Silver tungstate: A single-component bifunctional catalyst for carboxylation of terminal alkynes with CO2 in ambient conditions

Article abstract of DOI:10.1039/c4gc01638f

Silver tungstate was successfully developed as a bifunctional catalyst for the ligand-free carboxylation of various terminal alkynes with electron-withdrawing or electron-donating groups under atmospheric pressure of carbon dioxide (CO₂) at room temperature. In this protocol, dual activation - i.e., the terminal alkyne activated by silver, and CO₂ activation by the tungstate anion - was verified using nuclear magnetic resonance spectroscopy, and means that this reaction can be run under ambient conditions. Notably, this protocol can be applied to the preparation of a phenylacrylate derivative by a cascade reaction using phenylacetylene, CO₂ and benzylamine as starting materials.

Full text of DOI:10.1039/c4gc01638f

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Silver tungstate: a single-component bifunctional
catalyst for carboxylation of terminal alkynes with
CO2 in ambient conditions†
Cite this: DOI: 10.1039/c4gc01638f
Chun-Xiang Guo, Bing Yu, Jia-Ning Xie and Liang-Nian He*
Silver tungstate was successfully developed as a bifunctional catalyst for the ligand-free carboxylation of
various terminal alkynes with electron-withdrawing or electron-donating groups under atmospheric
pressure of carbon dioxide (CO2) at room temperature. In this protocol, dual activation – i.e., the terminal
alkyne activated by silver, and CO2 activation by the tungstate anion – was verified using nuclear magnetic
resonance spectroscopy, and means that this reaction can be run under ambient conditions. Notably, this
protocol can be applied to the preparation of a phenylacrylate derivative by a cascade reaction using
phenylacetylene, CO2 and benzylamine as starting materials.
Received 22nd August 2014,
Accepted 15th September 2014
DOI: 10.1039/c4gc01638f
www.rsc.org/greenchem
Carboxylic acids, especially unsaturated ones such as
acrylic acid, are important starting materials/intermediates in
1. Introduction
The design of efficient methods to conveniently form carbon– organic synthesis and are popular skeletons in a wide number
carbon bonds is exciting and challenging.1 Recently, because of natural products, agrochemicals, or pharmaceutically rele-
of the advantages of being nontoxic, renewable, abundant and vant compounds such as Lipitor®, Blopress, Prandin®,
economical, carbon dioxide (CO2) is considered more and Vancomycin, and so on.6 Traditional synthesis of carboxylic
more to be an ideal C1 synthon for production of value-added acids commonly employs strong oxidants or toxic cyanides
chemicals in both academic and pharmaceutical laboratories.2 via cumbersome operations. In this context, convenient
However, compared with traditional C1 sources, (e.g., carbon approaches to preparing carboxylic acid derivatives with CO2
monoxide and phosgene), CO2 is in its highest oxidation state as a carboxylative reagent under mild conditions have attracted
and the molecule is thermodynamically stable and kinetically more and more attention. Indeed, a renaissance has recently
inert. In this respect, high-energy active reagents, low-energy been witnessed in the carboxylation of organotin,7 organo-
targets, powerful catalysts, extra energy or high CO2 pressure zinc,8 and organoboron9 reagents with CO2. However, the
are usually required for successful CO2 incorporation.3 In par- benefits associated with CO2 are more than offset by unavoid-
ticular, utilization of CO2 under mild conditions will inevitably ably using such expensive organometallic reagents. Therefore,
rely on its activation. For example, transition metal mediated using the insertion of CO2 into active C–H bonds to give the
CO2 fixation and conversion has been successfully applied to corresponding carboxylic acids has been used.
the synthesis of natural products and biologically active mole-
Since Inoue et al. reported the first carboxylation of term-
cules. Although significant achievements have been accom- inal alkynes and CO2,10 remarkable progress has been made in
plished, chemical conversion of CO2 at atmospheric pressure the field of the Ag(I)/Cu(I)-catalyzed carboxylation of terminal
and room temperature remains a great challenge.4 In this alkynes and CO2.2e,11 Silver salts have been widely employed as
context, bifunctional catalysts able to simultaneously activate CuC triple-bond activators in organic synthesis,12 and several
both CO2 and the substrate are promising candidates for important studies about the silver-catalyzed carboxylation of
highly efficient CO2 chemical conversion under milder terminal alkynes with CO2 have been reported. The AgI cata-
conditions.5
lytic system was developed by Lu et al. with a relatively high
CO2 pressure and the catalytically active species was investigated
by density functional theory (DFT) studies.13 Gooßen et al.
found that the AgBF4-catalyzed carboxylation reactions could
be conducted under atmospheric pressure of CO2 in dimethyl
sulfoxide (DMSO) at 50 °C.14 With the poly-NHC-Ag (NHC,
N-heterocyclic carbene) catalytic system developed by Zhang
et al., the carboxylation of terminal alkynes under ambient
State Key Laboratory and Institute of Elemento-Organic Chemistry, Collaborative
Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University,
Tianjin, 300071, People’s Republic of China. E-mail: heln@nankai.edu.cn;
Fax: (+86) 22-23503878; Tel: (+86) 22-23503878
†Electronic supplementary information (ESI) available: Characterization pro-
ducts and copies of the NMR spectra. See DOI: 10.1039/c4gc01638f
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Scheme 1 Metal-catalyzed carboxylation of terminal alkynes with CO2.
CO2 pressure and room temperature was achieved.15 Unfortu-
nately, rigorous reaction conditions, such as the extra addition
of ligands or high CO2 pressure, are commonly indispensable
to the previously mentioned catalytic reactions.
a
Table 1 Ag(I)-promoted carboxylation of 1a with CO2
In this paper, we would like to report on the use of a simple
bifunctional silver tungstate (Ag2WO4) catalyst for the direct
carboxylation of terminal alkynes without utilization of any
ligand or additive. The synergistic effect of the silver cation
being able to activate the C–C triple bond and the tungstate
anion able to activate the CO2 molecule allows the reaction to
perform smoothly at room temperature and under the atmos-
pheric pressure of CO2 without the extra addition of ligand, as
shown in Scheme 1.
Entry
Catalyst (mol%)
Additive (mol%)
Yieldb (%)
1
2
3
4
5
6
7
8
Ag2WO4 (5)
Ag2WO4 (2.5)
Ag2WO4 (1)
Ag2WO4 (2.5)
Ag2WO4 (2.5)
AgCl (5)
AgBr (5)
AgI (5)
AgOAc (5)
Ag2CO3 (2.5)
AgOTf (5)
—
—
—
Ph3P (3)
DABCO (3)
—
—
—
—
—
—
—
—
—
—
—
71
76
46
46
66
45
43
52
26
27
42
22
26
4
9
10
11
12
13
14
15
16
17
AgBF4 (5)
2. Results and discussion
AgO2CCF3 (5)
Ag2O (2.5)
AgNO3 (5)
Na2WO4 (2.5)
AgNO3 (5)
54
<1
72
In a prototype experiment designed to test the validity of the
concept outlined previously (Scheme 1), 5 mol% of Ag2WO4
was used as a catalyst for the carboxylation of phenylacetylene
1a in the presence of n-butyl iodide and caesium carbonate
(Cs2CO3), with atmospheric pressure of CO2 in dimethylform-
amide (DMF) at room temperature. The desired product, butyl
3-phenylpropiolate 2a was obtained at a 71% yield as a color-
less oil (entry 1, Table 1). As shown in Table S1 (see ESI†), it
was found that CO2, Cs2CO3 and silver(I) catalyst are prerequi-
sites for performing the carboxylation. In addition, 2.5 mol%
Ag2WO4 was found to be enough for the reaction to run
smoothly (entries 1–3). Furthermore, the presence of the
ligand had a negative effect on the catalyst activity. For
example, the yield decreased to 46% and 66% in the presence
of triphenyl phosphine (Ph3P) and 1,4-diazabicyclo[2.2.2]-
octane (DABCO), respectively (entries 4 and 5). A high catalyst
loading and the addition of extra ligand resulted in a lower
yield, which might be tentatively be ascribed to the decarboxy-
lative activity of the Ag(I) salt.16
Na2WO4 (2.5)
a Reaction conditions: phenylacetylene (0.0511 g, 0.5 mmol), catalyst
(molar percentage to 1a), Cs2CO3 (0.1955 g, 0.6 mmol), nBuI (0.1104 g,
0.6 mmol), DMF (3 mL), CO2 (99.999%, balloon), room temperature,
12 h. b Determined by gas chromatography (GC) with biphenyl as the
internal standard.
54% to 72%. However, Na2WO4 itself showed no catalytic
activity under the reaction conditions (entries 15–17). This
implied that the tungstate anion could efficiently activate CO2.
As a result, Ag2WO4 was determined to be the most active
silver catalyst for the carboxylation of terminal alkynes with
CO2.
Furthermore, the presence of a base could strongly affect
the carboxylation (entries 1–4, Table 2). Similarly to previous
research,17 Cs2CO3 could play an essential role in the carboxy-
lation reaction. Other inorganic bases, such as K2CO3, KOH,
CsF, t-BuOLi, CsOAc, and NaNH2 (Table S1†), gave unsatisfac-
tory yields under identical conditions. In addition, several
organic bases were also examined. Among them (Table S1†),
only 1,5,7-triazabicyclo[4.4.0]dec-1-ene (TBD) was effective
Subsequently, several silver salts were investigated. Most of
the tested silver compounds exhibited poor to moderate
activity without any additive at room temperature (entries
6–14). Interestingly, when Na2WO4 was added to the AgNO3-
catalyzed system, the yield of 2a dramatically improved from
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Table 2 Optimization of the reaction conditionsa
Further prolonging the reaction time to 24 h gave 2a in quanti-
tative yield (entries 1, 16, 17).
With the optimized reaction conditions to hand, we evalu-
ated the scope of the substrate in this reaction. All the pro-
ducts were characterized by conventional spectroscopic
methods and by comparison with the data available in the lit-
erature.17,19 The reaction conditions were broadly adapted to a
series of terminal alkynes 1a–1n and the corresponding alkyl
2-alkynoates 2a–2n were obtained in good to excellent yields,
as summarized in Table 3. Both electron-donating and elec-
tron-withdrawing group substituted phenylacetylenes were suc-
cessfully carboxylated with this method (entries 2–9). For 1,4-
diethynylbenzene, biscarboxylative product 2ja was formed as
the major product with 64% yield and 24% yield of the mono-
carboxylative product 2jb, which was also detected (entry 10).
Aliphatic alkynes (entry 11) and heteroaromatic acetylenes
(entries 12–15) were also found to be suitable substrates. In
agreement with the DFT calculation, 3-ethynylpyridine showed
lower activity when compared with 2-ethynylpyridine (entry 12
vs. 13), indicating that the acidity of the alkyne proton strongly
impacts on the activity.20
Entry Base (eq.)
nBuI (eq.) Solvent
DMF
Time (h) Yieldb (%)
1
2
3
4
Cs2CO3 (1.2) 1.2
K2CO3 (1.2) 1.2
tBuOLi (1.2) 1.2
TBD (1.2) 1.2
12
12
12
12
12
12
12
12
76
5
2
DMF
DMF
DMF
PC
DMC
DEC
THF
21
<1
<1
<1
<1
<1
<1
25
81
82
76
78
91
>99
5
6
7
8
Cs2CO3 (1.2) 1.2
Cs2CO3 (1.2) 1.2
Cs2CO3 (1.2) 1.2
Cs2CO3 (1.2) 1.2
Cs2CO3 (1.2) 1.2
Cs2CO3 (1.2) 1.2
Cs2CO3 (1.2) 1.2
Cs2CO3 (1.2) 1.2
Cs2CO3 (1.2) 1.2
Cs2CO3 (1.2) 1.5
Cs2CO3 (1.2) 2.0
Cs2CO3 (1.2) 1.2
Cs2CO3 (1.2) 1.2
9
1,4-Dioxane 12
10
11
12
13
14
15
16
17
DMSO
MeCN
DMF
DMF
DMF
DMF
DMF
DMF
12
12
12
12
12
12
18
24
a Unless otherwise stated, the reactions were carried out with 0.5 mmol
1a in 3 mL solvent in the presence of base and nBuI, at room
temperature. b GC yield with biphenyl as the internal standard.
The reactivity of the alkylating reagent was also examined.
Bromoalkanes such as n-BuBr and EtBr gave 2a in excellent
yields (entries 15 and 16). Even n-BuCl with poor activity21
also worked well in this protocol to give 2a with a 59% yield
(entry 17).
To explore the reaction mechanism, the interactions of
2−
WO4 with CO2 were examined by NMR spectroscopy. In the
13C-NMR spectrum, a new signal appeared at δ = 157.8 ppm
when CO2 was absorbed by a suspension of deuterated DMSO
(DMSO-d6) and Ag2WO4 (Fig. 2), indicating that the interaction
of the tungstate anion with CO2 {Ag+ [WO4]2−-CO2} leads to the
2
activation of the CO2 molecule.
A catalytic cycle for the single-component bifunctional
carboxylation catalytic system is proposed, as delineated
in Scheme 2. On the basis of the coordination of silver with
a C–C triple bond, phenylacetylene 1a is converted to silver
acetylide II with the aid of a base, and CO2 is simultaneously
activated by tungstate to form WO42−-CO2 adduct III. Sub-
sequently, insertion of III into the sp-hybridized Ag–carbon
bond forms the silver phenylpropiolate IV with release of
WO42−. Finally, IV is esterified in situ by using n-BuI to
produce the ester 2a.
The effectiveness of the Ag2WO4-catalyzed carboxylation of
terminal alkynes and CO2 provides a convenient approach for
the synthesis of carboxylic acid derivatives and various useful
chemicals under the very mild reaction conditions. For
example, β-enaminoesters, widely used in organic syntheses,
were traditionally prepared from β-ketoesters.22 Here, we have
successfully prepared such a β-enaminoester through the carb-
oxylation and hydroamination sequence from a terminal
alkyne in one pot as demonstrated in Scheme 3. The Ag(I)-cata-
lyzed carboxylation of 1a with CO2 gave 2-alkynoate 2ab, which
further reacted with benzylamine, resulting in the formation
of ethyl (Z)-3-(benzylamino)-3-phenylacrylate 3a in 80% iso-
lated yield. The configuration of the enamino double bond
Fig. 1 Proton-nuclear magnetic resonance
(
13C-NMR) spectrum of
13
C
-labeled product 2ac.
carbonyl
(entry 4), suggesting that the reaction goes through the
TBD-CO2 intermediate pathway.18
To prove the origin of the carboxylate, an isotope labeling
experiment was performed. The carboxylation of 1a was con-
ducted using labeled 13CO2, and the 13C-labeled product 2ac
was obtained as shown in Fig. 1. This result clearly de-
monstrated that CO2 was the carboxyl source of this
transformation.
Type of solvent is another important influencing factor for
this transformation. Organic carbonates – e.g., propylene car-
bonate (PC), dimethyl carbonate (DMC), or diethyl carbonate
(DEC) – were found to be inefficient (entries 5–7, Table 2).17
Other conventional solvents did not work at all (entries 8–10)
or gave poor yield (entry 11) compared with DMF (entry 1).
Therefore, DMF was chosen as the reaction medium for
further investigations. On the other hand, an excess amount of
Cs2CO3 and n-BuI hardly affected the reaction (entries 12–15).
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Table 3 Ag2WO4-catalyzed carboxylation of various terminal alkynesa
Entry
RCuCH
R1X
Product
Yieldb (%)
1
2
nBuI
nBuI
96
80
3
4
5
nBuI
nBuI
nBuI
82
81
84
6
nBuI
nBuI
nBuI
nBuI
nBuI
80
75
68
91
7
8
9
10
64
24
11
12
nBuI
nBuI
60
84
13
14
nBuI
nBuI
70
90
15
16
17
nBuBr
EtBr
92
92
59
nBuCl
a Reaction conditions: Substrate (1.0 mmol), Ag2WO4 (0.0116 g, 0.025 mmol), Cs2CO3 (0.3910 g, 1.2 mmol; 2.4 mmol for entry 10), alkyl halide
(1.2 mmol; 2.4 mmol for entry 10), DMF (3 mL; 5 mL for entry 10), CO2 (99.999%, balloon), 25 °C, 24 h. b Isolated yield.
was identified by NMR spectroscopy and compared with those for the direct carboxylation of terminal alkynes with atmos-
previously reported.23
pheric CO2 at room temperature, as confirmed using 13CO2.
This protocol also features a halogen-free catalyst and a simple
process with low energy consumption and avoidance of com-
plicated preparation of the catalyst. The origin of the carboxy-
late was corroborated by the isotope labeling experiment. Dual
3. Conclusions
In summary, we have developed a convenient ligand-free Ag(I)- activation of the alkyne substrate by silver(I) and CO2 by tung-
based bifunctional catalytic system, i.e., silver tungstate, which state could account for such carboxylation working under
can simultaneously activate both CO2 and a C–C triple bond extremely mild conditions. Furthermore, the utility of this
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4. Experimental details
The general procedure for the carboxylation of terminal
alkynes is described here.
The terminal alkyne (1.0 mmol), Ag2WO4 (0.0116 g,
0.025 mmol), Cs2CO3 (0.3910 g, 1.2 mmol), nBuI (0.2208 g,
1.2 mmol) and DMF (3 mL) were added in a 10 mL Schlenk
flask. The flask was capped and sealed. Then a gas-exchanging
process was conducted using the “freeze–pump–thaw”
method. The reaction mixture was stirred at 25 °C for 24 h
under an atmosphere of CO2 (99.999%, balloon). After the
reaction, water was added to the mixture, which was extracted
with acetic ether five times. The combined organic layer was
washed with saturated aqueous NaCl and then dried over
MgSO4. Then the solvent was removed under reduced pressure
to get a crude product and the residue was purified by column
chromatography (silica gel, petroleum ether–EtOAc) to give the
desired product. The products were further identified by NMR
and mass spectroscopy (see the ESI†), which are consistent
with those reported in the literature and were in good agree-
Scheme 2 The dual activation pathway for the present Ag2WO4-cata-
lyzed carboxylation of terminal alkynes with CO2.
1
ment with the assigned structures. 2a. Colorless oil; H-NMR
(400 MHz, deuterated chloroform (CDCl3)) δ (ppm): 7.58–7.57
(d, J = 4.0 Hz, 2H), 7.45–7.34 (m, 3H), 4.23 (t, J = 6.7 Hz, 2H),
1.73–1.66 (m, 2H), 1.48–1.38 (m, 2H), 0.95 (t, J = 7.4 Hz, 3H);
13C-NMR (100 MHz, CDCl3) δ (ppm): 154.1, 132.9, 130.5, 128.5,
119.6, 86.0, 80.7, 65.9, 30.4, 19.0, 13.6; EI-MS, m/z (%): 129.15
(100), 201.20 (11.72) [M+].
Acknowledgements
We are grateful to the National Natural Sciences Foundation
of China (no. 21172125, 21121002, 21472103), Specialized
Research Fund for the Doctoral Program of Higher Education
(20130031110013), MOE Innovation Team (IRT13022) of China
for financial support.
Fig. 2 13C-NMR study: (a) DMSO-d6 under atmospheric CO2 (1 atm,
25 °C); (b) Ag2WO4 in DMSO-d6 under atmospheric CO2 (1 atm, 25 °C).
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