Development of triarylamine mediator having ionic-tag and its application to electrocatalytic reaction in ionic liquid

Article abstract of DOI:10.1016/j.electacta.2012.05.049

Novel triarylamine (Ar₃N) mediators bearing an ionic-tag moiety were synthesized from 4,4′-dibromotriphenylamine. Their electrochemical properties were investigated by cyclic voltammetry both in organic solvent and ionic liquid HF salt. The electrocatalytic reactions such as deprotection and fluorodesulfurization of dithioacetals were successfully carried out using these Ar₃N mediators in an undivided cell at room temperature.

Full text of DOI:10.1016/j.electacta.2012.05.049

Electrochimica Acta 77 (2012) 47–53
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Development of triarylamine mediator having ionic-tag and its application to
electrocatalytic reaction in ionic liquid
Kohta Takahashi, Takashi Furusawa, Takahiro Sawamura, Shunsuke Kuribayashi,
Shinsuke Inagi, Toshio Fuchigami∗
Department of Electronic Chemistry, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
a r t i c l e i n f o
a b s t r a c t
Novel triarylamine (Ar3N) mediators bearing an ionic-tag moiety were synthesized from 4,4ꢀ-
dibromotriphenylamine. Their electrochemical properties were investigated by cyclic voltammetry both
in organic solvent and ionic liquid HF salt. The electrocatalytic reactions such as deprotection and
fluorodesulfurization of dithioacetals were successfully carried out using these Ar3N mediators in an
undivided cell at room temperature.
Article history:
Received 21 January 2012
Received in revised form 10 May 2012
Accepted 11 May 2012
Available online 6 June 2012
© 2012 Elsevier Ltd. All rights reserved.
Keywords:
Electrochemical fluorination
Electrochemical deprotection
Mediator
Electrocatalytic reaction
Ionic liquid
1. Introduction
one successful approach for reusable mediator [20]. We previously
prepared a polystyrene-supported iodobenzene (PSIB) mediator
The development of efficient methods for selective fluorina-
tion of organic compounds is becoming increasingly important
in order to produce novel medicines, agrochemicals and func-
tional materials [1–6]. Electrochemical partial fluorination is a
promising approach to introduce fluorine atoms into a specific
position of organic compounds such as heteroatom compounds
[7–10]. The reaction involves the electrochemical oxidation of
substrates, followed by a nucleophilic attack of fluoride anion
used as a supporting electrolyte. However, passivation that forms
a non-conducting polymer film at anodic surface often occurs
and suppresses anodic current, resulting in decrease of yield
and current efficiency [11]. The indirect electrolysis using medi-
ators can solve the problems [12,13]. One of the most promising
anodic mediator, triarylamine (Ar3N) has been widely used [14].
For example, anodic deprotection of dithioacetal [15], anodic
gem-difluorodesulfurization of dithioacetal [16], anodic monofluo-
rodesulfurization of phenylthio ␤-lactams [17] and arylthio esters
[18] have been reported.
which was effective in combination with chloride mediator for
anodic fluorination [21]. The PSIB mediator could be recovered
with a simple filtration after electrolysis. As for Ar3N, Steck-
han and co-workers reported a polymer electrolyte-supported
Ar3N used as both electrolyte and mediator [22]. However, this
polymer electrolyte-supported Ar3N mediator was decomposed
by the elimination of Ar3N moiety from polymer chain during
electrolysis.
Mediators bearing ionic-tag show good compatibility to polar
solvents especially ionic liquid [23–28]. For example, 2,2,6,6-
tetramethylpiperidine 1-oxyl (TEMPO) derivative having ionic-tag
could be used repeatedly for chemical oxidation of alcohols in
ionic liquid [29,30]. In our previous report, we prepared a reusable
iodobenzene derivative, in which an imidazolium tag was intro-
duced [31]. The ionic-tag strategy makes the mediators stay in polar
solvent even after extraction of products with non-polar organic
solvents; therefore this is still promising for development of prac-
tical mediators.
However, mediators are usually discarded after electrolysis,
which is far from atom economy. Recent progress in reusable medi-
ators is remarkable [19]. For example, polymer-supported system is
In this paper, we report on novel Ar3N-based mediators
(Med-1 and Med-2) bearing ionic-tag moiety, which imparts
compatibility to ionic liquid. Electrochemical properties of the
mediators in organic solvent and ionic liquid HF salts and their
mediatory use for electrocatalytic reaction such as deprotec-
tion and difluorodesulfurization of dithioacetal compounds were
investigated.
∗
Corresponding author. Tel.: +81 45 924 5406; fax: +81 45 924 5406.
E-mail address: fuchi@echem.titech.ac.jp (T. Fuchigami).
0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2012.05.049

48
K. Takahashi et al. / Electrochimica Acta 77 (2012) 47–53
2. Experimental
2.1. Measurements
2.3.3. Synthesis of Med-1
To a stirred solution of 4-[bis(4-bromophenyl)amino]benzyl
alcohol (3), (0.60 g, 1.4 mmol) and triethylamine (0.55 mL,
4.2 mmol) in CH2Cl2 (2.0 mL), trifluoromethanesulfonyl chloride
(0.48 mL, 4.2 mmol) in CH2Cl2 (2 mL) was added at 0 ◦C and the
mixture was stirred for 2 h. The mixture was washed with water
and extracted with CHCl3. The organic layer was dried over Na2SO4,
and the solvent was removed under reduced pressure. The residue
was washed with Et2O (5× 30 mL) to give compound Med-1
(0.65 g, 0.97 mmol, 71%) as white solid. M.p. 128–131 ◦C; 1H NMR
(270 MHz, CDCl3, ppm): ı = 7.42 (d, J = 8.6 Hz, 4H), 7.21 (d, J = 8.6 Hz,
2H), 7.03 (d, J = 8.6 Hz, 2H), 6.98 (d, J = 8.6 Hz, 4H), 4.25 (s, 2H), 3.23
(q, J = 8.1 Hz, 6H), 1.42 (t, J = 8.1 Hz, 9H); 13C NMR (67.8 MHz, CDCl3,
ppm): ı = 150.24, 147.02, 134.64, 133.69, 127.74, 123.94, 121.94,
1H, 13C and 19F NMR spectra were determined on a JEOL EX270
(1H: 270.05 MHz, 13C: 67.8 MHz 19F: 254.05 MHz) spectrometer
using CDCl3 as a solvent. The chemical shifts for 1H, 13C and 19F
NMR were given in ı (ppm) from internal TMS, CDCl3, and monoflu-
orobenzene (−36.5 ppm), respectively. The product yields were
determined by 19F NMR using monofluorobenzene as an inter-
nal standard. Cyclic voltammetry measurements were carried out
using ALS 600A Electrochemical Analyzer. The preparative electrol-
ysis was performed using Metronix Corp. constant current power
supply model 5944 monitored with coulomb/amperehour meter
HF-201. High resolution mass spectra were recorded on JEOL The
MStaion JMS-700. Melting points were determined using Yanako
Micro Melting Point Apparatus MP-500P.
121.63 (q, J = 319 Hz), 117.81, 60.79, 53.31, 8.07; 19F NMR (254 MHz,
+
CDCl3, ppm): ı = −1.70 (s, 3F); HRMS Calcd for C25H29Br2N2
:
515.0697, found: 515.0710, CF3O3S−: 148.9520, found: 148.9520.
2.3.4. Synthesis of
2.2. Materials
1-{4-[bis(4-bromophenyl)amino]benzyl}-3-methylimidazolium
trifluorometanesulfonate (4)
Unless otherwise stated, all reagents and chemicals were
obtained from commercial sources and used without further purifi-
cation. Dry solvents were perchased and used as received.
Trifluoromethanesulfonyl chloride (1.17 mL, 11.1 mmol)
in CH2Cl2 (8.0 mL) was added to the solution of 4-[bis(4-
bromophenyl)amino]benzyl alcohol (3) (1.60 g, 3.69 mmol)
and 1-methylimidazole (0.911 g, 11.1 mmol) in CH2Cl2 (8.0 mL) at
0 ◦C and the reaction mixture was stirred for 2 h. The mixture was
washed with water and extracted with CHCl3. The solvent was
evaporated. The residue was washed with Et2O (5× 30 mL) to give
compound 4 (1.37 g, 2.17 mmol, 59%) as dark yellow solid. M.p.
101.1–103.7 ◦C; 1H NMR (270 MHz, CDCl3, ppm): ı = 10.64 (s, 1H),
7.39–7.28 (m, 4H), 7.15 (d, J = 5.7 Hz, 2H), 7.03 (d, J = 8.6 Hz, 2H),
6.93 (d, J = 8.6 Hz, 4H), 5.45 (s, 2H), 4.07 (s, 3H); 13C NMR (67.8 MHz,
CDCl3, ppm): ı = 147.76, 145.78, 145.68, 133.57, 131.27, 127.14,
125.26, 124.80, 124.72, 122.33, 120.41 (q, J = 320 Hz), 116.23, 52.64,
35.47; 19F NMR (254 MHz, CDCl3): ı = −1.87 (s, 3F); HRMS Calcd
for C23H20Br2N3+: 496.0024, found: 496.0030, Calcd for CF3O3S−:
148.9520, found: 148.9517.
2.3. Synthesis of
4-[bis(4-bromophenyl)amino]benzyltriethylammonium
trifluoromethanesulfonate (Med-1) and
1-{4-[bis(4-bromophenyl)amino]benzyl}-3-methylimidazolium
bis(trifluoromethanesulfonyl)amide (Med-2)
2.3.1. Synthesis of 4-[bis(4-bromophenyl)amino]benzaldehyde
(2) [32]
POCl3 (14.0 mL, 155 mmol) in DMF (12.0 mL, 155 mmol) was
added to the solution of 4,4ꢀ-dibromotriphenylamine (1) (2.50 g,
6.20 mmol). The mixture was stirred at 95 ◦C for 1 h and cooled
to room temperature. Aqueous NaOH was then added to the mix-
ture. After stirring for 30 min, the product was extracted with
CHCl3. The organic layer was dried over Na2SO4, and the solvent
was evaporated. The residue was purified by column chromatog-
raphy over silica gel with CH2Cl2/hexane (5:1) to give compound 2
(2.42 g, 5.61 mmol, 90%) as yellow solid. M.p. 155–156 ◦C; 1H NMR
(270 MHz, CDCl3, ppm): ı = 9.84 (s, 1H), 7.71 (d, J = 8.6 Hz, 2H), 7.45
(d, J = 8.6 Hz, 4H), 7.02 (d, J = 8.6 Hz, 6H), 13C NMR (67.8 MHz, CDCl3,
ppm): ı = 190.10, 152.09, 144.79, 132.73, 131.19, 129.92, 127.20,
120.24, 117.86.
2.3.5. Synthesis of Med-2
Lithium
bis(trifluoromethanesulfonyl)amide
(1.50 g,
5.23 mmol) in water (50 mL) was added to the solution of 3-
{4-[bis(4-bromophenyl)amino]benzyl}-1-methyl-imidazolium
trifluoromethanesulfonate (4) (1.10 g, 1.74 mmol) in CHCl3
(3.0 mL), and the reaction mixture was stirred for 3 h at room
temperature. Then the organic layer was washed with water, and
the solvent was evaporated. The residue was washed with Et2O
(5× 30 mL) to give compound Med-2 (0.860 g, 1.11 mmol, 64%) as
yellow solid. M.p. 74.7–78.5 ◦C; 1H NMR (270 MHz, CDCl3, ppm):
ı = 8.76 (s, 1H), 7.36–7.24 (m, 8H), 7.03 (d, J = 8.6 Hz, 2H), 6.93 (d,
J = 8.6 Hz, 4H), 5.24 (s, 2H), 3.92 (s, 3H); 13C NMR (67.8 MHz, CDCl3,
ppm): ı = 147.56, 145.50, 145.42, 135.22, 132.16, 129.91, 126.50,
125.70, 123.58, 121.87, 119.37 (q, J = 320 Hz), 115.87, 52.49, 35.85;
19F NMR (254 MHz, CDCl3, ppm): ı = −2.32 (s, 6F); HRMS Calcd for
2.3.2. Synthesis of 4-[bis(4-bromophenyl)amino]benzyl alcohol
(3)
NaBH4 (0.284 g, 6.88 mmol) in EtOH (13.0 mL) and 1.0 M
NaOH solution (1.40 mL) were added to the solution of 4-[bis(4-
bromophenyl)amino]benzaldehyde (2) (2.42 g, 5.61 mmol) in THF
(16 mL). The mixture was stirred for 1 h at room temperature. Aque-
ous HCl was added to the mixture and the mixture was stirred
for further 30 min. Then the product was extracted with CHCl3.
The organic layer was dried over Na2SO4, and the solvent was
evaporated. The residue was purified by column chromatography
over silica gel with CHCl3 to give compound 3 (2.20 g, 5.07 mmol,
90%) as pale yellow solid. M.p. 90–91 ◦C; 1H NMR (270 MHz, CDCl3,
ppm): ı = 7.34 (d, J = 8.9 Hz, 4H), 7.27 (d, J = 8.3 Hz, 2H), 7.05 (d,
J = 8.3 Hz, 2H), 6.93 (d, J = 8.9 Hz, 4H), 4.66 (d, J = 5.7 Hz, 2H), 1.62
(t, J = 5.7 Hz, 1H); 13C NMR (67.8 MHz, CDCl3, ppm): ı = 146.38,
146.36, 136.04, 132.31, 128.44, 125.37, 124.47, 115.50, 64.84; IR
(KBr): ꢀ = 3214, 2934, 2871, 1574, 1493 cm−1; HRMS Calcd for
C19H15NOBr2: 430.9520, found: 430.9529.
−
23H20Br2N3+: 496.0024, found: 497.9987. Calcd for C2F6NO4S2
:
C
279.9173, found: 279.9175.
2.3.6. Synthesis of dithioacetals (7a–7c)
Dithioacetals, 7a–7c were prepared from the corresponding
ketones and ethane dithiol in the presence of BF3-2(AcOH) at room
temperature [33].
2.4. Cyclic voltammetry
Cyclic voltammetry was carried out at 25 ◦C in anhydrous MeCN
containing 0.6 M tetraethylammonium perchlorate (Et4NClO4,
TEAP) using a platinum disk working electrode (ꢁ = 0.16 cm) and

K. Takahashi et al. / Electrochimica Acta 77 (2012) 47–53
49
Scheme 1.
a platinum plate counter electrode (1 cm × 1 cm). Potentials were
referred to a saturated calomel electrode (SCE) using saturated KCl
salt bridge for the junction between SCE and the MeCN solution.
In the case of measurement in ionic liquid HF salt, potentials were
referred to Ag wire with external ferrocene/ferrocenium (Fc/Fc+).
Next, 4-[bis(4-bromophenyl)amino]benzaldehyde (2) was reduced
using NaBH4 to form 4-[bis(4-bromophenyl)amino]benzyl alcohol
(3). The compound 3 was reacted with trifluoromethanesul-
fonyl chloride in the presence of triethyamine (Et3N) to form
the corresponding sulfonyl ester, which was immediately
reacted with Et3N to provide 4-[bis(4-bromophenyl)amino]
benzyltriethylammonium trifluoromethanesulfonate (Med-1).
2.5. General procedure for indirect anodic deprotection of
dithioacetal using Med-1 and Med-2
In
a
similar way, 1-{4-[bis(4-bromophenyl)amino]benzyl}-
3-methylimidazolium
trifluoromethanesulfonate (4) was
synthesized by using 1-methylimidazole instead of triethy-
lamine as cation part. Since 4 was too hygroscopic to handle under
air, the anion part of 4 was changed to more hydrophobic anion
like bis(trifluoromethanesulfonyl)amide anion. As expected, the
obtained Med-2 was easy to handle under air.
Constant current electrolysis was carried out with platinum
anode and cathode (1 cm × 1 cm) in 0.6 M TEAP/25 mM H2O/MeNO2
(3.0 mL) containing dithioacetal 7 (0.1 mmol) and Ar3N mediator
(10 mol%) using an undivided cell at room temperature. After the
charge was passed, the solution was passed through a short col-
umn filled with silica gel using EtOAc as an eluent to remove
salts. The product 8a was identified by the comparison with 19F
NMR (−29.17 ppm) and mass spectra of the authentic sample. The
yields were determined by 19F NMR using monofluorobenzene
(−36.50 ppm) as an internal standard.
3.2. Cyclic voltammetry (CV) analysis for Med-1 and Med-2
Cyclic voltammograms of Med-1 and Med-2 in 0.6 M
TEAP/MeCN showed a reversible redox wave, and their oxi-
dation peak potentials were 1.16 V and 1.13 V, respectively as
shown in Fig. 1. Their diffusion coefficients could be estimated
from the gradient of the linear plot of anodic peak current vs.
square root of scan rate based on the Randles–Sevcik formula for a
reversible process (Eq. (1)) [35,36].
2.6. General procedure for indirect anodic fluorodesulfurization
of dithioacetal using Med-1 and Med-2
Constant current electrolysis was carried out with platinum
anode and cathode (1 cm × 1 cm) in 0.3 M Et3N-5HF/MeNO2 (3 mL)
containing dithioacetal 7 (0.1 mmol) and Ar3N mediator (10 mol%)
using an undivided cell at room temperature. After the charge was
passed, the solution was passed through a short column filled with
silica gel using EtOAc as an eluent to remove HF salts. The difluori-
ꢀ
ꢁ
nF
RT
Ipa = 0.44nF
1/2 × AC0D1/2v1/2
(1)
where Ipa is the oxidation peak current [A], n is the number of
reaction electrons, A is the area of electrode [cm2], C0 is the con-
centration of substance [mol cm−3], D is the diffusion coefficient
[cm2 s−1] and v is scan rate [mV s−1]. The estimated diffusion coef-
ficients of Med-1 and Med-2 in MeCN were 7.4 × 10−6 cm2 s−1 and
5.5 × 10−6 cm2 s−1, respectively (Fig. 2).
nated products 9a–9c were identified by the comparison with 19
F
NMR (9a: ı = −9.73, −34.01, 9b: ı = −11.98, 9c: ı = −12.21 ppm) and
mass spectra of authentic samples [34]. The yields were determined
by 19F NMR using monofluorobenzene as an internal standard.
Med-1 and Med-2 were easily soluble in ionic liquid HF salt due
to the introduction of ionic-tag. CV analysis for Med-1 and Med-2
was also carried out in neat ionic liquid HF salt. Cyclic voltammo-
grams of Med-1 and Med-2 in Et3N-5HF showed a reversible redox
wave, and their oxidation peak potentials were 0.77 V and 0.73 V vs.
Fc/Fc+, respectively (Fig. 3). Their estimated diffusion coefficients in
Et3N-5HF were 2.7 × 10−6 cm2 s−1 and 2.6 × 10−6 cm2 s−1, respec-
tively. These values are less than half of those in MeCN (Fig. 4).
3. Results and discussion
3.1. Synthesis of Med-1 and Med-2
Ar3N mediator bearing ionic-tag was synthesized according
to Schemes 1 and 2. First, a formyl group was introduced to
4,4ꢀ-dibromotriphenylamine (1) using Vilsmeier–Haack reaction.
Scheme 2.

50
K. Takahashi et al. / Electrochimica Acta 77 (2012) 47–53
Fig. 1. Cyclic voltammograms of Med-1 (left) and Med-2 (right) (3 mM) in 0.6 M TEAP/MeCN (5 mL) at 25 ◦C at several scan rates; solid circle (᭹): 10 mV s−1, solid square
(ꢀ): 50 mV s−1, solid diamond (ꢁ): 100 mV s−1, cross (x): 200 mV s−1, circle (ꢁ): 300 mV s−1, square (ꢂ): 400 mV s−1, diamond (♦): 1000 mV s−1
.
50
40
30
20
10
0
50
40
30
20
10
0
Med-1
Med-1
Med-2
Med-2
1
0
0.2
0.4
0.6
0.8
1
1.2
0
0.2
0.4
0.6
0.8
1.2
v1/2 / V1/2s-1/2
v1/2 / V1/2s-1/2
Fig. 2. Plots of Ipa (Med-1: circle; ꢁ, Med-2: square; ꢂ) in 0.6 M TEAP/MeCN (5 mL)
as a function of v1/2
Fig. 4. Plots of Ipa (Med-1: circle; ꢁ, Med-2: square; ꢂ) in Et3N-5HF as a function
of v1/2
.
.
Diffusion coefficient of tris(p-bromophenyl)amine in MeCN
the diffusion coefficients of Med-1 and Med-2 in HF salt ionic liquid
and ionic liquid, 1-butyl-3-methylimidazolium hexafluorophos-
were little smaller than those of ordinary molecular solvents like
phate [BMIm][PF6] were 1.3 × 10−5 cm2 s−1 and 2.8 × 10−8 cm2 s−1
,
MeCN. This is because of low viscosity of Et3N-5HF (1.30 mPa s−1
)
respectively [37]. In general, diffusion coefficient of organic
molecules in ionic liquid is ca. 500–1000 times lower than that in
MeCN due to the highly viscous nature of ionic liquid [38]. However,
[39–41]. Therefore, it was found that such low viscous HF salt ionic
liquids like Et3N-5HF can be used as electrolytic reaction media
similarly to molecular organic solvents.
Fig. 3. Cyclic voltammograms of Med-1 (left, 3 mM) and Med-2 (right, 3 mM) in Et3N-5HF (5 mL) at 25 ◦C referenced to Ag wire with external ferrocene/ferrocenium (Fc/Fc+,
3 mM) at several scan rates; solid circle (᭹): 10 mV s−1, solid square (ꢀ): 50 mV s−1, solid diamond (ꢁ): 100 mV s−1, cross (x): 200 mV s−1, circle (ꢁ): 300 mV s−1, square (ꢂ):
400 mV s−1, diamond (♦): 1000 mV s−1
.

K. Takahashi et al. / Electrochimica Acta 77 (2012) 47–53
51
Fig. 5. Cyclic voltammgrams of Med-1 (left, 3 mM) and Med-2 (right, 3 mM) in 0.6 M TEAP/MeNO2 (5 mL), (solid circle; ᭹) at 100 mV s−1, (solid square; ꢀ) in the presence of
7a (15 mM) at 100 mV s−1, (diamond; ♦) in the presence of 7a (15 mM) at 300 mV s−1 (left) or 200 mV s−1 (right), (square; ꢂ) in the presence of 7a (15 mmol) at 1000 mV s−1
.
Fig. 6. Cyclic voltammgrams of Med-1 (left, 3 mM) and Med-2 (right, 3 mM) in Et3N-5HF (5 mL) using Ag wire with external ferrocene/ferrocenium (Fc/Fc+, 3 mM), (solid
circle; ᭹) at 100 mV s−1, (solid square; ꢀ) in the presence of 7a (15 mM) at 100 mV s−1, (diamond; ♦) in the presence of 7a (15 mM) at 1000 mV s−1
.
Ar3N derivatives have been utilized as an electrocatalytic medi-
ator for electrochemical oxidative deprotection of dithioacetal
compounds [14,15]. Therefore, CV measurements for Med-1 and
Med-2 were carried out in the absence and the presence of dithioac-
etal 7a (15 mM) (Epa = 2.30 V vs. SCE, 1.79 V vs. Fc/Fc+) as a model
substrate in MeNO2 and Et3N-5HF, respectively (Figs. 5 and 6). In
both media, the oxidation peak currents for Med-1 and Med-2 were
markedly increased in the presence of 7a, while the re-reduction
peaks completely disappeared at a scan rate of 100 mV s−1. Even
at a scan rate of 300 mV s−1, the re-reduction peaks mostly dis-
ca. 80% (Entry 4). In sharp contrast, constant-potential electrolysis
of 7a at this potential did not proceed at all in the absence of Med-1.
Next, electrocatalytic oxidative fluorodesulfurization of
dithioacetal 7a using Med-1 and Med-2 in MeNO2 containing ionic
liquid HF salt, Et3N-5HF was carried out. As shown in Table 2,
fluorodesulfurization without mediator resulted in the fluorinated
product 9a in low yield (Entries 1–3). On the other hand, in the
presence of a mediator, the yield of 9a increased appreciably
compared to the case without mediator (Entries 4–9). However,
the yield of 9a was up to 54% due to the formation of considerable
amount of ketone 8a as a by-product. According to the reaction
mechanism previously proposed [15,42], the cation intermediate
generated by the oxidation of 7a is attacked by a fluoride anion to
provide fluorinated product 9a. However, the nucleophilic attack
by contaminating water is also possible to give by-product 8a.
In order to reduce the contaminating water from the reaction
system, we conducted the electrolysis in neat ionic liquid HF salt,
Et3N-5HF without organic solvent. In this system, ionic liquid, Et3N-
5HF works not only as a fluorine source and electrolyte, but also as
reaction media [43]. Due to the acidic nature of the HF salt, the
contaminating water would be protonated to be less nucleophilic
as reported previously [44]. Moreover, these mediators are well
compatible with the ionic liquid HF salt. The results are shown in
Table 3.
appeared. However, at extremely high scan rate like 1000 mV s−1
,
the re-reduction peaks appeared. Therefore, the enhanced anodic
currents seem to be typical catalytic currents.
3.3. Mediatory use of Med-1 and Med-2 for electrocatalic
reaction
On the basis of the results of the CV measurements above, the
macroscale electrocatalytic oxidative deprotection of dithioacetal
7a was carried out using Med-1 as a mediator. The results are sum-
marized in Table 1.
The deprotection reaction using Med-1 in the absence of base
did not proceed well (Entry 1), whereas the deprotection was
achieved in the presence of base to give the corresponding ketone,
8a in good yield (Entry 2). Interestingly, constant-potential electrol-
ysis at 1.10 V vs. SCE provided 8a in favorable yield even without
base (Entry 3). In the presence of base, the yield of 8a increased to
Although, the fluorodesulfurization of 7a did not take place
efficiently in the absence of a mediator, the desired fluorination
using the mediators proceeded well (Entries 2–6). Especially, when

52
K. Takahashi et al. / Electrochimica Acta 77 (2012) 47–53
Table 1
Electrocatalytic oxidative deprotection of dithioacetal 7a using mediator, Med-1.a
Entry
Base
Methodb
Yield of 8a (%)c
Recovery of 7a (%)c
1
2
3
4
–
C. C.
C. C.
C. P.
C. P.
42
81
71
78
29
3
22
12
NaHCO3
–
NaHCO3
a
Reaction conditions: 7a (0.1 mmol) and base (0.5 mmol) in 0.6 M Et4NClO4/Solvent (3 mL)
b
C. C. = constant current electrolysis at 5 mA/cm2.
C. P. = constant potential electrolysis at 1.10 V vs. SCE.
c
Determined by 19F NMR.
Table 2
Electrocatalytic difluorodesulfurization of dithioacetal 7a in MeNO2 containing Et3N-5HF using mediators.a
Entry
Mediator
Electricity (F/mol)
Yield (%)b
Recovery of 7a (%)b
9a
8a
1
2
3
4
5
6
7
8
9
–
2
4
6
4
6
8
2
3
4
27
35
39
41
53
54
35
44
52
10
8
21
3
0
32
19
0
41
30
19
12
19
25
24
16
15
16
Med-1
Med-2
a
Reaction conditions: 7a (0.1 mmol) in 0.3 M Et3N-5HF/MeNO2 (3 mL).
b
Determined by 19F NMR.
Table 3
Electrocatalytic difluorodesulfurization of dithioacetal 7 in ionic liquid, Et3N-5HF using mediators.a
Entry
Substrate
Oxidation potential (V vs. SCE)
Mediator
Electricity (F/mol)
Yield (%)b
Recovery of 7 (%)b
9
8
1
2
3
4
5
6
7
8
7a: X = F
7a: X = F
7a: X = F
7a: X = F
7a: X = F
7a: X = F
7b: X = Cl
7c: X = H
2.30
2.30
2.30
2.30
2.30
2.30
2.30
2.20
–
4
3
4
2
3
4
6
4
30
72
53
61
74
8
9
21
12
8
8
8
Med-1
Med-1
Med-2
Med-2
Med-2
Med-2
Med-2
Trace
26
14
80
73
9
Trace
c
c
–
–
34 (45d)
–
–
c
c
a
Reaction conditions: 7 (0.1 mmol) in 0.3 M Et3N-5HF (3 mL).
Determined by 19F NMR.
Not determined.
b
c
d
PEG (2.5%) was added.

K. Takahashi et al. / Electrochimica Acta 77 (2012) 47–53
53
Med-2 was used as mediator, desired 9a was obtained in good yield
suppressing the formation of 8a (Entry 6).
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Furthermore, 7b having an electron-withdrawing Cl group at the
para-position of the benzene ring underwent electrocatalytic fluo-
rodesulfurization to provide the desired fluorinated product 9b in
good yield (Entry 7). On the other hand, non-substituted 7c pro-
vided 9c in moderate yield even in addition of PEG to enhance the
nucleophilicity of fluoride anion (Entry 8) [45,46]. This suggested
that the difluorodesulfurization of dithioacetal substrates was
strongly affected by substituents on the benzene ring. Previously,
we reported similar results, namely, that electron-withdrawing
groups markedly promoted the anodic fluorodesulfurization due to
the high electrophilic reactivity of the anodically generated benzyic
cation intermediates [42].
The reusability of Med-2 in anodic fluorodesulfurization of 7a
as a model substrate was investigated under the same conditions
as Entry 6 in Table 3. The product in the ionic liquid Et3N-5HF was
readily extracted with hexane, and the ionic liquid containing Med-
2 was reused for next runs. However, the yield of 9a decreased
from 80% to 51% and 48% for the second and the third electrolyses,
respectively. The recovered mediator, Med-2 showed a slightly less
positive redox peaks in the cyclic voltammogram compared to the
original one. However, the real structure of the recovered media-
tor could not be established. Therefore, the decrease of the yield
seems to be as due to the partial modification of the mediator dur-
ing the electrolysis. Although the yield of the fluorination product
decreased when Med-2 was reused, it was demonstrated that the
mediator having an ionic-tag moiety could be recyclable to some
extent.
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Society 124 (2002) 926.
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1311.
4. Conclusion
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In conclusion, novel Ar3N mediators containing ionic-tags
such as ammonium and imidazolium moieties were synthesized
from 4,4ꢀ-dibromotriphenylamine. The introduction of the ionic-
tag enhanced the solubility of the mediators in ionic liquid HF
salt, Et3N-5HF. Electrocatalytic reaction such as deprotection and
fluorodesulfurizaiton of dithioacetals using these mediators in
molecular solvent, MeNO2 and ionic liquid HF salt, Et3N-5HF was
successfully carried out to produce the desired deprotected and
difluorinated products, respectively. Since the fixation of the medi-
ators in ionic liquid HF salt was possible even after extraction with
a less polar organic solvent like hexane, the recycle use of the medi-
ators was successfully demonstrated to some extent.
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