Copper supported on H+-modified manganese oxide octahedral molecular sieves (Cu/H-OMS-2) as a heterogeneous biomimetic catalyst for the synthesis of imidazo[1,2-a]-N-heterocycles

Article abstract of DOI:10.1039/c5cy01433f

Copper supported on acid-modified manganese oxide octahedral molecular sieves (Cu/H-OMS-2) was prepared and found to be a versatile catalyst for the oxidative synthesis of 3-aroylimidazopyridines with a broad substrate scope. Cu/H-OMS-2 that was character

Full text of DOI:10.1039/c5cy01433f

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Copper supported on H+-modified manganese
oxide octahedral molecular sieves (Cu/H-OMS-2)
as a heterogeneous biomimetic catalyst for the
synthesis of imidazoij1,2-a]-N-heterocycles†
Cite this: DOI: 10.1039/c5cy01433f
a
a
b
c
a
*
Xu Meng, Jinqi Zhang, Baohua Chen, Zhenqiang Jing and Peiqing Zhao
Copper supported on acid-modified manganese oxide octahedral molecular sieves (Cu/H-OMS-2) was
prepared and found to be a versatile catalyst for the oxidative synthesis of 3-aroylimidazopyridines with a
broad substrate scope. Cu/H-OMS-2 that was characterized by BET, XRD, XPS, FTIR, TEM, SEM, H2-TPR
and O2-TPD techniques could also be used to synthesize 3-aroylimidazopyrimidines and applied in one-
pot, three-component reactions of ketones, aldehydes and 2-aminopyridines. The catalytic system
employs low loading Cu as the catalytic metal and support H-OMS-2 as the electron-transfer mediator
(ETM) to sequentially lower the redox energy barrier, which generates a low-energy pathway and enables
the reaction to proceed in a biomimetic way. Moreover, Cu/H-OMS-2 could be reutilized 4 times with a
slight decrease in the catalytic activity.
Received 28th August 2015,
Accepted 21st October 2015
DOI: 10.1039/c5cy01433f
www.rsc.org/catalysis
mediator (ETM)8 at the same time can interact with the
supported catalytic metal (M) to lower the energy barrier of
Introduction
Within the past decades, OMS-2, microporous tunnel-
structured manganese oxide octahedral molecular sieves, has
attracted great interest in catalysis as well as materials and
environmental science.1 In particular, OMS-2 has been treated
as a versatile redox catalyst in various oxidations because it
has superior properties, such as excellent structural stability,
mixed valence, large surface areas, electron-conducting prop-
erties and oxygen reduction abilities.2,3 Meanwhile, modified
OMS-2 via ion exchange has also been prepared and applied
in oxidations.4 Moreover, OMS-2 has been employed as a sup-
port of the supported catalysts in heterogeneous catalysis,
which was fruitfully used in CO oxidation,5 VOC combustion6
and the removal of HCBz.7 Therefore, developing useful and
sustainable catalytic systems by the use of OMS-2 is desirable
and challenging.
an oxidation in theory as long as the redox potential of OMS-
2 is between the potential of Mred/Mox and green oxidant O2/
H2O (Scheme 1).9 In this way, electrons of an oxidative trans-
formation can rapidly transfer from the substrate to the oxi-
dant in a biomimetic way by the use of ETM, which enhances
the catalytic efficiency, lowers the catalytic metal loading,
enables O2 to be an efficient oxidant and produces water as
the sole by-product. On the basis of this strategy, we previ-
ously developed the heterogeneous biomimetic CuOx/OMS-2-
catalyzed synthesis of 3-iodoimidazopyridines using air as the
oxidant under a low-energy pathway.9a Likewise, Mizuno's
group previously reported a biomimetic homocoupling of
alkynes catalyzed by CuIJOH)x/OMS-2.9b Our continued inter-
est in this field prompts us to discover other efficient and
novel OMS-2-based catalytic systems that are able to sequen-
tially decrease the redox energy barriers in oxidative
transformations.
From the perspective of redox potential, mixed valent
OMS-2 that works as a support and an electron-transfer
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou
Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese
Academy of Sciences, Lanzhou 730000, China. E-mail: zhaopq@licp.cas.cn;
Fax: + 96 931 8277008; Tel: + 86 931 4968688
b State Key Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou, 730000, China
c Suzhou Institute of Nano-Tech and Nano-Bionic (SINANO), Chinese Academy of
Sciences, Suzhou 215123, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy01433f
Scheme 1 The oxidations involve the use of ETM under a long-energy
pathway.
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Imidazoij1,2-a]pyridines are very important structural
spectrometry (AAS) showed that the quantities of Cu in the
sample was 1.58 wt%. The BET surface areas and porosities
of Cu/H-OMS-2 were determined by N2 adsorption–desorp-
tion at 77 K, and the results showed that the BET surface
area is 87 m2 g−1, the pore volume is 0.4 cm3 g−1 and the pore
size is 136 Å. X-ray diffraction (XRD) was employed to charac-
terize OMS-2, H-OMS-2 and Cu/H-OMS-2 (Fig. 1, left). XRD
patterns showed that the diffraction peaks of H-OMS-2 and
Cu/H-OMS-2 were the same as that of OMS-2, which means
both Cu/H-OMS-2 and its support are typical cryptomelane
materials (JCPDS file #29-1020). No signals due to copper
metal (cluster) or copper oxide were observed, which indi-
cates that the copper oxide was low loading and highly dis-
persed on H-OMS-2. Notably, the diffraction peaks of Cu/H-
OMS-2 and H-OMS-2 became slightly narrower and sharper
compared with OMS-2, which suggests that their crystallite
sizes increased. The lattice vibrational behaviour of the Cu/H-
OMS-2 was studied by FTIR spectroscopy to detect the effect
of Cu and H+ substitution on the spectral features of OMS-2.
As shown in Fig. 1 (right), we obtained similar IR spectra
from OMS-2, H-OMS-2 and Cu/H-OMS-2. All samples
displayed four characteristic bonds at 714 cm−1, 606 cm−1,
521 cm−1 and 465 cm−1 with comparable relative intensities
that can be ascribed to Mn–O lattice vibration modes in
MnO6 octahedra.16 Additionally, TEM images showed that
H-OMS-2 and Cu/H-OMS-2 (for the SEM image, see the ESI,†
Fig. S1) have a typical nano-rod morphology,4 which means
the acid modification and the supported copper do not
change the structure of OMS-2 (Fig. 2).
The redox ability of Cu/H-OMS-2 was measured by means
of H2-TPR, while support H-OMS-2 and OMS-2 were also
examined for comparison. For all OMS-2-based materials, the
reduction of MnO2 to MnO proceeds in two different steps
(MnO2 to Mn3O4 to MnO) generally, but these processes over-
lap sometimes.5a As shown in Fig. 3, the overlapping peaks
(a) from 369 °C to 395 °C are ascribed to the reduction of
MnO2 to MnO of OMS-2. For H-OMS-2, the two reduction
peaks (b and b′) of MnO2 which were demonstrated at 345 °C
and 387 °C shifted to lower temperatures, suggesting that the
reducibility of H-OMS-2 was improved by H+ modification.
Furthermore, two new peaks (α and β) appeared at tempera-
tures in the range of 250–300 °C, which are ascribed to the
partial reduction of MnO2 species derived from the interac-
tion between H+ and MnO2.5a Specifically, the α peak
motifs and have been found in various biologically active
and pharmaceutical compounds because they are antiviral,
antimicrobial, antitumour, anti-inflammatory, antiparasitic,
hypnotic, etc.10 Many commercial drugs involve imidazo-
pyridines, like zolpidem, zolimidine, alpidem, saripidem and
necopidem.11 Especially, 3-aroylimidazoij1,2-a]pyridines exhibit
excellent anticancer properties.12 The most traditional method
of obtaining 3-aroylimidazoij1,2-a]pyridines is functionalization
of imidazoij1,2-a]pyridines in three steps, including
formylation, Grignard reaction and oxidation.12a Most recently,
homogeneous Cu-catalyzed oxidative cyclizations for the syn-
thesis of 3-aroylimidazoij1,2-a]pyridines from chalcones and
2-aminopyridines via C–N bond forming were developed.13
Nevertheless, the aforementioned homogeneous catalysis gen-
erally produces a small amount of by-products, like Michael
adducts and another cyclized product, naphthyridines,14
because of the high level of activity derived from its uniformity
on a molecular level and solubility in the reaction medium.
In addition, the separation, recovery and recycling of cata-
lysts are relatively hard to achieve in homogeneous reac-
tions.15 So, there is an incentive to develop a heterogeneous
process able to produce 3-aroylimidazoij1,2-a]pyridines by an
environmentally friendly concept while minimizing the by-
products and wastes.
As a part of our continuing efforts on developing efficient
heterogeneous catalytic systems and their applications in
organic transformations,3e,9a we now present the study of
immobilized copper on H+-modified OMS-2 as a catalyst
(Cu/H-OMS-2) that can sequentially decrease the energy
barrier of the oxidation and generate a low-energy pathway in
the synthesis of 3-aroylimidazoij1,2-a]pyridines in air (Scheme 2).
The heterogeneous system can also tolerate 2-aminopyrimidine
as a substrate and offer 3-aroylimidazoij1,2-a]pyrimidines in
good yields. Furthermore, the desired heterocycles can be
obtained via one-pot, three-component reactions of alde-
hydes, ketones and aminopyridines using Cu/H-OMS-2.
Results and discussion
Cu/H-OMS-2 was prepared via wet impregnation using
CuIJNO3)2·3H2O as the copper precursor in deionized water.
Elemental analysis of the catalyst by atomic absorption
Scheme 2 Cu-catalyzed multistep oxidation using H-OMS-2 as an
ETM for the synthesis of imidazoij1,2-a]pyridines and imidazoij1,2-
a]pyrimidines.
Fig. 1 XRD (left) and IR spectra (right) of OMS-2, H-OMS-2 and Cu/H-
OMS-2.
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interaction compared with the interaction between oxygen
atoms and MnIJIV), so two different oxygen species were
released at middle temperature and high temperature,
respectively, during TPD.
The oxidative cyclization between 2-aminopyridine and
chalcone was selected as a model reaction to test the catalytic
activity of Cu/H-OMS-2 (Table 1). From a synthetic point of
view, the copper-catalyzed cyclization of 1a and 2a will lead
to 3-aroylimidazoij1,2-a]pyridine as the desired product with
3aa and another product, naphthyridine, which comes from
a competitive cyclization, as by-products.14 Michael adduct
3aa actually is the intermediate during the formation of 3a,
which means the high yield of 3a depends on the efficient
formation of 3aa firstly and its subsequent intermolecular
oxidative cyclization. So, we screened different parameters,
such as the catalyst, the solvent and the temperature, to
determine the best catalytic system for obtaining high yield
of 3-aroylimidazoij1,2-a]pyridine. In the initial experiment,
OMS-2 was treated with reactants in DCB at 80 °C in air for
20 h, which did not show any catalytic activity (Table 1,
entry 1). When we switched the catalyst to Cu/OMS-2 under
the same conditions, 3a and 3aa were both obtained in low
yields (Table 1, entry 2). However, the formation of 3a failed
to proceed using CuIJOH)x/OMS-2 as a catalyst (Table 1, entry
3). An attempt to employ Cu/H-OMS-2 (12 mg, 0.7 mol%, Cu:
1.58 wt%) in the synthesis of 3-aroylimidazoij1,2-a]pyridine
Fig. 2 TEM images of H-OMS-2 (A) and CuO/H-OMS-2 (B).
Fig. 3 H2-TPR of OMS-2, H-OMS-2 and Cu/H-OMS-2.
probably corresponds to the reduction of MnO6-□ (where □
stands for oxygen vacancies) and the β peak likely corre-
sponds to the reduction of MnO6-K+ and MnO6-H+. For Cu/H-
OMS-2, four reduction peaks (c, c′, α′ and β′) all shifted to
lower temperatures compared with support H-OMS-2 and
OMS-2, implying further enhanced redox ability. In particu-
lar, the α′ peak generally corresponds not only to the reduc-
tion of well-dispersed CuO particles but also to the partial
reduction of MnO2 species. The improved reduction of MnO2
is due to the spillover hydrogen from copper atoms to man-
ganese oxides. It is believed that β′ peak corresponds to the
combined reduction of large CuO particles and MnO2.5a Cu/
H-OMS-2 has lager α′ and β′ peaks and more obvious c and c′
peaks, which probably indicates that Cu2+–O–Mn4+ entities
were formed at the interface between CuO and H-OMS-2 and
the mobility of oxygen was promoted by the electronic delo-
calization effect between Cu and Mn species.5b,7 Further-
more, the XPS of Mn 2p from H-OMS-2 and Cu/H-OMS-2 also
indicates that there is an interaction between copper and H-
OMS-2 (for detailed discussion, see the ESI,† Fig. S2). The
phenomenon of electron transfer between the catalytic metal
copper and the electron-transfer mediator H-OMS-2 probably
can be proved from these observations.
Then, O2-TPD technology was used to analyze the oxygen
species of Cu/H-OMS-2. As shown in Fig. S3 (see the ESI†),
very similar peaks from the desorption of H-OMS-2 and Cu/
H-OMS-2 were obtained, and the majority of oxygen species
can be ascribed to lattice oxygen.7 Specifically, the similar
and clear shoulder peaks at around 500 °C can be ascribed to
surface oxygen species or the labile oxygen species. The peaks
at around 600 °C and 750 °C result from the transformation
of cryptomelane to Mn2O3 and further to Mn3O4.17 Oxygen
atoms in the framework bound to MnIJIII) have a weaker
Table 1 Optimization of reaction conditionsa
Yieldb (%)
T
Entry
Catalyst
Solvent
[°C]
3a
3aa
1
OMS-2
Cu/OMS-2
DCB
DCB
DCB
DCB
DCB
DMSO
MeNO2
Toluene
80
80
80
0
15
0
10
23
8
8
13
0
41
89
12
10
<5
42
0
76
<5
<5
0
28
<5
18
<5
12
<5
25
59
0
0
0
0
0
16
0
0
40
45
2c
3d
4
5
6
CuIJOH)x/OMS-2
Cu/H-OMS-2
Cu/H-OMS-2
Cu/H-OMS-2
Cu/H-OMS-2
Cu/H-OMS-2
Cu/H-OMS-2
Cu/H-OMS-2
Cu/H-OMS-2
Cu/H-OMS-2
Cu/H-OMS-2
Cu/H-OMS-2
Cu/H-OMS-2
Cu/H-OMS-2
Cu/H-OMS-2
CuO + H-OMS-2
CuO
80
100
100
100
100
100
100
100
100
100
100
100
70
7
8
9e
HOAc
10
11f
12f
13f
14f
15f,g
16f
17f
18f,h
19f,i
Cl2CHCHCl2
Cl2CHCHCl2/HOAc
Xylene/HOAc
Dioxane/HOAc
DMF/HOAc
Cl2CHCHCl2/HOAc
Cl2CHCHCl2/HOAc
Cl2CHCHCl2/HOAc
Cl2CHCHCl2/HOAc
Cl2CHCHCl2/HOAc
120
100
100
a
Reaction conditions: 1a (0.6 mmol), 2a (0.4 mmol), catalyst (12
b
c
mg), solvent (1.2 mL), air, 20 h. Isolated yields. 12 mg of Cu/OMS-
2 (Cu: 1.65 wt%) was used. d 12 mg of CuIJOH)x/OMS-2 (Cu: 1.45 wt%)
e
f
was used. For 3 h. 0.1 mL of HOAc was used to comprise the
mixed solvent. Under O2. 1 mol% of CuO and 12 mg of H-OMS-2
were used. i 0.4 mmol of CuO was used.
g
h
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Table 2 Synthesis of 3-aroylimidazoij1,2-a]pyridines catalyzed by Cu/H-
was also unfruitful, and 3a was produced in 10% yield with
18% yield of 3aa (Table 1, entry 4). To our delight, increasing
the reaction temperature to 100 °C enhanced the yield of 3a
to 23% with a trace amount of 3aa when Cu/H-OMS-2 was
used as a catalyst (Table 1, entry 5). Next, a lot of solvents
were examined to further better the yield of 3a at 100 °C with
Cu/H-OMS-2 (Table 1, entries 6–10). It was found that apolar
Cl2CHCHCl2 gave the highest yield of 3a (41%) in the absence
of by-products and HOAc provided 0% yield of 3a but 59%
yield of 3aa within 3 h in the presence of Cu/H-OMS-2
(Table 1, entries 9 and 10). Therefore, we envisioned that the
mixed solvent of HOAc and Cl2CHCHCl2 probably could effi-
ciently transform 3aa and give 3a in high yield because HOAc
is suited for the generation of Michael adduct 3aa and able
to enhance the electrophilic ability of copper while
Cl2CHCHCl2 is superior for the oxidative cyclization. As
expected, when we added 0.1 mL of HOAc into 1.2 mL of
Cl2CHCHCl2 as a mixed solvent in the reaction, the desired
product 3a was obtained in 89% yield without 3aa and other
by-products (Table 1, entry 11). Subsequently, a series of
mixed solvents containing HOAc were investigated, but the
results were not satisfying (Table 1, entries 12–14). Also, we
found that the oxygen atmosphere did not better the yield of
the reaction (Table 1, entry 15). Next, the reaction tempera-
ture was changed and the results demonstrated that the reac-
tion did not proceed below 80 °C and higher temperature did
not improve the reaction (Table 1, entries 16 and 17). Finally,
control experiments of the catalysts were run in the mixed
solvent in air at 100 °C. The physical mixture of bulk CuO
and H-OMS-2 only gave Michael adduct 3aa as the main
product, while 3a was barely observed by the use of a stoi-
chiometric amount of bulk CuO in the reaction (Table 1,
entries 18 and 19). These observations suggest that the highly
dispersed copper species on H-OMS-2 play a key role in the
oxidative cyclization and there is an interaction between
supported copper and H-OMS-2. As a result of the optimiza-
tion, we concluded that the best reaction conditions for cycli-
zation of 2-aminopyridine and chalcone in a mixed solvent of
Cl2CHCHCl2/HOAc involve the use of Cu/H-OMS-2 as the cata-
lyst at 100 °C for 20 h with air as the oxidant.
OMS-2a
Product
R1
R2
R3
Yieldb (%)
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
3s
3t
3u
3v
3w
—
—
H
H
H
H
H
H
H
H
4-Cl
4-Cl
H
2-Cl
4-OMe
H
H
76
82
88
<5
79
65
45
84
60
72
71
<5
70
66
64
69
53
39
45
42
55
52
0
4-Cl
4-Cl
H
H
4-OMe
3,4-diOMe
H
4-Me
H
4-Cl
H
4-Cl
H
4-Cl
4-Cl
4-Me
4-Cl
4-Cl
4-Cl
4-Cl
4-Cl
H
H
4-NO2
4-F
H
4-Cl
2-Cl
4-Cl
H
H
3-Me
3-Me
4-Me
4-Me
5-Me
5-Me
5-Me
5-Me
4-CF3
3-Br
5-Br
4-Cl
5-Cl
5-COOMe
3-CN
4-Cl
H
4-F
4-Cl
4-Cl
4-Cl
4-Cl
4-Cl
H
H
H
0
a
Reaction conditions: 1 (0.6 mmol), 2 (0.4 mmol), Cu/H-OMS-2
(12 mg, 0.7 mol%), Cl2CHCHCl2 (1.2 mL), HOAc (0.1 mL), 100 °C,
20 h, air. b Isolated yields.
importantly, many multi-halogen-substituted 3-aroylimidazo-
ij1,2-a]pyridines were obtained in 42–64% yields, which pro-
vides opportunities to further functionalize these halogen-
substituted products using coupling techniques.18 Neverthe-
less, 2-aminopyridines with sensitive groups, like –COOMe
and –CN, failed to proceed under the present system because
the HOAc of the solvent decomposed these substrates
quickly.
In order to further study the generality of this heteroge-
neous process, 2-aminopyrimidine 5 was used as a substrate
to synthesize 3-aroylimidazoij1,2-a]pyrimidines. Gratifyingly,
2-aminopyrimidine worked very well with chalcones and good
yields of 3-aroylimidazoij1,2-a]pyrimidines 4a–4d were
obtained fruitfully (Table 3). To the best of our knowledge,
this is the first example that a heterogeneous copper-based
material catalyzed the synthesis of 3-aroylimidazoij1,2-
a]pyrimidines between 2-aminopyrimidine and chalcones via
oxidative C–N bond forming.13d However, steric hindrance
affected the reactions significantly (Table 3, 4e) and the
electron-rich substrate, dimethoxychalcone, did not partici-
pate in the reaction at all. Unfortunately, 2-aminopyrazine
and 2-aminothiazole were not tolerant under the present cata-
lytic system.
With the optimized conditions in hand, we investigated
the substrate scope of the cyclization (Table 2). Firstly, vari-
ous substituted chalcones were tolerable in the reactions and
gave the corresponding 3-aroylimidazoij1,2-a]pyridines in mod-
erate to good yields (Table 2, 3a–3j). The Michael adduct by-
products were not observed in all cases. Generally, chalcones
with electron-withdrawing substitutes offered higher yields of
desired products than electron-rich chalcones did. Sterically
hindered chalcone, like 2-chlorochalcone, did not proceed in
the reaction (Table 2, 3e). On the other hand, a number of
substituted 2-aminopyridines smoothly participated in the
reactions and provided desired products in good yields with-
out the observation of Michael adducts. Specifically,
2-aminopyridines with electron-donating groups showed
higher activity in comparison with the ones with electron-
withdrawing groups, such as halogens (Table 2, 3k–3w). More
From a synthetic point of view, to make this heteroge-
neous approach to 3-aroylimidazoij1,2-a]pyridines more
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Table 4 The recyclability of Cu/H-OMS-2a
Table 3 Synthesis of 3-aroylimidazoij1,2-a]pyrimidines catalyzed by Cu/
H-OMS-2a
Run
1
2
3
4
5
Isolated yield (%)
82
78
70
65
48
a
Reaction conditions: 2-aminopyridine (0.6 mmol), chalcone
(0.4 mmol), Cu/H-OMS-2 (12 mg, 0.7 mol%), Cl2CHCHCl2 (1.2 mL),
HOAc (0.1 mL), 100 °C, 20 h, air.
4a, 76%
4b, 82%
4e, 25%
4c, 81%
the catalyst, which showed 0.5 ppm of copper leached from
Cu/H-OMS-2. The leached copper from the catalyst perhaps
incurs the loss of the catalytic activity. Furthermore, a hot fil-
tration experiment (for experiment detail, see the ESI,†
Scheme S1) showed that the leached copper species in the
solution is catalytically inactive and cannot trigger the reac-
tion at all. Therefore, it is believed that the observed catalysis
is derived from the solid catalyst rather than the leached cop-
per species, and the catalyst is heterogeneous in nature.
N.R.
4d, 46%
a
Reaction conditions: 5 (0.6 mmol), 2 (0.4 mmol), Cu/H-OMS-2
(12 mg, 0.7 mol%), Cl2CHCHCl2 (1.2 mL), HOAc (0.1 mL), 100 °C,
20 h, air, isolated yields.
attractive, we investigated their formation via a one-pot,
three-component reaction sequence that avoids the isolation
of the chalcone intermediate. In Cl2CHCHCl2, acetophenone
reacted with aldehyde catalyzed by Cu/H-OMS-2 in the pres-
ence of 2-aminopyridine for 2 h at 100 °C. Then, HOAc was
added into the mixture and the reaction was run for 20 h
again in air. In this way, this one-pot synthesis gave the
corresponding 3-aroylimidazoij1,2-a]pyridines in 52–75%
yields without using any additives (Scheme 3).
Conclusions
In conclusion, we have employed Cu/H-OMS-2 as an efficient
catalyst in the heterogeneous biomimetic oxidative synthesis
of 3-aroylimidazoij1,2-a]pyridines and 3-aroylimidazoij1,2-
a]pyrimidines. The reactions proceed in a mixed solvent of
Cl2CHCHCl2 and HOAc in the presence of a very low amount
of catalyst (Cu: 0.7 mol%) and air as the oxidant without the
use of bases and ligands. Moreover, the catalyst can also be
applied in one-pot reactions of ketones, aldehydes and
2-aminopyridines. More importantly, the results from H2-TPR
prove that the redox ability of Cu/H-OMS-2 is significantly
improved because of the electronic interaction between Cu
and H-OMS-2. For the catalyst, the low loading Cu is used as
a catalytic metal and H-OMS-2 is used as an ETM as well as
an excellent support, which sequentially decreases the redox
energy barrier during the oxidative transformation to make
the electrons transfer rapidly. Finally, the highly dispersed
Cu/H-OMS-2 is naturally heterogeneous and recyclable.
Finally, recovery and recyclability of the heterogeneous
catalyst
were
studied
by
the
reaction
between
2-aminopyridine and 4,4′-dichlorochalcone using 0.7 mol% of
Cu/H-OMS-2 in the mixed solvent at 100 °C for 20 h in air.
After each run, Cu/H-OMS-2 was easily isolated by centrifuga-
tion, then washed by water and finally dried at 110 °C for
the next experiment. The image of TEM demonstrated that
the morphology of the used Cu/H-OMS-2 is not changed and
it is stable on the structure (see the ESI,† Fig. S4). The experi-
mental results showed that Cu/H-OMS-2 could be reutilized
four times with an increasing decrease in the catalytic activity
and the activity of the catalyst significantly dropped to 48%
after four runs (Table 4). After the first run, inductively
coupled plasma-atomic emission spectroscopy (ICP-AES) was
conducted to analyze the reaction solution after filtration of
Experimental section
Unless otherwise noted, all reagents were purchased from
commercial suppliers and used without further purification.
Metal salts were commercially available and were used
directly. All experiments were carried out under air condi-
tions. Flash chromatography was carried out with Merck sil-
ica gel 60 (200–300 mesh). Analytical TLC was performed
with Merck silica gel 60 F254 plates, and the products were
visualized by UV detection. 1H NMR and 13C NMR (400 and
100 MHz, respectively) spectra were recorded in CDCl3.
Chemical shifts (δ) are reported in parts per million using
TMS as the internal standard, and spin–spin coupling
Scheme 3 One-pot synthesis of 3-aroylimidazoij1,2-a]pyridines from
ketone, aldehyde and 2-aminopyridine.
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constants ( J) are given in hertz. Cu/OMS-2 (ref. 9a) (2.0 wt%
theoretical loading, actual loading is 1.65 wt%) was synthe-
sized by wet impregnation in deionized water, and CuIJOH)x/
OMS-2 (ref. 9b) (2.0 wt% theoretical loading, actual loading is
1.45 wt%) was made by deposition–precipitation in water.
4 h followed by calcination at 350 °C in air for 2 h. The black
powder Cu/OMS-2 (2.0 wt% theoretical loading, the actual
loading is 1.58 wt%) was characterized by ICP-OES, BET,
XRD, XPS, FTIR, TEM, SEM, H2-TPR and O2-TPD techniques.
General procedure for Cu/H-OMS-2-catalyzed 3-aroylimidazo-
Catalyst characterization methods
ij1,2-a]pyridines synthesis
The crystal phase and composition of catalysts were deter-
mined by powder X-ray diffraction using an X-Pert PRO X-ray
diffractometer with CuKα radiation in the 2θ range of 10–
90°. Infrared spectra of the materials were recorded with cal-
cined powders dispersed in KBr (2 mg of the sample in 300
mg of KBr) using a Perkin-Elmer One FTIR spectrometer with
a resolution of 4 cm−1 operating in the range 500–2000 cm−1
with 4 scans per spectrum. The morphologies of the samples
were characterized by using a TF20 transmission electron
microscope and SM-5600LV scanning electron microscope.
Cu/H-OMS-2 (12 mg, 0.7 mol%), 2-aminopyridine (0.6 mmol),
chalcones (0.4 mmol) and Cl2CHCHCl2 (1.2 mL)/HOAc (0.1
mL) were added to a flask with a bar. The flask was stirred at
100 °C for 20 h in air. After cooling to room temperature, the
mixture was diluted with ethyl acetate and filtered. The fil-
trate was removed under reduced pressure to get the crude
product, which was further purified by silica gel chromatog-
raphy (petroleum/ethyl acetate = 4/1 as the eluent) to yield
the corresponding product. The identity and purity of the
products were confirmed by 1H and 13C NMR spectroscopic
analyses.
Nitrogen
adsorption–desorption
measurements
were
performed at 76 K using an ASAP 2020M analyzer utilizing
the BET model for the calculation of specific surface areas.
The reducibility of the catalysts was measured by the hydro-
gen temperature-programmed reduction (H2-TPR) technique.
50 mg of OMS-2, H-OMS-2 or Cu/H-OMS-2 was placed in a
quartz reactor that was connected to a TPR apparatus, and
the reactor was heated from r.t. to 550 °C with a heating rate
of 10 °C min−1. The reducing atmosphere was a mixture of
Acknowledgements
We gratefully acknowledge the National Natural Science Foun-
dation of China (Grant No. 21403256, 21573261, and
21372102) and the Suzhou Industrial Technology and Innova-
tion Project (Grant No. SYG201531).
H2 and N2 with a total flow rate of 30 mL min−1 and the Notes and references
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