Pd-TPPTS catalyzed Mizoroki-Heck coupling in halogen-free ionic liquids [Rmim][p-CH3C6H4SO3]

Article abstract of DOI:10.1016/j.catcom.2011.01.008

A highly efficient system composed of Pd-TPPTS [TPPTS: trisodium salt of tri(m-sulphonylphenyl)phosphine] and halogen-free ionic liquid ([Rmim][p-CH ₃C₆H₄SO₃], R = methyl, ethyl, n-butyl, n-hexyl, n-octyl, n-dodecyl) has been established for Heck coupling of aryl halides with styrene. Most of the investigated substrates could give the complete conversions (> 95%) with the catalyst of 1 mol% at 110 °C. The resulting products can be easily separated from the ionic liquids by simple liquid-liquid extraction, and the catalyst immobilized by ionic liquids can be consecutively run five times without significant loss in catalytic activity.

Full text of DOI:10.1016/j.catcom.2011.01.008

Catalysis Communications 12 (2011) 748–752
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Pd-TPPTS catalyzed Mizoroki–Heck coupling in halogen-free ionic liquids
[Rmim][p-CH3C6H4SO3]
⁎
Juan Wei, Hai-Yan Fu, Rui-Xiang Li , Hua Chen, Xian-Jun Li
Key Lab of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A highly efficient system composed of Pd-TPPTS [TPPTS: trisodium salt of tri(m-sulphonylphenyl)phosphine]
and halogen-free ionic liquid ([Rmim][p-CH3C6H4SO3], R=methyl, ethyl, n-butyl, n-hexyl, n-octyl, n-dodecyl)
has been established for Heck coupling of aryl halides with styrene. Most of the investigated substrates could
give the complete conversions (N95%) with the catalyst of 1 mol% at 110 °C. The resulting products can be
easily separated from the ionic liquids by simple liquid-liquid extraction, and the catalyst immobilized by
ionic liquids can be consecutively run five times without significant loss in catalytic activity.
© 2011 Elsevier B.V. All rights reserved.
Received 2 November 2010
Received in revised form 20 December 2010
Accepted 9 January 2011
Available online 19 January 2011
Keywords:
Heck coupling
Palladium
Ionic liquid
Aryl halide
1. Introduction
catalyzed the Heck coupling to reduce effectively the loss of catalyst
caused with organic solvent extraction. Furthermore, as the low
The transition metal palladium-catalyzed Heck coupling [1] is one of
the most powerful reactions in the synthesis of natural products,
pharmaceutical, and agrochemical derivatives [2,3]. Since the availabil-
ity and the economic viability, aryl chlorides are the most attractive
starting materials among aryl halids [4], but its reaction must be
performed in the presence of electron-rich and sterically bulky tertiary
phosphine ligands under harsh conditions [5]. In view of the increased
demand for environmental benign reaction process, numerous efforts
have been made to investigate this reaction in non-conventional
reaction media [6], such as water [7], ionic liquids [8], and supercritical
CO2 [9], etc. Especially, the development of homogeneous/heteroge-
neous catalysts in ionic liquids leads to a variety of different catalyst
systems for the Heck coupling [10], such as hybrid P,O-ligand [11], P,N-
ligand [12], palladacycles [13], N-heterocyclic carbenes (NHCs) [14,15],
and electron-rich monodentate phosphine [16], polymeric system in
water [17], ligand-free palladium nanoparticles [18]. Herein, subse-
quent experimental and theoretical trials suggest that Pd-catalyzed the
Heck coupling in ionic liquids shows the stable and recycling
advantages. However, the challenging problems are how to improve
the solubility of complexes or ligands in the ionic liquid, to decrease
the loading of transition metal, and to reduce the loss of transition
metal complex caused by organic solvent extraction while workup, etc.
In this paper, our attention is focused on studying water-soluble
Pd-TPPTS [TPPTS: trisodium salt of tri(m-sulphonylphenyl)phosphine]
solubility of TPPTS and its complexes in [bmim]PF6 and [bmim]BF4
would cause the slow reaction rate [19,20], anionic groups in [bmim]
BF4 and [bmim]PF6 are exchanged with sulfonate aromatic group to
increase the similarity of ionic liquid with TPPTS and to improve the
solubility of TPPTS and Pd-TPPTS species. It is found that Pd-TPPTS in the
resulting ionic liquids 1-alkyl-3-methylimidazolium p-toluenesulfonate
([Rmim][p-CH3C6H4SO3], R=methyl, n-ethyl, n-butyl, n-hexyl, n- octyl,
n-dodecyl) (see Fig. 1) is highly efficient system for the Heck coupling.
2. Experimental
2.1. Materials and methods
1-Methylimidazole (Alfa, 99%), 1-butanol (Alfa, 99%), 1-octanol
(Alfa, 99%) and the other reagents were commercially available and
used as received. TPPTS, ionic liquids [Rmim][p-CH3C6H4SO3] were
synthesized according to known methods in our laboratory. The
conventional ionic liquids [bmim]Br, [bmim]BF4 and [bmim]PF6 were
prepared according to the literatures [21]. The transmission electron
microscopy (TEM) observation was taken on a Tecnai G2 F20 S-TWIN
at an accelerating voltage of 200 kV. And the IR spectra were recorded
on a NEXUS 670.
2.2. Typical procedure for the Heck coupling of bromobenzene with
styrene
Catalyst precursor PdCl2 (0.02 mmol, 3.5 mg), ionic liquid [bmim]
⁎
Corresponding author. Tel./fax: +86 28 85412904.
E-mail address: liruixiang@scu.edu.cn (R.-X. Li).
[p-CH3C6H4SO3] (1.1 g) and TPPTS (0.08 mmol, 45.4 mg) were placed
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.01.008

J. Wei et al. / Catalysis Communications 12 (2011) 748–752
749
N
N
SO
3
R
R = methyl, ethyl, n-butyl, n-hexyl, n-octyl, n-dodecyl
Fig. 1. The structure of ionic liquid [Rmim][p-CH3C6H4SO3].
Scheme 1. The Heck coupling of bromobenzene with styrene in [bmim]
[p-CH3C6H4SO3].
in a dried flask. The flask was placed in preheated oil bath at 110 °C
and stirred for 5 min under inert atmosphere to give a pale yellow
solution as the catalyst. Subsequently, bromobenzene (2 mmol,
0.21 mL), styrene (2.4 mmol, 0.28 mL), Et3N (2.8 mmol, 0.39 mL)
and H2O (0.2 mL) were added into the pre-formed catalyst, and then
reacted at 110 °C for 2 h. At the end of reaction, the solution was
cooled and extracted with ethyl ether (3×10 mL). The product
was concentrated under vacuum. The conversion and selectivity were
analyzed by GC Agilent-6890 N equipped with a capillary column
SE-30 (30 m×0.25 mm). The isolated products were further deter-
mined by 1H and 13C NMR (Bruker Avance II-400, 400 MHz for 1H
NMR, and 100 MHz for 13C NMR).
under reaction conditions and promoted the catalytic reaction [22].
Many chemists have proved that Pd nanoparticles are highly active to
C–C coupling [23–25], but they must be effectively protected with the
suitable reagent, such as PEG [26]. It is also found that the imidazole
ring of ionic liquid could form the carbene complex with Pd species
and stabilize the active Pd species. Our result, a lot amount of Pd black
precipitated quickly without TPPTS, did not support the formation of
the carbene complex in this system. When 2 equiv. of TPPTS added
into the reaction media, the catalytic system not only gave a high
yield of 93.5%, but also the formation of Pd black was obviously
inhibited (Table 2, entry 2). The result indicated that the water-
soluble catalytic species of Pd-TPPTS were formed by reacting PdCl2
with TPPTS in situ or the active Pd species could be efficiently
stabilized with TPPTS in [bmim][p-CH3C6H4SO3]. A small amount of
the reacted sample was transferred to a carbon-coated copper grid
and analyzed with TEM (see Fig. 2). The size of TPPTS-stabilized
Pd particles is about 5–10 nm with irregular shape. In order to prove
whichever the true active species was nanoparticles or molecule
species. Mercury poisoning experiment, which is an effective method
to distinguish nanoparticles and molecule species [27], revealed a
typical character of nanoparticles catalysis in this system because the
activity of this system was completely lost as soon as mercury was
added into the system. An induction period of 40 min in the kinetic
curve (Fig. 3) further supported that Pd nanoparticles acted as the
catalytically active specie. Interestingly, a complete conversion was
obtained at 110 °C for 2 h. Using the highly reactive iodobenzene as
the substrate and palladium of 2 mol% as the catalyst, Earle et al. got
a yield of near 100% in [C6py]Cl at 100 °C for 24 h [28]. Iranpoor
et al. used an imidazolium-based phosphinite ionic liquid (IL-OPPh2)
as a ligand for the Heck coupling of bromobenzene with styrene,
but it needed a high Pd loading of 3 mol% [11]. Shreeve et al. inves-
tigated the catalytic performance of Pd complex bearing pyrazolyl-
functionalized N-heterocyclic carbene ligand for the Heck coupling of
bromobenzene with n-butyl acrylate. Although the amount of
palladium complex was up to 2 mol%, the system only gave a yield
of 49% at 120 °C for 12 h [14]. This simple and efficient system
compares well with previously reported catalysts in ionic liquid. If the
amount of TPPTS continually increased, the active site of Pd could be
occupied by TPPTS and the reaction rate decreased a little (Table 2,
entries 3 and 4).
2.3. Catalyst recycles
Successive runs were carried out as following procedures. At the
end of reaction, the solution was cooled and extracted with ethyl
ether (3×10 mL). And then the residual organic solvent in ionic liquid
immobilized catalyst was evaporated under vacuum at 70 °C for 1 h.
The remaining liquid phase was added with fresh bromobenzene
(2 mmol, 0.21 mL), styrene (2.4 mmol, 0.28 mL), Et3N (2.8 mmol,
0.39 mL), and H2O (0.2 mL) for the next run.
3. Results and discussion
Firstly, the performance of different palladium precursors was
tested in ionic liquid [bmim][p-CH3C6H4SO3]. The results shown in
Table 1 indicated that PdCl2 was the most effect precursor at the
same conditions. Besides, when the reaction time was prolonged,
[Pd(C3H5)Cl]2 and Pd(OAc)2 precursors also gave the satisfying yields
(Scheme 1, Table 1, entries 3 and 6). These results revealed that
[bmim][p-CH3C6H4SO3] is a good solvent for Heck coupling and the
investigated palladium precursors could show the good catalytic
performance in it.
According to the results in Table 2, if PdCl2 was used as a precursor
and no TPPTS was introduced in this system, lots of palladium black
was quickly generated and a low yield of 27.6% was given (Table 2,
entry 1). The low yield demonstrated that PdCl2 could be transformed
into the active species without TPPTS and catalyzed this reaction in
the water-soluble ionic liquid [bmim][p-CH3C6H4SO3]. Dupont et al.
reported that active Pd nanoparticles could be formed in ionic liquid
According to the reported results, the solubility of ligand in ionic
liquid and the change of the cation or anion ions in ionic liquids
Table 1
The catalytic performance of different palladium precursors for Heck coupling of
a
bromobenzene with styrene
.
Table 2
a
Effect of molar ratio of Pd to P on the Heck coupling of bromobenzene with styrene
.
b
Entry
Catalyst precursor
Yield (%)
Entry
1
Pd/P (molar ratio)
Yieldb (%)
1
PdCl2
99.6
15.6
81.5
15.3
63.8
88.0
2
[Pd(C3H5)Cl]2
[Pd(C3H5)Cl]2
Pd(OAc)2
PdCl2
–
27.6
93.5
83.0
78.7
99.6
3c
4
2
3
4
1: 2
1: 3
1: 4
1: 4
c
d
e
5d
6e
f
Pd(OAc)2
5
a
a
bromobenzene 2 mmol, styrene 1.2 equiv., NEt3 1.4 equiv., H2O 0.2 mL, catalyst
bromobenzene 2 mmol, styrene 1.2 equiv., NEt3 1.4 equiv., H2O 0.2 mL, PdCl2 1 mol%,
1 mol%, P/Pd=4 (molar ratio), [bmim][p-CH3C6H4SO3] 1.1 g, temperature 110 °C, time
[bmim][p-CH3C6H4SO3] 1.1 g, temperature 110 °C, time 1.5 h.
b
2 h.
GC analysis.
Pd black precipitated largely.
Pd black precipitated slightly.
Pale yellow solution.
b
c
GC-MS analysis.
4 h.
TPPDS was added.
3.5 h.
c
d
d
e
e
f
Reaction time, 2 h.

750
J. Wei et al. / Catalysis Communications 12 (2011) 748–752
Fig. 2. TEM micrographs of Pd nanoparticles dispersed in [bmim][p-CH3C6H4SO3].
also enhanced from 14.8% to 99.6% with increasing the chain-length of
100
80
60
40
20
0
alkyl from C1 to C4 (Table 3, entries 1, 2 and 3). Obviously, the low
solubility of substrates in ionic liquids led to the mass transfer of
substrates becoming the determining step of the reaction rate. In
addition, the chain-length of alkyl in imidazolium ring increased too
much, it would lead to the solubility decrease of water-soluble
palladium-phosphine species in ionic liquid, so that the reaction
conversion decreased from 99.6% to 15.3% (Table 3, entry 6). The
similar phenomenon was also observed in the asymmetric hydroge-
nation of aromatic ketones [29]. Compared with the traditional ionic
liquids, the novel system, especially in [bmim][p-CH3C6H4SO3], gave
much higher catalytic activity (Table 3, entries 7, 8 and 9). It is
reasonably suggested that there is a matched relationship between
the chain-length of alkyl in imidazolium and the solubility of Pd-
TPPTS or substrates. The matched relationship is to maintain the
solubility of substrates as well as the reasonable solubility of Pd-TPPTS
species in the ionic liquid.
40
50
60
70
80
90
100
110
120
130
Time (min)
It is reported that water deeply affects the reaction when RTILs are
used as a solvent [30–32]. If we used a dry ionic liquid as solvent, we
obtained a low conversion (Table 4, entry 1). Added a small amount of
water, the catalytic system showed a high activity due to the
improvement of the solubility of water-soluble catalyst (Table 4,
entries 2 and 3). When the amount of water was added too much, the
activity of this system was inhibited due to the hydrophobicity of
substrates (Table 4, entries 4, 5 and 6). Similarly, the effect of water
on this reaction was also observed in other ionic liquids.
Fig. 3. The kinetic test for the Heck coupling of bromobenzene with styrene.
would greatly influence the catalytic performance. Therefore, the
effect of the structure of ionic liquid on the Heck coupling was also
investigated. The results listed in Table 3 showed that the catalytic
activity was highly sensitive to the structure of the ionic liquids. Since
the hydrophobicity of ionic liquid and the solubility of organic
substrates would be improved simultaneously with increasing the
chain-length of alkyl in imidazolium ring, the reaction conversions
To extend the scope of the protocol, we applied these optimum
conditions to screen a wide array of substrates (Scheme 2, Table 5). It
was found that the catalytic system is suitable to all kinds of aryl
Table 3
Effect of different ionic liquid cation and anion on the Heck coupling of bromobenzene
Table 4
a
with styrene
.
Effect of different water volume on the Heck coupling of bromobenzene with styrene a
.
b
b
Entry
Ionic liquids
Yield (%)
Entry
Water volume (mL)
Yield (%)
1
2
3
4
5
6
7
8
9
[mmim][ p-CH3C6H4SO3]
[emim][ p-CH3C6H4SO3]
[bmim][ p-CH3C6H4SO3]
[hmim][ p-CH3C6H4SO3]
[omim][ p-CH3C6H4SO3]
[dmim][ p-CH3C6H4SO3]
[bmim]Br
14.8
29.0
99.6
94.7
83.1
15.3
5.9
1
2
3
4
5
6
No water
0.1
0.2
0.3
0.4
5.0
87.2
99.6
53.7
7.0
c
d
0.6
No IL
8.8
5.2
7
[bmim]BF4
[bmim]PF6
b5.0
55.2
a
bromobenzene 2 mmol, styrene 1.2 equiv., NEt3 1.4 equiv., PdCl2 1 mol%, P/Pd=4
(molar ratio), [bmim][p-CH3C6H4SO3] 1.1 g, temperature 110 °C, time 2 h.
a
b
bromobenzene 2 mmol, styrene 1.2 equiv., NEt3 1.4 equiv., H2O 0.2 mL, PdCl2 1 mol%,
P/Pd=4 (molar ratio), ionic liquids 1.1 g or 1 mL, temperature 110 °C, time 2 h.
GC analysis.
Pd black precipitated.
c
b
d
GC analysis.
Pd black precipitated largely.

J. Wei et al. / Catalysis Communications 12 (2011) 748–752
751
100
80
Scheme 2. The Heck coupling of halides with styrene in [Rmim][p-CH3C6H4SO3].
60
40
20
0
bromides. Not only did the activated bromides with electron-
withdrawing substituents convert efficiently into the desired pro-
ducts, but the inactive bromides with electron-donating substituents
also exhibited high reactivity in this reaction system. In addition,
chlorobenzene and activated aryl chlorides with acetyl and trifluor-
omethyl groups and N-heterocycle substrates could convert into
desired products in low to high yields under the relatively mild
conditions. These results are better than that in many conventional
and functioned ionic liquids. Such that, Hagiwara et al. obtained a low
product yield of 29% by reacting p-acetylbromobenzene with styrene
over Pd/C of 3 mol% in [bmim]PF6 [33], and T. Liu et al. got only
stilbene of 41% in functioned ionic liquid [bmim][TPPTMS] under the
1
2
3
4
5
Runs
Fig. 4. Recycling results for the Heck coupling of bromobenzene with styrene.
conditions of S/C=100 and 140 °C [34]. It was noteworthy that a
small amount of DMF was introduced to improve the solubility
and mass transfer of o-bromotoluene and p-bromotoluene in this
hydrophilic system. We also demonstrated that the catalyst immobi-
lized in [bmim][p-CH3C6H4SO3] was stable and could be recycled (see
Fig. 4). After this system was run five times, its activity had no
significant loss and the product was separated from the ionic liquid by
means of a simple liquid–liquid extraction with ethyl ether at the end
of each run. Considering the salt produced in the reaction, each
recycling need to extend reaction time for one more hour [35].
Table 5
a
Heck coupling of halides with styrene in ionic liquid
.
b
b
Substrates
Time Yield
Substrates
Time
(h)
Yield
(%)
(h)
(%)
c
18
N99.9
24
94.4
Br
Br
MeO
Br
Br
c
18
18
N99.9
N99.9
24
N99.0
F3C
F3C
4. Conclusion
I
Br
In summary, we successfully applied Pd-TPPTS as the catalyst and
water-soluble ionic liquid [Rmim][p-CH3C6H4SO3] as solvent for the
Mizoroki–Heck coupling and obtained the high conversion and good
selectivity. The resulting products can be easily separated from the
catalyst system by extraction with ethyl ether. The catalyst immobi-
lized in ionic liquid can be recoverable and reused five times without
significant loss of the conversion and selectivity.
d
15 min N99.9
CF3
C l
e
24
N99.9
22
45.0
H3COC
F3C
Br
Br
C l
e
e
24
24
98.0
86.3
22.5
24
25.8
15.6
Appendix A. Supplementary data
H3COC
Br
Supplementary data to this article can be found online at
doi:10.1016/j.catcom.2011.01.008.
C l
NC
Br
Br
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24
24
24
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91.2
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