Periodic Mesoporous Organosilicas as Efficient Nanoreactors in Cascade Reactions Preparing Cyclopropanic Derivatives

Article abstract of DOI:10.1002/asia.201900043

In this work, three organosilica precursors functionalized with carbamate moieties were synthesized by condensing of 3-isocyanatopropyltriethoxysilane and coupling regents of either hydroquinone (HQ), bisphenol A (BPA), or 1,1′-bi-2-naphthol (BN). These organosilica precursors were covalently bonded in the framework of periodic mesoporous organosilicas by co-condensation and hydrolysis with tetraethyl orthosilicate (TEOS) under hydrothermal treatment. The compositions and physical properties were characterized with FTIR, XRD, thermogravimetric/differential thermal analysis (TG/DTA), ²⁹Si NMR, ¹³C NMR spectroscopies, SEM, TEM, and BET technologies. These characterizations suggest that three different structures were formed as the result of different sizes and compositions of the organosilica precursors. The three mesoporous organosilicas were applied as heterogeneous catalysts in the one-pot cascade Knoevenagel and Michael cyclopropanic reactions for the synthesis of cyclopropanic derivatives and showed excellent activity and selectivity. The highest conversion was obtained with mesoporous catalyst (MC)-HQ owing to its ordered mesostructure, highest surface area, and weakest stereo effect of the organic linking groups compared with MC-BAP and MC-BN. This methodology employed cheaper and more easily obtainable raw materials as reagents over the traditional alkene additive system and these heterogeneous catalysts exhibit superior performance and recyclability than typical homogeneous organic catalysts.

Full text of DOI:10.1002/asia.201900043

CHEMISTRY
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Accepted Article
Title: Periodic mesoporous organosilicas as efficient nanoreactors in
the cascade reaction for preparation of cyclopropanic derivatives
Authors: Hang Huo, Yanqiu Jiang, Tingting Zhao, Junlei Wang, Defeng
Li, Xianzhu Xu, and Kaifeng Lin
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Periodic mesoporous organosilicas as efficient nanoreactors in
the cascade reaction for preparation of cyclopropanic derivatives
Hang Huo, Yanqiu Jiang*, Tingting Zhao, Junlei Wang, Defeng Li, Xianzhu Xu and Kaifeng Lin*[a]
Abstract: In this work, three organosilica precursors functionalized
cyclopropanic products can be obtained. Even sometimes, the
noble metallic active sites of catalysts can be lost or inactive
during the recycling reactions, thus leading to inevitable waste
although the dosage of this kind of catalysts is catalytic.
Furthermore, the alkene reactants are usually synthesized from
the raw carboxylic acid derivatives via Knoevenagel or Michael
addition reactions. From the raw carboxylic acid derivatives to
the final cyclopropane chemicals, more procedures of
purification have to be operated, resulting lower atomic
efficiency and more environmental pollution problems.
In order to promote the atomic efficiency and decrease the
economic cost or undesired intermediate separation process, it
needs to develop a new kind of catalyst that can connect the
two-step reactions in tandem and can be recovered easily from
the system for further recycling. Taking this consideration in
mind, it is known that heterogeneous catalyst can realize this
goal and mesoporous silica material can be a good candidate as
this kind of solid nanoreactors. The mesoporous silica has to be
functionalized with proper catalytic species in the skeleton to
accommodate the cyclopropanic reactions. It is well known that
the periodic mesoporous organosilicas (PMO) possess high
surface area, uniform pore structure, narrow pore size
distribution and robust hydrothermal stability, these materials
have been manifested to show impressive heterogeneous
catalytic ability[7].Compared with the traditional functionalized
mesoporous silica, the catalytic active species are
homogeneously inserted in the framework of PMO materials
instead of on the outer surface or at the mouth of the pores and
the pore channels of PMO materials are unblocked and open for
the mass transfer of the reactants during the catalytic process.
What’s more, the incorporated active species can be easily
adjusted according to the demands of the catalytic reaction, so
the PMO materials were selected as the ideal matrix for this kind
of cascade reaction.
with carbamate moieties were synthesized by condensing of 3-
isocyanatopropyltriethoxysilane
and
coupling
regents
of
hydroquinone (HQ), bisphenol A (BPA) and 1,1’-bi-2-naphthol (BN),
respectively. These organosilica precursors were covalently bonded
in the framework of periodic mesoporous organosilicas via co-
condensation and hydrolysis with tetraethoxysilane (TEOS) under
hydrothermal treatment. The compositions and physical properties
were characterized with FT-IR, XRD, TG/DTA, 29Si NMR, 13C NMR,
SEM, TEM and BET technologies. These characterizations suggest
that three different structures were formed as the result of different
sizes and compositions of the organosilica precursors. The three
mesoporous organosilicas were performed as heterogeneous
catalysts in the one pot cascade Knoevenagel and Michael
cyclopropanic reactions for the synthesis of cyclopropanic
derivatives and showed excellent activity and selectivity. The highest
conversion was obtained by MC-HQ owing to its ordered meso-
structure, highest surface area and weakest stereo effect of organic
linking groups compared with MC-BAP and MC-BN. This
methodology employed more cheap and easily obtained raw
materials as reagents over traditional alkene additive system and
these heterogeneous catalysts exhibit superior performance and
recyclability than typical homogeneous organic catalysts.
Introduction
Cyclopropanic chemicals commonly exist in natural products
and biologically active compounds[1]. Additionally, due to the
strained ring force and abundant substituent groups bonded on
the three membered ring, cyclopropane intermediates play an
important role in medicine synthesis. It has been reported that
the cyclopropane moiety has been selected as one of the most
popular applied motives in the new drug synthesis[2]. During the
past few decades, various methodologies and strategies for the
elaboration of donor-acceptor cyclopropanes containing
chemicals have been explored[3] and some exciting results have
been achieved. Looking into the cyclopropane synthetic
reactions, it can be seen that almost parts of these reactions are
proceeded with alkenes as the initial reactants, and
organic/inorganic base[4], metallic nanoparticles[5] or coordinated
complexes[6] as catalysts. But these catalysts have some
inevitable defects. For example, these homogeneous catalytic
reactions usually need post treatment, and by a series of
separation of catalyst and the by-product, the final purified
To connect the Knoevenagel and Michael addition reactions,
proper reactants should be considered. It is well known that
aldehyde and active hydrogen compounds are usually used for
the Knoevenagel reaction, so it is necessary to select a proper
reactant for the Michael addition reaction. Compared with the
methodologies in the synthesis of donor-acceptor cyclopropanic
derivatives, such as the Simmons–Smith reaction[8],
cyclopropanation via various ylides[9], diazomethane[10]
, Au
carbenoids[3a, 11], etc., it is obvious that the Michael addition of
activated α-bromo ester to alkene is a much easier and
convenient approach, which is originally pioneered by Gaunt and
co-workers[12], thus, α-bromo ester was selected as one of the
reactants in our methodology. Investigating into the mechanism
of this kind of reaction[1b], it can be concluded that most of this
kind of cyclopropanic process involve the generation of
ammonium ylide and carbanion intermediate, and these species
are the key factors in the activation of the reaction. Firstly, under
the inducement of organic/inorganic catalyst or visible light, the
bromomalonate radicals or ylides are afforded. Subsequently,
taking off a proton by solid base generates the corresponding
[a]
H. Huo, T. Zhao, J. Wang, Dr. Y. Jiang, D. Li and Prof. X. Xu, K. Lin
School of Chemistry and Chemical Engineering
Harbin Institute of Technology
Harbin, Heilongjiang, 150001, P. R. China
E-mail: linkaifeng@hit.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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malonate carbanion intermediated. Finally, Michael addition of
the malonate carbanion to α, β-unsaturated alkene and SN2-type
intramolecular addition nucleophilic displacement furnish the
final cyclopropane product. Thus, the mesoporous silica must be
designed that can catalyze the Knoevenagel and Michael
cyclopropanic reactions in tandem. Modifying the mesoporous
silica with co-condensation of functional organic active moieties
can achieve this goal owing to the abundant silicon hydroxyl on
its surface. This method can realize more homogeneously
distribution of organic moieties and can’t block the meso-pores.
This kind of materials have been widely used in catalytic
reactions. Enlightened by these intricate works and our former
molecularly imprinting materials[13], we aim to address the
challenge of designing a cascade-reaction nanoreactor made of
hybrid mesoporous silicas and realize the high performance and
well reutilization of the catalysts.
In this work, coupling regents of hydroquinone (HQ),
bisphenol
A
(BPA) and 1,1’-bi-2-naphthol (BN) were
nucleophilically added to the 3-isocyanatopropyltriethoxysilane,
and three types of organosilica precursors denoted as HQP,
BPAP and BNP respectively were prepared. These organosilica
precursors were co-condensed and hydrolyzed with tetraethyl
orthosilicate (TEOS) to synthesis hybrid mesoporous materials
via hydrothermal treatment. Three absolutely different
morphologies and structures of the materials were obtained as
the result of the difference in sizes and structures of the organic
coupling regents. Then the application of the three materials as
heterogeneous catalysts was investigated in the one-pot
cascade of Knoevenagel and Michael cyclopropanic reactions.
These materials showed superior catalytic performance
compared with unfunctionalized silica and some homogeneous
organic chemicals. In addition, the structural factors of the
catalysts and the reaction conditions that impact the catalytic
performance were also discussed.
Figure 1 FT-IR spectra of the mesoporous catalysts of MC-HQ (black), MC-
BPA (red) and MC-BN (blue)
The TG/DTA analysis was carried out to examine the thermal
stability and organic content of the mesoporous catalyst.
Figure 2 shows the TG profiles of the three samples recorded in
the air atmosphere. For the cases of MC-HQ and MC-BPA, the
fluctuations of the profiles before 280oC are associated with
slightly weight loss that can be the result of the evaporation of
physiosorbed water, solvent or some other volatile substrates on
the surface of the materials. Owing to the thermal instability of
carbamate group, two steps of weight loss between 300oC and
700oC can be ascribed to the decomposition of the incorporated
organosilica. The first step of weight loss is the thermal
decomposition of carbamate into organic linking groups and
isocyanate group, and the second step of weight loss is the
Results and Discussion
Chemical compositions and structural properties
The introduction of the functional organic active sites was
monitored with FT-IR spectroscopy, as shown in Figure 1. The
FT-IR peaks at 1720 cm-1 are assigned to the stretching
vibrations of C=O bonds in carbamate group formed by the
condensation of isocyanato and hydroxyl. Other peaks at 1500
cm-1 can be seen for the three samples, which can be assigned
to the stretching vibrations of C-C bond in benzene or
naphthalene ring originated from the organic coupling parts,
indicating that the organosilica precursor with a large size of the
organic linking group as naphthalene is not limited to participate
in the co-condensation with TEOS instead of being a separate
phase. The peaks observed at 1100 cm-1 are characteristic
peaks of asymmetry vibrations of Si-O-Si bonds and the
absorption peaks between 800 cm-1 and 1000 cm-1 are bending
and symmetric stretching vibrations of Si-O-Si bonds.
Figure 2 TG analysis of the mesoporous catalysts MC-HQ (black), MC-BPA
(red) and MC-BN (blue)
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removal of residue of propyl isocyanate group. Overall, the total
mass losing for MC-HQ, MC-BPA and MC-BN can be up to 18%,
41% and 31%, respectively. This result indicates that only part of
the organosilica precursor with the naphthalene linking group
can be incorporated into the skeleton of the material according
to its corresponding molecular weight.
To acquire a more comprehensive information of the
composition of the mesoporous materials, 29Si CP NMR and 13
C
CP/MAS NMR were carried out for MC-HQ. As seen in Figure 3
of 29Si CP NMR spectrum, it shows two T silicon sites and three
Q silicon sites. The peaks centered at -41.7 ppm and -50.9 ppm
can be ascribed to T2 [SiR(OH)(OSi)2] and T3 [SiR(OSi)3]
organosilicon species. The absence of T0 [SiR(OH)3] species
suggests that both the two ends of the organosilica precursor
were incorporated into the framework of the material, instead of
anchoring in the pores as a pendant part. The signals at -76.2
ppm, -85.1 ppm and -94.5 ppm can be corresponded to Q2
Figure 4. XRD diffractions of the mesoporous catalysts of MC-HQ (black),
MC-BPA (red) and MC-BN (blue)
[Si(OH)2(OSi)2], Q3 [Si(OH)(OSi)3] and Q4 [Si(OSi)4] silicon
species. The peak area ratio of T/Q silicon species is about
17.6%, which is close to the silicon molar concentration ratio
(14.5%) between HQP and TEOS, indicating that this synthesis
condition is efficiency for the incorporation of organosilicon in the
mesoporous framework. In the 13C CP/MAS NMR spectrum, the
signal centered at 157 ppm can be ascribed to –C=O in the
carbamate group. The peaks in the range of 0-55 ppm for Si-
1CH2-2CH2-3CH2- illustrate the incorporation of the organolisilica
fragment[14]. The signals at 148.5 ppm and 121.8 ppm can be
attributed to C species in the phenol ring. The results of the
compositional characterizations suggest that the organosilica
precursor can be successfully introduced into the skeleton of the
material under hydrothermal condition.
The small angel X-Ray diffraction (XRD) patterns of the three
materials are shown in Figure 4. The XRD spectra of MC-HQ
exhibits three well resolved diffraction peaks at 2θ values around
0.9o, 1.6o and 1.8o, which can be assigned to [100], [110] and
[200] diffraction facets of highly ordered two dimensional
hexagonal mesostructure with space group of p6mm. For the
XRD diffraction of sample MC-BPA, only one broad diffraction
peak can be seen at 2θ value centered at 1.2o, which indicates
that just disordered meso-structure are formed during the
hydrothermal condition. This may be attributed to the larger size
of organic linking group can have disruptive effect on self-
assemble of P123 in solution. However,
a sharp narrow
diffraction peak at 2θ values around 0.5o can be observed for the
material of MC-BN, which can be indexed as the first diffraction
peak of mesoporous material. This result is in accordance with
the report that the addition of salt facilitate micelle self-assembly
to form ordered structure even a large size of organosilica
precursor was used[15]. Furthermore, all of these structural
information agrees well with the TEM results in the next.
Scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) analysis were investigated to
Figure 3 Solid state 29Si NMR (top) and 13C CP/MAS NMR (down) spectra of
MC-HQ.
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Table 1. Physicochemical properties of the hybrid mesoporous catalysts.
Surface area
(m2g-1)
Pore sizea
(nm)
Pore volume
(cm3g-1)
Sample
MC-HQ
MC-BPA
MC-BN
649
457
288
6.8
3.6
0.81
0.40
0.86
2.9; 52.9
a The pore size distribution was calculated from the adsorption branch with the
BJH method.
replicable morphologies with the current synthetic conditions.
The TEM images in Figure 5 indicates that MC-HQ clearly
display two dimensional hexagonal meso-structure with uniform
pore size, and parallel and perpendicular to the pore axis pores
can be noticed in the [100] and [110] directions. The pore
diameter is about 7.0 nm. However, a larger size of organosilica
precursor BPAP leads to the formation of disordered mesopores.
From Figure 5f the cellular mesostructured of MCF characteristic
large pores can be distinguished. All of these results well
evidenced the result of XRD and nitrogen adsorption tests.
For the sake of evaluating the physicochemical properties of
the catalysts, nitrogen adsorption-desorption analysis was
performed. All of the adsorption-desorption isotherms (as shown
in Figure 6) show a type of IV curve at relative pressure P/P0
region between 0.4 and 1.0, which can be denoted as the typical
property of ordered mesoporous materials. Table 1 summarizes
the textural properties of the three mesoporous catalysts. The
surface area ranging from 288 to 649 m2 g-1 directly relate to the
pore structures and ordering of the pores. The disordered
mesoporous structure is more likely to decrease the surface
area and the lowest surface area of MC-BN is mainly caused by
the occupation of hollow cavities in mesocellular foam structure.
Moreover, the surface area is also related to the polymerization
degree of the material, as mentioned in the TG/DTA
characterization analysis, only part of the BNP organosilica
precursor participated in the construction of the matrix, so lower
polymerization degree of the silica precursor inclines to minimize
the surface area. Two pore size distributions were obtained for
the material MC-BN, which is identical to the mesocellular
siliceous foam structure evidenced by the TEM result. The whole
pore volume in the range of 0.4 to 0.86 cm3g-1 also suggests
some differences exist in the shapes of the mesopores.
Figure 5 SEM images of MC-HQ (a), MC-BPA (b), MC-BN (c), and TEM
images of MC-HQ (a), MC-BPA (b) MC-BN (c), respectively.
examine the morphology, pore size distribution and pore
structure. Figure 5 shows the SEM images of the three materials,
and it is obvious that MC-HQ possesses noodle like morphology
with uniform particle size and smooth surface, which is similar to
that of SBA-15 types of material. Whereas MC-BPA shows
irregular bulk particles with random shapes and dispersed
particle sizes, and these observations agree well with the
disordered XRD result. MC-BN exhibits cellular foam
morphology with clearly uniform hollow cavities. The different
shapes of the materials are mainly as the result of different
molecular size/structure and synthesis condition. The smaller
size of the organosilica precursor, the better orderliness of the
material will be obtained. At the same time, the existence of F
ion makes the hydrolysis and condensation processes of silica
Catalytic performance in cascade cyclopropanic reactions
The cyclopropane with a rigid three membered cyclopropanic
ring are very available molecular structure for a large number of
medical and natural product, as they can be the fundamental
matrix for the ring-opening reaction to prepare a series of new
substances. Well, cascade reaction can be served as an
efficient approach to synthesis the cyclopropanic derivatives in
one operation, as it can be regarded as one of the basic issues
related to the synthetic efficiency. So we designed a carbamate
catalyzed cascade approach to synthesis functionalized
cyclopropanic derivatives in a single process. After the elaborate
species
reversible
and
favors
to
recombine
at
thermodynamically stable area to form ordered foam
mesostructure. To investigate the replicable morphologies of the
materials, higher molar ratios of organosilica precursors and
TEOS were used to synthesize the materials, and the SEM
images are listed in Figure S1. Nearly all the same morphologies
as before were obtained of the three materials, indicating the
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Table
2
The catalytic results of the cascade reactions catalyzed by
organosilica precursors and ordinary organic bases1
Conversion7
(a) (%)
Yield7 (c)
(%)
Yield7
(d) (%)
Selectivity
(d) (%)
Entry
1
Condition
Blank
14.9
2.4
0.9
-
-
-
2
2
Na2CO3
1.5
-
3
Na2CO3
MC-HQ3
MC-HQ4
MC-HQ5
MC-HQ
HQP
28.6
74.1
18.2
19.0
73.7
22.1
20.9
21.4
34.6
63.2
23.3
73.8
18.1
18.7
5.8
1.5
-
28.5
-
4
5
-
-
6
-
-
7
58.0
2.5
2.0
2.1
10.0
15.8
85.4
57.8
57.1
46.7
49.0
48.7
8
17.8
17.4
16.9
14.2
30.7
9
BPAP
10
11
12
BNP
Et3N
Morpholine
2,2,6,6-
tetramethylp
iperidine
SBA15-
APTES-
grafted
13
14
40.0
90.2
-
25.9
65.4
64.9
87.2
15.2
1. All of the reactions were completed in 5 hours with 40 mg catalyst and 212
mg Na2CO3 in 2 mL of toluene unless especially mentioned. The assignments
of the products were confirmed via GC-MS.
2. The reaction was performed with solid base of Na2CO3 at the time of 10
minutes.
3. The reaction was carried out without Na2CO3 and interrupted at the time of
5 hours.
4. The reaction was carried out without Na2CO3 and interrupted at the time of
10 minutes.
5. The reaction was carried out with Na2CO3 and interrupted at the time of 10
minutes.
Figure 6 Nitrogen adsorption/desorption isotherms plot (top) and pore size
distribution profiles (down) of MC-HQ (A), MC-BPA (B) and MC-BN (C)
highest conversion can be reached at the time of 5 hours. Owing
to the differences in polarity and boiling point of solvent, the best
conversion of aldehyde was obtained in solvent of toluene. So
the conditions with the addictive of Na2CO3 reach to the peak
conversions in 5 hours in the solvent of toluene, can be selected
as the optimal reaction conditions. The catalytic results in Table
2 show that the reaction almost can’t proceed successfully
without any catalyst. In order to judge whether the inorganic
base of Na2CO3 alone can promote the course of the reaction,
two comparative experiments were at the reaction time of 10
minutes and 5 hours performed (entry 2 and 3) and only 2.4%
and 28.6% of conversions were obtained. This result suggests
that the inorganic base of Na2CO3 alone hardly acts as a
species of catalytic active site in this reaction. While, only the
middle product of α, β-unsaturated alkene can be detected if
merely MC-HQ was added (entry 4), thus it can be concluded
that Na2CO3 and mesoporous catalysts work together in this
choose of optimized raw materials and reactive conditions,
aldehyde and bromomalonate ester were selected as
nucleophilic substrates, and Na2CO3 was selected as acid
scavenger. From the raw reagents of aldehyde and malononitrile,
the mediate product of α, β-unsaturated alkene is synthesized
via Knoevenagel condensation, then the Michael addition of
bromomalonate ester functionalized as nucleophilic reagent is
added to α, β-unsaturated alkene and yields the cyclopropanic
product, which can be seen as Michael addition reaction. So it
realizes the one-pot cascade of Knoevenagel and Michael
cyclopropanic reactions for the synthesis of cyclopropanic
derivatives.
At first, in order to evaluate the influence of conditions on the
reaction, a system with different solvent and catalysts are
developed. As displayed in Table S2, the conversion of
aldehyde gradually rises up with reaction time goes by, and the
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system. But which step does Na2CO3 play a role in the cascade
reaction? Compared with the results in entry 5 and entry 6, the
conversion obtained from MC-HQ and Na2CO3 is very close to
that obtained from MC-HQ without Na2CO3 at the time of 10
minutes. During this period only the first step of the cascade
reaction could be proceeded and few of the cyclopropanic
product was detected, which suggests that Na2CO3 affects the
second step of the cascade reaction and nearly has no impact
on the first step. According to the previous report that the role of
obtained. Another similar example is carried out with solid SiO2
nanoparticles with the diameter around 470 nm, and nearly
19.7% of conversion was obtained (Entry 8 of Table S2). These
results suggest that the pure mesoporous silica matrixes are
only supporter of active species and the carbamate groups that
is covalently bonded in the framework of mesoporous silica are
real active sites for the reactions. Compared with the catalytic
results of three ordinary organic bases and the corresponding
homogeneous organic moieties from the catalysts of
organosilica precursors, one can find that the conversion of
aldehyde, and the yield/selectivity of product are much lower
than that obtained by the heterogeneous catalysts. This is
mainly benefited from the effect of the mesoporous structure as
nano-sized reactor as well as the highly dispersed active sites
on the surface of the mesoporous hybrid materials with high
surface areas. Moreover, traditional amino functionalized
mesoporous silica of SBA15-APTES-gragted that was
synthesized via post grafting method (the detailed synthesis
process was illustrated in the Supporting Information and the
nitrogen adsorption-desorption isotherms, XRD diffraction
pattern were illustrated in Table S1, Figure S3 and Figure S4,
respectively) was also used as catalyst for comparison. Nitrogen
contents in the materials of SBA15-APTES-gragted and MC-HQ
were measured to be 3.4 and 2.0%, respectively. Taken into
account of these nitrogen contents, the turnover number (TON)
of MC-HQ is calculated to be 12.9 (calculated based on the
activity at the reaction time of 5 h), which is higher than that of
SBA15-APTES-grafted (9.3; calculated based on the activity at
the reaction time of 5 h). The detailed catalytic comparison
between MC-HQ and SBA15-APTES-grafted was listed in Table
S3. This result clearly indicates that the active species in the
framework of MC-HQ are more accessible to the reactant
because the pores of SBA15-APTES-grafted might be blocked
due to the presence of terminal amino groups anchored in the
mesopores and pore mouths.
With the optimal reaction conditions in hand, the catalytic
performance was evaluated on the three mesoporous catalysts
that contain different organic moieties in the skeleton, and the
results are listed in Table 3. As it can be seen, the catalysts
display excellent conversions of the reactant, and
yield/selectivity of the product. The maximum conversion of the
aldehyde can be as high as 93.7%, and the maximum yield and
selectivity of the product can be as high as 82.4% and 99.9%,
respectively. Compared with the same substrate catalyzed by
different catalysts, the activity trend of the catalysts is MC-HQ >
MC-BPA > MC-BN. The reason should be that in subject to the
limitation effect of the nano-sized catalysts, the activity of the
catalysts is directly depended on the BET specific surface of the
catalyst. Higher specific surface generally favors more exposed
active sites, leading to higher activity of the catalyst. What’s
more, the size of the substrate has significant effect on the
activity of the catalysts on molecular-level, which is mainly
attributed to the stereo effect by considering that larger size of
reactant or coupling reagent are more disadvantage to interact
with the active species. Influenced by the mass transfer during
the catalytic process, a smaller size of the substance is more
favorable to promote the proceeding of the reaction.
the addictive Na2CO3 is mainly to neutralize the liberated HBr[16]
.
Then, to evaluate whether the pure mesoporous silica matrix
is active in this catalytic system, all of the three mesoporous
catalysts were treated with calcination in muffle furnace at the
temperature of 550oC for 3 hours, and the incorporated
functional organic species can be completely removed but the
meso-structure can’t be destroyed. The calcinated materials
were denoted as MC-HQ-C, MC-BPA-C and MC-BN-C,
respectively, and their FT-IR and XRD characterizations are
illustrated in Figure S2. These materials were also carried out in
the cyclopropanic cascade reaction, and the result listed in entry
9 to 11 of Table S2 reveals that the calcinated unfunctionalized
mesoporous materials show quite weak activity toward this
reaction system, and only around 20% of conversion can be
Table
3 One-pot cascade synthesis of cyclopropanic derivatives with
heterogeneous hybrid mesoporous silicas 1
Conversion2
(a) (%)
Yield2
(c) (%)
Yield2
(d) (%)
Selectivity
(d) (%)
Reagent a
Catalysts
MC-HQ
MC-BPA
MC-BN
MC-HQ
MC-BPA
MC-BN
MC-HQ
MC-BPA
MC-BN
MC-HQ
MC-BPA
MC-BN
MC-HQ
MC-BPA
MC-BN
93.7
10.5
9.5
33.8
5.8
15.8
26.3
6.3
5.7
2.7
6.9
-
72.3
72.1
27.1
58.0
26.3
8.9
86.9
88.1
53.0
85.4
62.0
40.3
99.9
98.7
88.7
96.4
97.6
99.0
45.4
36.9
28.5
91.3
84.9
73.7
58.3
48.4
88.8
88.6
85.7
77.1
74.9
74.3
57.4
56.4
31.8
82.4
81.8
73.6
67.8
73.1
73.6
25.3
9.1
-
1.5
31.8
21.1
3.1
1. The reactions were completed in 5 hours with 40 mg catalyst and 212 mg
Na2CO3. The assignment of the products was confirmed via GC-MS.
2. The numbers of conversion and yield were calculated via GC with n-butanol
as the standard substance at the time of 5h.
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The substrate with an electron withdrawing substitute displays
higher conversion, yield and selectivity than that of the one with
an electron donor group, which evidence the nucleophilic
addition mechanism during the cascade reactions[17]. The
mechanism of the cascade reaction was proposed based on the
obtained experimental results, as illustrated in Scheme 1.
Comparing the catalytic results between the materials with and
without calcination treatment, one can conclude that the catalytic
active species are mainly from the nitrogen species in
carbamate groups. For the first step of Knoevenagel reaction,
the process was proceeded with the nucleophilic of carbamate
to benzaldehyde, and subsequently added to malononitrile to
generate mediate production of α, β-unsaturated alkene, which
can be evidenced from the result that only the mediate product
can be obtained if only MC-HQ was added. What’s more, during
the first step, the silanol groups can provide a proton to the
middle addictive species, that may be the reason why lower
conversions were obtained by the organosilica precursors. The
second step of Michael cyclopropanic reaction involves the
nucleophilic of carbamate to diethylbromomalonate to form the
ion pair, and subsequently generate an amine ylide in the
presence of solid inorganic base of Na2CO3. Michael addition of
the ylide to the intermediate product of α, β-unsaturated alkene
originating from the first step obtains the malonate carbanion,
which undergoes an intramolecular SN2-type substitute to yield
the final cyclopropane product with the persistent regeneration
of the active sites. Thus, with the addition of another solid base
of Na2CO3, the functionalized mesoporous catalysts realize the
one-pot cascade reaction for the synthesis of cyclopropanic
Figure 7 Recyclability tests of the mesoporous catalyst MC-HQ in the cascade
Knoevenagel and Michael cyclopropanation reaction
products from the crude and cheap easily obtained reagents.
The recyclability potential is an important parameter for the
heterogeneous catalysts. So the recycling experiments were
performed, and the mesoporous catalyst of MC-HQ was
selected as a model catalyst and anisaldehyde was selected as
the substrate. Figure 7 shows the catalytic results about the
conversion of aldehyde, yield and selectivity of cyclopropanic
product after five times recycling by regenerating in inert
atmosphere at 200oC. The conversion of aldehyde and the yield
of the cyclopropanic product are slightly decreased accompany
the recycling experiments proceed. The selectivity of the
cyclopropanic product almost keep the same as the original
number. These results suggest that this kind of organic
functionalized mesoporous catalysts present excellent catalytic
activity and recyclability at least for five times.
Conclusions
In summary, three different functionalized organosilica
precursors were synthesized and then co-condensed and
hydrolyzed with TEOS to prepare periodic mesoporous
organosilicas via hydrothermal method. The material of MC-HQ
showed the highest surface area with ordered two dimensional
hexagonal structure, and the materials of MC-BPA and MC-BN
showed disordered and cellular foam structure with lower
surface area respectively. Such materials were applied as nano-
reactors in the heterogeneous catalysis of one-pot cascade
Knoevenagel and Michael cyclopropanic reactions and showed
excellent activity and selectivity. The catalytic results indicate
that the ordered meso-structure, higher surface area and smaller
size of organic linking groups are more favorable to the
exposure of active sites to improve catalytic performance. The
three catalysts showed superior catalytic activity over some
homogeneous organic catalysts and they can be selected as
Scheme 1 Mechanism of the one pot cascade reaction.
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excellent nano-reactor candidates. Moreover, these mesoporous
catalysts showed perfect recyclability without obvious active loss
at least after five recycling runs. This work opens a door for the
efficient synthesis of valuable donor-acceptor cyclopropanation
derivatives from easy obtained and cheap raw reagents, which
may have potential application in medical preparation and
refined chemistry.
Preparation of organosilica HQP
The preparation procedure was referred to our former
reported method[13], except dibutyltin dilaurate was applied as
catalyst to accelerate the reaction in the present work. The
synthetic route was illustrated in Scheme 2. In a typical modified
procedure, under N2 atmosphere, to a 100 mL of dried round
bottom flask, 15 mmol of hydroquinone (HQ, 1.65 g, 1 eq.) and
31.5 mmol of (3-isocyanatopropyl) triethoxysilane (ICPTES,
Experimental Section
Materials and methods
All of the chemical regents were analytical grade and used as
received without further purification. Reagents of (3-
Isocyanatopropyl) triethoxysilane (ICPTES), hydroquinone,
bisphenol A, 1,1’-bi-2-naphthol tetraethoxysilane (TEOS)
,
methyl orthosilicate (TMOS), all species of aldehyde,
malononitrile, and diethylbromomalonate were purchased from
Macklin Industrial Corporation. Pluronic P123 (P123,
EO20PO70EO20) was purchased from Aldrich. Anhydrous THF
and toluene were obtained via treating with activated 4 Å
molecular sieves for at least one week and were used without
further purification.
Scheme 2 Synthesis of HQP/BPAP/BNP with HQ/BPA/BN and ICPTES in a
stoichiometric ratio
alkoxysilane monomer; 7.4 mL, 2.1 eq.) were dissolved in 20 mL
of absolute tetrahydrofuran (THF), and one drop of dibutyltin
dilaurate worked as catalyst was added to the system to boost
the condensation process. Then the mixture was heated to
reflux and the reaction was proceeded for at least 24 hours. The
proceeding of the nucleophilic addition was tested by thin layer
chromatography (TLC). When the condensational reaction was
finished, the solvent of THF was removed with distillation under
vacuum. The white product in powder was obtained by
ultrasonic treatment in dried hexane. Drying in the oven for
overnight at the temperature of 60oC, the yield of the product
was calculated at 91%. Its 1H NMR spectra image is depicted in
Figure S5.
Instrument
Solution 1H NMR characterization of HQP, BPAP and BNP
were carried out on Bruker-400 MHz spectrometer at 25oC. The
samples were dissolved in the solvent of CDCl3 and the
chemical shift was referenced to the regent of TMS. Solid state
13C CP-MAS NMR spectra was recorded on a Bruker BioSpin
400 MHz spectrometer equipped with magic angle spin prober in
a 4-mm ZrO2 rotor with 4.25 μs of p/2 pulse, 3 s of repetition
delay, 1 ms of contact time and 2000 times of scanning. The 29Si
CP-MAS NMR experiment was performed at the condition of
3000 times of scanning, 10 ms of contact cycling,15 s of
repeating time and 3.5 μs of p/2 pulse. The samples were
placed in a ZrO2 rotator and covered with a KeI-F lid. 13C and
29Si chemical shifts were externally referenced to the
resonances of tetramethylsilane. The FT-IR spectra was
collected with Perkin-Elmer Spectrum 100 spectrometer and the
background was deducted with a KBr pellet. The small angel X-
ray patterns were measured on a Bruker (Germany) instrument
using Cu-Kα radiation (λ= 1.5418 Å) with the condition of the
testing voltage at 40 kV and the current at 30 mA. The typical
physico-chemical properties were examined by N2 adsorption-
desorption test on ASAP 2000 instrument, and the surface area
was computed based on the BET method. The pore diameter
was determined by the BJH model. The analysis of N elemental
content was recorded on PerkinElmer PE 2400II. TG/DTA
analysis was carried out in the air condition on TA-SDT Q600
analyzer, and the tested temperature was between 30oC and
800oC. The scanning electronic micrographs were recorded on
Hitachi (Japan) SU8010 instrument and the telescope electronic
micrographs were obtained on Hitachi (Japan) H-7650 electron
microscope.
Preparation of organosilica BPAP
The preparation procedure was similar to that of HQP. The
detailed process was as following: Under N2 atmosphere, to
a100 mL dried round bottom flask, 15 mmol of bisphenol A (BPA,
3.42 g,
1 eq.) and 31.5 mmol of (3-isocyanatopropyl)
triethoxysilane (ICPTES, alkoxysilane monomer; 7.4 mL, 2.1
eq.) were dissolved in 20 mL of absolute tetrahydrofuran (THF),
and one drop ofdibutyltin dilaurate worked as catalyst was
added to the system to boost the condensation process. Then
the mixture was heated to reflux and the reaction was
proceeded for at least 24 hours. The proceeding of the
nucleophilic addition was tested by thin layer chromatography
(TLC). When the condensational reaction was finished, the
solvent of THF was removed with distillation under vacuum. The
white product in powder was obtained by ultrasonic treatment in
dried hexane. Drying in the oven for overnight at the
temperature of 60oC, the yield of the product was calculated at
87%. Its 1H NMR spectra image is depicted in Figure S6.
Preparation of organosilica BNP
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The preparation procedure was similar to that of HQP. The
detailed process was as following: under inert atmosphere, to a
dried round bottom flask, 5 mmol of 1,1’-bi-2-naphthol (BN, 1.43
g, 1 eq.) and 10.5 mmol of (3-isocyanatopropyl) triethoxysilane
(ICPTES, alkoxysilane monomer; 2.36 mL, 2.1 eq.) were
dissolved in 20 mL of absolute tetrahydrofuran (THF), and one
drop of dibutyltin dilaurate worked as catalyst was added to the
system to boost the condensation process. Then the mixture
was heated to reflux and the reaction was proceeded for at least
24 hours. The proceeding of the nucleophilic addition was tested
by thin layer chromatography (TLC). When the condensational
reaction was finished, the solvent of THF was removed with
distillation under vacuum. The product in oil was obtained by
ultrasonic treatment in dried hexane. Drying in the oven for
overnight at the temperature of 60oC, the yield of the product
was calculated at 90%. Its 1H NMR spectra image is depicted in
Figure S7.
Preparation of the mesoporous catalyst MC-BPA
The mesoporous catalyst MC-BPA was synthesized similar to
that of MC-HQ except the silica precursor was replaced with
organosilica of BPAP and TEOS. The amphiphilic triblock
copolymer P123 was applied as templating regent. In
a
representative run for MC-BPA, 0.84 g of P123, 2.44 g of NaCl,
6.96 g of deionized water and 20.88 g of 2 M HCl were mixed
together and stirred at room temperature until a homogeneous
transparent template solution was obtained. Then, to 15 g of the
templating solution, 160 mg of BPAP homogeneouly dispersed
in 0.65 mL of TEOS was added, the reaction was proceeded at
room temperature for 24 h. Finally, the mixture was transferred
into a PTFE autoclave and aged at 80°C under static condition
for 24 h in oven. The structure directing agent of P123 was
removed by continuously extraction in acidic ethanol
(ethanol/HCl; 80:5, v/v) under refluxing for one day. The white
mesoporous catalyst of MC-BPA was obtained after washing
with water and dried in oven at 60oC for overnight.
Preparation of the mesoporous catalysts
The mesoporous catalysts were prepared via a gel-sol method
with P123 as structure directing reagent, TEOS and
HQP/BPAP/BNP as silica precursors, followed by extracting in
acid ethanol solution, and the three mesoporous catalysts are
denoted as MC-HQ, MC-BPA and MC-BN, respectively. It is well
known that a large size of organic linking group is difficult to
incorporate into the framework of the material, as they can some
or what participate the assemble process of structure directing
reagent to form an irregular micelle, and thus yields disordered
or nonporous material. As we expected, disordered meso-
structures were obtained for the organosilica precursors of
BPAP and BNP if the same synthesis route was applied as that
of HQP, so a new strategy was selected for BNP to get different
particle morphology from that of HQP and BPAP.
Preparation of the mesoporous catalyst MC-BN
The mesoporous catalyst MC-BN was synthesized via sol-gel
method, and organosilica of BNP and TEOS were selected as
silica precursor, amphiphilic triblock copolymer P123 was
selected as structure directing agent. The synthetic method was
based on the literature reported before[14]. In a representative
run for MC-BN, 0.5 g of P123, 1.0 g of NaCl, and 0.15 mL of
NH4F solution (0.25M) were mixed with 1.5 mL of diluted HCl
solution (0.1M) and 13.5 mL of deionized water. The mixture
was stirred at 40°C with vigorous stirring until a homogeneous
solution was obtained. Then, the silica precursors of TMOS
(0.608 g), BNP (0.39 g) and absolute ethanol (2.00 g) was
slowly added to the above solution. The resultant mixture was
preceded at 40°C for 20h with vigorous stirring and then aged at
80°C under static condition in oven for 24 h. The template of
P123 was removed by continuously extraction in acidic ethanol
(ethanol/HCl; 80:5, v/v) under refluxing for one day. The white
mesoporous catalyst of MC-BN was obtained after washing with
water and dried in oven at 60oC for overnight.
Preparation of the mesoporous catalyst MC-HQ
The mesoporous catalyst MC-HQ was synthesized via sol-gel
method, and organosilica of HQP and TEOS were selected as
silica precursor, amphiphilic triblock copolymer P123 was
applied as templating regent. For the sake of acquiring a
mesoporous silica material with highly ordered structure, an
assisted regent of NaCl was added in the preparation[18]. In a
representative run for MC-HQ, 0.84 g of P123, 2.44 g of NaCl,
6.96 g of deionized water and 20.88 g of 2 M HCl were mixed
together and stirred at room temperature until a homogeneous
transparent template solution was obtained. Then, to 15 g of the
templating solution, 130 mg of HQP homogeneously dispersed
in 0.7 mL of TEOS was added, the reaction was proceeded at
room temperature for 24 h. Then, the mixture was transferred
into a PTFE autoclave and aged at 80°C under static condition
for 24 hours in oven. The structure directing agent of P123 was
removed by continuously extraction in acidic ethanol
(ethanol/HCl; 80:5, v/v) under refluxing for one day. The white
mesoporous catalyst of MC-HQ was obtained after washing with
water and dried in oven at 60oC for overnight.
Preparation of MC-HQ-C, MC-BPA-C and MC-BN-C materials
To remove all of the functional organosilicas incorporated in
the skeleton, the three materials of MC-HQ, MC-BPA and MC-
BN were calcined in a muffle furnace at the temperature of
550oC during 3 hours with the rate of 2oC/min and the calcinated
pure silica based matrixes were denoted as MC-HQ-C, MC-
BPA-C and MC-BN-C, respectively. During this period, the
functional organic species were buried thoroughly, that can be
evidenced by FT-IR spectrum, as shown in Figure S2. The
calcination treatment can’t have an obvious effect on the porous
structure of the three materials, and XRD characterization in
Figure S2 suggests that almost the same diffraction peaks can
be observed after the calcination.
Catalytic procedure of the one pot cascade reaction
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1
mmol
of
aldehyde,
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This work was supported by the National Natural Science
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
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Three carbamate functionalized
Hang Huo, Yanqiu Jiang*, Tingting
Zhao, Junlei Wang, Defeng Li,
Xianzhu Xu and Kaifeng Lin*
organosilica
were
covalently
incorporated in the framework of
mesoporous silica, and these
materials
catalytic
heterogeneous Knoevenagel and
Michael cyclopropanic cascade
reactions.
exhibited
performance
excellent
toward
Page No *. – Page No *.
Periodic mesoporous organosilicas
as efficient nanoreactors in the
cascade reaction for preparation of
cyclopropanic derivatives
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