An efficient and recyclable AgNO3/ionic liquid system catalyzed atmospheric CO2 utilization: Simultaneous synthesis of 2-oxazolidinones and α-hydroxyl ketones

Article abstract of DOI:10.1016/j.jcat.2020.11.011

Oxazolidinones and α-hydroxyl ketones are two series of fine chemicals that have been generally utilized in biological, pharmaceutical, and synthetic chemistry. Herein, a AgNO₃/ionic liquid (IL) catalytic system was developed for the simultaneous synthesis of these compounds through the atom-economical three-component reactions of propargyl alcohols, 2-aminoethanols, and CO₂. Notably, this system behaved excellent catalytic activity with the lowermost metal loading of 0.25 mol%. Meanwhile, it is the first reported metal-catalyzed system that could efficiently work under atmospheric CO₂ pressure and be recycled at least five times. Evaluation of the green metrics proved the AgNO₃/IL-catalyzed processes to be relatively more sustainable and greener than the other Ag-catalyzed examples. Further mechanistic investigations revealed the derivative active species of N-heterocyclic carbene (NHC) silver complexes and CO₂ adducts generated during the process. Subsequently, their reactivity in this reaction was assessed for the first time, which was finally identified as beneficial for the catalytic activity.

Full text of DOI:10.1016/j.jcat.2020.11.011

Journal Pre-proofs
An Efficient and Recyclable AgNO3/Ionic Liquid System Catalyzed Atmos-
pheric CO2 Utilization: Simultaneous Synthesis of 2-Oxazolidinones and α-
Hydroxyl Ketones
Minchen Du, Yanyan Gong, Chao Bu, Jia Hu, Yongxing Zhang, Cheng Chen,
Somboon Chaemchuen, Ye Yuan, Francis Verpoort
PII:
S0021-9517(20)30457-7
https://doi.org/10.1016/j.jcat.2020.11.011
YJCAT 13970
DOI:
Reference:
To appear in:
Journal of Catalysis
Received Date:
Revised Date:
Accepted Date:
1 September 2020
6 November 2020
10 November 2020
Please cite this article as: M. Du, Y. Gong, C. Bu, J. Hu, Y. Zhang, C. Chen, S. Chaemchuen, Y. Yuan, F.
Verpoort, An Efficient and Recyclable AgNO3/Ionic Liquid System Catalyzed Atmospheric CO2 Utilization:
Simultaneous Synthesis of 2-Oxazolidinones and α-Hydroxyl Ketones, Journal of Catalysis (2020), doi: https://
doi.org/10.1016/j.jcat.2020.11.011
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An Efficient and Recyclable AgNO3/Ionic Liquid System
Catalyzed Atmospheric CO2 Utilization: Simultaneous
Synthesis of 2-Oxazolidinones and α-Hydroxyl Ketones
Minchen Du,[a,c] Yanyan Gong,[b] Chao Bu,[a,c] Jia Hu,[a,c] Yongxing Zhang,[a,c] Cheng
Chen,[a] Somboon Chaemchuen,[a] Ye Yuan,*[a] Francis Verpoort*[a,c,d,e]
[a] State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,
Wuhan University of Technology, Wuhan 430070, P.R. China
[b] State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of
Technology (Shandong Academy of Sciences), Jinan 250353, P.R. China
[c] School of Material Science and Engineering, Wuhan University of Technology, Wuhan
430070, P.R. China
[d] National Research Tomsk Polytechnic University, Lenin Avenue 30, Tomsk 634050, Russian
Federation
[e] Global Campus Songdo, Ghent University, 119 Songdomunhwa-Ro, Yeonsu-Gu, Incheon,
Korea
Correspondence
authors
E-mail:
fyyuanye@whut.edu.cn;francis.verpoort@ghent.ac.kr
Abstract
Oxazolidinones and α-hydroxyl ketones are two series of fine chemicals that have
been generally utilized in biological, pharmaceutical, and synthetic chemistry. Herein,

a AgNO3/ionic liquid (IL) catalytic system was developed for the simultaneous
synthesis of these compounds through the atom-economical three-component
reactions of propargyl alcohols, 2-aminoethanols, and CO2. Notably, this system
behaved excellent catalytic activity with the lowermost metal loading of 0.25 mol%.
Meanwhile, it is the first reported metal-catalyzed system that could efficiently work
under atmospheric CO2 pressure and be recycled at least five times. Evaluation of the
green metrics proved the AgNO3/IL-catalyzed processes to be relatively more
sustainable and greener than the other Ag-catalyzed examples. Further mechanistic
investigations revealed the derivative active species of N-heterocyclic carbene (NHC)
silver complexes and CO2 adducts generated during the process. Subsequently, their
reactivity in this reaction was assessed for the first time, which was finally identified
as beneficial for the catalytic activity.
Keywords
carbon dioxide utilization; silver catalysis; ionic liquid; multicomponent reaction;
heterocycles
1. Introduction
With modern industry development, carbon dioxide (CO2) has attracted more and
more attention as its unignorable influence on the greenhouse effect [1-4]. While in
the aspect of synthetic chemistry, CO2 is considered as an ideal C1 source for its
merits, such as nontoxicity, economy, renewability, and abundance [5-8]. Based on

this, the transformation of CO2 into organics has become a highly promising area in
modern green and sustainable chemistry [9-12]. Up to now, numerous strategies have
been developed to effectively utilize CO2 for the synthesis of valuable chemicals
[13-21] such as alcohols [22-25], carboxylic acids & esters [26-31], amides [32-36],
carbonates [31, 37-51] and carbamates [52-56]. Among them, oxazolidinones and
α-hydroxyl ketones have gained a lot of interest for their wide applications in
biological, pharmaceutical, and synthetic chemistry [57]. For example, oxazolidinone
skeletons exist in multitudinous drugs like linezolid, eperezolid, zolmitriptan, etc. [58]
Meanwhile, they are applied to prepare chiral amino acids, dipeptide,
(α,α)-di-substituted amine and other fine chemicals [59]. On the other hand,
α-hydroxyl ketones such as HMPP (2-hydroxy-2-methylpropiophenone) and HCPK
(1-hydroxycyclohexyl phenyl ketone) act as photo-initiators in the curing system of
ultraviolet photosensitive resin [60, 61]. However, the traditional methods for
preparing these two series of compounds still suffered from several drawbacks, such
as harsh reaction conditions, low atom economy and harmful reagents such as
phosgene [62], Hg salts, and H2SO4 [63-65]. These disadvantages limited their further
utilization in modern green and sustainable industry.
Based on this, several pathways aimed to establish an economical and
environmentally-friendly synthesis of oxazolidinones and α-hydroxyl ketones,
including the cyclization of propargylamine with CO2 [66-71], three-component
reactions of propargyl alcohols/primary amine/CO2 [72-76], direct condensation of

2-aminoalcohols with CO2 [77-79], the cycloaddition of aziridines with CO2 [80-83]
for preparing oxazolidinones, and CO2-cocatalysed hydration of propargyl alcohols
[84-86], reduction of diketones or oxidation of diols [87], α-H oxidative
hydroxylation of ketones [88] for producing α-hydroxyl ketones. In particular, the
three-component reactions of propargyl alcohols, CO2 and 2-aminoethanols attracted
our interests because this strategy could simultaneously produce both oxazolidinones
and α-hydroxyl ketones in a one-pot manner with 100% atom-economy under
relatively mild and green conditions. In this area, He and co-workers [89] developed a
silver salt/phosphine ligand catalytic system for this three-component cascade
reaction
in
which
5
mol%
of
Ag2CO3
and
10
mol%
of
9,9-Dimethyl-4,5-bis(diphenylphosphino)xanthene (Xantphos) were employed. This
catalytic system could smoothly convert various substrates into the corresponding
products under 1 MPa of CO2 in CHCl3. Subsequently, they reported a catalytic
system consisting of 5 mol% Ag2O and 30 mol% of 1,1,3,3-tetramethylguanidine
(TMG) for this reaction, which could proceed successfully under 1 MPa of CO2 in
CH3CN with a TON up to 1260 [90]. Further investigations showed that TMG could
activate propargyl alcohols, 2-aminoethanols, and CO2 remarkably. Furthermore, a
Cu(I) system with considerable catalytic activity was also established [56]. Desired
products could be obtained in the presence of 5 mol% CuI as the catalyst, 5 mol%
1,10-phen (1,10-phenanthroline) as the ligand and 10 mol% t-BuOK as additives
under 0.5 MPa of CO2. Very recently, a task-specific ionic liquid (IL),
1,5,7-triazabicylo[4.4.0]dec-5-ene trifluoroethanol ([TBDH][TFE]) was designed and

synthesized through an anion-exchange resin, which could promote the cascade
reaction under 0.1 MPa of CO2 [91]. Although remarkable progress has been achieved,
the existing systems still suffered from several drawbacks. For the most studied
metal-catalyzed systems, which were considered to be potential for industrial
applications due to their simple, stable and relatively economical components, high
metal loadings, harsh reaction conditions, poor recyclability, usage of volatile
solvents, additions of ligands and/or bases still blocked their practical utilizations. On
the other hand, the only example of a metal-free strategy generally employed a
commercially unavailable IL ([TBDH][TFE]), whose synthesis required a relatively
rare organic base and anion-exchange resins. This also limited its large-scale
applications in industry. Despite this, it was worth noting that these two strategies
were complementary in certain aspects, which inspired us that the combination of
these two strategies might create catalytic systems that brought in the dual merits
from both metal- and metal-free systems.
Herein, we employed a commercially available and easily accessible ionic liquid,
namely 1-ethyl-3-methylimidazolium acetate ([C2C1im][OAc]) together with AgNO3
for the simultaneous synthesis of 2-oxazolidinones and α-hydroxyl ketones through
the three-component reactions of propargyl alcohols, 2-aminoethanols, and CO2 under
atmospheric CO2 pressure without any additives or traditional volatile solvents.
Diverse desired products could be smoothly produced with a lowermost level of metal
loading (0.25 mol%). Particularly, this catalytic system could be effectively recycled

and reused, which also exhibited excellent metrics in the evaluation of its green and
sustainable level.
2. Experimental details
2.1. Materials
Ag2WO4 and AgSbF6 were obtained from Alfa Aesar and Macklin, respectively,
while the other silver salts were obtained from Aladdin. [C2C1im][OAc],
[C4C1im][OAc] and [C2C1im][ClO4] were purchased from Sigma-Aldrich, while other
imidazole-derived ionic liquids and [N4444][OAc] were purchased from Aladdin. The
CO2 (99.999%) was supplied by Wuhan Xiangyun Industry and Trade Co., Ltd.
2-(Benzylamino)ethanol, diethanolamine, as well as all kinds of propargyl alcohols
were purchased from Macklin. [DBUH]-derived ILs and other 2-aminoethanol
derivatives were synthesized according to the reported literature [89-92]. The
aromatic aldehydes and 2-aminoethanols utilized for the synthesis were purchased
from Macklin. Raw materials of DBU, acetic acid, 1H-imidazole, and
2-methylimidazole were purchased from Aladdin. Drying agents (Na2SO4), NaBH4,
and the organic solvents (CH3OH, CH2Cl2, Et2O, petroleum ether, and ethyl acetate)
used in the processes of reaction, extraction and purification were acquired from
Sinopharm Chemical Reagent Co., Ltd. 1,3,5-Trimethoxybenzene used as the internal
standard was obtained from Aladdin. Deuterated solvents used for NMR analysis
were purchased from CIL (Cambridge Isotope Laboratories). All the above-mentioned
materials were directly used without further purification.

2.2. General analytic methods
1H NMR spectra were recorded on a Bruker Avance III HD 500 MHz spectrometer
with internal standard TMS (δ = 0 ppm) as the reference. 13C NMR spectra were
recorded at 126 MHz in CDCl3 (δ = 77.00 ppm) or DMSO-d6 (δ = 39.90 ppm) with
the solvent peaks as references. The data were given as chemical shifts (ppm) and
coupling constants (Hz), respectively. HRMS was conducted using a Bruker Daltonics
micro TOF-QII mass spectrometry instrument with the data of high-resolution mass
given in per charge (m/z).
2.3. General procedures for the synthesis of 2-oxazolidinones and α-hydroxyl ketones
The synthesis of 2-oxazolidinones and α-hydroxyl ketones was performed in a 15 ml
Schlenk tube. AgNO3 (0.0125 mmol, 0.25 mol%), [C2C1im][OAc] (6 mmol),
2-aminoethanols (5 mmol) and propargyl alcohols (7.5 mmol) were first added. Then
the system was purged with СО2 three times and the mixture was stirred at 60°C
under 0.1 MPa of CO2 for 12 h. Afterwards, the mixture was extracted with diethyl
ether (5 × 10 mL) and the upper layers were collected and concentrated under vacuum
to give the raw products, which were further purified by column chromatography on
silica gel using petroleum ether/ethyl acetate (v/v, 5:1–1:1) as the eluent. When the
recyclability of the catalytic system was investigated, the lower layer was recycled
and reused directly for the next round after drying under vacuum at 60°C for 4 h.

3. Results and Discussion
Table 1. Screening of Ag salts in catalytic systems. a, b
Yields/%
Entry
[Ag] salts
Ionic liquids
3a
/
4a
/
1
2
/
/
AgNO3
/
/
/
/
3
[C2C1im][OAc]
[C2C1im][OAc]
[C2C1im][OAc]
[C2C1im][OAc]
[C2C1im][OAc]
[C2C1im][OAc]
[C2C1im][OAc]
[C2C1im][OAc]
[C2C1im][OAc]
[C2C1im][OAc]
[C2C1im][OAc]
[C2C1im][OAc]
[C2C1im][OAc]
/
/
4
AgCl
76
87
65
66
68
75
88
94
74
70
39
57
74
85
65
65
66
74
87
93
73
68
36
53
5
AgBr
6
AgI
7
Ag2CO3
Ag2WO4
Ag3PO4
AgOAc
AgNO3
C2AgF3O2
Ag2O
8
9
10
11
12
13
14
15
AgPF6
AgBF4

16
AgSbF6
[C2C1im][OAc]
71
69
a Reaction conditions: [Ag] (0.0125 mmol, 0.25 mol% based on 1a), [C2C1im][OAc] (6 mmol), 1a
(5 mmol), 2a (7.5 mmol), CO2 (0.1 MPa), 60°C, 12 h.
b Yields were determined by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as the internal
standard based on 1a.
The study initially proceeded by screening the optimal catalytic system of Ag salts
and ILs in the reaction of 2-(benzylamino)ethanol (1a) and 2-methylbut-3-yn-2-ol (2a)
under 0.1 MPa of CO2 at 60oC for 12 h (Table 1). Firstly, a blank experiment showed
that the reaction couldn’t proceed without catalysts (entry 1). Similarly, the absence of
any component in the catalytic system led to a failure of the reaction (entries 2-3).
Satisfactorily, when introducing both Ag salts and ILs into the reaction, desired
2-oxazolidinone 3a and α-hydroxyl ketone 4a were eventually obtained (entry 4).
Further investigations indicated that most of the silver salts including AgCl, AgBr,
AgI, Ag2CO3, Ag2WO4, Ag3PO4, AgOAc, AgNO3, C2AgF3O2, Ag2O, AgPF6, AgBF4
and AgSbF6 could efficiently catalyze the reaction in combination with [C2C1im][OAc]
(entries 4-16). Among them, AgNO3 exhibited the best catalytic performance for the
three-component reaction (entry 11). These differences of Ag salts might be attributed
to their negative anions which probably influenced their properties. For example, Ag
salts like Ag2O, Ag2CO3, etc. showed lower solubility and dissociation than AgOAc,
AgNO3, etc., which might be responsible for their relatively poor catalytic
performances (entries 7, 13 vs. 10, 11). Additionally, the anions also affected the

stability of the silver salts. AgNO3 is considered to be relatively stable when exposed
to the light, while AgPF6 or AgBF4 is quite easy to decompose under the same
condition, thus possibly leading to the low yields (entries 14, 15 vs.11).
Table 2. Screening of ILs in catalytic systems. a, b
N
N
N
N
N
N
N+
H
cation
anion
[N4444
]
[C2C1im]
[DBUH]
[C4C1im]
-N
N
OAc-
(CF3SO2)2N- (EtO)2PO2
Br-
-
-
-
-
-
[Im]
Yields/%
CF3SO3
NO3
ClO4
HSO4
Entry
[Ag] salts
Ionic liquids
3a
94
73c
7
4a
93
70c
4
1
AgNO3
[C2C1im][OAc]
2
3
4
5
6
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
[C2C1im][NTf2]
[C2C1im][Et2PO4]
[C2C1im]Br
42
/
43
/
[C2C1im][OTf]
[C2C1im][NO3]
6
5
6
5

7
8
9
AgNO3
AgNO3
AgNO3
[C2C1im][ClO4]
[C2C1im][HSO4]
[C4C1im][OAc]
/
/
/
/
78
54
44 c
36
20
77
53
48 c
33
23
10
AgNO3
[DBUH][OAc]
11
12
AgNO3
AgNO3
[DBUH][Im]
[N4444][OAc]
a Reaction conditions: AgNO3 (0.0125 mmol, 0.25 mol% based on 1a), ILs (6 mmol), 1a (5 mmol),
2a (7.5 mmol), CO2 (0.1 MPa), 60°C, 12 h.
b Yields were determined by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as the internal
standard based on 1a.
c 10 mL of CH3CN was added.
Afterwards, the influences of different ionic liquids on the catalytic activity were
examined (Table 2). First of all, the anion OAc− in the [C2C1im][OAc] IL was
−
−
−
−
replaced respectively by (CF3SO2)2N−, Et2PO4 , Br−, CF3SO3 , NO3 , ClO4 , and
−
−
HSO4 (entries 2-8). It could be clearly observed that only the Et2PO4 gave a
moderate yield of 42%. At the same time, other neutral or acidic anions behaved quite
poor activity, indicating that the basic anions were more favorable and OAc−
exhibited superior activity for this reaction. Similar conclusions could also be
obtained in the experiments of [DBUH]-derived ILs (entry 10 vs. 11). On the other
hand, the cation of [C2C1im][OAc] was replaced by a similar n-butyl substituted
imidazole cation, resulting in a slight decrease of the yield (entry 9). Particularly, the

catalytic performance of the [DBUH][OAc] was much lower than the
imidazole-derived ILs (entry 10 vs. 1, 9). This was probably due to the fact that
[DBUH]+ cation is a H-bond donor, which can potentially interact with the partially
deprotonated propargyl alcohol hence decrease its nucleophilicity and its interaction
with CO2. Besides, another acetate IL employing [N4444]+ as the cation also gave a
poor result (entry 12). In conclusion, the system of AgNO3/[C2C1im][OAc] was
proved to behave the optimal catalytic activity for this reaction.
Table 3. Screening of the optimal reaction conditions. a. b
Yields/%
3a
AgNO3
/mol% c
0.10
[C2C1im][OAc]
Temperature
Entry
1a:2a
/equiv. c
1.2
/oC
60
60
60
60
60
60
60
25
40
4a
70
93
90
91
73
86
83
/
1
2
3
4
5
6
7
8
9
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
69
94
93
94
71
89
86
/
0.25
1.2
0.50
1.2
1.00
1.2
0.25
0.6
0.25
0.9
0.25
1.5
0.25
1.2
0.25
1.2
31
32

10
11
12
13
0.25
0.25
0.25
0.25
1.2
1.2
1.2
1.2
80
60
60
60
1:1.5
1:1
95
69
80
91
91
70
79
90
1:1.2
1:2
a CO2 (0.1 MPa), 12 h.
b Yields were determined by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as the internal
standard based on 1a.
c Based on the amount of 1a.
Considering the amounts of catalytic components, temperature, and the ratio of the
substrates (1a: 2a) might affect the reaction, these condition parameters were further
screened (Table 3). Firstly, when the silver loading rose from 0.1 mol% to 0.25 mol%,
the yield increased significantly. However, higher Ag loadings of 0.5 mol% and 1.0
mol% would not contribute more for the yield (entries 1-4), indicating 0.25 mol% of
AgNO3 was suitable for this reaction (entry 2 & Part 5.1 of supporting information).
Similarly, the varying amounts of IL would influence the catalytic activity and the
optimal amount was tuned as 1.2 equiv. of 1a (entry 2 vs. entries 5-7 & Part 5.2 of
supporting information). Subsequently, the influence of temperature was also studied.
At room temperature, no reaction occurred. However, with an increase in temperature,
the desired products would be successively obtained in rising yields. When the
temperature reached 60°C, 3a and 4a could be produced in more than 90% yield.
Although the higher temperature of 80°C would give a slightly better yield, we still

selected 60°C as the best condition considering the energy-consumption and the
volatility of α-hydroxyl ketones (entry 2 vs. entries 8-10). Surprisingly, changing the
ratio of substrates would make a difference for the experimental result, which
revealed that a higher equivalent of 2a would help shift the equilibrium towards the
desired products (entry 2 vs. entries 11-13). Finally, the optimal conditions were fixed
as AgNO3 (0.25 mol%)/[C2C1im][OAc] (1.2 equiv. of 1a) at 60°C under atmospheric
CO2 pressure with the ratio of 1:1.5 (1a:2a). Particularly, it is worth mentioning that
this is the first report that could efficiently work under atmospheric CO2 pressure for
the metal-catalyzed systems. Meanwhile, 0.25 mol% is the lowest level of metal
loading reported by far.
Based on the optimal catalytic system as well as its best reaction conditions, the scope
of the substrates was explored afterwards, as shown in Table 4. Delightedly, various
propargyl alcohols and 2-aminoethanols with diverse substituents could be smoothly
converted into the desired oxazolidinones and α-hydroxyl ketones with the catalysis
of the AgNO3/[C2C1im][OAc] system under 0.1 MPa of CO2. Particularly, the
structures of the propargyl alcohols greatly influenced the reactions, and high yields
could be easily obtained in the cases of substrate 2 bearing less sterically hindered
substituents (entries 1, 2, 3). While for those propargyl alcohols with bulky groups
such as cyclohexyl or phenyl group, prolonged reaction time was required to reach the
comparable yields (entries 4, 5). This might be because the bulky substituent group
would hinder the effective nucleophilic attack of the adjacent hydroxyl oxygen on the

carbon of the CO2 molecule, thus leading to the lower reactivity of the corresponding
propargyl alcohol. On the other hand, different 2-aminoethanols substituted by the
benzyl or its derived groups also performed different reactivity. Electron-donating
groups such as -CH3 connected on the aromatic rings would facilitate the reactions
while a specific electron-withdrawing group of -NO2 largely limited the reactivity of
the corresponding substrate (entries 6, 7 vs. 10). This was possibly because the
electron-donating groups would increase the electron density of the benzene ring, thus
might enhance the nucleophilicity of the nitrogen atom. While for the strong
electron-withdrawing group such as O2N- (entry 10), its electron effect would give an
opposite impact. In addition, an alkyl substituted 2-aminoethanol 1g was also tried
and satisfactory yields of 2-oxazolidinones (94%) and α-hydroxyl ketones (95%) were
smoothly obtained (entry 11). Finally, a gram-scale experiment of 1a and 2a was
conducted under its optimal reaction conditions, in which the AgNO3/[C2C1im][OAc]
system still gave satisfactory yields for grams of substrates (entry 1, condition c).
Table 4. Screening of the substrates. a, b
Yield/%
Entry
Product 3
Product 4
Time/h
12
3
4
O
94
93
O
1
N
O
93[c]
91[c]
OH

3a
4a
O
O
94
98
2
24
N
OH
O
90[d]
90[d]
3a
3a
3a
3a
3b
3c
4b
4c
O
O
O
O
O
3
4
5
6
7
8
24
36
36
12
18
18
90
85
93
N
N
N
O
O
O
OH
OH
95
89
4d
O
91
OH
OH
OH
OH
84[d]
Ph
4e
4a
4a
O
O
97
97
O
O
O
N
O
91[d]
94
98
90
97
N
O
O
N
O
OCH3

3d
3e
4a
4a
O
O
N
9
12
97
92
O
OH
Cl
O
O
O
N
10
11
24
24
25
94
22
95
O
OH
OH
NO2
3f
4a
4a
O
OH
N
O
3g
a
AgNO3 (0.0125 mmol, 0.25 mol% based on 1a), IL (6 mmol), 2-aminoethanols (1) (5 mmol),
propargyl alcohols (2) (7.5 mmol).
b Yields were determined by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as the internal
standard based on 1.
c
Gram-scale experiment: 1a (10 mmol, 1.5122 g), 2a (15 mmol, 1.2618 g), AgNO3 (0.25 mol%
based on 1a), IL (12 mmol, 2.0425 g).
d Isolated yield.
Having identified the excellent catalytic activity and the broad substrate scope of the
AgNO3/[C2C1im][OAc] system, we next studied its recyclability, which was treated as
another key metric for evaluating its potential in practical applications. Herein, the
recycling experiments of the AgNO3/[C2C1im][OAc] system were performed using 1a
and 2a as the model substrates. To our delight, this system could efficiently catalyze
the three-component reaction at least 5 times with robust recyclability and reusability
(as shown in Fig. 1& Part 4 of supporting information), which is the first report
among the metal-catalyzed systems.

Oxazolidinone
Hydroxyl ketone
93%
89%
88%
87%
94%
Y
86%
94%
93%
91%
100%
86%
80%
60%
40%
20%
0%
1
2
3
4
5
Recycle round
Fig. 1. Reusability of the AgNO3/[C2C1im][OAc] system.
For the modern industry, the green and sustainable chemical process level have
obtained more and more attention, which could be assessed by the quantitative green
metrics [93, 94]. In order to evaluate the performance of the AgNO3/[C2C1im][OAc]
system in this aspect, several representative metrics were calculated for this system
and other reported Ag-catalyzed systems based on several common substrates (as
shown in Fig.2 & Part 2 of supporting information). Firstly, the atom economy (AE)
values of all these processes were 100%, indicating the three-component reaction was
a theoretically ideal way to produce oxazolidinones and hydroxyl ketones. However,
AE is a theoretical number that disregards the substances that do not appear in the
chemical equation (e.g., solvents or auxiliary chemicals). Therefore, the E-factor,
which takes waste from all auxiliary components into account, was examined
subsequently. To our delight, the E-factors of the AgNO3/[C2C1im[OAc]-catalyzed

processes showed much lower values than those of the references, implying these
processes exhibited weaker negative effects on the environment. Subsequently,
calculations of carbon efficiency (CE) and reaction mass efficiency (RME) were
successively performed, in which the AgNO3/[C2C1im][OAc]-catalyzed substrates
gave higher values, indicating higher catalytic efficiency of the corresponding system.
Finally, the metrics of mass intensity (MI) and mass productivity (MP) which reveal
the relationship between product mass and total input mass, were also evaluated. The
obtained data showed that the AgNO3/[C2C1im][OAc]-catalyzed processes also
exhibited better performances than the others in this assessment. These
abovementioned calculation results proved the AgNO3/[C2C1im][OAc]-catalyzed
three-component reactions to be relatively more sustainable and greener than the other
processes accomplished by the reported Ag-catalyzed systems.
（a）
(b)

(c)
Fig. 2. Green metrics evaluation for the Ag-catalyzed systems.
(a. Ag2CO3/Xantphos system [89]; b. Ag2O/TMG system [90]; c. this work)
4. Mechanistic investigation
4.1 Investigations on the activation of the hydroxyl protons in propargyl alcohols
Regarding the mechanism, it has been reported that the three-component reactions of

propargyl alcohols, CO2, and 2-aminoethanols are cascade reactions [56, 89-91], in
which the cyclization between CO2 and propargyl alcohols occurs first to generate the
key intermediary α-alkylidene cyclic carbonates. Then, the carbonates react with
2-aminoethanols to give the final oxazolidinones and hydroxyl ketones through the
nucleophilic ring-opening reaction (Fig. 3a). In the first cyclization step, the activation
of the hydroxyl groups in propargyl alcohols is regarded as the crucial process, which
initiates the vital bonding between CO2 and substrates, thus introducing the inert CO2
molecular into the catalytic process [45, 49, 90, 95]. Therefore, investigations on the
components which could accomplish this activation are of great importance.
Generally, this activation is achieved by forming the hydrogen bonds of the hydroxyl
groups with certain basic species, which could be indicated by the shape and chemical
1
shift of the hydroxyl proton signal in H NMR [48, 96]. Therefore, Pure 2a, the
mixture of 2a and the basic amine 1a (1.5:1), 2a and the basic IL [C2C1im][OAc]
1
(1.5:1.2), were prepared in DMSO-d6 and directly checked by H NMR (Fig. 3b). A
sharp peak at δ = 5.31 ppm could be observed in pure 2a, representing the unactivated
hydroxyl proton (Fig. 3b (1)). When 1a was added, the peak remained sharp,
revealing 1a had no noticeable effect on the activation (Fig. 3b (2)). Nevertheless, in
the mixture of 2a and [C2C1im][OAc], the hydroxyl proton was found to show a
distinctively different signal which was broad and shifted from its previous position,
implying the activation of the hydroxyl group was achieved in this system (Fig. 3b
(3)). Consequently, the [C2C1im][OAc] was the key component that could accomplish
the activation of the hydroxyl group individually.

(a)
(b)
Fig. 3. (a) The cascade procedures for the three-component reactions of propargyl alcohols, CO2
and 2-aminoethanols. (b) The activation of hydroxyl protons in propargyl alcohols.
4.2 Proposed catalytic mechanism of the AgNO3/[C2C1im][OAc] system

Subsequently, the catalytic cycle of AgNO3/[C2C1im][OAc] system for the reaction
was speculated, as shown in Scheme 1. Firstly, the basic acetate ion activates the
hydroxyl group in substrate 2, inducing its formation of the C-O bond with CO2
(intermediate II). Subsequently, the Ag species activates the triple bond to promote
the bonding of the oxygen anion to the carbon in the alkynyl group, resulting in the
formation of a five-membered ring III. Then, the catalysts are released and the cyclic
carbonate IV is produced. Afterwards, the nitrogen atom in substrate 1 attacks the
carbonyl group of the carbonate IV to produce intermediate V, followed by the
tautomerization of the enol to the ketone. Eventually, the corresponding
oxazolidinone is formed through the intramolecular cyclization of intermediate VI
with one hydroxyl ketone molecular released from the skeleton.
Scheme 1. Proposed catalytic mechanism of the AgNO3/[C2C1im][OAc] system.

4.3 Exploration for the N-heterocyclic carbene-CO2 (NHC-CO2) adducts
Based on this general mechanism, we further explored more catalytic features of the
AgNO3/[C2C1im][OAc] system. It was worth noting that besides the general physical
absorption of CO2, this specific [C2C1im][OAc] IL might behave extra chemical
absorption ability. When CO2 was introduced into the [C2C1im][OAc] system in some
cases, the OAc− would take the protons from the imidazole rings and free NHC
carbenes were generated in situ, which would capture and activate CO2 by forming
NHC-CO2 adducts (Scheme 2) [97]. Herein, we explored whether this extra CO2
source existed in the reaction and participated in the catalytic process.
Scheme 2. Generation of NHC-CO2 adducts.
Following the reported literature [97], we first prepared the fresh NHC-CO2 adducts
in the corresponding IL by introducing CO2 into the pure [C2C1im][OAc] for several
hours. Once becoming unclear, the solution was degassed and sampled in DMSO-d6
13
for C NMR. A series of similar peaks (Fig. 4 (2), marked with red color and ▲)
appeared close to the original peaks of [C2C1im][OAc] (Fig. 4 (1), marked with green
color), representing the imidazole carbon signals of the NHC-CO2. More importantly,

a peak around δ = 154 ppm was clearly observed (Fig. 4 (2), 7*), which represented
the extra carboxylic carbon of the NHC-CO2. This peak could be used as the
characteristic peak for the detection of the NHC-CO2 [97]. Subsequently, we took the
reaction mixture of 1a and 2a catalyzed by the AgNO3/[C2C1im][OAc] system for 13C
NMR (Fig. 4 (3)). It could be clearly observed that the characteristic peak at δ =
154.22 ppm appeared, indicating the existence of the NHC-CO2 adducts. Moreover,
other peaks of the imidazole carbon signals of the NHC-CO2 could also be
successively found in this spectrum (Fig. 4 (2) vs. (3)). These results further
confirmed the existence of the NHC-CO2 adducts in the reaction process.
Fig. 4. The detection of NHC-CO2 adducts.
Afterwards, model reaction I (Fig. 5 Model I) without CO2 was performed to further
explore the reactivity of this extra CO2 source, in which the freshly prepared and
degassed 3 mmol [C2C1im][OAc]/NHC-CO2 mixture (containing 0.45 mmol of
NHC-CO2. Fig. S1 of Part 1.1, supporting information) was introduced instead of the

normal [C2C1im][OAc]. After several hours, the reaction mixture was directly
13
sampled for C NMR. In the spectrum, the signals of 3a could be clearly detected
(Fig. S2 of Part 1.1, supporting information), indicating the NHC-CO2 adducts might
participate in the reaction individually without CO2. Moreover, 17% of 3a was
obtained, namely 0.45 mmol of NHC-CO2 was converted to 0.42 mmol of 3a,
revealing this transformation was almost stoichiometric. Subsequently, model
reactions II and III (Fig.5 Model II vs. III) varied in the ILs (freshly prepared
[C2C1im][OAc]/NHC-CO2 vs. normal [C2C1im][OAc]) were conducted under 1 bar of
CO2 with the same amount of AgNO3 and substrates. After 20 minutes, model
reaction II that generating NHC-CO2 in advance, gave a better yield than model
reaction III. These results suggested that the existence of this extra CO2 source was
probably beneficial for the catalytic reaction.
Fig. 5. Model reactions for the exploration of NHC-CO2 adducts

4.4 Exploration for the NHC-Ag complexes
Having verified the existence of the NHC-CO2 adducts as well as its reactivity, we
were inspired to further investigate a similar hypothesis about the active Ag species.
Similarly, we speculated that in this catalytic reaction, the AgNO3 might combine
with the imidazole rings assisted by the basic OAc− anions (Scheme 3), giving the
NHC-Ag complexes [29, 56, 86, 98, 99]. Herein, we successively explored the
existence of the complexes as well as the catalytic activity.
Scheme 3. Generation of NHC-Ag complexes.
In order to explore the NHC-Ag complexes in the catalytic process, model reaction
IV (Fig. 7 Model IV) was performed. Once the reaction was finished, the mixture was
checked directly by HRMS. Obviously, the spectrum clearly showed two signals of
216.98911 and 327.07304, which were found to meet the calculated masses of the
NHC-Ag complexes (Fig. 6). Other isotopic peaks generated due to the silver were
also observed, which further confirmed the existence of the desired complexes.

Fig. 6. Detection of the NHC-Ag complexes during the catalytic process.
Subsequently, we compared the catalytic activity of the NHC-Ag complexes with the
normal AgNO3 through the model reaction IV and V (Fig. 7a). Particularly for V,
AgNO3 and [C2C1im][OAc] were firstly mixed and stirred for several hours to in situ
generate the desired NHC-Ag complexes, which could be indicated by the HRMS
spectrum (Fig. S3 of Part 1.2, supporting information). Once finished, this mixture

was directly used for model reaction V. Meanwhile, model reaction IV with the
same amount of AgNO3, [C2C1im][OAc], substrate 1a and 2a under the same
conditions were simultaneously started. Both of these reactions were monitored by 1H
NMR to respectively obtain their kinetic curves (Fig. 7b). Obviously, a higher
conversion could be invariably obtained by model reaction V at any time of the
reaction process, although the same amount of Ag source was employed. This result
indicated that the NHC-Ag complexes were probably more active catalytic species
than the normal AgNO3. This might be one reason for the superior catalytic activity of
the AgNO3/[C2C1im][OAc] system even with the lowermost Ag loading employed.
(a)
(b)