Synthesis of structured porous polymers with acid and basic sites and their catalytic application in cascade-type reactions

Article abstract of DOI:10.1021/cm400123d

We described the use of one porous polymeric aromatic framework (PPAF) with a 9,9′-spiro-bisfluorene unit as a support in heterogeneous catalysis. The material was functionalized with acid and base active sites and used as a bifunctional catalyst in a model cascade reaction. This catalyst was recycled up to eight times with only a small loss of activity.

Full text of DOI:10.1021/cm400123d

Article
pubs.acs.org/cm
Synthesis of Structured Porous Polymers with Acid and Basic Sites
and Their Catalytic Application in Cascade-Type Reactions
Estíbaliz Merino,† Ester Verde-Sesto,‡ Eva M. Maya,‡ Marta Iglesias,§ Felix Sanchez,*,†
́
́
and Avelino Corma*,||
†Instituto de Química Organ
́
ica General, CSIC, C/Juan de la Cierva 3, 28006 Madrid, Spain
‡Instituto de Ciencia y Tecnología de Polímeros, CSIC, C/Juan de la Cierva 3, 28006 Madrid, Spain
§Instituto de Ciencia de Materiales de Madrid, CSIC, C/Sor Juana Ines
́
de la Cruz 3, Cantoblanco 28049 Madrid, Spain
||Instituto de Tecnología Química, UPV-CSIC, Avenida de los Naranjos s/n, 46022 Valencia, Spain
S
* Supporting Information
ABSTRACT: We described the use of one porous polymeric aromatic framework (PPAF) with
a 9,9′-spiro-bisfluorene unit as a support in heterogeneous catalysis. The material was
functionalized with acid and base active sites and used as a bifunctional catalyst in a model
cascade reaction. This catalyst was recycled up to eight times with only a small loss of activity.
KEYWORDS: porous aromatic frameworks, bifunctionalized solids, heterogeneous catalysis, organocatalysis, one-pot tandem reaction
PAFs with polar organic groups (−OH, −COOH, −NH2)
guided by quantum chemical calculations for CO2, H2, and N2
adsorption and have simulated the behavior of these materials
using the Grand Canonical Monte Carlo emthod.21 To the best
of our knowledge, there are no examples of multifunctionaliza-
tion (Figure 1) of this type of material and their use in catalysis.
Heterogenization of acid and base homogeneous catalysts is
an attractive concept to carry out multistep cascade reactions
for the synthesis of complex organic molecules.22−24 Although
homogeneous acids and bases will rapidly neutralize when used
together in one reactor, this can be prevented by fixing them on
solid supports and tailoring spatial separation between the two
INTRODUCTION
■
Nature’s strategy of employing multistep cascade reaction for
the synthesis of complex and bioactive organic molecules in
living systems has been a source of inspiration for designing
artificial catalysts. Most of the time, a multistep chemical
process involves multisite catalysts. However, by performing
the consecutive reaction steps in one pot, costly intermediate
separations and purification processes will be avoided. There-
fore, also from an energy savings point of view, the design of
solid catalysts with well-defined multisites are of interest to
achieve more-intensive chemical processes. Efforts are being
made today to prepare such multifunctional inorganic and
hybrid organic solid catalysts for performing one-pot multistep
reactions.1−5 Normally, those multisite catalysts are formed by
well-structured microporous and mesoporous organic−inor-
ganic hybrid materials such as metal-organic frameworks
(MOFs) and periodic mesoporous organosilicas (PMOs).
Sometimes, it could also be of interest to have purely
multisite-organic microporous and mesoporous structured
materials. This could, in principle, be achieved by suitable
functionalization of covalent organic frameworks (COFs),6−13
including the porous aromatic frameworks (PAFs) with a
diamond-like structure.14,15
Figure 1. Bifunctionalized structure of the unit of PPAF.
Recently, organic porous materials have attracted consid-
erable attention, because of potential applications in storage,16
separation,17 and catalysis.18−20 Thus, Jiang et al. have designed
Received: January 11, 2013
Revised: February 27, 2013
Published: February 27, 2013
© 2013 American Chemical Society
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antagonist functional sites.25 Heterogeneous multifunctional
catalysts can provide a continuous range of functional groups
and offer advantages, such as an enhancement of reactivity and
stability of antagonist functional groups.26−28 Bifunctional
materials of different nature have been described.29−39 One
example can be represented by the zeolitic hybrid organic−
inorganic materials with acid sites located in zeolitic counter-
part and base centers in the organic component. They are used
as bifunctional catalysts in a cascade reaction.2 In this paper, we
report for the first time, the simultaneous functionalization of
porous polymeric aromatic frameworks (PPAFs) with acid and
base groups and their application to an acid−base catalyzed
tandem reaction. We demonstrate that the incorporation of
acid and basic groups in the same porous aromatic framework
results in an excellent catalyst for one-pot cascade reaction
(hydrolysis of an acetal and a Knoevenagel condensation),
avoiding the inconvenience observed with homogeneous
catalysts where the acid and base sites are cancelled. It will
be shown that the bifunctional acid−base mesoporous material
is stable and robust, allowing several recycles with a small loss
of activity.
subjected to refluxing until a yellow color was obtained. Finally, the
sample was filtrated and washed with tetrahydrofuran (THF) and
diethyl ether (Et2O). After drying in vacuo, a pale yellow powder was
obtained.
Catalytic Testing. The reactions were carried out in a micro
reaction vessel (5 mL). Benzaldehyde dimethyl acetal (4) (54 μL),
malononitrile (29 mg), and dodecane (41 μL) were dissolved in 2 mL
of toluene and 25 μL of water, and then the mixture was stirred at 90
°C under argon with 20 mg of PPAF−SO3H−NH2. The samples of
the reaction mixture were analyzed by gas chromatography (GC)
(Konik, Model HCGC 5000B) using a 15-m KAP-5 capillary column
and a flame ionization detector (FID). After the completion of the
reaction, the bifunctionalized material was separated by filtration and
washed with toluene, an AcOH/AcONa buffer (pH 4), water, THF,
and Et2O, and then was reused for the above one-pot tandem reaction.
Characterization Methods. Chemical Composition. The carbon,
nitrogen, sulfur, and hydrogen contents were determined in a LECO
Model CHNS-932 analyzer (see Table 1). Thermogravimetric and
Table 1. Elemental Analysis of PPAF and Functionalized
PPAFs
support
PPAF-I
%C
%H
%N
%S
87.04
72.64
58.22
62.4
4.38
4.70
2.92
4.7
EXPERIMENTAL DETAILS
PPAF-SO3H
4.93
2.99
2.56
■
PPAF was prepared using an adaptation to a literature method.40 (See
PPAF-SO3H-NO2
PPAF-SO3H-NH2
PPAF-NO2
6.93
7.18
7.38
6.92
Scheme 1.) Here, 259 mg (0.316 mmol) of 2,2′,7,7′-tetraiodo-9,9′-
64.31
60.85
3.11
4.90
PPAF-NH2
Scheme 1. Synthesis of PPAF Support with Idealized
Structures for Solid Materials
differential thermal analyses (TGA-DTA) were conducted in an air
stream with a TA Instruments Model TA-Q500 analyzer. The samples
were heated under an air stream from 40 °C to 850 °C with a heating
rate of 10 °C/min. Fourier Transform Infrared (FTIR) spectra were
recorded on a Perkin−Elmer Spectrum One spectrometer and are
reported in terms of the frequency of absorption (cm−1). Ultraviolet−
visible light (UV-vis) spectra were recorded on a Shimadzu Model UV-
80 2401PC apparatus and are reported in terms of wavelength (nm).
Nuclear magnetic resonance (NMR) spectra were recorded with a
Bruker Model AV300 spectrometer (Larmor frequencies of 75 and
300 MHz for 13C and 1H, respectively) for liquids and a Bruker AV400
WB spectrometer (Larmor frequencies of 400 and 100 MHz, using 4-
mm MAS probes spinning at a rate of 10 kHz for 13C solid-state magic
angle spinning nuclear magnetic resonance (MAS NMR) measure-
ments. The 13C CP-MAS spectra were obtained using a contact time
of 3.5 ms and a relaxation time of 4 s. The number of scans used for
the 13C CP-MAS spectra was 1024.
Textural Characterization. Nitrogen adsorption isotherms were
measured at 77 K, using a Micromeritics ASAP 2020 instrument. Prior
to measurement, the samples were degassed for 12 h at 200 °C. The
pore size average was determined using the Barrett−Joyner−Halenda
(BJH) method.
Wide-angle X-ray scattering (WAXS) was carried out using a Bruker
D8 Advance diffractometer. Data were collected stepwise over the
angular region of 1° ≤ 2θ ≤ 65°, using steps of 0.5 s/step
accumulation time, a Vantec detector, and Cu Kα radiation (λ = 1.542
Å).
Scanning electron microscopy (SEM) micrographs were obtained
with a Hitachi Model SU-8000 microscope operating at 0.5 kV. The
samples were prepared directly by dispersing the powder onto a
double-sided adhesive surface.
spirobisfluorene (2),41 105 mg (0.632 mmol) of 1,4-phenylenedibor-
onic acid (3), 41 mg (0.158 mmol) of triphenylphosphine, 117 mg
(1.39 mmol) of sodium bicarbonate, and 3.5 mg (0.016 mmol) of
palladium acetate are suspended in a mixture of dimethylformamide
(DMF) (2 mL) and water (0.5 mL). The mixture was degassed by
argon bubbling. Microwave heating was performed in a computer-
controlled CEM Discover microwave with temperature and pressure
control. Initial heating was performed at a power input of 75 W. After
the pressure has reached ∼10 bar, the heating was stopped until the
temperature cooled to 60 °C and the pressure disappears. After that
period, heating was continued to reach the reaction temperature
(∼145 °C). The reaction was conducted at that temperature and a
pressure of ∼7 bar for 5 min. After cooling to room temperature, the
mixture was filtrated and the crude product was washed with DMF and
water. The solid was suspended in a mixture of water (H2O, 100 mL),
hydrochloric acid (HCl, 1 mL), and nitric acid (HNO3, 2 mL) and was
RESULTS AND DISCUSSION
■
The preparation of PPAF is carried out by combining 1,4-
benzenediboronic acid (3) with the tetrabromo monomer (1)
or tetraiodo monomer (2), easily available from 9,9′-spiro-
bisfluorene. Thus, 2,2′,7,7′-tetrabromo-9,9′-spiro-bisfluorene
(1) or 2,2′,7,7′-tetraiodo-9,9′-spiro-bisfluorene (2) reacts with
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commercial 1,4-benzenediboronic acid (3) by Suzuki coupling,
using microwave heating40 (Scheme 1, regular geometry
optimized by molecular modeling using Materials Studio 6.0
with a “universal” force field). A series of six reactions,
quantitative in all the cases, are carried out with each material
and the crude reactions are mixed and purified together.
The PPAFs are characterized by solid-state 13C NMR, FT-
IR, and elemental analysis. The solid-state 13C NMR (Figure 2)
additives. More-accurate values are obtained from EDX
analysis, which also confirms that the iodo, palladium, and
phosphor contents are <1% (see Figures S22 and S23 in the
Supporting Information).
The thermal stability is evaluated by TGA. In a first run, a
weight loss is observed at <100 °C, which is attributed to the
thermosorption of water, as a consequence of the highly cross-
linked structure formed. This behavior has been previously
observed in porous polymers and in interpenetrated organic
frameworks.42,43 Thus, thermograms in a N2 atmosphere are
recorded after a previous heating of the samples at 120 °C for
10 min (see Figure 4a). Both polymers show a similar
Figure 2. 13C NMR spectra of PPAFs.
of the materials shows a peak at 68 ppm that is due to the
quaternary spiranic carbon, two intense peaks at 122 and 129
ppm assigned to the aromatic tertiary carbons, and a peak at
142 ppm corresponding to quaternary aromatic carbons. In
both cases, the 13C NMR of PPAFs derived from tetrabromo
(PPAF-Br) or tetraiodo (PPAF-I) monomers are similar,
which indicates that the chemical structure is identical in both
cases.
FT-IR spectroscopy shows the typical bands of aromatic
compound at ∼3000 and 1590 cm−1, which are due to C−H
stretching and aromatic CC double bonds, respectively. The
superposition of the spectra of the materials synthesized from
tetrabromo and tetraiodo monomers is also identical (see
Figure 3).
The carbon contents determined by elemental analysis are
less than the expected theoretical values; this phenomenon is
something that is generally observed in porous aromatic
polymers,39 probably because of the difficulty of these materials
to be burned completely, even in the presence of oxide
Figure 4. Thermogravimetric analysis (TGA) of PPAF-Br and PPAF-
I in (a) a N2 atmosphere and (b) an air atmosphere.
decomposition pattern with high residue at 900 °C. However,
thermograms that were registered in an air atmosphere show
different degradation patterns. Thus, the thermogram of PPAF-
Br (Figure 4b) shows a decomposition pattern in several steps,
whereas the PPAF-I exhibits only a one-step degradation
pattern (Figure 4b). The differential thermogravimetric analysis
(DTA) in air of PPAF-I shows a unique peak, whereas PPAF-
Br shows two peaks, at 425 and 535 °C, and a shoulder at lower
temperature. These results indicate that the degradation
mechanisms are quite different, depending on the starting
tetra-halogen, which could suggest differences in the polymer
network formed. The material derived from tetraiodo monomer
Figure 3. Fourier Transform Infrared (FTIR) spectra of PPAFs.
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2 appears to be a more-homogeneous structure, since the
differential thermogravimetric curve contains fewer peaks.
However, when tetrabromo monomer 1 is used, a porous
polymer with a greater number of branches with lower length
could be obtained. In fact, the initial decomposition temper-
ature of PPAF-Br is lower than for that formed from tetraiodo
monomer 2. PPAF-Br is probably more irregular and has more
branches than the one prepared from tetraiodo derivative 2.
The small carbonaceous residues for the materials in an air
atmosphere are consistent with the results previously noted.
PPAF-Br has shorter branches and decomposes completely
before PPAF-I does.
Figure 6. SEM micrographs of PPAF-Br and PPAF-I.
simple treatment with chlorosulfonic acid in chloroform,
yielding PPAF-SO3H (see Scheme 2). In the IR spectrum,
In order to identify differences in the networks formed, wide-
angle X-ray scattering (WAXS) are recorded for the materials.
The WAXS patterns of both polymer networks are identical
(Figure 5), indicating an amorphous texture.
Scheme 2. Synthesis of PPAF-SO3H-NH2
the characteristic bands of the SO3H group appear between
1224 cm−1 and 1124 cm−1.44 The material PPAF-SO3H
contains ∼1.54 mmol g−1 of sulfonic groups (by elemental
analysis), corresponding to 0.8 units of sulfonic acid per
monomer unit. Based on the expected reactivity, we supposed
that the sulfonic groups were located preferably on more
activated phenyl rings. This fact has little influence on catalytic
behavior.
Figure 5. X-ray diffractograms of PPAF-Br and PPAF-I.
The porosity of the PPAFs is experimentally studied by BET
via nitrogen sorption. The isotherms obtained indicate
extensive mesoporosity for the polymers, with average pore
sizes of 3−6 nm (Table 2) and specific surface areas of 407−
Next, base sites were introduced by functionalization of
PPAF-SO3H with amino groups in two steps (Scheme 2). First,
nitration of PPAF-SO3H with nitric acid in trifluoroacetic acid
(TFA) yields the nitro-functionalized PPAF-SO3H-NO2 with
NO2 groups located on nonsulfonated aromatic rings. In the IR
spectrum, the bands corresponding to nitro group appear at
1529 and 1348 cm−1. PPAF-SO3H-NO2 contains ∼4.95 mmol
g−1 of nitrogen calculated from elemental analysis, correspond-
ing to 3.36 units of nitro groups per repeat unit, and the
amount of sulfur was similar to the that of the precursor (0.93
mmol g−1). Second, the nitro groups were reduced with solid
SnCl2·2H2O in THF yields PPAF-SO3H-NH2 (Scheme 2).
The course of the reaction is followed by IR analysis, and it was
considered finished when the bands corresponding to the nitro
groups disappeared (1529 cm−1 and 1348 cm−1). Elemental
analysis of PPAF-SO3H-NH2 revealed an amount of nitrogen
(5.12 mmol g−1) and sulfur (0.8 mmol g−1) that corresponded
to 2.94 units of amino group per unit of material, similar to
those found in the precursor PPAF-SO3H-NO2 (4.95 mmol
g−1 nitrogen and 0.93 mmol g−1 sulfur). It is also possible to
functionalize the material only with amino groups. The basic
sites in PPAF-NH2 are introduced by functionalization of
PPAF in two steps in a manner similar to that in the case of
PPAF-SO3H-NH2 (Scheme 3). First, the nitration of PPAF
with nitric acid in TFA gives the nitro-functionalized PPAF-
Table 2. Textural Properties of PPAF-Br and PPAF-I
a
surface area
pore volume
average pore size
(nm)
support
(m2 g−1
)
(cm3 g−1
)
PPAF-Br
PPAF-I
407
580
0.17
0.42
6.0
3.0
a
Estimated by the Barrett−Joyner−Halenda (BJH) method.
580 m2/g. The PPAF-Br shows similar specific surface area to
that reported by Thomas et al. for this polymer (450 m2/g);40
however, in both cases, the specific surface areas are lower than
the corresponding material prepared from tetraiodo monomer
2. These results are in agreement with the assumption that less-
reactive PPAF-Br has more branches or a smaller number of
closed structures. Tetraiodo monomer 2 (PPAF-I) yields a
slightly more regular structure, and larger BET surface area,
than tetrabromo monomer 1 (PPAF-Br).
Scanning electron microscopy (SEM) images of the materials
show the typical porous polymer morphology with some
spherical structures and some agglomerates (see Figure 6).
The functionalization with acid groups, of the PPAF
prepared from the tetraiodo monomer, was carried out by
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Scheme 3. Synthesis of PPAF-NH2
NO2. Again, the IR spectrum shows the corresponding bands of
the nitro group appear at 1528 and 1345 cm−1. PPAF-NO2
contains 5.27 mmol g−1 of nitrogen. In the second step, the
reduction of nitro groups with solid SnCl2·2H2O yields PPAF-
NH2 (Scheme 3). The elemental analysis of PPAF-NH2 reveals
4.94 mmol g−1, which is an amount of nitrogen that is close to
that found in the precursor PPAF-NO2.
The N2 adsorption measurements for the starting meso-
porous support (PPAF-I) show an isotherm with a hysteresis
loop at relatively high pressure, which is characteristic of
mesoporous materials. The BET surface area was >580 m2 g−1,
the pore volume was 0.42 cm3 g−1 (Table 3), and the material
Figure 8. WAXS pattern of PPAF-I (black line) and PPAF-SO3H-
NH2 (gray line).
to isolate the product from the reaction reagents and
subproducts. In many cases, the catalysts used in different
steps are incompatible and their use in a one-pot reaction
becomes impossible. For these reasons, it is very interesting to
be able to control the functionalization of the solid support
with different groups and to carry out the consecutive steps in a
one-pot system, avoiding isolation and purification of the
intermediates, resulting in a simpler, more-intensive process
and reducing the waste products and energy.
To demonstrate the concept, we have applied the bifunc-
tional PPAF-SO3H-NH2 catalyst in a cascade reaction.45−47
This reaction involves two steps: an acid-catalyzed hydrolysis of
acetal 4 to give benzaldehyde and the subsequent base-
catalyzed Knoevenagel reaction to yield 2-benzylidenemalono-
nitrile (6). The bifunctional material is able to catalyze the
tandem reaction described in Scheme 4, and the pure product
(6) is isolated after a simple filtration.
Table 3. Textural Properties of PPAF and Functionalized
PPAFs
a
surface area (m2
g−1
pore volume
pore diameter
(nm)
support
PPAF-I
)
(cm3 g−1
)
580
409
416
310
0.42
0.14
0.38
0.17
2.9
4.8
8.6
7.1
PPAF-SO3H
PPAF-NH2
PPAF-SO3H-
NH2
a
Estimated using the BJH method.
was stable up to 555 °C, as measured by TGA. The nitrogen
adsorption isotherm for PPAF-SO3H-NH2 is shown in Figure
7. The BET surface area was 310 m2 g−1, and the pore volume
Scheme 4. PPAF-SO3H-NH2 Catalyzed One-Pot Tandem
Reaction of Benzaldehyde Dimethylacetal (4) and
Malononitrile
■
▼
Figure 7. N2 adsorption isotherms of ( ) PPAF-I, ( ) PPAF-NH2,
(
▲
●
) PPAF-SO3H, and ( ) PPAF-SO3H-NH2.
was 0.17 cm3 g−1, which was lower than for PPAF-I. When
PPAF-I was functionalized with one sulfonic group by a repeat
unit, the BET surface area decreased to 409 m2 g−1. In the case
of PPAF-NH2, which contains 2.5 amino groups per unit, the
BET surface area was 416 m2 g−1.
The WAXS pattern of PPAF-I (Figure 8) is very similar to
that of PPAF-SO3H-NH2.
Sometimes, the synthesis of organic molecules with
interesting properties involves several primary and consecutive
steps. Each step is carried out with different reagents and
catalysts, and, after each reaction step, it is normally necessary
The bifunctional PPAF-SO3H-NH2 converts 4 to 6 with
100% yield (by GC) after 1 h (se Table 4, entry 1). When a
mixture of monoderivatized PPAF-SO3H + PPAF-NH2 is used
as a catalyst, product 6 is obtained with 91% yield and 9% yield
of benzaldehyde 5 was observed (see Scheme 5, as well as
Table 4, entry 2). The use of a porous organic polymer catalyst
with no functionalities (PPAF-I) does not lead to the
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a
Amberlysts IR-120 and A-21 increased by a factor of 5, the yield
increases to 33% (Table 4, entry 11). These results indicate that
our bifunctionalized PPAF are more active catalyst than a
mixture of strong acid and base Amberlites, probably as a
consequence of their mesoporous nature, which allows easier
access of reagents to most catalytic centers.
When the reaction is scaled-up to 1.1 mmol of benzaldehyde
dimethyl acetal (4), the product 6 (GC yield =100%), after
workup and purification by flash chromatography, was isolated
in 83% yield. We carry out the kinetic profile of reaction
between benzaldehyde 5 and malononitrile catalyzed by PPAF-
NH2 and PPAF-SO3H-NH2 (Figure 9) to study if the presence
Table 4. Tandem Deprotection−Aldol Reaction
Conversion (%) Yield GC (%)
entry
catalyst
4
5
6
1
2
PPAF-SO3H-NH2
PPAF-SO3H + PPAF-NH2
PPAF-I
100
100
0
100
91
9
3
4
PPAF-SO3H
99
74
25
5
PPAF-NH2
b
6
PPAF-SO3H + aniline
100
100
96
95
7
50
75
76
67
0
50
25
20
28
7
c
7
PTSA + PPAF-NH2
d
8
PTSA + aniline
e
9
PTSA + aniline
10
11
IR-120 + A-21
f
IR-120 + A-21
33
0
33
a
Reaction conditions: benzaldehyde dimethyl acetal (0.36 mmol),
malononitrile (0.43 mmol), catalyst (10 mol % referred to base sites),
b
toluene (2 mL) + H2O (25 μL), 90 °C, 1 h. PPAF-SO3H (10 mol %)
c
+ aniline (12 mol %). PTSA (30 mol %) + PPAF-NH2 (27 mol %).
d
e
PTSA (10 mol %) + aniline (10 mol %). PTSA (10 mol %) + aniline
f
(8 mol %). IR-120 (50 mol %) + A-21 (50 mol %).
Scheme 5. PPAF-SO3H + PPAF-NH2 Catalyzed One-Pot
Tandem Reaction of Benzaldehyde Dimethylacetal (4) and
Malononitrile
Figure 9. Kinetics of PPAF-SO3H-NH2- and PPAF-NH2-catalyzed
reactions between benzaldehyde and malononitrile.
of acid accelerates the second reaction. The turnover frequency
(TOF) is 0.52 min−1 for the PPAF-NH2-catalyzed reaction and
0.57 min−1 when the reaction is carried out with PPAF-SO3H-
NH2. Thus, we can conclude that the sulfonic acid group has
only a small effect on the second step of the cascade reaction.
The functionalized PPAFs are treated with acid and base to
test if changes are observed in the IR spectra. When PPAF-
SO3H is soaked with an aqueous solution of NaOH (10%), IR
spectra are very similar, with a detected decrease in the
intensity of the band associated to sulfonic group (see Figure
S1a in the Supporting Information). When the sample was
treated with buffer AcONa/AcOH (pH 4), no difference with
the IR spectrum of the precursor is observed (see Figure S1b in
the Supporting Information). Also, PPAF-NH2 is treated with
an aqueous solution of AcOH (50%) (see Figure S1c in the
Supporting Information) and with an aqueous solution of HCl
(10%) (Figure S1d in the Supporting Information) and no
changes in the IR spectra are observed. The IR spectrum of the
bifunctional material PPAF-SO3H-NH2 does not change when
it is soaked with an aqueous solution of HCl (10%) (see Figure
S1e in the Supporting Information).
The recyclability of PPAF-SO3H-NH2 was examined after
isolating the bifunctional catalyst from the reaction via simple
filtration and washing it with toluene and pentane. As traces of
base were detected, the solution was neutralized with an
aqueous buffer solution of AcOH/AcONa. With this treatment
(pH 4), the catalyst can be recycled at least eight times with
only a small loss of activity, as can be seen in Figure 10.
To investigate possible leaching effects with the correspond-
ing homogeneous reaction, the solid catalyst was removed after
formation of product 6 (Table 4, entry 3). When the sequence
is carried out only in the presence of PPAF-SO3H (Table 4,
entry 4), benzaldehyde 5 and a 25% yield of product 6 are
observed, while in the presence of PPAF-NH2, no reaction is
observed (Table 4, entry 5). These facts indicate that each
active site promotes one reaction, i.e., the acid centers catalyze
the hydrolysis of the acetal and the base centers catalyze the
Knoevenagel condensation between the resulting benzaldehyde
5 and malononitrile, without cross-interferences. When the
reaction is carried out with 10 mol % PPAF-SO3H and aniline,
the product 6 is obtained in 50% yield, and benzaldehyde 5
(50% yield) is also detected (Table 4, entry 6). The mixture of
PTSA and PPAF-NH2 gives rise to 25% yield of 2-
benzylidenemalononitrile 6 and a 75% yield of benzaldehyde
5 (Table 4, entry 7). The use of a mixture of PTSA and aniline
yields mainly benzaldehyde product 5, and product 6 is
obtained in low yield (Table 4, entries 8 and 9). When the
cascade reaction is carried out with 10 mol % Amberlite IR-120
(with sulfonic acid groups) and Amberlyst A-21 (with weak
base), product 6 was obtained only in 7% yield (Table 4, entry
10). When the reaction is carried out with the loading of
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ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
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AUTHOR INFORMATION
Corresponding Author
*E-mail: felix-iqo@iqog.csic.es.
■
(31) Alauzun, J.; Mehdi, A.; Reye,
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
We acknowledge to the Direccion
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■
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General de Investigacion
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C02-02 and Consolider-Ingenio 2009-CSD-0050-MULTI-
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of “Junta de Ampliacion de Estudios” program.
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