Iron-catalyzed synthesis of 2-arylbenzo[b]furans

Article abstract of DOI:10.1080/00397911.2011.610550

An iron-catalyzed procedure was employed to achieve both the Sonogashira cross-coupling and intramolecular o-arylation of o-iodophenols and aryl acetylenes/1-substituted-2-trimethylsilyl acetylenes. A variety of 2-arylbenzo[b]furans were synthesized in mo

Full text of DOI:10.1080/00397911.2011.610550

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Iron-Catalyzed Synthesis of 2-
Arylbenzo[b]furans
Jianguo Yang a , Guodong Shen b & Dingben Chen a b
a College of Pharmaceutical and Chemical Engineering, Taizhou
University, Taizhou, Zhejiang, China
b Department of Chemistry, Xi Xi Campus, Zhejiang University,
Hangzhou, Zhejiang, China
Accepted author version posted online: 22 Feb 2012.Version of
record first published: 21 Dec 2012.
To cite this article: Jianguo Yang , Guodong Shen & Dingben Chen (2013): Iron-Catalyzed Synthesis of
2-Arylbenzo[b]furans, Synthetic Communications: An International Journal for Rapid Communication
of Synthetic Organic Chemistry, 43:6, 837-847
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Synthetic Communications1, 43: 837–847, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2011.610550
IRON-CATALYZED SYNTHESIS OF
2-ARYLBENZO[b]FURANS
Jianguo Yang,1 Guodong Shen,2 and Dingben Chen1,2
1College of Pharmaceutical and Chemical Engineering, Taizhou University,
Taizhou, Zhejiang, China
2Department of Chemistry, Xi Xi Campus, Zhejiang University,
Hangzhou, Zhejiang, China
GRAPHICAL ABSTRACT
Abstract An iron-catalyzed procedure was employed to achieve both the Sonogashira
cross-coupling and intramolecular o-arylation of o-iodophenols and aryl acetylenes/
1-substituted-2-trimethylsilyl acetylenes. A variety of 2-arylbenzo[b]furans were synthesized
in moderate to good yields under the catalysis of 5% FeCl3 and 10% 1,10-phenanthroline.
Keywords 2-Arylbenzo[b]furan; intramolecular o-arylation; iron-catalyzed; one pot;
Sonogashira reaction; synthesis
INTRODUCTION
Benzo[b]furans are privileged organic compounds of medicinal significance
because of their biological and therapeutic activities,[1] such as antitumor
properties,[1b] anti-fungal properties,[2] inhibition of protein phosphatase 1B and
5-HT2 and 5-HT3 antagonist activity,[3] and inhibition of 5-Lipoxygenase
(5-LO).[4] They constitute key structural motifs in a wide range of natural products
and thus are appealing targets in drug synthesis.[5]
Benzo[b]furans are classically prepared via multistep reactions with particular
substrates. More recently, palladium- or copper-based cross-coupling using a tandem
Sonagashira coupling-cyclization reaction have been developed.[6] These protocols
offer increased functional group tolerance and improved yields of the desired ben-
zo[b]furans. However, based on the concerns of green chemistry, some environmen-
tally unfriendly metal catalysts should be avoided as far as possible in synthetic
chemistry. In recent years, several groups have focused on developing iron-catalyzed
Received June 18, 2011.
Address correspondence to Jianguo Yang, College of Pharmaceutical and Chemical Engineering,
Taizhou University, Taizhou 317000, Zhejiang, China. E-mail: yjg@tzc.edu.cn
837

838
J. YANG, G. SHEN, AND D. CHEN
cross-coupling reactions and other organic syntheses.[7] Compared to other metal cat-
alysts, iron-based methods have economic and environmentally benign attractiveness
and hence remain the reactions of choice in large and industrial-scale applications.
Herein we report a palladium-free synthesis of 2-arylbenzo[b]furans via an
iron(III)-catalyzed cross-coupling reaction of o-iodophenols with aryl acetylenes
and subsequent ring closure in situ using FeCl3 (5% mmol)=1,10-phenanthroline
(10% mmol) as the catalyst and Cs2CO3 as base in xylene.
RESULTS AND DISCUSSION
Initially, o-iodophenol and phenylacetylene were selected as the model sub-
strates (Table 1) and catalyzed by FeCl3. Different ligands, tempreatures, bases,
and solvents were examined to set up standard reaction conditions. Ligands L1–
L7 (Fig. 1) were tested to find the most active one. Initially, all experiments were car-
ried out with 5 mol% FeCl3 and 10 mol% ligand at 145 ꢀC in xylene with a reaction
time fixed to 36 h. From the results in Table 1, it is obvious that 1,10-phenanthroline
Table 1. Synthesis of 2-arylbenzo[b]furan via iron(III)-catalyzed coupling of o-iodophenol and phenyl-
acetylene in different conditionsa
Entry
Ligand
Base
Solvent
Time (h)
T (ꢀC)
Yield (%)b
1
2
L1
L2
L3
L4
L5
L6
L7
L2
L2
L2
L2
L2
L2
–
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
–
xylene
xylene
xylene
xylene
xylene
xylene
xylene
xylene
xylene
xylene
DMSO
NMP
36
36
36
36
36
36
36
36
24
36
24
24
24
36
36
36
36
36
36
36
145
145
145
145
145
145
145
145
145
120
145
145
145
145
145
145
145
145
145
145
73
82
44
0
3
4
5
0
6
50
41
58c
35
40
0
7
8
9
10
11
12
13
14
15
16
17
18
19
20
0
n-PrCN
xylene
xylene
xylene
xylene
xylene
xylene
xylene
0
25
18d
0
–
L2
L2
L2
L2
L2
K2CO3
Et3N
31
30
44
35
NaOH
t-BuOK
aReaction conditions: FeCl3 (0.025 mmol), ligand (0.05 mmol), o-iodophenol (0.5 mmol), phenylacety-
lene (0.75 mmol), and base (2.0 mmol) in solvent (2.0 mL) under N2.
bIsolated yield.
cFe2O3 (0.025 mmol) was used.
dNo FeCl3 was used.

IRON-CATALYZED SYNTHESIS OF 2-ARYLBENZO[b]FURANS
839
Figure 1. Examined ligands.
is the most efficient ligand (Table 1, entries 1–7) and gave the coupling product in
82% yield. When L-proline and 2-(dimethylamino)acetic acid were used as the
ligand, no product was detected (entries 4 and 5). The acidic ligands may have an
inhibitory effect on the reaction. With 1,10-phenanthroline as a ligand, both reduc-
ing the reaction time and lowering the temperature resulted in decreased yields
(entries 9 and 10). Fe2O3 is inferior to FeCl3 (entry 8). Besides xylene, a number
of other solvents such as dimethylsulfoxide (DMSO), NMP, and n-PrCN were also
surveyed. It was found that the use of the polar solvents suppressed the reaction
entirely (entries 11–13). Interestingly, the product was obtained in 25% yield in the
absence of the ligand and 18% without the metal catalyst and ligand (entries 14
and 15). Various bases were then screened (entries 16–20). Cs2CO3 was the optimal
one. Other bases such as K2CO3, NaOH, t-BuOK, and Et3N were less effective. The
reaction cannot proceed without the base.
With the optimized procedure in hand, the scope of this reaction was further
explored with different substrates, and the results are summarized in Table 2. Aryl
acetylenes with different functional groups were able to couple with 4-substituted-
o-iodophenols in moderate to good yields. The steric hindrance of both substrates
affected the product yields. Compared with 4-ethynylbenzonitrile, the yield of
2-ethynylbenzonitrile typically decreased (Table 2, entries 6, and 7). Electron-
donating groups on o-iodophenols were unfavorable to the product yields (entries
10 and 11). Para- and meta-substituted aryl acetylenes all could be coupled to
o-iodophenol in moderate to good yields (entries 2–5 and 7–9). Furthermore, several
1-substituted-2-trimethylsilyl acetylenes could be coupled with o-iodophenols and
gave a better effect than aryl acetylenes (Table 3).
According to the experimental results, we speculate that the reaction process
mechanism is as described in Scheme 1. First of all, we propose that initial step in
the mechanism is oxidative addition of the 2-iodophenol to Fe(II), giving an aryl
iron(III) intermediate I, followed by replacement between intermediate I and reac-
tion of alkyne with Fe(III) to afford intermediate II, then the reductive elimination
of intermediate II to give the desired product III, and finally, the cyclizations proceed
by an anti attack of the electrophile and the oxygen of the hydroxyl group on the
alkyne to produce an 2-arylbenzo[b]furans.
CONCLUSION
In summary, we have reported a synthetic protocol for 2-arylbenzo[b]furans
from o-iodophenols and aryl acetylenes=1-substituted-2-trimethylsilyl acetylenes

840

841

842
J. YANG, G. SHEN, AND D. CHEN
Table 3. Synthesis of 2-arylbenzo[b]furans via iron(III)-catalyzed coupling of o-iodophenols and various
1-substituted-2-trimethylsilyl acetylenesa
Entry
1
o-Iodophenol
TMS acetylene
Product
Yield (%)b
80
1a
3b
2
3
1a
1a
3 g
72
60
3f
4
5
6
1a
1d
1a
3e
70
64
80
4e
3a
7
8
8
9
1b
1c
1d
1e
4a
4a
4a
4a
3j
3k
3l
50
52
76
85
3m
aReaction conditions: FeCl3 (0.025 mmol), 1,10-phenanthroline (0.05 mmol), o-iodophenols (0.5 mmol),
1-substituted-2-trimethylsilyl acetylene (0.75 mmol), and Cs2CO3 (2.0 mmol) in xylene (2.0 mL) under N2.
bIsolated yield.
using the FeCl3=1,10-phenanthroline as the catalyst system. This protocol tolerates a
lot of functional groups that can be used to further functionalize the benzo[b]furan
ring. This synthetic procedure is palladium-free, economical, and environmentally
benign and avoids the use of expensive and=or air-sensitive additives. Its attractive-
ness makes it the reaction of choice in large and industrial-scale applications.
EXPERIMENTAL
All reagents and solvents were pure analytical-grade materials purchased from
commercial sources and were used without further purification, if not stated other-
wise. All 4-substituted-o-iodophenols and 1-substituted-2-trimethylsilyl acetylenes

IRON-CATALYZED SYNTHESIS OF 2-ARYLBENZO[b]FURANS
843
Scheme 1. Working mechanism.
were prepared according to the known literature.[8] All melting points are uncor-
rected. The NMR spectra were recorded in CDCl3 on a 400-MHz instrument with
tetramethylsilane (TMS) as internal standard. Thin-layer chromatography (TLC)
was carried out with 0.2-mm-thick silica-gel plates (GF254). Visualization was
accomplished by ultraviolet (UV) light. The columns were hand packed with silica
gel 60 (200–300). All reactions were carried out in an over-dried Schlenk tube
equipped with a magnetic stir bar under N2 atmosphere. Unknown compound
was additionally confirmed by 13C NMR, elemental analysis, and high-resolution
mass spectra. Mass spectra were obtained using electron impact (EI) ionization.
General Procedure
An over-dried Schlenk tube was charged with FeCl3 (0.025 mmol), base
(2.0 mmol), and ligand (0.05 mmol). The tube was vacuumed for 20 min and back-
filled with N2. Xylene (2.0 mL) was added under N2 and stirred for 0.5 h at room
tempreature. Then aryl acetylene or trimethylsilyl acetylene (0.75 mmol) and o-iodo-
phenol (0.5 mmol) were added under N2 atomsphere. The reaction mixture was stir-
red for 36 h at 145 ꢀC. The resulting suspension was cooled to room temperature and
filtered through a pad of filter paper with the help of 50 mL of ethyl acetate. The fil-
trate was concentrated, and the residue was purified by chromatography to afford
the products.
2-Phenylbenzofuran (3a)[9b,c]
1
A white solid; mp 119–120 ꢀC; H NMR (400 MHz, CDCl3=TMS): d 7.05 (s,
1H), 7.24–7.33 (m, 2H), 7.38 (t, J ¼ 7.6 Hz, 1H), 7.48 (t, J ¼ 8.0 Hz, 2H), 7.55 (d,
J ¼ 8.0 Hz, 1H), 7.61 (d, J ¼ 8.0 Hz, 1H), 7.89 (d, J ¼ 7.6 Hz, 1H). 13C NMR
(100 MHz, CDCl3=TMS): 155.84, 154.80, 130.40, 129.14, 128.72, 128.49, 124.86,

844
J. YANG, G. SHEN, AND D. CHEN
124.20, 122.87, 120.85, 111.12, 101.24. MS (EI) m=z 194 (Mþ); m=z (EI) 194.0732
(Mþ: C14H10O requires 194.0732).
2-p-Tolyl-benzo[b]furan (3b)[9a,b,c]
1
A white solid; mp 124–125 ꢀC; H NMR (400 MHz, CDCl3=TMS): d 2.41 (s,
3H), 6.98 (s, 1H), 7.20–7.28 (m, 4H), 7.53 (d, J ¼ 8.0 Hz, 1H), 7.58 (d, J ¼ 8.0 Hz,
1H), 7.78 (d, J ¼ 8.4 Hz, 2H). 13C NMR (100 MHz, CDCl3=TMS): 156.13, 154.70,
138.54, 132.35, 129.44, 127.68, 124.83, 123.95, 122.81, 120.69, 111.04, 100.50,
21.34. MS (EI) m=z 208 (Mþ); m=z (EI) 208.0885 (Mþ: C15H12O requires 208.0884).
2-(4-Propyl-phenyl)-benzo[b]furan (3c)
1
A white solid; mp 79–80 ꢀC; H NMR (400 MHz, CDCl3=TMS): d 0.97 (t,
J ¼ 7.4 Hz, 3H), 1.68 (m, J ¼ 7.6 Hz, 2H), 2.63 (t, J ¼ 7.6 Hz, 2H), 6.97 (s, 1H),
7.20–7.28 (m, 4H), 7.51 (d, J ¼ 8.0 Hz, 1H), 7.57 (d, J ¼ 8.0 Hz, 1H), 7.78 (d,
J ¼ 8.4 Hz, 1H). 13C NMR (100 MHz, CDCl3=TMS): 156.23, 154.79, 143.42,
126.37, 128.91, 127.98, 124.89, 123.99, 122.85, 120.75, 111.11, 100.59, 37.90, 24.47,
13.83. MS (EI) m=z 236 (Mþ); m=z (EI) 236.1200 (Mþ: C17H16O requires 236.1201).
2-(4-Methoxy-phenyl)-benzo[b]furan (3d)[9]
1
A white solid; mp 147–148 ꢀC; H NMR (400 MHz, CDCl3=TMS): d 6.91 (s,
1H), 7.00 (d, J ¼ 8.8 Hz, 2H), 7.20–7.28 (m, 2H), 7.50 (d, J ¼ 8.4 Hz, 1H), 7.55 (d,
J ¼ 7.2 Hz, 1H), 7.82 (d, J ¼ 9.2 Hz, 2H). 13C NMR (100 MHz, CDCl3=TMS):
159.95, 156.64, 154.68, 129.49, 126.41, 123.73, 123.32, 122.83, 120.57, 114.24,
110.98, 99.67, 55.35. MS (EI) m=z 224 (Mþ); m=z (EI) 224.0837 (Mþ: C15H12O2
requires 224.0837).
2-(4-Bromophenyl)benzo[b]furan (3e)
1
A white solid; mp 149–150 ꢀC; H NMR (400 MHz, CDCl3=TMS): d 7.02 (s,
1H), 7.22–7.32 (m, 2H), 7.51 (d, J ¼ 8.4 Hz, 1H), 7.56–7.59 (m, 3H), 7.72 (d,
J ¼ 8.8 Hz, 2H). 13C NMR (100 MHz, CDCl3=TMS): 154.81, 154.70, 131.90,
129.31, 128.96, 126.30, 124.54, 123.04, 122.42, 120.95, 111.14, 101.77. MS (EI)
m=z 272 (79Br, Mþ), 274 (81Br, Mþ); m=z (EI) 271.9837 (Mþ: C14H9OBr requires
271.9836).
2-(2-Cyanophenyl)benzo[b]furan (3f)
1
A white solid; mp 72–73 ꢀC; H NMR (400 MHz, CDCl3=TMS): d 7.30 (d,
J ¼ 7.6 Hz, 1H), 7.37 (t, J ¼ 7.6 Hz, 1H), 7.43 (t, J ¼ 7.6 Hz, 1H), 7.56 (d, J ¼ 8.0 Hz,
Hz, 1H), 7.67–7.73 (m, 3H), 7.78 (d, J ¼ 8.0 Hz, 1H), 8.12 (d, J ¼ 8.4 Hz, 1H). 13C
NMR (100 MHz, CDCl3=TMS): 154.56, 151.11, 134.34, 132.99, 132.87, 128.66,
128.10, 128.96, 125.64, 123.32, 121.87, 118.81, 111.20, 107.97, 106.71. MS (EI)
m=z 219 (Mþ); m=z (EI) 219.0684 (Mþ: C15H9ON requires 219.0684).

IRON-CATALYZED SYNTHESIS OF 2-ARYLBENZO[b]FURANS
845
2-(3-Cyanophenyl)benzo[b]furan (3g)
1
A white solid; mp 112–113 ꢀC; H NMR (400 MHz, CDCl3=TMS): d 7.11 (s,
1H), 7.27 (d, J ¼ 7.6 Hz, 1H), 7.35 (d, J ¼ 7.6 Hz, 1H), 7.54–7.63 (m, 4H), 8.06 (d,
J ¼ 8.0 Hz, 1H), 8.14 (s, 1H). 13C NMR (100 MHz, CDCl3=TMS): 154.97, 153.14,
131.63, 131.46, 129.59, 128.66, 128.59, 128.17, 125.20, 123.32, 121.31, 118.43,
113.10, 111.31, 103.11. MS (EI) m=z 219 (Mþ); m=z (EI) 219.0684 (Mþ: C15H9ON
requires 219.0684).
2-(3-Fluorophenyl)benzo[b]furan (3h)[9d]
A white solid; mp 76–78 ꢀC;1H NMR (400 MHz, CDCl3=TMS): d 7.02–7.07 (m,
2H), 7.23–7.26 (m, 1H), 7.31 (t, J ¼ 7.8 Hz, 1H), 7.38–7.43 (m, 1H), 7.52-7.65 (m,
4H).13C NMR (100 MHz, CDCl3=TMS): 163.05 (d, J ¼ 246 Hz), 154.83, 154.48 (d,
J ¼ 3 Hz), 132.48 (d, J ¼ 8 Hz), 130.33 (d, J ¼ 9 Hz), 128.86, 124.67, 123.06, 121.06,
120.48 (d, J ¼ 3 Hz), 115.27 (d, J ¼ 22 Hz), 111.73 (d, J ¼ 24 Hz), 111.20, 102.27;
MS (EI) m=z 212 (Mþ); m=z (EI) 212.0638 (Mþ: C14H9OF requires 212.0637).
2-(3-Nitrophenyl)benzo[b]furan (3i)
1
A yellow solid; mp 133–134 ꢀC; H NMR (400 MHz, CDCl3=TMS): d 7.18 (s,
1H), 7.27 (t, J ¼ 7.2 Hz, 1H), 7.35 (d, J ¼ 7.2 Hz, 1H), 7.55–7.64 (m, 3H), 8.14–8.19
(m, 2H), 8.69 (s, 1H). 13C NMR (100 MHz, CDCl3=TMS): 155.01, 153.04, 148.66,
132.05, 130.21, 129.78, 128.57, 125.31, 123.36, 122.75, 121.36, 119.57, 111.35,
103.50. MS (EI) m=z 239 (Mþ); m=z (EI) 239.0583 (Mþ: C14H9O3N requires
239.0582).
5-Methyl-2-phenyl-benzo[b]furan (3j)[9b,c]
1
A white solid; mp 130–131 ꢀC; H NMR (400 MHz, CDCl3=TMS): d 2.44 (s,
3H), 6.95 (s, 1H), 7.09 (d, J ¼ 8.0 Hz, 1H), 7.25–7.46 (m, 5H), 7.85 (d, J ¼ 7.6 Hz,
2H). 13C NMR (100 MHz, CDCl3=TMS): 155.89, 153.24, 132.26, 130.53, 129.22,
128.68, 128.33, 125.46, 124.77, 120.64, 110.58, 101.00, 21.28. MS (EI) m=z 208
(Mþ); m=z (EI) 208.0887 (Mþ: C15H12O requires 208.0888).
5-tert-Butyl-2-phenyl-benzo[b]furan (3k)[9c]
A white solid; mp 103–104 ꢀC;1H NMR (400 MHz, CDCl3=TMS): d 1.43 (s,
9H), 7.03 (s, 1H), 7.37–7.49 (m, 5H), 7.62 (s, 1H), 7.89 (d, J ¼ 7.6 Hz, 2H). 13C
NMR (100 MHz, CDCl3=TMS): 155.95, 153.05, 145.90, 130.60, 128.81, 128.70,
128.31, 124.78, 122.19, 117.03, 110.38, 101.43, 34.66, 31.79. MS (EI) m=z 250
(Mþ); m=z (EI) 250.1358 (Mþ: C18H18O requires 250.1358).
5-Chloro-2-phenyl-benzo[b]furan (3l)[9c,d]
1
A white solid; mp 155–156 ꢀC; H NMR (400 MHz, CDCl3=TMS): d 6.96 (s,
1H), 7.22–7.24 (m, 1H), 7.36–7.39 (m, 1H), 7.42–7.48 (m, 3H), 7.54–7.55 (m, 1H),

846
J. YANG, G. SHEN, AND D. CHEN
7.85 (d, J ¼ 7.2 Hz, 2H). 13C NMR (100 MHz, CDCl3=TMS): 157.37, 153.22, 130.56,
129.93, 128.98, 128.85, 125.03, 124.78, 120.39, 112.11, 100.77. MS (EI)
m=z 228 (35Cl, Mþ), 230 (37Cl, Mþ); m=z (EI) 228.0342 (Mþ: C14H9OCl requires
228.0342).
5-Fluoro-2-phenyl-benzo[b]furan (3 m)
A white solid; mp 175–177 ꢀC; 1H NMR (400 MHz, CDCl3=TMS): d 6.97–7.02
(m, 1H), 7.23 7.85 (dd, J1 ¼ 2.8 Hz, J2 ¼ 8.4 Hz, 1H), 7.37 (d, J ¼ 8.0 Hz, 1H),
7.42–7.47 (m, 3H), 7.85 (d, J ¼ 6.8 Hz, 2H). 13C NMR (100 MHz, CDCl3=TMS):
159.25 (d, J ¼ 237 Hz), 157.62, 151.03, 130.04, 129.96 (d, J ¼ 10 Hz), 128.82,
128.76, 124.92, 111.80 (d, J ¼ 22 Hz), 111.2 (d, J ¼ 4 Hz), 106.24 (d, J ¼ 26 Hz),
101.33 (d, J ¼ 5 Hz); MS (EI) m=z 212 (Mþ). Anal. calcd. for C14H9FO: C, 79.23;
H, 4.27. Found: C, 79.23; H, 4.26.
5-Chloro-2-(4-bromophenyl)-benzo[b]furan (3n)
1
A white solid; mp 175–177 ꢀC; H NMR (400 MHz, CDCl3=TMS): d 6.97 (s,
1H), 7.37–7.49 (m, 2H), 7.55 (d, J ¼ 2.4 Hz, 1H), 7.58 (d, J ¼ 8.4 Hz, 2H), 7.71
(d, J ¼ 8.8 Hz, 2H). 13C NMR (100 MHz, CDCl3=TMS): 156.18, 153.19, 131.98,
130.31, 128.81, 128.61, 126.39, 124.67, 122.97, 120.44, 112.07, 101.23. MS (EI) m=z
308 (35Cl, 81Br, Mþ), 306 (35Cl, 79Br, Mþ), 310 (37Cl, 81Br, Mþ). Anal. calcd. for
C14H8BrClO: C, 54.67; H, 2.62. Found: C, 64.66; H, 2.63.
ACKNOWLEDGMENTS
This work was financially supported by the Natural Science Foundation of
China (No. 21272169), the Specialized Research Fund for the Doctoral Program
Foundation of Higher Education of China (No. 2060335036), and Natural Science
Foundation of Zhejiang Province for founding (No. Y406049).
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