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Aphosphine-free heterogeneous SuzukiЦMiyaura reaction of aryl bromides catalyzed by MCM-41-supported tridentate nitrogen palladium complex under air.

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Full Paper
Received: 15 July 2011
Revised: 14 September 2011
Accepted: 3 October 2011
Published online in Wiley Online Library: 29 November 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1854
A phosphine-free heterogeneous Suzuki–
Miyaura reaction of aryl bromides catalyzed by
MCM-41-supported tridentate nitrogen
palladium complex under air
Hong Zhaoa*, Guodong Dingb, Li Xua and Mingzhong Caib
MCM-41-supported tridentate nitrogen palladium(II) complex [MCM-41-3 N-Pd(II)] was conveniently synthesized from
commercially available and cheap 3-(2-aminoethylamino)propyltrimethoxysilane via immobilization on MCM-41, followed by
reacting with pyridine-2-carboxaldehyde and PdCl2. It was found that this palladium complex is an excellent catalyst for the
Suzuki–Miyaura coupling reaction of aryl bromides on two points: (i) the use of 5 10 4 mol equiv. of MCM-41-3 N-Pd(II) under
air afforded the coupling products efficiently after easy workup; (2) the catalyst can be reused many times without loss of catalytic
activity. Copyright © 2011 John Wiley & Sons, Ltd.
Keywords: supported catalyst; C–C coupling reaction; functionalized MCM-41; tridentate nitrogen palladium complex; heterogeneous
catalysis
Introduction
Appl. Organometal. Chem. 2011, 25, 871–875
Results and Discussion
Although phosphine ligands stabilize palladium and influence its
reactivity, the simplest and cheapest palladium catalysts are of
* Correspondence to: Hong Zhao, School of Chemistry and Chemical Engineering,
Guangdong Pharmaceutical University, Guangzhou 510006, People’s Republic of
China. E-mail: zhaohong1001@sina.com
a
School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical
University, Guangzhou 510006, People’s Republic of China
b
Department of Chemistry, Jiangxi Normal University, Nanchang 330022,
People’s Republic of China
Copyright © 2011 John Wiley & Sons, Ltd.
871
Suzuki coupling is a particularly important reaction in organic
chemistry, since it is the most powerful tool for constructing a biaryl
structure, which is found in many biologically active compounds,
liquid crystals and EL materials.[1] However, when homogeneous
palladium catalysts such as Pd(PPh3)4, Pd(PPh3)2Cl2 are used in
the synthesis of fine chemical products, we must address the problem of residual metal in the product.[2] Furthermore, Pd, which is
becoming increasingly expensive, is wasted and cannot be reused.
In contrast, heterogeneous catalysts can be easily separated from
the reaction mixture by simple filtration and reused in successive
reactions, provided that the active sites have not become deactivated. Heterogeneous catalysis also help to minimize wastes
derived from reaction workup, contributing to the development
of green chemical processes.[3] From the standpoint of environmentally benign organic synthesis, development of immobilized
palladium catalysts is challenging and important.[4] Recently,
palladium immobilized on cross-linked polystyrene[5] or silica gels[6]
have been used for the Suzuki reaction. However, these catalysts
have generally suffered from limited mass transfer and lower activity than the homogeneous ones, in addition to leaching of the
catalytic species from the surface of the support,[7] and some of
these studies have been related to polymer-supported phosphine
palladium catalysts.[5,8] It is known that the catalysts containing
phosphine ligands at higher temperatures are unstable.[9] Furthermore, the procedure for preparing the polymer-supported
phosphine palladium complexes is rather complicated since the
synthesis of the phosphine ligands requires multi-step sequences.
The development of phosphine-free heterogeneous palladium
catalysts having high activity and good stability is a topic of enormous importance. Our approach was guided by three imperatives:
(i) the polymeric ligand should be easily accessible, (ii) starting from
readily available and cheap reagents, and (iii) the polymeric
palladium catalyst should be air stable, which should allow its storage in normal bottles with unlimited shelf life. Developments on
the mesoporous material MCM-41 have provided a possible new
candidate for a solid support for immobilization of homogeneous
catalysts.[10] MCM-41 has a regular pore diameter of ~5 nm and a
specific surface area > 700 m2 g 1.[11] Its large pore size allows
passage of large molecules such as organic reactants and metal
complexes through the pores to reach to the surface of the channel.[12] Considering the fact that MCM-41 support has an extremely
high surface area and the catalytic palladium species is anchored
on the inner surface of the mesopore of the MCM-41, we expect that
MCM-41-supported palladium catalyst will exhibit high catalytic
activity and excellent reusability. To date, a few palladium complexes
on functionalized MCM-41 have been prepared and used in organic
reactions.[13] In continuing our efforts to develop greener synthetic
pathways for organic transformations, our new approach, described
in this paper, was to design and synthesize a new MCM-41supported tridentate nitrogen palladium(II) complex, which was used
as an effective palladium catalyst for the Suzuki reaction under air.
H. Zhao et al.
Scheme 1. Preparation of MCM-41-3 N-Pd(II) complex.
course phosphine-free systems, specifically when used in low
loading. A novel MCM-41-supported tridentate nitrogen palladium(II) complex [MCM-41-3 N-Pd(II)] was conveniently synthesized
from commercially available and cheap 3-(2-aminoethylamino)
propyltrimethoxysilane via immobilization on MCM-41, followed
by reacting with pyridine-2-carboxaldehyde and PdCl2 (Scheme 1).
Analysis of MCM-41-3 N-Pd(II) by X-ray diffraction indicated that, in
addition to an intense diffraction peak (100), two higher-order
peaks with lower intensities were also detected, and therefore the
chemical bonding procedure did not diminish the structural ordering of the MCM-41. Elemental analyses and X-ray photoelectron
spectroscopy (XPS) were used to characterize the MCM-41supported tridentate nitrogen palladium(II) complex. The N:Pd
molar ratio of MCM-41-3 N-Pd(II) was determined to be 5.97. The
XPS data for MCM-41-3 N-Pd(II), MCM-41-3 N, and metal Pd are
listed in Table 1. It can be seen that the binding energies of Si2p
and O1s of MCM-41-3 N-Pd(II) are similar to those of MCM-41-3 N.
However, the difference of N1s binding energies between MCM41-3 N-Pd(II) and MCM-41-3 N is 0.9 eV. The binding energy of
Pd3d5/2 in MCM-41-3 N-Pd(II) is 0.6 eV less than that in PdCl2, but
2.3 eV larger than that in metal Pd. These results show that a coordination bond between N and Pd is formed in MCM-41-3 N-Pd(II).
First, we carried out the Suzuki–Miyaura coupling of phenylboronic
acid (1.5 equiv.) with 4-bromoanisole using the MCM-41-3 N-Pd(II) as
the catalyst under various conditions to identify the better reaction
conditions. The results are summarized in Table 2. For the bases
Table 1. XPS data for MCM-41-3 N-Pd(II), MCM-41-3 N and metal Pda
Sample
MCM-41-3 N-Pd(II)
MCM-41-3 N
PdCl2
Metal Pd
a
Pd3d5/2
N1s
Si2p
O1s
Cl2p
337.5
400.6
399.7
103.3
103.2
533.3
533.4
199.2
338.1
335.2
199.3
The binding energies are referenced to C1s (284.6 eV) and the energy differences were determined with an accuracy of 0.2 eV.
Table 2. Coupling reaction of 4-bromoanisole with phenylboronic acid in the presence of several bases and solventsa
Entry
1
2
3
4
5
6
7
8
9
10
11
12
13c
14c
Base
K3PO4
K3PO4
K3PO4
Na2CO3
Na2CO3
Na2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Solvent
Xylene
DMF
Dioxane
Xylene
DMF
Dioxane
Xylene
DMF
Dioxane
Xylene
Xylene
Xylene
Xylene
Xylene
MCM-41-3 N-Pd(II) (mol%)
Time (h)
Yieldb (%)
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.1
0.05
0.5
0.05
0.005
2
2
2
1
1.5
1.5
1
1.5
1
2.5
6
0.5
6
48
81
76
74
91
82
89
96
85
93
94
95
96
95
52
All reactions were performed using 1.0 mmol 4-bromoanisole, 1.5 mmol phenylboronic acid, 2.0 mmol base in 3.0 ml solvent at 60 C under Ar.
Isolated yield based on the 4-bromoanisole used.
c
Under air.
a
b
872
wileyonlinelibrary.com/journal/aoc
Copyright © 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 871–875
A phosphine-free heterogeneous Suzuki–Miyaura reaction
evaluated (K3PO4, Na2CO3, and K2CO3), K2CO3 was found to be the
most effective. Among the solvents used (DMF, dioxane, and xylene),
xylene was the best choice. Increasing the amount of palladium
catalyst could shorten the reaction time, but did not increase the
yield of 4-methoxybiphenyl (entry 12). The low palladium concentration usually led to a long period of reaction, which was consistent
with our experimental result (entry 10). The coupling reaction also
proceeded smoothly even with 0.05 mol% of palladium catalyst, to
afford 4-methoxybiphenyl in excellent yield after 6 h (entry 11). It
was found that the coupling reaction under air can give the same
result as under Ar (entry 13). However, when 0.005 mol% of the
catalyst was used, the reaction was too slow and 4-methoxybiphenyl
was obtained in only 52% yield after 48 h (entry 14). Taken together,
an excellent result was obtained when the reaction was carried out
with 0.05 mol% of the catalyst using K2CO3 as base in xylene at
60 C under air (entry 13).
To examine the scope for this heterogeneous Suzuki reaction, we have investigated the reactions using a variety of
Scheme 2. Suzuki coupling of aryl bromides with arylboronic acids.
arylboronic acids and a wide range of aryl bromides as substrates under optimized reaction conditions (Scheme 2) and
the results are listed in Table 3. As shown in the table, the
Suzuki reactions of aryl bromides with phenylboronic acid
proceeded very smoothly at 60 C to afford the corresponding
coupled products in excellent yield (entries 1–3). The reactions of sterically hindered 2-methylbromobenzene and bulky
1-bromonaphthalene with phenylboronic acid provided high
yields of the desired biaryls 3d and 3e, respectively (entries
4 and 5). The coupling reactions of heteroaryl bromides such as
2-bromothiophene and 2-bromopyridine with phenylboronic acid
also gave the corresponding coupled products 3f and 3g in 90%
and 88% yields, respectively (entries 6 and 7).
The optimized reaction conditions were also applied to the
coupling reactions of substituted phenylboronic acids with a
variety of aryl bromides, and the results are also summarized in
Table 3. Various electron-donating and electron-withdrawing
groups such as –CH3, –OCH3, –Ph, –Cl, –CN, –NO2, –CF3, –COCH3,
and –CO2CH3 on both aryl bromides and arylboronic acids were
well tolerated, to give the desired unsymmetrical biaryls in
good to excellent yield (entries 8–19). The reactions of bulky
1-naphthylboronic acid with aryl bromides also gave the desired
coupled products 3e, 3t and 3u in good yield (entries 20–22).
A favorable effect of electron-withdrawing substituents is
normally observed in palladium-catalyzed reactions.[14] With
Table 3. Heterogeneous Suzuki reaction of aryl bromides with arylboronic acids catalyzed by MCM-41-3 N-Pd(II)a
Entry
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26c
Ar
4-CH3COC6H4
4-CH3OC6H4
4-O2NC6H4
2-CH3C6H4
1-Naphthyl
2-Thienyl
2-Pyridyl
4-CH3OC6H4
4-CH3COC6H4
4-CH3OCOC6H4
2-Thienyl
4-O2NC6H4
4-CH3C6H4
4-ClC6H4
4-CH3C6H4
4-ClC6H4
4-PhC6H4
2-Thienyl
2-Pyridyl
Ph
4-CH3C6H4
4-ClC6H4
3-CH3C6H4
Ph
4-ClC6H4
2-CH3C6H4
Ar1
Ph
Ph
Ph
Ph
Ph
Ph
Ph
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-CH3OC6H4
4-CH3OC6H4
4-NCC6H4
4-NCC6H4
4-CH3C6H4
4-CH3C6H4
4-CH3C6H4
4-CH3C6H4
1-Naphthyl
1-Naphthyl
1-Naphthyl
2-CH3C6H4
2-CF3C6H4
2-CH3C6H4
2-CH3C6H4
Time (h)
Product
Yield(%)b
5
6
4
12
8
6
6
6
5
5
7
4
6
5
6
6
6
8
7
10
12
10
24
24
20
24
3a[17]
3b[17]
3c[17]
3d[17]
3e[18]
3f[19]
3g[19]
3h[20]
3i[21]
3j[22]
3k[19]
3l[20]
3m[20]
3n[23]
3o[20]
3p[20]
3q[24]
3r[19]
3s[19]
3e
3t[18]
3u[18]
3v[17]
3w[17]
3x[17]
3y[21]
93
95
94
86
88
90
88
93
94
96
90
94
90
88
91
95
93
91
89
83
81
85
83
80
87
Trace
a
Appl. Organometal. Chem. 2011, 25, 871–875
Copyright © 2011 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc
873
Reactions were carried out with 1 mmol aryl bromide, 1.5 mmol arylboronic acid, 0.0005 mmol palladium catalyst, and 2 mmol K2CO3 in 3.0 ml
xylene at 60 C under air.
b
Yield of isolated product 3 based on the aryl bromide.
c
At 120 C.
H. Zhao et al.
our catalyst, however, electron-withdrawing groups in aryl
bromides have relatively little effect on the Suzuki coupling
reaction. Substituted chlorobenzenes were inert under the
same conditions, giving traces of cross-coupling products. To
further illustrate that chloroarenes were inert in the reaction system, 4-chloro-4′-methylbiphenyl was selectively produced in
95% yield in the coupling reaction of (4-methylphenyl)boronic
acid with 1-bromo-4-chlorobenzene (entry 16). The coupling
reactions of sterically hindered arylboronic acids with aryl
bromides could also proceed smoothly, affording the desired
coupled products 3v–3x in good yield after 20–24 h (entries
23–25). However, the reaction of 2-bromotoluene with
2-methylphenylboronic acid was very slow even at 120 C
and only a trace of desired coupled product 3y was formed after
24 h (entry 26). The present method provides a quite general
route for the synthesis of unsymmetrical biaryls having various
functionalities.
In order to determine whether the catalysis was due to the
MCM-41-3 N-Pd(II) complex or to a homogeneous palladium
complex that comes off the support during the reaction and then
returns to the support at the end, we performed the hot filtration
test.[15] We focused on the coupling reaction of 4-bromoanisole
with phenylboronic acid. We filtered off the MCM-41-3 N-Pd(II)
complex after 1.5 h of reaction time and allowed the filtrate to
react further. The catalyst filtration was performed at the reaction
temperature (60 C) in order to avoid possible recoordination or
precipitation of soluble palladium upon cooling. We found that,
after this hot filtration, no further reaction was observed and no
palladium could be detected in the hot filtered solution by
atomic absorption spectroscopy (AAS). This result suggests that
the palladium catalyst remains on the support at elevated
temperatures during the reaction.
This heterogeneous palladium catalyst can be easily recovered
by simple filtration. We also investigated the possibility of reusing
the catalyst by using the coupling reaction of 4-bromoanisole
with phenylboronic acid under air. In general, the continuous
recycle of resin-supported palladium catalysts is difficult owing
to leaching of the palladium species from the polymer supports,
which often reduces their activity within a five-recycle run. However, when the reaction of 4-bromoanisole with phenylboronic
acid was performed even with 0.05 mol% of MCM-41-3 N-Pd(II)
under air, the catalyst could be recycled 10 times without any loss
of activity. The reaction promoted by the 10th recycled catalyst
gave 3b in 93% yield (Table 4, entry 2). The average yield of 3b
in consecutive reactions promoted by the first through the tenth
recycled catalyst was 94% (entry 3). The high stability and
excellent reusability of the catalyst should result from the
chelating action of tridentate nitrogen ligand on palladium and
the mesoporous structure of the MCM-41 support.
Conclusion
We have developed a novel, phosphine-free, practical and
economic catalyst system for the Suzuki–Miyaura coupling reaction
of aryl bromides with arylboronic acids by using MCM-41supported tridentate nitrogen palladium(II) complex as catalyst.
The advantages of our heterogeneous catalytic system over others
are as follow: (i) the preparation of the MCM-41-3 N-Pd(II) is simple
and convenient from commercially available reagents; (ii) the
reaction conditions are very mild, i.e. only 0.05 mol% palladium
catalyst, air atmosphere and lower temperature (60 C); (iii) excellent performance and reusability of the catalyst.
Experimental
All chemicals were reagent grade and used as purchased. The
mesoporous material MCM-41 was prepared according to literature procedure.[16] All coupling products were characterized by
comparison of their spectra and physical data with authentic
samples. IR spectra were determined on a PerkinElmer 683 instrument. 1H NMR spectra were recorded on a Bruker AC-P400
(400 MHz) spectrometer with tetramethylsilane as an internal
standard in CDCl3 as solvent. 13 C NMR spectra were recorded
on a Bruker AC-P400 (100 MHz) spectrometer in CDCl3 as solvent.
Palladium content was determined by inductively coupled
plasma atom emission spectroscopy (Atomscan 16, TJA Corporation). Melting points are uncorrected. X-ray powder diffraction
patterns were obtained on Damx-rA (Rigaka). X-ray photoelectron
spectra were recorded on an XSAM 800 spectroscope (Kratos).
Preparation of MCM-41-3 N
A solution of 1.54 g 3-(2-aminoethylamino)propyltrimethoxysilane
in 18 ml dry chloroform was added to a suspension of 2.2 g of the
MCM-41 in 180 ml dry toluene. The mixture was stirred for 24 h at
100 C. The solid was then filtered and washed with CHCl3
(2 20 ml), and dried under vacuum at 160 C for 5 h. The
dried white solid (1.725 g) was then reacted with pyridine-2carboxaldehyde (0.251 g, 2.34 mmol) in 10 ml dry ethanol at 30 C
for 48 h. The solid product was filtered, washed with ethanol
(3 20 ml) and diethyl ether (20 ml), and dried under vacuum at
Table 4. Suzuki reaction of 4-bromoanisole with phenylboronic acid catalyzed by recycled catalyst under air
Br
MeO
10 mmol
874
Entry
1
2
3
+
PhB(OH)2
K2CO3, xylene, air, 60 oC, 6 h
MeO
Ph
3b
15 mmol
Catalyst cycle
1st
10th
1st to 10th consecutive
wileyonlinelibrary.com/journal/aoc
0.005 mmol MCM-41-3N-Pd(II)
(1st-10th use)
Isolated yield (%)
95
93
av. 94
Copyright © 2011 John Wiley & Sons, Ltd.
TON
1900
1860
Total of 18 800
Appl. Organometal. Chem. 2011, 25, 871–875
A phosphine-free heterogeneous Suzuki–Miyaura reaction
120 C for 5 h to obtain 1.874 g of hybrid material MCM-41-3 N. The
nitrogen content was found to be 2.65 mmol g 1 by elemental
analysis.
Preparation of MCM-41-3 N-Pd(II)
[4]
[5]
In a small Schlenk tube, 1.756 g of the above-functionalized
MCM-41 (MCM-41-3 N) was mixed with 0.143 g (0.81 mmol) PdCl2
in 40 ml dry acetone. The mixture was refluxed for 72 h under
an argon atmosphere. The product was allowed to cool, then
filtered. The yellow solid was washed with distilled water
(3 20 ml) and acetone (2 20 ml) and dried under vacuum at
60 C for 5 h to give 1.802 g of a yellow palladium complex
[MCM-41-3 N-Pd(II)]. The nitrogen and palladium contents were
found to be 2.27 mmol g 1 and 0.38 mmol g 1, respectively.
[6]
[7]
[8]
General procedure for Suzuki reaction of aryl bromides with
arylboronic acids
A mixture of aryl bromide (1.0 mmol), arylboronic acid
(1.5 mmol), potassium carbonate (2.0 mmol), xylene (3 ml) and
the MCM-41-3 N-Pd(II) complex (1.4 mg, 0.0005 mmol of Pd) was
stirred at 60 C under air for 4–24 h. The mixture was cooled and
filtered. The MCM-41-3 N-Pd(II) complex was washed with distilled
water (2 10 ml), dioxane (2 10 ml) and Et2O (2 10 ml), and
reused in the next run. The filtrate was poured into a saturated
aqueous NaCl solution (50 ml) and extracted with methylene
chloride (2 50 ml). The extracts was washed with water
(3 30 ml) and dried over MgSO4. After removal of the solvent,
the residue was purified by column chromatography on silica gel.
Compounds 3a–3x are known compounds and were characterized by comparing their 1H, 13 C NMR, and IR spectra with
those found in the literature.
Acknowledgments
We gratefully acknowledge the financial support of this work by
the National Natural Science Foundation of China (Project No.
20862008).
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
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air, suzukiцmiyaura, complex, reaction, mcm, supported, aryl, bromide, catalyzed, tridentate, free, heterogeneous, palladium, nitrogen, aphosphine
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