An environmentally friendly, chemoselective, and efficient protocol for the preparation of coumarin derivatives by Pechman condensation reaction using new and reusable heterogeneous Lewis acid catalyst polystyrene-supported GaCl 3

Article abstract of DOI:10.1016/j.crci.2013.01.006

An environmentally benign Pechman protocol for the one-pot synthesis of 4-substituted coumarins through the condensation reactions of phenol substrates with β-ketoesters using polystyrene-supported GaCl₃ (PS-GaCl₃) as a highly active and reusable solid Lewis acid catalyst under mild and heterogeneous conditions in good to excellent yields is described. This new protocol is easy, simple, cost-effective, chemoselective, and in addition has the advantages of easy availability, stability, reusability and eco-friendly character of the catalyst, high to excellent yields, simple experimental and work-up procedure.

Full text of DOI:10.1016/j.crci.2013.01.006

C. R. Chimie 16 (2013) 271–278
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Full paper/Memoire
An environmentally friendly, chemoselective, and efficient protocol for
the preparation of coumarin derivatives by Pechman condensation
reaction using new and reusable heterogeneous Lewis acid catalyst
polystyrene-supported GaCl3
*
Ali Rahmatpour , Sara Mohammadian
Polymer Science and Technology Division, Research Institute of Petroleum Industry (RIPI), 14665-1137 Tehran, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
An environmentally benign Pechman protocol for the one-pot synthesis of 4-substituted
Received 27 June 2012
coumarins through the condensation reactions of phenol substrates with
b-ketoesters
Accepted after revision 2 January 2013
Available online 23 February 2013
using polystyrene-supported GaCl3 (PS–GaCl3) as a highly active and reusable solid Lewis
acid catalyst under mild and heterogeneous conditions in good to excellent yields is
described. This new protocol is easy, simple, cost-effective, chemoselective, and in
addition has the advantages of easy availability, stability, reusability and eco-friendly
character of the catalyst, high to excellent yields, simple experimental and work-up
procedure.
Keywords:
Coumarin
Polymer-supported catalyst
Phenols
ß 2013 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Gallium trichloride
1. Introduction
catalysts, all factors that are important in industry [6]. A
large number of polymer-supported Lewis acid catalysts
Conventional, toxic and polluting BrØnsted and Lewis
acid catalysts, such as H2SO4, HCl, CF3CO2H, AlCl3, FeCl3,
TiCl4, ZrCl4 are very effective catalysts for a wide variety of
organic reactions [1], but one of the major problems
associated with these homogeneous catalysts is the
recovery of catalyst from the reaction medium. In recent
years, there have been intense efforts to replace these
catalysts with eco-friendly reusable heterogeneous cata-
lysts. The use of polymer-supported catalysts in organic
transformations has been receiving extraordinary atten-
tion and the design of functionalized polymers carrying
catalytically active metal species has attracted consider-
able interest [2–5]. Immobilization of catalysts on solid
support improves the available active sites, stability,
hygroscopic properties, handling, and reusability of
have been prepared by immobilization of the catalysts on
polymer via coordination or covalent bonds [7]. 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 in various organic reactions. Polystyrene is one
of the most widely studied heterogeneous and polymeric
supports due to its environmental stability and hydropho-
bic nature which protects water-sensitive Lewis acids from
hydrolysis by atmospheric moisture until it is suspended in
an appropriate solvent where it can be used in a chemical
reaction [8]. It is well known that gallium trichloride is a
strong Lewis acid and an important catalyst in organic
transformations. However, it easily hydrolyzes in air, so
that its use, storage, and separation from the reaction
mixture are inconvenient and difficult. Polystyrene-sup-
ported gallium chloride, PS–GaCl3, which is a tightly bound
* Corresponding author.
E-mail address: rahmatpoura@ripi.ir (A. Rahmatpour).
1631-0748/$ – see front matter ß 2013 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2013.01.006

272
A. Rahmatpour, S. Mohammadian / C. R. Chimie 16 (2013) 271–278
and stable complex between anhydrous GaCl3 and
polystyrene-divinylbenzene copolymer beads, has been
described for the first time by Ruicheng et al. [9]. The use of
PS–GaCl3 complex catalyst has several advantages over
conventional Lewis acid catalysts like its cost-effective-
ness, ease of handling, recyclability, and tunable Lewis
acidity.
and the investigation of its catalytic activity. To the best of
our knowledge, this is only the second example of PS–
GaCl3 as a catalyst for any synthetic transformation.
2. Experimental
2.1. Materials and instruments
Coumarin and its derivatives are the important class of
naturally occurring oxygen heterocyclic compounds hav-
ing a distinct and important place in the realm of natural
and synthetic organic chemistry as these compounds
display useful and diverse biological properties, viz.
antibacterial, antiviral, anticancer, anti-HIV [10–12], and
also have been used as additives in food, cosmetics, optical
brightening agents, and laser dyes [13–15]. Due to these
applications and properties, a variety of methods have
been developed to synthesize coumarins [16–20]. Despite
these developments, the Pechman reaction, a two-compo-
All chemical reagents were obtained from Fluka and
Merck chemical companies and were used without further
purification. Cross-linked polystyrene (7–8% divinylben-
zene, mesh size: 20–80) was prepared via suspension
polymerization as reported in the literature [42,43]. PS/
AlCl3 was also prepared as previously reported [42]. The
products were isolated and identified by comparing their
physical and spectral data with those in the literature [21–
32]. 1H and 13C NMR spectra were recorded on a Bruker
DPX-250 Avance spectrometer at 250.13 MHz. FT-IR
spectra of the samples were recorded from 400 to
4000 cmꢀ1 on a Unicam Matteson 1000 spectrophotome-
ter. UV spectra were taken using a Pharmacia Biotech
Ultraspec 3000 model 80-2106-20 spectrometer. Mass
spectra were recorded on a Fisons instrument. Elemental
analyses were performed using a Heraeus CHN-O-Rapid
analyzer by RIPI and the results agreed favorably with
calculated values. The capacity of the catalyst was
determined by the Mohr titration method and atomic
absorption technique using a Philips atomic absorption
instrument. Reaction monitoring and purity determination
of the products were accomplished by TLC on silica gel
nent (activated phenols and
b-ketoesters) coupling under
homogeneous acid catalysis, is a very well established
method for the preparation of substituted coumarin rings
[21–32]. However, this reaction usually involves the use of
non-reusable homogeneous BrØnsted, Lewis, and mineral
acids in an excess amount are necessary in the classical
preparations, resulting in environmental pollution. The
present drive therefore is towards the development of
more effective, non-stoichiometric, preferably recyclable
heterogeneous catalysts. For eco-friendly reasons, some
solid-phase catalysts such as a microporous polymer based
on Nafion resin/silica [33], Well–Dawson heteropolyacid
[34], montmorillonite KSF [35], calcined Mg–Al hydro-
talcite [36], sulfated zirconia [37], ion exchange resins [38],
polyaniline sulfate salt [39], zeolites [40], etc. [41] as
alternatives to homogeneous acids have been employed
for the preparation of coumarins. Nevertheless, substituent
diversity in the synthesis of 4-substituted coumarins
remains a challenge, and restricted reaction conditions are
often involved in the preparation. Therefore, there is still
demand for the introduction of mild, selective and
environmentally benign methods, especially using recy-
clable heterogeneous catalysts for this transformation. The
search continues for finding a better catalyst in terms of
operational simplicity, economic viability, and greater
selectivity.
polygram SILG/UV254 plates or by
a Shimadzu Gas
Chromatograph GC-16A instrument with a flame ioniza-
tion detector.
2.2. Preparation of PS–GaCl3
Anhydrous GaCl3 (4.50 g) was added to polystyrene (7–
8% divinylbenzene, mesh size: 20–80, 40 g) in carbon
disulfide (5 mL) as the reaction medium. The mixture was
stirred using a magnetic stirrer under reflux condition for
1 h, cooled, and then water (50 mL) was cautiously added
to hydrolyze the excess GaCl3. The mixture was stirred
until the bright red color disappeared, and the polymer
became yellow. The polymer beads were collected by
filtration and washed with water (300 mL) and then with
ether (30 mL) and chloroform (30 mL). The catalyst was
dried in a vacuum oven overnight at 50 8C before use. The
chlorine content of PS–GaCl3 was 4.17% analyzed by the
Mohr titration method [46] and the loading capacity of
GaCl3 on the polymeric catalyst or the amount of GaCl3
In continuation of our recent works on the use of
polymeric Lewis acid catalysts in synthetic transforma-
tions [42–45], herein we wish to report the preparation of
polystyrene-supported gallium trichloride as a stable,
highly active and reusable heterogeneous Lewis acid
catalyst in the Pechman coumarin synthesis (Scheme 1),
3
R
O
O
PS/GaCl (10 mol%)
OH
3
1
2
1
2
3
+ H O + C2H5OH
2
O
OC H
2 5
R ,R
R ,R
+
R
C H OH, Reflux
2 5
O
2
2
1
1
CH , OCH , Ar, OH, NH
; R : OH
3
3
3
2
R :
3
R : CH
,
3
CH Cl, Ph, cyclic 1,3-ketoester (5- and 6-membered ring)
2
Scheme 1. PS–GaCl3-catalyzed Pechman coumarin synthesis.

A. Rahmatpour, S. Mohammadian / C. R. Chimie 16 (2013) 271–278
273
complexed with polystyrene was calculated to be
0.391 mmol/g [47].
derivative. The spent catalyst from different experiments
was washed with chloroform and ether, dried and used
again.
2.3. General experimental procedure for the synthesis of
coumarins (3)
2.4. The spectral data for new compounds
In a round-bottom flask (25 mL) equipped with a
condenser and a magnetic stirrer, a stirring mixture of the
(Table 2, 3e, entry 5): White solid; m.p.: 210–213 8C; IR
(KBr):
(250 MHz, DMSO-d6): d = 10.15 (s, 1H), 7.32–7.36 (m,
n ;
= 3180 (OH), 1680 (C5O) cmꢀ1 1H NMR
phenolic substrate (1,1 mmol),
b
-ketoester (2,1 mmol)
and PS–GaCl3 (2.55 g, 1 mmol of GaCl3) in ethanol (5 mL)
was heated under reflux for an appropriate time as
indicated by TLC (Tables 1 and 2). After completion of the
reaction, the reaction mixture was cooled to room
temperature and then hot ethanol (5 mL) was added to
the mixture and the catalyst was collected by filtration
and then washed with chloroform and ether (2 ꢁ 5 mL).
The filtrate was concentrated on a rotary evaporator under
reduced pressure and the crude product obtained was
washed with water (10 mL ꢁ 2), and purified by recrys-
tallization from ethanol to afford the pure coumarin
5H), 6.72 (s, 1H), 6.46 (s, 1H), 5.95 (s, 1H), 3.02 (s, 3H);
[Elemental analysis, Found: C, 76.08, H, 4.78, C16H12O3
requires C, 76.19, H, 4.76%].
(Table 2, 3t, entry 20): White solid; m.p.: 264–265 8C; IR
(KBr):
1376, 1301, 1160, 820, 722 cmꢀ1
DMSO-d6): 1.64–1.66 (m, 4H, CH2), 2.33–2.36 (m, 2H,
n
= 3185 (OH), 2925, 2726, 2360, 1637 (C5O), 1460,
;
1H NMR (250 MHz,
d
CH2), 3.03–3.05 (m, 2H, CH2), 6.15 (s, 1H, Ar-H), 6.24 (s, 1H,
Ar-H), 10.12 (s, OH), 10.37 (s, OH); [Elemental analysis,
Found: C, 67.33, H, 5.16, C13H12O4 requires C, 67.24, H,
5.18%].
Table 1
Optimization of reaction conditions and comparison of the condensation reaction of resorcinol (1, 1 mmol) with ethyl acetoacetate (2, 1 mmol) catalyzed by
PS–GaCl3 with the other catalysts used for this reaction.
Entry
Solventa
Catalyst (mol %)
Time (h)
Yieldb (%)
Ref.
1
Toluene
PS–GaCl3 (10)
PS–GaCl3 (10)
PS–GaCl3 (10)
PS–GaCl3 (10)
PS–GaCl3 (10)
PS–GaCl3 (10)
None
2 h
26
20
14
15
77
NRe
NR
62
94
94
NR
61
51
77
NR
58
59
54
58
67
36
41
51
47
71
95
77
94
90
91
80
92
72
99
90
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
[38]
2
Cyclohexane
CH2Cl2
2 h
3
2 h
4
CH3Cl
2 h
5
CH3CN
2 h
6
H2O
2 h
7
C2H5OH
2 h
8
C2H5OH
PS–GaCl3 (5)
50 min
50 min
50 min
2 h
9
C2H5OH
PS–GaCl3 (10)
PS–GaCl3 (15)
PS (10)
10
11c
12c
13d
14e
15f
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
C2H5OH
C2H5OH
C2H5OH
Toluene-GaCl3 complex (10)
PS–GaCl3 (10)
PS–GaCl3 (10)
PS–GaCl3 (10)
PS/AlCl3 (10)
75 min
50 min
50 min
20 min
2 h
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
SiO2/AlCl3 (10)
AlCl3 (10)
2 h
C2H5OH
2 h
C2H5OH
BiCl3 (10)
2 h
C2H5OH
ZnCl2 (10)
2 h
C2H5OH
FeCl3 (10)
2 h
C2H5OH
I2 (10)
2 h
C2H5OH
SnCl4 (10)
2 h
C2H5OH
KAl(SO4)2.12H2O (10)
TiCl4 (10)
2 h
C2H5OH
2 h
Toluene, reflux
Toluene, reflux
Toluene, reflux
Toluene, reflux
None, 170 8C
Toluene, reflux
Toluene, reflux
None, 170 8C
n-Hexadecane, 160 8C
None, 175 8C
Amberlyst
2 h
Zeolite
4 h
[40]
Montmorillonite K-10 (30 wt%)
Montmorillonite KSF (30 wt%)
SO4/ZrO2
8 h
[35b]
10 h
3 h
[35b]
[37]
W/ZrO2
6 h
[51]
Nafion/silica
2 h
[33]
Polyaniline sulfate salt (95 mg)
SiO2/MeSO3H (0.25 g)
Metal ion-exchanged
ZAPO-5
6 h
[39]
2 h
[52]
6 h
[53]
a
All reactions were carried out under reflux conditions with 10 mol% of catalyst in 5 mL of solvent.
Isolated yield.
b
c
d
e
f
PS and the toluene–GaCl3 complex were used as catalysts, respectively.
Two reactions were carried out in ethanol at 45, 60 8C, respectively.
NR: no reaction.
Catalyst was removed by filtration after 20 min.

274
A. Rahmatpour, S. Mohammadian / C. R. Chimie 16 (2013) 271–278
Table 2
PS–GaCl3-catalyzed synthesisa of coumarins via Pechman condensation of phenol substrates with
b-ketoesters.
Entry
1
Phenol substrate
b
-Ketoester
Product
Time (min)
Yieldb (%)
m.p.(8C)
Ref.
CH
3
CH3COCH2CO2 C2H5
50
85
55
9
184–186
[31]
(3a)
(3b)
HO
HO
HO
OH
OH
OH
HO
O
O
O
Ph
2
3
PhCOCH2CO2C2H5
87
90
253–255
180–182
[25]
[25]
HO
O
CH Cl
2
ClCH2COCH2CO2C2H5
(3c)
HO
O
O
OH
OH
CH
3
CH3
CH3
4
5
CH3COCH2CO2C2H5
60
90
90
82
258–260
210–213
[25]
(3d)
CH
3
HO
O
O
OH
O
Ph
PhCOCH2CO2C2H5
–
(3e)
HO
HO
CH
3
OH
OH
O
CH
3
6
CH3COCH2CO2C2H5
60
84
137–139
[28]
(3f)
HO
HO
CH
3
O
O
O
CH
3
Ph
7
PhCOCH2CO2C2H5
90
83
284–285
[28]
HO
OH
(3g)
CH
3
O
CH
3
OH
OH
OH
OH
CH
3
8
CH3COCH2CO2C2H5
PhCOCH2CO2C2H5
ClCH2COCH2CO2C2H5
45
70
55
96
90
91
283–285
242–244
186–188
[31]
(3h)
(3i)
(3j)
HO
HO
HO
HO
HO
HO
OH
OH
OH
O
O
O
O
OH
OH
Ph
9
[23b]
[23c]
O
CH Cl
2
10
O

A. Rahmatpour, S. Mohammadian / C. R. Chimie 16 (2013) 271–278
275
Table 2 (Continued)
Entry
Phenol substrate
b-Ketoester
Product
Time (min)
Yieldb (%)
m.p.(8C)
Ref.
CH
3
11
CH3COCH2CO2C2H5
65
82
241–243
[25]
HO
OH
(3k)
HO
OH
O
O
OH
CH
3
12
13
CH3COCH2CO2C2H5
100
78
81
131–133
149–151
[35]
[35]
(3l)
OH
OH
CH
3
CH
3
O
O
CH
CH
3
3
CH
3
CH3COCH2CO2C2H5
60
(3m)
O
O
CH
3
14
CH3COCH2CO2C2H5
CH3COCH2CO2C2H5
CH3COCH2CO2C2H5
CH3COCH2CO2C2H5
CH3COCH2CO2C2H5
70
85
84
45
84
71
160–162
165–166
80–82
[23c]
[23c]
[21]
(3n)
CH
3O
OH
CH O
3
O
O
CH
CH O
3
3
CH O
3
15
85
(3o)
OH
O
O
CH
3
16c
300
(3p)
OH
O
O
CH
3
17
60
222–224
154–156
[24]
(3q)
NH
OH
2
NH
2
O
O
OH
CH
3
18c
100
[23b]
(3r)
O
O
O
19
70
60
81
82
243–245
264–265
[54]
CO2C2H5
(3s)
HO
HO
OH
OH
HO
HO
O
O
OH
O
OH
CO C H
2 5
2
20
–
(3t)
O
O
a
Reaction conditions: phenol (1 mmol),
Yields refer to isolated pure products which were characterized by comparison of their m.p., IR, 1H and 13C NMR spectra with those of authentic samples
b
-ketoester (1 mmol), PS–GaCl3 (0.1 mmol), C2H5OH (5 mL), Reflux.
b
prepared by reported procedures.
c
The reaction was performed using 20 mol% PS–GaCl3.

276
A. Rahmatpour, S. Mohammadian / C. R. Chimie 16 (2013) 271–278
3. Results and discussion
10 mol% of PS–GaCl3 was sufficient to drive the reaction
completely and excessive amount of 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 heterogeneous 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,
while 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 61% (Table 1, entry 12). Also, PS–GaCl3 was found
to be a more effective catalyst than PS/AlCl3 and SiO2/AlCl3
in terms of reaction time and the yield of the product 3a
under identical conditions (Table 1, entries 16, 17). We also
tested the catalytic activity of different homogeneous
catalysts for comparing the efficiency of PS–GaCl3, and we
obtained low to moderate yields in ethanol (Table 1,
entries 18–25). All the reactions were run in the same
reaction conditions, and similar amounts of catalysts
(10 mol%) were used. As shown in Table 1, PS–GaCl3
catalyzes the reaction better than other catalysts, and
satisfactory results were obtained only with PS–GaCl3
(Table 1, entry 9). In addition, the catalytic efficiency of PS–
GaCl3 was compared with that of the reported heteroge-
neous catalytic systems in similar reactions as shown in
Table 1, entries 26–35. It is worth mentioning that our
method is more efficient and simpler than the existing
methods.
By considering the excellence of polystyrene-divinyl-
benzene (7–8%) copolymer beads (PS) as an effective
supporting material for immobilization of Lewis acid
previously reported [42,43,45] by our group, we decided
to use PS beads as a support for the heterogenization of
gallium trichloride. PS–GaCl3 was prepared by addition of
anhydrous gallium trichloride to polystyrene (7–8%
divinylbenzene) in carbon disulfide under reflux condi-
tions. The loading capacity of the polymeric catalyst
obtained by gravimetric method and checked by atomic
absorption technique was 0.391 mmol GaCl3/g of complex
beads catalyst [46]. The data obtained by these two
techniques showed, within the limits of experimental
error, that the catalyzing species are in the form of GaCl3
supported on the polymeric support. The UV spectrum of
the solution of the 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 with gallium
trichloride. The 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 1500–
1560 and 400–800 cmꢀ1, 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 et al. [48], because the Lewis acid GaCl3 is
complexed with the benzene rings of the polystyrene and
the GaCl3 is stabilized due to the decreased mobility of the
benzene rings hindered by the long polystyrene chain. The
PS–GaCl3 complex is a non-hygroscopic, water tolerant,
and especially 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 appreciable loss of its activity.
The lifetime and leaching of active metal species into
the solution are important issues to consider when
heterogeneous catalysts are used, particularly for practical
applications of the reaction. Our preliminary investiga-
tions demonstrated that PS–GaCl3 catalyst (a stable
p
complex) is very stable to air and moisture. Moreover, in a
separate experiment the catalyst 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). On the other hand,
after each run the filtrates were used for determination of
catalyst leaching (gallium content), which showed a
negligible release of GaCl3 by atomic absorption or ICP
measurement. The capacity of the catalyst after six uses
was 0.378 mmol of GaCl3 per gram. Therefore, we may
conclude that PS–GaCl3 is stable, that no significant
leaching of Lewis acid moieties is operating under our
reaction conditions, that any gallium species that leached
into the reaction mixture is not an active homogeneous
catalyst, and that the observed catalysis is truly heteroge-
neous in nature.
To study the feasibility of the PS–GaCl3-catalyzed
Pechman condensation, the reaction of resorcinol (1,3-
dihydroxybenzene) with ethyl acetoacetate was selected
as a model to screen the best conditions. The swelling
property of cross-linked resin (PS) in organic solvents is an
important factor for effective solid-phase reactions [49].
Our initial experiments focused on the selection of solvent.
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 non-polar solvents. Ethanol
was chosen as the solvent system for this conversion,
because of its swellability with the catalyst and miscibility
with the substrates. No reaction occurred when H2O was
used as a 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 [50]. 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 80 8C (Table 1, entries 9, 13,
14). Consequently, we chose 80 8C as the optimal
temperature for the model reaction. Next, the amount of
the catalyst was examined and we found that the yields
were obviously affected with different amount of catalyst.
No reaction was observed in the absence of catalyst, while
With the optimal conditions in hand, the substrate
scope was investigated. We subjected a series of monohy-
dric and polyhydric phenols to react with a variety of
b-
ketoesters to obtain the corresponding 4-substituted
coumarins (Table 2). A wide range of structurally varied
phenols reacted smoothly to give the corresponding
coumarins in good yields. The remarkable feature of this
improved protocol is the wide stability of a variety of
functional groups, such as ether, hydroxyl, amino, etc.,
under the present reaction conditions. It is worth

A. Rahmatpour, S. Mohammadian / C. R. Chimie 16 (2013) 271–278
277
Table 3
2-carbethoxy cyclo pentanone, and also 2-carbethoxy
cyclohexanone. It was found that R3 can be different
groups and in all these cases, good yields of the
corresponding coumarin derivatives were obtained (Table
2, entries 2, 3, 5, 7, 9, 10, 19, 20). Nevertheless, the steric
effect of R3 had a negative influence on this reaction.
The experimental procedure with this polymeric
catalyst is very simple and the catalyst can be removed
easily by filtration. Hence, there will not be any unneces-
sary acidic waste streams to create environmentally
hazardous pollution and in all the cases, no chro-
matographic separation or cumbersome reaction work-
up is necessary to get the pure compounds.
Reusability of aPS–GaCl3 in the synthesis of 3ab.
Run
1
2
3
4
5
6
Yieldc (%)
94
50
92
50
90
50
88
60
86
60
84
60
Time (min)
a
The capacity of the catalyst after six uses was 0.378 mmol GaCl3 per
gram.
b
Reaction conditions: resorcinol (1 mmol), ethyl acetoacetate
(1 mmol), PS–GaCl3 (0.1 mmol), C2H5OH (5 mL), reflux.
c
Isolated yield.
mentioning that substrates like phenol and cresols, which
failed to react in many of the protocols reported in the
literature, showed better reactivity, giving moderate to
good yields, under these reaction conditions (Table 2,
entries 12, 13, 16). From the experimental results,
substrates having electron-donating groups in the para
position to the site of electrophilic substitution (the meta
position to the phenolic–OH) facilitated the condensation
and gave maximum yields in a short period of time. The
resonance effects of these substituents support formation
of the reactive polarized carbocation in the ortho position.
An alkyl group is not strong enough to furnish the
activation needed and thus gives a moderate yield (Table
2, entries 12, 13). In contrast, electron-withdrawing
groups, such as salicylaldehyde and 2-chloro phenol
inhibit the reaction. Phenol required a higher amount of
catalyst and longer reaction duration, as no electron-
donating group is present. Another important feature is
that no detectable demethylation was observed in the case
of 3-methoxy and 4-methoxy phenol (Table 2, entries 14,
15). In the case of 1-naphthol, a higher amount of catalyst
and longer reaction time was required to obtain the
corresponding coumarin derivative due to presence of
another phenyl moiety (Table 2, entry 18). Finally, to
generalize the protocol, we also attempted the condensa-
The main objective and potential benefits of supporting
a homogeneous Lewis acid on to a polymer support include
enhancing the life and facilitating the separation of the
resulting heterogeneous catalyst from the reagents and
reaction products, and its recyclability for repeated use. It
is critical that catalyst recovery should be simple and
efficient, and that the recovered catalyst should retain its
original reactivity through multiple cycles. To test this, we
have examined the reusability of PS–GaCl3. After the
reaction of resorcinol with ethyl acetoacetate was com-
plete, the catalyst was recovered by filtration and
thoroughly washed with chloroform, ether, and dried
carefully for consecutive runs. The results are shown in
Table 3. The recovered catalyst was used in a new reaction
batch of the reactants. The catalyst showed at least six
cycle reusability without showing noticeable deterioration
in catalytic activity.
Selectivity of the catalysts for chemical transformations
is important, especially, when the catalyst is used in multi-
step synthesis. The difference in reactivity of the PS–GaCl3
catalyst towards phenols gave us the impetus to study
chemoselective reactions. When an equimolar mixture of
resorcinol and hydroquinone or phenol or p-hydroxyben-
zaldehyde was reacted with ethyl acetoacetate under the
same reaction conditions, only the resorcinol converted
predominantly to coumarin products (Scheme 2).
tion reaction using a further variety of
b-ketoesters such
as 4-chloroethyl acetoacetate, benzoyl acetoacetate,
CH
CH
3
3
HO
+
HO
PS/GaCl (10 mol%)
3
CH COCH CO C H
2 2 5
+
O
+
3
2
C H OH, Reflux
2 5
OH
HO
HO
HO
OH
O
(96 %)
O
O
50 min
(1 mmol)
(1 mmol)
(1 mmol)
(4 %)
CH
3
CH
3
PS/GaCl (10 mol%)
3
CH COCH CO C H
2 2 5
+
+
OH
+
3
2
C H OH, Reflux
2 5
OH
HO
O
O
(100 %)
O
O
(1 mmol)
70 min
(1 mmol)
(1 mmol)
(0 %)
CH
3
CH
3
OHC
+
OH
OHC
+
O
PS/GaCl (10 mol%)
3
CH COCH CO C H
2 2 5
+
3
2
C H OH, Reflux
2 5
OH
HO
HO
O
(100 %)
O
O
70 min
(1 mmol)
(1 mmol)
(1 mmol)
(0 %)
Scheme 2. Competitive reaction in Pechman coumarin synthesis catalyzed by PS–GaCl3.

278
A. Rahmatpour, S. Mohammadian / C. R. Chimie 16 (2013) 271–278
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b-ketoesters. The mild reaction
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and easy preparation and handling (as a bench top
catalyst) of the polymeric catalyst, its ability to tolerate
a wide variety of substitutions in both components, and
the simple experimental and product isolation procedures
are the significant advantages of the present method. In
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