One-pot multicomponent synthesis of thieno[2,3-: B] indoles catalyzed by a magnetic nanoparticle-supported [Urea]4[ZnCl2] deep eutectic solvent

Article abstract of DOI:10.1039/d0ra00773k

In this study, we have developed the synthesis of thieno[2,3-b]indole dyes via a multicomponent reaction of cheap and available reagents such as sulfur, acetophenones, and indoles using a magnetic nanoparticle-supported [Urea]₄[ZnCl₂] deep eutectic solvent as a green catalyst. The synthesis of a series of diversely functionalized thieno[2,3-b]indole has been successfully performed in a one-pot reaction. Among a total of 25 compounds synthesized, there are 21 new compounds with full characterization such as FT-IR, ¹H and ¹³C NMR, HRMS (ESI). Due to the deep eutectic solvent coated surface of the magnetic nanoparticles, the catalyst could be recovered by an external magnet and reused in five consecutive runs without a considerable decrease in catalytic activity.

Full text of DOI:10.1039/d0ra00773k

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One-pot multicomponent synthesis of thieno[2,3-
b]indoles catalyzed by a magnetic nanoparticle-
supported [Urea]4[ZnCl2] deep eutectic solvent†
ab
Cite this: RSC Adv., 2020, 10, 9663
The Thai Nguyenab and Phuong Hoang Tran
*
In this study, we have developed the synthesis of thieno[2,3-b]indole dyes via a multicomponent reaction of
cheap and available reagents such as sulfur, acetophenones, and indoles using a magnetic nanoparticle-
supported [Urea]4[ZnCl2] deep eutectic solvent as a green catalyst. The synthesis of a series of diversely
functionalized thieno[2,3-b]indole has been successfully performed in a one-pot reaction. Among a total
Received 24th January 2020
Accepted 28th February 2020
1
of 25 compounds synthesized, there are 21 new compounds with full characterization such as FT-IR, H
and 13C NMR, HRMS (ESI). Due to the deep eutectic solvent coated surface of the magnetic
nanoparticles, the catalyst could be recovered by an external magnet and reused in five consecutive runs
without a considerable decrease in catalytic activity.
DOI: 10.1039/d0ra00773k
rsc.li/rsc-advances
scope, and the inability to recycle catalysts remain a crucial
challenge to industrial application.
1. Introduction
The development of new catalysts is necessary for the
advancement of organic synthesis.13–15 Over the last decade,
several synthetic approaches have been reported in the eld by
employing deep eutectic solvents as catalysts.16–20 However,
signicant drawbacks including difficult catalyst recovery,
tedious product separation, and the requirement of greater
stoichiometric amounts of DES as dual catalyst/solvent limited
their widespread application.15,21–23 Hence, the idea for using
supported DES always exists in our mind, which combines the
prominent feature of DES and the benets of heterogeneous
catalysts, including ease of handling, separation, and recycling.
Magnetic nanoparticles (MNPs) have recently been shown to be
catalyst supports due to their easy preparation, low cost, high
stability, high surface area, and simple recovery by magnetic
forces.24 Due to these unique properties, many MNPs supported
catalysts have been employed in magnetic catalysts in organic
syntheses,23–31 biomass conversion,32 and biodiesel produc-
tion.33 Thus, we focused our attention on the combination of
the attractive properties of MNPs and DESs to produce a novel
catalyst which can be highly dispersed in the reaction mixture
(like a homogenous catalyst) and easily separated by a magnet
and reused (like a heterogeneous catalyst).34
Thieno[2,3-b]indoles are common structural motifs in a variety
of biological activities and therapeutically useful compounds
leading to potential applications in treating epilepsy, neuro-
logical diseases,1 plant-growth regulatory properties,2 battery
devices,3–7 and polymer conductors.4 Due to the potential
applications of thieno[2,3-b]indoles, the development of new
and efficient synthetic protocols for these compounds has
received considerable attention. Traditionally, thieno[2,3-b]
indoles are synthesized using functionalized 2-alkynylphenyli-
sothiocyanates, 1-alkylisatins, and indole-2-thiones as starting
materials, and require several steps to afford the desired prod-
ucts (Scheme 1).8–10 Although these classical approaches have
been extensively used, there is ongoing research for new
methods that have simpler work-ups, are atom-economical with
good to excellent yields, use recyclable catalysts and use readily
inexpensive starting materials. To date, only two reports have
described the synthesis of thieno[2,3-b]indoles via a one-pot
multicomponent reaction which provides a greener approach
than the classical methods. Li and coworkers reported the
Brønsted acid-catalyzed three-component synthesis of thieno
[2,3-b]indoles starting from indoles, alkenes or alkynes, and
sulfur powder.11 Ni and coworkers demonstrated the synthesis
of 3-substituted thieno[2,3-b]indoles via cascade cyclization by
Brønsted acid-promoted annulation of indoles, ketones, and
sulfur powder.12 However, the low yields, narrow substrate
Because of the concerns about the development of a novel
catalyst in green and sustainable chemistry, we report here the
one-pot three-component synthesis of thieno[2,3-b]indoles
from readily available indoles, acetophenones, and sulfur
powder using a magnetically recyclable DES@MNP catalyst. The
outstanding features of the current work are good to excellent
yield, high regioselectivity, a broad substrate scope, and a recy-
clable catalyst.
aDepartment of Organic Chemistry, Faculty of Chemistry, University of Science, Ho Chi
Minh City 721337, Vietnam. E-mail: thphuong@hcmus.edu.vn; Tel: +84 903706762
bVietnam National University, Ho Chi Minh City 721337, Vietnam
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra00773k
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Scheme 1 Approaches to thieno[2,3-b]indoles.
2.2. General experimental procedure for the synthesis of 2-
phenyl-8H-thieno[2,3-b]indole
2. Experimental
2.1. Preparation of the magnetic nanoparticle-supported
Indole (1.0 mmol, 117 mg), acetophenone (2.0 mmol, 240 mg),
and elemental sulfur power (5.0 mmol, 160 mg) were added
Lewis acidic deep eutectic solvent (DES@MNP)
Synthesis of silica-coated Fe3O4 nanoparticles (Fe3O4@-
SiO2@(CH2)3Cl). The magnetic nanoparticle-supported deep
eutectic solvent catalyst was prepared following the procedure
shown in Scheme 1. Firstly, the magnetic nanoparticle Fe3O4
was synthesized by the coprecipitation method reported in the
previous literature. The mixture of FeCl2 and FeCl3 was dis-
solved in 200 mL water under ultrasonic irradiation. The
potassium hydroxide solution (1.0 M) was added slowly until pH
12.0. Aer 120 min sonication, Fe3O4 was washed with water
and ethanol to pH 7.5, separated by a magnetic and drying at
80 ꢀC for 12 h. Fe3O4 was coated with a layer of silica by reuxing
mixture Fe3O4 (1.0 g), 3.0 mL ammonia solution, and 3.0 mL of
tetraethylorthosilicate (TEOS) in 100 mL ethanol for 8 h. Next,
Fe3O4@SiO2 (5.0 g) and (3-chloropropyl)triethoxysilane (4.820 g,
20.0 mmol) in 100 mL toluene were reuxed for 12 h. The
prepared Fe3O4@SiO2@(CH2)3Cl was separated using a perma-
nent magnet, followed by washing with ethanol and dried at
into
a round-bottom ask precharged with DES@MNP
(10 mol%, 30 mg) in DMF (1.5 mL). The mixture reaction was
then stirred at 140 ꢀC for 12 h under the opening atmosphere.
Aer completion of the reaction as checked by TLC, and the
reaction mixture dissolved in 5.0 mL ethyl acetate. The
DES@MNP catalyst was separated out by magnet and washed
further with anhydrous acetone (3 ꢁ 5.0 mL) and ethanol (3 ꢁ
5.0 mL) for the next experiments. The reaction mixture was
diluted with water (15.0 mL) and extracted with EtOAc (3 ꢁ
20.0 mL). The organic layer was dried over sodium sulfate, and
the solvent was recovered under reduced pressure. Next, the
residue was puried by column chromatography on silica gel
using dichloromethane/petroleum ether ¼ 2 : 3 as the eluent
to afford the pure product. The product identity was
conrmed by melting point, FT-IR, 1H NMR, 13C NMR, and
MS.
ꢀ
80 C for 6 h.
Synthesis of DES. The [Urea]4[ZnCl2] DES was synthesized by
adding urea (1.2 g, 20.0 mmol) to zinc chloride (0.680 ꢀg, 5.0
mmol) and stirring to afford a transparent liquid at 100 C for
30 min.
3. Results and discussion
3.1. Catalyst characterization
Initially, the DES@MNP was synthesized following the
procedure presented in Scheme S1 (see ESI†).35 First, Fe3O4
was prepared by a coprecipitation method similar to a previ-
ously reported literature, followed by a SiO2 coating procedure
afforded Fe3O4@SiO2. Subsequently, Fe3O4@SiO2 was func-
tionalized using (3-chloropropyl)triethoxysilane by surface
capping method to provide Fe3O4@SiO2@(CH2)3Cl. The
precursor [Urea]4[ZnCl2] deep eutectic solvent was prepared
Synthesis of DES@MNP. Fe3O4@SiO2@(CH2)3Cl (3.0 g) and
DES [Urea]4[ZnCl2] (1.880 g, 5.0 mmol) were added round-
ꢀ
bottomed ask and stirred vigorously at 100 C for 18 h. Aer
completion of the reaction, the DES@MNP was collected by an
external magnet and washed with ethanol (3 ꢁ 20.0 mL) and
ꢀ
dried under reduced pressure at 60 C for 6 h.
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by mixing of ZnCl2 with urea (1 : 4 molar ratio) at 100 ꢀC until addition, the XRD pattern of the coated nanoparticles func-
a clear solution was obtained. Finally, the treatment of DES tionalized DES was carried out to conrm the crystalline nature.
with Fe3O4@SiO2@(CH2)3Cl in acetone at 100 ꢀC for 18 h The XRD pattern (Fig. S5, ESI†) of the DES@MNP showed the
provides the DES@MNP catalyst. The structure of DES@MNP diffraction peak positions and the relative intensities of the
was characterized by various techniques including FT-IR, peaks, which matched well with the standard Fe3O4 sample
Raman, TGA, SEM, TEM, XRD, VSM, EDX and ICP-MS (see (JCPDS le no. 19-0629). The SEM image of DES@MNP revealed
the ESI†).
that the catalyst was obtained in uniform nanometer-sized
FT-IR spectra of Fe3O4@SiO2 magnetic nanoparticles showed particles (Fig. S6a, ESI†). TEM image showed a well-dened
peaks at around 580, 1095, 1645, and 3419 cmꢂ1, which are core–shell structure with relatively nanosize distribution,
attributed to the Fe–O–Fe, Si–O–Si and O–H groups (Fig. S1, a dark Fe3O4 core surrounded by a grey silica shell, and the
ESI†). For DES-supported Fe3O4@SiO2, additional peaks were average size of the obtained particles are 15–25 nm (Fig. S6b,
observed for C]C, C–N, N–H, which conrms successful ESI†). The chemical composition of DES@MNP was recorded by
immobilization (Fig. S1, ESI†). The characteristic peaks of energy-dispersive X-ray spectroscopy (EDX), conrming the
DES@MNP were at the same wavenumbers with the precursors, presence of C, O, N, Fe, Zn, Si, and Cl elements in the structure
with a small shi due to the interaction between DES and the of the catalyst (Fig. S7, ESI†). The EDX revealed that the mass
support. The Raman peak at 485 cmꢂ1 can be interpreted as percent of Zn was 26.69%. This calculation was in good agree-
stretching vibration of the Zn–Cl bond (Fig. S2, ESI†). The ment with ICP-MS data for Zn analysis reporting the content of
stability of the DES@MNP was proved by the thermogravimetric elemental Zn as 277 600 ppm, corresponding to Zn2+ loading of
(TG) analysis. DES@MNP showed a sharp decrease in weight 4.10 mmol gꢂ1
.
ꢀ
(40%) was observed at 250ꢂ650 C, which is attributed to the
organic composition graing. Thus, the DES@MNP is stable at
around 250 C (Fig. S3, ESI†).
3.2. The activity of DES@MNP for the synthesis of 2-phenyl-
8H-thieno[2,3-b]indole
ꢀ
The magnetic properties of the DES@MNP and the neat
Fe3O4 were recorded by vibrating sample magnetometry
(Fig. S4, ESI†). The saturation magnetization of these samples
was found to decrease from 69.2 to 30.8 emu gꢂ1 due to the
silica coating and the organic graing on the surface of Fe3O4.
However, it was sufficient for the recovery of the DES@MNP
from the reaction mixture using an external magnet. In
In a preliminary survey of reactions, DES@MNP was found to be
catalytically active in a one-pot multi-component reaction of
indole, acetophenone, and sulfur as a model reaction (Table 1).
Previously, this reaction can also be accomplished with acetic
acid as a homogeneous catalyst.12 However, the method suffers
from difficulties in the recovery of the catalyst, moderate yield,
high temperature for longer reaction time. These drawbacks
Table 1 Screening of different catalysts on the synthesis of 2-phenyl-8H-thieno[2,3-b]indole
Entrya
Type of catalysts
Brønsted acids
Catalyst (mol%)
Yieldb (%)
1
2
3
4
5
6
7
8
H2SO4
HCl
H3PO4
CH3COOH
CF3COOH
AlCl3
FeCl3
ZnCl2
SnCl4
CuCl2
Fe3O4
Fe3O4@SiO2
Fe3O4@SiO2@(CH2)3Cl
[Urea]4[ZnCl2]
Fe3O4@SiO2@(CH2)3–[Urea]4[ZnCl2]
72
70
67
75
80
73
57
80
65
42
—
—
—
82
87
Lewis acids
9
10
11
12
13
14
15
Control experiment
a
Reaction conditions: acetophenone (2.0 mmol), indole (1.0 mmol), sulfur (5.0 mmol) and catalysis (10 mol%) were heated in DMF (1.5 mL) at
b
140 ꢀC for 12 h. Yield of 2-phenyl-8H-thieno[2,3-b]indole was isolated by column chromatography (dichloromethane/petroleum ether).
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Table 2 Substrate scope for the synthesis of 2-phenyl-8H-thieno[2,3-b]indole derivatives
Entrya
Indoles 1
Aromatic ketones 2
Products 3
Yieldb (%)
1
2
3
4
5
3a
3b
3c
3d
3e
87
81
83
77
65
6
3f
59
7
3g
3h
3i
80
77
78
80
71
75
75
78
80
79
8
9
10
11
12
13
14
15
16
3j
3k
3l
3m
3n
3o
3p
17
3q
75
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Table 2 (Contd.)
Entrya
18
Indoles 1
Aromatic ketones 2
Products 3
Yieldb (%)
3r
3s
3t
78
81
89
19
20
21
22
23
24
3u
3v
3w
3x
80
81
81
87
25
3y
54
a
Reaction conditions: aromatic ketones (2.0 mmol), indoles (1.0 mmol), sulfur (5.0 mmol) and DES@MNP (10 mol%) were heated in DMF (1.5 mL)
b
at 140 ꢀC for 12 h. Yield of 2-phenyl-8H-thieno[2,3-b]indole was isolated by column chromatography.
can be overcome by using DES@MNP as a magnetically recov- the reaction yield as well as selectivity and made the recovery of
erable catalyst. To our delight, the expected product was ob- the catalyst much more comfortable by using an external
tained in excellent yield (87%) at 140 ꢀC for 12 h when magnet. The reaction proceeds effectively because the nano-
DES@MNP was employed. The reactions were also tested using catalyst possesses a high surface area increasing the adsorp-
Brønsted acids as the catalysts at 140 C for 12 h in DMF; the tion of reactants on the surface of the active Zn2+ sites on the
ꢀ
desired product was obtained in 67–72% yield (Table 1, entries DES@MNP. Interestingly, DES@MNP showed a higher yield in
1–5). Next, several Lewis acids were screened in the model comparison with HCOOH catalyst (75% yield) which was re-
reaction (Table 1, entries 6–10). Among these Lewis acids, ZnCl2 ported by Ni and coworkers.12
was shown to be a suitable catalyst that provided the desired
The additive DMF plays a vital role in the yield and regio-
product in 80% yield. At this point, we revealed that ZnCl2 could selectivity control for the thieno[2,3-b]indole synthesis.12 Next,
play an essential role in promoting the condensation of indole, we turned our attention to the optimization of the additives to
sulfur, and acetophenone and this is the reason we developed obtain the major corresponding regioselective product where
the new catalyst with ZnCl2 active sites. Additionally, the control the phenyl substituent was attached at the C-2 position in the
experiment in the absence of catalyst did not generate the ex- thieno[2,3-b]indole. The results are presented in Table S1 (ESI†).
pected product. The support Fe3O4, Fe3O4@SiO2, and Fe3O4@- Among these additives, DMF was the best additive under the
SiO2@(CH2)3Cl were also not reactive under these conditions current method. The reaction proceeds efficiently with DABCO
(Table 1, entries 11–13). The typical [Urea]4[ZnCl2] DES provided or N-ethylmorpholine to provide the desired product in 75 and
the desired product in 82% yield, but the homogeneous DES 81%, respectively.
was difficult to recover and reuse with the loss of the catalytic
When solid catalysts promote organic synthesis, the solvent
activity. The combination of nanoparticle and DES increased plays a vital role in the reaction rate, depending on the nature of
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ꢀ
these catalysts. The solvent effect on the catalytic activity of temperature to 140 C led to a signicantly improved reaction
DES@MNP was further investigated, and the results are shown yield of 2-phenyl-8H-thieno[2,3-b]indole. The reaction time was
in Table S2 (ESI†). The three-component cyclization reaction also screened and the best yield was observed at 12 h (please see
was carried out at 140 ꢀC, using acetophenone, indole, and in ESI, Table S4†). The reactant molar ratio was investigated in
sulfur molar ratio of 1 : 2 : 5, in the presence of 10 mol% the model reaction at 140 ꢀC for 12 h. It was found that the
DES@MNP catalyst and 0.3 mL DMF as an additive, in 1.5 mL reaction yield was signicantly affected by the reactant molar
organic solvent. A signicant drop in reaction yield was ratio. The results revealed that the use of fewer than 4 equiva-
observed for the other solvents, whereas a higher reaction lents of sulfur gave rise to the low yields of the desired product.
activity was observed in DMF as a double role of additive and However, when 4 or 5 equivalents of sulfur were employed, the
solvent. It was proposed that the DMF solvent could form the yields dramatically increased.
active precursor when interacted with sulfur and acetophenone,
affording reaction rate enhancement (please see the proposed method, various indoles and acetophenones were examined. As
mechanism in Scheme S2†). shown in Table 2, the process was highly useful for the prepa-
To further dene the scope and limitations of the present
Next, the effect of reaction time and temperature were ration of thieno[2,3-b]indole derivatives (Fig. 1). Acetophenone
investigated (ESI, Table S3 and S4†). The three-component bearing various substituents at the para position could
reaction was conducted in DMF for 12 h, using acetopheno- smoothly proceed under the present method provided the
ne : indole : sulfur molar ratio of 1 : 2 : 5 in the presence of desired product in good to excellent yields. When acetophenone
10 mol% of DES@MNP catalyst. It was shown that no expected and acetophenone bearing electron-donating groups were
product was detected for the reaction at 80 ꢀC. The low yield of employed to couple with indole, the desired product was ob-
the product was obtained at 100 ꢀC. Increasing the reaction tained in higher yields than 4-uoroacetophenone and 2-
Fig. 1 The structure of various 2-phenyl-8H-thieno[2,3-b]indole compounds.
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acetylthiophene. To our delight, good to excellent yields were
observed when the methyl, methoxyl, halo, and nitro groups
were located at the C-5 in the NH-unprotected indole ring.
Indoles with halogens were all reactive, but 5-bromoindole
produced the product in slightly higher yields than 5-uo-
roindole and 5-chloroindole. 5-Nitroindole, which is known to
deactivate many organic reactions in which it is used, was found
to be a feasible reagent in our method producing 5-nitro-2-
phenyl-8H-thieno[2,3-b]indole in 54% yield. It is noteworthy
that these unprotected 1H-indoles worked well in this protocol
to provide the desired product in high yields without the need
for protection of the NH position when compared with previous
literature.12 Additionally, signicant regioselectivities of prod-
ucts were observed in over 95% making the isolation of prod-
ucts more efficient. Thus, the novel DES@MNP is a highly active
catalyst for the synthesis of thieno[2,3-b]indole derivatives.
Although several homogeneous catalysts in our research
such as CF3COOH, ZnCl2, [Urea]4[ZnCl2] also exhibited high
activity in the reaction (please see in Table 1). However, the
main drawback is the difficulty in the recovery and reuse of
these homogeneous catalysts. The prominent feature of the
current method is the ease of separation of DES@MNP by an
external magnet. The DES@MNP was recovered and reused over
ve consecutive runs, by repeatedly separating the DES@MNP
catalyst from the reaction mixture by a magnet, washed with an
organic solvent, dried under vacuum in 30 min, and then reused
for next cycle. Interestingly, the reaction still afforded 83% yield
of the desired product in the h run (Fig. 2). Remarkably, the
structure of the recovered DES@MNP catalyst could be main-
tained under the current method, as conrmed by FT-IR
(Fig. S8, ESI†).
Fig. 3 Leaching test.
reaction mixture without catalyst was then stirred for further
8 h and analyzed by GC-MS. It was found that no additional
desired product was obtained in the absence of DES@MNP
catalyst (Fig. 3). Thus, the leaching of zinc from the catalyst to
the solution was negligible. Additionally, the content of Zn
species in reaction ltrate was recorded by ICPMS, and the
result was measured less than 1 ppm of Zn. Obviously, there
was no contribution from the leaching of active Zn species in
the reaction solution, and the synthesis of thieno[2,3-b]indole
could only occur in the presence of the DES@MNP.
3.3. Reaction mechanism research
The active sites on the heterogeneous catalyst can dissolve
into the liquid phase during a reaction and act as a homoge- To determine the mechanism insights for the synthesis of
neous catalyst. To conrm whether active zinc species dis- thieno[2,3-b]indole via a multicomponent reaction, some
solved from the DES@MNP promoted to the formation of the mechanistic experiments were carried out. According to the
desired product (A), the leaching test was investigated. The above control experiments and previous literature, a proposed
model reaction was stopped aer
8 h, separation of mechanism was depicted in Scheme 2. Assisted by DES@MNP,
DES@MNP from the reaction mixture by a magnet. The the reaction proceeds via the initial formation of 1-(dimethy-
lamino)-2-phenyl-1H-thiiren-1-ium (A), which is a resonance
to form A0 (Willgerodt–Kindler reaction). The intermediate (A)
reacts with indole via a ring-opening addition, followed by
ring closure and then eliminated one molecule of dimethyl-
amine to afford the target product. To check the conform-
ability of the plausible mechanism for the same reaction with
the absence of indole catalyzed by DES@MNP, we also carried
out the reaction of acetophenone, elementary sulfur, and DMF
under optimized conditions. The (A0) product was obtained in
a yield of 65%. Next, the indole was then subjected to the
reaction mixture and the desired product was achieved in 87%
yield (ESI†). Thus, it is not doubtful that the multicomponent
synthesis of thieno[2,3-b]indole reported herein must also
undergo formation the precursor (A) and the DMF solvent
plays an important role in the reaction mechanism. Although
the reaction mechanism is not clear now, the protocol
possesses attractive merits including simply and efficiently
recyclable catalyst, wide scope of substrates, high yield, and
Fig. 2 Recycling experiments.
high regioselectivity.
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Scheme 2 Proposed reaction mechanism.
4. Conclusion
References
In conclusion, we have developed a catalyst for the synthesis of
thieno[2,3-b]indole via the one-pot multicomponent reaction of
indoles, acetophenones, and sulfur. The corresponding prod-
ucts are smoothly formed in high yields and regioselectivity
under mild conditions. More importantly, we have described
the rst example of using DES@MNP-catalyzed the thieno[2,3-b]
indole synthesis via multicomponent reaction with cheap and
readily available reactants. DES@MNP was shown to be the
extremely powerful catalyst to activate the reaction leading to
total conversions even various substrates bearing electron-rich
or electron-poor substituents. Further development and appli-
cations of multicomponent reactions of direct construction of
thienoindole from elemental sulfur and indoles are currently
under investigation in our laboratory. We hope that DES@MNP
will nd their applications in many organic syntheses, espe-
cially in green and sustainable syntheses of biologically active
compounds.
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Conflicts of interest
9 R. A. Irgashev, A. A. Karmatsky, G. L. Rusinov and
V. N. Charushin, Beilstein J. Org. Chem., 2015, 11, 1000–1007.
10 H. Z. Boeini, Helv. Chim. Acta, 2009, 92, 1268–1272.
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There are no conicts to declare.
Acknowledgements
This research is funded by Vietnam National Foundation for
Science and Technology Development (NAFOSTED) under grant
number 104.01-2019.26. We thank MSc Duy-Khiem Nguyen
Chau (North Carolina State University, USA) and MSc Mai
Hoang Ngoc Do (IPH-HCM) for their help.
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2017, 19, 5553–5558.
13 L. Wu, A. Mendoza-Garcia, Q. Li and S. Sun, Chem. Rev.,
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