Immobilization of 1,5,7-triazabicyclo [4.4.0] dec-5-ene over mesoporous materials: An efficient catalyst for Michael-addition reactions under solvent-free condition

Article abstract of DOI:10.1016/j.apcata.2011.03.010

Immobilization of 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD, a bicylic guanidine base) over mesoporous material like SBA-15 has been found to be an excellent catalyst for Michael-addition of β-nitro styrene with malonate. The reactions were performed und

Full text of DOI:10.1016/j.apcata.2011.03.010

Applied Catalysis A: General 397 (2011) 250–258
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Applied Catalysis A: General
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Immobilization of 1,5,7-triazabicyclo [4.4.0] dec-5-ene over mesoporous
materials: An efficient catalyst for Michael-addition reactions under
solvent-free condition
Pranjal Kalitaa,∗,1, Rajiv Kumarb
a Iowa State University, Department of Chemistry, Ames, Iowa 50011, USA
b Innovation Center, TATA Chemicals Ltd., Ghotavde Phata, Urawde Road, Pirangut Industrial Area, Gat No 1139/1, Mulshi, Pune 412108, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Immobilization of 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD, a bicylic guanidine base) over mesoporous
material like SBA-15 has been found to be an excellent catalyst for Michael-addition of ␤-nitro styrene
with malonate. The reactions were performed under solvent-free condition at 373 K for 9 h. A wide variety
of Michael donors and acceptors were investigated. Among them, high yield of Michael product was
obtained for the reaction between p-Cl-nitrostyrene with dimethyl malonate.
Received 13 November 2010
Received in revised form 2 March 2011
Accepted 5 March 2011
Available online 12 March 2011
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Mesoporous materials
1,5,7-triazabicyclo [4.4.0] dec-5-ene
Michael-addition
␤-nitro styrene and malonate
1. Introduction
Michael-addition to nitroalkenes has been developed as a powerful
tool in organic synthesis, because Michael adducts are versatile
There are many organic superbases [1] such as 1,5,7-
triazabicyclo [4.4.0] dec-5-ene (TBD), 3,4,6,7,8-hexahydro-1-
methyl-2H-pyrimido [1,2-a] pyrimidine (MTBD), 1,4-diazabicyclo
[2,2,2] octane (DABCO), 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU)
and tetramethylguanidine (TMG), etc. Among the various organic
superbases, 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD) has been
widely utilized in organic synthesis because of its high pKa value
[2]. For example, TBD promotes various organic reactions such
as Wittig reaction [3], nitroaldol (Henry) reaction [4], dialkyl
phosphate addition to carbonyl compounds [4] and the addi-
tion of azoles to ␣, ␤-unsaturated nitriles and esters [5], and
Baylis–Hillman reactions [6].
Several authors have reported Michael-addition of ␤-
nitrostyrene to malonates [7], 1,3-dicarbonyl compounds [8],
ketones [9], aldehydes [10], N-heterocycles [11] and indoles
[12], using homogeneous catalyst. Very recently, Sartori reported
[13] the similar type of reaction using silica supported organic
bases including TBD under continuous-flow system condition. The
building blocks for agricultural and pharmaceutical compounds.
For example, the Michael-addition of ␤-nitrostyrene to diethyl
malonate produced Michael adducts.
The immobilization of 1,5,7-triazabicyclo [4.4.0] dec-5-ene
(TBD) in MCM-41 was first reported by Jacobs and co-workers
and then catalyst was examined for Michael-addition of ␣, ␤-
unsaturated ketone with malonate and Knoevenagel condensation
[14]. Recently, immobilization of 1,5,7-triazabicyclo [4.4.0] dec-
5-ene (TBD) in SBA-15 was reported by reported by Srivastava
and then catalytic activity was examined for Mukaiyama-aldol and
Michael reaction [15].
This paper deals with an effort towards developing green pro-
tocol for synthesis of diethyl 2-(3-nitro-phenylethyl) malonate and
its derivatives by Michael-addition of ␤-nitrostyrene (nitro-alkene)
and malonate using TBD immobilized mesoporous MCM-41/SBA-
15 catalysts. The use of single step solventless conditions in
combination with heterogeneous catalysts represents one of the
main aspects of green chemical methods. For catalytic model reac-
tion, ␤-nitrostyrene and diethyl malonate was chosen as starting
material as shown in reaction Scheme 1. The effect of different
mesoporous materials as support, effect of amount of catalyst,
effect of temperature, recyclability of the catalyst and different sub-
strates have been studied in this paper. A wide variety of Michael
donors and acceptors were investigated. Among them, high yield
of Michael product was obtained for the reaction between p-Cl-
∗
Corresponding author.
E-mail addresses: pkalita@iastate.edu (P. Kalita),
rajiv.kumar@tatachemicals.com (R. Kumar).
1
Work carried out at Catalysis Division, National Chemical Laboratory, Pune
411008, India.
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.03.010

P. Kalita, R. Kumar / Applied Catalysis A: General 397 (2011) 250–258
251
Scheme 1. Michael-addition of ␤-nitrostyrene and diethyl malonate.
nitrostyrene with dimethyl malonate. The product was successfully
isolated by column chromatography and identified by 1H NMR
spectroscopy.
triazabicyclo [4.4.0] dec-5-ene in MCM-41 [14,19] and SBA-15
material, 3 g of vacuum dried MCM-41 and SBA-15 was allowed
to react with 4.5 mmol of 3-glycidoxy propyl trimethoxy silane
(Scheme 2) in dry toluene at refluxing temperature for 24 h. Then,
the samples were cooled to room temperature, filtered, washed
with acetone and dried. This material is designated as glycidy-
lated MCM-41/SBA-15 (Scheme 2). After that, the glycidylated
MCM-41/SBA-15 (1.0 g) was allowed to react with 2.2 mmol of
1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD) in dry toluene (15 ml) at
298 K for 10 h. The excess TBD was removed by soxhlet extraction
with DCM. The sample was designated by MCM-41/SBA-15-TBD
(Scheme 2) and stored under vacuum.
2. Experimental
2.1. Materials
Fumed silica (surface area = 384 m2 g−1, Sigma Aldrich, USA),
poly(ethylene glycol)-block-poly(propylene glycol)-poly(ethylene
glycol) (P123, average molecular weight 5800, Aldrich, USA),
tetraethyl orthosilicate (TEOS, Aldrich, USA) were employed as
a starting material. Cetyltrimethylammonium bromide (CTMABr,
Loba Chemie, India) was used as a structure directing agent,
a 25 wt% aqueous solution of tetramethylammonium hydroxide
(TMAOH, Loba Chemie, India), 3-glycidoxypropyl trimethoxysilane
(Aldrich, USA), 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD, Aldrich,
USA) were used without further purification.
2.5. General procedure for Michael-addition of ˇ-nitrostyrene to
malonate
The catalytic liquid-phase reaction was performed in a two-
necked round bottom flask with
a water condenser under
vigorously stirring in N2 atmosphere. The catalyst was preacti-
vated at 393 K in a vacuum oven and subsequently used for the
reactions under dry conditions. In a typical procedure, a mixture
of ␤-nitrostyrene (10 mmol) and diethyl malonate (10 mmol) was
added to a preactivated catalyst (0.2 g). The reaction mixture was
stirred magnetically at 373 K for a period of 12 h. The progress
of the reaction was monitored by gas chromatography (Varian
model-CP-3800) equipped with capillary column and flame ioniza-
tion detector (FID) as well as by thin layer chromatography (TLC).
After completion of the reaction, the catalyst was separated by
centrifugation. The filtrate was concentrated and corresponding
product was purified through column chromatography using sil-
ica gel (100–200 mesh), petroleum ether: ethyl acetate (3:1) and
confirmed through GC, GC-MS, 1H NMR, 13C NMR. The 1H NMR
spectra were recorded in a 200 MHz using CDCl3 as solvent.
1H NMR (200 MHz, CDCl3) ı ppm: 1.03 (t, 3H), 1.25 (t, 3H),
3.84 (d, 1H), 4.04 (q, 2H), 4.16–4.26 (m, 3H), 4.88–4.98 (d q, 2H),
7.20–7.31 (m, 5H).
2.2. Synthesis of Si-MCM-41 material
The hydrothermal synthesis was carried out in autoclave
according to reported procedure [16]. The molar gel composition of
the synthesis gel was: 1 SiO2:0.30 TMAOH:0.25 CTMABr:125 H2O.
In a typical synthesis of a Si-MCM-41 sample, 3.0 g of fumed silica
was slowly added to 5.47 g of TMAOH (25 wt%) in 10.0 g of water
under vigorous stirring. Subsequently, an aqueous solution of 4.55 g
of CTMABr dissolved in 30.0 g of water. The remaining 72.5 g of
water was added and the stirring was continued for 15 min. Finally,
the synthesis gel was taken in teflon-lined auto-clave for 48 h. The
materials thus obtained were filtered, washed thoroughly first with
deionized water and then with acetone, and dried at 353 K. All the
samples were calcined at 773 K for 8 h in the presence of air.
2.3. Synthesis of SBA-15 material
13C NMR (100 MHz, CDCl3) ı ppm: 13.7, 13.9, 42.9, 54.9, 61.9,
62.1, 77.2,77.6, 128.0, 128.3, 128.9, 136.2, 166.8, 167.4.
Mesoporous silica SBA-15 was synthesized according to
the reported procedure [17,18]. In a typical synthesis, 10 g of
amphiphilic triblock copolymer, poly(ethylene glycol)-block-
poly(propylene glycol)-block-poly(ethylene glycol) (average
molecular weight = 5800, Aldrich Co.), was dispersed in 75 ml of
water and 300 ml of 2 M HCl solution while stirring. 21.25 g of
tetraethyl orthosilicate (TEOS, Aldrich Co.) was added to it. The
gel was continuously stirred at 313 K for 24 h, and then finally
crystallized in a teflon-lined autoclave at 373 K for 48 h. After
hydrothermal treatment, the solid product was filtered, washed
with deionized water, acetone and dried in air at room tempera-
ture. The solid product (SBA-15) was calcined in air at 773 K for
6 h.
2.6. Characterization techniques
The powder X-ray diffractograms of as-synthesized and calcined
samples were recorded on a Rigaku Miniflex◦diffractometer (Cu-K␣
˚
radiation, ꢀ = 1.54054 A) in 2ꢁ range 1.5–10 at a scanning rate of
1◦ min−1 for MCM-41, and 1.5–60◦ at a scanning rate of 2◦ min−1 for
MCM-41. The PXRD data for SBA-15 materials were collected on a
PAN alytical X’pert Pro instrument using Bragg-Brentano geometry
in 2ꢁ ranges 0.5–5.0◦ at a scanning rate of 1◦ min−1 (ꢀ = 1.5416 A).
˚
The specific surface area (SBET) and mesoporosity were checked
by N2 sorption at 77 K using NOVA 1200 Quantachrome equip-
ment. The samples were evacuated at 573 K before N2 sorption.
The surface area was calculated from linear part of BET (Brunauer-
Emmet-Teller) equation and the method of Barret-Joyner-Halenda
(BJH) was employed to determine the pore-size distribution (PSD).
Elemental analysis for C, H and N to measure the TBD loading in the
catalysts were recorded by an EA 1108 elemental analyzer (Carlo-
2.4. Immobilization of 1,5,7-triazabicyclo [4.4.0] dec-5-ene in
MCM-41 and SBA-15 material
Siliceous SBA-15 and MCM-41 were obtained by the above
procedure and the solid residual templates were removed by
calcinations. In a typical procedure for immobilization of 1,5,7-

252
P. Kalita, R. Kumar / Applied Catalysis A: General 397 (2011) 250–258
Scheme 2. Immobilization of 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD) over MCM-41 and SBA-15 materials.
√
alysts were recorded by an EA 1108 elemental analyzer (Carloba
instruments).
tion a0 = 2d1 0 0
/
3 (Table 1) is in good agreement with the values
reported by others authors [17,18].
Fourier transform infrared (FTIR) spectra recorded for functional
group detection in the range of 400–4000 cm−1 on a Shimadzu FTIR
8201 PC and KBr pallet was taken as reference. The IR spectra were
recorded at different temperatures and 300 scans were collected
for each spectrum.
The scanning electron microscopic (SEM) images were recorded
on a Philips Model XL 30 instrument. The samples were loaded on
stubs and sputtered with thin gold film to prevent surface charg-
ing and also to protect from thermal damage from the electron
beam, prior to scanning. The samples were dispersed on Holey
carbon grids and Transmission Electron Microscopic (TEM) images
were scanned on a JEOL Model 1200 EX instrument operated at an
accelerating voltage of 100 kV.
The solid state MAS and CP MAS NMR spectra were recorded
on a Bruker MSL 300 NMR spectrometer. The finely powdered
samples were placed in 7.0 mm zirconia rotors and spun at
8 kHz. The resonance frequencies of 29Si and 13C were 59.63,
and 75.47 MHz, respectively. The chemical shifts were determined
using 3-(trimethylsilyl) propane-1-sulfonic acid (ı = 0 ppm from
TMS) and adamantane (ı = 28.7 ppm from TMS) as the reference
compounds for 29Si and 13C, respectively.
3.2. Porosity measurements
Fig. 2 shows N2 adsorption–desorption isotherm and the cor-
responding pore size distribution curve for the calcined SBA-15
and SBA-15-TBD samples. The nitrogen adsorption/desorption
isotherms of both the samples are of type IV isotherm (Fig. 2)
and exhibit a H1 hysteresis loop, which is typical of mesoporous
solids [17,18]. Furthermore, the adsorption branch of each isotherm
showed a sharp inflection at a relative partial pressure value of
about 0.55–0.64. This is the characteristic of capillary condensation
within uniform pores. The position of the inflection point indicates
mesopore structure, and the sharpness of these steps indicates
the uniformity of the mesopore size distribution. A good match
between the points of inflection on the adsorption branch of both
isotherms suggests that samples have similar pore sizes (Fig. 2). The
low mesopore volume was observed in case of SBA-15-TBD because
the mesopores were partially blocked by functionalized organic
base (Table 1). The organic-functionalized materials show some
loss of surface area and a pronounced reduction in the pore volume.
˚
The calculated ‘pore diameter’ is reduced from a value of 27.6 A and
˚
˚
˚
67.1 A in the parent MCM-41 or SBA-15 to about 26.0 A and 58.5 A
in the MCM-41-TBD or SBA-15-TBD catalysts, respectively.
3. Result and discussion
3.3. Elemental microanalyses
3.1. Powder X-ray diffraction
The results microanalyses of carbon, hydrogen and nitrogen
contents of the catalysts are shown in Table 1. The nitrogen content
was assigned to the loading of TBD over MCM-41/SBA-15 materi-
als. The output amount was calculated with respect to nitrogen
content.
The X-ray diffraction patterns of (a) MCM-41 and (b) MCM-41-
TBD materials are shown in the Fig. 1. The typical hexagonal phase
(p6mm) of MCM-41 [main (1 0 0) peak with weak (1 1 0), (2 0 0)
and (2 1 0) reflections] is clearly visible in all the samples. These
results indicate the reflection of highly ordered mesoporosity even
after incorporation of organic functional group. However, a slight
decrease in the peak intensities was observed in the case of the
organo base functional group (TBD) loaded samples, which might
be due to partial filling of organic group inside the mesopores.
Fig. 1 shows the XRD profiles of pure SBA-15 and organo base
functionalized SBA-15 materials. All the samples showed very
similar XRD patterns. The samples showed three well-resolved
diffraction peaks due to (1 0 0), (1 1 0) and (2 0 0) reflection in the 2ꢁ
range of 0.5–5◦ that could be indexed according to a 2D hexagonal
p6mm symmetry [17,18]. Organic base incorporation did not alter
the long-range ordering of the mesoporous structure. Inter-planar
spacing (d1 0 0) and unit cell parameter (a0) of various functional-
ized SBA-15 materials are listed in Table 1. The d-spacing (d1 0 0),
estimated from the position of the low-angle peak is in the range
of 9.8–10.4 nm. The unit cell parameter calculated using the equa-
3.4. FTIR-spectroscopy
Fig. 3 represents the FTIR spectra of (a) SBA-15, (b) glyci-
dated SBA-15 and (c) SBA-15-TBD catalysts. The FTIR spectroscopy
confirms the presence of the organic groups. The peaks around
1230, 1090, 804 and 465 cm−1 are the typical Si
O Si band
attributed to the condensed silica network present in all sam-
ples. The Si OH vibration band at 970 cm−1 decreases after the
first grafting suggesting the successful anchoring reaction between
Si OH and 3-glycidoxypropyl trimethoxysilane. The vibrational
frequency of Si OH decreases from 970 to 952 cm−1 by organo-
functionalization of 3-glycidoxypropyl trimethoxysilane and TBD
over calcined SBA-15 sample. The IR peaks at 3325 cm−1 and
3300 cm−1 are due to free N H bond. In the case of TBD func-

P. Kalita, R. Kumar / Applied Catalysis A: General 397 (2011) 250–258
253
4000
3500
3000
2500
2000
1500
1000
500
3500
a
A
B
3000
a
b
2500
b
2000
1500
1000
500
0
0
0
2
4
6
8
10
0
1
2
3
4
5
2θ (Degree)
2θ (Degree)
Fig. 1. Powder X-ray diffraction pattern of A: (a) MCM-41 and (b) MCM-41-TBD and B: (a) SBA-15 and (b) SBA-15-TBD catalysts.
Table 1
Physiochemical properties of MCM-41/SBA-15-TBD catalysts.
a
SBET (m2/g)
d100 (A)
ao (A)
Pore diameter
(A)
Pore volume
˚
˚
Catalysts
TBD (mmol
input)
Elemental analysis
TBD (mmol)
(output)
(cm3/g)
˚
C
H
N
MCM-41
MCM-41-TBD
SBA-15
–
2.2
–
–
3.3
–
–
0.92
–
980
363
733
391
37.9
37.5
96.6
91.6
43.8
43.4
111.7
105.9
27.6
26.0
67.1
58.5
1.01
0.25
0.95
0.52
11.9
16.0
3.9
4.1
SBA-15-TBD
2.2
3.8
0.97
√
/ 3.
a
a0, unit cell parameter = 2d100
tionalized SBA-15 sample, these peaks are absent indicating that
TBD is attached with porous material. Characteristic peaks, due to
and C N bonds mixed with the C H bending modes of methylene
groups appear in the 1400–1270 cm−1 range. The absorption bands
in the 1100–880 and 800–450 cm−1 regions are connected with
C
H stretching vibrations of propyl spacer and cycloalkane ring,
were appeared in the range of 2863–2950 cm−1. The spectrum c
stretching and deformational skeleton modes of rings and O H
displays two new peaks at 1541 cm−1 and 1465 cm−1, which are
deformation mode at 1300–1400 cm−1
.
attributed to C C and C N bands, respectively. The peak of C
N
is usually observed at 1000–1300 cm−1. In the 1236–1310 cm−1
regions, the dominant contribution to the frequencies of the vibra-
tional forms in which the different deformations of C H bonds of
3.5. Scanning electron microscopy
Fig. 4 represents the scanning electron microscopy (SEM)
images of calcined SBA-15 sample (A) and TBD immobilized SBA-15
methylene groups occurs are located. The stretching modes of C
C
5
Adsorption
Desorption
700
600
500
400
300
200
100
0
A
B
4
a
b
a
3
2
1
0
b
0.0
0.2
0.4
0.6
0.8
1.0
0
100
200
300
Relative Pressure, P/P0
Pore Diameter (Å)
Fig. 2. (A) N2 adsorption–desorption isotherms and (B) corresponding pore size distribution curves for (a) calcined SBA-15 and (b) SBA-15-TBD catalysts.

254
P. Kalita, R. Kumar / Applied Catalysis A: General 397 (2011) 250–258
3.6. Transmission electron microscopy
c
Transmission electron microscopy (TEM) images of calcined (A)
SBA-15 and (B) SBA-15-TBD samples show well-ordered hexagonal
arrays of mesopores with 2D p6mm hexagonal structure (Fig. 5).
The ordered mesoporous structure of the SBA-15 was slightly
affected by functionalization of TBD over calcined SBA-15 sample
[15,17–18,20–23].
952
962
1465
1541
b
3.7. Solid state 29Si CP MAS NMR spectrum
806
970
a
Fig. 6 shows the 29Si MAS NMR spectrum of the SBA-15-TBD.
It was observed from NMR spectrum that the strong resonances
at ı ≈ −102 and −110 ppm are due to the presence of of Q3
[(SiO)3 Si OH] and Q4 [(SiO)3 Si
O Si ] species, respectively,
present in the silicate framework of SBA-15-TBD materials. The
presence of close-packed conformation of organic group is also
manifested from the spectrum of the SBA-15-TBD sample. The two
additional signals at ı ≈ −59 and −68 ppm are assigned to terminal
(T2) and cross-linked (T3) siloxane groups, respectively, attached
with pendant organic groups. These results suggest that the co-
condensation process, the organic functional groups are anchored
to the surface walls.
4000
3000
2000
1000
Wavenumber (cm-1)
Fig. 3. FTIR spectra of (a) SBA-15, (b) glycidated SBA-15 and (c) SBA-15-TBD cata-
lysts.
3.8. Solid-state 13C CP MAS NMR spectrum
The 13C CP MAS NMR spectrum of SBA-15-TBD catalyst is shown
in Fig. 7. The three distinct 13C signals observed at ı ≈ 9, 22 and
41 ppm, were assigned to C1, C2 and C3 atoms, respectively, of the
propyl chain attached with SBA-network (Scheme 2). The char-
acteristic peaks in the cycloalkane viz. C N (ı = 158 ppm (C10)),
sample (B). It consists of many rope-like domains with relatively
uniform sizes, which are aggregated into grain-like macrostruc-
tures [18,22,23]. After organofunctionalization by TBD, SBA-15
shows a similar particle morphology, which reflects the stability
of the macroscopic structure.
Fig. 4. SEM micrographs of (A) calcined SBA-15, and (B) SBA-15-TBD catalysts.
Fig. 5. TEM micrographs of (A) calcined SBA-15, and (B) SBA-15-TBD catalysts.

P. Kalita, R. Kumar / Applied Catalysis A: General 397 (2011) 250–258
255
-102
C
N bond (ı = 73 ppm, (C6, C4), ı = 46 ppm (C7, C9)) and C C bond
(ı = 22 ppm (C8)) were also observed (Fig. 7).
3.9. Catalytic activities
3.9.1. Effect of reaction time
-110
Fig. 8 A shows the effect of reaction time on conversion
of ␤-nitrostyrene over MCM-41-TBD and SBA-15-TBD catalyst
in Michael-addition of ␤-nitrostyrene with diethyl malonate
(Scheme
1 1to produce corresponding diethyl 2-(2-nitro-1-
-68
phenylethyl) malonate. Ethanol (EtOH) was used as a solvent
in these experiments at 353 K. Although, each experiment was
continued for ca. 12 h, there was only marginal increase in the con-
version after 9 h of the reaction. The individual experiments were
performed for MCM-41-TBD and SBA-15-TBD catalyst under N2
atmosphere. It was observed that the conversion of ␤-nitrostyrene
was reached 65 and 71% within 9 h over the MCM-41-TBD and
SBA-15-TBD catalyst, respectively (Fig. 8 A).
-59
-200
-150
-100
-50
0
Chemical shift (ppm)
The catalytic activity was also examined for Michael-addition
of ␤-nitrostyrene and diethyl malonate over (a) MCM-41-TBD and
(b) SBA-15-TBD catalyst under solvent free system at 353 K. The
reaction data are plotted as function of time in Fig. 8B. Under sol-
vent free system, 67 and 75% conversion of ␤-nitrostyrene were
obtained within 9 h over the MCM-41-TBD and SBA-15-TBD cata-
lyst, respectively (Fig. 8B). The selectivity towards the product over
both catalysts was found to be 100%. It was observed that there was
marginal increase in the conversion of ␤-nitrostyrene under sol-
vent free condition (vis-à-vis in the presence of solvent, ethanol)
over the MCM-41-TBD and SBA-15-TBD catalyst (Table 2). It was
observed that SBA-15-TBD catalyst exhibited higher activity com-
pared to that of MCM-41-TBD under same reaction (Table 1) and
this is because of the larger pore size of SBA-51 materials. Therefore,
for further studies were carried out using SBA-15-TBD catalyst.
Fig. 6. 29Si CP MAS NMR spectrum of SBA-15-TBD catalyst.
22
73
46
41
9
158
152
3.9.2. Effect of catalyst amount
To optimize the amount of catalyst for the Michael-addition
of ␤-nitrostyrene with diethyl malonate (Scheme 1), the reaction
was carried out under solvent free condition over SBA-15-TBD
catalyst for 12 h using varying amount of catalyst (3.5–13.3 wt%
with respect to ␤-nitrostyrene). The conversion of ␤-nitrostyrene
was found to increase from 35 to 81% when catalyst amount was
increased from 0.05 (3.3 wt%) to 0.2 g (13.3 wt%) (Fig. 9curves a–d).
200
150
100
50
0
-50
Chemical shift (ppm)
Fig. 7. 13C CP MAS NMR spectrum of SBA-15-TBD catalyst.
90
90
A
B
80
70
60
50
40
30
20
10
0
80
b
b
a
70
60
50
40
30
20
10
a
0
0
3
6
9
12
15
0
3
6
9
12
15
Reaction time (h)
Reaction time (h)
Fig. 8. Effect of reaction time on conversion for Michael-addition of ␤-nitrostyrene and diethyl malonate over (a) MCM-41-TBD and (b) SBA-15-TBD catalyst with (A) ethanol
as solvent at 353 K and (B) no solvent at 353 K.

256
P. Kalita, R. Kumar / Applied Catalysis A: General 397 (2011) 250–258
Table 2
90
Michael-addition of ␤-nitrostyrene with diethyl malonate over MCM-41-TBD and
SBA-15-TBD catalyst at different temperaturea.
d
c
Catalysts
Solvent
Temperature (K)
Conv.b (mol%)
TONc
75
60
45
30
15
MCM-41-TBD
MCM-41-TBD
SBA-15-TBD
SBA-15-TBD
SBA-15-TBD
SBA-15-TBD
SBA-15-TBD
Ethanol
No solvent
Ethanol
No solvent
No solvent
No solvent
No solvent
353
353
353
353
373
333
298
63
67
71
75
81
50
23
33.9
36.0
36.3
38.4
41.4
25.6
11.7
b
a
a
Reaction condition: ␤-nitrostyrene (10 mmol), malonate (10 mmol), reaction
time 9 h, catalyst amount = 0.2 g.
b
Conversion (Conv.) with respect to ␤-nitrostyrene and based on GC analysis.
TON is given as moles of ␤-nitrostyrene transformed per mole of nitrogen.
c
The product selectivity was always obtained ca. 100%. It is rea-
sonable to assume that the interaction of the reactant molecules
with the active sites of the SBA-15-TBD catalyst is commensurate
with increase in the amount of catalyst in the reaction mixture
and hence the availability of a higher number of active sites. How-
ever, the reaction showed only marginal increase in the conversion
from 76 to 81% with further increasing the catalyst amount from
0.15 to 0.20 g (curves c and d). Hence, about 10 wt% catalysts is
required amount for significant Michael-addition of ␤-nitrostyrene
with diethyl malonate.
0
0
3
6
9
12
15
Reaction time (h)
Fig. 10. Plot of conversion vs reaction time for Michael-addition of ␤-nitrostyrene
with diethyl malonate over SBA-15-TBD catalyst at different temperature. Curves
(a) 298 K, (b) 333 K, (c) 353 K and (d) 373 K.
3.9.3. Effect of temperature
version at higher temperatures clearly shows that this reaction
demands high-activation energy. It is also evident that the active
sites are activated more at higher temperatures, which leads to
a higher rate of attack of nucleophile (malonate) to electrophile
(nitrostyrene). Consequently, the conversion of ␤-nitrostyrene
increases with an increase in the reaction temperature from 298 to
373 K (Fig. 10curves a–d). Again, the product selectivity was found
almost ∼100% in each case.
The effect of temperature was examined for Michael-addition
of ␤-nitrostyrene with diethyl malonate under solvent free con-
dition over the SBA-15-TBD catalyst for 12 h (Fig. 10). Individual
experiments were performed at each temperature under identical
reaction conditions. The progress of the reaction was monitored
by gas chromatography. The rate of ␤-nitrostyrene conversion was
increased from 23 to 81% with increasing temperature from 298
to 373 K as expected (curves a–d). This indicates that the cata-
lyst is quite stable and the reaction was free from the diffusion
limitation at higher temperatures. Furthermore, the increased con-
3.9.4. Recycle studies
In order to check the recyclability and stability of the catalyst,
the Michael-addition of ␤-nitrostyrene with diethyl malonate was
carried out for three consecutive reaction cycles using SBA-15-TBD
catalyst. After the reaction, the catalyst was filtered from hot reac-
tion mixture, washed with dichloromethane and used for three
successive times without any further activation. The reactants were
taken with respect to the amount of the catalyst recovered after
each reaction cycle. All reactions were carried out under solvent-
free condition at 373 K for 9 h. The conversion and turn over number
(TON) obtained for the period of recycle studies of the SBA-15-TBD
catalyst are given in Table 3. The conversion was decreased from
81 to 76% in the first recycle, and then it decreases slowly upto 69%
(3rd recycle) as shown in Table 3. The conversion decreased due
to loss of nitrogen content (due to leaching) in the solid catalyst as
confirmed by elemental analysis (Table 3). Nevertheless, the TON
90
d
c
75
60
b
45
a
30
Table 3
Recycle studies of SBA-15-TBD catalyst for Michael-addition of ␤-nitrostyrene and
diethyl malonate under solvent free condition.
15
0
Recycle no.
Elemental analysis
(wt%)
Conv.a
(mol%)
TONb
Selectivity
(%)
C
H
N
0
3
6
9
12
Fresh
1st
2nd
3rd
16.0
15.4
14.3
11.9
3.8
3.6
3.1
2.8
4.1
3.9
3.6
3.2
81
77
71
69
41.4
41.0
41.1
41.5
100
100
100
100
Reaction time (h)
Fig. 9. Plot of conversion vs reaction time for Michael-addition of ␤-nitrostyrene
with diethyl malonate using different amount of SBA-15-TBD catalyst under sol-
vent free condition at 373 K. Curves (a) 0.05 g (3.3 wt%), (b) 0.1 g (6.6 wt%), (c) 0.15 g
(10 wt%) and (d) 0.2 g (13.3 wt%).
a
Conversion (Conv.) with respect to ␤-nitrostyrene and based on GC analysis.
TON is given as moles of ␤-nitrostyrene transformed per mole of nitrogen.
b

P. Kalita, R. Kumar / Applied Catalysis A: General 397 (2011) 250–258
257
Table 4
Table 5
Michael-addition of ␤-nitrostyrene with different malonate over SBA-15-TBD
Michael-addition of different nitrostyrene with different malonate over SBA-15-TBD
catalysta.
catalysta.
Entry
Malonate
Conv.c
(mol%)
Product
Entry Malonate
Nitrostyrene Conv.b
(mol%)
Product
1.
Diethyl malonate
81
1.
2.
3.
Diethyl
malonate
p-OMe-
NO2
Styrene
79
84
84
91
2.
3.
Dimethyl malonate
84
86
Diethyl
malonate
p-Cl-NO2
Styrene
Dimethyl
methylmalonate
Dimethyl
malonate
p-OMe-
NO2
Styrene
4.
5.
Dimethyl
methoxymalonate
89
65
0
Diethyl
ethylmalonate
4.
Dimethyl
malonate
p-Cl-NO2
Styrene
6.b
Diethyl
phenylmalonate
a
Reaction condition: nitrostyrene (10 mmol), malonate (10 mmol), no solvent,
reaction temperature 373 K, reaction time 9 h, catalyst amount = 0.2 g.
a
b
Reaction condition: ␤-nitrostyrene (10 mmol), malonate (10 mmol), no solvent,
Conversion (Conv.) with respect to ␤-nitrostyrene and based on GC analysis.
reaction temperature 373 K, reaction time 9 h, catalyst amount = 0.2 g.
b
Reaction time 24 h.
Conversion (Conv.) with respect to ␤-nitrostyrene and based on GC analysis.
c
of ␤-nitrostyrene with diethyl phenylmalonate (Entry 6, 0%) even
after the reaction was carried out for 24 h. This is mainly because
of the presence of more sterically hindered phenyl group in ␣-
substituted malonate (Entry 6). Hence, it is not possible to make
nucleophilic center in ␣-substituted malonate. Therefore, attack of
nucleophile to electrophile did not occur in this reaction.
was found to remain unchanged when calculated on the basis of
the N remaining in the solid catalysts after these consecutive test
runs.
3.9.6. Michael-addition of different nitrostyrenes with different
malonates
3.9.5. Michael-addition of ˇ-nitrostyrene with different
malonates
The Michael-addition was also carried out with different
nitrostyrenes and different malonates under identical reaction
condition and these results are summarized in Table 5. The con-
version obtained was 79% and 84% for the reactions of diethyl
malonate with p-OMe-NO2 styrene (Entry 1) and p-Cl-NO2 styrene
(Entry 2), respectively. Similarly, the reaction of dimethyl malonate
with p-OMe-NO2 styrene (Entry 3) and p-Cl-NO2 styrene (Entry
4) gave conversion of 84 and 91%, respectively. High conversion
was observed when the reaction was carried out with p-Cl-NO2
styrene (Entry 2, 4) and diethyl malonate as well as with dimethyl
malonate. This is mainly because of the presence of electron-
withdrawing group (-Cl) in the 4-position of nitrostyrene (Entry 2
and 4). Since, chlorine has more electron-withdrawing power than
methoxy (–OMe) group, so, the electropositive character will be
high at ␤-carbon in nitro styrene. Hence, attack of nucleophile to
electrophile will be more facile which leads to the more conversion
of the reaction.
The different substrates were examined for the Michael-
addition of ␤-nitrostyrene with different malonates over SBA-15-
TBD catalyst. All the reactions were performed under solvent free
condition at 373 K for 9 h in N2 atmosphere. The corresponding
results are summarized in Table 4. The product selectivity of all
the reactions was observed ca. 100%. As compared to dimethyl
malonate (Entry 2), less conversion was obtained when diethyl
malonate was used (Entry 1). This is because of the less elec-
tron delocalization in diethyl malonate (Entry 1) and due to the
more electron donating nature of ethoxy group in it, which results
in the less electronegative ␣-substituted malonate. Hence, the
attack of nucleophile to electrophile will be less facile in diethyl
malonate than dimethyl malonate. Slightly less conversion was
obtained in the case of dimethyl methylmalonate (Entry 3) than
dimethyl methoxymalonate (Entry 4). This is mainly because of the
more electron-donating group (–OMe group) present in dimethyl
methoxymalonate (Entry 4). Since, methoxy group (–OMe) is
more electron-donating than methyl group, so, the formation of
nucleophile in ␣-substituted malonate will be more in dimethyl
methoxymalonate (Entry 4) than dimethyl methylmalonate (Entry
3). In the case of Entry 5, the significantly low conversion was
obtained for the reaction of ␤-nitrostyrene and diethyl ethyl-
malonate. However, no conversion was observed for the reaction
4. Conclusions
To conclude, a well-organized and environmentally friendly
catalyst has been successfully synthesized by a facile two-step
route immobilization and a methodology has been developed for

258
P. Kalita, R. Kumar / Applied Catalysis A: General 397 (2011) 250–258
carbon–carbon bond formation reaction such as Michael-addition
of ␤-nitrostyrene to malonate. This methodology is applicable to
wide range of substrates making it a useful addition reaction for the
organic chemists. Since, 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD)
is inexpensive and commercially available, so converting homoge-
neous TBD into heterogeneous TBD catalyst is a better option as it
can be easily filtered as well as can be recycled for several times.
The loss of activity is observed to some extent after three recycles,
which may be because of the decrease of nitrogen content and loss
of crystallanity of the SBA-15-TBD catalyst.
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