Syntheses of trisila analogues of allyl chlorides and their transformations to chlorocyclotrisilanes, cyclotrisilanides, and a trisilaindane

Article abstract of DOI:10.1021/ja711169w

The rearrangements of (chlorosilyl)disilenes R₂(Cl)Si-(Tip)Si= SiTip₂ (5a,b: Tip = 2,4,6-/Pr₃C₆H₂, a: R = Me, b: R = Ph) quantitatively yield the isomeric chlorocyclotrisilanes (6a,b). The disilene precursors 5a,b are, in turn, accessible from the reactions of the disilenide Tip₂Si=Si(Tip)Li (1), that is, a disila analogue to vinyl anions, with dichlorosilanes R₂SiCl₂. This novel approach to cyclotrisilanes potentially allows for the facile variation of the substitution pattern and grants access to the first anionic derivatives; while the rearrangement of 5a,b to 6a,b is slow at room temperature and additionally requires the presence of THF or other n-donors, reduction of 5b with lithium instantly yields the corresponding cyclotrisilanide (7b) without detection of any open-chained isomer. Heating of a neat sample of 5b to 150°C provides a completely characterized 1,2,3-trisilaindane derivative (13), strongly supporting the intermediacy of a disilanyl silylene species that inserts into an ortho-CH bond of the phenyl substituents. The X-ray diffraction studies on single crystals of 6a,b and 7b reveal that the Si-Si bond distance in cyclotrisilanes depends significantly on the electronegativity of the opposing silicon atom's substituents, which is rationalized by density functional theory (DFT) calculations on model systems.

Full text of DOI:10.1021/ja711169w

Published on Web 03/01/2008
Syntheses of Trisila Analogues of Allyl Chlorides and Their
Transformations to Chlorocyclotrisilanes, Cyclotrisilanides,
and a Trisilaindane
Kai Abersfelder and David Scheschkewitz*
Institute of Inorganic Chemistry, Julius-Maximilians-UniVersity Wu¨rzburg, Am Hubland,
D-97074 Wu¨rzburg, Germany
Received December 17, 2007; E-mail: scheschkewitz@mail.uni-wuerzburg.de
Abstract: The rearrangements of (chlorosilyl)disilenes R2(Cl)Si-(Tip)SidSiTip2 (5a,b: Tip ) 2,4,6-iPr3C6H2,
a: R ) Me, b: R ) Ph) quantitatively yield the isomeric chlorocyclotrisilanes (6a,b). The disilene precursors
5a,b are, in turn, accessible from the reactions of the disilenide Tip2SidSi(Tip)Li (1), that is, a disila analogue
to vinyl anions, with dichlorosilanes R2SiCl2. This novel approach to cyclotrisilanes potentially allows for
the facile variation of the substitution pattern and grants access to the first anionic derivatives; while the
rearrangement of 5a,b to 6a,b is slow at room temperature and additionally requires the presence of THF
or other n-donors, reduction of 5b with lithium instantly yields the corresponding cyclotrisilanide (7b) without
detection of any open-chained isomer. Heating of a neat sample of 5b to 150 °C provides a completely
characterized 1,2,3-trisilaindane derivative (13), strongly supporting the intermediacy of a disilanyl silylene
species that inserts into an ortho-CH bond of the phenyl substituents. The X-ray diffraction studies on
single crystals of 6a,b and 7b reveal that the Si-Si bond distance in cyclotrisilanes depends significantly
on the electronegativity of the opposing silicon atom’s substituents, which is rationalized by density functional
theory (DFT) calculations on model systems.
Chart 1. Some Isomers of Si3H6
Introduction
Since the synthesis of the first stable derivative by Masamune
et al. in 1983,1 cyclotrisilanes have attracted considerable interest
as the smallest homonuclear silacycles, both from a fundamental
and applied point of view.2 Due to the inherent ring strain,
cyclotrisilanes usually fragment easily under photochemical or
thermal conditions and are therefore valuable precursors for the
generation of silylenes and disilenes, as has been shown by the
groups of Masamune, Weidenbruch, Belzner, and others in
various contributions.1,3 Mechanistic studies using combined
experimental and theoretical approaches investigated the forma-
tion of elemental silicon films by pyrolysis of silane SiH4 and
disilane Si2H6 and the possible involvement of the parent
cyclotrisilane I as an intermediate.4 Computational efforts have
been devoted to neutral and the (experimentally elusive) anionic
cyclotrisilanes,5 particularly as examples of small hydrogenated
silicon clusters, which are assumed to be band-gap-determining
components of amorphous silicon.5b,d
in energy than the open-chained silyl disilene II. A plausible
intermediate for the interconversion of I and II could be disilanyl
silylene III, which is, in turn, 18.8 kcal mol-1 higher in energy
than I.6d While a large number of stable disilenes bearing silyl
(3) (a) Masamune, S.; Tobita, H.; Murakami, S. J. Am. Chem. Soc. 1983, 105,
6524. (b) Watanabe, H.; Okawa, T.; Kato, M.; Nagai, Y. J. Chem. Soc.,
Chem. Commun. 1983, 781. (c) Murakami, S.; Collins, S.; Masamune, S.
Tetrahedron Lett. 1984, 25, 2131. (d) Scha¨fer, A.; Weidenbruch, M. J.
Organomet. Chem. 1985, 282, 305. (e) Dewan, J. C.; Murakami, S.; Snow,
J. T.; Collins, S.; Masamune, S. J. Chem. Soc., Chem. Commun. 1985,
892. (f) Scha¨fer, A.; Weidenbruch, M.; Saak, W.; Pohl, S. Angew. Chem.
1987, 99, 806; Angew. Chem., Int. Ed. Engl. 1987, 26, 776. (g) Weiden-
bruch, M.; Flintjer, B.; Pohl, S.; Saak, W. Angew. Chem. 1989, 101, 89;
Angew. Chem., Int. Ed. Engl. 1989, 28, 95. (h) Weidenbruch, M.; Brand-
Roth, B.; Pohl, S.; Saak, W. Angew. Chem. 1990, 102, 93; Angew. Chem.,
Int. Ed. Engl. 1990, 29, 90. (i) Sakakibara, A.; Kabe, Y.; Shimizu, T.; Ando,
W. J. Chem. Soc., Chem. Commun. 1991, 43. (j) Belzner, J. J. Organomet.
Chem. 1992, 430, C51. (k) Weidenbruch, M.; Kroke, E.; Marsmann,
H.; Pohl, S.; Saak, W. J. Chem. Soc., Chem. Commun. 1994, 1233. (l)
Belzner, J.; Ihmels, H.; Kneisel, B. O.; Herbst-Irmer, R. J. Chem. Soc.,
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Pohl, S.; Marsmann, H. Organometallics 1995, 14, 5695. (n) Belzner, J.;
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Weidenbruch, M.; Marsmann, H. Organometallics 2000, 19, 4938. (q)
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2002, 21, 636. (r) Kira, M.; Iwamoto, T.; Maruyama, T.; Kuzuguchi, T.;
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In sharp contrast to the cyclopropane-propene case, cyclo-
trisilane I (see Chart 1) is considered the global minimum of
the Si3H6 hypersurface.6 It was found to be 7.8 kcal mol-1 lower
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9
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J. AM. CHEM. SOC. 2008, 130, 4114-4121
10.1021/ja711169w CCC: $40.75 © 2008 American Chemical Society

Transformations of Trisila Analogues of Allyl Chlorides
A R T I C L E S
substituents is known,7 the silyl disilene-cyclotrisilane rear-
rangement, to the best of our knowledge, has never been
observed experimentally, unlike the related silyl silylene-
disilene interconversion.8 In fact, the synthesis of cyclotrisilanes
is usually accomplished by means of Wurtz-type reductive
coupling of substituted dihalosilanes.9 Since this approach is
evidently restricted to homoleptic systems in most cases, only
a few unsymmetrical derivatives have been prepared from
unsaturated cyclotrisilenes10 or dianionic disilanes11 as starting
materials.
Scheme 1. Diverging Reactivity of Disilenide 1 toward
Chlorosilanes13b,c (Tip = 2,4,6-iPr3C6H2)
Recently, disilenides, that is, disila analogues to vinyl anions,
have become available in pure form.12 In the meantime, a
number of applications of such disilenides as reagents for the
transfer of SidSi double bonds to organic and inorganic
substrates has emerged.13 First evidence for the nucleophilicity
of disilenide 1 was derived from its quantitative conversion to
monosilyl disilene 2 by treatment with Me3SiCl.12a While
reaction of 1 with bulkily substituted TipSiCl3 yielded 3, a
compound featuring a difunctionalized trisila allyl moiety,13b
addition of 4 equiv of 1 to silicon tetrachloride led to the
formation of the unusual cluster 4 exhibiting one vertex free of
additional substituents.13c These diverging results (open chain
vs tricyclic derivative) gave a hint at the importance of the
substitution pattern of the electrophile employed and prompted
us to investigate the reactivity of disilenide 1 toward chlorosi-
lanes bearing substituents with intermediate steric demand (see
Scheme 1).
Here, we report on the synthesis of trisila analogues of allyl
chlorides, that is, chlorosilyl disilenes, and their reduction with
lithium to cyclotrisilanides, the first anionic homonuclear three-
membered silacycles,14 rather than open-chained allylic anions.
Moreover, the tetrahydrofuran-promoted rearrangement of chlo-
rosilyl disilenes yields chlorocyclotrisilanes, not only allowing
the first observation of the silyl disilene-cyclotrisilane isomer-
ism but also the full characterization of both isomers. Addition-
ally, the thermal isomerization of the diphenyl-substituted
chlorosilyl disilene affords a trisilaindane derivative, providing
firm evidence for a transient disilanylsilylene that undergoes
intramolecular CH insertion. To complement the experimental
data, the structural features of neutral and anionic cyclotrisilanes
are rationalized by DFT calculations on model systems.
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2003, 22, 4721. (d) Xu, W.; Yang, Y.; Xiao, W. J. Phys. Chem. A 2004,
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Experimental Section
General. All manipulations were carried out under a protective
atmosphere of argon applying standard Schlenk techniques or in a dry
box. Ethereal solvents were refluxed over sodium/benzophenone;
toluene over sodium; pentane, hexane, and (deuterated) benzene over
sodium/potassium alloy. All solvents were distilled and stored under
argon and degassed prior to use. Tip2SidSi(Tip)Li (1) was prepared
following our published procedure.12a Chlorosilanes were purchased
from Aldrich, stored over Mg turnings, and distilled prior to use. Finely
dispersed lithium powder was prepared by melting lithium rods
(Aldrich) in paraffin oil under argon, dispersing it with an Ultra Turrax
(Ika) for 3 min, filtering the cooled suspension, and rinsing three times
with pentane before drying in vacuum. The NMR spectra were recorded
on a Bruker Avance 200 (1H, 200.13 MHz), on a Bruker DRX 300
(7Li, 116.64 MHz), and on a Bruker AV 500 (1H, 500.13 MHz; 11B,
160.46 MHz; 13C, 125.76 MHz; 29Si, 99.36 MHz) FT-NMR spectrom-
(10) (a) Iwamoto, T.; Kabuto, C.; Kira, M. J. Am. Chem. Soc. 1999, 121, 886.
(b) Lee, V. Y.; Matsuno, T.; Ichinohe, M.; Sekiguchi, A. Heteroat. Chem.
2001, 12, 223.
(11) Fischer, R.; Konopa, T.; Baumgartner, J.; Marschner, C. Organometallics
2004, 23, 1899.
1
eter. H and 13C{1H} NMR spectra were referenced to external TMS
via the residual protons of the deuterated solvent (1H) or the solvent
7
itself (13C). Li{1H} NMR spectra were referenced to external LiCl-
(12) (a) Scheschkewitz, D. Angew. Chem. 2004, 116, 3025; Angew. Chem., Int.
Ed. 2004, 43, 2965. (b) Ichinohe, M.; Sanuki, K.; Inoue, S.; Sekiguchi, A.
Organometallics 2004, 23, 3088. (c) Inoue, S.; Ichinohe, M.; Sekiguchi,
A. Chem. Lett. 2005, 34, 1564. (d) Kinjo, R.; Ichinohe, M.; Sekiguchi, A.
J. Am. Chem. Soc. 2007, 129, 26.
(13) (a) Nguyen, T.; Scheschkewitz, D. J. Am. Chem. Soc. 2005, 127, 10174.
(b) Abersfelder, K.; Gu¨clu¨, D.; Scheschkewitz, D. Angew. Chem. 2006,
118, 1673; Angew. Chem., Int. Ed. 2006, 45, 1643. (c) Scheschkewitz, D.
Angew. Chem. 2005, 117, 3014; Angew. Chem., Int. Ed. 2005, 44, 2954.
(d) Inoue, S.; Ichinohe, M.; Sekiguchi, A. Angew. Chem. 2007, 119, 3410;
Angew. Chem., Int. Ed. 2007, 46, 3346. (e) Bejan, I.; Gu¨clu¨, D.; Inoue, S.;
Ichinohe, M.; Sekiguchi, A.; Scheschkewitz, D. Angew. Chem. 2007, 119,
3413; Angew. Chem., Int. Ed. 2007, 46, 3349. (f) Bejan, I.; Scheschkewitz,
D. Angew. Chem. 2007, 119, 5885; Angew. Chem., Int. Ed. 2007, 46, 5783.
(aq) and 29Si{1H} NMR spectra to external TMS. Microanalyses for
C, H, and N were performed on a Leco CHNS-932 elemental analyzer.
UV-vis spectra were recorded on a Shimadzu UV-Mini 1240. Melting
points were determined under argon in closed NMR tubes and are
uncorrected.
1-Chloro-1,1-dimethyl-2,3,3-tris(2,4,6-triisopropylphenyl)-2-
trisilene, 5a. Via a cannula, 10 mL of hexane is added to 2.00 g (2.344
(14) Cyclodisilagermanides have been reported: Lee, V. Y.; Sekiguchi, A. Chem.
Lett. 2004, 33, 84.
9
J. AM. CHEM. SOC. VOL. 130, NO. 12, 2008 4115

A R T I C L E S
Abersfelder and Scheschkewitz
mmol) of disilenide 1. The orange solution is cooled to -45 °C, and
0.33 g (2.557 mmol) of Me2SiCl2 is added. After 5 min, the reaction
mixture is brought to -10 °C and left stirring for another 10 min.
Following subsequent warming to room temperature, the solution is
separated from precipitated LiCl by filtration. All volatiles are removed
in vacuum, and the solid residue is dissolved with approximately 5
mL of hexane. After 16 h at 0 °C, 0.58 g of 6a is separated from the
mother liquor, the volume of which is slightly reduced, leading to a
second crop of crystals predominantly composed of 6a (0.20 g). The
remaining solution is evaporated to dryness in vacuum, yielding 0.98
g (55%) of 5a as an orange powder of 90-95% purity (contains 5-10%
of 6a), as estimated from the NMR spectra. 1H NMR (500 MHz, C6D6,
298 K): δ 7.09, 7.06, 6.99 (each s, each 2H, Tip-H), 4.29, 4.00, 3.78
(each hept., each 2H, iPr-CH), 2.744, 2.736, 2.64 (each hept., each
1H, iPr-CH), 1.34, 1.30 (br), 1.18, 1.17, 1.07, 0.96 (each d, altogether
54H, iPr-CH3), 0.45 (s, 6H, SiCH3). 13C NMR (125 MHz, C6D6, 298
K): δ 156.55, 155.36, 155.15, 151.86, 151.16, 150.84 (Tip-Co/p),
134.81, 134.20, 129.38 (Tip-Ci), 122.51, 122.27, 121.96 (Tip-CH),
38.32, 36.76, 34.83, 34.78, 34.44 (iPr-CH), 25.44 (br), 24.65, 24.13,
24.08, 23.87 (iPr-CH3), 6.92 (SiCH3). 29Si NMR (99.36 MHz, C6D6,
298 K): δ 103.0 (1J(Si,Si) ) 123 Hz, SiTip2), 39.9 (1J(Si,Si) ) 123
and 84 Hz, SiTip), 26.2 (1J(Si,Si) ) 84 Hz, SiMe2).
δ -26.5 (SiTip), -35.4 (SiMe2), -64.2 (SiTip2). Exact Mass (ESI-
MS) (m/z) calcd for C47H76Si3Cl+ (MH+): 759.49379; found: 759.49374.
Anal. Calcd for C47H75Si3Cl: C, 74.30; H, 9.95. Found: C, 74.10; H,
9.88.
1-Chloro-2,2-diphenyl-1,3,3-tris(2,4,6-triisopropylphenyl)cyclo-
trisilane, 6b. A solution of 500 mg (0.57 mmol) of 5b in 5 mL of
THF is left at room temperature overnight, leading to quantitative
conversion to 6b. Crystallization from a hexane solution saturated at
50 °C affords 325 mg (65%) of slightly yellow crystals (mp 158-61
°C) of 6b. Single crystals are grown at -30 °C from THF solution. 1H
NMR (500 MHz, C6D6, 298 K): δ 8.18 (d, 2H, Ph-H), 7.26-7.20
(m, 4H, Tip- and Ph-H), 7.11-6.82 (m, 10H, Tip- and Ph-H), 4.04,
3.96, 3.89, 3.55 (each hept., altogether 6H, iPr-CH), 2.79, 2.74, 2.71
(each hept., each 1H, iPr-CH), 1.59, 1.48, 1.43, 1.41 (each d, each
3H, iPr-CH3), 1.29 (d, 6H, iPr-CH3), 1.23, 1.22, 1.17, 1.16, 1.15,
1.14, 1.09, 0.96, 0.75, 0.61, 0.56, 0.52 (each d, each 3H, iPr-CH3).
13C NMR (125 MHz, C6D6, 298 K): δ 156.96, 156.45, 156.19, 154.12,
152.40, 151.44, 150.54 (Tip-Co/p), 138.68, 138.12 (Ph-CH), 135.94,
135.01 (Ph-Ci), 132.34, 131.10, 130.22 (Tip-Ci), 129.45, 128.95,
128.27, 127.67 (Ph-CH), 123.20, 122.97, 122.11, 122.08, 122.06,
121.39 (Tip-CH), 37.43, 36.84, 36.82, 36.68, 36.32, 34.74, 34.70,
34.62, 34.46 (iPr-CH), 28.00, 27.79, 27.18, 26.97, 25.81, 25.42, 24.64,
24.61, 24.48, 24.31, 24.11, 24.07, 23.97, 23.93, 23.48, 21.63 (iPr-
CH3). 29Si NMR (99.36 MHz, C6D6, 298 K): δ -28.4 (SiTip), -45.8
(SiPh2), -62.6 (SiTip2). Anal. Calcd for C61H87ClSi3: C, 76.63; H, 9.17.
Found: C, 76.43; H, 9.23.
1-Chloro-1,1-diphenyl-2,3,3-tris(2,4,6-triisopropylphenyl)-2-
trisilene, 5b. Via a syringe, 1.74 g (6.87 mmol) of neat Ph2SiCl2 is
added to a solution of 5.00 g (5.86 mmol) of disilenide 1 in 40 mL of
toluene at -78 °C. The color of the reaction mixture gradually
intensifies to deep red while it is brought to room temperature overnight.
Toluene is removed under vacuum and replaced by 70 mL of pentane.
The solution is filtered, and solvents are distilled off, affording an orange
oil, which is crystallized at 0 °C from a saturated pentane solution,
1-Lithio-2,2-diphenyl-1,3,3-tris(2,4,6-triisopropylphenyl)cyclot-
risilane, 7b‚Li+‚Et2O. Via a cannula, 10 mL of Et2O is added to 1.50
g (1.70 mmol) of 5b and 0.15 g (21.6 mmol) of lithium powder at
-80 °C. The resulting mixture is stirred overnight. All volatiles are
removed in vacuum, and the solid is residue digested with hot hexane
(50 °C). The solution is filtered while warm, and its volume reduced
to 15 mL. Prematurely precipitating product is dissolved by gentle
heating. Keeping the temperature at 25 °C for 24 h yields 1.04 g (72%)
of orange crystals. Slightly yellow crystals of pure 7b‚Li+‚Et2O (mp
199 °C dec) are obtained by a second crystallization from hexane (0.73
1
yielding 3.35 g (65%) of 5b as orange blocks (mp 134 °C, dec). H
NMR (500 MHz, C6D6, 298 K): δ 7.72 (t, 2H, Ph-H), 7.70 (d, 2H,
Ph-H), 7.00-6.90 (m, 12H, Tip, and Ph-H), 4.15, 3.97, 3.74 (each
hept., each 2H, iPr-CH), 2.70, 2.68, 2.66 (each hept., each 1H, iPr-
CH), 1.30-0.96 (br, 24H, iPr-CH3), 1.144, 1.142, 1.095 (each d, each
6H, iPr-CH3), 0.93 (d, 12H, iPr-CH3). 13C{1H} NMR (125 MHz,
C6D6, 298 K): δ 156.06, 155.05, 154.50, 151.41, 150.99, 150.98 (Tip-
1
g, 50%). H NMR (500 MHz, C6D6, 298 K): δ 8.34 (d, 2H, Ph-H),
Co/p), 136.67 (Ph-Ci), 135.93 (Ph-CH), 135.26, 134.73 (Tip-Ci),
7.34 (t, 2H, Ph-H), 7.28-7.22 (m, 5H, Tip- and Ph-H), 7.07 (br,
1H, Tip-H), 7.02 (d, 1H, Tip-H), 6.96-6.88 (m, 5H, Tip- and Ph-
H), 4.64, 4.59 (br), 4.38, 4.21, 4.14, 3.88, 2.88, 2.85 (each hept.,
altogether 8H, iPr-CH), 2.70 (q, 4H, Et2O), 2.59 (hept., 1H, iPr-
CH), 1.69 (br), 1.51 (br), 1.39, 1.380 (br), 1.377, 1.34 (each d, each
3H, iPr-CH3), 1.31 (d, 6H, iPr-CH3), 1.279, 1.275, 1.15, 1.07, 1.06,
0.89 (br), 0.64 (br) (each d, each 3H, iPr-CH3), 0.63 (t, 6H, Et2O),
0.56, 0.52, 0.35 (each d, each 3H, iPr-CH3). 13C NMR (125 MHz,
C6D6, 298 K): δ 158.48 (br), 156.61 (br), 155.56, 155.16, 154.84,
154.15 (br), 150.05 (br), 149.79, 146.51 (Tip-Co/p), 144.49, 142.87,
139.78 (br), 139.77 (Tip and Ph-Ci), 138.74 (Ph-CH), 138.30 (br,
Tip- or Ph-Ci), 137.79, 127.66, 127.60, 127.05, 126.98 (Ph-CH),
123.69, 122.89, 122.68, 121.10, 120.65 (br), 119.03 (br) (Tip-CH),
65.64 (THF), 36.40, 36.39, 36.05 (br), 35.74, 34.84, 34.79, 34.45 (br),
34.32, 33.58 (iPr-CH), 27.04 (br), 26.82, 26.79, 26.09, 24.91, 24.80,
24.60, 24.53, 24.47, 24.44, 24.32, 24.06, 23.91, 23.75 (br), 23.74, 23.50,
22.37 (br) (iPr-CH3), 14.05 (THF). 29Si NMR (99.36 MHz, C6D6, 298
130.18 (Ph-CH), 129.21 (Tip-Ci), 127.97 (Ph-CH), 122.52, 122.24,
121.91 (Tip-CH), 38.55, 37.83, 37.70, 34.74, 34.62, 34.40 (iPr-CH),
24.73, 24.49 (each br, each iPr-CH3), 24.06, 24.00, 23.86 (iPr-CH3).
29Si NMR (99.36 MHz, C6D6, 298 K): δ 109.1 (1J(Si,Si) ) 120 Hz,
SiTip2), 35.5 (1J(Si,Si) ) 120 and 82 Hz, SiTip), 11.8 (1J(Si,Si) ) 82
Hz, SiPh2). UV-vis (hexane) λmax (ꢀ): 427 nm (24800 L mol-1 cm-1).
Anal. Calcd for C57H79Si3Cl: C, 77.45; H, 9.01. Found: C, 77.22; H,
9.06.
1-Chloro-2,2-dimethyl-1,3,3-tris(2,4,6-triisopropylphenyl)cyclo-
trisilane, 6a. Via a syringe, 0.93 g (7.21 mmol) of neat Me2SiCl2 is
added to a solution of 4.00 g (4.69 mmol) of disilenide 1 in 40 mL of
toluene at -78 °C. The color of the reaction mixture quickly intensifies
to deep red. After stirring at room temperature overnight, the color of
the product mixture changes to light yellow. The solution is filtered,
and the volatile components are distilled off. Crystallization at room
temperature from a hexane solution saturated at 60 °C yields 2.18 g
(61%) of 6a as light-yellow blocks (mp 184 °C). 1H NMR (400.2 MHz,
C6D6, 298 K): δ 7.20, 7.18, 7.15, 7.02, 6.95 (each s, altogether 6H,
Tip-H), 4.21 (br), 4.09, 3.74 (br), 3.61, 3.43 (br), 2.79, 2.76, 2.70
(each hept., altogether 9H, iPr-CH), 1.51 (br), 1.49, 1.42 (br), 1.41,
1.39, 1.37, 1.31, 1.24 (br), 1.17, 1.14, 1.13 (each d, altogether 45H,
iPr-CH3), 0.83 (s, 3H, SiCH3), 0.67 (br, 3H, iPr-CH3), 0.51 (br, 6H,
iPr-CH3), 0.23 (s, 3H, SiCH3). 13C{1H} NMR (100.6 MHz, C6D6, 298
K): δ 156.77, 156.67, 156.00, 155.94, 155.75, 154.18, 152.23, 150.57,
150.23 (Tip-Co/p), 132.91, 131.20, 130.80 (Tip-Ci), 122.59, 122.22,
121.98, 121.79, 121.73, 121.00 (Tip-CH), 37.78, 36.90, 36.57, 36.34,
35.67, 34.90, 34.83, 34.59, 34.49 (iPr-CH), 27.74, 27.68, 26.98, 26.85,
26.45, 24.77, 24.63, 24.51, 24.05, 24.02, 23.99, 23.60, 22.50 (iPr-
CH3), -2.48, -4.43 (SiCH3). 29Si NMR (99.36 MHz, C6D6, 298 K):
1
K): δ -47.8 (1Si, SiPh2), -76.4 (1Si, SiTip2), -128.0 (1Si, J(Si,Li)
≈ 50 Hz, SiTipLi). MS-CI, negative mode (isobutane) (m/z): 849
(M--Li+‚Et2O + H+).
[Lithium(C2.2.1)]-2,2-diphenyl-1,3,3-tris(2,4,6-triisopropylphenyl)-
cyclotrisilan-1-ide, 7b‚[Li+ (C2.2.1)]. A solution of 124 mg (0.43
mmol) of cryptand 2.2.1 in 1 mL of toluene is added to 350 mg (0.41
mmol) of 7b‚Li+‚Et2O in 1 mL of toluene. About 0.5 mL of hexane is
added by a pipet. The resulting precipitate is dissolved by gentle
warming to 60 °C in an oil bath. Heating is turned off, and the clear
orange solution is left to cool down to room temperature overnight.
Decanting the mother liquor and drying in vacuum yields 7b‚[Li+
1
(C2.2.1)] as orange crystals. H NMR (500 MHz, C6D6, 298 K): δ
9
4116 J. AM. CHEM. SOC. VOL. 130, NO. 12, 2008

Transformations of Trisila Analogues of Allyl Chlorides
A R T I C L E S
Table 1. Selected Crystallographic Details for Compounds 5b, 6a,b, 7b‚Li+‚Et2O, 7b‚[Li+ (C2.1.1)], and 13
5b
6a
6b
7b
‚
Li+
‚
Et2O
7b
‚
[Li+ (C2.1.1)]
13
formula
temperature
crystal system
space group
a, Å
b, Å
c, Å
C233H328Cl4Si12
98(2) K
monoclinic
P21/c
31.0675(8)
18.4074(5)
39.2606(9)
90°
99.287(1)°
90°
22157.7(10)
4
C47H75ClSi3
173(2) K
triclinic
P-1
14.135(1)
17.7858(13)
20.6446(15)
69.997(1)°
75.569(1)°
89.749(2)°
4704.4(6)
4
C61H87ClOSi3
193(2) K
orthorhombic
P 21 21 21
10.8740(5)
15.4908(8)
34.2768(17)
90°
90°
90°
5773.8(5)
4
1.100
C64H96LiOSi3
173(2) K
triclinic
P-1
12.9877(9)
13.6085(10)
19.7857(14)
99.423(2)°
100.153(2)°
112.956(1)°
3063.6(4)
2
C78H115LiN2O4Si3
97(2) K
triclinic
P-1
13.0569(4)
16.6258(4)
17.0696(5)
84.847(1)°
86.065(1)°
85.138(1)°
3670.28(18)
2
C75H115ClSi3
173(2) K
triclinic
P-1
13.3185(8)
15.7123(9)
18.0837(11)
78.6948(14)°
75.7526(13)°
76.4736(13)°
3527.4(4)
2
R
â
g
V, Å3
Z
d, g cm-3
µ, mm-1
F(000)
1.082
0.168
7848
1.073
0.187
1664
1.054
0.115
1066
1.118
0.113
1348
1.070
0.144
1248
0.166
2080
θmax
25.03°
427594
39112
25.03°
37758
16623
1.123
25.03°
46564
10194
1.307
26.37°
27554
12481
1.031
26.37°
232386
14989
25.03°
28117
12409
1.107
refl.total
refl.unique reflections
S
1.054
1.025
R1 (I > 2σ)
wR2 (all data)
0.0508
0.1376
0.0645
0.1483
0.0671
0.1548
0.0524
0.1307
0.364
0.0901
0.0696
0.1660
8.66 (d, 2H, Ph-H), 7.44-7.28 (m, 6H, Tip- and Ph-H), 7.12-6.82
(m, 8H, Tip- and Ph-H), 4.96, 4.93 (br), 4.66, 4.49, 4.28 (each hept.,
altogether 6H, iPr-CH), 2.99-2.92 (m, 6H, iPr-CH and OCH2), 2.91
(s, 4H, OCH2), 2.84-2.72 (m, 9H, iPr-CH and OCH2), 1.96 (ddd,
4H, NCHH), 1.90 (t, 4H, NCH2), 1.81 (ddd, 4H, NCHH), 1.66, 1.63
(br), 1.57, 1.49, 1.38 (br), 1.37, 1.28-1.20 (br), 1.05 (br), 1.03, 0.74,
0.72, 0.68 (each d, altogether 54H, iPr-CH3). 7Li NMR (194.37 MHz,
C6D6, 298 K): δ -1.11. 13C NMR (125 MHz, C6D6, 298 K): δ 155.26,
155.15, 154.87, 154.84, 154.64, 149.26, 148.70, 148.07, 147.74, 146.69,
146.21, 145.50, 143.19 (Tip-Ci/o/p), 139.80, 137.94, 128.00, 126.63,
126.28, 125.88, 124.83 (Ph-CH), 121.67, 121.17, 120.59, 120.51,
118.93, 118.47 (Tip-CH), 68.30, 66.46, 65.37 (OCH2), 51.41, 50.04
(NCH2), 36.42, 36.19, 35.92, 35.0 (br), 34.99, 34.92, 34.74, 34.62,
34.54, 34.5 (br) (iPr-CH), 27.60, 27.40, 27.24, 26.87 (br), 25.22, 25.19,
25.05, 24.93, 24.90, 24.71, 24.56, 24.44, 23.76, 23.61 (iPr-CH3). 29Si
NMR (99.36 MHz, v): δ -42.0 (1Si, SiPh2), -74.1 (1Si, SiTip2),
-123.0 (1Si, SiTip).
transferred to the cold nitrogen stream at the goniometer. A sphere of
data was collected on a Bruker Smart Apex II with CCD detector (5b
and 7b‚[Li+ (C2.1.1)]: Bruker Smart Apex II with rotating anode source
and CCD detector). The cells were determined accurately on a
representative set of frames. Data reduction was carried out with
SAINT+ 6.22, and a semiempirical absorption correction was
applied with SADABS. Structures were solved by direct methods or
Patterson synthesis. The disorder of some isopropyl groups and/or
cocrystallized solvent molecules in 5b, 6a, 6b, and 7b‚Li+‚Et2O
was modeled by applying split positions and appropriate positional
restraints. H atoms attached to silicon were located in the difference
electron map and refined isotropically; all other H-atom positions were
calculated and refined using a riding model. Refinements (ShelXL-
97) proceeded smoothly to yield the crystallographic parameters
summarized in Table 1.
Theoretical Calculations. All calculations were carried out with
the Gaussian 98 program package.15 The B3LYP functional was applied
with the 6-31G(d,p) basis set. The natural bond orbital (NBO) analyses
were done with NBO 5.0 as implemented in Gaussian.16 The molecular
orbitals were visualized and plotted with the MOLEKEL program.17
1-Chloro-1-phenyl-2,2,3-tris(2′,4′,6′-triisopropylphenyl)-1,2,3-
trisilaindane, 13. A neat sample of 5b (200 mg, 0.226 mmol) is heated
under vacuum to 150 °C for 30 min. The colorless residue is dissolved
in a minimum amount of hexane at 60 °C and left at room temperature
overnight. Decanting of the mother liquor and drying in vacuum yields
170 mg of 9 (85%) as colorless plates (mp. 136 °C). Single crystals
are grown from cyclohexane at 25 °C. 1H NMR (500 MHz, C6D6, 298
K): δ 8.05 (m, 2H, Ph-H), 7.80, 7.40 (each d, 1H, Ph-H), 7.36, 7.32
(each s, 1H, Tip-H), 7.23 (m, 3H, Ph-H), 7.07, 7.04, 7.01 (each s,
each 1H, Tip-H), 7.00 (m, 1H, Ph-H), 6.91 (m, 1H, Ph-H), 6.80 (s,
Results and Discussion
Disilenide 1 reacts with dichlorosilanes R2SiCl2 in toluene
or THF at room temperature in 2 h to yield the chlorosilyl-
substituted disilenes 5a,b (a: R ) Me, b: R ) Ph) in essentially
quantitative yield. Both 5a and 5b were fully characterized by
multinuclear NMR spectroscopy; only the diphenyl derivative
5b, however, was isolated as orange blocks (mp 134 °C) in
65% yield by crystallization from a concentrated pentane
solution at 0 °C (see Scheme 2).
The chemical shifts in the 29Si NMR spectra compare well
to those of monosilyl-substituted disilenes 2 and 3, recently
reported by us (Table 2).12a,13b The well-resolved 29Si-29Si
coupling constants together with 2D NMR spectra (29Si,1H)
allow for the unambiguous assignment of the signals of 5a,b;
the resonances at intermediate field show two sets of 29Si
1
1H, Tip-H), 6.41 (s, 1H, J(Si,H) ) 194 Hz, SiH), 4.54, 4.04, 3.62,
3.44, 3.07, 2.79, 2.78, 2.63, 2.40, (each hept., each 1H, iPr-CH), 1.91,
1.63, 1.59, 1.43, 1.22, 1.20, 1.14, 1.08, 1.05, 0.96, 0.94, 0.67, 0.41,
0.19, 0.17 (each d, altogether 54H, iPr-CH3). 13C NMR (125 MHz,
C6D6, 298 K): δ 157.54, 157.46, 156.38, 156.19, 154.69, 152.41,
151.25, 150.19, 149.89 (Tip-Co/p), 147.63, 143.59 (Ph-Ci), 137.63,
136.55, 134.24 (Ph-CH), 133.53 (Tip-Ci), 133.30 (Ph-Ci), 132.19
(Tip-Ci), 130.45, 129.85, 128.95, 127.87 (Ph-CH), 127.15 (Tip-Ci),
123.53, 123.22, 123.19, 122.76, 122.19, 121.45 (Tip-CH), 39.48, 38.39,
37.70, 36.62, 35.70, 34.61, 34.49, 34.40, 34.25 (iPr-CH), 31.91
(hexane), 27.67, 26.70, 25.92, 25.78, 25.28, 25.17, 24.84, 24.74, 24.68,
24.08, 24.06, 24.04, 24.01, 23.95, 23.94, 23.78, 23.66 (iPr-CH3), 23.00,
14.33 (hexane). 29Si NMR (99.36 MHz, v): δ 5.7, (SiCl), -42.0 (1Si,
SiH), -50.6 (1Si, SiTip2). Anal. Calcd for C63H93ClSi3: C, 78.00; H,
9.66. Found: C, 77.37; H, 9.44.
(15) Frisch, M. J., et al. Gaussian 98, g03, revision B.04, Gaussian, Inc.:
Pittsburgh, PA, 2003.
(16) Glendening, E. D.; Badenkoop, J. K.; Reed, A. E.; Carpenter, J. E.;
Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.G; Theoretical
Chemistry Institute, University of Wisconsin: Madison, WI, 2001.
(17) Flu¨kiger, P.; Lu¨thi, H. P.; Portmann, S.; Weber, J.; MOLEKEL 4.0; Swiss
National Supercomputing Centre CSCS: Manno, Switzerland, 2000.
Crystallography. Single crystals of 5b, 6a, 6b, 7b‚Li+‚Et2O, 7b‚
[Li+ (C2.2.1)], and 13 were coated with perfluorinated polyether and
9
J. AM. CHEM. SOC. VOL. 130, NO. 12, 2008 4117

A R T I C L E S
Abersfelder and Scheschkewitz
Scheme 2. Syntheses of Chlorosilyl Disilenes 5a,b and Their
Rearrangement and Reduction to Cyclotrisilanes 6a,b and
Cyclotrisilanides 7a,b (Tip ) 2,4,6-iPr3C6H2)
Table 2. Comparison of 29Si NMR and UV-Vis Data of Chlorosilyl
Disilenes 5a and 5b with 2,12a 3,13b and 1121
5a
5b
212a
313b
1121
δ29Si1, ppma
δ29Si2, ppmb
δ29Si3, ppmc
1JSi1,Si2, Hz
1JSi2,Si3, Hz
103.0
39.9
26.2
123
109.1
35.5
11.8
120
82
427
97.7
50.9
-8.3
116
75
99.1
54.7
11.7
102.7d
96.0d
-3.2
Figure 1. Molecular structure of 5b in the solid state at the 50% probability
level. Only one of four independent molecules is displayed. Cocrystallized
pentane, disordered iPr groups, and protons were removed for clarity.
84
λmax, nm
411
46820
ꢀ, L mol-1cm-1
24800
Chart 2. Symmetrical Disilyl Disilenes (E)-10, (Z)-10,20 and
Chlorosilyl Disilene 1121 (Tip ) 2,4,6-iPr3C6H2) for Comparison
a SiTip2. b SiTip. c Si3 ) Silyl-Si. d Tentatively assigned.
satellites and are hence attributable to the silyl-substituted silicon
atoms of the SidSi double bonds. The coupling constants
derived from the 29Si satellites of the low-field signals are
approximately 1.5 times larger than those for the high-field
shifts, which reflects the higher s character of the SidSi double
bond compared to that of the Si-Si single bond. Both 5a and
5b show very similar 29Si resonances for the corresponding
double bond atoms, with considerable differences between the
chemical shifts of the two silicon atoms of one double bond.
This difference is significantly more pronounced than that in
(dichloro)silyl-substituted 313b and trimethylsilyl disilene 212a
(5a: ∆δ ) 63.1; 5b: ∆δ ) 73.6; 2: ∆δ ) 46.8, 3: ∆δ )
44.4 ppm) but less than that in disilyldiaryl disilenes of the A2-
SidSiB2 type.18 The underlying large chemical shift anisotropies
of unsymmetrically substituted disilenes were subject to detailed
theoretical studies.19
slightly shorter than the usual single bond (234 pm),22 which
could be attributed to π back donation into the silyl group’s σ*
orbitals. As expected in terms of higher group electronegativity
and lesser steric bulk,7 the trans-bent angles θ (defined as the
angle between the SidSi bond vector and the normal vector to
the plane given by the sp2 silicon atom and the pending
substituent atoms) are higher for the chlorosilyl-substituted
silicon atoms (θ ) 9.5, 17.6, 16.6, and 25.0°) compared to those
of the SiTip2 moieties of the same molecule (θ ) 3.9, 10.8,
4.5, and 17.3°).
Both chlorosilyl disilenes 5a,b isomerize to chlorocyclotrisi-
lanes 6a,b at room temperature, albeit under differing rates and
conditions. While a solution of diphenyl(chloro)silyl-substituted
5b in C6D6 does not show any significant changes of the NMR
spectroscopic data for several weeks, its rearrangement to 6b
is complete after 2 h in n-donating tetrahydrofuran. Presumably
due to the less bulky dimethyl(chloro)silyl group, the derivative
5a is much more prone to cyclization even in the absence of
donor solvents, which explains our failure to obtain it as a pure
sample. While 5b is unaffected by excess Ph2SiCl2, the presence
of excess Me2SiCl2 accelerates the rearrangement of 5a to such
an extent that the stoichiometry of the reaction of the dichlo-
rosilane with disilenide 1 has to be carefully respected in order
to be able to isolate 5a at all.
Because 5a could not be isolated in sufficiently pure form,
meaningful UV-vis spectroscopic data could only be obtained
for 5b. Its longest wavelength absorption is observed at λmax
)
427 nm (ꢀ ) 24800 L mol-1 cm-1), which is considerably red
shifted compared to λmax values of symmetrical (aryl)(silyl)-
disilenes ((E)-10: 394 nm; (Z)-10: 398 nm)20 but similar to
that of bis(chlorosilyl)-substituted disilene 1121 (Table 2 and
Chart 2).
An X-ray diffraction study on single crystals of 5b proved
the constitution deduced from the spectroscopic data (Figure
1). Remarkably, 5b crystallizes as four independent molecules
in the asymmetric unit, which show significantly differing Sid
Si bond lengths ranging from 217.23(8) to 219.00(8) pm. The
Si-Si single bond distances (231.60(8)-232.70(8) pm) are
The 29Si NMR spectra of the rearrangement products cyclo-
trisilanes 6a,b, show signals between -26.5 and -64.2 ppm at
a relatively high field, as typically observed for cyclotrisilanes
(Table 3).2c While the chemical shifts for the Si(Cl)Tip and
(18) Ichinohe, M.; Arai, Y.; Sekiguchi, A.; Takagi, N.; Nagase, S. Organome-
tallics 2001, 20, 4141.
(19) Auer, D.; Strohmann, C.; Arbuznikov, A. V.; Kaupp, M. Organometallics
2003, 22, 2442.
(20) (a) Archibald, R. S.; van den Winkel, Y.; Millevolte, A. J.; Desper, J. M.;
West, R. Organometallics 1992, 11, 3276. (b) Wiberg, N.; Niedermeyer,
W.; Polborn, K. Z. Anorg. Allg. Chem. 2002, 628, 1045.
(21) Boomgarden, S.; Saak, W.; Marsmann, H.; Weidenbruch, M. Z. Anorg.
Allg. Chem. 2002, 628, 1745.
(22) Weidenbruch, M. In The Chemistry of Organic Silicon Compounds;
Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 2001; Vol. 3,
Chapter 5.
9
4118 J. AM. CHEM. SOC. VOL. 130, NO. 12, 2008

Transformations of Trisila Analogues of Allyl Chlorides
A R T I C L E S
Table 3. Comparison of 29Si NMR and Structural Data of Cyclotrisilanes 6a,b and Cyclotrisilanides 7a,b‚Li+‚Et2O
6a
6b
7a
‚
Li+
‚
Et2Oe
7b
‚
Li+
‚
Et2O
7b‚
[Li+ (C2.1.1]
δ29Si1, ppma
δ29Si2, ppmb
δ29Si3, ppmc
Si1-Si2, pm
Si1-Si3, pm
Si2-Si3, pm
-64.2
-62.6
-78.2d
-76.4
-128.0
-47.8
-74.1
-123.0
-42.0
-26.5
-28.4
-121.6d
-39.0d
-35.4
-45.8
235.8(1)f
230.7(1)f
237.7(1)f
236.1(1)
233.9(1)
240.9(1)
237.27(7)
238.32(5)
230.24(6)
233.60(7)
232.18(5)
233.87(5)
a SiTip2. b SiTip. c Si3 ) SiR2. d Tentatively assigned. e Prepared by reduction of 6a, not isolated. f Intermediate value.
Figure 2. Molecular structures of 6a (left) and 6b (right) in the solid state at the 30% probability level. Only one of two independent molecules is displayed
(6a). Cocrystallized THF (6b), disordered iPr groups, and protons were removed for clarity.
SiTip2 moieties are almost identical for 6a and 6b, the
resonances of the SiR2 fragments differ significantly, which was
also observed in the case of 5a,b.
first example of an anionic cyclotrisilanide14,24 was formed as
the only detectable product and isolated as its lithium salt 7b‚
Li+‚Et2O in 50% yield. The structure determination by single-
crystal X-ray diffraction (Figure 3) reveals the presence of a
close contact ion pair, in which Li1 strongly coordinates to the
negatively charged silicon atom and shows additional contacts
to Si2 and one ipso-carbon atom (Si1-Li1 258.0(4), Si2-Li1
289.8(4), Li1-C31 245.5(4) pm). This contact ion pair clearly
remains intact in C6D6 solution, as proven by the quartet splitting
of the 29Si NMR signal at -128.0 ppm due to coupling to the
7Li nucleus (1J(29Si,7Li) ≈ 50 Hz).
The structural features of neutral cyclotrisilanes 6a,b as
determined by X-ray diffraction studies on single crystals (Figure
2) exhibit highly varying endocyclic Si-Si bond distances from
230.67(11) to 240.95(13) pm. The bond lengths of homoleptic
cyclotrisilanes were subject to discussion due to relatively large
variations depending on the substitution pattern; it has been
suggested that these variations are mainly attributable to large
steric strain induced by the bulky substituents.1,9a In a detailed
theoretical study on substituted derivatives of the carbon
congener, that is, cyclopropane C3H5X, by Schleyer and Clark
et al., a significant effect of the electronic nature of the
substituent X was demonstrated.23 They convincingly showed
that σ-accepting groups lengthen the opposing and, at the same
time, shorten the adjacent endocyclic C-C bonds of cyclopro-
panes. Apparently, a similar trend can be observed in the case
of cyclotrisilanes as the longest bond in both cases, 6a and 6b,
is the one opposing the chlorine-substituted silicon atom
(Table 3).
In contrast to the corresponding chlorocyclotrisilane 6b, the
endocyclic Si-Si bond lengths of 7b‚Li+‚Et2O are all in the
range expected for single bonds (Table 3), corroborating the
above assumption that more σ-donating substituents should
lead to shorter opposing Si-Si bonds in cyclotrisilanes.
However, not only the Si2-Si3 bond distances of 7b‚Li+‚Et2O
are shorter than the corresponding bond length of 6b, but also,
the Si1-Si3 distance is shorter. In order to eliminate the
possibility that the coordination of the Li+ counterion is
responsible for the observed relatively short endocyclic
bonds of 7b, we also prepared the solvent-separated ion pair
by addition of cryptand 2.2.1 (C2.2.1) to a benzene solution of
7b‚Li+‚Et2O. The solid-state structure (Figure 3) of 7b‚[Li+
(C2.2.1)] shows no close contact of the cation to the anion
7b. While the Si1-Si3 bond length of 7b‚[Li+ (C2.2.1)] is
Given the slow isomerization of chlorosilyl disilene 5b, a
trisila analogue of allyl chlorides, we investigated its reduction
with finely dispersed lithium powder in Et2O at room temper-
ature. Contrary to our expectations, the anticipated trisila
analogue to allyl lithiums could not be observed. Instead, the
(24) An anion of the composition Si3H5- was detected in a mass spectrometric
study: Operti, L.; Rabezzana, R.; Vaglio, G. A. Rapid Commun. Mass
Spectrom. 2006, 20, 2696.
(23) Clark, T.; Spitznagel, G. W.; Klose, R.; Schleyer, P. v. R. J. Am. Chem.
Soc. 1984, 106, 4412.
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A R T I C L E S
Abersfelder and Scheschkewitz
Figure 3. Molecular structures of 7b‚Li+‚Et2O (left) and 7b‚[Li+ (C2.2.1)] (right) in the solid state at the 30 and 50% probability levels, respectively.
Cocrystallized hexane (7b‚Li+‚Et2O), disordered iPr groups, disordered coordinating Et2O, and protons were removed for clarity.
Scheme 3. Proposed Mechanisms for Thermal and
THF-Promoted Isomerizations of Chlorosilyl Disilenes (Tip )
2,4,6-iPr3C6H2)
Figure 4. Contour plots of the HOMOs of model cyclotrisilanes Me5Si3X,
with X ) Li (left) and Cl (right). Color coding: Si (pink), Cl (green), Li
have been reported for both stable and transient silylenes.26 As
(light-blue), C (dark gray), H (white).
an aryl-silyl-substituted silylene, 12 most certainly would have
a singlet ground state,27 which should be further stabilized by
a Lewis acid-base interaction of the vacant pz orbital with THF
or other n-donors. An n-donating solvent would thereby not only
catalyze the formation of cyclotrisilanes 6 but possibly lower
the reactivity of 12 to such an extent that competing reactions
such as CH insertions would be disfavored.
indeed much closer to that of 6b, the Si2-Si3 is practically
unaffected by the detachment of the counterion.
In order to clarify the bonding situation in cyclotrisilanes 6
and 7 in a manner independent of the steric strain induced by
the bulky ligand set, we carried out DFT calculations on the
permethylated model compounds Me5Si3X (X ) Li, Cl) at the
B3LYP/6-31G(d,p) level of theory. The Si2-Si3 bond opposing
the heterosubstituted silicon atom is considerably shorter for X
) Li (232.7 pm) than that for X ) Cl (237.5 pm). Conversely,
the adjacent Si-Si bonds are shorter in the case of the more
electronegative chloro substituent (Si1-Si2/3 (X ) Li): 238.3;
(X ) Cl): 235.3 pm). As can be seen from the plots in Figure
4, the HOMO with respect to Si2-Si3 is bonding for the
lithiated and antibonding cases of the chlorinated cyclotrisilane.
Consequently, the increase in electronegativity amended by the
chloro substituent reduces the Wiberg bond indices of the Si2-
Si3 bond from 0.9718 for X ) Li to 0.9240 for X ) Cl, as
well as the NLMO/NPA indices from 1.0648 for X ) Li to
1.0117 for X ) Cl. Indeed, the few crystallographically
characterized chlorinated cyclotrisilanes apart from 6a,b all
exhibit longer endocyclic Si-Si bonds opposing the chlorinated
silicon atoms as well.10
Not too surprisingly, all attempts to trap a silylene intermedi-
ate 12b with bis(trimethylsilyl)acetylene in either the presence
or absence of THF were unsuccessful, the intramolecular
reaction likely being much more rapid than the intermolecular
one with the trapping agent (Scheme 3). Heating of 5b slightly
above its melting point (mp 134 °C) in the absence of n-donors,
however, yields a single product without the SidSi double bond
but contrary to cyclotrisilane 6b with a silicon-bonded proton,
as evidenced by a signal in the 1H NMR spectrum at δ ) 6.41
ppm with a typical 1J(Si,H) coupling constant of 194 Hz.28 The
constitution of the product as a 1,2,3-trisilaindane29,30 derivative
(25) Review on silyl migrations: Kira, M.; Iwamoto, T. In The Chemistry of
Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley:
Chichester, U.K., 2001; Vol. 3, Chapter 16.
(26) (a) Belzner, J.; Dehnert, U.; Ihmels, H.; Hu¨bner, M.; Mu¨ller, P.; Uso´n, I.
Chem.sEur. J. 1998, 4, 852. (b) Belzner, J.; Dehnert, U.; Scha¨r, D.; Rohde,
B.; Mu¨ller, P.; Uso´n, I. J. Organomet. Chem. 2002, 649, 25. (c) Ishida, S.;
Iwamoto, T.; Kabuto, C.; Kira, M. Nature 2003, 421, 725. (d) Ishida, S.;
Iwamoto, T.; Kabuto, C.; Kira, M. Silicon Chem. 2003, 2, 137.
(27) Two very bulky electropositive tBu3Si groups were shown to be necessary
in order to stabilize the triplet state below the singlet by experiment and
theory: (a) Sekiguchi, A.; Tanaka, T.; Ichinohe, M.; Akiyama, K.; Tero-
Kubota, S. J. Am. Chem. Soc. 2003, 125, 4962. (b) Yoshida, M.; Tamaoki,
N. Organometallics 2002, 21, 2587.
Although the formation mechanism of the cyclotrisilanes 6a,b
remains speculative for now, we tentatively interpret the solvent
dependency of the rearrangement in terms of the formation of
transient disilanyl silylene 12 by 1,2-migration of the diphenyl-
(chloro)silyl moiety.25 The silylene 12 would likely insert into
the Si-Cl bond, keeping in mind that such insertion reactions
(28) Nagai, Y.; Ohtsuki, M.-A.; Nakano, T.; Watanabe, H. J. Organomet. Chem.
1972, 35, 81.
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Transformations of Trisila Analogues of Allyl Chlorides
A R T I C L E S
chlorine atom Cl1. The almost exclusive formation of the cis
stereoisomer 13 is proven by the NMR spectra of the crude
product mixture showing only very minor impurities apart from
the signals belonging to 13. This preference can be explained
by a pronounced staggering of the five-membered ring (dihedral
angles: Si1-Si2-Si3-C1 25.67(9)°; C2-Si1-Si2-Si3-
26.50(9)°), which avoids the unfavorable 1,3-diaxial interactions
that would necessarily be present in the trans stereoisomer.
Conclusion
We have described the synthesis of chlorosilyl disilenes 5a,b
from the reaction of a disilenide 1 with dichlorosilanes. While
5a,b rearrange to the isomeric cyclotrisilanes 6a,b in the
presence of THF (or possibly other n-donors), heating of 5b
above its melting point exclusively yields a trisilaindane
derivative 13. We propose that both rearrangements, thermal
and THF-promoted, proceed via a transient disilanyl silylene,
the reactivity of which might be tamed through stabilization of
its vacant pz orbital by a Lewis base. The reductions of 5b and
6a do not provide the anticipated open-chained trisila allylic
anions but furnish the first anionic cyclotrisilanes 7a,b. The
solid-state structures of neutral 6a,b and anionic 7b show that
bond lengths in cyclotrisilanes apart from steric strain also
depend significantly on the electronic properties of the substit-
uents. Supported by DFT calculations, we conclude that the
endocyclic bond distances of cyclotrisilanes are longer when
the σ-acceptor strength of the opposing substituent is high.
Given that various disilenides have been reported in the last
couple of years,12 the reactions described in this article should
be of broad applicability for the preparation of a large variety
of tailor-made asymmetric cyclotrisilanes, cyclotrisilanides, and
silaindane derivatives.
Figure 5. Molecular structure of 1,2,3-trisilaindane 13 in the solid state at
the 30% probability level. Cocrystallized cyclohexane, disordered iPr groups,
and most protons were removed for clarity.
13 is determined by X-ray diffraction on single crystals isolated
in 85% yield (mp 136 °C) by crystallization from a concentrated
solution in cyclohexane at room temperature (Figure 5).
While the intermediacy of silylene 12 in the formation of
cyclotrisilane 6a,b is hypothetical, the connectivity of 13, in
which the phenyl- and chloro-substituted silicon atom is attached
to the SiTip2 rather than to the SiTip moiety, as in the precursor
disilene 5b, provides strong evidence for a transient silylene
12b.31 Both Si-Si bond distances (Si1-Si2, 240.36(10); Si2-
Si3, 237.38(10) pm) are slightly longer than the usual Si-Si
single bond presumably due to the sterically demanding
substitution pattern. The structure in the solid state also reveals
the stereochemistry of the trisilaindane, in which the proton H1
is located on the same side of the five-membered ring as the
Acknowledgment. This work was supported by the DFG
(Sche 906/3-1 and 3-2), the FCI, and the Dr. Otto-Ro¨hm-
Foundation. The authors thank Prof. H. Braunschweig for
continuous support, Dr. K. Radacki for the recording of two of
the X-ray data sets, and Dr. R. Bertermann for NMR spectra.
(29) 1-Silaindanes: (a) Gilman, H.; Marrs, O. L. J. Org. Chem. 1964, 29, 3175.
(b) Benkeser, R. A.; Mozdzen, E. C.; Muench, W. C.; Roche, R. T.; Siklosi,
M. P. J. Org. Chem. 1979, 44, 1370. (c) Tsukuda, N.; Hartwig, J. F. J.
Am. Chem. Soc. 2005, 127, 5022. (d) Ruddy, D. A.; Berry, D. H.; Nataro,
C. J. Organomet. Chem. 2008, 693, 169.
(30) 1,2,3-Trisilaindane: Naka, A.; Hayashi, M.; Okazaki, S.; Ishikawa, M.
Organometallics 1994, 13, 4994.
(31) The thermally induced CH insertion reactions of disilenes reported have
not been associated with silylene intermediates: (a) Fink, M. J.; DeYoung,
D. J.; West, R.; Michl, J. J. Am. Chem. Soc. 1983, 105, 1070. (b) Wiberg,
N.; Niedermayer, W.; Polborn, K. Z. Anorg. Allg. Chem. 2002, 628, 1045.
Supporting Information Available: Crystallographic data in
CIF format, plots of NMR spectra, and computational details
with complete ref 15. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA711169W
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