Palladium nanoparticles catalyzed synthesis of benzofurans by a domino approach

Article abstract of DOI:10.1055/s-0034-1380463

Abstract Palladium nanoparticles (PdNPs) were used as a catalyst for the one-pot synthesis of a variety of benzofurans by Sonogashira cross-coupling reactions under ambient conditions. The catalyst could be recycled without significant loss in its activity.

Full text of DOI:10.1055/s-0034-1380463

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2015, 47, 1661–1668
paper
1661
P. K. Mandali, D. K. Chand
Paper
Synthesis
Palladium Nanoparticles Catalyzed Synthesis of Benzofurans by a
Domino Approach
Pavan Kumar Mandali
Dillip Kumar Chand*
R1
R1
R1
I
R3
R2
R3
Department of Chemistry, Indian Institute of Technology
Madras, Chennai, Tamil Nadu-600 036, India
dillip@iitm.ac.in
O
O
K2CO3, 60 °C
catalyst
OH
K2CO3, 60 °C
catalyst, Ph3P
R2
catalyst
PdNPs
Received: 23.01.2015
Accepted after revision: 02.03.2015
Published online: 08.04.2015
development of efficient strategies for the synthesis of ben-
zofurans is important.8 The domino approach towards the
synthesis of benzofurans by Sonogashira cross-coupling be-
tween 2-halophenols and arylacetylenes is attractive due to
the fact that this protocol provides better yields of the de-
sired compounds and also tolerates various functional
groups present in the substrates.9 Domino reactions pro-
vide efficient routes in synthetic organic transformations to
achieve complex molecules in one pot; this approach is effi-
cient because the separation of unstable intermediates is
unnecessary.10
DOI: 10.1055/s-0034-1380463; Art ID: ss-2015-z0047-op
Abstract Palladium nanoparticles (PdNPs) were used as a catalyst for
the one-pot synthesis of a variety of benzofurans by Sonogashira cross-
coupling reactions under ambient conditions. The catalyst could be re-
cycled without significant loss in its activity.
Key words palladium, nanostructures, catalysis, domino reaction,
cross-coupling
Palladium compounds are well used as catalysts in a va-
riety of organic transformations reactions.1 Among palladi-
um compounds, the use of palladium nanoparticles (PdNPs)
in catalysis has been a recent trend.2 Typical palladium
nanoparticles are stabilized by a suitable capping agent, e.g.
dendrimers, polymers, ionic liquids, and designed organic
ligands.3 However, ligand-free palladium nanoparticles (LF-
PdNPs) are also known to catalyze organic reactions,
though very limited in number.4 Ligand-free palladium
nanoparticles are probably stabilized by the coordinating
solvents that are used for their preparation. The C–C cou-
pling reactions, such as the Suzuki, Heck, and Sonogashira
reactions, have been well studied using a variety of ligand-
stabilized palladium nanoparticles.5 However, there are
fewer reports on the use of ligand-free palladium nanopar-
ticles for such coupling reactions. Although palladium
nanoparticles are used to catalyze Sonogashira reactions,
their use in the synthesis of benzofurans via Sonogashira
reactions is not well explored.5b To the best of our knowl-
edge there are only two reports on the use of palladium
nanoparticles as catalysts for the synthesis of benzofurans.6
Benzofurans exhibit a broad range of biological activity
against asthma, ulcers, type 2 diabetes, and some other dis-
orders. The unique benzofuran structure is also found in
the core of many natural products.7 For these reasons the
In our previous communications, we discussed the cata-
lytic activity of thoroughly characterized ligand-free palla-
dium nanoparticles towards Suzuki4d and Sonogashira
cross-coupling reactions.4a,11 In the present work, we would
like to discuss the role of ligand-free palladium nanoparti-
cles in the synthesis of a variety of benzofurans under mild
reaction conditions without using an inert atmosphere.
Triphenylphosphine is used as a co-ligand for the synthesis
of aryl-substituted benzofurans, hence the palladium
nanoparticles used for catalysis are not in the strictest
sense ligand-free. Symmetrical and unsymmetrical diben-
zofurans could be prepared by controlling the ratio of the
reactants. Along with benzofuran 1a, a rare type12 of substi-
tuted benzofuran 2, a new, fluorescent active compound is
isolated as a minor product. Such structures were found in
medicinally important compounds reported recently.12a
Condensation of 2-iodophenol with phenylacetylene
was carried out under various reaction conditions to opti-
mize the synthesis of 2-phenylbenzofuran (1a) (Table 1).
The results suggest that the reaction temperature and cata-
lytic loading play major roles in terms of product formation
as well as isolated yields. A slight increase in the catalyst
loading, i.e. from 2 to 2.5 mol%, gave a huge improvement in
the yield and reaction time (entry 2 vs. 5). Similarly, addi-
tion of triphenylphosphine increased the yield of 1a (entry
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 1661–1668

1662
P. K. Mandali, D. K. Chand
Paper
Synthesis
3 vs. 5). A new benzofuran derivative, 2-phenyl-3-(phenyl-
ethynyl)benzofuran (2), was isolated as a minor product
under the reaction conditions employed.
Arylacetylenes bearing a methyl group at ortho-, meta-,
or para-position are coupled smoothly with 2-iodophenol
to give the corresponding benzofurans 1b–d in good yields
(entries 2–4); the use of the 2-tolyl group also exemplifies
steric hindrance in the acetylene substrate. Both electron-
deficient and electron-rich arylacetylenes gave the corre-
sponding products in good yields. A heterocyclic acetylene
3-ethynylpyridine gave corresponding benzofuran 1h in
81% yield (entry 8). The coupling reactions between 2-io-
dophenols and arylacetylenes bearing electron-withdraw-
ing group like trifluoromethyl, cyano, fluoro, or bromo, pro-
ceeded smoothly and the products 1j–l,o were isolated in
high yields (entries 10–12 and 15). Notably, benzofurans
1o,p could be prepared in high yields even in cases where
electron-withdrawing groups are present in both 2-io-
dophenol and arylacetylene moieties (entries 15 and 16).
The isolated benzofuran 1a was subjected to ICP-OES analy-
sis which showed that no palladium is detected in the sam-
ple. This experiment indicates that the palladium content, if
any, is below the detection limit.
Table 1 Optimization of the Reaction Conditions for the Synthesis of
2-Phenylbenzofuran (1a)
I
O
PdNPs
K2CO3
OH
1a
+
+
Ph3P
O
2
Entry
PdNPs/Ph3P (mol%) Temp (°C) Time (h)
Yielda (%)
1a
31
2
Table 2 Synthesis of Aryl-Substituted Benzofurans
1
2/10
30
60
60
30
60
60
24
10
10
5
~5
~5
~5
~5
~5
–
R
I
2
2/20
40
41
72
79
25
3
2.5/–
2.5/20
2.5/20
2.5/20
OH
R
PdNPs (2.5 mol%)
Ph3P, K2CO3, 60 °C
+
Ar
4
O
5
5
1
Ar
6b
20
Entry
R
Ar
Time (h) Product Yielda (%)
a Isolated yield,
b Et3N was employed instead of K2CO3.
1b
2
H
H
H
H
H
H
H
H
H
H
H
H
H
Ph
5
1a
1b
1c
1d
1e
1f
79
81
70
72
78
86
81
81
68
75
81
79
87
86
87
88
2-MeC6H4
3-MeC6H4
4-MeC6H4
4-t-BuC6H4
3-H2NC6H4
4-H2NC6H4
3-pyridyl
3-HC≡CC6H4
3-F3CC6H4
4-NCC6H4
2,4-F2C6H3
4-BrC6H4
Ph
6
A series of benzofurans 1a–v were prepared by domino
reaction of 2-iodophenol or methyl 4-hydroxy-3-iodoben-
zoate with a variety of terminal aryl- or alkylacetylenes un-
der the optimized reaction condition [PdNPs (2.5 mol%),
Ph3P (20 mol%), 60 °C] (Scheme 1,Tables 2 and 3).
3
3
4
2
5
8
6
2.5
2
7
1g
1h
1i
8
2.5
5
R2
9c
R1
PdNPs (2.5 mol%)
10
4
1j
O
Ph3P, K2CO3, 60 °C
R1
I
11
12
13
14
15
16
3
1k
1l
R2
1a–p
4
R1
OH
3
1m
1n
1o
1p
PdNPs (2.5 mol%)
K2CO3, 60 °C
R3
R3
CO2Me
CO2Me
CO2Me
4
O
4-NCC6H4
4-BrC6H4
3
1q–v
R1 = H, CO2Me
3.5
R
R
2 = H, 4-t-Bu, 2-Me, 3-Me, 4-Me, 3-NH2, 4-NH2, 3-CF3, 4-CN, 4-Br
a Isolated yields after column chromatography.
3 = (CH2)3Me, (CH2)7Me, (CH2)4OH
b 2-Phenyl-3-(phenylethynyl)benzofuran (2) was isolated in 5% yield.
c 1,3-Di(benzofuran-2-yl)benzene (3) was isolated in 11% yield.
Scheme 1 Palladium nanoparticles catalyzed synthesis of benzofurans
by coupling of 2-iodophenols and terminal aryl- or alkylacetylenes
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 1661–1668

1663
P. K. Mandali, D. K. Chand
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Synthesis
I
PdNPs (2.5 mol%)
+
+
Ph3P, 60 °C
K2CO3
O
O
O
OH
3
1i
1 equiv
3 equiv
5 equiv
1.2 equiv
1 equiv
1 equiv
11%
41%
52%
68%
28%
31%
Scheme 2 Influence of the stoichiometry of the reactants in Sonogashira coupling reactions on the yield of 1,3-dibenzofuran 3
Coupling 2-iodophenol with 1,3-diethynylbenzene gave
a benzofuran derivative 1i bearing one unreacted acetylene
moiety in 68% yield together with the symmetrical 1,3-
dibenzofuran 3 in 11% yield (Table 2, entry 9, and Scheme
2). Our efforts to increase the yield of the symmetrical 1,3-
dibenzofuran 3 by reacting 1,3-diethynylbenzene with
three and five equivalents of 2-iodophenol gave the target-
ed compound 3 in 41 and 52% yields, respectively.
Several of the benzofurans 1f,g,i,k,m,o,p (Table 2, en-
tries 6, 7, 9, 11, 13, 15 and 16) contain functional groups
that are suitable for further derivatization. For instance, the
derivatizable benzofurans 1i and 1m are subjected to
Sonogashira and Suzuki coupling reactions, respectively,
using the same palladium nanoparticles and a protocol that
we had previously developed (see Schemes 4 and 5).4a,d
Thus, 2-(3-ethynylphenyl)benzofuran (1i) was coupled
with 2-iodophenols to synthesize symmetrical 3 and un-
symmetrical 4 dibenzofurans in excellent yields (Scheme
3).
The benzofuran 1i bearing an unreacted acetylene was
further coupled with 4-iodoanisole and sterically hindered
substrate 2-iodo-1,3-dimethylbenzene in the presence of
palladium nanoparticles (2 mol%) at room temperature for
one hour to afford the corresponding coupling products 5
and 6 (Scheme 4).
The coupling reaction between 2-iodophenol and 1-
bromo-4-ethynylbenzene afforded 2-(4-bromophenyl)ben-
zofuran (1m) (Table 2, entry 13) in good yield and this was
further derivatized using 3-tolylboronic acid as a coupling
partner for the Suzuki coupling reaction using a protocol
that we have previously developed (Scheme 5).4d
Alkylacetylenes, such as hex-1-yne, dec-1-yne and hex-
5-yn-1-ol, were coupled with two different 2-iodophenols
to give corresponding benzofurans 1q–v in good yields (Ta-
ble 3) without the addition of triphenylphosphine and
within short reaction times (Table 3).
The recycling of palladium nanoparticles was examined
for the preparation of benzofuran 1a. It was observed that
the catalyst could be reused for at least four catalytic cycles
R
I
PdNPs (2.5 mol%)
CO3, Ph3P
+
O
O
R
O
K2
60 °C, 3 h
OH
1i
3, R = H, 89%
4, R = CO2Me, 91%
Scheme 3 Coupling reactions between 2-(3-ethynylphenyl)benzofuran (1i) and iodophenols
I
R2
R2
PdNPs (2 mol%)
+
O
K2CO3, r.t.
1 h
O
R1
R2
1i
R2
R1
5, R1 = OMe, R2 = H, 96%
6, R1 = H, R2 = Me, 92%
Scheme 4 Derivatization of 2-(3-ethynylphenyl)benzofuran (1i) by Sonogashira coupling reactions
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 1661–1668

1664
P. K. Mandali, D. K. Chand
Paper
Synthesis
B(OH)2
PdNPs (2 mol%)
+
Br
K2CO3, r.t.
3 h
O
O
1m
7, 91%
Scheme 5 Suzuki coupling of 2-(4-bromophenyl)benzofuran (1m) and 3-tolylboronic acid
Table 3 Synthesis of Alkyl-Substituted Benzofurans
R1
R1
I
PdNPs (2.5 mol%)
K2CO3, 60 °C
R2
R2
+
O
OH
Entry
R1
R2
Bu
Time (h) Product
Yield (%)a
1
2
3
4
5
6
H
1
1q
1r
1s
1t
91
92
87
91
91
86
H
H
(CH2)7Me
(CH2)4OH
Bu
1
1.5
1
CO2Me
CO2Me
CO2Me
Figure 2 TEM images of palladium nanoparticles (a) before catalysis
and (b) after the 4th cycle of the preparation of benzofuran 1a
(CH2)7Me
(CH2)4OH
1
1u
1v
2
a Isolated yields.
As discussed, a seldom found type of benzofuran deriva-
tive 2 (Table 1) was isolated as a minor product during the
synthesis of 1a.12 Since, compound 2 is fluorescent active
and no photophysical studies have been reported in litera-
ture for such compounds, a preliminary investigation was
performed (Figure 3). The absorption spectrum of com-
pound 2 in chloroform shows two distinct absorption max-
ima at 306 and 320 nm. An emission maximum is observed
at 355 nm when compound 2 is excited at either 306 or 320
nm.
(Figure 1). No significant loss in the yield of the major prod-
uct was observed between cycles. The slight loss in the ac-
tivity of the catalyst can be ascribed to the agglomeration of
the palladium nanoparticles.13 This was established by ana-
lyzing the TEM of the catalyst before the 1st cycle and after
the 4th cycle (Figure 2). The sizes of the palladium nanopar-
ticles before catalysis fall in the range of 4.5–8 nm whereas
after the 4th cycle the calculated sizes were in the range of
12–18 nm.
Figure 3 UV-vis (left) and fluorescence (right) spectra of compound 2
(CHCl3)
Figure 1 Representation of the recyclability of palladium nanoparticles
as catalyst in the synthesis of benzofuran 1a
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 1661–1668

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P. K. Mandali, D. K. Chand
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Synthesis
In conclusion, we have demonstrated that palladium
nanoparticles catalyzed the one-pot synthesis of benzofu-
rans. A variety of aryl- and alkylbenzofurans were synthe-
sized in good yields. The catalyst could be recycled without
significant loss in its activity for at up to four catalytic cy-
cles for the synthesis of benzofuran 1a. A preliminary pho-
tophysical study of a new compound is presented.
(0.110 g, 0.5 mmol) or methyl 4-hydroxy-3-iodobenzoate (0.139 g,
0.5 mmol) and 2-(3-ethynylphenyl)benzofuran (0.109 g, 0.5 mmol).
Then, the mixture was stirred at 60 °C under aerobic conditions. The
reaction was monitored by TLC until complete consumption of the
terminal acetylene compound.
2-[3-(Arylethynyl)phenyl]benzofurans 5 and 6 by Sonogashira
Coupling Reactions; General Procedure
The PdNPs were prepared by using a protocol established from our
group.4a A freshly prepared solution of PdNPs (5 mL) was taken up in a
25-mL round-bottomed flask. To this solution, K2CO3 (0.138 g, 1
mmol was added followed by 4-iodoanisole (0.117 g, 0.5 mmol) or 2-
iodo-1,3-dimethylbenzene (0.116 g, 0.5 mmol) and 2-(3-ethynylphe-
nyl)benzofuran (0.109 g, 0.5 mmol). Then, the mixture was stirred at
r.t. in an open atmosphere. The reaction was monitored by TLC until
complete consumption of the starting materials.
All reagents were purchased from Sigma-Aldrich and purified sol-
vents were used to perform the reactions. The coupling reactions
were monitored by TLC using silica gel as a stationary phase. H and
1
13C NMR were recorded at r.t. with a Bruker Avance-400 and 500 MHz
FT NMR spectrometers. The mass spectra of the samples were ob-
tained from a Micromass Q-TOF mass spectrometer by electrospray
ionization method (ESI) and GC-MS recorded on Jeol GCMATE II GC-
MS mass spectrometer. High-resolution TEM images were obtained
from Jeol 3010 with a UHR pole piece operates at an accelerating volt-
age 300 kV. UV-Visible absorption spectrum was recorded on Jasco V-
630 spectrophotometer and fluorescence spectra were recorded on a
Jasco FP-6300 spectrofluorometer. ICP-OES measurements were car-
ried out on a Perkin Elmer Optima 5300 DV.
2-(3′-Methylbiphenyl-4-yl)benzofuran (7)
The PdNPs were prepared by using a protocol established from our
group.4d A freshly prepared solution of PdNPs (5 mL) was taken up in
a 25-mL round-bottomed flask. To this solution, K2CO3 (0.138 g, 1
mmol) was added followed by 2-(4-bromophenyl)benzofuran (0.136
g, 0.5 mmol) and 3-tolylboronic acid (0.081 g, 0.6 mmol). Then, the
mixture was stirred at r.t. in open atmosphere. The reaction was
monitored by TLC until complete consumption of the starting materi-
al.
Analytical data of all the new and known compounds are provided
and the spectra are collected in the Supporting Information. The new
benzofurans are 1f,l,o,p,s,u,v, 2–7. Analytical data of the previously
reported compounds are in good agreement with the literature data.
Recyclability of the Catalyst for Synthesis of Benzofuran 1a
Ligand-free Palladium Nanoparticles (LF-PdNPs); General Proce-
dure
We have investigated the recyclability of the catalytic system by
choosing 2-iodophenol and phenylacetylene as model substrates. Af-
ter one run, the mixture was centrifuged and the supernatant was de-
canted. The residue was washed with hexane and then MeOH (2 ×) to
extract the organic products. Then it was dried under reduced pres-
sure and redispersed in MeOH–MeCN (12.5 mL) to generate the cata-
lyst. The required amount of K2CO3 was added followed by the organic
substrates to carry out the next catalytic cycle. The regenerated cata-
lyst system was found to be reusable without any significant loss of
the catalytic activity for at least 4 cycles.
The LF-PdNPs were prepared by using a protocol established from our
group and also characterized.4a,d Pd(OAc)2 (0.0056 g, 0.025 mmol) was
dissolved in MeOH–MeCN (1:1, 12.5 mL) to give a yellow-colored
solution that was 2 mM in metal. The yellow solution was stirred at
r.t. for about 3 h whereupon brownish-black color developed due to
reduction of Pd(II) to Pd(0). The size of the thus-prepared PdNPs was
in the range 4.5–8 nm by TEM analysis.
2-Phenylbenzofuran (1a); Typical Procedure
2-Phenylbenzofuran (1a)8c
A freshly prepared solution of PdNPs (12.5 mL) was taken up in a 25-
mL round-bottomed flask. To this solution K2CO3 (0.276 g, 2 mmol)
and Ph3P (0.052 g, 0.2 mmol) were added followed by 2-iodophenol
(0.22 g, 1 mmol) and phenylacetylene (0.122 g, 1.2 mmol). Then, the
mixture was stirred at 60 °C under aerobic conditions. The reaction
was monitored by TLC until complete consumption of phenylacety-
lene. When the reaction was complete, the solvents were evaporated
and the residue was purified by column chromatography (hexane–
EtOAc). Yields for all compounds are calculated after column chroma-
tography. The catalyst could be recycled. The size of the PdNPs after
four catalytic cycles was found in the range of 12–18 nm by TEM anal-
ysis indicating some agglomeration.
White solid; yield: 153.3 mg (79%); mp 118 °C.
1H NMR (500 MHz, CDCl3): δ = 7.87–7.85 (d, J = 10 Hz, 2 H), 7.58–7.57
(s, 1 H), 7.53–7.51 (d, J = 10 Hz, 1 H), 7.45–7.42 (t, J = 8 Hz, 2 H), 7.35–
7.32 (t, 1 H), 7.29–7.26 (t, J = 8 Hz, 1 H), 7.23–7.20 (m, 1 H), 7.01 (s, 1
H).
13C NMR (125 MHz, CDCl3): δ = 156.06, 155.03, 132.65, 130.63,
128.92, 128.69, 125.07, 124.40, 123.07, 121.04, 111.32, 101.44.
2-(2-Tolyl)benzofuran (1b)14a
Colorless oil; yield: 169 mg (81%).
1H NMR (500 MHz, CDCl3): δ = 7.86–7.83 (m, 1 H), 7.62–7.60 (m, 1 H),
7.54–7.51 (m, 1 H), 7.32–7.30 (m, 4 H), 7.26–7.22 (m, 1 H), 6.899–
6.896 (d, J = 1.2 Hz, 1 H), 2.58 (s, 3 H).
13C NMR (125 MHz, CDCl3): δ = 155.82, 154.54, 135.99, 133.07,
131.41, 128.66, 128.32, 126.24, 125.80, 124.37, 122.93, 121.05,
111.24, 105.24, 22.06.
The reactions of 2-iodophenol or methyl 4-hydroxy-3-iodobenzoate
with arylacetylenes were carried out as given in the typical proce-
dure. However, reactions using alkylacetylenes did not require the
use of Ph3P.
Symmetrical and Unsymmetrical Dibenzofurans 3 and 4; General
Procedure (Scheme 3)
2-(3-Tolyl)benzofuran (1c)14a
A freshly prepared solution of PdNPs (6.2 mL) was taken up in a 25-
mL round-bottomed flask. To this solution, K2CO3 (0.138 g, 1 mmol)
and Ph3P (0.026 g, 0.1 mmol) were added followed by 2-iodophenol
Colorless solid; yield: 145.6 mg (70%); mp 76 °C.
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P. K. Mandali, D. K. Chand
Paper
Synthesis
1H NMR (500 MHz, CDCl3): δ = 7.67 (s, 1 H), 7.65–7.63 (d, J = 10 Hz, 1
H), 7.56–7.49 (m, 2 H), 7.50–7.48 (d, J = 10 Hz, 1 H), 7.32–7.29 (t, J = 6
Hz, 1 H), 7.27–7.23 (dt, 1 H), 7.21–7.19 (m, 1 H), 7.15–7.13 (d, J = 10
Hz, 1 H), 6.97 (s, 1 H), 2.40 (s, 3 H).
13C NMR (125 MHz, CDCl3): δ = 156.2, 154.9, 138.6, 130.5, 129.5,
129.3, 128.8, 125.7, 124.3, 123.0, 122.3, 121.0, 111.3, 102.3, 21.6.
2-(3-Ethynylphenyl)benzofuran (1i)14d
White solid; yield: 148 mg (68%); mp 80–82 °C.
1H NMR (500 MHz, CDCl3): δ = 8.01–8.00 (t, 1 H), 7.86–7.83 (m, 1 H),
7.61–7.58 (m, 1 H), 7.54–7.51 (m, 1 H), 7.48–7.45 (m, 1 H), 7.43–7.39
(m, 1 H), 7.32–7.28 (m, 1 H), 7.26–7.22 (m, 1 H), 7.052–7.050 (d, 1 H),
3.13 (s, 1 H).
13C NMR (125 MHz, CDCl3): δ = 155.09, 154.93, 132.12, 130.88,
129.16, 128.99, 128.66, 125.32, 124.76, 123.22, 122.90, 121.22,
111.39, 102.16, 83.34, 77.82.
HRMS: m/z [M + H]+ calcd for C15H13O: 209.0966; found: 209.0960.
2-(4-Tolyl)benzofuran (1d)8c
White solid; yield: 149.8 mg (72%); mp 127 °C.
2-[3-(Trifluoromethyl)phenyl]benzofuran (1j)14e
1H NMR (400 MHz, CDCl3): δ = 7.76–7.74 (d, J = 8 Hz, 2 H), 7.57–7.55
(d, J = 8 Hz, 1 H), 7.51–7.49 (d, J = 8 Hz, 1 H), 7.26–7.21 (m, 4 H), 6.96
(s, 1 H), 2.39 (s, 3 H).
13C NMR (100 MHz, CDCl3): δ = 156.3, 154.92, 138.74, 132.54, 129.63,
127.9, 125.04, 124.13, 122.99, 120.88, 111.23, 100.7, 21.52.
White solid; yield: 196.5 mg (75%); mp 65 °C.
1H NMR (400 MHz, CDCl3): δ = 8.10 (s, 1 H), 8.01–7.99 (d, J = 8 Hz, 1
H), 7.60–7.52 (m, 4 H), 7.33–7.30 (dt, 1 H), 7.26–7.23 (m, 1 H), 7.09 (s,
1 H).
13C NMR (100 MHz, CDCl3): δ = 155.2, 154.4, 131.4, 129.5, 129.0,
128.0, 125.1 (t, J= 16.5, 14 Hz), 123.4, 121.8, 121.7, 121.4, 111.5,
102.8.
HRMS: m/z [M + H]+ calcd for C15H13O: 209.0966; found: 209.0957.
2-(4-tert-Butylphenyl)benzofuran (1e)8c
HRMS: m/z [M + K]+ calcd for C15H9F3OK: 301.0243; found: 301.0247.
White solid; yield: 195 mg (78%); mp 128–130 °C.
1H NMR (500 MHz, CDCl3): δ = 7.81–7.79 (d, J = 10 Hz, 2 H), 7.58–7.50
(m, 2 H), 7.48–7.46 (d, J = 10 Hz, 2 H), 7.29–7.19 (m, 2 H), 6.98–6.97
(d, J = 5 Hz, 1 H), 1.34 (s, 9 H).
4-(Benzofuran-2-yl)benzonitrile (1k)14f
White solid; yield: 195.2 mg (81%); mp 142–144 °C.
1H NMR (500 MHz, CDCl3): δ = 7.96–7.94 (d, J = 10 Hz, 2 H), 7.74–7.72
(d, J = 10 Hz, 2 H), 7.64–7.62 (dd, J = 7.5 Hz, 1 H), 7.55–7.53 (dd, J = 8
Hz, 1 H), 7.37–7.34 (m, 1 H), 7.29–7.27 (m, 1 H), 7.18 (d, 1 H).
13C NMR (125 MHz, CDCl3): δ = 156.31, 154.97, 151.95, 129.50,
127.88, 125.87, 124.89, 124.12, 122.97, 120.89, 111.26, 100.81, 34.91,
31.39.
13C NMR (125 MHz, CDCl3): δ = 155.41, 153.71, 134.63, 132.78,
128.81, 125.71, 125.28, 123.59, 121.65, 118.89, 111.68, 111.58,
104.49.
HRMS: m/z [M + Na]+ calcd for C15H9NONa: 242.0582; found:
242.0588.
HRMS: m/z [M + H]+ calcd for C18H19O: 251.3495; found: 251.3495.
2-(3-Aminophenyl)benzofuran (1f)
Brown solid; yield: 179 mg (86%); mp 124–126 °C.
1H NMR (400 MHz, CDCl3): δ = 7.58–7.56 (d, J = 8 Hz, 1 H), 7.51–7.49
(d, J = 8 Hz, 1 H), 7.29–7.20 (m, 5 H), 6.97 (s, 1 H), 6.69–6.67 (m, 1 H),
3.08 (s, 2 H).
2-(2,4-Difluorophenyl)benzofuran (1l)
White solid; yield: 181.7 mg (79%); mp 131–133 °C.
13C NMR (100 MHz, CDCl3): δ = 156.28, 154.96, 146.89, 131.59,
129.89, 129.39, 124.30, 123.01, 120.99, 115.70, 111.48, 111.26,
101.43.
1H NMR (500 MHz, CDCl3): δ = 8.05–7.99 (m, 1 H), 7.62–7.60 (m, 1 H),
7.53–7.51 (m, 1 H), 7.33–7.29 (m, 1 H), 7.27–7.23 (m, 1 H), 7.18–7.17
(m, 1 H), 7.03–6.92 (m, 2 H).
13C NMR (125 MHz, CDCl3): δ = 163.74 (d, J = 12 Hz), 161.74 (d, J = 12
Hz), 160.89 (d, J = 12 Hz), 158.86 (d, J = 12 Hz), 154.26, 149.16, 129.36,
128.21 (dd, J = 10 Hz), 124.88, 123.24, 121.46, 112.05 (dd, J = 21.25
Hz), 106.12 (d, J = 12.5 Hz), 104 (t, J = 25 Hz).
19F NMR (470 MHz, CDCl3): δ = –108.39 (d, J = 8.93 Hz), –109.32 (d, J =
8.93 Hz).
GC-MS: m/z = 232 [M + 2 H]+.
HRMS: m/z [M + H]+ calcd for C14H12NO: 210.0919; found: 210.0929.
2-(4-Aminophenyl)benzofuran (1g)14b
Brown solid; yield: 169 mg (81%); mp 149–151 °C.
1H NMR (400 MHz, CDCl3): δ = 7.68–7.66 (d, J = 8 Hz, 2 H), 7.53–7.46
(m, 2 H), 7.23–7.18 (m, 2 H), 6.81 (s, 1 H), 6.75–6.73 (d, J = 8 Hz, 2 H),
3.83 (s, 2 H).
13C NMR (100 MHz, CDCl3): δ = 156.83, 154.66, 147.08, 129.80,
126.53, 123.49, 122.84, 121.21, 120.46, 115.19, 110.99, 98.71.
2-(4-Bromophenyl)benzofuran (1m)8c
White solid; yield: 236 mg (87%); mp 156–158 °C.
2-(3-Pyridyl)benzofuran (1h)14c
1H NMR (400 MHz, CDCl3): δ = 7.74–7.72 (d, J = 10 Hz, 2 H), 7.59–7.56
(m, 3 H), 7.52–7.50 (d, J = 10 Hz, 1 H), 7.32–7.27 (m, 1 H), 7.23–7.21
(m, 1 H), 7.03 (d, 1 H).
HRMS: m/z [M + Na]+ calcd for C14H9BrONa: 294.9734; found:
294.9733.
White solid; yield: 157 mg (81%); mp 82 °C.
1H NMR (400 MHz, CDCl3): δ = 9.12 (s, 1 H), 8.59–8.57 (d, J = 8 Hz, 1
H), 8.14–8.12 (d, J = 8 Hz, 1 H), 7.63–7.61 (d, J = 8 Hz, 1 H), 7.56–7.54
(d, J = 8 Hz, 1 H), 7.40–7.37 (dd, 1 H), 7.35–7.31 (dt, 1 H), 7.27–7.24 (t,
1 H), 7.13 (s, 1 H).
13C NMR (100 MHz, CDCl3): δ = 155.24, 153.05, 149.41, 146.56,
132.04, 128.90, 126.79, 125.10, 123.77, 123.42, 121.36, 111.47,
102.90.
Methyl 2-Phenylbenzofuran-5-carboxylate (1n)14g
White solid; yield: 216.7 mg (86%); mp 154–156 °C.
1H NMR (500 MHz, CDCl3): δ = 8.32 (dd, J = 1 Hz, 1 H), 8.02–8.00 (dd,
J = 2.5 Hz, 1 H), 7.88–7.86 (m, 2 H), 7.55–7.53 (m, 1 H), 7.48–7.44 (m,
2 H), 7.40–7.36 (m, 1 H), 7.07 (d, J = 1.5 Hz, 1 H), 3.95 (s, 3 H).
HRMS: m/z [M + H]+ calcd for C13H10NO: 196.0762; found: 196.0756.
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13C NMR (125 MHz, CDCl3): δ = 167.45, 157.56, 157.53, 130.02,
129.39, 129.18, 129.02, 126.20, 125.47, 125.21, 123.45, 111.15,
101.66, 52.26.
1H NMR (400 MHz, CDCl3): δ = 8.02 (d, J = 1.2 Hz, 1 H), 7.94–7.92 (dd,
J = 8 Hz, 1 H), 7.42–7.40 (m, 1 H), 6.43 (d, J = 0.8 Hz, 1 H), 3.92 (s, 3 H),
2.80–2.76 (t, 2 H), 1.77–1.70 (quint, 2 H), 1.47–1.38 (quint, 2 H), 0.98–
0.94 (t, J = 7.2 Hz, 3 H).
13C NMR (100 MHz, CDCl3): δ = 167.66, 161.48, 157.44, 129.19,
125.15, 124.87, 122.73, 110.65, 102.34, 52.16, 29.77, 28.27, 22.42,
13.92.
HRMS: m/z [M + H]+ calcd for C16H13O3: 253.0865; found: 253.0867.
Methyl 2-(4-Cyanophenyl)benzofuran-5-carboxylate (1o)
White solid; yield: 240 mg (87%); mp 152–154 °C.
HRMS: m/z [M + H]+ calcd for C14H17O3: 233.1178; found: 233.1167.
1H NMR (500 MHz, CDCl3): δ = 8.29 (d, J = 1.5 Hz, 1 H), 8.02–7.99 (dd,
J = 7.5 Hz, 1 H), 7.90–7.88 (d, J = 10 Hz, 2 H), 7.69–7.67 (d, J = 10 Hz, 2
H), 7.51–7.49 (d, J = 10 Hz, 1 H), 7.16 (d, J = 1.5 Hz, 1 H), 3.89 (s, 3 H).
Methyl 2-Octylbenzofuran-5-carboxylate (1u)
Colorless oil; yield: 262 mg (91%).
13C NMR (125 MHz, CDCl3): δ = 167.15, 157.82, 155.15, 134.01,
132.87, 128.83, 127.33, 126.03, 125.47, 124.08, 118.11, 112.26,
111.46, 104.69, 52.39.
1H NMR (400 MHz, CDCl3): δ = 8.20 (d, J = 1.6 Hz, 1 H), 7.94–7.92 (dd,
J = 8.2 Hz, 1 H), 7.42–7.40 (d, J = 10 Hz, 1 H), 6.42 (d, J = 0.8 Hz, 1 H),
3.92 (s, 3 H), 2.78–2.74 (t, J = 7.5 Hz, 2 H), 1.78–1.70 (quint, J = 7.5 Hz,
2 H), 1.30–1.25 (m, 10 H), 0.89–0.86 (t, J = 6.8 Hz, 3 H).
HRMS: m/z [M + H]+ calcd for C17H12NO3: 278.0817; found: 278.0825.
13C NMR (100 MHz, CDCl3): δ = 167.63, 161.50, 157.43, 129.19,
124.88, 122.72, 110.63, 102.32, 52.12, 31.97, 29.42, 29.32, 28.57,
27.67, 22.78, 14.20.
Methyl 2-(4-Bromophenyl)benzofuran-5-carboxylate (1p)
White solid; yield: 290 mg (88%); mp 180–182 °C.
1H NMR (500 MHz, CDCl3): δ = 8.33–8.32 (d, J = 5 Hz, 1 H), 8.04–8.02
(dd, J = 10 Hz, 1 H), 7.75–7.73 (d, J = 10 Hz, 2 H), 7.60–7.58 (d, J = 10
Hz, 2 H), 7.54–7.52 (d, J = 10 Hz, 1 H), 7.08 (s, 1 H), 3.95 (s, 3 H).
13C NMR (125 MHz, CDCl3): δ = 167.34, 157.55, 156.41, 132.25,
129.22, 128.96, 126.65, 126.51, 125.67, 123.57, 123.25, 111.20,
102.22, 52.30.
HRMS: m/z [M + H]+ calcd for C18H25O3: 289.1804; found: 289.1797.
Methyl 2-(4-Hydroxybutyl)benzofuran-5-carboxylate (1v)
Yellow oil; yield: 278 mg (86%).
1H NMR (400 MHz, CDCl3): δ = 8.19 (d, J = 1.6 Hz, 1 H), 7.94–7.91 (dd,
J = 8.8 Hz, 1 H), 7.41–7.39 (d, J = 8.8 Hz, 1 H), 6.44 (d, J = 0.8 Hz, 1 H),
3.91 (s, 3 H), 3.71–3.68 (t, J = 6.5 Hz, 2 H), 2.83–2.80 (t, J = 7.5 Hz, 2 H),
1.86–1.82 (m, 2 H), 1.69–1.67 (t, J = 10 Hz, 2 H), 1.32 (s, 1 H).
13C NMR (100 MHz, CDCl3): δ = 167.63, 160.87, 157.43, 129.07,
125.24, 122.77, 110.65, 102.60, 62.56, 52.14, 32.20, 28.28, 27.97,
23.98.
GC-MS: m/z = 329 [M – H]+, 331 [M – H + 2]+.
2-Butylbenzofuran (1q)7c
Yellow oil; yield: 158 mg (91%).
1H NMR (500 MHz, CDCl3): δ = 7.48–7.47 (m, 1 H), 7.43–7.41 (m, 1 H),
7.23–7.16 (m, 2 H), 6.383–6.381 (d, 1 H), 2.80–2.76 (t, 2 H), 1.78–1.71
(quint, 2 H), 1.49–1.40 (quint, 2 H), 0.99–0.95 (t, 3 H).
HRMS: m/z [M + H]+ calcd for C14H17O4: 249.1127; found: 249.1120.
13C NMR (125 MHz, CDCl3): δ = 159.88, 154.75, 129.17, 123.13,
122.47, 120.26, 110.82, 101.87, 29.93, 28.28, 22.42, 13.94.
HRMS: m/z [M + H]+ calcd for C12H15O: 175.1123; found: 175.1128.
2-Phenyl-3-(phenylethynyl)benzofuran (2)
Yellow solid; yield: 14.7 mg (~5%); mp 150–152 °C.
1H NMR (400 MHz, CDCl3): δ = 8.37–8.35 (d, J = 8.0 Hz, 2 H), 7.78–7.76
(m, 1 H), 7.66–7.63 (m, 2 H), 7.55–7.50 (m, 3 H), 7.44–7.40 (m, 4 H),
7.37–7.33 (m, 2 H).
13C NMR (100 MHz, CDCl3): δ = 156.45, 153.68, 131.67, 130.33,
130.07, 129.31, 128.81, 128.62, 128.58, 126.19, 125.50, 123.53,
120.51, 111.34, 99.37, 96.61, 81.33.
2-Octylbenzofuran (1r)9e
Yellow oil; yield: 211 mg (92%).
1H NMR (500 MHz, CDCl3): δ = 7.48–7.46 (d, J = 10 Hz, 1 H), 7.42–7.40
(d, J = 10 Hz, 1 H), 7.22–7.16 (m, 2 H), 6.37 (s, 1 H), 2.78–2.75 (t, J = 7.5
Hz, 2 H), 1.77–1.71 (quint, J = 7.5 Hz, 2 H), 1.32–1.27 (m, 10 H), 0.90–
0.87 (t, J = 7.5 Hz, 3 H).
HRMS: m/z [M + H]+ calcd for C22H15O: 295.1123; found: 295.1131.
13C NMR (125 MHz, CDCl3): δ = 159.93, 154.74, 129.17, 123.12,
122.47, 120.27, 110.82, 101.86, 31.99, 29.47, 29.35, 28.60, 27.84,
22.80, 14.24.
1,3-Di(benzofuran-2-yl)benzene (3)
Yellow solid; yield: 137 mg (89%); mp 162–164 °C.
1H NMR (500 MHz, CDCl3): δ = 8.38–8.37 (t, J = 2 Hz, 1 H), 7.86–7.84
(dd, J = 6 Hz, 2 H), 7.63–7.61 (m, 2 H), 7.59–7.57 (m, 2 H), 7.55–7.52 (t,
J = 8 Hz, 1 H), 7.34–7.30 (m, 2 H), 7.28–7.24 (m, 2 H), 7.152–7.150 (d,
J = 1 Hz, 2 H).
13C NMR (125 MHz, CDCl3): δ = 155.25, 155.12, 131.24, 129.48,
129.28, 125.06, 124.68, 123.20, 121.41, 121.19, 111.41, 102.11.
4-(Benzofuran-2-yl)butan-1-ol (1s)
Colorless oil; yield: 165 mg (87%).
1H NMR (500 MHz, CDCl3): δ = 7.48–7.47 (m, 1 H), 7.41–7.39 (m, 1 H),
7.22–7.16 (m, 2 H), 6.39 (d, J = 1 Hz, 1 H), 3.71–3.68 (t, J = 6.5 Hz, 2 H),
2.83–2.80 (t, J = 7.5 Hz, 2 H), 1.87–1.81 (m, 2 H), 1.69–1.67 (t, J = 10
Hz, 2 H), 1.32 (s, 1 H).
HRMS: m/z [M + K]+ calcd for C22H14O2K: 349.0631; found: 349.0624.
13C NMR (125 MHz, CDCl3): δ = 159.25, 154.77, 129.05, 123.28,
122.56, 120.35, 110.86, 102.20, 68.67, 62.73, 32.27, 28.31, 24.11.
HRMS: m/z [M + H]+ calcd for C12H15O2: 191.1072; found: 191.1073.
Methyl 2-[3-(Benzofuran-2-yl)phenyl]benzofuran-5-carboxylate
(4)
Yellow solid; yield: 167 mg (91%); mp 161–163 °C.
1H NMR (400 MHz, CDCl3): δ = 8.36–8.34 (m, 2 H), 8.05–8.03 (dd, J =
6.8 Hz, 1 H), 7.87–7.83 (m, 2 H), 7.63–7.52 (m, 4 H), 7.34–7.31 (m, 1
H), 7.27–7.24 (m, 1 H), 7.18 (s, 1 H), 7.15 (s, 1 H), 3.96 (s, 3 H).
Methyl 2-Butylbenzofuran-5-carboxylate (1t)15
Colorless oil; yield: 211 mg (91%).
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13C NMR (100 MHz, CDCl3): δ = 167.41, 157.62, 156.98, 155.31,
155.12, 131.34, 130.64, 129.52, 129.30, 129.23, 126.44, 125.60,
125.50, 125.15, 124.76, 123.59, 121.50, 121.22, 111.42, 111.24,
102.32, 102.25, 52.30.
Ed. 2005, 44, 7852. (c) Moreno-Manas, M.; Pleixats, R. Acc.
Chem. Res. 2003, 36, 638. (d) Polshettiwar, V.; Asefa, T. Nanoca-
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(3) (a) Huang, Y.; Zheng, Z.; Liu, T.; Lü, J.; Lin, Z.; Li, H.; Cao, R. Catal.
Commun. 2011, 14, 27. (b) Durand, J.; Teuma, E.; Malbose, F.;
Kihn, Y.; Gomez, M. Catal. Commun. 2008, 9, 273. (c) Narayanan,
R.; El-Sayed, M. A. J. Am. Chem. Soc. 2003, 125, 8340. (d) Astruc,
D.; Ornelas, C.; Diallo, A. K.; Ruiz, J. Molecules 2010, 15, 4947.
(e) Mieczyńska, E.; Borkowski, T.; Cypryk, M.; Pospiech, P.;
Trzeciak, A. M. Appl. Catal., A 2014, 470, 24. (f) Huang, Y.; Gao,
S.; Liu, T.; Lü, J.; Lin, X.; Li, H.; Cao, R. ChemPlusChem 2012, 77,
106.
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Chem. Rev. 2002, 102, 3757.
HRMS: m/z [M + H]+ calcd for C24H17O4: 369.1127; found: 369.1134.
2-{3-[(4-Methoxyphenyl)ethynyl]phenyl}benzofuran (5)
White solid; yield: 155 mg (96%); mp 138–140 °C.
1H NMR (400 MHz, CDCl3): δ = 8.03–8.02 (t, J = 2 Hz, 1 H), 7.82–7.80
(m, 1 H), 7.61–7.59 (m, 1 H), 7.55–7.48 (m, 5 H), 7.43–7.39 (t, J = 6 Hz,
1 H), 7.32–7.22 (m, 2 H), 7.058–7.056 (d, J = 0.8 Hz, 1 H), 6.90–6.88 (d,
J = 8 Hz, 2 H), 3.84 (s, 3 H).
13C NMR (100 MHz, CDCl3,): δ = 159.92, 155.26, 155.09, 133.29,
131.49, 130.83, 129.24, 128.97, 127.99, 124.65, 124.48, 124.41,
123.17, 121.17, 115.32, 114.21, 111.36, 102.01, 90.09, 87.82, 55.47.
GC-MS: m/z = 324 [M]+.
Anal. Calcd for C23H16O2·0.5H2O: C, 82.86; H, 5.14. Found: C, 82.50; H,
4.60.
(5) (a) Balanta, A.; Godard, C.; Claver, C. Chem. Soc. Rev. 2011, 40,
4973. (b) Chinchilla, R.; Nájera, C. Chem. Soc. Rev. 2011, 40, 5084.
(c) Teratani, T.; Ohtaka, A.; Kawashima, T.; Shimomura, O.;
Nomura, R. Synlett 2010, 2271.
2-{3-[(2,6-Dimethylphenyl)ethynyl]phenyl}benzofuran (6)
(6) (a) Saha, D.; Dey, R.; Ranu, B. C. Eur. J. Org. Chem. 2010, 6067.
(b) Ohtaka, A.; Teratani, T.; Fujii, R.; Ikeshita, K.; Kawashima, T.;
Tatsumi, K.; Shimomura, O.; Nomura, R. J. Org. Chem. 2011, 76,
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103, 893. (b) Thevenin, M.; Thoret, S.; Grellier, P.; Dubois, J.
Bioorg. Med. Chem. 2013, 21, 4885. (c) Khan, M. W.; Alam, M. J.;
Rashid, M. A.; Chowdhury, R. Bioorg. Med. Chem. 2005, 13, 4796.
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Commun. 2010, 46, 9049. (b) Takeda, N.; Miyata, O.; Naito, T. Eur.
J. Org. Chem. 2007, 1491. (c) Wang, X.; Liu, M.; Xu, L.; Wang, Q.;
Chen, J.; Ding, J.; Wu, H. J. Org. Chem. 2013, 78, 5273.
White solid; yield: 148 mg (92%); mp 98–100 °C.
1H NMR (500 MHz, CDCl3): δ = 8.03–8.02 (t, J = 1.5 Hz, 1 H), 7.84–7.82
(dt, J = 8 Hz, 1 H), 7.61–7.59 (m, 1 H), 7.56–7.53 (m, 2 H), 7.46–7.43 (t,
J = 8 Hz, 1 H), 7.32–7.29 (m, 1 H), 7.26–7.23 (m, 1 H), 7.17–7.14 (m, 1
H), 7.10–7.08 (m, 3 H), 2.56 (s, 6 H).
13C NMR (125 MHz, CDCl3): δ = 155.21, 155.11, 140.56, 131.54,
130.91, 129.23, 129.02, 128.11, 127.77, 126.89, 124.69, 124.67,
124.62, 123.20, 122.91, 121.19, 111.40, 102.11, 97.47, 87.80, 21.34.
HRMS: m/z [M + H]+ calcd for C24H19O: 323.1358; found: 323.4157.
2-(3′-Methylbiphenyl-4-yl)benzofuran (7)
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A.; Neuville, L.; Genet, J.-P. Adv. Synth. Catal. 2004, 346, 1733.
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Park, H. J. Bull. Korean Chem. Soc. 2013, 34, 2645. (h) Zanardi, A.;
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White solid; yield: 128 mg (91%); mp 162–164 °C.
1H NMR (400 MHz, CDCl3): δ = 7.94–7.92 (d, J = 8 Hz, 2 H), 7.69–7.67
(d, J = 8 Hz, 2 H), 7.60–7.58 (m, 1 H), 7.54–7.52 (m, 1 H), 7.46–7.44 (m,
2 H), 7.37–7.34 (t, J = 8 Hz, 1 H), 7.31–7.27 (td, J = 8 Hz, 1 H), 7.25–
7.21 (m, 1 H), 7.20–7.18 (d, J = 8 Hz, 1 H), 7.06 (d, 1 H), 2.44 (s, 3 H).
13C NMR (100 MHz, CDCl3): δ = 155.90, 155.10, 141.54, 140.58,
138.63, 129.45, 128.93, 128.49, 127.92, 127.62, 125.44, 124.42,
124.26, 123.11, 121.03, 111.32, 101.51, 21.70.
HRMS: m/z [M + H]+ calcd for C21H17O: 285.1279; found: 285.1290.
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A. M.; Wu, L.; Wu, D. J.; Barrios, A. M.; Bottini, N.; Cato, A. C. B.;
Zhang, Z.-Y. J. Med. Chem. 2013, 56, 4990. (b) Mehta, S.; Larock,
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D.; Wan, W.-C.; Deng, X.-Y.; Li, Y.; Yang, L.-J.; Li, L.; Zhang, H.-B.
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Acknowledgment
D.K.C. thanks CSIR, India for financial support. P.K.M. thanks IIT
Madras for a fellowship.
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0034-1380463.
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References
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© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 1661–1668