"click" post-synthetic modification of metal-organic frameworks with chiral functional adduct for heterogeneous asymmetric catalysis

Article abstract of DOI:10.1039/c2dt12153k

Two enantiomeric Zn-MOFs, having l- or d-proline chiral functionality, were achieved through in situ click reactions by modifying two opposite chiral adducts within the same pre-assembled achiral MOFs, respectively. Both of them exhibited remarkable catal

Full text of DOI:10.1039/c2dt12153k

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PAPER
“Click” post-synthetic modification of metal–organic frameworks with chiral
functional adduct for heterogeneous asymmetric catalysis†
Wenting Zhu, Cheng He, Pengyan Wu, Xiao Wu and Chunying Duan*
Received 10th November 2011, Accepted 29th November 2011
DOI: 10.1039/c2dt12153k
Two enantiomeric Zn-MOFs, having L- or D-proline chiral functionality, were achieved through in situ
click reactions by modifying two opposite chiral adducts within the same pre-assembled achiral MOFs,
respectively. Both of them exhibited remarkable catalytic activities in the relative asymmetric aldol
reactions, and led to the formation of opposite enantiomorphs. The significant advantage of this approach
not only includes the conversion of chemically and thermally robust MOFs into enantiomeric chiral
material having catalytically active sites, but also involves the potential applications in asymmetric
transformations with desirable chiralities of a special enantiomorph.
MOFs can be converted to numerous catalytically active chiral
materials for a variety of asymmetric transformations by attach-
Introduction
Metal–organic frameworks (MOFs), known as coordination
polymers, are hybrid solids with infinite network structures built
from organic bridging ligands and inorganic connecting points.1
They represent a unique platform for the design of functional
materials using both molecular and crystal engineering by com-
bining highly versatile organic synthetic mythologies and well-
established strategies for the synthesis of inorganic solids.2 In
particular, analogues of homogeneous asymmetric catalysts can
be incorporated into the MOF synthesis, allowing the integration
of the selectivity of well-defined, single site catalysts into the
MOF micropores to perform or enhance the shape, size and
enantioselectivities beyond those observed in homogeneous
solution.3 And the ability to separate and reuse a heterogeneous
catalyst would be highly attractive in large scale reactions, where
separation and waste disposal can be costly. Chiral MOFs thus
are desirable compounds for heterogeneous asymmetric catalysts,
and the tenability, versatility and original flexibility of MOFs
place them at the frontier between zeolites and enzymes.4,5
ing special catalytic units, potentially to overcome the limitation
of the precatalyst approach.
On the other hand, the copper(I) catalyzed azide–alkyne
cycloaddition reaction, a so called “click reaction”,10 has
received a great deal of utility in diverse fields.11 This reaction
proceeds with excellent efficiency, with high yield and high
specificity in the presence of various functional groups and has
been applied for the generation of achiral MOFs with various
desired and tunable properties by attaching clickable groups in
the reticular MOFs.12 Herein, we described the generation of
two enantiomeric solids Zn-MOF1 and Zn-MOF2, through an
in situ click reaction between the external alkynes of a pre-
assembled achiral framework and azide groups with specially
asymmetric catalytic sites, L-N-2-azidomethylpyrrolidine
(L-AMP) or D-N-2-azidomethylpyrrolidine (D-AMP), respect-
ively, for the application on asymmetric aldol reactions
(Scheme 1).13
Several well-established strategies have been used to construct
homochiral MOFs.6 In a typical precatalyst approach, enantio-
pure organic moieties as cross-linking or chiral co-ligands were
used to control the stereochemistry for the generation of homo-
chiral MOFs.7 The strategy requires extensive efforts to syn-
thesize every framework. In an alternative and complementary
post-synthetic modification strategy,8 chiral auxiliary units con-
taining asymmetric catalytic sites were modified chemically
inside the reassembled achiral framework.9 A significant advan-
tage of this approach is that chemically and thermally robust
State Key Laboratory of Fine Chemicals, Dalian University of
Technology, Dalian, 116012, China. E-mail: cyduan@dlut.edu.cn
†Electronic supplementary information (ESI) available. CCDC reference
number 826186. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt12153k
Scheme 1 Schematic drawing of alkyne modified MOF (Zn-DPYI)
and chiral azide (L- or D-2-azidomethylpyrrolidine) as substrates for the
click reaction.
3072 | Dalton Trans., 2012, 41, 3072–3077
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Fig. 1 The asymmetry unit in the crystal structure of Zn-DPYI. Hydro-
gen atoms, solvent molecule and the disordered part are omitted for
clarity. Selected bond lengths (Å): Zn(1)–O(2B) 2.034(3), Zn(1)–O(1)
2.064(3), Zn(1)–O(3C) 2.135(3), Zn(1)–N(1) 2.142(4), Zn(1)–N(2A)
2.151(4), Zn(1)–O(4C) 2.311(3). Symmetry code A: x, y, 1 + z; B:
1 − x, 2 − y, 1 − z; C: 1 − x, 1 − y, 1 − z.
Fig. 2 Crystal structure of Zn-DPYI showing the positions of the
clickable alkyne moieties that are exposed within the 1D channels (a)
and the coordination mode of the zinc ions (b). Cyan spheres exhibit the
alkyne moieties and the red, gray, blue and green spheres represent the
oxygen, carbon, nitrogen and zinc atoms, respectively, H atoms and
lattice water molecules are omitted for clarity. (c) CD spectra of bulk
crystalline solids of Zn-MOF1 (black line) and Zn-MOF2 (red line),
showing the opposite Cotton effects of the two compounds.
Results and discussion
Synthesis and structural characterization of Zn-DPYI
Ligand dimethyl-5-(prop-2-ynyloxy)isophthalic acid (H2DPYI)
was synthesized according to the literature.14 Solvothermal reac-
tion of H2DPYI, 4,4-dipyridine and ZnCl2 (1.5 : 1 : 1 stoichi-
ometry) in a methanol aqueous solution gave compound
Zn-DPYI in a high yield (65%). Elemental analyses and powder
X-ray analysis proved the pure phase of the bulky sample. Zn-
ClO4 at 323 K in CH3OH. The click reaction was monitored by
IR spectra after isolation of the MOFs with the removal of
unreacted azide and solvents by repeated washing and drying in
vacuum. The alkyne stretching band at 3275 cm−1 decreased and
completely disappeared after 24 h (Fig. 3). Without azides, no
decrease of the alkyne band was observed under the same con-
ditions. Coupled with the NMR spectra of the reactants and pro-
ducts (Fig. 4),15 these results suggested the progressing of the
click reaction in the Zn-DPYI network (the post-synthetic yield
is about 80%). On the other hand, the C–O band at 1412 cm−1
was not changed during the reaction conditions, which indicated
that the MOF framework constructed by zinc ions and carboxy-
lates was preserved during the click reaction. Moreover, most of
the clicked Zn-DPYI crystals still kept their shape with the main
peaks in the XRD pattern being maintained after the click reac-
tion. These results suggested that the click reaction proceeded
without dissolution and reconstruction of the MOF in CH3OH.
ˉ
DPYI crystallized in an achirality space group P1 with one zinc
ion, one DPYI ligand, one 4,4′-dipyridine molecule as well as
half a solvent water molecule in a unit (Fig. 1). Each zinc(II) ion
was coordinated by two oxygen atoms in one bidentate carboxyl
group, two monodentate carboxyl groups from two different
DPYI ligands and two nitrogen donors from two different
4,4-dipyridine linkers.
As shown in Fig. 2b, two zinc(II) atoms were connected by
two bridged COO− moieties into a dimeric moiety. The dimeric
moieties were connected by pairs of DPYI ligands alternatively
to form a one-dimensional infinite ribbon. Adjacent ribbons
were further linked together by 4,4′-dipyridine molecules to con-
solidate the two-dimensional sheet. These sheets were stacked
parallel to the bc plane through weak interactions forming a non-
interpenetrating channel-like framework with a 7.5 × 8.0 Å
cross-section along the a-axis. The alkyne moieties were well-
positioned within the channels as clickable sites. They were
able to react with chiral-functional azide in situ to form the
1,3-dipolar cycloaddition product.
Aldol reactions catalyzed by Zn-MOF1 and Zn-MOF2
The catalytic activities of post-modified Zn-MOF1 in asym-
metric aldol reactions between various aromatic aldehydes and
cyclohexanone were assessed (Table 1). First, Zn-MOF1 cata-
lyzed an aldol reaction between 4-nitrobenzaldehyde and cyclo-
hexanone at room temperature in the presence of methanol and
water solvents with a decent yield (75%) and a fairly high enan-
tioselectivity (ee 71%). A control experiment with unmodified
Zn-DPYI showed less than 10% conversion and almost
undetectable ee value after 7 d (possibly catalyzed by the mild
Lewis acidic zinc centers). Interestingly, at the same reaction
Synthesis and structural characterization of Zn-MOFs
The block crystals of Zn-DPYI were treated with an excess
amount of the L-AMP adduct in the presence of Cu(CH3CN)4
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Table 1 Aldol reactions catalyzed by Zn-MOF1a
Entry Ar
Catalyst
Yield (%)b ee (%)c
1
2
3
4
4-Nitrophenyl
Zn-MOF1 75
70
n.d
26
65
n.d.
29
Zn-DPYI
L-AMP
<10
40
3-Nitrophenyl
Zn-MOF1 45
Zn-DPYI
L-AMP
12
45
2-Nitrophenyl
Zn-MOF1 22
48
Zn-DPYI
L-AMP
<10
17
n.d.
<10
n.d
n.d
n.d
Fig. 3 Monitored click reaction of Zn-DPYI and L- or D-AMP by IR
spectra.
3-Formyl-1-phenylene-
(3,5-di-tert-butylbenzoate) Zn-DPYI
L-AMP
Zn-MOF1 Trace
Trace
24
a Reactions conditions: 25 °C for seven days in 1 : 1 solution of
methanol and water using 0.5 mmol aldehyde and 5 mmol of
cyclohexanones with the catalyst about 0.045 mmol (9% mol) and co-
catalyst HAC 0.07 mmol (14% mol). b Isolated yield based on
aldehydes. c Value represents the anti-isomer; n.d. = not determined.
performed. While the reaction between 3-formyl-1-phenylene-
(3,5-di-tert-butylbenzoate), which is larger than the window size
of Zn-MOF1, and cyclohexanone was completed within 7 d, no
1
signals corresponding to the aldol product were found in the H
NMR spectra of the reaction mixture under the same experimen-
tal conditions.
Fig. 4 1H NMR spectra of the chiral compound synthesized from
dimethyl-5-(prop-2-ynyloxy)isophthalic acid (H2DPYI) and L-AMP(a),
as-synthetic MOF Zn-DPYI (b) and the post-synthetic Zn-MOF1 (c) in
the presence of DCl and DMSO-d6, respectively. The stars indicate the
signal corresponding to the triazole derivative.
The IR spectrum of the catalyst impregnated with a dichloro-
methane solution of cyclohexanone exhibited one broad C–O
stretch at 1706 cm−1. The small red shift from 1713 cm−1 (free
cyclohexanon) suggested the absorbance of cyclohexanone and
possible activation of the substrate in the channels of Zn-
MOF1.17 1H NMR of Zn-MOF1 after impregnation with a
dichloromethane solution of 4-nitrobenzaldehyde confirmed the
absorption of substrates with the molar ratio of about 1 : 1
(Fig. 5). The PXRD pattern of the guest-included Zn-MOF1 was
virtually the same as those of pristine Zn-MOF1, indicating the
maintenance of the structural integrity and open channels of
porous MOFs during the guest sorption process. Noticeably, the
removal of Zn-MOF1 by filtration after only 4 d completely shut
down the reaction, affording only 9% (from 36 to 45%)
additional conversion upon stirring for another 3 d. The exper-
iment suggests that Zn-MOF1 is a true heterogeneous asym-
metric catalyst. Solids of Zn-MOF1 can be easily isolated from
the reaction suspension by simple filtration alone and can be
reused at least three times with a slight decrease in the reactivity
and enantioselectivity.
conditions when performed under homogeneous conditions with
L-AMP acting as a catalyst resulted in much lower ee value
(26%). The better enantioselectivity in the case of the hetero-
geneous catalysts may originate from the restricted movement of
substrates in the confined microporous systems in combination
with multiple chiral inductions.4a
While the smooth reaction between cyclohexanone and
4-nitrobenzaldehyde suggested that the window size of Zn-
MOF1 is large enough to allow such large substrates to pass
through, the reaction between cyclohexanone and 3-nitrobenzal-
dehyde only gave 45% yield in the same experimental con-
ditions. Since the corresponding chiral compound exhibited
almost the same catalytic activity for both reactions, and in most
cases,16 both aldol reaction mentioneds exhibited almost the
same reaction activity, the lower yield in entry 2 compared to
that in entry 1 is possibly attributed to the molecules of 3-nitro-
benzaldehyde are wider than those of molecules of 4-nitro-
benzaldehyde. And the size selectivity might be one of the
indicators that the aldol reactions occur mostly in the cavities of
the catalyst, and not on the external surface. To make sure that
the aldol reactions occur mostly in the cavities of the catalyst,
and not on the external surface, a size selectivity study was
Importantly, the post-synthetic modification approach was able
to achieve the catalyst Zn-MOF2 having the same lattice pattern
but with the opposite handedness using D-AMP instead of
L-AMP as a chiral adduct to display the similar click reaction.
And as shown in Fig. 2c, the CD spectrum of bulk crystals of
Zn-MOF2 exhibited the exact opposite Cotton effects to those of
Zn-MOF1. While most homochiral porous solids have been pre-
pared so far via self-assembly having enantiopure organic linkers
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Table 2 Aldol reactions catalyzed by Zn-MOF1 and Zn-MOF2a
Entry
Ar
Catalysts
Yield (%)
ee (%)
1
2
3
4-Nitrophenyl
3-Nitrophenyl
2-Nitrophenyl
Zn-MOF1
Zn-MOF2
Zn-MOF1
Zn-MOF2
Zn-MOF1
Zn-MOF2
75
76
45
49
22
19
70
−73
65
−64
48
−73
a Reactions conditions were same with those in Table 1.
filtration. Yield: 65% (based on the metal salt). Anal calc. for
C42H30Zn2N4O11: C, 56.21; H, 3.37; N, 6.24%. Found: C,
55.31; H, 3.29; N, 6.29%. 1H NMR (400 MHz, DCl, DMSO-d6)
for Zn-DPYI: δ = 3.66 (S, 1H, HV), 4.96 (s, 2H, HIV), 7.73
(s, 2H, HI, HIII), 8.11 (s, 1H, HII), 8.69 (d, 4H, 4,4′-bpy), 9.23
(d, 4H, 4,4′-bpy). IR (solid KBr pellet ν/cm–1): 3441.73 br,
3275.78 s, 2113.72 w, 1620.79 s, 1608.41 s, 1586.89 s, 1558.76
s, 1485.85 w, 1454.02 m, 1412.63 s, 1386.83 s, 1319.75 w,
1268.76 w, 1243.81 w, 1213.74 w, 1127.50 w, 1105.04
w,1063.04 w, 1048.60 m, 813.69 m, 784.13 m, 727.08 m,
632.96 m.
Fig. 5 1H NMR spectra of Zn-MOF1 (DCl, DMSO-d6, top),
Zn-MOF1 adsorbed 4-nitrobenzaldehyde (DCl, DMSO-d6, middle), and
4-nitrobenzaldehyde (DMSO-d6, bottom). The red stars exhibit the
signals of the aldehyde.
and metal ions as the starting materials, the post-synthesized
modification approach with an easy clickable chiral moiety is
quite significant. This is because the homochirality of the
product is easily controlled by the chirality of the enantiopure
adduct. As shown in Table 2, the catalysts having opposite chir-
ality exhibited almost the same catalytic activity, but gave oppo-
site enantioselective products.18 It is expected that our approach
would benefit the design and synthesis of efficient enantioselec-
tive catalysts in the synthesis of fine chemicals and pharmaceuti-
cals, since the enantiomorphs having different chirality exhibited
different optical properties and biological activities.
Preparation of Zn-MOFs
In a typical synthesis, the freshly dried Zn-DPYI (200 mg,
0.22 mmol) was placed into a vial (25 mL capacity) with 10 mL
MeOH, L- or D-AMP (305 mg, 2.4 mmol) and an amount of
Cu(CH3CN)ClO4 (45 mg, 0.22 mmol) and the mixture was
stirred continuously for 24 h. After decantation, the supernate
was removed. The solid was washed three times by MeOH
(× 8 mL) and three times by CH2Cl2 (× 8 mL) in order to
remove unreacted substrates. The solid was then dried under
vacuum at room temperature to yield the final yellow compound.
1H NMR spectra suggested that the efficiency of the click reac-
tion is about 80% (400 MHz, DCl, DMSO-d6): δ = 1.72
(m, 0.8H, H8), 1.98 (m, 1.6H, H8, H9), 2.12 (m, 0.8H, H9),
3.15–3.25 (m, 1.6H, H10), 3.66 (br, about 0.2H, HV), 4.00
(m, 0.8H, H7), 4.85 (m, 1.6H, H6), 4.96 (br, 0.4H, HIV), 5.33
(s, 1.6H, H4), 7.73 (s, 0.4H, HI, HIII), 7.78 (s, 1.6H, H1, H3),
8.10 (s, 1H, H2, HII), 8.45 (s, 0.8H, H5), 8.79 (br, 4H, 4,4′-
bpy), 9.37 (br, 4H, 4,4′-bpy). IR (solid KBr pellet ν/cm–1):
3441.22 br, 1610.40 s, 1576.82 s, 1452.80 m, 1415.53 m,
1384.37 s, 1262.94 w, 1220.20 w, 1047.55 m, 813.57 m, 783.03
w, 727.35 m, 645.46 m.
Experimental
Instruments and reagents
1H NMR spectra were measured on a Varian INOVA 400 M
spectrometer. The elemental analyses of C, H and N were per-
formed on a Vario EL III elemental analyzer. The powder XRD
diffractograms were obtained on a Riguku D/Max-2400 X-ray
Diffractometer with Cu sealed tube (λ = 1.54178 Å). Thermo-
gravimetric analysis (TGA) was carried out at a ramp rate of
5 °C min−1 in a nitrogen flow with a Mettler-Toledo TGA/
SDTA851 instrument. FT-IR spectra were recorded as KBr
pellets on JASCO FT/IR-430. The CD spectra were measured on
JASCO J-810. HPLC analysis was performed on Agilent 1100
using a ChiralPak AD-H column purchased from Daicel Chemi-
cal Industries, Ltd. Products were purified by flash column
chromatography on 200–300 mesh silica gel and Kromasil Lc-80
using SiO2 chromatographic column.
Preparation of Zn-DPYI
A
mixture of dimethyl-5-(prop-2-ynyloxy)isophthalic acid
(H2DPYI) (330 mg, 1.5 mmol), ZnCl2 (136 mg, 1 mmol) and
4,4-dipyridine (156 mg, 1 mmol) were dissolved in the presence
of H2O and methanol with NaOH (pH = 7.0) in a screw capped
vial. The resulting mixture was placed in an oven at 100 °C for
3 days. Colorless block crystals with were obtained after
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Click reaction of H2DPYI
2.15–2.08 (m, 1H), 1.86–1.79 (m, 1H), 1.66–1.52 (m, 3H),
1.44–1.37 (m, 1H).
The same protocol as above. Dimethyl-5-(prop-2-ynyloxy)
isophthalic acid (H2DPYI) (110 mg, 0.5 mmol) was dissolved in
10 mL MeOH, L-AMP (89 mg, 0.7 mmol) and an amount of
Cu(CH3CN)ClO4 (61 mg, 0.3 mmol) and the mixture was stirred
continuously for 24 h. The mixture was concentrated under
vacuum and the organics were extracted by CH2Cl2 and washed
with water, saturated NaHCO3(aq) and brine. After drying over
Catalyzed by Zn-MOF1: Enantiomeric excess was determined
by HPLC with a Chiralpak AD-H column (hexane/2-propanol =
92 : 8), 210 nm, 1 mL min−1, tR1 = 51.644 min (major), area (%)
= 82.2903; tR2 = 66.431 min (minor), area (%) = 17.7097.
Catalyzed by Zn-MOF2: Enantiomeric excess was determined
by HPLC with a Chiralpak AD-H column (hexane/2-propanol =
92 : 8), 210 nm, 1 mL min−1, tR1 = 55.312 min (minor), area (%)
= 17.8229; tR2 = 70.265 min (major), area (%) = 82.1771.
1
Na2SO4, the solvent was removed under reduced pressure. H
NMR of the chiral ligand synthesised from dimethyl-5-(prop-2-
ynyloxy)isophthalic acid (H2DPYI) and L-AMP: (400 MHz,
DMSO-d6): δ = 1.66–1.71 (m, 1H), 1.87–1.95 (m, 2H),
2.07–2.10 (m, 1H), 3.15–3.20 (m, 2H), 3.97 (m, 1H), 4.77–4.79
(m, 2H), 5.32 (m, 2H), 7.70 (S, 2H), 8.10 (S, 1H), 8.37 (S, 1H).
anti-2-(Hydroxy-(o-nitrophenyl)methyl)cyclohexan-1-one19
1H NMR (400 MHz, CDCl3): δ = 7.87, 7.84 (d, 1H), 7.79, 7.77
(d, 1H), 7.66–7.62 (m, 1H), 7.45–7.43(m, 1H), 5.46, 5.44
(d, 1H), 4.18 (s, 1H), 2.77–2.73 (m, 1H), 2.48–2.44 (m, 1H),
2.40–2.30 (m, 1H), 2.14–2.06 (m, 1H), 1.90–1.81 (m, 1H),
1.66–1.52 (m, 4H).
Typical procedure for the aldol reaction using the catalyst in
heterogeneous manner
Catalyzed by Zn-MOF1: Enantiomeric excess was determined
by HPLC with a Chiralpak AD-H column (hexane/2-propanol =
95 : 5), 254 nm, 1 mL min−1, tR1 = 80.233 min (major), area (%)
= 73.7532; tR2 = 85.589 min (minor), area (%) = 26.2468.
Catalyzed by Zn-MOF2: Enantiomeric excess was determined
by HPLC with a Chiralpak AD-H column (hexane/2-propanol =
95 : 5), 254 nm, 1 mL min−1, tR1 = 71.871 min (minor), area (%)
= 13.6598; tR2 = 77.743 min (major), area (%) = 86.3402.
To a mixture of ketone (5 mmol) and aromatic aldehyde
(0.5 mmol) catalyst (about 0.045 mmol) was added and the
resulting mixture was stirred at rt for seven days. After com-
pletion of the reaction, it was extracted with EtOAc (3 × 2 mL).
The combined organic extracts were dried over Na2SO4 and
evaporated to dryness. Diastereoselectivity and conversion were
determined by 1H NMR analysis. The crude product was column
chromatographed over silica gel (200–300 mesh) and Kromasil
Lc-80 using SiO2 chromatographic column to afford pure aldol
product. The enantiomeric excess (ee) was determined by HPLC
analysis. The catalyst was washed repeatedly with EtOAc
(3 × 2 mL), dried and reused when required.
X-Ray crystallographic characterization
Intensity data of Zn-DPYI was collected on Bruker Smart
APEX II-CCD single crystal X-ray diffractometer equipped with
graphite-monochromated MoKα radiation (λ = 0.71073 Å) using
the SMART and SAINT programs.21 The structure was solved
by direct methods and refined on F2 by full-matrix least-squares
methods with SHELXTL-97.22 Crystal data of Zn-DPYI:
anti-2-(Hydroxy-(p-nitrophenyl)methyl)cyclohexan-1-one19
1H NMR (400 MHz, CDCl3): δ = 8.23, 8.21 (d, 2H), 7.53, 7.51
(d, 2H), 4.92, 4.90 (d, 1H), 4.08 (s, 1H), 2.62–2.59 (m, 1H),
2.53–2.49 (m, 1H), 2.42–2.30 (m, 1H), 2.17–2.08 (m, 1H),
1.86–1.82 (m, 1H), 1.61–1.50 (m, 3H), 1.44–1.33 (m, 1H).
ˉ
C42H30Zn2N4O11, M = 897.44, triclinic, space group P1, colour-
less block, a = 9.4392(7), b = 10.0591(7), c = 11.4120(9) Å,
α = 100.49(1)°, β = 106.83(1)°, γ = 103.43(1)°, V = 972.0(1)
Å3, Z = 1, Dc = 1.533 g cm−3, μ(Mo-Kα) = 1.302 mm−1, T
= 293(2)K. 3411 unique reflections [Rint = 0.0480]. Final R1
[with I > 2σ(I)] = 0.0576, wR2 (all data) = 0.1568. GOOF =
1.025. CCDC number 826186. Non-hydrogen atoms were aniso-
tropically refined, except for the disordered solvent molecules.
Hydrogen atoms were geometrically fixed and refined by the use
of a riding model. Four carbon atoms of the phenyl ring and
the prop-2-ynyloxy groups were disordered into two parts with
the site occupied factor (s. o. f.) of each part being fixed on 0.5,
respectively.
Catalyzed by Zn-MOF1
Enantiomeric excess was determined by HPLC with a Chiralpak
AD-H column (hexane/2-propanol = 90 : 10), 254 nm, 1 mL
min−1, tR1 = 57.116 min (minor), area (%) = 14.7683; tR2
=
75.654 min (major), area (%) = 85.2317.
Catalyzed by Zn-MOF2
Enantiomeric excess was determined by HPLC with a Chiralpak
AD-H column (hexane/2-propanol = 90 : 10), 254 nm, 1 mL
Conclusions
min−1, tR1 = 51.586 min (major), area (%) = 86.3812; tR2
=
68.169 min (minor), area (%) = 13.6188.
In a summary, we reported a new strategy to achieve enantio-
meric metal organic frameworks Zn-MOFs through in situ click
reactions to modify two opposite chiral adducts within the same
pre-assembled achiral MOFs. Both two enantiomeric Zn-MOFs,
having L- or D-proline chiral functionality, exhibited remarkable
catalytic activities in the relative asymmetric aldol reactions, and
led to the formation of opposite enantiomorphs, respectively.
anti-2-(Hydroxy-(m-nitrophenyl)methyl)cyclohexan-1-one20
1H NMR (400 MHz, CDCl3): δ = 8.21–8.15 (m, 2H), 7.68, 7.66
(d, 1H), 7.55–7.51 (m, 1H), 4.91, 4.89 (d, 1H), 4.12 (s, 1H),
2.66–2.60 (m, 1H), 2.53–2.49 (m, 1H), 2.42–2.33 (m, 1H),
3076 | Dalton Trans., 2012, 41, 3072–3077
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The significant advantage of this approach not only includes the
conversation of chemically and thermally robust MOFs into
enantiomeric chiral material having catalytically active sites, but
also involves the potential applications in asymmetric trans-
formations with desirable chiralities of a special enantiomorph.
Further studies will focus on the modification of the achiral
MOFs towards other enantioselective versions of these and other
reactions, as well as on investigating the potential catalytic
activities arising upon the metal ions through exchange of Zn(II)
for other reactive metal centers.
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Acknowledgements
We gratefully acknowledge financial support from the NSFC
(21171029 and 21025102).
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