Synthesis, structure, and photochemistry of 2,3,5,6-tetrasilyl-1,4-benzoquinones and related compounds

Article abstract of DOI:10.1246/bcsj.75.2571

2,3,5,6-Tetrasilyl- and 2,3,5,6-tetragermyl-1,4-benzoquinones were obtained by oxidation of the corresponding 1,4-dimetaloxybenzene derivatives with pyridinium chlorochromate. X-ray crystallographic analysis revealed that the quinone ring of 2,3,5,6-tetrakis(trimethylsilyl)-1,4-benzoquinone (1a) was distorted into a chair form. The deformed quinone ring allowed a σ(C_quinone-Si)-π* electronic transition in 1a. Photolysis of 1a resulted in an isomerization, creating a ketene derivative, 4-carbonyl-2,3,5,5-tetrakis(trimethylsilyl)-2-cyclopenten-1-one.

Full text of DOI:10.1246/bcsj.75.2571

c. Jpn.
© 2002 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 75, 2571–2577 (2002) 2571
e Chemical
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Headline Articles
Synthesis, Structure, and Photochemistry of 2,3,5,6-Tetrasilyl-1,4-benzo-
quinones and Related Compounds
02
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Synthesis of Tetrasilyl-1,4-benzoquinones
S. Tsutsui et al.
Shinobu Tsutsui,# Kenkichi Sakamoto,*,†,## Keisuke Ebata,† Chizuko Kabuto,† and Hideki Sakurai*,†
Photodynamics Research Center, The Institute of Physical and Chemical Research (RIKEN),
519-1399 Aoba, Aramaki, Aoba-ku, Sendai 980-0845
02
02
†Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578
(Received May 22, 2002)
2,3,5,6-Tetrasilyl- and 2,3,5,6-tetragermyl-1,4-benzoquinones were obtained by oxidation of the corresponding
1,4-dimetaloxybenzene derivatives with pyridinium chlorochromate. X-ray crystallographic analysis revealed that the
quinone ring of 2,3,5,6-tetrakis(trimethylsilyl)-1,4-benzoquinone (1a) was distorted into a chair form. The deformed
quinone ring allowed a σ(Cquinone-Si)–π* electronic transition in 1a. Photolysis of 1a resulted in an isomerization, creat-
ing a ketene derivative, 4-carbonyl-2,3,5,5-tetrakis(trimethylsilyl)-2-cyclopenten-1-one.
Silyl groups perturb various π-electron systems electronical-
ly and sterically.1 Fully silylated π-electron systems such as
tetrasilylethene, hexasilylbenzene, pentasilylcyclopentadien-
yl, hexasilylbicyclopropenyl, and tetrasilylcyclobutadiene have
previously been reported.2–5 These compounds exhibit quite
unusual properties due to the perturbation of silyl groups. 1,4-
Benzoquinone is one of the most interesting fundamental π-
electron systems due to properties such as high electron affini-
ty.6–8 In the present study we report the preparation and prop-
erties of 2,3,5,6-tetrasilyl-1,4-benzoquinones and related com-
pounds as new fully silylated π-electron systems.9
Results and Discussion
Preparation of Silyl- and Germyl-1,4-benzoquinones.
The synthesis of 2,3,5,6-tetrakis(trimethylsilyl)-1,4-benzo-
quinone (1a) was carried out as shown in Scheme 1. Chloranil
Scheme 1.
was reacted with trimethylchlorosilane and magnesium10 using
a mixture of THF and HMPA as the solvent (THF/HMPA =
10/1) to give 1,4-bis(trimethylsiloxy)-2,3,5,6-tetrakis(trimeth-
ylsilyl)benzene (2a) in 51% yield. Compound 2a was also ob-
tained by the reaction of 2,3,5,6-tetrabromo-1,4-benzoquinone
with trimethylchlorosilane in the presence of magnesium in
THF, even though the yield was lower (18%). The compound
was then oxidized using pyridinium chlorochromate (PCC),
producing 1a (72%) as deep red crystals.11 2,3,5-Tris(trimeth-
ylsilyl)-1,4-benzoquinone (3a) was obtained in 9% yield as a
by-product of the reaction. Preparation of 1b, 1c, and 1e was
Chart 1.
achieved using similar procedures. This method was applica-
#
ble to the preparation of 2,5-disilyl- and 2,5-digermyl-1,4-ben-
zoquinones (4), 2,6-disilyl-1,4-benzoquinones (5), and 2-silyl-
Tohoku University
RIKEN
##

2572 Bull. Chem. Soc. Jpn., 75, No. 12 (2002)
HEADLINE ARTICLES
1,4-benzoquinones (6) (Chart 1). However, attempts to pre-
pare 2,3,5,6-tetrakis(dimethylphenylsilyl)-1,4-benzoquinone
1,3-dichloro-1,1,3,3-tetramethyl-1,3-disilapropane in the pres-
ence of magnesium in THF provided tricyclic dimethoxyben-
zene (7) and bicyclic dimethoxybenzene (8) in 5 and 3%
yields, respectively. Oxidation of 7 and 8 using cerium(Ⅳ)
ammonium nitrate (CAN) in CH3CN provided 1d and 9 in 37
and 69% yields, respectively.
and
2,3,5,6-tetrakis(t-butyldimethylsilyl)-1,4-benzoquinone
under similar conditions were not successful.
In addition, we have found that 1,4-dimethoxybenzene de-
rivatives are useful precursors of silyl-1,4-benzoquinone deriv-
atives.12 Thus, preparation of tricyclic 2,3,5,6-tetrasilyl-1,4-
benzoquinone (1d) was performed as shown in Scheme 2. The
reaction of 2,3,5,6-tetrabromo-1,4-dimethoxybenzene with
Scheme 2.
Fig. 2. ORTEP drawings of 1b: upper, top view; below, side
view.
Table 1. Selected Bond Lengths (Å), Bond Angles (deg),
and Torsion Angles (deg) for 1a with Their Esd’s in Paren-
theses
Bond lengths
Si1–C2
Si2–C3
O1–C1
C1–C2
1.910(2)
1.911(2)
1.225(4)
1.491(4)
C1–C6
C2–C3
C3–C1ꢀ
1.499(4)
1.354(4)
1.499(3)
Bond angles
O1–C1–C2
O1–C1–C3ꢀ
C2–C1–C3ꢀ
C1–C2–C3
C2–C3–C1ꢀ
118.0(2)
118.2(2)
123.4(2)
116.9(2)
116.3(2)
Si1–C2–C1
Si1–C2–C3
Si2–C3–C1ꢀ
Si2–C3–C2
111.8(2)
131.3(2)
116.3(2)
Torsion angles
Si1–C2–C3–Si2 −17.3(4) O1ꢀ–C1ꢀ–C3–Si2 −29.1(3)
O1–C1–C2–Si1 27.9(3) C1–C2–C3–C1ꢀ −20.0(3)
Fig. 1. ORTEP drawings of 1a: upper, top view; lower, side
view.

S. Tsutsui et al.
Bull. Chem. Soc. Jpn., 75, No. 12 (2002) 2573
Table 2. Selected Bond Lengths (Å), Bond Angles (deg), and
Torsion Angles (deg) for 1b with Their Esd’s in Parenthe-
Table 3. Selected Averaged Bond Lengths (Å) of 1,4-Ben-
zoquinones
ses
Bond lengths
Si1–C2
Si2–C3
Si3–C5
Si4–C6
O1–C1
O2–C4
1.891(3)
1.896(3)
1.892(3)
1.895(3)
1.214(4)
1.215(4)
C1–C2
C1–C6
C2–C3
C3–C4
C4–C5
C5–C6
1.491(4)
1.499(4)
1.353(4)
1.500(4)
1.500(4)
1.349(4)
R
C–C
CwC
CwO
Ref.
Me3Si
HMe2Si
i-Pr2(Ow)P
Me
1.503
1.498
1.494
1.492
1.490
1.474
1.354
1.351
1.349
1.341
1.344
1.334
1.225
1.215
—
1.232
1.211
1.222
This Work
This Work
13a
Bond angles
13b
13c
13d
O1–C1–C2
O1–C1–C6
C2–C1–C6
C1–C2–C3
C2–C3–C4
O2–C4–C3
O2–C4–C5
C3–C4–C5
C4–C5–C6
118.4(3)
118.9(2)
122.7(2)
118.4(2)
118.5(2)
118.8(2)
118.6(2)
122.6(3)
118.4(2)
C1–C6–C5
Si1–C2–C1
Si1–C2–C3
Si2–C3–C2
Si2–C3–C4
Si3–C5–C4
Si3–C5–C6
Si4–C6–C1
Si4–C6–C5
118.6(2)
113.1(2)
128.4(2)
129.2(2)
112.3(2)
112.5(2)
129.0(2)
112.3(2)
129.0(2)
Cl
H
quinone ring (Fig. 2). The torsion angles for Si1–C2–C3–Si2,
Si3–C5–C6–Si4, C1–C2–C3–C4, and C4–C5–C6–C1 were
4.9, 0.6, 6.0, and 1.4°, respectively. The Si–H bond of 1b was
directed not towards the CwO group but towards another Si–H
bond of the vicinal silyl group. The intramolecular distances
of SiH≥HSi of the vicinal silyl groups were 2.09–2.20 Å; this
is shorter than the sum of two van der Waals radii for hydrogen
(2.4 Å). Also, angles for Si–CwC of 1b were large (128.2–
129.2°). These results indicate that even the less-hindered
dimethylsilyl groups interact with each other sterically, but the
steric hindrance is insufficient to distort the quinone ring.
Physical Properties of 2,3,5,6-Tetrasilyl-1,4-benzoqui-
nones and Related Compounds. The electronic absorption
maxima and IR absorption frequencies of the CwO unit in si-
lyl- and germyl-1,4-benzoquinones are summarized in Table 4.
In the electronic absorption spectra of all the 1,4-benzoqui-
nones in hexane, three bands were observed at about 250, 300,
and 500 nm. The first and second bands are assignable to π–π*
transitions and the third one is assignable to a n–π* transition.
In addition, 1a showed a broad band centered at 403 nm (ε =
123) assignable to a σ(Cquinone-Si)–π* transition. This transi-
tion is usually forbidden for planar quinones. In the distorted
structure of 1a, however, the π-orbitals of the quinone ring and
the σ(Cquinone-Si) bonds are not orthogonal to each other and
hence the σ–π* transition is allowed. A similar σ–π* transi-
tion was observed at 417 nm (ε = 54) for 3a. Since a similar
steric repulsion between vicinal trimethylsilyl groups occurs in
3a, its quinone ring should be similarly distorted. In contrast,
2,5-bis(trimethylsilyl)-1,4-benzoquinone (4a) having a planar
ring showed no σ–π* transition.7c The σ–π* band was also ob-
served for 1b (λmax = 426 nm, ε = 33) and 1d (λmax = 433 nm,
ε = 10), but these absorptivities were smaller than that of 1a.
These results indicate that absorptivity of the σ–π* band in
these systems depends on the planarity of the quinone ring.
The IR spectrum of 1a showed a peak at 1618 cm−1 assign-
able to the carbonyl group. Other 2,3,5,6-tetrasilyl- and
2,3,5,6-tetragermyl-1,4-benzoquinones also showed peaks at
about 1620 cm−1, such values are much lower than those of
unsubstituted 1,4-benzoquinone (1668 and 1656 cm−1).14
With increasing number of silyl and germyl substituents, the
peak shifted to lower frequencies, as shown in Table 4. These
results showed that silyl and germyl groups acted as electron-
donating substituents.
Torsion angles
Si1–C2–C3–Si2
Si3–C5–C6–Si4
C1–C2–C3–C4
C4–C5–C6–C1
−4.9(5)
−0.6(5)
−6.0(5)
1.4(5)
O1–C1–C2–Si1
O1–C1–C6–Si4 −10.6(4)
O2–C4–C5–Si3
Si2–C3–C4–O2
13.2(4)
2.0(4)
1.1(4)
X-ray Crystallographic Analysis of 1a and 1b.
Crystal
structures of 1a and 1b were revealed by X-ray crystallograph-
ic analysis. Single crystals of 1a and 1b suitable for X-ray
crystallographic analysis were obtained from hexane solutions.
ORTEP drawings of 1a and 1b are shown in Figs. 1 and 2, re-
spectively. The selected bond lengths, bond angles, and tor-
sion angles of 1a and 1b are summarized in Tables 1 and 2, re-
spectively. In Table 3, the bond lengths in the quinone ring of
1a and 1b are shown and compared with those of other 1,4-
benzoquinones.13
Figure 1 shows that the quinone ring of 1a was significantly
distorted into a chair conformation by the steric repulsion be-
tween the pairs of vicinal trimethylsilyl groups. The torsion
angles for Si1–C2–C3–Si2 and C1–C2–C3–C1ꢀ were 17.3 and
20.0°, respectively; the sp2 carbon atoms C2 and C3 remained
in near-planar geometries. As shown in Table 3, bond lengths
of 1a for C1–C2 (1.503 Å) and C2–C3 (1.354 Å) were slightly
longer than the corresponding lengths for unsubstituted 1,4-
benzoquinone (1.474 and 1.334 Å, respectively).13d The bond
angles of 1a for Si1–C2–C3 and Si2–C3–C2 were 131.3 and
131.1°, respectively, suggesting steric repulsion between vici-
nal trimethylsilyl groups. The angle between the planes de-
fined by C1–C2–C3ꢀ and C2–C3–C2ꢀ–C3ꢀ was 19.1°, larger
than the corresponding planes in 2,3,5,6-tetraphenyl-1,4-ben-
zoquinone (7.0°)13e and 2,3,5,6-tetrakis(diisopropylphosphor-
yl)-1,4-benzoquinone (17.8°).13a The angle between the C1–
O1 bond and the C1–C2–C3ꢀ plane of 1a (7.2°) was also larger
than the corresponding angle in 2,3,5,6-tetrakis(diisopropy-
lphosphoryl)-1,4-benzoquinone (1.3°).13a
In contrast to the structure of 1a, 1b displays an almost pla-
nar quinone ring with silicon atoms almost coplanar to the

2574 Bull. Chem. Soc. Jpn., 75, No. 12 (2002)
HEADLINE ARTICLES
Table 4. Physical Properties of Silyl- and Germyl-1,4-benzoquinones
UV (hexane) λmax/nm (ε)
Quinone
IR ν/cm−1 (CwO)
π–π*
n–π*
n–π*
527 (29)
553 (48)
531 (35)
536 (37)
562 (18)
539 (94)
526 (67)
504 (25)
499 (20)
499 (26)
508 (28)
501 (27)
497 (24)
492 (16)
511 (11)
1a
1b
1c
1e
1d
3a
3e
4a
4b
4c
4d
4f
265 (9260), 289 (4620), 327 (sh, 190)
266 (12300), 322 (sh, 300)
266 (8140), 291 (4750)
403 (122)
426 (33)
403 (119)
405 (90)
433 (10)
1618
1624
1620
1620
266 (9290), 286 (7790), 327 (sh, 223)
266 (10300), 315 (sh, 354)
1626
258 (11400), 273 (sh, 5990), 328 (723) 417 (54)
1635, 1625
1635, 1628
1645
1645
1643
1643
1643
1651, 1626
1653, 1628
1643
263 (8120), 328 (sh, 490)
252 (14600), 296 (sh, 360)
252 (14000), 298 (390)
252 (13400), 306 (sh, 385)
255 (12800), 295 (sh, 968)
255 (12400), 306 (sh, 396)
253 (14500), 303 (562)
252 (13700), 306 (497)
250 (13700), 312 (962)
418 (33)
5a
5b
9
ent. Irradiation of 1a at the n–π* band (λ > 500 nm) also gave
10, even though the reaction was very slow. Usually, irradia-
tion of 1,4-benzoquinones gives n-π* triplet excited states,
which undergo radical reactions such as hydrogen abstraction.6
However, no such reaction was observed in the photolysis of
1a.
Experimental
Apparatus. 1H, 13C, and 29Si NMR spectra were recorded on
a Bruker AC300-P FT-NMR spectrometer at 300, 75.4, and 59.6
MHz, respectively. Mass spectra and high-resolution mass spectra
were obtained on a JEOL JMS D-300 mass spectrometer. Elec-
tronic absorption spectra were recorded on a Milton Roy Spec-
tronic 3000 Array spectrometer. Gas-liquid chromatography
(GLC) analysis was carried out with a Shimadzu GC-8A gas chro-
matograph. IR spectra were recorded on a JEOL JIR-3505/3510
system.
Scheme 3.
Materials. THF, Et2O, and benzene were dried over sodium
benzophenone ketyl and distilled just before use. HMPA was
dried over LiAlH4 and distilled before use. CH3CN and CH2Cl2
were dried over CaH2 and distilled before use. Hexane (for UV
spectra), C6D6, CDCl3, Me3SiCl, HMe2SiCl, PhMe2SiCl,
(H2CwCH)Me2SiCl, chloranil, Mg, and PCC were purchased from
Aldrich and used without further purification. 2,3,5,6-Tetrabro-
mo-1,4-benzoquinone was purchased from Tokyo Kasei Kogyo
Co., Ltd. and used without further purification. Me3GeCl was pur-
chased from AZmax Co., Ltd. and used without further purifica-
tion. CAN, NaHCO3, and Na2SO4 were purchased from Nacalai
tesque Inc. and used without further purification. 2-Bromo-1,4-
benzoquinone,16a 2,5-dibromo-1,4-benzoquinone,16b 2,6-dibromo-
Photochemistry of 1a. The distorted structure of 1a led
to a novel photochemical reactivity of quinone. Photolysis of
1a (λ > 250 nm) in hexane resulted in a transformation of the
quinone ring: an isomer of 1a, 4-carbonyl-2,3,5,5-tetrakis(tri-
methylsilyl)-2-cyclopenten-1-one (10) was obtained in 21%
yield as pale yellow crystals (Scheme 3). Compound 10 was
isolated by column chromatography on silica gel using hexane/
benzene as the eluent, although 10 gradually decomposed in
air. The structure of 10 was determined by NMR, IR, and mass
spectra. The IR spectrum of 10 showed two peaks at 2089 and
1630 cm−1, which are assignable to the ketene and ketone moi-
eties, respectively. 13C NMR and 29Si NMR spectra of 10 indi-
cated that 10 has three kinds of trimethylsilyl groups.
Biradical or zwitterionic species are plausible intermediates
in this reaction, as shown in Scheme 3.15 Attempts to trap the
intermediates by t-BuOH and acetone failed. 2,5-Bis(trimeth-
ylsilyl)-1,4-benzoquinone (4a), 2,6-bis(trimethylsilyl)-1,4-
benzoquinone (5a), 2,3,5,6-tetrakis(dimethylsilyl)-1,4-benzo-
quinone (1b) and 2,3,5,6-tetrakis(trimethylgermyl)-1,4-benzo-
quinone (1e) did not undergo a similar photoreaction. Thus,
isomerization is driven by the large steric hindrance between
silyl substituents and finishes with migration of a silyl substitu-
1,4-benzoquinone,16c
1,4-dimethoxy-2,3,5,6-tetrabromoben-
16e
zene,16d and (ClMe2Si)2CH2 were made following the proce-
dures described in the literature.
Preparation of 1,4-Bis(trimethylsiloxy)-2,3,5,6-tetrakis(tri-
methylsilyl)benzene (2a). Method A: To a THF (100 mL) so-
lution of Me3SiCl (47.1 g, 434 mmol), Mg (10.6 g, 43.5 mmol),
and HMPA (9.5 mL) was added a THF (150 mL) solution of chlo-
ranil (11.8 g, 48.1 mmol) at room temperature. After stirring at
room temperature for 1 night, the mixture was hydrolyzed with a
saturated aqueous solution of NaHCO3. The organic layer was

S. Tsutsui et al.
Bull. Chem. Soc. Jpn., 75, No. 12 (2002) 2575
separated, and the aqueous layer was extracted with hexane. The
organic layer and the extracts were combined, washed with water
and brine, dried over Na2SO4, and filtered. The filtrate was con-
centrated under reduced pressure, and the residue was chromato-
graphed on silica gel using hexane/benzene as the eluent to give
2a (13.2 g, 24.3 mmol, 51%). 2a: colorless crystals; mp 240 °C;
1H NMR (C6D6) δ 0.18 (s, 18H), 0.42 (s, 36H); 13C NMR (C6D6) δ
0.94, 4.04, 141.97, 162.21; 29Si NMR (C6D6) δ −5.75, 19.23;
HRMS found: m/z 542.2740, calculated for C24H54O2Si6:
542.2737; Anal. Found: C, 53.04; H, 9.96%, Calcd for
C24H54O2Si6: C, 53.07; H, 10.02%.
Method B: To a THF (80 mL) solution of Me3SiCl (37.2 g,
342 mmol) and Mg (8.14 g, 33.5 mmol) was added a THF (150
mL) solution of 2,3,5,6-tetrabromo-1,4-benzoquinone (23.6 g,
55.6 mmol) at room temperature. After stirring at room tempera-
ture for 2 days, a workup similar to that in Method A described
above gave 2a (5.56 g, 10.2 mmol, 18%).
12H), 0.54 (d, J = 3.8 Hz, 24H), 4.91 (sept., J = 3.8 Hz, 4H), 5.17
(sept., J = 2.9 Hz, 2H); 13C NMR (C6D6) δ −1.57, −1.26, 138.40,
161.14; 29Si NMR (C6D6) δ −20.76, 6.22; IR (KBr) ν/cm−1 2160,
2141 (Si–H); HRMS found: m/z 458.1815, calculated for
C18H42O2Si6: 458.1801; Anal. Found: C, 47.02; H, 8.96%, Calcd
for C18H42O2Si6: C, 47.10; H, 9.22%.
Preparation of 2,3,5,6-Tetrakis(dimethylsilyl)-1,4-benzo-
quinone (1b). A mixture of 2b (5.83 g, 12.7 mmol), PCC (5.48
g, 25.4 mmol), and CH2Cl2 (180 mL) was stirred under reflux for
1 h. A workup similar to that for 1a gave 1b (4.11 g, 12.1 mmol,
95%). 1b: purple crystals; mp 103 °C; 1H NMR (C6D6) δ 0.35 (d,
J = 3.5 Hz, 24H), 4.62 (sept., J = 3.5 Hz, 4H); 13C NMR (C6D6) δ
−3.20, 162.55, 192.15; 29Si NMR (C6D6) δ −22.09; UV (hexane)
λmax/nm (ε) 266 (12300), 322 (sh, 300), 426 (33), 553 (48); IR
(CCl4) ν/cm−1 2156 (Si–H), 1624 (CwO); HRMS found: m/z
340.1167, calculated for C14H28O2Si4: 340.1167; Anal. Found: C,
49.10; H, 8.13%, Calcd for C14H28O2Si4: C, 49.35; H, 8.28%.
Preparation of 2,5-Bis(dimethylsilyl)-1,4-benzoquinone
(4b). The compound was prepared from 2,5-dibromo-1,4-ben-
zoquinone using a procedure similar to that for 1b. 4b: 52% yield;
orange crystals; mp 28–29 °C; 1H NMR (C6D6) δ 0.13 (d, J = 3.8
Hz, 12H), 4.28 (sept., J = 3.8 Hz, 2H), 6.70 (s, 2H); 13C NMR
(C6D6) δ −4.85, 145.20, 149.57, 189.27; 29Si NMR (C6D6) δ
−18.63; UV (hexane) λmax/nm (ε) 252 (14000), 298 (390), 499
(20); IR (CCl4) ν/cm−1 2131 (Si–H), 1645 (CwO); HRMS found:
m/z 224.0695, calculated for C10H16O2Si2: 224.0689; Anal. Found:
C, 53.64; H, 7.22%, Calcd for C10H16O2Si2: C, 53.52; H, 7.19%.
Preparation of 2,6-Bis(dimethylsilyl)-1,4-benzoquinone
(5b). The compound was prepared from 2-bromo-1,4-benzo-
quinone using a procedure similar to that for 1a. 5b: 34% yield;
an orange oil; 1H NMR (C6D6) δ 0.08 (d, J = 3.8 Hz, 12H), 4.25
(sept., J = 3.8 Hz, 2H), 6.73 (s, 2H); 13C NMR (C6D6) δ −4.81,
144.43, 150.92, 184.72, 193.79; 29Si NMR (C6D6) δ −18.47; UV
(hexane) λmax/nm (ε) 252 (13700), 306 (497), 492 (16); IR (CCl4)
ν/cm−1 2131 (Si–H), 1653, 1628 (CwO); HRMS found: m/z
224.0689, calculated for C10H16O2Si2: 224.0689; Anal. Found: C,
53.39; H, 6.53%, Calcd for C10H16O2Si2: C, 53.52; H, 7.19%.
Preparation of 2-(Dimethylsilyl)-1,4-benzoquinone (6b).
The compound was prepared from 2-bromo-1,4-benzoquinone us-
ing a procedure similar to that for 1b. 6b: 83% yield; an orange
Preparation of 2,3,5,6-Tetrakis(trimethylsilyl)-1,4-benzo-
quinone (1a). A CH2Cl2 (2 mL) solution of 2a (1.01 g, 1.86
mmol) and PCC (0.825 g, 3.83 mmol) was stirred under reflux for
1.5 h. The resulting mixture was concentrated and chromato-
graphed on silica gel using hexane as the eluent to give 1a (0.529
g, 1.34 mmol, 72%) and 2,3,5-tris(trimethylsilyl)-1,4-benzoqui-
none (3a) (56 mg, 0.17 mmol, 9.3%). 1a: deep red crystals; mp
182 °C; 1H NMR (C6D6) δ 0.29 (s, 36H); 13C NMR (C6D6) δ 1.54,
167.50, 198.99; 29Si NMR (C6D6) δ −5.36; UV (hexane) λmax/nm
(ε) 207 (18900), 265 (9260), 289 (4620), 327 (sh, 190), 403 (122),
527 (29); IR (CCl4) ν/cm−1 1618 (CwO); HRMS found: m/z
396.1794, calculated for C18H36O2Si4: 396.1792; Anal. Found: C,
54.38; H, 9.01%, Calcd for C18H36O2Si4: C, 54.48; H, 9.14%. 3a:
1
deep red crystals; mp 140–140.5 °C; H NMR (C6D6) δ 0.10 (s,
9H), 0.28 (s, 9H), 0.33 (s, 9H), 6.64 (s, 1H); 13C NMR (C6D6) δ
−1.94, 1.76, 1.82, 144.21, 153.08, 163.30, 168.36, 189.88,
196.17; 29Si NMR (C6D6) δ −4.71, −4.45, −4.31; UV (hexane)
λ
max/nm (ε) 258 (11400), 273 (sh, 5990), 328 (723), 417 (54), 539
(94); IR (CCl4) ν/cm−1 1635, 1625 (CwO); HRMS found: m/z
324.1393, calculated for C15H28O2Si3: 324.1397; Anal. Found: C,
55.70; H, 8.55%, Calcd for C15H28O2Si3: C, 55.50; H, 8.69%.
Preparation of 2,6-Bis(trimethylsilyl)-1,4-benzoquinone
(5a).7f The compound was prepared from 2,6-dibromo-1,4-ben-
zoquinone using a procedure similar to that for 1a. 5a: 7.9%
1
oil; H NMR (C6D6) δ 0.07 (d, J = 3.8 Hz, 6H), 4.22 (sept., J =
1
yield; orange crystals; mp 54.5–55.5 °C; H NMR (C6D6) δ 0.08
3.8 Hz, 1H), 6.06 (d, J = 1.8 Hz, 1H), 6.07 (s, 1H), 6.62 (d, J =
1.8 Hz, 1H); 13C NMR (C6D6) δ −4.90, 136.09, 136.99, 143.98,
149.38, 185.85, 190.34; 29Si NMR (C6D6) δ −18.38; IR (CCl4)
ν/cm−1 2131 (Si–H), 1666, 1650 (CwO); HRMS found: m/z
166.0450, calculated for C8H10O2Si: 166.0434; Anal. Found: C,
58.09; H, 5.93%, Calcd for C8H10O2Si: C, 57.80; H, 6.06%.
Preparation of 2,3,5,6-Tetrakis(dimethylvinylsilyl)-1,4-ben-
zoquinone (1c). The compound was prepared from 2,3,5,6-tet-
rabromo-1,4-benzoquinone using a procedure similar to that for
1b. 1c: 3.3% yield; deep red crystals; mp 71.5 °C; 1H NMR
(C6D6) δ 0.38 (s, 24H), 5.68 (dd, J = 3.4, 20.4 Hz, 4H), 5.91 (dd,
J = 3.4, 14.6 Hz, 4H), 6.38 (dd, J =14.6, 20.4 Hz, 4H); 13C NMR
(C6D6) δ −0.03, 133.02, 139.30, 166.54, 198.33; 29Si NMR
(C6D6) δ −12.96; UV (hexane) λmax/nm (ε) 266 (8140), 291
(4750), 403 (119), 531 (35); IR (CCl4) ν/cm−1 1620 (CwO);
HRMS found: m/z 444.1793, calculated for C22H36O2Si4:
444.1793; Anal. Found: C, 59.22; H, 7.94%, Calcd for
C22H36O2Si4: C, 59.40; H, 8.16%.
(s, 18H), 6.75 (s, 2H); 13C NMR (C6D6) δ −1.84, 143.55, 153.37,
185.29, 194.40; 29Si NMR (C6D6) δ −3.86; UV (hexane) λmax/nm
(ε) 253 (14500), 303 (562), 497 (24); IR (CCl4) ν/cm−1 1651,
1626 (CwO); MS (70 eV) m/z 252 (M+).
Preparation of 2-(Trimethylsilyl)-1,4-benzoquinone (6a).7e
The compound was prepared from 2-bromo-1,4-benzoquinone us-
ing a procedure similar to that for 1a. 6a: 7.4% yield; orange
crystals; mp 63.5–64 °C; 1H NMR (C6D6) δ 0.06 (s, 9H), 6.07 (m,
2H), 6.61 (s, 1H); 13C NMR (C6D6) δ −1.96, 135.93, 137.28,
143.24, 151.42, 186.11, 190.59; 29Si NMR (C6D6) δ −3.73; UV
(hexane) λmax/nm (ε) 246 (15400), 297 (630), 470 (28); IR (CCl4)
ν/cm−1 1662, 1651 (CwO); MS (70 eV) m/z 180 (M+).
Preparation of 1,4-Bis(dimethylsiloxy)-2,3,5,6-tetrakis(di-
methylsilyl)benzene (2b).
To a THF (50 mL) solution of
HMe2SiCl (15.3 g, 161 mmol) and Mg (3.93 g, 157 mmol) was
added a THF (70 mL) solution of 2,3,5,6-tetrabromo-1,4-benzo-
quinone (7.38 g, 17.4 mmol) at room temperature. After the mix-
ture was stirred at room temperature for 8 days, a workup similar
to that for 2a gave 2b (2.50 g, 5.46 mmol, 31%). 2b: colorless
crystals; mp 112–112.5 °C; 1H NMR (C6D6) δ 0.13 (d, J = 2.9 Hz,
Preparation of 2,5-Bis(dimethylvinylsilyl)-1,4-benzoquino-
ne (4c). The compound was prepared from 2,5-dibromo-1,4-
benzoquinone using a procedure similar to that for 1b. 3c: 41%

2576 Bull. Chem. Soc. Jpn., 75, No. 12 (2002)
HEADLINE ARTICLES
yield; orange crystals; mp 110.5–111 °C; 1H NMR (C6D6) δ 0.21
(s, 12H), 5.64 (dd, J = 3.6, 20.2 Hz, 2H), 5.90 (dd, J = 3.6, 14.6
Hz, 2H), 6.18 (dd, J =14.6, 20.2 Hz, 2H), 6.74 (s, 2H); 13C NMR
(C6D6) δ −3.66, 134.10, 135.93, 145.18, 150.30, 189.71; 29Si
NMR (C6D6) δ −11.66; UV (hexane) λmax/nm (ε) 252 (13400),
306 (sh, 385), 499 (26); IR (CCl4) ν/cm−1 1643 (CwO); MS (70
eV) m/z 276 (M+); HRMS found: m/z 276.0997, calculated for
C14H20O2Si2: 276.1002; Anal. Found: C, 60.54; H, 7.43%, Calcd
for C14H20O2Si2: C, 60.82; H, 7.29%.
Preparation of 2,5-Bis(dimethylphenylsilyl)-1,4-benzoqui-
none (4f). The compound was prepared from 2,5-bromo-1,4-
benzoquinone using a procedure similar to that for 1b. 4f: 35%
yield; orange crystals; mp 115–117 °C; 1H NMR (C6D6) δ 0.39 (s,
12H), 6.65 (s, 2H), 7.1–7.2 (m, 6H), 7.4–7.5 (m, 4H); 13C NMR
(C6D6) δ −3.29, 128.54, 129.87, 134.58, 135.45, 145.70, 150.29,
189.66; 29Si NMR (C6D6) δ −8.75; UV (hexane) λmax/nm (ε) 255
(12800), 295 (sh, 968), 508 (28); IR (CCl4) ν/cm−1 1643 (CwO);
HRMS found: m/z 376.1314, calculated for C22H24O2Si2:
376.1315; Anal. Found: C, 70.01; H, 6.36%, Calcd for
C22H24O2Si2: C, 70.16; H, 6.42%.
(ε) 250 (13700), 312 (962), 511 (11); IR (CCl4) ν/cm−1 1643
(CwO); HRMS found: m/z 236.0685, calculated for C11H16O2Si2:
236.0689; Anal. Found: C, 55.60; H, 6.85%, Calcd for
C11H16O2Si2: C, 55.88; H, 6.82%.
Preparation of 1,4-Bis(trimethylgermyloxy)-2,3,5,6-tetra-
kis(trimethylgermyl)benzene (2e). To a THF (20 mL) solution
of Me3GeCl (2.87 g, 18.7 mmol) and Mg (0.460 g, 18.9 mmol)
was added a THF (15 mL) solution of 2,3,5,6-tetrabromo-1,4-ben-
zoquinone (1.30 g, 3.06 mmol) at room temperature. After the
mixture was stirred at room temperature for 36 h, a workup simi-
lar to that for 2a gave 2e (84.8 mg, 1.05 × 10−4 mol, 3.4%). 2e:
1
colorless crystals; mp 264–265 °C; H NMR (C6D6) δ 0.31 (s,
18H), 0.58 (s, 36H); 13C NMR (C6D6) δ 1.56, 4.42, 142.33,
163.23; Anal. Found: C, 35.71; H, 6.64%, Calcd for C24H54Ge6O2:
C, 35.57; H, 6.71%.
Preparation of 2,3,5,6-Tetrakis(trimethylgermyl)-1,4-ben-
zoquinone (1e). A mixture of 2e (72.0 mg, 8.90 × 10−5 mol),
PCC (39.0 mg, 0.181 mmol), and CH2Cl2 (15 mL) was stirred un-
der reflux for 1 h. A workup similar to that for 1a gave 1e (40.5
mg, 6.98 × 10−5 mol, 79%) and 2,3,5-tris(trimethylgermyl)-1,4-
benzoquinone (3e) (4.1 mg, 9.0 × 10−4 mol, 10%). 1e: deep red
Preparation of 2,8-Dimethoxy-4,4,6,6,10,10,12,12-octameth-
yltricyclo[7.3.0.03,7]dodeca-4,6,10,12-tetrasila-1,3(7),8-triene
(7). To a THF (100 mL) solution of (ClMe2Si)2CH2 (13.6 g, 67.6
mmol) and Mg (3.29 g, 13.6 mmol) was added to a THF (50 mL)
solution of 2,3,5,6-tetrabromo-1,4-dimethoxybenzene (10.0 g,
22.0 mmol) at room temperature. After the mixture was stirred at
room temperature for 1 day, a workup similar to that for 2a gave 7
(0.439 g, 1.11 mmol, 5.1%) and 2,5-dimethoxy-7,9-disila-7,7,9,9-
tetramethyl-bicyclo[4.3.0]nona-l,3,5-triene (8) (0.194 g, 0.729
1
crystals; mp 220 °C; H NMR (C6D6) δ 0.43 (s, 6H); 13C NMR
(C6D6) δ l.93, 169.03, 193.89; UV (hexane) λmax/nm (ε) 266
(9290), 286 (7790), 327 (sh, 223), 405 (90), 536 (37); IR (CCl4)
ν/cm−1 1620 (CwO); HRMS found: m/z 579.9571, calculated for
C18H36O274Ge4: 579.9563; Anal. Found: C, 37.75; H, 6.21%, Cal-
cd for C18H36Ge4O2: C, 37.60; H, 6.31%. 3e: deep red crystals;
mp 128–130 °C; 1H NMR (C6D6) δ 0.23 (s, 9H), 0.43 (s, 9H), 0.49
(s, 9H), 6.72 (s, 1H); 13C NMR (C6D6) δ −2.04, 2.19, 2.25,
143.46, 156.46, 165.74, 169.63, 187.34, 192.74; UV (hexane)
1
mmol, 3.3%). 7: colorless crystals; mp 195–196.5 °C; H NMR
(C6D6) δ −1.03 (s, 4H), 0.47 (s, 24H), 3.55 (s, 6H); 13C NMR
(C6D6) δ −0.19, 1.38, 62.72, 145.78, 166.85; 29Si NMR (C6D6) δ
7.84; HRMS found: m/z 394.1630, calculated for C18H34O2Si4:
394.1636; Anal. Found: C, 54.56; H, 8.53%, Calcd for
C18H34O2Si4: C, 54.76; H, 8.68%. 8: colorless crystals; mp 117.5–
120.5 °C; 1H NMR (C6D6) δ −0.05 (s, 2H), 0.52 (s, 12H), 3.37 (s,
6H), 6.51 (s, 2H); 13C NMR (C6D6) δ −2.10, 0.73, 55.19, 111.39,
139.95, 158.32; 29Si NMR (C6D6) δ 10.38; HRMS found: m/z
266.1155, calculated for C13H22O2Si2: 266.1158; Anal. Found: C,
58.69; H, 8.19%, Calcd for C13H22O2Si2: C, 58.59; H, 8.32%.
Preparation of 4,4,6,6,10,10,12,12-Octamethyl-4,6,10,12-
tetrasilatricyclo[7.3.0.03,7]dodeca-1(9),3(7)-diene-2,8-dione
(1d). To a CH3CN (20 mL) solution of 7 (0.147 g, 0.373 mmol)
was added a H2O (20 mL) solution of CAN (1.71 g, 3.72 mmol) at
room temperature. After the mixture was stirred under reflux for
12 h, a workup similar to that for 1a gave 1d (0.050 g, 0.14 mmol,
λmax/nm (ε) 263 (8120), 328 (sh, 490), 418 (33), 526 (67); IR
(CCl4) ν/cm−1 1635, 1628 (CwO); HRMS found: m/z 455.9707,
calculated for C15H28O272Ge3: 455.9752.
Preparation of 1,4-Bis(trimethylgermyloxy)-2,5-bis(tri-
methylgermyl)benzene (11e). To a THF (5 mL) solution of
Me3GeCl (1.00 g, 6.53 mmol) and Mg (0.258 g, 10.6 mmol) was
added a THF (5 mL) solution of 2,5-dibromo-1,4-benzoquinone
(0.390 g, 1.47 mmol) at room temperature. After the mixture was
stirred for 18 h at room temperature and under reflux for 2.5 h, a
workup similar to that for 2a gave 11e (0.293 g, 0.507 mmol,
34%). 11e: colorless crystals; mp 160–161.5 °C; 1H NMR (C6D6)
δ 0.38 (s, 18H), 0.56 (s, 18H), 6.88 (s, 2H); 13C NMR (C6D6) δ
−0.95, 1.56, 121.79, 132.65, 156.77; HRMS found: m/z
573.9758, calculated for C18H3872Ge4O2: 573.9755; Anal. Found:
C, 37.44; H, 6.34%, Calcd for C18H38Ge4O2: C, 37.47; H, 6.64%.
Preparation of 2,5-Bis(trimethylgermyl)-1,4-benzoquinone
(4e). A mixture of 11e (121 mg, 2.10 × 10−4 mol), PCC (91.0
mg, 4.22 × 10−4 mol), and CH2Cl2 (10 mL) was stirred at room
temperature for 1 h. A workup similar to that for 1a gave 4e (71.3
mg, 2.10 × 10−4 mol, 99%). 4e: orange crystals; mp 190.5–191
1
37%). 1d: purple crystals; mp 254.5–255 °C; H NMR (C6D6) δ
−0.39 (s, 4H), 0.35 (s, 24H); 13C NMR (C6D6) δ −3.57, 0.01,
166.75, 191.15; 29Si NMR (C6D6) δ 12.95; UV (hexane) λmax/nm
(ε) 266 (10300), 315 (sh, 354), 433 (10), 562 (18); IR (CCl4)
ν/cm−1 1626 (CwO); HRMS found: m/z 364.1161, calculated for
C16H28O2Si4: 364.1167; Anal. Found: C, 52.90; H, 7.80%, Calcd
for C16H28O2Si4: C, 52.69; H, 7.74%.
1
°C; H NMR (C6D6) δ 0.26 (s, 18H), 6.75 (s, 2H); 13C NMR
(C6D6) δ −1.99, 143.89, 154.91, 188.50; UV (hexane) λmax/nm (ε)
255 (12400), 306 (sh, 396), 501 (27); IR (CCl4) ν/cm−1 1643
(CwO); HRMS found: m/z 339.9901, calculated for
C12H20O272Ge2: 339.9905; Anal. Found: C, 42.14; H, 6.01%, Cal-
cd for C12H20Ge2O2: C, 42.20; H, 5.90%.
Preparation of 7,7,9,9-Tetramethyl-7,9-disilabicyclo[4.3.0]-
nona-1(6),3-diene-2,5-dione (9). To a CH3CN (10 mL) solution
of 8 (85.8 mg, 3.22 × 10−4 mol) was added a H2O (12 mL) solu-
tion of CAN (0.752 g, 1.63 mmol) at room temperature. After the
mixture was stirred at room temperature for 3 h, a workup similar
to that for 1a gave 9 (52.6 mg, 0.223 mmol, 69%). 9: pale orange
X-ray Crystallographic Analysis of 1a.17,18 Diffraction data
were collected on a Rigaku Denki AFC-5R diffractometer with a
rotating anode (45 kV, 200 mA) with graphite-monochromatized
Cu-Kα radiation (λ = 1.54178 Å). Crystals for X-ray analysis
were obtained from a hexane solution. The data collection was
carried out at 170 K on a crystal of dimensions 0.25 × 0.20 ×
1
crystals; mp 81–81.5 °C; H NMR (C6D6) δ −0.43 (s, 2H), 0.28
(s, 12H), 6.12 (s, 2H); 13C NMR (C6D6) δ −3.92, −0.17, 137.53,
164.36, 189.14; 29Si NMR (C6D6) δ 13.47; UV (hexane) λmax/nm

S. Tsutsui et al.
Bull. Chem. Soc. Jpn., 75, No. 12 (2002) 2577
0.25 mm3. A total of 1929 reflections with 2θmax = 128° were col-
lected. Crystal data: MF = C18H36O2Si4; MW = 396.83; orthor-
hombic; a = 12.184(1), b = 18.696(2), c = 10.562(1) Å; V =
2405.9(3) Å3; space group = Pbca; Z = 4; Dc = 1.10 g/cm3. The
structure was solved by the direct method and refined anisotropi-
cally for Si, O, and C atoms and isotropically for H atoms. All cal-
culations were performed using the UNICS ꢁ package system and
K. Ebata, K. Sakamoto, Y. Nakadaira, and C. Kabuto, Chem. Lett.,
1988, 965.
3
a) H. Sakurai, K. Ebata, C. Kabuto, and A. Sekiguchi, J.
Am. Chem. Soc., 112, 1799 (1990). b) A. Sekiguchi, K. Ebata, Y.
Terui, and H. Sakurai, Chem. Lett., 1991, 1417. c) A. Sekiguchi,
Y. Sugai, K. Ebata, C. Kabuto, and H. Sakurai, J. Am. Chem. Soc.,
115, 1144 (1993).
RANTAN81 program. The final R factor was 0.0376 (Rw
0.0469) for 1735 reflections with Fo > 3σ(Fo).
=
4
1675.
5
K. Sakamoto, T. Saeki, and H. Sakurai, Chem. Lett., 1993,
X-ray Crystallographic Analysis of 1b.17,18 Diffraction data
were collected on a Rigaku Denki AFC-5R diffractometer with a
rotating anode (45 kV, 200 mA) with graphite-monochromatized
Mo-Kα radiation (λ = 0.71069 Å). Crystals for X-ray analysis
were obtained from a hexane solution. The data collection was
carried out at 150 K on a crystal of dimensions 0.20 × 0.20 × 0.3
mm3. A total of 5062 reflections with 2θmax = 60° were collected.
Crystal data: MF = C14H28O2Si4; MW = 340.72; monoclinic; a =
9.452(4), b = 17.702(5), c = 12.690(4) Å; V = 2046.5(24) Å3;
space group = P21/c; Z = 4; Dc = 1.11 g/cm3. The structure was
solved by the direct methods and refined anisotropically for Si, O,
and C atoms and isotropically for H atoms. All calculations were
performed using the UNICS ꢁ package system and RANTAN81
program. The final R factor was 0.0628 (RW = 0.0626) for 3967
reflections with Fo > 3σ(Fo).
A. Sekiguchi, M. Tanaka, T. Matsuo, and H. Watanabe, An-
gew. Chem., Int. Ed., 40, 1675 (2001).
Reviews for quinones. a) “The Chemistry of the Quinonoid
6
Compounds Part 1 and Part 2,” ed by S. Patai, John Wiley & Sons,
Bristol (1974). b) “The Chemistry of the Quinonoid Compounds,
Part 1 and Part 2,” ed by S. Patai and Z. Rappoport, John Wiley &
Sons, New York, (1988), Vol. 2.
7
a) K. Sakamoto and H. Sakurai, J. Am. Chem. Soc., 113,
1466 (1991). b) H. Sakurai, J. Abe, and K. Sakamoto, J. Photo-
chem. Photobiol., A, 65, 111 (1992). c) H. Bock and H. Alt, An-
gew. Chem., Int. Ed. Engl., 6, 941 (1967). d) G. D. Cooper, B.
Williams, and C. P. Lape, J. Org. Chem., 26, 4171 (1961). e) H.
Hashimoto, Yakugaku Zasshi, 87, 535 (1967). f) N. S.
Vasileiskaya, L. V. Gorbunova, O. N. Mamysheva, and G. N.
Bortnikov, Izv. Akad. Nauk. SSSR, Ser. Khim., 1972, 2755. g) L. D.
Foland, J. O. Karlsson, S. T. Perri, R. Schwabe, S. L. Xu, S. Patil,
and H. W. Moore, J. Am. Chem. Soc., 111, 975 (1989).
Photoisomerization of 1a. A hexane (7 mL) solution of 1a
(101 mg, 2.55 × 10−4 mol) in a Pyrex tube (φ 11 mm) was irradi-
ated (λ > 250 nm) for 20 h at room temperature. The mixture was
concentrated under reduced pressure, and the residue was chro-
matographed on silica gel using hexane/benzene as the eluent to
give 4-carbonyl-2,3,5,5-tetrakis(trimethylsilyl)-2-cyclopenten-1-
one (10) (21 mg, 5.3 × 10−5 mol, 21%). 7 mg (1.8 × 10−5 mol,
7%) of 1a was recovered. 10: Pale yellow crystals; mp 56–60 °C;
1H NMR (C6D6) δ 0.13 (s, 18H), 0.19 (s, 9H), 0.39 (s, 9H); 13C
NMR (C6D6) δ −1.81, 0.91, 1.06, 47.51, 55.03, 149.10, 173.30,
197.60, 206.72; 29Si NMR (C6D6) δ −7.6, −3.1, 4.7; IR (KBr,
cm−1) 2089 (CwCwO), 1630 (CwO); HRMS found: m/z 396.1791,
calculated for C18H36O2Si4: 396.1792; UV (hexane) λmax/nm (ε)
281 (6340), 338 (6500).
Supporting Information Available: Ordering information is
given on any current masthead page. Tables of atomic coordi-
nates, anisotropic thermal parameters, bond lengths and angles,
and observed and calculated structure factors for compounds 1a
and 1b (40 pages) are available. The crystallographic data for two
crystals are deposited as Document No. 75054 at the Office of the
Editor of Bull. Chem. Soc. Jpn. Crystallographic data have been
deposited at the Cambridge Crystallographic Data Center, 12
Union Road, Cambridge CB2 1EZ, UK and copies can be ob-
tained on request, free of charge, by quoting the publication cita-
tion and the deposition numbers 192669 and 192670.
8
É. P. Oleinik, O. N.Mamysheva, and L.V. Gorbunova,
Dokl. Akad. Nauk SSSR, 289, 1137 (1986).
For the preliminary results of 2,3,5,6-tetrakis(trimethylsi-
9
lyl)-1,4-benzoquinone, see: K. Sakamoto, S. Tsutsui, K. Ebata, C.
Kabuto, and H. Sakurai, Chem. Lett., 2000, 226.
10 R. Calas, J. Organomet. Chem., 200, 11 (1980).
11 J. P. Willis, K. A. Z. Gogins, and L. L. Miller, J. Org.
Chem., 46, 3215 (1981).
12 P. Jacob, III, P. S. Calley, A. T. Shuigin, and N. Castagnoli,
Jr., J. Org. Chem., 41, 3627 (1976).
13 a) K. Kobiro, M. Shi, and Y. Inoue, Chem. Lett., 1999, 633.
b) D. Rabinovich and G. M. J. Schmidt, J. Chem. Soc. B, 1967,
131. c) K. J. van Weperen and G. J. Visser, Acta Crystallogr., Sect.
B, 28, 338 (1972). d) F. van Bolhuis and C. Th. Kiers, Acta Crys-
tallogr., Sect. B, 34, 3697 (1978). e) H. Bock, A. John, M. Kleine,
C. Naether, and J. W. Bats, Z. Naturforsch., B: Chem. Sci., 49, 529
(1994).
14 E. Charney and E. D. Becker, J. Chem. Phys., 42, 910
(1965).
15 K. Schaffner and M. Demouth, “Photochemical Rearrange-
ment of Conjugated Cyclic Dienes,” in “Rearrangements in
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