Silica gel-supported polyphosphoric acid (PPA/SiO2): An efficient and reusable heterogeneous catalyst for facile synthesis of 14-Aryl-14H-dibenzo[a,j]xanthenes under solvent-free conditions

Article abstract of DOI:10.1002/cjoc.201190081

A simple, efficient and green method for the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes by a one-pot condensation reaction of β-naphthol and aryl aldehydes using silica gel-supported polyphosphoric acid (PPA/SiO₂), an effective and reusable

Full text of DOI:10.1002/cjoc.201190081

FULL PAPER
Silica Gel-Supported Polyphosphoric Acid (PPA/SiO ): An
2
Efficient and Reusable Heterogeneous Catalyst for Facile
Synthesis of 14-Aryl-14H-dibenzo[a,j]xanthenes
under Solvent-free Conditions
Khojastehnezhad, Amir
Davoodnia, Abolghasem*
Bakavoli, Mehdi
Tavakoli-Hoseini, Niloofar
Zeinali-Dastmalbaf, Mohsen
Department of Chemistry, Faculty of Sciences, Islamic Azad University, Mashhad Branch, Mashhad, Iran
A simple, efficient and green method for the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes by a one-pot con-
densation reaction of β-naphthol and aryl aldehydes using silica gel-supported polyphosphoric acid (PPA/SiO ), an
2
effective and reusable catalyst, under solvent-free conditions is described. The present methodology offers several
advantages, such as a simple procedure with an easy work-up, short reaction times, high yields, and the absence of
any volatile and hazardous organic solvents.
Keywords 14-Aryl-14H-dibenzo[a,j]xanthenes, PPA/SiO , heterogeneous catalysis, green chemistry, solvent-free
2
conditions
10-12
Introduction
agricultural bactericide activity,
13
anti-inflammatory
14
effect, and antiviral activity. There are several
methods reported for the synthesis of xanthenes and
dibenzoxanthenes, such as palladium catalyzed cycliza-
The principles of green chemistry have been intro-
duced to eliminate or at least to reduce the use of haz-
ardous materials, such as H2SO4 or H3PO4 in chemical
processes. Cleaner technologies could become possible
by making use of the environmental friendly materials,
such as the use of solid acids. These catalysts have
15
tion of polycyclic aryltriflate esters, and intramolecular
16
trapping of benzynes by phenols.
14-aryl-14H-dibenzo[a,j]xanthenes and their analogues
were prepared by reaction of β-naphthol with
Furthermore,
1-3
many advantages over liquid acid catalysts. They are
17
18
2-naphthol-1-methanol, formamide, carbon monox-
not corrosive but environmentally benign, presenting
fewer disposal problems. Thus the development and use
of solid green catalysts are very important in organic
syntheses.
19
ide as well as by condensation of β-naphtol with alde-
20-24
hydes using various acid catalysts.
Many of these
methodologies, however, suffer from disadvantages,
such as unsatisfactory yields, expensive catalysts, long
reaction times, toxic organic solvents, and harsh reaction
conditions. Therefore, the development of simple, effi-
cient, high-yielding, and environmentaly friendly meth-
ods using new catalysts for the synthesis of these com-
pounds is still necessary.
Recently, solid-supported reagents, such as the silica
gel-supported acids, have gained considerable interests
in organic synthesis because of their unique properties,
such as high efficiency due to more surface area, more
stability and reusability, low toxicity, greater selectivity
4-6
and ease of handling. Although the catalytic applica-
As part of our current studies on the development of
tions of silica supported reagents for organic synthesis
have been established, to the best of our knowledge,
there is no published report on the use of PPA-SiO2 in
the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes.
Xanthenes and dibenzoxanthenes’ derivatives are
important classes of organic compounds with a large
number of naturally occurring, as well as synthetic de-
rivatives, and occupy a prominent position in medicinal
new routes for the synthesis of organic compounds us-
25-33
ing reusable catalysts,
herein we wish to report a
green and rapid method for the synthesis of
14-aryl-14H-dibenzo[a,j]xanthenes by condensation of
β-naphthol with aryl aldehydes using PPA/SiO2, as a
reusable catalyst, under solvent-free conditions (Scheme
1).
7
chemistry. A number of these compounds have been
Experimental
considered as dyes and fluorescent visualization materi-
8,9
als for biomolecules and laser technologies. Further-
All chemicals were available commercially and used
without additional purification. The catalyst was synthe-
more, these compounds have been investigated for their
*
E-mail: adavoodnia@mshdiau.ac.ir; Tel.: 0098-511-8435000; Fax: 0098-511-8424020
Received August 12, 2010; revised October 29, 2010; accepted November 13, 2010.
Chin. J. Chem. 2011, 29, 297— 302
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
297

Khojastehnezhad et al.
FULL PAPER
Scheme 1
CH), 6.85 (t, J＝8.7 Hz, 2H, arom-H), 7.46 (t, J＝7.8
Hz, 2H, arom-H), 7.48—7.55 (m, 4H, arom-H), 7.62 (td,
J＝7.7, 1.2 Hz, 2H, arom-H), 7.84 (d, J＝8.9 Hz, 2H,
arom-H), 7.87 (d, J＝8.1 Hz, 2H, arom-H), 8.37 (d, J＝
8.5 Hz, 2H, arom-H).
1
3e (Ar＝3-HOC6H4): H NMR (CDCl3) δ: 4.55 (s,
1H), 6.42—6.60 (m, 2H), 6.84 (s, 1H), 7.06 (t, J＝8.2
Hz, 1H), 7.21 (d, J＝7.8 Hz, 1H), 7.34—7.45 (m, 4H),
7.52—7.60 (m, 2H), 7.73—7.81 (m, 4H), 8.36 (d, J＝
8.5 Hz, 2H).
＋
34
sized according to the literature. The amount of H in
the PPA/SiO2 was determined by acid-base titration.
Melting points were recorded on an electrothermal type
9100 melting point apparatus. The IR spectra were ob-
tained using a 4300 Shimadzu spectrophotometer as
1
3f (Ar＝4-HOC6H4): H NMR (CDCl3) δ: 5.21 (sbr.
1H), 6.44 (s, 1H), 6.56 (d, J＝8.2 Hz, 2H), 7.37 (t, J＝
9.8 Hz, 2H), 7.43 (d, J＝7.6 Hz, 2H), 7.50 (d, J＝8.8 Hz,
2H), 7.61 (t, J＝7.7 Hz, 2H), 7.75 (d, J＝6.4 Hz, 2H),
7.89 (d, J＝8.5 Hz, 2H), 8.33 (d, J＝8.2 Hz, 2H).
1
KBr disks. The H NMR (500 MHz) spectra were re-
corded with a Bruker DRX500 spectrometer.
General procedure for the synthesis of 14-aryl-14H-
1
3g (Ar＝4-MeOC6H4): H NMR (CDCl3) δ: 3.65 (s,
dibenzo[a,j]xanthenes 3a — 3j using PPA/SiO as
catalyst
2
3H, OCH3), 6.49 (s, 1H, CH), 6.70 (d, J＝8.8 Hz, 2H,
arom-H), 7.40—7.47 (m, 4H, arom-H), 7.51 (d, J＝8.9
Hz, 2H, arom-H), 7.61 (td, J＝8.3, 1.1 Hz, 2H, arom-H),
7.81 (d, J＝8.9 Hz, 2H, arom-H), 7.86 (d, J＝8.0 Hz,
2H, arom-H), 8.42 (d, J＝8.5 Hz, 2H, arom-H).
A mixture of β-naphthol 1 (2 mmol), aromatic alde-
hyde 2a—2j (1 mmol), and PPA/SiO2 (0.030 g, 0.015
＋
mmol of H ) was heated in the oil bath at 120 ℃ for
1
30—40 min. During the procedure, the reaction was
monitored by TLC. Upon completion, the reaction mix-
ture was cooled to room temperature and hot ethanol
was added. The catalyst was dissolved in hot ethanol
and filtered off. The product was then collected from the
filtrate after cooling to room temperature and recrystal-
lized from ethanol to give compounds 3a—3j in high
yields.
3h (Ar＝4-MeC6H4): H NMR (DMSO-d6) δ: 2.04 (s,
3H, CH3), 6.66 (s, 1H, CH), 6.92 (d, J＝8.0 Hz, 2H,
arom-H), 7.45 (t, J＝7.4 Hz, 2H, arom-H), 7.48 (d, J＝
8.1 Hz, 2H, arom-H), 7.54 (d, J＝8.8 Hz, 2H, arom-H),
7.61 (t, J＝7.2 Hz, 2H, arom-H), 7.91 (d, J＝8.9 Hz, 2H,
arom-H), 7.92 (d, J＝7.7 Hz, 2H, arom-H), 8.65 (d, J＝
8.5 Hz, 2H, arom-H).
1
3i (Ar＝3-O2NC6H4): H NMR (CDCl3) δ: 6.63 (s,
1H, CH), 7.32 (t, J＝8.0 Hz, 1H, arom-H), 7.47 (t, J＝
7.4 Hz, 2H, arom-H), 7.55 (d, J＝8.9 Hz, 2H, arom-H),
7.65 (t, J＝7.4 Hz, 2H, arom-H), 7.80—7.92 (m, 6H,
arom-H), 7.34 (d, J＝8.5 Hz, 2H, arom-H), 8.45 (s, 1H,
arom-H).
Recycling and reusing of the catalyst
Due to the fact that the catalyst was dissolved in hot
ethanol, it could therefore be recycled by a simple filtra-
tion. The separated catalyst was washed with cold etha-
nol, dried at 100 ℃ under vacuum for 2 h and was re-
used in another reaction. The catalyst could be reused at
least three times without significant loss of activity.
1
3j (Ar＝4-O2NC6H4): H NMR (CDCl3) δ: 6.63 (s,
1H, CH), 7.47 (t, J＝7.5 Hz, 2H, arom-H), 7.54 (d, J＝
8.9 Hz, 2H, arom-H), 7.63 (t, J＝7.6 Hz, 2H, arom-H),
7.70 (d, J＝8.6 Hz, 2H, arom-H), 7.84—7.91 (m, 4H,
arom-H), 8.02 (d, J＝8.6 Hz, 2H, arom-H), 8.31 (d, J＝
8.4 Hz, 2H, arom-H).
1
H NMR data for compounds 3a— 3j
1
3a (Ar＝Ph): H NMR (CDCl3) δ: 6.53 (s, 1H, CH),
7.04 (t, J＝7.4 Hz, 1H, arom-H), 7.19 (t, J＝7.6 Hz, 2H,
arom-H), 7.45 (t, J＝7.0 Hz, 2H, arom-H), 7.54 (d, J＝
8.9 Hz, 2H, arom-H), 7.58 (d, J＝7.3 Hz, 2H, arom-H),
7.62 (td, J＝7.0, 1.2 Hz, 2H, arom-H), 7.83 (d, J＝8.9
Hz, 2H, arom-H), 7.87 (d, J＝8.0 Hz, 2H, arom-H), 8.44
(d, J＝8.5 Hz, 2H, arom-H).
Results and discussion
PPA-SiO2 was prepared according to the literature
34
procedure. Initially, the synthesis of compound 3a was
selected as a model reaction to optimize the reaction
conditions. The reaction was carried out by heating a
mixture of β-naphthol (2 mmol) and benzaldehyde (1
mmol) under various conditions.
1
3b (Ar＝4-BrC6H4): H NMR (CDCl3) δ: 6.39 (s, 1H,
CH), 7.19 (d, J＝8.0 Hz, 2H, arom-H), 7.32—7.40 (m,
4H, arom-H), 7.42 (d, J＝8.8 Hz, 2H, arom-H), 7.53 (t,
J＝7.5 Hz, 2H, arom-H), 7.74 (d, J＝8.8 Hz, 2H,
arom-H), 7.78 (d, J＝7.9 Hz, 2H, arom-H), 8.26 (d, J＝
8.3 Hz, 2H, arom-H).
The efficiency of the reaction is affected mainly by
the amount of PPA/SiO2 (Table 1). No product was ob-
tained in the absence of the catalyst (Entry 1) indicating
that the catalyst is necessary for the reaction. Increasing
the amount of the catalyst increased the yields of the
product 3a. The optimal amount of PPA/SiO2 was 0.03
g (Entry 4); increasing the amount of the catalyst be-
yond this value did not increase the yield noticeably
(Entries 5, 6).
1
3c (Ar＝4-ClC6H4): H NMR (CDCl3) δ: 6.51 (s, 1H,
CH), 7.14 (d, J＝8.3 Hz, 2H, arom-H), 7.22—7.55 (m,
6H, arom-H), 7.62 (t, J＝7.6 Hz, 2H, arom-H), 7.84 (d,
J＝8.9 Hz, 2H, arom-H), 7.87 (d, J＝8.0 Hz, 2H,
arom-H), 8.35 (d, J＝8.4 Hz, 2H, arom-H).
1
3d (Ar＝4-FC6H4): H NMR (CDCl3) δ: 6.53 (s, 1H,
298
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Chin. J. Chem. 2011, 29, 297— 302

Efficient and Reusable Catalyst for Facile Synthesis of 14-Aryl-14H-dibenzo[a,j]xanthenes
a
Table 1 Effect of PPA/SiO amount on the model reaction
2
Table 2 Synthesis of 14-phenyl-14H-dibenzo[a,j]xanthene 3a
in the presence of PPA/SiO (0.03 g) in different solvents
2
a
Time/h Yield /%
Entry
Solvent
MeOH
EtOH
Temperature/℃
1
2
3
4
6
64
78
5
5
Trace
65
CH CN
3
81
5
82
b
Yield /%
H O
2
100
120
5
None
90
Entry
Catalyst/g
None
0.01
Time/min
Solvent-free
0.5
1
2
3
4
5
6
60
30
30
30
30
30
—
65
81
90
90
a
The yields were calculated based on benzaldehyde and refer to
the pure isolated product.
0.02
0.03
Table 3 Synthesis of 14-phenyl-14H-dibenzo[a,j]xanthene 3a
in the presence of PPA/SiO (0.03 g) at different temperatures in
2
0.04
solvent-free conditions
0.05
91
a
Yield /%
a
b
2 mmol β-naphthol and 1 mmol benzaldehyde at 120 ℃; the
Entry
Temperature/℃
Time/min
yields were calculated based on benzaldehyde and refer to the
pure isolated product.
1
2
3
4
5
80
30
30
30
30
30
72
81
86
90
90
100
110
120
140
Furthermore, the reaction was carried out in different
solvents and under solvent-free conditions. As shown in
Table 2, the yields of the reaction under solvent-free
conditions were greater and the reaction times were
generally shorter than the conventional methods.
a
The yields were calculated based on benzaldehyde and refer to
the pure isolated product.
The same model reaction in presence of 0.03 g of the
catalyst was carried out at different temperatures in sol-
vent-free conditions to assess the effect of temperatures
on the reaction yield. It was observed that yield is a
function of temperature since the yield was increased as
the reaction temperature was raised (Table 3). At 120
℃, the product 3a was obtained in excellent yield.
Therefore, all subsequent reactions were carried out at
120 ℃ in presence of 0.03 g of PPA/SiO2 under sol-
vent-free conditions.
reacted successfully and gave the expected products in
excellent yields and short reaction times. The type of
aldehyde had no significant effect on the reaction. The
results are shown in Table 4.
Reusability of the catalyst was also investigated. For
this purpose, the same model reaction was again studied
under optimized conditions. After the completion of the
reaction, the reaction mixture was cooled to room tem-
perature, and hot ethanol was added. The catalyst was
separated by simple filtration, dried at 100 ℃ under
vacuum for 2 h, and reused for a similar reaction. As
shown in Figure 1, the catalyst could be reused at least
three times without significant loss of activity.
In order to evaluate the generality of this model reac-
tion, we prepared a range of 14-aryl-14H-dibenzo-
[a,j]xanthenes under optimized reaction conditions. In
all cases, aromatic aldehydes with substituents carrying
either electron-donating or electron-withdrawing groups
a
Table 4 Preparation of 14-aryl-14H-dibenzo[a,j]xanthenes using PPA/SiO (0.03 g) as catalyst
2
b
c
Yield /%
Entry
Ar
Product
Time/min
m.p. (Lit. m.p.)/℃
184—186
1
30
90
21
(182—183)
3a
296—298
2
35
86
21
(295—296)
3b
Chin. J. Chem. 2011, 29, 297— 302
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299

Khojastehnezhad et al.
FULL PAPER
Continued
b
c
Yield /%
Entry
Ar
Product
Time/min
m.p. (Lit. m.p.)/℃
290—291
3
40
89
21
(287—288)
3c
238—240
4
5
6
35
30
35
90
88
90
21
(239—240)
3d
243—245
21
(242—243)
3e
138—140
21
(139—140)
3f
202—205
7
40
89
21
(202—203)
3g
228—230
8
40
88
21
(227—228)
3h
210—212
9
40
86
21
(210—211)
3i
a
b
1
2 mmol β-naphthol and 1 mmol aryl aldehyde at 120 ℃ under solvent-free conditions. All the products were characterized by H
aryl aldehyde and refer to the pure isolated products.
c
NMR and IR spectral data and comparison of their melting points with those of authentic samples. The yields were calculated based on
300
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Chin. J. Chem. 2011, 29, 297— 302

Efficient and Reusable Catalyst for Facile Synthesis of 14-Aryl-14H-dibenzo[a,j]xanthenes
β-carbon of the α,β-unsaturated carbonyl group in acti-
vated intermediate II yields the intermediate III, which
finally converts to the product 3 by a cyclocondensation
reaction. Under these conditions, attempts to isolate the
intermediates I, II and III failed after careful monitor-
ing of reactions.
Conclusion
In conclusion, we have reported a simple new cata-
lytic method for the synthesis of 14-aryl-14H-dibenzo-
[a,j]xanthenes by one-pot condensation reaction of
β-naphthol and aryl aldehydes using PPA/SiO2 as an
efficient, reusable, and green heterogeneous catalyst
under solvent-free conditions. The catalyst can be recy-
cled after a simple work-up, and reused at least three
times without substantial reduction in its catalytic activ-
ity. High yields, short reaction times, easy work-up and
absence of any volatile and hazardous organic solvents
are just a few of the advantages in this procedure.
Figure 1 Reusability of PPA/SiO for model reaction
2
Although we did not investigate the reaction mecha-
nism, to show the catalyst’s role, a plausible mechanism
for the present reaction was proposed as depicted in
Scheme 2. The addition of nucleophiles to the aldehydes
is promoted by the protonation of the carbonyl group
using a Brønsted acid, enhancing the electrophilicity of
this moiety. Therefore, it is proposed that PPA/SiO2≡
HA facilitates the formation of intermediate I such that
after dehydration, intermediate II is produced. Nucleo-
philic attack of another molecule of β-naphthol at the
Acknowledgments
The authors are thankful to the Islamic Azad Univer-
sity, Mashhad Branch for financial support. A. Davood-
nia would also like to thank Dr. Shahriar Khadem for
his valuable advices.
Scheme 2
53.
7
8
9
Wang, H. K.; Morris-Natschke, S. L.; Lee, K. H. Med. Res.
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Chin. J. Chem. 2011, 29, 297— 302