Copper(0) Nanoparticles Supported on Al2O3 as Catalyst for Carboxylation of Terminal Alkynes

Article abstract of DOI:10.1007/s10562-017-2127-0

Abstract: The copper particles, supported on Al₂O₃ were for the first time used as heterogeneous, highly active, easily available, and recyclable copper catalyst of direct carboxylation of various terminal alkynes with CO₂

Full text of DOI:10.1007/s10562-017-2127-0

Catal Lett
DOI 10.1007/s10562-017-2127-0
Copper(0) Nanoparticles Supported on Al2O3 as Catalyst
for Carboxylation of Terminal Alkynes
Grigoriy N. Bondarenko1 · Ekaterina G. Dvurechenskaya1 ·
Eldar Sh. Magommedov1 · Irina P. Beletskaya1
Received: 1 March 2017 / Accepted: 20 June 2017
© Springer Science+Business Media, LLC 2017
Abstract The copper particles, supported on Al2O3 were
for the first time used as heterogeneous, highly active, eas-
ily available, and recyclable copper catalyst of direct car-
boxylation of various terminal alkynes with CO2 (2 bar)
in the presence of cesium carbonate, allowing for the
synthesis of alkyl 2-alkynoates. The catalyst contains large-
scale CuNPs covered by oxide layer and less prone to ero-
sion, resulting in its high selectivity and efficiency unusual
for the particles of this size.
Graphical Abstract
Keywords Carboxylation · Terminal alkyne · CuNPs ·
Supported nanoparticles · Heterogeneous catalysis
1 Introduction
Recently, reactions involving CO2 as the most available
and cheap source of C1 attract attention of many research
groups [1, 2]. Many routes of CO2 application in fine
organic synthesis such as reactions with alkenes [3–5],
alkynes [6–8], dienes and allenes [9–11], aromatic [12, 13]
and heteroaromatic compounds [14, 15] have already been
found. However, taking into account kinetic and thermo-
dynamic stability of the molecule, it is evident that either
* Irina P. Beletskaya
beletska@org.chem.msu.ru
1
Department of Chemistry, Lomonosov Moscow
State University, 1 Leninskie Gory, GSP-2, Moscow,
Russia 119992
Vol.:(0123456789)
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G. N. Bondarenko et al.
active substrates, primarily organometallic compounds
[16], or active catalysts [17] should necessarily be used in
the reactions. In the last case, availability and recyclability
are important in addition to the catalyst efficiency.
for more active substrates fantastic values of TON and TOF
can be obtained under mild conditions [36, 37].
Thus, the advantages of the heterogeneous catalysts
(easy separation of the catalyst from the reaction product
and recyclability) are accompanied by the high catalytic
activity quite comparable with that of the homogeneous
catalysts, whereas such a high activity is not observed for
the last in the absence of toxic and/or expensive ligands.
Copper(II) nanooxide was successfully used as a cata-
lyst in the C–S cross-coupling [38, 39], arylation of active
methylene compounds [40] and syntheses of thiol esters
in ionic liquid [41] and flavanones [42]. In addition, nano-
CuO can be applied as sensor [43]. In the most cases, the
use of complicated ligands is not required for successful
reactions in the presence of these systems.
Naturally, organic molecules functionalization involving
CO2 cannot affect such processes as global warming and
can hardly solve the problem of binding greenhouse vapors,
but the use of nontoxic carbon dioxide in organic synthesis
as an alternative for phosgene or isocyanate seems to be an
important area of its application.
Since the reactions involving CO2 molecule are greatly
variable, it is evident that different catalytic systems would
be needed for each specific reaction [18]. For example,
the carboxylation of epoxides needs another catalytic sys-
tem than the activation of С–Н bonds. The copper and sil-
ver salts turned out to be optimum catalysts for the widely
studied carboxylation of terminal alkynes [19]. Silver com-
plexes with carbene ligands efficiently catalyse the carbox-
ylation processes and exhibit high yields in mild conditions
[20–22].
CuNPs can catalyze “click” process but the authors
note that the reactions involved a film containing copper
oxide(s) on the CuNPs surface [44, 45], which is thermo-
dynamically more stable than the non-oxidized metallic
copper surface [46]. The observed activity of these cata-
lysts significantly exceeds the catalytic activity of CuO and
Cu2O.
Saegusa and coauthors in 1974 pioneered this field [23]
in describing stoichiometric alkyne carboxylation in the
presence of the copper salts and showed [24] reversibility
of the reaction even at 35°C, which can partially be avoided
by “capturing” the formed carboxylate by alkyl halide.
Only 20 years later, in 1994, Inoue and coworkers used cat-
alytic amounts (4 mol%) of the copper(I) and silver(I) salts
in the carboxylation of terminal acetylenes [25]. The inten-
sive search for mild reaction conditions (room temperature,
CO2 pressure of 1 bar) and, if possible, for avoiding revers-
ibility has started only in the XXI century. Different cop-
per salts [26, 27] and their complexes with various ligands
[28–30] were tested in the reaction and the systems with
carbenes CuCl-polyNHC were most efficient [30]. Exclu-
sive importance of base and of DMA and DMF as solvent
was also reported [25]. There are examples of the reactions
in ionic liquids [31] and supercritical CO2 [27].
Similar particles, as well as nanoparticles CuNPs/Cu2O,
exhibit synergic properties [47] and, hence, can be used as
efficient catalysts. For example, the Cu/Cu2O nanoparticles
can manifest high activity in the photocatalytic reactions
due to the properties of the corresponding redox pair [48,
49]. The calculated data for carbon dioxide adsorption on
the copper(I) oxide show that the geometry of the adsorb-
ate molecule is influenced by its coordination at the surface
copper and oxygen atoms [50]. The electron transfer due to
the coordinatively unsaturated copper atoms occurs when
the carbon dioxide molecule gains bent form [51].
In continuation of our work with transition metal nano-
particles [52, 53] we decided to try copper NPs in alkyne
carboxylation. We focused on a catalyst based on CuNPs
covered by a thin layer of copper oxide assuming that
alkyne would interact with the copper surface not covered
by oxides (for example, in crystal defects), while carbon
dioxide, on the contrary, would be adsorbed on copper
oxide. And this symbiotic microreactor would provide a
good selectivity of direct carboxylation.
It should be mentioned that the system CuCl-polyNHC
[30] was heterogeneous and manifested considerable advan-
tages since it catalyzed the reactions even with electron-
withdrawing alkynes (for example, nitrophenylacetylene) in
the presence of small catalyst amounts (0.5 mol%); how-
ever, the authors [30] did not perform recycling. Another
heterogeneous system consisting of CuBr on active carbon
was recycled five times without a significant loss in activity
[32].
2 Results and Discussion
The chemistry of metallic nanoparticles has recently
become very important in synthesis, catalysis, and medi-
cine [33]. The catalytic properties of the nanoparticles were
especially pronounced in the palladium catalysis where the
variation of the support and reaction conditions made it
possible to carry out reactions of poorly substrates such as
aryl chlorides [34, 35] in the absence of any ligands, while
CuNPs obtained by the method earlier proposed by us for
AuNPs [54] (impregnation of the support with an aque-
ous solution of CuSO4 followed by evaporation and heat-
ing at 300°C in a hydrogen flow) gave rather large (up to
50 nm) copper crystallites which turned out to be active
in the carboxylation of para-methoxyphenylacetylene
1a in DMF at 60°C, 2 atm. CO2 and Cs2CO3 as a base
1 3

Copper(0) Nanoparticles Supported on Al2O3 as Catalyst for Carboxylation of Terminal…
Scheme 1 CuNPs catalyzed carboxylation of 4-methoxy-1-ethynylbenzene
97
these catalysts under standard conditions (Scheme 1). On
100
80
60
40
20
0
86
the other hand, the Sonogashira reaction and azide–alkyne
addition (“click” reaction, CuAAC) [45] which can be per-
formed with the small nanoparticles did not occur in the
presence of CuNPs/Al2O3 (50 nm) obtained by our method
mentioned above.
76
42
42
40
40
32
32
13
The SEM, XPS, and XRD studies were carried out for
the most efficient catalysts. The copper particles in the
CuNPs/Al2O3 catalyst sample observed in the SEM image
(Fig. 2a) are fairly large (about 50–100 nm) (Fig. 2b),
whereas the crystallites in the CuNPs/C and CuNPs/GrOx
samples are larger and beyond the nano dimensionality
range (~250–500 nm) (Fig. 2b, c), and the smaller effective
surface area of these particles results in decrease of their
catalytic activity. These catalysts are also difficult to recy-
cle and we excluded them from further investigations.
The powder XRD pattern of the CuNPs/Al2O3 sample
(Fig. 3) exhibits characteristic peaks (marked by blue) of
the metallic phase of CuNPs corresponding to 2θ values
of 43.4°, 50.5°, and 74.2° and signals of the aluminoxide
support (2θ values of 14.5°, 28.2°, 38.3°, 45.8°, 48.8°,
49.2°, 51.7°, 55.2°, 60.5°, 64.2°, 64.9°, 66.9°, 67.7°,
72.0°, 72.2°, and 79.2° marked by red) corresponding
to the typical spectrum of the amorphous sandwich-like
form of alumina, boehmite, formed under high-temper-
ature conditions of catalyst calcination under hydrogene
atmosphere. The oxygen ions located in the octahedral
layers are bound to the aluminum ions only, whereas
the external oxygen ions are also linked to the hydrogen
ions [55]. The layered structure of boemite and presence
Fig. 1 Screening of the catalyst activity in the carboxylation of
4-methoxy-1-ethynylbenzene
(Scheme 1, Fig. 1). CuNPs/Al2O3 catalyzed the reaction to
give the corresponding ester 2a in an almost quantitative
yield. The catalysts on the carbon supports also showed
high activity: the yields of the product for CuNPs/C and
CuNPs/GrOx (GrOx-graphite oxide) were 76 and 86%,
respectively. These yields significantly exceeded both the
yields obtained using other supports (Cu/CaCO3, Cu/TiO2,
and Cu/SiO2) and those for unsupported CuO and Cu2O,
as well as the yields for nano-Cu2O/SiO2 (note that the last
three exhibited similar catalytic activity) (Fig. 1).
Our attempts to use CuNPs supported on the surface
of TiO2, Al2O3 and zeolite prepared according to a known
method [44] that gives small particles (~1.5 nm) active in
click reactions [45] were unsuccessful, and we failed to
perform the carboxylation of alkyne 1a in the presence of
Fig. 2 SEM micrograph of the CuNPs in CuNPs/Al2O3 (a), CuNPs/C (b) and CuNPs/GrOx (c)
1 3

G. N. Bondarenko et al.
Fig. 3 X-ray diffraction patterns of the powder sample of CuNPs/Al2O3
О–Н–О–Н–О bonds on its surfaces promote better
adsorption of metal ions, which makes it the best support
for our catalyst.
Note that the reaction was successfully performed by
using the special nanoparticles and supports as well as cer-
tain base and solvent. We showed that K2CO3, Bu4NOAc
and DABCO widely used in copper catalysis were quite
inactive as bases in our case. The results indicated were
obtained using two equivalents of Cs2CO3 (Fig. 1) (using
only 1 equiv. of base sharply decreased the yield of
the product). As it was shown [56] the reaction can be
The presence of an oxide layer with different Cu(I)/
Cu(II) ratios on the catalyst surface was shown by XPS
(Fig. 4): 41/59% for CuNPs/C and 51/49% for CuNPs/
Al2O3 (signal of Cu+ from 930 to 935 eV and that of Cu2+
from 931 to 947 eV).
Fig. 4 XPS spectra of the surface of the CuNPs/C (a) and CuNPs/Al2O3 (b) catalysts
1 3

Copper(0) Nanoparticles Supported on Al2O3 as Catalyst for Carboxylation of Terminal…
performed in non-catalytic conditions in the presence
of cesium carbonate only but requires high temperature
(120°C), whereas our reaction conditions are much milder.
We also confirmed the importance of CuNPs catalysis by
carrying out the reaction in the presence of Cs2CO3 and
support (Al2O3) only and obtaining a negligible yield (<5%
within 6 h). Except for DMF and DMA, all other tested
solvents turned out to be inactive. In the case of CuNPs/C,
the DMF and DMA solvents showed similar results (86
and 89%, respectively, within 6 h). The reaction catalyzed
by CuNPs/Al2O3 turned out to be more sensitive to the
solvent nature, and the yield close to quantitative (97%)
was obtained in DMF only. Therefore, all other catalysts
were also tested in the Cs2CO3 (2 equiv.)—DMF system
(Scheme 1).
our case in the absence of CO2 under standard conditions,
product 2а decarboxylated almost completely within 6 h.
Our attempt to reduse CO2 pressure to 1.5 atm. resulted
in decrease of alkynoate yield to 75% what probably caused
by reversibility of the process.
The study of the temperature dependence was also car-
ried out for the most promising catalyst CuNPs/Al2O3
(Fig. 6). At 80°C the maximum conversion was achieved
within 1.5 h, at 70°C it took 2 h, and at 60°C—16 h
(Fig. 5). At temperatures below 40°C, the reaction was
very slow and the yield of the product did not exceed 20%
even in 16 h. It should be mentioned that at 80 and 70°C
after reaching the maximum the yield began to decrease
(Fig. 6), which was also observed for the carbon supports
[32] and can be explained by an increase in the decarboxy-
lation rate with temperature.
Kinetic studies of three selected catalysts (Fig. 5)
showed that the kinetic curves resembled each other, as a
whole, but the carbon supports exhibited a specific feature
already noted in previous publications [32], namely, the
yield relatively rapidly reached a maximum after which
began to decrease with time. This effect is explained by
the equilibrium between the carboxylation and backward
decarboxylation [24]. The study of the latter process, which
has also been performed previously [57], showed that, in
We proposed the mechanism for the direct catalytic
carboxylation of terminal alkynes on the surface of
nanocrystalline copper covered by copper oxides (Fig. 7).
Terminal alkyne appears to be coordinated on the cop-
per surface and carbon dioxide is adsorbed on Cu2O sur-
face [49, 50]. To check the possibility of leaching from
the catalyst surface, a hot centrifugation test was car-
ried out. It showed that the reaction did not occur in the
Fig. 5 Comparison of the
kinetic profiles for the catalysts
CuNPs/Al2O3 (filled rhombus),
CuNPs/C (filled circle), and
CuNPs/GrOx (filled triangle)
in 4-methoxy-1-ethynylbenzene
1a carboxylation. Experimental
conditions: alkyne/Cs2CO3/
BuBr/catalyst=1/2/1.2/0.05;
CO2 pressure: 2 atm.; DMF,
60°C. Kinetic profile of butyl
3-(4-methoxyphenyl)propionate
2a decarboxylation performed
in the absence of CO2 (filled
square)
100
80
60
40
20
0
Cu/Al2O3
Cu/GrOx
Cu/C
decarboxylaꢀon
0
2
4
6
8
10
12
14
16
t, h
100
80
60
40
20
0
Fig. 6 The reaction kinetic
data of butyl 2-alkynoate 2a
yield versus time in 4-methoxy-
1-ethynylbenzene 1a carboxyla-
tion catalyzed by CuNPs/Al2O3
at 80°C (filled rhombus), 70°C
(filled square) and 60°C (filled
triangle). Experimental condi-
tions: alkyne/Cs2CO3/BuBr/
catalyst=1/2/1.2/0.05; CO2
pressure: 2 atm.; DMF. Hot cen-
trifugation test (filled circle)
80oC
70oC
60oC
hot centrifugaꢀon
0
60
120
180
240
300
360
t, min
1 3

G. N. Bondarenko et al.
Fig. 7 Mechanism proposed for direct alkyne carboxylation on the surface of nanocrystalline copper covered by the thin oxide layer
centrifugate of the reaction mixture sampled after 2 h
from reaction start with a new portion of the base and
heating for 4 h (Fig. 6). The absence of copper traces in
the centrifugate was also confirmed by inductively cou-
pled plasma mass spectroscopy analysis (ICP/MS).
According to the SEM data, the isolated catalyst sam-
ples contain a significant amount of adsorbed salts on its
surface even after washing with an aqueous-alcohol solu-
tion (Fig. 8). Washing of the catalyst by other solvents
except DMF influenced dramatically its recyclization,
probably because traces of alcohols and oxygen were
destroying the catalyst surface. Therefore, the CuNPs/
Al2O3 catalyst was recycled with washing it by DMF
only.
The yield of the product upon recycling was determined
within kinetic-controlled region at the time of half com-
pletion τ1/2 (2 h, 60°С). The results of these experiments
presented in Fig. 9 show that the catalyst activity changes
insignificantly after five cycles.
Fig. 8 SEM micrograph of the CuNPs/Al2O3 sampled from the reac-
tion
The carboxylation of a series of terminal alkynes was
carried out under the found optimum conditions (2 equiv.
Cs2CO3, 5 mol% CuNPs/Al2O3, 1.2 equiv. BuBr, 2 atm.
CO2, 60°C, DMF) (Fig. 10). Whereas for alkynes 1a–d
reaction was completed after 6 h, for alkynes 1e–m needed
at least 16 h.
Thus in the case of phenylacetylene 1b and aromatic
alkynes with electron-donating substituents 1a, c–d, the
corresponding esters 2a–d were obtained in high yields of
78–92%. However, in the case of 4-tert-butyl-substituted
phenylacetylene 1e, the reaction time had to be increased
to 16 h to obtain an almost quantitative yield of 97%,
1 3

Copper(0) Nanoparticles Supported on Al2O3 as Catalyst for Carboxylation of Terminal…
which can be explained by a larger volume of the tert-butyl
70
60
50
40
30
20
10
0
60
group having weak electron-donor properties and impeding
adsorption on the catalyst surface. Less reactive alkynes
with the electron-withdrawing substituents, aliphatic alkyne
and heteroaromatic (thienyl, pyrazolyl, quinolil-substituted)
alkynes reacted somewhat more slowly: the corresponding
products 2f–m were formed within 16 h in 61–96% yields.
Both the elongation of the reaction time and an increase
in the temperature and CO2 pressure to 80°C and 4 atm.,
respectively, were needed for heteroaromatic alkyne 1l and
aliphatic alkyne 1m. However in all cases, the reactions
proceeded without by-product formation.
55
54
52
51
1
2
3
4
5
No of cycles CuNPs used
Fig. 9 Recovery of NPs Cu/Al2O3 in 4-methoxy-1-ethynylben-
zene 1a carboxylation at 60°C for 2 h with a negligible loss in cata-
lytic efficiency
Fig. 10 Direct carboxylation of terminal alkynes with CO2 catalyzed by CuNPs/Al2O3
1 3

G. N. Bondarenko et al.
3.2.1 Preparation of CuO/SiO2 Catalyst
3 Experimental
To 0.125 g CuSO4 dissolved in water 0.95 g of SiO2 was
added. The mixture was stirred for 1 h, 1 М KOH solu-
tion was then added to alkaline the reaction. The precipi-
tate was centrifuged and calcined in air to dark-brown
colour.
3.1 Materials and Methods
All starting materials were purchased from Sigma
Aldrich except graphite oxide, 97% purchased from NPO
Unichemtech (http://www.unichimtek.ru) and 4-ethy-
nyl-1-methyl-1H-pyrazole obtained by Bestmann–Ohira
method [58]. Solvents were dried and deoxygenated using
standard procedures. Carbon dioxide gas 99,99% purity
was used. Carboxylation reactions were performed in
15 mL capacity glass low pressure reactor RLP25ML
(http://www.openscience.ru) equipped with gas feeding
system, magnetic stirrer and manometer. For all products
NMR spectra were recorded on a Bruker Avance 400 and
3.3 Carboxylation Reactions
3.3.1 General Procedure of Alkyne Carboxylation
To a low pressure reactor equipped with magnetic stirrer
alkyne (0.15 mmol), Cs2CO3 (0.30 mmol), copper cata-
lyst (0.0075 mmol), n-BuBr (0.18 mmol) and DMF (2 mL)
were added. CO2 (2 atm.) was introduced into the reac-
tion mixture by triply purging the reactor with CO2. The
reaction mixture was stirred at 60°C for 6 h or 16 h. After
the reaction, the mixture was cooled to room temperature
and diluted with DMF. After centrifugation, the organic
phase was concentrated under reduced pressure and puri-
fied by column chromatography on silica gel (eluent petro-
leum ether/EtOAc 20:1). Isolated product was dried under
vacuum.
Agilent 400MR spectrometers (1H NMR 400 MHz and 13
C
NMR 100.6 MHz) at ambient temperature in CDCl3 and
DMSO-d6. Chemical shifts are reported in ppm relative
1
to SiMe4 in the H NMR spectra and to chemical shifts of
the solvent peaks as a secondary standard in the 13C NMR
spectra. ESI-TOF spectra were recorded with a Thermo
Scientific Orbitrap Elite instrument. Column chromatog-
raphy was carried out on Merck silica gel 60 (40–63 µm)
with mixture of petroleum ether (60–90°C) and ethylac-
etate 20:1 as eluent. For the catalytic carboxylation reac-
tions, the reaction mixtures were analyzed by gas chroma-
tography, using a Chromatec Crystal 5000 GC instrument.
The melting point was measured using capillary sealed at
one end in an Electrothermal 9100 melting point indicator.
Target-oriented approach was utilized for the optimization
of the analytic measurements [59]. Before measurements
the samples were mounted on a 25 mm aluminum speci-
men stub and fixed by carbon conductive paste. Samples
morphology was studied under native conditions to exclude
metal coating surface effects [60]. The observations were
carried out using Hitachi SU8000 field-emission scanning
electron microscope (FE-SEM). Images were acquired in
secondary electron mode at 1, 2, 3, 5 and 15 kV accelerat-
ing voltage and at working distance 2–9 mm. X-ray pho-
toelectron spectra (XPS) were recorded on a Kratos Axis
Ultra DLD. The X-ray diffraction data were collected on an
X-ray diffractometer DRON-3 (Co-Kα radiation, θ/2θ scan-
ning technique). Detection of copper in reaction mixture
was performed on inductively coupled plasma mass spec-
trometer (ICP-MS)—element-2 (Thermo Fisher Scientific).
3.3.2 Spectral Characteristics of Carboxylation Products
3.3.2.1 Butyl 3-(4-methoxyphenyl)propionate 2a White
1
solid, H NMR (400 MHz, CDCl3, δ): 7.54 (d, J=8.8 Hz,
2H), 6.88 (d, J=8.8 Hz, 2H), 4.22 (t, J=6.8 Hz, 2H), 3.83
(s, 3H), 1.72–1.65 (m, 2H), 1.47–1.38 (m, 2H), 0.95 (t,
J=7.3 Hz, 3H). 13C NMR (100 MHz, CDCl3, δ): 161.4,
154.5, 134.9, 114.2, 111.4, 86.9, 80.1, 65.8, 55.4, 30.5, 19.1,
13.7. The spectroscopic data matched those reported in the
literature [CAS: 1359848-70-6].
3.3.2.2 Butyl 3-phenylpropionate 2b Colorless oil, 1H
NMR (400 MHz, CDCl3, δ): 7.59–7.56 (m, 2H), 7.46–7.41
(m, 1H), 7.38–7.34 (m, 2H), 4.23 (t, J=6.8 Hz, 2H), 1.73–
1.65 (m, 2H), 1.47–1.38 (m, 2H), 0.95 (t, J=7.4 Hz, 3H).
13C NMR (100 MHz, CDCl3, δ): 154.2, 132.9, 130.5, 128.5,
119.6, 86.0, 80.7, 65.9, 30.5, 19.0, 13.6. The spectroscopic
data matched those reported in the literature [CAS: 80220-
93-5].
3.2 Preparation of Cu/Support Catalysts
3.3.2.3 Butyl 3-(p-tolyl)propionate 2c Light yellow solid,
1H NMR (400 MHz, CDCl3, δ): 7.48 (d, J=8.2 Hz, 2H),
7.17 (d, J=8.0 Hz, 2H), 4.22 (t, J=6.7 Hz, 2H), 2.36 (s, 3H)
1.72–1.65 (m, 2H), 1.47–1.38 (m, 2H), 0.95 (t, J=7.4 Hz,
3H). 13C NMR (100 MHz, CDCl3, δ): 154.3, 141.2, 132.9,
129.3, 116.5, 86.6, 80.3, 65.8, 30.5, 21.7, 19.0, 13.6. The
To 0.125 g CuSO4 dissolved in water 0.95 g of support was
added. The mixture was stirred for 1 h, the solvent was then
removed in vacuo. The dried catalyst was heated at 300 оС
for 2–3 h in a stream of hydrogen (15–20 mL min−1) using
a tubular furnace.
1 3

Copper(0) Nanoparticles Supported on Al2O3 as Catalyst for Carboxylation of Terminal…
spectroscopic data matched those reported in the literature
[CAS: 1359848-69-3].
3.3.2.10 Butyl 3-(3-thienyl)propionate 2j Yellow oil,
1H NMR (400 MHz, CDCl3, δ): 7.73–7.72 (m, 1H), 7.31–
7.29 (m, 1H), 7.22–7.20 (m, 1H), 4.21 (t, J=6.7 Hz, 2H),
1.71–1.64 (m, 2H), 1.46–1.36 (m, 2H), 0.94 (t, J=7.4 Hz,
3H). 13C NMR (100 MHz, CDCl3, δ): 154.2, 133.6, 130.1,
126.0, 118.8, 81.5, 80.7, 65.9, 30.4, 19.0, 13.6. The spectro-
scopic data matched those reported in the literature [CAS:
1359848-76-2].
3.3.2.4 Butyl 3-mesitylpropionate 2d Colorless oil,
1H NMR (400 MHz, CDCl3, δ): 6.87 (s, 2H), 4.23 (t,
J=6.8 Hz, 2H), 2.43 (s, 6H), 2.28 (s, 3H), 1.73–1.66 (m,
2H), 1.48–1.38 (m, 2H), 0.96 (t, J=7.4 Hz, 3H). 13C NMR
(100 MHz, CDCl3, δ): 154.6, 142.4, 140.4, 127.9, 116.5,
88.2, 84.5, 65.7, 30.5, 21.5, 20.8, 19.1, 13.7. HRMS: Calcd.
for C16H20O2: 244.1505, found: 244.1498.
3.3.2.11 Butyl
3-(1-methyl-1H-pyrazol-4-yl)propionate
1
2k Yellow oil, H NMR (400 MHz, CDCl3, δ): 7.68 (s,
1H), 7.64 (s, 1H), 4.19 (t, J=6.7 Hz, 2H), 3.89 (s, 3H),
1.69–1.62 (m, 2H), 1.45–1.35 (m, 2H), 0.93 (t, J=7.4 Hz,
3H). 13C NMR (100 MHz, CDCl3, δ): 154.3, 143.5, 135.0,
99.9, 82.7, 79.2, 65.7, 39.3, 30.4, 19.0, 13.6. HRMS: Calcd.
for C11H14N2O2: 206.1133, found: 206.1140.
3.3.2.5 Butyl
3-(4-(tert-butyl)phenyl)propionate
2e Colorless oil, 1H NMR (400 MHz, CDCl3, δ): 7.53 (d,
J=8.2 Hz, 2H), 7.39 (d, J=8.2 Hz, 2H), 4.22 (t, J=6.7 Hz,
2H), 1.73–1.65 (m, 2H), 1.47–1.38 (m, 2H), 1.3 (s, 9H), 0.95
(t, J=7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3, δ): 154.4,
154.2, 132.9, 125.6, 116.5, 86.6, 80.3, 65.9, 35.0, 31.0, 30.5,
19.1, 13.7. HRMS: Calcd. for C17H22O2: 258.1643, found:
258.1648.
3.3.2.12 Butyl 3-(quinolin-3-yl)propionate 2l Brown
1
solid, m.p. 92–93°C, H NMR (400 MHz, DMSO-d6, δ):
8.79 (s, 1H), 8.03 (t, J=9.1 Hz, 2H), 7.92 (s, 1H), 7.86 (t,
J=7.3 Hz, 1H), 7.68 (t, J=7.5 Hz, 1H), 4.20 (t, J=6.8 Hz,
2H), 1.64–1.49 (m, 2H), 1.38–1.29 (m, 2H), 0.87 (t,
J=7.4 Hz, 3H). 13C NMR (100 MHz, DMSO-d6, δ): 153.5,
151.7, 147.3, 141.8, 132.1, 129.0, 128.6, 128.1, 126.5,
112.8, 83.3, 83.1, 66.1, 29.9, 18.7, 13.6. HRMS: Calcd. for
C16H15NO2: 253.1161, found: 206.1155.
3.3.2.6 Butyl
3-(4-(trifluoromethyl)phenyl)propion-
1
ate 2f Colorless oil, H NMR (400 MHz, CDCl3, δ):
7.70 (d, J=8.1 Hz, 2H), 7.64 (d, J=8.3 Hz, 2H), 4.25 (t,
J=6.7 Hz, 2H), 1.73–1.66 (m, 2H), 1.48–1.39 (m, 2H),
0.95 (t, J=7.5 Hz, 3H). 13C NMR (100 MHz, CDCl3, δ):
153.7, 133.1, 125.6-125.5, 83.8, 82.3, 66.2, 30.4, 19.0, 13.6.
HRMS: Calcd. for C14H13F3O2: 270.0965, found: 270.0959.
1
3.3.2.13 Butyl oct-2-ynoate 2m Colorless oil H NMR
3.3.2.7 Methyl 4-(3-butoxy-3-oxoprop-1-yn-1-yl)benzoate
2g Colorless oil, 1H NMR (400 MHz, CDCl3, δ): 8.04 (d,
J=8.3 Hz, 2H), 7,65 (d, J=8.3 Hz, 2H), 4.24 (t, J=6.7 Hz,
2H), 3.92 (s, 3H), 1.73–1.65 (m, 2H), 1.48–1.38 (m, 2H),
0.95 (t, J=7.3 Hz, 3H). 13C NMR (100 MHz, CDCl3,
δ): 166.1, 153.8, 132.8, 131.6, 129.6, 124.2, 84.5, 82.7,
66.1, 52.4, 30.4, 19.0, 13.6. HRMS: Calcd. for C15H16O4:
260.1038, found: 260.1042.
(400 MHz, CDCl3, δ): 4.13 (t, J=6.7 Hz, 2H), 2.30 (t,
J=7.2 Hz, 2H), 1.66–1.53 (m, 4H), 1.42–1.26 (m, 6H),
0.93–0.86 (m, 6H). 13C NMR (100 MHz, CDCl3, δ): 154.0,
89.5, 73.1, 65.6, 31.0, 30.4, 27.2, 22.1, 19.0, 18.6, 13.8,
13.6. The spectroscopic data matched those reported in the
literature [CAS: 127794-10-9].
3.4 Recyclization Studies and Leaching Experiments
3.3.2.8 Butyl 3-(4-cyanophenyl)propionate 2h Colorless
For the studies of the catalysts recyclization, the follow-
ing procedure was used. After 2 h from the start of the car-
boxylation reaction, the reactor was allowed to rest, organic
phase was decanted to separate it from the solid catalyst.
The solid residue was twice washed with DMF. Combined
organic layers were concentrated and analyzed by GLC. A
fresh charge of reagents was introduced into reactor and
the reaction was carried as described previously. This was
repeated for every recycle reaction.
1
oil, H NMR (400 MHz, CDCl3, δ): 7.66 (s, 4H), 4.24 (t,
J=6.7 Hz, 2H), 1.72–1.65 (m, 2H), 1.47–1.37 (m, 2H), 0.95
(t, J=7.3 Hz, 3H). 13C NMR (100 MHz, CDCl3, δ): 153.5,
133.2, 133.2, 124.5, 117.8, 113.9, 83.7, 83.1, 66.3, 30.4,
19.0, 13.6. The spectroscopic data matched those reported
in the literature [CAS: 1359848-71-7].
3.3.2.9 Butyl 3-(4-fluorophenyl)propionate 2i Yellow
1
oil, H NMR (400 MHz, CDCl3, δ): 7.59–7.56 (m, 2H),
The possibility of leaching from the catalyst surface
was studied by centrifugation of the hot reaction mixture
at 333 K by abruptly stopping the reaction at ∼50% conver-
sion. The collected centrifugate of the reaction mixture was
heated for 4 h with CO2 (2 atm.) and a new portion of the
base filtered and no reaction was observed. The filtrate of
the mixture was analyzed by ICP-MS (inductively coupled
7.08–7.04 (m, 2H), 4.22 (t, J=6.7 Hz, 2H), 1.72–1.65 (m,
2H), 1.47–1.38 (m, 2H), 0.95 (t, J=7.3 Hz, 3H). 13C NMR
(100 MHz, CDCl3, δ): 165.1, 162.6, 154.1, 135.3, 116.2,
85.0, 80.6, 66.0, 30.4, 19.0, 13.6. The spectroscopic data
matched those reported in the literature [CAS: 1449127-55-
2].
1 3

G. N. Bondarenko et al.
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plasma for atomic emission spectroscopy) investigation and
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4 Conclusions
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The first CuNPs catalyzed direct carboxylation reaction
of terminal alkynes was carried out. A comparison of the
activity of a number of oxide- and carbon-based supports
was performed. The best of the tested catalytic supports,
i.e., highly active, selective, easily available, and recycla-
ble was the Cu/Al2O3 containing medium and large CuNPs
particles (up to 50–100 nm) covered by the thin oxide layer.
The catalyst was successfully applied in the carboxylation
of various terminal alkynes with CO2 affording the cor-
responding alkyl 2-alkynoates in good yields. A possible
mechanism was proposed for the reaction that, according to
leaching tests, occurred on the catalyst surface.
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Acknowledgements This work was supported by Russian Science
Foundation (Grant No. 14-33-00001). Electron microscopy, XPS and
X-ray characterization were performed in the Department of Struc-
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