Rearrangement/fragmentation reactions of oligosilanes with aluminum chloride

Article abstract of DOI:10.1021/om1011165

Reinvestigation of the Lewis acid catalyzed rearrangement of some open-chain permethyloligosilanes with the Al(Fe)Cl₃ catalyst system exhibited several cases of additional reactivity: namely, a fragmentation/ cyclization reaction. Introduction of (trimethylsilyl)methyl substituents into the oligosilane substrates strongly facilitated this reaction, yielding cyclic or bicyclic carbacyclosilanes. Investigations concerning the composition of the catalyst system indicated that the incorporation of about 0.1% FeCl₃ into the AlCl₃ lattice provided an effective catalyst.

Full text of DOI:10.1021/om1011165

ARTICLE
pubs.acs.org/Organometallics
Rearrangement/Fragmentation Reactions of Oligosilanes with
Aluminum Chloride
Harald Wagner, Judith Baumgartner,* and Christoph Marschner*
Institut f€ur Anorganische Chemie, Technische Universit€at Graz, Stremayrgasse 9, A-8010 Graz, Austria
Peter Poelt
Institute for Electron Microscopy, Technische Universit€at Graz, Steyrergasse 17, A-8010 Graz, Austria
S Supporting Information
b
ABSTRACT: Reinvestigation of the Lewis acid catalyzed rearrangement of some open-
chain permethyloligosilanes with the Al(Fe)Cl3 catalyst system exhibited several cases
of additional reactivity: namely, a fragmentation/cyclization reaction. Introduction of
(trimethylsilyl)methyl substituents into the oligosilane substrates strongly facilitated
this reaction, yielding cyclic or bicyclic carbacyclosilanes. Investigations concerning the
composition of the catalyst system indicated that the incorporation of about 0.1% FeCl3
into the AlCl3 lattice provided an effective catalyst.
’ INTRODUCTION
until the most stable ion is obtained. Eventually remethylation of
the ion concludes the rearrangement process.13
Over the last few years our group has been strongly involved in
the study of the chemistry of oligosilanes.1,2 In particular, the
development of a convenient synthetic access to structurally
diverse oligosilanyl anions3À6 allowed the preparation of a huge
variety of previously unknown small polysilanes.1,2
’ RESULTS AND DISCUSSION
The rearrangement processes of methylated oligosilanes can
1
conveniently be monitored by NMR spectroscopy. H NMR
Some 40 years ago Ishikawa and Kumada discovered the
AlCl3-catalyzed rearrangement of linear methylated oligosilanes
to branched isomers.7,8 This chemistry can be considered to be
complementary to our oligosilanyl anion chemistry, as it pro-
duces the strongly branched oligosilanes9,10 that are required as
precursors for effectively stabilized oligosilanyl anions.
We became aware of this potential during the synthesis of an all-
silicon analogue of the adamantane structure.11 In a subsequent
study we investigated AlCl3-catalyzed rearrangement reactions of
already branched cyclosilanes.12 One of the main conclusions we
could draw from this work was that these rearrangement reactions
always lead to substituted cyclopentasilanes.12
More recently, we showed that the introduction of trimethyl-
germyl groups into oligosilanes followed by AlCl3-catalyzed
rearrangement exclusively led to the formation of isomers with
germanium atoms possessing the highest possible silylation
degree.13 In the course of this study a computational investiga-
tion of the reaction mechanism demonstrated its similarity to the
well-known WagnerÀMeerwein reaction: starting with the ab-
straction of a methyl group from the oligosilane, a silylium ion
forms, which undergoes a series of 1,2-methyl and -silyl shifts
gives a good impression of the degree of conversion and in
addition can be used to judge the molecular symmetry of the
formed products from the number and integration of methyl
resonances. 29Si NMR spectroscopy provides even more insight.
Not only is the direct methylation degree of silicon atoms
reflected by their chemical shift but also to a lesser extent some
information about the neighboring silicon atoms can be ob-
tained. In general the rearrangement process leads to highly
branched structures, and by analysis of the corresponding 29Si
NMR spectra we can distinguish between nonmethylated
(δ(SiSi4): ca. À110 to À140 ppm), monomethylated (δ(MeSiSi3):
ca. À70 to À100 ppm), dimethylated (δ(Me2SiSi2):ca.À25 to À60
ppm), and trimethylated silicon atoms (δ(Me3SiSi): ca. À5 to À20
ppm). In addition, tetraalkylated silicon atoms resonate close to a
chemical shift value of 0 ppm. A high degree of branching of a
neighboring silicon atom usually results in an additional downfield
shift. How general this assignment scheme is can be estimated by
studying Schemes 5, 6, and 8À13 of this report. 29Si NMR shifts of
Received: November 29, 2010
Published: July 11, 2011
r
2011 American Chemical Society
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Scheme 1. Cyclization Reaction of a Branched Methylated Nonasilane To Give 1,1,3-Tris(trimethylsilyl)heptamethylcyclopen-
tasilane
Scheme 2. Rearrangement/Cyclization Reaction of a Branched Methylated Octasilane To Give Branched Isomers and Further
To Give 1,1-Bis(trimethylsilyl)octamethylcyclopentasilane
Scheme 3. Cyclization/Fragmentation Reaction of a
Branched Methylated Tetradecasilane To Give 1,1,3-Tris-
(trimethylsilyl)Heptamethylcyclopentasilane and 2,2-
Bis(trimethylsilyl)octamethyltetrasilane
the involved oligosilanes are shown with the compounds. In addition,
a short discussion of the chemical shift behavior of silicon atoms
connected to methylene groups is given after the synthetic part.
Although much of the relevant analytical data required to
describe the course of reaction can be obtained from NMR
spectroscopy, reactions can also be followed by GC/MS analysis.
While MS fragmentation patterns of rearranged oligosilanes are
usually indistinguishable from the starting material, the retention
behavior often changes and the method is suitable to detect
fragmentation/cyclization reactions. Isolated products are finally
subjected to standard characterization techniques such as NMR
and UV spectroscopy, elemental analysis or HRMS, and (if
possible) X-ray single-crystal diffraction analysis.
For our studies concerning the rearrangement of branched
cyclosilanes12 we used a cosublimate of AlCl3 and FeCl3 as
described by Blinka and West14 as the catalyst. Attempts to use
plain AlCl3 proved to give less reproducible results in comparison
to those for the cosublimate. However, the fact that in the original
papers by Ishikawa, Kumada, and co-workers7À10 only the use of
AlCl3 was reported prompted us to study the rearrangement of
some materials isomeric with their noncyclic oligosilane sub-
strates with the Al(Fe)Cl3 cosublimate catalyst.
Acyclic Methylated Oligosilanes. In the course of our studies
we found that in general Al(Fe)Cl3-catalyzed reactions of branched
oligosilanes which are isomeric with materials already studied by the
Kumada group7À10 led to the same results. However, a few reactions
showed an additional twist.
For example, the AlCl3-mediated rearrangement of n-eicosa-
methylnonasilane was reported to yield 2,2,5-tris(trimethylsilyl)
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undecamethylhexasilane (1) and 2,2,4,4-tetrakis(trimethylsilyl)-
octamethylpentasilane in a ratio of 4:1 in high yield.9 Explicitly it
was noted that no other isomers were detected by spectroscopic
analysis.9 In contrast to that, the rearrangement of 2,2,5-tris-
(trimethylsilyl)undecamethylhexasilane (1) with Al(Fe)Cl3 led
to the almost quantitative formation of 1,1,3-tris(trimethylsilyl)
heptamethylcyclopentasilane (2)10,12 and tetramethylsilane
(Scheme 1).
Similarly Ishikawa et al. reported that the rearrangement
of n-octadecamethyloctasilane gave a mixture of 2,2,4-tris-
(trimethylsilyl)nonamethylpentasilane (4) and 2,2,3,3-tetrakis-
(trimethylsilyl)hexamethyltetrasilane (5) in a ratio of 81:19.9
Our attempts to rearrange the isomeric compound 2,5-bis-
(trimethylsilyl)dodecamethylhexasilane (3) with Al(Fe)Cl3 re-
vealed initial formation of the two aforementioned compounds 4
and 5 in addition to 2,2-bis(trimethylsilyl)dodecamethylhexasilane
(6) and 1,1-bis(trimethylsilyl)octamethylcyclopentasilane (7).10,12
However, after a prolonged reaction time the cyclic compound
7 and tetramethylsilane became the major reaction products
(Scheme 2).
A related result was obtained when 2,2,9,9-tetrakis(trimethylsilyl)
octadecamethyldecasilane (8) was subjected to the rearrangement
conditions with Al(Fe)Cl3. After initial formation of 2,2,5,8,8-
pentakis(trimethylsilyl)pentadecamethylnonasilane (9) again
the occurrence of 1,1,3-tris(trimethylsilyl)pentamethylcyclo-
pentasilane (2) was observed, this time accompanied by 2,2-
bis(trimethylsilyl)octamethyltetrasilane9 (10) as an elimination
product (Scheme 3).
As this desilylative cyclization behavior was never mentioned
in the Ishikawa/Kumada group reports,7À10 we attribute it to
the enhanced reactivity of Al(Fe)Cl3 compared to that of
commercially available AlCl3. Mechanistically the reaction can
be interpreted as a migration/fragmentation reaction. We
assume that the cyclization occurs via a 1,5-silyl shift. In the
course of this shift the cationic charge is not retained in the
molecule, as in the usual rearrangement reaction, but is
eliminated as a trimethylsilylium ion which upon methylation
with [MeAlCl3]À is converted to tetramethylsilane. There
seems to be a steric component of this reaction, as it was not
possible to cyclize 2,2,5,5-tetrakis(trimethylsilyl)decamethyl-
hexasilane, which is likely sterically too hindered at the 5-posi-
tion to undergo a nucleophilic attack at the silylium ion
(Scheme 4).
Acyclic Methylated Oligosilanes Containing One Me3-
SiCH2 Group. In a recent study we investigated the conforma-
tional behavior of acyclic methylated oligosilanes containing
(trimethylsilyl)methyl groups.15 These compounds provided
us with the opportunity to study their rearrangement behavior.
Initial expectations to obtain homogeneous product distributions
were not too high, as it was demonstrated by Blinka and West
that the introduction of an ethyl group into an otherwise
methylated oligosilane led to a scrambling of ethyl groups upon
rearrangement to give non-, mono-, di-, tri-, and higher ethylated
products.14 We found the same behavior after introducing single
phenyl groups into methylated oligosilanes. However, after
preliminary results showed a clean rearrangement course, it
was decided to study the reactions of a series of branched
Scheme 4. Mechanistic Rationale for the Cyclization Behavior
Involving a 1,5-Silyl Migration
Scheme 5. Rearrangement Reactions of (Trimethylsilyl)methyl-Substituted Tri-, Tetra-, and Pentasilanesa
a 29Si NMR chemical shifts (ppm) of substrates and rearrangement products have been added to the structures.
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Scheme 6. Cyclization Reactions of (Trimethylsilyl)methyl-Substituted Hexa-, Hepta-, and Octasilanesa
a 29Si NMR chemical shifts (ppm) of substrates and cyclized products have been added to the structures.
Scheme 7. Mechanistic Rationale for the Cyclization Behavior of a (Trimethylsilyl)methyl-Substituted Hexasilane Involving a
1,5-Methylene Shift
compounds of the general composition Me3SiCH2(SiMe2)n
SiMe3, with n ranging from 2 to 8.
2-(trimethylsilyl)-1-[(trimethylsilyl)methyl)]hexamethyltrisilane
(14) (Scheme 5). Starting material 13 and product 14 both
contain isotetrasilane structures. Again a 1,2-shift of the
(trimethylsilyl)methyl group moved the more bulky substituent
from the branching point of the silane to a terminal position. Steric
interaction at the branching point is thus diminished. In a similar
way the next member of the series, 2-(trimethylsilyl)-2-
[(trimethylsilyl)methyl]octamethyltetrasilane (15), was converted
to 2,2-bis(trimethylsilyl)-3-[(trimethylsilyl)methyl]pentamethyl-
trisilane (16) (Scheme 5). The driving force behind this
The series starts with the linear trisilane 11, where the
(trimethylsilyl)methyl substituent is located in the 2-posi-
tion. The rearrangement involved a 1,2-shift of this group
to a terminal silicon atom, thus minimizing steric interac-
tion and affording 1-[(trimethylsilyl)methyl]heptamethyl-
trisilane (12) as the product (Scheme 5). The rearrange-
ment of the next higher homologue, 2-(trimethylsilyl)-2-[(tri-
methylsilyl)methyl]hexamethyltrisilane (13), proceeded to
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Scheme 8. Cyclization Reaction of a (Trimethylsilyl)methyl-Substituted Branched Nonasilane To Give an Intermediate
Monocyclic Molecule and Further To Give a Bicyclic Moleculea
a 29Si NMR chemical shifts (ppm) of substrates and cyclized products have been added to the structures.
Scheme 9. Cyclization Reaction of a Bis[(trimethylsilyl)methyl]-Substituted Branched Octasilane To Give a Bicyclic Moleculea
a 29Si NMR chemical shifts (ppm) of substrates and cyclized products have been added to the structures.
Scheme 10. Cyclization Reaction of a Bis[(trimethylsilyl)methyl]-Substituted Branched Nonasilane To Give a Bicyclic Moleculea
a 29Si NMR chemical shifts (ppm) of substrates and cyclized products have been added to the structures.
transformation is the typical branching mechanism which favors
the formation of neopentasilanyl units such as in 16.
All reactions depicted in Scheme 5 show the expected
rearrangement behavior, featuring a series of 1,2-shifts to the
most stable silylium ion, yielding the most stable branched
oligosilane isomers.
The next higher homologues 17À19 possess enough formal
dimethylsilylene units to exhibit a cyclization reaction behavior
(Scheme 6) similar to what was already outlined above for
compounds 1, 3, and 8. It is interesting to see that in compounds
21 and 22 the branching points are located in 1,2-relationships.
While a 1,3-disilyl substitution pattern would be sterically less
hindered, it would also be associated with a lower silylation
degree at the branching points.
For cyclooligosilane rearrangement reactions a general trend
favoring the formation of five-membered rings was observed.12
This trend was also found for the reactions of 17À19
(Scheme 6). In all cases carbacyclopentasilanes were formed.
The incorporated methylene group was always connected to
dimethylsilylene units and was never attached to a branching
point of the oligosilane backbone (Scheme 6).
It seems that the first step of these cyclization processes is a
rearrangement similar to what was observed for 11, 13, and 15.
The preferentially formed acyclic intermediate would feature the
(trimethylsilyl)methyl group at a terminal position. Such a
structure can then undergo a cyclization reaction in a way similar
to that pointed out above. The feasibility of this can be judged
from the fact that in none of the rearrangement reactions of
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17À19 could any intermediate be detected by NMR spectros-
copy, indicating that once the reaction started, it would invariably
lead to the desilylative cyclization. Scheme 7 depicts the assumed
cyclization path for compound 17.
second series of substrates containing two of these substituents
was investigated. Rearrangement of the first two compounds of
this series, (Me3Si)2Si(CH2SiMe3)2 (26) and [(Me3Si)2Si(CH2-
SiMe3)]2 (27), did not give clean reactions. In contrast to this,
the next higher homologue, 28, gave a single product. Elimina-
tion of 2 equiv of tetramethylsilane led to the bicyclic compound
3,7-dicarba-1,5-bis(trimethylsilyl)octamethylbicyclo[3.3.0]octasilane
(29) (Scheme 9).
Increasing the number of dimethylsilylene units by one unit
led to a reaction which was less clean but still formed one major
product that could be isolated and characterized. The structure of
the resulting compound is related to 29 with the additional SiMe2
unit inserted into the central SiÀSi bond, thus representing 3,7-
dicarba-1,5-bis(trimethylsilyl)decamethylbicyclo[3.3.1]nonasilane
(31) (Scheme 10).
Rearrangement of 2,9-bis(trimethylsilyl)-2,9-bis[(trimethylsilyl)-
methyl]octadecamethyldecasilane (32), where in comparison to 30
the number of dimethylsilylene units was increased by another three
units, was also not clean.
Cyclic Methylated Oligosilanes Containing Me3SiCH2 Sub-
stituents. Considering the enhanced cyclization potential of
compounds containing (trimethylsilyl)methyl groups, we as-
sumed that cyclic oligosilanes with this substituent should exhibit
a pronounced tendency to form bicyclic compounds under
rearrangement conditions. The reaction of the easily available
cyclohexasilane 33 confirmed this assumption, and 3-carba-1,5-
bis(trimethylsilyl)decamethylbicyclo[3.2.1]octasilane (34) was
obtained (Scheme 11).
The next higher homologue, 23, was expected to follow a similar
path. Spectroscopic evidence suggested initially a related cycliza-
tion. Supposedly a carbacyclohexasilane was formed to avoid
attachment of the methylene group to a disilylated silicon atom.
In addition to this product the formation of another compound
was detected right from the beginning of the reaction, which
eventually became the only product. After isolation, structural and
spectroscopic characterization this product was found to be
3-carba-1-(trimethylsilyl)undecamethylbicyclo[3.2.1]nonasilane
(25), which was formed by elimination of another 1 equiv of
tetramethylsilane (Scheme 8). It was surprising to find 25 as the
final product, as we previously recognized the formation of
tetrasilylated silicon atoms to be a major driving force. Whereas
compound 24 possesses two of these structural elements, only
one of them is found in the structure of 25. An explanation for
this apparent problem would be that, while the outcome of most of
the rearrangements is governed by the thermodynamic stability of
the formed cations, the elimination of tetramethylsilane would
introduce a nonreversible kinetic component to the reaction.
Acyclic Methylated Oligosilanes with Two Me3SiCH2
Groups. After the investigation of the rearrangement behavior
of oligosilanes containing one (trimethylsilyl)methyl group a
Scheme 11. Cyclization Reaction of a (Trimethylsilyl)methyl-
Substituted Tris(trimethylsilyl)cyclohexasilane To Give a
Bicyclic Moleculea
Also, the rearrangement of a mixture of cis- and trans-35
followed a selective pathway leading to the already known
bicyclic compound 31 (see above) (Scheme 12).
The rearrangement of 36, which contains three (trimethyl-
silyl)methyl substituents, led cleanly to the formation of 29
(Scheme 13). This was unexpected, for it was assumed that the
product would contain three methylene units corresponding to
the three (trimethylsilyl)methyl groups of the starting material.
However, different from most of the reactions described so far,
the elimination of tetramethylsilane was not observed but rather
that of bis(trimethylsilyl)methane. The course of reaction is
therefore similar to what was observed with compound 8, where
also a more complex silylium ion was eliminated.
Finally, the reactions of 1,1,4,4-tetrakis[(trimethylsilyl)me-
thyl]octamethylcyclohexasilane (37) and the bicyclic compound
1-(trimethylsilyl)-4-[(trimethylsilyl)methyl]dodecamethylbicy-
clo[2.2.2]octasilane (38) did not give clean conversions to single
products.
a 29Si NMR chemical shifts (ppm) of substrates and cyclized products
have been added to the structures.
Scheme 12. Cyclization Reaction of an Isomeric Mixture of 1,4-Bis[(trimethylsilyl)methyl]-Substituted 1,4-Bis(trimethylsilyl)-
cyclohexasilanes To Give a Bicyclic Molecule with Two Methylene Bridgesa
a 29Si NMR chemical shifts (ppm) of substrates and cyclized products have been added to the structures.
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Scheme 13. Cyclization Reaction of a Tris[(trimethylsilyl)-
methyl]-Substituted (Trimethylsilyl)cyclohexasilane To Give
a Bicyclic Molecule with Two Methylene Bridgesa
a 29Si NMR chemical shifts (ppm) of substrates and cyclized products
have been added to the structures.
Figure 2. Molecular structure and numbering of 35-trans. Selected
bond lengths (Å) and bond angles (deg) with esd's: Si(1)ÀSi(2) =
2.3484(18), Si(1)ÀSi(3) = 2.3658(13), Si(2)ÀSi(3A) = 2.3541(15),
Si(3)ÀSi(2A) = 2.3541(15), Si(5)ÀC(1) = 1.860(3); Si(2)ÀSi(1)ÀSi-
(3) = 107.03(6), Si(1)ÀSi(2)ÀSi(3A) = 111.58(6), Si(2A)ÀSi(3)ÀSi-
(1) = 111.45(5).
Figure 1. Molecular structure and numbering of 33. Selected bond
lengths (Å) and bond angles (deg) with esd's: Si(1)ÀC(12) = 1.913(2),
Si(1)ÀSi(7) = 2.3572(9), Si(1)ÀSi(6) = 2.3640(10), Si(1)ÀSi(2) =
2.3683(9), Si(2)ÀSi(3) = 2.3443(10), Si(8)ÀC(12) = 1.877(2); Si-
(6)ÀSi(1)ÀSi(2) = 110.83(3), Si(3)ÀSi(2)ÀSi(1) = 111.29(3), Si-
(2)ÀSi(3)ÀSi(4) = 112.29(3), Si(3)ÀSi(4)ÀSi(5) = 110.95(3),
Si(6)ÀSi(5)ÀSi(4) = 112.68(3), Si(5)ÀSi(6)ÀSi(1) = 112.94(3).
Figure 3. Molecular structure and numbering of 37. Selected bond
lengths (Å) and bond angles (deg) with esd's: Si(1)ÀC(2) = 1.889(3),
Si(1)ÀC(1) = 1.897(3), Si(1)ÀSi(3A) = 2.3570(11), Si(1)ÀSi(2) =
2.3592(12), Si(2)ÀSi(3) = 2.3495(11), Si(3)ÀSi(1A) = 2.3571(11);
Si(3A)ÀSi(1)ÀSi(2) = 105.83(4), Si(3)ÀSi(2)ÀSi(1) = 119.18(4),
Si(2)ÀSi(3)ÀSi(1A) = 109.49(5).
X-ray Crystallography. The cyclohexasilanes 33, 35-trans,
and 37 as well as the bicyclic compounds 25, 29, 31, 34, and 38
could be subjected to crystal structure analyses.
The molecular structure of 33 (Figure 1), which crystallizes
in the monoclinic space group C2/c, resembles that obtained
for 1,1,4,4-tetrakis(trimethylsilyl)octamethylcyclohexasilane.16
Both structures are rare cases of six-membered rings in a stable
twist conformation. Both 35-trans (Figure 2) and 37 (Figure 3)
crystallize in the monoclinic space group P21/c, and their six-
membered rings show the expected chair conformation. SiÀSi
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Figure 4. Molecular structure and numbering of 38. Selected bond
lengths (Å) and bond angles (deg) with esd's: Si(1)ÀC(16) = 1.909(3),
Si(1)ÀSi(7) = 2.3537(10), Si(1)ÀSi(6) = 2.3583(11), Si(1)ÀSi(2) =
2.3599(10), Si(2)ÀSi(3) = 2.3767(10), Si(3)ÀSi(4) = 2.3373(10), Si-
(4)ÀSi(5) = 2.3367(10), Si(4)ÀSi(8) = 2.3422(9), Si(10)ÀC(16) =
1.855(3); Si(7)ÀSi(1)ÀSi(6) = 107.89(4), Si(7)ÀSi(1)ÀSi(2) =
106.26(3), Si(6)ÀSi(1)ÀSi(2) = 106.75(3), Si(1)ÀSi(2)ÀSi(3) =
108.17(4), Si(4)ÀSi(3)ÀSi(2) = 111.87(4), Si(5)ÀSi(4)ÀSi(3) =
106.81(4), Si(5)ÀSi(4)ÀSi(8) = 108.03(4), Si(3)ÀSi(4)ÀSi(8) =
107.59(4), Si(4)ÀSi(5)ÀSi(6) = 107.12(4), Si(5)ÀSi(6)ÀSi(1) =
113.13(3), Si(1)ÀSi(7)ÀSi(8) = 108.45(4), Si(4)ÀSi(8)ÀSi(7) =
110.81(4).
Figure 6. Molecular structure and numbering of 34. Selected bond
lengths (Å) and bond angles (deg) with esd's: Si(1)ÀSi(5) =
2.3372(10), Si(1)ÀSi(2) = 2.3389(11), Si(2)ÀSi(7) = 2.3459(12),
Si(2)ÀSi(3) = 2.3538(11), Si(3)ÀSi(4) = 2.3722(11), Si(4)ÀSi(5) =
2.3498(10), Si(5)ÀSi(6) = 2.3455(12), Si(6)ÀC(9) = 1.882(2), Si-
(7)ÀC(9) = 1.877(2); Si(5)ÀSi(1)ÀSi(2) = 100.51(4), Si(1)ÀSi-
(2)ÀSi(7) = 106.82(4), Si(1)ÀSi(2)ÀSi(3) = 100.35(4), Si(7)À
Si(2)ÀSi(3z) = 108.11(4), Si(2)ÀSi(3)ÀSi(4) = 104.73(4), Si(5)À
Si(4)ÀSi(3) = 105.42(4), Si(1)ÀSi(5)ÀSi(9) = 114.10(4), Si(1)À
Si(5)ÀSi(6) = 107.20(4), Si(1)ÀSi(5)ÀSi(4) = 100.21(4), Si(6)À
Si(5)ÀSi(4) = 107.28(4), Si(7)ÀC(9)ÀSi(6) = 121.32(13).
Figure 5. Molecular structure and numbering of 25. Selected bond
lengths (Å0 and bond angles (deg) with esd's: Si(1)ÀSi(5) = 2.344(3),
Si(1)ÀSi(7) = 2.346(3), Si(1)ÀSi(2) = 2.354(3), Si(2)ÀSi(3) =
2.356(3), Si(3)ÀSi(4) = 2.355(3), Si(4)ÀSi(5) = 2.336(3), Si(4)ÀSi-
(6) = 2.346(3), Si(6)ÀC(10) = 1.865(6), Si(7)ÀC(10) = 1.892(6);
Si(5)ÀSi(1)ÀSi(7) = 107.91(10), Si(5)ÀSi(1)ÀSi(2) = 101.05(10),
Si(7)ÀSi(1)ÀSi(2) = 108.35(10), Si(1)ÀSi(2)ÀSi(3) = 103.96(10),
Si(4)ÀSi(3)ÀSi(2) = 105.08(10), Si(5)ÀSi(4)ÀSi(6) = 108.70(10),
Si(5)ÀSi(4)ÀSi(3) = 100.94(10), Si(6)ÀSi(4)ÀSi(3) = 108.54(10),
Si(4)ÀSi(5)ÀSi(1) = 98.18(10), Si(6)ÀC(10)ÀSi(7) = 121.0(3).
Figure 7. Molecular structure and numbering of 29. Selected bond
lengths (Å) and bond angles (deg) with esd's: Si(1)ÀC(3) = 1.876(3),
Si(1)ÀSi(2) = 2.3625(10), Si(2)ÀSi(6) = 2.3508(12), Si(2)ÀSi(3) =
2.3719(11), Si(3)ÀSi(4) = 2.3573(11), Si(3)ÀSi(7) = 2.3653(11),
Si(4)ÀC(3) = 1.884(3), Si(6)ÀC(11) = 1.881(3); C(3)ÀSi(1)À
C(2) = 111.55(12), C(3)ÀSi(1)ÀC(1) = 109.04(12), C(3)ÀSi(1)À
Si(2) = 105.22(9), Si(6)ÀSi(2)ÀSi(1) = 111.95(5), Si(6)ÀSi(2)À
Si(3) = 98.03(4), Si(1)ÀSi(2)ÀSi(3) = 99.89(4), Si(4)ÀSi(3)ÀSi(7) =
111.32(4), Si(4)ÀSi(3)ÀSi(2) = 98.11(4), Si(7)ÀSi(3)ÀSi(2) =
100.02(4), Si(1)ÀC(3)ÀSi(4) = 113.21(13), Si(6)ÀC(11)ÀSi(7) =
113.09(13).
bond distances in all three cyclohexasilanes are in the normal
range between 2.35 and 2.37 Å, and the same is true for the SiÀC
distances, except for that between the ring silicon atoms and the
attached CH2 groups in 33 and 35-trans. These distances are
somewhat elongated to a value of 1.91 Å, whereas they are
around 1.89 Å for 37. The two CH2ÀSiMe3 groups of 35-trans
do not adopt the expected equatorial but rather axial positions, as
was reported for similar compounds in the literature.17
Compound 38 (Figure 4) crystallizes in the monoclinic space
group P21/c, whereas the parent compound without the CH2
spacer was found to crystallize in a tetragonal space group due to
the high degree of symmetry.18 The molecule consists of a
regular [2.2.2]cyclooctasilane framework with all SiÀSi bond
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lengths within a range of 2.34À2.36 Å and all SiÀSiÀSi bond
angles close to the ideal tetrahedral angle (106À113°). The
distance between the silicon atoms and the CH2 spacer is again at
1.91 Å longer than all the other SiÀC distances (1.89 Å).
The [3.2.1]-3-carba-cyclooctasilane compounds 25 (Figure 5)
and 34 (Figure 6) both crystallize in the triclinic space group P1.
For both structures no elongation of the bonds between the Si
atoms and the CH2 spacer was observed, but in the case of 34 the
exo-methyl groups next to the spacer are somewhat longer
(Si(6)ÀC(7) and Si(7)ÀC(10) both 1.90 Å). In contrast to
this, SiÀSi bond lengths are all fairly short, in the ranges of
2.34À2.37 Å for 34 and 2.34À2.35 Å for 25.
Compound 29 (Figure 7) also crystallizes in the triclinic space
group P1. All SiÀSi bonds, even the central one, Si(2)ÀSi(3), are
in the range of regular bond lengths (2.34À2.37 Å). The
previously studied all-silicon compound 1,5-bis(trimethylsilyl)
dodecamethylbicyclo[3.3.0]octasilane19 exhibited a cisoid con-
figuration of the two rings with a torsion angle of the two SiMe3
substituents along the bridge of 40.3°.19 In 29 the replacement of
two SiMe2 units by CH2 led to diminished strain, resulting in a
torsion angle of 14.5°.
The parent all-silicon compound of 31 (Figure 8), hexadeca-
methylbicyclo[3.3.1]nonasilane, was found to crystallize in the
monoclinic space group P21/c with two molecules in the asym-
metric unit,20 while 31 crystallized in the triclinic space group P1.
In the all-silicon compound the two six-membered rings featured
a chair and a twist conformation. The replacement of two SiMe2
units by CH2 also changed this system, in which both rings have
perfect chair conformations.
1
NMR Spectroscopy. H, 13C, and 29Si NMR spectra of the
compounds described in this study were measured. To some
extent the spectroscopic characteristics of oligosilanes with
attached (trimethylsilyl)methyl units have already been dis-
cussed recently.15 As 29Si NMR spectra are the most significant
for structural assignments of oligosilanes, a short discussion
seems appropriate. Especially cyclosilanes with endocyclic
methylene groups were found to exhibit some interesting
behavior. For compound 20 (Chart 1), with two SiMe2 groups
attached to the CH2 unit, we would expect their 29Si resonances
around À9.0 ppm, which is typical for a trimethylsilyl group
attached to a silicon atom. In fact, we find indeed one resonance
(À9.0 ppm) (for the SiMe2 group attached to another SiMe2) in
the expected region, whereas the other resonance (which is
connected to a quaternary silicon atom) is shifted downfield to
+0.6 ppm (Chart 1). For compound 21 the situation is similar,
Figure 8. Molecular structure and numbering of 31. Selected bond
lengths (Å) and bond angles (deg) with esd's: Si(1)ÀC(10) = 1.884(3),
Si(1)ÀSi(2) = 2.3552(11), Si(2)ÀSi(7) = 2.3404(12), Si(2)ÀSi(3) =
2.3534(11), Si(3)ÀC(5) = 1.882(3), Si(4)ÀSi(5) = 2.3522(11), Si-
(5)ÀSi(7) = 2.3382(13), Si(5)ÀSi(6) = 2.3556(11), Si(6)ÀC(10) =
1.877(3); Si(7)ÀSi(2)ÀSi(3) = 106.86(5), Si(7)ÀSi(2)ÀSi(1) =
106.22(4), Si(3)ÀSi(2)ÀSi(1) = 109.43(5), Si(7)ÀSi(5)ÀSi(4) =
106.51(4), Si(8)ÀSi(5)ÀSi(4) = 111.15(5), Si(7)ÀSi(5)ÀSi(6) =
106.79(5), Si(4)ÀSi(5)ÀSi(6) = 109.75(4), Si(5)ÀSi(7)ÀSi(2) =
104.12(5), Si(4)ÀC(5)ÀSi(3) = 122.29(14), Si(6)ÀC(10)ÀSi(1) =
123.51(14).
Chart 1. 29Si NMR Chemical Shifts (ppm) of Selected Compoundsa
aData used in the discussion are shown in boldface type.
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Figure 9. SEM images of the catalyst particles: (left) backscattered electron image, image width 1.2 mm; (right) secondary electron image, image
width 11.5 μm.
of the products was a frequently occurring side reaction. For this
reason they recommended treatment of the reaction mixture
with methyl Grignard reagent in order to obtain permethylated
products. Large catalyst amounts, which are common if the
reaction does not start properly, were also reported to lead to
polymerization or decomposition. Blinka and West14 pointed out
two types of catalysts that worked in their hands: either sublimed
technical AlCl3 (with a specified FeCl3 content of 0.1%) or a
cosublimate of pure AlCl3 and 1% FeCl3. They further noticed
that pure sublimed AlCl3, nonsublimed technical AlCl3, and a
mixture of pure AlCl3 and 1% FeCl3 (without prior cosublima-
tion) did not show catalytic activity.14
For these reasons it seemed a worthwhile goal to establish
conditions which allow a reliable start of the reaction with rea-
sonably small amounts of catalyst. To find these optimized
conditions, we used the known rearrangement reaction of
Figure 10. EDX spectrum for the left-hand image in Figure 9.
dodecamethylcyclohexasilane to (trimethylsilyl)nonamethyl-
cyclopentasilane10,14 as a test system. Cosublimates of AlCl3
and FeCl3 were prepared with 1%, 3%, 5%, and 10% FeCl3
content. The 1% FeCl3 product had a pale yellow color, whereas
the other products were bright yellow. The almost identical
colors and the fact that the 5% and 10% batches led to a larger
sublimation residue (of FeCl3) seemed to indicate that these
samples have a similar stoichiometry. This was supported by the
fact that there was no marked difference in the reactivity of these
two samples. The minimum catalyst amount to achieve complete
conversion was for all samples between 0.03 and 0.09 mol equiv.
Experiments to study the activity of the catalyst were conducted
in dichloromethane at room temperature. This solvent was found
useful for an easy rearrangement of substrates but occasionally
led to chlorination side reactions at longer reaction times. For
difficult cases, therefore, cyclohexane was the preferred solvent,
even though it requires elevated temperatures.
As additional alternative catalysts a cosublimate of AlBr3 and
FeBr3 and samples of technical AlBr3 and Al(OTf)3 were studied.
The AlBr3/FeBr3 cosublimate was of equal value to the AlCl3/
FeCl3 system in dichloromethane with respect to both minimum
catalyst amount and reaction time, whereas technical AlBr3
required the use of at least 0.15 mol equiv for complete
conversion. However, the visible inhomogeneity of this material
made conclusive statements difficult. Finally, Al(OTf)3 did not
show any activity as a catalyst at all. In conclusion, the system
reported by Blinka and West14 was found to be the best one.
To find a mechanistic rationale for the requirement of the
FeCl3 cosublimation procedure, we were interested in the type of
but in this case the first SiMe2 (now attached to a tertiary Si
atom) resonance is now observed at À3.3 ppm while the second
resonance is further shifted to +2.5 ppm. For the symmetrical
compound 22 both SiMe2 groups (attached to quaternary Si
atoms) were found to resonate at +0.6 ppm.
Analysis of the SiMe2CH2 resonances of acyclic compounds
12 (À15.1 ppm) and 16 (À8.1 ppm) showed also that a higher
silylation degree of the attached silicon atom causes a downfield
shift (Chart 1). However, the extent of this shift seems also to be
connected to some type of steric strain, which is particularly
effective in the cyclopentasilanyl unit. This is seen by a compar-
ison of the bicyclic compounds 29, 31, and 34. While the last two
(31, 34) exhibit SiMe2CH2 resonances in the expected range
around À8.0 ppm, for 29, which has structural features resem-
bling those of 20À22, again a marked downfield shift to +2.9
ppm was observed (Chart 1).
The influence of ring strain on SiMe2 groups of homocyclic
polysilanes can be seen also from a comparison of 1,1,3,3-
tetrakis(trimethylsilyl)hexamethylcyclopentasilane14,21 (SiMe2SiMe2
À24.8 ppm) and 1,1,4,4-tetrakis(trimethylsilyl)octamethylcyclo-
hexasilane6 (SiMe2SiMe2 À40.0 ppm).
Catalyst. It is somewhat peculiar that some of our results are
different from the findings of Ishikawa, Kumada, and co-
workers.7À10 The additional cyclization reactivity observed by
us is most likely caused by the modified catalyst. While Ishikawa
et al. reported the use of commercially available AlCl3, they also
noticed that on use of larger amounts of the catalyst chlorination
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incorporation of the FeCl3 into the AlCl3 lattice. As it is not clear
in what way this incorporation occurs, we decided to study the
material by X-ray powder diffraction, electron microscopy (SEM,
EDXS), and atomic emission spectroscopy (ICP-OES). The
powder diffraction measurement of a sample prepared by
cosublimation of AlCl3 and 1% FeCl3 did not indicate any
presence of FeCl3 at all and gave a pattern indistinguishable
from that of pure AlCl3. The virtually identical cell constants thus
suggested a very low concentration of FeCl3, which likely is
incorporated in a very regular way into the lattice.
nature of these compounds restricts the conformational flexibil-
ity of the polysilane skeleton, making them interesting building
blocks for the construction of longer chains with all-transoid
conformation.22À27
Investigations concerning the composition of the catalyst
system indicated that the cosublimation of AlCl3 even with
10% FeCl3 leads to the incorporation of only about 0.1% FeCl3
into the AlCl3 lattice.
’ EXPERIMENTAL SECTION
For the SEM measurements two samples were chosen. The
first one was a cosublimate of AlCl3 and 3% FeCl3, which was
heated in refluxing cyclohexane for several hours in order to
mimic the rearrangement conditions. As the EDXS measure-
ments of this sample did not indicate the presence of any iron
(see the Supporting Information for details), we tried to increase
the FeCl3 content by preparing a sample by cosublimation of
AlCl3 and 10% FeCl3. The SEM images of Figure 9 show the size
and shape of the particles of this sample. Rod-shaped particles
with a length of up to several 100 μm and spherical particles with
diameters of approximately 1 μm can be seen. EDXS was again
used for the chemical analysis of the particles and to detect
whether any strong accumulation of iron somewhere at the
particles or at specific particles could be found. The detection
limit of EDXS is around 0.1 wt %, the lateral resolution is around
1 μm, and the analysis depth is also around 1 μm. Thus, in the
case of the big particles the EDXS analysis definitely corresponds
to a surface analysis. No iron could be detected, either in an
overall analysis or in the analysis of individual particles (see
Figure 10). Thus, the iron concentration in the catalyst particles
was, despite the high initial concentration of 10% FeCl3, below
the detection limit of EDXS. ICP-OES analysis eventually proved
that the iron concentration in the sample was close to 0.1 wt %.
When the results from EDXS and ICP-OES are combined, it
follows that the iron is homogenously distributed both across the
individual particles and also across the whole sample. No local
accumulation of iron could be detected. This was also confirmed
by the backscattered electron images (see Figure 10 left), where the
contrast increases with increasing mean atomic number and there-
fore iron-enriched areas should appear much brighter than the rest
of the surface. The small contrast variations observed in the images
proved to be only due to variations in the particle topography.
Quantitative evaluation of the spectra showed that in case of the
large particles the atom ratio Al:Cl corresponds within measurement
accuracy to AlCl3. Additionally some oxygen could be detected.
Much higher oxygen concentrations were found in the small
spherical particles. Possibly, despite great care during the transfer
of the samples, air admission could not be completely avoided.
General Remarks. All reactions involving air-sensitive compounds
were carried out under an atmosphere of dry nitrogen or argon using
either Schlenk techniques or a glovebox. Solvents were dried using a
column solvent purification system. All chemicals bought from different
suppliers were used without further purification.
1H (300 MHz), 13C (75.4 MHz), and 29Si (59.3 MHz) NMR spectra
were recorded on a Varian INOVA 300 spectrometer. If not noted
otherwise, C6D6 was used for all samples or, in the case of reaction
samples, they were measured with a D2O capillary in order to provide an
external lock frequency signal. To compensate for the low isotopic
abundance of 29Si, the INEPT pulse sequence was used for the am-
plification of the signal.28,29 Elementary analysis was carried out using a
Heraeus Vario Elementar instrument . EI high-resolution mass spectra
(HRMS) were obtained using a Waters GCT premier instrument.
2,5-Bis(trimethylsilyl)dodecamethylhexasilane (3),30 2,2,3,3-tetrakis
(trimethylsilyl)hexamethyltetrasilane (5),3,31 2,2,9,9-tetrakis(trimethy-
lsilyl)octadecamethyldecasilane (8),32 2-(trimethylsilyl)-2-[(trimethyl-
silyl)methyl]hexamethyltrisilane (13),15 2-trimethylsilyl-2-[(trimethyl-
silyl)methyl]octamethyltetrasilane (15),15 2,2,3-tetrakis(trimethylsilyl)-
3-[(trimethylsilyl)methyl]hexamethyltetrasilane (18),15 2,2,5,tris(trim-
ethylsilyl)-5-[(trimethylsilyl)methyl]decamethylhexasilane (23),15 2,2-
bis[(trimethylsilyl)methyl]hexamethyltrisilane (26),15 2,3-bis(trimeth-
ylsilyl)-2,3-bis[(trimethylsilyl)methyl]hexamethyltetrasilane (27),15
2,5-bis(trimethylsilyl)-2,5-bis[(trimethylsilyl)methyl]decamethylhexa-
silane (28),15 2,6-bis(trimethylsilyl)-2,6-bis[(trimethylsilyl)methyl]do-
decamethylheptasilane (30),15 2,9-bis(trimethylsilyl)-2,9-bis[(trime-
thylsilyl)methyl]octadecamethyldecasilane (32),15 2,2,5,5-tetrakis(tri-
methylsilyl)decamethylhexasilane,21,33,34 2,2,3-tris(trimethylsilyl)hep-
tamethyltetrasilane,35 2,2,4,4-tetrakis(trimethylsilyl)octamethylpentasi-
lanes,3 pentamethylchlorodisilane,36 1,1,4,4-tetrakis(trimethylsilyl)oc-
tamethylcyclohexasilane,6 and 1,4-bis(trimethylsilyl)dodecamethylbicy-
clo[2.2.2]octasilane6 were prepared by employing published procedures.
X-ray Structure Determinations. For X-ray structure analyses
the crystals were mounted onto the tip of glass fibers, and data collection
was performed with a Bruker-AXS SMART APEX CCD diffractometer
using graphite-monochromated Mo KR radiation (0.710 73 Å). The
2
data were reduced to Fo and corrected for absorption effects with
SAINT37 and SADABS,38 respectively. The structures were solved by
direct methods and refined by full-matrix least-squares methods
(SHELXL97).39 If not noted otherwise, all non-hydrogen atoms were
refined with anisotropic displacement parameters. All hydrogen atoms
were located in calculated positions to correspond to standard bond
lengths and angles. All diagrams were drawn with 30% probability
thermal ellipsoids, and all hydrogen atoms were omitted for clarity.
Unfortunately, the obtained crystal quality of some substances was poor.
This fact is reflected by quite high R and low θ values. Crystallographic
data for compounds 25, 29, 31, and 33 are given in Table 1 and for
compounds 34, 35-trans, 37, and 38 in Table 2.
’ CONCLUSION
On investigation of some rearrangement reactions of
branched oligosilanes with catalytic amounts of Al(Fe)Cl3, a
novel cyclization reaction was found in addition to the already
known rearrangement behavior.7À12,14 The introduction of
(trimethylsilyl)methyl substituents into oligosilanes strongly
facilitates the rearrangement/cyclization behavior leading to
rearranged branched oligosilanes and in the case of a sufficient
number of silicon atoms to carbacyclosilanes with endocyclic
methylene units. Depending on the number of (trimethyl-
silyl)methyl groups and the structure of the starting material,
either mono- or bicyclic compounds were obtained. The bicyclic
Crystallographic data (excluding structure factors) for the structures
of compounds 25, 29, 31, 33, 34, 35-trans, 37, and 38 reported in this
paper have been deposited with the Cambridge Crystallographic Data
Center as Supplementary Publication Nos. CCDC-720006 (33),
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Table 1. Crystallographic Data for Compounds 25, 29, 31, and 33
25
29
31
33
empirical formula
Mw
Si8C15H44
449.22
Si8C16H46
463.25
Si9C18H52
541.40
Si10C21H62
595.61
temp (K)
size (mm)
cryst syst
space group
a (Å)
100(2)
100(2)
100(2)
100(2)
0.30 Â 0.22 Â 0.20
triclinic
0.34 Â 0.28 Â 0.20
triclinic
0.32 Â 0.22 Â 0.18
triclinic
0.35 Â 0.30 Â 0.10
monoclinic
P1
P1
P1
C2/c
9.793(2)
9.6431(19)
10.100(2)
15.544(3)
75.42(3)
9.3855(19)
11.547(2)
16.262(3)
71.75(3)
16.278(3)
b (Å)
9.793(2)
10.248(2)
c (Å)
15.025(3)
89.15(3)
46.368(9)
R (deg)
β (deg)
γ (deg)
V (Å3)
90
80.67(3)
87.67(3)
80.80(3)
94.92(3)
88.36(3)
78.90(3)
73.88(3)
90
1421(2)
1438(2)
1603(2)
7706(3)
Z
2
2
2
8
F
calcd (g cmÀ3
)
1.050
1.070
1.080
1.027
abs coeff (mmÀ1
)
0.377
0.375
0.378
0.351
F(000)
492
508
572
2624
θ range (deg)
2.08 < θ < 25.00
10 251/4975
99.2
1.17 < θ < 24.00
11 183/5711
97.3
1.32 < θ < 26.36
12 830/6442
98.5
1.76 < θ < 26.37
30 040/7837
99.7
no. of collected/unique rflns
completeness to θ (%)
no. of data/restraints/params
goodness of fit on F2
4975/0/222
0.97
5711/0/231
1.09
6442/0/260
1.10
7837/0/300
1.16
final R indices (I > 2σ(I))
R1 = 0.080, wR2 = 0.133
R1 = 0.170, wR2 = 0.166
0.57/-0.42
R1 = 0.049, wR2 = 0.132
R1 = 0.052, wR2 = 0.135
0.73/-0.43
R1 = 0.054, wR2 = 0.131
R1 = 0.061, wR2 = 0.136
0.59/-0.35
R1 = 0.043, wR2 = 0.098
R1 = 0.050, wR2 = 0.100
0.60/-0.22
R indices (all data)
largest diff peak/hole (e/Å3)
720005 (35-trans), 720009 (37), 720007 (38), 720003 (34), 720002
(31), 720004 (29), and 720008 (25). Copies of the data can be obtained
free of charge at http://www.ccdc.cam.ac.uk/products/csd/request/.
SEM/EDXS: Analysis of the Catalyst. The catalyst particles from
the cosublimate of AlCl3 and FeCl3 with 5% FeCl3 were investigated by
energy dispersive X-ray spectrometry (EDXS: Noran Voyager, Thermo
Fisher Scientific Inc., Waltham, MA, USA) in the scanning electron
microscope (SEM: Leo DSM 982 Gemini, Carl Zeiss SMT Inc.,
Oberkochen, Germany). The samples were mounted in the glovebox,
where they had also been synthesized, on an adhesive attached to a
copper plate. Subsequently they were transferred in an airtight specimen
holder to a carbon coater, where they were coated with a thin carbon
layer (thickness ∼20 nm) and finally transferred again in the airtight
specimen holder to the SEM. Both the carbon coater and the SEM are
equipped with purpose-built airlocks to avoid contact between sample
and air as far as possible. The coating of the samples with a thin
conductive carbon layer is necessary to avoid charging of the electrically
insulating specimens during electron irradiation in the SEM.
General Procedures for the Preparation of Silyl Anions.
Method A for Monoanions in THF. The corresponding oligosilane together
with KOtBu (1À1.05 equiv) was dissolved in a minimum amount of THF.
The completeness of the conversion could be confirmed by 29Si NMR
spectroscopy and was usually achieved within a few hours.
Method B for Dianions in THF. The corresponding oligosilane
together with KOtBu (2À2.05 equiv) was dissolved in a minimum
amount of THF. The completeness of the conversion could be
confirmed by 29Si NMR spectroscopy and could require, depending
on the starting material, some days. Heating to 60 °C allows to increase
the reaction rate.
equiv) was dissolved in a minimum amount of benzene. The complete-
ness of the conversion could be confirmed by 29Si NMR spectroscopy
and should be achieved within 1 h.
Method D for Dianions in Benzene. The corresponding oligosilane
together with KOtBu (2 to 2.05 equiv) and 18-crown-6 (2À2.05 equiv)
was dissolved in a minimum amount of benzene. The completeness of
conversion could be confirmed by 29Si NMR spectroscopy and was
usually complete within 18 h.
For the derivatization of the obtained anions, the obtained solutions
were treated with an electrophile and after complete conversion sub-
jected to an aqueous workup, which included washing with dilute H2SO4
and saturated NaHCO3 solution. Extraction of the product with pentane
was followed by drying over Na2SO4 and removal of the solvents.
Synthesis of Starting Materials. 2,2,5-Tris(trimethylsilyl)unde-
camethylhexasilane (1). Compound 1 was obtained from 2,2,5,5-
tetrakis(trimethylsilyl)decamethylhexasilane (395 mg, 0.65 mmol) and
KOtBu (72 mg, 0.64 mmol) using method A and dimethyl sulfate (61
μL, 0.64 mmol). Colorless crystals (350 mg, 98%) were obtained from
pentane (mp 68À71 °C). NMR data (δin ppm): 29Si, À9.8, À12.1, À31.9,
À33.4, À81.3, À129.1; 1H, 0.47 (s, 6H), 0.42 (s, 6H), 0.31 (s, 27H), 0.24
(s, 18H), 0.23 (s, 3H); 13C, 3.5, 1.1, 1.0, À1.1, À11.1. Anal. Calcd for
C20H60Si9 (553.46): C, 43.40; H, 10.93. Found: C, 43.26; H, 10.91.
2-[(Trimethylsilyl)methyl]heptamethyltrisilane (11). Compound 11
was obtained from tris(trimethylsilyl)methylsilane (465 mg, 1.77 mmol)
and KOtBu (219 mg, 1.95 mmol) using method A and (trimethylsilyl)
methyl chloride (224 mg, 1.82 mmol), yielding a colorless oil
1
(468 mg, 96%). NMR data (δ in ppm): 29Si, 1.7, À16.3, À47.3; H,
0.21 (s, 3H), 0.15 (s, 18H), 0.09 (s, 9H), À0.17 (s, 2H); 13C, 1.7, À1.0,
À3.8, À6.0. Anal. Calcd for C11H32Si4 (276.72): C, 47.75; H, 11.66.
Found: 46.67; H, 10.96. HRMS for C11H32Si4: m/z calcd 276.1581,
found 276.1595.
Method C for Monoanions in Benzene. The corresponding oligosi-
lane together with KOtBu (1À1.05 equiv) and 18-crown-6 (1À1.05
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Table 2. Crystallographic Data for Compounds 34, 35-trans, 37, and 38
34
35-trans
37
38
empirical formula
Mw
Si9C17H50
507.38
Si10C22H64
Si10C24H68
637.68
Si10C19H56
565.54
609.63
temp (K)
size (mm)
cryst syst
space group
a (Å)
150(2)
100(2)
100(2)
100(2)
0.38 Â 0.25 Â 0.20
triclinic
0.33 Â 0.28 Â 0.16
monoclinic
0.30 Â 0.28 Â 0.20
monoclinic
0.30 Â 0.30 Â 0.15
monoclinic
P1
P21/c
P21/c
P21/c
9.866(2)
9.7457(19)
9.7384(19)
19.710(4)
b (Å)
12.667(3)
13.420(3)
72.56(3)
17.373(4)
23.464(5)
10.492(2)
c (Å)
11.849(2)
9.820(2)
18.068(4)
R (deg)
β (deg)
γ (deg)
V (Å3)
90
90
90
89.26(3)
97.06(3)
114.63(3)
101.12(3)
86.28(3)
90
90
90
1597(2)
1991(2)
2040(2)
3667(2)
Z
2
2
2
4
F
calcd (g cmÀ3
)
1.055
1.017
1.038
1.025
abs coeff (mmÀ1
)
0.378
0.341
0.335
0.366
F(000)
556
672
704
1260
θ range (deg)
1.59 < θ < 26.37
12 807/6436
98.4
2.09 < θ < 25.00
13 146/3466
98.8
1.74 < θ < 26.37
16 073/4171
100
2.11 < θ < 26.38
28 568/7477
99.8
no. of collected/unique rflns
completeness to θ (%)
no. of data/restraints/params
goodness of fit on F2
6436/0/251
1.14
3466/0/155
1.05
4171/0/164
1.09
7477/0/280
1.05
final R indices (I > 2σ(I))
R1 = 0.045, wR2 = 0.103
R1 = 0.050, wR2 = 0.105
0.55/-0.22
R1 = 0.060, wR2 = 0.152
R1 = 0.089, wR2 = 0.173
0.88/-0.44
R1 = 0.053, wR2 = 0.112
R1 = 0.073, wR2 = 0.120
0.63/-0.26
R1 = 0.047, wR2 = 0.116
R1 = 0.053, wR2 = 0.120
1.45/-0.38
R indices (all data)
largest diff peak/hole (e/Å3)
2,3-Bis(trimethylsilyl)-2-[(trimethylsilyl)methyl]heptamethyltetra-
silane (17). Compound 17 was obtained from 2,2,3-tetrakis-
(trimethylsilyl)-3-[(trimethylsilyl)methyl]hexamethyltetrasilane (18;
406 mg, 0.80 mmol) and KOtBu (89 mg, 0.79 mmol) using method
A and dimethyl sulfate (101 mg, 0.80 mmol). After aqueous workup 17
was obtained as a colorless oil (315 mg, 88%). NMR data (δ in ppm):
29Si, 1.4, À11.8, À12.8, À76.1, À78.9; 1H, 0.32 (s, 3H), 0.30 (s, 18H),
0.27 (s, 18H), 0.19 (s, 2H), 0.14 (s, 9H); 13C, 2.3, 1.9, 1.7, 0.4, À8.5.
UV absorption (pentane, λmax in nm (ε in MÀ1 cmÀ1)): 216 (1.4 Â
104). Anal. Calcd for C17H50Si7 (451.18): C, 45.26; H, 11.17. Found:
C, 45.26; H, 10.75.
mp 131À137 °C). NMR data (δ in ppm): 29Si, 1.2, À7.8, À8.7, À11.9,
À37.2, À40.0, À79.0, À131.8; 1H, 0.41 (s, 6H), 0.39 (s, 6H), 0.38 (s,
6H), 0.37 (s, 6H), 0.33 (s, 9H), 0.32 (s, 9H), 0.31 (s, 9H), 0.16 (s, 9H),
0.06 (s, 2H); 13C, 4.0, 3.9, 2.5, 1.9, À0.2, À1.0, À2.0, À3.3, À6.1. UV
absorption (pentane, λmax in nm (ε in MÀ1 cmÀ1)): 206 (4.8 Â 104),
249 (s, 6 Â 7 Â 103), 267 (s, 2.7 Â 102). Anal. Calcd for C21H62Si10
(595.57): C, 42.35; H, 10.49. Found: C, 41.11; H, 10.49. HRMS for
C21H62Si10: m/z calcd 594.2545, found 594.2549.
cis-/trans-1,4-Bis(trimethylsilyl)-1,4-bis[(trimethylsilyl)methyl]oct-
amethylcyclohexasilane (35). The reaction was carried out analogously
to the preparation of 33 using the dianion prepared from 1,1,4,4-
tetrakis(trimethylsilyl)octamethylcyclohexasilane (424 mg, 0.73 mmol)
and KOtBu (164 mg, 1.46 mmol) (Method B) and chloromethyltri-
methylsilane (179 mg, 2 equiv). The mixture of cis- and trans-35 (430
mg, 97%) was obtained as a colorless solid. By recrystallization with
Et2O/acetone the trans isomer was separated (mp 157À164 °C). Data
for the trans-35 isomer are as follows. NMR data (δ in ppm): 29Si, 1.4,
À10.9, À36.9, À77.2; 1H, 0.41 (s, 12H), 0.39 (s, 12H), 0.32 (s, 18H),
0.16 (s, 18H), 0.09 (s, 4H); 13C, 2.7, 2.0, À1.1, À3.2, À5.7. UV
absorption (pentane, λmax in nm (ε in MÀ1 cmÀ1)): 211 (3.8 Â 104),
269 (s, 4.5 Â 103). Data for the cis-2 isomer are as follows. NMR data (δ
2,3,3-Tris(trimethylsilyl)-2-[(trimethylsilyl)methyl]octamethylpen-
tasilane (19). Compound 19 was obtained from 2,2,3-tetrakis-
(trimethylsilyl)-3-[(trimethylsilyl)methyl]hexamethyltetrasilane (18; 696
mg, 1.37 mmol) and KOtBu (158 mg, 1.38 mmol) using method A and
chloropentamethyldisilane (228 mg, 1.37 mmol). After aqueous workup
colorless crystals (720 mg, 93%) of 19 were obtained after removing the
solvent (mp 138À140 °C). NMR data (δ in ppm): 29Si, 1.2, À10.0, 13.1,
1
À13.2, À38.2, À69.7, À115.2; H, 0.39 (s, 6H), 0.35 (s, 18H), 0.33 (s,
18H), 0.20 (s, 9H), 0.14 (s, 9H), 0.09 (s, 2H); 13C, 4.6, 3.3, 2.1, 1.3, 0.8, 0.1.
UV absorption (pentane, λmax in nm (ε in MÀ1 cmÀ1)): 216 (1.9 Â 104).
Anal. Calcd for C21H62Si9 (567.49): C, 44.45; H, 11.01. Found: C, 44.46;
H, 10.83.
1
in ppm): 29Si, 1.3, À11.9, À38.2, À78.6; H, 0.37 (s, 12H), 0.35 (s,
12H), 0.30 (s, 18H), 0.15 (s, 18H), 0.06 (s, 4H); 13C, 2.5, 1.3, À2.2,
À2.7, À6.1. Anal. Calcd for C22H64Si10 (609.60): C, 43.35; H, 10.58.
Found: C, 43.92; H, 10.55.
1,1,4-Tris(trimethylsilyl)-4-[(trimethylsilyl)methyl]octamethylcyclo-
hexasilane (33). To an ice-cold solution of (chloromethyl)trimethyl-
silane (116 mg, 0.94 mmol, 1.0 equiv) in THF (5 mL) were added the
monoanion of 1,1,4,4-tetrakis(trimethylsilyl)octamethylcyclohexasilane
(550 mg, 0.94 mmol) and KOtBu (106 mg, 0.94 mmol ) (Method A)
dropwise over a period of 10 min. After it was stirred for 16 h at room
temperature, the reaction mixture was subjected to an aqueous workup.
Recrystallization from Et2O/acetone yielded colorless 33 (230 mg, 41%;
1-(Trimethylsilyl)-1,4,4-tris[(trimethylsilyl)methyl]octamethylcyclo-
hexasilane (36). The reaction was carried out analogously to the
preparation of 33 using the monoanion obtained from 35 (334 mg,
0.54 mmol), KOtBu (62 mg, 1.0 equiv), and 18-crown-6 (145 mg,
1.0 equiv) (Method C) and chloromethyltrimethylsilane (68 mg,
1 equiv). Compound 36 (285 mg, 83%) was obtained as a colorless
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solid after recrystallization with Et2O/acetone (mp 103À107 °C). NMR
data (δ in ppm): 29Si, 1.4, 1.3, À11.0, À36.1, À37.1, À40.0, À78.0; 1H,
0.40 (s, 6H), 0.37 (s, 6H), 0.36 (s, 6H), 0.35 (s, 6H), 0.32 (s, 9H), 0.32
(s, 9H), 0.22 (s, 2H), 0.19 (s, 9H), 0.18 (s, 9H), 0.16 (s, 9H), 0.13 (s,
2H), 0.09 (s, 2H); 13C, 2.6, 2.4, 2.3, 2.0, À0.1, À0.7, À1.5, À2.9, À3.2,
À4.3, À5.7. UV absorption (pentane, λmax in nm (ε in MÀ1 cmÀ1)): 233
(s, 1.2 Â 104), 255 (s, 5.8 Â 103). Anal. Calcd for C23H66Si10 (623.63):
C, 44.30; H, 10.67. Found: C, 44.81; H, 10.76.
1,1,4,4-Tetrakis[(trimethylsilyl)methyl]octamethylcyclohexasilane
(37). The reaction was carried out analogously to the preparation of
35 using the dianion obtained from 35 (493 mg, 0.79 mmol), KOtBu
(183 mg, 1.63 mmol), and 18-crown-6 (432 mg, 1.63 mmol) (Method
D) and chloromethyltrimethylsilane (203 mg, 1.65 mmol). Compound
37 (461 mg, 89%) was obtained as a colorless solid after recrystallization
with Et2O (mp 173À179 °C). NMR data (δ in ppm): 29Si, 1.4, À36.6,
À40.5; 1H, 0.36 (s, 24H), 0.19 (s, 36H), 0.16 (s, 8H); 13C, 2.3, À1.2,
À3.8. UV absorption (pentane, λmax in nm (ε in MÀ1 cmÀ1)): 231 (s,
8.7 Â 103), 261 (s, 1.2 Â 103). Anal. Calcd for C24H68Si10 (637.66): C,
45.21; H, 10.75. Found: C, 45.23; H, 10.74.
1-(Trimethylsilyl)-4-[(trimethylsilyl)methyl]dodecamethylbicyclo-
[2.2.2]octasilane (38). The reaction was carried out analogously to the
preparation of 33 using the monoanion of 1,4-bis(trimethylsilyl)do-
decamethylbicyclo[2.2.2]octasilane (641 mg, 1.16 mmol) and KOtBu
(131 mg, 1.17 mmol) (Method A) and chloromethyltrimethylsilane
(147 mg, 1.20 mmol, 1.03 equiv). Compound 38 (630 mg, 96%) was
obtained as a colorless solid (mp 168À175 °C). NMR data (δ in ppm):
29Si, 1.0, À6.1, À37.8, À39.9, À79.2, À131.5; 1H, 0.38 (s, 18H), 0.36 (s,
18H), 0.31 (s, 9H), 0.15 (s, 9H), 0.05 (s, 2H); 13C, 3.7, 1.5, À0.9, À2.4,
À2.4. Anal. Calcd for C19H56Si10 (565.50): C, 40.35; H, 9.98. Found: C,
39.99; H, 9.97.
General Procedure for the Rearrangement Reactions.
Solutions of the corresponding oligosilanes (50À1000 mg) and approxi-
mately 0.15 equiv of Al(Fe)Cl3 in cyclohexane in a Schlenk tube with a
Teflon screw cap were heated to 80 °C. After 16 or 24 h the rate of
conversion was checked by 29Si NMR spectroscopy. If no or insufficient
conversion was observed, additional Al(Fe)Cl3 was added. After com-
pletion of the reaction, acetone (10À20 mL) was added and the white
precipitate that formed was removed by either filtration or centrifuga-
tion. After removal of the solvent the residue was subjected to crystal-
lization conditions. In the case of air-sensitive compounds the workup
procedure was carried out under a nitrogen atmosphere and simple
filtration through a path of Celite. Especially when higher catalyst loads were
used, chlorinated byproducts were detected by mass spectroscopy. Treat-
ment of these mixtures with methylmagnesium bromide or methyllithium
could be used to obtain homogeneous product distributions.
After 16 h a mixture of 4 (48%), 5 (12%), 6 (34%), and 7 (6%) was
detected. Another batch of Al(Fe)Cl3 (25 mg, 0.30 equiv) was added,
and almost no change of composition was detected after an additional 24
h. After 6 days a mixture of compounds, with 6 (30%) and 7 (30%) being
the main products, was observed. Additional Al(Fe)Cl3 (25 mg, 0.30
equiv) was added. After another 6 days compound 7 and tetramethylsi-
lane were found to be the main products along with numerous smaller
signals in the NMR spectrum. An increasingly worse signal/noise ratio
indicates some decomposition process. Assignment of the structure of
compound 6 was based on the 29Si NMR data (δ in ppm, cyclohexane,
D2O capillary): 29Si, À9.5, À14.8, À32.8, À37.8, À42.0, À129.1.10,12
Repeating the reaction with 5 gave essentially the same result.
Rearrangement of 2,9,9-Tetrakis(trimethylsilyl)octadecamethylde-
casilane (8) to 2,2,5,9,9-Pentakis(trimethylsilyl)pentadecamethylno-
nasilane (9) and Further to 1,1,3-Tris(trimethylsilyl)pentamethylcy-
clopentasilane (2) and 2,2-Bis(trimethylsilyl)octamethyltetrasilane
(10). Following the general procedure, the reaction of 8 (379 mg,
0.45 mmol) was started with Al(Fe)Cl3 (20 mg, 0.33 equiv) at 80 °C in
cyclohexane. Two additional batches of Al(Fe)Cl3 (2 Â 25 mg, 0.83
equiv) were required before extensive conversion of 8 to 9 was observed.
Addition of another batch of Al(Fe)Cl3 (25 mg, 0.42 equiv) led to
further conversion of 8 but was also accompanied by the formation of
additional products. Workup allowed the isolation of 9 (46 mg, 13%)
still contaminated with some 17% of 8. Purification was achieved by
recrystallization from acetone/isopropyl alcohol and further from
diethyl ether at À30 °C (mp 89À92 °C). NMR data of 9 (δ in ppm):
29Si, À9.8, À11.7, À30.6, À32.7, À72.6, À128.5; 1H, 0.56 (s, 6 H), 0.55
(s, 6H), 0.55 (s, 6H), 0.51 (s, 6H) 0.34 (s, 54H), 0.31 (s, 9H), 0.29 (s,
3H); 13C, 3.5, 1.6, 1.4, 1.3, À0.1, À0.2, À8.6. UV absorption (pentane,
λmax in nm (ε in MÀ1 cmÀ1)): 216 (6.1 Â 104); 258 (4.8 Â 104); 270
(2.4 Â 104). Anal. Calcd for C30H90Si14 (844.24): C, 42.68; H, 10.74.
Found: C, 41.90; H, 10.11.
If the reaction was not stopped, complete conversion of 8 and 9 was
observed. In addition to a small amount of side products compounds 2
and 10 in approximately equal quantities were the major reaction
products.
Rearrangement of 2-[(Trimethylsilyl)methyl]heptamethyltrisilane
(11) to 1-[(Trimethylsilyl)methyl]heptamethyltrisilane (12). The reac-
tion was carried out by following the general procedure, using 11 (186
mg, 0.672 mmol) and Al(Fe)Cl3 (2 Â 25 mg, 0.56 equiv) in cyclohexane
at 80 °C. After 48 h no conversion was detected and a second batch of
Al(Fe)Cl3 was added. Clean and complete conversion to 12 was
detected after 120 h. After workup and removal of solvent 12 (173
mg, 93%) was obtained as a colorless oil. NMR data (δ in ppm): 29Si, 1.3,
À15.1, À16.3, À48.2; 1H, 0.21 (s, 6H), 0.15 (s, 9H), 0.15 (s, 6H), 0.09
(s, 9H), À0.19 (s, 2H); 13C, 1.8, 1.7, À0.5, À1.2, À6.8. Anal. Calcd for
C11H32Si4 (276.72): C, 47.75; H, 11.66. Found: C, 44.04; H, 10.26.
Rearrangement of 2-(Trimethylsilyl)-2-[(trimethylsilyl)methyl]hexa-
methyltrisilane (13) to 2-(Trimethylsilyl)-1-[(trimethylsilyl)methyl]
hexamethyltrisilane (14). The reaction was carried out by following
the general procedure, using 13 (421 mg, 1.26 mmol) and Al(Fe)Cl3
(120 mg, 0.72 equiv) in cyclohexane. Completion was reached after 24 h.
14 (380 mg, 90%) could be obtained as a colorless oil. NMR data (δ in
ppm): 29Si, 1.3, À11.3, À12.9, À86.5; 1H, 0.29 (s, 6H), 0.21 (s, 18H),
0.13 (s, 3H), 0.10 (s, 9H), À0.08 (s, 2H); 13C, 3.7, 1.7, 1.2, 0.6, À13.1.
Anal. Calcd for C13H38Si5 (334.87): C, 46.63; H, 11.44. Found: C,
44.66; H, 10.86.
Synthesis of the Catalysts. A mixture of AlCl3 and FeCl3 (3%)
was sublimed at a temperature of 100 °C and a pressure of 0.3À0.5 mbar
for 3À4 h. A bright yellow powder was obtained, which was stored under
nitrogen. For the characterization experiments a sample of the sublimate
was refluxed for 12 h in cyclohexane in order to mimic the reaction
conditions (see Figures S2 and S3 in the Supporting Information). ICP
analysis of this sample indicated a value of 0.36 mg of Fe/g (0.104%). For all
other measurements a sublimate (AlCl3 2.441 g, FeCl3 297 mg (10%)) was
used which according to ICP contained 1.30 mg of Fe/g (0.37%).
Rearrangement of 2,2,5-Tris(trimethylsilyl)undecamethylhexasil-
ane (1) to 1,1,3-Tris(trimethylsilyl)heptamethylcyclopentasilane (2).
Following the general procedure, the reaction of 1 (386 mg, 0.70
mmol)) was carried out with Al(Fe)Cl3 (25 mg, 0.27 equiv) for 12 h
in cyclohexane. After 16 h complete conversion was detected and
compound 210,12 (303 mg, 93%) was isolated.
Rearrangement of 2-(Trimethylsilyl)-2-[(Trimethylsilyl)methyl]oct-
amethyltetrasilane (15) to 2-(Trimethylsilyl)-2-[((trimethylsilyl)me-
thyl)dimethylsilyl]hexamethyltrisilane (16). The reaction was carried
out by following the general procedure using 15 (240 mg, 0.611 mmol)
and Al(Fe)Cl3 (20 mg, 0.25 equiv) in cyclohexane. Completion was
reached after 16 h. 16 (227 mg, 95%) could be obtained as a colorless oil.
NMR data (δ in ppm): 29Si, 1.2, À8.1, À10.3, À132.8; 1H, 0.36 (s, 6H),
Rearrangements of 2,5-Bis(trimethylsilyl)dodecamethylhexasilane
(3) and 2,2,3,3-Tetrakis(trimethylsilyl)hexamethyltetrasilane (5). Fol-
lowing the general procedure, the reaction of 3 (332 mg, 0.63 mmol) was
started with Al(Fe)Cl3 (25 mg, 0.30 equiv) at 80 °C in cyclohexane.
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0.27 (s, 27H), 0.09 (s, 9H), 0.02 (s, 2H); 13C, 6.4, 3.5, 2.9, 1.8. Anal.
Calcd for C15H44Si6 (393.03): C, 45.84; H, 11.28. Found: C, 46.28;
H, 11.21.
243 (1.8 Â 104). Anal. Calcd for C17H50Si9 (507.35): C, 40.25; H, 9.93.
Found: C, 39.67; H, 9.71. HRMS for C17H50Si9: m/z calcd 506.1836,
found 506.1824.
Rearrangement of Tetrakis(trimethylsilyl)-1-[(trimethylsilyl)met-
hyl]-2-methyldisilane (17) to 1,1-Bis(trimethylsilyl)-3-carba-2,2,4,4,-
5,5-hexamethylcyclopentasilane (20). The reaction was carried out by
following the general procedure using 17 (253 mg, 0.561 mmol) and
Al(Fe)Cl3 (60 mg, 0.80 equiv) in cyclohexane. Completion was reached
after 36 h. 20 (186 mg, 91%) could be obtained as a colorless liquid.
NMR data (δ in ppm): 29Si, 0.6, À8.8, À9.0, À34.6, À136.7; 1H, 0.35 (s,
6H), 0.30 (s, 6H), 0.27 (s, 18H), 0.20 (s, 6H), À0.08 (s, 2H); 13C, 7.1,
3.2, 1.6, 0.4, À2.2. Anal. Calcd for C13H38Si6 (362.96): C, 43.02; H,
10.55. Found: C, 37.67; H, 8.71.
Rearrangement of Pentakis(trimethylsilyl)[(trimethylsilyl)methyl]
disilane (18) to 1,1,5-Tris(trimethylsilyl)-3-carba-2,2,4,4,5-penta-
methylcyclopentasilane (21). The reaction was carried out by following
the general procedure using 18 (227 mg, 0.446 mmol) and Al(Fe)Cl3
(20 mg, 0.34 equiv) in cyclohexane. Completion was reached after 16 h.
White crystalline 21 (68 mg, 36%) could be obtained after recrystalliza-
tion from pentane/2-propanol at 0 °C (oil at room temperature). NMR
data (δ in ppm): 29Si, 2.5, À3.3, À8.1, À9.9, À11.5, À78.8, À131.5; 1H,
0.34 (s, 3H), 0.31 (s, 3H), 0.29 (s, 3H), 0.28 (s, 9H), 0.27 (s, 9H), 0.26
(s, 3H), 0.25 (s, 9H), 0.24 (s, 3H), À0.02 (d, 1H, J = 13 Hz), À0.11 (d,
1H, J = 13 Hz); 13C, 7.5, 4.3, 3.6, 3.4, 3.3, 1.8, 0.5, 0.2, À7.4. Anal. Calcd
for C15H44Si7 (421.11): C, 42.78; H, 10.53. Found: C, 42.00; H, 10.24.
Rearrangement of 1-Pentamethyldisilanyltetrakis(trimethylsilyl)-
2-[(trimethylsilyl)methyl]disilane (19) to 1,1,5,5-tTetrakis(trimethylsi-
lyl)-3-carba-2,2,4,4-cyclopentasilane (22). The reaction was carried out
by following the general procedure using 19 (252 mg, 0.444 mmol) and
Al(Fe)Cl3 (50 mg, 0.84 equiv) in cyclohexane. Completion was reached
after 24 h. White crystalline 22 (110 mg, 52%) could be obtained after
recrystallization from methanol/2-propanol at À30 °C (oil at room
temperature). NMR data (δ in ppm): 29Si, 1.6, À9.1, À128.7; 1H, 0.31
(s, 36H), 0.31 (s, 12H), À0.05 (s, 2H); 13C, 8.7, 4.0, 3.5. Anal. Calcd for
C17H50Si8 (479.27): C, 42.60; H, 10.52. Found: C, 39.95; H, 9.61.
HRMS for C17H50Si8: m/z calcd 478.2067, found 478.2099.
Rearrangement of 35 or 1,1,5,5-Tetrakis(trimethylsilyl)-1,5-bis-
[(trimethylsilyl)methyl]hexamethylpentasilane (30) to 3,7-Dicarba-
1,5-bis(trimethylsilyl)decamethylbicyclo[3.3.1]nonasilane (31). The
reaction was carried out by following the general procedure using 35
(425 mg, 0.697 mmol) and Al(Fe)Cl3 (60 mg, 0.65 equiv) in cyclohex-
ane. Completion was reached after 24 h. White crystalline 31 (280 mg,
77%) could be obtained after recrystallization from benzene at room
temperature (mp 189À195 °C). NMR data (δ in ppm): 29Si, À7.2,
0.29 (s, 18H), À0.04 (d, 2H, J = 13 Hz), À0.10 (d, 2H, J = 13 Hz); 13C,
11.0, 5.9, 3.9, 3.6, 3.3. UV absorption (pentane, λmax in nm (ε in
MÀ1 cmÀ1)): 216 (3.1 Â 104), 244 (2.1 Â 104). Anal. Calcd for
C18H52Si9 (521.38): C, 41.47; H, 10.05. Found: C, 41.47; H, 10.00.
Starting from 30 (362 mg, 0.519 mmol) and Al(Fe)Cl3 (125 mg, 1.81
equiv), complete conversion was reached after 4 days but some
unidentified byproducts were detected in this case.
Rearrangement of 36 or 1,1,4,4-Tetrakis(trimethylsilyl)-1,4-bis-
[(trimethylsilyl)methyl]tetramethyltetrasilane (28) to 3,7-Dicarba-
1,5-bis(trimethylsilyl)decamethylbicyclo[3.3.0]octasilane (29). The
reaction was carried out by following the general procedure using 36
(153 mg, 0.251 mmol) and Al(Fe)Cl3 (15 mg, 0.45 equiv) in cyclohex-
ane. Completion was reached after 16 h, and white crystalline 29 (65 mg,
56%) was obtained after recrystallization from dichloromethane at À30 °C
(sublimation from 280 °C). NMR data (δin ppm): 29Si, 2.9, À8.7, À127.9;
1H, 0.36 (s, 12H), 0.33 (s, 18H), 0.31 (s, 12H), À0.04 (d, 2H, J = 13 Hz),
À0.26 (d, 2H, J = 13 Hz); 13C, 9.3, 3.5, 3.2, 3.1. UV absorption (pentane,
λmax in nm (ε in MÀ1 cmÀ1)): 224 (2.1 Â 104). Anal. Calcd for C16H46Si8
(463.22): C, 41.49; H, 10.01. Found: C, 39.65; H, 9.81.
1
À8.4, À31.3, À132.8; H, 0.59 (s, 6H), 0.33 (s, 12H), 0.32 (s, 12H),
Starting from 28 (130 mg, 0.203 mmol) and Al(Fe)Cl3 (30 mg, 1.11
equiv), 29 (44 mg, 47%) was obtained after 48 h.
’ ASSOCIATED CONTENT
Rearrangement of Pentakis(trimethylsilyl)[(trimethylsilyl)methyl]-
2,2,3,3-tetramethyltetrasilane (23) to 1-(Trimethylsilyl)-3-carbaun-
decamethylbicyclo[3.2.1]octasilane (25). The reaction was carried
out by following the general procedure using 23 (291 mg, 0.465 mmol)
and Al(Fe)Cl3 (60 mg, 0.97 equiv) in cyclohexane. After 24 h a 1:1
mixture of 25 and 1,1,5,5-tetrakis(trimethylsilyl)-3-carbahexamethylcy-
clohexasilane (24) (NMR data (δ in ppm, D2O capillary): 29Si À8.1,
À9.6, À30.2, À132.6.) could be observed. Complete conversion was
reached after 48 h. White crystalline 25 (110 mg, 53%) could be
obtained after recrystallization from pentane/2-propanol (mp 100À
110 °C). NMR data (δ in ppm): 29Si, À6.0, À6.9, À8.9, À27.9, À33.7,
À39.4, À77.8, À129.6; 1H, 0.54 (s, 3H), 0.39 (s, 3H), 0.37 (s, 3H), 0.36
(s, 3H), 0.33 (s, 3H), 0.31 (s, 3H), 0.30 (s, 3H), 0.29 (s, 9H), 0.28 (s,
3H), 0.26 (s, 3H), 0.24 (s, 3H), 0.05 (d, 1H, J = 13 Hz), À0.16 (d, 1H, J =
13 Hz); 13C, 8.6, 5.3, 3.6, 3.4, 2.5, 1.0, 0.1, À1.1, À1.1, À1.8, À3.7, À4.0,
À14.1. Anal. Calcd for C15H44Si6 (449.20): C, 40.11; H, 9.87. Found: C,
38.52; H, 9.65.
Rearrangement of 33 to 3-Carba-1,5-bis(trimethylsilyl)decame-
thylbicyclo[3.2.1]octasilane (34). The reaction was carried out by
following the general procedure using 33 (202 mg, 0.339 mmol) and
Al(Fe)Cl3 (20 mg, 0.44 equiv) in cyclohexane. Completion was reached
after 16 h, and white crystalline 34 (117 mg, 68%) could be obtained
after recrystallization from dichloromethane at room temperature (mp
162À169 °C). NMR data (δ in ppm): 29Si, À6.2, À7.3, À22.5, À31.4,
À126.8; 1H, 0.63 (s, 3H), 0.43 (s, 3H), 0.37 (s, 6H), 0.36 (s, 6H), 0.32
(s, 6H), 0.30 (s, 6H), 0.29 (s, 18H), 0.02 (d, 1H, J = 13 Hz), À0.14
(d, 1H, J = 13 Hz); 13C, 9.7, 5.4, 3.6, 3.4, 2.6, 0.8, À1.0, À2.1. UV
absorption (pentane, λmax in nm (ε in MÀ1 cmÀ1)): 218 (4.9 Â 104),
S
Supporting Information. CIF files giving X-ray crystal-
b
lographic data for 25, 29, 31, 33, 34, 35-trans, 37, and 38 and
figures giving additional experimental details of catalyst charac-
terization. This material is available free of charge via the Internet
at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: christoph.marschner@tugraz.at (C.M.); baumgartner@
tugraz.at (J.B.).
’ ACKNOWLEDGMENT
This study was supported by the Austrian Fonds zur F€orderung
der wissenschaftlichen Forschung (FWF) via the projects P-18538 and
I-669. We wish to thank Prof. Dr. Christian L. Lengauer (Institut f€ur
Mineralogie und Kristallographie, Universit€at Wien À Geozentrum)
for powder XRD measurements of Al(Fe)Cl3 samples.
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