Fluoroboric acid adsorbed on silica-gel-catalyzed synthesis of 14-aryl-14H-dibenzo[a,j]xanthene derivatives

Article abstract of DOI:10.1080/00397910802412842

A simple and efficient procedure has been developed for the preparation of 14-aryl-14H-dibenzo[a,j]xanthene derivatives using fluoroboric acid adsorbed on silica gel as a heterogeneous catalyst. The methodology involves the one-pot condensation reaction o

Full text of DOI:10.1080/00397910802412842

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Fluoroboric Acid Adsorbed
on Silica-Gel–Catalyzed
Synthesis of 14-Aryl-14H-
dibenzo[a,j]xanthene
Derivatives
Yu-Heng Liu a , Xu-Ye Tao a , Li-Qin Lei a & Zhan-Hui
Zhang a
a College of Chemistry and Material Science, Hebei
Normal University, Shijiazhuang, China
Published online: 27 Jan 2009.
To cite this article: Yu-Heng Liu , Xu-Ye Tao , Li-Qin Lei & Zhan-Hui Zhang (2009):
Fluoroboric Acid Adsorbed on Silica-Gel–Catalyzed Synthesis of 14-Aryl-14H-
dibenzo[a,j]xanthene Derivatives, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 39:4, 580-589
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Synthetic Communications1, 39: 580–589, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802412842
Fluoroboric Acid Adsorbed on
Silica-Gel–Catalyzed Synthesis of
14-Aryl-14H-dibenzo[a,j]xanthene Derivatives
Yu-Heng Liu, Xu-Ye Tao, Li-Qin Lei, and Zhan-Hui Zhang
College of Chemistry and Material Science, Hebei Normal University,
Shijiazhuang, China
Abstract: A simple and efficient procedure has been developed for the preparation
of 14-aryl-14H-dibenzo[a,j]xanthene derivatives using fluoroboric acid adsorbed
on silica gel as a heterogeneous catalyst. The methodology involves the one-pot
condensation reaction of b-naphthol and aryl aldehydes under solvent-free
conventional heating conditions. The present approach offers several advantages
such as shorter reaction times, simple work-up, excellent yields, low cost, and mild
reaction conditions. The catalyst is easily recoverable and reusable without loss of
its catalytic activity.
Keywords: Aldehyde, HBF4-SiO2, heterogeneous catalyst, b-naphthol, solvent-
free conditions, xanthene
Xanthene and its derivatives are known as an important class of
heterocyclic compounds. They have been widely used as leuco-dye,[1] in
laser technology,[2] and fluorescent materials.[3] They have been reported
to possess diverse biological and therapeutic properties such as antibacter-
ial, antiviral, and anti-inflammatory activities.[4] These compounds are
also utilized as antagonists for the paralyzing action of zoxazolamine
and in photodynamic therapy.[5] As a result, various synthetic approaches
have been developed for the preparation of such compounds including
cyclohydration, trapping of benzynes by phenols, and cyclocondensation
Received June 1, 2008.
Address correspondence to Zhan-Hui Zhang, College of Chemistry and
Material Science, Hebei Normal University, Shijiazhuang 050091, China. E-mail:
zhahui@126.com
580

14-Aryl-14H-dibenzo[a,j]xanthene Derivatives
581
between 2-hydroxy aromatic aldehydes and 2-tetralone. In addition,
14-aryl-14H-dibenzo[a,j]xanthenes and related products are prepared by
reaction of 2-naphthol and formamide, 2-naphthol-1-methanol, carbon
monoxide, or aldehydeacetals.[6] In spite of potential utility of aforemen-
tioned routes for the synthesis of xanthene derivatives, many of these
methods suffer from one or more disadvantages, such as low yields,
prolonged reaction time, and lack of easy availability of the starting mate-
rials. Recently, several improved procedures were reported for the synth-
esis of 14H-dibenzo[a,j]xanthene by the reaction of 2-naphthol with
aldehydes in the presence of protic acids such as toluenesulfonic acid[7]
and methanedulfonic acid;[8] Lewis acids such as KAl(SO4)2ꢀ12H2O,[9]
Ce(SO4)2,[10] LiBr[11] and iodine;[12] and heterogeneous catalysts such as
sulfamic acid,[13] NaHSO4ꢀSiO2,[14] WO3=ZrO2,[14] Amberlyst-15,[15]
perchloric acid–silica,[16] cation-exchange resin,[17] and Dowex-50W.[18]
Other catalysts used for this transformation are potassium dodecatungsto-
cobaltate trihydrate (K5CoW12O40ꢀ3H2O),[19] sulfonyl-functionalized ionic
liquid,[20] and wet cyanuric chloride.[21] However, some of these catalysts
are toxic, difficult to prepare, and expensive. Thus, the quest for less expen-
sive, environmentally benign, and easily handled catalyst and mild reaction
conditions for the synthesis of xanthenes is a major challenge.
In recent years, solid-supported catalysts, in which the catalysts are
absorbed onto an insoluble inorganic or organic support, have been
applied widely in organic synthesis.[22] Silica-supported fluoroboric acid
as a recyclable solid acid catalyst was prepared by adsorbing fluoroboric
acid on silica gel.[23] It has been demonstrated to be an efficient catalyst
for various useful chemical transformations, such as the ring opening
of epoxides with nitrogen heterocycles,[24] conversion of aldehydes
into acetals[25] or acylals,[26] chemoselective protection of carbonyl
compounds, transthioacetalization of O,O- and S,O-acetals,[27] Mannich-
type reaction,[28] thia-Michael addition,[29] and the synthesis of 1,5-
benzodiazepines[30] and thiiranes.[31] In continuation of our work to
develop new synthetic methodologies,[32] we report for the first time a
facile and efficient synthetic strategy for the preparation of 14-aryl-
14H-dibenzo[a,j]xanthene derivatives using HBF4-SiO2 as heterogeneous
catalyst under solvent-free conditions (Scheme 1).
Scheme 1. Synthesis of 14-aryl-14H-dibenzo[a,j]xanthene derivatives.

582
Y.-H. Liu et al.
To prove the catalytic activity of HBF4-SiO2 in the synthesis of
14-aryl-14H-dibenzo[a,j]xanthene derivatives, we initially conducted a
blank reaction, in which 4-chlorobenzaldehyde (1q) was reacted with
b-naphthol (2) at room temperature and 120 ꢁC without catalyst under
solvent-free conditions (Table 1, entries 1 and 2). The results showed that
the reactions were very sluggish, and no product formation was observed.
However, addition of a catalytic amount of HBF4-SiO2 resulted in the
product 3m. The effect of amount of catalyst on the yield and rate was
also investigated. It was found that 0.2 g (10 mmol%) of catalyst was
enough for a fairly high yield. The lesser amounts gave a low yield even
after long reaction time, and the more amounts could not cause the
obvious increase in the yield of product. Meanwhile, we also tested the
Table 1. Reaction of 4-chlorobenzaldehyde (1m) and b-naphthol (2) under
different reaction conditions
Entry
Catalyst
No catalyst
No catalyst
Temperature (ꢁC)
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
9
25
120
120
80
100
120
120
120
111
83
12
5
5
2
1.5
1
3.5
1
1
1
1
0
0
5
SiO2
HBF4-SiO2 (10 mol%)
HBF4-SiO2 (10 mol%)
HBF4-SiO2 (10 mol%)
HBF4-SiO2 (5 mol%)
HBF4-SiO2 (20 mol%)
HBF4-SiO2 (10 mol%)
HBF4-SiO2 (10 mol%)
HBF4-SiO2 (10 mol%)
85
90
95
85
95
0b
0c
0d
10
11
78
aIsolated yield.
b5 mL toluene were added.
c5 mL ClCH2CH2Cl were added.
d5 mL EtOH were used.

14-Aryl-14H-dibenzo[a,j]xanthene Derivatives
583
effect of reaction temperature on the catalyzed reaction. When the
reaction was carried out at 120 ꢁC, the maximum yield was obtained in
a short reaction period (Table 1, entry 6). Some common organic solvent
such as toluene, 1,2-dichloroethane, and ethanol were also tested for this
reaction, and the results indicated that no product was observed. We
chose to perform this reaction at 120 ꢁC in the presence of 10 mmol%
of catalyst under solvent-free conditions.
After optimizing the reaction conditions, we next examined the
generality and substrate scope of this methodology, and the results are
summarized in Table 2. The reaction was amenable to a wide variety
Table 2. HBF4-SiO2 catalyzed preparation of 14-aryl-14H-dibenzo[a,j]xanthene
derivatives under solvent-free conditions
MP (ꢁC)
Entry Aldehydes
Time (h) Yield (%)a Found Reported
a
b
c
d
e
f
PhCHO
1.5
3
3
3
3
2
2
4
2
1.5
2
1
1
2
1.5
3
1
1
3.5
1
92
93
90
93
92
93
94
87
95
92
90
94
95
91
95
88
94
95
89
90
94
95
90
93
95
182–183 181–183[19]
3-MeC6H4CHO
4-MeC6H4CHO
4-Me2CHC6H4CHO
2-MeOC6H4CHO
4-MeOC6H4CHO
iPrC6H4CHO
4-OHC6H4CHO
3-FC6H4CHO
4-FC6H4CHO
2-ClC6H4CHO
3-ClC6H4CHO
4-ClC6H4CHO
2,4-Cl2C6H3CHO
2,6-Cl2C6H3CHO
2-BrC6H4CHO
198–200
198[12]
234–236 238–240[19]
240–241 239–240[20]
258–260
260[7]
212–213 213–215[19]
240–241 239–240[20]
201–202 200–202[10]
g
h
i
261–263
229–230
215–217
259[13]
238[9]
215[7]
j
k
l
210–212 209–211[19]
301–303 300–302[19]
252–253 253–255[19]
265–266 267–268[20]
m
n
o
p
q
r
190–192
192–193
296–298
289–291
215–216
310–312
268–270
336–338
192[7]
190[13]
297[7]
293[13]
211[7]
310[7]
3-BrC6H4CHO
4-BrC6H4CHO
s
2-NO2C6H4CHO
3-NO2C6H4CHO
4-NO2C6H4CHO
3-NO2-4-OHC6H3CHO
4-CNC6H4CHO
2-Naphthalene-carbaldehyde
Anthracene-9-carbaldehyde
t
u
v
w
x
y
1
1
2
1.5
1.5
199–201 198–200[10]
233–235
aIsolated yield.

584
Y.-H. Liu et al.
of substituents on aryl aldehydes. It is evident that substituents in the
aromatic ring of aldehydes have a tremendous effect on the reaction
process. Electron-withdrawing groups (such as halide, nitro, and cyano)
in the aromatic ring of aldehydes accelerated the reaction, whereas
electron-donating groups (such as methyl, methoxy, and hydroxyl)
retarded the reaction. Generally, ortho-substituted benzaldehydes
required longer reaction times, which were probably attributed to the
steric hindrance (Table 2, entries 1k, 1n, 1o, 1p, and 1s). 2-Naphthalene-
carbaldehyde and anthracene-9-carbaldehyde (Table 2, entries 1x and 1y)
were examined under the same condition to give the corresponding
products in excellent yields. However, heterocyclic aldehydes such as
furan-2-carbaldehyde and thiophene-2-carbaldehyde did not undergo
the condensation reaction with b-naphthol in the presence of HBF4-
SiO2 to produce the desired products. The reaction conducted with
phenol instead of b-naphthol also did not afford any product.
The reusability of the catalyst was studied. After completing the model
reaction, the catalyst was removed by simple filtration, washed with
CHCl3, and subjected to a second run of the reaction process with the same
substrate. The results of the first experiment and subsequent ones were
almost consistent in yields after three runs (95%, 92%, and 90%).
In conclusion, we have developed a simple and efficient method for
the synthesis of 14-aryl-14H-dibenzo[a,j]xanthene derivatives by the
one-pot condensation reaction of b-naphthol and aryl aldehydes using
HBF4-SiO2 as a heterogeneous catalyst under solvent-free conditions.
The short reaction times, simple workup, high yields, mild reaction
conditions, and utilization of an inexpensive and reusable catalyst are
feature of this new procedure. We believe that this procedure provides
a valuable addition to current methodologies.
EXPERIMENTAL
Melting points were determined on an X-4 apparatus and are uncor-
rected. IR spectra were obtained using Shimadzu FTIR-8900 spectro-
meter. 1H NMR spectra were recorded with a Varian Mercury Plus
400 spectrometer using TMS as internal standard. Elemental analyses
were performed on Vario EL III CHNOS elemental analyzer.
Preparation of Tetrafluoroboric Acid Adsorbed on Silica Gel (HBF4-SiO2)
The catalyst system HBF4-SiO2 was prepared following the originally
reported method.[23] A mixture of silica gel (26.7 g, 300–400 mesh) and

14-Aryl-14H-dibenzo[a,j]xanthene Derivatives
585
40% aq. HBF4 (3.3 g, 8.25 mL, 15 mmol) in diethyl ether (75 ml) was
stirred for 3 h. The mixture was concentrated, and the residue dried under
vacuum at 100ꢀC for 72 h to afford HBF4-SiO2 (0.5 mmol HBF4=g) as a
free-flowing power.
General Procedure for the Preparation of 14-Aryl-14H-dibenzo[a,j]
xanthene Derivatives
To a mixture of aldehydes (1 mmol) and b-naphthol (2 mmol), HBF4-
SiO2 (0.20 g, 0.1 mmol) was added, and the mixture was heated at
120 ꢀC for an appropriate time (monitored by thin-layer chromatogra-
phy, TLC). After completion of the reaction, the mixture was cooled to
room temperature and washed with CHCl3 (15 ml). The solvent was
evaporated, and the crude product was recrystallized from EtOH to
afford the pure product. The products were characterized by IR and
1H NMR spectral data, and their melting points were compared with
those of authentic samples and found to be identical.
Representative Spectral Data
14-(3-Nitro-4-hydroxyphenyl)-14H-dibenzo[a,j]xanthene (3v)
Yellow solid; mp 268–270 ꢀC; IR (KBr): 3296, 3068, 1624, 1593, 1535,
1
1483, 1451, 1336, 1255, 1244, 1172, 968, 804 cmꢁ1; H NMR (400 MHz,
CDCl3) d ¼ 6.51 (s, 1H), 6.89 (d, J ¼ 8.8 Hz, 1H), 7.44 (t, J ¼ 7.6 Hz,
2H), 7.49 (d, J ¼ 8.8 Hz, 2H), 7.60–7.66 (m, 3H), 7.82 (d, J ¼ 9.2 Hz,
2H), 7.85 (d, J ¼ 8.4 Hz, 2H), 8.27 (s, 1H), 8.29–8.30 (m, 2H), 10.35 (s,
1H) ppm; 13C NMR (100 MHz, CDCl3) d ¼ 37.03, 116.05, 118.29,
120.66, 122.13, 122.31, 124.78, 127.44, 129.25, 129.32, 129.71, 131.19,
131.27, 133.17, 137.65, 148.99, 153.93 ppm. Anal. calcd. for C27H17NO4:
C, 77.32; H, 4.09; N, 3.34. Found: C, 77.50; H, 3.92; N, 3.20.
14-(4-Cyanophenyl)-14H-dibenzo[a,j]xanthene (3w)
Incarnadine needles; mp 336–338 ꢀC; IR (KBr): 2873, 1635, 1591, 1558,
;
1506, 1458, 1398, 1245, 1179, 960, 810 cmꢁ1 1H NMR (400 MHz,
CDCl3) d ¼ 6.54 (s, 1H), 7.42–7.63 (m, 10H), 7.84 (t, J ¼ 8.8 Hz, 4H),
8.27 (d, J ¼ 8.8 Hz, 2H) ppm; 13C NMR (100 MHz, CDCl3) d ¼ 38.21,
110.48, 116.13, 118.26, 122.21, 122.32, 124.77, 127.38, 123.10, 129.29,
129.75, 131.25, 132.62, 148.99, 150.24 ppm. Anal. calcd. for C28H17NO:
C, 87.71; H, 4.47; N, 3.65. Found: C, 87.52; H, 4.28; N, 3.50.

586
Y.-H. Liu et al.
14-Anthracen-9-yl-14H-dibenzo[a,j]xanthene (3y)
White solid, 233–235 ꢁC; IR (KBr): 3082, 2898, 1635, 1529, 1458, 1402,
1
1348, 1255, 1141, 966, 825 cmꢂ1; H NMR (400 MHz, CDCl3) d ¼ 6.60
(s, 1H), 7.27–7.63 (m, 10H), 7.81–7.86 (m, 8H), 8.30 (d, J ¼ 8.8 Hz,
2H), 8.42 (s, 1H) ppm; 13C NMR (100 MHz, CDCl3) d ¼ 37.88, 116.08,
118.34, 118.80, 121.84, 121.96, 122.15, 122.33, 122.69, 124.82, 127.46,
129.33, 129.75, 129.81, 131.25, 134.53, 147.15, 148.42, 148.99 ppm. Anal.
calcd. for C35H22O: C, 91.68; H, 4.84. Found: C, 91.82; H, 4.66.
ACKNOWLEDGMENTS
We are grateful for financial support from Hebei Normal University
(L20061314), the Nature Science Foundation of Hebei Province
(B2008000149), and the Natural Science Foundation of Hebei Education
Department (2006318).
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