Anhydrous Conditions Enable the Catalyst-Free Carboxylation of Aromatic Alkynes with CO2 under Mild Conditions

Article abstract of DOI:10.1002/hlca.201900258

The direct carboxylation of aromatic alkynes with CO₂, a cheap and widely available C1 source, is the most attractive method for the synthesis of carboxylic acids. Here we show that direct carboxylation of terminal alkynes can be simply performed in near-quantitative yield in four hours with anhydrous Cs₂CO₃ under mild conditions without need of a metal catalyst.

Full text of DOI:10.1002/hlca.201900258

HELVETICA
chimica acta
Accepted Article
Title: Anhydrous conditions enable the catalyst-free carboxylation of
aromatic alkynes with CO2 under mild conditions.
Authors: Marinella Mazzanti, Davide Toniolo, Felix D. Bobbink, and
Paul J. Dyson
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To be cited as: Helv. Chim. Acta 10.1002/hlca.201900258
Link to VoR: http://dx.doi.org/10.1002/hlca.201900258
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10.1002/hlca.201900258
Helvetica Chimica Acta
HELVETICA Accepted Article
Anhydrous conditions enable the catalyst-free carboxylation of aromatic
alkynes with CO2 under mild conditions.
Davide Toniolo,a Felix D. Bobbink, a Paul J. Dysona and Marinella Mazzanti*,a
a Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL) 1015 Lausanne (Switzerland). E-mail :
marinella.mazzanti@epfl.ch.
Abstract: The direct carboxylation of aromatic alkynes with CO2, a cheap and widely available C1 source, is the most attractive method for the synthesis of
carboxylic acids. Here we show that direct carboxylation of terminal alkynes can be simply performed in near-quantitative yield in 4 hours with anhydrous
Cs2CO3 under mild conditions without need of a metal catalyst
Keywords: alkynes • carbon dioxide fixation • propiolic acids • C-H activation • cesium carbonate
Introduction
Results and Discussion
Phenylacetylene was taken a model substrate and since the reaction is
reversible, even under mild conditions, the product is converted in situ into
the corresponding ester by reaction with 1-iodobutane (Table 1).[15],[23]
Initially, the reaction was conducted in DMSO at 40 °C for 24 h under 1 atm.
CO2 in presence of two equivalents of Cs2CO3. A non-negligible yield of 33%
was obtained (Table 1, entry 1) in line with previous results showing that the
reaction proceeds to some extent under very mild conditions even in the
absence of a catalyst.[24] We found that, on increasing the temperature to
60 °C and then to 80 °C the yields increases up to 99 % with an isolated yield
of 91% (Table 1, entries 2, 3, Figure S1). The increase in yield with
temperature is attributed to the increased solubility of the base in DMSO
with temperature, and to faster deprotonation kinetics.[25] To demonstrate
the ability of the base to deprotonate (at least partially) the acidic C-H bond,
the reaction between phenylacetylene and Cs2CO3 was monitored by 1H
NMR spectroscopy (Figure S2), at room temperature. The signal assigned
The utilization of CO2 as a C1 source is attractive due to its low toxicity,
low price and growing abundance in the atmosphere.[1],[2],[3],[4] Recent years
have seen the emergence of many reactions building complexity with
CO2;[5],[6] examples include the synthesis of cyclic carbonates[7],[8],[9]
methylated amines[10]
,
formylated amines[11]
,
propiolic carboxylic
acids,[12],[13],[14] esters[15],[16],[17] as well as numerous heterocycles.[18] Due to
the high thermodynamic stability of CO2, it is generally considered that
robust catalytic tools are required for its efficient transformations. Of the
reactions mentioned above, the carboxylation of alkynes is interesting due
to the importance of propiolic acid derivatives in medicinal chemistry.[15]
Many studies have been carried out to optimize the conditions for the
carboxylation of terminal alkynes using a broad range of homogeneous and
heterogeneous catalysts based on transition or rare earth metals. (Scheme
1).[15],[19],[20],[21] In all the reported metal catalysed reactions, a base is also
required to promote deprotonation of the alkyne, and the utilization of
Cs2CO3 in DMF or DMSO appears to be a privileged system. Base promoted
direct carboxylation of terminal alkynes by Cs2CO3 was reported to occur in
the absence of catalyst only when using significantly higher temperatures
and CO2 pressure (120 ° C and 2.5 bar of CO2) compared to the metal-
catalysed reaction.[22] Here, we show that under rigorous anhydrous
conditions dry Cs2CO3 allows for the convenient base-promoted, catalyst-
free carboxylation of aromatic alkynes under mild conditions and in
excellent yields.
to the aliphatic proton disappeared after
1 h at 80 °C, confirming
deprotonation of the alkyne. In presence of water (1 :2 with respect to
Cs2CO3) only partial deprotonation is observed (Figure S3). Indirectly, these
results suggest that the rate determining step of the C-H carboxylation
reaction is the deprotonation of the alkyne. The reaction of the
deprotonated alkyne with 13CO2 resulted in the instantaneous formation of
the 3-phenylpropiolate salt, detected by 13C NMR spectroscopy
(appearance of a signal at 155.8 ppm assigned to the carboxylic group,
Figure S5).
The reaction does not proceed when it is conducted in DMF or THF (Table
1 entries 4 and 5), where Cs2CO3 is markedly less soluble, indicating that the
solubility of Cs2CO3 is important for good activity, presumably due to the
deprotonation rate that is accelerated when the base is soluble.[26],[27] Low
yields are also obtained with sub-stoichiometric amounts of anhydrous
Cs2CO3 (Table 1, entry 6).
Scheme 1 Carboxylation of phenylacetylene using Cs2CO3 as the base and CO2 as the
carboxylation reagent.
1
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10.1002/hlca.201900258
Helvetica Chimica Acta
HELVETICA Accepted Article
Remarkably, at 80 °C the time required for
a near-quantitative
[a]Yields determined by GC using n-decane as internal standard. [b]Isolated
product. [c]0.2 eq. of Cs2CO3. [d]Without CO2.
conversion of the substrate is much shorter compared to those previously
reported in literature (14-24 h), even if catalysts are used and/or harsher
conditions are applied.[28],[26],[27],[22] Notably, high yields are only reached
after 12 h when the AgPF6 catalyst was used in DMF with stoichiometric
amounts of Cs2CO3 and 2 atm. CO2.[20] The time-dependent yields measured
at 80 °C show that the reaction is essentially first order with respect to the
substrate with an estimated kinetic constant of 2 h-1, and reaches the
maximum yield after approximately 3 h (Figure 2, Table 1, entries 7-9). To
ensure that the carboxylic group is derived from CO2 and not Cs2CO3, an
experiment was conducted in absence of CO2 and no reaction was observed
(Table 1, entry 10).
Our results under catalyst-free conditions, show that the low to modest
yields reported for the catalyst-free reaction in DMSO can be easily
optimized by increasing the temperature to 80°C to afford the product in
near-quantitative yield. Since the reproducibility of carboxylation reaction
promoted by weak bases as Cs2CO3 is likely to be affected by the presence
of water, even under forcing conditions (high temperature,
overpressure),[29] we investigated the effect of stoichiometric amounts of
water in the used mild conditions.
Addition of carefully controlled amounts of water (0.1, 0.2 and 1 eq. with
respect to Cs2CO3) to the system leads to a significant reduction in yield
(Table 2, entries 1-3, 80, 43 and <1 %, respectively). Hence, to ensure
reproducible high yields under catalyst-free conditions, water must be
excluded from the reaction mixture, and anhydrous solvents and reagents
must be used. The detrimental effect of water is not unique to metal free
carboxylation reactions. Notably, to emphasize the general detrimental
effect of water on Cs2CO3 promoted reactions we performed the metal
catalysed reaction, using the performant AgPF6 under the reported
conditions,[20] but in the presence of H2O (1 eq.). Under these conditions, 4
% product was obtained (Table 2, entry 4).
These results indicate that rigorously anhydrous conditions (thoroughly
dried solvents, anhydrous Cs2CO3 and reagents, rigorous Schlenk
techniques) should be used in base promoted carboxylation reactions.
Figure 2. Time-dependent yields for the catalyst-free carboxylation of
phenylacetylene. Reaction condition: PhCCH (28 mg, 0.28 mmol), Cs2CO3 (183 mg,
0.56 mmol, 2 eq.), CO2 (1 atm.), DMSO (1 mL), 80 °C.
Table 2. Effect of water on the catalysed and catalyst-free reaction. Reaction
conditions: phenylacetylene (0.28 mmol), CO2 (1 atm), DMSO (1 mL).
Product trapped with butyl iodide (1.1 eq.). Yields determined by GC using
n-decane as internal standard.
Table 1. Optimization of reaction conditions. Reaction conditions:
phenylacetylene (0.28 mmol), CO2 (1 atm), Cs2CO3 (183 mg, 0.56 mmol, 2
eq.), DMSO (1 mL). Product trapped with butyl iodide (1.1 eq.). [a] Yields
determined by GC using n-decane as internal standard. [b] Isolated product.
[c] 0.2 eq. of Cs2CO3. [d] Without CO2.
Entry
Notes
T [°C]
Time [h]
Yield [%]
1
2
3
4
0.1 eq H2O
0.2 eq H2O
1 eq H2O
80
80
80
80
4
4
4
4
80
43
< 1
Entry
Solvent
T [°C]
Time [h]
Yield [%][a]
1
2
DMSO
DMSO
DMSO
DMF
40
60
80
80
80
80
80
80
80
40
24
4
33
AgPF6 (4 %), 1
eq H2O
< 1
30
3
4
> 99, 91[b]
4
4
4
The catalytic activity of transition metal ions based on copper or silver in
DMF (where Cs2CO3 is not able to deprotonate terminal alkynes) reported
by other authors can be rationalized by the ability of those metals to
coordinate the substrate, weakening the C-H bond, and accelerating the
deprotonation of the alkyne, as proposed in computational studies.[30],[31]
However, our results strongly suggest that the carboxylation of aromatic
terminal alkynes in DMSO essentially proceeds via an uncatalysed acid-
base reaction and the addition of catalyst is not crucial.
5
THF
4
< 1
6
6[c]
7
DMSO
DMSO
DMSO
DMSO
DMSO
4
2
95
83
66
< 1
8
1
9
0.5
24
10[d]
2
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10.1002/hlca.201900258
Helvetica Chimica Acta
HELVETICA Accepted Article
The catalyst-free carboxylation was explored for various phenyl
acetylene derivatives under the reaction conditions optimized for phenyl
acetylene. Quantitative yields were obtained in four hours for aromatic
alkynes with electron withdrawing groups in the aromatic ring and for both
p-methyl phenyl acetylene and m-methoxy phenyl acetylene. (Table 3,
Entry 1- 7), while good yields were instead achieved for less acid alkynes
(Table 3, Entry 7 - 10).
Table 3. Carboxylation of phenyl acetylene derivatives. Reaction conditions: alkyne
(0.28 mmol), CO2 (1 atm), DMSO (1 mL). Product trapped with butyl iodide (1.1 eq.).
Yields determined by GC using n-decane as internal standard. [a]Isolated yield.
Entry
Alkyne
Time [h]
Yield [%]
> 99, 92[a]
1
4
Scheme 2 Proposed steps for the uncatalysed carboxylation of PhCCH using CO2 as in
the absence (top) and presence (bottom) of water.
2
3
4
4
> 99
> 99
Conclusions
In conclusion, we demonstrated that carboxylation of phenylacetylene
can be promoted in quantitative yield under anhydrous conditions using
Cs2CO3 as the base in DMSO under mild conditions and in short reaction
times (80 °C, 1 atm. CO2, 4 h). When moisture is present in the reaction
mixture, even reported catalysts such as AgPF6 are unable to catalyse the
reaction. Hence, the C-H carboxylation of alkynes proceeds mainly via an
acid-base mechanism. We believe that these results can be generalized to
many base-promoted CO2 reactions.
> 99, 95[a]
> 99
4
5
4
4
Experimental Section
6
4
> 99
General methods.
Loading of reagents for carboxylation was carried out under an inert argon
atmosphere using an MBraun glovebox equipped with a purifier unit or
using standard Schlenk-type techniques. Glassware was dried overnight at
150 °C prior to use. Unless otherwise noted, reagents were acquired from
commercial sources and used without further purification. The solvents and
the reagents were purchased from Sigma Aldrich or Eurisotop (deuterated
solvents) in their anhydrous form, conditioned under argon and dried with
molecular sieves (4 Å) for two weeks. Water content in DMSO was tested
using quantitative 1H NMR spectroscopy (see supporting information).
Cs2CO3 was purchased from Sigma Aldrich and dried under vacuum (10-3
bar) for 5 days at 60 °C prior to use. Shorter drying times result in the
deleterious presence of water and reduced catalytic performance ( Table S1,
see supporting information). 1H and 13C NMR experiments were carried out
using NMR tubes adapted with J. Young valves. The NMR spectra were
recorded on Bruker 400 or 800 MHz. 1H chemical shifts are reported in ppm
and were measured relative to residual solvent peaks, which were assigned
relative to an external TMS standard set at 0.00 ppm. Gas chromatography-
mass spectrometry (GC-MS) of liquid samples was performed on an Agilent
7890B Gas Chromatograph together with Agilent 7000C MS triple quad
7
4
4
> 99
88
8
9
4
4
75
10
60
The mechanism of the base-promoted transformation is therefore
postulated to require the following steps: deprotonation of the alkyne by
the carbonate base, insertion of CO2, nucleophilic attack of the carboxylate
anion onto the alkyl halide in the final step. Figure 3 summarizes the key
mechanistic steps and postulates the main deactivation pathway due to
water, i.e. acidification of the reaction mixture inhibiting the deprotonation
of the alkyne.
3
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10.1002/hlca.201900258
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detector using He as the carrier gas. The yields were determined by the
internal standard method using n-decane as the internal standard.
4.17-4.20 (t 2H), 1.62-1.69 (m, 2H), 1.37-1.47 (m, 2H), 0.93-0.97 (t, 3H)
+
(Figure S1). HRMS (APPI/LTQ-Orbitrap) m/z calc. for C13H15O2 [M + H]+:
203.1067; Found 203.1064.
Typical procedure for the base-promoted carboxylation reaction.
The synthesis of butyl 3-(4-fluorophenyl)propiolate and 3-(4-
methoxyphenyl)propiolate was performed using the same conditions
except for the final extraction of the ester from water which was performed
using CH2Cl2 instead of ether (three times with 4 mL).
To ensure reproducibility with respect to temperature and time, four
reactions were conducted simultaneously.[32] Four 25 mL round bottom
flasks were each charged with 183 mg (0.56 mmol, 2 eq) of anhydrous
Cs2CO3, 30 μL of phenyl acetylene (0.28 mmol, 1 eq), and 1 mL of DMSO.
The individual reaction mixtures were stirred to afford pale yellow
suspensions. The mixtures were frozen and then 1 atm CO2 was introduced
to each flask after removing the argon atmosphere under mild vacuum.
Subsequently, the mixtures were quickly warmed to 80 °C in a pre-heated
oil bath under continuous stirring at 400 rpm for 4 hours. Than each mixture
was cooled to 40 °C and 35 μL of 1-iodobutane (0.3 mmol, 1.1 eq) were
added under air. After 1 h, 53 μL of n-decane (0.28 mmol, 1 eq) were added
as internal standard together with 4 ml of AcOEt. The suspension was
filtered to remove the cesium carbonate and hydrogen carbonate formed
during the reaction and the filtrate was analyzed by GC-MS/FID.
1H NMR spectrum of butyl 3-(4-fluorophenyl)propiolate (CD3CN, 400
MHz, 298 K): δ = 7.64-7.68 (m, 2H), 7.17-7.22 (m, 2H), 4.20-4.23 (t, 2H), 1.63-
1.70 (m, 2H), 1.36-1.45 (m, 2H), 0.93-0.96 (m, 3H). HRMS (ESI/QTOF) m/z
calc. for C13H14FO2+ [M + H]+: 221.0978; Found 221.0977
1H NMR spectrum of butyl 3-(4-methoxyphenyl)propiolate (CD3CN, 400
MHz, 298 K): δ = 7.33-7.37 (t, 1H), 7.18-7.20 (d, 1H, J = 8 Hz), 7.15 (s, 1H), 7.08-
7.10 (m, 1H), 4.20-4.23 (t, 2H), 3.80 (s, 3H) 1.63-1.70 (m, 2H), 1.36-1.46 (m,
2H), 0.93-0.97 (t, 3H). HRMS (ESI/QTOF) m/z calc. for C14H16O3Ag+ [M + Ag]+:
339.0150; Found 339.0163 (addition of Ag+ is required to observe the peack).
For the reaction conduced with other substrates the same procedure
described above was used, except that an equimolar amount of other
acetylene derivatives was employed.
Supplementary Material
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/MS-number.
For the reactions conduced with different solvents, temperature,
amount of base or time, the same procedure described above was used,
except for that other solvents, other temperature, other amount of base or
other times were employed.
Acknowledgements
This work was supported by Ecole Polytechnique Fédéral de Lausanne
(EPFL), from the Swiss National Science Foundation grant number 200021_
178793 and by Swiss Competence Center for Heat and Electricity Storage
(PJD).
For the reaction conduced in presence of AgPF6 the same procedure
described above was used, except that AgPF6 was added to the reaction
flask under argon atmosphere before the addition of CO2.
Author Contribution Statement. DT, FDB, PD, and MM planned the
research and wrote the manuscript. MM supervised the research in all
aspects. DT performed and analyzed all the experiments, F.D.B carried out
and analyzed the GC -MS experiments;
For the reaction conduced without CO2 the same procedure described
above was used, except that CO2 was not added to the flasks.
For the reactions conduced in presence of water the same procedure
described above was used, except that H2O was added to the flasks trough
a septum outside of the glovebox.
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HELVETICA Accepted Article
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5
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HELVETICA Accepted Article
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