Nano tetraimine Pd(0) complex as an efficient catalyst for phosphine-free Suzuki reaction in water and copper-free Sonogashira reaction under aerobic conditions

Article abstract of DOI:10.1002/aoc.3486

A nano tetraimine Pd(0) complex catalyst was successfully used as an efficient heterogeneous catalyst for the phosphine-free palladium-catalysed Suzuki coupling reaction in water at 80 °C. This nano tetraimine Pd(0) complex was also used for copper-free Sonogashira reaction in dimethylformamide at 100 °C. The catalyst was easily recovered from the reaction mixture by centrifugation and reused for at least six cycles without any significant loss in its catalytic activity. Analysis of the reaction mixture using inductively coupled plasma analysis showed that leaching of palladium from the catalyst was negligible. The reactions can be performed efficiently for aryl iodides, bromides and also chlorides.

Full text of DOI:10.1002/aoc.3486

Full paper
Received: 14 December 2015
Revised: 2 February 2016
Accepted: 9 March 2016
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3486
Nano tetraimine Pd(0) complex as an efficient
catalyst for phosphine-free Suzuki reaction in
water and copper-free Sonogashira reaction
under aerobic conditions
Zeinab Mandegani, Mozaffar Asadi* and Zahra Asadi
A nano tetraimine Pd(0) complex catalyst was successfully used as an efficient heterogeneous catalyst for the phosphine-free
palladium-catalysed Suzuki coupling reaction in water at 80°C. This nano tetraimine Pd(0) complex was also used for copper-
free Sonogashira reaction in dimethylformamide at 100 °C. The catalyst was easily recovered from the reaction mixture by centri-
fugation and reused for at least six cycles without any significant loss in its catalytic activity. Analysis of the reaction mixture using
inductively coupled plasma analysis showed that leaching of palladium from the catalyst was negligible. The reactions can be
performed efficiently for aryl iodides, bromides and also chlorides. Copyright © 2016 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: nano tetraimine Pd(0) complex; Suzuki reactions; Sonogashira reactions; phosphine-free and copper-free; heterogeneous catalyst
catalysts is the stabilization of an active catalyst, which is highly
related to ligand stability, chelation or steric shielding of the metal
Introduction
Palladium is one of the most critical metals in catalysis and it is gen-
centre and the strength of the metal–ligand bond. Therefore, tran-
sition metal complexes containing nitrogen-donor ligands have
received much attention and can stabilize various oxidation states
of metals by strong electron-donating ligands to the metal
erally used to catalyse a wide variety of carbon–carbon bond
[1]
forming reactions. Palladium complexes of a large number of
[2]
[3]
[4]
[5]
[6]
phosphorus, carbene, oxime, imine and other ligands
have been reported as catalysts for such coupling.
[
39,40]
centre.
In addition, in the Sonogashira coupling reaction, the
Nitrogen-donor ligands were pioneered at the end of the nine-
teenth century. These ligands perform a highly significant role in
formed copper acetylides very much tend to undergo homo-
[
41,42]
coupling reaction (Glaser coupling).
Thus, several synthetic
[
7]
the field of coordination chemistry. Nitrogen coordination com-
pounds such as imidazole, benzimidazole, imines, pyridine or
methodologies have been designed to eliminate copper and phos-
[43–45]
phine from reaction mixtures.
Most of the palladium catalysts
[
8,9]
pyrazole as well as thioether or mercapto groups
have been
that have been used in the Suzuki and Sonogashira reactions are
[46–49]
designed to produce different kinds of transition metal complexes.
Some of these complexes have been shown to possess some
homogeneous.
Although homogeneous palladium catalysts
often offer higher activities for these reactions, they suffer from
disadvantages such as difficulties in product separation, handling
sensitive ligands and catalyst recovery, and recycling of expensive
ligands and palladium complexes. These problems can be solved
by the use of heterogeneous catalysts consisting of supported
palladium complexes. These heterogeneous catalysts can be simply
recovered from the reaction mixture by simple filtration or centrifu-
[
10]
characteristics of proteins
as antibacterial, antifungal, pesticidal and catalytic activities.
Carbon–carbon bond formation is among the most important
or to exhibit biocidal activities such
[
11]
[12,13]
[
14]
reactions in organic chemistry. Transition metals or their com-
plexes have been applied as catalysts and promoters in many of
these reactions. To catalyse a large variety of carbon–carbon
bond forming reactions, palladium is one of the most essential
transition metals which is commonly used. Various kinds of
cross-coupling reactions have been investigated; among them cou-
pling reactions such as the Heck–Mizoroki reaction,
the Sonogashira reaction,
the Negishi reaction
interesting examples of C–;C bond formation.
Various types of palladium catalytic systems have been reported
for these valuable cross-coupling reactions.
[15]
[
50–58]
gation and then they can be reused in further reactions.
Some
[
1]
heterogeneous catalysts have been supported on carbon
[
59]
[60]
[61]
nanofibers, montmorillonite, magnetic mesoporous silica,
[
16–18]
[62]
[63]
the Suzuki
the Kumada
and the Stille reaction are
metal oxides and zeolites.
[
19,20]
[21–24]
reaction,
reaction,
Recently various heterogeneous catalysts have been reported,
[25]
[22,26]
such as polymer-supported N-heterocyclic carbene–palladium
[
27]
[64]
catalyst for Sonogashira reactions,
poly(vinyl chloride)-
[28–35]
Phosphine
ligands have been used for the promotion of these reactions; how-
ever, they are sensitive to air and moisture. Thus air-free and dry
conditions pose significant inconvenience for synthetic applica-
*
Correspondence to: Mozaffar Asadi, Department of Chemistry, College of Sciences,
Shiraz University, Shiraz 71454, Iran. E-mail: asadi@susc.ac.ir
[
36–38]
tions of phosphine ligands.
A key feature for phosphine-free
Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.

Z. Mandegani, M. Asadi and Z. Asadi
[
65]
supported Pd(II) complex for Sonogashira reactions, Pd(II) Schiff
base complex supported on multiwalled carbon nanotubes for
in ethanol (10 ml) under nitrogen atmosphere. The mixture was
refluxed for 3 h to complete the reaction. After that, the resulting
solid product was centrifugation and washed with hot ethanol
(3× 2 ml) and water (3 × 2 ml) and dried at 80 °C overnight. Yield:
0.64 g (92.7%).
[
66]
Suzuki and Sonogashira reactions,
feather keratin for Suzuki reactions,
palladium supported on
palladium particles from
/oleic acid for
PCP-pincer palladium nanoparticles
supported on modified Merrifield resin for carbon–carbon cross-
[
67]
oxime-derived palladacycle supported on Fe
Sonogashira reactions,
O
3 4
[
68]
The catalyst was characterized using FT-IR spectroscopy, X-ray
diffraction, X-ray photoelectron spectroscopy, UV–visible spectros-
copy and elemental analysis. FT-IR spectra of pure N,N-bisimine
ligand and its nano tetraimine Pd(0) complex were obtained. The
[69]
coupling reactions
and HypoGel-supported palladium
[
70]
nanocatalysts for Suzuki reactions. The features of these catalysts
have made them important from environmental and economic
points of view due to their facile recycling and ease of separation.
Very recently, we have introduced a novel nano tetraimine Pd(0)
complex as a heterogeneous catalyst for the Heck–Mizoroki cou-
ꢀ
1
spectrum of the N,N-bisimine ligand exhibits a band at 1643 cm
due to azomethine ν(C¼;N) stretch. From the ICP and atomic ab-
sorption spectroscopy results, the amount of palladium is
0.78 mmol per gram of the complex.
[
71]
pling reaction of aryl halides with n-butyl acrylate in water.
Herein, we describe another application of this heterogeneous
nano tetraimine Pd(0) complex for the phosphine-free Suzuki and
copper-free Sonogashira coupling reactions.
General procedure for Suzuki reaction using nano tetraimine
Pd(0) complex as catalyst
To a 10 ml flask equipped with a magnetic stirring bar was added a
mixture of nano tetraimine Pd(0) complex (0.2 mol%, 0.003 g, con-
tains 0.0025 mmol of palladium), NaOH (2 mmol), phenylboronic
acid (1.2mmol), aryl halide (1.0mmol) and water (3.0 ml) which
was heated at 80 °C. The completion of the reaction was monitored
by TLC or GC. After that, the resulting hot reaction mixture was fil-
tered quickly through a thick cellulose filter paper under reduced
pressure and washed with diethyl ether (2× 5 ml). The combined
Experimental
General
All chemicals were purchased from Merck, Fluka or Acros and were
used without any further purification. The progress of the reactions
was followed by TLC using silica gel polygrams SIL G/UV 254 plates
or by GC using a Shimadzu GC-14A gas chromatograph, equipped
with a flame ionization detector and a 3 m length glass column
packed with DC-200 stationary phase and nitrogen as the carrier
gas. Fourier transform infrared (FT-IR) spectroscopic analysis of sam-
ples was conducted using a Shimadzu FT-IR 8300 spectrophotome-
2 4
organic phase was separated and dried over anhydrous Na SO
and evaporated under vacuum. The resulting crude product was
purified by flash chromatography to give the desired pure product
in high to excellent (78–96%) isolated yields.
1
13
ter, with samples and KBr pressed to form tablets. H NMR and
NMR spectra were recorded with a Bruker Avance DPX 250 MHz
spectrometer in CDCl or DMSO-d solvents using tetramethylsilane
C
General procedure for copper-free Sonogashira reaction using
tetraimine Pd(0) complex as catalyst
3
6
as an internal standard. Mass spectra were obtained at 70 eV. The
amount of palladium nanoparticles supported on N,N-bisimine li-
gand was measured using an inductively coupled plasma (ICP)
analyser (Varian, Vista-pro) and atomic absorption spectroscopy.
All yields refer to the isolated products. Elemental analysis was
done with a 2400 series PerkinElmer analyser. Melting points were
determined with a Buchi 510 instrument in open capillary tubes
and are uncorrected. All products were identified by comparison
of their spectral data and physical properties with those of authen-
tic samples.
Phenylacetylene (1.2mmol) was added to a suspension of aryl
halide (1.0mmol), K CO (2.0mmol) and palladium complex
2 3
(0.4mol%, 0.005 g, contains 0.004 mmol of palladium) in
dimethylformamide (DMF; 3 ml) in a10 ml reaction flask. The reac-
tion mixture was stirred at 100 °C for an appropriate time. The
progress of the reaction was monitored by TLC or GC. After comple-
tion of the reaction and separation of catalyst by simple filtration,
15ml of water was added and the mixture was extracted with
diethyl ether (2× 5 ml). The organic phase was washed with water
(2× 10 ml) and dried over anhydrous Na SO . Then, the solvent
2 4
was removed under reduced pressure. The resulting crude product
was purified by flash chromatography to give the desired cross-
coupling products in excellent (70–96%) isolated yields.
Procedure for preparation of N,N-bisimine ligand (3)
Ethylenediamine (2.0mmol, 0.13 ml) was dissolved in 10 ml of eth-
anol and added dropwise to a stoichiometric amount of 4-
chlorobenzaldehyde (4.0 mmol, 0.56 g) dissolved in 5 ml of ethanol.
The crude mixture was stirred at room temperature for 10h to give
ligand 3 as a white precipitate. The resulting product was separated
by filtration and washed with ethanol (2×5 ml) and then dried in
vacuum. The crude product was recrystallized from ethanol to ob-
tain the pure product.
General procedure for recycling of catalyst in Suzuki and
Sonogashira reactions
In a 5 ml flask, iodobenzene (2mmol, 0.2 ml) was reacted with
phenylboronic acid (2.4 mmol, 240 mg) in neat water (6 ml) in the
presence of nano tetraimine Pd(0) complex (6mg, contains
0.0050 mmol of palladium) and NaOH (4 mmol, 160 mg) at 80 °C.
After completion of the reaction as determined using GC and TLC
analysis and cooling the mixture to room temperature, the black
solid catalyst was separated by centrifugation and washed with
diethyl ether to remove any remaining organic compound. The
catalyst was dried in a vacuum oven and was used again for the
next run in another batch of the reaction. This recycling was
repeated for six runs without appreciable loss of activity of the
catalyst.
General procedure for preparation of nano tetraimine Pd(0)
complex (4)
The synthesis of the nano tetraimine Pd(0) complex catalyst was
[
71]
conducted according to a procedure previously reported. The
nano tetraimine Pd(0) complex was prepared by reacting a mixture
2
of Pd(OAc) (1.0mmol, 0.22 g) and a solution of 3 (2.0mmol, 0.61g)
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Copyright © 2016 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2016)

Nano tetraimine Pd(0) catalyst for Suzuki and Sonogashira reactions
For the Sonogashira reaction, after completion of the reaction
of iodobenzene (2.0mmol, 0.02ml) with phenylacetylene
corresponding coupling product is obtained in 95% yield (Table 1,
entry 1). When the amount of catalyst is decreased from 0.004 to
0.001g, the yield of product is decreased to 28% (Table 1, entry
12). The effect of temperature on the activity of the catalyst was also
studied by carrying out the model reaction at various temperatures;
the best result is obtained at 80 °C (Table 1, entry 1). Furthermore,
we also investigated the use of various protic and aprotic solvents
such as DMF, N-methyl-2-pyrrolidone (NMP), dimethylsulfoxide
(DMSO), CH CN, EtOH, EtOH–H O, tetrahydrofuran and toluene at
(
2.4 mmol, 0.26 ml) and K
the presence of nano tetraimine Pd(0) complex (10mg, contains
.0080 mmol of palladium), the reaction mixture was centrifuged
2 3
CO (4.0mmol, 0.55 g) in DMF (6ml) in
0
to separate solid material and washed with diethyl ether. The cat-
alyst was dried and reused for a similar reaction. The recycling of
the catalyst was repeated for six runs without appreciable loss of
its catalytic activity.
3
2
90°C (Table 1, entries 1–11), with excellent yields of the product be-
ing obtained in water (Table 1, entry 1).
Large-scale Suzuki and Sonogashira reactions
Furthermore, under these optimized conditions, we investigated
the usefulness of the catalyst in the Suzuki reaction involving cross-
coupling of an aryl halide with phenylboronic acid. Table 2 illus-
trates that the reaction is effective in the presence of a wide variety
of functional groups on aryl iodides, aryl bromides and aryl chlo-
rides, giving good to excellent conversions to the corresponding
products.
We also studied large-scale Suzuki and Sonogashira reactions. For
this goal, according to the general procedure, iodobenzene
(10 mmol, 1.0 ml) and 2.0mol% (0.05 g) of the Pd(0) complex were
reacted with phenylboronic acid (12 mmol, 1.2g) and NaOH
(
(
20 mmol, 0.8 g) in water (30 mL) and with phenylacetylene
12 mmol, 1.3 ml) and K CO (20 mmol, 2.76 g) in DMF (30 ml), re-
2
3
spectively, for Suzuki and Sonogashira reactions. After workup,
the desired coupled products were obtained in 90% yield (1.36g,
Aryl halides with electron-donating groups (Table 2, entries 2,
7–9, 13) are less active than those with electron-withdrawing
ꢀ
ꢀ
1
1
1
54.2 g mol ) for Suzuki reaction and 86% yield (1.53 g,
groups (Table 2, entries 3, 5, 6, 11, 12). As expected, the coupling
of phenylboronic acid with iodides or bromides leads to the de-
sired products in moderate to high yields (Table 2, entries 1–9).
In particular, the reaction of aryl iodide substituted with
electron-withdrawing groups affords high yield after 0.5 h at
1
78.23 g mol ) for Sonogashira reaction. The catalyst was also sep-
arated as described above and reused for the next run.
Results and discussion
100 °C (Table 2, entry 3). In the case of deactivated bromides, lon-
ger reaction time is required for the reaction to produce good
yields (Table 2, entries 7–9). The catalyst effect is also evident
for chlorides, where only those substituted with electron-
withdrawing groups give reasonable yields for the coupling prod-
uct (Table 2, entries 11 and 12). Non-activated aryl chlorides gave
a low yield in coupling reactions under similar conditions (Table 2,
entry 13).
In this study, ligand 3 was synthesized from 4-chlorobenzaldehyde
(
1) and 1,2-ethandiamine (2) in ethanol at room temperature.
Ligand 3 was used for the synthesis of nano tetraimine Pd(0) com-
plex 4 in the presence of Pd(OAc) under nitrogen atmosphere in
2
[
71]
ethanol reflux (Scheme 1).
Catalytic application of nano tetramine in Suzuki
cross-coupling reactions
Catalytic application of nano tetramine in Sonogashira
coupling reaction
To explore the catalytic activity, the Suzuki cross-coupling reaction
of aryl halides with phenylboronic acid was conducted. The rate of
the coupling reaction is dependent on a variety of parameters such
as solvent, base, temperature and catalyst loading. For optimization
of the reaction conditions, we chose the cross-coupling of 4-
iodobenzene (1.0mmol) with phenylboronic acid (1.2mmol), under
aerobic conditions, as a model reaction using the nano tetraimine
Pd(0) complex (0.2 mol%, 0.003 g) as catalyst in water at 80 °C for
Upon screening the reaction conditions for the copper-free
Sonogashira coupling with the nano tetraimine Pd(0) complex cat-
alyst, we find that the reaction efficiency is enhanced when K CO is
employed as base. Thus, the coupling of iodobenzene (1 mmol) and
phenylacetylene (1.2mmol) was carried out in DMF with the nano
tetraimine Pd(0) complex (0.4mol%, 0.005 g) at 100°C for 3 h in
2
3
the presence of 2 mmol of various bases. Bases including K CO ,
4
h in the presence of 2 mmol of base. The catalyst and the effect
2
3
Na CO , Cs CO , Et N, CsOH and NaOH were tested (Table 3, entries
of solvent, base, temperature and also the amount of catalyst on
the reaction were examined (Table 1). As evident from Table 1, of
the screened bases, NaOH shows the best result, and the
2
3
2
3
3
6
, 9–13) and, among them, K CO proves to be the most efficient
2 3
and gives a GC yield of 94% (Table 3, entry 6). We further evaluated
the effects of temperature and amount of catalyst on the reaction.
As evident from Table 3 (entries 20, 21 and 1–4), both reducing the
amount of catalyst and lowering the temperature lead to a de-
crease in the yield. So, based on these results, we selected K CO3
2
as the base and DMF as solvent in the presence of 0.005 g of the
catalyst at 100°C as the best conditions for the Sonogashira
reaction.
Under our optimized reaction conditions, we accomplished the
Sonogashira-type cross-coupling of a wide range of structurally
diverse aryl halides (Table 4). As expected, the coupling of
phenylacetylene with iodides or bromides leads to the desired prod-
ucts in moderate to high yields (Table 4, entries 1–8). In particular,
the reaction of aryl iodides substituted with electron-withdrawing
groups affords high yield after 0.5 h at 100 °C (Table 4, entry 2). In
the case of deactivated bromides, longer reaction time is required
Scheme 1. Route for synthesis of nano tetraimine Pd(0) complex.
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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Z. Mandegani, M. Asadi and Z. Asadi
a
Table 1. Optimization of conditions for Suzuki reaction using nano tetraimine Pd(0) complex as catalyst
b
Entry
Solvent
Catalyst amount (g)
Base
Temperature (°C)
Yield (%)
1
2
3
4
5
6
7
8
9
H
2
O
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
None
0.001
0.002
0.004
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
None
80
95
76
80
81
87
51
23
83
66
85
0
EtOH
Reflux
80
EtOH–H
EtOH–H
EtOH–H
2
O (3:1 v/v)
2
O (2:1 v/v)
2
O (1:1 v/v)
80
80
CH
3
CN
80
Toluene
NMP
80
80
DMSO
DMF
80
10
11
12
13
14
15
16
18
19
20
21
22
23
24
80
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
O
O
O
O
O
O
O
O
O
O
O
O
O
140
80
28
72
95
Trace
55
79
74
82
36
17
81
94
80
80
140
80
K
K
3
PO
CO
CO
4
2
3
80
Na
2
3
80
KOH
80
NaOAc
NaOH
NaOH
NaOH
80
r.t.
70
90
a
Reaction conditions: iodobenzene (1.0 mmol), phenylboronic acid (1.2 mmol), base (2.0 mmol), solvent (3.0 ml), 4 h.
b
Isolated yield.
a
Table 2. Suzuki coupling of aryl halides with phenylboronic acid in the presence of nano tetraimine Pd(0) complex
b
Entry
Ar
X
Time (h)
Isolated yield (%)
Ref.
1
2
3
4
5
6
7
8
9
C
6
H
5
I
I
I
1
95
90
96
88
90
87
84
86
82
80
83
81
78
72
72
72
73
73
50
50
17
72
72
72
73
72
4-OMeC
6
H
4
2
4-NO
2
C
6
H
4
0.5
3
C
6
H
5
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
4-NO
4-ClC
2
C
6
H
4
2
6
H
4
2.5
4
4-MeC
2-MeC
6
H
4
6
H
4
4
4-OMeC
H
6 4
4
10
11
12
13
C
6
H
5
8
4-NO
4-CNC
4-MeC
2
C
6
H
4
3
6
H
4
3.5
12
6 4
H
a
Reaction conditions: ArX (1.0 mmol), phenylboronic acid (1.2 mmol), NaOH (2.0 mmol) in the presence of Pd complex 0.2 mol% (0.003 g) in H
2
O (3.0 ml)
at 80 °C.
b
Isolated yield.
for the reaction to produce good yields (Table 4, entries 4–8). The
catalyst effect is also evident for chlorides, where only those
substituted with electron-withdrawing groups give reasonable yield
of coupling product (Table 4, entries 12 and 13). Non-activated aryl
chlorides give a low yield in coupling reactions under similar condi-
tions (Table 4, entries 10 and 11).
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Nano tetraimine Pd(0) catalyst for Suzuki and Sonogashira reactions
a
Table 3. Optimization of reaction conditions for Sonogashira reaction catalysed by nano tetraimine Pd(0) complex
b
Entry
Solvent
Catalyst amount (g)
Base
Temperature (°C)
Yield (%)
1
2
3
4
5
6
7
8
9
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
NMP
None
0.001
0.002
0.003
0.004
0.005
0.006
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
K
K
K
K
K
K
K
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
3
130
100
100
100
100
100
100
130
100
100
100
100
100
100
100
Reflux
Reflux
Reflux
Reflux
r.t.
Trace
17
3
3
38
3
75
3
90
3
94
3
94
None
Na CO
Cs CO
Et
Trace
82
2
3
10
11
12
13
14
15
16
17
18
19
20
21
22
2
3
86
3
N
44
CsOH
NaOH
62
38
K
2
K
2
K
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
59
3
87
3
CH CN
3
31
Toluene
K
2
3
28
2
H O
K
2
K
2
K
2
K
2
K
2
3
32
EtOH
DMF
DMF
DMF
3
37
3
11
3
9
79
3
110
94
a
Reactions were run in 5 ml of solvent with 1.0 mmol of iodobenzene, 1.2 mmol of phenylacetylene and 2 mmol of base for 3 h.
b
Isolated yield.
would be useful to introduce the nano tetraimine Pd(0) complex
as an active catalyst for mild conditions in high yield, short time
and low temperature of these reactions in comparison to other het-
erogeneous catalyst systems.
Table 4. Sonogashira coupling of aryl halides with phenylacetylene in
the presence nano tetraimine Pd(0) complex
a
The possibility of the recycling of the catalyst is very important
and makes it useful for commercial applications. Thus, we
investigated the recovery and reusability of the catalyst using
iodobenzene with phenylboronic acid (Suzuki reaction; Fig. 1) and
iodobenzene with phenylacetylene (Sonogashira reaction; Fig. 2)
as model substrates under optimized conditions. After completion
of the reactions, the solid catalyst was separated by centrifugation
and was reused for subsequent reactions under similar reaction
conditions. Six consecutive cycles of the reaction show that the
catalyst does not lose its activity and can be completely recycled.
The amounts of the nano tetraimine Pd(0) leaching during the
reactions under the optimized conditions were determined. The
palladium content in the Suzuki and Sonogashira reactions media
after each reaction cycle was measured using ICP analysis and the
results are provided in Figs. 3 and 4. The ICP analyses show that
leaching is negligible for both reactions.
b
Entry
Ar
X
Time (h)
Yield (%)
Ref.
1
2
3
4
5
6
7
8
9
C
6
H
5
I
I
I
0.75
0.5
2
94
96
91
89
90
93
87
89
78
73
71
84
87
72
72
72
17
72
17
17
72
73
74
17
74
17
4-NO
2 6 4
C H
4-OMeC
6 4
H
C
6
H
5
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
2.5
2.5
1.5
3.5
3
4-ClC
4-NO
4-MeC
3-NC
6 4
H
2 6 4
C H
6
H
4
H
5 4
C
6
H
4
6
10
11
12
13
6
4-MeC H
4
10
12
5
4-OMeC
4-CNC
4-NO
6 4
H
6
H
4
C
2 6
H
4
3
To determine the nature of the palladium species responsible
for the observed reactions and to measure the extent of palladium
leaching after the reactions, we used the hot filtration test. For this
aim, we have studied the coupling reaction of iodobenzene with
phenylboronic acid under optimized conditions. The hot reaction
mixture was filtered after 36% conversion of iodobenzene (GC)
to remove the catalyst. Continuation of the reaction of the
resulting filtrate under the same conditions showed 41% conver-
sion (GC) of iodoobenzene after 5 h. This result shows that the
a
Reaction conditions: ArX (1.0 mmol), phenylacetylene (1.2 mmol) and
CO (2.0 mmol) in the presence of nano tetraimine Pd(0)
complex (0.4 mol%, 0.005 g) in DMF (3.0 ml) at 100 °C.
Isolated yield.
K
2
3
b
Several methods for Suzuki and Sonogashira reactions have been
reported with various heterogeneous catalysts. For comparison,
several results are summarized in Table 5. In our investigation it
Appl. Organometal. Chem. (2016)
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Z. Mandegani, M. Asadi and Z. Asadi
Table 5. Comparison of efficiency of nano tetraimine Pd(0) complex with that of some reported heterogeneous catalysts for phosphine-free Suzuki and
copper-free Sonogashira reactions
Entry Catalyst
Suzuki
Sonogashira
Ref.
Time (h)
1
Condition
Yield (%) Time (h)
Condition
Yield (%)
a
a
1
2
Nano tetraimine
Pd(0) complex
FDU-NHC/Pd(II)
H
2
O/NaOH/80 °C/0.2 mol%
95
—
—
0.75
3
DMF/K
2
3
CO /100 °C/0.4 mol%
94
This work
64
a
—
—
DMA/K
2
3
CO /100 °C/1.0 mol%,
94
CuI 10 mol%
Solvent free/Et N/r.t./1.0 mol%
b
3
4
PVC-dtz-Pd(II)
Pd Schiff
—
—
3
1
3
99
65
66
a
a
2
2
DMF/H O (1:1 v/v)
99
2
H O/Et
3
N/90 °C/1.2 mol%
95
base@MWCNTs
Feather keratin-Pd
Fe /oleic acid/Pd
/K CO /60 °C/0.1 mol%
2
3
a
5
6
7
8
5
—
1
H
2
O/NaOH/75 °C/0.55 mol%
99
—
6
—
—
67
68
69
70
b
O
3 4
—
—
EtOH/K
DMF/K
2
CO
CO
3
/100 °C/0.05 mol%
100
a
a
PCP-pincer Pd
DMF/K
O/K
2
CO
CO
3
/80 °C/3.0 mol%
/90 °C/12 mol%
96
88
3
2
3
/120 °C/3.0 mol%
90
a
HypoGel-supported Pd
6
H
2
2
3
—
—
—
a
Isolated yield.
b
Conversion determined by GC.
ꢀ1
Figure 3. Palladium content (mg mol ) of fresh catalyst and catalyst used
for up to six cycles in Suzuki reaction.
Figure 1. Recycling of catalyst for Suzuki reaction. Reaction conditions:
iodobenzene (1.0 mmol), phenylboronic acid (1.2 mmol), NaOH (2.0 mmol.),
catalyst (0.2 mol%), H O (3.0 ml), 1 h.
2
ꢀ1
Figure 4. Palladium content (mg mol ) of fresh catalyst and catalyst used
for up to six cycles in Sonogashira reaction.
and workup, the amount of leaching was determined using ICP
analysis. The amount of palladium leaching after the first run is only
0.23%, and after six repeat recycles is 4.14%.
Figure 2. Recycling of catalyst for Sonogashira reaction. Reaction
conditions: iodobenzene (1 mmol), phenylacetylene (1.2 mmol), catalyst Conclusions
0.2 mol%), K CO (2.0 mmol) in DMF (3.0 ml) at 100 °C, 0.75 h.
(
2
3
We have developed a novel, practical and economic nano
tetraimine Pd(0) complex catalyst system for cross-coupling reac-
tions. This catalyst demonstrated high catalytic activities in Suzuki
and Sonogashira reactions in comparison to similar systems for
synthesizing various biphenyl and alkyne derivatives with high
yield under mild reaction conditions in a heterogeneous system.
Moreover, the catalyst offers notable advantages such as facile
amount of leaching of the catalyst into the reaction mixture is low
and confirms that the catalyst acts heterogeneously in the
reaction.
In order to get more information about the leaching of palla-
dium, the reaction of iodobenzene with phenylboronic acid as
model reaction was studied. After completion of the reactions
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Nano tetraimine Pd(0) catalyst for Suzuki and Sonogashira reactions
[
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Supporting Information
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version of this article at the publisher’s web site.
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