Spiro[2,5-dibenzo-1,4-disilacyclohexa-2,5-diene-1,7′-2′, 5′- dibenzo-1′,4′,7′-trisilanorbornadiene]: Unusual dimerization of silyl-substituted 9,10-disila(dewar anthracene)

Article abstract of DOI:10.1080/10426501003771385

The reduction of 9,10-dibromo-9,10-dihydro-9,10-disilaanthracene (3) with 2.2 equiv. of KC8 in THF afforded the unexpected spiro[2,5-dibenzo-1,4- disilacyclohexa-2,5-diene-1,7′-2′,5′-dibenzo-1′, 4′,7′-trisilanorbornadiene] (4) through the dimerization of

Full text of DOI:10.1080/10426501003771385

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Phosphorus, Sulfur, and Silicon and the
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Spiro[2,5-dibenzo-1,4-
disilacyclohexa-2,5-diene-1,7′-2′,5′-
dibenzo-1′,4′,7′-trisilanorbornadiene]:
Unusual Dimerization of Silyl-Substituted
9,10-Disila(Dewar Anthracene)
Norio Nakata a , Sayaka Masubuchi a & Akira Sekiguchi a
a Department of Chemistry, Graduate School of Pure and Applied
Sciences , University of Tsukuba , Tsukuba, Ibaraki, Japan
Published online: 27 May 2010.
To cite this article: Norio Nakata , Sayaka Masubuchi & Akira Sekiguchi (2010) Spiro[2,5-dibenzo-1,4-
disilacyclohexa-2,5-diene-1,7′-2′,5′- dibenzo-1′,4′,7′-trisilanorbornadiene]: Unusual Dimerization
of Silyl-Substituted 9,10-Disila(Dewar Anthracene), Phosphorus, Sulfur, and Silicon and the Related
Elements, 185:5-6, 957-963, DOI: 10.1080/10426501003771385
To link to this article: http://dx.doi.org/10.1080/10426501003771385
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Phosphorus, Sulfur, and Silicon, 185:957–963, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426501003771385
SPIRO[2,5-DIBENZO-1,4-DISILACYCLOHEXA-2,5-DIENE-
1,7ꢀ-2ꢀ,5ꢀ- DIBENZO-1ꢀ,4ꢀ,7ꢀ-TRISILANORBORNADIENE]:
UNUSUAL DIMERIZATION OF SILYL-SUBSTITUTED
9,10-DISILA(DEWAR ANTHRACENE)
Norio Nakata, Sayaka Masubuchi, and Akira Sekiguchi
Department of Chemistry, Graduate School of Pure and Applied Sciences,
University of Tsukuba, Tsukuba, Ibaraki, Japan
The reduction of 9,10-dibromo-9,10-dihydro-9,10-disilaanthracene (3) with 2.2 equiv. of KC8
in THF afforded the unexpected spiro[2,5-dibenzo-1,4-disilacyclohexa-2,5-diene-1,7ꢀ-2ꢀ,5ꢀ-
dibenzo-1ꢀ,4ꢀ,7ꢀ-trisilanorbornadiene] (4) through the dimerization of the transient 9,10-
disila-9,10-Dewar anthracene (5). The formation of 5 was demonstrated by low-temperature
NMR spectroscopy.
Keywords Dewar anthracene; disilaanthracene; silicon; spiro compound; X-ray crystallogra-
phy
INTRODUCTION
Photochemical reactions of aromatic compounds are well known for forming inter-
esting valence isomers.1 The photolysis of anthracenes is known to result in the formation
of the 9,10-Dewar anthracenes or the corresponding [4 + 4] dimers.2 Although a number of
[4 + 4] dimers of anthracene derivatives have been structurally characterized,3 the crystal
structures of 9,10-Dewar anthracenes are little known because of their thermal instability.
The only known example of an X-ray crystallographic analysis of 9-tert-butyl-9,10-Dewar
anthracene was reported by Angermund et al.4 On the other hand, there has been much
interest in the chemistry of silaaromatic compounds in recent decades.5 Recently, Shinohara
et al. succeeded in the synthesis and characterization of a stable 9-silaanthracene using a
bulky substituent.6 They also carried out the photolysis of this 9-silaanthracene to form the
corresponding Dewar-type isomer, 9,10-Dewar 9-silaanthracene; however, it could not be
isolated because of its thermal instability.7
Received 12 November 2008; accepted 20 November 2008.
Dedicated to Professor Naomichi Furukawa on the occasion of his 70th birthday.
This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Areas “Advanced
Molecular Transformations of Carbon Resources” from the Ministry of Education, Culture, Sports, Science, and
Technology, Japan. We thank Dr. Kazuhiko Sato in Research Institute for Innovation in Sustainable Chemistry,
National Institute of Advanced Industrial Science and Technology (AIST) for the helpful cooperation.
Address correspondence to Akira Sekiguchi, Department of Chemistry, Graduate School of Pure and Applied
Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan. E-mail: sekiguch@chem.tsukuba.ac.jp
957

958
N. NAKATA ET AL.
Meanwhile, we reported a simple new synthetic method for a silyl-substituted 1,4-
disila(Dewar benzene) derivative together with its unique photochemical reaction8 and
reduction with lithium metal to give the 1,4-disilacyclohexa-2,5-diene-1,4-diide, represent-
ing a nonaromatic 8π-electron cyclic system.9 We have now extended our chemistry to
the 9,10-disilaanthracene system, and we have found an unexpected product resulting from
9,10-disila-9,10-Dewar anthracene. In this article, we report the spectroscopic observa-
tion of 9,10-disila-9,10-Dewar anthracene generated by the reduction of 9,10-bis(di-tert-
butylmethylsilyl)-9,10-dibromo-9,10-dihydro-9,10-disilaanthracene (3) with KC8 at low
temperature and its unusual dimerization to give a spiro compound, spiro[2,5-dibenzo-1,4-
disilacyclohexa-2,5-diene-1,7ꢁ-2ꢁ,5ꢁ-dibenzo-1ꢁ,4ꢁ,7ꢁ-trisilanorbornadiene] (4).
RESULTS AND DISCUSSION
At first, 9,10-dihydro-9,10-disilaanthracene derivative 1 bearing two anisyl groups
(Ani = 4-methoxyphenyl) was prepared by modification of the literature method10 as a
4:3 mixture of syn and anti isomers (Scheme 1). Dearylation/triflation of 1 with CF3SO3H
t
at −78◦C followed by reaction with Bu2MeSiNa at room temperature gave the silyl-
substituted 9,10-dihydro-9,10-disilaanthracene derivative 2 in 20% yield. Finally, careful
bromination of 2 with Br2 resulted in the quantitative formation of the corresponding
dibromide 3, which is a suitable precursor for a 9,10-disila-Dewar anthracene derivative.
H
Ani
Si
Br
H
Ani Br
Si
Br
Br
(a)
(b)
Si
(74%)
Ani
H
1 (77%)
Br SiMetBu2
H
SiMetBu2
Si
Si
Si
(c)
(d)
Si
tBu2MeSi
H
tBu2MeSi Br
2 (20%)
3 (99%)
(a) 1) nBuLi, THF, Et2O, -110 °C; 2) AniSiHCl2, THF.
(b) 1) nBuLi, THF, -78 °C; 2) AniSiHCl 2, THF.
(c) 1) CF3SO3H, toluene, -78 °C; 2) tBu2MeSiNa, THF, RT.
(d) Br2, CH2Cl2, 0 °C.
Scheme 1

DIMERIZATION OF 9,10-DISILA(DEWAR ANTHRACENE)
959
The reduction of 3 with KC8 (2.2 equiv) in THF at −78◦C gradually afforded
a red-brown reaction mixture due to the formation of 9,10-disila-9,10-Dewar an-
thracene 5. However, compound 5 was unstable at room temperature, and it formed an
unexpected spiro compound, 1ꢁ,4,4,4ꢁ-tetrakis(di-tert-butylmethylsilyl)spiro[2,5-dibenzo-
1,4-disilacyclohexa-2,5-diene-1,7ꢁ-2ꢁ,5ꢁ-dibenzo-1ꢁ,4ꢁ,7ꢁ-trisilanorbornadiene] (4), which
was obtained in 81% yield as colorless crystals (Scheme 2). The structure of 4 was
t
t
Bu MeSi SiMe Bu
2
2
t
Br SiMe Bu
2
Si
Si
KC
8
Si
t
SiMe Bu
°
THF, –78 C to RT
2
Si
Si
Si
Bu MeSi
t
Bu MeSi Br
2
t
2
3
4 (81%)
Scheme 2
unambiguously determined by NMR spectral data and X-ray crystallographic analysis
(Figure 1). The crystal structure of 4 showed that the 9,10-dihydro-9,10-disilaanthracene
framework containing Si3 and Si4 atoms adopts a somewhat twisted structure, probably
t
because of the steric influence of the bulky Bu2MeSi groups. This is reflected in the
chemical shifts of the 29Si nuclei of the ring atoms. Thus, the 29Si NMR spectrum showed
five signals at −44.0, −30.1, 9.6, 10.4, and 25.0 ppm, of which the signal at highest field
(−44.0 ppm) can be assigned to the spiro silicon atom (Si3). The most deshielded signal
at 25.0 ppm can be assigned to the Si4 atom, the geometry around which is distorted, as
shown by the stretched bond lengths of Si4–Si7 (2.4597(11) Å) and Si4–Si8 (2.4733(12)
Å), as well as by the increased bond angle of Si7–Si4–Si8 (124.25(4)◦). The 29Si signal due
to the Si1 and Si2 atoms was observed at −30.1 ppm, and the signals due to the tBu2MeSi
substituents were found at 9.6 and 10.4 ppm.
Although the mechanism to form 4 is not clear at present, one might be able to
postulate the intermediacy of an extremely reactive biradical species 5ꢁ, which is generated
by the cleavage of the strained central Si–Si bond in Dewar derivative 5 (Scheme 3), as found
in the 1,4-disila-Dewar benzene derivative.8 Thus, the formation of 4 can be explained by
dimerization of 5ꢁ through stepwise Si–Si bond formation to give a biradical intermediate
t
7 and the subsequent migration of a Bu2MeSi group. In contrast, no dimerization of 5ꢁ
t
to give 6 occurred, probably because of the steric repulsion between the bulky Bu2MeSi
groups.
Next, we spectroscopically observed Dewar-type species 5 by a low-temperature
NMR measurement. When the reduction of 3 with 2.2 equiv of KC8 was performed in
THF-d8 at −30◦C, the reaction was completed within 10 h, and the generation of 5 was
1
confirmed by NMR spectra at 243 K. Thus, the H NMR resonances of 5 were observed
at 0.38, 1.21, 7.09, and 7.43 ppm. The 29Si NMR spectrum of 5 showed two signals at
−14.9 and 11.4 ppm, which are assignable to the ring silicon atom and tBu2MeSi groups,
respectively. This assignment was reasonably supported by comparison with the observed

960
N. NAKATA ET AL.
SiMetBu2
SiMetBu2
SiMetBu2
Si
SiMetBu2
Si
Si
Si
Si
Si
KC8
tBu2MeSi
3
Si
Si
tBu2MeSi
SiMetBu2
SiMetBu2
6
5'
5
SiMetBu2
Si
Si
Si
Si
tBu2MeSi
SiMetBu2
4
tBu2MeSi
7
Scheme 3
chemical shifts of silyl-substituted 1,4-disila(Dewar benzene) (−26.3 for skeletal Si and
t
11.0 ppm for Bu2MeSi).8 Compound 5 is thermally unstable, similar to 9,10-Dewar-9-
silaanthracene.7 Thus, the signals of the dimeric product 4 appeared upon warming to room
temperature with concomitant disappearance of the NMR signals due to 5.
Figure 1 ORTEP drawing of 4 (30% thermal ellipsoids). Hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Si1–Si3 = 2.4377(12), Si1–Si5 = 2.4000(12), Si2–Si3 = 2.4178(11), Si2–Si6 =
2.3966(12), Si4–Si7 = 2.4597(11), Si4–Si8 = 2.4733(12), Si5–Si1–Si3 = 126.13(4), Si6–Si2–Si3 = 129.02(5),
Si1–Si3–Si2 = 77.93(4), Si7–Si4–Si8 = 124.25(4).

DIMERIZATION OF 9,10-DISILA(DEWAR ANTHRACENE)
961
EXPERIMENTAL
All experiments were performed using high-vacuum line techniques or in an argon
atmosphere using an MBRAUN MB 150B-G glove box. All solvents were dried and
degassed over a potassium mirror in vacuum prior to use. NMR spectra were recorded with
Bruker AC-300FT NMR (1H NMR at 300.1 MHz, 13C NMR at 75.5 MHz, 29Si NMR at 59.6
MHz) and AV-400FT NMR (1H NMR at 400 MHz, 13C NMR at 100.6 MHz, 29Si NMR
at 79.5 MHz) spectrometers. High-resolution mass spectra were measured with Bruker
Daltonics micrOTOF-TU mass spectrometer with APCI (atmospheric pressure chemical
ionization method). tBu2MeSiNa was prepared according to the reported method.11
9,10-Bis(4-methoxyphenyl)-9,10-dihydro-9,10-disilaanthracene (1)
n
1.57 M BuLi solution in hexane (24.0 mL, 37.7 mmol) was added to a solution
of bis(2-bromophenyl)-4ꢁ-methoxyphenylsilane (7.88 g, 17.6 mmol) in THF (120 mL) at
−78◦C under an argon atmosphere. After being stirred for an additional 1 h, a solution of
4-methoxyphenyldichlorosilane (7.33 g, 35.4 mmol) in THF (6 mL) was added dropwise
to the mixture. Then the reaction mixture was allowed to warm to room temperature, was
refluxed for 4 h and quenched with water. The mixture was diluted with Et2O, and the organic
layer was washed with brine and dried over Na2SO4. After the solvent was evaporated, the
crude product was purified by column chromatography on SiO2 (hexane:Et2O = 1:1) to
give 9,10-dihydro-9,10-disilaanthracene 1 as a mixture of syn and anti isomers (5.75 g,
77%, 3:4 mixture as estimated by 1H NMR). Mp 145–146◦C; 1H NMR (C6D6): δ = 3.22
(s, 3H, OMe), 3.25 (s, 3H, OMe), 5.74 (s, 1H, SiH), 5.76 (s, 1H, SiH), 6.70–6.78 (m, 4H,
arom-H), 7.15 (brs, 4H, arom-H), 7.46–7.54 (m, 4H, arom-H), 7.70 (brs, 4H); 13C NMR
(C6D6): δ = 54.5, 114.4, 114.5, 129.2, 136.1, 137.8, 138.0, 141.7, 161.8; 29Si NMR (C6D6):
δ = −29.9, −30.7; Anal. Calcd for C26H24Si2O2: C, 73.54; H, 5.70. Found: C, 73.70; H,
6.10.
9,10-Bis(di-tert-butylmethylsilyl)-9,10-dihydro-9,10-disilaanthracene (2)
CF3SO3H (1.2 g, 8.0 mmol) was added to a solution of 1 (1.70 g, 4.0 mmol) in
toluene (75 mL) at −78◦C under an argon atmosphere and stirred for 5 h. Then the reaction
mixture was allowed to warm to room temperature, and a solution of tBu2MeSiNa (1.6 g,
8.8 mmol) in THF (10 mL) was added dropwise to the mixture. The mixture was diluted
with hexane, and the organic layer was washed with brine and dried over Na2SO4. After
the solvent was evaporated, compound 2 was obtained as colorless crystals (0.41 g, 20%).
1
t
Mp 183–184◦C; H NMR (C6D6): δ = 0.04 (s, 6H, Me), 0.96 (s, 36H, Bu), 5.52 (s, 2H,
SiH), 7.15 (dd, J = 3.3, 5.5 Hz, 4H, arom-H), 7.77 (dd, J = 3.3, 5.5 Hz, 4H, arom-H); 13C
NMR (C6D6): δ = −6.1, 21.1, 29.6, 127.8, 136.8, 141.5; 29Si NMR (C6D6): δ −47.7, 1.7.
Anal. Calcd for C30H52Si4: C, 68.62; H, 9.98. Found: C, 68.43; H, 9.94.
9,10-Bis(di-tert-butylmethylsilyl)-9,10-dibromo-9,10-dihydro-9,10-
disilaanthracene (3)
Br2 (0.16 g, 1.0 mmol) was added to a solution of 2 (0.24 g, 0.46 mmol) in CH2Cl2
(5 mL) at 0◦C. Then the reaction mixture was allowed to warm to room temperature and
was stirred for 30 min. After the solvent was evaporated, the crude product was washed

962
N. NAKATA ET AL.
with hexane to give anti-9,10-dibromo-9,10-dihydro-9,10-disilaanthracene 3 as colorless
1
crystals (0.31 g, 99%). Mp 233–234◦C. H NMR (C6D6): δ = 0.16 (s, 6H, Me), 0.92 (s,
36H, tBu), 7.15 (dd, J = 3.3, 5.6 Hz, 4H, arom-H), 8.10 (dd, J = 3.3, 5.6 Hz, 4H, arom-H);
13C NMR (C6D6): δ = −6.5, 22.0, 29.8, 129.3, 137.4, 142.3; 29Si NMR (C6D6): δ = −8.2,
1.8. Anal. Calcd for C30H50Br2Si4: C, 52.77; H, 7.38. Found: C, 52.34; H, 7.57.
Reduction of 9,10-Dibromo-9,10-dihydro-9,10-disilaanthracene
3 with KC8
Potassium graphite (88 mg, 0.64 mmol) and 3 (200 mg, 0.29 mmol) were placed in a
glove box into a glass tube with a magnetic stirring bar, then dry THF (2 mL) was vacuum
transferred into the tube. After one day of stirring at room temperature, the solvent was
removed in vacuo, and then hexane was added. The product was purified by column chro-
matography on SiO2 (toluene) to give 1ꢁ,4,4,4ꢁ-tetrakis(di-tert-butylmethylsilyl)spiro[2,5-
dibenzo-1,4-disilacyclohexa-2,5-diene-1,7ꢁ-2ꢁ,5ꢁ-dibenzo-1ꢁ,4ꢁ,7ꢁ-trisilanorbornadiene] (4)
as colorless crystals (120 mg, 81%). Mp > 300◦C; 1H NMR (C6D6): δ = 0.21 (s, 6H, Me),
0.44 (s, 6H, Me), 1.06 (s, 36H, tBu), 1.16 (s, 36H, tBu), 6.09 (d, J = 7.7 Hz, 2H, arom-H),
6.64 (dd, J = 7.7, 7.7 Hz, 2H, arom-H), 6.92 (dd, J = 7.7, 7.7 Hz, 2H, arom-H), 7.32 (dd,
J = 3.5, 5.4 Hz, 4H, arom-H), 8.00 (d, J = 7.7 Hz, 2H, arom-H), 8.69 (dd, J = 3.5, 5.4
Hz, 4H, arom-H); 13C NMR (C6D6): δ = −3.6, −1.2, 21.8, 23.1, 31.5, 31.6, 127.0, 127.3,
128.3, 135.7, 135.9, 139.4, 144.0, 144.8, 150.9; 29Si NMR (C6D6): δ = −44.0, −30.1,
9.6, 10.4, 25.0. HRMS (APCI, toluene as solvent) m/z calcd for C60H101Si8 [(M+H)+]
1045.6052, found 1045.6070.
Observation of 9,10-Bis(di-tert-butylmethylsilyl)-9,10-disila-9,10-Dewar
Anthracene (5) by Low Temperature NMR Spectroscopy
Dry THF-d8 (0.5 mL) was vacuum transferred to a mixture of potassium graphite (13
mg, 0.097 mmol) and 3 (30 mg, 0.044 mmol) in an NMR tube, and then the NMR tube was
sealed under vacuum. The reaction was complete in 11 h at −30◦C, and 9,10-disila-9,10-
1
Dewar anthracene 5 was formed (77%) together with 4 (23%), as estimated by H NMR
1
t
spectroscopy. H NMR (243 K, THF-d8): δ = 0.38 (s, 6H, Me), 1.21 (s, 36H, Bu), 7.09
(m, 4H, arom-H), 7.43 (m, 4H, arom-H); 13C NMR (243 K, THF-d8): δ = −6.4, 21.3, 30.0,
128.4, 133.7, 157.1; 29Si NMR (243 K, THF-d8): δ = −14.9, 11.4.
X-Ray Crystallographic Analysis of Compound 4
Single crystals of 4 suitable for X-ray diffraction were obtained from a toluene
solution. The intensity data were collected at 120 K on a Mac Science DIP2030 Image Plate
Diffractometer with a rotating anode (50 kV, 90 mA) employing graphite-monochromatized
MoKα radiation (λ = 0.71071 Å). The structure was solved by direct methods (SIR97)12
and refined by full-matrix least-squares procedures on F2 for all reflections (SHELX-97).13
Crystal data for 4: C60H100Si8, MW = 1046.12, monoclinic, space group P21/n (no. 14),
a = 15.2150(10), b = 22.4140(16), c = 19.5000(8) Å, β = 108.339(4)◦, V = 6312.3(7)
Å3, Z = 4, Dcalc = 1.101 g cm−3, R1 (I > 2σ(I)) = 0.0518, wR2 (all data) = 0.1079 for
12406 reflections and 641 parameters, GOF = 1.040. CCDC No. 698632 for 4 contains
supplementary crystallographic data. These data can be obtained free of charge via the

DIMERIZATION OF 9,10-DISILA(DEWAR ANTHRACENE)
963
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK (fax:
(+44)1223–336-033; e-mail: deposit@ccdc.cam.ac.uk).
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