl halide
1 equiv.) and 5 mL of EtOH (volume ratio of H O:EtOH=
2
1
mmol of bromoarene is used.
-Nitrobiphenyl (yellow crystals): reaction between 1,4-
1
:1) were successively added. Then the solution containing
4
the PdNPs was added. The suspension was allowed to stir
bromonitrobenzene and phenylboronic acid is followed by
flash chromatography with 95% petroleum ether/5% di-
chloromethane as mobile phase and silica as stationary
phase; yield: 197 mg (99%) when 1 mmol of bromoarene is
used.
under N or air. After the reaction time (see Table 1), the
2
reaction mixture was extracted twice with diethyl ether
(
Et O; all the reactants and final products are soluble in
2
Et O), the organic phase was dried over Na SO , and the
2
2
4
solvent was removed under vacuum.
4
-Biphenylcarboxaldehyde (light yellow crystals): reac-
In parallel, the reaction was checked using TLC in only
petroleum ether as eluent in nearly all the cases, and
tion between bromobenzaldehyde and phenylboronic acid
was followed by flash chromatography with 95% petroleum
ether/5% dichloromethane as mobile phase and silica as sta-
tionary phase; yield: 164 mg (90%) when 1 mmol of bro-
moarene is used.
p-Terphenyl (white powder): reaction between 1,4-dibro-
mobenzene and phenylboronic acid was followed by simple
flash chromatography with petroleum ether as mobile phase
and silica as stationary phase; yield: 226 mg (908%) when
1
H NMR. Purification by flash chromatography column was
conducted with silica gel as stationary phase and petroleum
ether as mobile phase. Another purification procedure con-
sists in cooling the Schlenk flask at the end of the reaction.
The product precipitates, and a simple filtration allows one
to obtain the product that is then washed with a cold solu-
tion of H O/EtOH (this is not the case of all the bromoar-
2
enes). With 1 ppm of catalyst, the reuse of the catalyst
shows a very slow reaction, which indicates that after 24 h
hours at 808C the catalyst is deactivated in solution. The
aqueous phase obtained after filtration cannot be reused,
because the polymer-stabilized catalyst is not stable for
a long time.
1
mmol of bromoarene is used.
-Acetylbiphenyl (white powder): reaction between 4-
4
bromoacetophenone and phenylboronic acid was followed
by flash chromatography with 95% petroleum ether/5% di-
ethyl ether as mobile phase and silica as stationary phase;
yield: 195 mg (99%) when 1 mmol of bromoarene is used.
After each reaction, the Schlenk flask was washed with
a solution of aqua regia (3 volumes of hydrochloric acid for
1
volume of nitric acid) in order to remove traces of Pd. All
the reactions have been carried out with 1 mmol of bro-
moarene. Three other reactions with 15 mmol of bromoar-
enes have been conducted in order to check the efficiency
of the system on a multigram scale. The yields obtained with
Acknowledgements
The staff of the CESAMO department of the Univ. Bordeaux
15 mmol of bromobenzene, 1,4-nitrobenzene or 4-bromoace-
(
elemental analyses and mass spectra), Paul Coupillaud,
tophenone are the same as that obtained with 1 mmol.
LCPO, Univ. Bordeaux (SEC analyses) and financial sup-
port from the Univ. Bordeaux 1 and Toulouse 3, the CNRS
and the Ministꢀre de l’Enseignement Supꢁrieur et de la Re-
cherche (PhD grant to CD) are gratefully acknowledged.
General Procedure for the Reduction of 4-
Nitrophenol
À5
In a beaker, 7 mg of 4-nitrophenol (5.03ꢃ10 mol) were
À3
mixed with 195 mg of NaBH4 (5.13ꢃ10 mol) in 20 mL References
water, 1 mL of PdNPs was added to the reaction mixture
(
0.2 mol%), and the reaction was completed in 80 seconds.
Alternatively, 0.5 mL of the total solution was diluted with
.5 mL of water before the reaction started in order to
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Adv. Synth. Catal. 0000, 000, 0 – 0
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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asc.wiley-vch.de
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Adv. Synth. Catal. 0000, 000, 0 – 0
ÝÝ
These are not the final page numbers!

FULL PAPERS
Efficient Click-Polymer-Stabilized Palladium Nanoparticle
Catalysts for Suzuki–Miyaura Reactions of Bromoarenes
and Reduction of 4-Nitrophenol in Aqueous Solvents
11
Adv. Synth. Catal. 2013, 355, 1 – 11
Christophe Deraedt, Lionel Salmon, Jaime Ruiz,
Didier Astruc*
Adv. Synth. Catal. 0000, 000, 0 – 0
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
11
ÞÞ
These are not the final page numbers!