Highly efficient and environmentally friendly preparation of 14-Aryl-14H dibenzo[a,j]xanthenes catalyzed by tungsto-divanado-phosphoric acid

Article abstract of DOI:10.1016/S1872-2067(11)60387-2

A rapid and efficient procedure for the preparation of various aryl-14H-dibenzo[a,j]xanthenes was reported. The method developed produced excellent yields via one-pot condensation of β-naphthol with various aryl-aldehydes in the presence of Keggin vanadiu

Full text of DOI:10.1016/S1872-2067(11)60387-2

CHINESE JOURNAL OF CATALYSIS
Volume 33, Issue 6, 2012
Online English edition of the Chinese language journal
Cite this article as: Chin. J. Catal., 2012, 33: 962–969.
ARTICLE
Highly Efficient and Environmentally Friendly Preparation of
14-Aryl-14H-dibenzo[a,j]xanthenes Catalyzed by
Tungsto-divanado-phosphoric Acid
Reza TAYEBEE*, Shima TIZABI
Department of Chemistry, School of Sciences, Hakim Sabzevari University, Sabzevar 96179-76487, Iran
Abstract: A rapid and efficient procedure for the preparation of various aryl-14H-dibenzo[a,j]xanthenes was reported. The method de-
veloped produced excellent yields via one-pot condensation of ꢀ-naphthol with various aryl-aldehydes in the presence of Keggin vanadium
substituted heteropolyacid, H5PW10V2O40, as catalyst under solvent free conditions. The present methodology therefore offered several
advantages but not limited to excellent yields (82%–98%), short reaction times (30–50 min), mild reaction conditions, simple work-up, as
well as the utilization of cheap and environmentally benign catalyst in the absence of organic solvents.
Key words: xanthene; one-pot; condensation; aldehyde; ꢀ-naphthol; heteropolyacid; Keggin; solvent free
Xanthene derivatives are biologically important compounds
with wider biological applicability including anti-depressants,
antibacterial, and antiviral agents [1]. These compounds also
have antagonist activity towards paralyzing action of zoxa-
zolamine [2] and in photodynamic therapy [3]. Because with
ampicilin and chlotrimazole, xanthenediones also have poten-
tial antimicrobial activity against Staphlococcus aureus and
Candida albicans [4]. Different procedures have been used to
produce xanthenes and benzoxanthenes, and this includes
the reaction of aryloxymagnesium halides with triethylortho-
formate [5], cyclodehydration [6], intra molecular phenyl
carbonyl coupling reactions of benzaldehydes and acetophe-
nones [7], cyclization of polycyclic aryltriflate esters [8], and
cyclocondensation of 2-hydroxy aromatic aldehydes with
2-tetralone [9]. However, many of the aforementioned proce-
dures have drawbacks such as long reaction times, unsatisfac-
tory yields, harsh reaction conditions, and excessive use of
reagents and catalysts. This paper therefore reports a novel and
convenient method for the production of these pharmaceuti-
cally important compounds in large quantities for utilization in
testing their pharmacological activities.
various homogeneous and heterogeneous organic transforma-
tions in applications ranging from fine chemical industries to
pharmaceuticals and food industries [10,11]. These compounds
possess a strong Brönsted acid activity approaching the super
acidic region, a region that is more potent than many mineral
acids and conventional solid acids such as amorphous SiO2,
Al2O3, H3PO4/SiO2, HX, and HY zeolites [12]. The advantages
afforded by applying Keggin-type heteropolyacids reactions
include strong acidities, lower proportion of side reactions, and
production of non toxic wastes. Following the diverse appli-
cations of heteropolyacids in organic synthesis, we have ex-
tended their excellent catalytic applications in the preparation
of bis-indolyl-methanes [13], acetylation of alcohols [14],
Biginelli reaction [15], oxidation of aromatic amines [16], and
tetrahydropyranylation of alcohols [17]. The use of heter-
opolyacids, as catalysts for fine organic synthetic processes, is
to develop and synthesize antioxidants, medicinal preparations,
vitamins, and biologically active substances as reported pre-
viously [18].
Heteropolyacids are generally constructed from their pri-
mary, secondary, and tertiary building blocks. The primary
unit, which is the most common, is a thermally stable poly
nuclear structure, and is known as Keggin unit. The catalytic
Heteropoly acids are water soluble complex metal-oxo
structures which have been extensively exploited as catalysts in
Received 4 January 2012. Accepted 17 February 2012.
*Corresponding author. Tel: +98-571-4410310; Fax: +98-571-4410300; E-mail: rtayebee@yahoo.com
This work was supported by the National Basic Research Program (973 Program, 2003CB615804); the National Natural Foundation of China (20673054).
Copyright © 2012, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(11)60387-2

Reza TAYEBEE et al. / Chinese Journal of Catalysis, 2012, 33: 962–969
function of the Keggin category has attracted much attention
dissolved in 50 ml of boiling water and mixed with disodium
hydrogen phosphate (Na2HPO4, 3.55g, 25 mmol), already
dissolved in 50 ml of water. After the resulting solution was
cooled to room temperature, concentrated sulfuric acid (5 ml,
17 mol/L, 85 mmol) was added to the mixture to give a red
solution. Sodium tungstate dihydrate (Na2WO4·2H2O, 82.5 g,
250 mmol) was dissolved in 100 ml of water and added to the
red solution with vigorous stirring, followed by the slow addi-
tion of concentrated sulfuric acid (42 ml, 17 mol/L, 714 mmol).
Extraction of the solution with diethyl ether (500 ml), followed
by evaporation in air, produced H5PW10V2O40 as a crystalline,
orange-red solid (yield, 74%). Calculated (Found): P, 0.98
(1.05); W, 58.24 (58.12); V, 3.22 (3.16); H2O, 17.12 (17.26).
FT-IR (KBr): 1070(s), 980(vs), 885(s), 788(vs) cm–1.
among heteropoly compounds synthesis. They have the general
formula L8-xXM12O40, where L is a counter cation (proton,
Group I and II metals, and transition metals with the oxidation
state x), X is a heteroatom such as P5+, Si4+, etc. which is func-
tioned as central atom in a tetrahedral arrangement of oxygen
atoms surrounded by 12 oxygens in a octahedra shape includ-
ing the addenda atoms, and M is the addenda atom (most
commonly Mo6+ or W6+). Accordingly, the oxygen atoms in the
Keggin unit can be classified as a central oxygen atoms, two
types of bridging oxygen atoms, and terminal oxygen atoms
[19,20]. Therefore, the Keggin structure is formed by a central
tetrahedron XO4 surrounded by 12 octahedra of MO6. The
structural properties of these compounds could provide in-
trigueing opportunities for the catalyst design and can be con-
trolled by a proper choice of the polyanion.
In the current work a highly versatile, highly efficient, and
environmental friendly method for the synthesis of biologically
active 14-substituted-14-H-dibenzo[a,j]xanthene derivatives is
reported. The reaction is via three-component condensation
reaction of readily available starting materials, ꢀ-naphthol and
aldehydes, in the presence of a catalytic amount of
H5PW10V2O40 as a Keggin-type vanadium substituted heter-
opolyacid under solvent-free condition (Scheme 1). The ex-
perimental procedure is remarkably simple, rapid, high yield-
ing, and requires non toxic organic solvents or inert atmos-
phere.
1.2 General procedure for the synthesis of aryl-14-H-
dibenzo[a,j]xanthenes
A mixture of ꢀ-naphthol (2 mmol), aldehyde (1 mmol), and
H5PW10V2O40 (1 mol%) was heated to 100 ºC for the appro-
priate time. Progress of the reaction was monitored by TLC
utilizing petroleum ether and ethyl acetate with a molar ratio of
2:1 acts as mobile solvent. After completion of the reaction, the
mixture was cooled and washed with CHCl3 (10 ml) to remove
the unreacted aldehyde. Then, the solvent was evaporated and
the crude product was recrystallized from ethanol to afford the
corresponding pure product.
1 Experimental
1.3 Spectral data for selected dibenzoxanthenes [22,23]
Reagents and starting materials were purchased from com-
mercially available vendors and were used as received. All
products were identified by comparison of their spectral and
physical data with those previously reported. Progress of the
reactions was monitored by Thin Layers Chromatography
(TLC) and Infrared analysis. Infrared spectra were recorded
(KBr pellets) on a 8700 Shimadzu Fourier Transform spec-
14-Phenyl-14H-dibenzo[a,j]xanthenes: Pale yellow solid;
1H NMR (CDCl3): ꢀ 8.40 (d, J = 8.4 Hz, 2H), 7.82 (d, J = 7.9
Hz, 2H), 7.79 (d, J = 8.8 Hz, 2H), 7.58 (t, J = 7.7 Hz, 2H), 7.53
(d, J = 7.5 Hz, 2H), 7.49 (d, J = 8.8 Hz, 2H), 7.41 (t, J = 7.5 Hz,
2H), 7.15 (t, J = 7.5 Hz, 2H), 7.00 (t, J = 7.5 Hz, 1H), 6.49 (s,
1H); 13C NMR: 148.7, 145.0, 131.3, 131.0, 128.8, 128.5, 128.1,
126.8, 126.5, 126.3, 124.2, 122.6, 117.9, 117.4, 38.2; IR (KBr,
cm–1): 3068, 3020, 2885, 1620, 1590, 1512, 1488, 1457, 1402,
1252, 1080, 1025, 965, 825, 745, 700; EI-MS: m/z (%) = 358
(M+); Anal. Calcd for C27H18O (%): C, 90.47; H, 5.06; Found:
C, 90.41; H, 5.14.
1
trophotometer. H and 13C NMR spectra were recorded on a
Bruker AVANCE 300-MHz instrument using TMS as an in-
ternal reference. The catalyst H5PW10V2O40 was prepared and
characterized according to published procedures as follows.
14-(4-Chlorophenyl)-14H-dibenzo[a,j]xanthenes:
Brown
solid; 1H NMR (CDCl3): ꢀ 8.29 (d, J = 8.46 Hz, 2H), 7.83–7.77
(m, 4H), 7.58–7.54 (m, 2H), 7.47 (s, 1H), 7.45–7.38 (m, 5H),
7.10–7.07 (m, 2H), 6.44 (s, 1H); 13C NMR: 156.0, 147.8,
1.1 Preparation of 10-tungsto-2-vanadophosphoric acid
H5PW10V2O40·30H2O [21]
Sodium metavanadate (NaVO3, 12.2 g, 100 mmol) was
X
H
O
H
OH
H5PW10V2O40 (0.5 mol%)
Solvent-free, 100 oC
+
2
X
O
Scheme 1. The synthesis of biologically active 14-substituted-14H-dibenzo[a,j] under solvent-free condition.

Reza TAYEBEE et al. / Chinese Journal of Catalysis, 2012, 33: 962–969
132.8, 131.2, 129.3, 128.8, 128.3, 127.0, 126.8, 126.5, 124.7,
model reactions. The Heteropolyoxometalate catalysts intro-
duced in Table 1 could be structurally divided into three im-
portant subclasses: Keggin, Wells-Dawson, and Preyssler. As
shown in Table 1, both the Keggin and Wells-Dawson salts
showed comparable catalytic activity; whereas, the Preyssler
salt, K12.5Na1.5[NaP5W30O110], was inactive. Moreover, substi-
tution of vanadium instead of Mo6+ and W6+ in the Keggin
H3PM12O40 increased the catalytic activity of heteropolyacid.
Di-vanado-tungsto(molybdo)phosphoric acids, H5PW10V2O40
and H5PMo10V2O40, indicated better catalytic activity than
H3PW12O40 and H3PMo12O40. At the same time, H5PW10V2O40
was a little more reactive than H5PMo10V2O40.
Clearly, the nature and relative position of the constituent
transition elements in the structural framework [PM12O40]3– (M
= W or Mo) strongly affected the acid-base and redox behavior
of the Keggin type heteropoly anions at the atomic and mo-
lecular level [24,25]. As shown in Table 1, H3PW12O40 and
H5PW10V2O40 exhibited higher catalytic activity than molyb-
denum counterparts and might be attributed to the slightly
higher acidity of the tungsten heteropolyacids. However, it
should be mentioned that the present protocol was not entirely
acid catalyzed. The effect of vanadium on the structure, acidity,
and catalytic performance of heteropolyoxometallates had
been studied extensively [26–28]. Particularly, it was found
that acidity of heteropolyacids diminished by replacing MoVI or
WVI atoms with VV [29–31]. Although, H5PW10V2O40 is con-
siderably a weaker acid than H3PW12O40, which behaved better
than H3PW12O40 and had led to 95% conversion rate after 80
min.
119.3, 118.2, 117.8, 33.5; IR (KBr, cm–1): 3050, 2925, 1620,
1595, 1456, 1431, 1396, 1242, 1060, 960, 824, 778, 695;
EI-MS: m/z (%) = 392 (M+); Anal. Calcd for C27H17ClO (%):
C, 82.54; H, 4.36; Found: C, 82.46; H, 4.44.
14-(4-Nitrophenyl)-14H-dibenzo[a,j]xanthenes:
Yellow
solid; 1H NMR (CDCl3): ꢀ 8.29 (2H, d, J = 8.4 Hz),7.99 (2H, d,
J = 8.7 Hz), 7.86 (2H, d, J = 4.1 Hz), 7.82 (2H, d, J = 5.4 Hz),
7.67 (2H, d, J = 8.8 Hz), 7.61 (2H, t, J = 5.6 Hz),7.51 (2H, d, J
= 8.9 Hz), 7.44 (2H, t, J = 7.9 Hz), 6.60 (1H, s); IR (KBr, cm–1):
3070, 2930, 1621,1591, 1614, 1457, 1400, 1340, 1200, 1140,
1105, 1013,964, 851, 827, 808, 742, 690. EI-MS: m/z (%) =
403 (M+); Anal. Calcd for C27H17NO3 (%): C, 80.38; H, 4.25;
N, 3.47; Found: C, 80.30; H, 4.35; N, 3.55.
14-(3-Chlorophenyl)-14H-dibenzo[a,j]xanthenes:
Brown
solid; 1H NMR (CDCl3): ꢀ 8.30 (d, J = 8.4 Hz, 2H), 7.86 (d, J =
8.6 Hz, 2H), 7.76 (d, J = 9.0 Hz, 2H), 7.60 (t, J = 7.0 Hz, 2H),
7.50 (d, J = 8.9 Hz, 2H), 7.48–7.43 (m, 4H), 7.10 (t, J = 8.0 Hz,
1H), 6.96 (d, J = 8.7 Hz, 1H), 6.45 (1H, s); 13C NMR: 148.5,
146.8, 134.5, 131.2, 131.0, 129.7, 129.1, 128.8, 128.2, 127.1,
126.8, 126.4, 124.5, 122.4, 118.1, 116.4, 37.8; IR (KBr, cm–1):
3053, 2926, 1622, 1590, 1508, 1455, 1430, 1398, 1245, 1064,
959, 815, 775, 745, 690; EI-MS: m/z (%)= 392 (M+); Anal.
Calcd for C27H17ClO (%): C, 82.54; H, 4.36; Found: C, 82.48;
H, 4.42.
1
14-(4-Bromophenyl)-14-H-dibenzo[a,j]xanthenes: H NMR
(CDCl3): ꢀ 8.30 (s, 1H), 8.28 (s, 1H), 7.83 (s, 1H), 7.81 (s, 1H),
7.79 (s, 1H), 7.77 (s, 1H), 7.56 (t, J = 6.96 Hz, 2H), 7.47 (s,
1H), 7.45 (s, 1H), 7.42 (s, 1H), 7.40 (s, 1H), 7.38 (s, 1H), 7.36
(s, 1H), 7.24 (d, J = 2.93 Hz, 2H), 6.43 (s, 1H); 13C NMR:
148.66, 143.95, 131.55, 131.22, 131.03, 129.83, 129.07,
128.88, 126.88, 124.34, 122.36, 120.18, 117.97, 116.63, 37.41.
IR (KBr, cm–1): 3030, 1624, 1586.
It should be mentioned that substitution of two vanadium
atoms in H3PM12O40 did not change the basic Keggin structure.
Substitution of PV with SiIV in the vanadium substituted
H5PW10V2O40, led to the formation of less acidic H7SiW9V3O40
heteropolyacid [32]. This compound was slightly less effective
than the former and led to 86% yield after 90 min.
2 Results and discussion
Table 1 Effect of the kind of heteropolyacid catalyst in the synthesis of
14-(4-chlorophenyl)-14H-dibenzo[a,j]xanthene
2.1 Effect of catalyst concentration
Content Time Yield Selectivity
Heteropolyacid
It had been experimentally shown that mono-valent vanadyl
cations could be substituted instead of the protons in the het-
eropolyacid structure [20]. It was also revealed in the same
findings that vanadium did not change the structural frame-
work of the acid, but strongly affected the physicochemical
properties and catalytic activity of the Keggin heteropolyacid
[20]. Presumably, the presence of strong acidic protons and the
existence of an effective electron acceptor such as the vana-
dium atom in H5PW10V2O40 would enhance the catalyst activity
in these studies.
To understand the catalytic efficacy of H5PW10V2O40, its
activity was compared with structurally similar heteropolya-
cids in the preparation of 14-(4-chlorophenyl)-14H-
dibenzo[a,j]xanthene. As shown in Table 1, different acidic and
anionic heteropolyoxometalates were examined in different
(%)
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
2
(min)
80
(%)
95
86
92
96
90
<5
92
86
92
98
(%)
100
>98
>98
100
> 98
—
H5PW10V2O40·xH2O
H7SiW9V3O40·xH2O
H5PMo10V2O40·xH2O
H5SiW9Mo2VO40·xH2O
H3PMo12O40·xH2O
K12.5Na1.5[NaP5W30O110]·xH2O
H6P2W18O62·xH2O
H3PW12O40·xH2O
H3PW12O40·xH2O
H3PW12O40·xH2O
90
90
180
100
480
90
80
60
30
>98
>98
>98
100
5
Reaction conditions: 2 mmol ꢀ-naphthol, 1 mmol 4-chlorobenzaldehyde,
100 ºC, solvent free. The progress of the reactions was monitored as de-
scribed in the experimental section by TLC. The product yield refers to
isolated pure substance.

Reza TAYEBEE et al. / Chinese Journal of Catalysis, 2012, 33: 962–969
Table
3
Effect of different solvents in the preparation of
H5SiW9Mo2VO40 as a mixed metal Si-substituted Keggin type
heteropolyacid, showed distinctly low reactivity and resulted in
96% of the after 180 min. However, it was difficult to propose
a clear mechanism for the different reactivity pattern observed
for the heteropolyacids mentioned above. Undoubtedly, there is
a complex relationship between the catalytic activity and
structure of the polyanion. The acid strength of a heteropolya-
cid as well as its catalytic activity could be altered by changing
the constituent hetero and addenda atoms of the polyanion.
Obviously, substitution of molybdenum and tungsten with
other transition metal cations strongly affected the catalytic
properties of the Keggin heteropolyacids. In the case of vana-
dium, rebuilding of the heteropolyanion might occur and va-
nadyl salts would be formed during the catalytic route. The
higher reactivity of H5PW10V2O40 than H5PMo10V2O40 could
further be explained by considering structural reformation of
the latter during the reaction progress.
Optimization of the reaction conditions was performed by
conducting the reaction of 4-chlorobenzaldehyde and
ꢀ-naphthol using different amounts of H5PW10V2O40 (0%,
0.1%, 0.5%, 1%, and 2%) under solvent free condition at 100
ºC (Table 2). The best results were obtained using 1% of
H5PW10V2O40 as the optimum amount. Lower amounts of the
catalyst decreased the conversion rate, whereas, higher
amounts did not improve the reaction time and yield. Of note
the product yield was found to be low in the absence of cata-
lyst.
14-(4-chlorophenyl)-14H-dibenzo[a,j]xanthene
Solvent
Free
Ethanol
Acetonitrile
Ethyl acetate
Toluene
Reflux temperature (ºC) Time (h)
Yield (%)
95
<10
100
80
1.3
16
8
90
<10
90
8
55
120
13
<15
Reaction conditions were the same as in Table 1 and 0.5 mol% H5PW10V2
O40 was used.
2.3 Effect of reaction temperature
The effect of different temperatures on the reaction was also
investigated for the synthesis of 14-(4-chlorophenyl)-14H-
dibenzo[a,j]xanthene in the presence of H5PW10V2O40 (0.5
mol%) under solvent free condition (Fig. 1). The results re-
vealed that percent yield was enhanced by increasing reaction
temperature from 25 to 100 ºC after 2 h. The product (95%)
was obtained at 100 ºC after 80 min, whereas, by decreasing the
temperature to 80 ºC, the conversion was reduced to 90% after
2 h. Even though, the maximum yield (> 98%) was obtained at
temperatures higher than 100 ºC during shorter times (< 1 h),
100 ºC was selected as the optimum temperature for all the
reactions.
2.4 Condensation of different aromatic aldehydes with
ꢀ-naphthol
2.2 Effect of solvent
The applicability of the target protocol was demonstrated by
applying a wide range of substituted aryl aldehydes to synthe-
size the corresponding products in high to excellent yields
(Table 4). High yield transformations were achieved without
any significant amounts of undesirable side products. Unlike
some previously reported methods, the present method did not
require toxic reagents or organic solvents to produce the
The condensation reaction to yield 14-(4-chlorophenyl)-
14H-dibenzo[a,j]xanthene was tested in different solvents such
as ethanol, acetonitrile, ethyl acetate, and toluene, and the
results were compared to the solvent free products. As shown
in Table 3, the order of different solvents over product yield
was as the follows of solvent free >> ethyl acetate > acetonitrile
> ethanol ~ toluene. Since the more polar nature of the solvent
is concentrated within the active centers of the catalyst, ethanol
and acetonitrile were found to be more suitable to yield the
desired xanthenedione. The less polar solvents such as toluene
showed low product yield whereas dimethyl sulfoxide and
dimethylformamide did not catalyze any reaction due to their
nature to retard the condensation of the intermediates.
100
80
60
40
20
0
Table 2 Synthesis of 14-(4-chlorophenyl)-14H-dibenzo[a,j]xanthene
using different amounts of H5PW10V2O40 catalyst
Catalyst
Content (%) Time (min) Yield (%) Selectivity (%)
H5PW10V2O40
H5PW10V2O40
H5PW10V2O40
H5PW10V2O40
H5PW10V2O40
—
0.1
0.5
1.0
2.0
600
540
80
30
30
17
79
95
98
97
~50
> 90
100
100
100
25
35
45
55
65
75
85
95
105
Temperature (oC)
Fig. 1.
Effect of reaction temperature on the preparation of
14-(4-chlorophenyl)-14H-dibenzo[a,j]xanthene after 2 h.
Reaction conditions were the same as in Table 1

Reza TAYEBEE et al. / Chinese Journal of Catalysis, 2012, 33: 962–969
Table 4 Synthesis of different xanthene derivatives catalyzed by 0.5 mol% H5PW10V2O40
Melting point (ºC)
Aldehyde
Product
Time (h)
Yield (%)
Reference
[33]
Observed
Reported
O
H
C
1 (0.5)
67 (98)
182–183
181–183
O
NO2
O
H
C
0.8 (0.5)
1 (0.5)
72 (98)
45 (88)
212–213
199–200
211–212
—
[33]
—
NO
2
O
O
H
C
Me
Me
O
Br
O
H
C
1 (0.5)
1 (40)
40 (91)
40 (92)
295–297
309–310
296
310
[33]
[34]
Br
C
O
NO2
O
H
NO2
O
O
Cl
H
C
1 (40)
66 (87)
95 (98)
75 (85)
35 (82)
189–190
289–290
213–214
190–91
210–211
289–290
215
[35]
[33]
[36]
[36]
Cl
O
O
Cl
H
C
1.3 (30)
0.7 (35)
1.2 (50)
O
Cl
O
H
C
Cl
Cl
O
O
Br
H
C
190–192
Br
O
Reaction conditions were the same as in Table 1, and 0.5 mol% H5PW10V2 O40 was used. All products were characterized by IR, 1H NMR, melting point.
aryl-14H-dibenzo[a,j]xanthene derivatives. A wide range of
aromatic aldehydes was employed and all benzoxanthenes
were obtained in high excellent yields based on the present
general method that tolerated both electron-withdrawing and
electron-donating constituents. The current findings revealed
that the reaction conditions were mild enough and did not
damage moieties susceptible to cleavage in strongly acidic
reaction media. The presence of electron-donating and elec-
tron-withdrawing groups in the aromatic ring of the aldehydes
did not affect the reaction progress such that the respective
xanthene derivatives were afforded in high yields. Therefore,
the present method was general and included a variety of
functional groups. However, para-substituted aldehydes re-
sulted in better yields compared with the ortho-substituents,
because of the more steric hindrances of the later.
2.5 Advantages of H5PW10V2O40 over some well-known
high-valent metal/non-metal oxides
Table 5 described the advantages of H5PW10V2O40 in com-
parison with other catalysts used for the preparation of
14-(4-chlorophenyl)-14H-dibenzo[a,j]xanthene under the re-
ported optimum reaction conditions. Evidently, H5PW10V2O40
is the best catalyst concerning catalyst content, reaction time,

Reza TAYEBEE et al. / Chinese Journal of Catalysis, 2012, 33: 962–969
2.7 A plausible reaction pathway
Table 5 Comparing the catalytic activity of H5PW10V2O40 with some
metal/non-metal oxides in the preparation of 14-(4-chlorophenyl)-
14H-dibenzo[a,j]xanthene
A scheme of the reported reaction mechanisms for the
catalytic synthesis of dibenzoxanthenes [46] is outlined in
Scheme 2. Activation of the carbonyl group of aldehyde by
H5PW10V2O40, facilitates nucleophilic attack of ꢀ-naphthol and
formation of the corresponding carbocation in the first step.
This carbocation was then transformed to an
aryl-methanebisnaphthol in the second step, and ultimately it
was converted to the desired product via dehydration of
bis(naphtholyl)methane species.
Catalyst
—
Content (%) Time (h) Yield (%) Selectivity (%)
—
0.5
20
20
70
20
20
20
50
10
1.3
2
17
95
~50
100
100
100
>70
n.d.
n.d.
n.d.
n.d.
H5PW10V2O40
ZrCL4
99
ZrOCl2
KH2PO4
ZrO2
ZnO
SiO2
1
6
9
18
24
6
99
48
<5
<5
<5
<10
Et3NHH2PO4
2.8 Spectroscopic evidence for the incorporation of
vanadium into the Keggin structure and
the stability of H5PW10V2O40
and yield. Among the examined additives, ZrOCl2 occupied the
next position and 20% of this catalyst provided complete
conversion after 1 h. Meanwhile, ZrCL4 showed less activity
and led to the same result under similar reaction condition after
2 h, whereas, ZrO2 was ineffective. KH2PO4 (70%) also
showed little catalytic activity and gave 48% of conversion
after 6. The ionic liquid [Et3NH]H2PO4, ZnO, and SiO2 were
inefficient in the aforementioned catalytic transformation.
The specific structure of Keggin H5PW10V2O40 gave rise to
four types of oxygen atoms in the fingerprint region of IR
spectrum in 700–1200 cm–1. These characteristic IR bands
appeared at ca. 1088 ꢁ(P–O), 998 ꢁ(W–Od), 890 ꢁ(W–Ob–M),
and 803 ꢁ(W–Oc–M, where M = W and V) [49].
Comparison of the characteristic IR bands of H5PW10V2O40
with those of H3PW12O40 (ca. 1075, 980, 885, and 810 cm–1)
showed a slight shift to lower wave numbers. This shift is
related to the influence of vanadium on the W–O bond, and
confirmed the entrance of vanadium into the primary structure
of the Keggin anion [26]. Moreover, appearance of a broad
P–Oi band in FT-IR spectrum of H5PW10V2O40 confirmed the
formation of vanadium included in the Keggin structure [50].
Furthermore, an intense charge-transfer UV-Vis absorption
band at ~200–300 nm appeared for various Keggin type het-
eropoly compounds, which was due to the charge-transfer of
the terminal and bridged oxygen atoms that octahedrally sur-
rounded the metal atoms (M–O and V–O) in the heteropoly
cage [51,52]. The presence of vanadium in the Keggin struc-
ture resulted in broadening of the LMCT band [53].
2.6 Comparing the catalytic activity of H5PW10V2O40 with
other reported catalysts
In order to further validate our work, the current protocol
was compared with the data in the literature based on the
catalysts content, temperature, reaction time, and percentage
yields (Table 6). Although, some of the additives catalyzed the
reaction at lower temperature, they required longer reaction
times and/or higher catalyst content. Short-reaction time (30
min), mild reaction condition, and environmentally benign
catalyst are distinct advantages of the current methodology.
Table 6 Comparison of the catalytic efficiency of H5PW10V2O40 with some reported catalysts in synthesis of dibenzoxanthenes
Catalyst
Catalyst (%)
Time (h)
0.5–0.8
6–12
2–5
6–12
73
3–4
2–7
1.2–6.5
2–4
3–7
Yield (%)
82–98
90–95
85–95
90–95
60–90
82–91
70–96
70–95
75–89
87–95
81–97
80–94
65–97
80–92
Condition
solvent free, 100 ºC
solvent free, 125 ºC
solvent free, 90 ºC
solvent free, 125 ºC
HOAc, 80 ºC
Reference
this work
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[35]
[45]
[46]
[47]
[48]
H5PW10V2O40
SelectfluorTMa
I2
Sulfamic acid
H2SO4:HOAc
KAl(SO4)2·12H2O
Sc[N(SO2C8F17)2]3
Fe(HSO4)3
Montmorillonite K10
Yb(OTf)3
FeCl3
RuCl3·nH2O
CoPy2Cl2
Polystyrene/AlCl3
1
10
10
10
1:4 (v:v)
50
1
10
0.3 g
1
5
H2O, 100 ºC
perfluorodecalin, 110 ºC
1,2-dichloroethane (reflux)
solvent free, 120 ºC
[bipy]BF4, 110 ºC
solvent free, 100 ºC
ethanol, 100 ºC
10
1–4
1.5–8
1.5–4
10
10
20
neat condition, 85 ºC
CH3CN, reflux
a1-(Chloromethyl)-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane-bis(tetrafluoroborate).

Reza TAYEBEE et al. / Chinese Journal of Catalysis, 2012, 33: 962–969
+
O
H
H PW V O
5
10
2
40
OH
H
+
X
+
-H
H
+
O
OH
OH
HO
+
OH
H
X
HO
H
X
X
H
+
X
H
-H O
2
OH
H
+
OH
HO
Ar
Ar
+
O
OH
X
+
-H
X
H
H
+
OH
2
OH
OH
-H O
2
OH
+
H
Scheme 2. Suggested reaction pathway for the catalytic synthesis of dibenzoxanthenes.
The stability and deactivation of H5PW10V2O40 under the
reaction conditions were evaluated by reusing the recycled
catalyst after completion of the first run. When the reaction was
repeated using the recovered catalyst, the resultant conversion
was close to the value obtained for the first run (Fig. 2). The
Keggin structure for H5PW10V2O40 could be used as an indi-
cator for the stability and reusability of the catalyst. As shown
in Fig. 3, the IR spectrum of the recovered H5PW10V2O40 after
the 9th run revealed that the Keggin structure was retained
almost completely.
100
80
60
40
20
0
(2)
(1)
1
2
3
4
5
6
7
8
9
1400
1200
1000
800
600
400
Wavenumber (cmꢀ1)
Number of run
Reusability of H5PW10V2O40 in the synthesis of
14-(4-chlorophenyl)-14H-dibenzo[a,j]xanthene.
Fig. 2.
Fig. 3. FT-IR spectra of fresh (1) and recycled H5PW10V2O40 after the
9th run (2).

Reza TAYEBEE et al. / Chinese Journal of Catalysis, 2012, 33: 962–969
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3 Conclusions
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In this paper a highly efficient, one-pot, and green method-
ology for the synthesis of aryl-14H-dibenzo[a,j]xanthenes. The
condensation of an aromatic aldehyde and ꢀ-naphthol in the
presence of a catalytic amount of H5PW10V2O40 under solvent
free conditions was produced in excellent yields. The present
methodology offers several advantages which include simple
procedure, low cost, easy work-up, short reaction time,
avoiding organic solvent, and using mild reaction conditions.
29 Jolivet J P, Metal Oxide Chemistry and Synthesis. Chichester:
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Acknowledgements
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Partial financial support from the Research Council of
Sabzevar Tarbiat Moallem University is greatly appreciated.
Special thanks to Dr. Davoudi for English editing of the
manuscript.
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