Homocoupling of arylboronic acids catalyzed by a simple hydrophilic palladium(II) complex in aqueous media

Article abstract of DOI:10.2174/1570178612666150115001813

Homocoupling of arylboronic acids has been successfully carried out by using the inexpensive hydrophilic palladium( II) complex PdCl₂(NH₂CH₂COOH)₂ as catalyst in i-PrOH/H₂O (v/v=1:2) under aerobic atmosphere without elevated heating to give rise to symmetrical biaryls in moderate to good yields. The aqueous media, room temperature reaction and the low amounts of catalyst (0.5 mol%) show a practical and environmentally benign protocol. In addition, enhancement of yield was observed in the presence of 0.5 equiv. p-toluenesulfonyl chloride. Furthermore, the byproducts of this reaction were not observed while biaryl is the only product, which results in much more facile in the separation of the product symmetrical biaryls from arylboronic acids.

Full text of DOI:10.2174/1570178612666150115001813

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Letters in Organic Chemistry, 2015, 12, 205-209
205
Homocoupling of Arylboronic Acids Catalyzed by a Simple Hydrophilic
Palladium(II) Complex in Aqueous Media
*
Mengping Guo , Liang Qi, Meiyun Lv, Xiuling Zhou, Hui Liang and Sanbao Chen
Institute of Coordination Catalysis, Engineering Center of Jiangxi University for Lithium Energy, Key Laboratory of
Jiangxi University for Applied Chemistry and Chemical Biology, Yichun University, Yichun 336000, China
Received July 07, 2014: Revised December 24, 2014: Accepted January 13, 2015
Abstract: Homocoupling of arylboronic acids has been successfully carried out by using the inexpensive hydrophilic pal-
ladium(II) complex PdCl (NH CH COOH) as catalyst in i-PrOH/H O (v/v=1:2) under aerobic atmosphere without ele-
2 2 2 2 2
vated heating to give rise to symmetrical biaryls in moderate to good yields. The aqueous media, room temperature reac-
tion and the low amounts of catalyst (0.5 mol%) show a practical and environmentally benign protocol. In addition, en-
hancement of yield was observed in the presence of 0.5 equiv. p-toluenesulfonyl chloride. Furthermore, the byproducts of
this reaction were not observed while biaryl is the only product, which results in much more facile in the separation of the
product symmetrical biaryls from arylboronic acids.
Keywords: Hydrophilic catalyst, homocoupling of arylboronic acids, aqueous media, green chemistry.
INTRODUCTION
the latest challenges for modern chemists [34-35]. In this
respect, the development of catalysis in aqueous media
seems particularly suitable for the homocoupling of arylbo-
ronic acids due to the excellent stability of boronic acids in
aqueous media. Despite the aqueous systems have been used
previously for this homocoupling [29-31], no hydrophilic
catalysts have been reported for this reaction in aqueous me-
dia. For this purpose, we expended all our efforts on the de-
velopment of hydrophilic catalysts that can promote the ho-
mocoupling of arylboronic acids to prepare symmetrical aryl
biphenyls. In this communication we wish to report the use
The symmetrical biaryl motifs are common in the struc-
tures of many natural products, functional molecules and
active ligands employed in catalysis [1-5]. The homocou-
pling of arylboronic acid derivatives represents a widely
used approach to symmetrical biaryls [6-8], this is mainly
due to commercial availability of a wide range of arylboronic
acids [9-11]. So far, There has been a number of reports on
homocoupling of arylboronic acids, where the catalysts and
ligands were found essential to make the reaction proceed
smoothly, for example, Au [6-7, 12-17], Cu [18-20], Cr [21],
V [22], and Rh [5] can be used to mediate biaryl formation
from arylboronic acids. However, Pd-based catalysts are
most frequently used for the homocoupling of arylboronic
acids. Although phosphine-based ligands have remained to
be the most popular selection in the palladium-catalyzed
homocoupling of arylboronic acids [23-26], the drawbacks
of the catalyst, such as sensitivity to air, high costs, and tox-
icity, limit their application. The application of alternative
ligands such as N-heterocyclic carbenes [27], nitrogen
ligands [28-29], as well as ligand free systems [30-33], in the
palladium-catalyzed homocoupling of arylboronic acids has
opened new opportunities. The development of new catalytic
systems that perform at mild reaction temperatures in short
times using low catalyst loadings is still interesting. In addi-
tion, from environmental and economic points of view, water
is an abundant, nontoxic, noncorrosive and nonflammable
solvent, and the use of water as reaction medium is one of
of easily accessible, cheap and simple Glycine-ligated PdCl
to prepare PdCl (NH CH COOH) -catalyzed homocoupling
2
2
2
2
2
reaction of arylboronic acids in aqueous media at room tem-
perature under ambient atmosphere using low catalyst load-
ings.
RESULTS AND DISCUSSION
2 2 2 2
With the water-soluble catalyst PdCl (NH CH COOH)
in hand, we initially wanted to investigate the Suzuki-
Miyaura cross coupling reaction of aryl bromides with aryl-
boronic acids in pure water at room temperature under an air
atmosphere as Suzuki-Miyaura cross coupling reaction in
pure water with improved yield and simplify the reaction
protocol still remains a major challenge [36]. The investiga-
tion was first carried out with 4-bromoanisole and phenylbo-
ronic acids as the model reaction using 0.5 mol%
2 2 2 2
PdCl (NH CH COOH) as the catalyst in pure water under
ambient atmosphere. Surprisingly, when the reaction was
observed using phenylboronic acid as the only substrate,
PdCl (NH CH COOH) as the catalyst, K CO as the base
and pure water as the solvent without following by the addi-
tion of 4-bromoanisole, all the product was symmetrical
biphenyl which was confirmed by NMR. Therefore, this re-
*
Address correspondence to this author at the Institute of Coordination
Catalysis, Engineering Center of Jiangxi University for Lithium Energy,
Key Laboratory of Jiangxi University for Applied Chemistry and Chemical
Biology, Yichun University, Yichun 336000, China;
Tel: +86 0795 3200535; Fax: +86 0795 3200535;
E-mail: guomengping65@163.com
2
2
2
2
2
3
1
875-6255/15 $58.00+.00
© 2015 Bentham Science Publishers

2
06 Letters in Organic Chemistry, 2015, Vol. 12, No. 3
Guo et al.
a
Table 1. Initial screening for solvent, the amount of catalyst PdCl
2
(NH
2
CH
2
COOH)
2
and base effects on homocoupling reaction .
Cat.
Solvent, base
B(OH)2
b
c
Entry
Solvent
PdCl
2
(NH
2
CH
2
COOH)
2
(mol %)
Base
Yield (%)
1
2
3
4
5
6
7
8
9
H
2
O
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.1
1.0
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
K
K
K
K
K
K
K
K
K
2
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
59
31
76
67
90
50
45
80
92
65
55
58
60
71
30
32
42
21
2
Methanol/H O
Ethanol/H
2
2
O
O
n-PrOH/H
2
i-PrOH/H O
n-Butanol/H
2
O
i-PrOH
i-PrOH/H
i-PrOH/H
i-PrOH/H
i-PrOH/H
i-PrOH/H
i-PrOH/H
i-PrOH/H
i-PrOH/H
i-PrOH/H
i-PrOH/H
i-PrOH/H
2
2
2
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
O
O
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
Na
NaF
NaOH
PO
KOH
COONa
HCOONa
NEt
PO
2 3
CO
K
3
4
CH
3
3
KH
2
4
a
Reaction conditions: phenylboronic acid (1 mmol), base (1 mmol), 0.1-1.0 mol% of cat. PdCl
2.5 h.
2
(NH
2
CH
2
COOH)
2
, solvent/water 3 mL (v/v = 1/2), under air, room temperature,
1
b
2 2 2 2
Amounts of cat. PdCl (NH CH COOH) based on phenylboronic acid.
c
Isolated yield.
sult demonstrated that the homocoupling reaction of phenyl-
boronic acid was performed in the present protocol. Unfor-
tunately, only moderate homocoupling yield of 59% was
achieved when used phenylboronic acid as substrate. Con-
sidering better solubility of arylboronic acids in alcohols, we
investigated the influence of various alcohols and water (1:
prove the homocoupling yield significantly (entry 9) and
decreasing the catalyst loading to 0.1 mol% resulted in rela-
tively low homocoupling yield (entry 8). These results indi-
cated that the proper catalyst loading is 0.5 mol% to arylbo-
ronic acids (entry 5) and the catalyst is stable under the reac-
tion conditions for long time.
2
) on the reactions (Table 1, entries 2-6). Among the solvent
With the optimized protocol, a variety of electronically
and structurally diverse arylboronic acid was broadened in
air. The results of the arylboronic acid homo-coupling study
are summarized in Table 2. Phenylboronic acid without any
substituent group on the ring gave higher yield and shorter
reaction time than that of containing groups in air without
any additive (entry 3). For arylboronic acids, containing
tested, methanol, ethanol, propanol, isopropanol and n-
butanol, isopropanol was found to be the best (entry 5). The
use of pure isopropanol led to lower yield (entry 7), which
showed that the water could accelerate the oxidative dimeri-
zation of phenylboronic acid.
Based on these results, we tested different base effects on
homocoupling reaction such as KH
CH COONa, KOH, PO4, NaOH,
Na CO (entries 5, 10-18). It can be seen that K
best choice of base and homocoupling of phenylboronic acid
gave the highest yield (90%) (entry 5). Further, the effect of
2
PO4, NEt3, HCOONa,
CO3, NaF and
CO was the
3
groups at the para position such as CH , Cl, F and OMe af-
3
K
3
K
2
ford moderate to good yields of biaryl products in air without
any additive (entries 1-2, entries 4-5). On the other hand, in
the absence of an additive, homo-coupling occurs but the
yield is poor (entries 6-10). As was the case with previous
findings [29], addition of 0.5 equiv. p-toluenesulfonyl chlo-
ride improves the yield dramatically (96%, entry 5). The
catalyst system that we showed is suitable for non-steric hin-
dered arylboronic acids (entries 1-2, 4-6, 8-10), and for steric
2
3
2
3
loading of PdCl
ning the homocoupling of arylboronic acids in a 1:2 mixture
of i-PrOH/H O and K CO as base was investigated (entries
-9). Increasing catalyst loading to 1.0 mol% did not im-
2 2 2 2
(NH CH COOH) on this reaction by run-
2
2
3
8

Homocoupling of Arylboronic Acids Catalyzed
Letters in Organic Chemistry, 2015, Vol. 12, No. 3 207
a
Table 2. Homocoupling of various arylboronic acids .
0
.5 mol%PdCl (NH CH COOH)
2 2 2 2
B(OH)2
i-PrOH/H O =1/2, K CO , rt, air
2
2
3
R
R
R
b
Entry
ArB(OH)
2
Product
Yield (%)
8
1
1
H
3
C
B(OH)
2
H
3
C
CH
3
c
8
6
6
6
2
Cl
B(OH)
2
Cl
Cl
c
6
6
d
3
B(OH)2
90
6
3
4
5
MeO
B(OH)2
MeO
OMe
c
7
3
7
0
F
B(OH)2
F
F
c
9
6
F
F
F
F
F
3
2
6
7
B(OH)
2
c
6
9
F
F
F
F
2
3
c
F
B(OH)2
F
Et
F
36
3
8
Et
B(OH)2
8
9
Et
c
7
4
3
9
n-Pr
B(OH)2
n-Pr
n-Pr
c
6
9
a
2 3 2 2 2 2 2
Reaction conditions: arylboronic acid (1 mmol), K CO (1 mmol), 0.5 mol% of cat. PdCl (NH CH COOH) , i-PrOH /H O 3 mL (v/v = 1/2), under air, room temperature. All the
reactions were carried out for 20 h.
b
Isolated yield.
-Toluenesulfonyl chloride (0.5 mmol).
2.5 h.
c
ꢀ
d
1
hindered arylboronic acids, only 36% isolated yield could be
obtained (entry 7). Furthermore, the byproducts of this reac-
tion were not observed while biaryl is the only product,
which results in much more facile in the separation of the
product biaryls from arylboronic acids.
loading and easy removal from the reaction mixture com-
pared with other methods reported in the literature[29, 31].
Efforts to understand the reaction mechanism and to catalyze
other carbon-carbon bond forming reactions of organoboron
compounds are in progress.
CONCLUSION
EXPERIMENTAL SECTION
General
In summary, we demonstrate the first simple hydrophilic
palladium (II) complex PdCl (NH CH COOH) as an effec-
2 2 2 2
All the homocoupling reaction of arylboronic acids were
performed without the protection of inert gas. All chemicals
employed in the reaction were analytical grade, obtained
commercially from Aldrich or Alfa Aesar and used as re-
ceived without any prior purification. Analytical thin-layer
chromatography was performed using glass plates with 200-
tive catalyst for homocoupling reactions of arylboronic acids
to synthesize symmetrical biaryls. Our catalytic system of-
fers a mild, simple and environmentally benign alternative to
the existing protocols since the homocoupling reaction is
proceeded in aqueous media under aerobic atmosphere with-
out elevated heating using low amounts of PdCl
2 2
(NH
4
00 mesh silica gel impregnated with a fluorescent indicator
CH COOH) (0.5 mol%) without giving side products. The
2
2
(
254 nm). Petroleum ether is used as the eluting solvent for
use of hydrophilic palladium(II) catalyst makes this aqueous
system very attractive in view of its low cost, low catalyst
1
13
the preparative thin layer chromatography. H NMR,
C

2
08 Letters in Organic Chemistry, 2015, Vol. 12, No. 3
Guo et al.
1
NMR spectra were recorded on a Bruker Avance III (400
MHz) spectrometer using tetramethylsilane as the internal
3, 5, 3’, 5’-Tetrafluorobiphenyl [19]: H NMR (CDCl
7.1-7.2 (6Harom, m, 6CH).
3
):
):
ꢁ
H
H
standard and CDCl
3
as the solvent.
1
2
, 4, 2’, 4’-Tetrafluorobiphenyl [38]: H NMR (CDCl
3
ꢁ
6.74 (4Harom, m, 4CH), 7.35 (2Harom, m, 2CH).
Synthesis of Catalyst PdCl
2
(NH
2
CH
2
COOH)
2
The catalyst PdCl (NH
was prepared according to recently reported literature proce-
dures summarized in Scheme 1 [35].
2
2
CH COOH)
2
2
used in this work
CONFLICT OF INTEREST
The authors confirm that this article content has no
conflict of interest.
CH CH OH
3
2
PdCl +2NH CH COOH
PdCl (NH CH COOH)
2 2 2 2
2
2
2
CH COOH
3
RT
ACKNOWLEDGEMENTS
We are grateful to the National Natural Science Founda-
tion of China (No. 21063015, No. 21363026), Jiangxi Pro-
vincial Natural Science Foundation of China (No. 20114
BAB203012) and the Key Science and Technology plan of
Yichun City (No. [2010] 24) for their financial support.
Scheme 1. Synthesis of the catalyst.
Typical Experimental Procedure for Homocoupling
Reaction of Aryl Boronic Acids
To a stirred i-PrOH /H
arylboronic acid (1.0 mmol), PdCl
2
O 3 mL (v/v = 1/2) solution of
(NH CH COOH) (0.5
SUPPLEMENTARY MATERIALS
2
2
2
2
mol%), potassium carbonate (1 mmol) and ꢀ-
Toluenesulfonyl chloride (0.5 mmol). The solution was
stirred at room temperature under ambient atmosphere for 20
h. Ethyl acetate (3ꢀ25 mL) was added to extract the product.
Supplementary material is available on the publisher’s
web site along with the published article.
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was isolated by preparative thin layer chromatography on
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1
silica gel. The purified products were identified by H NMR,
1
3
[2]
[3]
C NMR spectroscopy. All the biaryl products are known
compounds, Spectral data for symmetrical biaryls:
1
4
, 4’-Dimethylbiphenyl [29]: H NMR (CDCl
3
): ꢁ
H
2.45
[4]
(
6H, s, 2CH
d, J=8.0 Hz, 4CH); C NMR (CDCl
29.5, 136.7, 138.3.
3
), 7.29 (4Harom, d, J=7.2 Hz, 4CH), 7.54 (4Harom,
13
C
): ꢁ 21.11, 126.8,
[5]
3
1
[6]
1
Biphenyl[29]: H NMR (CDCl
3 H
): ꢁ 7.66 (4Harom, d,
J=7.2 Hz, 4CH), 7.49 (4Harom, d, J=7.6 Hz, 4CH), 7.42
1
3
(
3 C
2Harom, t, J=7.2 Hz, 2CH); C NMR (CDCl ): ꢁ 127.2,
[
7]
8]
1
27.3, 128.8, 141.3.
, 4’-Dimethoxybiphenyl [29]: H NMR (CDCl
.87 (6H, s, 2CH ), 6.99 (4Harom, d, J=8.8 Hz, 4CH), 7.51
1
4
3 H
): ꢁ
[
3
3
1
3
(
1
4Harom, d, J=8.8 Hz, 4CH); C NMR (CDCl
14.2, 127.7, 133.5, 158.7.
3
): ꢁ
C
55.3,
[
9]
1
4
, 4’-Difluorobiphenyl [30]: H NMR (CDCl
3
): ꢁ
H
7.43
(
4Harom, d, J=8.8 Hz, 4CH), 7.50 (4Harom, d, J=8.8 Hz, 4CH);
1
3
[10]
C NMR (CDCl
3
): ꢁ
C
128.1, 129.1, 133.7, 138.4.
1
4
, 4’-Diethylbiphenyl [37]: H NMR (CDCl
3
): ꢁ
H
1.28
[
11]
(
2
6H, t, J=7.6 Hz, 2CH
CH CH ), 7.26 (4Harom, d, J=8.0 Hz, 4CH), 7.50 (4Harom, d,
J=8.4 Hz, 4CH); C NMR (CDCl
28.2, 138.6, 143.0.
2 3
CH ), 2.69 (4H, q, J=7.6 Hz,
2
3
1
3
3
): ꢁ
C
15.6, 28.5, 126.9,
[12]
1
1
4
, 4’-Dipropylbiphenyl [39]: H NMR (CDCl
6H, t, J=7.2 Hz, 2CH CH CH ), 1.63-1.72 (4H, m,
CH CH CH ), 2.62 (4H, t, J=7.2 Hz, 2CH CH CH ), 7.23
4Harom, d, J=8.0 Hz, 4CH), 7.50 (4Harom, d, J=8.0 Hz, 4CH);
3
3 H
): ꢁ 0.97
(
2
2
2
3
[
13]
2
2
3
2
2
3
(
1
C NMR (CDCl
41.5.
3
): ꢁ
C
13.9, 24.6, 37.7, 126.8, 128.8, 138.6,
1
[14]
1
4
, 4’-Dichlorobiphenyl [30]: H NMR (CDCl
3
): ꢁ
H
7.43
(
4Harom, d, J=8.0 Hz, 4CH), 7.50 (4Harom, d, J=8.0 Hz, 4CH).

Homocoupling of Arylboronic Acids Catalyzed
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