Tungstic acid functionalized mesoporous SBA-15: A novel heterogeneous catalyst for facile one-pot synthesis of 2-amino-4H-chromenes in aqueous medium

Article abstract of DOI:10.1039/c3dt50947h

A new highly ordered mesoporous tungstic acid functionalized SBA-15, TAFMC-1 has been synthesized via post-synthesis modification of mesoporous SBA-15 with (3-chloropropyl) triethoxysilane followed by substitution reaction of chlorine atom of the 3-chloropropyl group by tungstic acid group under refluxing conditions in n-hexane. The tungstic acid functionalized mesoporous silica material has been characterized by using small angle powder X-ray diffraction, N₂ sorption, HR-TEM, FE-SEM, FT-IR and solid state MAS NMR studies. TAFMC-1 catalyzes the facile one-pot catalytic three-component condensation reaction of resorcinol, aromatic aldehyde and malononitrile for the synthesis of a diverse range of 2-amino-4H-chromenes in aqueous medium. This heterogeneous catalyst can be recycled very efficiently for six consecutive reaction cycles without significant loss of catalytic activity.

Full text of DOI:10.1039/c3dt50947h

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Tungstic acid functionalized mesoporous SBA-15:
A novel heterogeneous catalyst for facile one-pot
synthesis of 2-amino-4H-chromenes in aqueous
medium
Cite this: Dalton Trans., 2013, 42, 10515
Sudipta K. Kundu, John Mondal and Asim Bhaumik*
A new highly ordered mesoporous tungstic acid functionalized SBA-15, TAFMC-1 has been synthesized
via post-synthesis modification of mesoporous SBA-15 with (3-chloropropyl) triethoxysilane followed by
substitution reaction of chlorine atom of the 3-chloropropyl group by tungstic acid group under
refluxing conditions in n-hexane. The tungstic acid functionalized mesoporous silica material has been
characterized by using small angle powder X-ray diffraction, N2 sorption, HR-TEM, FE-SEM, FT-IR and solid
state MAS NMR studies. TAFMC-1 catalyzes the facile one-pot catalytic three-component condensation
reaction of resorcinol, aromatic aldehyde and malononitrile for the synthesis of a diverse range of
2-amino-4H-chromenes in aqueous medium. This heterogeneous catalyst can be recycled very efficiently
for six consecutive reaction cycles without significant loss of catalytic activity.
Received 10th April 2013,
Accepted 13th May 2013
DOI: 10.1039/c3dt50947h
www.rsc.org/dalton
sclerosis, Huntington’s disease, Parkinson’s disease, AIDS
associated dementia and Down’s syndrome as well as for the
Introduction
Recently, an increasing attention has been focused for the treatment of schizophrenia and myoclonus.9 Synthesis of
development of new synthetic routes in chemical processes these important class of compounds have made significant
employing nontoxic solvent, reagents and catalysts. In order to impact in the field of cosmetics, pigments and potential bio-
execute the reactions under nontoxic conditions the multicom- degradable agrochemicals.10
ponent reaction (MCR) strategy has been adopted by research-
Owing to their versatile applications, researchers are keener
ers.1 The simplicity of the one-pot procedure, accessibility of a to synthesize amino chromene derivative over various Lewis
variety of compounds, conducting reaction without isolation acid and base catalysts such as NaOH,11 K2CO3,12 MgO,13
of intermediate, reduction of time and energy are the major TiCl4,14 InCl3,15 heteropolyacid and Et3N.16 Although, most of
advantages of the MCR type reactions compared with the con- these catalytic methods showed high activity and selectivity,
ventional reaction procedures.2 Because of these advantages, these catalysts are homogeneous in nature. The main problem
multicomponent coupling reactions are preferred to tune the traditionally associated with these homogeneous catalysts
synthetic reaction steps for combinatorial synthesis.3 Various includes separation of the catalyst from the reaction mixture
heterocyclic compounds such as benzopyrans, benzoxanthene, after the reaction. Reusability and recovery of the most expens-
benzochromene can be developed by applying this one-pot ive catalysts plays a decisive role for the sustainable develop-
multicomponent reaction strategy. Benzochromenes are the ment of any catalytic procedure. Catalyst separation has
most important class of biologically active heterocyclic com- become the most influential, powerful and practical technique
pounds which have attracted an emerging interest in recent in the field of heterogeneous catalyst and separation of the
research areas due to their antimicrobial,4 anti-inflammatory,5 active catalysts that are very important to recycle/reuse are
antibacterial,6 antiviral7 and anticancer8 activities. In addition, hugely acknowledged in the chemical industry. So the develop-
they can be employed as the building blocks of several human ment of heterogeneous catalysts that are employed to perform
drugs, which are used for the treatment of neurodegenerative the most efficient and economically feasible catalytic processes
diseases, including Alzheimer’s disease, amyotrophic lateral synthesized through the heterogenisation of homogeneous
catalytic sites on the solid supports is highly desirable.
Recently mesoporous material has witnessed a remarkable
growth in academia and industry in this context. Exception-
ally high surface area, tunable pore size distribution, high
Department of Materials Science, Indian Association for the Cultivation of Science,
Jadavpur 700 032, India. E-mail: msab@iacs.res.in
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hydrothermal and mechanical stability make them a potential under continuous stirring. Then it was refluxed for 24 h under
candidate in the field of heterogeneous catalysis.17 Meso- nitrogen atmosphere. After completion of the reaction, the
porous materials are hugely acknowledged as a platform for reaction mixture was allowed to cool at room temperature.
solid support, where various active sites can be effectively Then it was filtered and washed thoroughly with toluene
anchored. The size, shape and connectivity of the existing (10 mL), dichloromethane (10 mL) and diethylether (10 mL),
pores can be tuned in the organically functionalized meso- respectively. The white colored sample was dried at room
porous materials via some chemical reactions, where the metal temperature.
atom or active sites can be covalently anchored with the func-
Synthesis of TAFMC-1. Tungstic acid functionalized SBA-15
tional groups present at the surface of the catalysts.18 Thus, (TAFMC-1) was prepared by reaction with SBA-15-Cl and
considerable attention has been paid to design the organic– sodium tungstate dihydrate. 0.5 g of SBA-15-Cl in 15 mL
inorganic hybrid mesoporous silica, where the active metal n-hexane was placed in a 50 mL round bottom flask. Then
and catalytic sites can be covalently grafted with the organic 0.658 g of sodium tungstate dihydrate was added to the dis-
functional groups so that no leaching could take place during persed SBA-15-Cl solution and then it was refluxed for 12 h
the course of the reaction. Recently, we have developed triazine under a nitrogen atmosphere. After completion of reaction it
functionalized mesoporous organocatalyst (TFMO-1) which was cooled to room temperature and filtered through suction
exhibited excellent catalytic activity in one-pot three com- followed by washing thoroughly with n-hexane (10 mL) and
ponent condensation reaction under solvent-free conditions to distilled water (10 mL), respectively. The sample was dried and
develop 2-amino-4H-chromene.19
then stirred in the presence of 15 mL aqueous HCl medium so
Herein, we wish to report the synthesis of a robust and non- that the pH of the solution comes down to ca. 4.0 in 1 h
air sensitive tungstic acid functionalized mesoporous SBA-15 (Scheme 1). Finally the mixture was filtered, washed with dis-
silica
by
post-grafting
approach
over
mesoporous tilled water and dried at room temperature.
General procedure for the synthesis of 2-amino-4H-chro-
SBA-15 material. The catalyst has been developed by the
surface modification technique of mesoporous SBA-15 silica menes. A mixture of aromatic aldehyde (1 mmol), malono-
by using (3-chloropropyl)triethoxysilane in toluene solvent. nitrile (1 mmol, 66 mg), resorcinol (1 mmol, 110 mg) and
Then the chloro group has been substituted by a substitution catalyst TAFMC-1 (30 mg) in water was refluxed in an oil bath
reaction using sodium tungstate in n-hexane solvent. The for an appropriate period of time. After completion of reaction
tungstic acid functionalized SBA-15 heterogeneous catalyst has (monitored by TLC), the reaction mixture was allowed to cool
been thoroughly characterized by XRD, FT-IR, HRTEM, FE to room temperature and 5 mL ethyl acetate was poured into
SEM, N2 sorption, 13C and 29Si solid state MAS NMR studies. the reaction mixture and then it was stirred for 30 min at room
The catalyst showed excellent catalytic activity for performing temperature. The catalyst was separated by filtration and it was
one-pot three component condensation reaction of resorcinol, washed with ethyl acetate by several times. The filtrate was
malononitrile and aromatic aldehyde in aqueous medium for washed with water (3 × 30 mL) and then dried over anhydrous
the synthesis of a diverse range of 2-amino-4H-chromene Na2SO4 and evaporated. The crude product was recrystallized
derivatives in very high yields (80–88%).
from ethanol to afford pure product.
Instrumentation. Powder X-ray diffraction patterns of
SBA-15, SBA-15-Cl and TAFMC-1 samples were recorded on a
Bruker D-8 Advance SWAX diffractometer operated at a voltage
of 40 kV and a current of 40 mA and calibrated with a standard
silicon sample, using Ni-filtered Cu-Kα (λ = 0.15406 nm). FT-IR
Experimental section
Synthesis of tungstic acid functionalized mesoporous SBA-15
(TAFMC-1)
Synthesis of mesoporous SBA-15. In a typical synthesis,
2.0 g of P123 was dissolved in 60 mL of 2.0 M aqueous HCl
and 15.0 g of distilled water under stirring at room tempera-
ture. Then 4.25 g of tetraethyl orthosilicate (TEOS) was added
dropwise to the solution of P123 in acid media at 40 °C. After
TEOS was added, the mixture was stirred for 24 h and then
transferred into a Teflon-lined autoclave and kept at 100 °C
for another 24 h. Finally the solid product was recovered by
filtration, washed with distilled water, and air-dried. The
template was removed by calcination of the sample at 500 °C
in air for 5 h to get the final product SBA-15.
General procedure for the synthesis of 3-chloropropyl func-
tionalized SBA-15. 0.2 g calcined SBA-15 in 15 mL dry toluene
was placed in a 50 mL round bottom flask. Then 0.6 mmol
(0.144 g) (3-chloropropyl)triethoxysilane taken in 5 mL dry
toluene was added dropwise to the dispersed SBA-15 solution
Scheme 1
10516 | Dalton Trans., 2013, 42, 10515–10524
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spectra of the samples were recorded using a Nicolet MAGNA DMSO-d6) δ 21.30, 57.01, 102.72, 112.91, 114.49, 121.26,
FT-IR 750 spectrometer series II. N2 adsorption/desorption iso- 127.89, 129.04, 129.71, 130.77, 146.28, 149.39, 157.59, 160.74.
therms were obtained by using a Beckman coulter SA 3100
2-Amino-3-cyano-7-hydroxy-4-(4-fluorophenyl)-4H-chromene
surface area analyzer at 77 K. JEOL JEM 6700F field emission (Table 1, entry 3, 4c). Yellow solid: 1H NMR (500 MHz, DMSO-
scanning electron microscope (FE SEM) was used for the deter- d6) δ 9.69 (s, 1H), 7.17–7.20 (m, 2H), 7.07–7.13 (m, 2H), 6.89 (s,
mination of morphology of powder samples. The pore struc- 2H), 6.76–6.78 (d, 1H), 6.47–6.49 (m, 1H), 6.40 (s, 1H), 4.65 (s,
ture was visualized by using a JEOL JEM 2010 high resolution 1H); 13C NMR (500 MHz, DMSO-d6) δ 56.20, 102.18, 112.43,
transmission electron microscope (HR TEM) operated at an 113.52, 115.12, 115.40, 120.52, 129.27, 129.86, 142.55, 148.80,
accelerating voltage of 200 kV. 1H and 13C NMR (solution) 157.13, 160.18.
experiments were carried out on a Bruker DPX-300 NMR
2-Amino-3-cyano-7-hydroxy-4-(4-chlorophenyl)-4H-chromene
spectrometer. The solid state MAS NMR spectra of the samples (Table 1, entry 4, 4d). Yellow solid: 1H NMR (300 MHz, DMSO-
were taken in a 500 MHz Bruker Avance II spectrometer.
d6) δ 9.71 (s, 1H), 7.35–7.37 (d, 2H), 7.18–7.19 (d, 2H), 6.89 (s,
2H), 6.77–6.79 (d, 1H), 6.48–6.50 (d, 1H), 6.41 (s, 1H), 4.66 (s,
1H); 13C NMR (300 MHz, DMSO-d6) δ 56.45, 102.81, 113.05,
113.77, 121.06, 129.13, 129.85, 130.46, 131.81, 145.89, 149.40,
157.79, 160.82.
1H and 13C NMR chemical shifts for different 2-amino-4H-
chromene products
2-Amino-3-cyano-7-hydroxy-4-(4-bromophenyl)-4H-chromene
(Table 1, entry 5, 4e). Yellow solid: 1H NMR (500 MHz, DMSO-
d6) δ 9.72 (b, 1H), 7.46–7.51 (m, 2H), 7.12–7.14 (m, 2H), 6.90 (s,
2H), 6.77–6.79 (d, 1H, J = 10 Hz), 6.48–6.50 (m, 1H), 6.41 (s,
1H), 4.65 (s, 1H); 13C NMR (500 MHz, DMSO-d6) δ 56.38,
102.78, 113.03, 113.65, 120.26, 121.00, 130.20, 130.42, 132.01,
146.27, 149.37, 157.78, 160.79.
2-Amino-3-cyano-7-hydroxy-4-(4-phenyl)-4H-chromene
(Table 1, entry 1, 4a). Yellow solid: 1H NMR (500 MHz, DMSO-
d6) δ 9.69 (s, 1H), 7.29–7.31 (m, 2H), 7.16–7.21 (m, 3H), 6.85 (s,
2H), 6.79–6.81 (m, 1H), 6.47–6.49 (d, 1H), 6.41 (s, 1H), 4.61 (s,
1H); 13C NMR (500 MHz, DMSO-d6) δ 56.97, 102.81, 113.00,
114.38, 121.23, 127.24, 127.98, 129.19, 130.51, 146.97, 149.50,
157.70, 160.88.
2-Amino-3-cyano-7-hydroxy-4-(2-bromophenyl)-4H-chromene
(Table 1, entry 6, 4f). Yellow solid: H NMR (300 MHz, DMSO-
2-Amino-3-cyano-7-hydroxy-4-(4-methylphenyl)-4H-chromene
(Table 1, entry 2, 4b). Yellow solid: 13C NMR (300 MHz,
1
Table 1 Synthesis of 2-amino-4H-chromenes in aqueous mediuma
Entry
1
Aromatic aldehyde
Time (h)
12
Yieldb (%)
86
TONc
77
Product
Conversion (%)
95
4a
2
3
4
5
6
14
12
15
12
12
80
82
80
83
79
72
73
72
75
72
4b
4c
4d
4e
4f
92
90
92
92
88
7
11
81
73
4g
90
8
9
12
14
85
78
76
71
4h
4i
95
86
10
14
77
69
4j
86
a Reaction conditions: Ar-CHO: 1.0 equiv., resorcinol: 1.0 equiv. (110 mg), malononitrile: 1.0 equiv. (66 mg), solvent: water, reaction temperature:
100 °C (oil bath), catalyst: 30 mg. b Isolated yield of pure product. c Turn over number (TON) = moles of substrate converted per mole of active site.
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d6) δ 9.75 (b, 1H), 7.56–7.60 (m, 1H), 7.30–7.36 (m, 1H),
7.12–7.18 (m, 2H), 6.91 (s, 1H), 6.71–6.74 (d, 1H, J = 9 Hz),
6.44–6.49 (m, 1H), 6.40–6.41 (m, 1H), 5.14 (s, 1H); 13C NMR
(300 MHz, DMSO-d6) δ 55.14, 79.14, 102.28, 112.53, 120.15,
122.27, 128.45, 128.81, 129.10, 130.97, 132.82, 144.55, 148.86,
157.36, 158.42, 160.38.
2-Amino-3-cyano-7-hydroxy-4-(3-bromophenyl)-4H-chromene
(Table 1, entry 7, 4g). Yellow solid: 1H NMR (500 MHz, DMSO-
d6) δ 9.74 (s, 1H), 7.40–7.42 (d, 1H, J = 10 Hz), 7.34 (s, 1H),
7.27–7.30 (t, 1H), 7.17–7.19 (d, 1H, J = 10 Hz), 6.93 (s, 2H),
6.81–6.83 (d, 1H, J = 10 Hz), 6.49–6.52 (m, 1H), 6.415–6.419 (d,
1H, J = 2 Hz), 4.68 (s, 1H); 13C NMR (500 MHz, DMSO-d6) δ
56.33, 102.91, 113.17, 113.60, 121.03, 122.43, 127.18, 130.21,
130.49, 130.58, 131.52, 149.45, 149.71, 157.91, 160.98.
2-Amino-3-cyano-7-hydroxy-4-(4-nitrophenyl)-4H-chromene
(Table 1, entry 8, 4h). Chocolate colour solid: 1H NMR
(500 MHz, DMSO-d6) δ 9.80 (b, 1H), 8.29–8.31 (m, 2H),
7.44–7.46 (d, 2H), 7.02 (s, 2H), 6.79–6.81 (d, 1H), 6.44–6.53 (m,
2H), 4.86 (s, 1H); 13C NMR (500 MHz, DMSO-d6) δ 55.34, 79.62,
102.89, 112.82, 113.11, 118.66, 120.74, 124.44, 129.17, 130.39,
146.80, 154.20, 157.99, 160.90.
Fig. 1 Small angle powder XRD pattern of SBA-15-Cl (a), tungstic acid function-
alized SBA-15 (b), TAFMC-1 mesoporous catalyst after using six times (c).
2-Amino-3-cyano-7-hydroxy-4-(2-nitrophenyl)-4H-chromene
(Table 1, entry 9, 4i). Chocolate colour solid: 1H NMR
(300 MHz, DMSO-d6) δ 9.97 (s, 1H), 8.66–8.68 (m, 1H),
8.45–8.48 (m, 1H), 8.27–8.30 (m, 1H), 8.11–8.12 (m, 1H), 7.82
(s, 2H), 7.71–7.74 (m, 1H), 7.59–7.61 (m, 1H), 7.27–7.33 (m,
1H), 5.97 (s, 1H); 13C NMR (300 MHz, DMSO-d6) δ 55.82,
103.02, 106.78, 112.52, 113.32, 120.63, 124.31, 128.79, 130.20,
132.00, 134.06, 140.00, 149.36, 158.19, 159.04, 160.97.
2-Amino-3-cyano-7-hydroxy-4-(thiophenyl)-4H-chromene
(Table 1, entry 10, 4j). Chocolate colour solid: 1H NMR
(300 MHz, DMSO-d6) δ 9.74 (s, 1H), 7.33–7.35 (m, 1H),
6.86–6.97 (m, 3H), 6.50–6.54 (m, 1H), 6.38 (s, 2H), 6.15–6.18
(m, 1H), 4.97 (s, 1H); 13C NMR (300 MHz, DMSO-d6) δ 56.50,
102.21, 106.21, 112.42, 113.54, 120.50, 124.02, 126.75, 129.86,
148.56, 151.49, 157.34, 158.44, 160.34.
Results and discussion
Fig. 2 N2 adsorption/desorption isotherm of the mesoporous catalyst
TAFMC-1 at 77 K temperature. Pore size distributions estimated through NLDFT
method is shown in the inset.
The small angle powder X-ray diffraction (PXRD) patterns of
3-chloropropyl functionalized SBA-15 (SBA-15-Cl) and tungstic
acid-functionalized mesoporous SBA-15 (TAFMC-1) are shown
in the Fig. 1. As seen in Fig. 1a SBA-15-Cl displays three charac- sample shows a typical type IV isotherm with a very large hys-
teristic peaks at the 2θ values of 1.00, 1.69 and 1.93, which teresis loop in the 0.6 to 0.8 P/P0 of N2. The BET surface areas
could be attributed for the 100 (strong), 110 (weak) and 200 for the pure 3-chloropropyl functionalized mesoporous SBA-15
(weak) reflections respectively, corresponding to the 2D-hexa- and TAFMC-1 are 680 m2 g−1 and 590 m2 g−1, respectively. A
gonal mesostructure. When SBA-15-Cl was functionalized with considerable decrease in the BET surface area upon covalent
tungstic acid then a considerable decrease in the intensity and grafting of tungstic acid at the surface of 3-chloropropyl-func-
shift in peak positions are observed, however 2D-hexagonal tionalized SBA-15 material suggests that tungstic acid groups
ordering has been retained (Fig. 1b). This decrease in the have been anchored at the surface of mesopores.21 The pore
intensities of the peaks reveals that tungstic acid has been suc- size distribution of the TAFMC-1 has been evaluated by
cessfully incorporated at the surface of the functionalized employing non-local density functional theory (NLDFT) shown
mesoporous SBA-15 material.20
in the inset of Fig. 2 suggests that uniform distribution of
N2 adsorption/desorption isotherms of the tungstic acid- large mesopores throughout the sample with the pore dimen-
functionalized TAFMC-1 at 77 K is shown in the Fig. 2. The sions ca. 8.1 nm. The pore dimension for the SBA-15-Cl
10518 | Dalton Trans., 2013, 42, 10515–10524
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Fig. 4 FTIR spectra of Na2WO4·2H2O (a), reused catalyst (b), SBA-15-Cl (c), and
mesoporous catalyst TAFMC-1 (d).
Fig. 3 HR TEM images of TAFMC-1 (A, parallel to pore axis, FFT pattern is
shown in the inset), (B, perpendicular to pore axis), HR TEM images of TAFMC-1
after using four times (C, parallel to pore axis), (D, perpendicular to pore axis).
indicates incorporation of 3-chloropropyl organic moiety in
the pore channel of SBA-15. The FT-IR spectra of TAFMC-1 dis-
plays a peak at 966 cm−1, which could be attributed to the
stretching vibration of WvO.23 The peak centered at
1041 cm−1, corresponding to the bending vibration of W–OH,
cannot be resolved due to the overlap with the absorbance of
the Si–O–Si stretch in the 1000–1246 cm−1 range. The peak
appeared in the range of 3439 cm−1 can be assigned to the
hydroxyl stretching of the hydrogen bonded internal silanol
groups. The characteristic bands of SiO2 related to the Si–O
stretching vibrations appeared at ca. 1080 and 799 cm−1. The
bending vibrations of the surface silanols and Si–O unit can be
material was 9.6 nm, which has been distinctly reduced to
8.1 nm after grafting of tungstic acid. The considerable
decrease in the pore dimension clearly suggests that tungstic
acid molecule has been anchored into the pore channel of the
mesoporous SBA-15-Cl material. Calculated pore volume of
TAFMC-1 is 1.024 cc g−1, which is considerably large. This
together with the pore width of 8.1 nm is good enough to carry
out the one-pot three component condensation reaction over
TAFMC-1.
recognized by the presence of the bands at 950 and 475 cm−1
,
The HR TEM images of TAFMC-1 are shown in Fig. 3. This
electron microscopic image clearly suggests that uniformly
ordered mesopores of dimensions ca. 7.2–7.5 nm have been
arranged in a honeycomb like hexagonal array throughout the
sample. The FFT diffractogram shown in the inset of Fig. 3
further suggests 2D-hexagonal pore channels. The TEM image
(Fig. 3B perpendicular to the pore axis) also illustrates that the
channel directions are parallel to the thickness of the plate,
and the 110 reflection plane of the sample is clearly viewed in
this image. From these TEM images we can conclude that the
TAFMC-1 has ordered mesopores with a 2D-hexagonal struc-
ture and the hexagonally ordered mesoporous structure of
SBA-15 is preserved after tungstic acid functionalization.22
In order to evaluate incorporation of organic groups and
tungsten moieties into the pore channel of SBA-15, FT-IR
spectra of the respective SBA-15-Cl and TAFMC-1 have been
analyzed. The FT-IR spectra of sodium tungstate dihydrate,
SBA-15-Cl, TAFMC-1 and reused catalyst are shown in Fig. 4.
The absorptions at 3439, 2978, 1631, 1080, 966, 799, 573,
475 cm−1 appear in the TAFMC-1 indicates the presence of
respectively. FE SEM image (Fig. 5) of the mesoporous catalyst
TAFMC-1 illustrates irregular particle morphology of the
catalyst.
Solid state 13C CP MAS and 29Si MAS NMR data provides
the information about the chemical environment around C
and Si in the hybrid frameworks of the mesoporous catalyst
TAFMC-1. In 13C CP MAS NMR (shown in Fig. 6), there are
three peaks with chemical shift at 12.6 ppm, 29.2 ppm and
50.5 ppm respectively, indicating the presence of three types of
carbon. These characteristic signals can be attributed to the
aliphatic propyl group attached with the Si, C3, C2 and C1
respectively (Fig. 6). 29Si MAS NMR spectrum of TAFMC-1 cata-
lyst (Fig. 7) exhibits three broad peaks with chemical shift at
−92.107 ppm, −101.520 ppm and −111.143 ppm, which could
be attributed to the T3 [C-Si(OSi)3], Q3 [(SiO)3Si(OH)], Q4
[Si(OSi)4] species respectively.24 These spectroscopic results
suggest that the organic propyl moiety incorporated in the
hybrid silica framework.
Titrimetric estimation of –OWO3H group loading
chloropropyl and WO4 groups. An absorbance at 2978 cm−1
,
could be assigned to the methylene stretching vibrations of We have calculated the loading of –OWO3H group in the
the propyl chain of organosilane moiety. This data clearly framework of TAFMC-1 by a back titration method adding a
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Fig. 5 FE SEM image of the catalyst TAFMC-1.
Fig. 7 29Si MAS NMR spectrum of the catalyst TAFMC-1.
Scheme 2
Fig. 6 13C CP-MAS NMR spectrum of the catalyst TAFMC-1.
known strength of NaOH solution. It is assumed that all
–OWO3H groups would be replaced by –OWO3−Na+ during
NaOH treatment. The excess NaOH was back titrated with a
known strength of oxalic acid. The strength of NaOH solution
was kept very low (0.01 N) in order to avoid the consumption
of the base in the hydrolysis of the mesoporous silica frame-
works. This titrimetric experimental data suggests that 1 g
TAFMC-1 catalyst contains 0.37 mmol –OWO3H groups.
Catalysis
Fig. 8 Effect of the catalyst loading in one-pot three component condensation
reaction.
Effect of the amount of catalyst. It was found that the con-
densation reaction among aromatic aldehyde (1), malono-
nitrile (2) and resorcinol (3) produces 2-amino-4H-chromenes
(4) in the presence of mesoporous TAFMC-1 catalyst in
aqueous medium under refluxing conditions (Scheme 2).
When the reaction was carried out in the absence of any cata-
lyst no product was formed. In order to evaluate the optimum
amount of catalyst required for the one-pot three component
condensation reaction, the reaction was carried out in the
presence of varying amounts of catalyst from 0.01 g to 0.06 g
in aqueous medium under refluxing conditions (Fig. 8). From
Fig. 8 we can conclude that 0.03 g of TAFMC-1 is the optimum
amount of catalyst needed to conduct this condensation reac-
tion. When the amount of catalyst has been increased more to
perform this catalytic reaction no further increase of product
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Table 2 Effect of the solvent in the TAFMC-1 catalyzed condensation of resorci-
nol, malononitrile, and benzaldehyde
Solvent
Yielda,b
Water
86
23
75
78
15
12
10
30
8
Acetonitrile
Methanol
Ethanol
DMF
THF
DCM
Solvent-free
n-Hexane
a All the reactions were carried out under reflux condition. Reaction
condition: Ph-CHO: 1.0 equiv. (106 mg), resorcinol: 1.0 equiv.
(110 mg), malononitrile: 1.0 equiv. (66 mg). Solvent-free reaction was
carried out at 100 °C. b Isolated yields.
Fig. 9 Effect of reaction temperature for the one-pot three component
condensation reaction.
yield has been observed. On the other hand, the product yield
and the rate of condensation reaction is decreased with
decreasing amount of catalyst compared to that required for
the optimum amount of catalyst.
Table 3 Screening of the different catalysts for the synthesis of 2-amino-4H-
chromenesa
Temperature
(°C)
Yield
Solvent (%)
Effect of solvent. To determine the best reaction conditions,
we have carried out the condensation of benzaldehyde,
malononitrile and resorcinol over TAFMC-1 in the presence of
various solvents such as water, acetonitrile, methanol, ethanol,
DMF, THF, DCM, n-hexane under refluxing conditions
(Table 2). For solvents like acetonitrile, DMF, THF, DCM and
n-hexane Knoevenagel condensation adduct was formed as a
major product and the desired product was formed only in
23%, 15%, 12%, 10% and 8% yields, respectively. But when
the reaction was carried out in polar protic solvents such as
water, methanol, ethanol, yield of the desired product was
increased to 86%, 75% and 78%, respectively. Here the
product is more polar than the reactants so when we have
used polar protic solvents the yield of the products was
increased. The fate of reaction was also investigated under
solvent-free conditions. But under this condition only a small
amount of the desired product was obtained, which indicates
that water is the optimum solvent for the completion of this
one-pot three component condensation reaction.
Effect of the reaction temperature. The activity of catalyst
TAFMC-1 for the synthesis of the chromene derivatives in
aqueous medium increases with increasing temperature. The
effect of reaction temperature has been investigated in the con-
densation reaction of benzaldehyde, malononitrile and resorci-
nol under aqueous medium at different temperatures (Fig. 9).
When the reaction was performed at room temperature (25 °C)
for 24 h in aqueous medium, no product was found. As the
temperature of the reaction was increased to 40 °C, the yield of
the product was 22%. Further, we performed the reaction at
three different temperatures such as 60, 80 and 100 °C. From
the activity profile shown in Fig. 9 it is clear that the yield of
the product is a maximum 86% at 100 °C.
Entry Catalyst
1
2
3
4
5
6
7
8
No catalyst
25
25
50
100
100
100
100
100
None
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
0
0
0
No catalyst
No catalyst
No catalyst
FeCl3
AlCl3·6H2O
SBA-15
SBA-15-Cl
SBA-15-Cl with aqueous HCl 100
Na2WO4·2H2O
H2WO4
TAFMC-1
8
18
12
5
5
9
12
45
40
80
10
11
12
100
100
100
a Reaction condition: 4-chlorobenzaldehyde (1 mmol, 140 mg),
resorcinol (1 mmol, 110 mg), malononitrile (1 mmol, 66 mg), solvent
free or solvent (water, 5 mL), catalyst (30 mg).
SBA-15, a control experiment for this condensation reaction
has been performed in the absence of any catalyst. The con-
densation reaction was carried out at different temperatures
under solvent-free or aqueous medium in the absence of any
catalyst (Table 3, entries 1–3). Under this condition no yield
corresponding to the expected condensation product has been
observed and starting materials remain unreacted. When the
condensation reaction was performed at 100 °C temperature in
the absence of any catalyst then only 8% product yield was
obtained (Table 3, entry 4). This yield could be due to the
higher reaction temperature where thermal condensation reac-
tion becomes more facile. We have employed various homo-
geneous Lewis acid catalysts like FeCl3 or AlCl3·6H2O in order
to compare the catalytic activity of TAFMC-1. But very poor
yields of the product were obtained (Table 3, entries 5 and 6).
In the presence of SBA-15 and SBA-15-Cl only very small
amounts of the products are generated in aqueous medium at
100 °C temperature (Table 3, entries 7 and 8). Then the con-
densation reaction was conducted by employing SBA-15-Cl in
Control experiment for the three component condensation
reaction. In order to understand the nature of bonding
of tungsten with the organically functionalized mesoporous
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the presence of aqueous HCl (Table 3, entry 9). A small
increase in the product yield is observed which may be due to
the presence of aqueous HCl. Homogeneous phase
Na2WO4·2H2O (Table 3, entry 10) has been utilized as the cata-
lyst for carrying out this condensation reaction. Only 45%
yield of the corresponding product has been obtained. The
reaction has also been carried out in the presence of H2WO4
catalyst (Table 3, entry 11). But no appreciable increase in the
product yield has been obtained. When our mesoporous het-
erogeneous catalyst TAFMC-1 has been employed for this con-
densation reaction then the yield of the product was
dramatically increased to 80% (Table 3, entry 12). This result
clearly suggests that the mesoporosity plays a significant role
in this condensation reaction. The surface area and pore size
for this mesoporous TAFMC-1 catalyst is large enough to allow
the diffusion of large organic molecules and helps them to
interact with each other for performing this one-pot three com-
ponent condensation reaction. Aromatic aldehydes attached
with both electron-withdrawing and electron donating groups
participated in the one-pot three component condensation
reaction very smoothly with very good efficiency (Table 1, see
the NMR data in the Experimental section). Thus, the nature
and position of substituted groups present in the aromatic
ring do not show any pronounced effect on the reaction. The
condensation reaction was performed for the both meta-substi-
tuted aromatic aldehyde (Table 1, entry 7) and as well as
heteroaromatic aldehyde such thiophene-2-carbaldehyde
(Table 1, entry 10) without any difficulty. Several sensitive func-
tional groups such as –Cl, –Br and –NO2 attached to the aro-
matic ring are also compatible for this reaction. The turn over
numbers (TONs) for the different reactions vary for different
aldehydes, suggesting high catalytic activity of TAFMC-1 gene-
rated from the tungsten species incorporated into the porous
Table 4 Recycling potential of mesoporous catalyst TAFMC-1 in the one-pot
three component condensation reaction
No. of cyclesa
Fresh
Run 1
Run 2
Run 3
Run 4
Run 5
Yieldb (%)
Time (h)
TONc
86.0
12.0
77
86.0
12.0
76
84.0
12.0
76
84.0
12.0
75
84.0
12.0
75
82.0
12.0
74
a Reaction condition: benzaldehyde = 1.0 equiv. (106 mg), resorcinol =
1.0 equiv. (110 mg), malononitrile = 1.0 equiv. (66 mg), aqueous
medium at 100 °C temperature. b Isolated yield of product. c Turn over
number = moles of substrate converted per mole of active site.
reaction cycles. In all cases, consistent catalytic activity over
mesoporous TAFMC-1 has been observed, establishing the fact
that the catalyst can be recycled and reused without any con-
siderable loss of catalytic activity (Table 4). A very small
amount of product has been observed, which could be attribu-
ted to the loss of catalyst recovery during the course of
reaction.
Leaching test
In order to check any leaching of metal from the solid catalyst
we have performed an in situ filtration technique. In this tech-
nique the catalyst (0.03 g) was stirred in 5 mL water for 24 h at
100 °C temperature, after filtration the filtrate was taken in a
50 mL round bottom flask and it was evaporated to dryness.
Then 1.0 mmol of each of the benzaldehyde, resorcinol and
malononitrile were taken in the same round bottom flask in
aqueous medium at 100 °C for 12 h. However, in this case no
product was found, suggesting that no leaching of the tung-
sten metal occurred before the filtration.
Hot filtration test
channel of organically functionalized SBA-15. All the conver- Furthermore, in order to check any leaching of W metal into
sions of the products (%) are listed in Table 1. The conden- the solution during the reaction, a hot-filtration test was per-
sation reaction was also performed with different active formed with TAFMC-1 for the condensation reaction. In such
methylene compounds, such as malononitrile, ethyl cyano- technique, benzaldehyde (1 mmol, 106 mg) was allowed to
acetate and diethyl malonate in the presence of the TAFMC-1 react with resorcinol (1 mmol, 110 mg) and malononitrile
catalyst in aqueous medium using benzaldehyde and resorci- (1 mmol, 66 mg) in aqueous medium with 0.03 g catalyst. The
nol as the representative case. Malononitrile has been con- reaction was performed at 100 °C and after 6 h under hot con-
sidered as the best suited active methylene compound to ditions the reaction mixture was immediately filtered. At this
conduct this one-pot three component condensation reaction stage the yield of the product was 52% (confirmed by 1H
as the highest yield of the desired product has been obtained NMR). After that the reaction was continued for a further 6 h
in this case. The
W content in the fresh catalyst is at the same temperature as the filtrate. However, no increase
0.0019 mmol g−1, as measured by ICP-OES analysis.
of the product yield beyond 52% was observed after 12 h reac-
tion time. This result clearly confirms the heterogeneous
nature of the mesoporous catalyst TAFMC-1 and no leaching of
Reusability of the catalyst
The reusability of the mesoporous TAFMC-1 in the one-pot W metal occurs during the course of reaction. ICP-OES analy-
three component condensation reaction was examined using sis has also been performed for the reused catalyst after the six
benzaldehyde, malononitrile and resorcinol as reference. After repetitive catalytic reaction cycles. After the six reaction cycle
completion of the reaction, the catalyst was separated from the the W content was marginally decreased to 0.0016 mmol g−1
reaction mixture by filtration, washed with dichloromethane (this decrease is within the experimental error of chemical
followed by diethyl ether. Then the recovered catalyst was analysis). This data clearly revealed the fact that the tungsten
dried at room temperature for 12 h and heated in the oven at metal has been covalently bonded with the organic groups
75 °C for activation of the catalyst. After recovery of the catalyst present in the mesoporous channel of the catalyst and the
it has been employed for a further six additional repetitive catalytic reactions are purely heterogeneous in nature.
10522 | Dalton Trans., 2013, 42, 10515–10524
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Characterization of reused catalyst
Acknowledgements
The reused TAFMC-1 catalyst was further characterized by
small angle powder XRD, TEM, FT-IR studies in order to inves-
tigate if any further change takes place in the catalyst after
reaction. Small angle powder XRD pattern of the reused meso-
porous catalyst displays (Fig. 1) a characteristic mesophase,
which clearly reveals that the hexagonal mesostructure of the
SKK and JM thank CSIR, New Delhi for their respective junior
and senior research fellowships. This work is partly funded by
DST Unit on Nanoscience.
reused catalyst are carefully preserved during the course of the References
catalytic reaction. In Fig. 1c it is quite evident that a small
1 M. M. Heravi, B. Baghernejad and H. A. Oskooie, J. Chin.
shift in the peak position for the reused catalyst is observed.
After six repetitive reaction cycles the expansion in pore-wall
for the reused catalyst could occur due to the hydrolysis of
silica species present at the catalyst surface. As a result
d-spacing for the reused catalyst is increased. This increase in
the d-spacing is responsible for the shift in the peak position
for the reused catalyst. The TEM images of the reused hetero-
geneous catalyst after the fourth catalytic cycle suggest that the
ordered mesostructure of the reused catalyst remains
unaffected after the reaction (Fig. 3). We took the FT-IR spectra
of the catalyst after the sixth catalytic cycle (Fig. 4), where the
strong absorption band at 966 cm−1 is observed, which corres-
ponds to the presence of WvO bond after use of the catalyst.
The above results revealed that the mesoporous heterogeneous
catalyst was very stable during the one-pot three component
condensation reaction.
Plausible reaction pathway. The possible reaction pathway
for the synthesis of 2-amino-4H-chromenes using meso-
porous catalyst having an acidic site involved the Knoevena-
gel condensation reaction between aromatic aldehyde and
malononitrile at the first step, then Michael addition
occurred between the Knoevenagel product and resorcinol,
which is followed by intra-molecular cyclization to yield
2-amino-4H-chromenes. Here W(VI) increases the electro-
philicity of the aldehydic carbon atom followed by dehy-
dration to give the Knoevenagel product.25 Again W(VI)
increases the electrophilicity of –CN group so that Michael
addition can occur and followed by intra-molecular cycliza-
tion and tautomerization to yield the 2-amino-4H-chromene
derivatives.19,26
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