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J. Chen et al. / Molecular Catalysis 445 (2018) 163–169
exposed to a trichlorosilane/Argon stream, and the time-related
spectra were collected every minute during the adsorption pro-
cess. Then the samples were swept by pure Argon at 30 ◦C for
1 h to examine the strength of adsorption. The spectra obtained
were quantified by employing the Kubelka-Munk function. MIL-
101(Cr) and CMIL-101 was degassed at 423 K and 393 K respectively
for 12 h before measuring the BET surface area on a Tri Star II
Plus instrument at 77 K. The powder X-ray diffraction (XRD) pat-
terns were obtained using a PANalytical X’Pert PRO diffractometer
with Cu K␣ radiation operated at 40 kV, 40 mA. Scanning electron
microscopy (SEM) images were obtained using Hitachi SU-8000
instrument. Elemental analysis results were achieved on the Ele-
mentar Vario MICRO elemental analyzer. 1H NMR and 13C NMR
spectra were measured on a Bruker Avance III device with a fre-
quency of 400 MHz. TOF MS was measured on a Waters GCT Premier
Time of Flight mass spectrometer. Enantiomeric excess value of
the amine products was measured on Waters equipment using
Daicel Chiralpak IB column with hexane/isopropyl alcohol solvent
systems. HPLC methods were calibrated with the corresponding
racemic samples.
Scheme 1. Structures of N-formyl group modified Lewis bases.
modified Lewis bases (1, Scheme 1) [16,17,54–56] were efficient
homogeneous catalyst for the reduction of imines. We anticipated
that the immobilization of such species inside the pores of MIL-
101(Cr) would possibly generate a chiral MOF for the reduction of
imines with trichlorosilane, upon which the confined environment
of MOFs may also benefit the enantioselectivity of the homoge-
neous modulators. In this work, we report that chiral MIL-101(Cr)
decorated with a pyridyl modified L-proline derivative (2) acts as a
chiral catalyst for asymmetric reduction of ketimines with compa-
rable catalytic performance to its counterpart.
2. Experimental section
3. Results and discussion
2.1. Preparation of catalysts
3.1. Characterization of MIL-101(Cr)
2.1.1. Preparation of metal-organic frameworks
MIL-101(Cr) [41] and MIL-101(Cr)-NH2 [57] were synthesized
MIL-101(Cr) was synthesized and purified according to the
method described in literature [41]. The resulted green powder
was characterized by various techniques. The SEM images in Fig. S1
demonstrated that MIL-101(Cr) was octahedron with a size around
200 nm. The crystallinity of MIL-101(Cr) was confirmed by PXRD,
which matches with the simulated experimental patterns of MIL-
101(Cr) (Fig. S2). The surface area and pore volume of MIL-101(Cr)
was determined by the N2 adsorption/desorption isotherm (Fig. S3)
and displayed in Table S1. The two bands at 1630 and 1398 cm−1 on
the IR spectrum (Fig. S4) are in agreement with the stretching vibra-
tion of carboxyl group. Thermogravimetric analysis in nitrogen (Fig.
S5) revealed that MIL-101(Cr) was stable up to 275 ◦C.
2.1.2. Construction of chiral MIL-101(Cr)
Before post-modification, MIL-101(Cr) was activated under vac-
uum at 150 ◦C for 24 h to generate open metal sites. The activated
MIL-101(Cr) and chiral ligand 2 (Scheme 1) was mixed together
in dry chloroform, and the heterogeneous mixture was stirred,
refluxed under N2 atmosphere for 24 h. After the mixture was
cooled to room temperature (r.t.), the solid was collected by cen-
trifugation. After that, the resulted solid was washed thoroughly
with dichloromethane and ethanol to eliminate trace amount of
free organic ligand 2. The modified MOFs (denoted as CMIL-101
hereafter) was dried under vacuum before use.
3.2. MIL-101(Cr) catalyzed reduction of imines
2.2. General procedure for the catalytic reduction of imines with
trichlorosilane
as well as MIL-101(Cr)-NH2 were also studied (Table 1). NaCl
(Table 1, entry 4) demonstrated a negligible catalytic effect as
compared with the control experiment (Table 1, entry 1). When
sodium benzoate (Table 1, entry 3) was employed as the catalyst,
a slight increase of reaction rate was resulted, suggesting that ben-
zoate is superior to chloride in activating the substrates. In the
entry 2), a dramatic increase in reaction rate was observed, which
may be attributed to a synergistic activation effect of Cr3+ and
terephthalate. Along with this line, we are pleased to find that MIL-
101(Cr) (Table 1, entry 5), consists of the same components as with
chromium terephthalate, is the best catalyst for this reaction. The
Cr3+ clusters and terephthalate ligands were regularly arranged
inside the pores of MIL-101(Cr), which may not only functional-
but also facilitate the diffusion of substrates into the ordered pores.
Therefore, an enhanced yield was observed with MIL-101(Cr) as the
catalyst.
All the reduction reactions of imines were performed in oven-
dried Schlenk flasks. The mixture of imine 3 (0.2 mmol) and the
MOF catalyst in anhydrous solvent was cooled to 0 ◦C under a
nitrogen atmosphere, trichlorosilane (60 L, 0.6 mmol) was added
dropwise to the mixture. After the reaction completed, the cat-
alyst was separated by centrifugation immediately, and washed
with ethyl acetate for three times. The supernatant was combined
and saturated solution of NaHCO3 (10 mL) was added. The mixture
was extracted with ethyl acetate (3 × 20 mL). The combined organic
layer was washed with brine and dried over anhydrous MgSO4, fil-
tered and evaporated. The crude product was purified using column
chromatography on silica gel eluted with a mixture of petroleum
ether and ethyl acetate.
2.3. Characterization
The FTIR spectra were recorded on a Nicolet 6700 FT-IR spec-
trometer in the range of 400–4000 cm−1. The trichlorosilane
adsorbed diffuse reflectance Fourier transform infrared spec-
troscopy (DRIFTS) of solid catalysts were recorded on Nicolet iS50
FT-IR spectrometer. The samples were preheated in situ at 100 ◦C for
5 min and cooled to 30 ◦C under Argon stream. The spectrum of each
solid was collected and used as the blank spectrum. The sample was
The amine functionalized MIL-101(Cr) (Table 1, entry 6) was
also effective for the reduction reaction, but an inferior yield was
resulted. It may be caused by the decrease on BET surface area and