Asymmetric photocatalysis over robust covalent organic frameworks with tetrahydroquinoline linkage

Article abstract of DOI:10.1016/S1872-2067(20)63572-0

The asymmetric photocatalytic organic synthesis (APOS) process is a sustainable and environmentally benign method for the production of optically active chemicals with sunlight as an energy source. However, it still lacks efficient semiconductors with tun

Full text of DOI:10.1016/S1872-2067(20)63572-0

Chinese Journal of Catalysis 41 (2020) 1288–1297
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Asymmetric photocatalysis over robust covalent organic
frameworks with tetrahydroquinoline linkage
Chunzhi Li a,b, Yinhua Ma b,c, Haoran Liu a, Lin Tao a,b, Yiqi Ren a,b, Xuelian Chen a, He Lia,*,
Qihua Yang a,#
Dedicated to Prof. Can Li on the occasion of his 60th birthday.
a State Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
b University of Chinese Academy of Sciences, Beijing 100049, China
c State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
A R T I C L E I N F O
A B S T R A C T
Article history:
The asymmetric photocatalytic organic synthesis (APOS) process is a sustainable and environmen-
tally benign method for the production of optically active chemicals with sunlight as an energy
source. However, it still lacks efficient semiconductors with tunable band structures and has a low
recycling stability. Herein, we report the synthesis of tetrahydroquinoline-linked covalent organic
frameworks (QH-COFs) with irreversible tetrahydroquinoline linkage as efficient semiconductors
for the visible-light-driven asymmetric α-alkylation of aldehydes by merging with a chiral second-
ary amine. Up to 94% ee was obtained over QH-COFs, and the activity of QH-COFs was significantly
higher than those of inorganic semiconductors (e.g., TiO2, BiVO4, and WO3) under similar conditions,
which is mainly attributed to their narrow band gap and suitable band edge. As far as we know,
QH-COFs are the most active semiconductors for asymmetric α-alkylation of aldehydes ever re-
ported. The QH-COFs were prepared via a one-pot Povarov cascade imine formation and cycloaddi-
tion reaction using Sc(OTf)3/Yb(OTf)3 as Lewis acid catalysts. Attributed to the tetrahydroquinoline
linkage, QH-COFs showed extremely high recycling stability, which made practicals application
possible. This work not only opens up a new avenue for asymmetric photocatalysis but also pro-
vides an efficient and general method for the construction of robust COFs.
Received 27 November 2019
Accepted 23 December 2019
Published 5 August 2020
Keywords:
Photocatalysis
Covalent organic frameworks
Tetrahydroquinoline linkage
Asymmetric catalysis
Ultrastability
© 2020, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
catalysis approach consisting of organic dyes/inorganic semi-
conductors and chiral catalysts for light absorption and chiral
The asymmetric photocatalytic organic synthesis (APOS)
process is a sustainable and environmentally benign method
for the production of optically active chemicals with sunlight as
an energy source [1–3]. Most APOS processes employ a dual
induction, respectively. In general, organic dyes confront prob-
lems with narrow absorption bands and low photostability,
while the band gap and band edge of inorganic semiconductors
are difficult to be tuned [4,5]. Furthermore, the efficiency of
* Corresponding author. Tel: +86-411-82463019; Fax: +86-411-84694447; E-mail: lihe@dicp.ac.cn
# Corresponding author. Tel: +86-411-84379552; Fax: +86-411-84694447; E-mail: yangqh@dicp.ac.cn
This work was supported by the National Natural Science Foundation of China (21733009, 21621063), and the Strategic Priority Research Program
of Chinese Academy of Sciences (XDB17020200).
DOI: 10.1016/S1872-2067(20)63572-0 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 41, No. 8, August 2020

Chunzhi Li et al. / Chinese Journal of Catalysis 41 (2020) 1288–1297
1289
APOS is strongly related to the textural structure and surface
properties of the semiconductors, because the interphases of
the semiconductors with the asymmetric catalysts and the re-
actants play an important role in achieving efficient electron
transfer and chiral induction [6,7]. Therefore, the development
of semiconductors with designable band structures, textural
structures, and surface properties is of extreme importance for
APOS.
reaction, Scheme 1) with Lewis acids as catalysts, such as
Yb(OTf)3 and Sc(OTf)3 [43–45]. More importantly, the Povarov
cascade reaction has a wide substrate scope. Previous reports
also demonstrated the efficiency of M(OTf)3 (M = Sc, Eu, In, Yb,
Y, etc.) for the formation of COFs with imine linkage [46].
Therefore, the cascade condensation and cycloaddition reac-
tions may provide a general method for the synthesis of ul-
trastable COFs with irreversible linkages via the judicious se-
lection of the Lewis acids.
Herein, we report an efficient and general method for the
synthesis of ultrastable tetrahydroquinoline-linked COFs
(QH-COFs) via cascade condensation and cycloaddition reac-
tions with Sc(OTf)3/Yb(OTf)3 as the catalysts. As far as we
know, this is the first example of the one-pot synthesis of COFs
via cascade reactions using aldehydes and amines in the pres-
ence of transformation reagents. The successful formation of a
tetrahydroquinoline linkage was confirmed by FT-IR and 15N
NMR spectroscopies of the QH-COFs. The QH-COFs exhibited
extremely high stability under the acidic/basic conditions and
light irradiation. More interestingly, the QH-COFs with visi-
ble-light absorption expanding to more than 560 nm could
efficiently catalyze the visible-light-driven asymmetric alkyla-
tion of aldehydes in combination with a chiral secondary amine
to afford both high yield and high ee for a large substrate scope.
Covalent organic frameworks (COFs) [7–14] with periodi-
cally ordered structures, high surface areas, and tunable band
gaps and band edges are potential organic semiconductors in
the field of photocatalysis and have been successfully used for
photocatalytic H2 production [7,15–21], CO2 reduction [22–25],
and organic synthesis [26–32]. The COFs with reversible C=N
or B-O-B as a linkage are not stable enough in acid/base me-
dium and under light irradiation, which is one of the biggest
obstacles to their applications. Intensive efforts have been de-
voted to improving the stability of COFs by replacing the re-
versible chemical bond with an irreversible bond [29,32–42].
For example, the direct cascade transformation of the imine
linkage of COFs with phenolic hydroxyl to oxazole linkage [32]
and the post-synthesis conversion of imine-linked COFs to am-
ide-linked and quinoline-linked COFs have been reported
[33,34]. Recently, Yaghi’s groups successfully constructed
highly stable dioxin-linked and olefin-linked COFs via an irre-
versible nucleophilic aromatic substitution reaction and
Knoevenagel condensation reaction, respectively [35,36].
However, the reported stable COFs synthesized via the
one-pot approach are only limited to some special monomers
[32,36–38]. This not only makes the monomer synthesis tedi-
ous but also limits the universality of the synthesis method. It is
highly desirable to use independent transformation reagents
instead of attaching them to the monomers. Through a litera-
ture survey, we found that tetrahydroquinoline derivatives
could be formed by cascade condensation and cycloaddition
reactions of amine, aldehyde, and alkene (Povarov cascade
2. Experimental
2.1. Synthesis of QH-COF-1
A
10 mL high-pressure flask was charged with
1,3,5-tris(p-formylphenyl)benzene (32.5 mg, 0.083 mmol),
benzidine (23.0 mg, 0.125 mmol), ethyl vinyl ether (72 mg, 1
mmol). A mixture of 1,2-dichlorobenzene and n-butyl alcohol
(4:1 v/v, 2.5 mL) was added, and the resulting suspension was
sonicated at room temperature until the monomers were fully
dispersed. Sc(OTf)3 (2.5 mg, 0.005 mmol) and Yb(OTf)3 (8.0 mg,
Sc(OTf)3
Yb(OTf)3
a)
+
Tetrahydroquinoline model compound
Imine model compound
b)
Sc(OTf)3/
Yb(OTf)3
+
Sc(OTf)3
+
Sc(OTf)3/
Yb(OTf)3
COF-1
Scheme 1. (a) The construction of imine and tetrahydroquinoline model compounds. (b) Schematic of the construction of QH-COF-1 and QH-COF-2
with tetrahydroquinoline linkage via Povarov cascade reaction and COF-1 with imine linkage.

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Chunzhi Li et al. / Chinese Journal of Catalysis 41 (2020) 1288–1297
0.013 mmol) were added, and the resulting suspension was
further sonicated for 30 s. The flask was left standing for an-
other 1 h at room temperature, after which it was degassed
through three freeze-pump-thaw cycles. The flask was charged
with N2 and sealed under positive N2 pressure; thereafter, it
was placed without stirring in a 120 °C pre-heated oil bath for
three days. The solid product obtained after filtration was
washed with methanol several times and extracted by Soxhlet
extraction using methanol for 1 day, followed by a supercritical
CO2 drying process to obtain a brown solid, which was denoted
as QH-COF-1.
line-linked COF (QH-COF-1) via a one-pot reaction of
1,3,5-tris(p-formylphenyl)benzene with benzidine in the pres-
ence of EVE (Scheme 1b). Initially, the synthesis conditions for
QH-COF-1 were screened, and the results of powder X-ray dif-
fraction (PXRD) studies show that the QH-COF-1 with the best
crystallinity was obtained when a mixture of Sc(OTf)3 and
Yb(OTf)3
was
used
as
the
catalyst
in
o-dichlorobenzene/n-butanol (4/1, v/v) at 120 °C under N2
atmosphere (Fig. S1). A control sample, COF-1, with an
imine-linked framework was also synthesized with
1,3,5-tris(p-formylphenyl)benzene and benzidine as monomers
using Sc(OTf)3 as the catalyst in the absence of EVE.
2.2. Synthesis of QH-COF-2
The chemical composition of QH-COF-1 was characterized
by FT-IR, 13C NMR, and 15N NMR spectroscopies, and the results
were compared with those of COF-1 and model compounds
(Figs. 1a–c). In the FT-IR spectrum of COF-1, the characteristic
vibration of C=N at 1622 cm–1 matched well with the vibration
of C=N in the imine model compound at 1621 cm–1, suggesting
the formation of an imine linkage for COF-1 [47]. QH-COF-1
displays a FT-IR spectrum quite different from that of COF-1.
The characteristic vibration of C=N disappeared completely
accompanied by the appearance of a new peak at 3410 cm–1
assigned to the secondary N–H vibration, and the peaks in the
range of 2962–2870 cm–1 were attributed to the C–H vibrations
of the ethylene group. The FT-IR spectrum of QH-COF-1 is al-
most identical to that of the THQ model compound with a slight
red-shift of N–H vibration (3410 versus 3340 cm–1), possibly
due to the formation of intramolecular hydrogen bonds among
THQ model compounds. The FT-IR results suggest the success-
ful formation of tetrahydroquinoline linkage in QH-COF-1 via
the one-pot Povarov cascade reactions.
In the 13C CP-MAS NMR spectrum of QH-COF-1, the signals
at 13–70 ppm are derived from the carbons of alkyl and alkoxy
groups formed via the cycloaddition reaction of imine and EVE,
referenced with the corresponding THQ model compound. The
aromatic carbons appeared in the range of 149–115 ppm. The
weak signal at 156 ppm could be assigned to the carbon of the
quinoline ring possibly formed by the oxidation during the
Soxhlet extraction process (confirmed by 15N NMR studies,
discussed later). No signals in the range of 13–70 ppm could be
observed in the 13C CP-MAS NMR spectrum of COF-1. With the
13C NMR spectrum of the corresponding imine model com-
pound as a reference, the signals at 158 ppm and in the range of
150–116 ppm in the 13C CP-MAS NMR spectrum of COF-1 could
be assigned, respectively, to C=N and aromatic carbons (Fig. S2)
[48].
QH-COF-2 was synthesized in a similar way to QH-COF-1
with 1,3,5-tris(p-formylphenyl)benzene and 1,3,5-tris(4-amin-
ophenyl)benzene as monomers in the presence of ethyl vinyl
ether.
2.3. General procedures of the asymmetric photocatalysis
An oven-dried 15 mL flat-bottomed flask equipped with a
piece of quartz glass and magnetic stir bar was charged with a
semiconductor (30 mg), the corresponding bromide (0.809
mmol, 1.0 equiv.), and (2R,5S)-2-tert-butyl-3,5-dimethylimi-
dazolidin-4-one triflate (0.162 mmol, 0.2 equiv.). The flask was
charged with N2, and 2.0 mL of dry DMF was added using a
syringe, followed by the corresponding aldehyde (1.62 mmol,
2.0 equiv.) and 2,6-lutidine (1.62 mmol, 2.0 equiv.). The flask
was sealed with parafilm and placed approximately 10 cm from
a 30 W blue light LED (460–470 nm). After the reaction was
complete (TLC analysis), the mixture was poured into a sepa-
ratory funnel containing 5 mL of ethyl acetate and 5 mL of H2O.
The layers were separated and the aqueous layer was extracted
with ethyl acetate (3  5 mL). The combined organic layers
were dried with Na2SO4 and concentrated in vacuo. The crude
product was purified by column chromatography. The enanti-
1
omeric excess was determined by H NMR analysis of the cor-
responding diastereomeric acetals obtained by derivatization
with (2S, 4S)-2,4-pentanediol (for details, see supplementary
methods in SI).
3. Results and discussion
3.1. Synthesis and characterizations
Firstly, benzaldehyde and benzidine were used as model
substrates to explore the feasibility of the Povarov reaction for
the formation of the tetrahydroquinoline model compound
(Scheme 1a). The imine model compound could be facilely ob-
tained by the reaction of benzaldehyde and benzidine in the
presence of 0.02 equiv. Sc(OTf)3 under ambient conditions.
Next, the imine model compound was reacted with ethyl vinyl
ether (EVE) in the presence of Yb(OTf)3 under N2 to afford the
tetrahydroquinoline (THQ) model compound.
To further confirm the formation of the tetrahydroquinoline
linkage in QH-COF-1, 15N labeled QH-COF-1 and COF-1 were
synthesized using 15N enriched benzidine as a monomer (for
details, see SI). The signal at 75 ppm assigned to the secondary
amine in the 15N CP-MAS NMR spectrum of 15N enriched
QH-COF-1 clearly confirmed the formation of a tetrahydro-
quinoline linkage, which is consistent with the liquid 15N NMR
spectrum of the 15N enriched THQ model compound. The weak
signal at 311 ppm could be assigned to the nitrogen of quino-
line, as discussed above [49]. Only the signal indicative of C=N
at 325 ppm together with a weak signal at 54 ppm assigned to
Inspired by the successful formation of THQ model com-
pound, we attempted to synthesize
a tetrahydroquino-

Chunzhi Li et al. / Chinese Journal of Catalysis 41 (2020) 1288–1297
1291
c)
b)
a)
Secondary amine : 75 ppm
THQ
COF-1
THQ
QH-COF-1
QH-COF-2
Secondary amine : 75 ppm
Quinoline : 311 ppm
QH-COF-1
QH-COF-1
Secondary amine : 75 ppm
THQ model compound
Imine model compound
Quinoline : 307 ppm
QH-COF-2
200
QH-COF-2
50
200
150
100
0
400
300
100
0
4000 3500 3000 2500 2000 1500 1000
Wavenumbers (cm-1)
Chemical shift (ppm)
Chemical shift (ppm)
d)
e)
Experimental
Refinement
Simulation
Difference
Experimental
Refinement
Simulation
Difference
QH-COF-1
QH-COF-2
5
10
15
20
25
30
35
40
5
10
15
20
25
30
35
40
2 Theta (o)
2 Theta (o)
Fig. 1. (a) FT-IR spectra, (b) 13C NMR spectra, and (c) 15N NMR spectra of the model compounds and COFs. (d, e) Experimental PXRD patterns (black),
Pawley-refined (red), and predicted eclipsed structure (magenta) of QH-COFs (The difference plots are presented in green).
unreacted terminal amine appeared in the 15N CP-MAS NMR
spectrum of 15N enriched COF-1 with the liquid 15N NMR spec-
trum of the 15N enriched imine model compound as a reference
(Fig. S3) [40,50].
ble S2).
The N2 sorption isotherms of QH-COF-1 and COF-1 meas-
ured at –196 °C after activation in vacuo are of type IV, which is
typical for mesoporous materials (Fig. S6). The Brunau-
er–Emmett–Teller (BET) surface areas of QH-COF-1 and COF-1
were calculated to be 674 and 1308 m2 g–1, respectively (Table
S3). QH-COF-1 and COF-1 have total pore volumes of 0.43 and
0.76 cm3 g–1. respectively. The nonlocal density-functional the-
ory gave rise to a narrow pore size distribution with average
pore sizes of 2.2 and 2.7 nm for QH-COF-1 and COF-1, respec-
tively. Scanning electron microscopic (SEM) images revealed
that all the COFs are composed of irregularly shaped particles
with a particle size of 1 m (Fig. S7).
The PXRD pattern of QH-COF-1 presents excellent crystal-
linity with three diffraction peaks at 2.42°, 4.16°, and 4.84°,
assigned to (100), (110), and (200) reflections, respectively
(Fig. 1d). Both the eclipsed and staggered models of QH-COF-1
were constructed using a Material Studio software (Fig. S4).
The simulation of the PXRD patterns showed that the structure
of QH-COF-1 was consistent with the eclipsed model. Pawley
refinement afforded optimize parameters of a = b = 43.751 Å, c
= 4.334 Å (Table S1), which provided good agreement factors
(Rwp = 4.25%, Rp = 2.90%). The cell parameter (c) value (usual-
ly expresses the distance between layer to layer) was signifi-
cantly larger than the value of the traditional imine COF. This is
possibly due to the conformational flexibility of the secondary
amine linkages, which weakens the π-π interaction between
the adjacent COF layers. Similar to QH-COF-1, the PXRD pat-
terns of COF-1 also presented high crystallinity with three dif-
fraction peaks at 2.46°, 4.26°, and 4.91°, assigned to the (100),
(110), and (200) reflections, respectively (Fig. S5). COF-1 was
also consistent with the eclipsed model, and Pawley refinement
afforded the optimize parameters of a = b = 44.493 Å, c = 3.900
Å with good agreement factors (Rwp = 5.95%, Rp = 4.58%) (Ta-
The results of all the above characterizations confirmed that
QH-COF-1 with high crystallinity was successfully formed via
one-pot cascade reactions.
3.2. Ultrastability of QH-COF-1
In general, stability is critical for COFs; therefore, the ther-
mal and chemical stabilities of QH-COF-1 were investigated and
compared with those of COF-1. The thermogravimetric analysis
(TGA) showed that both QH-COF-1 and COF-1 are thermally
stable up to 400 °C in air, suggesting the high thermal stability
of both COFs (Fig. S8). To our delight, the PXRD patterns, N2

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Chunzhi Li et al. / Chinese Journal of Catalysis 41 (2020) 1288–1297
a)
b)
500
400
300
200
100
0
QH-COF-1
In 12 M HCl for 3d
QH-COF-1
In 98 wt% H2SO4 for 3d
In TFA for 3d
In 12 M HCl for 3d
In 98 wt% H2SO4 for 3d
In TFA for 3d
In 14 M NaOH for 3d
In 14 M NaOH for 3d
0.0
0.2
0.4
0.6
0.8
1.0
5
10
15
20
25
30
35
40
Relative pressure (P/P0)
2 Theta (o)
d)
c)
In 14 M NaOH for 3d
In TFA for 3d
In 98 wt% H2SO4 for 3d
In 12 M HCl for 3d
QH-COF-1
In 12 M HCl for 3d
400
300
200
100
0
3500
3000
2500
2000
1500
1000
Wavenumbers (cm-1)
Chemical shift (ppm)
Fig. 2. (a) PXRD patterns, (b) N2 isotherms (the isotherms have been shifted vertically), and (c) FT-IR spectra of QH-COF-1 measured after treatment
in acid or base medium. (d) Solid-state 15N NMR spectrum of 15N enriched QH-COF-1 measured after treatment in 12 mol L–1 HCl.
sorption isotherms, FT-IR spectra, and SEM images of
QH-COF-1 after treatment in TFA, 12 mol L–1 HCl, 14 mol L–1
NaOH, and even concentrated H2SO4 (98%) remained almost
unchanged, indicating that QH-COF-1 could tolerate strong acid
and base conditions (Figs. 2, S9). Furthermore, the 15N NMR
spectrum of 15N enriched QH-COF-1 after treatment in 12 mol
L–1 HCl for 3 days was almost identical to that of the fresh one,
which further confirmed the excellent chemical stability of
QH-COF-1. The weak signal at ~210 ppm was possibly derived
from the protonated nitrogen of the secondary amine. On the
contrary, the imine-linked COF (COF-1) was destroyed almost
completely in acid or base solutions under similar conditions,
indicating that the imine-linked structure was not stable under
harsh acid or base conditions. The above results confirmed the
ultrastability of QH-COF-1 under harsh conditions. As far as we
know, QH-COF-1 is one of the most stable COFs ever reported.
line-linked COFs, 1,3,5-tris(p-formylphenyl)benzene and
1,3,5-tris(4-aminophenyl)benzene were used as monomers for
the synthesis of QH-COF-2 in a similar way to that of QH-COF-1
(Scheme 1). QH-COF-2 exhibits similar a PXRD pattern to that
of QH-COF-1 with three diffraction peaks at 4.12°, 7.02°, 8.20°,
assigned to the (100), (110), and (200) reflections, respectively
(Fig. 1e). The simulation results showed that QH-COF-2 was
consistent with the eclipsed model, and the Pawley refinement
afforded optimize parameters of a = b = 26.380 Å, c = 5.484 Å
with good agreement factors (Rwp = 5.13%, Rp = 3.95%) (Table
S4). The FT-IR (–NH–, 3405 cm–1; –OCH2CH3, 2870–2960 cm–1),
solid-state 13C CP-MAS NMR (alkyl and alkoxy, 13–68 ppm),
and solid-state 15N CP-MAS NMR (–15NH–, 75 ppm) spectra of
QH-COF-2 confirmed the successful formation of a tetrahydro-
quinoline linkage in QH-COF-2 (Fig. 1). The weak signals at 307
and 200 ppm are assigned to the nitrogen of quinoline and the
nitrogen coordinated with a metal salt, respectively. Similar to
QH-COF-1, QH-COF-2 was also chemically stable in TFA, 12 mol
L–1 HCl, 14 mol L–1 NaOH, and concentrated H2SO4 (98%) as
evidenced by the PXRD results (Fig. S10). The successful for-
mation of QH-COF-2 suggests that the one-pot cascade con-
densation and cycloaddition reaction is a versatile method for
the synthesis of ultrastable COFs with tetrahydroquinoline
linkage.
3.3. Universality of the one-pot cascade reactions for the syn-
thesis of ultrastable COFs
Theoretically, most of the monomers employed for the
synthesis of imine-linked COFs could be used for the
construction of tetrahydroquinoline-linked COFs under
suitable conditions. To explore the universality of this one-pot
cascade method for the construction of tetrahydroquino-

Chunzhi Li et al. / Chinese Journal of Catalysis 41 (2020) 1288–1297
1293
1.5
1.0
0.5
0.0
-2
c)
2.0
1.5
1.0
0.5
0.0
a)
COF-1
QH-COF-1
QH-COF-2
b)
-1.33
COF-1
QH-COF-1
QH-COF-2
-1.07
-1.08
-1.06
-1
Ru(I)/
Ru(II)
-0.49
0
1
0.84
Ru(II)*/
Ru(I)
1.40
1.49
1.52
2
COF-1
QH-COF-2
QH-COF-1
3
300
400
500
600
700
800
0
50
100
150
200
Time (s)
Wavelength (nm)
f)
e)
d)
LUMO: -2.31 eV
LUMO: -2.51 eV
LUMO: -2.87 eV
HOMO: -4.75 eV
QH-COF-1: LUMO - HOMO = 2.44 eV
HOMO: -4.99 eV
QH-COF-2: LUMO - HOMO = 2.48 eV
HOMO: -5.47 eV
COF-1: LUMO - HOMO = 2.6 eV
Fig. 3. (a) UV-vis spectra, (b) transient photocurrent responses under visible-light irradiation, (c) CB and VB position (vs. SCE, referenced with
molecular photocatalyst [Ru(bpy)3]2+ and the redox potential of 2-bromoacetophenone) [6,53], and (d–f) pictorial representation of HOMO and LUMO
orbitals and their energy levels of QH-COF-1, COF-1, and QH-COF-2.
transport efficiency in QH-COF-1 is higher. QH-COF-1 afforded
3.4. Asymmetric photocatalysis
much higher photocurrent under visible-light irradiation than
those of QH-COF-2 and COF-1, which could be attributed to the
narrow band gap and enhanced charge transportation efficien-
cy. The conduction band (CB) levels of QH-COF-1, QH-COF-2,
and COF-1 were respectively located at –1.06, –1.07, and –1.08
eV vs. SCE, respectively, as determined by cyclic voltammetry
measurement (Fig. S14). The estimated band gaps, band struc-
tures, and energy levels of QH-COF-1, QH-COF-2, and COF-1 are
summarized in Fig. 3c. The above results suggest that the band
gap could be narrowed by the formation of a tetrahydroquino-
line linkage and that the band edge could be finely tuned by
varying the chemical composition of the monomers.
The merging of photo-redox catalysis with asymmetric ca-
talysis has attracted great research interest over the past dec-
ades [51–60]. One of the milestone work focused on combining
photocatalysis with asymmetric organocatalysis was reported
by MacMillan and co-workers [51]. To date, most asymmetric
photocatalysis processes employ [Ru(bpy)3]2+ or organic dyes
as photocatalysts. The narrow light absorption range, difficult
recyclability, photobleaching, and low anti-photo corrosion
ability are the main shortcomings for the molecular photosen-
sitizer. The potentials of visible-light responsive QH-COF-1,
QH-COF-2, and COF-1 materials in the field of photocatalysis
were investigated in asymmetric MacMillan reactions consid-
ering their suitable band structures, based on the previous
reports using (2R,5S)-2-tert-butyl-3,5-dimethylimidazolidin-
4-one triflate as a chiral organocatalyst (Table 1).
Diffuse reflectance ultraviolet-visible (UV-vis) spectroscopy
showed that all the COFs could efficiently absorb visible light
(Fig. 3a). In comparison with the COF-1 spectrum, the
QH-COF-1 spectrum exhibited a noticeable red-shift of the
absorption edge from ~480 nm to >560 nm. The spectrum of
QH-COF-2 showed an absorption edge extending to more than
550 nm. The optical band gaps of QH-COF-1, QH-COF-2, and
COF-1 were estimated to be 2.46, 2.56, and 2.60 eV from
Kubelka-Munk plots, respectively (Fig. S11). The corresponding
THQ and imine model compounds only absorb UV light and
exhibit considerably broad band gaps (Fig. S12). The band gaps
of QH-COF-1, QH-COF-2, and COF-1 were further calculated
using
the
self-consistent-charge
density-functional
tight-binding (SCC-DFTB) method based on the DFTB + Materi-
al Studio Software (Fig. 3; for details, see SI). The calculated
results showed that the band gaps of QH-COF-1 and COF-1 are
almost similar to the experimental data. For QH-COF-2, the
calculated band gap is relatively narrower than the experi-
mental data. This is possibly due to the fact that precise model-
ing is difficult for QH-COF-2 with moderate crystallinity. All the
COFs show a noticeable photocurrent under visible-light irra-
diation, suggesting the efficiency of the separation of the pho-
togenerated charges in COFs (Fig. 3b). Electrochemical imped-
ance spectroscopy (EIS) was exploited to investigate the elec-
trical conductivity of the COFs (Fig. S13). The semicircular ra-
dius on the EIS Nyquist plot of QH-COF-1 was smaller than
those of QH-COF-2 and COF-1, indicating that the charge
QH-COF-1 could efficiently catalyze the MacMillan reaction
to afford the corresponding product with a 75% isolation yield

1294
Chunzhi Li et al. / Chinese Journal of Catalysis 41 (2020) 1288–1297
high stability, QH-COF-1 could be recovered without notable
Table 1
weight loss after the photocatalytic reaction, further confirming
the superiority of tetrahydroquinoline-linked COFs in photoca-
talysis.
The catalytic performances of different types of photocatalysts in the
asymmetric α-alkylation of aldehydes.
Inorganic photocatalysts, BiVO4, WO3, and TiO2, could also
catalyze the asymmetric MacMillan reaction under similar con-
ditions. The ee value over BiVO4 and TiO2 could reach up to
91%, while WO3 afforded a slightly lower ee value of 88%. All
the inorganic photocatalysts screened here afforded much
lower product yields than QH-COF-1. The photogenerated elec-
trons and holes are both involved in the asymmetric MacMillan
reaction based on the reaction mechanism (discussed later). An
efficient photocatalyst should be able to absorb visible light and
possess suitable redox properties of electrons and holes. The
visible-light responsive BiVO4 and WO3 have relatively low
electron reducing power [61], while TiO2 with stronger redox
properties has a wide band gap [62]. Consequently, they are
not efficient photocatalysts for the asymmetric MacMillan reac-
tion. As far as we know, QH-COF-1 is the most active organic
semiconductor for the asymmetric MacMillan reaction. The
superiority of QH-COF-1 for the organic asymmetric photoca-
talysis is mainly related to its narrow band gap and suitable
band edge.
Photocatalyst
None
Yield a (%)
ee b (%)
—
—
—
91
84
91
91
91
88
91
—
—
N.D.c
QH-COF-1d
THQ
N.D.
N.D.
75
30
60
80
20
18
QH-COF-1
COF-1
QH-COF-2
[Ru(bpy)3]Cl2·6H2O e
BiVO4
WO3
TiO2 (P25)
QH-COF-1 (TEMPO) f
QH-COF-1 (DIPEA) g
27
< 3
< 3
Reaction
conditions:
2-bromoacetophenone
(0.809
mol),
n-butyraldehyde (1.62 mmol), 2,6-dimethlpyridine (1.62 mmol),
(2R,5S)-2-tert-butyl-3,5-dimethylimidazolidin-4-one triflate salt (0.162
mmol), photocatalyst (30 mg), anhydrous DMF (2 mL), irradiation with
30 W blue LEDs, 12 h. a Yield of isolated product based on bromide.
b
Determined by 1H NMR analysis of the diastereometric acetals
To investigate the reaction mechanism of the asymmetric
photocatalysis, we performed a series of controlled experi-
ments. In the absence of photocatalysts or without light, no
product was detected, confirming that the asymmetric MacMil-
lan reaction indeed involves the photocatalytic process. With
THQ as the photocatalyst, no product was detected under simi-
lar conditions, suggesting that it was not the tetrahydroquino-
line fragments but QH-COFs that acted as the photocatalyst.
When the reaction was performed in the absence of a chiral
secondary amine, no product was obtained. This implies the
QH-COFs cannot catalyze the condensation of a secondary
amine with an aldehyde. In the presence of 2 equiv. of TEMPO
(radical scavenger) or DIPEA (hole scavenger), the yield of iso-
lated chiral products decreased to less than 3%, suggesting that
radical species [63] and oxidative holes both participated in the
photocatalytic process. Further, the radical species produced
by 2-bromoacetophenone were captured and separated in the
obtained by derivatization with (2S, 4S)-2,4-pentanediol. c N.D. = not
detected. d Light off. e [Ru(bpy)3]Cl2·6H2O (8.09 μmol). f TEMPO (1.62
mmol). g DIPEA (1.62 mmol).
and 91% ee. Under similar conditions, [Ru(bpy)3]2+ afforded an
80% yield with 91% ee. The almost comparable catalytic per-
formances of QH-COF-1 and [Ru(bpy)3]2+ suggested that
QH-COF-1 is an efficient photocatalyst for the asymmetric
MacMillan reaction. QH-COF-2 afforded less yield than
QH-COF-1 (60% versus 75%), possibly due to its wider band
gap and reduced charge separation efficiency. COF-1 afforded a
much lower yield (30% versus 75%) and ee value (84% versus
91%) than QH-COF-1, possibly due to its low stability under
photocatalytic conditions. After one cycle, less than 20% of the
initial COF-1 amount could be recovered and most COF-1 de-
composed during the photocatalytic process. Due to its ultra-
Aldehyde
e-
CB
Chiralcatalyst
Excitation
Enamine
Charge
Separation
Product
Chiral Catalytic
Cycle
α-amino
radical
VB
h+
Iminiumion
Scheme 2. Proposed mechanism for the photocatalytic asymmetric α-alkylation of aldehydes in the presence of QH-COF-1.

Chunzhi Li et al. / Chinese Journal of Catalysis 41 (2020) 1288–1297
1295
QH-COF-1 produced an almost identical PXRD pattern, FT-IR
spectrum, and SEM image compared with those of the fresh
one, showing the structural and morphological stability under
light irradiation (Figs. 5, S9, and S15). This suggests that
QH-COFs have ultrahigh photostability, which makes them
superior to most reported COFs.
S1
S2
S3
S4
80% yield, 94% ee
70% yield, 92% ee
75% yield, 91% ee
80% yield, 91% ee
4. Conclusions
In summary, we successfully developed an efficient and
general method to build QH-COFs as robust photocatalysts via
the one-pot Povarov reaction. QH-COFs could endure strong
acidic/basic conditions and light irradiation, which is attributed
to the irreversible tetrahydroquinoline linkage. The formation
of tetrahydroquinoline linkage could effectively narrow the
band gap of QH-COFs to widen the absorption spectrum. The
QH-COFs exhibited high activity, enantioselectivity, and
excellent recyclability in the photocatalytic asymmetric
MacMillan reaction by merging with a chiral secondary amine.
Theoretically, all imine-linked COFs that have been reported
could be converted into stable tetrahydroquinoline-linked
structures under optimized conditions, considering that the
functional groups for further transformation are no longer
required to be attached to the monomers. Our work would
shed light on the designable synthesis of robust COFs with
novel irreversible linkages. This research also demonstrates
the promising applications of ultrastable COFs with designable
band structures, porosity, and lipophilicity in the field of
visible-light-driven APOS.
S5
S6
S7
65% yield, 85% ee
91% yield, 92% ee
90% yield, 92% ee
Fig. 4. The substrate scope of asymmetric photocatalysis over
QH-COF-1 (S1–S5: 20 h, S6–S7: 6 h).
presence of 1 equiv. of TEMPO and 2 equiv. DIPEA (for details,
see SI). The above-controlled experiments indicated that the
QH-COF acted as an electron and hole provider. Based on the
above results, the schematic diagram of the photocatalytic re-
action process is outlined in Scheme 2. Under light irradiation,
the photogenerated electrons transferred from the CB of
QH-COF-1 to -bromocarbonyl substrate to form an elec-
tron-deficient alkyl radical. In the following step, an α-amino
radical was produced by the addition of the electron-deficient
alkyl radical to the enamine formed by the condensation of the
chiral secondary amine with the aldehyde. The hole in the VB of
QH-COF-1 would remove a single electron from a sacrificial
quantity of α-amino radical to form the iminiumion that de-
composed into the catalyst and product to complete the cycle
[51,53].
Acknowledgments
As shown in Fig. 4, a series of 2-bromoacetophenone deriva-
tives with both electron-withdrawing and electron-donation
substituents and diethyl bromomalonate could be smoothly
reacted with aldehyde for the production of corresponding
alkylation products in the presence of QH-COF-1 with high iso-
lated yield (65%–91%) and ee value (85%–94%). The wide
substrate scope of the asymmetric photocatalysis validates the
generality of QH-COFs as a privileged photocatalyst.
We are grateful to Prof. X. Feng and Mr. P. Shao for the sup-
port on the structural modeling of COFs. H. Li thanks Ms. X. Tao
for helpful discussions.
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Chunzhi Li et al. / Chinese Journal of Catalysis 41 (2020) 1288–1297
Graphical Abstract
Chin. J. Catal., 2020, 41: 1288–1297 doi: 10.1016/S1872-2067(20)63572-0
Asymmetric photocatalysis over robust covalent organic
frameworks with tetrahydroquinoline linkage
Chunzhi Li, Yinhua Ma, Haoran Liu, Lin Tao, Yiqi Ren, Xuelian Chen,
He Li*, Qihua Yang *
Dalian Institute of Chemical Physics, Chinese Academy of Sciences;
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新型超稳定四氢喹啉连接的共价有机骨架材料的制备及其不对称光催化性能
李纯志a,b, 马银华b,c, 刘浩然a, 陶 琳a,b, 任亦起a,b, 陈雪莲a, 李 贺a,*, 杨启华a,#
a中国科学院大连化学物理研究所催化基础国家重点实验室, 辽宁大连116023
b中国科学院大学, 北京100049
c中国科学院大连化学物理研究所分子反应动力学国家重点实验室, 辽宁大连116023
摘要: 不对称光催化有机合成是一种可持续的、环境友好的以太阳光为能源生产光学活性化学品的方法. 绝大多数不对称
光催化有机合成体系是由有机染料或无机半导体作为捕光材料与手性催化剂耦合构成. 然而通常面临有机染料吸收谱带
窄、光腐蚀严重, 以及无机半导体材料的能带结构和带隙难以调控等问题. 直到目前, 仍然缺乏具有可调节带隙结构和高
循环稳定性的半导体材料. 最近, 一类通过共价键连接形成、具有晶态有序结构的共价有机骨架材料(COFs)被报道用于光
催化有机合成. 但多数COFs材料均由可逆的化学键(如B–O–B键、–C=N键等)构成, 其化学稳定性和光稳定性有待提升. 目
前已报道的一些稳定COFs均基于特殊的单体结构或通过后修饰策略制备, 其合成路线过长并需要对单体进行额外的修饰.
本文中, 我们发展了一种具有普适性的通过不可逆四氢喹啉连接的COFs材料的制备方法, 并考察了该材料在不对称光催
化反应中的活性和稳定性.
我们以1,3,5-三(4’-醛基苯基)苯、1,3,5-三(4’-氨基苯基)苯(或联苯胺)和乙烯基乙醚为原料, 以三氟甲磺酸钪和三氟甲磺
酸镱为催化剂, 通过一锅法波瓦罗夫反应制备了两种以不可逆四氢喹啉连接的新型共价有机骨架材料QH-COFs.
QH-COFs材料表现出了极高的化学稳定性, 在浓硫酸、浓盐酸、三氟乙酸或氢氧化钠溶液中浸泡三天, 其比表面积和结晶
性均没有发生明显变化. QH-COFs(作为半导体)和手性二级胺(为手性催化剂)在可见光下可高效催化醛的不对称α烷基化
反应, ee值高达94%. 由于QH-COFs具有窄的禁带宽度和适合的能带结构, 在相同条件下, QH-COFs的活性明显优于无机半
导体(如二氧化钛、钒酸铋、三氧化钨等). QH-COFs是迄今为止报道的在可见光下催化醛的不对称α烷基化反应活性最高
的半导体材料. 由于四氢喹啉结构的存在, QH-COFs具有极高的循环稳定性, 在循环五次之后, 活性和ee值没有发生明显
下降, 并且其仍保持高结晶性. 理论上所有可构筑亚胺COFs的单体均可通过该策略制备出稳定四氢喹啉结构的COFs材料,
且无需对单体作任何修饰. 该工作不仅为不对称光催化开辟了一条新途径, 同时也为构筑稳定的COFs材料提供了一种高
效、普适性方法.
关键词: 光催化; 共价有机骨架材料; 四氢喹啉; 不对称催化; 超高稳定性
收稿日期: 2019-11-27. 接受日期: 2019-12-23. 出版日期: 2020-08-05.
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基金来源: 国家自然科学基金(21733009, 21621063); 中国科学院战略性先导科技专项(XDB17020200).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).