Tungstophosphoric acid nanoparticles supported on polyamic acid: A mild and recoverable heterogeneous catalyst for the selective synthesis of mono and bulky bis(1,8-dioxooctahydroxanthene)s under solvent-free conditions

Article abstract of DOI:10.1080/10426507.2015.1100185

A series of 1,8-dioxooctahydroxanthene derivatives and some crowded bis(1,8-dioxooctahdroxanthene) were selectively synthesized using tungstophosphoric acid nanoparticles supported on polyamic acid (TPA/PAA) as a new catalyst in solvent-free conditions. T

Full text of DOI:10.1080/10426507.2015.1100185

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Tungstophosphoric acid nanoparticles
supported on polyamic acid: a mild and
recoverable heterogeneous catalyst for the
selective synthesis of mono and bulky bis(1,8-
dioxooctahydroxanthene)s under solvent-free
conditions
Masoud Nasr-Esfahani, Zahra Rafiee & Hassan Kashi
To cite this article: Masoud Nasr-Esfahani, Zahra Rafiee & Hassan Kashi (2016):
Tungstophosphoric acid nanoparticles supported on polyamic acid: a mild and
recoverable heterogeneous catalyst for the selective synthesis of mono and bulky bis(1,8-
dioxooctahydroxanthene)s under solvent-free conditions, Phosphorus, Sulfur, and Silicon and
the Related Elements, DOI: 10.1080/10426507.2015.1100185
To link to this article: http://dx.doi.org/10.1080/10426507.2015.1100185
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Date: 20 January 2016, At: 11:43

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Tungstophosphoric acid nanoparticles on polyamic acid
Tungstophosphoric acid nanoparticles supported on polyamic acid: a mild and recoverable
heterogeneous catalyst for the selective synthesis of mono and bulky bis(1,8-
dioxooctahydroxanthene)s under solvent-free conditions
Masoud Nasr-Esfahani1,*, Zahra Rafiee1 and Hassan Kashi1
1Department of Chemistry, Yasouj University, Yasouj 75918-74831, Iran
*E-mail: manas@mail.yu.ac.ir
Abstract
series of 1,8-dioxooctahydroxanthene derivatives and some crowded bis(1,8-
A
dioxooctahdroxanthene) were selectively synthesized using tungstophosphoric acid nanoparticles
supported on polyamic acid (TPA/PAA) as a new catalyst in solvent-free conditions. The high
purity products were isolated and catalyst was easily separated in simple work-up and was
recycled several times without loss of reactivity under the described reaction conditions. The
reaction is characterized by short reaction time, high efficiency and environmentally friendly
reaction conditions.
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Keywords
Tungstophosphoric acid, Bis(1,8-dioxooctahdroxanthene), Polyamic acid, Supported catalyst,
Heterogeneous catalyst
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Introduction
Xanthene’s derivatives especially 1,8-dioxooctahydroxanthenes are an important class of
heterocyclic compounds. They have been found in a large number of naturally occurring
molecules. The synthesis of these compounds has attracted the attention of organic chemists due
to their wide ranging biological and therapeutic properties including antiviral,1 antibacterial2 and
photodynamic therapy.3 The xanthenes are pH sensitive fluorescent materials and they were used
for the visualization of biomolecular assemblies.4 Von Bayer synthesized xanthene and some its
derivatives by condensation reaction of resorcinol and phthalic anhydride in the presence of
sulfuric acid at 1871.5 After this report, many efforts were performed for development of
synthetic methods of 1, 8- dioxooctahydroxanthenes via condensation reactions of aldehydes and
1,3-cyclohexanediones. Several synthetic procedure have been reported including solvent or
solvent-free conditions,6-9 ultrasound and microwave-assisted,10-12 activation with
homogeneous13-17 or heterogeneous catalysts.18-25 Many of these catalysts such as H2SO4, HCl,
AlCl3, InCl3 and BF3 are homogeneous and commonly used for organic synthesis carried out in
laboratories and industries. However, some of these catalysts are toxic, corrosive, volatile and it
is often difficult to separate them from reaction mixture. In order to overcome all the drawbacks
in the use of environmentally harmful homogeneous catalysts, solid heterogeneous catalysts have
been tested such as montmorillonite K10,18 silica sulfuric acid,19 cellulose sulfonic acid,20 silica-
supported preyssler nano particles (SPNP),21 Dowex-50W,22 ZrOCl2.8H2O,23 VSA24 and
polyethylene glycol (PEG).25
Catalysis based on heteropoly acids (HPAs) have been used for synthesis of many organic
compounds.26-28 The major disadvantages of heteropoly acids are extremely hard recovery, water
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solubility and recyclability which always limit their practical utilization. Application of
heteropoly acids supported on polymer as heterogeneous catalysts in organic synthesis seems to
be the most attractive ways to avoid the above mentioned problems and referred to as eco-benign
catalysts for their non-polluting properties. In general, heteropoly acids supported on polymer
offer high atom efficiency, higher surface area, lower coordinating sites, easy product
purification, and simple reusability.29
In continuation of our interest in the application of heterogeneous catalysts to development of
useful synthetic methods,30 herein we report a simple and highly efficient route for the synthesis
of 1,8-dioxooctahydroxanthene derivatives using tungstophosphoric acid nanoparticles supported
on polyamic acid (TPA/PAA) as a new recoverable catalyst with high catalytic activity under
solvent-free conditions.
Results and Discussion
Characterization and stability of the catalyst
Initially, tungstophosphoric acid supported on polyamic acid (TPA/PAA) was prepared as a new
heterogeneous catalyst. FT-IR is a powerful technique for the study of surface interactions
between HPA and organic support. Figures 1a and 1b show the FT-IR spectra of pure TPA and
PAA, respectively.
In polyoxometalate molecules four types of addendum-oxygen connections are present that are
responsible for the appearance of peaks;31 oxygen atoms connecting bridge between the
heteroatom X and addendum M (common both octahedron coordination of the addendum and
heteroatom X polyhedron) (Oa), oxygen atoms connecting bridge between two metal addenda
atoms M located in two different sets of triplets M3O13 (Ob), oxygen atoms connecting bridge
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between two addenda atoms in the same group M3O13 (Oc) and terminal oxygen atoms belonging
to the MO6 octahedron (Od).
Pure acid compound displays infrared bands at 1080 (P-Oa), 1007-990 (three bond related to
symmetric and asymmetric vibrations of W-Od), 890 (W-Ob-W) and 810 cm-1 (W-Oc-W).32 In
addition, a broad, intense band centered around 3450 cm-1 (O-H stretching) and a weak
absorption at 1640 cm-1 (δH2O bending) indicates the presence of water. The FT-IR spectrum of
the 30% w/w of TPA on PAA shows four bands in the range of 1065-634 cm-1 related to Keggin
bands of heteropoly acid in TPA/PAA (Fig. 1c).
However, small shifts of νW=Od (980 cm-1) and νW-Oc-W (634 cm-1) vibrations were registered
indicating interactions of the support with the most external atoms Od and Oc of the Keggin
anion.
XRD for pure PAA and TPA/PAA are shown in Figure 2. According to the results, PAA is
totally amorphous in nature, which does not show any sharp diffraction peaks. While, sharp
diffraction peaks are observed in the XRD pattern of catalyst. A number of prominent Bragg
reflections reveal that the resultant particles of TPA have an orthorhombic structure.33 The size
of the TPA particles was also determined from X-ray line broadening using the Debye-Scherrer
formula (D = kλ/βcosθ). For the (210) reflection, the average size of the TPA particles was
estimated to be around 35 nm.
Figure 3 shows the morphology as obtained by FE-SEM micrographs of (a) Pure PAA and (b)
TPA/PAA which demonstrated a homogeneous nanostructure and confirmed that the surface of
TPA/PAA catalyst is in nanoscale with average size of 40 nm, confirming the results calculated
from Debye-Scherrer equation.
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The transmission electron microscopy (TEM) image of the TPA NPs/PAA was in good
agreement with FE-SEM and XRD analysis and successfully confirmed the supporting of
heteropoly acid nanoparticles on the polymer (Fig. 4).
Also energy dispersive X-ray spectroscopy (EDAX) results confirm the presence of phosphorus
and tungsten in the supported polymer (Figure 5).
The UV diffusion/reflectance spectra of heteropoly acid (TPA) and heteropoly acid supported on
polymer (TPA NPs/PAA) are exhibited at figure 6. The heteropoly acid spectrum show two
bonds at 222 and 263 nm (Fig. 6a) that can be assigned to internal transition of TPA. After
supporting of TPA on PAA, it was found that the same bonds appeared but with red shift by 2-12
nm (Fig. 6b). The similarity of electronic spectra of TPA and TPA/PAA well confirm retention
of the Keggin unit of TPA. On the other hand, the observed red shifts can be safely attributed to
interactions of TPA with functional groups of PAA.
Since the catalyst may be used at different temperatures, therefore, the thermal resistance should
also be investigated. For this purpose, the thermogravimetric analysis (TGA) was used, it was
observed that the TPA/PAA (fig. S1(2)) and polyamic acid (fig. S1 (1)) until to 300 °C remain
unchanged, but at higher temperatures up to 600 °C decomposed around 40 percent. The
remaining mass upper than 600 °C is attributed to weight percentage of polymer left
undecomposed under nitrogen atmosphere.34 From these data it is clear that supporting of
tungstophosphoric acid on polyamic acid, no change in thermal stability was caused. As shown
in Figure S 1 (Supplemental Materials), the weight losses% of PAA and TPA/PAA are the same.
From this behavior, it is obvious that this weight loss% is related to PAA not to TPA. TPA is
thermally stable solid but supported PAA on it is thermally decomposed similar to PAA alone in
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this range of temperature. TG diagrams also show the supported PAA is decomposed slower than
free PAA so that its main decomposition initiates at higher temperature (about 50 °C). This
means that TPA enhances thermal stability of supported PAA.
Based on the functional groups in the polymer structure (NH, C=O) and TPA (OH, P=O), it
seems that hydrogen bonding and some other polar forces are the most interaction between PAA
and TPA.35
Effect of solvent and catalyst concentration on the synthesis of 1,8-dioxooctahdroxanthene
In order to evaluate the catalytic activity of TPA/PAA, the reaction of benzaldehyde (2a) and
5,5-dimethyl-1,3-cyclohexanedione (dimedone) (1) was carried out as a model reaction. Reaction
various parameters such as solvent, catalyst loading, molar ratio of reactants, and temperature
were optimized.
For comparison of the results in solvent and solvent-free conditions, the condensation of
benzaldehyde (1 mmol) and dimedone (2 mmol) was studied in the presence of TPA/PAA (0.03
g of 30% w/w tungstophosphoric acid supported on polymer) as model reaction. As shown in
table 1, among the selected solvents, such as ethanol, acetonitrile and chloroform and a solvent-
free system, the best result was obtained after 30 min under solvent-free conditions in excellent
yield (Table 1, entries 4, 11-13).
For investigation of the effect of various amounts of TPA loading on PAA in the synthesis of
1,8-dioxooctahydroxanthene, different weight percents of heteropoly acid were used. Table 1
shows differences in catalytic activity among catalysts having 20-40 wt.% of TPA on PAA
(entries 3-5). Lowering the loading of the supported tungstophosphoric acid causes the reduction
of catalytic activity. Also no increments in the reaction rate and yield were observed by
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increasing the amount of acid on PAA from 30 to 40 wt.%. Since 30 wt.% of TPA was the best
catalyst loading, it was used to study the effect of various parameters on yields.
The catalyst concentration was varied over a range of 0.02-0.04 g (0.031-0.062 mmol of H+) on
the basis of the total volume of the reaction mixture (Table 1, entries 4, 9, 10). The results show
the effect of catalyst concentration on the reaction of benzaldehyde and dimedone. The yield of
the corresponding 1,8-dioxooctahydroxanthene increased with increasing catalyst concentration
from 0.031 to 0.047 mmol of H+. Further addition of catalyst had no noticeable effect on the
yield because there is an excess of catalyst sites over what is actually required by the reactant
molecules. Therefore, in all further reactions 0.047 mmol of H+ (equal to 0.016 mol% of TPA)
were used for 30 wt.% of tungstophosphoric acid (entry 4).
Moreover, the effect of temperature on the reaction rate as well as on yields of products was
investigated. The results showed that faster reactions are observed on increasing the temperature
to 100 ºC, meanwhile the product yield is decreased at higher temperatures (Table 1, entries 4, 6-
8). To improve the yields, in other investigations, the reaction is performed using different
quantities of reagents. The best results were obtained with a 1:2 ratio of benzaldehyde to
dimedone.
Hence, performing the reaction in solvent-free conditions and in the presence of 0.03 g of the
catalyst at 100 °C was determined as the optimal conditions. Using the optimized reaction
conditions, the scope and limitations of this method were evaluated via variety of aromatic
aldehydes. In general, aromatic aldehydes bearing electron-donating or electron-withdrawing
groups reacted smoothly with dimedone and within a short reaction time to generate 1,8-
dioxooctahydroxanthene derivatives with good to excellent yields (Table 2).
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Also, due to stereoelectronic effects, presence of substituent on 2-position of the benzaldehyde
ring causes decreasing of the rate of the condensation reaction (Table 2, entries 2, 6).
Surprisingly, it was observed that both mono- and bis-dioxooctahydroxanthenes can be
selectively obtained by this catalytic system. As shown in Table 2, mono- and bis-
dioxooctahydroxanthenes were produced from dialdehydes 4 and 7 (a bulky molecule) in 55-
90% yields with appropriate reaction times (25-35 min). The preparation of mono-
dioxooctahydroxanthenes from dialdehydes is of great interest because the remaining formyl
group can be converted to other functional groups.
Amines with three different substituents are chiral but racemization via pyramidal N-inversion is
often too fast for separation of enantiomers at room temperature. In general, the barrier to
nitrogen inversion decreases as the bulkiness of the substituents increases because of
destabilizing steric repulsion in the pyramidal ground state. Inversion at the sp3-hybridized
nitrogen occurs via a planar transition state. An increase in the p-character of the nitrogen bonds
therefore prevents N-inversion.41 Various factors have been identified that affect the activation
energy for this process. These include the nature of the inversion center itself, electronegativity
of substituents, steric bulk of substituents, conjugation, the presence of lone pairs on substituents,
and angular constraints.42
Due to these scientific principles, the optical rotation of 8 was investigated as a bulky chiral
amine and the change in orientation of monochromatic plane-polarized light (+23.5° observed
rotation using an amine solution with 0.3 g/ml concentration in CHCl3 as solvent) was
surprisingly observed. Although, the enantiomeric purity of the amine unknown.
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The dialdehyde 7 was synthesized using the Vilsmeyer reaction according to the literature
(Scheme 3).43 1-Bromobutane was reacted with diphenylamine in the presence of sodium
hydroxide in DMSO by heating at 110 °C to give N-butyl-N-phenylaniline (10). Under nitrogen
atmosphere, freshly distilled POCl3 was added to DMF at 0 °C, then the amine 10 was added to
this solution, and the resulting mixture was stirred at 95 °C. After cooling and basified with
sodium hydroxide solution, the resulting mixture was extracted and purified by column
chromatography to give 4,4'-(butylazanediyl)dibenzaldehyde (7).
It is noteworthy that the catalyst was recovered simply by filtration without any acidic or basic
workup even after its five uses. After completion the reaction of benzaldehyde and dimedone, the
catalyst was recovered from the reaction mixture and recovered catalyst was washed with
chloroform and ethanol and dried. The recovered catalyst was added to fresh substrates under the
same experimental conditions for five runs without a noticeable decrease in the product yield and
its catalytic activity (Figure S 2). Also FT-IR spectra of fresh and used TPA/PAA catalyst
confirmed that the structure of the catalyst remained unchanged after recycling.
As TPA/PAA is not soluble in ethanol, no catalyst leaching as well as no contribution of
homogeneous catalysis in the course of reaction was expected. To rule out the contribution of
homogeneous catalysis, the standard leaching experiment was conducted. A model reaction in
refluxing ethanol was preceded for 10 min in the presence of catalyst and then catalyst was
removed by filtration. The reaction was allowed to proceed without catalyst. There was no
change in the progress of the reaction even after 6 h, indicating that no leaching has been
occurred and therefore no homogeneous catalyst was involved.
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Comparative results
In order to show the ability of our method with respect to previous reports, some of our results in
comparison to some other methods are summarized in Table 3. As shown, the yield/time ratio of
the present method is better or comparable with the other reported results.
Experimental
Chemicals were purchased from Merck or Aldrich. IR spectra were recorded from KBr discs on
a JASCO FT-IR-680. NMR spectra were taken with a Bruker 400 MHz Ultrashield spectrometer
at 400 MHz (1H) and 100 MHz (13C) using CDCl3 or DMSO-d6 as the solvent with TMS as the
internal standard. Energy dispersive X-ray spectroscopy analyses (EDAX) were carried out on a
PHILIPS XL30, operated at a 20KV accelerating voltage. X-ray diffraction analysis was carried
out using a D8 ADVANCE, Bruker X-ray diffractometer using Cu-Kα radiation (λ = 1.5406 A˚).
Surface morphology and particle size were studied using a KYKY-EM-32000 SEM instrument.
Thermal degradation of polymers was carried out in the range 28-800 °C at a heating rate of 10
°C/min under nitrogen atmosphere using thermogravimetric analyzer-TG-209 F3, Netzsch,
Germany. The optical rotation obtained with Jasco p-1030 polarimeter. The melting points were
determined using an electrothermal digital melting point apparatus and are uncorrected. The
Supplemental Materials contains sample IR, 1H and 13C spectra for compounds 8 and 9 (Figures
S 3 – S 8)
Synthesis of polymer
3,5-Diamino-1,2,4-triazole (11, 0.50 g, 5.05 mmol) and freshly distilled NMP (4 mL) were added
to a 50 mL three necked round-bottomed flask equipped with a high-power electromagnetic
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stirrer and N2 inlet. A clear diamine solution formed after stirring for 5 min. Then, 3,3',4,4'-
benzophenonetetracarboxylic dianhydride (BTDA) 12 (1.10 g, 5.05 mmol) was added
immediately, followed by additional NMP (2 mL) to adjust the solid content of the mixture to be
20 wt%. The mixture was stirred at room temperature for 24 h to afford an almost colorless,
highly viscous solution. The inherent viscosity of the resulting PAA was 0.75 dl g-1, which was
measured in DMF at a concentration of 0.5 g dl-1 at 25 °C (Scheme 4).35
Preparation of TPA/PAA
PAA (5.0 g) in DMF (10 mL) was stirred at 50 °C. Tungstophosphoric acid (1.5 g) was dissolved
in deionized water and impregnated drop wise into mixture and it was stirred for 0.5 h at 50 °C
and then 24 h in room temperature. After evaporating the solvent in reduced pressure, a brown
solid was obtained. This solid was filtered and washed with distilled water. This was repeated
until the pH of effluent water neutralized. The resulting was titrated by sodium hydroxide
solution and determined the supported acid is 98 percent. The catalyst is 30% w/w from
heteropoly acid relative to PAA, with respect to the initial amount of polymer and acid. IR (KBr,
cm-1): 3320, 3064, 2925, 1785, 1727, 1589, 1450, 1346, 1292, 1098, 1065, 980, 836, 634.
Synthesis of N-butyl-N-phenylaniline (10)
1-Bromobutane (2.7 g, 0.02 mol), diphenylamine (2.0 g, 0.012 mol) and sodium hydroxide (4.0
g, 0.1 mol) were added in DMSO (30 mL), followed by heating at 110 C for 12 h. After cooling
to room temperature, the resulting mixture was extracted with ethyl acetate/water and then dried
with Na2SO4. The solvent was evaporated and the resulting crude liquid was purified by column
chromatography on neutral alumina by using hexane as solvent to give colorless liquid with yield
80%.43
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Synthesis of 4,4'-(butylazanediyl)dibenzaldehyde (7)
Freshly distilled POCl3 (23.1 mL, 25 eq) was added dropwise to DMF (17.6 mL, 23 eq) under N2
atmosphere at 0 C, and then it was stirred for 1 h. Compound 10 (2.23 g, 9.9 mmol) was added
to the above solution, and the resulting mixture was stirred for 4 h at 95 C. After cooling to room
temperature, the mixture was poured into a beaker containing ice-cube, and basified with 4 M
NaOH solution. The resulting mixture extracted with EtOAc/brine.
After evaporating the organic solvent, the crude product was purified by column chromatography
on neutral alumina using a mixture of ethylacetate/hexane (1:4, v/v), to give a pale green liquid
(2.5 g, yield = 90%).
General procedure for the synthesis of 1,8-dioxooctahydroxanthene
In a 25 mL flask, mixture of aldehyde (1 mmol), 5,5-dimethyl-1,3-cyclohexanedione (2 mmol)
and TPA/PAA (0.03 g) was heated in an oil bath at 100 °C under solvent-free conditions. After
completion of the reaction as monitored by TLC, the resulting solid was cooled; chloroform
added and stirred for 5 min. Then, the catalyst was filtered. The filtrate was kept overnight to
obtain the solid as crude product. The mixture was filtered and the crude product recrystallized
from EtOH to afford the pure product. The removed catalyst was washed with ethanol-
chloroform mixture and dried prior to reuse in subsequent reaction.
4-(butyl(4-(3,3,6,6-tetramethyl-1,8-dioxo-2,3,4,5,6,7,8,9-octahydro-1H-xanthen-9-
yl)phenyl)amino)benzaldehyde (8): Orange solid; 55% yield; mp: 72-74˚C; αD20 = +23.5° (c =
0.3 gr/ml, CHCl3); IR (KBr, cm-1): 2963, 2935, 2871, 1728, 1663, 1623, 1509, 1359, 1195, 1145;
1H NMR (400 MHz, CDCl3)  (ppm): 9.91 (1H, s, CHO), 7.81 (2H, d, J = 8.4 Hz, Ar-H), 7.09
(2H, d, J = 8.5 Hz, Ar-H), 6.91 (2H, d, J = 8.5, Ar-H), 6.76 (2H, d, J = 8.5 Hz, Ar-H), 4.73 (1H,
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s, CH), 3.53 (2H, t, J = 7.5 Hz, N-CH2), 2.48 (4H, s, 2×CH2), 2.19 (4H, s, 2×CH2), 1.51 (2H, m,
CH2), 1.25 (2H, m, CH2), 1.11 (6H, s, 2×CH3), 1.01 (6H, s, 2×CH3), 0.88 (3H, t, J = 7.3 Hz,
CH3); 13C NMR (100 MHz, CDCl3) δ (ppm): 201.2 (–C=O, aldehyde), 197.5 (C=O), 162.9
(C=C), 148.7, 138.1, 130.9, 128.7, 127.3, 113.8 (Ar-C), 112.8 (C=C), 50.1 (2×CH2), 46.1 (CH2),
40.9 (2×CH2), 37.7 (CH), 31.9 (2×C(CH3)2), 29.1 (CH2), 28.2 (4×CH3), 20.1 (CH2), 13.8 (CH3);
Anal. Calcd. C34H39NO4: C 77.68; H 7.48; N 2.66; O 12.17; found: C 77.78, H 7.41, N 2.73.
9,9'-((butylazanediyl)bis(1,4-phenylene))bis(3,3,6,6-tetramethyl-3,4,5,6,7,9-hexahydro-1H-
xanthene-1,8(2H)-dione) (9): Green-blue solid; 87% yield; mp: 225-227 °C; IR (KBr, cm-1):
1
2958, 2930, 1665, 1623, 1509, 1359, 1197, 1137; H NMR (400 MHz, CDCl3)  (ppm): 7.09
(4H, d, J = 8.5 Hz, Ar–H), 6.76 (4H, d, J = 8.5 Hz, Ar-H), 4.70 (2H, s, CH), 3.53 (2H, t, J = 7.5
Hz, N–CH2), 2.48 (8H, s, 4×CH2), 2.23 (8H, s, 4×CH2 ), 1.53 (2H, m, CH2), 1.26 (2H, m, CH2),
1.11 (12H, s, 4×CH3), 1.01 (12H, s, 4×CH3), 0.88 (3H, t, J = 7.3 Hz, CH3); 13C NMR (100 MHz,
CDCl3) δ (ppm): 196.6 (C=O), 162.1 (C=C), 146.2, 136.4, 128.8, 120.3 (Ar-C), 115.8 (C=C),
50.8 (4×CH2), 40.9 (CH2), 32.2 (4×CH2), 30.8 (2×CH), 29.6 (4×C(CH3)2), 29.2 (CH2), 27.5
(8×CH3), 20.2 (CH2), 13.9 (CH3); Anal. Calcd. C50H59NO6: C 77.99; H 7.72; N 1.82; O 12.47;
found: C 78.11, H 7.67, N 1.86.
Conclusions
In this study tungstophosphoric acid nanoparticles supported on polyamic acid (TPA/PAA) was
prepared as a new and heterogeneous catalyst and was successfully used for the improved
selective synthesis of 1,8-dioxooctahydroxanthenes and bis(1,8-dioxooctahdroxanthene)s under
solvent-free conditions. The mono- and bis-dioxooctahydroxanthenes as bulky systems can be
obtained by this catalytic system in good to excellent yields. The mild reaction conditions, rapid
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conversion, high yields, simple experimental procedure, stability of the catalyst are notable
advantages of the present method. Most importantly, we did not use organic solvents in reaction
process, and it helps the development of green technology.
Acknowledgments
We are grateful to the Yasouj University for financial assistance.
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Table 1. Optimization of the model reactiona
Entry
1
(Wt.%) Catalyst (g)
TPA (0.007)
Conditions Time (min.) Yieldb (%)
Neat/100 C
Neat/100 C
Neat/100 C
Neat/100 C
Neat/100 C
Neat/80 C
30
60
45
30
35
30
30
30
50
35
60
60
60
75
Trace
78
2
-
3
(20%) TPA/PAA (0.03)
(30%) TPA/PAA (0.03)
(40%) TPA/PAA (0.03)
(30%) TPA/PAA (0.03)
(30%) TPA/PAA (0.03)
(30%) TPA/PAA (0.03)
(30%) TPA/PAA (0.02)
(30%) TPA/PAA (0.04)
(30%) TPA/PAA (0.03)
(30%)TPA/PAA (0.03)
(30%)TPA/PAA (0.03)
4
93
5
89
6
70
7
Neat/90 C
85
8
Neat/110 C
Neat/100 C
Neat/100 C
EtOH/reflux
CH3CN/reflux
CHCl3/reflux
80
9
80
10
11
12
13
90
80
70
55
aReaction condition: dimedone (2 mmol), benzaldehyde (1 mmol),
bIsolated yields.
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Table 2. Synthesis of 1,8-dioxooctahydroxanthenes catalyzed by TPA/PAA under solvent-
free conditionsa
entry
1
Aldehyde
C6H5
Product Time(min) Yield(%)b Mp (ºC)c Ref.
30
35
30
45
35
35
35
75
50
50
45
60
30
30
35
93
80
90
80
90
87
85
65
85
80
75
60
85
90
55
202-204
217-219
173-145
224-227
183-184
226-228
241-243
250-253
168-170
226-228
62-64
36
37
38
37
10
37
36
36
36
36
37
39
40
12
-
3a
3b
3c
3d
3e
3f
3g
3h
3i
2
4-CH3C6H4
CH3
3
4
2-ClC6H4
3-ClC6H4
4-ClC6H4
4-CH3OC6H4
2-NO2C6H4
3-NO2C6H4
4-NO2C6H4
Furyl
5
6
7
8
9
10
11
12
13
14
15
3j
3k
3l
Thienyl
197-198
211-213
274-276
72-74
4-CHOC6H4
4-CHOC6H4
5
6d
8
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16
9d
25
87
225-227
-
aReaction conditions: dimedone (2 mmol), aldehyde (1 mmol), TPA/PAA (0.03 g), 100 °C
bMelting points are uncorrected
cIsolated yield
ddimedone (4 mmol), TPA/PAA (0.06 g), 100 °C.
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Table 3. Comparison of the efficiency of various catalysts in the synthesis of 3,3,6,6-
tetramethyl-9-phenyl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (3a)
entry
catalyst
Cellulose Sulfonic Acid
Montmorillonite-K10
Dowex-50W
conditions
Solvent-free,110 °C
Solvent-free,120 °C
Solvent-free,100 °C
H2O, US-assisted,30 °C
H2O/Reflux
Time(min) Yield(%) Ref.
1
2
3
4
5
6
7
300
120
180
60
94
82
87
89
90
87
93
20
18
22
10
14
13
-
p-DBSA
DABCO-bromine
InCl3·4H2O
60
Ionic liquid,80 °C
Solvent-free,100 °C
240
30
TPA/PAA NPS
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Figure 1. FT-IR spectra of TPA (a), PAA (b) and TPA/PAA (c)
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Figure 2. XRD curves of (a) Pure PAA (b) TPA/PAA.
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Figure 3. FE-SEM micrographs of (a) Pure PAA (b) TPA/PAA.
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Figure 4. Transmission electron microscopy (TEM) image of TPA/PAA
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Figure 5. Energy dispersive X-ray spectroscopy (EDAX) pattern of TPA/PAA
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