A new pathway to pyrrolo[1,2-a]quinoxalines via solvent-free one-pot strategy utilizing FeMoSe nanosheets as efficient recyclable synergistic catalyst

Article abstract of DOI:10.1016/j.jcat.2019.07.008

FeMoSe nanocatalyst was synthesized using solvothermal approach from readily available precursors, namely Mo(CO)₆, FeSO₄ and Se, and characterized by spectroscopic and microscopic analyses. The FeMoSe material offered high catalytic

Full text of DOI:10.1016/j.jcat.2019.07.008

Journal of Catalysis 377 (2019) 163–173
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Journal of Catalysis
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A new pathway to pyrrolo[1,2-a]quinoxalines via solvent-free one-pot
strategy utilizing FeMoSe nanosheets as efficient recyclable synergistic
catalyst
Tuong A. To a, Chuc T. Nguyen b, My H.P. Tran a, Thai Q. Huynh a, Tung T. Nguyen a, Nhan T.H. Le a,
Anh D. Nguyen b, Phong D. Tran b, , Nam T.S. Phan a,
⇑
⇑
a Faculty of Chemical Engineering, HCMC University of Technology, VNU-HCM, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Viet Nam
b University of Science and Technology of Ha Noi, Viet Nam Academy of Science and Technology, 18 Hoang Quoc Viet, Ha Noi, Viet Nam
a r t i c l e i n f o
a b s t r a c t
Article history:
FeMoSe nanocatalyst was synthesized using solvothermal approach from readily available precursors,
namely Mo(CO)6, FeSO4 and Se, and characterized by spectroscopic and microscopic analyses. The
FeMoSe material offered high catalytic activity in the synthesis of pyrrolo[1,2-a]quinoxalines via
solvent-free one-pot strategy starting from 2-nitroanilines. Compared to previous works, the significant
advantages of our protocols are: (i) solvent-free simple, direct synthesis; (ii) using phenylacetic acids as
both Brønsted acid catalysts and reactants; (iii) no requirement of any external oxidants or reductants;
and (iv) recyclable heterogeneous catalyst for cyclization. To our best knowledge, synthesis of pyrrolo
[1,2-a]quinoxalines using such a synergistic bimetallic system was not previously reported.
Ó 2019 Elsevier Inc. All rights reserved.
Received 11 February 2019
Revised 24 June 2019
Accepted 1 July 2019
Keywords:
Pyrrolo[1,2-a]quinoxalines
One-pot
Synergistic
Selenide
Molybdenum
Solvent-free
1. Introduction
pyrrolo[1,2-a]quinoxalines, resulting in a one-step shorter synthe-
sis compared with the anilines. Later, the group of Sanz and co-
Quinoxaline derivatives have been considered as precious hete-
rocyclic scaffolds, found in a plethora of natural products and syn-
thetic compounds [1–3]. More particularly, pyrrolo[1,2-a]
quinoxalines, a unique fused class of the family, have gained
remarkable attention due to their large spectrum of biological
and pharmacological activities [4–7]. Use of pyrrolo[1,2-a]quinox-
alines in synthesis of bioactive dipyrrolo[1,2-a]quinoxalines is also
known [8–10]. Accordingly, numerous attempts have been devoted
to the synthesis of these valuable fused heterocycles. Most of the
methods rely on using 2-pyrrolyl anilines which then couple with
aldehydes, via Pictet-Spengler mechanism, or synthetic equiva-
lences [11–15]. Although a staggering amount of efforts have been
spent to develop a mild and convenient method for the coupling-
intramolecular cyclization-oxidation sequence, multistep synthe-
sis could be arguably the most notorious challenging of this
approach. In 2012, Pereira reported an iron-mediated reductive
coupling of 2-pyrrolyl nitroarenes and alcohols [16]. The method
marked a first example of using nitro derivatives to afford
workers presented a domino reduction/Pictet-Spengler reaction
from 1-(2-nitrophenyl)-1H-pyrroles and glycols [17]. The authors
employed a catalytic amount of molybdenum(VI) complex, instead
of stoichiometric iron powder, to facilitate the quinoxaline synthe-
sis. We speculated that a combination of iron and molybdenum
active centers may bring some compelling activities with regard
to nitro reduction. Synthesis of pyrrolo[1,2-a]quinoxalines using
such a synergistic bimetallic selenide system has been not investi-
gated yet, to the best of our knowledge.
Molybdenum disulfide and molybdenum diselenide are known
to be among the best noble-metal-free catalysts for hydrogen evo-
lution and hydrodesulfurization [18–23]. Introduction of a first-
row transition metal like Fe, Co, and Ni into MoS2 or MoSe2
resulted in a significant enhancement of its catalytic activities.
The added metal could induce the creation of M-MoS2 or M-
MoSe2 phase (where M is Fe, Co, Ni). In that case, a promoting
effect is discussed [24–26]. Alternatively, composite material like
MoS2-CoS2 [27,28] or MoSe2-CoSe2 [29,30] is generated upon the
introduction of a Co precursor during the MoS2 or MoSe2 synthesis.
In the latter cases, a synergetic operation causes the enhanced
catalytic activity. To the best of our knowledge, while the sulfide/
selenides of transition metals were investigated for the
⇑
Corresponding authors.
E-mail addresses: tran-dinh.phong@usth.edu.vn (P.D. Tran), ptsnam@hcmut.edu.
vn (N.T.S. Phan).
https://doi.org/10.1016/j.jcat.2019.07.008
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

164
T.A. To et al. / Journal of Catalysis 377 (2019) 163–173
hydrodesulfurization to remove thiol compounds from petroleum,
their use as catalyst for organic reactions is still rare in the litera-
ture. Only few examples of using bimetallic sulfide like Co-Mo-S
for hydrogenation [31,32] and thiolation [33] have been reported.
In this work, we would like to report a new protocol to achieve
pyrrolo[1,2-a]quinoxalines via a solvent-free one-pot synthesis
starting from 2-nitroanilines with a nano bimetallic selenide cata-
lyst, namely the iron-molybdenum selenide (Scheme 1). 1-(2-
Nitrophenyl)-1H-pyrroles could be firstly obtained from the reac-
tions of 2-nitroanilines and 2,5-dimethoxytetrahydrofuran
through an acid-catalyzed Clauson-Kaas reaction. Without isola-
tion, the intermediates then coupled with phenylacetic acids in
the presence of the iron-molybdenum selenide catalyst to afford
pyrrolo[1,2-a]quinoxalines. It should be noted that phenylacetic
acid could be used as a catalyst for the first step, thus circumvent-
ing any need of an extra acid catalyst. We believe that our catalyst,
comprising iron and selenide, would facilitate the formation of
active benzylic species since decarboxylation of activated C(sp3)–
H bonds in acetic acids promoted by first row metals or group 6
atoms is known [34–36]. Different from previous methods, our
conditions offer salient benefits which are: (i) solvent-free one-
pot protocol using directly simple, cheap 2-nitroanilines as starting
materials; (ii) using phenylacetic acids as both Brønsted acid cata-
lysts and coupling reagents; (iii) eliminating the need of any exter-
nal oxidants or reductants; and (iv) recyclable heterogeneous
catalyst.
dimethyl formamide was added Mo(CO)6 (39.6 mg, 0.15 mmol),
FeSO4ꢀ7H2O (39.0 mg, 0.15 mmol), and Se (35.55 mg, 0.45 mmol).
The mixture was sonicated at room temperature for approximately
30 min to obtain a clear solution, then transferred into a 100 mL
Teflon-lined autoclave. It was heated and kept at 200 °C for 10 h
in an oven. Once the reaction was over, the mixture was cooled
to room temperature. Dark brown solid product, here after named
FeMoSe, was collected by centrifugation at 10,000 rpm for 5 min.
The obtained product was intensively washed with deionized
(DI) water, ethanol, CS2, and diethyl ether, then dried under vac-
uum before characterization and use.
2.2. Catalyst characterization
Morphology of the FeMoSe powder was characterized by a Field
emission scanning electron microscopy (SEM) using a Hitachi S-
4800 equipment. It was further characterized using transmission
electron microscopy (TEM) (Hitachi H7650, operated at 100 kV).
Powder X-ray diffraction was collected on a XPERT- PRO PANalyt-
ical X-ray diffractometer using the Cu Ka 0.15406 nm X-ray source.
We employed inductively coupled plasma mass spectrometry (ICP-
MS) analysis and atomic absorption spectroscopy (AAS) analysis to
identify the chemical composition of the FeMoSe nanopowder. ICP-
MS measurement was conducted on a Perkin Elmer Nexion 2000.
Sample preparation was done by (i) first completely dissolving
0.05 g FeMoSe nanopowder in 15 mL aqua regia and then (ii) dilut-
ing with water to obtain a solution of 100 mg/L. The AAS measure-
ment was conducted on an AAS 3300 Perkin Elmer equipment. The
sample was prepared by the same procedure as that for the ICP-MS
analysis to obtain solution of 2000 mg/L. Chemical nature of the
FeMoSe powder was characterized by X-ray photoelectron spec-
troscopy (XPS) using an ULVAC PHI 500 (Versa Probe II) equipment
2. Experimental section
2.1. Catalyst synthesis
All chemicals purchased from Sigma Aldrich were used as
received without any further purification. To 40 mL of N,N0-
equipped with a monochromatic Al Ka (1486.6 eV) X-ray source.
Scheme 1. Common and new strategies to synthesize pyrrolo[1,2-a]quinoxalines.

T.A. To et al. / Journal of Catalysis 377 (2019) 163–173
165
The background pressure in the analysis chamber was ca. 10ꢁ10
Torr. The surface and high-energy resolution scans were recorded
with pass energies of 150 and 20 eV, respectively.
-
FeSe2 crystal domain was not observed. However, in the powder
XRD analysis, characteristic diffraction patterns of FeSe2 phase
were found together with those of Se impurity (Fig. 1e). Patterns
of MoSe2 phase were not observable, likely because of very thin
MoSe2 nanosheets. The chemical composition of the FeMoSe
nanopowder was assayed by a combination of ICP-MS and AAS
analyses. It showed a Fe:Mo:Se molar ratio of 2.02:1:4.34. This
composition was rather low in Mo content compared with the ini-
tial precursor molar ratio used of 1:1:3. This could be rationalized
based on the fact that a considerable amount of Mo was still
remained in the DMF solution after collecting the FeMoSe
nanopowder.
2.3. Catalytic studies
In a typical experiment, a mixture of 2-nitroaniline (27.6 mg,
0.2 mmol),
2,5-
dimethoxytetrahydrofuran
(2,5-DMTHF)
(27.7 mg, 0.21 mmol), and phenylacetic acid (68.0 mg, 0.5 mmol)
was added into a 8 mL screw-cap pressurized vial. The reaction
mixture was stirred at 80 °C for 1 h. 1,4-Diazabicyclo[2.2.2]
octane (DABCO) (33.7 mg, 0.3 mmol) and FeMoSe catalyst
To further identify the chemical environment of elements
within the FeMoSe nanopowder, XPS analysis was used. The Fe
envelop was fitted into three doublets with Fe2p3/2 peaked at
707.5, 711.3, and 714.8 eV (Fig. 2a). The Fe2p3/2 707.5 eV signal is
attributed to the Fe2+ within the FeSe2 [37]. Assignment for the
Fe2p3/2 711.3 eV signal is more complicated. Fe2+ within the FeSe2/
C composite was found at Fe 2p3/2 binding energy of 711.5 eV [38].
However, it is also assignable to Fe oxides species that could be
generated due to an oxidation while handling the FeMoSe
nanopowder in air-exposed condition. The Fe2p3/2 714.8 eV is
characteristics for the satellite of Fe2+ species. The Mo3d envelop
was fitted to three doublets with Mo3d5/2 peaked at 228.4, 230.2,
and 232.4 eV (Fig. 2b). The main signal at 228.4 eV is attributed
to the MoIV within MoSe2 layers [39], whereas the higher binding
energy signals are assignable to MoV and MoVI species, respectively
[40,41]. These species could be either oxo or oxide species gener-
ated due to a partial oxidation of FeMoSe during the growth or
handling in air. The Se envelop was well fitted for a set of three
species, namely Se2- selenide ligand found at 53.9 eV, Se0 impurity
found at 55.1 eV, and oxidized Se like SeO23- or SeO42- at 58.1 eV
(Fig. 2c) [42,43]. Thus, putting together these available results,
we conclude that the resultant FeMoSe consists of layered MoSe2
and FeSe2 phases. Some impurities like Se element, oxo and/or
oxide of molybdenum and iron were also incorporated. Neverthe-
less, attempts to remove these impurities were not successful.
(2.8 mg, 5 lmol) were then added into the mixture. The reaction
was consequently stirred at 140 °C for additional 4 h. After that,
diphenyl ether (34.0 mg, 0.2 mmol) as internal standard was added
to the reaction mixture, and the reaction was quenched by NaHCO3
solution (5% wt/wt, 2 mL). Organic compounds were extracted into
ethyl acetate (3 ꢂ 5 mL). The organic phase was dried over anhy-
drous Na2SO4, filtered, and analyzed by GC. The desired product,
pyrrolo[1,2-a]quinoxaline, was purified by column chromatogra-
phy (dichloromethane/hexane = 2:1). The structure of product
was also confirmed by 1H NMR and 13C NMR. To investigate the
recyclability of catalyst, the FeMoSe was separated from the reac-
tion mixture by simple centrifugation, washed thoroughly with
excess amounts of methanol, dried under strong vacuum for 2 h,
and then used for next catalytic experiments.
3. Results and discussion
3.1. Catalyst characterization
As revealed by SEM analysis, the FeMoSe nanopowder obtained
from the solvothermal reaction of FeSO4ꢀ7H2O, Mo(CO)6, and Se
with molar ratio of 1:1:3 consisted of nanosheets (Fig. 1a, b). Some
of these sheets curved and stacked together, generating large size
nanoparticles. TEM analysis clearly evidenced the typical MoSe2
layered structure with layer-layer spacing of 0.65 nm (Fig. 1b, d).
Fig. 1. SEM images (a, b), TEM images (c, d) and XRD patterns (f) recorded on the as-prepared FeMoSe nanopowder and SEM image recorded on the reused FeMoSe
nanosheets catalyst (e).

166
T.A. To et al. / Journal of Catalysis 377 (2019) 163–173
Fig. 2. XPS analysis conducted on the as-prepared FeMoSe nanopowder (a, b, c) and on the reused FeMoSe nanopowder catalyst (d, e, f).
3.2. Catalytic studies
with 3a. Since the reactant for the second step 3a also functioned
as a Brønsted acid catalyst for the first step, no additional acid cat-
alyst was required for the Clauson-Kaas transformation. Moreover,
in the second step, the redox process occurred readily without the
need of an external oxidant or reductant. This approach would
shorten the synthetic scheme as well as minimize wastes produced
from the purification of intermediates and excess reagents. Nota-
bly, the use of phenylacetic acids as redox balanced coupling part-
ners with 2-nitroanilines in the synthesis of 2-substituted
benzimidazoles is known [44].
Initially, the synthesis of 4-phenylpyrrolo[1,2-a]quinoxaline
(4aa) from 2-nitroaniline (1a), 2,5-DMTHF, and phenylacetic
acid (3a) was explored (Table 1). The transformation proceeded
in two steps: (i) The Clauson-Kaas reaction between 1a and
2,5-DMTHF using catalytic amount of 3a to produce 1-(2-
nitrophenyl)-1H-pyrrole
(2a),
and
(ii)
the
domino
decarboxylation/intramolecular cyclization/oxidation between 2a
and 3a in the presence of the bimetallic iron molybdenum selenide
catalyst to afford 4aa. Different from previous synthetic protocols
[11–17], the intermediate 2a was not isolated but directly coupled
Reaction conditions were optimized to improve the yield of the
desired pyrrolo[1,2-a]quinoxaline product (Table 1). Screening
Table 1
Screening of reaction conditions for the one-pot transformation.a
Entry
additive
additive amount (equiv.)
FeMoSe amount (mol%)
temperature (°C)
yieldb (%)
1
2
3
4
5
6
7
8
Pyridine
3-Picoline
N-methylmorpholine
DABCO
N-methylpiperidine
DIPEA
DABCO
DABCO
DABCO
DABCO
2
2
2
2
2
2
0
1
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
0
140
140
140
140
140
140
140
140
140
140
140
140
120
140
140
7
28
38
62
59
30
6
54
77
7
50
88
33
82
88
9
10
11
12c
13c
14c,d
15c,e
DABCO
DABCO
DABCO
DABCO
1
2.5
2.5
2.5
2.5
DABCO
a
Reaction conditions for the first step: 1a (0.2 mmol), 3a (0.4 mmol), 2,5-dimethoxytetrahydrofuran (2,5-DMTHF) (0.21 mmol), 80 °C, 1 h. The second step was conducted
for 16 h unless noted.
b
GC yield.
3a (0.5 mmol).
c
d
The second step was conducted for 3 h.
The second step was conducted for 4 h. DABCO:1,4-diazabicyclo[2.2.2]octane; DIPEA: diisopropyl ethyl amine.
e

T.A. To et al. / Journal of Catalysis 377 (2019) 163–173
167
conditions of the first step indicated that 1a was fully converted to
2a after 1 h at 80 °C with 1.05 equivalents of 2,5-DMTHF. There-
fore, the investigation focused on the second step with regard to
additive, additive amount, catalyst loading, and temperature. The
impact of additive on the formation of 4aa was initially explored
(entries 1–5). A series of different amines were used for the trans-
formation, and DABCO emerged as the best candidate, producing
4aa in 62% yield (entry 4). We hypothesized that the amine could
be involved in the decarboxylation or the Pictet-Spengler typed
rearomatization step [45,46]. The amount of DABCO displayed a
remarkable impact on the transformation (entries 7–9). Noted that
only 6% yield of 4aa was obtained in the absence of base, while
using 1.5 equivalents of DABCO afforded 77% yield (please see
the Supporting Information for full optimization). The quantity of
FeMoSe catalyst was crucial with regard to the formation of 4aa
(entries 10 and 11). Using less than 2.5 mol% of the catalyst plum-
metted the yield of 4aa. Only 7% yield was detected in the absence
of FeMoSe, proving the importance of the catalyst. A clean mixture
was obtained if 2.5 equivalents of phenylacetic acid was used
(entry 12). Running the reaction at lower temperature (than
140 °C) afforded low yield of 4aa (entry 13). Lastly, 4 h was enough
for the second step to get a considerable amount of 4aa (entries 14
and 15). With the conditions in hand, we attempted to conduct the
reaction on a large scale. In the 2 mmol run, 74% yield of 4aa was
obtained, showing that the reacion is scalable (Scheme 2).
As the synthesis of 4aa from 1a, 2,5-DMTHF, and 3a via one-pot
strategy utilizing the FeMoSe catalyst was conducted in liquid
phase, the potentiality that 4aa might be formed via homogeneous
catalysis was investigated. In some cases, the solid catalyst might
be partially soluble in the reaction medium during the catalytic
experiment, and leached species might be active for the transfor-
mation. In order to clarify if leached species, if any, contributed
to the formation of 4aa, a control experiment was consequently
performed. The reaction was carried out under standard conditions
for 4 h. After that, the reaction mixture was diluted by ethyl acet-
ate, and then analyzed by GC. The solid catalyst was removed by
filtration. The filtrate was subsequently concentrated under vac-
uum prior to be used for the next catalytic experiment. Following
this, 1a, 2,5-DMTHF, and 3a were added, and the transformation
was allowed to proceed under standard conditions in the absence
of utilizing the FeMoSe catalyst. The formation of 4aa during the
course of the reaction was monitored by GC. To our expect, only
4% yield of 4aa was detected for the transformation after 4 h
(Fig. 3). The results verified that the one-pot reaction of 1a, 2,5-
DMTHF, and 3a to form 4aa using the FeMoSe catalyst was truly
heterogeneous.
Fig. 3. Leaching test.
2,5-DMTHF, 2.5 equivalents of 3a, 1.5 equivalents of DABCO, in
the presence of 2.5 mol% catalyst as previously described. Upon
completion of the experiment, the FeMoSe powder was separated
from the reaction mixture by a simple centrifugation, washed thor-
oughly with excess amounts of methanol, dried under strong vac-
uum for 2 h, and reused in the next catalytic run under standard
conditions. It was noted that the FeMoSe catalyst was reusable
without a major loss of catalytic activity. Certainly, 88% yield of
4aa was obtained in the 6th catalytic run (see Fig. 4).
Morphology and chemical composition of the FeMoSe catalyst
collected after catalytic assay were characterized to gain insights
into its eventual changes during the catalytic transformation.
SEM analysis revealed the similar morphology (Fig. 1e) to that
recorded for the as-prepared FeMoSe catalyst (Fig. 1a, b). FeMoSe
nanosheets as well as its agglomeration in form of large nanopar-
ticles were remained after being used in the catalysis. However,
To highlight significant aspects of using the FeMoSe catalyst for
the synthesis of 4aa from 1a, 2,5-DMTHF, and 3a via one-pot strat-
egy, the reusability of the catalyst was studied. Previously, the cat-
alysts used in the synthesis of 4aa from 2a were not recyclable
[11–17]. Therefore, exploring a recyclable heterogeneous catalyst
for the formation of 4aa to facilitate product purification and to
minimize waste would add a prominent value to the one-pot
strategy. The reaction was conducted using 1.05 equivalent of
Fig. 4. Reuse of the FeMoSe catalyst in the synthesis of 4-phenylpyrrolo[1,2-a]
quinoxaline.
Scheme 2. Synthesis of 4-phenylpyrrolo[1,2-a]quinoxaline at 2 mmol scale.

168
T.A. To et al. / Journal of Catalysis 377 (2019) 163–173
Scheme 3. Control experiments.
NH2
NO2
+
Ph
H
COO
H
Ph
COOH
OMe
- MeOH
1a
MeO
OMe
H
MeO
MeO
O
O
O
OMe
H
OMe
O
O
O
H
O
NH
N
N
NH
H
H
NO2
NO2
NO2
NO2
Ph
N
Ph
COOH
[Se]
N
O
N
3a
[Mo]
OH
NO2
2a
O
FeMoSe
NO2
[Fe]
N
Ph
N
A
Ph
H
N
N
[Se]
- FeMoSe
product
[Mo]
N
dissociation
[Fe]
B
Scheme 4. Plausible mechanism.

T.A. To et al. / Journal of Catalysis 377 (2019) 163–173
169
Table 2
Synthesis of pyrrolo[1,2-a]quinoxalines via one-pot strategy using FeMoSe catalyst.a
Entry
1
Reactant 1
Reactant 3
Product 4
Yieldb
(%)
85
80
75
1a
1a
3a
3b
4aa
4ab
2
3
1a
1a
1a
1a
1a
3c
3d
3e
3f
4ac
4ad
4ae
4af
4
5
6
7
82
80
70
72
3g
4ag
(continued on next page)

170
T.A. To et al. / Journal of Catalysis 377 (2019) 163–173
Entry
8
Reactant 1
Reactant 3
Product 4
Yieldb
(%)
73
83
1a
1a
3h
3i
3ah
4ai
9
10
11
65
43
1a
1a
3j
4aj
4aj
3k
12
trace
1a
1a
3l
4al
13
14
trace
89
3m
4am
1b
1c
3a
3a
4ba
4ca
15
91

T.A. To et al. / Journal of Catalysis 377 (2019) 163–173
171
Entry
16
Reactant 1
Reactant 3
Product 4
Yieldb
(%)
85
82
80
1d
1e
1f
3a
3a
3a
4da
4ea
4fa
17
18
a
First step conditions: 1 (0.2 mmol); 2,5-DMTHF (0.21 mmol); 3 (0.5 mmol); 80 °C; 1 h. Intermediate 2 was not isolated.
Second step conditions: FeMoSe (5 mol), DABCO (0.3 mmol); 140 °C; 4 h; under argon atmosphere.
bYields are isolated yields. Please see the Supporting Information for details.
l
the chemical composition was changed to Fe:Mo:Se of 0.96:1:5.23,
representing a significant lower Fe content. Consisting with this
result, XPS analysis showed a change in the Fe core level. Content
of the lower binding energy species (Fe2p3/2 of 707.7 eV) decreased
while that of the higher binding energy species (Fe 2p3/2 of
711.2 eV) increased (Fig. 2d). The same trend was recorded for
the Mo species. The signal corresponding to MoIV species
(Mo3d5/2 228.4 eV) decreased whereas that of MoVI species
(Mo3d5/2 232.3 eV) increased (Fig. 2e). No significant oxidation
was observed for Se ligand as the amount of oxidized Se species
type SeO23- and SeO24- at 58.3 eV was not changed compared with
the as-prepared FeMoSe catalyst (Fig. 2f). These results suggest
the oxidation occurred on the Fe and Mo centers but not on the
Se ligand within the FeMoSe catalyst during the catalytic opera-
tion. The fact that Se ligand was intact could be crucial to maintain
the structure of FeMoSe catalyst and therefore to make it
recyclable.
A few experiments were carried out to study the mechanism of
the reaction and presented in Scheme 3. Using solely either the FeII
salt or Se0 was not effective for the formation of product 4aa (i,
Scheme 3). Interestingly, a moderate yield was obtained when a
mixture of FeSO4ꢀ7H2O and Se was used. The use of Mo(0), as Mo
(CO)6, had a negative effect on the formation of 4aa. This suggested
an unlikely mechanism that involved a low-valent molydenum-
FeMoSe catalyst to form the species A. Decarboxylation presum-
ably occurred on selenide sites [49], which was then intramolecu-
larly oxidized by an adjacent nitro group followed by
a
condensation to generate the species B [50]. The Pictet-Spengler
typed cyclization followed by dissociation of FeMoSe catalyst
afforded the product.
The scope of the reaction with regard to phenylacetic acids was
then studied and presented in Table 2. Both electron-rich (entries
1,2) and electron-poor (entries 3–8, 10) phenylacetic acids succes-
fully coupled with 2-nitroaniline. A relatively rare example of
ortho-substituted phenylacetic acid was attempted, yielding a
sterically hindered product (entry 7) [35]. Unlike a previous reduc-
tive method for decarboxylation of phenylacetic acids [51], reduc-
tion of nitro functionality was not observed under our conditions
(entry 8). 4-Thiophenyl pyrrolo[1,2-a]quinoxaline was obtained
in 83% yield (entry 9), thus showing the compatibility of our con-
ditions with heterocycles. The transformation was used to derive
a fused quinoxaline (entry 11). It should be noted that the presence
of (hetero)aryl ring adjacent to acetic acid is crucial, since styry-
lacetic acid failed to give the product (entry 12). We also attempted
other carboxylic acids for the transformation. Cinnamic acids were
inactive (entry 13), as our conditions unlikely afforded an imine
intermediate before decarboxylation occurred [51]. We next inves-
tigated the effect of 2-nitroaniline derivatives on the transforma-
tion. Generally, good to excellent yields of the products were
obtained (entries 14–18). Functionalities such as methoxy, chloro,
and fluoro groups were tolerant of reaction conditions.
mediated nitrosoarene formation followed by a 6p electrocycliza-
tion [47,48]. At this stage, we speculated the intramolecular
cyclization possibly proceeded through the formation of imine fol-
lowed by electrophilic addition as a common Pictet-Spengler
mechanism. Such an imine intermediate was obtained if a 2-
nitro-1,10-biphenyl was used (ii, Scheme 3). Because the nucle-
ophilicity of benzene ring is weaker than that of pyrrole, cycliza-
tion did not occur in this case. With the results in hand, we
proposed a mechanism as presented in Scheme 4. Formation of
2-nitropyrrole 2a from 1a following the classical Clauson-Kaas is
known. The intermediate 2a then adsorbed onto the surface of
4. Conclusions
In this work, FeMoSe nanosheets composed of MoSe2 and FeSe2
phases were synthesized using readily available precursors via a
solvothermal synthesis. The FeMoSe nanosheets exhibited high
activity in the synthesis of pyrrolo[1,2-a]quinoxalines via
solvent-free one-pot strategy starting from 2-nitroanilines. Our

172
T.A. To et al. / Journal of Catalysis 377 (2019) 163–173
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protocol offers significant advantages over previous methods. The
transformation proceeded readily via one-pot strategy under
solvent-free condition utilizing commercially available 2-
nitroanilines as starting materials. Moreover, phenylacetic acids
functioned as both Brønsted acid catalysts and reactants, thus no
additional acid catalyst was required for the Clauson-Kaas step in
the reaction sequences. Furthermore, the second step progressed
via auto redox mechanism, and no external oxidant or reductant
was needed. The FeMoSe was reused 6 times and still afforded
the product in good yield. The fact that valuable pyrrolo[1,2-a]
quinoxalines could be produced via solvent-free one-pot strategy
utilizing a recyclable catalyst would attract significant attention
from the chemical industry.
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Acknowledgments
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We would like to thank the financial support from the Viet Nam
National Foundation for Science and Technology Development
(NAFOSTED) for catalyst synthesis and characterization via Project
No. 103.99-2015.46, and for catalytic studies via Project No.
104.05-2018.13. We thank Dr. Truong Quang Duc from Tohoku
University, Japan for helps in XPS analysis.
Appendix A. Supplementary material
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