Wells-Dawson heteropolyacid as reusable catalyst for sustainable synthesis of flavones

Article abstract of DOI:10.1016/j.apcata.2011.07.011

The behavior of Wells-Dawson (H₆P₂W ₁₈O₆₂·24H₂O, WD) acid, both bulk and supported on silica, for the cyclodehydration reaction of 1-(2-hydroxyphenyl)-3- phenyl-1,3-propanedione to obtain flavone was studied in heterogeneous conditions, using toluene as solvent. Catalytic experiences with bulk and supported catalysts with different WD acid loadings and reuse of the catalysts were done. The catalytic activity of supported catalysts was higher than that of the bulk catalyst, and their activity was almost constant after three reaction cycles. The following reaction conditions: reflux toluene and 1% mmol of WD supported on silica (0.4 g WD acid by gram of silica) were used for the preparation of five flavones. Yields above 85% were obtained in 4-5 h.

Full text of DOI:10.1016/j.apcata.2011.07.011

Applied Catalysis A: General 404 (2011) 68–73
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Wells–Dawson heteropolyacid as reusable catalyst for sustainable synthesis of
flavones
Daniel O. Bennardia, Gustavo P. Romanellia,b, Ángel G. Sathicqb,∗, Juan C. Autinoa,
Graciela T. Baronettic, Horacio J. Thomasd
a Cátedra de Química Orgánica, Facultad de Ciencias Agrarias y Forestales, UNLP, Calles 60 y 119, B1904AAN La Plata, Argentina
b Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. J.J. Ronco” (CINDECA), Departamento de Química, Facultad de Ciencias Exactas, UNLP-CCT-CONICET. Calle 47 N◦ 257,
B1900AJK La Plata, Argentina
c Departamento de Ingeniería Química, Facultad de Ingeniería, Universidad de Buenos Aires, Ciudad Universitaria, C1428BG Buenos Aires, Argentina
d Planta Piloto Multipropósito PlaPiMu, UNLP-CICPBA, Camino Centenario y 508,1987 Manuel B. Gonnet, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
The behavior of Wells–Dawson (H6P2W18O62·24H2O, WD) acid, both bulk and supported on silica, for the
cyclodehydration reaction of 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione to obtain flavone was
studied in heterogeneous conditions, using toluene as solvent. Catalytic experiences with bulk and sup-
ported catalysts with different WD acid loadings and reuse of the catalysts were done. The catalytic
activity of supported catalysts was higher than that of the bulk catalyst, and their activity was almost
constant after three reaction cycles. The following reaction conditions: reflux toluene and 1% mmol of
WD supported on silica (0.4 g WD acid by gram of silica) were used for the preparation of five flavones.
Yields above 85% were obtained in 4–5 h.
Received 1 March 2011
Received in revised form 8 July 2011
Accepted 12 July 2011
Available online 22 July 2011
Keywords:
Acid catalysis
Heteropolyacids
Wells–Dawson
Flavones
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
opments are being carried out in basic research as well as in
fine chemistry processes [6]. HPAs possess a very strong acidity,
It is well known that the use of conventional liquid and Lewis
acids such as H2SO4, HF, HCl, AlCl3, and ZnCl2 poses risks in
handling, containment, disposal and regeneration due to their cor-
rosive and toxic nature. The processes used in the industry for the
manufacture of specialty chemicals in the field of pharmaceutical,
agrochemical and flavor/fragrance products are largely based on
stoichiometric organic synthesis, but they lead to a large quantity of
byproducts [1]. For this reason, there is an urgent need to eliminate
the aggressive mineral acid-catalyzed processes [2].
The use of solid (heterogeneous) catalysts in organic synthe-
sis and in the industrial manufacture of chemicals is increasingly
important since they provide green alternatives to homogeneous
catalysts [3,4]. Different solid acid catalysts such as inorganic
oxides, mixed oxides, including alumina, silica, titania, zirconia,
zeolites, clays, and supported reagents have been used [5].
but theyexhibit the characteristic features of bulk-type catalysis
(pseudo-liquid bulk type), in which the reagent molecules are
absorbed between the polyanions (rather than in a polyanion) in
the ionic crystal by replacing crystallization water or expanding
the lattice, and the reaction occurs there [7].
The reactions catalyzed by both heterogeneous and homoge-
neous systems have been reviewed by many researchers. The
reactions in which they can be used, from dehydration, cycliza-
tion or esterification up to amine oxidation or olefin epoxidation,
may find wide applications in fine chemical production, such as
fragrances, pharmaceuticals and food [8–14].
Although there are many structural types of HPAs, the major-
ity of the catalytic applications use the most common commercial
Keggin-type and then the Wells–Dawson HPAs especially for acid
catalysts, owing to their availability and chemical stability [15,16].
The general formula of the Wells–Dawson heteropolyanion is
Catalysis by heteropolyacids (HPAs) and related compounds
is a field of increasing importance worldwide. Numerous devel-
[(Xn+)2M18O62 (16−2n)−, where Xn+ represents a central atom, such
]
as phosphorous(V), arsenic(V), sulfur(VI), and fluorine surrounded
by a cage of M addenda atoms, such as tungsten(VI), molybde-
num(VI) or a mixture of elements, each of them composing MO6
(M-oxygen) octahedral units. The structure, known as ␣ isomer,
∗
Corresponding author. Tel.: +54 221 421 1353; fax: +54 221 421 1353.
E-mail address: agsathicq@quimica.unlp.edu.ar (Á.G. Sathicq).
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.07.011

D.O. Bennardi et al. / Applied Catalysis A: General 404 (2011) 68–73
69
Table 1
OH
O
O
Preparation of substituted flavones using 0.4WDSiO2 catalyst.a
O
O
H+
Entry
Flavone
Time (h)
Yield (%)
TOFb
R
R
O
Scheme 1. cyclodehydration of 1-(2-hydroxyphenyl)-1,3-diketones.
O
1
2
3
4.5
4.5
4.5
87
86
85
19.3
19.1
18.9
O
possesses two identical “half units” of the central atom surrounded
by nine octahedral units XM9O31 linked through oxygen atoms
[17]. Recently, we have used Wells–Dawson heteropolyacids in a
range of processes such as preparation of heterocycles [18,19] and
protection/deprotection of organic functional groups [20–22].
Flavones constitute a large number of natural products with
many medicinal applications. Because of their broad range of
biological activities this class of molecule has been extensively
investigated, and more than 4000 chemically unique flavonoids
have been isolated from plants [23]. They exhibit diverse biological
and pharmacological activities, for example, antibacterial, antifun-
gal [24,25], anticancer [26], gastro-protective [27], antioxidant [28]
and anti-HIV [29]. Furthermore, it has been reported that some
flavones have a repelling property against some phytophagous
insects and a subterranean termite (Coptotermes sp.) acting as an
antifeedant [30].
Various strategies have been developed to synthesize flavones.
One of the most commonly used methods consists of the cyclodehy-
dration of 1-(2-hydroxyphenyl)-1,3-diketones [31]. This is usually
a catalyzed reaction (Scheme 1) and it has been performed in dif-
ferent media. Some reaction conditions employed were the use of
excess of sulfuric acid in glacial acetic acid [32], cationic exchange
resins in isopropanol [33], CuCl2 in ethanol [31], and ionic liq-
uid under microwave irradiation [34]. Recently, we reported a
more suitable method for their synthesis, including heteropoly-
acids [35–37] and-carbon supported triflic acid [38].
Cl
Cl
O
O
O
O
H3C
H3C
O
O
4
5
5
5
86
87
17.2
17.4
O
O
Br
Br
O
O
6
5
5
83
82
16.6
16.4
O
7
The main goal of the present work was to study the catalytic
activity of both bulk and silica-supported WD in the reaction of
cyclization of 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione to
flavone.
a
Toluene, reflux. Catalyst: 1% mmol.
b
Turnover frequency (product mol s × WD mol s−1 × h−1).
2.3. General procedure for the preparation of substituted flavones
2. Experimental
All the starting 1,3-diketones were prepared following a method
described elsewhere [21]. A mixture of 1,3-diketone (0.5 mmol) dis-
solved in 3 ml toluene and catalyst (1% mmol) was refluxed with
stirring for the indicated time (see Table 1). The reaction was fol-
lowed by TLC using TLC aluminum sheets (silica gel 60 F254 Merck).
When the reaction time was over, the catalyst was filtered and
washed twice with toluene (1 ml). The extracts were combined and
washed with NaOH 3 M, then with H2O, and dried with anhydrous
sodium sulfate. The organic solution was concentrated in vacuum.
All the solid crude products were recrystallized from methanol.
Products were identified by 1H and 13C NMR with a Bruker Avance
DPX-400 instrument, operating at 400 MHz for 1H and 100 MHz
for 13C NMR, and by comparison with authentic samples prepared
using sulfuric acid as catalyst.
2.1. Preparation of catalysts
H6P2W18O62·24H2O (WD) was synthesized as described else-
where [39] from an aqueous solution of ˛/ˇ K6P2W18O62·10H2O
salt. The WD silica-supported catalyst was obtained by wet impreg-
nation of Grace Davison silica (Grade 59, specific area = 250 m2/g)
with an aqueous solution of the synthesized WD. Catalysts contain-
ing 0.1, 0.2, 0.4 and 0.6 g/g of WD acid were prepared (0.1WDSiO2,
0.2WDSiO2, 0.4WDSiO2 and 0.6WDSiO2). Then, samples were dried
at room temperature in a vacuum desiccator for 8 h.
2.2. Catalyst characterization
Bulk and supported catalysts were characterized by 31P MAS-
NMR measurements. The 31P MAS-NMR spectra were recorded
in Bruker MSL-300 equipment operating at frequencies of
121.496 MHz. A sample holder of 5 mm diameter and 17 mm in
height was used. The spin rate was 2.1 kHz, and several hundreds of
pulse responses were collected. Chemical shifts were expressed in
parts per million with respect to 85% H3PO4 as an external standard
for 31P-NMR.
2.4. Catalytic tests
All the reactions were carried out in glass tubes (10 ml),
by adding the corresponding catalyst to a solution of 0.5 mmol
of 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione in toluene,
with stirring and heating at the indicated temperature (range:
65–110 ◦C). Samples (30 ␮l each) were withdrawn with a syringe
at prescribed intervals. Each sample was diluted with toluene

70
D.O. Bennardi et al. / Applied Catalysis A: General 404 (2011) 68–73
to
a volume of 10 ml. Then the absorbance values of 1-
(2-hydroxyphenyl)-3-phenyl-1,3-propanedione contained in the
samples were determined at ꢀ = 420 nm using a UV/VIS spectrom-
eter LAMBDA 35, Perkin Elmer. The absorbance values of standard
solutions of 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione in
toluene were determined at ꢀ = 420 nm to build the calibration plot.
The activity of bulk and supported catalysts for the
cyclodehydration reaction of 1-(2-hydroxyphenyl)-3-phenyl-
1,3-propanodione to obtain flavone was studied in heterogeneous
conditions, using toluene as reaction solvent. Conversion to flavone
as a function of the time was determined by means of UV–vis
spectroscopy. In order to estimate the possible catalyst solubi-
lization, additional tests were performed. The 0.4WDSiO2 sample
(60 mg) was refluxed in toluene (3 ml) for 5 h, filtered and dried in
vacuum till constant weight. Loss of mass was not detected. The
refluxed toluene was used as solvent for attempting the reaction
without adding the catalyst. After 20 h flavone was not detected,
and the starting material was quantitatively recovered. Presence
of W species in the filtrate was not detected by ICP.
0.4 WDSiO2
0.6 WDSiO2
-25
-20
-15
-10
-5
0
5
δ, ppm
Fig. 1. 31P-NMR spectra of 0.4 and 0.6 WDSiO2.
3. Results and discussion
nals close to −12 and −11 ppm are detected. As has been reported
in detail in our previous work [40] for silica supported samples
up to 0.2WDSiO2, these signals could be related to the presence of
H6P2W18O62 species strongly interacting with the Si–OH groups of
the support, and to lacunary species such as P2W21O71−6, respec-
tively. These results and those obtained in a previous work [40]
indicate that WD keeps its heteropolyoxoanion structure after the
impregnation and drying steps when it is supported on SiO2 using
a WD loading in the range 0.1 to 0.6gWD/g SiO2.
3.1. Characterization of bulk and supported WD acid
In a previous work [18], bulk WD acid was characterized by
31P MAS-NMR. It is known that this pure acid has two equivalent
phosphorus atoms, and consequently it has only one peak in the 1
P
MAS-NMR spectrum. The chemical shift obtained for synthesized
WD was only one peak at −12.8 ppm (using 85% H3PO4 as external
reference). The 31P MAS-NMR spectra of silica-supported samples
(0.4WDSiO2 and 0.6WDSiO2, Fig. 1) show a main peak with a chemi-
cal shift of −12.7 ppm, which indicates that after the impregnation
and drying, the acid is supported keeping its Dawson structure,
independent of the loading. In both samples, two new small sig-
On the other hand, DRX spectra for all the samples only showed
the pattern of the support indicating that there exists a good dis-
persion of WD acid on the silica [40].
Scheme 2.

D.O. Bennardi et al. / Applied Catalysis A: General 404 (2011) 68–73
71
100
100
80
60
40
20
0
80
0.1 WD/SiO2
60
0.2 WD/SiO2
0.4 WD/SiO2
0.6 WD/SiO2
40
0.2WDSiO2
0.4WDSiO2
0.6WDSiO2
0.1WDSiO2
SiO2
20
0
0
50
100
150
200
250
300
0
50
100
150
200
250
300
Time (min)
Time (min)
Fig. 3. Conversion of 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione to flavone
vs. time for supported catalyst maintaining the initial reagent to active phase ratio
constant.
Fig. 2. Conversion of 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione to flavone
vs. time for supported catalysts (100 mg) with different acid ratios.
3.2. Catalyst activity
For the first, we used bulk and supported catalysts, and four cat-
alysts with different charges of WD were prepared. Measurements
at different reaction temperatures were also made. The variation of
conversion as a function of time was also analyzed, and additional
runs were made in order to see if the supported catalyst presents a
deactivation process.
The probable mechanism for the reaction of cyclodehydration
of 1,3-diketones has been described in the bibliography [42,43]. By
using 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione as reagent
(Scheme 2), the intramolecular nucleophilic attack leading to the
ring closure is catalyzed by acidic sites; this addition is followed by
elimination of water and regeneration of protons. In our case the
acidic sites are provided by the WD.
Fig. 2 shows the results obtained for the cyclodehydration
reaction of 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione to
flavone using supported catalysts with different WD loadings
(0.1WDSiO2, 0.2WDSiO2, 0.4WDSiO2 and 0.6WDSiO2).
Additional tests carried out with the support as catalyst indi-
cate that the support has a very low catalytic activity, only 20%
after 5 h of reaction. In this figure a noticeable increase in catalytic
activity for supported catalysts can be observed. An increase in WD
loading produces a conversion increment following the sequence
0.1WDSiO2 < 0.2WDSiO2 < 0.4WDSiO2 ≈ 0.6WDSiO2. For the high-
est WD content the catalytic activity is similar to 0.4WDSiO2
sample.
In a previous paper [41] the pseudo liquid or surface-type behav-
ior of Wells–Dawson acid in organic reactions in liquid phase was
analyzed theoretically and experimentally. As previously shown
+
[31], H5O2 species, bridged to secondary structures, built the ter-
tiary heteropolyanion structure and then a cage-type structure was
obtained. This three-dimensional framework was connected with
all its neighbors, so all its acid active sites inside the cage could take
part in catalysis by the diffusion of reagents from the solution.
Through theoretical calculations and experimental results we
have shown that there is a direct relationship between polarity of
organic molecules and the ability to diffuse inside the catalyst cage
or framework. A nonpolar molecule does not diffuse inside and only
interacts with surface acidic sites, but a polar molecule can move
into the cage and then all internal acidic sites are available. When
diffusion occurs, the number of active sites grows sharply. In this
way, the higher the diffusion is, the closer the behavior of a bulk
catalyst to a supported one results.
The 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione has two
carbonyl groups in ß position so it is possible to believe that it is a
polar molecule. Then we propose two different types of reactions,
the first one in order to see if the catalyst shows a pseudo-liquid or
surface-type behavior, and the second one to analyze the influence
of different substituent groups and their positions on the substrate
reactivity.
40
100
b
a
383 K
378 K
368 K
80
60
32
24
16
40
338 K
343 K
353 K
20
0
368 K
383 K
100
150
200
250
300
0
50
100
150
200
250
300
Time (min)
Time (min)
Fig. 4. Conversion of 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione to flavones vs. time, at different reaction temperatures, (a) 0.4 WDSiO2 catalyst, (b) bulk catalyst.

72
D.O. Bennardi et al. / Applied Catalysis A: General 404 (2011) 68–73
100
80
60
40
20
-12.6 ppm
0.4 WDSiO2
1st use
2nd use
3rd use
0
50
100
150
Time (min)
200
250
300
-60
-40
-20
0
20
40
60
δ, ppm
Fig. 5. Reuse of 0.4WDSiO2. Conversion of 1-(2-hydroxyphenyl)-3-phenyl-1,3-
propanedione vs. time.
Fig. 6. 31P-NMR spectra of 0.4 WDSiO2 after the third reaction cycle.
Wells–Dawson structure indicating that after the reaction cycles
the structure of the heteropolyoxoanion keeps intact.
However, the small peaks (close to −12 and −11 ppm) in the
used sample (Fig. 1), asigned to the H6P2W18O62 species strongly
interacting with the Si–OH groups of the support, and to lacunary
species such as P2W21O71−6, for the fresh sample have not been
detected.
On the other hand, catalytic tests keeping the ratio between ini-
tial reagent and amount of active phase constant were done (Fig. 3).
For this purpose we used a variable mass of catalyst. It can be
observed that the conversion values are similar for samples with
low acid content in the range 0.1–0.2. In the same way, for the high
acid content in the range 0.4–0.6 the conversion values are similar.
These results may indicate that the acid dispersion is higher in the
samples with the lower acid content.
This fact would be indicating that during the procedure of wash-
ing before each new reaction cycle these species are lost.
3.2.1. Temperature effect
3.2.3. Preparation of substituted flavones
The effect of temperature in the range 65–110 ◦C on the reac-
tion of 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione to obtain
flavone, using 0.4 WDSiO2, was studied. This catalyst showed the
best catalytic conversion by gram of catalyst (see Fig. 2).
The following reaction conditions: reflux toluene and 1% mmol
0.4WDSiO2 catalyst were employed for the preparation of sub-
stituted flavones. The obtained results are listed in Table 1. The
experiments were conducted until the 1,3-diketone was 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 unchanged
starting materials were recovered nearly quantitatively. No stere-
oelectronic effects on the yields due owing to the substituent were
observed.
Fig. 4 shows that the conversion is higher as temperature
increases. Fig. 4a shows the conversion of 1-(2-hydroxyphenyl)-
3-phenyl-1,3-propanedione to flavones versus time, at different
reaction temperatures using 0.4 WDSiO2 catalyst, being obtained
conversions higher than 90% at reflux in toluene in 3 h.
Fig. 4b shows the conversion of 1-(2-hydroxyphenyl)-3-phenyl-
1,3-propanedione to flavones versus time, using a bulk catalyst.
Comparing the conversion values of supported and bulk cata-
lysts at different temperatures it was observed that the difference
between them were not so high as expected considering that the
amount of employed catalyst was always the same and the sup-
ported ones have a well disperse active phase. So, these results
could be explained in terms of pseudo-liquid behavior of bulk cata-
lyst, as we described before. The difference with supported ones can
be explained in terms of diffusion of reagents and products inside
and outside crystallites on supported catalysts.
4. Conclusions
In this work the performance of catalysts with Wells–Dawson
(H6P2W18O62·24H2O) structure was studied in the cyclodehydra-
tion reaction of 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione
to obtain flavone. The tests were carried out in heterogeneous
conditions, at different reaction temperatures. The activity of the
supported catalysts was higher than that of the bulk catalyst.
Moreover, the activity of the supported catalyst remained almost
constant after three cycles; it can be separated and recovered for
further use.
The utilization of these solid catalysts provided interest-
ing yields (above 85%) in the cyclization reaction of 1-
(2-hydroxyphenyl)-3-phenyl-1,3-propanediones to substituted
flavones, in reaction times of 4.5–5 h.
We assume that the difference in conversion between bulk and
supported catalysts can be explained considering that they have
microporous and mesoporous structures respectively, with 0.5 and
2.5 nm pore radius, so the diffusion factor in the whole reaction has
a different influence on both systems.
3.2.2. Catalyst reuse
The catalytic behavior after three reaction cycles of 0.4WDSiO2
catalyst was studied according to the protocol described in detail
in the experimental section. Fig. 5 shows the conversion after each
reaction cycle. All conversions are similar, indicating that the cat-
Acknowledgements
We thank Agencia Nacional de Promoción Científica y Tecnológ-
ica (Argentina), Universidad Nacional de La Plata, CONICET, and
CIC for financial support. GPR, AGS, GTB and HJT are members of
CONICET
alytic activity remains constant after three reaction cycles. The 31
P
MAS-NMR spectra of silica-supported sample (0.4WDSiO2 Fig. 6)
after three reaction cycles shows only one peak corresponding to

D.O. Bennardi et al. / Applied Catalysis A: General 404 (2011) 68–73
73
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