Synthesis, characterization and catalytic properties of magnetic nanoparticle supported guanidine in base catalyzed synthesis of α-hydroxyphosphonates and α-acetoxyphosphonates

Article abstract of DOI:10.1016/j.apcata.2013.07.001

Magnetic nanoparticle Fe₃O₄-immobilized guanidine (MNPs-Guanidine) as a novel magnetically interphase nanocatalyst was synthesized and characterized. MNPs-Guanidine catalyzed the synthesis of α-hydroxyphosphonates from aldehydes and

Full text of DOI:10.1016/j.apcata.2013.07.001

Applied Catalysis A: General 467 (2013) 7–16
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Synthesis, characterization and catalytic properties of magnetic
nanoparticle supported guanidine in base catalyzed synthesis of
␣-hydroxyphosphonates and ␣-acetoxyphosphonates
Amin Rostamia,b,∗, Bahareh Atashkara, Darush Moradia
a Department of Chemistry, Faculty of Science, University of Kurdistan, Zip Code 66177-15175, Sanandaj, Iran
b Research Centre for Medicinal Plant Breeding and Improvement, University of Kurdistan, Sanandaj, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Magnetic nanoparticle Fe3O4-immobilized guanidine (MNPs-Guanidine) as a novel magnetically inter-
phase nanocatalyst was synthesized and characterized. MNPs-Guanidine catalyzed the synthesis of
␣-hydroxyphosphonates from aldehydes and dimethyl phosphite in solvent-free condition at 80 ◦C. The
synthesis of ␣-acetoxyphosphonates through a one-pot reaction of aldehydes, dimethyl phosphite and
acetic anhydride was also achieved using this catalyst in PEG at room temperature. The catalyst was
recycled up to 10 times with little loss of activity.
Received 22 January 2013
Received in revised form 30 June 2013
Accepted 2 July 2013
Available online xxx
Keywords:
© 2013 Elsevier B.V. All rights reserved.
␣-Hydroxyphosphonates
␣-Acetoxyphosphonates
Magnetic nanoparticles Fe3O4
Guanidine
Basic catalyst
1. Introduction
The
synthesis
of
␣-hydroxyphosphonates
and
␣-
acetoxyphosphonateshas attracted much attention due to their
potential biological activities with broad applications as synthetic
intermediates [16–30].
The use of environmentally benign, sustainable, and efficiently
reusable catalysts provides both economical and ecological benefits
[1–3].
Guanidines are important classes of compounds that have many
uses within organic chemistry commonly as organic bases [31].
With the recent increase in interest in organocatalysis, they have
also been shown to act as nucleophilic catalysts [32]. However the
major disadvantage of catalysts based on guanidines is their sepa-
ration from the products, which needs solid–liquid or liquid–liquid
techniques in many reactions. This drawback can be overcome by
immobilization these catalysts on magnetic nanoparticle (MNPs).
In continuation of our studies on environmentally benign chem-
ical processes [33–35], in the present work we disclose that
MNPs-Guanidine can be used as a novel magnetic interphase
nanocatalyst for the synthesis of ␣-hydroxyphosphonates and ␣-
acetoxyphosphonate.
Magnetic separation from the reaction mixture is simple, eco-
nomical, and promising for industrial applications [4–6]. This
strategy is typically more effective than filtration or centrifugation
[7]. Among the various magnetic nanoparticles as the core magnetic
support, Fe3O4 nanoparticles are arguably the most extensively
studied because of their intrinsic properties such as high surface
area, low toxicity and superparamagnetic behavior [8,9,5,10–13].
In addition to that, all ferrites are metal oxides, presenting a large
number of hydroxyl groups on the surface of their particles [14].
This characteristic allows building well-defined shells of different
materials around the ferrite core or, when functional materials are
targeted, grafting functional groups suitable for the supporting of
all kinds of actuators, ligands and/or catalysts by covalent bonds
[15].
2. Experimental
2.1. General remarks
∗
Corresponding author at: Department of Chemistry, Faculty of Science, Univer-
The materials were purchased from Merck and Fluka and were
used without any additional purification. All reactions were mon-
itored by thin layer chromatography (TLC) on gel F254 plates.
Melting points were obtained in open capillary tubes and also were
sity of Kurdistan, Zip Code 66177-15175, Sanandaj, Iran. Tel.: +98 9183730910;
fax: +98 8716624004.
E-mail addresses: a.rostami@uok.ac.ir, a rostami372@yahoo.com,
arostami372@gmail.com (A. Rostami).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.07.001

8
A. Rostami et al. / Applied Catalysis A: General 467 (2013) 7–16
Scheme 1. (a) Aqueous ammonia, N2, rt, 30 min; (b) (3-chloropropyl)-triethoxysilane, ethanol/water, 40 ◦C, 8 h; (c) guanidine hydrochloride, NaHCO3, dry toluene, reflux,
28 h.
measured on an Electrothermal 9100 apparatus. The X-ray pow-
der diffraction (XRD) data were collected on an X’Pert MPD. Philips
diffractometer with Cu Ka radiation source (ꢀ = 1.54050 A) at 40 kV
Na2SO4 and then evaporation of dichloromethane under reduced
pressure gave the pure products in 62–98% yields.
˚
voltage and 40 mA current. The SEM image was obtained by VEGA
TESCAN. The thermogravimetric analysis (TGA) was carried out on
a Bähr STA 503 instrument (Germany) under air atmosphere, heat-
ing rate 10 ◦C/min. The magnetic measurements were carried out
in a vibrating sample magnetometer (VSM, BHV-55, Riken, Japan)
at room temperature.
2.6. General procedure for the preparation of
˛-acetoxyphosphonate derivatives
MNPs-Guanidine (0.06 g) was added to a mixture of dimethyl
phosphite (0.110 g, 1 mmol), aldehyde (1 mmol) and acetic anhy-
dride (0.306 g, 3 mmol) in PEG (2 mL) at room temperature and
stirred for the appropriate time. The progress was monitored
by TLC. After completion of the reaction, the catalyst was sepa-
rated by an external magnet and the mixture was washed with
diethylether/water = 1:1 (3 × 20 mL). The combined organics were
washed with brine (5 mL) and dried over anhydrous Na2SO4. The
evaporation of diethylether under reduced pressure gave the pure
products in 87–98% yields.
2.2. Preparation of large-scale the magnetic Fe3O4 nanoparticles
(MNPs)
FeCl3·6H2O (4.865 g, 0.018 mol) and FeCl2·4H2O (1.789 g,
0.0089 mol) were added to 100 mL deionized water and sonicated
until the salts dissolved completely. Then, 10 mL of 25% aque-
ous ammonia was added quickly into the reaction mixture in one
portion under N2 atmosphere at room temperature followed by
stirring about 30 min with mechanical stirrer. The black precipitate
was washed with doubly distilled water (five times).
3. Results and discussions
3.1. Characterization of MNPs-Guanidine
The process of the preparation guanidine-functionalized mag-
netic Fe3O4 nanoparticles is shown in Scheme 1.
2.3. Preparation of MNPs coated by
(3-chloropropyl)-trimethoxysilane (MNPs-CPTMS)
MNPs-Guanidine was characterized using a variety of different
techniques. The XRD pattern of MNPs-Guanidine is shown in Fig. 1a.
A weak broad band (2ꢁ = 18–22◦) appeared in MNPs-Guanidine pat-
tern which could be assigned to the amorphous silane shell formed
around the magnetic cores [36]. The lattice parameter was calcu-
lated for the prepared particles and compares these values with the
standard parameters for magnetite.
The interlayer spacing (dh k l), calculated using the Bragg equa-
tion, agrees well with the data for standard magnetite (Table 1).
Fig. 1b shows the SEM image of the synthesized guanidine
loaded magnetite nanoparticles. It was confirmed that the cata-
lyst was made up of uniform nanometer-sized particles less than
17 nm.
One indication of bond formation between the nanoparticles
and the catalyst can be inferred from thermogravimetric analy-
sis (TGA). TGA curve of the MNPs-Guanidine show the mass loss
of the organic functional group as it decompose upon heating
(Fig. 2a). The weight loss at temperatures below 200 ◦C is due to
the removal of physically adsorbed solvent and surface hydroxyl
groups [38,39]. Organic groups have been reported to desorb at
temperatures above 260 ◦C. The curve shows a weight loss about
10% from 260 to 600 ◦C, resulting from the decomposition of organic
spacer grafting to the MNPs surface.
The obtained MNPs powder (1.5 g) was dispersed in 250 mL
ethanol/water (volume ratio, 1:1) solution by sonication for
30 min, and then CPTMS (99%, 2.5 mL) was added to the mixture.
After mechanical stirring under N2 atmosphere at 33–38 ◦C for
8 h, the suspended substance was separated with centrifugation
(RCF = 13,200 × g for 30 min). The settled product was re-dispersed
in ethanol by sonication. The final sample was separated by an
external magnet and washed five times with ethanol. The product
stored in a refrigerator to use.
2.4. Preparation of guanidine-functionalized magnetic Fe3O4
nanoparticles (MNPs-Guanidine)
The MNPs-CPTMS (1 g) was dispersed in dry toluene (6–8 mL)
by ultrasonication for 10 min. Subsequently, guanidine hydrochlo-
ride (0.382 g, 0.004 mmol) and sodium bicarbonate (0.672 g,
0.008 mmol) were added and the mixture was refluxed for 28 h.
Then, the final product was separated by magnetic decantation
and washed twice by dry CH2Cl2, EtOH and CH2Cl2 respectively
to remove the unattached substrates. The product was stored in a
refrigerator until use.
EDX spectrum shows the elemental composition of the MNPs-
Guanidine (Fig. 2b). Elemental analysis results showed that the
2.5. General procedure for the preparation of
˛-hydroxyphosphonate derivatives
Table 1
Interlayer spacings (dh k l) for MNPs-Guanidine.
MNPs-Guanidine (0.03 g) was added to a mixture of dimethyl
phosphite (0.110 g, 1 mmol) and aldehyde (1 mmol) at 80 ◦C and
stirred for 90–120 min. The progress was monitored by TLC. After
completion of the reaction, the catalyst was separated by an exter-
nal magnet and the mixture was washed with CH2Cl2 (2 × 5 mL)
and decanted. The combined organics were dried over anhydrous
˚
Sample
dh k l (A)
1
2
3
4
5
6
Standard Fe3O4 [37]
MNPs-Guanidine
2.96
2.97
2.53
2.53
2.09
2.08
1.71
1.71
1.61
1.61
1.48
1.48

A. Rostami et al. / Applied Catalysis A: General 467 (2013) 7–16
9
Fig. 1. (a) XRD pattern and (b) SEM image of the MNPs-Guanidine.
carbon, hydrogen, and nitrogen content of the MNPs-Guanidine
was 5.7, 0.5, and 0.9 (wt%), respectively. The loading of the guani-
dine function on the magnetic nanoparticles was determined by
elemental analysis of nitrogen as 0.22 mmol/g.
MNPs-Guanidine is shown in Fig. 4a and b, respectively. As
expected, the bare MNPs, showed the higher magnetic value (sat-
uration magnetization, Ms) of 74.3 emu g−1 [42], the Ms value of
MNPs-Guanidine is decreased due to the silica coating and the layer
of the grafted catalyst (47.4 emu g−1). It has been reported that the
Fe3O4 nanoparticles with a value of coercivity (Hc) lower than 20 Oe
could be called superparamagnetic. MNPs and MNPs-Guanidine
have an Hc of 16.09 and 14.32 Oe, respectively and the remanent
magnetization (Mr) of ∼1.47 and 1.01 emu g−1, respectively. As a
result, the modified MNPs have a typical superparamagnetic behav-
ior [43–45] and can be efficiently attracted with a small magnet.
Fig. 3 shows Fourier transform infrared (FTIR) spectra for MNPs,
MNPs-CPTMS, and MNPs-Guanidine. The FTIR spectrum for the
MNPs alone shows a stretching vibration at 3418 cm−1 which incor-
porates the contributions from both symmetrical and asymmetrical
modes of the O H bonds which are attached to the surface iron
atoms [9]. The bands at low wave numbers (≤700 cm−1) come
from vibrations of Fe O bonds of iron oxide, in which for the
bulk Fe3O4 samples appear at 570 and 375 cm−1 but for Fe3O4
nanoparticles at 624 and 572 cm−1 as a blue shift, due to the size
reduction [38,40,41]. The presence of an adsorbed water layer is
confirmed by a stretch for the vibrational mode of water found at
1620 cm−1. The FTIR spectra of MNPs-CPTMS and MNPs-Guanidine
show Fe O vibrations in the same vicinity. The introduction of
CPTMS to the surface of MNPs is confirmed by the bands at 1005 and
Table 2
Optimization of the reaction conditions for the reaction of dimethyl phosphite
(1 mmol) with benzaldehyde (1 mmol).
Entry
Solvent
Catalyst (mg)
Time (min)
Conversion (%)
1128 cm−1 assigned to the Fe
O Si and C Cl stretching vibrations
1
2
3
4
5
6
7
8
9
Solvent-free, 80 ◦
Solvent-free, 80 ◦
Solvent-free, 80 ◦
Solvent-free, 80 ◦
Solvent-free, 80 ◦
Solvent-free, 80 ◦
Solvent-free, rt
H2O
C
C
C
C
C
C
None
40
30
20
15
10
15
15
15
>1440
115
120
120
120
380
380
380
380
380
No reaction
100
100
100
100
100
Trace
Trace
No reaction
No reaction
respectively. Reaction of MNPs-CPTMS with guanidine produces
MNPs-Guanidine in which the presence of guanidine moiety is
asserted with 1443 and 3381 cm−1 bands corresponding to the C
and N H stretches respectively.
N
Superparamagnetic particles are beneficial for magnetic sep-
aration; the magnetic properties of MNPs and MNPs-Guanidine
were characterized by a vibrating sample magnetometer (VSM).
The room temperature magnetization curves of MNPs and
PEG
CH3CN
10
15

10
A. Rostami et al. / Applied Catalysis A: General 467 (2013) 7–16
Fig. 2. (a) TGA profile and (b) EDX spectrum of the MNPs-Guanidine.
3.2. Application of MNPs-Guanidine for the synthesis of
˛-hydroxyphosphonates derivatives
aromatic aldehydes 1 with dimethyl phosphite 2 under solvent-free
conditions at 80 ◦C (Scheme 2).
The investigation of the reaction conditions for the model
hydrophosphonylation reaction between dimethyl phosphite
(1 mmol) and benzaldehyde (1 mmol) in terms of the catalyst
amount, reaction time and product yield demonstrated that 15 mg
MNPs-Guanidine was tested as basic magnetically sepa-
rable heterogeneous nanocatalyst for the synthesis of the
␣-hydroxyphosphonates (3a-p) from reaction of wide range of
Scheme 2. MNPs-Guanidine catalyzes the preparation of ␣-hydroxyphosphonate derivatives.

A. Rostami et al. / Applied Catalysis A: General 467 (2013) 7–16
11
Table 3
MNPs-Guanidine (15 mg) catalyzed the synthesis of ␣-hydroxyphosphonates from reaction of aldehydes (1 mmol) and dimethyl phosphite (1 mmol) in solvent-free conditions
at 80 ◦C.
Entry
Aldehyde
Product
Time (min)
Yield (%)a
CHO
OH
OMe
OMe
P
O
3a
120
85
62
NO2 OH
CHO
OMe
OMe
NO2
P
O
3b
75
120
90
CHO
OH
OMe
OMe
P
O
OMe
CHO
MeO
3c
89
93
OH
OMe
OMe
P
O
Me
Me
Br
3d
CHO
OH
OMe
OMe
P
O
Br
3e
3f
90
74
92
Cl
OH
CHO
OMe
OMe
Cl
Br
P
O
150
CHO
CHO
OH
OMe
Br
OMe
P
O
3g
3h
210
240
70
83
OH
OMe
OMe
P
O
Cl
Cl
CHO
OH
OMe
OMe
P
O
F
F
3i
3j
210
85
88
OH
CHO
OMe
OMe
P
O
Cl
Cl
Cl
Cl
90

12
A. Rostami et al. / Applied Catalysis A: General 467 (2013) 7–16
Table 3 (Continued)
Entry
Aldehyde
Product
Time (min)
Yield (%)a
OH
OMe
OMe
P
CHO
Cl
Cl
O
Cl
Cl
3k
3l
90
98
77
OH
OMe
OMe
P
CHO
CHO
OH
O
OH
180
OH
OMe
OMe
MeO
O
P
OMe
OH
P
O
CHO
3m
240
120
70
90
O
P
HO
CHO
OMe
OMe
3n
3o
OH
OMe
OMe
CHO
P
O
120
90
70
93
OH
OMe
OMe
O
O
P
CHO
O
3p
a
All the products are known and were characterized by IR, 1H NMR and 13C NMR comparisons with those of authentic samples [24–27].
of the catalyst under solvent-free conditions at 80 ◦C was optimal
for the desired reaction (Table 2, entry 5).
With the optimal conditions, the generality and the appli-
cability of this method was further examined for the synthesis
of ␣-hydroxyphosphonates from the reaction of dimethyl phos-
phite with structurally diverse aromatic aldehydes including
different types of substituted benzaldehydes, cinnamaldehyde, 1-
naphthaldehyde, and terephthaldehyde (Table 3). The results are
summarized in (Table 3).
The feasibility of repeated use of MNPs-Guanidine was also
investigated for the reaction of dimethyl phosphite with benz-
aldehyde. We found that this catalyst demonstrated excellent
recyclability. The catalyst can be efficiently recovered easily and
rapidly from the product by exposure to an external magnet (Fig. 5).
To remove the residual product, the remaining magnetic nanoparti-
cles were further washed with the EtOH, air-dried and used directly
for the next round of reaction without further purification. The
recycled catalyst was used for up to 10 runs with little loss of
activity (Table 4).
In order to learn the efficiency and greenness of this method,
we compared our obtained results for hydrophosphonylation of
benzaldehyde with some data from the literature. We have found
many of the previously reported methodologies suffer from one or
more disadvantages such as using volatile and toxic organic sol-
vents [24–29] and prolonged reaction times [26–29]. In addition,
Fig. 3. FTIR spectra of MNPs (blue), MNPs-CPTMS (black) and MNPs-Guanidine (red).
(For interpretation of the references to color in this figure caption, the reader is
referred to the web version of the article.)

A. Rostami et al. / Applied Catalysis A: General 467 (2013) 7–16
13
Table 4
Recycling of MNPs-Guanidine for the synthesis of 3aa and 5a.b
Cycle
Conversion (%) 3a
Conversion (%) 5a
1
2
3
4
5
100
100
100
100
100
96
100
100
100
100
97
6
95
7
96
95
8
90
90
9
85
83
10
80
78
a
Reaction conditions: benzaldehydes (1 mmol), dimethyl phosphite (1 mmol),
MNPs-Guanidine (15 mg), solvent-free, 80 ◦C, 120 min.
b
Reaction conditions: benzaldehyde (1 mmol), dimethyl phosphite (1 mmol)
acetic anhydride (3 mmol), MNPs-Guanidine (60 mg), PEG, rt 5 min.
the number of catalyst recycles is increased when the magnetic
nanoparticles is used as alternative catalyst support. We believe
the present method to be an improvement with respect to other
procedures.
3.3. Application of MNPs-Guanidine in synthesis of
˛-acetoxyphosphonates derevatives
Owing to the success of MNPs-Guanidine for carrying out dif-
ferent reactions, we have studied the possibility of applying this
new catalyst for the synthesis of ␣-acetoxyphosphonates in PEG at
ambient temperature (Scheme 3).
To obtain the optimal reaction conditions, we evaluated the
influence of solvent and different amounts of catalyst on the
time and product yield in the reaction of benzaldehyde (1 mmol),
dimethyl phosphite 2 (1 mmol) and acetic anhydride 4 (3 mmol),
which was used as a model. In the absence of any catalyst, no
reaction was observed even after prolonged reaction time. When
the catalyst was added, the times were reduced. However 60 mg
of MNPs-Guanidine in PEG was optimal for the desired reaction.
Also, inferior results were obtained with H2O, CH3CN and under
solvent-free conditions.
Fig. 4. (a) Hysteresis loop of the MNPs and (b) MNPs-Guanidine at room tempera-
ture (left inset: the magnified field from −30 to 30 Oe).
Fig. 5. Image showing MNPs-Guanidine can be separated by applied magnetic field. A reaction mixture in the absence (left) or presence of a magnetic field (right).
Scheme 3. MNPs-Guanidine catalyzes the synthesis of the ␣-acetoxyphosphonate derivatives.

14
A. Rostami et al. / Applied Catalysis A: General 467 (2013) 7–16
Table 5
MNPs-Guanidine (60 mg) catalyzed the synthesis of ␣-acetoxyphosphonates from aldehyde (1 mmol), dimethyl phosphite (1 mmol) and acetic anhydride (3 mmol) in PEG
at ambient temperature.
Entry
Aldehyde
Product
Time (min)
Yield (%)a
CHO
OAc
OMe
OMe
P
O
5a
5
7
98
92
NO2 OAc
OMe
CHO
CHO
OMe
NO2
P
O
5b
OAc
OMe
O2N
OMe
P
NO2
O
5c
6
93
CHO
OAc
OMe
OMe
P
O
NO2
O2N
5d
5
99
CHO
OAc
OMe
OMe
P
O
Me
Me
Me
5e
5f
10
91
89
OAc
OMe
CHO
OMe
Me
P
O
15
CHO
CHO
OAc
OMe
MeO
OMe
P
O
OMe
5g
8
94
90
OAc
OMe
OMe
P
O
OMe
CHO
MeO
MeO
5h
12
OAc
OMe
OMe
P
O
OMe
OMe
CHO
OMe
5i
13
10
92
97
OAc
OMe
OMe
P
O
Br
Br
5k

A. Rostami et al. / Applied Catalysis A: General 467 (2013) 7–16
15
Table 5 (Continued)
Entry
Aldehyde
Product
Time (min)
Yield (%)a
Cl
OAc
OMe
CHO
OMe
Cl
P
O
5l
15
96
CHO
OAc
OMe
Br
OMe
P
O
Br
Br
5m
5n
6
8
89
95
Br
OAc
OMe
CHO
CHO
OMe
P
O
OAc
OMe
OMe
P
O
F
F
5o
5
6
94
95
OAc
CHO
OMe
OMe
P
O
Cl
Cl
Cl
Cl
5p
a
All the products are known and were characterized by IR, 1H NMR and13C NMR with those of authentic samples [30].
Subsequently, a series of differently ␣-acetoxyphosphonates
derivatives were prepared successfully under optimal conditions.
These results are listed in Table 5. In all cases, up to quantitative
yields in reasonable reaction times were obtained.
We also investigated the recycling of the catalyst using model
reaction of benzaldehyde (1 mmol) with dimethyl phosphite
(1 mmol) and acetic anhydride (3 mmol). The recovered catalyst
was reused for at least 10 runs with little loss of activity (Table 4).
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In summary, we have synthesized the first MNPs-Guanidine
for use as a magnetically heterogeneous basic nanocatalyst. The
catalyst is easily synthesized and can catalyze the synthesis of ␣-
hydroxyphosphonates and ␣-acetoxyphosphonates with good to
high yields in different conditions. The characteristic aspects of
this catalyst are rapid, simple and efficient separation by using an
appropriate external magnet, which minimizes the loss of catalyst
during separation and reusable for several times with little loss
of activity. In addition, MNPs-Guanidine couples the advantages of
heterogeneous and homogeneous guanidine-based systems, which
make it a promising material for industrial.
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