Enhanced enantioselectivity in heterogeneous manganese-catalyzed asymmetric epoxidation with nanosheets modified amino acid Schiff bases as ligands by modulating the orientation and the arrangement order

Article abstract of DOI:10.1016/j.jcat.2021.08.015

Catalytic enantioselective epoxidation of olefins plays an important role in the production of optically-active epoxy. Transition-metal complexes prove efficient for the catalytic epoxidation of un-functionalized olefins by employing privileged chiral lig

Full text of DOI:10.1016/j.jcat.2021.08.015

Journal of Catalysis 402 (2021) 22–36
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Enhanced enantioselectivity in heterogeneous manganese-catalyzed
asymmetric epoxidation with nanosheets modified amino acid Schiff
bases as ligands by modulating the orientation and the arrangement
order
Zhe An, Yuanzhong Tang, Yitao Jiang, Hongbo Han, Qi Ping, Wenlong Wang, Yanru Zhu, Hongyan Song,
⇑
Xin Shu, Xu Xiang, Jing He
State Key Laboratory of Chemical Resource Engineering & Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology,
Beijing 100029, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Catalytic enantioselective epoxidation of olefins plays an important role in the production of optically-
active epoxy. Transition-metal complexes prove efficient for the catalytic epoxidation of un-
functionalized olefins by employing privileged chiral ligands with rotational symmetry and bulky sub-
stituents, which usually require complicated multistep synthesis and are often very expensive. Here, this
work proposes an efficient strategy to promote the enantioselectivity of Mn(III)-catalyzed indene epox-
Received 28 May 2021
Revised 3 August 2021
Accepted 4 August 2021
Available online 11 August 2021
idation, which simply utilizes the nanosheets of layered double hydroxides (LDHs) to modify
a-amino
Keywords:
Asymmetric epoxidation
Spatial effects
Layered double hydroxides
Intercalation
Mn (III) catalysis
acid derivatives as chiral ligands, achieving an enantioselectivity of more than 91.0%, much higher than
that (<35%) in the previous reports using chiral ligands without any rotational-symmetry. By virtue of
‘‘flexible” interlayer spaces of LDHs, intercalated L-tert-leucine, L-valine, L-leucine, and L-phenylalanine
anions have further been derived in situ to L-tert-leucine, L-valine, L-leucine, and L-phenylalanine
Schiff base anions. Over the Mn(III) coordinated with L-phenylalanine Schiff base anion intercalated
LDHs, an indene conversion of 81.0
1.0%, a chem-selectivity of 89.0
1.0%, and an enantiomeric
excesses (ee) value of 94.5 0.5% for (1S,2R)-1,2-epoxyindane have been afforded. The spatial effects
of LDH nanosheets on the enantioselectivity have been investigated, focusing on the interlayer orienta-
tion and arrangement order of intercalated amino acid derivative anions. The strategy has been proved
valid for the epoxidation of functionalized olefins, which is more challenging, including allylic alcohols,
chalcone, and chromene derivatives.
Ó 2021 Elsevier Inc. All rights reserved.
1. Introduction
pany [8]. Lately, asymmetric catalytic epoxidation has been
developed as a mainstream method due to the shortened catalytic
Catalytic enantioselective epoxidation of olefins plays an
important role in organic synthesis [1,2], because optically-active
epoxy is not only a common block in natural products [3,4] but also
a valuable organic intermediate to produce bioactive or pharma-
ceutical molecules [5,6]. (1S,2R)-1,2-epoxyindane can be used to
synthesize (1S,2R)-1-amino-2-indanol, a key intermediate for syn-
thesis of Indinavir, a well-known medicine for AIDS [7]. For the
preparation of enantiopure (1S,2R)-1,2-epoxyindane in industry,
the bromoperoxidase and dehydrogenase are employed as cata-
lysts to convert indene to (1S,2R)-1,2-epoxyindane by Merck Com-
term and low overall cost [9–25]. Groves et al. [10] reported the
first asymmetric indene epoxidation in 1990 with a chiral vaulted
binaphthyl Fe-porphyrin, providing an enantiomeric excess (ee)
value of 20% and a (1S,2R)-1,2-epoxyindane yield of 73%. Where-
after, many efforts have been devoted to the development of metal
complexes composing of C2-symmetric chiral ligands originating
from cyclohexanediamine, including Mn-salen derivatives [11–
18], Ti-salen derivatives [19–22], Mo-bis-hydroxamic acids (BHA)
[23], and di-m-oxo Ti-salen derivatives [24,25], with ee value
improved to 55 ~ –99%. But those effective metal complexes with
bulky and complicated chiral ligands usually require complicated
multistep synthesis and are often very expensive. Elaborately
⇑
Corresponding author at: Box 98, 15 Beisanhuan Dong Lu, Beijing 100029, PR
China.
E-mail address: hejing@mail.buct.edu.cn (J. He).
https://doi.org/10.1016/j.jcat.2021.08.015
0021-9517/Ó 2021 Elsevier Inc. All rights reserved.

Z. An, Y. Tang, Y. Jiang et al.
Journal of Catalysis 402 (2021) 22–36
designed strategies, which are facile but could effectively afford
remarkable enantioselectivity, are highly desired.
epoxyindane have been obtained with a significant improvement
compared with the homogeneous counterpart (20%) in the asym-
metric indene epoxidation. The strategy proposed in this work also
proves valid for the epoxidation of functionalized olefins, including
allylic alcohols, chalcone, and chromene derivatives.
Heterogeneous catalytic chiral epoxidation, in which the cata-
lysts can be separated simply by filtration and recycled easily,
has been bestowed more potential applications in industry [9].
Early reported heterogeneous asymmetric epoxidation catalysts
have been prepared by grafting chiral Mn(III)-Salen complex onto
aminopropyl-[26,27] or mercaptopropylsilyl-functionalized meso-
porous materials [28] through covalent interaction, producing
decreased enantioselectivity in the indene epoxidation compared
with the neat Mn(III)-salen complex. Later, heterogeneous chiral
metal–organic frameworks have been in situ synthesized based
on Mn-salen-derived bridging ligands, achieving an enantioselec-
tivity in the indene epoxidation comparable with the homoge-
neous analogue [29,30]. Recently, enhanced enantioselectivity
has been reported with chiral Mn(III)-salen complex on phenoxy-
modified zirconium poly(styrene-phenylvinylphosphonate)phos
phate (ZPS-PVPA) [31] or crystalline aluminium oligo-styrenyl
phosphonatehydrogen phosphate (AlSPP) [32] through Mn(III)-O
axial coordination, and on zinc poly(styrenephenylvinylphospho
nate)-phosphate (ZnPS-PVPA) [33] or aluminium 3-bromopropyl
phosphonate-phosphate (AlPS-BrPPAS) [34] through Mn(III)-N
axial coordination. The spatial effects on the enantioselectivity
have been originated from the microenvironment improvement
[32]. More recently, Farokhi et al. [35] have reported a chiral Mn
(III)-porphyrin complex covalently bonded onto mesoporous
SBA-15, achieving an ee value of >99% for (1S,2R)-1,2-
epoxyindane at an indene conversion of >99%, benefiting from
the confinement effects of mesopores. In these heterogeneous cat-
alytic systems, bulky and complicated chiral ligands have still been
involved, with more elaborate grafting steps on the well-designed
functionalized mesoporous materials. Our previous work [36–41]
proposed a simple but vigorous strategy to enhance enantioselec-
tivity by using the nanosheets of layered double hydroxides (LDHs)
2. Experimental
2.1. Materials
L-phenylalanine, L-leucine, L-tert-leucine, L-valine, n-hexane,
and ethyl acetate were purchased from Sigma-Aldrich. Al(NO3)3-
Á9H2O, Zn(NO3)3Á6H2O, NaOH, KOH, CuI, N,N-dimethylformamide
(DMF), Na2CO3, ethanol, n-butanol, and dichloromethane
were purchased from Sinopharm Reagent. Salicyladehyde, 3-
chloro-3-methylbutene, N,N-dimethylaniline, 4-nitrophenol, 4-
methoxyphenol, Mn(III) acetylacetonate, 4-nitrocinnamyl alcohol,
indene, 1,1,1,2-tetrachloroethane, chalcone, and iodobenzene were
purchased from Alfa-Aesar. Anhydrous ethanol was purchased
from Aladdin. All the reagents and commercial chemicals were of
analytical purity and used as received without further purification.
2.2. Preparation and synthesis
2.2.1. Amino acids intercalated LDHs
L-t-leucine, L-valine, L-leucine, and L-phenylalanine anions
intercalated Zn/Al-LDHs (Zn/Al-AA-LDHs, AA = t-Leu, Val, Leu,
and Phe) were prepared by the reported co-precipitation method
[53,54]. Typically, an aqueous solution (1 M) of Zn(NO3)2Á6H2O
and Al(NO3)3Á9H2O was added dropwise to an amino acid solution
(50 mM) of 200 mL with a molar ratio of Zn2+/Al3+/amino acid
anions molar ratio equal to 2/1/1 under stirring. The pH value of
the solution was maintained at 9 by dropwise adding NaOH solu-
tion (1 M) under stirring. The suspension was stirred for 6 h at
313 K in N2 atmosphere. The resulting suspension was filtrated,
washed thoroughly with de-carbonated deionized water and anhy-
drous alcohol, and dried in a vacuum oven at 293 K, producing Zn/
Al-AA-LDHs.
as the rigid and giant substituent of natural a-amino acids, as the
ligands of metal sites in vanadium-catalysed asymmetric epoxida-
tion of allylic alcohols or zinc-catalysed asymmetric aldol reaction,
or directly the catalyst for asymmetric aldol reaction. Remarkable
enantioselectivity improvement has been established, originating
from both steric and H-bonding effects as revealed by theoretical
calculations [38–40]. In our recent work [41], enhancement of
enantioselectivity was further achieved in the asymmetric epoxi-
2.2.2. Amino acid Schiff base intercalated LDHs
The amino acid Schiff base intercalated LDHs (Zn/Al-AA-sal-
LDHs, AA = t-Leu, Val, Leu, and Phe) were prepared by in situ
derivatization of Zn/Al-AA-LDHs according to the reported homo-
geneous derivatization procedure [45,55]. Typically, 0.5 g of Zn/
Al-AA-LDHs were added into 15 mL of anhydrous ethanol under
stirring. Then, a methanolic solution (15 mL) of salicylaldehyde
(2.0 mmol) was added dropwise into the above suspension, fol-
lowed by refluxing for 5 h under stirring. The resulting suspension
was centrifugated, washed thoroughly with anhydrous alcohol
until the supernatant became colorless, and then dried in a vacuum
oven at 293 K, producing Zn/Al-AA-sal-LDHs. Zn/Al-AA-sal-LDHs as
ligands were then subjected to in situ coordinate with manganese
centers in the subsequent catalytic reaction.
dation of
a,b-unsaturated aldehydes by the ordered arrangement
of interlayer amines.
Here, an efficient strategy to promote the enantioselectivity of
manganese-catalyzed indene epoxidation, which utilizes a-amino
acid derivatives modified with nanosheets of LDHs as chiral
ligands. Amino acid derivatives are well known as low-cost and
readily-available ligands for metal-catalyzed reaction [42,43]. But
in the asymmetric epoxidation of un-functionalized olefins, few
reports [44–47] have employed
a-amino acid or a-amino acid
derivatives as ligands for metal active sites, while only racemic
product [44,45] or chiral epoxides with an enantioselectivity of
lower than 35% [46,47]. LDHs, a natural and/or synthetic anionic
compound with positively-charged brucite-like layers, exchange-
able interlayer anions, and ‘‘flexible” interlayer spaces [48–52],
provides opportunity to enhance the asymmetric induction of sim-
2.2.3. Dispersion of Zn/Al-Phe-sal-LDHs
Zn/Al-Phe-sal-LDHs were dispersed through a simple liquid-
exfoliation process [56]. Typically, 50 mg of Zn/Al-Phe-sal-LDHs
were dispersed into 100 mL of n-butanol under N2 protection.
The suspension was stirred for 24 h to obtain a colloidal solution,
denoted as dispersed-Zn/Al-Phe-sal-LDHs.
ple
a-amino acids through guest–host electrostatic interaction
[36–41]. In the present work, the enantioselectivity of
manganese-catalysed epoxidation has been improved by modulat-
ing the interlayer orientation and arrangement order of LDHs
nanosheets modified amino acid Schiff base as ligands. An enan-
tioselectivity of >91.0% in the epoxidation, much higher than
observed with each homogeneous counterpart. Especially, with L-
phenylalanine Schiff base anions intercalated LDHs as ligand for
2.2.4. Amino acid Schiff base potassium
For control, pristine amino acid Schiff base potassium (AA-sal,
AA = t-Leu, Val, Leu, and Phe) -was prepared by the reported
method [45,57]. Typically, a methanolic solution (15 mL) of
manganese centres, an ee value of 94.5
0.5% for (1S,2R)-1,2-
23

Z. An, Y. Tang, Y. Jiang et al.
Journal of Catalysis 402 (2021) 22–36
salicylaldehyde (5.5 mmol) was added into a mixture methanolic
solution (15 mL) of amino acid (5.5 mmol) and KOH (6.6 mmol)
under stirring. The obtained bright yellow solution was refluxed
for 2 h under ambient condition and the mixture was then dried
by rotary evaporator. The resulting solid was then dissolved with
a mixture of n-hexane (10 mL) and ether (10 mL) followed by fil-
tration to remove the insoluble amino acid. The solution was fur-
ther dried by rotary evaporation, producing bright yellow
powder, denoted as AA-sal. Each AA-sal was identified with 1H
and 13C NMR technique.
and 30 mA) at a scanning rate of 10°/min with a step size of
0.04°. The lattice parameters a and b of LDHs was determined from
the XRD patterns. Elemental analysis of Zn, Al, and Mn was per-
formed on an inductively coupled plasma (ICP) atomic emission
spectrophotometry (Shimadzu ICPs-7500) by dissolving the sam-
ples in dilute HNO3. Elemental analysis of C, H, and N was per-
formed on an Elementar Co. Vario elemental analyzer. Based on
the ICP and elemental analysis of samples, the chemical composi-
tions were determined, as displayed in Table S1. Fourier transform
infrared (FT-IR) spectra were recorded on a Bruker Vector 22 FT-IR
spectrometer with a resolution of 1 cmÀ1 using the standard KBr
pellet method. 1H and 13C NMR spectra were recorded in DMSO-
D6 or CDCl3 on Bruker Avance 400 MHz NMR spectrometer. The
solid state 13C CP/MAS NMR spectra were recorded on a Bruker
Avance-400 M solid-state spectrometer equipped with a commer-
cial 4 mm MAS NMR probe at frequencies of 100.56 MHz. X-ray
photoelectron spectroscopic (XPS) characterization was performed
on Shimadzu KRATOS AXIS SUPRA at a pressure of 2 Â 10À9 Torr.
C1s peak at 284.8 eV was used as calibration. Transmission elec-
tron microscope (TEM) images were taken on Tecnai G2 F30 STWIN
electron microscope operated at 300 kV. Mapping scanning of Mn
element was carried out using energy-dispersive X-ray mode.
Atomic Force Microscope (AFM) images were taken on a Digital
Instruments NanoScope IV atomic force microscopy in tapping-
mode using a Si tip cantilever with a force constant of 20 N/cm.
The colloidal solution was deposited on Si wafer and then
lyophilized.
t-Leu-sal: 1H NMR (400 MHz; DMSO-D6): d = 0.96 (9H, s), 3.34
(1H, s), 6.57–6.67 (2H, m), 7.21–7.26 (2H, m), 8.27 (1H, s) ppm; 13
C
NMR (100 MHz; DMSO-D6): d = 27.7, 34.0, 81.8, 115.3, 117.7,
119.5, 132.5, 133.3, 162.7, 168.2, 170.4 ppm.
Val-sal: 1H NMR (400 MHz; DMSO-D6): d = 0.84 (6H, dd, J = 9.7,
13.5 Hz), 2.22–2.27 (1H, m), 3.40–3.42 (1H, d, J = 6.0 Hz), 6.61–6.70
(2H, m), 7.20–7.30 (2H, m), 8.28 (1H, s) ppm; 13C NMR (100 MHz;
DMSO-D6): d = 18.6, 20.7, 31.3, 78.2, 115.8, 118.1, 119.1, 132.3,
133.1, 163.7, 167.2, 171.2 ppm.
Leu-sal: 1H NMR (400 MHz; DMSO-D6): d = 0.85–0.88 (6H, dd,
J = 9.7, 13.5 Hz), 1.47–1.70 (3H, m), 3.70 (1H, dd, J = 4.1, 13.5 Hz)
6.64–6.71 (2H, m), 7.20–7.32 (2H, m), 8.36 (1H, s) ppm; 13C NMR
(100 MHz; DMSO-D6): d = 22.2, 23.9, 25.2, 43.6, 70.8, 116.2,
118.3, 118.8, 132.2, 132.9, 163.3, 166.4, 171.6 ppm.
Phe-sal: 1H NMR (400 MHz; DMSO-D6): d = 2.90–3.30 (1H, m),
3.28 (1H, dd, J = 4.1, 13.5 Hz), 3.85 (1H, dd, J = 4.1, 9.7 Hz), 6.65–
6.73 (2H, m), 7.08–7.20 (7H, m), 8.10 (1H, s) ppm; 13C NMR
(100 MHz; DMSO-D6): d = 19.0, 40.7, 56.5, 74.6, 117.3, 117.8,
118.7, 126.2, 128.5, 129.7, 132.0, 132.6, 140.3, 163.9, 164.3,
171.8 ppm.
2.4. Structural model and molecular dynamics (MD) simulation
method
2.2.5. Chromene derivatives as reactants
Chromene derivatives as reactants were synthesized following
the improved literature method [58]. Typically, 3-chloro-3-
methylbutene (10 mmol), 4-methoxyphenol (5 mmol), K2CO3
(10 mmol), and CuI (0.1 mmol) were added into 10 mL of DMF
under N2 protection. The suspension was stirred for 10 h at
338 K and then dried by rotary evaporator. The obtained solid
was dissolved in n-hexane (10 mL), followed by filtration to
remove the insoluble substrates. The solution was further dried
by rotary evaporation. The crude product was purified by column
chromatography (ethyl acetate/n-hexane (v/v) = 1:8), obtaining a
light brown oil. Then, 5 mmol of such purified product was added
into a DMF solution (10 mL) of N,N-dimethylaniline (5 mmol)
under stirring. The mixture was stirred for 10 h at 418 K and then
dried by rotary evaporator. The crude product was purified by
column chromatography (ethyl acetate/n-hexane (v/v) = 1:10)
Structural models were estimated by Materials Studio Program
including the van der Waals radii. All the simulations were per-
formed using the Forcite module in the Material Studio software
package following our previous work [41]. The molecular dynam-
ics (MD) method was used to simulate the arrangement of amino
acid Schiff base in the interlayer space of Zn/Al-LDHs. The lattice
containing 18 Zn atoms and 9 Al atoms was built in the light of
each [AlO6] octahedron surrounded by 6 [ZnO6], because the ratio
of Zn to Al is 2:1, which ensures that Al atoms will not occupy
adjacent octahedron. According to XRD pattern, the lattice param-
eters of the LDH layer are a = 3.149 and b = 3.154 Å. On the basis
of the model of the host layer, a supercell was thus constructed
with lattice parameter a = 28.285 Å, b = 9.437 Å, which was trea-
ted as P1 symmetry and all of lattice parameters were considered
as independent variables during the simulation. Then, to maintain
the whole system electrically neutral and match the chemical
compositions derived by ICP and elemental analysis (Table S1),
4 L-tertiary leucine Schiff base anions, 5 nitrate anions, and 14
water molecules were introduced into the simulated supercell
randomly for Zn1.94/Al-t-Leu-sal0.40-LDHs, 4 L-valine Schiff base
anions, 5 nitrate anions, and 16 water molecules for Zn1.94/Al-
Val-sal0.39-LDHs, 6 L-leucine Schiff base anions, 3 nitrate anions,
and 26 water molecules for Zn1.78/Al-Leu-sal0.62-LDHs, and 6 L-
phenylalanine Schiff base anions, 2 nitrate anions, and 24 water
molecules for Zn1.78/Al-Phe-sal0.73-LDHs. All MD simulations were
performed by the LDHFF force field developed by Yan et al.
[59,60]. When energy minimization was applied on the initial
models, MD simulations were performed in an isothermal-
isobaric (NPT) ensemble with the temperature of 298 K and the
pressure of 0.1 MPa. Temperature and pressure control were per-
formed using the Andersen and Berendsen method, respectively.
The simulation time-step was set to be 1 fs and the total simula-
tion time was 200 ps.
to obtain
a
yellow solid, producing 6-methoxy-2,2-
dimethylchromene. 6-nitro-2,2-dimethylchromene was synthe-
sized using 4-nitrophenol as substrate following the similar
method. Each reactant was identified with 1H and 13C NMR
technique.
6-nitro-2,2-dimethylchromene: 1H NMR (400 MHz; CDCl3):
d = 1.31 (6H, s), 5.54 (1H, d, J = 4.0 Hz), 6.10 (1H, d, J = 8.0 Hz),
6.59–6.62 (m, 3H) ppm; 13C NMR (100 MHz; CDCl3): d = 36.9,
55.3, 124.2, 127.3, 127.5, 127.7, 128.2, 132.9, 136.4, 147.8, 148.1,
154.1 ppm.
6-methoxy-2,2-dimethylchromene: 1H NMR (400 MHz;
CDCl3): d = 1.31 (6H, s), 5.53 (1H, d, J = 4.0 Hz), 6.10 (1H, d,
J = 8.0 Hz), 6.59–6.64 (3H, m) ppm; 13C NMR (100 MHz; CDCl3):
d = 35.9, 61.6, 126.0, 128.4, 128.5, 141.2, 152.9, 157.1, 175.2 ppm.
2.3. Characterizations
The powder X-ray diffraction (XRD) patterns were taken on a
Shimadzu XRD-6000 diffractometer with Cu K
a radiation (40 kV
24

Z. An, Y. Tang, Y. Jiang et al.
Journal of Catalysis 402 (2021) 22–36
2.5. Catalytic evaluation and catalyst recycling
3. Results and discussion
Zn/Al-AA-sal-LDHs and AA-sal (AA = t-Leu, Val, Leu, and Phe),
in situ coordinated to Mn(acac)3, were respectively used to cat-
alyze the asymmetric epoxidation of indene. Typically, Zn/Al-AA-
sal-LDHs (containing 0.01 mmol of AA-sal) and acetylacetone man-
ganese (0.01 mmol) was dispersed in 2 mL of dichloromethane and
stirred at 20 °C for 3 h, producing Mn(III)-Zn/Al-Phe-sal-LDHs.
Then iodosylbenzene (0.2 mmol) and indene (0.1 mmol) were
added into the suspension. The mixture was stirred at 20 °C for
24 h. The reaction progress was monitored by TLC (hexane/ethyl
acetate (v/v) = 8/1). The reaction mixture was quenched by satu-
rated sodium sulfite solution. The solid catalyst was recycled by fil-
tration, washed with 5 mL of CH2Cl2 for three times, dried in a
vacuum oven at 313 K, and used directly for the next catalytic reac-
tion. The left reaction mixture was extracted with CH2Cl2 for three
times. The combined organic phase was dried with anhydrous
sodium sulfate and concentrated using a rotary evaporator, giving
the crude product. 1,1,2,2-tetrachloroethane (0.1 mmol) was added
to the crude product as internal standard for product quantifica-
tion. The conversion, product selectivity and yield were deter-
mined by 1H NMR. The crude product was further subjected to
flash chromatography (silica gel: ethyl acetate/n-hexane (v/
v) = 1/8) to afford the desired product. The yield for the isolated
product was determined by weighing. The enantioselectivity was
determined by chiral OB-H column (n-hexane/isopropanol (v/
v) = 97/3). In order to characterize the Mn(III)-Zn/Al-Phe-sal-
LDHs, the solid catalyst was obtained by simple filtration after
in situ coordination.
3.1. Structure charcterizations
L-t-leucine, L-valine, L-leucine, and L-phenylalanine anions
intercalated Zn/Al-LDHs (Zn/Al-AA-LDHs, AA = t-Leu, Val, Leu,
and Phe) were first prepared for in situ synthesis of the corre-
sponding amino acid Schiff base anions intercalated Zn/Al-LDHs
(Zn/Al-AA-sal-LDHs) as ligands. Fig. 1 (left column) shows the
XRD patterns of Zn/Al-AA-LDHs and Zn/Al-AA-sal-LDHs, which all
display the regular (00l) reflections characteristic [48] of layered
structure for LDHs. The basal spacing (d003)could be calculated
from the (003) reflections in XRD patterns, respectively. For Zn/
Al-t-Leu-LDHs and Zn/Al-t-Leu-sal-LDHs, the basal spacing is cal-
culated as being 0.86 and 0.87 nm, respectively (Fig. 1A). The in-
situ synthesis of Zn/Al-t-Leu-sal-LDHs from Zn/Al-t-Leu-LDHs has
no adverse on the intercalated structure and only results in a slight
increase in the basal spacing. Slight increase in the basal spacing
from 0.85 to 0.87 nm is also observed for Zn/Al-Val-sal-LDHs
(Fig. 1B), and from 1.71 to 1.73 nm for Zn/Al-Leu-sal-LDHs
(Fig. 1C), respectively. While for Zn/Al-Phe-sal-LDHs, a visible
increase (0.26 nm) on the basal spacing from 1.73 to 1.99 nm
(Fig. 1D) is observed. Subtracting the brucite-like layer thickness
(0.48 nm) [61] from the basal spacing [48,51], the gallery height
is estimated to be 0.39, 0.39, 1.23, and 1.51 nm for Zn/Al-t-Leu-
sal-LDHs, Zn/Al-Val-sal-LDHs, Zn/Al-Leu-sal-LDHs, and Zn/Al-Phe-
sal-LDHs. The structural models of homogeneous amino acid Schiff
base anions were simulated by Materials Studio Program including
the van der Waals radii following our previous work [36–41]. In
light of the dimensional sizes (Fig. 1, middle column), a monolayer
horizontal arrangement of interlayer amino acid Schiff base anions
for Zn/Al-t-Leu-sal-LDHs and Zn/Al-Val-sal-LDHs, and an interdig-
itated bilayer vertical arrangement for Zn/Al-Leu-sal-LDHs and Zn/
Al-Phe-sal-LDHs have been concluded, as illustrated in Fig. 1 (right
column). The orientation angle, which is defined as the angle
between orientated plane of the amino acid Schiff base and the
LDH layer, is thus computed to be 3°, 10°, 60°, and 80° for Zn/Al-
t-Leu-sal-LDHs, Zn/Al-Val-sal-LDHs, Zn/Al-Leu-sal-LDHs, and Zn/
Al-Phe-sal-LDHs, respectively. The larger orientation angle of Zn/
In the large-scale asymmetric epoxidation, Zn/Al-Phe-sal-LDHs
(containing 0.01 mmol of Phe-sal) and acetylacetone manganese
(1 mmol) was dispersed in 200 mL of dichloromethane. After the
suspension was stirred at 20 °C for 3 h, iodosylbenzene (20 mmol)
and indene (10 mmol) were added. The mixture was stirred at
20 °C for 24 h, followed by the similar procedure as mentioned
above.
The substrates were further expanded to allyl alcohol, chalcone,
and chromene derivatives with the same experimental operation
as mentioned above. Each product was identified with 1H and 13C
NMR technique.
Al-Phe-sal-LDHs is deduced to originate from the p-p conjugation
(1S,2R)-1,2-epoxyindane: 1H NMR (400 MHz; DMSO-D6):
d = 2.99–3.04 (1H, m), 3.22–3.27 (1H, m), 4.14 (1H, dd, J = 4.1,
9.7 Hz), 4.30 (1H, s), 7.11–7.23 (4H, m) ppm; 13C NMR
(100 MHz; DMSO-D6): d = 18.6, 20.7, 31.3, 78.2, 115.8, 118.1,
119.1, 132.3, 133.1 ppm.
between benzene rings on the side chain of the phenylalanine and
the attached salicylidene, which also well accounts for the struc-
tural re-arrangement after in situ synthesis of Zn/Al-Phe-sal-
LDHs from Zn/Al-Phe-LDHs (Fig. 1D). According to the ICP results
(Table S1), ZnII/AlIII molar ratio of Zn/Al-AA-sal-LDHs was deter-
mined in a narrow range from 1.78 to 1.94, remaining similar to
Zn/Al-AA-LDHs. Based on the ICP and CHN elemental analysis,
the proposed formula and abbreviations were given in Table S1,
showing NOÀ3 as co-intercalated anions to compensate the charge
of the brucite-like layer. The N content in Zn/Al-AA-LDHs is consid-
ered as a 100% N content. According to the proposed formula,
about 91–97% of the intercalated amino acid anions in Zn/Al-AA-
LDHs was derivatized to the amino acid Schiff base anions in Zn/
Al-AA-sal-LDHs.
(2S,3S)-2,3-epoxy-3-(4-nitrophenyl)-1-propanol: 1H NMR
(400 MHz; CDCl3): d = 3.20 (1H, dt), 3.86 (1H, dd, J = 12.8,
3.5 Hz), 4.06–4.10 (2H, m), 7.45–7.47 (2H, m, J = 8.8 Hz), 8.21–
8.23 (2H, d, J = 8.8 Hz,) ppm; 13C NMR (100 MHz; CDCl3):
d = 54.3, 60.6, 63.0, 123.8, 126.4, 127.6, 128.1, 144.4, 147.7 ppm.
(3R,4R)-3,4-epoxy-2,2-dimethyl-6-nitrochromane: 1H NMR
(400 MHz; DMSO-D6): d = 2.04 (6H, s), 3.51 (1H, d, J = 12.0 Hz),
4.68 (1H, d, J = 4.0 Hz), 7.00–7.10 (3H, m) ppm; 13C NMR
(100 MHz; DMSO-D6): d = 18.6, 20.7, 31.3, 78.2, 115.8, 118.1,
119.1, 132.3, 133.1, 163.7, 167.2 ppm.
To confirm the formation of Schiff base, the 13C NMR spectra of
pristine amino acid potassium salts (AA-sal) and the 13C CP/MAS
NMR spectra of the corresponding Zn/Al-AA-sal-LDHs are dis-
played in Fig. 2. For pristine t-Leu-sal (Fig. 2A, a), the resonances
at 27.73, 34.01, 81.82, 115.32, 117.70, 119.55, 132.48, 133.27,
162.67, 168.18, and 170.31 ppm, are indexed to the chemical shift
for C-4/-5/-6, C-3, C-2, C-12, C-10, C-8, C-9, C-11, C-7, C-13, and C-1
atoms in t-Leu-sal, respectively, according to previous literatures
[62,63]. For Zn/Al-t-Leu-sal-LDHs (Fig. 2A, b), the characteristic
resonances that mentioned above are also observed, indicating
the formation of L-t-Leucine Schiff base anions through the
(3R,4R)-3,4-epoxy-2,2-dimethyl-6-methoxychromane:
1H
NMR (400 MHz; DMSO-D6): d = 2.04 (6H, s), 3.53 (1H, d,
J = 12.0 Hz), 4.09–4.15 (3H, m), 4.68 (1H, d, J = 8.0 Hz), 7.00–7.10
(3H, m) ppm; 13C NMR (100 MHz; DMSO-D6): d = 18.6, 20.7,
31.3, 78.2, 115.8, 118.1, 119.1, 132.3, 133.1, 158.2, 163.7, 167.2,
171.2 ppm.
(2S,3R)-2,3-epoxy-1,3-diphenylcyclopropan-1-one: 1H NMR
(400 MHz; CDCl3): d = 3.31 (2H, m), 6.85–7.05 (10H, m) ppm; 13C
NMR (100 MHz; CDCl3): d = 53.0, 114.3, 114.5, 114.9, 115.1,
124.5, 131.0, 1311, 139.0, 139.1, 164.00, 164.6, 166.5, 170.5 ppm.
25

Z. An, Y. Tang, Y. Jiang et al.
Journal of Catalysis 402 (2021) 22–36
Fig. 1. XRD patterns of (A) L-t-leucine, (B) L-valine, (C) L-leucine, and (D) L-phenylalanine anions intercalated Zn/Al-LDHs (a) and Zn/Al-AA-sal-LDHs after in-situ
derivatization (b), and the schematic illustrations of the corresponding pristine AA-sal anions and Zn/Al-AA-sal-LDHs. The dimensional sizes and the orientation angle were
computed by Materials Studio Program including the van der Waals radii. ( Zn
Al
N
C
O
H).
in situ synthesis within the interlayer space. In addition, the reso-
nance corresponding to C-1 atom in carboxyl anions is resolved to
appear at 179.02 ppm for Zn/Al-t-Leu-sal-LDHs, much higher than
that (170.3 ppm) for C-1 atom in t-Leu-sal, which could originate
from the strong electrostatic interaction between the carboxyl
anions and the LDH layers. Similarly, the formation of L-Valine,
L-Leucine, or L-Phenylalanine Schiff base anions in Zn/Al-Val-sal-
LDHs, Zn/Al-Leu-sal-LDHs, or Zn/Al-Leu-sal-LDHs has been identi-
fied in Fig. 2B, 2C, and 2D, respectively.
LDHs (c) have been first recorded and shown in Fig. 3. For t-Leu
(Fig. 3A, a), the asymmetric ( COOas) and symmetric vibrations
COOs) of carboxylate groups are resolved [64–66] at 1562 and
1332 cmÀ1 with a tCOO of 230 cmÀ1. The band assigned [66] to
the NH2 scissoring deformation (
NH2) appears at 1623 cmÀ1. The
t
(t
D
q
intercalation of L-t-leucine anions into the interlayer of LDHs
results in an obvious blue shift of both tCOOas and tCOOs bands to
1603 and 1353 cmÀ1 with an increased
DtCOO from 230 to
250 cmÀ1 (Fig. 3A, b), identifying the electrostatic interactions
between the LDH layer and L-t-Leucine anions in monodentate
mode, according to the report in literatures [64,61]. The NH2 scis-
FT-IR spectra have been employed to identify the intercalated
structure and the coordinated structure. The FT-IR spectra of pris-
tine AA potassium salt (a), Zn/Al-AA-LDHs (b), and Zn/Al-AA-sal-
soring deformation
(qNH2) has hardly changed, appearing at
26

Z. An, Y. Tang, Y. Jiang et al.
Journal of Catalysis 402 (2021) 22–36
Fig. 2. 13C CP/MAS NMR spectra of pristine (A) t-Leu-sal, (B) Val-sal, (C) Leu-sal, and (D) Phe-sal (a) and the corresponding intercalated Zn/Al-AA-sal-LDHs (b).
1626 cmÀ1. The absorption band at 1384 cmÀ1 assigned to the
vibration of NOÀ3 groups are also observed, indicating a coexistence
of NOÀ3 and amino acid anions, in accordance with the calculated
chemical compositions (Table S1). After in situ derivatization of
Zn/Al-t-Leu-LDHs with salicylaldehyde, the qNH2 band vanishes
and new absorption bands assigned [45,67,68] to C@N stretching
and Mn(III)-AA-sal (f) as comparisons have been also shown in
Fig. 3. Compared with Zn/Al-t-Leu-sal-LDHs (Fig. 3A, c), the tC=N
and tAr-O bands for Mn(III)-Zn/Al-t-Leu-sal-LDHs (Fig. 3A, d) after
Mn coordination red shift to 1626 and 1259 cmÀ1, suggesting the
coordination of C@N and ArAO with Mn(III) centers, consistent
with previous report [45]. The coordination also leads to a slight
shift of tCOOas and tCOOs bands to 1599 and 1352 cmÀ1 for Mn
vibration (tC=N) and ArAO stretching vibration (tAr-O) appear at
1636 and 1266 cmÀ1, demonstrating the formation of the Schiff
base (Fig. 3A, c). The tCOOas and tCOOs bands remain unchanged
at 1603 and 1354 cmÀ1, indicating that in situ synthesis of Schiff
base has no destruction on the electrostatic interaction between
(III)-Zn/Al-t-Leu-sal-LDHs, but still close to the DtCOO for Zn/Al-t-
Leu-sal-LDHs (Fig. 3A, c), indicating that the COOÀ groups might
participate in the coordination with Mn(III) centers and no visible
influence on the electrostatic interaction between the COOÀ groups
and the LDHs layers occurs. For control, after coordination, the tC=N
the brucite-like layer and L-t-leucine anions. Similarly, the
DtCOO
values of L-Val (Fig. 3B, a), L-Leu (Fig. 3C, a), and L-Phe (Fig. 3D,
a) are calculated to be 238, 222, and 221 cmÀ1, respectively. The
tCOOas and tCOOs bands of carboxylate groups are resolved at
and t
Ar-O bands at 1644 and 1271 cmÀ1 for t-Leu-sal (Fig. 3A, e) red
shifts to 1637 and 1260 cmÀ1 for Mn(III)-t-Leu-sal (Fig. 3A, f), sup-
porting the coordination structure for Mn(III)-Zn/Al-t-Leu-sal-
LDHs proposed above. Similarly, after coordination with Mn(acac)3,
red shifts are resolved for the tC=N and tAr-O bands to 1626 and
1242 cmÀ1 for Mn(III)-Zn/Al-Val-sal-LDHs (Fig. 3B, d), 1630 and
1257 cmÀ1 for Mn(III)-Zn/Al-Leu-sal-LDHs (Fig. 3C, d), and 1630
and 1260 cmÀ1 for Mn(III)-Zn/Al-Phe-sal-LDHs (Fig. 3D, d), respec-
tively, reasonably identifying the coordination of C@N and ArAO
with Mn(III) centers. Red shifts of tCOOas and tCOOs bands are
observed for Mn(III)-Zn/Al-Val-sal-LDHs (Fig. 3B, d), Mn(III)-Zn/
Al-Leu-sal-LDHs (Fig. 3C, d), and Mn(III)-Zn/Al-Phe-sal-LDHs
(Fig. 3D, d) after Mn(III) coordination, respectively, confirming
the coordination of COOÀ groups to Mn(III) centers. For control,
by comparing the FT-IR spectra of pristine Val-sal (Fig. 3B, e) to
Mn(III)-Val-sal (Fig. 3B, f), Leu-sal (Fig. 3C, e) to Mn(III)-Leu-sal
(Fig. 3C, f), and Phe-sal (Fig. 3D, e) to Mn(III)-Phe-sal (Fig. 3D, f),
respectively, the co-coordination of C@N, ArAO, and COOÀ groups
has been identified in each case. In addition, by comparison with
the FI-IR spectra of Mn(acac)3 (Fig. S1), an absorption band at
1508 cmÀ1 for Mn(III)-t-Leu-sal (Fig. 3A, f) assigned [69] to the
vibration of the C@C in the acetylacetonate arises, which could
be clearly distinguished from a set of vibrations assigned [70] to
aromatic C@C group in the range of 1510–1430 cmÀ1 for
1598 and 1351 cmÀ1 with a
D
tCOO of 247 cmÀ1 for Zn/Al-Val-
LDHs (Fig. 3B, b), at 1588 and 1355 cmÀ1 with a
233 cmÀ1 for Zn/Al-Leu-LDHs (Fig. 3C, b), and at 1598 and
1351 cmÀ1 with tCOO of 247 cmÀ1 for Zn/Al-Phe-LDHs
(Fig. 3D, b), respectively. The electrostatic interactions between
the intercalated -amino acid anions and the brucite-like layer
are also identified as monodentate mode in the light of the
increased COO values after intercalation. After in situ derivatiza-
tion of the intercalated amino acid anions, the appearance of tC=N
DtCOO of
a
D
a
D
t
and t
Ar-O bands are resolved at 1632 and 1250 cmÀ1 for Zn/Al-Val-
sal-LDHs (Fig. 3B, c), at 1636 and 1262 cmÀ1 for Zn/Al-Leu-sal-
LDHs (Fig. 3C, c), and at 1636 and 1262 cmÀ1 for Zn/Al-Phe-sal-
LDHs (Fig. 3D, c), respectively. The FT-IR results confirm the forma-
tion of the Schiff base in the interlayer spaces, well supporting the
XRD (Fig. 1) and the 13C CP/MAS NMR (Fig. 2) results.
The coordination of Mn (III) centers with Zn/Al-AA-sal-LDHs as
ligands was carried out by in situ incorporating Mn(acac)3 in
dichloromethane, achieving Mn(III)-Zn/Al-AA-sal-LDHs. To deter-
mine the coordination of Mn (III) centers, the solid was obtained
by simple filtration after in situ incorporation. The FT-IR spectra
of Mn(III)-Zn/Al-AA-sal-LDHs (d) as well as pristine AA-sal (e)
27

Z. An, Y. Tang, Y. Jiang et al.
Journal of Catalysis 402 (2021) 22–36
Fig. 3. FT-IR spectra of pristine (A) L-t-leucine, (B) L-valine, (C) L-leucine, and (D) L-phenylalanine potassium salt (a), and the corresponding Zn/Al-AA-LDHs (b), Zn/Al-AA-sal-
LDHs after in-situ derivatization (c), and Mn(III)-Zn/Al-AA-sal-LDHs after Mn (III) coordination (d), as well as the as-prepared AA-sal potassium salt (e) and Mn(III)-AA-sal (f)
as comparisons. The broad band between 1750 and 1500 cmÀ1 has been deconvoluted into two separated bands filled in olive and wine using the Gaussian fitting method.
t-Leu-sal (Fig. 3A, e). It demonstrates the additional coordination
by the acetylacetone groups in Mn(III)-t-Leu-sal (Fig. 3A, f).
The N 1 s1/2 XPS spectra for Mn (III)-AA-sal and Mn (III)-Zn/Al-
AA-sal-LDHs (Fig. S2) have been recorded to confirm the N species
that coordinated to Mn(III) centers. In Zn/Al-AA-sal-LDHs (dashed
short), three N species including NOÀ3 , N-H, and N@C [45,71,72] are
observed. After Mn (III) coordination (solid), the peaks of NOÀ3 and
N-H remain at the same position, but the peak of N@C shifts to
lower binding energy, indicating that only N@C groups contributed
to the coordination with Mn (III) centers. Moreover, in the high res-
olution Mn 2p3/2 XPS spectra (Fig. S3), the higher Mn 2p3/2 binding
energy [73,74] for Mn(III)-Zn/Al-AA-sal-LDHs (black) than Mn(III)-
AA-sal (red) indicates a stronger interaction between Mn (III) cen-
ters and Zn/Al-AA-sal-LDHs than that between Mn (III) centers and
AA-sal. The distribution of Mn centers was then explored for Mn
(III)-Zn/Al-Val-sal-LDHs (Fig. 4A) and Mn(III)-Zn/Al-Phe-sal-LDHs
(Fig. 4B) as examples by TEM technology with Mn area scanning
mappings. The white brightened dots representing Mn element
are observed homogeneously throughout the hexagonal crystallite.
It indicates that the strategy on in situ coordination of interlayer
28

Z. An, Y. Tang, Y. Jiang et al.
Journal of Catalysis 402 (2021) 22–36
Fig. 4. TEM images and Mn area scanning mappings of (A) Mn(III)-Zn/Al-Val-sal-LDHs and (B) Mn(III)-Zn/Al-Phe-sal-LDHs. (C) The schematic illustrations of the coordination
structures of pristine Mn(III)-AA-sal and Mn(III)-ZnAl-AA-sal-LDHs. ( Zn
Al
O
H).
amino acid Schiff base with the Mn (III) centers could guarantee
the homogeneous coordination, so as that the reactants could fur-
ther access the interlayer catalytic sites to undergo the asymmetric
epoxidation of indene. According to the elemental analysis of Mn
(III)-AA-sal and Mn(III)-Zn/Al-AA-sal-LDHs (Table S1), the molar
employed for the asymmetric indene epoxidation (Table 1). The
catalytic reaction was first carried out in dichloromethane at
20 °C for 26 h. Over Mn(III)-AA (Entries 1–4), an indene conversion
of < 20% and an ee value of <5% were observed for 1-amino-2-
indanol (1a) with a selectivity of around 50%, similar to the previ-
ous reports [44,45]. While over both Mn(III)-AA-sal and Mn(III)-Zn/
Al-AA-sal-LDHs (Entries 5–12), (1S,2R)-1-amino-2-indanol (1a)
was determined as the main product with a selectivity of 80–
ratios of Mn and
N were both determined as being 1:1
(Table S1). Combined with the FT-IR results and the previous
report [45], the possible coordination structures for Mn(III)-AA-
sal and Mn(III)-Zn/Al-AA-sal-LDHs are given in Fig. 4C.
90%, with
a
small amount of racemic 2-(2-oxoethyl)-
benzaldehyde (1b) as by-product. Over Mn(III)-t-Leu-sal (Entry
5), an ee value of 72% for (1S,2R)-1-amino-2-indanol at an indene
conversion of 28% was obtained. Over Mn(III)-Zn/Al-t-Leu-sal-
LDHs (Entry 6), an ee value of 94% at an indene conversion of
18% was achieved. The ee value has been improved as expected
through the modification by LDH nanosheets to L-t-Leucine Schiff
3.2. The orientation and arrangement order of nanosheet modified
amino acid Schiff base anions and enantioselectivity enhancement
Pristine amino acid potassium, AA-sal, and Zn/Al-AA-sal-LDHs
as ligands were in situ coordinated with Mn(acac)3 and directly
29

Z. An, Y. Tang, Y. Jiang et al.
Journal of Catalysis 402 (2021) 22–36
Table 1
Manganese-catalyzed asymmetric indene epoxidation using pristine amino acid potassium salt, AA-sal, and Zn/Al-AA-sal-LDHs as ligands.
Entry
Catalyst
Solvent
Con. (%)a
Sel. (%)a
1a
1H NMR yield (%)a
Isolated yield (%)b
ee (%)c
D
ee (%)d
1b
1
2
3
4
5
6
7
8
Mn(III)-t-Leu
Mn(III)-Val
Mn(III)-Leu
Mn(III)-Phe
Mn(III)-t-Leu-sal
Mn(III)-Zn/Al-t-Leu-sal-LDHs
Mn(III)-Val-sal
Mn(III)-Zn/Al-Val-sal-LDHs
Mn(III)-Leu-sal
Mn(III)-Zn/Al-Leu-sal-LDHs
Mn(III)-Phe-sal
Mn(III)-Zn/Al-Phe-sal-LDHs
Mn(acac)3
–
Zn/Al-Phe-sal-LDHs
Mn(III)-Zn/Al-Phe-sal-LDHs
Mn(III)-Phe-sal
Mn(III)-Zn/Al-Phe-sal-LDHs
Mn(III)-dispersed-Zn/Al-Phe-sal-LDHs
Mn(III)-Zn/Al-Phe-sal-LDHs
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
n-butanol
n-butanol
n-butanol
CH2Cl2
16
20
20
18
28
18
40
30
52
38
90(88)
82(80)
20
13
13
<1
48
56
50
47
50
84
80
85
83
88
84
93(92)
90(88)
23e
–
–
–
90
89
44
50
53
50
16
20
15
17
9
N.D.
N.D.
N.D.
N.D.
N.D.
14
N.D.
23
N.D.
32
N.D.
73
N.D.
–
–
–
<5
<5
<5
<5
72
94
67
93
52
91
20(22)
95(94)
–
–
–
–
10
75
74
95
–
–
–
–
–
22
–
26
–
39
–
75
–
–
–
12
12
9
24
14
34
24
46
32
84(81)
74(71)
6
–
–
–
43
37
43
72
9
12
16
10
11
12
13
14
15
16f
17
18
19
20g
7(8)
10(12)
77
100
100
–
10
10
11
11
–
–
65
64
75
N.D.
N.D.
N.D.
71
42
48
80
89
89
Reaction conditions: Mn(acac)3 (0.01 mmol), ligand (0.01 mmol of amino acid Schiff base), solvent (2 mL), 20 °C, 3 h. Then, indene (0.1 mmol), PhIO (0.2 mmol), 20 °C, 26 h. a
Determined by 1H NMR, b Determined by isolated product. c Determined by chiral OB-H. d e Racemic 1a. f Without PhIO. g Reaction in large scale, indene
ee = eeheter-eehomo
D
.
(10 mmol). The data in parentheses are reproduced results. N.D. represents not determined.
base anions. More significant improvement on the ee value was
observed on Mn(III)-Zn/Al-Val-sal-LDHs (93%, Entry 8) than on
Mn(III)-Val-sal (67%, Entry 7), Mn(III)-Zn/Al-Leu-sal-LDHs (91%,
Entry 10) than Mn(III)-Leu-sal (52%, Entry 9), and Mn(III)-Zn/Al-
Phe-sal-LDHs (95%, Entry 12) than Mn(III)-Phe-sal (21%, Entry
36 h, which is independent on the reaction process, superior to
the ee value of 20 % over Mn(III)-Phe-sal.
To elucidate the enantioselectivity improvement by the LDH
nanosheets modification, the intercalation orientation of the inter-
layer amino acid Schiff base anions in Zn/Al-AA-sal-LDHs has been
11), respectively. The obtained ee value in this work is much higher
than the results (<35%) in the previous reports[44–47] using
amino acid or -amino acid derivatives as ligands for metal active
considered firstly. Fig. 6A shows the profiles of the Dee value in the
asymmetric indene epoxidation by Mn(III)-Zn/Al-AA-sal-LDHs
towards the orientation angle (h) of the interlayer amino acid Schiff
a
-
a
sites. Moreover, it is the first report on the asymmetric epoxidation
of un-functionalized olefins in such high enantioselectivity cat-
alyzed by the transition metal centres coordinated with the chiral
ligands without any rotational-symmetry. The ee value improve-
base anions. Along with increasing orientation angle, the Dee value
firstly exhibits an approximate linear increase and then presents
an abnormal increase over Mn(III)-Zn/Al-Phe-sal-LDHs, demon-
strating that the ee improvement is significantly dependent on
the orientation angle. It further confirms that the spatial effect of
the LDH nanosheet to orientated modify amino acid Schiff base
as rigid ligand on the enantioselectivity in the indene epoxidation.
Moreover, over Mn(III)-Zn/Al-Phe-sal-LDHs, besides the intercala-
tion orientation, another factor might further contribute the enan-
tioselectivity enhancement.
Our previous work [41] reported that the arrangement order of
the interlayer anions within the 2D LDH interlayer space could
obviously influence the spatial effect of the LDH nanosheet on
the enantioselectivity. The arrangements of interlayer anions in
Zn/Al-AA-sal-LDHs were simulated by molecular dynamics (MD)
(Fig. 7). The side views of the simulated arrangement of interlayer
amino acid Schiff base anions, nitrate anions and water molecules
are given in Fig. 7A. The elements along the c-staking axis in the
projection view, including Zn (black), O (red), and H (green) atoms
in the brucite-like layer, O atoms (blue) in interlayer H2O mole-
cules and amino acid Schiff base, C atoms of phenyl ring in salicyli-
dene (purple) and other skeleton C atoms (orange) of the
interlayer amino acid Schiff base were profiled (Fig. 7B). Since
the phenyl ring in salicylidene group of the amino acid Schiff base
ment (Dee) is calculated as being 22, 26, 39, and 75%, increasing
in the order of Mn (III)-Zn/Al-t-Leu-sal-LDHs (Entry 6), Mn(III)-
Zn/Al-Val-sal-LDHs (Entry 8), Mn(III)-Zn/Al-Leu-sal-LDHs (Entry
10), and Mn(III)-Zn/Al-Phe-sal-LDHs (Entry 12), respectively. For
control, the reaction was performed with Mn(acac)3, and an indene
conversion of 20% to 2-(2-oxoethyl)-benzaldehyde (1b) at a selec-
tivity of 77% and racemic 1-amino-2-indanol (1a) at a selectivity of
23% was obtained (Entry 13). It confirms that the establishment of
the (1S,2R)-1-amino-2-indanol benefits from the coordination of
LDH nanosheet modified amino acid Schiff base to the Mn(III) cen-
ters. In the reactions without catalyst (Entry 14) and catalyzed by
Zn/Al-Phe-sal-LDHs only (Entry 15), an indene conversion of 13%
to 2-(2-oxoethyl)-benzaldehyde (1b) as the only product was
obtained. While in the reaction catalyzed by Mn(III)-Zn/Al-Phe-
sal-LDHs without PhIO, almost no reaction was detected (Entry
16). It indicates the 2-(2-oxoethyl)-benzaldehyde as by-product
originates from the oxidation of indene by PhIO, in accordance
with the previous report [75,76]. Fig. 5A shows the profile of the
ee value of (1S,2R)-1,2-epoxyindane catalyzed by Mn(III)-Zn/Al-
Phe-sal-LDHs and Mn(III)-Phe-sal as a function of reaction time.
The ee value over Mn(III)-Zn/Al-Phe-sal-LDHs keeps at 95% in
between layers has a
p-p conjugation at a certain distance, its
30

Z. An, Y. Tang, Y. Jiang et al.
Journal of Catalysis 402 (2021) 22–36
Fig. 5. (A) The ee value of (1S,2R)-1,2-epoxyindane, (B) the indene conversion, and (C) the selectivity of (1S,2R)-1,2-epoxyindane as a function of reaction time in the
asymmetric indene epoxidation catalyzed by Mn(III)-Phe-sal (hollow symbol) and Mn(III)-Zn/Al-Phe-sal-LDHs (solid symbol).
groups (Fig. 7C) is estimated from Fig. 7A. For Zn/Al-t-Leu-sal-LDHs
(a) and Zn/Al-Val-sal-LDHs (b) in a monolayer horizontal arrange-
ment of interlayer amino acid Schiff base anions, a similar mono-
distribution is resolved with a maximum distance at 6.47 and
5.91 Å, respectively. While for Zn/Al-Leu-sal-LDHs (c) and Zn/Al-
Phe-sal-LDHs (d) in an interdigitated bilayer vertical arrangement,
a dual-distribution is obtained with the maximum at 6.49/7.65 Å,
and at 5.68/7.58 Å, respectively, in which the latter is much nar-
rower than the former. Both of the profile distance along c-
stacking axis and ab-plane indicate that the arrangement of inter-
layer L-phenylalanine Schiff base anions is better ordered than that
of interlayer L-leucine Schiff base anions, accounting for the addi-
tional enhancement of ee in the asymmetric epoxidation besides
the intercalation orientation, as illustrated in Fig. 6A.
3.3. Diffusion limitation of 2D confined space and catalytic activity
As shown in Table 1, over Mn(III)-t-Leu-sal after 26 h, an indene
conversion of 28% with a (1S,2R)-1-amino-2-indanol selectivity of
84% was obtained (Entry 5). Over Mn(III)-Zn/Al-t-Leu-sal-LDHs,
an indene conversion of 18% with a (1S,2R)-1-amino-2-indanol
selectivity of 80% was achieved (Entry 6). Similar decreases in
the indene conversion observed on Mn(III)-Zn/Al-Val-sal-LDHs
(30% and 83%, Entry 8) compared with Mn(III)-Val-sal (40% and
85%, Entry 7), Mn(III)-Zn/Al-Leu-sal-LDHs (38% and 84%, Entry
10) with Mn(III)-Leu-sal (52% and 88%, Entry 9), and Mn(III)-Zn/
Al-Phe-sal-LDHs (81
1.0% and 89
1.0%, Entry 12) with Mn
(III)-Phe-sal (89 1.0% and 92.5 0.5%, Entry 11), respectively.
The diffusion limitation of the 2D confinement space of the hetero-
geneous Mn(III)-Zn/Al-AA-sal-LDHs is inferred to cause such
decrease in the indene conversion. To reduce the diffusion limita-
tion, Zn/Al-Phe-sal-LDHs was dispersed through a simple liquid-
exfoliation process [56], affording the dispersed-Zn/Al-Phe-sal-
LDHs. After being dispersed in n-butanol with continuous stirring
under N2 atmosphere for 24 h, the suspension becomes transpar-
ent colloid, showing obvious Tyndall effect [77] (Fig. S4, A). As
shown in the XRD pattern (Fig. S4, B), the absence of (003) reflec-
tion and the retention of the (110) and (113) reflections provide
another evidence on the high dispersity of the dispersed-Zn/Al-
Phe-sal-LDHs without obvious layer destruction. Furthermore,
AFM image (Fig. S4, C) proves that the thickness of the obtained
nanosheets in dispersed-ZnAl-Phe-sal-LDHs is located in a range
of 4.8–5.2 nm, which is consistent with the thickness of double
or three layers of Zn/Al-Phe-sal-LDHs (d003 = 1.99 nm, Fig. 1D).
Then, Phe-sal, ZnAl-Phe-sal-LDHs, or dispersed-ZnAl-Phe-sal-
LDHs was coordinated with Mn(III) centers in n-butanol and then
directly employed in the asymmetric indene epoxidation at 20 °C
in n-butanol (Table 1, Entries 17–19). Over the homogeneous Mn
(III)-Phe-sal (Entry 17), an ee value of only 10% at an indene con-
Fig. 6. The profiles of
Dee value over Mn(III)-Zn/Al-AA-sal-LDHs in the asymmetric
epoxidation of (A) indene and (B) 4-nitrocinnamol towards the orientation angle of
the interlayer amino acid Schiff base anions in the corresponding Zn/Al-AA-sal-
LDHs.
arrangement could reflect the order degree to a certain extent. The
distribution range of the phenyl rings in salicylidene groups along
the c-stacking axis was 3.0 Å for Zn/Al-t-Leu-sal-LDHs (a), 2.5 Å for
Zn/Al-Val-sal-LDHs (b), 7.6 Å for Zn/Al-Leu-sal-LDHs (c), and 5.9 Å
for Zn/Al-Phe-sal-LDHs (d), respectively. The narrower distribution
range of the phenyl rings in salicylidene groups in Zn/Al-AA-LDHs
means the better arrangement order of interlayer AA-sal anions in
the similar intercalated orientation. Moreover, the distance
between two adjacent centroids of the phenyl rings in salicylidene
31

Z. An, Y. Tang, Y. Jiang et al.
Journal of Catalysis 402 (2021) 22–36
Fig. 7. (A) The side view of interlayer amino acid Schiff base anion, NOÀ3 and H2O molecules ( Zn
Al
N
C
O
H), (B) atomic density projection of c axis (The red,
black, and green lines represent the Zn, O, and H atoms in the brucite-like layer, respectively; the blue line represents the O atoms in interlayer H2O molecules and amino acid
Schiff base; the purple and orange lines represent the C atoms of phenyl ring in salicylidene groups and other skeleton C atoms of the interlayer amino acid Schiff base), and
(C) the distribution of distance between two adjacent centroids of the phenyl rings in salicylidene groups for (a) Zn/Al-t-Leu-sal-LDHs, (b) Zn/Al-Val-sal-LDHs, (c) Zn/Al-Leu-
sal-LDHs, and (d) Zn/Al-Phe-sal-LDHs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
version of 48% with a (1S,2R)-1-amino-2-indanol selectivity of 90%
is obtained. Over Mn(III)-Zn/Al-Phe-sal-LDHs (Entry 18), a signifi-
cantly enhanced ee value to 75% at an indene conversion of 42%
with a (1S,2R)-1-amino-2-indanol selectivity of 89% is achieved.
Over Mn(III)-dispersed-Zn/Al-Phe-sal-LDHs (Entry 19), similar ee
value (74%) and product selectivity (89%) was observed to that over
Mn(III)-Zn/Al-Phe-sal-LDHs (Entry 18), but the indene conversion
increases to 48% approaching the homogeneous counterpart (Entry
17). Through the dispersion process, the diffusion limitation of the
2D confined space could be reduced. It verifies the spatial effect of
the LDH nanosheet as rigid substitutes for the L-phenylalanine
Schiff base to enhance the enantioselectivity. After dispersion,
the orientation of the interlayer L-phenylalanine Schiff base anions
is deduced to be well retained due to the strong interaction
between L-phenylalanine Schiff base anions as discussed in Fig. 1.
Fig. 5B and 5C show the profiles of the indene conversion and
the selectivity of (1S,2R)-1,2-epoxyindane as a function of the reac-
tion time by Mn(III)-Zn/Al-Phe-sal-LDHs and Mn(III)-Zn/Al-Phe-
sal. The indene conversion gradually increases along with increas-
ing reaction time and reaches an equilibrium at 24 h for Mn(III)-
Zn/Al-Phe-sal-LDHs and at 20 h for Mn(III)-Zn/Al-Phe-sal, respec-
tively (Fig. 5B). The selectivity-time profiles show a similar ten-
dency with the conversion-time profile (Fig. 5C), demonstrating
the dependency relationship on the indene conversion as men-
32

Z. An, Y. Tang, Y. Jiang et al.
Journal of Catalysis 402 (2021) 22–36
tioned above. The decrease on the (1S,2R)-1-amino-2-indanol
selectivity over each Mn(III)-Zn/Al-AA-sal-LDHs could be thus rea-
sonably explained. The calculated 1H NMR yield of (1S,2R)-1-
amino-2-indanol catalyzed by Mn(III)-Zn/Al-Phe-sal-LDHs are
determined to be 74%, which is close to the determined yield of
73% for the isolated product (Table 1, Entry 12). In order to detect
the potential of the catalyst’s industrial applications, the large-
scale asymmetric epoxidation reaction with an indene input of
10 mmol was performed over Mn(III)-Zn/Al-Phe-sal-LDHs (Table 1,
Entry 20). A (1S,2R)-1-amino-2-indanol yield of 71% with an ee
value of 95% were obtained.
6), a 4-nitrocinnamol conversion of 72% holds the same level with
that (70%) by Mn(III)-Phe-sal (Entry 5). Especially for Mn(III)-Zn/
Al-Phe-sal-LDHs (Entry 8), a 4-nitrocinnamol conversion of 92%
was obtained, much higher for that (78%) by Mn(III)-Phe-sal (Entry
7). The interlayer hydrogen-bonding environment of LDHs are thus
deduced to be able to provide an additional substrate enrichment
on 4-nitrocinnamol [78]. Along with the increasing interlayer spac-
ing, the enrichment effect becomes prominent beyond the diffu-
sion
nitrocinnamaldehyde conversion is achieved over the Mn(III)-Zn/
Al-Phe-sal-LDHs with total (2S,3S)-2,3-epoxy-3-(4-nitrophe
limitation.
Therefore,
a
visibly
enhanced
4-
a
nyl)-1-propanol yield of 82%. Moreover, the selectivity of (2S,3S)-
2,3-epoxy-3-(4-nitrophenyl)-1-propanol catalyzed by Mn(III)-Zn/
Al-AA-sal-LDHs holds the same level with the corresponding
3.4. Substrate scope and catalyst reusability
The asymmetric reaction was then extended to the epoxidation
of allyl alcohol, as shown in Table 2. Over homogeneous Mn(III)-t-
Leu-sal, Mn(III)-Val-sal, Mn(III)-Leu-sal, and Mn(III)-Phe-sal, an ee
value of 53, 44, 26, and 18% for (2S,3S)-2,3-epoxy-3-(4-nitrophe
nyl)-1-propanol was afforded (Entries 1, 3, 5, and 7). While over
Mn(III)-Zn/Al-t-Leu-sal-LDHs, Mn(III)-Zn/Al-Val-sal-LDHs, Mn(III)-
Zn/Al-Leu-sal-LDHs, and Mn(III)-Zn/Al-Phe-sal-LDHs, an improved
ee value of 58%, 50%, 37%, and 60% was obtained (Entries 2, 4, 6,
homogeneous Mn(III)-AA-sal, with a small amount of 4-
nitrocinnamaldehyde detected as the main by-product.
The substrate scope of Mn(III)-Zn/Al-Phe-sal-LDHs catalyst was
then explored to chromene and
a,b-unsaturated aldehydes
(Table 2, Entries 9–11). For chromene substrates, both electron-
withdrawing (Entry 9) and electron-donating substituents (Entry
10) are well tolerated. A yield of 62% and an ee value of 80% for
(3R,4R)-3,4-epoxy-2,2-dimethyl-6-nitrochromane are obtained
(Entry 9); a yield of 54% and an ee value of 89% for (3R,4R)-3,4-e
poxy-2,2-dimethyl-6-methoxychromane are obtained (Entry 10).
and 8) with the corresponding
Dee value of 5, 6, 11, and 42%.
LDH nanosheets as the rigid substitutes to modify the amino acid
Schiff base as ligands for Mn(III) centers could also supply obvious
spatial effect on the enantioselectivity enhancement in the epoxi-
dation of allyl alcohol. As shown in Fig. 6B, the ee improvement
in the epoxidation of allyl alcohol is significantly dependent on
the orientation angle, displaying the similar relationship with that
in the epoxidation of allyl alcohol. The enantioselectivity enhance-
ment by modulating the orientation angle and arrangement order
has been further verified.
The 4-nitrocinnamol conversion was detected to be 40% for Mn
(III)-Zn/Al-t-Leu-sal-LDHs (Entry 2) and 48% for Mn(III)-Zn/Al-Val-
sal-LDHs (Entry 4), much lower than that for Mn(III)-Val-sal (52%,
Entry 1) and Mn(III)-t-Leu-sal (68%, Entry 3). It is in good agree-
ment with the result in the asymmetric indene epoxidation, which
could be reasonably originated from the diffusion limitation in the
2D interlayer spacing. While for Mn(III)-Zn/Al-Leu-sal-LDHs (Entry
For
a,b-unsaturated aldehydes, a yield of 56% and an ee value of
48% for (2S,4R)- 2,3-epoxy-1,3-diphenylcyclopropan-1-one are
obtained (Entry 11).
Mn(III)-Zn/Al-Phe-sal-LDHs have been recycled in the asym-
metric indene epoxidation and reused without any treatment. As
shown in Fig. 8A, the ee value and the indene conversion of the cat-
alyst well remain at 95% and 80% in six catalytic runs. A slight
decline of the (1S,2R)-1-amino-2-indanol selectivity from 90 to
84% was observed. The leaching of Mn, Zn, Al, C, H, and N elements
from solid catalyst and in reaction solution in six catalytic runs was
displayed in Fig. 8B. Almost no leaching of Zn, Al, and N from solid
catalyst or the spent reaction solution was detected, indicating that
LDH layers and intercalated L-phenylalanine Schiff base anions did
not leach into the reaction solution. Only 1.2% of Mn dosage was
detected to leach from the solid in six runs, which is close to the
Table 2
Substrate scope for Mn(III)-Zn/Al-AA-sal-LDHs-catalyzed the asymmetric epoxidation.
Entry
Substrate
Product
Catalyst
Con. (%)a
Sel. (%)a
1H NMR Yield(%)a
ee (%)b
D
ee (%)c
1
2
3
4
5
6
7
8
9
Mn(III)-t-Leu-sal
Mn(III)-Zn/Al-t-Leu-sal-LDHs
Mn(III)-Val-sal
Mn(III)-Zn/Al-Val-sal-LDHs
Mn(III)-Leu-sal
Mn(III)-Zn/Al-Leu-sal-LDHs
Mn(III)-Phe-sal
52
40
68
48
70
72
78
92
74
74
72
77
76
86
86
90
90
86
38
29
52
37
60
62
70
83
64
53
58
41
50
26
47
18
60
80
–
5
–
9
–
21
–
42
–
Mn(III)-Zn/Al-Phe-sal-LDHs
Mn(III)-Zn/Al-Phe-sal-LDHs
10
11
Mn(III)-Zn/Al-Phe-sal-LDHs
Mn(III)-Zn/Al-Phe-sal-LDHs
66
67
84
85
55
57
89
48
–
–
Reaction conditions: Mn(acac)3 (0.01 mmol), ligand (0.01 mmol of amino acid Schiff base), CH2Cl2 (2 mL), 20 °C, 3 h. Then, substrate (0.1 mmol), PhIO (0.2 mmol), 20 °C, 26 h. a
b
Determined by 1H NMR, Determined by chiral OB-H/OJ-H. c Dee = eeheter-eehomo
.
33

Z. An, Y. Tang, Y. Jiang et al.
Journal of Catalysis 402 (2021) 22–36
solution. Considering the proposed formula of Mn(III)-Zn/Al-Phe-
sal-LDHs (Table S1), the C and N leaching might be originated from
the acetylacetone as co-ligands, which caused the un-coordination
of Mn centers. As a result, a declined selectivity of (1S,2R)-1-
amino-2-indanol from 95% and 80% in six catalytic runs occurred
(Fig. 8A). According to the previous literatures on the epoxidation
mechanism [78], the Mn(III) centers firstly proceed an oxidation
to Mn (V) = O by leaving one acetylacetone ligand. After one cat-
alytic cycle, Mn (V)
= O recovers to Mn(III) by the re-
coordination of the left acetylacetone ligand. The leaching of acety-
lacetone might be attributed to the un-accomplished re-
coordination in one catalytic cycle.
3.5. Proposed heterogeneous catalytic mechanism
According to the previous reports by Jacobsen’s group
[79,11,80], the enantioselectivity in the homogeneous reaction is
originated from the side-on perpendicular attack by the reacting
olefin at the oxygen of the formed Mn (V) = O moiety. Taking the
asymmetric indene epoxidation as an example, the preferred con-
former through upper right-hand approach (from front to page) is
given in Fig. 9A, in which the phenyl ring of indene points is paral-
lel to the phenyl ring. The bulky substituents (R) on the amino acid
Schiff base backbone could minimize stereochemical communica-
tion in the disfavored transition state. Over the heterogeneous
Mn(III)-Zn/Al-t-Leu-sal-LDHs and Mn(III)-Zn/Al-Val-sal-LDHs in a
monolayer horizontal arrangement of interlayer amino acid Schiff
base anions with an orientation angle of 3° and 10°, respectively
(Fig. 1A and 1B), the attack of indene to form the disfavored tran-
sition state has been restricted by the LDH nanosheet, and the
steric hindrance becomes more serious along with the increasing
orientation angle (Fig. 9B). While over Mn(III)-Zn/Al-Leu-sal-
LDHs and Mn(III)-Zn/Al-Phe-sal-LDHs in an interdigitated bilayer
vertical arrangement of interlayer amino acid Schiff base anions
with an orientation angle of 60° and 80°, respectively (Fig. 1C
and 1D), indene with its phenyl ring plane nearly perpendicular
to LDH nanosheets attacks the oxygen of the formed Mn (V) = O
moiety, in which the steric hindrance supplied by LDH nanosheets
is more significant than that in horizontal orientation (Fig. 9C). For
Mn(III)-Zn/Al-Phe-sal-LDHs with an orientation angle of 80°, the
steric hindrance of LDH nanosheets has almost cut off the disfa-
vored attacked pathway, which is thought to completely replace
the enrichment effect, thereby leading to a sharp increase in the
Fig. 8. (A) Reusability of Mn(III)-Zn/Al-Phe-sal-LDHs in the asymmetric indene
epoxidation and (B) the total leaching of Mn, Zn, Al, C, H, and N from solids and in
solution in six catalytic runs.
total loss of 1.5% in the reaction solution. 6.2% and 5.8 of C and H
dosages were also leached from the solid in six runs. Similar results
in the total loss of 6.5%, and 6.2% were detected in the reaction
Fig. 9. Proposed mechanism of the asymmetric indene epoxidation catalyzed by (A) pristine Mn(III)-AA-sal and the heterogeneous Mn(III)-Zn/Al-AA-sal-LDHs (B) in a monolayer
horizontal arrangement and (C) in an interdigitated bilayer vertical arrangement of interlayer amino acid Schiff base anions. ( Mn
N
C
O
substituent R).
34

Z. An, Y. Tang, Y. Jiang et al.
Journal of Catalysis 402 (2021) 22–36
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In conclusion, LDH nanosheets as rigid substituents to modify
amino acid Schiff base as ligands have promoted the enantioselec-
tivity in the Mn(III)-catalyzed asymmetric epoxidation. The enan-
tioselectivity enhancement depends on the orientation angle and
the arrangement order of the intercalated amino acid Schiff base
anions within the LDH interlayer space. 72.5
1.5% of yield,
89.0 1.0% of selectivity and 94.5 0.5% of ee for (1S,2R)-1,2-
epoxyindane have been afforded in the asymmetric indene epoxi-
dation catalyzed by the Mn(III)-Zn/Al-Phe-sal-LDHs at an orienta-
tion angle of 80°. Substrate scope and catalyst reusability have
been investigated elaborately, further supporting the heteroge-
neous catalytic strategy. The catalytic mechanism on the spatial
synergy of LDH nanosheets has been proposed. Nevertheless, there
remain great challenges that are worth of further in-depth investi-
gation. One is to ameliorate the efficacy of heterogeneous asym-
metric catalysis including the selectivity and the substrate
conversion. The other is the lack of the theoretical investigation
on the role of the LDH layers including both spatial and electronic
factors.
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Declaration of Competing Interest
[21] Y. Shimada, S. Kondo, Y. Ohara, K.T. Matsumoto, T. Katuski, Titanium-Catalyzed
Asymmetric Epoxidation of Olefins with Aqueous Hydrogen Peroxide:
Remarkable Effect of Phosphate Buffer on Epoxide Yield, Synlett 15 (2007)
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[22] A. Berkessel, M. Brandenburg, E. Leitterstorf, J. Frey, J. Lex, M. Schäfer, A
Practical and Versatile Access to Dihydrosalen (Salalen) Ligands: Highly
Enantioselective Titanium In Situ Catalysts for Asymmetric Epoxidation with
Aqueous Hydrogen Peroxide, Adv. Synth. Catal. 349 (2007) 2385–2391.
[23] A.U. Barlan, W. Zhang, H. Yamamoto, Development and Application of
Versatile Bis-Hydroxamic Acids for Catalytic Asymmetric Oxidation,
Tetrahedron 63 (2007) 6075–6087.
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
Financial supports from the National Key R&D Program of China
(2017YFB0307203), NSFC (21673016, 21521005), and the Funda-
mental Research Funds for the Central Universities (XK1802-6)
are gratefully acknowledged. The authors are grateful for the sup-
port of the Beijing Engineering Center for Hierarchical Catalysts.
[24] K. Matsumoto, Y. Sawada, B. Saito, K. Sakai, T. Katsuki, Construction of Pseudo-
Heterochiral and Homochiral Di-l-oxotitanium(Schiff base) Dimers and
Enantioselective Epoxidation Using Aqueous Hydrogen Peroxide, Angew.
Chem. Int., Ed. 44 (2005) 4935–4939.
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Angew. Chem. Int. Ed. 45 (2006) 3478–3480.
[26] K. Yu, L.L. Lou, C. Lai, S.J. Wang, F. Ding, S.X. Liu, Asymmetric Epoxidation of
Unfunctionalized Olefins Catalyzed by Mn(III) Salen Complex Immobilized on
MCM-48, Catal. Commun. 7 (2006) 1057–1060.
[27] R.I. Kureshy, T. Roy, N.H. Khan, S.H.R. Abdi, A. Sadhukhan, H.C. Bajaj,
Immobilized Dimeric Chiral Mn(III) salen Complex on Short Channel
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Non-Functionalized Alkenes, Tetrahedron 68 (2012) 6314–6322.
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Activity by Controlling Framework Catenation and Varying Open Channel
Sizes, J. Am. Chem. Soc. 132 (2010) 15390–15398.
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2021.08.015.
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