Tungstophosphoric acid supported on zirconia: A recyclable catalyst for the green synthesis on quinoxaline derivatives under solvent-free conditions

Article abstract of DOI:10.1080/10426507.2012.710678

A green, simple, and fast procedure has been developed for the preparation of quinoxaline derivatives by a condensation of 1,2-diamines with 1,2-dicarbonyl compounds in the presence of zirconium oxide modified with tungstophosphoric acid (H₃PW1

Full text of DOI:10.1080/10426507.2012.710678

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Tungstophosphoric Acid Supported
on Zirconia: A Recyclable Catalyst for
the Green Synthesis on Quinoxaline
Derivatives under Solvent-Free
Conditions
Alexis A. Sosa a , Toa S. Rivera a , Mirta N. Blanco a , Luis R. Pizzio a
& Gustavo P. Romanelli a
a Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr.
Jorge J.J. Ronco” (CINDECA), Departamento de Química, Facultad
de Ciencias Exactas, UNLP-CCT La Plata CONICET , 1900 , La Plata ,
Argentina
Accepted author version posted online: 17 Jul 2012.Published
online: 22 Jul 2013.
To cite this article: Alexis A. Sosa , Toa S. Rivera , Mirta N. Blanco , Luis R. Pizzio & Gustavo P.
Romanelli (2013) Tungstophosphoric Acid Supported on Zirconia: A Recyclable Catalyst for the Green
Synthesis on Quinoxaline Derivatives under Solvent-Free Conditions, Phosphorus, Sulfur, and Silicon
and the Related Elements, 188:8, 1071-1079, DOI: 10.1080/10426507.2012.710678
To link to this article: http://dx.doi.org/10.1080/10426507.2012.710678
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Phosphorus, Sulfur, and Silicon, 188:1071–1079, 2013
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2012.710678
TUNGSTOPHOSPHORIC ACID SUPPORTED ON
ZIRCONIA: A RECYCLABLE CATALYST FOR THE GREEN
SYNTHESIS ON QUINOXALINE DERIVATIVES UNDER
SOLVENT-FREE CONDITIONS
Alexis A. Sosa, Toa S. Rivera, Mirta N. Blanco, Luis R. Pizzio,
and Gustavo P. Romanelli
Centro de Investigacio´n y Desarrollo en Ciencias Aplicadas “Dr. Jorge J.J. Ronco”
(CINDECA), Departamento de Qu´ımica, Facultad de Ciencias Exactas,
UNLP-CCT La Plata CONICET, 1900 La Plata, Argentina
GRAPHICAL ABSTRACT
Abstract A green, simple, and fast procedure has been developed for the preparation of
quinoxaline derivatives by a condensation of 1,2-diamines with 1,2-dicarbonyl compounds
in the presence of zirconium oxide modified with tungstophosphoric acid (H3PW12O40) as
a heterogeneous catalyst, in a solvent-free medium using conventional heating. Quinoxaline
derivatives were formed in short-time periods and excellent yields (65–100%). The reaction
work-up is very simple and the catalyst can be easily separated from the reaction mixture and
reused several times in subsequent reactions without appreciable loss of the catalytic activity.
Supplementary materials are available for this article. Go to the publisher’s online edition of
Phosphorus, Sulfer, and Silicon and the Related Elements for the following free supplemental
files: Additional text.
Keywords Quinoxalines; tungstophosphoric acid; mesoporous zirconia; ecofriendly process;
solvent-free condition
INTRODUCTION
The need for greener techniques in the synthesis of different chemical compounds
leads to the use of different environmentally friendly reaction conditions; among them, the
replacement of pollutant inorganic acid catalysts, such as sulfuric or hydrochloric acids,
by reusable solid acids is yet very necessary.1 The application of solid acids in organic
Received 8 May 2012; accepted 30 June 2012.
The authors thank L. Osiglio and G. Valle for their experimental contribution and CONICET, ANPCyT,
and National University of La Plata for the financial support. Luis R. Pizzio, Mirta N. Blanco, and Gustavo P.
Romanelli are members of CONICET.
Address correspondence to Centro de Investigacio´n y Desarrollo en Ciencias Aplicadas “Dr. Jorge J. J.
Ronco” (CINDECA), Departamento de Qu´ımica, Facultad de Ciencias Exactas, UNLP-CCT La Plata CONICET-
CCT, 47 N◦ 257, 1900 La Plata, Argentina. E-mail: gpr@quimica.unlp.edu.ar
1071

1072
A. A. SOSA ET AL.
transformations has an important role, because these materials have many advantages,
such as the ease of handling, decreased plant corrosion, and more environmentally safe
waste-disposal procedures.1
In particular, catalysis by heteropolyacids (HPA) and related compounds is a field
of increasing significance worldwide. Many developments have been carried out both
in basic research and in fine chemistry processes.2 The HPA are well-defined molecular
arrangements with remarkable and useful applications. Their main technological property
is their reusability in heterogeneous catalysis. Diverse electronic and molecular structures
of HPA lead to their application in different areas, such as medicine and materials science,
among others. Within the wide field of structural possibilities that the HPA have shown,3
those that present the Keggin-type primary structure deserve to be mentioned. The HPA with
Keggin structure are polynuclear complexes mainly constituted by molybdenum, tungsten,
or vanadium as polyatoms (M), and phosphorus, silicon, or germanium as central atom or
heteroatom (X). The Keggin structure is formed by a central XO4 tetrahedron, surrounded
by 12 MO6 octahedra. They could be either multielectron oxidants or strong acids, with an
acid strength higher than that of the classical acids. Recently, our research group reported
green catalytic acid and oxidation procedures using Keggin HPA.4–8
Zirconia is an interesting material to be used as catalyst support due to its thermal
stability in different atmospheres. Its acid properties can be modified by addition of cationic
or anionic substances, such as sulfate or tungstate.9 The addition of Keggin heteropolyacids
to modify them has been studied to a lesser extent.10 We recently reported a procedure
for the 14-aryl-14H-dibenzo[a,j]xanthenes using recyclable mesoporous zirconia modified
with tungstophosphoric acid.11
On the other hand, quinoxaline derivatives are a very important class of nitrogen-
containing heterocycles as they constitute useful intermediates in organic synthesis.12 This
structure plays an important role in the design of a number of heterocyclic compounds
with different biological activities, making this type of compounds important in the field
of medicine as antitumor, anticonvulsant, antimalarial, anti-inflammatory, antiamoebic,
antioxidant, antidepressant, antiprotozoal, antibacterial, and anti-HIV agents.12–15 Quinox-
alines are important in the pharmaceutical industry, with antibiotics such as echinomycin,
levomycin, and actinoleutin having quinoxaline as part of their structure.16
The most used method for the preparation of quinoxaline is the condensation of a
1,2-dicarbonylic compound with a 1,2-diamino compound. In general, this procedure needs
high temperature, the use of a strong acid catalyst, and long reaction times.17
A variety of catalysts were tested in these reactions such as acetic acid,18 io-
dine,19 CuSO4·5H2O,20 nickel nanoparticles,21 montmorillonite K10,22 ionic liquids,23 nano
TiO2,24 Al2O3,25 zirconium(IV)-modified silica gel,26 cerium(IV) ammonium nitrate,27
iron-exchanged molybdophosphoric acid,28 silica-bonded S-sulfonic acid,1 among others.
However, some of these methods suffer from unsatisfactory yields, problem of recyclability
of the traditional acid catalysis, harsh reaction conditions, which limit their use in friendly
processes.
As part of our research project directed toward the development of highly suitable
methods and the synthesis of diverse heterocyclic compounds, we herein disclose a new,
one-pot synthesis of quinoxaline derivatives from 1,2-diaminobenzenes and 1,2-dicarbonyl
compounds catalyzed by mesoporous zirconia modified with tungstophosphoric acid un-
der solvent-free conditions with the aim to obtain high yields with an environmentally
advantageous procedure (Scheme 1).

TUNGSTOPHOSPHORIC ACID SUPPORTED ON ZIRCONIA
1073
Scheme 1
RESULTS AND DISCUSSION
Catalyst Characterization
As a result of the addition of PW12 to the zirconia matrix, the FT-IR spectra of the
PW12ZrO2 sample displayed a new set of bands overlapped to the zirconia wide band.
The presence of the P–Oa, W–Od, and W–Ob–W stretching vibrations of the [PW12O40]3−
anion can be clearly observed. In addition, bands assigned to the [PW11O39]7− anion are
also observed in the catalyst if a comparison with the spectrum of the sodium salt of the
lacunary anion is made, which presents bands at 1100, 1046, 958, 904, 812, and 742 cm−1
in agreement with the literature.29
,
The 31P MAS-NMR spectra of PW12ZrO2 displayed a wide band with maximum at
around −14 ppm, accompanied by a shoulder at −12 ppm. They may be attributed to the
[PW12O40]3− anion and to the [P2W21O71]6− dimeric species, respectively.30 The downfield
shift and the increase of the line width observed, compared to tungstophosphoric acid
(−15.3 ppm), can be attributed to the interaction between the anion and the zirconia matrix.31
The acidity measurements of the catalyst by means of potentiometric titration with
n-butylamine let us to estimate the number of acid sites and their acid strength. It was
suggested that the initial electrode potential (Ei) indicates the maximum acid strength of
the sites, and the value of meq amine/g solid where the plateau is reached indicates the
total number of acid sites. The acid strength of these sites may be classified according to
the following scale: Ei > 100 mV (very strong sites), 0 < Ei < 100 mV (strong sites),
−100 < Ei < 0 (weak sites), and Ei < −100 mV (very weak sites).32,33 The acid strength
of the modified sample PW12ZrO2 (Ei = 397 mV) was markedly higher than that of the
unmodified zirconia (Ei = 140 mV), but lower than that of bulk PW12 (Ei = 620 mV).34 The
lower acid strength of the PW12ZrO2 sample compared to bulk PW12 could be assigned to
the fact that the protons in the H3PW12O40·6H2O are present as H+(H2O)2 species, whereas
in the PW12ZrO2 sample they are interacting with the oxygen of Zr–OH groups or in a
higher hydration state (H+(H2O)n), as previously reported.35
Synthesis of Quinoxaline Derivative
As a continuation of our interest in the application of HPA catalysts for the de-
velopment of a useful synthetic methodology, a simple and efficient approach for the
preparation of quinoxaline derivatives is here reported, using mesoporous zirconia mod-
ified with tungstophosphoric acid as an eco-friendly catalyst with high catalytic activity
under solvent-free conditions at 80 ◦C.
The reaction between 1,2-diaminobenzene and benzyl (PhCOCOPh) was first studied
(Table 1, entry 1) to screen the reaction conditions, in order to determine the best conditions.

1074
A. A. SOSA ET AL.
a
Table 1 Synthesis of quinoxaline derivatives catalyzed by PW12ZrO2
Entry
1
Product
Time (min)
5
Yieldsb (%)
Product (ref.)
[1,34,36]
100 (100, 99, 99)c
2
5
99
[1,34,36]
3
4
5
10
20
91
92
65
[1,34]
[34]
300
[1,34,36]
6
7
120
95
93
[34]
3
[19,36]
8
9
10
3
94
78
[19,36]
[19]
(Continued on next page)

TUNGSTOPHOSPHORIC ACID SUPPORTED ON ZIRCONIA
1075
Table 1 Synthesis of quinoxaline derivatives catalyzed by PW12ZrO2a (Continued)
Entry
10
Product
Time (min)
5
Yieldsb (%)
88
Product (ref.)
[36]
11
5
95
[36]
aAll reactions were performed at 1 mmol scale using 1 mmol % of PW12ZrO2 in solvent-free conditions at 80
◦C.
bColumn-isolated yields.
cFirst, second, and third reuse.
The influence of the reaction temperature, the reaction time, the molar ratio of substrates, and
the amount of catalyst were all examined. For a typical experiment, 1,2-diaminobenzene,
◦
benzyl, and the catalyst were mixed at 80 C. The condensation was completed in the
formation of quinoxaline ring after 5 min (Table 1, entry 1, 100%).
The excellent preliminary result encouraged us to extend the generality of the novel
catalytic system to various 1,2-diamines and 1,2-dicarbonyls. Most of the reaction took
5 min to be completed at 80 ◦C. The experiments were run until benzyls were consumed
or until no changes in the composition of the reaction mixture were observed. In all the
cases, the desired products were obtained with high selectivity, almost free of secondary
products.
The result presented in Table 1 shows that electron-donating groups at the phenyl ring
of the 1,2-diamine favored the formation of the product (Table 1, entry 2) to give quantitative
yields. In contrast, electron-withdrawing groups such as chloro and bromo gave slightly
lower yields (91–92%) in a longer time, 10 and 20 min, respectively (Table 1, entries 3 and
4). The presence of a nitro group, a strong electron-withdrawing group, reduced the yield to
65% (Table 1, entry 5), and the reaction was not complete. However, the unchanged starting
materials were recovered nearly quantitatively (Table 1, entry 6). In Table 1, entry 6 and, it
can be observed that, when a 2,3-diamine pyridine was used as substrate, a pyrazine was
obtained as reaction product in excellent yields (95%). On the other hand, the substituents at
the 1,2-diketone had no significant effects on the yield to the corresponding products. Since
only symmetric 1,2-diketones were used for the condensation reaction, no regioisomers
were generated as the products (Table 1, entries 7–11).
The possibility of recycling the catalyst (PW12ZrO2) was examined. For this reason,
the reaction of 1,2-diaminobenzene and benzyl was studied in solvent-free conditions at
80 ◦C in the presence of PW12ZrO2. When the reaction was complete, the reaction mixture
was cooled to room temperature (20 ◦C), toluene (5 mL) was added, and then the mixture
was stirred for 15 min, filtered to separate the catalyst, and the catalyst was dried in a
vacuum oven at room temperature for 4 h prior to reuse. No appreciable loss of catalytic
activity was observed after four cycles (Table 1, entry 1).

1076
A. A. SOSA ET AL.
Scheme 2
Niknam and coworkers1 proposed that the reaction follows a mechanism of acid-
catalyzed condensation reactions. In our case, PW12ZrO2 catalyst acts as a Brønsted acid
and the mechanism can be rationalized in four stages: (1) the coordination of the diketone
to the acid sites of PW12ZrO2, (2) the nucleophilic attack on the carbonyl intermediate, (3)
the dehydration to give a carbocation intermediate, and (4) the elimination of a proton to
give the quinoxaline product (Scheme 2).1
CONCLUSIONS
In conclusion, tungstophosphoric acid supported on zirconia (PW12ZrO2), which can
simply be prepared from commercially available and relatively cheap starting materials, is
an efficient, thermally stable, and easily recoverable catalyst for the synthesis of quinoxaline
derivatives. The present procedure provides a simple, efficient, selective, and recyclable
methodology for the preparation of quinoxaline derivatives in high yields with an easy
work-up procedure. The catalyst can be recovered and reused over several reaction cycles
without considerable loss of reactivity. Moreover, this methodology introduces a practical
and viable green technology for quinoxaline preparation. We are currently exploring further
applications of this solid to other types of heterocycles.

TUNGSTOPHOSPHORIC ACID SUPPORTED ON ZIRCONIA
1077
EXPERIMENTAL
General
All chemical reagents and solvents were obtained from Merck and Aldrich and were
used without further purification. All products are known compounds and were identified
by comparison of their physical and spectral data with those of the authentic samples.
The purity of the substances and the progress of the reaction were monitored by thin
layer chromatography (TLC) on silica gel, and yields refer to isolated products. Flash
column chromatography was performed with 230–400 mesh silica gel. Melting points
of the compounds were determined in sealed capillary tubes on a Bioamerican Bs 448
apparatus and were uncorrected. The 13CNMR and 1H NMR spectra were recorded at room
temperature on Varian-200 spectrometer using TMS as internal standard.
Synthesis of Supported Catalysts
Zirconium propoxide (26.6 g) was mixed with absolute ethanol (336.1 g) and stirred
for 10 min to obtain a homogeneous solution under N2 at room temperature. Then,
0.5 mL of HCl 0.28 M aqueous solution was dropped slowly into the above mixture
to catalyze the sol–gel reaction. After 3 h, an appropriate amount of polyethylene glycol
(PEG)–alcohol–water solution (1:5:1 weight ratio) was added to the hydrolyzed solution
under vigorous stirring to act as pore-forming agent. The amount of added solution was
fixed in order to obtain a PEG concentration of 10% by weight in the final material. A
tungstophosphoric acid (H3PW12O40·6H2O) (PW12) solution, of adequate concentration in
order to obtain a PW12 concentration of 60% w/w in the solid, was added together with the
template addition (PW12ZrO2 sample). The gel was then kept in a beaker at room temper-
ature to dryness. The solid was ground into powder and extracted with distilled water for
three periods of 8 h, in a system with continuous stirring, to remove PEG. Afterwards, the
solid was thermally treated at 100 ◦C for 24 h.
Catalyst Characterization
In a previous paper, we reported the full characterization of the catalysts by X-ray
diffraction (XRD), Fourier transformed infrared spectroscopy (FT-IR), the 31P magic angle
spinning-nuclear magnetic resonance (31P MAS-NMR), and the acidity of the solids was
estimated by means of potentiometric titration. We present here only the result necessary
to confirm the catalyst structure.37
General Procedure for the Synthesis of Quinoxalines Using
PW12ZrO2 Catalyst
A mixture of the corresponding benzil (1.2 mmol), diamine, and PW12ZrO2 (1 mmol
%, ca 30 mg) was stirred at 80 ◦C for the appropriate time according to Table 1. Completion
of the reaction was indicated by TLC. The reaction mixture was cooled to 25 ◦C, toluene
(5 mL) was added, and then the mixture was stirred for 15 min and filtered to separate
the catalyst, which was subsequently washed twice with toluene (3 mL). The combined
toluene extracts were washed twice with water (5 mL), dried over anhydrous Na2SO4, and

1078
A. A. SOSA ET AL.
evaporated in vacuo. The solid obtained was recrystallized from ethyl alcohol to afford the
pure quinoxaline derivative.
Recycling of the Catalyst
After the reaction, the catalyst was filtered, washed thoroughly with toluene, dried
◦
in a vacuum oven (80 C, 4 h), and reused for the next reaction, following the procedure
described above.
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