Copper containing nanosilica thioalated dendritic material: A recyclable catalyst for synthesis of benzimidazoles and benzothiazoles

Article abstract of DOI:10.1002/aoc.3937

In this paper, the design and characterization of a new heterogeneous catalyst by incorporation of copper ions into the nanosilica modified by thiole–based dendrimer are reported. The prepared catalyst was characterized by FT–IR, TGA, elemental analysis,

Full text of DOI:10.1002/aoc.3937

Received: 4 April 2017
DOI: 10.1002/aoc.3937
Revised: 9 May 2017
Accepted: 23 May 2017
F U L L PA P E R
Copper containing nanosilica thioalated dendritic material: A
recyclable catalyst for synthesis of benzimidazoles and
benzothiazoles
Maryam Zakeri | Majid Moghadam
| Valiollah Mirkhani | Shahram Tangestaninejad
|
Iraj Mohammadpoor‐Baltork | Zari Pahlevanneshan
Department of Chemistry, Catalysis
In this paper, the design and characterization of a new heterogeneous catalyst by
incorporation of copper ions into the nanosilica modified by thiole–based dendrimer
are reported. The prepared catalyst was characterized by FT–IR, TGA, elemental
analysis, FE–SEM, TEM, XPS and ICP–OES techniques. This material was used
as catalyst in the synthesis benzimidazoles and benzothiazoles by the reaction of
substituted benzaldehydes with 1,2–diaminobenzene or 2–aminothiophenol,
respectively. The advantages of the present catalytic system are high yields, mild
conditions and short reaction times. On the other hand, this new synthesized catalyst
was recycled very well and reused several times without significant loss of its
catalytic activity.
Division, University of Isfahan, Isfahan
1746‐73441, Iran
8
Correspondence
Majid Moghadam, Valiollah Mirkhani and
Shahram Tangestaninejad, Department of
Chemistry, Catalysis Division, University of
Isfahan, Isfahan, 81746‐73441 Iran.
Email: moghadamm@sci.ui.ac.ir;
mirkhani@sci.ui.ac.ir; stanges@sci.ui.ac.ir
KEYWORDS
benzimidazole, benzothiazole, dendritic polymer, nanosilica
1
| INTRODUCTION
to be the most important part of these catalytic reactions. The
mentioned metals have been employed to catalyze both C–C
Benzimidazoles and benzothiazoles are of considerable
and C–X (X = heteroatom) synthesis from the starting C–H
[1–3]
[34–36]
importance in biological applications.
These compounds
bond.
Recently, much more attention has been attracted
are crucial core structures used to produce drugs and
materials. They show medicinal activities such as antican-
into reducing the use of these precious metals in catalytic reac-
tions due to their cost and limited availability. Copper is a great
[4]
[5]
[6]
[7]
cer, antiulcer, antihypertensive, antibacterial, and
substitution in this case because of its low toxicity and low
[
8,9]
[37,38]
enzyme inhibition.
They have also been applied in
price
This metal is the key catalyst in different useful
[
10]
[11]
dyes,
ence.
chemosensing,
Considering the importance of these heterocycles
fluorescence, and corrosion sci-
methods for the synthesis of biological molecules and chemical
[
12–14]
[39–42]
compounds. For example, it is applied for N–arylation,
[47,48]
[
43,44]
[45,46]
in vital applications, proposing of milder, novel, sustainable
and cheaper processes for the design and assembly of the
benzimidazoles and benzothiazoles is imperative.
S–arylation
O–arylation
and C–arylation.
It is
also reported as an efficient catalyst for the Suzuki–
[
49,50]
[51]
Miyaura
and Sonogashira cross–coupling reactions
These compounds have been synthesized previously in
different conditions, like using various types of
Recent advances in the chemistry of dendrimers, make
them largely suitable for catalysis. Dendrimers are very
well–defined, mono–sphere, three–dimensional polymers.
However, dendrimers architecture makes them able to host
metals in their uncountable cavities and also by coordination
to their functional groups. These composites are also
[
15–17]
[18]
[19]
catalysts,
solvent free
or catalyst free
reactions.
The direct functionalizing of covalent bond using transi-
tion metal catalysts has been attracted lots of attentions due
[
20–22]
[23,24]
to its developing applicability.
Ruthenium
as precious metals used
[
25–28]
[29–33]
[52,53]
rhodium
and palladium
excellent scaffolds for catalyst structures.
Appl Organometal Chem. 2017;e3937.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.3937

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ZAKERI ET AL.
Recently, we reported the application of triazine dendritic
materials containing Pd, Cu, Bi, Au and Ru in organic
synthesis.
In continuation of our previous works on the use of den-
dritic materials, here, we wish to report the preparation and
characterization of a thiol–based dendritic material contain-
ing Cu(II) species. These copper species are the key point
for the catalytic synthesis of benzmidazoles and
benzothiazoles from corresponding aldehydes (Scheme 1).
emission − scanning electron microscope (FE − SEM).
Transmission electron microscopy (TEM) measurements
were carried out on a Philips CM10 analyser operating at
100 kV. The nano − silica supported thiolated dendritic poly-
mer (G2 or nSTDP) was prepared according to our recently
[
54–61]
[
64]
reported procedure.
2
.2 | Synthesis of nanosilica triazine
dendrimer containing cu(II) species (cu(II)–
TD@nSiO )
2
2
| EXPERIMENTAL
.1 | General remarks
In a round bottomed flask, CuBr ·2H O (0.4 mmol) was
2
2
added to a stirring mixture of nano–silica supported dendri-
mer (G2) (0.5 g) in DMF (10 ml). The slurry mixture was
stirred at room temperature for 12 h, and filtered by a
Buchner funnel with a sintered glass disc. The solid material
was washed continuously with acetone (3 × 10 ml) and then
dried in a vacuum oven at 50 °C. Finally, Cu(II)–TD@nSiO2
catalyst was obtained as a greenish blue solid.
2
All reagents were purchased from Merck, Fluka and Sigma–
Aldrich. All reagents and solvents were used without further
purification. The activated nano − silica was prepared as
[62,63]
described in the literature.
FT − IR spectra were
recorded on a Jasco 6300D instrument in the range of
−
1
4
00–4000 cm . The UV–vis diffuse reflectance spectra of
the samples were obtained by a JASCO V–670 spectropho-
tometer. Thermal gravimetric analysis (TGA) was recorded
2
.3 | Typical procedure for synthesis of
on a Mettler TG50, under air flow at a consistent heating of
benzimidazoles or benzothiazoles catalyzed by
1
5
°C min− in the range of 0–600 °C. Elemental analysis
cu(II)–TD@nSiO2
In a round–bottomed flask equipped with a condenser and a
magnetic stirrer, a mixture of aldehyde (1 mmol), 1,2–
phenylenediamine or 2–aminithiophenol (1 mmol) and
was obtained on a LECO CHNS − 932 analyzer. X–ray pho-
toelectron spectra (XPS) were recorded on an XPS–Auger
Perkin Elmer 8025–BesTec electron spectrometer. This
instrument includes an ultra–high vacuum chamber, a
hemi–spherical electron energy analyzer and an X–ray source
providing unfiltered Kα radiation from its Al anode
Cu(II)–TD@nSiO (5 mg, containing 0.0012 mmol Cu(II))
2
in EtOAc (2 mL) was prepared and stirred at 50 °C. The
progress of the reaction was tracked by TLC (eluent: n–hex-
ane/EtOAc, 4:1). At the end of the reaction, the reaction mix-
ture was cooled to room temperature and EtOAc/EtOH (3:2,
(hν = 1486.6 eV). The pressure of the main spectrometer
chamber during data acquisition was maintained at ca.
−7
10
Pa. The binding energy (BE) scale was calibrated by
1
5 ml) was added. The catalyst was filtered and washed with
using the peak of adventitious C 1 s, setting it to 284.5 eV.
The accuracy of the BE scale was ±0.1 eV. The copper con-
tent of the catalyst was measured by an inductively coupled
plasma optical emission spectrometry (ICP − OES), via a Jar-
rell − Ash 1100 ICP analyzer. Scanning electron microscopy
images were performed on a Hitachi S − 4700 field
EtOAc (3 × 5 ml). The filtrate was evaporated and the product
was purified by recrystallization from EtOAc or EtOH
(depends on the polarity of the product).
3
3
| RESULTS AND DISCUSSION
.1 | Preparation and characterization of the
catalyst, Copper(II) encapsulated in nanosilica
thiolated dendric polymer
The TD@nSiO dendrimer was prepared and characterized
2
[
64]
according to our recently published literature.
The G2–
dendrimer with SH groups on its outer periphery is suitable
for encapsulation of Cu(II). As shown in Scheme 2, the
copper (II) particles immobilized on nano − silica thiolated
dendritic polymer (Cu(II)–TD@nSiO ) were obtained by
2
the reaction of CuBr ·2H O with nSTDP in DMF at room
2
2
SCHEME 1 Synthesis of benzimidazoles and benzothiazoles
temperature. Providing evidence of copper presence, copper
content of the catalyst was measured by ICP, which showed
catalyzed by cu(II)–TD@nSiO
2

ZAKERI ET AL.
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2
SCHEME 2 Preparation of the Cu(II)–TD@nSiO catalyst

4
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ZAKERI ET AL.
a value of 0.254 mmol per gram of Cu(II)–TD@nSiO2
catalyst.
The FT − IR spectrum of G2 and Cu(II)–TD@nSiO2
Figure S1a and b) were in a good accordance which shows
(
almost no specific change in dendrimer structure after
hosting Cu(II) particles. This is due to the low copper loading
of the catalyst which shows no obvious change compared to
the support.
The diffuse reflectance UV–vis spectra of G2 and Cu(II)–
TD@nSiO are compared in Figure S2. The new broad bands
2
of low intensity centred at 700 and ∼850 nm (Figure S2b)
which are usually assigned to a d–d transitions, indicating
that the solid is
TD@nSiO2.
a
copper(II) compound, Cu(II)–
Also an intense peak at ∼310 nm is assigned
to the moderate ligand–to–metal charge transfer transition
[
65]
[
66]
from p orbitals of the donor atoms to d orbitals of Cu(II).
π
FIGURE 2 (a) TEM image of cu(II)–TD@nSiO
size distribution histogram of cu(II)–TD@nSiO
2
and (b) particle
2
Figure 1a and b present the FE − SEM images of nSTDP
and Cu(II)–TD@nSiO . The size and surface morphologies
2
of the nSTDP and Cu(II)–TD@nSiO are uniform, as it shows
2
in the images, the nSiO particles are spherical and the aver-
2
age diameters are in the range of 50 to 70 nm. The energy dis-
persive X − ray (EDX) results, collected from SEM analysis,
proved the presence of sulfur in the nSTDP and also copper
element in the Cu(II)–TD@nSiO (Figures S3 and S4).
2
Figure 2a, shows the transmission electron microscopy
(TEM) image of the copper containing dendrimer. The dark
colored area of the photo is because of higher electron den-
[
67]
sity of copper in comparison with nanosilica.
Also, the
dendrimers agglomerate and local higher densities of copper
salts are observed. The histogram of size distribution showed
that the matrix has an average diameter of about 15–30 nm
(Figure 2b).
X–ray photoelectron spectroscopy (XPS) is a powerful
tool for assessing the chemical composition of macromole-
cules, ligands and mixed valent complexes via investigation
of electron properties of the species formed on the surface,
such as the electron environment, oxidation state, and the
[
68]
binding energy of the core electron of the metal.
The XPS spectrum of catalyst from 0.0 to 1200.0 eV is
shown in Figure 3a. In this spectrum, the characteristic bind-
ing energies of C 1 s, Si 2p, O 1 s, S 2 s, S 2p and Cu 2p are
identified. In general, the binding energy at 102.40 eV is due
FIGURE 1 FE–SEM images of: a) nSTDP and b) cu(II)–TD@nSiO
2

ZAKERI ET AL.
5 of 10
to Si 2p in the nSiO and the binding energy at 532.1 eV is
2
[
69]
attributed to O 1 s in Si–O network.
Furthermore, the
presence of Cu(II) ions was also confirmed by X–ray photo-
electron spectroscopy. The XPS analysis of Cu(II) in the
catalyst shows two intense peaks at 932–935 eV and
9
52–955 eV, corresponding to Cu 2p3/2 and Cu 2p1/2 peaks.
Each of the Cu 2p peaks in the spectrum of the catalyst was
fitted using two Gaussian–Lorentzian peaks with energies
of 934.32 and 932.99 eV for Cu 2p3/2, and 953.89 and
9
52.72 eV for Cu 2p1/2 (Figure 3b). The peak at 934.32 eV
(90.36%) corresponds to Cu(II) ions while the lower peak
at 932.99 eV (9.64%) corresponds either to Cu(I) or Cu(0),
though the chemical states of Cu(I) and Cu(0) are not distin-
[
70]
guishable by XPS analysis. The presence of reduced forms
[
71]
of Cu(II) has been described by Hwang and Woo
who
have reported that Cu(II) is reduced to the lower oxidation
states under X–ray irradiation in Ultrahigh vacuum (by calci-
nation followed by heating in He). The peaks corresponding
to sulfur are also clearly observed in XPS elemental survey
of the catalyst at 224.8 eV (S 2 s) and 164.7 eV (S 2p3/2),
[
69]
respectively (Figure 3a).
All above observations confirm that the thiolated
dendritic polymer is an appropriate host and ligand for
copper(II) particles.
3
.2 | Catalytic experiments
FIGURE 3 The XPS spectra of catalyst: (a) the elemental survey
spectrum, (b) showing cu 2p3/2 and cu 2p1/2 binding energies
The catalytic activity of the prepared catalyst was investi-
gated in the synthesis of benzimidazoles and benzothiazoles.
TABLE 1 Optimization of the conditions for synthesis of benzimidazoles and benzothiazoles
a
Entry
ZH
Catalyst (mg)
Solvent
T (°C)
Time (min)
Yield (%)
1
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
2
–
EtOAc
50
15
–
2
3
4
5
6
7
8
9
2
2
2
2
2
2
2
2
2
2
2
2
n–SiO
TD@nSiO
Cu(II)@nSiO
2
(5)
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
–
50
50
50
50
50
50
50
50
50
50
50
25
15
15
15
15
15
15
15
15
15
15
15
15
8
10
84
100
100
–
2
(5)
(5)
2
Cu(II)–TD@nSiO
Cu(II)–TD@nSiO
Cu(II)–TD@nSiO
Cu(II)–TD@nSiO
Cu(II)–TD@nSiO
Cu(II)–TD@nSiO
Cu(II)–TD@nSiO
Cu(II)–TD@nSiO
Cu(II)–TD@nSiO
2
2
2
2
2
2
2
2
2
(10)
(5)
(5)
(5)
(5)
(5)
(5)
(5)
(5)
n–hexane
THF
45
65
93
25
86
75
10
11
12
13
3
CH CN
2
H O
DMF
EtOAc
(
Continues)

6
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ZAKERI ET AL.
TABLE 1 (Continued)
Entry
ZH
Catalyst (mg)
Solvent
T (°C)
Time (min)
Yield (%)a
14
15
16
17
18
19
20
21
NH
NH
NH
SH
SH
SH
SH
SH
2
Cu(II)–TD@nSiO
Cu(II)–TD@nSiO
Cu(II)–TD@nSiO
Cu(II)–TD@nSiO
Cu(II)–TD@nSiO
Cu(II)–TD@nSiO
Cu(II)–TD@nSiO
Cu(II)–TD@nSiO
2
2
2
2
2
2
2
2
(5)
(5)
(5)
(10)
(5)
(5)
(5)
(5)
EtOAc
75
15
100
2
2
EtOAc
50
50
25
25
50
25
25
10
20
15
15
15
10
20
84
100
96
EtOAc
–
–
–
–
–
96
96
86
96
a
Isolated yield.
a
2
TABLE 2 Synthesis of benzimidazoles catalyzed by cu(II)–TD@nSiO
Entry
R
Ar
Product
Time (min)
Yield (%)b
TOF (h−1
)
1
H
Ph
20
95
2398
2
3
4
5
H
H
H
H
4–cl–C
6
H
4
15
18
35
35
100
94
3333
2611
1379
1293
6 4
4–Br–C H
4–me–C
6
H
4
96
9–Anthracenyl
90
6
7
4–cl
1–Naphthyl
1–Naphthyl
55
50
90
94
819
943
4–me
8
9
H
3–Pyridyl
3–Indolyl
80
80
90
88
563
551
4–me
a
2
Reaction conditions: aldehyde (1 mmol), 1,2–phenylenediamine (1 mmol), Cu(II)–TD@nSiO (5 mg, 0.0012 mmol Cu(II)), EtOAc (2 ml), 50 °C.
b
Isolated yield.

ZAKERI ET AL.
7 of 10
a
2
TABLE 3 Synthesis of benzothiazoles catalyzed by cu(II)–TD@nSiO
Entry
Ar
Product
Time (min)
Yield (%)b
TOF (h−1
)
1
Ph
20
90
2272
3200
2247
2722
1364
2
3
4
5
4–ClC
4–BrC
4–MeC
3–MeOC
6
H
4
15
20
18
35
96
89
98
95
6 4
H
6 4
H
6 4
H
6
1–Naphthyl
50
92
923
7
8
3–NO
2
C
6
H
4
100
80
85
95
426
595
3–Indolyl
a
2
Reaction conditions: aldehyde (1 mmol), 2–aminothiophenol (1 mmol), Cu(II)–TD@nSiO (5 mg, 0.0012 mmol Cu(II)), 25 °C.
b
Isolated yield.
3
.2.1 | Synthesis of benzimidazoles and
benzothiazole can be obtained in the presence of 5 mg cata-
lyst at room temperature within 15 min under solvent–free
conditions.
In order to explore the scope and generality of the pre-
pared catalyst, the optimized reaction conditions were then
applied for the synthesis of a series of benzimidazoles and
benzothiazoles. As shown in Table 2, aromatic aldehydes
bearing electron–donating and electron–withdrawing groups
reacted efficiently with 1,2–phenylenediamines in the pres-
benzothiazoles catalyzed by cu(II)–TD@nSiO
2
Synthesis of benzimidazoles and benzothiazoles, typically
require careful optimization of reaction parameters such as
amount of Cu(II)–TD@nSiO catalyst, solvent, temperature,
2
and reaction time (Table 1).
For identifying the exact optimized conditions for the
synthesis of benzimidazoles, the reaction of 4–
chlorobenzaldehyde with 1,2–phenylenediamine was
selected as a model in the presence of Cu(II)–TD@nSiO2
as catalyst. The model reaction was first carried out in the
absence of the catalyst. No product was observed under these
conditions (Table 1, entry 1). The reaction was then per-
formed in the presence of nano–SiO , TD@nSiO , Cu(II)
ence of catalytic amount of Cu(II)–TD@nSiO at 50 °C to
2
TABLE 4 Comparison of the results obtained in the synthesis of the
benzimidazole. Reaction of benzaldehyde with 1,2–phenylenediamine
2
2
catalyzed by cu(II)–TD@nSiO
2
with those obtained by the recently
[
59]
@
nSiO2
and Cu(II)–TD@nSiO (Table 1, entries 2–5).
2
reported catalysts
The results confirmed that Cu(II)–TD@nSiO was an effi-
2
TOF
cient catalyst in this reaction. The optimized conditions were
obtained applying 5 mg of catalyst (containing 0.0012 mmol
Cu(II)) in EtOAc at 50 °C within 15 min (Table 1, entry 6).
To find the optimized conditions for the synthesis of
benzothiazoles, 2–aminothiophenol (1 mmol) was treated
with 4–chlorobenzaldehyde (1 mmol) under conventional
conditions. After some preliminary experiments (Table 1,
entries 17–21), it was found that the best yield of the desired
−1
Catalyst
Reaction conditions
Methanol/O /r.t
EtOAc/ 50 °C
2
CH CN/reflux/N atm
(h
)
Ref
Cu(OH)
2
2
2.45
670.6
0.55
[72]
Copper dendrimer
Copper triflate
[38]
[73]
[74]
3
GLR–G2–cu polymer Ethanol/ air/ 30 °C
Cu(II)–TD@nSiO EtOAc/ 50 °C
1342
2
2398 This work

8
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ZAKERI ET AL.
a
b
TABLE 5 Recycling of the cu(II)–TD@nSiO
2
catalyst in the benzimidazole and benzothiazole synthesis in 15 min
c
d
Yield (%)
Cu(II) leached (%)
Run
Benzimidazole
Benzothiazole
Benzimidazole
Benzothiazole
1
100
96
0.8
1
2
3
4
5
97
95
93
93
90
90
90
90
0.5
0.4
0
0.6
0
0
0
0
a
Reaction conditions: aldehyde (1 mmol), 1,2–phenylenediamine (1 mmol), Cu(II)–TD@nSiO
2
(5 mg, 0.0012 mmol Cu(II)), EtOAc (2 mL), 50 °C.
b
Reaction conditions: aldehyde (1 mmol), 2–aminothiophenol (1 mmol), Cu(II)–TD@nSiO
2
(5 mg, 0.0012 mmol Cu(II)), 25 °C.
c
Isolated yield.
d
Measured by ICP analysis.
afford the corresponding benzimidazoles in high yields
a negligible amount of copper (less than 1%) has been
leached in the first three runs in the benzimidazole synthesis
and in the first two runs in the benzothiazole synthesis.
(94–100%) (Table 2, entries 1–4). Under the same reaction
conditions, polycyclic aldehydes such as 9–
anthracenecarboxaldehyde and 1–naphthaldehyde afforded
the corresponding benzimidazoles in 90–94% yields
4
| CONCLUSIONS
(Table 2, entries 5–7). It is worth mentioning that heterocyclic
aldehydes such as 3–pyridinecarboxaldehyde and indole–3–
carboxaldehyde took part smoothly in the reaction to give
the desired products in 88–90% yields (Table 2, entries 8, 9).
To further widen the applicability of the present method-
ology, the synthesis of benzothiazoles was investigated in the
We have developed an efficient and scalable multi–step pro-
cess for the synthesis of a new thiolated dendrimer which
was precisely characterized. The obtained dendrimer was
encapsulated by copper (II) and successfully utilized for the
catalytic synthesis of benzimidazoles and benzothiazoles
from corresponding aldehyde, diamine and aminothiophenol.
The reusability of the catalyst was also investigated. Com-
pared to deposited copper on nanosilica surface, our catalyst
shows a better stability and recyclability.
presence of Cu(II)–TD@ nSiO . As shown in Table 3, treat-
2
ment of a variety of aldehydes with 2–aminothiophenol in the
presence of catalytic amount of Cu(II)–TD@nSiO at room
2
temperature afforded the corresponding benzothiazoles in
85–96% yields.
The results of the present study were compared with
some results reported in the benzimidazole synthesis
catalysed by different copper containing catalysts. From the
provided data in Table 4, this catalytic system is superior
compared to the others.
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2

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[
[
[
[
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