Facial preparation of sulfonic acid-functionalized magnetite-coated maghemite as a magnetically separable catalyst for pyrrole synthesis

Article abstract of DOI:10.1002/cctc.201300623

The synthesis, characterization, an Copyright

Full text of DOI:10.1002/cctc.201300623

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DOI: 10.1002/cctc.201300623
Facial Preparation of Sulfonic Acid-Functionalized
Magnetite-Coated Maghemite as a Magnetically Separable
Catalyst for Pyrrole Synthesis
Hajar Mahmoudi and Abbas Ali Jafari*[a]
Dedicated to Professor Habib Firouzabadi on the occasion of his 70th birthday
The synthesis, characterization, and catalytic performance of
the new sulfonic acid supported on the magnetic nanoparti-
cles with maghemite coating were reported. The morphology,
particle size, structure, magnetic properties, and the formation
of nanoparticles with narrow size distribution were investigat-
ed. Chemical analysis was performed by using TEM, wide-angle
XRD, FTIR spectroscopy, and X-ray fluorescence, and magnetic
measurements were performed by using vibrating sample
magnetometry. The catalyst was found to be active for the syn-
thesis of pyrroles. The nanometer size range of these particles
facilitates the catalytic process owing to the increased surface
area available for the reaction. The easy separation of the cata-
lyst by an external magnet from liquid-phase reactions and its
usability for at least nine consecutive trials without any de-
crease in activity are additional advantages.
Introduction
The heterogenization of the active catalytic molecules is one
of the efficient ways to overcome the problem of isolation and
separation of a homogeneous catalyst.[1–4] Heterogeneous cata-
lysts can be recovered through filtration or centrifugation.
However, these methods are time-consuming and may cause
catalyst loss. Furthermore, conventional separation methods
may become inefficient with particle sizes less than 100 nm. In
line with this, magnetic nanoparticles are of great interest to
researchers because with a large surface-to-volume ratio rela-
tive to bulk materials,[5,6] these nanoparticles demonstrate high
activity and selectivity (like a homogeneous catalyst) and can
be easily separated and recovered. Magnetically supported cat-
alysts can be recovered with an external magnet owing to the
paramagnetic character of the support, and the catalysts can
be reused in another cycle. Considering the iron oxide nano-
particles, the iron atoms on the surface act as Lewis acids and
coordinate with molecules that donate lone pair electrons and
functionalize the iron oxide surface hydroxyl groups. These hy-
droxyl groups are amphoteric and may react with acids or
bases.[7] The coatings also provide stability to nanoparticles.
Iron oxide nanoparticles can be coated with silica, gold, or
gadolinium(III).[8–13] Magnetically separable catalysts can be
used for designing surface functionalities along with catalyst
preparation.
Sulfonic acid catalysts are widely used in various industries;
however, it is often difficult to isolate and separate the final
product after the reaction is complete. Notably, even if it is
possible to separate the catalyst from the reaction mixture,
trace amounts of the catalyst are likely to remain in the final
product. In recent years, with the aim of switching to increas-
ingly efficient and benign processes, the immobilization of sul-
fonic acid on the magnetic support as the reusable catalyst
that avoids the use of toxic reagents and reduces the time-
consuming wasteful separations[14–16] is a challenging task.
However, there are several factors that limit the use of these
catalysts. For instance, the applicable reagents are expensive
and have harsh preparation conditions. Hence, one of the
main goals of the present work is to overcome these draw-
backs through the design of a new one-pot preparation of sul-
fonic acid supported on the magnetic nanoparticles with mag-
netic maghemite (\g-Fe2O3) coating as a powerful recoverable
catalytic system.
The pyrrole ring system is a useful structural element in me-
dicinal chemistry[17] and has found broad application in the de-
velopment of, for example, antibacterial, antiviral, anti-inflam-
matory, antitumoral, and antioxidant drugs.[18] They are
a highly versatile class of intermediates in the synthesis of nat-
ural products and in heterocyclic chemistry[19] and are widely
used in materials science.[20] Thus, many synthetic methods
have been developed for the preparation of these com-
pounds.[21] Of the many methods developed, the Paal–Knorr[22]
and Clauson–Kass[23] reactions are still considered to be the
most attractive methods for the synthesis of pyrroles. In these
methods, 1,4-dicarbonyl or 2,5-dimethoxytetrahydrofuran com-
pounds are converted to pyrroles through the reaction with
primary amines in the presence of various promoting
[a] H. Mahmoudi, Prof. A. A. Jafari
Chemistry Department
Yazd University
P.O. BOX 89195-741, Yazd (Iran)
Fax: (+98)351-8210644
E-mail: jafari@yazd.ac.ir
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300623.
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agents.[24] However, some of these methods often suffer from
certain drawbacks such as the use of hazardous organic sol-
vents, high-cost reagents, relatively expensive or stoichiometric
amounts of catalysts, tedious workup, which leads to the gen-
eration of large amounts of toxic metal-containing waste, and
low product yields. Therefore, the development of facile and
environmentally friendly methods for the synthesis of pyrroles
is necessary for organic synthesis.
Results and Discussion
Magnetite (Fe3O4) nanoparticles with a mean size of 10–
15 nm[25] were prepared by using the coprecipitation technique
[Eq. (1)].
Figure 1. XRD spectra of a) Fe3O4 and b) Fe3O4@Fe2O3ÀSO3H.
mination of the structural order. The XRD patterns of Fe3O4
shown in Figure 1a are in good agreement with a cubic struc-
ture, whereas the XRD patterns shown in Figure 1b confirm
the conversion of the Fe3O4 core structure to Fe3O4@g-Fe2O3
during the sulfonation process. These observations confirm
that the magnetic particles change from Fe3O4 to
Fe3O4@g-Fe2O3.
In this way, Fe3O4 is not very stable and is sensitive to the oxi-
dation. Fe3O4 is then transformed into g-Fe2O3 in the presence
of oxygen. Notably, oxidation in air is not the only way to
transform Fe3O4 into g-Fe2O3. According to Equation (2), vari-
ous electrons or ions transferred are involved, depending on
the pH of the suspension. Under acidic and anaerobic condi-
tions, the surface Fe2+ ions desorb as hexa-aqua complexes in
the solution, whereas under basic conditions, the oxidation of
Fe3O4 involves the oxidation–reduction of the Fe3O4 surface.[26]
The FTIR spectra of the synthesized nanoparticles with and
without SO3H loading are shown in Figure 2. For the bare mag-
netic nanoparticles (Figure 2), the vibration band at 557 cmÀ1
is the typical IR absorbance induced by structure of the FeÀO
Herein, we design a one-pot method for the preparation of
sulfonic acid supported on the magnetic nanoparticles with
g-Fe2O3 coating as a powerful recoverable catalytic system
[Eq. (3)].
Figure 2. FTIR spectra of Fe3O4 and Fe3O4@g-Fe2O3ÀSO3H.
vibration. The three new bands appeared at 1200–1250, 1010–
1100, and 650 cmÀ1 corresponding to the O=S=O asymmetric
and symmetric stretching vibrations and SÀO stretching vibra-
tion of the sulfonic groups (ÀSO3H), respectively. The increase
in the intensities of the band at 3000–3600 cmÀ1 confirms that
the sulfonic groups functionalized the surface of the magnetic
nanoparticles.
The analysis of the XRF results (Table 1) of the catalyst con-
firms the presence of sulfonic acid on the catalyst surface and
demonstrates the existence of Fe2O3 on the outer layer of the
catalyst (Figure 3).
The magnetically separable sulfonic acid nanocatalysts were
characterized by using FTIR spectroscopy, XRD, X-ray fluores-
cence (XRF), SEM, TEM, potentiometric titration, and vibrating
sample magnetometry.
The TEM and SEM images of Fe3O4@g-Fe2O3ÀSO3H are
shown in Figure 4. These images demonstrate the uniform-
sized particles (mean size range: 10–15 nm) with spherical
morphology.
The XRD spectra of the Fe3O4 and Fe3O4@g-Fe2O3ÀSO3H ma-
terials are shown in Figure 1, which are also used for the deter-
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Table 1. Results of XRF analysis.
Compound
Concentration [wt%]
Fe2O3
SO3
Cl
42.65
20.26
1.66
V2O5
Al2O3
MnO
CaO
CuO
ZnO
LOI[a]
Total
0.073
0.041
0.034
0.029
0.020
0.009
35.48
100.26
Figure 5. Potentiometric titration curves of a) Fe3O4 and b) Fe3O4@g-Fe2O3À
[a] Loss on ignition (T=10008C, t=2 h).
SO3H.
which generates very strong acid sites (E=600 mV) on the
magnetic particle surface. These results indicate that Fe3O4@g-
Fe2O3ÀSO3H is a very strong solid acid with a high density of
acid sites. The acid sites in the samples are approximately six
times larger than those in Nafion,[28] which is a highly active
and stable perfluorosulfonate ionomer.
To determine whether the particles demonstrate magnetic
properties, the sulfonic acid-loaded magnetic particles were
dispersed in water, which resulted in a brown suspension.
Within approximately 5 s of the application of an external
magnet, the sulfonic acid-functionalized magnetic nanoparti-
cles were completely collected onto the stirrer bar or the side
of the cuvette wall and the solution became clear and trans-
parent (Figure 6).
Figure 3. XRF analysis of Fe3O4@g-Fe2O3ÀSO3H.
Figure 6. Separation of Fe3O4@g-Fe2O3ÀSO3H with an external magnet.
Figure 4. a) TEM and b) SEM images of Fe3O4@g-Fe2O3ÀSO3H.
The magnetization curves of Fe3O4 and Fe3O4@g-Fe2O3ÀSO3H
were recorded at room temperature (Figure 7). The two mea-
sured samples demonstrate a superparamagnetic behavior, as
confirmed by zero coercivity and remanence on the magneti-
zation loop. The saturation magnetization value of Fe3O4@g-
Fe2O3ÀSO3H is 55 emugÀ1 (Figure 7b), which is a slightly lower
The amount of sulfonic acid groups on the magnetic nano-
particles with g-Fe2O3 coating was calculated by using the neu-
tralization titration method and was found to be in the range
of 3–3.2 mmolgÀ1. The surface acidity of Fe3O4@g-Fe2O3ÀSO3H
was calculated by using the potentiometric titration method
with n-butyl amine. To interpret the results, the acid strength
was assigned according to the following scale: maximum acid
strength (MAS) @100 mV, very strong acid sites; 0<MAS<
100 mV, strong acid sites; À100 mV<MAS<0 mV, weak acid
sites; and MAS !M->100 mV, very weak acid sites.[27] A re-
markable difference in acid strength between pure Fe3O4 and
Fe3O4@g-Fe2O3ÀSO3H (in the millivolt range) can be seen in
Figure 5. A pure magnetic particle is a weakly acidic oxide (E=
2 mV); however, it is modified through the ÀSO3H addition,
than that of the uncoated magnetic particles (ꢀ60 emugÀ1
;
Figure 7a) but is nearly twice that of Fe3O4@g-Fe2O3ÀSO3H[16]
(ꢀ28 emugÀ1; Figure 7c).
To show its catalytic activity, Fe3O4@g-Fe2O3ÀSO3H was used
as a catalyst in the Paal–Knorr and Clauson–Kaas pyrrole syn-
thesis reactions. Aniline and 2,5-heptadione was chosen as
model substrates for the optimization of the Paal–Knorr reac-
tion and the identification of the best amount of the catalyst.
By screening loading of the catalyst and considering the
effect of the solvent and temperature on the reaction rate, it
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Table 3. Synthesis of pyrrole derivatives with Fe3O4@g-Fe2O3ÀSO3H.[a]
Entry
1
Product
t [h]
Yield [%]
91
1.5
2
3
0.5
94
97
Figure 7. Magnetization loops of a) Fe3O4, b) Fe3O4@g-Fe2O3ÀSO3H, and
c) \g-Fe2O3ÀSO3H.[16] All the data are recorded at RT.
1.45
was found that only 0.05 g of Fe3O4@g-Fe2O3ÀSO3H was suffi-
cient to condense aniline (1 mmol) with 2,5-heptadione
(1.2 mmol) at room temperature under solvent-free conditions
(Table 2).
4
5
1
2
93
90
Table 2. Optimization of the reaction conditions.
6
7
0.45
1
95
92
Entry
Catalyst [mol%]
Solvent
T [8C]
t [h]/Conv. [%]
1
2
3
4
5
6
7
8
–
–
–
–
–
25
25
25
25
25
25
25
25
25
50
80
8/20
8/100
4/100
1.5/100
1.15/100
5/100
3/100
6/100
24/100
1/100
1/100
Fe3O4 (5)
8
9
0.5
3
98
93
Fe3O4@g-Fe2O3ÀSO3H (3)
Fe3O4@g-Fe2O3ÀSO3H (7)
Fe3O4@g-Fe2O3ÀSO3H (10)
Fe3O4@g-Fe2O3ÀSO3H (7)
Fe3O4@g-Fe2O3ÀSO3H (7)
Fe3O4@g-Fe2O3ÀSO3H (7)
Fe3O4@g-Fe2O3ÀSO3H (7)
Fe3O4@g-Fe2O3ÀSO3H (7)
Fe3O4@g-Fe2O3ÀSO3H (7)
–
CH3CN
Et2O
EtOAc
n-hexane
–
9
10
11
–
10[b]
0.5
96
[a] Reaction conditions: amine (1 mmol), 2,5-heptadione (1.2 mmol), and
Fe3O4@g-Fe2O3ÀSO3H (0.05 g) were mixed, followed by stirring at RT.
After this finding, the general applicability of the method
was studied through the reaction of structurally diverse
amines with hexane-2,5-dione under similar reaction condi-
tions. With use of this method, aromatic and aliphatic amines
easily reacted with hexane-2,5-dione and gave N-substituted
pyrroles in good to excellent yields. Aliphatic amines reacted
more efficiently than aromatic amines and gave N-alkyl pyr-
roles in excellent yields. The results are summarized in Table 3.
The possibility of recycling the magnetic catalyst was also
examined. For this purpose, the reaction of aniline and 2,5-
heptadione in the presence of Fe3O4@Fe2O3ÀSO3H was studied.
After completion of the reaction, diethyl ether (10 mL) was
added to the mixture, the catalyst was collected onto the reac-
tion vessel with an external magnet, and the reaction solution
was decanted. The resulting catalyst was washed with diethyl
ether to remove the residual product, dried under vacuum,
and reused in a subsequent reaction. Fe3O4@g-Fe2O3ÀSO3H was
reused directly without any deactivation even after nine con-
secutive runs (Figure 8).
Notably, the stability of Fe3O4@g-Fe2O3ÀSO3H nanoparticles
makes the storage of this catalyst convenient. The reaction of
aniline and 2,5-heptadione in the presence of Fe3O4@Fe2O3À
SO3H was studied at different times, and it was found that
these particles are stable for more than 2 months (Figure 9 and
Table 4).
The Clauson–Kaas reaction is another attractive route to pro-
duce N-substituted pyrrole. Aniline and 2,5-dimethoxytetrahy-
drofuran were chosen as the model substrates for the optimi-
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Table 5. Synthesis of pyrrole derivatives with Fe3O4@g-Fe2O3ÀSO3H.[a]
Entry
1
Product
t [min]
Yield [%]
90
8
2
15
93
Figure 8. Recovery of the Fe3O4@g-Fe2O3ÀSO3H catalyst in 1.5 h.
3[b]
20
30
92
90
4[b]
5
6
7
8
6
8
6
8
90
95
91
94
9
20
30
90
90
Figure 9. Stability of the Fe3O4@g-Fe2O3ÀSO3H catalyst.
Table 4. Optimization of the reaction conditions.
10
[a] Reaction conditions: amine (1 mmol), 2,5-dimethoxytetrahydrofuran
(1.2 mmol), and Fe3O4@g-Fe2O3ÀSO3H (0.07 g) were irradiated under sol-
vent-free conditions; [b] Using 2 equiv. 2,5-dimethoxytetrahydrofuran.
Entry
Catalyst [mol%]
t [min]
Conversion [%]
(Table 5). Notably, in the case of strong electron-deficient
amine, a longer reaction time was needed.
1
2
3
4
5
6
–
5
5
15
8
5
2
20
40
100
100
100
100
Fe3O4 (5)
Fe3O4@g-Fe2O3ÀSO3H (5)
Fe3O4@g-Fe2O3ÀSO3H (10)
Fe3O4@g-Fe2O3ÀSO3H (15)
Fe3O4@g-Fe2O3ÀSO3H (20)
Conclusions
The Fe3O4@g-Fe2O3ÀSO3H nanoparticles prepared by using the
simple method are the efficient and magnetically recyclable
heterogeneous catalysts for pyrrole synthesis. In contrast to
other catalysts, the magnetically separable sulfonic acid cata-
lysts can be easily removed from the reaction mixture and the
separation and purification of the products do not require
a complicated process. The preparation of magnetically separ-
able sulfonic acid catalysts is easier, faster, and cheaper than
that of other catalysts. Moreover, this catalyst is a solid acid
catalyst, which is environmentally friendly as well.
[a] MW=microwave radiation.
zation of the Clauson–Kaas reaction conditions. The best reac-
tion condition was achieved under microwave irradiation in
the presence of 0.07 g of the catalyst under solvent-free
conditions.
Encouraged by this result, we have performed the reactions
of various amines with 2,5-dimethoxytetrahydrofuran to probe
the scope and reactivity of the new catalyst and the results are
summarized in Table 5. Reactions with most of the examined
amines containing strong electron-deficient (p-NO2, m-NO2,
and m-COCH3) or strong electron-donating (p-MeO) anilines
gave the corresponding pyrroles in good to excellent yields
Experimental Section
Raw materials for the reaction were purchased from Merck and
Fluka and were used without any further purification. The reaction
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progress was followed by using TLC. The reaction product was de-
termined from IR, 1H NMR, and 13C NMR analysis. The whole pro-
cess was performed with TLC-Card Silica Gel-G/UV 254 nm; analyti-
cal balance Mitller/Colleg 150; OVEN/Model: U30/W:800-Shimaz
CO; FTIR: Spectrometer-Spectrum RX 1 Perkin–Elmer AVE (4000–
400 cmÀ1); 1H NMR and 13C NMR: Bruker Avance 400 MHz spec-
trometer; pH meter: Horiba model:f-IIE.
Method for the reuse of the catalyst
Fe3O4@g-Fe2O3ÀSO3H (0.15 g) was added to the mixture of aniline
(3 mmol, 0.27 mL) and 2,5-heptadione (3.3 mmol, 0.47 mL). The re-
sulting mixture was stirred at RT until aniline disappeared (1.5 h).
Absolute ethanol (5 mL) was added to the reaction mixture, and
the resulting mixture was stirred for 2 min. Fe3O4@g-Fe2O3ÀSO3H
was separated with an external magnet. Fe3O4@g-Fe2O3ÀSO3H was
washed 3 times with absolute ethanol (3ꢁ5 mL) and dried at RT.
The same catalyst was used in 9 cycles for pyrrole synthesis.
Preparation of the Fe3O4 nanoparticles
The Fe3O4 nanoparticles were prepared by using the published
method with a slight modification.[24] First, FeCl3·6H2O (20 mmol,
5.40 g) and FeCl2·4H2O (10 mmol, 2.00 g) were added to deionized
water (40 mL) and the resulting mixture was vigorously stirred
under nitrogen gas for 30 min. Then, the mixture was heated to
608C. Finally, ammonium hydroxide (15 mL, 28 wt%) was added
quickly to the solution and the solution immediately turned black.
The reaction mixture was kept at 608C for 2 h. The black precipi-
tate was separated with an external magnet.
Acknowledgements
We thank the Research Council of Yazd University for financial
support. H.M. is grateful to Mohammad Javad Faghihi for his
kind support during this work.
Keywords: heterogeneous catalysis
properties · nanocatalysis · pyrroles
·
iron
·
magnetic
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General method for the Paal–Knorr reaction
Fe3O4@Fe2O3ÀSO3H (0.05 g) was added to a mixture of amine
(1 mmol, 0.09 mL) and 2,5-heptadione (1.2 mmol, 0.17 mL). The re-
sulting mixture was stirred for 1.5 h at RT. The reaction progress
was monitored by using TLC (n-hexane/ethyl acetate eluent 8:2).
After completion of the reaction, absolute ethanol (5 mL) was
added to the reaction mixture and the resulting mixture stirred for
2 min. Fe3O4@g-Fe2O3ÀSO3H was separated by using a permanent
magnet, and the reaction solution was decanted. Fe3O4@g-Fe2O3À
SO3H was washed 3 times with absolute ethanol (3ꢁ5 mL). After
the evaporation of ethanol, pure products were obtained in good
to excellent yields.
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General method for the Clauson–Kaas reaction
Amine
(1 mmol),
2,5-dimethoxytetrahydrofuran
(1.2 mmol,
0.15 mL), and the catalyst (0.07 g) were irradiated in a microwave
under solvent-free conditions. The reaction progress was moni-
tored by using TLC (n-hexane/ethyl acetate eluent 8:2). After com-
pletion of the reaction, absolute ethanol (5 mL) was added to the
mixture and the resulting mixture was stirred for 2 min. Fe3O4@g-
Fe2O3ÀSO3H was separated with an external magnet and then
washed 3 times with absolute ethanol (3ꢁ5 mL), after which pure
products were obtained.
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Received: July 28, 2013
Published online on && &&, 0000
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FULL PAPERS
H. Mahmoudi, A. A. Jafari*
Magnetic aura: The catalytic applica-
tion of sulfonic acid-functionalized mag-
netite-coated maghemite as a powerful
magnetically separable catalyst in pyr-
role synthesis has been investigated.
The catalyst can be recovered and
reused several times in Paal–Knorr and
Clauson–Kaas reactions.
&& – &&
Facial Preparation of Sulfonic Acid-
Functionalized Magnetite-Coated
Maghemite as a Magnetically
Separable Catalyst for Pyrrole
Synthesis
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 8
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These are not the final page numbers!