Electrochemical behaviour of 9-methylcaffeinium iodide and in situ electrochemical synthesis of hymeniacidin

Article abstract of DOI:10.1016/j.electacta.2018.05.073

9-Methylcaffeinium iodide, a bio-based salt obtained by reaction of caffeine with methyl iodide, is an imidazolium salt. The electrochemical behaviour of 9-methylcaffeinium iodide was studied by means of cyclic voltammetry, differential pulse voltammetry and electrolysis. Its behaviour revealed to be very similar to that of common imidazolium salts. In fact, its cathodic reduction yielded the corresponding N-heterocyclic carbene, which was evidenced by its reaction products with dioxygen and with sulfur, although in low amounts. In fact, this electrogenerated carbene was very unstable and prone to add water, yielding a ring opening product (hymeniacidin) in high yield. Hymeniacidin is a natural product from the marine sponge Hymeniacidon sp. The voltammetric behaviour of isolated hymeniacidin confirmed the in situ formation of this ring opening product, by comparison of the voltammetric peak potentials of starting caffeinium salt and hymeniacidin. This study allowed to determine that hymeniacidin derives from NHC, and not by hydrolysis of the caffeinium salt.

Full text of DOI:10.1016/j.electacta.2018.05.073

Electrochimica Acta 280 (2018) 71e76
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journal homepage: www.elsevier.com/locate/electacta
Electrochemical behaviour of 9-methylcaffeinium iodide and in situ
*
electrochemical synthesis of hymeniacidin
a
a
b
,
**
, Marta Feroci a
, *
Fabiana Pandolfi , Leonardo Mattiello , Daniela Zane
a
Dept. Basic and Applied Sciences for Engineering (SBAI), Sapienza University of Rome, via Castro Laurenziano, 7, 00161, Rome, Italy
CNR ISMN Istituto per lo Studio dei Materiali Nanostrutturati, UOS Sapienza, Via del Castro Laurenziano 7, 00161 Roma, Italy
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 April 2018
Received in revised form
9-Methylcaffeinium iodide, a bio-based salt obtained by reaction of caffeine with methyl iodide, is an
imidazolium salt. The electrochemical behaviour of 9-methylcaffeinium iodide was studied by means of
cyclic voltammetry, differential pulse voltammetry and electrolysis. Its behaviour revealed to be very
similar to that of common imidazolium salts. In fact, its cathodic reduction yielded the corresponding N-
heterocyclic carbene, which was evidenced by its reaction products with dioxygen and with sulfur,
although in low amounts. In fact, this electrogenerated carbene was very unstable and prone to add
water, yielding a ring opening product (hymeniacidin) in high yield. Hymeniacidin is a natural product
from the marine sponge Hymeniacidon sp. The voltammetric behaviour of isolated hymeniacidin
confirmed the in situ formation of this ring opening product, by comparison of the voltammetric peak
potentials of starting caffeinium salt and hymeniacidin. This study allowed to determine that hyme-
niacidin derives from NHC, and not by hydrolysis of the caffeinium salt.
10 May 2018
Accepted 11 May 2018
Available online 22 May 2018
Keywords:
9-Methylcaffeinium iodide
Cyclic voltammetry
Differential pulse voltammetry
Electrochemical reduction
Hymeniacidin
© 2018 Published by Elsevier Ltd.
1. Introduction
leads to the formation of the corresponding carbene [2,3]. The
reactivity (as nucleophile or base) of this NHC is strongly influenced
Caffeine
(1,3,7-trimethyl-3,7-dihydro-1H-purine-2,6-dione,
by the substituents of the imidazole ring, and thus the presence of
the amide moiety of the caffeine structure could give this molecule
a particular reactivity [4,5].
Scheme 1) is a widespread natural product found in seeds, nuts,
leaves of plants from South America and East Asia [1]. Among them,
the most famous are coffee, tea, cocoa plants. Caffeine is thus a bio-
based, renewable reagent and its use as starting material in
chemical synthesis is highly recommended. Caffeine structure can
be divided into two substructures, a pyrimidinedione ring and an
imidazole ring. In particular, we envisaged in the presence of the
imidazole ring the possibility of transforming caffeine in a bio-
based imidazolium salt, which could be used in organic synthesis
as N-heterocyclic carbene (NHC) precursor (Scheme 1). In fact, the
deprotonation (chemical or electrochemical) of the CeH between
the two nitrogen atoms in the imidazole ring of an imidazolium salt
Although the electrochemical behaviour of caffeine has been
extensively studied [6] due to the importance of this molecule, no
paper on the electrochemical behaviour of the corresponding caf-
feinium salt is present in the literature. Nevertheless, caffeinium
salts are molecules known for decades and sparingly used as
chemical synthons [7e9], but mainly as precursors of ligands in Pd
[10e12], Pt [13], Ru [14], Ag [15,16], Au [17] complexes, in some
cases yielding complexes with biological activity.
In no case the free NHC derived from caffeinium salts has been
evidenced. Scope of this paper is the study of the electrochemical
behaviour of 9-methylcaffeinium iodide (1,3,7,9-tetramethyl-2,6-
dioxo-2,3,6,9-tetrahydro-1H-purin-7-ium iodide, Scheme 1) by
voltammetric techniques and electrolysis, in order to establish the
possibility to obtain the corresponding NHC by cathodic reduction.
*
Although 9-methylcaffeinium iodide, caffeine, hymeniacidin and theacrine are
not IUPAC names, we decided to use them throughout the paper due to their
widespread and common use. In any case, the corresponding IUPAC names are
reported in the paper.
2. Experimental
*
Corresponding author.
*
* Corresponding author.
E-mail addresses: daniela.zane@ismn.cnr.it (D. Zane), marta.feroci@uniroma1.it
9-Methylcaffeinium iodide (1,3,7,9-tetramethyl-2,6-dioxo-
(
M. Feroci).
2,3,6,9-tetrahydro-1H-purin-7-ium iodide) was synthesized by the
https://doi.org/10.1016/j.electacta.2018.05.073
013-4686/© 2018 Published by Elsevier Ltd.
0

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F. Pandolfi et al. / Electrochimica Acta 280 (2018) 71e76
Scheme 1. Synthesis of 9-methylcaffeinium iodide from caffeine, and possible NHC formation.
method reported in the literature [15]. Voltammetric measure-
ments were performed as previously described [18], using a 492/
GC/3 (glassy carbon) Amel microelectrode. Differential pulse vol-
tammetric (DPV) analysis was carried out using an AMEL 2551
potentiostat galvanostat, using a 492/GC/3 Amel microelectrode
and elaborating data with VApeak 2016. Constant potential elec-
166 (7%), 128 (19%), 82 (62%), 67 (100%), 42 (51%).
3. Results and discussion
4 4
The cyclic voltammetry (CV) of a DMF- 0.1 M Et NBF solution of
ꢀ
9-methylcaffeinium iodide on a GC electrode shows a cathodic peak
at - 1,76 V vs Ag/AgCl (Fig. 1A), while there is no anodic peak
attributable to the cation (Fig. 1B), only the anodic peaks relative to
iodide anion at þ 0.66 and þ 0.92 V vs Ag/AgCl [23].
trolyses were performed under a nitrogen atmosphere, at 25 C,
using an Amel 2053 potentiostat-galvanostat equipped with an
Amel 731 integrator. The cell was a divided glass one separated
through a porous glass plug filled up with a layer of gel (i.e., methyl
ꢁ
3
cellulose 0.5% vol dissolved in DMF-Et
4
NBF
spirals (apparent area 0.8 cm ) were used as both cathode and
anode. Catholyte: 10 mL of DMF-0.1 M Et NBF containing
.5 mmol of 9-methylcaffeinium iodide. Anolyte: 2 mL of DMF-
.1 M Et NBF
4
1.0 mol dm ); Pt
2
4
4
0
0
4
4
. The constant potential was kept at ꢁ2.0 V vs Ag/
AgCl. At the end of the electrolysis (98 C, 2.0 F), the solvent of the
catholyte was evaporated under reduced pressure. Flash column
chromatography (eluent: ethyl acetate-methanol 80:20) gave pu-
rified hymeniacidin (N-(1,3-dimethyl-6-(methylamino)-2,4-dioxo-
1,2,3,4-tetrahydropyrimidin-5-yl)-N-methylformamide) in 88%
yield (with respect to starting 9-methylcaffeinium iodide). Its
spectral data were in accordance with those reported in the liter-
ature [19].
When, after the electrolysis, the catholyte was treated with
elemental sulfur and oxygen under ultrasound irradiation [20],
1
(
3
,3,7,9-tetramethyl-7,9-dihydro-1H-purine-2,6,8(3H)-trione
Theacrine, NHCO [21], Fig. 3) and 1,3,7,9-tetramethyl-8-thioxo-
,7,8,9-tetrahydro-1H-purine-2,6-dione (NHCS [22], Fig. 3) were
isolated in low amount along with hymeniacidin. Their spectral
data were in accordance with those reported in the literature.
Hymeniacidin
(N-(1,3-dimethyl-6-(methylamino)-2,4-dioxo-
1
,2,3,4-tetrahydropyrimidin-5-yl)-N-methylformamide), mixture
1
of rotamers: yellow oil. H NMR (200 MHz, CD
rotamer: 7.93 (s,1H, CHO), 5.6 (bs,1H, 4-NHMe), 3.37 (s, 3H, 3-NMe),
.19 (s, 3H, 1-NMe), 2.89 (s, 3H, 5-NMe), 2.87 (d, J ¼ 5.0 Hz, 3H, 4-
NHMe); minor rotamer: 8.16 (s, 1H, CHO), 5.4 (bs, 1H, 4-NHMe),
.36 (s, 3H, 3-NMe), 3.18 (s, 3H, 1-NMe), 2.89 (s, 3H, 5-NMe), 2.83 (d,
3
CN) d, major
3
3
13
J ¼ 5.0 Hz, 3H, 4-NHMe). C NMR (50.3 MHz, CD
rotamer: 166.6 (CHO), 161.8 (C6), 153.2 (C4), 150.9 (C2), 94.8 (C5),
3.0 (5-NMe), 31.2 (4-NMe), 30.4 (3-NMe), 27.5 (1-NMe); minor
rotamer: 165.1 (CHO), 159.9 (C6), 152.5 (C4), 150.9 (C2), 92.2 (C5),
3
CN) d, major
3
3
6.6 (5-NMe), 31.2 (4-NMe), 30.5 (3-NMe), 27.5 (1-NMe). MS (ESI),
þ
m/z (M þ Na ): 248.53.
1
,3,7,9-tetramethyl-7,9-dihydro-1H-purine-2,6,8(3H)-trione
Theacrine, NHCO) and 1,3,7,9-tetramethyl-8-thioxo-3,7,8,9-
tetrahydro-1H-purine-2,6-dione (NHCS), in z8:1 mixture:
NHCO: 1H NMR (200 MHz, CDCl
, 3.74 (s, 3H), 3.65 (s, 3H),
(
3
) d
Fig. 1. CV of a DMF- 0.1 M Et NBF solution of A (red): 9-methylcaffeinium iodide
4
4
ꢁ
3
3
.60 (s, 3H), 3.39 (s, 3H). MS (EI), m/e (relative intensity): 224 (65%,
(5 ꢂ 10 M), potential scan: 0.0 to ꢁ2.5 to 0.0 V; B (violet): 0.0 to þ1.6 to 0.0 V. C
(black): 0.0 to ꢁ2.2 to þ1.6 to 0.0 V; D (blue): 0.0 to ꢁ1.5 to þ1.6 to 0.0 V. E (green):
þ.
M ), 166 (16%), 139 (19%), 82 (79%), 67 (100%), 42 (49%).
ꢁ
3
1
Hymeniacidin 1 ꢂ 10 M. Potential scan: 0.0 to ꢁ2.1 to þ1.6 to 0.0 V. GC working
NHCS: H NMR (200 MHz, CDCl
3
)
d, 3.82 (s, 3H), 3.74 (s, 3H), 3.61
ꢁ1
electrode, Ag/AgCl reference electrode;
n
¼ 0.2 V s ; RT, N
2
atmosphere. (For inter-
þ.
(
s, 3H), 3.24 (s, 3H). MS (EI), m/e (relative intensity): 240 (45%, M ),
pretation of the references to colour in this figure legend, the reader is referred to the
Web version of this article.)

F. Pandolfi et al. / Electrochimica Acta 280 (2018) 71e76
73
The electrodic process at ꢁ1.76 V generates a product which is
oxidised at þ0.05 V, as can be seen in Fig. 1CeD, in which the
negative potential limit is shifted from ꢁ1.5 V to ꢁ2.2 V; no
oxidation peak is present when the reversal potential is more
positive than the reduction peak potential of caffeinium salt.
Moreover, when caffeinium salt is reduced, a second oxidation peak
appears at þ 1.20 V (vide infra).
The electrochemical behaviour of 9-methylcaffeinium iodide is
very similar to the one of other imidazolium salts [3], for which the
cathodic reduction generates the corresponding N-heterocyclic
carbene (NHC) which, in turn, is oxidised at a potential near 0 V.
Following this hypothesis, in order to better understand the elec-
trochemical behaviour of 9-methylcaffeinium iodide, we carried
out the cathodic reduction of 9-methylcaffeinium iodide in DMF-
0
.1 M Et
4
NBF
4
solution, at the potential of ꢁ2.0 V on a Pt cathode. At
the end of the electrolysis (2.0 F), a ring opening product, hyme-
niacidin, was isolated in 88% yield (Scheme 2), whose oxidation
peak potential is þ1.20 V (Fig. 1E).
Hymeniacidin is a natural product, isolated for the first time by
Capon and coworkers [19] from the Southern Australian marine
sponge Hymeniacidon sp. Although this sponge metabolite revealed
not active against nematocides (the object of Capon study), its
biological activity is still under study.
The formation of hymeniacidin from the cathodic reduction of
9
-methylcaffeinium iodide seems to be an indirect confirmation of
ꢁ3
Fig. 2. CV of a DMF- 0.1 M Et
under N (red) or CO
(black) atmosphere. Potential scan: 0.0 to ꢁ2.1 to 1.8 to 0.0 V. GC
working electrode, Ag/AgCl reference electrode;
4
NBF
4
solution of 9-methylcaffeinium iodide (5 ꢂ 10 M)
the formation of NHC during the electrolysis. In fact, Capon [19]
reported the chemical synthesis of hymeniacidin by reaction of 9-
methylcaffeinium iodide with a base (ammonium hydroxide). The
action of the base should be the deprotonation of the CeH between
the two nitrogen atoms (in the imidazolium ring) and formation of
2
2
ꢁ
1
n
¼ 0.2 V s . RT. (For interpretation of
the references to colour in this figure legend, the reader is referred to the Web version
of this article.)
2
the corresponding NHC. NHC reaction with H O yields the ring
corresponding NHC (Scheme 2). This NHC in the presence of CO
forms an adduct NHCeCO which subtracts NHC from the oxidation
and from the reaction to hymeniacidin (Scheme 3).
A further confirmation of a NHC intermediate in this electrolysis
derives from a paper of Bredereck and coworkers [25] dated 1959,
in which the reaction between water and this NHC (represented
with a triple bond between an imidazole nitrogen atom and the
carbene carbon atom) led to the formation of hymeniacidin.
Last, the formation of NHC by electrochemical reduction of the
caffeinium salt was confirmed by the isolation of 1,3,7,9-
2
opening and hymeniacidin formation. In order to exclude that the
formation of hymeniacidin derives from mere hydrolysis of the
caffeinium salt, we studied the stability of 9-methylcaffeinium io-
2
dide in water: after 1 week in D
was evidenced.
2
O at RT, no decomposition product
Another hint on the strict correlation between NHC and
hymeniacidin formation was obtained by carrying out the cyclic
voltammetries of 9-methylcaffeinium iodide in the presence of
carbon dioxide. As seen before (Fig. 1C), when 9-methylcaffeinium
iodide is reduced (negative scan of the CV), a product is formed
which is oxidised at þ0.05 V (which we attribute to NHC) and a
tetramethyl-7,9-dihydro-1H-purine-2,6,8(3H)-trione
(NHCO,
Theacrine -a biologically active compound- Fig. 3) and 1,3,7,9-
tetramethyl-8-thioxo-3,7,8,9-tetrahydro-1H-purine-2,6-dione
second oxidation peak is evident at þ1.20 V (hymeniacidin). If CO
2
is added to the solution, the CV alters considerably (Fig. 2). In fact, if
the reversal potential is less negative than CO reduction (so that
(NHCS, Fig. 3) by reaction of electrochemically reduced 9-
2
methylcaffeinium iodide and sulfur and dioxygen. These two
compounds should derive from the corresponding NHC by reaction
with dioxygen and sulfur, as reported in the literature for other
imidazole carbenes [20].
It is thus possible to infer that the reduction of 9-
methylcaffeinium iodide leads to the formation of the corre-
sponding carbene, whose oxidation peak potential is þ0.05 V
carbon dioxide is not electrochemically reduced), a new peak is
formed during the positive scan at ꢁ0.22 V and no peak attributable
to hymeniacidin is present.
In analogy with the electrochemical behaviour of other imida-
zolium cations [24], it can be hypothesized that the cathodic
reduction of caffeinium salt leads to the formation of the
Scheme 2. Electrochemical reduction of 9-methylcaffeinium iodide yielding hymeniacidin.

74
F. Pandolfi et al. / Electrochimica Acta 280 (2018) 71e76
Scheme 3. Electrochemical reduction of 9-methylcaffeinium iodide in the presence of carbon dioxide. Hypothesis of reactivity.
O
N
O
N
N
N
N
N
O
S
N
O
O
N
NHCO
NHCS
Fig. 3. Products obtained by reaction of electrochemically reduced 9-methylcaffeinium
iodide and elemental sulfur and dioxygen.
(Fig. 1b, black curve, first oxidation peak). This carbene evolves
towards the formation of hymeniacidin, whose oxidation peak
potential is þ1.20 V (Fig. 1C, black curve, fourth oxidation peak, and
Fig. 1E).
Any attempt to carry out the benzoin condensation starting
from benzaldehyde and cathodically reduced 9-methylcaffeinium
iodide failed, confirming the unstability of the corresponding
NHC (or the too weak nucleophilicity of the carbene site).
In order to better understand and identify the oxidation peaks
appearing during the CV anodic sweep, differential pulse voltam-
metric analysis (DPV) was carried out. In this case, sharper peaks
Fig. 4. DPV scan for caffeinium iodide ( ) and hymeniacidin (C). DMF-0.1 M Et
4
NBF
4
,
ꢁ
3
ꢁ1
;
solute: 5 ꢂ 10 M; GC working electrode. Scan rate: 100 mV s
potential
range: ꢁ2.1 V to þ1.5 V vs Ag/AgCl, RT, N atmosphere.
2
10 s (C). A direct correlation between the time at the reductive
(
with respect to CV ones) were obtained.
potential of ꢁ2.1 V and peak II and IV heights is evident, confirming
that the products corresponding to oxidation peaks II and IV derive
from the reduction of caffeinium salt.
The study of the effect of the sweep rate on the faradaic current
intensity and corresponding peak height is reported in Fig. 7.
Although peak I seems to be independent of the scan rate, for peak
II (highlighted in Fig. 8a) it is evident that there is variation with the
sweep rate, while for peak IV (highlighted in Fig. 8b) no correlation
is found, probably due to the presence, close to the electrode sur-
face, of species resulting from iodide oxidation process.
In Fig. 4, the DPV sweeps for caffeinium salt ( ) and hymenia-
cidin (C) (from ꢁ2.1 V to 1.6 V vs Ag/AgCl) are reported.
Peak I of caffeinium salt (ꢁ1.72 V) is attributable to its reduction
reaction, as showed in Scheme 2, leading to the carbene formation.
The oxidation of the NHC, as already discussed, produces a sharp
peak (II) centered at ~ 0 V. In the next region of potential (III) a
series of peaks related of the iodide oxidation is well evident [23].
Finally the comparison between the scan of caffeinium salt and
hymeniacidin for peak IV (1.10 V) provides confirmation of hyme-
niacidin as a product from caffeinium salt.
The confirmation that the peak I indicates the reduction process
of caffeinium ion is found in the voltammetry in Fig. 5, where the
negative current occurring during the anodic sweep from ꢁ2.1 V is
maintained till ꢁ1.4 V, value of potential corresponding to the sign
inversion of current in DPV scan.
Analyzing the carbene oxidation peak (highlighted in Fig. 8a), a
good linear dependence (confirmed by a correlation value close to
1) between the current intensity and the square root of the scan
rate was found (Fig. 9), indicating the formation of NHC as a process
ꢁ1
under diffusion control in the range 10e100 mV s . It is worth
ꢁ1
mentioning that for 10 mV s only a very small signal is observed,
In Fig. 6, the DPV curve for caffeinium iodide ( ) is reported
along with the DPV obtained after a pre-electrolysis at ꢁ2.1 V for
ꢁ1
while for scan rates exceeding 100 mV s
found.
no correlation was

F. Pandolfi et al. / Electrochimica Acta 280 (2018) 71e76
75
ꢁ
3
ꢁ3
Fig. 5. Linear Scan Voltammetry for caffeinium iodide (5 ꢂ 10 M). DMF-0.1 M
Fig. 7. Effect of scan rate on DPV for caffeinium iodide (5 ꢂ 10 M) with 10 s pre-
electrolysis at ꢁ2.1 V. DMF-0.1 M Et NBF , GC working electrode. Scan rate: from 10
to 400 mV s ; potential range: ꢁ2.1 V to þ1.5 V vs Ag/AgCl, RT, N atmosphere.
GC working electrode. Scan rate: 100 mV s 1
ꢁ
;
potential range: ꢁ2.1
V
4
4
Et
4
NBF
4
,
ꢁ
1
to þ1.5 V vs Ag/AgCl, RT, N
2
atmosphere.
2
ꢁ
3
Fig. 6. DPV scan for caffeinium iodide (5 ꢂ 10 M) without pre-electrolysis ( ) and
with 10 s pre-electrolysis at ꢁ2.1 V (C). DMF-0.1 M Et NBF , GC working electrode.
Scan rate: 100 mV s ; potential range: ꢁ2.1 V to þ1.5 V vs Ag/AgCl, RT, N atmosphere.
4
4
ꢁ
1
2
ꢁ3
Fig. 8. Effect of scan rate on DPV for caffeinium iodide (5 ꢂ 10 M) with 10 s pre-
electrolysis at ꢁ2.1 V. DMF-0.1 M Et NBF , GC working electrode. Scan rate: from 10
atmosphere.
4
4
ꢁ
1
to 400 mV s ; potential range: ꢁ2.1 V to þ1.5 V vs Ag/AgCl, RT, N
2
4
. Conclusions
Caffeine, a natural product, can be easily transformed into the
Selected potential ranges from Fig. 7, corresponding to peak II (graph A, left) and peak
IV (graph B, right).
corresponding imidazolium salt (9-methylcaffeinium iodide) by
reaction with methyl iodide. Scope of this work was the study of the
electrochemical behaviour of 9-methylcaffeinium iodide (never
reported in the literature) by means of cyclic voltammetry, differ-
ential pulse voltammetry and electrolysis.
The interpretation of the voltammetric peaks and their inter-
relation allowed to describe the cathodic process for 9-
methylcaffeinium iodide. Confirmation came from isolated prod-
ucts after electrolysis.
reaction products with elemental sulfur and dioxygen. Moreover,
the cathodic reduction of the caffeinium iodide gave a ring opening
product, hymeniacidin (a natural product), in high yield. This work
proved that hymeniacidin derives from hydrolysis of electro-
generated carbene and not from hydrolysis of caffeinium salt.
Hymeniacidin was obtained in high yield (88%) by simple elec-
trolysis of 9-methylcaffeinium iodide in DMF solution. This is the
first electrochemical synthesis of this natural product (previously
isolated from a marine sponge). Although the cathodic reduction of
In fact, the cathodic reduction of the caffeinium iodide yielded
the corresponding carbene, whose presence was confirmed by its
9-methylcaffeinium iodide leads to the formation of the corre-
sponding carbene (in analogy with the behaviour of other

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F. Pandolfi et al. / Electrochimica Acta 280 (2018) 71e76
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ꢁ
3
Fig. 9. DPV for caffeinium iodide (5 ꢂ 10 M) with 10 s pre-electrolysis at ꢁ2.1 V.
15] A. Kascatan-Nebioglu, A. Melaiye, K. Hindi, S. Durmus, M.J. Panzner,
L.A. Hogue, R.J. Mallett, C.E. Hovis, M. Coughenour, S.D. Crosby, A. Milsted,
D.L. Ely, C.A. Tessier, C.L. Cannon, W.J. Youngs, Synthesis from caffeine of a
mixed N-Heterocyclic carbene-silver acetate complex active against resistant
respiratory pathogens, J. Med. Chem. 49 (2006) 6811e6818. https://doi.org/
ꢁ
1
4
DMF-0.1 M Et NBF
4
, GC working electrode. Scan rate: from 10 to 400 mV s ; potential
atmosphere. Linear dependence of the
range: ꢁ2.1 V to þ1.5 V vs Ag/AgCl, RT, N
2
current intensity of peak II upon the square root of the scan rate. Data from Fig. 7.
10.1021/jm060711t.
[
16] T.A. Aweda, O. Ikotun, T. Mastren, C.L. Cannon, B. Wright, W.J. Youngs,
C. Cutler, J. Guthried, S.E. Lapi, The use of 111 Ag as a tool for studying bio-
logical distribution of silver-based antimicrobials, Med. Chem. Commun. 4
imidazolium salts), this NHC seems to be either too unstable or too
weak nucleophile to catalyse the benzoin condensation (a classical
NHC-catalysed organic reaction). Further work is currently under-
way in our laboratory to verify the possibility to use this NHC in
other organocatalysed reactions.
(2013) 1015e1017. https://doi.org/10.1039/c3md00082f.
[
17] B. Bertrand, L. Stefan, M. Pirrotta, D. Monchaud, E. Bodio, P. Richard, P. Le
Gendre, E. Warmerdam, M.H. de Jager, G.M.M. Groothuis, M. Picquet, A. Casini,
Caffeine-based Gold(I) N-;Heterocyclic carbenes as possible anticancer
agents: synthesis and biological properties, Inorg. Chem. 53 (2014)
2
296e2303. https://doi.org/10.1021/ic403011h.
Declarations of interest
None.
[
18] M. Feroci, I. Chiarotto, M. Orsini, F. Pandolfi, D. Zane, A. Inesi, Electrogenerated
N-Heterocyclic olefin (NHO): stability and catalytic ability, ChemElectroChem
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Acknowledgements
The authors acknowledge Sapienza University of Rome for
financial support and Mr. Marco Di Pilato for his help with the
voltammetric analysis.
[
[
[
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