Electrochemical carboxylation of 2-iodoaromatic compounds

Article abstract of DOI:10.1081/SCC-200025608

The electrochemical reduction of a series of 2-iodoaromatic substituted compounds in acetonitrile under an atmospheric pressure of carbon dioxide leads to ortho-substitued aromatic carboxylic acid. The maximum yield of acid obtained by electrolysis performed in a diaphragm-less cell at low temperature (5°C) with a sacrificial magnesium anode and by the use of a low current density was slightly higher than 70%.

Full text of DOI:10.1081/SCC-200025608

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Electrochemical Carboxylation
of 2‐Iodoaromatic Compounds
B. Haouas Korbi a , N. Adhoum a & K. Boujlel a
a Laboratoire de Chimie Analytique et
d'Electrochimie, Faculté des Sciences de Tunis ,
Campus Universitaire , El Manar 2092, Tunis,
Tunisia
Published online: 12 Jan 2011.
To cite this article: B. Haouas Korbi , N. Adhoum & K. Boujlel (2004)
Electrochemical Carboxylation of 2‐Iodoaromatic Compounds, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 34:14, 2557-2564, DOI: 10.1081/SCC-200025608
To link to this article: http://dx.doi.org/10.1081/SCC-200025608
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 14, pp. 2557–2564, 2004
Electrochemical Carboxylation of
2-Iodoaromatic Compounds
*
B. Haouas Korbi, N. Adhoum, and K. Boujlel
´
Laboratoire de Chimie Analytique et d’Electrochimie, Faculte des
Sciences de Tunis, Campus Universitaire, Tunis, Tunisia
ABSTRACT
The electrochemical reduction of a series of 2-iodoaromatic substituted
compounds in acetonitrile under an atmospheric pressure of carbon
dioxide leads to ortho-substitued aromatic carboxylic acid. The maxi-
mum yield of acid obtained by electrolysis performed in a diaphragm-
less cell at low temperature (58C) with a sacrificial magnesium anode
and by the use of a low current density was slightly higher than 70%.
Key Words: Electrochemical carboxylation; 2-Iodoaromatic substituted
compounds; Carbon dioxide.
*
Correspondence: K. Boujlel, Laboratoire de Chimie Analytique et d’Electrochimie,
´
Faculte des Sciences de Tunis, Campus Universitaire, El Manar 2092, Tunis,
Tunisia; E-mail: khaled.boujlel@fst.rnu.tn.
2557
DOI: 10.1080/SCC-200025608
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

2558
Korbi, Adhoum, and Boujlel
INTRODUCTION
The C–X bond of organic halides can be cleaved through electrochemical
reduction under suitable conditions. In most cases and in particular with sub-
stituted aromatic or aliphatic halides, the generated carbanion after two elec-
tron transfer can react with electrophile derivatives as carbon dioxide to lead
the corresponding carboxylic acids.[1–6] Nevertheless, this reaction maybe
competed with the protonation reaction by residual water.[7,8]
On the other hand, previous work has shown the possibility of electroche-
mical carboxylation of a large variety of compounds such as aldehydes and
[9]
ketones
or gem-dihalides.[10] In the present paper, we report a promising
procedure for the synthesis of ortho-substituted benzoic acid derivatives by
reductive carboxylation of 2-iodoaromatic compounds (Sch. 1) in aprotic
media. Coulometric and analytical results are discussed.
RESULTS AND DISCUSSION
Cyclic voltammetry was carried out in acetonitrile (ACN) solution with
tetrabutylammonium tetrafluoroborate (TBATF4) as supporting electrolyte.
The voltammograms of compounds 1a, 1b, and 1c show one irreversible
wave (Fig. 1).
This wave is attributed to the cleavage of the C–I bond after two electrons
transfer.[11] The compounds 1d and 1e exhibit two irreversible peaks. The first
one corresponds to the cleavage of C–I bond, while the second is attributed to
the reduction of the carbonyl function.[12]
Table 1 shows that the first peak potentials Ep of 1e is less cathodic than
that of the other studied compounds, owing to the electron withdrawing effect
of the carboxylic substituent.
In CO2 saturated acetonitrile solution, only a slight increase of the catho-
dic peak was observed at a scan rate of 0.05 V sec21 (Fig. 2).
Scheme 1.

Electrochemical Carboxylation of 2-Iodoaromatic Compounds
2559
Figure 1. Cyclic voltammogram of 1a in ACN–TBAP 0.1 M. Hanging mercury drop
electrode (area ꢀ 1 mm2). Reference electrode: Ag/AgI/I2 0.1 M. Concentration: C ¼
5 ꢁ 1023 M. Sweep rate: v ¼ 50 mV sec21
.
This result suggests the occurrence of an interaction between the reduced
iodoaromatic compounds and carbon dioxide.
In fact Ep varies linearly with the logarithm of the scan rate (Fig. 3), the
average slope @Ep/@log v is equal to 60 mV per decade and the value of the
peak width is about 120 mV at a scan rate of 0.1 Vsec21
.
These data are in agreement with a reduction controlled by the kinetic of
the heterogeneous electron transfer. The lower value for the transfer coeffi-
cient a (a ꢀ 0.2–0.4) indicates that the reduction potential is more negative
than the standard potential of the rate determining electron transfer.
Table 1. Characteristic of cyclic voltammetric curves of some o-iodoaromatic
compounds in ACN–TBAP 0.1 M.
First peak
Ep (V) vs. SCE
Second peak
Ep (V) vs. SCE
Substrates
Ip (mA)
Ip (mA)
1a
1b
1c
1d
1e
21.60
21.61
21.75
21.70
21.08
3.1
2.9
3.5
3.2
2.8
—
—
—
—
—
21.85
21.77
—
6.30
3
Note: Hanging mercury drop electrode. Reference electrode: SCE 3 M. Concentration:
C ¼ 5 ꢁ 1023 M. Sweep rate: v ¼ 100 mV sec21
.

2560
Korbi, Adhoum, and Boujlel
Figure 2. Cyclic voltammogram of 1b in the presence of CO2 in ACN–TBAP 0.1
M. Hanging mercury drop electrode (area ꢀ 1 mm2). Reference electrode: Ag/AgI/
I2 0.1 M. Sweep rate: v ¼ 50 mV sec21. Concentration: C ¼ 3 ꢁ 1023 M.
So, the reduction of the 2-iodoaromatic compounds followed a mechan-
ism where electron transfer and cleavage of the carbon–halogen bond take
place simultaneously at the working electrode (Eq. 1, Sch. 2).
A fast reduction of the radical entity may occur at the working potential
owing to the very negative potential required for the reduction of the sub-
strates (Eq. 2, Sch. 2).
The anion maybe trapped by the carbon dioxide to give the corresponding
carboxylate (Eq. 3, Sch. 2).
Figure 3. Variation of the peak potential of 1b in ACN–TBAP 0.1 M as a function of
the scan rate range. Hanging mercury drop electrode. Reference electrode: Ag/AgI/I2
0.1 M. Concentration: C ¼ 2.5 ꢁ 1023 M.

Electrochemical Carboxylation of 2-Iodoaromatic Compounds
2561
Scheme 2.
However, the chemical yield is never equal to 100% because of the com-
peting protonation reaction by residual proton impurities.
The preparative electrolysis of compound 1a–1d conducted in aceto-
nitrile solution under a slow stream of carbon dioxide gas with magnesium
anode, gave the corresponding aromatic carboxylic acid in good yield after
consumption of 2 faraday per molecule. This confirms the above suggested
mechanism. The anodic process which consists of oxidation of magnesium
anode may contribute to the stabilisation of the electrogenerated carboxylate
anion by neutralisation of the Lewis acidity of Mg2þ. Indeed when a platinum
anode is used instead of a magnesium or zinc electrode, a lower yield of ortho-
benzoic acid is achieved (Table 2).
In the same conditions, the macroelectrolysis of 1e leads only to benzoic
acid because of a rapid autoprotonation reaction of the electrogenerated anion
by the substrate.
On the other hand, we investigated the effect of some parameters like the
current density, the temperature, and the number of transferred electron on
the yield of the corresponding anthranilic acid and o-methoxy benzoic acid
followed by HPLC method.
Table 2. Effect of the anode nature on the yield of some orthobenzoic
acids get after reductions of 1a and 1b in presence of carbon dioxide.
Yield of anthranilic
acid (%)
Yield of o-methoxy
benzoic acid (%)
Anode nature
Platinum
Aluminum
Magnesium
Zinc
5
6.2
44.60
45.20
62
47.80
74.31
51.10

2562
Korbi, Adhoum, and Boujlel
Table 3. Variation of the yield of anthranilic
acid with the temperature.
Temperature
(8C)
Yield of anthranilic
acid (%)
5
10
20
62
56
59
The experimental results showed (Table 3), a slight decrease of the elec-
trocarboxylation yield upon increasing the temperature. This behaviour maybe
ascribed to a decrease of solubility of carbon dioxide, since the CO2 dissol-
ution reaction is exothermic.[13]
Moreover, the electrocarboxylation yield (Table 4) is increased by lower-
ing the current density.
Aromatic acids have been identified by spectroscopic methods and their
spectral characteristics are summarised below:
2a. Yield: 59%; m.p: 1448C (lit. 144–1468C); IR nC55O ¼ 1680 cm21
;
1
NMR: H, 7.2 (s, 2H); 6.8–8.0 (m, 4H); 11 (s, 1H); 13C, 111, 116, 116.58,
131.84, 133.99, 150.80, 170.70.
2b. Yield: 63%; m.p: 998C (lit. 98–1008C); IR nC55O ¼ 1672 cm21
;
1
NMR: H, 3.60 (s, 3H); 7.4–7.9 (m, 4H); 10.90 (s, 1H); 13C, 53, 126.8,
127.8, 128.15, 128.75, 130.8, 140, 171.
2c. Yield: 54%; IR nC55O ¼ 1672 cm21; NMR: 1H, 2.2 (s, 1H); 4.6
(s, 2H); 7.2–7.5 (m, 4H); 10.50 (s, 1H); 13C, 65, 126.87, 127.18, 127.81,
128.42, 128.71, 140.96, 170.50.
2d. yield: 69%; IR nC55O ¼ 1666 cm21; NMR: 1H, 3.3 (s, 4H); 7.3–7.85
(m, 4H); 13C, 61.5, 127.3, 124.7, 128.15, 128.85, 131.3, 134.1, 166.3.
2e. Yield: 75%; m.p: 2068C (lit. 206–2088C); IR nC55O ¼ 1713 cm21
;
1
NMR: H, 11.50 (s, 1H); 7.4–8.20 (m, 5H); 13C, 128.31, 128.59, 129.33,
130.24, 133.65, 133.90, 172.49.
Table 4. Effect of the current density on the yield of anthranilic and
o-methoxy benzoic acids.
Current density
(mA cm22
Yield of anthranilic
acid (%)
Yield of o-methoxy
benzoic acid (%)
)
5
10
20
63.10
62
55
74.31
68.37
61.56

Electrochemical Carboxylation of 2-Iodoaromatic Compounds
EXPERIMENTAL
2563
The IR spectra (CHCl3) were determined on a Perkin–Elmer 1000 PC
spectrophotometer. The NMR spectra were recorded on a Brucker AC 300
MHz spectrometer in CDCl3 containing tetrametylsilane as an internal
standard.
All the experiments are performed in ACN purchased from Fluka and was
dried over anhydrous magnesium sulphate MgSO4, distilled under vacuum
and kept over activated alumina. The supporting electrolyte is 0.1M
TBATF4 (Fluka). It was used without further purification.
The cyclic voltammetry measurements were performed using a PAR
scanning potentiostat model 362. The cell was a three-electrode type with
hanging mercury drop as a working electrode, a platinum wire as counter elec-
trode and SCE 3 M or Ag/AgI/I2 0.1 M system as a reference electrode.
Macroscale electrolysis is carried out in an undivided cell on a stirred
mercury pool as a cathode and a magnesium anode. The current density is con-
trolled by a Tacussel-type PJT 35-2 potentiostat and the amount of electricity
is measured with a Tacussel IG5 integrator. All the electrolyses were carried
out under continuous bubbling of carbon dioxide in the solution containing 0.1
M of the substrate (the corresponding mass of substrate, about 1 g was dis-
solved in 50 mL of a solution of acetonitrile containing the supporting electro-
lyte). At the end of the electrolysis, the catholyte was acidified by HCl 2M
solution, then extracted by diethyl oxide and the organic solution was dried
over MgSO4. The yields obtained analytically are followed by HPLC using
Perkin–Elmer liquid chromatography equipped with C18 column and an
auto control UV detector. The eluting solution was 70% acetonitrile–30%
phosphate buffer.
ACKNOWLEDGMENT
Authors wish to thank SERST (Lab-CH02) for financial support and
assistance.
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