Asymmetric Cascade Catalysis with Chiral Polyoxometalate-Based Frameworks: Sequential Direct Aldol and Epoxidation Reactions

Article abstract of DOI:10.1002/cctc.201700160

Catalytic asymmetric cascade reactions in which the substrates are transferred through well-choreographed consecutive independent steps by a single catalyst have received increasing interest. By incorporating a chiral organocatalyst pyrrolidine, an oxidat

Full text of DOI:10.1002/cctc.201700160

Accepted Article
Title: Asymmetric Cascade Catalysis with Chiral Polyoxometalate-
based Frameworks: Sequential Direat Aldol and Epoxidation
Reactions
Authors: Qiuxia Han, Wenwen Li, Shugai Wang, Jiachen He, and
Mingxue Li
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ChemCatChem
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Asymmetric Cascade Catalysis with Chiral Polyoxometalate-
based Frameworks: Sequential Direct Aldol and Epoxidation
Reactions
†
, †
*[a]
Wenwen Li, Qiuxia Han,* Shugai Wang, Jiachen He and Mingxue Li
Abstract: Catalytic asymmetric cascade reactions in which the
substrates are transferred through well-choreographed
consecutive independent step by a single catalyst have received an
increasing interest. By incorporating chiral organocatalyst
Enantiomerically enriched epoxy ketones are highly useful
intermediates and versatile building blocks in asymmetric
organic transformations, in which the carbonyl groups as
a
[
5]
a
additional moieties are susceptible to further transformations.
pyrrolidine, an oxidation catalyst polyoxometalate and a Lewis acid
catalyst copper(II) into a single porous coordination network (PCN),
two enantiomorphs CuWPYIs with novel Kagomé lattice are
achieved and applied in the conversion of aldehyde with ketone into
value-added chiral epoxy ketone via a one-pot procedure. The chiral
pyrrolidine organocatalysts interact with carbonyl group of ketones
throughout the reaction, which is beneficial to achievement of
compatibility between reaction intermediates and synergy of the
multiple catalytic cycles, thus driving the direct aldol/epoxidation
cascade reaction in orderly and asymmetrical way. Moreover, ‘two-
center catalysis’ principle is well demonstrated in the direct
aldol/epoxidation cascade reactions.
Therefore, a variety of powerful homogeneous organic and
organometallic chiral catalysts have been explored for highly
efficient and reliable asymmetric epoxidation of-unsaturated
[
6]
aldehydes/ketones to date. However, the general synthetic
procedure for the asymmetric syntheses of chiral epoxy ketones
requires two mechanistically distinct catalytic processes: 1) the
condensation of aldehydes with ketones to yield -
unsaturated carbonyl compounds, and 2) the asymmetric
epoxidation of unsaturated carbonyl compounds to afford
optical epoxy ketones. It will be a straightforward and valuable
synthetic tool for preparing enantiomerically enriched epoxy
aldehydes/ketones from readily available light aldehydes and
ketones by a well-choreographed consecutive discrete catalytic
[
7]
step with multiple transformations in a same catalyst.
Introduction
Catalytic asymmetric cascade reactions have aroused
considerable interest because of avoiding time-consuming,
costly protecting-group manipulations as well as the isolation of
[
1]
reaction intermediates. In recent years, many of new cascade
reactions utilizing the chiral organocatalysts with one reaction
center in a suitable ligand environment have been extensively
[
2]
investigated. Compared with the one reaction center in many
catalytic systems, ‘two-center catalysis’ is the extension of this
strategy in the field of organocatalytic cascade reactions. ‘Two-
center catalysis’ possesses significant advantages in both high
selectivity and reactivity because the reactants are bound and
[
3]
oriented at two different centers (see Scheme 1a). This
discovery can provide a new method to design catalysts for the
other cascade reactions. However, the biggest challenge for the
successful implementation of ‘two-center catalysis’ is to realize
[
4]
the consistency of the different catalyzed reaction rates.
Scheme 1. (a) The schematic representation of the typical ‘two-center
catalysis’ pattern. (b) The schematic representation of synthetic procedure of
CuWPYIs, showing the constitutive of the chiral PCNs and the cascade
catalytic process for production of chiral epoxy ketones directly from
aldehydes and ketones.
,
*
[
a] Prof.Q. Han,* W. Li, S. Wang, J. He, Prof. M. Li
Henan Key Laboratory of Polyoxometalate Chemistry, Institute of
Molecular and Crystal Engineering, School of Chemistry and Chemical
Engineering, Henan University, Kaifeng, 475004, P. R. China
Fax: (+86)-378-23881589
E-mail: qiuxia_han@163.com, limingxue@henu.edu.cn.
Chiral PCNs as the typical crystalline materials with infinite
network structures and a chiral template have attracted much
attention for their unique applications in asymmetric catalysis.
The key features of the chiral PCNs are their abilities not only to
incorporate multiple modified catalytic sites simultaneously in the
building block of PCNs but also to position substrates in
†
The author Q. X. Han and W. W. Li contributed equally.
[
8]
Supporting information for this article is given via a link at the end of
the document.
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ChemCatChem
10.1002/cctc.201700160
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proximity to each other for enhancing their reactivity through a
synergistic cooperation between different catalysis sites.
groups coordinated with copper ions and tightly surrounded the
[
9]
3−
PW12O
40
anions in a fan configuration. The arrangement
Beyond that, the most important problem in the cascade
catalytic processes is how to achieve the compatibility between
reaction intermediates and the consistency of the multiple
patterns of the PYI groups in the CuW−PYI1 and CuW−PYI2
were the mirror image inversion (see Figure 1b). Compared with
our previous structures, the pyrrolidine N atoms were not
protonated and no hydrogen bonds were found between the
[10]
catalytic cycles.
3–
Polyoxometalates (POMs) are the appropriate candidates to
design and construct tailored PCNs due to their versatility,
accessibility, excellent physical properties and potential
pyrrolidine N atoms and the imbedded PW12
O
40
anion in
[
13]
CuW−PYI1. As a result, the bared pyrrolidine N atoms were
potentially functionalized as asymmetric catalytic active sites to
stabilize the potential transition state in both asymmetric direct
aldol reaction and epoxidation of ,- unsaturated carbonyl
[
11]
applications in the catalysis. A promising way to combine the
organic and inorganic catalytic components synergistically is to
introduce effective oxidation catalyst POMs into the appropriate
chiral PCNs, that would be to increase the stability and the
[
14,6a]
compounds.
PW12
Moreover, the unique redox property of
3–
O40 could act as the cooperative catalytic sites to activate
oxidants for generating the epoxidation.
[
12]
[15]
efficiency of the corresponding functions. Recently, we have
successfully developed the asymmetric dihydroxylation and
asymmetric
epoxidation/cycloaddition
tandem
reactions
catalyzed solely by POM-based chiral metal-organic
[
13]
frameworks. Based on our previous work, this paper reported
two new enantiomorphs of POM-based PCNs (POM/PCNs),
CuWPYI1 and CuWPYI2, which were prepared by Keggin-
3−
type PW12O40 anions, copper(II) ions and L- or D-pyrrolidine-2-
yl-imidazole (PYI) under solvothermal conditions. CuWPYIs
displayed two-dimensional (2D) Kagomé topologies. In the
network the chiral organocatalyst pyrrolidine, oxidation catalyst
polyoxometalate and Lewis acid catalyst copper(II) were orderly
distributed and spatial matching, which will be beneficial for the
high efficiency and enantioselectivity in the conversion of
aldehyde and ketone into value-added epoxy ketone via a one-
pot procedure (see Scheme 1b).
3
−
Figure 1. (a) Plot of the connections between the PW12O40 and the copper
Results and Discussion
centers of CuW−PYI1. (b) Representation of the mirror image structures of
CuW−PYI1 (left) and CuW−PYI2 (right). (c) The 2D open sheet of CuW−PYI1
viewed down the c-axis. (color scheme: carbon, orange; nitrogen, blue; copper,
Structural Description.
3
-
green; PW12O40 , polyhedron), H atoms are omitted for clarity. (d) A schematic
representation of the 2D Kagomé topological network of CuW−PYI1.
CuW−PYI1 was synthesized by the solvothermal reaction of
3 2 3 2 2
H PW12O40·7H O, Cu(NO ) ·3H O and L−BCIP in a mixed
solvent of alcohol and water with a yield of 68%. CuW−PYI2 was
prepared similarly to CuW−PYI1, except that using D−BCIP in
place of L−BCIP. Single crystal structural analysis revealed that
both CuW−PYI1 and CuW−PYI2 shared the same cell
The crystal structure of CuW−PYI1 could be described as a
D Kagomé lattice topology (see Figure 1c). The copper ions as
2
a quasi-planar 4-connected building unit were positioned at the
3−
lattice points and bridged by the PW12
O
40
ligands, thereby
dimensions and crystallized in a chiral space group R
3
. Both the
generating the trigonal and large hexagonal windows within the
layer (Cu···Cu diagonal distances are of 19.96 Å in hexagonal
asymmetric unit of CuW−PYI1 and CuW−PYI2 consisted of two
3−
2+
PW12
O
40 anions and three [Cu(PYI)
2
]
cations (see Figure S1,
3
−
windows) (see Figure 1d). Each PW12
equilateral triangle ligand coordinated with three copper ions by
terminal oxygen atoms of the same {W 10} clusters. It was
12 40
worth mentioning that the PW O anions were imbedded in
O
40
anion as an
S2). In the crystal structure of CuW−PYI1, there were two
crystallographically independent copper ions, which coordinated
with two nitrogen atoms from two L−PYI1 units and two oxygen
3
O
3−
3−
atoms from different PW12O40 anions and adopted a distorted
the triangular cavities and alternately distributed on the each
side of the 2D layer (see Figure S3), thus realizing catalysis
sites distributed at the molecular scale, which should avail to
contact with the substrate molecules. The corresponding
structures of CuW−PYI2 were shown in Figure S4-S7 in the
Supporting Information.
quadrilateral geometry (see Figure 1a). Because the axial
positions of the copper ion were empty, the special coordination
mode would help to enhance the Lewis acidity of the copper(II)
center and afford the possibility of CuW−PYIs as Lewis acid
[
14]
catalysts.
Besides, the copper ions were well-positioned
within the channels, which was beneficial to interact with
substrate molecules that entered into the framework channels.
Chiral PYI groups were in-situ generated with the
butoxycarbonyl of BCIP removed in the reaction. The PYI
The layer of CuW−PYI1 displayed a honeycomb-like pattern,
and the layers packed in an eclipsed arrangement along the c-
3
axis to furnish 1D chiral channels with porosity of 8.5% (749.1 Å ,
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ChemCatChem
10.1002/cctc.201700160
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[
16]
calculated from PLATON analysis ) (see Figure 2a,b). The
accessible pores provided the possibility of adsorption suitable
substrates within the channels. As can be seen from the Figure
EPR spectroscopic studies of CuW−PYI1 were conducted on
crystalline samples with microwave frequency 9.45 GHz at 100
K to gain more information about the nature of the copper
catalysts. The complex exhibited an anisotropic EPR spectrum
with poorly resolved copper hyperfine features and a narrower
transition at the effective g value of 2.098 (see Figure 3c), which
was consistent with a quadrilateral geometry for the copper(II)
complex and in accordance with the crystal structure of
2
1
c, the Kagomé lattice exhibited an interlayer separation of
3−
0.40 Å, which can exactly accommodate one PW12
in diameter) into the cavities.
Remarkably, the first layer and the third layer interacted through
the OO interactions in concerted manner with OO
distances in the expected range from 3.08 to 3.40 Å, thus
O
40
molecule (about 10.40
Å
a
[
20]
CuW−PYI1.
characteristic of d
axial symmetry of Cu coordination sphere.
The EPR parameter
g
>
2.0023 was
3−
2 2
furnishing the sextuplet PW12
O
40 embrace (see Figure 2d). The
x
-y orbital of unpaired electron ground state in
II
[21]
suitable interlayer separation of 20.56 Å between the first and
the third layer can exactly hold the middle layer. To the best of
our knowledge, the metal-organic frameworks and inorganic
Kagomé compounds have been extensively reported.
However, the POM-based compounds with Kagomé lattice
To mimic the
catalysis process, we also studied the EPR of CuW−PYIs in the
catalysis process with ketones (100 mmol), aldehydes (10
mmol) and t-butylhydroperoxide (TBHP) (20 mmol). EPR spectra
of recovered solids from the catalytic system seemed identical
with that of the fresh catalyst. The results indicated that the
[
17]
[
18]
topology are still scarce.
II
valence state of Cu maintained in the catalytic process, which
supported our proposed mechanism of ‘two-center catalysis’ in
cascade asymmetric direct aldol/epoxidation reactions by
CuW−PYIs.
Figure 2. (a) Layers of CuW−PYI1 pack along the crystallographic c-axis. (b)
The stacking pattern of CuW−PYI1 showing the 1D channels along the c axis
3
40
−
with PW12
O
anions omitted. (c) Layers of CuW−PYI1 pack along the
crystallographic b-axis. The middle layer (green) is held in place by direct
O…O interactions with the first and third layers. (d) Sextuplet POM embrace.
The middle layer has been deleted.
Powder XRD, CD and EPR characterization.
The PXRD of CuW−PYIs showed the good correspondence
between the calculated and the experimental patterns
suggesting the high purity of the bulk samples (see Figure 3a,
Figure S8). CD of the bulk sample of CuW−PYI1 presented two
Cotton effects at 248 nm (θ = 32.0 mdeg) and 320 nm (θ = -56.7
mdeg) (Figure 3b), respectively. The Cotton effect at 248 nm
arose from the asymmetric groups, which had an about 12 nm
blue-shifted relative to the chiral PYI precursor. The other new
strong Cotton effect at about 320 nm was assigned to the
characteristic oxygen-to-tungsten charge-transfer bands of
Figure 3. (a) PXRD diagrams of CuW−PYI1. (b) CD spectra of crystals of
CuWPYI1 (red line) and CuWPYI2 (black line), respectively. Inset: CD
spectrum of L−PYI. (c) X-band EPR powder spectrum of CuW−PYI1 (black)
and CuW−PYI1 acquired from the cascade reaction system (red) at 110 K with
9.45 GHz. Reaction conditions: ketones (200 mmol), aldehydes (20 mmol),
CuW−PYIs (0.2 mmol), TBHP (40 mmol), 40 °C, 3 days. (d) IR spectra of
fresh and TBHP-oxidized CuW−PYI1. (e) IR spectra of cinnamaldehyde, fresh
and cinnamaldehyde-incorporated CuW−PYI1, respectively. (f) IR spectra of
benzaldehyde, fresh and benzaldehyde-incorporated CuW−PYI1, respectively.
3−
[19]
PW12O
40 anion. The CD spectrum of CuW−PYI2 was almost
a mirror image of CuW−PYI1 giving opposite Cotton effects at
48 nm (θ = -24.4 mdeg) and 320 nm (θ = 73.2 mdeg), which
showed that they were enantiomers.
Asymmetric epoxidation.
2
The asymmetric epoxidation reaction was initially employed
using cinnamaldehyde and TBHP oxidant, along with CuW−PYIs
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ChemCatChem
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(
0.1% mol ratio) in a heterogeneous reaction at 40 °C for 120 h,
that the formation of hydrogen bonds played a key role in driving
the catalytic reaction to proceed in an enantioselective manner.
The higher enantioselectivity of epoxy ketones obtained by
catalyst CuW−PYIs benefited from the formation of covalent
bonding between the chiral amino group of PYI and the carbonyl
group of , - unsaturated carbonyl compounds, which provided
additional steric orientation and drove the catalysis occurred in a
enantioselective manner. Meanwhile, the excellent catalytic
activities were attributed to the synergetic interactions between
the chiral amines and the inorganic POM clusters.
which showed a satisfactory yield (80.5%) and an excellent
enantioselectivity (85.8% ee) for (2R,3S)-3-Phenyloxirane-2-
carbaldehyde (Table 1, entry 1). As shown in Table 1 (entries 2-
5
), when the catalysts were applied to the other , -unsaturated
ketones, which also converted to the corresponding chiral epoxy
ketones with decent yields and enantioselectivities. The control
experiment showed that L−PYI as homogenous catalyst with
one organocatalytic reaction center could initiate the reaction
and afford the (2S,3R)-configured product with a 35.6% yield
and a 38.4% ee under similar reaction conditions because it
Mechanistically, we deduced that the formation of iminium
(IM) intermediates between the chiral amino groups of PYIs and
the carbonyl groups of , - unsaturated carbonyl compounds
initially activated the substrates and enforced the catalysis to
[
6a]
takes place by an iminium mechanism. The use of L−PYI and
CuHPW12 40 as homogeneous catalyst also obtained a 56.5%
O
conversion and a 35.7% ee. The blank experiment under
identical conditions gave only 7% of yield for 3-Phenyloxirane-2-
carbaldehyde. The higher efficiency and enantioselectivity
catalyzed by CuW−PYI1 may be attributed to the orderly
distribution and spatial matching of chiral PYI moieties and
[
24,6a]
proceed in an enantioselective manner;
double bond directly attacked the peroxooxygen groups in a
spiro fashion responsible for the epoxidation of
next, the olefin
cinnamylaldehydes and followed by hydrolysis of the IM
3
40
−
[11e]
PW12
O
anions that simultaneously effectively bound and
intermediates.
oriented with cinnamylaldehyde and TBHP, respectively.
One-Pot Asymmetric Cascade Reactions.
Table 1. Yields and enantiomer excess (ee) in the asymmetric epoxidation
reactions of olefins.
[
a]
Proline compounds are particularly favorable organocatalysts
for the catalytic cascade reactions due to their flexibility and
[
2a]
diversity of combinations of EN and IM activation. Substantial
experiments have demonstrated that they can effectively
promote aldehydes and ketones reaction by formation of EN
intermediates between the pyrrolidine moieties of proline and the
[
b]
[c]
Entry
Substrate
Yield [%]
Ee [%]
1
2
R
R
H
carbonyl groups of ketones providing the enantiomerically
1
2
3
4
5
6
H
H
H
80.5(82.6)
78.5(77.6)
75.4(78.7)
82.5(83.8)
76.5(77.6)
73.8(71.9)
85.8(-80.4)
76.8(-85.4)
79.4(-70.8)
78.5(-81.2)
74.5(-75.8)
35.4(-36.7)
[14,24b]
enriched direct aldol products.
It is clear from the crystal
CH
phenyl
CH
phenyl
3
structures of CuW−PYIs that the chiral directors, Lewis acid and
oxidation catalyst coexistence within a confined space, which
4-NO
4-NO
2
3
2
would provide
a particular environment for the efficient
styrene
conversion of aldehyde with ketone into the chiral epoxy ketone
via the combination of EN and IM activation in the asymmetric
cascade direct aldol/epoxidation reaction.
[
(
1
a] Reaction conditions: substrates (5 mmol), CuW−PYIs (0.05 mmol), TBHP
70% in decane, 10 mmol), 40 C, 120 h. [b] The yields were determined by
H NMR spectroscopy. [c] The ee values were determined by chiral HPLC on
a Chiralcel AD column. The yields and ee values catalyzed by CuW−PYI2
Table 2. Yields and enantiomer excess (ee) in the direct aldol/epoxidation
[
a]
cascade reactions between aldehydes and ketones.
were listed in the parentheses.
-
1
As shown in Figure 3d, a new (O-O) band at 827 cm was
found in the IR spectrum of the catalyst CuWPYI1 impregnated
with TBHP, which confirmed that the active intermediate
[22]
1
2
[b]
[c]
peroxotungstate was formed during the epoxidation process.
Entry
Sub. A (R )
Sub. B(R )
Yield [%]
ee [%]
-
1
7
8
9
H
68.5(66.6)
67.5(65.6)
69.4(68.7)
72.5(70.8)
70.5(74.5)
78.5(-72.9)
75.4 (-70.6)
79.4(-73.3)
76.4(-77.5)
77.3(-79.4)
The C=O stretching vibration at 1672 cm in the IR spectrum of
the catalyst impregnated with CH Cl solution of
cinnamaldehyde was presented, which had a red-shifted about
4-CH
4-Cl
3
a
2
2
CH
3
10
4-NO
H
2
-
1
-1
1
1 cm relative to the 1683 cm of the free cinnamaldehyde.
11
12
13
The results suggested cinnamaldehyde molecules were
absorbed in the cavity of CuWPYI1 and activated by pyrrolidine
nitrogen atoms (Figure 3e). In contrast to the previously reported
4-CH
3
67.8(64.3)
69.2(66.6)
71.3(69.8)
78.4(-75.6)
75.3(-77.4)
86.2(-78.7)
Phenyl
4-Cl
14
4-NO
2
[
13]
results,
CuW−PYIs were poor catalysts in asymmetric
[
a] Reaction conditions: ketones (50 mmol), aldehydes (5 mmol), CuW−PYIs
0.05 mmol), TBHP (10 mmol), 40 C, 120 h. [b] The yields were determined
by H NMR spectroscopy. [c] The ee values were determined by chiral HPLC
on a Chiralcel AD columns. The yields and ee values catalyzed by CuW−PYI2
were listed in the parentheses.
epoxidation of styrene under the same reaction conditions,
which may be attributed to no formation of hydrogen bonds
between the terminal oxygen atoms of the POM and the
(
1
[
23]
pyrrolidine nitrogen atoms (Table 1, entry 6).
These results
provided an experimental support for our previous speculation
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As shown in Table 2 (entries 7-14), the cascade reactions
could be smoothly completed within a single workup stage by a
reaction mixture of benzaldehyde, acetone and TBHP with
catalyst CuW−PYI1 at 40 C. The target (2R, 3S)-3-
Phenyloxirane-2-carboxaldehyde was obtained in a 68.5% yield
and a 78.5% ee. The control experiment showed under identical
condition without CuW−PYI1, no reaction happened. CuW−PYI1
solids could be easily recycled from the reaction suspension by
filtration, and be reused at least three runs with slight decrease
in yield (from 68.5% to 65.6% yield) and enantioselectivity (from
basic centers can activate the carbonyl groups of ketones
leading to the nucleophilic EN intermediates (A), meanwhile Cu
II
ions as Lewis-acidic centers were available to activate
benzaldehyde molecules resulting in the electrophile
[
14b]
carbocations (B).
Then the nucleophilic EN intermediates
attacked the electrophile carbocations furnishing the - hydroxyl
IM intermediates (C), and further converted into the , -
unsaturated IM intermediates (D) smoothly under heating
conditions. Simultaneously, TBHP molecules were activated by
POMs forming peroxotungstate intermediates (E); then, the
olefin double bond directly attacked the peroxooxygen groups in
a spiro fashion forming the epoxidation of cinnamylaldehydes (F);
lastly, the IM intermediates hydrolyzed furnishing the chiral
products (Scheme 2) and the catalytic cycle was accomplished.
In conclusion, the orderly, asymmetrical and efficient conversion
of aldehyde with ketone into value-added chiral epoxy ketone in
the direct aldol/epoxidation cascade reaction was largely
attributed to the chiral pyrrolidine organocatalysts, which
interacted with carbonyl groups of ketones throughout the
reaction, thus achieving of compatibility between the reaction
intermediates and synergy of the multiple catalytic cycles.
78.5% to 74.2% ee). In order to verify the heterogeneity, the
experiment was carried out by gravity filtration to remove the
catalyst after reaction 60 hours. The results demonstrated the
reaction nearly was terminated in the next another 60 hours,
which suggested that CuW−PYI1 was a true heterogeneous
catalyst. The index of PXRD patterns of the CuW−PYI1 bulky
samples filtered from the catalytic reaction after three runs was
agreed well with the fresh samples, which indicated the
crystallinity maintained (Figure 3a). As expected, CuW−PYI2
solids exhibited similar catalytic activities and gave products with
opposite chirality in the asymmetric cascade direct
aldol/epoxidation reaction. In addition, to test the generality of
the catalysis, some other benzaldehydes bearing an electron-
withdrawing or electron-attracting substituent and ketones were Conclusions
screened. The results showed that all of the aldehydes and
ketones were converted to the corresponding chiral epoxy
ketones with decent yields and enantioselectivities.
In summary, we have developed a new strategy to construct
functionalized chiral polyoxometalate materials and explored
them in the asymmetric cascade reactions for preparing optical
epoxy ketones from the readily available light aldehydes and
ketones. CuWPYIs exhibited the unusual 2D Kagomé topology.
In the crystal structures of CuW−PYIs, the chiral director, Lewis
acid and oxidation catalyst coexist within a confined space,
which provided
a particular environment for the efficient
conversion of aldehyde with ketone into the chiral epoxy ketone
via the combination of EN and IM activation in the asymmetric
cascade direct aldol/epoxidation reaction. The results
demonstrated that the chiral POM-based PCNs were excellent
catalysts for creating enantiomeric compounds by a well-
choreographed consecutive discrete catalytic step with multiple
transformations.
Experimental Section
General Methods and Materials.
Scheme 2. The diagram for potential mechanism of ‘two-center catalysis’ in
cascade asymmetric direct aldol/epoxidation reactions by CuW−PYIs.
Elemental analyses (EA) of C, H and N were performed on a Vario EL
III elemental analyzer. ICP analyses were performed on a Jarrel-AshJ-
A1100 spectrometer. FT-IR spectra were recorded from a solid sample
pelletized with KBr on JASCO FT/IR-430. Powder XRD diffractograms
were obtained on a Riguku D/Max-2400. Circular dichroism (CD) spectra
were obtained from a solid sample pelletized with KBr on JASCO J-810.
EPR spectra were obtained at 110K using a Bruker ESP-380E FTEPR
spectrometer operating at 9.44 GHz (X-band) with 100 kHz modulation,
-
1
One broad C-O stretching vibration at 1690 cm was
presented in the IR spectrum of the catalyst CuW−PYI1
impregnated with benzaldehyde in acetonitrile solution, which
-
1
-1
had a red shift of 11 cm from 1701 cm of the free
The result unambiguously
demonstrated the absorbance of benzaldehyde and possible
activation of the substrate by the unsaturated Cu in the channel
of CuW−PYI1. CuW−PYIs belonged to a particular class of the
two-center catalysts, in which the chiral amine groups as Lewis-
[
14a]
benzaldehyde (see Figure 3f).
1
13
microwave power 2.04 mW and modulation amplitude 1 G. H and
C
II
1
NMR spectra were recorded on a Varian INOVA-400 MHz type ( H, 400
13
MHz; C, 400 MHz) spectrometer. The chemical shifts are reported in
1
3
ppm relative to CDCl (δ = 7.26) for H NMR and relative to the central
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ChemCatChem
10.1002/cctc.201700160
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1
3
CDCl
on a PerkinElmer 241 polarimeter. HPLC analysis was performed on
Agilent 1150 using ChiralPAk OD−H column or AD−H column
purchased from Daicel Chemical Industries, Ltd. Products were purified
by flash column chromatography on 200-300 mesh silica gel, SiO
3
resonance (δ = 77.0) for C NMR. Optical rotation was measured
paper. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
a
Catalysis.
2
.
General Procedure for the Organocatalytic Epoxidation of α,β-
Unsaturated Aldehydes/Ketones. The catalyst (0.05 mmol, 1 mol %)
was added to a solution of the α,β-unsaturated aldehydes or ketones (5
mmol) followed by the addition of TBHP (70% in decane) (10 mmol, 2
equiv.) at 40 C. After reaction five days the crude reaction mixture was
passed through a silica gel FC column (ether/pentane) to give the
corresponding α,β-epoxy aldehydes/ketones. The stereochemistry of the
chiral amine-catalyzed asymmetric epoxidation reactions was established
by comparison of the optical rotation of 3-Phenyloxirane-2-
Synthesis. All reagents were used as purchased without further
purification. L− and D− N-tert-butoxy-carbonyl-2-(imidazole)-1-pyrrolidine
[25]
[26]
(
L− or D−BCIP), and H
3
PW12
O
40nH
2
1
O
were prepared according to
the literatures and confirmed by IR and HNMR, respectively.
Synthesis of CuW − PYI1. CuW − PYI1 was synthesized by the self-
assembly under hydrothermal conditions (see Scheme S1):
H
3
PW12
O
40·7H
mmol) and L−N-tert-butoxy-carbonyl-2-(imidazole)-1-pyrrolidine (L−BCIP)
50.0 mg, 0.2 mmol) was added in mixed water (4 mL) and alcohol (4
2 3 2 2
O (200 mg, 0.067 mmol), Cu(NO ) ·3H O (72.5 mg, 0.3
[6a,30]
23
carboxaldehyde with the literatures
{[a]
D
: +14.3 (c 0.48, CHCl
3
) for
3
-20.4 (c = 0.55, CHCl )
25
(
(2R,3S)-3-Phenyloxirane-2-carbaldehyde; [α]
D
mL) and stirred at pH 3.0 for 6 h, then sealed in a 25 mL Teflon-lined
autoclave and maintained at 130 °C for 72 h (see Scheme S1). After
cooling the autoclave to room temperature, yellow block single crystals of
CuW−PYI1 were separated, washed with water and air-dried. (Yield: ca.
for (2S,3R)-3-Phenyloxirane-2-carbaldehyde}, which revealed that the
stereochemistry of the 3-Phenyloxirane-2-carboxaldehyde derived were
(2R,3S) and (2S,3R) by CuW−PYI1 and CuW−PYI2, respectively.
6
C
6
8% based on
Cu
4.39; Found: C 8.43, H 1.12, N 3.64, P 0.92, Cu 2.76, W 64.40. IR
H
3
PW12
O
40·7H
2
O). EA and ICP calcd (%) for
Typical one-pot procedure for the transformation of aldehydes with
ketones to chiral epoxy ketones using the CuW−PYIs. The aldehydes
48
H
78
N
18
P
2
3 24
W O80:C 8.41, H 1.15, N 3.68, P 0.90, Cu 2.78, W
(5 mmol) and ketones (50 mmol) were added in 2 mL solvent of DMSO
3
−
(KBr): four characteristic asymmetric vibrations resulting from PW12O
40
and stirred for 15 mins, the catalyst (0.05 mmol) and TBHP (70% in
decane, 10 mmol) were added and the mixture was stirred for several
days at 40C. The mixture was extracted with ethyl acetate (2mL×3),
-
1
-1
-1
t c
anions: (W=O ) (922 cm ),(W-Ob) (887 cm ), (W-O ) (794 cm ), and
-1

(P-O
a
) (1078 cm ); four characteristic vibrations resulting from PYI
-
1
-1
molecules: (N-O) (1229 cm ), (ring) (1473 cm ), (C-H, in plane)(1181
cm ), and (N-O) (841 cm ).
and the organic layer was dried with anhydrous MgSO
4
, filtered, and
-
1
-1
concentrated to give pure epoxy ketones after column chromatography
1
(
ether/pentane). The conversion was directly determined by H NMR
Synthesis of CuW−PYI2. The preparation of CuW−PYI2 was similar to
that of CuW−PYI1, except that using D−BCIP in place of L−BCIP. (Yield:
analysis of the reaction solution. The ee was determined by chiral HPLC
analysis.
ca. 65% based on
Cu
4.39; Found: C 8.40, H 1.13, N 3.67, P 0.89, Cu 2.79, W 64.43. IR
H
3
PW12
O
40·7H
2
O). EA and ICP calcd (%) for
C
6
48
H
78
N
18
P
2
3 24
W O80: C 8.41, H 1.15, N 3.68, P 0.90, Cu 2.78, W
3
−
Acknowledgements
(
KBr): four characteristic asymmetric vibrations resulting from PW12O
40
-
1
-1
-1
t c
anions: (W=O ) (923 cm ),(W-Ob) (888 cm ), (W-O ) (795 cm ), and
-1

(P-O
a
) (1079 cm ); four characteristic vibrations resulting from PYI
This work was supported by the National Natural Science
Foundation of China (Nos. 21601048, U1304201and 21671055),
the China Postdoctoral Science Foundation (2015M580626), the
Natural Science Foundation Project of Henan province
-
1
-1
molecules: (N-O) (1230 cm ), (ring) (1474 cm ), (C-H, in plane)
-
1
-1
(1182 cm ), and (N-O) (842 cm ).
X-ray Crystallographic Analysis.
(162102210171, 162300410012).
Crystals with dimensions 0.30×0.24×0.18 mm for CuW−PYI1 and
Keywords: polyoxometalate • porous coordination networks •
asymmetric catalysis • two-center catalysis • cascade
0
.28×0.25×0.21 mm for CuW−PYI2 were stuck on a glass fiber, and
intensity data were collected at 153 K on a Bruker Smart APEX II CCD
diffractometer with graphite-monochromated Mo-Kα radiation (λ
.71073 Å), respectively. Routine Lorentz polarization and Multi-scan
=
0
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absorption correction were applied to intensity data. Their structures
were determined and the heavy atoms were found by direct methods
[27]
using the SHELXTL-97 program package. The remaining atoms were
2
found from successive full-matrix least-squares refinements on F and
[28,29]
Fourier syntheses.
Positions of the hydrogen atoms attached to
carbon and nitrogen atoms were geometrically placed. All hydrogen
atoms were refined isotropically as a riding mode using the default
SHELXTL parameters. For CuW−PYI1, of 128 reflections, 5729 unique
reflections (Rint = 0.0672) were considered observed [I>2σ(I)]. The final
cycle of refinement including atomic coordinates and the anisotropic
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converged to R = 0.0576 and wR = 0.1395 [I>2σ(I)]. CCDC 1478772
and 1478773 contain the supplementary crystallographic data for this
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= 0.0611 and wR
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= 0.1680 [I>2σ(I)].
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Entry for the Table of Contents
FULL PAPER
Two enantiomorphs CuW−PYIs with
novel Kagomé lattice were prepared
Prof. W. Li, Q. Han,* S. Wang, J. He,
Prof. M. Li
*
from
oxidation catalyst polyoxometalate
and Lewis acid catalyst. They
showed the satisfactory yields and
enantioselectivities in the cascade
Direct aldol/ epoxidation reactions by
synergism of the multiple catalytic
sites.
a
chiral organocatalyst, an
Page No. – Page No.
a
Asymmetric Cascade Catalysis with
Chiral Polyoxometalate-based
frameworks: Sequential Direct aldol
and Epoxidation Reactions
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