Enantioselective Reductive Homocoupling of Allylic Acetates Enabled by Dual Photoredox/Palladium Catalysis: Access to C2-Symmetrical 1,5-Dienes

Article abstract of DOI:10.1021/jacs.1c06271

Transition-metal-catalyzed reductive coupling reactions have emerged as powerful protocols to construct C-C bonds. However, the development of enantioselective C(sp3)-C(sp3) reductive coupling remains challenging. Herein, we report a highly regio-, diastereo-, and enantioselective reductive homocoupling of allylic acetates through cooperative palladium and photoredox catalysis using diisopropylethylamine or Hantzsch ester as a homogeneous organic reductant. This straightforward protocol enables the stereoselective construction of C(sp3)-C(sp3) bonds under mild reaction conditions. A series of C2-symmetrical chiral 1,5-dienes were easily prepared with excellent enantioselectivities (up to >99% ee), diastereoselectivities (up to >95:5 dr), and regioselectivities (up to >95:5 rr). The resultant chiral 1,5-dienes can be directly used as chiral ligands in asymmetric synthesis, and they can be also transformed into other valuable chiral ligands.

Full text of DOI:10.1021/jacs.1c06271

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Article
Enantioselective Reductive Homocoupling of Allylic Acetates
Enabled by Dual Photoredox/Palladium Catalysis: Access to
C2‑Symmetrical 1,5-Dienes
Hong-Hao Zhang, Menghan Tang, Jia-Jia Zhao, Changhua Song, and Shouyun Yu*
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ABSTRACT: Transition-metal-catalyzed reductive coupling reac-
tions have emerged as powerful protocols to construct C−C bonds.
However, the development of enantioselective C(sp3)−C(sp3)
reductive coupling remains challenging. Herein, we report a highly
regio-, diastereo-, and enantioselective reductive homocoupling of
allylic acetates through cooperative palladium and photoredox
catalysis using diisopropylethylamine or Hantzsch ester as a
homogeneous organic reductant. This straightforward protocol
enables the stereoselective construction of C(sp3)−C(sp3) bonds
under mild reaction conditions. A series of C2-symmetrical chiral
1,5-dienes were easily prepared with excellent enantioselectivities (up to >99% ee), diastereoselectivities (up to >95:5 dr), and
regioselectivities (up to >95:5 rr). The resultant chiral 1,5-dienes can be directly used as chiral ligands in asymmetric synthesis, and
they can be also transformed into other valuable chiral ligands.
1,5-Dienes are valuable building blocks in organic syn-
thesis11 and are also frequently found in numerous bioactive
substances and natural products.12 Transition-metal-catalyzed
allyl−allyl coupling reactions have been proved to be powerful
protocols for the synthesis of chiral 1,5-dienes.13,14 In this field,
significant advances have been made by the Morken group
through Pd-catalyzed cross-coupling of allylic electrophiles and
allyl boronates.14a−d Recently, we have been interested in the
synthesis of C2-symmetrical chiral 1,5-dienes, which are widely
used as ligands in transition-metal-catalyzed asymmetric
transformations (Figure 1c).15 One of the most straightforward
methods accessing 1,5-dienes is reductive homocoupling of
two allylic electrophiles.16 The most formidable challenge is
the selectivity issue (Figure 1b). In the case of homocoupling
of two allylic electrophiles, there are up to five kinds of
selectivity issues (chemoselectivity, regioselectivity, Z/E
selectivity, diastereoselectivity, and enantioselectivity). The
construction of a C2-symmetrical chiral 1,5-diene out of up to
10 isomers regio-, diastereo-, and enantioselectively is an
unsolved synthetic problem to date.
INTRODUCTION
■
The selective formation of C−C bonds remains one of the
foremost challenges in organic synthesis. This is particularly
true for the construction of C(sp3)−C(sp3) bonds.1 Tran-
sition-metal-catalyzed coupling reactions between alkyl electro-
philes and alkyl nucleophiles have been recognized as some of
the most powerful and straightforward protocols to construct
C(sp3)−C(sp3) bonds efficiently and stereoselectively (Figure
1a, left).2 These traditional coupling reactions generally require
preformed alkyl metals or metalloids acting as nucleophiles.3
The reductive coupling of two alkyl electrophiles in the
presence of a terminal reductant can provide a potential
strategy to overcome these synthetic disadvantages (Figure 1a,
right).4 Historically, Wurtz coupling has been an important
method for the homocoupling of two sp3 carbons. However,
the use of highly reactive alkali metals somewhat limited its
synthetic applications.5 In the past few decades, significant
progress has been achieved in the transition-metal-catalyzed
cross- or homocoupling of two electrophiles.4 For example,
Weix,6 Gong,7 Reisman,8 and others9 have established a variety
of nickel-catalyzed reductive couplings of alkyl electrophiles.
Despite advances, most transition-metal-catalyzed reductive
couplings extensively require stoichiometric metal reductants,
such as Mn(0) or Zn(0), to turn over the catalysts.4 The
development of new reduction systems would be highly
desirable, and great challenges still remain. Another unsolved
challenge in this field is the control of stereoselectivity.10
Highly enantioselective reductive C(sp3)−C(sp3) coupling has
not yet been reported.
Visible-light photoredox catalysis has emerged as a powerful
tool to construct C−C bonds under mild and environmentally
Received: June 17, 2021
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© XXXX American Chemical Society
A

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Figure 2. Rationale for photoredox/Pd-cocatalyzed reductive
homocoupling of allylic electrophiles (LG = leaving group, PC =
photocatalyst).
conditions with the help of a reductant. Meanwhile, oxidative
addition of the allylic electrophile to a Pd(0) complex delivers
the Pd(II)-π-allyl species B. The allylic radical A is then
trapped by the π-allyl palladium complex B to provide the
Pd(III) complex C. Reductive elimination of the Pd(III)
complex C gives a C2-symmetrical chiral 1,5-diene as a
homocoupling product (Figure 2).
RESULTS AND DISCUSSION
■
To test our aforementioned hypothesis, we initiated our
studies with racemic allylic acetate (1a) as an allylic
electrophile. With a combination of Pd2(dba)3 (1.25 mol
%)/(R,R)-DACH-phenyl Trost ligand (L1, 3 mol %) and
Ir(ppy)2(dtbbpy)PF6 (I, 1 mol %) as an initial dual catalytic
system, Hantzsch ester (HE) as a reductant, and K2CO3 as a
base, the reaction proceeded smoothly under the irradiation of
45 W blue LEDs, affording the desired reductive homocou-
pling product (2a) in a good yield (86% yield determined by
GC analysis; Table 1, entry 1). However, poor regio-,
diastereo-, and enantioselectivities were observed (33% ee,
49:51 dr, 29:71 rr). In addition, the over-reduction product 1-
allyl-4-methoxybenzene could also be detected by GC-MS,
which indicated that the chemoselectivity issue of this reaction
was also a great challenge. After an evaluation of chiral ligands,
it was found that axially C2 symmetrical chiral diphosphine
ligands with different scaffolds, such as BINAP (L2), MeO-
BIPHEP (L3), and GARPHOS (L4), improved the regio- and
stereoselectivities remarkably (entries 2−4). When the steri-
cally more demanding GARPHOS L5 was employed (entry 5),
excellent diastereo- and enantioselectivities (>95:5 dr, >99%
ee) were achieved, although the regioselectivity (84:16 rr)
needed to be improved further. We then turned our attention
to the reductants and found that diisopropylethylamine
(DIPEA) as the reductant had positive effects on the regio-
and diastereoselectivities of this reaction (entries 6 and 7).
Other photocatalysts, such as Ir(ppy)3 (II), Ru(bpy)3Cl2 (III),
and eosin Y (IV), were evaluated, but lower yields and
regioselectivity were obtained in all cases (entries 8−10). To
our delight, the replacement of K2CO3 with Cs2CO3 gave
improved yields and regioselectivity (90% GC yield, 94:6 rr,
entry 12). Finally, an increase in concentration of this reaction
resulted in a further improved yield (96% GC yield and 92%
Figure 1. Transition-metal-catalyzed C(sp3)−C(sp3) couplings.
benign conditions.17 The photoredox cycle could serve as a
milder alternative for oxidation or reduction through single-
electron-transfer (SET) processes. Dual photoredox and
transition-metal catalysis has been applied to the area of
reductive reactions, enabling the coupling of organohalides
with the use of a homogeneous organic reducing agent.18,19
Despite this progress, enantioselective versions are still
limited.19l,o,20 Herein, we describe a dual photoredox/Pd
catalytic protocol for regio-, diastereo-, and enantioselective
reductive homocoupling of allylic electrophiles (Figure 2).
This cooperative catalysis strategy precludes preformed
organometallic reagents and stoichiometric metal-based
reductants used in the traditional C(sp3)−C(sp3) coupling
and reductive coupling reactions. This approach allows the
direct and efficient synthesis of C2-symmetrical chiral 1,5-
dienes.
We have recently achieved enantioselective photoredox/Pd-
cocatalyzed allylic alkylation of the racemic allylic acetates with
4-alkyl-1,4-dihydropyridines.21,22 During this investigation, we
could observe a mixture of the reductive homocoupling of
allylic acetates as the side products. 4-Alkyl-1,4-dihydropyr-
idines serve as both the alkylation reagents for the allylic
alkylation and the reductants for the reductive homocoupling.
On the basis of these observations, it is believed that
enantioselective homocoupling of allylic acetates can be
achieved by careful selection of a reductant and catalytic
system. Mechanistically, an allylic radical A can be generated
from the corresponding allylic electrophile under photoredox
B
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a
Table 1. Optimization of Reaction Conditions
b
c
b
b
entry
PC
ligand
[H]
HE
HE
HE
HE
HE
base
yield (%)
ee (%)
dr
rr
1
2
3
4
5
6
7
8
I
I
I
I
I
I
I
II
III
IV
I
I
I
L1
L2
L3
L4
L5
L5
L5
L5
L5
L5
L5
L5
L5
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K3PO4
Cs2CO3
Cs2CO3
86
80
83
82
88
74
76
63
49
50
85
90
33
74
80
49:51
89:11
90:10
90:10
>95:5
>95:5
>95:5
88:12
>95:5
94:6
29:71
64:36
70:30
75:25
84:16
90:10
83:17
41:59
48:52
34:66
94:6
88
>99
>99
>99
>99
>99
>99
>99
>99
>99
DIPEA
TEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
9
10
11
12
>95:5
>95:5
>95:5
94:6
>95:5
d
e
13
96 (92)
a
Reaction conditions unless specified otherwise: a solution of 1a (0.2 mmol), reductant (0.3 mmol), base (0.3 mmol), Pd2(dba)3 (1.25 mol %),
b
ligand (3 mol %), and photocatalyst (1 mol %) in CH3CN (4.0 mL) was irradiated by 45 W blue LEDs for 12 h. Yields, diastereomeric ratios (dr),
and regiomeric ratios (rr) were determined by GC analysis. Enantiomeric excess (ee) values were determined by HPLC on a chiral stationary
phase. The reaction was run in 2.0 mL of CH3CN. Isolated yield.
c
d
e
isolated yield) and stereoselectivity (>99% ee, >95:5 dr, and
>95:5 rr, entry 13).
unsymmetric 1,3-disubstituted allyl reagents in Pd-catalyzed
asymmetric allylation: (a) suppression of β-H elimination side
reactions24 and (b) dynamic kinetic asymmetric trans-
formation (DyKAT) of internal allyl electrophiles.25 To meet
these challenges and expand the scope of this reductive
homocoupling further, polysubstituted allyl acetates were
explored (Figure 4). We began with substrate 3a. Under the
previous optimal reaction conditions, the desired internal 1,5-
diene 4a was obtained with excellent enantio-, regio- and E/Z
selectivities (>99% ee, >95:5 rr, E:Z >95:5), but the yield
(36%) and diastereoselectivity (80:20 dr) were not satisfac-
tory. Fortunately, when HE was used as a reductant instead of
DIPEA, an improved yield (64%) and diastereoselectivity
(89:11 dr) were observed. A series of 1-methyl-3-phenylallyl
acetates with various substituents at different positions of the
phenyl ring were then examined. The reaction proceeded
smoothly to deliver chiral internal 1,5-dienes 4b−j in moderate
to high yields (63−87%) and excellent enantio-, regio-, and E/
Z-selectivities (>99% ee, >95:5 rr, E:Z >95:5). The alkyl group
could be ethyl (3k), propyl (3l), and pentyl (3m). The length
of the alkyl group had little effect on the reactivity and
selectivity. Cyclic allyl acetates were also suitable for the
reaction and provided corresponding products 4n−p in good
yields (78%, 68%, and 66%, respectively) and excellent
enantioselectivities (>99%, 96%, and 94% ee, respectively).
Furthermore, reductive homocoupling of 1,2,3-trisubstituted
allyl acetate 3q also proceeded well to deliver the desired
After establishing the optimal reaction conditions, we
investigated the scope and limitations of this dual photo-
redox/Pd-catalyzed enantioselective reductive allylic homo-
coupling. As shown in Figure 3, this strategy was applicable to
a wide range of allylic acetates bearing electronically distinct
substituents at different positions of the aryl moiety, offering
the corresponding C2-symmetrical chiral 1,5-dienes 2a−y in
moderate to high yields (45−95%), good diastereoselectivity
(85:15 to >95:5 dr), and excellent regio- and enantioselectiv-
ities (>95:5 rr and 96 to >99% ee). Specifically, allylic acetates
with electron-donating groups delivered higher reactivity and
diastereoselectivity in comparison to those with electron-
withdrawing groups. In general, para- and ortho-substituted
substrates reacted similarly, while lower yields and dr values
were obtained for meta-substituted substrate. Furthermore,
heterocycles and polycyclic aromatics, as well as heteroar-
omatics, which have significance in medicinal chemistry, were
also applicable. The S,S configuration of 2t was determined
unambiguously by X-ray crystallography.23
Monosubstituted allyl substrates have been widely used in
transition-metal-catalyzed enantioselective allyl−allyl coupling
reactions, while unsymmetrical 1,3-disubstituted allyl sub-
strates have been less studied.14 In the latter case, chiral
internal 1,5-dienes with Z/E selectivity would be obtained.
Two major challenges may prevent the use of racemic
C
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Figure 3. Scope of monosubstituted allyl acetates. Reaction conditions: a solution of 1 (0.2 mmol), DIPEA (0.3 mmol), Cs2CO3 (0.3 mmol),
Pd2(dba)3 (1.25 mol %), ligand L5 (3 mol %), and photocatalyst I (1 mol %) in CH3CN (2.0 mL) was irradiated by 45 W blue LEDs for 12 h.
Isolated yields are presented, and ee values were determined by HPLC on a chiral stationary phase. The diastereoselectivity and regioselectivity
1
were determined by H NMR. Unless otherwise noted, rr > 95:5.
product 4q with good yield (60%) and selectivity (90% ee).
Additionally, when L2 was used as the ligand, 1,3-diphenylallyl
acetate 3r (80% yield and 92% ee) was also a valid substrate
for this transformation.
To illustrate the utility of this dual catalytic enantioselective
reductive homocoupling, preparative-scale reactions were
performed. As shown in Figure 5a, under our established
conditions, the C2-symmetrical chiral 1,5-diene 2s was
obtained on synthetically useful scales with good yields (78%
for 4 mmol scale and 66% yield for 10 mmol scale) and
excellent stereoselectivities (>99% ee, >95:5 dr, and >95:5 rr).
Furthermore, the resulting C2-symmetrical chiral 1,5-dienes are
synthetically valuable building blocks and could be expediently
converted into a series of synthetically valuable optically active
compounds. The derivatizations of 2s were then explored. As
shown in Figure 5b, hydrogenation of 2s could be achieved
D
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Figure 4. Scope of polysubstituted allyl acetates. Reaction conditions: a solution of 3 (0.2 mmol), HE (0.3 mmol), Cs2CO3 (0.3 mmol), Pd2(dba)3
(1.25 mol %), ligand L5 (3 mol %), and photocatalyst I (1 mol %) in CH3CN (2.0 mL) was irradiated by 45 W blue LEDs for 12 h. Isolated yields
are presented, and ee values were determined by HPLC on a chiral stationary phase. The diastereoselectivity and regioselectivity were determined
1
by H NMR. Unless otherwise noted, rr > 95:5 and E/Z >95:5. Legend: (a) DIPEA was used; (b) L2 was used.
efficiently using catalytic Pd/C. Hydroboration of 2s and
subsequent Suzuki cross-coupling gave 6. The C2-symmetrical
chiral 1,6-diol 7 and 1,4-diol 9 could be synthesized from 2s
through hydroboration/oxidation or ozonolysis/reduction
without affecting optical purity. In addition, 7 and 9 could
be efficiently transformed into C2-symmetrical chiral 1,6-
diphenyl phosphinohexane 8 and 1,4-diphenyl phosphinobu-
tane 10, respectively. These results indicated the great
potential of the chiral dienes in the development of valuable
ligands and organocatalysts. Given that chiral dienes are highly
effective ligands for transition-metal-catalyzed enantioselective
reactions, the use of 2t as a new chiral diene ligand was
investagated for the Rh-catalyzed asymmetric arylation of 2-
cyclohexenone 11 and N-tosyl imine 14 with phenylboronic
acids 12 and 15 (Figure 5c). To our delight, the desired
products were generated in good enantioselectivities (13, 90%
ee; 16, 82% ee). This result demonstrates that this class of
chiral 1,5-dienes can be developed as new types of chiral diene
ligands.
(Figure 6a). We investigated on−off switching of the blue
LEDs of the reaction (Figure 6b). The reaction progressed
smoothly upon irradiation with light, while the reaction paused
during the “light-off” periods. These results reveal that this
reductive homocoupling undergoes a photoredox catalysis
process. Stern−Volmer quenching experiments indicate that
DIPEA quenches the photoexcited catalyst I (Figure 6c).
The isomeric allylic acetates 1a and 1a′ (Figure 7a, eq 1), as
well as an equimolar mixture of 1a and 1a′ (Figure 7a, eq 2),
gave similar results, respectively. In addition, we synthesized π-
allylpalladium complex 17, which was subjected to this
homocoupling, providing reductive homocoupling product
2b in 53% yield (Figure 7a, eq 3). These phenomena
demonstrate that a (π-allyl)palladium complex might be the
key intermediate of this reaction. The intermediacy of the
allylic radical was uncovered by radical trapping experiments
(Figure 7b). When the radical trapping reagent 2,2,6,6-
tetramethylpiperidine-N-oxyl radical (TEMPO) was added to
the model reaction of 1a, the reaction efficiency decreased
dramatically, and the allyl-TEMPO adduct 18 was isolated
(Figure 7b, eq 4). Meanwhile, 18 could not be detected in the
absence of photocatalysis and light irradiation, which revealed
To gain additional insight into the reaction, we performed
several control experiments (Figure 6). The photocatalyst, the
palladium catalyst, and light were all essential to this reaction
E
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Figure 6. Control experiments and Stern−Volmer fluorescence
quenching experiments.
On the basis of the aforementioned results and previous
published work on photoredox/Pd cocatalysis,27 a plausible
mechanism is depicted in Figure 8. DIPEA (E1/2 = +0.68 V vs
SCE)28 quenches visible-light-excited Ir(III)* (E1/2(IrIII*/IrII)
= +0.66 V vs SCE)26 to generate a low-valent Ir(II) complex
and the radical cation DIPEA•+.19d,j,n,28 Meanwhile, Pd(0)
oxidatively adds to the allylic acetate 1 to give a (π-allyl)PdII
complex A. The single-electron reduction of A by an Ir(II)
complex can generate the (π-allyl)PdI complex B,27i which can
equilibrate with an allylic radical species C and a Pd(0)
species.27c,e The second oxidative addition of another allylic
acetate 1 to B would give the bis(π-allyl)PdIII complex D
equilibrated with bis(η1-allyl)PdIII complex D′ (path a).14a−d
Alternatively, allylic radical species C is trapped by (π-allyl)PdII
complex A to generate D/D′ (path b).21,27a−e,g,h,j,n Reductive
elimination14a−d from D/D′ yields the homocoupling product
2 and the Pd(I) species E.21,27f,m,n,29 Finally single-electron
reduction of E most likely by the Ir(II) complex would
regenerate Pd(0).21,27i,m,n The Pd(III) intermediate D/D′
could also be reduced to a bis(allyl)PdII species and then
undergo a reductive elimination14a−d to deliver the final
product 2 (for more detailed discussions, see Figures S7 and
S8).27h,k
Figure 5. Scaled-up experiments and synthetic applications.
that the radical trapping product was not formed through the
directly nucleophilic addition of TEMPO to the formed (π-
allyl)palladium complex (Figure 7b, eq 5). In the presence of
TEMPO, the allyl radical trapping 19 was also observed in the
reaction of (π-allyl)palladium complex 17 (Figure 7b, eq 6).
These results suggest that an allylic radical might be generated
from the (π-allyl)palladium complex with the assistance of light
and the photocatalyst. In order to further confirm this
conclusion, the redox potentials of allyl acetate 1b (E1/2
−1.43 V vs SCE) and (π-allyl)palladium complex 17 (E1/2
=
=
−0.95 V vs SCE) were measured (for more details, see Figures
S5 and S6). The redox potentials indicate that the allyl radical
is more likely generated from (π-allyl)palladium complex 17,
which is more easily reduced than allylic acetate 1b by the
Ir(II) complex (E1/2(IrIII/IrII) = −1.51 V vs SCE)26 to give the
allylic radical.
CONCLUSION
■
In summary, we have described a highly regio-, diastereo-, and
enantioselective reductive homocoupling of allylic acetates
through cooperative palladium and photoredox catalysis.
DIPEA or Hantzsch ester (HE) was used as the reductant.
F
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This dual catalytic protocol allows access to C2-symmetrical
chiral 1,5-dienes in optically pure form. The resultant chiral
1,5-dienes can be directly used as chiral ligands in asymmetric
synthesis, and they can be also transformed into other valuable
chiral ligands. Further mechanistic studies, cross-coupling
variants, and synthetic applications of this photoredox/Pd-
cocatalyzed enantioselective reductive coupling are underway
in our laboratory.
ASSOCIATED CONTENT
■
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c06271.
General experimental procedures and characterization
1
data abd H NMR, 13C NMR, 19F NMR, and 31P NMR
spectral data (PDF)
Accession Codes
CCDC 2049849 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
Shouyun Yu − State Key Laboratory of Analytical Chemistry
for Life Science, Jiangsu Key Laboratory of Advanced Organic
Materials, Chemistry and Biomedicine Innovation Center
(ChemBIC), School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210023, People’s Republic of
China; orcid.org/0000-0003-4292-4714;
Email: yushouyun@nju.edu.cn
Authors
Hong-Hao Zhang − State Key Laboratory of Analytical
Chemistry for Life Science, Jiangsu Key Laboratory of
Advanced Organic Materials, Chemistry and Biomedicine
Innovation Center (ChemBIC), School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210023,
People’s Republic of China
Figure 7. Mechanistic investigations.
Menghan Tang − State Key Laboratory of Analytical
Chemistry for Life Science, Jiangsu Key Laboratory of
Advanced Organic Materials, Chemistry and Biomedicine
Innovation Center (ChemBIC), School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210023,
People’s Republic of China
Jia-Jia Zhao − State Key Laboratory of Analytical Chemistry
for Life Science, Jiangsu Key Laboratory of Advanced
Organic Materials, Chemistry and Biomedicine Innovation
Center (ChemBIC), School of Chemistry and Chemical
Engineering, Nanjing University, Nanjing 210023, People’s
Republic of China
Changhua Song − State Key Laboratory of Analytical
Chemistry for Life Science, Jiangsu Key Laboratory of
Advanced Organic Materials, Chemistry and Biomedicine
Innovation Center (ChemBIC), School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210023,
People’s Republic of China
Figure 8. Proposed mechanism.
Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Natural Science
Foundation of China (21971110, 21732003, and 22001120)
and the Natural Science Foundation of Jiangsu Province
(BK20200297) is acknowledged.
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