The direct carboxylation of terminal alkynes with carbon dioxide

Article abstract of DOI:10.1039/c0gc00819b

A simple and straight-forward method has been developed for direct carboxylation of terminal alkynes using CO₂ as the C1 carbon feedstock. The direct C-H bond functionalization is achieved with Cs ₂CO₃ as the base and in the absence of transition metal catalyst. Various propiolic acids were synthesized in good to excellent yields with a wide substrate scope and a good functional groups tolerance. This is a more atom- and step-economic protocol with great potential in practical application.

Full text of DOI:10.1039/c0gc00819b

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PAPER
The direct carboxylation of terminal alkynes with carbon dioxide
Yu Dingyi and Zhang Yugen*
Received 16th November 2010, Accepted 28th February 2011
DOI: 10.1039/c0gc00819b
A simple and straight-forward method has been developed for direct carboxylation of terminal
alkynes using CO2 as the C1 carbon feedstock. The direct C–H bond functionalization is achieved
with Cs2CO3 as the base and in the absence of transition metal catalyst. Various propiolic acids
were synthesized in good to excellent yields with a wide substrate scope and a good functional
groups tolerance. This is a more atom- and step-economic protocol with great potential in
practical application.
the well-known hydrolysis of bromide and relate derivatives,
Introduction
and the oxidation of preoxidized substrates, such as alcohols or
aldehydes.14 Despite the efficiency of these conventional proce-
dures, their major drawbacks include severe reaction conditions
and restrictions of organometallic reagent that dramatically
limits the synthesis of a wide scope of functionalized propiolic
acids. In addition, copper catalyzed coupling CO2 with terminal
alkyne and alkyl halide to form propiolic acid esters were also
reported.15 We have recently disclosed a copper-NHC catalyzed
CO2 directly coupling with C–H bond of terminal alkynes,
a system performed under ambient conditions and tolerant
to wide range of functional groups (Scheme 1A).11 In our
previous report, it was shown that strong base, like KOtBu,
can directly promote the carboxylation of terminal alkynes at
ambient conditions, but the reaction did not work on electron-
deficient aryl alkynes or alkyl alkynes.11 However, this result
evoked our interests on the investigation of bases on promoting
carboxylation of alkynes under various conditions (Scheme 1B).
By far, the most attractive carbon source is carbon dioxide as it
is abundant, renewable and cheap.1 As such, using CO2 as C1
carbon feedstock in organic synthesis has attracted increased
attention.2 Significant efforts have been conducted toward
converting alkynes,3 allenes,4 alkenes5 and allyl halide6 into
carboxylic acid and derivatives with CO2 via a transition metal
center. Due to the high thermodynamic and kinetic stability
of CO2, carbon nucleophiles involving direct CO2 coupling
reactions are mostly limited to metal activated hydrocarbons
or organometallic reagents.7 More interestingly, metal catalyzed
CO2 carboxylation of certain unsaturated hydrocarbons, such
as styrenes and aryl halides have also been developed when
certain organometallic reducing reagents were applied.8 Very
recently, several groups reported Au/Cu catalyzed CO2 direct
carboxylation of activated arenes,9 aromatic heterocycles10 and
terminal alkynes11 through C–H activation. These are the first
examples of carboxylation of carbon nucleophiles with CO2
in the absence of organometallic reagents. In contrast to the
recent developments of metal catalyzed CO2 coupling reactions,
progress on direct base promoted CO2 couplings are still very
limited.12 From the organic synthesis point of view, base pro-
moted reactions also provide a very important complimentary
methodology to transition metal catalyst systems. Herein, we
report a Cs2CO3 promoted direct C–C coupling between CO2
and terminal alkynes
The ubiquity of alkynyl carboxylic acids, as one of the most
important types of compounds in medicinal chemistry as well as
the tremendous utility as a synthon in organic synthesis, makes
them particularly attractive targets for pharmaceutical and fine-
chemicals synthesis.13 A plethora of well-established methods
for the preparation of alkynyl carboxylic acids includes CO2
insertion into the metal-carbon bond of organometallic reagents,
Scheme 1 Protocols for synthesis of substituted propiolic acids.
Results and discussion
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The
Nanos, Singapore, 138669. E-mail: ygzhang@ibn.a-star.edu.sg;
Fax: +65 64789081; Tel: +65 68247162
We initially chose 1-ethynylbenzene (1a) as a model compound
to test the direct carboxylation reaction with carbon dioxide. In
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Table 1 Effect of pressure and temperature on carboxylation of 1-
the presence of 1.2 mmol Cs2CO3, 1a (1 mmol) was treated with
CO2 (2.5 atm) in N,N-dimethylformamide (DMF) at 120 ◦C for
14 h, and subsequent acid hydrolysis afforded 3-phenylpropiolic
acid (1b) in an excellent yield. Encouraged by this result, the
reaction conditions were further investigated. It was found that
both the reaction temperature and CO2 pressure are very crucial
for this base promoted carboxylation reaction. The optimized
reaction temperature range is between 120 to 160 ◦C. In this
temperature range, the 1b yield could reach more than 93%
with 2.5 atm of CO2 (entries 11–13, Table 1). Under lower
a
ethynylbenzene with CO2
Entry Pressure (atm) T/◦C Reaction Time (hours) Yield of 1b (%)c
1b
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0 (no CO2)
1
1.5
2.5
4
120
120
120
120
120
120
25
14
14
14
14
14
14
14
14
14
14
14
14
14
14
14
0 (0)
20 (25)
60 (64)
95 (99)
96 (99)
96 (100)
0 (0)
◦
temperature (60 to 100 C), the reaction became slow and can
not be completed (entries 8–10, Table 1). A low product yield
(30%) was obtained at 80 ◦C even with over pressured CO2
atmosphere (7 atm) (entry 16, Table 1). There was no reaction
observed at room temperature with 2.5 atm CO2 and Cs2CO3
(entry 7, Table 1). However, as the reaction temperature was
raised to over 160 ◦C, the propiolic acid yield dramatically
dropped to less than 10% (entries 14–15, Table 1). This is
because the propiolic acid product 1b is not stable at higher
temperatures. It may decompose to terminal alkynes 1a through
de-carboxylation process when it is over heated.11,15b Under the
overheated condition, the yield of propiolic acid dropped and the
conversion also dropped in the same range. No other byproduct
was observed. A control experiment was conducted by using
propiolic acid as starting material under the same conditions
with CO2 (2.5 atm) in DMF at 180 ◦C for 14 h. A mixture
of propiolic acid and terminal alkyne (ratio is about 8 : 2) was
obtained. As the reaction temperature was fixed at 120 ◦C, it was
found that the CO2 pressure is linearly related to the reaction
rate. As shown in Fig. 1, the reaction was completed in 14 h
under 2.5 atm, 7 h under 4 atm and 3 h under 7 atm of CO2.
Kinetics studies of this reaction showed that the 1b was produced
linearly to reach the maximum yield in the time range related
to CO2 pressure (Fig. 1). When the CO2 pressure was less than
1.5 atm, the reaction became very slow (entries 2–6, Table 1).
Control reaction in the absence of CO2 showed no acid product,
suggesting that the COO moiety in the product did not originate
from Cs2CO3 (entry 1, Table 1).
7
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
7
60
80
5 (7)
20 (25)
40 (44)
95 (99)
93 (99)
10 (15)
10 (13)
30 (34)
100
140
160
180
200
80
a Unless otherwise noted, reaction conditions are as follows: 1-
ethynylbenzene (1.0 mmol), Cs2CO3 (1.2 mmol), CO2, DMF (5 mL),
14 h. b Reaction was under N2 in the absence of CO2. c Isolated yields.
Conversions are in parentheses.
best base for this direct carboxylation reaction of terminal
alkyne with CO2. Among those bases we screened, Na2CO3,
NaOCH3, NaOAc, NaOH, KOH and triethylamine (TEA)
did not give any acid product, while K2CO3, K3PO4 and 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) resulted in low conver-
sions (Table 2). Other caesium sources, such as CsF, CsOH and
CsF + Na2CO3, were inactive to this reaction. In addition, DMF
is th◦e best solvent for this reaction. Under the conditions of
120 C and 2.5 atm of CO2, reaction in DMF gave 95% yield
of 1b while reaction in dimethylsulfoxide (DMSO), CH3CN
and tetrahydrofuran (THF) gave 58%, 20% and 5% yields
respectively. This is because DMF is not only a good solvent
for Cs2CO3, but also a good solvent for CO2 (DMF is a weak
base).2f No product was observed as the reaction was carried out
in CH2Cl2 and toluene under the same conditions (Table 2).
With the optimized reaction conditions of 1.2 equivalent
Cs2CO3, 2.5 atm CO2 and 120 ◦C in DMF, the alkyne substrate
scope was screened (Table 3). For aromatic alkynes with or
without electron donating groups, the corresponding alkynyl
carboxylic acids were obtained in 87–96% yields (isolated) under
standard conditions. Good yields (80–87%) were also achieved
for less active aromatic alkynes with electron withdrawing group
and alkyl alkynes.11,16 For both alkyl- and aryl-substituted
alkynes, the transformations proceeded smoothly without any
side product formation. It must be noted that this Cs2CO3
promoted reaction system is not sensitive to a variety functional
groups, such as –OH, –CHO, CN, NO2 etc. It provides a
useful tool for synthesis of highly functionalized propiolic acids
directly from terminal alkynes and CO2 without transition metal
involvement. The proposed mechanism involved an uphill C–H
cleavage of terminal alkyne with base, followed by C–C bond
formation with CO2.12
Fig. 1 Kinetic study of direct carboxylation of 1a with CO2. Reaction
conditions: 1-ethynylbenzene (1.0 mmol), Cs2CO3 (1.2 mmol), CO2 (2.5
atm, 4 atm or 7 atm), DMF (5 mL), 120 ◦C. The reactions were quenched
at each time point by cooling down and adding water. Yields were of
isolated product.
Cs2CO3 is one of the most commonly used bases for many
coupling reactions.12 Here, Cs2CO3 also performed as the
In comparison to the Cu catalyzed system11 (Scheme 1), this
Cs2CO3 system has its own advantages. Although it requires
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Table 2 Effect of bases and solvents on carboxylation of 1-
Table 3 Direct carboxylation of terminal alkynes with CO2
a
ethynylbenzene with CO2
Entry
1
Alkynes
Reaction time (h)
14
Isolated yields (%)
Time
(hours)
Entry
Base
Solvent
Yield of 1b (%)b
95
96
87
90
92
93
1
2
3
4
5
6
7
8
Na2CO3
NaOCH3
NaOAc
K2CO3
K3PO4
Cs2CO3
NaOH
KOH
TEA
DBU
CsOH
CsF + Na2CO3
CsF
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
tBuOK
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
CH3CN
THF
14
14
14
14
14
14
14
14
14
14
14
14
14
14
14
14
14
14
14
0 (0)
0 (0)
0 (0)
16 (20)
26 (30)
95 (99)
0 (0)
0 (0)
0 (0)
2
3
4
5
6
14
14
16
16
14
9
10
11
12
13
14
15
16
17
18
19c
30 (33)
0 (0)
0 (0)
0 (0)
55 (58)
20 (24)
5 (8)
0 (0)
0 (0)
7
8
18
18
90
92
CH2Cl2
Toluene
DMF
96 (98)
9
24
24
24
20
80
85
83
87
a Unless otherwise noted, reaction conditions are as follows: 1-
ethynylbenzene (1.0 mmol), base (1.2 mmol), CO2 (2.5 atm), solvent
(5 mL), 120 ◦C. b Isolated yields. Conversion yield are in parentheses.
c Reaction conditions: 1-ethynylbenzene (1.0 mmol), base (1.2 mmol),
solvent (5 mL), normal pressure (1 atm) and room temperature (25 ◦C).
10
11
12
elevated temperature and pressurized CO2 conditions, the
Cs2CO3 system is not sensitive to minor moisture and oxygen in
the reaction system. In addition, the Cs2CO3 system is cleaner as
the reaction only delivers the desired acid product. It represents
an excellent complimentary method to Cu catalyst system for
the direct synthesis of propiolic acids with terminal alkynes and
CO2.
13
14
20
16
88
85
15
24
85a
Conclusions
In summary, a simple and straight-forward method for carboxy-
lation of terminal alkynes with CO2 was described here. Various
propiolic acids were synthesized in good to excellent yields with
a wide substrate scope and good functional groups tolerance.
The direct C–H bond functionalization is achieved with Cs2CO3
as the base and in the absence of transition metal catalyst. The
current protocol is more atom- and step-economic with great
potential in practical application.
Reaction conditions: alkynes (1.0 mmol), Cs2CO3 (1.2 mmol), CO2 (2.5
atm), DMF (5 mL), 120 ◦C. a 1,3-Phenylenedipropynoic acids.
reported in the following order: chemical shift in ppm (d)
(multiplicity were indicated by br (broadened), s (singlet), d
(doublet), t (triplet), q (quartet), m (multiplet), mc (centered
multiplet)); coupling constants (J, Hz); integration; assignment.
Experimental
General procedure for carboxylation of the terminal alkynes (1b
as example)
All solvents were purchased from Sigma–Aldrich (99.8%).
Alkynes were used without purification from commercial sup-
pliers, unless otherwise indicated. Chemical yields referred
to the pure isolated substances. Gas chromatography-mass
spectrometry (GC-MS) was performed with Shimadzu GC-
2010 coupled with GCMS-QP2010. H and 13C NMR spectra
were obtained using a Brucker AV-400 (400 MHz) spectrometer.
Chemical shifts were reported in ppm from tetramethylsilane
with the solvent resonance as the internal standard. Data were
1.0 mmol of terminal alkynes (1a, 102 mg) and Cs2CO3
(1.2 mmol) were added to the DMF (5 mL) in a pressure
reactor (25 mL). CO2 was then introduced into the reaction
mixture under stirring. The reaction mixture was stirred at
120 ◦C for 14 h under 2.5 atm. After completion of the reaction,
the reaction mixture was cooled to room temperature and
transferred to the potassium carbonate solution (2 N, 5 mL)
under stirring for 30 min. The mixture was washed with
1
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dichloromethane (3 ¥ 5 mL) and the aqueous layer was acidified
with concentrated HCl to pH = 1, then extracted with diethyl
ether (3 ¥ 5 mL). The combined organic layers were dried with
anhydrous Na2SO4, filtered and the solution was concentrated
in vacuum affording pure product 1b. Data for 1H and 13C
NMR of acids were all conducted in d6–DMSO and consistent
with the data in reported literatures.
4,4-Dimethylpent-2-ynoic acid (8b)24
1H NMR (400 MHz, DMSO): d = 13.69 (br, s, 1H, -COOH),
1.28 (s, 9H, CH3); 13C NMR (100 MHz, DMSO): d = 154.88
(-COOH), 95.20, 73.08, 30.15, 27.42. Ms (ESI): m/z: 126.0.
3-(4-Cyanophenyl)propiolic acid (9b)25
1H NMR (400 MHz, DMSO): d = 13.72 (br, s, 1H, -COOH), 7.95
3-Phenylpropiolic acid (1b)17
(d, J = 8.4 Hz, 2H, Ar–H), 7.82 (d, J = 8.8 Hz, 2H, Ar–H); 13
C
NMR (100 MHz, DMSO): d = 154.46 (-COOH), 133.78, 133.35,
124.35, 118.64, 113.57, 85.03, 82.69. Ms (ESI): m/z: 171.0.
1H NMR (400 MHz, DMSO): d = 13.62 (br, s, 1H, -COOH), 7.62
(d, J = 7.2 Hz, 2H, Ar–H), 7.55 (t, J = 7.4 Hz, 1H, Ar–H), 7.47
(t, J = 7.6 Hz, 2H, Ar–H); 13C NMR (100 MHz, DMSO): d =
154.94 (-COOH), 133.22, 131.56, 129.66, 119.50, 84.98, 82.31.
Ms (ESI): m/z: 146.0.
3-(4-Nitrophenyl)propiolic acid (10b)26
1H NMR (400 MHz, DMSO): d = 13.75 (br, s, 1H, -COOH),
8.28 (d, J = 8.8 Hz, 2H, Ar–H), 7.91 (d, J = 8.8 Hz, 2H, Ar–H);
13C NMR (100 MHz, DMSO): d = 154.28 (-COOH), 149.02,
134.30, 136.00, 124.48, 85.49, 82.16. Ms (ESI): m/z: 191.0.
3-p-Tolylpropiolic acid (2b)18
1H NMR (400 MHz, DMSO): d = 13.66 (br, s, 1H, -COOH),
7.49 (d, J = 7.6 Hz, 2H, Ar–H), 7.25 (d, J = 7.6 Hz, 2H, Ar–H),
2.33 (s, 3H, CH3); 13C NMR (100 MHz, DMSO): d = 155.05
(-COOH), 141.78, 133.19, 130.25, 116.48, 85.47, 82.02, 21.77.
Ms (ESI): m/z: 160.0.
3-(4-Formylphenyl)propiolic acid (11b)27
1H NMR (400 MHz, DMSO): d = 13.77 (br, s, 1H, -COOH),
10.06 (s, 1H, CHO), 7.97 (d, J = 8.4, 2H, Ar–H), 7.84 (d, J =
8.4, 2H, Ar–H); 13C NMR (100 MHz, DMSO): d = 193.26
(CHO), 154.65 (-COOH), 137.46, 133.78, 130.28, 125.24, 84.91,
83.26. HRMS (ESI), calcd for C10H6O3 (M+-O) 173.0239, found
173.0235.
3-(Bi-phenl)propiolic acid (3b)19
1H NMR (400 MHz, DMSO): d = 13.63 (br, s, 1H, -COOH),
7.76 (m, 2H, Ar–H), 7.71 (m, 4H, Ar–H), 7.47 (m, 2H, Ar–H),
7.41 (m, 1H, Ar–H); 13C NMR (100 MHz, DMSO): d = 154.94
(-COOH), 142.92, 139.30, 133.88, 129.73, 128.95, 127.78,
127.45, 118.40, 84.94, 82.96. Ms (ESI): m/z: 222.0.
3-(4-(Hydroxymethyl)phenyl)propiolic acid (12b)11
1H NMR (400 MHz, DMSO): d = 13.65 (br, s, 1H, -COOH),
7.58 (d, J = 8.4, 2H, Ar–H), 7.40 (d, J = 8.8, 2H, Ar–H), 5.37 (br,
s, 1H, OH), 4.54 (s, 2H, CH2); 13C NMR (100 MHz, CDCl3): d =
154.97 (-COOH), 146.49, 133.09, 127.29, 117.54, 85.37, 82.00,
62.95. HRMS (ESI), calcd for C10H8O3 (M+-O) 176.0473, found
176.0471.
3-(4-Methoxyphenyl)propiolic acid(4b)20
1H NMR (400 MHz, DMSO): d = 13.70 (br, s, 1H, -COOH),
7.57 (d, J = 8.4 Hz, 2H, Ar–H), 7.01 (d, J = 8.8 Hz, 2H, Ar–H),
3.83 (s, 3H, CH3); 13C NMR (100 MHz, DMSO): d = 161.66,
155.00 (-COOH), 135.11, 115.20, 110.94, 85.76, 81.50, 55.90. Ms
(ESI): m/z: 176.0
3-(3-Hydroxyphenyl)propiolic acid (13b)28
1H NMR (400 MHz, DMSO): d = 13.61 (br, s, 1H, -COOH),
7.94 (br, s, 1H, OH), 7.24–7.28 (m, 1H, Ar–H), 7.02–7.05 (m,
1H, Ar–H), 6.92–6.94 (m, 2H, Ar–H); 13C NMR (100 MHz,
DMSO): d = 157.90, 154.80 (-COOH), 130.74, 123.84, 120.20,
119.02, 118.93, 85.00, 81.66. HRMS (ESI), calcd for C9H6O3
(M+-O) 162.0317, found 162.0310.
3-(4-Chlorophenyl)propiolic acid (5b)21
1H NMR (400 MHz, DMSO): d = 13.72 (br, s, 1H, -COOH),
7.65 (d, J = 8.8 Hz, 2H, Ar–H), 7.54 (d, J = 8.8 Hz, 2H, Ar–H);
13C NMR (100 MHz, DMSO): d = 154.74 (-COOH), 136.40,
134.94, 129.83, 118.41, 83.65, 83.11. Ms (ESI): m/z: 180.0.
3-(Naphthalen-1-yl)propiolic acid (14b)29
3-(Thiophen-3-yl)propiolic acid (6b)22
1H NMR (400 MHz, DMSO): d = 13.68 (br, s, 1H, -COOH),
8.22 (m, 1H, Ar–H), 8.12 (m, 1H, Ar–H), 8.04 (m, 1H, Ar–H),
7.93 (m, 1H, Ar–H), 7.70 (m, 1H, Ar–H), 7.62 (m, 1H, Ar–H),
7.57 (m, 1H, Ar–H),; 13C NMR (100 MHz, DMSO): d = 154.97
(-COOH), 133.50, 133.35, 133.23, 132.03, 129.38, 128.66,
127.75, 126.21, 125.39, 116.85, 87.03, 82.99. Ms (ESI): m/z:
196.0.
1H NMR (400 MHz, DMSO): d = 13.62 (br, s, 1H, -COOH),
8.18–8.20 (m, 1H, Ar–H), 7.68–7.71 (m, 1H, Ar–H); 7.31–7.34
(m, 1H, Ar–H); 13C NMR (100 MHz, DMSO): d = 155.01
(-COOH), 135.48, 130.51, 128.36, 118.47, 82.13, 80.89. Ms
(ESI): m/z: 152.0.
Hept-2-ynoic acid (7b)23
1H NMR (400 MHz, DMSO): d = 13.75 (br, s, 1H, -COOH),
2.37 (t, J = 6.8 Hz, 2H, CH2), 1.59 (m, 2H, CH2), 1.58 (m, 2H,
CH2), 0.95 (t, J = 6.4, 3H, CH3); 13C NMR (100 MHz, DMSO):
d = 158.09 (-COOH), 92.61, 72.72, 29.45, 22.00, 18.64, 13.42.
Ms (ESI): m/z: 126.0.
Phenyl-1,3-dipropilic acid (15b)30
1H NMR (400 MHz, DMSO): d = 13.79 (br, s, 1H, -COOH),
7.87 (s, 1H, Ar–H), 7.78 (d, J = 7.2 Hz, 2H, Ar–H), 7.57 (t, J =
7.8 Hz, 1H, Ar–H); 13C NMR (100 MHz, DMSO): d = 154.67
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(-COOH), 136.45, 135.26, 130.49, 120.56, 83.14, 83.02. Ms
(ESI): m/z: 214.0.
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Acknowledgements
This work was supported by the Institute of Bioengineering
and Nanotechnology (Biomedical Research Council, Agency
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