An ester appending multifunctional ionic liquid for Pd(II) catalyzed Heck reaction

Article abstract of DOI:10.1016/j.catcom.2010.10.004

A basic, ester functionalized, imidazolium based ionic liquid (IL2), 3-Methyl-1-(ethoxycarbonylmethyl)imidazolium hydroxide was designed eyeing in situ generation and stabilization of palladium nanoparticles for palladium catalyzed Heck reaction of haloarenes and olefins. This phosphine-free Pd-IL catalyst demonstrated excellent activity and reusability at relatively low reaction temperature (80 °C) for a wide spectrum of haloarenes including chloroarenes. The relatively low reaction temperature could be attributed to in situ generation of uniform and spherical palladium nanoparticles (~ 5 nm).

Full text of DOI:10.1016/j.catcom.2010.10.004

Catalysis Communications 12 (2010) 273–276
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
An ester appending multifunctional ionic liquid for Pd(II) catalyzed Heck reaction
a
a
a
a
b,
Anand D. Sawant , Dilip G. Raut , Nitin B. Darvatkar , Uday V. Desai , Manikrao M. Salunkhe ⁎
a
Department of Chemistry, Shivaji University, Vidyanagar, Kolhapur-416 004, India
Central University of Rajasthan, JLN Marg, Jaipur-302 017, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 May 2010
Received in revised form 24 September 2010
Accepted 4 October 2010
Available online 28 October 2010
A basic, ester functionalized, imidazolium based ionic liquid (IL2), 3-Methyl-1-(ethoxycarbonylmethyl)
imidazolium hydroxide was designed eyeing in situ generation and stabilization of palladium nanoparticles
for palladium catalyzed Heck reaction of haloarenes and olefins. This phosphine-free Pd-IL catalyst
demonstrated excellent activity and reusability at relatively low reaction temperature (80 °C) for a wide
spectrum of haloarenes including chloroarenes. The relatively low reaction temperature could be attributed to
in situ generation of uniform and spherical palladium nanoparticles (~5 nm).
Keywords:
Heck
©
2010 Elsevier B.V. All rights reserved.
IL
Palladium
Nanoparticles
1
. Introduction
potential of ILs as reaction media and as catalyst for some organic
transformations. In one of our recent findings, we have demonstrated
[7] the importance of ionic liquid as superior reaction media for
cycloaddition reaction catalyzed by copper nanoparticles. Since the
first report [8], there are a number of reports on Heck reaction in
ionic liquid with very recent reports [9,10] of use of palladium
nanoparticles in ionic liquids. It is now well-versed that nanoparticles
provide enhanced catalytic activity due to increased surface area and
rotational degrees of freedom [11]. In general, the larger the surface
area (i.e. smaller size), the greater the catalytic activity. Thus, smaller
NPs are preferable, although they tend to be less stable and are prone
to aggregation ultimately leading to less effective catalysts. Agglom-
eration of nanoparticles can be avoided by use of stabilizing agents.
However, these stabilizing agents have substantial effects on their
catalytic activity [12]. Ionic liquids have been shown to provide
“electrostatic” stabilization for metal nanoparticles and also more
surface area for the reaction to take place [13,14]. Multiphase systems
of ionic liquid-nanoparticles facilitate easy recovery of nanoparticles.
Recently, Wang et al. [15] reported a novel ethanolamine based
quaternary ammonium salt for Heck reaction. The ionic liquid
showed very high activity with reusability of palladium. Taking into
account advantages of use of ionic liquids in synthesis of nanopar-
ticles and as an attempt to circumvent the concerns of traditional
Heck reaction, we decided to design and synthesize an ionic liquid
(IL) that can act as base, ligand and reaction media and also provide
stability to in situ generated palladium nanoparticles. Herein we
report an ester appending imidazolium based basic ionic liquid (2)
for Heck reaction of olefins and aryl halides. IL2 is designed, eyeing in
situ generation and stabilization of the palladium nanoparticles. The
IL2 showed excellent activity, reactivity and reusability for the Heck
reaction.
The palladium-catalyzed Heck arylation of olefins is one of the
most important carbon–carbon bond forming processes in synthetic
organic chemistry [1–3]. The availability of a wide spectrum of
functional groups in both the substrates makes this reaction more
compatible for total synthesis. At present there are some concerns
with respect to this reaction viz. activation of C–Cl bond, development
of phosphine-free palladium catalyst, high reaction temperature
(
usually above 100 °C) and recyclability of the catalytic system.
Even though the arylchlorides are easily available starting materials,
they require drastic reaction conditions (N140 °C), use of phosphine
ligands and hindered amines [4]. Phosphine ligands act as stabilizers
for in situ generated catalytically active Pd(0) complexes. However,
due to high cost, air sensitivity and environmental concerns related to
phosphine containing systems, recently development of phosphine-
free catalyst has received considerable attention.
Green and cost-effective catalyst systems with high efficiency and
selectivity are the cornerstones of contemporary synthetic chemistry.
In the last decade the field of ionic liquids (ILs) has emerged as a
promising field to address concerns of organic chemists. These ILs
being non-volatile and thermally stable became popular as sub-
stituents to conventional volatile organic solvents. Apart from this,
their chemical and physical properties can be tuned by modifying
cation or anion, which has further popularized them as a catalyst and
as conducive reaction media. Our group has [5,6] explored the
⁎
Corresponding author.
E-mail address: mmsalunkhe@hotmail.com (M.M. Salunkhe).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.10.004

274
A.D. Sawant et al. / Catalysis Communications 12 (2010) 273–276
2
. Experimental
3. Results and discussion
2
.1. Materials
3.1. Synthesis and application of IL to Heck reaction
Palladium acetate, aryl halides, olefins, ethyl bromoacetate and 1-
With the aim to improve the catalytic activity by in situ generation
and stabilization of palladium nanoparticles, we designed and
synthesized IL 2 for Heck reaction. In our earlier work on ionic liquid
mediated synthesis of copper nanoparticles and their use in 1–3
dipolar cycloaddition reaction, it was observed that only imidazole
containing ILs could not provide sufficient stability to nanoparticles
making the use of stabilizing agents mandatory. These stabilizing
agents may have profound effect on course of reaction, further
jeopardizing the whole procedure. It is now a well established fact
that mechanism of Heck reaction involving palladium nanoparticles
proceeds via the formation of soluble palladium complexes. Taking
into account these aspects we decided to design a carbonyl
functionalized ionic liquid. In one of the earlier reports [16], the
ester functionalized ILs were found to have advantages of biodegrad-
ability and analogous behavior to their alkyl side chain analogues (like
methyl imidazole were obtained from Aldrich and used as received.
All other reagents were AR grade and used as received. Ionic liquid (2)
was prepared as indicated in Scheme 1. Progress of reaction was
monitored by TLC (Silica gel 60 F254). The formed IL2 was
characterized with ESI-MS, 1H, C and
13
1
H
13
C HETCOR NMR and
1
13
HRMS. Products of Heck reaction were characterized by H and
C
NMR analysis. The isolated palladium nanoparticles were character-
ized by TEM analysis.
2
.2. General procedure for preparation of IL 2
Ionic liquid 1 was prepared by the following reported method [16].
To a stirred solution of 1-methylimidazole (8.0 mL, 100 mmol) in THF
50 mL) at −5 °C under a nitrogen atmosphere was added dropwise
ethyl bromoacetate (13.4 mL, 120 mmol). The reaction mixture was
stirred vigorously at −5 °C for 1 h, then at r.t. for 3 h. The THF top
phase was decanted and the IL washed with diethyl ether (3×15 mL),
then residual solvent removed in vacuo. The product was dried at
(
6
[bmim]PF , [bmim]OH etc.). However, from some recent reports [17–
2
2], we came to the conclusion that an ionic liquid with ester function
can best fit for the stabilization of nanoparticles. Keeping these points
in mind an IL was prepared starting with ethyl bromoacetate and 1-
methyl imidazole. Addition of ethyl bromoacetate was carried out
slowly at lower temperature and the reaction mixture was then
stirred at room temperature for 3 h to get the ionic liquid 1. IL 1 was
then subjected to metathesis with potassium hydroxide to get the
desired IL 2. Post-reaction analysis (ESI-MS) of ionic liquid wards off
the possibility of hydrolysis of appending ester function (at elevated
reaction temperature). The stability of IL after reaction was confirmed
by NMR analysis of the recovered IL (after reaction).
6
0 °C under vacuum for 72 h to give a clear viscous hygroscopic oil as
ionic liquid 1 (IL1).
Solid potassium hydroxide (2.3 g, 40 mmol) was then added to
ionic liquid 1 (10.4 g, 40 mmol) in dry dichloro methane and stirred
vigorously for 5 h. The precipitated salt (KBr) was filtered off, and the
filtrate was evaporated to leave the crude ionic liquid 2 (IL2) as a
viscous liquid. It was washed with ether (2×20 mL) and dried (80 °C)
under inert atmosphere for 5 h to get the pale yellow coloured viscous
The effect of source of palladium on the Heck reaction was tested
for reaction of p-iodoanisole and styrene. Among the screened
palladium salts as illustrated in Table 1, Pd(OAc) was found effective
2
1
13
1
13
liquid for use. This ionic liquid 2 was characterized by H, C, H
HETCOR NMR, ESI-MS and HRMS analysis.
C
for Heck reaction in IL 2. A temperature of 80 °C was found to be
optimum for an 8 h reaction time. The applicability and generality of
this Pd-IL 2 catalytic system for Heck reaction of olefins and aryl
halides were studied with a wide array of substrates. Representative
results of the coupling reaction are summarized in Table 2. In order to
investigate the effect of substituent group on arylhalide, various
substituted arylhalides were used. The coupling reaction for arylha-
lides with both electron deficient and electron rich substituents,
underwent smoothly as indicated in Table 2. There was no any
substantial effect of substituent on reaction. Even though iodoarenes
and bromoarenes gave excellent yields in stipulated time of 8 h, the
arylchlorides showed some reluctance and gave moderate yields
(Entries 16–19 Table 2). Though the general experimental procedure
was based on 1 mmol scale, 50 mmol reactions produced similar
results. Initially when the ionic liquid was stirred with palladium
acetate at 80 °C, within 10 min the reaction mixture turned black. This
blackening of the reaction mixture at the beginning prompted us to
isolate the palladium. The isolated palladium was characterized by
transmission electron microscopy (TEM). From TEM images, gener-
ation of palladium nanoparticles with average size of 5 nm and
2
.3. General procedure for Heck reaction
ToIL2 (0.2 g) in a 50 ml round bottom flaskwasadded the palladium
acetate (2.3 mg) and stirred at 80 °C for 10 min. After that aryl halide
and olefin (1 mmol each) were introduced in the flask and stirred for 8 h
at 80 °C. After completion of reaction, ethyl ether was added and the
organic layer was washed with water and dried over sodium sulfate. The
organic layer was evaporated under reduced pressure and the crude
product was purified by column chromatography.
Br
O
BrCH COOC H
5
1
2
2
+
N
N
N
N
Acetonitrile
O
0
5
C
KOH DCM
O
2
+
N
N
O
-
HO
Table 1
Palladium salts for Heck reaction.
No
Catalyst
Yielda (%)
X
R
IL 2
1
2
3
4
5
Palladium powder (b1 μ)
Pd/C
40
56
71
88
95
+
R
Pd(OAc)2
R'
R'
Pd(PPh ) Cl
2
0
3
2
8
0 C
PdCl
Pd(OAc)
2
X= Cl, Br, I
R= Ar, CO R1
2
2
a
Isolated yields. Reaction conditions: p-Iodoanisole (1 mmol), styrene (1 mmol), Pd
Scheme 1. Synthesis and application of IL 2 for Heck reaction.
source (0.01 mmol), IL 2 (0.2 g), 80 °C, 8 h.

A.D. Sawant et al. / Catalysis Communications 12 (2010) 273–276
275
Table 2
Table 3
Palladium-catalyzed Heck reaction in IL2.
Reusability of the catalyst.
Entry
Ar
X
R
Yield %
Cycle
TON
1
2
3
4
5
6
7
8
9
C
6
H
5
I
I
I
I
I
I
I
I
C
C
C
6
6
6
H
H
H
5
5
5
94
98
96
94
98
95
95
96
98
94
93
93
95
92
85
89
86
83
1
2
3
4
5
96.19
95.23
93.80
92.85
90.47
p-NO
p-CH
C
p-CH
p-CH
C
p-CH
2-C10
p-CNC
p-CH
p-CH
p-CH
p-C
C
p-CH
p-CNC
p-C
2
C
OC
6
H
5
3
6
H
5
H
6 5
COOBu
COOBu
COOMe
COOMe
COOMe
COOBu
COOBu
COOBu
3
OC
OC
6
H
H
5
3
6
5
Reaction conditions : 10 mmol of each 4-Iodoanisole and styrene, 0.1 mmol (23 mg) of
Pd(OAc) , 2 g of IL., 80 °C, 8 h.
TON (turn over number) [23]=mmol of product/ mmol of catalyst.
H
6 5
2
3
C
6
H
5
H
7
I
10
11
12
13
14
15
16
17
18
6
H
5
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
3
3
3
COC
COC
COC
6
6
6
H
H
H
5
5
5
3
.2. Characterization
6 5
C H
COOMe
COOBu
The IL 2 was characterized by ESI-MS (+ve mode), H, C, H 13C
1
13
1
6
H
5
COC
6
H
5
5
H
6 5
6 5
C H
HETCOR NMR and HRMS. ESI-MS analysis was done on Bruker
Daltonics ESI-MS with ion trap detector. The ESI-MS analysis was
carried out by dissolving the IL2 in methanol. Peak in positive mode at
69.0 confirmed the presence of desired cation. The H and C NMR
were recorded in CDCl (Aldrich 225789, 99.8 atom% D) on Bruker
Avance II spectrometer. H and C NMR spectra were recorded at
00 MHz and 75.4 MHz respectively. In H NMR of the IL2 there was
an ambiguity about peak of OH at δ 3.61. However from the
3
OC
6
H
6
H
5
COOBu
COOBu
COOBu
5
6
H
5
COC
6
H
1
13
1
2
Yields are isolated yields; olefin (1 mmol), aryl halide (1 mmol) and Pd (OAc) (2.3 mg)
in IL 2 (0.2 g) at 80 °C for 8 h.
3
1
13
1
3
1
13
H
C
HETCOR NMR (Fig. 2), as there was no correlation for this proton, this
peak could be assigned for OH anion. Analysis of palladium
nanoparticles with transmission electron microscopy (TEM) was
performed on a JEOL JEM 2010 transmission electron microscope
operating at 200 kV with nominal resolution of 0.25 nm. From TEM
images and SEAD pattern it is clear that uniformly distributed
spherical nanoparticles were formed at an average size of 5 nm.
uniform spherical shape was observed (Fig. 1). This in situ generation
of palladium nanoparticles can be attributed to the dual role of IL 2 as
a reductant and a stabilizing agent. TEM analysis of palladium after
second and third reuses confirmed the regeneration of palladium
nanoparticles.
Reusability of catalyst is an important aspect from a commercial
point of view. The reusability of catalyst was tested using p-
iodoanisole and styrene as substrates. After completion of reaction,
the reaction mixture was extracted with diethyl ether (3×30 ml) to
obtain the desired product. The catalyst left in the reaction vessel was
dried under vacuum (3 h). This dried catalyst-IL system was reused
for the next run. It was observed that there was no any substantial loss
of catalytic activity even after the fifth run as indicated by TON
4
. Conclusion
We have described a meticulously designed multifunctional IL 2 as
base, ligand and reaction media for palladium-catalyzed Heck
reaction. The present IL2-Pd catalyst system showed excellent activity
for a wide range of haloarenes, including chloroarenes. No use of
phosphine ligand, no need of inert atmosphere, use of environmen-
tally benign reaction media of IL, relatively low reaction temperature
and reusability are the advantages of this IL2-Pd catalyst system. In
situ generation of uniform and spherical nanoparticles (5 nm) of
palladium may be responsible for reactions at relatively low
temperature (80 °C). Further investigation on preparation of some
(
Table 3). Although kinetic measurements or TOF50 values should be
a better way of assessing the true reusability of the catalyst, [24] the
current results clearly indicate that our system has a significant
potential in this regard. The catalyst leaching was tested after each run
and from ICP analysis it was found that palladium leaching was not
more than 1% after the fifth run.
Fig. 1. TEM image of palladium nanoparticles.
Fig. 2. 1H 13C HETCOR NMR of IL2.

276
A.D. Sawant et al. / Catalysis Communications 12 (2010) 273–276
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