Mesoporous poly(melamine-formaldehyde): A green and recyclable heterogeneous organocatalyst for the synthesis of benzoxazoles and benzothiazoles using dioxygen as oxidant

Article abstract of DOI:10.1002/cctc.201402628

A simple, highly efficient, and sustainable strategy for the synthesis of benzoxazole and benzothiazole derivatives was developed by using inexpensive, green, readily available, and recyclable mesoporous poly(melamine-formaldehyde) as a green, heterogeneous organocatalyst. The corresponding substituted benzoxazoles and benzothiazoles were obtained in good to excellent yields by aerobic oxidation of o-substituted aminobenzenes with various aldehydes under dioxygen atmosphere. The catalyst can be completely recovered through simple filtration to be reused more than six times without significant loss of catalytic activity.

Full text of DOI:10.1002/cctc.201402628

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DOI: 10.1002/cctc.201402628
Mesoporous Poly(melamine–formaldehyde): A Green and
Recyclable Heterogeneous Organocatalyst for the
Synthesis of Benzoxazoles and Benzothiazoles Using
Dioxygen as Oxidant
Daoshan Yang,[a, b] Peng Liu,[a, b] Ning Zhang,[a, b] Wei Wei,[a, b] Mingbo Yue,[a, b] Jinmao You,[a, b]
and Hua Wang*[a, b]
A simple, highly efficient, and sustainable strategy for the syn-
thesis of benzoxazole and benzothiazole derivatives was devel-
oped by using inexpensive, green, readily available, and recy-
clable mesoporous poly(melamine–formaldehyde) as a green,
heterogeneous organocatalyst. The corresponding substituted
benzoxazoles and benzothiazoles were obtained in good to ex-
cellent yields by aerobic oxidation of o-substituted aminoben-
zenes with various aldehydes under dioxygen atmosphere. The
catalyst can be completely recovered through simple filtration
to be reused more than six times without significant loss of
catalytic activity.
Introduction
N-Heterocycles widely occur in natural products and biological-
ly active molecules. Especially, they are often considered as the
privileged targets by pharmaceutical industries and medicinal
chemists. As a consequence, the ongoing interest for develop-
ing versatile and more efficient approaches for the synthesis of
heterocycles has always been a thread in the synthetic com-
munity. As the important N-heterocycles, benzoxazole and
benzothiazole derivatives are found in a variety of natural
products and pharmacological agents. For example, UK1 is
a
structurally unique bisbenzoxazole (natural product),[1]
NSC693638 is used as an anticancer agent,[2] IDD552 is a well-
known aldol reductase inhibitor,[3] and RWJ-51084 is a potent
cationic trypsin precursor[4] (Figure 1). In the past few decades,
many significant methods to construct the two important
motifs have been subsequently developed. Traditionally, the
synthesis of benzoxazoles involves two approaches. One is the
metal-catalyzed intramolecular O-arylation of o-haloanilides.[5]
Another method mainly involves the condensation of 2-amino-
phenol derivatives with carboxylic acids or carboxylic halides
under strong-acid/high-temperature conditions.[6] These meth-
ods are successful, however, the uneasily available precursors
and undesired byproducts can limit their wide applications,
Figure 1. Benzoxazole and benzothiazole derivatives of related natural prod-
ucts and drugs.
which thus stimulates chemists to search for more effective
processes.
Aldehydes are important and common building blocks, and
they are easily prepared from readily available materials. Re-
cently, condensation of o-aminophenol with aldehydes under
oxidative conditions has caught considerable attention. In this
regard, the special reaction conditions, such as using undesira-
ble stoichiometric oxidants, toxic metal catalysts, the use of
strong acid, and the residual of the trace amounts of metal
catalyst in the end products, may limit their wide applica-
tions.[7] Therefore, it would be highly desirable to develop
a more effective and environmentally friendly process.
In view of the principles of green chemistry, the proposal of
the reaction conditions, media and catalysts, is still challeng-
ing.[8] Optimization of existing chemical transformations to-
[a] D. Yang, P. Liu, N. Zhang, W. Wei, M. Yue, J. You, Prof. Dr. H. Wang
Shandong Province Key Laboratory of Life-Organic Analysis
School of Chemistry and Chemical Engineering, Qufu Normal University
Jingxuan Road, Qufu 273165, Shandong (China)
E-mail: huawang_qfnu@126.com
[b] D. Yang, P. Liu, N. Zhang, W. Wei, M. Yue, J. You, Prof. Dr. H. Wang
Key Laboratory of Pharmaceutical Intermediates and
Analysis of Natural Medicine, Qufu Normal University
Qufu 273165, Shandong (China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402628.
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gether with the development of practical, environmentally
friendly processes depend greatly on improvement of catalyst
performance.[9,10] In particular, the recovery of catalysts after
the catalytic reaction and reusing them to minimize waste pro-
duction represent the central idea of the green chemistry
movement. Recently, heterogeneous organocatalysts have
drawn great interest in organocatalysis owing to their potential
advantages over homogeneous catalysts, such as efficient ac-
tivity, ease in recovery, and potential reusability.[11] Among
them, porous organic polymers (POPs), a new class of porous
materials with a cross-linked amorphous organic framework,
have emerged as a sustainable catalyst support owing to their
advantages in synthetic diversity, pore size controllability, high
specific surface area, easy pore-surface modifiability and facile
separation by filtration.[12]
Scheme 1. Synthesis of mPMF.
Very recently, poly(melamine–formaldehyde) (mPMF) materi-
al has been successfully synthesized by Zhang and co-workers,
and there is great progress in its application in CO2 capture,[13]
toxic metal removal,[14] and organic catalytic synthesis.[15] Im-
portantly, the aminal (ÀNHÀCH2ÀNHÀ) and triazine groups ex-
isting in the mPMF organic frameworks can have dual roles in
Brønsted acidity and Lewis basicity. Therefore, mPMF can act
as a bifunctional organocatalyst, which can activate the car-
bonyl functional groups by means of double hydrogen bond-
ing (Figure 2). For green and sustainable chemistry, there is an
Figure 3. Characterization of mPMF by a) TEM and b) SEM. Scale bar-
s=a) 100 nm (inset: 50 nm) and b) 500 nm.
mPMF could be expected to achieve high catalysis efficiency in
the synthetic reactions of benzoxazole derivatives.
Initially, 2-aminophenol (1a) and 4-methylbenzaldehyde (2b)
were chosen as the model substrates to optimize the reaction
conditions including the amounts of catalyst, solvents, and re-
action temperatures under dioxygen atmosphere (Table 1).
First, six solvents were tested in the presence of mPMF
(10 mg), and xylenes gave the highest yield (95%), Interesting-
ly, the absence of solvent could also afford 3b in 80% yield
(Table 1, entries 1–7). If the amount of the catalyst was
changed to 2.0 mg from 10.0 mg, the reaction yield decreased,
only 31% yield was provided (entries 6, 9, 10). Control experi-
ments confirmed that only a trace amount of target product
was observed in the absence of the catalyst (entry 8). We at-
tempted different temperatures (compare entries 6, 11–14),
and 1108C was optimal. The reaction under air also gave
a good yield (79%, entry 15). Therefore, the standard reaction
condition for the mPMF-catalyzed synthesis of benzoxazole de-
rivatives is as follows: A catalyst quantity of 10 mg of mPMF
particles and xylenes as the solvent under oxygen atmosphere.
We then investigated the scope of mPMF-catalyzed reactions
of substituted 2-aminophenols (1) with benzaldehydes (2)
under the optimized catalytic conditions determined above. As
shown in Table 2, most of the examined substrates provided
good to excellent yields of products. For the substituted ben-
zaldehydes, substrates with electron-withdrawing groups, such
as 4-fluorobenzaldehyde, 4-chlorobenzaldehyde, and 4-bromo-
benzaldehyde (Table 2, 3c–f), gave better yields than the ben-
zaldehyde substrate with the electron-donating substituent
(3b). For the substituted 2-aminophenols, electron-withdraw-
Figure 2. Chemical structure and bifunctional motif of mPMF.
increasing demand for the use of dioxygen as an oxidant
owing to its natural, inexpensive, and environmentally friendly
characteristics. Herein, we describe the use of green, efficient,
recyclable mPMF as a heterogeneous organocatalyst for the
synthesis of substituted benzoxazoles and benzothiazoles
using dioxygen as oxidant.
Results and Discussion
Poly(melamine–formaldehyde) (mPMF) was prepared according
to the literature procedure and the chemical process to mPMF
is described in the Experimental Section and Scheme 1.[13–15] As
can be seen from the TEM images (Figure 3a), mPMF has
a foam-like interconnected mesoporous network structure. It
consists of numerous spherical aggregates of submicron-sized
particles, as evidenced by SEM (Figure 3b). The unique meso-
porous particle structure with high surface-to-volume ratio of
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zole (3c’) in a very low yield. To explore the reason
for this transformation, control experiments were per-
formed. Upon treatment of benzene-1,2-diamine only
under the standard conditions, the reaction detected
by TLC was messy and no benzene-1,2-diamine was
recovered, and the results showed that benzene-1,2-
diamine could be oxidized into other substrates in
this transformation. The mPMF-catalyzed domino re-
actions could tolerate some functional groups such
as alkyl group, CÀF bonds, CÀCl bonds, CÀBr bonds,
which could be used for further modifications at the
substituted positions.
Table 1. mPMF-catalyzed condensation of 2-aminophenol (1a) with 4-methylbenzal-
dehyde (2b) leading to 2-p-tolylbenzo[d]oxazole (3b): optimization of conditions.[a]
Entry
Solvent
Temp. [oC]
Yield [%][b]
1
2
H2O
EtOH
100
80
0
0
3
4
CH3CN
THF
80
80
0
0
5
6
7
8
toluene
xylenes[c]
–
110
110
110
110
110
110
25
60
90
100
110
110
90
95
80
10[d]
31[e]
64[f]
0
40
65
74
79[g]
38[h]
We also studied the recyclability of the catalyst.
For this, we investigated the mPMF-catalyzed cycliza-
tion of 2-aminophenol (1a) with benzaldehyde (2a)
under the optimized conditions. After completion of
the reaction, the reaction mixture was cooled to
room temperature, and the catalyst was filtered from
the reaction mixture, washed with ethanol, and dried
at 1708C for 2 h and then used directly for further
catalytic reactions. No significant loss of catalytic per-
formance was observed as illustrated in Figure 4.
Further, we explored the synthetic applicability of
the method. As shown in Scheme 2, the gram-scale
reaction was performed in the usual laboratory setup
with a one-neck round-bottomed flask fitted with
a dioxygen balloon, and the reaction afforded 3a in
xylenes
xylenes
xylenes
xylenes
xylenes
xylenes
xylenes
xylenes
xylenes
9
10
11
12
13
14
15
16
[a] Reaction conditions: 2-aminophenol (1a, 0.75 mmol), benzaldehyde (2b,
0.5 mmol), catalyst (10.0 mg), solvent (0.5 mL) reaction time, 10 h; under oxygen at-
mosphere. [b] Isolated yield. [c] Mixture of o-, m-, and p-xylene. [d] Without catalyst.
[e, f] In the presence of catalyst (2.0 mg; 5.0 mg; respectively). [g] Under air condi-
tions. [h] Using melamine as the catalyst.
ing as well as electron-donating substituents did not signifi-
cantly affect the catalytic activity. The steric hindrance in the
benzaldehydes significantly affected the catalytic efficiency
(3b’). Although aromatic aldehydes displayed high reactivity,
aliphatic ones were poor substrates. If aliphatic aldehydes such
as butyraldehyde, heptanal, and cyclohexanecarbaldehyde
were used as the substrates under the optimal reaction condi-
tions, no desired product was obtained. The reason might be
the weak hydrogen bonding between aliphatic aldehyde and
mPMF (see the reaction mechanism in Scheme 3). Under simi-
lar conditions, the methodology was extended to the synthesis
of various benzothiazoles from o-aminothiophenol. The results
are also summarized in Table 2. Unfortunately, treatment of
benzene-1,2-diamine (1g) with benzaldehyde (2a) under the
optimized conditions produced 2-phenyl-1H-benzo[d]imida-
92% yield. This example clearly demonstrates the practical
aspect of this newly developed method.
To explore the mechanism on mPMF-catalyzed condensation
of o-aminophenols with aldehydes, a control experiment was
Scheme 2. Synthesis of 3a on a gram scale.
Figure 4. Recycling of the mPMF catalyst.
Figure 5. Control experiments using different amine catalysts.
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tion activity. In contrast, mPMF provided su-
perior activity. The results indicated that
mPMF containing of the high density of
aminal (ÀNHÀCH2ÀNHÀ) groups and triazine
rings plays the dual roles of Brønsted acidity
and Lewis basicity. The interaction between
the triazine rings and the hydroxyl on o-ami-
nophenol could shorten the distance be-
tween the two molecules, so as to make it
suitable for nucleophilic attack.
Table 2. mPFM-catalyzed synthesis of benzoxazoles and benzothiazoles, showing products and
isolated yields.[a]
We also calculated the structures and
bonding energies of mPMF system and mela-
mine system using Gaussian 03 program
package[16] (Figure 6). Calculations were con-
ducted in the framework of DFT by use of
the hybrid B3LYP functional[17] combined
with the standard basis set 6-31G(d,p). The
calculated structure confirmed that benzalde-
hyde formed two H bonds with mPMF and
only one with melamine. Hydrogen bonding
interaction played a dominant role between
the reactants and the catalyst, and the hy-
drogen bonding energy in the mPMF system
(23.27 kcalmolÀ1) was higher than that in the
melamine system (19.65 kcalmolÀ1, Figure 6).
On the basis of these results above and ac-
cording to the previous report, a possible
mechanism is thus proposed as illustrated in
Scheme 3. First, coordination of mPFM to the
benzaldehyde (binding and activation), fol-
lowed by the nucleophilic attack of amino of
o-aminophenol gives (E)-2-(benzylideneami-
no)phenol (I) with elimination of water, and I
transforms into II in the presence of mPMF,
which presents dual functionalities of Brønst-
ed acidity and Lewis basicity. The intermedi-
ate II then subsequently undergoes aromati-
zation by dioxygen oxidation under the reac-
tion conditions to afford the desired
products.
Conclusion
We have developed a simple, green, and effi-
cient strategy for the synthesis of benzoxa-
zoles and benzothiazoles. The couplings
were performed by using readily available
starting materials (o-substituted aminoben-
zene and various aldehydes), recyclable het-
erogeneous organocatalyst mPMF as the cat-
alyst, and dioxygen as the green oxidant, im-
portantly, organic oxidizing agents, strong
acids, or bases were not necessary. The pres-
ent method shows eco-friendly, economical,
[a] Reaction conditions: o-substituted aminobenzene (1, 0.75 mmol), benzaldehyde (2,
0.5 mmol), catalyst (10 mg), solvent (0.5 mL) under oxygen atmosphere.
performed as shown in Figure 5. Treatment of o-aminophenol
(1a) with benzaldehyde in the presence of other small amine-
based molecules under dioxygen for 36 h gave very low reac-
and practical advantages over the previous methods. Further
applications of mPFM in organic transformations are in prog-
ress in our laboratory.
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chromatography on silica gel by
using petroleum ether/ethyl ace-
tate as the eluent to provide the
desired product (3).
Recycling of the mPMF catalyst
After completion of the reaction,
the reaction mixture was trans-
ferred to a centrifugal tube, and
then was filtered after centrifuga-
tion. The residue solid was washed
with ethanol (3ꢁ5 mL) to remove
all traces of product or reactant
present, dried at 1708C in an oven
for 2 h to provide the recycling
mPMF about 9.84 mg. The recov-
ered solid catalyst was used for
further catalytic reactions.
Acknowledgements
The authors gratefully acknowl-
edge the financial support from
the National Natural Science
Foundation of China (nos.
21302110,
21375075,
and
Scheme 3. Proposed mechanism for the direct transformation.
21302109), the Taishan Scholar
Foundation of Shandong Prov-
ince, the Natural Science Founda-
tion of Shandong Province (ZR2013BQ017 and ZR2013BQ015),
the Project of Shandong Province Higher Educational Science
and Technology Program (J13LD14).
Keywords: green chemistry · mesoporous materials · nitrogen
heterocycles · organocatalysis · oxidation
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Received: August 9, 2014
Revised: August 30, 2014
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D. Yang, P. Liu, N. Zhang, W. Wei, M. Yue,
J. You, H. Wang*
&& – &&
Mesoporous Poly(melamine–
formaldehyde): A Green and
Recyclable Heterogeneous
Organocatalyst for the Synthesis of
Benzoxazoles and Benzothiazoles
Using Dioxygen as Oxidant
Going round in heterocycles: Benzoxa-
zole and benzothiazole derivatives are
synthesized by using mesoporous poly-
(melamine–formaldehyde) as a green
and recoverable catalyst. This heteroge-
neous organocatalytic method can pro-
vide a novel and efficient strategy for
the synthesis of other N-heterocycles.
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