A simple and efficient procedure has been developed for the preparation of aryl-14H-dibenzo[a,j]xanthenes by a one-pot condensation of 2-naphthol and aryl aldehydes, in the presence of mesoporous zirconia modified with tungstophosphoric acid (ZrTPA60BT100), to be used as a heterogeneous catalyst in a solvent-free medium using conventional heating. The present approach offers the advantages of clean reaction, simple methodology, short reaction time, and high yield. The reaction work-up is very simple and the catalyst can be easily separated from the reaction mixture and reused several times in subsequent reactions without appreciable loss of the catalytic activity.Centro de Investigación y Desarrollo en Ciencias Aplicada

Blanco, Mirta Noemí

Pizzio, Luis René

Rivera Vélez, Toa Sumak

Romanelli, Gustavo Pablo

Green Chemistry Letters and Reviews

English

El Servicio de Difusión de la Creación Intelectual

RESEARCH LETTER
Green catalytic synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes using recyclable mesoporous
zirconia modified with tungstophosphoric acid
Toa S. Rivera, Mirta N. Blanco, Luis R. Pizzio and Gustavo P. Romanelli*
Departamento de Quı´mica, Facultad de Ciencias Exactas, Centro de Investigacio´n y Desarrollo en Ciencias Aplicadas ‘‘Dr. Jorge
J.J. Ronco’’ (CINDECA), UNLP-CCT La Plata CONICET, 47 N8 257, 1900 La Plata, Argentina
(Received 4 November 2011; final version received 23 December 2011)
A simple and efficient procedure has been developed for the preparation of aryl-14H-dibenzo[a,j]xanthenes by a
one-pot condensation of 2-naphthol and aryl aldehydes, in the presence of mesoporous zirconia modified with
tungstophosphoric acid (ZrTPA60BT100), to be used as a heterogeneous catalyst in a solvent-free medium using
conventional heating. The present approach offers the advantages of clean reaction, simple methodology, short
reaction time, and high yield. The reaction work-up is very simple and the catalyst can be easily separated from
the reaction mixture and reused several times in subsequent reactions without appreciable loss of the catalytic
activity.
R CHO +
OH
OR2 2H2O
ZrTPA60BT100
TPA bulk
130°C, solvent-free
+
Keywords: mesoporous zirconia; heteropolyacids; 14-aryl-14H-dibenzo[a,j]xanthenes; heterogeneous
catalysts; solvent-free synthesis
1. Introduction
The preparation of organic compounds involving a
greener process and under solvent-free conditions has
been investigated worldwide due to stringent envir-
onmental and economic regulations. The implemen-
tation of several transformations in a single
manipulation is highly compatible with the goals of
Green Chemistry (13).
In the last two decades, inorganic solid-catalyzed
organic transformations have gained much impor-
tance due to the proven advantage of heterogeneous
catalysis, such as simplified product isolation, mild
reaction conditions, high selectivities, easy catalyst
recovery and reuse, and reduction in generation of
waste byproducts (46).
Particularly, the catalysis by heteropolyacids
(HPAs) and related compounds is a field of increasing
importance worldwide. Numerous developments are
being carried out in basic research as well as in fine
chemistry processes (7). Due to the strong acidic
properties of solid HPAs, in the last decades, HPAs
have found applications as useful and versatile acid
catalysts for some acid-catalyzed reactions (8,9). They
can be used in bulk or supported form in both
homogeneous and heterogeneous systems. Further-
more, HPAs have several advantages, including
high flexibility in modification of acid strength,
environmental compatibility, nontoxicity, and ease
of handling (10).
On the other hand, zirconia is an interesting
material to be used as catalyst support due to its
thermal stability in different atmospheres. Its acid
properties can be modified by the addition of cationic
or anionic substances, such as sulfate or tungstate
(1113). The addition of Keggin HPAs to modify
them has been studied to a lesser extent (1417).
The most common methods that can be used to
obtain zirconia are the sol-gel method, the micellar
*Corresponding author. Email: gpr@quimica.unlp.edu.ar
Green Chemistry Letters and Reviews
Vol. 5, No. 3, September 2012, 433437
ISSN 1751-8253 print/ISSN 1751-7192 online
# 2012 Toa S. Rivera, Mirta N. Blanco, Luis R. Pizzio and Gustavo P. Romanelli
http://dx.doi.org/10.1080/17518253.2012.665947
http://www.tandfonline.com
technique, or mechanochemical synthesis (1618).
Zirconia has been frequently prepared by the micellar
method, while the sol-gel method from an alkoxide
has been less used (16,18).
On the other hand, xanthenes and benzoxanthene
derivatives have received considerable interest from
the pharmaceutical industry due to their wide range
of interesting biological and therapeutic properties
(19) such as anti-inflammatory (20), antibacterial
(21), antiviral (22), CNS stimulating (23) activity as
well as their use in photodynamic therapy (PDT: a
method of treating tumors by the combined use of
photosensitizer and light) (24) and for antagonism of
the paralyzing action of zoxazolamine (25). The other
useful applications of these heterocycles are as
dyes (26), fluorescent materials for visualization of
biomolecules (27), luminescent sensors (28), and in
laser technologies (29).
One of the best reported protocols for the
synthesis of 14-aryl-14H-dibenzo-[a,j]xanthenes in-
volves the mixing of 2-naphthol with aldehydes in
the presence of a catalyst (30), such as AcOH-H2SO4
(31), p-TSA (32), iodine (33), Amberlist-15 (34),
P2O5/Al2O3 (35), silica sulfuric acid (36),
Dowex-50W (37), ionic liquid (38), bulk (39), and
silica-supported (40) HPAs.
As reported by Kumar and co-workers (41), some
of these methods show varying degrees of success as
well as limitations such as long reaction times, low
yields, use of toxic solvents, and harsh reaction
conditions. Thus, the development of an alternative
clean procedure is highly needed for the synthesis of
benzoxanthenes and relative compounds. In this
direction our research group has experience in the
friendly synthesis of heterocycle compounds using
HPAs, for example, coumarins (42), dihydropyrimi-
dinones (43), azlactones (44), and flavones (45).
As part of our research project directed towards
the development of highly suitable methods and the
synthesis of diverse heterocyclic compounds, we
herein disclose a new, one-pot synthesis of 14-
aryl-14H-dibenzo[a,j]-xanthenes from aldehydes and
2-naphthol catalyzed by mesoporous zirconia mod-
ified with tungstophosphoric acid under solvent-free
conditions with the aim to obtain high yields with an
environmentally advantageous procedure (Scheme 1).
2. Results and discussion
In continuation of our interest in the application of
heteropolyacid catalysts for the development of a
useful synthetic methodology, we wish to report a
simple and efficient approach for the preparation of
14-aryl-14H-dibenzo[a,j]xanthenes using mesoporous
zirconia modified with tungstophosphoric acid as an
eco-friendly catalyst with high catalytic activity under
solvent-free conditions at 100 8C.
Initially we conducted blank experiments without
the presence of a catalyst. In order to obtain the
optimal reaction temperature, four temperatures (80,
100, 120 and 130 8C) were tested. The experimental
reaction conditions were: 1.2 mmol of benzaldehyde,
2 mmol of 2-naphtol, solvent-free conditions, and a
reaction time of 1.5 h. No reaction was observed at 80
and 100 8C (Table 1, entries 1 and 2). A temperature
increase gives low substrate conversion. The yield of
14-phenyl-14H-dibenzo[a,j]xanthene for a reaction
time of 1.5 h at 120 8C was only 15% (Table 1, entry
3), whereas at 130 8C the yield was 32% (Table 1,
entry 4). It was also observed that for a reaction time
of 5 h, the yield decreased 2% when the reaction
temperature was 130 8C (Table 1, entry 5).
Afterwards, the catalytic activity of the bulk
H3PW12O40 (TPA) was tested in the preparation of
1-phenyl-14H-dibenzo[a,j]xanthene. The experimen-
tal reaction conditions were 1.2 mmol of benzalde-
hyde, 2 mmol of 2-naphthol, 50 mg of bulk catalyst,
and solvent-free conditions. No reaction was
observed at 80 8C and only traces were obtained at
100 8C for a reaction time of 1 h (Table 1, entries 6
and 7, respectively). The yields of 1-phenyl-
14H-dibenzo[a,j] xanthene increased notably for a
reaction time of 1 h at 130 8C. In this case the yield
was 90% (Table 1, entry 9).
R CHO +
OH
OR2 2H2O
ZrTPA60BT100
130°C, solvent-free
+
1 2
3
R = aryl
(1.2 mmol)             (2 mmol)
Scheme 1. Synthesis of aryl-14H-dibenzo[a,j]xanthenes with ZrTPA60BT100 as catalyst.
434 T.S. Rivera et al.
Finally, we studied the catalytic activity of the
mesoporous zirconia modified with tungstophospho-
ric acid prepared in the present work. The
ZrTPA60BT100 was tested in the preparation of
1-phenyl-14H-dibenzo[a,j]xanthene. The experimen-
tal reaction conditions were 1.2 mmol of benzalde-
hyde, 2 mmol of 2-naphthol, 85 mg of supported
catalyst, and solvent-free conditions. No reaction was
observed at 80 8C and only a 10% yield of product
was obtained at 100 8C for a reaction time of 3 h
(Table 1, entries 10 and 11). The yield of 1-phenyl-
14H-dibenzo[a,j]xanthene increased notably for a
reaction time of 1 h at 130 8C. In this case, the yield
was 99% (Table 1, entry 13), being noticeably more
active than the bulk catalyst.
The reusability of the catalyst was checked by
separating the mesoporous zirconia modified with
tungstophosphoric acid catalyst from the reaction
mixture by simple filtration, washing with toluene,
and drying in a vacuum oven at room temperature for
4 h prior to reuse in subsequent reactions without
significant loss in product yields (Table 1, entries 14,
15, and 16).
After optimization of the reaction conditions, we
studied the generality of this condition to other
substrates. Using this method, different substrates
were reacted with 2-naphthol to produce the
corresponding 14-aryl-14H-dibenzo[a,j]xanthenes at
130 8C in solvent-free conditions. The results are
summarized in Table 2. Several aromatic aldehydes
with different functional groups were subjected to the
condensation reaction, and the desired products were
synthesized in good to excellent yields. The substi-
tuted functional groups in the aromatic ring of the
aldehyde affect the yield and the reaction time. In
comparison with electron-withdrawing groups in the
aldehyde, we found that the presence of electron-
donating groups in the aldehyde decreased both the
reaction rate and the yields of products (Table 2,
entries 8, 9, and 10).
3. Experimental
All chemical reagents and solvents were obtained
from Aldrich and were used without further purifica-
tion. All products are known compounds and were
identified by comparison of their physical and
spectral data with those of the authentic samples.
Melting points were measured on a Bioamerican Bs
448 apparatus and were uncorrected. 1H-NMR and
13C-NMR spectra were recorded on a Varian 200
MHz. The purity of the substances and the progress
of the reaction were monitored by thin layer chro-
matography (TLC) on silica gel, and yields refer to
isolated products.
3.1. Preparation of catalyst
Zirconium propoxide (Aldrich, 26.6 g) was mixed
with absolute ethanol (Merck, 336.1 g) and stirred for
10 min to obtain a homogeneous solution under N2 at
room temperature. Then 0.47 cm3 of 0.28 M HCl
aqueous solution was dropped slowly into the afore-
mentioned mixture to catalyze the sol-gel reaction.
Table 1. Reaction between 2-naphthol and benzaldehyde
under different conditions.
Entry Catalizador
Temperature
(8C)
Time
(h)
Yielda
(%)
1 Without
catalyst
80 1.5 
2 100 1.5 
3 120 1.5 15
4 130 1.5 32
5 130 5 30
6 TPA bulkb 80 5 
7 100 1 Traces
8 100 3 10
9 130 1 90
10 ZrTPA60BT100
c 80 5 
11 100 3 10
12 120 2 82
13 130 1 99
14 130d 1 98
15 130d 1 98
16 130d 1 96
aThe yields refer to the isolated product.
bTPA bulk50 mg.
cZrTPA60BT10084 mg.
dFirst, second, and third reuse.
Table 2. Synthesis of aryl-14H-dibenzo[a,j]xanthenes with
ZrTPA60BT100 as catalyst.
a
Entry Aromatic aldehyde
Time
(h)
Yieldb
(%)
1 Benzaldehyde 1 99
2 4-Methylbenzaldehyde 1 94
3 4-Methoxybenzaldehyde 1 93
4 4-Florobenzaldehyde 2 98
5 4-Chlorobenzaldehyde 2 96
6 4-Bromobenzaldehyde 2 93
7 2-Chlorobenzaldehyde 2 89
8 2,4,6-
Trimethoxybenzaldehyde
5 60
9 2-Naphthaldehyde 3 78
10 1-Naphthaldehyde 3 81
aReaction conditions: aldehyde, 1.2 mmol; 2-naphthol, 2 mmol;
catalyst: ZrTPA60BT100, 85 mg, 130 8C.
bThe yields refer to the isolated product.
Green Chemistry Letters and Reviews 435
After 3 h, an appropriate amount of
PEGalcoholwater (1:5:1 weight ratio) solution
was added to the hydrolyzed solution under vigorous
stirring to act as template. The amount of added
solution was fixed in order to obtain a template
concentration of 10% by weight in the final material.
A TPA solution (whose concentration was fixed in
order to obtain TPA concentration of 60% w/w in the
solid) was added together with the addition of PEG
(sample ZrTPA60B).
The gel was kept in a beaker at room temperature
up to dryness. The solid was ground into powder and
extracted with distilled water for three periods of 8 h,
in a system with continuous stirring, to remove PEG.
Afterwards, the solids were calcined at 100 8C for
24 h.
3.2. Typical procedure for aryl-14H-dibenzo[a,j]
xanthenes synthesis using ZrTPA60BT100 catalyst
A mixture of the aldehyde (1.2 mmol), 2-naphthol
(2 mmol), and ZrTPA60BT100 (85 mg) was stirred at
130 8C for the appropriate time according to Table 1.
Completion of the reaction was indicated by TLC
The reaction mixture was cooled to 25 8C, toluene (5
cm3) was added, and then the mixture was stirred for
15 min and filtered to separate the catalyst, which was
subsequently washed twice with toluene (3 cm3). The
combined toluene extracts were washed twice with
water (5 cm3), dried over anhydrous Na2SO4 and
evaporated in vacuo. The solid obtained was recrys-
tallized from ethyl alcohol to afford the pure 14-aryl-
14H-dibenzo[a,j]xanthenes derivatives. After the re-
action, the catalyst was filtered, washed thoroughly
with toluene, dried in a vacuum oven (room tem-
perature, 4 h) and reused for the next reaction,
following the procedure described earlier.
3.3. Representative spectral 1H-NMR and 13C-NMR
data
14-(4-Methylphenyl)-14H-dibenzo[a,j]xanthenes
(Table 1, entry 2): Mp: 237239 8C. 1H-NMR
(200 MHz, CDCl3): d 2.15 (s, 3H), 6.37 (s, 1H),
6.92 (d, J9.5 Hz, 2H), 7.307.78 (m, 12H), 8.33
(d, J9.4Hz, 2H). 13C-NMR (50 MHz, CDCl3): d
19.2, 35.2, 115.8, 116.3, 121.3, 122.8, 125.3, 126.5,
127.6, 129.3, 129.3, 129.4, 129.5, 134.0, 140.8, 146.6.
14-(4-Methoxylphenyl)-14H-dibenzo[a,j]xanthenes
(Table 1, entry 3): 211214 8C. 1H-NMR (200 MHz,
CDCl3): d 3.55 (s, 3H), 6.42 (s, 2H), 6.64 (d, J9.6
Hz, 2H), 7.307.82 (m, 12H), 8.35 (d, J9.6 Hz, 2H).
13C-NMR (50 MHz, CDCl3): d 36.5, 53.1, 114.3,
117.4, 118.5, 123.6, 124.2, 127.3, 128.1, 129.2, 129.5,
131.5, 133.8, 137.4, 149.1, 158.1.
14-(4-Chlorophenyl)-14H-dibenzo[a,j]xanthenes
(Table 1, entry 5): 288290 8C. 1H-NMR (200 MHz,
CDCl3): d 6.44 (s, 1H), 7.088.32 (m, 16H).
13C-NMR (50 MHz, CDCl3): 37.4, 116.8, 118.2,
122.5, 125.4, 126.9, 128.7, 129.0, 129.5, 129.6, 131.1,
131.3, 132.0, 143.5, 148.7.
4. Conclusion
In conclusion, ZrTPA60BT100 as a noncorrosive,
environmentally friendly and stable heterogeneous
material has been proved to be a very efficient
catalyst for the synthesis of dibenzoxanthene
derivatives. A convenient and green process for the
synthesis of aryl-14H-dibenzo[a,j]xanthenes by con-
densation of various aromatic aldehydes with
2-naphthol using ZrTPA60BT100 as a heterogeneous
acid catalyst in a solvent-free medium using
conventional heating at 130 8C has been developed.
In addition, short reaction times, excellent yields, a
straightforward procedure, and relative nontoxicity
of the catalyst are other noteworthy advantages of the
present method. Finally, this solid acid catalyst can
be recovered and reused at least four times with
negligible loss in its activity. Further applications of
ZrTPA60BT100 in green organic transformations are
currently in progress in our laboratory.
Acknowledgements
The authors thank Agencia Nacional de Promocio´n
Cientı´fica y Tecnolo´gica, CONICET, and Universidad
Nacional de La Plata for financial support, and L. Soto,
L. Osiglio, D. Pen˜a, and N. Firpo for their experimental
collaboration in the experimental measurements.
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Green Chemistry Letters and Reviews 437


Green catalytic synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes using recyclable mesoporous zirconia modified with tungstophosphoric acid

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Green catalytic synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes using recyclable mesoporous zirconia modified with tungstophosphoric acid

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