Microporous polyurethane material for size selective heterogeneous catalysis of the Knoevenagel reaction

Article abstract of DOI:10.1039/c6cc02578a

The first polyurethane material which is microporous (BET surface area of 312 m² g^-1) is prepared by solvothermal synthesis and acts as highly efficient and recyclable heterogeneous catalyst in the Knoevenagel condensation showing si

Full text of DOI:10.1039/c6cc02578a

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Microporous polyurethane material for size selective
heterogeneous catalysis of the Knoevenagel reaction
Sandeep Kumar Dey,‡ Nader de Sousa Amadeu and Christoph Janiak*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The first polyurethane material which is microporous (BET surface Knoevenagel and Henry reactions and thus, addressing the
area of 312 m2 g‒1) is prepared by solvothermal synthesis and acts problems of recyclability and high loading common with
as highly efficient and recyclable heterogeneous catalyst in the homogeneous base catalysts such as alkali metal hydroxides
Knoevenagel condensation showing size selectivity, and in the and organic amines.8 Further, a wide range of other porous
Henry reaction showing substrate selectivity under mild reaction solid catalysts have also been investigated for Knoevenagel
conditions.
reaction.9 Yet, only a couple of examples of POF-catalysed
Knoevenagel reactions are known in literature.10 Its variant the
nitroaldol (Henry) reaction (Scheme 1) has not been reported
with POF catalysts, to the best of our knowledge.
The development of functional porous organic frameworks
(POFs) assembled from simple molecular building blocks have
gained considerable attention in recent times,1 mainly because
of their potential applications in gas storage and separation,2
catalysis,3 optoelectronics4 and nanotechnology.5 Considering
a number of covalent linkages that can be derived by applying
different synthetic reactions of a wide variety of molecular
building blocks, it has become increasingly important to
develop new functional POFs having high specific surface area,
catalytically active pore surface, and remarkable chemical and
thermal stability. Although impressive progress has been
achieved in the field of heterogeneous catalysis using modified
zeolites and metal-organic frameworks (MOFs),6 improving the
functions of the pores in POFs for better catalytic performance
and selectivity still remains a major challenge.
HO
2
OH
+
3
OCN
NCO
150 °C, 72 h, DMSO
OH
O
O
O
O
NH
HN
NH
O
HN
O
O
O
MPU =
microporouspolyurethane
Knoevenagel condensation
MPU
O
O
R2
R2
O
5 mol%
O
O
+
NH
NH
CN
CN
THF
50 °C
R1
R1
(60-99%)
OH
There has been, so far, no report on porous polyurethanes
(Scheme 1) as catalysts, to the best of our knowledge.
Nitroaldol (Henry) reaction
MPU
5 mol%
The Knoevenagel condensation (Scheme 1) is one of the most
useful and widely employed carbon-carbon bond formation
reactions enjoying widespread application in synthetic organic
chemistry for obtaining fine chemicals and heterocyclic
compounds of biological importance.7 Several amine and
amide functionalized MOFs have been employed as
heterogeneous solid catalyst to promote both the
HN
HN
CH3
NO2
O
O
O
O
+
NO2
O
O
MeOH
50 °C
(77-95%)
O
EWG
O
EWG
O
HN
NH
NH
O
HN
O
O
O
Scheme 1 Solvothermal synthesis of the microporous polyurethane (MPU) material,
with the MPU catalysed Knoevenagel condensation reaction (R1 = various electron
donating and withdrawing substituents, see Table 1 and R2 = -CN/-COOMe) and the
nitroaldol (Henry) reaction (EWG = electron withdrawing group, see Table 2) (product
yields in parentheses).
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Porous polyurethanes have received considerable attention in uptake at P/Po < 0.10 is associated with the filling of
recent years, because of their high elastance, biocompatibility micropores. H4 loops are often found with aggregated crystals
and biodegradability essential for soft tissue engineering of zeolites, some mesoporous zeolites, and micro-mesoporous
applications.11 They have also been used in enzyme carbons.13 Non-local density functional theory (NLDFT)14 pore
immobilization and in biosensor applications.12 In our attempt size distribution using a slit pore model indicates that a
to assess the, so far unknown catalytic activities of porous significant fraction of the pores of the MPU originates from
polyurethane, herein we report
a
new microporous pores with a diameter less than 20 Å (micropores by
polyurethane (MPU), synthesized by an unprecedented definition).13 Accordingly, the pore size distribution curve
solvothermal route for MPUs, based on the condensation of showed micropores centred at a diameter of 12 Å and 16 Å,
phloroglucinol and 1,4-phenylenediisocyanate and its catalytic and a small fraction of mesopores between 20-40 Å (Fig. 1b).
efficiency in the Knoevenagel and Henry reactions under mild Larger micropores of 16 Å give a possible indication towards
conditions (Scheme 1). Due to its mild basic nature and ability the formation of hexagonal pore structure in the MPU
to form hydrogen bonds, the urethane (carbamate) group can material, while smaller micropores of 12 Å could be the result
act as a catalytic driving force to prompt several important of hexagonal pore shrinking due to the inclusion of DMSO
base-catalysed reactions.
within the porous framework. The total pore volume was
The microporous polyurethane (MPU) material was calculated to be 0.30 cm3 g‒1.
synthesized solvothermally by the condensation of
phloroglucinol and 1,4-phenylenediisocyanate in dimethyl
sulfoxide at 150 °C (Scheme 1, see ESI† for details). The
successful formation of the MPU was verified by Fourier
transform infrared (FT-IR) and solid-state 13C CP/MAS NMR
spectroscopy. The FT-IR spectrum of the MPU showed
disappearance of the stretching band of isocyanate (N=C=O) at
2274 cm‒1 and concomitant emergence of the carbonyl (C=O)
stretching band at 1633 cm‒1 (Fig. S2, ESI†) indicating complete
Fig. 1 (a) Nitrogen sorption isotherm at 77 K, and (d) NLDFT pore size distribution
curve of the activated MPU using a slit pore model.
condensation between 1,4-phenylenediisocyanate and
phloroglucinol. Solid-state 13C NMR spectrum further
confirmed the presence of carbonyl carbon of the urethane
moiety at δ = 156 ppm, whereas other peaks at 150, 134, 125,
and 109 ppm were assigned to carbon atoms of the aryl rings
(Fig. S3, ESI†).
Over the past few years, the use of POFs as heterogeneous
catalyst for various organic reactions has been of continuous
growing interest.3 For the Knoevenagel reaction, two 3D
microporous imine-COFs (BF-COFs) have recently been shown
to catalyse the condensation reaction in benzene with
selectivity for benzaldehyde only.10a Further, an amide
functionalized microporous organic polymer (Am-MOP) has
also been shown to catalyse the Knoevenagel reaction in THF,
albeit giving only three examples of benzaldehyde
derivatives.10b A POF-catalysed Henry reaction has not been
reported till date, to the best of our knowledge.
The PXRD pattern of the activated MPU (Fig. S4, ESI†)
indicated the partial ordered nature of the framework as
reflected from the presence of several peak maxima in the
diffractogram, notably in the 2θ range of 20° to 30° at 20.3° (d
= 4.37 Å), 21.4° (d = 4.14 Å), 26.2° (d = 3.39 Å), 27.8° (d = 3.20
Å) and 29.9° (d = 2.99 Å). SEM images revealed that the MPU
crystallizes as small proliferated flakes, which are aggregated
into an overall sponge like morphology (Fig. S5, ESI†). The SEM
images also showed the presence of scarcely distributed
spherical particles, a likely indication towards the initial
formation of the MPU spheres which eventually burst out
forming the proliferated flakes during the solvothermal
synthesis. Energy dispersive X-ray spectrometric (EDX)
measurement showed the presence of sulfur which strongly
suggests the inclusion of DMSO molecules within the pores of
the framework (Fig. S6, ESI†). CHNS analysis revealed the
Encouraged by these results, the new MPU was assessed for its
catalytic activity in the Knoevenagel reaction using a model
reaction, wherein benzaldehyde reacts with malononitrile in
the presence of 5 mol% of activated MPU in tetrahydrofuran at
50 °C to give 98% yield of the desired α,β-unsaturated product
within 14 h. In the absence of the MPU catalyst, only 5%
desired product was formed under the same optimal reaction-
condition. The reaction when carried out at room temperature
(25 °C) did not proceed to completion, and yielded 90% of
desired product even after 24 h. The quantitative analysis of
the Knoevenagel product formation was monitored by 1H NMR
spectroscopy at regular intervals of time. The detailed kinetic
study showed that the MPU catalysed Knoevenagel reaction is
completed within 14 h at 50 °C in THF (Fig. S15, ESI†).
presence of
accounts for
∼
∼
2.0 wt% of sulfur in the activated MPU, which
6.6 wt% of DMSO of the total weight of the
polymer (Table S2, ESI†). This result was supported by the
thermogravimetric analysis (TGA) of the activated MPU, which
showed 6.2% mass loss beginning at 180 °C till 280 °C, beyond
which the polymer starts to decompose (Fig. S7, ESI†).
In general, excellent conversions above 95% to the desired
products were observed for most of the aromatic aldehyde
substrates except for 9-anthracenecarboxaldehyde and
biphenyl-4-carboxaldehyde (Table 1). This suggests that the
A nitrogen sorption measurement at 77 K showed permanent
microporosity of the MPU with a BET surface area of 312 m2 g‒
1. The nitrogen sorption isotherm is a composite of Types I and
II with an H4 hysteresis loop (Fig. 1a). The pronounced gas
2 | J. Name., 2012, 00, 1-3
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Further, the model Knoevenagel reaction performed in the
presence of 5 mol% of homogeneous base catalysts such as,
pyrrole and piperidine/pyridine, in THF at 50 °C yielded 67%
and 98% respectively (Fig. S56 and S57, ESI†), of the desired
product within 10 h. Also, imidazole/2-methylimidazole acts as
a highly efficient homogeneous catalyst in the Knoevenagel
reaction.16 However, unlike heterogeneous solid catalysts,
these homogeneous catalysts could not be recycled and
reused, and therefore they are practically undesirable.
Table 1 Catalytic data of MPU catalysed Knoevenagel condensation reactions.
Aromatic aldehydes
Yield (%) [TON]c of α, β-unsaturated
product
R2 = ―CNa
R2= ―COOMeb
[19.6]
benzaldehyde
4-nitrobenzaldehyde
3-nitrobenzaldehyde
2-nitrobenzaldehyde
4-carboxybenzaldehyde
4-chlorobenzaldehyde
4-fluorobenzaldehyde
4-tolualdehyde
98
99
99
99
99
97
98
95
99
80
0
60
95
95
90
86
89
77
63
45
75
0
[12.0]
[19.8]
[19.8]
[19.8]
[19.8]
[19.4]
[19.6]
[19.0]
[19.8]
[16.0]
[0]
[19.0]
[19.0]
[18.0]
[17.2]
[17.8] The catalytic activity of the MPU has also been assessed in the
[15.4]
[12.6]
[9.0]
[15.0]
[0]
Knoevenagel condensation of methylcyanoacetate with
aromatic aldehydes under the same optimal reaction
conditions in THF. It has been observed that the yields are
significantly reduced for benzaldehyde (60%) and its
derivatives with electron donating methyl and methoxy groups
(Table 1). Whereas, excellent conversions above 80% have
been observed for most of the benzaldehyde derivatives with
electron withdrawing groups. As expected, reaction of
methylcyanoacetate with 9-anthracenecarboxaldehyde and
biphenyl-4-carboxaldehyde yielded no desired product due to
their larger molecular size. These results strongly suggest that
the MPU catalysed Knoevenagel reactions are closely
dependent on pore size.
4-anisaldehyde
2-naphthalenealdehyde
9-anthracenealdehyde
biphenyl-4-carboxaldehyde
[0]
0
0
[0]
a general conditions: aldehyde (1.0 mmol), malononitrile (72 mg, 1.1 mmol) in the
presence of 5 mol% (18 mg) of MPU in tetrahydrofuran at 50 °C.
b
general conditions: aldehyde (1.0 mmol), methylcyanoacetate (110 mg, 1.1
mmol) in the presence of 5 mol% (18 mg) of MPU in tetrahydrofuran at 50 °C.
c yields corresponding also aldehyde conversion are based on 1H NMR analysis of
the isolated product obtained after 14 h of reaction time (Fig. S21-S49, ESI†). TON
(Turnover number) = Moles of desired product formed/moles of catalyst.
Increasing the molar reagent ratio of benzaldehyde and
malononitrile to 1:2 with 5 mol% of MPU at 50 °C in THF
yielded 99% of the desired product after 8 h (Fig. S16, ESI†).
Thus, the reagent ratio is also an important factor that should
also be taken into consideration in the MPU catalysed
Knoevenagel condensation.
reaction occurs within the pores of the MPU which cannot
possibly accommodate substrates of larger dimensions.
Nevertheless, if the reaction has occurred, the products might
have been trapped within the pores due to their larger size as
compared to the reactants and thus, no more reaction could
occur. It should be noted that, a comparatively decreased yield
of 80% has been observed with 2-naphthaldehyde given its
larger molecular size (5.6 Å x 7.5 Å) as compared to
benzaldehyde (5.0 Å x 5.6 Å) and its derivatives. While the
molecular sizes of biphenyl-4-carboxaldehyde (4.3 Å x 10.4 Å)
Table 2 Catalytic data of the MPU catalysed Henry (nitroaldol) reactions.
Aromatic aldehydes
benzaldehyde
Yield (%) [TON] of β-nitroalkanola,b
00
94
95
94
97
90
77
20
[0]
4-nitrobenzaldehyde
3-nitrobenzaldehyde
2-nitrobenzaldehyde
4-cyanobenzaldehyde
4-carboxybenzaldehyde
4-chlorobenzaldehyde
4-tolualdehyde
[18.8]
[19.0]
[18.8]
[19.4]
[18.0]
[15.2]
[4.0]
and 9-anthracenecarboxaldehyde (6.4
Å x 9.6 Å) are
comparatively smaller than the larger micropores (16 Å) of the
MPU, but their Knoevenagel products having larger molecular
dimensions may not get out of the micropores if the reaction
has occurred.
It is important to mention here that the reaction between
benzaldehyde derivatives and malononitrile (1:1.1 molar ratio)
can proceed without any catalyst in methanol/ethanol at 50 °C
within 10 h (yield > 90% yield). However, the background
a
general conditions: aldehyde (1.0 mmol), nitromethane (610 mg, 10 mmol) in
the presence of 5 mol% (18 mg) of activated MPU in methanol at 50 °C.
b yields, corresponding also aldehyde conversion are based on 1H NMR analysis of
reaction is comparatively much slower in the case of the the isolated product obtained after 10 h of reaction time (Fig. S50-S55, ESI†). TON
(Turnover number) = Moles of desired product formed/moles of catalyst.
reaction between benzaldehyde derivatives and methylcyano-
acetate (1:1.1 molar ratio) in methanol, and the yields are
To further explore the catalytic efficiency of the MPU towards
significantly improved using 5 mol% the MPU catalyst (Table
other C‒C bond formation reactions, we have studied the
S4, ESI†). High background reaction rate has also been
Henry (nitroaldol) reaction of several aromatic aldehydes with
observed in acetonitrile (MeCN) for the reaction between
nitromethane in the presence of 5 mol% of activated MPU in
benzaldehyde and malononitrile giving a yield of 55% within 10
methanol at 50 °C (Table 2). It was observed that, excellent
h at 50 °C. These observations suggested that protic solvents
conversions above 90% to the desired β-nitroalkanol products
and nitrogen containing aprotic solvents (such as, MeCN and
were
obtained
with
nitrobenzaldehyde(s),
4-
DMF) are not generally suitable for catalyst evaluation in the
Knoevenagel reaction. Recent studies have shown that DMF as
well as water can significantly promote the Knoevenagel
condensation with quantitative yield.15
cyanobenyaldehyde and 4-carboxybenzaldehyde within 10 h
(Fig. S17, ESI†). However, no reaction has occurred with
benzaldehyde, 2-naphthaldehyde, 4-anisaldehyde and
biphenyl-4-carboxaldehyde under the same optimal reaction
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and Henry reactions and the possibility of leaching of some of
the active sites from the solid catalyst to the liquid phase has
been ruled out (see ESI† for details).
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In conclusion, we have reported a catalytically active new
microporous polyurethane material with sponge like
morphology obtained by solvothermal synthesis in DMSO. Due
to its confined microporosity and the presence of mildly basic
urethane (carbamate) functional groups along the pore walls,
the MPU showed catalytic efficiency with pronounced size
selectivity in the Knoevenagel condensation of aromatic
aldehydes with malononitrile or methylcyanoacetate. In the
related nitroaldol (Henry) reaction involving nitromethane as
the nucleophile, the catalyst showed high substrate selectivity
as significant yields of β-nitroalkanol product could only be
obtained with benzaldehyde derivatives having electron
withdrawing groups. Overall, this work represents the first
example of heterogeneous catalysis by a porous polyurethane
material which will encourage the development of new
functional porous polyurethanes for the catalysis of several
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