Poly(ethylene glycol)-functionalized imidazolium salts-palladium-catalyzed Suzuki reaction in water

Article abstract of DOI:10.1039/c2gc16486h

Three water-soluble imidazolium salts bearing poly(ethylene glycol) moieties directly attached to an N-atom of imidazole have been synthesized via a simple synthetic method, which could be served as N-heterocyclic carbene precursors for the palladium-catalyzed Suzuki reaction. The catalytic system generated in situ from a source of Pd(OAc)₂, a precursor of imidazolium salt, and a base of triethylamine is able to smoothly perform the Suzuki reaction of a variety of substrates in water. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2gc16486h

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COMMUNICATION
Poly(ethylene glycol)-functionalized imidazolium salts–palladium-catalyzed
Suzuki reaction in water†
Ning Liu, Chun Liu* and Zilin Jin
Received 21st November 2011, Accepted 11th December 2011
DOI: 10.1039/c2gc16486h
enhanced water solubility, easy separation from the products by
decantation, and so on.16 The synthesis of a PEG-decorated
phosphine ligand goes back to the earliest works reported by
Schurig and Bayer in 1976.17 Until the 1990s, Bergbreiter and
co-workers synthesized PEG-bound phosphine and pincer-type
SCS ligands for the rhodium-catalyzed hydrogenation and palla-
dium-catalyzed cross-coupling reaction.18 Since then, a variety
of PEG-functionalized phosphine,19 salen,20 cinchona alkaloid,21
diamino-oligothiophene,22 diphenylethylenedi-amine,23 dipyri-
dyl,14a,24 porphyrin,25 tartrate ester,26 and BINOL27 ligands have
been developed.28 Recently, Uozumi et al. utilized a series of
polystyrene–PEG-supported chiral phosphine ligands for an
aqueous Suzuki reaction with high stereoselectivity.29
The nature of the strong electron-donating abilities of N-
heterocyclic carbene (NHC) ligands facilitates the ligand tighter
binding to the metal, which significantly enhances the catalyst
lifetime and efficiency.30 Thus, NHC ligands have been applied
in versatile catalytic transformations due to their water or air
stability, unique electron-donating abilities and the stability of
the resulting metal complexes.31 The groups of Herrmann,32
Nolan,33 Çetinkaya,34 Y. S. Lee,35 Organ,36 Karimi,37 Glorius,38
Hagiwara,39 H. M. Lee,40 and Kirschning41 have synthesized
numerous NHC ligands for the Suzuki reaction. Since the report
of the first water-soluble sulfonate-functionalized NHC ligand,42
there have been a number of sulfonate-, carboxylate- and
ammonium-functionalized NHC ligands used in the palladium-
catalyzed cross-coupling reactions.43
To date, there are only a few examples of PEG-modified NHC
ligands,11a,43a although PEG moieties are widely used to functio-
nalize phosphine ligands. In 2006, Grubbs and co-workers
reported the first example of a PEG-decorated ruthenium NHC
complex for aqueous olefin metathesis.44 PEG-decorated NHC
ligands have also been used as organocatalysts in the Stetter
reaction and redox esterification.45 Tsuji and co-workers reported
a series of palladium-catalysts bearing tetraethylene glycol and/
or long-chain alkyl groups for the Suzuki reaction.46 As a result,
the palladium-catalysts bearing tetraethylene glycol groups were
apparently more effective than the catalysts with long-chain
alkyl groups. However, to the best of our knowledge, the palla-
dium-catalyzed aqueous Suzuki reaction promoted by the PEG-
bound NHC ligand has not been reported.
Three water-soluble imidazolium salts bearing poly(ethylene
glycol) moieties directly attached to an N-atom of imidazole
have been synthesized via a simple synthetic method, which
could be served as N-heterocyclic carbene precursors for the
palladium-catalyzed Suzuki reaction. The catalytic system
generated in situ from a source of Pd(OAc)2, a precursor of
imidazolium salt, and a base of triethylamine is able to
smoothly perform the Suzuki reaction of a variety of sub-
strates in water.
Introduction
Water is an attractive solvent for chemical reactions for reasons
of low cost, non-flammability, non-toxicity, and environmental
concerns.1 Over the past decade, many organic reactions that
were conventionally believed to occur only in organic solvents
have been successfully performed using water as an environmen-
tally-benign reaction medium.2 Among the developed reactions,
the Suzuki reaction is an excellent example and has strongly
benefited from aqueous chemistry.3 A large number of strategies
for the Suzuki reaction in water have been developed, including
the addition of organic co-solvents4 or surfactants,5 microwave-
heating,1a,6 ultrasonic-irradiation,7 ligand-free methodology,8
and the use of water-soluble catalysts.9 Among them, the appli-
cation of water-soluble ligands/palladium as catalysts in the
aqueous Suzuki reaction has attracted increasing attention, not
just for the sake of enhancing water solubility of the catalyst, but
for facilitating the catalyst recovery by separating the water
phase as well.10 The typical strategy for obtaining a water-
soluble catalyst is to modify a ligand with a water-soluble func-
tional
group,
including
carboxylate,11
sulfonate,4b,12
ammonium,13 and polyoxyethylene groups.14 Since Casalnuo-
vo’s15 initial report of the palladium-catalyzed cross-coupling
reactions in aqueous media catalyzed by TPPMS [sodium 3-
(diphenylphosphino)benzenesulfonate]/Pd(OAc)2, a lot of water-
soluble palladium catalysts have been developed for a range of
cross-coupling reactions.3a,10a
After being functionalized with PEG [poly(ethylene glycol)]
moieties, a catalyst could possess a lot of advantages, such as
State Key Laboratory of Fine Chemicals, Dalian University of
Technology, Linggong Road 2, 116024 Dalian, China.
E-mail: chunliu70@yahoo.com; Tel: + 86 411 84986182
†Electronic supplementary information (ESI) available: See DOI:
10.1039/c2gc16486h
We have a long-standing interest in developing efficient
aqueous catalytic systems for palladium-catalyzed cross-coupling
reactions.47 Encouraged by previous results, we report herein a
new and simple synthetic method for the preparation of three
592 | Green Chem., 2012, 14, 592–597
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Scheme 1 The synthetic path of imidazolium salts 1–3.
new water soluble imidazolium salts containing PEG moieties,
which have been successfully employed in the palladium-cata-
lyzed Suzuki reaction in water. This study gives an opportunity
to disclose the catalytic performance differences of imidazolium
salts containing PEG moieties from other water-soluble groups
functionalized NHC ligand precursors described in the literature
for the palladium-catalyzed Suzuki reaction.
Results and discussion
The synthetic path for the imidazolium salts 1–3 is depicted in
Scheme 1. The MeOPEG550OSO2CH3 was firstly prepared via a
reaction of MeOPEG550OH with methanesulfonyl chloride using
Et3N as a base in toluene. The key step in the synthesis was a
nucleophilic substitution between MeOPEG550OSO2CH3 and
substituted imidazoles to generate the desired imidazolium salts
1–3 under solvent-free conditions.
All imidazolium salts are water-soluble and air-stable. The
imidazolium salts, 1–3, were characterized by means of NMR
spectroscopy and high-resolution mass spectrometry. Fig. 1
shows the mass assignments for the imidazolium cations of 1–3.
As illustrated in Fig. 1, the average separation between the peaks
in the distribution is 44.03 m/z, which is in exact agreement with
the theoretical value of 44.03 m/z for the ethoxyl group repeat
unit. In Fig. 1a, the peaks of imidazolium cation of 1 correspond
very well to the masses expected for polyoxyethylene chains
with an imidazole end group, where the corresponding theoreti-
cal values are 449.2857, 493.3120, 537.3382, 581.3644,
625.3906, 669.4168, 713.4430, 757.4692, 801.4955 and
845.5217 m/z, consistent with polymerization degrees of 8, 9,
10, 11, 12, 13, 14, 15, 16 and 17, respectively.
Fig. 1 Q-TOF mass spectra of imidazolium cations of 1 (a), 2 (b), and
3 (c).
Table 1 The effect of base on the Suzuki reactiona
Entry
Base
Yield (%)b
1
2
3
4
5
6
7
8
KOH
Na2CO3
K3PO4·7H2O
K2CO3
Et3N
30
21
19
10
71
37
7
6
The next investigation is to study the catalytic performance of
the synthesized imidazolium salts as NHC precursors in the pal-
ladium-catalyzed Suzuki reaction in water. It is known that the
nature of the base is an important factor for determining the
efficiency of the Suzuki cross-coupling reaction. Therefore, the
influence of various bases was firstly investigated for the
Pd(OAc)2/1 catalyzed Suzuki coupling reaction of 4-bromoani-
sole with phenylboronic acid in water. The results in Table 1
show that Et3N (Table 1, entry 5) is the best choice as compared
to the other bases. The activity towards cross-coupled product
was decreased when Et3N was replaced with morpholine,
CH3CH2ONa, piperidine, 1,2-diaminocyclohexane, DMEDA (N,
N′-dimethylethanediamine), and TMEDA (N,N,N′,N′-tetramethyl-
ethanediamine) (Table 1, entries 7, 8, 9, 11, 12, and 13). Interest-
ingly, (nPr)3N (Table 1, entry 10) was inefficient as well,
although it is more sterically hindered, indicating that the steric
effect of the base does not play a significant role in this system.
Only DABCO (1,4-diazabicyclo[2.2.2]octane) showed relatively
DABCO
Morpholine
CH3CH2ONa
Piperidine
(nPr)3N
1,2-Diaminocyclohexane
DMEDA
9
trace
trace
no reaction
no reaction
no reaction
10
11
12
13
TMEDA
a Reaction conditions: 0.5 mmol of 4-bromoanisole, 0.75 mmol of
phenylboronic acid, 1 mmol of Et3N, 0.25 mol% Pd(OAc)2, ligand 1/Pd
= 4 : 1 (molar ratio), 100 °C, 5 min, 1 mL H2O. b Isolated yield.
high activity (Table 1, entry 6), which is known as a typical
organic base used in the palladium-catalyzed cross-coupling
reactions. The excellent performance of Et3N (Table 1, entry 5)
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without an inert atmosphere. As aryl-substituted pyridines are
the most common N-heteroaryl units in pharmaceutically active
compounds, we further investigated the Suzuki reaction of 3-
pyridyl bromides with arylboronic acids. As shown in Table 2
(entries 9 and 10), this protocol is applicable to the couplings
between 3-pyridyl bromides and phenylboronic acid. Encour-
aged by such promising results, we next turned our attention to
the Suzuki coupling of aryl chlorides. The activation of 4-chloro-
nitrobenzene required the addition of tetrabutylammoniumbro-
mide (TBAB) as a phase transfer catalyst, an 88% isolated yield
was obtained (Table 2, entry 11). These results showed that the
activity of Pd(OAc)2/3 in the coupling of 4-chloronitrobenzene
is similar with the results recently reported by Godoy and co-
workers.43a However, the Suzuki coupling of 4-chloronitroben-
zene was difficult to proceed in the absence of TBAB under the
same reaction conditions.
Fig. 2 Kinetic profile of Pd(OAc)2/1 (round), 2 (square), and 3 (tri-
angle) catalyzed Suzuki reaction in water. Reaction conditions:
0.5 mmol of 4-bromoanisole, 0.75 mmol of phenylboronic acid, 1 mmol
of Et3N, 0.2 mol% Pd(OAc)2, Ligand/Pd = 4 : 1 (molar ratio), 80 °C,
1 mL H2O.
Conclusion
In conclusion, three new NHC precursors bearing PEG moieties
have been synthesized, which are readily accessible from inex-
pensive and commercial materials. The in situ-generated cata-
lysts from palladium acetate and water-soluble imidazolium salts
1–3 demonstrated excellent catalytic activities towards the
Suzuki reaction of aryl halides bearing a wide range of func-
tional groups, which were varied in their electronic, steric and
dissolution properties, while using water as the sole medium.
Further work to explore the real catalytic species in this reaction
system and to develop more efficient ligands is currently under
investigation in our laboratory.
is supposedly due to Et3N having a much higher tendency to re-
coordinate to the active NHC-Pd species, elongating the lifetime
of active species in solution.48 Among inorganic bases, the stron-
ger base KOH (Table 1, entry 1) gave better results than the
weaker bases (Table 1, entries 2–4).
To evaluate the catalytic performance of the NHC precursors
1–3, a kinetic study of the Suzuki reaction between 4-bromoani-
sole and phenylboronic acid was performed in the presence of
Pd(OAc)2 and Et3N at 80 °C in water. Fig. 2 demonstrates the
kinetic curves of the reaction time vs. the isolated yield using in
situ-generated catalysts from Pd(OAc)2 and imidazolium salts
1–3. As shown in Fig. 2, Pd(OAc)2/3 has a relatively shorter
induction period and a higher catalytic activity than those of
Pd(OAc)2/1 and Pd(OAc)2/2. These results indicate that the
enhancement of the “steric-bulk” of the ligand attached to the
metal center is crucial to the success of the palladium-catalyzed
Suzuki reaction.
Experimental
General
Aryl halides and arylboronic acids were purchased from Alfa
Aesar. Other chemicals were obtained commercially and used
without any prior purification. NMR spectra were recorded on a
Brucker Advance II 400 spectrometer using TMS as internal
standard (400 MHz for H NMR and 100 MHz for 13C NMR).
1
The scope of the catalytic system was explored with a range
of aryl halides and arylboronic acids in the presence of 0.5 mol
% Pd(OAc)2 at 100 °C in 1 mL H2O. As illustrated in Table 2,
the results showed that the Suzuki coupling of aryl bromides
with arylboronic acids took place smoothly to give a nearly
quantitative yield of biaryls (Table 2, entries 1–6). The electronic
nature of substituents bearing aryl bromides is initially investi-
gated. Various 4-substituted aryl bromides bearing either elec-
tron-donating groups or electron-withdrawing groups such as
methoxy, nitro, cyano and acetyl groups, provided the corre-
sponding products in excellent yields in short reaction times
(Table 2, entries 1–5). Furthermore, sterically hindered aryl
bromide was also highly reactive in the reaction system, afford-
ing the desired cross-coupling product in a high yield (Table 2,
entry 6). The hydrophilic aryl bromides with hydroxyl or car-
boxyl groups were successfully coupled with 4-methylphenyl-
boronic acid in excellent yields (Table 2, entries 7 and 8). It is
noteworthy that trace amounts of homocoupling by-product was
observed, although cross-coupling reactions were carried out
Mass spectroscopy data (ESI-MS) of the compounds were col-
lected on a UPLC/Q-TOF mass spectrometer. All products were
isolated by short chromatography on a silica gel (200–300 mesh)
column using petroleum ether (60–90 °C), unless otherwise
noted. Compounds described in the literature were characterized
by 1H NMR spectra compared to reported data.
Synthesis of MeOPEG550OSO2CH3
A solution of methanesulfonyl chloride (4.6 g, 0.04 mol) in dry
toluene (50 mL) was added dropwise to a mixture of
MeOPEG550OH (22 g, 0.04 mol), Et3N (4.1 g, 0.04 mol) in dry
toluene (100 mL) under N2. The mixture was stirred at 0 °C for
24 h and then the white solid by-product was removed by fil-
tration. The filtrate was evaporated under vacuum to afford
MeOPEG550OSO2CH3 (22.9 g, yield 89%) as a colorless liquid.
1H NMR (400 MHz, CDCl3, TMS): δ 4.39–4.36 (m, 2H,
CH2OSO2), 3.78–3.53 (m, 49H, PEG-H), 3.37 (s, 3H,
PEG-OCH3), 3.09 (s, 3H, OSO2CH3) ppm.
594 | Green Chem., 2012, 14, 592–597
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Table 2 Suzuki reaction between aryl halides and aryl boronic acidsa
Entry
1
Aryl halide
Product
Time (min)
5
Yield (%)b
97
2
3
4
5
6
5
96
95
97
96
98
5
10
10
20
7
8
9
10
10
30
96
93
95
10
30
98
88
11c
480
a Reaction conditions: 0.5 mmol of aryl halide, 0.75 mmol of arylboronic acid, 1 mmol of Et3N, 0.5 mol% Pd(OAc)2, ligand 3/Pd = 4 : 1 (molar ratio),
100 °C, 1 mL H2O. b Isolated yield. c 0.75 mmol TBAB, 1 mol% Pd(OAc)2.
Synthesis of imidazolium salts 1, 2, and 3
(NCH2), 39.55 (OSO2CH3), 23.49 (CH2), 10.71 (CH3) ppm; MS
(ESI) m/z [M]+: 433.1993, 477.2220, 521.2362, 565.2465,
609.2604, 653.2875, 697.3015, 741.3147, 785.3329, 829.3502,
873.3594, 917.3660.
A mixture of MeOPEG550OSO2CH3 (1.6 g, 2.5 mmol) and the
respective substituted imidazole (3.0 mmol) was heated in a
sealed tube at 100 °C for 24 h. The resulting solution was iso-
lated by short chromatography on a silica gel (200–300 mesh)
column using CH2Cl2 and CH3OH.
Imidazolium salt 3. Yield: 1.69 g, 88%, a pale yellow liquid.
1H NMR (400 MHz, CDCl3, TMS): δ 10.13 (s, 1H, CHimid),
7.64 (s, 1H, CHimid), 7.25 (s, 1H, CHimid), 4.75 (heptet, J = 8.0
Hz, 1H, NCH), 4.60 (t, J = 4.4 Hz, 2H, NCH2), 3.90-3.53 (m,
47H, PEG–H), 3.38 (s, 3H, PEG–OCH3), 2.80 (s, 3H,
OSO2CH3), 1.61 (d, J = 8.0 Hz, 6H, 2CH3) ppm; 13C NMR: δ
136.51 (Cimid), 123.79 (Cimid), 118.99 (Cimid), 71.86–69.21
(CPEG), 58.96 (OCH3), 53.03 (NCH), 49.54 (NCH2), 39.51
(OSO2CH3), 22.95 (2CH3) ppm; MS (ESI) m/z [M]+: 345.2864,
389.3203, 433.3504, 477.3857, 521.4127, 565.4488, 609.4761,
653.5029, 697.5443, 741.5858, 785.6030, 829.6337, 873.6565,
917.7091, 961.7439.
1
Imidazolium salt 1. Yield: 1.57 g, 87%, a yellow liquid. H
NMR (400 MHz, CDCl3, TMS): δ 9.76 (s, 1H, CHimid), 7.68 (s,
1H, CHimid), 7.41 (s, 1H, CHimid), 4.52 (t, J = 4.0 Hz, 2H,
NCH2), 4.01 (s, 3H, NCH3), 3.88–3.54 (m, 52H, PEG–H), 3.38
(s, 3H, PEG–OCH3), 2.79 (s, 3H, OSO2CH3) ppm; 13C NMR: δ
137.95 (Cimid), 123.55 (Cimid), 122.92 (Cimid), 71.86–69.06
(CPEG), 58.96 (OCH3), 49.51 (CH2), 39.53 (OSO2CH3), 36.24
(NCH3) ppm; MS (ESI) m/z [M]+: 449.2745, 493.2917,
537.3089, 581.3403, 625.3712, 669.3821, 713.4109, 757.4518,
801.4578, 845.5190, 889.5416.
Imidazolium salt 2. Yield: 1.22 g, 65%, a pale yellow liquid.
1H NMR (400 MHz, CDCl3, TMS): δ 9.86 (s, 1H, CHimid), 7.69
(s, 1H, CHimid), 7.42 (s, 1H, CHimid), 4.55 (t, J = 4.4 Hz, 2H,
NCH2), 4.24 (t, J = 8.0 Hz, 2H, NCH2), 3.89–3.54 (m, 49H,
PEG–H), 3.38 (s, 3H, PEG–OCH3), 2.77 (s, 3H, OSO2CH3),
1.95 (sextet, J = 8.0 Hz, 2H, CH2), 0.99 (t, J = 8.0 Hz, 3H, CH3)
ppm; 13C NMR: δ 137.66 (Cimid), 123.67 (Cimid), 121.31 (Cimid),
71.87–69.17 (CPEG), 58.97 (OCH3), 51.27 (NCH2), 49.51
General procedure for the Suzuki reactions
All Suzuki reactions were carried out without an inert atmos-
phere. A mixture of aryl halide (0.5 mmol), arylboronic acid
(0.75 mmol), Et3N (101 mg, 1 mmol), Pd(OAc)2 (0.56 mg,
0.0025 mmol), and imidazolium salts (7.2 mg, 0.01 mmol) in
H2O (1 mL) was allowed to react in a sealed tube at 100 °C. The
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activation of 4-chloronitrobenzene required the addition of
TBAB (0.75 mmol) as a phase transfer catalyst. The reaction
mixtures was added to brine (15 mL) and extracted three times
with ethyl acetate (3 × 15 mL). The solvent was concentrated
under vacuum and the product was isolated by short chromato-
graphy on a silica gel (200–300 mesh) column.
Acknowledgements
The authors thank the financial support from the National
Natural Science Foundation of China (20976024, 21076034,
20923006), the Fundamental Research Funds for the Central
Universities (DUT11LK15), Special Research Foundation of
Dalian University of Technology for Retired Professors (Z. Jin:
DUTTX2011103), and the Ministry of Education (the Program
for New Century Excellent Talents in University).
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