Electrosynthesis of imidazolium carboxylates

Article abstract of DOI:10.1021/ol401949f

Synthesis of imidazolium carboxylate compounds was efficiently achieved by electrochemical reduction of imidazolium precursors under very mild conditions.

Full text of DOI:10.1021/ol401949f

ORGANIC
LETTERS
2013
Vol. 15, No. 17
4410–4413
Electrosynthesis of Imidazolium Carboxylates
†
,†
‡
†
ꢀ ꢁ
Guillaume de Robillard, Charles H. Devillers,* Doris Kunz, Helene Cattey,
Eric Digard,†,‡ and Jacques Andrieu*,†
ꢀ
Institut de Chimie Moleculaire de l’Universite de Bourgogne, UMR CNRS 6302,
Universite de Bourgogne, BP 47870, 21078 DIJON Cedex, France, and Institut fur
ꢀ
ꢀ
€
€
Anorganische Chemie, Eberhard Karls Universitat Tubingen, Auf der Morgenstelle 18,
€
D-72076 Tu€bingen, Germany
charles.devillers@u-bourgogne.fr; jacques.andrieu@u-bourgogne.fr
Received July 10, 2013
ABSTRACT
Synthesis of imidazolium carboxylate compounds was efficiently achieved by electrochemical reduction of imidazolium precursors under very
mild conditions.
Although thermodynamically stable, N-heterocyclic
carbenes (NHCs) are very reactive species and, thus, need
to be stored under exclusion of air and moisture generally
at low temperatures. They are commonly used in many
applications such as ligands for organometallic catalysts,
organic catalysis, material and drug syntheses, therapeu-
tics, and electrochemistry.1 Currently, a large majority of
NHCs are synthesized from their corresponding imidazo-
lium salts by deprotonation with a strong base (n-BuLi,
tBuOK, NaH), which usually requires special conditions
(low temperature, inert atmosphere).2 Numerous works are
devoted to the synthesis of stable masked carbenes such
as imidazolium-2-carboxylate,3,4 hydrogen carbonate,5
2-alkoxy,6 2-cyano,7 2-trichloromethyl,8,9 2-pentafluoro-
benzene,7,10 2-thioisocyanates,11 and metal-stabilized
NHCs8,12 which can regenerate in situ free carbenes by
thermal activation. Among these protected carbenes,
synthesis of imidazolium-2-carboxylate is of particular
interest due to the almost free cost of CO2. These zwitter-
ionic compounds react with numerous transition metals3c,e,4
and electrophiles3a,c,13 and are also useful building blocks to
generate halide-free ionic liquids or imidazolium salts.3 Two
different pathways are currently described for the synthesis
of imidazolium-2-carboxylate compounds. The first one
†
(6) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1,
953.
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Universite de Bourgogne.
‡
€
€
Eberhard Karls Universitat Tubingen.
(7) Bittermann, A.; Baskakov, D.; Herrmann, W. A. Organometallics
2009, 28, 5107.
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(1) (a) Nair, V.; Bindu, S.; Sreekumar, V. Angew. Chem., Int. Ed.
2004, 43, 5130. (b) Enders, D.; Balensiefer, T. Acc. Chem. Res. 2004, 37,
534. (c) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (d)
Arduengo, A. J.; Bertrand, G. Chem. Rev. 2009, 109, 3209. (e) Feroci,
M.; Chiarotto, I.; Orsini, M.; Sotgiu, G.; Inesi, A. Adv. Synth. Catal.
2008, 350, 1355.
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Scholl, M.; Choi, T.-L.; Ding, S.; Day, M. W.; Grubbs, R. H. J. Am.
Chem. Soc. 2003, 125, 2546.
(2) Arduengo, A. J. Acc. Chem. Res. 1999, 32, 913.
€
(10) Bittermann, A.; Harter, P.; Herdtweck, E.; Hoffmann, S. D.;
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427. (b) Holbrey, J. D.; Reichert, W. M.; Tkatchenko, I.; Bouajila, E.;
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Herrmann, W. A. J. Organomet. Chem. 2008, 693, 2079.
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2006, 47, 8529. (d) Jellen, F. S. R. Dissertation, Erlangen, 2003. (e) Duong,
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R. H. J. Am. Chem. Soc. 2007, 129, 12834.
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47, 2883. (b) Liu, B.; Liu, X.; Chen, C.; Chen, C.; Chen, W. Organome-
tallics 2012, 31, 282.
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B.; Tkatchenko, I. Eur. J. Inorg. Chem. 2007, 2007, 4877. (b) Picquet, M.;
Poinsot, D.; Stutzmann, S.; Tkatchenko, I.; Tommasi, I.; Wasserscheid,
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r
10.1021/ol401949f
Published on Web 08/12/2013
2013 American Chemical Society

(named “B” as basic pathway) consists of deprotonation of
an imidazolium salt at low temperature under an inert
atmosphere generating the free carbene intermediate.3a,c
After filtration and removal of NaX or KX salts (with
X = halides) on Celite, passing CO2 into the carbene contain-
ing solution generally leads to precipitation of the carbox-
ylate adduct. The second method (named “DMC”) lies on
the direct one-pot N-methylation and C-carboxylation of
N-monoalkylatedimidazole precursorsusing dimethylcar-
bonate (DMC) as the reactant.3b In contrast to route B, this
7 h procedure requires a high temperature and pressure,
typically 130 °C at 10 bar, and is limited to the methylation
of the remaining unsubstituted nitrogen. Given the weak
acidity of the NCHN proton of the imidazolium ring (pKa
∼20ꢀ24),14 an interesting alternative toward the formation
of the carboxylate may arise from its electrochemical reduc-
tion leading to the carbene and finally, after CO2 addition, to
the desired adduct (Scheme 1). Numerous reports have
demonstrated the possibility to electrogenerate the carbene
intermediate from its imidazolium salt. These carbenes have
been trapped with different electrophiles15 or metals11 pro-
duced by anodic dissolution.
Figure 1. Molecular structure of the investigated imidazolium
salts.
Scheme 1. Plausible Formation Mechanism of Imidazolium
Carboxylate
Figure 2. CVs of imidazolium salts in DMF containing 0.1 M
TBAPF6 (under Ar, WE: Pt, Ø = 1 mm, ν = 0.1 V sꢀ1, c = 2.0 ꢁ
10ꢀ3 M).
Despite several publications reporting on the potential
electrogeneration of transient carboxylate species,16 to the
best of our knowledge, their preparative isolation has never
been reported although electrochemistry usually allows
working under mild conditions.17 Thus, we report here a
new pathway (named “E” as electrochemical) toward the
formation of imidazolium-2-carboxylate compounds or
their bicarbonate evolution product by electroreduction of
the corresponding imidazolium salts 1Hþ,Iꢀ, 2Hþ,Iꢀ, 3Hþ,
BF4ꢀ, and 4Hþ,BF4ꢀ (Figure 1). Furthermore this original
approach will be confronted with the above-mentioned
chemical methods.
Cyclic voltammograms (CV) of these four imidazolium
salts reveal one irreversible monoelectronic reduction at
Epc = ꢀ1.83, ꢀ1.70, ꢀ1.77, and ꢀ1.82 V/SCE, respectively.
A similar potential, i.e. ꢀ2.2 V/SCE, has been previously
reported by Clyburne and co-workers for the monoelectro-
nic reduction of 1,3-bis(2,4,6-trimethylphenyl) imidazolium
chloride. These authors have previously ascribed the irrever-
sibility of this reduction process to a consecutive reaction
which leads to the formation of H2 and free carbene.19 For
1Hþ,Iꢀ and 2Hþ,Iꢀ, two successive anodic oxidations are
observed at Epa = 0.39 (ꢀ1e) and 0.75 V/SCE (ꢀꢀ0.5e) as
expected for iodide salts. Additionally, 4Hþ,BF4 exhibits
one irreversible oxidation at Epa = 1.42 V/SCE. The higher
intensity of this peak as compared to the monoelectronic
reduction one (ipa/ipc = 1.56) implies that more than 1 F
molꢀ1 is abstracted during this oxidation process, in agree-
ment with an electrochemicalꢀchemicalꢀelectrochemical
(ECE) mechanism. However, the oxidation product(s) was-
(were) not studied further. Similarly, in acetonitrile, a solvent
allowing higher oxidation potentials to be reached compared
After 1Hþ,Iꢀ, 2Hþ,Iꢀ, 3Hþ,BF4ꢀ, and 4Hþ,BF4ꢀ were
synthesized according to known procedures,18 voltam-
metric studies were undertaken in DMF (Figure 2).
(14) Higgins, E. M.; Sherwood, J. A.; Lindsay, A. G.; Armstrong, J.;
Massey, R. S.; Alder, R. W.; O’Donoghue, A. C. Chem. Commun. 2011,
47, 1559.
(15) Feroci, M.; Orsini, M.; Inesi, A. Adv. Synth. Catal. 2009, 351,
2067.
(16) (a) Zhao, S.-F.; Wu, L.-X.; Wang, H.; Lu, J.-X.; Bond, A. M.;
Zhang, J. Green Chem. 2011, 13, 3461. (b) Wu, L.-X.; Wang, H.; Xiao, Y.;
Tu, Z.-Y.; Ding, B.-B.; Lu, J.-X. Electrochem. Commun. 2012, 25, 116.
(17) (a) Moeller, K. D. Tetrahedron 2000, 56, 9527. (b) Lund, H. J.
Electrochem. Soc. 2002, 149, S21. (c) Sperry, J. B.; Wright, D. L. Chem.
Soc. Rev. 2006, 35, 605. (d) Yoshida, J.; Kataoka, K.; Horcajada, R.;
Nagaki, A. Chem. Rev. 2008, 108, 2265. (e) Devillers, C. H.; Dime,
A. K. D.; Cattey, H.; Lucas, D. Chem. Commun. 2011, 47, 1893. (f)
Dime, A. K. D.; Devillers, C. H.; Cattey, H.; Habermeyer, B.; Lucas, D.
Dalton Trans. 2012, 41, 929.
(18) Nonnenmacher, M.; Kunz, D.; Rominger, F.; Oeser, T. Chem.
Commun. 2006, 1378.
(19) Gorodetsky, B.; Ramnial, T.; Branda, N. R.; Clyburne, J. A. C.
Chem. Commun. 2004, 1972.
Org. Lett., Vol. 15, No. 17, 2013
4411

to DMF, additional irreversible oxidation processes are
observed at Epa = 2.43, 2.26, 2.01 V/SCE for 1Hþ,Iꢀ,
2Hþ,Iꢀ, and 3Hþ,BF4ꢀ, respectively (see Supporting
Information).
Table 1. Comparison of the Efficiency of the Three Different
Methods for Carboxylate Synthesis (See Experimental Details
in Supporting Information)
Electrosyntheses of imidazolium carboxylates were car-
ried out at rt in DMF or CH3CN (0.1 M TBAPF6 or
TEAPF6) on a large area platinum electrode. As for the
chemical deprotonation pathway, electrolyses were first
conducted in two steps, i.e. reduction of the imidazolium
salts under Ar and then bubbling of CO2 into the solution
at atmospheric pressure. However, following this proce-
dure no carboxylate was obtained. Indeed, working at rt
drastically increases the reactivity of carbenes or radicals
which might be able to react during the time scale of the
electrolysis with themselves, the supporting electrolyte,
and/or the solvent. Nevertheless, in the particular case of
2Hþ,Iꢀ, the imidazolium salt with a hydrogen carbonate
product isolated yield (%)
starting product
1Hþ,Iꢀ
methoda
E (1.50)
CO2
HCO3
1Hþ,HCO3
ꢀ
ꢀ
1CO2 77b
ꢀ
ꢀ
17b
1Hþ,Iꢀ
B
1CO2 81
1CO2 90
1-methyl-imidazole
2Hþ,Iꢀ
DMC
E
2Hþ,HCO3
56 (1.50), 57d
(1.25), 66c,d
(1.25)
2Hþ,Iꢀ
B
2CO2 23b
2CO2 27b
2Hþ,HCO3
70b
2Hþ,HCO3
55b
ꢀ
ꢀ
1-methyl-benzimidazole DMC
counterion 2Hþ,HCO3 was produced. The latter is
ꢀ
3Hþ,BF4ꢀ
E (1.14)
3CO2 66
3CO2 55
3CO2 53
4CO2 84
4CO2 83
ꢀ
known to be the product of hydrolysis of the desired
carboxylated material (2CO2).5
3Hþ,BF4
B
imidazo[1,5-a]pyridine
DMC
E (1.50)
B
With the hope of preventing side product formation,
electrolyses were then performed directly under a CO2
atmosphere by setting the working electrode at a potential
corresponding to the first reduction step of imidazolium
salts under these conditions. The current dropped to the
residual current after an uptake of 1.1ꢀ1.5 mol equiv of
electrons, in agreement with a one-electron transfer me-
chanism. Then, the CVs of the electrolyzed solutions no
longer show the initial reduction peaks, which gives evi-
dence for complete conversion of the reactants. After
evaporation of the solvent, the solid residues were washed
with THF to remove the supporting electrolyte, Iꢀ or
BF4ꢀ, and were dried under vacuum. Good to high yields
were obtained (Table 1), validating the high interest of the
electrochemical route. Indeed, this procedure is easy and
highly selective, works at ambient temperature and atmo-
spheric pressure, and is thus safe and energy-saving. Be-
sides, workup is very simple, and though not experienced,
the supporting electrolyte from the cathodic compartment
could be potentially recycled. In the case of 1Hþ,Iꢀ and
2Hþ,Iꢀ, the carboxylate adducts consecutively react to the
respective bicarbonate salts upon workup under nonabso-
lute conditions. This hydrolysis was also observed for
“DMC” and “B” chemical pathways in the particular case
of 2Hþ,Iꢀ. The absence of oxidation peaks at Epa = 0.39
and 0.75 V/SCE proves that these products are not con-
taminated by iodide impurities which are known to inhibit
certain organometallic catalysts.
4Hþ,BF4ꢀ
ꢀ
4Hþ,BF4
a E: Electrochemical (rt, 1 atm CO2); values in brackets correspond to
number of Faradays consumed per mole of imidazolium salts; B:
deprotonation with a strong base; DMC: N-methylation and C-carboꢀx-
ylation using dimethylcarbonate. b Mixture of CO2 and HCO3
.
ꢀ
c Electrolysis performed at ꢀ40 °C. d Performed in two steps (reduction
then bubbling of CO2).
Very few X-ray structure analyses of imidazolium bi-
carbonate salts have been reported to date.4,20 Numerous
short contacts (<sum of van der Waals radii) are observed
in the crystal. In particular, HCO3ꢀ interacts with the slightly
acidic H1, H8A, and H9A protons of the benzimidazolium
moiety (C1ꢀH1 O3 = 3.094(2), 142°;C8ꢀH8A O1 =
3 3 3
3 3 3
3.343(2), 160°; C9ꢀH9A O1 = 3.181(2), 122°) and forms
3 3 3
a common H-bridged dimer with a second bicarbonate
molecule (not shown).
Indeed, after keeping the electrolyzed solution under a
CO2 atmosphere during the night, single crystals of 2Hþ,
ꢀ
HCO3 suitable for X-ray diffraction analysis were ob-
tained (Figure 3).
Figure 3. Ortep view of 2Hþ,HCO3ꢀ. Thermal ellipsoids are scaled
to the 50% probability level.
(20) (a) Gurau, G.; Rodrıguez, H.; Kelley, S. P.; Janiczek, P.; Kalb,
R. S.; Rogers, R. D. Angew. Chem., Int. Ed. 2011, 50, 12024. (b) Abu-
€
Rayyan, A.; Abu-Salem, Q.; Kuhn, N.; Maichle-Moßmer, C.; Steimann,
Due to the absence of 2CO2 upon electrochemical
reduction of 2Hþ,Iꢀ under CO2 at rt, the same reaction
was performed at ꢀ40 °C. Under these conditions, 2CO2
M.; Henkel, G. Z. Anorg. Allg. Chem. 2008, 634, 823. (c) Choi, Y.-S.;
Shim, Y. N.; Lee, J.; Yoon, J. H.; Hong, C. S.; Cheong, M.; Kim, H. S.;
Jang, H. G.; Lee, J. S. Appl. Catal. A: Gen. 2011, 404, 87. (d) Bridges,
N. J.; Hines, C. C.; Smiglak, M.; Rogers, R. D. Chem.;Eur. J. 2007, 13,
5207.
ꢀ
was still not detected but 2Hþ,HCO3 was produced in
4412
Org. Lett., Vol. 15, No. 17, 2013

identified by X-ray diffraction analyses. The chloride
counteranion necessarily comes from the deuterated sol-
vent which is known to contain a significant level of free
chloride anion.
Scheme 2. Electrochemical Formation of the Neutral σ-Dimer
Common chemical “B” and “DMC” pathways were
then undertaken to compare the efficiency of these well-
known methods with the electrochemical pathway (Table 1).
It should be mentioned that the “DMC” method cannot be
performed with compound 4Hþ,BF4ꢀ due to the presence of
fully substituted N-atoms, demonstrating in this particular
case its limitation. Comparable yields were obtained support-
ing the high interest in electrosynthesis. Moreover, deproto-
nation of imidazolium salts with tBuOK only works with
2Hþꢀ4Hþ. With 1Hþ, a stronger base such as NaH is
needed to generate the intermediate carbene demonstrating
the lack of universality for the chemical method.
In conclusion, electrosynthesis of imidazolium carbox-
ylates has been achieved for the first time. This very simple,
versatile, environmentally benign (recycling of the support-
ing electrolyte provided), safe, and novel route proceeds at
room temperature and atmospheric pressure of CO2. It leads
to the synthesis of pure carboxylate or hydrogenocarbonate
compounds in yields comparable to those of the common
DMC or the basic deprotonation pathway, but with a
broader substrate spectrum. We now want to extend the
scope of this method to more unusual azolium salts.
66% yield along with a white deposit which covered the
platinum working electrode. This intriguing solid was
directly dissolved in deuterated benzene for further
1
NMR analyses. Its initial H NMR spectrum was very
simple, denoting the high symmetry of this compound. Its
molecular structure can be unambiguously attributed to
the σ-dimer depicted in Scheme 2.21 Noteworthy, this
compound corresponds to the bis-aminal of glyoxal with
N,N0-dimethyl-1,2-phenylenediamine. Two multiplets cen-
tered at 6.80 (4 H6, see attribution in Scheme 2) and
6.33 ppm (4 H5) and two singlets located at 4.17 (2 H2)
and 2.48 ppm (12 H(Me)) are observed.
The 1H,13C HSQC experiment reveals a correlation
between the H2 signal at 4.17 ppm and the C2 signal at
91.1 ppm proving that this proton is bound to a C-atom.
1
1
Moreover, the Hꢀ H NOESY experiment confirms the
spacial proximity of this H2 proton (4.17 ppm) and the
N-methyl group(2.48ppm) aswellas thatofthe proton H5
(6.33 ppm) and the N-methyl group (2.48 ppm). Addition-
ally, the doublet (4J(Me/H2) = 0.4 Hz, 4.28 ppm) initially
observed for methyl protons of 2Hþ,Iꢀ is absent on this
electrogenerated product which indicates a disruption of
the conjugation between these protons. This rare radicalꢀ
radical coupling product has been predicted to exhibit the
lowest energy conformation by quantum chemical
calculations.22 Thus the absence of 2ꢀCO2 stems from its
subsequent hydrolysis to 2Hþ,HCO3 and also from the
direct coupling of the electrogenerated intermediate radi-
cal. The σ-dimer is not stable under air over a longer period
(<1 h) and thus underwent a consecutive oxidation reac-
tion in CDCl3, finally providing 2Hþ,Clꢀ as the major
product which crystallized in the NMR tube and was
Acknowledgment. The authors would like to thank the
Centre National de la Recherche Scientifique, the Conseil
ꢀ
Regional de Bourgogne, and the Universite de Bourgogne
for their financial support. G.D.R. acknowledges a PhD
ꢀ
ꢁ
support grant from the Ministere de l’Enseignement
Superieur et de la Recherche. D.K. thanks the BMBF
ꢀ
€
and MWK Baden-Wurttemberg for financial support
(“Professorinnenprogramm”) and Verena Gierz (Eberhard
€
Karls Universitat Tubingen) for providing a sample of
€
ꢀ
4Hþ,BF4
.
1
Supporting Information Available. Copies of H, 13C,
and 2D NMR spectra for all new compounds, details on
experimental procedures, characterization data, HR-ESI
mass spectra, IR spectra, X-ray structures of 2Hþ,
HCO3 and 2Hþ,Clꢀ. This material is available free of
ꢀ
(21) (a) Lane, G. H. Electrochim. Acta 2012, 83, 513. (b) Feroci, M.
Int. J. Org. Chem. 2011, 1, 191.
(22) Kroon, M. C.; Buijs, W.; Peters, C. J.; Witkamp, G.-J. Green
charge via the Internet at http://pubs.acs.org.
Chem. 2006, 8, 241.
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 17, 2013
4413