Silica-catalyzed carboxylative cyclization of propargylic amines with CO2

Article abstract of DOI:10.1016/j.tetlet.2020.152557

By employing only silica as a catalyst, the carboxylative cyclization of a propargylic amine with CO₂ proceeded to afford the corresponding 2-oxazolidinone. MCM-41, which was a mesoporous silica, was found to be the most effective silica for th

Full text of DOI:10.1016/j.tetlet.2020.152557

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Silica-catalyzed carboxylative cyclization of propargylic amines with CO2
Hideaki Matsuo, Jun-Chul Choi, Tadahiro Fujitani, Ken-ichi Fujita
PII:
S0040-4039(20)31050-9
DOI:
Reference:
https://doi.org/10.1016/j.tetlet.2020.152557
TETL 152557
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Tetrahedron Letters
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Revised Date:
Accepted Date:
7 September 2020
7 October 2020
11 October 2020
Please cite this article as: Matsuo, H., Choi, J-C., Fujitani, T., Fujita, K-i., Silica-catalyzed carboxylative
cyclization of propargylic amines with CO2, Tetrahedron Letters (2020), doi: https://doi.org/10.1016/j.tetlet.
2020.152557
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Hideaki Matsuo, Jun-Chul Choi, Tadahiro Fujitani, Ken-ichi Fujita*

1
Tetrahedron Letters
Silica-catalyzed carboxylative cyclization of propargylic amines with CO2
Hideaki Matsuo, Jun-Chul Choi, Tadahiro Fujitani, Ken-ichi Fujita*
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
By employing only silica as a catalyst, the carboxylative cyclization of a propargylic amine with
CO2 proceeded to afford the corresponding 2-oxazolidinone. MCM-41, which was a mesoporous
silica, was found to be the most effective silica for this purpose. Moreover, after the reaction, the
MCM-41 catalyst was recovered by filtration and could be reused over ten times without
deactivation.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Carbon dioxide fixation
Cyclization
Silica
Keyword_4
Keyword_5
Strategies for the capture and transformation of carbon dioxide
(CO2) into value-added chemicals have gained tremendous
attention, because CO2 can be used as a nontoxic, non-
flammable, abundant, and renewable resource.1 Moreover, CO2 is
one of the most attractive C1 building blocks to displace toxic
reagents such as phosgene and carbon monoxide.2 Although CO2,
as the most oxidized carbon derivative, is much less reactive, its
application to organic synthesis has been considered one of the
most challenging research topics. In particular, the cyclization of
unsaturated organic molecules with CO2 is an atom-economical
approach to produce cyclic carbonates and oxazolidinones, which
are the essential building blocks for plastics and antibiotics,
respectively.3
One of the promising approaches for transforming CO2 is
through the carboxylative cyclization of propargylic amines with
CO2 to provide 2-oxazolidinones.4 Recently, various
investigations of the carboxylative cyclization of propargylic
amines with CO2 catalyzed by metal nanoclusters5 and by
organometallic complexes of transition metals such as
ruthenium,6 silver,7 and gold8 have been conducted. It is also
We have examined the carboxylative cyclization of a
propargylic amine 1a with CO2 to provide a 2-oxazolidinone 2a
through the use of various solid catalysts, as shown in Table 1.15
First, when a toluene solution of propargylic amine 1a (0.4
mmol) was stirred at 120 ºC for 5 h in a sealed autoclave under
3.0 MPa of CO2 using Q-6 (80 mg), an amorphous silica, as a
catalyst, the corresponding 2-oxazolidinone 2a was obtained in a
58% chemical yield (Table 1, entry 1). Next, by employing
mesoporous silicas, such as SBA-15 and MCM-41, as catalysts, it
was found that MCM-41 afforded the highest chemical yield of
2a (77%; Table 1, entry 3).16 In contrast, neither basic alumina
nor acidic alumina exhibited catalytic activities (Table 1, entries
4 and 5). When hydrotalcite was used as a catalyst, 2a was
obtained in a low chemical yield (24%; Table 1, entry 6). When
no catalyst was used, 2a was not obtained (Table 1, entry 7).
These results indicate that the carboxylative cyclization of 1a
with CO2 in entries 1–3 in Table 1 could be catalyzed by silica.
Table 1. Carboxylative cyclization of the propargylic amine
1a with CO2 using various solid catalysts.a
reported that
a number of metal-free catalysts, such as
superbases,9 N-heterocyclic carbenes,10 triethanolamine,11 and
ammonium fluoride,12 have been developed for this reaction. In
addition, in order to easily recover a catalyst by filtration after the
reaction, the use of silica (SiO2)-bound catalytic active species,
such as a ruthenium complex and organic bases, has been
reported.13 We report herein that silica can be used alone to
catalyze the carboxylative cyclization of propargylic amines with
CO2 to provide 2-oxazolidinones.14 In a screening of various
silicas as catalysts, MCM-41 was found to be the most effective
catalyst for the carboxylative cyclization of propargylic amines
with CO2. Further, it was found that the MCM-41 catalyst could
be recovered by filtration and reused without deactivation.
Entry
Solid catalyst
Silica (Q-6)
Yield (%)b
1
2
3
4
5
6
7
58
66
77
1
0
24
0
Silica (SBA-15)
Silica (MCM-41)
Basic alumina
Acidic alumina
Hydrotalcite
none

2
Tetrahedron Letters
a Reaction conditions: 1a (0.4 mmol), solid catalyst (80 mg), toluene (1 mL;
of 5.0 MPa according to the reaction conditions indicated in
Table 4. By carrying out the carboxylative cyclization of 1a at
120 ºC for 10 h, the 2-oxazolidinone 2a was obtained in an 84%
chemical yield (Table 4, entry 1). In the case of the cyclization of
N-methyl propargylic amine 1b, which has a low boiling point,
we used a sealed autoclave containing the reaction mixture
pressurized with CO2 of 5.0 MPa at room temperature before
heating to 110 ºC.17 As a result, the carboxylative cyclization of
1b with CO2 proceeded at 110 ºC to provide the corresponding 2-
oxazolidinone 2b in a 57% chemical yield (Table 4, entry 2).
Even by the introduction of a methyl group at R2 in 1, the
carboxylative cyclization of the propargylic amine 1c proceeded
to provide the corresponding 2-oxazolidinone 2c in a 93%
chemical yield by carrying out the reaction at 140 ºC for 20 h
(Table 4, entry 3). In contrast, by the introduction of a phenyl
group at R1 in 1, the carboxylative cyclization of 1d gave a poor
chemical yield (10%; Table 4, entry 4).18 On the other hand,
when a trifluoromethyl or a cyano group was introduced at the
phenyl group in R1, the chemical yields of the corresponding 2-
oxazolidinones 2e and 2f slightly increased due to the high
reactivity of the carbon–carbon triple bond owing to the
introduction of electron-withdrawing groups10a,12a (46% and 33%;
0.4 M based on 1a), carried out in a sealed autoclave at 120 ºC for 5 h under
pressurized CO2 (3.0 MPa).
b
1
Determined by the integration of H NMR absorptions with reference to an
internal standard.
Encouraged by these results, we subsequently optimized the
amount of catalyst, as shown in Table 2. By employing various
amounts of MCM-41 as a catalyst, the carboxylative cyclization
of a propargylic amine 1a with CO2 was carried out under the
same reaction conditions and scale as shown in Table 1. As a
result, when 40 mg of MCM-41 was used as a catalyst, the
corresponding 2-oxazolidinone 2a was obtained in the highest
chemical yield (84%; Table 2, entry 2).
Table 2. Optimization of the amount of the MCM-41 catalyst
in carboxylative cyclization of the propargylic amine 1a with
CO2.a
Entry
MCM-41
80 mg
40 mg
Yield (%)b
Table 4. Carboxylative cyclization of various propargylic
amines 1 for the synthesis of 2.a
1
2
3
77
84
77
20 mg
a The reaction conditions and scale were identical to those in Table 1.
b
1
Determined by the integration of H NMR absorptions with reference to an
internal standard.
Next, by employing the optimized amount of the MCM-41
catalyst, we examined two of the reaction conditions—namely,
the CO2 pressure and reaction temperature—as shown in Table 3.
We first performed the carboxylative cyclization of the
propargylic amine 1a in toluene at 120 ºC for 3 h under various
CO2 pressures (1.0–5.0 MPa). When the reaction was carried out
under CO2 pressure of 5.0 MPa, the 2-oxazolidinone 2a was
obtained in the highest chemical yield (70%; Table 3, entry 4).
We then examined the reaction temperature under CO2 pressure
of 5.0 MPa. When the reaction was carried out at 100 ºC, the
chemical yield of 2a was very low (4%; Table 3, entry 5). On the
other hand, even when the reaction was carried out at 140 ºC, the
chemical yield of 2a was almost the same as that at 120 ºC (71%;
Table 3, entry 6).
Temp.
(ºC)
Time
(h)
Yield
(%)b
84 (84)c
Entry
1
Substrate
1a
1b
120
110
10
10
2d
57 (56)c
3
4
5
6
1c
1d
1e
1f
140
120
120
100
20
4
93 (91)c
10
10
5
46
33
7
1g
1h
140
140
20
20
6
0
Table 3. Carboxylative cyclization of the propargylic amine
1a with CO2 under various reaction conditions.a
8d
a Reaction conditions: 1 (0.4 mmol), MCM-41 (40 mg), toluene (1 mL; 0.4 M
based on 1), carried out in a sealed autoclave under the conditions indicated
in the table.
b
1
Determined by the integration of H NMR absorptions with reference to an
internal standard.
c
Isolated yield. Purified with silica gel column chromatography (hexane–
Entry
CO2 (MPa)
Temp. (ºC)
120
Yield (%)b
ethyl acetate as eluents).
Pressurized with CO2 of 5.0 MPa at room temperature before heating to the
indicated temperature.
d
1
2
3
4
5
6
1.0
3.0
4.0
5.0
5.0
5.0
42
55
64
70
4
120
120
120
100
140
71
a
Reaction conditions: 1a (0.4 mmol), MCM-41 (40 mg), toluene (1 mL; 0.4
M based on 1a), carried out in a sealed autoclave for 3 h under the indicated
reaction conditions.
Determined by the integration of H NMR absorptions with reference to an
b
1
internal standard.
Figure 1. Structure of 2-oxazolones 3e and 3f.
We then performed the MCM-41-catalyzed carboxylative
cyclization of various propargylic amines 1 under CO2 pressure

3
Table 5. Catalyst recycling in carboxylative cyclization of the propargylic amine 1b with CO2.a,b
No. of run
Yield (%)c
1
68
2
74
3
78
4
79
5
81
6
80
7
81
8
81
9
79
10
80
11
81
12
80
13
78
14
77
15
76
a
Reaction conditions: 1b (4 mmol), MCM-41 (400 mg), toluene (5 mL; 0.8 M based on 1b), carried out at 110 ºC in a sealed autoclave for 10 h under
pressurized CO2.
b Pressurized with CO2 of 5.0 MPa at room temperature before heating to 110 ºC.
c Determined by the integration of 1H NMR absorptions with reference to an internal standard.
Table 4, entries 5 and 6, respectively). Only in these cases, the
corresponding 2-oxazolones 3e and 3f were also obtained in
slight chemical yields, respectively (Figure 1; 3e: 4%, 3f: 2%). 2-
Oxazolones 3e and 3f appeared to be obtained by the
tautomerization of the generated 2-oxazolidinones 2e and 2f,
respectively.10a,19 When we introduced the methyl group in R1,
the corresponding 2-oxazolidinone 2g was obtained in a low
chemical yield (6%, Table 4, entry 7).20 When a primary amine
1h was used as a substrate, the corresponding 2-oxazolidinone 2h
was not obtained (Table 4, entry 8).
Finally, the reusability of the MCM-41 catalyst was examined
by use of the N-methyl propargylic amine 1b, as shown in Table
5.21 In this experiment, it was found that the MCM-41 catalyst
was recovered by filtration of the reaction mixture, and could be
reused over ten times without deactivation to afford the
corresponding 2-oxazolidinone 2b in a fair chemical yield.
Figure 2 shows a proposed mechanism for the silica-catalyzed
carboxylative cyclization of the propargylic amine 1. First, the
propargylic amine 1 reacts with CO2 to form the corresponding
carbamic acid 4 in situ.8g,12b It is considered that the thus-obtained
carbamic acid 4 was activated by silica-surface OH– interaction
with the carbon–carbon triple bond, as shown in 5.22 Then, the
reaction, the MCM-41 catalyst was recovered by filtration and
could be reused over ten times without deactivation. We are
currently trying to apply a silica catalyst to other chemical
transformations of CO2. The results will be reported in due
course.
Acknowledgment
This work was based on results obtained from a project
(JPNP16010) commissioned by the New Energy and Industrial
Technology Development Organization (NEDO).
Supplementary Data
Supplementary data associated with this article can be found,
in the online version , at http://
.
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Figure 2. Proposed mechanism of the silica-catalyzed
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In summary, by employing only silica as a catalyst, the
carboxylative cyclization of propargylic amines with CO2
proceeded to afford the corresponding 2-oxazolidinones. MCM-
41, which was a mesoporous silica, was the most effective
catalyst for the reaction, providing a 2-oxazolidinone derivative
in a maximum chemical yield of 93%. Moreover, after the

4
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15. Carboxylative cyclization of a propargylic amine 1a with CO2:
General Procedure. To a toluene solution (1 mL) of propargylic
amine 1a (0.4 mmol) in a stainless steel autoclave was added a
solid catalyst (80 mg) under an argon atmosphere. The autoclave
was sealed, heated to 120 ºC and then pressurized with CO2 of 3.0
MPa. The resulting mixture was magnetically stirred at 120 ºC for
Title: Silica-catalyzed carboxylative cyclization of
propargylic amines with CO2
Highlights
• Silica catalyzed the carboxylative cyclization of a
propargylic amine with CO2.
• MCM-41 was the most effective silica catalyst for
the reaction.
• The MCM-41catalyst was recovered by filtration
and could be reused over ten times.
5
h. After the autoclave was cooled in an ice bath and
depressurized, a solid catalyst was separated by filtration under a
vacuum, followed by washing with dichloromethane. The
chemical yield of the 2-oxazolidinone 2a was determined by the
integration of 1H NMR absorption with reference to an internal
standard (2-benzyloxynaphthalene), which was added to the
filtrated dichloromethane solution.
16. By measuring the nitrogen adsorption-desorption isotherm of
silicas, it was found that MCM-41 has the largest BET specific
surface area, which is shown in the supplementary data.
17. The pressure of CO2 at 110 ºC was raised to 6.0 MPa.
18. Recovery of the propargylic amine 1d could not be detected. By
prolonging the reaction time, the chemical yield of
a 2-
oxazolidinone 2d decreased, probably due to the thermal
decomposition of 2d.
19. Fujita, K.; Sato, J.; Yasuda, H. Synlett 2015, 26, 1106–1110.
20. In Figure 2, by the introduction of the methyl group in R1, it is
assumed that the nucleophilic ring-closure process from 5 to 2 is
probably retarded due to the electron-donating methyl group.
21. Catalyst recycling in the carboxylative cyclization of 1b with CO2:
To a toluene solution (5 mL) of the propargylic amine 1b (277
mg, 4.01 mmol) in a stainless steel autoclave was added MCM-41
(402 mg) under an argon atmosphere. The autoclave was sealed,
pressurized with CO2 of 5.0 MPa at room temperature, and then
heated to 110 ºC (CO2: 6.6 MPa). The resulting mixture was
magnetically stirred at 110 ºC for 10 h. The autoclave was cooled
in an ice bath and depressurized. MCM-41 was separated by
filtration under a vacuum, followed by washing with 2-propanol
and dichloromethane successively, under an argon atmosphere.
After drying MCM-41 under reduced pressure, it was reused
according to the above reaction procedure. The chemical yield of
the 2-oxazolidinone 2b was determined according to the method,
which was similar to that of Table 1.15
22. Recently, silica-surface OH– interaction with the carbon–carbon
double bond was confirmed.; Fang, Y.; Lakey, P. S. J.; Riahi, S.;
McDonald, A. T.; Shrestha, M.; Tobias, D. J.; Shiraiwa, M.;
Grassian, V. H. Chem. Sci. 2019, 10, 2906–2914.
Silica-catalyzed carboxylative cyclization of
propargylic amines with CO2