PEG modification effect of silica on the Suzuki-Miyaura coupling reaction using silica-immobilized palladium catalysts

Article abstract of DOI:10.1007/s10562-011-0583-5

The Pd phosphine complex catalysts immobilized onto polyethylene glycol (PEG)-modified silica were prepared in order to clarify the effect of the PEG modification on the Suzuki-Miyaura coupling reaction in organic solvents. For the reaction of ethyl p-bro

Full text of DOI:10.1007/s10562-011-0583-5

Catal Lett (2011) 141:866–871
DOI 10.1007/s10562-011-0583-5
PEG Modification Effect of Silica on the Suzuki–Miyaura
Coupling Reaction Using Silica-immobilized Palladium Catalysts
Shun-ya Onozawa
Kaori Saitou Toshiyasu Sakakura
Hiroyuki Yasuda
•
Norihisa Fukaya
•
•
•
Received: 23 December 2010 / Accepted: 19 March 2011 / Published online: 19 April 2011
Ó Springer Science+Business Media, LLC 2011
Abstract The Pd phosphine complex catalysts immobi-
lized onto polyethylene glycol (PEG)-modified silica were
prepared in order to clarify the effect of the PEG modifi-
cation on the Suzuki–Miyaura coupling reaction in organic
solvents. For the reaction of ethyl p-bromobenzoate and
phenylboronic acid in the presence of potassium carbonate
in toluene, the PEG-modified silica-immobilized Pd cata-
lysts exhibited much higher activities than the catalysts
without PEG modification.
strategy to enhance the activity of immobilized catalysts is
modifying the surface of catalyst supports with polymers
that have a high affinity for substrates and/or reaction
media [5–11].
The palladium-catalyzed Suzuki–Miyaura coupling
reaction between organoboranes and organic electrophiles
is an important carbon–carbon bond forming reaction in
organic synthesis [12–14]. A number of studies about the
design and preparation of immobilized catalysts for the
Suzuki–Miyaura reaction have been reported [15–21].
Since the reaction takes place in aqueous organic media,
modifying supports with amphiphilic polymers such as
polyethylene glycol (PEG) effectively promotes the reac-
tion. Uozumi et al. [22, 23] and Lee et al. [24] have
demonstrated that polystyrene-immobilized Pd complexes
in which PEG is employed as a linker of the complex
efficiently catalyze the Suzuki–Miyaura reaction in water.
Lee et al. [10] have also developed PEG-modified chitosan-
supported Pd nanoparticle catalysts for the reaction in
water. Ma et al. [11] have proposed the catalytic system
consisting of Pd(PPh ) and PEG-coated mesoporous sil-
Keywords Immobilized Pd phosphine complex Á
Polyethylene glycol Á Suzuki–Miyaura coupling reaction Á
Mesoporous silica Á Heterogeneous catalysis
1
Introduction
Immobilization of molecular catalysts such as transition
metal complex catalysts onto organic polymers or inor-
ganic porous materials has attracted much interest in recent
organic synthesis because immobilized catalysts are
advantageous in terms of their recovery from reaction
mixtures by simple filtration and potential for application
to flow reaction systems [1–4]. However, heterogenization
of molecular catalysts generally sacrifices catalytic per-
formance, including the activity and/or selectivity. One
3
n
ica. The excellent performances of these catalysts are
owing to the water-compatible nature of the PEG-modified
supports. On the other hand, the effect of PEG modification
on the catalytic activity for the Suzuki–Miyaura reaction in
organic solvents has not yet been clarified. Because the
reaction employs both hydrophobic compounds (organic
halides) and hydrophilic compounds (inorganic bases) as
starting materials, we hypothesized that the amphiphilic
PEG modification would efficiently concentrate both sub-
strates near catalytic active sites, and thus lead to an
improved catalytic activity even in organic solvents. In this
study, we prepared an immobilized catalyst in which Pd
phosphine complexes and PEG groups were separately
grafted onto the surface of silica supports, and verified the
S. Onozawa (&) Á N. Fukaya Á K. Saitou Á T. Sakakura Á
H. Yasuda (&)
National Institute of Advanced Industrial Science and
Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi,
Tsukuba, Ibaraki 305-8565, Japan
e-mail: s-onozawa@aist.go.jp
H. Yasuda
e-mail: h.yasuda@aist.go.jp
1
23

PEG Modification Effect of Silica-Immobilized Palladium Catalysts
867
effect of the PEG modification on the Suzuki–Miyaura
coupling reaction in toluene.
was then added and stirred for 15 h under refluxing. After
cooling, the precipitate was filtered off and the solvent was
removed under a vacuum. Allylated compound was
obtained as a white solid (9.3 g). Allylated compound
(1.96 g) and H PtCl Á6H O (1.7 mg) were dissolved in
2
Experimental
2
6
2
THF (20 mL) and stirred at room temperature. Then, tri-
ethoxysilane (0.67 g) was added dropwise and the solution
was stirred at 60 °C for 15 h. After cooling, the solvent
was removed under a vacuum. The crude PEG-function-
alized triethoxysilane 1 was purified by the reprecipitation
method using toluene and hexane at 0 °C. The resulting
white solid was dried under a vacuum to afford 1 (1.39 g).
2
.1 General
All catalyst preparation and reactions were carried out
under a nitrogen atmosphere. Chemicals and dry solvents
were purchased from Sigma–Aldrich Co., Tokyo Chemical
Industry Co., Ltd., and Wako Pure Chemical Industries,
Ltd., and used without further purification. Ethyl 4-bro-
mobenzoate and 4-tert-butyltoluene were degassed by
freeze–pump–thaw (FPT) cycles. Palladium (II) acetate
was supplied by N.E. CHEMCAT Co., while amorphous
silica CARiACT Q-3 was supplied by Fuji Silysia Chem-
ical Ltd. Ordered mesoporous silica SBA-15 was prepared
according to the literature [25, 26]. 3-(Diphenylphos-
phino)propyltrimethoxysilane (2) was synthesized accord-
ing to the literature [27].
1
H NMR (400 MHz, CDCl ): d 0.55–0.56 (m, 2H, SiCH ),
3
2
1.17 (t, J = 7.2 Hz, 9H, CH CH ), 1.60–1.69 (m, 2H,
2
3
CH CH CH ), 3.33 (s, 3H, OCH ), 3.36–3.68 (brs, 250H,
2
2
2
3
1
OCH CH ), 3.76 (q, J = 7.2 Hz, 6H, OCH CH ). C NMR
3
2
2
2
3
(100 MHz, CDCl ): d 10.6, 18.5, 22.9, 59.1, 70.1, 70.6,
3
72.0, 72.6, 73.1.
2.2.2 Preparation of Modified Silica Supports
Nitrogen adsorption/desorption isotherms were mea-
sured at -196 °C using a Bel Japan BELSORP-mini II
analyzer. Specific surface areas were calculated by the
BET method. Pore size distributions were calculated from
the adsorption branch of nitrogen isotherms using the BJH
Typically, silica (2 g) was dried under a vacuum at 80 °C
for 3 h. After cooling, diglyme (20 mL) and PEG com-
pound 1 (1 g) were added. The resulting mixture was
stirred at 110 °C for three days. The suspension was fil-
tered, and the solid residue was washed successively with
THF, toluene, chloroform, and methanol. The resulting
solid was dried under a vacuum at 80 °C for 3 h. To a
suspension of the PEG-modified silica (1 g) in dry toluene
(10 mL) was added 2 (0.4 mL). The reaction mixture was
refluxed for three days. The suspension was filtered, and
1
3
31
method. Solid–state C and P CP/MAS NMR spectra
were recorded on a Bruker AVANCE 400WB spectrometer
equipped with a 4 mm CP/MAS probe head. Liquid NMR
spectra were recorded on a JEOL JNM-Lambda 400
spectrometer. Transmission electron microscopy (TEM)
measurements were carried out using a JEOL JEM-2010F
with an acceleration voltage of 200 kV. Elemental com-
positions of carbon and phosphorous were determined by a
CE Instruments EA 1112 elemental analyzer and a Shi-
madzu EDX-800HS energy dispersive X-ray spectrometer
the Ph P-grafted silica was washed successively with tol-
2
uene, chloroform, and methanol. The resulting solid was
dried under a vacuum at 80 °C for 3 h.
2.2.3 Preparation of Palladium Catalysts
(
EDX), respectively. GC analysis was performed on a
Shimadzu GC-14B gas chromatograph with a TCD
equipped with an OV-101 column. Palladium concentra-
tions in reaction solutions were measured using a Shima-
dzu ICPS-8100 inductively coupled plasma-optical
emission spectrometer (ICP-OES).
Typically, the modified silica PEG-Q-PPh2 (0.5 g) was
dried under a vacuum at 80 °C for 3 h prior to use in
complexation. After cooling, THF (4 mL) and palladium
acetate (8.3 mg) were added. The mixture was stirred at
room temperature for 20 h. Then, the solvent was evapo-
rated, and the resulting solid was dried under a vacuum at
room temperature for 3 h.
2
.2 Catalyst Preparation
2
.2.1 Preparation of PEG-Functionalized Triethoxysilane
(
2.3 Suzuki–Miyaura Coupling Reactions
1)
The Suzuki–Miyaura coupling reaction of ethyl 4-bro-
mobenzoate and phenylboronic acid was carried out in a
Schlenk flask (30 mL) under a nitrogen atmosphere. Typi-
cally, a mixture of ethyl 4-bromobenzoate (3 mmol), phen-
ylboronic acid (3.3 mmol), potassium carbonate (6 mmol),
4-tert-butyltoluene (0.5 mL, internal standard), and the
Commercially available poly(ethylene glycol) methyl ether
(
(
(
average Mn of 2,000, 10 g) was dissolved in toluene
50 mL). The solution was added to a suspension of NaH
132 mg) in toluene (30 mL) at room temperature, and the
mixture was stirred for 30 min. 3-Bromopropene (0.67 g)
123

8
68
S. Onozawa et al.
catalyst (Pd: 0.05 mol% of aryl bromide) was stirred in a
Table 1 summarizes the loading amounts of the PEG group
and the phosphine ligand determined by combining ele-
mental analysis of carbon and EDX measurements of
phosphorus. The molar ratios of the phosphine ligand to the
PEG group for PEG-Q-PPh2 and PEG-SBA-PPh2 were
about seven and four, respectively. Although the phosphine
ligand was introduced onto the silica surface after PEG
modification, more phosphine ligand was loaded. Com-
pared to the PEG group, about three times as much TEG
group was loaded onto Q-3. The loading amounts of the
phosphine ligand on the silica supports modified with PEG
or TEG were 70–55% of the loading amounts on unmod-
ified silicas. Table 1 includes the textural properties of the
silica supports determined by nitrogen adsorption mea-
surements. Modifying the silica surface with PEG, TEG, or
phosphine ligands largely decreased the surface area and
the pore volume; the surface areas of PEG-Q-PPh2 and
PEG-SBA-PPh2 were approximately 15% of Q-3 and 20%
of SBA-15, respectively. Modification also narrowed the
average pore size of SBA-15 from 7.1 to 4.8 nm.
solvent (3 mL) at 100 °C. The reaction was performed for
0
.5 or 5 h. Product yield was determined by GC analysis.
3
Results and Discussion
Amorphous silica (CARiACT Q-3, abbreviated as Q-3) and
ordered mesoporous silica (SBA-15) were used as silica
supports. The PEG compound, which contained triethoxy-
silyl moiety (1) to modify the silica surface, was prepared in
two steps from poly(ethylene glycol) methyl ether (average
Mn of 2,000) [11]. Reactions of Q-3 and SBA-15 with 1 in
diglyme at 110 °C for three days gave PEG-modified silicas
PEG-Q and PEG-SBA, respectively (Scheme 1). To support
the palladium complexes on PEG-modified silicas, a phos-
phorus ligand was introduced onto PEG-Q and PEG-SBA.
The reaction of PEG-Q with 3-(diphenylphosphino)propyl-
trimethoxysilane (2) in toluene while refluxing for three days
gave silica support PEG-Q-PPh2 onto which the 3-(diphen-
ylphosphino)propyl group was grafted. The reaction of
PEG-SBA with 2 gave silica support PEG-SBA-PPh2.
For comparison, silica supports without PEG modification,
Q-PPh2 and SBA-PPh2, as well as a silica support modified
with tri(ethylene glycol), TEG-Q-PPh2 were also prepared.
The silica-supported Pd catalysts were prepared by
reacting the modified silicas with palladium acetate in THF
at room temperature for 20 h. The amount of palladium
acetate was adjusted so that the molar ratio of phosphorous in
the supports to Pd was six. In addition to the peaks due to
phosphine (d -15 ppm) and phosphine oxide (d 40 ppm),
1
3
Figure 1 shows the C CP/MAS NMR spectrum of
PEG-Q-PPh2. The signals at d 13–32, 58, and 70 ppm
corresponded to trimethylene, terminal methoxy, and
dimethylene groups of the grafted PEG group, respectively
3
1
the P CP/MAS NMR spectrum of the resulting Pd/PEG-Q-
PPh2 catalyst exhibited a new peak at d 23 ppm, which is
indicative of a Pd-P bond [30, 31] (Fig. 2). This observation
suggests the formation of palladium phosphine complexes
on PEG-Q-PPh2. On the other hand, the TEM images of Pd/
PEG-SBA-PPh2 reveal the presence of small amounts of Pd
metal aggregates (3–5 nm) in the mesopores and on the
external surface of the PEG-SBA-PPh2 support, indicating
that Pd partially exists as Pd nanoparticles in the Pd/PEG-
SBA-PPh2 catalyst (Fig. 3). The reduction of palladium
[
28, 29]. The peaks at d 13–32 ppm contained the reso-
nance due to trimethylene group in 2, whereas the peak at d
1 ppm was due to the unreacted methoxy group of 2. The
5
peaks observed at d 125–135 ppm were assigned to the
phenyl groups of the grafted phosphine ligand. The results
clearly show that both the PEG group and the phosphine
ligand are successfully loaded onto the Q-3 surface.
1
Fig. 1 C CP/MAS NMR spectrum of PEG-Q-PPh2
3
Scheme 1 Preparation of PEG-modified silica supports
1
23

PEG Modification Effect of Silica-Immobilized Palladium Catalysts
869
Table 1 Physical properties of silica and modified silica supports
–
1
PEG group (mmol g )
–1
P group (mmol g )
a
2
–1
S (m g )
b
3
–1
V (cm g )
c
D (nm)
Support
Ph
2
Q-3
–
–
716
112
220
101
926
183
404
0.50
0.13
0.15
0.09
0.89
0.34
0.54
–
PEG-Q-PPh2
TEG-Q-PPh2
Q-PPh2
0.059
d
0.18
0.43
0.54
0.78
–
–
–
–
–
SBA-15
–
7.1
4.8
6.2
PEG-SBA-PPh2
SBA-PPh2
a
0.098
–
0.42
0.68
Specific surface area (BET)
b
c
d
0
Pore volume (P/P = 0.99)
Average pore size (BJH)
Loading amount of TEG group
acetate by phosphorous ligands to generate unstable Pd(0)
species possibly caused the formation of Pd nanoparticles
PEG was used as a solvent, the effect of the PEG modifi-
cation was negligible (entries 8 vs. 9). Therefore, the PEG
groups in the presence of an ionic compound like potas-
sium carbonate in a non-polar solvent such as toluene may
attract ionic compounds and assist in its dissociation near
the active Pd sites, which accelerates the Pd-catalyzed
reaction. In the case of the reaction in diglyme, inorganic
bases that have a high solubility in diglyme may be easily
accessible to the hydrophilic surface of silica surface,
which reduces the PEG modification effect. The reaction
using the Pd/Q-PPh2 catalyst was conducted while adding
the same amount of poly(ethylene glycol) methyl ether
(average Mn of 2,000) (1 mg) as the amount of PEG in the
Pd/PEG-Q-PPh2 catalyst. However, an additive effect was
not observed; the yields after 30 min and 5 h were almost
the same as those for Pd/Q-PPh2 (entries 2 vs. 10).
Moreover, the addition of 10 mg of poly(ethylene glycol)
methyl ether greatly decreased the yields (entries 2 vs. 11).
Therefore, it is likely that the role of the immobilized PEG
groups in the neighborhood of the Pd species is important
for promoting the reaction.
[
32, 33].
The Suzuki–Miyaura coupling reaction of ethyl p-bro-
mobenzoate and phenylboronic acid using the silica-sup-
ported Pd catalysts was carried out in the presence of
potassium carbonate in toluene (Scheme 2). The catalyst
was adjusted to 0.05 mol% based on Pd. Table 2 summa-
rizes the results. For both Q-3 and SBA-15, modifying the
silica surface with PEG remarkably enhanced the catalytic
activity (entries 1 vs. 2 and 3 vs. 4). Additionally, SBA-15
was a more effective catalyst support than Q-3 (entries 1
vs. 3 and 2 vs. 4). Modifying the surface of Q-3 with TEG
also improved the catalytic activity (entries 2 vs. 5).
For the reaction using the Pd/PEG-Q-PPh2 and Pd/Q-
PPh2 catalysts, potassium carbonate was replaced with an
organic base like triethylamine. However, the effect of the
PEG modification was not observed (entries 6 vs. 7).
Additionally, when diglyme having the similar structure as
After the reaction using the Pd/PEG-Q-PPh2 catalyst in
toluene was carried out for 5 h, the catalyst was separated
by filtration. Then Pd leaching into the reaction solution
was measured by ICP-AES. The Pd content in each filtrate
was below the detectable limit (\0.5 ppm), indicating the
supported Pd species is relatively stable on the PEG-Q-
PPh2 support. The recyclability of the Pd/PEG-Q-PPh2
catalyst was also examined. After the reaction in toluene
was done for 5 h, the catalyst was separated by vacuum
filtration, rinsed successively with by toluene, water, and
ethanol under a nitrogen atmosphere. Then it was dried in a
vacuum at room temperature, and reused in a second
reaction. However, the catalytic activity of the recovered
Pd/PEG-Q-PPh2 notably decreased (the first reaction: 52%
yield, the second reaction: 18% yield). Because the
potassium salt of boronic acid and/or potassium bromide
3
1
Fig. 2 P CP/MAS NMR spectra of (a) Pd/PEG-Q-PPh2 and
b) PEG-Q-PPh2
(
123

8
70
S. Onozawa et al.
Fig. 3 TEM images of Pd/PEG-SBA-PPh2
Scheme 2 Suzuki–Miyaura coupling reaction using silica-immobi-
lized Pd catalysts
Table 2 Suzuki-Miyaura coupling reaction using silica-immobilized
Pd catalysts
a
Entry
Catalyst
Solvent
Base
Yield (%)
Scheme 3 Suzuki–Miyaura coupling reaction of various aryl
bromides
0
.5 h
5 h
1
2
3
4
5
6
7
8
Pd/PEG-Q-PPh2
Pd/Q-PPh2
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Diglyme
Diglyme
Toluene
Toluene
K
K
K
K
K
2
2
2
2
2
CO
CO
CO
CO
CO
3
3
3
3
3
20
4
52
24
89
48
68
8
significant loss in the catalytic activity (the first reaction:
Pd/PEG-SBA-PPh2
Pd/SBA-PPh2
Pd/TEG-Q-PPh2
Pd/PEG-Q-PPh2
Pd/Q-PPh2
53
30
34
5
82% yield, the second reaction: 81% yield, the third reac-
tion: 75% yield).
The Suzuki–Miyaura coupling reaction in toluene using
the Pd/PEG-SBA-PPh2 catalyst was applicable to various
aryl bromides bearing electron-donating and electron-
withdrawing substituents at their para-positions, giving
the corresponding biaryl compounds in good yields
(Scheme 3).
NEt
NEt
3
3
7
8
b
Pd/PEG-Q-PPh2
Pd/Q-PPh2
K
K
K
K
2
CO
2
CO
2
CO
2
CO
3
3
3
3
53
55
3
75
b
9
81
c
1
1
a
0
Pd/Q-PPh2
25
3
d
1
Pd/Q-PPh2
2
Each yield was determined by GC using 4-tert-butyltoluene as an
4
Conclusions
internal standard
b
Yield at a time of 3 h
c
We prepared Pd phosphine complex catalysts immobilized
onto amorphous and mesoporous silica supports in which
the surface was modified with PEG. For the Suzuki–
Miyaura coupling reaction in toluene, the PEG-modified
silica-immobilized Pd catalysts exhibited much higher
activities than the catalysts without PEG modification. The
immobilization of PEG groups in the neighborhood of the
Pd species seems important for promoting the reaction.
These results show that modifying the surface of catalyst
1
was added
mg of poly(ethylene glycol) methyl ether (average Mn: 2,000)
d
1
0 mg of poly(ethylene glycol) methyl ether was added
co-produced during the reaction have a low solubility in
toluene, they possibly remain on the recovered catalyst,
resulting in the reduced catalytic activity. Actually, when
the reaction was performed in water, the recovered
Pd/PEG-Q-PPh2 catalyst could be recycled twice without a
1
23

PEG Modification Effect of Silica-Immobilized Palladium Catalysts
871
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Acknowledgments This research was financially supported by the
Project of ‘‘Development of Microspace and Nanospace Reaction
Environment Technology for Functional Materials’’ of New Energy
and Industrial Technology Development Organization (NEDO),
Japan. The authors thank Ms. Michiyo Yoshinaga for assistance with
catalytic tests.
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