An efficient and eco-friendly synthesis of 14-aryl-14H-dibenzo[a,j] xanthenes using H4[SiW12O40] as a heterogeneous and reusable catalyst under solvent-free conditions

Article abstract of DOI:10.1016/j.cclet.2011.10.003

A simple and efficient catalytic protocol for the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes via the one-pot condensation of β-naphthol and aryl aldehydes using silicotungstic acid (H ₄[SiW₁₂O₄₀]) is reported. The pres

Full text of DOI:10.1016/j.cclet.2011.10.003

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Chinese Chemical Letters 23 (2012) 17–20
www.elsevier.com/locate/cclet
An efficient and eco-friendly synthesis of 14-aryl-14H-
dibenzo[a,j]xanthenes using H4[SiW12O40] as a heterogeneous
and reusable catalyst under solvent-free conditions
*
S. Allameh , A. Davoodnia, A. Khojastehnezhad
Department of Chemistry, Faculty of Sciences, Islamic Azad University, Mashhad Branch, Mashhad 91735-413, Iran
Received 26 May 2011
Available online 8 November 2011
Abstract
A simple and efficient catalytic protocol for the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes via the one-pot condensation of
b-naphthol and aryl aldehydes using silicotungstic acid (H4[SiW12O40]) is reported. The present method offers several advantages
such as high to excellent yields, short reaction times, recovery and reusability of catalyst, mild reaction conditions and easy workup
procedures.
# 2011 S. Allameh. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Dibenzo[a,j]xanthenes; Reusable catalyst; Solvent-free conditions; b-Naphthol; Aryl aldehydes
Solid acids have attracted much attention in organic synthesis owing their easy work-up procedures, easy filtration
and minimization of cost and waste generation due to reuse and recycling of these catalysts [1]. The application of
heteropolyacids as catalytic materials is growing continuously in the catalytic field. These compounds possess unique
properties such as well-defined structure, Brønsted acidity, possibility to modify their acid–base and redox properties
by changing their chemical composition (substituted HPAs), ability to accept and release electrons, high proton
mobility, being environmentally benign, ease of reusability, excellent thermal and chemical stability and presenting
fewer disposal problems [2]. Because of their stronger acidity, they generally exhibit higher catalytic activity than
conventional catalysts for example mineral acids, ion exchange resins, mixed oxides zeolites, etc. [3]. They are used as
industrial catalysts for several liquid phase reactions [4–7].
The synthesis of xanthenes, especially benzoxanthenes, has received significant attention in recent years because of
their wide range of biological and pharmaceutical properties such as antiviral [8], antibacterial [9] and anti-
inflammatory [10] activities as well as efficacy in photodynamic therapy [11] and antagonists for the paralyzing acting
of zoxazolamine [12]. Furthermore, these compounds can be used as dyes [13] and pH-sensitive fluorescent materials
[14]. Literature reveals number of synthetic methods for preparation of xanthenes and benzoxanthenes including
cyclodehydrations [15], trapping of benzynes by phenols [16], cyclocondensation between 2-hydroxyaromatic
aldehydes and 2-tetralone [17] and the reaction of b-naphthol with aldehydes or acetals under acidic conditions [18].
* Corresponding author.
E-mail addresses: sadegh_allameh@yahoo.com, allameh0161@mshdiau.ac.ir (S. Allameh).
1001-8417/$ – see front matter # 2011 S. Allameh. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2011.10.003

18
S. Allameh et al. / Chinese Chemical Letters 23 (2012) 17–20
Ar
OH
+
H4SiW12O40
Ar-CHO
2
solvent - free
O
2 a -j
1
3 a -j
Scheme 1.
Although various procedures are reported for synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes, some drawbacks such
as low yields, prolonged reaction time, use of costly reagents or catalysts and use of toxic organic solvents, exist. Thus,
the development of an environmentally benign methodology for the synthesis of benzoxanthene derivatives is in great
demand.
It is therefore of interest to examine the behavior of H4[SiW12O40] as catalyst in solvent-free conditions for
synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes. To the best of our knowledge, condensation of b-naphthol and aryl
aldehydes in the presence of a catalytic amount of H4[SiW12O40] for the synthesis of 14-aryl-14H-
dibenzo[a,j]xanthenes has not been reported in literature. Herein we report the use of H4[SiW12O40] for synthesis
of benzoxanthene derivatives (Scheme 1).
Initially, in order to find the optimal amount of catalyst, the reaction of benzaldehyde (1 mmol) with b-naphthol
(2 mmol) under solvent-free condition at 110 8C in the presence of various amount of H4[SiW12O40] were used as a
model reaction. The best result has been obtained at 0.015 mmol (0.045 g) of catalyst (Table 1).
The generality of this process was demonstrated by the wide range of substituted aryl aldehydes to synthesize the
corresponding products in high to excellent yields (Table 2). Unlike some previously reported methods, the present
method does not require toxic organic solvents to produce the 14-aryl-14H-dibenzo[a,j]xanthene derivatives. All the
products were characterized by NMR, IR and melting point and also by comparison with the data reported in literature.
Concerning the reaction mechanism, we proposed that a carbocation is initially formed, aryl-methanebisnaphthols
are then formed in the second step, which then undergo dehydration to give the final product (Scheme 2).
Table 1
Comparison of the amount of H4[SiW12O40] and yields for synthesis of 14-phenyl-14H-dibenzo[a,j]xanthenes.
Entry
Catalyst (g)
Time (min)
Yield (%)
1
2
3
4
5
0
60
45
30
15
15
Nil
40
60
95
96
0.015
0.030
0.045
0.060
Table 2
Synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes using H4[SiW12O40] under solvent-free conditions.
Entry
Ar
Product
Time (min)
Yield%
Mp (8C)
Lit. Mp (8C)
1
2
C6H5
3a
3b
3c
3d
3e
3f
15
12
12
14
18
20
18
20
13
10
95
97
93
94
92
93
93
91
90
93
183–185
296–298
290–291
238–240
243–245
138–140
202–205
228–230
210–212
310–313
182–183 [19]
295–296 [19]
287–288 [19]
239–240 [19]
242–243 [19]
139–140 [19]
202–203 [19]
227–228 [19]
210–211 [19]
310–311 [19]
4-Br–C6H4
4-Cl–C6H4
4-F–C6H4
3
4
5
3-HO–C6H4
4-HO–C6H4
4-MeO–C6H4
4-Me–C6H4
3-NO2–C6H4
4-NO2–C6H4
6
7
3g
3h
3i
8
9
10
3j

S. Allameh et al. / Chinese Chemical Letters 23 (2012) 17–20
19
OH
Ar
OH
Ar
+
H
H
+
O
H+
O
OH
H+
OH
Ar
H
Ar
H+
-
-H2O
OH
Ar
Ar
O
-H2O
H+
-
OH OH
Scheme 2.
We also investigated the reusability of the catalyst. For this purpose after completion of the model reaction, the
mixture was cooled to 25 8C and cold ethanol was added. The catalyst is soluble in cold ethanol and could therefore be
recycled as the filtrate. The catalyst was recovered by evaporation of the ethanol and reused in three reactions with the
same substrate without appreciable reduction in the catalytic activity. The results of the first experiment and
subsequent experiments were almost consistent in yields (91, 88 and 84%).
1. Experimental
Melting points were determined on an Electrothermal type 9100 melting point apparatus. IR spectra were recorded
1
using a 4300 Shimadzu spectrophotometer with KBr plates. H NMR spectra were recorded on a Bruker DRX500
spectrometer. 13C NMR spectra were determined on a Bruker AC 100 spectrometer. The products (3a–3j) were
isolated and characterized by comparison of physical and spectral data with those of known samples.
1.1. General procedure for the preparation of 14-aryl-14H-dibenzo[a,j]xanthenes (3a–3j)
A mixture of b-naphthol 1 (2 mmol), aromatic aldehyde 2a–j (1 mmol) and H4[SiW12O40] (0.045 g) was heated on
the oil bath at 110 8C for 10–20 min. After completion of reaction (monitored by TLC petroleum ether/ethylacetate, 2/
1) the reaction mass was cooled to room temperature and cold ethanol was added. The product was filtered and
recrystallized from ethanol to give compounds (3a–j) in high yields.
1.2. 14-Phenyl-14H-dibenzo[a,j]xanthene (3a)
1H NMR (500 MHz, CDCl3): d 6.53 (s, 1H, CH), 7.04 (t, 1H, J = 7.4 Hz, Ar–H), 7.19 (t, 2H, J = 7.6 Hz, Ar–H),
7.45 (t, 2H, J = 7.0 Hz, Ar–H), 7.54 (d, 2H, J = 8.9 Hz, Ar–H), 7.58 (d, 2H, J = 7.3 Hz, Ar–H), 7.62 (td, 2H, J = 7.0,
1.2 Hz, Ar–H), 7.83 (d, 2H, J = 8.9 Hz, Ar–H), 7.87 (d, 2H, J = 8.0 Hz, Ar–H), 8.44 (d, 2H, J = 8.5 Hz, Ar–H). 13
C
NMR (100 MHz, CDCl3): d 38.2, 117.4, 118.2, 122.8, 124.4, 126.53, 126.9, 128.4, 128.6, 128.9, 129.0, 131.2, 131.6,
145.2, 148.8. IR (KBr disc) y: 3074, 3020, 2886, 1622, 1591, 1513, 1455, 1430, 1401, 1251, 1152, 1078, 1028, 962,
857, 827, 743, 700 cmÀ1
.
1.3. 14-(4-Chlorophenyl)-14H-dibenzo[a,j]xanthene (3c)
1H NMR (500 MHz, CDCl3): d 6.51 (s, 1H, CH), 7.14 (d, 2H, J = 8.3 Hz, Ar–H), 7.22–7.55 (m, 6H, Ar–H), 7.62 (t,
2H, J = 7.6 Hz, Ar–H), 7.84 (d, 2H, J = 8.9 Hz, Ar–H), 7.87 (d, 2H, J = 8.0 Hz, Ar–H), 8.35 (d, 2H, J = 8.4 Hz, Ar–H).
13C NMR (100 MHz, CDCl3): d 39.9, 117.5, 118.2, 123.8, 125.1, 127.6, 128.9, 129.2, 129.8, 130.2, 131.2, 131.3,
131.4, 145.0, 148.5. IR (KBr) y: 3066, 2922, 1622, 1590, 1514, 1456, 1397, 1245, 1237, 1209, 1140, 1065, 959, 812,
775, 746 cmÀ1
.

20
S. Allameh et al. / Chinese Chemical Letters 23 (2012) 17–20
1.4. 14-(4-Methylphenyl)-14H-dibenzo[a,j]xanthene (3h)
1H NMR (500 MHz, CDCl3): d 2.04 (s, 3H, CH3), 6.66 (s, 1H, CH), 6.92 (d, 2H, J = 8.0 Hz, Ar–H), 7.45 (t, 2H,
J = 7.4 Hz, Ar–H), 7.48 (d, 2H, J = 8.1 Hz, Ar–H), 7.54 (d, 2H, J = 8.8 Hz, Ar–H), 7.61 (t, 2H, J = 7.2 Hz, Ar–H), 7.91
(d, 2H, J = 8.9 Hz, Ar–H), 7.92 (d, 2H, J = 7.7 Hz, Ar–H), 8.65 (d, 2H, J = 8.5 Hz, Ar–H). 13C NMR (100 MHz,
CDCl3): d 20.2, 37.1, 117.3, 117.4, 117.5, 123.3, 124.3, 126.7, 127.7, 128.4, 128.8, 130.8, 135.9, 142.5, 147.8, 147.9.
IR (KBr disc) y: 3466, 3070, 3020, 2904, 1622, 1590, 1510, 1398, 1245, 1111, 1077, 961, 811, 778, 741 cmÀ1
.
1.5. 14-(3-Nitrophenyl)-14H-dibenzo[a,j]xanthene (3i)
1H NMR (500 MHz, CDCl3): d 6.63 (s, 1H, CH), 7.32 (t, 1H, J = 8.0 Hz, Ar–H), 7.47 (t, 2H, J = 7.4 Hz, Ar–H),
7.55 (d, 2H, J = 8.9 Hz, Ar–H), 7.65 (t, 2H, J = 7.4 Hz, Ar–H), 7.80–7.92 (m, 6H, Ar–H), 7.34 (d, 2H, J = 8.5 Hz, Ar–
H), 8.45 (s, 1H, Ar–H). 13C NMR (100 MHz, CDCl3): d 37.8, 115.9, 118.2, 121.8, 122.1, 122.8, 124.7, 127.3, 129.2,
129.6, 129.7, 131.1, 134.4, 147.0, 148.3, 148.8. IR (KBr disc) y: 3038, 1610, 1582, 1515, 1246, 810, 800 cmÀ1
.
1.6. 14-(4-Nitrophenyl)-14H-dibenzo[a,j]xanthene (3j)
1H NMR (500 MHz, CDCl3): d 6.63 (s, 1H, CH), 7.47 (t, 2H, J = 7.5 Hz, Ar–H), 7.54 (d, 2H, J = 8.9 Hz, Ar–H),
7.63 (t, 2H, J = 7.6 Hz, Ar–H), 7.70 (d, 2H, J = 8.6 Hz, Ar–H), 7.84–7.91 (m, 4H, Ar–H), 8.02 (d, 2H, J = 8.6 Hz, Ar–
H), 8.31 (d, 2H, J = 8.4 Hz, Ar–H). 13C NMR (100 MHz, CDCl3): d 36.4, 116.2, 117.7, 123.1, 123.6, 124.7, 127.2,
128.7, 129.6, 130.7, 130.8, 135.1, 145.9, 148.1, 152.6. IR (KBr) y: 2925, 1590, 1516, 1335, 1236, 828 cmÀ1
.
2. Conclusion
It should be noted that, this method is effective for the preparation of 14-aryl-14H-dibenzo[a,j]xanthenes form
condensation of b-naphthol and aryl aldehydes using H4[SiW12O40]. Comparison of the obtained results shows that
aromatic aldehydes with electron-withdrawing groups are reactants, the reaction time is shorter than that with
electron-donating groups.
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