Construction of Pyridine-Based Chiral Ionic Covalent Organic Frameworks as a Heterogeneous Catalyst for Promoting Asymmetric Henry Reactions

Article abstract of DOI:10.1021/acs.orglett.1c00175

The difficulties in the separation of products from the reaction mixture and the recovery of the organic cationic ionic liquids (OCILs) catalysts still need to be addressed. Post modification of the pyridine unit in the covalent organic framework (COF) vi

Full text of DOI:10.1021/acs.orglett.1c00175

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Letter
Construction of Pyridine-Based Chiral Ionic Covalent Organic
Frameworks as a Heterogeneous Catalyst for Promoting Asymmetric
Henry Reactions
Minghui Chen, Jiabin Zhang, Chenxi Liu, Hongrui Li, Hewei Yang, Yaqing Feng, and Bao Zhang*
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ABSTRACT: The difficulties in the separation of products from the
reaction mixture and the recovery of the organic cationic ionic liquids
(OCILs) catalysts still need to be addressed. Post modification of the
pyridine unit in the covalent organic framework (COF) via the
formation of pyridinium salts with the chiral bromoacetate led to the
chiral ionic COF with OCIL “immobilized”, which was utilized as a
heterogeneous catalyst for asymmetric Henry reactions with high
yield and excellent stereoselectivity.
As a proof of concept, the resultant chiral OCIL-based
rganic cationic ionic liquids (OCILs) have offered a
Opromising template as an efficient homogeneous COFs as heterogeneous catalysts were employed to catalyze
asymmetric Henry addition15 reactions of nitromethane with
substituted benzaldehydes. Even though the heterogeneous
catalysts involving metal complex,8 sieve,16 organic polymer,9
and etc. have been utilized for Henry reaction, these complexes
always had rather low yield (15−82.4%) and stereoselectivity
(7−40% ee). Interestingly, with the heterogeneous catalysts
involving the chiral COFs first employed for the enantiose-
lective Henry reactions, excellent yields and stereoselectivity
(up to 97% yield and 92% ee, respectively) were realized.
Furthermore, the catalysts can be easily recovered and recycled
without a notable loss of the catalytic performance. The
proposed catalytic mechanism suggested that the synergistic
effect between the pyridinium salts and the COF template
played a critical role in promoting the enantioselective Henry
reactions. Herein, we report our preliminary studies on the first
examples of chiral OCIL-based COFs as heterogeneous
catalysts in asymmetric synthesis.
catalyst.1 However, the procedure for the most OCIL-involved
reactions is complicated, in particular for the separation of
products from the reaction mixture and recovery of OCILs.
Furthermore, the recycling of OCILs is normally not
environmentally benign.2,3 “Immobilization” of OCILs via
the host−guest encapsulation seems to be an optimal strategy,
which can not only remain the catalytic function of OCILs, but
also simplify the workup procedure leading to a “greener”
process. Introduction into the rigid supports such as porous
materials and polymers has offered a possible strategy to
immobilize OCILs.4−6 However, the OCILs-immobilized
catalysts reported so far had rather low catalytic perform-
ance.7−9 Thus, it is still a challenge for the immobilization of
OCILs, which can facilitate the heterogeneous catalytic
procedure with high catalytic performance.
In an effort to immobilize OCILs and make chiral covalent
organic framework (COF)-based heterogeneous catalysts,10 we
prepared the pyridine-based COFs via the condensation of the
predesigned monomer 2,4,6-(4′-triaminophenyl) pyridine
(TPP) with 2-hydroxyl tribenzaldehyde (HTD). So far, three
strategies for the preparation of chiral COFs were summarized
in literature including postsynthesis, direct synthesis, and chiral
induction synthesis.11−14 Herein, the chiral auxiliary was
introduced by the post modification of pyridine unit leading
to the pyridinium salts in COFs. The ionized COFs remained
the highly crystalline structure with the chiral auxiliary
arranged in an ordered manner in the framework.
The solvothermal synthesis of TPP-HTD COF starting from
TPP and HTD via imine formation was performed in mixed
solvent systems with acetic acid as the catalyst (Figure 1a). It
Received: January 17, 2021
Published: February 24, 2021
https://dx.doi.org/10.1021/acs.orglett.1c00175
© 2021 American Chemical Society
Org. Lett. 2021, 23, 1748−1752
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Figure 2. (a) FT-IR spectra of TPP, TPP-HTD COF, and CCLSM-1.
(b) Raman spectra of CCLSM-1.
absorptions at 1460 cm−1 are characteristic for the vibrations of
aromatic CN groups in pyridine units. The FT-IR spectrum
of CCLSM-1 also showed the vibration absorptions of
pyridinium C−N bonds at 1280 cm−1 and the ester C−O
units at 1050 cm−1, which were absent in the spectrum of TPP-
HTD COF, indicating that the pyridinium salts were
successfully formed by the post modification of the pyridine
moieties in TPP-HTD COF with the bromoacetate derivative.
The Raman spectrum of CCLSM-1 (Figure 2b) exhibited the
characteristic out-of-plane vibrations of methylene at 1335
cm−1 and the antisymmetric vibration of C−N (pyridine) at
1449 cm−1 further suggesting the presence of the chiral
prolinol group introduced into COFs. The N2 sorption
measurement at 77 K was performed to analyze the pore
characters of TPP-HTD COF and chiral ionic COF, CCLSM-
1 (see Figure S4 and the corresponding discussions). The
structures of TPP-HTD COF and CCLSM-1 were further
characterized by 13C NMR in their solid states (Figures S5 and
S6). Furthermore, the zeta potential and potential distribution
of TPP-HTD COF and CCLSM-1 were obtained in the Figure
S7, which displayed the significant potential increase from the
TPP-HTD COF to the ionic CCLSM-1. The thermal stability
of the TPP-HTD COF and CCLSM-1 was investigated by the
thermogravimetric analysis (TGA), and the results showed that
the as-synthesized ionized COF had the similar thermal
stability with the TPP-HTD COF (Figure S8). And the TPP-
HTD COF has good chemical stability (Figure S9). Especially,
CCLSM-1 exhibited Cotton effects in the solid-state circular
dichroism (CD) spectra (Figure S10). The X-ray photo-
electron spectroscopy (XPS) spectra provided an insight into
the presence of different nitrogen forms in the TPP-HTD COF
and CCLSM-1, and the bromine element in the ionic CCLSM-
1. The results indicate the successfully partial formation of
pyridinium salts in CCLSM-1 which are in good agreement
with the pore size distribution results obtained for CCLSM-1
(see Figure S12, Table S3, and the corresponding discussions).
The scanning electron microscopy (SEM) images of TPP-
HTD COF and CCLSM-1 suggested that the TPP-HTD COF
was synthesized as island-like sheets and the as-synthesized the
chiral ionic COF, CCLSM-1, was obtained as spherical
particles with diameters of ∼400 nm probably due to the
presence of the electrostatic interaction between layers (Figure
3). The high resolution transmission electron microscopy
(HRTEM) image of CCLSM-1 displayed two different types
of lattice fringe spacing (Figure 3: D1 and D2). The lattice
fringe spacing with a value of 0.306 nm actually represented
the interlamellar spacing of TPP-HTD COF and the one with
0.244 nm suggested that of CCLSM-1, respectively, which
were both in good agreement with those obtained from PXRD
refinement.
Figure 1. (a) Synthesis of TPP-HTD COF and CCLSM-1. (b,c)
PXRD Patterns of TPP-HTD COF and CCLSM-1. (d) Representa-
tion of AA and AB Stacking Modes of TPP-HTD COF.
was found that the mixed solvent containing 1,2-dichlor-
obenzene and n-butanol in a ratio of 1:5 (v/v) resulted in the
construction of TPP-HTD COF with a high crystallinity
successfully (Table S1 and Figure S1). The post modification
of TPP-HTD COF with (S)-prolinol bromoacetate in
acetonitrile at room temperature leading to the generation of
pyridinium salts furnished the chiral ionic COF, CCLSM-1
(the immobilized OCILs based on COFs).
The powder X-ray diffraction (PXRD) analysis was
performed to determine the crystalline structure of TPP-
HTD COF (Figures 1b, S2, and S3). As shown in Figure 1b,
the experimental PXRD pattern of TPP-HTD COF displayed
five prominent diffraction peaks at 2θ = 5.47°, 9.55°, 11.08°,
14.63° and 25.35°, which could be assigned to the (100),
(110), (200), (3−10), and (101) facets of the crystalline,
respectively. The Pawley-refined PXRD profile of AA stacking
is highly consistent with the experimental pattern with RwpTOP
= 4.37%. Thus, comparison of the experimental and the
simulated PXRD patterns suggested that TPP-HTD COF
adopted the eclipsed AA stacking mode (Figure S4 and Figure
1d). Interestingly, the PXRD pattern of the chiral ionic COF,
CCLSM-1, indicated the good crystallinity of the post
synthesized product and analogous diffraction facets to its
precursor was observed. However, due to the presence of the
chiral auxiliary in the channel of the 2D COFs, the pore size of
CCLSM-1 is expected to be reduced, and thus, it was shown
that the main peaks in the PXRD image of CCLSM-1 shifted
0.16° toward the right side (Figure 1c).
Compared to that of TPP, the Fourier Transform Infrared
(FT-IR) spectra of TPP-HTD COF and CCLSM-1 (Figure
2a) exhibited a new absorption band at 1620 cm−1, which
corresponded to the stretching vibrations of CN groups,
indicative of the presence of the Schiff-base linkages in the
TPP-HTD COF and chiral ionic COF, CCLSM-1. The
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Table 1. Recycling Performance of CCLSM-1 in
a
Asymmetric Henry Reactions
b
c
cycle
residual weight (mg)
time (h)
yield (%)
% ee
1
2
3
4
5
26.0
25.9
25.9
25.8
25.7
24
24
24
24
24
97
92
90
89
89
89
96.2
95.5
95
95
a
All reactions were carried out with 0.5 mmol p-nitrobenzaldehyde
b
and 2 mL nitromethane. Isolated yield after chromatographic
purification. Determined by chiral HPLC.
c
demonstrated in Figure S13. As is shown, almost no weight
loss of the heterogeneous catalysts can be observed and the
yield of the nitroaldol product remained excellent (above 95%)
after 5 cycles. The enantioselectivities only slightly reduced
(shown in cycles 3−5). The overall results suggested that the
immobilized OCIL in COF exhibited excellent recyclability as
shown in Figure S13.
The influence of the amount of the chiral moiety introduced
at the pyridine nitrogen of the initial COF on the catalytic
performance under the same reaction conditions have also
been evaluated in Figure S14 (see the detailed discussions in
SI).
Figure 3. (a,b) SEM images of CCLSM-1 and TPP-HTD COF. Inset:
zoomed-in image within the annotated area. (c) HRTEM images of
CCLSM-1. Inset: the lattice of CCLSM-1. Right: the distributions of
the lattice distance on the line D1 and D2.
To further understand the role of the immobilized OCIL in
COF played in the promoting the Henry reaction and the
sense of the asymmetric induction imparted, a plausible
catalytic cycle was proposed involving the transition model
(Figure 4), which is based on the previously reported steric
and electronic considerations.17−19
Next, the asymmetric catalytic performance of the resulted
OCIL-based chiral COF catalyst was investigated. The
asymmetric Henry addition reaction of nitromethane with a
representative series of aromatic aldehydes involving CCLSM-
1 as heterogeneous catalysts (10 mol %) was evaluated at rt.
Nitromethane was employed both as the reactant and the
solvent. The catalytic results with different substrates are
summarized in Table S5.
As shown in Table S5 (entry 1), in the presence of CCLSM-
1, the Henry reaction of nitromethane with benzaldehyde
furnished the desired nitroaldol product in good yield (85%)
with decent enantioselectivities (69% enantiomeric excess, ee).
The aromatic aldehydes (entries 2−7) bearing electron-
withdrawing moieties under the same conditions led to the
nitroaldols in good to excellent yields (72−97%) with good to
excellent enantioselectivities obtained (72−92% ee). The
highest yield of 97% was realized for 4-nitro benzaldehyde
with the greatest enantioselectivity (92% ee). Interestingly, it
was shown that in the presence of an electron-donating unit at
the para-position of the aromatic aldehyde, the nitroaldol
products were obtained in a poor yield with low
enantioselectivities (entries 7 and 8). For a comparison, the
Henry reaction of nitromethane with 4-nitro benzaldehyde in
the absence of CCLSM-1 was investigated, and it was shown
that only trace amounts of the nitroaldol product resulted
(entry 8). In the presence of TPP-HTD COF, the precursor to
CCLSM-1, under the same conditions, the desired product was
only obtained in 12%.
As is shown above, the catalytic performance of the
immobilized OCIL remained and thus it would be of interest
to further examine the recyclability of the heterogeneous
OCIL-based COF catalysts. The recyclable performance of
CCLSM-1 was then investigated by evaluating the Henry
reaction of nitromethane with 4-nitro benzaldehyde. After the
reaction, the catalysts were recovered by simple filtration and
dried for the second cycle. The residual weights of dry
CCLSM-1 catalyst were recorded after each cycle, and up to 5
cycles performed. The results are summarized in Table 1 and
Figure 4. Plausible catalytic mechanism of CCLSM-1 for Henry
addition reaction.
As shown, the ionic pyridinium bromide has a resonance
structure which can be stabilized by the two-dimensional
framework with widely expanded π-conjugations. It is believed
that the chiral OCIL-based COF actually played a dual role in
promoting the asymmetric Henry addition reactions. First,
nitromethane was deprotonated by the residual basic pyridine
unit in the framework. The static charge interaction between
the negative charged oxygen of the nitromethane and the
pyridinium unit can favorably occur from the bottom due to
the steric interaction above induced by the presence of the
chiral prolinol moiety. The nucleophilic attack of the
nitromethane to Si-face of the aromatic aldehyde followed by
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Letter
protonation furnished the nitroaldol products mainly with the
absolute R configuration.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00175
In summary, “immobilization” of OCILs in COF was
realized by the post modification of the pyridine-based COF
via the formation of pyridinium salts with the chiral prolinol
bromoacetate derivative leading to the highly crystalline chiral
ionic COFs, CCLSM-1. The resultant chiral ionic COF
material as a heterogeneous catalyst not only remained the
catalytic function of OCILs, but also simplify the workup
procedure leading to a “greener” process, as demonstrated in
asymmetric Henry reactions. Up to 97% yield and 92% ee were
realized, and the catalysts can be readily recovered and
recycled without loss of the catalytic performance. A plausible
catalytic cycle was proposed to further understand the role that
the immobilized OCIL in COF played in promoting the Henry
reaction and the sense of the asymmetric induction imparted.
The results suggested that the immobilization of OCILs by the
post modification of COFs is an optimal strategy, which can
facilitate the heterogeneous catalytic procedure involving
OCILs. Expanding of this methodology in catalyzing various
asymmetric organic reactions is in progress.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (Grant No. 22078241). The
authors would like to express their gratitude to Mr. Yuyao
Zeng at School of Chemical Engineering and Technology,
Tianjin University, for his help in HR-TEM measurement.
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00175.
Characterization data of all the monomers, linkages and
the COFs such as liquid and solid NMR, TGA, EDS,
CD spectra and crystal data of COFs and the HPLC
profile and 1H NMR characterization of catalytic
product (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Bao Zhang − School of Chemical Engineering and Technology,
Tianjin University, Tianjin 300350, PR China; Guangdong
Laboratory of Chemistry and Fine Chemical Industry Jieyang
Center, Guangdong Province 522000, PR China;
orcid.org/0000-0003-4846-0594; Email: baozhang@
tju.edu.cn
Authors
Minghui Chen − School of Chemical Engineering and
Technology, Tianjin University, Tianjin 300350, PR China;
Guangdong Laboratory of Chemistry and Fine Chemical
Industry Jieyang Center, Guangdong Province 522000, PR
China
Jiabin Zhang − School of Chemical Engineering and
Technology, Tianjin University, Tianjin 300350, PR China
Chenxi Liu − School of Chemical Engineering and Technology,
Tianjin University, Tianjin 300350, PR China
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Hewei Yang − School of Chemical Engineering and
Technology, Tianjin University, Tianjin 300350, PR China
Yaqing Feng − School of Chemical Engineering and
Technology, Tianjin University, Tianjin 300350, PR China;
Guangdong Laboratory of Chemistry and Fine Chemical
Industry Jieyang Center, Guangdong Province 522000, PR
China; Collaborative Innovation Center of Chemical Science
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