Silica grafted polyethylenimine as heterogeneous catalyst for condensation reactions

Article abstract of DOI:10.1016/j.apcata.2011.03.054

Primary amine groups were attached to a silica surface by using α,ω-diamines derivatives and (3-glycidyloxypropyl)-trimethoxysilane activation. The same activation was used to graft polyethylenimine, which also contains secondary and tertiary amine groups. These silica aminated structures were tested as heterogeneous catalysts in nitroaldol condensation with nitromethane, the derivative with the polyethylenimine moiety being the more active catalyst. This catalyst also showed efficiency in the Knoevenagel condensation of benzaldehydes with ethyl cyanoacetate under very mild reaction conditions and showed much the same efficiency when used in consecutive reaction runs. A reaction mechanism with participation of the several amine groups of the catalysts is discussed.

Full text of DOI:10.1016/j.apcata.2011.03.054

Applied Catalysis A: General 399 (2011) 126–133
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Silica grafted polyethylenimine as heterogeneous catalyst for condensation
reactions
Sonia M. Ribeiro, Arménio. C. Serra, A.M. d’A. Rocha Gonsalves∗
Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade de Coimbra, Rua Larga, P-3000 Coimbra, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Primary amine groups were attached to a silica surface by using ␣,␻-diamines derivatives and (3-
glycidyloxypropyl)-trimethoxysilane activation. The same activation was used to graft polyethylenimine,
which also contains secondary and tertiary amine groups. These silica aminated structures were tested
as heterogeneous catalysts in nitroaldol condensation with nitromethane, the derivative with the
polyethylenimine moiety being the more active catalyst. This catalyst also showed efficiency in the Kno-
evenagel condensation of benzaldehydes with ethyl cyanoacetate under very mild reaction conditions
and showed much the same efficiency when used in consecutive reaction runs. A reaction mechanism
with participation of the several amine groups of the catalysts is discussed.
Received 20 October 2010
Received in revised form 23 March 2011
Accepted 27 March 2011
Available online 1 April 2011
Keywords:
Polyethylenimine
Silica
Heterogeneous catalysts
Knoevenagel condensation
© 2011 Published by Elsevier B.V.
1. Introduction
tionalizations is the placement of amino groups near silanols in
order to get efficient acid–base activation processes.
The search for green chemical processes [1] with high efficiency
but at the same time with reduced uses of energy and materials,
as well as the lowest generation of waste has increased the inter-
est for supported catalytic processes. Also, the cost associated with
product isolation from the reaction mixture is an important fea-
ture in the choice of a heterogeneous process [2–5]. Supported
catalytic processes are therefore attractive to answer the above
requirements. Searching for a catalytic process can be tricky if
the reaction requires an acid–base activation. General homoge-
neous conditions make difficult the compatibilization between
acid and basic sites but this can be circumvented using ionic
liquids [6]. In this particular case, heterogeneous supports with
acid and base groups located separately can work cooperatively
and be an advantageous solution [7]. This acid–base cooperativity
which is common in enzymatic systems [8] can also be achieved
using heterogeneous organic–inorganic bifunctional catalysts
[9,10].
The Knoevenagel condensation [15] and the nitroalkene synthe-
sis from nitroaldol condensation [16] are carbon–carbon forming
reactions that originate valuable compounds with biological activ-
ity [17–21]. Both reactions can be catalyzed by homogeneous
catalysts [22–31] but heterogeneous catalysts have been also suc-
cessfully used [32–39].
In this work various aminofunctionalized silica materials were
prepared with a terminal amine group separated from the silica
structure by a carbon chain of variable length. The effect of this
separation on the catalytic activity of the material as a heteroge-
neous catalyst for carbon–carbon condensations was studied. In
addition, grafting a polyethylenimine structure at the silica surface
originates a much more active heterogeneous catalyst.
2. Experimental
All solvents were purified before use according to the literature
procedures. Ethyl cyanoacetate (98% purity, Fluka), ninhydrin
(95% purity, Fluka), anisaldehyde (98% purity, Riedel-de Haën), 4-
chlorobenzaldeyhde (98% purity, Fluka), benzaldeyhde (98% purity,
Merck), cinammaldehyde (98%, BDH), 4-methylbenzaldehyde
Silica is an inexpensive, easily available and resistant material
that can be superficially modified with organic groups in order to
modify the acid–base properties or covalently anchor different cat-
alysts [7–12]. An analogous more developed approach is the use
of synthesized mesoporous silica incorporating different organic
functionalities [13,14]. An interesting capacity of these silica func-
(97%
purity),
4-nitrobenzaldehyde
(98%
purity),
4-
hydroxybenzaldehyde (98% purity), 3,4-dimetoxybenzaldehyde
(99% purity), 2-furfuraldehyde (99% purity, freshly distilled),
pyrrole-2-carboxaldehyde (98% purity), 1-naphthaldehyde
(97% purity), cyclohexanone, 3-aminopropylltrimethoxysilane
(97% purity), (3-glycidyloxypropyl)-trimethoxysilane (98%
∗
Corresponding author. Fax: +351 239 826069.
E-mail address: arg@qui.uc.pt (A.M.d’A.R. Gonsalves).
0926-860X/$ – see front matter © 2011 Published by Elsevier B.V.
doi:10.1016/j.apcata.2011.03.054

S.M. Ribeiro et al. / Applied Catalysis A: General 399 (2011) 126–133
127
purity), 1,12-diaminododecane (98% purity), 1,6-diaminohexane,
polyethyleneimine (low molecular weight typical Mw-800) were
used as purchased from Aldrich. Silica gel type 60, 35–70 mesh,
with particle size of 0.2–0.5 mm was purchased from Fluka.
was carried out using a Supelcowax (30 m × 0.25 mm) capil-
lary column on a Hewlett-Packard 5890A instrument with a
Hewlett-Packard 3396A integrator. GC analysis was run at 100 ◦C
(2 min)/20 ◦C min−1/220 ◦C (20 min); detector temperature 250 ◦C,
injector temperature 220 ◦C. After completion, the catalyst was fil-
tered off and washed with chloroform. The filtrate was evaporated
and the residue was washed with cold hexane. The product was
analyzed by NMR, IR spectroscopy and mass spectrometry. NMR
spectra were recorded on a 400 MHz Bruker spectrometer. J values
are given in Hertz. The IR spectra were obtained at room tem-
perature in transmission mode using Thermo Nicolet 6700 FTIR
spectrometer. Mass spectra were obtained on a HP 5973 MSD appa-
ratus by electronic impact at 70 eV.
2.1. Catalysts preparation
2.1.1. Synthesis of the aminoalkylated silica derivatives
ASC3: To a stirred solution of 70 ml dry toluene containing 10% of
3-aminopropyltrimethoxysilane was added 45 g of SiO2 (silica gel
type 60, 35–70 mesh) activated as described at 120 ◦C. The resulting
mixture was refluxed overnight. The amine functionalized silica
ASC3 was isolated by filtration and washed with tetrahydrofuran
and methanol, then dried under vacuum for several days [40]
ASC6.6, ASC6.12 and ASCPEI [41]: To a stirred solution of 80 ml
dry toluene containing 3.0 mmol of diamine (1,6-diaminohexane
for ASC6.6, 1,12–diaminododecane for ASC6.12, polyethylenimine
for ASCPEI); 1.0 mmol of (3-glycidyloxypropyl)-trimethoxysilane
was added. The resulting mixture was allowed to react at 80 ◦C for
24 h. After this time, 1.5 g of activated SiO2 (Silica gel type 60, 35–70
mesh) and 5 ml of ethanol were added to that stirred solution and
was maintained at 80 ◦C for 24 h. The amine functionalized silicas
were isolated by filtration and washed with methanol and ethanol.
Then, they were refluxed with ethanol during 1 h, filtered again and
dried at 40 ◦C for several days.
SilPEI: To a stirred solution of 80 ml dry toluene, containing
3.0 mmol of polyethylenimine, 1.5 g of activated SiO2 (Silica gel
type 60, 35–70 mesh) and 5 ml of ethanol were added and was
maintained at 80 ◦C for 24 h. The silica product obtained was iso-
lated by filtration and washed with methanol and ethanol. Then,
it was refluxed with ethanol during 1 h, filtered again and dried at
40 ◦C for several days
The structure of the products was established by comparison
with data shown in the literature and the product characteristics
are identical to that previously reported. The conversion and selec-
tivity were determined by 1H NMR (for the dinitro and nitroalcohol
derivatives see Refs. [44,45]:
(E)-1-(4-Methoxyphenyl)-2-nitroethene(3a)
[46];
(E)-1-(4-
Chlorophenyl)-2-nitroethene (3b) [47]; (E)-1-pheny-2-nitroethene
(3c) [23]; (E)-1-(4-Methylphenyl)-2-nitroethene (3d) [48]; (E)-1-(4-
Hydroxyphenyl)-2-nitroethene (3e) [48].
2.3.2. General procedure for the Knoevenagel condensation
reaction
In a typical experiment, a mixture of the aldehyde (5.23 mmol),
ethyl cyanoacetate (5.23 mmol) and 50 mg of catalyst in ethanol
(6 ml) was stirred at 43 ◦C. The reaction was monitored by GC or
TLC. After completion of the reaction, the catalyst was filtered off
and washed with chloroform. The filtrate was evaporated and the
residue was washed with cold hexane. The product was analyzed by
NMR, IR and mass spectroscopy. The structure of the products was
established by comparison with data described in the literature.
2.2. Catalysts characterization
Ethyl
(8a);
(E)-2-cyano-3-(4-methoxyphenyl)-2-propenoate
(E)-3-(4-chlorophenyl)-2-cyano-2-propenoate
Ethyl
Ethyl
2.2.1. Measurement of total nitrogen content
Total nitrogen content was determined using a Fisons Instru-
ments EA1108-CHNS-0 apparatus.
(8b);
Ethyl
Ethyl
Ethyl
Ethyl
(E)-2-cyano-3-phenyl-2-propenoate
(8c);
(8d);
(8f);
(8g);
Ethyl
(E)-2-cyano-3-(4-methylphenyl)-2-propenoate
(E)-2-cyano-3-(4-nitrophenyl)-2-propenoate
(E)-2-cyano-3-(1-naphthyl)-2-propenoate
2.2.2. Quantification of amines group by ninhydrin test
(E)-2-cyano-3-(2-furyl)-2-propenoate
(8i);
The procedure employed followed a previously report for
amine-functionalized silica material [42,43]. Typically, 50 mg of the
catalyst samples was mixed with 5 ml of ethanolic ninhydrin solu-
tion (0.175 M). The solution was stirred at 90 ◦C for 25 min. It was
cooled and centrifuged for 10 min. The absorbance of the super-
natant solution was monitored and the amount of aminogroups
estimated from a calibration curve made with different concentra-
tions of 3-aminopropylltrimethoxysilane.
(E)-2-cyano-3-(1H-pyrrol-2-yl)-2-propenoate (8j); Ethyl (2E,4E)-2-
cyano-5-phenyl-2,4-pentadienoate (8k) [30].
Ethyl (E)-2-cyano-3-(3,4-dimethoxyphenyl)-2-propenoate (8h)
[49]: 1H NMR (400 MHz, CDCl3) ␦ (ppm): ␦ 1.40 (t, J = 7 Hz, 3 H),
3.96 (s, 6 H), 4.37 (q, J = 7 Hz, 2 H), 6.94(d, J = 8.2 Hz, 1 H), 7.47(d,
J = 8.2 Hz, 1 H), 7.80 (s, 1 H), 8.15 (s, 1 H). 13C NMR (100.613 MHz,
CDCl3) ␦ (ppm): 14.21, 56.05, 56.15, 62.44, 99.43, 110.96, 111.67,
116.35, 124.63, 127.87, 149.29, 153.68, 154.65, 163.08.
Ethyl cyano (cyclohexylidene) acetate (10c) [27]: 1H NMR
(400 MHz, CDCl3) ␦ (ppm): 1.33 (t, J = 7.2 Hz, 3 H), 1.66–1.80 (m,
6 H), 2.66 (t, J = 6 Hz, 2 H), 2.98 (t, J = 6 Hz, 2 H), 4.27 (q, J = 7.2.0 Hz,
2 H); 13C NMR (100.613 MHz, CDCl3) ␦ (ppm): 13.95, 25.49, 28.16,
28.49, 31.45, 36.74, 61.15, 101.88, 161.83, 179.84. EI-MS: m/z (%)
193 (58, M+), 165 (75), 148 (62), 137 (100), 121 (81), 109 (27), 93
(37), 80 (17), 65 (12).
2.2.3. N2 adsorption/desorption isotherms
The N2 adsorption/desorption isotherms were obtained in an
ASAP 2000 (Micrometrics Instrument Corporation) using liquid
nitrogen for analysis gas. The samples were pre-treated in situ
under vacuum at 295/296 K until vacuum stabilization at a pressure
below 10 micrometers of mercury. The surface area was calcu-
lated by the BET method. The pore volume was measured using
the adsorption curve. The pore size distribution (PSD) were deter-
minated from the desorption isotherm using BJH model.
3. Results and discussion
2.3. Catalytic tests
3.1. Catalyst preparation
2.3.1. General procedure for aldehyde/nitroalkane condensations
In a typical experiment, a mixture of the aldehyde (5 mmol)
and catalyst (50 mg) in the nitroalkane (2 ml) was stirred at
95 ◦C. The reaction was monitored by gas chromatography.
The gas chromatography with flame ionization detector (gc–fid)
Siliceous materials with supported amines have been largely
tested as heterogeneous catalysts for carbon–carbon condensation
reactions. Several kinds of silica supports can be used to incorpo-
rate the amine functionalities, such as synthesized mesoporous
silica [50–54] or MCM-41 silica structures [55–60]. However,

128
S.M. Ribeiro et al. / Applied Catalysis A: General 399 (2011) 126–133
Scheme 1. Strategies for grafting aminoderivatives into silica structures.
an easier approach is to use commercial silica and incorporate
the amine functionality by reaction of the silanol groups with
aminotriethoxysilane derivatives. First works with aminopropyl
groups showed activity for Knoevenagel reactions [61,62]. More
recently the same approach was followed using silica-alumina sup-
ports and catalysts proved to be efficient in Michael additions
and nitroaldol condensations. The incorporation of only diethy-
lamino groups or these in the presence of primary amine groups
modulates the activity of the heterogeneous catalyst [63–66].
The effect of the structure of the amine groups grafted into sil-
ica materials has been studied for mesoporous silica materials
[67,68,43,53,69] in which the catalytic process occurs at the sur-
face, but also within pores and channels. These different catalytic
environments can originate misleading contributions of the amine
groups to the catalytic process. In a different previous work, we
prepared a series of silica based photosensitizers with different
chain spacers and studied the contribution of the silica structure
to reaction efficiency [41]. Taking into account this experience
and to enlighten the cooperative effect of the amine groups and
silica surface on catalytic activity we decided to graft into the
silica surface several kinds of aminoalkylgroups in which the
amine functionality is increasingly separated from the silica silanol
groups. For the smaller carbon chain catalyst ASC3 we reacted com-
mercial silica (Kieselgel 60) with 3-aminopropyltrimethoxysilane
following a described procedure [40]. To increase the carbon
chain length it was necessary to use the silica activation with
(3-glycidyloxypropyl)-trimethoxysilane [70] followed by reaction
with 1,6-hexanediamine or 1,12-dodecanediamine giving the cor-
responding aminated silica catalysts ASC6.6, and ASC6.12. As
pointed out in other works with mesoporous silicas as catalysts,
the type of amine group (primary, secondary or tertiary) can influ-
ence the catalytic activity. The literature describes simultaneous
grafting of primary and tertiary amine groups in silica structures
using the silane activation approach [64]. We envisaged another
strategy by grafting in the silica surface a polyamine, such as
polyethylenimine [71], which presents the three kinds of amine
groups and proved to be efficient catalysts for hydrolysis reac-
tions [72–74] (Scheme 1). The presence of these different amine
groups, with different acid–base behavior, could originate inter-
esting catalytic effects. The possibility of simple polyethylenimine
adsorption to silica surface was taken into consideration by treating
poliethyleneimine with activated silica gel in the same conditions
used in the case of amine grafted samples and the corresponding
catalyst was also prepared (SilPEI).
Evidence for the presence of the amine groups in the silica struc-
tures is obtained by IR spectra evidence (Fig. 1). The reduction in
Fig. 1. IR region corresponding to C–H stretching vibrations. (a) silica; (b) ASC3; (c) ASC6.6; (d) ASC6.12 and (e) ASCPEI.

S.M. Ribeiro et al. / Applied Catalysis A: General 399 (2011) 126–133
129
Table 1
Total nitrogen and active amine groups (by ninhydrin test) of the catalysts.
450
ASC3
ASC3
ASCPEI
ASCPEI
400
350
300
250
200
150
100
50
Catalysts
% N by elemental analysis
% N by ninhydrin test
ASC3
1.29
1.07
0.880
4.80
3.45
0.540
0.068
0.048
0.702
1.36
ASC6.6
ASC6.12
ASCPEI
SilPEI
intensity of the broad band around 3500 cm−1 (O–H stretching)
can be due to decrease of the silica hydrophilicity by reaction of
the silanols with trimethoxylsilyl groups [58] which is more evi-
dent in the ASC6.12 catalyst. Also, reduction of absorbed water
due to increased hydrophobicity can explain this behavior [75]. The
existence of bands around 2940 cm−1 and 2850 cm−1 due to C–H
stretching, visible in the ASC catalysts shows the presence of the
amine carbon chain bonded to silica.
0
0,0
0,2
0,4
0,6
0,8
1,0
Relative Pressure
Mesopore
volumea
(cm3/g)
Surface area
BET(m2/g)
Average pore
diameter (nm)
Catalyst
The total amine groups were assessed by elemental analysis and
those groups which are accessible to catalysis were estimated by
the ninhydrin tests (Table 1).
ASC3
ASCPEI
312
256
3.9
3.9
0.599
0.420
Elemental analysis of the aminated silica catalysts shows that
ASCPEI shows the highest nitrogen content, as expected due
to the polyamine incorporation. The SilPEI catalyst shows also
high nitrogen content and evidences the adsorption ability of
polyethylenimine to silica surface. The values for the nitrogen con-
tent for the other catalysts show that amine incorporation is lower
and inversely related to the length of the carbon chain. The nin-
hydrin test is a measured of the active amine groups mainly the
primary ones [42]. Despite having moderate value for amine incor-
poration, ASC6.6 and ASC6.12 catalysts show very low values for
the active amine groups, which can be explained by the double
reaction of the two terminal amino groups with two siloxane acti-
vated surface places creating diamine bridges [42,43]. Nitrogen
adsorption–desorption isotherms for the ASC3 and ASCPEI and cat-
alyst characteristics are presented in Fig. 2.
a
From BJH adsorption
Fig. 2. Nitrogen adsoption/desorption isotherms and surface characteristics for
ASC3 and ASCPEI catalysts.
introduction of the aminated structures [77]. Between the two cata-
lysts ASCPEI presents lower values for surface area due to its higher
value of amine bonding.
3.2. Nitroaldol condensation
The activity of the different aminated silica catalysts was
first tried in the reaction of p-methoxybenzaldehyde (1a) with
nitromethane (2) (Table 2). For all the experiments the isolation
process consists of the catalyst isolation by filtration and evap-
oration of the excess of nitroalkane. The ASC3 catalysts showed
good selectivity for the nitroalkene product (3) with high yields.
Relatively to the characteristics of the commercial silica-gel
used as starting reagent [76] the catalysts ASC3 and ASCPEI shows
low values for surface area and pore diameter as expected for the
Table 2
Results for the catalytic activity of amine silica catalysts in nitroaldol condensationa.
NO2
OH
Catalyst
X
X
X
+
X
+
NO2
O
NO2
NO2
CH3NO2
(1)
(2)
(3)
(5)
(4)
X=OCH3 (1a) X=CH3 (1d)
X=Cl
X=H
(1b) X=OH (1e)
(1c)
Entry
Aldehyde
Catalyst
Time (h)
Conversion (%)
Relative yield of products (%)b
3
4
5
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1a
1a
1b
1c
1d
1a
1b
1a
1b
1a
1b
1c
1d
1e
–
24
8
7
7
7
24
24
24
24
4
0
97
98
99
99
16
69
12
59
95
95
99
95
93
–
–
1
1
1
4
1
3
–
7
12
7
11
11
16
–
–
1
1
1
1
10
1
18
3
6
ASC3
ASC3
ASC3
ASC3
ASC6.6
ASC6.6
ASC6.12
ASC6.12
ASCPEI
ASCPEI
ASCPEI
ASCPEI
ASCPEI
99
98
98
95
98
87
99
75
85
87
86
84
81
6
3
5
4
3
5
3
a
Reaction conditions: 1a to 1e (5 mmol), 2 (2 ml), catalyst (50 mg), 95 ◦C.
Determined by 1H NMR spectroscopy, based on 1.
b

130
S.M. Ribeiro et al. / Applied Catalysis A: General 399 (2011) 126–133
Table 3
Results for the catalytic activity of ASCPEI catalyst in Knoevenagel condensation of aldehydesa.
X
H
ASCPEI
+
NC
CO2Et
CHO
CO2Et
(8)
NC
X
Ethanol
(7)
(1)
Entry
Aldehyde
Time (h)
Yield (%)b
m.p. (lit.)
CHO
1
1c
1c
1c
1c
1c
1a
1b
1d
1f
1.5
5
97
96
96
98
98
95
96
99
99
46–47 ◦C (49–50 ◦C)c
CHO
CHO
CHO
CHO
2d
3e
4f
5g
6
45–47 ◦
46–48 ◦
45–48 ◦
45–48 ◦
C
C
C
C
4
2
2
H3CO
Cl
CHO
CHO
CHO
CHO
1.5
2.5
3
79–80 ◦C (79–81 ◦C)c
87–88 ◦C (89–90 ◦C)c
89–91 ◦C (90–92 ◦C)c
166–167 ◦C (170–171 ◦C)c
7
H3C
O2N
8
9
1
CHO
10
1 g
3
98
73–76 ◦C (73–74 ◦C)c
H3CO
H3CO
CHO
11
12
13
1 h
1i
2
1
98
97
152–153 ◦C (155–157 ◦C)h
91 ◦C (89–91 ◦C)c
CHO
CHO
O
N
1j
3
4
98
84
137–138 ◦C(135–137 ◦C)c
–
CH CHCHO
14
1k
a
Reaction conditions: Aldehyde (5.2 mmol), ethyl cyanoacetate (5.2 mmol), ethanol (6 ml), ASCPEI (50 mg), 43 ◦C.
b
c
d
e
f
Isolated yield.
See Ref. [29].
Toluene as solvent.
Room temperature.
1st reutilization of the catalyst of entry 2.
2nd reutilization of the catalyst.
See Ref. [70].
g
h
Pre-activation of the catalysts seems to be important to promote a
Another explanation is the reaction of the terminal primary amino
groups of the carbon chain with siloxane surface groups forming
bridges [42] and blocking the participation of these groups in cat-
alytic process. The low value for the ninhydrin test for ASC6.6 and
ASC6.12 may confirm this hypothesis. Catalyst ASCPEI with the
polyamine grafted at silica surface is the most active with aldehyde
1a and was tried with other aldehydes (1b–1e). The main prod-
uct is the nitroalkene with variable amounts of dinitro derivative
4 and nitroalcohol 5. The higher values from the ninhydrin test for
ASCPEI means a larger number of active amine groups and explain
the higher catalytic efficiency relatively to ASC3. The reason for the
catalytic success of ASCPEI can also be related to the organoamine
functionality of ASCPEI which is probably flexible enough to allow
proximity between amine and silanol groups and cooperativity
faster reaction. A reaction with aldehyde 1a catalyzed by a sample
of ASC3, without heating activation, only gives sluggish conver-
sion after 19 h. Analysis of the effect of the carbon chain on the
activity of the catalysts shows that the catalyst with the smaller car-
bon chain, ASC3, is the most active. Catalysts ASC6.6 and ASC6.12
shows low conversion values even after 24 h reaction. These results
and the observation of a better catalyst activity observed with pre-
activated samples confirm the involvement of the silanol groups
in the catalytic process as suggested by others [10,54,56,65,68,78].
The catalysts with the primary amine group, which are involved
in catalytic process, apart from the silica surface (ASC6.6 and
ASC6.12) gives much slower catalysis probably due to the more
difficult involvement of surface silanol groups in catalytic process.

S.M. Ribeiro et al. / Applied Catalysis A: General 399 (2011) 126–133
131
Table 4
Results for the catalytic activity of ASCPEI catalyst in Knoevenagel condensation of ketonesa.
R1
R2
R1
ASCPEI
Ethanol
+
NC
CO2Et
C O
CO2Et
NC
R2
(7)
(10)
(9)
Entry
Ketone
(9)
Reaction temperature (◦C)
Time (h)
Conversionb (%)
1
2
3
4
9a
9a
9b
9b
R1 = CH3R2 = isobutyl
R1 = CH3R2 = isobutyl
R1 = CH3 R2 = Phenyl
R1 = CH3 R2 = Phenyl
43
78
43
78
24
24
24
24
17
12
0
0
O
5
9c
78
24
75
a
Reaction conditions: ketone (5.2 mmol), ethyl cyanoacetate (5.2 mmol), ethanol (6 ml), ASCPEI (50 mg).
b
Determined by 1H NMR spectroscopy.
between groups is working [43]. Relatively to selectivity and com-
(entry 14) the reaction stops after 4 h and the product with E,E
stereochemistry [30] is isolated with a small amount of the starting
reagent. Looking at the IR spectrum of the recovered catalyst new
bands of medium intensity appear at 1743 and 702 cm−1 which
cannot be related to the described deactivation process by amide
formation between amine groups of the catalyst and ethyl cyanoac-
paratively to ASC3, ASCPEI presents a lower selectivity for the
product 3 and originates a greater amount of the dinitro deriva-
tive 4. This difference can be explained by the existence of tertiary
amine groups in the structure of ASCPEI which helps the proton
abstraction in nitromethane (2) favoring the Michael addition to
the formed nitroalkene molecule [64].
etate which correspond to a band at lower frequency (1690 cm−1
)
Catalytic activity of ASCPEI was further explored using anisalde-
hyde and the less active nitroethane [50]. In this case the catalyst
also showed activity, the nitroalkene product with E geometry
[79,80] being isolated in 89% yield. Recyclability of the catalyst
was tried in consecutive runs using p-anysaldehyde as substrate
and nitromethane. Unfortunately the first reutilization showed
only 29% of conversion indicating that some catalyst deactivation
occurred. The IR spectrum of this material shows new sharp bands
at 1384 and 1558 cm−1 probably due to the presence of a nitro
groups.
[62,67,81]. This deactivation is caused by the reaction product since
a new sample of catalyst left in contact with the product 8k origi-
nates new IR bands at 1741 and 702 cm−1. The value obtained by the
deactivated catalyst (1743 cm−1) is closer to the carbonyl frequency
of the ethyl cyanoacetate, 1755 cm−1 [81] which may indicate that
this molecule participates in the deactivation process.
The possibility of some catalytic effect by the adsorbed
polyethylenimine was analyzed using SilPEI catalyst giving 97% of
product 3c after 7 h of reaction. This is a reduced catalytic activ-
ity compared with ASCPEI. The difference is more evident from the
reutilization experiment in which SilPEI originates 97% of the same
product but in 13.5 h.
3.3. Knovenagel condensation
Simple comparisons with others silica based systems namely
mesoporous materials can be misleading since differences in
reaction conditions as solvent and temperature can originate
incorrect conclusions. However, taking into account the same
substrate and analyzing the conversion of substrate versus
reaction time, ASCPEI can be more active than silica grafted 1,8-
bis(dimethylaminonaphtalene) [54], aminopropyl-functionalized
mesoporous SBA-15 [58] or amine functionalized MCM-41 [59] but
seems less active than aminopropyl functionalized AlMCM-41 [69].
As expected for steric reasons, the Knoveanagel condensation
with ketones is much slower [27,35]. With ASCPEI catalyst no reac-
tion was observed after 24 h under conditions of Table 2. However,
at higher temperature no conversion for acetophenone is observed,
sluggish conversion occurs for methyl isobutylketone and there is
a reasonable conversion for cyclohexanone (Table 4).
The Knovenagel condensation requires the same kind of
acid–base activation but mechanistically the involved species are
quite different and the products formed are less reactive than nitro
alkenes. This prompted us to try the ASCPEI catalyst on the conden-
sation of ethyl cyanoacetate (7) with several aromatic aldehydes as
showed in Table 3.
The ASCPEI catalyst was found to be a very efficient catalyst for
the synthesis of ␣-cyanocinnamates (8). The reaction goes fast in
very mild conditions and in some cases the product soon begins
to precipitate from the reaction media. The great advantage of this
catalytic system is that products are obtained by simply evapora-
tion of the solvent after filtering the catalyst. Purity of the obtained
products is relatively high, based on NMR spectra and melting point
values. A blank experiment without catalyst and aldehyde 1c gives
21% of conversion after 4 h.
Even at room temperature reactions go to completion, albeit
slightly slower (entry 3). Analysis of the products shows that the
reaction occurs in a stereoselective manner originating the exclu-
sively the E isomer. Changing the solvent to non polar toluene gives
slower reaction (entry 2) relatively to that in ethanol which can
be indicative of a stabilization of the transition states by the more
polar solvent [54]. Another advantage of this catalyst is that reuti-
lization can be done without loosing the catalytic activity (entries 4
and 5). Using the best reaction conditions, the catalytic efficiency of
ACSPEI was also observed for other substituted aromatic aldehydes
(entries 6–11). Heteroaromatic aldehydes give the corresponding
products in good yields (entries 12 and 13). With cinnamaldehyde
The mechanism of the catalyzed reaction, in which the silanol
groups and the different amine groups of the catalyst work coop-
eratively, can be visualized in Fig. 3 [69,82–84].
At the catalyst surface the aldehyde group is activated by silanol
groups favoring the NH2 group attack forming the correspond-
ing imine. It is also possible that the presence of secondary amine
groups in the catalyst gives the possibility of iminium ion activation.
Catalyst structure with tertiary amine groups promotes the abstrac-
tion of a proton from ethyl cyanoacetate. Attack of the anion to the
imine, followed by proton elimination, originates the product and
regenerates the catalyst. Besides stereochemical reasons, ketones
which do not form the imine do not suffer this kind of activation,
also contributing to a much slower reaction.

132
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The authors would like to thank Chymiotechnon, FCT-
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