Highly efficient synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes Catalyzed by carbon-based solid acid under solvent-free conditions

Article abstract of DOI:10.1080/00397910902792630

Carbon-based solid acid, a strong solid acid catalyst, was prepared readily by sulfonation and carbonization of aromatic compounds such as naphthalene in sulfuric acid at 250°C. This reusable and ecofriendly carbon-based solid acid highly efficiently cond

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Highly Efficient Synthesis
of 14-Aryl-14H-
dibenzo[a,j]xanthenes
Catalyzed by Carbon-Based
Solid Acid Under Solvent-Free
Conditions
a
a
Valiollah Mirkhani , Majid Moghadam , Shahram
a
a
Tangestaninejad , Iraj Mohammadpoor-Baltork
&
a
Mohammad Mahdavi
a
Chemistry Department, Catalysis Division,
University of Isfahan, Isfahan, Iran
Version of record first published: 12 Nov 2009
To cite this article: Valiollah Mirkhani, Majid Moghadam, Shahram Tangestaninejad,
Iraj Mohammadpoor-Baltork & Mohammad Mahdavi (2009): Highly Efficient Synthesis
of 14-Aryl-14H-dibenzo[a,j]xanthenes Catalyzed by Carbon-Based Solid Acid Under
Solvent-Free Conditions, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 39:24, 4328-4340
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1
Synthetic Communications , 39: 4328–4340, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902792630
Highly Efficient Synthesis of 14-Aryl-14H-
dibenzo[a,j]xanthenes Catalyzed by
Carbon-Based Solid Acid Under Solvent-Free
Conditions
Valiollah Mirkhani, Majid Moghadam, Shahram Tangestaninejad,
Iraj Mohammadpoor-Baltork, and Mohammad Mahdavi
Chemistry Department, Catalysis Division, University of Isfahan,
Isfahan, Iran
Abstract: Carbon-based solid acid, a strong solid acid catalyst, was prepared
readily by sulfonation and carbonization of aromatic compounds such as
ꢀ
naphthalene in sulfuric acid at 250 C. This reusable and ecofriendly carbon-based
solid acid highly efficiently condensed b-naphtol with aromatic aldehydes under
solvent-free conditions, and the corresponding aryl-14-H-dibenzo[a,j]xanthenes
were obtained in excellent yields. The catalyst can be reused several times without
loss of its activity.
Keywords: Naphthol, solid acid catalyst, solvent-free, sulfonated carbon,
xanthene
INTRODUCTION
Xanthenes and dibenzoxanthenes are important products used as dyes
and fluorescent visualization materials for biomolecules and laser tech-
[
1,2]
nologies.
gated for agricultural bactericide activity,
Furthermore, recently these compounds have been investi-
[
3–5]
[6]
photodynamic therapy,
[7]
[8]
anti-inflammatory effect, and antiviral activity. Many procedures
Received September 2, 2008.
Address correspondence to Valiollah Mirkhani, Chemistry Department,
Catalyst Division, University of Isfahan, Isfahan 81746-73441, Iran. E-mail:
mirkhani@sci.ui.ac.ir
4
328

Carbon-Based Solid Acid Catalyst
4329
have been reported for synthesis of xanthenes, including palladium-
[
catalyzed cyclization of polycyclic aryltriflate esters, intramolecular
9]
[10]
trapping of benzynes by phenols,
reaction of aryloxymagnesium
halides with triethylorthoformate, and condensation of b-naphtol with
[11–20]
a wide variety of aldehydes by acid catalysts.
However, these meth-
ods suffer from disadvantages such as poor yields, expensive reagents,
long reaction times (16 h to 5 days), toxic organic solvents, strongly acidic
conditions, and production a lot of waste salt as a by-product.
Traditional homogeneous acid catalysts such as H SO , HCl, H PO ,
2
4
3
4
or CH COOH are toxic, corrosive, and hard to remove from the reaction
3
medium. Environmental pollution is a worldwide problem. Recyclable
solid acids can be used to help solve this problem. Heterogeneous cata-
lysts have several advantages in comparison with their homogeneous
counterparts. Easy separation, easy handling, and recyclability of the
catalyst or the possibility of continuous operation of the reactor are
attractive features for any industrial application. The replacement of
environmentally hazardous catalysts with the use of solid acid catalysts
is one of the innovative trends, and preparation of cheap, recyclable, non-
toxic, strong solid acids is an attractive area in green chemistry. This kind
of solid acid must be as strong, active, and stable as sulfuric acid. A good
material for this purpose should have high stability and numerous strong
protonic acid sites. Although many inorganic-oxide solid acids (such as
zeolites and niobic acid, which provide acidic OH groups) and organic
acids (such as cationic exchange resins and perfluorosulfonated ionomers
with SO H groups as strong acidic sites) have been studied extensively,
3
these materials are expensive and the activity of acidic sites is much less
than liquid inorganic acids. Therefore, adequate performance in acid-
catalyzed reactions in aqueous media has not been obtained. Carbon-
based solid acid is a promising alternate for sulfuric catalyst. However,
sulfonation of carbon in concentrated sulfuric acid is very difficult, and
finally a mellitic acid is produced. Therefore, the sulfonation of carbon
nanotubes has been investigated for preparation of sulfonated carbon,
but carbon nanotubes are very expensive and the degree of sulfonation
[
21]
[22]
is very low. Okamura et al. used pyrrolyzed D-glucose for prepara-
tion of sulfonated carbon as a solid acid, and Zong et al. used this solid
acid for efficient production of biodiesel from fatty acids and waste oils.
[
23]
Michikazu Hara and coworkers
reported the synthesis of a carbon-
based solid acid with a high density of sulfonic acid (SO H). Such
3
carbon-based solid acids can be readily prepared by heating aromatic
compounds such as naphthalene in sulfuric acid under N at 200–
2
ꢀ
[22–30]
3
00 C.
In this synthesis, the sulfonation of the aromatic compounds
is the first stage of the reaction. The resulting sulfonated aromatic com-
pounds are incompletely carbonized, which results in the formation of a

4330
V. Mirkhani et al.
Scheme 1. Preparation of 14-aryl-14-H-dibenzo[a,j]xanthenes.
solid with a nominal sample composition of CH0.35O0.35S0.14. The solid
powder was insoluble in water and organic solvents even at boiling tem-
peratures. Here, we report this carbon-based solid acid as an efficient and
promising alternate heterogeneous solid acid catalyst for preparation of
1
4-aryl-14-H-dibenzo[a,j]xanthenes by condensation of 2-naphtol with a
wide variety of aldehydes under solvent free and reflux conditions
Scheme 1).
(
EXPERIMENTAL
Instruments
Melting points were measured on an Electrothermal 9100 apparatus.
1
Nuclear magnetic resonance ( H NMR) spectra were recorded on a
Bruker DRX-300 Avance spectrometer using tetramethylsilane (TMS)
as an internal standard (CDCl solution). Fourier transform–infrared
3
spectra (FT-IR) were recorded on a Nicolet Impact 400D instrument.
X-ray diffraction patterns (XRD) were obtained on a Bruker D8 Avance
instrument.
All of the products were characterized by a comparison of their spec-
12–20]
[
tral and physical data with those of authentic samples.
All yields
refer to isolated products. The purity of the substances and the progress
of the reactions were monitored by thin-layer chromatography (TLC).
General Procedure for Preparation of Carbon-Based Solid Acid
In a typical procedure for the preparation of carbon-based solid acid,
[
23]
based on Hara’s method,
concentrated H SO (98 wt%). The solution was heated for 15 h in
2
naphthalene (20 g) was added to 100 mL of
2
4
ꢀ
50 C under N . After that, excess sulfuric acid was removed from the
2
mixture by vacuum distillation, which resulted a black solid. The solid
was ground and washed with boiling distilled water until no sulfate ion
was detected in the filtrates by BaCl . After that, the product was dried
2
ꢀ
ꢀ
in an oven at 100 C and then heated at 300 C to remove the trapped
sulfuric acid. The solid was insoluble in water and organic solvents.

Carbon-Based Solid Acid Catalyst
4331
The obtained catalyst is a sulfonated carbon solid acid with nominal sample
composition (CH0.4O0.3S0.12) with an amorphous structure. Reaction is
sensitive to oxygen, because the oxygen oxidized the carbon, and therefore
the yield of the product and degree of sulfonation decreased. At cooler
temperatures, the reaction times increased, and at hotter temperatures,
H SO decomposed rapidly. Reaction times more than 15 h cannot affect
2
4
the product yield and sulfonation degree. Under the optimized reaction
conditions, it is possible to prepare a reproducible carbon-based solid acid.
After preparation of the catalyst several times under optimized conditions,
no significant changes were observed in the sulfonation degree.
General Procedure for the Preparation of Aryl-14-H-
dibenzo[a,j]xanthenes in 1,2-Dichloroethane Under Reflux Conditions
A mixture of b-naphthol (2 mmol), aromatic aldehyde (1 mmol), and
highly sulfonated carbon (20 mg) in 1,2-dichloroethane (10 mL) was
heated under reflux for appropriate time according to Table 1. The pro-
gress of the reaction was monitored by TLC. After completion of the
reaction, the catalyst was filtered and washed with hot ethanol. The
organic solvent was evaporated, and the crude product was recrystallized
from EtOH to afford the pure product.
General Procedure for the Preparation of Aryl-14-H-
dibenzo[a,j]xanthenes Under Solvent-Free Conditions
In the solvent-free procedure, a mixture of aromatic aldehyde (1 mmol),
b-naphtol (2 mmol), and carbon-based solid acid (20 mg) was heated and
Table 1. Synthesis of 3b catalyzed by carbon-based solid
a
acid in different solvents
b
Entry
Solvent
Time (h)
Yield (%)
1
2
3
4
5
6
7
CH OH
3
10
10
12
9
12
13
0.5
65
60
55
94
50
10
96
CH CH OH
3
2
CH
ClCH
3
CN
2 2
CH Cl
CHCl3
DMF
Solvent-free
a
Reaction conditions: 4-chlorobenzaldehyde (1 mmol), b-
naphthol (2 mmol), catalyst (20 mg), reflux.
b
Isolated yields.

4332
V. Mirkhani et al.
ꢀ
stirred at 85 C in an oil bath for the appropriate time according to
Table 1. The progress of the reaction was monitored by TLC. After com-
pletion of the reaction, hot ethanol (10 mL) was added, and the reaction
mixture was filtered. The organic solvent was evaporated, and the crude
product was recrystallized from ethanol (EtOH) to afford the pure
product.
RESULTS AND DISCUSSION
Characterization of Catalyst
The density of ꢁSO H groups on sulfonated carbon was determined by
3
potentiometric titration by NaOH (0.01 N), and it was in agreement with
elemental analysis (CH0.4O0.3S0.12). The neutralization titration showed
ꢁ
1
that the amount of SO H attached to carbon is 4.75 mmol g
.
The XRD pattern (Fig. 1) exhibits two broad and weak diffraction
3
ꢀ
ꢀ
peaks (2h ¼ 10–30 , 35–50 ), which are attributed to amorphous carbon.
The x-ray diffractogram is similar to char because the sulfonic acid
groups were attached to edges of isotropic carbon sheets and the solid
acid exhibits broad diffraction peaks of amorphous carbon.
Optimization of Conditions
First, we examined the effect of different solvents on the synthesis of
4-chlorophenyl derivative of xanthene. Among the different solvents, such
as methanol, ethanol, acetonitrile, chloroform, and 1,2-dichloroethane,
Figure 1. XRD pattern of sulfonated carbon.

Carbon-Based Solid Acid Catalyst
4333
Figure 2. Effect of catalyst amount on the synthesis of 14-(4-chlorophenyl)-14H-
dibenzo[a,j]xanthene.
and a solvent-free system (Table 1), the best yields were obtained under
solvent-free conditions and in the refluxing 1,2-dichloroethane.
The optimization of catalyst amount was carried out in the reaction
of 4-chlorobenzaldehyde (1 mmol) with b-naphthol (2 mmol) to afford
ꢀ
the product 3b under solvent-free conditions at 85 C. As can be seen
from Fig. 2, the best results were obtained with 20 mg of catalyst under
solvent-free conditions, and the corresponding xanthene derivative was
obtained in good yield (96% isolated yield).
The optimum temperature was obtained using the same reaction
under these conditions. The results showed that the best results were
ꢀ
obtained at 85 C (Table 2).
Table 2. Effect of temperature on the solvent-free
a
synthesis of 3b
Temp.
ꢀ
Yield
b
(%)
Entry
( C)
1
2
3
4
5
6
70
80
85
90
95
100
65
92
96
90
85
80
a
Reaction conditions: 4-chlorobenzaldehyde (1 mmol),
b-naphthol (2 mmol), catalyst (20 mg), 30 min.
Isolated yield.
b

4334
V. Mirkhani et al.
Synthesis of 14-Aryl-14H-dibenzo[a,j]xanthene Derivatives Under
Solvent-Free Conditions and in 1,2-Dichloroethane
The synthesis of substituted xanthenes was carried out in the presence of
catalytic amounts of carbon-based solid acid in the reaction of various
aromatic aldehydes with b-naphthol under solvent-free conditions at
ꢀ
8
the contents with a small magnet in an oil bath. In a typical experimental
5 C. The mixing method was done easily in a large test tube by stirring
procedure, a mixture of b-naphthol (2 mmol) and aromatic aldehyde
(1 mmol) was heated in the presence of a catalytic amount of carbon-
based solid acid (20 mg), and the corresponding xanthene derivatives
were obtained in good to excellent isolated yields (Table 3). The obtained
results showed that the nature of the substituents in the aromatic ring has
no significant effect on the product yield. Benzaldehyde and other aro-
matic aldehydes containing electron-withdrawing and electron-donating
groups were converted efficiently to the corresponding dibenzoxanthenes
in excellent yields. The catalyst plays a crucial role in this reaction. In the
absence of the catalyst, the reaction of 4-chlorobenzaldehyde with b-
naphthol produced less than 10% of the corresponding xanthene after a
1
prolonged time. All products were characterized by melting point, H
NMR, and IR spectral data.
These reactions were carried out in refluxing 1,2-dichloroethane in
the presence of carbon-based solid acid as catalyst. The results (Table 3)
showed that in comparison with solvent-free system the reaction times
increased, but the product yields were comparable to solvent-free
conditions.
To show the catalyst’s role, a probable mechanism has been pro-
posed in Scheme 2.
The efficiency of carbon-based solid acid (reaction time, yield, and
reaction conditions) was compared with other catalysts in the synthesis
of 14-aryl-14H-dibenzo[a,j]xanthene derivatives (Table 4). It is clear that
the present method is superior in terms of reaction times, product yields,
and catalyst amounts.
Reusability of the Catalyst
We found that carbon-based solid acid shows good catalytic activity for
preparation of 14-aryl-14-H-dibenzo[a,j]xanthenes and could be recov-
ered and recycled several times without significant loss of activity. At
the end of each reaction, the catalyst was filtered, washed consecutively
ꢀ
with hot ethanol and diethyl ether, dried at 180 C for 1 h, and reused
in another reaction. The results showed that this solid acid catalyst can

Carbon-Based Solid Acid Catalyst
4335
Table 3. Synthesis of aryl-14-H-dibenzo[a,j]xanthenes catalyzed by carbon-based
solid acid
Solvent-free
procedure
Solvent
procedure
Mp
ꢀ
( C)
Time Yield Time Yield
a
a
Entry
a
R
(min) (%)
(h)
(%)
Found Reported
35
30
30
92
96
94
9
89
182
285
311
193
287
312
b
8
94
92
b
c
10
d
e
f
30
45
40
92
93
90
12
13
14
91
90
89
292
258
236
293
260
237
g
45
30
92
93
11
30
91
93
204
226
205
228
h
i
30
93
9
92
189
190
(
Continued )

4336
V. Mirkhani et al.
Table 3. Continued
Solvent-free
procedure
Solvent
procedure
Mp
ꢀ
( C)
Time Yield Time Yield
a
a
(%)
Entry
j
R
(min) (%)
(h)
Found Reported
30
35
94
93
8
93
294
237
296
238
k
8
90
a
Isolated yields.
b
ꢀ
Reaction temperature 125 C.
be reused several times (eight times were checked) without loss of its
catalytic activity or leaching (Table 5). By TLC monitoring the reaction
mixtures, no impurities were detected in the products only after use of the
1
catalyst. This subject was proved by comparing melting point, H NMR,
and IR spectral data of products with authentic samples. On the other
hand, the black color of the catalyst is a characteristic sign of leaching
Scheme 2. Suggested transformation mechanism.

Carbon-Based Solid Acid Catalyst
4337
Table 4. Comparison of catalytic activity of carbon-based solid acid with pre-
viously reported catalysts in the synthesis of 14-aryl-14H-dibenzo[a,j]xanthene
derivatives
Yield
(%)
Time
(h)
Catalyst
Conditions
Ref.
ꢀ
Carbon-based solid acid Solvent-free, 85 C
Carbon-based solid acid 1,2-Dichloroethane (reflux) 89–94
90–96 0.5–0.75
8–14
80–96 2.5–6
—
—
12
12
14
15
16
ꢀ
p-Toluene sulfonic acid
p-Toluene sulfonic acid
Sulfamic acid
Solvent-free, 125 C
1,2-Dichloroethane (reflux) 85–95 15–24
ꢀ
Solvent-free, 125 C
ꢀ
90–95
60–90
78–91
6–12
73
1–2
H
2
SO
Dowex-50 W
4
AcOH, 80 C
Solvent-free, 100 C
ꢀ
or a homogeneous reaction. In all reactions, after completion, the filtered
solvent was perfectly light. After the catalyst was filtered, fresh aldehyde
and b-naphthol were added to the filtrates. Execution of reaction under
the same conditions reveals that no catalyst had been leached during
the reaction. More experimental studies about leaching were done by
boiling carbon-based solid acid in various organic solvents and hot filtra-
tion after boiling. In all of these experiments, no weight loss or leaching
were seen. We are confident that the carbon-based solid acid is insoluble
in organic solvents.
Table 5. Reusability of the catalyst in the solvent-free synthesis
a
of 3b
Temp.
ꢀ
Yield
b
(%)
Run
( C)
1
2
3
4
5
6
7
8
85
85
85
85
85
85
85
85
96
94
96
96
95
96
95
95
a
Reaction conditions: 4-chlorobenzaldehyde (1 mmol), b-
naphtol (2 mmol), catalyst (20 mg), 30 min.
b
Isolated yield.

4338
V. Mirkhani et al.
CONCLUSION
In conclusion, we have described a very simple and convenient procedure
for the synthesis of 4-substituted 14-H-dibenzo[a,j]xanthene derivatives
catalyzed by noncorrosive, environmentally benign carbon-based solid
acid in solvent and under solvent-free conditions. In addition, it is possi-
ble to synthesize interesting biologically products under solvent-free
media, which is less expensive and less toxic than those with organic sol-
vents. Also, the catalyst is recyclable and could be reused without loss of
its activity.
ACKNOWLEDGMENT
The financial support of this work by the Center of Excellence of
Chemistry of University of Isfahan is acknowledged.
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