Efficient chemical fixation of CO2 promoted by a bifunctional Ag2WO4/Ph3P system

Article abstract of DOI:10.1039/c3gc42406e

An efficient heterogeneous silver-catalyzed reaction for construction of the α-methylene cyclic carbonate motif was developed through carboxylative assembly of propargyl alcohols and CO₂. Such a CO₂ fixation protocol proceeded smoothly with only 1 mol% of Ag₂WO ₄ and 2 mol% of PPh₃ as well as atmospheric CO₂ at room temperature under solvent-free conditions, in an environmentally benign and low energy manner along with an easy operating procedure. Notably, up to 98% isolated yields of carbonates could be attained with exclusive chemo-selectivity. In addition, the dual activation capacity of Ag ₂WO₄ towards both the propargylic substrate and CO ₂ is based on which cooperative catalytic mechanism by the silver cation and the tungstate anion is proposed. Recycling trials on carboxylative cyclization of propargyl alcohols and CO₂ illustrate that the catalyst can be reused at least 4 times with retention of high catalytic activity and selectivity. Especially, it allows the direct and effective application in the one-pot synthesis of various oxazolidinones bearing exocyclic alkenes and carbamates in moderate to high yields upon the alternative introduction of primary or secondary amines.

Full text of DOI:10.1039/c3gc42406e

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Efficient chemical fixation of CO2 promoted by a
bifunctional Ag2WO4/Ph3P system†
Cite this: Green Chem., 2014, 16,
1633
Qing-Wen Song,‡a Bing Yu,‡a Xue-Dong Li,a Ran Ma,a Zhen-Feng Diao,a
Rong-Guan Li,b Wei Li*b and Liang-Nian He*a
An efficient heterogeneous silver-catalyzed reaction for construction of the α-methylene cyclic car-
bonate motif was developed through carboxylative assembly of propargyl alcohols and CO2. Such a CO2
fixation protocol proceeded smoothly with only 1 mol% of Ag2WO4 and 2 mol% of PPh3 as well as atmos-
pheric CO2 at room temperature under solvent-free conditions, in an environmentally benign and low
energy manner along with an easy operating procedure. Notably, up to 98% isolated yields of carbonates
could be attained with exclusive chemo-selectivity. In addition, the dual activation capacity of Ag2WO4
towards both the propargylic substrate and CO2 is based on which cooperative catalytic mechanism by
the silver cation and the tungstate anion is proposed. Recycling trials on carboxylative cyclization of pro-
pargyl alcohols and CO2 illustrate that the catalyst can be reused at least 4 times with retention of high
catalytic activity and selectivity. Especially, it allows the direct and effective application in the one-pot
synthesis of various oxazolidinones bearing exocyclic alkenes and carbamates in moderate to high yields
upon the alternative introduction of primary or secondary amines.
Received 24th November 2013,
Accepted 19th December 2013
DOI: 10.1039/c3gc42406e
www.rsc.org/greenchem
remains a daunting challenge presumably due to limitations
existing in those known processes such as tedious reaction
Introduction
With increasing awareness of the ever-growing CO2 level in the procedures, harsh reaction conditions, or low yields.4 There-
aerosphere, great efforts have been devoted to development of fore, development of effective methodologies using CO2 as a
strategies towards the reduction of CO2 emissions.1 The chemi- feedstock under mild conditions particularly low pressure of
cal fixation of CO2 is highly attractive and effective as a CO2 (ideally at 1 atm) could be still highly desirable.
straightforward method to recycle CO2 into value-added pro-
One of the most promising examples of CO2 fixation is
ducts due to the great potential of CO2 as an abundant, non- carboxylative cyclization of propargyl alcohols with CO2 to
toxic, easily available and renewable C1 building block in access α-alkylidene cyclic carbonates, which stands for an
organic synthesis.2 However, only a small proportion of the important class of heterocyclic compounds with a wide range
total abundance of CO2 is currently being consumed for of applications in organic synthesis (Scheme 1).5 In this
chemical production due to its low reactivity as a result of its context, numerous catalytic systems including Ru,6 Pd,7 Co,8
high thermodynamic stability and kinetic inertness. Conse- Cu,9 Ag,10 Fe,11 phosphines,12 N-heterocyclic carbenes
quently, vigorous nucleophiles like Grignard reagents and (NHC),13 K2CO3/crown ethers,14 bicyclic guanidines,15 silver-
organolithium reagents, high-energy starting materials such as NHC complexes,16 or N-heterocyclic olefin/CO2 adducts17
small ring compounds and organometallic reagents, or drastic have been developed. Moreover, reactions conducted in
reaction conditions are commonly required for reactions invol- ionic liquids18 or using the electrochemical method19 have
ving CO2.3 Despite the significant advances in the past few also been reported. However, those procedures pose a number
decades, converting CO2 into readily usable chemicals still of drawbacks, in particular, use of complicated catalysts,
aState Key Laboratory and Institute of Elemento-Organic Chemistry, Collaborative
Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University,
Tianjin, 300071, P. R. China. E-mail: heln@nankai.edu.cn
bKey Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),
College of Chemistry, Nankai University, Tianjin, 300071, P. R. China.
E-mail: weili@nankai.edu.cn
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3gc42406e
Scheme 1 Chemical fixation of CO2 into α-alkylidene cyclic
‡Authors with equal contributions.
carbonates.
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a
Table 1 Ag(I)-catalyzed reaction of propargyl alcohol 1a with CO2
Scheme 2 Chemical fixation of CO2 through the dual activation
pathway.
requirement of higher CO2 pressure and avoidance of moist-
ure. These challenges can be surmounted, to our delight,
through the rational design of a high performance catalytic
system. Consequently, a robust catalytic system is believed to
be essential to find novel protocols for chemical fixation of
CO2 to value-added products in a cost-effective, practically
viable, and environmentally benign manner.
Entry
Catalyst
Additive
Time/h
Yieldb/%
1
2
3
4
5
6
7
8
—
DBU
Ph3P
CuCl
AgNO3
AgOAc
Ag2WO4
AgI
Ag2WO4
Ag2WO4
Ag2WO4
Ag2WO4
Ag2WO4
Ag2WO4
Ag2WO4
Ag2WO4
Ag2WO4
Ag2WO4
Ag2WO4
Ag2WO4
—
—
—
—
6
6
6
6
6
6
6
6
6
6
6
3
3
3
3
12
12
12
6
0
0
0
0
0
<1
45
0
2
0
0
85
0
<1
82
>99
98
82
>99
>99
Ph3P
Ph3P
Ph3P
Ph3P
DBU
2,2′-Bipyridine
TMEDA
Ph3P
L1
L2
L3
Ph3P
Ph3P
Ph3P
Ph3P
Ph3P
Silver salts are widely employed as activators of the CuC
triple bond in organic synthesis.20 For the reaction of propar-
gyl alcohols with CO2, the Yamada group10 and the Jiang
group16 have developed a silver-based procedure for the
synthesis of the α-alkylidene cyclic carbonates from CO2.
However, a stoichiometric amount of the base or high CO2
pressure was required in those Ag-catalyzed systems. On the
other hand, Mizuno et al.21 reported that [WO4]2− could acti-
vate a CO2 molecule by forming the [WO4]2−/CO2 adduct,
which thus could allow the CO2 transformation to proceed
smoothly under mild conditions. In this context, we envisaged
that a novel bifunctional catalytic species containing Ag+ and
[WO4]2−, i.e. Ag2WO4, could be particularly efficient for the
reaction of propargylic alcohols with CO2. The [WO4]2− anion
would activate both CO2 and the O–H bond of propargylic alco-
hol,21a which could favor the generation of the carboxylate
intermediate. The carboxylate intermediate would then be
trapped by the silver-activated CuC triple bond to afford the
stable α-alkylidene cyclic carbonate as shown in Scheme 2.
Herein, we would like to report Ag2WO4/Ph3P as a highly
efficient and robust bifunctional catalyst system for chemical
fixation of atmospheric CO2 with propargyl alcohols to provide
α-alkylidene cyclic carbonates under neutral solvent-free con-
ditions. Notably, this catalytic system was also proved to be
effective for the three-component coupling reaction of pro-
pargylic alcohols, amines and CO2. The present system can
eliminate the need for large quantities of caustic base or flam-
mable organic solvents.
9
10
11
12c
13c
14c
15c
16
17d
18e
19f
20g
6
a Unless otherwise specified, the reactions were performed with
2-methylbut-3-yn-2-ol (0.421 g, 5.0 mmol), [Ag+] or other catalysts
(2 mol%), additive (2 mol%), CO2 balloon, 50 °C. DBU = 1,8-
diazabicyclo[5.4.0]undec-7-ene;
TMEDA
=
tetramethylethylene-
diamine. b GC yields were given using biphenyl as the internal
standard. c Additive (1 mol%), 0.3 MPa. d 25 °C. e Ag2WO4 (16.3 mg,
0.7 mol%), Ph3P (18.4 mg, 1.4 mol%). f 0.5 MPa. g Ag2WO4 (58.0 mg,
2.5 mol%), Ph3P (65.5 mg, 5 mol%).
and AgI were also screened in the presence of Ph3P as an addi-
tive (entries 5–8). Gratifyingly, the α-alkylidene cyclic carbon-
ates 2a were obtained in a yield of 45% by employing 1 mol%
of Ag2WO4 in conjunction with 2 mol% of Ph3P at 50 °C for
6 h (entry 7). The results encouraged us to further optimize the
reaction conditions by using Ag2WO4 as a catalyst. The reaction
did not occur using DBU, 2,2′-bipyridine, or TMEDA as an
additive (entries 9–11). Then several bidentate phosphine
ligands were screened under 0.3 MPa of CO2 (entries 13–15).
Those results show that L1 and L2 were not effective, whereas
L3 gave 82% yield of the target product. Gratifyingly, a simple
Ag2WO4/Ph3P catalytic system gave 85% yield of 2a by increas-
ing CO2 to 0.3 MPa (entry 12). Based on this result, a quantita-
tive yield was successfully reached in the presence of 0.1 MPa
CO2 at 50 °C or 25 °C by further prolonging the reaction time
to 12 h (entries 16 and 17). With lower loading of the catalyst,
the reaction at 50 °C for 12 h afforded 2a in a yield of 82%
(entry 18). Furthermore, it was found that higher CO2 pressure
or catalyst loading was needed for the full conversion in 6 h
(entries 19 and 20).
Results and discussion
Several metal compounds were initially investigated for the
reaction of 2-methylbut-3-yn-2-ol (1a), as the model substrate
under a CO2 balloon without a solvent at 50 °C for 6 h
(Table 1). The reaction without a catalyst and an additive was
carried out only to recover the starting material quantitatively
(entry 1, Table 1). The catalysts such as DBU, Ph3P, and CuCl
were demonstrated to be ineffective under the given conditions
(entries 2–4). Different silver salts like AgNO3, AgOAc, Ag2WO4,
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a
Table 2 Carboxylative cyclization of propargyl alcohols with CO2
Entry
1
Substrate
Product
t/h
Yieldb/%
96
12
Fig. 1 Catalyst recycling for the carboxylation of 1a with CO2 (0.5 MPa)
at 50 °C for 6 h.
2
12
97
(entry 6), and 1,1-cyclopentylene showed low activity (entry 7),
probably due to the steric hindrance and ring strain. The
internal propargyl alcohol 1h required higher CO2 pressure
(1 MPa) and temperature (80 °C) to afford the corresponding
carbonate in 82% yield after 36 h (entry 8). However, the
secondary alcohol 1i failed in giving the cyclic carbonate but
the subsequent alcoholysis product 2i was formed (entry 9).
Notably, the reaction of propargyl alcohol 1a with CO2
(0.5 MPa) at 50 °C was continuously run to investigate the con-
stancy of the catalyst activity. In each cycle, the heterogeneous
catalyst could be easily separated by extraction with hexane.
After drying with nitrogen, the catalyst was reused for the next
run. As shown in Fig. 1, the catalyst can be reused at least 4
times without significant loss of catalytic activity.
The successful transformation of propargyl alcohols to
α-alkylidene cyclic carbonates through CO2 fixation under
extremely mild conditions prompted us to expand potential
applications of this Ag2WO4/Ph3P system to the synthesis of
α-alkylidene cyclic carbamates via the three-component reac-
tion15,22 of propargylic alcohols, primary amines and CO2
under mild conditions as listed in Table 3. Pleasingly, the
transformation was performed smoothly with the catalysis of
Ag2WO4 in the presence of 0.5 MPa CO2 at 50 °C for 12 h.
When 1,1-dimethylpropargylic alcohol was used, and amines
bearing n-propyl, n-butyl, cyclohexyl, and benzyl groups
respectively were subjected to the reaction conditions, the
3
12
12
12
24
24
90
98c
4
96
94
0
5
6
7d
35
8d
36
12
82
9
78
5-methyleneoxazo-lidin-2-one
derivatives
(4a–4d)
were
89c
obtained in good to excellent yields. In the case of aniline, the
undehydrated product 4e was afforded in 31% yield. Interest-
ingly, another five-membered ring was constructed via intra-
molecular dehydration to furnish 4f in 91% yield by employing
ethanolamine as a substrate. Reactions of propargylic alcohols
with a range of substituents including 1-methyl, 1-ethyl,
1-phenyl, and 1-n-pentyl groups gave the desired products
(4g–4i). Moreover, the steric hindrance of the tertiary butyl
group of the amine significantly diminishes the reactivity of
the carbamate species, which failed to provide the intramole-
cular cycloaddition product but β-oxopropylcarbamate 4k was
formed. Disappointingly, the corresponding carbamate 4l was
not obtained with internal propargyl alcohol as a starting
material.
a Unless otherwise specified, the reactions were performed with 1
(5.0 mmol), Ag2WO4 (23.2 mg, 1 mol%), Ph3P (26.2 mg, 2 mol%),
25 °C. b Isolated yield. c 50 °C. d 1 MPa of CO2 and 80 °C were used.
Subsequently, the substrate scope of this reaction was inves-
tigated under 0.1 MPa of CO2 at 25 °C (Table 2). Propargyl alco-
hols with a variety of alkyl and aryl substituents at the
propargylic position (1a–1e) were effective substrates to give
the corresponding carbonates 2a–2e in good to excellent yields
even under the conditions of atmospheric CO2 (entries 1–5,
Table 2), whereas 1-isopropyl propargyl alcohol lost reactivity
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Table 3 Three-component reaction of propargylic alcohols, primary
a
amines and CO2
Scheme 4 Stepwise synthesis of typical oxazolidinone and
β-oxopropylcarbamate.
a Unless otherwise stated, the reactions were conducted on a 5 mmol
scale of propargylic alcohol and amine with Ag2WO4 (23.2 mg,
1 mol%) and Ph3P (26.2 mg, 2 mol%) under 0.5 MPa of CO2 at 50 °C
for 12 h. Isolated yield was given.
Scheme 5 Proposed mechanism for the Ag2WO4/Ph3P-catalyzed
three-component reactions.
alcohol, 1-methyl-1-phenyl propargylic alcohol, 1-ethyl-1-
methyl propargylic alcohol, and 1-ethynylcyclohexanol, were
also found to be good substrates. Both cyclic and linear sec-
ondary aliphatic amines afforded the corresponding product
with good to excellent yields. Unfortunately, the reaction using
the aromatic amines cannot furnish the desired products.
Although we are quite confident that the oxazolidinone gene-
rated from the three-component coupling reaction between
propargylic alcohols, primary amines and CO2 proceeds via a
tandem aminolysis of α-alkylidene cyclic carbonates/intramole-
cular cyclization,22c,23b the individual steps of this process
were further examined as depicted in Scheme 4. In the
absence of the amine, the expected carbonates 2a were pro-
duced in 98% GC yield under the reaction conditions. Sub-
sequently, equimolar primary and secondary amines were
added into the reaction mixture, respectively. The corres-
ponding products 4b and 6a were isolated in reasonable yields
after 0.5 h.
Scheme 3 The Ag2WO4/Ph3P-catalyzed three-component reaction of
propargylic alcohols, secondary amines and CO2. Isolated yield was
given.
Similarly, secondary amines were also applicable to the
three-component reaction under identical reaction conditions,
which afforded β-oxopropylcarbamates as products.11,22a,23
Various representative β-oxopropylcarbamates synthesized are
depicted in Scheme 3. In most cases, the reaction was per-
formed with an approximately equimolar amount of reactants
and proceeded to completion within 12 h at 50 °C. Aliphatic
propargylic alcohols, including 1,1-dimethyl propargylic
Consequently, a plausible mechanism of the Ag2WO4/Ph3P-
catalyzed three-component reactions is proposed as illustrated
in Scheme 5. The carboxylative cyclization of the propargyl
alcohol with CO2 affords the carbonate 2 as an intermediate,
which then undergoes a nucleophilic attack by a primary or
secondary amine to generate the carbamate species. In the
presence of
a
secondary amine, the corresponding
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β-oxopropylcarbamate 6 can be obtained via tautomerization.
In the case of a primary amine, further intramolecular nucleo-
philic attack followed by dehydration affords the oxazolidinone
4.
Notes and references
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Conclusions
In summary, we have described a sustainable and cost-com-
petitive catalytic process promoted by the Ag2WO4/Ph3P system
for efficient chemical fixation of CO2 to produce α-alkylidene
cyclic carbonates, α-alkylidene cyclic carbamates, and β-oxo-
propylcarbamates under mild conditions, respectively. The
scope of the reaction is broad, with a variety of propargylic
alcohols undergoing carboxylative cyclization. Moreover, the
three-component reactions of propargylic alcohols, amines
and CO2 are also well performed with this protocol. In
addition, this procedure can eliminate the need for large quan-
tities of caustic base and flammable organic solvents, yielding
the strategy to prepare value-added α-alkylidene cyclic carbon-
ates, α-alkylidene cyclic carbamates, and β-oxopropylcarba-
mates from both environmental and economical points of
view. Importantly, the catalytic system is not sensitive to air
and minor moisture, which may enhance the possibility of fix-
ation of CO2. Such findings may be of great interest for devel-
oping alternative access to chemical fixation of CO2 with the
preparation of the CO2-based products under mild conditions.
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Experimental section
General procedure for the synthesis of α-alkylidene cyclic car-
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and propargylic alcohols (5 mmol) were added to
a
Schlenk tube equipped with a magnetic stir bar. A CO2 balloon
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gel using petroleum ether–ethyl acetate as an eluent to give the
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as a colourless oil. 1H NMR (CDCl3, 400 MHz) δ 4.75 (d, J = 4.0
Hz, 1H), 4.31 (d, J = 4.0 Hz, 1H), 1.59 (s, 6H) ppm. 13C NMR
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China (no. 21172125 and 21376123), Specialized Research
Fund for the Doctoral Program of Higher Education
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