Suzuki-miyaura reaction in water, catalyzed by palladium nanoparticles stabilized by pluronic F68 triblock copolymer

Article abstract of DOI:10.1134/S1070428011040014

Palladium nanoparticles stabilized by Pluronic F68 triblock copolymer effectively catalyzed Suzuki-Miyaura reaction in water. The reactions with water-soluble aryl iodides and aryl bromides containing electron-withdrawing or electron-donating substituent occurred at room temperature. The catalytic efficiency was found to depend on the size of palladium nanoparticles and their morphology. Pleiades Publishing, Ltd., 2011.

Full text of DOI:10.1134/S1070428011040014

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2011, Vol. 47, No. 4, pp. 475–479. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © A.N. Kashin, I.P. Beletskaya, 2011, published in Zhurnal Organicheskoi Khimii, 2011, Vol. 47, No. 4, pp. 479–483.
Suzuki–Miyaura Reaction in Water, Catalyzed by Palladium
Nanoparticles Stabilized by Pluronic F68 Triblock Copolymer
A. N. Kashin and I. P. Beletskaya
Faculty of Chemistry, Moscow State University, Vorob’evy gory 1, Moscow, 119992 Russia
e-mail: ankashin@org.chem.msu.ru
Received November 8, 2010
Abstract—Palladium nanoparticles stabilized by Pluronic F68 triblock copolymer effectively catalyzed
Suzuki–Miyaura reaction in water. The reactions with water-soluble aryl iodides and aryl bromides containing
electron-withdrawing or electron-donating substituent occurred at room temperature. The catalytic efficiency
was found to depend on the size of palladium nanoparticles and their morphology.
DOI: 10.1134/S1070428011040014
Reactions leading to formation of new carbon–
carbon bonds, such as Suzuki–Miyaura and Mizuroki–
Heck reactions, are very important for organic synthe-
sis, and they permanently attract researchers’ attention,
especially if these reactions are carried out in water as
the most accessible and ecologically safe solvent. It
was proposed to catalyze such reactions in aqueous
medium by metal nanoparticles, in particular by palla-
dium nanoparticles stabilized by micelles formed by
surfactants and amphiphilic block copolymers [1].
Examples of the latter are micelles containing poly-
styrene-b–poly(ethylene oxide) (PS–PEO) [2], poly-
(ethylene oxide)-b–poly(2-vinylpyridine) (PEO–P2VP)
[3], polystyrene-b–poly(4-vinylpyridine) (PS–P4VP)
[1, 4, 5], and poly(ethylene oxide)-b–poly(isopropyl-
ene oxide)-b–poly(ethylene oxide) (PEO–PPO–PEO)
[6]. To ensure efficient recycling of the catalyst, PS–
P4VP micelles were immobilized on polystyrene-b–
poly(methyl methacrylate) microspheres [5].
determined only in dimethylformamide. Pluronic F68
is characterized by an intermediate length of PEO frag-
ments among the above triblock copolymers. It is
fairly hydrophilic and convenient for studying Suzuki–
Miyaura reactions in water.
The catalyst stabilized by Pluronic F68 was pre-
pared according to the procedure described previously
for the catalyst with Pluronic P123 [6], by reduction of
Na2PdCl4 in the presence of Pluronic F68. According
to the transmission electron microscopy (TEM) data,
the obtained catalyst contained mainly cubooctahedral
PdNPs with an average diameter of 5.4 nm and rela-
tively low dispersion (Fig. 1). The position of reflexes
(111, 200, and 220) in the electron diffraction pattern
indicated crystalline nature of PdNPs [6, 8–11].
We examined Suzuki–Miyaura reactions of phenyl-
boronic acid PhB(OH)2 with water-soluble aryl iodides
and aryl bromides containing an electron-withdrawing
(m-COOH) or electron-donating substituent (p-OH) in
water in the presence of potassium hydroxide as base
(Scheme 1). The catalyst showed a high efficiency, and
the reaction occurred at room temperature in the pres-
ence of an optimal amount of KOH (5 equiv). At a pal-
ladium concentration of 1 mol %, the reaction with
m-iodobenzoic acid was almost complete in 0.5 h
(Table 1, run no. 1), and with m-bromobenzoic acid, in
10 h (Table 1, run no. 6). In the reaction with p-iodo-
phenol, the conversion attained 95% in 4 h (run no. 8),
whereas the conversion of p-bromophenol was only
60% even in 24 h (run no. 10). The reactions were
selective, and the yield approached the conversion of
the initial aryl halide. Biphenyl was formed as by-
We previously showed that palladium nanoparticles
(PdNPs) stabilized by PS–PEO and cetylpyridinium
chloride (CPC) micelles [2] efficiently catalyzed
Suzuki–Miyaura reactions with water-soluble reagents
in water [7]. We now propose to stabilize PdNPs with
the aid of accessible Pluronic F68 triblock copolymer
(PEO76–PPO30–PEO76). Among triblock copolymers
studied previously [Pluronic PL64 (PEO13–PPO30–
PEO13), Pluronic P103 (PEO17–PPO60–PEO17),
Pluronic P123 (PEO19–PPO69– PEO19), and Pluronic
F108 (PEO132–PPO50–PEO132)], only Pluronic P123
was used as PdNPs stabilizer in Suzuki–Miyaura reac-
tions [6]. However, the catalytic activity of PdNPs was
475

KASHIN, BELETSKAYA
(a)
476
Scheme 1.
Hlg
B(OH)2
R
+
1 equiv
Pd (1 mol %)
1.2 equiv
KOH (5 equiv)
H2O, 20°C
R
Ia, Ib
(b)
Hlg = I, Br; R = m-COOH (a), p-HO (b).
product (5–12%) via oxidation of phenylboronic acid
with atmospheric oxygen [12]. It should be empha-
sized that the examined reactions in water occurred
very easily at room temperature. By contrast, analo-
gous reactions catalyzed by Pluronic P123-stabilized
palladium species in DMF required heating at 90°C
over a period of 8 h to attain quantitative yield of
cross-coupling products [6]. Presumably, the difference
in the reaction conditions is related primarily to good
solubility of KOH in water and poor solubility of
Cs2CO3 in DMF.
50 nm
Diameter, nm
Fig. 1. (a) Transmission electron micrograph of palladium
nanoparticles and (b) their size distribution.
Table 1. Suzuki–Miyaura reaction of phenylboronic acid
with aryl halides in water in the presence of palladium
nanoparticles stabilized by Pluronic F68 block copolymer
micellesa
The catalyst stabilized by Pluronic F68 was more
active than that based on palladium nanoparticles
stabilized by PS–PEO/CPC micelles [7]. In the first
case, the reaction with m-iodobenzoic acid was com-
plete in 30 min at room temperature (Table 1, run
no. 1), whereas in the second case as long reaction
time as 4 h was insufficient to complete the process.
Even stronger difference in the activity of the above
catalysts was observed in the reaction with p-iodo-
phenol. The reaction in water in the presence of PdNPs
stabilized by Pluronic F68 was complete in 4 h at room
temperature (see above), while the reaction with the
same substrate, catalyzed by 3.5-nm palladium nano-
particles with a surface coated by 46% with perthiolat-
ed β-cyclodextrin, in aqueous acetonitrile (1:1) was
not complete even on heating for 2 h under reflux [13].
Run
no.
Time, Yield,b Conversion
Aryl halide (ArX)
h
%
of ArX,b %
01 m-Iodobenzoic acid
00.5
00.5
01.0
01.0
93
69
95
72
98
75
98
80
89
98
44
95
91
60
0c2c
03
0d4d
0e5e
04.0 85 [7]
06 m-Bromobenzoic acid 10.0
93
30
86
07 p-Iodophenol
01.0
04.0
08
0e9e
Using the reaction of PhB(OH)2 with m-iodoben-
zoic acid in water as model process, we examined the
effect of the size of PdNPs and their morphology on
the catalytic activity. Apart from cubooctahedral cata-
lyst with a particle size of 5.4 nm, we prepared other
catalysts which contained mainly the following types
of palladium nanoparticles: (a) trigonal, palladium con-
centration ~70%, particle size ~25 nm [8]; (b) hexa-
gonal, ~90%, 10–20 nm [8]; (c) cubooctahedral, ~90%,
8–10 nm [9]; and (d) cubooctahedral, >90%, 1.7 nm
[2]. The reactions were carried out under similar con-
04.0 68 [7]
24.0 49
10 p-Bromophenol
a
Reaction conditions: 0.075 mmol of ArX, 0.09 mmol of
PhB(OH)2, 0.38 mmol of KOH, 1 mol % of Pd, 0.25 ml of water;
temperature 20°C.
According to the GLC data; biphenyl was also formed as by-
product, yield 5–12%.
b
c
d
e
In the presence of 0.5 mol % of Pd.
KOH, 0.23 mmol.
In the presence of palladium nanoparticles stabilized by PS–
PEO/CPC micelles [7].
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SUZUKI–MIYAURA REACTION IN WATER
477
Table 2. Suzuki–Miyaura reaction of phenylboronic acid
with m-iodobenzoic acid in water, catalyzed by palladium
nanoparticles with different morphologiesa
ditions in the presence of the same amount of Pluronic
F68 (Table 2). The results showed that at 20°C small
cubooctahedal PdNPs with a size of 1.7 or 5.4 nm
(Table 2; run nos. 9, 10), regardless of their mor-
phology, were more active than larger PdNPs with
a size of 8–25 nm (Table 2; run nos. 1, 4). High cata-
lytic activity of small palladium particles is generally
rationalized in terms of developed metal surface and
a larger number of defects in the crystal lattice.
Run
no.
Morphology
of PdNPs
Pd,
mol %
Yield,b
%
T, °C Time, h
01 Trigonal
2
1
1
2
1
1
20
60
80
20
60
80
60
80
20
20
4
03.5
29.0
98.0
0<3.5<
03.6
91.0
92.0
95.0
93.0
85.0
02
5
03
5
Large palladium particles catalyzed the model
reaction only on heating (Table 2; run nos. 3, 6, 8).
However, at 80°C PdNPs catalyzed the reaction so
effectively that it was almost complete in 5 h, regard-
less of the catalyst morphology. Nevertheless, we suc-
ceeded in detecting the effect of morphology of PdNPs
on their catalytic activity at an intermediate tempera-
ture, 60°C (Table 2; run nos. 2, 5, 7). As follows from
the results of run nos. 2 and 5 (Table 2), hexagonal
PdNPs are less active (yield 3.6% in 5 h) than trigonal
(29% in 5 h), though the latter are larger than hexa-
gonal particles. On the other hand, the catalytic activity
of cubooctahedral PdNPs remains as high as at 80°C
(Table 2, run no. 7); presumably, this is related to their
lower size as compared to PdNPs of the other type.
This factor complicates analysis of the effect of mor-
phology of PdNPs on the catalytic activity.
04 Hexagonal
4
05
5
06
5
5
07 Cubooctahedral
08
1
5
c09c
d10d
0.0.5
4
a
Reaction conditions: 0.075 mmol of ArX, 0.09 mmol of
PhB(OH)2, 0.38 mmol of KOH, 0.25 ml of water, 30 mg
(3.6 μmol, 4.8 equiv) of Pluronic F68.
b
c
GLC data.
Palladium nanoparticles were prepared in water in the presence
of Pluronic F68; particle size 5.4 nm.
In the presence of palladium nanoparticles stabilized by PS–
PEO/CPC micelles [7]; particle size 1.7 nm.
d
by PS–PEO/CPC-stabilized PdNPs, palladium par-
ticles with a size of 1.7 nm after the first cycle are
converted into particles with a size of 1.9 nm and
agglomerates with a size of 2.4–3.1 nm; the latter are
characterized by a definite structure, and they consist
of small particles [7].
Obviously, under the examined conditions trigonal
PdNPs are more active in the Suzuki–Miyaura reaction
than hexagonal PdNPs with a similar size. The activity
of PdNPs is determined by the relative free energy
which depends on the number of palladium atoms on
the edges and apices of the corresponding nano-
particles [14, 15]. An indirect support in favor of
higher free energy of trigonal PdNPs is the fact that
palladium leaching (in the presence of O2 and FeCl3) is
accompanied by transformation of trigonal palladium
particles into thermodynamically more stable hexa-
gonal [8] or cubooctahedral species [9].
In the Suzuki–Miyaura reaction catalyzed by
PdNPs stabilized by Pluronic F68 micelles we ob-
served decrease of the size of PdNPs from 5.4 to
4.3 nm. The size of PdNPs also decreased from 5.4 to
4.1 nm in the absence of PhB(OH)2, i.e., as a result of
reaction of PdNPs with m-iodobenzoic acid. On the
other hand, in the presence of p- and o-iodobenzoic
acids, the size of PdNPs increased from 5.4 to 6.3 and
9.2 nm, respectively.
The catalytic activity of PdNPs in the Suzuki–
Miyaura reaction depends not only on their initial size
and morphology but also on their transformations
during the process. As we showed previously [7],
Suzuki–Miyaura reaction is accompanied by change of
the size of PdNPs, and increase in the size of PdNPs is
accompanied by “crunching.” There are published
data, according to which the size of palladium particles
in Suzuki–Miyaura reactions and related processes
with participation of aryl halides decreases [16, 17] or
increases [7, 18]. For example, in the reaction of
phenylboronic acid with m-iodobenzoic acid catalyzed
Enlargement of PdNPs both during Suzuki–
Miyaura reaction and in reaction with aryl halide in the
absence of PhB(OH)2 is the result of known process
involving growth of some species at the expense of
dissolution of other smaller species that are more
catalytically active (Ostwald ripening process). Initial
PdNPs decrease in size due to partial dissolution by the
action of reagents or oxygen. Here, aryl halide can act
as palladium carrier via leaching of palladium from
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 4 2011

KASHIN, BELETSKAYA
(a)
478
The size of PdNPs can also increase through ag-
glomeration of initial palladium particles to form large
spherical particles. Examination of the latter by TEM
showed that they consist of smaller particles. Spherical
agglomerates can be formed during the synthesis of
PdNPs [10, 20]. We were the first to observe their
formation in water from 5.4-nm palladium particles in
the presence of 1.2 equiv of m-iodobenzoic acid and
3 equiv of KOH. After 3 h, initial PdNPs are converted
into globules with a size of ~50 nm (Fig. 2). These
globules should be considered to be intermediate
species between the initial palladium particles (Fig. 1)
and very large aggregates which are formed after
repeated use of the catalyst, e.g., when the next cycle is
initiated after loading of new portions of the reactants
without preliminary isolation of the product (Fig. 3). It
should be noted that such aggregates consist of palla-
dium particles with a size fairly similar to the size of
initial PdNPs, which are stuck together.
(b)
1 μm
Thus Pluronic F68 triblock copolymer is an effec-
tive stabilizer for palladium nanoparticles which cata-
lyze Suzuki–Miyaura reactions in water. The activity
of this catalytic system is sufficient to ensure success-
ful reaction at room temperature not only for water-
soluble aryl iodides containing an electron-withdraw-
ing or electron-donating substituent but also for the
corresponding aryl bromides. The catalytic activity
was found to depend on the size of palladium nano-
particles and their morphology. In particular, the most
active under the examined conditions were trigonal
PdNPs. The catalytic activity can change during the
process as a result of transformations of the catalyst.
Fig. 2. Transmission electron micrograph of (a) agglomerat-
ed palladium nanoparticles and (b) globule under strong
magnification.
small PdNPs with formation of ArPdX species (direct
oxidative addition; for analysis of published data and
experimental proofs, see [19]). The reverse reaction,
reductive elimination of palladium from ArPdX occurs
on the surface of larger PdNPs; as a result, the size of
PdNPs increases [7].
EXPERIMENTAL
The reaction mixtures were analyzed by GLC on
a Kristalyuks 4000M chromatograph equipped with
a flame-ionization detector (Chrompack CP-Sil 8 CB
capillary column, 25 m×0.25 mm; oven temperature
programming from 120 to 300°C at a rate of 25 deg×
min–1; injector and detector temperature 310°C).
Transmission electron microscopic studies were per-
formed using a Carl Zeiss LEO912 AB Omega micro-
scope.
Commercially available (Lancaster, Aldrich) aryl
halides, phenylboronic acid, block copolymers, poly-
(N-vinylpyrrolidin-2-one) (M 55000), and calibration
standard for GLC were used without additional purifi-
cation. In the synthesis of palladium nanoparticles with
different morphologies, the reactants were supplied
with the aid of an ABU12 autoburette with a TTT1c
titrator (Radiometer).
100 nm
Fig. 3. Transmission electron micrograph of palladium nano-
particles after repeated use as catalyst in Suzuki–Miyaura
reaction.
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SUZUKI–MIYAURA REACTION IN WATER
479
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tion. A glass 5-ml test tube equipped with a magnetic
stirrer was charged with 0.075 mmol of the corre-
sponding aryl halide, 0.09 mmol of phenylboronic
acid, 0.38 mmol of potassium hydroxide, and 0.25 ml
of water. The catalyst (0.1 ml of a solution containing
a required amount of palladium) was added with the
aid of a chromatographic syringe, and this moment
was taken as the reaction start. When the reaction was
complete, the mixture was diluted with 2 ml of a satu-
rated aqueous solution of sodium chloride, 5–7 μl of
hexadecane C16H34 (internal standard) was added, and
the mixture was extracted with methylene chloride
(3 × 2 ml). The extract was dried over anhydrous
sodium sulfate and analyzed by GLC.
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forming TEM studies. This study was performed under
financial support by the Russian Foundation for Basic
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