Copper(I)@carbon-catalyzed carboxylation of terminal alkynes with CO2 at atmospheric pressure

Article abstract of DOI:10.1021/acscatal.5b00764

Activated carbon supported CuBr was found to be an efficient catalyst for the carboxylation of terminal alkynes under atmospheric pressure of CO₂ using ethylene carbonate as solvent at 80 °C for only 2 h, as verified with ¹³CO₂. Various terminal alkynes could react smoothly with CO₂ and organic halides under the reaction conditions to afford the corresponding carboxylic esters. In addition, the catalyst can be easily recovered and reused at least five times without significant loss of activity.

Full text of DOI:10.1021/acscatal.5b00764

Research Article
pubs.acs.org/acscatalysis
Copper(I)@Carbon-Catalyzed Carboxylation of Terminal Alkynes with
CO2 at Atmospheric Pressure
Bing Yu,† Jia-Ning Xie,† Chun-Lai Zhong,† Wei Li,*,‡ and Liang-Nian He*,†
†State Key Labora‡tory and Institute of Elemento-Organic Chemistry, Collaborative Innovation Center of Chemical Science and
Engineering, and Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry,
Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, People’s Republic of
China
S
* Supporting Information
ABSTRACT: Activated carbon supported CuBr was found to
be an efficient catalyst for the carboxylation of terminal alkynes
under atmospheric pressure of CO2 using ethylene carbonate
as solvent at 80 °C for only 2 h, as verified with 13CO2. Various
terminal alkynes could react smoothly with CO2 and organic
halides under the reaction conditions to afford the
corresponding carboxylic esters. In addition, the catalyst can
be easily recovered and reused at least five times without
significant loss of activity.
KEYWORDS: carbon dioxide, alkynes, carboxylation, copper, heterogeneous catalysis
conditions with high efficiency is an important subject of
research.
INTRODUCTION
■
Carbon dioxide is an abundant, ubiquitous, cheap, and nontoxic
gas in nature. However, the concentration of CO2 in the
atmosphere has risen sharply since the Industrial Revolution,
primarily due to the combustion of fossils fuels.1 As the
principal greenhouse gas thought to be causing global warming,
CO2 has gained considerable attention in recent years.
Consequently, a number of methods for CO2 capture and
storage (CCS) have been developed for mitigation of carbon
emission.2−6 On the other hand, catalytic transformation of
CO2 into value-added compounds can be regarded as a
desirable alternative to CCS.7
On the other hand, alkynyl carboxylic acids and their
derivatives have been extensively applied in organic synthesis.22
For example, alkyl 2-alkynoates have been reported as an
important feedstock for the construction of heterocycles,
including pyrroles,23 oxazole,24 indole,25 oxindole,26 benzofur-
ans,27 furans,28 quinolines,29 indene,30 and benzazepines,31 as
depicted in Scheme 1. In this regard, several protocols have
been developed to prepare carboxylic acids/esters via
carboxylation of terminal alkynes with CO2,32−42 which avoid
the use of oxidant and toxic carbonylating reagent in the
oxidative carbonylation of alkynes.43
In fact, CO2 transformation on a large scale is still limited
today due to its inherent thermodynamic stability and kinetic
inertness. Traditionally, this challenge can be surmounted
through the application of noncatalytic processes involving
Grignard reagent or organolithium reagents. However, sensitive
prefunctionalized substrates are required and the functional
group compatibility is also questionable. Therefore, a catalytic
procedure for CO2 transformation under mild conditions with a
broad substrate scope is highly attractive and remains a
challenge.
In recent years, C−H bond functionalization has attracted
much attention for the direct, concise construction of chemical
bonds.8−12 In particular, C−H bond carboxylation using CO2
as a carboxylative reagent for C−C bond construction has
emerged to synthesize carboxylic acid derivatives.13−21
However, these methods generally suffer from some drawbacks:
e.g. tedious reaction procedures, extended periods of time, and
the formation of side products. Therefore, catalytic function-
alization of C−H with CO2 (ideally 1 atm of CO2) under mild
In previous reports, a catalyst based on Cu or Ag is able to
activate the triple bond of alkynes in most cases for the
carboxylation with CO2. On the other hand, heterogeneous
catalytic systems are favorable for industrial applications
because of the ease of handling and purification of products.
However, procedures employing heterogeneous catalysts have
been rarely disclosed. For example, Zhang and co-workers
developed a heterogeneous catalytic system for terminal alkyne
carboxylation by using N-heterocyclic carbene polymer
supported silver nanoparticle as an efficient catalyst.44 Very
recently, a zeolite-type metal−organic framework MIL-101
supported silver nanoparticle was employed for the first time as
a catalyst for the coupling of CO2 and terminal alkynes.45 The
heterogeneous catalysts showed good catalytic activity and
good reusability for the carboxylation.
Received: January 17, 2015
Revised: May 17, 2015
© XXXX American Chemical Society
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a
Scheme 1. Use of Alkyl 2-Alkynoates as Synthons in Organic
Synthesis
Table 1. Carboxylation of Phenylacetylene with CO2
b
entry
cat.
cat. loading (mol %)
T (°C)
yield (%)
65
1
2
3
4
5
6
CuCl@C
CuBr@C
CuI@C
2
2
2
80
80
78
76
80
c
d e f
80
13, 3, 4, 7
CuBr@C
CuBr@C
CuBr@C
CuBr@C
CuBr@C
2
2
2
2
5
60
52
33
77
75
100
80
g
7
h
8
80
h
i
9
80
90, 93
a
Reaction conditions unless otherwise specified: phenylacetylene (51
mg, 0.5 mmol), Cs2CO3 (196 mg, 0.6 mmol), n-BuI (110 mg, 0.6
mmol), EC (3 mL), CO2 (99.999%, balloon), indicated amount of
b
catalyst (based on Cu), 15 h. The yields were determined by GC with
c
d
biphenyl as internal standard. Without catalyst. Without base.
e
f
Without CO2. Equal amount of the activated carbon as a catalyst
versus entry 9. 24 h. 2 h. The recovered catalyst was used.
g
h
i
In 2013, we developed a ligand-free process for the CuI-
catalyzed carboxylation of terminal alkyne with ambient CO2
using ethylene carbonate (EC) as solvent.46 In this protocol,
external ligand is avoided by using EC as solvent. However, the
reusability of the transition-metal catalyst is poor under
traditional conditions. Herein, we report an effective heteroge-
neous catalytic system using activated carbon supported CuBr
for terminal alkyne carboxylation with CO2 in EC under mild
conditions. In addition, the catalyst is stable to air and moisture
and can be easily recovered and reused.
90% (entry 9). Interestingly, the recovered catalyst gave a 93%
yield of 2a in the second run, probably because the Cu(II)
species was partially reduced under the reaction conditions, as
verified by XPS measurements (vide infra).
Under the optimal conditions, the control experiment was
conducted with isotopically labeled 13CO2 as a carboxylative
reagent. As delineated in Scheme 2, the result examined by 13
C
NMR and high resolution mass spectrometry (HRMS)
confirmed that CO2 was the only carboxylative reagent, rather
than Cs2CO3 and ethylene carbonate.
RESULTS AND DISCUSSION
■
Taking into account the high stability and easy availability of
activated carbon,47−49 we initiated our study by using activated
carbon as a support to prepare heterogeneous copper catalysts
by using the impregnation method. Different copper salts
(CuX, X = Cl, Br, I) were supported on activated coconut-shell
carbon (100−200 mesh), which was easily available and
economically feasible (see the Supporting Information). The
copper contents of CuCl@C, CuBr@C, and CuI@C,
determined by graphite furnace atomic absorption spectrom-
etry, were found to be 17.4, 12.3, and 8.3 wt %, respectively.
The exploratory experiments started using phenylacetylene
(1a) as the model substrate in the presence of n-BuI under
atmospheric pressure of CO2. As shown in Table 1, 2 mol % of
the catalyst, i.e. CuCl@C, CuBr@C, and CuI@C, was
employed to promote the carboxylation under 80 °C for 15
h (entries 1−3). Among these, CuBr@C was the most efficient
and delivered the desired product 2a in 78% yield (entry 2).
Control reactions established that, in the absence of any copper
catalyst, base, or CO2, no satisfactory yield was observed (entry
4). Decreasing the reaction temperature to 60 °C slowed the
reaction, affording a 52% yield of 2a (entry 5). When the
reaction was conducted at 100 °C, the yield of 2a was only
33%, which was presumably due to the decarboxylation of the
intermediate in the equilibrium (entry 6).33 Subsequently, the
reaction was investigated at 80 °C for a different period of time.
However, improved yield was not observed by extending the
reaction time to 24 h (entry 7). Gratifyingly, a 75% yield of 2a
was obtained even when the reaction time was reduced to 2 h
(entry 8). Furthermore, an increase in the amount of CuBr@C
to 5 mol % caused an increase in the yield of this reaction to
Scheme 2. Carboxylation of Terminal Alkynes with 13CO2
under Standard Conditions
Having established the optimal conditions, we then
converted a variety of commercially available terminal alkynes
into the corresponding esters to further explore the scope of
this procedure under ambient CO2 pressure, as summarized in
Table 2. Electron rich group substituted phenylacetylenes 1b−e
(e.g., R1 = 4-Me, 4-Et, 4-nC5H11, 4-MeO) were converted to the
corresponding alkynoates 2b−e in 77−81% yield (Table 2,
entries 2−5). Electron-withdrawing groups substituted at the
meta or ortho position (1f−i) were also tolerable and gave
good yields (entries 6−9). The reactions with heterocyclic
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a
Table 2. Carboxylation of Terminal Alkynes Catalyzed by CuBr@C
a
Standard reaction conditions: terminal alkyne (1.0 mmol), catalyst (5 mol %, based on Cu), Cs2CO3 (0.3910 g, 1.2 mmol), n-BuI (0.2208 g, 1.2
b
mmol), ethylene carbonate (3 mL), CO2 (99.999%, balloon), 80 °C, 2 h. Isolated yield.
alkynes, such as 2-ethynylthiophene (1j), 2-ethynylpyridine
(1k), and 3-ethynylpyridine (1l), were also practical, providing
the corresponding products 2j−l in yields of 72%, 70%, and
66%, respectively (entries 10−12). The aliphatic alkyne 1-
octyne (1m) was also examined under the standard conditions.
To our delight, the desired product was also obtained in 75%
yield (entry 13). Other organic halides, including MeI and EtBr,
were also employed in this procedure. As a result, reasonable
yields (81% and 78%) were observed by using MeI and EtBr as
alkylating reagents (entries 14 and 15).
In a final set of experiments, we further assessed the stability
and reusability of the CuBr@C catalyst in the carboxylation
reaction, because it is crucial to confirm that the highly active
catalyst is recyclable. As is shown in Figure 1, there was no
discernible loss in activity or selectivity. The results were in
accordance with ICP experiments because only trace amounts
of copper (87 ppb) were detected in the resultant solution
collected by hot filtration after 1 h.
Figure 1. Recovery and reuse of CuBr@C catalyst for the
carboxylation of terminal alkynes with CO2.
Moreover, the reaction with the solution after hot filtration at
approximately 50% yield essentially stopped, strongly suggest-
ing that the reaction likely proceeded on the heterogeneous
surface, as illustrated in Figure 2.
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Furthermore, the electronic state of the Cu element in the
catalyst was analyzed by X-ray photoelectron spectroscopy
(XPS), as shown in Figure 4.
Figure 2. Leaching test indicating no contribution from homogeneous
catalysis of the active species leaching into the resultant solution.
The CuBr@C catalyst before and after the reaction was
characterized by Brunauer−Emmett−Teller (BET) surface area
analysis and scanning electron microscopy (SEM). No
significant changes were observed (see Table S1 in the
Supporting Information). On the other hand, the reaction
kinetics were measured as shown in Figure 3. It was observed
Figure 4. XPS patterns of high-resolution Cu 2p of (a) as-prepared
CuBr@C catalyst and (b) recovered CuBr@C catalyst.
Figure 4a shows the binding energies of Cu 2p for the
CuBr@C catalyst as prepared. The distinct peaks at 933.8 and
953.1 eV are assigned to Cu 2p3/2 and Cu 2p1/2, respectively.
The satellite peak at 940.2 eV proves the presence of Cu(II)
species, probably arising from slight oxidation of Cu+ during the
catalyst preparation.50
In addition, the XPS spectrum of Cu 2p for the recycled
catalyst is shown in Figure 4b. Two distinct peaks at 932.8 and
953.0 eV are assigned to Cu 2p3/2 and Cu 2p1/2, respectively.
The very weak, in fact negligible, satellite peak means that
copper mainly exists as Cu+ after the carboxylative reaction.
Generally, the carboxylation of terminal alkynes with CO2 was
catalyzed by cuprous salts rather than Cu(II).32 In this case, it
may be reasoned that the Cu(II) species on the surface of the
catalyst were reduced to Cu+ under the reaction conditions,47
which may give slightly higher performance in the second and
third runs.
Figure 3. Reaction kinetics profile of the carboxylation reaction: (a)
effects of the supporter size; (b) effects of stirring speed; (c) reaction
kinetic plots by using the fresh and recovered CuBr@C catalyst,
respectively. Reaction conditions unless otherwise notified: 100−200
mesh carbon as the support, 1500 rpm, 80 °C.
CONCLUSIONS
■
In conclusion, a CuBr@C-catalyzed carboxylation of terminal
alkynes with 1 atm of CO2 for the synthesis of alkyl 2-
alkynoates has been developed. The readily available
heterogeneous catalyst CuBr@C could be easily recovered
and reused without significant loss of activity, which makes this
protocol very practical and easy to handle. A range of electron-
rich and electron-poor terminal alkynes could undergo the
carboxylation reaction efficiently with CuBr@C as catalyst
within 2 h.
that similar plots were obtained for the reactions using different
sizes of the activated carbon as supporter and different stirring
speeds (Figure 3a,b). Surprisingly, the reaction could be slightly
improved when the recovered catalyst was employed (Figure
3c). However, catalytic deactivation of the recovered catalyst
appears in common heterogeneous catalysis.51
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b00764.
■
S
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1
Experimental procedures, characterization data, and H
and 13C NMR spectra for all products (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail for W.L.: weili@nankai.edu.cn.
*E-mail for L.-N.H.: heln@nankai.edu.cn
Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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We are grateful to the National Natural Sciences Foundation of
China, the Specialized Research Fund for the Doctoral Program
of Higher Education (20130031110013), the MOE Innovation
Team (IRT13022) of China, and the “111” Project of the
Ministry of Education of China (project No. B06005) for
financial support.
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