Efficient and Recyclable Cobalt(II)/Ionic Liquid Catalytic System for CO2 Conversion to Prepare 2-Oxazolinones at Atmospheric Pressure

Article abstract of DOI:10.1002/cjoc.201900346

Converting CO₂ into value-added chemicals represents a promising way to alleviate the CO₂ derived environmental issues, for which the development of catalysts with high efficiency and recyclability is very desirable. Herein, the catalytic system by combining cobalt source and ionic liquid (IL) has been developed as the efficacious and recyclable catalyst for the carboxylative cyclization of propargylic amine and CO₂ to prepare 2-oxazolinones. In this protocol, various propargylic amines were successfully transformed into the corresponding 2-oxazolinones with CoBr₂ and diethylimidazolium acetate ([EEIM][OAc]) as the catalyst under atmospheric CO₂ pressure. It is worth noting that the turnover number (TON) of this transformation can be up to 1740, presumably being attributed to the cooperative effect of the cobalt and IL. Furthermore, the existence of IL enables the catalytic system to be easily recycled to 10 times without losing its activity.

Full text of DOI:10.1002/cjoc.201900346

CO2 conversion, carboxylative cyclization, cobalt catalysis, ionic liquid,
sustainable chemistry
中
国 化 学
Efficient and Recyclable Cobalt(II)/Ionic Liquid Catalytic System for CO2
Conversion to Prepare 2-Oxazolinones at Atmospheric Pressure
Zhi-Hua Zhou, Kai-Hong Chen,* and Liang-Nian He*
State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China
Cite this paper: Chin. J. Chem. 2019, 37, 1223—1228. DOI: 10.1002/cjoc.201900346
Summary of main observation and conclusion Converting CO2 into value-added chemicals represents a promising way to alleviate the CO2 derived
environmental issues, for which the development of catalysts with high efficiency and recyclability is very desirable. Herein, the catalytic system by com-
bining cobalt source and ionic liquid (IL) has been developed as the efficacious and recyclable catalyst for the carboxylative cyclization of propargylic
amine and CO2 to prepare 2-oxazolinones. In this protocol, various propargylic amines were successfully transformed into the corresponding 2-oxazo-
linones with CoBr2 and diethylimidazolium acetate ([EEIM][OAc]) as the catalyst under atmospheric CO2 pressure. It is worth noting that the turnover
number (TON) of this transformation can be up to 1740, presumably being attributed to the cooperative effect of the cobalt and IL. Furthermore, the
existence of IL enables the catalytic system to be easily recycled to 10 times without losing its activity.
recover from products with the assistance of low-polar solvents.
Recently, Han’s group reported ILs as catalysts can promote the
Background and Originality Content
Utilization of carbon dioxide (CO2) as an ideal carbon source to
reaction of propargylic amines with CO2 efficiently, in which the
ILs are reusable.[7a,7e] Based on all of these, we questioned
prepare value-added chemicals is of great significance in view of
the environmental and resource factors.[1] However, the highest
whether the introduction of ILs into transition-metal catalysts
oxidation state of carbon in CO2 earns it high thermodynamic
might give them reusability while maintaining their high efficien-
stability and kinetic inertness, resulting in the necessity of harsh
cy.
reaction conditions in most CO2 conversions. Therefore, the devel-
As one of the naturally abundant metals, cobalt compounds
opment of catalysts for efficient CO2 transformation has become
have recently been developed as cost-effective catalysts for the
cyclization of propargylic amine with CO2.[6u] Unfortunately, a rela-
an important aspect of CO2 chemistry. In this regard, a great deal
of catalytic systems involving homogeneous and heterogeneous
tively high amount of cobalt catalyst is needed in this system to
catalysts have already been designed and developed.[2] In general,
ensure the efficient transformation. Moreover, the catalyst cannot
homogeneous and heterogeneous catalysts are known for high
be recycled. In this work, to further develop recyclable and more
catalytic efficiency and easy recovery, respectively, and the combi-
efficient cobalt catalysis, cobalt compounds and ILs were well
nation of these two separated advantages of catalysts will be con-
combined and used for the carboxylative cyclization of propargylic
ducive to CO2 conversion on a large scale. Therefore, developing
amines with CO2 (Scheme 1). With this catalyst, amazing perform-
catalysts characterized with high-performance and easily recycling
ance was exhibited and the highest turnover number (TON), to
for CO2 transformation is meaningful and desirable.
the best of our knowledge, of 1740 was obtained. A series of
Among multifarious CO2 transformation, the carboxylative cy-
propargylic amines can be cyclized under atmospheric CO2 pres-
clization of propargylic amines with CO2 represents a typical ex-
sure to afford the corresponding 2-oxalizaliones in good to excel-
ample of using CO2 as a raw material to synthesize valuable
lent yields. Notably, this catalytic system is easily recovered and its
2-oxazolidones,[3] which are common substructures in pharma-
original activity can be remained after the catalyst has been recy-
cled for 10 times.
ceuticals[4] and found in asymmetric catalysis as chiral auxiliaries.[5]
Methodologies by employing transition metals[6] and organo-
catalysts[7] for this transformation have been developed. Notably,
Results and Discussion
Yamada’s group achieved this transformation under mild condi-
tions, in particular low CO2 pressure, by using silver(I) salts as
catalysts.[6h,6o] With IPr-gold(I) (IPr = 1,3-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene) as catalysts, catalytic mechanism involving
nucleophilic cyclization of the carbamate at the alkyne unit acti-
vated by gold is proven through the isolation of alkenylgold(I)
complexes.[6e,6r] These results show that the activation of C≡C
triple bond by transition metals is favorable for the carboxylative
cyclization.[6c] Unfortunately, the reusability is always a significant
drawback for the transition-metal catalysts.
Initially, we choose CoBr2 and 1-butyl-3-methylimidazolium
Scheme 1 Recyclable Co/IL system for carboxylative cyclization of pro-
pargylic amines with CO2
Ionic liquids (ILs),[8] as a kind of green raw material, have the
advantages of homogeneous and heterogeneous catalysis, i.e.
high efficiency and recyclability. In detail, tunable structures of ILs
make them easy to dissolve the substrates, benefiting homoge-
neous catalysis; meanwhile, their high polarity is beneficial to
*E-mail: khchen@nankai.edu.cn; heln@nankai.edu.cn
For submission: https://mc.manuscriptcentral.com/cjoc
For articles: https://onlinelibrary.wiley.com/journal/16147065
† Dedicated to Professor Mingyuan He on the occasion of his 80th birthday.
Chin. J. Chem. 2019, 37, 1223－1228
© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1223

Zhou, Chen & He
Concise Report
chloride ([BMIM][Cl]) as the catalytic system to promote the car-
boxylative cyclization of N-benzylprop-2-yn-1-amine (1a) with CO2
and a 14% yield of 3-benzyl-5-methyleneoxazolidin-2-one (2a) was
obtained (Table 1, entry 1). Tuning the IL component by employ-
ing the synergistic effect between CoBr2 and [EEIM][OAc]. There-
fore, CoBr2 and [EEIM][OAc] were selected as the optimal catalytic
system for further investigation.
Investigating the reaction parameters such as the amount of
catalyst, temperature, the amount of IL and reaction time was
subsequently proceeded. 2 mol% of CoBr2 is most suitable, at
which 2a can be generated in quantitative yields (entries 5, 11 and
12). Temperature has an obvious effect on this reaction, and sig-
nificantly reduced yield of 2a was observed when reaction pro-
ceeded at 40 oC (entry 12 vs. 14). Halving the dosage of
[EEIM][OAc], yield of 2a was reduced to 52% (entry 15). In addi-
tion, it is inappropriate to shorten the reaction time to 6 h (entry
16). Finally, the optimal reaction conditions were determined as

ing other anions such as BF4 and OAc instead of Cl, increased
yields of 2a were observed (entries 2 and 3). Gratifyingly, the
combination of CoBr2 with 1-butyl-3-methylimidazolium acetate
[BMIM][OAc] promoted the complete conversion of 1a with 100%
selectivity to give the desired product 2a (entry 3). These results
indicate cobalt catalysis is sensitive to the anion of IL, and the
weakly basic acetate anion shows the excellent compatibility with
cobalt catalyst. Compared with anions, the cations of ILs have no
significant effect on the reaction (entries 3—5). Then, the type of
cobalt compounds was investigated. All elected cobalt salts in-
cluding CoBr2, CoCl2, CoI2 and Co(OAc)2 showed high activity for
this transformation in the presence of diethylimidazolium acetate
[EEIM][OAc] (entries 5—8). The catalytic efficiency of various co-
balt salts is further evaluated by reducing their amount to 1 mol%
(relative to 1a) (entries 5—8, data in parentheses), and results
showed that the highest catalytic efficiency was obtained when
using CoBr2 as the catalyst (entry 5, data in parenthesis). Further
exploration by employing CoBr2 or [EEIM][OAc] individually as the
catalyst only afforded low yields of 2a (entries 9 and 10), indicat-
o
follows: 2 mol% of CoBr2, 2 equivalent of [EEIM][OAc], 60 C and
10 h.
Subsequently, the diverse propargylic amines were examined
to react with CO2 under the optimized reaction conditions. As
shown in Table 2, this protocol is quite universal and a host of
propargylic amines could all be perfectly applicative. Benzyl de-
rived propargylic amines with electron-donating and electron-
withdrawing groups on the aryl underwent the carboxylative cy-
clization to afford the corresponding 2-oxazolidines in high yields
(entries 2—8). Halogens such as chlorine and bromine are toler-
ated in this system and reactions of 1b and 1c with CO2 proceeded
to generate the desired 2b and 2c in 92% and 91% yields, respec-
tively (entries 2 and 3). The position of substituent on the aryl ring
has an effect on the reactivity of propargylic amine. For example,
meta-methyl substituted propargylic amine 1h showed lower
activity than those with methyl group in ortho- and para-positions
(entry 8 vs. 6 and 7). Compared with N-benzylprop-2-yn-1-amine
(1a), the introduction of a methyl group on benzyl position (1i) or
replacing the benzyl with a cyclohexyl group (1j) results in the
lowered reactivity of propargylic amine (entries 9 and 10), which
may be owing to the impediment of fixation of CO2 induced by
increase of steric hindrance around the nitrogen. Moreover, phe-
nylethyl substituted propargylic amine 1k also underwent this
cyclization smoothly, affording the desired 2k in 91% yield (entry
11). Internal propargylic amine, for example 1l, gave a moderate
yield of the targeted 2-oxazolinone (entry 12), indicating the ad-
verse effect of the presence of phenyl at the terminal alkyne on
the reactivity of propargylic amine. In addition, nitrogen on
N-(prop-2-yn-1-yl)aniline (1m) and propargylic amines with elec-
tron-withdrawing protecting groups such as –Ts, –Boc and –Ac
(1n—1p) exhibit relatively weak nucleophilicity, resulting in the
failure of transformation (entries 13—16).
Table 1 Co/IL catalytic system for reaction of propargylic amine 1a and
a
CO2
Entry Co salt
IL
1a Conversionb/%
2a Yieldb/%
1
2
CoBr2
CoBr2
CoBr2
CoBr2
CoBr2
CoCl2
CoI2
[BMIM]Cl
[BMIM]BF4
[BMIM]OAc
[EMIM]OAc
[EEIM]OAc
[EEIM]OAc
[EEIM]OAc
[EEIM]OAc
—
37
65
14
30
3
>99 (84c)
>99
99 (82c)
94
4
5
>99 (88c)
>99 (30c)
>99 (45c)
>99 (56c)
>99
99 (86c)
99 (29c)
99 (45c)
99 (55c)
33
6
7
8
Co(OAc)2
CoBr2
—
9
10
11d
12e
13e,f
14e,g
15e,h
16e,i
[EEIM]OAc
[EEIM]OAc
[EEIM]OAc
[EEIM]OAc
[EEIM]OAc
[EEIM]OAc
[EEIM]OAc
24
20
CoBr2
CoBr2
CoBr2
CoBr2
CoBr2
CoBr2
>99
99
>99
96
To further evaluate the efficiency of this catalytic system, the
reaction of 1a and CO2 was magnified to a gram scale accompa-
nying with the amount of CoBr2 reduced to 0.05 mol%. As depict-
ed in Scheme 2, an 87% yield of 2a was obtained with a TON of
1740.
>99
99
26
26
56
52
74
69
Scheme 2 Gram scale experiment
After that, the reusability of the catalyst was investigated,
which was rarely mentioned in previous work.[6d,6p,6v,6x,7a] 1a was
chosen as the model substrate to investigate the recyclability of
the catalytic system, in which CoBr2 and [EEIM][OAc] were reused
after extracting 2a with hexane and removing the residual hexane
under vacuum. As shown in Figure 1, the catalytic activity of the
CoBr2/[EEIM][OAc] nearly remained after ten successive runs,
a Reaction conditions: 1a (0.5 mmol, 72.6 mg), [Co] (10 mol% relative to
o
1
1a), IL (1 mmol), 60 C, 9 h, CO2 balloon. b Determined by H NMR with
c
1,3,5-trimethoxybenzene as internal standard. CoBr2 (1 mol%). d CoBr2 (5
e
f
o
mol%). CoBr2 (2 mol%). 10 h. g 40 C. h [EEIM][OAc] (0.5 mmol, 92.1 mg).
i 6 h.
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Chin. J. Chem. 2019, 37, 1223－1228

Cobalt(II)/Ionic Liquid Catalytic System for CO2 Conversion
Chin. J. Chem.
a
Table 2 Co/IL catalytic system for carboxylative cyclization of various propargylic amines with CO2
Entry
1
Substrate
Product
Yieldb/%
99 (88c)
Entry
9
Substrate
Product
Yieldb/%
79
1a
1b
2a
2b
1i
1j
2i
2j
2
3
92 (83c)
91 (80c)
10
11
72
91
1c
2c
1k
1l
2k
2l
1d
2d
4
99
12
51
1e
1f
2e
2f
1m
1n
1o
1p
2m
2n
2o
2p
5
6
7
8
98 (90c)
13
14
15
16
0
0
0
0
84
1g
1h
2g
2h
87
72
a Reaction conditions: propargylic amines (0.5 mmol), CoBr2 (2 mol%, 2.2 mg), [EEIM][OAc] (2 eq., 184.2 mg), CO2 balloon, 60 oC, 10 h. b Determined by 1H
NMR with 1,3,5-trimethoxybenzene as internal standard. c Isolated yield.
indicating the catalytic system could maintain its primitive per-
formance and can be recycled. Further investigation on the cata-
lytic system by comparing their FT-IR spectra before and after the
tenth recovery experiment, showed no obvious changes of char-
acteristic peaks (Figure S1, see Supporting Information).
To gain insight into the reaction mechanism, the interaction
between cobalt salt and IL was studied via 1H NMR (Figure 2). The
disappearance of signal of [OAc] after mixing CoBr2 and
[EEIM][OAc] (Figures 2a and 2b) indicates the existence of a pos-
sible coordination between them, which has been proven to be
favorable to enhance the activity of cobalt catalysis.[6u] Further
investigation found that combination of Co(OAc)2 and [EEIM]Br as
the catalyst cannot give the comparable result to that with CoBr2
and [EEIM][OAc],[9] indicating the multifunctional role of basic
[EEIM][OAc]. The down-shifted signal of 2-H of imidazole (Figures
2c and 2e) with synchronously up-shifted signal of N-H of 1a in 1H
NMR spectrum (Figures 2d and 2e) indicates the activation of
propargylic amine by IL. Therefore, a feasible mechanism for the
cobalt-catalyzed carboxylative cyclization was illustrated in
Scheme 3. In this catalytic system, IL can be used to cooperate
with cobalt to enhance its activity,[6u] and as a base at the same
[10]
time to capture CO2 and activate the amino group of propar-
gylic amine to facilitate the CO2 fixation. After CO2 capture, CoBr2
stabilized carbamate intermediate I was formed. Further intra-
molecular cyclization of I generated the intermediate II as the
precursor of 2-oxazolinone, which gave the targeted 2-oxazo-
linone after a protonation process.
Figure 1 Recyclability of the CoBr2/[EEIM][OAc] system. Reaction condi-
tions: 1a (0.5 mmol, 72.6 mg), 60 C, 12 h, CO2 balloon. Yield of 2a was
determined by NMR with 1,3,5-trimethoxybenzene as internal standard.
o
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1225

Zhou, Chen & He
Concise Report
mmol), [EEIM][OAc] (184.2 mg, 1 mmol), and propargylic amine
(0.5 mmol) were added to a 10 mL Schlenk tube. Then, the mix-
o
ture was stirred at 60 C for 10 h with a CO2 balloon (about 1 L).
When finished, the resultant mixture was analyzed by 1H NMR
with 1,3,5-trimethoxybenzene as internal standard and then puri-
fied by column chromatography with petroleum ether (60—90 oC)
and ethyl acetate as eluents (20 : 1—10 : 1) to obtain the desired
2-oxazolinone (2).
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.201900346.
Acknowledgement
This work was financially supported by National Natural Sci-
ence Foundation of China (Nos. 21672119, 21975135), and the
China Postdoctoral Science Foundation (No. 2018M641624).
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Chin. J. Chem. 2019, 37, 1223－1228
© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
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Zhou, Chen & He
Concise Report
[9] With 1 mol% Co(OAc)2 and 2 equivalent [EEIM]Br as the catalyst, a 52%
yield of 2a was obtained from the reaction of 1a with CO2, which is
less than the generated 86% yield of 2a in the presence of 1 mol%
CoBr2 and 2 equivalent [EEIM][OAc] (Table 1, entry 5, data in paren-
theses).
[10] Gurau, G.; Rodríguez, H.; Kelley, S. P.; Janiczek, P.; Kalb, R. S.; Rogers,
R. D. Demonstration of Chemisorption of Carbon Dioxide in
1,3-Dialkylimidazolium Acetate Ionic Liquids. Angew. Chem. Int. Ed.
2011, 50, 12024–12026.
Manuscript received: August 28, 2019
Manuscript revised: September 25, 2019
Manuscript accepted: September 26, 2019
Accepted manuscript online: October 4, 2019
Version of record online: XXXX, 2019
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© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2019, 37, 1223－1228