Palladium nanoparticles supported in a polymeric membrane: An efficient phosphine-free "green" catalyst for Suzuki-Miyaura reactions in water

Article abstract of DOI:10.1039/c4ra01104j

Palladium(0) nanoparticles supported on a polymeric membrane, CA/Pd(0), were found to be a highly efficient "dip catalyst" (heterogeneous catalyst) for Suzuki-Miyaura cross-coupling reactions. Iodo-, bromo- and electron-poor chloroarenes coupled with phenylboronic acid under eco-friendly conditions (i.e., phosphine-free and with water as the solvent) to give excellent yields. The CA/Pd(0) was prepared initially via the synthesis of Pd(0) by hydrogen decomposition of Pd(acac)₂ dissolved in BMI·BF₄ ionic liquid at 75°C for 1.0 hour to yield a black suspension (nanoparticles with a diameter of 2.7 ± 0.4 nm). These nanoparticles were washed with acetone and dried under reduced pressure. The Pd(0) nanoparticles were subsequently added to a cellulose acetate solution with acetone to generate the CA/Pd(0) polymeric membrane. The CA/Pd(0) "dip catalyst" was characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), electron-dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra01104j

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Palladium nanoparticles supported in a polymeric
membrane: an efficient phosphine-free “green”
catalyst for Suzuki–Miyaura reactions in water†
Cite this: RSC Adv., 2014, 4, 13446
Vinıcius W. Faria,a Deyvid G. M. Oliveira,b Marcia H. S. Kurz,b Fabio F. Gonçalves,b
´
´
b
´
a
*
*
Carla W. Scheeren and Gilber R. Rosa
Palladium(0) nanoparticles supported on a polymeric membrane, CA/Pd(0), were found to be a highly
efficient “dip catalyst” (heterogeneous catalyst) for Suzuki–Miyaura cross-coupling reactions. Iodo-,
bromo- and electron-poor chloroarenes coupled with phenylboronic acid under eco-friendly conditions
(i.e., phosphine-free and with water as the solvent) to give excellent yields. The CA/Pd(0) was prepared
initially via the synthesis of Pd(0) by hydrogen decomposition of Pd(acac)2 dissolved in BMI$BF4 ionic liquid
at 75 ꢀC for 1.0 hour to yield a black suspension (nanoparticles with a diameter of 2.7 ꢁ 0.4 nm). These
nanoparticles were washed with acetone and dried under reduced pressure. The Pd(0) nanoparticles were
subsequently added to a cellulose acetate solution with acetone to generate the CA/Pd(0) polymeric
membrane. The CA/Pd(0) “dip catalyst” was characterised by X-ray diffraction (XRD), scanning electron
microscopy (SEM), electron-dispersive spectroscopy (EDS) and transmission electron microscopy (TEM).
Received 7th February 2014
Accepted 27th February 2014
DOI: 10.1039/c4ra01104j
www.rsc.org/advances
contain palladium.11 However, the majority of catalytic systems
operate under drastic conditions in an inert atmosphere with
Introduction
toxic phosphines and an absence of water, which makes these
systems not environmentally friendly. More recently,
researchers have developed new eco-friendly catalytic systems
for Suzuki–Miyaura cross-coupling reactions using palladium
nanoparticles (Pd NPs).7 Microwave,12 sonication (ultrasound)13
as well as less-toxic bases and solvents are used in “green”
Suzuki–Miyaura catalytic systems;7 however, reports of systems
in which water is used as solvent are still scarce.14 Our group has
been investigating the use of ionic liquids to stabilise nano-
particles, with particular focus on optimising new catalytic
systems for eco-friendly hydrogenation reactions.2,15 Recently,
our efforts have been focused on the immobilisation of nano-
particles in biopolymers, particularly cellulose acetate.2,16 We
believe that this approach has great potential in the develop-
ment of heterogeneous catalysts for cross-coupling reactions.
Herein, we report the development of Pd NPs supported on
polymeric membrane, CA/Pd(0), as “dip catalysts” and their use
in a catalyst system for the Suzuki–Miyaura reaction in water.
Cellulose is among the most abundant renewable organic
resources and is also easily biodegradable; it therefore represents
a relatively low contamination risk to the environment. Cellulose
acetate (CA) exhibits excellent biocompatibility, and these prom-
ising materials are thus suitable for the immobilisation of both
biological compounds1 and nanoparticles of transition metals.2
The ideal support for a catalyst, for example, should be inert,
stable and resistant to mechanical force. Other properties should
also be considered, including the shape, distribution, pore size
and expandability. Increased stability, selectivity and activity of a
catalyst are oen obtained through a combination of immobili-
sation techniques and the proper selection of the support.
Numerous studies have been conducted on the use of cellulose
matrices for adsorption and covalent-bond immobilisation.1,3–5
Palladium-nanoparticle-based catalysts are oen used in
organic syntheses.6–8 Suzuki–Miyaura and Heck–Mizoroki cross-
couplings are versatile reactions in organic chemistry that are
classically catalysed by palladium salts with auxiliary ligands
(phosphines),9 palladacycles10 or others organometallics that
Experimental
1. Synthesis and isolation of Pd(0) nanoparticles
a
´
Escola de Quımica e Alimentos, Universidade Federal do Rio Grande – FURG, Campus
´
Carreiros, Av. Italia km 8, Bairro Carreiros, CEP 96203-900, Rio Grande – RS, Brasil.
Palladium nanoparticles were prepared by simple hydrogen
decomposition (4 atm H2, constant pressure) of 30 mg (0.28
mmol) of Pd(acac)2 dissolved in 1-n-buthyl-3-methyl-
E-mail: carlascheeren@gmail.com
b
´
Escola de Quımica e Alimentos, Universidade Federal do Rio Grande – FURG, Campus
Santo Antonio da Patrulha, Rua Barao do Cahy, 125, Cidade Alta, CEP 95500-000,
ˆ
˜
ꢀ
imidazolium tetrauoroborate (BMI$BF4) ionic liquid at 75 C
ˆ
Santo Antonio da Patrulha – RS, Brasil. E-mail: gilberrosa@furg.br
for 1.0 hour to yield a black suspension. Acetone (15 mL) was
added, and centrifugation of this mixture yielded nanoparticles
† Dedicated to Prof. Roberto Fernando de Souza for his outstanding contribution
in the development of Brazilian Catalysis: a great master who le us too soon.
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with a diameter of 2.7 ꢁ 0.4 nm; these nanoparticles were the Pd(0) NPs supported in the polymeric membrane, the
washed with acetone and dried under reduced pressure.
material was immobilised in resin, sliced with an ultramicro-
tome and placed on a carbon-coated copper grid.
2. Preparation of a polymeric membrane containing Pd(0)
nanoparticles
6. Suzuki–Miyaura cross-couplings
6.1 Experimental. All reactions were conducted under an
Cellulose acetate (10.0 g) was addedto a reaction ask containing
90 mL of acetone, and the mixture was allowed tostand for 24 h at
room temperature under a dry nitrogen atmosphere. Aer a
viscous syrup was formed, 10.0 mg (0.095 mmol) Pd(0) nano-
particles were added to 5.0 g of the syrup. The mixtures were
magnetically stirred until a homogeneous phase was obtained.
We prepared lms of the polymeric membrane, designated here
as CA/Pd(0) (cellulose acetate/Pd NPs), by spreading the homo-
geneous phase over a glass plate. The thickness was controlled to
be 20 mm through the use of a spacer. The solvent was evaporated
in an open atmosphere for 2 min. A similar method was used to
prepare a blank CA polymeric membrane, in which no Pd(0)
nanoparticles were used. The polymeric membrane containing
Pd(0) nanoparticles (10 mg of Pd in 907 mg of thin lm) were
used in the Suzuki–Miyaura reactions.
´
air atmosphere in a Goes “dip catalyst” reactor (GDCR, Fig. 1).
K2CO3, CsF, MgSO4, Et2O, 1,4-dioxane, ethanol, and DMF were
purchased from Synth. Phenylboronic acid and aryl halides
were purchased from Sigma-Aldrich. All chemicals were used
without further purication. NMR spectra were recorded on a
Varian XL300 spectrometer. Mass spectra were obtained on a
Shimadzu QP-5050 GC/MS (EI, 70 eV). Gas chromatography was
performed on a Perkin Elmer Clarus 400 GC equipped with a
ame ionization detector (FID) and a 30 m capillary column
with a dimethylpolysiloxane stationary phase.
6.2 Typical experiment for the Suzuki–Miyaura cross-
coupling reaction. A GDCR (Fig. 1) was charged with K2CO3
(279 mg, 2 mmol), phenylboronic acid (187 mg, 1.5 mmol), aryl
halide (1 mmol), polymer thin lm “dip catalyst” CA/Pd(0)
(97 mg, 660 mm2 for [Pd] ¼ 1 mol%; 907 mg of thin lm con-
tained 10 mg of Pd(0) nanoparticles), and distilled water (10
3. X-ray powder diffraction analysis (XRD)
ꢀ
mL). The reaction mixture was stirred at 100 C for 24 h. The
The phase structures of the Pd(0) nanoparticles prepared in
BMI$BF4 were characterised by XRD. For the XRD analysis, the Pd
NPs were isolated as a ne powder and placed in a sample holder.
The XRD experiments were performed on a SIEMENS D500
diffractometer equipped with a curved graphite crystal mono-
solution was then allowed to cool to r.t. and was subsequently
extracted with Et2O (2 ꢂ 5 mL). The organic layer was dried over
MgSO4, ltered, and concentrated in vacuo; the crude material
was subsequently puried by ash chromatography on silica
gel. The corresponding biaryl products were characterised by 1H
and 13C NMR and by GC-MS.
4-Methoxybiphenyl.18 White solid, mp 81–83.5 ꢀC (lit. 77–78.5
ꢀC). 1H NMR (300 MHz, CDCl3) d 7.58–7.53 (m, 3H), 7.45–7.40 (m,
2H), 7.34–7.26 (m, 2H), 7.01–6.98 (m, 2H), 3.86 (s, 3H). 13C NMR
(75.4 MHz, CDCl3) d 159.4, 141.1, 134.0, 129.0, 128.4, 127.0,
126.9, 114.5, 55.6. GC-MS (IE, 70 eV) m/z (%): 184 (100, M+), 169
(55), 141 (47), 115 (34), 185 (13), 63 (11), 139 (10), 76 (10).
Biphenyl.19 White solid. 1H NMR (300 MHz, CDCl3) d 7.83–7.30
(m, 10H). 13C NMR (75.4 MHz, CDCl3) d 141.2, 128.7, 127.2, 127.1.
˚
chromator and a Cu Ka radiation source (l ¼ 1.5406 A). The
diffraction data were collected at room temperature in Bragg–
Brentano q–2q geometry. The X-ray source was operated at 40 kV
and 20 mA, and the samples were scanned over the range
between 20ꢀ and 90ꢀ. The diffractograms were obtained with a
constant step of D2q ¼ 0.05ꢀ. The Bragg reections were indexed
via a pseudo-Voigt prole tting using the FULLPROF program.17
4. Scanning electron microscopy (SEM) and EDS elemental
analysis
The morphology of the membrane polymeric CA/Pd(0) was
examined and the electron-dispersive spectroscopy (EDS) anal-
ysis was performed using a JEOL model JSM 5800 operated at 10
or 20 kV and with a magnication of 1000ꢂ.
5. Transmission electron microscopy analysis (TEM)
TEM analyses were performed using a JEOL JEM1200EXII oper-
ated at 120 kV. A 20 mm objective aperture and a slightly under
focused (Df z ꢃ300 nm) objective lens were used to obtain the
bright-eld TEM images. The morphology and electron diffrac-
tion (ED) patterns of the isolated Pd NPs and supported in the
polymeric membrane, were analysed by TEM. The samples were
prepared by deposition of the Pd(0) NPs from an isopropanol
suspension at room temperature onto a carbon-coated copper
grid. The histograms of the nanoparticle size distribution were
obtained from diameter measurements of approximately 300
particles and were reproduced in different regions of the Cu grid;
a spherical nanoparticle shape was assumed. For the analyses of Fig. 1 Goes “dip catalyst” reactor (GDCR).
´
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nebulisation chamber; gas nebulisation ¼ 0.7 L minꢃ1; auxiliary
gas ¼ 0.2 L minꢃ1; plasma gas ¼ 15 L minꢃ1; pump ow ¼
4-Methylbiphenyl.19 White solid, mp 40–41 C (lit. 42–45 C).
1H NMR (300 MHz, CDCl3) d 7.62–7.26 (m, 9H), 2.42 (s, 3H). 13
ꢀ
ꢀ
C
1.2 mL minꢃ1
.
NMR (75.4 MHz, CDCl3) d 141.7, 141.5, 138.6, 130.0, 129.1,
129.0, 128.9, 128.8, 128.3, 128.2, 127.5, 124.6, 21.8. GC-MS (IE,
70 eV) m/z (%): 168 (100, M+), 167 (58), 82(38), 165 (25), 152 (24),
153 (18), 169 (14), 166 (9).
Results and discussion
Synthesis and characterisation of the polymeric membrane,
4-Cyanobiphenyl.20 White solid, mp 89–92 ꢀC (lit. 86–87 ꢀC). 1H
NMR (300 MHz, acetone-d6) d 7.77–7.74 (m, 4H), 7.57–7.47 (m,
5H). 13C NMR (75.4 MHz, acetone-d6) d 145.8, 139.1, 132.9, 129.4,
128.9, 128.0, 127.4, 118.9, 111.1. GC-MS (IE, 70 eV) m/z (%): 179
(100, M+), 178(25), 76(21), 151(16), 180(15),89(14),51(10), 63(9).
4-Nitrobiphenyl.19 Brown solid, mp 109–111.5 ꢀC (lit. 112–113
CA/Pd(0)
The Pd(0) NPs were prepared by the decomposition of Pd(acac)2
dispersed in 1-n-buthyl-3-methylimidazolium tetrauoroborate
ionic liquid (BMI$BF4) at 75 ꢀC under 4 atm of hydrogen. These
NPs were isolated from the BMI$BF4 ionic liquid and were
characterised by XRD, SEM-EDS and TEM analyses. The XRD
pattern of the Pd(0) NPs (Fig. 2) conrmed that the product was
1
ꢀC). H NMR (300 MHz, CDCl3) d 8.32–8.29 (m, 2H), 7.76–7.73
(m, 2H), 7.65–7.62 (m, 2H), 7.54–7.27 (m, 3H). 13C NMR (75.4
MHz, CDCl3) d 147.9, 147.3, 139.0, 129.5, 129.2, 128.0, 127.7,
124.4. GC-MS (IE, 70 eV) m/z (%): 152 (100), 199 (95, M+), 169
(37), 151 (30), 76 (28), 141 (27), 153 (26), 51 (26).
2-Methylbiphenyl.20 Oil. 1H NMR (300 MHz, CDCl3) d 7.41–7.20
(m, 9H), 2.24 (s, 3H). 13C NMR (75.4 MHz, CDCl3) d 141.9, 135.3,
131.5, 130.3, 129.8, 129.2, 128.4, 128.0, 127.2, 126.7, 125.7, 20.4.
GC-MS (IE, 70 eV) m/z (%): 168 (100, M+), 167 (91), 82(55), 153
(41), 165 (40), 152 (32), 51 (15), 63 (14).
7. Inductively coupled plasma optical emission
spectrometry (ICP-OES)
Palladium present in the aqueous solution was measured using
a Perkin Elmer Optima 4300 DV ICP-OES. Operating conditions:
emission line Pd ¼ 340, 458 nm; concentric nebuliser; cyclonic
Fig. 4 TEM micrograph of the Pd(0) NPs (left) and a histogram (right)
illustrating the particle size distribution.
Fig. 5 TEM micrograph of a CA/Pd(0) polymeric membrane (20 mm)
(A) and TEM micrograph after repeated runs of the Suzuki–Miyaura (B).
Table 1 Surface areas and pore volumes a 20 mm membrane of pure
cellulose acetate (CA) and a polymeric membrane containing nano-
particles, CA/Pd(0)
Fig. 2 XRD pattern of Pd(0) NPs synthesised in BMI$BF4 ionic liquid.
Entry
Sample
M(0) (mg)
aSbet (m2 gꢃ1
)
1
2
CA
CA/Pd(0)
—
10
38
113
Fig. 3 SEM micrograph illustrating the distribution of Pd(0) NPs in the
polymeric membrane (left) and an EDS pattern showing the detection Scheme 1 The Suzuki–Miyaura cross-coupling reaction investigated
of Pd metal (right).
in this work.
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Table
2 Optimisation of reaction conditions for Suzuki–Miyaura
crystalline and that the mean diameter was 3.2 ꢁ 0.5 nm, as
estimated by the Debye–Scherrer equation using the full-width
at half-maximum (FWHM) of the (111), (200), (220), (311), and
(222) planes.
cross-coupling of CA/Pd(0) with phenylboronic acid and 4-
bromoanisolea
Entry
1
Base
CsF
Solvent
Yield (%)
10
Remarks
A SEM micrograph of a cross-section of the CA/Pd(0) poly-
meric membrane is shown in Fig. 3. The CA/Pd(0) lm appears
to have a compact structure, and the Pd(0) NPs, represented by
the clear points (BSE method), are homogeneously distributed
over the polymeric membrane. EDS analysis of the Pd(0) NPs
(Fig. 3, right) indicated the presence of palladium metal.
TEM analysis was also performed on the prepared samples.
The NPs were irregularly shaped, with a monomodal size
distribution of 2.7 ꢁ 0.4 nm (Fig. 4). They were dispersed in
cellulose acetate (CA) solution. The homogeneous solution thus
formed was spread over a glass plate, and lms of 20 mm
thickness were obtained using a spacer. The lms of the poly-
meric membrane, CA/Pd(0), were characterised, and their
catalytic properties were investigated in Suzuki–Miyaura
reactions.
1,4-Dioxane
Highly hygroscopic
base. Film dissolved.
Film dissolved.
Film dissolved.
Film preserved.
Film preserved.
Film preserved.
Film preserved.
2
3
4
5
6
7
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
1,4-Dioxane
DMF
Ethanol
H2O
H2O
H2O
17
22
49
70
96b (90)
68b,c
a
Reaction conditions: 4-bromoanisole (1 mmol), phenylboronic acid
(1.5 mmol), CA/Pd(0) (97 mg, 1 mol%), base (2 mmol), solvent (10
mL), 24 h (time not optimised), 50 ꢀC, yields determined by GC
b
c
(average of two runs). 100 ꢀC. 0.5 mol% of CA/Pd(0). The isolated
yield is stated in parentheses.
Table 3 Suzuki–Miyaura cross-coupling of CA/Pd(0) with phenylboronic acid and different aryl halidesa
Entry
1
ArX
[Pd] (mol%)
0.5
Product
Time (h)
2
Yield (%)
94
TOF (hꢃ1
94
)
TOFd (hꢃ1
209
)
2
3
4
5
6
7
8
0.5
0.5
0.5
1.0
1.0
1.0
1.0
3
3
95
97
63
65
141
144
12
24
51
89
9
7
19
17
12
24
67
99
6
4
12
9
24
24
24
92
4
4
4
9
9
9
95
45b, 37b,c
1.0
2.0
24
48
38b, 23b,c
31b, 19b,c
2
0
4
1
9
10
1.0
24
36b, 10b,c
2
1
a
Reaction conditions: ArX (1 mmol), phenylboronic acid (1.5 mmol), CA/Pd(0) (0.5–2.0 mol%), K2CO3 (2 mmol), water (10 mL), 100 ꢀC, isolated
yields (average of two runs). GC yields (average of two runs). With 0.2 mmol of tetrabutylammonium bromide. Corrected TOF (hꢃ1),
b
c
d
considering only the amount of exposed surface metal atoms (45%).
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A TEM micrograph of the polymeric membrane containing of CA/Pd(0). Electron-rich compounds, however, were found to
the Pd(0) NPs (Fig. 5A) shows that the metal particles are be unsuitable partners in this context, even with the addition of
homogeneously distributed over the entire membrane. This tetrabutylammonium bromide (entries 9 and 10). Paradoxically,
result clearly indicates that the use of the support for immobi- the addition of tetrabutylammonium bromide caused
lisation the NPs does not substantially change the aggregation decrease in cross-coupling product yield compared with those
and size distribution of the NPs.
reported by other authors.7 Increases in both the catalyst load
a
However, aer using the lm CA/Pd(0) (Fig. 5B) is possible to (2ꢂ) and reaction time were ineffective in facilitating product
observe that occurs metal agglomeration (black points in TEM formation (entry 9). For the same reaction (Table 3, entry 1),
image), which can cause loss in catalytic activity aer some Radhakrishnan found high TOF (ꢄ104 hꢃ1), however, used very
recycling reactions.
small piece of the catalyst lm (14 ppb of Pd).7 The smaller size
The surface area of the polymeric membrane (CA) was 38 m2 of the lm is likely to enhance the accessibility of Pd(0) NPs
gꢃ1 (ꢁ10%). In the case of the polymeric membrane containing contributing to the enhanced TON and TOF.
Pd(0) NPs, the surface areas was 113 m2 gꢃ1 (ꢁ10%). This
To conduct a preliminary investigation on the recycling
increased surface area indicates that the presence of small and capability of CA/Pd(0), we chose the reaction of 4-iodoanisole
stable Pd(0) NPs induces an augmentation in the polymeric lm with phenylboronic acid (Table 3, entry 3). Approximately 100%
surface area, possibly due to the occupation of the free pores of yield was obtained in the rst run in 3 h. The catalyst polymeric
the membrane structure (compare entries 1 and 2, Table 1). The membrane was removed, washed, dried, and reinserted for the
N2 adsorption–desorption isotherms at very low relative pres- next run (see Experimental section). Aer six recycles, the yields
sures (Pie/Po < 0.2) exhibited high adsorption, thereby conrm- varied nonlinearly (Fig. 6). This result may indicate that the
ing the presence of microporous structures.
polymeric membrane suffered deformation during the pro-
longed reaction time. Additionally, aer the last run (number
6), only 17% of the cross-coupling product was found, which
was somewhat lower than that reported for other systems.7
Fig. 6 shows a loss of catalytic activity (cf. entry 1 and entries
Catalytic properties of the polymer thin lm “dip catalyst” CA/
Pd(0)
The catalytic properties of CA/Pd(0) were evaluated for the 2–3); aer the fourth recycle, activity again increased. This fact
Suzuki–Miyaura cross-coupling of 4-bromoanisole with phenyl- may be related to the leaching of the supercial Pd NPs,
boronic acid (Scheme 1). The reaction conditions, such the resulting in the release of the catalytically active Pd(0) atoms.
solvent(1,4-dioxane, DMF, ethanol, orwater), base(CsForK2CO3)
Thus, we investigated the possibility that Pd(II) was present
and temperature, was optimised using 1 mol% of Pd in the in the solution by ICP-OES because the solution exhibited a
polymer thin lm (Table 2). The bases and solvents were chosen yellow colour aer the rst recycle (i.e., the same colour as Pd
from previous work that supported our investigation.7,10a–d
salts). Pd(II) was not detected by ICP-OES; however, the limit of
The results outlined in Table 2 indicate that the best result detection was high (0.51 mg Lꢃ1). To further investigate the
(entry 5) is obtained with the use of K2CO3 as the base and water presence of Pd(II) in aqueous solution, we added a fresh charge
as the solvent. This result is surprising and represents a Suzuki– of reagents in the absence of a catalyst in the solution that
Miyaura cross-coupling reaction that is free of phosphine resulted from the rst recycle. Aer 3 h of reaction, biphenyl
ligands and quaternary ammonium salts. When the same
reaction was repeated at 100 ꢀC (entry 6), the reaction yield
increased to 96%. When the amount of CA/Pd(0) catalyst was
reduced to 0.5 mol%, the yield decreased to 68% (entry 7).
Aer optimising the catalytic system for the Suzuki–Miyaura
reaction, we performed the cross-coupling reactions using
different aryl halides (Table 3).
When smaller quantities of CA/Pd(0) were used than that
required with the chloro- and bromoarenes, we obtained
excellent yields for the coupling of the more reactive aryl iodides
and a wide variety of functional groups were tolerated on the
aromatic ring (Table 3). The coupling reaction of iodobenzene
(entry 1) occurred in only 2 h; however, a long reaction time was
needed to couple deactivated aryl bromides and chlorides.
Table 3 also shows that both electron-rich and electron-poor
aryl bromides were efficiently coupled in the presence of CA/
Pd(0) (entries 4–6), furnishing the cross-coupled products in
excellent yields. As the main goal of our efforts with the Suzuki–
Miyaura catalytic system was to work with aryl chlorides
because they are the cheapest aryl halides, we performed reac-
tions using different chloroarenes and obtained very good
Fig. 6 Yield obtained after 3 h of reaction time in repeated runs of the
Suzuki–Miyaura cross-coupling of phenylboronic acid with 4-iodoa-
nisole in water using K2CO3 as the base and the same piece of thin-film
yields with electron-poor aryl chlorides (entry 7) using 1 mol% catalyst.
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Fig. 7 SEM micrographs show that morphological structure of the polymeric membrane changes: (left) before recycling and (right) after
recycling in the Suzuki–Miyaura experiments.
was recovered in 39% yield, which clearly indicates the presence
of Pd(II) in the aqueous solution. This result explains the poor
Notes and references
recyclability of the CA/Pd(0) thin polymeric membrane.
Fig. 7 shows the morphological structure of the polymeric
membrane. The temperature applied during the Suzuki–
Miyaura reactions apparently causes a thermal deformation
with consequent reduction of the porosity of the polymeric
membrane. This deformation affects the reaction yield because
it makes the polymeric membrane more compact. The defor-
mation in the structure of the polymeric membrane, possibly
renders the nanoparticles of Pd(0) (supported inside the pores
of the membrane) inaccessible (Fig. 7B), this fact combined
with the metal aglomeration (Fig. 5B), thus, can be associate
with loss of catalytic activity.
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preparation of palladium nanoparticles dispersed in cellulose
acetate, CA/Pd(0), as an eco-friendly “dip catalyst” for Suzuki–
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the coupling of phenylboronic acid with a wide variety of
functional groups on the aromatic ring of iodo- and bromoar-
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unsuitable reagents in this context, giving modest results. The
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undergo leaching and because the temperature used in the
Suzuki–Miyaura reactions induces a thermal deformation of the
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Acknowledgements
This work was supported by grants from CNPq, FAPERGS and
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CAPES (Brazil). Thanks are also due to Prof. Fabio Andrei Duarte
(IQ/UFSM) for assistance with the ICP-OES analyses.
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