Granulated copper oxide nano-catalyst: A novel and efficient catalyst for C-N cross-coupling of amines with iodobenzene

Article abstract of DOI:10.1007/s00706-011-0506-6

Nano-structured CuO granules catalyze the C-N cross-coupling of amines with iodobenzene in excellent yields. The reaction is simple, efficient, and operates in air under ligand-free conditions. Springer-Verlag 2011.

Full text of DOI:10.1007/s00706-011-0506-6

Monatsh Chem (2011) 142:801–806
DOI 10.1007/s00706-011-0506-6
ORIGINAL PAPER
Granulated copper oxide nano-catalyst: a novel and efficient
catalyst for C–N cross-coupling of amines with iodobenzene
•
Seyed Javad Ahmadi Sodeh Sadjadi
•
•
Morteza Hosseinpour Mojtaba Abdollahi
Received: 14 November 2010 / Accepted: 14 April 2011 / Published online: 31 May 2011
Ó Springer-Verlag 2011
Abstract Nano-structured CuO granules catalyze the
C–N cross-coupling of amines with iodobenzene in
excellent yields. The reaction is simple, efficient, and
operates in air under ligand-free conditions.
protocol is the use of stoichiometric amounts of copper or
copper salts, which results in the production of large quan-
tities of waste, making this method environmentally
unfriendly [2]. To overcome these limitations, much atten-
tion has recently focused on developing catalytic systems to
reduce the environmental impact of the processes [3–6].
Copper complexes containing electron-rich ligands have
been studied considerably for the cross-coupling of nitrogen
nucleophiles with aryl halides [7–10]. Most of the systems
involve a homogeneous process, and the ligand chelated with
the metal plays a crucial role in the catalysis.
Keywords Iodobenzene Á CuO nanoparticles Á
Alginate Á Nano-structured CuO granules Á
Ligand-free conditions
Introduction
Very recently, the employment of nanocrystalline metal
oxides as catalysts in organic synthesis has attracted much
attention [11]. CuO nanoparticles have been studied for
C–N, C–O, and C–S bond formations via cross-coupling
reactions of nitrogen, oxygen, and sulfur nucleophiles with
aryl halides [12, 13].
The synthesis of N-arylamines and N-arylheterocycles is an
active area in organic synthesis because of the occurrence
of these moieties in biologically important natural prod-
ucts, pharmaceuticals, and their applications in materials
research. Among the various strategies developed to date,
the copper-catalyzed Ullmann reaction has proven to be the
most convenient synthetic route for installing an N-aryl
functionality [1].
Despite their great chemical reactivity, nano-catalysts in
general and nano metal oxides in particular suffer from
having an inappropriate physical form that makes their
practical applications often unfeasible or at least formida-
ble. Industrial processes, especially fixed bed operations,
often require casting of nano powder catalysts into a gran-
ulated structure. In order to shape green bodies from
powders, many methods can be used, such as pressing, slip
casting, tape casting, injection molding, and gel casting. Gel
casting has been widely studied for the last decade [14–16].
Alginate is a type of gelling polysaccharide, which can
be dissolved in water at room temperature (RT) and then
gelled after casting by cross-linking with divalent metal
ions [17]. Several reports have demonstrated the gel casting
of micron and submicron powders by using sodium algi-
nate [18–20].
However, the synthetic scope of this reaction is strongly
limited by the insolubility of copper(I) salts in organic sol-
vents, the high reaction temperatures required (*200 °C),
and the sensitivity of the substituted aryl halide to the harsh
reaction conditions applied. Another major drawback of this
S. J. Ahmadi Á S. Sadjadi (&)
Nuclear Fuel Cycle School, Nuclear Science and Technology
Research Institute, End of North Karegar Ave,
P.O. Box 1439951113, Tehran, Iran
e-mail: sadjadi.s.s@gmail.com
M. Hosseinpour Á M. Abdollahi
Department of Energy Engineering,
Sharif University of Technology, Azadi Ave,
P.O. Box 113658639, Tehran, Iran
We herein describe a method for the preparation of
spherical granules of CuO nano-catalyst that preserve a
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S. J. Ahmadi et al.
I
R
N
Nano-structured
CuO granules
R'
+ RR'NH
KOH
1
2
3
Scheme 1
great portion of their initial specific surface area by using
sodium alginate. The separation of nano-structure catalysts
is often very difficult; in particular, when the particles are
too fine, they should pass through any filter available.
However, after algination, nanoparticles were fixed into a
granulated structure so we could separate them very easily
by filtration. The prepared catalysts were then used in the
Ullmann cross-coupling of amines with iodobenzene under
ligand-free conditions (Scheme 1).
Result and discussion
Fig. 1 XRD patterns of CuO particles prepared under supercritical
conditions and bulk CuO
Catalyst preparation
Granulated copper oxide nano-catalyst was prepared via a
three-step procedure. First, copper oxide nanoparticles
were synthesized by hydrothermal decomposition of cop-
per nitrate under supercritical water conditions. Second, the
nanoparticles were immobilized in the polymeric matrix of
sodium alginate. Third, high temperature calcination under
an airstream entirely removed the carbonaceous materials
and resulted in a pebble-type catalyst of high porosity.
The X-ray diffraction (XRD) patterns of nano-sized and
bulk CuO are shown in Fig. 1. All the XRD peaks are
indexed to the monoclinic crystal system of CuO. The
average size of the obtained CuO particles shown in Fig. 2
is 5 nm. The crystallite size was also calculated by X-ray
line-broadening analysis using the Scherrer equation; we
found that the average CuO crystallite size was 8 nm. The
mean surface area of the CuO catalyst was 32.5 m2/g from
Brunauer–Emmett–Teller (BET) analysis.
Fig. 2 Transmission electron micrographs of the CuO nanoparticles
The first step in the synthesis of the nano-structured
CuO granules was the preparation of the aqueous CuO
nanoparticle suspension. Fabrication of the granules (MSs)
was then effected by mixing of the CuO suspension with
sodium alginate solution, and dropwise injection of the
obtained sol into BaCl2 solution through a 0.50-mm med-
ical needle to gelify and form granules (Fig. 3). The size of
the droplets and thus granules was adjusted by mean of a
pneumatic cutting system which blew an airstream around
the injecting needle. Particles obtained by the dripping
method were calcined at different temperature [600 °C
(MS1), 700 °C (MS2), 800 °C (MS3)]. With respect to
particle morphology, the calcination temperature played an
important role in determining the surface characteristics of
the granules.
Table 1 summarizes the results of BET analysis of the
final granulated products. The maximum surface area was
presented by MS1, which was calcined at the lowest tem-
perature. The XRD patterns of the nano-structured CuO
granules are shown in Fig. 4. All the XRD peaks are
indexed to the monoclinic crystal system of CuO. The
XRD pattern of the copper oxide granules formed at
800 °C shows relatively sharp peaks with the highest
intensity as shown in Fig. 4. This is mainly due to the
growth of crystallite size. Figure 5 shows the shape, size,
and morphology of different microspheres that have
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Granulated copper oxide nano-catalyst
803
Fig. 3 Schematic diagram of the apparatus used for preparation of
the nano-structured CuO granules
Table 1 Surface area of granules at different condition
Entry
Temperature (°C)
Time (h)
BET (m2 g-1
)
MS1
MS2
MS3
600
700
800
6
6
6
18.74
9.16
5.65
Fig. 4 XRD patterns of nano-structured CuO granules a MS1, b MS2,
c MS3
Fig. 5 SEM images of a MS1, b MS2, c MS3 (scale size 1 lm),
d shape of the nano-structured CuO granules (scale size 200 lm)
undergone different thermal treatments. The size of all
microspheres is in the range of 300–500 lm.
activity of the nano-structured CuO granules. We attemp-
ted various reaction conditions including different copper
sources, bases, and solvents to optimize the Ullmann
coupling.
Catalytic reactions
The cross-coupling reaction of aniline with iodobenzene
was chosen as a model reaction to study the catalytic
Several readily available copper(II) salts such as
CuSO4Á5H2O and Cu(OAc)2ÁH2O were screened for the
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804
S. J. Ahmadi et al.
Ullmann coupling reaction of iodobenzene with aniline at
110 °C using 1.26 mol% of copper salt as the catalyst and
1 equiv of KOH in dimethyl sulfoxide (DMSO) under air.
The experiments showed that commercially available
copper sources catalyze this reaction to afford the corre-
sponding product 3a in low yields of 33–48% (Table 2,
entries 1, 2). The size of CuO plays an important role in
terms of yields and reaction times. When the reaction was
conducted with bulk CuO, a poor yield of the product was
obtained (Table 2, entry 3).
Table 2 C–N cross-coupling of aniline with iodobenzene in the
presence of copper(II) compounds
Entry
Catalyst
Time (h)
Yield (%)
1
2
3
4
5
6
CuSO4Á5H2O
Cu(OAc)2ÁH2O
CuO bulk
Nano-CuO
MS1
10
10
10
2
48
33
55
96
93
78
2
MS2
5
We compared the catalytic activity of CuO nanoparti-
cles with that of the nano-structured CuO granules (MS1,
MS2, MS3). The highest yield was obtained with CuO
nanoparticles and surprisingly CuO granules (MS1) gave
comparable yields to CuO nanoparticles (Table 2). The
high surface area of the copper oxides is supposed to be
important for the catalytic performance. Although the
granulation of nanoparticles by the immobilization–calci-
nation method reduces their specific surface area from 32.5
to 18.74 m2/g, the surface area is still much larger than that
of bulk CuO (0.49 m2/g).
Catalyst (1.26 mol%), aniline (1.2 mmol), iodobenzene (1 mmol),
KOH (1 mmol), and DMSO (1 cm3) were stirred at 110 °C
Table 3 C–N cross-coupling of aniline with iodobenzene in different
solvents and temperatures
Entry
Solvent
Temperature (°C)
Time (h)
Yield (%)
1
2
3
4
5
6
7
DMSO
110
110
110
110
120
90
2
8
93
56
35
95
95
21
–
Toluene
DMF
10
2
Solvent free
Solvent free
Solvent free
Solvent free
2
Although it is possible to slightly increase the yield of
the reaction by using CuO nanoparticles, the nano-struc-
tured CuO granule-catalyzed reaction has the advantages of
easy product purification, efficient recycling of the catalyst,
and minimization of metal oxide traces in the product.
Thus, we chose CuO granules (MS1) as the catalyst for this
reaction.
8
RT
10
CuO granules (1.26 mol%), aniline (1.2 mmol), iodobenzene
(1 mmol), KOH (1 mmol), and solvent (1 cm3)
Table 4 C–N cross-coupling of aniline with iodobenzene in the
presence of different bases
To study the effect of the solvent on the yield of this
reaction, the model reaction was carried out in various
solvents and under solvent-free conditions by using
1.26 mol% of CuO granules (MS1) as the catalyst. As
shown in Table 3 the best results in terms of yield and time
were achieved under solvent-free conditions.
Entry
Base
Time (h)
Yield (%)
1
2
3
4
KOH
2
5
95
45
–
K2CO3
NEt3
10
10
The same reaction was carried out at temperatures
ranging from 90 to 120 °C, with an increment of 10 °C
each time. The yield of product 3a was increased, and the
reaction time was shortened when the temperature was
increased from 90 to 110 °C. Temperatures above 110 °C
had no obvious influence on this reaction. Therefore, a
temperature of 110 °C was chosen for all further reactions.
Virtually no product was detected when a mixture of
iodobenzene and aniline was stirred at room temperature. A
temperature of at least 90 °C is required for appreciable
conversion to product.
Pyridine
–
CuO granules (1.26 mol%), aniline (1.2 mmol), iodobenzene
(1 mmol), and base (1 mmol) were stirred at 110 °C under solvent-
free conditions
solvent-free conditions. The coupling reaction of iodo-
benzene with various amines was carried out using this
catalyst system, and the desired amination products were
obtained in good to high yields. As shown in Table 5,
anilines having electron-donating groups showed greater
reactivity than those with electron-withdrawing groups.
These reaction conditions were also suitable for the cross-
coupling of alkyl amines and N-heterocyclic compounds
with iodobenzene. As nano-structured CuO granules are a
heterogeneous catalyst, the recyclability of the catalyst in
this coupling reaction was examined. The coupling of
aniline with iodobenzene was chosen as a model reaction.
After each cycle, the catalyst was recovered by simple
We also investigated the coupling of iodobenzene with
aniline using different bases (Table 4). Among the bases
we selected, KOH gave the best result, and K2CO3 was less
efficient. The application of NEt3 and pyridine as base
failed to yield the desired product.
The scope of the copper-catalyzed aryl amination reac-
tion was explored by using CuO granules (MS1) as the
copper source and 1 equiv of KOH as the base under
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Granulated copper oxide nano-catalyst
805
5
Table 5 C–N cross-coupling of amines with iodobenzene
Table 6 Recycling of CuO granules
Entry
a
Amine
Time (h)
2
Yield (%)
95
Run
1
2
3
4
Time (h)
2
2
3
3
3
NH2
Yield (%)
95
93
89
86
84
portion of their initial specific surface area. The resulting
granules were used as a catalyst for the efficient C–N cross-
coupling reaction of amines with iodobenzene. The scope
of the reaction using our catalyst is quite broad; various
aryl, alkyl, and N-heterocyclic amines react with iodo-
benzene. Furthermore, nano-structured CuO granules can
be recovered and recycled by simple filtration of the
reaction solution and reused for at least five consecutive
trials without a decrease in their activity.
b
c
10
76
92
NH2
O2N
2
NH2
d
e
6
7
91
89
Experimental
O
NH
Batchwise supercritical hydrothermal synthesis of CuO
nanoparticles
NH
Copper(II) nitrate trihydrate (Merck AG, for synthesis) was
used as the precursor for synthesis of nano copper oxide.
Preparation of CuO took place in a stainless steel
(316 dm3) autoclave that was able to endure a working
temperature and pressure of 550 °C and 610 bar, respec-
f
4
90
NH2
tively. The concentration of Cu(NO3)2 was 0.05 mol dm-3
,
g
h
10
3
91
93
and the heating period was about 2 h. The synthesis was
carried out at 500 °C to accelerate the hydrolysis reactions
and thus shorten the fabrication period. To maintain a
suitable safety margin, the 200-cm3 stainless steel auto-
clave was loaded with only 60–80 cm3 of the solution. The
autoclave was removed from the furnace, quenched with
cold water, and the CuO nanoparticles were recovered from
the discharged solution by high speed centrifugation at
14,000 rpm for about 60 min. The produced nanoparticles
were then washed three times in the same centrifuge with
ultrapure water, and then dried at ambient temperature.
NH2
N
H
i
20
87
H
N
N
Preparation of nano-structured CuO granules
CuO granules (MS1, 1.26 mol%), amine (1.2 mmol), iodobenzene
(1 mmol), and KOH (1 mmol) were stirred at 110 °C under solvent-
free conditions
CuO nanoparticles (2 g) were dispersed in 5 cm3 deionized
water and the mixture was stirred to form an aqueous CuO
suspension, and the reaction mixture was exposed to
ultrasonic irradiation for 30 min. Afterwards 14 cm3 of
gelatin aqueous solution (sodium alginate, 3.6 wt%) was
added, and the mixture was stirred mechanically for 2 h.
The resulting alginate–CuO nanoparticle solution was
sprayed through a thin inner nozzle (Ø = 0.5 mm) into a
0.1 mol dm-3 barium chloride solution with pressurized
air blown from an outer space of the nozzle (Fig. 3). The
filtration, washing with deionized water and ethanol, and
then drying in vacuo. The recovered CuO granules were
used directly in the next cycle. The recycling results are
listed in Table 6 and show that the catalyst was still highly
efficient after the fifth cycle.
In conclusion, we have developed a new method of
granulation of CuO nanoparticles that preserves a great
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S. J. Ahmadi et al.
resultant beads were allowed to stand in the solution for 3 h
at room temperature on a magnetic stirrer, and then they
were washed three times with pure water and eventually
dried at 400 °C for 10 h. The dried granules were trans-
ferred into a muffle furnace and calcined at 600 °C (MS1),
700 °C (MS2), and 800 °C (MS3) for 6 h to decompose and
burn all of the organic materials.
All products were known and characterized by com-
parison of their physical and spectra data with those
already reported [13, 21–24].
Acknowledgments The authors are thankful to the Nuclear Science
and Technology Research Institute for the partial financial support.
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CuO granules (MS1, 1.26 mol%) were added to a stirred
solution of amine (1.2 mmol), iodobenzene (1 mmol), and
KOH (1 mmol). The reaction mixture was heated at 110 °C
for the appropriate time (Table 5) and then allowed to cool to
room temperature. Diethyl ether (2 cm3) and 2 cm3 water
were then added to the reaction mixture and the catalyst was
removed by high speed centrifugation (for CuO nano-cata-
lyst) or simple filtration (for CuO granules). The reaction
mixture was further extracted with diethyl ether
(3 9 5 cm3). The combined organic phases were washed
with brine and dried over sodium sulfate. The solvent was
removed by rotary evaporation to give a residue which was
purified on silica gel column chromatography using ethyl
acetate and hexane as eluent.
123