A simple and efficient protocol for a palladium-catalyzed ligand-free Suzuki reaction at room temperature in aqueous DMF

Article abstract of DOI:10.1039/c0gc00176g

A convenient, effective and mild protocol has been developed for the palladium-catalyzed ligand-free Suzuki reaction of aryl bromides with arylboronic acids in aqueous N,N-dimethylformamide (DMF) in the presence of K₂CO₃ and a catalytic amount of PdCl₂ in air at room temperature. It is noteworthy that the volume ratio of water-DMF and base play important roles in the reaction, and various functional groups are tolerated under the optimized conditions. Furthermore, this protocol could be extended to the cross-couplings of nitrogen-based heteroaryl halides with arylboronic acids in moderate to excellent yields.

Full text of DOI:10.1039/c0gc00176g

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PAPER
A simple and efficient protocol for a palladium-catalyzed ligand-free Suzuki
reaction at room temperature in aqueous DMF†
Chun Liu,* Qijian Ni, Fanying Bao and Jieshan Qiu
Received 4th June 2010, Accepted 15th February 2011
DOI: 10.1039/c0gc00176g
A convenient, effective and mild protocol has been developed for the palladium-catalyzed
ligand-free Suzuki reaction of aryl bromides with arylboronic acids in aqueous
N,N-dimethylformamide (DMF) in the presence of K2CO3 and a catalytic amount of PdCl2 in air
at room temperature. It is noteworthy that the volume ratio of water–DMF and base play
important roles in the reaction, and various functional groups are tolerated under the optimized
conditions. Furthermore, this protocol could be extended to the cross-couplings of nitrogen-based
heteroaryl halides with arylboronic acids in moderate to excellent yields.
Sajiki and co-workers have described a Pd/C-catalyzed ligand-
Introduction
free system for the Suzuki reaction in a mixture of water
The palladium-catalyzed Suzuki cross-coupling reaction of
and ethanol at room temperature.26 However, the procedures
aryl halides with arylboronic acids has become an extremely
important method for the construction of carbon–carbon bonds,
mentioned above were performed either with heating or over a
long reaction time. Very recently, Del Zotto et al. developed a
in particular for the formation of biaryls, due to the broad
fast protocol for the Suzuki reaction in aqueous ethylene glycol
functional group tolerance and the low toxicity associated with
monomethyl ether, which is the only example of a highly efficient
boron compounds.1,2 The general palladium-catalyzed Suzuki
and ligand-free Suzuki reaction in air at room temperature.27
reaction employs phosphine ligands, which are often water-
There is still considerable room for the development of simple
and/or air-sensitive.3–6 Although some modified ligands could
and efficient reaction systems for such transformations.
work in water, and even in air, the tedious multi-step synthesis
Water–DMF mixtures are widely used as solvents in organic
and high cost of these ligands restrict their applications.7–11 Nu-
reactions.28–33 Artok and co-workers34,35 have employed aqueous
merous attempts have been made to develop ligand-free catalytic
DMF for the Pd(II)-NaY zeolite-catalyzed Suzuki reaction of
systems for the Suzuki reaction.12 However, most systems are
aryl halides with arylboronic acids using a high palladium
operated with the aid of additives, such as surfactants13–15 or
loading for a long reaction time at room temperature. We
polymers.12
report herein that a fast and efficient Suzuki reaction of aryl
With consideration of economy, safety and environment
bromides with arylboronic acids can be achieved in the presence
problems, the use of water as a solvent for palladium-catalyzed
of PdCl2, K2CO3 and aqueous DMF at room temperature in
coupling reactions such as Ullman, Suzuki, Hiyama and Stille
couplings in air was first examined by Li and co-workers.16 The
study of Suzuki reactions in water or aqueous–organic mixtures
has received much attention since then.17–23 Due to the ability
to dissolve bases in water for activating arylboronic acids, the
air. Moreover, this simple catalytic system can be applied to the
cross-couplings of N-heteroaryl halides with arylboronic acids
at room temperature after replacing K2CO3 with K3PO4·7H2O.
reaction proceeds much faster in an aqueous medium. Zhang
et al. reported that the palladium-catalyzed Suzuki reaction
Results and discussion
of aryl bromides with arylboronic acids occurred smoothly in
aqueous acetone24 or PEG/H2O25 in the absence of a ligand.
Optimization of reaction conditions
As reported in the literature, a suitable amount of water
is very important for improving the reactivity of Suzuki
reactions.17,25 We initially studied the impact of the volume
ratio of water–DMF on the Suzuki reaction. The coupling of 4-
bromoanisole (0.5 mmol) and phenylboronic acid (0.75 mmol)
was chosen as the model reaction in 4 mL water–DMF with
PdCl2 (0.0025 mmol, 0.44 mg) and K2CO3 (1 mmol) at room
temperature in air. No coupling took place when just DMF was
State Key Laboratory of Fine Chemicals, Dalian University of
Technology, Linggong Road 2, 116024, Dalian, China.
E-mail: chunliu70@yahoo.com; Fax: +86 411-84986182; Tel: +86
411-84986182
† Electronic supplementary information (ESI) available: Experimental
detail and characterization of cross-coupling products. See DOI:
10.1039/c0gc00176g
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3 min, resulting in a high TOF of 4000 h-1 (Table 2, entry
Table 1 Optimization of the reaction conditions for the Suzuki reaction
of 4-bromoanisole with phenylboronic acida
3). However, substituted arylboronic acids containing electron-
withdrawing groups showed a slightly lower reactivity than those
containing electron-donating or neutral groups, due to the effect
of these substituents on the nucleophilicity on the arylboronic
acid. Therefore, 4-fluoro-phenylboronic acid completed the
reaction in a longer time than arylboronic acids with electron-
donating groups (Table 2, entries 5–7 vs. entries 9–11). Besides,
the protocol tolerated the presence of functional groups in
the ortho position of the aryl bromide and arylboronic acid.
In comparison to the non-sterically-hindered substrates, the
ortho-substituted substrates were coupled in good to excellent
yields in almost the same reaction time (Table 2, entries 12–
15), even sometimes in a shorter reaction time (Table 2, entry
17). The most challenging combination was the cross-coupling
of 2-bromoanisole with 2-methoxy-phenylboronic acid, which
gave a quantitative yield in 10 min (Table 2, entry 18). To the
best of our knowledge, this is the most effective protocol for
the synthesis of such products.40,41 Overall, because of the high
activity of this catalytic system, both steric and electronic effects
made only a small impact on the reaction rate. Thus, the scope
of this protocol is quite broad.
As far as we know, the ligand-free Suzuki reaction of
heterocycle-containing substrates usually proceeds with addi-
tional heating due to its low reactivity.42–44 Only one paper,27 to
the best of our knowledge, has reported the cross-coupling of N-
heteroaryl halides with arylboronic acids at room temperature
in aqueous media. However, the limitation was that only a
moderate yield of the target product was obtained when 2-
bromopyridine reacted with phenylboronic acid. The present
protocol was also extended to the synthesis of nitrogen-based
heterobiaryls. The results are summarized in Table 3. Noticeably,
the Suzuki reaction of 2-bromopyridine with phenylboronic acid
using K3PO4·7H2O as the base was more effective than using
K2CO3 under the same conditions (Table 3, entries 1 and 2),
affording the product in 81% yield. Thus, the coupling reactions
of arylboronic acids containing electron-donating groups, such
as methyl or methoxy, afforded a slightly higher yield than
that of phenylboronic acid using a PdCl2 loading of 1.5 mol%
(0.0075 mmol, 1.32 mg; Table 3, entries 3 and 4), while an
arylboronic acid bearing electronic-withdrawing groups, such
as a fluoro group, merely converted in a moderate yield (Table 3,
entry 5). It is worth noting that the coupling of 5-bromo-2-
methoxypyridine with phenylboronic acid gave the product in
98% yield. Compared with the reported methods,45,46 this is the
most efficient catalytic system for such a transformation. In
addition, the electronic effect had little influence on the reactivity
of the arylboronic acid (Table 3, entry 6). Both electron-rich
and electron-deficient arylboronic acids completed the reaction
in quantitative yields within 1 h (Table 3, entries 7 and 8).
However, the reaction between 5-bromo-2-methoxypyridine and
2-methylphenylboronic acid gave only 52% yield in 3 h due to the
effect of steric hindrance (Table 3, entry 9). Another noteworthy
result was that the couplings of 2-chloropyrazine with 4-
methylphenyl boronic acid or phenylboronic acid afforded high
yields (Table 3, entries 10 and 11) after a prolonged period.
To the best of our knowledge, this is the first example where a
Suzuki reaction of an N-heteroaryl chloride could be performed
smoothly in an aqueous medium at room temperature in air.
Entry
[Pd]
Base
Time/min
Yield (%)b
1
2c
3
4
5
6
7
8
9
PdCl2
PdCl2
Pd(OAc)2
Pd/C
Pd2(dba)3
PdCl2
PdCl2
PdCl2
PdCl2
K2CO3
K2CO3
K2CO3
K2CO3
5
120
5
5
5
5
5
5
5
99
84
94
11
trace
18
17
16
20
K2CO3
K3PO4·7H2O
NaOH
CH3ONa
KF·2H2O
a Reaction conditions: 4-bromoanisole (0.5 mmol), phenylboronic acid
(0.75 mmol), [Pd] (0.0025 mmol), base (1 mmol), DMF–H2O (2 mL/2
mL), room temperature, in air. The reaction was monitored by TLC.
b Isolated yield. c PdCl2 (0.0005 mmol, 0.088 mg).
used as the solvent. Upon changing the volume ratio of water to
DMF from 3 : 1 to 2 : 1, the yield of the product increased from 31
to 81%. A quantitative coupling yield (99%) was obtained when
the volume ratio of water–DMF reached 1 : 1. Upon further
increasing the water content, the yield dropped sharply to trace
in neat water. Hence, the ratio 1 : 1 was the best choice, combined
with the maximum water content.
The next investigation in this study was to optimize the
conditions of the Suzuki coupling reaction in terms of the
palladium species and bases in the same model reaction (see
Table 1). The results indicate that PdCl2 (Table 1, entry 1) and
Pd(OAc)2 (Table 1, entry 3) have a preferable catalytic activity,
while the zero-valent pre-catalysts Pd/C (Table 1, entry 4) and
Pd2(dba)3 (Table 1, entry 5) showed a rather poor catalytic
activity under the same reaction conditions. The reason for this
might be that both Pd/C and Pd2(dba)3 are not activated at room
temperature, which is consistent with our reported results.23,36
Besides, the amount of PdCl2 could be decreased even down to
0.0005 mmol, giving an isolated yield of 84% (Table 1, entry 2).
Further studies focused on the effect of bases. The experi-
mental data presented in Table 1, entries 1, 6–9 showed that
K2CO3 gave the highest yield (99%) in 5 min, resulting in a
turnover frequency (TOF) of up to 2400 h-1 (Table 1, entry
1). However, the other bases, such as K3PO4·7H2O, NaOH,
CH3ONa and KF·2H2O, which are commonly used in Suzuki
reactions,37–39 gave disappointing results (<20%). The results
reveal that K2CO3 was the exclusive base for activating this
catalytic system.
Scope and limitations of substrates
We then explored the scope and limitations of substrates for
this protocol under the optimized conditions. As shown in
Table 2, in general, all the reactions gave biaryl derivatives
in high yield. Various 4-substituted aryl bromides bearing
either electron-donating or electron-withdrawing groups, such as
methyl, methoxy, cyano and acetyl, provided the corresponding
products in good to excellent yields (Table 2, entries 1–4). For
example, 4-bromobenzonitrile afforded a quantitative yield in
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Table 2 Suzuki reactions of aryl bromides with arylboronic acidsa
Entry
1
Ar–Br
R
H
Product
Time/min
5
Yield (%)b
99
2
3
4
H
H
H
15
3
85
99
99
10
5
6
4-CH3
3-CH3
6
6
94
99
7
4-OCH3
4-OCH3
4-F
15
25
25
14
99
97
99
99
8
9
10
4-F
11
12
4-CHO
H
120
15
76
72
13
H
6
97
14
15
16
H
6
99
99
99
4-CH3
2-CH3
5
12
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Table 2 (Contd.)
Entry
17
Ar–Br
R
Product
Time/min
5
Yield (%)b
2-OCH3
99
18
2-OCH3
10
99
a Reaction conditions: aryl bromides (0.5 mmol), arylboronic acids (0.75 mmol), K2CO3 (1 mmol), PdCl2 (0.0025 mmol, 0.44 mg), DMF–H2O
(2 mL/2 mL), room temperature, in air. The reaction was monitored by TLC. b Isolated yield.
Table 3 The Suzuki reaction of N-heteroaryl halides with arylboronic acidsa
Entry
1
N-Heteroaryl halides
Product
Time/h
12
Yield (%)b
55c
2
3
4
12
12
12
81
98
83
5
6
12
1
68
98
7
0.75
99
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Table 3 (Contd.)
Entry
8
N-Heteroaryl halides
Product
Time/h
1
Yield (%)b
95
9
3
52
10
11
24
24
83
88
a Reaction conditions: heteroaryl halide (0.5 mmol), arylboronic acid (0.75 mmol), PdCl2 (0.0075 mmol, 1.32 mg), K3PO4·7H2O (1 mmol), DMF–H2O
(2 mL/2 mL), rt, in air. The reaction was monitored by TLC. b Isolated yield. c K2CO3 (1 mmol) instead of K3PO4·7H2O.
The effect of atmosphere on the Suzuki reactions
The role of oxygen in the reaction and further applications
of this system in other transformations are currently under
investigation in our laboratory.
Inspired by two recently reported oxygen-promoted systems,
36,47
PEG-400/Pd(OAc)2
and Pd(OAc)2/i-PrOH/H2O,44 we fur-
ther examined the effect of different atmospheres on the Suzuki
reaction. The results are illustrated in Table 4. We observed that
all the reactions carried out under aerobic conditions worked
better than those under nitrogen. However, the atmosphere had
a different effect on the Suzuki reaction of different substrates.
For example, the cross-coupling of 4-bromobenzonitrile with 4-
methoxyphenylboronic acid resulted in a 97% isolated yield in
air (Table 4, entry 1a) and 96% isolated yield in nitrogen (Table 4,
entry 1b) after the same reaction time (25 min).
Noticeably, the Suzuki reaction between 2-bromopyridine and
phenylboronic acid using K3PO4·7H2O as the base in open air
resulted in an 81% isolated yield in 12 h (Table 4, entry 4a),
while the isolated yield decreased to 16% in nitrogen (Table 4,
entry 4b). Although the mechanism involving oxygen in the
reaction is unclear, we propose that a peroxo-palladium complex
might be formed in the presence of oxygen48,49 that accelerates
the oxidative addition step due to the enhanced electron density
of the palladium.
Experimental section
General remarks
Unless otherwise noted, all the reactions were carried out in air.
All aryl halides and arylboronic acids were purchased from Alfa
Aesar, Avocado and used without purification. NMR spectra
were recorded on a Varian Inova 400 spectrometer. Chemical
shifts are reported in ppm relative to TMS. Mass spectroscopy
data of the product were collected on an MS-EI instrument. 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
1
characterized by H NMR spectra and compared to reported
data.
General procedure for the Suzuki reaction of aryl bromides with
arylboronic acids
A mixture of aryl bromide (0.5 mmol), arylboronic acid
(0.75 mmol), PdCl2 (0.0025 mmol, 0.44 mg), K2CO3 (1 mmol),
distilled water (2 mL) and DMF (2 mL) was stirred at room
temperature in air for the indicated time. The mixture was added
to brine (15 mL) and extracted four times with diethyl ether (4 ¥
15 mL). The solvent was concentrated under vacuum and the
product isolated by short chromatography on a silica gel (200–
300 mesh) column.
Conclusions
In summary, we have developed a simple and highly active
ligand-free protocol for the palladium-catalyzed Suzuki cou-
pling of aryl bromides with arylboronic acids in air in aqueous
DMF at room temperature. It is noteworthy that a wide range
of groups could be tolerated. Moreover, this protocol could
be extended to the synthesis of nitrogen-based heterobiaryls.
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Table 4 The effect of atmosphere on the Suzuki reactiona
Entry
1a
Ar–Br
R
Atmosphere
Air
Time/min
25
Yield (%)b
4-OMe
97
1b
2a
N2
Air
25
25
96
99
4-F
H
2b
3a
N2
Air
25
25
94
77
3b
4a
N2
Air
25
720
37
81
H
4b
5a
N2
Air
720
720
16
83
4-OMe
5b
N2
720
72
a Reaction conditions: aryl bromide (0.5 mmol), arylboronic acid (0.75 mmol), K2CO3 (1 mmol) (1 mmol K3PO4·7H2O for 4a, 4b, 5a and 5b), PdCl2
(0.0025 mmol, 0.44 mg) (0.0075 mmol, 1.32 mg for 4a, 4b, 5a and 5b), DMF–H2O (2 mL/2 mL), room temperature. b Isolated yield. c The DMF and
H2O were de-gassed.
General procedure for the Suzuki Reaction of N-heteroaryl
halides with arylboronic acids
Fund of DUT (JG0916) and the Program for New Century
Excellent Talents in University.
PdCl2 (0.0075 mmol, 1.32 mg) and K3PO4·7H2O (1 mmol) were
used instead of PdCl2 (0.0025 mmol, 0.44 mg) and K2CO3 (1
mmol) in the above procedure, respectively.
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