Preparation and application of SBA-15-supported palladium catalyst for Suzuki reaction in supercritical carbon dioxide

Article abstract of DOI:10.1039/c004250a

The external and internal surfaces of SBA-15 were modified by (MeO) ₃SiPh and a phosphine ligand, (MeO)₃SiCH ₂CH₂CH₂SCH₂C₆H ₄PPh₂, before and after the template Pluronic P123 copolymer was removed, respectively. Palladium was then tethered within the cavity of the mesoporous material Ph-SBA-15-PPh₃via a ligand-exchange reaction to provide a new supported palladium catalyst Ph-SBA-15-PPh ₃-Pd. This catalyst was demonstrated to be a robust and active catalyst in Suzuki reaction of a wide range of aryl bromides with arylboronic acids in supercritical carbon dioxide. After the reaction mixtures were treated at 90 °C for 24 h, the crude coupling products were obtained as crystalline solids when the reaction temperature was lowered to room temperature and the carbon dioxide was then slowly released. The pure products can be obtained by simple recrystallization in good to excellent yields. The Ph-SBA-15-PPh ₃-Pd catalyst has low leaching loss and can be reused at least 7 times without loss of activity.

Full text of DOI:10.1039/c004250a

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www.rsc.org/greenchem | Green Chemistry
Preparation and application of SBA-15-supported palladium catalyst for
Suzuki reaction in supercritical carbon dioxide
Xiujuan Feng,* Mei Yan, Tao Zhang, Ying Liu and Ming Bao*
Received 15th March 2010, Accepted 11th August 2010
DOI: 10.1039/c004250a
The external and internal surfaces of SBA-15 were modified by (MeO)3SiPh and a phosphine
ligand, (MeO)3SiCH2CH2CH2SCH2C6H4PPh2, before and after the template Pluronic P123
copolymer was removed, respectively. Palladium was then tethered within the cavity of the
mesoporous material Ph-SBA-15-PPh3 via a ligand-exchange reaction to provide a new supported
palladium catalyst Ph-SBA-15-PPh3-Pd. This catalyst was demonstrated to be a robust and active
catalyst in Suzuki reaction of a wide range of aryl bromides with arylboronic acids in supercritical
carbon dioxide. After the reaction mixtures were treated at 90 ◦C for 24 h, the crude coupling
products were obtained as crystalline solids when the reaction temperature was lowered to room
temperature and the carbon dioxide was then slowly released. The pure products can be obtained
by simple recrystallization in good to excellent yields. The Ph-SBA-15-PPh3-Pd catalyst has low
leaching loss and can be reused at least 7 times without loss of activity.
preparation of the immobilized palladium catalyst (SBA-15-
Introduction
SH-Pd), which was found to be an active and stable catalyst
The Suzuki reaction of organoboron reagents with organic
for Suzuki reaction in water with virtually no leaching of Pd.
halides represents one of the most versatile and straightforward
The heterogeneity tests including hot filtration tests and three-
methods for the construction of new carbon–carbon bonds
phase tests have demonstrated that the catalysis occurs on the
in the synthesis of fine chemicals, agrochemicals, and active
pharmaceutical intermediates as well as functional materials.1
surface or in the pores of the silicate.
Inspired by the work of Crudden et al. as mentioned above,
It is well known that when the Suzuki reaction is performed in
we attempted the Suzuki reaction of 1-bromo-4-nitrobenzene
conventional organic solvent using a homogeneous palladium
with phenylboronic acid utilizing the SBA-15-SH-Pd catalyst in
catalyst there are often some drawbacks: it is difficult to
supercritical carbon dioxide, and the desired coupling product,
separate the coupling product from the catalyst and any organic
4-nitrobiphenyl, was obtained in 60% yield under the opti-
solvent, the expensive palladium catalyst cannot be recycled,
mized reaction conditions. This result indicated that SBA-15-
and the organic solvent could potentially cause environmental
supported palladium catalyst can be used in Suzuki reaction
contamination due to its volatility. Green solvents, water,2 ionic
in supercritical carbon dioxide, and encouraged us to design
liquids,3 and supercritical carbon dioxide,4 have recently been
and prepare a new SBA-15-supported palladium catalyst to
used in Suzuki reactions to achieve environmentally benign
increase the reaction yield. Prompted by the work of Kerton
processes. Furthermore, the Suzuki reactions have been success-
and co-workers,9d who demonstrated that polydimethylsiloxane-
fully performed under solvent-free conditions.5 To accomplish
derived phosphine ligand (PDMS-PPh2) based nanopalladium
the recycled use of expensive palladium species, many heteroge-
catalyst can be used in Suzuki reaction of 4-iodotoluene with
neous palladium catalysts such as polymer-supported palladium
phenylboronic acid in supercritical carbon dioxide to provide a
catalysts,6 SBA-15 or SBA-16/Pd,7 and polyurea-encapsulated
coupling product in excellent yield, we prepared a new SBA-15-
palladium catalyst8 as well as others,9 have been prepared for
supported palladium catalyst. The principle of the preparation
Suzuki reactions. In many cases, both the green solvent and
is illustrated in Scheme 1. At first, the external surface of
heterogeneous palladium catalyst were used together in the
SBA-15 was modified by (MeO)3SiPh before the template was
Suzuki reaction. However, it has been clearly demonstrated that
removed, and the internal surface was then modified by a
heterogeneous palladium catalysts act as reservoirs for highly
phosphine ligand, (MeO)3SiCH2CH2CH2SCH2C6H4PPh2, after
active soluble forms of Pd in many cases, and this behavior may
lead to the leaching of active catalyst species.
Crudden and co-workers have more recently reported ex-
cellent results.7e In their research thiol-modified mesoporous
the template was removed. Finally, palladium was tethered
within the cavity of the mesoporous material Ph-SBA-15-PPh3
via a ligand-exchange reaction. This newly formed SBA-15-
supported palladium catalyst (Ph-SBA-15-PPh3-Pd) may act
material (SBA-15-SH) was employed as a support for the
as a nanoreactor, in which the catalysis will occur. Even if
the palladium species breaks off from one phosphine ligand,
it may attach to another phosphine ligand inside of the SBA-
15. Therefore, the Ph-SBA-15-PPh3-Pd catalyst should be very
stable, and could be reused several times without loss of activity.
As expected, the Ph-SBA-15-PPh3-Pd catalyst exhibited high
State Key Laboratory of Fine Chemicals, Dalian University of
Technology, Dalian, 116012, China.
E-mail: xiujuan.feng@163.com, mingbao@dlut.edu.cn;
Fax: +86-411-39893687; Tel: +86-411-39893862
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Scheme 1 Schematic illustration for the preparation of the SBA-15 supported palladium catalyst.
catalytic activity in Suzuki reaction of various aryl bromides
with arylboronic acids in supercritical carbon dioxide. Here we
wish to report our results.10
Results and discussion
Preparation and characterization of the catalyst
Ph-SBA-15-PPh3-Pd
According to the process as illustrated in Scheme 1, the meso-
porous materials Ph-SBA-15 and Ph-SBA-15-PPh3 as well as
the desired catalyst Ph-SBA-15-PPh3-Pd were synthesized. Fig.
1 shows the UV-visible spectra of the SBA-15 and Ph-SBA-15.
Compared with SBA-15, Ph-SBA-15 has an obvious absorption
peak of the benzene ring at 263 nm which revealed that the
external surface of SBA-15 has been successfully modified with
(MeO)3SiPh.
Fig. 2 N2 adsorption-desorption isotherms for Ph-SBA-15 and Ph-
SBA-15-PPh3-Pd.
material. As shown in Fig. 3, the pore size distribution of catalyst
Ph-SBA-15-PPh3-Pd was very narrow. The average pore size of
the catalyst Ph-SBA-15-PPh3-Pd was ca. 5.2 nm calculated by
Barrett–Joyner–Halenda (BJH) method.
Fig. 1 UV-visible spectra of SBA-15 and Ph-SBA-15.
The N2 sorption isotherms of Ph-SBA-15 and Ph-SBA-
15-PPh3-Pd are displayed in Fig. 2. Both materials exhibit
a type-IV isotherm pattern with an H2 hysteresis loop,
which is characteristic of the mesoporous structure. Although
the mesoporous structure of the catalyst Ph-SBA-15-PPh3-
Pd was still maintained, the surface area and pore volume
showed a significant decrease, which indicated that the ligand
(MeO)3SiCH2CH2CH2SCH2C6H4PPh2 was introduced into the
interior of Ph-SBA-15. Therefore the palladium species were
immobilized on the modified internal surface of the mesoporous
Fig. 3 Pore size distribution of catalyst Ph-SBA-15-PPh3-Pd.
The ordered mesoporous structure of Ph-SBA-15, Ph-SBA-
15-PPh3, and Ph-SBA-15-PPh3-Pd were further confirmed by
the XRD pattern determination (Fig. 4). All the three curves
obtained displayed an intense peak and two weak peaks,
respectively, which matched well with the pattern of SBA-15
silica reported in the literature.11 The prominent peaks should
be indexed as (100) diffraction peaks, and the others as (110) and
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Fig. 4 XRD patterns of Ph-SBA-15, Ph-SBA-15-PPh3, and Ph-SBA-
15-PPh3-Pd.
(200) diffraction peaks. After a phosphine ligand was introduced
into the interior of Ph-SBA-15, the XRD pattern of parent SBA-
15 silica is still kept very well. However, due to filling of the
grafting organic functional groups in the pore of Ph-SBA-15, a
decrease in the intensity of diffraction peaks is observed.
The scanning electron microscopy image (SEM) of catalyst
Ph-SBA-15-PPh3-Pd (Fig. 5) shows rope-like domains aggregat-
ing to wheat-head like macrostructures, similar to that of parent
SBA-15, indicating that modification of external and internal
surfaces does not influence the morphology of the mesoporous
silica SBA-15. The transmission electron microscopy (TEM)
shows well-ordered hexagonal pore channels in catalyst Ph-
SBA-15-PPh3-Pd (Fig. 6, upper). The TEM image also reveals
arrays in catalyst Ph-SBA-15-PPh3-Pd when viewed perpendic-
ular to the channel axis (Fig. 6, lower).
Fig. 6 TEM image of Ph-SBA-15-PPh3-Pd: (upper) taken along the
parallel channels; (lower) vertical to the parallel channels.
in the model reaction of 1-bromo-4-nitrobenzene with phenyl-
boronic acid were first investigated, and the results of which are
shown in Table 1. Among the bases examined, K3PO4·3H2O gave
the best result (99% yield, entry 7). The other bases including
organic bases and inorganic bases, such as diisopropyl ethyl
amine (DIPEA), MeCO2NBu4, NEt3, MeCO2K, and KOH,
gave low to moderate yields or no reaction (entries 1–6 and
8–11). When the Pd loading was decreased from 0.8 mol% to
0.6 mol%, a decreased reaction yield was obtained (87%, entry
12). Therefore, K3PO4·3H2O was employed as a base and the
0.8 mol% of Pd loading was chosen in the subsequent study.
Suzuki reactions of a wide range of aryl bromides with
arylboronic acids were tested, the catalysis results of which
are summarized in Table 2. The reactions of aryl bromides
bearing electron-withdrawing groups such as NO2, CH3CO, and
CHO at the para position, respectively, with phenylboronic acid
proceeded smoothly to furnish biphenyls in good to excellent
yields (84–99%, entries 1–3). Even aryl bromide bearing OH,
an electron-donating group, at the para position, the coupling
reaction of which with phenylboronic acid gave also excellent
yield (95%, entry 4). 3-Bromopyridine, a brominated aromatic
heterocycle, was also treated with phenylboronic acid, and 61%
reaction yield was obtained (entry 5, 3-bromopyridine was
recovered in 23% yield). Finally, the MeO or F substituted
arylboronic acid was employed in the coupling reaction of
Fig. 5 SEM image of Ph-SBA-15-PPh3-Pd.
Finally, inductively coupled plasma (ICP) analysis indicated
that the ratio of Pd on the catalyst Ph-SBA-15-PPh3-Pd was 1.04
wt%.
Suzuki coupling reaction catalyzed by Ph-SBA-15-PPh3-Pd
The Suzuki cross-coupling reaction between aryl bromides and
arylboronic acids was chosen to evaluate the catalytic activity
and stability of the catalyst Ph-SBA-15-PPh3-Pd in supercritical
carbon dioxide.12,13 The influences of base and catalyst loading
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Table 1 Effects of base and catalyst loading on the Suzuki coupling
reaction of 1-bromo-4-nitrobenzene with phenylboronic acid catalyzed
by Ph-SBA-15-PPh3-Pd in supercritical carbon dioxidea
(83% and 86%, respectively, entries 7 and 8). Except for the
3,4-difluoro-4¢-nitro-1,1¢-biphenyl14 in entry 8, other coupling
products are known compounds and their 1H NMR data were
consistent with those reported in the literature.15–19
Interestingly, after the reaction mixtures were treated at 90 ◦C
for 24 h, the crude coupling products were obtained as crystalline
solids in all cases when the reaction temperature was lowered
to room temperature and the carbon dioxide was then slowly
released. And the pure products could be obtained by simple
recrystallization in good to excellent yields. Fig. 7 shows the
crystalline state and 1H-NMR spectrum of crude and pure
coupling product, 4-acetylbiphenyl. Compared with the 1H-
NMR spectrum of pure 4-acetylbiphenyl (Fig. 7, right), the 1H-
NMR spectrum of crude 4-acetylbiphenyl (Fig. 7, left) indicated
that the crude product has fairly high purity.
The stability of the catalyst Ph-SBA-15-PPh3-Pd was then
investigated, the results of the repeated use of the catalyst for the
coupling reaction of 4-bromoacetophenone with phenylboronic
acid are summarized in Table 3. Using the fresh catalyst, 97%
yield was obtained (run 1). The use of the recovered catalyst also
produced an excellent yield of the coupling product (run 2, 96%).
Again, the catalyst was recovered and used repeatedly. Even
when the catalyst was reused for the seventh time, the coupling
reaction of 4-bromoacetophenone with phenylboronic acid still
proceeded smoothly to give the desired product in excellent yield
(run 7, 96%). Extremely low leaching of the catalyst is observed,
at the end of the reaction only 0.009% of the initially added Pd
was lost, as determined by ICP analysis. These results clearly
Entry
Base
Pd (mol%)
Yield (%)b
1
2
3
4
5
6
7
8
DIPEA
MeCO2NBu4
NEt3
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.6
58
56
47
49
55
18
99
K2CO3
K3PO4·7H2O
K3PO4
K3PO4·3H2O
K3PO4·H2O
KOH
9
9
NRc
NRc
16
10
11
12
NaF
MeCO2K
K3PO4·3H2O
87
a Reaction conditions: 1-bromo-4-nitrobenzene (0.5 mmol), phenyl-
boronic acid (1.5 equiv.), base (2.0 equiv.), Ph-SBA-15-PPh3-Pd (Pd: 0.6-
1.0 mol%), scCO2, 90 ◦C, 20 MPa, 24 h. b Isolated yields. c No reaction.
1-bromo-4-nitrobenzene. The reaction of 1-bromo-4-nitrobenzene
with 4-methoxyphenylboronic acid also proceeded smoothly to
offer the desired biaryl in 91% yield (entry 6). High yields were
obtained from the reactions of 1-bromo-4-nitrobenzene with
4-fluorophenylboronic acid or 3,4-difluorophenylboronic acid
Fig. 7 (Left) Crystalline state and 1H NMR spectrum of crude 4-acetylbiphenyl. (Right) Crystalline state and 1H NMR spectrum of pure 4-
acetylbiphenyl.
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Table 2 Suzuki coupling reaction of aryl bromides with arylboronic acids catalyzed by Ph-SBA-15-PPh3-Pda
Entry
1
Aryl bromide
Arylboronic acid
Product
Yield (%)b
99
2
97
3
4
5
6
84
95
61c
91
7
8
83
86
a Reaction conditions: aryl bromides (0.5 mmol), arylboronic acids (1.5 equiv.), K3PO4·3H2O (2.0 equiv.), Ph-SBA-15-PPh3-Pd (Pd: 0.8 mol%),
scCO2, 90 ◦C, 20 MPa, 24 h. b Isolated yields. c 3-Bromopyridine was recovered in 23% yield.
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Table 3 Recycled use of the Ph-SBA-15-PPh3-Pd catalyst for Suzuki
homogeneous Pd is released outside of the mesoporous material,
reaction of 4-bromoacetophenone with phenylboronic acida
then the supported aryl bromide can be converted to produce
a biaryl compound. When the three-phase test was performed
under standard conditions (K3PO4·3H2O, PhB(OH)2, scCO2,
0.8 mol% Pd, 90 ◦C, 48 h), 4-bromobenzoic acid was recovered
in almost quantitative yield upon cleavage from the support,
and its coupling product, 4-phenylbenzoic acid, was obtained in
trace level, which was considered that due to the small amount
of leaching Pd. The coupling product, 4-phenylacetophenone,
derived from the soluble 4-bromoacetophenone substrate was
obtained in 74% isolated yield. These results clearly indicate
that the catalyst Ph-SBA-15-PPh3-Pd is very stable and almost
all of the catalysis occurred inside of the mesoporous material.
Run
Yield (%)b
Run
Yield (%)b
1
2
3
4
97
96
98
99
5
6
7
97
97
96
a Reaction conditions: 4-Bromoacetophenone (0.5 mmol), phenyl-
boronic acid (1.5 equiv.), K3PO4·3H2O (2.0 equiv.), Ph-SBA-15-PPh3-Pd
(Pd: 0.8 mol%), scCO2, 90 ◦C, 20 MPa, 24 h. b Isolated yields.
Conclusions
In conclusion, we present a new method for the preparation
of SBA-15-supported palladium catalyst (Ph-SBA-15-PPh3-
Pd) for the Suzuki coupling reaction in supercritical carbon
dioxide. In the prepared catalyst Ph-SBA-15-PPh3-Pd, the Pd
species were anchored inside of the mesoporous materials, which
therefore acted as nanoreactors. Our study on the catalyst
reuse suggests that the catalyst Ph-SBA-15-PPh3-Pd was very
stable and can be reused at least 7 times without loss of
catalytic activity. The high catalytic activity of Ph-SBA-15-
PPh3-Pd may be attributed to the flexible chain between the
triphenylphosphine ligand and SBA-15, which could make the
catalysis of supported palladium species is similar to those of a
homogeneous catalyst. The use of supercritical carbon dioxide
as a green solvent not only prevents potential environmental
contamination, but also prevents the leaching of the Pd species
from their support. Further investigations of other C–C bond
forming reactions using Ph-SBA-15-PPh3-Pd as a catalyst are
now in progress.
indicate that this catalyst Ph-SBA-15-PPh3-Pd is very stable and
can be used repeatedly without loss of activity. In addition, as a
comparative test, we examined the cross-coupling reaction of 4-
bromoacetophenone with phenylboronic acid using catalyst Ph-
SBA-15-PPh3-Pd in ethanol, and found that the desired cross-
coupling product was also obtained in excellent yield (99%).
However, the Pd leaching of the catalyst (22%) in ethanol is
much larger than that in supercritical CO2 (Scheme 2).
Scheme 2 Cross-coupling reaction of 4-bromoacetophenone with
phenylboronic acid catalyzed by Ph-SBA-15-PPh3-Pd in ethanol.
Experimental
Reagents and materials
Pluronic P123 copolymer (EO20PO70EO20) and 1-bromo-4-
nitrobenzene were purchased from Sigma-Adrich Company.
Tetraethylorthsilicate (TEOS) was obtained from Beijing Chem-
ical Reagents Company, China. Pd2(dba)3 was purchased
from Shanghai Zealandchem Co., Ltd., China. Trimethoxy-
phenylsilane, 4-bromobenzaldehyde, 4-methylbenzenesulfonic
acid, chlorodiphenylphosphine, and KBH4 were obtained
from Shanghai Chemical Reagent Company of Chinese
Medicine Group. 3-(Trimethoxysilyl)propane-1-thiol, bro-
mobenzene, 4-bromotoluene, 1-bromo-4-fluorobenzene, 1-
bromo-3,4-difluorobenzene, and 4-bromoanisole were ob-
tained from ABCR GmbH & Co. KG, Germany. 4-
Bromoacetophenone was purchased from Alfa Aesar China
(Tianjin) Co., Ltd. 4-Bromophenol and 3-bromopyridine were
purchased from Acros Organics.
Scheme 3 Three-phase test in the Suzuki reaction with catalyst Ph-
SBA-15-PPh3-Pd.
In an attempt to ascertain whether the catalysis occurred
inside or outside of the catalyst Ph-SBA-15-PPh3-Pd, a three-
phase test was utilized (Scheme 3). This test, developed by
Rebek and co-workers20 and used by Corma,21 Davies,22 and
others,7e,23 is considered to be a definitive test for the presence of
catalytically active homogeneous metal species. In our case, if the
catalyst remains immobilized inside of the mesoporous materi-
als, no transformation should be observed for the anchored aryl
bromide, because the solid substrate could not enter the cavities
of the mesoporous materials to access the catalyst species. If
The arylboronic acids including phenylboronic acid, 4-
methoxyphenylboronic acid, 4-fluorophenylboronic acid, and
3,4-difluorophenylboronic acid were synthesized from the cor-
responding aryl bromides (bromobenzene, 4-bromoanisole,
1-bromo-4-fluorobenzene, and 1-bromo-3,4-difluorobenzene)
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according to the modified method (Scheme 4).24 A solution of
an arylmagnesium bromide (0.2 mol, in 80 mL of THF) was
added dropwise to a solution of n-butyl borate (0.2 mol) in
Preparation of mesoporous material Ph-SBA-15-PPh3
Mesoporous material Ph-SBA-15-PPh3 was synthesized accord-
ing to the modified method (Scheme 1).26 A mixture of Pluronic
P123 (EO20PO70EO20, 4.0 g), diluted HCl solution (6 wt%, 120 g),
and deionized water (30 mL) was stirred at room temperature
until the template Pluronic P123 was dissolved completely.
After the temperature of the resulting solution was increased to
40 ◦C, 8.4 g of tetraethylorthsilicate was added dropwise to the
solution. After being stirred for 24 h, the resultant suspension
was transferred into an autoclave. The autoclave was placed
under static conditions at 100 ◦C for 24 h. After the temperature
was reduced to room temperature, the precipitated solid was
isolated by a filtration, washed with deionized water (100 mL)
and ethanol (25 mL) successively, and dried at 80 ◦C under
vacuum for 5 h.
◦
80 mL of dry THF at -78 C under nitrogen atmosphere with
vigorous stirring. The resulting mixture was continuously stirred
for 3 h at the same temperature of -78 ◦C. Then the reaction
mixture was allowed to warm slowly to room temperature. After
the reaction mixture was continuously stirred for 10 h at room
temperature, an aqueous solution of HCl (6 wt%) was added
dropwise until the pH of the mixture was reduced to ca. 5.
The resulting mixture was filtered under reduced pressure, and
then the filtrate obtained was extracted with ether (50 mL ¥ 3).
The extracts were combined, and evaporated to remove ether,
THF, and n-BuOH to obtain a crude arylboronic acid. The
recrystallisation of crude arylboronic acids from water afforded
pure arylboronic acids as white needle crystals in 42–64% yields.
The solid (2.0 g) obtained was treated with trimethoxyphenyl-
silane (2.0 mL) in toluene (50 mL) in the presence of pyridine
(2.0 mL) at 115 ◦C for 2 h. After the mixture was cooled to
room temperature, the precipitated white solid was isolated
by filtration, washed with ethanol, ethyl ether, and acetone
successively, and dried at 80 ◦C under vacuum for 5 h. Then
the template Pluronic P123 was removed by extraction with
supercritical carbon dioxide to obtain the mesoporous material
Ph-SBA-15.27
The mesoporous material Ph-SBA-15 (1.5 g) obtained
was treated with ligand (MeO)3SiCH2CH2CH2SCH2C6H4PPh2
(1.5 g) in toluene (80 mL) in the presence of pyridine (1.5 mL)
at reflux under N2 atmosphere for 24 h. The mixture was slowly
cooled to room temperature, and then the precipitated white
solid was isolated by a filtration, washed with methanol, ethyl
ether, acetone, and hexane successively, and dried at 80 ◦C under
vacuum for 5 h. The mesoporous material Ph-SBA-15-PPh3 was
eventually obtained as white solid powders.
Scheme 4 Preparation of arylboronic acids.
Ligand (MeO)3SiCH2CH2CH2SCH2C6H4PPh2 was synthe-
sized as described below (Scheme 5). A mixture of K2CO3
(2.0 g 14.5 mmol), 3-(trimethoxysilyl)propane-1-thiol (1.8 mL,
10 mmol), and THF (20 mL) was stirred at 70 ◦C for
2 h, and then to which a solution of (4-(chloromethyl)-
phenyl)diphenylphosphine25 (3.7 g, 12 mmol) in THF (30 mL)
was added dropwise. The resulting mixture was continuously
stirred at 70 ◦C for 18 h, then the mixture was cooled to room
temperature and filtered to remove solid inorganic salts, and the
filtrate was evaporated to remove solvent under reduced pres-
sure. Pure (MeO)3SiCH2CH2CH2SCH2C6H4PPh2 was obtained
as a colorless viscous liquid (2.2 g, 48%) by a chromatography
on silica gel column [petroleum ether (60–90 ◦C)/ethyl acetate =
10 : 1]. 1H NMR (400 MHz, CDCl3): d 0.72 (t, J = 8.2 Hz, 2H),
1.66–1.70 (m, 2H), 2.45 (t, J = 7.4 Hz, 2H), 3.54 (s, 9H), 3.68
(s, 2H), 7.23–7.32 (m, 14H); 13C NMR (100 MHz, CDCl3): d
8.7, 22.7, 34.5, 36.0, 76.9, 77.2, 77.5, 128.6, 128.7, 128.8, 129.1,
129.2, 133.7, 133.9, 134.1, 137.3, 137.4, 139.5; 31P NMR (161
MHz, EtOAc): d -8.63; HRMS (EI) calcd for C25H31O3PSSi:
470.1501 [M]+; found: 470.1511.
Preparation of catalyst Ph-SBA-15-PPh3-Pd
Mesoporous materials Ph-SBA-15-PPh3 (1.0 g) was treated with
a solution of Pd2(dba)3 (0.1 g) in CH2Cl2 (50 mL) at room
temperature under N2 atmosphere for 24 h. Such palladium
species were tethered within the cavity of the mesoporous
materials Ph-SBA-15-PPh3 via a ligand-exchange reaction. The
content of Pd in the mesoporous materials was determined by
ICP after the resultant khaki solid was filtrated and washed with
◦
CH2Cl2 (10 mL¥2) as well as dried at 80 C under vacuum for
2 h.
General procedure for Suzuki coupling reaction
The supported catalyst Ph-SBA-15-PPh3-Pd (0.8 mol% Pd), aryl
bromide (0.5 mmol), arylboronic acid (0.75 mmol, 1.5 equiv.),
and K3PO4·3H2O (1.0 mmol, 2 equiv.) were placed in a 25 mL
stainless steel pressure vessel with a magnetic stir bar under
nitrogen atmosphere. The vessel was purged with carbon dioxide
three times, and then 15 g of carbon dioxide was filled into
◦
the vessel. It was then heated to 90 C for 24 h. The pressure
increased to 20 MPa at this temperature. The vessel was allowed
to cool to room temperature and was then vented into ethyl
acetate (30 mL). The resultant crystalline solid (crude coupling
product) was purified by a simple recrystallization process. The
Scheme 5 Synthesis of phosphine ligand having (MeO)3Si moiety.
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catalyst Ph-SBA-15-PPh3-Pd was collected, washed with water,
ethanol and dichloromethane, then dried in a vacuum and
reused.
a D/Max-2400 diffractometer using Cu-Ka-ray radiation (l =
1.5405 A) operating at 40 kV and 100 mA. The scanning electron
˚
microscopy (SEM) images were obtained using a JSM-5600LV
microscope and transmission electron microscopy (TEM) with
a Tecnai 20 microscope. The amount of Pd was obtained by
inductively coupled plasma (ICP) atomic emission spectrometry
(Optima 2000 DV). 1H, 13C, and 31P NMR spectra were recorded
in CDCl3 or ethyl acetate solution on a Varian Inova-400
Three-phase test
Silica gel supported aryl bromide was prepared according
to the literature procedure.7d A solution of 3-aminopropyl
triethoxysilane (5.1 mL, 22.0 mmol) in dry THF (10 mL) was
added dropwise to a solution of 4-bromobenzoyl chloride (4.83
g, 22.0 mmol) and triethyl amine (3.2 mL, 22.0 mmol) in
dry THF (50 mL) at -15 ◦C under nitrogen atmosphere. The
resulting mixture was warmed to room temperature, and stirred
for 2 h. Then the solid was removed via filtration and the solvent
was removed under vacuum at room temperature, giving 8.5 g of
the desired product, 4-bromobenzamide 3-propyltriethoxysilane
as a beige solid.
1
spectrometer (400 MHz for H, 100 MHz for 13C, 161 MHz
for 31P). The chemical shifts are reported in ppm downfield (d)
from Me4Si. High resolution mass spectra were recorded on a Q-
TOF mass spectrometry (Micromass, England) equipped with
Z-spray ionization source.
Acknowledgements
We are grateful to the National Natural Science Foundation
of China (20603005) for financial support. This work was
partly supported by the Program for Changjiang Scholars and
Innovative Research Teams in Universities (IRT0711).
A mixture of 4-bromobenzamide 3-propyltriethoxysilane (8.5
g, 21.9 mmol) thus obtained and pyridine (2.3 mL, 29.4 mmol)
was added dropwise to a suspension of silica (2.0 g) in dry toluene
(50 mL) under nitrogen atmosphere. The resulting mixture was
refluxed for 24 h. And then the suspension was filtered and
Soxhlet extracted with dichloromethane for 24 h. The resulting
solid was dried under vacuum at room temperature, giving 2.4 g
of white powder, and the content of aryl bromide was 1.64 mmol
g-1 determined by elemental analysis.
A mixture of 4-bromoacetophenone (0.25 mmol), phenyl-
boronic acid (0.38 mmol, 1.5 equiv.), K3PO4·3H2O (0.38 mmol,
1.5 equiv), silica gel supported aryl bromide (192 mg), catalyst
Ph-SBA-15-PPh3-Pd (0.8 mol% Pd), and carbon dioxide (15◦g)
in a 25 mL stainless steel pressure vessel was treated at 90 C
for 48 h. And then the mixture was allowed to cool to room
temperature. 15 mL of ethyl acetate was added into the vessel to
dissolve the coupling product, 4-phenylacetophenone, derived
from the soluble 4-bromoacetophenone substrate. The resulting
mixture was filtered, and the filtrate was evaporated to remove
solvent under reduced pressure. Pure 4-phenylacetophenone was
obtained in 74% isolated yield by a chromatography on silica gel
column [petroleum ether (60–90 ◦C)/ethyl acetate = 20 : 1]. The
separated solid was washed with ethanol, and further extracted
with dichloromethane, and was then hydrolyzed with 2 M KOH
in ethanol–water (1.68 g, 10 mL of EtOH, 5 mL of H2O) at 90 ◦C
for 3 days. The resulting solution was neutralized with aqueous
HCl (10 wt%), extracted with dichloromethane followed by ethyl
acetate, concentrated, and the resulting mixture was analyzed by
1H NMR.
Notes and references
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2 Suzuki reaction was carried out in water. For recent reviews, see:
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General characterization
UV-vis absorption spectrum was performed on the HP 8453
ultraviolet and visible spectrophotometry. N2 physical adsorp-
tion was carried out on a Micrometrics ASAP2020 volumetric
adsorption analyzer (before the measurements, samples were
degassed at 433 k for 6 h). The Brunauer–Emmett–Teller
(BET) surface area was evaluated from data in the relative
pressure range from 0.05 to 0.25. The total pore volume of
each sample was estimated from the amount adsorbed at the
highest P/P0 (above 0.99). Pore diameters were determined from
the adsorption branch using Barrett–Joyner–Halenda (BJH)
method. X-Ray diffraction (XRD) patterns were measured on
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MHz, CDCl3):d 8.31 (d, J = 8.8 Hz, 2H), 7.68 (d, J = 8.8 Hz, 2H),
7.47–7.28 (m, 3H); 13C NMR (100 MHz, CDCl3):d 116.6, 118.3,
123.7, 124.4, 124.5, 124.5, 127.9, 127.9, 136.0, 145.5, 147.5, 149.8,
152.2; IR (KBr): n 3075, 1512, 1345, 852, 752 cm-1; HRMS (EI)
calcd for C12H7F2NO2: 235.0445 [M]+; found: 235.0451.
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type C–C bond forming reaction, the Stille reaction was first
examined to evaluate the catalytic activity and stability of the Ph-
SBA-15-PPh3-Pd catalyst. After the reaction mixture was treated for
24 h under standard conditions, we found the desired Suzuki reaction
did not occur at all, and the starting materials were recovered. It was
considered that the organotin reagents such as allyl tributylstannane
and phenyl tributylstannane could not enter the cavities of the
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1766 | Green Chem., 2010, 12, 1758–1766
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The Royal Society of Chemistry 2010
©