Carboxylation of terminal alkynes promoted by silver carbamate at ambient pressure

Article abstract of DOI:10.1039/c9nj02203a

Transition metal carbamates constitute a class of compounds with unique properties, however their catalytic potential has been sparingly explored so far. The easily available silver N,N-dimethylcarbamate, Ag(O₂CNMe₂), worked as a catalyst in the carboxylation reaction of terminal alkynes with CO₂ at atmospheric pressure. Different reaction parameters (solvent, base, temperature, time and the amount of catalyst) were investigated in order to establish the optimal conditions.

Full text of DOI:10.1039/c9nj02203a

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Carboxylation of terminal alkynes promoted by
silver carbamate at ambient pressure†
Cite this:DOI: 10.1039/c9nj02203a
ab
ab
ab
Giulio Bresciani,
Fabio Marchetti
and Guido Pampaloni
*
Transition metal carbamates constitute a class of compounds with unique properties, however their
catalytic potential has been sparingly explored so far. The easily available silver N,N-dimethylcarbamate,
Ag(O2CNMe2), worked as a catalyst in the carboxylation reaction of terminal alkynes with CO2 at
atmospheric pressure. Different reaction parameters (solvent, base, temperature, time and the amount
of catalyst) were investigated in order to establish the optimal conditions.
Received 29th April 2019,
Accepted 11th June 2019
DOI: 10.1039/c9nj02203a
rsc.li/njc
catalysts due to the tendency of such metal centers to activate
carbon–carbon triple bonds,8 thus various Cu(I)9 and Ag(I)
Introduction
The use of carbon dioxide as a nontoxic and inexpensive C1 complexes,10 copper conjugated polymers11 and silver nano-
synthon to obtain valuable chemicals represents a foremost particles12 have been proposed in this regard. Nevertheless, also
topic of contemporary chemistry research, in view of developing dinuclear Mo–alkoxides,13 lanthanide amidates14 and metal free
new convenient and sustainable synthetic processes.1 Among systems15 have been evaluated. Several of the efficient molecular
the large variety of studied organic reactions exploiting CO2, the metal compounds require CO2 pressure 41 atm and temperatures
carboxylation of terminal alkynes to afford propiolic acids is one above 100 1C in order to gain acceptable conversion values.16
of the most intriguing ones in terms of added-value products.
Metal N,N-dialkylcarbamates are easily accessible and cost
Indeed, propiolic acids are useful intermediates to produce effective materials of general formula M(O2CNR2)n, being usually
polymers and molecules of pharmaceutical interest, such as obtained by treatment of a range of metal chloride precursors
flavones, coumarins, spirobenzofuranones and vinyl sulfide.2 with dialkylamines in the presence of CO2 at atmospheric
The classical synthetic strategy to access propiolic acids involves pressure (eqn (1)).17
the oxidation of propargylic alcohols3 or aldehyde derivatives,4
MCln + nCO2 + 2nNHR2 - M(O2CNR2)n + n(NH2R2)Cl
and the carboxylation of terminal alkynes represents an alter-
(1)
native and more sustainable route. The latter involves alkyne
deprotonation by a suitable base, with the resulting carbanion A limited number of transition metal carbamates have found
adding CO2 to finally afford propiolic acids (Scheme 1).2
application as catalytic precursors in various reactions, such
Saegusa and co-workers reported the first example of a as the hydrogenation of alkenes18 and the polymerization of
metal-mediated carboxylation of terminal alkynes using a stoi- alkenes19 and cyclic esters.20
chiometric amount of copper(I) or silver(I) tert-butoxides.5 The
Despite the fact that such a class of versatile metal com-
first catalytic reaction was published 20 years later exploiting plexes display interesting chemical–physical properties17,21 and
silver(I) and copper(I) salts in the presence of an excess of constitute in turn an intrinsic form of CO2 activation, their
K2CO3, at ambient CO2 pressure at 100 1C.6 Since this pioneering employment in metal promoted carboxylation reactions is still
work, attention paid to carboxylation reactions of alkynes has in its infancy. However, we recently reported that Fe(O2CNEt2)3
exponentially increased, and several catalytic systems have been is effective in the synthesis of cyclic carbonates by CO2 addition
developed to reach satisfying conversions under mild conditions.7 to epoxides at ambient temperature and pressure, in the presence
It is believed that copper and silver species may work as optimal of NBu4Br as a co-catalyst.22 Remarkably, the proposed catalytic
system takes advantage of a dynamic pre-activation of carbon
dioxide, incorporated within the metal structure as a carbamato
a
`
Universita di Pisa, Dipartimento di Chimica e Chimica Industriale,
ligand, as it has been outlined by DFT calculations and NMR
spectroscopy. Analogously, carbamates of tin, aluminum and
copper are active in the epoxide–CO2 coupling process under
mild conditions.23
Via Moruzzi 13, I-56124 Pisa, Italy. E-mail: guido.pampaloni@unipi.it
b CIRCC, via Celso Ulpiani 27, I-70126 Bari, Italy
† Electronic supplementary information (ESI) available: NMR data (Table S1). See
DOI: 10.1039/c9nj02203a
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Scheme 1 Base(B)-assisted synthesis of propiolic acids by carboxylation of terminal alkynes.
Table 2 Yield of phenylpropiolic acid at different temperatures using
Herein, we explore for the first time the catalytic potential of
copper and silver carbamates, showing their performance in
the carboxylation reactions of terminal alkynes.
Ag(O2CNMe2) as the catalysta
Entry
Temperature [1C]
Yieldb [%]
1
2
3
4
25
50
80
10
60
20
19
Results and discussion
120
We selected silver and copper carbamates AgI(O2CNR2), R = Me,
Et, CuI(O2CNiPr2) and CuII(O2CNEt2)2 in order to assess their
catalytic activity in the formation of phenylpropiolic acid from
phenylacetylene and CO2, selected as a model process. The
reactions were carried out in a solvent (THF, DMSO or DMF) for
24 h at 50 1C and 1 atm, in the presence of a base (KOH or
Cs2CO3), see Table 1.
a
Reaction conditions: phenylacetylene (0.22 mL, 2.0 mmol), Ag(O2CNMe2)
b
1 mol%, Cs2CO3 3.0 mmol, DMF 10 mL, p(CO2) = 1 bar, t = 24 h. Referred
to isolated products after work-up (see Experimental for details).
Table 3 Yield of phenylpropiolic acid using different amounts of catalysta
Entry
Amount of Ag(O2CNMe2) [mol%]
Yieldb [%]
The silver carbamate Ag(O2CNMe2) is the best catalyst,
leading to the isolation of phenylpropiolic acid in 60% yield,
after work-up. DMF and Cs2CO3 were revealed to be the optimal
solvent/base combination. A blank experiment (Table 1, entry 11)
showed that a significant lower yield is achieved under the same
conditions but in the absence of the catalyst. Since the silver
carbamate exhibited a higher catalytic activity with respect to copper
complexes, we used the former for additional studies. First, we
investigated the effect of temperature (Table 2).
The results reported in Table 2 indicate that temperatures
above 50 1C are detrimental in terms of yield; this trend might
be associated with a very rapid decrease of CO2 solubility in
DMF over 50 1C.24
1
2
3
4
0.5
1
2
39
60
77
53
5
a
Reaction conditions: phenylacetylene (0.22 mL, 2.0 mmol), Cs2CO3
b
3.0 mmol, DMF 10 mL, p(CO2) = 1 bar, t = 24 h. Referred to isolated
products after work-up (see Experimental for details).
The data indicate a favourable effect of the increase of
catalyst amount up to 2 mol% (Table 3, entry 3). However, a
further increase to 5 mol% determines a negative effect.
The reactions carried out in DMSO or THF under the
optimized conditions afforded slightly lower yields compared
to the reaction performed in DMF (Table 4). On the other hand,
replacing the methyl groups with ethyl ones within the carba-
mato ligand resulted in moderate lowering of the product yield
(Table 4, entry 4).
The influence of the amount of catalyst on the product yield
was evaluated from a minimum of 0.5 mol% to a maximum of
5 mol% (Table 3).
Having established that Ag(O2CNMe2), in combination with
Cs2CO3, is an effective system for the carboxylation of pheny-
lacetylene, we decided to expand the scope of the present study
to other terminal alkynes (Table 5).
Table 1 Carboxylation of phenylacetylene catalyzed by Ag(I), Cu(I) and
Cu(II) carbamatesa
The results show that, in general, phenylacetylenes are prone
to carboxylation, leading to the isolation of the corresponding
Entry
Catalyst
Base
Solvent
Yieldb [%]
Table 4 Yield of phenylpropiolic acid under various conditionsa
1
2
3
4
5
6
7
8
Cu(O2CNEt2)2
Ag(O2CNMe2)
Cu(O2CNEt2)2
Ag(O2CNMe2)
Ag(O2CNMe2)
Ag(O2CNMe2)
Cu(O2CNiPr2)
Cu(O2CNEt2)2
Ag(O2CNMe2)
Ag(S2CNEt2)
—
KOH
KOH
KOH
KOH
THF
THF
—
—
2
Yieldb [%]
DMSO
DMSO
DMF
DMSO
DMF
DMF
DMF
DMF
DMF
Entry
Solvent
Temperature [1C]
1
KOH
—
45
37
20
60
54
15
1c
2
3
4
5
6
Ag(O2CNMe2)
Ag(O2CNMe2)
Ag(O2CNMe2)
Ag(O2CNEt2)
Ag(O2CNMe2)
Ag(O2CNMe2)
DMF
DMSO
THF
DMF
DMF
DMF
50
50
50
50
80
50
35
66
72
65
20
77
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
9
10
11
a
Reaction conditions: phenylacetylene (0.22 mL, 2.0 mmol), catalyst
a
Reaction conditions: phenylacetylene (0.22 mL, 2.0 mmol), catalyst 1 mol%, 2 mol%, Cs2CO3 3.0 mmol, solvent 10 mL, T = 50 1C, p(CO2) = 1 bar,
b
base 3.0 mmol, solvent 10 mL, T = 50 1C, p(CO2) = 1 bar, t = 24 h. b Referred to t = 24 h. Referred to isolated products after work-up (see Experimental
isolated products after work-up (see Experimental for details).
c
for details). Cs2CO3 4.0 mmol.
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Table 5 Yield of propiolic acids obtained by carboxylation of terminal
alkynes using Ag(O2CNMe2) as the catalyst
only traces of the other possible monoacidic product were
detected by H NMR.
1
It is presumable that the Ag(O2CNMe2)-mediated formation
of propiolic acid proceeds through the generally accepted
catalytic cycle reported in Scheme 2.1a However, a CO2 dynamic
exchange (see the Introduction)22 might be working too.
Conclusions
The use of carbon dioxide as a convenient C1 synthon in organic
synthesis is a topic of great interest due to environmental and
economic issues, and in this context the carboxylation of terminal
alkynes represents an attractive strategy to afford propiolic acids.
On the other hand, transition metal carbamates are easily
accessible materials representing an intrinsic form of CO2 activation
and possessing unique properties. However, the catalytic potential
of transition metal carbamates remains largely investigated.
Herein, we have assessed silver and copper complexes containing
carbamato ligands as catalysts for the carboxylation of a series of
terminal alkynes at ambient CO2 pressure. Ag(O2CNMe2), in
combination with Cs2CO3 in DMF solution, was found to be the
most efficient system, and the produced propiolic acids were
isolated in moderate to good yields after work-up.
Experimental section
General experimental details
All the operations were carried out under an atmosphere of
pre-purified nitrogen. The reaction vessels were oven dried at
140 1C prior to use, evacuated (10À2 mmHg) and then filled with
propiolic acids in satisfying yields, irrespective of the position nitrogen. CO2 (99.99%) was purchased from Rivoira (Chivasso,
and nature of the substituents. Likewise, the reaction worked Italy). Deuterated solvents and organic and inorganic reactants were
well with alkynes containing electron-withdrawing groups, i.e. commercial products (Sigma Aldrich, TCI Europe or Strem) of the
trimethylsilylacetylene and methyl propiolate. Lower yields were highest purity available and stored under a nitrogen atmosphere as
achieved with alkynes substituted with fused aromatic rings, received. Solvents (Sigma Aldrich) were distilled before use over
alkyl chains and a pyridine moiety.
appropriate drying agents. Cu(O2CNEt2)2,25 Cu(O2CNiPr2)26 and
Interestingly, the reaction with 1,4-diethynylbenzene supplied Ag(O2CNMe2)26 were prepared according to literature procedures.
almost quantitative selectivity for the diacid derivative, since Infrared spectra (solid state) were recorded at room temperature on
Scheme 2 Possible mechanism of Ag-promoted carboxylation of terminal alkynes.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
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The University of Pisa (Fondi di Ateneo 2017) is gratefully
acknowledged for financial support.
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