Electrochemical selective incorporation of CO2 into terminal alkynes and diynes

Article abstract of DOI:10.1002/1099-0690(200107)2001:13<2507::aid-ejoc2507>3.0.co;2-p

Electrochemical incorporation of CO₂ into terminal alkynes and diynes on silver cathodes occurred selectively, to afford monocarboxylic acid derivatives in good yields. The electrolyses were carried out in one-compartment cells fitted with magnesium anodes, under mild conditions.

Full text of DOI:10.1002/1099-0690(200107)2001:13<2507::aid-ejoc2507>3.0.co;2-p

FULL PAPER
Electrochemical Selective Incorporation of CO into Terminal Alkynes and
2
Diynes
[
a]
[b]
[b]
Frank Köster, Erkhardt Dinjus, and Elisabet Du n˜ ach*
Dedicated to Prof. E. G. Jäger on the occasion of his 65th birthday
Keywords: Alkynes / Carbon dioxide fixation / Carboxylic acids / Electrochemistry
Electrochemical incorporation of CO
2
into terminal alkynes
lyses were carried out in one-compartment cells fitted with
and diynes on silver cathodes occurred selectively, to afford magnesium anodes, under mild conditions.
monocarboxylic acid derivatives in good yields. The electro-
Introduction
have also been reported.[12,13] Thus, terminal alkynes re-
gioselectively afforded α,β-unsaturated carboxylic acids in a
reductive hydrocarboxylation-type reaction, when
Carbon dioxide is an abundant and low-cost carbon
source for the production of fuels and organic chemicals.[
The development of novel and selective reactions capable
of forming CϪC bonds from CO and organic substrates
constitutes an important topic, avoiding the use of toxic CO
1]
2ϩ
Ni(bpy)3 ·2BF was used as the catalyst precursor (bpy ϭ
4
,2Ј-bipyridine).[14]
2
2
The electrocarboxylation of terminal alkynes in the pres-
ence of Ni(cyclam)Br resulted in the corresponding alkyn-
2
or phosgene, and using CO as an environmentally
friendly reagent.
[15]
2
ylcarboxylic acid, in a stoichiometric process.
The elec-
[
2]
trochemical, uncatalyzed carboxylation of diphenylacetyl-
Of the selective and catalytic reactions involving carbon
dioxide as a C-1 building block for the synthesis of fine
chemicals, electrochemical methods have been shown to
have a wide range of applications.[ The direct electrochem-
ical reduction of carbon dioxide requires relatively negative
[16]
ene has also been reported, but a nonselective mixture of
reduced and carboxylated compounds was obtained.
3]
reduction potentials (Ϫ2.2 V vs. SCE in an aprotic me- Results and Discussion
[
4]
dium), and the use of several catalytic systems has been
[
3]
reported.
The fixation of CO into organic substrates may involve
We report here a simple electrochemical carboxylation
procedure for the selective conversion of terminal alkynes
into α,β-alkynyl carboxylic acids under mild conditions
2
[5]
its reactivity in nucleophilic reactions or its activation by
coordination to low-valent transition metal complexes. In
this case, new CϪC bonds can be formed by simultaneous
activation by the metal center, both of the organic substrate
and of the carbon dioxide.[
We have been particularly interested in the incorporation
of CO into unsaturated hydrocarbons, such as alkynes and
diynes. Ni complexes have been reported to mediate the
CϪC coupling of disubstituted alkynes and CO for the
(
Scheme 1). The electrocarboxylations were carried out in
DMF, in single-compartment cells fitted with magnesium
[17,18]
anodes.
Electrolyses were performed at room temper-
6Ϫ8]
ature and at a CO pressure of 0.5 atm. Up to now, the
2
reported selective electrocarboxylation processes involving
2
nonactivated, unsaturated hydrocarbons needed the pres-
0
[12,13]
ence of a catalytic system.
We report here that, de-
2
pending on the nature of the cathodic material used, the
carboxylation reaction of terminal alkynes does not need
formation of α,β-unsaturated carboxylic acids in stoichi-
ometric reactions at carbon dioxide pressures around 1
the presence of metal catalysts to afford selective CO in-
2
[
9]
atm. At higher CO pressures (p
Ն 50 atm), the syn-
2
CO2
corporation.
thesis of pyrones from alkynes[ and diynes
10]
[11]
has been
achieved.
Nickel(II)-catalyzed electrochemical reactions for the
fixation of carbon dioxide into unsaturated hydrocarbons
[
a]
Forschungszentrum Karlsruhe, Institut für Technische Chemie
CPV,
Postfach 3640, 76021 Karlsruhe, Germany
Laboratoire Ar oˆ mes, Synth e` ses et Interactions, Universit e´ de
Nice-Sophia Antipolis,
[
b]
06108 Nice cedex 2, France
Scheme 1. Electrochemical carboxylation of terminal alkynes
Eur. J. Org. Chem. 2001, 2507Ϫ2511
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WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
1434Ϫ193X/01/0713Ϫ2507 $ 17.50ϩ.50/0
2507

F. Köster, E. Dinjus, E. Du n˜ ach
FULL PAPER
a) Influence of the Cathodic Material
higher carboxylation yields and selectivities than those in
acetonitrile.
Some preliminary tests were carried out on the electrore-
ductive behavior of 1-nonyne (1a) in the presence of CO2
at different cathodic materials. Silver plate, nickel foam, Pt
foil, or carbon fiber cathodes were used. The results of the
electrocarboxylations are presented in Table 1.
2
Table 2. Influence of the solvent and of the CO pressure in the
electrocarboxylations of 1a and 1b, with Mg/Ag electrode couples
Table 1. Influence of the cathodic material on the electrocarb-
oxylation of 1-nonyne (1a)
Entry
Substrate
Solvent
CO
2
Yield of carboxylic
pressure acid methyl esters
[atm]
of 2a or 2b
1
2
3
4
5
6
1a
1a
1a
1b
1b
1b
DMF
5
2a 5%
2a 90%
2a 10%
2b 5%
2b 90%
2b 5%
DMF
MeCN
DMF
DMF
MeCN
0.5
0.5
5
0.5
0.5
c) Influence of Carbon Dioxide Pressure
We performed the electrocarboxylation reactions of 1a
and 1b in DMF at 5 atm of CO and at 0.5 atm of CO ,
and the results are shown in Table 2.
Entry[a] Cathode Products Yield (%)
Selectivity for 2a
among the carboxylic
acids (methyl esters)
2
2
Electrolyses were performed with 1 mmol of 1 at constant
current (50 mA) over 3 h, which corresponds to a passage
of 5.4 F/mol of 1. The carboxylation reaction should theor-
etically consume 1 F/mol of 1. The electrolyses thus oc-
curred with faradic yields for carboxylic acids 2 and 3 in the
range of 15Ϫ20% at a pressure of 0.5 atm. The incomplete
conversions after the passage of 1 F/mol of 1a are ac-
counted for by a different electroreduction reaction, which
1
2
Ag
Ni
1a
5
90
95%
2
a
1a
35
55
5
2
3
a
a
85%
3
4
Pt
C
1a
35
5
40
2
3
a
a
8%
in this system corresponds to the direct reduction of CO2
(
see Scheme 3).
1a
75
20
Ϫ
In reactions performed at 5 atm of CO , the yields of 2a
2
3
a
or 2b were very low after 3 h of electrolysis. These low yields
indicate an important parallel carbon dioxide reduction
process, as confirmed by the formation of oxalic acid as the
[
a]
For experimental details, see Exp. Sect.
Interestingly, 1a was electrochemically carboxylated on a main product in these electrolyses. This CO
silver plate cathode to afford 2-decynoic acid (2a) in 90% side reaction increased with higher CO pressures, as shown
yield (Entry 1), whereas under the same conditions, but on by the results in Table 2. For Entries 1 and 4, the faradic
a nickel foam cathode, the same substrate produced a mix- yield of CO reductive coupling to oxalate was as high as
ture of 2a and 2-decenoic acid (3a) with incomplete conver- 90Ϫ95%. We found that a CO pressure of 0.5 atm was the
homocoupling
2
2
2
2
sion (Entry 2). On a platinum surface, only 5% of 2a was best compromise for alkyne reactivity versus CO
obtained, the main product being the reduced carboxylic duction on silver surfaces.
acid 3a (mixture of cis and trans) formed in 40% yield
electrore-
2
(
Entry 3). The electrocarboxylation of 1a on a carbon fiber d) Cyclic Voltammetry
cathode (Entry 4) afforded a low degree of conversion and
reductive carboxylation to 3a in 20% yield.
Studies by cyclic voltammetry were carried out with di-
yne 1b and CO . Figure 1 and 2 present the behavior of 1b
2
The same reactivity trends could be observed in cases of
electrocarboxylations of 1,8-nonadiyne (1b). The second
triple bond of the starting material was not reduced, nor
carboxylated. The electrochemical process showed a very
strong and interesting chemoselectivity for monofunctional-
ization of a symmetrically disubstituted substrate.
in DMF containing tetrabutylammonium tetrafluoroborate
(
TBABF ) for two different diyne concentrations, in the ab-
4
sence and in the presence of CO2.
In the absence of carbon dioxide (Figure 1), a nonrevers-
ible reduction peak of 1b was observed at Ϫ1.9 V vs. Ag/
AgCl (curve a), the intensity of which increased upon fur-
ther addition of the diyne (curve b). In the presence of CO2
b) Influence of the Solvent
(
Figure 2, curve c), the same irreversible behavior could be
The influence of the solvent in the electrocarboxylations observed with a further reduction peak at Ϫ2.5 V, presum-
of 1a and 1b in DMF and acetonitrile was examined. As ably due to the reduction of carbon dioxide under these
shown in Table 2, the electrolyses in DMF resulted in much conditions.
2508
Eur. J. Org. Chem. 2001, 2507Ϫ2511

2
Electrochemical Selective Incorporation of CO into Terminal Alkynes and Diynes
FULL PAPER
Table 3. Electrocarboxylation of terminal acetylenic derivatives on
a silver cathode surface
Figure 1. Cyclic voltammograms of a DMF solution (10 mL) con-
4
taining TBABF (1 ), at 20 °C, at 100 mV/s on a carbon fiber
working electrode, reference electrode Ag/AgCl; curve a) addition
of 1b (0.1 mmol); curve b) addition of 1b (0.2 mmol)
Figure 2. Cyclic voltammograms of a DMF solution (10 mL) con-
4
taining TBABF (1 ), at 20 °C, at 100 mV/s on a carbon fiber
working electrode, reference electrode Ag/AgCl; curve a) addition
of 1b (0.1 mmol); curve c) same as curve a) but after addition of
2
CO at 20 °C, 1 atm
(
1i) (Entry 9). The main carboxylation product for both
e) Electrocarboxylation of Alkynes and Diynes
substrates arose from single CO incorporation into the ter-
2
Several alkyne and diyne derivatives were carboxylated minal position of the triple bond. Further carboxylation of
using an Mg anode and an Ag cathode as the pair of elec- the double bond occurred only at low ratios.
trodes, in DMF at room temperature and at a CO pressure
Phenylacetylene was very reactive under the reaction con-
2
of 0.5 atm. The results are presented in Table 3. The ob- ditions and produced a nonselective mixture of mono- and
tained carboxylic acids were esterified in the reaction mix- dicarboxylated compounds, together with some reduced
ture by the presence of methyl iodide, in order to facilitate products. The reactivity of internal triple bonds versus ter-
the extraction and the analysis of the products as the cor- minal triple bonds was also examined. Thus, internal 4-oc-
responding methyl esters.
tyne (1j) exhibited no reactivity under the standard reaction
The reactivity trends already noted for 1a and 1b (Entries conditions and could be quantitative recovered at the end
and 2) were also observed for the electrocarboxylation of the electrolysis.
1
of 1-octyne (1c) and 1,7-octadiyne (1d) (Entries 3 and 4).
Excellent yields of the corresponding monocarboxylic acids
were obtained in all cases. Cyclohexyl acetylene (1e) pro-
duced 3-cyclohexylpropionic acid (2e) in 80% yield (Entry
f) Mechanistic Aspects
For the one-compartment cell electrolysis procedure used
here, we have, at the anode, the oxidation of the magnesium
rod into Mg2 ions in solution (Scheme 2). At the cathode,
the reduction of the organic substrate takes place.
ϩ
5
).
The presence of an ether function in 1-methoxy-3-butyne
1f) was compatible with the reaction conditions and 5-me-
thoxypentynoic acid (2f) was formed in 80% yield (Entry
). However, no CO incorporation occurred in the case of
(
6
2
the analogous unprotected 3-butyn-1-ol. Dipropargyl ether
(
1g) also underwent a single CO incorporation, affording
2
dialkynyl monocarboxylic acid 2g in 90% yield (Entry 7).
The selectivity of alkyne carboxylation versus alkene
carboxylation was examined in the case of 3-methylbut-3-
Scheme 2. Electrochemical reaction at the electrodes and in solu-
en-1-yne (1h) (Entry 8), and of 3-butynyl 4-pentenyl ether tion
Eur. J. Org. Chem. 2001, 2507Ϫ2511
2509

F. Köster, E. Dinjus, E. Du n˜ ach
FULL PAPER
The absence of reactivity observed for internal alkynes
and the selectivity in the monocarboxylation of α,ω-diynes
prompted us to propose a mechanism in which the first step
is the deprotonation of the terminal alkyne by direct elec-
trochemical reduction on the silver surface, as the cathodic
reduction step. This deprotonation, with H2 evolution,
should be followed by reaction of the acetylide with CO2
acting as an electrophile. In the presence of the MgII ions,
issuing from the anodic oxidation, the coupling reaction af-
fords the alkynylcarboxylate as a magnesium salt.
In the case of α,ω-diynes, the first terminal alkyne cath-
odic reduction and carboxylation produces an Mg2ϩ mono-
carboxylate. The presence of the anionic carboxylate species
should disfavor their approach towards the cathode and
therefore the further reduction of its remaining alkyne unit. Scheme 4
This fact could explain the high selectivity towards diyne
monocarboxylation.
odic material. This factor plays an important role in the
Furthermore, the lack of carboxylation of 3-butyn-1-ol
can be explained similarly. For this substrate, initial depro-
tonation of the alcohol function to the corresponding al-
koxide occurs before the alkyne reduction. Thus, the nega-
tively charged alkoxide inhibits further reduction of the ter-
minal triple bond. As a cathodic side reaction, the direct
CO electroreductive side reaction, and therefore on the fa-
radic yield of the carboxylation.
2
Conclusions
electroreduction of CO in DMF forms oxalate, through the
2
In conclusion, we have developed a very simple electro-
chemical method that selectively affords monoalkynylcar-
boxylic acids from alkynes and diynes. The reaction does
not need the presence of a catalyst and can be carried out
under very mild pressure and temperature conditions. The
carboxylation reaction is chemoselective: It permits mono-
functionalization of α,ω-diynes but not the carboxylation
of internal triple bonds.
[4]
radical anion of CO (Scheme 3). In solution, magnesium
2
oxalate is formed. At the end of the electrolysis, these oxal-
ate ions, in the presence of methyl iodide, are esterified to
dimethyl oxalate, which could be found in the reaction mix-
tures.
Interestingly, terminal alkynes have for the first time
demonstrated selective reduction of the terminal CϪH
bond on silver cathodes. The change in the cathodic mat-
erial modified the mechanism and the chemoselectivity of
Scheme 3. Oxalate formation as a cathodic side reaction
It is interesting to compare the results of the uncatalyzed the reaction, changing the nature of the carboxylic acid
electrocarboxylation on Ag cathodes with those of the ana- formed in the carboxylation process.
logous nickel-catalyzed reactions on carbon fiber cathodes.
Nickel(II) in association with 2,2Ј-bipyridine has been re-
ported to catalyze CO incorporation into terminal al- Experimental Section
2
[
12]
[13]
kynes
or diynes,
yielding α,β-unsaturated acids 4 in
a reductive hydrocarboxylation-type reaction, using an Mg General: All solvents were dried and degassed by standard
anode. In this process, the cathodic reaction involves reduc- methods. DMF was freshly distilled from calcium hydride before
II
0
electrolyses.
tion of Ni to Ni , which is followed by the formation of
oxanickelacycles as intermediates in the CϪC bond forma-
tion, with further recycling of the Ni species (Scheme 4).
4 4
Electrochemical Procedure: A DMF (25 mL) solution of nBu NBF
II
(
0.3 mmol) and the alkyne (1 mmol) was electrolyzed in a single-
In the case of an Ag cathode, the use of NiϪbpy as the compartment cell fitted with an Mg anode and a silver cathode
catalytic system in the electrochemical carboxylation of 1c under carbon dioxide (0.5 atm) and nitrogen at room temperature.
produced 4c as the main compound in 30% yield, together The electrodes were connected to a DC power supply and a current
of 50 mA was applied between them for 3 h. The consumption of
with 2c and dimers and trimers of 1c. These results indicate
1
was monitored by GC analysis of aliquots withdrawn from the
that a change in the reaction conditions, such as the nature
of the cathodic material, may strongly influence the results
of the electrochemical reaction. In the case of an Ag cath-
ode, there is a strong interaction between the surface and
the electrogenerated acetylide, which can explain the high
reaction selectivity.
2
reaction mixture. The apparent current density was 0.25 A/dm (ap-
plied voltage ca. 3Ϫ15 V). The faradic yields were in the range of
3
0Ϫ40%. The reaction mixture was esterified directly in DMF by
addition of anhydrous CO (4 mmol) and methyl iodide
12 mmol) and stirring the mixture at 50 °C for 12 h. The solution
was hydrolyzed with 20 mL of 0.1  HCl solution and extracted
K
2
3
(
On the other hand, CO is adsorbed and/or reduced dif- with Et O. The organic layer was washed with H O, dried with
2
2
2
ferently and selectively, according to he nature of the cath- MgSO
4
, and concentrated. The methyl esters corresponding to the
2510
Eur. J. Org. Chem. 2001, 2507Ϫ2511

2
Electrochemical Selective Incorporation of CO into Terminal Alkynes and Diynes
FULL PAPER
[
[
[
4]
5]
6]
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1
13
and H and C NMR. The products are known compounds and
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microelectrodes (Tacussel). All potentials are quoted with respect
to Ag/AgCl electrode at room temperature, which correspond to
[
[
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[10]
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ϩ
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ϩ
Ϫ
4
[11]
nBu
ried out using a stabilized constant current supply (Sodilec, EDL
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4
N BF
. Controlled constant intensity electrolyses were car-
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2
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Received January 6, 2001
[O01027]
Eur. J. Org. Chem. 2001, 2507Ϫ2511
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