One-pot formic acid dehydrogenation and synthesis of benzene-fused heterocycles over reusable AgPd/WO2.72 nanocatalyst

Article abstract of DOI:10.1039/c8ta09342c

Using nanoparticles (NPs) to catalyze multiple chemical reactions in one-pot and to achieve high-yield syntheses of functional molecules/materials is an important direction in NP chemistry, catalysis and applications. In this article, we report a nanocomposite of AgPd NPs anchored on WO_2.72 nanorods (NRs) (denoted as AgPd/WO_2.72) as a general catalyst for formic acid dehydrogenation and transfer hydrogenation from Ar-NO₂ to Ar-NH₂ that further reacts with aldehydes to form benzene-fused heterocyclic compounds. The AgPd/WO_2.72 catalysis is Ag/Pd dependent and Ag₄₈Pd₅₂ is the most active composition for the multiple chemical reactions. The high activity of AgPd/WO_2.72 arises from strong interfacial interaction between AgPd and WO_2.72, resulting in AgPd lattice expansion and electron polarization from AgPd to WO_2.72. The syntheses proceed in one-pot reactions among formic acid, 2-nitrophenol (or 2-nitroaniline, or 2-nitrothiophenol) and aldehydes in dioxane/water (2/1 v/v) at 80-90 °C, leading to one-pot syntheses of benzoxazoles, benzimidazoles and benzothiazoles that are key ring structures present in functional compounds for pharmaceutical, optical and polymer applications.

Full text of DOI:10.1039/c8ta09342c

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PAPER
One-pot formic acid dehydrogenation and
synthesis of benzene-fused heterocycles over
reusable AgPd/WO2.72 nanocatalyst†
Cite this: J. Mater. Chem. A, 2018, 6,
23766
Chao Yu,‡a Xuefeng Guo, ‡a Bo Shen, a Zheng Xi, a Qing Li, b Zhouyang Yin,a
a
a
a
a
*
Hu Liu,
Michelle Muzzio,a Mengqi Shen,
Junrui Li,
Christopher T. Seto
a
*
and Shouheng Sun
Using nanoparticles (NPs) to catalyze multiple chemical reactions in one-pot and to achieve high-yield
syntheses of functional molecules/materials is an important direction in NP chemistry, catalysis and
applications. In this article, we report a nanocomposite of AgPd NPs anchored on WO2.72 nanorods
(NRs) (denoted as AgPd/WO2.72) as a general catalyst for formic acid dehydrogenation and transfer
hydrogenation from Ar-NO2 to Ar-NH2 that further reacts with aldehydes to form benzene-fused
heterocyclic compounds. The AgPd/WO2.72 catalysis is Ag/Pd dependent and Ag48Pd52 is the most
active composition for the multiple chemical reactions. The high activity of AgPd/WO2.72 arises from
strong interfacial interaction between AgPd and WO2.72, resulting in AgPd lattice expansion and
electron polarization from AgPd to WO2.72. The syntheses proceed in one-pot reactions among
formic acid, 2-nitrophenol (or 2-nitroaniline, or 2-nitrothiophenol) and aldehydes in dioxane/water
(2/1 v/v) at 80–90 ꢀC, leading to one-pot syntheses of benzoxazoles, benzimidazoles and
benzothiazoles that are key ring structures present in functional compounds for pharmaceutical,
optical and polymer applications.
Received 26th September 2018
Accepted 4th November 2018
DOI: 10.1039/c8ta09342c
rsc.li/materials-a
available precursors in a green chemistry environment without
compromising the synthetic yield and selectivity.
1 Introduction
Catalytic dehydrogenation of small molecules to release
hydrogen (H2) is an attractive approach to organic hydrogena-
tion reactions without using pure H2 as a reactant.15,16 The
common H2 storage materials explored for H2 formation or
hydrogenation reactions include conventional alcohols and
hydrides.17–19 Recently, ammonia borane (NH3$BH3) and formic
acid (HCOOH) have attracted a lot of attention as H2 storage
materials.20–30 HCOOH as a H2 carrier is especially appealing
since it is easily available from oxidation of biomass, or
hydrogenation of CO2. Recently, AgPd and AuPd alloy nano-
particles (NPs) were found to be active and stable for HCOOH
dehydrogenation into H2 and CO2, and to initiate transfer
hydrogenation to RNO2 to give RNH2.31–36 Especially when AgPd
NPs were coupled to oxygen-decient WO2.72, they catalyzed
one-pot reactions among HCOOH, nitrophenol and aldehydes
in H2O/dioxane, providing a clean and safe method for
preparing benzoxazoles.32
Benzene-fused heterocycles, are common core structures found
in compounds prepared for biomedical, sensor and optical
applications.1–5 Some typical examples of these structures are
benzoxazoles where X ¼ O, NH or S. Once properly functional-
ized for polymer formation, the ring structure is also a key
component of a class of rigid organic polymers, such as poly-
benzoxazoles, that are important for ballistic ber, ame
retardant and electronic textile applications.6–9 Conventional
chemistry leading to the synthesis of benzoxazoles oen
requires long reaction times (up to 24 h), harsh reaction
conditions (over 130 ꢀC), reactive additives (H2O2), and toxic
solvents.10–14 These conditions can make it challenging to
prepare aromatic ring structures with controlled functionali-
zation to tune their chemical and physical properties. Ideally,
the synthesis could proceed using a one-pot reaction with easily
Despite the exciting synthetic potential demonstrated in the
AgPd/WO2.72 catalyzed one-pot reactions to yield benzoxazoles,
there are two questions that remain unanswered: (1) why are
these AgPd/WO2.72 nanocomposites more active than AgPd NPs
(AgPd is still active but with lower efficiency)? (2) can this one-
pot strategy be extended to produce other aromatic analogues,
as shown in Fig. 1, without signicantly changing the reaction
aDepartment of Chemistry, Brown University, 324 Brook St. Providence, RI 02912, USA.
E-mail: ssun@brown.edu; christopher_seto@brown.edu
bSchool of Materials Science and Engineering, Huazhong University of Science and
Technology, Wuhan 430074, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ta09342c
‡ These authors contributed equally to this work.
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ꢀ
rst heated to 120 C in a gentle Ar ow and magnetic stirring
for 30 min to remove air and moisture from the reaction system.
The clear dark green solution was then heated to 280 ꢀC in
ꢀ
30 min (when the reaction temperature reached 200 C, the Ar
gas was switched to form an Ar blanket over the reaction system
and the solution started to turn to blue). The reaction solution
was kept at 280 ꢀC for 10 h before it was cooled to room
temperature. The product was separated from the solution by
adding 80 mL ethanol and centrifuging at 9500 rpm for 8 min.
Then 20 mL hexane was added to re-disperse the synthesized
NRs and centrifugation (6000 rpm, 4 min) was applied to
precipitate any undispersed material. The product in the
dispersion was then precipitated by adding 35 mL ethanol fol-
lowed by centrifugation. The NRs were puried again with
15 mL hexane and 35 mL ethanol. Aer separation by centri-
fugation, the NRs were dispersed in hexane for further use.
2.2.2 Synthesis of AgPd/WO2.72 nanocomposites. Under
a gentle nitrogen ow, 0.084 g of Ag(Ac) (0.5 mmol), 0.15 g of
Pd(acac)2 (0.5 mmol) and 0.34 g of WO2.72 NRs (1.5 mmol) were
magnetically stirred in 4.5 mL of OAc, 0.5 mL of OAm, and
10 mL of ODE. The mixture was heated to 80 ꢀC to generate
a homogeneous solution and kept fꢀor 1 h. Then the solution was
heated to 180 C at a rate of 5–7 C minꢁ1. The reaction was
ꢀ
Fig. 1 The schematic illustration of Ag48Pd52/WO2.72-catalyzed one-
pot syntheses of benzoxazoles (where X ¼ O, NH, S).
allowed to proceed for 20 min and cooled to room temperature.
100 mL of isopropanol was added, and the product was sepa-
rated by centrifugation at 9500 rpm for 15 min to remove the
organic impurities and precursor residues. The product was
redispersed in 10 mL of hexane and then recollected by adding
40 mL of ethanol and centrifugation at 9500 rpm for 8 minutes.
The products were washed twice with hexane/ethanol (v/v ¼
1 : 15) and then suspended in 30 mL of acetic acid and the
suspension was stirred at 60 ꢀC for 10 h. 30 mL of ethanol was
added and the mixture was centrifuged at 9500 rpm for 8
minutes. This ethanol washing procedure was repeated for
three times, yielding Ag48Pd52/WO2.72, which was dispersed in
hexane for future use. The composition of AgPd/WO2.72 was
controlled by the molar ratio of metal precursors. For instance,
with the total amount of metal precursor xed at 1.0 mmol,
Ag78Pd22/WO2.72, Ag57Pd43/WO2.72, Ag48Pd52/WO2.72, Ag38Pd61/
WO2.72 and Ag20Pd80/WO2.72 were formed in the molar ratio
from 3 : 1, 2 : 1, 1 : 1, 1 : 2 and 1 : 3 respectively.
2.2.3 Decomposition of FA. A two-necked reaction ask (25
mL) containing a Teon-coated stir bar was placed on
a magnetic stirrer and thermostated to 50.0 ꢂ 1 ꢀC by using
a constant temperature bath. A gas burette lled with water was
connected to one neck of the ask (the other neck was sealed) to
measure the volume of gas mixture evolved from the reaction.
9.64 mL of aqueous suspension of the catalyst was transferred
into the reaction ask, and 0.36 mL (9 mmol) of FA was added
into the stirred (800 rpm) solution. The volume of gas mixture
evolved was measured by recording the volume of water dis-
placed. The reaction was considered to cease when hydrogen
conditions? This paper aims to answer these two questions by
studying what makes AgPd/WO2.72 more active than simpler
AgPd NPs, and by demonstrating that the one-pot synthesis can
indeed be applied not only to benzoxazoles, but also to other
structures where X ¼ NH and S (Fig. 1). The reactions provide
a general approach to C–X bond formation and high yield
production of benzene-fused heterocyclic compounds.
2 Experimental
2.1 Material
The NP synthesis was carried out using standard airless
procedures and commercially available reagents. All reagents
were used as received. Oleylamine (OAm, >70%), 1-octadecene
(ODE, technical grade, 90%), oleic acid (OA, 90%), formic acid
(FA, 95%), nitro compounds 2-nitrophenol, 2-nitrothiophenol,
2-nitroaniline and various benzaldehydes were from Sigma-
Aldrich. Tungsten(IV) chloride, 97% (WCl4), silver(I) acetate
(Ag(Ac)2, 97%), and palladium(II) acetylacetonate (Pd(acac)2,
95%) were from Strem Chemicals. 4,6-Dinitroresorcinol was
purchased from Fisher Scientic. These chemicals were used
without further purication. The deionized water was obtained
from a Millipore Autopure System.
2.2 Preparation and characterization
2.2.1 Synthesis of WO2.72 NRs. 81 mg WCl4 (0.25 mmol), gas generation was no longer observed.
5 mL ODE, 3 mL OAm were rst mixed in a 20 mL vial through
2.2.4 CO2 removal from the H2 and CO2 mixture. To
sonication to form a dark brown solution and transferred into quantify the molar ratio of CO2 to H2 in the gas mixture
a 100 mL four-neck round bottom ask. 10 mL ODE and 6 mL generated during the AgPd/WO2.72 catalyzed dehydrogenation
OAc were then added into the ask. The mixture solution was of aqueous FA solution (10 mL of 900 mM), the gas burette
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system was modied by placing a NaOH trap (10 M NaOH
solution) between the jacketed reactor and gas burette. The gas
generated during the reaction was passed through the NaOH
trap and CO2 was captured, leaving only H2. The gas volume
change observed before and aer the NaOH trap experiment
was used to quantify CO2 and H2.
3 Results and discussion
The AgPd/WO2.72 catalyst contains AgPd NPs perched on WO2.72
nanorods (NRs), as shown in Fig. 2a. WO2.72 NRs were prepared
by reacting WCl4 with oleic acid (OA) and oleylamine (OAm) in
1-octadecene (ODE) at 280 ꢀC,37 while AgPd/WO2.72 was ob-
tained by growing AgPd NPs at 180 ꢀC in the presence of the
WO2.72 NRs.38 Under these reaction conditions, the molar ratios
of Ag/Pd and the mass ratios of AgPd/WO2.72 were controlled by
varying the amounts of the Ag- and Pd-precursors and WCl4,
and were determined using inductively coupled plasma-atomic
emission spectroscopy (ICP-AES). We prepared Ag78Pd22/
WO2.72, Ag57Pd43/WO2.72, Ag48Pd52/WO2.72, Ag38Pd61/WO2.72 and
Ag20Pd80/WO2.72, with a xed ratio of AgPd to WO2.72 of 3 : 5 mol
molꢁ1. Representative TEM images of Ag48Pd52/WO2.72 (Fig. 2a)
show that the NPs have an average diameter of 2.3 ꢂ 0.1 nm and
“perch” on 40 ꢄ 5 nm WO2.72 NRs. High-resolution TEM
(HRTEM) images of the composite (inset in Fig. 2a) show that
the WO2.72 NRs have a good crystalline structure with an
2.2.5 General procedure for catalytic formation of primary
amines. Ar-NO2 (1 mmol), Ag48Pd52/WO2.72 (3 mol%), dioxane (6
mL) and water (3 mL) were stirred for 5 min in a 50 mL ask at
ꢀ
50.0 C. Next, FA (4 mmol) was added to the reaction mixture,
and the ask was sealed by a balloon. The solution was stirred at
ꢀ
800 rpm at 50.0 C for a pre-set reaction time. The progress of
the reaction and the yields of the amine compounds were
determined by GC-MS with benzyl ether as an internal standard.
2.2.6 General procedure for catalytic one-pot synthesis of
benzoazoles. 2-Nitrobenzene substrate (1 mmol), aldehyde (1.2
mmol), Ag48Pd52/WO2.72 (3 mol%), dioxane (6 mL) and water (3
mL) were stirred for 5 min in a 25 mL ask at 80 ꢀC. Next, FA (4
mmol) was added to the reaction mixture, and the ask was
sealed by a balloon. The solution was stirred at 800 rpm at 80 ꢀC
for 8 h. Aer completion of the reaction, a small amount of ethyl
acetate was added and the catalyst was removed by centrifuga-
tion at 9500 rpm and washed three times with water and
methanol. Then the catalyst was dried before further use. The
solvent was removed under vacuum and the residue was puri-
ed by column chromatography (hexane/ethyl acetate ¼ 8 : 1) to
give the product.
2.2.7 Characterization. Samples for transmission electron
microscopy (TEM) and high-resolution TEM (HRTEM) analyses
were prepared by depositing a single drop of diluted NP
dispersion/suspension on amorphous-carbon-coated copper
grids. Images were obtained by a JEOL 2010 TEM (200 kV). X-ray
powder diffraction (XRD) patterns of the samples were collected
on a Bruker AXS D8-Advanced diffractometer with Cu Ka radi-
˚
interfringe distance of 3.8 A, which corresponds to the (010)
˚
interplanar spacing of monoclinic WO2.72 (3.78 A, JCPDS no. 65-
1291). The Ag48Pd52 NPs attached to the WO2.72 NRs have an
˚
interfringe distance of 2.5 A, which is larger than (111) inter-
˚
planar spacing of 2.3 A observed for independent Ag48Pd52 NPs
(Fig. S1†). X-ray diffraction (XRD) of the Ag48Pd52 NPs exhibit
a wide (111) peak around 40ꢀ (Fig. 2b). Once attached to WO2.72
,
the (111) peak shis to a smaller angle, indicating that AgPd
NPs coupled to WO2.72 have their lattice expanded. The (111)
peaks of AgPd/WO2.72 show Ag/Pd-composition-dependent
shis (Fig. 2b) and the corresponding lattice parameters lie
along the theoretical line based on Vegard's law (Fig. S2†),
conrming the formation of an alloy structure of AgPd, not
a core/shell structure. As a comparison, WO2.72 does not show
obvious pattern changes. X-ray photoelectron spectroscopy
(XPS) was further used to characterize the AgPd NPs, WO2.72
NRs and the AgPd/WO2.72 composite structure (Fig. 3a–b). Once
attached to WO2.72, the Ag 3d and Pd 3d binding energies of the
AgPd NPs increase slightly. The Ag 3d peaks shi less (from
368.1/374.1 eV to 368.2/374.2 eV) than the Pd 3d ones (from
336.4/341.8 eV to 336.8/342.1 eV). In contrast, the binding
energy of W 4f in the composite structure is down-shied
˚
ation (l ¼ 1.5406 A). The compositions of the AgPd NPs and the
molar ratio of AgPd/WO2.72 were measured by inductively
coupled plasma-atomic emission spectroscopy (ICP-AES). For
ICP-AES analyses, the dried NPs were dissolved in warm aqua
regia (ꢃ70 ꢀC, 30 min) to ensure the complete dissolution of
metal into the acid. The solution was then diluted with 2%
HNO3 solution. The measurements were carried out on a JY2000
Ultrace ICP-AES equipped with a JY-AS 421 auto sampler and
2400 g mmꢁ1 holographic grating. TLC analysis was done using
silica gel TLC with 60 F254 indicators. The analyses of the one-
pot reaction products were carried out by GC-MS using an Agi-
lent 6890 GC coupled to a 5973 Mass spectrometer detector with
a DB-5 (Agilent) fused silica capillary column (L ꢄ I.D. 30 m ꢄ
0.25 mm, df 0.25 mm) and helium as carrier gas. The gas chro-
matograph was temperature programmed from 65 ꢀC (3 min
initial time) to 300 ꢀC at 6 ꢀC minꢁ1 (isothermal for 20 min nal
time). The mass spectrometer was operated in the electron
impact mode at 70 eV ionization energy. NMR spectra were
recorded using a Bruker Avance III Ultra-Shield 400 MHz spec-
trometer (400 MHz for 1H, 100 MHz for 13C) at 296 K. Chemical
shis are reported in ppm relative to the residual solvent signal,
coupling constants (J) are reported in Hz.
Fig. 2 (a) TEM image of the Ag48Pd52/WO2.72 nanocomposite. Inset:
HRTEM image of Ag48Pd52/WO2.72. (b) XRD pattern of the Ag48Pd52
WO2.72 and Ag48Pd52 NPs synthesized without WO2.72. Inset: XRD
patterns of Ag78Pd22/WO2.72 Ag57Pd43/WO2.72 Ag48Pd52/WO2.72
Ag38Pd61/WO2.72 and Ag20Pd80/WO2.72
/
,
,
,
.
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Fig. 4 (a) Plot of time vs. volume of gas generated from FA dehy-
drogenation catalyzed by nanocomposites with different Ag48Pd52
/
WO2.72 molar ratio ([AgPd] ¼ 3.1 mM, [FA] ¼ 900 mM, T ¼ 323 K). Inset:
The dehydrogenation TOF values of nanocomposites with different
Ag48Pd52/WO2.72 molar ratios. (b) Plot of time vs. volume of gas
generated from FA dehydrogenation catalyzed by Ag48Pd52 on
different supports ([AgPd] ¼ 3.1 mM, [FA] ¼ 900 mM, T ¼ 323 K). Inset:
TOF values of Ag48Pd52 on different supports.
Fig. 3 (a–c) XPS spectra of Ag48Pd52/WO2.72 and Ag48Pd52 (Ag 3d, Pd
3d and W 4f). (d) CO stripping voltammetry of the Ag48Pd52/WO2.72
composites, Ag48Pd52 and commercial Pd obtained in 0.1 M HClO4
with a scan rate of 20 mV sꢁ1. The solid line is from the first scanning
cycle and the dashed line is from the second scanning cycle.
the continued increase of TOF, rather it decreases its value due
to apparent over-loading of the catalyst. At [FA] ¼ 900 mM and
[AgPd]/WO2.72 ¼ 3/5 mol molꢁ1, it shows the best activity with
a TOF value reaching 1718 hꢁ1. This TOF is among the highest
ever reported for Pd-based catalysts (Table S1†).29,50–56 We also
tested the effects of different inorganic supports, such as
conventional carbon (C), silica (SiO2), graphene (G), or
commercial WO3 and WO2.9 on the Ag48Pd52 catalysis at the 3/5
[AgPd]/[support] ratios (Fig. 4b). These composites are all less
active than Ag48Pd52/WO2.72, suggesting that AgPd coupling to
WO2.72 favors the dehydrogenation of FA.
(Fig. 3c). These observations indicate that the electron density
of AgPd is decreased in the composite structure, and that Pd is
affected more than Ag by the AgPd–WO2.72 interactions.37,39–43
The interaction between AgPd and WO2.72 induces charge
polarization from AgPd to WO2.72, which should increase the
Ag48Pd52/WO2.72 could further catalyze transfer hydrogena-
tion from HCOOH to R-NO2 to give R-NH2 at 50 ꢀC. Using
nitrobenzene as a model substrate, we optimized the reaction
conditions: 1 mmol nitrobenzene, 3.0 mmol FA, 3 mol%
Ag48Pd52/WO2.72 in 9 mL H2O/dioxane (1 : 2 v/v). Under these
conditions the dehydrogenation of FA and hydrogenation of
nitrobenzene were complete within 30 min, and aniline was
isolated in a 98% yield. Using similar reaction conditions,
a variety of substituted nitrobenzene derivatives could be
reduced to the corresponding anilines in yields above 90%
within 2 h (Table S2†). Ag/Pd-dependent studies indicate that
Ag48Pd52/WO2.72 is the best catalyst for FA dehydrogenation,
and transfer hydrogenation to reduce nitro groups (Fig. S6†).
We further extended this reaction to a one-pot synthesis of
benzoxazoles (Table 1) starting from 2-nitrophenol and an
aldehyde. Various benzoxazoles (entries 1–8) were obtained in
>90% yield at 80 ꢀC within 8 h. The electronic properties of
substituents at either the para- or meta-positions of the benz-
aldehyde had little inuence on the reaction outcome. However,
substituents at the ortho position decreased the yield somewhat
(entries 9–10), likely due to steric hinderance. The good yield
(91%) of the thiophene derivative (entry 11) shows that the
reaction is tolerant to substrates containing a sulfur atom.
However, replacing the aromatic aldehyde with an aliphatic
aldehyde does lower the yield (entry 12).
Lewis acidity of the AgPd NP, and the Lewis basicity of WO2.72
,
facilitating AgPd binding of electrons and WO2.72 absorption of
protons. We rst studied the CO binding properties of the AgPd
surface via the CO stripping test (Fig. 3d). From Pd to AgPd, the
CO stripping peak is negatively shied and broadened, indi-
cating that CO is oxidized more easily on the AgPd surface than
on the Pd surface. Once coupled with WO2.72, the CO stripping
peak is much weaker and more negatively shied, demon-
strating the improved CO-tolerance on the AgPd surface. We
then evaluated the catalytic properties of AgPd/WO2.72 for for-
mic acid (FA) dehydrogenation. We found that the conditions,
previously optimized in aqueous solution (10 mL of 900 mM FA,
323 K), were still suitable for the current catalyst system. Gas
chromatography (GC) analyses (Fig. S3†) show that the gas
mixture consists of only CO2 and H2, and no CO is present.
Using a NaOH trap to remove CO2 from the gas mixture, we
obtained the H2 volume (Fig. S4†). Our studies show that AgPd/
WO2.72 catalyzes FA decomposition in a CO-free pathway, and
that the strong AgPd–WO2.72 interaction makes AgPd more
electrophilic, favoring the activation of FA to generate HCOO*
via O–H bond cleavage or *COOH via C–H bond cleavage.27,40–49
CO2 and H2 are formed via the AgPd-promoted decomposition
of HCOO* and *COOH, which essentially blocks the CO-
forming pathway.
Ag48Pd52-catalyzed dehydrogenation of FA was evaluated at
323 K with [AgPd] ¼ 3.1 mM. Fig. 4a and S5† show that 3/5
molar ratio and 900 mM are necessary to reach optimum cata-
lytic activity. Adding more Ag48Pd52 and WO2.72 does not lead to
To determine if other steps in this multi-step reaction, in
addition to FA dehydrogenation and reduction of the nitro
group, were dependent on the structure and composition of the
catalyst, we examined the synthesis of 2-phenylbenzoxazole
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Table
1 Ag48Pd52/WO2.72-catalyzed one-pot condensation of 2-
the rst two hours of the reaction, before it is transformed more
slowly to the product. Other AgPd-catalysts were not as efficient
as Ag48Pd52/WO2.72 for both the condensation of 2-amino-
phenol and benzaldehyde to give the imine intermediate, and
also the cyclization and dehydrogenation of the intermediate to
give the nal benzoxazole product. Among the catalysts studied,
Ag is the least active catalyst for both of these processes. Taken
together, these synthetic data indicate that; (1) FA dehydroge-
nation, transfer hydrogenation from Ar-NO2 to Ar-NH2, and Ar-
NH2 reaction with aldehydes to form benzoxazoles are all
dependent on the AgPd composition and identity of the solid
support of the catalyst, (2) Ag48Pd52/WO2.72 is the most effective
catalyst for all of these reaction steps, and (3) the reactions are
not signicantly affected by the electronic character of substit-
uents on the aldehyde, but are inuenced by the steric charac-
teristics of the aldehyde.
nitrophenol with various aldehydesa
The Ag48Pd52/WO2.72 catalyst was not only active, but also
stable under the one-pot reaction conditions. For example, in
the formation of 2-phenylbenzoxazole, aer the reaction was
complete the catalyst was precipitated by adding ethyl acetate,
isolated and puried by washing with water/methanol, and re-
used. Aer 5 rounds of catalyst reuse, the catalyst showed
little drop in activity, and the yield of the nal product remained
above 95% for all ve reactions. In addition, the catalyst showed
no obvious changes in morphology (Fig. S7†).
a
Reaction conditions: 2-nitrophenol (1 mmol), aldehyde (1.2 mmol),
Ag48Pd52/WO2.72 (3 mol%), dioxane (6 mL), water (3 mL) and FA (4
mmol), 8 h.
The AgPd/WO2.72 catalyzed one-pot reaction was not limited
to the formation of benzoxazoles, but could also be extended to
the synthesis of other benzene-fused heterocycles (Fig. 6). By
replacing 2-nitrophenol with 2-nitroaniline, N-methyl-2-
nitroaniline or 2-nitrothiophenol, we obtained the corre-
sponding benzimidazoles or benzothiazole in >83% yield using
the same one-pot reaction conditions (13–15). By raising the
starting from 2-aminophenol and benzaldehyde. The reaction
was performed using the same conditions as shown in Table 1,
and we used GC-MS to monitor the concentrations of the 2-
aminophenol starting material, the imine intermediate, and the
benzoxazole product. The most efficient catalyst for synthesis of
2-phenylbenzoxazole starting from 2-aminophenol is Ag48Pd52/
WO2.72 (Fig. 5a). 2-Aminophenol was almost completely
consumed aer 3 h, and the reaction was essentially complete
aer 7 h. Fig. 5b shows the accumulation and subsequent
consumption of the imine intermediate (black circles) formed
by dehydration of the amino group of 2-aminophenol with
benzaldehyde. This intermediate quickly accumulates during
ꢀ
reaction temperature from 80 to 90 C, the yields were further
improved to >94% (13–15 in parentheses). It is notable that the
Fig.
6 (a) Ag48Pd52/WO2.72-catalyzed synthesis of 2-phenyl-
benzimidazole, 1-methyl-2-phenylbenzimidazole and 2-phenyl-
Fig. 5 AgxPdy/WO2.72-catalyzed condensation of 2-aminophenol and benzothiazole. Reaction conditions: substrate (1 mmol), benzaldehyde
benzaldehyde. (a) Time-dependent formation of 2-phenyl- (1.2 mmol), Ag48Pd52/WO2.72 (3 mol%), dioxane (6 mL), water (3 mL)
benzoxazole (rectangles) starting from 2-aminophenol (triangles). (b) and FA (4 mmol), 8 h. The yields in parentheses were obtained at 90 ꢀC.
Time-dependent formation and consumption of the imine interme- (b) Ag48Pd52/WO2.72-catalyzed synthesis of phenylene-bis-benzox-
diate (circles). Note: (1) Ag, (2) Ag78Pd22, (3) Ag57Pd43, (4) Ag48Pd52, (5) azole, 2,6-diphenyl-benzobis(oxazole) and 2,5-bis(benzoxazol-2-yl)
Ag39Pd61, (6) Ag20Pd80 and (7) Pd.
thiophene.
23770 | J. Mater. Chem. A, 2018, 6, 23766–23772
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yield of 2-phenylbenzothiazole remains high at 94%. The AgPd/ a general approach to benzene-fused heterocyclic compounds
WO2.72 catalyst is not negatively affected by the thiol group in for pharmaceutical and functional polymer applications.
the 2-nitrothiophenol starting material. This outcome is
different than what has been reported previously for other Pd-
based nanocatalysts, which are typically poisoned by
thiols.57–60 This observation suggests that the presence of the There are no conicts of interest to declare.
WO2.72 helps to protect the AgPd catalyst from poisoning.
Conflicts of interest
The one-pot reaction could be further extended to
prepare phenylene-bis-benzoxazole (16) and 2,6-diphenyl-
Acknowledgements
benzobis(oxazole) (17) from 2-nitrophenol/terephthalaldehyde The work was supported in part by the U.S. Army Research
ꢀ
and 4,6-dinitroresorcinol/benzaldehyde under the same 80 C Laboratory and the U.S. Army Research Office under grant
reaction conditions with >90% yields, making the reaction very W911NF-15-1-0147, Strem Chemicals, and the Office of Vice
promising for fabricating linear p-conjugated polymers (work President of Research of Brown University. M.M. is supported
ꢀ
in progress). By elevating the temperature to 100 C, the reac- by the National Science Foundation Graduate Research
tion was also suitable for condensation of two equivalents of 2- Fellowship, under Grant No. 1644760.
nitrophenol with thiophene-2,5-dicarbaldehyde to give 2,5-
bis(benzoxazol-2-yl)thiophene (18) (Fig. S8†), the known
commercial uorescent brightener 185. This brightener is
Notes and references
normally prepared from the reaction between 2-aminophenol
and thiophene-2,5-dicarboxylic acid in toluene and N-methyl-2-
pyrrolidone at 185 ꢀC with boric acid as an additive.61 Compared
with this conventional approach, our one-pot reaction is milder
and produces the brightener in a modestly good yield. This 63%
yield is somewhat lower than what we have observed for the
other reactions reported herein. This reduced yield is not due to
poisoning of the catalyst by the sulfur atom of thiophene.
Rather, it is caused by reduction of one of the aldehyde groups
in thiophene-2,5-dicarbaldehyde to the corresponding methyl
group. During synthesis of bis-benzoxazole 18, we also observe
formation of the byproduct compound 19 in 21% yield. The
study of this reduction of thiophene aldehyde is out of the scope
of this paper and will be investigated separately.
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