Immobilization of copper(II) in organic-inorganic hybrid materials: A highly efficient and reusable catalyst for the classic Ullmann reaction

Article abstract of DOI:10.1055/s-2008-1067107

The immobilization of copper(II) in organic - inorganic (silica gel) hybrid materials as catalysis for the homocoupling of aryl halides (classic Ullmann reaction) has been described. The homocoupling of aryl iodides, bromides and chlorides underwent smoothly in the presence of a 3-(2-aminoethylamino)propyl- functionalized silica gel immobilized copper(II) catalyst. The protocol involved the use of DMSO as the solvent and potassium fluoride as the base. The reactions generated the corresponding homocoupling products in good to excellent yields. Furthermore, the silica-supported copper(II) could be recovered and recycled by simple filtration and used for five consecutive trials without loss of its reactivity. Thieme Stuttgart.

Full text of DOI:10.1055/s-2008-1067107

PAPER
2007
Immobilization of Copper(II) in Organic–Inorganic Hybrid Materials:
A Highly Efficient and Reusable Catalyst for the Classic Ullmann Reaction
Silica
G
el Immobi
i
lizedC
a
opper(II)
C
n
atalyst for th
g
e
U
llmannReactio
W
u,a Lei Wang*a,b
a
Department of Chemistry, Huaibei Coal Teachers College, Huaibei, Anhui 235000, P. R. of China
Fax +86(561)3090518; E-mail: leiwang@hbcnc.edu.cn
b
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, P. R. of China
Received 29 February 2008; revised 11 April 2008
classic Ullmann reaction, that is, reductive homocoupling
of aryl halides, in the presence of catalytic amount or
stoichiometric amount of only copper(II).
Abstract: The immobilization of copper(II) in organic–inorganic
(silica gel) hybrid materials as catalysis for the homocoupling of
aryl halides (classic Ullmann reaction) has been described. The
homocoupling of aryl iodides, bromides and chlorides underwent
smoothly in the presence of a 3-(2-aminoethylamino)propyl-
functionalized silica gel immobilized copper(II) catalyst. The pro-
tocol involved the use of DMSO as the solvent and potassium fluo-
ride as the base. The reactions generated the corresponding
homocoupling products in good to excellent yields. Furthermore,
the silica-supported copper(II) could be recovered and recycled by
simple filtration and used for five consecutive trials without loss of
its reactivity.
The high costs of the transition metal catalysts coupled
with toxic effects associated with many transition metals
has led to an increased interest in immobilizing catalysts
onto a support. This class of supported reagents can facil-
itate both the isolation and recycling of the catalyst by
filtration, thus providing environmentally cleaner pro-
cesses.14 Recent examples are the palladium complexes
immobilized onto amphiphilic polystyrene supports for
effecting Suzuki coupling reaction in an aqueous medium
as well as a modified aminomethyl polystyrene-supported
palladium reagent for use as a catalyst in a range of Sono-
gashira coupling reactions.15 The development of organic
reagents grafted onto silica gel has attracted increasing at-
tention in recent years because the industry seeks more
environmentally friendly chemical manufacturing pro-
cesses.16 Recently, we have developed a silica-supported
copper(I)-catalyzed Sonogashira coupling reaction and a
1,3-dipolar cycloaddition reaction of terminal alkynes and
alkyl halides with sodium azide in one-pot, and a silica-
supported copper(II)-catalyzed Ullmann diaryl etherifica-
tion.17
Key words: classic Ullmann reaction, homocoupling, aryl halides,
immobilization of copper(II), organic–inorganic hybrid materials
Biaryls are widely used as chiral phosphine ligands,1
monomers of conductive polymer,2 and as important in-
termediates in the synthesis of natural products and phar-
maceutics.3 As for the synthesis of symmetrical biaryls,
the main method is the classic Ullmann reaction, which
however, is carried out at high temperatures (>200 °C)
and cannot tolerate many functional groups for a long pe-
riod of time. In addition, these reactions often require the
use of stoichiometric amounts of copper reagents [copper
powder or copper(I) salt, such as copper(I) thiophene-2-
carboxylate, CuTC] leading to problems of waste dispos-
al.4 To overcome these drawbacks, more and more atten-
tion has been paid to palladium-catalyzed homocoupling
reactions. These are usually conducted in the presence of
the reducing agents, such as zinc,5 formate salts,6 hydro-
quinone (HQ),7 hydrogen,8 alcohols,9 carbon monox-
ide,10a or ascorbic acid.10b Nevertheless, the use of
expensive palladium and noncommercial sophisticated
phosphines limits the attractiveness of this method for
large-scale or industrial applications.7,11
Here, we wish to report that the 3-(2-aminoethylami-
no)propyl-functionalized silica gel immobilized cop-
per(II) catalyst (Scheme 1) was used as an effective
catalyst for the classic Ullmann reaction of aryl iodides,
bromides, and chlorides in the presence of potassium
fluoride in DMSO. The reaction generated the corre-
sponding symmetrical biaryls in good to excellent yields.
It is important to note that the silica-supported copper(II)
catalyst could be recovered and recycled by a simple fil-
tration of the reaction mixture and used for five consecu-
tive trials without significant loss of its reactivity.
In the last decade, there were notable improvements in the
copper-catalyzed Ullmann-type reaction of C–C bond for-
mation.12 Recently, Buchwald and Ma et al. developed al-
ternative approaches to copper(I)-catalyzed Ullmann-type
reaction of C–N, C–O, and C–S bond formation.13 How-
ever, to the best of our knowledge, there is no report on the
The synthesis of the organic–inorganic hybrid material
immobilized copper catalyst was reported by our group.17c
It was readily prepared through a two-step procedure. The
silica gel (100–200 mesh, Aldrich) was reacted with 3-(2-
aminoethylamino)propyltrimethoxysilane in anhydrous
toluene at 120 °C for 24 hours to afford the 3-(2-amino-
ethylamino)propyl functionalized silica gel. The organic–
inorganic hybrid materials then reacted with copper(II)
acetate in ethanol at room temperature for 4 hours to gen-
erate the silica-supported copper(II) catalyst.
SYNTHESIS 2008, No. 13, pp 2007–2012
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x
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Advanced online publication: 21.05.2008
DOI: 10.1055/s-2008-1067107; Art ID: F04708SS
© Georg Thieme Verlag Stuttgart · New York

2008
Q. Wu, L. Wang
PAPER
O
O
O
O
O
O
Si
Si
HN
HN
NH2
Cu(II)
NH2
silica-supported
copper(II)
SiO2
(20 mol%)
X
+
X
KF, DMSO, 130 °C
R
R
R
R
X = I, Br, Cl
Scheme 1
Table 1 Effect of Copper Source on the Classic Ullmann Reactiona
In our initial screening experiments, the classic Ullmann
reaction of 4-iodoanisole catalyzed by the silica-support-
ed copper(II) catalyst was chosen as a model reaction.
When we searched for a homocoupling protocol of 4-io-
doanisole, we observed that 4-iodoanisole could undergo
self-coupling in the presence of 20 mol% of a silica-sup-
ported copper(II) catalyst and 2 equivalents of K2CO3 in
DMF at 130 °C to generate the corresponding homocou-
pling product in 47% yield. Encouraged by this result, we
continued our research to improve the yield of product by
optimization of the reaction conditions.
Cu source
MeO
I
MeO
OMe
KF, DMSO
Entry
Copper source (mol%)
No
Yield (%)b
1
2
0
25
41
45
42
73
92
93
71
47
CuI (20)
3
CuCl2 (20)
4
Cu(OAc)2 (20)
In the presence of potassium fluoride, the classic Ullmann
reaction could be catalyzed by either Cu(I) or Cu(II) in the
absence of any palladium. However, no reaction occurred
in the absence of any copper and palladium catalyst. As
can be seen from Table 1, silica-supported Cu(II) cata-
lysts were superior. Among silica-supported Cu(II) cata-
lysts, which were prepared from different copper(II)
sources, silica-supported Cu(II) catalyst from copper(II)
acetate was found to be the most effective one (Table 1,
entries 1–7). This may be the effect of the anion in silica-
supported Cu(II) catalysts on its catalytic activity. Thus,
silica-supported Cu(II) from Cu(OAc)2 was used as a cat-
alyst in the following investigation on account of its high
efficiency, easy separation, and recycling. In addition, the
5
Silica-supported Cu(I)c (20)
Silica-supported Cu(II)d (20)
Silica-supported Cu(II)e (20)
Silica-supported Cu(II)e (30)
Silica-supported Cu(II)e(10)
Silica-supported Cu(II)e (5)
6
7
8
9
10
a 4-Iodoanisole (2.00 mmol), Cu catalyst (containing 0.20 mmol of
Cu), KF (2.00 mmol) in DMSO (4 mL) at 130 °C for 20 h.
b Isolated yields.
c Catalyst prepared from CuI.
d Catalyst prepared from CuCl2.
amount of silica-supported Cu(II) from Cu(OAc)2 was e Catalyst prepared from Cu(OAc)2.
also examined and the optimized loading of Cu(II) on sil-
ica was found to be in the range of 20–30 mol% (Table 1,
entries 7–10). We tested several different bases for the
classic Ullmann reactions catalyzed by the silica-support-
ed copper(II) in DMSO. KF was found to be more effec-
tive than other bases, such as K2CO3, Cs2CO3, K3PO4,
KOAc, and KOH (Table 2, entries 1–6).
ed copper(II) catalyst, the reactions were generally
completed in DMSO at 130 °C under stirring for 20 hours.
When the reaction was carried out in DMSO at 120 °C,
the reaction was over in 43 hours. Meanwhile, when the
reaction was performed in DMSO at 140 °C, the reaction
was complete in 12 hours. Because the organic compound
usually decomposes at such a high reaction temperature, a
reaction temperature of 130 °C was found to be optimal.
Thus, the optimized reaction conditions for this Ullmann
reaction are the silica-supported copper(II) catalyst (20
mol%) and KF (2 equiv) in DMSO at 130 °C for 20 hours.
We then turned our attention to investigate the effect of
solvent on the coupling reaction. When the reactions were
conducted in DMF and DMSO, good to excellent yields
(78% and 92%, respectively) of products were isolated.
No desired homocoupling product was observed during
the reactions in EtOH, THF, and toluene (Table 3,
entries1–5).
We have investigated the reactions using a variety of aryl
iodides, bromides, and chlorides as the substrates under
the present reaction conditions and the results are summa-
rized in Table 4. Electron-neutral, electron-rich and elec-
tron-poor aryl iodides, as well as electron-poor aryl
During the course of optimization of the reaction condi-
tions, when using a 20 mol% loading of the silica-support-
Synthesis 2008, No. 13, 2007–2012 © Thieme Stuttgart · New York

PAPER
Silica Gel Immobilized Copper(II) Catalyst for the Ullmann Reaction
2009
Table 2 Effect of Base on the Classic Ullmann Reactiona
Table 4 Silica-Supported Copper(II) Catalyzed Classic Ullmann
Reactiona
silica-supported
Cu(II) catalyst
MeO
I
Entry Substrate
1
Product
Yield
(%)b
MeO
OMe
DMSO, base
Base
Entry
Yield (%)b
93
90
92
I
1
2
3
4
5
6
KF
92
70
68
56
61
84
K2CO3
Cs2CO3
K3PO4
KOAc
KOH
2
Me
I
Me
Me
3
MeO
I
MeO
NC
OMe
NC
CN
4
91
96
93
91
I
a 4-Iodoanisole (2.0 mmol), silica-supported Cu(II) catalyst (contain-
ing 0.20 mmol of Cu), base (2.0 mmol) in DMSO (4 mL) at 130 °C
5
O2N
I
O2N
O2N
NO2
for 20 h.
b Isolated yields.
O2N
NO2
6
Table 3 Effect of Solvent on the Classic Ullmann Reactiona
I
silica-supported
Cu(II) catalyst
MeO
I
MeO
OMe
7
NC
Br
Br
Br
NC
CN
KF, solvent
Entry
Solvent/temp (°C)
EtOH/78
Yield (%)b
8
94
90
85
81
82
68
O2N
Br O2N
NO2
1
2
3
4
5
0
0
9
THF/70
Ac
Ac
Ac
Toluene/110
DMF/130
0
10
Br
78
92
DMSO/130
11
Me
Me
Me
a 4-Iodoanisole (2.0 mmol), silica-supported Cu(II) catalyst (contain-
ing 0.20 mmol of Cu), KF (2.0 mmol) in solvent (4 mL) at the tem-
perature indicated in Table 3 and stirring for 20 h.
b Isolated yields.
12
MeO
OMe
MeO
Br
13
Cl
bromides underwent homocoupling reactions very well to
generate the corresponding products in excellent yields
under standard reaction conditions (Table 4, entries 1–9).
For electron-neutral and electron-rich aryl bromides, good
yields of the desired products were obtained (Table 4, en-
tries 10–12). Fortunately, electron-poor aryl chlorides re-
acted under self-coupling under the present reaction
conditions to generate the corresponding biaryls in good
yields (Table 4, entries 14–15), and a moderate yield of
product was obtained when chlorobenzene was used as
substrate (Table 4, entry 13). However, when 2-bromopy-
ridine was used as starting material, only 47% of homo-
coupling product was obtained (Table 4, entry 16).
14
78
76
47
O2N
Cl O2N
NO2
15
Ac
Cl
Ac
Ac
N
N
N
16
Br
a Aryl halide (2.0 mmol), silica-supported Cu(II) catalyst (containing
0.20 mmol of Cu), KF (2.0 mmol) in DMSO (4 mL) at 130 °C and stir-
ring for 20 h.
b Isolated yields.
be recovered, recycled and used for five consecutive trials
without loss of activity (Table 5).
The recyclability of the silica-supported copper(II) was
also surveyed. After the reaction, the solution was vacuum
filtered using a sintered glass funnel and the catalyst was
washed with CH2Cl2, Et2O, and hexane, respectively. Af-
ter drying, the catalyst can be reused directly without fur-
ther purification. Thus, the silica-supported copper(II) can
Generally, the classic Ullmann reaction was catalyzed ei-
ther by Cu(0) or Cu(I). Under the present reaction condi-
tions, dimethyl sulfoxide might be decomposed by KF
into complicated decomposition products in small
Synthesis 2008, No. 13, 2007–2012 © Thieme Stuttgart · New York

2010
Q. Wu, L. Wang
PAPER
Table 5 Successive Trials by Using Recoverable Cu(II) Catalysta
g), and 3-(2-aminoethylamino)propyltrimethoxylsilane (AAPTS,
10 mL). The suspension was refluxed for 24 h and filtered. The solid
was washed with acetone (10 mL) and CH2Cl2 (10 mL), respective-
ly, and dried under reduced pressure at 60 °C; yield: 3.38 g.
reused silica-
supported Cu(II)
MeO
OMe
MeO
I
KF, DMSO
FT-IR (KBr): 1088 (Si–O), 2941 cm–1 (C–H).
Entry
Reused Catalyst
Fresh
Yield (%)b
Anal. Found: C, 6.56; H, 1.55; N, 2.56, corresponding to 0.914
mmol/g of 2-aminoethylaminopropyl groups based on the nitrogen
percentage.
1
2
3
4
5
92
90
91
89
88
First reuse
Copper(II)-Immobilized Silica Using Cu(OAc)2: In a small
Schlenk tube, the immobilized silica (1 g, 0.914 mequiv
CH2CH2CH2NHCH2CH2NH2/g) was mixed with Cu(OAc)2 (0.110
g, 0.6 mmol) in anhyd EtOH (5 mL). The mixture was stirred for 4
h under N2 at r.t. The suspension was filtered and the solid was
washed with acetone (5 mL) and MeOH (5 mL), respectively. The
solid was dried under reduced pressure at room temperature for 16
h, leading to a blue powder (1.05 g).
Second reuse
Third reuse
Fourth reuse
a 4-Iodoanisole (2.0 mmol), reused silica-supported Cu(II) catalyst
(containing 0.20 mmol of Cu), KF (2.0 mmol) in DMSO (4 mL) at
130 °C stirring for 20 h.
FT-IR (KBr): 1047 (Si–O), 2932 cm–1 (C–H).
b Isolated yields.
Anal. by ICP (atomic %): Cu, 2.55.
Copper(II)-Immobilized Silica Using CuCl2: In a small Schlenk
tube, the immobilized silica (1 g, 0.914 mequiv
amounts at 130 °C. As one of the decomposition product,
dimethyl sulfide could reduce copper(II) into copper(I). CH2CH2CH2NHCH2CH2NH2/g) was mixed with CuCl2 (0.081 g,
0.6 mmol) in anhyd EtOH (5 mL). The mixture was stirred for 4 h
We also observed that the blue silica-supported copper(II)
under N2 at r.t. The suspension was filtered and the solid was
catalyst gradually became a dark brown powder under the
washed with acetone (5 mL) and MeOH (5 mL), respectively. The
solid was dried under reduced pressure at room temperature for 16
h, leading to a blue powder (1.05 g).
FT-IR (KBr): 1045 (Si–O), 2934 cm–1 (C–H).
present reaction conditions. Also we noted that the source,
anion, and storage time of silica-supported copper(I) cat-
alyst affected its activity.
In summary, we have developed an efficient and econom-
ic catalyst system for the classic Ullmann reaction of aryl
halides by using the organic–inorganic (silica) hybrid ma-
terials-supported copper(II) as the catalyst in DMSO. The
homocoupling reactions of aryl iodides, bromides and
chlorides generate the corresponding coupling products in
Anal. by ICP (atomic %): Cu, 2.58.
Copper(I)-Immobilized Silica Using CuI: In a small Schlenk
tube, the immobilized silica (1 g, 0.914 mequiv
CH2CH2CH2NHCH2CH2NH2/g) was mixed with CuI (0.349 g, 1.1
mmol) in anhyd EtOH (5 mL). The mixture was stirred for 4 h under
N2 at r.t. The suspension was filtered and the solid was washed with
good to excellent yields at the present reaction conditions. acetone (5 mL) and MeOH (5 mL), respectively. The solid was
dried under reduced pressure at r.t. for 16 h, leading to a brown pow-
der (1.22 g).
FT-IR (KBr): 1039 (Si–O), 2928 cm–1 (C–H).
Furthermore, the silica-supported copper(II) catalyst can
be recovered and recycled by a simple filtration of the re-
action solution and used for five consecutive trials without
decrease in activity.
Anal. by ICP (atomic %): Cu, 5.06.
Classic Ullmann Reaction; General Procedure
Melting points were recorded on a WRS-2B melting point apparatus
and were uncorrected. All H NMR spectra were recorded on a
Under N2, an oven-dried round-bottomed flask was charged with
the silica-supported copper(II) catalyst (500 mg, containing 0.2
mmol of Cu), KF (116 mg, 2.0 mmol), aryl halide (2.0 mmol), and
DMSO (4.0 mL). The mixture was refluxed at 130 °C for 20 h. Af-
ter cooling to r.t., the mixture was filtered under reduced pressure
using a sintered glass funnel and washed with CH2Cl2 (2 × 3 mL).
The combined organic extracts were dried (Na2SO4), filtered, con-
centrated, and the residue was purified by flash chromatography on
silica gel to give the desired homocoupled product (Table 4).
1
Bruker 300 MHz or a Bruker 400 MHz FT-NMR spectrometer.
Chemical shifts were given as d value with reference to TMS as the
internal standard. Products were purified by flash column chroma-
tography on 230–400 mesh silica gel. The chemicals were pur-
chased from commercial suppliers (Aldrich, USA and Shanghai
Chemical Company, China) and were used without purification pri-
or to use.
Silica-Immobilized Copper(II) and Copper(I) Catalysts
These catalysts were prepared in the following steps from commer-
cial silica gel according to the literature.17c
Biphenyl
Mp 66–68 °C (Lit.18 mp 70 °C).
IR (KBr): 3033, 1479, 1169, 728, 696 cm–1.
Activation of Silica: Silica (10 g, 100–200 mesh, Aldrich) was intro-
duced in a round-bottomed flask equipped with a reflux condenser.
Concd H2SO4 (80 mL) and HNO3 (15 mL) were added and the mix-
ture was heated in an oil bath at 140 °C for 24 h. The suspension was
filtered and the white powder was washed with distilled H2O until
pH 7 was attained. The solid was again washed with acetone (30
mL), MeOH (30 mL), and CH2Cl2 (30 mL), respectively, and dried
under vacuum at 150 °C for 48 h.
1H NMR (CDCl3, 300 MHz): d = 7.36 (t, J = 7.3 Hz, 2 H), 7.46 (t,
J = 7.5 Hz, 4 H), 7.61 (d, J = 7.8 Hz, 4 H).
4,4¢-Dimethylbiphenyl
Mp 119–121 °C (Lit.19 mp 121 °C).
IR (KBr): 3022, 2915, 1501, 1113, 804 cm–1.
1H NMR (CDCl3, 300 MHz): d = 2.38 (s, 6 H), 7.22 (d, J = 7.8 Hz,
4 H), 7.47 (d, J = 8.1 Hz, 4 H).
Immobilized Silica: In a 50 mL round-bottomed flask were intro-
duced successively anhyd toluene (20 mL), activated silica gel (3
Synthesis 2008, No. 13, 2007–2012 © Thieme Stuttgart · New York

PAPER
Silica Gel Immobilized Copper(II) Catalyst for the Ullmann Reaction
2011
4,4¢-Diacetylbiphenyl
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IR (KBr): 3079, 1525, 1349, 1103, 857, 699 cm–1.
1H NMR (CDCl3, 300 MHz): d = 6.73 (t, J = 8.0 Hz, 2 H), 7.00 (d,
J = 7.5 Hz, 2 H), 7.33 (d, J = 8.1 Hz, 2 H), 7.52 (t, J = 1.5 Hz, 2 H).
2,2¢-Bipyridine
Mp 68–70 °C (Lit.18 mp 70–71 °C).
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IR (KBr): 3053, 1579, 1452, 1250, 1040, 757 cm–1.
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2 H), 8.30–8.42 (m, 2 H), 8.68–8.69 (m, 2 H).
The Recyclability of the Silica-Supported Copper Catalyst
After the coupling reaction, the mixture was vacuum filtered using
a sintered glass funnel and washed with CH2Cl2 (3 mL), Et2O (3
mL), EtOH (3 mL), and hexane (3 mL), respectively. After drying
in an oven, the catalyst can be reused directly without further puri-
fication.
Acknowledgment
We gratefully acknowledge the financial support from the National
Natural Science Foundation of China (No. 20772043, 20572031),
and the Key Project of Science and Technology, Department of
Education, Anhui, China (No. ZD2007005–1).
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