Hollow Hyper-Cross-Linked Nanospheres with Acid and Base Sites as Efficient and Water-Stable Catalysts for One-Pot Tandem Reactions

Article abstract of DOI:10.1021/acscatal.6b03631

A task-specific functionalized hyper-cross-linked polymer (HCP) with hollow spherical structure was synthesized by an easily accessible Friedel-Crafts reaction-based approach. A harmonious coexistence of acid (sulfonic acid) and base (amine) sites on a microporous organic material was achieved. The acid-base bifunctional HCP catalyst (HCP-A-B) structure was fully characterized by many physicochemical methods. In the subsequent tandem reactions (hydrolysis/Henry and hydrolysis/Knoevenagel reactions), the HCP-A-B catalyst displayed high catalytic efficiency and chemical stability toward water or organic solvent. These HCP-A-B catalyst characteristics led to the development of a previously unreported transformation of 2-ethoxy-3,4-dihydropyran derivative to a 2-cyclohexen-1-one derivative through a tandem reaction involving water-assisted ring-opening hydrolysis of the dihydropyran, an intramolecular aldol reaction, and a dehydration reaction. In all of these reactions, the bifunctional HCP-A-B catalyst can be recovered and reused more than 10 times without significant loss of activity.

Full text of DOI:10.1021/acscatal.6b03631

Research Article
pubs.acs.org/acscatalysis
Hollow Hyper-Cross-Linked Nanospheres with Acid and Base Sites as
Efficient and Water-Stable Catalysts for One-Pot Tandem Reactions
Zhifang Jia,†,∥,‡ Kewei Wang,†,‡,∥ Bien Tan,*,† and Yanlong Gu*,†,§
†Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material
Chemistry and Service Failure, and School of Chemistry and Chemical Engineering, Huazhong University of Science and
Technology, Wuhan 430074, China
‡Department of Chemistry and Chemical Engineering, Shanxi Datong University, Datong 037009, China
§State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Lanzhou 730000, China
S
* Supporting Information
ABSTRACT: A task-specific functionalized hyper-cross-
linked polymer (HCP) with hollow spherical structure was
synthesized by an easily accessible Friedel−Crafts reaction-
based approach. A harmonious coexistence of acid (sulfonic
acid) and base (amine) sites on a microporous organic
material was achieved. The acid−base bifunctional HCP
catalyst (HCP−A−B) structure was fully characterized by
many physicochemical methods. In the subsequent tandem
reactions (hydrolysis/Henry and hydrolysis/Knoevenagel
reactions), the HCP−A−B catalyst displayed high catalytic
efficiency and chemical stability toward water or organic
solvent. These HCP−A−B catalyst characteristics led to the
development of a previously unreported transformation of 2-
ethoxy-3,4-dihydropyran derivative to a 2-cyclohexen-1-one derivative through a tandem reaction involving water-assisted ring-
opening hydrolysis of the dihydropyran, an intramolecular aldol reaction, and a dehydration reaction. In all of these reactions, the
bifunctional HCP−A−B catalyst can be recovered and reused more than 10 times without significant loss of activity.
KEYWORDS: hollow sphere, hyper-cross-linked polymers (HCPs), water-stable, bifunctionalized catalyst, cyclohexenone,
tandem reactions
unsolved. These problems are low stability of the catalyst,
unsatisfactory catalytic performance resulting from weak acidity
or alkalinity and ineffective mass transfer of substrate in the
support, and cost increase because of the tedious preparation
procedure. A bifunctional catalyst is an ideal option for
establishing a multistep continuous-flow system. However,
numerous reactions are associated with water generation.
Moreover, water produced in the upstream reactions is difficult
to remove from the continuous-flow system. Therefore, the
water stability of the bifunctional catalyst is crucial.11
Accordingly, highly stable organic porous materials constructed
with aromatic building blocks would be a good choice. This
speculation has been seemingly confirmed by the use of a
porous aromatic framework12 in the preparation of water-stable
acid−base catalyst. In this illustration, although the obtained
acid−base catalyst reportedly possessed water stability, its
durability was unsatisfactory because the catalyst was reused
only twice. Additionally, the expensive catalyst (Ni(COD)2)
INTRODUCTION
■
A multifunctional catalyst improves the overall efficiency of
synthetic reactions,1 and thus, the ability to design and control
the synthesis of multifunctional heterogeneous catalysts is of
interest for modern multistep organic synthesis, in which
different catalysts and sometimes those with antagonistic
catalytic sites are required. Through tandem reactions, the
use of multifunctional catalysts in fine-chemical synthesis not
only simplifies the operational procedure by omitting the step
of isolating the intermediate but also offers us an effective
means to develop new reactions that cannot be attained with
other methods.2 Considerable effort has been focused in this
direction. A well-known example is the bifunctional acid−base
catalyst, in which the acid and base sites are located at different
places on the same support.3 To date, numerous acid−base
bifunctional catalysts4 have been prepared using several
materials, including metal−organic frameworks,5 mesoporous
organosilicas,6,7 MFI zeolite,8 cross-linked micelles,9 and
Pickering emulsions.10 Although the acidic and basic sites are
anchored on the same support, to maximize the catalytic
activity and facilitate the practical utilization of these materials
for developing a useful new reaction, some problems remain
Received: December 21, 2016
Revised: April 7, 2017
© XXXX American Chemical Society
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and harsh conditions were required in the synthesis of the
catalyst support. Therefore, an efficient, highly durable, and
water-stable acid−base organic material catalyst prepared by an
easily accessible approach is needed.
RESULTS AND DISCUSSION
■
As shown in Figure 1, preparing the bifunctional catalyst
(HCP−A−B) involves three steps. First, a microporous organic
support (HCP), a hollow nanosphere, is synthesized using a
SiO2@PS19 composite as a precursor. Second, HCP is modified
by using acetyl sulfate as the precursor for the acid sites,
affording HCP−A. In this step, because of the spherical
structure of HCP, it is very likely that most of the sulfonation
reactions occur on the external shell of the sphere by control of
the amount of acetyl sulfate and manipulation of the reaction
conditions as described in the Experimental Section. Third,
Friedel−Crafts-type reaction-induced decoration of the HCP−
A polymer occurs through the use of 2-(hydroxymethyl)-
isoindoline-1,3-dione as an amine precursor, yielding HCP−A−
B. Because of the deactivation effect of the sulfonic groups21 on
the benzene rings, the Friedel−Crafts reactions of 2-
(hydroxymethyl)isoindoline-1,3-dione on the benzene rings
bearing sulfonic groups are prevented. Therefore, the amine
precursors react preferentially with the active benzene rings of
the microsphere.
Hyper-cross-linked polymers (HCPs), a novel class of
microporous organic polymers, possess numerous promising
and unique properties, such as high surface area, low-cost
synthesis, hydrophobic skeleton, and high thermal stability.13
Because these materials can be easily prepared through the one-
step Friedel−Crafts reaction of an aromatic precursor with an
external cross-linking reagent, fine control of the pore structure
of HCPs by changing the monomer is possible.14 Moreover, the
aromatic ring as the building block and the rigid linkage
methylene segment ensure that this material has substantial
chemical stability even in water, acidic, or basic media.15 All of
these advantages allow HCPs to be applied in multiple areas,
including gas storage and separation,16 drug release,17
catalysis,18 and removal of heavy metal ions from wastewater.19
HCPs are intrinsically ideal choices for synthesizing solid
catalysts because of their unique properties. In particular, HCPs
have been utilized successively by our team as supporting
materials to implement the immobilization of metal-complex
catalysts.20 We envisaged that if two antagonistic species were
covalently bonded onto the different aromatic rings of an HCP,
then their interaction may be minimized thanks to the rigid
structure of carrier. A new acid−base bifunctional HCP catalyst
(HCP−A−B) synthesized by an easily accessible Friedel−
Crafts reaction-based approach (Figure 1) is presented herein.
All of the obtained HCPs were subjected to spectroscopic
characterizations. The Fourier transform infrared spectroscopy
(FT-IR) spectra in Figure 2 show the characteristic peaks of the
Figure 2. FT-IR spectra of HCP materials.
−CH2− stretching band at 2981 and 2870 cm−1 and C−H
modes of the benzene ring at around 1610−1430 and 3030
cm−1 for all of the HCPs. The disappearance of characteristic
peak for Si−O−Si at about 1097 cm−1 in HCP−A−B infers
that the template SiO2 was fully etched and the hollow
structure was successfully obtained. The absorption peaks at
approximately 1382, 1198, and 1037 cm−1 in both HCP−A−B
and HCP−A are assigned to the asymmetric and symmetric
stretching vibrations of S−O from sulfonic acid groups.22 The
broad, strong band ranging from 3200 to 3650 cm−1 in both
HCP−A−B and HCP−A may be associated with the −OH
stretching vibration. The broad, strong absorption band ranging
from 3200 to 3500 cm−1 is associated with the −NH2 stretching
vibration in HCP−A−B. These results prove the successful
introduction of sulfonic and amine groups into the hollow
organic porous polymer.
Figure 1. Schematic illustration of HCP−A−B. (NHPI = 2-
(hydroxymethyl)isoindoline-1,3-dione; TfOH = triflic acid; TFA =
trifluoroacetic acid).
The acidic (sulfonic acid) and basic (benzylamine) sites of this
catalyst peacefully coexist in a hollow microsphere. Thus, a
compatibility of antagonistic catalytic sites on an organic
microporous material has been successfully achieved. This
hollow spherical structure not only offers a combination of two
antagonistic microenvironments but also enables rapid diffusion
of the substrate, thereby maximizing the catalytic efficiency of
the acid−base catalyst. The obtained bifunctional material was
used as an efficient catalyst for two tandem reactions, namely,
hydrolysis/Henry and hydrolysis/Knoevenagel reactions, in
which good catalytic performance and stability in organic
solvent or water are displayed. With this catalyst, a hitherto-
unreported water-assisted tandem reaction of a 2-ethoxy-3,4-
dihydropyran derivative through a ring opening/aldol/dehy-
dration sequence has been established for the first time, thereby
providing a new method to synthesize 2-cyclohexen-1-one
derivatives, which are important synthetic intermediates.
The thermal stability of HCP−A−B was investigated by
thermogravimetric analysis (TGA). The TGA curves (Figure
S1 in the Supporting Information) clearly show that the initial
decomposition temperature of the HCP−A−B catalyst is close
to 400 °C. Thus, the organic skeleton of HCP−A−B possesses
excellent thermal stability.
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The scanning electron microscopy (SEM) images of HCPs
in Figure 3 show that the introduction of organic functional
respectively. Evidently, the BET surface areas decreased slightly
after the acid and base functional groups were introduced into
the support. This decrease can be attributed to partial pore
filling by the functional groups. The mass increase is also
partially responsible. However, the BET surface area of HCP−
A−B is sufficiently high for catalytic uses. As shown in Figure
5a, the adsorption and desorption isotherms of HCP−A,
Figure 3. (a−e) SEM images of (a) HCP, (b) HCP−A, (c) HCP−A−
B, (d) HCP−B, and (e) recovered HCP−A−B. Scale bars are 200 nm.
(f−j) TEM images of (f) HCP, (g) HCP−A, (h) HCP−A−B, (i)
HCP−B, and (j) recovered HCP−A−B. Scale bars are 200 nm.
groups to HCP did not distinctly influence the morphology and
particle size. Figure 3c shows the perfect spherical shape of
HCP−A−B with a uniform size distribution, flat surfaces, and a
microsphere diameter of 300−330 nm. Figure 3h shows the
hollow nanosphere structure of HCP−A−B as confirmed by
transmission electron microscopy (TEM), with an inner
diameter of approximately 100−120 nm and a spherical shell
with a thickness of approximately 100−110 nm. Moreover, the
elements N and S of the HCP−A−B catalyst from the sulfoacid
and amine groups can be clearly observed from the TEM
elemental mapping images of HCP−A−B (Figure 4). Sulfoacid
Figure 5. (a) Nitrogen adsorption−desorption isotherms and (b) pore
size distributions calculated using density functional theory methods
for HCP, HCP−A, HCP−A−B, and HCP−B.
HCP−B, and HCP−A−B are similar. All of the sorption
isotherms display steep nitrogen gas uptake at low relative
pressure (P/P0 < 0.01), which reflects abundant micropore
structure. A sharp rise at high pressure (P/P0 = 0.9−1.0)
indicates the presence of macropores in these materials. These
results agree with the pore size distributions of these three
materials (Figure 5b).
The bifunctional HCP−A−B catalyst design aims at
combining acid- and base-catalyzed reactions to establish a
one-pot tandem reaction. The first tandem reaction catalyzed
by HCP−A−B is acetal hydrolysis and Henry reaction (Table
2). Compared with acyclic acetals, the thermal stability of cyclic
acetals is so superior23 that a strong acid is required to enable
the hydrolysis reaction to occur. Thus, a cyclic acetal, 2-(2-
bromophenyl)-1,3-dioxolane (1), was used in the proof-of-
concept study. Initially, no product was detected when the
support HCP was employed in this reaction (entry 1).
Although HCP−A can catalyze the first reaction, 2 is hardly
converted (entry 2) because of the low efficiency resulting from
the lack of basic sites. Similarly, no product was generated when
HCP−B was used as a catalyst because the hydrolysis reaction
cannot occur (entry 3). Acidic and basic sites are both needed
to ensure that the one-pot tandem reaction will be successful.
Thus, HCP−A−B was used as a catalyst. As expected, 1 was
converted completely, and the desired product 3 was achieved
in nearly quantitative yield (entry 4). However, the same
Figure 4. TEM elemental mappings of (a) C, N, and S, (b) N, (c) C,
and (d) S for HCP−A−B.
and amine were successfully introduced into the HCP−A−B
catalyst. In addition, elemental analysis (EA) revealed that the S
and N loadings of HCP−A−B were 1.02 and 1.19 wt %,
respectively (Table 1).
The surface areas and porous properties of the HCPs were
analyzed by nitrogen sorption analysis. The Brunauer−
Emmett−Teller (BET) surface areas of HCP, HCP−A, and
HCP−A−B (Table 1) were 626, 598, and 487 m2/g,
Table 1. Physicochemical Data for the Obtained HCP Materials
a
b
c
d
d
entry
catalyst
HCP
SBET (m2/g)
Smicro (m2/g)
Vtotal (cm3/g)
S content(wt %)
N content (wt %)
1
2
3
4
626
598
487
586
360
397
294
394
0.48
0.42
0.35
0.41
0
0
HCP−A
HCP−A−B
HCP−B
2.71
1.02
0
0
1.19
1.52
a
b
c
Surface area calculated from the nitrogen adsorption isotherm by using the BET method. Micropore area derived using the t-plot method. Total
d
pore volume at P/P0 = 0.99. Obtained by elemental analysis.
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a
deactivate the acid and base sites, thereby diminishing the
recyclability.
Table 2. One-Pot Tandem Hydrolysis/Henry Reaction
The good stability of the HCP−A−B catalyst in organic
solvent inspired us to examine its water stability. The tandem
hydrolysis/Knoevenagel/dehydration reaction of 1 and malo-
nonitrile is a typical paradigm of cooperative catalysis in which
both the acid and base catalysts are required. All of the reported
catalytic systems for this tandem reaction were operated in
organic solvents.24 Thus, the possibility of using water as the
solvent in this reaction was determined. As shown in Scheme 1,
c
yield (%)
0
b
entry
catalyst
conv. of 1 (%)
2
3
1
2
3
4
HCP
trace
100
trace
100
100
35
0
HCP−A
HCP−B
HCP−A−B
HCP−A + HCP−B
HCP−A + Bn−NH2
HCP−B + p-TSA
p-TSA + Bn−NH2
HCP−A−B
97
trace
0
trace
0
Scheme 1. One-Pot Tandem Hydrolysis/Knoevenagel
Reaction Catalyzed by HCP−A−B
96
d
e
5
21
72 (95)
18
6
7
8
12
9
trace
trace
0
trace
trace
94
10
f
9
100
a
Conditions: 1 (0.3 mmol), solid catalyst (40 mg), liquid catalyst (10
a
b
The yield shown in parentheses was obtained in the reuse of the
mol %), CH3NO2 (1.5 mL), 80 °C, 6 h. Determined by using gas
chromatography. Isolated yields. HCP−A (15 mg) and HCP−B (31
mg). 10 h. The catalyst was used in the 18th run.
c
d
catalyst in the 20th run.
e
f
treatment of 1 in water at room temperature for 8 h in the
presence of HCP−A−B provided the desired product 4 in 95%
yield. In a 10 mmol-scale reaction, a similar yield of 4 (93%)
was obtained when the catalyst was reused in the 20th run
under optimal conditions. The catalyst could also be recovered
and reused 19 times with no significant loss of product yield for
this reaction. By EA measurement, no significant leaching of
acid and base sites was observed after 20 runs. The present
bifunctional catalyst not only exhibits good catalytic perform-
ance but also presents excellent stability in water. Numerous
multistep tandem reactions generally produce water, acid, or
base as a byproduct or intermediate. Thus, the stability of
HCP−A−B under acidic and basic aqueous conditions was also
investigated. HCP−A−B was stirred in basic (NaOH, 10 wt %)
and acidic (HCl, 10 wt %) aqueous solutions sequentially. After
the alkali and acid treatments, the BET specific surface area
showed no significant change. The SEM and TEM images show
that the hollow sphere of material remained significantly
unchanged (Figure S3). To investigate further the stability of
the catalyst, the recovered HCP−A−B catalyst that had been
treated by acid and base was also used to catalyze the one-pot
tandem hydrolysis/Knoevenagel reaction under the same
conditions. The product 4 was obtained in 91% yield (Scheme
S2). HCP−A−B possesses good stability under acidic and basic
conditions. All of these results imply that the HCP catalyst
should be a suitable candidate for combining acid- and base-
catalyzed reactions even under aqueous conditions.
Acid−base bifunctional catalyst synthesis can be used to
develop new reactions through a simplified operational
procedure. However, the practical use has been limited. We
then tried to use the HCP−A−B catalyst in the development of
new reactions for organic synthesis. 2-Cyclohexen-1-one and its
derivatives are important synthetic intermediates that are
widely used in medicines, pesticides, perfumes, and the polymer
industry.25 These compounds have often been used to
synthesize substituted phenols,26 steroid compounds, and the
antipyretic analgesic ibuprofen.27 The present synthetic
methods for 2-cyclohexen-1-ones mainly include the following:
(1) selective oxidation of cyclohexenes28 or cyclohex-2-enols,29
(2) aldol-type condensation of 5-oxohexanals,30 and (3)
elimination of hydrogen halide from α-halogenated cyclo-
hexanones.31 However, these methods are often hindered by
reaction over a physical mixture composed of the monofunc-
tional catalysts HCP−A and HCP−B reached only 72% yield
under identical conditions. The yield decrease is likely due to
the long distance between the HCP−A acid sites and HCP−B
base sites, consequently decreasing the efficiency of the mass
transfer over these catalysts. As a proof, a 95% yield was
obtained by increasing the reaction time to 10 h (entry 5).
Interestingly, when HCP−A was used along with a
homogeneous base, benzylamine (Bn−NH2), 3 was obtained
in only 18% yield (entry 6). Only unreacted starting materials
were recovered when HCP−B was combined with a
homogeneous acid, p-toluenesulfonic acid (p-TSA) (entry 7).
The combination of Bn−NH2 and p-TSA was also not active
for the initiation of the model tandem reaction (entry 8).
Neutralization of the acid and base should be avoided to obtain
good catalytic performance.
The recyclability of HCP−A−B was also examined. At the
end of the reaction, the recovered catalyst was washed with
ethyl acetate and dried under vacuum conditions. The solid was
then used for the next run. HCP−A−B can be recycled a
minimum of 18 times without a significant yield decrease (entry
9). The leaching of active species from HCP−A−B was
investigated through a hot filtration experiment. The catalyst
was isolated after 3 h of reaction. The liquid reaction mixture
was divided into two equal parts. The first part was subjected to
a workup procedure, in which the yield of 3 reached 75%. The
second part was allowed to react for the next 3 h under the
same conditions without the addition of catalyst. Finally, we
found that the yield of 3 was nearly unchanged (78%). The
acid−base active sites of the solid catalyst were relatively robust
in the organic solvent. It should be noted that the structure of
the hollow microspheres is also important to ensure that the
catalyst has good recyclability. In addition, an amorphous
congener of HCP−A−B (Figure S2) was also prepared and
used in the same reaction (Scheme S1). Although almost the
same yield could be obtained with the fresh catalyst, the
catalytic activity was not retained during the reuse experiment.
Because of the spherical structure of the hollow HCP−A−B,
minimization of the acid−base neutralization between the
particles was conjectured. However, with its amorphous
congener, the interaction between the particles may partially
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poor selectivity, difficulty of product separation, and limited
availability of substrates. Therefore, an efficient and low-cost
synthetic strategy for preparing 2-cyclohexen-1-ones is
required. Recently, 2-alkoxy-3,4-dihydropyrans have emerged
as easily accessible and important building blocks for organic
synthesis.32 These compounds behave like a special class of
endocyclic acetals that can produce 5-oxohexanal compounds
via acid-catalyzed hydrolysis of the acetal. The products can
potentially be converted to cyclohexenones. However, a
literature survey showed that the synthesis of 2-cyclohexen-1-
ones through the intramolecular aldol/dehydration of 5-
oxohexanals under acidic conditions is often plagued by low
yield, which results from poor selectivity of the reaction.33 The
base catalyst is empirically preferable for this type of reaction.
Therefore, an acid−base bifunctional material should be a good
catalyst for the synthesis of 2-cyclohexen-1-ones from 2-alkoxy-
3,4-dihydropyrans.
We attempted to investigate the feasibility of converting a 2-
alkoxy-3,4-dihydropyran, 5a, into a 2-cyclohexen-1-one deriv-
ative by using HCP−A−B as a catalyst. Initially, the reaction
was performed in nitromethane under anhydrous conditions.
The desired product, 2-cyclohexen-1-one 6a, was formed at 100
°C, but the reaction proceeded reluctantly. The yield reached
only 8% after a 4 h reaction (Figure 6). Our attempt to increase
Table 3. Synthesis of 2-Cyclohexen-1-one 6a from 2-Ethoxy-
3,4-dihydropyran 5a
a
b
entry
catalyst
HCP−A−B
solvent
T (°C)
yield (%)
1
2
3
4
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
EtOH
100
100
100
100
100
100
80
85
0
HCP
HCP−A
HCP−B
44
0
c
d
5
e
HCP−A + HCP−B
HCP−A−B
HCP−A−B
HCP−A−B
HCP−A−B
HCP−A−B
HCP−A−B
HCP−A−B
HCP−A−B
HCP−A−B
HCP−A−B
63 (83)
71
6
7
8
49
DCE
60
trace
0
9
THF
60
10
11
12
CH3CN
DMSO
80
47
100
100
100
60
trace
22
toluene
CH3NO2
CH3NO2
CH3NO2
f
13
62
14
18
g
15
100
c
69
a
b
5a (0.5 mmol), solvent (2.5 mL). Isolated yields. HCP−A (25 mg)
and HCP−B (55 mg). 9 h. Catalyst (45 mg). Water (0.25 equiv). 2
d
e
f
g
h.
reaction time was increased to 9 h, a similar yield (83%) was
obtained (entry 5). The simultaneous existence of amino and
sulfonic groups in HCP−A−B can improve the selectivity of
the reaction. Additionally, compared with the cocatalyst of
HCP−A and HCP−B, HCP−A−B offers a shorter distance
between the acid and base centers to shorten the mass transfer
distance of substances and catalytic sites, thus favoring good
HCP−A−B catalytic performance. We scrutinized the effects of
various other parameters, such as the amount of HCP−A−B
catalyst, water, solvent, temperature, and reaction time. Finally,
the optimal conditions were confirmed as follows: HCP−A−B
(70 mg for a 0.5 mmol reaction), H2O (1.0 equiv), CH3NO2,
100 °C, and 4 h (entries 7−15).
A plausible mechanism is also proposed in Figure 7. The
initial event should be the activation of 5a with an acid catalyst
Figure 6. Kinetic curves for the transformation of 5a to 6a catalyzed
by HCP−A−B at 100 °C in CH3NO2. (Red line I: distilled water (1.0
equiv with respect to 5a) was added in CH3NO2; black line II:
absolute CH3NO2.)
the yield of 6a by doubling the reaction time failed. In this case,
the unreacted 5a could be recovered at the end of the reaction.
Since the first step of the synthesis is the hydrolysis of 5a, which
involves the incorporation of water, 1 equiv of water was added
to increase the reaction rate. This approach worked, and 6a was
formed in 85% yield under the same conditions. Thus, water is
necessary for completing the transformation of 2-alkoxy-3,4-
dihydropyran 5a to 6a under the above-mentioned conditions.
To understand the effect of both the acidic and basic sites on
this reaction, several other catalysts were also examined. When
the naked support HCP was used as catalyst, only unreacted
starting material was recovered (Table 3, entry 2). The yield
with HCP−A as the catalyst reached only 44% because of the
lack of selectivity (entry 3), whereas HCP−B was proven to be
inactive to initiate the synthetic reaction of 6a (entry 4). A
mixture of HCP−A and HCP−B could also promote this
reaction under the same conditions, but the obtained yield was
inferior compared with that of HCP−A−B. However, when the
Figure 7. Proposed mechanism for the transformation of 5a to 6a
catalyzed by HCP−A−B.
through the formation of intermediate A. A ring-opening
hydrolysis reaction occurs, thereby producing polycarbonyl
compound B.34 The amino group of HCP−A−B activates the
aldehyde carbonyl group of B through an imine formation
reaction. Intramolecular aldol condensation occurs, leading to
the formation of the target compound 6a. In this step, the acid
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Table 4. Synthesis of 2-Cyclohexen-1-one Derivatives Catalyzed by the HCP−A−B Catalyst
a
b
c
d
5a (0.5 mmol), CH3NO2 (2.5 mL). Isolated yields. The reaction was performed on a 10 mmol scale. The yield was obtained in the reuse of the
e
catalyst in the 15th run. 12 h.
groups of HCP−A−B perform a key role in facilitating the
dehydration of the aldol reaction product. Therefore, HCP−
A−B could act as an effective catalyst to promote the formation
of the 2-cyclohexen-1-one derivative.
smoothly, and good yields of the expected products 6c and 6d
were obtained. However, the introduction of an ether-
containing alkoxycarbonyl group at C3 of the dihydropyran
ring reduced the reaction yield, possibly because of the subtle
influence of the bulky group (6e). The reaction of 5-acetyl-2-
ethoxy-3,4-dihydropyran 5i proceeded well, and the expected
product 6f was obtained in 74% yield (entry 9). The methyl
group at C2 of the dihydropyran can also be changed to an
ethyl group without significantly affecting the reaction yield
(6g). Finally, the starting material 5k, which has a bigger
molecular size, was also investigated. The yield of product 6h
was only 46% even when the reaction time was increased to 12
h. The unsatisfactory yield of 6h would be associated with mass
transfer limitations to some extent because of the existence of
the abundant micropores in the catalyst. Notably, the reaction
products (α-ester cyclohexenones) have often been used as
starting materials to synthesize numerous pharmaceutical
Using the optimal conditions, we explored the scope of the
reaction substrate. As shown in Table 4, many 3,4-dihydropyran
molecules with ester or ether moieties were proven to be viable
substrates (5b−k). In comparison with 2-ethoxy-3,4-dihydro-
pyran 5a, 2-butoxy-3,4-dihydropyrans 5b and 5c gave slightly
decreased yields of 6a (entries 2 and 3). A similar tendency was
observed when compounds 5d and 5e were used as substrates
(entries 4 and 5). This tendency could be explained by the
difference between ethoxy and butoxy in terms of leaving
ability. The superior leaving ability of ethoxy conferred higher
reactivity to 2-ethoxy-3,4-dihydropyrans. When 3,4-dihydropyr-
ans with isopropoxycarbonyl and allyloxycarbonyl groups (5f
and 5g) were used as substrates, the reaction proceeded
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molecules,35 such as the angucycline antibiotic BE-23254,36 the
spiroimine core of (−)-gymnodimine A,37 and paecilomycine
A.38 Previous methods that have been used to access these
compounds are difficult.39 Therefore, the HCP−A−B-catalyzed
transformation of 2-substituted 3,4-dihydropyrans to α-ester
cyclohexenones offers an alternative route to access these
valuable molecules.
starting material 5a and the selectivity for product 6a were
similar to those of the fresh HCP−A−B catalyst. Thus, the
catalyst could be recycled at least 14 times without significant
loss of activity under the optimal conditions. All of these results
indicate that the present HCP−A−B is an effective and robust
catalyst for the synthesis of 2-cyclohexen-1-ones.
Although water is added in this reaction, the HCP−A−B
catalyst can be recycled uneventfully at the end of the reaction
because of its good water stability. To investigate the durability
of HCP−A−B, we increased the reaction scale to 10 mmol.
Under the optimal conditions for the synthesis of 2-cyclohexen-
1-one compounds, a similar yield was obtained (Table 4, entry
1). Notably, in a 10 mmol-scale reaction, the catalyst could be
recycled at least 14 times without significant loss of product
yield, as shown in Figure 8. The SEM and TEM images reveal
CONCLUSIONS
■
A hollow spherical-structured water-stable HCP−A−B catalyst
was synthesized through an easily accessible approach. In this
catalyst, the acidic and basic sites simultaneously coexist on the
microporous organic sphere. To obtain a proof of the concept,
the resulting acid−base catalyst was utilized in establishing one-
pot tandem reactions, including hydrolysis/Henry and
hydrolysis/Knoevenagel reactions. To apply the new concept,
a new water-assisted transformation of dihydrofurans to
cyclohexenones was developed using the water-stable catalyst
HCP−A−B through a tandem ring-opening hydrolysis/aldol/
dehydration reaction. In all of these reactions, the HCP−A−B
catalyst displayed not only good catalytic activity but also
excellent recyclability. Because of the inherent advantages of
HCP, such its easy preparation, good chemical stability, and
excellent water stability, this method may be adopted in the
future to produce useful task-specific materials.
EXPERIMENTAL SECTION
■
Instrumentation and Materials. IR spectra were recorded
on a Bruker EQUINOX 55 FT-IR spectrometer using KBr
pellets or neat liquid technology. TGA was performed under a
flow of nitrogen by heating from room temperature to 900 °C
at a rate of 10 °C min−1. The catalyst surface areas and N2
adsorption isotherms (77.3 K) were obtained using a
Micromeritics ASAP 2020 M surface area analyzer. Before
analysis, the samples were degassed at 110 °C for 8 h under
vacuum (10−5 bar). EA was determined using a vario MICRO
cube elemental analyzer (Elementar, Langenselbold, Germany).
Field-emission SEM was carried out on a Holland FEI
Instruments Sirion 200 microscope operating at 30 kV. TEM
images were obtained on a Tecnai G20 microscope (FEI
Corporation, Hillsboro, OR, USA) operated at an accelerating
voltage of 200 kV. 1H and 13C NMR spectra were recorded on
a Bruker AV-400 spectrometer. The GC conversions of the
starting materials were examined on a FULI 9790 gas
chromatograph. All of the chemicals were of reagent grade
and used as received without further purification. 3,4-
Dihydrofurans 5a−k were synthesized in accordance with the
previously reported procedure.40
Figure 8. Recycling of the HCP−A−B catalyst.
that the particle size (Figure 3e) and hollow sphere structure
(Figure 3j) of HCP−A−B were completely retained. HCP−A−
B leaching during the reaction was investigated. The
heterogeneous catalyst was isolated by centrifugation after 2 h
of reaction (ca. 65% yield), and the solution phase was allowed
to react further for 2 h under the same conditions. The yield of
6a was only 67%, which confirms that virtually no acid and base
groups leached into the reaction mixture. EA showed that the S
and N contents of the recovered HCP−A−B did not change
appreciably after 15 runs. To investigate further the activity of
the recovered catalyst, the reaction kinetics was also studied
over the catalyst recovered in the 14th run. As shown in Figure
9, when the recovered catalyst was used, the conversion of the
General Procedure for the Preparation of HCP−A−B.
The procedure mainly includes three steps, as follows. (1)
Preparation of hollow HCP: The precursor SiO2@PS was
synthesized according to the reported method (see the
Supporting Information).19 SiO2@PS (2.00 g) was added to
DCE (20 mL) and stirred for 12 h to make it swell.
Chloromethyl methyl ether (19.2 mmol, 1.46 mL) and ZnCl2
(19.2 mmol, 2.62 g) were then added to the mixture. After 24 h
of stirring at 55 °C, the mixture was centrifuged, and the
bottom solid was washed with water (25 mL × 3) and EtOH
(25 mL × 3). Chloromethyl-substituted hybrid material, SiO2@
PS−Cl, was thus obtained. The hyper-cross-linking reaction of
SiO2@PS−Cl is a Friedel−Crafts reaction, which can be carried
out in DCE (40 mL). The reaction was performed at 80 °C for
24 h using FeCl3 (19.5 mmol, 3.16 g) as the catalyst. The
Figure 9. Kinetic curves for the model transformation of 5a to 6a
catalyzed by fresh and recovered HCP−A−B under the optimal
conditions.
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obtained solid was then treated by Soxhlet extraction for 24 h
to provide SiO2@HCP. The SiO2 component of SiO2@HCP
was then etched by hydrofluoric acid. A hollow HCP was
obtained. (2) Surface modification of HCP to synthesize
HCP−A: A fresh DCE solution of acetyl sulfate19 (1.89 mL,
12.5 wt %) was added carefully into a mixture of HCP (0.4 g,
3.0 mmol) and DCE (20 mL) at 65 °C. After 2 h of stirring, the
mixture was quenched with EtOH (40 mL) and then
centrifuged, and the solid was washed with water and ethanol
(50 mL × 3). HCP−A was thus obtained. (Note: the molar
amount of HCP was calculated on the basis of the theoretical
monomer molecular weight and the actual weight of HCP; the
molar amount of acetyl sulfate was controlled to be
approximately 20% of the amount of HCP; swelling of HCP
should be avoided prior to the sulfonation, and the reaction
time should be no more than 2 h. (3) Synthesis of HCP−A−B:
All of the obtained HCP−A in the previous step and an amine
precursor, NHPI (1.90 mmol, 0.34 g), were mixed with
CF3CO2H (15 mL) in DCM (15 mL). Under an atmosphere of
N2, CF3SO3H (0.765 mL) was then added dropwise into the
mixture. Stirring at room temperature for 10 h followed. After
centrifugation, a yellow solid was obtained that was then
subjected to washing with EtOH (50 mL × 3). To protect the
sulfoacid groups, the yellow solid was treated with saturated
NaHCO3 (20% aq., 20 mL) to convert −SO3H into −SO3Na.
After 12 h of stirring at room temperature, the mixture was
filtered and washed with water until the filter liquor reached a
pH of 7. Then the obtained solid was treated with NH2NH2·
H2O (1 mL) under reflux conditions for 24 h. An orange solid
was obtained, which was then acidified to with HCl (10% aq.,
20 mL) for 3 h. HCP−A−B was obtained after several washes
with water until the pH of the filter liquor was 7.
General Procedure for the One-Pot Hydrolysis/Henry
Reaction. 2-(2-Bromophenyl)-1,3-dioxolane (70 mg, 0.3
mmol) and HCP−A−B (40 mg) were added to CH3NO2
(1.5 mL). The resulting solution was stirred at 80 °C for 6 h.
After reaction completion, the mixture was cooled to room
temperature and then centrifuged. The supernatant organic
phase was isolated, and the bottom solid was washed with ethyl
acetate (4.0 mL × 3). All of the organic phases were then
combined and isolated with preparative thin-layer chromatog-
raphy (TLC) (eluting solvent: ethyl acetate/petroleum ether =
1/5 v/v). The desired product 3 was obtained in 96% yield (66
mg).
General Procedure for the One-Pot Hydrolysis/
Knoevenagel Reaction. In a typical reaction, 2-(2-bromo-
phenyl)-1,3-dioxolane (70 mg, 0.3 mmol), malononitrile (24
mg, 0.36 mmol), and HCP−A−B (40 mg) were added to
distilled water (1.0 mL). The resulting solution was stirred at
room temperature for 8 h. After reaction completion, the
mixture was cooled to room temperature and then centrifuged.
The supernatant water phase was isolated and extracted with
ethyl acetate (2.0 mL × 3). The bottom solid was washed with
ethyl acetate (2.0 mL × 3). All of the organic phases were then
combined, concentrated, and isolated with preparative TLC
(eluting solvent: ethyl acetate/petroleum ether = 1/8 v/v). The
desired product 4 was obtained in 95% yield (66 mg).
centrifuged. The supernatant organic phase was isolated, and
the bottom solid was washed with ethyl acetate (4.0 mL × 3).
All of the organic phases were then combined, concentrated,
and isolated with preparative TLC (eluting solvent: ethyl
acetate/petroleum ether = 1/4 v/v). Then the product 6a was
obtained in 85% yield (71 mg). The synthesis procedures for
6b−h were similar to this method.
General Procedure for Recycling of the Catalyst. To
test the recyclability of the HCP−A−B catalyst, a mixture of the
reaction system was filtered at the end of the reaction. The
obtained solid catalyst was washed with ethyl acetate (2.0 mL ×
4). After 12 h of drying under vacuum at 60 °C, the catalyst was
subjected to the next run. The procedure for the reuse of the
catalyst in the other reactions was similar to this method.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b03631.
■
S
Synthesis of SiO2@PS, HCP−B, amorphous HCP−A−B,
and the starting materials 5a−k; NMR data for organic
1
products; Figures S1−S3 and Table S1; and H and 13C
NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: bien.tan@mail.hust.edu.cn.
*E-mail: klgyl@hust.edu.cn.
■
ORCID
Yanlong Gu: 0000-0001-5130-3026
Author Contributions
∥Z.J. and K.W. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National Natural Science Foundation of
China for financial support (21373093, 21474033, 21473064,
and 51273074) and also the Analytical and Testing Center of
HUST. The Chutian Scholar Program of the Hubei Provincial
Government and the Cooperative Innovation Center of Hubei
Province are also acknowledged. This work was also supported
by the Fundamental Research Funds for the Central
Universities of China (2014ZZGH019).
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