Copper-catalyzed oxidative arylation of heteroarenes under mild conditions using dioxygen as the sole oxidant

Article abstract of DOI:10.1002/chem.201100136

Dioxy-gently does it! A new copper-catalyzed oxidative cross-coupling reaction between heteroarenes and arylboronic esters has been developed. Uniformly high yields are obtained by using dioxygen as the sole oxidant under mild conditions (see scheme). Copyright

Full text of DOI:10.1002/chem.201100136

COMMUNICATION
DOI: 10.1002/chem.201100136
Copper-Catalyzed Oxidative Arylation of Heteroarenes under Mild
Conditions Using Dioxygen as the Sole Oxidant
[
a, b]
[a]
[a, b]
[a]
[b]
Fanzhi Yang,
Zhaoqing Xu,* Zhe Wang,
Zhengkun Yu,* and Rui Wang*
Arylation of heteroarene has been intensively studied
over the past few years because aryl-substituted aromatic
heterocycles are known to exhibit interesting biological ac-
tivities and are also useful as electronic materials. Numerous
methods have been reported to successfully construct this
motif by transition-metal-catalyzed cross-coupling and most
of them required expensive transition-metal catalysts such
[1,2]
as Rh and Pd.
It is ideal to replace these catalysts by
cheap metals. In 2007, Daugulis and co-workers first report-
ed a copper-catalyzed cross-coupling of heteroarenes and
[3a]
aryl iodide under high temperature. You and co-workers
modified a prior reported method by using aryl bromide as
the electrophile; however, the reaction needed to be kept at
Scheme 1. Two pathways for the arylation of heteroarene.
[3b]
high temperature for at least for one day. Very recently,
Miura and Itami reported the Ni-catalyzed cross-coupling
between heteroarenes and aryl halide, respectively. Their
protocols also suffered from harsh conditions; for example,
nal oxidant that serves in oxidative cross-coupling always in-
volves metal salts, organohalide, or benzoquinone, and so
on. Replacement of these oxidants by dioxygen is more
practical and more economical since this oxidant is very
cheap and produces no environmentally hazardous byprod-
[3c,d]
high temperatures and long reaction times.
Therefore,
[5]
finding a more practical and efficient method to achieve this
aim remains a challenge.
uct. The C-2 position of azole or thiazole is easily depro-
tonated under basic conditions. It may serve as a nucleo-
phile and cross-couple with another nucleophile under oxi-
dative coupling conditions (Scheme 1, B). Recently, Su and
coworkers reported the Cu-catalyzed oxidative amination of
Like the traditional transition-metal-catalyzed cross-cou-
pling reactions, previously reported catalytic methods for ar-
ylation of heteroarenes normally worked between heteroar-
ene nucleophiles and aryl halide electrophiles (Scheme 1,
A). Recently, remarkable progress has been made in oxida-
[6]
azoles and thiazoles by using O as the oxidant. Only a
2
few examples have been reported on oxidative arylation of
[4]
[7]
tive cross-coupling. Unlike the traditional cross-coupling
reactions, the oxidative cross-coupling needs two nucleophil-
ic partners in the reaction and the transformation includes
three steps: a two-stage transmetalation to high-valent
metal catalyst, reductive elimination, then reoxidation of the
catalyst to high valency by the external oxidant. The termi-
azoles and thiazoles. Miura and co-workers developed an
oxidative arylation and alkenylation of heteroarenes includ-
ing azole and thiazole with organosilicon reagents by a
[8]
nickel catalyst. You reported a Pd-catalyzed oxidative
cross-coupling reaction between heteroarenes and aryl bor-
[9]
onic acid. Both of them require metal salts and/or benzo-
[10]
[11]
quinone as oxidant. Very recently, Pd- and Ni-catalyzed
oxidative arylation of azoles and thiazoles with aryl boronic
acid by using dioxygen as oxidant have been achieved. The
reactions also proceeded under high temperature and/or for
a long reaction time. All above mentioned drawbacks have
made the prior methods less attractive. In our study, we
found that CuCl could very efficiently catalyze the oxidative
cross-coupling between heteroarenes and arylboronic esters
with good yields and selectivities. This reaction was com-
plete in only minutes under mild conditions by using dioxy-
gen as the sole oxidant.
[
a] F. Yang, Prof. Z. Xu, Z. Wang, Prof. Z. K. Yu
Dalian Institute of Chemical Physics
Chinese Academy of Sciences
Dalian 116023 (P.R. China)
Fax : (+86)411-84379227
E-mail: xuzq@dicp.ac.cn
zkyu@dicp.ac.cn
[
b] F. Yang, Z. Wang, Prof. R. Wang
Key Laboratory of Preclinical Study
for New Drugs of Gansu Province Department
Institute of Biochemistry and Molecular Biology
Lanzhou University, Lanzhou 730000 (P.R. China)
Fax : (+86)931-8911255
We initiated our investigation by using Pd as the catalyst
and dioxygen as the oxidant (Table 1, entry 1). Under var-
iant conditions, only low yields were obtained (details are
E-mail: wangrui@lzu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100136.
[12]
shown in the Supporting Information). We then switched
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6321

[
a]
Table 1. Optimizing the reaction conditions.
onic acid was used instead of the corresponding ester, the
yield sharply declined (Table 1, entry 12). A control reaction
was carried out by replacing the reaction atmosphere with
argon; this resulted in the reaction being completely shut
down and it revealed that the dioxygen was crucial to this
transformation (Table 1, entry 13). It is known that copper
can very efficiently catalyze heteroarene homocoupling
under aerobic conditions. However, our system has shown
excellent selectivity for the cross-coupling of heteroarene
and arylboronic ester. Only trace amounts (<2%) of ben-
zoxazole homocoupling product was observed under the op-
timal conditions (Table 1, entry 6).
We first investigated the substrate scope with respect to
arylboronic esters by using benzoxazole and benzothiazole
as coupling partners under the optimal conditions. As shown
in Table 2, uniformly high yields are obtained with a variety
of arylboronic esters. Electron-deficient or electron-rich as
well as heteroaryl boronic ester all provide high yields. It is
worth mentioning that when 4-chlorophenylboronic ester is
used as substrate, the reaction shows a very good selectivity
for reacting at boronic ester side and keeps the chlorine side
untouched (Table 2, entries 11 and 19). This gives the prod-
Entry
Cat.
Pd(OAc)
Solvent
T
[
Yield
[%]
o
[b]
C]
1
2
3
4
5
6
7
8
9
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
DMF
DMF
DMF
DMF
DMF
DMF
DMF
dioxane
tBuOH
Toluene
DMF
DMF
DMF
40
40
40
40
40
40
23
40
40
40
40
40
40
0
[13]
CuI
83
85
trace
33
93
67
72
45
0
CuCl
2
Cu
Cu
2
O
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
10
11
12
13
[
[
[
c]
d]
e]
47
32
0
[
(
[
[
a] All reactions were carried out by using 1 (0.4 mmol), compound 2
1.5 equiv), tBuOLi (3 equiv), catalyst (10 mol%), and solvent (2 mL).
b] Isolated product yields. [c] tBuONa (3 equiv) was used as the base.
d] The corresponding boronic acid was used instead of 2. [e] Reaction
was carried out under argon atmosphere.
the catalyst to copper salts. To
our delight, the copper catalyst
was very active in the reaction.
[a]
Table 2. Arylation scope with variety of arylboronic esters.
At 408C, under O atmosphere,
2
and with CuI (10 mol%) as the
catalyst, the reaction was com-
plete within 10 minutes and
gave the desired arylation prod-
uct in 83% yield (Table 1,
Entry
Product
Yield
Entry
Product
Yield
[%]
[
b]
[b]
[
%]
1
2
93
99
11
12
91
92
entry 2). CuCl2 provided
a
slightly higher yield whereas
CuO only led to the formation
2
of a trace amount of product
(
Table 1, entries 3 and 4).
3
82
13
98
Copper powder exhibited low
catalytic activity and only 33%
yield was obtained (Table 1,
entry 5). Under the same condi-
tions, by using CuCl as the cata-
lyst, the cross-coupling product
was isolated in 93% yield
4
5
81
91
14
15
93
87
6
7
78
83
16
17
85
81
(
Table 1, entry 6). The reaction
also proceeded under room
temperature to give the product
in moderate yield (Table 1,
entry 7). The yield heavily
relied on the choice of solvent;
8
9
80
18
19
77
73
dioxane
or
tert-butanol
(
tBuOH) only provided moder-
87
85
ate yield whereas toluene gave
no arylation product (Table 1,
entries 8–10). tBuOLi was supe-
rior to tBuONa in our test
1
0
[a] All reactions were carried out by using 1 (0.4 mmol), compound 2 (1.5 equiv), tBuOLi (3 equiv), and DMF
(
Table 1, entry 11). When bor- (2 mL). [b] Isolated product yields.
6322
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Chem. Eur. J. 2011, 17, 6321 – 6325

Copper-Catalyzed Oxidative Arylation of Heteroarenes
COMMUNICATION
[
a]
Table 3. Arylation scope with respect to heteroarenes.
the arylation of aryl-substituted thiazole. When 1,10-phenan-
throline (15 mol%) is added, 5-phenylthiazole is also active
and couples with phenylboronic ester in 82% yield (Table 2,
entry 9).
It is known that alkylborane could generate an alkyl radi-
Entry
Heteroarene
Product
Yield
[14]
[
b]
cal by reacting with dioxygen. Also, copper can catalyze
terminal alkyne homocoupling reactions under a dioxygen
atmosphere through a radical mechanism, for example, in
[
%]
[
[
c]
1
2
90
75
[15]
Glaser coupling. To gain insight into the reaction mecha-
nism, a model reaction was carried out with 2,2,6,6-tetrame-
thylpiperidine-1-oxyl (TEMPO, 1.5 equiv) as the additive
under standard conditions (Scheme 2). The arylation prod-
d]
3
4
5
98
92
85
Scheme 2. Arylation of benzoxazole with 2,2,6,6-tetramethylpiperidine-1-
oxyl (TEMPO) as an additive.
6
80
7
8
88
85
uct was obtained with similar yield. So, the possibility of a
radical pathway could be ruled out. The copper-catalyzed
oxidative cross-coupling of arylboronic acid and heteroatom
(
N or O), which is called the Chan–Evans–Lam reaction,
typically employ milder conditions than the analogous Ull-
mann and Buchwald–Hartwig reactions. The mechanism has
[
d]
9
82
I
III
[16]
been proved to involve a Cu to Cu pathway. Combining
[16]
[
(
a] All reactions were carried out by using 4 (0.4 mmol), compound 2
1.5 equiv), tBuONa (3 equiv), and DMF (2 mL), except as noted. [b] Iso-
lated product yields. [c] Reaction carried out at 608C. [d] 1,10-phenan-
throline (15 mol%), tBuOLi (3 equiv), and DMF (4 mL) were used and
the reaction carried out at 1008C.
the reported mechanisms for similar reactions and our ex-
perimental observations, a preliminary reaction mechanism
for CuCl-catalyzed cross-coupling reaction of benzoxazole
with phenylboronic ester might be proposed, as shown in
Scheme 3. CuCl is oxidized to CuCl by molecular oxygen,
2
uct good opportunities for further transformations, such as
transition-metal-catalyzed cross-coupling.
The scope of the reaction with respect to the heteroarenes
is presented in Table 3. Although a higher temperature is
needed, the reactions are still very efficient and are com-
plete within 20 minutes in good yields and with good selec-
tivities. With azole or substituted azole as the substrate,
tBuONa was superior to tBuOLi (Table 2, entries 1, 3–8),
whereas when thiazole or substituted thiazole is used as sub-
strate tBuOLi gives a better yield (Table 2, entries 2 and 9).
Azole can be monoarylated at 608C with very good selectiv-
ity. No diarylation product was observed in the system
Scheme 3. Proposed reaction mechanism for CuCl-catalyzed cross-cou-
pling reaction of benzoxazole with phenylboronic ester.
(
Table 2, entry 1). Thiazole is normally diarylated by previ-
[3a]
ously reported method. We found that by using 1,10-phe-
nanthroline as ligand (15 mol%), the thiazole can be selec-
tively monoarylated at C-2 position and only a trace amount
followed by a transmetalation from phenyl boronic ester to
CuCl , which forms an arylcopper(II) species A. A similar
2
aryl copper(II) species was proposed as an intermediate in
Cu-catalyzed oxidative coupling of arylboronic acids with
(
<2%) of diaryl product is obtained. When 5-phenylazole is
[16a]
used, electron-rich and electron-poor arylboronic esters all
give good results (Table 2, entries 3–5). Azole with either an
electron-withdrawing substituent or electron-donating sub-
stituent provides excellent conversion to the desired product
nucleophiles.
Recently, a well defined aryl copper ACHTUNGTRENNUNG( III)
species was isolated by Stahl group and proved to be the
key intermediate in the catalytic cycle for copper-catalyzed
[16b,c]
aerobic oxidative functionalization of arene CÀH bond.
In our system, we also propose that aryl copper(II) A is oxi-
(
Table 2, entries 6–8). A appropriate ligand is important for
Chem. Eur. J. 2011, 17, 6321 – 6325
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6323

Z. Xu, Z. Yu, R. Wang et al.
dized to aryl copper
A
C
H
T
U
N
T
E
N
N
(III) intermediate B by another equiva-
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2
2
002, 124, 5286–5287; h) T. Martin, C. Verrier, C. Hoarau, F. Mar-
intermediate C. Reductive elimination results the desired
cross-coupling product in addition to the recycled catalyst.
However, we could not completely rule out other possibili-
ties such as a pathway involving a [h -CuCl(O )]-complex-
mediated-process at this stage.
In summary, we have discovered a novel copper-catalyzed
oxidative heteroarene arylation reaction. Heteroarene com-
pounds such as benzoxazole and benzothiazole were easily
cross-coupled with arylboronic esters at 408C in minutes
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2
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boronic acid derivatives with azoles and thiazoles using O2
as sole oxidant under mild conditions.
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[
3a]
Representative procedure: Synthesis of 2-phenylbenzoxazole: tBuOLi
96 mg, 1.2 mmol), CuCl (4 mg, 0.04 mmol), and DMF (2 mL) were
added to a 25 mL Schlenk tube charged with dry oxygen. The mixture
1
7
5049; h) P. Baran, M. P. DeMartino, Angew. Chem. 2006, 118,
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was stirred until the solid dissolved, after which benzoxazole (1, 48 mg,
9
0
.4 mmol) and phenylboronic ester (2, 114 mg, 0.6 mmol) were added in
order. The mixture was stirred at room temperature for 2 minutes, and
then at 408C for 10 minutes. The reaction was quenched with water, ex-
tracted with ethyl acetate, dried over sodium sulphate and concentrated
in vacuo. After column chromatography (2% ethyl acetate in petroleum
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ether), 73 mg (93%) of 2-phenylbenzoxazole (3) was obtained as a white
1
solid. R
f
=0.32 (2% ethyl acetate in petroleum ether). H NMR (CDCl
3
,
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4
7
00 MHz): d 7.34–7.37 (m, 2H), 7.52–7.54 (m, 3H), 7.57–7.60 (m, 1H),
.78–7.80 (m, 1H), 8.25–8.28 ppm (m, 2H); C NMR (CDCl , 100 MHz):
3
1
3
d 110.7, 120.1, 124.8, 125.3, 127.2, 127.8 (2C), 129.0 (2C), 131.7, 142.0,
50.8, 163.2 ppm.
1
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[
[
1
Keywords: arylation
·
boronic ester
·
copper
·
2
cross-coupling · heterocycles
[
[
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6324
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 6321 – 6325

Copper-Catalyzed Oxidative Arylation of Heteroarenes
COMMUNICATION
1
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mechanism study of [h -Pd
A
H
U
G
R
N
U
G
3
)
2
(O
2
)]-catalyzed homocoupling of
Received: January 14, 2011
arylboronic acid has been reported by Jutand and co-workers: C.
Published online: April 27, 2011
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www.chemeurj.org
6325