Sulfonic acid-functionalized mesoporous silica nanoparticles (SAMSNs): A recoverable heterogeneous acid catalyst for green synthesis of dicoumarols

Article abstract of DOI:10.1007/s11164-014-1578-9

Sulfonic acid-functionalized mesoporous silica nanoparticles (SAMSNs) have been used as a highly efficient and recoverable heterogeneous acid catalyst for green synthesis of dicoumarols by reaction of different aldehydes with 4-hydroxycoumarin in aqueous

Full text of DOI:10.1007/s11164-014-1578-9

Res Chem Intermed
DOI 10.1007/s11164-014-1578-9
Sulfonic acid-functionalized mesoporous silica
nanoparticles (SAMSNs): a recoverable heterogeneous
acid catalyst for green synthesis of dicoumarols
•
•
•
Ebrahim Mehrasbi Yaghoub Sarrafi Amir Vahid
Heshmatollah Alinezhad
Received: 24 October 2013 / Accepted: 25 February 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract Sulfonic acid-functionalized mesoporous silica nanoparticles (SAMS-
Ns) have been used as a highly efficient and recoverable heterogeneous acid catalyst
for green synthesis of dicoumarols by reaction of different aldehydes with 4-hy-
droxycoumarin in aqueous media. High to excellent yields were obtained. This
mesoporous catalyst is readily recovered and can be reused at least six times without
significant loss of its catalytic activity.
Keywords Green synthesis ꢀ Dicoumarols ꢀ 4-Hydroxycoumarin ꢀ
Sulfonic acid-functionalized mesoporous silica ꢀ Aqueous medium
Introduction
Coumarins constitute a prominent class of organic compounds utilized in the
cosmetics, food, pharmaceutical, and agrochemical industries [1–3]. Among the
coumarin derivatives, dicoumarols have attracted much attention because of their
biological importance and wide-ranging pharmacological activity, for example
anticoagulant, antitumor, antihelminthic, insecticidal, hypnotic, antifungal, and HIV
protease inhibitory activity [3–12]. Dicoumarol, 3,30-methylenebis(4-hydroxycoum-
arin), a naturally occurring anticoagulant drug that functions as a vitamin K
antagonist, was first isolated from sweet clover (Melilotus alba and Melilotus
officinalis) [13, 14]. Neodymium(III) and lanthanum(III) complexes of dicoumarol
have substantial cytotoxic activity [15]. Notably, clinical trials have revealed that
E. Mehrasbi ꢀ Y. Sarrafi (&) ꢀ A. Vahid ꢀ H. Alinezhad
Department of Organic Chemistry, Faculty of Chemistry, University of Mazandaran,
47416 Babolsar, Iran
e-mail: ysarrafi@umz.ac.ir
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E. Mehrasbi et al.
dicoumarols are active against prostate cancer, malignant melanoma, and metastatic
renal cell carcinoma [16–18].
Synthesis of dicoumarols is usually achieved by condensation of 4-hydroxy-
coumarin with aldehydes. Attempts to synthesize of dicoumarols over such catalysts
as molecular iodine [19], Et2AlCl3 [20], DBU [21], MnCl2 [22], POCl3 in dry DMF
[23], SO3H-functionalized ionic liquids [24], [bmim][BF4] [25], tetrabutylammo-
nium bromide (TBAB) [26], phosphotungstic acid [27], Zn(proline)2 [28], and
sulfated titania [29] have been reported. Catalyst-free condensation reactions under
thermal and microwave irradiation have also been developed [30]. Although each of
these methods has merit, some have such disadvantages as long reaction time,
expensive reagents, harsh conditions, low product yields, and use of environmen-
tally harmful toxic catalysts. Moreover, the main disadvantage of almost all
methods is that the catalysts are damaged in the workup procedure and cannot be
recovered. Therefore, to avoid these limitations there is still a need for development
of a better process for synthesis of dicoumarol derivatives in terms of operational
simplicity, reusability of the catalyst, and economic viability.
Heterogeneous acid catalysts have attracted much attention in organic synthesis
because of numerous advantages which include reusability, environmental
compatibility, non-corrosiveness, and ease of separation, compared with their
hazardous and corrosive homogeneous counterparts [31–37]. Among the different
solid acid catalysts, mesoporous silica grafted with sulfonic acid has attracted
extensive interest because of its unique properties, including high surface area,
large pore volume, uniform hexagonal array, tunable pore diameter, and excellent
thermal stability [38–41]. Incorporation of sulfonic acid groups on porous silica
supports produces highly convenient solid acid catalysts with the advantages of
homogeneous catalysts [42]. For these reasons, development of acid-functional-
ized mesoporous silica as recoverable heterogeneous acid catalysts for the
synthesis of heterocyclic compound is a topic of current interest. In continuation
of our ongoing effort to develop new and eco-friendly synthetic methods [43–46],
we report herein a novel, environmentally benign, facile, and efficient method for
synthesis of dicoumarols using sulfonic acid-functionalized mesoporous silica
nanoparticles (SAMSNs) as a green and recoverable solid acid catalyst
(Scheme 1).
Scheme 1 Synthesis of dicoumarol derivatives catalyzed by SAMSNs
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Sulfonic acid-functionalized mesoporous silica nanoparticles
Experimental
General
All chemicals were purchased from Merck, Fluka, and Sigma–Aldrich. Reactions were
monitored by thin-layer chromatography (TLC). X-ray diffraction (XRD) patterns were
recorded on a Seifert TT 3000 diffractometer using Cu Ka radiation of wavelength
0.15405 nm. Diffraction data were recorded in the region 2h = 1–10° at intervals of
2h = 0.01°. A scanning rate of 1.0°/min was used. Scanning electron micrographs were
recorded using a Zeiss (Oberkochen, Germany) DSM 962. Samples were deposited on a
sample holder withadhesive carbonfoil and sputteredwith gold. Physisorptionof nitrogen
was measured at 77 K by use of a Belsorp-mini porosimeter. Pore-size distribution was
calculated from the desorption branch by use of the Barrett–Joyner–Halenda (BJH)
method [47]. Melting points were measured on an electrothermal 9100 apparatus. NMR
spectra were recorded with a Bruker DRX-400Avanceinstrument (400.1 MHzfor1Hand
100.6 MHz for 13C). IR spectra were recorded on an FT-IR Bruker vector 22
spectrometer. Elemental analysis was performed with a Heraeus CHN-O Rapid analyzer.
Preparation of sulfonic acid-functionalized mesoporous silica nanoparticles
(SAMSNs)
Well-ordered mesoporous silica nanoparticles (MSNs) were synthesized by a method
described elsewhere [46]. Typically, 3.0 g MSNs was evacuated at 150 °C then excess
3-mercaptopropyltrimethoxysilane (MPTS) in dry toluene was added. The mixture was
heated under reflux for 6 h and the solid was isolated by filtration, washed with dry
toluene, and dried in air. The –SH groups were converted into –SO3H groups by mild
oxidation with H2O2 (stirring for 24 h at 60 °C with excess oxidant). The solid was
isolated by filtration, washed with water and ethanol, acidified with 0.1 M H2SO4, and
thoroughly washed with water to remove all traces of liquid acid. The solid material was
finally dried at 60 °C overnight. The amount of sulfonic acid groups of the SAMSNs,
which was determined by acid–base titration, was found to be 0.83 mmol g-1
.
General procedure for synthesis of dicoumarols
Mixtures of 4-hydroxycoumarin (2 mmol), aldehyde (1 mmol), SAMSNs (0.025 g),
and water (3 mL) were stirred at 80 °C for different times (Table 5). After
completion of the reactions (as monitored by TLC), the mixtures were cooled to
room temperature. The crude products were extracted with dichloromethane and
concentrated to furnish the product.
Physical and spectral data of selected products
3,30-((4-Nitrophenyl)methylene)bis(4-hydroxy-2H-chromen-2-one) [19]
Yellow powder, mp 233–235 °C; 1H NMR (400 MHz, CDCl3): dH (ppm) 6.14 (1H,
s, CH), 7.43 (6H, m, ArH), 7.69 (2H, t, J = 7.6 Hz, ArH), 8.01 (1H, d, J = 7.6 Hz,
123

E. Mehrasbi et al.
ArH), 8.10 (1H, d, J = 7.6 Hz, ArH), 8.20 (2H, d, J = 9.2 Hz, ArH), 11.39 (1H, s,
OH), 11.59 (1H, s, OH). 13C NMR (100 MHz, CDCl3): dc (ppm) 36.52, 103.27,
104.76, 116.23, 116.67, 116.75, 116.82, 123.89, 124.47, 124.51, 125.17, 125.24,
127.58, 133.39, 143.37, 146.87, 152.32, 152.57, 164.85, 166.45, 167.01, 169.12.
Anal. Calcd. for C25H15NO8: C, 65.65; H, 3.31; N, 3.06. Found: C, 65.75; H, 3.39;
N, 3.11.
3,30-(Naphthalen-2-ylmethylene)bis(4-hydroxy-2H-chromen-2-one) 3m [3]
Off-white powder, mp 265–267 °C; 1H NMR (400 MHz, DMSO-d6): dH (ppm) 6.50
(1H, s, CH), 7.34 (3H, m, ArH), 7.41 (4H, m, ArH), 7.62 (3H, m, ArH), 7.80 (3H, m,
ArH), 7.91 (2H, d, J = 8 Hz, ArH). 13C NMR (100 MHz, DMSO-d6): (ppm) 36.86,
104.59, 116.48, 118.47, 124.23, 124.41, 124.86, 125.67, 126.26, 126.56, 127.68,
128.01, 128.05, 132.04, 132.39, 133.52, 138.29, 152.76, 165.25, 165.88. Anal.
Calcd. for C29H18O6: C, 75.32; H, 3.92. Found: 75.41; H, 3.96.
3,30-((2-Chloro-5-nitrophenyl)methylene)bis(4-hydroxy-2H-chromen-2-one) 3o
Off-white powder, mp 277–279 °C; IR (KBr): 3,495 (OH), 3,076 (OH), 1,688 and
1,612 (CO), 1,525 and 1,345 (NO), 763 (C–Cl) cm-1. 1H NMR (400 MHz, DMSO-
d6): dH (ppm) 6.21 (1H, s, CH), 7.44 (4H, m, ArH), 7.41 (1H, d, J = 8.8 Hz, ArH),
7.68 (2H, m, ArH), 8.08 (2H, m, ArH), 8.14 (1H, m, ArH), 8.37 (1H, m, ArH), 11.02
(1H, s, OH), 11.77 (1H, s, OH). 13C NMR (100 MHz, DMSO-d6): (ppm) 35.94,
116.74, 123.43, 124.56, 124.69, 125.23, 131.66, 133.38, 135.94, 140.37, 146.61.
Anal. Calcd. for C25H14ClNO8: C, 61.05; H, 2.87; N, 2.85. Found: C, 61.11; H,
2.91; N, 2.88.
Results and discussion
First, the mesoporous silica nanoparticles (MSNs) were prepared in accordance with
our previous work [46]. In the next step, the surface of the MSNs was functionalized
by use of 3-mercaptopropyltriethoxysilane (MPTS). The thiol groups were then
converted into sulfonic acid groups by mild oxidation with H2O2 (Fig. 1).
Sulfonated mesoporous silica nanoparticles were characterized by X-ray
diffraction (XRD), N2 adsorption/desorption, FT-IR spectroscopy, and scanning
electron microscopy (SEM). Powder X-ray diffraction patterns of MSNs and
SAMSNs are shown in Fig. 2.
For the MSNs five well-defined Bragg peaks at low angles are apparent; these can
be indexed to the (1 0 0), (1 1 0), (2 0 0), (2 1 0), and (3 0 0) reflections
corresponding to a hexagonal lattice (i.e. MCM-41). When the surface of
mesoporous silica is functionalized with the propylsulfonic acid groups a decrease
in the intensity of the peaks is observed, however the two-dimensional hexagonal
array of mesoporous structure is retained. The specific surface area and total pore
volume of the SAMSNs determined from N2 adsorption–desorption isotherms were
366 m-2 g-1 and 0.36 cm-3 g-1, respectively [47].
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Sulfonic acid-functionalized mesoporous silica nanoparticles
Fig. 1 Preparation of sulfonic acid-functionalized mesoporous silica nanoparticles (SAMSNs)
(100)
MSNs
(110)
(200)
SAMSNs
(210)
(300)
0
1
2
3
4
5
6
7
8
9
10
2 Theta/deg
Fig. 2 XRD patterns of the calcined MSNs and SAMSNs
FT-IR spectra of MSNs and SAMSNs are shown in Fig. 3. Anchoring of
propylsulfonic acid groups in the pore channels of the mesoporous materials is
apparent from the methylene stretching bands in the 2,850–2,950 cm-1 region.
Also, the SEM images (Fig. 4) showed that the sulfonic acid-functionalized MSNs
were present as uniform particles with spherical morphology.
To study the catalytic activity of the prepared SAMSNs, we examined the reaction
of 4-hydroxycoumarin with benzaldehyde, as simple model substrate, in aqueous
media, using a wide variety of catalysts. As indicated in Table 1, the best results, in
terms of yield and reaction time, was obtained by use of the SAMSNs catalyst (entry
10). The acid catalysts alum, sulfamic acid, PTSA, and Amberlist-15 gave moderate
yields of the corresponding dicoumarols (entries 2–5). The K10–catalyzed reaction
gave a much lower yield. The better catalytic activity of the SAMSNs may be because
of their higher surface area, which enhanced chemical accessibility of the substrate to
the anchored acid groups. When SiO2 and ZnO nanoparticles were used, moderate
123

E. Mehrasbi et al.
Fig. 3 IR spectra of MSNs and SAMSNs
yields of product were obtained after a longer reaction time (entries 7, 8). When the
reaction was conducted with MSNs, longer reaction times were required and
significantly lower yields were obtained. When the reaction was attempted without a
catalyst, only a trace of product was formed (entry 1).
We next studied the effect of the amount of SAMSNs on the model reaction. As
indicated in Table 2, the best results were obtained by use of 0.025 g SAMSNs; on
further increasing the amount of SAMSNs the yield of the reaction was almost
123

Sulfonic acid-functionalized mesoporous silica nanoparticles
Fig. 4 SEM images of SAMSNs
Table 1 Optimization of the catalyst for the synthesis of dicoumarols in water
O
O
O
O
O
O
O
2
+
H
H
OH
OH
OH
1a
2a
3a
Entry
Amount of catalysta (0.05 g)
Time
24 h
Yield (%)b
1
2
3
4
5
6
7
8
9
10
a
–
Trace
65
Alum
120 min
120 min
120 min
120 min
5 h
Sulfamic acid
PTSA
60
50
Amberlist-15
Montmorillonite K10
Nano-SiO2
Nano-ZnO
MSNs
57
33
5 h
55
5 h
60
5 h
42
SAMSNs
60 min
80
Reaction conditions: benzaldehyde (1 mmol) and 4-hydroxycoumarin (2 mmol) in H2O (3 mL) at room
temperature
b
Isolated yields
unaffected. The yield decreased when the amount of catalyst was reduced to 0.015 g
(Table 2, entry 4), and prolonging the reaction time did not improve the yield
(Table 2, entry 5).
During optimization of the reaction conditions, the effect of temperature on reaction
rate and product yield was investigated (Table 3). When the reaction was conducted at
temperaturesranging fromroom temperature to 100 °C, 80 °C was found to bethe most
suitable reaction temperature (Table 3, entry 3). The reaction proceeded sluggishly at
lower temperatures and lower yields of the product were obtained (Table 3, entries 1
123

E. Mehrasbi et al.
Yield (%)b
Table 2 Influence of the amount of SAMSNs in synthesis of dicoumarolsa
Entry
SAMSNs (g)
Time (min)
1
2
3
4
5
a
0.1
60
60
82
80
80
67
70
0.05
0.025
0.015
0.015
60
120
240
Reaction conditions: benzaldehyde (1 mmol) and 4-hydroxycoumarin (2 mmol) in H2O (3 mL) at room
temperature
b
Isolated yields
Table 3 Effect of temperature on the SAMSNs-catalyzed synthesis of dicoumarolsa
Entry
Temperature (°C)
Time (min)
Yield (%)b
1
2
3
4
a
RT
60
60
40
20
20
80
88
96
94
80
100
Reaction conditions: benzaldehyde (1 mmol) and 4-hydroxycoumarin (2 mmol) in H2O (3 mL) in the
presence of 0.025 g SAMSNs
b
Isolated yields
Table 4 Effect of solvent on the SAMSNs-catalyzed synthesis of dicoumarols
Entry
Solventa,b
Time (min)
Yield (%)c
1
2
3
4
5
6
a
THF
20
20
20
20
20
20
80
68
88
85
96
75
DMSO
EtOH
MeOH
H2O
Solvent-free
Reaction conditions: benzaldehyde (1 mmol) and 4-hydroxycoumarin (2 mmol) in the presence of
0.025 g SAMSNs at 80 °C
b
Solvent: 3 mL
c
Isolated yields
and 2). However, no increase in the rate of reaction was observed when the reaction
temperature was increased from 80 to 100 °C (Table 3, entry 4).
To improve the yields further, the reaction was performed in different solvents
and without solvent (Table 4). In polar protic solvents, for example MeOH and
EtOH, relatively high yield of the product was obtained. However, when the
reaction was performed in water, the product was obtained in excellent yield. The
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Sulfonic acid-functionalized mesoporous silica nanoparticles
Table 5 Synthesis of different dicoumarol derivatives 3 catalyzed by SAMSNs
Yield
(%)b,c
Producta
Time (min)
[Ref.]
[19]
Aldehydes
Entry
1
20
96
97
3a
10
10
[29]
2
3
3b
3c
92
98
[30]
[19]
4
10
3d
5
6
15
15
96
96
[29]
[30]
3e
3f
123

E. Mehrasbi et al.
Table 5 continued
Aldehydes
Yield
(%)b,c
Entry
Producta
Time (min)
[Ref.]
[26]
7
10
98
3g
8
9
15
15
92
95
[30]
[29]
3h
3i
10
30
88
[19]
3j
11
12
25
30
92
85
[29]
[19]
3k
3l
13
15
95
[3]
3m
123

Sulfonic acid-functionalized mesoporous silica nanoparticles
Table 5 continued
Yield
(%)b,c
Producta
Time (min)
25
[Ref.]
[19]
Aldehydes
Entry
14
88
3n
15
25
92
This work
3o
Reaction conditions: benzaldehyde (1 mmol) and 4-hydroxycoumarin (2 mmol) in the presence
a
of 0.025 g SAMSNs in H2O (3 mL) at 80 °C.
b Yields refer to pure isolated products.
c
The products characterized by the comparison of their spectral (1H NMR, 13C NMR and IR ) and
physical data with those reported in the literature.
100
90
80
70
60
50
40
30
20
10
0
1
2
3
4
5
6
Number of cycles
Fig. 5 Results from study of the reusability of the catalyst in the model reaction
hydrophobicity of the catalyst and organic starting materials in water might promote
their interaction with each other and smooth conversion to the desired product.
In brief, the optimum reaction conditions for SAMSNs-catalyzed synthesis of
dicoumarols were found to be benzaldehyde (1 mmol) and 4-hydroxycoumarin
(2 mmol) in the presence of 0.025 g SAMSNs at 80 °C in aqueous media.
123

E. Mehrasbi et al.
To assess the versatility of the method a series of aromatic aldehydes were studied
under the optimum reaction conditions; the results are listedin Table 5. In all cases, the
reactions gave the products in good to excellent yields in very short reaction times.
Aromatic aldehydes with electron-withdrawing or electron-donating substituents, for
example CN, NO2, halogens, and CH3, reacted smoothly under the optimized
conditions affording the corresponding products (3a–3i, 3k and 3m) in high yields
(Table 5), although lower yields of product were obtained from aromatic aldehydes
with strongly electron-donating substituents, for example p-OCH3 and p-OH. The
acid-sensitive substrate thiophene-2-carbaldehyde gave the expected good yield.
Recovery and reuse of catalysts is a very important aspect of green chemistry. To
investigate the recyclability of the catalyst, we conducted six successive cycles of
the model reaction under the optimum reaction conditions using SAMSNs recycled
from the previous run. The reaction proceeded smoothly even after six cycles,
without any increase of reaction time or marked loss of yield (Fig. 5).
Conclusion
In summary, we have successfully developed a solid acid catalyst, sulfonic acid-
functionalized mesoporous silica nanoparticles (SAMSNs), for synthesis of
dicoumarol derivatives by reaction of aldehydes and 4-hydroxycoumarin in aqueous
media. The SO3H-functionalized catalyst was synthesized by incorporation of
mercaptopropyl groups into mesoporous silica, then mild oxidation of SH to SO3H.
This catalyst can be recovered easily and reused over several reaction cycles
without substantial loss of reactivity.
Acknowledgments The authors acknowledge the University of Mazandaran for financial support of this
research.
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