A highly enantiomerically enriched lithiosilane by selective cleavage of a silicon-phenyl bond with lithium

Article abstract of DOI:10.1002/anie.200604636

(Chemical Equation Presented) On the path to chiral polysilanes, the synthesis and first structural characterization of the enantiomerically pure tetrasilane (R)-3 was achieved. A new synthetic strategy was used, by which the cleavage of a silicon-phenyl

Full text of DOI:10.1002/anie.200604636

Communications
DOI: 10.1002/anie.200604636
Chiral Silanes
A Highly Enantiomerically Enriched Lithiosilane by Selective
Cleavage of a Silicon–Phenyl Bond with Lithium**
Carsten Strohmann,* Christian Däschlein, Marco Kellert, and Dominik Auer
Preparative access to enantiomerically enriched lithio-
silanes[1,2] is very restricted and usually requires an
aromatic substituent on the silicon atom.[3] We reported
recently on the selective cleavage of the silicon–phenyl
bond in a diphenyl(aminomethyl)silane with lithium
metal (Scheme 1).[4] Building on this result, we won-
ꢀ
Scheme 2. Synthesis of (R)-3 by cleavage of the Si Si bond in (R)-4 with
lithium.
enantiomerically pure disilane (R)-3 is obtained with reten-
tion of the configuration over both reaction steps.[2c] Com-
Scheme 1. Selective cleavage of a silicon–phenyl bond in the diphenyl-
silane 1 with lithium and formation of the disilane rac-3.
ꢀ
ꢀ
pound (R)-3 comprises both a Si Si bond and a Si Ph bond as
potential reaction sites for conversions with lithium metal. In
ꢀ
initial conversions of rac-3 with lithium at 08C in THF the Si
Ph bond was cleaved selectively with the formation of rac-5,
and by subsequent reaction with Ph2MeSiCl the racemic
trisilane rac-6 was obtained. To be able to prepare the
corresponding highly enantiomerically enriched lithiosilane 5
and transform it under preservation of the stereoinformation,
it must have sufficiently high configurational stability in
solution. It was known from earlier experimental work that
the highly enantiomerically enriched lithiosilane 2 racemizes
within a few hours at temperatures above 08C, and thus its
synthesis was carried out at ꢀ788C.[2c,7] Then, to be able to
estimate the configurational stability of 5 in comparison to
2—in accompaniment to the experimental work—the inver-
sion barriers for an inversion process through a formally free
silyl anion in the gas phase were determined on the basis of
two model systems by means of quantum chemical calcula-
tions [MP2/6-31 + G(d)].[8] The model compound Me2PhSiꢀ
(model system for 2) with an inversion barrier of 135 kJmolꢀ1
is configurationally slightly more stable than the model
compound Me2(H3Si)Siꢀ (model system for 5) with
125 kJmolꢀ1.[9] Thus, no stable configuration can be assumed
in the experiment for 5 also at temperatures above 08C.
dered whether this reaction could also be used for the
synthesis of highly enantiomerically enriched lithiosilanes.
This new selective type of reaction appeared particularly
attractive for the synthesis of an enantiomerically enriched
lithiosilane that is not substituted with aryl groups. However,
ꢀ
it was not conclusive whether a Si Ph bond can be cleaved
with lithium under preservation of the stereoinformation at
the Si center. Herein we describe the synthesis of the highly
enantiomerically enriched lithiosilane 5 (see Scheme 3) by
ꢀ
cleavage of a Si Ph bond and conversion of 5 into highly
enantiomerically enriched oligosilanes.
Alkyl-substituted oligosilanes with four or more con-
nected Si centers with trans conformation are sought-after
model systems for an understanding of the electronic proper-
ties of corresponding polysilanes. Such oligosilanes are
hitherto not sufficiently characterized structurally, and
highly enantiomerically enriched systems with stereogenic
Si centers have until now not been structurally investi-
gated.[5,6]
The enantiomerically pure disilane (R)-4, described by us
in earlier work, can be converted into the enantiomerically
ꢀ
Therefore, the cleavage of the Si C bond of (R)-3 was first
carried out at ꢀ788C, but no reaction was observed. Only at a
ꢀ
pure lithiosilane 2 by cleavage of the Si Si bond with lithium
(Scheme 2).[2c] After subsequent trapping with Me3SiCl the
temperature of ꢀ508C was the Si C bond in (R)-3 completely
ꢀ
cleaved. After five hours reaction time 5 was treated
separately with Ph2MeSiCl and Me3SiSiMe2Cl at ꢀ788C,
and the silanes (R)-6 and (R)-7 were obtained after workup in
yields of 53% and 88% and with enantiomeric ratios of
ꢁ 98:2 (Scheme 3).
[*] Dr. C. Strohmann, C. Däschlein, M. Kellert, Dr. D. Auer
Institut für Anorganische Chemie
Universität Würzburg
Am Hubland, 97074 Würzburg (Germany)
Fax: (+49)931-888-4605
E-mail: mail@carsten-strohmann.de
The absolute configuration of (R)-3 was determined by
single-crystal X-ray structural analysis of the hydrochloride
(R)-3·HCl (Figure 1).[10] The absolute configuration of (R)-7
was elucidated by single-crystal X-ray structural analysis of
(R)-7·H2SO4 (Figure 2).[10] The conversion of (R)-3 into (R)-7
(cleavage and trapping reaction) thus runs with overall
retention of the configuration at the Si center. We suggest
[**] We thank the Deutsche Forschungsgemeinschaft and the Fonds der
Chemischen Industrie for the financial support of this work and
Wacker Chemie AG for the supply of special chemicals.
Supporting information for this article is available on the WWW
under http://www.angewandte.com or from the author.
4780
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 4780 –4782

Angewandte
Chemie
Both (R)-3·HCl and (R)-7·H2SO4 crystallize from Et2O in
the monoclinic crystal system in space group P21, whereby in
each case two molecules are contained in the asymmetric unit.
ꢀ
The Si Si bond lengths are in the usual range with values
between 2.342(2) and 2.371(2) Š. The tetrasilane (R)-
7·H2SO4 shows an all-trans conformation of the Si centers
for both molecules [torsion angles: Si1-Si2-Si3-Si4
ꢀ176.62(7)8, Si5-Si6-Si7-Si8 174.65(7)8]. A helical conforma-
tion, induced by the defined stereogenic center, as discussed
for chiral polysilanes, cannot thus be observed for the
tetrasilane (R)-7·H2SO4 in the crystal structure.[5]
It is known that poly- and oligosilanes show intense
absorption bands in the near-UV range which are generated
by their special electronic properties (delocalization of the
bonding s electrons along the silicon framework).[5,6,11] More
recent studies on linear oligosilanes show that this delocal-
ization is most effective for the anti conformation.[12] Such an
anti conformation was observed in the solid state for the salt
of the tetrasilane (R)-7·H2SO4. Even if it is not possible to
assume with certainty that the anti conformation also
predominates in solution, the UV absorption spectrum of
(R)-7 (293 K, cyclohexane) exhibits a maximum at 232 nm.
Thus, (R)-7 shows a UV absorption spectrum in the same
absorption region as that of the s!s* transition of the
tetrasilane Si4Me10 at 293 K (233 nm).[13,14] In current inves-
tigations we are attempting to elucidate the effect of chain
extension and protonation at the nitrogen center on the
conformation and the resulting electronic behavior.
Scheme 3. Selective cleavage of the silicon–phenyl bond in the phenyl-
silane (R)-3 with lithium and synthesis of the trisilane (R)-6 and
tetrasilane (R)-7.
An enantiomerically enriched lithiosilane has been syn-
ꢀ
thesized by cleavage of a Si C bond for the first time. With
Figure 1. Molecular structure and numbering scheme of (R)-3·HCl in
the crystal (Schakal plot; molecule 1 of two molecules). Selected bond
lengths [Š]: Si1-Si2 2.346(4), Si2-C4 1.884(7), Si2-C5 1.896(7).
this new preparative approach the first highly enantiomeri-
cally enriched, non-aryl-substituted lithiosilane 5 has been
obtained. Bond cleavage and reactions with chlorosilanes run
with overall retention of configuration. The first molecular
structure of an enantiomerically enriched tetrasilane in the
solid state could also be determined. This new synthetic
strategy allows the construction of (aminomethyl)-function-
alized, highly enantiomerically enriched oligosilanes and
provides a route to corresponding polysilanes.
Received: November 14, 2006
Published online: May 22, 2007
Keywords: chirality · lithiation · lithiosilanes · organosilanes ·
.
silanes
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the identical reaction path for the reaction of the enantio-
merically pure lithiosilane 5 with MePh2SiCl to the enantio-
merically pure compound 6 so that the R configuration
likewise arises for the corresponding reaction product.
Angew. Chem. Int. Ed. 2007, 46, 4780 –4782
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
4781

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