Iron oxide encapsulated by copper-Apatite: An efficient magnetic nanocatalyst for: N-Arylation of imidazole with boronic acid

Article abstract of DOI:10.1039/c9ra06991g

N-Arylation of imidazole was carried out with various arylboronic acids on iron oxide encapsulated by copper-Apatite (Fe₃O₄?Cu-Apatite), producing excellent yields. Firstly, the iron nanoparticles were prepared using a solvothermal m

Full text of DOI:10.1039/c9ra06991g

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Iron oxide encapsulated by copper-apatite: an
efficient magnetic nanocatalyst for N-arylation of
imidazole with boronic acid†
Cite this: RSC Adv., 2019, 9, 36471
a
a
ab
Othmane Amadine, * Younes Essamlali, Abdallah Amedlous
ab
and Mohamed Zahouily*
N-Arylation of imidazole was carried out with various arylboronic acids on iron oxide encapsulated by
copper-apatite (Fe @Cu-apatite), producing excellent yields. Firstly, the iron nanoparticles were
prepared using a solvothermal method, and then they were encapsulated by copper-apatite to obtain
magnetic Fe @Cu-apatite nanocatalysts. Several physico-chemical analysis techniques were used to
characterize the prepared nanostructured Fe @Cu-apatite catalyst. The prepared Fe @Cu-apatite
was used as a nanocatalyst for N-arylation of imidazole with a series of arylboronic acids with different
substituents to reaffirm the effectiveness of this magnetic nanocatalyst. The Fe @Cu-apatite
nanocatalyst can also be easily separated from the reaction mixture using an external magnet. More
importantly, the as-prepared Fe @Cu-apatite exhibited good reusability and stability properties in
3 4
O
3 4
O
3
O
4
3 4
O
Received 2nd September 2019
Accepted 1st November 2019
3
O
4
DOI: 10.1039/c9ra06991g
3 4
O
rsc.li/rsc-advances
successive cycles. However, there was a notable loss of its catalytic activity after multiple cycles.
contamination – toxic waste produced aer separation of the
catalyst, which cannot be easily recovered aer the reaction.
1
. Introduction
1
4
Over the years, N-arylheterocycles have gained prominence These disadvantages can be overcome by anchoring the metal
due to their presence in a wide variety of natural products and on suitable supports, which can be easily recovered, and then
bioactive compounds. They have also played an important role potentially be reusable with a minimal amount of product
1
,2
as building blocks in organic synthesis. There is a special contamination. Recently, diverse forms of heterogeneous
emphasis on N-aryl imidazole because it exhibits antipsy- catalysts have been developed for N-arylation, such as cellu-
3
,4
15
16
chotic, antiallergic, and herbicidal properties, as well as lose supported copper(0), polymer supported Cu(II), MCM-
1
7
others. The development of a mild and highly efficient method 41-immobilized bidentate nitrogen copper(I), CuO nano-
for the synthesis of N-arylheterocycles over classical Ullman particles and Cu-exchanged uorapatite. Magnetic separa-
type,
nucleophilic aromatic substitution reactions, or tion has also received a lot of attention as a solid catalyst
coupling with organometallic reagents has recently gained separation technology, since it can be very efficient and fast.
considerable attention in synthetic chemistry. But the harsh Numerous studies have focused on the immobilization of
conditions of these reactions, such as very high temperature copper catalytic systems on a magnetic medium based on iron
and strong bases have restricted and limited their applica- in order to separate them by the simple application of
1
8
19
5
,6
7
8
,9
st
20,21
tions. At the turn of the 21 century, recent development of N- a magnet.
In the same way, Alper and co-workers have
arylation with boronic acids unleashed the power of C- immobilized Cu(I) catalyzed on the surface of Fe
3 4
O
magnetic
heteroatom bond formation reaction due to the mild reac- nanoparticle-supported L-proline as a recyclable and recover-
tion conditions (room temperature, weak base and ambient able catalyst for N-arylation of heterocycles.
1
0–13
atmosphere).
However, the synthetic scope of this reaction
Calcium phosphate, such as hydroxyapatite and its derivates
is strongly limited due to some disadvantages, such as product appear very attractive due to their ion-exchange capability,
22
adsorption capacity, and acid–base properties. Hydroxyapatite
2
3,24
based materials have been used in the biomedical eld
and in some important chemical transformations.
lately
25–28
a
The latter
VARENA Center, MAScIR Foundation, Rabat Design, Rue Mohamed El Jazouli,
Madinat El Irfane, 10100-Rabat, Morocco. E-mail: m.zahouily@mascir.com; applications are related to its well-known ability to immobilize
o.amadine@mascir.com
divalent and trivalent metal ions, by partial cationic exchange
b
Laboratoire de Mat ´e riaux, Catalyse et Valorisation des Ressources Naturelles, URAC
29–33
with calcium.
Recently, the possibility to combine the
2
4, FST, Universit ´e Hassan II-Casablanca, Morocco
properties of apatite with other inorganic phases within core–
†
Electronic supplementary information (ESI) available: Complete experimental
shell nanostructures has attracted great attention. For instance,
procedures and spectral data the compounds listed in Table 4. See DOI:
0.1039/c9ra06991g
34
35
hydroxyapatite/TiO
2
,
hydroxyapatite/carbon nanotubes, and
1
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Fourier transformation infrared (FT-IR) spectra of samples in
KBr pellets were measured on a Bruker Vector 22 spectrom-
eter. Magnetic properties of Fe O and Fe O @Cu-apatite were
3
4
3 4
investigated in a MPMS-XL-7AC superconducting quantum
interference device (SQUID) magnetometer. The magnetic
Scheme 1 N-Arylation of imidazole with boronic acid by a nano- measurements were performed from ꢁ15 000 to 15 000 Oe at
catalyst (Fe @Cu-apatite).
3
O
4
3 4
room temperature. The element content of Fe O @Cu-apatite
materials was determined by an Elemental analysis was real-
ized using inductively coupled plasma atomic emission
1
hydroxyapatite/SiO
2
(ref. 36) core–shell nanoparticles have been spectrometry (ICP AES; Ultima 2 – Jobin Yvon). The H NMR
studied for different applications, such as photocatalytic spectra were recorded in CDCl or DMSO d6, using a Bruker
3
processes, uorescence imaging and drug delivery. Indeed, the Avance 600 spectrometer.
combination of hydroxyapatite with iron oxide particles could
be useful to design sorbents that combine large activity and easy
2
.2. Catalyst preparation
37,38
recovery via magnetic separation.
Hence, several iron oxide–
2.2.1. Preparation of Fe O nanoparticles. The magnetic
3
4
hydroxyapatite nanocomposites have been described in the
iron oxide nanoparticles used in this study were synthesized
using solvothermal method, adapting methodologies
described in the literature. In a typical procedure, 5.0 mmol
of the FeCl $6H O was dissolved in 20 mL of ethylene glycol,
and then 20 mmol of NaOH was added to the resultant mixture
with vigorous stirring. Then, the obtained EG solution was
transferred to a Teon-lined stainless-steel autoclave and
sealed. Aer reacting at 180 C for 6 h, the autoclave was
cooled down to ambient temperature and the mixture was
withdrawn from the reactor with a magnet bar to get the Fe O4
magnetic NPs, which were then washed with water and
39–45
literature.
been explored, and to the best of our knowledge, the use of
Fe @Cu-apatite as a heterogeneous catalyst for the N-aryla-
tion of imidazole has not been reported in literature. Fe is
3 4
However, the Fe O @Cu-apatite has only rarely
a
5
1
3 4
O
3
2
3 4
O
one of the cheaper magnetic oxide, which act in this study as
a high surface area framework that is coated by the growth of
Cu-apatite shell. The Fe O core has to facilitate the recycle of
3 4
ꢀ
nanocatalysts through magnetic separation. This present work
was achieved within our progressive program to develop an eco-
friendly and efficient approach for the synthesis of various
3
46–50
products.
This is the background upon which this work is
ꢀ
ethanol and dried at 40 C for 24 h.
situated upon, and the objective is based on the synthesis and
characterization of Fe O @Cu-apatite nanoparticles, and then
2.2.2. Fe
3 4 3 4
O @Cu-apatite nanocomposite. The Fe O @Cu-
3
4
apatite nanocomposite was prepared along a hydrothermal
route. Firstly, 0.30 g of Fe O NPs was dispersed in 100 mL
3 4
distilled water, and then 12 g of urea was added. Then, 10 mL of
their use as magnetic catalysts for the N-arylation of imidazole
with arylboronic acids (Scheme 1).
52
a solution produced from a mixture of Ca(NO ) $4H O (0.5904
3
2
2
2
. Experimental
g) and Cu(NO ) $4H O (0.604 g) was added into the above
3
2
2
solution in drops (molar ratio of Ca : Cu 1 : 1). Aer heating at
2
.1. Materials and apparatus
Copper(II) nitrate (Cu(NO $3H
FeCl $6H O), calcium nitrate (Ca(NO
ammonium hydrogen phosphate ((NH ) HPO ), sodium
ꢀ
9
0 C for 2 h, 10 mL of (NH
4
)
2
HPO
4
(0.1981 g) solution was also
3
)
2
2
O, $98.0%), ferric chloride
$4H O, $99.0%),
added in drops and the pH value of the solution was tuned to 10
with ammonia water (molar ratio of Ca : P 10 : 6). Aer 0.5 h,
the solution was transferred into a Teon-lined stainless-steel
(
3
2
3
)
2
2
4
2
4
acetate (CH COONa), ethylene glycol (EG), sodium hydroxide
3
ꢀ
autoclave, where it was sealed and heated at 165 C for 12 h.
(
3 2
NaOH), ammonia solution (NH $H O, 30%), poly (ethylene
Finally, the Fe O @Cu-apatite NPs were collected using
3
4
glycol) (PEG, 1000) were purchased from Aldrich Chemical
Company and were used as received without any further
purication. Deionized water was used in all experiments. X-
ray diffraction (XRD) patterns of the catalysts were obtained
at room temperature on a Bruker AXS D-8 diffractometer using
a magnet bar.
2.3. General procedure of imidazol's N-arylation with
arylboronic acid
Cu-Ka radiation in Bragg–Brentano geometry (q–2q). SEM and In a 50 mL round-bottomed ask, imidazole (1 mmol), phe-
STEM micrographs were obtained on a Tecnai G2 microscope nylboronic acid (1.2 mmol), K CO (1.5 mmol) and Fe O @Cu-
2
3
3 4
at 120 kV. The average diameter of sample was quantied from apatite (15 mol%) were added and stirred in MeOH under air at
ꢀ
the SEM image using ImageJ soware. The gas adsorption 60 C for the required time, monitoring by TLC. Aer comple-
2
data was collected using a Micromeritics 3Flex Surface char- tion, the mixture was diluted with H O and the product was
acterization analyzer, using nitrogen. Prior to nitrogen sorp- extracted with EtOAc (3 times). The combined extracts were
ꢀ
tion, all samples were degassed at 150 C overnight. The washed with brine (3 times) and dried over Na
2
SO
4
. The product
specic surface areas were determined from the nitrogen was puried using column chromatography (60–120 mesh silica
ꢀ
adsorption/desorption isotherms (at ꢁ196 C), using the BET gel, eluting with EtOAc–hexane). The structures of the prepared
1
(
Brunauer–Emmett–Teller) method. Pore size distributions products were conrmed by H NMR and assigned on the basis
were calculated from the N
2
adsorption isotherms with the of their spectral data in comparison with those reported in the
“
classic theory model” of Barret, Joyney and Halenda (BJH). literature.
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Table 1 Lattice parameter, crystallite size, BET surface area and pore
volume for all as-synthesized samples catalysts
a
b
c
Crystallite size
(nm)
S
BET
V
meso
D
meso
(nm)
2
ꢁ1
ꢁ1
Samples
(m g
)
(mL g
)
Fe
Fe
3
O
O
4
4
21
33
22
84
0.0305
0.1350
6.5391
7.1465
3
@Cu-apatite
a
b
Calculated by Debye–Scherer equation. Calculated from adsorption
c
2 2
of the N isotherm. Calculated from desorption of the N isotherm.
3 4 3 4
The FTIR spectra of Fe O and Fe O @Cu-apatite are pre-
sented in Fig. 2. This gure shows the FTIR spectrum of
Fe O @Cu-apatite nanoparticles, where there are observable
3
4
ꢁ1
peaks around 1020, and 1090, 964, 600, 561 and 470 cm
,
which are attributed to the asymmetric and symmetric
stretching mode of P–O group and bending mode of O–P–O
group, respectively. The characteristic bands at about 3571 and
3 4 3 4
Fig. 1 XRD patterns of the Fe O (a) and Fe O @Cu-apatite (b).
ꢁ
1
3
641 cm are related to the stretching vibrations of the OH
ꢁ1
group. The weak and broad bands at around 1470 cm and
ꢁ1
873 cm are most likely assigned to carbonate groups adsorbed
during the synthesis process. In addition, the ngerprint of iron
3 4 3 4
Fig. 2 FT-IR spectra of (a) Fe O and (b) Fe O @Cu-apatite.
3. Results and discussion
3
.1. Catalyst characterization
XRD was used to analyze the crystalline phases of the as-
prepared Fe and Fe @Cu-apatite samples (Fig. 1). The
Fig. 1a shows the characteristic diffraction peaks of Fe
nanoparticles observed clearly at 2q ¼ 30.2, 35.4, 43.3, 53.7,
7.3, and 62.98, which are attributed to the (220), (311), (400),
422), (511), and (440) Bragg reections of the face-centered
cubic lattice of Fe O nanoparticles, respectively (JCPDS no.
3
O
4
3 4
O
3 4
O
5
(
3
4
1
9-0629). In the XRD pattern of Fe O @Cu-apatite (Fig. 1b), we
3 4
can observe the presence of a secondary phase in which the
diffraction peaks correspond to calcium copper phosphate
Ca20Cu(PO
4
)
14 (JCPDS 49-1080). The average crystallite size of
@Cu-apatite was estimated using the Debye–
Fe and Fe O
3
O
4
3 4
Scherrer formula, which gave 21 and 33 nm for Fe
Fe O @Cu-apatite, respectively.
3 4
O and
Fig. 3 Nitrogen adsorption/desorption isotherms and BJH pore size
distribution of the (a) Fe and (b) Fe @Cu-apatite.
3
4
O
3 4
3 4
O
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oxide is assigned in the FTIR spectroscopy to the Fe–O
ꢁ1
stretching vibration, and detected towards 587 cm
.
The calculated texture parameters of Fe O and Fe O @Cu-
3
4
3 4
apatite are summarized in Table 1. The calculation of the BET
2
ꢁ1
surface area for Fe
0
O
3 4
3
O
4
is 22 m
g
and the pore volume is
3
ꢁ1
.0305 cm
Fe @Cu-apatite are 84 m g and 0.1350 cm g , respec-
tively. This change in surface properties was assumed to be due
to the coating of Cu-apatite on the surface of Fe nano-
g . The surface area and pore volume of the
2
ꢁ1
3
ꢁ1
3 4
O
particles. The rough surface provides more pores for nitrogen
adsorption. The nitrogen sorption isotherm of Fe O is type IV,
3
4
displaying a hysteresis loops type H3, indicating the meso-
porous nature of the material (Fig. 3a). For the Fe @Cu-
3 4
O
apatite, the nitrogen sorption isotherm is type (IV) with
a hysteresis loop type H3, which proves the existence of meso-
pores according to the IUPAC manifests (Fig. 3b).
3 4 3 4
Fig. 5 STEM images of (a and b) Fe O and (c and d) Fe O @Cu-
apatite.
The corresponding pore size distribution curve indicates
that Fe O and Fe O @Cu-apatite have a centralized pore size
distribution around 4 and 11 nm, respectively, which corre-
sponding to mesoporous materials (inset Fig. 3a and b).
3
4
3 4
Ca, P, Fe and Cu content were found to be 30.96, 18.63, 47,
1.83%, respectively.
3 4
STEM was used to analyze the microstructure of the Fe O
SEM was used to study the surface morphology of Fe
@Cu-apatite (Fig. 4). The micrographs show that Fe
composed of spherical nanoparticles. Furthermore, in the case
of Fe @Cu-apatite, some particles were observed on the
surface of Fe , which may be due to the coating of the
3
O
4
and
and Fe
observed that Fe
spherical particles. These Fe
cores for the growth of Cu-apatite shells to obtain the
Fe O @Cu-apatite core–shell nanostructures. Compared to the
3
O
4
@Cu-apatite materials. From Fig. 5a and b, it can be
NPS show a very good dispersion of
spheres were further used as
Fe
3
O
4
3
O
4
is
3 4
O
3 4
O
3 4
O
3 4
O
magnetic nanoparticles with Cu-apatite nanoparticles. From
the SEM images of Fe O , we determined that the average sizes
3 4
Fe O cores, the outside surfaces became coarse aer the
3
4
3
4
growth process, as shown in Fig. 5c and d. These results indi-
cate that the Cu-apatite nanoparticles were successfully loaded
and the relative standard deviation (SD) values for the diameters
of the spherical Fe
MNPs was 264 ꢂ 28 nm. Moreover, ICP
analysis was performed to determine the weight of element
composition of Fe @Cu-apatite. From the ICP analysis, the
3 4
O
onto the Fe
formation of core–shell nanostructures. We have also noticed
3 4
that Cu-apatite was still tightly anchored on the surface of Fe O
3 4
O sphere surfaces, which clearly demonstrates the
3 4
O
even aer the severe conditions under which the sample prep-
aration for STEM analysis were conducted (a long time of
mechanical stirring and sonication), suggesting the existence of
strong interactions between Cu-apatite and Fe
The magnetic properties of Fe and Fe
were investigated using SQUID from ꢁ15 000 to 15 000 Oe at
room temperature. As shown in Fig. the saturation
3 4
O .
3
O
4
3 4
O @Cu-apatite
6
Fig. 4 SEM images of (a and b) Fe
3 4 3 4
O , (c and d) Fe O @Cu-apatite and
diameter distribution of Fe nanoparticles (e).
3
O
4
3 4 3 4
Fig. 6 Magnetization curves of Fe O and Fe O @Cu-apatite.
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This superparamagnetism can make the magnetic nano-
particles dispersible in the solution with negligible magnetic
interactions between each other, which avoids magnetic clus-
tering. As shown in the Fig. 7, Fe O @Cu-apatite can be
3
4
dispersed in deionized water to form a stable brown suspension
before magnetic separation. However, when a magnet was
placed close to the reaction vessel for a while, it could be
observed that the synthesized samples were rapidly attracted to
the magnet side, and a nearly colorless solution was obtained.
Fig. 7 Photographs of the separation processes Fe
O
3 4
@Cu-apatite:
(
a) without external magnetic field, and (b and c) with external
3
.2. N-Arylation of imidazole over Fe
Firstly, the activities of some samples were screened, such as:
3 4
Fe , Cu-apatite, Fe @Cu-apatite and Cu-apatite@Fe O
3 4
O @Cu-apatite catalyst
magnetic field.
3
O
4
3
O
4
a
catalyzing imidazole N-arylation using phenyl boronic acid as
Table 2 Evaluation and screening of catalysts
a model reaction. The resultant obtained results are summa-
Yields (%) rized in Table 2. First of all, the reaction doesn't occur without
the addition of a catalyst (Table 2, entry 1), and the iron oxide is
b
Entry
Catalyst
1
2
3
4
5
—
n.r.
n.r.
98
also inactive in this reaction (Table 2, entry 2). However, the
reaction of N-arylation of imidazole was occurring when Cu-
Fe
Cu-apatite
Fe @Cu-apatite
3 4
Cu-apatite@Fe O
3 4
O
O
4
97
47
apatite was used as catalyst with a yield of 98% (Table 2, entry
3). These results can be explained by the presence of the copper,
which play an important role as catalytic element for enhancing
3
a
Reaction conditions: catalyst (5 mol%), arylboronic acids (1 mmol),
imidazole (1 mmol), base (A mmol), methanol (2 mL), reux for 6 h. the cross-coupling reaction. Additionally, coating iron oxide by
b
Isolated yield.
copper-apatite (Fe O @Cu-apatite) in order to separate them by
3
4
the simple application of a magnet has no signicant effect on
its catalytic activity (Table 2, entry 4) with a yield of 97%. On the
other hand, coating Cu-apatite by Fe O decrease their catalytic
3 4
ꢁ
1
magnetization (Ms) of Fe
3
O
4
is close to 57 emu g , a higher
@Cu-
core
was successfully coated with a Cu-apatite shell. Moreover, the
eld-dependent magnetization curves of Fe O and Fe O @Cu-
value than that obtained for the corresponding Fe
apatite (30.66 emu g ). These results indicate that Fe O
3 4
3 4
O
activity (Table 2, entry 5), this can be explain by the decrease of
the contact surface of the catalytic element and the reactants.
This result shows the importance of the catalytic system
developed in this work (Fe O @Cu-apatite) for the N-arylation
ꢁ1

3
4
3 4
3
4
apatite show negligible remanence (Mr) and coercivity (Hc),
indicating superparamagnetic behavior for both samples at
room temperature.
of imidazole.
The inuence of various reaction parameters, such as type of
solvent, nature of the base, reaction temperature, and catalyst
a
Table 3 N-Arylation of imidazole with arylboronic acid in various solvents and bases in the presence of Fe
3 4
O @Cu-apatite
ꢀ
b
b
Entry
Solvent
Base
Catalyst (mol%)
Temp. ( C)
Yield of 3a (%)
Yield of 4a (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
Ethanol
Water
K
K
K
Na
KOH
2
2
2
CO
CO
CO
3
20
20
20
20
20
20
20
20
20
20
15
10
5
Reux
Reux
Reux
Reux
Reux
Reux
70
60
50
40
60
87
66
97
94
88
84
95
93
89
75
93
84
72
—
—
—
—
—
—
—
—
—
—
—
—
—
3
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
3
2
CO
3
NaOH
K
2
K
2
K
2
K
2
K
2
K
2
K
2
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
0
1
2
3
60
60
a
b
Reaction conditions: arylboronic acid (1 mmol), imidazole (0.5 mmol), base (1 mmol), solvent (2 mL) for 6 h. Isolated yield.
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Table 4 N-Arylation of N(H)-heterocycles with aryl boronic acid in the yields were obtained by using K
3
CO as base. Otherwise, it was
2
a
3 4
presence of Fe O @Cu-apatite
noted that the presence of the base was necessary for the
achievement of the reaction as reported in the literature. This is
explained by the role of the base to activate the boronic acid in
the mechanism of the reaction.
The study of the inuence of temperature, in the case of
using MeOH, indicated that this reaction is very sensitive to
changes in temperature (Table 3, entries 7–10). According to
these studies we can conclude that temperature is a key
parameter in this type of reaction, and the optimum yield was
b
Samples
N(H)-heterocycle
Aryl
Yield (%)
1
2
3
4
5
6
7
8
9
Imidazole
Imidazole
Imidazole
Imidazole
Imidazole
Imidazole
Indole
Indole
Indole
Indole
Indole
C
6
H
5
89
87
85
83
92
78
91
86
88
85
96
72
4-CH
3-CH
2-CH
4-MeO–C H
6 4
4-NO
3
3
3
–C
–C
–C
6
6
6
H
H
H
4
4
4
ꢀ
obtained at 60 C. Furthermore, the decreasing the catalyst
loading to half results in a yield of 84% (Table 3, entry 12),
which demonstrate the inuence of the catalyst amount in this
type of reactions. Thus, the optimum conditions for this reac-
2
–C
6
H
4
6 5
C H
4-CH
3-CH
2-CH
4-MeO–C
4-NO –C H
3
3
3
–C
6
–C
6
–C
6
H
H
H
4
4
4
ꢀ
tion are at 60 C in MeOH in the presence of 15 mol% of
1
1
1
0
1
2
nanocatalyst with respect to imidazole and K
the base (Table 3, entry 11).
3
CO (1 mmol) as
2
6 4
H
Indole
2
6 4
With the optimized reaction conditions in hand, a variety of
@Cu-apatite (15 mol%), arylboronic acid (1 arylboronic acids were chosen as the substrates in this N-
a
Reaction conditions: Fe
3
O
4
2 3
mmol), N(H)-heterocycles (0.5 mmol), K CO (1 mmol), methanol (2
ꢀ
b
imidazole cross-coupling reaction and the results are shown in
Table 4. The substitution effects of arylboronic acid were
investigated. It was found that the reaction with phenylboronic
acids with an electron donating group afforded better yields
mL), 60 C for 6 h. Isolated yield.
(
(
Table 4, entries 1–5) than with electron withdrawing groups
Table 4, entries 6). Similar observation was made when indole
was used in place of imidazoles to obtain the corresponding N-
arylindole (Table 4, entries 8–10).
Another benet of this nanocatalyst system consists of its
ease of recyclability. The coupling of imidazole with arylboronic
acid was chosen as a model reaction for the reusability study
(Fig. 8). Aer the cross-coupling reaction, the recovered catalyst
was washed twice by dichloromethane and water, dried at room
temperature before being reused under similar conditions for
the next run. It has to be noted that the catalytic behavior of the
Fig. 8 Reuse performance of Fe
arylation of imidazole. Reaction conditions: Fe
10 mol%), arylboronic acid (1 mmol), imidazole (0.5 mmol), K
3
O
4
@Cu-apatite catalyst in the N-
@Cu-apatite
CO
Fe O @Cu-apatite catalyst remains nearly the same for the ve
O
3 4
3 4
(
2
3
(1 successive runs. Aer the 5th cycle, a notable loss of the cata-
lytic activity was observed. The N-arylation of N-arylheterocycles
ꢀ
mmol), methanol (2 mL), 60 C for 8 h.
over Fe
ported in the literature as tabulated in Table 5. From this, we
loading was investigated. The results obtained are summarized can show clearly that our Fe O @Cu-apatite catalyst exhibited
3 4
O @Cu-apatite was compared with other catalysts re-
3
4
in Table 3. Initially, with 20 mol% of the catalyst and K
a base, the screening of the effect of different solvents showed with other catalytic systems.
that H O may not be the optimal solvent, when the reaction was
In order to investigate the behavior of the Fe O @Cu-apatite
carried out in EtOH or MeOH, the yield of the product 1-phenyl- catalyst during recycling experiments, XRD and STEM analysis
H-imidazole was isolated in 87 and 97%, respectively (Table 3, were performed (Fig. 9). It was observed from the XRD pattern of
entry 1 and 3). Aer optimizing the solvent, the effect of the base the Fe O @Cu-apatite catalyst that, aer one catalytic cycle, the
2 3
CO as a best catalytic activity of N-arylation of imidazole comparing
2
3
4
1
3
4
was studied (Table 3, entries 4–6), and it was found that the best crystalline composition of the catalyst remained unchanged.
Table 5 Comparison of activity of different heterogeneous catalysts in the N-arylation reaction of imidazole
Catalyst
Reaction conditions
Yield (%)
Ref.
ꢀ
ꢀ
CuI-zeolite
Silica-supported copper(II)
Cu-FAP
Methanol, 65 C, 17 h
95
78–98
88
53
54
55
Methanol, 70 C, 6 h
Methanol, K
2
CO
3
, r.t., 6 h
Cu(0)-cellulose
Methanol, reux, 2.5 h
Methanol, 60 C, 6 h
98
96
15
ꢀ
Fe
3
O
4
@Cu-apatite
This work
36476 | RSC Adv., 2019, 9, 36471–36478
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36478 | RSC Adv., 2019, 9, 36471–36478
This journal is © The Royal Society of Chemistry 2019