Polystyrene-supported aluminum chloride: An efficient and recyclable green catalyst for one-pot synthesis of 14-aryl or alkyl-14h-dibenzo[a,j]xanthenes

Article abstract of DOI:10.1002/hc.20655

A new, mild, and efficient method has been developed for the one-pot synthesis of 14-aryl or alkyl-14H-dibenzo[a, j]xanthenes by condensation of 2-naphthol and aryl or alkyl aldehydes in the presence of a catalytic amount of cross-linked polystyrene-suppo

Full text of DOI:10.1002/hc.20655

Heteroatom Chemistry
Volume 22, Number 1, 2011
Polystyrene-Supported Aluminum Chloride:
An Efficient and Recyclable Green Catalyst for
One-Pot Synthesis of 14-Aryl or
Alkyl-14H-Dibenzo[a,j ]Xanthenes
Ali Rahmatpour and Jamal Aalaie
Research Institute of Petroleum Industry (RIPI), Polymer Science and Technology Division,
Tehran, 14665-1137, Iran
Received 14 July 2010; revised 9 October 2010
because of the unique reactivities and selectivities
ABSTRACT: A new, mild, and efficient method
that can be achieved as well as for the mild condi-
has been developed for the one-pot synthesis of 14-
tions used [2]. The most frequently used polymeric
aryl or alkyl-14H-dibenzo[a, j]xanthenes by conden-
support is polystyrene; Its hydrophobic nature pro-
sation of 2-naphthol and aryl or alkyl aldehydes in
tects water-sensitive Lewis acids from hydrolysis by
the presence of a catalytic amount of cross-linked
atmospheric moisture until it is suspended in an ap-
polystyrene-supported aluminum chloride (PS/AlCl3)
propriate solvent where it can be used in a chemical
as an ecofriendly heterogeneous catalyst. This poly-
reaction [3].
meric solid acid catalyst is stable and can be easily
Xanthenes, especially benzoxanthenes, are an
recovered and reused without appreciable change in
important and very useful class of intermediates in
ꢀ
C
its activity.
2010 Wiley Periodicals, Inc. Heteroatom
organic synthesis due to their wide range of biolog-
ical and pharmaceutical properties such as antibac-
terial [4], antiviral [5], anti-inflammatory activities
[6], and sensitizers in photodynamic therapy for de-
stroying the tumor cells [7], and also these hete-
rocyclic compounds widely used as leucodyes [8],
pH-sensitive fluorescent materials for visualization
of biomolecules [9], and in laser technology [10].
The reported methods for the synthesis of 14-aryl or
alkyl-14H-dibenzo[a, j]xanthenes involve the mixing
of 2-naphthol with aldehydes in the presence of an
acidic catalyst such as AcOH-H2SO4[11], pTSA [12],
sulfamic acid [13], molecular iodine [14], LiBr [15],
Amberlyst-15 [16], silica sulfuric acid [17,18a], silica
perchloric acid [18b], heteropoly acid [19,20], wet
cyanuric chloride [21], BF3·SiO2 [22], Yb(OTf)3[23a],
In(OTf)3 [23b], alum [24]. Montmorillonite K-10
[25], P2O5/Al2O3 [26], and ionic liquids [27,28]. In
spite of potential utility of aforementioned routes
for the synthesis of benzoxanthene derivatives, some
of these methods suffer at least from one or more
Chem 22:51–54, 2011; View this article online at wileyon-
linelibrary.com. DOI 10.1002/hc.20655
INTRODUCTION
The utility of polymer-supported catalysts is now
well recognized because of their ease of workup
and of separation of products and catalysts, from
the economical point of view, and in application to
industrial processes, recyclability, easier handling,
greater selectivity, enhanced stability, nontoxicity,
and noncorrosiveness [1]. In general, catalysts are
immobilized on polymers via coordinate or covalent
bonds. On the other hand, polymer-supported Lewis
acid catalyzed reactions are of great current interest
Correspondence to: Ali Rahmatpour; e-mail: rahmatpoura@
ripi.ir.
c
ꢀ
2010 Wiley Periodicals, Inc.
51

52 Rahmatpour and Aalaie
R
o
OH
+
Polystyrene/AlCl
(20 mol%)
3
2H O
+
RCHO
2
CH CN, reflux
3
(1 mmol)
(2 mmol)
R: aryl or alkyl
SCHEME 1
TABLE 1 Reaction between 2-Naphthol (2 mmol) and 4-
Chlorobenzaldehyde (1 mmol) Using Catalytic Amount of
PS/AlCl3 under Different Conditionsa
disadvantages, such as
a long reaction time,
relatively expensive reagents, use of toxic sol-
vents and catalysts, requirement of excess of
reagents/catalysts, harsh and strong acidic reaction
conditions, unsatisfactory yield, and tedious workup
procedures. In addition, some of these catalysts
are not recyclable and require workup of the reac-
tion mixture. Therefore, to avoid these limitations,
the development of simple, mild and environment
friendly approaches, especially using heterogeneous
catalysts for the preparation of these pharmaceuti-
cally important compounds, is still demanding.
In continuation of our interest in synthe-
sis of organic compounds from condensation
of phenols with dialdehydes [29], herein, we
wish to report an efficient and environmen-
tally benign method for the synthesis of 14-aryl
or alkyl-14H-dibenzo[a,j]xanthenes derivatives cat-
alyzed by polystyrene-supported aluminum chloride
(PS/AlCl3) (Scheme 1).
Entry
Solvent
THF
CHCl3
CH2Cl2
Conditions Time (h) Yield (%)
1
2
3
4
5
6
7
8
9
Reflux
Reflux
Reflux
6
6
6
6
3
1.5
8
0
Trace
Trace
Trace
66
ClCH2CH2Cl Reflux
CH3CH2OH
CH3CN
Toluene
Neat
Reflux
Reflux
Reflux
80◦C
92
Trace
Trace
Trace
6
Neat
100◦C
6
aThe yields refer to the isolated pure products.
It is worthy to note that in the absence of PS/AlCl3
catalyst, the reaction did not yield any product at
reflux temperature even after a long reaction time.
To find out whether the reaction takes place in the
solid matrix of PS/AlCl3 or whether released AlCl3 is
responsible for the condensation reaction, PS/AlCl3
was added to CH3CN and the mixture was stirred at
reflux temperature for 3 h. Then, the catalyst was
filtered off and the filtrate was analyzed for its alu-
minum content, which showed a negligible release
of AlCl3. The filtrate was found to be inactive for the
condensation reaction. This observation indicates
that PS/AlCl3 is stable under the reaction conditions,
and there is no leaching of Lewis acid moieties dur-
ing reactions.
After optimization of the reaction conditions,
we studied the generality of this condition to other
substrates. Using this method, different kinds of
aromatic and aliphatic aldehydes were reacted
with 2-naphthol to produce the corresponding
14-aryl or alkyl-14H-dibenzo[a, j]xanthenes at the
reflux temperature in acetonitrile. 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 high yields and a short
reaction time. The substituted functional groups on
the aromatic ring of the aldehyde affected the yield
RESULTS AND DISCUSSION
Cross-linked polystyrene-supported aluminum chlo-
ride (PS/AlCl3) was prepared by addition of anhy-
drous aluminum chloride to polystyrene (8% divinyl-
benzene) in carbon disulfide under reflux conditions
[2b]. The capacity of the polymeric catalyst was 0.48
mmol AlCl3 per gram catalyst [30]. Although AlCl3
is a water-sensitive, corrosive, and environmentally
harmful compound, PS/AlCl3 is a stable and water-
tolerant species. This polymeric catalyst is easy to
prepare, stable in air for a long time without any
change, easily recyclable, and reusable without ap-
preciable loss of its activity.
Initially, to optimize the reaction conditions, we
studied the reaction between 2-naphthol (2 mmol)
and 4-chlorobenzaldehyde (1 mmol) as a simple
model using catalytic amount of PS/AlCl3 (20 mol%)
under different conditions (Table 1). We found that
the best result was obtained when the reaction was
carried out in acetonitrile as reaction media (Table
1, entry 6). The use of 20 mol% of PS/AlCl3 was suf-
ficient to progress the reaction, and an increase in
the amount of catalyst did not improve the yield.
Heteroatom Chemistry DOI 10.1002/hc

Polystyrene-Supported Aluminum Chloride: An Efficient and Recyclable Green Catalyst 53
TABLE 2 Synthesis of 14-Aryl or Alkyl-14H-dibenzo[a,j ]-
EXPERIMENTAL
xanthenes Catalyzed by PS/AlCl3 (20 mol%) Using Different
Aldehydes and 2-Naphthola
Materials and Instruments
Time Yieldc
M.p. ( ◦C)
(lit. m.p) [Ref.]
All chemical reagents and solvents were obtained
from Fluka (Switzerland) and Merck (Germany) and
were used without further purification. Cross-linked
polystyrene (8% divinylbenzene) was prepared via
suspension polymerization. PS/AlCl3 was prepared
using cross-linked polystyrene as reported in the lit-
erature [2b]. The capacity of the catalyst was de-
termined by the Mohr titration method. All prod-
ucts were known compounds and were identified
by comparison of their physical and spectral data
with those of the authentic samples. Melting points
were measured on an Electrothermal 535 appara-
tus and were uncorrected. 1H NMR spectra were
recorded on a Bruker DPX-250 Avance spectrom-
eter at 250.13 MHz. IR spectra were recorded on
a Unicam Matteson 1000 spectrophotometer. Re-
action monitoring and purity determination of the
products were accomplished by TLC on silica-gel
polygram SILG/UV254 plates. All yields refer to iso-
lated products.
Entry
Rb
C6H5
4-MeC6H4
4-OMeC6H4
4-ClC6H4
(h)
(%)
1
2
3
4
5
6
7
8
2.5
2.5
2
1.5
2
1.5
2.5
2.5
2.5
3
89
88
86
92
90
93
91
90
84
85
82
80
184-185(185) [19]
228(229) [19]
202-203(204) [19]
289-290(289) [19]
214-215(215) [19]
310-311(310) [19]
213-214(213) [19]
297-298(297) [19]
261-262(263) [19]
149-150 (152) [28]
153-154 (155-157) [22]
152-153(153-154) [22]
2-ClC6H4
4-NO2C6H4
2-NO2C6H4
4-BrC6H4
4-OHC6H4
CH2CH3
CH2CH2CH3
CH(CH3)2
9
10
11
12
4
4
aReaction condition: aldehyde (1 mmol); 2-naphthol (2 mmol),
PS/AlCl3 (20 mol%); reflux in CH3CN.
bAll the products are known and were characterized by comparison
of their IR, 1H NMR, and physical data with those reported in the
literature.
cIsolated yields.
and reaction time. In comparison with electron-
withdrawing groups on the aryl aldehydes, we found
that the presence of electron-donating groups on the
aryl aldehydes decreased both the reaction rate and
yield of product (Table 2, entries 3,9). The workup
procedure is simple and includes filtration of the
mixture to separate the catalyst. Although AlCl3 is
a water-sensitive, corrosive, and environmentally
harmful compound, PS/AlCl3 is a very active, sta-
ble to air and moisture, nontoxic species, and it
can be quantitatively recovered by filtration and
reused.
Preparation of PS/AlCl3
Anhydrous AlCl3 (4.5 g) was added to polystyrene
(8% divinylbenzene, 3.5 g) in carbon disulfide
(25 mL) as the reaction medium. The mixture was
stirred using a magnetic stirrer under reflux condi-
tion for 50 min, cooled, and then water (40 mL) was
cautiously added to hydrolyze the excess AlCl3. The
mixture was stirred until the deep orange color dis-
appeared, and the polymer became light yellow. The
polymer beads were then filtered and washed with
water (300 mL) and then with acetone and diethyl
ether. The polymer was dried in a vacuum oven for
12 h at room temperature. The capacity of the poly-
meric catalyst based on its chloride content was 0.47
mmol AlCl3/g catalyst [30,31].
Based on the experimental results and by refer-
ring to the literature [32,33], the mechanism of the
dibenzoxanthene formation proceeds by the usual
pathway proposed using Lewis acids.
In conclusion, PS/AlCl3 as a noncorrosive, en-
vironmentally benign, and stable heterogeneous
polymer-supported Lewis acid has proved to be
a very efficient catalyst for the efficient synthesis
of dibenzoxanthene derivatives. The notable advan-
tages of this methodology are mild reaction condi-
tions, high yields, short reaction times, operational
simplicity, generality, easy workup and low cost,
easy preparation, and handling of the catalyst. In ad-
dition, the use of water-tolerant PS/AlCl3 has resulted
in a reduction in the unwanted and hazardous waste
that is produced during conventional homogeneous
processes. Finally, this polymeric solid acid catalyst
can be recovered and reused at least five times with
negligible loss in its activity.
General Experimental Procedure for the
Synthesis of 14-Aryl or
Alkyl-14H-dibenzo[a,j]xanthenes
In a round-bottom flask (50 mL) equipped with a
condenser and a magnetic stirrer, a solution of the
aldehyde (1 mmol) and 2-naphthol (2 mmol) in
CH3CN (10 mL) was prepared. PS/AlCl3 (20 mol%)
was added to the solution, and the reaction mixture
was stirred magnetically under reflux conditions
for the specified period of time. The progress of the
reaction was monitored by TLC. After completion of
the reaction, the catalyst was filtered off and washed
with CHCl3 (2 × 10 mL) to remove any adhering
xanthene product and the filtrate concentrated on a
Heteroatom Chemistry DOI 10.1002/hc

54 Rahmatpour and Aalaie
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