Nano n-propylsulfonated γ-Fe2O3 as magnetically recyclable heterogeneous catalyst for the efficient synthesis of β-phosphonomalonates

Article abstract of DOI:10.1016/j.apcata.2011.09.039

Nano n-propylsulfonated γ-Fe₂O₃ as a new sulfonated nanomagnetic iron oxide was synthesized directly through ring opening reaction of 1,3-propanesultone with nano magnetic γ-Fe₂O ₃ and used as magnetically recyclable heterogeneous catalyst for the efficient one-pot synthesis of β-phosphonomalonates. The catalyst was easily isolated from the reaction mixture by magnetic decantation using an external magnet and reused at least five times without significant degradation in the activity.

Full text of DOI:10.1016/j.apcata.2011.09.039

Applied Catalysis A: General 409–410 (2011) 162–166
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Nano n-propylsulfonated ␥-Fe2O3 as magnetically recyclable heterogeneous
catalyst for the efficient synthesis of ␤-phosphonomalonates
Sara Sobhani∗, Zahra Pakdin Parizi, Nasrin Razavi
Department of Chemistry, College of Sciences, Birjand University, Birjand 414, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Nano n-propylsulfonated ␥-Fe2O3 as a new sulfonated nanomagnetic iron oxide was synthesized directly
through ring opening reaction of 1,3-propanesultone with nano magnetic ␥-Fe2O3 and used as magnet-
ically recyclable heterogeneous catalyst for the efficient one-pot synthesis of ␤-phosphonomalonates.
The catalyst was easily isolated from the reaction mixture by magnetic decantation using an external
magnet and reused at least five times without significant degradation in the activity.
© 2011 Elsevier B.V. All rights reserved.
Received 26 July 2011
Received in revised form
28 September 2011
Accepted 29 September 2011
Available online 6 October 2011
Keywords:
Nanomagnetic iron oxide
Heterogeneous catalyst
␤-phosphonomalonates
1,3-propanesultone
1. Introduction
Magnetic nanoparticles (MNPs) have attracted much attention
by researchers in different areas including physics, biomedicine,
Phosphonates exhibit a wide range of notable biological prop-
erties which expand their applications as enzyme inhibitors,
metabolic probes [1–3], peptide mimetics [4], antibiotics, and
pharmacologic agents [5,6] in addition to their traditional roles
as intermediates in organic synthesis [7]. Extensive efforts have
been made to introduce convenient and efficient routs to synthe-
sis of phosphonates. Direct phosphorus-carbon bond formation
represents the most versatile and powerful technique for the
synthesis of phosphonates. Amongst the methods for P–C bond
formation, phospha-Michael addition, that is, the addition of
a phosphorous nucleophile to an electron-deficient alkene, has
evoked remarkable attention of organic chemists [8–22]. Synthe-
sis of ␤-phosphonomalonates by this method has been commonly
promoted by bases [8–13], BrØnsted/Lewis acids [14–16], tran-
sition metals [17,18], radical initiators such as AIBN [19,20] and
microwave radiation [21]. Even though, phospha-Michael addition
could be proceed by these methods, many of these reagents can-
not be reused, and in many instances, long reaction time, drastic
reaction conditions and sometimes, according to the nature of the
catalyst, tedious workup is needed. Therefore, the development of
a new method to overcome these shortcomings still remains as an
ongoing challenge for the synthesis of these significant scaffolds.
biotechnology, material science and catalysis, because of the large
ratio of surface area to volume, paramagnetic behavior and low
toxicity [23–25]. In recent years, MNPs acquired organic chemists’
attention as a new alternative to porous materials for supporting
catalytic transformations [26–34]. The magnetic nature of MNPs
allows a convenient method for removing and recycling MNPs-
supported catalysts by applying an appropriate magnetic field. In
contrast to filtration and centrifugation methods, this kind of sepa-
ration is not time-consuming and prevents the loss of solid catalyst
in the process. It also enhances products purity and optimizes oper-
ational costs. In addition, the MNPs-supported catalysts show high
dispersion and reactivity with a high degree of chemical stability.
On the other hand, n-propylsulfonated surface materials with
propanesulfonate moieties are organic–inorganic hybrid materials
that have been applied as effective solid acid catalysts in organic
transformations [35–38]. In these types of solid acid catalysts,
the reactive centers are highly mobile similar to that of homoge-
neous catalysts. At the same time these species have the advantage
of being recyclable in the same fashion as heterogeneous cata-
lysts. In general, synthesis of n-propylsulfonated surface materials
with propanesulfonate moieties was conducted by SH oxidation of
supported mercaptopropyl [37–39] or directly through ring open-
ing reaction of 1,3-propanesultone with surface hydroxyl groups
[35,36]. In both cases, sulfonic acid groups are introduced on the
surface via covalently bonds. However, in SH oxidation method,
imperfect oxidation of SH groups decreases the efficiency of the
catalyst.
∗
Corresponding author. Tel.: +98 561 2502009; fax: +98 561 2502009.
E-mail addresses: ssobhani@birjand.ac.ir, sobhanisara@yahoo.com (S. Sobhani).
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.09.039

S. Sobhani et al. / Applied Catalysis A: General 409–410 (2011) 162–166
163
Combining characteristic properties of organic–inorganic
hybrid materials and nanomagnetic iron oxides as sup-
a
porting material, herein, we report the synthesis of nano
n-propylsulfonated ␥-Fe2O3 (NPS-␥-Fe2O3) directly through ring
opening reaction of 1,3-propanesultone by nano magnetic ␥-Fe2O3.
We have also used these newly synthesized sulfonated nanomag-
netic iron oxide as a magnetically recyclable heterogenous catalyst
for the efficient one-pot synthesis of ␤-phosphonomalonates.
Scheme 1.
2. Experimental
3. Results and discussion
2.1. Synthesis of NPS-ꢀ-Fe2O3
Due to the reasonable needs to clean and green recovery of
the heterogenous catalyst, especially acid catalysts, we synthe-
sized NPS-␥-Fe2O3 as a new sulfonated nanomagnetic iron oxide
simply by a direct method through the ring opening reaction of 1,3-
propanesultone with nano magnetic ␥-Fe2O3 in refluxing toluene
(Scheme 1). The synthesized NPS-␥-Fe2O3 was then characterized
by different methods such as XRD, SEM, TEM and FT-IR. The amount
of sulfonic acid loaded on the surface of nano magnetic ␥-Fe2O3 was
determined by TG analysis and confirmed by ion-exchange pH, back
titration and elemental analysis.
The maghemite (␥-Fe2O3) nanoparticles were synthesized by a
reported chemical co-precipitation technique of ferric and ferrous
ions in alkali solution with minor modifications [41,42]. FeCl2·4H2O
(9.25 mmol) and FeCl3·6H2O (15.8 mmol) were dissolved in deion-
ized water (150 mL) under Ar atmosphere at room temperature.
A NH4OH solution (25%, 50 mL) was then added dropwise (drop
rate = 1 mL min−1) to the stirring mixture at room temperature
to reach the reaction pH to 11. The resulting black dispersion
was continuously stirred for 1 h at room temperature and then
heated to reflux for 1 h to yield a brown dispersion. The mag-
netic nanoparticles were then purified by a repeated centrifugation
(1730–3461 g, 20 min), decantation, and redispersion cycle 3 times.
The as-synthesized sample was heated at 2 ◦C min−1 up to 200 ◦C
and then kept in the furnace for 3 h to give a reddish-brown
powder. The sulfonation of maghemite nanoparticles was con-
ducted using the reaction of ␥-Fe2O3 with 1,3-propanesultone.
␥-Fe2O3 (6 g) was suspended in 600 mL of 0.1 M toluene solution
of 1,3-propanesultone and the colloidal solution was refluxed for
48 h. The n-propylsulfonated ␥-Fe2O3 was isolated and purified by
repeated washing (first in CH3CN and then in water) and centrifuga-
tion.
3.1. X-ray diffraction (XRD) analysis
Fig. 1 shows the XRD pattern of nano n-propylsulfonated ␥-
Fe2O3. Diffraction peaks at around 2ꢁ = 30.4◦, 35.8◦, 43.3◦, 54.0◦,
57.6◦, 63.2◦ corresponding to the (2 2 0), (3 1 1), (4 0 0), (4 2 2),
(5 1 1) and (4 4 0) are readily recognized from the XRD pattern.
The observed diffraction peaks agree with the cubic structure of
maghemite (JCPDS file No 39-1364) with a unit cell dimension of
˚
8.35 A and the space group of P4132 (2 1 3). The average crystallite
size was calculated to be 10.7 nm using the Scherrer equation in
which K = 0.9 and ꢂ = 0.154.
3.2. SEM and TEM
2.2. Ion-exchange pH analysis of the catalyst
The size and structure of the NPS-␥-Fe2O3 were also evaluated
using scanning electron microscopy (SEM) and transmission elec-
tron microscopy (TEM). According to the SEM (Fig. 2) and TEM
(Fig. 3), it was observed that the synthesized NPS-␥-Fe2O3 has nano
dimension ranging from ∼35 to 100 nm.
To an aqueous solution of NaCl (1 M, 25 mL) with a primary pH
5.93, the catalyst (500 mg) was added and the resulting mixture
was stirred for 2 h after which the pH of solution decreased to 2.25.
This is equal to a loading of 0.28 mmol SO3H g−1
.
2.3. Back titration analysis of the catalyst
2200
NaOH solution (10 mL, 0.1 M) was added to the catalyst (100 mg)
in an Erlenmeyer flask. Excess amount of the base was neu-
tralized by addition of HCl solution (0.1 M) to the equivalence
point of titration. The required volume of HCl to this point was
9.75 mL.
2000
)
2
1800
4
2
( 4
0
0
)
(
1600
1400
1200
1000
800
2.4. General procedure for the synthesis of
ˇ-phosphonomalonates
NPS-␥-Fe2O3 (1 mol%) was added to a mixture of aldehyde
(1 mmol), malononitrile (1 mmol) and triethyl phosphite (1 mmol).
The mixture was stirred at 60 ◦C for the appropriate time (Table 2).
The reaction was monitored by TLC. After completion, the reaction
mixture was diluted with EtOH. The catalyst was separated by an
external magnet, washed with EtOH, dried and re-used for a con-
secutive run under the same reaction conditions. Evaporation of the
solvent of the filtrate under reduced pressure gave a crude prod-
uct. The pure products (1–12) were isolated by chromatography on
silica gel eluted with n-hexane:EtOAc (1:2).
20
30
40
50
60
70
80
o
2 Theta ( )
Fig. 1. XRD spectra of NPS-␥-Fe2O3.

164
S. Sobhani et al. / Applied Catalysis A: General 409–410 (2011) 162–166
101
100
214° C
98.75%
99
98
97
96
95
449° C
96.92%
744° C
95.96%
100
200
300
400
500
600
700
800
Fig. 2. SEM image of NPS-␥-Fe2O3.
Temperature
Fig. 5. TGA diagram of NPS-␥-Fe2O3.
are related to the stretching of the S O bonds. A peak appeared at
3470 cm−1 is due to the stretching of OH groups in the SO3H.
3.4. Thermo-gravimetric analysis (TGA)
The thermal behavior of NPS-␥-Fe2O3 is shown in Fig. 5. A signif-
icant decrease in the weight percentage of the n-propylsulfonated
␥-Fe2O3 at about 150 ◦C is related to desorption of water molecules
from the catalyst surface. This was evaluated to be ∼1.3% accord-
ing to the TG analysis. In addition, the analysis showed two other
decreasing peaks. First peak appears at temperature around 300 ◦C
due to the decomposition of sulfuric acid and formation of sulfur
dioxide. This is followed by a second peak at 660 ◦C, correspond-
ing to the loss of the organic spacer group. According to the TGA,
the amount of sulfuric acid functionalized on ␥-Fe2O3 is evaluated
to be 0.22 mmol g−1. These results are in agreement with those of
ion-exchange pH, back titration and elemental analysis (S 0.721%
and C 0.928%).
Fig. 3. TEM image of NPS-␥-Fe2O3.
3.5. Catalytic activity of NPS-ꢀ-Fe2O3 for the one-pot synthesis of
ˇ-phosphonomalonates
3.3. FT-IR spectroscopy
As a part of our ongoing program directed towards the develop-
ment of new methods for the synthesis of phosphonate derivatives,
we have recently introduced micellar solution of sodium
stearate for the one-pot synthesis of ␤-phosphonomalonates
In FT-IR of NPS-␥-Fe2O3 (Fig. 4), characteristic adsorption bands
due to the stretching vibrations of Fe–O bond in ␥-Fe2O3 are
observed at 638.94 cm−1. The peaks placed in 1047 and 1220 cm−1
[43] and heterogeneous catalysts [H3PMo12O40
, HClO4/SiO2,
aminopropylated silica (AP-SiO2)] for the two-pot synthesis of ␤-
phosphonomalonates [44–46]. In this relation, in order to show
the merit of synthesized NPS-␥-Fe2O3 as magnetically recyclable
heterogeneous catalyst in organic reactions, we used this new
sulfonated nanomagnetic iron oxide for the one-pot synthesis of ␤-
phosphonomalonates directly from aldehydes, malononitrile and
trialkyl phosphites.
For this purpose, one-pot reaction of benzaldehyde, malonon-
itrile and triethyl phosphite was chosen to optimize the reaction
conditions such as temperature and solvent (Table 1). We found
that in the presence of NPS-␥-Fe2O3, the best yield of the cor-
responding ␤-phosphonomalonate was obtained at 60 ◦C under
solvent-free conditions (entry 1). In order to show the role of the
catalyst, similar reactions in the absence of the catalyst or in the
presence of nanomagnetic ␥-Fe2O3 were also examined. Under
these conditions, the reactions led to the formation of the desired
product in low yields after a long reaction time (entries 8 and 9).
Fig. 4. FT-IR spectra of ␥-Fe2O3 and NPS-␥-Fe2O3.

S. Sobhani et al. / Applied Catalysis A: General 409–410 (2011) 162–166
165
Fig. 6. (a) Reaction mixture, (b) Separation of NPS-␥-Fe2O3 from the reaction mixture by an external magnetic field.
Table 1
O
One-pot reaction of benzaldehyde, malononitrile and triethyl posphite under dif-
ferent conditions.
(EtO)2P
CN
CN
R
H
CN
CN
NPS- -Fe2O3 (1 mol%)
solvent-free, 60 ºC
P(OEt)3
O
+
+
Entry
Catalyst
Solvent
Time (h)
Yielda (%)
R
1
2
3
4
5
6
7
8
9
NPS-␥-Fe2O3
NPS-␥-Fe2O3
NPS-␥-Fe2O3
NPS-␥-Fe2O3
NPS-␥-Fe2O3
NPS-␥-Fe2O3
NPS-␥-Fe2O3
␥-Fe2O3
–
–
H2O
Toluene
n-Hexane
CH3CN
EtOH
–
1
1
3
24
24
24
24
24
24
90
65
51
84
52
71
65
62
64
b
R = aryl, heteroaryl, alkyl
1-12
Scheme 2.
of the present method (Scheme 2). The results of this study are
depicted in Table 2.
c
–
–
As shown in Table 2, differently substituted benzaldehydes with
electron-donating and electron-withdrawing groups underwent
successful Knoevenagel-phospha-Michael reaction with malonon-
itrile and triethyl phosphite (entries 1–8). The catalyst was
compatible with functional groups such as Cl, Br and O–Me. No
competitive nucleophilic methyl ether cleavage was observed in
the substrate possessing an aryl–O–Me group (entry 7), despite
the strong nucleophilicity of phosphites. Acid-sensitive aldehy-
des, such as furan-2-carbaldehyde and pyridine-3-carbaldehyde
underwent smooth reactions without any decomposition or poly-
merization under the present reaction conditions (entries 9 and
10). This method is also applicable for the synthesis of ␤-
phosphonomalonates from the reaction of triethyl phosphite with
aliphatic aldehydes and malononitrile (entries 11 and 12). The reac-
tion of ketones with malononitrile and triethyl phosphite was also
studied under the same conditions. However, the results were not
as positive as those presented above.
a
Isolated yield, conditions: benzaldehyde (1 mmol), malononitrile (1 mmol), tri-
ethyl phosphite (1 mmol), catalyst (1 mol%, except for entry 9), 60 ◦C (except for
entry 3).
b
Conditions: room temperature.
No catalyst.
c
It is important to note that the magnetic property of NPS-␥-
Fe2O3 facilitates its efficient recovery from the reaction mixture
during work-up procedure. In the presence of an external mag-
net, NPS-␥-Fe2O3 moved onto the magnet steadily and the reaction
mixture turned clear within 10 s (Fig. 6). The catalyst was isolated
by simple decantation. After washing with EtOH and then drying
for 30 min at 110 ◦C, NPS-␥-Fe2O3 was reused without any signif-
icant deactivation even after five runs. The average isolated yield
for five successive runs was 85.6%, which clearly demonstrates the
practical recyclability of this catalyst (Fig. 7).
The reaction of a variety of aldehydes with malononitrile and
triethyl phosphite was then investigated to confirm the generality
Furthermore, we have evaluated the generality of the presented
method for the one-pot synthesis of ␤-phosphonomalonates from
Table 2
One-pot synthesis of different ␤-phosphonomalonates catalyzed by NPS-␥-Fe2O3.
Entry Carbonyl compound
␤-phosphonomalonate Time (min) Yielda (%)
1
2
3
4
5
6
7
8
9
benzaldehyde
1
2
3
4
5
6
7
8
9
60
60
30
30
45
15
240
120
60
5
90
80
82
88
87
85
85
91
83
96
85
84
2-chlorobenzaldehyde
3-chlorobenzaldehyde
4-chlorobenzaldehyde
3-bromobenzaldehyde
4-bromobenzaldehyde
4-methoxybenzaldehyde
4-methylbenzaldehyde
furfural
10
11
12
3-pyridincarbaldehyde
butyraldehyde
heptanaldehyde
10
11
12
60
60
a
Isolated yield, conditions: aldehyde (1 mmol), malononitrile (1 mmol), triethyl
phosphite (1 mmol), catalyst (0.01 mmol), 60 ◦C, solvent-free conditions. All the
products were characterized by spectroscopic methods and compared with the
authentic spectra [13,47–49].
Fig. 7. Reusability of NPS-␥-Fe2O3 as a magnetically recyclable heterogeneous cat-
alyst for the synthesis of ␤-phosphonomalonate 1.

166
S. Sobhani et al. / Applied Catalysis A: General 409–410 (2011) 162–166
O
aldehydes and malononitrile. The magnetic catalyst was easily iso-
lated from the reaction mixture by magnetic decantation using an
external magnet and reused at least five times without significant
degradation in the activity.
X
Y
(R'O)2P
CHO
+
X
Y
NPS- -Fe2O3 (1 mol%)
solvent-free, 60 ºC
P(OR')3
+
Cl
Cl
Acknowledgement
Scheme 3.
We are thankful to Birjand University Research Council for their
support on this work.
Table 3
One-pot synthesis of ␤-phosphonomalonates from different in situ generated
Michael acceptors and trialkyl phosphites catalyzed by NPS-␥-Fe2O3.
Appendix A. Supplementary data
Entry
R
X
Y
Product
4
Time (h)
Yielda (%)
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2011.09.039.
1
2
3
4
5
6
Et
CN
CN
CN
CN
CO2Et
H
CN
CN
CN
CO2Et
CO2Et
NO2
0.5
2
3
8
24
24
88
85
80
75b
Me
iso-Pr
Et
Et
Et
13
14
15
–
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NPS-␥-Fe2O3
HClO4-SiO2
AP-SiO2
H3PMo12O40
Sodium stearate
1
1
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1
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90
30
42
35
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As it is shown in Table 3, the catalytic one-pot reaction of
4-chlorobenzaldehyde and malononitrile proceeded well with
trialkyl phosphites such as triethyl/trimethyl/tri-iso-propyl phos-
phite (entries 1–3). These results demonstrate that both the yields
and the reaction time are relatively independent of the phosphorus
compound. In addition to malononitrile, some other in situ gener-
ated Michael acceptors were also examinedto carry out the reaction
with triethyl phosphite (entries 4–6). The results showed that the
reaction involving ethyl cyano acetate worked well and the desired
product was obtained in 75% yield. However, no product was pro-
duced when diethyl malonate or nitromethane were used in this
one-pot reaction under the same conditions.
In order to show the unique catalytic behavior of NPS-␥-Fe2O3
in these reactions, we have performed one-pot reaction of ben-
zaldehyde, malononitrile and triethyl phosphite in the presence of a
catalytic amount of HClO4-SiO2, AP-SiO2, H3PMo12O40 and sodium
stearate (Table 4). As it is evident from Table 4, NPS-␥-Fe2O3 is the
most effective catalyst for this purpose, leading to the formation of
␤-phosphonomalonate (1) in a good yield.
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4. Conclusion
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In summary, NPS-␥-Fe2O3 as a new sulfonated nanomagnetic
iron oxide was synthesized directly through ring opening reac-
tion of 1,3-propanesultone with nano magnetic ␥-Fe2O3. The
synthesized NPS-␥-Fe2O3 was used as a magnetically recyclable
heterogeneous catalyst for the efficient one-pot synthesis of ␤-
phosphonomalonates from the reaction of phosphite esters with