Direct and indirect electrochemical generation of alkoxycarbenium ion pools from thioacetals

Article abstract of DOI:10.1016/j.tet.2009.09.020

Thioacetals were found to be effective precursors to generate and accumulate alkoxycarbenium ions based on direct and indirect cation pool methods. Alkoxycarbenium ions thus generated reacted with carbon nucleophiles such as allylsilanes and enol silyl et

Full text of DOI:10.1016/j.tet.2009.09.020

Tetrahedron 65 (2009) 10901–10907
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Tetrahedron
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Direct and indirect electrochemical generation of alkoxycarbenium
ion pools from thioacetals
1
*
Kouichi Matsumoto, Koji Ueoka, Shinkiti Suzuki, Seiji Suga , Jun-ichi Yoshida
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 July 2009
Received in revised form 22 August 2009
Accepted 7 September 2009
Available online 11 September 2009
Thioacetals were found to be effective precursors to generate and accumulate alkoxycarbenium ions
based on direct and indirect cation pool methods. Alkoxycarbenium ions thus generated reacted with
carbon nucleophiles such as allylsilanes and enol silyl ethers to give C–C bond formation products in
good yields.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
electrolysis.5 The cation pool method serves as a powerful tool not
only for mechanistic studies on highly reactive organic cations but
Alkoxycarbenium ions, carbocations stabilized by a neighboring
alkoxy group, are important intermediates in organic synthesis.1
Usually alkoxycarbenium ions are generated from acetals using
Lewis acids such as BF3–OEt2, SnCl4, and TiCl4 in the presence of
a nucleophile (Scheme 1). Although benzylic alkoxycarbenium ions
and di- and tri(alkoxy)carbenium ions are stable and are well
characterized spectroscopically,2 simple alkylalkoxycarbenium ions
are unstable and are difficult to characterize spectroscopically. The
generation processes for such unstable alkoxycarbenium ions are
reversible, and their equilibrium concentrations are usually very
low. In fact, extensive NMR studies on the reaction of acetals with
Lewis acids revealed the presence of Lewis acid–acetal complexes,
but alkoxycarbenium ions were not detected.3
also for rapid parallel synthesis. The method has been successfully
applied to alkoxycarbenium ions,6 in addition to N-acyliminium
ions,7 diarylcarbenium ions,8 organosilicon cations,9 and iodine
cations.10
a-Silyl ethers serve as effective precursors of alkoxy-
carbenium ion pools (Scheme 2). Silyl groups serve as effective
electroauxiliaries, which lower the oxidation potentials and control
the regiochemistry by virtue of selective C–Si bond cleavage.11
Alkylalkoxycarbenium ions thus generated are well characterized
by NMR spectroscopy, which exhibits very similar spectra to those
generated in super acid media.12 In the next step, the cations react
with carbon nucleophiles such as allylsilanes to give the corre-
sponding carbon–carbon bond formation products.
electrochemical
OR'
OR'
OR'
SiR'3
oxidation
OR'
Nu
Nu
Lewis Acid
OR'
Nu
OR'
OR'
OR'
Nu
low
R
R
R
R
R
R
R
L
R
temperature
reversible
irreversible
L = OR' etc.
Scheme 2.
Scheme 1.
We have been searching for alternative precursors of alkoxy-
carbenium ion pools and have been interested in thioacetals as
precursors because of the following reasons: 1) An arylthio group is
also an effective electroauxiliary for electrochemical oxidation.13 2)
Thioacetals can be easily prepared. In preliminary communica-
tions,14 we have reported generation of alkoxycarbenium ion pools
from thioacetals by low-temperature electrolysis (direct cation pool
method) and by the action of ArS(ArSSAr)þ, which is generated by
low-temperature electrolysis of ArSSAr (indirect cation pool
method) (Scheme 3). In this paper we report full details of these
studies.
The electrochemical method is also effective for generation of
alkoxycarbenium ions.4 Recently we have developed the ‘cation
pool’ method, in which organic cations are generated and accu-
mulated in the absence of nucleophiles by low-temperature
* Corresponding author. Tel.: þ81 75 383 2726; fax: þ81 75 383 2727.
E-mail address: yoshida@sbchem.kyoto-u.ac.jp (J.-i. Yoshida).
1
Present address: Division of Chemistry and Biochemistry, Graduate School of
Natural Science and Technology, Okayama University, 31-1 Tsushima-naka, Kita-ku,
Okayama 700-8530, Japan.
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.09.020

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K. Matsumoto et al. / Tetrahedron 65 (2009) 10901–10907
of the electrolysis should stay in the solution until the electrolysis is
direct cation pool method
-e
or
completed. This means the application of direct cation pool method
strongly depends on the stability of accumulated alkoxycarbenium
ions.15 During the accumulation, unstable alkoxycarbenium ions
might decompose. To solve this problem, we examined indirect
electrochemical method.16 In the first step, an active chemical re-
agent is generated by the electrolysis, which is allowed to react
with a precursor to generate an alkoxycarbenium ion pool in the
second step. The second reaction can be faster than the electrolysis
because it takes place in a homogeneous solution.
OMe
OMe
SAr
+
0.5 ArSSAr
R
R
electrogenerated
ArS(ArSSAr)+
alkoxycarbenium ion
pool
indirect cation pool method
Scheme 3.
2. Result and discussion
2.1. Direct cation pool method
We envisaged that ‘ArSþ’17 generated by the electrochemical
oxidation of ArSSAr might be suitable for the conversion of thio-
acetals to alkoxycarbenium ion pools. It is known in the literature
that ‘ArSþ’ 18 or its equivalent ArS(ArSSAr)þ19 serves as a powerful
and highly thiophilic reagent, though the nature of the generated
species has not yet been fully elucidated.20
Thus, the electrolysis of ArSSAr (Ar¼p-XC6H4) (oxidation po-
tential: X¼F: 1.47 V, X¼CH3: 1.34 V, X¼OMe: 1.47 V, X¼Cl: 1.27 V,
2 equiv based on thioacetal 1) was carried out in Bu4NBF4/CH2Cl2 at
ꢀ78 ꢁC using an H-type divided cell (Scheme 6, step 1). In the next
step, the electrogenerated ArS(ArSSAr)þ was reacted with thio-
acetal 1 to generate alkoxycarbenium ion 2 (step 2). This process
requires only 5 min at ꢀ78 ꢁC. In the third step, allyltrimethylsilane
(10 equiv) was added to obtain the final product 3.
At first, we examined the direct electrochemical method using
thioacetal 1 as a precursor of an alkoxycarbenium ion. Thus, a solution
of 1 was electrolyzed in Bu4NBF4/CD2Cl2 at ꢀ78 ꢁC using an H-type
divided cell (Scheme 4). After the electrolysis (1.4 F/mol), theresulting
solution was analyzed by NMR spectroscopy at ꢀ80 ꢁC. 1H and 13C
NMR spectra (ꢀ80 ꢁC) indicated the formation of alkoxycarbenium
ion(2)byselectivecleavageoftheC–Sbond.1HNMRexhibitedasignal
at
230.9 ppm due to methine carbon. These chemical shifts are quite
d
9.55 ppm due tothemethine proton.13C NMRexhibited a signalat
d
similar to those obtained by the direct electrochemical oxidation of
C8H17CH(OMe)SiMe3 (9.55 and 231.0 ppm),6a suggesting that the
alkoxycarbenium ion is efficiently generated and accumulated as
a single species in irreversible fashion. Though PhSSPh isgenerated by
the oxidation of 1, the oxidation potential of 1 (RDE decomposition
potential: 1.30 V vs. SCE) is lower than that of PhSSPh (1.42 V), in-
dicating that PhSSPh cannot be a mediator for the oxidation of 1.
-e
step 1
ArSSAr
0.67 ArS(ArSSAr)+
ArSSAr
0.67 F/mol
(Ar = p-XC6H4)
e
ArS(ArSSAr)+
anode
step 3
step 2
SiMe3
(10 eq)
4.95 ppm
230.9 ppm
OMe
OMe
SPh
OMe
OMe
5 min,
-78 °C
C8H17
C8H17
C8H17
OMe
SPh
-e
-78 °C
Bu4NBF4/CD2Cl2
1 eq
0.5 PhSSPh
+
3
C8H17
H
1
2
C8H17
1
Scheme 6.
9.55 ppm
2
alkoxycarbenium
ion pool
The results obtained under various conditions are summarized
in Table 1. Electrolysis with 1.34 F/mol based on 1 (0.67 F/mol based
on ArSSAr) gave the best yield of 3 (entry 2). 0.67 F/mol is the
theoretical amount of electricity to convert ArSSAr to ArS(ArSSAr)þ.
This means that 1.34 equiv of ArS(ArSSAr)þ based on 1 was formed,
because 2 equiv of ArSSAr based on 1 was used. Almost quantitative
yield of 3 (98%) suggests that alkoxycarbenium ion 2 was generated
almost quantitatively. High reactivity of ArS(ArSSAr)þ and homo-
geneity of the reaction system seem to be responsible for fast and
quantitative generation of 2. Use of 1.5 equiv of ArSSAr led to
a decrease in the yield of 3 (entry 5). Use of 3.0 equiv of ArSSAr also
led to a decrease in the yield of 3 (entry 6). Other ArSSAr (Ar¼C6H5,
p-CH3C6H4, p-MeOC6H4 and p-ClC6H4) were also effective, although
the yield of 3 depends on the nature of the substituent on the aryl
group (entries 7–10).
Scheme 4.
The reaction of 2 with allyltrimethylsilane gave the allylated
product 3 in 60% (Scheme 5). The electrochemical method was
found to be applicable to cyclic thioacetals (4 and 5), which were
effectively oxidized under similar conditions. After the electrolysis,
the resulting solutions were allowed to react with an allylsilane to
give the corresponding C–C bond formation products 6 and 7 in 82%
and 38% yields, respectively.
SiMe3
C8H17
OMe
H
OMe
SPh
OMe
60%
-e, -SPh
-78 °C
Bu4NBF4/CH2Cl2
C8H17
C8H17
1
3
2
+
0.5 PhSSPh
Table 1
SiMe3
Optimization of the indirect cation pool method
n
n
n
Entry
X
ArSSAr (equiv
based on 1)
Electricity (F/mol
based on ArSSAr)
Yield (%) of 3
-e, -SPh
-78 °C
Bu4NBF4/CH2Cl2
SPh
O
O
O
1
2
3
4
5
6
7
8
9
F
F
F
F
F
F
H
CH3
OMe
Cl
2
2
2
2
1.5
3
2
2
2
0.50
0.67
0.75
1.0
0.67
0.67
0.67
0.67
0.67
0.67
69
98
91
87
73
86
79
79
69
96
+
4 (n = 1)
5 (n = 2)
6 (n = 1)
7 (n = 2)
82%
38%
0.5 PhSSPh
Scheme 5.
2.2. Indirect cation pool method
Because electrochemical reactions take place only on the surface
of the electrode, alkoxycarbenium ions generated in an early stage
10
2

K. Matsumoto et al. / Tetrahedron 65 (2009) 10901–10907
10903
The formation of 2 was confirmed by NMR spectroscopy at
ꢀ80 ꢁC. A solution obtained by the reaction of 1 with the electro-
generated ArS(ArSSAr)þ (Ar¼p-FC6H4) exhibited signals at 9.53 and
4.92 ppm due to the methine proton and methyl protons, re-
spectively (1H NMR), and a signal at 230.6 ppm due to the methine
carbon (13C NMR) (Scheme 7a). These chemical shifts are quite
similar to those obtained by the direct electrochemical oxidation of
C8H17CH(OMe)SiMe3 (9.55, 4.95, and 231.0 ppm).6a Such similarity
in chemical shifts indicated that the sulfur-containing byproducts,
such as PhSSAr and ArSSAr, which should be present in the solution,
did not change the nature of alkoxycarbenium ion 2 appreciably.21
We also generated and analyzed an alkoxycarbenium ion by the
reaction of 5 with ArS(ArSSAr)þ (Ar¼p-FC6H4). The cation exhibited
a signalat9.86 ppmduetothemethineproton (1HNMR)andasignal
at 227.8 ppm due to the methine carbon (13C NMR) (Scheme 7b).
As described above, alkoxycarbenium ion 2 generated by the
indirect method exhibited NMR spectra similar to that generated by
the direct anodic oxidation of C8H17CH(OMe)SiMe3.6a Thus, we next
compared the thermal stability of 2 as follows: The cation pool
generated at ꢀ78 ꢁC was allowed to warm to a second temperature.
After being kept there for 30 min, the pool was allowed to react
with allyltrimethylsilane. The yield of 3 is plotted against the
temperature in Figure 1. The cation pool of 2 generated by the in-
direct method exhibited thermal stability similar to that generated
by the direct method of C8H17CH(OMe)SiMe3.6a
100
80
direct method
60
40
4.92 ppm
230.6 ppm
OMe
indirect method
OMe
SPh
ArS(ArSSAr)+
(a)
(b)
20
0
-78 °C
Bu4NBF4/CH2Cl2
C8H17
H
C8H17
9.53 ppm
1
2
-80
-60
-40
-20
0
20
alkoxycarbenium
ion pool
temperature (°C)
Figure 1. Thermal stability of alkoxycarbenium ion 2 generated by the direct method
and the present indirect method.
ArS(ArSSAr)+
227.8 ppm
9.86 ppm
-78 °C
Bu4NBF4/CH2Cl2
O
SPh
O
H
To test the applicability of the present indirect method, reactions
with several carbon nucleophiles were examined (Table 2). Allylsi-
lanes, enolsilyl ethers, andketenesilyl acetals, andenol acetatewere
5
alkoxycarbenium
ion pool
Scheme 7.
The detailed mechanism for the reaction of 1 with ArS(ArSSAr)þ
(step 2) has not been clarified as yet, but 2 seems to be generated
according to Scheme 8. Though the possibility of a single electron-
transfer mechanism cannot be ruled out, ionic mechanism seems to
be more plausible. In the ionic mechanism ArS(ArSSAr)þ acted as
a thiophilic Lewis acid.
Table 2
The reaction of alkoxycarbenium ion with various carbon nucleophilesa
Nu
Product
Yield (%)b
OMe
3
8
89c (98)c,d
SiMe3
SiMe3
C8H17
C8H17
OMe
Ar
91 (1.25:1)e
OMe
SPh
OMe
S
S
+
+
+
ArSSPh ArSSAr
Ar
S
C8H17
C8H17
2
1
Ar
OMe O
OMe O
OSiMe3
Ph
79
9
Scheme 8.
C8H17
C8H17
Ph
In order to get a deeper insight into the mechanism, reactions of 1
with other Lewis acids such as BF3–OEt2 and SnCl4 were examined.22
The resulting solutionwas analyzed by NMR spectroscopyat ꢀ80 ꢁC,
but alkoxycarbenium ion 2 was not detected at all.3 Presumably
these Lewis acids are not strong enough to generate 2 in a significant
concentration. The equilibrium between 1 and 2 lies to 1.
Much lower reactivity of BF3–OEt2 was confirmed by the fol-
lowing experiments. The reaction of thioacetal (1) with
ArS(ArSSAr)þ (Ar¼p-FC6H4)in 5 min followed by treatment with
allyltrimethylsilane in 1 min at ꢀ78 ꢁC gave 3 in 90% yield Eq .(1). In
contrast, the reaction using a BF3–OEt2 did not give 3 in an ap-
preciable amount under similar conditions Eq. (2).
OSiMe3
10
42 (2.45:1)e
OSiMe3
OMe
OMe O
11
67
C8H17
OMe
OMe O
OMe O
OAc
63f
12
C8H17
C8H17
O
O
43d
13
OMe
OMe
SiMe3
ArS(ArSSAr)+
SPh
5 min, -78 °C
O
a
C8H17
3
C8H17
ArSSAr (Ar¼p-FC6H4, 0.40 mmol) was oxidized electrochemically in 0.3 M
2 eq, -78 °C
1 min
then Et3N
ð1Þ
ð2Þ
Bu4NBF4/CH2Cl2 at ꢀ78 ꢁC using 0.67 F/mol of electricity. The solution thus obtained
was allowed to react with 0.20 mmol of thioacetal at ꢀ78 ꢁC for 5 min. Then
a nucleophile (0.50 mmol, 2.5 equiv) was added, and the resulting solution was
stirred at ꢀ78 ꢁC for 15 min and the reaction was quenched with Et3N (1 mL).
1
90 %
BF3-OEt2
1.1 eq
b
OMe
OMe
0 %
Isolated yield.
GC yield.
10 equiv of Nu was used.
Diastereomer ratio.
SiMe3
c
C8H17
SPh
C8H17
2 eq, -78 °C
d
5 min, -78 °C
e
3
1
1 min
then Et3N
f
The reaction time with a nucleophile was 30 min.

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K. Matsumoto et al. / Tetrahedron 65 (2009) 10901–10907
Table 3
Indirect generation of various alkoxycarbenium ion pools followed by reactions with carbon nucleophiles
Substrate Nu
Product
Yielda (%)
82b
Cl
F
OMe
OMe
SiMe3
3
14
C8H17
C8H17
C8H17
C8H17
C8H17
C8H17
S
OMe
OMe
OMe
S
83b
SiMe3
SiMe3
SiMe3
15
3
Me
OMe
S
72b
16
3
OMe
OMe
91c
17
18
Ph
SPh
Ph
OMe
OMe
20
19
SiMe3
SiMe3
68
SPh
86 (ca. 9:1)d
85 (ca. 7:1)d
4
6
O
SPh
SPh
O
SiMe3
5
7
O
O
a
b
c
Isolated yield.
GC yield.
The acetal was reacted with ArS(ArSSAr)þ for 10 min, and the resulting alkoxycarbenium ion was reacted with allyltrimethylsilane for 1 h.
Diastereomer ratio.
d
effective as carbon nucleophiles, and the corresponding C–C bond
formation products were obtained. 1,3-dicarbonyl compounds such
as acetylacetone were also effective to form carbon–carbon bond.
Finally, we examined generation of alkoxycarbenium ion pools
from several thioacetals. The p-ClC6H4S, p-FC6H4S, and p-MeC6H4S
groups were also effective as electroauxiliaries. Aryl and alkyl
substituted thioacetals, including cyclic substrates, were effective
for generation of alkoxycarbenium ion pools. The resulting alkoxy-
carbenium ion pools reacted with carbon nucleophiles such as
allylsilanes to give the corresponding C–C bond formation products
in good yields as shown in Table 3.
silica capillary. 1H and 13C NMR spectra were recorded in CDCl3 on
a Varian Gemini 2000 (1H 300 MHz, 13C 75 MHz), Varian MERCURY
plus-400 (1H 400 MHz, 13C 100 MHz), JEOL A-500 (1H 500 MHz, 13
C
125 MHz), or JEOL ECA-600P (1H 600 MHz, 13C 150 MHz) spec-
trometer with Me4Si as an internal standard unless otherwise
noted. Mass spectra were obtained on JEOL JMS SX-102A mass
spectrometer. IR spectra were measured with a SHIMADZU FTIR
1600 spectrometer. Thin layer chromatography (TLC) was carried
out by using Merck precoated silica gel F254 plates (thickness
0.25 mm). Flash chromatography was carried out on a column of
silica gel (Kanto Chem. Co., Silica Gel N, spherical, neutral, 40–
100 mm). Gel permeation chromatography (GPC) was carried out
on Japan Analytical Industry LC-908 equipped with JAIGEL-1H and
2H using CHCl3 as eluent. All reactions were carried out under Ar
atmosphere unless otherwise noted.
3. Conclusions
In conclusion, we have developed a method for generation and
accumulation of alkoxycarbenium ions from thioacetals by direct
low-temperature electrolysis. We have also developed a sequential
one-pot indirect method for generation and accumulation of
alkoxycarbenium ions. In the first step, ArSSAr is oxidized by low-
temperature electrolysis to generate and accumulate ArS(ArSSAr)þ,
which is allowed to react with a thioacetal to generate and accu-
mulate an alkoxycarbenium ion. The cation pool thus generated
reacts with various carbon nucleophiles such as allylsilanes and
silyl enol ethers. The direct and indirect methods for generation of
alkoxycarbenium ion pools from thioacetals add a new dimension
to organic cation chemistry and organic electrochemistry.
4.2. Materials
Dichloromethane was washed with water, distilled from P2O5,
redistilled from dried K2CO3 to remove a trace amount of acid, and
stored over molecular sieves 4A. Dry THF was used as obtained
(Kanto Chemical Co., Inc.). ArSSAr (Ar¼p-FC6H4) was prepared
according to the procedures in the literatures,23 and identified by
the comparison of its spectral data with that of authentic sample.24
4.3. Thioacetals
4. Experimental section
4.1. General remarks
1-Methoxy-1-phenylthiononane (1),13c 2-phenylthiotetrahydrofuran
(4)25 and 2-phenylthiotetrahydropyran (5)25 were prepared
according to the literature procedures.
GC analysis was performed on a gas chromatograph (SHIMADZU
GC-14B) equipped with a flame ionization detector using a fused
4.3.1. 1-Methoxy-1-(4-chlorophenylthio)nonane (14). To a solution
of 4-chlorobenzenethiol (5.19 g, 36.0 mmol) in tetrahydrofuran

K. Matsumoto et al. / Tetrahedron 65 (2009) 10901–10907
10905
(THF) (64 mL), was added triethylamine (7.2 mL, 52 mmol) and
chloromethyl methyl ether (3.3 mL, 43 mmol) at 0 ꢁC. After being
stirred at room temperature for 2 h, the reaction mixture was
partitioned between saturated aqueous NH4Cl and ether. The or-
ganic phase was separated, washed with saturated aqueous
NaHCO3, and was dried over MgSO4. The solvent was removed
under reduced pressure and the residue was purified by distillation
(130 ꢁC, 14 mm Hg), to obtain (4-chlorophenylthio)methyl methyl
ether (5.76 g, 30.5 mmol).
residue was purified by flash chromatography (hexane/EtOAc 50:1)
to obtain the title compound (4.43 g, 15.8 mmol, 83%): TLC Rf 0.42
(hexane/EtOAc 20:1); 1H NMR (300 MHz, CDCl3)
3H), 1.22–1.34 (m, 10H), 1.36–1.52 (m, 2H), 1.64–1.80 (m, 2H), 2.32
d
0.87 (t, J¼6.6 Hz,
(s, 3H), 3.46 (s, 3H), 4.54 (t, J¼6.6 Hz,1H), 7.09 (m, 2H), 7.35 (m, 2H);
13C NMR (75 MHz, CDCl3)
d 14.1, 21.1, 22.6, 26.2, 29.1, 29.2, 29.4, 31.8,
35.6, 55.4, 91.1, 129.4, 129.4, 134.0, 137.5; IR (neat) 2924, 1466, 1088,
644 cmꢀ1; LRMS (EI) m/z 280 (Mþ), 157 (Mþ-SC6H4p-CH3); HRMS
(EI) calcd for C17H28OS: 280.1861, found: 280.1859.
To a solution of (4-chlorophenylthio)methyl methyl ether
(1.51 g, 8.0 mmol) in THF (20 mL), was added butyllithium (1.54 M
in hexane, 6.4 mL, 10 mmol) at ꢀ78 ꢁC. The mixture was stirred at
ꢀ45 ꢁC for 3 h and was cooled to ꢀ78 ꢁC. 1-Iodooctane (2.62 g,
10.9 mmol) was added, and the mixture was stirred at ꢀ78 ꢁC for
20 min. Saturated aqueous NH4Cl was added. The organic materials
were extracted with ether, and the organic extracts were washed
with brine and dried over MgSO4. After removal of the solvent, the
residue was purified by flash chromatography (hexane) to obtain
the title compound (1.60 g, 5.3 mmol, 66%): TLC Rf 0.73 (hexane/
4.3.4. (Methoxy(phenylthio))methylbenzene (17). To a toluene so-
lution of benzaldehyde dimethyl acetal (786.8 mg, 5.17 mmol) and
thiophenol (612.0 mg, 5.55 mmol), was added BF3–OEt2 (736.0 mg,
5.19 mmol) at ꢀ78 ꢁC. The solution was stirred for 2 h. Dry pyridine
(2.4 mL) was added. The mixture was diluted with ether and was
washed with 1 M NaOH solution and water. After drying over
Na2SO4 and removal of solvent, colorless oil thus obtained was
purified by flash chromatography (hexane/EtOAc 50:1–10:1) to
obtain the title compound (757.8 mg, 3.29 mmol, 64%).26
EtOAc 20:1); 1H NMR (400 MHz, CDCl3)
d
0.87 (t, J¼6.8 Hz, 3H),
1.21–1.28 (m, 10H), 1.40–1.43 (m, 2H), 1.36–1.44 (m, 2H), 1.62–1.78
4.3.5. (Methoxy(phenylthio)methyl)cyclohexane (19). To a toluene
solution of cyclohexanecarbaldehyde dimethyl acetal (774.0 mg,
4.89 mmol) and thiophenol (600 mg, 5.45 mmol) was added BF3–
OEt2 (901.3 mg, 6.35 mmol) at ꢀ78 ꢁC. The solution was stirred for
2 h, and dry pyridine (0.5 mL) was added. The mixture was diluted
with ether and was washed with 1 M NaOH solution and water.
After drying by Na2SO4 and removal of solvent, colorless oil thus
obtained was purified by flash chromatography (hexane/EtOAc
50:1–10:1) and GPC to give the title compound (918.6 mg,
3.89 mmol, 79%).26
(m, 2H), 3.46 (s, 3H), 4.57 (t, J¼6.8 Hz, 1H), 7.23–7.26 (m, 2H), 7.36–
7.40 (m, 2H); 13C NMR (100 MHz, CDCl3)
d 14.2, 22.8, 26.3, 29.2,
29.3, 29.5, 31.9, 35.6, 55.4, 90.9, 128.7, 131.6, 133.5, 134.6; IR (neat)
2953, 1476, 1130, 1092, 623 cmꢀ1; LRMS (EI) m/z 300 (Mþ), 269
(Mþ-OCH3), 187 (Mþ-C8H17), 157 (Mþ-SC6H4 p-Cl); HRMS (EI) calcd
for C16H25OClS: 300.1315, found: 300.1313.
4.3.2. 1-Methoxy-1-(4-fluorophenylthio)nonane (15). To a solution
of pelargonaldehyde dimethyl acetal (4.03 g, 21.4 mmol) and p-
fluorothiophenol (2.30 mL, 21.5 mmol) in toluene (180 mL), was
added BF3–OEt2 (2.7 mL, 21.5 mmol) at ꢀ78 ꢁC. The solution was
stirred for 2 h. Saturated aqueous NaHCO3 was added. The reaction
mixture was partitioned between Et2O and saturated aqueous
NaHCO3. The organic phase was separated and washed with satu-
rated aqueous NaHCO3 and dried over MgSO4. After removal of the
solvent, the crude product was purified by flash chromatography
(hexane/EtOAc 30:1) to give the title compound (15) (4.23 g,
14.9 mmol, 70%): TLC Rf 0.28 (hexane/EtOAc 20:1); 1H NMR
4.4. Generation of alkoxycarbenium ions (direct cation pool
method)
4.4.1. Typical procedure. The anodic oxidation was carried out in an
H-type divided cell (4 G glass filter) equipped with a carbon felt
anode (Nippon Carbon JF-20-P7, ca. 320 mg, dried at 250 ꢁC/1
mm Hg for 1 h before use) and
a platinum plate cathode
(40 mmꢂ20 mm). In the anodic chamber was placed a solution of
1-methoxy-1-phenylthiononane (1) (94.9 mg, 0.356 mmol) in
0.3 M Bu4NBF4/CH2Cl2 (8.0 mL). In the cathodic chamber were
placed 0.3 M Bu4NBF4/CH2Cl2 (8.0 mL) and trifluoromethane-
sulfonic acid (150.7 mg, 1.00 mmol). The constant current elec-
trolysis (8 mA) was carried out at ꢀ78 ꢁC with magnetic stirring
until 2.5 F/mol of electricity was consumed.
(400 MHz, CDCl3)
d
0.87 (t, J¼6.8 Hz, 3H), 1.18–1.32 (m, 10H), 1.34–
1.46 (m, 2H), 1.58–1.74 (m, 2H), 3.47 (s, 3H), 4.51 (t, J¼6.4 Hz, 1H),
6.94–7.02 (m, 2H), 7.39–7.45 (m, 2H); 13C NMR (75 MHz, CDCl3)
d
14.1, 22.6, 26.2, 29.1, 29.2, 29.4, 31.8, 35.5, 55.5, 91.0, 115.8 (d,
J¼21.7 Hz), 127.9 (d, J¼3.4 Hz), 136.1 (d, J¼8.0 Hz), 162.7 (d,
J¼246.0 Hz); IR (neat) 2926, 1590, 1491, 830 cmꢀ1; LRMS (EI) m/z
284 (Mþ), 157 (Mþ-SC6H4p-F); HRMS (EI) calcd for C16H25FOS (Mþ)
284.1610, found 284.1605.
4.5. NMR analysis of alkoxycarbenium ion (2) based on direct
cation pool method
4.3.3. 1-Methoxy-1-(4-methylphenylthio)nonane (16). To a solution
of 4-methylbenzenethiol (6.13 g, 49.4 mmol) in THF (100 mL), was
added triethylamine (10 mL, 73 mmol) and chloromethyl methyl
ether (4.69 g, 58.3 mmol) at 0 ꢁC. After being stirred at room tem-
perature for 2 h, the reaction mixture was partitioned between
saturated aqueous NH4Cl and ether. The organic phase was sepa-
rated, washed with saturated aqueous NaHCO3, and was dried over
MgSO4. The solvent was removed under reduced pressure and the
residue was purified by distillation to obtain (4-methylphenylthio)-
methyl methyl ether (6.71 g, 39.9 mmol).
To a solution of (4-methylphenylthio)methyl methyl ether
(3.21 g, 19.1 mmol) in THF (45 mL), was added butyllithium (1.54 M
in hexane, 15 mL, 23.1 mmol) at ꢀ78 ꢁC. The mixture was stirred at
ꢀ45 ꢁC for 3 h and cooled to ꢀ78 ꢁC. 1-Iodooctane (5.90 g,
24.6 mmol) was added, and the mixture was stirred at ꢀ78 ꢁC for
30 min. Saturated aqueous NH4Cl was added. The organic materials
were extracted with ether, and the organic extracts were washed
with brine and dried over MgSO4. After removal of the solvent, the
1H and 13C NMR spectra were recorded in CD2Cl2 on a JEOL A-
500 spectrometer. Chemical shifts are reported using methylene
signals of CH2Cl2 at
d d
5.32 (1H NMR) and 53.80 (13C NMR) as
standards. The anodic oxidation was carried out in a divided cell
equipped with a carbon felt anode and a platinum plate cathode. In
the anodic chamber were placed a solution of 1-methoxy-1-phe-
nylthiononane (1) (49.9 mg, 0.187 mmol) in 3.7 mL of 0.3 M
Bu4NBF4/CD2Cl2. In the cathodic chamber were placed tri-
fluoromethanesulfonic acid (71.1 mg, 0.474 mmol) and 0.3 M
Bu4NBF4/CD2Cl2 (3.7 mL). The constant current electrolysis
(3.7 mA) was carried out at ꢀ78 ꢁC with magnetic stirring. After
1.4 F/mol of electricity was consumed, the reaction mixture of the
anodic chamber was transferred to a 5 mm
4 NMR tube with
a septum cap under Ar atmosphere at ꢀ78 ꢁC. The NMR measure-
ment was carried out at ꢀ80 ꢁC: 1H NMR (500 MHz, CD2Cl2, se-
lected)
selected)
d
4.95 (s, 3H), 9.55 (s, 1H); 13C NMR (125 MHz, CD2Cl2,
d
40.7 (CH3OCHCH2), 76.1 (CH3OCH), 230.9 (CH3OCH). In

10906
K. Matsumoto et al. / Tetrahedron 65 (2009) 10901–10907
the NMR spectra the other signals could not be assigned because of
overlap of the signals of Bu4NBF4 used as electrolyte. The signals
due to the thiophenyl group leaving from 1 were also observed: 1H
could not be assigned because of overlap with the signals of
Bu4NBF4 used as the electrolyte.
NMR
d
7.2–8.0 (m); 13C NMR
d
116.5, 119.0, 124.1, 130.3.
4.8.2. Alkoxycarbenium ion derived from 5. The anodic oxidation
was carried out in a divided cell equipped with a carbon felt anode
and a platinum plate cathode. In the anodic chamber was placed
a solution of ArSSAr (Ar¼p-FC6H4) (153.0 mg, 0.602 mmol) in
4.0 mL of 0.3 M Bu4NBF4/CH2Cl2/CD2Cl2 (10:1). In the cathodic
chamber were placed trifluoromethanesulfonic acid (80.2 mg,
0.534 mmol) and 0.3 M Bu4NBF4/CH2Cl2/CD2Cl2 (10:1) (4.0 mL).
The constant current electrolysis (4.0 mA) was carried out at ꢀ78 ꢁC
with magnetic stirring. After 0.67 F/mol of electricity was con-
sumed, the reaction mixture of the anodic chamber (0.8 mL) was
4.6. Reaction of alkoxycarbenium ion and carbon
nucleophiles
4.6.1. 4-Methoxy-dodec-1-ene (3) (a typical procedure of direct cat-
ion pool method). The electrolysis of 1 (94.9 mg, 0.356 mmol) was
carried out as described above. To the ‘cation pool’ thus generated
in the anodic chamber, was added allyltrimethylsilane (99.7 mg,
0.873 mmol) at ꢀ78 ꢁC and the reaction mixture was stirred for
15 min. The solvent was removed under reduced pressure and the
residue was quickly filtered through a short column (2ꢂ3 cm) of
silica gel to remove Bu4NBF4. The silica gel was washed with ether
(150 mL). The GC analysis of the combined filtrate indicated that 3
was formed in 60% yield (GC tR 6.3 min, column, OV-1;
0.25 mmꢂ25 m; initial oven temperature, 100 ꢁC; rate of temper-
ature increase, 10 ꢁC/min). The isolated product was identified by
the comparison of its spectral data with those of an authentic
sample.6a
transferred to a 5 mm
4 NMR tube equipped with a septum cap
under Ar atmosphere at ꢀ78 ꢁC. 2-(Phenylthio)tetrahydropyran (5)
(16.4 mg, 0.084 mmol) was added and the tube was shaken at the
same temperature. The NMR measurement was carried out at
ꢀ80 ꢁC. Chemical shifts are reported using methylene signals of
CH2Cl2 at d d
5.32 (1H NMR) and 53.80 (13C NMR) as standards. The
huge signal coming from CH2Cl2 is reduced by usual pulse tech-
niques27: 1H NMR (600 MHz, CH2Cl2/CD2Cl2 (10:1), selected)
d
1.88
(br t, J¼5.8 Hz, 2H), 2.14 (br s, 2H), 3.49 (br t, J¼5.2 Hz, 2H), 5.24 (br
s, 2H), 9.86 (s, 1H); 13C NMR (150 MHz, CH2Cl2/CD2Cl2 (10:1), se-
lected)
d 12.1, 19.5, 35.4, 82.5, 227.8. Other signal could not be
4.7. Generation of alkoxycarbenium ions (indirect cation pool
method)
assigned because of overlap with the signals of Bu4NBF4 used as the
electrolyte or CH2Cl2, which was used as the solvent.
4.7.1. Typical procedure. The anodic oxidation was carried out in an
H-type divided cell (4 G glass filter) equipped with a carbon felt
anode and a platinum plate cathode (40 mmꢂ20 mm). In the an-
odic chamber was placed a solution of ArSSAr (Ar¼p-FC6H4)
(101.9 mg, 0.401 mmol) in 0.3 M Bu4NBF4/CH2Cl2 (8.0 mL). In the
cathodic chamber were placed 0.3 M Bu4NBF4/CH2Cl2 (8.0 mL) and
trifluoromethanesulfonic acid (41.0 mg, 0.273 mmol). The constant
current electrolysis (8 mA) was carried out at ꢀ78 ꢁC with magnetic
stirring until 0.67 F/mol of electricity was consumed. To the anodic
chamber containing electrogenerated ArS(ArSSAr)þ, was added 1-
methoxy-1-phenylthiononane (1) (53.3 mg, 0.200 mmol) and the
mixture was stirred for 5 min at ꢀ78 ꢁC. The solution thus obtained
was used for the subsequent reaction.
4.9. Reaction of alkoxycarbenium ion and carbon
nucleophiles
4.9.1. 4-Methoxy-dodec-1-ene (3) (a typical procedure of indirect
cation pool method). The electrolysis of ArSSAr (Ar¼p-FC6H4)
(102 mg, 0.401 mmol) was carried out as described above. To the
ArS(ArSSAr)þ thus generated in the anodic chamber, was added 1-
methoxy-1-phenylthiononane (1) (52.7 mg, 0.198 mmol) at ꢀ78 ꢁC
and the reaction mixture was stirred for 5 min. Then allyl-
trimethylsilane (56.6 mg, 0.495 mmol) was added and the resulting
solution was stirred at ꢀ78 ꢁC for 15 min. The reaction was
quenched with Et3N. The solvent was removed under reduced
pressure and the residue was quickly filtered through a short col-
umn (2ꢂ3 cm) of silica gel to remove Bu4NBF4. The silica gel was
washed with ether (150 mL). The GC analysis of the combined fil-
trate indicated that the title compound was formed in 89% yield (GC
4.8. NMR analysis of alkoxycarbenium ion based
on indirect cation pool method
tR 7.1 min, column, CBP1; 0.22 mm
4
ꢂ0.25 mmꢂ25 m; initial oven
4.8.1. Alkoxycarbenium ion (2) derived from 1. 1H and 13C NMR
spectra were recorded in CH2Cl2/CD2Cl2 (10:1) on a JEOL ECA600P
(1H 600 MHz, 13C 150 MHz) spectrometer. The anodic oxidation was
carried out in a divided cell equipped with a carbon felt anode and
a platinum plate cathode. In the anodic chamber were placed
a solution of ArSSAr (Ar¼p-FC6H4) (81.9 mg, 0.322 mmol) in 4.0 mL
of 0.3 M Bu4NBF4/CH2Cl2/CD2Cl2 (10:1). In the cathodic chamber
were placed trifluoromethanesulfonic acid (32.4 mg, 0.216 mmol)
and 0.3 M Bu4NBF4/CH2Cl2/CD2Cl2 (10:1) (4.0 mL). The constant
current electrolysis (4.0 mA) was carried out at ꢀ78 ꢁC with mag-
netic stirring. After 0.67 F/mol of electricity was consumed, the
reaction mixture of the anodic chamber (0.8 mL) was transferred to
temperature, 100 ꢁC; rate of temperature increase, 10 ꢁC/min). The
isolated product was identified by the comparison of its spectral
data with those of an authentic sample.6a
The other products, 6,6a 7,6a 8,6a 9,6a 10,6a 11,6a 12,6a 13,6a 186c
and 206a were identified by the comparison of their spectral data
with those of authentic samples.
Acknowledgements
This work was financially supported in part by a Grant-in-Aid for
Scientific Research from the Japan Society for the Promotion of
Science. The authors thank Prof. Dr. Hendrik Zipse of Ludwig-
Maximilians-Universita¨t Mu¨nchen for fruitful discussions. K.M ac-
knowledges JSPS for financial support.
a 5 mm
4 NMR tube with a septum cap under Ar atmosphere at
ꢀ78 ꢁC. 1-Methoxy-1-phenylthiononane (1) (10.2 mg, 0.0383 mmol)
was added and the tube was shaken at the same temperature. The
NMR measurement was carried out at ꢀ80 ꢁC. Chemical shifts are
References and notes
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d
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CH2Cl2/CD2Cl2 (10:1), selected)
3.26 (t, J¼6.3 Hz, 2H), 4.92 (s, 3H),
9.53 (s, 1H); 13C NMR (150 MHz, CH2Cl2/CD2Cl2 (10:1), selected)
40.3 (CH3OCHCH2), 75.6 (CH3OCH), 230.6 (CH3OCH). Other signals
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