G Model
EA 26659 No. of Pages 5
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M. Tanigawa et al. / Electrochimica Acta xxx (2015) xxx–xxx
CDCl3. The chemical shifts for 1H, 13C and 19F NMR spectra were
given in (ppm) from internal TMS, CDCl3 and monofluoroben-
zene, respectively. Cyclic voltammetry was performed using BAS
ALS Instruments model 600 A. Preparative electrolysis experi-
ments were carried out with Metronnix Corp. (Tokyo) constant
current power supply model 5944 by monitoring electricity with
Hokutodenko Coulomb/Ampere-hour meter HF-201.
through a short column filled with silica gel using n-hexane as an
eluent to remove excess allyltrifluoroborate. The yield of 5 in the
eluent was estimated by 1H NMR using nitromethane as an internal
standard.
d
2.7. A typical procedure for anodic fluorination
A constant current (5 mA/cm2) was passed for anodic fluorina-
tion of 2 (1.0 mmol) in a plastic undivided cell equipped with
platinum anode and cathode (2 cm ꢀ 2 cm) containing 10 mL of 1 M
Et3N-3HF/DME. After electrolysis, the electrolytic solution was
passed through a short column with silica gel to remove salts. The
yield of 6 was estimated by 19F NMR using monofluorobenzene as
an internal standard.
2.2. Measurement of cyclic voltammetry
Cyclic voltammetry was carried out in a glass cell. A platinum
disk electrode (w= 0.8 mm) was used as a working electrode. A
platinum plate (1 cm ꢀ 1 cm) was used as a counter electrode. A
saturated calomel electrode was used as a reference electrode.
Electrolyte solutions for cyclic voltammetry were deoxygenated
with bubbling N2 gas before use.
3. Results and discussion
2.3. Materials
3.1.
b-Effect of trifluoroborate
Thioanisole (1), 4,4,5,5-tetramethyl-2-phenylsulfanylmethyl-
[1,3,2]dioxaborolane (2), and allyltrimethylsilane were purchased
and used without purification. Potassium phenylthiomethyltri-
fluoroborate [24] and tetra-n-butylammonium allyltrifluoroborate
[25] were prepared according to the literatures. The known
methoxylated, ethoxylated, acetoxylated, allylated and fluorinated
products (4a,4b, 4c, 5 and 6) were identified by comparison with
the spectral data of their authentic samples [26–28].
At first, cyclic voltammetry measurements of thioanisole (1),
4,4,5,5-tetramethyl-2-phenylsulfanylmethyl-[1,3,2]-dioxaboro-
lane (2) and tetra-n-butylammonium phenylthiomethyltrifluor-
oborate (3) were carried out (Fig. 1). The introduction of boron
atom into the
a-position of the sulfur atom of thioanisole did not
influence the oxidation peak potential. However, the oxidation
peak potential of 3 was markedly decreased by ca. 0.5 V. This
cathodic shift was caused by the existence of a trifluoroborate
moiety in the molecule.
2.4. Synthesis of tetra-n-butylammonium
phenylthiomethyltrifluoroborate (3)
DFT calculation was performed using Gaussian 03 suit of
programs [29] for further understanding the CV results. The
structures of boronate ester 2 and borate 3 were optimized using
the B3LYP/6-31G(d,p) method. The orbital diagrams were gener-
ated by using the GaussView program [30]. The highest occupied
molecular orbital (HOMO) of 3 was mainly located on the sulfur
To a stirred solution of potassium phenylthiomethyltrifluor-
oborate (5 mmol) in CH2Cl2 (10 mL), tetra-n-butylammonium
hydroxide (5 mmol) in water (40%) was added and the reaction
mixture was stirred for 2 h at room temperature. The mixture was
added brine and extracted with CH2Cl2. The organic phase was
dried over MgSO4 and evaporated under reduced pressure. A clear
crystal was obtained in 100% yield. 1H NMR (270 MHz, CDCl3,
atom and the C-B
overlapping of these two orbitals seemed to result in the marked
decrease of the oxidation potential. Such -effect of organoborane
s bond as shown in Fig. 1. The observed well-
b
compounds has never been observed so far although it is well-
ppm):
8H), 2.00 (q, J = 5.4 Hz, 2H), 1.65–1.53 (m, 8H), 1.48–1.35 (m, 8H),
0.98 (t, J = 7.2 Hz, 12H); 13C NMR (67.8 MHz, CDCl3, ppm):
143.4,
128.0, 125.1, 122.7, 58.5, 23.9, 19.7, 13.7; Anal. Calcd. for
23H43BF3NS: C, 63.73; H, 10.00; N, 3.23; S, 7.40. Found: C,
d
7.24–7.14 (m, 4H), 6.97 (t, J = 7.1 Hz, 1H), 3.19 (t, J = 8.3 Hz,
known in organosilicon chemistry [31–34]. Therefore, this is the
first example of
marked decrease of oxidation potentials of
compounds due to the -effect. However, they observed only slight
decrease of the oxidation potentials of organosulfur analogues.
They employed the -effect of a silyl group in electrochemical
b
-effect of organoboranes. Yoshida et al. found the
d
a-silylorganooxygen
b
C
63.89; H, 10.24; N, 3.17; S, 7.35. The ammonium salt shows higher
solubility in organic solvents than the precursor potassium salt.
b
2.5. A typical procedure for anodic methoxylation, ethoxylation and
acetoxylation
Constant current (5 mA/cm2) anodic oxidation of 2 or 3
(1 mmol) was carried out with a graphite anode (2 cm ꢀ 2 cm)
and a platinum cathode (2 cm ꢀ 2 cm) in 10 mL of alcohol with and
without 0.1 M Et4NOTs or in 0.5 M NaOAc/AcOH in an undivided
cell. After electrolysis, to the mixture was added water and
extracted with CHCl3. The organic phase was washed with an
aqueous solution of NaHCO3 and dried over MgSO4. The solution
was evaporated under reduced pressure and the crude product was
purified by silica gel column chromatography.
2.6. A typical procedure for anodic allylation
Constant current (5 mA/cm2) anodic allylation of 3 (0.2 mmol)
was carried out with a graphite anode (2 cm ꢀ 2 cm) and a
platinum cathode (2 cm ꢀ 2 cm) in 10 mL of tetra-n-butylammo-
nium allyltrifluoroborate (2.0 mmol)/MeNO2 in an undivided cell
at 50 ꢁC. After electrolysis, the electrolytic solution was passed
Fig. 1. Oxidation peak potentials (Epox) of sulfides, measured in Bu4NClO4/MeCN
and HOMO diagrams of 2 and 3. Isovalue is 0.02.
Please cite this article in press as: M. Tanigawa, et al., Electrochemical Properties and Reactions of Sulfur-Containing Organoboron Compounds,