Iodine-promoted efficient homocoupling of arylboronic acids in PEG-400 under Aerobic conditions

Article abstract of DOI:10.1002/ejoc.200900087

Iodine is able to catalyze the homocoupling of arylboronic acids in moderate to good yields in PEG-400 under aerobic conditions. This transition-metal-free catalytic system is readily available at low cost, which is practical and useful. It is noteworthy that a high activity of our catalytic system could be maintained under mild conditions (100 °C) by addition of water.

Full text of DOI:10.1002/ejoc.200900087

FULL PAPER
DOI: 10.1002/ejoc.200900087
Iodine-Promoted Efficient Homocoupling of Arylboronic Acids in PEG-400
under Aerobic Conditions
[a]
[b]
[a]
[c]
[a]
Jincheng Mao,* Qiongqiong Hua, Guanlei Xie, Zhigang Yao, and Daqing Shi*
Keywords: C–C coupling / Arylboronic acids / Aerobic conditions / Iodine / Biaryls
Iodine is able to catalyze the homocoupling of arylboronic
acids in moderate to good yields in PEG-400 under aerobic
conditions. This transition-metal-free catalytic system is
readily available at low cost, which is practical and useful. It
is noteworthy that a high activity of our catalytic system
could be maintained under mild conditions (100 °C) by ad-
dition of water.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
Introduction
The synthesis of biaryls through C–C bond-forming re-
actions is of great interest because of the prevalence of this
structural motif in a myriad of bioactive important natural
[
1]
products and pharmaceutically interesting compounds.
Various biaryl coupling methods have thus been developed
[
2]
in the past decades. Among them, cross-coupling between
arylboronic acids and aryl halides has proved effective and
has attracted much attention.[
2a]
Figure 1. Symmetrical biaryl moieties in natural products.
In contrast, there are very few preparations of symmetri-
cal biaryls through homocoupling reactions,[2b,3] although
symmetrical biaryl moieties are common in the structures
of many natural products (Figure 1). Crisamicin A, for ex-
ample, which has been isolated from a mud sample in the
formed to show the importance of oxidative coupling in
[
8]
the presence of dioxygen. With the increasing demand for
[
9a]
environmentally friendly methods, we
groups
and other
[
9b–9e]
have found that the Pd-catalyzed oxidative di-
Philippines, was found to be active primarily towards
Gram-positive bacteria.[4] Biphenomycin B, which was pre-
merization of arylboronic acids or esters can be performed
in aqueous media. In many cases, addition of certain oxi-
dants was necessary to acquire high yields of biaryls.[
Recently, Demir et al. reported that certain copper spe-
cies are able to mediate the homocoupling of arylboronic
viously isolated from culture filtrates of Streptomyces grise-
orubiginosus No. 43708, was found to be a highly potent
10]
antibiotic against Gram-negative, β-lactam-resistant bacte-
ria.[5] The discovery of these compounds’ important phar-
[
11]
acids in moderate to good yields. Corma and co-workers
described the supported-gold-catalyzed homocoupling of
phenylboronic acid with high levels of conversion and selec-
macological properties thus stimulated substantial interest
in homocoupling chemistry.[6]
The procedures commonly used for the preparation of
symmetrical biaryls rely on palladium-catalyzed dimeriza-
tion of arylboronic acids in the presence of various li-
[
12]
tivity.
Very recently, homocoupling of arylboronic acids
III[13]
[14]
mediated by Mn
and by Ag O/CrCl2
has been re-
2
[
7]
ported. Although considerable efforts have been made, the
development of other effective routes to these biaryls would
gands. Studies of the possible mechanism have been per-
[
15]
be highly desirable.
In this context, iodine-mediated
[
a] Key Laboratory of Organic Synthesis of Jiangsu Province, Col-
lege of Chemistry, Chemical Engineering and Materials Science,
Suzhou University,
homocoupling of arylboronic acids under aerobic condi-
tions will possibly provide a promising alternative.
Suzhou 215123, P. R.China
[
[
b] Department of Chemistry, Xuzhou Normal University,
Xuzhou 221116, P. R. China
c] Analytical & Testing Center, Suzhou University,
Suzhou 215123, P. R. China
Results and Discussion
Fax: +86-512-6588-0089
E-mail: jcmao@suda.edu.cn
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
One simple way to address demands for recyclability and
other environmental concerns is to immobilize a catalyst in
a liquid phase by dissolving it in a nonvolatile liquid of low
2262
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Eur. J. Org. Chem. 2009, 2262–2266

Homocoupling of Arylboronic Acids
toxicity, such as a poly(ethylene)glycol (PEG).[16] Accord-
We therefore performed the catalytic reactions at 140 °C.
ingly, we reported the first example of copper-powder-cata- By screening different bases, including KF, Cs CO , KOH,
2
3
lyzed reusable Suzuki–Miyaura coupling,[ which proved K CO , and K PO (Entries 7–11), we found that K CO
17]
2
3
3
4
2
3
capable of affording almost quantitative yields of coupling was the best one (Entry 10). Shortened times afforded dis-
products from aryl iodides. In the presence of iodine as a appointing results (Entry 12). Use of a higher loading of
necessary additive, it was also possible to perform smooth iodine was favorable for the synthesis of biphenyl (En-
coupling reactions involving aryl bromides or chlorides. tries 13–14), whereas with TBAB (nBu NBr) or LiCl as the
4
However, we do not know the exact role of the iodine in additive we were unable to achieve enhanced results (En-
the mechanism, and so for this paper a series of experiments tries 15–16). Use of more iodine or of a longer reaction time
were conducted to try to make this clear.
(48 h) afforded better yields (Entries 17–19). Finally, use of
The experimental data for the screening conditions are 1 equiv. of iodine afforded biphenyl almost quantitatively
listed in Table 1. Initially, the homocoupling of phenylbo- (97% yield, Entry 19). With the same reaction time, re-
ronic acid was chosen as the model reaction. Iodine-cata- duction of the reaction temperature or of the amount of
lyzed homocoupling reactions were conducted both under base both gave inferior results (Entries 20–21).
argon and in air (Table 1, Entries 1–2). As expected, the
Under the optimized conditions, various arylboronic ac-
reaction under aerobic conditions gave a higher yield of the ids were employed in the homocoupling reactions; the re-
desired product (Entry 2). At the same time, TLC showed sults are summarized in Table 2. With 4-fluorophenylbo-
that the use of common organic solvents, such as dimethyl- ronic acid as the substrate, an increase in the temperature
formamide, dimethyl sulfoxide, dioxane, ethanol, and from 140 °C to 150 °C led to an enhanced yield (Table 2,
dichloromethane, did not afford the biphenyl. The blank Entries 2–3). However, this modification was not suitable
experiment showed that without iodine, the reaction could for the reaction of 4-chlorophenylboronic acid (Entries 4–
not occur (Entry 3). Other halogenated reagents such as 5), and an even worse result was obtained with ethyl bromo-
bromine and NBS (N-bromosuccinimide) were also investi- acetate used as the additional oxidant (Entry 6), although
gated, but no catalytic reactions occurred (Entries 4–5). for the reaction of 3-methoxyphenylboronic acid this oxi-
Without K CO , we also could not obtain the desired prod- dant was beneficial (Entries 7–8). Such irregularity has also
2
3
[
10a]
uct (Entry 6).
been observed in Pd-catalyzed homocouplings.
Per-
forming of this reaction under oxygen gave a further im-
provement in the result (Entry 9).
With 2-methoxyphenylboronic acid as the substrate,
lower yields of biaryl were obtained even at higher tempera-
ture (Entries 10–11), which is possibly due to the steric sub-
stituent effect. This explanation was further supported by
the fact that the self-coupling product of 2-methylphenylbo-
ronic acid could not be acquired even if iodine (3 equiv.)
was employed (Entry 12).
3-Formylphenylboronic acid gave a 50% yield of the cor-
responding biaryl (Entry 13).
The homocoupling of para-substituted arylboronic acids
gave good results (Entries 14–15), whereas with thiophen-
Table 1. Screening of catalytic conditions for iodine-mediated
homocoupling of phenylboronic acid.[
a]
Entry
Additive
Base
Temp. [°C] Time [h] Yield [%][b]
1[c]
I
I
2
(20%)
(20%)
–
K
K
K
K
K
2
2
2
2
2
CO
CO
CO
CO
CO
–
3
3
3
3
3
110
110
110
110
110
110
140
140
140
140
140
140
140
140
140
140
140
140
140
100
140
24
24
24
24
24
24
24
24
24
24
24
12
24
24
24
24
48
48
48
48
48
6
11
n. r.
n. r.
n. r.
n. r.
10
n. r.
trace
31
30
20
42
44
43
40
54
73
97
75
7
2
2
[
[
[
[
d]
d]
d]
d]
3
4
5
6
Br
2
(20%)
3-ylboronic acid as the substrate only a moderate yield of
NBS (20%)
the desired product was obtained (Entry 16). To our sur-
prise, the same coupling reaction did not take place when
carried out under oxygen (Entry 17). In addition, the self-
coupling of 3-pyridinylboronic acid gave no product (En-
try 18).
I
2
I
2
I
2
I
2
I
2
I
2
I
2
I
2
I
2
I
2
I
2
I
2
I
2
(20%)
(20%)
(20%)
(20%)
(20%)
(20%)
(20%)
(30%)
(50%)
(30%)
(30%)
(30%)
(50%)
(100%)
(100%)
(100%)
7
KF
Cs CO
KOH
CO
PO
[
d]
8
2
3
9
1
0
1
2
3
4
K
2
3
1
1
1
1
K
3
4
Subsequently, we tried to apply the I /K CO /PEG-400
K
K
K
K
K
K
K
K
K
K
2
2
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
3
2
2
3
system for the homocoupling of other aryl reagents. As
shown in Scheme 1, no biphenyl was obtained when iodo-
benzene or trimethoxy(phenyl)silane were employed. How-
ever, this catalytic system was effective for various borane
sources, such as phenylboronic esters and potassium trifluo-
ro(phenyl)borate.
Couplings between two different arylboronic acids, to
provide unsymmetrical biaryls, could be conducted
smoothly under the standard conditions. As shown in
Scheme 2, as well as the unsymmetrical products, self-coup-
ling products of the two individual arylboronic acid compo-
nents were also obtained, as would be expected and as was
[
e]
1
1
5
6
[
f]
1
7
8
9
0
1
1
2
I
2
I
I
2
[
g]
21
2
[
(
[
a] Reaction conditions: 1a (0.8 mmol),
4.0 mmol), PEG-400 (2 mL), 100–140 °C, in air. [b] Isolated yield.
c] Under argon. [d] No reaction. [e] TBAB (10 mol-%) as the ad-
ditional additive. [f] LiCl (10 mol-%) as the additional additive.
g] K CO (0.8 mmol) was used.
2
I (20 mol-%), base
[
2
3
Eur. J. Org. Chem. 2009, 2262–2266
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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J. Mao, Q. Hua, G. Xie, Z. Yao, D. Shi
FULL PAPER
Table 2. Iodine-promoted homocoupling of various arylboronic ac-
[
a]
ids.
Scheme 1. Other aryl reagents investigated with the I
2 2 3
/K CO /
PEG-400 system.
Scheme 2. Unsymmetrical biaryls synthesized by our method.
were different; we believe that this might be due to the dif-
ference between the activities of the α- and the β-positions
on the naphthyl ring. Meanwhile, 4-(naphthalen-1-yl)phen-
ylboronic acid was also employed in the homocoupling re-
action. To our astonishment, only a trace of the desired
self-coupling product was found, and 1-phenylnaphthalene
was obtained in 70% yield. From these experimental data,
[
(
2 2 3
a] Reaction conditions: 1 (0.8 mmol), I (100 mol-%), K CO
4.0 mmol), PEG-400 (2 mL), 140 °C, 48 h, in air. [b] Isolated yield. we wondered whether the coupling reaction in the presence
COOEt (1 equiv.) as additive.
e] The catalytic reaction was performed under oxygen. [f] No reac-
[
[
c] Temperature 150 °C. [d] BrCH
2
of the I /K CO /PEG-400 system proceeds through a radi-
2
2
3
cal mechanism.
tion occurred even with iodine (8 equiv.). [g] No reaction.
In order to clarify the possible radical nature of this reac-
tion, the homocoupling of phenylboronic acid with the
confirmed by GC-MS. It is interesting to observe that the I /K CO /PEG-400 system was conducted in the presence
2
2
3
amount of cross-coupling product is approximately the sum of 4-OH-TEMPO (4-hydroxy-2,2,6,6-tetramethylpiperidin-
of the amounts of the two different homocoupling products. 1-oxyl, 10 mol-%, Scheme 4). GC-MS showed that no bi-
In the application of our methodology to two naph- phenyl was obtained. This suggests that once the phenylbo-
thylboronic acids (Scheme 3), GC-MS suggested that naph- ronic acid has decomposed to the phenyl radical, it will be
thalene was detectable besides the biaryl products. It was immediately quenched by 4-OH-TEMPO. Actually, it has
found that the ratios between the biaryl and naphthalene been reported that aryl radicals are generated from aryl-
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2009, 2262–2266

Homocoupling of Arylboronic Acids
2 2 3
Scheme 3. Homocoupling of several specific arylboronic acids with the I /K CO /PEG-400 system.
boronic acids in the presence of some oxidants.[13,18] In ad- were obtained up to a ratio of 5:1, at which we obtained
dition, Demir et al. observed that aryl radicals were ob- our best result, in the form of a 94% yield (Entry 5). How-
tained from arylhydrazines in the presence of the potassium ever, further increases in the ratio resulted in poorer results
[
19]
permanganate/carboxylic acid/organic solvent system.
(Entries 6–7). We can thus see that appropriate amounts of
water might accelerate the self-coupling reactions.
Table 3. Homocoupling of phenylboronic acid in aqueous solu-
tions.[
a]
Entry
Solvent [v/v]
Temp. [°C]
Yield [%][b]
1
2
3
4
5
6
7
H
2
O
100
100
100
100
100
100
100
trace
12
24
72
94
67
58
PEG-400/H
PEG-400/H
PEG-400/H
PEG-400/H
PEG-400/H
PEG-400/H
2
2
2
2
O (1:2)
O (1:1)
O (2:1)
O (5:1)
Scheme 4. Experiments for consideration of the possible mecha-
nism.
2
O (10:1)
O (20:1)
2
The question that then arises is: What is the oxidant in
2 2 3
[a] Reaction conditions: 1a (0.8 mmol), I (100 mol-%), K CO
our catalytic reaction? Consequently, hypervalent iodine, (2.0 mmol), solvent (2 mL), 100 °C, 48 h, in air. [b] Isolated yield.
such as iodobenzene diacetate, was employed to replace io-
dine. The result showed that biphenyl was obtained almost
quantitatively, and so we propose that iodine serves not only Conclusions
as the catalyst but also as the oxidant. We speculate that
We showed that iodine was able to promote the homo-
coupling of arylboronic acids in PEG-400 under aerobic
conditions in moderate to good yields. In comparison with
previously reported systems, this transition-metal-free cata-
lytic system is readily available and at low cost, unlike palla-
dium catalyst systems, which has particular relevance to the
fields of drug discovery and manufacture. It is noteworthy
that addition of water endowed our catalytic system with a
high activity under milder conditions. Further work is in
progress in this laboratory with the aims of extending the
application of these readily available catalytic systems in
other coupling transformations and of studying the mecha-
nisms of such iodine-mediated homocoupling reactions.
iodine is probably transformed into some hypervalent io-
dine species in this particular solvent (PEG-400) in the pres-
ence of oxygen from the air and of K CO in excess. In this
2
3
way, the putatively generated oxidant would perhaps result
in the decomposition of arylboronic acids at high tempera-
ture (140 °C), and the corresponding radicals could be pro-
duced. This could thus explain why the homocoupling
products and the decomposition products were obtained
during the self-coupling of arylboronic acids.
For interest in green chemistry, we then began to explore
the couplings in aqueous solution at lower temperature
(100 °C), and the relationship between volume ratio (PEG-
400/water) and the yield of biphenyl is shown in Table 3. It
was found that when only water was used, no product was
obtained, which may be attributed to the poor solubility
Experimental Section
of the phenylboronic acid in water (Table 3, Entry 1). With General Remarks: All reactions were carried out in air. All aryl-
increasing PEG-400/H O ratio, greater amounts of product boronic acids or esters and other chemical reagents were purchased
2
Eur. J. Org. Chem. 2009, 2262–2266
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2265

J. Mao, Q. Hua, G. Xie, Z. Yao, D. Shi
FULL PAPER
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Eur. J. Org. Chem. 2009, 2262–2266