On the reactivity and stability of electrogenerated N-heterocyclic carbene in parent 1-butyl-3-methyl-1H-imidazolium tetrafluoroborate: Formation and use of N-heterocyclic carbene-CO2 adduct as latent catalyst

Article abstract of DOI:10.1016/j.electacta.2013.07.115

A simple electrolysis (under galvanostatic conditions) of the room temperature ionic liquid 1-butyl-3-methyl-1H-imidazolium tetrafluoroborate, BMIm-BF₄, yields, after bubbling CO₂ into the catholyte, theadduct NHC-CO₂. The considerable stability of this NHC-CO ₂ adduct, at room temperature, in the parentionic liquid as solvent, has been compared with the one of free NHC in the same BMIm-BF4. The BMIm-BF4solution containing NHC-CO₂ adduct, suitably triggered (US irradiation or 120 °C), is able to releasefree NHC. The NHC-CO₂ adduct usefulness has been demonstrated using it as efficient latent catalyst, inBMIm-BF₄as solvent, in the benzoin condensation and in the oxidative esterification of cinnamaldehydewith benzyl alcohol.

Full text of DOI:10.1016/j.electacta.2013.07.115

Electrochimica Acta 109 (2013) 95–101
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
On the reactivity and stability of electrogenerated N-heterocyclic
carbene in parent 1-butyl-3-methyl-1H-imidazolium
tetrafluoroborate: Formation and use of N-heterocyclic carbene-CO2
adduct as latent catalyst
Marta Ferocia,∗, Isabella Chiarottoa, Stefano Vecchio Cipriotia, Achille Inesib,∗∗
a Department of SBAI, Sapienza University of Rome, via Castro Laurenziano, 7, I-00161 Rome, Italy
b Via Antelao, 9, I-00141 Rome, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 May 2013
Received in revised form 19 July 2013
Accepted 19 July 2013
Available online 27 July 2013
A simple electrolysis (under galvanostatic conditions) of the room temperature ionic liquid 1-butyl-3-
methyl-1H-imidazolium tetrafluoroborate, BMIm-BF4, yields, after bubbling CO2 into the catholyte, the
adduct NHC-CO2. The considerable stability of this NHC-CO2 adduct, at room temperature, in the parent
ionic liquid as solvent, has been compared with the one of free NHC in the same BMIm-BF4. The BMIm-
BF4 solution containing NHC-CO2 adduct, suitably triggered (US irradiation or 120 ◦C), is able to release
free NHC. The NHC-CO2 adduct usefulness has been demonstrated using it as efficient latent catalyst, in
BMIm-BF4 as solvent, in the benzoin condensation and in the oxidative esterification of cinnamaldehyde
with benzyl alcohol.
Keywords:
N-heterocyclic carbene
NHC-CO2 adduct
Cyclic voltammetry
Cathodic reduction
Carbon dioxide
© 2013 Elsevier Ltd. All rights reserved.
1. Introduction
reaction; moreover solvent and temperature could play a remark-
able role. Hypothesis of possible correlations have been reported
The synthesis of efficient catalysts and/or the set up of latent
catalysts (i.e. of systems, stable at room temperature and able, suit-
ably triggered, to release the catalyst) [1,2] may be regarded as a
significant target in modern organic chemistry.
by different authors [24–26].
Although some NHCs have been synthesized and isolated
[6,7,27,28], as a general rule NHCs (air and moisture sensitive) are
notably unstable even at room temperature and any hypothesis of
storage of NHCs can be ruled out. Therefore, in the NHCs catalyzed
syntheses, the generation in situ of the catalyst is often required.
On this subject, NHCs can be obtained from suitable pre-catalysts
(i.e. the parent azolium salts: imidazolium, thiazolium, oxazolium,
etc.) in organic solvents, in the presence of proper bases or reducing
agents (Scheme 1, reactions 1 and 2). Nevertheless, the use of bases
and reducing agents could be inconsistent with acidic substrates
or substrates containing reducible functional groups. To circum-
vent this difficulty, as well as to avoid the utilization of volatile and
toxic organic solvents, NHCs have been obtained by us via cathodic
reduction of imidazolium room temperature ionic liquids (RTILs)
in the absence of organic solvents, bases, etc. (Scheme 1, reaction
3). The double role of solvent and pre-catalyst of the imidazolium
ionic liquids has been frequently emphasized [29–37].
Since the pioneering investigations of the last century [3–7],
N-heterocyclic carbenes (NHCs) have been frequently utilized as
ligands in organometallic catalysis [8–10] and as organocatalysts
[11–17,9] in organic synthesis. Owing to the ability of NHCs to
induce the inversion of the polarity (umpolung) in the structure
of aldehydes, considerable results have been achieved in NHC cat-
alyzed procedures of carbon-carbon bond formation via generation
of NHC-aldehyde adduct (Breslow intermediate): benzoin conden-
sation, Stetter reaction, 1,2-addition reactions, etc. [11–13].
At present, considerable investigations are carried out to syn-
thesize further, more efficient NHCs, as well as to identify new
and more intriguing applications [18–23]. The structure of a
NHC strongly affects its efficiency as organocatalyst in a peculiar
As an alternative route, the possible set up of systems able to
catch and release NHCs has been investigated [38,39]. Intriguing
results have been obtained studying the reactivity of NHCs versus
carbon dioxide. NHC-CO2 adducts (imidazolium carboxylate, sta-
ble at room temperature) have been obtained, in organic solvents,
∗
Corresponding author. Tel.: +39 06 49766563; fax: +39 06 49766749.
Corresponding author.
E-mail addresses: marta.feroci@uniroma1.it (M. Feroci),
∗∗
achille.inesi@uniroma1.it (A. Inesi).
0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2013.07.115

96
M. Feroci et al. / Electrochimica Acta 109 (2013) 95–101
pre-catalyst as the most extensively utilized RTIL in organic syn-
base
Y
thesis) and to control the stability of this adduct in RTIL compared
to the one of the free NHC (i.e. 1-butyl-3-methylimidazol-2-
ylidene) in RTIL;
(1)
X
N
X
N
N
N
organic solvent
(C2 deprotonation)
R
R
R
R
R
R
H
H
H
- to verify the possible utilization of a CO2-saturated electrolyzed
BMIm-BF4 solution as a system able to catch electrogenerated
NHC (as stable, at room temperature, NHC-CO2 adduct) and to
release free NHC when heated at a suitable temperature, in the
absence of any base and organic solvent (Scheme 3, reaction 2);
- to ascertain the possible utilization of NHC-CO2 adduct as a latent
catalyst in NHC catalyzed reactions (e.g. the benzoin condensa-
tion – Scheme 3, reaction 3-and the oxidative esterification of
cinnamaldehyde – Scheme 3, reaction 4).
K
Y
Y
X
X
N
N
X
X
(2)
(3)
organic solvent
(reduction)
+ e
_
1
2
+
H2
The behavior of this electrogenerated system has been studied
by cyclic voltammetry, thermogravimetry and chemical reactivity,
using the NHC both as reagent and as catalyst.
cathodic reduction
2. Experimental
X: N-R, S, O
Y: BF4, PF6, Cl, Br, CH3SO4
2.1. Starting material
Ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate
(IoLiTec) was commercially available and used as pure compound
(impurities reported: water < 0.02%, chloride < 100 mg/kg), after
being kept at reduced pressure at 70 ◦C for 24 h. Sulfur, cinnamalde-
hyde and benzyl alcohol (Aldrich) were commercially available and
used as received. Benzaldehyde (Aldrich) was commercially avail-
able and used after distillation.
Scheme 1. Chemical and electrochemical generation of N-heterocyclic carbenes.
by simple bubbling CO2 in a solution containing imidazolium salt
and a suitable base (i.e. containing NHCs). NHC-CO2 adducts have
been isolated as white solids, stable at room temperature, and able,
heated at proper temperature, to release NHCs and carbon dioxide
(Scheme 2).
Therefore, carbon dioxide is able to catch NHC, at room tem-
perature, yielding a stable NHC-CO2 adduct, and to release NHC if
heated at higher temperature.
2.2. Instrumentation
1H and 13C NMR spectra were recorded using a Bruker AC
200 spectrometer using CDCl3 as internal standard. Voltammet-
ric measurements were performed with an Amel 552 potentiostat
equipped with an Amel 566 function generator and an Amel 563
multipurpose unit in a three-electrode cell; the curves were dis-
played on an Amel 863 recorder. A 492/GC/3 Amel microelectrode
was employed, using a Pt counter electrode and an Ag quasi refer-
ence electrode (q.r.e.), ꢀ = 0.2 V s−1. Electrolyses under galvanostatic
control were carried out (using an Amel 552 potentiostat equipped
with an Amel 771 integrator) in a two compartment home-made
glass cell. The anolyte and catholyte (1.0 and 2.0 ml, respectively)
were separated by a glass disk (porosity 3). The electrode sur-
face areas (flat Pt spirals, 99.9%) were ca 1.0 cm2 for the cathode
and 0.8 cm2 for the anode. The cell was kept at 60 ◦C, if not oth-
erwise specified. Infrared (IR) spectra were recorded on a Perkin
Elmer Spectrum-One spectrophotometer equipped with an ATR
detector, band frequencies are given in wave number (cm−1). Ther-
mogravimetry (TG) experiments were carried out at a heating rate
of 10 K min−1 on a Stanton-Redcroft 625 simultaneous apparatus at
flow rate of 50 ml min−1 using open cylindrical aluminum crucibles
with a cross sectional area of 2.0 × 10−5 m2.
The NHC-CO2 adducts could be regarded as “latent pre-
catalysts”. The stability of the adduct and the release-temperature,
extensively investigated via thermogravimetric analysis (TGA), are
strongly affected by the structure of the NHC and, for the NHC-
CO2/organic solvent solutions, by the nature of the solvent [38,39].
Recently, syntheses of cyclic carbonates, via reaction of carbon
dioxide (2.0–4.5 M Pa) and epoxides or propargylic alcohol, have
been reported [40–42]. The syntheses have been carried out in the
presence of NHC-CO2 adducts and the role of the adduct versus the
one of free NHC as catalyst has been discussed [43]. The utilization
of imidazolium hydrogen carbonate versus imidazolium carboxyl-
ate as organic precatalysts for NHC catalyzed reactions carried out
in organic solvents has been performed [44]. Last, the formation of
dialkyl carbonate from carbon dioxide and alcohols, in the presence
of electrogenerated NHC has been hypothesized [45].
Spurred from these results, as a continuation of our previous
investigations concerning the use, as catalysts, of electrogenerated
NHCs in the parent ionic liquid, we have studied the reactiv-
ity of CO2 in a pre-electrolyzed 1-butyl-3-methyl-1H-imidazolium
tetrafluoroborate BMIm-BF4 (i.e. RTIL 1a containing NHC 1b;
Scheme 3, reaction 1).
Aim of this investigation was:
2.3. Electrochemical synthesis of NHC and NHC-CO2 in BMIm-BF4
- to prove the actual formation of the NHC-CO2 adduct by simple
bubbling of CO2 in pre-electrolyzed BMIm-BF4 (chosen as model
A galvanostatic electrolysis was carried out on BMIm-BF4
(catholyte: 2 ml; anolyte: 1 ml) under N2 atmosphere at 60 ◦C.
The current density was 20 mA cm−2. After 97 C, the current was
switched off, the anodic compartment removed and the catholyte
kept as it was (obtaining the NHC-BMIm-BF4 solution) or CO2
was bubbled into the catholyte for 15 min (obtaining the NHC-
CO2-BMIm-BF4 solution). The two solutions were kept at room
temperature under air for the time reported in Table 1 or used
immediately after their preparation (Tables 2 and 3).
+ CO2
N
N
N
N
R
R
R
R
- CO2
CO2
Scheme 2. Catch and release of carbon dioxide by N-heterocyclic carbenes.

M. Feroci et al. / Electrochimica Acta 109 (2013) 95–101
97
(1)
(2)
BMIm-BF4
_
1
N
N
N
+
e
+
H2
N
N
N
2
Bu
Bu
Bu
Me
Me
H
1a
1b
+ CO2
BMIm-BF4
, - CO2
N
N
N
N
Bu
Bu
Me
2
BMIm-BF4
Me
Me
CO2
2
O
O
BMIm-BF4, 2
Ph
(3)
(4)
Ph
H
Ph
OH
O
O
BMIm-BF4, 2
+
Ph
H
Ph
OH
Ph
O
Ph
N
N
BMIm-BF4, 1:
Bu
Me
BF4
H
Scheme 3. Electrochemical generation of NHC-CO2 adduct and its use as latent catalyst.
Table 2
Reactivity of benzaldehyde with 2-BMIm-BF4 solutions under different activation
temperatures: benzoin condensation.a
Release conditionsb
Benzoin (yields, %)c
T = 60 ◦
C
C
C
–
49
75
85
72
63
80
T = 100 ◦
T = 120 ◦
T = 65 ◦C, literatured
T = 100 ◦C, literatured
US irradiation
Table 1
Reactivity of elemental sulfur with BMIm-BF4 solutions containing 1b (sample A) or
US irradiation, literatured
2 (sample B), freshly prepared or used after the time interval ꢁt*.a
a
Galvanostatic reduction of BMIm-BF4 (193 C), CO2 bubbling in the catholyte
(15 min), then, under open air and moisture conditions, 10 mmol of benzaldehyde
were added and the catholyte kept at the indicated temperature for 1 h.
Sampleb
ꢁt*/dayc
Products (yields, %)d
Imidazole-2-thione
Imidazole-2-one
b
Temperature at which the catholyte was kept for 1 h after the addition of benzal-
dehyde; in the case of ultrasound irradiation, the catholyte was irradiated for 30 min
at 22.5 kHz, 4 W.
A
B
A
B
A
B
A
B
A
B
A
B
0e
0
1
1
2
2
3
3
4
4
7
7
99
99
89
98
56
95
28
87
11
71
3
–
–
11
–
10
–
12
–
12
–
11
9
c
Yields of isolated products, based on starting benzaldehyde.
Yields taken from our published work [33].
d
Table 3
Reactivity of cinnamaldehyde and benzyl alcohol with adduct 2-BMIm-BF4 solutions
under two different activation conditions: oxidative esterification reaction.a
28
Release conditions
Ester (yields, %)b
a
Galvanostatic reduction of BMIm-BF4 (97 C), CO2 bubbling in the catholyte
(15 min), then, under open air and moisture conditions, 0.5 mmol of S8 were added
and the catholyte sonicated for 10 min.
T = 120 ◦C, 0.7 F/mol
65
91
T = 60 ◦C, 0.7 F/mol, literaturec
b
a
Sample A: electrolyzed BMIm-BF4; sample B: electrolyzed BMIm-BF4, subse-
Galvanostatic reduction of BMIm-BF4 (70 C, 0.7 F/mol aldehyde), CO2 bubbling
in the catholyte (15 min), then, under open air and moisture conditions, 1 mmol of
cinnamaldehyde and 3 mmol of benzyl alcohol were added and the catholyte kept
at the indicated temperature for 1 h.
quently saturated with CO2.
c
Days passed between the preparation of samples A and B and their use in this
synthesis (see Section 2).
d
b
Yields of isolated products, based on starting sulfur.
This result is identical to the one obtained by us in the published paper [51].
Yields of isolated benzyl hydrocinnamate, based on starting cinnamaldehyde.
Yields taken from our published work [31].
e
c

98
M. Feroci et al. / Electrochimica Acta 109 (2013) 95–101
Fig. 2. Cyclic voltammetric curves (vitreous carbon cathode, ꢀ = 0.2 V s−1, Ag as pseu-
doreference electrode, T = 60 ◦C) of BMIm-BF4 (curve a) and of BMIm-BF4 after 15 min
CO2 bubbling (curve b). At E < −2.0 V, the current reaches the upper limit of the
instrument.
Fig. 1. Cyclic voltammetric curves of BMIm-BF4 (vitreous carbon cathode,
ꢀ = 0.2 V s−1 Ag as pseudoreference electrode, T = 60 ◦C). Scanning reversed at
,
E = −2.5 V (curve a) or E = −1.8 V (curve b). At E < −2.0 V, the current reaches the
upper limit of the instrument.
as well as in organic solvents, yielding the electroinactive adduct
NHC-CO2 2 (Scheme 3, reaction 2). The stability of the NHC-CO2
adducts, i.e. their ability to relase free NHC and CO2, is strongly
affected by the temperature as well as by the structure of the NHC
and by the solvent (as previously reported) [38,39].
To inquire the stability, in the parent BMIm-BF4, of the adduct
2 with the temperature, we have compared the voltammetric
curves of BMIm-BF4 (at different temperatures) with those of CO2
–saturated BMIm-BF4.
2.4. Synthesis of 1-butyl-3-methylimidazole-2-thione
The NHC-BMIm-BF4 or NHC-CO2-BMIm-BF4 solution was added
to 0.5 mmol of elemental sulfur and the mixture subjected to ultra-
sound irradiation (4 W) for 10 min. Then usual workup [49] gave
the product in the yields reported in Table 1.
2.5. Benzoin condensation
In Fig. 3 the voltammetric curves for CO2 saturated BMIm-
BF4 solutions, recorded at different temperatures (60–120 ◦C) have
been reported. The ip∗ox. current increases on increasing of the tem-
perature. At T > 120 ◦C, the values of ipo∗x. and ipox. (not in Fig. 3) are
equal. As the decrease of ip∗ox. (with respect to ipox.) is related to
the formation of the electroinactive adduct 2, the catch of CO2 by
electrogenerated 1b in BMIm-BF4 is reversible and the adduct is
able to relase completely free NHC and CO2 at T > 120 ◦C (Scheme 3,
reaction 2).
The NHC-CO2-BMIm-BF4 solution was added to 5.0 mmol of
freshly distilled benzaldehyde. The solution was kept for 2 h under
stirring at 60 ◦C (or 100 ◦C or 120 ◦C). Otherwise the mixture sub-
jected to ultrasound irradiation (22.5 kHz, 4 W) for 30 min. Then
usual workup [33,36] gave the product in the yields reported in
Table 2.
2.6. Oxidative esterification of cinnamaldehyde
The NHC-CO2-BMIm-BF4 solution (after the consumption of
70 C) was added to 1.0 mmol of cinnamaldehyde and 3.0 mmol
of benzyl alcohol. The solution was kept for 1 h under stirring at
120 ◦C. Then usual workup [31] gave the product in the yields
reported in Table 3.
3.2. Analysis of electrolyzed BMIm-BF4 and of CO2-saturated
electrolyzed BMIm-BF4
To prove the initial hypothesis (i.e. the formation of the adduct
2), IR spectra have been recorded for a sample of BMIm-BF4
3. Results and discussion
3.1. Voltammetric behavior of BMIm-BF4: the role of CO2 and of
temperature
The voltammetric curves recorded at a glassy carbon cathode for
BMIm-BF4 under N2 bubbling (ꢀ = 0.2 V s−1, potential window from
0.0 V to −2.5 V, Ag pseudo reference electrode [46,47], 60 ◦C) show
a rapid increase in the cathodic current (Ered. ≤ −2.1 V) and an oxi-
dation peak (Epox. = −0.3 V) related to the reduction of the BMIm+
cation 1a to NHC 1b (Scheme 3, reaction 1) and to the oxidation
of 1b (electrogenerated at Epred) [48], respectively. This hypothesis
is confirmed by the fact that if the scanning is reversed at a less
negative potential than −2.1 V, the oxidation peak Epox. disappears
(Fig. 1).
The voltammetric curve of CO2-saturated BMIm-BF4 shows a
value of the current of the oxidation peak (ip∗ox.) significantly
smaller than the one (ipox.) in the absence of CO2 (Fig. 2).
These results suggest that the reaction of electrogenerated 1b
versus carbon dioxide in the parent ionic liquid can be operative
Fig. 3. Cyclic voltammetric curves (vitreous carbon cathode, ꢀ = 0.2 V s−1, Ag as pseu-
doreference electrode,) of BMIm-BF4 after 15 min CO2 bubbling. Curve a: T = 60 ◦C;
curve b: T = 80 ◦C; curve c: T = 100 ◦C; curve d: T = 120 ◦C. At E < −2.0 V, the current
reaches the upper limit of the instrument.

M. Feroci et al. / Electrochimica Acta 109 (2013) 95–101
99
of CO2, similarly to what it was found for other imidazolinium-2-
carboxylate derivatives [49,50].
3.3. Reactivity of electrolyzed BMIm-BF4 and of CO2-saturated
electrolyzed BMIm-BF4
The effective reactivity and stability of this particular NHC-CO2
adduct in BMIm-BF4 can be demonstrated using it as latent cata-
lyst, which releases NHC upon thermal or ultrasound activation;
the released NHC can be used as reagent or as catalyst in classical
chemical reactions.
Recently, we have reported that the reaction of NHCs, elec-
trogenerated via electrolysis of the parent 1,3-dialkylimidazolium
ionic liquid, with elemental sulfur under ultrasound irradia-
tion yields 1,3-dialkylimidazole-2-thiones in very high yields
(Scheme 4) [51].
According, to realize a comparison between the stability of
adduct 2 and free carbene 1b (both in BMIm-BF4), we have inves-
tigated the reactivity of sulfur with an electrolyzed BMIm-BF4
solution (containing 1b, sample A) and with an electrolyzed BMIm-
BF4 solution subsequently saturated with CO2 (containing the
adduct 2, sample B).
The two solutions were prepared and used after prefixed inter-
vals of time (ꢁt*); to them S8 was added and the mixture was
subjected to ultrasound irradiation. Usual workup gave the cor-
responding imidazole-2-thione. The yields of isolated thione are
strongly affected by the nature of the initial solution (thus contain-
ing 1b or 2) and by the time interval ꢁt* between the preparation
of the solution and its use (Table 1), thus reflecting the stability of
the reagent in the parent ionic liquid.
Fig. 4. IR spectra of BMIm-BF4 (curve a) and of electrolyzed BMIm-BF4 and subse-
quently saturated with CO2 (curve b).
From both the solutions (samples A and B) the corresponding
imidazole-2-thione was obtained. Nevertheless, the yield of thione
isolated from sample A fast decreased on increasing ꢁt*, according
to a significant instability of 1b in the parent ionic liquid at room
temperature (isolated imidazole-2-thione: 11%, ꢁt* = 4 days; 3%,
ꢁt* = 7 days). On the contrary, the decrease with ꢁt* of thione iso-
lated from the sample B is slow, according to a considerable stability
of the adduct 2 in the parent ionic liquid at room temperature.
The adduct is able (conveniently trigged) to release 1b with ele-
vated yields (71% of isolated imidazole-2-thione, ꢁt* = 4 days). As
the two solutions were stored without any precaution to preserve
them by air and moisture (in order to demonstrate the ability of the
ionic liquid solvent to protect the reagent because of its high vis-
cosity), imidazole-2-one was isolated in addition to the formation
of imidazole-2-thione in some cases, due to the reaction of free
carbene with oxygen [51]. This is the demonstration that no free
carbene is present in the sample with the adduct (no imidazole-2-
one formation within the first four days), confirming the stability
of 2 in BMIm-BF4. These results show that it is possible to produce
a 1b-BMIm-BF4 solution and protect the carbene under the adduct
2 form, in order to use it in a later time, without loss of reactivity
for the first 2–3 days (period in which the 1b-BMIm-BF4 solutions
lose more than half of their reactivity).
Fig. 5. TGA of BMIm-BF4 (curve a) and of electrolyzed BMIm-BF4 and subsequently
saturated with CO2 (curve b). Heating rate: 10 K min−1
.
electrolyzed and subsequently bubbled with CO2 for 15 min and
compared with the one of BMIm-BF4. The first sample displays the
distinct carbonyl stretching frequency of adduct 2, ꢀ = 1669 cm−1
,
absent in BMIm-BF4 (Fig. 4) [38].
In addition, electrolyzed BMIm-BF4 solution subsequently satu-
rated with CO2 (containing the adduct 2) and pure BMIm-BF4 were
monitored via thermogravimetric analysis (TGA) (Fig. 5). Starting
from about 120 ◦C the electrolyzed sample begins to decompose
in several steps, the first of which is ascribable to the evolution
In order to study the reactivity of 1b released from adduct 2
(in BMIm-BF4) as organocatalyst, we have synthesized the adduct
in ionic liquid and let it react with benzaldehyde in the benzoin
reaction (Scheme 3, reaction 3). The yields in benzoin rely on the
procedure of release of the catalyst (Table 2), indicating again that
+ S8
+ e
N
N
N
N
N
N
Bu
Bu
Bu
Me
Me
Me
or US
BMIm-BF4
H
S
Scheme 4. Electrogeneration and reaction of N-heterocyclic carbene with elemental sulfur.

100
M. Feroci et al. / Electrochimica Acta 109 (2013) 95–101
for the optimal release, a temperature of 100 ◦C or higher is neces-
sary, as a confirmation of the findings of TGA results (see Fig. 5).
The yields in benzoin (not optimized) are comparable to those
obtained in a previous work using a BMIm-BF4 solution of free car-
bene, indicating a good yield in the release of 1b from adduct 2
[33,36].
To confirm the ability of 2 to behave as latent organocatalyst,
we have used it in the oxidative esterification of cinnamaldehyde
with benzyl alcohol, reaction in which benzyl hydrocinnamic ester
is formed via umpolung of the aldehyde and internal reduction of
the double bond catalyzed by 1b (Scheme 3, reaction 4) [31]. The
results are reported in Table 3.
The not optimized yield, reported in this paper (electrogener-
ated NHC-CO2 adduct as latent catalyst), is smaller than the one
reported by us in a previous work (free electrogenerated NHC as
catalyst; 65% versus 91%) [31]. Nevertheless, this last result sug-
gests a possible general utilization of electrogenerated NHC-CO2
adduct as latent catalyst in the NHC-catalyzed organic reactions (it
should be noted, however, that the literature yields are relative to
reactions carried out at 60 ◦C, while the yield of this paper – first
line of Table 3 – is for a reaction carried out at 120 ◦C, so the yields
are not strictly comparable).
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4. Conclusions
The stability of electrogenerated N-heterocyclic carbene in the
parent ionic liquid BMIm-BF4 (studied by cyclic voltammetry) can
be enhanced by reaction with carbon dioxide to form an NHC-CO2
adduct. In this way a latent catalyst is obtained, able to re-produce
NHC upon thermal or ultrasonic stimulation. The production of NHC
from the adduct has been confirmed by reaction with elemental
sulfur to yield the corresponding imidazole-2-thione. A compari-
son between the behavior of an adduct-containing BMIm-BF4 and
an NHC-containing BMIm-BF4 has been carried out, both of freshly
prepared and of aged (days) solutions and the higher stability of the
adduct solution confirmed. This latent catalyst has been employed
in two exemplifying organocatalyzed reactions, the benzoin con-
densation and the oxidative esterification of cinnamaldehyde with
benzyl alcohol, showing the ability of the re-generated NHC to effi-
ciently catalyze these reactions.
Supplementary materials
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aldehydes in ionic liquids. The electrochemical route, Electrochimica Acta 89
(2013) 692.
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ionic liquid catalyzed by electrogenerated N-heterocyclic carbenes. Synthe-
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Communications 48 (2012) 5361.
Supplementary materials are available with the on-line version
only.
Acknowledgements
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The authors thank Mr. Marco Di Pilato for his support to the
experimental part of this work and Miur (Italy) for financial sup-
port.
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