Periodic mesoporous silica chloride (PMSCl) as an efficient and recyclable catalyst for the Pechmann reaction

Article abstract of DOI:10.1016/j.catcom.2011.05.019

Periodic mesoporous silica chloride (PMSCl) with 2D P₆mm hexagonal structure which can be prepared by a simple procedure is reported to act as an efficient and recyclable catalyst for the conversion of a variety of phenols into their corresponding coumarins through Pechmann reaction. The reactions were conducted under solvent-free conditions in good to excellent yields.

Full text of DOI:10.1016/j.catcom.2011.05.019

Catalysis Communications 12 (2011) 1432–1436
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Periodic mesoporous silica chloride (PMSCl) as an efficient and recyclable catalyst for
the Pechmann reaction
Babak Karimi ⁎, Hesam Behzadnia
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Gava Zang, Zanjan 45137-6731, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Periodic mesoporous silica chloride (PMSCl) with 2D P mm hexagonal structure which can be prepared by a
6
Received 31 January 2011
Received in revised form 13 May 2011
Accepted 19 May 2011
simple procedure is reported to act as an efficient and recyclable catalyst for the conversion of a variety of
phenols into their corresponding coumarins through Pechmann reaction. The reactions were conducted
under solvent-free conditions in good to excellent yields.
Available online 27 May 2011
©
2011 Elsevier B.V. All rights reserved.
Keywords:
Pechmann reaction
Coumarins
Periodic mesoporous silica chloride
PMSCl
1
. Introduction
sulfonic acid functionalized mesoporous Zr-TMS [30], and hetero-
polyacids supported on silica Ralt–Chemie [31] have been developed
and used by several groups to catalyze the Pechmann reaction with
different degrees of success. However, these protocols do not always
lead to satisfactory results, and they generally require longer reaction
times and higher reaction temperatures than the homogeneous
catalyst systems.
Therefore, it seems that there is still much room to develop new
methods based on recyclable catalysts under more appropriate
reaction conditions. Since the discovery of ordered mesoporous
silicates with honeycomb arrangement of hexagonal pores in 1990s
[32–34], new organic-inorganic nanoccomposites, based on ordered
mesoporous silica materials, have been widely used in the develop-
ment of functional materials which have been mainly due to their
exceptional properties such as high surface area and regular and
tunable pore sizes [35–41].
Recently, we developed a variety of water-tolerant sulfonic acid
based nanoreactors based on ordered nanoporous silica for the von
Pechmann reaction to satisfy both recyclability and reactivity [42]. We
found that among these nanoreactors, the material having both acidic
sites and phenyl groups, located inside the mesochannels of SBA-15,
was the most active catalyst. Despite the fact that this method
provided an interesting achievement in the field of Pechmann
reaction, but it required several steps for the synthesis of the catalysts.
Very recently, we have developed a new family of periodic
mesoporous silica chloride (PMSCl) which exhibits high thermal
stability, excellent 2D hexagonal symmetry, and high loading of Si–Cl
Coumarins occupy an important place in the realm of natural
products and synthetic organic chemistry. They are used as additives
in foods, perfumes, cosmetics, pharmaceuticals, and also in the
preparation of insecticides, optical brighteners, and dispersed fluo-
rescent and tunable laser dyes [1–5]. In addition, some coumarins
show a variety of bioactivities, such as inhibition of platelet
aggregation, antibacterial, anticancer, and inhibition of HIV-1 prote-
ase [1–5]. Although there are several routes for the formation of
coumarins such as Reformatsky [6], Wittig [7], Knoevenagel [8–10],
Perkin [11], and Baylis–Hillman reactions [12], Pechmann reaction is
one of the simplest methods which is based on inexpensive starting
materials [13,14].
This reaction is usually catalyzed by different Brønsted and Lewis
acids in a homogeneous pathway [15–23]. Although these methods
are quite effective in some instances, the stoichiometric use of strong
acids and the difficulty of separation of homogeneous catalysts from
the reaction products would lead to both environmental and
economical problems.
To address the above inherent limitations of homogeneous
systems, a diverse array of supported catalysts and solid acids, such
as Resins [24], Montmorillonite Clay [25], Nafion resin/silica compos-
ite [26], Amberlyst ion-exchange resins [27], silica supported
perchloric acid [28], nano-crystalline sulfated-zirconia [29], Benzyl-
2
moieties in comparison with their amorphous SiO Cl analogs [43].
Meanwhile in the same cluster article, we have also explored the use
⁎
Corresponding author.
E-mail address: karimi@iasbs.ac.ir (B. Karimi).
of PMSCl as an efficient and recyclable catalyst/reagent for the
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.05.019

B. Karimi, H. Behzadnia / Catalysis Communications 12 (2011) 1432–1436
1433
2.4. Pechmann condensation catalyzed by PMSCl
The PMSCl was used as catalyst for the Pechmann condensation of
a variety of phenols. In a typical reaction, the phenolic compound
10 mmol), ethyl acetoacetate (11 mmol) and PMSCl (~7 mol%) were
Scheme 1. Beckmann rearrangement of oximes using PMSCl [43].
(
placed in a glass flask equipped with a magnetic stirrer and condenser.
The mixture was heated at 130 °C with constant stirring for the
desired period as indicated in Table 2.
Beckmann rearrangement of oximes to the corresponding amides in
excellent yields (Scheme 1).
In continuation of our studies in this field, we envisioned that
PMSCl could also be efficiently used for the Pechmann reaction.
The reaction was monitored by TLC, and after completion of
reaction, the reaction mixture was diluted with ethanol. The crude
product was purified by column chromatography on silica gel using a
mixture of n-hexane and ethylacetate (hexane: ethylacetate; 10:1)
and was then recrystallized from hot ethanol to afford the pure
coumarin derivatives in yields stated in Table 2.
2
. Experimental
2
.1. Materials
2.5. Characterization
Tetraethyl orthosilicate (TEOS), hydrochloric acid (HCl 37%),
phenolic compounds, ethyl acetoacetate and solvents were obtained
commercially (Merck) and used without further purification. Thionyl
chloride was purchased from Fluka. Pluronic P123 (EO20PO70EO20
Transmission electron microscopy (TEM) images of the samples
were obtained with a Philips CM 200FEG microscope. The pore
structures of the prepared materials were verified by the nitrogen
sorption isotherm ([5.0.0.3] Belsorp, BEL Japan, Inc.). Prior to the
measurements, all the samples were degassed at 100 °C for 5 h. Pore
size distributions, pore volumes, surface areas, and BET isotherms
(
EO=ethylene Oxide, PO=Propylene Oxide), Mav =5800) was
purchased from Aldrich.
were determined from N
2
adsorption–desorption experiments by
2
.2. Preparation of the SBA-15
using Barrett–Joyner–Halenda (BJH) method, and Brunauer–Emmett–
Teller (BET) analysis. The percentage of Si–Cl bonds in sample surfaces
were determined by suspending the calcined SBA-Cls in deionized
water, acid-base titration and elemental analysis. The NMR spectra
were detected by Bruker NMR spectrometer (1 H frequency:
In a typical synthesis of SBA-15 [33,34], 1 mol TEOS was added to
2
an aqueous hydrochloric acid solution (5.9 mol HCl and 193 mol H O)
containing 0.017 mol of Pluronic P123 block copolymer. SBA-15 was
obtained from in situ hydrolysis and condensation of tetraethyl
orthosilicate (TEOS) in the presence of Pluronic P123 under acidic
conditions. The mixture was stirred at 40 °C for 20 h and aged at 80 °C
for 24 h without stirring. The surfactant was then extracted for 48 h
with ethanol using a soxhelet apparatus.
2
50 MHz, 13 C frequency: 62.9 MHz). FT-IR spectra were recorded
−
1
by using KBr pellets in 400–4000 cm
region.
3
. Results and discussion
The synthesis of PMSCl was easily achieved by direct chlorination
2
.3. Preparation of the PMSCl
of SBA-15 surface Si–OH groups. Because of its high thermal and
hydrothermal stability we chose SBA-15 as a precursor for prepara-
tion of chlorinated silica. SBA-15 was first obtained from in situ
hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in the
presence of Pluronic P123 as a structure directing agent under acidic
conditions, according to the literature [33,34]. The surface Si–OH
groups of the resulting surfactant-free SBA-15 was then chlorinated
The preparation of PMSCl was easily achieved by direct chlorina-
tion of SBA-15 surface Si–OH groups. Typically, 1 g of SBA-15 was
refluxed in 40 ml of thionyl chloride in a round bottomed flask
equipped with a condenser and a drying tube for 48 h. The remainder
thionyl chloride was first distilled off and the resulting PMSCl was
dried directly on flame at ~550 °C under dry argon atmosphere and
was then stored in a sealed vessel, under argon atmosphere.
2
using an excess of SOCl under reflux condition for 48 h, followed
by calcination at ~550 °C, to produce the corresponding periodic
Fig. 1. Nitrogen adsorption–desorption isotherms for A) PMSCl and B) recovered PMSCl; Inset: BJH pore size distributions obtained from the corresponding isotherms.

1434
B. Karimi, H. Behzadnia / Catalysis Communications 12 (2011) 1432–1436
Table 1
Characterization of materials.
61%, 45 min: 84%). For a better comparison of the catalytic per-
formance of PMSCl, a second set of individual catalytic experiments
were conducted by employing p-TSOH, H SO , SBA-15, amorphous
a
Pore sizeb
c
Cl-contentd
Entry
Compound
S
BET
V
P
2
4
silica chloride, and sulfonic acid functionalized mesoporous silica
under the same reaction conditions (Table 2, entries 1–8). This study
clearly shows that PMSCl exhibits higher activity in Pechmann
coupling reaction of resorcinol with ethyl acetoacetate under the
same reaction condition and the catalyst loading. Remarkably, it is
also clearly seen from these data that only low yields (68%) of the
corresponding 7-hydroxy-4-methyl coumarin were obtained when
1
2
3
SBA-15
PMSCl
870
682
408
63
53
59.8
0.87
0.79
0.61
–
3.4
2.2
SiO
BET surface area (m2 g−1).
BJH average pore diameter (Å).
2
Cl
a
b
c
3
−1
Total pore volume (cm g ).
d
Surface Si–Cl content (mmol g−1) determined by back titration using (0.01 M,
NaOH) after hydrolysis. The data was further confirmed with elemental analysis.
amorphous silica chloride (SiO
Table 2, entry 7).
Having established that PMSCl constitutes a highly active catalyst
2
Cl) [44–47], was employed as catalyst
(
system for Pechmann reaction, we then examined the condensation
of several other representative phenols with ethyl acetoacetate
which are listed in Table 2. As expected, electron rich phenols
underwent faster coupling with ethyl acetoacetate, giving essentially
excellent yields of the products (Table 2). For all data embodied in
Table 2, good to excellent conversions and selectivities were easily
obtained. Interestingly, 3-aminophenol also reacted to provide the
amino coumarin in good yield with high chemoselectivity (Table 2,
entry 17). One of the interesting feature of this procedure over most
of the previous heterogeneous and homogeneous systems is the
formation of 4-methylcoumarin in acceptable yield from the reaction
of unsubstituted phenol, with ethyl acetoacetate (Table 2, entry 20).
This is very surprising because no detectable conversion was
observed when SBA-15-Ph-PrSO H (7 mol%) and amorphous SiO Cl
mesoporous silica chlorides. The materials were then characterized by
surface analysis, elemental microanalysis, and transmission electron
microscopy (TEM) [43].
Nitrogen adsorption–desorption analysis of the resulting PMSCl
showed a clear type IV isotherm with sharp hysteresis loop (Fig. 1). The
capillary condensation step, located at relative pressure between 0.6
0
and 0.8 P/P , provided clear evidence for the preservation of ordered
mesopore structures in PMSCl during synthesis and even after
calcination at ~550 °C. This is also evident on the BJH curves, which
shows narrow pore size distributions for PMSCl in the mesoporous
region (Fig. 1, inset). This mesoporous silica chloride possessed a high
2
3
BET surface area of 682 m /g and total pore volume of 0.79 cm /g
(
Table 1, entry 2).
3
2
The TEM images of the chlorinated sample also provided further
were used as the catalysts for this transformation (Table 2, entries 21
and 22).
To study the reusability of PMSCl, after the first use of the catalyst
in the preparation of 7-hydroxy-4-methylcoumarin, the reaction
evidence for its highly ordered structure similar to the parent SBA-15
Fig. 2). The accurate images of PMSCl were recorded along and
perpendicular to the channel axis of the two-dimensional hexagonal
(
(
6
P mm) mesostructures.
mixture was diluted with CH Cl2 and the catalyst was recovered by
2
The back-titration method and elemental microanalysis were used
simple filtration under argon atmosphere and reused directly for the
for the determination of chlorine content in the materials. The
titration was carried out on the resulting HCl upon the treatment of
the materials with deionized water for 12 h (Table 1). Noteworthy, all
samples were preheated at 550 °C under dry argon atmosphere,
before performing hydrolysis and titration experiments to illustrate
the fact that the observed data for chlorine content related to Si–Cl
moieties throughout the structure.
In the next stage, we decided to evaluate the ability of periodic
mesoporous silica chloride as a recoverable catalyst for selective
preparation of coumarin derivatives via Pechmann coupling reaction.
Our investigation was initiated by conducting the condensation of
resorcinol with ethyl acetoacetate as a probe under solvent-free
conditions. We observed that the reaction was very efficient in the
presence of as low as 7 mol% of PMSCl and the corresponding
coumarin product was formed in excellent yield after 60 min (30 min:
next reaction run (Table 3).
It is also noteworthy that N adsorption–desorption analysis of the
recovered catalyst after the third reaction run showed no change in
2
the nanostructure of the catalyst (Fig. 1).
The precise mechanism for this reaction is not clear at this stage.
However, it seems that PMSCl surface Si–Cl bonds might act as a Lewis
acid. Therefore, a plausible mechanism for this reaction is illustrated in
Scheme 2. Although the possibility of in situ formation of a trace HCl
cannot be ruled out at this stage, the contribution of physically adsorbed
HCl in the catalytic process has minimized by the calcination of the
catalyst at 550 °C. Transesterification of the phenol and ethyl acetoace-
tate in the present of PMSCl will generate species 1. Alternatively, Michael
addition reaction generates a new ring to afford species 2. Rear-
omatisation leads to the formation of the coumarin skeleton 3. At last
stage PMSCl can be regenerated by dehydration. Further mechanistic
Fig. 2. TEM images of PMSCl.

B. Karimi, H. Behzadnia / Catalysis Communications 12 (2011) 1432–1436
1435
Table 2
Pechmann reactions of various phenols.
Entry
Catalyst
MCM-Pr-SO
SBA-15-Pr-SO
p-TsOH
R
Time min/[h]
Yielda [%]
1
2
3
4
5
6
7
8
9
3
H [48]
3-OH
3-OH
3-OH
3-OH
3-OH
3-OH
3-OH
3-OH
60
60
60
60
60
60
60
60
30 [42]
50 [42]
65 [42]
60 [42]
NR [42]
NR [42]
68
3
H [49]
H SO
2 4
Silica gel
SBA-15
SiO
2
Cl
SBA-15-Ph-Pr-SO
HCl
3
H [42]
90 [42]
57
3-OH
60
10
11
12
13
14
15
16
17
18
19
20
21
22
PMSCl
PMSCl
PMSCl
PMSCl
PMSCl
PMSCl
PMSCl
PMSCl
PMSCl
PMSCl
PMSCl
SBA-15-Ph-Pr-SO
3-OH
2,3-(OH)
3,5-(OH)
3-Me-5-OH
2-Me-3-OH
3-MeO
3-Me
3-NH
4-OH
60
94
98 (65)b
2
2
20 (10)
15 (7)
20 (10)
25
[6]
[8]
70
70
[3]
[7]
b
96 (68)
92 (57)b
96
87
80
72
70
75
45
NR
NR
2
1-naphthol
H
H
H
3
H
[7]
[7]
SiO
2
Cl
a
Isolated yields of pure products. The number in parenthesis refers to the yield of products after the indicated time in parenthesis in time column.
investigations on the detailed reaction mechanisms are currently in
progress and will be reported in due course.
Acknowledgement
The authors acknowledge IASBS Research Councils and Iran
National Science Foundation (INSF) for support of this work. We
also acknowledge Dr. Saeed Emadi from IASBS for reading the
manuscript.
4
. Conclusions
We developed a simple and practical method for the synthesis of a
wide range of coumarins through the use of periodic mesoporous
silica chlorides (PMSCl) with 2D P6mm Hexagonal structures.
Furthermore various coumarins were usually obtained in high purity
and excellent yields just by simple purification procedure. The catalyst
showed high stability during the reaction and could be recovered and
reused at least three reaction cycles.
Appendix A. Supplementary data
Supplementary data to this article can be found online at doi:10.
1016/j.catcom.2011.05.019.
Table 3
Recyclability of the Pechmann reaction catalyzed by PMSCl.
Entry
Run
Time min/[h]
Conversion (%)
98
Cl-contentc (mmol.g-1
)
1
2
3
4
5
6
1
2
3
4
2
3
20
40
[1]
[3]
20
20
3.4
2.7
1.8
1.2
2.7
1.9
a
90
85
50
65
a
a
b
b
48
a
Recovery based on the maximum chemical yields.
Recovery under the same reaction time.
b
c
Cl content (mmol g−1) of catalyst before each reaction cycles.

1436
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