Complex of [BMIm] PF6 with PEG1000: A high efficient and recycle system for palladium-catalyzed Suzuki cross-coupling and Heck reaction

Article abstract of DOI:10.1002/aoc.2860

A complex of [BMIm] PF₆ (1-Butyl-3-methylimidazolium hexafluorophosphate) with PEG₁₀₀₀ has been successfully applied and found to be efficient for Suzuki cross-coupling and Heck reaction using Pd(OAc)₂ as the catalyst under air atmosphere. Moreover, the isolation of the products is readily performed by the extraction of diethyl ether, and the catalytic system reported here can be recycled and reused several times. Copyright

Full text of DOI:10.1002/aoc.2860

Full Paper
Received: 8 January 2012
Revised: 12 March 2012
Accepted: 12 March 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.2860
Complex of [BMIm] PF6 with PEG1000: a high
efficient and recycle system for palladium-
catalyzed Suzuki cross-coupling and Heck
reaction
Xiang Liu, Yang Mao and Ming Lu*
A complex of [BMIm] PF6 (1-Butyl-3-methylimidazolium hexafluorophosphate) with PEG1000 has been successfully applied and
found to be efficient for Suzuki cross-coupling and Heck reaction using Pd(OAc)2 as the catalyst under air atmosphere. More-
over, the isolation of the products is readily performed by the extraction of diethyl ether, and the catalytic system reported
here can be recycled and reused several times. Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: complex; [BMIm] PF6; PEG1000; Suzuki cross-coupling; Heck reaction
results showed that these complexes could have potential
applications in catalysis, especially PEG-ILs. In this communica-
Introduction
In the past years, C&bond;C cross-coupling catalyzed by expensive
transition metal palladium complex along with ligands in organic
solvents was applied widely in the pharmaceutical and fine chemical
industries.[1–3] Unfortunately, the problem of homogeneous catalysis
is the difficulty in separating the costly catalyst from the reaction
mixture and the impossibility of reusing it in consecutive reactions.
The development of heterogeneous catalysts would be an attractive
solution to this problem owing to their easy separation and facile
recycling.[4–8] However, most of the heterogeneous catalysts exhibit
lower activity than their homogeneous counterparts. Considerable
research efforts have been devoted to the development of efficient
catalytic systems possessing the respective advantages of homoge-
neous and heterogeneous catalysis. Ionic liquids (ILs) have paved
the way for alternative and new synthetic strategies, because this
methodology allows not only high reaction activity but also the
possibility of recycling the catalyst.[9–13] Concerning C&bond;C
cross-coupling reactions carried out in ionic liquids consisting of
1,3-dialkylimidazolium cations, some studies have already been
reported.[14–16] Regrettably, however, these processes suffer from
some limitations: (i) the need for the use of toxic, expensive and
air-sensitive phosphine; [17–19] and (ii) performing abnormal method-
ology, such as ultrasound or microwave.[20–22] These limits are of
significant concern in large-scale organic syntheses. Moreover, this
reaction system requires a large amount of expensive ionic liquid.
It would be desirable to reduce the amount of ionic liquid phase
from an economic and toxicological viewpoint.[23–25] As a result,
the search for new readily available IL catalytic systems addressing
these drawbacks remains an impending goal.
tion, we would like to present a complex of [BMIm] PF6 with
PEG1000 (Scheme 1) for C&bond;C cross-coupling reactions. To
the best our knowledge, there is no report on the utility of this
complex, including metal palladium-catalyzed coupling reaction.
On the basis of pioneering work,[29] we explored the catalytic role
of the complex in the palladium-catalyzed Suzuki reaction and
Heck reaction. To our delight, this system exhibited excellent
catalytic activity and stability in the presence of Pd(OAc)2. In
addition, the desired products were obtained simply by extrac-
tion and the residual could be reused several times.
Results and Discussion
Using the complex of PEG1000–[BMIm] PF6 as the catalytic system
in the presence of inexpensive and convenient Pd(OAc)2, we
screened a series of general Pd-catalyzed C&bond;C bond-
forming reactions with the aim of studying the activity and
reusability of the system. Preliminary reactions were carried out
respectively in [BMIm] PF6, PEG1000 and PEG1000–[BMIm] PF6.
These results demonstrated that the Suzuki reaction (Table 1,
entries 10–11) are sluggish in the pure ionic liquid or PEG1000
under the same reaction condition. However, the use of
PEG1000–[BMIm] PF6 as the reaction medium furnished the best
yield of desired product in the coupling reaction.
In fact, PEG1000 plays a very important part in the coupling
reaction. On the one hand, used as the solvent, PEG1000 promotes
Polyethylene glycols (PEGs), as inexpensive, thermally stable,
recoverable, and non-toxic media for catalysts, have been attract-
ing increasing interest.[26,27] Recently, PEGs were chosen to form
complexes with metal cations, organocatalyst or ILs for some
reactions such as asymmetric Michael addition reactions.[28] The
*
Correspondence to: Ming Lu, College of Chemical Engineering, Nanjing University
of Science and Technology, Nanjing, Jiangsu, China. E-mail: luming302@126.com
College of Chemical Engineering, Nanjing University of Science and
Technology, Nanjing, Jiangsu, China
Appl. Organometal. Chem. 2012, 26, 305–309
Copyright © 2012 John Wiley & Sons, Ltd.

X. Liu et al.
as a phase-transfer catalyst. The use of palladium complexes
along with N-heterocyclic carbene as catalysts for C&bond;
C cross-coupling reactions is well known and the formation of
Pd-carbene species from the 1,3-disubstituted imidazolium salts
in situ under basic conditions has been demonstrated, which
are speculated to have implications on the success of the
catalytic reaction in this kind of IL. Firm evidence for the in situ
formation of a mixed palladium complex with [BMIm] BF4 at
room temperature had been reported by Mathews in studies of
the Suzuki reaction in ILs. Therefore, it is believed that the
PEG1000–[BMIm] PF6 catalytic system could enhance the coupling
rate and the yield of desired product.[30–32] It is noteworthy that
PEG1000–[BMIm] PF6 is not simply a mixture but a complex
formed by an ion–dipole interaction. Furthermore, the formation
of 1:1 self-assembled complex provided the stable structure
detected by Luo and his co-workers.[29]
HO
O
N
O
O
O
O
N
n-C4H9 PF6
O
O
O
HO
n
Scheme 1. Structure of PEG1000–[BMIm] PF6
ligand-free Suzuki–Miyaura reactions through reduction of Pd(II)
to Pd(0), obviating the need for phosphine reducing agents. On
the other hand, an excess of PEG could play a positive role in
stabilizing Pd(0). In addition, the far higher efficiency of
PEG1000–[BMIm] PF6 system over [BMIm] PF6 ionic liquid alone
can probably be attributed in part to the properties of PEG1000
From Table 1 (entries 12–13), it is clear that the efficiency of the
complex was influenced by the nature of the anion. For example,
the Suzuki reaction in PEG1000–[BMIm] Br or PEG1000–[BMIm] BF4
always afforded low conversion. Optimization studies were also
Table 1. Suzuki reaction of p-methoxybenzeneboronic acid with bromobenzene catalyzed by Pd(OAc)2 in given mediaa
Entry
1
Media
Base
Temperature / oC
Conversion b / %
Time / h
3.0
Yield c/ %
50
PEG1000-[BMIm] PF6
PEG1000-[BMIm] PF6
PEG1000-[BMIm] PF6
PEG1000-[BMIm] PF6
PEG1000-[BMIm] PF6
PEG1000-[BMIm] PF6
PEG1000-[BMIm] PF6
PEG1000-[BMIm] PF6
PEG1000-[BMIm] PF6
[BMIm] PF6
NaOAc
KOH
80
80
80
80
RT
60
110
80
80
80
80
80
80
60
75
92
100
10
85
94
78
96
40
81
88
60
2
3.0
70
3
K2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
3.0
85
4
3.0
96
5
3.0
trace
81
6
3.0
7
3.0
90
8
3.0
73d
92e
36f
9
3.0
10
11
12
13
3.0
PEG1000
3.0
65f
PEG1000-[BMIm]BF4
PEG1000-[BMIm]Br
3.0
80
3.0
55
a Reaction condition: 1.0 mmol of bromobenzene, 1.2 mmol of p-methoxybenzeneboronic acid, 2.0 mmol of base, 2.0 g of
media, 5 mmol% of Pd (OAc)2.
b Determined by GC analysis, therein biphenyl as an internal standard.
c Isolated yield.
d 10 mmol% of Pd(OAc)2.
e 30 mmol% of Pd(OAc)2.
f Cross-coupling reaction in [BMIm] PF6 or PEG1000 were carried out under the same condition.
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Appl. Organometal. Chem. 2012, 26, 305–309

Complex of [BMIm] PF6 with PEG1000 for Suzuki and Heck reaction
performed to determine how bases, temperature and the amount
of Pd(OAc)2 affect the coupling reaction. As demonstrated in
Table 1, the best yield was not obtained until the Suzuki reaction
was carried out at 80 ꢀC in the PEG1000–[BMIm] PF6 system using
Cs2CO3 as base and 5 mmol% Pd(OAc)2.
KOH, Na2CO3 and K2CO3 were less effective. Many aryl iodides
including electron-poor and electron-rich substrates could react
with terminal olefins within a much shorter time (approximately
2–3h)[8,20,33,34] and the yields of corresponding coupling products
were very satisfactory (Table 3, entries 1–9). It was worth noting
that the reaction can be conducted under mild conditions
(60 ꢀC) without the use of higher temperature or microware
previously reported. Encouraged by these results, we tried to
study the reaction activity between substituted aryl bromides
and terminal olefins. Compared to aryl iodides, it required a
longer reaction time and a higher amount of palladium acetate
to complete the Heck reaction. Furthermore, the results in Table 3
show that the electronic nature of the aryl bromides have a clear
effect on the coupling reactions. For example, the coupling
reaction of electron-rich 4-methyl-bromobenzene with methyl
acrylate provided 64% isolated yield (Table 3, entry 12), but for
electron-deficient 4-nitro-bromobenzene the desired coupled
product was obtained in 82% isolated yield (Table 3, entry 10).
Having established the scope of the new reaction system,
our attention was focused on the recycling of the system. To
demonstrate this issue, recycling experiments were conducted
for the Suzuki cross-coupling reaction of bromobenzene with
p-methoxybenzeneboronic acid, and for the Heck reaction of
iodobenzene and methyl acrylate. After the product was isolated
With the best conditions to hand, we determined to explore the
scope of Suzuki cross-coupling in PEG1000–[BMIm] PF6 with various
substrates, including aryl bromides and chlorides with electron-
withdrawing and electron-donating substituents. The results, given
in Table 2, indicate that PEG1000–[BMIm] PF6 in the presence of
palladium acetate without any ligands is among themost efficient
systems for C&bond;C cross-coupling ever reported. As expected, aryl
iodides and bromides give comparatively higher yields than chlorides,
and there is a beneficial influence of the presence of electron-
withdrawing substituents on the reaction product yield. These
features are in agreement with the general behavior of Pd catalysts.
To extend the application scope of the catalytic system, we
carried out another classic palladium-catalyzed C&bond;C cross-
coupling, namely the Heck reaction of aryl halides with terminal
olefins. Initially, we explored the conditions for the Heck reaction
by using iodobenzene and methyl acrylate (Table 3, entry 4) as
reactants in the Pd(OAc)2–PEG1000-[BMIm] PF6 catalytic system.
The results demonstrated that Et3N as the base was the best
choice for the cross-coupling reaction and other bases such as
a
Table 2. Suzuki coupling reaction of various aryl halides with p-methoxybenzeneboronic acid by Pd(OAc)2 in PEG1000-[BMIm] PF6
Entry
X
I
R
Conversion b / %
Time / h
2.0
Yield c/ %
98
100
100
100
99
1
2
H
2.0
97
I
4-CH3
4-NO2
4-OCH3
4-CH3
4-NO2
H
2.0
98
3
I
3.0
95
4
Br
Br
Br
Cl
Cl
Cl
Cl
100
100
60
3.0
95
5
3.0
96
6
8.0
55d
70 d
75 d
80 d
7
80
8.0
8
2-NO2
4-NO2
2,4-NO2
82
8.0
9
87
8.0
10
a
Reaction condition: 1.0 m mol of ArX, 1.2 mmol of
p-methoxybenzeneboronic acid , 2.0 mmol of Cs2CO3, 2.0 g of
PEG1000-[BMIm] PF6, 5 mmol% of Pd(OAc)2 , 80 oC (external).
b Determined by GC analysis, therein biphenyl as an internal standard.
c Isolated yield.
d 30 mmol% of Pd(OAc)2 .
Appl. Organometal. Chem. 2012, 26, 305–309
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X. Liu et al.
a
Table 3. Heck coupling reaction with various aryl halides with terminal olefins by Pd(OAc)2 in PEG1000-[BMIm] PF6
Entry
1
R1
4-NO2
4-NO2
4-NO2
H
R2
X
I
Time / h
Yield c / %
COOMe
COOEt
2
2
99
99
99
98
97
97
93
91
90
82b
70b
64b
2
I
3
COOn-Bu
COOMe
COOEt
I
2
I
2
4
I
2
5
H
COOn-Bu
COOMe
COOEt
I
2
6
H
I
3
7
4-CH3
4-CH3
4-CH3
4-NO2
H
I
3
8
COOn-Bu
COOMe
COOEt
I
3
9
Br
Br
Br
6
10
11
12
10
24
COOn-Bu
4-CH3
a Reaction condition: 1.0 m mol of ArX, 2.0 mmol of terminal olefins, 2.0 mmol of Et3N, 2.0 g of PEG-[BMIm] PF6, 5 mmol%
of Pd(OAc)2, 60 oC (external).
b 20 mmol% of Pd(OAc)2 , 110 oC(external).
c Isolated yield.
be decreased rapidly from the fifth run. With respect to recycling
in successive batch operations of the Heck reaction, the results
shown in Figure 1 show that catalytic activity was maintained
very well even after six cycles.
In summary, we have presented an active, air-stable and
recyclable approach for ligand-free Suzuki and Heck coupling
reactions using inexpensive Pd(OAc)2 as the palladium catalyst in
PEG1000–[BMIm] PF6 media. For the Suzuki reaction, a number of aryl
halides, including aryl bromides and chlorides, were coupled to
arylboronic acids smoothly and efficiently. Furthermore, the present
Pd(OAc)2–PEG1000–[BMIm] PF6 system were also desired for the
Heck reaction of aryl iodides and aryl bromides with terminal
olefins. Other excellent properties are simple separation of wanted
products and outstanding recyclability of the catalytic system.
Figure 1. Recycling of Pd(OAc)2–PEG–[BMIm] PF6 catalytic system. –●–,
Experimental
GC yield of Suzuki reaction between bromobenzene with p-methoxyben-
zeneboronic acid in every cycle; –■–, GC yield of Heck reaction between
iodobenzene and methyl acrylate in every cycle
General Information
All chemicals were from commercial sources without any
from the reaction mixture by ether (4 Â 15 ml), the solidified Pd
(OAc)2–PEG1000–[BMIm] PF6 was subjected to a second run by
charging with the same substrates without any regeneration or
addition of Pd(OAc)2. As demonstrated in Fig. 1, the reusability
of Pd(OAc)2–PEG1000–[BMIm] PF6 for the Suzuki reaction would
pretreatment. All reagents were of analytical grade. 1H NMR
spectra were recorded on a Bruker 500 MHz spectrometer with
tetramethylsilane (TMS) as an internal standard. Melting points
were recorded on a Buchi R-535 apparatus and are uncorrected.
GC analysis was performed on an Agilent GC-6820 gas
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Appl. Organometal. Chem. 2012, 26, 305–309

Complex of [BMIm] PF6 with PEG1000 for Suzuki and Heck reaction
(E)-Methyl 3-(4-nitrophenyl) acrylate
chromatograph equipped with a 30 m  0.32 mm  0.5 mm HP-
Innowax capillary column and a flame ionization detector. GC-
MS analyses were performed on a Saturn 2000 GC-MS instrument.
Rf = 0.4 (petroleum ether/ethyl acetate = 20:1, v/v). Chromatography
solvent petroleum ether/ethyl acetate = 50/1, v/v). 1H NMR
(500 MHz, CDCl3): s = 8.23–8.25 (m, 2H), 7.66–7.73 (m, 3H), 6.55 (d,
J = 16.1 Hz, 1H), 3.83 (s, 3H).
Preparation of PEG1000–[BMIm] PF6
[BMIm] PF6 (10.0 mmol) was added to a 100 ml methanol contain-
ing PEG1000 (10.0 mmol) and stirred at room temperature for
10 min. The solution was concentrated under vacuum to give
the desired product.
(E)-Ethyl 3-(4-nitrophenyl)acrylate
Rf = 0.4 (petroleum ether/ethyl acetate = 20:1, v/v). Chromatography
solvent petroleum ether/ethyl acetate = 50/1, v/v). M.p. 134.9–136.1 ꢀC;
1H NMR (500 MHz, DMSO): s =8.29–8.32 (m, 2H), 7.24–8.16 (m, 3H),
6.95 (d, J= 16.0 Hz, 1H), 4.25–4.30 (m, 2H), 1.33 (t, J= 7.1 Hz, 3H).
General Procedure for Suzuki Cross-Coupling Reaction
Under air atmosphere, a flask was charged with aryl halides
(1.0 mmol), p-methoxybenzeneboronic acid (1.2 mmol), Cs2CO3
(2.0 mmol), Pd(OAc)2 (5 mmol%) and PEG1000–[BMIm] PF6 (2.0 g).
The mixture was heated to 80 ꢀC for the indicated time. After
the reaction solution was cooled to room temperature, the
mixture was extracted four times with ether (4 Â 15 ml). The
combined ether phase was analyzed by GC-MS and then concen-
trated. Further purification of the product was achieved by flash
chromatography on a silica gel column.
Supporting Information
Supporting information may be found in the online version of
this article.
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