Chiral Cu(salen)-Based Metal-Organic Framework for Heterogeneously Catalyzed Aziridination and Amination of Olefins

Article abstract of DOI:10.1021/acs.inorgchem.6b02151

A homochiral 3D porous metal-organic framework was assembled from a chiral dicarboxylic acid-functionalized Cu(salen)-based catalyst and could serve as an efficient heterogeneous catalyst for aziridination and allylic amination of olefins. Besides easy separation and reuse of the catalyst, the chiral framework confinement could impart substrate size selectivity, enhance catalyst activity, and induce product enantioselectivity.

Full text of DOI:10.1021/acs.inorgchem.6b02151

Communication
pubs.acs.org/IC
Chiral Cu(salen)-Based Metal−Organic Framework for
Heterogeneously Catalyzed Aziridination and Amination of Olefins
Yan Liu,* Zijian Li, Guozan Yuan, Qingchun Xia, Chen Yuan, and Yong Cui*
School of Chemistry and Chemical Engineering and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong
University, Shanghai 200240, China
S
* Supporting Information
(DMF) and water (H2O) at 80 °C afforded single crystals of
ABSTRACT: A homochiral 3D porous metal−organic
[Cd4(CuL)4(DMF)4]·DMF·4H2O (1) in good yield (Figure 1).
framework was assembled from a chiral dicarboxylic acid-
functionalized Cu(salen)-based catalyst and could serve as
an efficient heterogeneous catalyst for aziridination and
allylic amination of olefins. Besides easy separation and
reuse of the catalyst, the chiral framework confinement
could impart substrate size selectivity, enhance catalyst
activity, and induce product enantioselectivity.
eterogenization of homogeneous catalysts is of great
Hinterest because of the ease of separation from the product
stream and reusability and has been attempted using polymeric
matrixes and porous inorganic oxides via ion exchange or
impregnation.1 Such hybrid catalysts afforded by these methods
are normally less effective than their homogeneous counterparts.
The isolation and uniformity of the catalysts were insufficient,
and the environment surrounding the catalysts could not be well
understood.2 If a molecular catalyst can be directly incorporated
onto the channel surface of a crystalline solid, a completely
isolated and uniform arrangement may be realized. In this
context, it is ideal for the integration of catalytically active
Figure 1. Synthesis of 1 from tetrameric [Cd4(O2C)8(DMF)4] units and
functions into metal−organic frameworks (MOFs) by using
functionalizing molecular catalysts as bridging linkers.3
Cu(L-H2), and a view of the 3D structure of 1 along the a axis (Cd, blue;
Cu, green; O, red; N, turquoise; C, gray; Cd4 units are shown as
polyhedra; only the O atoms of DMF are shown for clarity).
MOFs have good stability, high void volumes, and well-defined
tailorable cavities of uniform size.4 Such networks have shown
potential in stabilizing catalytic centers by isolating the sites in a
manner similar to that of the peptide architecture of enzymes in
biological systems and induce selectivity, regioselectivity, or
shape/size selectivity by creating an appropriate environment
around the metal center in the restricted space.5−7 Metallosalen
complexes have diverse applications in homogeneous catalysis.8
Chiral MOFs based on M(salen) have been explored as catalysts
for asymmetric transformations.7,9 However, the direct incorpo-
ration of chiral Cu(salen) into MOFs as heterogeneous catalysts
has not been reported. Herein we report the synthesis, structure,
and heterogeneous catalysis characteristics of a chiral Cu(salen)-
based MOF. For aziridination and allylic amination of olefins, the
immobilized Cu(salen) catalyst exhibits a much better catalytic
performance than its homogeneous counterparts as a result of
the framework confinement effect.
Single-crystal X-ray analysis revealed that 1 crystallizes in the
chiral space group C2. Four Cd atoms are bridged by four
bidentate and four tridentate carboxylate groups of eight CuL
units and four DMF to form a square-planar tetrameric
[Cd4(O2C)8(DMF)4] secondary building unit, with a C2 axis
passing through one metal center. There are three crystallo-
graphically independent Cd ions in 1, all of which adopt distorted
octahedral geometries by coordinating to six carboxylate O
atoms for Cd1, five carboxylate O atoms and one DMF for Cd2,
and four carboxylate O atoms and two DMF for Cd3. All of the
CuL ligands exhibit an exo-pentadentate coordination mode, and
each Cu adopts a distorted square-planar geometry (Figures S1
and S2). Each CuL ligand is thus linked to two Cd4 cores and
each Cd4 unit is linked by eight CuL ligands, forming a unique 3D
framework with 1D channels of ∼1.2 × 0.8 nm2 along the a axis,
which are filled with guest molecules DMF and H2O. The
The reaction of N,N′-bis(3-tert-butyl-5-(carboxyl)salicylide
(L-H4) and Cu(NO3)2·4H2O in methanol at room temperature
results in Cu(L-H2). [Cu(L-Me2)] was obtained in a way similar
to that for the ester L-H2Me2 of L-H4. Heating a mixture of CdI
and Cu(L-H2) (2:1 molar ratio) in N,N-dimethylformamide
Received: September 8, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b02151
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Inorganic Chemistry
Communication
topology of 1 can be described as a 2,8-c net with the point
methylstyrene was employed, 1 showed a remarkable selectivity
for the allylic amine rather than aziridine production (Table 1,
entries 8 and 9), probably because of the fact that these substrates
readily undergo nitrene insertion into allylic C−H bonds at
secondary C atoms.12c The allylic amines were obtained in 57
and 71% isolated yields, respectively, and no aziridination
product was detected. The starting PhINTs is recovered as
sulfonamide after column chromatography.
After completion of the reaction, simple filtration of the
mixture allowed separation of the solid-state catalyst in almost
quantitative yield (>98%), and the recovered solids could be used
for at least five cycles in the amination of styrene without
significant loss of catalytic activity (isolated yields 81, 81, 80, 78,
and 76%, respectively). PXRD showed that 1 remained highly
crystalline after five cycles. The BET surface area of the recycled
sample was found to be 137 m2/g. Moreover, a hot filtration test
showed no indication of catalysis by leached homogeneous
species, suggesting the heterogeneous nature of our catalyst
system. The leaching percentages of metal ions in each cycle
during recycling of the catalyst were ∼2.1% and 0.9% for Cu and
Cd, respectively, as determined by inductively coupled plasma
optical emission spectrometry analysis.
To ascertain whether the substrates are accessing the internal
active sites via open channels, we synthesized a series of styrene
derivatives of varying sizes. An obvious size-selectivity effect
consistent with the channel dimensions was observed (Table 1,
entries 5−7). The molecular dimensions of 3-methoxystyrene
allow it to diffuse swiftly through the pores, such that its
conversion to the aziridine derivative reaches 74%, while the
conversion yields for 3-benzyloxystyrene decreased to 66%
under similar conditions. In contrast, no aziridine product was
observed for the largest olefin 3-R0O-styrene, presumably
because it cannot access the catalytic sites as a result of its
large diameter. Such shape and size selectivity implied that
catalysis primarily occurs in the channels.
(Schlafli) symbol {44.1224}{4} when the tetrameric Cd unit and
̈
4
4
CuL ligand are treated as 8- and 2-connected nodes, respectively
(Figure S3).
The solvent-accessible void space of 1 was about ∼44%,
calculated using PLATON.10 The phase purity of the bulk sample
was established by a comparison of the observed and simulated
powder X-ray diffraction (PXRD) patterns. Thermogravimetric
analysis revealed that the guest molecules could be readily
removed in the temperature range from 100 to 220 °C and the
framework is stable up to 350 °C (Figure S4). Variable-
temperature PXRD experiments suggested that the network is
stable at least up to 280 °C (Figure S5). The enantiomeric nature
of 1 made from R and S enantiomers of Cu(L-H2) was
demonstrated by the mirror images in solid-state circular
dichroism spectra (Figure S6). The N2 adsorption measurement
provided a Brunauer−Emmett−Teller (BET) surface area of
195.6 m2/g (Figure S7).
Given that copper complexes are promising oxidation catalysts
in many important organic transformations,11 the catalytic
activity of 1 toward olefin aziridination was examined. Aziridines
are important intermediates in organic synthesis for many
nitrogen-containing pharmaceuticals and biologically active
compounds.12 The reaction was carried out with a 4:1 molar
ratio of styrene and [N-(p-tolylsulfonyl)imino]phenyliodinane
(PhINTs) in CH3CN at room temperature. A 2.3 mol %
loading of 1 led to 81% conversion of PhINTs after 12 h. We
found that 1 was an efficient catalyst for the reactions of a variety
of olefins including derivatives with electron-withdrawing or
-donating substituents (Table 1, entries 1−5). Despite good
transformation, we only observe fairly and specific asymmetric
induction. An aziridination product of styrene gave 8.6% ee,
while products of substituted styrenes were essentially racemic
(<2% ee). Notably, when α-methylstyrene or 4-chloro-α-
To identify the catalytically active sites, the isostructural
Ni(salen)-MOF7d to 1 was prepared and employed as the
catalyst. When styrene or α-methylstyrene and PhINTs were
treated with the Ni(salen) analogue, no desired product was
detected, thus indicating that the unsaturated Cu sites within 1
are active centers for the oxidation process indeed. A transient
CuNTs group is hypothesized to form on the open
coordination sites of the Cu(salen) core upon reaction with
PhINTs.13 Further research on the nature of the intermediate
is needed in order to investigate the mechanism of nitrogen-
transfer reactions.
Table 1. Catalyzed Aziridination and Amination of Styrene
and Its Derivatives by PhINTs
a
To compare the catalytic activities of the immobilized Cu
catalyst and homogeneous analogues, olefin aziridination and
amination catalyzed by Cu(L-H2) and its ester Cu(L-Me2) were
carried out. Under identical conditions, 9.2 mol % loading of
Cu(L-H2) or Cu(L-Me2) could also catalyze aziridination of
styrene or 4-bromostyrene and amination of α-methylstyrene or
4-chloro-α-methylstyrene, but the yields of ∼10% were much
lower than those by 1 and, moreover, no enantioselectivity was
observed for the aziridination product (Table 1, entries 1, 4, 8,
and 9). Obvious enhancement in the activity was observed for the
Cu(salen) units in 1 compared with homogeneous counterparts.
Such activity enhancement could presumably be attributed to the
site isolation of Cu(salen) units, thus avoiding the deactivation
caused by dimerization and/or oligomerization.14 Moreover,
because of the geometrical constraints imposed by chiral
channels around the CuII centers, 1 demonstrated specific
enantioselectivity in the aziridination reactions, which, however,
a
b
For reaction details, see the Supporting Information. Isolated yield
c
after column chromatography. The ee values (determined by HPLC)
were 8.6% for entry 1 and <2% for entries 2−5. 2.3 mol % loading of
1. 9.2 mol % loading of Cu(L-H2). 9.2 mol % loading of Cu(L-Me2).
d
e
f
B
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Communication
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performances has been observed in hosts based on mesoporous
and layered inorganic solids15 and has also received attention in
MOFs.3,7
In summary, we have constructed a chiral robust MOF by
using a chiral dicarboxylic acid-functionalized Cu(salen) ligand
and demonstrated that the MOF could catalyze aziridination and
allylic amination of olefins. Besides easy separation and reuse of
the catalyst, the chiral framework confinement could impart
substrate size selectivity, enhance catalyst activity, and induce
product enantioselectivity.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02151.
■
S
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Bryliakov, K. P.; Talsi, E. P.; Fedin, V. P.; Kim, K. A homochiral metal-
organic material with permanent porosity, enantioselective sorption
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Experimental details and spectral data (PDF)
Crystallographic file in CIF format (CIF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: liuy@sjtu.edu.cn.
*E-mail: yongcui@sjtu.edu.cn.
ORCID
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Y. A homochiral metal-organic framework as an effective asymmetric
catalyst for cyanohydrin synthesis. J. Am. Chem. Soc. 2014, 136, 1746−
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Park, S. M.; Seo, G.; Kim, K. Postsynthetic modification switches an
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K. K.; Cohen, S. M. Engineering a metal-organic framework catalyst by
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Yong Cui: 0000-0003-1977-0470
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Science
Foundation of China (Grants 21371119, 21431004, 21401128,
21522104, and 21620102001), the National Key Basic Research
Program of China (Grants 2014CB932102 and
2016YFA0203400), and the Shanghai “Eastern Scholar”
Program.
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