Y. Jia, et al.
MolecularCatalysis495(2020)111146
stemming from supported (salen)Mn(III) catalysts led to reduced cata-
lytic activities and enantioselectivities [16]. Supporting materials may
play a key role in catalysis as ligands, so their acid-base properties,
steric hindrances, electronic effects, and surface periodicities certainly
deserved comprehensive and careful discussions [17]. In general, there
(salen)Mn(III) complex catalysts.
were recorded on Shimadzu UV-1800, samples were fixed at 10-3 mol L-
1
in CH2Cl2 (Mn content as criteria), wavelength was ranged from
290 nm to 550 nm. ESI-HRMS of positive molecular ions were detected
on microOTOF-Q II, Bruker.Daltonics equipment. C, H, and N elemental
analyses were tested on an Elementar VarioEL III instrument. Metal ion
contents were measured by inductively coupled plasma atomic emis-
sion spectrometry (ICP-AES) on an ICPE-9000 spectrometer, Shimadzu,
according to standard working curve method. Optional rotations were
measured on Perkin-Elmer 341 with λ of 587 nm at 25 °C, and values
were formatted as absolute rotation [ ]2D5, where contents of samples
were controlled at 0.01 g mL−1 in CH2Cl2. The number (Mn)- and
weight (Mw)-average molecular weights and polydispersity indices
(PDI, Mw/Mn) of four polymeric salen ligands were measured on a gel
permeation chromatography (GPC, Waters 1515–2414) equipped with
a Styragel HT3 THF column, temperature was 40 °C, eluent was THF,
flow rate was 1 mL min-1, which was calibrated by polystyrene stan-
dard.
Currently, immobilization of homogeneous complexes into in-
soluble solids having ordered structures appeared to be a very pro-
mising method for the desing of advanced heterogeneous catalysts.
SBA-15 attracted interest from the community for more than one
decade as a well-ordered hexagonal mesoporous silicate featuring some
advantages as compared to classical MCM-41 including thicker pore
wall, larger pores, as well as unique internal connectivity among piled
silicate pipes [18]. Its combination with homogeneous catalysts usually
showed improbved activities than homogeneous counterparts [18].
SBA-15 was also successfully employed as supporting material for
several heterogeneous catalysts [19]. In practice, the anchoring of large
looked attractive but difficult [20]. Thus, axial coordination of supports
to manganese centers seemed to be another option for holding large
(salen)Mn(III) complexes [21], while non-covalent linkages were also
supposed that a (salen)Mn(III) polymer complex could be formed and
assembled within channels of zeolite using a “ship-in-a-bottle” strategy
[22]. Overall, immobilization of (salen)Mn(III) compounds into SBA-15
could create relevant systems for asymmetric epoxidation reactions.
In this work, four chiral (tartrate-salen)Mn(III) polymer complexes
were prepared as catalysts for asymmetric epoxidation reactions
(Fig. 1). SBA-15 was selected to construct a rigid supporting environ-
ment, in which a “ship-in-a-bottle” strategy was employed to assemble
(tartrate-salen)Mn(III) into SBA-15. Both homogeneous and hetero-
geneous catalysts pointed to the presence of synergetic effects among
chiral centers stemming from different blocks of catalyst. In addition,
sodium hypochlorite was unstable and corrosive, and alternatively
some solid oxidants including iodosylbenzene and 3-chloroperox-
ybenzoic acid were utilized in this work to establish a more sustainable
process for the future large-scale applications.
BET surface area, pore volume, pore radius, and pore size dis-
tribution were recorded on Micromeritics ASAP 2020, using N2 ad-
sorption isotherms at 77.35 K, and each sample was degassed at 150 °C
in vacuum before testing. Surface area was calculated on these iso-
therms using the multi-point Brunauer-Emmett-Teller (BET) method
based on adsorption data in the relative pressure P / P0 ranged from
0.06 to 0.3. Total pore volume was obtained from N2 adsorbed at P /
P0 = 0.97, both pore volume and pore radius were determined using
Barrett-Joyner-Halenda (BJH) method. Bulk density of sample was
detected on SOTAX TD2 density detector, CAMAG Corporation. Particle
size and zeta potential measurements were carried out in CH2Cl2 at
298 K on a Zetasizer Nano ZS90 sepctrometer, Malvern. The X-ray
diffraction (XRD) patterns of powdered samples were reported on
Shimadzu XRD-6000 (Cu-Ka1, λ 1.54059 Å), and diffraction data were
collected when 2θ angles ranged from 4° to 55° with 0.02° intervals. X-
ray photoelectron spectroscopy (XPS) were carried out on Kratos Axis
Ultra DLD, irradiation source was monochromatic Al Kα X-ray
(1486.6 eV). Scanning electron microscopy (SEM) was performed on
JSM-6700F, JEOL.
Thin layer chromatography (TLC) was conducted on glass plates
coated with GF254 silica gel, where coloration was performed in phos-
phomolybdic acid (PMA)/ethanol (5% mass percent) solution. Both
conversion and enantiomeric excess were determined by chiral HPLC
analysis, including a Waters chromatograph (system controller: Waters
1525, binary hplc pump; UV–vis detector: Waters 2998, photodiode
array detector; UV detection: 242 nm, determined after wavelength
scanning between 210 nm and 400 nm), equipped with a Daicel
Chiralcel OD-H column (150 mm × 4.6 mm; 5 μm particle; mobile
2. Experimental
2.1. Materials
2-tert-Butylphenol, tetrabutylammonium bromide (TBAB), paraf-
ormaldehyde, L-(+)-tartaric acid, D-(-)-tartaric acid, sodium L-
(+)-tartrate dihydrate, sodium D-(-)-tartrate dihydrate, styrene, α-
methylstyrene, trans-stilbene, indene, 1,2-diaminocyclohexane (mix-
ture of isomers), 3-aminopropyltrimethoxysilane (3-APTMS), Pluronic
P123 (average Mn, 5800), tetraethyl orthosilicate (TEOS), 3-chlor-
operoxybenzoic acid (mCPBA), inorganic salts, and HPLC-grade sol-
vents were totally purchased from Sigma-Aldrich Corporation without
purification. Regular solvents and silica gel of column and thin layer
chromatography were provided by local distributors, some sensitive
solvents were further purified in our laboratory. 3-tert-Butyl-5-chlor-
omethyl-2-hydroxybenzaldehyde [23], (R,R)-1,2-diammoniumcyclo-
hexane mono-(+)-tartrate salt [23], (S,S)-1,2-diammoniumcyclo-
hexane mono-(-)-tartrate salt [23], SBA-15 [24], Mn5 [25], and
phase: n-hexane / 2-propanol, 97 / 3, v / v; flow rate: 1.0 mL min−1
;
column temperature: 300 K; pressure: 5.0 MPa to 7.0 MPa; sample
concentration: 1.0 mg mL−1 in n-hexane; injection: 10 μL).
2.3. Synthesis of chiral dimeric salicylaldehyde (3)
As shown in Scheme 1, 3-tert-butyl-5-chloromethyl-2-hydro-
xybenzaldehyde (1, 3.25 g, 14.4 mmol) and sodium L-(+)-tartrate di-
hydrate (2a, 1.65 g, 7.2 mmol) were combined with dry triethylamine
(30 mL) into a round-bottomed flask (250 mL), and the orange solution
was heated at 110 °C for 3 h under vigorous stirring. Small crystals
(sodium chloride) gradually precipitated from the purple solution
during this procedure. After being cooled down to room temperature,
solvent was removed under reduced pressure, and residue was diluted
with CH2Cl2 (100 mL). Organic layer was thoroughly washed by water
(3 × 50 mL) and brine (3 × 50 mL), dried over anhydrous Na2SO4, and
filtered. After removal of solvent under rotary evaporation, residue was
further purified by column chromatography (SiO2, 200–300 mesh;
petroleum ether / ethyl acetate, 6 / 1, v / v, with a few drops of trie-
thylamine) to afford chiral dimeric salicylaldehyde (3a, yellow sticky
solid, 1.49 g, 39 % yield).
2.2. Characterization
H1 NMR were recorded on a Bruker ADVANCE III instrument
(400 MHz): spectral width in Hz (SWH) was 8223.6 Hz, dwell time
(DW) was 60.8 μs, temperature was 293.0 K. FT-IR data were collected
in potassium bromide pellets on a Bruker Tensor 27 spectrometer, wave
numbers were ranged from 400 cm−1 to 4000 cm-1. UV–vis spectra
2