Phosphine-free palladium acetate catalyzed Suzuki reaction in water

Article abstract of DOI:10.1021/jo050724z

Pd(OAc)₂ in a mixture of water and poly(ethylene glycol) (PEG) is shown to be an extremely active catalyst for the Suzuki reaction of aryl iodides and bromides. The reaction can be conducted under mild conditions (50 °C) without the use of a microwave or phosphine ligand in high yields. The isolation of the products is readily performed by the extraction of diethyl ether, and the Pd(OAc)₂-PEG can be reused without significant loss in activity.

Full text of DOI:10.1021/jo050724z

complete conversion of aryl bromides, albeit aryl iodides
delivered moderate yields in the same conditions. Very
recently, the Suzuki coupling reaction catalyzed by the
Pd(OAc)2-H2O-TBAB system was investigated under
conventional thermal conditions and microwave irradia-
tion.10,11 PEGs are known to be nontoxic, recoverable, and
inexpensive media of phase-transfer catalysts.12 The
Suzuki reaction of PEG-bound aryl halides has been
reported.13a However, the Suzuki coupling reaction for
non-PEG-bound aryl halides was only studied under
microwave conditions, and the yields of the PEG-bound
products were superior to those of non-PEG-bound trans-
formations.13 We herein report the application of
Pd(OAc)2-H2O-PEG system as an efficient catalytic
medium for the Suzuki coupling reaction. We found that
the ratio of water and PEG was pivotal to the Suzuki
reaction of non-PEG-bound aryl halides, and the conven-
tional thermal condition was more applicable to the
Pd(OAc)2-H2O-PEG system than the microwave ir-
radiation.
Phosphine-Free Palladium Acetate
Catalyzed Suzuki Reaction in Water
Leifang Liu, Yuhong Zhang,* and Yanguang Wang
Department of Chemistry, Zhejiang University,
Hangzhou 310027, P.R. China
yhzhang@zjuem.zju.edu.cn
Received April 12, 2005
Pd(OAc)2 in a mixture of water and poly(ethylene glycol)
(PEG) is shown to be an extremely active catalyst for the
Suzuki reaction of aryl iodides and bromides. The reaction
can be conducted under mild conditions (50 °C) without the
use of a microwave or phosphine ligand in high yields. The
isolation of the products is readily performed by the extrac-
tion of diethyl ether, and the Pd(OAc)2-PEG can be reused
without significant loss in activity.
The effect of PEG on the Suzuki reaction in water was
initially investigated. The coupling of 4-bromotoluene (1
mmol) and phenylboronic acid (1.5 mmol) was chosen as
the model reaction. The reaction was carried out in water
(3 g) in the presence of 1 mol % of Pd(OAc)2 using
The Suzuki coupling reaction is an important and
versatile method for the generation of unsymmetrical
biaryls from arylboronic acids and aryl halides in a single
step.1 As an intriguingly flexible reaction, it offers
considerable potential in the synthesis of natural prod-
ucts, herbicides, pharmaceuticals, and conducting poly-
mers.2 The traditional Suzuki reaction usually proceeds
using phosphine-based palladium catalysts, and there
has been considerable recent interest in the develop-
ment of new catalysts that are environmentally be-
nign and efficient. Some significant advances have been
made, including the use of palladium nanoparticles,3
water-soluble phosphines as ligands,4 microwave tech-
nology,5 nucleophilic carbene ligands,6 ionic liquids,7 and
so on.8
Water has clear advantages as a solvent for use in
chemistry because it is cheap, readily available, and
nontoxic. The phosphine-free Suzuki reaction involving
water as the cosolvent was reported by Wallow and
Novak in 1994.9a,b Later, Badone and co-workers devel-
oped an elegant method of Suzuki reaction by using water
as the solvent and palladium acetate as the catalyst.9c
In their experiments, 1 equiv of tetrabutylaminium
bromide (TBAB) was required as the promoter for the
(4) (a) Dupuis, C.; Adiey, K.; Charruault, L.; Michelet, V.; Savignac,
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Namboodiri, C. M. Green. Chem. 2000, 2, 120.
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Chem. 1998, 557, 93. (b) Navarro, O.; Kelly, R. A., III; Nolan, S. P. J.
Am. Chem. Soc. 2003, 125, 16194. (c) Alonso, D. A.; Najera, C.; Pacheco,
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Dai, M.; Li , A.; Chen, J.; Yang, Z. Org. Lett. 2004, 6, 3337.
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, W.; Chan, T. H. Org. Lett. 2003, 5, 5003. (b) Mathews, C. J.; Smithb,
P. J.; Welton, T. Chem. Commun. 2000, 1249. (c) Rajagopal, R.;
Jarikote, D. V.; Srinivasan, K. V. Chem. Commun. 2002, 616.
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2003, 606. (b) Mori, Y.; Seki, M. J. Org. Chem. 2003, 68, 1571. (c)
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Tsukuda, T.; Hirao, T. J. Org. Chem. 2002, 67, 2721. (f) Indolese, A.
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75, 61. (b) Wallow, T. I.; Novak, B. M. J. Org. Chem. 1994, 59, 5034.
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* To whom correspondence should be addressed. Fax: +86-571-
87951512. Tel: +86-571-87953253.
(1) (a) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457. (b)
Hassan, J.; Se´vignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M. Chem.
Rev. 2002, 102, 1359. (c) Miyaura, N. Top. Curr. Chem. 2002, 11.
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10.1021/jo050724z CCC: $30.25 © 2005 American Chemical Society
Published on Web 06/29/2005
6122
J. Org. Chem. 2005, 70, 6122-6125

was applicable to various aryl bromides and tolerant to
a broad range of functional groups, such as NO2, NH2,
OMe, OH, COMe, and CN (Table 2, entries 1-10). Both
the electron-rich and electron-deficient aryl bromides
delivered the products with high yields, and the reaction
was not affected by the aqueous conditions. Furthermore,
sterically demanding aryl bromide also gave good yields
(Table 2, entries 3 and 5). The coupling of phenylboronic
acid with aryl iodides in the Pd(OAc)2-H2O-PEG system
was superior and afforded almost quantitative yields
(Table 2, entries 11-13). This new catalyst system
toward the Suzuki reaction of aryl iodides is more
efficient than the published Pd(OAc)2-H2O-TBAB
system.9c The high efficiency of Pd(OAc)2-H2O-PEG
system is probably attributed to the properties of PEG
as cosolvent14 and PTC.
Encouraged by these results, we studied the activity
of aryl chlorides (Table 2, entries 14-19). The activated
4-nitrochlorobenzene was first applied as the Suzuki
coupling partner. Surprisingly, the coupling reaction
proceeded almost quantitatively and afforded the desired
biaryl with high yield after the increase of the temper-
ature to 120 °C for 3 h using 2 mol % of Pd(OAc)2 (Table
2, entry 14). The coupling yield of 4-chloroacetophenone
as the substrate dropped to 52%, although the reaction
time was prolonged to 28 h (Table 2, entry 15). In these
cases, however, the biphenyl was formed as a byproduct
from the self-coupling of phenylboronic acid as confirmed
by GC-MS and 1H NMR. To study the impact of reaction
conditions to the formation of biphenyl, we performed the
coupling reactions with bromotoluene and chlorotoluene
respectively at various reaction conditions. The results
showed that the excellent yield of Suzuki coupling
product with bromotoluene was always obtained, and the
increase of temperature (from 50 °C to 100 °C), reaction
time (0.5-2 h), and the amount of Pd(OAc)2 (1-5 mol %)
had a small influence to the formation of biphenyl. In
contrast, a very low yield of the desired 4-methylbiphenyl
was obtained in the case of deactivated chlorotoluene at
50 °C, and the byproduct of biphenyl was rapidly formed
(Table 2, entry 16). The biphenyl in the coupling of
chlorotoluene was markedly increased with the enhance-
ment of temperatures and the prolongation of reaction
times (Table 2, entries 17-19). These results illustrated
that the self-coupling of the phenylboronic acid occurred
rapidly when the Suzuki coupling reaction was slow in
the Pd(OAc)2-H2O-PEG system.15
FIGURE 1. Effect of PEG 2000 on the Suzuki coupling of
bromotoluene and phenylboronic acid in water.
Na2CO3 (2 mmol) as base at 50 °C for 0.5 h in air. In
the absence of PEG 2000, the result was not prom-
ising, and a very low yield was obtained (Figure 1).
However, the addition of incremental PEG 2000 led to a
very rapid increase in activity, and a maximum (99.5%,
GC yield) was observed when the amount of PEG 2000
was achieved 3.5 g. Further addition of PEG 2000 caused
the decreasing of the yields. It was worthy of note
that only trace of the desired 4-methyl-biphenyl pro-
duced using neat PEG 2000 as the solvent (Table 1,
entry 1). The byproduct of biphenyl from the self-coupling
of phenylboronic acid was not detected in all of the
reactions.
To compare the effect of PEG in organic solvents, the
Suzuki reaction was performed at the same conditions
in the mixture of PEG 2000 and various solvents,
including DMF, acetonitrile, acetone, toluene, dioxane,
methanol, ethanol, 1-propanol, and 2-propanol (Table 1).
The combination of PEG 2000 with nonpolar toluene and
aprotic polar solvents, such as DMF, acetonitrile, acetone,
and dioxane, afforded very little of the product (Table 1,
entries 2-6). On the other hand, the mixture of PEG
2000 with polar protic solvents, such as water, methanol,
ethanol, and 2-propanol, furnished the coupling product
in various yields. The solution of PEG 2000-water and
PEG 2000-methanol afforded the coupling product in
excellent yields (Table 1, entries 7 and 8), whereas PEG
2000-ethanol only delivered a moderate yield (Table 1,
entry 9). The very low yields were obtained in PEG 2000-
2-propanol and PEG 2000-1-propanol (Table 1, entries
10 and 11). These results show a profound solvent effect
on the reaction, and the water as well as methanol is
good solvent for the Suzuki reaction in the presence of
PEG.
The effect of the arylboronic acid partner on the Suzuki
reaction in water was also investigated (Table 3). It can
be seen that both active arylboronic acid with an electron-
donating group and deactivated arylboronic acid with an
electron-withdrawing group exhibited excellent reactivity
in high yields (Table 3, entries 1-12), although the
prolonged reaction time was required for deactivated
Of the bases tested, NaOAc was a poor base for the
coupling (Table 1, entry 14). The use of Na2CO3, K2CO3,
and K3PO4 delivered the product in high yields (Table 1,
entries 7, 12, and 13). KF, Et3N and KOH gave moderate
yields (Table 2, entries 15-17). We chose to use Na2CO3
for the remainder of the studies as it is less expensive.
In addition, the efficiency of various chain length PEGs
on the reaction was examined at the same reaction
conditions (Table 1, entries 18-21). PEG 2000 was
superior to PEG 600 and PEG 1000. The large chain
length PEGs such as PEG 4000 and PEG 6000 were
almost as good as PEG 2000.
(14) (a) Zaslavsky, B. Y. Aqueous Two-Phase Partitioning: Physical
Chemistry and Bioanalytical Applications; Marcel Dekker: New York,
1995. (b) Yalkowsky, S. H.; Banerjee, S. Aqueous Solubility: Methods
of Estimation for Organic Compounds; Marcel Dekker: New York,
1992.
(15) (a) Kong, K.-C.; Cheng, C.-H. J. Am. Chem. Soc. 1991, 113,
6313. (b) Campi, E. M.; Jackson, W. R.; Marcuccio, S. M.; Naeslund,
C. G. M. J. Chem. Soc., Chem. Commun. 1994, 2395. (c) Gillmann, T.;
Weeber, T. Synlett 1994, 649. (d) Song, Z. Z.; Wong, H. N. C. J. Org.
Chem. 1994, 59, 33. (e) Moreno-Man, M.; Pea`ez, M.; Pleixats, R. J.
Org. Chem. 1996, 61, 2346.
The scope of the reaction was studied with various aryl
halides (Table 2). It was seen that the catalytic system
J. Org. Chem, Vol. 70, No. 15, 2005 6123

TABLE 1. Suzuki Coupling Reaction of Bromotoluene with Phenylboronic Acid in Various Mediaa
entry
base
solventb
PEG
PEG/toluene
PEG/DMF
PEG/CH3CN
PEG/dioxane
PEG/acetone
PEG/H2O
PEG/methanol
PEG/ethanol
PEG/propanol
PEG/2-propanol
yieldc (%)
entry
base
solventb
yieldc (%)
1
2
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
11
32
24
32
15
21
97
97
72
40
41
12
13
14
15
16
17
18
19
20
21
K2CO3
K3PO4
NaOAc
KOH
KF
Et3N
Na2CO3
Na2CO3
Na2CO3
Na2CO3
PEG/H2O
95
96
47
70
86
76
64
75
94
90
PEG/H2O
3
PEG/H2O
4
PEG/H2O
5
PEG/H2O
6
PEG/H2O
7
PEG600/H2O
PEG1000/H2O
PEG4000/H2O
PEG6000/H2O
8
9
10
11
a Reaction conditions: bromotoluene (1.00 mmol), PhB(OH)2 (1.5 mmol), base (2 mmol). b The amount of water or solvent was 3 mL,
PEG was 3.5 g. c Isolated yields.
TABLE 2. Suzuki Reactions of Aryl Halides with Phenylboronic Acid Catalyzed by Pd(OAc)2-H2O-PEGa
entry
X
R1
time (h)
yieldd (%)
entry
X
R1
time (h)
yieldd (%)
1
2
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
4-OMe
3-OH
2-NH2
4-NH2
2-Me
0.5
0.5
0.5
0.5
0.5
0.5
0.3
0.3
0.3
0.25
95
98
94
98
95
97
97
98
98
96
11
12
13
14
15
16
17
18
19
I
4-OMe
H
0.5
0.4
0.25
3
97
I
99
3
I
4-NO2
4-NO2
4-COCH3
4-Me
99
4
Cl
Cl
Cl
Cl
Cl
Cl
98b (23)c
52b (41)c
22b (43)c
4b (8)c
5b (21)c
7b (23)c
5
28
28
0.5
1
6
4-Me
7
H
4-Me
8
4-COCH3
4-CN
4-NO2
4-Me
9
4-Me
3
10
a Reaction conditions: aryl halide (1 mmol), PhB(OH)2 (1.5 mmol), Na2CO3 (2 mmol), H2O/PEG 2000 ) 3:3.5 g. b The reaction temperature
was 120 °C, and the amount of Pd(OAc)2 was 2%. c The GC yield of biphenyl is shown in parentheses. d Isolated yields.
arylboronic acid (Table 3, entries 6-12). Homocoupled
product of arylboronic acid was not detected under the
reaction conditions, which made the isolation of the
product easier.
the activating effect of the ester linkage, the direct
bonding of the aryl halide with the PEG probably
enhances the reactivity of the Suzuki coupling under
microwave conditions.13a
Data in the literature showed that microwave condi-
tions were less efficient in the Suzuki reaction in PEG.
The reusability of PEG and catalyst was tested in the
coupling reaction of bromotoluene with phenyboronic
acid using 1 mol % of Pd(OAc)2. The product was iso-
lated by extraction with diethyl ether (4 × 15 mL), and
the solidified Pd(OAc)2-PEG was subjected to the sec-
ond run by charging with the same substrates without
any regeneration or addition of Pd(OAc)2. The yields of
the first three cycles were almost consistent (99%,
98%, 97%). In the fourth cycle, the yield dropped down
to 89%.
In summary, we have shown that Pd(OAc)2, in combi-
nation with PEG, can be used as a highly efficient
catalyst for the Suzuki coupling reaction of aryl halides
in water. The role of PEGs is extraordinary in the
catalysis relative to classical phase transfer agent of
13
To determine the effect of the Pd(OAc)2-H2O-PEG
system under microwave conditions, the mixture of
4-bromotoluene (1 mmol), phenylboronic acid (1.5 mmol),
Na2CO3 (2 mmol), water (3 g), and PEG 2000 (3.5 g) was
subjected to microwave irradiation in the presence of 1
mol % of Pd(OAc)2. In accordance with the results of
Blettner et al.,13a the conversion was very low (100 W, 4
min, 8%; 300 W, 8 min, 58%). It is known that PEG can
serve as the PTC since the poly(ethylene oxide) chains
in PEG can form complexes with the metal ions, similar
to crown ethers.16 In the Pd(OAc)2-H2O-PEG system,
it is possible that the PEG-metal cation complexes bring
the activated ArB(OH)3- to maintain the electro-neutral-
ity, which react with the palladium intermediates and
thus make the Suzuki reaction in water proceed rapidly.
However, if these large complexes are instable to micro-
wave irradiation, the catalytic efficiency of the Pd(OAc)2-
H2O-PEG system will decrease under microwave con-
ditions. In the case of PEG-bound aryl halides, besides
(16) (a) Totten G. E.; Clinton, N. A. J. Macromol. Sci. Rev. Macromol.
Chem. Phys. 1988, C28, 293. (b) Totten G. E.; Clinton, N. A.; Matlock,
P. L. J. Macromol. Sci. Rev. Macromol. Chem. Phys. 1998, C38, 77. (c)
Bernson, A.; Lindgren, J.; Huang, W.; Frech, R. Polymer 1995, 36, 4471.
(d) Rogers, R. D.; Bond, A. H.; Roden, D. M. Inorg. Chem. 1996, 35,
6964.
6124 J. Org. Chem., Vol. 70, No. 15, 2005

TABLE 3. Suzuki Coupling Reaction of Various
Arylboronic Acid with Aryl Halidesa
Experimental Section
General Procedure for Suzuki Reaction. A mixture of
Na2CO3 (0.212 g, 2 mmol), Pd(OAc)2 (2 mg, 1 mol %), PEG 2000
(3.5 g), and water (3 g) was heated to 50 °C with stirring.
Afterward, aryl halides (1 mmol) and arylboronic acid (1.5 mmol)
were added to the solution, and the reaction was carried out at
50 °C for the indicated time. After the reaction solution was
cooled to room temperature, the resulting suspension was
extracted four times with diethyl ether (4 × 15 mL). The residue
of the extraction was subjected to a second run of the Suzuki
reaction by charging with the same substrates (bromotoluene,
phenylboronic acid, water, and Na2CO3) under the same condi-
tions without further addition of Pd(OAc)2 or PEG. The combined
ether phase was analyzed by GC/MS and then concentrated.
Further purification of the product was achieved by flash
chromatography on a silica gel column. All of the products
prepared are known and were characterized by 1H NMR and
MS.
entry
X
R1
R2
time (min) yieldb (%)
1
2
Br
Br
H
H
4-OCH3 (a)
20
45
25
30
30
40
45
60
20
40
15
20
20
30
30
40
10
25
94
89
94
87
96
92
94
85
96
96
97
93
97
98
95
96
97
97
4-CF3 (b)
(a)
3
Br 4-CH3
Br 4-CH3
Br 2-CH3
Br 2-CH3
Br 4-OCH3
Br 4-OCH3
Br 4-COCH3 (a)
Br 4-COCH3 (b)
Br 4-NO2
Br 4-NO2
I
I
I
I
I
I
4
(b)
5
(a)
6
(b)
7
(a)
8
(b)
9
10
11
12
13
14
15
16
17
18
(a)
(b)
(a)
(b)
(a)
(b)
(a)
(b)
4-Methoxybiphenyl (Table 2, Entry 6). 1H NMR (500 MHz,
CDCl3, TMS): δ7.54 (q, 4 H, J ) 6.7 Hz), 7.42 (t, 2 H, J ) 7.7
Hz), 7.28 (t, 1 H, J ) 14.8 Hz), 6.98 (d, 2 H, J ) 4.3 Hz), 3.86 (s,
3 H). MS (EI) m/z: 184 (100) [M+], 169 (44), 141 (38), 115 (26),
63 (4).
H
H
4-OCH3
4-OCH3
4-NO2
4-NO2
a Reaction conditions: aryl halide (1 mmol), ArB(OH)2 (1.5
mmol), Na2CO3 (2 mmol), H2O/PEG 2000 ) 3:3.5 g. b Isolated
yields.
Acknowledgment. We thank the support of Natural
Science Foundation of China (No. 20373061)
Supporting Information Available: Experimental pro-
cedure and spectroscopic data (1H NMR and MS) for the
products. This material is available free of charge via the
Internet at http://pubs.acs.org.
tetrabutylammonium bromide. The application of various
aryl halides and arylboronic acids permits a number of
biaryls to be produced in a short reaction time under mild
conditions involving only air-stable chemicals.
JO050724Z
J. Org. Chem, Vol. 70, No. 15, 2005 6125