Pd Nanoparticles Immobilized on Nanosilica Triazine Dendritic Polymer: A Reusable Catalyst for the Synthesis of Mono-, Di-, and Trialkynylaromatics by Sonogashira Cross-Coupling in Water

Article abstract of DOI:10.1002/ejoc.201402503

Palladium nanoparticles immobilized on nano-silica triazine dendritic polymer (Pd_np-nSTDP) was found to be a highly effective catalyst for the Sonogashira cross-coupling of aryl halides (iodides, bromides, and chlorides) with aromatic and aliphatic terminal alkynes. This reaction was best performed in water as a green solvent in the presence of just 0.01 mol-% of the catalyst at room temperature. Efficient synthesis of V- and star-shaped polyalkynylated molecules with a benzene, pyridine, or pyrimidine central core was also achieved through Sonogashira cross-coupling of dihalo and trihalo aromatics with terminal alkynes in the presence of this catalytic system. The Pd_np-nSTDP catalyst was easily recovered and reused several times without significant loss of reactivity.

Full text of DOI:10.1002/ejoc.201402503

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FULL PAPER
DOI: 10.1002/ejoc.201402503
Pd Nanoparticles Immobilized on Nanosilica Triazine Dendritic Polymer:
A Reusable Catalyst for the Synthesis of Mono-, Di-, and Trialkynylaromatics
by Sonogashira Cross-Coupling in Water
Amir Landarani Isfahani,[a] Iraj Mohammadpoor-Baltork,*[a] Valiollah Mirkhani,*[a]
Ahmad R. Khosropour,[a] Majid Moghadam,[a] and Shahram Tangestaninejad[a]
Keywords: Heterogeneous catalysis / Water chemistry / Palladium / Nanoparticles / Cross-coupling / Dendrimers / Alkynes
Palladium nanoparticles immobilized on nano-silica triazine
dendritic polymer (Pdnp-nSTDP) was found to be a highly ef-
fective catalyst for the Sonogashira cross-coupling of aryl
halides (iodides, bromides, and chlorides) with aromatic and
aliphatic terminal alkynes. This reaction was best performed
in water as a green solvent in the presence of just 0.01 mol-
% of the catalyst at room temperature. Efficient synthesis of
V- and star-shaped polyalkynylated molecules with a benz-
ene, pyridine, or pyrimidine central core was also achieved
through Sonogashira cross-coupling of dihalo and trihalo
aromatics with terminal alkynes in the presence of this cata-
lytic system. The Pdnp-nSTDP catalyst was easily recovered
and reused several times without significant loss of reactivity.
and drawbacks of many of the previously reported methods.
Despite the enormous number of publications, the develop-
ment of efficient and greener protocols for the Sonogashira
cross-coupling reaction is still challenging and still desir-
able.
Since a large amount of waste in the environment is
attributed to the use of organic solvents,[10] there is growing
demand for improvement of organic reactions in environ-
mentally-friendly media.[11] During recent years, water has
gained special attention as a reaction medium for organic
synthesis, as it is nontoxic, nonflammable, cheap, and envi-
ronmentally benign.[12] Moreover, the chemical and physical
properties of water can lead to selectivity and/or reactivity
that cannot be attained in organic solvents.[13] Therefore,
the introduction of highly efficient and green protocols
using recoverable catalysts in water as the reaction medium
may be seen as a worthwhile goal.
In recent years, dendritic polymers have attracted signifi-
cant attention, due to their applications in different areas
such as drug delivery,[14] medicinal chemistry,[15] collar
cells,[16] and nanoscience.[17] A characteristic feature of den-
dritic polymers is the presence of internal cavities. These
cavities enable the polymers to act as molecular boxes for
the encapsulation of organic molecules, ions, or metal
nanoparticles. Such metal-containing dendritic polymers
are recognized to catalyse various organic reactions.[18]
Moreover, the immobilization of these metal-encapsulated
dendritic catalysts on solid supports makes them easily re-
coverable and reusable, which is very important from eco-
nomical and environmental points of view.[19]
Introduction
The Sonogashira cross-coupling reaction between a ter-
minal alkyne and an aryl or vinyl halide represents a valu-
able and facile synthetic strategy for the formation of a new
C(sp)–C(sp2) bond.[1] This reaction has been widely used in
the synthesis of natural products[2] and biologically active
molecules,[3] as well as in materials science.[4] Traditionally,
the Sonogashira reaction is carried out in the presence of
homogeneous palladium catalysts such as Pd(PPh3)4,
Pd(OAc)2, PdCl2(PPh3)2, and PdCl2(CH3CN)2.[5] In spite of
their great utility, these homogeneous catalysts suffer from
disadvantages such as difficulties in separation and recov-
ery, sensitivity to air, and the formation of alkyne homo-
coupling products (Glaser coupling).[6] Furthermore, con-
tamination of the desired product with palladium limits the
applicability of these homogeneous catalysts in large scale
operations, and can lead to serious environmental, econ-
omic, and safety problems, especially in the pharmaceutical
industry.[7] In order to overcome these problems and to im-
prove the environmental and economical sustainability of
the Sonogashira reaction, many heterogeneous palladium-
based catalytic systems have been developed.[8,9] However,
high temperatures, long reaction times, and the use of large
amounts of palladium and organic solvents are limitations
[a] Department of Chemistry, Catalysis Division, University of
Isfahan,
Isfahan 81746-73441, Iran
E-mail: imbaltork@sci.ui.ac.ir
mirkhani@sci.ui.ac.ir
http://sci.ui.ac.ir/imbaltork; http://sci.ui.ac.ir/mirkhani
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402503.
Recently, we reported the palladium nanoparticles immo-
bilized on nano-silica triazine dendritic polymer
Eur. J. Org. Chem. 0000, 0–0
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I. Mohammadpoor-Baltork, V. Mirkhani et al.
FULL PAPER
Scheme 1. Sonogashira cross-coupling catalysed by Pdnp-nSTDP.
(Pdnp-nSTDP) (Figure 1) as an efficient and reusable cata- cyclo[5.4.0]undec-7-ene), and DIPEA (N,N-diisopropyl-
lyst for Suzuki–Miyaura cross-coupling and Heck reac- ethylamine) were screened, and we found that DIPEA was
tions,[20a] and C–S cross-coupling reactions.[20b] In a contin- the most effective base both in terms of reaction time and
uation of our work on new applications of the Pdnp-nSTDP yield. To study the effect of the solvent, the model reaction
catalytic system, and also on the development of useful syn- was carried out in the presence of Pdnp-nSTDP/DIPEA in
thetic methodologies,[21] in this paper, we report an efficient various solvents (500 μL), including H2O, DMF, THF,
method for synthesis of mono-, di-, and trialkynylaromatic EtOH, H2O/DMF (1:1), and H2O/EtOH (1:1), and also un-
and heteroaromatic compounds by Sonogashira cross-cou- der solvent-free conditions at room temperature. The high-
pling of aryl halides with terminal alkynes catalysed by est yield and the shortest reaction time were obtained in
Pdnp-nSTDP in water as
(Scheme 1).
a
green reaction medium H2O. Finally, the effect of the catalyst loading on the yield
of the desired product in model reaction was investigated,
Table 1. Optimization of conditions for the Sonogashira cross-cou-
pling of bromobenzene with phenylacetylene catalysed by Pdnp-
nSTDP.[a]
Entry Base
Catalyst
[mol-% Pd]
Solvent
Time Yield
[h]
[%][b]
1
2
3
4
5
6
7
8
NaOH
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.012
0.008
0.006
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
10
10
10
8
8
7
8
5
5
6
6
6
8
6
5
10
10
12
34
35
86
80
65
50
95
29
90
71
78
49
60
95
65
61
Na2CO3
K2CO3
Et3N
tBuNH2
piperidine
DBU
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
Figure 1. The structure of the Pdnp-nSTDP catalyst.
9
–
Results and Discussion
10
11
12
13
14
15
16
17
H2O/DMF (1:1)
H2O/EtOH (1:1)
DMF
THF
EtOH
H2O
Sonogashira Cross-Coupling of Aryl Halides with Terminal
Alkynes Catalysed by Pdnp-nSTDP
To find the optimal reaction conditions, we examined the
Sonogashira cross-coupling between bromobenzene
(1 mmol) and phenylacetylene (1.1 mmol) in the presence of
the Pdnp-nSTDP catalyst as a model reaction (Table 1).
First, a wide range of bases including NaOH, Na2CO3,
H2O
H2O
[a] Reaction conditions: bromobenzene (1 mmol), phenylacetylene
(1.1 mmol), base (1 mmol), Pdnp-nSTDP, solvent (500 μL), room
K2CO3, NEt3, tBuNH2, piperidine, DBU (1,8-diazabi- temperature. [b] Isolated yield.
2
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Sonogashira Cross-Coupling in Water
and the highest yield was obtained using 0.01 mol-% of the a wide range of electronic and optoelectronic devices.[23–25]
palladium catalyst. Consequently, from the observations These compounds are suitable candidates for applications
summarized in Table 1, we conclude that 0.01 mol-% of the as liquid crystals,[26] non-linear optical materials,[27] light-
catalyst, DIPEA (1 mmol) as the base, in H2O (500 μL) at emitting materials,[28] and building blocks for two-dimen-
room temperature, are the most appropriate reaction condi- sional carbon networks.[23a,29] In addition, some of them
tions for this transformation (Table 1). It is important to have been used as core structures for dendritic materials[30]
mention that the amount of H2O has an effect on the prod- and functional dyes.[31] Thus, the development of new and
uct yield. When the experiments were carried out in 1 mL highly efficient protocols for their preparation is of practical
and 2 mL of H2O, the desired product was obtained in 88 importance and highly desirable.
and 85% yields, respectively, and a small amount of the
homocoupling product was detected.
Encouraged by the excellent results described above for
the Sonogashira cross-coupling of aryl halides with ter-
Under these optimized reaction conditions, various aryl minal alkynes, we went on to investigate the synthetic po-
iodides and bromides were cross-coupled with aromatic and tential of the Pdnp-nSTDP catalytic system for the synthesis
aliphatic terminal alkynes in the presence of Pdnp-nSTDP/ of V- and star-shaped molecules. The results are shown in
DIPEA in H2O at room temperature to give the desired Table 3. The Sonogashira cross-coupling of 2,6-dibromo-
products in high yields (Table 2, entries 1–11). Impressed pyridine with phenylacetylene, 4-chlorophenylacetylene,
with the results with aryl iodides and bromides, we tried to and/or 3-methoxyphenylacetylene proceeded smoothly in
further expand the scope of Pdnp-nSTDP catalyst in cou- the presence of Pdnp-nSTDP and DIPEA in H2O at room
pling reactions using aryl chlorides, which are less reactive temperature to give the desired dialkynylated V-shaped mo-
but cheaper and more readily available than aryl iodides lecules in 91–95% yields (Table 3, P1–P3). Under these con-
and bromides. The Sonogashira cross-coupling of activated ditions, trialkynylated star-shaped molecules were also ob-
and deactivated aryl chlorides with terminal alkynes pro- tained in high yields (85–92%) by Sonogashira cross-cou-
ceeded effectively to give the corresponding products in 87– pling of 1,3,5-tribromobenzene or 2,4,6-trichloropyrimidine
90% yields (Table 2, entries 12–14) under the same condi- with phenylacetylene and/or 3-methoxyphenylacetylene
tions, but these reactions required longer times. It is inter- (Table 3, P4–P7).
esting to note that this method is better than the many pre-
It is also important to note that the synthesis of trialk-
viously reported Sonogashira cross-coupling methods in ynylated star-shaped molecules can be achieved directly
terms of reaction times, reaction temperatures, and/or the from 4-haloacetophenones by Sonogashira cross-coupling
amount of the catalyst.[22]
and subsequent cyclotrimerization reactions. As shown in
Scheme 2, the Sonogashira reactions of 4-bromoacetophen-
one with phenylacetylene, 4-bromoacetophenone with 4-
chlorophenylacetylene, and 4-chloroacetophenone with 3-
methoxyphenylacetylene, gave the desired cross-coupled
products, which, upon cyclotrimerization in the presence of
H3PW12O40 (HPW),[32] gave the corresponding star-shaped
molecules (P8–P10) in high yields (Scheme 2).
In Table 4, the result for the coupling of iodobenzene
with phenylacetylene by this method is compared with
some results from the literature with several Pd catalysts in
copper-free Sonogashira reactions. Compared to most of
the previously reported methods, the yield with the Pdnp-
nSTDP catalyst is higher, the reaction time is shorter, and
the reaction temperature is lower. Moreover, the amount of
the Pdnp-nSTDP catalyst required is much lower, and the
turnover frequency (TOF) is higher, indicating the effi-
ciency of this method.
We carried out a recycling investigation of the catalyst in
the reaction of bromobenzene with phenylacetylene under
the optimized conditions. After completion of the reaction,
ethyl acetate (15 mL) was added, and the catalyst was sepa-
rated by centrifugation, dried, and reused for eight consecu-
tive trials without any appreciable loss in activity (Table 5).
After that, the activity of the catalyst decreased slightly, and
the desired product was obtained in 84 and 80% yields in
the ninth and tenth recycles, respectively. Furthermore, palla-
dium leaching from the Pdnp-nSTDP catalyst was deter-
Table 2. Sonogashira cross-coupling of aryl halides with aromatic
or aliphatic alkynes catalysed by Pdnp-nSTDP.[a]
Entry
X
R1
R2
Time [h] Yield [%][b]
1
2
3
4
5
6
7
8
I
I
I
I
H
Ph
Ph
Ph
n-butyl
4-ClC6H4
Ph
Ph
Ph
4-ClC6H4
n-butyl
pentyl
Ph
3-MeOC6H4
Ph
2
2
96
93
95
85
95
95
92
94
95
80
84
87
90
90
2-Me
4-MeO
4-MeO
4-MeO
H
4-MeO
Ac
Ac
Ac
H
H
2
2
2
5
5
5
5
5
I
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
9
10
11
12
13
14
5
10
10
10
Ac
4-MeO
[a] Reaction conditions: aryl halides (1 mmol), aryl acetylene
(1.1 mmol), DIPEA (1 mmol), Pdnp-nSTDP (0.01 mol-% Pd), H2O
(500 μL), room temperature. [b] Isolated yield.
Synthesis of V- and Star-Shaped Molecules Catalysed by
Pdnp-nSTDP
Highly conjugated polyalkynylated organic molecules are mined, and ICP analysis of the ethyl acetate solution showed
of significant interest due to their extensive applications in that the Pd content of the solution was less than 0.10 ppm.
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Table 3. Synthesis of V- and star-shaped molecules by Sonogashira cross-coupling catalysed by Pdnp-nSTDP.
[a] Isolated yield. [b] Reaction conditions: 2,6-dibromopyridine (1 mmol), arylacetylene (2.2 mmol), DIPEA (2 mmol), Pdnp-nSTDP
(0.02 mol-% Pd), H2O (1 mL), room temperature. [c] Reaction conditions: 1,3,5-tribromobenzene or 2,4,6-trichloropyrimidine (1 mmol),
arylacetylene (3.3 mmol), DIPEA (3 mmol), Pdnp-nSTDP (0.03 mol-% Pd), H2O (1 mL), room temperature.
Table 4. Comparison of the results of Sonogashira coupling reactions catalysed by Pdnp-nSTDP and reported results with some other Pd
catalysts.
Entry Catalyst
Condition
Time [h] (Yield [%][a]
)
TOF [h–1]
Ref.
1
2
3
4
5
6
Pdnp-nSTDP (0.01 mol-%)
DIPEA, H2O, r.t.
2 (96)
40 (100)
3 (95)
3 (90)
0.16 (92)
3 (85)
4800
2.5
26.4
428
575
28.3
this work
[33a]
DAB dendr/Pd(OAc)2, G2[b] (1 mol-%) Et3N, N2, 25 °C
PNP–SSS[c] (1.2 mol-%)
K2CO3, H2O, reflux
CH3CN, Et3N, 82 °C
Piperidine, 70 °C
[33b]
[33c]
[33d]
[33e]
Pd0–Mont.[d] (0.07 mol-%)
[e]
Silica–Pd (1 mol-%)
Polymer NHC–Pd complex (1 mol-%)[f] Cs2CO3, 60 °C
[a] Isolated yield. [b] Diaminobutane dendrimer/Pd(OAc)2. [c] Immobilization of Pd nanoparticles on a silica–starch substrate. [d] Pd0
nanoparticles into the nanopores of modified Montmorillonite. [e] Silica-supported palladium. [f] NHC = N-heterocyclic carbene.
4
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Sonogashira Cross-Coupling in Water
Scheme 2. Synthesis of star-shaped molecules from 4-haloacetophenones.
with a LECO CHNS-932 instrument. The Pd content of the cata-
lyst was determined by Jarrell-Ash 1100 ICP analysis. The Pdnp-
nSTDP catalyst was prepared according to our previously reported
method.[20a]
Table 5. The recycling of Pdnp-nSTDP in the Sonogashira cross-
coupling of bromobenzene with phenylacetylene.[a]
Cycle
1
2
3
4
5
6
7
8
9
10
Yield
[%][b]
95 93 94
94 92
92 91 90
84 80
General Procedure for the Sonogashira Cross-Coupling of Aryl Hal-
ides with Terminal Alkynes Catalysed by Pdnp-nSTDP: A mixture
of aryl halide (1 mmol), terminal alkyne (1.1 mmol), DIPEA
(1 mmol), and Pdnp-nSTDP (0.01 mol-% Pd) in H2O (500 μL) was
stirred at room temperature for the time indicated in Table 2. The
progress of the reaction was monitored by TLC (diethyl ether/ethyl
acetate, 6:1). After the reaction was complete, ethyl acetate (15 mL)
was added, and the catalyst was separated by centrifugation. The
organic phase was washed with H2O (2ϫ 10 mL), and dried with
anhydrous MgSO4, and the solvents were evaporated. The residue
was recrystallized from ether and ethyl acetate (3:1) to give the pure
product (Table 2).
[a] Reaction conditions: bromobenzene (1 mmol), phenylacetylene
(1.1 mmol), DIPEA (1 mmol), Pdnp-nSTDP (0.01 mol-% Pd), H2O
(500 μL), room temperature, 5 h. [b] Isolated yield.
Conclusions
In conclusion, we have developed an environmentally
friendly and efficient method for the Sonogashira cross-
coupling of aryl halides (iodides, bromides, and chlorides)
with aromatic and aliphatic terminal alkynes. The method
uses palladium nanoparticles immobilized on nano-silica
triazine dendritic polymer (Pdnp-nSTDP) as a catalyst, and
the reactions are run in the greenest solvent, water. This
catalytic system can also be used efficiently for the synthesis
of V- and star-shaped molecules. The catalyst can be reco-
vered and reused, the experimental procedure is simple and
green, and the yields are high, all of which combine to make
this method a valid alternative to existing reaction proto-
cols for the preparation of these fine chemicals.
General Procedure for Synthesis of V- and Star-Shaped Molecules
by Sonogashira Cross-Coupling Catalysed by Pdnp-nSTDP: A mix-
ture of 2,6-dibromopyridine, 1,3,5-tribromobenzene or 2,4,6-tri-
chloropyrimidine (1 mmol), terminal alkyne (2.2–3.3 mmol),
DIPEA (2–3 mmol), and Pdnp-nSTDP (0.02–0.03 mol-% Pd) in
H2O (1 mL) was stirred at room temperature for the time indicated
in Table 3. The progress of the reaction was monitored by TLC
(diethyl ether/ethyl acetate, 6:1). After the reaction was complete,
ethyl acetate (15 mL) was added, and the catalyst was separated by
centrifugation. The organic layer was washed with water (2ϫ
10 mL), and dried with anhydrous MgSO4. Evaporation of the sol-
vent followed by recrystallization of the crude product from ether
and ethyl acetate (1:1) gave the pure product (Table 3).
Experimental Section
General Remarks: Chemicals were purchased from Fluka and
Merck. Melting points was determined with a Stuart Scientific
SMP2 apparatus. FTIR spectra were recorded with a Nicolet-Im-
Synthesis of P8, P9, and P10 by Sonogashira Cross-Coupling and
Subsequent Cyclotrimerization: The synthesis was carried out in two
steps (Scheme 2). First, the reaction of the 4-haloacetophenone
with the terminal alkyne was carried out according to the general
procedure given above for Sonogashira cross-coupling in the pres-
ence of Pdnp-nSTDP (Table 2, entries 8, 9, and 13). This gave the
1
pact 400D spectrophotometer. H and 13C NMR spectra (400 and
100 MHz) were recorded with a Bruker Avance 400 MHz spec-
trometer using CDCl3 solvent. Elemental analysis was carried out
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I. Mohammadpoor-Baltork, V. Mirkhani et al.
FULL PAPER
S. B. Park, H. Alper, Chem. Commun. 2004, 1306–1307; d) A.
Modak, J. Mondal, M. Sasidharan, A. Bhaumik, Green Chem.
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trimerization of the 4-(arylethynyl)acetophenone was carried out
according to our previously reported procedure.[32] Thus, a mixture
of 4-(arylethynyl)acetophenone (1 mmol) and H3PW12O40 (15 mol-
%) was exposed to microwave irradiation (450 W, 90 °C) for 20 min.
The progress of the reaction was monitored by TLC (diethyl ether/
ethyl acetate, 6:1). After the reaction was complete, hot ethyl acet-
ate (10 mL) was added, and the catalyst was separated by filtration.
Evaporation of the solvent followed by recrystalization of the crude
material from ethyl acetate gave the pure product.
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Received: April 29, 2014
Published Online: ᭿
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7

Job/Unit: O42503
/KAP1
Date: 29-07-14 17:53:49
Pages: 8
I. Mohammadpoor-Baltork, V. Mirkhani et al.
FULL PAPER
Palladium Nanoparticles
Palladium nanoparticles immobilized on
nano-silica triazine dendritic polymer
(Pdnp-nSTDP) was found to be a highly ef-
fective catalyst for the Sonogashira cross-
coupling of aryl halides (iodides, bromides,
and chlorides) with aromatic and aliphatic
terminal alkynes. The reaction was best
performed in water as a green solvent in
the presence of just 0.01 mol-% of the cata-
lyst at room temperature.
A. Landarani Isfahani,
I. Mohammadpoor-Baltork,*
V. Mirkhani,* A. R. Khosropour,
M. Moghadam,
S. Tangestaninejad ............................ 1–8
Pd Nanoparticles Immobilized on Nano-
silica Triazine Dendritic Polymer: A Reu-
sable Catalyst for the Synthesis of Mono-,
Di-, and Trialkynylaromatics by Sonoga-
shira Cross-Coupling in Water
Keywords: Heterogeneous catalysis / Water
chemistry / Palladium / Nanoparticles /
Cross-coupling / Dendrimers / Alkynes
8
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