Cu-Pd/γ-Al2O3 catalyzed the coupling of multi-step reactions: Direct synthesis of benzimidazole derivatives

Article abstract of DOI:10.1039/c6ra13004f

The coupling of multi-step reactions catalyzed by a heterogeneous catalyst is an important path to accomplish some unconventional chemical transformations. Since the starting materials generated from previous steps were adsorbed on the catalyst, the activation energy of following steps was largely decreased, and thus the reaction conditions were more mild and environmental friendly. Catalyzed by a multifunctional Cu-Pd/γ-Al₂O₃ catalyst, the transfer hydrogenation and successive cyclization coupling reaction from o-nitroaniline and alcohol to afford benzimidazole derivatives in high yield was realized. The catalyst could be reused several times without loss of activity. The synergies of reforming hydrogenation of Cu-Pd bimetal and support acidity of γ-Al₂O₃ were responsible for this catalytic transformation.

Full text of DOI:10.1039/c6ra13004f

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PAPER
Cu-Pd/γ-Al O Catalyzed the Coupling of Multi-step Reaction:
2
3
Direct Synthesis of Benzimidazole Derivatives
Feng Feng, Jia Ye, Zheng Cheng, Xiaoliang Xu, Qunfeng Zhang,* Lei Ma, Chunshan Lu, Xiaonian Li*
The coupling of multi-step reaction catalyzed by heterogeneous catalyst is an important path to accomplish some
unconventional chemical transformations. Since the starting materials generated from previous steps were adsorbed on
the catalyst, the activation energy of following step was largely decreased, and thus the reaction conditions were more
Received 00th January 20xx,
Accepted 00th January 20xx
2 3
mild and environmental friendly. Catalyzed by multifunctional Cu-Pd/γ-Al O catalyst, the hydrogenation transfer and
successive cyclization coupling reaction from o-nitroaniline and alcohol to afford benzimidazole derivatives in high yield
DOI: 10.1039/x0xx00000x
was realized. The catalyst could be reused for several times without loss of activities. The synergies of reforming
www.rsc.org/
2 3
hydrogenation of Cu-Pd bimetal and support acidity of γ-Al O were responsible for this catalytic transformation.
Although this method is effective, the main drawback is high
pressure and temperature, equipment requirements, and at the
Introduction
The synthesis of complex organic compounds often
required multiꢀstep reaction, in which harsh reaction conditions
and complex catalysts were frequently used in several steps. In
recent decades, use of easily recyclable solid acid replacing
liquid acid in the acidꢀcatalyzed reaction was largely developed
same time transfer and transport of hydrogen is tedious and
3ꢀ8
dangerous. Recently Dumesic
reported a new hydrogen
production method based on transition or noble metalꢀcatalyzed
aqueousꢀreforming technology. Thus, this liquidꢀsolid two
phase catalytic transfer hydrogenation provides an efficient
route to replace the threeꢀphase catalytic hydrogenation for
certain reactions. Use of in situ produced and adsorbed
1
and considered as a major goal of green chemistry. The
reactants generated from previous step were in situ adsorbed on
the catalyst surface, which reduced the activation energy for the
subsequent steps of the catalyst acidic requirements (Fig. 1).
hydrogen on catalyst, nitro compounds are successfully reduced
9,10
to amines
by this method. And by tuning the acidꢀbase and
surface characteristics of support, the multiꢀstep successive
coupling reactions of adsorbed intermediates on catalyst may be
realized. According to these requirements, modified
multifunctional heterogeneous catalyst is an important means to
realize this kind of coupling reactions. Based on these, recently
C
Multiꢀstep reaction
*
*
E
F
D
A
B
I
II
*E
*F
*
A
B
*C
*D
*C
*D
*
A+*B *C+*D
*C+*D *E+*F
*
Coupling reaction
*
E
I
II
*F
A
*
A
+
*E
*F
*
A+*B *C+*D *E *F
B
*
B
the direct reductive
Nꢀalkylation of nitrobenzene catalyzed by
Fig. 1 Possible mechanism of coupling reaction
11
Raney Ni and synthesis of quinoline derivatives from
nitrobenzene and alcohol catalyzed by CuꢀPd/γꢀAl O was
2
3
The gasꢀliquidꢀsolid threeꢀphase catalytic hydrogenation
for the reduction of nitro compounds to amines is an important
1
2
successfully established by our group.
Heterocycles are ubiquitous intermediates for the synthesis
2
and conventional technology. The solvent, hydrogen, noble
1
3ꢀ15
16,17
of
pharmaceuticals,
dyes,
organic
functional
metal catalyst is essential in this catalytic transformation.
1
8,19
materials
. Among these, the fiveꢀmembered nitrogen
heterocycles, especially benzimidazole derivatives, are widely
2
0
13ꢀ15
applied in fungicidal and pharmaceutical
. Normally, the
condensation of ꢀphenylenediamine with acid derivatives or oꢀ
o
phenylenediamine with aldehydes in the presence of an
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appropriate oxidant is a typical way for the synthesis of
RSC Advances
Results and discussion
2
1ꢀ24
benzimidazole
. However, the traditional methods mostly
Firstly, monometallic or bimetal nanoparticles supported on γꢀ
suffer from use of strong inorganic acid, formation of large
amount of waste, difficult recycling of catalyst, and low atom
economy.
Al O were synthesized. Subsequently, ethanol and oꢀnitroaniline
2
3
was chosen as a model compound to evaluate the catalytic activity of
catalyst, and the results were listed in Table 1. Monometallic Pd (5%
Comply with the green chemistry trend, heterogeneous
2 3
weight amount) loaded on γꢀAl O as a catalyst gave very low
2
5ꢀ37
26,31,38,39
catalytic systems
and microwave technology
were
conversion and yield (entry 1). However, upon alloying palladium
with copper, activity of catalysts improved greatly, and different
Pd/Cu ratio does not result in significant difference in activity or
yield. 2ꢀmethylbenzimidazole was selectively obtained in 98.2%
yield catalyzed by Cu ꢀPd /γꢀAl O (entry 4). Since monometallic
applied for the synthesis of benzimidazoles recently, such as
2
6,35
27,28
30,31
using PꢀMo or PꢀW heteropoly acid,
modified zeolite,
2
9
mesoporous SiꢀAl material,
transition metal oxides,
3
6
alumina supported potassium fluoride and supported noble
5
5
2
3
3
7,39
40,41
metal
For instance, Gadekar reported that modified zeolite catalyst
gave 83~96% yield of benzimidazoles from
as catalysts, and using air or oxygen as oxidant
.
Cu/γꢀAl
2
O
3
was totally inactive for this reaction (entry 6), the
catalyst can be ascribed to
2
7
improved performance of CuꢀPd/γꢀAl
2
O
3
o
ꢀ
the modification of palladium properties by alloying with copper.
Changing the bimetal component to CuꢀNi, CuꢀPt, FeꢀPd, FeꢀPt, Znꢀ
Ni, the conversion and yield are disappointing (entries 7ꢀ11). Use of
5% weight amount of Zn and Pd loaded on γꢀAl O , only moderate
3
1
phenylenediamine and aldehydes. Rathod
reported that
benzimidazoles were obtained in 92~94% yield from
o
ꢀ
phenylenediamine and aromatic aldehyde using MoO /CeO ꢀ
3
2
2
3
ZrO
2
as catalyst under solvent free condition. Compared with
o
ꢀ
conversion and yield were obtained (entry 12). From these, it can be
concluded that Ni or Pt show lower dehydrogenation and
hydrogenation performance, compared with that of Pd.
phenylenediamines,
substrates. The reductive cyclization of
carbonyl derivatives or alcohols is another pathway for the
oꢀnitroanilines are more readily available
o
ꢀnitroaniline with
4
2ꢀ50
synthesis of benzimidazole.
in one pot including dehydrogenation of alcohols, successive
reduction of ꢀnitroaniline and cyclization simplified the
synthesis process obviously. Based on this concept, Sun
The direct continuous catalysis
Table 1 Evaluation of the catalytic activity of metal component
o
5
0
a
Entry
Catalyst
Pd/γꢀ Al
Con. (%)
Yield (%)
Selectivity (%)
reported the synthesis of benzimidazole derivatives using
methanol both as solvent and carbon source under supercritical
condition over copperꢀdoped porous metal oxide catalysts.
1
2
3
4
5
6
7
8
9
2
b
O
3
6.5
6.5
100
95.2
98.3
98.2
95.4
ꢀ
Cu
Cu
Cu
Cu
5
5
5
5
ꢀPd
ꢀPd
ꢀPd
1
3
5
/γꢀAl
/γꢀAl
/γꢀAl
2
O
3
3
23.0
69.4
100
100
ꢀ
23.5
18.7
63.0
21.3
12.0
73.5
21.9
68.2
98.2
95.4
ꢀ
22.5
14.6
12.2
8.7
2
O
O
2
3
ꢀPd10/γꢀAl
2
O
However,
the
yield
of
benzimidazole
and
Nꢀ
Cu/γꢀAl
CuꢀNi/γꢀAl
CuꢀPt/γꢀAl O
2 3
O
methylbenzimidazole were 82% and 11% respectively, and the
2
O
3
95.7
78.1
19.4
40.8
85.0
70.7
4
4
2
3
purify procedure was tedious. Selvam reported the oneꢀpot
photocatalytic synthesis of disubstituted benzimidazoles from
FeꢀPd/γꢀAl
FeꢀPt/γꢀAl
ZnꢀNi/γꢀAl
ZnꢀPd/γꢀAl
2 3
O
1
1
1
0
1
2
2 3
O
2
O
3
10.2
52.0
N
ꢀsubstituted 2ꢀnitroanilines or 1,2ꢀdiamines with good yield
2 3
O
(
40~96%) over PtꢀTiO catalyst using solar and UVꢀA light.
2
Reaction conditions: 1 g of catalyst and 5% loading for each metal, 6 g of
nitroaniline, 100 mL of ethanol, 50 mL of H O, 453 K, 3.5 MPa, 12 hrs,
00r/min. checked by HPLC. 1% loading for Pd.
oꢀ
2
The efficiency and yield of target product were found to be
higher in UV light than in solar light.
a
b
9
Herein we wish to disclose a convenient way to synthesize
benzimidazole derivatives in moderate to excellent yields from
Then different supports were estimated (Table 2). The 5%
weight amount of Cu and Pd loaded on αꢀAl or γꢀAl as the
catalysts were used to check the effect of different alumina carriers
on this reaction. The more acidic γꢀAl support gave better result
and yield of 2ꢀmethylbenzimidazole could be reached 98.2% (entry
). The selectivity and yield of 2ꢀmethylbenzimidazole are very low
2
O
3
2 3
O
2 3
oꢀnitroaniline and alcohol catalyzed by CuꢀPd/γꢀAl O . The
catalysts are rationally designed by the cooperative catalysis
concept, where the metal center acts as dehydrogenation and
hydrogenation site while the weak acid site acts for the
condensation process.
2
O
3
2
when MgO were used as a support, maybe because alkaline MgO
support is disadvantage for the condensation of aldehyde and oꢀ
2
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ARTICLE
phenylenediamine. Use of activated carbon as a support gave slightly prepared catalyst is slightly increased. The total pore volume and
high yield (entry 4). TiO2 or CeO2 as the supports gave only average pore size were reduced, compared with that of γꢀAl O . And
2
3
moderate selectivity, compared with that of γꢀAl
2
O
3
(entries 5ꢀ6).
the spent catalyst underwent a very slight loss of surface area and
pore volume. TEM and SEM (Fig. 3) results showed that the metal
Table 2 Effect of support on this reaction
particles were loaded on γꢀAl
the average size of metal particles was about 6.9 nm (based on TEM
3
results). Fig. 4 showed Xꢀray diffraction patterns of CuꢀPd/γꢀAl O .
2 3
O surface with high dispersion, and
a
Entry
Catalyst
CuꢀPd/αꢀAl
Conversion (%)
Yield (%)
42.2
Selectivity (%)
1
2
3
4
5
6
2
O
3
73.0
100
57.8
98.2
11.7
53.1
64.2
73.3
CuꢀPd/γꢀAl
CuꢀPd/MgO
CuꢀPd/C
2
O
3
98.2
2
59.5
72.9
62.8
95.6
8.5
The XRD spectrum showed that at 2θ=41.6°, 48.3° and 71.3°, which
were attributed to obvious and weak characteristic diffraction peaks
of CuꢀPd bimetal, respectively. The peak of elemental Pd was found
at 2θ=39.8°, while the peak of elemental copper could not be found
from the XRD spectrum. From these, it is concluded that there has a
38.7
CuꢀPd/TiO
CuꢀPd/CeO
2
40.3
2
70.1
Reaction conditions: 1 g of catalyst and 5% loading for each metal, 6 g of oꢀ
nitroaniline, 100 mL of ethanol, 50 mL of H
2
O, 453 K, 3.5 MPa, 12 hrs, 900
a
r/min. checked by HPLC
2 3
little elemental palladium existing on the surface of γꢀAl O support,
In addition, the recovery and reuse of the developed CuꢀPd/γꢀ
Al O catalyst could be achieved by a simple phase separation. The
whereas the existence of copper should be as an alloy of CuꢀPd.
Compared with the fresh catalyst, no obvious change was observed
for the spent catalyst XRD spectrum, which indicated that CuꢀPd/γꢀ
2
3
catalyst can be reused for at least 6 times without loss of activity
when ethanol and oꢀnitroaniline were used, as shown in Fig. 2. The
conversion of each time is 100% and the yield is 97.3%, 96.2%,
2 3
Al O kept constant crystalline phase after reused for 6 times.
According to these characterization and recycling test results, it
could be confirmed that CuꢀPd/γꢀAl O remained stable properties
9
7.5%, 95.3%, 96.2% and 95.8%, respectively. This demonstrated
2
3
that the component, structure and surface characteristic of catalyst
are stable after several times of uses.
and performances in this oneꢀpot synthesis process. In addition,
NH ꢀTPD analysis revealed that the acidity of prepared catalyst was
3
decreased (Fig. 5).
Conversion
Yield
100
Table 3 Physical structure parameters of catalysts
8
0
Sample
Pd
Cu
Surface
Pore
Average
pore
a
a
loading
loading
(wt%)
area
volume
60
2
ꢀ1
3 ꢀ1
(
wt%)
(m g )
(cm g ) diameter
(nm)
40
2 3
γꢀAl O
2
0
-
-
124.9
139.2
0.92
0.69
26.54
19.27
CuꢀPd/γꢀAl
Fresh)
CuꢀPd/γꢀAl
2
O
O
3
3
0
5
.58
.53
4.93
1
2
3
4
5
6
(
Cycle
2
5
4.75
129.6
0.61
18.83
(
Spent)
Fig. 2 Recycling of CuꢀPd/γꢀAl
Reaction conditions: 0.8 g of catalyst, 8 g of
mL of H O, 453 K, 3.5 MPa, 12 hrs, 900 r/min.
2
O
3
for the synthesis of 2ꢀmethylbenzimidazole.
a
Based on EDS results.
o
ꢀnitroaniline, 120 mL of ethanol, 80
2
a
b
Catalyst characterization
The prepared catalyst was characterized by different techniques
to give its structure information. The physical structure parameters
2 3 2 3 2 3
of γꢀAl O , fresh CuꢀPd/γꢀAl O and spent CuꢀPd/γꢀAl O (reused
for 6 times) were measured and results are listed in Table 3. The
EDS showed that the Pd and Cu content of fresh PdꢀCu/γꢀAl O was
2
3
2 3
Fig. 3 TEM (a) and SEM (b) images of CuꢀPd/γꢀAl O
around 5%, which was in accordance with the theoretical loading.
And the metal contents of spent catalyst were very close to fresh
catalyst, indicating no obvious metal components loss during the
reaction. The BET analysis disclosed that the specific surface area of
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dehydrogenation of dihydrobenzimidazole intermediate which would
5
9
♥
♥
be usually aided by the alumina supported Cu catalyst. According
to the analysis above, it can be concluded that this coupling reaction
system was catalyzed by the synergy effect of CuꢀPd/γꢀAl O
♥
♦
CuPd
Pd
♦
♦
c
♥
♥
2
3
♥
♥
b
catalyst’s multiple roles including dehydrogenation (Cu), transfer
hydrogenation (Pd) centers, and Lewis or Bronsted acid sites of γꢀ
a
2 3
Al O .
1
0
20
30
40
50
60
70
80
2
Theta (degree)
2 3 2 3
Fig. 4 XRD patterns of (a) γꢀAl O , (b) CuꢀPd/γꢀAl O (Fresh), and (c) CuꢀPd/γꢀ
Al
2
O
3
(Spent)
Scheme 1 Possible mechanism for the direct synthesis of 2ꢀmethylbenzimidazole
from ethanol and oꢀnitroaniline
a
b
Synthesis of derivatives
With the optimized conditions in hand, several types of oꢀ
nitroaniline and alcohol derivatives were tried (Table 4). The methyl
or methoxyl substituted oꢀnitroanilines can act as good partners.
However yields of the corresponding benzimidazole derivatives
were very low, when the chloro or fluoro substituted oꢀnitroaniline
was used as the starting material. The reason may be the nucleophilic
of amino group was decreased due to electronꢀwithdrawing ability of
3
00
400
500
600
700
800
900 1000
Temperature (K)
3 2 3 2 3
Fig. 5 NH ꢀTPD profiles of (a) γꢀAl O and (b) CuꢀPd/γꢀAl O
Possible reaction mechanism
4
4,46,50ꢀ53
Based on previous reports
and experimental results, the chloro or fluoro atom. Methanol, ethanol, nꢀpropanol, nꢀbutanol,
possible mechanism is shown as below (Scheme 1). benzyl alcohol were all appropriate candidates.
Dehydrogenation of ethanol is well known to be effectively
Table 4 Scope of substrate
5
0,54ꢀ58
catalyzed by Cu
which lead to acetaldehyde and active
H
N
NH
2
Cat.
hydrogen that likely be the key intermediates for successive reaction.
Subsequently the transfer hydrogenation reduction of oꢀnitroaniline
with the adsorbed active hydrogen by Pd gave oꢀ
2
R2
+
R CH OH
2
R1
NO2
H
2
O
R1
N
1
2
a
Entry
R
R
Cat. (g)
Conversion (%)
Yield (%)
1
0,12
1
2
3
4
5
6
7
8
9
H
H
0.8
0.8
0.8
1.2
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
100
100
100
100
100
100
100
100
100
100
100
15.8
10.5
22.9
23.8
96.5
97.4
96.2
98.7
100
phenylenediamine,
which then condensed with acetaldehyde on
H
Me
Et
2
1
Lewis or Bronsted acid center to give Schiff base. The desired
product was obtained after molecular nucleophilic cyclization and
H
H
nꢀPr
Ph
H
H
dehydrogenation on the surface of PdꢀCu/γꢀAl
2 3
O . Recently, the
Me
Me
Me
98.7
97.5
97.5
97.6
98.5
96.8
solid acid such as modified zeolite and transitionꢀmetal
Me
Et
27,28,30,31
oxides
condensation process. However, in this work relative weak acidic γꢀ
Al
condensation of oꢀphenylendiamine and acetaldehyde. It may be
ascribed to that the in situ formed ꢀphenylendiamine and
was reported to be used as the acid catalyst for this
MeO
MeO
MeO
Cl
H
1
0
Me
Et
2
3
O could also successfully act as the acid center for the 11
b
1
1
1
2
3
4
H
9.3
b
Cl
Me
H
5.8
o
b
F
17.5
b
acetaldehyde which adsorbed on the catalyst surface lower the 15
F
Me
13.9
Reaction conditions: 8 g of oꢀnitroaniline, 120 mL of alcohol, 80 mL of H
2
O, 453 K,
requirements of activation energy and the catalyst acidity.
a
3
.5 MPa, 12 hrs, 900 r/min; checked by HPLC
Finally, the corresponding benzimidazole product was obtained by
4
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To further establish the general utility of these adding aqueous NaHCO
ARTICLE
3
. After maintained at this condition for 5
transformations, oꢀnitroaniline was replaced with different nitroꢀ minutes, the solid was filtered and washed to neutral by distilled
compounds for the synthesis of other heterocyclic compounds (Table water, then washed by ethanol, distilled water, successively. The
5), affording the corresponding benzoxazole, benzothiazole and resulting solid was dried in vacuo at 383 K and then calcined at 533
quinoline compound. But yields of the corresponding heterocyclic K for 3 hours, finally reduced by H at 533 K for 2 hours to give the
2
compounds were relatively low.
catalyst.
Table 5 The synthesis of other heterocyclic compounds
Characterization of catalysts
Conversion Yield
Entry
Substrate
Alcohols
Product
The bulk composition of the samples was determined via
Energyꢀdispersive Xꢀray spectroscopy (EDS) on Noran VANTAGEꢀ
ESI spectrometer. The surface area and porosity analysis of catalysts
a
(
%)
(%)
1
ethanol
100
30.7
2
were measured with N physisorption at 77 K on a Micromeritics
ASAP 2020. Transmission electron microscope (TEM) analysis was
performed with a Tencnai G2 F30 SꢀTwin instrument operating at
2
3
4
propanol
ethanol
ethanol
100
95.2
100
36.7
5.3
3
00 kV. Average particle size was calculated by 300 particles which
were randomly measured. Scanning electron microscopy (SEM) was
analysed on a Hitachi Sꢀ4700 instrument at 15 kV. Xꢀray powder
diffraction (XRD) was obtained on a Thermo ARL SCINTAG X’
TRA diffractometer using Cu Ka radiation (λ= 0.15406 nm) at 45 kV
and 40 mA. The scanning range is from 2θ=10°ꢀ80°with a scanning
78.4
ꢀ
1
Reaction conditions: 0.8 g of catalyst, 8 g of substrate, 120 mL of alcohol, 80
rate of 28 min . The diffraction peaks obtained were referred to the
JCPDS cards. Prior to each experiment, catalyst (0.075 g) was
heated in a flow of He (30 ml/min) at 533 K for 40 min, followed by
cooling to 323 K under He flow. Then, the catalyst was exposed to a
a
mL of H
2
O, 453 K, 3.5 MPa, 16 hrs, 900 r/min; checked by HPLC.
Conclusions
flow of 100% NH
the catalyst was heated linearly at 10 K·min to 973 K in a flow of
He, and NH (m/e=17) in the outlet gas were analyzed by the mass
3
for 30 min. After being purged in He for 60 min,
In conclusion, the direct synthesis of benzimidazole from the oꢀ
ꢀ
1
nitroaniline and alcohol was successfully established by tuning the
types of metal and support. The combination of Cu and Pd was
responsible for the dehydrogenation of alcohol and hydrogenation of
3
spectrometer (BELMASS).
2 3
oꢀnitroaniline. The γꢀAl O carrier supplied the Lewis or Bronsted
Synthesis of benzimidazoles
acid center to promote the condensation of in situ formed oꢀ
phenylenediamine and aldehyde, which gave the desired
benzimidazole derivatives after oxidation dehydrogenation. The
method described here has the advantage of easily available starting
materials, high efficiency, and simple procedure.
The reaction was performed in a 500 mL stainless steel
autoclave, which was charged with 8 g of the nitroaromatic
compound,120 ml alcohol, 80 ml deionized water and 0.8 g of the
catalyst. The batch reactor was initially purged five times with pure
N
2
to replace the air in the system. The reactor was then heated to
the desired temperature at the desired N pressure and at a stirring
2
Experimental
rate of 900 rpm. After the complete conversion of the reactant, the
catalyst was filtered from the mixture for subsequent use, and the
corresponding product was separated from the aqueous phase using a
separatory funnel. The products were identified by gas
chromatographyꢀmass spectrometry (GCꢀMS, Agilent 5973N) and
were analyzed by HPLC using a Baseline C18 column (150mm×
Catalyst preparation
The CuꢀPd/γꢀAl
precursors of aqueous solution of Cu(NO
PdCl (0.05 gmetal/mL). After γꢀAl
distilled water for 10 minutes at 80 , H
dropwise followed by aqueous solution of Cu(NO
solution at 80 for 5 hours. Then pH value was adjusted to 8ꢀ9 by
2
O
3
catalyst was prepared by impregnation with
(0.05 gmetal/mL) and
support was stirred in
PdCl solution was added
, and kept the
3 2
)
H
2
4
2 3
O
℃
2
4
2
.1nm) with a mobile phase of ethanolꢀethyl acetateꢀwater (7:1:2 by
3 2
)
℃
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1
volume) at a 1.2 mL min constant flow. Quantitative analysis was 24 I. Bhatnagar and M. V. George, Tetrahedron, 1968, 24, 1293-
1
298.
conducted using the areaꢀnormalization method.
2
2
5 H. Sharghi, O. Asemani and S. M. H. Tabaei, J. Heterocycl.
Chem., 2008, 45, 1293-1298.
6 R. Fazaeli and H. Aliyan, Appl. Catal. A: Gen., 2009, 353, 74-
Recycling tests
7
9.
After a typical catalytic run (8 g oꢀnitroaniline, 120 mL ethanol, 27 L. S. Gadekar, B. R. Arbad and M. K. Lande, Chin. Chem. Lett.,
010, 21, 1053-1056.
2
8
2 2 3
0 mL H O, 0.8 g CuꢀPd/γꢀAl O catalyst, 453 K, 3.5 MPa, 900
2
2
3
3
8 K. W. Ahmad, S. A. Majid and T. Radha, Res. J. Chem. Environ.
2013, 17, 40-45.
9 M. A. Chari, D. Shobha, E. Kenawy, S. S. Al-Deyab, B. V. S.
Reddy and A. Vinu, Tetrahedron Lett., 2010, 51, 5195-5199.
r/min, 12 h), the catalyst was filtered, additionally washed with
alcohol and deionized water, and reused after dried in vacuo at 383
K.
0 R. V. Shingalapur and K. M. Hosamani, Catal. Lett., 2010, 137
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,
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1 S. B. Rathod, M. K. Lande and B. R. Arbad, Bull. Korean Chem.
Soc., 2010, 31, 2835-2840.
32 F. K. Behbahani, P. Ziaei, Z. Fakhroueian and N. Doragi,
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3 M. Rekha, A. Hamza, B. R. Venugopal and N. Nagaraju, Chin.
J. Catal., 2012, 33, 439-446.
4 A. Teimouri, A. N. Chermahini, H. Salavati and L. Ghorbanian,
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Acknowledgements
Financial support by the National Natural Science Foundation
3
3
3
3
3
3
3
4
4
of China (21406199 and 21476208) is gratefully acknowledged.
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9 D. Damodara, R. Arundhathi and P. R. Likhar, Adv. Synth.
Catal., 2014, 356, 189–198.
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The coupling of multi-step reaction catalyzed by heterogeneous catalyst is an important path to
accomplish some unconventional chemical transformations. Since the starting materials
generated from previous steps were adsorbed on the catalyst, the activation energy
requirements of following steps were largely decreased. Thus the reaction conditions were more
2 3
mild and environmental friendly. Catalyzed by multifunctional Cu-Pd/γ-Al O solid acid catalyst,
the transfer hydrogenation and successive cyclization coupling reaction from o-nitroaniline and
alcohol to afford benzimidazole derivatives in high yield was realized. The catalyst could be
reused for several times without loss of activities. The synergies of reforming hydrogenation of
Cu-Pd bimetal and support acidity of γ-Al O were responsible for this catalytic transformation.
2
3