Cycloaddition of Aziridine with CO2/CS2 Catalyzed by Amidato Divalent Lanthanide Complexes

Article abstract of DOI:10.1021/acs.joc.8b02924

This is the first time that the amidato lanthanide amides ({LⁿLn[N(SiMe₃)₂]THF}₂ (n = 1, Ln = Eu (1); n = 2, Ln = Eu (3), Yb (4); HL¹ = ^tBuC₆H₄CONHC₆H₃(ⁱPr)₂; HL² = C₆H₅CONHC₆H₃(ⁱPr)₂) and {L³Eu[N(SiMe₃)₂]THF}{L³₂Eu(THF)₂} (2) (HL³ = ClC₆H₄CONHC₆H₃(ⁱPr)₂)) were applied in the cycloaddition reactions of aziridines with carbon dioxide (CO₂) or carbon disulfide (CS₂) under mild conditions. The corresponding oxazolidinones and thiazolidine-2-thiones were obtained in good to excellent yields with good functional group tolerance.

Full text of DOI:10.1021/acs.joc.8b02924

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Article
Cycloaddition of Aziridine with CO2/CS2 Catalyzed
by Amidato Divalent Lanthanide Complexes
Yueqin Xie, Chengrong Lu, Bei Zhao, Qianyu Wang, and Yingming Yao
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b02924 • Publication Date (Web): 16 Jan 2019
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The Journal of Organic Chemistry
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Cycloaddition of Aziridine with CO2/CS2 Catalyzed by Amidato Divalent
Lanthanide Complexes
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Yueqin Xiea, Chengrong Lua,b, Bei Zhao*a,b, Qianyu Wang and Yingming Yaoa,b
Key Laboratory of Organic Synthesis of Jiangsu Province,a College of Chemistry, Chemical Engineering and Materials Sci-
ence,b Soochow University, Suzhou 215123, People’s Republic of China.
ABSTRACT: This is the first time that the amidato lanthanide amides ({LnLn[N(SiMe3)2]THF}2 (n = 1, Ln = Eu (1); n = 2, Ln =
Eu (3), Yb (4); HL1 = tBuC6H4CONHC6H3(iPr)2; HL2 = C6H5CONHC6H3(iPr)2) and {L3Eu[N(SiMe3)2]THF}{L3 Eu(THF)2} (2);
2
(HL3 = ClC6H4CONHC6H3(iPr)2)) were applied in the cycloaddition reactions of aziridines with carbon dioxide (CO2) or carbon
disulfide (CS2) under mild conditions. The corresponding oxazolidinones and thiazolidine-2-thiones were obtained in good to ex-
cellent yields with good functional group tolerance.
Introduction Oxazolidinones are the main component of a
class of antimicrobial agents. Also, oxazolidinones are raw
(salen)Co(I) complexes,5b-c small molecules,5d ILs,5e loaded
macromolecule,5f-h and mesoporous materials.5i-j Although
significant advantages have been made, most of them still
have some problems, for instance, high pressure (60 atm),
elevated temperature (120 C), relative high catalyst loading
(10 mol %) or toxic co-solvent (CH2Cl2) was required.
1
material in many transformations, including aldol condensa-
tion,2a,2b alkylation reactions,2c Diels-Alder cycloaddition reac-
tions,2d alcoholysis reaction,2e stereoselective oxidation.2f
Consequently, the synthesis of oxazolidinones has attracted
great attention. As the primary greenhouse gas, CO2 is a grow-
ing problem in climatic change and chemical transformation of
CO2 is worthy studying.3 Several methods using CO2, which
is often considered as an attractive C1 building block, for the
formation of oxazolidinones were developed, for example,
carbonylation of amino alcohols with phosgene/CO2, 4a reac-
tions of propargyl/acetylenic amines/epoxy amines with
CO2,4b-e coupling reactions between epoxides and phenyl car-
bamate,4f and multicomponent reaction using propargylamines,
arylhalides and CO2.4g The study of the aziridine/CO2 coupling
reaction to obtain oxazolidinones was also discovered. Alkali
metal halides were applied to the transformation.5a The reac-
tion can also be catalyzed by iodine in supercritical CO2 with
good outcome. However, the scope of substrates is limited to
o
The rare-earth metal complexes, especially amidato divalent
lanthanide amides,6 have high reactivities in many transfor-
mations, including forming 2,4-quinazolidinones from CO2
and 2-aminobenzonitriles,6b and carboxylation of terminal
alkynes with CO2.7a And also, rare-earth metal complexes
showed reactivities in the reaction of epoxides with CO2 under
mild condition.7b-7d It indicates that rare-earth metal complexes
do fairly well in the chemical fixation of CO2. Thus, starting
from CO2 and aziridines to realize the formation of oxazoli-
dinones catalyzed by rare-earth metal complexes is promising
and quite meaningful.
Thiazolidine-2-thiones, sulfur-substituted analogues to oxa-
zolidinones, are also useful intermediates in pharmaceutical
chemistry8 and some enantioselective reactions (e.g aldol reac-
tions,9a,9b alkylation reactions,9c cross-coupling reaction.9d).
The synthetic strategies for the construction of thiazolidine-2-
thione skeleton include the reaction of CS2 with amino alco-
hols,10a-10b the tandem umpolung addition and intramolecular
cyclization of bifunctional sulfur pronucleophiles on ar-
aliphatic
aziridines.
(Salen)chromium(III)/4-(N,N-
dimethylamino)pyridine was reported to be an effective cata-
lyst for the coupling of CO2 and aziridines, which successfully
extended the substrates to monosubstituted N-aryl aziridines.5b
Some other catalysts have also been exploited for the trans-
formation, such as Al-based, Cr-based metal complexes and
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ylpropiolates with a phosphine catalyst,10c reaction of diben-
Page 2 of 8
gather and activate CO2 molecule. 6b,13 The increasing yield of
74% of cycloaddition products 6a and 7a, with excellent regi-
oselectivity of 99:1 was obtained after 24 hours at 50 oC in the
presence of 2 mol% DBU (Table 1, entries 2). The same in-
creasing tendency was observed using complex 1 as the cata-
lyst, the overall yield of 6a and 7a up to 90% in the presence
of DBU (Table 1, entries 12 and 14). The yield of 6a was very
poor when another frequently used basic additive DABCO
was added instead of DBU (Table 1, entry 13). Obviously,
solvent free is optimal choice, because some representative
coordinating solvent (DME, DMSO), non-coordinating polar
solvent (CHCl3) were tested and did not lead to higher yields
(Table 1, entries 2 and 6-8). Complexes 1-4 and the precursor
Eu[N(SiMe3)2]2 were tested in the reaction. The amidato diva-
lent europium complex 1 was proved as the optimal catalyst
for cycloaddition of N-ethyl-2-phenylaziridine with CO2 (Ta-
ble 1, entries 2 and 9-12).
1
2
3
4
5
6
7
8
zoylacetylene/racemic α-chloro-β,γ-alkenoate esters with CS2
and amines,10d and the cyclization reaction with propargyla-
mine and CS2.10e Cycloaddition of CS2 with aziridines is a
most step- and atom-economic approach to construct thiazoli-
dine-2-thiones. Catalysts for this transformation involve or-
ganoantimony halide,10f tetrabutylammonium halide,5a tribu-
tylphosphine,10g functionalized ion-exchange resin,5g and
amine.10h The main drawbacks are relative limited substrates
and harsh reaction conditions, such as high temperature
(180 °C) and large dose of catalysts (25 mol%). Seeking for
high efficient catalyst to fulfill the cycloaddition of CS2 with
aziridines in mild condition is still necessary.
Though the properties of CS2 and CO2 are similar,11 CS2 is
considered to be more reactive owing to the weaker C=S dou-
ble bond.12 It is thus divinable that the reaction of more active
CS2 with aziridines catalyzed by the amidato divalent lantha-
nide amides is feasible.
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Table 1 Screening of conditions for the reaction of CO2
and N-ethyl-2-phenylaziridine[a]
Results and Discussion Four amidato divalent lanthanide
complexes were synthesized by simple metathesis of the pro-
ligand HL with appropriate lanthanide precursors
Ln[N(SiMe3)2]2 (Ln = Eu or Yb) in THF at 60 oC (Scheme 1).
Scheme 1. Synthesis of amidato divalent lanthanide amides
1-4
En
try
Cat.
T
Sovent[b Co-
Yield
[%][c]
Regio-
sel.[d]
]
[oC]
catalyst
1
2
30
50
60
80
100
50
50
50
50
50
50
-
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
30
74
69
56
36
5
99:1
99:1
98:2
98:2
96:4
99:1
98:2
99:1
99:1
99:1
99:1
3
3
3
3
3
3
3
3
4
2
-
3
-
4
-
5
-
6
DME
7
CHCl3
7
8
DMSO
10
75
83
34
9
-
-
-
10
11
Eu[N(Si
Me3)2]2
12
13
14
50
50
50
-
-
-
DBU
90
98:2
99:1
99:1
1
1
1
DABCO 11
78
The reaction of CO2 with N-ethyl-2-phenylaziridine 5a was
catalyzed by 1 mol% amidato divalent lanthanide complex 3
under ambient pressure. The preliminary result showed the
formation of four kinds of products in the model reaction.
Compounds 6a and 7a are the main products with the overall
yield of 62%, while compounds 8 and 9 are dimeric products
of N-ethyl-2-phenyl aziridine (Scheme 2). It is consistent with
the results of several cases.5g-h
-
[a] Reaction conditions: 5a (2.0 mmol), CO2 (1 atm); [b] Solvent
[c]
(2 mL);
Determined by GC with biphenyl as an internal
standard; [d] Molar ratio of 6a to 7a.
As shown in table 2, various aziridines react with CO2 effi-
ciently, and the target products with high yields and excellent
selectivities were obtained. The high yields of oxazolidinones
were gained when the groups attached to the nitrogen atom
have relative less steric hindrance (Table 2, entries 1-4). There
was no significant impact of the electronic effects of the sub-
stituents on the phenyl ring, and the yields of the correspond-
ing oxazolidinones are high as well (Table 2, entries 5-9).
However, the 54% yield of oxazolidinone 6k and 7k, without
substituent on N atom, decreased significantly. It may due to
the extreme instability of the starting material 5k. A reported
case of 6k can get a moderate yield of 80%, but 50 atmospher-
ic pressures was required.5f In addition, an excellent yield of
Scheme 2. Cycloaddition reaction of CO2 with N-ethyl-2-
phenylaziridine
To improve the yield of cycloaddition product, 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) was added to the reac-
tion system, which is widely regarded as a kind of reagent to
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6b (>90%) was obtained in previous reports, but the regiose-
yield was only 11% in the absence of catalysts (Table 3, entry
17). The results suggest that the divalent lanthanide amides
modified by appropriate amidato ligands promote the model
reaction of CS2 with N-ethyl-2-phenylaziridine 5a, and the
divalent europium amides perform better in the transformation.
Overall, the reaction catalyzed by complex 1 gave the best
result of 96% at 45 °C after 24 h reaction, while the product
10a can be only obtained in medium yield under higher tem-
perature (100 or 180 oC) in other cases. 10h
lectivities of 6b:7b was relatively poor (90:10 5b or 97:3 5i).
1
2
3
4
5
6
7
8
Table 2 Cycloaddition reaction of various aziridines with
[a]
CO2
9
Yield
[%][b]
Regio-
sel.[c]
Entry Aziridine
R1
R2
Scheme 3 A proposed mechanism for the cycloaddition of
CO2 and aziridines by amidato divalent lanthanide amides.
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11
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1
2
H
H
Et
n-propyl
n-butyl
i-butyl
Et
92
89
88
79
83
73
87
82
43
54
99:1
99:1
99:1
99:1
99:1
99:1
99:1
99:1
99:1
93:7
5a
5b
5c
5d
5f
3
H
4
H
5
Cl
6
Br
Et
5g
5h
5i
7
Methyl
t-butyl
Methoxy
H
Et
8
Et
9
Et
5j
10
H
5k
aReaction condition: 5a-k (2 mol), CO2 (1 atm), cat 1 (1 mol%),
DBU (2 mol%), solvent free; Isolated yield of product; c Molar
b
ratio of 6 to 7.
Based on the previous works,5b,14 the possible catalytic cycle
of the cycloaddition of aziridines and CO2 is proposed in
scheme 3. Firstly, the starting material aziridine is activated by
the europium amide through the coordination of the nitrogen
atom to the Lewis acidic europium center, resulting in the
formation of a Lewis acid-Lewis base complex A. The nucle-
ophilic attack of the DBU activated CO2 species B was favored
on the partial positive charged benzylic carbon, which lead to
the ring-opening of aziridine to produce intermediate C. Next,
an intramolecular nucleophilic attack occurs to realize cycliza-
tion product D, meanwhile DBU is released. Obviously, the
increased steric hindered R group attached to nitrogen atom
has negative effect on the nucleophilic addition. Finally, prod-
uct 6a was produced via the intermolecular replacing by a new
aziridine molecule, and intermediate A was regenerated.
Table 3 Screening of reaction conditions[a]
Encouraged by the successful reaction of N-ethyl-2-
phenylaziridine and CO2, the model reaction of CS2 with N-
ethyl-2-phenylaziridine 5a was catalyzed by 1 mol% amidato
divalent lanthanide complex 1 smoothly, which produced the
desired thiazolidine-2-thione 10a in 77% yield at 30 C for 24
hours. To our delight, after screening of the reaction tempera-
En-
try
Yield
[%][c]
Cat
T [oC]
t [h]
Solvent[b]
1
2
30
40
45
50
45
45
45
45
45
45
45
24
24
24
24
48
6
-
-
-
-
77
85
96
73
98
47
61
1
1
1
1
1
1
1
1
1
1
1
1
o
o
ture, the yield of 10a reached up to 96% at 45 C (Table 3,
3
entries 1-4). Results of evaluation of reaction time and sol-
vents proved that solvent free and 24 hours are the optimal
reaction conditions (Table 3, entries 5-12). Complexes 1-4 and
the precursor Eu[N(SiMe3)2]2 showed similar reactivities ten-
dency as that for the reaction of N-ethyl-2-phenylaziridine
with CO2.The central metal have striking difference in the
catalytic reactivities, for the 15% yield of 10a catalyzed by
ytterbium complex 4 is significantly lower than 76% of the
europium complex 3 (Table 3, entries 14-15). The divalent
precursors Eu[N(SiMe3)2]2 was also tested in the reaction,
which gave out 10a only 58% (Table 3, entry 16), while the
4
5
6
-
7
12
24
24
24
24
-
8
THF
Tol
9
7
10
11
DME
DMF
6
66
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12
13
14
15
16
17
45
45
45
45
45
45
24
24
24
24
24
24
CH3CN
77
41
76
15
58
11
1
1
2
3
4
5
6
7
8
-4
-5
-6
-7
y=1.6089x+1.6464
R2=0.9658
-
-
-
-
-
2
3
4
Eu[N(SiMe3)2]2
-
aReaction conditions: 5a (1 mmol), CS2 (5 mmol), cat (1
mol %); Solvent: 2 mL; Determined by GC with biphenyl as
an internal standard.
b
c
9
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The results of the cycloaddition reaction of various N-
substituted aziridines with CS2 catalyzed by complex 1 are
presented in Table 4. It shows that most of N-substituted aziri-
dines reacted with CS2 smoothly and the target products were
obtained in high to excellent yields. N-isobutyl-2-
phenylaziridine was found to be least reactive, which is proba-
bly due to the relatively high steric hindrance. An improved
yield of 83% of N-isobutyl-5-phenylthiazolidine-2-thione 10d
was obtained by increasing catalyst loading to 3 mol % (Table
4, entry 4). Aziridines bearing strong electron-donating groups
on the aromatic ring, such as t-butyl and methoxyl, exhibited
relatively low reactivities, and gave moderate yields of 70%
and 50%, respectively (Table 4, entries 9-10).
-5.0
-4.5
-4.0
-3.5
Ln[Cat]
Fig. 1 Plot of LnKapp versus Ln[Cat.]. Conditions: complex 1,
[5a]0 : [CS2]0 = 1:5, 45 °C, solvent free, determined by GC with
biphenyl as an internal standard.
Finally, the method applied in the gram scale synthesis of
compounds 6a and 10a were carried out in the presence of
catalyst 1, respectively. The outcomes are satisfying (Scheme
4).
Scheme 4 Gram scale synthesis of compounds 6a and 10a
Table 4 Cycloaddition reaction of various aziridines with
[a]
CS2
CONCLUSION
In summary, four amidato divalent lanthanide amides
were applied to catalyze the cycloaddition reaction of aziri-
dines with CO2 and CS2, respectively. The europium complex
1 {tBuC6H4CONC6H3(iPr)2Eu[N(SiMe3)2]THF}2 was proved
to be the most efficient catalyst for the cycloaddition reactions
of aziridines. Both oxazolidinones and thiazolidine-2-thiones
were prepared in high to excellent yield using 1 mol% of
complex 1 under mild reaction conditions. A plausible mecha-
nism of the cycloaddition of CO2 and aziridines by amidato
divalent lanthanide amides was raised. A kinetic study reveals
that it has a reverse-first-order reaction of aziridine in the reac-
tion of aziridines with CS2.
Entry Aziridine
R1
R2
Et
Yield [%][b]
1
2
H
95[c]
5a
5b
5c
5d
5e
5f
H
n-propyl
n-butyl
i-butyl
Bn
97
3
H
H
87
35 (83[d])
4
5
H
71
6
Cl
Et
89
7
Br
Et
85
5g
5h
5i
8
Methyl
t-butyl
Methoxyl
H
Et
85
9
Et
70
EXPERIMENTAL SECTION
10
11
Et
50
5j
General procedures. CO2 was supplied from Wu Jiang Mayer
industrial gas co., Ltd with a purity of >99.999%, CS2 was sup-
plied from Shanghai Kefei Industry Co., Ltd. Aziridines were
prepared according to the reported method.5h Four known amidato
divalent lanthanide amides, (LnLn[N(SiMe3)2]THF}2 (n = 1, Ln =
H
86
5k
aReaction conditions : 5a-k (1 mmol), CS2 (5 mmol), 48 h, solvent
b
c
d
free; Isolated yield of product; Reaction time: 24 h; cat 1 (3
mol %).
Eu (1);
n
=
2, Ln
=
Eu (3), Yb (4); HL1
=
tBuC6H4CONHC6H3(iPr)2; HL2 = C6H5CONHC6H3(iPr)2) and
To gain more insight into the cyclization reaction, the re-
action rate was measured. When N-ethyl-2-phenylaziridine 5a
was treated with 5-fold excess of CS2, the linear plot of
Ln([5a]0/[5a]t) versus time indicates a first-order reaction of
N-ethyl-2-phenylaziridine (SI.28). Furthermore, the rate order
of catalyst was determined to be 1.6, which implies that both
mono- and dinuclear species may exist in catalytic cycle (Fig.
1)
{L3Eu[N(SiMe3)2]THF}{L3 Eu(THF)2}
(2);
(HL3
=
2
ClC6H4CONHC6H3(iPr)2)) were prepared according to the litera-
ture methods.6a, 6b Solvents were distilled from sodium benzophe-
1
none ketyl under argon prior to use unless otherwise noted. H,
13C NMR spectra were obtained with a Bruker-400 spectrometer
in CDCl3. High resolution mass (HRMS) spectra were obtained
using Bruker ESI-TOF. GC analyses were performed on a thermo
trace 1300 gas chromatograph.
General procedure for the synthesis of aziridines.5h Dimethyl
sulfide (12.4 g, 0.2 mol) and bromine (32.0 g, 0.2 mol) were sepa-
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rately dissolved in dry dichloromethane (40 mL and 40 mL re-
1-ethyl-2-(4-methoxyphenyl)aziridine (5j). Colorless liquid. 1H
NMR (400 MHz, CDCl3): δ 7.20 – 7.10 (m, 2H), 6.90 – 6.68 (m,
2H), 2.50 – 2.35 (m, 2H), 2.26 (dd, J = 6.5, 3.4 Hz, 1H), 1.85 (t, J
= 6.1 Hz, 1H), 1.60 (d, J = 6.5 Hz, 1H), 1.21 (dt, J = 14.3, 7.1 Hz,
3H) ppm.
spectively). The bromine solution was then added dropwise over
30 min to an ice-cooled solution of dimethyl sulfide. During the
addition, light orange crystals of bromodimethyl sulfonium bro-
mide began to separate. Then, the crystals were collected by filtra-
tion and washed with dry ether and dried under vacuum. Olefin
(160 mmol) was added dropwise to the 160 mL acetonitrile solu-
tion of the orange crystals (35.56 g, 160 mmol) in ice-water bath.
During the addition, the white solid began to separate. The solu-
tion was stirred for 10 min after the addition of olefin was com-
pleted. The alkenyl sulphonium bromide were collected by filtra-
tion, dried under vacuum. To a stirred solution of alkenyl sulpho-
nium bromide (10 mmol) in 20 mL of water at r.t., amine (20-50
mmol) was added dropwise and the resulting mixture was stirred
overnight. The mixture was poured into 20 mL of saturated brine,
extracted with diethyl ether (3×20 mL), dried with MgSO4 and the
solvent evaporated under reduced pressure. Pure product was
obtained by distillation under reduced pressure.
1
2
3
4
5
6
7
8
2-phenylaziridine (5k).5h Colorless liquid. 1H NMR (400 MHz,
1
CDCl3): H NMR (400 MHz, CDCl3): δ 7.29 (m, 2H), 7.21 (m,
3H), 2.95 (m, 1H), 2.15 (m, 1H), 1.74 (m, 1H), 0.91 (br, 1H) ppm.
General procedure for the catalytic ring-opening reaction of
aziridines with CO2. All manipulations and reactions were con-
ducted under a purified argon atmosphere by using standard
Schlenk techniques. A 5 mL Schlenk tube under dried argon was
charged with complex 1 (0.02 mmol), aziridine (2 mmol) and a
stirring bar. After the catalyst was dissolved, 1,8-
diazabicyclo[5.4.0]-undec-7-ene (DBU, 0.04 mmol) was added
into the tube. Thereafter, purging the Schlenk flask with CO2
three times, then carbon dioxide was introduced with a balloon at
ambient pressure. The reaction mixture was stirred at oil bath of
50 °C for 48 h, then cooled down to room temperature, quenched
with ethyl acetate, then biphenyl (1 mmol) was added as an inter-
nal standard for the analysis of product yields by GC, The crude
mixture was purified by column chromatography on silica gel
(200−300 mesh, eluting with 20:1 petroleum ether/ethyl acetate to
give desired products).
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Characterization data of aziridines. 1-ethyl-2-phenylaziridine
1
(5a).5h Colorless liquid. H NMR (400 MHz, CDCl3): δ 7.33 –
7.14 (m, 5H), 2.49 – 2.38 (m, 2H), 2.28 (dt, J = 12.1, 6.1 Hz, 1H),
1.88 (d, J = 3.3 Hz, 1H), 1.63 (d, J = 6.6 Hz, 1H), 1.19 (t, J = 7.1
Hz, 3H) ppm.
2-phenyl-1-propylaziridine (5b).5h Colorless liquid. 1H NMR (400
MHz, CDCl3): δ 7.31 – 7.17 (m, 5H), 2.47 – 2.39 (m, 2H), 2.28
(dt, J = 11.0, 5.5 Hz, 1H), 1.88 (d, J = 3.3 Hz, 1H), 1.63 (d, J =
6.6 Hz, 1H), 1.19 (t, J = 7.1 Hz, 3H) ppm.
Characterization data of oxazolidinones.
3-ethyl-5-phenyloxazolidin-2-one (6a).5h Colorless liquid. Yield:
1
92%, 351.6 mg; H NMR (400 MHz, CDCl3): δ 7.42 – 7.31 (m,
1
1-butyl-2-phenylaziridine (5c).5h Colorless liquid. H NMR (400
5H), 5.53 – 5.38 (m, 1H), 3.91 (t, J = 8.7 Hz, 1H), 3.46 – 3.24 (m,
3H), 1.16 (t, J = 7.3 Hz, 3H) ppm. 13C NMR (100 MHz, CDCl3):
δ 157.2, 138.3, 128.3, 125.0, 73.8, 51.1, 38.4, 12.0 ppm. HRMS
(ESI-TOF) m/z: [M + H]+ calcd for C11H14NO2: 192.1019, found:
192.1016.
MHz, CDCl3): δ 7.38 – 7.24 (m, 5H), 2.56 (dt, J = 11.5, 7.3 Hz,
1H), 2.44 – 2.30 (m, 2H), 1.95 (d, J = 3.3 Hz, 1H), 1.67 (ddd, J =
21.2, 12.9, 6.6 Hz, 3H), 1.54 – 1.39 (m, 2H), 0.98 (t, J = 7.3 Hz,
3H) ppm.
5-phenyl-3-propyloxazolidin-2-one (6b).5h Colorless liquid. Yield:
89%, 365.1 mg; H NMR (400 MHz, CDCl3): δ 7.45–7.30 (m,
1-isobutyl-2-phenylaziridine (5d).5h Colorless liquid. 1H NMR
(400 MHz, CDCl3): δ 7.38 – 7.23 (m, 5H), 2.49 (dd, J = 11.6, 7.1
Hz, 1H), 2.36 – 2.28 (m, 1H), 2.11 (dd, J = 11.6, 6.5 Hz, 1H),
2.02 – 1.88 (m, 2H), 1.69 (d, J = 6.5 Hz, 1H), 1.01 (dt, J = 17.6,
8.8 Hz, 6H) ppm.
1
5H), 5.53–5.43 (m, 1H), 3.91 (t, J = 8.8 Hz, 1H), 3.42 (dd, J = 8.7,
7.4 Hz, 1H), 3.33–3.17 (m, 2H), 1.58 (h, J = 7.4 Hz, 2H), 0.93 (t,
J = 7.4 Hz, 3H) ppm. 13C NMR (100 MHz, CDCl3): δ 157.5,
138.4, 128.3, 125.0, 73.8, 51.6, 45.3, 20.1, 10.5 ppm. HRMS
(ESI-TOF) m/z: [M + Na]+ calcd for C12H15NNaO2: 228.0995,
found: 228.1003.
1
1-benzyl-2-phenylaziridine (5e).5h Colorless liquid. H NMR (400
MHz, CDCl3): δ 7.37 – 7.14 (m, 10H), 3.61 (dd, J = 31.1, 13.8 Hz,
2H), 2.46 (dd, J = 6.5, 3.3 Hz, 1H), 1.95 (d, J = 3.3 Hz, 1H), 1.80
(d, J = 6.5 Hz, 1H) ppm.
3-butyl-5-phenyloxazolidin-2-one (6c).5h Colorless liquid. Yield:
1
88%, 385.4 mg; H NMR (400 MHz, CDCl3): δ 7.42–7.26 (m,
2-(4-chlorophenyl)-1-ethylaziridine (5f).5h Colorless liquid. 1H
NMR (400 MHz, CDCl3): δ 7.22 (dt, J = 30.7, 6.2 Hz, 4H), 2.42
(q, J = 7.1 Hz, 2H), 2.25 (dd, J = 6.5, 3.2 Hz, 1H), 1.82 (d, J = 3.2
Hz, 1H), 1.64 (d, J = 6.5 Hz, 1H), 1.18 (t, J = 7.1 Hz, 3H) ppm.
5H), 5.50–5.35 (m, 1H), 3.95–3.82 (m, 1H), 3.45–3.15 (m, 3H),
1.53 (dt, J = 14.3, 7.2 Hz, 2H), 1.34 (dd, J = 14.6, 7.5 Hz, 2H),
0.97–0.86 (m, 3H) ppm. 13C NMR (100 MHz, CDCl3): δ 157.9,
138.9, 128.8, 125.5, 74.3, 52.1, 43.9, 29.4, 19.8, 13.7 ppm. HRMS
(ESI-TOF) m/z: [M + Na]+ calcd for C13H17NNaO2: 242.1151,
found: 242.1151.
2-(4-bromophenyl)-1-ethylaziridine (5g).5h Colorless liquid. 1H
NMR (400 MHz, CDCl3): δ 7.37 (t, J = 16.4 Hz, 2H), 7.11 (d, J =
8.4 Hz, 2H), 2.42 (q, J = 7.1 Hz, 2H), 2.25 (dd, J = 6.5, 3.2 Hz,
1H), 1.81 (t, J = 10.6 Hz, 1H), 1.63 (dd, J = 17.0, 4.7 Hz, 1H),
3-isobutyl-5-phenyloxazolidin-2-one (6d).5h Colorless liquid.
Yield: 79%, 346.2 mg; 1H NMR (400 MHz, CDCl3): δ 7.45–7.29
(m, 5H), 5.47 (d, J = 7.4 Hz, 1H), 3.90 (s, 1H), 3.40 (s, 1H), 3.08
(dd, J = 9.8, 7.5 Hz, 2H), 1.95–1.77 (m, 1H), 0.91 (dd, J = 9.0, 4.4
Hz, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ 157.8, 138.5, 128.4,
125.0, 73.8, 52.4, 51.3, 26.4, 19.4 ppm. HRMS (ESI-TOF) m/z:
[M + Na]+ calcd for C13H17NNaO2: 242.1151, found: 242.1161.
1.26 – 1.10 (m, 3H) ppm.
1
1-ethyl-2-(p-tolyl)aziridine (5h).5i Colorless liquid. H NMR (400
MHz, CDCl3): δ 7.13 (q, J = 8.1 Hz, 4H), 2.52 – 2.36 (m, 2H),
2.33 (s, 3H), 2.28 (dd, J = 6.5, 3.4 Hz, 1H), 1.89 (t, J = 5.3 Hz,
1H), 1.63 (d, J = 6.6 Hz, 1H), 1.20 (t, J = 7.1 Hz, 3H) ppm.
5-(4-chlorophenyl)-3-ethyloxazolidin-2-one (6f).5h White solid.
1
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Yield: 83%, 373.6 mg; H NMR (400 MHz, DMSO-d6): δ 7.50
2-(4-(tert-butyl)phenyl)-1-ethylaziridine (5i). Colorless liquid. H
(dd, J = 26.0, 8.4 Hz, 4H), 5.60 (t, J = 7.9 Hz, 1H), 4.06–3.86 (m,
1H), 3.51–3.38 (m, 1H), 3.26 (tq, J = 14.0, 7.0 Hz, 2H), 1.12 (t, J
= 7.2 Hz, 3H) ppm. 13C NMR (100 MHz, DMSO-d6): δ 157.2,
138.8, 133.6, 129.2, 128.4, 73.4, 50.9, 38.7, 12.8 ppm. HRMS
NMR (400 MHz, CDCl3):δ 7.37 – 7.29 (m, 2H), 7.24 – 7.14 (m,
2H), 2.52 – 2.36 (m, 2H), 2.32 – 2.24 (m, 1H), 1.90 (d, J = 3.3 Hz,
1H), 1.64 (t, J = 4.9 Hz, 1H), 1.32 (d, J = 7.4 Hz, 9H), 1.23 – 1.15
(m, 3H) ppm.
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(ESI-TOF) m/z: [M + Na]+ calcd for C11H12ClNNaO2: 248.0449,
HRMS (ESI-TOF) m/z: [M + H]+ calcd for C11H14NS2: 224.0562,
found: 224.0557. MP: 63.3-66.9 oC.
found: 248.0447. MP: 59.0-66.4 oC.
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5-(4-bromophenyl)-3-ethyloxazolidin-2-one (6g).15 White solid.
Yield: 73%, 392.7 mg; 1H NMR (400 MHz, DMSO-d6): δ 7.63 (d,
J = 8.4 Hz, 2H), 7.37 (d, J = 8.4 Hz, 2H), 5.62–5.48 (m, 1H), 3.96
(s, 1H), 3.41–3.37 (m, 1H), 3.29–3.17 (m, 2H), 1.08 (t, J = 7.2 Hz,
3H)ppm. 13C NMR (100 MHz, DMSO-d6): δ 161.9, 143.9, 136.88,
133.4, 126.9, 78.2, 55.6, 43.5, 17.5 ppm. HRMS (ESI-TOF) m/z:
[M + Na]+ calcd for C11H12BrNNaO2: 291.9944, found: 291.9938.
MP: 57.8-65.5 oC.
5-phenyl-3-propylthiazolidine-2-thione (10b). Yellow solid. Yield:
97%, 229.9 mg; H NMR (400 MHz, CDCl3): δ 7.47 –7.30 (m,
1
5H), 4.91–4.77 (m, 1H), 4.40 (dd, J = 11.6, 8.4 Hz, 1H), 4.08 (dd,
J = 11.6, 6.9 Hz, 1H), 3.89–3.67 (m, 2H), 1.82–1.62 (m, 2H), 0.97
(q, J = 7.1 Hz, 3H) ppm. 13C NMR (100 MHz, CDCl3): δ 195.3,
138.2, 128.7, 128.1, 126.8, 63.7, 50.3, 46.6, 19.7, 10.8 ppm.
HRMS (ESI-TOF) m/z: [M + H]+ calcd for C12H16NS2: 238.0719,
found: 238.0705. MP: 53.2-54.7 oC.
3-ethyl-5-(p-tolyl)oxazolidin-2-one (6h).5i White solid. Yield:
87%, 356.9 mg; 1H NMR (400 MHz, CDCl3): δ 7.21 (dd, J = 19.7,
8.1 Hz, 4H), 5.43 (t, J = 8.1 Hz, 1H), 3.89 (t, J = 8.7 Hz, 1H),
3.44–3.27 (m, 3H), 2.35 (s, 3H), 1.17 (td, J = 7.2, 0.5 Hz, 3H)
ppm. 13C NMR (100 MHz, CDCl3): δ 157.7, 138.6, 135.8, 129.5,
125.6, 74.4, 51.6, 38.9, 21.2, 12.6 ppm. HRMS (ESI-TOF) m/z:
[M + Na]+ calcd for C12H15NNaO2: 228.0995, found: 228.0998.
MP: 53.6-55.9 oC.
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3-butyl-5-phenylthiazolidine-2-thione (10c). Yellow solid. Yield:
87%, 218.4 mg; H NMR (400 MHz, CDCl3):δ 7.41– 7.29 (m,
1
5H), 4.84 (dd, J = 8.2, 7.0 Hz, 1H), 4.38 (dd, J = 11.6, 8.3 Hz,
1H), 4.05 (dt, J = 12.9, 6.4 Hz, 1H), 3.85 – 3.71 (m, 2H), 1.71–
1.58 (m, 2H), 1.43–1.31 (m, 2H), 0.95 (t, J = 7.4 Hz, 3H) ppm.
13C NMR (100 MHz, CDCl3): δ 195.5, 138.7, 129.1, 128.5, 127.3,
64.1, 49.1, 47.1, 28.8, 20.1, 13.8 ppm. HRMS (ESI-TOF) m/z: [M
+ H]+ calcd for C13H18NS2: 252.0875, found: 252.0866. MP: 53.4-
55.1 oC.
5-(4-(tert-butyl)phenyl)-3-ethyloxazolidin-2-one (6i). White liquid.
1
Yield: 82%, 405.3 mg; H NMR (400 MHz, CDCl3): δ 7.42 (d, J
3-isobutyl-5-phenylthiazolidine-2-thione (10d). Yellow solid.
Yield: 83%, 208.4 mg; 1H NMR (400 MHz, CDCl3): δ 7.38 –7.22
(m, 5H), 4.83–4.73 (m, 1H), 4.29 (dd, J = 11.6, 8.3 Hz, 1H), 3.98
(dd, J = 11.6, 7.1 Hz, 1H), 3.56 (qd, J = 13.4, 7.6 Hz, 2H), 2.04
(dp, J = 13.7, 6.9 Hz, 1H), 0.93–0.83 (m, 6H) ppm. 13C NMR
(100 MHz, CDCl3): δ 195.9, 138.0, 128.6, 128.1, 126.9, 64.3, 55.9,
46.8, 26.9, 19.7 ppm. HRMS (ESI-TOF) m/z: [M + H]+ calcd for
C13H18NS2: 252.0875, found: 252.0865. MP: 54.7-55.3 oC.
= 8.4 Hz, 2H), 7.30 (d, J = 8.3 Hz, 2H), 5.46 (t, J = 8.1 Hz, 1H),
3.91 (t, J = 8.7 Hz, 1H), 3.49–3.28 (m, 3H), 1.33 (s, 9H), 1.19 (t, J
= 7.2 Hz, 3H) ppm. 13C NMR (100 MHz, CDCl3): δ 157.3, 151.3,
135.3, 125.3, 124.9, 73.8, 51.0, 38.4, 30.8, 12.1 ppm. HRMS
(ESI-TOF) m/z: [M + Na]+ calcd for C15H21NNaO2: 270.1465,
found: 270.1464.
3-ethyl-5-(4-methoxyphenyl)oxazolidin-2-one (6j). White solid.
1
3-benzyl-5-phenylthiazolidine-2-thione (10e).10b Yellow solid.
Yield: 71%, 202.4 mg; 1H NMR (400 MHz, CDCl3): δ 7.40 –7.24
(m, 10H), 5.02 (dd, J = 33.4, 14.6 Hz, 2H), 4.86–4.74 (m, 1H),
4.23 (dd, J = 11.6, 8.4 Hz, 1H), 3.93 (dd, J = 11.7, 7.2 Hz, 1H)
ppm. 13C NMR (100 MHz, CDCl3): δ 196.5, 138.4, 134.9, 129.1,
129.0, 128.5, 128.4, 128.4, 127.3, 63.2, 52.7, 46.9 ppm. HRMS
(ESI-TOF) m/z: [M + Na]+ calcd for C16H15NNaS2: 308.0538,
found: 308.0544. MP: 115.2-118.7 oC.
Yield: 43%, 190.1 mg; H NMR (400 MHz, CDCl3): δ 7.30 (d, J
= 8.3 Hz, 2H), 6.93 (d, J = 8.2 Hz, 2H), 5.43 (t, J = 8.0 Hz, 1H),
3.89 (t, J = 8.6 Hz, 1H), 3.82 (s, 3H), 3.40 (ddd, J = 20.4, 14.9,
7.5 Hz, 3H), 1.19 (t, J = 7.1 Hz, 3H) ppm. 13C NMR (100 MHz,
CDCl3): δ 160.0, 157.7, 130.7, 127.2, 114.2, 74.3, 55.4, 51.6, 38.9,
12.6 ppm. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for
C12H15NNaO3: 244.0944, found: 244.0940. MP: 68.3-72.1 oC.
5-phenyloxazolidin-2-one (6k).5h Colorless liquid. Yield: 54%,
176.1 mg; 1H NMR (400 MHz, CDCl3): δ 7.47–7.31 (m, 5H),
5.75 (s, 1H), 5.63 (t, J = 8.2 Hz, 1H), 3.99 (t, J = 8.7 Hz, 1H),
3.60–3.50 (m, 1H) ppm. 13C NMR (100 MHz, CDCl3): δ 160.41,
138.5, 128.9, 127.4, 125.8, 78.0, 48.4 ppm. HRMS (ESI-TOF)
m/z: [M + Na]+ calcd for C9H9NNaO2: 186.0525, found: 186.0523.
5-(4-chlorophenyl)-3-ethylthiazolidine-2-thione (10f). Yellow
solid. Yield: 89%, 228.7 mg; 1H NMR (400 MHz, CDCl3): δ
7.30–7.21 (m, 4H), 4.73 (dd, J = 8.2, 6.5 Hz, 1H), 4.32 (dd, J =
11.6, 8.3 Hz, 1H), 3.94 (dd, J = 11.6, 6.5 Hz, 1H), 3.79 (ddt, J =
20.8, 13.6, 6.9 Hz, 2H), 1.19 (t, J = 7.2 Hz, 3H) ppm. 13C NMR
(100 MHz, CDCl3): δ 194.5, 137.0, 133.9, 128.8, 128.2, 62.9, 45.7,
43.7, 11.1 ppm. HRMS (ESI-TOF) m/z: [M + H]+ calcd for
C11H13ClNS2: 258.0172, found: 258.0163. MP: 91.4-92.9 oC.
General procedure for the catalytic ring-opening reaction of
aziridines with CS2. All manipulations and reactions were con-
ducted under a purified argon atmosphere by using standard
Schlenk techniques. A 5 mL Schlenk tube under dried argon was
charged with complex 1 (0.01 mmol), aziridine (1 mmol) and a
stirring bar. After the catalyst was dissolved, CS2 (5 mmol) was
added into the tube. The reaction mixture was stirred at oil bath of
5-(4-bromophenyl)-3-ethylthiazolidine-2-thione (10g). Yellow
solid. Yield: 85%, 255.8 mg; 1H NMR (400 MHz, CDCl3): δ 7.42
(d, J = 8.5 Hz, 2H), 7.22–7.16 (m, 2H), 4.77–4.63 (m, 1H), 4.32
(dd, J = 11.6, 8.3 Hz, 1H), 3.94 (dd, J = 11.6, 6.5 Hz, 1H), 3.87–
3.66 (m, 2H), 1.18 (t, J = 7.2 Hz, 3H) ppm. 13C NMR (100 MHz,
CDCl3): δ 194.4, 137.5, 131.8, 128.5, 122.0, 62.9, 45.8, 43.7, 11.2
o
45 C for 24 h, then cooled down to room temperature, quenched
with ethyl acetate, then biphenyl (0.5 mmol) was added as an
internal standard for the analysis of product by GC. The crude
mixture was purified by column chromatography on silica gel
(200−300 mesh, eluting with 20:1 petroleum ether/ethyl acetate to
give desired products).
ppm. HRMS
(ESI-TOF) m/z: [M +
Na]+ calcd for
C11H12BrNNaS2: 323.9487, found: 323.9474. MP: 103.6-105.1 oC.
3-ethyl-5-(p-tolyl)thiazolidine-2-thione (10h). Yellow solid. Yield:
1
85%, 201.5 mg; H NMR (400 MHz, CDCl3): δ 7.22 –7.17 (m,
2H), 7.09 (d, J = 8.0 Hz, 2H), 4.79–4.71 (m, 1H), 4.28 (dd, J =
11.5, 8.3 Hz, 1H), 3.97 (dd, J = 11.6, 7.1 Hz, 1H), 3.87–3.70 (m,
2H), 2.27 (s, 3H), 1.18 (d, J = 7.2 Hz, 3H) ppm. 13C NMR (100
MHz, CDCl3): δ 195.4, 138.5, 135.6, 129.8, 127.2, 63.6, 46.8,
44.2, 21.1, 11.7 ppm. HRMS (ESI-TOF) m/z: [M + H]+ calcd for
C12H16NS2: 238.0719, found: 238.0711. MP: 63.5-64.9 oC.
Characterization data of thiazolidine-2-thiones. 3-ethyl-5-
phenylthiazolidine-2-thione (10a).5g Yellow solid. Yield: 95%,
1
211.9 mg; H NMR (400 MHz, CDCl3): δ 7.46– 7.29 (m, 5H),
4.84 (dd, J = 8.2, 7.1 Hz, 1H), 4.38 (dd, J = 11.5, 8.4 Hz, 1H),
4.07 (dd, J = 11.6, 7.0 Hz, 1H), 3.87 (ddd, J = 14.2, 9.1, 6.9 Hz,
2H), 1.26 (t, J = 7.2 Hz, 3H) ppm. 13C NMR (100 MHz, CDCl3):
δ 194.7, 138.3, 128.7, 128.1, 126.8, 63.0, 46.5, 43.7, 11.2 ppm.
5-(4-(tert-butyl)phenyl)-3-ethylthiazolidine-2-thione (10i). Yellow
solid. Yield: 70%, 195.4 mg; 1H NMR (400 MHz, CDCl3): δ 7.34
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α,β-unsaturated imides mediated by a visible light photoredox catalyst.
(d, J = 8.4 Hz, 2H), 7.26 (d, J = 8.4 Hz, 2H), 4.80 (dd, J = 16.1,
8.3 Hz, 1H), 4.37–4.24 (m, 1H), 4.11–3.98 (m, 1H), 3.82 (tq, J =
13.7, 6.9 Hz, 2H), 1.28–1.19 (m, 12H) ppm. 13C NMR (100 MHz,
CDCl3): δ 195.0, 151.3, 135.0, 126.5, 125.6, 63.0, 46.3, 43.7, 34.2,
30.8, 11.2 ppm. HRMS (ESI-TOF) m/z: [M + H]+ calcd for
C15H22NS2: 280.1188, found: 280.1187. MP: 96.6-99.8 oC.
Chem. Commun. 2017, 53, 9136-9138. (d) Evans, D. A.; Chapman, K.
T.; Bisaha, J. Asymmetric diels-alder cycloaddition reactions with
chiral α, β-unsaturated N-acyloxazolidinones. J. Am. Chem. Soc. 1988,
110, 1238–1256. (e) Huang, P. Q.; Geng, H. Ni-catalyzed chemose-
lective alcoholysis of N-acyloxazolidinones. Green Chem. 2018, 1-7.
(f) Palomino, A. G.; Romea, P.; Urpí, F. Stereo selective oxidation of
titanium(IV) enolates with oxygen. Synthesis 2018, 50, 2721–2726.
[3] Sakakura, T.; Choi, J. C.; Yasuda, H. Transformation of carbon
dioxide. Chem. Rev. 2007, 107, 2365-2387.
[4] (a) Khusnutdinova, J. R.; Garg, J. A.; Milstein, D. Combining
low-pressure CO₂capture and hydrogenation to form methanol. ACS
Catal. 2015, 5, 2416−2422. (b) Liu, A. H.; Ma, R.; Song, C.; Yang, Z.
Z.; Yu, A.; Cai, Y.; He, L. N.; Zhao, Y. N.; Yu, B.; Song, Q. W.
Equimolar CO2 capture by N-substituted amino acid salts and subse-
quent conversion. Angew. Chem. Int. Ed. 2012, 51, 11306-11310. (c)
Hase, S.; Kayaki, Y.; Ikariya, T. NHC-gold(I) complexes as effective
catalysts for the carboxylative cyclization of propargylamines with
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3-ethyl-5-(4-methoxyphenyl)thiazolidine-2-thione (10j). Yellow
1
solid. Yield: 50%, 126.5 mg; H NMR (400 MHz, CDCl3):δ 7.21
(dd, J = 8.5, 2.6 Hz, 2H), 6.78 (dd, J = 8.4, 3.1 Hz, 2H), 4.75 (t, J
= 7.3 Hz, 1H), 4.37–4.19 (m, 1H), 4.00 –3.89 (m, 1H), 3.84–3.71
(m, 2H), 3.70 (d, J = 3.9 Hz, 3H), 1.16 (dd, J = 10.5, 7.1 Hz, 3H)
ppm. 13C NMR (100 MHz, CDCl3): δ 195.2, 159.6, 130.5, 128.5,
114.4, 63.6, 55.4, 46.6, 44.2, 11.7 ppm. HRMS (ESI-TOF) m/z:
[M + H]+ calcd for C12H16NOS2: 254.0668, found: 254.0669. MP:
54.8-55.3 oC.
9
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5-phenylthiazolidine-2-thione (10k). Yellow solid. Yield: 86%,
167.7 mg; 1H NMR (400 MHz, DMSO-d6): δ 10.35 (s,
1H)，7.51–7.37 (m, 5H)，5.44–5.26 (m, 1H)，4.32 (dd, J = 12.1,
8.1 Hz, 1H)，4.01 (dd, J = 12.1, 6.5 Hz, 1H) ppm. 13C NMR (100
carbon dioxide. Organometallics. 2013, 32, 5285−5288; (d) Gao, X.
T.; Gan, C. C.; Liu, S. Y.; Zhou, F.; Wu, H. H.; Zhou, J. Utilization of
CO2 as a C1 building block in a tandem asymmetric A3 coupling-
carboxylative cyclization sequence to 2-oxazolidinones. ACS Catal.
2017, 7, 8588−8593. (e) Lee, Y.; Choi, J.; Kim, H. Stereocontrolled,
divergent, Al(lll)-catalyzed coupling of chiral N‑aryl epoxy amines
and CO2. Org. Lett. 2018, 20, 5036−5039; (f) Birrell, J. A.; Jacobsen ,
E. N. A practical method for the synthesis of highly enantioenriched
trans-1,2-amino alcohols. Org. Lett, 2013, 15, 2895-2897. (g) Patricia,
G. D.; Fehr, L.; Rusconi, G.; Nevado, C. Palladium-catalyzed incor-
poration of atmospheric CO2 : efficient synthesis of functionalized
oxazolidinones. Chem. Sci. 2016, 7, 3914-3918.
MHz,
DMSO-d6):
δ
198.2，140.0，129.3，128.6，127.7，58.4，52.8 ppm. HRMS
(ESI-TOF) m/z: [M + H]+ calcd for C9H10NS2: 196.0249, found:
196.0256. MP: 54.8-55.3 oC.
ASSOCIATED CONTENT
Supporting Information
Supporting Information Available: Additional experimental
procedures and full spectroscopy data for all the products. This
materials is available free of charge via the Internet at
http://pubs.asc.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax: +86-512-65880305; Tel: +86-512-65880305; E-mail:
zhaobei@suda.edu.cn
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT
We gratefully acknowledge financial support from the National
Natural Science Foundation of China (Grant No. 21572151), the
project of scientific and technologic infrastructure of Suzhou
(SZS201708), and PAPD.
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