Salicylaldoxime-functionalized poly(ethylene glycol)-bridged dicationic ionic liquid ([salox-PEG1000-DIL][BF4]) as a novel ligand for palladium-catalyzed Suzuki-Miyaura reaction in water

Article abstract of DOI:10.1002/aoc.3038

Two novel salicylaldoxime-functionalized poly(ethylene glycol)-bridged dicationic ionic liquids ([salox-PEG₁₀₀₀-DIL][BF₄] and [salox-PEG₁₀₀₀-DIL][PF₆]) were prepared and characterized. [salox-PEG₁₀₀₀-DIL][BF₄] was found to be an efficient and recyclable ligand for palladium-catalyzed Suzuki-Miyaura reaction in water. The catalytic system could be easily recovered and reused for at least five runs only with slight decrease in its activity. Copyright

Full text of DOI:10.1002/aoc.3038

Full Paper
Received: 20 May 2013
Revised: 29 June 2013
Accepted: 30 June 2013
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3038
Salicylaldoxime-functionalized poly
(ethylene glycol)-bridged dicationic ionic
liquid ([salox-PEG1000-DIL][BF4]) as a novel
ligand for palladium-catalyzed Suzuki–Miyaura
reaction in water
Yinglei Wang, Jun Luo* and Zuliang Liu
Two novel salicylaldoxime-functionalized poly(ethylene glycol)-bridged dicationic ionic liquids ([salox-PEG1000-DIL][BF4] and
[salox-PEG1000-DIL][PF6]) were prepared and characterized. [salox-PEG1000-DIL][BF4] was found to be an efficient and
recyclable ligand for palladium-catalyzed Suzuki–Miyaura reaction in water. The catalytic system could be easily recovered
and reused for at least five runs only with slight decrease in its activity. Copyright © 2013 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: salicylaldoxime; poly(ethylene glycol); functionalized ionic liquid; Suzuki–Miyaura reaction; water
fields.[11] FILs have been used not only as alternative solvents but
also as reagents and/or catalysts in synthetic organic chemistry. Li
Introduction
The palladium-catalyzed Suzuki–Miyaura cross-coupling reaction
of aryl halides with aryl boronic acids is one of the most impor-
tant and powerful methods for the construction of C―C bonds
in organic synthesis.[1] Traditionally, Suzuki–Miyaura reactions
are catalyzed by palladium complexes with phosphine ligands
in organic solvents under an inert atmosphere. However,
phosphines are susceptible to oxidation during the reaction to
yield phosphine oxides and ‘palladium black’, which is catalyti-
cally inactive. At the same time, another main drawback of such
an approach is the difficulty in recovering and reusing the expen-
sive palladium complex. More recently, many coupling reactions
have been carried out with a phosphine-free catalytic system,
and various ligands such as N,N-ligand,[2] N,O-ligand,[3] O,O-ligand[4]
and N-heterocyclic carbenes (NHC)[5] have been employed in the
palladium-catalyzed Suzuki–Miyaura reaction. A large number of
strategies for the Suzuki–Miyaura reaction in water have also been
developed, including the addition of organic co-solvents,[6] the use
of water-soluble catalysts,[7] microwave heating[8] and ultrasonic ir-
radiation.[9] In view of the increasing demand for economical and
environmental benign processes in organic chemistry, it is highly
desirable to develop a versatile, cost-efficient and non-phosphine li-
gand for palladium-catalyzed Suzuki–Miyaura reaction to make the
catalyst recyclable.
and co-workers prepared a series of ionic liquid-supported Schiff
bases and used them as ligands in the Suzuki–Miyaura
reaction.[12] Wang and co-workers reported the use of palladium
nanoparticles supported on amine functional ionic liquid
modified magnetic nanoparticles as a recyclable catalyst for the
Suzuki–Miyaura reaction.[13]
Currently, poly(ethylene glycol)s (PEGs) are employed as
organic polymer-soluble supports for both synthesis and catalyst
immobilization, owing to their main advantages such as low cost,
non-volatility, non-toxicity, thermal stability and easy recyclabil-
ity.[14] Owing to their special chemical and physical properties,
various poly(ethylene glycol)-functionalized ionic liquids (PEG-
ILs) have been designed, synthesized and applied in organic
chemistry.[15] Bhanage and co-workers reported the use of PEG-
functionalized phosphonium salt as an efficient and recyclable
catalyst for the regioselective synthesis of 5-aryl-2-oxazolidinones
from CO2 and aziridines.[16] Liu and co-workers synthesized three
PEG-functionalized imidazolium salts which served as N-hetero-
cyclic carbine precursors for the palladium-catalyzed Suzuki–
Miyaura reaction; however, the catalyst could not be
recovered.[17]
Salicylaldoxime is a versatile ligand in coordination chemistry
and is mainly used for analytical purposes and separation
Ionic liquids (ILs) have attracted considerable attention for
application in organic transformation, owing to their favorable
properties in relation to green chemistry, including high thermal
and chemical stability, excellent electrical conductivity and
negligible vapor pressure.[10] In recent years, functionalized ionic
liquids (FILs), which incorporate functional groups as a part of the
cation and/or anion, have been developed and applied in various
*
Correspondence to: J. Luo, School of Chemical Engineering, Nanjing University
of Science and Technology, Nanjing 210094, China. Email: luojun@njust.edu.cn
School of Chemical Engineering, Nanjing University of Science and Technology,
Nanjing210094, China
Appl. Organometal. Chem. (2013)
Copyright © 2013 John Wiley & Sons, Ltd.

Y. Wang et al.
techniques.[18] More recently, salicylaldoxime has been used as
an efficient N,O-ligand for transition-metal-catalyzed reactions
such as N-arylation,[19] C-arylation reaction[20] and O-arylation.[21]
However, one drawback of those reactions is that the catalyst
could not be efficiently recovered. Recently, Naik and co-workers
prepared metal complexes of salicylaldoxime and salen
containing imidazolium ionic liquids with copper and manga-
nese, and then used these for biphasic catalysis and metal
extraction.[22]
In continuation of our studies on the synthesis and applica-
tions of PEG-bridged dicationic ionic liquids in synthetic organic
chemistry,[23] herein we report the preparation and characteriza-
tion of salicylaldoxime-functionalized PEG-bridged dicationic
ionic liquids ([salox-PEG1000-DIL][X]) (Scheme 1), which have also
been tested as ligands in the palladium-catalyzed Suzuki–
Miyaura reaction.
dropwise and stirred for another 12 h. After removal of the
solvent in vacuum, the residue was dissolved in CH2Cl2 and the
inorganic salts were filtered. The [salox-PEG1000-DIL][X] was finally
obtained after removal of the filtrate under vacuum.
1
[salox-PEG1000-DIL][BF4] (15.1 g, 87%) H NMR (500 MHz, DMSO-
d6) δ: 11.35 (s, 2H, ―N―OH), 10.30 (s, 2H, Ph―OH), 9.16 (s, 2H,
Im―H15,18), 8.29 (s, 2H, Ph―CH―N―OH), 7.77 (t, J= 1.6 Hz, 2H,
Im―H13,17), 7.74 (t, J=1.6Hz, 2H, Im―H14,16), 7.61 (d, J=2.1Hz,
2H, Ph―H2,8), 7.30 (dd, J=8.4, 2.1Hz, 2H, Ph―H6,12), 6.93 (d,
J= 8.4 Hz, 2H, Ph―H5,11), 5.33 (s, 4H, Ph―CH2―Im), 4.38–4.32 (m,
4H, Im―CH2CH2), 3.79–3.75 (m, 4H, Im―CH2CH2), 3.59–3.42 (m,
91H, (OCH2CH2)n); 13C NMR (126 MHz, DMSO-d6) δ: 156.65
(Ph―C4,10), 146.45 (Ph―CH―N―OH), 136.71 (Im―C15,18), 131.27
(Ph―C6,12), 128.10 (Ph―C2,8), 126.02 (Ph―C1,7), 123.59 (Im―C13,17),
122.60 (Im―C14,16), 119.37 (Ph―C3,9), 117.08 (Ph―C5,11), 70.23
((OCH2CH2)n), 70.07 ((OCH2CH2)n), 69.99 ((OCH2CH2)n), 68.52
(Im―CH2CH2), 51.95 (Ph―CH2―Im), 49.38 (Im―CH2CH2); IR (KBr,
cmÀ1): 3147.14, 2870.11, 1624.59, 1562.68, 1497.71, 1452.84,
1349.07, 1271.78, 1054.67, 947.00, 840.64, 804.42, 756.05, 726.25,
672.04, 643.00, 579.79, 541.58; ESI-MS, m/z: 693.4 (M++/2, n= 20),
715.6 (M++/2, n= 21), 737.8 (M++/2, n= 22); Anal. Calcd (%) for
C66H110O25N6B2F8 (1560.06, n= 21): C 50.77, H 7.10, N 5.38; Found:
C 51.08, H 7.54, N 4.97.
Experimental
General Remarks
All of the reagents and solvents are commercially available and
were used without further purification. 1H NMR and 13C NMR
were recorded on Bruker DRX 500 and tetramethylsilane (TMS)
was used as a reference. IR spectra were recorded in KBr disks
with a Shimadzu IR Prestige-21 FT-IR spectrometer. Mass spectra
were taken on an Agilent LC-MS 1100 series instrument in the
electrospray ionization (positive ESI) mode. TGA was performed
on a TGA/SDTA851e thermal analyzer (Mettler Toledo). Samples
were loaded into an aluminium oxide crucible and heated at a
rate of 20 °C minÀ1 from 50 to 600 °C under N2.
[salox-PEG1000-DIL][PF6] (15.6 g, 84%) 1H NMR (500 MHz,
DMSO-d6) δ: 11.37 (s, 2H, ―N―OH), 10.32 (s, 2H, Ph―OH), 9.16
(s, 2H, Im―H15,18), 8.29 (s, 2H, Ph―CH―N―OH), 7.77
(s, 2H, Im―H13,17), 7.74 (s, 2H, Im―H14,16), 7.75 (d, J = 13.9 Hz,
4H, Im―H13,14,16,17), 7.61 (d, J = 2.1 Hz, 2H, Ph―H2,8), 7.31 (dd,
J = 8.4, 2.1 Hz, 2H, Ph―H6,12), 6.93 (d, J = 8.4 Hz, 2H, Ph―H5,11),
5.33 (s, 4H, Ph―CH2―Im), 4.38–4.31 (m, 4H, Im―CH2CH2), 3.78–3.74
(m, 4H, Im―CH2CH2), 3.68–3.36 (m, 104H, (OCH2CH2)n); 13C NMR
(126 MHz, DMSO-d6) δ: 156.67 (Ph―C4,10), 146.48 (Ph―CH―N―OH),
136.71 (Im―C15,18), 131.27 (Ph―C6,12), 128.12 (Ph―C2,8), 126.01
(Ph―C1,7), 123.59 (Im―C13,17), 122.58 (Im―C14,16), 119.36 (Ph―C3,9),
117.07 (Ph―C5,11), 70.23 ((OCH2CH2)n), 70.07 ((OCH2CH2)n), 69.99
((OCH2CH2)n), 68.52 (Im―CH2CH2), 51.94 (Ph―CH2―Im), 49.38
(Im―CH2CH2); IR (KBr, cmÀ1): 3144.79, 3073.17, 2870.60, 1623.91,
1588.26, 1562.34, 1497.25, 1452.34, 1397.00, 1349.67, 1299.09,
1271.51, 1249.25, 1188.81, 1089.65, 1036.28, 950.15, 833.30, 754.78,
739.67, 672.36, 643.37, 616.52, 589.88, 580.54, 556.53, 541.47, 534.93,
526.10; ESI-MS, m/z: 693.6 (M++/2, n= 20), 715.8 (M++/2, n= 21), 737.6
Synthesis of [salox-PEG1000-DIL][BF4] and [salox-PEG1000-DIL][PF6]
To the solution of compound 4 in CH2Cl2 (100 ml) was added
NaBF4 (2.4 equiv., 2.9 g) or KPF6 (2.4 equiv., 4.9 g) and the reaction
mixture was stirred at room temperature for 48 h. After comple-
tion, the resulting mixture was filtered. After removal of the
filtrate under vacuum, methanol (30 ml) was added and hydroxyl-
amine hydrochloride (2.4 equiv., 1.8 g) in methanol (30 ml) was
added dropwise with constant stirring at 40 °C for 12 h. Subse-
quently, NaHCO3 (2.4 equiv., 2.2 g) in water (20 ml) was added
Scheme 1. Synthesis of salicylaldoxime-functionalized PEG-bridged dicationic ionic liquid ([salox-PEG1000-DIL][X]).
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Copyright © 2013 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2013)

Pd(OAc)2/[salox-PEG1000-DIL][BF4]-catalyzed Suzuki reaction in water
(M++/2, n= 22); Anal. Calcd (%) for C66H110O25N6P2F12 (1677.58,
n= 21): C 47.25, H 6.61, N 5.01; Found: C 47.85, H 6.83, N 4.78.
in toluene to afford the compound 4. Two methods for preparing
[salox-PEG1000-DIL][X] were studied. The first method was the classical
approach with anion exchange as the last step. However, this method
suffered from tedious workup and it was almost impossible to purify
the ILs. Alternatively, when the ClÀ of compound 4 was exchanged
with BFÀ4 or PFÀ6 first, and then reacted with hydroxylamine hydrochlo-
ride in the presence of NaHCO3, the target salicylaldoxime-functional-
ized PEG-bridged dicationic ionic liquids could be obtained in high
purity and were characterized by 1H NMR, 13C NMR, infrared and mass
spectrometry. The thermal properties of these dicationic ionic liquids
were determined by TGA. As shown in the TGA figure
(see supporting information), the salicylaldoxime-functionalized
PEG-bridged dicationic ionic liquid with BFÀ4 as anion is more stable
than that with PF-6 anion. In addition, the solubility of [salox-
PEG1000-DIL][X] was determined at room temperature. In general,
these liquids are immiscible with hexane, cyclohexane, petroleum
ether, diethyl ether, toluene, but miscible with dichloromethane,
acetone, methanol, ethanol, DMF and DMSO.
General Procedure for the Suzuki–Miyaura Reaction
Aryl halide (1.0 mmol), aryl boronic acid (1.2 mmol), Pd(OAc)2
(0.005 mmol), Et3N (2 mmol), [salox-PEG1000-DIL][BF4] (0.1 mmol)
and H2O (2 ml) were placed in a tube and sealed. The reaction
mixture was stirred at 100 °C for a certain time. After cooling to
room temperature, the reaction mixture was extracted with
diethyl ether. The combined organic extracts were dried over
anhydrous Na2SO4 and concentrated to give the crude product,
which was purified by column chromatography on silica gel
(200–300 mesh) using ethyl acetate–petroleum ether as eluent.
The products were identified by NMR; all data were compared
with those that reported in the literature.
To evaluate the catalytic efficiency of the catalyst, 4-
bromoacetophenone and phenylboronic acid were first chosen
as the model substrates for Suzuki–Miyaura reaction. Selected
results from our screening experiments are summarized in
Table 1. The salicylaldoxime-functionalized PEG-bridged
dicationic ionic liquid with different anion was first investigated.
It was observed that [salox-PEG1000-DIL][BF4] exhibited much
higher catalytic activity than [salox-PEG1000-DIL][PF6] (Table 1,
entries 1 and 2). Pd black precipitated and low yield was obtained
in the absence of ionic liquid (Table 1, entry 3). The amount of
catalyst loading was then studied. It was observed that 0.5mol%
Results and Discussion
The synthetic pathway for the salicylaldoxime-functionalized
PEG-bridged dicationic ionic liquids are described in Scheme 1.
They are readily prepared through a five-step procedure from
commercially available starting materials and reagents with good
yields. The PEG-1000 dichloride (1) and PEG-1000-bridged
diimidazolium compound (2) were prepared according to our previ-
ous reported method.23a 5-Chloromethyl-2-hydroxybenzaldehyde (3)
was synthesized by a modified synthetic procedure described in the
literature.[24] The quaternization of compounds 2 and 3 was conducted
Table 1. Screening reaction conditions for Pd-catalyzed Suzuki–Miyaura coupling reaction between 4-bromoacetophenone and
a
phenylboronic acid
Entry
[salox-PEG1000-DIL][X]
Pd(OAc)2 (mol%)
Base
t (h)
T (°C)
Yield (%)b
1
[salox-PEG1000-DIL][BF4]
[salox-PEG1000-DIL][PF6]
—
1.0
1.0
1.0
0.5
0.25
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
Et3N
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
9
100
100
100
100
100
100
100
100
100
100
100
100
100
100
90
94
90
2
Et3N
3
Et3N
29
4
[salox-PEG1000-DIL][BF4]
[salox-PEG1000-DIL][BF4]
[salox-PEG1000-DIL][BF4]
[salox-PEG1000-DIL][BF4]
[salox-PEG1000-DIL][BF4]
[salox-PEG1000-DIL][BF4]
[salox-PEG1000-DIL][BF4]
[salox-PEG1000-DIL][BF4]
[salox-PEG1000-DIL][BF4]
[salox-PEG1000-DIL][BF4]
[salox-PEG1000-DIL][BF4]
[salox-PEG1000-DIL][BF4]
[salox-PEG1000-DIL][BF4]
[salox-PEG1000-DIL][BF4]
[salox-PEG1000-DIL][BF4]
Et3N
94
5
Et3N
67
6
—
Trace
56
7
K2CO3
Na2CO3
Cs2CO3
K3PO4
NaOAc
Bu3N
DIPEA
Pyridine
Et3N
8
49
9
65
10
11
12
13
14
15
16
17
18
33
74
90
91
21
62
Et3N
80
48
Et3N
100
100
71
Et3N
6
28
aReaction conditions: 4-bromoacetophenone (1.0 mmol), phenylboronic acid (1.2 mmol), base (2 mmol), [salox-PEG1000-DIL][X] (0.1 mmol),
H2O (2 ml).
bGas chromatographic yield.
Appl. Organometal. Chem. (2013)
Copyright © 2013 John Wiley & Sons, Ltd.
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Y. Wang et al.
Pd(OAc)2 could afford 4-acetylbiphenyl in high yield (Table 1,
entries 4 and 5). Base was an important factor for the coupling
reaction. Only a trace amount of product was detected in the
absence of base (Table 1, entry 6). Among various bases examined,
some organic bases such as triethylamine, tributylamine and
diisopropylethylamine (DIPEA) were all effective for the catalysis,
but pyridine and inorganic bases such as K2CO3, Na2CO3, Cs2CO3,
K3PO4 and NaOAc resulted in lower yields (Table 1, entries 7–14).
Furthermore, low temperature or short time led to lower yields
(Table 1, entries 15–18).
One of the main aims of our study was to investigate the recy-
clability of the catalyst. For this purpose, the recycling of Pd(OAc)
2/[salox-PEG1000-DIL][BF4] was studied using the above model
reaction under optimized conditions. After completion of the
reaction, the reaction mixture was cooled to room temperature
and the product was extracted with diethyl ether. The resultant
residues were reused directly without further treatment for the
next run after being charged with fresh starting materials
(4-bromoacetophenone, 1.0 mmol; phenylboronic acid, 1.2 mmol;
triethylamine, 2.0 mmol). It was found that the Pd(OAc)2/[salox-
PEG1000-DIL][BF4] system could be recycled five times with a
slight decrease in its activity (Fig. 1).
To examine the scope of this method, we investigated the cou-
pling reaction using a variety of aryl halides and aryl boronic acids
under the optimized conditions. The results are summarized in
Table 2. The aryl bromides bearing either electron-withdrawing or
electron-donating groups could couple with phenylboronic acid
efficiently to afford the corresponding products in good to excellent
yields (Table 2, entries 1–12). Furthermore, the catalytic system could
tolerate a wide range of functional groups, including acetyl, halide,
aldehyde, nitro and cyano. The ortho-substituted aryl bromide gener-
ated the corresponding product in lower yield than the meta- and
para-substituted ones due to steric hindrance (Table 2, entries
10–12). Meanwhile, some substituted aryl boronic acids reacted
smoothly with aryl halides and generated the desired products in
high yields (Table 2, entries 13–16). Then, in order to test the
feasibility of this protocol for challenging substrates, we conducted
the coupling of aryl chlorides with phenylboronic acid. However,
poor yields were obtained, even when prolonging the time to 48 h
and elevating the temperature to 120 °C (Table 2, entries 17 and
18). When tetrabutylammonium bromide (TBAB) was added,
the coupling between 4-chloronitrobenzene and phenylboronic
acid could give a high yield. Here, the TBAB may play the role of
phase transfer catalyst or halogen exchange reagent.[25]
Table 2. Suzuki–Miyaura coupling reaction of aryl halide with aryl
boronic acid catalyzed by Pd(OAc)2/[salox-PEG1000-DIL][BF4] in watera
Entry
X
R1
R2
Product
Yield (%)b
1
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
4-CH3CO
4-Cl
H
9a
9b
9c
9d
9e
9f
92
93
94
93
96
91
94
90
89
88
86
80
91
89
86
85
7c
2
H
3
4-CHO
3-CHO
4-NO2
4-CN
H
4
H
5
H
6
H
7
4-CF3
H
H
9g
9h
9i
8
H
9
4-CH3
4-CH3O
3-CH3O
2-CH3O
H
H
10
11
12
13
14
15
16
17
18
H
9j
H
9k
9l
H
4-Cl
4-CH3
4-CH3
4-CH3O
H
9b
9i
H
4-CH3CO
4-CH3
H
9m
9n
9h
9e
4-NO2
H
18c
83d
aReaction conditions: aryl halide (1.0 mmol), aryl boronic acid
(1.2 mmol), Et3N (2 mmol), Pd(OAc)2 (0.005 mmol), [salox-
PEG1000-DIL][BF4] (0.1 mmol), H2O (2 ml), 12 h, 100 °C.
bIsolated yield.
c48 h, 120 °C.
d48 h, 120 °C, 1 mmol TBAB.
Conclusion
In summary, we have prepared two novel salicylaldoxime
functionalized PEG-bridged dicationic ionic liquids ([salox-
PEG1000-DIL][BF4] and [salox-PEG1000-DIL][PF6]), which could be
used as ligands for the palladium-catalyzed Suzuki–Miyaura reac-
tion. Using the Pd(OAc)2/[salox-PEG1000-DIL][BF4] system, a series
of substituted aryl halides could couple with aryl boronic acids to
afford the corresponding products in water with good to excel-
lent yields. Furthermore, the catalytic system could be easily
recycled at least five times without significant loss in activity.
100
80
60
40
20
0
94
93
90
Supporting information
86
81
Supporting information may be found in the online version of
this article.
Acknowledgments
We are grateful for financial support by the National Natural
Science Foundation of China (No. 21002050), the ‘Zijin Star
Project’ and the ‘Excellence Initiative Project’ of NUST.
1
2
3
4
5
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Figure 1. Recycling of Pd(OAc)2/[salox-PEG1000-DIL][BF4] in the Suzuki–
Miyaura coupling reaction of 4-bromoacetophenone and phenylboronic acid.
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Copyright © 2013 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2013)

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