A green route towards substituted 2-amino-4H-chromenes catalyzed by an organobase (TBD) functionalized mesoporous silica nanoparticle without heating

Article abstract of DOI:10.1007/s11164-016-2755-9

1,5,7-Triazabicyclo-[4,4,0]-dec-1-ene functionalized mesoporous silica nanoparticle promoted one-pot multicomponent synthesis of 2-amino-4H-chromenes from a phenolic component, different aldehydes and malononitrile was achieved in aqueous media at room te

Full text of DOI:10.1007/s11164-016-2755-9

Res Chem Intermed
DOI 10.1007/s11164-016-2755-9
A green route towards substituted 2-amino-4H-chromenes
catalyzed by an organobase (TBD) functionalized
mesoporous silica nanoparticle without heating
Bikash Karmakar1 Rajesh Nandi1
•
Received: 22 July 2016 / Accepted: 3 October 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract 1,5,7-Triazabicyclo-[4,4,0]-dec-1-ene functionalized mesoporous silica
nanoparticle promoted one-pot multicomponent synthesis of 2-amino-4H-chrome-
nes from a phenolic component, different aldehydes and malononitrile was achieved
in aqueous media at room temperature. The methodology promises advantages of
short reaction times, environmentally benign conditions, high yields, and opera-
tional convenience. The catalyst was reused several times without significant loss in
activity.
Keywords 2-Amino-4H-chromene Á Multicomponent reaction Á Organobase Á
Reusable Á Silica nanoparticle
Introduction
With the increase in environmental awareness worldwide, synthetic organic
chemists have become more concerned with adopting ‘green’ protocols in research
and development [1]. The most significant measures taken are to avoid the use of
hazardous solvents and harmful chemicals, to reduce the formation of by-products
as chemical wastes, promoting atom-economical reactions, and reusability of
reagents. [2, 3]. Accordingly, implementation of greener methods has become a
ubiquitous research topic. Multicomponent reactions (MCRs) have emerged as one
of the most potent strategies as a part of ‘green chemistry’ with ample opportunities
to assemble different medicinally privileged scaffolds in a single molecule for the
synthesis of bioactive pharmacophores [4–7].
&
Bikash Karmakar
bikashkarm@gmail.com
1
Department of Chemistry, Gobardanga Hindu College, Khantura, North 24 Parganas 743273,
India
123

B. Karmakar, R. Nandi
Chromenes are an important class of oxygenated heterocycles; among them, the
chemistry of 2-Amino-4H-chromene has received great interest over the last few
years. The scaffold functions as a significant structural component in naturally
occurring and synthetically made drug-like molecules, displaying a broad spectrum
of biological activities (Scheme 1). They are endowed with a wide range of
pharmaceutical aspects such as antiinfertility [8], anticancer [9, 10], antidepressant,
antihypertensive, antitubulin, antiviral and antioxidant qualities [11, 12]. They have
potential application in psoriatic and rheumatoid arthritis [13–15]. Chromenes are
known to activate potassium channels and inhibit phosphodiesterase IV and
dihydrofolate reductases [16–22]. They are the main constituents of many natural
products and potentially biodegradable agrochemicals [23, 24].
Several methods have been applied in the synthesis of 2-amino-4H-chromenes
from resorcinol, aldehydes and active methylene nitriles using catalysts like
piperazine [25], DBU [26], NEt3 [27], Et2NH [28], K2CO3 [29], glycine [30], L-
Proline [31], Fe(HSO4)3 [32], CuO–CeO2 nanocomposite [33], nano MgO [34],
hydrotalcite [35], Na2CO3/ball milling [36] etc.
Although earlier reported methods are efficient and offer relative advantages with
regard to one another, some of them are confronted with drawbacks regarding eco-
friendliness of reaction conditions, tedious work-ups, use of toxic and expensive
catalysts, high temperature conditions and recyclability of catalysts. However,
despite the merits of the reported methods, there is still scope for further
development on the environmental and economical impacts of the reaction.
In continuation of our efforts for the development of environmentally benign
synthetic methodologies for the production of various biologically important
moieties using mesoporous heterogeneous and reusable catalysts [37–43], we wish
to report for the first time the use of 1,5,7-triazabicyclo-[4,4,0]-dec-1-ene (TBD), a
cyclic guanidine base anchored by mesoporous silica nanoparticles (MSN) [44], in
the three-component expeditious synthesis of 2-amino-4H-chromene derivatives in
an aqueous medium at room temperature, requiring stirring only (Scheme 2).
Surface functionalization of nanomaterials has emerged as a powerful technique
for tuning physical and chemical properties [45–51]. Mesoporous silica usually have
OMe
Br
MeO
Me
MeO
N
CN
CN
CN
O
O
H2N
O
NH2
O
NH2
O
NH2
NH2
Antioxidant
Antimiotic, antitubulic
Crolibulin (Anticancer)
Scheme 1 Some biologically active 2-amino-4H-chromene scaffolds
123

A green route towards substituted 2-amino-4H-chromenes…
R
O
CN
CN
CN
TBD-MSN
Water, RT
+
RCHO
+
HO
NH2
HO
OH
Scheme 2 Green synthesis of substituted 2-amino-4H-chromenes
a high surface area, covered by silanol groups, which has made the functionalization
of the pore surface feasible by covalent bonding with TBD. The high surface area of
silica nanoparticles, strong basicity of the well dispersed organobase active sites and
reusability are the key factors behind making the catalyst very efficient as evident
from the excellent yield of the products in short time frames.
The aim of this work was the synthesis and physical characterization of organobase
functionalized mesoporous silica nanoparticles and assessment of its catalytic
performance in the synthesis of diverse functionalized 2-amino-4H-chromenes.
Experimental
Preparation of the catalysts
Preparation of mesoporous silica nanoparticle (MSN) and chloropropyl
functionalization (CPMSN)
MSN was prepared based on the MCM-41 preparation strategy. The surfactant and
structure directing template CTAB (Cetyl Triethyl Ammonium Bromide) was
dissolved in 2(M) NaOH at 80 °C. Then TEOS (Tetra Ethyl Ortho Silicate) was
added and stirred at the same temperature setting in a condenser for 2 h. The
solution was cooled, filtered and dried for 24 h at 100 °C. The surfactant was
removed by washing with methanolic HCl at 60 °C for 6 h. Finally, it was filtered
and dried at 100 °C for another 6 h to obtain a fine white powder (MSN). The
powder was then dispersed in dry toluene and 3-chloropropyltrimethoxysilane was
added to it and refluxed for 24 h at 115 °C. The mass was then filtered and washed,
dried at 100 °C for 6 h to get the CPMSN (Scheme 3).
OH
OH
OH
O
O
O
MeO
Cl
Dry toluene
Reflux
Cl
Si
+
MeO Si
24 h
MeO
N
N
N
O
O
Cl
+
N
Dry toluene
Reflux
O
Si
Si
O
24 h
O
N
N
O
H
Scheme 3 Preparation of the catalyst TBD–MSN
123

B. Karmakar, R. Nandi
TBD substitution over CPMSN
The CPMSN was suspended in dry toluene and 1,5,7-triazabicyclo-[4,4,0]-dec-1-
ene (TBD) was added and the mixture was refluxed at 115 °C for 8 h. It was then
cooled, filtered and washed well with a solvent mixture of toluene, methanol and
water (1:1:1 by volume). Finally the material was dried in air for 12 h to get the
final TBD–MSN catalyst (Scheme 4).
General synthesis of 2-amino-4H-chromenes
A mixture of active phenol (1.0 eq), aldehyde (1.0 eq) and malononitrile (1.0 eq)
was dissolved in 5 mL water in the presence of 10 mg of MSN–TBD catalyst. The
mixture was then stirred for the requisite time. After completion (indicated by TLC,
solvent system 2:3 EtOAc/petroleum ether) the reaction mixture was filtered to
separate the precipitated product. The crude product was then redissolved in
ethanol, the catalyst was filtered out and the filtrate was recrystallized to obtain pure
1
products. Finally, the isolated compounds were characterized by mp, H NMR
spectroscopy, 13C NMR spectrometry and spectral analysis (C, H, and N).
Results and discussion
Physicochemical characterization of catalyst
The TBD–MSN catalyst was characterized by High Resolution Transmission
Electron Micrograph (HRTEM), FT-IR, Powder X-ray diffraction (XRD) and BET
surface area analysis. TEM images of the particles are shown in Fig. 2a, b.
OH
ꢀ
B
CN
X
R
H
CN
X
R
H
B
ꢀ
+
O
+
OH
B
ꢀ
= TBD-MSN
OH
OH
R
H2O
X
NH2
O
N
O
HO
O
H
R
ꢀ
B
R
NH
X
X
Scheme 4 Probable pathway of the reaction towards 2-amino-4H-chromene
123

A green route towards substituted 2-amino-4H-chromenes…
Dimensions of the particles was around 100 nm. A perfectly round shaped particle
is shown in Fig. 1a. No bulk aggregation was observed and all the particles are of
the same size. Each particle contains numerous microsized pores to make the
surface spongy as showed in Fig. 1b. The ordered porous morphology ensures high
surface area. BET measurement showed the surface area of the MSN to be
676 m2 g-1
.
FT-IR spectrum (Fig. 2) of the TBD–MSN exhibits sharp bands at 466, 798, and
1067 cm-1 which attributes to the condensed silica (Si–O–Si) network. Besides, a
shoulder band at 956 cm-1 can also be detected in the spectra, corresponding to the
stretching vibrations of :Si OH bonds. The covalent bonding of TBD moiety is
verified from the presence of sharp aliphatic N=C vibrations at 1634 cm-1 and C–H
bending and stretching modes at 1324, 1377, 1444, 2948, 2882 cm-1, respectively
[52].
Figure 3 represents the small angle X-ray diffraction pattern of TBD–MSN
material. In the analysis, a sharp peak appears at 2h = 0.4° and a small peak at
2h = 0.7°. These two correspond to Bragg reflections on the 100 and 110 planes,
respectively. These peaks are clear indications of mesoporosity, which remains
intact even after post-grafting.
Reaction conditions
A very simple protocol was followed in the reaction processes. A mixture of
resorcinol, aldehyde and malononitrile was stirred at room temperature in water
over TBD–MSN catalyst. Progress was checked by TLC, and after a characteristic
reaction time the product was obtained as precipitate in excellent yields. There was
no requirement of tedious work-ups and chromatographic separation in any of the
cases, and simple filtration sufficed to do the job. Moreover, no other additive was
necessary to promote the reaction. For further purification it was recrystallized from
hot aqueous ethanol.
Fig. 1 a, b HRTEM images of the globular mesoporous silica nanoparticles
123

B. Karmakar, R. Nandi
Fig. 2 FT-IR analysis of the TBD–MSN catalyst
1000000
100
800000
600000
400000
200000
0
110
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.425
Position (2θ)
Fig. 3 Small angle XRD pattern of the TBD–MSN catalyst
However, prior to this generalization, feasibility of the reaction conditions were
investigated (Table 1). To recognize the optimization conditions, a representative
reaction assembling resorcinol, 4-chlorobenzaldehyde and malononitrile, taken in
equimolar proportion (1.0 mmol each), and a series of catalysts (10 mg) as well as
solvents (5 mL), including catalyst-free and solvent-free conditions, were
employed. As per our ongoing research on eco-friendly solvent mediated reactions,
123

A green route towards substituted 2-amino-4H-chromenes…
Table 1 Optimization of the
reaction conditions
Entry
Catalyst
Solvent
Time (h)
Yielda (%)
b
1
–
Water
EtOH
Water
Water
Water
Water
Water
Water
MeOH
EtOH
CH3CN
CH2Cl2
Et2O
12
12
12
12
6
–
b
–
2
–
3
MSN
35
30
58
72
87
95
90
88
91
68
57
62
4
SiO2 gel
5
Basic Al2O3
AP–MSN
TBD–SiO2 gel
TBD–MSN
TBD–MSN
TBD–MSN
TBD–MSN
TBD–MSN
TBD–MSN
TBD–MSN
6
6
7
6
8
2
9
2.5
2.5
3
Reaction conditions: resorcinol,
4-chlorobenzaldehyde and
malononitrile 1 mmol each in
5 mL solvent at ambient
10
11
12
13
14
6
condition, catalyst 10 mg each
a
Isolated yield
6
–
3
b
Reaction failed
the reactants were initially taken in water and ethanol, respectively, and stirred in
absence of any catalyst. Even after a run of 12 h, the desired product was not
obtained (Table 1, entries 1, 2). The same proportion of reactants was treated in
water with various catalysts like unfunctionalized MSN, silica gel (230–400 mesh),
basic Al2O3, amino functionalized MSN, TBD–Silica gel and TBD–MSN. Only
35 % yield was produced with unfunctionalized silica nanoparticles (Table 1, entry
3). Silica gel (30 %) and basic alumina (58 %) also failed to promote the reaction up
to the mark (Table 1, entries 4, 5). Moderate yield was obtained with amino propyl
functionalized AP–MSN (Table 1, entry 6). However, TBD functionalized silica gel
improved the yield, affording 87 % product, but the most satisfactory result in terms
of yield as well as reaction time was achieved when TBD–MSN was used in the
reaction (Table 1, entries 7 and 8).
The next step was then to explore the effects of solvents on productivity. With the
best catalyst, TBD–MSN, different solvents were used keeping other factors intact.
Methanol and ethanol were almost comparable in solvation and polarity and thereby
also the yield (Table 1, entries 9, 10). Acetonitrile also proved to be a good solvent
but was not encouraging in view of green methodology (Table 1, entry 11).
Relatively less polar dichloromethane or diethyl ether could not improve the yield
significantly (Table 1, entries 12, 13). Even in solvent-free conditions, satisfactory
output was not obtained (Table 1, entry 14). The reaction was optimized to be
carried out in aqueous medium over TBD–MSN, affording complete conversion in a
short time. The marked polarity and high solvation character of water was the key
for superior behavior.
As the reaction involved Knoevenegel and Michael reactions as key steps, which
are typically base catalyzed reactions, a strong base catalyst resulted in high yields.
It is pertinent to mention that the high surface area of MSN upon which the bulky
TBD base was grafted definitely had immense influence over the yield.
123

B. Karmakar, R. Nandi
The conditions thus optimized were applied in the synthesis of diverse
functionalized aminochromenes by combining resorcinol, malononitrile and a wide
variety of aromatic and heterocyclic aldehydes in order to delineate the approach.
Details of the study are given in Table 2. Different functional electron-attracting
and electron-donating groups such as nitro, halogen, hydroxy, methoxy and nitrile,
substituted for benzaldehydes, were highly compatible under reaction conditions
and excellent yields were obtained in each case. Different active phenolic systems
such as catechol and naphthols were also equally productive when coupled with
malononitrile and 4-cholorobenzaldehyde. The reaction proceeded smoothly with
all the substrates within 1.2–2.5 h at room temperature, and that is the distinguish-
ing feature of TBD–MSN catalyst compared to the hitherto reported methods.
However, the reactions did not produce satisfactory yields while reacting with
aliphatic aldehydes, probably due to their low reactivity.
The advantage of the current simple and benign synthetic methodology may be
demonstrated by comparison with various previous reports (Table 3). The published
results clearly show that the present ambient and expeditious protocol furnishes
quantitative yields in water. Previous catalytic techniques were performed mostly at
elevated temperatures and with longer reaction times. They require costly
purification workups with hazardous waste formation, disposal of which impedes
catalyst recovery.
To obtain further pure crystallized product, the reaction mixture was dissolved in
hot ethanol, filtered to separate the catalyst and the filtrate was simply crystallized
after concentration, which furnished pure crystals of the products. Thus the method
is free from work up, column chromatography or chemical garbage disposal, and
hence satisfies the green protocol.
A probable mechanistic pathway has been elucidated in Scheme 3. The initial
reaction involves a base catalyzed Knoevenagel reaction between aldeyde and
methylene nitrile, which is further attacked by active phenol following the Michael
pathway. Finally cyclization and tautomerism leads to the substituted 2-amino-4H-
chromene.
Spectroscopic data for some of the characteristic compounds are given as-
(Table 2, entry 13) 2-amino-7-hydroxy-4-(3,4,5-trimethoxyphenyl)-4H-chromene-
3-carbonitrile 1H NMR (DMSO-d6, 300 MHz): d 3.79 (s, 3H, OCH3), 3.81 (s, 3H,
OCH3), 3.82 (s, 3H, OCH3), 4.58 (s, 1H, CH), 6.39 (s, 1H, aromatic), 6.47–6.51 (m,
3H, NH2 and 1H aromatic), 6.84–6.89 (m, 3H, aromatic), 9.67 (s, 1H, OH); 13C
NMR (DMSO-d6, 100 MHz): d 30.7, 57.2, 57.5, 59.8, 105.4, 106.2, 111.2, 113.5,
117.8, 128.3, 134.7, 135.8, 151.5, 154.6, 157.8, 178.0; Anal. Calcd. for
C19H18N2O5: C, 64.40; H, 5.12; N, 7.91. Found: C, 64.37; H, 5.19; N, 7.83.
LCMS m/z: 355.09 (M ? 1).
(Table 2, entry 17) 2-amino-7-hydroxy-4-(4-imidazoyl)-4H-chromene-3-carboni-
trile 1H NMR (DMSO-d6, 300 MHz): d 4.58 (s, 1H, CH), 6.34 (s, 1H, aromatic),
6.47 (d, 1H, aromatic), 6.72 (s, 2H, NH2), 6.8 (s, 1H, imidazoyl), 6.94 (d, 1H,
aromatic), 7.48 (s, 1H, imidazoyl), 9.6 (s, 1H, OH), 11.8 (s, 1H, NH); 13C NMR
(DMSO-d6, 100 MHz): d 30.2, 58.6, 104.8, 107.9, 110.6, 117.8, 120.1, 132.0, 134.9,
123

A green route towards substituted 2-amino-4H-chromenes…
Table 2 Synthesis of diversely functionalized 2-amino-4H-chromenes by coupling phenol derivatives,
different aldehydes and malononitrile
Ar
CN
CN
CN
TBD-MSN
Water, RT
+
ArCHO
+
O
NH2
OH
Entry
Phenol
R
Time (h)
Yielda (%)
MP (Lit./obs.)b
1
Resorcinol
Resorcinol
Resorcinol
Resorcinol
Resorcinol
Resorcinol
Resorcinol
Resorcinol
Resorcinol
Resorcinol
Resorcinol
Resorcinol
Resorcinol
Resorcinol
Resorcinol
Resorcinol
Resorcinol
Catechol
Ph
2.0
2.0
1.5
1.0
1.5
1.5
1.2
1.2
2.0
1.5
1.5
1.5
1.0
2.0
1.5
2.0
2.5
2.0
2.5
2.5
92
95
96
93
92
94
95
92
90
91
89
92
96
92
90
90
91
88
85
80
232/234 [36]
162/164 [33]
258/262 [33]
162/164 [33]
225/224 [33]
188/186 [33]
–/156
2
4-Cl-Ph
3
2,4-di Cl-Ph
2-NO2-Ph
4-Br-Ph
4
5
6
4-F-Ph
7
4-Br-2-F-Ph
4-CN-Ph
8
110/112 [33]
–/282
9
4-OMe-3-OH-Ph
2-OMe-Ph
3-OMe-Ph
3,4-diOMe-Ph
3,4,5-triOMe-Ph
2-Furyl
10
11
12
13
14
15
16
17
18
19
20
–/104
184/186 [28]
215/213 [31]
210/212 [26]
210/212 [34]
–/158
2-Pyrrolyl
3-Indolyl
–/208
4-Imidazoyl
4-Cl-Ph
–/274
–/178
2-Naphthol
1-Naphthol
4-Cl-Ph
208/210 [32]
230/232 [32]
4-Cl-Ph
Reaction conditions: resorcinol, aldehyde and malononitrile 1 mmol each in 5 mL water at room tem-
perature stirring, catalyst 10 mg
a
Isolated yield
b
MP in °C
155.8, 156.8, 177.3; Anal. Calcd. for C13H10N4O2: C, 61.41; H, 3.96; N, 22.04.
Found: C, 62.37; H, 3.91; N, 23.05.
Catalyst reusability study
The reusability study of the catalyst was done separately with a larger batch size.
Initially 0.5 g resorcinol was used with equivalent amount of 4-chlorobenzaldehyde
and malononitrile in presence of 50 mg catalyst. After completion of the reaction,
123

B. Karmakar, R. Nandi
Table 3 Comparison of the current methodology with hitherto reported results for the synthesis of
2-amino-4-chlorophenyl-7-hydroxy-4H-chromene-3-carbonitrile
Entry
Catalyst
Solvent
Time (min)
Condition/°C
Yield (%)
1
2
3
4
5
6
7
8
a
CuO–CeO2 Nano [18]
Glycine [15]
–
7
80
93
92
98
96
86
99
90
95
H2O
EtOH
EtOH
CH3CN
–
16
US/rt
L-Proline [16]
Et2NH [13]
4 h
4.5 h
3 h
5
Reflux
Reflux
Reflux
Ball milling/rt
60
Fe(HSO4)3 [17]
Na2CO3 [21]
Hydrotalcite [20]
MSN/TBDa
H2O
H2O
5 h
2 h
rt
Present work
Table 4 Reusability study of TBD–MSN catalyst in the condensation of resorcinol, malononitrile and
4-chlorobenzaldehyde at optimum conditions
Cycles
resorcinol (g)
Catalyst (mg)
Yield (%)a
1
2
3
4
5
a
0.5
0.4
0.3
0.2
0.1
50
40
30
20
10
95
93
93
88
80
Isolated yield
the catalyst was recovered by filtration over Whatmann filter paper, washed several
times with ethanol to remove any adhered product, dried at 100 C for 3 h and reused
in further cycles. The results are shown in Table 4. The catalyst was so efficient that
we could use it 5 times in succession without much change in catalytic activity, as
indicated in the change in yield of the product.
Conclusions
In conclusion, one-pot three component synthesis of substituted 2-amino-4H-
chromene derivatives has been reported. TBD anchored mesoporous silica
nanoparticle was found to be an efficient catalyst due to its applicability to a
wide range of starting materials, heterogeneity and high reusability. Moreover,
simple work-up, clean procedure, room temperature reaction, short reaction times,
and high yields of products distinguishes this procedure from the available methods.
Acknowledgments B.K. thanks University Grants Commission (New Delhi) for financial support under
Minor Research Project.
123

A green route towards substituted 2-amino-4H-chromenes…
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