Nano-CoFe2O4 supported molybdenum as an efficient and magnetically recoverable catalyst for a one-pot, four-component synthesis of functionalized pyrroles

Article abstract of DOI:10.1039/c3nj01368e

A novel magnetic nanoparticle CoFe₂O₄ supported Mo ([CoFe₂O₄@SiO₂-PrNH₂-Mo(acac) ₂]) was prepared and found to be a highly active and efficient catalyst for a one-pot synthesis of polysubstituted pyrroles via four-component reaction of aldehydes, amines, 1,3-dicarbonyl compounds and nitromethane. The easy recovery of the catalyst and reusability, broad substrate scopes, short reaction time, high yields of products and solvent-free conditions make this protocol practical, environmentally friendly and economically attractive. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3nj01368e

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Nano-CoFe2O4 supported molybdenum as an
efficient and magnetically recoverable catalyst
for a one-pot, four-component synthesis of
functionalized pyrroles†
Cite this: New J. Chem., 2014,
38, 2435
Bao-Le Li,a Mo Zhang,b Hai-Chuan Hu,a Xia Dub and Zhan-Hui Zhang*a
A novel magnetic nanoparticle CoFe2O4 supported Mo ([CoFe2O4@SiO2-PrNH2-Mo(acac)2]) was prepared
and found to be a highly active and efficient catalyst for a one-pot synthesis of polysubstituted pyrroles
via four-component reaction of aldehydes, amines, 1,3-dicarbonyl compounds and nitromethane.
The easy recovery of the catalyst and reusability, broad substrate scopes, short reaction time, high yields
of products and solvent-free conditions make this protocol practical, environmentally friendly and
economically attractive.
Received (in Montpellier, France)
6th November 2013,
Accepted 21st February 2014
DOI: 10.1039/c3nj01368e
www.rsc.org/njc
carbonates with b-enamino esters,15 (10) copper-catalyzed reac-
tion of amine with but-2-ynedioate,16 (11) reaction of a-azido
Introduction
Pyrroles are broadly found as important structural motifs in chalcones and 1,3-dicarbonyl compounds,17 and (12) via the
numerous natural products as well as bioactive compounds one-pot two-step procedure by the treatments of acetaldehyde,
such as porphyrins, bile pigments, coenzymes, and alkaloids.1 ethyl acetoacetate, sodium carbonate, piperidine, and iodine.18
They have also tremendous applications in materials science, In addition, multicomponent reactions have been proven to
medicinal chemistry and drug synthesis.2 Widespread synthetic be efficient methods for the synthesis of highly substituted
utilities coupled with the interesting biological activities have pyrroles.19 Despite these achievements, the further development
made them attractive synthetic targets. The most common of effective, environmentally benign and practical procedures to
routes for the construction of pyrrole rings are Clauson-Kaas synthesize functionalized pyrroles that minimize the use of
condensation,3 Hantzsch reaction,4 the Paal–Knorr synthesis5 special reagents, cost, time, and steps from readily available
and various cycloaddition methods.6 However, it remains and inexpensive materials is a very important subject.
difficult to prepare the structural variety of pyrroles by these
With the increasing demand for ‘‘green chemistry’’, the
methodologies. Consequently, conceptually novel and elegant design of efficient and recoverable supported heterogeneous
procedures have been devised for the construction of function- catalysts becomes one of the most important topics of research in
alized pyrroles, for example, (1) tandem one-pot addition and synthetic organic chemistry, material science, and engineering.
cyclization of the Blaise reaction intermediate and nitro- The catalyst immobilization enables the efficient recovery of the
olefins,7 (2) dehydrogenative coupling of b-amino alcohols with catalyst from the product without a troublesome separation
secondary alcohols,8 (3) the intermolecular Wacker-type reac- process. One of the most attractive alternatives to catalyst
tion of unactivated alkenes with amines,9 (4) rhodium-catalyzed supports are magnetic nanoparticles (MNPs), which have
reaction of furans with N-sulfonyl-1,2,3-triazoles,10 (5) oxidative witnessed increasing popularity due to their high surface areas
annulation of enamides with alkynes,11 (6) the coupling of and improved dispersability in the reaction medium. Specifi-
a-diazoketones with b-enaminoketones and esters,12 (7) oxidative cally, magnetically supported catalysts can be recovered using
alkyne annulations with electron-rich enamines,13 (8) tandem an external magnet due to the paramagnetic character of the
cyclization of alkynoates with amines,14 (9) cyclization of propargylic support. This makes the removal and recycling of the catalyst
much easier than filtration and centrifugation. Moreover, func-
a College of Chemistry and Material Science, Hebei Normal University,
Shijiazhuang 050024, China. E-mail: zhanhui@mail.nankai.edu.cn;
Fax: +86-311-80787431
b Huihua College, Hebei Normal University, Shijiazhuang 050091, China
† Electronic supplementary information (ESI) available: 1H NMR and 13C NMR
spectra of all new compounds. CCDC 949005. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c3nj01368e
tionalized MNPs can be easily prepared by appropriate struc-
tural surface modification and have been applied in a range of
organic transformations.20 Among various MNPs, cobalt ferrite
(CoFe2O4) with the general formula AB2O4 is one of the most
versatile magnetic materials as it has high saturation magneti-
zation, chemical stability, low toxicity, readily accessibility,
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Scheme 1 Synthesis of fuctionalized pyrroles in the presence of
CoFe2O4@SiO2-NH2-Mo(acac)2.
inexpensiveness and relatively high permeability.21 Meanwhile,
multicomponent reactions (MCRs) have been shown to provide
efficient access to structurally complex target compounds with
fascinating biological properties in a one-pot manner ensuring
atom economy, high overall yields and high selectivity, minimizing
waste, labor, and manpower, and less time consumption, and
obviation of complicated purification processes.22 The combi-
nation of the benefits of multicomponent reactions and environ-
mentally compatible catalysts can be considered to result in a
highly green synthesis design.
Considering the above subjects and continuing our efforts
toward the design of magnetic nanocatalysts23 and sustainable
synthesis development,24 herein, we report on the preparation
of a new type of magnetic nanoparticle-supported Mo complex
([CoFe2O4@SiO2-PrNH2-Mo(acac)2]) and its application in the syn-
thesis of N-protected functionalized pyrroles via four-component
reactions (4CRs) of amines, aldehydes, 1,3-dicarbonyl compounds
and nitromethane (Scheme 1).
Fig. 1 EDS spectrum of CoFe2O4@SiO2-NH2-Mo(acac)2.
mass spectrometry (ICP-MS) analysis. The presence of Fe, Co,
Si, P, O, C and Mo was observed in the EDX spectrum (Fig. 1).
Fig. 2 shows Fourier transform infrared (FT-IR) spectra
of CoFe2O4@SiO2, CoFe2O4@SiO2-PrNH2 and CoFe2O4@SiO2-
PrNH2-Mo(acac)2. The bands at 454, 1082, 1619 and 3438 cmÀ1
are characteristic of SiO2 absorption, which show evidence of
the formation of a silica shell. The presence of Co–O and Fe–O
bonds in the magnetic particles is confirmed by the charac-
teristic peak that appear at 597 cmÀ1. The intense and broad
peaks near 1084 and 3300 cmÀ1 in the three spectra are
characteristic absorption bands of the vibration of –OH bands,
which are overlapped with the –NH2 stretching vibration. The
band at 1559 cmÀ1 provides direct evidence to support the
existence of N–H deformation vibration. The appearance of
two new peaks near 948 and 917 cmÀ1 is due to Mo–O in the
CoFe2O4@SiO2-PrNH2-Mo(acac)2 material, which clearly indicates
that Mo(acac)2 has been anchored onto the CoFe2O4@SiO2. Two
bands are present at 2937 and 2857 cmÀ1, which are assignable to
stretching of alkyl C–H bonds.
The XRD pattern of CoFe2O4@SiO2-PrNH2-Mo(acac)2 is shown
in Fig. 3. A weak broad band (2y = 22 to 231) appeared in the
MNP-supported Mo complex pattern, which could be assigned
to the amorphous silane shell formed around the magnetic
cores. Furthermore, the diffraction peaks at 2y values of 18.21,
30.11, 35.51, 43.11, 57.01 and 62.61 were also observed, which
corresponded to the (111), (220), (311), (400), (511) and (440)
reflections, respectively. It is apparent that these diffraction
Results and discussion
The MNP-supported Mo complex ([CoFe2O4@SiO2-PrNH2-
Mo(acac)2]) was synthesized according to the procedure shown
in Scheme 2. At first, CoFe2O4 nanoparticles were prepared by a
chemical co-precipitation technique using FeCl3Á6H2O and
CoCl2Á6H2O as precursors according to the method reported
in the literature.25 The coating of a layer of silica on the surface of
CoFe2O4 nanoparticles was achieved by sonication of a CoFe2O4
suspension in an alkaline ethanol–water solution of tetra-
ethyl orthosilicate (TEOS). Particle surfaces were further func-
tionalized with 3-aminopropyltriethoxysilane (APTES) in
refluxing toluene to afford aminated-CoFe2O4@SiO2 MNPs.
The molybdenyl acetylacetonate complex was then anchored
to aminated-CoFe2O4@SiO2 to yield MNPs supported Mo
catalyst (CoFe2O4@SiO2-PrNH2-Mo(acac)2).
The Mo metal amount of the immobilized catalyst was found
to be 2.10 mmol gÀ1 Mo based on inductively coupled plasma
Fig. 2 IR spectra of the (a) CoFe2O4@SiO2, (b) CoFe2O4@SiO2-NH2 and
Scheme 2 Synthesis of CoFe2O4@SiO2-NH2-Mo(acac)2.
(c) CoFe2O4@SiO2-NH2-Mo(acac)2.
2436 | New J. Chem., 2014, 38, 2435--2442
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Fig. 3 XRD pattern of CoFe2O4@SiO2-NH2-Mo(acac)2.
Fig. 5 SEM image of CoFe2O4@SiO2-PrNH2-Mo(acac)2 (a) ‘‘fresh’’ catalyst
and (b) ‘‘recovered’’ catalyst after reusing 5 times.
Fig. 4 TEM image of CoFe2O4@SiO2-NH2-Mo(acac)2.
peaks are consistent with the standard CoFe2O4 with the cubic
spinel structure (JCPDS 22-1086), which revealed that the crystal
structure of the CoFe2O4 core was still well-maintained after
functionalization.
The TEM and SEM images of MNP-supported Mo complex
are shown in Fig. 4 and 5, respectively. The TEM image of the
catalyst shows that magnetic particles are highly aggregated.
The average size of these particles is about 20 nm, which shows
a close agreement with the values calculated by XRD data.
The SEM image of the supported catalyst confirms that these
nanoparticles are uneven-sized particles due to deposition of
silica on magnetic nanoparticles and most of the particles have
a quasi-spherical shape.
Fig. 6 TGA profile of CoFe2O4@SiO2-PrNH2-Mo(acac)2.
is due to the removal of physically adsorbed solvent and the
The nitrogen absorption–desorption measurements indicated surface hydroxyl group. Organic groups have been reported
that the catalyst had a specific surface area of 49 m2 gÀ1
to desorb at temperatures above 250 1C. The curve shows a
.
The thermal stability of the magnetic catalyst was deter- weight loss of about 35% from 250 1C to 450 1C, resulting from
mined under N2 atmosphere by thermogravimetric analysis the decomposition of the organic spacer grafting onto the
(TGA) (Fig. 6). The weight loss at temperatures below 200 1C MNPs surface.
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Table 1 Reaction of 4-chlorobenzaldehyde, aniline, acetylacetone and
nitromethane in different conditionsa
Temp
(1C)
Yield
(%)
Entry
Catalyst
Solvent
1
2
3
4
5
6
7
8
No
No
No
No
No
No
No
No
No
90
90
90
90
90
90
90
90
90
20
39
71
30
28
41
45
35
32
80
5
12
16
7
30
61
84
82
78
g-Fe2O3@SiO2-PW
g-Fe2O3@SiO2-PMo
g-Fe2O3@SiO2-TfOH
g-Fe2O3@HAP-SO3H
g-Fe2O3@SiO2-NHC-Cu(II)
g-Fe2O3@SiO2-NHC-Zn(II)
CoFe2O4
9
CoFe2O4@SiO2
No
10
11
12
13
14
15
16
17
CoFe2O4@SiO2-PrNH2-Mo(acac)2
CoFe2O4@SiO2-PrNH2-Mo(acac)2
CoFe2O4@SiO2-PrNH2-Mo(acac)2
CoFe2O4@SiO2-PrNH2-Mo(acac)2
CoFe2O4@SiO2-PrNH2-Mo(acac)2
CoFe2O4@SiO2-PrNH2-Mo(acac)2
CoFe2O4@SiO2-PrNH2-Mo(acac)2
CoFe2O4@SiO2-PrNH2-Mo(acac)2
CoFe2O4@SiO2-PrNH2-Mo(acac)2
CoFe2O4@SiO2-PrNH2-Mo(acac)2
(0.5 mol%)
H2O
THF
MeCN
MeOH
Toluene
No
No
No
No
No
90
Reflux
Reflux
Reflux
90
25
70
80
100
90
Fig. 7 Magnetization curves of (a) CoFe2O4@SiO2 and (b) CoFe2O4@SiO2-
PrNH2-Mo(acac)2.
The magnetic properties of the samples CoFe2O4@SiO2 and
CoFe2O4@SiO2-PrNH2-Mo(acac)2 were evaluated by vibrating 18
sample magnetometery (VSM) at room temperature. The VSM
19
magnetization curves of magnetic nanoparticles, before and
20
CoFe2O4@SiO2-PrNH2-Mo(acac)2
CoFe2O4@SiO2-PrNH2-Mo(acac)2
(1.5 mol%)
No
No
90
90
86
86
after functionalization, exhibit a superparamagnetic character 21
(Fig. 7). The magnetic saturation (Msat) values are 21.0 emu gÀ1
22b
CoFe2O4@SiO2-PrNH2-Mo(acac)2
No
90
87
and 25.0 emu gÀ1 for CoFe2O4@SiO2-PrNH2-Mo(acac)2 and
CoFe2O4@SiO2. The decrease in Msat for CoFe2O4@SiO2-PrNH2-
PMo is probably due to the existence of the large amount of non-
magnetic material [PrNH2-Mo(acac)2] on the surface of the CoFe2O4
particles. Even with this reduction in the saturation magnetization,
the prepared catalyst could still be efficiently separated from the
reaction mixtures with the help of an external magnetic force.
a
Reaction condition: 4-chlorobenzaldehyde (1 mmol), aniline (1 mmol),
acetylacetone (1 mmol), nitromethane (0.5 ml), catalyst (1.0 mol%),
solvent (2 ml), 5 h. The reaction was carried out in 50 mmol scale.
b
Under the optimal reaction conditions, the scope and limi-
tation of these 4CRs for the synthesis of pyrrole 5 were
The activity of the prepared catalyst was tested for the evaluated and the selected results are summarized in Table 2.
reaction of 4-chlorobenzaldehyde (1 mmol), aniline (1 mmol), Initially, a range of amines were treated with benzaldehydes,
acetylacetone (1 mmol) and nitromethane (0.5 ml) under solvent- acetylacetone and nitromethane. It was generally observed that
free conditions. As shown in Table 1, the reaction proceeded high to excellent yields of the products were obtained for
sluggishly in the absence of catalyst and offered only 20% yield of aniline bearing electron-rich or weakly electron-poor groups.
the expected product (Table 1, entry 1). To our delight, heating Strongly electron-poor aromatic amines such as 4-nitroaniline
the mixture at 90 1C led to the formation of product 5v in 86% decreased the reactivity and afforded the desired product in
yield in the presence of CoFe2O4@SiO2-PrNH2-Mo(acac)2 (Table 1, moderate yield. An amine with a larger aromatic group, such as
entry 20). Moreover, a higher reaction temperature (100 1C) does naphthyl, was also applicable to this reaction to afford the
not make an obvious difference in the yield of the product, but a desired product, albeit in relatively low yield. The reaction was
lower reaction temperature of 80 1C decreased the yield to 84% also very effective with 9H-fluoren-2-amine to form 5j in 89%
(Table 1, entry 17). The yield was greatly affected by the amount yield (entry 10). Moreover, different kinds of aliphatic amines
of catalyst loaded. When 0.5, 1.0 and 1.5 mol% of the catalyst also worked well in this reaction, leading to high turnover
was used, the yield varied from 78, 86 and 87%, respectively. frequencies (TOF) in a relatively short time. It should be noted
A number of common solvents were examined, but no better that although various aliphatic amines were good substrates
yield was obtained. It should be mentioned that the presence for this 4CRs in the presence of NiCl2,28a FeCl3
and I2,28c
28b
of nanoparticles of g-Fe2O3@SiO2-PW,26 g-Fe2O3@SiO2-PMo,27 aromatic amines were not favored reactants and the desired
g-Fe2O3@SiO2-TfOH,23a g-Fe2O3@HAP-SO3H,23c g-Fe2O3@SiO2- products were obtained in lower yields.
NHC-Cu(II),3b g-Fe2O3@SiO2-NHC-Zn(II),3b CoFe2O4, CoFe2O4@SiO2
Subsequently, a variety of aldehydes were evaluated in these
led to a decreased yield of product. Accordingly, performing the 4CRs. The substituents on the aromatic ring of benzaldehyde
reaction in the presence of 1.0 mol% of CoFe2O4@SiO2-PrNH2- show no significant influence on the reaction times and the
Mo(acac)2 at 90 1C under solvent-free conditions was chosen as yields of the products. Moreover, this reaction works well
optimal.
with heteroaromatic carbaldehydes such as 2-furaldehyde and
To demonstrate preparative utility, we also enlarged the 2-thiophenealdehyde, (Table 2, entries 26 and 27). 1-Naphth-
reaction scale to 50 mmol for the model reaction, and it was aldehyde was also a suitable partner and gave the desired
found that the reaction yield reached the same level as when it product 5ab in 80% yield (Table 2, entry 28). To our disappoint-
was performed in a small scale (entry 22).
ment, aliphatic aldehydes are not appropriate starting materials
2438 | New J. Chem., 2014, 38, 2435--2442
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Table 2 Synthesis of fuctionalized pyrroles 5
Time Yielda
TOFc
Entry Aldehyde
Amine
R3
R4
Product (h)
(%)
TONb (hÀ1
)
mp (1C)
1
2
3
4
5
6
7
8
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhNH2
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
4.0
5.0
4.0
5.5
4.0
6.5
90
86
92
90
80
82
50
80
48
83
88
90
91
87
91
90
90
86
90
81
87
86
85
83
85
60
71
80
80
75
78
70
80
80
88
89
85
88
80
82
80
78
80
90
86
92
90
80
82
50
80
48
83
88
90
91
87
91
90
90
86
90
81
87
86
85
83
85
60
71
80
80
75
78
70
80
80
88
89
85
88
80
82
80
78
80
22.5 106–107
17.2 Oil
3-OCH3C6H4NH2
4-CH3C6H4NH2
4-FC6H4NH2
4-ClC6H4NH2
4-BrC6H4NH2
4-NO2C6H4NH2
4-OCF3C6H4NH2
23
16.4 130–131
20 127–128
109–111
12.6 141–143
1.0 171–172
11.4 109–110
4.0 143–144
15.9 147–149
48
7.0
12.0
5.5
1.0
0.5
0.5
1.0
0.5
1.0
0.5
0.5
0.5
6.0
5.0
4.0
5.0
6.5
6.5
8.0
5.5
8.0
6.0
6.5
6.0
6.0
6.0
2.0
1.0
1.0
2.0
0.5
5.0
5.0
5.5
5.5
5.0
9
Naphthalen-1-amine Me Me
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
9H-Fluoren-2-amine
H2CQCHCH2NH2
PhCH2NH2
4-CH3C6H4CH2NH2
4-FC6H4CH2NH2
PhCH2CH2NH2
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
88
180
182
87
182
90
Oil
5l
87–88
100–102
98–100
91–92
Oil
5m
5n
5o
5p
5q
5r
5s
5t
5u
5v
5w
4-OHC6H4(CH2)2NH2 Me Me
PhCHO
PhCHO
Cyclopropanamine
Cyclopentanamine
Propan-1-amine
PhNH2
PhNH2
PhNH2
PhNH2
PhNH2
PhNH2
PhNH2
PhNH2
PhNH2
4-CH3C6H4CH2NH2
4-FC6H4NH2
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me Me
Me OMe
Me OEt
Me OCH2CH2OMe
180
172
180
72–74
74–76
Oil
4-FC6H4CHO
4-OCHMe2C6H4CHO
4-FC6H4CHO
4-ClC6H4CHO
4-BrC6H4CHO
4-NO2C6H4CHO
4-CF3C6H4CHO
Furan-2-carbaldehyde
Thiophene-2-carbaldehyde
1-Naphthaldehyde
Thiophene-2-carbaldehyde
Thiophene-2-carbaldehyde
5-Methylthiophene-2-carbaldehyde 4-FC6H4NH2
5-Methylthiophene-2-carbaldehyde 4-ClC6H4NH2
5-Methylthiophene-2-carbaldehyde 9H-Fluoren-2-amine
Thiophene-2-carbaldehyde
4-CH3C6H4CHO
4-FC6H4CHO
3-CF3C6H4CHO
4-CMe3C6H4CHO
PhCHO
13.5 Oil
17.4 112–113
21.5 105–106
17
90–93
5x
5y
5z
12.8 Oil
13.1 Oil
7.5 91–92
12.9 105–106
5aa
5ab
5ac
5ad
5ae
5af
5ag
5ah
5ai
5aj
5ak
5al
5am
5an
5ao
10
114–115
13.3 106–108
11.5 97–99
13
78–80
11.7 108–110
13.3 Oil
PhCH2NH2
PhCH2CH2NH2
PhCH2CH2NH2
PhCH2CH2NH2
Cyclopropanamine
PhNH2
PhNH2
PhNH2
PhNH2
PhNH2
40
88
89
115–116
105–106
111–112
42.5 73–74
176
16
Oil
Oil
PhCHO
PhCHO
PhCHO
PhCHO
16.4 Oil
14.5 Oil
14.2 Oil
Me OCH2CHQCH2 5ap
Et OMe 5aq
16
Oil
a
b
c
Isolated yield. Turnover numbers were simple calculated according to mol product/mol Mo in each case. Turnover frequencies [(mol product/
mol Mo)/time] were calculated at the time indicated in each case.
under these reaction conditions, a complex mixture was detected
and no desired product was isolated.
Finally, to further extend the high synthetic potential of this new
MCR, the synthesis of dipyrrole derivatives was also examined. As
Stimulated by these results, we then extended the scope of shown in Scheme 3, treatment of 4-chloroaniline (2 mmol), acetyl-
this reaction to several other b-ketoesters, such as methyl acetone (2 mmol) and nitromethane (1 ml) with 1,4-phthalaldehyde
acetoacetate, ethyl acetoacetate, 2-methoxyethyl acetoacetate, (1 mmol) under the above optimal conditions led to the formation
allyl acetoacetate and methyl 3-oxopentanoate. In general, the of the corresponding dipyrrole 6 in 75% yield. Likewise, the reaction
present method was also equally effective with b-ketoesters and of 4-benzaldehyde (2 mmol), benzene-1,4-diamine (1 mmol), acetyl-
led to the desired products in high yields (Table 2, entries 39–43). acetone (2 mmol) and nitromethane (1 ml) proceeded efficiently to
These successful results demonstrate that this procedure is extend- furnish the expected dipyrrole 7 in 78% yield.
able to a wide variety of substrates for construction of structural
variations of pyrroles.
The structures of the products were identified by their
FTIR, 1H NMR, 13C NMR spectra and elemental analysis.
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Scheme 3 Synthesis of dipyrroles.
Fig. 9 Reusability of catalyst.
the recycled catalyst with essentially no loss of catalytic activity
(Fig. 9). Furthermore, SEM images of both fresh and recovered
catalysts indicated that no significant change in the shape and
size of the support had occurred during the reaction and the
recycling stages (Fig. 5). This indicates that MNP-supported Mo
catalyst was very high chemical stable and could endure the
employed hydrothermal conditions.
Conclusions
In conclusion, a novel type of magnetically recoverable nano-
particle CoFe2O4 supported molybdenum catalyst was success-
fully prepared. The synthesized catalyst was confirmed using
XRD, FT-IR, TEM, SEM, TGA, ICP-MS, and VSM techniques. The
immobilized Mo complex was shown to be an efficient hetero-
geneous catalyst for a one-pot four-component coupling reaction of
amines, aldehydes, 1,3-dicarbonyl compounds and nitromethane.
A wide range of amines, aldehydes, 1,3-dicarbonyl compounds can
be tolerated, giving the structural diversity of pyrroles in good
to excellent yields. Morever, the catalyst could be easily recovered
by simple magnetic decantation and reused five times without
significant loss of activity, showing good potential for industrial
application.
Fig. 8 Single-crystal X-ray structural of compound 5h.29
Furthermore, the structure of compound 5h was unambiguously
confirmed by single crystal X-ray analysis (Fig. 8).
A plausible mechanism for the formation of pyrrole deriva-
tives is proposed in Scheme 4. The intermediates (Z)-4-(phenyl-
amino)pent-3-en-2-one (A) from aniline and acetylacetone and
(E)-1-chloro-4-(2-nitrovinyl)benzene (B) from 4-chlorobenzalde-
hyde and nitromethane could be generated in the presence of a
magnetic nanocatalyst and were separated in the reaction
process. These two intermediates reacted via Michael addition,
followed by intramolecular cyclization with the elimination of
nitroxyl (HNO) and water to lead to the final product 5v.
Finally, the catalyst recyclability has been tested in the model
reaction. After the completion of reaction, the catalyst was sepa-
rated magnetically from the reaction mixtures and washed with
ethyl acetate. The recovered catalyst was dried to remove residual
solvents, and then reused directly in the model reaction for the
next round without further purification. In addition, metal leach-
ing in the catalyst was determined and ICP-MS analysis of the
clear filtrate indicated that the Mo and Fe contents were 0.10 and
0.15 ppm, respectively. Five runs can be efficiently performed with
Experimental section
General methods
All solvents and chemicals were commercially available and
were used as purchased. The X-ray diffraction analysis was
performed on a PANalytical X’Pert Pro X-ray diffractometer.
Surface morphology and particle size were studied using a
Hitachi S-4800 SEM instrument. Transmission electron micro-
scope (TEM) observations were carried out using a Hitachi
H-7650 microscope at 80 KV. Elemental compositions were
determined using a Hitachi S-4800 scanning electron micro-
scope equipped with an INCA 350 energy dispersive spectro-
meter (SEM-EDS) presenting a 133 eV resolution at 5.9 keV. The
molybdenum content was determined using ICP-MS (an X
Series 2 spectrometer). Melting points were measured on an
X-4 digital melting point apparatus and are uncorrected. IR
spectra were obtained as KBr pellets or as liquid films on
1
KBr pellets using a Bruker-TENSOR 27 spectrometer. H NMR
(500 MHz) and 13C NMR (125 MHz) spectra were recorded on a
Bruker DRX-500 spectrometer using CDCl3 as the solvent and
Scheme 4 Plausible mechanism for the synthesis of pyrrole derivatives.
2440 | New J. Chem., 2014, 38, 2435--2442
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TMS as an internal standard. Elemental analyses were deter- References
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.
General procedure for the synthesis of functionalized pyrroles 5.
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Acknowledgements
We appreciate the National Natural Science Foundation of
China (Nos 21272053, 21072042 and 20872025) for financial 18 P. F. Xu, K. Z. Huang, Z. G. Liu, M. Zhou and W. B. Zeng,
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2442 | New J. Chem., 2014, 38, 2435--2442
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