Facile and efficient Suzuki-Miyaura coupling reaction of aryl halides catalyzed by Pd2(dba)3 in ionic liquid/supercritical carbon dioxide biphasic system

Article abstract of DOI:10.1016/j.molliq.2016.02.056

A general and efficient catalytic system for Pd₂(dba)₃-catalyzed ligand-free Suzuki cross-coupling reactions was developed. The cross-couplings of a wide range of aryl halides with various arylboronic acids were feasible under mild conditions in environmental friendly [hmim]NTf₂/supercritical CO₂ biphasic system employing K₂CO₃ as a base, which provided in good to high yields to the corresponding products. The catalytic system could be recycled and reused without significant loss of catalytic activity. Additionally, a possible mechanism of the Suzuki cross-coupling reaction is depicted in this paper.

Full text of DOI:10.1016/j.molliq.2016.02.056

Journal of Molecular Liquids 218 (2016) 429–433
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Journal of Molecular Liquids
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Short Communication
Facile and efficient Suzuki–Miyaura coupling reaction of aryl halides
catalyzed by Pd (dba) in ionic liquid/supercritical carbon dioxide
2
3
biphasic system
a
a,b,
a
Hong Bo Wang , Yu-Lin Hu ⁎, De-Jiang Li
a
College of Materials and Chemical Engineering, China Three Gorges University, Yichang 443002, People's Republic of China
Key Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials, China Three Gorges University, Yichang 443002, People's Republic of China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A general and efficient catalytic system for Pd (dba) -catalyzed ligand-free Suzuki cross-coupling reactions was
2
3
Received 1 December 2015
Received in revised form 31 January 2016
Accepted 17 February 2016
Available online xxxx
developed. The cross-couplings of a wide range of aryl halides with various arylboronic acids were feasible under
mild conditions in environmental friendly [hmim]NTf /supercritical CO biphasic system employing K CO as a
base, which provided in good to high yields to the corresponding products. The catalytic system could be recycled
and reused without significant loss of catalytic activity. Additionally, a possible mechanism of the Suzuki
cross-coupling reaction is depicted in this paper.
2
2
2
3
Keywords:
Suzuki coupling
Crown Copyright © 2016 Published by Elsevier B.V. All rights reserved.
Pd
Ionic liquid
scCO
2 3
(dba)
2
1
. Introduction
The carbon–carbon bond formation reaction is one of the most
nickel [14], rhodium [15], and other complexes [16–18], have been
developed for this conversion. However, these procedures still suffered
from the use of expensive reagents, difficulties in work up, environmen-
tal hazards, and difficulties in recycling of the catalyst. Sometimes,
complicated coordinating and expensive ligands are often required to
stabilize the active metal intermediates during the reactions.
Supercritical carbon dioxide (scCO
2
) is an abundant, inexpensive,
nontoxic and non-flammable reagent, and it has received considerable
attentions as a reaction medium for organic reactions due to the attrac-
tive physical and toxicological properties [19]. CO could be a particular-
2
ly advantageous reaction medium when it serves as both a reactant and
a solvent. In fact, the Suzuki–Miyaura reaction is very suitable for pro-
2
ceeding under supercritical conditions, because scCO possesses high
solvating power towards various organoboron compounds and organic
halides. A number of catalytic processes based upon the combination
fundamental transformations due to the large number of applications
of the resulting compounds in organic, pharmaceutical, and polymer
chemistry [1]. Therefore, there has been tremendous interest in
developing efficient methods for such a conversion, and a well known
method constitutes the Suzuki–Miyaura cross-coupling reaction. The
Suzuki–Miyaura cross-coupling reaction is a powerful tool for the
formation of carbon–carbon bond, which is characteristic of mild condi-
tions, high selectivity, and tolerance with a broad range of functional
groups [2]. Traditionally, the most common way for performing such a
transformation is palladium complexes-catalyzed cross-coupling reac-
tion between organoboron compounds and organic halides or triflates
[3,4]. However, the cost of reagents, removal of trace palladium from
late stage synthetic intermediate, and the use of expensive, difficult to
prepare and handle ligands are the major drawbacks of this method.
Moreover, in many cases, ligands are air and moisture sensitive, which
makes reaction setup too complex for straightforward scale-up and
makes ligand-free palladium applications the alternative of choice
from a green chemistry perspective. In addition to palladium catalysts,
a variety of transition metal complexes catalysts, such as iron [5,6],
manganese [7], cobalt [8,9], ruthenium [10], copper [11,12], gold [13],
2
of transition metals and scCO have been developed for such a conver-
sion [20–22]. However, these methods suffered from environmental
hazards, poor recovery of expensive metal catalysts and laborious work-
up procedures. Ionic liquids (ILs) have received considerable attention
as alternatives to the traditional organic solvents. Because of their
interesting physical and chemical properties, such as negligible vapor
pressure, unique permittivity, high thermal stability, good solvents for
both organic and inorganic substances, high electrical conductivity
and wide electrochemical window [23–25], ionic liquids have found
an application in many chemical reactions [26] and processes including
extraction and separation [27]. Up to now, examples of their application
in the Suzuki–Miyaura cross-coupling reaction were also reported
⁎
Corresponding author at: College of Materials and Chemical Engineering, China Three
Gorges University, Yichang 443002, People's Republic of China.
E-mail address: huyulin1982@163.com (Y.-L. Hu).
http://dx.doi.org/10.1016/j.molliq.2016.02.056
0167-7322/Crown Copyright © 2016 Published by Elsevier B.V. All rights reserved.

430
H.B. Wang et al. / Journal of Molecular Liquids 218 (2016) 429–433
the recovered catalyst and then recycled under identical reaction condi-
tions. The target substrates were characterized by Elemental analysis,
1
H NMR spectra or compared with their authentic samples. Spectral
characteristics of the products in Table 3 were provided as follows.
2
.2.1. 4-Methyl-1,1'-biphenyl (Table 3, entry 2)
1
H NMR (500 MHz, CDCl
m, Ar–H, 4H), 7.39–7.52 (m, Ar–H, 5H). Anal. Calcd. for C13
C, 92.79; H, 7.17. Found: C, 92.81; H, 7.19.
3 3
): δ (ppm) = 2.24 (s, CH , 3H), 7.25–7.36
(
12
H :
Scheme 1. The Suzuki cross-coupling reaction.
2
.2.2. 2-Methoxy-1,1'-biphenyl (Table 3, entry 3)
1
[28–32]. However, these reported protocols are still associated with one
3 3
H NMR (500 MHz, CDCl ): δ (ppm) = 3.84 (s, CH , 3H), 7.01–7.03
or more disadvantages, such as low catalyst activity, water or air sensi-
tivity, and the requirement for expensive transition metal additives.
Consequently, the search for new and environmentally benign synthetic
methodologies for Suzuki–Miyaura cross-coupling reaction that address
these drawbacks remains to be of value and interest.
(m, Ar–H, 2H), 7.35–7.43 (m, Ar–H, 2H), 7.53–7.58 (m, Ar–H, 5H).
Anal. Calcd. for C13
H, 6.57; O, 8.68.
H12O: C, 84.72; H, 6.54; O, 8.65. Found: C, 84.75;
2.2.3. 4-Methoxy-1,1'-biphenyl (Table 3, entry 4)
1
In view of the above-mentioned advantages of scCO
combination of scCO and ILs may provide a clean synthesis process in
which reaction and separation occur consecutively. Due to the strong
electrostatic forces between cations and anions, scCO dissolves well
in ILs, but ILs actually do not dissolve in scCO . The organic products
from a reaction in the IL-phase can, therefore, be extracted by supercrit-
ical CO , allowing only a small amount of cross-contamination between
the IL and scCO phases [33,34]. Considering the good physical and
toxicological properties of scCO and ILs, the main aim of this work is
2
and ILs, the
3 3
H NMR (500 MHz, CDCl ): δ (ppm) = 3.86 (s, CH , 3H), 6.98–7.01
2
(m, Ar–H, 2H), 7.23–7.26 (m, Ar–H, 1H), 7.39–7.42 (m, Ar–H, 2H),
7.51–7.55 (m, Ar–H, 4H). Anal. Calcd. for C13H12O: C, 84.74; H, 6.52;
O, 8.64. Found: C, 84.75; H, 6.57; O, 8.68.
2
2
2.2.4. 1,1'-Biphenyl-4-ol (Table 3, entry 5)
1
2
3
H NMR (500 MHz, CDCl ): δ (ppm) = 4.75 (s, OH, 1H), 6.88–6.92
2
(m, Ar–H, 2H), 7.26–7.29 (m, Ar–H, 1H), 7.37–7.53 (m, Ar–H, 6H).
2
Anal. Calcd. for C12
H, 5.92; O, 9.40.
H10O: C, 84.65; H, 5.91; O, 9.38. Found: C, 84.68;
to establish a new and efficient protocol for the Suzuki–Miyaura
cross-coupling reaction in the presence of ionic liquid. Herein, we report
an efficient, ligand-free and environmentally friendly protocol for
the Suzuki–Miyaura coupling between aryl halides and aryl boronic
2.2.5. 1,1'-Biphenyl-2-amine (Table 3, entry 6)
1
3 2
H NMR (500 MHz, CDCl ): δ (ppm) = 3.74 (s, NH , 2H), 6.73–6.81
acid catalyzed by tris(dibenzylideneacetone)dipalladium (Pd
with K CO as the base in the presence of scCO and 1-hexyl-3-
methylimidazolium bis(trifluoromethylsulfonyl) imide ([hmim]NTf
2 3
(dba) )
(m, Ar–H, 2H), 7.11–7.15 (m, Ar–H, 2H), 7.32–7.43 (m, Ar–H, 5H). Anal.
2
3
2
Calcd. for C12H11N: C, 85.15; H, 6.54; N, 8.26. Found: C, 85.17; H, 6.55;
2
)
N, 8.28.
biphasic system (Scheme 1). In addition, recyclability of the catalytic
system can be easily carried out up to six cycles.
2.2.6. 4-Nitro-1,1'-biphenyl (Table 3, entry 7)
1
H NMR (500 MHz, CDCl
7.63–7.66 (m, Ar–H, 2H), 7.71–7.75 (m, Ar–H, 2H), 8.28–8.31
m, Ar–H, 2H). Anal. Calcd. for C12 NO : C, 72.32; H, 4.54; N, 7.01;
3
): δ (ppm) = 7.44–7.51 (m, Ar–H, 3H),
2
. Experimental
(
H
9
2
2
.1. General remarks
O, 16.03. Found: C, 72.35; H, 4.55; N, 7.03; O, 16.06.
All the chemicals were from commercial sources without any
2.2.7. 1,1'-Biphenyl-4-carbaldehyde (Table 3, entry 8)
H NMR (500 MHz, CDCl ): δ (ppm) = 7.43–7.52 (m, Ar–H, 3H),
3
7.62–7.65 (m, Ar–H, 2H), 7.73–7.76 (m, Ar–H, 2H), 7.94–7.96
(m, Ar–H, 2H), 10.03 (s, CHO, 1H). Anal. Calcd. for C13
H, 5.51; O, 8.75. Found: C, 85.69; H, 5.53; O, 8.78.
1
pretreatment. All reagents were of analytical grade. The ionic liquids
were synthesized according to the literature procedure [25]. 1
NMR spectra was recorded on a Bruker 500-MHz spectrometer using
CDCl as the solvent with tetramethylsilane (TMS) as an internal stan-
H
H
10O: C, 85.68;
3
dard. High performance liquid chromatography (HPLC) experiments
were performed on a liquid chromatograph (Dionex Softron GmbH,
America), consisting of a pump (P680) and ultraviolet–visible light
detector (UVD) system (170 U). Elemental analysis was performed on
a Vario EL III instrument (Elmentar Anlalysensy Teme GmbH, Germany).
Table 1
a
Optimization studies for the Suzuki reaction of bromobenzene and phenylboronic acid.
Entry
Ionic liquid
Base
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
9
–
–
12
8
8
2
2
2
2
2
8
8
2
2
2
2
2
b20
58
42
78
92
65
87
72
48
59
84
75
68
80
72
[hmim]PF
[hmim]OTf
[hmim]BF
6
K
2
K
2
K
2
CO
CO
CO
3
3
3
2
.2. General procedure for the Suzuki reactions
4
The Suzuki reaction was carried out in a stainless-steel reactor
[hmim]NTf2
[hmim]Cl
K2CO3
equipped with a magnetic stirrer and automatic temperature control
system. Aryl halide (1 mmol), arylboronic acid (1.1 mmol), Pd (dba)
0.1 mol%), [hmim]NTf (5 mL) and K CO (1 mmol) were charged
into the reactor. CO was charged in the reactor and the pressure was
K
2
K
2
K
2
K
2
–
CO
CO
CO
CO
3
3
3
3
[bmim]NTf
2
2
3
[bpy]NTf
2
(
2
2
3
–
2
10
[hmim]NTf
[hmim]NTf
[hmim]NTf
[hmim]NTf
[hmim]NTf
[hmim]NTf
2
2
2
2
2
2
adjusted to 10 MPa. The reactor was heated at 80 °C for an appropriate
time (Table 3), and the pressure was kept constant during the reaction.
At the end of the reaction, the reactor was cooled and the carbon dioxide
was vented slowly. The products were analyzed by HPLC. The residue
was purified by preparative thin-layer chromatography on silica gel
11
Na
Et
KOAc
PO
KOH
2
CO
3
12
13
14
15
3
N
K
3
4
a
The reactions were carried out with bromobenzene (1 mmol), phenylboronic
(dba) (0.1 mol%), base (1 mmol), ionic liquid (5 mL), and
(10 MPa) at 80 °C.
Isolated yield.
(
200–300 mesh) to afford the desired pure product. The rest of the
ionic liquid and catalyst were recovered and reused directly without
further treatment. Fresh substrates and CO were then recharged to
acid (1.1 mmol), Pd
CO
2
3
2
b
2

H.B. Wang et al. / Journal of Molecular Liquids 218 (2016) 429–433
431
Table 2
3. Results and discussion
a
Screening of the reaction condition for the Suzuki coupling reaction.
Entry
[hmim]NTf
2
(mL)
Pd
2
(dba)
3
(mol%)
Time (h)
Yield (%)b
The initial study was carried out using the Suzuki reaction of
bromobenzene and phenylboronic acid so as to optimize the reaction
conditions, and the results are summarized in Table 1. The reaction
screening led to disappointing results in the absence of ionic
liquid and base, the reaction proceeded very slowly, and the yield was
no more than 20% after 12 h (Table 1, entry 1). The results mean
1
2
3
4
5
6
7
8
–
2
4
5
6
5
5
5
–
24
4
2
2
2
2
2
2
b5
72
85
92
92
75
87
92
0.1
0.1
0.1
0.1
0.05
0.08
0.12
that Pd
under supercritical conditions. The effects of different ionic liquids,
such as [hmim]PF , [hmim]OTf, [hmim]BF , [hmim]NTf , [hmim]Cl,
bmim]NTf , and [bpy]NTf , were tested in the reaction (Table 1,
entries 2–8), it was observed that [hmim]NTf demonstrated the
2 3
(dba) alone does not work effectively in the reaction
a
6
4
2
The reactions were carried out with bromobenzene (1 mmol), phenylboronic
CO (1 mmol), Pd (dba) catalyst, ionic liquid [hmim]NTf
(10 MPa) at 80 °C.
Isolated yield.
[
2
2
acid (1.1 mmol), K
and CO
2
3
2
3
2
,
2
2
b
best performance (Table 1, entry 5). The different effects of ILs
in the reaction may be attributed to their different abilities of
2 3
stabilizing and dissolving the substrates, Pd (dba) and the different
2
.2.8. 4-Fluoro-1,1'-biphenyl (Table 3, entry 9)
miscibilities of scCO . Under the same conditions, the IL who stabilizes
and dissolves them more easily will leads to a larger increase in the
2
1
H NMR (500 MHz, CDCl
.33–7.41 (m, Ar–H, 3H), 7.49–7.53 (m, Ar–H, 4H). Anal. Calcd.
F: C, 83.68; H, 5.25; F, 11.01. Found: C, 83.70; H, 5.27;
3
): δ (ppm) = 7.09–7.12 (m, Ar–H, 2H),
7
effective reactant concentration, which increases the encounter proba-
bilities between the substrate and reactive species, and so the higher
rate or yield of the reaction is observed. For a blank test (Table 1,
entry 9), a lower yield of the product was obtained when the same re-
action condition were used in the absence of ionic liquid. In addition,
the base is crucial for this reaction, and its lack leads to a much lower
yield (Table 1, entry 10). Besides K CO , we also tried to use other
for C12
F, 11.03.
H
9
2
.2.9. 1,1'-Biphenyl-4-carbonitrile (Table 3, entry 10)
1
H NMR (500 MHz, CDCl
3
): δ (ppm) = 7.37–7.52(m, Ar–H, 5H),
2
3
7
.85–7.89 (m, Ar–H, 4H). Anal. Calcd. for C13
H
9
N: C, 87.09; H, 5.04;
types of bases in this model reaction (Table 1, entries 11–15), and the
N, 7.81. Found: C, 87.12; H, 5.06; N, 7.82.
results showed that K CO demonstrated the best performance.
2
3
Scheme 2. Possible mechanism for the Suzuki reaction.

432
H.B. Wang et al. / Journal of Molecular Liquids 218 (2016) 429–433
7
2
2% in a shorter time (4 h) when only 2 mL was used (Table 2, entry
). The yield improved with increasing amount of [hmim]NTf
2
(
(
(
Table 2, entries 2–4), and reached maximum with 5 mL of [hmim]NTf
2
Table 2, entry 4). However, further addition the amount of [hmim]NTf
2
Table 2, entry 5), under the same conditions, the yield was not en-
hanced significantly. These experiments revealed that ionic liquid
hmim]NTf is crucial for the Suzuki reaction. On investigating the effect
of catalyst amount on the reaction, catalysts of 0.05, 0.08, 0.1, and
.12 mol% were utilized. The best results were obtained in 0.1 mol%
Pd (dba) (Table 2, entry 4).
The excellent catalytic abilities of [hmim]NTf
[
2
0
2
3
2
2 2 3
/scCO /Pd (dba)
suggest the Suzuki reaction has a particular mechanism. According to
the literatures [35–41] and the observations in our reactions, taking
the Suzuki cross-coupling reaction of bromobenzene and phenylboronic
acid as an example, a possible mechanism is proposed (Scheme 2).
Fig. 1. Repeating reactions using recovered catalytic system. The reactions were carried
out with bromobenzene (1 mmol), phenylboronic acid (1.1 mmol), K CO (1 mmol),
recovered catalytic system ([hmim]NTf , Pd (dba) ), in the presence of CO (10 MPa) at
0 °C for 2 h.
2
3
2
2
3
2
8
Initially, Pd
the metal carbene complexes Pd (NHC) NTf
oxidative addition of bromobenzene to Pd (NHC) NTf
palladium intermediate 1. Phenylboronic acid which is activated by
CO reacts with intermediate 1. After transmetallation reaction, the
2
(dba)
3
) could react with ionic liquid [hmim]NTf
[39–41], followed by the
to form aryl
2
to form
The Suzuki cross-coupling reaction of bromobenzene and
phenylboronic acid was then tested in the presence and absence of
catalyst and ionic liquid using K CO as base under scCO supercritical
conditions. The result demonstrated that the reaction proceeded very
slowly and the yield of biphenyl was no more than 5% after 24 h
2
2
2
3
2
K
2
3
intermediate 2 obtains. Finally, reductive elimination of intermediate
2 produces the desired product. The product was separated from
in the absence of [hmim]NTf
2 2 3
and Pd (dba) (Table 2, entry 1).
When the reaction was performed in the presence of the ionic liquid
reaction mixture by scCO
2
extraction during the reaction course.
2
[hmim]NTf , the product biphenyl could be obtained in a higher yield
Decompression of the supercritical mixture downstream gave the
Table 3
Suzuki reactions of aryl halides with arylboronic acids.
a
Entry
1
Aryl halide
Arylboronic acid
Product
Time (h)
2
Yield (%)b
92
2
3
2
2
89
88
4
2
87
5
6
2
2
91
90
7
8
9
2
2
2
97
95
96
10
11
12
13
14
15
2
6
6
6
1
1
98
79c
85c
76c
98
96
1
1
1
1
2
6
7
8
9
0
1
2
2
2
2
99
93
94
92
90
a
b
c
The reactions were carried out with aryl halide (1 mmol), arylboronic acid (1.1 mmol), K
Isolated yield.
The reaction was carried out at 95 °C.
CO
2 3
(1 mmol), Pd
2
(dba)
3
(0.1 mol%), [hmim]NTf
2
(5 mL), and CO
2
(10 MPa) at 80 °C.

H.B. Wang et al. / Journal of Molecular Liquids 218 (2016) 429–433
433
2 2 3
product free of [hmim]NTf /Pd (dba) . The combination of ionic liquid
Acknowledgments
and scCO provides superior advantages in product separation, catalyst
2
recycling and reuse of the reaction media over traditional organic
solvents [37,38].
We thank the National Natural Science Foundation of China
(no. 21506115) and the Natural Science Foundation of Jiangsu
Province (no. BK20140460) for the support of this research.
Under the supercritical conditions, a series of catalytic cycles were
run to investigate the constancy of the catalyst activity and recycle.
In each cycle, the catalyst system could be easily recovered by venting
carbon dioxide. The results illustrated in Fig. 1 show that the catalytic
system could be typically recovered and reused for subsequent
reactions with no appreciable decrease in yields and reaction rates.
Furthermore, the loading amount of the six times used catalyst was
determined by elemental analysis, it was found that (0.091 mol%) of
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1
4–16). These results are consistent with the well-established trend in
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[
[
[
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(
(
4-methoxyphenyl)boronic acid, (4-fluorophenyl)boronic acid, and
4-nitrophenyl)boronic acid using bromobenzene as a representative
(
(
aryl halide (Table 3, entries 17–20), excellent yields of the expected
products were obtained. On the basis of the rates and yields of reactions,
obviously, our protocol was found to be more effective in Suzuki cou-
pling of aryl iodides than that of both aryl bromides and aryl chlorides.
[
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4
. Conclusion
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(
[
[
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In conclusion, we developed a new and efficient catalytic system,
which consisted of [hmim]NTf and Pd (dba) under supercritical
2 2 3
carbon dioxide conditions. This catalytic system can accomplish
Suzuki-Miyaura coupling reaction between various aryl halides and
aryl boronic acids under relatively mild conditions. Simplicity of opera-
tion, high yields, easy isolation of products, and excellent recyclability
of the catalytic system are the attractive features of this methodology.
The scope, definition of the mechanism and synthetic application of
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