Upgrading CO2 by Incorporation into Urethanes through Silver-Catalyzed One-Pot Stepwise Amidation Reaction

Article abstract of DOI:10.1002/cjoc.201700572

One-pot two-step stepwise reaction of terminal propargylic alcohols, carbon dioxide, and primary/secondary amines for the effective synthesis of various urethanes through robust silver-catalysed C-O/C-N bond formation is reported. Catalytic activities were investigated by controlling catalyst loading, reaction pressure and time, and very high turnover number (turnover frequency) was obtained: 3350 (35 h^?1) with 0.01 mol% silver catalyst under 0.1 MPa, and up to 13360 (139 h^?1) with 0.005 mol% silver catalyst under 2.0 MPa at room temperature. The strategy was ingeniously regulated, and synchronously afforded a wide range of β-oxopropylcarbamate and 1,3-oxazolidin-2-one motifs in excellent yields and selectivity together with unprecedented high turnover number (TON) and turnover frequency (TOF) value.

Full text of DOI:10.1002/cjoc.201700572

Upgrading CO2 by incorporation into urethanes through sil-
ver-catalyzed one-pot stepwise amidation reaction
Qing-Wen Song,a Ping Liu,a Li-Hua Han,a Kan Zhanga* and Liang-Nian Heb*
a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan,
030001, P. R. China
b State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin, 300071, P. R. China
One-pot two-step stepwise reaction of terminal propargylic alcohols, carbon dioxide, and primary/secondary
amines for the effective synthesis of various urethanes through robust silver-catalysed C–O/C–N bond formation is
reported. Catalytic activities were investigated by controlling catalyst loading, reaction pressure and time, and very
high turnover number (turnover frequency) was obtained: 3350 (35 h-1) with 0.01 mol% silver catalyst under 0.1
MPa, and up to 13360 (139 h-1) with 0.005 mol% silver catalyst under 2.0 MPa at room temperature. The strategy
was ingeniously regulated, and synchronously afforded
a wide range of β-oxopropylcarbamate and
1,3-oxazolidin-2-one motifs in excellent yields and selectivity together with unprecedented high TON and TOF
value.
Keywords Carbon dioxide utilization, Homogeneous catalysis, C-N bond formation, Silver catalysis, Synthetic
method
Introduction
and turnover frequency (TOF) for breaking through the
bottleneck in the amidation reaction using CO2 as a
feedstock under mild conditions could be still highly
desirable; this represents a significant and challenging
area in both catalysis and sustainable chemistry.
As is well-known, α-alkylidene cyclic carbonates,
generally synthesized through carboxylative cyclization
of propargylic alcohols and CO2, are key intermediate
for the synthesis of β-oxopropylcarbamate and
1,3-oxazolidin-2-one motifs in the three-component
reaction of propargylic alcohols, amines, and CO2
(Scheme 1).
Over the last several decades, the chemical utiliza-
tion of the abundant, easily available, and renewable
CO2 feedstock as an integral part of the carbon cycle for
producing chemicals, polymers, and fuels has received
considerable attention.1-3 Although CO2 is thermody-
namic stability and kinetic inertness, recent progress in
the catalytic conversion of CO2 has been remarkable. In
ideal process, chemical fixation of CO2 as green
carbonyl source is of great significance as an alternative
to the CO and phosgene process for the synthesis of
valuable products, such as urea and its derivatives,
urethanes, and polyurethanes through C-N bond
formation.4 In this respect, urethanes, among the most
Scheme 1 Chemical fixation of CO2 with propargylic alcohols
and amines.
important
carbonyl
compounds
such
as
β-oxopropylcarbamate and 1,3-oxazolidin-2-one motifs,
are playing a significant role in organic synthesis and
pharmaceutical chemistry.5-10
Various catalysts/promoters have been developed to
allow the effective synthesis of urethanes from
commercial propargylic alcohols, primary/secondary
amines, and CO2, including transition metal complexes
(Ru,11,12 Fe,13 Cu,14-17 Ag,18-21 and La22) and
organocatalysts (organic base23,24 and ionic liquid25). In
these systems, transition metal catalysis has more
advantages such as using lower catalytic loading, higher
effeciency and selectivity with mild reaction conditions
than metal-free systems. Despite considerable progress
has been made in the past few decades, converting CO2
into readily useful chemicals still remains a daunting
challenge presumably due to some limitations such as
catalyst compatibility, harsh reaction conditions, low
yields or selectivity. Therefore, development of effec-
tive methodologies with high turnover number (TON)
Although it is thermodynamic favorable and effec-
tive for carboxylative cyclization and the reaction of
α-alkylidene cyclic carbonate with aliphatic amine, it is
of much difficulty for the one-pot three-component re-
action of propargylic alcohols, amines, and CO2 to go
smoothly with superior efficiency. Study revealed that
active center of transition metal catalyst is prone to be
restrained or poisoned by the electron-rich substrates
such as amines which lead to a decreased catalytic per-
formance and increased catalytic loading. Considering
the limitation of the convenient routes and mechanism
exploration in previous study, herein we became
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interested in developing an efficacious methodology for
the conversion of CO2 into valuable chemicals with
great enhanced efficiency on three-component reaction.
In this context, we hypothesized that one-pot stepwise
strategy namely α-alkylidene cyclic carbonates can be
efficiently produced in the first step as reactive
intermediate through carboxylative cyclization of pro-
pargylic alcohols with CO2, and subsequently amines
are introduced with the generation of aimed compounds.
Based on our knowledge in the area of CO2 chemistry
associated with propargylic alcohols,11-25 we herein
4H), 1.45 (s, 6H) ppm. 13C NMR (CDCl3, 100.6 MHz) δ
208.0, 153.8, 62.8, 46.0, 23.8, 23.6 ppm. GC-MS (EI,
70 eV) m/z (%) = 199.15 (1), 157.15 (2), 156.20 (19),
100.15 (1), 99.15 (8), 98.15 (100).
2-Methyl-3-oxobutan-2-yl
piperi-
dine-1-carboxylate (3b) Light brown oil. 1H NMR
(CDCl3, 400 MHz) δ 3.39 (m, 4H, 2N-CH2), 2.11 (s, 3H,
COCH3), 1.59-1.49 (m, 6H, -CH2CH2CH2-), 1.43 (s, 6H,
2CH3) ppm. 13C NMR (CDCl3, 100.6 MHz) δ 207.5
(C=O), 154.0 (N-C=O), 82.7, 44.6, 25.6, 24.0, 23.4,
23.2 ppm. GC-MS (EI, 70 eV) m/z (%) = 213.15 (1),
171.15 (2), 170.20 (15), 129.15 (1), 128.15 (12), 114.15
(1), 113.15 (8), 112.15 (100), 69.10 (46).
present
a favorable one-pot stepwise process to
synthesize β-oxopropylcarbamate and oxazolidinone
scaffolds from terminal propargylic alcohols, CO2, and
amines. As a result, high effeciency with unprecedented
TON and TOF values of the cascade three-component
reaction under low catalytic loading of silver catalyst
was obtained.
2-Methyl-3-oxobutan-2-yl
2-methylpiperidine-1-carboxylate (3c) 1H NMR
(CDCl3, 400 MHz) δ 4.42-4.36 (m, 1H), 3.94-3.90 (d,
1H), 2.91-2.85 (m, 1H), 2.10 (s, 3H), 1.67-1.32 (m, 6H),
1.44 (3H), 1.43 (3H), 1.16-1.14 (d, J = 7.2 Hz, 3H) ppm.
13C NMR (CDCl3, 100.6 MHz) δ 207.8, 154.3, 82.8,
46.5, 39.0, 30.0, 25.6, 23.64, 23.57, 23.4, 18.4 ppm.
GC-MS (EI, 70 eV) m/z (%) = 213.10 (1), 212.10 (2),
184.10 (14), 142.10 (10), 126.10 (100), 84.10 (13),
Experimental
Unless otherwise noted, carbon dioxide (99.999%),
commercially available propargylic alcohols and amines.
All starting materials were obtained from Aladdin and
Alfa Aesar, and used as received. All reactions were
carried out without any special precautions against air.
1H NMR spectra was recorded on 400 MHz spectrome-
ters using CDCl3 as solvent referenced to CDCl3 (7.26
ppm). 13C NMR was recorded at 100.6 MHz in CDCl3
(77.00 ppm). Multiplets were assigned as singlet, dou-
blet, triplet, doublet of doublet, multiplet and broad sin-
glet. Mass spectra were recorded on a Shimadzu
GCMS-QP2010 equipped with a RTX-5MS capillary
column at an ionization voltage of 70 eV. The data are
given as mass units per charge (m/z). High resolution
mass spectrometry was conducted using a Bruker mi-
crOTOF-Q III by ESI technique.
+
83.10 (8), 55.10 (17). HRMS (ESI): C12H22NO3 for
[M+H]+ calculated 228.1594, found 228.1614.
2-Methyl-3-oxobutan-2-yl
4-(hydroxymethyl)piperidine-1-carboxylate
(3d)
1
White solid powder. H NMR (CDCl3, 400 MHz) δ
4.17-4.13 (2H), 3.51 (d, J = 6.4 Hz, 2H), 2.79 (br, OH),
2.12 (s, 3H, CH3), 1.77-1.64 (m, 4H), 1.45 (s, 6H),
1.21-1.11 (m, 2H) ppm. 13C NMR (CDCl3, 100.6 MHz)
δ 207.8, 154.2, 83.1, 67.4, 44.0, 38.6, 28.6, 23.6, 23.5
ppm. GC-MS (EI, 70 eV) m/z (%) = 158.10 (5), 142.10
(100), 114.10 (10), 69.10 (10), 43.10 (22), 28.10 (26).
+
HRMS (ESI): C12H22NO4 for [M+H]+ calculated
244.1543, found 244.1545.
2-Methyl-3-oxobutan-2-yl
morpho-
line-4-carboxylate (3e) Orange solid. M.p. 75-77 oC 1H
NMR (CDCl3, 400 MHz) δ 3.62 (t, J = 4.0 Hz, 4H),
3.43 (t, J = 20.0 Hz, 4H), 2.09 (s, 3H), 1.42 (s, 6H) ppm.
13C NMR (CDCl3, 100.6 MHz) δ 207.2, 154.0, 83.3,
66.5, 44.5, 43.6, 23.5, 23.5 ppm. GC-MS (EI, 70 eV)
m/z (%) = 215.10 (1), 173.15 (2), 172.15 (18), 115.15
(6), 114.15 (100), 71.10 (2), 70.10 (49), 69.10 (2).
2-Methyl-3-oxobutan-2-yl diethylcarbamate (3f)
Straw yellow oil. 1H NMR (CDCl3, 400 MHz) δ
3.30-3.25 (q, 4H, CH2), 2.12 (s, 3H), 1.45 (s, 6H), 1.14
(m, 6H) ppm. 13C NMR (CDCl3, 100.6 MHz) δ 207.7,
154.6, 82.8, 41.8, 41.6, 23.6, 23.4, 14.1, 13.5 ppm.
GC-MS (EI, 70 eV) m/z (%) = 202.15 (1), 159.20 (2),
158.20 (16), 102.15 (3), 101.15 (6), 100.20 (100), 72.10
(47).
General
procedure
for
the
synthesis
of
β-oxopropylcarbamates
Taking the reaction of 2-methylbut-3-yn-2-ol (1a),
pyrrolidine, and CO2 as an example: a 75 mL stainless
steel reactor equipped with a stir bar was charged with
Ag2CO3 (2.8 mg, 0.01 mol%), (p-CH3OC6H4)3P (14.1
mg, 0.04 mol%), 1a (8.41 g, 100 mmol), and CHCl3 (2
mL). Next, the pressure was kept to 2 MPa CO2. Then,
the mixture was stirred at 25 °C for the desired time.
Excessive CO2 was carefully released after the reaction.
Open the reactor, pyrrolidine (7.11 g, 100 mmol) was
introduced, and subsequently stirred for another two
hours. The residue was flushed with CH3CN (5 mL),
and detected by GC method immediately. Finally, the
solvent was distilled off and the residue mixture was
purified by silica gel column chromatography to get the
β-oxopropylcarbamate 3a. The structure of products
was further identified by using NMR and GC-MS tech-
niques, which are consistent with those reported in the
literature.20,21
2-Methyl-3-oxobutan-2-yl dibutylcarbamate (3g)
1
Yellow oil. H NMR (CDCl3, 400 MHz) δ 3.20 (t, 4H),
2.11 (s, 3H), 1.52-1.47 (m, 4H), 1.43 (s, 6H), 1.34-1.24
(m, 4H), 0.92 (6H) ppm. 13C NMR (CDCl3, 100.6 MHz)
δ 207.7 (C=O), 155.0 (N-C=O), 82.8, 47.0, 46.7, 30.8,
30.1, 23.6, 23.3, 19.9, 13.8 ppm. GC-MS (EI, 70 eV)
m/z (%) = 258.25 (1), 215.20 (2), 214.20 (12), 173.20
(1), 172.20 (12), 157.20 (10), 156.20 (100), 101.15 (2),
100.15 (35), 88.10 (7), 87.15 (2), 86.15 (27), 85.10 (14).
2-Methyl-3-oxobutan-2-yl
pyrroli-
dine-1-carboxylate (3a) Colourless oil. 1H NMR
(CDCl3, 400 MHz) δ 3.37 (m, 4H), 2.14 (s, 3H), 1.87 (m,
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2-Methyl-3-oxobutan-2-yl
cyclohex-
155.6 (C=O), 150.9, 81.9, 79.0, 41.1, 28.3, 27.9, 19.9,
13.7 ppm. GC-MS (EI, 70 eV) m/z (%) = 183 (M+, 21),
168 (84), 128 (32), 97 (58), 96 (100), 84 (52), 82 (84).
3-Cyclohexyl-5,5-dimethyl-4-methyleneoxazolidin
-2-one (4b) White solid. M.p. 54-56 oC. 1H NMR
(CDCl3, 400 MHz) δ 4.19 (d, J = 4.0 Hz, 1H), 3.97 (d, J
= 4.0 Hz, 1H), 3.58-3.50 (m, 1H), 2.11-2.01 (m, 2H),
1.86-1.64 (m, 4H), 1.45 (s, 6H), 1.32-1.14 (m, 4H) ppm.
13C NMR (CDCl3, 100.6 MHz) δ 155.0 (C = O), 150.7,
81.1, 79.7, 53.7, 28.3, 27.9, 25.9, 25.1 ppm. GC-MS (EI,
70 eV) m/z (%) = 209.20 (10), 194.20 (3), 129.15 (8),
128.20 (100), 127.20 (25), 85.15 (4), 84.15 (31), 83.15
(5), 68.10 (9), 67.10 (16).
3-Benzyl-5,5-dimethyl-4-methyleneoxazolidin-2-o
ne (4c) Orange-yellow oil. 1H NMR (CDCl3, 400 MHz)
δ 7.33-7.25 (m, 5H, Ar-H), 4.64 (s, 2H), 4.03 (d, J = 4.0
Hz, 1H), 3.96 (d, J = 2.4 Hz, 1H), 1.51 (s, 6H) ppm. 13C
NMR (100.6 MHz, CDCl3) δ 155.8, 150.2, 135.2, 128.6,
127.6, 126.9, 82.2, 80.5, 45.0, 27.8 ppm. GC-MS (EI,
70 eV) m/z (%) = 217.15 (M+, 21), 172.20 (7), 91.10
(100).
3-Butyl-4-methylene-1-oxa-3-azaspiro[4.5]decan-
2-one (4d) White solid. M.p. 60-62 oC. 1H NMR
(CDCl3, 400 MHz) δ 4.04 (d, J = 4.0 Hz, 1H), 3.92 (d,
1H), 3.40 (t, J = 8.0 Hz, 2H), 1.83-1.30 (m, 14H), 0.90
(t, 3H) ppm. 13C NMR (CDCl3, 100.6 MHz) δ 155.8
(C=O), 150.8, 83.5, 79.2, 40.9, 36.8, 28.3, 24.6, 21.5,
19.8, 13.6 ppm. GC-MS (EI, 70 eV) m/z (%) = 223.20
(32), 181.20 (50), 168.20 (100), 137.20 (29), 136.20
(24), 112.10 (51), 95.15 (26), 67.10 (20).
yl(methyl)carbamate (3h) Colourless oil. 1H NMR
(CDCl3, 400 MHz) δ 3.88 (m, 1H), 2.78 (s, 3H), 2.11 (s,
3H), 1.79-1.75 (m, 2H), 1.65-1.62 (m, 2H), 1.44 (s, 6H),
1.36-1.30 (m, 2H) ppm. 13C NMR (CDCl3, 100.6 MHz)
δ 207.7, 155.0, 82.9, 54.8, 30.1, 28.4, 25.6, 25.4, 23.7,
23.4 ppm. GC-MS (EI, 70 eV) m/z (%) = 198.10 (25),
156.10 (15), 140.10 (100), 114.10 (14), 83.10 (98),
70.10 (16), 59.10 (19), 57.10 (19), 55.10 (21), 43.10
+
(17). HRMS (ESI): C13H24NO3 for [M+H]+ calculated
242.1751, found 242.1779.
1-Acetylcyclohexyl pyrrolidine-1-carboxylate (3i)
o
1
Colourless solid. M.p. 62-63 C. H NMR (CDCl3, 400
MHz) δ 3.45 (2H), 3.36 (2H), 2.12 (3H), 2.06-2.03 (2H),
1.89 (4H), 1.67-1.57 (5H), 1.54-1.44 (2H), 1.23 (1H)
ppm. 13C NMR (CDCl3, 100.6 MHz) δ 208.3 (C=O),
153.4 (N-C=O), 84.0, 46.0, 45.9, 30.9, 25.6, 25.1, 24.8,
23.6, 21.3 ppm. GC-MS (EI, 70 eV) m/z (%) = 196.15
(M+-43, 15), 98.15 (100).
1-Acetylcyclohexyl piperidine-1-carboxylate (3j)
Colourless oil. 1H NMR (CDCl3, 400 MHz) δ 3.45-3.36
(4H), 2.06 (s, 3H), 2.01-1.98 (2H), 1.64-1.38 (m, 13H),
1.24-1.12 (1H) ppm. 13C NMR (CDCl3, 100.6 MHz) δ
208.1 (C=O), 153.7 (N-C=O), 84.1, 45.2, 44.6, 30.8,
25.9, 25.0, 24.2, 23.4, 21.4 ppm. MS (EI, 70 eV) m/z (%)
= 210.20 (12), 112.15 (100), 69.10 (27).
3-Methyl-2-oxopentan-3-yl
pyrroli-
dine-1-carboxylate (3k) Straw yellow oil. 1H NMR
(CDCl3, 400 MHz) δ 3.36 (4H), 2.13 (3H), 1.96-1.87
(5H), 1.72-1.63 (1H), 1.45 (s, 3H), 0.88 (t, 3H) ppm. 13C
NMR (CDCl3, 100.6 MHz) δ 208.3(C=O), 153.4
(N-C=O), 84.0, 46.0, 45.9, 30.9, 25.6, 25.1, 24.8, 23.6,
21.3 ppm. GC-MS (EI, 70 eV) m/z (%) = 170.15 (M+-43,
17), 98.15 (100).
Results and discussion
Initially, we explored the carboxylative cyclization
of propargylic alcohols and CO2 via robust silver catal-
ysis to confirm the highest efficiency. As shown in
Scheme 2, the reaction of 1a and CO2 with affording
α-alkylidene cyclic carbonate (2a) disclosed distinct
results in different medium. Despite unclear mechanism,
we envisioned that solvent is of much importance to
promote the formation of well-defined silver complex
and stabilize it. In this protocol, almost quantitative
yield of 2a was obtained in trichloromethane at ambient
conditions.
Subsequently, we further examined the catalytic ac-
tivities of silver system with rich-electron phosphine
ligand. The use of 0.05 mol% of Ag2CO3 in combination
with PPh3 resulted in 62% conversion of propargylic
alcohol (1a) into the α-alkylidene cyclic carbonate (2a)
after 12 hours, whereas using (p-MeOC6H4)3P led to an
increase yield but (p-MeC6H4)3P show decrease trend as
the yield (Table 1, entries 1-3). These results indicated
that the phosphine ligand and substituent group in aro-
matic structure could both regulate the Lewis acidity of
the silver ion and the steric configuration of silver com-
plex for the effective activation and catalysis function.
Therefore, the steric and electron effect of phosphine
ligand are both the important factor for the catalytic ac-
tivity of silver complex. However, PCy3 and bidentate
General
procedure
for
the
synthesis
of
1,3-oxazolidin-2-ones
Taking the reaction of 2-methylbut-3-yn-2-ol (1a),
n-butylamine, and CO2 as an example: a 75 mL stainless
steel reactor equipped with a stir bar was charged with
Ag2CO3 (2.8 mg, 0.01 mol%), (p-CH3OC6H4)3P (14.1
mg, 0.04 mol%), 1a (8.41 g, 100 mmol), and CHCl3 (2
mL). Next, the pressure was kept to 2 MPa CO2. Then,
the mixture was stirred at 25 °C for the desired time.
Excessive CO2 was carefully released after the reaction.
Open the reactor, n-butylamine (7.31 g, 100 mmol),
PhCH3 (10 mL) and 2.0 g 4Å MS were introduced, and
o
subsequently stirred for another two hours at 120 C.
The residue was flushed with CH3CN (5 mL) and fil-
trated, and detected by GC method. Finally, the solvent
was distilled off and the residue mixture was purified by
silica gel column chromatography to get the
1,3-oxazolidin-2-one 4a. The structure of products was
further identified by using NMR and GC-MS techniques,
which are consistent with those reported in the litera-
ture.20
3-Butyl-5,5-dimethyl-4-methyleneoxazolidin-2-on
e (4a) Orange oil. 1H NMR (CDCl3, 400 MHz) δ 4.06 (d,
J = 4.0 Hz, 1H), 3.97 (d, J = 4.0 Hz, 1H), 3.42 (t, J = 8.0
Hz, 2H), 1.61-1.53 (m, 2H), 1.48 (s, 6H), 1.37-1.28 (m,
2H), 0.92 (t, 3H) ppm. 13C NMR (CDCl3, 100.6 MHz) δ
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phosphine
ligand
such
as

1,3-bis(diphenylphosphino)propane (Dppp) proved to
be little effective under the given conditions, whereas
Xantphos gave 44% 2a yield, suggesting a crucial role
of phosphine structure in promoting catalytic perfor-
mance (Table 1, entries 4-6).
Scheme 2 Solvent investigation.
Reaction conditions: 1a (0.421 g, 5 mmol), Ag2CO3 (6.9 mg, 0.5
mol%), PPh3 (26.2 mg, 2 mol%), solvent (1 mL), CO2 balloon, 2
o
h, 25 C. Yield was determined by GC with biphenyl as the in-
ternal standard.
a
Table 1 Carboxylic cyclization of propargylic alcohols with CO2
Entry Ligand P (MPa) T (oC) t (h) Yield (%)b TON TOF (h-1)
1
2
balloon
balloon
balloon
balloon
balloon
balloon
balloon
1.0
25
25
25
25
25
25
25
25
25
40
60
25
25
25
25
12
12
12
12
12
12
12
6
62
65
44
<1
1
1240
1300
880
103
108
73
L1
L2
L3
L4
L5
L6
L2
L2
L2
L2
L2
L2
L2
L2
L2
3
4
-
-
5
20
1
6
26
84
95
98
63
3
520
43
7c
1680
1906
1962
1260
60
140
318
327
105
5
8c
9c
2.0
6
10c
11c
12c,d
13c,d
14c,e
15c,f
balloon
balloon
balloon
balloon
2
12
12
48
96
96
96
17
34
31
67
1840
3350
12160
13360
38
35
127
2
139
aReaction conditions: 1a (1.6824 g, 20 mmol), Ag2CO3 (2.8 mg, 0.05 mol%), ligand (0.2 mol%), CO2 balloon, 25 oC, 12 h. bDetermined
by GC with biphenyl as the internal standard. cCHCl3 (2 mL). d1a (8.41 g, 100 mmol), Ag2CO3 (2.8 mg, 0.01 mol%), L2 (14.1 mg, 0.04
mol%). e1a (33.64 g, 400 mmol), Ag2CO3 (2.8 mg, 0.0025 mol%), L2 (14.1 mg, 0.01 mol%). f1a (16.82 g, 200 mmol), Ag2CO3 (2.8 mg,
0.005 mol%), L2 (14.1 mg, 0.02 mol%). TON = 2a (mol)/Ag2CO3 (mol); TOF = TON/time (hour).
Next, the survey of other reaction parameters was
conducted using (p-MeOC6H4)3P as the ligand. Increase
the pressure, the reaction ratio was greatly improved,
and almost quantitative yield was obtained with high
TON and TOF (Table 1, entries 7-9). The increase of
CO2 density contributes to the promotion of carboxyla-
tive cyclization of propargylic alcohols and CO2 ac-
cording to the Le Chartelier's principle. However, in-
crease reaction temperature is not beneficial for the re-
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action probably due to the instability of silver complex
in high temperature (entries 10 and 11). Under atmos-
pheric pressure of CO2, the TON value up to 3350 with
the loading of 0.01 mol% Ag2CO3 for 96 h (entry 13).
Furthermore, the TON value up to 13360 with the TOF
value of 139 h-1 was gained through adjusting the cata-
lyst usage (entry 15). In 2012, Nizuno group reported
(nBu4N)2WO4 catalyzed carboxylative cyclization with
TON value of up to 473 (TOF 20) which represented the

highest efficiency among the reported metal-free sys-
tems.26 In our previous work, a robust catalytic system
consisting of (nC7H15)4NBr and AgOAc was developed
for the synthesis of a-alkylidene cyclic carbonates with
unprecedentedly high turnover number up to 6024 (TOF
34) to date.27 Through the comparison with these excel-
lent catalytic systems, great advance in the work was
made. Similarity, the two-step strategy also gave aimed
urethanes with unprecedentedly high TON and TOF
value in excellent selectivity. Notably, the continuously
increased TON value (or yield) and the decreased TOF
value with prolonging the reaction time (entries 7, 12-15)
indicated that the catalyst still showed high activity even
though a tendency to lower catalytic rate.
mixture
H2/CO2
Ar/CO2
N2/CO2
air/CO2
H2/CO2
ratio
3:1
4:1
4:1
9:1
3:1
(%)b
11.8
7.0
(h-1)
10
6
1
2
118
70
3
6.3
63
5
4
4.6
46
4
5c
19.9
199
17
aReactions were carried out with 10 mmol 1a, 0.1 mol% Ag2CO3,
o
0.4 mol% L2, CHCl3 (1 mL) under balloon (5 L) at 25 C for 12
h. bDetermined by GC with biphenyl as the internal standard.
cAg2CO3 (28 mg, 1 mol%), L2 (141 mg, 4 mol%), CHCl3 (2 mL).
Notably, the reusability of silver complex in the car-
boxylic cyclization of propargylic alcohol with CO2 was
also demonstrated through a simple recycling method of
homogeneous silver complex catalyst (Scheme S1,
Supporting Information). The excellent results
(Conv./Yield: >98%) revealed that the recovered cata-
lyst still performed good catalytic activity after three
runs (Scheme S2).
Generally, in many robust transition metal systems,
inactive reaction atmosphere was required to guarantee
the promotion of conversion.28,29 Herein, several ex-
periments under various reaction atmospheres were
performed to investigate the tolerance. Under different
gas atmosphere such as reducing, oxidizing and inert
gas (Table 2, entries 1-5), the reaction was not inhibited
in all of the cases. It is worth noting that the yield re-
duced with the decreasing of CO2 content which further
indicates the concentration of CO2 is one of the most
key parameters during the conversion.
Having established the highly effective sil-
ver-catalyzed carboxylative cyclization protocol, we
then studied the new strategy for synthesis of
β-oxopropylcarbamates through C-N bond formation.
Initially, we performed the control experiments to com-
pare the reactivity. In traditional three-component reac-
tion of 1a, pyrrolidine, and CO2, only 4%
β-oxopropylcarbamate (3a) yield under atmospheric
pressure and 9% under 2 MPa were obtained (Scheme 3,
a), although the excellent yield and selectivity of car-
boxylative cyclization of propargylic alcohols with CO2,
and amidation of 2a with pyrrolidine (Scheme 3, b).
Gratifyingly, 3a was obtained in a yield of 81% with
significant advantage in TON and TOF value by em-
ploying one-pot stepwise method under the same reac-
tion conditions (Scheme 3, c). These results preliminary
Scheme 3 Control experiments.
With an optimized catalytic system in hand, we ex-
plored the scope of the three-component reaction by
employing a variety of secondary amines and typical
terminal propargylic alcohols under CO2 atmosphere
(Table 3). The one-pot stepwise reactions initially took
place via the formation of a-methylene cyclic car-
bonates and a subsequent aminolysis sequence. During
the procedure, no isolation process for the carbonate
intermediate was needed and the catalytic activity of
silver complex was also kept well without any inhibition
by introducing secondary amines separately. Various
representative β-oxopropylcarbamates synthesized are
depicted in Scheme 3. Under atmospheric pressure of
CO2, the yield was up to 32.7% and TON value of up to
3270 at a catalytic loading of 0.01 mol% at room tem-
perature for 96 h (entries 1 and 6). Under the same con-
ditions, the reaction was proceeded smoothly to give
good yield and TON value. Both cyclic and linear ali-
phatic secondary aliphatic amines afforded the corre-
sponding product with moderate to excellent yields. As
seen from the results, when increasing the steric hin-
drance of substrate such as 2-methylpiperidine and
n-dibutylamine, the reactive activity slightly declined
(entries 3 and 7). In addition, electronic effect is also
one of the most important factors to influence the reac-
tivity of amines such as 1-oxa-4-azacyclohexane and
N-methylbenzylamine (entries 5 and 8).
displayed
the
higher
production
of
β-oxopropylcarbamates with higher TON and TOF val-
ue than that of traditional effective systems.
Table 2 Fixation of CO2 under various gas atmospherea
Entry
Gas
Initial
Yield
TON
TOF
Table 3 Scope of the reaction of propargylic alcohols, CO2, and secondary aminesa
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Entry
Product 3
Yield (%)b TON TOF (h-1) Entry
Product 3
Yield (%)b TON TOF (h-1)
32.7c
3270
34
30.5c
3050
31
6
7
92.4
9240
96
91.2
9120
93
3f
60.8d
26.8e
11200
10720
117
112
56.0
64.5
5600
6450
57
66
1
3g
3a
8
3h
2
3
91.8
37.7
9180
3770
94
38
9
92.6
93.2
9260
9320
94
95
3b
3c
3i
3j
10
4
5
90.4
77.4
9040
7740
92
79
11
60.8
6080
62
3k
3d
3e
aUnless otherwise specified, the reactions in first step were performed with 1a (8.41 g, 100 mmol), Ag2CO3 (2.8 mg, 0.01 mol%), L2
(14.1 mg, 0.04 mol%), 2.0 MPa, CHCl3 (2 mL), 96 h, 25 oC; reaction conditions in second step were performed with equivalent amines at
ambient temperature for 2 h. bDetermined by GC with biphenyl as the internal standard. cCO2 balloon. d1a (16.82 g, 200 mmol), Ag2CO3
(2.8 mg, 0.005 mol%), L2 (14.1 mg, 0.02 mol%). e1a (33.64 g, 400 mmol), Ag2CO3 (2.8 mg, 0.0025 mol%), L2 (14.1 mg, 0.01 mol%).
Totally, the steric hindrance and electronic induction
effect reduced the nucleophilicity of amines, which fur-
ther depressed the activity of nucleophilic substrates.
For piperidin-4-ylmethanol bearing with two type func-
tional group, target product was obtained with excellent
selectivity (entry 4).
tremely mild conditions prompted us to expand poten-
tial applications of this one-pot stepwise reaction strat-
egy to the synthesis of 1,3-oxazolidin-2-one motifs via
the three-component reaction of propargylic alcohols,
primary amines and CO2 under mild conditions as listed
in Table 4.
Typical terminal propargylic alcohols with alkyl
substituents at the propargylic position (1b and 1c) un-
derwent the reaction smoothly under the identical reac-
tion conditions to afford the corresponding
β-oxopropylcarbamates (3i-k) in yields of 60.8%-93.2%
and TON value of up to 9320. Additionally, 1c also
showed a good reactivity in high yield and TON value.
The successful transformation of propargylic alco-
hols and secondary amines to β-oxopropylcarbamates
Table 4 Scope of the reaction of propargylic alcohols, CO2, and
primary aminesa
O
O
4
R1
R
N
RNH2
OH
R2
catalyst
25 oC
O
O
O
R1
R2
R1
R2
+
CO2
1
2
Stepwise in one-pot
with high efficiency through CO2 fixation under ex-
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Entry
1
Product 4
Yield (%)b TON TOF (h-1)
92.5
89.4
88.3
89.3
9250
8940
8830
8930
94
91
90
91
4a
2
3
4
4b
4c
4d
aUnless otherwise specified, the reactions in first step were per-
formed with 1a (8.41 g, 100 mmol), Ag2CO3 (2.8 mg, 0.01
mol%), L2 (14.1 mg, 0.04 mol%), 2.0 MPa, CHCl3 (2 mL), 96 h,
25 oC; reaction conditions in second step were performed with
equivalent amines, 10 mL PhCH3 and 2.0 g 4Å MS at 120 oC for
2 h. bDetermined by GC with biphenyl as the internal standard.
Scheme 4 Plausible mechanism.
Conclusions
In summary, a successful protocol of one-pot step-
wise three-component reaction of propargylic alcohols,
carbon dioxide, and primary/secondary amines is dis-
closed for the effective synthesis of various urethanes
with excellent TON value of up to 11,200 and TOF of
up to 117 h-1 at mild reaction conditions. The over-
whelming superiority of the stepwise strategy is verified
in comparison with the traditional one-pot method. This
approach also does not require high loading of silver
catalyst, high pressure and temperature, and no interme-
diate was required to be isolated. Notably, the good re-
usability of silver complex is demonstrated, and the
simple recycling method of homogeneous catalyst is
well developed. Finally, a broad amine substrate appli-
cation scope is been primarily demonstrated.
Pleasingly, the transformation was performed
smoothly with the catalysis of low loading silver cata-
lyst. Herein, the reflux conditions of toluene were used
with the assistance of 4Å molecular sieve for removing
the water generated from intramolecular dehydration.
When 1a was used, and amines bearing n-butyl, cyclo-
hexyl, and benzyl groups, respectively, were subjected
to the reaction conditions, the 1,3-oxazolidin-2-one de-
rivatives (4a–4c) were obtained in excellent yields.
Furthermore, reaction of propargylic alcohols with cy-
clohexyl substituent gave the desired products 4d
smoothly.
A plausible reaction mechanism is proposed as
shown in Scheme 4. Initially, the carboxylative cycliza-
tion of propargylic alcohol with CO2 affords the
α-alkylidene cyclic carbonate intermediate through in
situ formed silver complex catalysis,4 which then goes
through a nucleophilic ring-opening reaction after in-
troducing secondary amines to generate the corre-
sponding β-oxopropylcarbamates 3. Additionally, when
primary amines are employed, the carbamate intermedi-
ate can further take place intramolecular nucleophilic
ring-closing reaction and elimination reaction with the
generation of oxazolidinones 4.
Acknowledgement
We are grateful to the National Natural Science
Foundation of China (21602232), the Natural Science
Foundation for Youths of Shanxi (201701D221057), and
Project of “Utilization of Low Rank Coal” Strategic
Leading Special Fund, Chinese Academy of Sciences
(XDA-07070800, XDA-07070400).
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Entry for the Table of Contents
Page No.
Upgrading CO2 by incorporation into
urethanes
through
silver-catalyzed
one-pot stepwise amidation reaction
Qing-Wen Song; Ping Liu; Li-Hua Han; Kan
Zhang;* Liang-Nian He*
One-pot two-step procedure fixation of CO2 for urethanes synthesis with unprecedented
TON and TOF value through silver catalysis was described.
This article is protected by copyright. All rights reserved.