Silyl-substituted dithioles as candidates for new electron donors

Article abstract of DOI:10.1246/bcsj.74.1717

Molecular orbital calculations revealed that the introduction of a silyl group onto the 2-position of 1,3-dithiole caused significant decrease of the ionization potential and that 2-silyl-1,3-dithioles served as a pseudo π-electron system. The silyl group could also be utilized as a bridge for linking π-electron systems to achieve artificial stacking. These concepts led us to the design and synthesis of silyl-substituted 1,3-benzodithioles as candidates for new electron donors. The oxidation potential of monosilyl-substituted 1,3-benzodithiole was found to be significantly less positive than that of the parent 1,3-benzodithiole. The introduction of the second silyl group caused further decrease of the oxidation potential. Compounds composed of two 1,3-benzodithiole units linked by a silyl group were also synthesized. These showed comparable oxidation potentials to those of the corresponding monomeric compounds. The effect of stannyl-substitution onto the 2-position of 1,3-dithiole was found to be larger than that of silyl-substitution. The effects of silyl-1 and stannyl-substitution for related systems such as thioanisole and 1,3-dithiane were also studied.

Full text of DOI:10.1246/bcsj.74.1717

c. Jpn.
© 2001 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 74, 1717–1725 (2001) 1717
e Chemical
ciety of Ja-
n
e Chemical
ciety of Ja-
n
Silyl-Substituted Dithioles as Candidates for New Electron Donors
Silyl-Substituted Dithioles
H. Li et al.
Hongqi Li, Keiji Nishiwaki, Kenichiro Itami, and Jun-ichi Yoshida*
Department of Synthetic Chemistry & Biological Chemistry, Graduate School of Engineering, Kyoto University,
Yoshida, Kyoto 606-8501
01
17
25
(Received March 8, 2001)
SJA8
09-2673
078
Molecular orbital calculations revealed that the introduction of a silyl group onto the 2-position of 1,3-dithiole
caused significant decrease of the ionization potential and that 2-silyl-1,3-dithioles served as a pseudo π-electron system.
The silyl group could also be utilized as a bridge for linking π-electron systems to achieve artificial stacking. These con-
cepts led us to the design and synthesis of silyl-substituted 1,3-benzodithioles as candidates for new electron donors.
The oxidation potential of monosilyl-substituted 1,3-benzodithiole was found to be significantly less positive than that of
the parent 1,3-benzodithiole. The introduction of the second silyl group caused further decrease of the oxidation poten-
tial. Compounds composed of two 1,3-benzodithiole units linked by a silyl group were also synthesized. These showed
comparable oxidation potentials to those of the corresponding monomeric compounds. The effect of stannyl-substitution
onto the 2-position of 1,3-dithiole was found to be larger than that of silyl-substitution. The effects of silyl- and stannyl-
substitution for related systems such as thioanisole and 1,3-dithiane were also studied.
01
01
.2
.3
Since the electrical conductivity in the perylene-bromine
complex was first reported,1 significant interest in organic con-
ducting materials (organic metals) has emerged among scien-
tists in various fields. The finding of high electrical conductiv-
ity in the charge-transfer (CT) complex of tetrathiafulvalene
(TTF) with tetracyanoquinodimethane (TCNQ)2 was a high-
light of this field and thereafter various derivatives of TTF have
been synthesized and utilized as organic metals (Fig. 1). While
the largest body of work on electron donors has been devoted
to structural modifications of TTF,3 very limited study was
done to achieve the developments of new classes of donors,
though such study is strongly needed.4 During the course of
our study directed toward identifying new electron donors uti-
lizing unique electronic properties of silicon, we have recently
reported the synthesis and the electrochemical behavior of si-
lyl-substituted 1,3-dithioles as candidates for new electron do-
nors.5 In this paper we report the full details of this study.5a
Our approach towards a new class of electron donors is
based on the following concepts.
(1) Silicon promotes the electron transfer from a heteroatom
such as oxygen, nitrogen, and sulfur, at β-position (Fig. 2).
Theoretical and experimental studies revealed that this effect
was attributed to the rise of HOMO level by the interaction of a
carbon–silicon σ orbital with a nonbonding p orbital of the het-
eroatom.6 Such interaction also stabilizes the resulting radical
cation and this stabilization also facilitates the electron trans-
fer.
(2) Linking of donor π-systems with covalent bonds may be
Fig. 1. TTF and its derivatives for organic metals.
Fig. 2.

1718 Bull. Chem. Soc. Jpn., 74, No. 9 (2001)
Silyl-Substituted Dithioles
molecule and the radical cation were calculated at MP2/3-
21G* level (Table 1 and Fig. 4). The calculations indicated
that the introduction of silicon onto the methylene carbon of
1,3-dithioles caused significant decrease of the ionization po-
tential. This remarkable effect of silyl-substitution is attribut-
ed to the interaction between the carbon–silicon σ orbital and
the p orbitals of the sulfur atoms in the radical cation. Because
two sulfur atoms conjugate with the carbon-carbon double
bond, the system serves as a pseudo π-system. It is noteworthy
that the five-membered ring is almost flat in the cation radical
(Fig. 4, Bꢀ) to ensure the effective conjugation in the ring (Fig.
4). The introduction of the second silyl group causes further
decrease of the ionization potential. These results are consis-
tent with the results of electrochemical studies on the oxidation
potentials (vide infra). Because HOMO levels usually serve as
effective indices of the ionization potentials, the HOMO levels
of the neutral compounds were also determined by the calcula-
tions at HF/3-21G* level. Interestingly, the effect of silyl-sub-
stitution on the HOMO level was found to be quite different
from that on the ionization potential. This is presumably at-
tributed to the envelope-like conformation of the neutral mole-
cules. In 2-silyl-1,3-dithiole the silyl group occupies the pseu-
do equatorial position (Fig. 4, B). Thus the interaction be-
tween the carbon–silicon σ orbital and the p orbitals of neigh-
boring sulfur atoms is not effective in the neutral molecule be-
cause they are nearly perpendicular with each other. The enve-
lope conformation of 1,3-dithiole moiety suggested by the cal-
culation is consistent with the structure determined by the X-
ray analysis (vide infra). The slight decrease of the HOMO
level by the introduction of a silyl group indicates that the SiH3
group is somewhat electron-withdrawing, although the de-
tailed mechanism has not been clarified as yet. In 2,2-disilyl-
1,3-dithiole, the second silyl group occupies the pseudo axial
position (Fig. 4, Cꢀ) and the carbon–silicon σ orbital interacts
effectively with the sulfur atoms to cause a very small increase
of the HOMO level. Presumably the effect of the first silyl
group and that of the second silyl group compensate each oth-
Fig. 3.
effective for the construction of artificial stackings.7 It has
been established that the high conductivity of organic metals is
related to their crystal structures, in which the donors and the
acceptors form segregated stacks with extensive π-electron
overlap. Thus, achievement of the stacking of donor mole-
cules to make columns in the CT complexes seems essential
for the conductivity.8 C–Si bonds which are longer than C–C
bonds seem to be suitable for linking π-systems to achieve arti-
ficial stacking (Fig. 3).
Based on these concepts we envisioned that silicon could be
utilized both for enhancing the electron-donating ability of π-
systems containing heteroatoms and for linking the π-systems
to construct artificial stacking.
Results and Discussion
Molecular Orbital Calculations. In the study directed
toward a new class of electron donors utilizing the unique elec-
tronic property of group 14 elements, we focused our attention
on 1,3-dithiole, which constitutes a half of TTF, as a core
structure. Because ionization potential represents one of the
most important properties of electron donors, the effect of si-
lyl-substitution on the ionization potential of 1,3-dithiole was
first examined using molecular orbital calculations. Ionization
potentials based on the energy difference between the neutral
Table 1. Ionization Potentials of Silyl-Substituted Dithioles Obtained by Molecular
Orbital Calculations
Ionization potential/eVa)
HOMO level/eV
Compound
MP2/3-21G*
HF/3-21G*
PM3
7.23
−8.04
−9.06
6.97
6.80
−8.06
−7.98
−8.69
−8.41
a) Adiabatic ionization potentials were calculated based on the difference in
total energy between the neutral molecule and the radical cation, geometries
of which are fully optimized.

H. Li et al.
Bull. Chem. Soc. Jpn., 74, No. 9 (2001) 1719
Fig. 4. Structures of neutral molecules of 1,3-dithiole (A), 2-silyl-1,3-dithiole (B), 2,2-disilyl-1,3-dithiole (C) and their radical
cations (Aꢀ, Bꢀ, and Cꢀ) obtained by ab initio molecular orbital calculations (MP2/3-21G*).
er. Based on these results, we can conclude that the HOMO
levels obtained by HF/3-21G* calculations do not serve as
good indices of the ionization potentials. The results of the
semi-empirical calculations are interesting. The HOMO levels
obtained by PM3 calculations correlate well with the ioniza-
tion potentials based on the energy difference between the neu-
tral molecule and the radical cation obtained by MP2/3-21G*
calculations. This is presumably because the geometry optimi-
zation with PM3 calculations gives flat structures of neutral si-
lyl-substituted 1,3-dithioles, in which the carbon–silicon σ
orbtial interacts effectively with the p orbitals of the neighbor-
ing sulfur atoms. Therefore, the HOMO levels obtained by
PM3 calculations seem to serve as good indices of the ioniza-
tion potentials, although the structures of the neutral molecules
are not correct.
deprotonated with butyllithium in THF at about −23 °C.10
Treatment of the resulting anion with chlorotrimethylsilane
and chlorodimethylsilane gave 2-trimethylsilyl-1,3-ben-
zodithiole (2) and 2-dimethylsilyl-1,3-benzodithiole (3), re-
spectively. The second silyl group was introduced by the
deprotonation of 2 with butyllithium followed by the reaction
with chlorotrimethylsilane to give 2,2-bis(trimethylsilyl)-1,3-
benzodithiole (5). Compound 6 was obtained by the deproto-
nation of 3 with lithium diisopropylamide (LDA) and succes-
sive reaction with chlorodimethylsilane. Hydrosilanes 3 and 6
were converted into the corresponding fluorosilanes 4 and 7 by
CuF2-mediated fluorination.11
Silyl-linked 1,3-benzodithioles 8 and 9 were prepared as
follows. Deprotonation of 1,3-benzodithiole (1), followed by
the treatment with dichlorodimethylsilane, gave the desired si-
lyl-linked compound 8 only in very poor yield. Although the
use of dichlorodiisopropylsilane gave a better result, the yield
was still low. The use of difluorodiisopropylsilane, however,
led to the formation of the desired silyl-linked compound 9 in
satisfactory yield (Scheme 2).
Synthesis. As model compounds for electron donors
based on our hypothesis, we designed a series of silicon-sub-
stituted 1,3-benzodithioles and related compounds (2–13). Si-
lyl-substituted 1,3-benzodithioles were synthesized in a proce-
dure shown in Scheme 1. 1,3-Benzodithiole (1)9 was easily
Scheme 1. Synthesis of silyl-substituted 1,3-benzodithioles.

1720 Bull. Chem. Soc. Jpn., 74, No. 9 (2001)
Silyl-Substituted Dithioles
Scheme 2. Synthesis of silyl-linked 1,3-benzodithioles.
Scheme 3. Synthesis of 11.
Scheme 4. Synthesis of stannyl-substituted 1,3-benzodithioles.
tential of 2-trimethylsilyl-1,3-benzodithiole (2) was found to
be less positive than that of 1,3-benzodithiole (1); this result is
consistent with the ionization potentials obtained by the mo-
lecular orbital calculations. The oxidation potential of 2,2-
bis(trimethylsilyl)-1,3-benzodithiole (5) was less positive than
2, indicating that the introduction of the second silyl group
caused a further decrease of the oxidation potential. As can be
seen from the oxidation potentials of 3, 4, 6, and 7, the substit-
uents on silicon slightly affect the electron donating ability.11
The electrochemical behavior of the silyl-linked 1,3-ben-
zodithioles 8 and 9 was studied and the oxidation potentials are
listed in Table 2. The value of compound 8 was found to be
slightly more positive than that of the corresponding mono-
meric silyl-substituted 1,3-benzodithiole 2. The effect of
stacking of two benzodithiole rings, which might enhance the
electron donating ability of 8, was not observed. The reason
for the slight increase of the oxidation potential is not clear at
present. The introduction of more bulky substituents such as
isopropyl groups on silicon, which might increase the popula-
tion of the stacking conformer, did not change the oxidation
potential remarkably (compound 9). Therefore, the electro-
chemical study did not give any evidence for the stacking of
two benzodithiole rings.
Fig. 5. X-ray crystal structure of 11.
The synthesis of silyl-substituted benzo[1,2-d:4,5-
dꢀ]bis[1,3]dithiole 11 was simple and straightforward. Depro-
tonation of benzo[1,2-d:4,5-dꢀ]bis[1,3]dithiole (10)12 with 1.0
mol amt. of butyllithium, followed by the treatment with 1.0
mol amt. of chlorotrimethylsilane, gave the monosilylated
compound together with small amounts of 2,2-bis-, 2,6-bis-,
and 2,2,6-tris-silylated compounds, although they were not ful-
ly characterized. Repetition of the sequence of deprotonation
and silylation in one-pot led to the formation of 11, which was
purified with flash chromatography followed by recrystalliza-
tion (Scheme 3). X-ray crystal structure analysis revealed that
the structure of 11 is non-planar and that the 1,3-dithiole rings
constitute envelope-like conformations, as shown in Fig. 5, al-
though 1H NMR spectroscopy indicated that four Me3Si
groups are equivalent in solution.
Tin-substituted 1,3-benzodithioles 12 and 13 were also syn-
thesized in a similar manner. As shown in Scheme 4, deproto-
nation of 1,3-benzodithiole with butyllithium followed by the
reaction with tributylchlorostannane gave the monostannylated
compound 12, which was deprotonated with LDA. The result-
ing anion was allowed to react with tributylchlorostannane to
give the distannylated compound 13.
The
oxidation
potentials
of
benzo[1,2-d:4,5-
dꢀ]bis[1,3]dithiole (10) and 2,2,6,6-tetrakis(trimethylsilyl)ben-
zo[1,2-d:4,5-dꢀ]bis[1,3]dithiole (11), in which two 1,3-dithiole
units are connected to a benzene ring, were also determined
with rotating disk electrode voltammetry. The oxidation po-
tential of 11 was much less positive than that of 10, indicating
the remarkable effect of silyl-substitution. Presumably, the
four carbon–silicon σ orbitals interact effectively with the
neighboring p orbitals of the sulfur atoms in the radical cation.
This result coincides with the effect of silyl-substitution on
benzodithiole.
Electrochemistry. The oxidation potentials of silyl-sub-
stituted 1,3-benzodithiole derivatives were determined with ro-
tating disk electrode voltammetry (Table 2). The oxidation po-
It is interesting to compare the effect of silicon for the oxi-
dation of the benzodithiole system with that for the oxidation
of other systems such as thioanisole and 1,3-dithiane. As

H. Li et al.
Bull. Chem. Soc. Jpn., 74, No. 9 (2001) 1721
Table 2. Oxidation Potentials of Silyl-Substituted 1,3-Benzodithioles and Related
Compounds
Compound
Ed/V
1.07a)
Compound
Ed/V
0.63a)
0.80a)
0.77a)
0.65a)
0.90a)
0.85a)
0.81a)
1.04b)
0.82a)
0.64b)
a) Determined by rotating disk electrode voltammetry with a glassy carbon working
electrode in 0.1 M LiClO4/CH3CN vs SCE. b) Determined by rotating disk electrode
voltammetry with a glassy carbon working electrode in 0.1 M Bu4NClO4/CH2Cl2 vs
Ag/AgCl.
Table 3. Oxidation Potentials of Silyl-Substituted 1,3-Benzodithioles, Thioanisole, and 1,3-Dithianes
Compound
Ed/Va)
1.07c)
Compound
Ed/Va)
1.18c)
Compound
Ep/Vb)
1.18d)
0.80c)
0.63c)
1.13c)
1.04c)
0.99d)
0.70d)
a) Decomposition potential determined by rotating disk electrode voltammetry. b) Peak potential determined by
cyclic voltammetry. c) Present work (in 0.1 M LiClO4/CH3CN vs SCE). d) Ref. 13 (in 0.1 M LiClO4/CH3CN vs
Ag/AgNO3).
shown in Table 3, the introduction of a silyl group onto the me-
thyl carbon of thioanisole caused little decrease of the oxida-
tion potential. This is probably because of the flexible confor-
mation of (trimethylsilylmethylthio)benzene (14). The free ro-
tation around the sulfur-carbon bond may disfavor the effective
interaction between the carbon–silicon σ orbital with the p or-
bital of the sulfur. In the case of [bis(trimethylsilyl)methylth-
io]benzene (15), the conformation is still flexible and its oxida-
tion potential was much higher than that of 5.
also smaller, as reported by Glass,13 than that for 1,3-ben-
zodithiole. Probably the fixed geometry of 2-trimethylsilyl-
1,3-dithiane (16), in which the silyl group occupies the equato-
rial position, disfavors the interaction of the carbon–silicon σ
orbital with the p orbitals of the sulfur atoms, because they are
nearly in perpendicular orientation. The oxidation potential of
2,2-bis(trimethylsilyl)-1,3-dithiane (17) was, however, compa-
rable to that of 2,2-bis(trimethylsilyl)-1,3-benzodithiole (5).
The remarkable effect of the second silyl group is explained as
follows. The second silyl group occupies the axial position in
The effect of mono-silyl-substitution for 1,3-dithiane was

1722 Bull. Chem. Soc. Jpn., 74, No. 9 (2001)
Silyl-Substituted Dithioles
Table 4. Oxidation Potentials of Stannyl-Substituted 1,3-Benzodithioles, Thioanisole, and 1,3-Dithianes
Compound
Ed/Va)
1.07c)
Compound
Ed/Va)
1.18c)
Compound
Ep/Vb)
1.18d)
0.20c)
0.68c)
0.47c)
0.75d)
0.19d)
−0.02c)
a) Decomposition potential determined by rotating disk electrode voltammetry. b) Peak potential determined by
cyclic voltammetry. c) Present work (in 0.1 M LiClO4/CH3CN vs SCE). d) Ref. 13 (in 0.1 M LiClO4/CH3CN vs
Ag/AgNO3).
1,3-dithiane ring and this carbon–silicon σ orbital interacts ef-
fectively with the p orbitals of the sulfur atoms because they
are almost in the same plane.
The oxidation potentials of stannyl-substituted 1,3-ben-
zodithioles 12 and 13 were also determined with rotating disk
electrode voltammetry. Table 4 included the oxidation poten-
tials of stannyl-substituted 1,3-benzodithioles and some related
compounds. The stannyl-substitution caused dramatic de-
crease of the oxidation potential. The larger effect of tin com-
pared to that of silicon is attributed to the higher energy level
of the carbon–tin σ orbital than that of the carbon–silicon σ or-
bital. The smaller energy gap between the carbon–tin σ orbital
and the sulfur p orbitals gives rise to better orbital interac-
tion.6d The effect of mono-stannyl-substitution in 1,3-ben-
zodithiole was larger than that for thioanisole and 1,3-
Fig. 6.
dithiane.13 These results are quite similar to those for silicon
cases. The introduction of the second stannyl group on 1,3-
8100 spectrophotometer. Mass spectra were determined on a
dithiane, however, caused a larger decrease of the oxidation
JEOL JMS-SX102A spectrometer. Melting points were measured
potential than that for 1,3-benzodithiole. The axial carbon–tin
by a Tanako micro melting point apparatus.
bond in 1,3-dithiane interacts strongly with the sulfur p orbit-
als, which in turn favors the electron transfer.
Molecular Orbital Calculations. The ab initio calculations
were carried out using the GAUSSIAN 98 program (G98W)14 at
the HF/3-21G* and MP2/3-21G* level. The semi-empirical cal-
culations were carried out using Chem 3D program. All geome-
tries were fully optimized.
Electrochemistry. The oxidation potential measurements
were carried out by using rotating disk electrode voltammetry
with a Bioanalytical Systems BAS 100B and a Nikko Keisoku
RDE-1, using a glassy carbon working electrode, a platinum wire
counter electrode, and a saturated calomel electrode (SCE) refer-
ence in 0.1 M LiClO4/CH3CN at 5000 rpm. The sweep rate was
10 mV/s.
Conclusion
The research reported above has demonstrated the potential-
ity of new electron donors based on β-effects of group 14 ele-
ments such as silicon and tin in electron transfer. The interac-
tion of the carbon-group 14 element σ orbital with the neigh-
boring p orbital of the heteroatom plays a crucial role to en-
hance the electron donating ability. Further work is in progress
to synthesize various types of silicon- and tin-substituted π-
systems to construct artificial stacking (Fig. 6), and to explore
the unique properties of such electron donors.
Synthesis. 1,3-Benzodithiole (1),9 benzo[1,2-d:4,5-dꢀ]bis-
[1,3]dithiole (10),12 and (trimethylsilylmethylthio)benzene (14)15
were synthesized in modified procedures reported in the literature.
2-Trimethylsilyl-1,3-benzodithiole (2): A solution of 1.50
M butyllithium in hexane (1.0 mL, 1.50 mmol) was added at 0 °C
to a solution of compound 1 (220 mg, 1.43 mmol) in dry THF (3
mL) and the solution was stirred at 0 °C for 1.5 h. The organolith-
ium compound thus obtained was added at −20 °C to another
flask containing chlorotrimethylsilane (0.19 mL, 162 mg, 1.50
mmol) and THF (5 mL). After being stirred at −20 °C–rt for 3 h,
Experimental
General. Tetrahydrofuran (THF) was freshly distilled under
argon from sodium benzophenone ketyl prior to use. Other re-
agents were commercially available and were used without further
treatment. Flash chromatography was carried out using Kanto
60N silica gel. 1H, 13C, and 29Si NMR spectra were measured on a
Varian Gemini 2000 spectrometer at 300, 75, and 60 MHz, respec-
tively. Infrared (IR) spectra were obtained on a Shimadzu FTIR-

H. Li et al.
Bull. Chem. Soc. Jpn., 74, No. 9 (2001) 1723
the mixture was poured into water (20 mL) and extracted with t-
butyl methyl ether (20 mL × 2). The ether phase was separated
and dried over anhydrous magnesium sulfate. Evaporation of the
solvent and subsequent purification of the residue by flash chro-
matography with hexane as the eluent afforded 2 as pale yellow
CDCl3) δ 0.27 (d, 12H, J = 3.3 Hz), 4.13–4.16 (m, 2H), 6.91–7.09
(m, 4H); 13C NMR (75 MHz, CDCl3) δ −4.9, 32.4, 121.7, 125.6,
140.0; 29Si NMR (60 MHz, CDCl3) δ −9.89; IR (neat) 2959,
2901, 2855, 2126, 1891, 1566, 1509, 1443, 1250, 1115, 1030,
1013, 793, 739, 698, 673, 662, 633 cm−1; LRMS (EI) m/z (%) 270
(M+, 35), 255 (6), 211 (100), 196 (49), 183 (95), 177 (25), 151
(37), 137 (10), 121 (18), 91 (10), 77 (9), 59 (54); HRMS (EI) Cal-
cd for C11H18S2Si2: (M+), 270.0388. Found: m/z 270.0392.
Found: C, 48.56; H, 6.95%. Calcd for C11H18S2Si2: C, 48.83; H,
6.71%.
1
needles (211 mg, 65%): Mp 67.0–68.0 °C; H NMR (300 MHz,
CDCl3) δ 0.24 (s, 9H), 4.52 (s, 1H), 6.96–7.20 (m, 4H); 13C NMR
(75 MHz, CDCl3) δ −2.6, 39.5, 122.1, 125.5, 140.1; 29Si NMR
(60 MHz, CDCl3) δ 3.74; IR (KBr) 1561, 1441, 1431, 1260, 1123,
1109, 1030, 1013, 930, 845, 777, 758, 743, 702, 673, 637 cm−1
;
UV–vis (CH3CN, λmax/nm) 393, 290, 281, 233, 215; LRMS (EI)
m/z (%) 226 (M+, 18), 212 (3), 183 (7), 168 (3), 153 (100), 108
(6), 82 (3), 73 (16), 69 (6), 58 (4); HRMS (EI) Calcd for
C10H14S2Si: (M+), 226.0306. Found: m/z 226.0315. Found: C,
53.11; H, 6.08%. Calcd for C10H14S2Si: C, 53.09; H, 6.24%.
2-Dimethylsilyl-1,3-benzodithiole (3): The title compound
was synthesized in a similar manner to 2. Pure compound 3 was
obtained as pale yellow needles after purification by flash chroma-
2,2-Bis(fluorodimethylsilyl)-1,3-benzodithiole (7): The title
compound was synthesized in a similar manner to 4. Pure com-
pound 7 was obtained as yellow oil after purification by flash
1
chromatography with hexane as the eluent (33%): H NMR (300
MHz, CDCl3) δ 0.46 (d, 12H, JH–F = 8.1 Hz), 6.95–7.10 (m, 4H);
13C NMR (75 MHz, CDCl3) δ −2.9 (d, JC–F = 13.7 Hz), 31.5,
121.9, 126.1, 139.1; 29Si NMR (60 MHz, CDCl3) δ 23.90 (d, JSi–F
= 294.0 Hz); IR (neat) 2979, 1904, 1561, 1445, 1431, 1406, 1279,
1260, 1248, 1115, 1030, 1011, 787, 745, 668, 644 cm−1; LRMS
(EI) m/z (%) 306 (M+, 20), 229 (100), 210 (11), 183 (3), 153 (7),
77 (9); HRMS (EI) Calcd for C11H16F2S2Si2: (M+), 306.0200.
Found: m/z 306.0214.
1
tography with hexane as the eluent (80%): Mp 55.0–56.0 °C; H
NMR (300 MHz, CDCl3) δ 0.33 (d, 6H, J = 3.9 Hz), 4.19–4.22
(m, 1H), 4.55 (d, 1H, J = 1.8 Hz), 6.97–7.21 (m, 4H); 13C NMR
(75 MHz, CDCl3) δ −5.3, 36.9, 122.1, 125.5, 139.7; 29Si NMR
(60 MHz, CDCl3) δ −12.0; IR (KBr) 2153, 1561, 1441, 1281,
1253, 1123, 1109, 1017, 936, 787, 666, 631 cm−1; LRMS (EI) m/z
(%) 212 (M+, 30), 211 (4), 183 (4), 168 (2), 153 (100), 121 (3),
109 (2), 91 (2), 77 (9), 59 (3); HRMS (EI) Calcd for C9H12S2Si:
(M+), 212.0150. Found: m/z 212.0150. Found: C, 50.60; H,
5.71%. Calcd for C9H12S2Si: C, 50.89; H, 5.69%.
Bis(1,3-benzodithiol-2-yl)dimethylsilane (8): A solution of
1.50 M butyllithium in hexane (0.99 mL, 1.49 mmol) was added at
0 °C to a solution of compound 1 (230 mg, 1.49 mmol) in dry
THF (5 mL) and the solution was stirred at 0 °C for 1.5 h. Dichlo-
rodimethylsilane (0.090 mL, 96 mg, 0.74 mmol) was added to the
reaction mixture at −78 °C. After being stirred at −78 °C–rt for 5
h, the mixture was poured into water (10 mL) and extracted with t-
butyl methyl ether (10 mL × 2). The ether phase was separated
and dried over anhydrous magnesium sulfate. Evaporation of the
solvent and subsequent purification of the residue by flash chro-
matography with hexane as the eluent afforded 8 as colorless nee-
2-Fluorodimethylsilyl-1,3-benzodithiole (4): To a suspen-
sion of CuF2•2H2O (275 mg, 2.0 mmol) in CCl4 (5 mL) was added
a solution of 3 (106 mg, 0.5 mmol) in CCl4 (2 mL). The mixture
was stirred at room temperature for 24 h. The solids were re-
moved by filtration and the filtrate was evaporated. Flash chroma-
tography of the residue with hexane as the eluent gave 4 (25.3 mg,
1
dles (60 mg, 22%): Mp 132.0–133.0 °C; H NMR (300 MHz,
1
22%) as pale yellow needles: Mp 66.5–67.0 °C; H NMR (300
CDCl3) δ 0.43 (s, 6H), 4.66 (s, 2H), 6.98–7.21 (m, 8H); 13C NMR
(75 MHz, CDCl3) δ −5.3, 36.9, 122.2, 125.7, 139.2; 29Si NMR
(60 MHz, CDCl3) δ 3.93; IR (KBr) 1561, 1443, 1431, 1254, 1111,
1015, 932, 833, 797, 750, 671 cm−1; UV–vis (CH3CN, λmax/nm)
300, 238; LRMS (EI) m/z (%) 364 (M+, 11), 211 (5), 183 (31),
153 (100), 121 (10), 108 (5), 77 (21), 69 (5); HRMS (EI) Calcd
for C16H16S4Si: (M+), 363.9904. Found: m/z 363.9902. Found: C,
52.25; H, 4.41%. Calcd for C16H16S4Si: C, 52.70; H, 4.42%.
Bis(1,3-benzodithiol-2-yl)diisopropylsilane (9): This com-
pound was synthesized similarly to 8 in 37% (dichlorodiisopro-
pylsilane) and 66% (difluorodiisopropylsilane) yields. Purifica-
tion by flash chromatography with hexane as the eluent afforded
compound 9 as yellow oil: 1H NMR (300 MHz, CDCl3) δ 1.29 (d,
12H, J = 7.5 Hz), 1.49–1.57 (m, 2H), 4.96 (s, 2H), 6.99–7.25 (m,
8H); 13C NMR (75 MHz, CDCl3) δ 12.2, 18.6, 35.5, 122.2, 125.6,
139.4; 29Si NMR (60 MHz, CDCl3) δ 3.57; IR (neat) 2944, 2865,
1561, 1458, 1441, 1260, 1121, 1011, 922, 882, 808, 741, 675, 631
cm−1; LRMS (EI) m/z (%) 420 (M+, 6), 284 (3), 211 (8), 183 (19),
153 (100), 140 (24), 121 (9), 77 (28), 69 (12); HRMS (EI) Calcd
for C20H24S4Si: (M+), 420.0530. Found: m/z 420.0543.
MHz, CDCl3) δ 0.45 (d, 6H, JH–F = 7.5 Hz), 4.51 (s, 1H), 6.99–
7.21 (m, 4H); 13C NMR (75 MHz, CDCl3) δ −3.0 (d, JC–F = 13.7
Hz), 36.9 (d, JC–F = 15.9 Hz), 122.2, 125.7, 138.8; 29Si NMR (60
MHz, CDCl3) δ 24.46 (d, JSi–F = 290.4 Hz); IR (KBr) 1655, 1561,
1441, 1262, 1125, 1015, 941, 887, 808, 762, 743, 664 cm−1
;
LRMS (EI) m/z (%) 230 (M+, 76), 215 (7), 187 (4), 168 (9), 153
(100), 140 (39), 108 (15), 96 (11), 77 (62), 69 (20), 63 (10).
Found: C, 46.75; H, 4.72; F, 8.02%. Calcd for C9H11FS2Si: C,
46.92; H, 4.81; F, 8.25%.
2,2-Bis(trimethylsilyl)-1,3-benzodithiole (5): The title com-
pound was synthesized in a similar manner to 2 by using com-
pound 2 as the starting material. Pure compound 5 was obtained
as colorless needles after purification by flash chromatography
with hexane as the eluent (33%): Mp 67.0–67.5 °C; 1H NMR (300
MHz, CDCl3) δ 0.20 (s, 18H), 6.91–7.00 (m, 4H); 13C NMR (75
MHz, CDCl3) δ −1.2, 35.7, 121.1, 125.3, 140.7; 29Si NMR (60
MHz, CDCl3) δ 6.56; IR (KBr) 2951, 1565, 1445, 1406, 1248,
1115, 1034, 1015, 930, 781, 743, 714, 673, 631, 619 cm−1; UV–
vis (CH3CN, λmax/nm) 321, 244; LRMS (EI) m/z (%) 298 (M+,
16), 283 (5), 225 (100), 210 (19), 183 (15), 177 (3), 151 (9), 91
(3), 73 (48); HRMS (EI) Calcd for C13H22S2Si2: (M+), 298.0701.
Found: m/z 298.0716. Found: C, 52.12; H, 7.67%. Calcd for
C13H22S2Si2: C, 52.29; H, 7.43%.
2,2,6,6-Tetrakis(trimethylsilyl)benzo[1,2-d:4,5-dꢀ]bis[1,3]-
dithiole (11): To a solution of 10 (230 mg, 1.00 mmol) in THF
(4 mL) was added butyllithium (1.56 M in hexane, 0.64 mL, 1.00
mmol) at 0 °C and the mixture was stirred at this temperature for
0.5 h. Chlorotrimethylsilane (0.127 mL, 1.00 mmol) was added at
−23 °C, and the mixture was stirred at this temperature for 20 min
and then warmed to 0 °C. Butyllithium (1.56 M in hexane, 0.64
mL, 1.00 mmol) was added at 0 °C, and the mixture was stirred at
2,2-Bis(dimethylsilyl)-1,3-benzodithiole (6): The title com-
pound was synthesized in a similar manner to 5. Pure compound
6 was obtained as pale yellow oil after purification by flash chro-
matography with hexane as the eluent (86%): 1H NMR (300 MHz,

1724 Bull. Chem. Soc. Jpn., 74, No. 9 (2001)
Silyl-Substituted Dithioles
this temperature for 0.5 h. Chlorotrimethylsilane (0.127 mL, 1.00
mmol) was added at −23 °C, and the mixture was stirred at this
temperature for 10 min. Butyllithium (1.56 M in hexane, 0.64
mL, 1.00 mmol) was added at −23 °C, and the mixture was stirred
at this temperature for 0.5 h. Chlorotrimethylsilane (0.127 mL,
1.00 mmol) was added at −23 °C, and the mixture was stirred at
this temperature for 15 min. Butyllithium (1.56 M in hexane, 0.64
mL, 1.00 mmol) was added at −23 °C, and the mixture was stirred
at this temperature for 0.5 h. Chlorotrimethylsilane (0.127 mL,
1.00 mmol) was added at −23 °C, and the mixture was stirred at
this temperature for 15 min and then warmed to room tempera-
ture. The reaction mixture was then partitioned between brine and
ether. The organic phase was separated and dried over Na2SO4.
After evaporation of the solvent, the residue was purified via flash
chromatography (N2 flash) followed by recrystallization (CH2Cl2/
CH3CN) to obtain the title compound (135 mg, 26%): Mp 189.5–
193.0 °C; 1H NMR (300 MHz, C6D6) δ 0.19 (s, 36H), 6.38 (s, 2H);
13C NMR (75 MHz, C6D6) δ −1.3, 37.5, 114.1, 138.6; IR (KBr)
2953, 1317, 1252, 858 cm−1. HRMS (EI) Calcd for C20H38S4Si4:
(M+), 518.0934. Found: m/z 518.0916. Found: C, 46.00; H,
7.46%. Calcd for C20H38S4Si4: C, 46.28; H, 7.38%.
12H), 1.44–1.52 (m, 12H), 6.91–7.17 (m, 4H); 13C NMR (75
MHz, CDCl3) δ 11.4, 13.6, 27.5, 29.2, 121.7, 125.3, 143.0; IR
(neat) 2957, 2923, 2853, 1665, 1522, 1464, 1439, 1416, 1375,
1075, 961, 743, 673 cm−1; UV-vis (CH3CN, λmax/nm) 332, 254.
Found: C, 50.56; H, 7.71%. Calcd for C31H58S2Sn2: C, 50.84; H,
7.98%.
[Bis(trimethylsilyl)methylthio]benzene (15): A solution of
1.6 M butyllithium in hexane (1.7 mL, 2.7 mmol) was added at 0
°C to a solution of compound 14 (430 mg, 2.2 mmol) in dry THF
(10 mL). After being stirred at 0 °C for 30 min, the solution was
allowed to warm up to room temperature and stirred for further 2
h. Chlorotrimethylsilane (0.40 mL, 340 mg, 3.1 mmol) was added
at −78 °C and the mixture was stirred at −78 °C–rt for 3 h. The
product was partitioned between saturated brine and ether. The
organic phase was separated and dried over anhydrous magnesium
sulfate. Evaporation of the solvent and subsequent purification by
flash chromatography with hexane as the eluent afforded com-
1
pound 15 as colorless oil (573 mg, 97%): H NMR (300 MHz,
CDCl3) δ 0.19 (s, 18H), 1.58 (s, 1H), 7.11–7.17 (m, 2H), 7.25–
7.37 (m, 3H); 13C NMR (75 MHz, CDCl3) δ −0.2, 18.6, 124.8,
127.6, 128.6, 140.3; 29Si NMR (60 MHz, CDCl3) δ 3.26; IR (neat)
2953, 2899, 1582, 1478, 1439, 1250, 1086, 1024, 995, 843, 770,
737, 689 cm−1; LRMS (EI) m/z (%) 268 (M+, 25), 253 (25), 195
(3), 177 (14), 165 (41), 151 (10), 135 (20), 129 (7), 118 (6), 85
(7), 73 (100), 59 (8); HRMS (EI) Calcd for C13H24SSi2: (M+),
268.1137. Found: m/z 268.1120. Found: C, 58.25; H, 8.73%.
Calcd for C13H24SSi2: C, 58.14; H, 9.01%.
Crystallographic Analysis of 11. Crystal data: C20H38S4Si4,
Mr =518.1 g mol−1, colorless prismatic crystals, crystal size 0.40
× 0.30 × 0.50 mm, a = 12.521(1) Å, b = 12.521(1) Å, c =
18.622(3) Å, tetragonal, space group P421c [No. 114], Rigaku
AFC5 diffractmeter, λ= 0.71069 Å, scan mode ω−2θ, 2005 mea-
sured reflections (+h, +k, +l) [[sinq/l]max = 0.65–1 1327 ob-
served reflections [I > 2σ(I)], structure solved by direct methods,
final refinement least squares. R = 0.053, Rw = 0.043.
(Tributylstannylmethylthio)benzene (18):
A solution of
1.63 M butyllithium in hexane (13.8 mL, 22.5 mmol) was added at
0 °C to a solution of thioanisole (2.0 mL, 2.12 g, 17.1 mmol) in
dry ether (30 mL). The solution was refluxed under stirring over-
night then cooled to 0 °C. Chlorotributylstannane (6.7 mL, 8.04 g,
24.7 mmol) was added in one portion and the mixture was re-
fluxed for 4 h. After being cooled to room temperature, the reac-
tion mixture was partitioned between saturated brine and ether.
The organic phase was separated and dried over anhydrous mag-
nesium sulfate. Evaporation of the solvent and subsequent purifi-
cation of the residue by flash chromatography with hexane as the
eluent gave compound 18 as colorless oil (5.25 g, 74%): 1H NMR
(300 MHz, CDCl3) δ 0.96 (t, 9H, J = 7.2 Hz), 1.13–1.19 (m, 6H),
1.43–1.55 (m, 6H), 1.65–1.75 (m, 6H), 2.29 (s, 2H, JSn–H = 40.8
Hz), 7.19–7.21 (m, 2H), 7.37–7.43 (m, 3H); 13C NMR (75 MHz,
CDCl3) δ 7.8, 9.6, 13.6, 27.2, 28.9, 124.1, 125.1, 128.4, 142.5; IR
(neat) 2957, 2920, 2870, 2858, 1586, 1478, 1464, 1439, 1377,
1075, 1024, 737, 689 cm−1; LRMS (EI) m/z (%) 414 (M+, 12),
412 (8), 410 (5), 357 (100), 355 (75), 353 (40), 291 (51), 289 (37),
287 (22), 235 (73), 197 (33), 179 (92), 177 (83), 175 (53), 123
(43), 121 (28), 119 (20), 91 (59), 77 (6); HRMS (EI) Calcd for
C19H34SSn: (M+), 414.1403. Found: m/z 414.1393. Found: C,
55.18; H, 8.38%. Calcd for C19H34SSn: C, 55.22; H, 8.29%.
[Bis(tributylstannyl)methilthio]benzene (19): A solution of
1.63 M butyllithium in hexane (4.67 mL, 7.61 mmol) was added at
−78 °C to a solution of diisopropylamine (1.07 mL, 0.77 g, 7.61
mmol) in THF (15 mL) and the solution was stirred at −78 °C for
30 min. A solution of compound 18 (3.13 g, 7.57 mmol) in THF
(20 mL) was added and the solution was stirred at −78–−10 °C
for 3 h. Tributylchlorostannane (2.1 mL, 2.52 g, 7.74 mmol) was
added at −78 °C and the mixture was stirred at −78 °C for 1.5 h
and at room temperature overnight. Work-up as usual and subse-
quent purification by flash chromatography with hexane/ethyl ace-
tate (99:1, v/v) as the eluent gave compound 19 as colorless oil
(2.61 g, 49%): 1H NMR (300 MHz, CDCl3) δ 0.82–1.17 (m, 30H),
2-Tributylstannyl-1,3-benzodithiole (12):
A
solution of
1.50 M butyllithium in hexane (2.70 mL, 4.05 mmol) was added at
0 °C to a solution of compound 1 (595 mg, 3.86 mmol) in dry
THF (5 mL) and the solution was stirred at 0 °C for 1.5 h. The an-
ion thus obtained was added at −20 °C to another flask containing
tributylchlorostannane (1.10 mL, 1.32 g, 4.05 mmol) and THF (3
mL). After being stirred at −20 °C–rt for 3 h, the mixture was
poured into water (10 mL) and extracted with t-butyl methyl ether
(10 mL × 2). The ether phase was separated and dried over anhy-
drous magnesium sulfate. Evaporation of the solvent and subse-
quent purification of the residue by flash chromatography with
hexane as the eluent afforded compound 12 as colorless oil (856
mg, 50%): 1H NMR (300 MHz, CDCl3) δ 0.95 (t, 9H, J = 7.5 Hz),
1.16 (t, 6H, J = 7.8 Hz), 1.34 (q, 6H, J = 7.5 Hz), 1.56–1.64 (m,
6H), 4.43 (s, 1H), 6.97–7.27 (m, 4H); 13C NMR (75 MHz, CDCl3)
δ 10.3, 13.6, 27.2, 28.9, 31.0, 121.8, 125.1, 141.3; IR (neat) 2957,
2926, 2870, 2851, 1563, 1464, 1439, 1377, 1109, 1075, 741, 693,
673 cm−1; UV–vis (CH3CN, λmax/nm) 393, 230; LRMS (FAB) m/z
(%) 443 (M+, 6), 291 (29), 261 (5), 235 (30), 209 (11), 183 (9),
179 (55), 153 (100), 121 (4). Found: C, 51.76; H, 7.32%. Calcd
for C19H32S2Sn: C, 51.48; H, 7.28%.
2,2-Bis(tributylstannyl)-1,3-benzodithiole (13): A solution
of 1.50 M butyllithium in hexane (0.66 mL, 0.99 mmol) was add-
ed at −78 °C to a solution of diisopropylamine (0.14 mL, 100 mg,
0.99 mmol) in dry THF (5 mL) and the solution was stirred at −78
°C for 30 min. A solution of compound 12 (411 mg, 0.93 mmol)
in THF (2 mL) was added and the solution was stirred at −78–
−10 °C for 2 h. Tributylchlorostannane (0.32 mL, 383 mg, 1.18
mmol) was added at −78 °C and the mixture was stirred at −78
°C–rt for 3 h. Work-up as usual and subsequent purification by
flash chromatography with hexane as the eluent afforded product
1
13 as yellow oil (402 mg, 59%): H NMR (300 MHz, CDCl3) δ
0.88 (t, 18H, J = 7.2 Hz), 0.93–0.99 (m, 12H), 1.23–1.35 (m,

H. Li et al.
Bull. Chem. Soc. Jpn., 74, No. 9 (2001) 1725
1.25–1.35 (m, 12H), 1.37–1.58 (m, 12H), 2.30 (s, 1H, JSn–H
=
4th ed, Revised and Expanded, ed by H. Lund and O. Hammerich,
Marcel Dekker, New York (2001), p. 765.
36.9 Hz), 7.07–7.12 (m, 2H), 7.25–7.32 (m, 3H); 13C NMR (75
MHz, CDCl3) δ 3.0, 11.1, 13.6, 27.5, 29.1, 124.5, 126.8, 128.4,
143.2; IR (neat) 2957, 2924, 2872, 2853, 1583, 1478, 1464, 1420,
1375, 1073, 1024, 961, 735, 689, 664 cm−1; LRMS (EI) m/z (%)
704 (M+, 9), 702 (11), 700 (9), 647 (22), 645 (28), 643 (24), 591
(21), 589 (27), 587 (24), 533 (3), 475 (2), 413 (3), 356 (53), 354
(40), 352 (21), 291 (37), 289 (28), 287 (17), 243 (68), 229 (93),
227 (68), 225 (41), 179 (100), 177 (83), 175 (52), 153 (6), 123
(30), 91 (36), 69 (33), 57 (23); HRMS (EI) Calcd for C31H60SSn2:
(M+), 704.2460. Found: m/z 704.2435 Found: C, 52.76; H,
8.76%. Calcd for C31H60SSn2: C, 53.02; H, 8.61%.
7
a) K. Takimiya, A. Oharuda, A. Morikami, Y. Aso, and T.
Otsubo, Eur. J. Org. Chem., 2000, 3013. b) K. Takimiya, A.
Oharuda, A. Morikami, Y. Aso, and T. Otsubo, Angew. Chem., Int.
Ed., 37, 619 (1998). c) D. D. Graf, R. G. Duan, J. P. Campbell, L.
L. Miller, and K. R. Mann, J. Am. Chem. Soc., 119, 5888 (1997).
d) K. Matsuo, K. Takimiya, Y. Aso, T. Otsubo, and F. Ogura,
Chem. Lett., 1995, 523. e) K. Takimiya, Y. Shibata, K. Imamura,
A. Kashihara, Y. Aso, T. Otsubo, and F. Ogura, Tetrahedron Lett.,
36, 5045 (1995). f) C. Kaes, M. W. Hosseini, R. Ruppert, A. D.
Cian, and J. Fischer, Tetrahedron Lett., 35, 7233 (1994).
8
T. E. Phillips, T. J. Kistenmacher, J. P. Ferraris, and D. O.
Cowan, J. Chem. Soc., Chem. Commun., 1973, 471.
J. Nakayama, K. Fujiwara, and M. Hoshino, Bull. Chem.
Soc. Jpn., 49, 3567 (1976).
10 S. Ncube, A. Pelter, K. Smith, P. Blatcher, and S. Warren,
Tetrahedron Lett., 26, 2345 (1978).
This research was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports and
Culture. H. Li thanks JSPS (Japan Society for the Promotion
of Science) for the award of a postdoctoral fellowship.
9
11 J. Yoshida, H. Tsujishima, K. Nakano, T. Teramoto, K.
Nishiwaki, and S. Isoe, Organometallics, 14, 567 (1995).
12 F. C. Krebs, J. Larsen, K. Boubekur, and M. Foumigue,
Acta Chem. Scand., 47, 910 (1993).
References
1
H. Akamatu, H. Inokuchi, and Y. Matsunaga, Nature, 173,
168 (1954).
2
J. Ferraris, D. O. Cowan, V. Waltka Jr., and J. H. Perlstein,
13 R. S. Glass, A. M. Radspinner, and W. P. Singh, J. Am.
Chem. Soc., 114, 4921 (1992).
J. Am. Chem. Soc., 95, 948 (1973).
3
For recent review articles, see: a) M. R. Bryce, J. Mater.
14 Gaussian 98 (Revision A.6), M. J. Frisch, G. W. Trucks, H.
B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G.
Zakrzewski, J. A. Montgomery, R. E. Stratmann, J. C. Burant, S.
Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain,
O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B.
Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A.
Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D.
Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V.
Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A.
Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M.
Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W.
Wong, J. L. Andres, M. Head-Gordon, E. S. Replogle, and J. A.
Pople, Gaussian, Inc., Pittsburgh PA, 1998.
Chem., 5, 1481 (1995). b) M. Adam and K. Müllen, Adv. Mater.,
6, 439 (1994). c) M. R. Bryce, Chem. Soc. Rev., 20, 355 (1991).
d) V. Enkelmann, Angew. Chem., Int. Ed. Engl., 30, 1121 (1991).
4
For example, a) T. Otsubo, Y. Aso, and K. Kakimiya, J.
Syn. Org. Chem. Jpn., 54, 752 (1996). b) Y. Yamashita, J. Syn.
Org. Chem. Jpn., 47, 1108 (1989). c) Y. Misaki, H. Fujiwara, and
T. Yamabe, J. Org. Chem., 61, 3650 (1996).
5
a) K. Nishiwaki and J. Yoshida, Chem. Lett., 1996, 171. b)
K. Nishiwaki and J. Yoshida, Chem. Lett., 1996, 787.
a) J. Yoshida, T. Maekawa, T. Murata, S. Matsunaga, and S.
6
Isoe, J. Am. Chem. Soc., 112, 1962 (1990). b) J. Yoshida, Y.
Ishichi, K. Nishiwaki, S. Shiozawa, and S. Isoe, Tetrahedron Lett.,
33, 2599 (1992). c) J. Yoshida, Top. in Curr. Chem., 170, 39
(1994). d) J.Yoshida, J. Syn. Org. Chem. Jpn., 53, 53 (1995). e) J.
Yoshida and K. Nishiwaki, J. Chem. Soc., Dalton Trans., 1998,
2589. f) J. Yoshida and S. Suga in “Organic Electrochemistry,”
15 D. J. Ager and R. C. Cookson, Tetrahedron Lett., 21, 1677
(1980).