One-pot synthesis of 2,3-diarylbenzofurans via sequential iodocyclization and Pd-catalyzed Suzuki coupling reactions of 2-alkynylanisoles with boronic acids in water

Article abstract of DOI:10.1080/00397911.2014.930493

An efficient approach for the one-pot synthesis of 2,3-diarylbenzofurans via sequential iodocyclization and Pd-catalyzed Suzuki coupling reaction of 2-alkynylanisoles with boronic acids in water is reported. The protocol utilizes water as the solvent and

Full text of DOI:10.1080/00397911.2014.930493

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One-Pot Synthesis of 2,3-
Diarylbenzofurans via Sequential
Iodocyclization and Pd-Catalyzed Suzuki
Coupling Reactions of 2-Alkynylanisoles
with Boronic Acids in Water
Jiang-Sheng Han a b , Su-Qin Chen a , Ping Zhong a c & Xiao-Hong
Zhang a
a College of Chemistry and Materials Engineering , Wenzhou
University , Wenzhou , China
b ShanDong WeGo Pharmaceutical Co. Ltd., Wego Group , Weihai ,
China
c Oujiang College, Wenzhou University , Wenzhou , China
Accepted author version posted online: 17 Jun 2014.Published
online: 20 Aug 2014.
To cite this article: Jiang-Sheng Han , Su-Qin Chen , Ping Zhong & Xiao-Hong Zhang (2014) One-
Pot Synthesis of 2,3-Diarylbenzofurans via Sequential Iodocyclization and Pd-Catalyzed Suzuki
Coupling Reactions of 2-Alkynylanisoles with Boronic Acids in Water, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 44:21, 3148-3155,
DOI: 10.1080/00397911.2014.930493
To link to this article: http://dx.doi.org/10.1080/00397911.2014.930493
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Synthetic Communications1, 44: 3148–3155, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2014.930493
ONE-POT SYNTHESIS OF 2,3-DIARYLBENZOFURANS
VIA SEQUENTIAL IODOCYCLIZATION AND
Pd-CATALYZED SUZUKI COUPLING REACTIONS
OF 2-ALKYNYLANISOLES WITH BORONIC ACIDS
IN WATER
Jiang-Sheng Han,1,2 Su-Qin Chen,1 Ping Zhong,1,3 and
Xiao-Hong Zhang1
1College of Chemistry and Materials Engineering, Wenzhou University,
Wenzhou, China
2ShanDong WeGo Pharmaceutical Co. Ltd., Wego Group, Weihai, China
3Oujiang College, Wenzhou University, Wenzhou, China
GRAPHICAL ABSTRACT
Abstract An efficient approach for the one-pot synthesis of 2,3-diarylbenzofurans via
sequential iodocyclization and Pd-catalyzed Suzuki coupling reaction of 2-alkynylanisoles
with boronic acids in water is reported. The protocol utilizes water as the solvent and there
is no need to isolate the intermediate 3-iodine-2-arylbenzofurans, which exemplifies the
ideal of green chemistry. Various 2-alkynylanisoles and boronic acids can participate in
the reactions, providing a series of 2,3-diarylbenzofurans for drug discovery in moderate
to good yields.
Keywords 2-Alkynylanisole; boronic acid; 2,3-diarylbenzofuran; iodocyclization;
palladium; Suzuki coupling reaction
INTRODUCTION
Benzofuran derivatives are an important class of heterocyclic compounds and
they are found as a key structural unit in many natural products, pharmaceuticals,
and electroactive materials. For example, Anigopreissin A (Fig. 1) is the naturally
Received April 24, 2014.
Address correspondence to Xiao-Hong Zhang, College of Chemistry and Materials Engineering,
Wenzhou University, Wenzhou 325035, China. E-mail: kamenzxh@wzu.edu.cn
Color versions of one or more of the figures in the article can be found online at www.tandfonline.
com/lsyc.
3148

ONE-POT SYNTHESIS OF 2,3-DIARYLBENZOFURANS
3149
Figure 1. Anigopreissin A.
occurring resveratrol dimer isolated from root cultures of Anigozanthos preissii and
from rhizomes of Musa cavendish plants.[1] Viniferifuran[2] and its congener
gnetuhainin B[3] (Fig. 2) were first derived from Vitis vinifera Kyohou and lianas
of Gnetum hainanense, respectively. In addition, many benzofurans exhibit a wide
variety of pharmacological activities including anticancer, anti-inflammatory,
antifungal, anti-HIV, and antibacterial activities.[4]
Owing to their potential utility, there have been enduring efforts to develop
general and versatile methods for the synthesis of diversity-oriented benzofuran
derivatives, such as 3-aryl-2-benzoylbenzofurans,[5] 3-chalcogen benzofurans,[6]
3-aryl-2-arylamidobenzofurans,[7]
3-halo-
and
3-cyano
benzofurans,[8]
benzofuran-2-carboxylic acid,[9] 2-arylbenzofurans,[10] and so on. As part of our
ongoing efforts devoted to constructing benzofuran scaffold, our interest was
focused on the synthesis of 2,3-diarylbenzofurans.
Initially, 2,3-diarylbenzofurans can be synthesized through the reductive
cyclization of 2-benzyloxybenzophenone by low-valent titanium reagents,[11] which
were then improved by Kraus using the hindered phosphazene base P4-t-Bu in
benzene.[12] These two methods are limited to the use of uncommercial 2-benzylox-
ybenzophenone reactants, unstable titanium reagents, and the solvent of benzene. In
2005, Churruca et al. prepared a series of 2,3-diarylbenzofurans starting from
Figure 2. Viniferifuran and gnetuhainin B.

3150
J.-S. HAN ET AL.
1,2-diarylethanones and 1,2-dibromoarenes by means of both homogeneous and
polymer-anchored palladium catalysts,[13] but the high temperature (165 ꢀC) also
limited the application of this method.
The alternative synthesis of 2,3-diarylbenzofurans is the metal-catalyzed annu-
lation of o-alkynylphenol. For example, Hu et al. described the palladium=
bpy-catalyzed annulation of o-alkynylphenol with various aryl halides to generate
diversified 2,3-diarylbenzofurans.[14] The cyclization of a metal 2-ynyl phenolate to
a 3-metallobenzoheterole intermediate that reacts further with an electrophile to
afford the 2,3-diarylbenzofurans was reported by Nakamura et al.15 Moreover, a
one-pot method for the synthesis of 2,3-disubstituted benzofurans from commer-
cially available 2-iodophenols, terminal acetylenes, and aryl iodides has been
developed utilizing Sonogashira reaction conditions by Markina et al.[16] These
excellent works have the merits of good yields and wide substrate scopes as well
as the shortcomings of the environmental problems owing to the use of toxic solvent
CH3CN or the equivalent BuLi=ZnCl2. Some novel approaches for the 2,3-diaryl-
benzofurans have been developed via the Pd-catalyzed arylation of 2-arylbenzofuran
(benzofuran).[17] However, these methods required drastic conditions to realize
the C-H bond functionalization and the yield is relatively poor. At present, 2,3-
diarylbenzofurans were traditionally obtained via Pd-catalyzed Suzuki cross-
coupling reactions of 3-iodo-2-arylbenzofuran or 2,3-dibromobenzofuran with
boronic acids.[18] These reactions are dependent on forming the substrates of benzo-
furan halide, which require several extra synthetic steps and generate waste. The
overall process is neither atom economical nor green. Therefore, there is still a great
need for developing mild and environmentally benign reaction conditions to provide
the 2,3-diarylbenzofurans in good yields.
During our previous work,[6c] the by-products 3-iodo-2-arylbenzofurans were
discovered from the iodocyclization of 2-alkynylanisoles. So we envisioned that
2,3-diarylbenzofurans can be obtained via sequential iodocyclization and
Pd-catalyzed Suzuki coupling reaction of 2-alkynylanisoles with boronic acids in
one pot. After a series of trials, we are pleased to find that the reaction can proceed
smoothly when adopting water as the solvent in mild conditions. In this article, we
describe our results in detail.
RESULTS AND DISCUSSION
Initially, the reactants 2-phenylethynylanisole and iodine were added in a
Schlenk tube. After stirring for 8 h at room temperature, 2-phenylethynylanisole
was totally transformed to the intermediate 3-iodo-2-phenylbenzofurans by gas
chromatography–mass spectrometry (GC-MS). Then phenylboronic acid (2 equiv),
H2O (2 mL) different base (1 equiv), and catalyst (10 mol%) were added to screen
the optimal reaction conditions. It was found that Pd(PPh3)4 (Table 1, entry 4) gave
the best result among the screened Pd catalysts, such as PdCl2, Pd(OAc)2,
Pd(PPh3)2Cl2, Pd(TFA)2, Pd(dba)2, and Pd2(dba)3 (entries 1–3 and 5–7) when using
K3PO4 ꢁ 3H2O as base at 80 ꢀC. Subsequently, other bases including Na2CO3,
Cs2CO3, KOH, CsF, and t-BuOK were investigated, affording the 2,3-diphenylben-
zofuran (3aa) in less yield than K3PO4 ꢁ 3H2O (entries 8–12). The structure of 3aa
was also unambiguously confirmed by X-ray crystallography (Fig. 3). Controlled

ONE-POT SYNTHESIS OF 2,3-DIARYLBENZOFURANS
3151
Table 1. Reaction condition screening for the synthesis of 2,3-diphenylbenzofurana
Entry
Catalyst (10 mol%)
Base (1 equiv)
T (ꢀC)
Yieldb (%)
1
2
PdCl2
Pd(OAc)2
Pd(PPh3)2Cl2
Pd(PPh3)4
Pd(TFA)2
Pd(dba)2
K3PO4 ꢁ 3H2O
K3PO4 ꢁ 3H2O
K3PO4 ꢁ 3H2O
K3PO4 ꢁ 3H2O
K3PO4 ꢁ 3H2O
K3PO4 ꢁ 3H2O
K3PO4 ꢁ 3H2O
Na2CO3
80
80
80
80
80
80
80
80
80
80
80
80
50
100
62
58
84
95
65
80
85
91
85
53
35
78
65
96
3
4
5
6
7
Pd2(dba)3
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
8
9
Cs2CO3
KOH
10
11
12
13
14
CsF
t-BuOK
K3PO4 ꢁ 3H2O
K3PO4 ꢁ 3H2O
aReaction conditions: 2-phenylethynylanisole (0.5 mmol) and iodine (0.5 mmol) were added in a Schlenk
tube and stirred for 8 h at room temperature; then phenylboronic acid (1 mmol), base (0.5 mmol), catalyst
(10 mol%), and H2O (2 mL) were added and stirred for another 24 h under the corresponding temperature.
bThe yields were detected by GC-MS analysis.
experiments showed that no obvious improvement of the yield was observed by
increasing the temperature to 100 ꢀC, whereas the yield was dramatically decreased
to 65% at 50 ꢀC. Therefore, the optimal condition was identified as follows: boronic
acid (2 equiv), K3PO4 ꢁ 3H2O (1 equiv), Pd(PPh3)4 (10 mol%), and H2O (2 mL),
80 ꢀC, 24 h under air.
Figure 3. X-ray crystallography of 3aa.

3152
J.-S. HAN ET AL.
Table 2. Scope of boronic acids for the synthesis of 2,3-diarylbenzofuransa
Entry
Ar
Products
Yield (%)b
1
2
C6H5
4-CF3C6H4
4-FC6H4
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
84
74
79
80
90
86
55
38
74
41
55
66
84
3
4
4-ClC6H4
5
4-MeC6H4
4-EtC6H4
4-Ph-C6H4
3,5-diFC6H3
3,5-diMeC6H3
2-FC6H4
6
7
8
9
10
11
12
13
3aj
2-ClC6H4
2-MeC6H4
3-MeC6H4
3ak
3al
3am
aConditions: 2-phenylethynylanisole (0.5 mmol) and iodine (0.5 mmol) were added in a Schlenk tube and
stirred for 8 h at room temperature; then boronic acid (1 mmol), K3PO4 ꢁ 3H2O (0.5 mmol), Pd(PPh3)4
(10 mol%), and H2O (2 mL) were added and stirred for another 24 h at 80 ꢀC.
bIsolated yields.
With the optimized reaction in hand, the scope of arylboronic acids with
2-phenylethynylanisole was next discussed (Table 2). The results showed that the
reaction seemed to be sensitive to the electronic effects on the aromatic ring of
arylboronic acids. For example, arylboronic acids with 4-Me and 4-Et moieties
gave greater yields than their 4-CF3, 4-F, and 4-Cl equivalents (entries 2–6).
Similarly, substrates with 3,5-diMe and 2-Me (entries 9, 12) generated their corre-
sponding product in greater yields than those with 3,5-difluoro, 2-F, and 2-Cl
groups (entries 8, 10, and 11), respectively. In addition, steric hindrance from
arylboronic acids affected the efficiency dramatically. It is evident that arylboronic
acids bearing either electron-donating or electron-withdrawing substituents on C2,
C3, or C3,5 positions (entries 8–13) afforded the respective target molecules
in lower yields than phenylboronic acid (entry 1). To our delight, treatment of
4-biphenylboronic acid with 2-phenylethynylanisole also provided the compound
3ag in 55% yield (entry 7).
Subsequently, the scope of the 2-alkynylanisoles 1b–i for this reaction with boro-
nic acids was evaluated (Table 3) and various substituents of 2-alkynylanisoles have
slight influence on the reaction. 2-Alkynylanisoles that had electron-donating groups
at the terminal of the 1-ethynyl-2-methoxybenzene or on the anisole moiety favored
the coupling reactions more than those with electron-withdrawing groups. For
example, 4-methyl-2-phenylethynylanisole (1b), 2-(4-methylphenyl)ethynylanisole
(1c), and 4-methyl-2-(4-methylphenyl)ethynylanisole (1d) (entries 1–9) gave their

ONE-POT SYNTHESIS OF 2,3-DIARYLBENZOFURANS
3153
Table 3. Scope of 2-arylethynylanisoles and boronic acids for the synthesis of 2,3-diarylbenzofuransa
Entry
R1
Ar1
Ar2
Products
Yield (%)b
1
2
CH3
CH3
CH3
H
C6H5
C6H5
C6H5
C6H5
4-FC6H4
4-EtC6H4
C6H5
3ba
3bc
3bf
3ca
3cc
3cf
3da
3dc
3df
3ea
3ec
3ef
3fa
3ga
3ha
3ia
3ig
87
81
90
85
80
87
88
80
89
80
69
82
75
80
60
90
60
3
4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-FC6H4
4-FC6H4
4-FC6H4
C6H5
5
H
4-FC6H4
4-EtC6H4
C6H5
6
H
7
CH3
CH3
CH3
CH3
CH3
CH3
Cl
8
4-FC6H4
4-EtC6H4
C6H5
9
10
11
12
13
14
15
16
17
4-FC6H4
4-EtC6H4
C6H5
Cl
Cl
4-MeC6H4
4-FC6H4
4-Ph-C6H4
4-Ph-C6H4
C6H5
C6H5
H
C6H5
4-Ph-C6H4
H
aConditions: 2-phenylethynylanisole (0.5 mmol) and iodine (0.5 mmol) were added in a Schlenk tube and
stirred for 8 h at room temperature; then boronic acid (1 mmol), K3PO4 ꢁ 3H2O (0.5 mmol), Pd(PPh3)4
(10 mol%), and H2O (2 mL) were added and stirred for another 24 h at 80 ꢀC.
bIsolated yields.
corresponding products in slightly greater yields than 4-methyl-2-(4-fluorophenyl)
ethynylanisole (1e) and 4-chloro-2-arylethynylanisole (1f, 1g, 1h) (entries 10–15).
The steric hindrance at the terminal of the 1-ethynyl-2-methoxybenzene does not affect
the efficiency. The reaction of 4-biphenylethynylanisole (1i) with phenylboronic acid
and biphenylboronic acid can proceed smoothly, giving compounds 3ia and 3ig in
90% and 60% yields, respectively (entries 16 and 17).
CONCLUSIONS
In conclusion, we have developed a convenient and environmentally benign
protocol for the construction of 2,3-diarylbenzofurans via sequential iodocyclization
and Pd-catalyzed Suzuki coupling reaction of 2-alkynylanisoles with boronic acids
in water. The protocol utilizes water as the solvent, does not need isolation of the
intermediate 3-iodine-2-arylbenzofurans, and exemplifies the ideal of green chemis-
try. Various 2-alkynylanisoles and boronic acids can participate in the reactions,
providing a series of 2,3-diarylbenzofurans for drug discovery in moderate to good
yields.

3154
J.-S. HAN ET AL.
EXPERIMENTAL
Chemicals were either purchased or purified by standard techniques without
1
special instructions. H NMR and 13C NMR spectra were measured on 500-MHz
Brucker spectrometer, using CDCl3 as the solvent with tetramethylsilane (TMS) as
the internal standard at room temperature. Chemical shifts are given in d relative to
TMS, and the coupling constants J are given in hertz. Thin-layer chromatography
was performed using commercially prepared 60-mesh silica-gel plates, and visualiza-
tion was effected with short wavelength UV light (254 nm). All melting points are
uncorrected. All reagents were used directly as obtained commercially unless otherwise
noted. Column chromatography was performed using EM silica gel 60 (300–400
mesh). Known compounds are identified by comparison with authentic samples.
Typical Procedure for the Sequential Iodocyclization and
Pd-Catalyzed Suzuki Coupling Reaction of 2-Alkynylanisoles with
Boronic Acids in Water
2-Phenylethynylanisole (0.5 mmol) and iodine (0.5 mmol) were added in a
Schlenk tube and stirred for 8 h at room temperature; then boronic acid (1 mmol),
K3PO4 ꢁ 3H2O (0.5 mmol), Pd(PPh3)4 (10 mol%), and H2O (2 mL) were added and
stirred for another 24 h at 80 ꢀC. After cooling to room temperature and extracting
twice with EtOAc, the combined organic phase was washed with brine, dried over
MgSO4, filtered, and evaporated under reduced pressure. The crude product was pur-
ified by flash chromatography over silica gel with petroleum ether to afford the desired
product 2,3-diphenylbenzofuran (3aa) in 84% yield, mp 123–124 ꢀC. 1H NMR
(CDCl3, 500 MHz) d 7.65–7.67 (m, 2H), 7.55–7.56 (d, J ¼ 5 Hz, 1H), 7.45–7.51 (m,
5H), 7.38–7.42 (m, 1H), 7.28–7.34 (m, 4H), 7.22–7.25 (m, 1H); 13C NMR (125 MHz,
CDCl3) d 154.0, 150.5, 132.9, 130.7, 130.3, 129.8, 129.0, 128.4, 128.4, 127.6, 127.0,
124.7, 122.9, 120.0, 117.5, 111.1. Crystallographic data for 3aa: C20H14O, white prism,
˚
˚
formula weight 270.31, orthorhombic, P 21 21 21, a ¼ 7.9454(18) A, b ¼ 10.384(2) A,
c ¼ 17.007(4) A, V ¼ 1403.2(5) A , Z ¼ 4, Dcalcd ¼ 1.280 Mg=m3, R(int) ¼ 0.0172,
3
˚
˚
R1 ¼ 0.0300, wR2 ¼ 0.0874, GOF ¼ 1.218. CCDC No. 876668.
FUNDING
The authors thank the National Natural Science Foundation of China
(21302144).
SUPPORTING INFORMATION
Supplemental data for this article can be accessed on the publisher’s website.
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