Generation of brnsted acidic ionic liquid by keggin heteropoly acid and its application in the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes

Article abstract of DOI:10.1080/15533174.2011.568449

A new waterstable Brnsted acidic ionic liquid based on Keggin heteropoly acid was used as an environmentally benign catalytic medium in the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes under mild reaction conditions. The catalyst can be easily recovered

Full text of DOI:10.1080/15533174.2011.568449

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Generation of Brønsted Acidic Ionic Liquid by Keggin
Heteropoly Acid and its Application in the Synthesis
of 14-Aryl-14H-dibenzo[a,j]xanthenes
a
b
b
Majid M. Heravi , Fatemeh F. Bamoharram & Niloofar Tavakoli-Hoseini
a
Department of Chemistry, School of Sciences , Alzahra University , Vanak , Tehran ,
Iran
b
Department of Chemistry, Mashhad Branch , Islamic Azad University , Mashhad , Iran
Published online: 11 Jul 2011.
To cite this article: Majid M. Heravi , Fatemeh F. Bamoharram & Niloofar Tavakoli-Hoseini (2011) Generation
of Brønsted Acidic Ionic Liquid by Keggin Heteropoly Acid and its Application in the Synthesis of 14-Aryl-14H-
dibenzo[a,j]xanthenes, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41:6, 616-620,
DOI: 10.1080/15533174.2011.568449
To link to this article: http://dx.doi.org/10.1080/15533174.2011.568449
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41:616–620, 2011
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2011.568449
Generation of Brønsted Acidic Ionic Liquid by Keggin
Heteropoly Acid and its Application in the Synthesis of
1
4-Aryl-14H-dibenzo[a,j]xanthenes
1
2
2
Majid M. Heravi, Fatemeh F. Bamoharram, and Niloofar Tavakoli-Hoseini
1
Department of Chemistry, School of Sciences, Alzahra University, Vanak, Tehran, Iran
Department of Chemistry, Mashhad Branch, Islamic Azad University, Mashhad, Iran
2
acids and stronger acids than conventional solid acids such as
A new waterstable Brønsted acidic ionic liquid based on Keggin SiO2–Al2O3, H3PO4, HNO3, HF, H2SO4, and HX and HY ze-
heteropoly acid was used as an environmentally benign catalytic olites.[ These compounds have several advantages that make
3,4]
medium in the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes un-
der mild reaction conditions. The catalyst can be easily recovered
and reused without appreciable loss of activity.
[5–7]
them economically and environmentally attractive.
These
solid acids are “green” with respect to corrosiveness, safety,
quantity of waste, and separability. Moreover, while some of
the acid catalysts such as HCl, HNO3, H2SO4, etc., can pro-
duce chlorated, nitrated, sulfated, etc. by-products, HPAs do
not produce any of these by-products. HPAs have been exten-
sively studied as green acid catalysts for many reactions, and
found industrial applications in several processes.[ Many re-
actions catalyzed by Brønsted and Lewis acids, but, in the pres-
ence of HPAs, proceed more effectively under milder conditions
with greater selectivity, better yields, and shorter reaction times.
Among HPAs, the application of Keggin structures has been
Keywords Brønsted acidic ionic liquid, dibenzoxanthene, green
chemistry, Keggin heteropoly anion
8,9]
INTRODUCTION
Recently, ionic liquids (ILs) have gained much attention as
designer solvents and catalysts for a diversity of chemical ap-
plications. The reason for this interest is the enormous selection
of weakly bonding anion and cation combinations, which make
up this special class of low melting salts.
[
10–12]
extensively studied.
[
The Keggin structure with formula of
n−
XM12O40] is shown in Figure 1.
One of the most important ILs is SO3H-functionalized ionic
liquids. For synthesis of these ILs, first a N-alkylimidazole (or
N-alkylpyrrolidine, pyridine, etc.) is reacted with 1,4-butan(or
propan)sultone to obtain a solid zwitterion. This zwitterion is
then protonated with a Brønsted acid such as sulfuric acid and
phosphoric acid to obtain the corresponding ionic liquid. Al-
though the mentioned IL shows satisfactory behavior, it needs
Pursuing these studies, and in continuation of our pre-
[
13–15]
vious works on the applications of Keggin HPAs
and
ILs[
16–18]
in the synthesis of organic compounds, in the present
work for the first time, we used H3PW12O40 as a Brønsted
acid for the synthesis of an ionic liquid and then investi-
gated its catalytic activity in the synthesis of 14-aryl-14H-
dibenzo[a,j]xanthenes.
[
1,2]
hazardeus acids as starting materials.
To the best of our
The synthesis of xanthenes, especially dibenzoxanthenes, has
received significant attention in recent years because of their
wide range of biological and pharmaceutical properties, such as
knowledge, protonation of such zwitterions with heteropoly
acids has not yet been reported in the literature.
The principles of green chemistry have been introduced to
eliminate or reduce the use of hazardous materials such as
H2SO4 or H3PO4 in chemical processes. Cleaner technologies
could be possible by making use of environmentally friendly
materials involving the use of solid acids. It was shown that
heteropolyacids (HPAs) in the solid state are pure Brønsted
anti-viral,[ anti-bacterial, and anti-inflammatory activi-
19]
[20]
[21]
[
22]
ties, as well as efficacy in photodynamic therapy and antag-
onists for the paralyzing acting of zoxazolamine.[
23]
EXPERIMENTAL
All compounds were known, and their physical and spec-
troscopic data were compared with those of authentic samples
and found to be identical. Melting points were recorded on an
Received 15 September 2010; accepted 28 January 2011.
Address correspondence to Majid M. Heravi, Department of Chem-
istry, School of Sciences, Alzahra University, Vanak, Tehran, I. R. Iran. electrothermal type 9100 melting point apparatus. The IR spec-
E-mail: mmh1331@yahoo.com
tra were obtained on a 4300 Shimadzu spectrophotometer as
616

SYNTHESIS OF 14-ARYL-14H-DIBENZO[A,J]XANTHENES
617
FIG. 1. The structure of the Keggin heteropoly anion [XM12O4
0 n−
]
in polyhedral (left), ball-and-stick (middle), and space-filling (right) representations (color
figure available online).
1
KBr disks. The H NMR (500 MHz) spectra were recorded on
Bruker DRX500 spectrometer.
For preparation of this IL, first, pyridine was reacted with
1,4-butansultone to give a solid zwitterion (1). Interestingly,
when we mixed two solids including 1 and H3PW12O40, a
liquid formed after 20 min at 60 C. It is worthwhile to men-
tion that protonating of zwitterion (1) with H2SO4 as Brønsted
◦
Preparation of IL
Pyridine (0.2 mol) and 1,4-butane sultone (0.2 mol) were
charged into a 100 mL round-bottom flask. Then, the mixture
acid need, longer reaction time (6 h) and higher temperature
◦
[1,2]
◦
(80 C).
also required temperature. Figure 2A and 2B show the IR spectra
of H PW O and IL.
3 12 40
Therefore, H PW O not only reduced time, but
was stirred at 40 C for 10 h. The obtained white solid zwitterion
was washed repeatedly with ether to remove non-ionic residues
and dried in vacuum. Then, H3PW12O40, Keggin heteropoly
acid was added and the mixture stirred for 20 min at 60 C to
[
1]
3
12 40
◦
4
The [XM O ] (Keggin structure) consists of one XO tetra-
12 40
IV
IV
V
hedron (X = Si , Ge , P , . . .) surrounded by four M3O13 sets
form the IL.
VI
VI
(
M = W , Mo ) linked together through oxygen atoms (named
M-Ob-M). The XO4 tetrahedron is assumed to vibrate almost
independently from the rest of the anion. This assumption is
Preparation of 14-Aryl-14H-dibenzo[a,j]xanthene
Derivatives: General Procedure
A mixture of β-naphthol (2 mmol), aromatic aldehyde
1 mmol) and IL (0.5 mL) was stirred at 100 C for 3–14 min.
The progress of the reactions was monitored by TLC (ethyl
acetate/n-hexane 3/8). After cooling to room temperature, water
10 mL) was added and the mixture was stirred for 5 min. The
precipitated product was filtered and purified by crystallization
from ethanol. The IL was recovered by evaporation of the wa-
ter from the filtrate, washed with diethyl ether, dried at 50 C
V
particularly valuable for X = P . The symmetric and asym-
◦
metric stretching of the different kinds of M-O bonds are ob-
served in the following spectral regions: M-Od bonds (1000–960
(
−1
cm ), M-Ob-M bridges (inter bridges between corner-sharing
octahedra) (890–850 cm ), M-Oc-M bridges (“intra” bridges
between edge-sharing octahedra, (800–760 cm ). Only the
M-Od stretching can be considered as pure vibrations: the
−1
(
−1
◦
b c
stretching involving O or O atoms present some bend charac-
ter.[24,25]
under vacuum for 1 h and reused in another reaction without
appreciable reduction in the catalytic activity.
In our study, the IR spectrum of the formed IL (Figure 2B)
exhibits the characteristic frequencies of the Keggin structure
−1
in the range 1100–600 cm . Compared with the initial Keggin
structures (Figure 2A), the bands arising from the HPAs change
RESULTS AND DISCUSSION
1
4-Aryl-14H-dibenzo[a,j]xanthenes have been synthesized obviously either in intensity, or in position. Comparing the IR
from the reaction of various aldehydes with β-naphthol in the spectrum of the IL with the IR spectrum of H3[PW12O40] (Fig-
presence of a new and green IL based on Keggin heteropoly ure 2A and 2B), the vibrational band of the M-Oc-M of the
acid (Scheme 1).
Keggin is split from 793.88 into two bands, including 728.73
SCH. 1.

618
M. M. HERAVI ET AL.
FIG. 2. (A) IR spectrum of the Keggin, H3 [PW12 O40]; and (B) IR spectrum of IL formed via interaction of Keggin with 1.
and 820.20, due to the difference in the M-Oc-M bonds. The dicate that many of the bonds were strengthened and the oth-
vibrational band of the P-O is split from 1080.22 into two ers were weakened. It means that the HPA has a intraction
bands, including 1040.96 and 1079.43. This means that the with 1.
−1
inter bridges between edge-sharing octahedra (800–760 cm )
It is suggested that the Keggin heteropoly acid, as a Brønsted
and the P-O stretchings have been affected. These results in- acid, protonates the zwitterion to produce an acidic IL that

SYNTHESIS OF 14-ARYL-14H-DIBENZO[A,J]XANTHENES
619
TABLE 1
Comparison of efficiency of various catalysts in synthesis of dibenzoxanthenes
Catalyst
Conditions
Time (h)
Yield (%)
Ref.
◦
H2SO4
AcOH/80 C
73
15–24
6–12
2.5–5
0.5–2
0.5–1
0.5–2
6
60–90
83–95
90–95
82–95
80–94
74–91
50–99
None
[26]
[27]
[28]
[29]
[30]
[31]
[32]
p-Toluene sulfonic acid
Sulfamic acid
I2
Amberlyst-15
NaHSO4
ClCH2CH2Cl/reflux
Solvent-free/125 C
Solvent-free/90 C
Solvent-free/125 C
Solvent-free/90 C
Solvent-free/90 C
EtOH/reflux
Solvent-free/100 C
◦
◦
◦
◦
◦
Preyssler[NaP5W30O110]
Keggin[H3PW12O40]
this work
[33]
this work
◦
[MIMPS]HSO4
0.1–0.5
3–14(min)
85–96
87–98
◦
[
(CH2)4SO3HPY]Keggin
Solvent-free/90 C
can be used as catalyst in the synthesis of 14-aryl-14H- CONCLUSION
dibenzo[a,j]xanthenes.
In summary, a new and green Brønsted acidic IL based
Our efforts were directed toward investigating the catalytic on Keggin anion was successfully used as effective and green
behavior of this new IL as a green medium for synthesis of medium for the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes
14-aryl-14H-dibenzo[a,j]xanthenes. The synthesis of diben- under smooth conditions. The products were easily separated
zoxanthenes has been achived by the reaction of aldehydes with high yields. The IL catalyst was readily recycled and reused
with β-naphthol by the dehydration, and the results have to produce almost identical results. No organic solvent was used,
been compared with the other reported methods.[
26–33]
The resulting in eco-friendly process. The use of this new novel IL
results are shown in table 1. As can be seen, the IL based in this reaction provides a better and practical alternative to the
on Keggin and 1 catalyzed the reaction and reduced reaction existing procedures and provides great promise toward further
time significantly. Excellent yields and selectivity were useful applications. This process will pave a way for large scale
obtained with different substrates (Table 2). A comparison applications of synthesis of dibenzoxanthenes. Further applica-
between catalytic activity of Keggin and IL showed that tions for other reaction systems are currently under investiga-
the Keggin structure cannot catalyze reaction even after tion.
6
h.
In order to investigate the possibility of recycling of this
Brønsted acidic IL, a recovery experiment was conducted. The
IL could be reused at least four times with only slight reduction
in catalytic activity.
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TABLE 2
Synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes catalyzed by [(CH2)4SO3HPY] Keggin
◦
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3
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