Efficient Mukaiyama–Aldol Reaction with Aqueous Formaldehyde on a Hydrophobic Mesoporous Lewis Acid Polymer

Article abstract of DOI:10.1002/cctc.201701426

The design of robust water-compatible solid Lewis acid catalysts in the efficient utilization of aqueous formaldehyde is a challenging task. Herein, we describe an ytterbium triflate containing ordered mesoporous polymer (Yb(OTf)₂-MP) that acts as a highly active and selective Lewis acid catalyst for the Mukaiyama–aldol reaction with aqueous formaldehyde. The unique capacity of hydrophobic surface and ordered mesoporosity was able to stabilize the hydrophilic and hydrophobic reactants simultaneously and catalyze the reaction selectively by minimizing water interference at active sites. Accordingly, it enabled a broad range of silyl enol ethers to be used to create structurally diverse β-hydroxy carbonyl molecules efficiently. Notably, the process can be scaled up easily to achieve the gram-scale production of the key intermediate of natural product sarkomycin. Additionally, it is stable under aqueous conditions and can be recovered easily and used repeatedly at least six times.

Full text of DOI:10.1002/cctc.201701426

Accepted Article
Title: Efficient Mukaiyama-Aldol Reaction with Aqueous Formaldehyde
on a Hydrophobic Mesoporous Lewis Acid Polymer
Authors: Fang Zhang, Chao Liang, Zhen Wang, and Hexing Li
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To be cited as: ChemCatChem 10.1002/cctc.201701426
Link to VoR: http://dx.doi.org/10.1002/cctc.201701426
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10.1002/cctc.201701426
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Efficient Mukaiyama-Aldol Reaction with Aqueous Formaldehyde
on a Hydrophobic Mesoporous Lewis Acid Polymer
Fang Zhang*, Chao Liang, Zhen Wang, Hexing Li*
Abstract: The design of robust water-compatible solid Lewis acid
handling, and less toxic aqueous solution of formaldehyde
catalysts in the efficient utilization of aqueous formaldehyde is a
4
(formalin). Furthermore, a high percentage of water in aqueous
challenging task. Herein we described that an ytterbium triflate-
formaldehyde increases the environmental advantage for
containing ordered mesoporous polymer (Yb(OTf)2-MP) acts as a
highly active and selective Lewis acid catalyst for the Mukaiyama-
Aldol reaction with aqueous formaldehyde. The unique capacity of
hydrophobic surface and ordered mesoporosity was capable of
simultaneously stabilizing the hydrophilic and hydrophobic reactants
and selectively catalyzing the reaction by minimizing water
interference at active sites. Accordingly, it enabled a broad range of
silyl enol ethers engaged in this process efficiently to create
structurally diverse β-hydroxy carbonyl molecules. Notably, the
process can be easily scaled up to achieve gram scale production of
the key intermediate of natural product sarkomycin. Also, it is stable
in aqueous conditions and can be easily recovered and used
repetitively at least six times.
5
chemical synthesis since it is clean, safe and non-flammable.
However, the consequent challenges are the increased mass
transfer resistance and the additional competitive reactions as
well as the possible decomposition of active species. Therefore,
the successful examples of the utilization of aqueous
formaldehyde as C1 feedstock without any additives or organic
6
medium as co-solvent are very rare.
The Mukaiyama-Aldol reaction with formaldehyde provides
a useful and straightforward approach to β-hydroxy carbonyl and
particularly 1,3-ol homologous unit,
a
basic repeating
functionality featured in numerous biologically important
7
macrolides and polyketides. Inspired by the type II Aldolases in
Introduction
Nature, extensive efforts have been devoted to the development
of solid Lewis acids to realize this process via the enolate
The utilization of simple, cheap and abundant C1 molecules as
the feedstock in the environmentally benign chemical processes
8
activation mechanism. Nonetheless, the aqueous nature tends
to reduce the electrophilicity of formaldehyde. Importantly, the
traditional Lewis acids such as AlCl3 and TiCl4 seriously
decompose in the presence of water, thus limiting their
1
has attracted tremendous interest. Formaldehyde is one of the
most important single-carbon electrophiles and has been used in
2
many organic reactions and natural product synthesis.
9
applications in the activation of aqueous formaldehyde. To
Obviously, formaldehyde gas is carcinogen and not convenient
to handle, which causes the environmental and health issues
when it is used as the reactant in the chemical transformations.
Accordingly, the non-electrophilic oligomers such as
paraformaldehyde and trioxaneare are often used as the
precursors of formaldehyde. But, they have to convert back to
active formaldehyde form by thermal or acid activation. Thermal
cracking produces highly toxic, gaseous formaldehyde whereas
that rapidly re-polymerizes. Acid treatment inevitably causes the
corrosion, a large amount of chemical wastes and difficult
address these challenges, Kobayashi, Kim, Gin and our group
have come up an elegant solution by developing solid supported
lanthanide or scandium triflate and using surfactant-induced
micellar Lewis acid catalytic system promoted aqueous
10
formaldehyde-involved Mukaiyama-Aldol reactions. However,
these catalytic systems often suffer from the tedious preparation,
the relatively expensive catalyst support, the use of toxic organic
co-solvent or the unsatisfactory performances. Moreover, the
late stage modification of biologically interesting complex
molecules with the methodology remain elusive, in part because
of the small pore size and the increased diffusion hindrance of
the complicated three-phase systems. In this manuscript, we
described the synthesis of a novel ytterbium triflate-containing
hydrophobic ordered mesoporous polymer by incorporating
ytterbium triflate groups in the mesoporous phenol-formaldehyde
polymer framework. It can remarkably accelerate the
Mukaiyama-Aldol reactions with aqueous formaldehyde without
requiring organic solvents as co-solvents or any additives.
Notably, this catalyst displayed significantly better catalytic
3
operation. On the contrary, the ideal reagent is cheap, easily
Prof. Dr. F. Zhang, Mr. C. Liang, Mr. Z. Wang, Prof. Dr. H. Li
The Education Ministry Key Laboratory of Resource Chemistry and
Shanghai Key Laboratory of Rare Earth Functional Materials Shanghai
Normal University
Shanghai, 200234, China
Fax: (+)86-21-643222272
E-mail: zhangfang@shnu.edu.cn; HeXing-Li@shnu.edu.cn
Supporting information for this article is given via a link at the end of the
document.
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Scheme 1. Illustration of the preparation of Yb(OTf)2-MP.
13
performance than homogeneous analogue (Yb(OTf)3) and
Yb(OTf)2-containing polystyrene counterpart (Yb(OTf)2-PS).
Furthermore, the efficient modification of a variety of bioactive
complex structures was achieved. It also realized the gram scale
production of bioactive sarkomycin intermediate with high
catalytic efficiency. Finally, the robust catalyst can be recycled
and reused for six times without significant loss of catalytic
activity.
species were present in the trivalent state. But, Yb 4d5/2 and
4d3/2 binding energies in the Yb(OTf)3 compound were 187.6 and
201.1 eV, respectively. Clearly, the binding energies of Yb
element in Yb(OTf)2-MP was negatively shifted by 0.6 eV (Figure
1). It was probably explained as the replacement of the CF3SO3-
ligands in Yb(OTf)3 by the PhSO3Na- groups in Yb(OTf)2-MP,
thereby making Yb(III) more electron-rich. To further confirm this
coordination effect, we compared the S 2p binding energies of
Yb(OTf)3 and Yb(OTf)2-MP (Figure S1-2). One peak at 169.1 eV
11
corresponding to the OTf-groups could be found in Yb(OTf)3.
Results and Discussion
However, for Yb(OTf)2-MP, the relatively broad peak was
observed. The deconvolution result revealed that two peaks at
168.3 eV and 169.4 eV were observed, which were ascribed to
Ordered mesoporous phenol-formaldehyde polymer was
selected as the support not only because of high surface area
10
the benzenesulfonic acid and the OTf-groups, respectively. As
and ordered large pores, but also rich hydrophobic and modular
expected, the S 2p binding energy of Yb(OTf)2-MP shifted
positively by 0.30 eV in comparison with Yb(OTf)3. Therefore, we
concluded that the formation of the Yb(III) Lewis acids in
Yb(OTf)2-MP was through the chemical interaction between the
PhSO3Na- groups in the polymer framework and Yb(OTf)3
compound, as described in Scheme 1.
11
phenyl groups in the framework.
After sulfonation with
chlorosulfuric acid, sodium ion exchange and Yb(OTf)3
coordination treatments (Scheme 1), the phenyl moieties could
efficiently transformed to Yb(III) triflate species, resulting in the
formation of Lewis acid polymer with ordered mesoporous
structure (Yb(OTf)2-MP). During the preparation process, we
carefully investigated the sulfonation parameters (Table S1),
Yb(III) triflate amount and the solvents in the post-grafting
process (Table S2). Satisfactorily, the optimized catalyst was
achieved with 0.366 mmol/g Yb content, as determined by
inductively coupled plasma (ICP) analysis, which was the
highest reported loading for Yb(OTf)3-containing heterogeneous
12
catalysts so far. This result could be rationalized by the
periodically arranged coordinating groups (PhSO3Na-) in the
functionalized mesoporous polymer.
X-ray photoelectron spectroscopy (XPS) measurement of
the Yb(OTf)2-MP showed that the Yb 4d5/2 and 4d3/2 binding
energies were 187.0 and 200.5 eV, confirming that all Yb
Figure 1. Yb 4d XPS spectra of Yb(OTf)3 and Yb(OTf)2-MP samples.
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10.1002/cctc.201701426
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36.3 wt.% toluene. Such surprisingly increase with the
hydrophobic molecule indicated that the pore surface of
Yb(OTf)2-MP was highly hydrophobic. On the basis of these
results, we can therefore assure that our current approach could
effectively avoid the destruction of the original mesostrcture and
meanwhile retained the intrinsic hydrophobicity of phenol-
formaldehyde polymer. These unique chemical and structural
properties may offer unique catalytic features.
Figure 2. N2 sorption isotherms (a) and pore size distribution curves (inset of
1a), XRD patterns (b) of MP and Yb(OTf)2-MP, and TEM images (inset of 1b,
c) and EDX analysis (d) of Yb(OTf)2-MP.
Figure 3. Water and toluene vapor adsorption tests (a) and trimethyl(1-
phenylprop-1-enyloxy)silane reactant adsorption experiment in water of
Yb(OTf)2-MP.
N2 sorption isotherms (Figure 2a) of both MP and
Yb(OTf)2-MP showed the typical type-IV sorption curves with H2
hysteresis loops at the relative pressures at 0.4-0.8, indicating
We focused our initial investigation on the transformation
of aqueous formaldehyde and trimethyl(1-phenylprop-1-
enyloxy)silane as the reactants to explore the catalytic
performance of the hydrophobic mesoporous Lewis acid
polymer in water. As shown in Table S4, we firstly optimized the
catalytic active species by probing different Ln(OTf)2-MP
catalysts including La, Ce, Nb, Sm, and Yb and Sc(OTf)2-MP. All
these catalysts furnished moderate to good yield with the
desired adduct 3-hydroxy-2-methyl-1-phenylpropan-1-one and
hydrolyzed byproduct propiophenone. Clearly, Yb(OTf)2-MP
exhibited the highest catalytic activity and selectivity under the
same reaction conditions. This could be attibuted to its relativley
14
the presence of mesopores. Their BET surface areas and pore
2
3
volumes were 343, 319 m /g and 0.23, 0.20 cm /g, respectively
(Table S3). Particularly, the pore size distribution curves of
Yb(OTf)2-MP and MP were centered at 3.74 and 3.68 nm,
respectively (inset of Figure 2a). It is interesting to note that only
the slight decreases in surface area, pore volume and pore size
of Yb(OTf)2-MP compared to MP sample indicated that Yb(III)
Lewis acid species were homogeneously dispersed in the
polymer framework rather than located in the pore channel or
15
pore mouth. Figure 2b showed small angle X-ray diffraction
(XRD) patterns of MP and Yb(OTf)2-MP. Similar to MP sample,
o
17
Yb(OTf)2-MP displayed one intense peak at 2θ
=
0.80
strong Lewis acidity. Next, we carefully investigated the
characteristic of (100) reflection, together with one small peak at
reaction conditions of Yb(OTf)2-MP including catalyst loading
and the amount of aqueous formaldehyde as well as reaction
time and temperature (Figure 3 and S3-5). Gratifyingly, we found
o
2θ = 1.39 indicative of (110) reflection. This result demonstrated
that the ordered hexagonal structure (p6mm) could be well
preserved after transforming the phenyl groups to Yb(III) Lewis
that 7.5 mol% Yb(OTf)2-MP achieved 99% conversion and 96%
16
o
acid species. TEM images further confirmed that Yb(OTf)2-MP
selectivity at 20 C for 8.0
h
with 10 equiv. aqueous
had a two-dimensional (2D) array of one-dimensional channel
with uniform pore size (inset of Figure 2c-d). Furthermore, water
and toluene vapors were brought to absorb with Yb(OTf)2-MP
and their respective absorbed amounts were measured by the
weight changes of Yb(OTf)2-MP (Figure 3a). Interestingly,
Yb(OTf)2-MP only absorbed 4.86 wt.% water while it absorbed
formaldehyde (Figure 4). The conditions delivered the highest
reported yield by solid Lewis acid in pure water so far (Table S5).
Interestingly, the homogenous analog Yb(OTf)3 showed the
remarkable decrease in the conversion (54%) and the slight
reduction in the selectivity (93%). This result probably reflected
the increased mass transfer limitation in the homogeneous
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system, which is composed of water-soluble Yb(OTf)3,
hydrophobic substance and hydrophilic aqueous formaldehyde.
The interaction between active sites and the reactants was
partially restricted and thus afforded the inferior catalytic
efficiency. This was confirmed by the plots of conversion against
reaction time (Figure 4). Furthermore, the substance adsorption
experiments was designed to gain better insight into the unique
enhancement of Yb(OTf)2-MP. As shown in Figure 3b, the silyl
enol ether can be quickly adsorbed and the adsorption capacity
can reach to 14.3 mg/g within 1.0 h in water, which further
confirmed the hydrophobic ordered mesoporous structure of
Yb(OTf)2-MP was crucial for its high catalytic performance.
Moreover, the enhanced performance of Yb(OTf)2-MP could be
To confirm the above observations, we prepared the
control sample Yb(OTf)2-PS by reacting Yb(OTf)3 with PhSO3Na-
functionalzied polystyrene support. TEM image revealed that it
didn’t have the abundant pores (Figure S7). N2 sorption isotherm
2
verified that BET surface area was only 40 m /g and the
hysteresis loop at P/P0=0.8 was asigned to the accumulated
pores of the particles (Figure S8). Moreover, toulene vapor
absorption isotherm showed that the absorbed amount was 13.2
wt.% (Figure S9). As expected, it exhibited the lowest
conversion (49.2%) and selectivity (79.8%). The inferior
conversion was probably due to the low surface area and small
pore size, which could not efficiently adsorb the reactants to
interact with Yb(III) active sites. Thus, the reaction was mostly
happened on the outer surface. Accordingly, the significant
decrease in the selectivity was due to that the Yb(III) active sites
on the surface of Yb(OTf)2-PS were easily to interact with water
and therefore it caused the serious hydrolysis reaction of the
silyl enol ether.
18
explained based on the proposed reaction mechanism, as
shown in Figure S6. The aldehyde group in aqueous
formaldehyde was activated by Yb(III) via
σ
bonding
coordination and then trimethyl(1-phenylprop-1-enyloxy)silane
attacks the activated HCHO to complete the aldol addition
reaction. The competing hydrolysis of the silyl enol ether
promoted by the Yb(III) Lewis acid occurs simultaneously. Due
to the good solubility of homogeneous catalyst Yb(OTf)3, it exists
in the aqueous phase in this Mukaiyama-aldol addition reaction,
which impedes the hydrophobic silyl enol ether interaction with
the active species, resulting in the low conversion. On the
contrary, the unique combination of hydrophobic pore surface
and ordered mesopores of the Yb(OTf)2-MP can efficiently
adsorb both hydrophobic and hydrophilic reactants and
subsequently stablize water sensitive hydrophobic silyl enol
ether, and concentrate both reactnants in the active sites. These
unique properties delivered the remarkably increased reactivity
of Yb(III) compound. Furthermore, the hydrophobic nature of the
catalyst surface can also minimize the amount of water molecule
O
OTMS
R3
Yb(OTf)2-MP (7.5 mol%)
H2O, 20oC
R1
aq. HCHO +
R3
HO
R1
R2
R2
OTMS
OTMS
CO2Me
TMSO
H3CO
H3C
90% conversion
97% selectivity
99% conversion
96% selectivity
93% conversion
98% selectivity
OTMS
OTMS
OTMS
OTMS
S
N
Boc
90% conversion
98% selectivity
91% conversion 95% conversion
97% selectivity 92% selectivity
86% conversion
91% selectivity
OTMS
OTMS
OTMS
92% conversion
90% selectivity
O
19
around the active sites, leading to the high selectivity.
82% conversion
96% selectivity
61% conversion
95% selectivity
Figure 5. Substrate scope of Yb(Of)2-MP catalyzed aqueous formaldehyde-
participated reactions with different silyl enol ethers.
Next, we set out to examine the reaction scope by
examining a variety of silyl enol ethers. It was found that both
electron-rich and -deficient silyl enol ethers were smoothly
transformed into the corresponding products in good yield with
excellent selectivity. Silyl enol ethers bearing various functional
moieties including cyclohexene, naphthalene and indene were
Figure 4. Plots of conversion (a) and selectivity (b) against time over
Yb(OTf)2-MP, Yb(OTf)3 and Yb(OTf)2-PS.
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compatible with this protocol, affording the satisfactory yields.
Additionally, the molecular structures that are frequently found in
biologically active agents, including bicyclo[2.2.1]hept-2-ene and
steroid, could be applied with this addition reaction with the
moderate conversion and good selectivity, further highlighting
the high catalytic activity of our Yb(OTf)2-MP catalyst (Figure 5).
of extremely low amount of Yb element (less than 5.0 ppm)
could be detected in the reaction mixture.
O
OSiMe3
+
O
Yb(OTf)2-MP
H2O
3 steps
OH
HCHO
20 equiv.
COOH
Sarkomycin
10 mmol
Yield: 65%, 0.91 g
Figure 6. Yb(OTf)2-MP catalyzed synthesis of the intermediate of (R)-
Sarkomycin.
Figure 7. Recycle tests of Yb(OTf)2-MP catalyst in the the Mukaiyama-Aldol
reaction between aqueous formaldehyde and trimethyl(1-phenylprop-1-
enyloxy)silane.
In addition, this Yb(OTf)2-MP-catalyzed Mukaiyama-aldol
process proved to be scalable under the mild reaction conditions
(Figure 6). We were able to perform a 10-mmol-scale (1.82 g)
reaction by using 3-ethenyl-1-trimethylsiloxycyclopent-1-ene as
the substance with 10 mol% Yb(OTf)2-MP and 20 equiv.
aqueous formaldehyde at 20 ºC for 24 h in water to give the
corresponding 2-(hydroxymethyl)-3-vinylcyclopentanone (65%
yield, d.r. 7:3) for the first time, which is the important
Furthermore, the reuse and recycle ability of Yb(OTf)2-MP
catalyst was investigated in the Mukaiyama-Aldol reaction
between aqueous formaldehyde and trimethyl(1-phenylprop-1-
enyloxy)silane. It was demonstrated that Yb(OTf)2-MP showed
very good recyclability (Figure 7). There was no significant
activity loss after recycling six times and the yield still remained
very high. ICP analysis revealed that Yb content in the reused
Yb(OTf)2-MP after six cycles was almost the same (0.35
mmol/g). Meanwhile, low-angle XRD pattern and TEM image
(Figure S10) confirmed that ordered mesoporous structure of the
recycled Yb(OTf)2-MP was well preserved. Moreover, Yb XPS
spectrum of the recycled Yb(OTf)2-MP (Figure S11) revealed
that the Yb 4d5/2 and 4d3/2 binding energies were 187.0 and
200.5 eV, which were almost the same with the fresh Yb(OTf)2-
MP. The result demonstrated that the Yb(III) species in the
recycled Yb(OTf)2-MP was stable even after six time reuse.
However, Yb(OTf)2-PS lost its catalytic activity about 30% after
only 4 recycles (Figure S12). ICP analysis clearly confirmed that
the leaching Yb(III) species in the reaction mixture was about
20% in comparison with the fresh sample. Thus the excellent
recyclability of Yb(OTf)2-MP could be attributed to the unique
periodically arranged Yb(III) Lewis acids in the polymer skeleton,
which effectively stabilized the Yb(III) triflate and inhibited the
intermediate for the synthesis of natural product sarkomycin with
20
antibiotic and anticancer activity.
The scalability and
robustness of this environmentally friendly heterogeneous
transformation illustrated its potential application in the
manufacture of medicinally useful chemicals on an industrial
scale.
To confirm whether the heterogeneous Yb(III) Lewis acid
polymer or the leached Yb(III) active species in solution was the
real catalyst responsible for this reaction, the following control
21
experiments were carried out. Yb(OTf)2-MP catalyst was
filtered from the reaction mixture after 4.0 h until conversion
exceeded 60%, and the reaction was continued for an additional
10 h under the same reaction conditions. It was found that no
obvious change in silyl enol ether conversion or the increased
yield of any products was observed. This result ruled out the
possible catalytic role of the leaching homogeneous Yb(III)
active sites from the polymer framework. This result was
consistent with ICP-AES result, which showed that the presence
22
loss of active species.
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calibrated by using C1s = 284.6 eV as a reference. X-ray powder
Conclusions
diffraction (XRD) data were acquired on
diffractometer using Cu Kα radiation. Nitrogen sorption tests were
measured using Quantachrome NOVA 4000e analyzer. Specific
a Rigaku D/maxr B
In summary, we have developed a novel Yb(OTf)2-functinalized
hydrophobic ordered mesoporous polymer for the Mukaiyama-
aldol reaction with aqueous formaldehyde. It displays
remarkable catalytic behavior by the virtue of the unique
combination of hydrophobic pore surface and ordered
mesopores. The process is applicable for structurally diverse
silyl enol ethers to deliver corresponding products in high yield
and selectivity. It also can be used to synthesize the important
intermediate for synthesis of (R)-sarkomycin in gram scale.
Furthermore, the catalyst can be recycled and reused for at least
six times without loss of its activity. We anticipate that this
strategy will enable the transformation of a broad range of
reagents with aqueous formaldehyde to the high value-added
fine chemicals in an eco-friendly way.
a
surface areas (SBET) and average pore diameter (DP) are calculated by
using BET and BJH models, respectively. Transmission electron
microscopy (TEM) experiments were performed on a JEM-2011 electron
microscope. The water and toluene vapour absorption measurements
were tested on a Hiden Isochema IGA-002/3 intelligent gravimetric
analyzer. A dosed amount of high-purity vapor was directly introduced
into the sample chamber and then the weight change was recorded after
the stable equilibrium pressure was reached.
Catalytic tests. As a typical run of aqueous formaldehyde participated in
Mukaiyama-Aldol reactions, 1.0 mmol trimethyl-(1-phenylprop-1-
enyloxy)silane, 10 mmol HCHO aqueous solution (37% w/w), a certain
amount of Yb(OTf)2-MP catalyst containing 0.075 mmol Yb(III) and 3.0 ml
distilled water were mixed in a three-necked round flask equipped with a
condenser and a magnetic stirrer. The reaction was carried out at 20oC
for 12 h under mild stirring. The products were analyzed by high
performance liquid chromatography analyzer (HPLC, Agilent 6410 series
Triple Quad) equipped with Agilent C18 column. The reaction conversion
was calculated based on trimethyl(1-phenylprop-1-enyloxy)silane since
HCHO was greatly in excess. The reaction selectivity for the target
product of 3-hydroxy-2-methyl-1-phenylpropan-1-one was determined
based on its ratio to the byproduct propiophenone. The yield of the target
product was thus obtained by multiplying the conversion with the
selectivity. In all the activity tests, the reproducibility was checked by
repeating each result at least three times and was found to be within ±5%.
Experimental Section
Experimental Details. Synthesis of hydrophobic ordered mesoporous
Lewis Acid polymer (Yb(OTf)2-MP). Ordered mesoporous phenol-
formaldehyde polymer (MP) was synthesized by evaporation induced
self-assembly (EISA) approach according to the procedure reported
elsewhere.1 After vacuum drying at 105oC for 1.0 h, 1.0 MP sample was
introduced into 40 ml CH2Cl2 solution at 0oC and subsequently 20 ml
chlorosulfonic acid was added dropwise into the mixture with constant
stirring. After stirring at 0oC for 12 h, the reaction mixture was refluxed for
another 12 h. Then, the mixture was filtered, washed by ethanol for three
times and dried in vacuum. The obtained sulfonated mesoporous
polymer was re-dispersed in 50 ml NaOH aqueous solution (1.0 mol/l)
and then this mixture was heated to 80oC and stirred for 24 h. After that,
a certain amount of 1.0 mol/l NaOH aqueous solution was added until the
pH value was 7.0, followed by filtering, washing by water and drying in
vacuum. The functionalized mesoporous polymer was designated as
PhNaSO3-MP. Furthermore, 0.50 g NaSO3-MP was added into 15 ml
In order to determine Yb(OTf)2-MP catalyst recyclability, it was allowed to
settle down by high speed centrifugation after each run of reactions and
the clear supernatant liquid was decanted slowly. The residual Yb(OTf)2-
MP catalyst was reused with a fresh charge of water and reactant for
subsequent recycle under the same reaction conditions.
Reactants adsorption test. The adsorption performance of the
representative reactant trimethyl-(1-phenylprop-1-enyloxy)silane on
Yb(OTf)2-MP catalyst was determined through the reactant concentration
change before and after adsorption of Yb(OTf)2-MP. In a typical run for
adsorption of silicon enolate, 50 mg catalyst was soaked in 50 ml water
and oscillated at 25 °C for 12 h, followed by adding 200 ml aqueous
solution containing 10 mg/l trimethyl-(1-phenylprop-1-enyloxy)silane. The
solution was sampled at given time intervals and the concentration of the
left trimethyl-(1-phenylprop-1-enyloxy)silane in solution was determined
on a HPLC analyzer (Agilent 6410 series Triple Quad) equipped with
Agilent C18 column at the characteristic adsorption wavelength of 218
nm. The adsorption capacity was determined after reaching saturation
adsorption.
anhydrous ethanol, following by adding 0.224
g
ytterbium(III)
trifluoromethanesulfonate (Yb(OTf)3). After stirring for 24 h at 80oC, the
solid product was filtered and washed thoroughly with anhydrous
tetrahydrofuran and anhydrous ethanol to eliminate un-coordinated
Yb(III) salt, followed by vacuum drying at 80oC for 12 h, denoted as
Yb(OTf)2-MP.
Synthesis of Ln(OTf)2-MP (Ln= La, Ce, Nb and Sm) and Sc(OTf)2-MP
samples. These samples were prepared using the same procedure by
using La(OTf)3, Ce(OTf)3, Nb(OTf)3, Sm(OTf)3, and Sc(OTf)3 instead of
Yb(OTf)3.
Acknowledgements
This work is supported by the Natural Science Foundation of
China (21677098), PCSIRT (IRT-16R49) and Shanghai
Government (TP2016034).
Synthesis of Yb(OTf)2-PS sample. This sample was synthesized using
the same protocol by changing PhNaSO3-MP to PhNaSO3-PS support.
Keywords: C1 feedstock utilization • Aqueous formaldehyde •
Mukaiyama-Aldol reaction • Hydrophobic mesoporous polymer •
Water-compatible solid Lewis acid
Characterization. Lanthanides and scandium contents were measured
by inductively coupled plasma optical emission spectrometer (Varian
VISTA-MPX). The surface electronic states of ytterbium and sulfur
elements were analyzed by X-ray photoelectron spectroscopy (XPS,
Perkin-Elmer PHI 5000C ESCA). All the binding energy values were
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10.1002/cctc.201701426
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An ytterbium triflate-containing
hydrophobic mesoporous polymer
enables the efficient Mukaiyama-Aldol
reaction with aqueous formaldehyde
without any additives or organic medium
as co-solvent in water. This highly active
and selective heterogeneous
Fang Zhang*, Chao Liang, Zhen Wang,
Hexing Li*
Page No. – Page No.
Efficient Mukaiyama-Aldol Reaction
with Aqueous Formaldehyde on a
Hydrophobic Mesoporous Lewis Acid
Polymer
transformation provides the
environmentally friendly way to create a
series of structurally diverse β-hydroxy
carbonyl molecules. Notably, the
process can be easily scaled up to
achieve gram scale production. Also, it
is very stable in aqueous conditions and
can be easily recycled at least six times.
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