Polystyrene-supported GaCl3 as a highly efficient and reusable heterogeneous Lewis acid catalyst for the three-component synthesis of benzoxanthene derivatives

Article abstract of DOI:10.1007/s00706-013-0931-9

Naphthoquinone-based xanthenes, specifically 14-aryl-13H-dibenzo[a,i] xanthene-8,13(14H)-diones and 12-aryl-2,3,4,12-tetrahydro-1H-benzo[b]xanthene-1, 6,11-triones, were obtained in good to excellent yields by a simple, mild, and efficient three-component coupling procedure utilizing polystyrene-supported gallium trichloride as a highly active and eco-friendly heterogeneous Lewis acid catalyst. This reaction was also extended to the preparation of 14-aryl-2-hydroxy-13H-dibenzo[a,i]xanthene-8,13(14H)-diones. The experimental and work-up procedure is very simple and the catalyst can be easily separated from the reaction mixture and reused several times in subsequent reactions.

Full text of DOI:10.1007/s00706-013-0931-9

Monatsh Chem (2013) 144:1205–1212
DOI 10.1007/s00706-013-0931-9
ORIGINAL PAPER
Polystyrene-supported GaCl3 as a highly efficient and reusable
heterogeneous Lewis acid catalyst for the three-component
synthesis of benzoxanthene derivatives
Ali Rahmatpour
Received: 12 May 2012 / Accepted: 16 January 2013 / Published online: 17 May 2013
Ó Springer-Verlag Wien 2013
Abstract Naphthoquinone-based xanthenes, specifically
14-aryl-13H-dibenzo[a,i]xanthene-8,13(14H)-diones and
compounds in the field of drugs and pharmaceuticals
[9–11]. Besides, these heterocyclic molecules have been
widely used as leuco-dyes [12, 13], pH-sensitive fluores-
cent materials [14, 15], as well as in laser technologies
[16, 17]. Among the classical methods reported for the
synthesis of xanthenes, the most reliable, commonly used,
and straightforward approach involves the addition
reaction between ortho-quinonemethides and certain
nucleophiles [18, 19]. The naphthoquinone nucleus com-
prises of an important class of oxygenated heterocycles.
The growing interest in this class of compounds has been
associated with its diverse pharmacological properties and
industrial applications [20]. This moiety also occurs in
different natural products [21]. Owing to the diverse bio-
logical activity of molecules with naphthoquinone-type
structure, it is important to find a clean, mild, and versatile
synthetic approach that allows for the maximum amount of
flexibility to afford a range of xanthenes containing naph-
thoquinone fragments, dibenzo[a,i]xanthenedione and
benzo[b]xanthenetrione systems.
12-aryl-2,3,4,12-tetrahydro-1H-benzo[b]xanthene-1,6,11-tri-
ones, were obtained in good to excellent yields by a simple,
mild, and efficient three-component coupling procedure
utilizingpolystyrene-supported gallium trichloride asa highly
active and eco-friendly heterogeneous Lewis acid catalyst.
This reaction was also extended to the preparation of 14-aryl-
2-hydroxy-13H-dibenzo[a,i]xanthene-8,13(14H)-diones.
The experimental and work-up procedure is very simple and
the catalyst can be easily separated from the reaction mixture
and reused several times in subsequent reactions.
Keywords Polymer-supported catalyst Á
Multi-component reaction Á Benzoxanthene Á
Heterogeneous catalyst
Introduction
The developing of new multi-component reactions (MCRs)
and improving the known MCRs to produce elaborate
biologically active compounds are active areas of research
in organic, combinatorial, and medicinal chemistry [1–4].
One-pot multi-component reaction strategies offer signifi-
cant advantages over two-component and stepwise
reactions in several aspects by virtue of their convergence,
productivity, high atom economy, facile execution, and
high yields [5–8]. Xanthenes, especially benzoxanthene
derivatives, are an important class of biologically active
Replacement of conventional, toxic, and polluting
Bronsted and Lewis acid catalysts with eco-friendly reus-
able heterogeneous catalysts is an area of current interest.
Immobilization of catalysts on a solid support improves the
availability of active sites, stability, hygroscopic properties,
handling, and reusability of catalysts, all of which are
important factors in industry [22]. A large number of
polymer-supported Lewis acid catalysts have been
prepared by immobilization of the catalysts on polymers
via coordination or covalent bonds [23]. Such polymeric
catalysts are usually as active and selective as their
homogeneous or solution-phase counterparts while having
the distinguishing characteristics of being easily separable
from the reaction mixture, recyclability, easier handling,
non-toxicity, enhanced stability, and improved selectivity
A. Rahmatpour (&)
Polymer Science and Technology Division, Research Institute
of Petroleum Industry (RIPI), 14665-1137 Tehran, Iran
e-mail: rahmatpoura@ripi.ir
123

1206
A. Rahmatpour
in various organic reactions. Polystyrene is one of the most
widely studied heterogeneous and polymeric supports
owing to its environmental stability and hydrophobic nat-
ure which protects water-sensitive Lewis acids from
hydrolysis by atmospheric moisture until they are sus-
pended in an appropriate solvent where they can be used in
a chemical reaction [24].
Results and discussion
First, cross-linked polystyrene (7–8 % divinylbenzene,
mesh size 20–75) was prepared via suspension polymeri-
zation as reported in the literature [26–28]. Then, PS/GaCl3
was prepared by addition of anhydrous gallium chloride to
polystyrene (7–8 % divinylbenzene) in carbon disulfide
under reflux conditions. The loading capacity of the
polymeric catalyst obtained by gravimetric analysis and
checked by an atomic absorption technique was
0.391 mmol GaCl3 per gram of complex bead catalyst [37].
The data obtained by these two techniques showed, within
experimental error, that the catalyzing species are in the
form of GaCl3 supported on the polymeric matrix. The UV
spectrum of the solution of PS–GaCl3 complex in CS2
showed a new strong band at 470 nm, which is due to the
formation of a stable p ? p-type coordination complex
between the benzene rings in the polystyrene carrier and
gallium trichloride. The FT-IR spectrum of PS/GaCl3
showed new absorption peaks due to the C–C stretching
vibration and the C–H bending vibration of the benzene
ring at 1,500–1,560 and 400–800 cm-1, by which complex
formation was demonstrated. The structure of the PS–
GaCl3 complex is similar to that of the PS–AlCl3 complex
as suggested by Neckers and co-workers [38], because the
Lewis acid GaCl3 is complexed with the benzene rings of
the polystyrene and the GaCl3 is stabilized owing to the
decreased mobility of the benzene rings hindered by the
long polystyrene chain. The PS–GaCl3 complex catalyst is
a non-hygroscopic, water-tolerant, and particularly stable
species. In addition, this polymeric catalyst is easy to
prepare, stable in air for a long time (over 2 years) without
any change, easily recycled, and reused without apprecia-
ble loss of its activity.
It is well known that gallium trichloride is a strong
Lewis acid and an important catalyst in organic transfor-
mations. However, it easily hydrolyzes in air, so that its
use, storage, and separation from reaction mixtures are
inconvenient and difficult. Polystyrene-supported gallium
chloride (PS/GaCl3), which is a tightly bound and stable
complex between anhydrous GaCl3 and polystyrene–divi-
nylbenzene copolymer beads, was described for the first
time by Ruicheng and co-workers [25]. The use of the PS/
GaCl3 complex catalyst has several advantages over con-
ventional Lewis acid catalyst like its cost-effectiveness,
ease of handling (as a bench-top catalyst), recyclability,
and tunable Lewis acidity.
As part of our continuing interest in heterogeneously cata-
lyzed organic reactions [26–31], we report herein an efficient
and eco-friendly procedure for the synthesis of (a) 14-aryl-2-
hydroxy-13H-dibenzo[a,i]xanthene-8,13 (14H)-diones via the
three-component reaction of b-naphthol or 2,7-dihydroxy-
naphthalene, aromatic aldehydes, and 2-hydroxynaphthalene-
1,4-dione and (b) 12-aryl-2,3,4,12-tetrahydro-1H-benzo[b]
xanthene-1,6,11-triones via the three-component reaction of
5,5-dimethylcyclohexane-1,3-dione, aromatic aldehydes, and
2-hydroxynaphthalene-1,4-dione in the presence of a catalytic
amount of polystyrene-supported GaCl3 as a reusable hetero-
geneous Lewis acid catalyst (Scheme 1). To the best of our
knowledge, this is only the second example of PS/GaCl3 as a
catalyst for any organic transformation and there are only four
previous reports describing this quinoidal or naphthoquinone
moiety [32–36].
In order to find the most appropriate reaction conditions
and evaluate the potential of PS/GaCl3 as a heterogeneous
X
Scheme 1
X
OH
Ar
O
O
O
X = H (3a), OH (3b)
-2 H2O
X = H (5a-5j)
X = OH (5k-5m)
O
O
PS/GaCl3
(10 mol%)
HO
ArCHO
+
C2H5OH,
reflux
O
Ar
O
-2 H2O
1
2
6
O
4
O
O
O
123

Polystyrene-supported GaCl3 as a highly efficient and reusable heterogeneous Lewis acid catalyst
1207
catalyst for the preparation of 14-aryl-13H-dibenzo-
[a,i]xanthene-8,13(14H)-dione derivatives, the three-com-
ponent coupling reaction between benzaldehyde (1a),
2-hydroxynaphthalene-1,4-dione (2), and b-naphthol (3a)
was selected as a model system to screen the best condi-
tions. The results are summarized in Table 1. It was found
that among the tested solvents, polar solvents such as
CH3CN and C2H5OH were better than nonpolar solvents.
Ethanol was chosen as the solvent system for this con-
version, because of its swellability with the catalyst and
miscibility with the substrates. No reaction occurred when
H2O was used as solvent (Table 1, entry 6), maybe because
of the aggregation of the catalyst caused by its hydrophobic
nature, leading to inadequate access of substrates to the
active sites of the catalyst [39, 40]. Moreover, the best
results were obtained by carrying out the reaction with
1.0:1.0:1.0 molar ratios of b-naphthol, benzaldehyde, and
2-hydroxynaphthalene-1,4-dione. Further studies showed
that the reaction temperature has a great influence on the
model reaction and the best results were observed when the
reaction temperature was 78 °C (Table 1, entries 9, 13, 14).
Consequently, we chose 78 °C as the optimal temperature
for the model reaction. Next, the amount of catalyst was
examined and we found that the yields were obviously
affected by the different amount of catalyst. No reaction
was observed in the absence of catalyst, whereas 10 mol%
of PS/GaCl3 was sufficient to drive the reaction to com-
pletion and excess catalyst did not alter either reaction time
or yield of the product significantly (Table 1, entries 7–10).
The key role played by the Lewis acidity of the heteroge-
neous catalyst PS/GaCl3 was proved by employing the
cross-linked polystyrene beads (PS) (Table 1, entry 11) and
the GaCl3–toluene complex as catalysts under the same
experimental conditions. In fact, whereas in the former case
no reaction occurred, which indicated that polystyrene itself
did not promote the reaction, in the latter, the desired product
was isolated in low yield of 49 % (Table 1, entry 12). Also,
PS/GaCl3 was found to be a more effective catalyst than
PS/AlCl3 in terms of reaction time and the yield of the
product 5a under identical conditions (Table 1, entry 16).
The lifetime and leaching of active metal species into
solution are important issues to consider when heteroge-
neous catalysts are used, particularly for practical
applications of the reaction. Our preliminary investigations
demonstrated that PS/GaCl3 catalyst is very stable to air
and moisture. Moreover, in a separate experiment the cat-
alyst PS/GaCl3 was heated under reflux in ethanol for
20 min and then isolated by filtration. When reactants were
added to the filtrate (the catalyst-free liquid) and heated under
reflux for 2 h, no reaction took place (Table 1, entry 15). The
capacity of the catalyst after five uses was 0.375 mmol of
GaCl3 per gram. Therefore, we conclude that (a) PS/GaCl3
is stable and no significant leaching of Lewis acid occurs
under our reaction conditions, (b) any gallium species that
leached into the reaction mixture is not an active homo-
geneous catalyst, and (c) the observed catalysis is truly
heterogeneous in nature.
Table 1 PS/GaCl3-catalyzed condensation reaction of b-naphthol
(3a, 1 mmol), benzaldehyde (1a, 1 mmol), and 2-hydroxynaphtha-
lene-1,4-dione (2, 1 mmol) under different reaction conditions
Entry Solventa
Catalyst/mol% Time/h Yieldb/%
1
Toluene
Cyclohexane
CH2Cl2
10
10
10
10
10
10
–
4
36
2
4
22
3
4
12
4
CH3Cl
4
15
5
CH3CN
H2O
4
74
NRh
6
4
7
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
4
NR
8
5
2
61
9c
10
11d
12e
13f
14g
15i
16j
a
10
15
–
2
89, 88, 86, 83, 81
2
89
2
NR
49
–
2
However, one of the important points regarding the
heterogeneous catalysts is their deactivation and reusabil-
ity. To test this, a series of five consecutive runs were
carried out with the same catalyst PS/GaCl3 sample,
without noticeable decrease in the activity, furnishing the
corresponding product with 89, 88, 86, 83, and 81 %
isolated yields (Table 1, entry 9). This reusability demon-
strates the high stability and turnover of the polymeric
Lewis acid catalyst under the operating conditions. After
each reaction, the solid PS/GaCl3 catalyst was simply fil-
tered off, washed with chloroform and ether, and reused.
On the basis of the observations described above, we
conducted the same three-component coupling reactions
using a variety of aromatic aldehydes 1, 2-hydroxynaph-
thalene-1,4-dione (2), and b-naphthol (3a) in the presence
of PS/GaCl3 (10 mol%) under similar conditions. As
10
10
10
10
2
51
2
77
0.33
3
NR
59
All reactions were carried out under reflux conditions with
10 mol% of catalyst in 5 cm3 of solvent
b
Isolated yield
c
Catalyst was reused for five times
d,e
PS and the toluene–GaCl3 complex were used as catalysts,
respectively
f,g
Two reactions were carried out in ethanol at 45 and 60 °C,
respectively
h
NR no reaction
i
Catalyst was removed by filtration after 20 min
j
PS/AlCl3 was used as catalyst
123

1208
A. Rahmatpour
References
Table 2 Preparation of 14-aryl-13H-dibenzo[a,i]xanthene-8,13(14H)-dione derivatives 5 catalyzed by 10 mol% PS/GaCl3
Entry
Ar
Product
Time/h
Yield/%a
M.p./°C
Lit. m.p./°C
1
C6H5
5a
5b
5c
5d
5e
5f
2
89
87
88
90
86
90
88
89
85
87
92
93
90
296–298
254–255
276–278
283–285
278–280
[300
294–297
254–256
279–280
281–284
281–282
332–333
304–305
294–298
[35, 36]
[32, 33]
[32, 33]
[35, 36]
[32, 33]
[32, 33]
[32, 33]
[35, 36]
[35, 36]
–
2
4-Me-C6H4
4-MeO-C6H4
4-Cl-C6H4
2-Cl-C6H4
4-NO2-C6H4
3-NO2-C6H4
4-Br-C6H4
4-HO-C6H4
3,4-(MeO)2-C6H3
C6H5
2
3
2
4
2
5
3
6
3.5
3.5
3.5
3.5
2
7
5g
5h
5i
297–300
296–299
[300
8
b
9
–
10
11
12
13
5j
262–265
–
–
–
–
5k
5l
2
[300
–
4-Cl-C6H4
4-MeO-C6H4
2
[300
–
5m
2
[300
–
All reactions were run with 1 (1 mmol), 2 (1 mmol), and 3 (1 mmol) in 5 cm3 C2H5OH at reflux temperature
a
Isolated yield
b
Not reported
expected, satisfactory results were obtained, and the results
are summarized in Table 2. A range of substituted aromatic
aldehydes react with b-naphthol and 2-hydroxynaphtha-
lene-1,4-dione smoothly and give the corresponding
benzoxanthene derivatives 5a–5j in good to excellent
yields without formation of any by-products such as aryl-
5H-dibenzo[b,i]xanthene. Aromatic aldehydes bearing
electron-withdrawing and electron-donating substitutions
at ortho, meta, and para positions employed here were all
compatible with the reaction conditions and gave good
yields of 5. Both electron-rich as well as electron-deficient
substrates were found to be effective for this transforma-
tion. When this reaction was carried out with an aliphatic
aldehyde such as propionaldehyde and butyraldehyde in
the same conditions, TLC and 1H NMR spectra of the
reaction mixture showed a combination of starting mate-
rials and numerous products; the yield of the expected
product was very poor, which can be attributed to the aldol
condensation side reaction.
b-naphthol ring increased the electron density at the
a-position of b-naphthol. Thus, the nucleophilic attack at
the carbonyl group is favored. None of the reactions showed
the formation of any bis-condensation product. Even the
three-component reactions using double molar ratio of
aldehydes and 2-hydroxynaphthalene-1,4-dione did not
show formation of bis-condensation products. Condensa-
tion reactions of the initially formed 5 with aldehyde and
2-hydroxynaphthalene-1,4-dione to give bis-condensation
products were also unsuccessful. The reason that 2,7-
dihydroxynaphthalene did not give the dimer is probably
due to the steric effect on 5k–5m which prevents its further
condensation with aldehyde. This is similar to the reported
results on the condensation of dihydroxynaphthalenes [41].
Our efforts to prepare similar novel series of xanthene
derivatives starting from 1,5-, 1,6-, 1,7-dihydroxynaphtha-
lenes, p-cresole, and a-naphthol have been unsuccessful. It
is well known that the electron density at the benzylic C-1
position of b-naphthol (which is in conjunction with the
aromatic ring) is higher than that at the C-3 position. Thus
the regioselective formation of the ortho-quinonemethide
from this compound involving the C-1 and C-2 positions is
favored. In simple phenolic compounds such as p-cresole
and a-naphthol (which are weaker nucleophiles compared
to b-naphthol) the electron density at the ortho position of
the hydroxyl group is not sufficient for the reaction of these
compounds with the aldehydes leading to the formation of
the corresponding ortho-quinonemethides.
Next, we were interested to see whether or not the same
catalyst is useful for the three-component coupling reaction
of dihydroxynaphthalenes. Remarkably, when we treated
aldehydes 1 and 2-hydroxynaphthalene-1,4-dione with 2,7-
dihydroxynaphthalene (3b) using the same catalyst under
similar reaction conditions, the corresponding xanthene
derivatives were obtained in high yields (Table 2, entries
11–13). On the basis of the results listed in Table 2,
b-naphthol afforded slightly lower yields of the corre-
sponding products than the analogous reactions with 2,7-
dihydroxynaphthalene. An explanation for this behavior
might be the fact that the electron-releasing groups on the
The actual mechanism of the reaction is unclear. How-
ever, on the basis of the experimental results and by
referring to the literature [42–44], we propose that the
123

Polystyrene-supported GaCl3 as a highly efficient and reusable heterogeneous Lewis acid catalyst
1209
Scheme 2
O
HO
GaCl3
O
1
H
O
2
Ar
Ar
Ar
H
GaCl3
O
O
OH
O
GaCl3
-H2O
3
7
O
O
Ar
Ar
O
O
-H2O
O
OH HO
5
efficiency of PS/GaCl3, and we obtained low to moderate
yields in ethanol (Table 3). All the reactions were run in the
same reaction conditions, and similar amounts of catalysts
(10 mol%) were used. As can be seen in Table 3, PS/GaCl3
catalyzes the reaction better than other catalysts, and satis-
factory results were obtained only with PS/GaCl3 (entry 14).
To further explore the generality and potential of this
protocol for the synthesis of other quinone-based xanthenes,
we studied the reaction of 5,5-dimethylcyclohexane-1,3-
dione (4) instead of b-naphthol under the aforementioned
optimized conditions. The corresponding 2,3,4,12-tetrahy-
dro-1H-benzo[b]xanthene-1,6,11-trione derivatives 6 were
obtained in good yields and the optimized results are
summarized in Table 4.
Table 3 Comparison of the effect of the catalysts on the reaction of
benzaldehyde, 2-hydroxynaphthalene-1,4-dione, and b-naphthol
Entry
Catalyst
Time/h
Yielda/%
1
ZnCl2
4
4
4
4
4
4
4
4
4
4
4
4
4
2
36
48
34
49
41
53
28
64
56
71
61
58
62
89
2
AlCl3
3
FeCl3
4
NH4Cl
5
BiCl3
6
SnCl4
7
CoCl2
8
NH2SO3H
Al(HSO4)3
I2
9
10
11
12
13
14
Silica chloride
AlCl3ÁSiO2
p-TSA
The nature of the compounds 5 and 6 as 1:1:1 adducts
was apparent from their mass spectra, which displayed, in
each case, the molecular ion peak at appropriate m/z values.
PS/GaCl3
Reaction conditions: benzaldehyde (1a, 1 mmol), 2-hydroxynaph-
thalene-1,4-dione (2, 1 mmol), b-naphthol (3, 1 mmol), catalyst
(0.1 mmol), 5 cm3 ethanol, reflux
Conclusion
a
Isolated yield
In summary, a novel, simple, eco-friendly, and highly
efficient synthetic protocol for the synthesis of 14-aryl-
13H-dibenzo[a,i]xanthene-8,13(14H)-diones, 14-aryl-2-
hydroxy-13H-dibenzo [a,i]xanthene-8,13(14H)-diones, and
12-aryl-2,3,4,12-tetrahydro-1H-benzo[b]xanthene-1,6,11-tri-
ones has been described by one-pot and three-component
cyclocondensation reaction using polystyrene–GaCl3
complex as a reusable solid Lewis acid catalyst. The simple
work-up procedure, general applicability, environmentally
friendliness, high yields, mild operating conditions, low
cost, easy preparation, safe handling, and recyclability of
reaction proceeds via the initial formation of ortho-qui-
nonemethide intermediate 7 from aldehyde and b-naphthol.
Subsequent addition of 2 with the intermediate 7 followed
by elimination of water affords the desired product 5 as
depicted in Scheme 2.
Using conditions based on the optimized reaction
of benzaldehyde, 2-hydroxynaphthalene-1,4-dione, and
b-naphthol with PS/GaCl3 as catalyst, we also tested the
catalytic activity of different catalysts to compare the
123

1210
A. Rahmatpour
References
Table 4 Preparation of substituted benzo[b]xanthenetrione derivatives 6 catalyzed by PS/GaCl3
Entry
Ar
Product
Time/h
Yielda/%
M.p./°C
Lit. m.p./°C
1
2
3
4
5
6
7
8
9
C6H5
6a
6b
6c
6d
6e
6f
3.5
3.5
3.5
3
86
84
85
88
87
85
83
82
80
264–265
235–238
244–248
283–285
261–264
278–281
265–268
267–269
284–287
263–265
254–257
245–248
282–284
[35, 36]
[35, 36]
[35, 36]
[35, 36]
[35, 36]
[35, 36]
[35, 36]
[35, 36]
[35, 36]
4-Me-C6H4
4-MeO-C6H4
4-Cl-C6H4
2-Cl-C6H4
4-NO2-C6H4
3-NO2-C6H4
4-Br-C6H4
4-HO-C6H4
b
4
–
b
3
–
b
6g
6h
6i
3.5
3.5
3.5
–
268–270
285–288
All reactions were run with 1 (1 mmol), 2 (1 mmol), 4 (1 mmol), and PS/GaCl3 (0.1 mmol) in 5 cm3 ethanol at reflux temperature
a
Isolated yield
b
Not reported
the polymeric Lewis acid catalyst are features of this new
procedure.
became yellow. The polymer beads were collected by fil-
tration and washed with 300 cm3 water and then with
30 cm3 ether and 30 cm3 chloroform. The catalyst was
dried in a vacuum oven overnight at 50 °C before use. The
chlorine content of PS/GaCl3 was 4.17 % analyzed by the
Mohr titration method [37] and the loading capacity of
GaCl3 on the polymeric catalyst or the amount of GaCl3
complexed with polystyrene was calculated to be
0.391 mmol/g [45].
Experimental
All chemical reagents were obtained from Fluka or Merck
chemical companies and were used without further purifi-
cation. Cross-linked polystyrene (7–8 % divinylbenzene,
mesh size 20–75) was prepared via suspension polymeri-
zation as reported in the literature [26–28]. PS/AlCl3 was
prepared as reported previously [26–28]. FT-IR spectra
were run on a Shimadzu FTIR-8300 spectrophotometer.
UV spectra were recorded using a Pharmacia Biotech Ul-
traspec 3000 model 80-2106-20 spectrophotometer. 1H
NMR and 13C NMR spectra were recorded on a Bruker
Avance DPX instrument (250 MHz) using TMS as internal
standard. The capacity of the catalyst was determined by
the gravimetric (Mohr titration) method and an atomic
absorption technique using a Philips atomic absorption
instrument. The progress of the reactions was followed by
TLC on silica gel Polygram SIL/UV 254 plates. Elemental
analyses were performed using a Heraeus CHN-O-Rapid
analyzer at RIPI and the results agreed favorably with
calculated values.
General experimental procedure for preparation
of 5 and 6
In a 25-cm3 round-bottom flask equipped with a condenser
and a magnetic stirrer, a mixture of b-naphthol (1 mmol) or
2,7-dihydroxynaphthalene (1 mmol) or 5,5-dimethylcy-
clohexane-1,3-dione (1 mmol), aromatic aldehydes
(1 mmol), and 255 mg PS/GaCl3 (0.1 mmol) in 5 cm3
ethanol was reacted in an oil bath at reflux temperature
with stirring for the appropriate times (Tables 2, 4). The
progress of the reaction was monitored by TLC. After
completion of the reaction, the reaction mixture was cooled
to room temperature, 5 cm3 hot ethanol was added, and the
catalyst was collected by filtration and then washed with
chloroform and ether (2 9 5 cm3). The filtrate was con-
centrated on a rotary evaporator under reduced pressure
and the solid product was washed twice with water
(2 9 5 cm3) and recrystallized from ethanol to afford the
pure products. The spent catalyst from different experi-
ments was washed with chloroform and ether, dried, and
used again. The reactions using the recycled catalyst were
conducted in a similar manner. All of the products except
5j–5m are known compounds and were characterized by IR
and NMR spectroscopy data and their melting points that
agreed with reported values.
Preparation of PS/GaCl3
Anhydrous GaCl3 (4 g) was added to 8 g polystyrene
(7–8 % divinylbenzene, mesh size 20–75) in 30 cm3 car-
bon disulfide as the reaction medium. The mixture was
stirred using a magnetic stirrer under reflux conditions for
1 h, cooled, and then 50 cm3 water was cautiously added
to hydrolyze the excess GaCl3. The mixture was stirred
until the bright red color disappeared, and the polymer
123

Polystyrene-supported GaCl3 as a highly efficient and reusable heterogeneous Lewis acid catalyst
1211
14-(3,4-Dimethoxyphenyl)-13H-dibenzo[a,i]xanthene-
CH), 3.68 (s, 3H, OCH3) ppm; 13C NMR (62.5 MHz,
DMSO-d6): d = 178.43 (CO), 178.38 (CO), 158.34,
157.08, 147.21, 136.12, 135.15, 131.84, 131.14, 131.06,
130.91, 130.00, 129.52, 129.61, 129.28, 128.22, 127.61,
125.41, 124.51, 123.61, 117.09, 116.77, 113.80, 55.11,
34.22 (C-14) ppm.
8,13(14H)-dione (5j, C29H20O5)
ꢀ
M.p.: 262–265 °C; IR (KBr): m = 3,035, 1,698, 1,655,
1,636, 1,590, 1,542, 1,508, 1,498, 1,458, 1,289, 1,213,
1,181, 1,155 cm-1; MS: m/z = 448 (M?); 1H NMR
(250 MHz, DMSO-d6): d = 8.24 (d, 1H, J = 8.5 Hz),
8.08–8.16 (m, 2H), 7.71–7.81 (m, 3H), 7.53 (t, 1H,
J = 7.5 Hz), 7.35–7.52 (m, 3H), 6.85 (d, 1H, J =
2.8 Hz), 6.75 (d, 1H, J = 8.8 Hz), 6.55-6.62 (m, 1H),
6.10 (s, 1H, CH), 3.76 (s, 3H, OCH3), 3.65 (s, 3H, OCH3)
ppm; 13C NMR (62.5 MHz, DMSO-d6): d = 178.40 (CO),
178.14 (CO), 157.42, 153.63, 151.21, 147.05, 135.15,
133.42, 131.62, 131.81, 131.14, 130.20, 129.26, 129.12,
128.41, 127.31, 125.40, 124.62, 124.11, 117.45, 117.22,
116.75, 116.21, 113.10, 112.10, 56.94, 56.12, 34.48
(C-14) ppm.
Acknowledgments The author is grateful to TPC for providing
polystyrene.
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Yellow powder; m.p.: [300 °C; IR (KBr): m = 3,197,
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