Water-soluble palladium nanoparticles: Click synthesis and applications as a recyclable catalyst in Suzuki cross-couplings in aqueous media

Article abstract of DOI:10.1002/ejoc.201000671

A new PEG-tagged material, which was prepared by a threefold copper-catalyzed [3+2] cycloaddition (click chemistry), was found to act as an efficient stabilizer for palladium nanoparticles. The newly formed material proved to be active asa recyclable catalyst in Suzuki coupling the presence of polyether chains allowed for the catalytic runs to be conducted in aqueous media.

Full text of DOI:10.1002/ejoc.201000671

FULL PAPER
DOI: 10.1002/ejoc.201000671
Water-Soluble Palladium Nanoparticles: Click Synthesis and Applications as a
Recyclable Catalyst in Suzuki Cross-Couplings in Aqueous Media
Nereida Mejías,[a] Roser Pleixats,*[a] Alexandr Shafir,*[a] Mercedes Medio-Simón,[b] and
Gregorio Asensio[b]
Dedicated to the memory of Professor José Manuel Concellón Gracia
Keywords: Cross-coupling / Homogeneous catalysis / Nanoparticles / Palladium / Suzuki reaction
A new PEG-tagged material, which was prepared by a three-
fold copper-catalyzed [3+2] cycloaddition (click chemistry),
a recyclable catalyst in Suzuki coupling; the presence of
polyether chains allowed for the catalytic runs to be con-
was found to act as an efficient stabilizer for palladium nano- ducted in aqueous media.
particles. The newly formed material proved to be active as
in addition to their potential in sustainable green chemistry,
Introduction
may help simplify separation, recovery and recycling of the
catalyst, which is a scientific challenge of economic and en-
vironmental relevance.[1]
The development of chemical processes with minimal en-
vironmental impact has become an important challenge for
the chemistry community. One of the promising directions
in this field has been the development of industrial pro-
cesses that use water as a cheap, safe, and non-toxic reac-
tion medium. In particular, the development of water-based
catalytic processes has spurred the development of an entire
field of water-soluble catalytic systems. The majority of
these are discrete metal complexes supported by the ligands
that are suitably modified to impart greater hydrophilicity.
Commonly, such ligands are based on traditional phos-
phorus-, sulfur- and nitrogen-containing derivatives.[1] Most
often, efforts have been directed towards adapting tradi-
tional palladium-based catalytic processes, such as hydro-
genation, oxidation, as well as C–C, and C–heteroatom
bond-forming reactions to the aqueous medium.[2] Fre-
quently, the palladium-catalyzed Suzuki–Miyaura cross-
coupling[3] reaction, which is a powerful synthetic tool in
its own right, has served as a “sharpening stone”[4] for the
honing of other cross-coupling processes. In this context, a
relatively small but steadily growing number of publications
have been devoted to the catalytic applications of water-
soluble metal nanoparticles (NPs).[5] These developments,
In catalytic processes, metal NPs tend to be more reactive
than their particulate metal counterparts as a result of in-
creasing surface area with decreasing particle size. However,
a protective agent is necessary to prevent aggregation of
transition-metal NPs in solution towards the thermody-
namically favored bulk metal.[6] Such stabilization has been
provided by polymers, dendrimers, β-cyclodextrines, and
ionic and non-ionic surfactants, in addition to nitrogen-,
phosphorus-, and sulfur-based ligands, and several ionic
compounds. The nature of the protecting shield determines
to a great extent the solubility of the resulting metal colloid.
Thus, a judicious choice of stabilizer may render the mate-
rial soluble in organic, aqueous or even fluorous medium,
and may facilitate recycling. In 2001, some of us described
the use of fluorous-phase soluble palladium nanoparticles
as recoverable catalysts for C–C coupling reactions.[7] More
recently, we also reported that the use of 15-membered tri-
olefinic azamacrocycles bearing polyoxyethylenated chains
(Figure 1, product 1) led to the formation of water-soluble
metal NPs (Pd, Pt, Ru, Rh, and Au).[8] Of these, the Pd
NPs appeared to be promising catalysts in the Heck[8a] and
the Suzuki[8b] reactions in organic or aqueous media.
Furthermore, the material proved to be recoverable and was
reused in several catalytic runs. In addition to this work,
several other examples of metal NPs supported by PEG-
based moieties have been reported. Thus, poly(ethylene ox-
ide) units are present in some block copolymers and in
some surfactants that have been used as protecting agents
(Au, Pd and Pt NPs).[9] PEG-modified dendrimers[10] and
[a] Department of Chemistry, Universitat Autònoma de Barcelona,
08193 Cerdanyola del Vallès, Barcelona, Spain
Fax: +34-93-5811265
E-mail: roser.pleixats@uab.cat
alexandr.shafir@uab.cat
[b] Department of Organic Chemistry, Facultad de Farmacia,
Universidad de Valencia,
46100 Burjassot, Valencia, Spain
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000671.
View this journal online at
wileyonlinelibrary.com
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Water-Soluble Palladium Nanoparticles
PEG derivatives immobilized on silica[11] have also been de-
We report herein the synthesis of 2 and its use in the
scribed (Au and Pd NPs, respectively). A few reports[12] are formation of palladium NPs. We also report the application
available on the use of PEG-tagged compounds (such as of the newly prepared materials as recoverable catalysts for
bisphenol A derivatives, phosphane oxides, bipyridines, Fi- Suzuki–Miyaura coupling reactions of aryl iodides and bro-
scher carbenes, and oxime carbapalladacycles) as capping mides in aqueous media.
agents or direct metal sources for Pd and Au nanoclusters.
Although there are examples of metal NPs dispersed in un-
functionalized poly(ethylene glycols),[13] in most instances
the incorporation of amino[14] or thiol[15] functional groups
in the PEG matrix was found to improve the stabilizing
capacity for Au NP formation.
Results and Discussion
Preparation of Materials 2a and 2b
Two different chain lengths for the MeO-PEG-X compo-
nent were used in this investigation, one with n = 7 and the
second with n = 44. The preparation of compounds 2 (2a,
n = 7; 2b, n = 44) is summarized in Scheme 1.
Treatment of commercially available MeO-PEG-OH 3a
and 3b (n = 7 and n = 44, respectively) with methanesulf-
onyl chloride in the presence of triethylamine furnished the
corresponding mesylates 4a and 4b. Although initially a
procedure involving toluene as solvent and a large excess of
mesylating agent was applied,[19] we later found that on
larger scales (5 g for 3a and 10 g for 3b), the use of dichloro-
methane and only 2 equiv. of methanesulfonyl chloride[20]
was more convenient to give the corresponding mesylates
in 95 and 61% yield, respectively. These were then allowed
to react with sodium azide in dimethylformamide at 60 °C
Figure 1. PEG-tagged compounds 1 and 2.
Encouraged by the promise shown by PEG-tagged for 1 d to afford the corresponding azides 5a and 5b in good
macrocycles of type 1 in the formation of metal NPs,[8] we yields. The second coupling partner, 1,3,5-triethynylbenzene
sought to improve the structure by creating a stabilizer that (6), was obtained in two steps from 1,3,5-tribromobenzene
would be easier to prepare and that would lead to NPs with through 1,3,5-tris(trimethylsilylethynyl)benzene as de-
a better performance in the catalytic C–C coupling reac- scribed previously.[21]
tions in aqueous media. Given that system 1 featured a co-
Finally, threefold copper-catalyzed 1,3-dipolar cycload-
ordinating macrocycle flanked by three long polyoxyeth- dition between trialkyne 6 and either azide 5a or 5b (1:3.3
ylenated chains, a related PEG-tagged compound (Figure 1, molar ratio) under the standard click conditions[22] pro-
product 2) was devised. Its synthesis would be ac- vided the desired compounds 2a and 2b in nearly quantita-
complished through a copper-catalyzed Huisgen 1,3-dipolar tive yields. However, our initial attempts to remove the re-
cycloaddition between an azide and an alkyne (“click chem- sidual azide 5 (used in slight excess) using standard pro-
istry”).[16]
cedures encountered some difficulties due to the predomi-
The “click” [2+3] cycloaddition reaction has recently nance of the polyether chain in both the starting material
been applied to the synthesis of a number of materials in and the product, which led to poor separation. We, there-
polymer chemistry and material science, mainly as a result fore, sought to develop alternative procedures for obtaining
of their virtually quantitative yields, displaying high selec- 2a and 2b free of impurities.
tivity and functional-group tolerance in a wide range of
In the case of compound 2a (n = 7), purification was
substrates.[17] In the case of the proposed stabilizer 2, the based on the reactivity of organic azides with phosphanes
reaction would also provide potentially coordinating tri- (the Staudinger reaction). Thus, the crude reaction mixture
azole moieties, which would presumably enhance the ability was treated with a commercial polymeric triphenylphos-
of the material to stabilize metal NPs.[10c,18]
phane in tetrahydrofuran (THF) at 50 °C (Scheme 2) until
Scheme 1. Synthesis of stabilizers 2a and 2b.
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R. Pleixats, A. Shafir et al.
FULL PAPER
the IR spectrum showed complete disappearance of the these precedents, we opted to use this strategy for the prepa-
RN3 band at 2099 cm–1. At this point, the polymer-bound ration of palladium NPs with the newly obtained PEG-
phosphinimine byproduct and the unreacted phosphane based stabilizers 2 (Scheme 3, Table 1).
were removed by filtration, leaving behind pure 2a.
Scheme 3. Synthesis of palladium(0) NPs 2b-Pd.
As summarized in Table 1, a variety of conditions were
tested by using both 2a and 2b; in all cases, hydrogenation
with [Pd(dba)2][28] was performed in a stirred THF solution
under H2 (3 bar) at room temperature overnight.
Scheme 2. Removal of azide 5a by Staudinger trapping.
Factors such as the length of the PEG side chains, the
Although the same procedure could, in principle, be em-
Pd/2 ratio, and the concentration were all found to have an
ployed for the purification of compound 2b (MW ≈ 6300),
effect on the outcome (Table 1). Initial experiments were
we opted for the removal of excess azide 5b (n = 44, MW
performed by using 2b. Thus, an experiment involving
≈ 2000) by dialysis in water using a membrane with a molec-
ular weight cut-off (MWCO) of 3500 Da.[19]
0.016 mmol of 2b and a Pd/2b molar ratio of 1:1 led to
the generation of NPs with a mean diameter of 2.3 nm, as
determined by TEM (Table 1, Entry 1). A Pd/2b ratio of 2:1
did not improve the yield based on palladium, and the
mean size of the nanoparticles was 4.8 nm (Table 1, En-
Preparation of Palladium Nanoparticles
With compounds 2a and 2b in hand, we tested their suit- try 2). When the conditions described in Table 1, Entry 1
ability as stabilizers of palladium NPs. Although chemical were reproduced on a larger scale (0.064 mmol of 2b), a
reduction of PdII salts remains the most widely used material with similar mean particle size and with a similar
method for the synthesis of Pd0 nanoparticles,[5] a careful percentage of Pd was obtained with an improved yield
and laborious search for optimal reaction conditions is (Table 1, Entry 3). In all cases, the resulting homogeneous
often necessary. On the other hand, the organometallic ap- black suspension was filtered through a Milli-Pore filter to
proach (reduction and subsequent displacement of a ligand eliminate residual bulk palladium. The solvent was then
from an M0 organometallic precursor) developed by Chau- evaporated, and the black residue was washed thoroughly
dret and collaborators,[23] offers a more straightforward and with diethyl ether to remove the hydrogenated dibenz-
reliable alternative for the controlled synthesis of metal NPs ylideneacetone; the remaining insoluble material was then
with a narrow size distribution. The preparation of palla- examined by high-resolution transmission electron micro-
dium NPs stabilized by phosphorus- or nitrogen-based li- scopy (HRTEM) (Figure 2). In all cases, the presence of 2b
1
gands following the organometallic synthesis has been re- as stabilizer was confirmed by IR and H NMR analysis,
ported.[24] Very recently, this methodology has also been and the metal content in the samples was determined by
used for the synthesis of water-soluble ruthenium and plati- inductively coupled plasma (ICP) analysis. Electron diffrac-
num NPs by using 1,3,5-triaza-7-phosphaadamantane as tion (ED) confirmed the presence of palladium(0) with a
stabilizer.[25] In a collaboration with Chaudret and Phil- face-centered cubic (fcc) structure (Figure 2). The newly ob-
ippot, we have previously succeeded in the preparation of tained nanoparticulated materials were soluble in water,
ruthenium[26] and platinum[27] NPs in the presence of THF, dichloromethane and ethyl acetate, and insoluble in
heavily fluorinated protecting agents. Taking advantage of diethyl ether.
Table 1. Preparation of Pd NPs stabilized by polyoxyethylenated compounds 2 by decomposition of [Pd(dba)2] under H2 (3 bar)
(Scheme 3).[a]
Entry
Pd [mmol][b]
2 ([mmol])
Pd/2
[Pd][c]
Pd [%][d]
Yield [%][e]
Ø [nm][f]
1
2
3
4
5
0.016
0.032
0.064
0.015
0.145
2b (0.016)
2b (0.016)
2b (0.064)
2a (0.015)
2a (0.145)
1:1
2:1
1:1
1:1
1:1
0.0016
0.0016
0.0016
0.0015
0.0072
1.57
2.13
1.58
–
59.5
45.6
70.4
2.3Ϯ0.4
4.8Ϯ1.2
2.7Ϯ0.7
–
[g]
–
[g]
–
–
–
[a] Performed in THF at room temperature overnight. [b] Based on %Pd in [Pd(dba)2] determined by ICP. [c] In mol/L. [d] Determined
by ICP analysis. [e] Defined as (total Pd in nanoparticles)/(initial Pd)ϫ100. [f] Mean diameter determined by TEM. [g] Bulk Pd.
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Water-Soluble Palladium Nanoparticles
by using a 0.25% loading of palladium. Triethylamine in
water as solvent afforded good yields in the first cycle
(Table 2, Entries 1 and 2), but the recyclability was poor
(Table 2, Entries 3 and 4). After testing other bases and sol-
vent mixtures (Table 2, Entries 5–10), it was found that
good performances were achieved by using K2CO3 in water
at 100 °C (Table 2, Entry 5), K2CO3 in acetone/water (3:2)
at 60 °C (Table 2, Entry 8), and KOH in water at 60 °C
(Table 2, Entry 9). The catalyst was recycled (Table 2, En-
tries 5 and 8) by a simple procedure: the coupling product
was extracted with diethyl ether (in Entry 8, acetone was
previously removed), and the aqueous phase containing the
catalyst was used in the next run (1 equiv. of the base was
added and the corresponding amount of acetone when re-
quired). HRTEM analysis after the first cycle showed no
appreciable change in the nanoparticle size, although ICP
analysis of the organic fraction did show ppm levels of le-
ached Pd. The latter observation may explain the increase
in the reaction time upon recycling (see the Supporting In-
formation).
Figure 2. HRTEM image and particle-size distribution diagram for
2b-Pd (top); electron-diffraction pattern for 2b-Pd (bottom).
Next, we turned our attention to the coupling of aryl
bromides. Before testing the more challenging substrates,
we investigated the model Suzuki reaction between p-bro-
moacetophenone (7b) and phenylboronic acid (8b), to af-
ford 4-acetylbiphenyl (9bb) (Table 3). Several reaction con-
ditions (base, solvent, molar ratios of reagents) were tested
by using a multireactor with closed vessels at 100 °C, moni-
toring the GC yield of 9bb after 16 h (Table 3, Entries 1–
16). In most cases, the formation of 9bb was accompanied
by small amounts of biphenyl as a result of the homocou-
pling of 8b. This side-reaction was minimized by the use of
Despite the good stabilizing ability of 2b, the procedure
failed with 2a (n = 7), leading to the precipitation of bulk
metal (Table 1, Entries 4 and 5).
Assay of Palladium Nanoparticles as Recoverable Catalysts
in Suzuki–Miyaura Cross-Coupling
The activity and recyclability of palladium nanoparticles potassium and cesium carbonates in acetone/water (3:2)
2b-Pd was first tested in the Suzuki cross-coupling between with 1.5 equiv. of 8b (Table 3, Entries 14 and 15). When the
iodobenzene (7a) and p-methoxyphenylboronic acid (8a) to reaction progress was monitored over time (see the Sup-
give p-methoxybiphenyl (9aa; Table 2). Several bases, sol- porting Information), it was found that the use of either of
vent systems, reagent ratios, and temperatures were assayed these two bases in an acetone/water mixture (3:2) at 100 °C
Table 2. Suzuki cross-coupling between iodobenzene (7a) and p-methoxyphenylboronic acid (8a) catalyzed by 2b-Pd.[a]
Entry
Base
Solvent
7a/8a/base
T [°C]
t [h]
9aa [%][b]
1
2
3
4
5
NEt3
NEt3
NEt3
H2O
H2O
H2O
H2O
H2O
1:2:3
1:2:3
90
60
60
90
100
60
60
60
60
50
1
3
94
60
1:1.25:2.5
1:1.25:2.5
1:1.25:2.5
1:1.5:2.5
1:1.25:2.5
1:1.5:2.5
1:1.25:2.5
1:1.25:2.5
4/21/144
1.5/7
2.2/2.5
1
3
0.5/3.5/5.5
1
99/53/–[c]
83/–[c]
97/50[c,d]
70
NEt3
K2CO3
K2CO3
K2CO3
K2CO3
KOH
6
H2O
7
MeOH/H2O (3:1)
acetone/H2O (3:2)
H2O
70
8[e]
9
95/91/91[b,c]
93
10
KOH
THF/H2O (3:1)
168
67
[a] Using 7a (0.3 mmol) and Pd (0.25 mol-%) in solvent (4.5 mL); for details see Exp. Sect. and the Supporting Information. [b] Yields
of isolated product. [c] Yields for consecutive cycles. [d] Calculated based on NMR integration. [e] At triple the normal scale (i.e., based
on 0.9 mmol 7a).
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R. Pleixats, A. Shafir et al.
Table 3. Suzuki cross-coupling between p-bromoacetophenone (7b) and phenylboronic acid (8b) catalyzed by 2b-Pd.[a]
FULL PAPER
Entry
Base
Solvent
8b [equiv.]
9bb [%][b]
Ph–Ph [%]
1
2
3
4
5
6
7
KOH
NaOAc
CsOAc
K2CO3
Na2CO3
K2CO3
K3PO4
K2CO3
Cs2CO3
K2CO3
Cs2CO3
K2CO3
Cs2CO3
K2CO3
Cs2CO3
Cs2CO3
K2CO3
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
1.25
1.25
1.25
1.25
1.25
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
64
37
35
83
80
Ͼ99
85
Ͼ99
82
57
59
Ͼ99
Ͼ99
Ͼ99
Ͼ99
66
5.4
0.9
0.9
2.9
4.7
6.9
4.1
6.6
1.4
0.6
0.6
7.8
traces
–
8[c]
9[c]
10[c]
11[c]
12[c]
13[c]
14[c]
15[c]
16[c]
17[d]
THF/H2O (4:1)
THF/H2O (4:1)
DMF/H2O (4:1)
DMF/H2O (4:1)
acetone/H2O (3:2)
acetone/H2O (3:2)
EtOH/H2O (1:1)
acetone/H2O (3:2)
–
26.5
–
99 (3 h)
98 (5.5 h)
99 (10 h)
98 (24 h)
95 (20 h)[e]
[a] Performed in closed vessels (60 mL) in a multireactor with orbital stirring, with 7b (0.3 mmol), base (2.5 equiv.) and Pd (0.25 mol-%)
in solvent (4.5 mL). Unless stated otherwise, all reactions were run at 100 °C for 16 h. [b] GC yields corrected to internal undecane. [c]
Argon and degassed solvent. [d] Reaction performed at 60 °C. [e] Isolated yields in consecutive cycles. Quantitative GC yields at the
indicated times.
led to a complete reaction after 3 h, with the use of K2CO3 strates and was compatible with the presence of amino, hy-
leading to a slightly faster process. With this base, lowering droxy and carbonyl groups. However, in the case of 2-iodo-
the temperature to 60 °C still kept the reaction time under benzaldehyde (7f; Table 4, Entry 10), acetone had to be ex-
3 h (see the Supporting Information). Finally, under the op- cluded from the reaction medium, and neat water was used
timized reaction conditions, the catalyst 2b-Pd could be re- as a solvent to avoid the formation of aldol side-products.
cycled five times (Table 3, Entry 17), leading to good iso- Surprisingly, little or no selectivity was achieved for the
lated yields in all runs, albeit at the cost of an increase in monoarylation of 4-bromoiodobenzene (7h). Thus, use of
the reaction time for later runs.
1.5 equiv. of 8b at 60 °C led to a mixture of monosubsti-
Once the activity and recyclability of 2b-Pd NPs in aque- tuted and disubstituted compounds, 9hb and 10bhb
ous media had been established, the method was extended (Table 4, Entry 12). Lowering of the reaction temperature
to the Suzuki coupling of a range of other aryl and hetero- and the amount of boronic acid did not improve the selec-
aryl halides 7 with several aryl (heteroaryl) boronic acids 8. tivity (Table 4, Entry 13). On the other hand, a nearly quan-
The results are summarized in Table 4.
titative yield of the disubstituted compound 10bhb was ob-
Thus, under the optimized conditions (Table 2, Entry 7), tained under the standard conditions by using a slight ex-
excellent yields were achieved in the coupling of iodo- cess (2.5 equiv.) of phenylboronic acid (8b; Table 4, En-
benzene with arylboronic acids 8a and 8c (Table 4, Entries 1 try 14). Similarly, the coupling of substrates 7h and 7i led
and 2). A heteroaryl iodide (3-iodothiophene, 7c) was also to high yields of the doubly arylated products 10aha, 10chc,
successfully coupled with substituted aryl and heteroaryl and 10bib (Table 4, Entries 15–17). As was the case for 7f,
boronic acids (Table 4, Entries 3–6). Only in the case of 2- the presence of an aldehyde moiety in bromide 7j required
acetylthiophene-5-boronic acid (8e) was the reaction slug- just water as solvent to avoid aldol condensation (Table 4,
gish, giving a low yield and resulting in the formation of Entry 18). The use of water at 100 °C also proved to be
the protodeboronation product and unreacted 7c (Table 4, optimal for the coupling of the deactivated anisole deriva-
Entry 7). Other aryl iodides, with both the electron-donat- tives 7k and 7l (Table 4, Entries 19 and 20). The more chal-
ing and -withdrawing substituents, were allowed to react lenging aryl chloride 7m gave a 17% yield of the coupling
with arylboronic acid 8b (Table 4, Entries 8–11) to afford product with the phenylboronic acid under the reaction
the desired product in isolated yields ranging from 67 to conditions given in Table 4, Entry 21, with the mass balance
87%. The system performed well for ortho-substituted sub- being the recovered starting material.
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Water-Soluble Palladium Nanoparticles
Table 4. Suzuki cross-coupling between aryl or heteroaryl halides 7 and boronic acids 8 catalyzed by 2b-Pd.[a]
[a] Unless otherwise stated, the reactions were performed in closed vessels (10 mL) by using ArX (0.3 mmol), ArЈB(OH)2 (1.5 equiv.),
K2CO3 (2.5 equiv.), Pd (0.25mol-%), acetone/H2O (3:2, 4.5 mL), 60 °C. [b] Isolated yield. [c] Protodeboronation gave 2-acetylthiophene
as the main side-product. [d] Reaction performed in H2O (4.5 mL) at 60 °C. [e] By using ArB(OH)2 (1.0 equiv.). [f] Performed at room
temperature. [g] By using ArB(OH)2 (2.5 equiv.). [h] Reaction performed in H2O (4.5 mL) at 100 °C. [i] Performed with KOH (2.5 equiv.)
1
in H2O (4.5 mL) at 130 °C. [j] Yield based on H NMR integration.
the nanocatalyst for aryl iodides and bromides has also
Conclusions
been established (up to five cycles) giving rise to good iso-
Using an efficient protocol based on a threefold [2+3]
lated yields in successive runs.
click coupling, new PEG-tagged compounds 2a and 2b were
prepared. The latter material bearing longer polyether
chains was successfully used as a stabilizer for the synthesis
of water-soluble palladium NPs by the organometallic ap-
proach. The newly prepared nanomaterial 2b-Pd displayed
good activity as a catalyst in the Suzuki coupling of aryl
iodides, and the more challenging bromides, in aqueous me-
dia, providing good to excellent yields in the coupling of a
Experimental Section
General Remarks: PEG derivatives 3a and 3b were purchased from
Fluka and dried as described; substrate 7f was prepared according
to a published procedure,[29] and substrate 7g was synthesized by
using a copper-catalyzed halide-exchange reaction[30] as described
in the Supporting Information. Other substrates were purchased
range of aryl and heteroaryl substrates. The recyclability of from commercial sources and used as received. The preparation of
Eur. J. Org. Chem. 2010, 5090–5099
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R. Pleixats, A. Shafir et al.
FULL PAPER
Pd NPs was carried out in Fischer–Porter glassware. Water Milli-
Q was used for the preparation and purification of compounds; the
organic solvents were purchased from Scharlau. Dialysis membrane
was obtained from Spectrum Laboratories [Spectra Por 3 Regener-
sodium sulfate, and the solvent was evaporated to give 4a as a pale-
yellow oil (5.8 g, 95% yield). IR (ATR, neat): ν = 2869, 1454, 1348,
˜
1
1095, 917, 800 cm–1. H NMR (CDCl3, 360 MHz): δ = 3.05 (s, 3
H, SO2Me), 3.34 (s, 3 H, OMe), 3.51–3.52 (m, 2 H, CH2), 3.61–
ated Cellulose (RC)] with a formal molecular weight cut-off 3.62 (m, 26 H, CH2), 3.72–3.76 (m, 2 H, CH2CH2OMes), 4.33–
(MWCO) of 3500. When required, experiments were carried out
with standard high-vacuum and Schlenk techniques. The solvents
were dried and distilled just before use. Catalytic runs with orbital
stirring were carried out in a Heidolph synthesis system equipped
with 12 sealed 60 mL glass vessels. All NMR measurements were
carried out at the Servei de Ressonància Magnètica Nuclear at the
Universitat Autònoma de Barcelona. Spectra were recorded with
Bruker AC250 (250 MHz for 1H), Avance360 (360 MHz for 1H),
and AvanceII 400 MHz (400 MHz for 1H) spectrometers. MALDI-
TOF-MS analyses were performed at the Institute of Material
Science of Aragon (Unidad de Nuevos Materiales Orgánicos,
Facultad de Ciencias, CSIC-Universidad de Zaragoza) and at the
Proteomics and Bioinformatics facility at the Universitat Autò-
noma de Barcelona (a member of ProteoRed network) with a
Bruker mass spectrometer, model Ultraflex with Modus Reflection.
The ionization was performed with a laser source operating at
337 nm and at 25 kV. In the MALDI experiments, ditranol was
used as a matrix for products 4b, 5b, and 2b; DHB or matrix-free
conditions were used for the analysis of other compounds. Infrared
spectra were recorded with a Bruker Tensor 27 instrument
equipped with an ATR Golden Gate cell and a diamond window.
ICP measurements of palladium content were carried out at the
Serveis Cientificotècnics of the Universitat de Barcelona with a
multichannel Perkin–Elmer instrument, model Optima 3200 RL.
TEM analyses were performed at the Servei de Microscòpia of the
Universitat Autònoma de Barcelona, with a JEOL JEM-2010
model instrument operating at 200 kV. The TEM measurements
were carried out by sonication of the nanoparticulate material in
THF for several minutes; then a specially produced structureless
carbon support film, which has a thickness of 4–6 nm, was placed
in the solution for a few seconds and dried before observation. The
size distributions were determined by manual analysis of enlarged
micrographs by measuring a high number of particles on a given
grid to obtain a statistical size distribution and a mean diameter
(Gatan Digital Micrograph Program). Gas chromatographic
analysis was accomplished with an Agilent Technologies 7890A
GC system equipped with an Agilent HP-5 column
(30 mϫ0.320 mmϫ0.25 µm). The integrated areas and peak posi-
tions in the chromatograms were referenced internally to undecane
in Table 3, Entries 1–16. Mass spectral analysis of the Suzuki prod-
ucts were carried out with an Agilent 6850 GC system equipped
with a 5975C Triple Axis EI Mass Detector. Melting points were
determined with a Reichert brand melting-point apparatus. Alug-
ram SIL G/UV254 sheets (Macherey–Nagel) were used for TLC.
Column chromatography was carried out by using SDS brand silica
gel with a grain size of 35–70 µm and a pore size of 60 Å.
4.36 (m, 2 H, CH2OMes) ppm. MALDI-TOF MS: m/z range for
[M + Na] from 353 (5ϫCH2CH2O units) to 705 (13 CH2CH2O
units) separated by 44 Da (CH2CH2O); the most intense peak was
at 485 (8 CH2CH2O units).
Synthesis of Mesylate 4b: Commercial MeO-PEG-OH 3b (average
n = 44; 10.0 g, 5 mmol) was dried by heating at 80 °C overnight
under vacuum. The material was then dissolved in anhydrous
CH2Cl2 (140 mL), and triethylamine (2.1 mL, 1.53 g, 15.1 mmol)
was added. The mixture was cooled to 0 °C, and methanesulfonyl
chloride (0.8 mL, 1.184 g, 10.2 mmol) was added dropwise whilst
stirring for 15 min. Upon addition of all of the reagent, the mixture
was stirred at the same temperature for an additional 15 min, then
at room temp. for 3.5 h. At this point, the reaction mixture was
filtered through a plug of silica gel, and the plug was washed with
additional portions of CH2Cl2. The solvent was evaporated, and
the oily residue was triturated with diethyl ether to yield compound
4b as a white solid, which was isolated by filtration, washed and
dried under vacuum (6.29 g, 61%). IR (ATR, neat): ν = 2884, 1468,
˜
1
1342, 1102, 959, 843 cm–1. H NMR (CDCl3, 250 MHz): δ = 3.06
(s, 3 H, SO2CH3), 3.36 (s, 3 H, OCH3), 3.51–3.53 (m, 2 H, CH2),
3.62 (m, 174 H, CH2), 3.73–3.76 (m, 2 H, CH2CH2OMes), 4.35–
4.37 (m, 2 H, CH2OMes) ppm. MALDI-TOF MS: m/z range for
[M + Na] from 1806 (38 CH2CH2O units) to 2511 (54 CH2CH2O
units) separated by 44 Da (CH2CH2O); the most intense peak was
at 2114 (45 CH2CH2O units).
Synthesis of Azide 5a: Sodium azide (2.781 g, 42.34 mmol) was
added to a solution of 4a (1.806 g, 4.22 mmol) in dimethylform-
amide (400 mL), and the stirred mixture was heated at 60 °C for
24 h. The solvent was evaporated under reduced pressure, and the
white solid residue was partitioned between CH2Cl2 and water. The
organic phase was dried with anhydrous sodium sulfate, and the
solvent was evaporated to afford 5a as a pale-yellow oil (1.574 g,
100% yield). IR (ATR, neat): ν = 2865, 2099, 1452, 1286, 1098,
˜
1
942, 850 cm–1. H NMR (CDCl3, 250 MHz): δ = 3.38–3.40 (m, 5
H, OCH3 and CH2), 3.53–3.56 (m, 2 H, CH2), 3.64–3.69 (m, 28 H,
CH2) ppm. MALDI-TOF MS: m/z range for [M + Na] from 215
(2 CH2CH2O units, CH2CH2N3, OMe) to 655 (12 CH2CH2O units,
CH2CH2N3, OMe) separated by 44 Da (CH2CH2O), the most in-
tense peak was at 346 (5 CH2CH2O units, CH2CH2N3, OMe).
Synthesis of Azide 5b: Sodium azide (1.034 g, 15.75 mmol) was
added to a solution of 4b (3.275 g, 1.57 mmol) in dimethylform-
amide (200 mL), and the stirred mixture was heated at 60 °C for
24 h. The solvent was evaporated at reduced pressure, and the resi-
due was partitioned between CH2Cl2 and water. The organic phase
was dried with anhydrous sodium sulfate, and the solvent was evap-
orated. Upon addition of diethyl ether to the residue, azide 5b pre-
cipitated as a white solid, which was filtered, washed with diethyl
Synthesis of Mesylate 4a: Commercial MeO-PEG-OH 3a (average
n = 7; 5.0 g, 14.2 mmol) was dried by heating at 80 °C under
vacuum overnight. The material was then dissolved in anhydrous
CH2Cl2 (100 mL), and triethylamine (6 mL, 4.37 g, 43.2 mmol) was
added. The mixture was cooled to 0 °C, and methanesulfonyl chlo-
ride (2.2 mL, 1.49 g, 28.42 mmol) was added dropwise with stirring
for 15 min. When all of the reagent was added, the mixture was
stirred at 0 °C for an additional 15 min, and then stirred at room
temp. for 4 h. The reaction mixture was then filtered through a
plug of silica gel with CH2Cl2 as eluent and the solvent was evapo-
rated to afford an orange oil. This residue was partitioned between
CH2Cl2 and water, the organic phase was dried with anhydrous
ether, and dried (2.78 g, 87% yield). IR (ATR, neat): ν = 2881,
˜
2103, 1963, 1466, 1454, 1341, 1103, 958, 842 cm–1. 1H NMR
(CDCl3, 360 MHz): δ = 3.36–3.39 (m, 5H¸ OCH3 and CH2), 3.81–
3.41 (m, 178 H, CH2) ppm. MALDI-TOF MS: m/z range for [M
+ Na] from 1534 (32 CH2CH2O units, CH2CH2N3, OMe) to 2440
(48 CH2CH2O units, CH2CH2N3, OMe) separated by 44 Da
(CH2CH2O), the most intense peak was at 1666 (35 CH2CH2O
units, CH2CH2N3, OMe).
Synthesis of PEG-Tagged Stabilizer 2a: To a stirred solution of
azide 5a (0.576 g, 1.41 mmol) and 6 (0.065 g, 0.43 mmol) in a mix-
5096
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 5090–5099

Water-Soluble Palladium Nanoparticles
ture of tert-butayl alcohol (5.4 mL) and water (1 mL) maintained
under argon and protected from light, a solution of CuSO4·5H2O
(38.2 mg, 0.15 mmol, 12%) in water (2 mL) was added followed by
a solution of sodium ascorbate (57 mg, 0.29 mmol, 22%) in water
(3 mL). The stirred mixture was left at room temp. in the dark for
24 h; then the organic solvent was evaporated, and the remaining
Synthesis of Palladium(0) Nanoparticles Stabilized by 2b (2b-Pd):
Table 1, Entry 1. A Fischer–Porter apparatus equipped with a mag-
netic stir bar was charged with 2b (100 mg, 0.016 mmol) and
[Pd(dba)2] (9.8 mg, 0.016 mmol). The apparatus was closed, and
the contents were subjected to three evacuate–refill cycles with ar-
gon. Anhydrous THF (10 mL) was added and the system was
aqueous solution was extracted with CH2Cl2. The organic phase purged with H2 three times (bringing the pressure of H2 to 3 atm),
was dried with anhydrous sodium sulfate, and the solvent was and then the mixture was stirred at room temp. for 18 h. The hydro-
evaporated to give a pale-yellow oil. Purification of the crude mix- gen was evacuated, and the solution was filtered through a Milli-
ture was undertaken as follows. Anhydrous THF (3.5 mL) was
added to polymeric triphenylphosphane (125 mg) in a Schlenk
flask. The stirred mixture was heated at 50 °C for 1 h, then the
solvent was removed with a cannula. The operation was repeated
two more times. The solution of the crude mixture containing 2a
Pore filter. The solvent from the filtrate was evaporated and the
black solid residue was washed with diethyl ether and dried to af-
ford the nanoparticulate material (65 mg, 59.5% yield with respect
to initial palladium). The IR and 1H NMR spectra are identical to
those of 2b. Pd: 1.57% (ICP); size of the nanoparticles:
and excess 5a in anhydrous THF (4 mL) was added to the Schlenk (2.3Ϯ0.4) nm (TEM).
flask containing the previously washed polymeric triphenylphos-
Suzuki Cross-Coupling Between p-Bromoacetophenone (7b) and
phane, and the mixture was stirred at 50 °C until no azide signal
was observed by IR spectroscopy. The mixture was centrifuged to
remove the polymer, and the solvent from the supernatant solution
was evaporated to give pure 2a as a pale-yellow oil (0.589 g,
Phenylboronic Acid (8b) To Give p-Acetylbiphenyl (9bb) Catalyzed
by 2b-Pd Nanoparticles: Table 3, Entry 17. Given that 2b-Pd is air-
stable and for reasons of convenience, the cross-coupling reactions
were conducted in screw-top disposable tubes to prevent the loss
of solvent during heating.
99.7%). IR (ATR, neat): ν = 2867, 1457, 1258, 1096, 798 cm–1. 1H
˜
NMR (CDCl3, 400 MHz): δ = 3.33–3.35 (m, 15 H, OCH3 and
CH2OMe), 3.53–3.65 (m, CH2 of PEG), 3.92–3.94 (m, 6 H, CH2
CH2-triazole), 4.60–4.64 (m, 6 H, CH2-triazole), 8.18 (s, 3 H, tri-
azole-H), 8.32 (s, 3 H. ArH) ppm. 13C NMR (CDCl3, 100 MHz):
δ = 50.5, 59.06, 61.7, 69.6, 70.3, 70.6 (br.), 71.9, 72.6, 121.7, 122.3,
132.0, 147.1 ppm. MALDI-TOF MS: m/z range for [M + H] from
850 (9 CH2CH2O units, 3 CH2CH2, 3 OMe, benzenetriazole core)
to 1554 (25 CH2CH2O units, 3 CH2CH2, 3 OMe, benzenetriazole
core) separated by 44 Da (CH2CH2O); the most intense peak was
at 1202 (17 CH2CH2O units, 3 CH2CH2, 3 OMe, benzenetriazole
core).
A screw-top sealable tube was charged with a magnetic stir bar, p-
bromoacetophenone (7b; 58.5 mg, 0.3 mmol), phenylboronic acid
(8b; 55.9 mg, 0.45 mmol), K2CO3 (103.7 mg, 0.75 mmol) and 2b-
Pd NP (0.25 mol-% Pd). Acetone/water (3:2, 4.5 mL) was added,
and the tube was sealed and heated to 60 °C. The reaction was
monitored by gas chromatography until it was complete. The tube
was then allowed to cool to room temp., and the product was ex-
tracted with Et2O, reserving the catalyst-containing aqueous frac-
tion for subsequent runs. The organic fraction was concentrated,
and the residue was partitioned between water and CH2Cl2. The
resulting organic fraction was separated and washed with ad-
ditional water, dried with anhydrous Na2SO4, and the solvents were
evaporated. Compound 9bb was obtained in pure form as a white
solid (for yields see Table 3, Entry 17). The aqueous fraction was
reused for four more cycles, by each time adding p-bromoaceto-
phenone (58.5 mg, 0.3 mmol), phenylboronic acid (55.9 mg,
0.45 mmol), acetone (2.7 mL), and K2CO3 (41.8 mg, 0.3 mmol).
After each cycle, the reaction was treated in a manner analogous
to the first run.
Synthesis of PEG-Tagged Stabilizer 2b: To a stirred solution of
azide 5b (4.037 g, 1.98 mmol) and 6 (0.0905 g, 0.63 mmol) in a mix-
ture of tert-butyl alcohol (7.2 mL) and water (1.1 mL) maintained
under argon and protected from light, a solution of CuSO4·5H2O
(167.7 mg, 0.672 mmol, 36%) in water (3 mL) was added followed
by a solution of sodium ascorbate (261.5 mg, 1.13 mmol, 60%) in
water (7.2 mL). The stirred mixture was left at room temp. in the
dark for 2 h, the organic solvent was evaporated, and the remaining
aqueous solution was extracted with CH2Cl2. The organic phase
was dried with anhydrous sodium sulfate, and the solvent was evap-
orated to afford a pale-yellow solid. Purification of the crude mix-
ture was undertaken as follows. The mixture was suspended in
water, and the insoluble part was removed by centrifugation. The
aqueous solution was placed in a dialysis bag (MWCO 3500 mem-
brane) and introduced into pure water in a beaker provided with
magnetic stirring. Every hour, the aqueous solution from the bea-
ker was removed and replaced by fresh pure water. After 5 h, the
azide 5b was no longer observed in this aqueous solution (IR moni-
toring). The solvent from the aqueous solution of the dialysis bag
was evaporated under reduced pressure to afford pure 2b as a pale-
Supporting Information (see footnote on the first page of this arti-
cle): Spectral data for 2, 4, and 5. HRTEM and particle size distri-
bution histograms for 2b-Pd (Table 1, Entries 1–3); reaction pro-
gress plots, general procedure for Suzuki cross-couplings (Table 4)
and characterization data for the corresponding cross-coupling
products 9.
Acknowledgments
We acknowledge financial support from Ministerio de Educación
y Ciencia (MEC) and Ministerio de Ciencia e Innovación (MIC-
INN) of Spain (Projects CTQ2007-65720 and CTQ2009-07881/
BQU, and a Ramón y Cajal contract to A. Shafir), Consolider
Ingenio 2010 (Project CSD2007-00006) and Generalitat de Catalu-
nya (Project SGR2009-01441). We would like to thank Dr. Jesús
Orduna Catalán (Universidad de Zaragoza) and Silvia Bronsoms
(UAB) for the MALDI analyses, Emma Rossinyol for help with
TEM analyses and Amanda Alonso for help with ICP analyses.
yellow solid (3.609 g, 96% yield). IR (ATR, neat): ν = 2882, 1466,
˜
1341, 1101, 959, 842 cm–1. 1H NMR (CDCl3, 360 MHz): δ = 3.37–
3.40 (m, 15 H, OCH3 and CH2OMe), 3.42–3.45 (m, 6 H, CH2),
3.53–3.63 (m, CH2 of PEG), 3.81–3.84 (m, 6 H, CH2), 3.92–3.96
(m, 6 H, CH2CH2-triazole), 4.61–4.63 (m, 6 H, CH2-triazole), 8.19
(s, 3 H, triazole-H), 8.32 (s, 3 H, ArH) ppm. MALDI-TOF MS:
m/z range for [M + Na] from 5413 (112 CH2CH2O units, 3
CH2CH2, 3 OMe, benzenetriazole core) to 6865 (145 CH2CH2O
units, 3 CH2CH2, 3 OMe, benzenetriazole core) separated by 44 Da
(CH2CH2O); the most intense peak was at 6116 (128 CH2CH2O
units, 3 CH2CH2, 3 OMe, benzenetriazole core).
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Eur. J. Org. Chem. 2010, 5090–5099
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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