Confinement-Driven Enantioselectivity in 3D Porous Chiral Covalent Organic Frameworks

Article abstract of DOI:10.1002/anie.202013926

3D covalent organic frameworks (COFs) with well-defined porous channels are shown to be capable of inducing chiral molecular catalysts from non-enantioselective to highly enantioselective in catalyzing organic transformations. By condensations of a tetrahedral tetraamine and two linear dialdehydes derived from enantiopure 1,1′-binaphthol (BINOL), two chiral 3D COFs with a 9-fold or 11-fold interpenetrated diamondoid framework are prepared. Enhanced Br?nsted acidity was observed for the chiral BINOL units that are uniformly distributed within the tubular channels compared to the non-immobilized acids. This facilitates the Br?nsted acid catalysis of cyclocondensation of aldehydes and anthranilamides to produce 2,3-dihydroquinazolinones. DFT calculations show the COF catalyst provides preferential secondary interactions between the substrate and framework to induce enantioselectivities that are not achievable in homogeneous systems.

Full text of DOI:10.1002/anie.202013926

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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International Edition
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Accepted Article
Title: Confinement-Driven Enantioselectivity in 3D Porous Chiral
Covalent Organic Frameworks
Authors: Bang Hou, Shi Yang, Kuiwei Yang, Xing Han, Xianhui Tang,
Yan Liu, Jianwen Jiang, and Yong Cui
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202013926
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10.1002/anie.202013926
Angewandte Chemie International Edition
RESEARCH ARTICLE
Confinement-Driven Enantioselectivity in 3D Porous Chiral
Covalent Organic Frameworks
Bang Hou+, Shi Yang+, Kuiwei Yang, Xing Han, Xianhui Tang, Yan Liu,* Jianwen Jiang* and Yong Cui*
[*] B. Hou,[+] S. Yang, [+] X. Han, X. Tang, Prof. Y. Liu, Prof. Y. Cui
School of Chemistry and Chemical Engineering, Frontiers Science Center for Transformative Molecules and State Key Laboratory of Metal Matrix
Composites
Shanghai Jiao Tong University, Shanghai 200240 (China)
E-mail: liuy@sjtu.edu.cn, yongcui@sjtu.edu.cn.
K. Yang, Prof. J. Jiang
Department of Chemical and Biomolecular Engineering
National University of Singapore, 117576 Singapore
Email: chejj@nus.edu.sg
[+] These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: Covalent organic frameworks (COFs) show great potential
in heterogeneous catalysis, but the confinement effect of frameworks
on molecular catalysts has yet to be explored. Here we demonstrate
the utilization of 3D COFs with well-defined porous channels capable
of inducing chiral molecular catalysts from non-enantioselective to
highly enantioselective in catalyzing organic transformations, sharply
different from the typical methods for tailoring enantioselectivities by
varying steric and electronic properties of molecular catalysts. By
condensations of a tetrahedral tetraamine and two linear dialdehydes
derived from enantiopure 1,1'-binaphthol (BINOL), two chiral 3D
COFs with a 9-fold or 11-fold interpenetrated diamondoid framework
are prepared. Obviously enhanced Brønsted acidity was observed for
the chiral BINOL units that are uniformly distributed within the tubular
channels compared to the non-immobilized acids. This facilitates the
Brønsted acid catalysis of cyclocondensation of aldehydes and
anthranilamides to produce 2,3-dihydroquinazolinones. While
homogeneous BINOL controls display no enantioselectivity and/or
low activity, constraint of their conformations in CCOFs leads to up to
91% isolated yield with 97% ee. DFT calculations show COF catalyst
provide preferential secondary interactions between the substrate and
framework to induce enantioselectivities that are not achievable in
homogeneous systems.
methodology for the generation and enhancement of BINOL and
its derivatives is of great importance in asymmetric catalysis. Un-
functionalized BINOL and its derivatives, an important class of
moderate Brønsted acids,[6] can only promote limited number of
asymmetric
transformations
to
produce
satisfactory
enantioselectivites.[7] Here we demonstrated that 3D covalent
organic frameworks (COFs) can be utilized as an attractive
platform to manipulate enantioselectivities of molecular catalysts,
which are typically tailored by varying steric and electronic
properties of molecular catalysts.[8]
COFs are a class of network solids constructed by organic
molecules through strong covalent bonds in a process named
reticular synthesis.[9] An outstanding feature of COF chemistry is
that their robust porosity, stability, and chemical functionality can
be controlled by the judicious selection of organic building
blocks.[10] This characteristic has paved the way for their use in
diverse applications, including gas separation and storage,[11]
sensing,[12] catalysis,[13] and drug release.[14] Importantly, by
condensation of chiral and achiral monomers, a few chiral COFs
(CCOFs) can be prepared for enantioselective processes.[15-17] In
particular, we and others have shown that well-defined
homogeneous (pre)catalysts can be incorporated into COFs in a
systematic fashion to generate single site heterogeneous
catalysts with activities and selectivities rivaling those of their
homogeneous
analogs.[17]
Nevertheless,
examples
of
Introduction
enantioselective reactions catalyzed by COFs are still very limited
and the confinement effect of frameworks on molecular catalysts
has yet to be explored.[18] In this work, we reported that
incorporating non-enentioselective BINOLs into conformationally
rigid pores of 3D COFs can induce highly enantioselectivity in the
Since its first reported use in asymmetric catalysis by Noyori in
1979, optically pure 1,1'-binaphthol (BINOL), together with other
axially chiral biaryldiols, has become one of the most widely used
ligands/catalysts[1,2] and also an attractive platform for chiral
recognition and optics.[3,4] Remarkably, in the past decades, a
variety of effective catalysts with the BINOL scaffold have been
designed and utilized in numerous asymmetric catalytic organic
transformations, especially with the 3,3′-functionalized BINOL
derivatives.[5] In general, the enantioselectivity of reactions is
heavily dependent on the steric and electronic properties of
substituents in the 3,3'-positions of the BINOL rings, which have
a significant effect on substrate activation and structures of
transition states and intermediates.[1] So, exploitation of a new
catalytic
synthesis
of
the
practically
important
dihydroquinazolinones from aldehydes and anthranilamides.[19]
To this end, chiral linker 6,6'-dichloro-2,2'-dihydroxy-1,1'-
binaphthyl-4,4'-dialdehyde (BDA)
and
6,6'-dichloro-4'-(4-
formylphenyl)-2,2'-dihydroxy-1,1'-binaphthyl-4-aldehyde (BPDA),
each of which contains the dialdehyde primary functionality and
chiral 2,2'-dihydroxy secondary functionality, was chosen and
synthesized. By imine condensations of the chiral monomers and
tetra(p-aminophenyl)methane (TAM), a pair of 3D BINOL-based
CCOFs were prepared (Scheme 1). Crystal structures of the as-
1
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Scheme 1. Synthesis of the Two 3D CCOFs.
(b)
(a)
(c)
(d)
Figure 1. PXRD patterns of a) CCOF 15 and b) CCOF 16 with the experimental profiles in red, Pawley refined in black, calculated in blue, and the difference
between the experimental and refined PXRD patterns in dark green. N2 adsorption−desorption isotherms (77 K) and pore size distribution profiles (insert) of c)
CCOF 15 and d) CCOF 16.
2
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prepared CCOFs were determined by powder X-ray diffraction
and modeling studies, as well as pore size distribution analysis.
attributed to the (220), (130), (400), (240), (321), (600), (341),
(521), (451) and (361) reflection planes, respectively. For 16, the
first and most intense peak corresponding to the (200) reflection
plane appears at 4.71°, with other minor peaks at 3.33°, 4.75°,
6.66°, 7.45°, 9.40°, 17.74°, 19.02°, and 20.12°, attributed to the
(110), (200), (220), (310), (400), (521), (611) and (631) reflection
planes, respectively. Notably, some peaks after 2θ ˃ 10° in 15
were violent, while those of 16 showed low intensity, revealing the
impact of different linear linkers on the crystalline frameworks.
Also, PXRD patterns were calculated for the two COFs on the
other structures, but all of the calculated patterns did not match
the experimental patterns well (Figure S8 and S9). CCOFs 15 and
16 are thus proposed to have the architectures shown in Figure 3.
The BINOL units in combination with TAM induced the formation
of a diamond network with 1D open channels of 0.82 nm/1.05 nm
for 15 and 1.20 nm/1.52 nm for 16, respectively. The porosity and
surface areas were measured by N2 adsorption and desorption
analysis at 77 K (Figure 1c, d). Both CCOFs show a sharp uptake
at a low pressure of P/P0 < 0.05, indicative of their microporous
nature. The Brunauer-Emmett-Teller (BET) surface areas were
calculated to be 631 and 825 m2 g-1 for 15 and 16, respectively,
and the total pore volumes were 0.51 cm3 g-1 and 0.53 cm3 g-1 at
P/P0 = 0.99. The pore size distribution analysis were calculated
by using the nonlocal density functional theory (NLDFT) to give
rise to a mean pore width 8 Å, 11 Å for 15 and 12 Å, 15 Å for 16,
which are in good agreement with those of the proposed models
(Figure 1c and 1d).
The chemical stability of the CCOFs was examined by PXRD
and N2 sorption isotherms after one day of treatment in boiling
water, HCl (aqueous), and NaOH (aqueous). The experiment
shows that both CCOFs were stable in boiling water and
maintained good crystallinity with a small decrease in decreased
surface areas compared to the pristine samples (Figures S6 and
S7). It was found that CCOF 15 was stable in 0.1 M HCl (aqueous)
and 0.1 M NaOH (aqueous), whereas 16 was stable in 0.01 M HCl
(aqueous) and 0.01 M NaOH (aqueous), though a small decrease
in the signal-to-noise ratio of the PXRD peaks and a little bit
decrease of surface areas was observed (Figures S6 and S7). So,
CCOF 15 showed improved alkali resistance and antihydrolysis
capability relative to 16, consistent with that the shorter building
block could guarantee the rigidity of the framework better.[20]
The two isostructural COFs adopt
a 9-fold and 11-fold
interpenetrated diamondoid open framework with about 8 and 11
Å wide tubular channels, respectively. All BINOL hydroxyls in the
frameworks are periodically aligned within the channels can be
used
as
efficient
heterogeneous
catalysts
for
the
cyclocondensation of aldehydes and anthranilamides with high
enenatioselectivity. DFT calculations suggest that the 3D porous
framework offers a chiral confined microenvironment that dictates
enantioselectivity of the catalysis.
Results and Discussion
As shown in Scheme 1, CCOFs 15 and 16 were synthesized by
solvothermal reactions of enantiopure (R)-BDA (0.063 mmol) or
(R)-BPDA (0.063 mmol) and TAM (0.032 mmol) in 1.0 mL
methanol or 1.0 mL n-butanol and mesitylene (1:1 v/v) in the
presence of acetic acid (6 M, 0.1 mL) at 120 °C for 3 days, which
afforded yellow or orange polycrystalline solids in 86% and 79%
yields, respectively.
In the FT-IR spectra of 15 and 16, the characteristic C=O
stretching bands (1680 and 1686 cm-1) were barely present,
indicative of the consumption of the aldehydes. The appearance
of characteristic C=N stretching band (1625 and 1624 cm-1) was
observed, thus indicating the formation of imine linkages (Figure
S1). The 13C cross-polarization magic-angle spinning (CP/MAS)
NMR signals of CCOF can be explicitly assigned as the proposed
structure. Specifically, the typical signals at about 160 ppm
indicated the successful formation of imine bonds for CCOFs 15
and 16 (Figure S2). Circular dichroism (CD) spectra of CCOFs 15
and 16 made from R and S enantiomers of BINOL monomers are
mirror images of each other, which is indicative of their
enantiomeric nature (Figure S3). Thermal gravimetric analysis
(TGA) reveals that both COFs start to decompose at around
400 °C (Figure S4). Scanning electron microscopy (SEM) images
showed CCOF 15 possesses a rod-like morphology with an
average particle size of 2 μm while CCOF 16 possesses a slake
shaped crystals with an average particle size of 2 μm × 3 μm
(Figure S5).
The crystal structures of CCOFs 15 and 16 were determined by
powder X-ray diffraction (PXRD) analysis with Cu Kα radiation in
conjunction with structural simulations (Figure 1). Referring to the
previous report and considering the chiral structure of these COFs,
we have simulated several different structures based on the
reticular chemistry. After the geometrical energy minimization by
using the Materials Studio software package, the detailed
simulation (Figures S8 and S9) suggested that CCOFs 15 and 16
were proposed to adopt a 9-fold and 11-fold interpenetrated dia
topology with the chiral I41 space group, respectively. Full profile
pattern matching Pawley refinements for the CCOFs were carried
out and the refinement results yield unit cell parameters that are
nearly equivalent to the predictions with good agreement factors
(a = b = 30.97 Å, c = 7.57 Å, α = β = γ = 90°, Rwp = 2.51%, and Rp
= 1.88% for 15; a = b = 37.58 Å, c = 7.15 Å, α = β = γ = 90°, Rwp
= 2.64%, and Rp = 1.94% for 16).
CCOF 15 shows high crystallinity, exhibiting the first intense
peak at a low angle 5.70° (2θ), which corresponds to the (200)
reflection plane, along with minor peaks at 8.08°, 9.04°, 11.45°,
12.80°, 15.63°, 17.20°, 18.54°, 19.37°, 21.79°, and 22.54°,
Figure 2. The acidities of the CCOFs and related BINOL acids that are
determined by the Hammett indicator method or UV-vis spectrophotometric
titration method.
3
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(c)
(a)
(b)
(d)
(e)
(f)
(g)
(h)
Figure 3. Structural representations of the two COFs. a, b) An adamantine-like cage in CCOF 15 and its space-filling model. d, e) An adamantine-like cage in
CCOF 16 and its space-filling model. c, f) Interpenetration of nine and eleven diamond nets in 15 and 16. g, h) The 3D structure of CCOFs 15 and 16 viewed along
the c-axis. C gray; N blue; Cl green; H white; O red.
4
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Table 1. Asymmetric Acetalization of 2-Aminobenzamide and Aldehydesa
aReaction conditions: 2-aminobenzamides (0.1 mmol), aldehyde (0.12 mmol), catalyst (COF: 10 mol %, homogeneous catalyst: 10 mol%), MgSO4 (100 mg,
dehydrating agent), CH3CN (1.0 mL), 40 °C, 24 h . bIsolated yield. cDetermined by HPLC. dG1 = 1-pyreneyl, G2 = 9-phenanthrene, G3 = 4-anthracylphenyl, G4 = 3,5-
bis(3,5-di-tert-butylbenzyloxy)phenyl. eBDN and BPDN are derivatives of BDA and BPDA, respectively, whose aldehyde groups were protected with neopentylglycol.
The Brønsted acidity of a catalyst is of significant importance
for catalytic performances, and so we measured the Brønsted
acidity of CCOFs and related BINOL acids by the Hammett
indicator method.[21a] Also, the Brønsted acidity of BINOL
pharmacological activities such as antibiotic, antidefibrillatory,
vasodilatory and analgesic efficacy.
After screening various reaction conditions including catalyst
loading, reaction time, solvent, and temperature (Table S3), we
found that CCOF 15 can be an active catalyst for the
condensation and cyclization of 2-aminobenzamide with
aldehydes. Especially, 10 mol% loading of the COF catalyzed the
condensation/amine addition of 4-chloro-benzaldehyde to
produce the desired molecule with 91% yield and 97% ee in
CH3CN at 40 °C after 24 h (Table 1, entry 3). Under the optimized
conditions, we used other benzaldehyde derivatives to extend the
substrate scope. As shown in Table 1, all examined reactions
reached completion within 24 h and gave corresponding DHQZ
products in good to excellent enantioselectivities (71-97% ee) and
80-92% yields. It was shown that benzaldehydes bearing
electron-withdrawing groups with 2-aminobenzamide afforded the
DHQZ products in higher yields and ee values than the ones with
electron-rich substituents (Table 1, entries 2-8). In sharp contrast,
despite the isostructural porous structure, CCOF 16 exhibited
much lower enantioselectivity than CCOF 15 in promoting the
same reactions, presumably due to the larger channels that has
weaker enantioselective induction ability (Table 1, entries 14-15).
It is worth noting that the main product configuration catalyzed by
(R)-16 is opposite to the product catalyzed by (R)-15, consistent
with that the porous structures of solid catalysts play an important
role in the configuration of products.[23a] The enantioselectivity
monomers
was
measured
in
CH3CN
by
UV-vis
spectrophotometric titration method.[21b] As shown in Figure 2, all
monomers BDA, BINOL, and BPDA have weak acidity, with H0 >
6.8 and pKa values of 7.8, 8.1, and 8.9, respectively. The order of
acidity for three monomers is consistent with the installation of
different electron-withdrawing groups on BINOL skeleton. After
incorporating into COFs, the acidity of BINOL derivatives was
obviously enhanced (4.0 ≤ H0 ≤ 4.8 for both). The enhanced
acidity may be ascribed to condensations of aldehyde groups of
the BINOL linkers to amine groups of TAM, which can facilitate
electrons delocalize over a conjugated structure and prevent the
isolated BINOL hydroxyl groups forming intermolecular hydrogen
bonding and promoting proton transfer.[22] Further research is
underway to better understand the origin of the enhanced
Brønsted acidity of COFs.
Inspired by the highly crystalline nature and accessible pores of
the two CCOFs, as well as the chiral channels and the rich
hydroxyl protons active sites in frameworks, we have evaluated
their heterogeneous catalytic properties in the asymmetric
acetalization of 2-aminobenzamides with aldehydes, which is the
highly effective method for the synthesis of optically pure 2,3-
dihydroquinazolinone (DHQZ).[18,22] As a privileged scaffold, the
DHQZ family of compounds are of critical importance for
5
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(a)
(b)
(c)
(d)
Figure 4. a) PXRD patterns b) N2 adsorption-desorption isotherms of CCOF 15 upon treatment in different conditions. c) Plots of ee values and conversions with
10 mol % CCOF 15 and of BDN. d) Recycling results of CCOF 15 in the acetalization of 2-aminobenzamides with 4-chlorobenzaldehyde.
(a)
(b)
(c)
Figure 5. Relative Gibbs energy profiles at 40 C for the intramolecular amidation of imine to (S)- and (R)-DHQZ on a) (R)-BDA and b) (R)-CCOF 15. c) Simplified
structures for (S)-DHQZ production on (R)-BDA. The breaking/forming bonds are labelled in orange, and the hydrogen bonds in red (bond lengths in Å). A/A’: imine
in adsorbed state; TS1/TS1’: transition state for nucleophilic attack of the N atom of amide group on imine C; B/B’: cyclization intermediate; TS2/TS2’: transition
state for H shift from NH2 to N; (S)/(R)-DHQZ in adsorbed state. C: grey, H: white, O: red, N: blue, F: light blue and Cl: green.
6
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reversal may result from the chiral environment of the (R)-16
channel, similar to enzymatic catalysis in which the product
selectivity is controlled by the enzyme pocket.[24]
subjected to the reaction, 15 afforded the targeted product in 61%
yield, lower than the 79% yield obtained with 16. Furthermore, for
the
more
bulky
substrate
3,5-bis
(3,5-di-tert-butyl)
The different catalytic performances of the CCOFs and related
homogeneous catalysts were investigated. To eliminate the
influence of aldehydes on the catalytic reaction, monomers BDP
and BPDP were converted into BDN and BPDN by protected their
aldehyde groups with neopentylglycol. As shown in Table 1
(entries 21-29), with 10.0 mol% loading of BDN (the same loading
of BINOL as the COF catalyst), the reaction of anthranilamide with
seven different aromatic aldehydes proceeded smoothly,
affording the targeted products in 88-94% yields, but with no
enantioselectivity in all cases. However, when BPDN was used
as a catalyst, the reactions cannot take place at all, even with
prolonged reaction times (48 h). This is probably due to its
extremely weak acidity (pKa = 8.9) that cannot activate the
substrates. An in-depth investigation of the reason is still
underway. Control experiments showed that pure BINOL
exhibited similar catalytic performance to BDN in promoting the
above reactions (Table 1, entries 38-40). As mentioned above,
the presence of catalytic amount of CCOFs can afford the desired
chiral DHQZ in high yield and enantioselectivity for CCOF 15 and
moderate yield and enantioselectivity for CCOF 16. Thus, these
findings suggested that the porous frameworks containing special
chiral cavities constructed from chiral BINOL-derived monomers
and TAM are essential for enantioselective generation of chiral
DHQZ, while the homogenous catalysts are incapable of
providing stereocontrol on the products. Moreover, incorporation
of BPDA units into the COF can obviously enhance the Brønsted
acidity and endow them with catalytic activity.
To further understand the catalytic process of the COF and
homogeneous control, we monitored the dynamic process in the
synthesis of 2-(4-chlorophenyl)-2,3-DHQZ by CCOF 15 or BDN.
As shown in Figure 4c, the transformations displayed different
reaction kinetics. The reactions catalyzed by CCOF 15 and BDN
were completed in 12 h and 20 h, respectively. In particular, the
ee values of the products were around 97% from the beginning to
the end for CCOF 15, and the values were always 0% for BDN.
The COF-catalyzed reactions needed longer time can be ascribed
to slow mass diffusion in the porous solid catalyst.
Recycle experiments were conducted to examine the
heterogeneity and recyclability of the COF catalyst. Upon
completion of the catalytic reaction, CCOF 15 could be recovered
by centrifugation and reused at least for ten times without any
obvious loss of its activity and enantioselectivity (Figure 4d). The
PXRD pattern and N2 absorption and desorption isotherms
showed that CCOF 15 remained its crystallinity and porosity after
ten catalytic recycles, though there was a slightly decreased
signal-to-noise ratio and a little bit decrease of surface areas (436
m2g-1), as shown in Figures 4a and 4b.
benzyloxybenzaldehyde (14 × 21 Å2), only trace yield of the
product was detected catalyzed by CCOF 15, which was much
lower than the 85% yield obtained with BDN. This very low yield
is probably attribute to that the very bulky substrate cannot access
the catalytically active Brønsted acid sites in the CCOF cavity
through the windows (8 × 11 Å2) because of its large diameter.
Taken together, the above results indicate that the reaction mainly
occurs inside the pores
To rationalize the difference in the enantioselectivity of DHQZ
observed experimentally, we performed density functional theory
(DFT) calculations for the reaction catalyzed by (R)-BDA and (R)-
15 (Figures S17 and S18), respectively. Previous studies have
suggested the enantioselectivity of DHQZ is determined by the
intramolecular amidation of imine,[18] which is also the focus in our
calculations. The DFT calculations illustrated in Figures S18 and
S19 indicate the intramolecular amidation of imine consists of two
steps. A cyclization intermediate is firstly generated through
nucleophilic attack of the N atom of amide group on imine C, and
then DHQZ is produced along with H shift from NH2 to N. For
intramolecular amidation of imine on (R)-BDA (it was assumed
that the aldehyde groups of BDA did not involve the reactions),
the imine interacts with (R)-BDA through hydrogen bond between
the N atom of the reactant and the hydroxyl group of (R)-BDA, in
addition to π-π interaction. As shown in Figure 5a, the overall
barriers are predicted to be 48.4 and 48.0 kcal/mol for (S)- and
(R)-DHQZ, respectively, which indicates
a
rather low
enantioselectivity. For intramolecular amidation of imine on (R)-
15 (Figures S20 and S21), the imine is located in the channel and
interacts with (R)-15 also through hydrogen bond. The overall
barriers for (S)- and (R)-DHQZ production are computed to be
46.7 and 47.6 kcal/mol, respectively (Figure 5b). Since the barrier
difference on (R)-15 (0.9 kcal/mol) is obviously larger than that on
(R)-BDA (0.4 kcal/mol), a higher enantioselectivity thus can be
expected on (R)-15. This is qualitatively consistent with the
experimental observed trend. Therefore, the confinement effect in
(R)-15 makes intramolecular amidation of imine via Si face more
favourable, which is responsible for the preferential production of
(S)-DHQZ in (R)-15.
A
variety of chiral solid Brønsted acids including solid
phosphoric acid have been explored as heterogeneous catalysts
for the enantioselective synthesis of 2,3-
dihydroquinazolinone,[22,25] but in most cases, only moderate to
excellent enantioselectivities were Diol
observed.[25c]
organocatalysts typically cannot enantioselectively catalyzing
acetalization reactions of 2-aminobenzamide and aldehydes.[18]
Remarkably, the present 3D COFs enable the nonselective chiral
BINOL to enantioselectively catalyze the acetalization of 2-
aminobenzamides and aldehydes to produce enantiomeric purity
DHQZ. The ee values of this COF-based protocol are higher or
comparable well to those of the enantioselective MOF-phosphoric
acid heterogenous catalysts[18,22] and even homogeneous
phosphoric acid catalysts,[19] as summarized in Table S4. To the
best of our knowledge, this is the first report on utilization of the
COF platform to boost a homogeneous catalyst from completely
nonselective to highly selective.[18] Further research on using the
CCOF catalysts with different types of acid-active sites for more
important and challenging catalytic reactions is in progress.
To study the role of the CCOF channels in catalysis, several
aromatic aldehydes with different sizes were selected and
subjected to the reactions. As illustrated in Table 1, the BDN
promoted condensation and cyclization of 2-aminobenzamide
with the bulky aldehydes to produce DHQZ in high yields (Table
1, entries 30-33). In contrast, the yields of the reaction products
catalyzed by CCOF 15 gradually decreases when the sizes of the
aldehyde substrates increases (Table 1, entries 10-13). When 9-
phenanthrenecarbaldehyde, which has the size (9 Å x 11 Å)
between the channel diameters of CCOFs 15 and 16, was
7
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[7]
a) W.Y. Han, J. Zuo, X. M. Zhang, W. C. Yuan, Tetrahedron 2013, 69,
537; b) L. Bayeh, P. Q. Le, U. K. Tambar, Nature 2017, 547, 196.
D. J. Gorin, B. D. Sherry, F. D. Toste, Chem. Rev. 2008, 108, 3351.
a) A. P. Cꢀtꢁ, A. I. Benin, N. W. Ockwig, M. O’keeffe, A. J. Matzger, O.
M. Yaghi, Science 2005, 310, 1166; b) C. S. Diercks, O. M. Yaghi,
Science 2015, 355, 6328.
Conclusion
[8]
[9]
We have reported two 3D CCOFs with interpenetrated open
frameworks that were prepared by imine condensation of
tetrahedral tetraamine and chiral BINOL dialdehydes. The BINOL
hydroxyl groups that are periodically aligned within the tubular
channels exhibit greatly enhanced acidity relative to the free acids
and can function as heterogeneous Brønsted acid catalysts for
the asymmetric acetalization of aromatic aldehydes and 2-
aminobenzamide to generate the products with up to 93% yield
and 97% ee. In contrast, the corresponding homogeneous
controls display no enantioselectivity. The COFs catalysts display
high robustness and can be recycled multiple times without
deterioration of catalytic activity and enantioselectivity. DFT
calculations suggest that the induced enantioselectivity of BINOL
can be ascribed to the steric hindrance and confinement effect of
framework. By enantioselective induction under confinement, we
can expect that more novel heterogeneous asymmetric catalysts
can be designed and constructed from non-enatioselective
catalysts.
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Acknowledgements
The authors acknowledge the financial support of the National
Science Foundation of China (Grant Nos. 21620102001,
91856204, 91956124 and 21875136), the National Key Basic
Research Program of China (2016YFA0203400), Key Project of
Basic Research of Shanghai (19JC1412600, 17JC1403100 and
18JC1413200),
and
Shanghai
Rising-Star
Program
(19QA1404300), the China Postdoctoral Innovative Talent
Support Program (BX20190195) and the China postdoctoral
science foundation (2019M661483).
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Conflict of interest
The authors declare no conflict of interest.
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Keywords: covalent organic framework • porosity • catalysis •
chirality • crystal engineering
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10.1002/anie.202013926
Angewandte Chemie International Edition
RESEARCH ARTICLE
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9
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10.1002/anie.202013926
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
The chiral 3D COFs with interpenetrated open frameworks were synthesized by condensation of tetrahedral tetraamine and enantiopure
1,1'-binaphthol (BINOL) dialdehydes. The Brønsted acidity of BINOL hydroxyl groups that are periodically aligned within the CCOFs
was enhanced obviously compared to the non-immobilized acids. The resulting 3D COF was capable of inducing chiral molecular
catalysts from non-enantioselective to highly enantioselective under confinement effect in catalyzing important organic transformation
with high catalytic activity and good recyclability.
10
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