Synthesis of arylphosphonates catalyzed by Pd-imino-Py-γ-Fe2O3 as a new magnetically recyclable heterogeneous catalyst in pure water without requiring any additive

Article abstract of DOI:10.1039/c5ra27330g

A palladium-Schiff base complex immobilized covalently on γ-Fe₂O₃ (Pd-imino-Py-γ-Fe₂O₃) was synthesized and characterized by different methods such as XRD (X-ray diffraction), SEM (scanning electron microscopy), TEM (transmission electron microscopy), FT-IR (Fourier transform infrared spectroscopy), TGA (thermogravimetric analysis), ICP (inductively coupled plasma), XPS (X-ray photoelectron spectroscopy), VSM (vibrating sample magnetometer) and elemental analysis. It was used as a new magnetically recyclable heterogeneous Pd catalyst for the synthesis of arylphosphinates via C_sp²-P coupling reactions. A wide range of electrophilic benzenes were coupled successfully with triethylphosphite to generate the corresponding products in good to high yields in pure water without using any additive. The true heterogeneous Pd-imino-Py-γ-Fe₂O₃ could be reused simply with the aid of a magnetic bar for eight consecutive cycles without any drastic loss of its reactivity.

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Synthesis of arylphosphonates catalyzed by Pd-imino-Py-γ-Fe2O3
as a new magnetically recyclable heterogeneous catalyst in pure
water without requiring any additive
Received 00th January 20xx,
Accepted 00th January 20xx
Sara Sobhania and Zohreh Ramezani
DOI: 10.1039/x0xx00000x
A palladium-Schiff base complex immobilized covalently on γ-Fe2O3 (Pd-imino-Py-γ-Fe2O3) was synthesized and
characterized by different methods such as XRD (X-ray diffraction), SEM (scanning electron microscopy), TEM
(transmission electron microscopy), FT-IR (Fourier transform infrared spectroscopy), TGA (thermogravimetric analysis), ICP
(inductively coupled plasma), XPS (X-ray photoelectron spectroscopy), VSM (vibrating sample magnetometer) and
elemental analysis. It was used as a new magnetically recyclable heterogeneous Pd catalyst for the synthesis of
arylphosphinates via Csp2-P coupling reactions. A wide range of electrophilic benzenes were coupled successfully with
triethylphosphite to generate the corresponding products in good to high yields in pure water without using any additive.
The true heterogeneous Pd-imino-Py-γ-Fe2O3 could be reused simply with the aid of a magnetic bar for eight consecutive
cycles without any drastic loss of its reactivity.
www.rsc.org/
other tedious workup processes, as it prevents loss of the
nano-sized catalysts and enhances products purity. Moreover,
Introduction
The difficulty of separation of homogeneous catalysts from
reaction medium consumedly restricts their applications in
industry, especially in the drug and pharmaceutical industry
owing to the issue of metal contamination in the case of
metal-catalyzed synthesis.1 Heterogenization of homogeneous
MNPs have unique physical properties including high surface
area, surface modification ability and excellent thermal and
chemical stability.9 In recent years, MNPs as catalysts or
catalyst supports have been extensively used in a variety of
important
organic reactions including reductions,10
oxidations,11 C–C12 and C–X13 couplings, and multicomponent
reactions.14 Despite the considerable success achieved in the
field of magnetically separable nanocatalysts, there is a lack in
the application of these systems in the Csp2-P coupling
reactions.
catalysts with
a
solid support fabricating insoluble
heterogeneous catalytic systems is an efficient strategy in
order to achieve the isolation and separation of homogeneous
catalysts. In this context, numerous solid supports such as
mesoporous2 and amorphous silica,3 zeolites,4 metal oxides,5
carbon material6 and polymers7 have been adopted for
heterogenization of homogeneous catalysts. Although these
heterogeneous catalysts can be recovered by filtration or
centrifugation methods, these methods are time consuming,
and results in product contaminations due to the loss of
catalyst particles. In recent decades, magnetic nanoparticles
(MNPs) that have been studied extensively for various
biological and medical applications have emerged as smart and
promising supports for immobilization of catalysts because
supported catalysts on MNPs can be easily separated from the
reaction medium using an external permanent magnet.8
Magnetic separation is simple, economical and promising for
industrial applications. This separation technique is more
effective than time-consuming filtration, centrifugation or
Arylphosphonates are valuable intermediates in organic
synthesis15 and versatile ligands in catalytic reactions.16 They
also found numerous practical applications in medicinal and
polymer chemistry,17 designing fuel cell membranes18 and
building blocks in construction of metal-organic frameworks
(MOFs).19 Arylphosphonates have been used in designing new
materials with special optical properties,20 which have been
employed in solar cells.21 They have found applications in
nucleic acid chemistry and nucleic acid-based medicinal
chemistry. In particular, they are used as analogues superior to
natural
phosphates
in
construction
of
modified
oligonucleotides with potential application in the
antisense/antigen techniques (gene therapy).22 The
resemblance to carboxylic group, and especially to tetrahedral
intermediates formed during substitution at the carboxylic sp2-
carbon, makes arylphosphonates potent enzyme inhibitors23,24
and efficient antagonists of biologically active carboxylic
acids.25 The Michaelis-Arbuzov reaction is the most frequently
used method for the synthesis of phosphonates via C-P bond
formation.26 However, this approach is not applicable to the
synthesis of arylphosphonates, due to low reactivity of the
corresponding Csp2-halides in nucleophilic substitution. Access
aSara
Department of Chemistry, College of Sciences, University of Birjand, Birjand, Iran,
E-mail: ssobhani@birjand.ac.ir, sobhanisara@yahoo.com; Fax:+98 56 32202065;
Sobhani
(Corresponding
Author),
Zohreh
Ramezani
Tel: +98 56 32202065.
Electronic Supplementary Information (ESI) available: [Supporting information for
a link at the end of the document]. See
this article is given via
DOI: 10.1039/x0xx00000x
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to arylphosphonates was for many years possible only using a
few traditional methods.27 The major breakthrough in the area
of the synthesis of arylphosphonates came with a series of
reports from a group of Hirao on the palladium-catalyzed
cross-coupling of aryl halides with dialkylphosphites catalyzed
by Pd(PPh3)4.28 In the following years the scope of the classical
Hirao reaction was broadened by applying different transition
metals,29 ligands,30 coupling partners31 and solvents.32
However, despite of all of these improvements, Csp2-P bond
formation suffers from severe problems related to the
separation, recovery, and instability of the homogeneous
catalysts. To address these limitations and in continue of our
recent works on the development of new heterogeneous
which are attributed to the diffraction scattering of γ-Fe2O3
were readily recognized from the XRD pattern of Pd-imino-Py-
γ-Fe2O3 (Figure S2). The observed diffraction peaks was
indicated that γ-Fe2O3 mostly exist in face-centered cubic
structure. The average crystallite size of Pd-imino-Py-γ-Fe2O3
(16 nm) was estimated using the Debye-Scherrer formula.
The size and structure of Pd-imino-Py-γ-Fe2O3 were evaluated
using scanning electron microscopy (SEM) and transmission
electron microscopy (TEM). The SEM image of the supported
catalyst showed uniformity and spherical morphology of MNPs
(Figure S3). From the TEM image (Figure S4), Pd-imino-Py-γ-
Fe2O3 exhibits spherical morphology with relatively good
monodispersity. The average diameter of nanoparticles was
evaluated and it was 15 nm.
Figure S5 displays the FT-IR spectra of γ-Fe2O3 and Pd-imino-
Py-γ-Fe2O3. The band at around 561-633 cm−1 was attributed
to the stretching vibrations of Fe-O bond in γ-Fe2O3 and Pd-
imino-Py-γ-Fe2O3. In the spectrum of Pd-imino-Py-γ-Fe2O3, the
C-H stretching mode of methylene can be observed at 2930
cm-1. The absorption bands at 1510 and 1420 cm-1 were
allocated to the stretching vibration of C=C bond. The peak at
around 1260 cm-1 was related to C-O stretching vibration.
Peaks were appeared at 1580 and 1340 cm-1 in the spectrum
of Pd-imino-Py-γ-Fe2O3, corresponded to COO (in acetate) and
confirmed the presence of Pd in the catalyst. These results
indicated that Schiff base was successfully grafted onto the γ-
Fe2O3 MNPs.
catalysts,33 herein,
a
palladium-Schiff base complex
immobilized covalently on γ-Fe2O3 (Pd-imino-Py-γ-Fe2O3) is
introduced as a new magnetically separable Pd catalyst for the
synthesis of arylphosphonates in pure water without requiring
any additive.
Results and discussion
Synthesis and characterization of Pd-imino-Py-γ-Fe2O3
The synthesis of Pd-imino-Py-γ-Fe2O3 was performed by a
simple procedure recently reported by us (Scheme 1).33i In the
first stage, Schiff base immobilized on γ-Fe2O3 (imino-Py-γ-
Fe2O3) was synthesized by the reaction of chloro-
functionalized γ-Fe2O3 with iminopyridine. Then, the treatment
of imino-Py-γ-Fe2O3 with Pd(OAc)2 in dry acetone led to the
formation of Pd-Schiff base complex supported on γ-Fe2O3
MNPs (Pd-imino-Py-γ-Fe2O3).
For investigation of the stability of Pd-imino-Py-γ-Fe2O3,
thermogravimetric analysis was studied (Figure S6). The weight
loss below 110 °C for the sample is apparently owing to the
loss of adsorbed water. The weight loss between 123 and 813
°C was assigned to the decomposition of covalently bonded
ligand. Thus, the TGA curve confirmed the successful
functionalization of MNPs.
(MeO) Si
Cl
OH
OH
OH
O
O
O
3
γ-Fe2O3
γ-Fe2O3
Si
Cl
The load of functionality on γ-Fe2O3 was estimated by
elemental analysis (N = 0.75% and C = 6.25%) and ICP and it
was 0.26 mmol g-1.
toluen, reflux, 24 h
X-ray photoelectron spectrum (XPS) of Pd-imino-Py-γ-Fe2O3
showed the existence of Pd(II) in the catalyst with binding
energies at 338.6 and 343.5 eV corresponding to Pd 3d5/2 and
Pd 3d3/2, respectively (Figure S7a). Further signals at 401.6,
533.2, 102.5 and 285.0 eV were attributed to the N1s, O1s,
Si2p and C1s energy levels, respectively (Figure S7b). The
binding energy of 711.2 was related to Fe2p3/2 of γ-Fe2O3.
Magnetic properties of γ-Fe2O3 and Pd-imino-Py-γ-Fe2O3 were
examined by vibrating sampling magnetometry (VSM) at room
temperature (Figure S8). The saturation magnetization of γ-
Fe2O3 and Pd-imino-Py-γ-Fe2O3 was around 68.9 emu g-1. This
figure showed almost zero coercivity, which proved the
superparamagnetic properties of the synthesized magnetic
nanoparticles.
N
N
NaH, DMF
Pd(OAc)2
Acetone
O
γ-Fe2O3
Si
O
O
N
N
O
O
O
O
γ-Fe2O3
Si
O
N
N
Pd
AcO OAc
Scheme 1. Synthesis of Pd-imino-Py-γ-Fe2O3.
The structural features of the prepared material were
investigated by variety of techniques (Supporting
a
information) such as XRD (X-ray diffraction), SEM (scanning
electron microscopy), TEM (transmission electron microscopy),
FT-IR (Fourier transform infrared spectroscopy), TGA
(thermogravimetric analysis), VSM (vibrating sample
magnetometer), ICP (inductively coupled plasma), XPS (X-ray
photoelectron spectroscopy) and elemental analysis.
Synthesis of arylphosphonates catalyzed by Pd-imino-Py-γ-
Fe2O3 in pure water without requiring any additive
From the standpoint of green chemistry, development of more
environmentally benign conditions for the reaction such as the
use of water as reaction media would be desirable. Compared
to other organic solvents, water is a green, available, and more
The reflection planes of (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1)
and (4 4 0) at 2θ = 30.3˚, 35.7˚, 43.4˚, 53.8˚, 57.4˚ and 63.0˚,
2 | J. Name., 2012, 00, 1-3
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importantly cheap and safe solvent and has therefore
attracted a lot of attention in organic synthesis. To the best of
our knowledge, there are a few reports for the synthesis of
arylphosphonates via Csp2-P coupling reaction in water.34 The
reported methods suffer from one or more of the following
drawbacks such as use of toxic and/or unrecyclable metal
catalysts, unstable dialkyl phosphites in water, requiring an
inert atmosphere, additive, microwave irradiation, high
temperature and tedious work-up procedures leading to
copious amount of toxic wastes. Therefore, the development
of a new method to overcome these short comings still
remains an ongoing challenge for the synthesis of these
significant scaffolds.
After characterization of Pd-imino-Py-γ-Fe2O3, its catalytic
performance was investigated in the synthesis of
arylphosphonates in pure water without using any additive. To
find the optimized reaction conditions, the coupling reaction
of chlorobenzene (1 mmol) with triethylphosphite (2 mmol) in
the presence of Pd-imino-Py-γ-Fe2O3 was chosen as a model
reaction. The model reaction was performed in H2O (1 mL) at
100 °C using 0.07, 0.1 and 0.14 mol% of Pd-imino-Py-γ-Fe2O3
and Et3N (4 mmol) (Figure 1a). In the presence of 0.07 and 0.1
mol% of the catalyst, the yield of the product was increased
after 5 and 4 h, respectively, and then remained constant.
Higher yield was obtained in the presence of 0.1 mol% of the
catalyst. Increasing the amount of the catalyst to 0.14 mol%
did not show any effect on the yield of the reaction. The
influence of a variety of bases such as n-Pr3N, KF, KOAc, KOH
and Et3N on the progress of the reaction were studied (Figure
1b). Among the bases tested, Et3N was the best choice, giving
the desired product in higher yield (98%) and shorter reaction
time (4 h). A similar reaction in the absence of the base
produced a trace amount of the product in 8 h. The model
reaction was also studied in the presence of Pd-imino-Py-γ-
Fe2O3 (0.1 mol%) in 80 °C. The results of this study suggested
that 100 °C is the optimum temperature for this reaction
(Figure 1c). In the screening of different solvents and also
solvent-free conditions, H2O gave the highest catalytic
efficiency for this coupling reaction (Figure 1d). The model
reaction was examined in the presence of Pd(OAc)2 and also in
the absence of the catalyst (Figure 1e). The results showed the
importance of the presence of the catalyst for this reaction.
Coupling reaction did not take place in the absence of the
catalyst and the product was obtained in 48% yield after 6 h in
the presence of Pd(OAc)2.
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Table 1. Synthesis of arylphosphonates via Csp2-P coupling reaction
catalyzed by Pd-imino-Py-γ-Fe2O3 in pure water.
Entry
Electrophilic
benzene
Product
Time
[h]
Isolated
Yield
[%][a]
O
1
98
I
P(OEt)2
1
2
3
O
I
P(OEt)2
Figure 1. Effect of the amount of the catalyst (a), base (b), temperature (c),
solvent (d), and catalyst (e) on the coupling reaction of chlorobenzene with
triethylphosphite.
4
2
95
93
MeO
MeO
To explore the scope of this method, the Csp2-P coupling
O
reaction
of
various
electrophilic
benzenes
with
Br
P(OEt)2
triethylphosphite was studied under optimal reaction
conditions (Scheme 2, Table 1).
O
O
P(OEt)2
X
Br
P(OEt)2
Pd-imino-Py-γ-Fe2O3 (0.1 mol%)
4
7
83
P(OEt)3
H2O, NEt3, 100 o
C
MeO
MeO
R
R
Scheme 2. Synthesis of arylphosphonates via Csp2-P coupling reaction catalyzed
by Pd-imino-Py-γ-Fe2O3.
O
As indicated in Table 1, the reaction of triethylphosphite with
different aryl iodides and bromides containing electron
withdrawing or electron releasing groups proceeded well and
the desired products were isolated in 78-98% yields (Entries 1-
9). Different substituted aryl chlorides (far cheaper and more
widely available than aryl iodides and bromides) underwent
the coupling reaction to give the corresponding products in
good to high yields (Entries 10-13). Interestingly, diethyl 4-
chlorophenylphosphonate was achieved by the selective
replacement of iodine and bromine in 1-chloro-4-iodobenzene
and 1-boromo-4-chlorobenzene with triethylphosphite (Entries
14 and 16). The present catalytic system also offered
promising results for the regioselective coupling reaction of
1,4-diiodo/1,4-dibromobenzene with triethylphosphite to give
Br
P(OEt)2
5
6
6
6
92
85
O2N
O2N
O
Br
P(OEt)2
O
O
O
Br
P(OEt)2
7
8
6
5
95
89
Me
Me
diethyl
4-iodophenylphosphonate
and
diethyl
4-
O
bromophenylphosphonate in 98 and 95% yields, respectively
(Entries 15 and 17). Both halogens in 1,4-diiodo and 1,4-
dibromobenzene were replaced by twice amount of phosphite
ester under the present reaction conditions (Entries 18 and
19). This method was also applicable for electrophilic benzenes
such as aryl tosylate and benzene boronic acid and also for β-
bromostyrene as a vinyl bromide (Entries 20-22). However,
this catalytic system was not effective for the synthesis of
arylphosphonates from 1-bromo-4-fluorobenzene, 1-bromo-3-
fluorobenzene and 3-bromoanisole.
Br
P(OEt)2
Me
Me
O
Br
P(OEt)2
9
10
78
88
O
Cl
P(OEt)2
10
4
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O
O
P(OEt)2
Br
Cl
P(OEt)2
22
10.5
96
11
8
6
89
78
O2N
O2N
[a] Reaction conditions: electrophilic benzene (1 mmol), triethylphosphite
(2 mmol, except for entries 17 and 18), Et3N (4 mmol, except for entries 17
and 18), Pd-imino-Py-γ-Fe2O3 (0.1 mol%, except for entries 17 and 18), H2O
(1 mL), 100 ˚C. The products were characterized by 1H NMR and 13C NMR
(see supporting information). [b] Triethlphosphite (4 mmol), Et3N (8 mmol),
Pd-imino-Py-γ-Fe2O3 (0.2 mol%).
O
Cl
P(OEt)2
12
O
O
The recovery and reuse of the catalyst become an important
factor due to stringent economical and ecological demands for
sustainability. Therefore, the recyclability of Pd-imino-Py-γ-
Fe2O3 was investigated in the reaction of iodobenzene and
triethylphosphite under the present reaction conditions. After
completion of the reaction, the catalyst could be simply and
efficiently recovered from the reaction mixture with a
magnetic bar (Figure 2), washed three times with EtOAc, dried
under vacuum and reused in a subsequent reaction. More than
99% of the catalyst could be recovered from each run. By this
procedure, the catalyst could be reused without a significant
loss of its activity in a test of eight cycles (Figure 3). Pd leaching
of the catalyst was further investigated. ICP analysis of the
catalyst after eight runs showed that only a very small amount
(less than 1%) of Pd metal was removed from the catalyst.
O
Cl
P(OEt)2
13
6
2
2
5
3
93
98
98
91
95
Me
Me
Cl
I
O
I
P(OEt)2
14
Cl
O
I
P(OEt)2
15
I
O
Br
Br
P(OEt)2
16
Cl
Cl
O
P(OEt)2
17
Br
Br
Figure 2. (a) Reaction mixture, (b) Separation of Pd-imino-Py-γ-Fe2O3
from the reaction mixture by a magnetic bar.
O
P(OEt)2
Br
18[b]
15
57
(EtO)2P
Br
O
O
P(OEt)2
I
19[b]
(EtO)2P
9
78
I
O
O
OTs
P(OEt)2
20
21
6
5
75
80
Figure 3. Reusability of Pd-imino-Py-γ-Fe2O3 for the synthesis of diethyl
phenylphosphonate after 1 h.
O
B(OH)2
P(OEt)2
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FT-IR spectrum of the reused catalyst after eight runs indicated poisoning test showed that the observed catalysis was
that no detectable changes of the catalyst occurred during the heterogeneous in nature.
reaction and the recycling stages (Figure 4).
A plausible mechanism for the synthesis of arylphosphonates
catalyzed by Pd-imino-Py-γ-Fe2O3 is postulated in Scheme 3. Pd
catalyst was inserted into the aryl halide bond during the initial
oxidative addition. The produced palladium-halide reacted
with triethylphosphite. An Arbuzov type reaction was occurred
and ethylhalide was eliminated. The corresponding
arylphosphonates were produced by reductive elimination.
The Pd catalyst was regenerated and returned to the catalytic
cycle.
O
Ar-X
Ar-P(OEt)2
Pd
O
P(OEt)2
Pd
X
Ar
Pd
Ar
P(OEt)3
Pd P(OEt)2
Ar
Figure 4. FT-IR spectrum of the reused catalyst
O
Et
TEM micrographs of the catalyst after eight runs showed no
evidence of nanoparticle aggregation (Figure 5).
X
Pd
Pd-imino-Py-γ-Fe2O3
Scheme 3. A plausible mechanism for the synthesis of arylphosphonates
catalyzed by Pd-imino-Py-γ-Fe2O3
Finally, the catalytic activity of Pd-imino-Py-γ-Fe2O3 was
compared with the reported methods for the synthesis of
arylphosphonates via Csp2-P coupling reaction in water (Table
2).33j,34 It is apparent from Table 2 that Pd-imino-Py-γ-Fe2O3
provides significant advantages in respect of low catalyst
loading and higher TON and TOF compared with the other
catalytic systems. More importantly, our catalyst is a magnetic
heterogeneous catalyst and can be easily separated from the
reaction mixture using a magnetic bar. It also works in pure
water using triethylphosphite as a stable phosphite ester
without requiring any additive (SDS, TBAB, Zn, iso-propanol) or
activation by microwave irradiation.
Figure 5. TEM image of the reused catalyst
Different tests were performed to check if the active catalyst is
indeed heterogeneous. For this purpose, hot filtration test was
examined in the reaction of iodobenzene and
triethylphosphite in the presence of Pd-imino-Py-γ-Fe2O3
under optimized reaction conditions. The catalyst was isolated
from the reaction mixture after obtaining a conversion of
about 50%. Thereafter, the residual solution was continued to
stir. Any further progress was not observed even after 24 h.
Moreover, poisoning test was done in the presence of Hg(0).
This metal can bind to homogeneous Pd(II) species and
poisons it. The reaction of iodobenzene and triethylphosphite
with heterogeneous catalyst (Pd-imino-Py-γ-Fe2O3) showed no
decrease in reactivity upon the addition of Hg(0). Any Pd
detected in the remaining solution after the separation of the
catalyst in the hot filtration test and the results of the
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Table 2. Comparison of the present catalyst with other reported systems in water.
O
X
P(OR)2
P(OR)3
Or
HP(OR)2
EntryRef.
Phosphite ester
P(OEt)3
X
Catalyst [mol%]
Conditions
Time
[h]
Yield
[%]
TON
TOF
1this work
I, Br, Cl
Pd-imino-Py-γ-Fe2O3
(0.1)
H2O, Et3N, 100 C
̊
1-15
57-98
48-97
65-99
570-
980
38-980
OTs, B(OH)2
233
P(OEt)3
I, Br, Cl , OTs , Pd-2-ATP-γ-Fe2O3 (0.1)
B(OH)2
H2O, SDS, 100
C
1.5-15
1.5-6
480-
970
32-646.6
1.7-15
334a
P(OEt)3
I, Br, Cl
PdCl2 (4.4)
H2O, TBAB, n-
14.7-
22.5
P(O-i-Pr)3
Pr3N, 100 ̊C
434b
534c
634d
HP(O)(Ph)2,
HP(O)Ph2
I, Br
NiCl2.6H2O (10 and 20)
Pd/C (1)
H2O, Zn, 2,2'-
15 and 75-97
24
3.7-9.7
18-87
0.1-0.6
18-87
bipyridine, 70 °C
I, Br, Cl
I, Br, Cl
H2O, K2CO3,
Microwave,
1
18-87
180 ̊C
HP(O)(O-i-Pr)2,
Palladacycle (II)a (1)
H2O, TBAB, KF,
reflux, iso-PrOH,
N2
16
53-99
35-99
53-99
35-99
3.3-6.1
2.1-6.1
HP(O)(Ph)2
HP(O)(O-i-Pr)2
Palladacycle (I)/X-Phosb
(1)
65-99
65-99
4.0-6.1
a Cyclopalladated ferrocenylimine.
b 2-(Dicyclohexylphosphanyl)-2 ,4,6-tri-iso-propyl-1,1-biphenyl.
Experimental
Conclusions
A mixture of aryl halide (1 mmol), triethylphosphite (0.342 mL,
2 mmol), Et3N (0.556 mL, 4 mmol) and Pd-imino-Py-γ-Fe2O3
o
(0.0038 g, 0.1 mol%) in water (1 mL) was stirred at 100 C.
In summary, in this paper, a palladium-Schiff base complex
immobilized covalently on γ-Fe2O3 (Pd-imino-Py-γ-Fe2O3) was
synthesized and characterized by different methods such as
XRD, SEM, TEM, FT-IR, TGA, ICP, XPS, VSM and elemental
analysis. It was used as a new magnetically recyclable
heterogeneous Pd catalyst for the synthesis of
arylphosphinates via Csp2-P coupling reactions. A wide range of
electrophilic benzenes were coupled successfully with
triethylphosphite to generate the corresponding products in
pure water in good to high yields. Any Pd detected in the
remaining solution after the separation of the catalyst in the
hot filtration test and the results of the poisoning test showed
that the observed catalysis was heterogeneous in nature. The
true heterogeneous Pd-imino-Py-γ-Fe2O3 could be reused with
the aid of a magnetic bar for eight consecutive cycles without
any drastic loss of its reactivity. Altogether, the possibility to
perform the reaction in water without using any additive, at
the same time using a small amount of a heterogeneous
catalyst, efficient and simple reusability of the catalyst make
After stirring for an appropriate time (Table 1), the reaction
mixture was allowed to cool to room temperature. The
catalyst was separated by a magnetic bar, washed with EtOAc,
dried and re-used for a consecutive run under the same
reaction conditions. The remaining solution was washed 3
times with EtOAc (5 mL). The solvent of the combined organic
phase was then removed under reduced pressure and the
residue was purified by means of column chromatography on
silica gel with n-hexane and EtOAc (1:1).
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Journal Name
Morimoto, T. Yoshino, T. Yukawa, G. Lu, S. Matsunaga and
M. Shibasaki, Angew. Chem. Int. Ed., 2008, 47, 9125; (e) L. P.
He, H. L. Mu, B. X. Li and Y. S. Li, J. Polym. Sci. Part A: Polym.
Chem., 2010, 48, 311.
this protocol an environmentally compatible process for the
synthesis of a variety of arylphosphonates.
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Long, Polymer, 2006, 47, 4085; (b) Z. Jin and B. L. Lucht, J.
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Acknowledgements
We gratefully acknowledge the University of Birjand Research
Council for the financial support of this study.
Phys., 2007, 208, 1324
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Arylphosphonates were synthesized in the presence of Pd-imino-Py-γ-Fe2O3 in pure water without using any additive.
O
N
N
Pd
AcO
OAc
Si
OO
O
O
X
2
P(OEt)2
3
R
R
P(OEt)3
X= I, Br, Cl