A new silica based substituted piperidine derivative catalyzed expeditious room temperature synthesis of homo and hetero bis-Knoevenagel condensation products

Article abstract of DOI:10.1016/j.catcom.2011.05.033

A new silica based piperidine derivative has been designed, synthesized and characterized by solid state carbon 13 CP MAS NMR, BET surface area analysis, IR, TGA studies, elemental analysis and pH experiment. This has been efficiently utilized as a recycl

Full text of DOI:10.1016/j.catcom.2011.05.033

Catalysis Communications 12 (2011) 1496–1502
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Catalysis Communications
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Short Communication
A new silica based substituted piperidine derivative catalyzed expeditious room
temperature synthesis of homo and hetero bis-Knoevenagel condensation products
⁎
Chhanda Mukhopadhyay , Suman Ray
Department of Chemistry, University of Calcutta, 92 APC Road, Kolkata-700009, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new silica based piperidine derivative has been designed, synthesized and characterized by solid state
carbon 13 CP MAS NMR, BET surface area analysis, IR, TGA studies, elemental analysis and pH experiment. This
has been efficiently utilized as a recyclable catalyst for both homo and hetero bis-Knoevenagel condensation
products in aqueous-ethanol. This is the first report of the synthesis of hetero bis-Knoevenagel condensation
products by our designed catalyst.
Received 29 March 2011
Received in revised form 25 May 2011
Accepted 31 May 2011
Available online 12 June 2011
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Silica based piperidine catalyst
Solid state carbon 13 CP MAS NMR
BET surface area analysis
TGA studies
Homo and hetero bis-Knoevenagel products
Aqueous-ethanol
1. Introduction
The non-formation of bis-hetero product with most of the earlier
reported catalysts is due to the enhanced reactivity of active
The increasing demands of environmental legislation promote the
chemical industries to minimize waste production in chemical
manufacture [1]. The use of heterogeneous catalyst, in particular
organosilane [2] having a covalently anchored organic spacer to create
organic–inorganic hybrid catalysts, have greatly developed in differ-
ent areas of organic synthesis due to their environmental compati-
bility combined with good yield and selectivities. In fact, the last
decade has witnessed a growing interest in the building up of
organic–inorganic hybrid catalysts using several types of supports and
immobilization strategies [3–8]. Our present work is directed to
immobilize 1-(2-chloroethyl)piperidine onto the silica support and to
apply it towards the synthesis of homo and hetero-bis-Knoevenagel
condensation products. Interestingly, 1-(2-chloroethyl)piperidine
chemistry may provide straightforward possibilities for catalyst
design, together with a simple immobilization procedure compared
to other less-functionalized organocatalysts.
In our present study, we report the formation of hetero-
Knoevenagel-dicondensation as well as homo-dicondensation prod-
ucts. This is the first report of the synthesis of hetero-Knoevenagel-
dicondensation products. Here lies the novel application of our
catalyst towards such synthesis. In addition, the synthesis of homo-
bis-Knoevenagel condensation products is very rare. It is mainly for
this reason that we took up the idea of synthesizing bis-homo and the
challenge of preparing hetero-Knoevenagel-dicondensation products.
methylene compounds towards the benzene-dicarboxaldehydes.
The same active methylene compound attacks at both aldehyde
functionalities simultaneously thereby resulting in the formation of
homo-dicondensation products. Our strategy was to decrease the
reactivity of active methylene compounds, so that, after addition of
one of the active methylene compounds, the mono-Knoevenagel-
condensation intermediate could be isolated. Then, adding the other
active methylene compound, it would be possible to prepare hetero-
dicondensation product. The subsequent discussion describes the
unique property of our catalyst over previously reported basic
catalysts in isolating the mono-Knoevenagel-condensation
intermediate.
2. Results and discussion
Scheme1: Onthebasisof thehigh advantagesof heterogeneous solid
catalysts, we designed the synthesis of the following new silica based
basic catalyst containing substituted piperidine as basic unit (Scheme 1).
2.1. Characterization of the catalyst
The catalyst (4) was characterized by solid state carbon 13 CP
MAS NMR (Fig. 1), BET analysis (Fig. 2), TGA studies, IR, elemental
analysis and pH experiment. The normal solution phase 13C NMR
spectra of 3-(mercaptopropyl)-trimethoxysilane (1) showed peaks
at δ 50.3 (–OMe), 27.3 (b,c) and 8.03 (a), and that of 1-(2-chloroethyl)
piperidine (3) showed peaks at δ 57.7 (d), 53.5 (f), 36.4 (e), 22.4 (g),
⁎
Corresponding author. Tel.: +91 33 23371104.
E-mail address: cmukhop@yahoo.co.in (C. Mukhopadhyay).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.05.033

C. Mukhopadhyay, S. Ray / Catalysis Communications 12 (2011) 1496–1502
1497
a
M eO
c
S i
M eO
S H
b
O M e
(1)
6(N)HCl
H 2O
O H
O H
O H
S iO 2
O
S iO 2
Dry toluene
Reflux,18h.
O H
Reflux for
24h.
d
e
C l
N
f
f
.H C l
g
g
h
a
c
O
(3)
S iO 2
S i
S H
O
O
b
Dry toluene
Et3N(excess)
(2)
Reflux, 18h.
a'
c'
d'
O
S iO 2
e'
S i
S
O
O
b'
N
f'
f'
+
E t3N .H C l
g'
(4)
g'
h'
Scheme 1. Preparation of silica based substituted piperidine catalyst (4).
21.5 (h). However, in the product (Fig. 1) obtained by the reaction
of 3-(mercaptopropyl)-trimethoxysilane (1) and 1-(2-chloroethyl)
piperidine (2), the peak at δ 57.7 (d) vanished and appeared at 24.0
along with (b, c, g, h) since bond formation has taken place through
carbon (d) and atom S and the peak at 36.4 for (e) showed slight
deviation and appeared at 34.4. The other peaks appeared more or
less in same position. When the solid state silica based substituted
piperidine catalyst (4) is prepared the peak at δ 50.3 for –OMe
vanished due to substitution of –OMe by the silanol group. In the
catalyst the remaining peaks for carbons showed very slight
deviations and appeared at δ 53.7 for (f′), 34.6 for (e′), a very broad
peak at 23.5 (b′, c′, d′, g′, h′), and at 11.8 for (a′). This confirms the
structure of the prepared silica based substituted piperidine catalyst.
In addition to structural confirmation, quantitative determination of
covalently anchored substituted piperidine group onto the surface of
catalyst (4) was performed by elemental analysis and ion-exchange pH
analysis and TGA. The elemental analysis of mercaptopropyl silica [9–
15] (2) showed the carbon and sulfur content to be 2.49% and 2.15%
respectively [16]. From this 0.67 mmol/g loading of the mercaptopropyl
group on the silica surface is obtained. In the silica based substituted
piperidine catalyst (4) the carbon, sulfur and nitrogen content was
found to be 7.71%, 2.05% and 0.90% respectively which corresponds to a
loading of 0.64 mmol/g. Therefore 95.55% conversion of the mercapto
group to the S-substituted piperidine unit is achieved. To check the basic
property of the prepared catalyst (4), the nitrogen atom of piperidine
unit in the catalyst (4) was protonated with diluted HCl (details in the
Fig. 1. Comparison solid state carbon-13 CP MAS NMR spectrum of the prepared catalyst (4) with the homogeneous analog of the grafted unit of catalyst (4).

1498
C. Mukhopadhyay, S. Ray / Catalysis Communications 12 (2011) 1496–1502
size or pore area and (iii) particle size measurement. The Brunnauer–
Emmett–Teller surface area was obtained using N2 adsorption–
desorption isotherm (Fig. 2) and found out to be 116 m2/g. Pore size
and pore volume is calculated to be 11.6 nm and 0.3256 cm3/g.
Scheme 2: As a part of our research program concerning the
Knoevenagel condensation [17] we report here, a very simple and
efficient method in which a variety of active methylene compounds
were allowed to react either with benzene-1, 4-dicarboxaldehyde or
with benzene-1, 3-dicarboxaldehyde in 2:1 mole ratio (active
methylene compounds:dialdehyde) and 5 mg of silica based
substituted piperidine catalyst in aqueous-ethanol at room temper-
ature with stirring (Scheme 2).
a) N adsorption-desorption isotherm
2
SR- 4
Adsorption
200
Desorption
BET Surface Area = 116 m2g-1
150
100
50
It was exciting to observe that, except a few cases, all the reactions
occurred rapidly and were completed in few minutes giving excellent
yield of bis-Knoevenagel products. In most of the cases the products
precipitated out from the medium once their formation started.
0.2
0.4
0.6
0.8
1.0
Relative pressure [P/P0]
2.2. Role of catalyst
b) Pore size distribution curve
0.003
11.6 nm
Pore size distribution
(Cylindr. pore,
2.2.1. Our strategy
Initially we thought that, by control addition of one of the active
methylene compounds, it is possible to obtain the intermediate in
which one of the aldehyde functionality is free. The electrophilicity of
one free aldehyde group in the mono-Knoevenagel condensation
product would be much less than the di-aldehyde because of the net
extensive conjugation with the olefinic bond formed on one side. So in
presence of limited amount of one active methylene compound, it is
possible to prepare the mono Knoevenagel condensation intermedi-
ate where, one aldehyde group remains free. Then, on adding the
other active methylene compound and keeping it for some time, it
would be possible to get the bis-hetero-dicondensation products.
NLDFT adsorption branch
model)
Pore Volume = 0.3256 cc/g
0.002
0.001
0.000
10
20
30
40
50
60
70
2.2.2. Reason of failure with previously reported catalysts
Pore Width (nm)
Homogeneous bases are soluble in aqueous ethanol and behaved
as comparatively stronger bases and thus the active methylene
compounds attacked at both aldehyde functionalities and mono
Knoevenagel condensation intermediate cannot be isolated. Therefore,
the reaction failed with NaOH, NaOAc in presence of AcOH, piperidine,
ethylpiperidine, triethylamine, 1-(2-chloroethylpiperidine) etc.
Fig. 2. a) N2 adsorption–desorption isotherm, and b) Pore size distribution curve of
silica based substituted piperidine (4).
supplementary material) and when 1 g of the protonated species was
placed in 100 ml saturated NaCl solution, the pH of the resultant
solution dropped to 3.50 since ion exchange occurred between sodium
ions and protons. From this ion-exchange pH analysis the same catalyst
loading was obtained as that from elemental analysis. These results are
in good agreement with the result obtained from thermogravimetric
analysis from which 95.57% conversion of the mercapto group to the S-
substituted piperidine unit and a loading of catalyst of 0.67 mmol/g is
obtained.
2.2.3. Unique property of our catalyst over the previously reported
catalysts
Our catalyst is of moderate basicity, since it is a heterogeneous
catalyst in aqueous-ethanolic medium. This is the greatest advantage
of our catalyst (4) and without this catalyst we were unable to
synthesize the hetero-dicondensation product. Initially it generates
the carbanion donor from the active methylene compounds by
abstracting one proton from it. The carbanion thus generated
coordinate with sulfur atom of the heterogeneous catalyst (4) or
Physical characterization of silica surface was done by (i) specific
surface area (m2 g−1), (ii) specific pore volume; distribution of pore
O
O Si
O
SiO2
S
X
N
H
E
CHO
NC
(5 mg)
X
NC
(6)
(4)
Y
+
+
NC
H
CHO
[EtOH (2ml)+H2O(2ml)]
room temperature, stirring
E
(7)
(1 mmol)
(sequential addition)
(5)
(1mmol)
NC
(1 mmol)
(8)
Y
Scheme 2. Synthesis of bis-homo and hetero-Knoevenagel condensation products [except for Table 1, entries 10, 11 where, either (6) or (7) is rhodanine] (for various X and Y, refer
to Table 1).

C. Mukhopadhyay, S. Ray / Catalysis Communications 12 (2011) 1496–1502
1499
forms salt like [\R2NH]+[CH(CN)2]− with the protonated nitrogen of
the catalyst (4) and thereby goes out of the homogeneous reaction
medium (Fig. 3). Therefore, the concentration of carbanion cannot
increase largely in the homogeneous reaction medium and this
renders the isolation of mono-Knoevenagel-condensation intermedi-
ate possible.
The desired product was obtained in excellent yield and at room
temperature only with catalyst (4). So, this proves that, a combined
effect of silica and the side chain is essential for the catalytic activity.
To study the performance of other catalysts the reaction of benzene-1,
4-dicarboxaldehyde with cyanoacetamide and ethylcyanoacetate was
taken as model reaction and the results are summarized in Table 2.
Again, the contribution from the inorganic porous structure of silica
to the formation of the mono-Knoevenagel condensation intermediate
is also justified. Knoevenagel condensation involves aldehydes and
products both having relatively large dimensions and solid catalysts
having small pores would impose a shape selective limitation to the
adsorption and desorption of the bulky molecules, [18] which then
makes the reaction slower. Due to the slow reaction rate the active
methylene compounds do not attack at both aldehyde functionalities
and mono-Knoevenagel condensation intermediate can be isolated.
The intermediate of rhodanine (Table 1, entry 10) with benzene-1,
4-dicarboxaldehyde has been isolated and characterized.
The model reaction was also carried out separately in the presence
of silica or MPS (2). No product was observed with silica as catalyst.
For the other case, the final product was obtained in very low yields.
2.2.4. Mechanism
Here, the surface basic sites of the catalyst produce carbanion from
active methylene compounds which attack on the aldehyde and form
condensation product by simple neucleophillic substitution reac-
tion [19]. However some notable features involving heterogeneous
catalyst and the choice of solvent must be addressed. Here, the active
methylene compounds and aldehydes remain in the homogeneous
reaction medium in aqueous-ethanol. Since, catalyst is heteroge-
neous, it remains out of the homogeneous reaction medium. Here,
water brings the active methylene compounds to the lone pair of
electrons of piperidine nitrogen of the catalyst through hydrogen-
bonding (Fig. 3) and thereby favoring the ionization into carbanion
donor [20,21]. The carbonyl oxygen coordinates with the silanol [21]
O
[CH(CN)X]
NHR2
O
O
Si
S
OR
S
O
H
H
N
OH
O
NR2
H
O Si
O
S
C
O
O
O
NHR2
CH
Si
OH
X
CH(CN)X
OH
Water brings the active methylene compound
towards the lone pair of electron of nitrogen.
The carbanion forms salt like this or coordinate
with sulphur and thereby goes out of the
homogeneous reaction medium with the catalyst.
CHO
CHO
R
N
H
R
O
O
O
O
NR2
Si
H
S
O
O
Si
H
S
X
O
O
C
H
H
OH
O
CN
C
H
C
X
CH
CN
CHO
CHO
-H2O
X
C
CN
X
C
H
NC
+
H
NC
C
C
Y
O
O
O
NR2
Si
S
The same sequence of
steps are repeated.
CHO
C
CN
C
H
Y
OH
Solid lines are pure covalent bonds and dotted lines are H-bond or other weak interactions.
Fig. 3. Mechanism showing the role of catalyst in the bis homo and hetero Knoevenagel dicondensation product formation.

1500
C. Mukhopadhyay, S. Ray / Catalysis Communications 12 (2011) 1496–1502
Table 1 (continued)
Table 1
Synthesis of homo and hetero-bis-Knoevenagel condensation products with solid silica
based substituted piperidine derivative catalyst (4).
Entry products
Diastereo
selectivity
Time
(min)
Yields (%)
isolated
References
Entry products
Diastereo
selectivity
Time
(min)
Yields (%)
isolated
References
CONH2
CN
1
–
12
97
[22]
H
CN
H
CN
6
Ea
30
94
–
H
NC
H NOC
2
CN
H
CN
CO2Et
CN
H
CN
CN
H
7
8
9
Ea
30
35
50
90
85
92
–
2
–
13
97
[25]
H
NC
NC
CN
H
CN
CO2Et
CN
H
CONH2
CN
H
Ea (92%)
[24]
3
Ea (52.7%)
Za (47.3%)
Ea (56.5%)
15
94
–
NC
H
CO2Et
NC
H
CN
CONH2
CN
H
CN
Ea
–
H
CONH2
NC
H
CO2Et
NC
H
CN
CN
CN
H
CN
CN
H
10
Ea
25
94
–
O
4
15
93
–
H
H
HN
S
S
H2NOC
CN
CONH
CN
CN
2
H
H
CN
Za (43.5%)
H
11
Ea
40
85
–
NC
CONH2
O
H
S
HN
CONH2
CN
H
H NOC
CN
2
5
Ea
30
95
[23]
a
The E/Z ratio was obtained by 1H NMR integration of the crude reaction products.
C
H
CONH2

C. Mukhopadhyay, S. Ray / Catalysis Communications 12 (2011) 1496–1502
1501
Table 2
Study of performance of different catalysts towards hetero-dicondensation product formation.a
Entry
Catalysts
Time (min.)
Product yieldsb (%)
CONH2
CONH
2
CO2Et
H
H
H
NC
NC
NC
CN
H
CN
CO2Et
CO Et
2
H
H
CONH2
CN
Entry 5
Entry 9
Entry 8
1
2
3
4
5
6
7
8
9
NaOH (1 eqv.)
NaOAc (1 eqv)
25
30
35
35
36
40
2 h
60
50
41
38
47
36
34
20
0
42
37
48
12
13
22
0
0
0
0
34
33
40
0
17
92
Piperidine (0.2 eqv)
Ethylpiperidine (0.2 eqv)
Et3N (0.2 eqv)
1-(2-chloroethyl)-piperidine (0.2 eqv)
Silica (5 mg)
MPS (5 mg)
25
0
22
0
cSilica based substituted catalyst (4) (5 mg)
a
Reaction condition: Benzene-1,4-dicarboxaldehydes (1 mmol), cyanoacetamide (1 mmol), ethylcyanoacetate (1 mmol), different catalysts, aqueous ethanol [(2+2 ) ml], room
temperature, stirring.
b
Isolated yield.
c
The TON for silica based substituted piperidine catalyst (4) (Table 2, entry 9) is 287.5 and TOF is 345/h.
group on silica surface increasing the electrophilicity of the carbonyl
carbon and thereby making it possible to carry out the reaction at
room temperature.
also the reaction time is extended (Table 4). This is probably due to
loss of some amount of catalyst in the time of filtration.
This is the first report of the synthesis of the bis-hetero-Knoevenagel
product. Both homo and hetero-bis-Knoevenagel products could be
prepared in excellent yields and thus the protocol is a very general one.
In this context, we would like to mention that due to amorphous
nature of all the products, it was not possible to do X-ray crystal
structure analysis of any of them.
2.2.5. Study on optimization of catalyst loading and choice of solvent
To study the effect of catalyst loading on Knoevenagel condensa-
tion the reaction of benzene-1, 4-dicarboxaldehyde with malononi-
trile was chosen as model reaction. The results show clearly that silica
based substituted piperidine (4) is an effective catalyst for this
transformation and 5 mg of the catalyst was the optimum usage under
this condition and the yields did not increase largely with higher
amount of catalyst. It should be noted that, the yield was best with
EtOH-water [(2+2)ml]. The yield decreased substantially when the
reaction was conducted in the presence of other solvents, such as
CH2Cl2, ACN, and THF etc. (Table 3).
The NMR spectra of some selected compounds are given in the
Supplementary material.
3. Conclusion
In conclusion, we have shown that our designed and synthesized
silica based substituted piperidine (4) prepared from commercially
available and cheap starting materials, catalyzed very efficiently the
formation of both the bis-homo and hetero-Knoevenagel condensa-
tion products at room temperature. The catalyst shows high thermal
stability and was recovered and reused without any noticeable loss of
activity. Mild reaction condition, simplicity of procedure, general
applicability, high yields makes this method practical for multistep
synthesis.
2.2.6. Recycling experiment
The possibility of recycling the catalyst was examined using the
reaction of benzene-1, 4-dicarboxaldehyde with cyanoacetamide
under optimized conditions. The recycled catalyst could be used at
least eight times without any further treatment. However yield
decreased to 84% in 8th run in comparison with 94% in first run and
Table 3
Optimization of the amount of silica based substituted piperidine catalyst (4) under various solvents for the synthesis of 2-cyano-3-[4-(2,2-dicyano-vinyl)-phenyl]-acrylonitrile
(Table 1, entry 1).a
Entry
Amount of catalyst (mg)
Solvent (ml)
Time (min)
Yieldsb (%)
TON
TOF/h
1
2
3
4
5
6
7
8
0
01
02
03
04
05
08
10
20
05
05
05
05
[H20 (2)+EtOH (2)]
[H20 (2)+EtOH (2)]
[H20 (2)+EtOH (2)]
[H20 (2)+EtOH (2)]
[H20 (2)+EtOH (2)]
[H20 (2)+EtOH (2)]
[H20 (2)+EtOH (2)]
[H20 (2)+EtOH (2)]
[H20 (2)+EtOH (2)]
CH2Cl2 (5)
10
10
10
10
10
10
10
10
10
10
10
10
10
0
23
44
66
80
92
92
92
91
43
50
50
64
–
–
71.88
431.25
825.00
1237.5
1500.00
1725.00
1725.00
1725.00
1706.25
806.25
937.50
937.50
1200.00
137.50
206.25
250.00
287.50
287.50
287.50
284.34
134.34
156.25
156.25
200.00
9
10
11
12
13
THF (5)
ACN (5)
EtOH (5)
a
Reaction conditions: Benzene-1,4-dicarboxaldehyde (1 mmol), malononitrile (2 mmol), room temperature, different solvents, stirring.
Isolated yield.
b

1502
C. Mukhopadhyay, S. Ray / Catalysis Communications 12 (2011) 1496–1502
Table 4
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5
6
7
8
30
30
33
35
35
38
39
43
95
91
91
89
89
88
85
84
a
Reaction condition: Benzene-1,4-dicarboxaldehydes (1 mmol), cyanoacetamide
(2 mmol), catalyst (4) (5 mg), aqueous ethanol [(2+2) ml], room temperature,
stirring.
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Acknowledgements
One of the authors (SR) thanks the Council of Scientific and
Industrial Research, New Delhi for his fellowship (JRF). We thank the
CAS Instrumentation Facility, Department of Chemistry, University of
Calcutta for spectral data. We also acknowledge grant received from
UGC funded major project, F. no. 37-398/2009 (SR) dated 11-01-2010.
Moreover, NMR Research Centre, IISc, Bangalore-560012 is gratefully
acknowledged for the solid state carbon-13 CP-MAS NMR spectrum.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at doi:10.
1016/j.catcom.2011.05.033.