Lanthanum(III) triflate supported on nanomagnetic γ-Fe 2O3: A new magnetically recyclable heterogeneous Lewis acid for the one-pot synthesis of β-phosphonomalonates

Article abstract of DOI:10.1039/c4ra00543k

Lanthanum(iii) triflate supported on nanomagnetic γ-Fe ₂O₃ was synthesized and characterized by HRTEM, XRD, ICP, FT-IR, TGA and VSM. It was applied as a magnetically recyclable heterogeneous Lewis acid catalyst for the efficient one-pot synthesis of β- phosphonomalonates via tandem Knoevenagel-phospha-Michael reaction. The catalyst was easily separated from the reaction mixture by magnetic decantation using an external magnet and reused ten times. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra00543k

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Lanthanum(III) triflate supported on nanomagnetic
g-Fe2O3: a new magnetically recyclable
heterogeneous Lewis acid for the one-pot synthesis
of b-phosphonomalonates†
Cite this: RSC Adv., 2014, 4, 13071
*
Sara Sobhani and Zahra Pakdin-Parizi
Lanthanum(III) triflate supported on nanomagnetic g-Fe2O3 was synthesized and characterized by HRTEM,
XRD, ICP, FT-IR, TGA and VSM. It was applied as a magnetically recyclable heterogeneous Lewis acid
catalyst for the efficient one-pot synthesis of b-phosphonomalonates via tandem Knoevenagel–
phospha-Michael reaction. The catalyst was easily separated from the reaction mixture by magnetic
decantation using an external magnet and reused ten times.
Received 19th January 2014
Accepted 24th February 2014
DOI: 10.1039/c4ra00543k
www.rsc.org/advances
supports affords improved recycling and facile use in synthetic
schemes. Several techniques for the immobilization of these
Introduction
catalysts on insoluble matrixes such as organic polymers or
inorganic materials have been developed.9 These immobilized
catalysts have been recycled without any loss of catalytic activity.
However, while the embedded catalysts have very good activity,
their recovery by ltration or centrifugation methods is time
consuming and may cause loss of the catalyst. Therefore, there is
a need for the design and preparation of new version of hetero-
geneous RE(OTf)3 to circumvent this problem. One way to attain
this goal is to immobilize these catalysts onto magnetic nano-
particles (MNPs) such as iron oxide. Supported catalysts on MNPs
can be easily collected by using an external magnet without
additional centrifugation or ltration of the sample. Magnetic
separation of nanoparticles is simple, economical, and prom-
ising for industrial applications. To the best of our knowledge,
synthesis of RE(OTf)3 supported on MNPs have not yet been
investigated.
To benet the valuable applications of MNPs and unique
properties of RE(OTf)3 as Lewis acids, and in continues of our
recent works on the development of new heterogeneous cata-
lysts,10 in this paper, we have synthesized lanthanum(III) triate
supported on g-Fe2O3@SiO2 [g-Fe2O3@SiO2–La(OTf)2] as a new
catalyst (Scheme 1). We have also used the synthesized
g-Fe2O3@SiO2–La(OTf)2 as a magnetically recyclable heteroge-
neous Lewis acid for the efficient one-pot synthesis of
b-phosphonomalonates.
Among organophosphorous compounds, phosphonates are an
important class of molecules. They have broad application in
medicinal chemistry,1 material chemistry,2 and catalysis.3 There
are several methods for the synthesis of phosphonates. Among
them, phospha-Michael addition has evoked remarkable
attention by organic chemists.4 These protocols proceed
through two-pot procedure in which P–C and C–C bond
formations occurred in separate steps. It is worth to note that
scarce reports are available for the one-pot synthesis of
b-phosphonomalonates via tandem Knoevenagel–phospha-
Michael reaction.4d,5 Even though, phospha-Michael addition
could be proceed by these methods, most of the existing
protocols suffer from limitations such as low yields, long reac-
tion times, using hazardous organic solvents or unrecyclable
catalysts and tedious work-up procedures leading to copious
amount of toxic wastes. So, it is still preferable to follow an eco-
friendly procedure that applies an efficient and reusable catalyst
for the one-pot synthesis of b-phosphonomalonates.
In the last two decades, rare earth metal triates [RE(OTf)3]
including lanthanide triates [Ln(OTf)3] have been introduced as
promising mild, powerful and selective Lewis acids for a variety
of functional group transformations.6 While most of Lewis acids
are decomposed or deactivated in the presence of water or protic
solvents, RE(OTf)3 are quite stable in aqueous media7 and can be
recovered from the reaction mixture by aqueous extraction and
reused.8 As a consummate goal in industrial and economical
catalyst development, immobilization of RE(OTf)3 on solid
Experimental
Department of Chemistry, College of Sciences, University of Birjand, Birjand, Iran.
E-mail: ssobhani@birjand.ac.ir; sobhanisara@yahoo.com; Fax: +98 (561) 2502065;
Tel: +98 (561) 2502065
Chemicals were purchased from Merck Chemical Company.
NMR spectra were recorded on a Bruker Avance DPX-250 and
400 in CDCl3 as solvent and TMS as internal standard. The
purity of the products and the progress of the reactions were
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra00543k
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followed by washing three times with EtOH and diethyl ether
ꢁ
and dried at 100 C in vacuum for 24 h.
Synthesis of aminopropylated g-Fe2O3@SiO2 (ref. 13)
The prepared g-Fe2O3@SiO2 (3.5 g) was sonicated in dry toluene
(50 mL) for 30 min. 3-Aminopropyltriethoxysilane (3.5 mL) was
added to the dispersed g-Fe2O3@SiO2 in toluene under
mechanical stirring, slowly heated to 105 ꢁC and kept at this
temperature for 24 h. The solid was separated by an external
magnet, washed three times with ethanol and dried under
vacuum.
Scheme 1 Synthesis of g-Fe2O3@SiO2–La(OTf)2.
Synthesis of Schiff base supported on g-Fe2O3@SiO2
(g-Fe2O3@SiO2–L)
accomplished by TLC on silica-gel polygram SILG/UV254 plates.
HRTEM analysis was performed using HRTEM microscope
(Philips CM30). FT-IR spectra were recorded on a Shimadzu
Fourier Transform Infrared Spectrophotometer (FT-IR-8300). IR
spectra were run on a Perkin Elmer 780 instrument. Mass
spectra were recorded on a Shimadzu GCMS-QP5050A. Ther-
mogravimetric analysis (TGA) was performed using a Shimadzu
thermogravimetric analyser (TG-50). Power X-ray diffraction
(XRD) was performed on a Bruker D8-advance X-ray diffrac-
tometer with Cu Ka (l ¼ 0.154 nm) radiation. Room tempera-
ture magnetization isotherms were obtained using a vibrating
sample magnetometer (VSM, LakeShore 7400). The content of
La in the catalyst was determined by ICP-OES VISTA-PRO
inductively coupled plasma analyzer.
The synthesized aminopropylated g-Fe2O3@SiO2 (3 g) was
sonicated in absolute ethanol (50 mL) for 30 min. Salicylalde-
hyde (3 mL) ꢁwas added slowly to the sonicated mixture and
stirred at 80 C for 24 h. The resulting mixture was cooled to
room temperature. The solid was separated by an external
magnet, washed with ethanol and dried at 50 ꢁC in oven under
vaccum.
Synthesis of g-Fe2O3@SiO2–La(OTf)2
g-Fe2O3@SiO2–L (2 g) was sonicated in dry acetonitrile (30 mL)
for 30 min. Lanthanum(III) triate (0.5 g) was added to the
dispersed mixture in acetonitrile under mechanical stirring.
The reaction mixture was stirred at room temperature for 24 h.
The solid was separated by an external magnet and washed
three times with acetonitrile. It was then dried under vaccum at
Synthesis of g-Fe2O3 MNPs
ꢁ
90 C overnight to furnish g-Fe2O3@SiO2–La(OTf)2.
g-Fe2O3 MNPs were synthesized by a reported chemical co-
precipitation technique of ferric and ferrous ions in alkali
solution with minor modications.11 FeCl2$4H2O (1.99 g) and
FeCl3$6H2O (3.25 g) were dissolved in deionized water (30 mL)
under Ar atmosphere at room temperature. A NH4OH solution
(0.6 M, 200 mL) was then added drop wise (drop rate ¼ 1 mL
minꢀ1) to the stirring mixture at room temperature to reach the
reaction pH to 11. The resulting black dispersion was continu-
ously stirred for 1 h at room temperature and then heated to
reux for 1 h to yield a brown dispersion. The magnetic nano-
particles were then separated by an external magnet and
washed with deionized water until it was neutralized. The as-
synthesized sample was heated at 2 ꢁC minꢀ1 up to 250 ꢁC and
then kept in the furnace for 3 h to give a reddish-brown powder.
General procedure for the synthesis of
b-phosphonomalonates
A mixture of aldehyde (1 mmol), malononitrile (1 mmol), triethyl
phosphite (1 mmol) and g-Fe2O3@SiO2–La(OTf)2 (0.08 g, 1 mol%)
was stirred at room temperature for an appropriate time
(Table 2). The reaction mixture was diluted with EtOAc. The
catalyst was separated by an external magnet, washed with
EtOAc, dried and re-used for a consecutive run under the same
reaction conditions. Evaporation of the solvent of the ltrate
under reduced pressure gave the crude products. The pure
products (1–14) were isolated by chromatography on silica gel
eluted with n-hexane–EtOAc (1 : 2).
Results and discussion
Catalyst synthesis and characterization
Synthesis of silica-coated magnetite nanoparticles
(g-Fe2O3@SiO2)12
The synthesized g-Fe2O3 (3.5 g) was suspended in deionized The synthetic procedure of g-Fe2O3@SiO2–La(OTf)2 is outlined
water (40 mL)–ethanol (160 mL) and sonicated at 40 ꢁC for 1 h. in Scheme 1. At rst, g-Fe2O3 MNPs were synthesized by a
Concentrated solution of ammonia (3.5 mL) was added slowly chemical co-precipitation technique of ferric and ferrous ions in
to the sonicated mixture and stirred at 40 ꢁC for 30 min. alkali solution.11 The magnetic nanoparticles were then allowed
Subsequently, tetraethyl orthosilicate (TEOS, 2.0 mL) was to react with an alkaline solution of tetraethyl orthosilicate
charged to the reaction vessel, and the mixture was continu- (TEOS) to give g-Fe2O3@SiO2. The outer shell of silica not only
ꢁ
ously stirred at 40 C for 24 h. The silica-coated nanoparticles improved the dispersibility but also provided suitable sites (Si–
(g-Fe2O3@SiO2) were collected by
a
permanent magnet, OH groups) for further surface functionalization. The resulting
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g-Fe2O3@SiO2 was then functionalized with 3-amino-
propyltriethoxysilane followed by condensation with salicyl-
aldehyde to give Schiff base supported on g-Fe2O3@SiO2
(g-Fe2O3@SiO2–L). Finally, the reaction of La(OTf)3 with
g-Fe2O3@SiO2–L in dry acetonitrile led to the formation of
g-Fe2O3@SiO2–La(OTf)2.
The size and structure of g-Fe2O3@SiO2–La(OTf)2 were eval-
uated using high resolution transmission electron microscopy
(HRTEM). HRTEM image demonstrated spherical-like shape for
g-Fe2O3@SiO2–La(OTf)2 with diameter of ca. 13 nm (Fig. 1). It
also showed the core–shell structure of the particles and
conrmed the presence of the silica shell, which has a uniform
thickness of 2 nm (Fig. 1b).
The crystalline structure of g-Fe2O3 and g-Fe2O3@SiO2– Fig. 3 TGA diagram of g-Fe2O3@SiO2–La(OTf)2.
La(OTf)2 were characterized by X-ray diffraction (XRD) (Fig. 2).
The XRD pattern of g-Fe2O3 showed characteristic peaks
matched with those of standard g-Fe2O3 nanoparticles (JCPDS
components attached to the surface. According to the TGA,
the amount of organic components functionalized on
g-Fe2O3@SiO2 calculated to be 0.13 mmol gꢀ1. The ICP analysis
showed that 0.12 mmol of lanthanum was anchored on 1.0 g of
g-Fe2O3@SiO2–La(OTf)2.
˚
le no. 04-0755) with a unit cell dimension of 8.35 A and space
group of P4132 (213). The same set of characteristic peaks was
observed in the XRD pattern of g-Fe2O3@SiO2–La(OTf)2, which
indicated the stability of the crystalline phase of nanoparticles
during the subsequent surface modication. The deduced
average size of g-Fe2O3@SiO2–La(OTf)2 from Scherrer's formula
was about 13.5 nm.
Thermogravimetric analysis (TGA) was used to study the
thermal stability of the catalyst. The thermal behaviour of
g-Fe2O3@SiO2–La(OTf)2 is shown in Fig. 3. Two weight loss
stages were observed in the TGA curve. The rst region, which
occurred below 147 ꢁC was corresponded to the loss of trapped
water from the catalyst. In the second stage, weight loss was
about 8.3 wt%, which could be attributed to the loss of organic
FT-IR spectra of g-Fe2O3 and g-Fe2O3@SiO2–La(OTf)2 are
shown in Fig. 4. The band at around 620–650 cmꢀ1 was assigned
to the stretching vibrations of Fe–O bond in g-Fe2O3 and
g-Fe2O3@SiO2–La(OTf)2. The intense band in the range of 1000–
1300 cmꢀ1 was related to the vibration of Si–O–Si bond. The
peaks positioned at around 1500–1610 and 1660 cmꢀ1 in the
FT-IR spectrum of g-Fe2O3@SiO2–La(OTf)2 were related to
the stretching vibrations of C]C and C]N bonds, respectively.
The band at 1440 cmꢀ1 was assigned to the stretching vibration
of S]O, which attributed to the sulfonate group of lanthanu-
m(III) triate loaded on MNPs. The strong and broad band in the
range of 2800–3700 cmꢀ1 corresponded to Si–OH groups and
adsorbed water that overlapped with C–H stretching vibration
of phenyl and CH2 groups. A band at 1650 cmꢀ1 in the spectra of
g-Fe2O3 and g-Fe2O3@SiO2–La(OTf)2 was also due to the OH
vibration of adsorbed water.
The magnetization curves of g-Fe2O3 and g-Fe2O3@SiO2–
La(OTf)2 were measured at room temperature with a vibrating
sample magnetometry (VSM). As shown in Fig. 5, no reduced
remanence and coercivity were detected, indicating both
Fig. 1 HRTEM images of g-Fe2O3@SiO2–La(OTf)2.
Fig. 2 XRD patterns of g-Fe2O3 and g-Fe2O3@SiO2–La(OTf)2.
Fig. 4 FT-IR spectra of g-Fe2O3 and g-Fe2O3@SiO2–La(OTf)2.
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temperature (Table 1, entries 1–6). We found that in the pres-
ence of 1 mol% of g-Fe2O3@SiO2–La(OTf)2, the corresponding
b-phosphonomalonate was obtained in the best yield at room
temperature under solvent-free conditions (entry 1). When
0.5 mol% of the catalyst was used, the yield of the desired
product was decreased to 83% (entry 6).
In order to show the role of the catalyst, similar reactions in
the absence of the catalyst and in the presence of nanomagnetic
g-Fe2O3 and aminopropylated g-Fe2O3@SiO2 were also exam-
ined. These reactions led to the formation of the desired
product in lower yields than that obtained in the presence of
g-Fe2O3@SiO2–La(OTf)2 (entries 7–9). La(OTf)3 catalyzed a
similar reaction as well as g-Fe2O3@SiO2–La(OTf)2 (entry 10).
This observation showed that the catalylic activity of La(OTf)3
did not changed aer its supporting on the surface of
nanoparticles.
Fig. 5 Magnetization curves of g-Fe2O3 and g-Fe2O3@SiO2–La(OTf)2.
To conrm the generality of the present method, we next
examined the reaction of a variety of aldehydes with malono-
nitrile and triethyl phosphite under the optimized reaction
conditions. The results of this study are depicted in Table 2.
As indicated in Table 2, reactions of benzaldehyde and
different substituted benzaldehydes containing electron-
donating and electron-withdrawing groups with malononitrile
and triethyl phosphite proceeded well to give the corresponding
b-phosphonomalonates in good to high yields (entries 2–10).
Acid-sensitive aldehydes such as pyridine-3-carbaldehyde and
furan-2-carbaldehyde underwent successful tandem Knoevena-
gel–phospha-Michael reaction without any decomposition or
polymerization (entries 11 and 12). This catalytic system was
also successfully applied for the reaction of aliphatic aldehydes
with malononitrile and triethyl phosphite and produced the
desired products in good yields (entries 13 and 14).
unmodied and La(OTf)2-modied g-Fe2O3 are super-
paramagnetic. The value of saturation magnetic moments of
g-Fe2O3 and g-Fe2O3@SiO2–La(OTf)2 are 68.6 and 66.5 emu gꢀ1
,
respectively.
Evaluation of catalytic activity of g-Fe2O3@SiO2–La(OTf)2 in
the synthesis of b-phosphonomalonates via tandem
Knoevenagel–phospha-Michael reaction
In continuous of our recent research programs on the devel-
opment of novel methods for the synthesis of phosphonate
derivatives,10b–f,14 in this paper, we have studied the applicability
of g-Fe2O3@SiO2–La(OTf)2 as a new magnetically recyclable
heterogeneous Lewis acid for the one-pot synthesis of b-phos-
phonomalonates directly from aldehydes, malononitrile and
trialkyl phosphites.
It is important to note that the magnetic property of
g-Fe2O3@SiO2–La(OTf)2 facilitates efficient recovery of the
catalyst from the reaction mixture during work-up procedure.
At rst, one-pot reaction of benzaldehyde, malononitrile and
triethyl phosphite was chosen to optimize the reaction condi-
tions such as solvents and molar ratio of the catalyst at room
Table 1 One-pot reaction of benzaldehyde, malononitrile and triethyl phosphite under different conditions
Entry
Catalyst
Solvent
Time (h)
Yielda (%)
1
2
3
4
5
6
7
8
9
10
g-Fe2O3@SiO2–La(OTf)2
g-Fe2O3@SiO2–La(OTf)2
g-Fe2O3@SiO2–La(OTf)2
g-Fe2O3@SiO2–La(OTf)2
g-Fe2O3@SiO2–La(OTf)2
g-Fe2O3@SiO2–La(OTf)2
—
1
1
1
1
1
1
24
1
1
1
91
42
76
78
67
83
46
62
66
92
H2O
EtOH
CHCl3
Toluene
—
—
—
—
—
b
c
—
g-Fe2O3
d
e
Aminopropylated g-Fe2O3@SiO2
La(OTf)3
a
Isolated yield, conditions: benzaldehyde (1 mmol), malononitrile (1 mmol), triethyl phosphite (1 mmol), catalyst (1 mol%, except for entries 6–9),
room temperature. b Catalyst ¼ 0.5 mol%. c No catalyst. d Catalyst ¼ 0.08 g. e Catalyst ¼ 0.002 g (1 mol%, 0.6 mmol of amine was anchored on 1.0 g
of catalyst).
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Table
2 One-pot synthesis of different b-phosphonomalonates
catalyzed by g-Fe2O3@SiO2–La(OTf)2
Entry
Carbonyl compound
Product
Time (h)
Yielda (%)
Fig. 7 Reusability of g-Fe2O3@SiO2–La(OTf)2 as a magnetically recy-
clable heterogeneous catalyst for tandem Knoevenagel–phospha-
Michael reaction of benzaldehyde, malononitrile and triethyl phosphite
at room temperature after 1 h.
1
2
3
4
5
6
7
8
Benzaldehyde
1
2
3
4
5
6
7
8
1
2
1
2
1
1
3
1
1
2
1
2
3
3
90
92
86
81
87
80
83
85
89
87
96
84
81b
77b
4-Chlorobenzaldehyde
3-Chlorobenzaldehyde
2-Chlorobenzaldehyde
4-Bromobenzaldehyde
3-Bromobenzaldehyde
4-Methoxybenzaldehyde
3-Methoxybenzaldehyde
4-Hydroxybenzaldehyde
4-Methylbenzaldehyde
3-Pyridincarbaldehyde
Furfural
9
9
10
11
12
13
14
10
11
12
13
14
Butyraldehyde
Heptanaldehyde
a
Isolated yield, conditions: aldehyde (1 mmol), malononitrile (1 mmol),
mol%), room
triethyl phosphite (1 mmol), catalyst (0.08 g,
1
temperature (except for entries 13 and 14), solvent-free conditions. All
the products were characterized by spectroscopic methods and
compared with the authentic samples (see ESI and ref. 4c and 5b–d).
Fig.
La(OTf)2.
8 HRTEM image after five times reuse of g-Fe2O3@SiO2–
b
Reaction temperature: 60 ꢁC.
(Fig. 8) with the fresh catalyst showed the fact that the
morphology of the catalyst remained intact aer ve times
recovering. FT-IR spectrum (Fig. 9) of the catalyst aer ten times
reuse was compared with that of the fresh catalyst (Fig. 4). In the
FT-IR spectrum of used catalyst, besides the corresponding
peaks to g-Fe2O3@SiO2–La(OTf)2, bands at 835, 1078 and 2233
cmꢀ1 were also observed. These peaks are related to P–O, P]O
and CN bonds in the product (1) and showed that a gradual
decrease in the catalytic activity of g-Fe2O3@SiO2–La(OTf)2
during ten times reuse might be due to the catalyst poisoning by
the product.
Aer performing the reaction of benzaldehyde, malononitrile
and triethyl phosphite, EtOAc was added to the reaction
mixture. The catalyst was separated by using an external magnet
(Fig. 6), washed with EtOAc, dried 30 min at 100 ꢁC and reused
for ten runs (Fig. 7). Signicant loss of the catalytic activity of
the catalyst was not observed aer ve times reuse. The
comparison of HRTEM images of used g-Fe2O3@SiO2–La(OTf)2
The generality of the method was also evaluated for the one-
pot synthesis of b-phosphonomalonates from different in situ
generated Michael acceptors and trialkyl phosphites (Table 3).
Fig.
6 Separation of g-Fe2O3@SiO2–La(OTf)2 from the reaction
mixture by an external magnet.
Fig. 9 FT-IR spectrum of g-Fe2O3@SiO2–La(OTf)2 after ten times reuse.
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Table One-pot synthesis of b-phosphonomalonates from 4-
chlorobenzaldehyde with different in situ generated Michael acceptors
and trialkyl phosphites catalyzed by g-Fe2O3@SiO2–La(OTf)2
Paper
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Entry
R0
X
Product
Time (h)
Yielda (%)
1
2
3
4
Et
CN
CN
CN
CO2Et
2
2
1
2
1
92
87
72
84b
Me
iso-Pr
Et
15
16
17
a
Isolated yield, 4-chlorobenzaldehyde (1 mmol), active methylene group
(1 mmol), trialkyl phosphite (1 mmol), catalyst (0.08 g, 1 mol%), room
temperature (except for entry 4), All the products were characterized by
spectroscopic methods and compared with the authentic samples
b
(see ESI and ref. 4c, 5b–e and 15). d.r. ¼ 50 : 50, according to NMR,
reaction temperature: 60 ꢁC.
As it is obvious from Table 3, one-pot reaction of 4-chlor-
obenzaldehyde and malononitrile with various trialkyl phos-
phites gave the corresponding b-phosphonomalonates in
72–92% yields. In addition to malononitrile, ethyl 2-cyanoace-
tate as an in situ generated Michael acceptor was also examined
to carry out the reaction with triethyl phosphite and the desired
product was obtained in 84% yield (entry 4).
H.
Sharghi,
S.
Ebrahimpourmoghaddam
and
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In summary, in this paper, lanthanum(III) triate supported on
nanomagnetic g-Fe2O3@SiO2 was synthesized. It was success-
fully applied as a new magnetically recyclable heterogeneous
Lewis acid for the one-pot synthesis of b-phosphonomalonates
via tandem Knoevenagel–phospha-Michael reaction under
solvent-free conditions. The catalyst was easily isolated from the
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Acknowledgements
We are thankful to University of Birjand Research Council for
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