Synthesis of benzofurans from terminal alkynes and iodophenols catalyzed by recyclable palladium nanoparticles supported on N,O-dual doped hierarchical porous carbon under copper- and ligand-free conditions

Article abstract of DOI:10.1016/j.cattod.2018.04.036

We herein report that a stable and recyclable heterogeneous catalyst, consisting of Pd nanoparticles supported on N,O-dual doped hierarchical porous carbon derived from naturally available and renewable biomass-bamboo shoots, which allows for highly efficient one-pot tandem reaction of o-iodophenols with terminal alkynes to synthesize biologically active 2-benzofuran derivatives under copper- and ligand-free conditions. The catalyst performed superior catalytic performance compared with the analogous Pd/C and supported on other metal oxides under otherwise identical conditions. A broad set of aryl and alkyl terminal alkynes can be effectively coupled /cyclized with various o-iodophenols to afford their corresponding 2-benzofurans in good to high yields with good tolerance of multiple functional groups. Moreover, the catalyst showed good stability and could be reused several times without significant loss of activity.

Full text of DOI:10.1016/j.cattod.2018.04.036

CatalysisTodayxxx(xxxx)xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Synthesis of benzofurans from terminal alkynes and iodophenols catalyzed
by recyclable palladium nanoparticles supported on N,O-dual doped
hierarchical porous carbon under copper- and ligand-free conditions
Guijie Ji, Yanan Duan, Shaochun Zhang, Yong Yang⁎
CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101 PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Supported palladium
nanoparticles
Tandem catalysis
Alkynes
O-Iodophenols
Benzofuran derivatives
We herein report that a stable and recyclable heterogeneous catalyst, consisting of Pd nanoparticles supported on
N,O-dual doped hierarchical porous carbon derived from naturally available and renewable biomass-bamboo
shoots, which allows for highly efficient one-pot tandem reaction of o-iodophenols with terminal alkynes to
synthesize biologically active 2-benzofuran derivatives under copper- and ligand-free conditions. The catalyst
performed superior catalytic performance compared with the analogous Pd/C and supported on other metal
oxides under otherwise identical conditions. A broad set of aryl and alkyl terminal alkynes can be effectively
coupled /cyclized with various o-iodophenols to afford their corresponding 2-benzofurans in good to high yields
with good tolerance of multiple functional groups. Moreover, the catalyst showed good stability and could be
reused several times without significant loss of activity.
1. Introduction
Given their immense pharmaceutical utility, the synthesis of ben-
Benzofuran derivatives, which structurally exist in numerous nat-
ural products and synthetic bioactive compounds, have attracted much
attention in the chemical and pharmaceutical industries [1,2]. Parti-
cularly, 2-arylbenzofurans with wide distribution in nature possessing a
number of biological activities and currently present in regularly pre-
scription or potential drugs. For instance, Vibsanol [3] is used as an
inhibitor for lipid peroxidation, Machicendiol [4] is used for treatment
of cancer, cardiovascular disease, migraines, dementia and anxiety, and
Benzbromarone [5] is a nonpurine xanthine oxidase inhibitor used for
the treatment of gout.
zofurans has been extensively explored and numerous conventional
methods have been developed over the past few decades. Among all
methods investigated, one-pot tandem transition-metal-catalyzed in-
termolecular Sonogashira cross-coupling followed by cyclization of o-
halogenated phenols with terminal alkynes represents one of the most
powerful and appealing methods owing to its convenience and en-
vironmental benefits. In this regard, a number of metal-based catalysts
including Cu [6–12], Pd [13–23], Zn [24,25], and Au [26] have been
employed for the transformation since the seminal work developed by
Castro et al. [6] To date, the majority of those previous tandem reac-
⁎
Corresponding author.
E-mail address: yangyong@qibebt.ac.cn (Y. Yang).
https://doi.org/10.1016/j.cattod.2018.04.036
Received 30 December 2017; Received in revised form 19 March 2018; Accepted 19 April 2018
0920-5861/©2018ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:Ji,G.,CatalysisToday(2018),https://doi.org/10.1016/j.cattod.2018.04.036

G. Ji et al.
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tions were prevailingly operated by soluble homogenous metal com-
plexes, which is a particularly significant drawback for their application
in pharmaceutical industry because they are expensive, cannot be re-
cycled, and are difficult to separate from the product mixture. On the
other hand, the current demand for green and sustainable transforma-
tion of organic molecules enhances the importance of heterogeneous
catalysts that can facilitate the recycling of precious metals and help to
prevent contamination of the products by toxic metals. Actually, there
are a few heterogeneous metal-based catalysts that have been exploited
for the transformation. For example, Ramon and co-workers [19] re-
ported an impregnated Cu or Pd-Cu on magnetite catalyst for synthesis
of benzofurans. Later on, a Pd catalyst supported on nanosize carbon
balls was shown to be efficient catalyst for benzofurans formation by
Park and co-workers [20]. Very recently, Bäckvall and co-workers [21]
developed a recyclable Pd nanoparticles immobilized on siliceous me-
socellular foam for efficient synthesis of benzofurans. An in-situ gen-
eration of Pd nanoparticles in porous covalent organic framework was
also fabricated and showed good activity for this reaction by Banerjee
and co-workers [22]. Despite such great progress, in most cases, both
homogeneous and heterogeneous catalysts generally suffer from one or
more drawbacks such as the use of toxic and expensive ligands, re-
quirement of the second co-catalyst like Cu salts, limited substrate
scope, and poor chemoselectivity. Therefore, it is highly desirable to
develop a new heterogeneous catalytic system that is capable of per-
forming such transformation via tandem processes under copper and
ligand-free conditions.
In the course of our continuing efforts in developing green catalysis
for sustainable synthesis, we have developed a highly efficient and re-
cyclable supported Pd nanoparticles (NPs) on N,O-dual doped hier-
archical porous carbon (Pd/N,O-Carbon) derived from naturally re-
newable biomass for chemoselective semihydrogenation and
hydrosilylation of a wide range of alkynes to versatile and valuable
compounds[27,28]. Intrigued by the catalytic utility of the Pd/N,O-
Carbon, we became more interesting in the development of innovative
catalytic processes that enable one-pot tandem reaction of o-iodophe-
nols with terminal alkynes. Herein, we report the application of such
catalyst Pd/N,O-Carbon for the successful tandem Sonogashira/cycli-
zation reaction of o-iodophenols with terminal alkynes to construct 2-
benzofuran derivatives under ligand- and copper-free conditions. The
present catalytic system shows high efficiency and broad substrate
scope of both o-iodophenols and terminal alkynes with good tolerance
of functional group and can be easily recycled for successive uses
without significant loss in activity and selectivity.
2.2. Catalyst preparation
2.2.1. Preparation of hierarchical porous carbon (N,O-carbon)
The porous heteroatom doped carbon materials were prepared in
two-steps including hydrothermal treatment and carbonization process.
The fresh bamboo shoots were obtained from Anhui Taiping Test
Centre, International Centre for Bamboo and Rattan, Anhui Province,
China. The Bamboo shoots were firstly cut into slices and dried at 70 °C
for 24 h. The dried bamboo shoots (2 g) were then ground into powders
and transferred in a 100 mL Teflon-inner stainless-steel autoclave cou-
pled with 20 ml of deionized water. The autoclave was then heated to
180 °C and maintained for 6 h. The resulting brown solids were filtered,
washed by distilled water thoroughly to remove any residual of soluble
metal ions such as Ca2+ or K+ (detected by ion chromatography), dried
at room temperature under vacuum for 24 h. After that, the hydrochars
were calcined at 850 °C for 4 h in N2 flow with a heating rate of
5 °C min−1. The black powder heteroatom doped porous carbons were
obtained and denoted as N,O-Carbon.
2.2.2. Preparation of Pd nanoparticles supported on N,O-carbon
The porous heteroatom doped carbon supported Pd materials with
metal loading of 2.0 wt % were prepared by ultrasound-assisted re-
duction method. 0.15 g of N,O-Carbon was dispersed in 100 mL deio-
nized water under ultrasonic condition for 30 min. Then 3 g of 0.268 wt
% of Pd(NO3)2 aqueous solution was added dropwise into the mixture
and stirred vigorously for 1 h. After that, the pH value of solution was
adjusted to 11 using ammonia solution. The resulting solution was put
into 80 °C oil bath with subsequent addition of 0.2 mL of hydrazine
solution (85%), which was further stirred vigorously for another 4 h.
Finally, the product was filtered and washed with deionized water and
dried at 105 °C under vacuum for 12 h. The as-prepared sample was
denoted as Pd/N,O-Carbon and the Pd loading was determined by ICP-
AES to be 1.64 wt%. For comparison, Pd NPs supported on commercial
activated carbon (commercially available from Fujian Xinsen Carbon
Coop., with surface area of 1580 m2 g−1) and other supports were
prepared with the same procedure.
2.3. Typical procedure for one-pot tandem reaction between o-iodophenols
and terminal alkynes
General produce: o-iodophenol (0.5 mmol), alkyne (1.0 mmol) and
base (1.0 mmol) were added into a 10 mL dry Schlenk tube under Ar,
then anhydrous DMF (5 mL) was injected into the mixture using syr-
inge. Then the solution stirred at preheated oil bath (160 °C). The re-
action was monitored by TLC and GC.The mixture was cooled down to
room temperature after full conversion, then diluted with di-
chloromethane and washed with water three times. The organic layer
was separated and washed with brine followed by drying with anhy-
drous Na2SO4. The filtrate was concentrated in vacuo to afford the
crude product, which was purified by flash column chromatography on
silica gel (petroleum ether).
2. Materials and methods
2.1. Materials
Unless otherwise noted, all reagents were purchased commercially
from Sigma-Aldrich, or Aladdin and used as received without further
purification. The fresh bamboo shoots were obtained from Anhui
Taiping Test Centre, International Centre for Bamboo and Rattan,
Anhui Province, China. The commercially available activated carbon
was purchased from Fujian Xinsen Carbon Coop., Shaowu, Fujian. All
operations were carried out in an argon atmosphere using glovebox and
Schlenk techniques unless otherwise specified. Anhydrous tetra-
hydrofuran (THF), hexanes and toluene were obtained from an argon
purged solvent purification system comprised of columns of activated
alumina and molecular sieves. Anhydrous N,N’-dimethylformamide
(DMF), acetonitrile (CH3CN), dimethyl sulfoxide (DMSO) and 1,4-di-
oxane were purchased from Aladdin and used without further pur-
ification.
2-Phenylbenzofuran (3a): The title compound was prepared ac-
cording to the general procedure. The product was obtained as white
solid (92%) by using petroleum ether as the column eluent on silica gel.
1H NMR (400 MHz, CDCl3) δ 7.88 (d, J = 7.2 Hz, 1 H), 7.59 (d, J
= 7.3 Hz, 1 H), 7.54 (d, J = 7.8 Hz, 1 H), 7.46 (t, J = 7.6 Hz, 1 H),
7.36 (t, J = 7.4 Hz, 1 H), 7.32–7.21 (m, 1 H), 7.03 (s, 1 H). 13C NMR
(101 MHz, CDCl3) δ 155.90, 154.87, 130.46, 129.20, 128.76, 128.52,
124.91, 124.24, 122.91, 120.88, 111.15, 101.28. These spectroscopic
data matched that previously reported [29].
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column eluent on silica gel. 1H NMR (400 MHz, CDCl3) δ 7.61–7.57 (m,
1 H), 7.56 – 7.51 (m, 1 H), 7.48–7.45 (m, 1 H), 7.44 – 7.41 (m, 1 H),
7.36 (t, J = 7.9 Hz, 1 H), 7.29 (td, J = 7.7 Hz, J = 1.3 Hz, 1 H), 7.24
(td, J = 7.5, 1.1 Hz, 1 H), 7.03 (d, J = 0.7 Hz, 1 H), 6.91 (ddd, J = 8.2,
2.6, 0.8 Hz, 1 H), 3.90 (s, 3 H). 13C NMR (101 MHz, CDCl3) δ 159.96,
155.76, 154.87, 131.79, 129.87, 129.18, 124.35, 122.97, 120.95,
117.55, 114.50, 111.20, 110.16, 101.65, 55.40. These spectroscopic
data matched that previously reported [2].
2-(o-Tolyl)benzofuran (3b): The title compound was prepared
according to the general procedure. The product was obtained as white
solid (81%) by using petroleum ether/DCM (4 : 1) as the column eluent
on silica gel. 1H NMR (400 MHz, CDCl3) δ 7.88–7.81 (m, 1 H), 7.61 (dd,
J = 7.5, 0.8 Hz, 1 H), 7.55–7.50 (m, 1 H), 7.33–7.27 (m, 4 H), 7.24 (td,
J = 7.3, 1.1 Hz, 1 H), 6.89 (d, J = 0.7 Hz, 1 H), 2.58 (s, 3 H). 13C NMR
(101 MHz, CDCl3) δ 155.64, 154.37, 135.81, 131.24, 129.92, 129.17,
128.49, 128.14, 126.08, 124.20, 122.76, 120.89, 111.08, 105.08,
21.91. These spectroscopic data matched that previously reported [2].
2-(4-Methoxyphenyl)benzofuran (3g): The title compound was
prepared according to the general procedure. The product was obtained
as white solid (90%) by using petroleum ether as the column eluent on
silica gel. 1H NMR (400 MHz, CDCl3) δ 7.80 (d, J = 8.8 Hz, 1 H), 7.55
(d, J = 6.6 Hz, 1 H), 7.50 (d, J = 8.0 Hz, 1 H), 7.30–7.17 (m, 1 H),
6.98 (d, J = 8.8 Hz, 1 H), 6.88 (s, 1 H), 3.86 (s, 3 H). 13C NMR
(101 MHz, CDCl3) δ 159.97, 156.04, 154.69, 129.48, 126.41, 123.72,
123.34, 122.81, 120.55, 114.24, 110.97, 99.66, 55.36. These spectro-
scopic data matched that previously reported [2].
2-(m-Tolyl)benzofuran (3c): The title compound was prepared
according to the general procedure. The product was obtained as white
solid (87%) by using petroleum ether as the column eluent on silica gel.
1H NMR (400 MHz, CDCl3) δ 7.61 (s, 1 H), 7.58 (d, J = 7.8 Hz, 1 H),
7.51–7.46 (m, 1 H), 7.43 (d, J = 8.2 Hz, 1 H), 7.24 (t, J = 7.7 Hz, 1 H),
7.19 (td, J = 7.8, 1.4 Hz, 1 H), 7.16–7.11 (m, 1 H), 7.08 (d, J = 7.6 Hz,
1 H), 6.91 (s, 1 H), 2.34 (s, 3 H). 13C NMR (101 MHz, CDCl3) δ 156.09,
154.83, 138.42, 130.36, 129.36, 129.24, 128.67, 125.51, 124.14,
122.87, 122.12, 120.83, 111.11, 101.16, 21.48. These spectroscopic
data matched that previously reported [2].
2-(4-Fluorophenyl)benzofuran (3h): The title compound was
prepared according to the general procedure. The product was obtained
as white solid (90%) by using petroleum ether as the column eluent on
silica gel. 1H NMR (400 MHz, CDCl3) δ 7.88–7.81 (m, 1 H), 7.60–7.56
(m, 1 H), 7.53–7.49 (m, 1 H), 7.31–7.26 (m, 1 H), 7.26–7.21 (m, 1 H),
7.18–7.11 (m, 1 H), 6.96 (s, 1 H). 13C NMR (101 MHz, CDCl3) δ 162.88
(d, J = 248.8 Hz), 155.01, 154.84, 129.17, 126.80, 126.76 (d, J
= 8.1 Hz), 124.28, 123.01, 120.88, 115.88 (d, J = 22.0 Hz), 111.13,
100.99 (d, J = 1.4 Hz). These spectroscopic data matched that pre-
viously reported [30].
2-(p-Tolyl)benzofuran (3d): The title compound was prepared
according to the general procedure. The product was obtained as white
solid (90%) by using petroleum ether as the column eluent on silica gel.
1H NMR (400 MHz, CDCl3) δ 7.77 (d, J = 8.2 Hz, 1 H), 7.57 (d, J
= 7.3 Hz, 1 H), 7.52 (d, J = 7.5 Hz, 1 H), 7.30–7.19 (m, 2 H), 6.97 (s,
1 H), 2.41 (s, 1 H). 13C NMR (101 MHz, CDCl3) δ 156.18, 154.76,
138.58, 129.47, 129.33, 127.74, 124.88, 123.97, 122.83, 120.72,
111.07, 100.54, 21.38. These spectroscopic data matched that pre-
viously reported [2].
2-(2-Chlorophenyl)benzofuran (3i): The title compound was
prepared according to the general procedure. The product was obtained
as colorless oil (77%) by using petroleum ether as the column eluent on
silica gel. 1H NMR (400 MHz, CDCl3) δ 8.07 (dd, J = 7.9, 1.7 Hz, 1 H),
7.65 (d, J = 7.6 Hz, 1 H), 7.56–7.49 (m, 3 H), 7.41–7.23 (m, 4 H). 13C
NMR (101 MHz, CDCl3) δ 154.17, 151.95, 131.34, 130.91, 129.08
(2 C), 128.99 (2 C), 126.99, 124.92, 123.00, 121.50, 111.09, 107.39.
These spectroscopic data matched that previously reported [4].
2-(2-Methoxyphenyl)benzofuran (3e): The title compound was
prepared according to the general procedure. The product was ob-
tained as white solid (87%) by using petroleum ether as the column
eluent on silica gel. 1H NMR (400 MHz, CDCl3) δ 8.13–8.05 (m, 1 H),
7.60 (d, J = 7.5 Hz, 1 H), 7.52 (d, J = 8.1 Hz, 1 H), 7.39–7.18 (m,
4 H), 7.09 (dd, J = 10.9, 4.2 Hz, 1 H), 7.02 (d, J = 8.3 Hz, 1 H), 4.01
(s, 3 H). 13C NMR (101 MHz, CDCl3) δ 156.48, 153.86, 152.14, 129.77,
129.22, 127.05, 124.08, 122.61, 121.01, 120.77, 119.36, 111.01,
110.81, 106.29, 55.45. These spectroscopic data matched that pre-
viously reported [2].
2-(3-Chlorophenyl)benzofuran (3j): The title compound was
prepared according to the general procedure. The product was obtained
as white solid (84%) by using petroleum ether as the column eluent on
silica gel. 1H NMR (400 MHz, CDCl3) δ 7.84 (t, J = 1.7 Hz, 1 H), 7.71
(dt, J = 7.6, 1.4 Hz, 1 H), 7.61–7.56 (m, 1 H), 7.51 (dd, J = 8.2, 0.7 Hz,
1 H), 7.35 (t, J = 7.8 Hz, 1 H), 7.33 – 7.27 (m, 2 H), 7.26–7.20 (m, 1 H),
7.02 (d, J = 0.8 Hz, 1 H). 13C NMR (101 MHz, CDCl3) δ 154.93,
154.31, 134.84, 132.16, 130.03, 128.90, 128.40, 124.88, 124.75,
123.12, 122.92, 121.12, 111.24, 102.36. These spectroscopic data
matched that previously reported [4].
2-(3-Methoxyphenyl)benzofuran (3f): The title compound was
prepared according to the general procedure. The product was obtained
as colorless oil (91%) by using petroleum ether/DCM (2/1) as the
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(101 MHz, CDCl3) δ 155.18, 152.21, 147.71, 145.06, 132.96, 128.61,
127.26, 125.28, 124.10, 123.42, 121.36, 111.39, 103.43. These spec-
troscopic data matched that previously reported [2].
2-(4-Chlorophenyl)benzofuran (3k): The title compound was
prepared according to the general procedure. The product was obtained
as white solid (87%) by using petroleum ether as the column eluent on
silica gel. 1H NMR (400 MHz, CDCl3) δ 7.74–7.70 (m, 2 H), 7.51 (dd,
J = 4.4, 3.9 Hz, 1 H), 7.44 (d, J = 8.1 Hz, 1 H), 7.37–7.32 (m, 2 H),
7.25–7.20 (m, 1 H), 7.18–7.14 (m, 1 H), 6.94 (s, 1 H). 13C NMR
(101 MHz, CDCl3) δ 154.89, 154.77, 134.30, 129.05, 129.03, 128.98,
126.12, 124.55, 123.09, 120.99, 111.19, 101.74. These spectroscopic
data matched that previously reported [23].
2-(Cyclohex-1-en-1-yl)benzofuran (3p): The title compound was
prepared according to the general procedure. The product was obtained
as colorless oil (75%) by using petroleum ether as the column eluent on
silica gel. 1H NMR (600 MHz, CDCl3) δ 7.50 (d, J = 7.5 Hz, 1 H), 7.42
(d, J = 8.1 Hz, 1 H), 7.24–7.20 (m, 1 H), 7.19–7.15 (m, 1 H), 6.62 (td,
J = 4.1, 2.4 Hz, 1 H), 6.50 (s, 1 H), 2.39 (dtd, J = 6.3, 4.2, 2.1 Hz, 2 H),
2.30–2.25 (m, 2 H), 1.83–1.76 (m, 2 H), 1.73–1.67 (m, 3 H). 13C NMR
(151 MHz, CDCl3) δ 157.48, 154.41, 129.16, 127.16, 126.09, 123.77,
122.45, 120.52, 110.69, 100.02, 25.40, 24.92, 22.33, 22.11. These
spectroscopic data matched that previously reported [32].
2-(4-Bromophenyl)benzofuran (3l): The title compound was
prepared according to the general procedure. The product was obtained
as white solid (87%) by using petroleum ether as the column eluent on
silica gel. 1H NMR (400 MHz, CDCl3) δ 7.76 – 7.70 (m, 1 H), 7.61–7.55
(m, 2 H), 7.52 (d, J = 8.0 Hz, 1 H), 7.33–7.28 (m, 1 H), 7.25–7.21 (m,
1 H), 7.03 (d, J = 0.7 Hz, 1 H). 13C NMR (101 MHz, CDCl3) δ 154.89,
154.77, 131.96, 129.39, 129.03, 126.36, 124.60, 123.10, 122.49,
121.01, 111.20, 101.84. These spectroscopic data matched that pre-
viously reported [31].
4-(Benzofuran-2-yl)butanenitrile (3q): The title compound was
prepared according to the general procedure. The product was obtained
as yellow oil (57%) by using petroleum ether as the column eluent on
silica gel. 1H NMR (400 MHz, CDCl3) δ 7.44–7.40 (m, 1 H), 7.36–7.32
(m, 1 H), 7.19–7.08 (m, 2 H), 6.40 (d, J = 0.8 Hz, 1 H), 2.88 (t, J
= 7.2 Hz, 2 H), 2.34 (t, J = 7.1 Hz, 2 H), 2.04 (p, J = 7.1 Hz, 2 H). 13
C
NMR (101 MHz, CDCl3) δ 156.19, 154.78, 128.49, 123.65, 122.68,
120.46, 119.08, 110.82, 103.41, 27.08, 23.62, 16.42. These spectro-
scopic data matched that previously reported [33].
2-(3-Bromophenyl)benzofuran (3m): The title compound was
prepared according to the general procedure. The product was obtained
as white solid (83%) by using petroleum ether as the column eluent on
silica gel. 1H NMR (600 MHz, CDCl3) δ 7.92 (t, J = 1.7 Hz, 1 H), 7.67
(d, J = 7.8 Hz, 1 H), 7.49 (d, J = 7.7 Hz, 1 H), 7.43 (d, J = 8.2 Hz,
1 H), 7.37 (dd, J = 7.9, 0.8 Hz, 1 H), 7.24–7.18 (m, 2 H), 7.15 (t, J
= 7.4 Hz, 1 H), 6.93 (s, 1 H). 13C NMR (151 MHz, CDCl3) δ 154.92,
154.13, 132.39, 131.30, 130.27, 128.87, 127.76, 124.75, 123.35,
123.11, 122.94, 121.11, 111.24, 102.38. These spectroscopic data
matched that previously reported [31].
5-Chloro-2-phenylbenzofuran (3r): The title compound was pre-
pared according to the general procedure. The product was obtained as
white solid (90%) by using petroleum ether as the column eluent on
silica gel. 1H NMR (600 MHz, CDCl3) δ 7.85 (d, J = 7.4 Hz, 2 H), 7.55
(d, J = 1.8 Hz, 1 H), 7.48–7.42 (m, 3 H), 7.38 (t, J = 7.4 Hz, 1 H), 7.24
(dd, J = 8.7, 2.0 Hz, 1 H), 6.96 (s, 1 H). 13C NMR (151 MHz, CDCl3) δ
157.38, 153.24, 130.56, 129.95, 128.98, 128.84, 128.47, 125.04,
124.38, 120.40, 112.10, 100.78. These spectroscopic data matched that
previously reported [33].
2-(2-Bromophenyl)benzofuran (3n): The title compound was
prepared according to the general procedure. The product was obtained
as colorless oil (65%) by using petroleum ether as the column eluent on
silica gel. 1H NMR (400 MHz, CDCl3) δ 7.90 (dd, J = 7.9, 1.7 Hz, 1 H),
7.63 (dd, J = 8.0, 1.2 Hz, 1 H), 7.59 – 7.53 (m, 1 H), 7.48 – 7.43 (m,
2 H), 7.35 (dt, J = 7.9, 2.6 Hz, 1 H), 7.28 – 7.22 (m, 1 H), 7.21 – 7.09
(m, 2 H). 13C NMR (101 MHz, CDCl3) δ 154.29, 153.12, 134.26, 131.02,
129.81, 129.40, 128.80, 127.48, 124.84, 122.97, 121.45, 120.76,
111.11, 107.02. These spectroscopic data matched that previously re-
ported [31].
Methyl 2-phenylbenzofuran-5-carboxylate (3s): The title com-
pound was prepared according to the general procedure. The product
was obtained as white solid (73%) by using petroleum ether as the
column eluent on silica gel. 1H NMR (400 MHz, CDCl3) δ 8.32 (d, J
= 1.6 Hz, 1 H), 8.02 (dd, J = 8.6, 1.7 Hz, 1 H), 7.89–7.85 (m, 2 H), 7.54
(d, J = 8.6 Hz, 1 H), 7.50–7.44 (m, 2 H), 7.41–7.36 (m, 1 H), 7.07 (s,
1 H), 3.95 (s, 3 H). 13C NMR (101 MHz, CDCl3) δ 167.31, 157.43,
157.40, 129.90, 129.26, 129.05, 128.90, 126.07, 125.34, 125.09,
123.32, 111.02, 101.53, 52.13. These spectroscopic data matched that
previously reported [34].
3-(Benzofuran-2-yl)pyridine (3o): The title compound was pre-
pared according to the general procedure. The product was obtained as
white solid (81%) by using DCM/EA (3/1) as the column eluent on
silica gel. 1H NMR (400 MHz, CDCl3) δ 9.13 (d, J = 1.6 Hz, 1 H), 8.59
(dd, J = 4.9, 1.3 Hz, 1 H), 8.26 – 8.18 (m, 1 H), 7.63 (d, J = 7.8 Hz,
1 H), 7.56 (d, J = 8.2 Hz, 1 H), 7.47 (dd, J = 8.0, 5.0 Hz, 1 H), 7.38 –
7.32 (m, 1 H), 7.30 – 7.27 (m, 1 H), 7.17 (d, J = 0.6 Hz, 1 H). 13C NMR
5-(tert-Butyl)-2-phenylbenzofuran (3t): The title compound was
prepared according to the general procedure. The product was obtained
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Table 1
Reaction condition optimization.a
Entry
Base
Solvent
Temperature /oC
Reaction time /h
Conversion /%b
Selectivity /%b
1
2
3
4
5
6
7
8
K2CO3
Na2CO3
Cs2CO3
KOH
K3PO4
NaOtBu
NEt3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
CH3CN
Toluene
DMSO
DMF
DMF
DMF
160
160
160
160
160
160
160
160
160
160
160
150
140
120
100
3
5
100
100
90
100
100 (92)c
0
64
100
0
100
100
100
100
0
60
40
94
89
96
–
25
0
–
66
71
94
98
–
10
4.5
8
3.5
5
5.5
5
6.5
5
DBU
9
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
10
11
12
13
14
15
8
11
8
DMF
8
0
–
a
Reaction conditions: o-iodophenol (0.5 mmol), phenylacetylene (1.0 mmol), Pd/N,O-Carbon (10 mg, 0.3 mol% of Pd), solvent (5 mL) under argon atmosphere.
Determined by GC-FID using dodecane as the internal standard.
Isolated yield.
b
c
(d, J = 7.5 Hz, 2 H), 7.77 (d, J = 8.2 Hz, 1 H), 7.48 (t, J = 7.4 Hz,
2 H), 7.41 (t, J = 7.4 Hz, 1 H), 7.24–7.18 (m, 2 H). 13C NMR (151 MHz,
CDCl3) δ 159.69, 148.90, 148.00, 145.84, 129.62, 129.57, 128.89,
125.30, 118.74, 117.87, 102.30. These spectroscopic data matched that
previously reported [29].
2.4. Characterization
The X-ray diff ;raction (XRD) patterns of all the catalysts were ob-
tained on a Bruker D8 Advance X-ray diff ;raction diff ;ractometer
equipped with Cu Ka radiation (λ = 1.5147 Å). The morphology of
catalysts was examined by a H-7600 transmission electron microscopy
(TEM), a Tecnai G2 F30 high-resolution TEM (HRTEM) and a FEI
Tecnai G2 F20 scanning transmission electron microscopy (STEM).
Nitrogen adsorption–desorption data were obtained on a Micromeritics
ASAP 2020 static volumetric sorption analyzer. The specific surface
area of the samples was calculated by the Brunauer-Emmet-Teller (BET)
method. The micropore volume was calculated by t-plot method. The
pore size distributions were determined by non-local density functional
theory (NLDFT). The X-ray photoelectron spectroscopy (XPS) data was
collected on an ESCALAB 250Xi (Thermo Scientific, UK) instrument
equipped with a monochromatized Al Ka line source. All the binding
energies obtained were calibrated based on the C 1 s peak at 284.8 eV.
The elemental composition analysis of the catalysts was conducted on
Vario El elemental analyzer. Ion Chromatography was conducted on a
Thermo Scientific Dionex ICS-5000 equipped with CS-12 column with
methanesulfonic acid (20 mM) as an eluent. Raman spectra were ob-
tained on a Horiba Jobin Yvon LabRAM HR800 Raman spectrometer
Fig. 1. Comparison of catalytic performance for one-pot tandem reaction of o-
iodophenol and phenylacetylene with various supported catalysts.
Reaction conditions: o-iodophenol (0.5 mmol), phenylacetylene (1.0 mmol),
catalyst (10 mg, 0.3 mol% of Pd), DMF (5 mL), 8 h under argon atmosphere.
Determined by GC-FID using dodecane as the internal standard.
as white solid (82%) by using petroleum ether as the column eluent on
silica gel. 1H NMR (400 MHz, CDCl3) δ 7.90 (d, J = 7.3 Hz, 2 H), 7.63
(d, J = 1.8 Hz, 1 H), 7.52–7.45 (m, 3 H), 7.42–7.35 (m, 2 H), 7.03 (s,
1 H), 1.44 (s, 9 H). 13C NMR (101 MHz, CDCl3) δ 156.01, 153.13,
145.95, 130.67, 128.89, 128.73, 128.34, 124.83, 122.23, 117.07,
110.43, 101.48, 34.70, 31.84. These spectroscopic data matched that
previously reported [23].
system using
a 532 nm wavelength laser at room temperature.
Inductively coupled plasma atomic emission spectroscopy (ICP-AES)
was conducted on a PerkinElmer Optima 5300 DV instrument. Gas
chromatography analysis was performed on an Agilent HP-7890 in-
strument with a flame ionization detector (FID) and an HP-5MS capil-
lary column (30 m, 0.25 mm i.d., 0.25 μm film thicknesses) using he-
lium as the carrier gas. Gas chromatography-mass spectrometry
analysis was carried out on an Agilent HP-7890 instrument with an
Agilent HP-5975 with triple-axis detector and HP-5 capillary column
using helium carrier gas. NMR spectra were from a Bruker DRX-400, or
DRX-600, instrument and calibrated using residual non-deuterated
solvent (CDCl3: δH = 7.26 ppm, δC = 77.16 ppm; C6D6: δH = 7.16 ppm,
δC = 128.06 ppm) as an internal reference.
2-Phenylfuro[3,2-b]pyridine (3u): The title compound was pre-
pared according to the general procedure. The product was obtained as
white solid (79%) by using petroleum ether as the column eluent on
silica gel. 1H NMR (600 MHz, CDCl3) δ 8.52 (d, J = 4.8 Hz, 1 H), 7.90
5

G. Ji et al.
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3. Results and discussion
the selective formation of 3a in high efficiency. Further investigation
showed that excess of 2a (at least 2 equivalents with respect to 1a) is
required to achieve 3a in high yield with excellent selectivity (see Table
S2 in Supporting information).
From the outset, the reaction of o-iodophenol (1a) with phenyla-
cetylene (2a) was chosen as benchmark reaction to assess whether the
catalyst Pd/N,O-Carbon could enable one-pot sequential Sonogashira-
cyclization under ligand- and copper-free conditions. The reaction was
initially performed in the presence of 0.3 mol% (10 mg) of Pd@N,O-
Carbon, K2CO3 in DMF at 160 °C. We pleased to observe that the re-
action proceeded efficiently and 1a was completely converted to the
target product 2-phenylbenzofuran 3a in 60% selectivity (Table 1, entry
1). Encouraged by this result, we decided to further optimize the con-
ditions to improve the reaction efficiency and a set of factors including
bases, solvents and reaction temperature were subsequently screened.
Selected results are compiled in Table 1. Firstly, the reaction was per-
formed with various bases such as Na2CO3, K2CO3, Cs2CO3, K3PO4,
NaOtBu, KOH, DBU (1,8-Diazabicyclo[5.4.0]undec-7-ene), and TEA
(triethylamine). Under otherwise identical conditions, K3PO4 with 2
equivalents with respect to 1a was found to be the best choice to give
superior catalytic performance in terms of both activity and selectivity
(full conversion with up to 96% selectivity) among all bases in-
vestigated (entries 2–8) (see Table S1 in the Supporting information).
Other bases showed either lower reactivity or poorer selectivity or both.
Subsequently, the reaction was conducted at varying temperatures
(from 100 to 160 °C). An increase of reaction temperature considerably
enhances the reaction rate without affecting the selectivity to 3a,
whereas the reaction completely shut down when the reaction tem-
perature was below 130 °C even with extending the reaction times
(entries 12–15). Next, the reaction was performed in various organic
solvents including DMF, DMSO, toluene, and CH3CN (entries 9–12).
Among all solvents screened, DMF was found to be the best choice for
For comparison, commercially available Pd/C (10 wt% of Pd) and
the catalysts prepared from commonly used activated carbon (AC with
surface area of 1580 m2 g−1) and other supports such as Al2O3, TiO2,
CeO2 and ZrO2 with roughly equal Pd loading and same preparation
procedure were employed for the reaction under otherwise identical
conditions (Fig. 1). In most cases, those catalysts gave roughly equal
reaction efficiency but with considerably lower selectivity to 3a. In
sharp contrast, the reaction efficiency and chemoselectivity to 3a of the
commercial Pd/C without any heteroatom-doping was significantly
lower than that of Pd/N,O-Carbon under identical conditions, in-
dicating the intrinsic nature of N,O-dual-doped hierarchical porous
carbon played a crucial role in catalysis (see Table S4, Figs. S1 & S2 in
the Supporting information). The reaction proceed slowly with either
the bare support N,O-Carbon as catalyst or no catalyst, while only de-
halogenated product phenol was produced with 23% conversion of 1a.
Such results demonstrate the essential role of the catalyst in this tandem
reaction. Token together, the optimized reaction conditions involve
K3PO4 as the base, DMF as the solvent, in the presence of 0.3 mol% of
the catalyst Pd/N,O-Carbon at 160 °C reaction temperature.
Having identified the optimal reaction conditions, we next in-
vestigated the substrate scope to explore the versatility of this ligand-
and copper-free Pd-catalyzed one-pot tandem reaction protocol
(Table 2). In general, the catalyst Pd/N,O-Carbon enabled the one-pot
tandem reaction of a diverse array of terminal alkynes with 1a, pro-
viding the corresponding 2-benzofuran derivatives in moderate to high
yields. Aryl alkynes bearing either electron-rich or electron-poor groups
Table 2
Substrate scopes.a
6

G. Ji et al.
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4. Conclusions
In summary, we have developed a highly efficient and robust het-
erogeneous supported Pd NPs on N,O-dual-doped hierarchical porous
carbon for one-pot tandem reaction of o-iodophenols with terminal
alkynes. The catalyst enabled the sequential intermolecular
Sonogashira cross-coupling followed by cyclization of o-iodophenols
with terminal alkynes to synthesize a range of 2-benzofuran derivatives
under copper- and ligand-free conditions. A broad set of terminal al-
kynes and o-iodophenols could be efficiently transformed with good
functional groups tolerance. The catalyst also demonstrates strong
stability, can be easily recovered for successive uses. The process re-
presents a green methodology for synthesizing biologically valuable
heterocyclic compounds.
Acknowledgements
We gratefully acknowledge the start-up financial support from
Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT),
Chinese Academy of Sciences (Grant No. Y6710619KL) and 13th-Five
Key Project of the Chinese Academy of Sciences (Grant No.
Y7720519KL).
Fig. 2. Recycling experiments for the catalyst (A) Pd/AC and (B) Pd/N,O-
Carbon.
were effectively transformed, and substituents in the para-, meta-, and
ortho- positions of the aryl ring are compatible with the reaction con-
ditions. However, substrates with ortho- substituents (2b and 2e) ap-
peared to be less reactive than their para- or meta-substituted analo-
gues, possibly due to the steric effect. Halogen-substituted aryl alkynes
(2h-n) underwent the reaction in moderate to high yields, and deha-
logenation products were not observed. Heteroaromatic alkyne, 2-3-
ethynylpyridine (2o) proceeded smoothly to deliver the corresponding
2-benzofurans products (3o) in 81% yield. Besides, not only terminal
aryl alkynes but also terminal alkyl alkynes (2p-r) even with bearing
functional groups e.g. eOH, eCN are also suitable substrates and un-
derwent the sequential Sonogashira/cyclization tandem reaction under
the optimized conditions to afford the product in 57–75%yields. It
appears that the present catalytic system has a broad substrate scope as
well as functional group tolerance for the terminal alkynes.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.cattod.2018.04.036.
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7

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