A simple and efficient approach for the palladium-catalyzed ligand-free Suzuki reaction in water

Article abstract of DOI:10.1039/c2gc36098e

A general and efficient protocol is described for the palladium-catalyzed ligand-free and aerobic Suzuki reaction in water in the absence of any additive. The results demonstrate that the base played a crucial role in the high efficiency. The Pd(OAc)₂/(i-Pr)₂NH/H₂O system showed the highest catalytic activity towards the Suzuki reaction of a wide range of aryl halides bearing hydrophilic or hydrophobic groups.

Full text of DOI:10.1039/c2gc36098e

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A simple and efficient approach for the palladium-catalyzed ligand-free
Suzuki reaction in water
Chun Liu,* Yixia Zhang, Ning Liu and Jieshan Qiu*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A general and efficient protocol is described for the
palladium-catalyzed ligand-free and aerobic Suzuki reaction
reaction, and a series of ligand-free systems have been
reported. At the moment, we are interested in developing a
1
5
in water in the absence of any additive. The results 50 simple and general protocol for the palladium-catalyzed
ligand-free and aerobic Suzuki reaction in pure water. Herein,
we describe fast and efficient catalytic system of
Pd(OAc) /(i-Pr) NH/H O for the cross-couplings of a wide
demonstrated that the base played a crucial role for the high
efficiency. The Pd(OAc) /(i-Pr) NH/H O system showed the
highest catalytic activity towards the Suzuki reaction of a
wide range of aryl halides bearing hydrophilic groups or
hydrophobic groups.
a
1
0
2
2
2
2
2
2
range of aryl halides with arylboronic acids without any
additive.
5
5
Results and discussion
Introduction
Optimization of reaction conditions
1
2
2
3
3
4
4
5
0
5
0
5
0
5
The chemical industry plays an important role in human
development. Nowadays, chemists have an increasing interest
to develop ‘green’ processes as the sustainability has become
a prior issue in every area of human activity. Among the 12
principles of Green Chemistry, the use of solvents is a key
Initially, the cross-coupling of 4-bromoanisole with phenyl
boronic acid in 1 mL water under air was chosen as a model
reaction for screening base. The results are summarized in
Table 1. Inorganic bases such as K CO , NaOH and
1
6
6
7
7
8
8
0
5
0
5
0
5
2
3
aspect since it gives rise to toxicity, hazard, pollution and
K PO ⋅7H O, which are soluble in water and have been
3
4
2
2
waste treatment issues. In
a
recent survey, most of
13-
successfully used in efficient ligand-free catalytic systems,
pharmaceutical companies are approaching green chemistry
and considering water as a potentially useful and safe
15
demonstrated low activity in the present protocol,
respectively (Table 1, entries 1-3). Subsequently, a series of
organic bases were examined (Table 1, entries 4-18).
Triethylamine, a typical organic base in the palladium-
3
solvent. Water is an abundant, nontoxic, noncorrosive and
nonflammable solvent, and the use of water as a sole reaction
4
9
a,16
medium is one of the latest challenges for modern chemists.
catalyzed Suzuki reaction,
was the first organic base tested
So far, a lot of efforts have been made to carry out organic
in this system, providing a positive result of an 83% isolated
yield of the cross-coupled product in 30 min (Table 1, entry 4).
N,N-Diisopropylethylamine provided
triethylamine (Table 1, entry 5). Tripropylamine and
tributylamine (Table 1, entries 6 and 7) were inefficient,
although they are more sterically hindered, indicating that the
steric effect of the base does not play a significant role in this
system. Diethylamine, dipropylamine, diisopropylamine and
dicyclohexylamine, each of which has a hydrogen atom
bonded to nitrogen in common, gave different results (Table
5
reactions in water. Among the developed transformations, the
palladium-catalyzed Suzuki reaction of aryl halides with aryl
boronic acids is an excellent example, which is one of the
most versatile and powerful tools for the construction of
a similar result as
6
biaryls. As an industrialized example, the palladium-
catalyzed Suzuki reaction has been successfully applied to the
production of compounds such as losartan,
a Merck
7
antihypertensive drug. Although the Suzuki reaction has
8
strongly benefited from aqueous chemistry, most of
halogenated substrates are insoluble in water, resulting in a
poor reactivity. To solve this problem, numerous attempts
1
, entries 8-11). It is clear that the most efficient base in the
present catalytic system is diisopropylamine, which provided
a 96% isolated yield in 5 min, resulting in a TOF up to 4600
9
have been made, such as synthesizing water-soluble ligands,
1
0
adding surfactants and/or phase-transfer agents, using co-
solvents or supporting by microwave or ultrasound, even
-1
h . Dicyclohexylamine gave a result of 15% isolated yield in
1
1
12
the first 5 min, but it afforded the product in 95% in 30 min.
Comparatively lower yields were abtained with the bases of
propylamine or isopropylamine (Table 1, entries 12 and 13).
Cyclic amine morpholine, piperazine and pyrrolidine were
also examined in this catalytic system, respectively (Table 1,
entries 14-16), only piperazine gave a moderate yield of 47%.
1
3
utilizing an inorganic salt as a promotor.
In 1989, Beletskaya and Bumagin et al. for the first time
realized a palladium-catalyzed ligandless Suzuki reaction of
aryl halides containing a -OH or -COOH group in water under
1
4
an argon atmosphere. Since 2007, our laboratory have
focused on the palladium-catalyzed ligand-free Suzuki
1
2
N ,N -Dimethylethane-1,2-diamine
and
DBU
(1,8-
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1

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diazabicyclo[5.4.0]undec-7-ene) were tested, resulting in trace
Recently, we have observed that the palladium-catalyzed
amounts of product (Table 1, entries 17 and 18). In this simple 25 Suzuki reaction could be promoted by oxygen in aqueous
8
d,12a
catalytic system, the reason for the high efficiency is
medium.
of atmosphere on the cross-coupling of 4-bromoanisole with
phenylboronic acid using 0.25 mol% Pd(OAc) as a precatalyst at
For further verification, we next studied the impact
1
7
supposed that diisopropylamine plays not only as a base but
also as ligand, which has much higher tendency to
coordinate to palladium to form active species as Boykin et al.
5
a
2
100 °C in water. As the reaction proceeded very fast using (i-
30 Pr) NH as base, which made it difficult to distinguish the reaction
1
8
reported.
2
2 3
rate in air and N , we chose Et N as the base to investigate the
influence of atmosphere. The kinetic studies were performed
Table 1 The effect of base on the cross-coupling of 4-bromoanisole with
phenylboronic acid
a
1
0
under air and N , respectively, as illustrated in Fig. 1. In the first
2
3
min, 46% and 63% yields were reached in air and nitrogen,
3
4
4
5
0
5
respectively. While an 82% yield was obtained in 5 min under air,
higher than 71% yield in nitrogen in the same reaction time. The
reason for the difference between the kinetics under air and
nitrogen is proposed due to continuous reoxidation of some
palladium species under air preventing agglomeration toward
inactive palladium black. As a consequence the system under air
would appear to be more stable. On the contrary, the system tends
to deactivate by formation of inactive palladium species due to
b
Entry
Base
CO
NaOH
PO
Yield (%)
1
2
3
4
5
6
7
8
9
K
2
3
18
24
25
83
81
23
25
29
30
K
3
4
2
⋅7H O
Et
3
N
(i-Pr)
2
NEt
3
(n-Pr) N
agglomeration under nitrogen. The presence of O is proposed to
2
3
(n-Bu) N
suppress both catalyst activation and deactivation processes,
Et
(n-Pr)
(i-Pr)
Dicyclohexylamine
n-PrNH
i-PrNH
2
NH
1
9
leading to the above mentioned results as Adrio et al. reported.
2
NH
c
1
0
2
NH
96
c
11
15 , 95
1
1
1
1
1
1
2
3
4
5
6
7
8
2
Trace
20
2
Morpholine
Piperazine
Pyrrolidine
35
47
Trace
1
2
N ,N -Dimethylethane-1,2-diamine Trace
DBU Trace
1
a
Reaction conditions: 4-bromoanisole (0.5 mmol), phenylboronic acid
0.75 mmol), 0.25 mol% Pd(OAc) , base (1.0 mmol), H O (1.0 mL), 100
(
2
2
b
c
°
C, 30 min, in air. Isolated yield. 5 min.
The next investigation in this study was to explore the effect of
palladium species (Table 2) on the model coupling reaction of 4-
bromoanisole with phenylboronic acid. The results indicated that
Pd(OAc) , PdCl and PdCl (CH CN) exhibited high catalytic
Fig. 1 Yield vs. time curves of the Suzuki reaction of 4-bromoanisole
with phenylboronic acid under air or nitrogen. Reaction conditions: 4-
bromoanisole (0.5 mmol), phenylboronic acid (0.75 mmol), 0.25 mol%
Pd(OAc) , Et N (1.0 mmol), H O (1.5 mL), 100 °C.
1
5
2
2
2
3
2
activity (Table 2, entries 1-3). However, zerovalent palladium
sources such as Pd (dba) or Pd/C gave low yields of product
5
0
2
3
2
3
2
(
Table 2, entries 4 and 5), which was consistent with our reported
1
5b,15f
results.
Scope and limitations of substrates
2
0
Table 2 The effect of palladium species on the cross-coupling of 4-
bromoanisole with phenylboronic acid
With the optimized conditions at hand, we further studied the
generality of the cross-coupling reactions between aryl halides
a
5
5
and arylboronic acids in the presence of 0.25 mol% Pd(OAc)
2
and two equivalent diisopropylamine at 100 °C in water. The
results are shown in Table 3. Various 4-substituted aryl
bromides bearing either electron-donating or electron-
withdrawing groups, such as -OMe, -NH , -CN, -COCH and -
b
Entry
[Pd]
Yield (%)
1
2
3
4
Pd(OAc)
2
96
94
95
33
30
PdCl
2
2
2
3
PdCl
(CH
3
CN)
2
60 NO , provided the corresponding products in good to
2
excellent yields (Table 3, entries 1-5). For example, 4-
bromonitrobenzene afforded a quantitative yield in 5 min,
Pd
2
(dba)
3
5
a
Pd/C
-
1
resulting in a high TOF up to 4752 h (Table 3, entry 5). Aryl
bromides with a carboxyl or hydroxyl group were successfully
65 coupled with p-tolylboronic acid in excellent yields (Table 3,
entries 6 and 7). This catalytic system could tolerate aryl
Reaction conditions: 4-bromoanisole (0.5 mmol), phenylboronic acid
0.75 mmol), 0.25 mol% [Pd], (i-Pr) NH (1.0 mmol), H O (1.0 mL), 100
(
2
2
b
°C, 5 min, in air. Isolated yield.
2
| Journal Name, [year], [vol], 00–00
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Table 3 The Suzuki reaction of aryl halides with aryl boronic
acids
halides with hydrophilic groups or hydrophobic groups. The
yield of coupling of 4-bromoanisole with 4-fluorophenyl
boronic acid was higher than that of the cross-coupling of 1-
bromo-4-fluorobenzene with (4-methoxyphenyl) boronic acid
under the same reaction conditions (Table 3, entries 8 and 9).
The cross-coupling reaction also proceeded very fast even
increasing the steric effect of aryl bromide or arylboronic acid
(Table 3, entries 10-14, 16 and 17). However, increasing the
steric effect in both aryl bromide and arylboronic acid could
decrease the reactivity of the cross-coupling, an 81% isolated
yield was obtained in 30 min between 2-bromobenzonitrile
and o-tolylboronic acid (Table 3, entry 15). As aryl-
15 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. It is clear that this protocol is applicable to
the cross-couplings between 3-pyridyl bromides and
phenylboronic acid (Table 3, entries 18 and 19). Encouraged
by such promising results, we next turned our attention to the
Suzuki reaction of aryl chlorides. To our surprise, the present
catalytic system could activate 4-chloronitrobenzene for the
cross-coupling with phenylboronic acid, resulting in a 75%
isolated yield of product in 60 min (Table 3, entry 20), which
is the successful example of the Suzuki reaction of aryl
chloride in pure water under ligand-free and aerobic
conditions without any additive. Moreover, reducing the
palladium loading down to 0.1 mol%, a 74% isolated yield
was still obtained in 60 min.
a
5
Time/ Yield
min (%)
Entry Aryl halide
Product
b
1
2
3
4
5
5
5
5
5
5
96
95
99
98
99
1
0
6
7
8
5
96
97
97
2
2
3
0
5
0
5
10
9
10
10
83
98
1
1
1
1
0
1
2
3
60
10
10
96
92
98
Even increasing the loading of 4-bromoanisole up to 5
mmol under the optimized conditions, a 95% isolated yield
was reached in 10 min.
Conclusion
3
5
In conclusion, a simple and general protocol has been developed
for the palladium-catalyzed ligand-free and aerobic Suzuki
reaction in pure water without any additive. Further work to
explore the exact function of (i-Pr) NH in the system and the
application of this approach are currently under investigation in
our laboratory.
1
1
4
5
10
30
96
81
2
4
0
Experimental
General remarks
Aryl halides and arylboronic acids were purchased from Alfa
Aesar. Other chemicals were obtained commercially and used
without any prior purification. H NMR spectra were recorded on
a Bruker AvanceII 400 spectrometer using TMS as internal
standard. All products were isolated by short chromatography on
a silica gel (200-300 mesh) column using petroleum ether (60-90
1
1
6
7
60
25
95
96
1
4
5
°
C), unless otherwise noted. Compounds described in the
1
1
8
9
0
120 90
1
5
0
literature were characterized by H NMR spectra compared to
reported data.
30
60
98
75
General Procedure for the Suzuki Reaction
2
All Suzuki reactions were carried out under air. A mixture of aryl
halide (0.5 mmol), arylboronic acid (0.75 mmol), base (1.0
a
Reaction conditions: aryl halide (0.5 mmol), arylboronic acid (0.75
, (i-Pr) NH (1.0 mmol), H O (1.0 mL), 100
C, in air. Isolated yield.
mmol), 0.25 mol% Pd(OAc)
°
2
2
2
b
55 mmol), Pd(OAc)
2
2
(0.25 mol%, 0.28 mg), H O (1.0 mL) was
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Science Foundation of China (21076034, 20923006, 20836002),
the Fundamental Research Funds for the Central Universities
(DUT11LK15), and the Ministry of Education (the Program for
New Century Excellent Talents in University). We are also
grateful to the referees for the advices on the kinetics study.
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