Highly efficient mesoporous base catalyzed Knoevenagel condensation of different aromatic aldehydes with malononitrile and subsequent noncatalytic Diels-Alder reactions

Article abstract of DOI:10.1016/j.molcata.2010.11.039

Knoevenagel condensation and [4 + 2] cycloaddition reactions are very important class of reactions in synthetic organic chemistry. We have prepared amino-functionalized mesoporous silica through co-condensation of 3-aminopropyltriethoxy-silane (APTES) alo

Full text of DOI:10.1016/j.molcata.2010.11.039

Journal of Molecular Catalysis A: Chemical 335 (2011) 236–241
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Journal of Molecular Catalysis A: Chemical
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Highly efficient mesoporous base catalyzed Knoevenagel condensation of
different aromatic aldehydes with malononitrile and subsequent noncatalytic
Diels–Alder reactions
John Mondal, Arindam Modak, Asim Bhaumik∗
Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 August 2010
Received in revised form 25 October 2010
Accepted 28 November 2010
Available online 9 December 2010
Knoevenagel condensation and [4 + 2] cycloaddition reactions are very important class of reactions in
synthetic organic chemistry. We have prepared amino-functionalized mesoporous silica through co-
condensation of 3-aminopropyltriethoxy-silane (APTES) along with tetraethylorthosilicate (TEOS) in
presence of a cationic surfactant CTAB hydrothermally. Small angle powder XRD, HR TEM, FE SEM, N2
sorption and FT IR spectroscopic tools are used to characterize the 2D-hexagonal mesostructure and to
identify the presence of surface –NH2 groups in amino-functionalized mesoporous silica material. Our
experimental results reveal that amino-functionalized mesoporous silica is an efficient base catalyst for
the Knoevenagel condensation of different aromatic aldehydes with malononitrile to ␣,␤-unsaturated
dicyanides under very mild reaction condition and in the presence of ethanol solvent. The isolated ␣,␤-
unsaturated dicyanides obtained through the condensation reaction further react very efficiently with
cyclopentadiene to form a series of Diels–Alder cycloaddition products in excellent yields in the absence
of any catalyst.
Keywords:
Amino-functionalization
Base catalysis
Diels–Alder cycloaddition
Knoevenagel condensation
Mesoporous materials
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
high catalytic activity in base catalyzed Knoevenagel condensa-
tion. Mesoporous zeolitic structures [11] or surface modification in
the presence of alkali metal cation [12], oxinitride [13], tetraalky-
lammonium cation [14], N,N-dimethyldecylamine [15], etc. has
been found an alternative strategy of generating a highly active
catalytic site necessary for base catalyzed Knoevenagel condensa-
tion. Amino-functionalized mesoporous silica synthesized through
co-condensation of 3-aminopropyltriethoxysilane and tetraethy-
lorthosilicate has been utilized in base catalyzed Knoevenagel
condensation for the synthesis of ␣,␤-unsaturated carboxylic acids
in recent times [16–22]. These organically functionalized meso-
porous silicas are particularly effective when the reactions are
carried out in polar solvents like ethanol or DMF. Parinda and
Rath [23] have prepared amino-functionalized mesoporous sil-
ica by co-condensation method, which showed high malonic
ester conversion and selectivity for cinnamic acid. However,
diethylmalonate (DEM) or ethyl cyanoacetate (ECA) are mostly
used as active methylene compounds in the previous reports
of Knoevenagel reactions over functionalized mesoporous silicas,
which leads to corresponding unsaturated carboxylic acid or ␣-
cyanocarboxylate as the products. Decarboxylation occurs in the
former case is undesirable as it reduces the fruitful utilization of the
carbon atoms. Thus further functionalization of the Knoevenagel
reaction product through Diels–Alder [4 + 2] cycloaddition reac-
tion [24,25] can provide a new strategy in designing novel target
molecules.
Ordered mesoporous materials having exceptionally high sur-
face areas and tunable pores of nano-scale dimensions gained
increasing interests over the years and found numerous poten-
tial applications in different frontier areas like adsorption [1],
metal ion uptake [2], catalysis [3,4], sensing [5] and so on. In
order to functionalize the surface of mesoporous silica and thus
to explore their applications in different catalytic transformations
a large number of organosilanes have been used as precursors
during synthesis or post-synthesis functionalization for the prepa-
rations of organically modified mesoporous materials [6–8]. Thus,
organic–inorganic hybrid mesoporous materials containing active
functional groups grafted at the surface of the mesopores can be
designed based on the particular requirement of the active sites in
a catalytic reaction.
Knoevenagel reaction is the condensation of an aromatic alde-
hyde and a compound containing active methylene group, which
proceeds in the presence of a catalytic amount of strong base
[9]. Srasra et al. [10] has incorporated nitrogen species in micro-
porous zeolites through ammonia treatment and observed their
∗
Corresponding author.
E-mail address: msab@iacs.res.in (A. Bhaumik).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.11.039

J. Mondal et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 236–241
237
In this context it is pertinent to mention that for the Kno-
evenagel condensation reactions with high catalytic activities are
mostly observed over a large number of organic solvents, like
DMSO, which is toxic and environmentally hazardous for disposal.
Whereas, ethanol is a common organic solvent and it can be easily
disposable. Herein, we describe a new route to synthesize ␣,␤-
unsaturated dicyano compounds via Knoevenagel condensation
of aromatic aldehydes and malononitrile catalyzed by amino-
functionalized mesoporous material in presence of ethanol solvent.
␣,␤-Unsaturated dicyano compounds formed in the initial conden-
sation step can act as a good dienophile in Diels–Alder cycloaddition
reaction with cyclopentadiene, which can be used as a building
block for designing novel target molecules.
a
b
2. Experimental
2.1. Synthesis of amino-functionalized mesoporous silica
Organosilane precursor 3-aminopropyltriethoxy-silane (APTES,
Aldrich) along with tetraethylorthosilicate (TEOS, Aldrich) in 1:5
molar ratio were used for the synthesis of amino-functionalized
mesoporous silica. In a typical synthesis procedure cetyltrimethy-
lammonium bromide (CTAB, Loba Chemie, 0.0075 mol 2.73 g) was
dissolved in an aqueous solution of tartaric acid (0.6 g in 30 g
H2O) under vigorous stirring condition. The resulting mixture was
stirred for about 30 min to obtain a clear solution. Then 1.105 g
APTES (0.005 mol) was added into the solution. After 30 min of stir-
ring 5.245 g TEOS (0.025 mol) was added in the reaction mixture
and it was stirred for about 2 h to obtain a gel. After 2 h stirring
2 M NaOH solution was added dropwise into the gel until the pH
reached ca. 12.0. The gel was aged overnight with continuous stir-
ring. Then the gel was heated in a closed stainless steel autoclave
at 373 K for 48 h. After the hydrothermal treatment the slurry was
filtered and washed several times with water and dried under
vacuum. The template CTAB was removed from as-synthesized
sample by extracting solid material three times with ammonium
acetate–ethanol solution.
2
4
6
8
10
2
in degree
Fig. 1. XRD patterns of as-synthesized (a) and template-free (b) amino-
functionalized mesoporous silica.
dicyanides was mixed with the 2 ml freshly distilled cyclopentadi-
ene taken in 5 ml dry THF in a round bottom flask. The mixture was
cooled to 273 K and then a pinch of hydroquinone was added and
the reaction mixture was stirred vigorously for 4–7 h at 273 K. After
the reaction was completed, solvent THF has been removed by the
rotary evaporator and the solid product was dried in the air. The
condensation as well as cycloaddition products were characterized
by the 1H and 13C NMR spectroscopy.
3. Results and discussion
3.1. Characterizations of the catalyst
The powder XRD patterns of the as-synthesized and template-
free amino-functionalized mesoporous silica are shown in Fig. 1. As
2.2. Characterization techniques
Powder X-ray diffraction patterns were recorded on a Bruker
D-8 Advance SWAX diffractometer operated at 40 kV voltage and
40 mA current. The instrument has been calibrated with a standard
silicon sample, using Ni-filtered Cu K␣ (ꢀ = 0.15406 nm) radiation.
Nitrogen adsorption/desorption isotherms were obtained by using
a Bel Japan Inc. Belsorp-HP at 77 K. Prior to gas adsorption, sample
was degassed for 4 h at 393 K under high vacuum conditions. A JEOL
JEM 6700F field emission scanning electron microscope was used
for the determination of morphology of the particles. FT IR spectra
of the samples were recorded using a Nicolet MAGNA-FT IR 750
Spectrometer Series II.
150
0.014
2.7 nm
100
0.007
2.3. Knoevenagel condensation and Diels–Alder cycloaddition
reactions
50
In a typical synthesis procedure for Knoevenagel condensation
1 mmol aromatic aldehyde was mixed with the 1 mmol malonon-
itrile in 5 ml absolute ethanol taken in a round bottom flask. Then
20 mg template-free amino-functionalized mesoporous silica cat-
alyst was added in it and the reaction mixture was stirred at room
temperature for ca. 5–7 h. After that reaction mixture was filtered
and the solid was washed with the ethanol and dried. The solvent
was removed by a rotary evaporator to obtain the solid prod-
uct residue. Then it was crystallized from absolute ethanol. For
the Diels–Alder cycloaddition reaction 1 mmol of ␣,␤-unsaturated
0.000
2
4
6
8
Pore diameter (nm)
0
0.0
0.2
0.4
0.6 0.8
1.0
Relative pressure (P/P0)
Fig. 2. N2 adsorption/desorption isotherms of amino-functionalized mesoporous
silica. Adsorption points are marked by filled circles and those of the desorption are
by open circles. BJH pore size distribution is shown in the inset.

238
J. Mondal et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 236–241
100
75
50
25
Fig. 3. FE SEM image of amino-functionalized mesoporous silica.
0
seen from the patterns that the as-synthesized material is meso-
scopically ordered, showing three diffractions for the 1 0 0, 1 1 0,
2 0 0 planes corresponding to 2D-hexagonal mesophase, whereas
in the template-free sample 1 1 0 and 2 0 0 peak intensities are
considerably reduced. The N2 sorption isotherms of template-free
amino-functionalized mesoporous silica are shown in Fig. 2. These
isotherms can be classified as type IV characteristic of small meso-
pores with no sharp capillary condensation step [26,27]. Pore size
distribution of this sample estimated by employing the NLDFT
method is shown in the inset of Fig. 2, suggesting the existence
of mesopores having dimension of ca. 2.7 nm in this material. The
BET surface area and pore volume of this material are 392 m2 g−1
and 0.2 ml g−1, respectively. Estimated peak pore width of ca.
2.7 nm is good for carrying out the Knoevenagel condensation reac-
tion of the bulky organic aldehydes with malononitrile inside the
nano-channels. FE SEM image of the amino-functionalized meso-
porous silica sample is shown in Fig. 3. As seen from the figure
that this material is composed of very tiny spherical nanoparticles
of ca. 25–40 nm in diameter. These small nanoparticles are self-
aggregated to form uniform large spherical particles of dimension
1
2
3
4
5
Number of cycles
Fig. 5. Recycling efficiency of amino-functionalized mesoporous silica catalyst.
ca. 300–500 nm in size. CHN chemical analysis results revealed the
loading of –NH2 groups for this amino-functionalized mesoporous
material is 3.09 mmol g−1, which is considerably good to carry out
the base catalyzed reactions. Further, the presence of N–H bending
vibration at 690 cm−1 and –NH2 symmetric bending vibration at
1557 cm−1, in the FT IR spectrum suggested successful grafting of
3-aminopropyl group onto the surface of the amino-functionalized
mesoporous materials [23].
3.2. Catalysis
The amino-functionalized mesoporous material is used as cat-
alyst in the liquid phase Knoevenagel condensation reaction
(Fig. 4) over various electron-withdrawing and electron-donating
aromatic aldehydes. Yields of various ␣,␤-unsaturated dicyano
products obtained in this reaction are given in Table 1. As seen
from the table that reaction proceeds almost exclusively at room
temperature (298 K). For all the substrates, very high yields of the
condensation products are obtained within 4–7 h reaction time.
We have tested the catalytic efficiency of amino-functionalized
mesoporous material in five repetitive reaction cycles for the reac-
tion between benzaldehyde and malononitrile. We have plotted
the recycling efficiency of the catalyst for these five consecutive
catalytic cycles for the condensation reaction between benzalde-
hyde and malononitrile (Fig. 5). It was found that the catalyst
can be efficiently recycled and reused for five repeating cycles
without much loss of efficiency. This indicates that our amino-
functionalized mesoporous silica is a very efficient catalyst for
Knoevenagel condensation. Electron withdrawing group present in
the para position of the aromatic ring increases the yield (Fig. 1)
by activating the aldehyde group due to –I effect. When –NO2
group is present in the ortho position of the aromatic ring the
yield of the product has been minimized. This could be attributed
to the steric hindrance at the reaction site. On the other hand
the yield for the –OH group containing substrate decreases a lit-
tle bit due to the +R effect of the –OH group present in the ring.
Further, we have carried out Knoevenagel condensation over a
well known solid base catalyst Mg–Al–hydrotalcite [28] for the
CN
CHO
CN
Amino functionalized
mesoporous silica
NC
CH2
+
NC
Absolute Ethanol
25 °C, 4-7 h
R
R
I
CN
CN
CN
CN
Dry THF, 0°C
Under Ar atm.
+
Hydroquinone
Stirred for 4-7 h
R
R
II
Fig. 4. Reaction pathway for the Knoevenagel condensation reactions over amino-
functionalized mesoporous silica.

J. Mondal et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 236–241
239
Table 1
Table 2
Knoevenagel condensation of different aldehydes with malononitrile over amino-
Diels–Alder [4 + 2] cycloaddition reactions of dicyano derivatives with
cyclopentadiene.
functionalized mesoporous silica.a
Entry Aldehyde
Time (h) Yield (%) Product
TOFb (h−1
)
Entry
Dicyano compound
Time (h)
Yield (%)
Product
CN
CN
CHO
CN
CN
CN
CN
CN
1
6
90
5.23
1
5
75
CN
CN
CN
CHO
CN
CN
2
5
95
4.53
4.04
4.31
2
3
4
4
4
6
80
82
75
NO2
NO2
CHO
NO2
NO2
CN
CN
CN
CN
NO2
CN
CN
3
6
80
NO2
NO2
O2N
CN
CHO
CN
CN
CN
CN
CN
4
6
84
CO2H
CHO
CO2H
CO2
H
CO2H
CN
CN
CN
CN
CN
CN
5
5
82
5.56
5
7
70
CH3
CH3
CH3
CH3
CN
CN
CHO
CN
CN
CN
CN
6
7
80
3.68
6
7
70
HO
OH
CN
OH
OH
CHO
CN
reaction between benzaldehyde and malononitrile for compari-
son. The yield of the condensation product was 65%. However,
for our amino-functionalized mesoporous material the yield was
90%. From this data it is clear that mesoporosity in our amino-
functionalized mesoporous material facilitates this condensation
reaction. Moreover, when the reaction between benzaldehyde and
malononitrile is carried out in absence of catalyst no condensation
product formed. This result suggests that Knoevenagel condensa-
tion reactions described herein are purely catalytic in nature.
7
6
65
0.68
1H and 13C NMR chemical shifts are given in supporting materials.
TOF = turn over frequency = moles of substrate converted per mole of –NH2
group (loading of –NH2 groups in amino functionalized mesoporous sil-
a
b
ica = 3.09 mmol g−1).
3.3. Mechanism of condensation
In Fig. 6 we have shown a pleasurable mechanism for the
base catalyzed Knoevenagel condensation of different aromatic

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J. Mondal et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 236–241
Fig. 6. Proposed mechanism for the Knoevenagel condensation over amino-functionalized mesoporous silica.
aldehydes with malononitrile. Here the –NH2 group of amino-
functionalized mesoporous silica picks up one proton from the
active methylene group of malononitrile, which is acidic in nature
due to presence of two strong electron withdrawing cyano groups.
The conjugate base generated here is stable due to conjugation with
the cyano groups. Now, this anion makes nuleophilic attack to the
carbonyl carbon atom of the substituted aromatic aldehydes. The ␤-
hydroxyl compound (carbon atom attached with the cyano group
designated as ␣) is generated when −ve charge over oxygen takes
up one proton from the catalyst and the catalyst is regenerated. The
␤-hydroxyl compound leads to the formation of ␣,␤-unsaturated
dicyano compounds via the elimination of one molecule
of water.
previously reported methods for the preparation of bicyclo [2.2.1]
hept-5-ene compounds.
These results suggest that strategy described herein can be uti-
lized for the synthesis of new bridge headed compound, which
can be further reacted to form desired target molecule. Moreover,
reduction of the cyano group leads to the formation of primary
amine group that can be easily condensed with any aldehydes and
generates the imine bond, which has a wide application in biolog-
ical chemistry as an antioxidant. For the reaction between DEM
and benzaldehyde over amino-functionalized mesoporous silica
[23] the product ester hydrolyzed and decarboxylate instantly. But
here as we have used malononitrile as active methylene compound,
only dicyano compounds are formed. Thus the catalytic reaction
described here provides the homologation of carbon, which is very
important in organic synthesis.
3.4. Diels–Alder cycloaddition reaction
The ␣,␤-unsaturated dicyano compounds obtained herein, can
behave as a good dienophile for [4 + 2] Diels–Alder cycloaddition
reaction [29,30] due to presence of two cyano groups, by which a
cyclic bridge headed compound can be obtained on reaction with a
conjugated diene (Fig. 4). When the ␣,␤-unsaturated dicyano com-
pound I (Fig. 4) reacts with cyclopentadiene observed yield of the
cyclic bridge headed compound II is 87%. For other aldehydes the
[4 + 2] Diels–Alder cycloaddition product yield varies from 76 to
85%. It is interesting to note that the products of Knoevenagel con-
densation catalyzed by amino-functionalized mesoporous material
are unsaturated dicyano compounds. These compounds undergo
facile [4 + 2] cycloaddition reaction with cyclopentadiene within a
few hours to generate 3-phenylsubstituted bicyclo [2.2.1] hept-5-
ene-2,2-carbonitrile derivatives (Table 2). It is pertinent to mention
that these types of bicyclic compounds are very rarely mentioned in
the literature. Only one synthetic methodology has been reported,
which is connected with hydroquinone derivative [31]. However,
it took very long time for the completion of reaction and crystal-
lization involved carcinogenic benzene solvent. Similar analogous
bicyclo [2.2.1] hept-5-ene derivatives are also synthesized in ben-
zene solvent [32]. Similar reaction has also been carried out under
ultrasound condition for 24 h for the completion of reaction [33].
Alternatively, the reaction can be performed in the presence of
catalytic amount of CuSO4 and ZnI2 [34]. But in our case no such
organic solvent or catalyst was needed and bicyclic compound was
obtained from only the removal of THF solvent. Further, the reac-
tion takes 4–7 h for the completion under noncatalytic conditions.
In this reaction hydroquinone acts as an anti-oxidant. It protects
the dienophiles to undergo rapid polymerization under the reaction
conditions. Thus our synthetic procedure is more efficient than the
4. Conclusions
From the above experimental results we can conclude that
amino-functionalized mesoporous silica synthesized through co-
condensation of APTES and TEOS is an efficient base catalyst in the
Knoevenagel condensation of different aromatic aldehydes with
malononitrile for the synthesis of ␣,␤-unsaturated dicyanides in
very good yields. ␣,␤-Unsaturated dicyano compounds obtained
herein, behaves excellent dienophile for the Diels–Alder cycloaddi-
tion reaction with cyclopentadiene under noncatalytic conditions,
which can be an efficient strategy for the synthesis of a bridge
headed functionalized target molecules.
Acknowledgement
JM and AM thank CSIR, New Delhi for their respective junior
research fellowships.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2010.11.039.
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