A new magnetically recoverable heterogeneous palladium catalyst for phosphonation reactions in aqueous micellar solution

Article abstract of DOI:10.1002/aoc.3392

A new heterogeneous palladium complex of 2-aminothiophenol supported on nanomagnetic γ-Fe₂O₃ was synthesized and characterized using various methods. The catalyst was used as a magnetically recoverable heterogeneous palladium catalyst for phosphonation reactions via C-P bond formation. Using this method, a wide range of electrophilic benzenes was coupled successfully with phosphite esters (triethyl/tri-isopropyl/triphenylphosphite and diethyl/di-isopropyl/diphenylphosphite) in aqueous micellar solution to generate the corresponding arylphosphonates in good to high yields. The catalyst was separated using an external magnet and reused for six consecutive cycles without any significant loss of its reactivity.

Full text of DOI:10.1002/aoc.3392

Full paper
Received: 17 August 2015
Accepted: 5 September 2015
Published online in Wiley Online Library: 26 October 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3392
A new magnetically recoverable heterogeneous
palladium catalyst for phosphonation reactions
in aqueous micellar solution
Sara Sobhani* and Zohre Zeraatkar
A new heterogeneous palladium complex of 2-aminothiophenol supported on nanomagnetic γ-Fe2O3 was synthesized and
characterized using various methods. The catalyst was used as a magnetically recoverable heterogeneous palladium catalyst
for phosphonation reactions via C–P bond formation. Using this method, a wide range of electrophilic benzenes was coupled
successfully with phosphite esters (triethyl/tri-isopropyl/triphenylphosphite and diethyl/di-isopropyl/diphenylphosphite) in
aqueous micellar solution to generate the corresponding arylphosphonates in good to high yields. The catalyst was sepa-
rated using an external magnet and reused for six consecutive cycles without any significant loss of its reactivity.
Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: palladium; heterogeneous catalyst; iron oxide; phosphonation reaction; aqueous micelle
considered as versatile ligands in catalytic reactions or building blocks
Introduction
in polymer sciences.[9] In comparison with the phosphorus com-
In the past few decades, much attention has been paid to research
dedicated to the development of environmentally compatible
processes.[1] The major drive towards this initiative is avoiding
waste formation by using heterogeneous catalysts. Hetero-
genization of homogeneous catalysts by their immobilization on
various insoluble supports would simplify catalyst removal and
minimize the amount of waste formed.[2] Among these solid sup-
ports, magnetic nanoparticles (MNPs) are some of the best
supporting material because of their magnetic nature, which allows
facile recovery and recycling of the catalyst without use of the tra-
ditional filtration method.[3] Moreover, MNPs have unique physical
properties including high surface area, surface modification ability
and excellent thermal and chemical stability.[4] Another approach
to an environmentally benign and economical process is the re-
placement of volatile organic solvents by ‘green’ reaction media.[5]
In this regard, water is the most preferred solvent[6] because it is
considerably safe, readily available, non-toxic, non-flammable, sus-
tainable and cheap. Apart from the economic and environmental
benefits, water also exhibits unique physical and chemical activity
that leads to unique selectivity compared with organic solvents.
However, many reaction substrates have poor solubility in water,
while solubility is generally considered as a prerequisite for
reactivity.[7] One approach to deal with this problem is to add inex-
pensive surfactants into aqueous media to form micelles, burying
water-insoluble organic substrates within hydrophobic cores. In
recent years, many methods have been developed for various
chemical reactions in aqueous micelles.[8]
pounds containing N–P, S–P or O–P linkages, organophosphonates,
which have C–P bonds, are synthetically more important due to
the high resistance of their C–P linkage to chemical hydrolysis, ther-
mal decomposition and photolysis.[10] The first powerful approach
to C–P bond formation involved the direct cross-coupling of aryl
halides with dialkylphosphites catalysed by palladium (Hirao
reaction).[11] After the pioneering work of Hirao et al. in 1980, signif-
icant progress has been achieved on improvements and modifica-
tions of the reaction conditions such as using different substrates as
coupling partners,[12] ligands[13] and metals.[14] However, despite of
all of these improvements, C–P couplings still have a limitation due
to the problems associated with the separation and recovery of the
homogeneous catalyst, which might result in undesirable metal
contamination of the products. Therefore, the immobilization of a
homogeneous catalyst on a suitable supporting material to enable
efficient recovery and recycling of the catalyst could be the subject
of intense research in this area.
As part of our efforts directed towards the development of new
heterogeneous catalysts for organic reactions,[15] we have recently
introduced a palladium complex of bisiminopyridine (BIP) sup-
ported on γ-Fe2O3@SiO2 nanoparticles (Pd-BIP-γ-Fe2O3@SiO2) as
the first magnetic heterogeneous catalyst for the synthesis of
arylphosphonates.[16] In this connection, herein we report the syn-
thesis of a new palladium complex of 2-aminothiophenol (2-ATP)
supported on γ-Fe2O3 (Pd-2-ATP-γ-Fe2O3). This newly synthesized
catalyst was characterized using various methods such as scanning
Transition metal-catalysed C–P bond-forming reactions are
powerful and reliable tools for the synthesis of various types of
phosphorus compounds such as phosphonates, phosphine oxides
and phosphines. Some of these compounds have wide biological
activities and material science applications, and some have been
*
Correspondence to: Sara Sobhani, Department of Chemistry, College of Sciences,
University of Birjand, Birjand, Iran. E-mail: ssobhani@birjand.ac.ir
Department of Chemistry, College of Sciences, University of Birjand, Birjand, Iran
Appl. Organometal. Chem. 2016, 30, 12–19
Copyright © 2015 John Wiley & Sons, Ltd.

New magnetically recoverable Pd catalyst for C–P bond formation
electron microscopy (SEM), transmission electron microscopy
(TEM), Fourier transform infrared (FT-IR) spectroscopy, thermogra-
vimetric analysis (TGA), inductively coupled plasma (ICP) analysis,
X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS),
vibrating sample magnetometry (VSM) and elemental analysis,
and was used as a magnetically recoverable heterogeneous palla-
dium catalyst for the phosphonation reactions of aryl halides with
triethyl/tri-isopropyl/triphenylphosphites and diethyl/di-isopropyl/
diphenylphosphites in aqueous micellar solution.
temperature. The solid was separated using an external magnet,
washed several times with methanol (3 × 20 ml) and dried in an
oven at 80°C overnight under vacuum.
General procedure for phosphonation of aryl halides with
phosphite esters
Pd-2-ATP-γ-Fe2O3 (3.5 mg, 0.001 mmol, 0.1 mol%) was added to a
stirred mixture of aryl halide (1 mmol), phosphite ester (1.5 mmol)
and Et3N (2 mmol) in aqueous micellar solution of sodium
dodecylsulfate (SDS; 5 ml, 1 CMC, 8.1 × 10ꢀ3 M) and heated at
100°C. The reaction was monitored using TLC. After an appropri-
ate time, the reaction mixture was cooled to room temperature.
The catalyst was separated using an external magnet from the re-
action mixture, washed with water and EtOAc, dried for 30 min at
100°C and reused for a subsequent run under the same reaction
conditions. The organic compound was extracted with ethyl ace-
tate (3 × 10 ml) from aqueous layer and dried over anhydrous
Na2SO4, filtered and concentrated in vacuum. The crude organic
mixture was then purified using silica gel column chromatography
with n-hexane–ethyl acetate (3:1) as eluent to afford the desired
product.
Experimental
Chemicals were purchased from Merck Chemical Company. NMR
spectra were recorded in CDCl3 with a Bruker Avance DPX-400 in-
strument using tetramethylsilane as internal standard. The purity
of the products and the progress of the reactions were monitored
using TLC on silica-gel Polygram SILG/UV254 plates. FT-IR spectra
were recorded with a JASCO FT-IR 460 plus spectrophotometer.
TEM analysis was performed using a Philips CM30 microscope.
The morphology of the products was determined using a SEM in-
strument (model s4160, Hitachi, Japan) at an accelerating voltage
of 15 kV. TGA was performed using a Shimadzu thermogravimetric
analyser (TG-50). Elemental analysis was carried out with a Costech
4010 CHN elemental analyser. Power XRD was performed using an
X’Pert Pro MPD diffractometer with Cu Kα (λ = 0.154 nm) radiation.
Surface analysis spectroscopy of the catalyst was performed with
an ESCA/AES system. This system was equipped with a concentric
hemispherical (CHA) electron energy analyser (Specs model EA10
plus) suitable for XPS. Room temperature magnetization isotherms
were obtained using VSM (Lake Shore 7400). The content of palla-
dium in the catalyst was determined with an OPTIMA 7300DV ICP
analyser.
Results and discussion
Synthesis and characterization of Pd-2-ATP-γ-Fe2O3
The chemistry of nitrogen–sulfur chelating ligands has witnessed a
great deal of interest because of their stability, chemical and elec-
trochemical activities and biological relevance.[17] It is also well
known that chelating nitrogen–sulfur donors can stabilize metal
ions in unusual oxidation states and uncommon coordination num-
bers in complexes.[18] In the work reported in this paper, supported
2-ATP was used as a nitrogen–sulfur chelating ligand to synthesize
a new heterogeneous palladium complex (Pd-2-ATP-γ-Fe2O3). The
synthetic procedure is described in Scheme 1. This is the first report
of the use of 2-ATP as a chelating ligand for a palladium catalyst.
First, chloro-functionalized γ-Fe2O3 was synthesized by the reaction
of γ-Fe2O3 MNPs with 3-chloropropyltrimethoxysilane in refluxing
dry toluene. Pd-2-ATP-γ-Fe2O3 was obtained by the reaction of
chloro-functionalized γ-Fe2O3 with 2-ATP followed by complex for-
mation with Pd(OAc)2. The synthesized Pd-2-ATP-γ-Fe2O3 was fully
characterized using SEM, TEM, FT-IR spectroscopy, TGA, ICP, XRD,
XPS, VSM and elemental analysis.
Synthesis of chloro-functionalized γ-Fe2O3
The synthesized γ-Fe2O3 (2 g)[16] was sonicated in dry toluene
(25 ml) for 30 min. 3-Chloropropyltrimethoxysilane (11 mmol,
2 ml) and triethylamine (1.4 mmol, 0.2 ml) were added to
the γ-Fe2O3 dispersed in toluene. The mixture was slowly
heated to 105°C and stirred at this temperature for 48 h. The
resulting chloro-functionalized γ-Fe2O3 was separated using an
external magnet and washed sequentially with toluene, deionized
water and ethanol (3 × 20 ml), and dried in an oven at 90°C under
vacuum.
The morphology of Pd-2-ATP-γ-Fe2O3 was studied using SEM
and TEM (Fig. 1). SEM and TEM images indicate that Pd-2-ATP-γ-
Fe2O3 has uniform and spherical morphology. The particle size
distribution of Pd-2-ATP-γ-Fe2O3 was also evaluated using TEM,
the average diameter of the particles being 8.2 nm.
Synthesis of 2-ATP-γ-Fe2O3
Chloro-functionalized γ-Fe2O3 (2 g) was sonicated in dry
dimethylformamide (DMF; 20 ml) for 30 min. A solution of 2-ATP
(30 mmol, 3.2 ml) in DMF (20 ml) and NaOH (30 mmol, 1.2 g) was
added dropwise to the chloro-functionalized γ-Fe2O3 dispersed
in DMF and was refluxed for 48 h under argon atmosphere. The
resulting light-brown solid was separated using an external
magnet, repeatedly washed with DMF, ethanol and methanol
(3 × 20 ml) and finally dried in an oven at 90°C under vacuum.
Synthesis of Pd-2-ATP-γ-Fe2O3
2-ATP-γ-Fe2O3 (2 g) was sonicated in methanol (30 ml) for 30 min. A
solution of Pd(OAc)2 (0.6 mmol, 0.134 g) in methanol (10 ml) was
added to the 2-ATP-γ-Fe2O3 dispersed in methanol and stirred
under reflux for 10 h. The reaction mixture was cooled to room
Scheme 1. Synthesis of Pd-2-ATP-γ-Fe2O3.
Appl. Organometal. Chem. 2016, 30, 12–19
Copyright © 2015 John Wiley & Sons, Ltd.
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S. Sobhani and Z. Zeraatkar
ATP-γ-Fe2O3, the C–N stretching vibration is red-shifted by 8 cmꢀ1
compared with the C–N stretching vibration in the spectrum of
2-ATP-γ-Fe2O3 and appears at 1230–1260 cmꢀ1. The band observed
at around 1617 cmꢀ1 is related to OAc in Pd-2-ATP-γ-Fe2O3.
The thermal behaviour of 2-ATP-γ-Fe2O3 is shown in Fig. 3. A
significant decrease in the weight percentage at around 100°C is
related to the desorption of water molecules from the catalyst
surface. This is evaluated to be ca 0.3% according to the TGA. The
organic parts decompose completely at around 829°C. The amount
of 2-ATP functionalized on γ-Fe2O3 is calculated to be 0.3 mmol gꢀ1
according to the elemental analysis (N = 0.43% and C = 3.26%).
These results are in agreement with those obtained from TGA of
2-ATP-γ-Fe2O3.
ICP analysis of Pd-2-ATP-γ-Fe2O3 shows that 0.28 mmol of Pd is
loaded on 1 g of the catalyst.
As presented in Fig. 4, 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°
are readily recognized from the XRD pattern of Pd-2-ATP-γ-Fe2O3.
The observed diffraction peaks indicate that γ-Fe2O3 MNPs
mostly exist in face-centred cubic structure. A weak peak appears
at 2θ = 39.9° corresponding to (1 1 1) reflection of the supported
Pd (Fig. 4(b)).
XPS was carried out to establish the oxidation state of Pd in the
catalyst (Fig. 5). The peaks at binding energies of 337.4 and 342.5
eV corresponding to Pd 3d5/2 and Pd 3d3/2, respectively, confirm
the presence of Pd(II) in the catalyst.[19]
The magnetic properties of the samples containing a magne-
tite component were studied using VSM at 300 K. Figure 6 shows
the absence of a hysteresis phenomenon and indicates that Pd-2-
ATP-γ-Fe2O3 shows superparamagnetism at room temperature.
The saturation magnetization values for γ-Fe2O3 and Pd-2-ATP-
γ-Fe2O3 are 68.9 and 65.2 emu gꢀ1, respectively. The slight
decrease of the saturation magnetization of Pd-2-ATP-γ-Fe2O3 is
due to the immobilization of Pd complex of 2-ATP on the surface
of γ-Fe2O3 MNPs.
Figure 1. (a, b) SEM images, (c, d) TEM images and (e) particle size
distribution histogram for Pd-2-ATP-γ-Fe2O3.
FT-IR spectra of chloro-functionalized γ-Fe2O3, 2-ATP-γ-Fe2O3 and
Pd-2-ATP-γ-Fe2O3 are shown in Fig. 2. The bands at around
576–624 and 2913 cmꢀ1 are assigned to the stretching vibrations
of Fe–O and CH2 bonds, respectively. In the FT-IR spectra of 2-ATP-
γ-Fe2O3 and Pd-2-ATP-γ-Fe2O3, the bands observed at 1509, 1590
and 1408–1436 cmꢀ1 can be attributed to C¼C stretching vibrations.
Strong broad peaks at about 1030–1110 cmꢀ1 are assigned to the
stretching vibrations of Si–O bond. The bands at around 3328 and
3446 cmꢀ1 are due to the N–H vibration. In the spectrum of Pd-2-
Catalytic activity of Pd-2-ATP-γ-Fe2O3 in phosphonation of
electrophilic benzenes with phosphite esters in water
In order to optimize the reaction conditions, the phosphonation
of iodobenzene with triethylphosphite (1.5 equiv.) in the
Figure 2. FT-IR spectra of (a) chloro-functionalized γ-Fe2O3, (b) 2-ATP-γ-
Fe2O3 and (c) Pd-2-ATP-γ-Fe2O3.
Figure 3. TGA diagram of Pd-2-ATP-γ-Fe2O3.
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Appl. Organometal. Chem. 2016, 30, 12–19

New magnetically recoverable Pd catalyst for C–P bond formation
(a)
(b)
Figure 6. Magnetization curves of γ-Fe2O3 and Pd-2-ATP-γ-Fe2O3.
Table 1. Phosphonation of idobenzene with triethylphosphite
catalysed by Pd-2-ATP-γ-Fe2O3 under various conditions
Entry
Base
Et3N
Surfactant
CTAB
Time (h) Isolated yield (%)a
1
1.5
1.5
2
80
2
Et3N
Triton X-100
82
3
Et3N
Sodium stearate
95
4
Et3N
SDS
—
1.5
4 (24)
3
97
5
Et3N
75 (75)
92
6
(n-Pr)3N
(i-Pr)2NH
KOH
SDS
SDS
SDS
SDS
SDS
SDS
SDS
SDS
SDS
SDS
SDS
SDS
Figure 4. XRD patterns of (a) γ-Fe2O3 and (b) Pd-2-ATP-γ-Fe2O3.
7
2.5
3
87
8
75
9
K2CO3
NaOAc
Et3N
3
82
10
11b
12c
13d
14e
15f
16g
17h
3
70
1.5
1.5
2
97
Et3N
97
Et3N
92
Et3N
2
83
Et3N
2.5
1.5
24
95
Et3N
97
Et3N
Trace
aReaction conditions: iodobenzene (1 mmol), triethylphosphite (1.5 mmol,
except for entries 15, 16), base (2 mmol), Pd-2-ATP-γ-Fe2O3 (1 mol%,
except for entries 11–13, 17), 100°C (except for entry 14), H2O (5 ml).
bPd-2-ATP-γ-Fe2O3 (0.5 mol%).
cPd-2-ATP-γ-Fe2O3 (0.1 mol%).
dPd-2-ATP-γ-Fe2O3 (0.08 mol%).
eTemperature = 80°C.
fTriethylphosphite (1 mmol).
gTriethylphosphite (2 mmol).
hNo catalyst.
Figure 5. XPS spectrum of Pd-2-ATP-γ-Fe2O3.
presence of Pd-2-ATP-γ-Fe2O3 (1 mol%) and Et3N in water at
100°C was chosen as a model reaction (Table 1). In the initial
study, the effect of various types of surfactants was evaluated.
As evident from Table 1, surfactants cetyltrimethylammonium
bromide (CTAB) as a cationic micelle and Triton X-100 as a neu-
tral micelle at their critical micellar concentration (CMC) in water
produce the desired product in 80 and 82% yields, respectively
(entries 1 and 2). The yield of the product is enhanced when
the reaction is performed in anionic micellar solutions (entries 3
and 4). The product is obtained in a shorter reaction time in
SDS in comparison with sodium stearate. The model reaction
was examined in the absence of a surfactant, the product being
obtained in 75% yield after 4 h. The yield of the product in the
absence of any surfactant does not increase even after 24 h
Appl. Organometal. Chem. 2016, 30, 12–19
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S. Sobhani and Z. Zeraatkar
(entry 5). Therefore, to push the reaction forward, using a surfactant
in water is necessary. Some other reaction conditions such as base,
molar ratio of catalyst and temperature were also optimized using
the model reaction in aqueous micellar solution of SDS (entries
6–14). The best result is obtained using Et3N as a base in the pres-
ence of 0.5 and 0.1 mol% of the catalyst at 100°C (entries 11 and
12). The lower amount of catalyst was chosen as the optimized
amount of Pd-2-ATP-γ-Fe2O3 (entry 12). Similar reaction in the pres-
ence of lower amount of triethylphosphite proceeds in a longer re-
action time (entry 15). Increasing the amount of phosphite ester
to 2 equiv. does not have any effect on the yield of the product
(entry 16). To show the role of the catalyst in this reaction, the
model reaction was examined in the absence of the catalyst, with
a trace amount of product being obtained after 24 h (entry 17).
To explore the scope of this method, the phosphonation of
various electrophilic benzenes with a variety of phosphite esters
was studied under the optimized reaction conditions (Table 1,
entry 12). The results of these studies are summarized in Table 2.
As is evident from Table 2, the catalytic reactions of iodobenzene
with triethyl-, tri-isopropyl- and triphenylphosphite proceed well
and the desired products are isolated in 97, 94 and 91% yields,
respectively (entries 1–3). The possibility of using dialkylphosphites
instead of trialkylphosphites for the phosphonation of
iodobenzene in micellar solution of SDS was also studied. The re-
sults show that the present method is applicable for the
phosphonation of iodobenzene with diethyl-, di-isopropyl and
diphenylphosphite as phosphite esters without using any additive
(entries 4–6). It is worth noting that dialkylphosphites are unstable
in water and reversibly produce phosphorous acid and their corre-
sponding alcohols. In previous reports addition of a suitable alco-
hol to the reaction mixture to avoid the side reaction in water
was necessary.[20] Phosphonation reactions of aryl halides (iodides,
bromides and chlorides) bearing either electron-withdrawing
or electron-donating groups with triethylphosphite proceed
well and the corresponding products are generated in 54–
94% yields (entries 7–20). In addition to halobenzenes, some
other electrophilic benzenes such as aryl tosylate and
`benzeneboronic acid were also examined to carry out the reac-
tion with triethylphosphite (entries 21 and 22). The results show
that the desired product is obtained in 71 and 75% yields, re-
spectively. Under the present reaction conditions, the coupling
reaction of dihalobenzenes with triethylphosphite proceeds well
and halophenylphosphonate is obtained in high yields (entries
23 and 25). Both halogens in dihalobenzenes are replaced by
twice the amount of phosphite ester under the present reaction
conditions (entries 24 and 26).
Table 2. Phosphonation of electrophilic benzenes with various phos-
phite esters catalysed by Pd-2-ATP-γ-Fe2O3
Entrya
X
R1
R2 Time (h) Isolated yield (%)b TON
To investigate the catalytic effect of SDS aqueous micellar solu-
tion in the phosphonation reaction, we performed two experi-
ments to make a comparison of conversion versus time in pure
water and SDS aqueous micellar solution (1 CMC) (Fig. 7(a)). In both
pure water and SDS aqueous micellar solution, the reaction pro-
ceeds quickly at first and then slows down over the time in pure
water. This would imply that the reaction rate is predominantly
based on the concentration of the substrates. It can also be con-
cluded that the reaction takes place in the proximity of the head
group of the surfactants, where the polarity is very similar to that
of water. Meanwhile, due to the very large interfacial area, the reac-
tion takes place more easily in a micelle droplet with respect to its
functioning as a micro- or nanoreactor.[23]
According to the results, a tentative mechanism of the
phosphonation reaction in aqueous micelles is illustrated in
Fig. 7(b). At first the Pd catalyst is inserted into the aryl halide
bond via an oxidative addition in the oil–water interface and
the reaction continues in the hydrophobic core of the micelle
droplets. Then, triethylphosphite reacts with the produced pal-
ladium halide. The corresponding arylphosphonates are pro-
duced after ethyl halide elimination by an Arbuzov-type
reaction and reductive elimination. The products are left in
the hydrophobic core and the regenerated Pd catalyst returns
to the hydrophilic environment.
The recovery and reusability of Pd-2-ATP-γ-Fe2O3 were investi-
gated in the reaction of iodobenzene with triethylphospite under
the present reaction conditions. After the reaction was completed
and allowed to cool, the catalyst was separated using an external
magnet from the reaction mixture (Fig. 8(b)), washed with water
and EtOAc, dried for 30 min at 100°C and reused for a subse-
quent run under the same reaction conditions.
The average isolated yield of the product for six consecutive runs
is 95.3%, which clearly demonstrates the practical reusability of this
catalyst (Fig. 9). It is worth mentioning that Pd-2-ATP-γ-Fe2O3
1[21]
2[21]
3[21]
4[21]
5[21]
6[21]
7[21]
8[21]
9[21]
10[21] Br
11[21] Br
12[21] Br
13[21] Br
14[21] Br
15[21] Cl
16[21] Cl
17[21] Cl
18[21] Cl
19[21] Cl
20[11] Cl
I
—
—
—
—
—
—
Et
iPr
Ph
Et
iPr
Ph
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
1.5
2
97
94
91
89
87
81
93
94
92
90
93
90
65
93
91
89
82
68
92
54
71
75
92
73c
91
48c
970
940
910
890
870
810
930
940
920
900
930
900
650
930
910
890
820
680
920
540
710
750
920
730
910
480
I
I
2
I
3
I
3
I
3
I
4-Cl
2.5
2
I
4-OMe
—
Br
2.5
3
4-Cl
4-OMe
4-Me
4-NO2
4-CN
—
2
2
5
3
4
4-Me
2-Me
4-NO2
4-CN
3
4
6
4
4-COMe Et
8
21[21] OTs
22[21] B(OH)2
23[22]
24[11]
25[21] Br
26[11] Br
—
—
Et
Et
Et
Et
Et
Et
6
7
I
4-I
4-I
2.5
I
10
4.5
15
4-Br
4-Br
aAll products are known compounds.
bReaction conditions: electrophilic benzene (1 mmol), trialkylphosphite
(1.5 mmol, except for entries 4–6, 24 and 26), dialkylphosphite (1.5
mmol, entries 4–6) Et3N (2 mmol), Pd-2-ATP-γ-Fe2O3 (0.1 mol%),
SDS (1 CMC), H2O (5 ml), 100°C.
cTriethlphosphite (3 mmol).
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Appl. Organometal. Chem. 2016, 30, 12–19

New magnetically recoverable Pd catalyst for C–P bond formation
(a)
Figure 9. Reusability of Pd-2-ATP-γ-Fe2O3 as a magnetically recyclable catalyst.
(b)
The amount of Pd leached after six runs was measured using ICP.
It is observed that only a very small amount (less than 1%) of Pd is
leached. CHN analysis of Pd-BIP-γ-Fe2O3@SiO2 was carried out after
six times of reuse and any ligand leaching is not detected. A slight
agglomeration is observed from TEM images of the catalyst after six
times of reuse (Fig. 10).
To check whether Pd is leached out from the solid catalyst during
coupling reaction or whether the catalyst is stable, phosphonation
of iodobenzene with triethylphosphite was investigated. After com-
pletion of 30% of the coupling reaction at 100°C, the catalyst was
separated using a magnet and the remaining solution was stirred
for 10 h. In the absence of the catalyst, no increase in the conver-
sion value is detected. These results suggest that the catalyst is sta-
ble and can tolerate the present reaction conditions without
leaching of a significant quantity of Pd.
A comparison of the catalytic activity of Pd-2-ATP-γ-Fe2O3 with
that of our recently reported catalyst (Pd-BIP-γ-Fe2O3@SiO2) shows
that Pd-2-ATP-γ-Fe2O3 is more efficient (turnover number (TON) =
540–970) than Pd-BIP-γ-Fe2O3@SiO2 (TON = 33–47.5)[16] for the syn-
thesis of arylphosphonates. Furthermore, in order to compare the
catalytic activity of Pd-2-ATP-γ-Fe2O3 with that of Pd-BIP-γ-Fe2-
O3@SiO2 under our present reaction conditions, the coupling of
iodobenzene with triethylphosphite catalysed by Pd-BIP-γ-Fe2-
O3@SiO2 (0.1 mol%) in micellar solution of SDS (1 CMC) at 100°C
was performed. Under these conditions, the reaction leads to the for-
mation of the desired product in 10 and 49% yields after 1.5 and 24
h, respectively. These results show that Pd-BIP-γ-Fe2O3@SiO2 (TON =
490) is not as effective as Pd-2-ATP-γ-Fe2O3 (TON = 970) for the
phosphonation of iodobenzene under the same reaction conditions.
The catalytic activity of Pd-2-ATP-γ-Fe2O3 was also compared
with that of various reported Pd catalysts in the phosphonation re-
action (Table 3). From Table 3 it is apparent that Pd-2-ATP-γ-Fe2O3
Figure 7. (a) Comparison of conversion versus time in pure water and SDS
aqueous micellar solution. (b) Tentative mechanism of the phosphonation
reaction in aqueous micelles catalysed by Pd-2-ATP-γ-Fe2O3.
Figure 8. (a) Reaction mixture. (b) Separation of Pd-2-ATP-γ-Fe2O3 from the
reaction mixture using a magnetic bar.
is also reusable in sequential condensations by the addition of new
samples of iodobenzene and triethylphosphite to the reaction
mixture.
Figure 10. TEM images of Pd-2-ATP-γ-Fe2O3 after six times of reuse.
Appl. Organometal. Chem. 2016, 30, 12–19
Copyright © 2015 John Wiley & Sons, Ltd.
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S. Sobhani and Z. Zeraatkar
Table 3. Catalytic activity of Pd-2-ATP-γ-Fe2O3 compared with some other catalysts reported for the phosphonation reaction
Entry Phosphite ester
X
Catalyst (mol%)
PdCl2 (4.4)
Conditions
Time (h) Yield (%)
TONa
1
P(OEt)3
I, Br, Cl
H2O, TBAB, n-Pr3N, 100°C
1.5–6
65–99[21]
14.7–22.5
P(O-iPr)3
2
3
4
5
HP(O)(O-iPr)2
HP(O)Ph2
I, Br, Cl
I, Br, Cl
I, Br, OTf
Br
Palladacycle (1)
Pd/C (1)
H2O, TBAB, KF, reflux, iPrOH, N2
H2O, K2CO3, MW, 180°C
16
1
53–99[20]
18–86[24]
63–98[25]
53–99
18–86
HP(O)(OEt)2
HP(O)(OEt)2
HP(O)(OMe)2
HP(O)(O-iPr)2
HP(O)(OEt)2
HP(O)Ph2
Pd(OAc)2/dppfb (10)
THF, Et3N, KOAc, 68°C
1.5–32
0.17 72–96[26]
6.3–9.8
14.4–19.2
Pd(pph3)4 (5)
THF, Cs2CO3 or Et3N, MW, 120°C
6
7
8
I, Br
Pd(OAc)2 (5 or 10)
Solvent free, Et3N, MW, 150–200°C
0.03–0.25 27–93[27]
3–18.6
25–43
PhP(O)(OEt)(Oct) I, Br, Cl, OTf Pd/Xantphos (2)
HP(O)(OEt)2
Toluene/EGc, iPr3N, 110°C
24
2
50–86[28]
34–97[29]
HP(O)(Et)2
I
Pd2(dba)3d/Xantphos (0.5) 1,4-Dioxane, Et3N, 24°C
68–194
HP(O)(Me)2
HP(O)(Ph)2
HP(O)(Cy)(Bn)
HP(O)(OEt)2
HP(O)(iPr)2
HPO(OEt)2
HP(O)(OEt)2
HP(O)(OBu)2
HP(O)Ph2
9
B(OH)2
I, Br, OTf
Br
Pd(OAc)2/dmphene (2)
Pd(OAc)2/dppf (1)
Pd(OAc)2/PPh3 (2)
Pd(OAc)2 (5 or 10)
DMF, P-BQf, MW, 100°C
0.5
51–90[30]
27–92[31]
80–93[32]
25.5–45
27–92
10
11
12
DMF, 110°C or MeCN, reflux, (iPr)2NEt
EtOH, (cyclohexyl)2MeN, reflux
Solvent free, Et3N, MW, 150–200°C
24
16
40–46.5
5.5–18.2
Br
0.08–0.25 41–95[33]
13
P(OEt)3
I, Br, Cl
OTs
Pd-2-ATP-γ-Fe2O3 (0.1)
H2O, SDS, 100°C
1.5–8
51–97 (This work) 510–970
P(OPh)3
HP(O)(OEt)2
HP(O)(OPh)2
HP(O)(O-iPr)2
HP(O)(O-iPr)2
B(OH)2
aTON = mol of product/mol of catalyst.
b1,1′-Bis(diphenylphosphino)ferrocene.
cEthylene glycol.
dTris(dibenzylideneacetone).
e2,9-Dimethyl-1,10-phenanthroline.
fp-Benzoquinone.
provides significant advantages in terms of TON compared with the
other catalytic systems. More importantly, our catalyst is a magnetic
heterogeneous catalyst and can be easily separated from the reac-
tion mixture using an external magnet. It is worth noting that
although there are many reports of using various heterogeneous
Pd catalyst for C–C bond coupling reactions, the synthesis of
arylphosphonates catalysed by heterogeneous Pd catalyst is rare
in the literature (Table 3, entry 3).[24] Pd-2-ATP-γ-Fe2O3 is the first
magnetically recoverable Pd catalyst which is used for efficient
C–P bond formation reactions in aqueous micellar solution.
heterogeneous Pd catalyst for C–P bond formation of a wide range
of electrophilic benzenes with phosphite esters (triethyl/tri-isopropyl/
triphenylphosphite and diethyl/di-isopropyl/diphenylphosphite)
in aqueous micellar solution. Efficient and simple reusability of
the catalyst and the use of water as an eco-friendly solvent make
this protocol an environmentally compatible process for the syn-
thesis of arylphosphonates in good to high yields.
Acknowledgement
Financial support of this project by the University of Birjand Re-
search Council is appreciated.
Conclusions
In summary, a new heterogeneous Pd complex of 2-ATPsupported
on γ-Fe2O3 was synthesized and characterized using various
methods such as SEM, TEM, FT-IR spectroscopy, TGA, ICP, XRD,
XPS, VSM and elemental analysis. The catalyst was successfully
(TON = 540–970) used as the first magnetically recoverable
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