Mesoporous poly-melamine-formaldehyde (mPMF)-a highly efficient catalyst for chemoselective acetalization of aldehydes

Article abstract of DOI:10.1039/c3gc40297e

A mesoporous poly-melamine-formaldehyde polymer with a high surface area, good porosity and a high density of amine and triazine functional groups was investigated as a highly efficient hydrogen-bonding catalyst. This porous organic polymer was found to be highly effective in catalyzing chemoselective acetalization of aldehydes, without the consumption of any dehydrating agents. The turnover frequency of mesoporous poly-melamine-formaldehyde is hundreds of times higher than melamine monomer, and this high efficiency is due to the high density of aminal (-NH-CH₂-NH-) groups and triazine rings in the polymer network, which provides an inherently powerful system with multiple hydrogen bonds. This unique characteristic imparts mesoporous poly-melamine-formaldehyde polymer with a very high activity as a heterogeneous organocatalyst. The polymer is also low cost, and easy to be synthesized and recycled.

Full text of DOI:10.1039/c3gc40297e

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Mesoporous poly-melamine-formaldehyde (mPMF) – a
highly efficient catalyst for chemoselective
acetalization of aldehydes†
Cite this: Green Chem., 2013, 15, 1127
Received 11th February 2013,
Accepted 25th March 2013
DOI: 10.1039/c3gc40297e
Mei Xuan Tan, Liuqun Gu, Nannan Li, Jackie Y. Ying* and Yugen Zhang*
www.rsc.org/greenchem
A mesoporous poly-melamine-formaldehyde polymer with a high framework and then introducing the catalytic centers via
surface area, good porosity and a high density of amine and tri- ligands,5 and (iii) directly using catalytic centers as monomers
azine functional groups was investigated as a highly efficient or building blocks in the synthesis of porous materials.6
hydrogen-bonding catalyst. This porous organic polymer was Although these strategies have been studied for many years,
found to be highly effective in catalyzing chemoselective acetali- their inherent drawback in catalytic activity as compared to the
zation of aldehydes, without the consumption of any dehydrating original small molecular catalysts has remained a challenge.
agents. The turnover frequency of mesoporous poly-melamine- Herein we report a novel mesoporous poly-melamine-formal-
formaldehyde is hundreds of times higher than melamine dehyde (mPMF) material that exhibits superb activity in cata-
monomer, and this high efficiency is due to the high density of lyzing the acetalization of aldehydes, even when compared to
aminal (–NH–CH2–NH–) groups and triazine rings in the polymer melamine. The high activity of mPMF is attributed to its con-
network, which provides an inherently powerful system with mul- densed network structure with a multifunctional hydrogen
tiple hydrogen bonds. This unique characteristic imparts meso- bonding system.
porous poly-melamine-formaldehyde polymer with a very high
Protection of carbonyl compounds such as aldehydes and
activity as a heterogeneous organocatalyst. The polymer is also ketones by acetal formation via reaction with alcohols or diols
low cost, and easy to be synthesized and recycled.
is a common and useful technique for multistep synthesis in
drug design, organic and carbohydrate chemistry, and the
pharmaceuticals, cosmetics and fragrances industries.7 Many
types of catalysts, including conventional acids (e.g. HCl, tri-
fluoroacetic acid (TFA) and p-toluenesulfonic acid (p-TSA)8),
solid acids,9 functionalized silica,10 acidic polymers,11 Lewis
acids,12 metal catalysts,13 small organic molecules,14 and
natural materials15 (such as kaolinitic clay), have been
reported to catalyze the acetal protection of carbonyls.
Although acetalization has been widely reported and well-
investigated, it suffers from drawbacks such as the use of a
corrosive acid catalyst,16 the need for an excess amount of
drying agents, poor chemoselectivity, and incompatibility with
substrates containing acid-sensitive functional groups. There
is increasing interest in developing an acetalization protocol
that is mild, greener, chemoselective, cost-effective and atom-
efficient, with ease of catalyst recycling and no excessive use of
water scavenging reagents.17
Introduction
The search for alternative technologies that are greener, safer
and environmentally friendly is a research priority, especially
for the chemical and pharmaceutical industries. In particular,
the use of recyclable catalysts for organic synthesis to mini-
mize waste production and optimize catalyst efficiency is of
great interest.1,2 Porous organic polymers (POPs), a class of
highly crosslinked amorphous polymers possessing nano-
pores, have recently emerged as a versatile platform for cata-
lysts due to their high structural stability. The bottom-up
synthesis for POPs provides an opportunity to design poly-
meric frameworks with various functionalities for use as cata-
lysts or ligands.3 Current strategies for the design of porous
heterogeneous catalysts include: (i) surface modification and
catalyst immobilization,4 (ii) incorporation of ligands into the
Results and discussion
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos,
Singapore 138669, Singapore. E-mail: jyying@ibn.a-star.edu.sg,
ygzhang@ibn.a-star.edu.sg; Fax: (+65)6478-9020
Recently, Kotke and Schreiner reported the acetalization of
carbonyl compounds catalyzed via double hydrogen bonding
using an electron-deficient thiourea derivative in the presence
†Electronic supplementary information (ESI)
available. See DOI:
10.1039/c3gc40297e
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of trialkyl orthoformate as a water scavenger.18 The electron-
deficient thiourea catalyst was designed to be capable of
‘partial protonation’ of carbonyl compounds via hydrogen
bonding. Herein we propose the use of melamine, which has a
low pKa value (5.3) and both hydrogen donating and accepting
sites, as a catalyst for aldehyde acetalization. Our preliminary
studies demonstrated melamine-catalyzed cinnamaldehyde
acetalization with methanol (Table S1†), but the activity was
very low. The reaction was sluggish at room temperature over
10 mol% melamine (<5% yield for 24 h). At 60 °C, 31% yield
was obtained in 24 h without using any water scavenger.
Although this melamine-catalyzed acetalization reaction was
not practical due to the low activity, it convinced us of the
potential of a more powerful hydrogen-bonding system for
catalyzing acetalization reactions.
Table 1 Acetalization of trans-cinnamaldehyde over mPMFa
Entry mPMF (mg) Alcohol Temp. (°C) Time (h) Yieldb (%)
1
2
CH3OH 25
8
24
8
8
8
65
68
64
84
0
46
62
83
85
77
81
2
2
2
—
2
CH3OH 40
CH3OH 25
CH3OH 25
3c
4
5
PDO
25
4
8
24
8
8
6
7
8
2
10
50
PDO
PDO
PDO
40
25
25
8
a Reaction conditions: 1 mmol of trans-cinnamaldehyde in 25 mmol of
methanol or 13 mmol of PDO. b Gas chromatography (GC) yield.
c Trimethyl orthoformate (1.65 eq.) was used.
Specifically, we have designed mPMF with the goal of deriv-
ing a high density of triazine and aminal (–NH–CH2–NH–)
functional groups. By mixing melamine and paraformaldehyde
in hot DMSO,19 we derived a POP with a high surface area of
930 m2 g−1 and an average pore size of 15.7 nm (see ESI
Fig. S1–S4†). A foam-like interconnected mesoporous network
structure was observed under transmission electron micro-
scopy (TEM), while scanning electron microscopy (SEM)
revealed that the mPMF consisted of aggregates of submicron-
sized spherical particles (Fig. 1(b) and (c)). mPMF was rich in
aminal (–NH–CH2–NH–) and triazine groups (Fig. 1(a)), which
would provide a powerful and highly dense hydrogen-bonding
system. The high surface area and mesoporous structure of
mPMF would be highly desirable for applications in hetero-
geneous catalysis.
Preliminary studies showed that the reaction of aldehyde
with methanol to form dimethyl acetal proceeded well over the
mPMF catalyst (Table 1, entries 1–3). trans-Cinnamaldehyde
was chosen as the model substrate as it contains an α,β-un-
saturated CvC bond, which is sensitive to acidic conditions,
and can undergo carbon–carbon double bond isomerization.
mPMF was found to catalyze the formation of dimethyl acetal
from trans-cinnamaldehyde and methanol without cis–trans
isomerization.20 The reaction reached equilibrium with 65%
yield in 8 h at room temperature using 2 mg of mPMF
(Table 1, entry 1). mPMF was superior in activity to melamine.
Further increase in reaction time or reaction temperature did
not lead to increased yield (Table 1, entries 1 and 2), probably
due to the presence of the water by-product. The addition of
trimethyl orthoformate as a water scavenger increased the
yield to 84% (Table 1, entry 3). Without the mPMF catalyst, no
reaction was observed after 8 h (Table 1, entry 4).
When acetalization was conducted in 1,3-propanediol
(PDO) to form 1,3-dioxane-protected cinnamaldehyde, a good
yield of 83% was achieved within 24 h at room temperature
without the use of drying agents or water scavengers (Table 1,
entry 5). The equilibrium was shifted to the product due to
the formation of a more stable 6-membered cyclic 1,3-dioxane
structure, instead of dimethyl acetal, which would be more
prone to hydrolysis. When the reaction temperature was
increased to 40 °C, a yield of 85% was attained in a shorter
reaction time of 8 h (Table 1, entry 6).
For a reaction to be feasible for industrial application, a
recyclable heterogeneous catalyst and a short reaction time
would be highly attractive.21 Table 1 shows that a product yield
of 77% could be achieved in 8 h at room temperature over
10 mg of mPMF (entry 7). This yield could be further increased
to 81% when 50 mg of mPMF was used (Table 1, entry 8).
Furthermore, it was found that the reaction rate could be
remarkably increased by increasing the reaction temperature.
Fig. 2 shows that 90% yield of acetal was obtained in less than
1 h at 60 °C. The short reaction time associated with the use of
mPMF allowed for the possibility of industrial application in
flow reactors. Hence, the stability of the mPMF catalyst was
further examined. mPMF was a solid powder that could be
easily filtered and separated from the reaction mixture. This
Fig. 2 Effect of reaction temperature on acetalization of trans-cinnamaldehyde
with mPMF. Reaction conditions: 1 mmol of trans-cinnamaldehyde in 1 ml of
PDO, 50 mg of the mPMF catalyst.
Fig. 1 (a) Chemical structure, (b) transmission electron microscopy (TEM) and
(c) scanning electron microscopy (SEM) images of mPMF.
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Table 2 Acetalization of various substrates in PDO over the mPMF catalysta
Entry
1
Substrate
Temp. (°C)
25
Yieldb (%)
94 (90)
2
3
4
5
25
40
85 (78)
76 (74)
48 (75)
Fig. 3 Recycling of the mPMF catalyst. Reaction conditions: 2.5 mmol of trans-
cinnamaldehyde in 2.5 ml of PDO, 5 mg of mPMF, 25 °C, 48 h.
40
60
40
60
80
40
60
80
Trace
71
97 (94)
Trace
58
97 (90)
heterogeneous organocatalyst exhibited excellent stability in
recycling, with no loss in catalytic activity over at least 5 runs
(Fig. 3). The use of the mPMF catalyst is advantageous over
conventional acetalization protocols in industrial reactors
because it is non-corrosive, does not require the use of a
co-solvent, is compatible with acid-sensitive substrates, has
minimal chemical waste, and provides ease of catalyst recovery
and reuse. While polymer-supported catalysts and reagents
have been around for decades,22 they still have a long way to
go before they move to industrial applications due to their
high costs. As remarked by Nicolaou and Snyder, “Every syn-
thetic technology … has a drawback. For solid-supported
reagents, it is their cost.”23 In this case, however, the ease of
synthesis, low cost of production, high activity, high stability,
and mild reaction conditions make mPMF a promising catalyst
for large-scale acetalization of aldehydes.
The substrate scope for acetalization in PDO over mPMF
was examined (Table 2). The reaction was found to be chemo-
selective to aldehydes, and did not occur with ketones
(Table 2, entries 14 and 15). Thus, this mild protocol could be
useful for the protection of aldehydes in the presence of
ketones and acid-sensitive groups. Generally, the protocol
worked well for various aryl aldehydes (Table 2, entries 1 to
10). For the more challenging substrates with electron-with-
drawing groups, higher reaction temperatures (e.g. 80 °C) were
required to give satisfactory yields (>95%) (Table 2, entries 5
and 6). Good yields were also obtained at 40 °C for derivatives
of trans-cinnamaldehyde, such as 4-nitrocinnamaldehyde
(Table 2, entry 11) and α-methyl-trans-cinnamaldehyde
(Table 2, entry 12). Importantly, the reaction also worked well
for aliphatic aldehydes. As an example, 95% of acetal yield was
achieved for hydrocinnamaldehyde at 60 °C (Table 2, entry 13).
The mechanism of mPMF and related molecules in the
acetalization of aldehydes was studied. Firstly, melamine and
other small molecules with similar hydrogen bonding func-
tionalities were examined as catalysts in the model reaction
(Table S1†). With 3-aminopyridine, which has an amine group
in close proximity to the nitrogen on the pyridine ring, a
moderate product yield of 53% was obtained at 60 °C after
24 h. With 4-aminopyridine, almost no reaction was found.
Reaction over 2-aminopyridine resulted in decomposition,
6
7
60
99 (98)
8
60
80
54
96 (90)
9
40
40
97 (94)
10
84 (79)c
11
12
40
87 (73)
25
40
52
97 (94)
13
14
15
40
60
29
95 (87)
d
40
—
d
40
—
a Reaction conditions: 1 mmol of substrate in 13 mmol of PDO, 2 mg
of mPMF catalyst, 24 h, unless otherwise specified. b GC yield; isolated
yield in parentheses. c Reaction time: 8 h. d No reaction.
yielding a dark brown mixture. With 2-aminopyrimidine as a
catalyst, 47% yield was obtained. The 2-pyridinol catalyst led to
a moderate yield of 52%, despite its high pKa value of 11.7.
The results here suggested that hydrogen bonding activation
could play a major role in aldehyde activation (Fig. S5†) and
acetal formation. The acetalization reaction proceeded with
2-pyridinol, but not with 4-aminopyridine probably because
the latter did not favor the synergistic double hydrogen
bonding model with its amino group in the para-position
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Fig. 6 Proposed mechanism for aldehyde activation over the mPMF catalyst via
synergistic double hydrogen bonding.
Fig. 4 1H NMR spectra of (a) trans-cinnamaldehyde and (c) melamine when
these two compounds are mixed, (b) pure trans-cinnamaldehyde, and (d) pure
melamine in CDCl3.
the acetalization reaction. Density functional theory (DFT)
modeling at B3LYP/6-31G(d,p) level also indicated favorable
hydrogen bonding between trans-cinnamaldehyde with mela-
mine (Fig. 5(a)) and mPMF (Fig. 5(b)). Interestingly, the hydro-
(Fig. S5(c)†). Small molecules with both hydrogen bond
donation and accepting sites in adjacent position might bring
the reactants together and promote the acetalization reaction
(Fig. S5(a) and (b)†). The presence of hydrogen bonding was
also observed in nuclear magnetic resonance (NMR) studies
(Fig. 4 and S6†). The NMR spectrum of the mixture of trans-
cinnamaldehyde and melamine in CDCl3 showed a downfield
shift in the doublet peak of aldehyde (–CHO) proton from
9.630–9.655 ppm to 9.703–9.723 ppm. At the same time, both
a downfield shift and a broadening of melamine (–NH2)
protons from 1.566 ppm to 1.712 ppm in CDCl3 were observed
(Fig. 4). The downfield shift and broadening of proton peaks
indicated the presence of hydrogen bonding interactions
between the aldehyde substrate and melamine.
gen bonding energy in the mPMF system (38.19 kcal mol−1
)
was much higher than that in the melamine system (7.76 kcal
mol−1). This might also explain the superior activity of mPMF
as compared with melamine. Fig. 6 illustrates the proposed
mechanism for aldehyde activation over the mPMF catalyst.
mPMF consisted of a high density of aminal (–NH–CH2–NH–)
groups and triazine rings, which presented dual functional-
ities of Brønsted acidity and Lewis basicity. This unique
characteristic of mPMF enabled multiple types of hydrogen
bonding interactions. The synergistic effect of these hydrogen
bonding interactions promoted the acetalization reaction in a
highly efficient manner.
Generally, the reactions using small amine-based catalysts
were sluggish (Table S1†). In contrast, mPMF provided
superior activity (Tables 1 and 2). Structurally, the mPMF cata-
lyst contained a high density of triazine rings and aminal
groups. It could provide two types of hydrogen bonding
models for both aldehyde and alcohol (Fig. 5 and 6). One
involved a bridged hydrogen bonding between the oxygen
atom of the aldehyde and the two –NH functional groups of
the aminal group; the other involved hydrogen bonding
between methanol and triazine. The electron-deficient aminal
groups in mPMF might have partially protonated carbonyl
compounds via hydrogen bonding.18 The formation of hydro-
gen bonding activated the carbonyl substrate and stabilized
the negative charge of the carbonyl oxygen in the transition
state during acetalization. At the same time, alcohols were also
stabilized and activated by triazine groups via hydrogen
bonding interactions. Such a synergistic hydrogen bonding
effect could be the reason for the superior activity of mPMF in
Conclusions
mPMF-catalyzed acetalization of aldehydes is a green and
chemoselective protocol. The reaction was highly efficient,
compatible with acid-sensitive substrates, and did not require
any dehydrating agents. A variety of substrates including ali-
phatic and aromatic aldehydes were reacted with good-to-
excellent yields. A synergistic hydrogen bonding activation
mechanism for the acetalization reaction was proposed. The
high density of aminal (–NH–CH2–NH–) groups and triazine
rings in the mPMF network and the inherently powerful
multiple hydrogen bonding system were responsible for the
high activity of this novel POP, as compared to its melamine
monomer. In addition, mPMF was attractive for its low cost,
ease of synthesis and excellent reusability. This material
demonstrated great potential for the application of function-
ally tailored POPs as highly efficient heterogeneous catalysts.
Experimental
General information
GC-mass spectrometry (GC-MS) was performed with Shimadzu
GC-2010 coupled with GCMS-QP2010. 1H and 13C NMR spectra
were obtained using a Brucker AV-400 (400 MHz) spectrometer.
Nitrogen sorption analysis was conducted on a Micromeritics
Tristar 3000 at 77 K. Elemental analysis (C, H, N, S) was
performed on Elementar Vario Micro Cube.
Fig. 5 DFT modeling for trans-cinnamaldehyde with (a) melamine (binding
energy = 7.76 kcal mol−1) and (b) mPMF (binding energy = 38.19 kcal mol−1).
1130 | Green Chem., 2013, 15, 1127–1132
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Synthesis and characterization of mPMF
Acknowledgements
Melamine (0.378 g, 3 mmol) and paraformaldehyde (1.8 eq,
0.162 g, 5.4 mmol) were mixed with 3.36 ml (overall concen-
tration of 2.5 M) of dimethyl sulfoxide (DMSO) in a 15 ml
Teflon container secured in a steel reactor. The reaction
mixture was heated to 120 °C in an oven for 1 h. The reactor
This work was funded by the Institute of Bioengineering and
Nanotechnology (Biomedical Research Council, Agency for
Science, Technology and Research, Singapore).
was then carefully removed from the oven for stirring on a References
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General acetalization procedure
The glassware used was dried in the oven at 100 °C. 2 mg of
the mPMF catalyst was used with 1 mmol of carbonyl substrate
and 1 ml of a solvent (methanol (anhydrous, 99.8%) or PDO
(98%)). The reaction mixture was stirred at room temperature
or heated to reaction temperature, and monitored by GC-MS.
The product was extracted using CH2Cl2, washed with H2O
and brine, and dried over Na2SO4. The organic layer was con-
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