Hexakis(trialkylsilyl)cyclotrisilanes and photochemical generation of bis(trialkylsilyl)silylenes

Article abstract of DOI:10.1039/b107905k

Hexakis(trialkylsilyl)cyclotrisilanes 1a (trialkylsilyl = tert-butyldimethylsilyl) and 1b (trialkylsilyl = diisopropylmethylsilyl) were synthesized by the reductive coupling of the corresponding 2,2-dibromotrisilanes. Their molecular structures were confi

Full text of DOI:10.1039/b107905k

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Hexakis(trialkylsilyl)cyclotrisilanes and photochemical generation
of bis(trialkylsilyl)silylenes
Mitsuo Kira, Takeaki Iwamoto, Toyotaro Maruyama, Tsuyoshi Kuzuguchi, Dongzhu Yin,
Chizuko Kabuto and Hideki Sakurai
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku,
Sendai 980-8578, Japan
Received 4th September 2001, Accepted 23rd January 2002
First published as an Advance Article on the web 8th March 2002
Hexakis(trialkylsilyl)cyclotrisilanes 1a (trialkylsilyl = tert-butyldimethylsilyl) and 1b (trialkylsilyl = diisopropyl-
methylsilyl) were synthesized by the reductive coupling of the corresponding 2,2-dibromotrisilanes. Their molecular
structures were confirmed by X-ray crystallography. In the absence of trapping reagents, photolysis of cyclotrisilanes
1a and 1b gave the corresponding stable tetrakis(trialkylsilyl)disilenes quantitatively. Irradiation of cyclotrisilanes
1a and 1b in (Z)-2-butene and (E)-2-butene afforded exclusively the corresponding (Z)- and (E)-2,3-dimethyl-1-
silacyclopropanes, respectively. Stereospecific formation of the cycloadducts indicates that the bis(trialkylsilyl)-
silylenes generated from cyclotrisilanes 1a and 1b would be singlets in the ground state.
2a and 2b toward (E)- and (Z)-2-butenes indicates that these
Introduction
silylenes are singlets in the ground state, being consistent with
One of the most intriguing research aims in the chemistry of
the results of the high-level calculations by Apeloig et al.2b
silicon divalent species (silylenes)1 is the generation of triplet
silylenes and the investigation of their properties. Although a
number of theoretical and experimental studies have been
devoted to the design and generation of the triplet silylenes,
they have never been observed so far even as the excited triplet
silylenes.2,3 On the basis of the previous studies, a promising
strategy for the triplet silylenes has been suggested to be the
introduction of sterically bulky σ-donating and/or π-accepting
substituents on the central silicon atom. The bulky substituents
widen the apex angle and cause a decrease in the HOMO(n)–
LUMO(3pπ) gap in the ground state singlet silylenes. The
Results and discussion
σ-donating and π-accepting substituents will elevate HOMO(n)
and lower LUMO(3pπ) orbital levels, respectively, and hence,
decrease the HOMO–LUMO gap. One of the prospective
substituents for triplet silylenes will therefore be sterically
bulky trialkylsilyl groups. In their theoretical paper, Grev,
Schaefer and Gaspar have stated “a winning strategy for
the experimental design of a triplet ground-state silylene is to
produce, by whatever means, a bis(trialkylsilyl)silylene with
alkyl being somewhat larger than methyl”.2a More recently,
Apeloig et al. have shown theoretically that (tBu3Si)2Si: has a
triplet state multiplicity in the ground state with a singlet–triplet
energy difference (∆EST) of Ϫ7.1 kcal molϪ1 at the BLYP/
DZVP-ECP level.2b Experimentally, Gaspar et al. have recently
generated (iPr3Si)2Si: by the photolysis of (iPr3Si)3SiH and
found the silylene adds to (Z)- and (E)-2-butenes to afford
stereospecifically the corresponding (Z)-and (E)-2,3-dimethyl-
silacyclopropanes, respectively.3
Syntheses of hexakis(trialkylsilyl)cyclotrisilanes
Prerequisite compounds for the synthesis of cyclotrisilanes 1a
and 1b, 2,2-dibromohexaalkyltrisilanes 3a and 3b were pre-
pared according to the method shown in Scheme 1. Treatment
During the course of our study on silyl-substituted silylenes4
and disilenes,5 we have independently investigated the reactivity
of bis(trialkylsilyl)silylenes generated by photolysis of the corre-
sponding hexakis(trialkylsilyl)cyclotrisilanes. Since the chem-
istry of cyclotrisilanes has attracted much attention over the
past two decades because of their unique physical and chemical
properties arising from the highly-strained three-membered
ring structure,6 we first discuss the synthesis and structure of
novel hexakis(trialkylsilyl)cyclotrisilanes 1a and 1b, and then,
the photolysis of these compounds affording the corresponding
bis(trialkylsilyl)silylenes 2a and 2b. Stereospecific addition of
Scheme 1
of a mixture of dichlorodiphenylsilane and a trialkylchloro-
silane with lithium powder in THF gave the corresponding
t
2,2-diphenyltrisilane; 4a (82% yield, R3Si = BuMe2Si) and 4b
i
(88% yield, R3Si = Pr2MeSi).7 Bromodephenylation of 4a and
4b with dry hydrogen bromide in the presence of a catalytic
amount of aluminium bromide provided 3a (95% yield) and 3b
(78% yield), respectively.
Cyclotrisilanes 1a and 1b were prepared by the reductive
dehalogenation of the corresponding 2,2-dibromotrisilanes 3a
DOI: 10.1039/b107905k
J. Chem. Soc., Dalton Trans., 2002, 1539–1544
This journal is © The Royal Society of Chemistry 2002
1539

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and 3b with lithium naphthalenide in 57 and 35% yields,
of the cyclotrisilane ring followed by reflection in a plane
perpendicular to the axis. The asymmetric units of 1b contain
two crystallographically independent molecules. In the single
crystal of 1a, one molecule of p-xylene was incorporated for
each cyclotrisilane in the lattice; no solvent was incorporated in
the crystal of 1b. Figs. 1 and 2 show ORTEP drawings of 1a
and 1b and their selected bond lengths and angles are summar-
ized in Table 1.
respectively [eqn. (1)].9
(1)
Cyclotrisilanes 1a and 1b are both quite stable to oxygen,
water and alcohols in the solid state in contrast to hexakis-
(triethylsilyl)cyclotrisilane 1c8b and hexakis(trimethylsilyl)-
cyclotrisilane 1d.10 Cyclotrisilanes 1a and 1b were stored for
more than three years without any decomposition. Protection
of the three-membered ring by bulky tert-butyldimethylsilyl
and diisopropylmethylsilyl groups would be responsible for the
high stability of 1a and 1b.
Molecular structure of cyclotrisilanes 1a and 1b in the solid state
Single crystals of 1a and 1b suitable for X-ray crystallographic
analysis were obtained by recrystallization from p-xylene and
hexane, respectively. Cyclotrisilanes 1a and 1b are almost equi-
lateral triangles, although they have no crystallographic C3 axis
perpendicular to the cyclotrisilane ring. In a single crystal of 1a
two structures, shown in Fig. 1, were found at the same site with
a ratio of 63 : 37. These two structures are enantiomeric and
overlap with each other through 180Њ rotation about the C3 axis
Fig. 2 ORTEP drawings of cyclotrisilane 1b. Thermal ellipsoids are
shown at the 30% level. Hydrogen atoms are omitted for clarity. The
two crystallographically independent molecules in the asymmetric unit
are shown in (a) and (b).
The averaged length of the endocyclic Si–Si bonds is 2.420 Å
for 1a and 2.390 Å for 1b, both of which are much longer than
that for hexakis(trimethylsilyl)cyclotrisilane 1d [2.357 Å (av.)];10
the severe steric repulsion between two vicinal bulky trialkyl-
silyl substituents would be responsible for the larger endocyclic
Si–Si bond distances in 1a and 1b. In addition, the Si(ring)–
Si(substituent) bond lengths and Si(ring)–Si(ring)–Si(sub-
stituent) angles for 1a and 1b are significantly larger than those
for 1d, probably due to the same reason; the bond lengths
are 2.423–2.439, 2.387–2.412 and 2.3575(6) Å and the angles
are 125.70–127.30Њ, 121.78–126.47Њ and 121.17–122.50Њ for 1a,
1b and 1d,10 respectively.
UV spectra of cyclotrisilanes 1a and 1b
Cyclotrisilanes 1a and 1b show two major absorption bands in
their UV-vis spectra (Fig. 3); λmax/nm (ε/dm3 molϪ1 cmϪ1) 264
(6.0 × 104) and 372 (1.3 × 103) for 1a and 255 (6.1 × 104) and 349
(1.3 × 103) for 1b. The band maxima at around 350 nm for 1a
and 1b are found at much longer wavelengths than those for
peralkylcyclotrisilanes and less hindered hexakis(triethylsilyl)-
cyclotrisilane 1c; the λmax/nm values are 310, 323, 328, 340 and
335 for [(neopentyl)2Si]3,11a (iPr2Si)3,6c [(diethylmethyl)2Si]3,11b
Fig. 1 ORTEP21 drawings of cyclotrisilane 1a. Thermal ellipsoids are
shown at the 30% level. Hydrogen atoms are omitted for clarity. Two
structures with 63 and 37% site occupancies are shown in (a) and (b),
respectively.
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J. Chem. Soc., Dalton Trans., 2002, 1539–1544

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Table 1 Selected bond lengths (Å) and angles (Њ) for cyclotrisilanes 1a and 1b
Cyclotrisilane 1a (R3Si = tBuMe2Si)
Si(1)–Si(2)
Si(1)–Si(3)
Si(2)–Si(5)
Si(7)–Si(7*)
Si(7)–Si(9)
2.4227(12)
2.4354(11)
2.4263(11)
2.418(2)
Si(2)–Si(2*)
Si(2)–Si(4)
Si(6)–Si(7)
Si(6)–Si(8)
Si(7)–Si(10)
2.4183(15)
2.4333(12)
2.423(2)
2.4301(18)
2.4244(18)
2.4389(19)
Si(2)–Si(1)–Si(2*)
Si(3)–Si(1)–Si(3*)
Si(2)–Si(1)–Si(3*)
Si(1)–Si(2)–Si(4)
Si(2*)–Si(2)–Si(4)
Si(7)–Si(6)–Si(7*)
Si(8)–Si(6)–Si(8*)
Si(7)–Si(6)–Si(8*)
Si(6)–Si(7)–Si(9)
Si(7*)–Si(7)–Si(9)
59.88(5)
107.41(7)
127.17(3)
127.05(4)
115.78(5)
59.86(7)
107.28(11)
127.30(5)
126.78(7)
115.90(8)
Si(1)–Si(2)–Si(2*)
Si(2)–Si(1)–Si(3)
Si(4)–Si(2)–Si(5)
Si(1)–Si(2)–Si(5)
Si(2*)–Si(2)–Si(5)
Si(6)–Si(7)–Si(*7)
Si(7)–Si(6)–Si(8)
Si(9)–Si(7)–Si(10)
Si(6)–Si(7)–Si(10)
Si(7*)–Si(7)–Si(10)
60.06(2)
114.93(3)
107.83(4)
114.61(4)
125.84(5)
60.07(4)
114.94(5)
107.96(7)
114.64(6)
125.70(9)
Cyclotrisilane 1b (R3Si = iPr2MeSi)
Si(1)–Si(2)
Si(2)–Si(3)
Si(1)–Si(5)
Si(2)–Si(7)
Si(3)–Si(9)
Si(10)–Si(12)
Si(10)–Si(13)
Si(11)–Si(16)
Si(12)–Si(15)
2.3892(8)
2.3871(9)
2.3973(8)
2.3997(8)
2.4007(8)
2.3835(9)
2.4118(8)
2.4018(8)
2.3967(8)
Si(1)–Si(3)
Si(1)–Si(4)
Si(2)–Si(6)
Si(3)–Si(8)
Si(10)–Si(11)
Si(11)–Si(12)
Si(10)–Si(14)
Si(11)–Si(17)
Si(12)–Si(18)
2.3957(8)
2.4111(8)
2.3918(8)
2.3962(8)
2.3902(8)
2.3932(8)
2.3936(8)
2.3987(8)
2.4092(8)
Si(2)–Si(1)–Si(3)
Si(1)–Si(3)–Si(2)
Si(2)–Si(1)–Si(5)
Si(3)–Si(1)–Si(5)
Si(1)–Si(2)–Si(6)
Si(3)–Si(2)–Si(6)
Si(6)–Si(2)–Si(7)
Si(1)–Si(3)–Si(9)
59.85(2)
59.94(2)
Si(1)–Si(2)–Si(3)
Si(2)–Si(1)–Si(4)
Si(3)–Si(1)–Si(4)
Si(4)–Si(1)–Si(5)
Si(1)–Si(2)–Si(7)
Si(3)–Si(2)–Si(7)
Si(1)–Si(3)–Si(8)
Si(2)–Si(3)–Si(8)
60.21(2)
124.88(3)
112.15(3)
111.87(3)
126.47(3)
116.61(3)
124.43(3)
112.19(3)
111.79(3)
59.77(2)
112.09(3)
125.65(3)
110.97(3)
125.19(3)
113.52(3)
110.56(3)
122.81(3)
111.45(3)
116.39(3)
122.76(3)
110.53(3)
126.27(3)
109.73(3)
116.47(3)
123.26(3)
60.18(2)
Si(2)–Si(3)–Si(9)
Si(8)–Si(3)–Si(9)
Si(11)–Si(10)–Si(12)
Si(10)–Si(12)–Si(11)
Si(11)–Si(10)–Si(14)
Si(12)–Si(10) Si(14)
Si(10)–Si(11)–Si(16)
Si(12)–Si(11)–Si(16)
Si(16)–Si(11)–Si(17)
Si(10)–Si(12)–Si(18)
Si(11)–Si(12)–Si(18)
Si(10)–Si(11)–Si(12)
Si(11)–Si(10)–Si(13)
Si(12)–Si(10)–Si(13)
Si(13)–Si(10)–Si(14)
Si(10)–Si(11)–Si(17)
Si(12)–Si(11)–Si(17)
Si(10)–Si(12)–Si(15)
Si(11)–Si(12)–Si(15)
Si(15)–Si(12)–Si(18)
60.05(2)
123.92(3)
116.00(3)
117.19(3)
121.78(3)
110.85(3)
124.74(3)
118.57(3)
and 1b are observed at Ϫ156.5 and Ϫ148.6 ppm, respectively,
which are more deshielded than that of 1c (Ϫ174 ppm).8b The
29Si resonances for these hexakis(trialkylsilyl)cyclotrisilanes
appear at very high field compared to those of known hexa-
alkyl- and hexaaryl-cyclotrisilanes,6c although the reason
remains unclear.12,14 The lower field 29Si resonances in 1a and
1b than that in 1c8b would be attributed to the widening of
the Si(substituent)–Si(ring)–Si(substituent) angles15 and
elongation of the ring Si–Si bond, which lower the σ
π*
transition energies.
Photochemical generation of bis(trialkylsilyl)silylenes from 1a
and 1b
Photolysis of hexakis(trialkylsilyl)cyclotrisilanes to give the
corresponding silylene and disilene was first reported by
Matsumoto, Sakamoto and Nagai.8b As we have previously
reported,5a irradiation of hexane solutions of 1a and 1b with
a high-pressure mercury arc lamp in the absence of any trap-
ping reagent at room temperature gave isolable disilenes 5a
and 5b, respectively, in quantitative yields [eqn. (2)]. Monitor-
Fig. 3 UV-vis spectra of hexakis(trialkylsilyl)cyclotrisilanes 1a (R3Si =
tBuMe2Si) and 1b (R3Si = iPr2MeSi) in hexane at rt.
(tBu2Si)3 11c and 1c.8b Elongation of the ring Si–Si bonds due to
steric repulsion between the bulky trialkylsilyl substituents as
well as the electron-donating effects of the trialkylsilyl substit-
uents may decrease the HOMO–LUMO gap in a cyclotrisilane
and lead to a red shift of the σ
σ* transition.12,13
(2)
29Si NMR spectra
The 29Si resonances due to the endocyclic silicon nuclei in 1a
J. Chem. Soc., Dalton Trans., 2002, 1539–1544
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ing the photolysis of 1a and 1b in benzene-d6 by NMR
spectroscopy proved quantitative formation of 5a and 5b;
no other products were observed by NMR spectroscopy during
the photoreaction.
The intermediate bis(trialkylsilyl)silylenes 2a and 2b were
trapped by 2-butenes to give the corresponding silacyclo-
propanes in a stereospecific syn-addition manner.3,16 Typically,
photolysis of 1a in (E)-2-butene at room temperature in an
NMR tube with a high-pressure mercury arc lamp gave the
Preparations
Chlorodiisopropylmethylsilane.20 To a THF (200 cm3) solu-
tion of isopropylmagnesium chloride [prepared by the reaction
of isopropyl chloride (83.2 g, 1.06 mol) with magnesium
(24.3 g, 1.00 mol)] in the presence of cuprous cyanide (0.5 g,
5.6 mmol), was added trichloromethylsilane (72.2 g, 0.48 mol)
with stirring. After completion of the exothermic reaction,
the mixture was refluxed for an hour. The THF was then
distilled off and diethyl ether (1000 cm3) was introduced. After
the resulting salt was removed by filtration, the solvent was
distilled off. The title compound (44.2 g, 0.27 mol) was
obtained in 56% yield after distillation from the residual oil;
bp 72 ЊC/45 Torr.
corresponding
silacyclopropane,
(E)-1,1-bis(tert-butyldi-
methylsilyl)-2,3-dimethylsilirane [(E)-6a] as a sole silylene
adduct in 86% yield together with the corresponding disilene
5a (24%) and its hydrate 7a (42%) (Scheme 2).17,18 On the other
1,3-Di(tert-butyl)-1,1,3,3-tetramethyl-2,2-diphenyltrisilane
4a. To a suspension of lithium powder (2.12 g, 305 mmol) in
THF (200 cm3) was added a mixture of dichlorodiphenylsilane
(14.9 g, 58.9 mmol) and tert-butylchlorodimethylsilane (26.6 g,
177 mmol) in THF (80 cm3) at 0 ЊC under sonication. After
about 80% of the lithium was consumed, the reaction mixture
was allowed to warm to room temperature and stirred over-
night. The reaction mixture was then hydrolyzed with water.
The organic layer was dried over anhydrous sodium sulfate and
then the solvent was distilled off. The title compound (20.0 g,
48.5 mmol) was obtained in 82% yield by Kugelrohr distillation.
4a: a colorless oil; bp 85 ЊC/ 0.02 Torr; δH (CDCl3) 0.23 (12 H, s,
4 SiMe), 0.65 (18 H, s, 2 tBu), 7.26–7.32 (6 H, m, Ph), 7.60–7.65
(4 H, m, Ph); δC (CDCl3) Ϫ3.2 (SiMe), 19.4 (CMe3), 27.8
(CMe3), 127.6, 128.1, 136.5, 136.6 (Ph); δSi (CDCl3) Ϫ41.4
(SiPh2), Ϫ5.73 (tBuMe2Si); m/z 412 (0.9, Mϩ), 355 (23),
167 (37), 149 (100); HRMS found, 412.2435. C24H40Si3 requires
412.2438.
Scheme 2
hand, photolysis of 1a in (Z)-2-butene gave exclusively the
(Z)-adduct, (Z)-1,1-bis(tert-butyldimethylsilyl)-2,3-dimethyl-
silirane [(Z)-6a] (82%) together with 5a (22%) and 7a (32%).
Similar stereospecific retention during addition of the starting
olefin was observed in the photolysis of 1b in (E)- and (Z)-2-
butenes giving (E)- and (Z)-6b, respectively. Photolysis of
cyclotrisilanes 1a and 1b in a 3-methylpentane matrix at 77 K
gave no evidence for the generation of the corresponding
bis(trialkylsilyl)silylenes; in matrices, a facile recombination of
the initial photoproducts, bis(trialkylsilyl)silylene and tetrakis-
(trialkylsilyl)disilene, may take place.
The stereospecific syn-addition of both bis(tert-butyldi-
methylsilyl)silylene 2a and bis(diisopropylmethylsilyl)silylene
2b is indicative that they are singlets in the ground state, being
consistent with the theoretical results for 2a,2b which is a singlet
in the ground state with a singlet–triplet energy difference of
1.5 kcal molϪ1 at the BLYP/DZVP-ECP level, while Gaspar
et al. pointed out that triplet silylenes possibly undergo stereo-
specific syn-addition to olefins.3c,19
1,1,3,3-Tetraisopropyl-1,3-dimethyl-2,2-diphenyltrisilane 4b.
The title compound (3.87 g, 8.80 mmol, 88%) was prepared
following to the same method as for 4a using lithium powder
(0.367 g, 52.8 mmol), dichlorodiphenylsilane (2.543 g,
10.1 mmol) and chlorodiisopropylmethylsilane (5.055 g, 30.7
mmol). 4b: a colorless oi1; bp 120 ЊC/0.02 Torr; δH (CDCl3) 0.21
(6 H, s, 2 SiMe), 0.89 (12 H, d, J 7.1 Hz, 4 CH3), 0.91 (12 H, d,
J 7.1 Hz, 4 CH3), 0.95–1.20 (4 H, m, 4 CH), 7.25–7.31 (6 H, m,
Ph), 7.56–7.62 (4 H, m, Ph); δC (CDCl3) Ϫ6.4 (SiCH3), 13.6
(CHMe2), 19.2 (CHMe2), 19.7 (CHMe2), 127.5, 128.0, 136.5,
137.1 (Ph); δSi (CDCl3) Ϫ3.88 (iPr2MeSi), Ϫ39.0 (SiPh2); m/z
440 (16, Mϩ), 269 (63), 234 (58), 227 (81), 59 (100); HRMS
found, 440.2747. C26H44Si3 requires 440.2751.
2,2-Dibromo-1,3-di(tert-butyl)-1,1,3,3-tetramethyltrisilane 3a.
To a mixture of 4a (12.0 g, 29.0 mmol) and a catalytic amount
of anhydrous aluminium bromide in benzene (100 cm3) was
introduced dry hydrogen bromide gas until the exothermic
reaction was complete. Acetone (0.5 cm3) was added and then
the resulting acetone–aluminium bromide complex was
removed by decantation. After solvent was distilled off, the title
compound (11.5 g, 27.6 mmol, 95%) was obtained by distil-
lation under reduced pressure. 3a: a colorless oil; bp 80 ЊC/0.02
Experimental
All operations were performed in oven- or flame-dried glass-
ware under an atmosphere of dry nitrogen or argon. All
solvents were distilled from appropriate drying agents before
use. H (300 MHz), 13C (75 MHz), and 29Si (59 MHz) NMR
1
were recorded on a Bruker AC300P NMR spectrometer. 1H and
t
13C NMR chemical shifts were referenced to residual H and
1
Torr; δH (CDCl3) 0.29 (12 H, s, 4 SiMe), 1.08 (18 H, s, Bu);
13C of the solvents; chloroform-d (1H δ 7.24 and 13C δ 77.0),
benzene-d6 (1H δ 7.15 and 13C δ 128.0). 29Si NMR chemical
shifts were relative to Me4Si in ppm. Mass spectra and high
resolution mass spectral data were obtained on a JEOL JMS
MS-600W mass spectrometer. UV-visible spectra were recorded
on a Milton Roy SP3000 spectrometer. GC analysis was
conducted using a Shimadzu 8A gas chromatograph (Silicon
SE-30 1 m glass column with 5 mm o.d.).
Lithium powder, magnesium, cuprous cyanide, 2-chloro-
propane, (E)-2-butene, (Z)-2-butene, trichloromethylsilane,
dichlorodiphenylsilane, tert-butylchlorodimethylsilane were
commercially available and used as supplied.
δC (CDCl3) Ϫ5.9 (SiMe), 19.1 (CMe3), 28.0 (CMe3); δSi (CDCl3)
1.0 (tBuMe2Si), 23.4 (SiBr2); m/z 418 [2, Mϩ(79Br8lBr)], 224 (9),
222 (9), 115 (14), 73 (100); HRMS found, 416.0026.
C12H30Si379Br2 requires 416.0022.
2,2-Dibromo-1,3-dimethyl-1,1,3,3-tetraisopropyltrisilane 3b.
The title compound (13.4 g, 30.0 mmol, 78%) was prepared
from 4b (16.9 g, 38.5 mmol) according to the same method
as for the preparation of 3a. 3b: a colorless oil; 90 ЊC/0.02
Torr; δH (CDCl3) 0.24 (6 H, s, 2 SiMe), 1.12 (12 H, d, J 6.7 Hz,
4 CH3), 1.14 (12 H, d, J 7.4 Hz, 4 CH3), 1.27–1.42 (4 H, m,
4 CH); δC (CDCl3) Ϫ8.9 (SiMe), 12.8 (CHMe2), 19.0,
1542
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19.3 (CHMe2); δSi (CDCl3) 0.71 (iPr2MeSi), 24.7 (SiBr);
m/z 448 [2, Mϩ(81Br81Br)], 446 (4), 444 (2), 129 (100), 73 (66),
59 (91); HRMS found, 444.0337. C14H34Si379Br2 requires
444.0335.
Photoreactions of cyclotrisilanes 1a and 1b
Photolysis in the absence of trapping agents. In an argon
atmosphere, cyclotrisilane 1a (50 mg, 0.065 mmol) and
degassed hexane (6 cm3) were placed in a Pyrex Schlenk tube.
The resulting suspension was irradiated with a high-pressure
mercury lamp (500 W) for a day at room temperature. The
precipitated 1a was gradually dissolved as the reaction
proceeded and the solution changed from light yellow to
orange. Evaporation of the solvent gave the corresponding
tetrasilyldisilene 5a as a light orange solid. NMR analysis
showed the complete consumption of 1a without any other
noticeable products. Similarly, photolysis of 1b in hexane gave
tetrasilyldisilene 5b quantitatively.
Hexakis(tert-butyldimethylsilyl)cyclotrisilane 1a. To a sus-
pension of lithium naphthalenide [prepared by mixing lithium
(250 mg, 36.0 mmol) and naphthalene (5.310 g, 41.4 mmol)
in THF (80 cm3)] was added a solution of 2,2-dibromotrisilane
3a (6.55 g, 15.7 mmol) in THF (20 cm3) at Ϫ78 ЊC. Warming
the mixture to room temperature and then stirring overnight
gave a dark red solution. The resulting mixture was hydrolyzed,
and then washed with water and brine. The organic layer
was dried over anhydrous sodium sulfate. After removal of
solvent and washing the resulting solid with ethanol, pale
yellow microcrystals of the title compound (2.310 g, 2.98
mmol) were obtained in 57% yield. 1a: light yellow crystals;
mp 191 ЊC (Found: C, 55.75; H, 11.82%. C36H90Si9 requires
C, 55.73; H, 11.69%); δH (C6D6) 0.47 (36 H, s, 12 SiMe), 1.20
(54 H, s, tBu); δC (C6D6) 3.4 (SiMe), 22.3 (CMe3), 30.4 (CMe3);
δSi (THF-d8) Ϫ156.5 (ring Si), 8.05 (tBuMe2Si); m/z (%)
774 (0.02, Mϩ), 716 (0.1), 73 (100), 59 (8); λmax/nm (hexane) 264
(ε/dm3 molϪ1 cmϪ1 6.0 × 104), 372 (1.3 × 103).
Photolysis of cyclotrisilane 1a in (E )-2-butene. Cyclotrisilane
1a (10.0 mg, 1.28 × 10Ϫ5 mol) and degassed (E)-2-butene (6
cm3) were placed in an NMR tube equipped with a stopcock.
The resulting suspension was then irradiated with a high-
pressure mercury lamp for 3 hours at room temperature. The
remaining solid 1a dissolved and the color changed to reddish
orange as the reaction proceeded. After evaporation of excess
(E)-2-butene, the residual solid was dissolved in dry benzene-d6,
1
and the NMR tube was sealed. The H, 13C and 29Si NMR
spectra showed 90% conversion of 1a. The mixture included
Hexakis(diisopropylmethylsilyl)cyclotrisilane 1b. To a suspen-
sion of lithium naphthalenide [prepared by mixing lithium
(73 mg, 10.5 mmol), naphthalene (1.36 g, 10.6 mmol) in
THF (30 cm3)] was added a solution of 3b (1.74 g, 3.91 mmol)
in THF (10 cm3) at Ϫ78 ЊC. Similar work-up as described
above gave the title compound (395 mg, 1.38 mmol, 35%) as
colorless crystals. 1b: colorless crystals; mp 221 ЊC (Found: C,
58.85; H, 12.17%. C42H102Si9 requires C, 58.66; H, 11.95%);
δH (CDCl3) 0.16 (18 H, s, 6 SiMe), 1.04 (36 H, d, J 7.4 Hz,
6 CHMe2), 1.10 (36 H, d, J 7.4 Hz, 6 CHMe2), 1.39 (12 H, sept,
J 7.4 Hz, 12 CHMe2); δC (CDCl3) Ϫ4.2 (Me), 16.4 (CHMe2),
19.5, 20.7 (CHMe2); δSi (CDCl3) Ϫ148.6 (ring Si), 10.8
(iPr2MeSi); m/z 858 (16.4, Mϩ), 815 (16.8), 615 (55.5), 557 (100);
λmax/nm (hexane) 255 (ε/dm3 molϪ1 cmϪ1 6.1 × 104), 349
(1.3 × 103).
(E)-1,1-bis(tert-butyldimethylsilyl)-2,3-dimethylsilirane
6a
(3.1 mg, 9.9 × 10Ϫ6 mol, 86%), tetrakis(tert-butyldimethyl-
silyl)disilene 5a (1.4 mg, 2.8 × 10Ϫ6 mol, 24%), and its hydrate
7a (2.6 mg, 4.8 × 10Ϫ6 mol, 42%).9,18 The yields were determined
by comparison of the 1H NMR integrals to those of the methyl
protons of 1,2,4,5-tetramethylbenzene (2.9 mg, 0.022 mmol),
which was added to the mixture after photolysis. Within the
limits of the 1H NMR measurements, only (E)-silirane formed.
(E)-6a; δH (C6D6) 0.12 (6 H, s, SiMe), 0.18 (6 H, s, SiMe),
0.72–0.82 (2 H, m, CHMe), 1.01 (18 H, s, tBu), 1.48–1.51 (6 H,
m, CHMe); δC NMR (C6D6) Ϫ1.8 (SiMe), Ϫ1.7 (SiMe), 18.8
(SiCMe3), 18.9 (ring C), 19.8 (ring Me), 27.9 (CMe3); δSi NMR
(C6D6) Ϫ144.1 (ring Si), 2.8 (tBuMe2Si); HRMS found,
314.2277. C16H38Si3 requires 314.2281.
Photolysis of cyclotrisilane 1a in (Z )-2-butene. Following the
same procedure used for the reaction of 1a in (E)-2-butene,
photolysis of 1a (10.0 mg, 1.28 × 10Ϫ5 mol) was carried out in
(Z)-2-butene. The NMR spectra of the reaction mixture
showed that the photolysis conversion was 75% and the mixture
X-Ray crystal structures of cyclotrisilanes 1a and 1b
Single crystals of 1a and 1b suitable for X-ray diffraction study
were obtained by recrystallization from p-xylene and hexane,
respectively. X-Ray data were collected on a Rigaku/MSC
Mercury CCD diffractometer with graphite monochromated
Mo-Kα radiation (λ 0.71073 Å). The data were corrected for
Lorentz and polarization effects. The structures were solved by
direct methods and refined by full-matrix least squares against
F 2 using all data (G. M. Sheldrick, SHELXL-97, Program for
the Refinement of Crystal Structures, University of Göttingen,
Germany, 1997).
included
(Z)-1,1-bis(tert-butyldimethylsilyl)-2,3-dimethyl-
silirane (Z)-6a (2.5 mg, 7.9 × 10Ϫ6 mol mmol, 82%), disilene
5a (1.1 mg, 2.1 × 10Ϫ6 mol, 22%), and its hydrate 7a (1.7 mg,
3.2 × 10Ϫ6 mol mmol, 33%). (Z)-6a: δH (C6D6) 0.07 (6 H, s,
2 SiCH3), 0.20 (6 H, s, 2 SiCH3), 0.98 (9 H, s, tBu), 1.01 (9 H, s,
tBu), 1.25–1.30 (2 H, m, 2 CH), 1.43–1.45 (6 H, m, 2 CH3);
δC (C6D6) Ϫ3.4, Ϫ0.8 (SiMe), 13.1 (ring C), 13.2 (ring Me),
19.4, 20.6 (SiCMe3), 27.86, 27.89 (SiCMe3); δSi (C6D6) Ϫ154.5
(ring Si), 0.6, 4.5 (tBuMe2Si); m/z 314 (71.4, Mϩ), 258 (96.7),
141 (39.8), 84 (100); HRMS found, 314.2314. C16H38Si3 requires
314.2281.
Crystal data for 1a. C36H90Si9ؒC8H10, M = 882.04, mono-
clinic, a = 18.223(2), b = 15.815(2), c = 11.7534(4) Å, β =
123.9562(7)Њ, V = 2809.7(5) Å3, T = 150 K, space group C2 (no.
5), Z = 2, µ(Mo-Kα) = 2.4 cmϪ1, 13441 reflections measured,
6579 unique (Rint = 0.023). The final R1 and wR2 values were
0.0372 [I > 2σ(I )] and 0.0978 (for all data), respectively.
Photolysis of cyclotrisilane 1b in (E )-2-butene. Following the
same procedure used for the photolysis of 1a in 2-butenes,
photolysis of 1b (10.0 mg, 1.16 × 10Ϫ5 mol mmol) was carried
out in (E)-2-butene. The NMR spectra of the reaction mixture
showed that the photolysis conversion was 68% and the mixture
Crystal data for 1b. C42H102Si9, M = 860.05, triclinic, a =
11.3299(5), b = 22.755(1), c = 24.229(1) Å, α = 64.743(3), β =
77.141(3), γ = 89.051(2)Њ, V = 5485.9(5) Å3, T = 150 K, space
included
(E)-1,1-bis(diisopropylmethylsilyl)-2,3-dimethyl-
group P1 (no. 2), Z = 4, µ(Mo-Kα) = 2.4 cmϪ1, 31145 reflections
silirane (E)-6b (2.2 mg, 6.4 × 10Ϫ6 mol, 81%), tetrakis(diiso-
proprylmethylsilyl)disilene 5b (0.81 mg, 1.4 × 10Ϫ6 mol, 18%)
and its hydrate 7b (2.6 mg, 4.4 × 10Ϫ6 mol, 56%).9,18 (E)-6b:
δH (C6D6) 0.05 (6 H, s, 2 SiMe), 0.68–0.72 (2H, m, 2 CHMe),
0.92–1.10 (4H, m, 4 CHMe2), 1.04–1.14 (24 H, m, 4 CHMe2),
1.47–1.50 (6 H, m, 2 CHMe); δC (C6D6) Ϫ6.9 (SiMe), 14.0
(SiCHMe2), 18.3 (ring C), 18.5 (ring Me), 18.7 (SiCHMe2);
¯
measured, 17805 unique (Rint = 0.045). The final R1 and wR2
values were 0.050 [for I > 2σ(I )] and 0.104 (for all data),
respectively.
CCDC reference numbers 170529 and 170530.
See http://www.rsc.org/suppdata/dt/b1/b107905k/ for crystal-
lographic data in CIF or other electronic format.
J. Chem. Soc., Dalton Trans., 2002, 1539–1544
1543

View Article Online
δSi (C6D6) Ϫ147.8 (ring Si), 7.2 (iPr2MeSi); m/z 342 (33.2, Mϩ),
286 (100), 160 (35.6), 73 (22.1); HRMS found, 342.2590.
C18H42Si3 requires 342.2594.
C. Kabuto, J. Am. Chem. Soc., 1996, 118, 10303; (i) T. Iwamoto
and M. Kira, Chem. Lett., 1998, 277; (j) T. Iwamoto, C. Kabuto and
M. Kira, J. Am. Chem. Soc., 1999, 121, 886
. Correction:
T. Iwamoto, C. Kabuto and M. Kira, J. Am. Chem. Soc., 200, 122,
12614; (k) T. Iwamoto, M. Tamura, C. Kabuto and M. Kira,
Science, 2000, 290, 504–506.
Photolysis of cyclotrisilane 1b in (Z )-2-butene. Following the
same procedure used for the photolysis of 1a in 2-butenes,
photolysis of 1b (10.0 mg, 1.16 × 10Ϫ5 mol) was carried out in
(Z)-2-butene. The NMR spectra of the reaction mixture
showed that the photolysis conversion was 81% and the mixture
6 For reviews on small ring compounds containing heavier Group 14
elements, see: (a) M. Weidenbruch, Comments Inorg. Chem., 1986, 5,
247; (b) T. Tsumuraya, S. A. Batcheller and S. Masamune, Angew.
Chem., Int. Ed. Engl., 1991, 30, 902; (c) M. Weidenbruch, Chem.
Rev., 1995, 95, 1479; (d ) E. Hengge and R. Janoschek, Chem. Rev.,
1995, 95, 1495; (e) M. Driess and H. Grützmacher, Angew. Chem.,
Int. Ed. Engl., 1996, 35, 828; ( f ) A. Sekiguchi and S. Nagase, in The
Chemistry Organic Silicon Compounds, vol. 2, ed. Z. Rappoport and
Y. Apeloig, John Wiley & Sons Ltd., New York, 1998, part 1, ch. 3,
pp. 119–152; (g) M. Weidenbruch, Eur. J. Inorg. Chem., 1999, 373.
included
(Z)-1,1-bis(diisopropylmethylsilyl)-2,3-dimethyl-
silirane (Z)-6b (1.9 mg, 5.6 × 10Ϫ6 mol, 60%), a trace amount
of disilene 5b (<1%), and its hydrate 7b (3.1 mg, 5.2 × 10Ϫ6 mol,
55%). (Z)-6b: δH (C6D6) Ϫ0.03 (3 H, s, SiCH3), 0.10 (3 H, s,
SiCH3), 0.90–1.01 (4 H, m, 4 CHMe2), 1.05–1.13 (24 H, m,
4 CHMe2), 1.33–1.40 (2 H, m, 2 CHMe), 1.43–1.45 (6 H, m,
2 CHMe); δC (C6D6) Ϫ8.2, Ϫ6.1 (SiMe), 12.4, 13.2 (SiCHMe2),
17.0 (ring Me), 18.5 (ring C), 18.8, 19.8 (SiCHMe2); δSi (C6D6)
Ϫ158.1 (ring Si), 4.7, 9.4 (iPr2MeSi); m/z 342 (30.6, Mϩ), 286
(100), 160 (40.1), 73 (31.3); HRMS found, 342.2621. C18H42Si3
requires 342.2594.
7 Structurally similar 2,2-diphenyltrisilanes Ph2Si(SiR3)2 (R3Si
=
EtMe2Si, Et2MeSi, and Et3Si) were prepared by reaction of Ph2SiCl2
with the corresponding trialkylchlorosilane with magnesium in the
presence of HMPA as reducing agent.8a Under similar reaction
conditions, neither 4a nor 4b have been obtained.
8 (a) H. Matsumoto, N. Yokoyama, A. Sakamoto, Y. Aramaki,
R. Endo and Y. Nagai, Chem. Lett., 1986, 1643; (b) H. Matsumoto,
A. Sakamoto and Y. Nagai, J. Chem. Soc., Chem. Commun., 1986,
1768.
9 Interestingly, the major product of the reductive debromination
of 3a and 3b depends on the reaction conditions. The reactions of
2,2-dibromotrisilanes 3a and 3b with sodium dispersion at room
temperature in toluene gave the corresponding tetrakis(trialkyl-
silyl)disilenes 5a and 5b in high yield.5a.
Acknowledgements
This work was supported by the Ministry of Education, Cul-
ture, Sports, Science, and Technology of Japan (Grants-in-Aid
for Scientific Research (B) No. 11440185 (M. K. and T. I.) and
Encouragement of Young Scientists No. 12740336 (T. I.)).
10 K. W. Klinkhammer, in Organosilicon Chemistry III, ed. N. Auner
and J. Weis, Wiley-VCH, Weinheim, 1997, pp. 82–85.
11 (a) H. Watanabe, T. Okawa, M. Kato and Y. Nagai, J. Chem. Soc.,
Chem. Commun., 1983, 781; (b) S. Masamune, H. Tobita and
S. Murakami, J. Am. Chem. Soc., 1983, 105, 6524; (c) A. Schäfer,
M. Weidenbruch, K. Peters and H.-G. von Schnering, Angew.
Chem., Int. Ed. Engl., 1984, 23, 302.
12 The CIS (configuration interaction, single) calculations [CIS/6-
31G(d,p)//B3LYP/6-31G(d)] for hexakis(trihydrosilyl)cyclotrisilane
References and notes
1 For reviews on silylenes, see: P. P. Gaspar and R. West, in The
Chemistry of Organic Silicon Compounds, vol. 2, ed. Z. Rappoport
and Y. Apeloig, Wiley & Sons Ltd., New York, 1998, part 3, ch. 32,
pp. 2463–2569; Y. Apeloig, in The Chemistry of Organic Silicon
Compounds, ed. S. Patai and Z. Rappoport, Wiley, Chichester, 1989,
part 1, ch. 2, pp. 167–184; M. Haaf, T. M. Schmedake and R. West,
Acc. Chem. Res., 2000, 33, 704.
2 For theorerical calculations on triplet silylenes, see: (a) R. S. Grev,
H. F. Schaefer III and P. P. Gaspar, J. Am. Chem. Soc., 1991, 113,
5638; (b) M. C. Holthausen, W. Kock and Y. Apeloig, J. Am. Chem.
Soc., 1999, 121, 2623 and references cited therein.
3 For attempted generation of triplet silylenes, see: (a) Y.-S. Chen and
P. P. Gaspar, Organometallics, 1982, 1, 1416; (b) Y. Apeloig,
D. Bravo-Zhivotovski, I. Zharov, V. Panov and G. W. Sluggett,
J. Am. Chem. Soc., 1998, 120, 1398; (c) P. P. Gaspar, A. M. Beatty,
T. Chen, T. Haile, D. Lei, W. R. Winchester, J. Braddock-Wilking,
N. P. Rath, W. T. Klooster, T. F. Koetzle, S. A. Mason and
A. Albinati, Organometallics, 1999, 18, 3921; (d ) P. Boudjouk,
U. Samaraweera, R. Sooriyakumaran, J. Chrusciel and K. R.
Anderson, Angew. Chem., Int. Ed. Engl., 1988, 27, 1355; (e) D. H.
Pae, M. Xiao, M. Y. Chiang and P. P. Gaspar, J. Am. Chem. Soc.,
1991, 113, 1281; ( f ) W. Ando, M. Fujita, H. Yoshida and
A. Sekiguchi, J. Am. Chem. Soc., 1988, 110, 3310; (g) S. Zhang, P. E.
Wagenseller and R. T. Conlin, J. Am. Chem. Soc., 1991, 113, 4278 .
After the submission of this paper, Gaspar et al. have reported
the intramolecular hydrogen abstraction of (tBu3Si)(iPr3Si)Si:
and (tBu3Si)2Si: as evidence for the triplet nature of these silylenes;
(h) P. Jiang and P. P. Gaspar, J. Am. Chem. Soc., 2001, 123, 8622 .
See also; (i) N. Wiberg, Coord. Chem. Rev., 1997, 163, 217; (j)
N. Wiberg and W. Niedermayer, J. Organomet. Chem., 2001, 628,
57.
4 M. Kira, T. Maruyama and H. Sakurai, Chem. Lett., 1993, 1345. See
also: M. Kira, T. Maruyama, H. Sakurai, presented at the XXV
Silicon Symposium, Los Angeles, April 1992, abstract no. 72P; K. E.
Banks, Y. Wang and R. T. Conlin, presented at the XXV Silicon
Symposium, Los Angeles, April 1992, abstract no. 7.
5 (a) M. Kira, T. Maruyama, K. Ebata, C. Kabuto and H. Sakurai,
Angew. Chem., Int. Ed. Engl., 1994, 33, 1489; (b) R. West, J. D.
Cavalieri, J. J. Buffy, C. Fry, K. W. Zilm, J. C. Duchamp, M. Kira,
T. Iwamoto, T. Müller and Y. Apeloig, J. Am. Chem. Soc., 1997, 119,
4972; (c) M. Kira, T. Iwamoto and H. Sakurai, Bull. Chem. Soc.
Jpn., 1998, 71, 274; (d ) M. Kira, S. Ohya, T. Iwamoto, M. Ichinohe
and C. Kabuto, Organometallics, 2000, 19, 1817; (e) M. Kira and
T. Iwamoto, J. Organomet. Chem., 2000, 611, 236; ( f ) M. Kira,
T. Ishima, T. Iwamoto and M. Ichinohe, J. Am. Chem. Soc., 2001,
123, 1676; (g) M. Kira, T. Iwamoto, D. Yin, T. Maruyama and
H. Sakurai, Chem. Lett., 2001, 910; (h) M. Kira, T. Iwamoto and
(D3) showed that the two σ
σ* transition bands appear at 205.5
and 193.2 nm with very different oscillator strengths of 0.0166 and
0.5886, respectively, where the σ and σ* orbitals are symmetric
about the reflection in the cyclotrisilane ring plane. It is possible
however that several exocyclic orbitals (π and π*) which are
antisymmetric about the reflection may participate to the electronic
transitions, while they are symmetry forbidden for D3 cyclotrisilane.
The CIS calculations showed the longest absorption band at 230.0
nm (5.391 eV) with zero oscillator strength (σ
π* transition).
13 Matsumoto, Nagai and their coworkers have mentioned the steric
effects of the silyl substituents on the UV-vis spectra of
cyclotetrasilanes.6c,8a
.
14 For solid-state NMR of cyclotrisilanes, see: J. D. Cavalieri, R. West,
J. C. Duchamp and K. W. Zilm, J. Am. Chem. Soc., 1993, 115, 3770;
R. West, J. D. Cavalieri, J. Duchamp and K. W. Zilm, Phosphorus,
Sulfur, Silicon Relat. Elem., 1994, 93–94, 213.
15 Theoretical calculations have shown that the endocyclic silicon
nuclei in cyclotrisilanes are deshielded on increasing the substituent–
Si(ring)–substituent angles: J. A. Tossell, D. C. Winkler and J. H.
Moore, Chem. Phys., 1994, 185, 297; G. Magyarfalvi and P. Pulay,
Chem. Phys. Lett., 1995, 241, 393.
16 M. Ishikawa, K.-I. Nakagawa and M. Kumada, J. Organomet.
Chem., 1979, 178, 105; V. J. Tortorelli and M. Jones, Jr., J. Am.
Chem. Soc., 1980, 102, 1425; D. Seyferth, D. C. Annarelli and
D. P. Duncan, Organometallics, 1982, 1, 1288; V. J. Tortorelli,
M. Jones, Jr., S.-H. Wu and Z.-H. Li, Organometallics, 1983, 2, 759.
17 Stereochemistry of the resulting siliranes was determined by
comparing the proton chemical shifts with those for the siliranes
reported by Conlin et al.: S. Zhang and R. T. Conlin, J. Am. Chem.
Soc., 1991, 113, 4272.
18 The yields of the siliranes 6a and 6b tetrasilyldilenes 5a and 5b and
their hydrates 7a and 7b were determined by assuming that
photolysis of a cyclotrisilane gives the corresponding silylene and
disilene with a ratio of 1 : 1. No reaction was observed during the
irradiation of tetrakis(trialkylsilyl)disilenes 5a and 5b in 2-butenes,
indicating that the disilenes did not undergo further dissociation to
the corresponding silylenes under the photochemical conditions.
19 P. P. Gaspar and G. S. Hammond, in Carbene Chemistry, 1st edn.,
ed. W. Kirmse, Academic Press, New York, 1964, ch. 12, pp. 235–274.
20 M. Weidenbruch, G. Schiffer, G. Haegele and W. Peters,
J. Organomet. Chem., 1975, 90, 145.
21 M. N. Burnett and C. K. Johnson, ORTEP3, Report ORNL-6895,
Oak Ridge National Laboratory, Oak Ridge, TN, 1996.
1544
J. Chem. Soc., Dalton Trans., 2002, 1539–1544