Cleavage of Unactivated Si?C(sp3) Bonds with Reed's Carborane Acids: Formation of Known and Unknown Silylium Ions

Article abstract of DOI:10.1002/anie.201805637

An efficient method for the benzenium-ion-mediated cleavage of inert Si?C(sp³) bonds is reported. Various tetraalkylsilanes can thus be converted into the corresponding counteranion-stabilized silylium ions. The reaction is chemoselective in the case of hexamethyldisilane. Computations reveal a mechanism with backside attack of the proton at one of the alkyl groups. Several activated Si?C(spⁿ) bonds (n=3–1) react equally well, and the procedure can be extended to the generation of stannylium ions.

Full text of DOI:10.1002/anie.201805637

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Accepted Article
Title: Cleavage of Unactivated Si-C(sp3) Bonds with Reed's Carborane
Acids: Clean Formation of Known and Unknown Silylium Ions
Authors: Qian Wu, Zheng-Wang Qu, Lukas Omann, Elisabeth Irran,
Hendrik Klare, and Martin Oestreich
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201805637
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10.1002/anie.201805637
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Cleavage of Unactivated Si‒C(sp3) Bonds with Reed’s Carborane
Acids: Formation of Known and Unknown Silylium Ions
Qian Wu, Zheng-Wang Qu,* Lukas Omann, Elisabeth Irran, Hendrik F. T. Klare, and Martin Oestreich*
Dedicated to Professor Douglas W. Stephan on the occasion of his 65th birthday
Abstract: An efficient method for the benzenium-ion-mediated
Scheme 1. Reported methods for Si–C(sp3) bond cleavage. DMSO = dimethyl
sulfoxide. Tf = trifluoromethanesulfonyl; sc = supercritical.
cleavage of inert Si–C(sp3) bonds is reported. By this, various
tetraalkylsilanes are converted into the corresponding counteranion-
stabilized silylium ions. The reaction is chemoselective in the case of
Inspired by the literature precedence, we entertained the
idea of employing Reed’s carborane acids[6] such as
H+[CHB11H5Br6]‒[7] as well as cognate protonated benzene
[C6H6·H]+[CHB11H5Br6]‒[8] for the cleavage of rather inert Si–
C(sp3) bonds (Scheme 2).[9] Unlike the reported methods,[2,5] its
successful implementation would enable the generation of
counteranion-stabilized silylium ions[10] directly from completely
unfunctionalized precursors. This strategy would be
complementary to the common preparations of silylium ions[11]
by either hydride abstraction from hydrosilanes[12] or the allyl-
leaving-group approach of sterically hindered systems.[13] We
dislcose here the chemoselective heterolysis of unactivated Si–
C(sp3) bonds by protonation to arrive at “exceptionally” clean
silylium ions. Computations also reveal a rather unexpected
mechanism of the bond-breaking event.
hexamethyldisilane. Computations reveal
a
mechanism with
backside attack of the proton at one of the alkyl groups. Likewise,
several activated Si–C(spn) bonds (n = 3–1) react equally well, and
the protocol can be extended to the generation of stannylium ions.
Me4Si is considered inert, and that holds true for any quaternary,
fully alkyl-substituted silane. Examples of cleavage of
unactivated Si–C(sp3) bonds are exceedingly rare and
essentially limited to the smallest member Me4Si [Scheme 1, Eq.
(1) and Eq. (2)]. Both hard oxygen nucleophiles[1] and acids[2]
have been shown to convert Me4Si into an oxygenated silane
with release of methane.[3,4] Itami and co-workers found that
tetraalkylsilanes are rapidly degraded to the corresponding
alkanes in supercritical water at high temperature and pressure
[Scheme 1, Eq. (3)];[5] the addition of mineral acid accelerates
these reactions.
Me4Si + KOtBu
Me4Si + TfOH
Alkyl4Si + H2O
Me3SiOtBu + CH4
Me3SiOTf + CH4
Alkyl–H
(1)
(2)
(3)
DMSO
25 °C
CHCl3 or CH2Cl2
25 °C
scH2O
w/ or w/o HCl
27 MPa
390 °C
broad range of
tetraalkylsilanes
silicon-containing
waste not characterized
Scheme 2. Representative carborane acids and their planned use in the
generation of counteranion-stabilized silylium ions.
[*]
Dr. Q. Wu, L. Omann, Dr. E. Irran,+ Dr. H. F. T. Klare, Prof. Dr. M.
Oestreich
Institut für Chemie, Technische Universität Berlin
Strasse des 17. Juni 115, 10623 Berlin (Germany)
E-mail: martin.oestreich@tu-berlin.de
We began our investigation with H+[CHB11H5Br6]‒ and
quickly found that tetraalkylsilanes are indeed converted into the
corresponding counteranion-stabilized silylium ions. However,
residual water in H+[CHB11H5Br6]‒ and difficulties with its removal
were detrimental to the purity of the obtained silylium ion-like
species. We therefore turned toward easy-to-purify benzenium
ion [C6H6·H]+[CHB11H5Br6]‒. Our reasoning was that it would
make no difference to start with either acid as the protonation is
performed in benzene. Gratifyingly, [C6H6·H]+[CHB11H5Br6]‒
transformed several tetraalkylsilanes into counteranion-
stabilized trialkylsilylium ions of remarkable purity with gas
evolution (Table 1); nBu4Si was included into the survey as it did
Homepage: http://www.organometallics.tu-berlin.de
Dr. Z.-W. Qu
Mulliken Center for Theoretical Chemistry
Institut für Physikalische und Theoretische Chemie
Rheinische Friedrich-Wilhelms-Universität Bonn
Beringstrasse 4, 53115 Bonn (Germany)
E-mail: qu@thch.uni-bonn.de
Homepage: http://www.thch.uni-bonn.de/tc
[+]
X-ray crystal-structure analysis.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201805637
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atoms are omitted for clarity); selected bond length (Å) and angles (°): Si–Br:
2.444(4); Si–Si: 2.354(5); Me‒Si‒SiMe3: 115.5(5); Me‒Si‒SiMe3: 116.0(6);
Me‒Si‒Me: 116.3(8).
allow for the conclusive verification of the released alkane, that
is n-butane, by 1H NMR spectroscopy (entry 3; see the
Supporting Information for details). The chemoselectivity of 1°
over 2° and 3° alkyl substitution, respectively, was also
demonstrated and is explained by steric hindrance (entries 4
and 5). Strikingly, chemoselective cleavage of the Si–C(sp3) was
also possible in the presence of the less stable Si‒Si bond (bond
dissociation enthalpies[14] for Me3Si–Me = 395 ± 4 kJ/mol versus
Me3Si–SiMe3 = 333 ± 6 kJ/mol). Again, we attribute this to the
sterically more accessible methyl group. This enabled the
formation of the previously unavailable silyl-substituted silylium
ion-like [Me2(Me3Si)Si]+[CHB11H5Br6]‒ (Scheme 3, top); its
molecular structure was established by X-ray diffraction
(Scheme 3, bottom). Conversely, dodecamethylcyclohexasilane
did not yield the targeted silylium ion; a complex mixture was
obtained, probably as a result of rearrangement reactions.[15]
To highlight the generality of the method, we subjected
representative trialkylsilanes decorated with leaving groups
known to be cleaved by protons (Table 2).[16] Activated Si–C(sp3)
bonds as in allyl-[2,17] and surprisingly benzylsilanes[18] cleanly
afforded the counteranion-stabilized trialkylsilylium ions (entries
1‒3). Likewise, vinyl-[19] and arylsilanes[20] underwent
protodesilylation (entries 4 and 5). These reactions are initiated
by protonation of the unsaturated unit and involve the
intermediacy of β-silicon-stabilized carbocations.[21] In turn, the
cleavage of an Si‒C(sp) bond by protonation would pass through
a β-silicon-substituted vinyl cation.[22] Such reaction was possible
but less clean. For example, alkynyl-substituted trimethylsilane
led to
a
complex mixture but [Me3Si]+[CHB11H5Br6]‒ was
nevertheless isolated in pure form after precipitation and
washing with 1,2-Cl2C6D4 (entry 6).
Table 1: Protonation of unactivated tetraalkylsilanes.
Table 2: Protonation of activated trialkyl(organo)silanes.
Entry
Silane
Alkane
Silylium ion
δ (29Si) [ppm][a]
1
2
3
4
5
Me4Si[b]
Et4Si
Me‒H (methane)
Et‒H (ethane)
Me3Si+
93.5
Entry
Silane
LG‒H
Silylium
ion
δ (29Si)
Et3Si+[c]
100.0
98.0
[ppm][a]
nBu4Si
nBu‒H (n-butane)
Me‒H (methane)
Me‒H (methane)
nBu3Si+
iPr2MeSi+
tBuMe2Si+
1
2
3
4
5
6
(allyl)Me3Si
(allyl)iPr3Si
BnMe3Si
Allyl‒H (propene)
Allyl‒H (propene)
Bn‒H (toluene)
Me3Si+
iPr3Si+
Me3Si+
Me3Si+
Me3Si+
Me3Si+
93.1
103.6
93.5
93.4
93.3
93.6
iPr2Me2Si
tBuMe3Si
100.9
98.0
[a] Determined by 1H/29Si HMQC NMR spectroscopy (500/99 MHz, 298 K,
optimized for J = 7 Hz) in 1,2-Cl2C6D4. [b] 2.0 equiv used due to the volatility of
Me4Si. [c] The solid state 29Si NMR spectrum (CPMAS) of Et3Si+[CHB11H5Br6]–
has been reported: δ (29Si) = 111.8 and 106.2 ppm.[10b]
Me3(vinyl)Si
Me3PhSi
Vinyl‒H (ethylene)
Ph‒H (benzene)
[b]
Me3(nBuC≡C)Si
—
[a] Determined by 1H/29Si HMQC NMR spectroscopy (500/99 MHz, 298 K,
optimized for J = 7 Hz) in 1,2-Cl2C6D4. [b] Formation of hex-1-yne not verified.
LG = leaving group.
The method was also applicable to the cleavage of
unactivated Sn–C(sp3) bonds and, hence, to the synthesis of
trialkylstannylium ions[13,23] stabilized by the counteranion
(Scheme 4). Treatment of nBu4Sn with [C6H6·H]+[CHB11H5Br6]‒
gave [nBu3Sn]+[CHB11H5Br6]‒ with a highly deshielded tin atom.
Its 119Sn chemical shift of δ 476.8 ppm is in accordance with
repor-ted [nBu3Sn]+[CMeB11Me11]‒ at δ 454.3 ppm.[23b]
Scheme 3. Chemoselective protonation of disilane (top) and molecular
structure of the silyl-substituted silylium ion-like Me2(Me3Si)Si+[CHB11H5Br6]‒
(bottom; thermal ellipsoids are shown at the 50% probability level; hydrogen
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10.1002/anie.201805637
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Scheme 4. Protonation of tetraalkylstannane for the generation of the
corresponding stannylium ion.
Q.W. gratefully acknowledges the Alexander von Humboldt
Foundation for a postdoctoral fellowship (2017–2019), and L.O.
thanks the Fonds der Chemischen Industrie for a predoctoral
fellowship (2015‒2017). Z.-W.Q. thanks Prof. Dr. Stefan Grimme
for his support as well as funding through the Deutsche
Forschungsgemeinschaft (Leibniz Prize to S.G.). M.O. is
indebted to the Einstein Foundation (Berlin) for an endowed
professorship.
Density functional theory (DFT) calculations were performed
at the PW6B95-D3/def2-QZVP
+ COSMO-RS(benzene) //
TPSS-D3/def2-TZVP + COSMO(benzene) level of theory[24] to
provide insight into the mechanism of the protonation of Me4Si
induced by Wheland complex A+ (Scheme 5). In solution, A+
exists as
a separate ion together with the counteranion
[CHB11H5Br6]‒. The proton of A+ attacks at one of the methyl
groups in Me4Si either opposite to the silicon atom through
TSAB+ or from the side of the Si–C(sp3) bond through TSAB’+.
The former scenario proceeds with inversion of the configuration
at the carbon atom and is kinetically more favorable (∆∆G‡ = 5.6
kcal/mol). That step releases CH4 and the silylium ion B+ and is
exergonic by 11.5 kcal/mol. The same applies to the protonation
of the Sn–C(sp3) bond yet with a lower barrier of 13.2 kcal/mol
(see the Supporting Information for details).
Keywords: Brønsted acids • carboranes • density functional
calculations • protonation • silylium ions
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Suggestion for the Entry for the Table of Contents
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Q. Wu, Z.-W. Qu,* L. Omann, E. Irran,
H. F. T. Klare, M. Oestreich*
Page No. – Page No.
Cleavage of Unactivated Si‒C(sp3)
Bonds with Reed’s Carborane Acids:
Formation of Known and Unknown
Silylium Ions
Watch your back! The benzenium ion [C6H6·H]+[CHB11H5Br6]‒ enables the
cleavage of inert Si–C(sp3) bonds, yielding counteranion-stabilized silylium ions. As
shown for Me4Si, the protonation proceeds with inversion at the carbon atom (see
scheme). The method is broadly applicable to tetraalkylsilanes and extends to
various activated Si–C(spn) bonds (n = 3–1). The related stannylium ions can be
made in the same way.
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