DOI: 10.1002/anie.201101139
Silicon(II) Compounds
The Pentamethylcyclopentadienylsilicon(II) Cation as a Catalyst for
the Specific Degradation of Oligo(ethyleneglycol) Diethers**
´
Kinga Leszczynska, Andreas Mix, Raphael J. F. Berger, Britta Rummel, Beate Neumann, Hans-
Georg Stammler, and Peter Jutzi*
Dedicated to Professor Hansgeorg Schnꢀckel on the occasion of his 70th birthday
Covalent silicon(IV) compounds of the type R3SiX (R =
organic substituent, X = electronegative group) have long
been used as catalysts for several organic reactions, mainly in
carbon–carbon bond-forming processes (such as Diels–Alder
reactions, aldol condensations).[1] Some ionic silicon(IV)
compounds containing R3Si+ cations have also been applied
of fluoroalkyl groups in the presence of excess triethylsilane.[6]
The closely related compound 2 containing a triethylsilyl
cation weakly interacting with a halocarbollide anion[7] was
used as an intermediate catalyzing several hydrodefluorina-
tion processes.[8] Donor-stabilized cations in compounds of
type 3 catalyze the deoxygenation of ketones with hydro-
silanes.[9] The hydrogen-bridged disilyl cation in compound 4,
comprising an electrophilic silicon center and the hydrogen
source in one molecule, catalyzes the hydrodefluorination of
alkyl and benzyl fluorides.[10] Finally, the ferrocenyl-stabilized
R3Si+ cation in compound 5 catalyzes selectively several
Diels–Alder reactions even at low temperature;[11] further-
more, it is the catalytic species in the reduction of ketones to
alkyl( silyl) ethers.[12] Only very recently, the role of ionic
silicon(IV) compounds in catalysis was comprehensively
reviewed by Klare and Oestreich.[13]
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as catalysts in C C bond formation reactions and in some
other transformations.[2] The most important examples are
collected in Scheme 1.
During the last decade, the first ionic silicon(II) com-
pounds that are stable at room temperature have been
prepared that contain electrophilic RSi+ cations with R =
Me5C5 (Cp*; 6),[14] iPr5C5,[15] and HC(CMeNAryl)2 (Aryl =
2,6-iPr2C6H3)[16] and B(C6F5)4 as the anion. Herein we
À
describe the function of the half-sandwich Cp*Si+ cation
(Scheme 1) as a catalyst for a novel type of a catalyzed
reaction, namely for the controlled degradation of oligo-
(ethylene glycol) dialkyl and disilyl ethers and of cyclic
diethers (crown ethers). Oligo(ethylene glycol) diethers serve
as conducting liquid organic electrolytes in lithium batter-
ies.[17]
Scheme 1. Organosilicon cations in catalysis.
Compound 1, containing the benzene-stabilized triethyl-
silyl cation,[3] was used as a catalyst for the hydrosilylation of
1,1-diphenylethene[4] and later as a catalyst in aldol and in
Diels–Alder reactions.[5] Furthermore, compound 1 was
chosen as the catalytic component in the hydrodefluorination
In the reaction of the compound [Cp*Si]+B(C6F5)4 (6)
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with dimethoxyethane (DME), which is regarded as the first
member in the series of oligo(ethylene glycol) dimethyl
ethers, we were able to isolate the crystalline, highly air- and
moisture sensitive and thermolabile 1:1 complex [Cp*Si-
´
[*] Dr. K. Leszczynska, Dr. A. Mix, Dr. R. J. F. Berger, B. Rummel,
(dme)]+B(C6F5)4 (7) [Eq. (1)]. Brief treatment of 7 with
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B. Neumann, Dr. H.-G. Stammler, Prof. Dr. P. Jutzi
Faculty of Chemistry, University of Bielefeld
Universitꢀtstrasse 25, 33615 Bielefeld (Germany)
Fax: (+49)521-106-6026
vacuum from an oil pump resulted in the reverse reaction.
E-mail: peter.jutzi@uni-bielefeld.de
[**] We thank Prof. R. West, University of Wisconsin, for a gift of the
compounds Me3SiO(CH2CH2O)3Me and Me3SiO-
(CH2CH2O)3SiMe3. Financial support of the Deutsche Forschungs-
gemeinschaft, the University of Bielefeld, and the Fonds der
Chemischen Industrie is gratefully acknowledged. We thank the staff
of Prof. N. Mitzel for technical support.
Despite the instability of complex 7, we could ascertain its
structure by X-ray analysis (Figure 1).[18] The DME molecule
coordinates rather weakly in an asymmetric bidentate fashion
to the silicon atom, and the Cp* ligand shows some deviation
Supporting information for this article, including experimental
details, time-dependent NMR spectra, and computational data, is
from h5 bonding towards h3 bonding. The average Si C
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Angew. Chem. Int. Ed. 2011, 50, 6843 –6846
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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