Photocatalyzed facile synthesis of 2,5-diaryl 1,3,4-oxadiazoles with polyaniline-?g-C3N4-TiO2 composite under visible light

Article abstract of DOI:10.1016/j.tetlet.2018.03.005

PANI (polyaniline)-g-C₃N₄-TiO₂ composite was prepared and found to be efficient for the synthesis of 2,5-diaryl 1,3,4-oxadiazoles under visible light. This reaction involved decarboxylation and cyclization from α-keto acids with acylhydrazines, and a broad scope of substrates were tolerated to provide the desired products in moderate to good yields. Control experiments indicated that a radical pathway was involved in the present photocatalytic reaction and a synergistic effect may exist in the ternary composite. Moreover, this semiconductor photocatalyst could be readily recovered and showed good reusability with only slight decrease in the catalytic activity after six consecutive runs.

Full text of DOI:10.1016/j.tetlet.2018.03.005

Accepted Manuscript
Photocatalyzed facile synthesis of 2,5-diaryl 1,3,4-oxadiazoles with polyaniline
−g-C3N4−TiO2 composite under visible light
Liang Wang, Yaoyao Wang, Qun Chen, Mingyang He
PII:
S0040-4039(18)30302-2
https://doi.org/10.1016/j.tetlet.2018.03.005
TETL 49774
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
1 February 2018
27 February 2018
2 March 2018
Please cite this article as: Wang, L., Wang, Y., Chen, Q., He, M., Photocatalyzed facile synthesis of 2,5-diaryl 1,3,4-
oxadiazoles with polyaniline−g-C3N4−TiO2 composite under visible light, Tetrahedron Letters (2018), doi: https://
doi.org/10.1016/j.tetlet.2018.03.005
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Graphical Abstract
Photocatalyzed facile synthesis of 2,5-diaryl
−g
1,3,4-oxadiazoles with polyaniline
Leave this area blank for abstract info.
‑
C3N4−TiO2 composite under visible light
Liang Wang,* Yaoyao Wang, Qun Chen, and Mingyang He*

1
Tetrahedron Letters
jour nal homepage: www.elsevier. com
Photocatalyzed facile synthesis of 2,5-diaryl 1,3,4-oxadiazoles with
−g
polyaniline -C3N4−TiO2 composite under visible light
Liang Wang,* Yaoyao Wang, Qun Chen, and Mingyang He*
School of Petrochemical Engineering, Jiangsu Key Laboratory of Advanced Catalytic Materials & Technology, Changzhou University, Changzhou, 213164, P. R.
China.
*Corresponding author. Tel.: +86-519-86330263; fax: +86-519-86330251. E-mail address: liangwang@cczu.edu.cn; hemingyangjpu@yahoo.com
ARTICLE INFO
ABSTRACT
Article history:
Received
PANI (polyaniline)-g-C3N4-TiO2 composite was prepared and found to be efficient for the
synthesis of 2,5-diaryl 1,3,4-oxadiazoles under visible light. This reaction involved
decarboxylation and cyclization from α-keto acids with acylhydrazines, and a broad scope of
substrates were tolerated to provide the desired products in moderate to good yields. Control
experiments indicated that a radical pathway was involved in the present photocatalytic reaction
and a synergistic effect may exist in the ternary composite. Moreover, this semiconductor
photocatalyst could be readily recovered and showed good reusability with only slight decrease
in the catalytic activity after six consecutive runs.
Received in revised form
Accepted
Available online
Keywords:
photocatalysis
2009 Elsevier Ltd. All rights reserved.
2,5-diaryl 1,3,4-oxadiazoles
PANI-g-C3N4-TiO2 composite
radical reaction
2,5-Diaryl 1,3,4-oxadiazoles have received much attention
owing to their wide applications in pharmaceutical chemistry,
synthetic chemistry and material science.1 They have shown
significant biological properties, including antimicrobial, anti-
inflammatory, antifungal, and antiviral properties. They are also
useful surrogates in synthetic chemistry, and their unique
optoelectronic properties make them popular in material science.
Generally, 2,5-diaryl 1,3,4-oxadiazoles can be synthesized via
dehydrative cyclization of 1,2-diacylhydrazines2 and oxidative
cyclization of N-acylhydrazones.3 Alternatively, transition-
metal-catalyzed coupling reaction between aryl halides and 2-
substituted 1,3,4-oxadiazole4 and the Huisgen 1,3,4-oxadiazole
protocol5 could also deliver the desired products. Recently,
radical mediated the synthesis of 2,5-diaryl 1,3,4-oxadiazoles
have also been realized.6 However, there are still some
drawbacks associated with the reported procedures including the
use of a large excess amount of acids or oxidants, the use of
metal catalysts as well as harsh reaction conditions. In some
cases, the starting materials were not readily available. Thus, the
development of a facile and green approach for the synthesis of
2,5-diaryl 1,3,4-oxadiazoles is still highly desirable.
attempts on modifications of TiO2 have also been made in order
to narrow its wide band gap (3.2 eV, anatase) and reduce its high
recombination rate of photoinduced electron−hole pairs.8 While
acknowledging these contributions, it should also be pointed out
that most of these heterogeneous photocatalysts are mainly used
for photo-degradations. Their application in synthetic chemistry
have been rarely reported yet. Recently, we focused on the
heterogeneous photocatalysis and some organic transformations
have been successfully realized.9-11 In continuation of our
interest in heterogeneous photocatalysis, herein, we reported the
−g
PANI -C3N4−TiO2 composite-catalyzed synthesis of 2,5-diaryl
1,3,4-oxadiazoles via decarboxylation and cyclization from α-
keto acids with acylhydrazines under visible light at room
temperature (Scheme 1).
Scheme 1 PANI−g-C3N4−TiO2 composite-catalyzed synthesis of 2,5-diaryl
1,3,4-oxadiazoles.
Recently, photoredox reactions has emerged as a useful and
fascinating synthetic technology.7 Particularly, heterogeneous
photocatalysts have become a rapidly growing class of catalysts
for organic transformations due to their low-cost, stability and
reusability. Among the reported semiconductor composites,
titanium dioxide (TiO2)-based composites are the most popular
because TiO2 is cheap, readily available and nontoxic. Many
The g-C3N4, PANI-g-C3N4 and PANI−g-C3N4−TiO2
composites were prepared according to our previous report11 and
experiment details were summarized in Supplementary Material.
These as-prepared composites were well characterized by X-ray
−visible light
photoluminescence spectroscopy (PL), transmission electron
diffraction (XRD), ultraviolet
(UV-vis),

2
Tetrahedron Letters
microscope (TEM) and BET analyses. Briefly, comparison of
activity of the composite, and a synergistic effect may exist in
the ternary composite. The bases also showed significant
influence on the reaction. Switching the K2CO3 to other bases
such as Na2CO3, KHCO3, and KOH afforded inferior yields
(entries 10~12), and the reaction did not occur when Et3N was
utilized as the base (entry 13). Other solvents were also surveyed
(entries 14~18). Among which, DMF gave the best yield (86%),
while reactions in acetonitrile, methanol, dichloromethane
afforded rather poor yields. Expectedly, no reactions occurred in
the absence of either a photocatalyst or visible light (entries 19
and 20). Moreover, no desired product was observed when the
reaction was performed under nitrogen atmosphere, indicating
the air was necessary. Finally, the survey on the catalyst loading
showed that 40 mg of catalyst was sufficient (entry 20).
the XRD patterns clearly indicates that these diffraction peaks
are assigned to the PANI, g-C3N4 and TiO2, respectively (Figure
S1, Supplementary Material). The UV−vis spectra shows that
−g
the introduction of PANI and g-C3N4 in PANI -C3N4−TiO2
composite can significantly enhance its visible-light absorption
(Figure S2, Supplementary Material). The PL spectra shows that
PANI−g-C3N4−TiO2 composite has the lowest intensity,
−hole pairs can be
indicating the photo-induced electron
effectively separated (Figure S3, Supplementary Material). TEM
and HRTEM images of PANI−g-C3N4−TiO2 composite show
that TiO2 nanoparticles disperse well, and both TiO2 and PANI
are physically supported on g-C3N4 (Figure S4, Supplementary
−g
Material). Moreover, PANI -C3N4−TiO2 with different weight
percentages of PANI are measured by the BET, and
With the optimized conditions in hand, we started to explore
the generality of this protocol (Figure 1).12
2
−g
PANI(40%) -C3N4−TiO2 has the highest surface area (128 m
g−1, Table S1, Supplementary Material).
O
O
N
N
PANI(40%)-g-C3N4-TiO2
With these semiconductor composites in hand, we next started
to optimize the reaction conditions using benzohydrazide 1a and
phenylglyoxylic acid 2a as the model substrates (Table 1).
Ar'
NH2
+
HO
Ar
Ar'
Ar
N
K2CO3, DMF, air
Visible light
O
H
O
3
2a
1
Table 1. Optimization of reaction conditionsa
R = H, 3a, 86%;
R = CH3, 3b, 87%;
R = OCH3, 3c, 82%;
R
R = CH3, 3j, 84%;
R = OCH3, 3k, 83%;
R = Cl, 3l, 82%;
N
N
N
N
Ph
Ph
R = F, , 75%;
3d
O
O
R
R = Cl, 3e, 84%;
R = Br, 3f, 82%;
R = CF3, 3g, 72%;
R = Br, 3m, 80%
R = CN,
, 65%;
3h
R = COOEt, 3i, 73%;
Entry
1
Catalyst
base
Solvent
DMSO
Yield (%)b
40
PANI(20%)-g-C3N4-TiO2
K2CO3
N
N
N N
N
N
N
N
O
S
Ph
Ph
2
3
PANI(40%)-g-C3N4-TiO2
PANI(60%)-g-C3N4-TiO2
PANI(80%)-g-C3N4-TiO2
PANI(40%)-g-C3N4
g-C3N4-TiO2
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
77
63
Ph
Ph
O
O
O
O
4
60
31
3n, 85%
3o, 88%
, 73%
3p
3q, 71%
5
6
20
18
7
g-C3N4
N
N
N
N
N
N
8
PANI(40%)-TiO2
TiO2(Anatase)
PANI(40%)-g-C3N4-TiO2
22
NRc
69
O
Ph
Ph
Ph
Ph
O
9
10
O
Cl
OCH3
3e, 78%
3
, 85%
, 81%
3c
3b
11
PANI(40%)-g-C3N4-TiO2
KHCO
DMSO
65
3
N
N
N
N
N
N
12
13
14
15
16
17
18
19
20
PANI(40%)-g-C3N4-TiO2
PANI(40%)-g-C3N4-TiO2
PANI(40%)-g-C3N4-TiO2
PANI(40%)-g-C3N4-TiO2
PANI(40%)-g-C3N4-TiO2
PANI(40%)-g-C3N4-TiO2
PANI(40%)-g-C3N4-TiO2
-
KOH
Et3N
DMSO
DMSO
DMF
51
NRc
86
O
Ph
Ph
O
O
O
CF3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
, 65%
3p
3j, 81%
, 71%
3g
MeCN
MeOH
dioxane
CH2Cl2
DMF
32
trace
trace
24
N
N
N N
NRc
O
O
H3CO
H3CO
Cl
PANI(40%)-g-C3N4-TiO2
DMF
NRc,d, NRc,e
,
80f, 87g
3r, 78%
3s, 75%
aReaction conditions:1a (0.5 mmol), 2a (0.5 mmol), base (2 equiv), and
photocatalyst (40 mg), solvent (5 mL), irradiation with a 14 W CFL under air
at room temperature for 24 h. bIsolated yields. cNo reaction. dReaction
without light. eReaction under N2. f30 mg catalyst was used. g60 mg catalyst
was used.
Figure 1 Reaction scope. Reaction conditions: 1 (0.5 mmol), 2 (0.5 mmol),
K2CO3 (2 equiv), and PANI(40%)-g-C3N4-TiO2 (40 mg), DMF (5 mL),
irradiation with a 14 W CFL under air at room temperature for 24 h. Isolated
yields.
Generally, all of the investigated acylhydrazines coupled with
α-keto acids smoothly to provide the 1,3,4-oxadiazoles in
moderate to good yields (65~87%). Initially, different
acylhydrazines were evaluated. To our delight, acylhydrazines
with substituents including methyl, methoxyl, fluoro, chloro,
bromo, trifluoromethyl, cyano, and ester groups at para-position
survived well under the reaction conditions to afford the desired
products in moderate to good yields (3a~3i). Notably, the C-
halogen bonds were not affected, enabling the further
modification of the initial products. Moreover, steric hindrance
was negligible, and the ortho- or meta-substituted starting
materials also gave the desired products in good yields (3j~3n,
80~85%). 1-Naphthohydrazide, furan-2-carbohydrazide, and
thiophene-2-carbohydrazide also showed good reactivity to
As shown from the results, a 40% yield of product 3a could be
obtained in the presence of PANI(20%)-g-C3N4-TiO2 in DMSO
in the presence of 2 equiv. of K2CO3 (Table 1, entry 1). The
loadings of PANI significantly affected the catalytic activity and
composite with 40 wt% of PANI gave the best yield (77%, entry
2), which might be due to its largest surface area since the it
could be able to expose more active catalytic sites during the
reaction. By contrast, using PANI-g-C3N4, g-C3N4-TiO2, pure g-
C3N4 and PANI-TiO2 as the photocatalyst gave the desired
product in 31%, 20%, 18% and 22% yields, respectively (entries
5~8). Expectedly, no reaction was observed when TiO2 (anatase)
was used alone (entry 9). These results indicated that the
addition of PANI and g-C3N4 could enhanced the photocatalytic

3
deliver products 3o~3q in 88%, 73% and 71% yields,
respectively.
and holes (h+) under visible light. Meanwhile, intermediate 5
was generated via condensation of 1a and 2a followed by
deprotonation. The intermediate 5 was then oxidized by h+ to
provide radical intermediate I, which undergoes decarboxylation
and intramolecular cyclization to give intermediate III.13 Finally,
oxygen free radical anion or air oxidizes the intermediate III to
release the target product 3a.
To further explore the scope of this protocol, a series of α-
keto acids were then investigated. It was pleased to find that α-
keto acids with different substituents such as methyl, methoxy,
chloro, and trifluoromethyl also survived well, giving the desired
2,5-diaryl-1,3,4-oxadiazoles in 71~85% yields. Steric hindrance
also showed negligible influence on the reaction. Moreover, α-
keto acid containing a furan moiety finished the desired product
in 65% yield. Finally, a series of unsymmetrical 2,5-aryl 1,3,4-
oxadiazoles were successfully synthesized in good yields via the
present protocol (3r and 3s), indicating the practicality of this
protocol.
Plausible mechanism
H
H
N
Ph
N
Ph
N
N
O
COO
O
COO
h
Ph
Ph
I
The reusability of such photocatalyst were also evaluated.
Using synthesis of 3a as example, the catalyst was recovered by
centrifugation after reaction. It was washed by ethyl acetate and
water, and then dried. Results showed that only slight decrease in
the catalytic activity was observed after six consecutive runs
(86%, 86%, 84%, 85%, 80%, and 78%). The recycled catalyst
was also subjected to XRD and TEM analyses, and no obvious
change in the structure was observed.
Some control experiments were then performed to gain
insight into the mechanism (Figure 2). It was found that the
reaction did not occur when 2 equiv. of radical scavenger,
2,2,6,6-tetramethylpiperidine N-oxide (TEMPO) was added
(Figure 2, a). This result suggested the reaction probably
-CO2
PC
K2CO3
H
N
Ph
N
O
1a + 2a
Ph
Ph
II
e
O2
O2
N
NH
3a
Ph
O
III
HO2
Proposed catalytic mode
followed
a
radical
reaction pathway.
When N'-
benzoylbenzohydrazide 4 was used as the starting material, no
desired product was observed (Figure 2, b), indicating that 4 was
not the intermediate. However, treatment of 5 under the standard
conditions, product 3a was obtained in 89% yield (Figure 2, c).
Finally, the result showed that continuous irradiation of visible
light is essential for this reaction (Figure 2, d). All these results
indicated that a radical pathway was involved in the present
photocatalytic reaction.
O
O
N
N
Standard conditions
PC
Ph
NH2
+
HO
(a)
Ph
Ph
Ph
N
H
TEMPO
O
(2 equiv.)
O
Figure 3. Plausible catalytic mode
2a
3a
1a
, NR
The possible catalytic mode is also proposed and discussed.
Considering the edge potentials of PANI (π*-orbital: -2.14 eV;
π-orbital: +0.62 eV)14 are more negative than the VB and CB
potentials of g-C3N4 (-1.13 eV and +1.57 eV, respectively)15 as
well as those of TiO2 (-0.29 eV and +2.91 eV, respectively), we
assume that the excited electrons migrate to the CB of TiO2
while the holes are collected by the π-orbital of PANI. Thereby,
the photogenerated electron–hole pairs are efficiently separated,
and the recombination process is hindered. This hypothesis is
probably account for its good catalytic activity. However, we
still cannot determine the real catalytic sites at the current stage
since both g-C3N4-TiO2 and PANI-TiO2 can promote this
reaction (Table 1, entry 5 and 6). The possibility that the
oxidation reaction takes place at the VB of g-C3N4 and the PANI
works as a conductor is still cannot be excluded out.
O
N
N
H
N
Standard conditions
(b)
Ph
Ph
Ph
N
Ph
Ph
Ph
O
H
O
O
4
3a
, NR
H
N
N
N
N
Standard conditions
(c)
(d)
Ph
O
Ph
COOH
3a
,89%
5
In summary, PANI-g-C3N4-TiO2 composite was prepared and
successfully applied in the synthesis of 2,5-diaryl 1,3,4-
oxadiazoles under visible light. This reaction involved
decarboxylation and cyclization from α-keto acids with
acylhydrazines. A broad scope of substrates survived the
reaction conditions to afford the desired products in moderate to
good yields. Control experiments indicated that a radical
pathway was involved in the present photocatalytic reaction and
a synergistic effect may exist in the ternary composite. Moreover,
this semiconductor photocatalyst could be readily recovered and
showed good reusability with only slight decrease in the catalytic
Figure 2. Control experiments
Based on the above results, a plausible catalytic mode of this
semiconductor photocatalyst (PC) was proposed (Figure 3).
Initially, the composite absorbs photons and excites electrons (e-)

4
Tetrahedron Letters
activity being observed after six consecutive runs. A possible
reaction mechanism and catalytic mode were also proposed.
Dai, Y.; Lu, J. ACS Appl. Mater. Interfaces 2015, 7, 9023.
(h) Obregón, S.; Zhang, Y.; Colón, G. Appl. Catal. B 2016,
184, 96.
9
Wang, L.; Wang, C.; Liu, W.; Chen, Q.; He, M.
Tetrahedron Lett. 2016, 57, 1771.
Acknowledgments
10 Liu, W.; Wang, C.; Huang, Y.; Chen, Q.; Wang, L.; He, M.
Synth. Commun. 2016, 46, 1268.
11 Liu, W.; Wang, C.; Wang, L. Ind. Eng. Chem. Res. 2017,
56, 6114.
We gratefully acknowledge financial support from the
National Natural Science Foundation of China (21302014 and
21676030), the Jiangsu Key Laboratory of Advanced Catalytic
Materials and Technology (BM2012110), the Priority Academic
Program Development (PAPD) of Jiangsu Higher Education
Institutions, and the Advanced Catalysis and Green
Manufacturing Collaborative Innovation Center of Changzhou
University of Changzhou University.
12 General procedure for synthesis of 2,5-diaryl 1,3,4-
oxadiazoles and reuse of the catalyst: A sealed tube
equipped with a magnetic stir bar was charged with
acylhydrazine 1 (0.5 mmol), α-keto acid 2 (0.5 mmol),
K2CO3 (1 mmol), PANI-g-C3N4-TiO2 (40 mg) and DMF
(5.0 mL). The mixture was then irradiated with a 14 W
CFL and stirred at room temperature (25 °C) for 24 h. The
distance of the reaction vial from the light is about 5
centimeters. After reaction, the mixture was diluted with
EtOAc (10 mL) and H2O (5 mL), and the solid catalyst was
recovered by centrifugation. The aqueous phase was
extracted with EtOAc (5 mL × 3). The collected organic
extracts were dried on Na2SO4, filtered and evaporated to
dryness. The crude was purified by flash chromatography
on silica gel using a mixture of PE/EA (20:1) to give the
pure product 3. The recovered catalyst was then washed
with ethanol and deionized water, dried under vacuum, and
reused for the next run.
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8

5
Highlights
ꢀ PANI−g-C3N4−TiO2 composite-catalyzed the
radical synthesis of 2,5-diaryl 1,3,4-oxadiazoles
is described.
ꢀ The reaction conditions are mild and green, and
the yields are satisfactory.
ꢀ The catalyst shows good stability and
photocatalytic performance, and can be reused
for six consecutive runs.
ꢀ A possible catalytic mode was also proposed