Electrogenerated base-promoted synthesis of N-benzylic rhodanine and carbamodithioate derivatives

Article abstract of DOI:10.1080/17415990903191752

Electrogenerated magnesium-associated cyanomethyl anions/bases obtained from the electrochemical reduction of acetonitrile and the oxidation of a sacrificial magnesium rod were successfully used to promote the addition of carbon disulfide to primary benzylic amines. Alkylation with ethyl 3-bromopropionate acid ester or with ethyl 2-bromoacetate acid ester yields the corresponding ring-opened carbamodithioate compounds or cyclic rhodanine derivatives, respectively. The effect of the amount of electrogenerated base on the yield of reaction was also evaluated.

Full text of DOI:10.1080/17415990903191752

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Electrogenerated base-promoted
synthesis of N-benzylic rhodanine and
carbamodithioate derivatives
Khalil Tissaoui a , Noureddine Raouafi a & Khaled Boujlel a
a Laboratoire de Chimie Analytique et Electrochimie, Faculté des
Sciences de Tunis, Université Tunis El-Manar, Campus Universitaire
2092, Tunis, Tunisia
Published online: 14 Aug 2009.
To cite this article: Khalil Tissaoui , Noureddine Raouafi & Khaled Boujlel (2010): Electrogenerated
base-promoted synthesis of N-benzylic rhodanine and carbamodithioate derivatives, Journal of
Sulfur Chemistry, 31:1, 41-48
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Journal of Sulfur Chemistry
Vol. 31, No. 1, February 2010, 41–48
Electrogenerated base-promoted synthesis of N-benzylic
rhodanine and carbamodithioate derivatives
Khalil Tissaoui, Noureddine Raouafi* and Khaled Boujlel
Laboratoire de Chimie Analytique et Electrochimie, Faculté des Sciences de Tunis, Université Tunis
El-Manar, Campus Universitaire 2092 Tunis, Tunisia
(Received 17 June 2009; final version received 18 July 2009)
Electrogenerated magnesium-associated cyanomethyl anions/bases obtained from the electrochemical
reduction of acetonitrile and the oxidation of a sacrificial magnesium rod were successfully used to promote
the addition of carbon disulfide to primary benzylic amines. Alkylation with ethyl 3-bromopropionate acid
ester or with ethyl 2-bromoacetate acid ester yields the corresponding ring-opened carbamodithioate com-
pounds or cyclic rhodanine derivatives, respectively. The effect of the amount of electrogenerated base on
the yield of reaction was also evaluated.
Keywords: electrogenerated bases/anions; rhodanines; carbamodithioates; electrosynthesis; cyclization;
heterocycles
1. Introduction
Alternative paths to environmentally unfriendly preparation methods of organic compounds
gains new interest if polluting and hazardous solvents and corrosive reagents are to be avoided.
Recently, an efficient and convenient one-pot multicomponent electrochemical synthesis of
tetrahydrobenzo[b]pyran and 2-amino-4H-chromenes derivatives, starting, respectively, from
malononitrile, aryl aldehydes and dimedone and from malononitrile, aryl aldehydes and resorcinol
without the involvement of an added base were reported (1, 2). Inesi and colleagues have devel-
oped a new approach for the synthesis of new families of carbonates, carbamates and oxazolinones
under very mild conditions. Alcohols, amines, aminoalcohols, haloamides, carbon dioxide and
strong basic species generated during the electrolysis from organic solvents are commonly known
as electrogenerated bases (EGBs) (3–6). In the last two decades, numerous papers reported the
generation and use of strong basic species from organic aprotic solvents such as dimethyl sulfox-
ide, dimethyl formamide, acetonitrile, etc., and their application in organic synthesis. One of the
most striking advantages is the excellent control of the base strength depending on the choice of
the solvent. Furthermore, the amount of EGB is a function of the imposed current density and the
duration of the electrolysis, which can be also adjusted (7, 8).
*Corresponding author. Email: noureddine.raouafi@gmail.com
ISSN 1741-5993 print/ISSN 1741-6000 online
© 2010 Taylor & Francis
DOI: 10.1080/17415990903191752
http://www.informaworld.com

42 K. Tissaoui et al.
On the other hand, it is well established that the electrochemical reduction of acetonitrile
with or without the addition of probases such as halobenzene, azobenzene, cyanomethyl
triphenylphosphonium or arsonium cations and phenylsulfonyl acetonitrile (9–11) in the presence
of a quaternary ammonium salt as a supporting electrolyte yields the corresponding cyanomethyl
anion, a strong basic entity, with a pKa value of ca. 32 (12), capable of removing a weak acidic
proton. In most cases, the generated anions are known to react once or several times with aceto-
nitrile, leading to aminocrotonitrile, pyrimidine and triazine anions, which could also serve as
EGBs (13, 14). These anions can also react as nucleophilic entities upon subsequent addition to
electrophiles, giving C- and N-functionalized products. The use of a magnesium rod as a sacrificial
anode frequently leads to cyanomethyl electrogenerated species, which are stabilized as Grignard-
type anions associated with the anodically generated magnesium cations. In a previous work, we
established that in the case of acetonitrile as a solvent, the dimerization of the cyanomethyl anion
occurred through a nucleophilic addition of the EGB to the nitrile group of the acetonitrile. The
aminocrotonitrile anions led to ethyl 1-cyanoprop-1-en-2-ylcarbamodithioate as a by-product via
the reaction with carbon disulfide and ethyl iodide during the synthesis of carbamodithioates (15).
Herein, we report the electrochemical synthesis of benzylic carbamodithioates (2) and (2ꢀ)
and rhodanines (3), starting from various benzylic amines, carbon disulfide and bromoalkyl acid
ester. These families of products may display a large spectrum of applications, such as antibacterial
agents for the inhibition of RNA transcription for Staphylococcus epidermidis or Staphylococcus
aureus, which are the major cause of nosocomial infections (16), reducing the activity of tyrosine
phosphatase (PRL-3) enzyme which could be associated with colorectal cancer proliferation (17),
and chelating agents for the coordination and detection of noble metals (18).
R
S
S
R
O
S
R
O
N
Ar
S
S
N
H
Ar
O
S
N
H
Ar
O
O
(2')
2
3
2. Results and discussion
Electrolysis of dry acetonitrile, under galvanostatic conditions, was conducted in an undivided
electrochemical cell cooled at −20 ◦C, fitted with a magnesium rod as a sacrificial anode and a
stainless steel grid cathode, maintained under an inert nitrogen atmosphere. When the desired
amount of base was formed, the electrolysis of acetonitrile was stopped. The amount of EGB
was calculated with respect to the quantity of amine to be used. At the end of the electrolysis,
the amine was introduced, followed by the addition of carbon disulfide. The color of the solution
turned to dark red immediately. Finally, the bromoalkyl acid ester was added, and the solution was
allowed to reach room temperature and kept under continuous stirring overnight. Final work-up
and column chromatography over silica gel afforded the products 2 and 3 (Table 1) in good yields
(Scheme 1).
A stabilized dithiocarbamate intermediate was formed during the reaction of carbon disulfide
and the amine. The EGB (cyanomethyl and aminocrotonitrile species) was essential to promote
the reaction. The dithiocarbamate anion was probably stabilized by ion-pairing with magnesium
cations generated during the oxidation of the sacrificial magnesium electrode. Alkylation of the
intermediate with ethyl 3-bromopropionate acid ester gave only the acyclic dithiocarbamate acid
ester (2); no cyclic products were formed, even if three to four equivalents of EGB were used with
respect to the introduced amine. Instead, a complex mixture of products was obtained. In one case,
the consumption of one equivalent of the EGB per mole of amine yielded acyclic intermediate

Journal of Sulfur Chemistry 43
Table 1. Synthesized carbamodithioates (2) and rhodanines (3).
Entry
Starting amine
Electrophile
Product
Qa
Yieldb (%)
S
CO2Et
CO2Et
NH2
N
S
S
1
A
2.2
41
H
2a
S
N
H
NH2
Cl
2
3
4
5
A
A
A
A
2.2
2.2
1.1
2.2
54
59
79
49
2b
Cl
S
S
N
CO2Et
CO2Et
N
N
H
S
S
NH2
2c
N
H
NH2
2d
S
O
O
O
N
S
2 CO2Et
CO2Et
NH2
H
2e
O
S
N
S
NH2
6
7
8
A
B
B
2.2
2.2
2.2
65
68
60
2f
H
O
O
S
N
S
NH2
NH2
3a
O
O
Cl
S
N
S
3b
Cl
S
N
NH2
S
9
B
B
B
2.2
2.2
2.2
78
67
3c
Cl
Cl
O
S
N
S
S
NH2
10
11
3d
O
N
NH2
S
80
3e
O
O
O
(Continued)

44 K. Tissaoui et al.
Table 1. Continued.
S
N
S
NH2
NH2
12
B
B
B
B
2.2
2.2
2.2
2.2
55
59
75
49
3f
O
S
N
N
N
S
13
3g
O
S
O
O
O
N
NH2
S
14
3h
O
O
S
N
NH2
S
15
3i
O
O
O
Notes: A, ethyl 2-bromopropionate; B, ethyl 2-bromoacetate. aThe consumed quantity of electricity represents the number of
faradays per mole of amine. bThe yield was calculated in regard to the starting amine.
Scheme1. EGB-promotedelectrosynthesisofring-openedandcycliccarbamodithioates2and3.
carbamodithioates (2ꢀ) when a hindered amine was used (entry 12) and ethyl 2-bromoacetate acid
ester was added as an alkylating agent. When the amount of base reached 2.2 equivalents of EGB,
the intermediate (2ꢀ) readily underwent in situ ring closure to give product 3. A series of products
2 and 3 were synthesized by applying the latter conditions, i.e. consumption of 2.2 faraday of
electrons per mole of amine, to form the same amount of EGB, followed by a subsequent addition
of amine, carbon disulfide and bromoalkyl acid ester (Table 1).
The improvement of the reaction yield could be probably attributed to two major factors: (i) the
replacement of a divided electrochemical cell by an undivided cell allowed to avoid an important
ohmic drop and thus permitted the application of higher current densities; (ii) the nature of the
electrode material was also important. In fact, the oxidation of graphite carbon did not produce
stabilizing cations for the electrogenerated anions/bases. It is noteworthy that when a small
quantity of the same amine was used, the electrolysis time of acetonitrile was shortened and less
basic species were re-protonated affording higher yields of compound 3a (Tables 1 and 2).
The consumed amount of electrons, i.e. number of faradays per mole of amine being propor-
tional to the number of mole of the EGB, affected the yield of the reaction. For this reason,
benzylamine was chosen as model compound to study this influence. A standard procedure con-
sists of the electrolysis of the acetonitrile by passing through the desired number of faradays
per mole, and then benzylamine was introduced, followed half an hour later by carbon disulfide
and finally ethyl 2-bromoacetate. The solution was stirred overnight and followed by a standard
work-up to yield product 3a (Table 2).

Journal of Sulfur Chemistry 45
Table 2. Variation of the yield of 3a as a function of
the consumed number of faradays per mole of amine.
Entry
Q (faraday/mole)
Yield of 3a (%)
1
2
3
4
5
6
7
8
9
0.5
1.0
1.5
1.8
2.0
2.5
3.0
4.0
5.0
67
76
82
84
86
88
90
94
96
From the obtained results, we noticed that the yield of the reaction increased with the amount
of the used EGB, i.e. the number of faradays per mole. For an amount of the base equal to
2.0–2.5 equivalents with respect to benzylamine, the yield reached 86–88%, which was a good
compromise between the reaction time and the consumed energy. In fact, when a five-fold excess
of base was used per mole of amine, the yield of was only ameliorated by 8–10%.
3. Conclusion
When compared with classical synthetic methods, the present one using EGBs shows several
advantages such as (i) no additional base required because of the in situ formation of the base,
(ii) milder reaction conditions and use of cheap and readily available reagents, (iii) fair to very
good yields, (iv) no hazardous chemicals and polluting by-products resulting from the addition of
base or probase, (v) no sophisticated electrochemical instrumentation and cell needed, (vi) easy
work-up procedure.
4. Experimental
Melting points were determined with an Electrothermal 9100 apparatus. 1H NMR and 13C NMR
spectra were recorded using a Bruker Advance 250 or 300 MHz spectrometer in CDCl3 with
tetramethylsilane as an internal standard. Elemental analyses were performed by Microanal-
yse Service of the ICSN, Gif sur Yvette, Paris, France. All the products have been previously
described (19–24).
4.1. Electro-organic synthesis of ethyl 3-(benzylcarbamothioylthio)propanoate (2)
and 3-benzyl-2-thioxothiazolidin-5-one (3)
In a typical reaction, acetonitrile (90 ml) solution of tetrabutylammonium tetrafluoroborate
(0.01 M) as a supporting electrolyte in an undivided cell fitted with a sacrificial magnesium rod as
an anode and a stainless steel grid (20 cm2) as a cathode was subjected to electrolysis at a constant
current density (I = 80 mA/cm2, 7.0 h, 2.2 faraday/mole with respect to the quantity of amine to
be added). The cell was cooled to −20 ◦C by immersing it in Lauder refrigerating system. During
the electrolysis, the system was maintained under inert atmosphere by continuous bubbling of
argon. The electrolysis was stopped after the formation of 2.2 equivalents of cyanomethyl anion
with respect to the amine. First, 10.0 mmol of the amine was introduced, and 1 h later, 2.2 equiv-
alents of carbon disulfide were added, and the solution turned immediately from pale yellow to

46 K. Tissaoui et al.
dark red. For the second set of experiments summarized in Table 2, 5.0 mmol of benzylamine,
11.0 mmol of CS2 and 11.0 mmol of ethyl 2-bromoacetate were used. After 1 h of stirring, 2.2
equivalents of bromoalkyl acid ester were added. The cooling bath was removed and the solution
was allowed to reach room temperature and kept under continuous stirring overnight to ensure
the tandem alkylation–cyclization reaction where it is possible. The excess of acetonitrile was
removed in rotatory evaporator and the residue was diluted in 50 ml of water and extracted three
times with 20 ml of diethyl ether. The organic layer was washed with 20 ml of water and dried over
anhydrous magnesium sulfate. The solvent was removed and the residue was purified by column
chromatography on silica gel 60. A mixture of ethyl acetate/hexane (3:7) was used as eluent.
The resulting yellowish solid was filtered and recrystallized from chloroform. All the products
are known and were characterized by 1H NMR and 13C NMR spectroscopy, melting point and,
in some cases, elemental analysis.
4.1.1. Ethyl 3-(benzylcarbamothioylthio)propanoate (2a) (19)
1
3
Yield: 41%, m.p.: 152–153 ◦C; H NMR (300 MHz, CDCl3), δ (ppm): 1.28 (3H, t, JH−H
=
3
3
6.2 Hz); 2.78 (2H, t, CH2, JH−H = 7.5 Hz); 3.52 (2H, t, CH2, JH−H = 7.4 Hz); 4.14 (2H, q,
CH2, 3JH−H = 6.2 Hz); 4.9 (2H, s, CH2); 7.32–7.44 (5H, CHarm); 13C NMR (250 MHz, CDCl3),
δ (ppm): 14.2; 30.2; 34.3; 51.1; 60.9; 126.7; 127.4; 128; 128.2; 128.8; 136.3; 172.1; 197.4.
4.1.2. Ethyl 3-(2-chlorobenzylcarbamothioylthio)propanoate (2b) (19)
Yield: 54%, m.p.: 178–179 ◦C; 1H NMR (300 MHz, CDCl3), δ (ppm): 1.25 (3H, t, CH3, 3JH−H
=
7.1 Hz); 2.78 (2H, t, CH2, 3JH−H = 6.7 Hz); 3.5 (2H, t, CH2, 3JH−H = 6.7 Hz); 4.14 (2H, q, CH2,
3JH−H = 7.2 Hz); 5.04 (2H, s, CH2); 7.4 (4H, CHarm); 13C NMR (250 MHz, CDCl3), δ (ppm):
30.3; 34.3; 48.6; 60.8; 127.1; 126.7; 127.9; 129.5; 130.7; 133.8; 171.9; 197.8.
4.1.3. Ethyl 3-(pyridin-2-ylmethylcarbamothioylthio)propanoate (2c) (19)
Yield: 43%, m.p.: 267–268 ◦C; H NMR (300 MHz, CDCl3), δ (ppm): 1.28 (3H, t, JH−H
=
1
3
3
3
6.2 Hz); 2.78 (2H, t, CH2, JH−H = 7.6 Hz); 3.52 (2H, t, CH2, JH−H = 7.4 Hz); 4.14 (2H, q,
3
CH2, JH−H = 6.2 Hz); 4.9 (2H, s, CH2); 8.1 (4H, CHarm); 13C NMR (250 MHz, CDCl3), δ
(ppm): 14.2; 30.2; 34.3; 51.1; 60.9; 126.7; 127.4; 128; 128.2; 128.8; 136.3; 172.1; 197.4.
4.1.4. Ethyl 3-(1-phenylethylcarbamothioylthio)propanoate(2d) (19)
Yield: 79%, m.p.: 87–88 ◦C; 1H NMR (300 MHz, CDCl3), δ (ppm): 1.24 (3H, t, 3JH−H = 6.1 Hz);
1.52(2H, d, CH3, 3JH−H = 7.0 Hz);2.74(2H, t, CH2, 3JH−H = 7.1 Hz);3.28(2H, t, CH2, 3JH−H
=
6.9 Hz); 4.15 (2H, q, CH2, 3JH−H = 7.0 Hz); 5.47 (1H, q, CH, 3JH−H = 6.8 Hz); 7.01–7.28 (5H,
CHarm);6.29(1H, br, NH);13CNMR(75 MHz, CDCl3), δ (ppm):14.5;20.5;31.7;32.2;53.6;61.7;
128.1; 129.0; 129.9; 142.1; 172.0; 205.2; Elemental analysis (found/calculated): C: 56.61/56.53;
H: 6.40/6.44; N: 4.75/4.71.
4.1.5. Ethyl 3-(furan-2-ylmethylcarbamothioylthio)propanoate (2e) (19)
Yield: 49%, m.p.: 148–149 ◦C; 1H NMR (300 MHz, CDCl3), δ (ppm): 1.27 (3H, t, CH3, 3JH−H
=
3
3
7.3 Hz); 2.78 (2H, t, CH2, JH−H = 6.6 Hz); 3.56 (2H, t, CH2, JH−H = 6.5 Hz); 4.16 (2H, q,
3
CH2, JH−H = 7.2 Hz); 6.98 (3H, CHarm); 13C NMR (300 MHz, CDCl3) : δ (ppm) 14.2; 30.3;
34.7; 43.8; 60.9; 108.8; 110.6; 143.0; 149.2; 172.0; 197.4.

Journal of Sulfur Chemistry 47
4.1.6. Ethyl 3-(benzo[d][1,3]dioxol-5-ylmethylcarbamothioylthio)propanoate (2f) (19)
Yield: 65%; m.p.: 155–156 ◦C; 1H NMR (300 MHz, CDCl3), δ (ppm): 1.28 (3H, t, CH3, 3JH−H
=
7.2 Hz); 2.78 (2H, t, CH2, 3JH−H = 6.7 Hz); 3.58 (2H, t, CH2, 3JH−H = 6.7 Hz); 4.18 (2H, q, CH2,
3JH−H = 7.2 Hz); 5.92 (2H, s, CH2); 6.78 (3H, CHarm13C NMR (300 MHz, CDCl3): δ (ppm) 14.1;
34.3; 37.8; 50.2; 61.3; 101.4; 108.5; 121.6; 129.9; 148.3; 171.9; 197.1.
4.1.7. 3-Benzyl-2-thioxothiazolidin-4-one (3a) (20)
1
Yield: 68%, m.p.: 80–81 ◦C; H NMR (250 MHz, CDCl3), δ (ppm): 3.92 (2H, s, CH2); 5.15
(2H, s, CH2); 7.34 (5H, CHarm); 13C NMR (250 MHz, CDCl3), δ (ppm): 35.5; 47.9; 128.5; 128.4;
129.1; 135.0; 174.7; 201.3 CI-MS: 224; HRMS: 224.0194; Elemental analysis (found/calculated):
C: 54.28/54.8; H: 3.97/4.03; N: 6.19/6.27; S: 28.54/28.69.
4.1.8. 3-(2-Chlorobenzyl)-2-thioxothiazolidin-4-one (3b) (21)
Yield: 60%, m.p.: 73–74 ◦C; 1H NMR (250 MHz, CDCl3), δ (ppm): 4.08 (2H, s, CH2); 5.3 (2H,s,
CH2); 7.27 (4H, CHarm); 13C NMR (250 MHz, CDCl3), δ (ppm): 35.5; 45.4; 126.8; 126.9; 128.8;
129.8; 131.6; 135; 173.4; 200.5.
4.1.9. 3-(4-Chlorobenzyl)-2-thioxothiazolidin-4-one (3c) (21)
1
Yield: 78%, m.p.: 126–127 ◦C; H NMR (300 MHz, CDCl3), δ (ppm): 3.95 (2H, s, CH2); 5.11
(2H, s, CH2); 7.29 (4H, CHarm); 13C NMR (300 MHz, CDCl3), δ (ppm): 35.4; 46.9; 128.2; 130.5;
133.1; 134.1; 173.7; 200.9.
4.1.10. 3-(4-Methylbenzyl)-2-thioxothiazolidin-4-one (3d) (21)
Yield: 67%, m.p.: 71–72 ◦C; 1H NMR (250 MHz, CDCl3), δ (ppm): 2.31 (3H, s, CH3); 3.97 (2H,
s, CH2); 5.12 (2H, s, CH2); 7.21 (4H, CHarm); 13C NMR (300 MHz, CDCl3), δ (ppm): 21.3; 38.6;
45.6; 128.1; 128.8; 133.5; 136.4; 171.2; 202.3.
4.1.11. 3-(4-Methoxybenzyl)-2-thioxothiazolidin-4-one (3e) (21)
Yield: 80%, m.p.: 98–99 ◦C; 1H NMR (300 MHz, CDCl3), δ (ppm): 3.78 (2H, s, CH2); 3.96 (3H,
s, CH3); 5.1 (2H, s, CH2); 7.18 (4H, CHarm); 13C NMR (300 MHz, CDCl3), δ (ppm): 35.4; 47.1;
55.2; 113.8; 126.9; 130.8; 159.5; 173.9; 201.1.
4.1.12. 3-(1-Phenylethyl)-2-thioxothiazolidin-4-one (3f) (22)
1
3
Yield: 55%, m.p.: 109–110 ◦C; H NMR (250 MHz, CDCl3), δ (ppm) 1.83 (3H, d, JH−H
=
7.2 Hz); 3.70 (2H, s); 6.37 (1H, d, CH, 3JH−H = 7.1 Hz); 7.32 (5H, CHarm); 13C NMR (250 MHz,
CDCl3), δ (ppm): 15; 34; 55.1; 127.5; 127.8; 128.1; 138; 173.4; 202.1. Elemental analysis
(found/calculated): C: 55.72/55.67; H: 4.61/4.67; N: 5.94/5.90.

48 K. Tissaoui et al.
4.1.13. 3-(Pyridin-2-ylmethyl)-2-thioxothiazalidin-4-one (3g) (23)
1
Yield: 59%, m.p.: 114–115 ◦C; H NMR (300 MHz, CDCl3), δ (ppm): 4.12 (2H, s, CH2); 5.29
(2H, s, CH2); 7.87 (4H, CHarm); 13C NMR (300 MHz, CDCl3), δ (ppm): 38.6; 50.2; 120.9; 124.1;
139.6; 148.6; 156.1; 171.2; 202.3.
4.1.14. 3-(Benzo[d][1,3]dioxol-5-ylmethyl)-2-thioxothiazolidin-4-one (3h) (24)
Yield: 75%, m.p.: 104–105 ◦C; H NMR (300 MHz, CDCl3), δ (ppm): 3.97 (2H, s, CH2); 5.09
1
(2H, s, CH2); 5.94 (2H, s, CH2); 6.95 (3H, CHarm); 13C NMR (300 MHz, CDCl3), δ (ppm): 38.6;
45.9; 101.2; 107.7; 109.2; 122.8; 134.9; 145; 146.8; 171.2; 202.3.
4.1.15. 3-(Furan-2-ylmethyl)-2-thioxothiazolidin-4-one (3i) (21)
1
Yield: 49%, m.p.: 118–120 ◦C; H NMR (300 MHz, CDCl3), δ (ppm): 3.88 (2H, s, CH2); 4.92
(2H, s, CH2); 5.814 (2H, s, CH2); 6.85 (3H, CHarm); 13C NMR (300 MHz, CDCl3), δ (ppm): 37.3;
44.8; 100.6; 106.4; 108.4; 120.3; 133.6; 144.9; 147.3; 173.7; 204.7.
Acknowledgements
The authors gratefully acknowledge the Tunisian “Ministère de l’Enseignement Supérieure, de la Recherche et la Tech-
nologie” for financial support (Lab CH-02). Bernd Schöllhorn is acknowledged for his valuable help during the preparation
of the manuscript.
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