Nano-SnCl4·SiO2 - A versatile and efficient catalyst for synthesis of 14-aryl- or 14-alkyl- 14H-dibenzo[a·j]xanthenes

Article abstract of DOI:10.1007/s10593-012-1066-3

2-Naphthol and various aldehydes were used for the synthesis of 14-aryl- or 14-alkyl-14H-di-benzo[a,j]xanthene in the presence of nano-SnCl ₄·SiO₂ under solvent-free condition. Nano-SnCl ₄·SiO₂ is an efficient,

Full text of DOI:10.1007/s10593-012-1066-3

Chemistry of Heterocyclic Compounds, Vol. 48, No. 6, September, 2012 (Russian Original Vol. 48, No. 6, June, 2012)
NANO-SnCl4·SiO2 – A VERSATILE AND EFFICIENT
CATALYST FOR SYNTHESIS OF 14-ARYL- OR 14-ALKYL-
14H-DIBENZO[a,j]XANTHENES
B. F. Mirjalili1*, A. Bamoniri2, and M. A. Mirhoseini1
2-Naphthol and various aldehydes were used for the synthesis of 14-aryl- or 14-alkyl-14H-di-
benzo[a,j]xanthene in the presence of nano-SnCl4·SiO2 under solvent-free condition. Nano-SnCl4·SiO2
is an efficient, readily available, and reusable catalyst for this reaction resulting in good to excellent
yields.
Keywords: 14-aryl- or 14-alkyl-14H-dibenzo[a,j]xanthenes, nano-SnCl4·SiO2, 2-naphthol, reusable
catalyst, solvent-free conditions.
Tin tetrachloride is a powerful Lewis acid which is, however, a highly volatile, corrosive liquid that is
difficult to handle. In the presence of moisture it is readily hydrolyzed to produce HCl. Silica-supported SnCl4
[1-2] is a mild solid Lewis acid, which could promote acid-catalyzed organic reactions. It does not need special
precautions for preparation, handling, or storage. It can be stored at ambient temperatures for months without
losing its catalytic activity. This catalyst was previously applied for the one-pot synthesis of β-acetamido
ketones [3], acylals [4], and silylation of hydroxyl group [5].
Benzoxanthenes possessing therapeutic and pharmacological properties, such as antibacterial [6], anti-
inflammatory [7], and antiviral [8] activities, are important compounds in organic synthesis. Furthermore, these
heterocyclic derivatives are used as sensitizers in photodynamic therapy [9], as leuco-dyes in laser technology
[10], as antagonists of the paralyzing action of zoxazolamine [11], and in fluorescent materials for visualization
of biomolecules [12]. 14-Aryl- or 14-alkyl-14H-dibenzo[a,j]xanthenes are prepared via condensation of
2-naphthol with various aldehydes. Previously, catalysts such as Mg(HSO4)2 [13], silica chloride [14], TaCl5
[15], SiO2–PrSO3H [16], Sc[N(SO2C8F17)2]3 [17], 1-methyl-3-(3-sulfopropyl)imidazolium hydrogen sulfate
(MIMPS)HSO4 [18], Fe(HSO4)3 [19], and BiCl3 [20]were applied for the above-mentioned reaction.
In continuation of our investigations on the application of solid acids in organic synthesis [3, 21–27], we
have studied the synthesis of these compounds in the presence of nano-silica-supported tin chloride as an acidic
catalyst. Nano-SnCl4·SiO2 was prepared via the reaction of nano-SiO2 with SnCl4. The morphology of
nanoparticles was observed with SEM. The particle sizes of the commercial nano-silica gel and synthesized
35% nano-SnCl4·SiO2 (1) were about 24 nm and 42 nm, respectively (Fig. 1).
_______
*To whom correspondence should be addressed, e-mail: fmirjalili@yazduni.ac.ir.
1College of Science, Yazd University, P.O. Box 8915813149, Yazd, Iran.
2University of Kashan, P.O. Box 87317-51167, Kashan, Iran; e-mail: bamoniri@kashanu.ac.ir.
_________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 923-927, May, 2012. Original
article submitted July 27, 2011.
856
0009-3122/12/4806-0856©2012 Springer Science+Business Media New York

Fig. 1. SEM photograph of (a) nano-SiO2 and (b) 35% nano-SnCl4·SiO2 (1).
Because the catalyst 1 produces HCl in water, we determined its acidic capacity by titration. We have
found that 1 g of catalyst produced 4.95 mmol of H+. Also, the loading amount of Sn on 1 g of 50% SnCl4·SiO2
(2) was determined by the addition of hot water and separation of the obtained Sn(OH)4 as a floating solid. The
calculated loading amount of Sn in the catalyst is 65.6 mg·g-1. The percent of SnCl4·SiO2 was calculated
TABLE 1. Preparation of 14-(4-Bromophenyl)-14H-dibenzo[a,j]xanthene
under Various Conditions*
O
OH
Catalyst
2
+
H
O
Entry Ref.
T, °C
90
Time, h
Solvent
Catalyst (g)
Yield, %
1
—
—
—
—
—
—
—
—
—
—
—
—
—
1
—
—
SiO2 (0.1)
10
65
74
93
20
30
35
—
80
70
72
92
82
2
70
1
30% SnCl4·SiO2 (0.05)
50% SnCl4·SiO2 (0.05)
50% SnCl4·SiO2 (0.05)
50% SnCl4·SiO2 (0.05)
50% SnCl4·SiO2 (0.05)
50% SnCl4·SiO2 (0.05)
50% SnCl4·SiO2 (0.05)
SnCl4 (0.5 ml)
3
70
1
—
4
90
1
—
5
reflux
reflux
reflux
reflux
90
2
CHCl3
EtOAc
H2O
EtOH
—
6
1.5
2
7
8
2
9
1
10
11
12
13
90
0.7
0.7
0.2
1
—
20% nano-SnCl4·SiO2 (0.015)
35% nano-SnCl4·SiO2 (0.01)
35% nano-SnCl4·SiO2 (0.015)
90
—
90
—
90
—
35% nano-SnCl4·SiO2 (0.015),
2nd run
14
—
90
1
—
35% nano-SnCl4·SiO2 (0.015),
70
3rd run
15
16
17
18
[13] 60
[15] 100
[16] 125
[17] 110
0.3
1
—
—
—
Mg(HSO4)2
TaCl5
97
95
98
93
0.3
5
SiO2–PrSO3H
perfluoro- Sc[N(SO2C8F17)2]3
decalin
19
20
21
[18] 100
[19] reflux
[20] 110
0.1
4
—
(ClCH2)2
—
(MIMPS)HSO4
Fe(HSO4)3
BiCl3
93
85
96
0.1
_______
* The ratio of 2-naphthol (mmol) : aldehyde (mmol) : 35% nano-SnCl4·SiO2 (g) is
2:1:0.015.
857

according to the assumed silica gel-bonded SnCl3. The chemical absorption of SnCl4 to silica gel is partial, and
the number of bonded chlorine atom to any Sn is unknown. Therefore, the calculated amounts of Sn and HCl
are not equivalent.
In this research, we have applied various SnCl4·SiO2 and nano-SnCl4·SiO2 catalysts for the synthesis of
14-aryl- or 14-alkyl-14H-dibenzo[a,j]xanthenes. The reaction of benzaldehyde with 2 equivalents of 2-naphthol
was examined to optimize the reaction conditions (Table 1). The best run in the presence of 50% SnCl4·SiO2 (2)
was achieved in a solvent-free regime at 90°C (Entry 4).
We have repeated the above-mentioned reactions with 35% nano-SnCl4·SiO2 and found that the activity
of the catalyst 1 is 3.3 times higher compared to bulk SnCl4·SiO2, requiring only 0.015 g. Moreover, the
reactions in the presence of 35% nano-SnCl4·SiO2 were completed in a shorter time (Entry 12). The reactions
were done in an open vessel without any N2 protection. The reaction when conducted with 1-naphthol instead of
2-naphthol did not afford any product. To examine the reusability of 35% nano-SnCl4·SiO2 (1) in a solvent-free
condition after each consequent run, the product was dissolved in CHCl3 and filtered through Whatman paper.
The catalyst residue was washed with acetone, dried at 100°C, and used once again. Treatment with acetone
removes the tar from the catalyst surface more efficiently (Entries 13 and 14). The catalyst 1 was found to be
reusable, although a gradual decline was observed in its activity. All of the obtained products were known
substances characterized by their IR and 1H NMR spectra and physical properties in the literature.
According to the obtained best conditions, we have applied 2-naphthol and various aldehydes for the
synthesis of 14-aryl- or 14-alkyl-14H-dibenzo[a,j]xanthene derivatives (Table 2).
TABLE 2. Synthesis of 14-Aryl- or 14-Alkyl-14H-dibenzo[a,j]xanthenes
in the Presence of 35% nano-SnCl4·SiO2 at 90°C for 40 min*
R
OH
35% nano-SnCl4·SiO2
RCHO
2
+
Neat, 90°C, 40 min
O
Mp, °C
R
Yield, %
Measured
Reported [Ref.]
Et
151-150
210-211
297-298
227-229
288-289
311-312
202-204
214-215
184-185
155-156
152-153
136-137
267-269
150-152 [13]
210-211 [13]
297 [13]
81
93
92
89
90
95
85
90
80
73
75
85
89
3-O2NC6H4
4-BrC6H4
4-MeC6H4
4-ClC6H4
4-O2NC6H4
4-MeOC6H4
2-ClC6H4
Ph
227-229 [13]
290 [20]
311-312 [13]
203-205 [13]
214-216 [13]
184 [20]
i-Bu
155-157 [13]
151-153 [17]
139-141 [16]
269-270 [16]
n-Bu
4-HOC6H4
2,4-ClC6H3
_______
* The ratio of 2-naphthol (mmol) : aldehyde (mmol) : 35% nano-SnCl4·SiO2 (g) is
2:1:0.015.
858

The applicability of the present method to a large-scale process was examined with 20 mmol of
2-naphthol and 10 mmol of benzaldehyde under thermal conditions, which gave 14-phenyldibenzo[a,j]xanthene
in 79% yield.
Thus, 35% nano-SnCl4·SiO2 was applied for the preparation of 14-aryl- or 14-alkyl-14H-dibenzo-
[a,j]xanthene derivatives in a simple and straightforward protocol. Short reaction time, high yields, simplicity of
operation, easy work-up, and catalyst reusability are advantages of this method.
EXPERIMENTAL
Melting points were determined on a Buchi melting point B-540 B.V.CHI apparatus and were
uncorrected. SEM of nano particles was performed with a VEGA/TESCAN scanning electron microscope. The
progress of the reactions of 2-naphthol and aldehydes was monitored by TLC using silica gel F 254 80 TLC
plates, with EtOAc–hexane 1:4 as eluent. The commercial nano-silica gel was purchased from Aldrich
company. All other chemicals were purchased from Sigma–Aldrich and Merck and were used without any
additional purification.
Preparation of 35% Nano-SnCl4·SiO2 (1). To a mixture of nano-silica gel (0.65 g) and CHCl3 (5 ml),
SnCl4 (0.35 g, 0.16 ml, 1.34 mmol) was added dropwise. The resulting suspension was stirred for 1 h at room
temperature, filtered, washed with chloroform, and dried at room temperature.
The synthesis of 20% nano-SnCl4·SiO2 was performed in a similar manner by mixing SnCl4 (0.2 g,
0.77 mmol) and nano-silica gel (0.8 g) in CHCl3 (5 ml).
Preparation of 50% SnCl4·SiO2 (2). To a mixture of silica gel (0.5 g) and CHCl3 (5 ml), SnCl4 (0.5 g,
0.22 ml, 1.92 mmol) was added dropwise. The resulting suspension was stirred for 1 h at room temperature,
filtered, washed with chloroform, and dried at room temperature.
The synthesis of 30% SnCl4·SiO2 was performed in a similar manner by mixing SnCl4 (0.3 g, 1.15
mmol) and silica gel (0.7 g) in CHCl3 (5 ml).
14-Aryl or 14-Alkyl-14H-dibenzo[a,j]xanthenes (General Method). A mixture of 2-naphthol (0.288 g,
2 mmol), aldehyde (1 mmol), and 35% nano-SnCl4·SiO2 (0.015 g) was heated at 90°C. After completion of the
reaction, the product was dissolved in CHCl3 and filtered to recover the catalyst, which was then washed with
hot EtOH. The filtrate was cooled to obtain crude 14-aryl- or 14-alkyl-14H-dibenzo[a,j]xanthene derivatives,
which were recrystallized from 2-PrOH–CHCl3, 8:2.
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