A phosphine-free carbonylative cross-coupling reaction of aryl iodides with arylboronic acids catalyzed by immobilization of palladium in MCM-41

Article abstract of DOI:10.1039/c0gc00138d

The phosphine-free heterogeneous carbonylative cross-coupling of aryl iodides with arylboronic acids under an atmospheric pressure of carbon monoxide was achieved in anisole at 80 °C in the presence of a 3-(2-aminoethylamino) propyl-functionalized MCM-41-immobilized palladium(ii) complex [MCM-41-2N-Pd(ii)], yielding a variety of unsymmetrical biaryl ketones in good to high yield. This heterogeneous palladium catalyst exhibited higher activity and selectivity than PdCl₂(PPh₃)₂, can be recovered and recycled by a simple filtration of the reaction solution, and used for at least 10 consecutive trials without any decrease in activity. Our system not only avoids the use of phosphine ligands, but also solves the basic problem of palladium catalyst recovery and reuse.

Full text of DOI:10.1039/c0gc00138d

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PAPER
A phosphine-free carbonylative cross-coupling reaction of aryl iodides with
arylboronic acids catalyzed by immobilization of palladium in MCM-41†
Mingzhong Cai,* Jian Peng, Wenyan Hao and Guodong Ding
Received 22nd May 2010, Accepted 1st November 2010
DOI: 10.1039/c0gc00138d
The phosphine-free heterogeneous carbonylative cross-coupling of aryl iodides with arylboronic
acids under an atmospheric pressure of carbon monoxide was achieved in anisole at 80 ◦C in the
presence of a 3-(2-aminoethylamino)propyl-functionalized MCM-41-immobilized palladium(II)
complex [MCM-41–2N–Pd(II)], yielding a variety of unsymmetrical biaryl ketones in good to high
yield. This heterogeneous palladium catalyst exhibited higher activity and selectivity than
PdCl2(PPh3)2, can be recovered and recycled by a simple filtration of the reaction solution, and
used for at least 10 consecutive trials without any decrease in activity. Our system not only avoids
the use of phosphine ligands, but also solves the basic problem of palladium catalyst recovery and
reuse.
the direct synthesis of biaryl ketones from carbon monoxide,
Introduction
aryl halides and arylboronic acids since a wide variety of
Biaryl ketones are important moieties in many biologically
functionalities can be tolerated on either partner and the
active molecules, natural products and pharmaceuticals,
arylboronic acids are generally nontoxic and thermally-, air-
and many methods for the preparation of them have been
and moisture-stable.10 Several groups have reported a variety
reported.1 One general approach for the synthesis of biaryl
of palladium-based homogeneous catalytic systems such as
ketones is the Friedel–Crafts acylation of substituted aromatic
PdCl2(PPh3)2,11 Pd(PPh3)4,12 Pd(OAc)2-imidazolium salts,13
rings.2 The crucial disadvantage of traditional Friedel–Crafts
acylation is the use of more than a stoichiometric amount
of aluminium trichloride, which is incompatible with many
Pd(OAc)2/N,N-bis(2,6-diisopropylphenyl)dihydroimidazolium
chloride,14 Pd(tmhd)2/Pd(OAc)2 (tmhd = 2,2,6,6-tetramethyl-
3,5-heptanedionate)15 and N-heterocyclic carbene palladium
functional groups and generates a large amount of waste.
complexes16 for this reaction. However, the problem with
Furthermore, the formation of ortho and para isomers with
the untunable regioselectivity results in separation problems
and makes biaryl ketones with a meta substituent difficult
to access. The transition metal-catalyzed three-component
homogeneous catalysis is the difficulty of separating the catalyst
from the reaction mixture and the impossibility of reusing it in
consecutive reactions. In contrast, heterogeneous catalysts can
be easily separated from the reaction mixture by simple filtration
cross-coupling reaction between arylmetal reagents, carbon
and reused in successive reactions, provided that the active sites
monoxide and aryl electrophiles has provided a straightforward
have not become deactivated. The high costs of the transition
and convenient route for the synthesis of unsymmetrical biaryl
metal catalysts coupled with the toxic effects associated with
ketones.3 Various arylmetal reagents including magnesium,4
many transition metals has led to an increased interest in
aluminium,5 silicon,6 tin,7 zinc8 and indium9 compounds
immobilizing catalysts onto a support. Heterogeneous catalysis
have been reported to undergo carbonylative cross-coupling
also helps to minimize wastes derived from reaction workup,
reactions. However, the known protocol is limited due to the
contributing to the development of green chemical processes.17
formation of side products ensuing from direct coupling of
So far, supported palladium catalysts have successfully been
the aryl groups without carbon monoxide insertion. Among
used for the Heck reaction,18 the Suzuki–Miyaura reaction,19
carbonylative cross-coupling reactions, the Suzuki carbonylative
coupling reaction is one of the most promising methods for
the Sonogashira reaction20 and the Stille reaction,21 etc. In
spite of the significant advances in this area, there are very few
reports that employ heterogeneous palladium complexes as
catalysts for carbonylative cross-coupling reactions.
Department of Chemistry, Jiangxi Normal University, Nanchang,
Developments on the mesoporous material MCM-41 pro-
330022, P. R. China. E-mail: caimzhong@163.com; Fax: +86
vided a new possible candidate for a solid support for the
791-812-0388
immobilization of homogeneous catalysts.22 MCM-41 has a
† Electronic supplementary information (ESI) available: Further exper-
regular pore diameter of ca. 5 nm and a specific surface
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imental data. See DOI: 10.1039/c0gc00138d
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Scheme 1
area > 700 m2 g-1.23 Its large pore size allows the passage of
large molecules such as organic reactants and metal complexes
through the pores to reach to the surface of the channels.24 It is
generally believed that the high surface area of heterogeneous
catalysts results in their high catalytic activity. Considering
the fact that the 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 support,
we expect that an MCM-41-supported palladium catalyst will
exhibit a high activity and good reusability. To date, a few
palladium complexes on functionalized MCM-41 supports have
been prepared and successfully used in organic reactions.25 In
our previous work, we reported the synthesis of an MCM-41-
supported bidentate phosphine palladium complex and found
that this palladium complex is an efficient and recyclable catalyst
for the carbonylative cross-coupling reaction of aryl halides with
sodium tetraphenylborate26 and the carbonylative Stille cross-
coupling reaction.27 However, the procedure for preparing the
MCM-41-supported bidentate phosphine palladium complex
was rather complicated, since the synthesis of the bidentate
phosphine ligand required a multi-step sequence. Therefore,
the development of phosphine-free heterogeneous palladium
catalysts having a high activity and a good stability is a topic
of enormous importance. 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 3-(2-aminoethylamino)propyl-functionalized MCM-41-
immobilized palladium complex, which was used as an effective,
phosphine-free palladium catalyst for the carbonylative cross-
coupling reaction of aryl halides with arylboronic acids under
an atmospheric pressure of carbon monoxide.
Table 1 XPS data for MCM-41–2N–Pd(II), MCM-41–2N, PdCl2 and
metal Pda
Sample
Pd3d5/2
337.4
N1s
Si2p
O1s
Cl2p
MCM-41–2N–Pd(II)
MCM-41–2N
PdCl2
400.5
399.6
103.4
103.2
533.1
533.1
199.1
199.2
338.1
335.2
Metal Pd
a The binding energies are referenced to C1s (284.6 eV) and the energy
differences were determined with an accuracy of 0.2 eV.
The X-ray powder diffraction (XRD) analysis of the MCM-41–
2N–Pd(II) indicated that, in addition to an intense diffraction
peak (100), two higher order peaks, (110) and (200), 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 3-(2-aminoethyl-
amino)propyl-functionalized MCM-41-immobilized palladium
complex. The N : Pd mole ratio of the MCM-41–2N–Pd(II) was
determined to be 4.94. The XPS data for MCM-41–2N–Pd(II),
MCM-41–2N, PdCl2 and metal Pd are listed in Table 1. It can
be seen that the binding energies of Si2p and O1s of MCM-41–
2N–Pd(II) are similar to those of MCM-41–2N, and the binding
energy of Cl2p of MCM-41–2N–Pd(II) is similar to that of PdCl2.
However, the difference of the N1s binding energies between
MCM-41–2N–Pd(II) and MCM-41–2N is 0.9 eV. The binding
energy of Pd3d5/2 in MCM-41–2N–Pd(II) is 0.7 eV less than that
in PdCl2, but 2.2 eV larger than that in metallic Pd. These results
show that a coordination bond between N and Pd is formed.
The carbonylative cross-coupling reaction of phenylboronic
acid (1.1 equiv.) with 4-iodoanisole under an atmospheric
pressure of carbon monoxide was chosen as a model reaction,
and the influences of various reaction parameters, such as
reaction temperature, solvent, palladium catalyst quantity, Pd
loading and base, on the reaction were tested. The results are
summarized in Table 2. For the temperatures evaluated [40, 60,
80 and 100 ◦C], 80 ◦C was found to be the most effective. Other
temperatures, such as 60 and 100 ◦C, were substantially less
effective, and no carbonylative cross-coupling reaction occurred
at 40 ◦C (Table 2, entry 1). We then turned our attention to
Results and discussion
Although phosphine ligands stabilize palladium and influ-
ence its reactivity, the simplest and cheapest palladium cata-
lysts are of course phosphine-free systems, specifically when
used in low loadings. A novel 3-(2-aminoethylamino)propyl-
functionalized MCM-41-immobilized palladium complex
[MCM-41–2N–Pd(II)] was very conveniently synthesized
from commercially available and cheap 3-(2-aminoethyl-
amino)propyltrimethoxysilane via immobilization on MCM-41,
followed by reacting it with palladium chloride (Scheme 1).
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Table 2 The carbonylative cross-coupling of phenylboronic acid with 4-iodoanisole under different conditionsa
Yield (%)b
Entry
Solvent
Base
Catalyst amount (mol%)
Temp./◦C
Time/h
A
B
1
2
3
4
5
6
7
8
Anisole
Anisole
Anisole
Anisole
Dioxane
o-Xylene
Toluene
DMF
Anisole
Anisole
Anisole
Anisole
Anisole
Anisole
Anisole
Anisole
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
K3PO4
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
2
2
2
2
2
2
2
2
2
2
2
4
1
0.5
2
2
40
60
80
100
80
80
80
80
80
80
80
80
80
80
80
80
24
24
8
0
0
21
4
6
6
8
7
17
6
23
18
4
36
91
88
85
82
78
25
86
62
54
89
87
86
89
90
6
10
13
16
24
10
12
18
5
9
10
11
12
13
14
15c
16d
20
40
8
5
6
5
4
7
a Reaction conditions: phenylboronic acid (1.1 mmol), 4-iodoanisole (1.0 mmol), CO (1 atm), base (3 mmol) and solvent (5 mL). b Isolated yield.
c Catalyst with Pd loading of 0.54 mmol Pd g-1. d Catalyst with Pd loading of 0.24 mmol Pd g-1.
Table 3 The synthesis of unsymmetrical biaryl ketonesa
investigate the effect of solvents on the carbonylative cross-
coupling reaction. When the reaction was conducted in less polar
Entry Ar
Ar1
Time/h Product Yield (%)b
solvents, such as anisole, dioxane, o-xylene or toluene, good
to high yields of carbonylative cross-coupling product A were
isolated, and anisole was found to be the best choice. However,
the carbonylative cross-coupling product was obtained in low
yield in polar solvents such as DMF (Table 2, entry 8). Bases
affected the selectivity of the reaction. Cs2CO3 and K3PO4, which
have been utilized in Suzuki–Miyaura coupling reactions of
organoboron compounds,10 had a strong tendency to produce a
direct coupling product, 4-methoxybiphenyl (18–23%) (Table 2,
entries 10 and 11). K2CO3 (3 equiv.) suspended in anisole is the
most efficient to yield 4-methoxybenzophenone in 91% yield,
which, however, was still accompanied by 4-methoxybiphenyl
(4%) (Table 2, entry 3). Increasing the amount of palladium
catalyst could shorten the reaction time, but did not increase
the yield of 4-methoxybenzophenone (Table 2, entry 12). The
low palladium concentration usually led to a long period of
reaction, which was consistent with our experimental results
(Table 2, entries 13 and 14). MCM-41–2N–Pd(II) complexes
with different Pd loadings were also tried as the catalyst in order
to assess the Pd loading effect on activity. However, similar
results were obtained (Table 2, entries 3, 15 and 16). Thus,
the optimized reaction conditions for this carbonylative cross-
coupling reaction are MCM-41–2N–Pd(II) (2 mol%) in anisole
using K2CO3 as the base at 80 ◦C under an atmospheric pressure
of carbon monoxide for 8 h (Table 2, entry 3).
1
2
3
4
5
6
7
8
4-CH3OC6H4
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
8
6
5
6
15
9
12
24
8
9
6
8
7
5
9
6
7
6
5
3a
3b
3c
3d
3e
3f
3g
3h
3i
91
86
82
85
74
90
89
75
80
82
84
83
84
83
89
86
83
85
79
90
87
85
64
81
86
4-CH3COC6H4
4-O2NC6H4
3-O2NC6H4
2-CH3OC6H4
4-H2NC6H4
2-H2NC6H4
1-Naphthyl
2-Thienyl
9
Ph
Ph
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
3-Pyridinyl
4-ClC6H4
2-Thienyl
3j
3k
3l
4-CH3C6H4
4-CH3C6H4
3-CH3OCOC6H4 4-CH3C6H4
3m
3n
3o
3p
3q
3r
3s
3t
3u
3v
3w
3x
3y
4-O2NC6H4
4-CH3OC6H4
4-CH3COC6H4
4-BrC6H4
4-CH3C6H4
4-CH3C6H4
4-CH3C6H4
4-CH3C6H4
4-CH3OCOC6H4 4-CH3C6H4
4-O2NC6H4
4-CH3OC6H4
4-ClC6H4
4-BrC6H4
2-CF3C6H4
2-Thienyl
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
8
6
7
24
8
3-NCC6H4
7
a Reactions were carried out with arylboronic acid (1.1 mmol), aryl
iodide (1.0 mmol), CO (1 atm), K2CO3 (3 mmol) and 2 mol% palladium
catalyst in anisole (5 mL) at 80 ◦C. b Isolated yield.
To examine the scope of this heterogeneous carbonylative
cross-coupling reaction, we have investigated the reactions using
a variety of arylboronic acids and a wide range of aryl iodides
as substrates under the optimized reaction conditions (Scheme
2). The results are outlined in Table 3. As shown in Table 3,
the carbonylative cross-coupling reaction of phenylboronic acid
with a variety of aryl iodides proceeded smoothly under mild
conditions, affording the corresponding carbonylative coupling
products 3a–j in good to high yields. Various electron-donating
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unsymmetrical biaryl ketones in good to high yields (Table 3,
entries 11–25). The method provides a quite general route for
the synthesis of unsymmetrical biaryl ketones having various
functionalities. The results above prompted us to investigate the
reaction of aryl bromides, but aryl bromides were not reactive
under the conditions optimized for the iodides, and thus 4-
bromoiodobenzene could be converted into a bromophenyl
ketone selectively (Table 3, entries 17 and 22). Even though
NaI or KI (3 equiv.) were used as the additive as reported
by Miyaura,11a the carbonylative cross-coupling reaction of
aryl bromides with arylboronic acids in anisole at 80 or
100 ◦C afforded traces of carbonylative coupling products after
24 h. A comparison with homogeneous analogous catalysts,
such as PdCl2/H2N(CH2)2NH2, was also made in order to
assess the immobilization effect on activity. When 2 mol% of
PdCl2/H2N(CH2)2NH2 (1 : 1) or PdCl2/H2N(CH2)2NH2 (1 : 2)
was used as the catalyst, the carbonylative cross-coupling of
phenylboronic acid with 4-iodoanisole in anisole using K2CO3
as the base at 80 ◦C afforded 3a in 64 and 57% yields,
respectively, and the direct coupling product 4-methoxybiphenyl
was formed in 13 and 17% yields. We then prepared a 3-
(2-aminoethylamino)propyl-functionalized amorphous silica-
supported palladium complex [SiO2–2N–Pd(II)] according to
a literature procedure20d for comparison with MCM-41–2N–
Pd(II); the palladium content was 0.36 mmol g-1. It was found
that the carbonylative cross-coupling of phenylboronic acid with
4-iodoanisole using 2 mol% of SiO2–2N–Pd(II) as the catalyst
and K2CO3 as the base also proceeded smoothly in anisole at
Scheme 2
and electron-withdrawing groups, such as –OCH3, –NH2, –
NO2 and –COCH3, on the aryl iodide were well tolerated.
Electron-donating groups, such as –OCH3 and –NH2, on the
iodoaryl partner gave excellent results. The presence of a strong
electron-withdrawing group, such as –NO2, is known to promote
the direct coupling, producing the biaryl. For example, the
carbonylative cross-coupling reaction of triphenylalane with
4-iodonitrobenzene was reported to provide a 41% yield of 4-
nitrobenzophenone and a 55% yield of 4-nitrobiphenyl,5 and
an analogous reaction with tributyltin hydride resulted in only
a 9% yield of 4-nitrobenzaldehyde with an accompanying 84%
yield of nitrobenzene.28 This heterogeneous palladium catalyst
exhibits a higher activity and selectivity than PdCl2(PPh3)2.
For example, the carbonylative cross-coupling reaction of 4-
iodonitrobenzene with phenylboronic acid in the presence of
2 mol% of MCM-41–2N–Pd(II) in anisole using K2CO3 as the
base at 80 ◦C for 5 h gave an 82% yield of carbonylative coupling
product 3c, along with only 12% yield of 4-nitrobiphenyl (Table
3, entry 3). The same reaction in the presence of 3 mol% of
PdCl2(PPh3)2 in anisole using K2CO3 as the base at 80 ◦C for
5 h gave 3c in 50% yield and 4-nitrobiphenyl in 35% yield.11a
The same reaction in the presence of 2 mol% of a phosphine-
free palladium◦complex [Pd(tmhd)2] in anisole using K2CO3 as
the base at 80 C for 6 h afforded 3c in 72% yield, but the use
of carbon monoxide under pressure (100 psi) was required.15
The reactions of sterically-hindered 2-iodoanisole and bulky 1-
iodonaphthalene with phenylboronic acid also provided good
yields of desired biaryl ketones 3e and 3h under the optimized
reaction conditions, respectively (Table 3, entries 5 and 8).
The carbonylative cross-coupling reactions of heteroaryl iodides
such as 2-iodothiophene and 3-iodopyridine with phenylboronic
acid gave corresponding heteroaryl ketones 3i and 3j in 80
and 82% yields, respectively (Table 3, entries 9 and 10). We
also performed the carbonylative cross-coupling reaction of
phenylboronic acid with 4-iodoanisole under a CO pressure of
100 psi in the presence of 2 mol% of MCM-41–2N–Pd(II) in
anisole using K2CO3 as the base at 80 ◦C; desired product 3a
was obtained in 93% yield after 7 h, which is slightly higher than
that obtained from the reaction under an atmospheric pressure
of CO.
◦
80 C, giving 3a in 83% yield after 10 h. The catalytic activity
of SiO2–2N–Pd(II) was lower than that of MCM-41–2N–Pd(II)
due to the faster diffusion of reactants and products in MCM-41
than in amorphous silica, since the pores are more regular in the
former and MCM-41 has a higher surface area than amorphous
silica.
In order to determine whether the catalysis was due to the
MCM-41–2N–Pd(II) complex or to a homogeneous palladium
complex that is released from the support during the reaction
and then re-attached to the support at the end, we performed
a hot filtration test.29 We focused on the carbonylative coupling
reaction of 3-iodonitrobenzene with phenylboronic acid. We
filtered off the MCM-41–2N–Pd(II) complex after a 1 h reaction
time and allowed the filtrate to react further. Catalyst filtration
was performed at the reaction temperature (80 ◦C) in order
to avoid the 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 to reuse
the catalyst in the carbonylative cross-coupling reaction of 4-
iodoanisole with phenylboronic acid. In general, the continuous
recycling of resin-supported palladium catalysts is difficult
owing to leaching of the palladium species from the polymer
supports, which often reduces their activity within five recycles.30
However, when the reaction of 4-iodoanisole with phenylboronic
acid was performed, even with 2 mol% of MCM-41–2N–Pd(II),
The optimized reaction conditions were also applied to the
carbonylative cross-coupling of substituted phenylboronic acids
such as 4-methylphenylboronic acid and 4-chlorophenylboronic
acid with a variety of aryl iodides. The results are sum-
marized in Table 3. Various electron-donating and electron-
withdrawing groups, such as –CH3, –OCH3, –Br, –Cl, –CN,
–NO2, –CF3, –COCH3 and –CO2CH3, on both aryl iodides
and arylboronic acids were well tolerated to give the desired
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Table 4 The carbonylative cross-coupling reaction of 4-iodoanisole with phenylboronic acid catalyzed by recycled catalyst
Entry
Catalyst cycle
Isolated yield (%)
TON
1
2
3
1st
10th
91
89
av. 90
45.5
44.5
Total of 450
1st to 10th consecutive
the catalyst could be recycled 10 times without any loss of
activity. The reaction promoted by the 10th recycled catalyst
gave 3a in 89% yield (Table 4, entry 2). The average yield of
3a in consecutive reactions promoted by the 1st through to the
10th recycled catalyst was 90% (Table 4, entry 3). The palladium
content of the catalyst was determined by ICP to be 0.32 mmol
g-1 after ten consecutive runs; only 3% of the palladium had been
lost from the MCM-41 support. The high stability and excellent
reusability of the catalyst should result from the chelating action
of the bidentate 2-aminoethylamino ligand on the palladium and
the mesoporous structure of the MCM-41 support. The result is
important from a practical point of view. The recovered catalyst
was analyzed by XPS. It was found that the binding energies of
Si2p, O1s and N1s of the recovered catalyst were similar to those of
fresh catalyst. However, the binding energy of Pd3d5/2 in the used
catalyst was 335.7 eV and the binding energy of Cl2p could not be
detected. These results indicate that the reduction of the starting
palladium(II) complex to the lower valent state [Pd(0)] had taken
place during the carbonylative cross-coupling reaction. The high
catalytic activity, excellent reusability and easy accessibility of
MCM-41–2N–Pd(II) make it a highly attractive heterogeneous
palladium catalyst for the parallel solution phase synthesis of
diverse libraries of compounds.
mesoporous material MCM-41 was easily prepared according
to a literature procedure.31
Synthesis of 3-(2-aminoethylamino)propyl-functionalized
MCM-41-immobilized palladium complex
[MCM-41–2N–Pd(II)]
A
solution of 1.54
g
of 3-(2-aminoethylamino)pro-
pyltrimethoxysilane in 18 mL of dry chloroform was added to
a suspension of 2.2 g of the MCM-41 in 180 mL of dry toluene.
The mixture was stirred for 24 h at 100 ◦C. Then, the solid was
filtered and washed with CHCl3 (2 ¥ 20 mL) and dried in a
vacuum at 160 ◦C for 5 h. The dried white solid was then soaked
in a solution of 3.1 g of Me3SiCl in 100 mL of dry toluene at
room temperature under stirring for 24 h. Then, the solid was
filtered, washed with acetone (3 ¥ 20 mL) and diethyl ether (3 ¥
20 mL), and dried in a vacuum at 120 ◦C for 5 h to obtain
3.49 g of hybrid material MCM-41–2N. The nitrogen content
was found to be 1.84 mmol g-1 by elemental analysis.
In a small Schlenk tube, 3.3 g of the above-functionalized
MCM-41 (MCM-41–2N) was mixed with 0.226 g (1.27 mmol)
of PdCl2 in 50 mL of dry acetone. The mixture was refluxed for
72 h under an argon atmosphere. The solid product was filtered
by suction, washed with acetone, distilled water and acetone
successively, and dried at 70 ◦C/26.7 Pa under Ar for 5 h to give
3.47 g of the yellow palladium complex [MCM-41-2N-Pd(II)].
The nitrogen and palladium contents were found to be 1.63 and
0.33 mmol g-1, respectively. Catalysts with different Pd loadings
could be prepared by varying the feed ratios of PdCl2 to MCM-
41–2N.
Experimental
All chemicals were of reagent grade and used as purchased.
All solvents were dried and distilled before use. The products
were purified by flash chromatography on silica gel. A mix-
ture of EtOAc and hexane was generally used as the eluent.
All carbonylative coupling products were characterized by
comparison of their spectra and physical data with authentic
samples. IR spectra were determined on a Perkin-Elmer 683
Typical procedure for the carbonylative Suzuki coupling reaction
A 50 mL round-bottomed flask equipped with a gas inlet tube,
a reflux condenser and a magnetic stirring bar was charged
with MCM-41–2N–Pd(II) (60 mg, 0.02 mmol Pd), aryl iodide
(1.0 mmol), arylboronic acid (1.1 mmol) and K2CO3 (3 mmol).
The flask was flushed with carbon monoxide, and anisole (5 mL)
was then added. After being stirred at 80 ◦C for 5–24 h under CO
(1 atm), the reaction mixture was cooled to room temperature
and diluted with diethyl ether (50 mL). The mixture was vacuum
filtered using a sintered glass funnel and washed with diethyl
ether (2 ¥ 5 mL). The ethereal solution was washed with water
(3 ¥ 20 mL), dried over anhydrous magnesium sulfate and
concentrated under reduced pressure. The residue was purified
1
instrument. H NMR spectra were recorded on a Bruker AC-
P400 (400 MHz) spectrometer with TMS as an internal standard
in CDCl3 as the solvent. 13C NMR spectra were recorded
on a Bruker AC-P400 (100 MHz) spectrometer in CDCl3 as
the solvent. Melting points are uncorrected. The palladium
content was determined by inductively coupled plasma atom
emission Atomscan16 (ICP-AES, TJA Corporation). X-Ray
powder diffraction was undertaken on a Damx-rA (Rigaka)
instrument. X-Ray photoelectron spectra were recorded on an
XSAM 800 (Kratos) instrument. Microanalyses were measured
by using a Yanaco MT-3 CHN microelemental analyzer. The
194 | Green Chem., 2011, 13, 190–196
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by flash column chromatography on silica gel (hexane–ethyl
acetate = 10 : 1).
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The recyclability of the MCM-41–2N–Pd(II)
After carrying out the reaction, the mixture was vacuum filtered
using a sintered glass funnel, and the residue washed with
distilled water (3 ¥ 5 mL), ethanol (2 ¥ 5 mL) and diethyl ether
(2 ¥ 5 mL), respectively. After being dried in an oven, the catalyst
could be reused directly without further purification.
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Conclusions
In summary, we have developed
a novel, phosphine-
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free, practical and economic catalyst system for the
carbonylative Suzuki–Miyaura cross-coupling reaction by
using 3-(2-aminoethylamino)propyl-functionalized MCM-41-
supported palladium as the catalyst in anisole under an atmo-
spheric pressure of carbon monoxide. This novel heterogeneous
palladium catalyst can be conveniently prepared by a simple two-
step procedure from commercially available and cheap reagents,
it exhibits a higher activity and selectivity than PdCl2(PPh3)2,
and can be reused at least 10 times without any decrease in its ac-
tivity. The carbonylative cross-coupling reaction of aryl iodides
with arylboronic acids catalyzed by the MCM-41–2N–Pd(II)
complex under an atmospheric pressure of carbon monoxide
provides a better and practical procedure for the synthesis of a
variety of unsymmetrical biaryl ketone compounds.
Acknowledgements
We thank the National Natural Science Foundation of China
(Project no. 20862008) and the Natural Science Foundation
of Jiangxi Province in China (2008GQH0034) for financial
support.
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