Diphenylphosphorylated PEG (DPPPEG) as a new support for generation of nano-Pd(0) as catalyst for carbon-carbon bond formation via copper-free Sonogashira and homocoupling reactions of aryl halides in PEG

Article abstract of DOI:10.1007/s13738-014-0469-8

In this study, synthesis and application of diphenylphosphorylated PEG₂₀₀ (DPPPEG₂₀₀) are described. Herein, we report a very simple procedure for the preparation of DPPPEG₂₀₀ as a stable solid through the reaction of PEG₂₀₀ with ClPPh₂. This compound was used as a very suitable ligand for the in situ generation of nano-Pd(0) particles through its reaction with PdCl₂ as a pre-catalyst. Isolation of this catalyst is very simple through addition of diethyl ether to the reaction mixture and centrifugations. Full characterization of the nano-Pd(0)/DPPPEG200 was performed by XRD spectra, UV-Vis spectra, and also by TEM image. This nano-complex was used as an efficient catalyst for copper-free Sonogashira and homocoupling reactions of aryl halides. The sonogashira reaction of aryl halides was conducted at 80 °C in PEG. However, the homocoupling reaction was performed at 100 °C for aryl iodides and activated aryl bromides and at 130 °C for deactivated aryl bromides and aryl chlorides in PEG. The catalyst was recovered and recycled for four consecutive runs.

Full text of DOI:10.1007/s13738-014-0469-8

J IRAN CHEM SOC
DOI 10.1007/s13738-014-0469-8
ORIGINAL PAPER
Diphenylphosphorylated PEG (DPPPEG) as a new support
for generation of nano-Pd(0) as catalyst for carbon–carbon bond
formation via copper-free Sonogashira and homocoupling
reactions of aryl halides in PEG
•
Nasser Iranpoor Habib Firouzabadi
•
Asma Riazi
Received: 29 April 2014 / Accepted: 8 May 2014
Ó Iranian Chemical Society 2014
Abstract In this study, synthesis and application of
diphenylphosphorylated PEG200 (DPPPEG200) are descri-
bed. Herein, we report a very simple procedure for the
preparation of DPPPEG200 as a stable solid through the
reaction of PEG200 with ClPPh2. This compound was used
as a very suitable ligand for the in situ generation of nano-
Pd(0) particles through its reaction with PdCl2 as a pre-
catalyst. Isolation of this catalyst is very simple through
addition of diethyl ether to the reaction mixture and cen-
trifugations. Full characterization of the nano-Pd(0)/DPP-
PEG200 was performed by XRD spectra, UV–Vis spectra,
and also by TEM image. This nano-complex was used as
an efficient catalyst for copper-free Sonogashira and ho-
mocoupling reactions of aryl halides. The sonogashira
reaction of aryl halides was conducted at 80 °C in PEG.
However, the homocoupling reaction was performed at
100 °C for aryl iodides and activated aryl bromides and at
130 °C for deactivated aryl bromides and aryl chlorides in
PEG. The catalyst was recovered and recycled for four
consecutive runs.
Introduction
Palladium-catalyzed formation of carbon–carbon bonds
through homo- and cross-coupling reactions has recently
emerged as a powerful method in an organic synthesis [1].
The palladium-catalyzed homocoupling of aryl halides [2–
8] and coupling of aryl halide with terminal alkyne (So-
nogashira–Hagihara coupling) [9–11] are recognized as the
most successful method for forming a C(sp2)–C(sp2) and
C(sp)–C(sp2) carbon–carbon bonds, respectively. These
reactions have been widely employed in the synthesis of
natural products, biologically active molecules, and mate-
rials science [12–17]. Significant research has been devoted
toward the identification of environmentally friendly cross-
coupling protocols. For this effort, over the past few years,
green media such as water, PEG, and ionic liquids [18–35]
have been under much attention for the replacement of
organic solvents.
Applications of nanoparticle-based materials for various
reactions are increasingly attracted chemists as new types
of homogeneous or heterogeneous catalysts in different
media [36–41]. Their high economic efficiency due to their
large surface-to-volume ratio and enhancement of the
reaction rate are considered as the general advantages of
these nano-catalysts in catalytic systems [42, 43]. Recently,
palladium nanoparticles have been explored as catalyst in
the cross-coupling reactions [44–56]. Our group was also
interested in this research field [26, 57, 58]. Polymer-sup-
ported reagents and catalysts have attracted considerable
attention as they offer alternative methods to separate and
reuse expensive and often toxic chemicals. In this regards,
selecting PEG as scaffold has considerable advantages,
including low toxicity, environmentally tested benignity,
and special properties arising from its cation complexation
ability. In addition, PEG is readily commercially available
Keywords Diphenylphosphorylated PEG200 Á Nano-
Pd(0) Á Sonogashira reaction Á Homocoupling reaction Á
Aryl halides
N. Iranpoor (&) Á H. Firouzabadi (&) Á A. Riazi
Chemistry Department, College of Sciences,
Shiraz University, Shiraz, Iran
e-mail: iranpoor@susc.ac.ir
H. Firouzabadi
e-mail: firouzabadi@susc.ac.ir
123

J IRAN CHEM SOC
at low cost, being formed from ethylene oxide by a simple
ring epoxide aperture [59]. The properties of PEG offer the
possibility for immobilizing and recycling of a homoge-
neous catalyst [60–68]. Immobilization of phosphine group
on PEGs with the aim of improving the efficiency of these
compounds as suitable polymeric ligands for preparation of
recyclable catalysts [69–71] has been achieved. The use of
PEG-bound phosphine ligands in organic synthesis has
generated considerable interest from both economical and
environmental points of view. Since those initial reports in
the 1960s on preparation of PEG-bound phosphines as the
first soluble polymeric ligands [72], many other supported
phosphines have been prepared [73]. In 1994, Ruckenstein
and Hong introduced [74] a,x-bis(diphenylphosphino)-
poly(ethylene glycol) (DPP-PEG) as a ligand and its
Rh(I) carbonyl complex as a liquid polymer phase catalyst.
After that, in 2010, Bhanage et al. [75] reported the use of
a,x-bis(diphenylphosphino)-poly(ethylene glycol) (DPP-
PEG) in coordination to Rh(I) and/or Pd(II) as catalyst for
limited examples of hydroformylation reaction, hydroge-
nation reaction, and Heck and Sonogashira reactions. In
continuation of our recent works on ligands containing
phosphitite group [25, 58, 76–80], herein, we report a very
simple procedure for the preparation of this ligand and its
application for the in situ generation of nano-Pd(0) for
catalyzing Sonogashira and also homo-coupling reactions
of aryl halides in an environmentally friendly protocol.
HCl. The reaction mixture was washed with 50 mL of dry
Et2O and dried under vacuum. Bis-(diphenylphosphory-
lated) PEG200 was obtained as a white solid (2.78 gr,
98 %), which was stored in a capped bottle. The reagent
can be kept without any change for months. In order to
determine the amount of active phosphorus content of the
reagent, DPPPEG200 was reacted with excess of iodine in
acetonitrile and stirred for 1 h under reflux condition. On
the basis of titration of unreacted iodine with an aqueous
solution of sodium thiosulfate (0.1 M), the amount of
active phosphorus content was determined to be 2.0 mmol
for 1.0 mmol of PEG200.
1H NMR (250 MHz, DMSO):
4.53–4.70 (m, 16H, –CH2O), 7.44–7.75 (m, 20H, aro-
matic); 13C NMR (62.9 MHz, DMSO): 68.3 (Ph2PO–CH2),
71.1 (OCH2–), C6H5– group (127.9, 130.6, 130.8, 131.0);
IR (KBr disk) m (cm-1) 3070(w), 2876(m), 1589(s),
1481(s), 1435(s), 1180(s), 1126(s), 1064(s), 956(m),
694(m).
General procedure for the sonogashira coupling
reaction of aryl halides and phenylacetyene
in the presence of PdCl2/DPPPEG200 in PEG200
A round-bottom flask was charged with phenylacetylene
(1.5 mmol, 0.16 mL), NaOH (1.5 mmol, 100 mg), PEG200
(2 mL), and aryl halide (1.0 mmol). To this mixture,
3.0 mol% of PdCl2 and 8.0 mol% of DPPPEG200 were
added. The reaction mixture was placed in an 80 °C oil
bath and stirred to complete. The completion of the reac-
tion was monitored by TLC. After completion of reaction,
the mixture was cooled to room temperature, and the cat-
alyst was removed by addition of Et2O and then centrifu-
gated. The isolated catalyst was dried and reused. The
filtered solution was evaporated reduced pressure, and the
expected product was purified by column chromatography
over silica gel [60 Merck (230–240 mesh)] using petroleum
ether/ethyl acetate (5:1) as eluent to afford the product with
high purity in 58–95 % yield.
Experimental
All chemicals were purchased from Merck, Fluka, or Acros
Chemical Companies and used without any further purifi-
cation. NMR spectra were recorded with a BrukerAvance
DPX-250 Spectrometer (1H NMR 250 MHz and 13C NMR
62.9 MHz) in CDCl3 with TMS as the internal standard.
UV–Vis spectra were recorded by PerkinElmer, Lambda
25, UV/Vis spectrometer to visualize the conversion of
Pd(II) to Pd(0). All products are known compounds, and
their spectral data along with their references are given in
the next Section.
General procedure for the homocoupling of aryl halides
in the presences of PdCl2/DPPPEG200 in PEG200
Preparation of diphenyl phosphorylated PEG200
To a flask containing DPPPEG200 (0.08 mmol, 0.05 g) and
PdCl2 (0.04 mmol, 6.0 mg) in PEG200, aryl halides
(1.0 mmol) and Et3N (2.0 mmol, 0.20 g) were added and
stirred to complete. In the case of aryl iodides and activated
aryl bromides, the reaction mixture was placed in a 100 °C
oil bath and in the case of less and deactivated aryl bro-
mides and chlorides; the reaction mixture was placed in a
130 °C oil bath (Table 7). GC and TLC of the reaction
Under an argon atmosphere, a flask containing PEG200
(5.0 mmol, 1.0 g) was added with Ph2PCl (3.7 ml,
20 mmol) at r.t and stirred slowly with a mechanical stirrer
for 30 min. After this time, the white precipitates appeared,
°
and the mixture was then heated to 80 C, while it was
stirring under atmosphere of argon for 3 h to remove all
123

J IRAN CHEM SOC
mixture showed the completion of the reaction. After
completion of reaction, the mixture was cooled to room
temperature, and the catalyst was removed by addition of
Et2O and then centrifugated. Evaporation of the solvent of
organic layer followed by chromatography on a short col-
umn of silica gel using n-hexane/ethyl acetate (5/1) as
eluent gave the coupled product in 65–95 % yield
(Table 7).
Table 1 Sonogashira reaction of iodobenzene with phenyl acetylene
using PdCl2 (3 mol%) in the presence of different DPPPEGs
(6.0 mol%)
catalyst, PEG200
NaOH, 80oC
I
Entry
Nano-catalyst
Time (min)
Conversion (%)a
1
2
3
4
5
6
7
PdCl2/DPPPEG200
PdCl2/DPPPEG400
PdCl2/DPPPEG1000
PdCl2/DPPPEG2000
PdCl2/DPPPEG4000
PdCl2/DPPPEG6000
PdCl2
10
100
100
100
70
15
25
4 h
10 h
10 h
10 h
Results and discussion
60
Catalysts characterization
50
45
In continuation of our studies on the use of different
phosphorous-based ligands in organic synthesis [18–28,
76–80], we reported that different bis-(diphenylphosphi-
nite) PEGs (DPPPEGs) can act as cheap, easily prepared,
and stable phosphor(III) containing reagents and can be
used as new sources of filterable phosphinite ligands [81].
The phosphoylated PEGs consist of two –PPh2 groups that
attached to the two terminal –OH groups in each PEG
molecules. PEGs with different molecular weights were
phosphorylated easily through their reaction with ClPPh2.
In general, for the preparation of DPPPEG200, chlorodi-
phenyl phosphine reacted with PEG200 in a mole ratio of
4:1 under an argon atmosphere at room temperature to give
white precipitates appeared and then the mixture was stir-
red for another 4 h at 80 °C. The phosphorylated PEGs
were obtained as stable solids in excellent yields after
addition of diethyl ether to the cold mixture (Scheme 1)
[81].
Reaction conditions 3 mol% of PdCl2, 6.0 mol% of DPPPEG200
1.0 mmol of iodobenzene, 1.5 mmol of phenyl acetylene, and
1.5 mmol of base
,
a
Conversion yields were determined by GC analysis
acetylene using PdCl2 as pre-catalyst. Among these
phosphorylated PEGs, DPPPEGs2000,
were
4000, and 6000
found to give incomplete reactions with 50–60 % con-
version (Table 1, Entries 4–6). As a controlled reaction,
this reaction was also performed with Pd(II) in the
absence of any DPPPEG. A very slow reaction occurred
and only 45 % of the coupled product was obtained
after 10 h (Table 1, entry 7). Since, the results of
Table 1 clearly showed that DPPEG200 offers the
shortest reaction time; therefore, it was chosen as the
most suitable one for further studies. The reason for its
more reactivity compared to those with higher molecular
PEGs could well be due to the more accessibility of
complexed Pd(0) with phosphorylated PEGs having
shorter chain length.
The obtained diphenylphosphorylated PEGs (DPP-
PEGs) were found to be very suitable ligands and also
reducing agents in the reaction with PdCl2 to generate
in situ nano-Pd(0) in its complex form with DPPPEGs.
In order to compare the reactivity of these ligands for
reduction of Pd(II) to Pd(0), we examined the catalytic
activity of PdCl2/DPPPEGs (Mw = 200, 400, 600,
1,000, 4,000, 6,000) on the Sonogashira reaction of iodo
benzene with phenyl acetylene in the presence of
sodium hadroxide as a base and PEG200 as the solvent
at 80 °C. Results (Table 1) showed that DPPPEG200,
400, and 1,000 are more efficient and suitable ligands
for the coupling reaction of iodobenzene and phenyl
In our recent report for the Heck-Mizoroki reaction, we
reacted PdCl2 with DPPPEG200 without using any solvent
in the presence ot triethyl amine as base and obtained
Pd(0)/DPPPEG200 in about 8.3 nm. Since in this work,
PEG was used as the solvent, and the optimized base was
found to be NaOH, our preliminary investigations were
focused on the characterization of the resulting Pd(0)/
DPPPEG200 complex. For preparation of Pd(0) complex,
we used PdCl2 as pre-catalyst and reacted it with DPP-
PEG200 in the presence of NaOH as a base in PEG200 as the
solvent at 80 °C. Transmission electron microscopy (TEM)
1. rt., 30min, under Ar
2. 80 oC,4h
Ph2PO
OPPh2
HO
Mw: 200, 400, 1000
Scheme 1 Synthesis of diphenylphosphorylated PEGs (DPPPEG)
OH
Ph2PCl
O
O
n
n
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J IRAN CHEM SOC
Fig. 1 Transmission electron microscopy (TEM) of a Pd(0)/DPPPEG200, b Recycled Pd(0)/DPPPEG200 from the coupling reaction between
iodobenzene and phenyl acetylene in PEG200 at 80 °C
250
(111)
200
150
100
(200)
(220)
70
(311)
50
0
(222)
90
20
30
40
50
60
80
100
2-Theta-Scale
Fig. 2 XRD pattern of Pd(0)/DPPPEG200 complex
image of the obtained catalyst shows that Pd nanoparticles
are formed with the size of *6 nm (Fig. 1).
The UV spectrum of the obtained Pd(0)/DPPEG200
complex in DMF shows the characteristic absorption band
of Pd(0) at 430 nm. [81].
Interestingly, it was observed that the reaction occurs
very efficiently without the need to add any copper salt
as co-catalyst. The best condition was obtained when we
used PEG200 (2.0 ml) as the solvent and NaOH (2.0
equimolar) as base. Decreasing the amount of NaOH
from 2.0 to 1.0, 1.5 mmol increased the reaction time
(Table 2, entries 11, 12, 13).
The XRD spectrum of the catalyst obtained from
PEG200 is shown in Fig. 2 which shows the harectyeristic
XRD pattern of Pd(0).
The optimized amount of the catalyst was determined by
performing a set of reactions with different mol% of PdCl2
and DPPPEG200 as ligand and reducing agent. The shortest
reaction time was obtained using 3.0 mol% of PdCl2 and
6.0 mol% of the ligand (Table 3).
Sonogashira reaction
For the optimization of reaction conditions, the reaction
of 4-iodotoluene with phenylacetylene was chosen as a
model reaction. In an attempt to boost reaction conver-
sion, solvent, base, reaction temperature, and also the
stoichiometry of phenyl acetylene were evaluated in the
presence of in situ generated nano-catalyst using PdCl2
(3.0 mol%, 0.0044 g)/DPPPEG200 (6.0 mol%, 0.056 g) as
catalyst system. Optimization of the model reaction with
respect to bases and solvents was investigated in PEG,
NMP, DMSO, and H2O as the solvents in the presence
of NaOH, KOAc, DBU, K3PO4, n-Pr3N, and K2CO3 as
bases at different reaction temperatures (Table 2).
We then applied our optimized reaction conditions
(0.06 mmol of DPPPEG200, 0.03 mmol of PdCl2) in
PEG200 (2.0 mL) as the solvent at 80 °C to the reaction of a
wide array of aryl iodides, bromides, and chlorides with
phenyl acetylene in the presence of NaOH as the base. The
desired coupled products were easily isolated with extrac-
tion into diethyl ether, and the catalyst was removed by
filtration or centrifugation. The results are shown in
Table 4.
Reusability of Pd(0)/DPPPEG200 was also studied since
this factor is an important process from different aspects
123

J IRAN CHEM SOC
reused for four runs for the same reaction under the opti-
mized condition without any decreasing in its reactivity
(Table 5).The TEM image of the recycled catalyst is shown
in Fig. 1b. As it was mentioned before, analysis of the
crude reaction mixture for the first reaction indicates a
leaching of 3 % of the palladium.
Table 2 Study of different parameters on the reaction of 4-iodotol-
uene and phenylacetylene
I
PdCl2/DPPPEG200
H3C
solvent, base, Δ
H3C
Entry Solvent Base (mmol)
Temperature Time
(oC)
Yielda
(%)
(h)
In order to find whether the catalyst was acting in a
truly heterogeneous manner or not, hot filtration test was
performed. In a typical experiment, DPPPEG200, PdCl2,
NaOH, iodo benzene, and phenylacetylene were taken in
a round-bottomed flask and stirred at 80 °C for 5 min. Gc
analysis at this stage showed 50 % conversion. The cat-
alyst was then filtered off. ICP analysis of the filtered
solution showed to contain 12 % palladium. The experi-
ment was continued with the filtered solution, and the
reaction was found to be completed after 8 h. Compari-
son of the amounts of leached Pd after 5 min (hot fil-
tration test, 12 %) with that obtained after the first run
(3.0 %) clearly shows that the majority of the leached
1
2
3
4
5
–
n-Pr3N (1.5)
K2CO3 (1.5)
Cs2CO3 (1.5)
NaOAc (1.5)
130
130
130
130
130
3.5
8
100
100
100
80
NMP
NMP
NMP
NMP
6.5
8
NaOAc (1.5),
n-Bu4NBr (1)
8
100
6
EtOH
H2O
K2CO3 (2.0)
NaOH (2.0)
NaOH (2.0)
80
80
7.5
24
6
100
30
7
8
DMF
80
100
60
9
DMSO NaOH (2.0)
PEG200 NaOH (2.0)
80
12
10
110
5
100
(min)
20
(min)
45
(min)
11
12
13
PEG200 NaOH (2.0)
PEG200 NaOH (1.5)
PEG200 NaOH (1.0)
80
80
80
100
100
100
palladium was reabsorbed by the phosphorylated PEG200
,
and the main pathway for the reaction is a heterogeneous
route.
2
Homocoupling reaction
Reaction conditions 4-Iodotoluene (1.0 mmol), phenylacetylene
(1.5 mmol), base (1.0–2.0 mmol), PdCl2(0.0044 g, 3.0 mol%),
DPPPEG200 (6.0 mol%, 0.038 g), and PEG200 (2.0 mL)
In this part of our research, we decided to apply this cat-
alyst for the homocoupling of aryl halides. For optimiza-
tion of this reaction, we chose the homocoupling of
4-iodotoluene as the model reaction and studied the effects
of different parameters such as the amount of catalyst,
amounts of ligand, solvent, base, and temperature. Initially,
we investigated the effect of different solvents such as
PEG, NMP, DMF, DMSO, TBAB, CH3CN, EtOH, and
H2O in the presence of NaOH, KOAc, Et3N, and K2CO3 as
bases on the homocoupling reaction catalyzed by the
PdCl2/DPPPEG200 system (0.006 g of PdCl2, 0.051 g of
DPPPEG200) at 80, 100, and 130 °C. We observed that
among the different solvents, similar to the Sonogashira
reaction, PEG200 is the best solvent and among the dif-
ferent bases, Et3N could be chosen as the most suitable one
(Table 6). The more efficiency of Et3N as base is probably
due to its solubility in most organic solvents as well as its
high reducing ability.
a
Conversion yield is based on GC analysis
Table 3 Optimization of the amounts of ligand and PdCl2 in the
reaction of 4-iodotoluene and phenylacetylene
Entry PdCl2
(mmol)
Ligand
(mmol)
Time
(h)
Coversion yielda
(%)
1
2
3
4
5
6
7
8
0.0025
0.005
0.01
0.006
0.015
0.03
0.04
0.06
0.04
0.06
0.08
24
24
3
20
50
70
0.02
2
100
100
0.02
2
0.03
45 min 100
15 min 100
15 min 100
0.03
0.03
Reaction conditions 1.0 mmol of 4-Iodotoluene, 1.5 mmol of phen-
ylacetylene, and 1.5 mmol of NaOH, 80 °C
Afterward, different amounts of PdCl2 were also
checked under the above mentioned reaction conditions. As
the results show, the yield of reaction increases signifi-
cantly with increasing the catalyst loading till 4.0 mol%
PdCl2 and then increases smoothly with higher catalyst
loading (Table 7, entries 4). Therefore, 4.0 mol% catalyst
was selected as a suitable amount. Moreover, we
a
Conversion yield is based on GC analysis using n-octane as internal
standard
such as environmental concerns, costs of the catalyst, and
its toxicity. The catalyst was easily recovered from the
reaction mixture of iodobenzene and phenyl acetylene and
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J IRAN CHEM SOC
Table 4 Sonogashira Reaction
of Phenylacetylene with Aryl
Halides
X
PdCl2/DPPPEG200
NaOH, PEG 200, 80oC
R
R
3
1
2
Entry
1
Substrate
Product
Time (h)
10min
Yield a(%)
95
I
1a
Ph
3a
Ph
I
2
3
15min
25min
90
95
1b
3b
I
Ph
Ph
H3CO
1c
3c
H3CO
I
I
H2N
4
5
6
1.45
78
92
70
1d
3d
H2N
Ph
25min
20min
1e
1f
3e
I
Ph
3f
O2N
O2N
O2N
Br
Ph
8
3
90
1g
3g
O2N
NC
Br
Ph
NC
9
1.5
2
84
83
75
1h
1i
3h
Br
3i
10
11
Ph
N
N
Br
Ph
Cl
1.45
1j
3j
Cl
Br
Ph
12
2
58
1k
1l
3k
Br
Ph
13
14
1
1
72
78
3l
Br
Ph
1m
3m
123

J IRAN CHEM SOC
Yield a(%)
70
Table 4 continued
Entry
15
Substrate
Product
3n
Time (h)
1.5
Br
Ph
S
1n
S
3o
Br
Ph
16
17
2
3
72
66
1o
Ph
Br
3p
1p
Br
Ph
18
19
HO
4
70
75
1q
3q
HO
Cl
Ph
O2N
NC
5.5
1r
3r
O2N
Reaction conditions aryl halide
(1.0 mmol), phenylacetylene
(1.5 mmol), NaOH (2.0 mmol),
PdCl2 (3.0 mol%), ligand
Cl
Ph
20
4
64
1s
3s
3t
NC
(6 mol%), and PEG
(2.0 mL) at mentioned
temperatures
200
Ph
Cl
21
22
5
5
94b
95b
a
Isolated yield
1t
b
GC conversion
Ph
All products are known
Cl
compounds and were identified
by comparison of their physical
or spectroscopic data with those
for known samples. [57, 58, 82]
3u
1u
Table 5 Recycling of Pd catalyst for the sonogashira reaction of iodobenzene with phenylacetylene
Run
Time (min)
Conversion yield (%)
1
2
3
4
10
10
15
15
95
90
88
85
Reaction conditions Iodobenzene (1.0 mmol), phenylacetylene (1.5 mmol), NaOH (2.0 mmol), PdCl2 (4.4 mg, 3.0 mol%), DPPPEG200 (3.8 mg,
6.0 mol%), and PEG200 (2.0 mL) at 80 °C temperature
investigated the effect of different reaction temperatures,
and the best yield was obtained when the reaction was
performed at 100 °C for aryl iodides, aryl bromides, and
130 °C for aryl chlorides (Table 8).
To determine the scope of homocoupling with this cat-
alytic system, we applied the optimized conditions to
varieties of substituted aromatic iodides, bromides, and
chlorides. The results are shown in Table 8. As it is
observed, higher yields of biaryls were obtained for aryl
iodides than for aryl bromides and for all those substrates
carrying electron-withdrawing groups on the aryl ring. Aryl
chlorides were relatively unreactive under these conditions,
and very low yields were obtained for these substrate at long
times. Thus, in order to improve the reaction condition for
aryl chlorides, we have to add n-Bu4NBr that led to increase
the yield and decrease the reaction time. This feature may
be interpreted by the fact that in the presence of n-Bu4NBr,
decomposition of nano-Pd(0) to Pd black is diminished.
123

J IRAN CHEM SOC
Table 6 Optimization studies for the homocoupling reaction of 4-iodotoluene with respect to different bases and the solvents using PdCl2
(0.006 g, 4.0 mol%)/DPPPEG200 (0.05 g, 8.0 mol%) as pre-catalyst in different solvents
Entry
Solvent
Base (mmol)
Temperature (oC)
Time (h)
Yielda (%)
1
–
n-Et3N (1.5)
n-Et3N (1.5)
n-Et3N (1.5)
n-Et3N (1.5)
n-Et3N (1.5)
K2CO3 (1.5)
NaOH (1.5)
n-Et3N (1.5)
KOAc (2.0)
NaOH (2.0)
NaOH (2.0)
n-Et3N (1.5)
n-Et3N (1.0)
130
130
130
80
30 min
100
40
25
30
10
–
2
DMSO
NMP
1
3
1
4
EtOH
H2O
2
5
100
80
3
6
EtOH
H2O
3
7
100
130
130
130
100
100
100
3
–
8
DMF
2
50
100
70
65
100
100
9
n-Bu4NBr
PEG200
PEG200
PEG200
PEG200
20
10
11
12
13
2
3
15 min
40
Reaction conditions 4-iodotoluene (1.0 mmol), phenylboronic acid (1.5 mmol), PdCl2 (0.006 g, 4.0 mol%), DPPPEG200 (0.05 g, 8.0 mol%),
base (1.0–2.0 mmol), and PEG200 (2.0 mL)
a
Conversion yield is based on GC analysis
Table 7 Optimization of the amounts of ligand and PdCl2 in the Homocoupling reaction of 4-Iodotoluene
Entry
PdCl2 (mmol)
Ligand (mmol)
Time (h)
Coversion yielda (%)
1
2
3
4
5
6
0.01
0.02
0.03
0.04
0.04
0.04
0.02
0.04
0.06
0.08
0.06
0.1
24
10
45
24
2
100
100
100
100
15 min
1
15 min
Reaction conditions 4-iodotoluene (1.0 mmol), Et3N (1.5 mmol), and PEG200 (2 mL), 100 °C
a
Conversion yield based on GC or TLC analysis
We, also, studied the reusability of the catalyst in the
homocoupling reaction of 4-iodotoluene, and the results are
shown in Table 9.
halides. In this regard, we have introduced a new appli-
cation for phosphite ligand in which -PPh2 anchored to
the PEG200 as a easily available ligand that can be
complexed to PdCl2 in order to reduce Pd(II) to Pd(0) and
generate in situ Pd(0) nanoparticles. The insolubility of
Pd(0) supported PEG200 provides a very simple workup
procedure, and the catalyst can be simply recovered by
filtration. The stability, ease of formation, and recycla-
bility of the catalyst can be considered as other advan-
tages of this system.
Conclusion
In conclusion, we have developed an efficient and envi-
ronmentally friendly catalyst protocol for the homocou-
pling and Sonogashira cross-coupling reactions of aryl
123

J IRAN CHEM SOC
Table 8 Homocoupling of aryl
halides with PdCl2/DPPPEG200
system using Et3N as base in
PEG200
X
DPPPEG/PdCl2
R
R
Pr3N, PEG200, 130°C
R
X= I, Br, Cl
Ar-X
Entry
1
Productb
Time
Yield (%)
95
I
10 min
1a
2a
MeO
Me
I
MeO
Me
OMe
2
15min
95
1c
2c
I
Me
3
4
5
15min
2
93
70
1b
2b
O2N
NO2
O2N
Me
I
1x
2x
Br
Me
NC
Me
2.45
90
1m
2m
NC
Br
CN
6
7
1.30
5
89
68
1h
2h
O2N
Br
O2N
NO2
1g
2g
Br
2i
8
9
5
2
65a
92
N
1i
N
N
Br
1l
2l
Br
10
1.45
90
S
S
2n
S
1n
Br
N
N
Reaction conditions substrate
(1.0 mmol), PdCl2 (0.006 g,
4 mol%), DPPPEG200 (0.05 g,
8 mol%), Et3N (1.5 mmol), and
PEG200 (2 mL) at 100 °C
N
11
12
3
2
95
93
N
N
2y
N
1y
Br
Cl
Cl
Cl
2j
1j
a
The reaction was performed
by addition of Bu4NBr at
13
14
15
7
7
9
85a
82a
85a
NC
Me
CN
1s
NC
Cl
2s
130 °C
b
Cl
All products are known
1t
2t
compounds and were identified
by comparison of their physical
or spectroscopic data with those
for known samples [83–92]
Me
Cl
Me
1u
2u
Table 9 Recycling of Pd Catalyst for the Homocoupling Reaction of 4-Iodotoluene with with PdCl2/DPPPEG200 system using Et3N as base in
PEG200
Run
Time (min)
Conversion Yield (%)
1
2
3
4
15
20
20
25
93
90
90
88
Reaction conditions substrate (1.0 mmol), PdCl2 (0.006 g, 4 mol%), DPPPEG200 (0.05 g, 8 mol%), Et3N (1.5 mmol), and PEG200 (2 mL) at
100 °C
123

J IRAN CHEM SOC
Acknowledgments The authors are thankful to the research council
of Shiraz University and Iran National Elite Foundation for the sup-
port of this work.
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