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9230 Chem. Commun., 2011, 47, 9230–9232 This journal is c The Royal Society of Chemistry 2011
Cite this: Chem. Commun., 2011, 47, 9230–9232
Hantzsch reaction on free nano-Fe2O3 catalyst: excellent reactivity
combined with facile catalyst recovery and recyclabilityw
Nadiya Koukabi,aEskandar Kolvari,*
bArdeshir Khazaei,*
aMohammad Ali Zolfigol,*
a
Behzad Shirmardi-Shaghasemicand Hamid Reza Khavasi
d
Received 7th May 2011, Accepted 27th June 2011
DOI: 10.1039/c1cc12693h
A magnetic nanoparticle catalyst was readily prepared from
inexpensive starting materials which catalyzed the Hantzsch
reaction. High catalytic activity and ease of recovery from the
reaction mixture using an external magnet, and several reuse
times without significant losses in performance are additional
eco-friendly attributes of this catalytic system.
Transition-metal catalyzed organic reactions are often con-
sidered to follow the principles of ‘‘Green Chemistry’’;
i.e. these catalyzed reactions consume a minimum of energy
and reagents or auxiliaries and minimize waste. Nanocatalysts
are considered to be a bridge between homogeneous and hetero-
geneous catalysis.1 With the development of nanochemistry it
has been possible to prepare ‘‘soluble’’ analogous of hetero-
geneous catalysts, materials that might have properties inter-
mediate between those of bulk and single particles due to high
surface areas and high densities of active sites.2 Unfortunately,
unsupported nanoparticles are usually unstable and the
coagulation of the nanoparticles during reaction is frequently
unavoidable.3 Thus, till now, the investigation of ‘‘free’’
nanoparticles as catalysts has been rare, although it is
an important tool to gain a fundamental understanding of
catalysis.4,5 Clearly, the development of ‘‘free’’ nanoparticles
with tunable catalytic activity is of great significance for both
academia and industry. However, the recycle problem must be
addressed before nanocatalytic processes can be scaled-up, due
to the fact that nanoparticles, which include nano-scaled metal
catalysts and supports, are difficult to separate from the
reaction mixture, which can lead to the blocking of filters
and valves by the nanoparticle catalyst. Currently, a method used
to address this problem is the use of magnetic nanoparticles,
a route that has attracted wide research interest for its unique
physical properties.6 They possess advantage of being
magnetically recoverable, thereby eliminating the requirement
for either solvent swelling before or catalyst filtration after
the reaction.7–9 In comparison with other transition metals
extensively used, iron-based catalysts are cheap, non-toxic,
environmentally friendly and abundant. In the past decades,
various iron salts have been applied as Lewis acids (Fe3+) in
homogeneous catalysis and different catalytically active iron
complexes were also reported.10 In heterogeneous catalysis,
iron oxides have been frequently used as catalysts and
supports in bulk industrial processes, usually at high temperature
(4300 1C) and pressure.11,12 At the onset of this study, no
example of the iron oxide nanoparticle had been reported for
the multicomponent reaction. Multicomponent reactions13
allow the creation of several bonds in a single operation and
are attracting increasing attention as one of the most powerful
emerging synthetic tools for the creation of molecular diversity
and complexity.14 In recent years, much attention has been
focused on the synthesis of 1,4-dihydropyridine compounds,
due to their significant biological activity.15–18 We were, thus,
intrigued by the possibility of applying nanotechnology to the
design of a novel, active, recyclable, and magnetically recoverable
‘‘free’’ nano-iron oxide as a Lewis-acid catalyst for the synthesis
of 1,4-dihydropyridine compounds under mild reaction conditions
for the first time.
For initial optimization of the reaction conditions and the
identification of the best iron source, temperature, and amount
of the catalyst, benzaldehyde 1, ethyl acetoacetate 2a and
ammonium acetate 3 were chosen as model substrates (Scheme 1).
By screening a wide range of iron sources, we found that the
product 4a could be obtained in yields ranging from 51 to 95% in
any oxidation state of iron salt (0, II, III; Table 1). As expected,
bulk Fe2O3 displayed only low activity (Table 1, entry 2). Next, we
Scheme 1 Synthesis of Hantzsch 1,4-dihydropyridines catalyzed by
‘‘free’’ nano-g-Fe2O3.
a Faculty of Chemistry, Bu-Ali Sina University,Hamedan 6517838683, Iran. E-mail: [email protected],[email protected]; Fax: +988118257407
bDepartment of Chemistry, Faculty of Science Semnan University,Semnan, Iran. E-mail: [email protected]; Fax: +982313354082
cDepartment of Chemistry, Payam Noor University, Hamedan, IrandDepartment of Chemistry, Shahid Beheshti University, Tehran, Iranw Electronic supplementary information (ESI) available: Generalexperimental details for starting materials and instruments, catalysismeasurement: XRD pattern, and XPS spectra and also elementalanalysis, spectral data of all compounds and literature references forknown compounds. CCDC 823102. For ESI and crystallographic datain CIF or other electronic format see DOI: 10.1039/c1cc12693h
ChemComm Dynamic Article Links
www.rsc.org/chemcomm COMMUNICATION
This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 9230–9232 9231
studied nanoparticle-sized Fe2O319 because of its super-
paramagnetic property,20 hence recovery and recycling of the
catalyst could be easily achieved. In the presence of nano-
Fe2O3, in which the majority of the particles is 14 nm in size
(Fig. 1a) the conversion reached 95% (Table 1, entry 5). The
corresponding catalytic activity is higher than the corresponding
bulk Fe2O3.
The crystal structure of the iron oxide g-Fe2O3 is reported as an
inverse spinel structure (Scheme 2),19,21,22 in which all the iron
cations are in the trivalent state, and the charge neutrality of the
cell is guaranteed by the presence of cation vacancies. The unit cell
of maghemite can be represented as (Fe3+)8[Fe3+
5/6&1/6]16O32,
where the brackets (), [] and & designate tetrahedral, octahedral
and vacant sites, respectively.23
X-Ray diffraction (XRD) powder patterns confirm that
bulk and nano-Fe2O3 have the same crystal strucure.24 From
the binding energies derived from X-ray photoelectron spectro-
scopy (XPS), it is clear that the surface iron ions are trivalent in
both samples.25
The reason for the improved activity of nano-iron oxide
most probably originates from the nanometre size of the bulk
iron oxide. In general, nanoscale heterogeneous catalysts
should offer higher surface areas, low-coordinated sites, and
surface vacancies, which are responsible for the higher catalytic
activity.26 Theoretically, it can be assumed that with a decrease
of the particle size down to a ‘‘molecular’’ level, the nanocatalyst
behaves as a homogeneous system in which the catalytic
activity is not controlled by the surface area of the catalyst
but rather governed by the concentration.27 The most important
significance of these results is that ‘‘free’’ nano-Fe2O3, but not
immobilized nano-Fe2O3, is highly active, selective, and stable
by merely controlling the particle size. Variation of the
amount of catalyst (0.15 mmol) and temperature (90 1C)
revealed an increased conversion of reactants and a product
yield of 98%. In summary, the optimal conditions for the
‘‘free’’ nano-Fe2O3 catalyzed Hantzsch reaction involved a
combination of nano-Fe2O3 (0.15 mmol), benzaldehyde 1a
(1 mmol), ethyl acetoacetate 2a (2 mmol), ammonium acetate
3 (1.5 mmol) at 90 1C under solvent-free conditions. In view
of these results, we then selected the optimized reaction
conditions to determine the scope of this ‘‘free’’ nano-Fe2O3
catalyzed reaction. A wide range of aromatic, aliphatic and
heteroaromatic aldehydes were subjected to react with 2a, 2b
in the presence of ammonium acetate 3 and 0.15 mmol of
nano-Fe2O3 to generate 4, 528 (Scheme 1) and the results are
summarized in Table 2. The aryl group substituted with
different groups and same groups located at different positions
of the aromatic ring does not show much effect on the
formation of the final product and afford the expected
products 4, 5 in good to high yields. The products were
characterized by IR, 1H NMR and 13CNMR spectroscopy,
and also by comparison with authentic samples. Product 4g
was structurally determined by X-ray single crystal diffraction
(see ESIw). This superparamagnetic property of nano-Fe2O3
made the isolation and reuse of this catalyst very easy. After
completion of the reaction, the mixture was triturated
with ethyl acetate. In the presence of a magnetic stirrer bar,
nano-Fe2O3 moved onto the stirrer bar steadily and the
reaction mixture turned clear within 10 s. The catalyst can
be isolated by simple decantation (see ESIw). After being
washed with acetone and dried in air, the nano-Fe2O3 can be
directly reused without any deactivation even after five rounds
of synthesis of product 4a (Table 3).
The characterization of the nano-Fe2O3 before and after
reuse five times showed the same particle size by transmission
electron microscopy (TEM; Fig. 1b) and the same crystal
structure by XRD. The only difference is visible from XPS,
which showed lower peak intensity after the fifth use (see
ESIw). This is due to the increased carbon content of
the surface. The C/Fe ratio rose from 1.51 atom% in fresh
nano-Fe2O3 to 2.10 atom% after the fifth use in the synthesis
Table 1 Screening of iron sources for the Hantzsch synthesis of1,4-dihydropyridinea
Entry Catalyst (0.15 mmol) t/min Yieldb (%)
1 Fe 25 512 Bulk-Fe2O3 40 703 FeCl2�4H2O 60 634 FeCl3�6H2O 35 805 Nano-g-Fe2O3 25 95
a Benzaldehyde–ethyl acetoacetate–NH4OAc= 1 : 2 : 1.5, solvent-free,
80 1C. b Yields refer to isolated products.
Fig. 1 TEM images of nano-g-Fe2O3 before use (a) and after reuse
five times (b).
Scheme 2 Proposed mechanism for ‘‘free’’ nano-g-Fe2O3 catalyzed
Hantzsch synthesis.
9232 Chem. Commun., 2011, 47, 9230–9232 This journal is c The Royal Society of Chemistry 2011
of product 4a. We believe that this is also the possible reason
for the high stability of the nano-Fe2O3 presented herein. A
thin layer of carbon is formed during the reaction which prevents
significant coagulation of the nano-Fe2O3. Obviously, carbon-
containing deposits cover the iron oxide particles partly during
reaction which, however, seems not to be detrimental to the
activity. The proposed mechanism for the synthesis of
1,4-dihydropyridines involves Lewis-acid catalyzed cyclo-
condensation of intermediates A and B, generated respectively
by Knoevenagel condensation of one equivalent of ethyl
acetoacetate with aldehyde and reaction of a second equivalent
of ethyl acetate with ammonia generated from ammonium
acetate (Scheme 2).
In conclusion, unsupported ‘‘free’’ nano-Fe2O3 has been
shown to be a stable yet highly active catalyst for preparing a
variety of 4-substituted-1,4-dihydropyridines from the one-pot
three-component condensation reaction. The catalytic research on
novel approaches toward nanomaterials should be improved
to enhance organic synthesis. For that purpose, magnetic
catalyst provides a new way for continuous processes, because
of its simple recyclability. From a scientific point, our results
expand the application of ‘‘free’’ nanoparticles. They should
be helpful to understand the advantageous combination of the
properties of homogeneous and heterogeneous catalysis and
the development of new catalytic systems.
Notes and references
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2 L. N. Lewis, Chem. Rev., 1993, 93, 2693–2730.3 C. N. R. Rao, A. A. Muller and K. Cheetham, The Chemistry ofNanomaterials: Synthesis and Applications, Wiley-VCH,Weinheim, 2004, vol. 1, pp. 555–562.
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5 B. P. S. Chauhan, J. S. Rathore and N. Glloxhani, Appl. Organomet.Chem., 2005, 19, 542–550.
6 M. Faraji, Y. Yamini and M. Rezaee, J. Iran. Chem. Soc., 2010, 7,1–37.
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10 E. B. Bauer, Curr. Org. Chem., 2008, 12, 1341–1369.11 B. Moens, H. DeWinne, S. Corthals, H. Poelman, R. De Gryse,
V. Meynen, P. Cool, B. F. Sels and P. A. Jacobs, J. Catal., 2007,247, 86–100.
12 T. Riedel, H. Schulz, G. Schaub, J. Jun and K. Lee, Top. Catal.,2003, 26, 41–51.
13 B. Jiang, T. Rajale, W. Wever, S. J. Tu and G. Li, Chem.–Asian J.,2010, 5, 2318–2335.
14 Some reviews on diversity-oriented organic synthesis:(a) S. L. Schreiber, Science, 2000, 287, 1964–1969; (b) M. D. Burkeand S. L. Schreiber, Angew. Chem., Int. Ed., 2004, 43, 46–58.
15 P. P. Mager, R. A. Coburn, A. J. Solo, D. J. Triggle and H. Rothe,Drug Design Disc., 1992, 8, 273–289.
16 R. Miri, K. Javidnia, H. Sarkarzadeh and B. Hemmateenejad,Bioorg. Med. Chem., 2006, 14, 4842–4849.
17 M. A. Zolfigol and M. Safaiee, Synlett, 2004, 827–828.18 M. A. Zolfigol, P. Salehi andM. Safaiee, Lett. Org. Chem., 2006, 3,
153–156.19 Y. K. Sun, M. Ma, Y. Zhang and N. Gu, Colloids Surf., A, 2004,
245, 15–19.20 Z. Xu, C. Shen, Y. Tian, X. Shi and H. J. Gao, Nanoscale, 2010, 2,
1027–1032.21 K. V. S. Ranganath and F. Glorius, Catal. Sci. Technol., 2011, 1,
13–22.22 Y. Cudennec and A. Lecerf, Solid State Sci., 2005, 7, 520–529.23 R. Grau-Crespo, A. Y. Al-Baitai, I. Saadoune and N. H.
De Leeuw, J. Phys.: Condens. Matter, 2010, 22, 255401.24 H. Zhu, Y. Ma, H. Yang, C. Ji, D. Hou and L. Guo, J. Phys.
Chem. Solids, 2010, 71, 1183–1186.25 P. Li, et al., Phys. D: Appl. Phys., 2011, 44, 075003.26 G. Pacchioni, Surf. Rev. Lett., 2000, 7, 277–306.27 Y. Zhao and K. Aoki, Chem. Phys. Lett., 2006, 430, 117–120.28 General procedure for the synthesis of 1,4-dihydropyridine
compounds: All reactions were carried out in an oil-bath (oil-bathtemperature 90 1C). The aldehyde (1 mmol), b-keto compound(2 mmol), ammonium acetate (1.5 mmol) and nano-Fe2O3
(0.15 mmol) were added to a glass reactor (ca. 25 mL). Thereaction mixture was vigorously stirred. After completion of thereaction (monitored by TLC), the mixture was cooled to roomtemperature and triturated with ethyl acetate (10 mL). In thepresence of a magnetic stirrer bar, nano-Fe2O3 moved onto thestirrer bar steadily and the reaction mixture turned clear within10 s. The catalyst can be isolated by simple decantation.The reaction mixture was treated with brine, extracted withethyl acetate (2�20 mL). After evaporation of the solvent, thecrude product was recrystallized from EtOH–H2O to give a puresolid.
Table 2 ‘‘Free’’ nano-g-Fe2O3 catalyzed Hantzsch synthesis of1,4-dihydropyridinesa
Entry R1 R2 Product t/min Yieldb (%)
1 Ph OEt 4a 15 982 p-MeC6H4 OEt 4b 25 903 p-ClC6H4 OEt 4c 15 924 p-HOC6H4 OEt 4d 20 815 p-O2NC6H4 OEt 4e 32 946 p-FC6H4 OEt 4f 25 937 p-CNC6H4 OEt 4g 40 908 p-BrC6H4 OEt 4h 30 889 o-MeOC6H4 OEt 4i 15 9010 m-ClC6H4 OEt 4j 15 9111 4-HO-3-MeOC6H3 OEt 4k 20 8712 n-Pr OEt 4l 20 9213 2-Furyl OEt 4m 12 8914 Ph OMe 5a 15 9515 o-MeOC6H4 OMe 5b 10 9416 p-O2NC6H4 OMe 5c 20 9617 p-MeC6H4 OMe 5d 25 9318 p-ClC6H4 OMe 5e 12 9019 p-HOC6H4 OMe 5f 10 8820 p-FC6H4 OMe 5g 20 8521 p-NCC6H4 OMe 5h 35 9122 p-BrC6H4 OMe 5i 40 9423 o-ClC6H4 OMe 5j 15 9624 4-HO-3-MeOC6H3 OMe 5k 19 8925 n-Pr OMe 5l 15 8026 2-Furyl OMe 5m 25 8627 m-O2NC6H4 OMe 5n 30 91
a All products were characterized by IR, 1H NMR and 13C NMR
spectroscopic data, and melting points. b Yields refer to isolated
products.
Table 3 Reuse of ‘‘free’’ nano-g-Fe2O3 in the synthesis of1,4-dihydropyridinesa
Run 1 2 3 4 5
Yieldb (%) 98 98 96 96 92
a Benzaldehyde–ethyl acetoacetate–NH4OAc= 1 : 2 : 1.5, solvent-free,
90 1C, 0.15 mmol ‘‘free’’ nano-g-Fe2O3; the weight loss of catalyst after
5 runs was 9 wt%. b Isolated yields.