Preparation of Si-centered chiral silanes by direct α-lithiation of methylsilanes

Article abstract of DOI:10.1002/chem.200902551

The direct α-lithiation of methyl-substituted silanes as an efficient method for the preparation and elaboration of Si-chiral compounds is reported. Deprotonation of chiral oligosilanes occurs selectively and with high yields at the methyl group of the stereogenic silicon center, even in the presence of multiple methylsilyl or methylgermyl substituents. Computational studies have confirmed this preference as a consequence of pre-coordi-nation of the lithiating agent by the amino side-arm and repulsion effects in the corresponding transition state. This complexation is also obvious from Xray structure analyses of the α-lithiated silanes, which exhibit intriguing structure formation patterns differing in the type of aggregation and the amount of alkyllithium used. An alternative route to Si-chiral compounds is also presented, which involves desymmetrization of dimethylsilanes mediated by a chiral side-arm. Structure analyses and computational studies have shown that the diastereoselectivity of this α-lithiation is influenced by the selectivity of the formation of the stereogenic nitrogen upon complexation of the alkyllithium.

Full text of DOI:10.1002/chem.200902551

DOI: 10.1002/chem.200902551
Preparation of “Si-Centered” Chiral Silanes by Direct a-Lithiation
of Methylsilanes
Christian Dꢀschlein, Viktoria H. Gessner, and Carsten Strohmann*[a]
Abstract: The direct a-lithiation of
methyl-substituted silanes as an effi-
cient method for the preparation and
elaboration of Si-chiral compounds is
reported. Deprotonation of chiral oli-
gosilanes occurs selectively and with
high yields at the methyl group of the
stereogenic silicon center, even in the
presence of multiple methylsilyl or
methylgermyl substituents. Computa-
tional studies have confirmed this pref-
erence as a consequence of pre-coordi-
nation of the lithiating agent by the
amino side-arm and repulsion effects in
the corresponding transition state. This
complexation is also obvious from X-
ray structure analyses of the a-lithiated
silanes, which exhibit intriguing struc-
ture formation patterns differing in the
type of aggregation and the amount of
alkyllithium used. An alternative route
to Si-chiral compounds is also present-
ed, which involves desymmetrization of
dimethylsilanes mediated by a chiral
side-arm. Structure analyses and com-
putational studies have shown that the
diastereoselectivity of this a-lithiation
is influenced by the selectivity of the
formation of the stereogenic nitrogen
upon complexation of the alkyllithium.
Keywords: chirality · lithium · orga-
nolithium compounds · organosili-
con · structure elucidation
Introduction
fully understood or even remain unexplored due to the ex-
tremely limited number of suitable preparation methods.
Silanes with a stereogenic silicon center are accessible
through three general methodologies.
Functionalized silicon compounds are well known for their
applications in various fields of chemistry, for example as
protecting or activating groups in organic synthesis.[1] In
recent years, the use of chiral silicon systems has become of
particular interest due to their application as auxiliaries in
stereoselective synthesis.[2] In this context, silicon groups
with a stereogenic silicon center have attracted a great deal
of attention owing to the high selectivities that can be ach-
ieved in a variety of reactions, such as silicon-to-element
(especially carbon and silicon) chirality transfers,[3] Mannich
reactions,[4] cycloaddition reactions,[5] Friedel–Crafts alkyla-
tions,[6] and aldol reactions,[7] as well as in the kinetic resolu-
tion of chiral secondary alcohols.[8] Despite this high synthet-
ic potential of silicon-chiral silanes in preparative chemistry,
many of their properties and applications are as yet not
i) The first method is the separation of stereoisomers by
the formation of diastereomers.[9] These diastereomeric
silanes can then be separated by standard resolution
methods, principally chromatography and crystallization.
In the case of crystallization, there have been examples
of target silanes that already possessed two or more ste-
reocenters (diastereomers) and thus did not require any
additional reaction step for their separation.[10] For com-
pounds with only one stereocenter (enantiomers), the
formation of diastereomers by the addition of a chiral
auxiliary has proved to be the method of choice. One
recent example is the treatment of aminomethyl-func-
tionalized silanes with mandelic acid,[11] which resulted
in quaternization of the nitrogen in the side-arm of the
molecule to form diastereomeric salts with different
crystallization properties. However, although this
method has generally proved to be highly efficient, it is
severely limited as only a few systems with suitable crys-
tallization behavior have as yet been identified.
[a] Dr. C. Dꢀschlein, Dr. V. H. Gessner, Prof. Dr. C. Strohmann
Anorganische Chemie, Technische Universitꢀt Dortmund
Otto-Hahn-Strasse 6, 44227 Dortmund (Germany)
Fax : (+49)231-755-7063
E-mail: mail@carsten-strohmann.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902551: Crystallographic
data of the presented compounds and aggregates; full computational
details.
ii) The second methodology for the synthesis of Si-centered
chiral silanes is the transformation of stereochemically
pure compounds to new chiral molecules. In particular,
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the cleavage of silicon–silicon or silicon–carbon bonds
with elemental lithium, thereby forming enantiomeri-
cally pure lithiosilanes, has been used for their prepara-
tion.[11–13] Subsequent trapping of the lithiated species
with electrophiles such as chlorosilanes or alkyl halides
results in the formation of novel stereochemically pure
silicon-chiral compounds. However, the lithiosilanes in-
volved can potentially lose stereo-information during
the reaction process due to their relatively low configu-
rative stabilities.[14] Although they are significantly more
stable than the corresponding carbon compounds, selec-
tive transformations to stereochemically pure trapping
products can only be accomplished at low temperatures
(À308C and lower) at which racemization is inhibited.
iii) Recently, some initial reports have appeared concerning
the use of chiral auxiliaries such as (R)-BINAP or (R,S)-
JOSIPHOS for the synthesis of silicon-chiral silanes.
The treatment of achiral reactants with metal salts and
the chiral ligand results in the formation of enantiomeri-
cally enriched silanes. However, the generality of appli-
cation of this method is still less than adequate and thus
the potential of the reaction is as yet extremely limit-
ed.[15] Scheme 1 summarizes the three described meth-
ods for the preparation of enantiomerically enriched
and pure Si-chiral silanes.
complishment of a variety of trapping reactions. The stereo-
chemical course of the presented deprotonation reactions
has been proved by X-ray diffraction analyses of the lithiat-
ed intermediates. Further structural analyses in combination
with computational examinations have provided insight into
the reaction mechanism and the observed selectivities.
Results and Discussion
In recent years, our group has reported several synthetic
pathways to enantiomerically pure tri- and tetrasilanes, as
well as silagermanes, based on trapping reactions of enantio-
merically pure lithiosilanes with chlorosilanes and chloroger-
manes (see Figure 1 for selected examples).[11,12a,b]
As part of our studies on stereochemically enriched and
pure silanes, we present herein the synthesis of Si-chiral
compounds by direct a-lithiation of methyl groups at silicon.
Two different synthetic routes are presented, namely the se-
lective transformation of an already stereochemically pure
Si-chiral species to a new chiral molecule and the diastereo-
selective a-lithiation of a dimethylsilane via a chiral side-
arm. The potential of this method is underlined by the ac-
Figure 1. Selected examples of highly enantiomerically enriched di- and
trisilanes as well as one silagermane that could be synthesized via enan-
tiomerically pure lithiosilanes.
However, due to the lack of configurative stability of the
intermediate lithiosilanes, further variants for direct trans-
formations of the enantiomerically enriched silanes without
a labile intermediate are highly desired. The best method
would be a direct functionalization of the substituents
bonded to the silicon without the involvement of the central,
stereogenic silicon atom. As all hitherto reported highly
enantiomerically enriched silanes have possessed at least
one methyl group at each silicon (or germanium) atom,
direct functionalization by deprotonation seemed to be a
promising strategy. Although such direct deprotonation of
methyl groups at silicon atoms in silanes is in general highly
disfavored,[16] so-called “side-arm complexation” of amino-
functionalized silanes proved to be the method of choice for
enabling this reaction.[17] The general concept for this depro-
tonation reaction is based on Klumppsꢂ idea[18] of oxygen-
and nitrogen-assisted lithiations, and it has recently been
used for the synthesis of functionalized a-lithiated silanes
with additional stabilizing groups close to the lithiated
center (e.g., benzylsilanes).[19] To the best of our knowledge,
this reaction has not yet been studied in relation to oligosi-
lanes with a stereogenic silicon center, and in this regard we
especially wanted to address two central questions: i) Can a
simple methyl group in an oligosilane be directly deproton-
ated by treatment with alkyllithium bases? ii) If so, can we
also accomplish regio- and stereoselective deprotonation re-
Scheme 1. Common methodologies for the preparation of silicon-chiral
silanes.
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4049

C. Strohmann et al.
actions in molecules with more than one methyl group at
the silicon atom?
and (R)-9,[21] under full preservation of the stereo-informa-
tion of the chiral silicon center with e.r. values >99:1. Thus,
the following two results may be noted: 1) Treatment of the
disilane 3 with tert-butyllithium results in selective deproto-
nation of the methyl group at the stereogenic silicon center,
and this may be exploited for selective functionalization.
2) As the stereogenic silicon center is not involved in the re-
action, enantiomerically pure, functionalized silanes are
easily accessible.
Synthesis of a-functionalized, enantiomerically pure silanes
by selective a-lithiation of enantiomerically pure reactants:
The trimethylsilyl-substituted disilane 3 seemed to be partic-
ularly suitable for these investigations. This compound pos-
sesses four methyl groups that, in principle, might be depro-
tonated. Furthermore, the steric demand of the whole
system is relatively low, and thus the necessary approach of
the alkyllithium base to the silane should be possible. To
evaluate the potential of the lithiation reaction for the func-
tionalization of such silicon compounds, the racemic silane
rac-3 was chosen as a first starting system. Thus, a solution
of rac-3 in n-pentane was treated with one equivalent of
tert-butyllithium for 5 h at room temperature. tBuLi was
chosen as it is the strongest commonly used alkyllithium
base. Trimethylchlorostannane was subsequently added to
the reaction mixture at À788C. To examine the reaction pro-
cess and the formed compounds, the crude product was ana-
lyzed by NMR and GC/MS. Only one a-stannyl-functional-
ized silane could be identified. The treatment of 3 with tert-
butyllithium had resulted in selective deprotonation of the
methyl group at the stereogenic silicon center, even though
there are four possible reactive methyl groups in the mole-
cule. The formed a-stannyl-functionalized silane rac-8 could
be isolated after work-up in an overall yield of 77% (see
Scheme 2). Likewise, the analogous trapping reaction of the
a-lithiated silane with paraformaldehyde as electrophile re-
sulted in the formation of the b-hydroxysilane rac-9 in 85%
yield.
Scheme 3. Synthesis of the enantiomerically pure, a-functionalized silanes
(R)-8 and (R)-9.
Based on these promising results pertaining to the selec-
tive deprotonation of the disilane (R)-3, we sought to
expand the method to the related trimethylgermyl-substitut-
ed silagermane (R)-6. Therefore, a solution of (R)-6 in n-
pentane was treated with one equivalent of tert-butyllithium
for 5 h at room temperature and the intermediate was sub-
sequently trapped with trimethylchlorostannane at À788C.
NMR and GC/MS analyses of the crude product confirmed
that selective deprotonation of the methyl group at the ste-
reogenic silicon center had also occurred in the silagermane
(R)-6. After work-up, the a-stannyl-functionalized silager-
mane (R)-11 was isolated in an overall yield of 95% (see
Scheme 4).
Analogous reactions were performed with the trisilane
rac-4. However, we changed the trapping reagents from par-
aformaldehyde and trimethylchlorostannane to deuterium
oxide and dimethyldichlorogermane to broaden the range of
trapping reagents used. As the main focus of this reaction
concerned the divergent trapping reagents, it was only car-
ried out using the racemic compound rac-4, which is more
Scheme 2. Synthesis of the racemic, a-functionalized silanes rac-8 and
rac-9.
Following the successful application of direct lithiation to
the racemic compound, the enantiomerically pure disilane
(R)-3 was subjected to the whole reaction sequence to ex-
amine the feasibility of this method for the preparation of
a-functionalized enantiomerically pure silanes. The same re-
gioselectivity was observed in the a-lithiation of (R)-3, with
exclusive deprotonation of the methyl group at the stereo-
genic silicon center (see Scheme 3). Detailed NMR studies
revealed the formation of both trapping products, (R)-8[20]
Scheme 4. Synthesis of the enantiomerically pure, a-functionalized sila-
germane (R)-11.
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Routes to Silicon–Chiral Silanes
FULL PAPER
easily accessible. Nevertheless, we postulate the same reac-
tivity for the enantiomerically pure trisilane. Compared to
the disilane 3, the phenyl group is substituted by the signifi-
cantly more sterically demanding diphenylmethylsilyl group.
In total, five different methyl groups are present in the mol-
ecule, resulting in three different possible reactive centers.
This raised the question as to whether the regioselective de-
protonation would still be possible despite the steric conges-
tion of the system. Therefore, after stirring with tBuLi for
5 h, the intermediate was trapped with either D2O or
Me2GeCl2. In both cases, NMR and GC/MS analyses of the
crude product confirmed selective deprotonation of the trisi-
lane at exclusively one position (see Scheme 5), the methyl
group at the stereogenic silicon center.
The lithiated disilane (rac-7)4 crystallized from n-pentane
in the tetragonal crystal system with space group P421/c as a
tetrameric structure with a central lithium tetrahedron with
an average side length (Li–Li distance) of 2.72(8) ꢃ (see
Figure 2). As is typical for such tetrameric alkyllithiums, the
Li3 triangles are m3-capped by the carbanionic centers, result-
ing in three contacts between each deprotonated carbon
atom and the lithium atoms.[22] The Li–C distances are be-
tween 2.249(5) and 2.416(5) ꢃ, and are thus in the typical
range for oligomeric alkyllithium compounds, as exemplified
by (tBuLi)4.[22] The lithium atoms are coordinated by the
aminomethyl side-arm of the disilane unit, which stabilizes
the whole system by completing the coordination sphere of
the lithium.
Figure 2. Molecular structure of the lithiated silane (rac-7)4 in the crystal.
All hydrogen atoms except those on the lithiated carbon centers, as well
as the numbering scheme of the hydrogen atoms and most of the carbon
atoms, have been omitted for clarity. Selected bond lengths [ꢃ] and
Scheme 5. Synthesis of the enantiomerically pure, a-functionalized silanes
rac-13 and 14.
À
À
À
À
angles [8]: C1 Si1 1.844(3), C2 Si1 1.898(3), C1 Li 2.249(5), C1 Li’
À
À
À
À
2.301(5), C1 Li’’ 2.416(5), C11 N1 1.479(4), C11 Si1 1.906(3), Li N1
À
À
À
À
2.160(5), Li C1’ 2.301(5), Li C1’’’ 2.416(5), Li Li’ 2.466(9), Li Li’’
Overall, these lithiation reactions prove that the treat-
ment of aminomethyl-functionalized oligosilanes with tBuLi
is a facile and useful method for the regioselective deproto-
nation of the methyl group at the stereogenic silicon center
and thus for the elaboration of enantiomerically pure, a-
functionalized systems.
À
À
À
2.849(8), Li Li’’’ 2.849(8), Li Si1 3.103(5), Si1 Si2 2.3677(11); Si1-C1-Li
98.12(17), N1-Li-C1 97.8(2); symmetry operations to generate equivalent
atoms: ’: Àx+1, Ày+1, z; ’’: y, Àx+1, Àz+2; ’’’: Ày+1, x, Àz+2.
Surprisingly, treatment of (R)-3 with two equivalents of
tBuLi also resulted in the formation of a tetrameric struc-
ture, albeit with a different composition. In this case, a
mixed aggregate of tBuLi and the lithiated disilane was
formed, in which the central Li4 tetrahedron consisted of
two molecules of the a-lithiated disilane and two molecules
of the sterically less bulky tBuLi (see Figure 3).[23] [(R)-7·
(tBuLi)]2 crystallized from n-pentane in the hexagonal crys-
tal system, space group P64. In this structure, the average
side length (Li–Li distance) of the central Li4 tetrahedron
amounts to 2.59(9) ꢃ, which is significantly shorter than that
in the structure of the racemic disilane. This bond shorten-
ing is due to the decreased spatial demand upon the incor-
poration of the tBuLi units instead of the disilane molecules.
Again, the Li–C contacts (2.290(8)–2.299(6) ꢃ) are in the
typical range for this class of compounds. Consequently, the
use of either one or two equivalents of tBuLi results in se-
lective deprotonation of the methyl group at the stereogenic
Structural studies: To obtain a profound and detailed insight
into the reaction process, the lithiated intermediates were
crystallized and their structures were determined by X-ray
diffraction analysis. To this end, one equivalent of each oli-
gosilane was treated with one equivalent of tert-butyllithium,
and the mixture was stirred for 5 h at room temperature and
then stored at À788C (À308C in the case of the lithiation of
trisilane 4). In this way, single crystals of the lithiated disi-
lane (rac-7)4 as well as of the lithiated trisilane (rac-12)2
could be isolated. In addition, all compounds were analo-
gously treated with two equivalents of tBuLi, which yielded
the different crystalline solids [(R)-7·ACTHNUTRGEN(UNG tBuLi)]2 and [(R)-10·
(tBuLi)]2. Interestingly, three different types of molecular
structures could be identified in the X-ray diffraction analy-
ses.
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C. Strohmann et al.
Figure 3. Molecular structure of the lithiated silane [(R)-7·
crystal. The numbering scheme of the hydrogen atoms has been omitted
for clarity. Selected bond lengths [ꢃ] and angles [8]: C1 Si1 1.835(3),
N
Figure 4. Molecular structure of the lithiated silagermane [(R)-10·
(tBuLi)]2 in the crystal. The numbering schemes of the hydrogen atoms
and some of the carbon atoms have been omitted for clarity. Selected
À
À
À
À
À
À
C2 Si1 1.892(3), C1 Li2 2.213(7), C1 Li1’ 2.275(6), C1 Li1 2.299(6),
bond lengths [ꢃ] and angles [8]: C1 Si 1.832(5), C2 Si 1.877(5), C11 Si
À
À
À
À
À
À
À
C11 N1 1.477(4), C11 Si1 1.922(3), C17 Li2 2.290(8), C17 Li2’ 2.295(9),
1.910(5), C1 Li1 2.291(10), C1 Li1’ 2.265(9), C17 Li2 2.263(14), C17
À
À
À
À
À
À
À
C17 Li1 2.471(8), Li1 N1 2.203(7), Li1 C1’ 2.275(6), Li1 Li2 2.512(8),
Li2’ 2.266(13), C11 N 1.485(6), Li1 N 2.189(9), Li1 Li2 2.505(12), Li1
À
À
À
À
À
À
Li1 Li2’ 2.668(8), Li1 Li1’ 2.760(11), Li2 C17’ 2.295(9), Li2 Li2’
Li2’ 2.651(12), Li1 Li1’ 2.747(16), Li2 Li2’ 2.388(19), Li2 Li1’,
À
À
À
2.419(13), Li2 Li1’ 2.668(8), Si1 Si2 2.3331(15); Si1-C1-Li1 95.4(2), N1-
C11-Si1 114.4(2); symmetry operations to generate equivalent atoms:
’: Àx+2, Ày+1, z.
2.651(12), Si Ge 2.3606(18); Si-C1-Li1 95.0(3), N-C11-Si 113.8(3); sym-
metry operations to generate equivalent atoms: ’: Àx+2, Ày+1, z.
silicon center. However, different structures are formed.
With an excess of tBuLi, the sterically demanding disilane
unit is replaced by the smaller tert-butyl unit.
Crystallization experiments with the lithiated silagermane
rac-10 using one as well as two equivalents of tBuLi also re-
sulted in the formation of crystalline solids. However, phase
transformations during the selection of the crystals prevent-
ed their measurement. Nevertheless, after treatment of the
enantiomerically pure silagermane with two equivalents of
tBuLi resulted in the formation of the same crystalline solid
of the lithiated system. In contrast to the tetrameric struc-
tures described above, (rac-12)2 forms a dimeric structure
that does not incorporate tBuLi. Evidently, the increased
steric demand of the trisilane permits only the formation of
this dimeric structure. Its central structural motif consists of
an Li–C–Li–C four-membered ring, which, in analogy to
those in other dimeric lithium organyls,[24] is slightly deviat-
ed from planarity (sum of angles: 357.788). The Li–C con-
tacts are significantly smaller (2.142(4), 2.175(4) ꢃ) than
those in the hitherto discussed lithiated silanes, but they are
in the range of monomeric lithium organyls (contacts small-
er than 2.20 ꢃ).[22] This can easily be explained with refer-
ence to the coordination sphere of the lithium. As shown in
Figure 5, the lithium atoms in (rac-12)2 have a coordination
number of only three (cf. four in the lithiated disilane and
silagermane). This threefold coordination results in in-
creased cationic character and thus in a stronger electrostat-
ic attraction between the carbanionic carbon and the short-
ened Li–C contact.
Figure 6 shows the three different structural motifs;
Figure 7 summarizes the determined structural motifs of the
a-lithiated silanes. Overall, the structure formation is deter-
mined by the steric demand of the system and the amount
of tBuLi used. In general, a structure with a central Li4 tet-
rahedron, comparable to that in (tBuLi)4, is preferred. In-
creasing steric demand results in the formation of mixed ag-
gregates incorporating two lithium organyls, and finally in
the formation of a dimeric structure after a further increase
in the bulkiness of the substituents at the central silicon.
In each of the isolated systems, the Si–C distance to the
lithiated carbon center is the shortest of all of the Si–C dis-
tances in the respective molecule {for example, (rac-7)4:
tBuLi, a crystalline solid could be isolated. [(R)-10·ACHTNUTRGNEUNG(tBuLi)]2
crystallized from n-pentane in the hexagonal crystal system,
space group P64, and was found to possess the same mixed
structure (Figure 4), with two molecules of the silagermane
being replaced by smaller tBuLi units, as described above
for the enantiomerically pure disilane [(R)-7·ACHTUNGRTNEUNG(tBuLi)]2.
The average side length (Li–Li distance) of the central Li4
tetrahedron is 2.562(13) ꢃ and is therefore in the same
range as that of the mixed aggregate of the above-described
disilane (2.59(9) ꢃ) and significantly shorter than that of the
lithiated disilane without tBuLi (2.72(8) ꢃ). The Li–C con-
tacts (2.263(14)–2.291(10) ꢃ) are in the typical range for
this class of compounds. In contrast to what was found for
the lithiated disilane 7, only crystals of the mixed aggregate
could be identified for the lithiated silagermane, even after
the usage of a sub-stoichiometric amount of tBuLi. Thus, it
seems that the formal substitution of the SiMe3 unit in 3 by
the sterically more demanding GeMe3 unit in 6 prevents the
crystallization of a structure without incorporated tBuLi.
Only the exchange of the lithiated silagermane by the small-
er tBuLi enables the successful crystallization of [(R)-10·
(tBuLi)]2, even if there is a deficiency of tBuLi.
Finally, we also succeeded in isolating the lithiated trisi-
lane (rac-12)2. The use of either one or two equivalents of
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Routes to Silicon–Chiral Silanes
FULL PAPER
and can be attributed to a polarizing effect (a-effect) be-
tween the lithiated carbon and the neighboring silicon.[19c]
A
second similarity of all of the lithiated silanes concerns the
aminomethyl side-arm. In all of the determined solid-state
structures, the amino side-arm is seen to be coordinated to
the adjacent lithium of the Li4 tetrahedron or of the Li–C–
Li–C four-membered ring, respectively. This coordination
stabilizes all of the solid-state structures, while, at the same
time, is also the reason for the successful deprotonation of
these systems, which would otherwise be impossible. This
complexation also suggests a pre-coordination of the lithium
organyl in the transition state. According to the complex-in-
duced proximity effect (CIP effect), this results in the ap-
proach of the reactive groups (alkyllithium base and methyl
group) and thus enables the deprotonation.
Figure 5. Molecular structure of the lithiated silane (rac-12)2 in the crys-
tal. The numbering scheme of the hydrogen atoms has been omitted for
À
À
clarity. Selected bond lengths [ꢃ] and angles [8]: C4 N 1.482(3), C4 Si2
À
À
À
À
1.930(2), C10 Si2 1.836(2), C10 Li’ 2.142(4), C10 Li 2.175(4), Li N
Theoretical studies: In the experiments, all of the silanes
showed selective lithiation of the methyl group at the ste-
reogenic silicon atom, thus indicating a strong preference
for this process compared to potential alternative deproto-
nation reactions. To gain insight into the energetic situation
of the deprotonation reactions and to evaluate the different
reaction barriers, DFT studies were performed. For exam-
ple, the deprotonation of disilane 3 at the SiMe group
(SiMe-ED, SiMe-TS) and at the SiMe3 group (SiMe3-ED,
SiMe3-TS) were calculated (see Figure 8). At first, a mono-
meric model system was assumed, with tBuLi coordinated
À
À
À
À
2.052(4), Li C10’ 2.142(4), Li Li’ 2.348(7), Li Si2 3.004(4), Si1 Si2
2.3426(12), Si2 Si3 2.3601(12); N-C4-Si2 114.52(14), Si2-C10-Li
96.64(14), N-Li-C10 101.69(17), N-Li-Si2 66.36(11), C10-Li-Si2 37.37(8),
C4-N-Li 98.63(16); symmetry operations to generate equivalent atoms:
’: Àx, y, Àz+1/2.
À
by the piperidino side-arm (according to the CIP effect[25]
)
and dimethyl ether present to complete the coordination
sphere. Fourfold coordination of the lithium could be ex-
cluded due to the spatial overload of the system. The geo-
metries of the stationary points were initially optimized
using density functional theory with the hybrid B3LYP func-
tional and the 6-31+G(d) basis set.[26] To identify transition-
state structures, frequency calculations were carried out at
the same level. Both transition states are depicted in
Figure 8.
The calculations on this monomer-based mechanism
showed a strong preference for deprotonation of the SiMe
group compared to that of the SiMe3 unit. While SiMe-TS
has an associated barrier of only 60 kJmolÀ1, the deprotona-
tion of the SiMe3 unit requires 84 kJmolÀ1. The overall pref-
Figure 6. Drawing of the molecular structure of the lithiated silanes; El=
Si (disilane) or Ge (silagermane).
Figure 7. Comparison of the different structural motifs of the lithiated si-
lanes.
1.844(3) ꢃ, [(R)-7·ACHTUNGTRENNUNG(tBuLi)]2: 1.835(3) ꢃ, [(R)-10·ACHTUNGTRNEN(UGN tBuLi)]2:
1.832(5) ꢃ, (rac-12)2: 1.836(2) ꢃ}. Such a shortened bond
has recently been observed in an a-lithiated benzylsilane
Figure 8. Transition states of the a-lithiation of disilane 3; selective lithia-
tion of the methyl group (SiMe-TS, left), selective lithiation of the trime-
thylsilyl group (SiMe3-TS, right), B3LYP/6-31+G(d).
Chem. Eur. J. 2010, 16, 4048 – 4062
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4053

C. Strohmann et al.
erence of 24 kJmolÀ1 is consistent with the experimentally
observed selective deprotonation. This energy difference
can be rationalized in terms of the spatial situations in the
corresponding transition states and starting systems. In the
starting system of the deprotonation of the SiMe3 group,
there is already a strong repulsion between the bulky SiMe3
group and the spatially demanding tert-butyl moiety of the
organolithium reagent. Approach of the reactants in the
transition state leads to a further increase in the repulsion
between these groups, thus resulting in the experimentally
observed selective deprotonation of the SiMe group. This
strong preference is also evident in a dimer-based mecha-
nism. The formation of such a dimeric model system with its
Li–C–Li–C four-membered ring is reasonable as numerous
organolithium compounds are known to form such dimeric
structures. This mechanism was assumed as in the experi-
ments no coordinating solvent was used. The dimeric transi-
tion states were modeled with trimethylamine replacing the
second piperidino side-arm (see Figure 9). Calculations on
this dimer-based mechanism yielded a barrier of only
76 kJmolÀ1 for deprotonation of the SiMe group. For depro-
tonation of the SiMe3 group, no stationary point could be lo-
cated due to the spatial overload of this system. Overall, the
calculations are consistent with the experimental observa-
tions and account for the observed selectivity in terms of re-
pulsion effects in the corresponding transition states.
readily and selectively a-deprotonated at a methyl group,
thereby allowing the facile introduction of stereo-informa-
tion into the molecule.[27,28] Thus, dimethylsilane (R,R)-16
was synthesized by a trapping reaction of the a-lithiated
(R,R)-TMCDA with the corresponding chlorosilane
(Scheme 6) and was obtained in 62% yield after work-up
and distillation.
Scheme 6. Synthesis of dimethylsilane (R,R)-16 via a-lithiated (R,R)-
TMCDA.
For selective lithiation of the dimethylsilane, a solution of
(R,R)-16 in n-pentane was treated with tert-butyllithium at
low temperature and the intermediate was trapped with
either trimethyl- or tributyltin chloride (Scheme 7). Prior to
Scheme 7. Asymmetric lithiation of dimethylsilane (R,R)-16 and trapping
with trimethyltin chloride to afford stannane 18.
work-up, the reaction mixtures were examined by NMR
spectrometry to determine the selectivity of the deprotona-
tion reaction. This revealed diastereoselectivities between
70:30 and 80:20, depending on the reaction temperature
(Table 1), with yields of 18 of around 70% after work-up.
Table 1. Reaction conditions and selectivities of the trapping of 17.
Figure 9. Dimer-based transition state of the a-lithiation of the SiMe
group of disilane 3; B3LYP/6-31+G(d).
Lithiation/trapping temperature [8C]
d.r.
À78 to À70/À70
À110 to À75/À75
À110 to À78/À78
À110 to À78/À78
crystals + solution
crystals + solution
crystals + solution
crystals
70:30
74:26
80:20
80:20
Synthesis of a-functionalized, enantiomerically pure silanes
by diastereoselective a-lithiation of
a dimethylsilane:
Having established the conditions for direct deprotonation
of methyl-substituted silanes, we tried to expand this reac-
tion to the desymmetrization of a non-chiral silicon center,
that is, to the preparation of Si-centered chiral silanes by se-
lective a-lithiation of one specific methyl group of a dimeth-
yl-substituted silane. For this purpose, a silane with a chiral
side-arm for the transfer of stereo-information was synthe-
sized. As starting material, we selected the C2-symmetric
The decrease in selectivity with increasing reaction tempera-
ture can be attributed to the kinetic control of such deproto-
nation reactions. The moderate asymmetric induction of the
TMCDA side-arm is in accordance with the low selectivities
observed in asymmetric deprotonation reactions mediated
by this ligand.[29]
chiral
(1R,2R)-N,N,N’,N’-tetramethylcyclohexane-1,2-dia-
To understand the moderate induction of the amino side-
arm used, crystallographic investigations of the intermediate
mine [(R,R)-TMCDA, (R,R)-15], since it is known to be
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Routes to Silicon–Chiral Silanes
FULL PAPER
lithiated silane were performed at different reaction temper-
atures. From a reaction mixture obtained by carrying out
the lithiation at À1208C and warming to À788C, crystals of
the diastereomerically pure silane (R,R,SSi)-17 were isolated.
In contrast, when deprotonation was carried out at a rela-
tively high temperature (À408C), crystals of a 1:1 aggregate
of both possible diastereomers were formed upon cooling to
À788C. The crystal structures of the two different lithiated
molecules are depicted in Figures 10 and 11. Figure 12
Figure 11. Molecular structure of the 1:1 aggregate of both silane diaste-
reomers (R,R,RSi)- and (R,R,SSi)-17. All hydrogen atoms of the amino
side-arm except those at the stereogenic carbon centers and those of the
solvent molecule have been omitted for clarity. Selected bond lengths
À
À
À
[ꢃ] and angles [8]: C1 Si1 1.877(3), C18 Si1 1.810(3), C8 Si1 1.907(4),
À
À
À
À
C18 Li2 2.199(7), C18 Li1 2.239(6), C19 Si2 1.900(4), C26 Si2 1.910(4),
À
À
À
À
C36 Si2 1.806(4), C36 Li2 2.205(7), C36 Li1 2.271(7), Li1 N1 2.133(6),
À
À
À
Li1 N2 2.152(6), Li2 N3 2.068(6), Li2 N4 2.194(6); Li2-C36-Li1 67.4(2),
Li2-C18-Li1 68.1(2), C18-Li1-C36 109.7(3), C18-Li2-C36 113.8(3).
Figure 10. Molecular structure of diastereomerically pure silane (R,R,SSi)-
17. All hydrogen atoms of the amino side-arm except those at the stereo-
genic carbon centers and those of the solvent molecule have been omit-
À
ted for clarity. Selected bond lengths [ꢃ] and angles [8]: C17 Si1
À
À
À
À
1.878(3), C18 Si1 1.787(4), C18 Li1’ 2.190(8), C18 Li1 2.315(6), N1 Li1
À
À
À
À
2.149(7), N2 Li1 2.137(7), Li1 C18’ 2.190(8), Li1 Li1’ 2.525(13), C10
Si1 1.891(3), C11 Si1 1.893(4); Si1-C18-Li1’ 127.3(3), Si1-C18-Li1
100.3(2), Li1’-C18-Li1 68.1(3), N2-Li1-N1 84.1(3), N2-Li1-C18’ 136.9(3),
N2-Li1-C18 95.2(2), N1-Li1-C18 120.6(3), C18’-Li1-C18 109.6(3), Li1’-
C18-Li1 68.1(3); symmetry operations to generate equivalent atoms:
Àx+1, y, Àz+1.
À
Figure 12. Molecular structures of racemic silanes (R,R,RSi)- and
(R,R,SSi)-17.
shows a schematic representation of both structures. Both
the diastereomerically pure product and the 1:1 aggregate of
the two diastereomers of the lithiated silane form dimeric
compounds in the crystal. The pure silane (R,R,SSi)-17 crys-
tallizes in the monoclinic crystal system, space group C2,
while the 1:1 aggregate crystallizes in the orthorhombic crys-
tal system, space group P212121. Both molecular structures
include an additional solvent molecule (pentane, hexane).
The asymmetric unit of the 1:1 aggregate contains one mole-
cule of the dimer, while that of the diastereomerically pure
compound contains only one half of the dimer, which is as-
sembled to the dimer through C2 symmetry. The central
structural motif of both dimers is formed by an Li–C–Li–C
four-membered ring, as has been observed in the case of
rac-12, and is typical of dimeric alkyllithium com-
pounds.[22,24] The lithium atoms exhibit fourfold coordina-
tion, with contacts to the metalated carbon atoms and both
nitrogen atoms of the diamino side-arm. The Li–C distances
vary between 2.190(8) and 2.315(13) ꢃ in the (R,R,SSi)-
silane and between 2.199(7) and 2.271(7) ꢃ in the aggregate,
and are thus comparable to those in known oligomeric orga-
nolithiums.[22,24] The same is true for the lithium–nitrogen
contacts. The silicon atoms have tetrahedral coordination
spheres, with almost ideal angles ranging from 102.2(2) to
116.0(2)8. However, the Si–C distances vary significantly,
with the shortest contacts to the lithiated carbon atoms
((R,R,SSi)-silane: 1.787(4) ꢃ; 1:1 aggregate: 1.810(3) and
1.806(4) ꢃ). This shortening has already been observed in
the a-lithiated oligosilanes (see above) and can be attribut-
ed to the polarizing effect (a-effect) between the lithiated
carbon and the silicon.[19c]
The fact that the diastereomerically pure lithiated inter-
mediate could be isolated from the reaction mixture
prompted us to investigate whether an increase in the dia-
stereoselectivity might be achieved by carrying out a trap-
ping reaction with exclusively the crystalline solid. However,
after work-up, the same diastereomeric ratio (80:20) was ob-
tained as with the whole reaction mixture. This indicated
Chem. Eur. J. 2010, 16, 4048 – 4062
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4055

C. Strohmann et al.
that both the (R,R,SSi)-silane and the 1:1 aggregate were
present in the solid and that no enrichment of the selectivity
could be achieved by crystallization.
neglected due to strong repulsion of particular groups. For
some of these structures, preliminary calculations at a lower
level did not even show a stationary point. Figure 13 depicts
the four most favored transition states (two for each diaste-
reomer of 17), which additionally differ in the configuration
at the nitrogen atom. The calculations showed that the three
transition states all had energies in the same range, albeit
with the most favored one, (RSi,RN)-19-TS, leading to the
RSi-configured product. At first glance, this seems to be in
contradiction to the experiments, whereby the SSi product
was probably the main isomer, as this could be isolated
from the diastereomerically enriched crystallization
medium. However, one also has to consider the configura-
tion at the nitrogen. While the RN-configured transition
states (Figure 13, lower part) show an energetic difference
of only 2 kJmolÀ1, their SN-configured counterparts differ in
energy by 9 kJmolÀ1 (Figure 13, upper part). This suggests
that with the RN configuration, the reaction leading to both
diastereomers, that is, the formation of a 1:1 mixture of
(R,R,SSi)-17 and (R,R,RSi)-17, may be expected. On the con-
trary, the SN configuration would lead to a preference for
the silane (R,R,SSi)-17. Most interestingly, these calculated
results on the isomeric transition states are consistent with
the molecular structures of 17. Thus, the diastereomerically
pure silane (R,R,SSi)-17 showed selective SN configuration
with regard to the conformation of the silyl unit, which was
also identified as the most favored configuration from the
theoretical point of view. On the contrary, the 1:1 aggregate
of the two diastereomers of 17 exhibited selective RN config-
Theoretical studies of the diastereoselective a-lithiation: To
gain insight into the mechanistic features of the asymmetric
deprotonation of dimethylsilane (R,R)-16, computational
studies were performed to evaluate the reaction barriers as-
sociated with lithiations of both diastereotopic methyl
groups. For the deprotonation, a mechanism comparable to
that for the piperidinomethyl-substituted silane 3 involving
a pre-coordinated species was assumed, in accordance with
the complex-induced proximity effect (CIPE).[25] As a model
system, the monomeric adduct (R,R)-19 was chosen, in
which the tert-butyllithium molecule is
pre-coordinated by the (R,R)-TMCDA
moiety. The formation of such an inter-
mediate during the lithiation reaction is
reasonable, as an analogous monomeric
tBuLi·ACTHNUGTRNEUNG(R,R)-TMCDA adduct is formed
during the deprotonation reaction of the
amine itself.[24] In adduct 19, the nitrogen
atom of the side-arm becomes stereogenic upon pre-coordi-
nation of the alkyllithium, so that different isomers have to
be considered for the calculation of the transition states (19-
TS) as well as of the pre-coordinated adducts 19. In the ex-
perimentally observed intermediate a-lithiated silanes
(R,R,RSi)- and (R,R,SSi)-17, there is no distinct preference
for one configuration. While the diastereomerically pure
(R,R,SSi)-17 (Figure 10) exhib-
its an SN configuration, the ni-
trogen of the 1:1 aggregate of
both diastereomers (Figure 11)
has an RN configuration. In the
SN configuration, the sterically
more demanding silyl substitu-
ent adopts the pseudo-equato-
rial position of the five-mem-
bered ring that is formed upon
the coordination of tBuLi.
First, we performed DFT
studies on the transition states
at the
B3LYP/6-31+G(d)
level. In principle, two transi-
tion states have to be consid-
ered for both diastereomeric
products
(R,R,SSi)-17
and
(R,R,RSi)-17, which differ in
the configuration at this ste-
reogenic nitrogen (RN and SN).
Additionally, further conform-
ers are possible by rotation of
the NCH2Si moiety. For each
diastereomer (RSi and SSi),
three respective conformers
and isomers were calculated.
Further conformers could be
Figure 13. Calculated structures and energies of the diastereomeric transition states of the a-lithiation of the
two methyl groups of dimethylsilane (R,R)-16, B3LYP/6-31+G(d).
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Routes to Silicon–Chiral Silanes
FULL PAPER
uration. Again, the conformations of the RSi- and SSi-silanes
were in accordance with the calculated structures.
Consequently, the experimental and theoretical observa-
tions suggest that the stereochemical outcome of the a-lith-
iation of silane (R,R)-16 is determined by the configuration
of the stereogenic nitrogen atom of the pre-coordinated
tBuLi adduct 19. As the diastereomeric reaction pathways
are associated with almost identical reaction barriers, this si-
multaneously implies that the greater the energetic differ-
ence between the RN and SN configurations of 19, the higher
the diastereoselectivity of the deprotonation. Subsequent
DFT calculations (at the B3LYP/6-31+G(d) level) of the
different isomers of 19 revealed a preference of 3 kJmolÀ1
for the RN configuration. Again, calculations were per-
formed on several conformers (see the Supporting Informa-
tion). The three most favored isomers are depicted in
Figure 14.
Overall, the energy difference of 3 kJmolÀ1 between the
RN and SN configurations of 19 is the crucial criterion for the
stereochemical outcome of the deprotonation. This small
difference of 3 kJmolÀ1 befits the moderate selectivities ob-
served experimentally. The low reaction barriers of around
60 kJmolÀ1 confirm that the a-lithiation will proceed even at
low reaction temperatures. Additional calculations taking
into account dispersion effects by using the M052X func-
tional and the 6-31+G(d) basis set were indicative of the
same tendencies as the DFT calculations, albeit with an ele-
vated preference (DH=9 kJmolÀ1) for the RN configuration
(see the Supporting Information). The energies obtained are
also listed in Table 2. Again, the most favored isomers and
conformers have the same structures as those observed in
the isolated crystals. The calculated transition states also
point to the formation of a 1:1 mixture (DDH° =2 kJmolÀ1)
via the RN-configured intermediate 19 and a preference for
the SSi-diastereomer 17 (DDH° =15 kJmolÀ1) when starting
from the SN-configured adduct 19. Contrary to the calcula-
tions using the B3LYP functional, the transition state lead-
ing to the SSi-silane showed the lowest barrier (DH° =
56 kJmolÀ1) under the consideration of dispersion effects.
The energetic features of the a-lithiation of silane (R,R)-16
are summarized in Figure 15. Overall, the calculations re-
flect the experimental observations (moderate selectivities,
path of the reaction) rather well. The apparent impact of
the configuration at the stereogenic nitrogen atom on the
stereoselectivity suggests that higher selectivities may be
achieved by using dimethylsilanes for which there are higher
energy differences between the isomers of the pre-coordi-
nated tBuLi adducts.
Figure 14. Calculated structures of the isomers and conformers of the
pre-coordinated tBuLi adduct 19, B3LYP/6-31+G(d).
Table 2. Relative energies [kJmolÀ1] of the isomers and conformers of
the precoordinated tBuLi adduct 19.
Compound
B3LYP/6-31+G(d)
M052X/6-31+G(d)
SN-Ed-1
SN-Ed-2
SN-Ed-3
RN-Ed-1
RN-Ed-2
RN-Ed-3
0
4
8
5
14
3
0
8
14
9
24
19
Conclusion
silanes and silagermanes were selectively deprotonated at
the stereogenic silicon center. Computational studies have
pointed to a mechanism involving pre-complexation of the
alkyllithium by the piperidinomethyl side-arm and the in-
duction of regioselectivity due to repulsion effects in the
corresponding transition states. The crystal structures of the
In conclusion, we have presented the direct a-lithiation of
methylsilanes as a powerful and efficient method for the
generation of Si-chiral compounds. Firstly, this method has
been successfully applied to the functionalization of already
Si-chiral silanes. In this approach, all of the examined oligo-
Chem. Eur. J. 2010, 16, 4048 – 4062
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4057

C. Strohmann et al.
tile compounds in vacuo, the crude product was purified by bulb-to-bulb
distillation (oven temperature: 1718C, pressure: 2.2ꢄ10À5 mbar); yield:
1.19 g, 2.62 mmol (77%). 1H NMR (500.1 MHz, C6D6): d=0.18 (m, 1H;
2
2
SiCH2Sn), 0.21 (s, JH,117Sn =25.8 Hz, JH,119Sn =27.0 Hz, 9H; Sn
ACHTUGNTRNEN(UNG CH3)3), 0.23
(m, 1H; SiCH2Sn), 0.28 (s, 9H; Si(CH3)3), 1.30–1.40 (m, 2H; NCCCH2),
AHCTUNGTRENNUNG
1.50–1.60 (m, 4H; NCCH2C), 2.35–2.45 (m, 4H; NCH2CC), 2.35, 2.50
2
(AB system, JAB =14.2 Hz, 2H; SiCH2N), 7.25–7.35 (m, 3H; arom. H-m,
H-p), 7.58–7.62 ppm (m, 2H; arom. H-o); {1H}13C NMR (125.8 MHz,
C6D6): d=À10.23 (1C; SiCH2Sn), À7.15 (1J
Sn =158.2 Hz, 1J
ACHTNUGTRNEN(NUG CH3)3), 24.2 (1C; NCCCH2),
=
13C,119Sn
13C,117
165.5 Hz, 3C; Sn
G
26.6 (2C; NCCH2C), 49.7 (1C; SiCH2N), 59.1 (2C; NCH2CC), 128.1
(2C; C-m, C6H5), 128.6 (1C; C-p, C6H5), 134.2 (2C; C-o, C6H5),
140.3 ppm (1C; C-i, C6H5); {1H}29Si NMR (99.4 MHz, C6D6): d=À19.2
(1Si; SiCH2N), À18.9 ppm (1Si; Si
ACHTNUGTRNEN(NUG CH3)3); GC/EI-MS: tR =8.20 min
[808C (2 min)–108CminÀ1–2808C (5 min)]; m/z (%): 455 (1) [M+], 440
(1) [MÀCH3+], 382 (6) [MÀSi
G
ACHTUNGTRENNUNG
[MÀCH2Sn
(CH3)3+]+, 165 (2) [Sn
ACHUTGTNRENNUG CAHTUNGTRENNUNG
Figure 15. Energetic features of the a-lithiation of silane (R,R)-16,
M052X/6-31+G(d).
(1) [SiACHTUNGTRNEUNG
(CH3)+]; elemental analysis (%) calcd for C19H37NSi2Sn: C 50.2, H
8.21, N 3.08; found: C 50.6, H 8.41, N 3.19.
Determination of the enantiomeric ratio of rac-8 with (R)-mandelic acid:
The enantiomeric ratio was determined in the presence of three equiva-
lents of (R)-mandelic acid. Therefore, rac-8 (15.0 mg, 33.0 mmol) was
added to a solution of (R)-mandelic acid (15.1 mg, 99.0 mmol) in CDCl3
(500 mL). 1H NMR (500.1 MHz, C6D6): d=À0.13 (m, 1H; SiCH2Sn, D1),
corresponding a-lithiated silanes exhibit interesting structur-
al motifs and types of aggregation. These have been found
to be crucially influenced by the spatial demand of the
silane and the amount of alkyllithium used, with the forma-
tion of mixed aggregates incorporating tert-butyllithium
being observed. Secondly, direct deprotonation has also
been applied to the formation of Si-chiral compounds by de-
symmetrization of dimethylsilanes. Here, the use of a chiral
(R,R)-TMCDA side-arm resulted in moderate diastereose-
lectivities. On the basis of X-ray analyses and computational
studies, the observed selectivities may be ascribed to the for-
mation of a stereogenic nitrogen center upon the pre-coordi-
nation of the alkyllithium to the silane. The greater the en-
ergetic difference between the different configurations of
the stereogenic nitrogen, the more selective the reaction.
We are currently seeking to optimize the diastereoselective
deprotonation by varying the substituents and the chiral
side-arm of the silane.
À0.11 (m, 1H; SiCH2Sn, D2), À0.08 (s, 9H; Sn
9H; Sn
SiCH2Sn, D2), 0.34 (s, 9H; Si
N
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
1.20–1.35 (m, 4H; NCCCH2, D1 & D2), 1.40–1.60 (m, 8H; NCCH2C, D1
& D2), 2.35–2.45, 2.50–2.65 (m, 6H each; NCH2CC & SiCH2N, D1 &
D2), 4.85 (s, 2H; CHOH), 7.25–7.35 (m, 6H; arom. H-m, H-p), 7.58–
7.62 ppm (m, 4H; arom. H-o); OH, NH not located; {1H}13C NMR
(125.8 MHz, C6D6): d=À10.8 (2C; SiCH2Sn, D1 & D2), À7.2 (6C,
1J
Sn =161.6 Hz, 1J
Sn =168.0 Hz; Sn
13C,119
G
&
D2), À1.38,
13C,117
À1.36 (3C each, 1J Si =21.8 Hz; Si
G
& D2), 22.4 (2C;
13C,29
NCCCH2, D1 & D2), 23.8 (4C; NCCH2C, D1 & D2), 48.9 (2C; SiCH2N,
D1 & D2), 57.0 (4C; NCH2CC, D1 & D2), 74.3 (2C; CHOH), 126.5
(4C; C-m, C6H5CHOH), 126.7 (2C; C-p, C6H5CHOH), 127.8 (4C; C-o,
C6H5CHOH), 128.0 (4C; C-m, C6H5Si), 129.0 (2C; C-p, C6H5Si), 134.1
(4C; C-o, C6H5Si), 137.1 (2C; C-i, C6H5Si), 142.7 (2C; C-i, C6H5CHOH),
177.7 ppm (2C; COO); {1H}29Si NMR (99.4 MHz, C6D6): d=À20.0 (2Si;
SiCH2N), À19.12, À19.10 ppm (1Si each; Si
ACHTUGNTREN(UNNG CH3)3, D1 & D2).
Synthesis of (R)-8: The synthesis of (R)-8 was analogous to that of rac-8.
The analytical data of the product were consistent with those of rac-8.
Yield: 1.35 g, 2.96 mmol, 86%; [a]2D0 =24.5 (c=0.40 in cyclohexane).
Determination of the enantiomeric ratio of (R)-8 with (R)-mandelic
acid: The enantiomeric ratio was determined in the presence of three
equivalents of (R)-mandelic acid. Therefore, (R)-8 (15.0 mg, 33.0 mmol)
was added to a solution of (R)-mandelic acid (15.1 mg, 99.0 mmol) in
CDCl3 (500 mL). 1H NMR (500.1 MHz, C6D6): d=À0.13 (m, 1H;
Experimental Section
Experimental details: All experiments were carried out under a dry,
oxygen-free argon atmosphere using standard Schlenk techniques. The
solvents used were dried over sodium and distilled prior to use. (R,R)-
TMCDA was prepared according to a literature method.[30] 1H, 13C, and
29Si NMR spectra were recorded on Bruker DRX-300 and AMX-500
spectrometers at 228C. Signal assignments were corroborated with the
aid of additional DEPT-135, C,H-COSY, and H,H-COSY experiments.
All chemical shifts are given in ppm on the d scale. All spin-spin coupling
constants (J) are given in Hertz (Hz).
SiCH2Sn), À0.08 (s, 9H; Sn
9H; Si
NCCH2C), 2.35–2.65 (m, 6H; NCH2CC
(CH3)3), À0.23 (m, 1H; SiCH2Sn), 0.05 (s,
AHCTUNGTRENNUNG
&
CHOH), 7.30–7.35 (m, 3H; arom. H-m, H-p), 7.40–7.45 ppm (m, 2H;
arom. H-o); OH and NH not located; {1H}13C NMR (125.8 MHz, C6D6):
d=À10.9 (1C; SiCH2Sn), À7.2 (3C, 1J
Sn =161.8 Hz, 1J
=
13C,119Sn
13C,117
169.1 Hz; Sn
G
ACHTUGNTRENN(UNG CH3)3), 22.1 (1C;
13C,29
NCCCH2), 23.4 (2C; NCCH2C), 48.8 (1C; SiCH2N), 56.7 (2C;
NCH2CC), 74.3 (1C; CHOH), 126.4 (2C; C-m, C6H5CHOH), 126.6 (C-p,
C6H5CHOH), 127.8 (2C; C-o, C6H5CHOH), 128.1 (2C; C-m, C6H5Si),
129.1 (1C; C-p, C6H5Si), 133.8 (1C; C-i, C6H5CHOH), 134.1 (2C; C-o,
C6H5Si), 137.0 (1C; C-i, C6H5Si), 177.5 ppm (1C; COO); {1H}29Si NMR
Synthesis of rac-8: A solution of rac-3 (1.00 g, 3.43 mmol) in n-pentane
(10 mL) was combined with a 1.7m solution of tBuLi in n-pentane
(2.04 mL, 3.45 mmol) at À788C. After warming to room temperature, the
reaction mixture was stirred for 5 h. Thereafter, one equivalent of
Me3SnCl (0.69 g, 3.45 mmol) was added at À788C and the solution was
allowed to warm to room temperature once more. After removal of all
volatiles in vacuo, the residue was suspended in 2m NaOH (35 mL) and
extracted with diethyl ether (5ꢄ30 mL). The combined organic layers
were extracted with 2m HCl (5ꢄ20 mL) and the combined aqueous
layers were subsequently adjusted to pH 12 with NaOH. Finally, the
aqueous layer was extracted with diethyl ether (5ꢄ30 mL) and the com-
bined organic layers were dried over Na2SO4. After removal of all vola-
(99.4 MHz, C6D6): d=À20.1 (1Si; SiCH2N), À19.1 (1Si; Si
ACHTUGNTRNEN(UNG CH3)3); e.r. >
99:1.
Synthesis of rac-9: A solution of rac-3 (2.50 g, 8.57 mmol) in n-pentane
(15 mL) was combined with a 1.5m solution of tBuLi in n-pentane
(6.28 mL, 9.43 mmol, 1.1 equiv) at À788C. After warming to room tem-
perature, the reaction mixture was stirred for 5 h. Thereafter, one equiva-
lent of paraformaldehyde (283 mg, 9.43 mmol) was added at À788C and
4058
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ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 4048 – 4062

Routes to Silicon–Chiral Silanes
FULL PAPER
the solution was allowed to warm to room temperature once more and
stirred for 24 h. After the addition of H2O (169 mg, 9.43 mmol) and stir-
ring for 24 h, all volatiles were removed in vacuo, and the residue was
suspended in a mixture of n-pentane (12 mL) and diethyl ether (15 mL)
and freed of all salts. After the removal of all volatile compounds in
vacuo, rac-9 could be isolated. Yield: 2.66 g, 8.27 mmol (85%). 1H NMR
(1C; SiCH2N), 59.0 (2C; NCH2CC), 128.1 (2C; C-m, C6H5Si), 128.8 (1C;
C-p, C6H5Si), 134.1 (2C; C-o, C6H5Si), 139.9 ppm (1C; C-i, C6H5Si);
{1H}29Si NMR (99.4 MHz, C6D6): d=À12.4 ppm (1Si; SiCH2N); GC/EI-
MS: tR =9.99 min [808C (2 min)–108CminÀ1–2808C (5 min)]; m/z (%):
486 (1) [(MÀMe)+], 382 (30) [(MÀSn)+], 98 (100) [(H2C=NC5H8)+]; ele-
mental analysis (%) calcd for C19H37NSiGeSn: C 45.7, H 7.48, N 2.81;
found: C 46.2, H 7.65, N 2.97.
(300.1 MHz, C6D6): d=0.16 (s, 9H; SiACTHNUTRGNEUNG(CH3)3), 1.20–1.30 (m, 2H;
NCCCH2), 1.45–1.60 (m, 4H; NCCH2C), 1.55–1.60 (m, 2H;
SiCH2CH2OH), 2.25–2.40 (m, 4H; NCH2CC), 2.01, 2.12 (AB system,
2JAB =14.4 Hz, 2H; SiCH2N), 4.17 (br, 2H; SiCH2CH2OH), 6.30–6.40 (br,
1H; SiCH2CH2OH), 7.20–7.35 (m, 3H; arom. H), 7.45–7.55 ppm (m, 2H;
Synthesis of rac-13: The synthesis of rac-13 was analogous to that of rac-
8. The residue was suspended in n-pentane (10 mL) and freed of all salts.
Thereafter, the crude product was purified by bulb-to-bulb distillation
(oven temperature: 2208C, pressure: 1.0ꢄ10À3 mbar); yield: 79.0 mg,
0.19 mmol (79%). 1H NMR (300.1 MHz, C6D6): d=0.22 (s, 9H; Si-
arom. H); {1H}13C NMR (75.5 MHz, CDCl3): d=À1.8 (3C; Si
ACHTUGNRTNE(NUNG CH3)3),
AHCTUNGTRENNUNG
(CH3)3), 0.43 (t, 1JHD =2.01 Hz, 2H; SiCH2D), 0.87 (s, 3H; SiSiSiCH3),
16.6 (1C; SiCH2CH2OH), 23.8 (1C; NCCCH2), 25.9 (2C; NCCH2C), 46.7
(1C; SiCH2N), 58.3 (2C; NCH2CC), 58.5 (1C; SiCH2CH2OH), 128.2
(2C; C-m, C6H5), 128.8 (1C; C-p, C6H5), 134.2 (2C; C-o, C6H5),
137.4 ppm (1C; C-i, C6H5); {1H}29Si NMR (59.6 MHz, CDCl3): d=À21.8
1.30–1.40 (m, 2H; NCCCH2), 1.50–1.60 (m, 4H; NCCH2C), 2.33 (d,
2JAB =1.28 Hz, 2H; SiCH2N), 2.35–2.40 (m, 4H; NCH2CC), 7.30–7.40 (m,
6H; arom. H-m, H-p), 7.75–7.80, 7.80–7.85 ppm (m, 2H each; arom. H-
o); {1H}13C NMR (75.5 MHz, C6D6): d=À6.88 (t, 1JCD =18.7 Hz, 1C;
(1Si; SiCH2N), À19.1 ppm (1Si; Si
ACHTUGNRTNE(NUNG CH3)3).
Determination of the enantiomeric ratio of rac-9 with [Eu
enantiomeric ratio was determined in the presence of an excess of tris[3-
(heptafluoropropylhydroxymethylene)-(+)-camphorato]europium(III),
[Eu(hfc)3]. Therefore, rac-8 (15.0 mg, 46.6 mmol) was added to a solution
of [Eu
(hfc)3] (30 mg) in CDCl3 (500 mL). 1H NMR (300.1 MHz, CDCl3):
d=0.32 (s, 9H; Si(CH3)3, D2), 0.33 (s, 9H; Si(CH3)3, D1), 1.00–1.10 (m,
(hfc)3]: The
SiCH2D), À3.00 (1C; SiCH3), À0.70 (3C; Si
ACHTUGNTRENN(UNG CH3)3), 24.2 (1C;
NCCCH2), 26.6 (2C; NCCH2C), 48.9 (1C; SiCH2N), 58.8 (2C;
NCH2CC), 128.12, 128.13 (2C each; C-m, C6H5Si), 129.03, 129.04 (1C
each; C-p, C6H5Si), 135.39, 135.51 (2C each; C-o, C6H5Si), 138.09,
138.19 ppm (1C each; C-i, C6H5Si); {1H}29Si NMR (59.6 MHz, C6D6): d=
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
G
U
À46.6 (1Si; SiCH2D), À18.7 (1Si; SiCH3), À15.4 ppm (1Si; Si
ACHTUNGTRENNUNG(CH3)3);
2H; SiCH2CH2OH, D1), 1.30–1.40 (m, 2H; NCCH2C, D2), 1.45–1.55 (m,
4H; NCCCH2, D1 & D2), 1.50–1.55 (m, 2H; SiCH2CH2OH, D2), 1.55–
1.65 (m, 2H; NCCH2C, D1), 1.95–2.10 (m, 4H; NCCH2C, D1 & D2),
2.35–2.45 (m, 8H; NCH2CC), 2.95–3.15 (D2), 3.15–3.25 (D1) (m, 2H
each; SiCH2N), 3.70–4.20 (br, 4H; SiCH2CH2OH, D1 & D2), 7.30–7.55
(m, 6H; arom. H), 7.85–7.95 ppm (m, 4H; arom. H); OH not located;
{1H}13C NMR (75.5 MHz, CDCl3): d=À1.78 (D1), À1.77 (D2) (3C each;
GC/EI-MS: tR =11.35 min [808C (2 min)–108CminÀ1–2808C (5 min)];
m/z (%): 412 (1) [M+], 397 (2) [(MÀMe)+], 339 (5) [(MÀSiMe3)+], 215
(18) [(MÀSiMePh2)+], 98 (100) [(H2C=NC5H10)+].
Synthesis of 14: The synthesis of 14 was analogous to that of rac-8. The
residue was suspended in n-pentane (10 mL), freed from all salts, diluted
with acetone (5 mL), and stored at À788C for 24 h. Thereafter, the
formed nearly solid residue was separated from the solution at low tem-
perature and the whole process was repeated twice more. Finally, all vol-
atile compounds were removed in vacuo; yield: 420 mg, 0.45 mmol
(75%). 1H NMR (500.1 MHz, C6D6): d=0.28, 0.29 (s, 9H each; Si-
SiACHTUNGTRENNUNG(CH3)3), 16.9 (1C; SiCH2CH2OH, D2), 17.2 (1C; SiCH2CH2OH, D1),
24.12 (2C; NCCCH2, D1 & D2), 26.37 (D1), 26.43 (D2) (2C each;
NCCH2C), 47.4 (D1), 47.5 (D2) (1C each; SiCH2N), 58.7 (2C;
SiCH2CH2OH, D1 & D2), 59.2 (D2), 59.3 (D1) (2C each; NCH2CC),
128.1 (D1), 128.3 (D2) (2C each; C-m, C6H5), 128.51 (D2), 128.57 (D1)
(1C each; C-p, C6H5), 134.45 (D2), 134.53 (D1) (2C each; C-o, C6H5),
137.6 (D2), 137.7 ppm (D1) (1C each; C-i, C6H5); {1H}29Si NMR
(59.6 MHz, CDCl3): d=À20.9 (1Si; SiCH2N, D1), À20.7 (1Si; SiCH2N,
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
NCCCH2), 1.55–1.65 (m, 8H; NCCH2C), 2.40–2.50 (m, 8H; NCH2CC),
2.426, 2.521 (AB system, 2JAB =13.9 Hz, 2H; SiCH2N), 2.430, 2.523 (AB
system, 2JAB =13.9 Hz, 2H; SiCH2N), 7.30–7.40 (m, 12H; arom. H-m, H-
p), 7.85–7.95 ppm (m, 8H; arom. H-o); {1H}13C NMR (125.8 MHz, C6D6):
D2), À19.1, À19.0 ppm (1Si each; Si
ACHTUGNTREN(UNNG CH3)3, D1 & D2).
Synthesis of (R)-9: The synthesis of (R)-9 was analogous to that of rac-9.
The analytical data of the product were consistent with those of rac-9.
Yield: 950 mg, 2.95 mmol, 91%; [a]2D0 =10.0 (c=0.30 in cyclohexane).
d=À2.38 (2C; CH2Ge
N
49.4 (2C; SiCH2N), 59.0 (4C; NCH2CC), 128.08, 128.12 (4C each; C-m,
C6H5Si), 128.99, 129.04 (2C each; C-p, C6H5Si), 135.64, 135.71 (4C each;
C-o, C6H5Si), 138.44, 138.48 ppm (2C each, C-i, C6H5Si); {1H}29Si NMR
(99.4 MHz, C6D6): d=À41.8 (2Si; SiCH2N), À18.8 (2Si; SiCH3),
Determination of the enantiomeric ratio of (R)-9 with [Eu
enantiomeric ratio was determined in the presence of an excess of tris[3-
(heptafluoropropylhydroxymethylene)-(+)-camphorato]europium(III),
[Eu(hfc)3]. Therefore, (R)-8 (15.0 mg, 46.6 mmol) was added to a solution
of [Eu
(hfc)3] (30 mg) in CDCl3 (500 mL). 1H NMR (300.1 MHz, CDCl3):
d=0.40 (s, 9H; Si(CH3)3), 0.95–1.05 (m, 2H; SiCH2CH2OH), 1.45–1.55
ACHTUNGRTENN(UNG hfc)3]: The
AHCTUNGTRENNUNG
À15.9 ppm (2Si; Si
ACHTNUGRTNE(NUGN CH3)3); GC/EI-MS: tR =13.61 min [808C (2 min)–
ACHTUNGTRENNUNG
108CminÀ1–2808C (5 min)]; m/z (%): 924 (2) [M+], 909 (4) [(MÀMe)+],
ACHTUNGTRENNUNG
851 (11) [(MÀSiMe3)+], 826 (3) [(MÀH2C=NC5H10)+], 728 (100)
ACHTUNGTRENNUNG
[(MÀ(H2C=NC5H10)2+], 514 (3) [{(Ph2MeSi)
ACHTUNGNERTN(UGN SiMe3)(CH2NC5H10)]Si-
(m, 2H; NCCCH2), 1.55–1.70, 1.85–2.00 (m, 2H each; NCCH2C), 2.15–
2.35 (m, 4H; NCH2CC), 2.95–3.15 (m, 2H; SiCH2N), 3.40–3.60 (br, 2H;
SiCH2CH2OH), 7.45–7.60 ppm (m, 5H; arom. H); OH not located;
AHCNUTRTGENU[GNN CH2GeACHTUNTRGENNNUG ACHTNUGRTNEUN(NG SiMe3)(CH2NC5H10)]Si(CH2Ge=
(CH3)2]]+, 498 (2) [{(Ph2MeSi)
CH2)+}, 197 (3) [(Ph2MeSi)+], 98 (100) [(H2C=NC5H10)+]; elemental
analysis (%) calcd for C48H78NSi6Ge: C 62.4, H 8.51, N 3.03; found: C
61.7, H 8.48, N 3.63.
{1H}13C NMR (75.5 MHz, CDCl3): d=À1.64 (3C; Si
ACHTUGNTRENN(UNG CH3)3), 18.1 (1C;
SiCH2CH2OH), 24.2 (1C; NCCCH2), 26.8 (2C; NCCH2C), 48.1 (1C;
SiCH2N), 58.9 (1C; SiCH2CH2OH), 59.7 (2C; NCH2CC), 128.7 (2C; C-
m, C6H5), 129.2 (1C; C-p, C6H5), 134.8 (2C; C-o, C6H5), 138.0 ppm (1C;
C-i, C6H5); {1H}29Si NMR (59.6 MHz, CDCl3): d=À19.9 (1Si; SiCH2N),
General conditions for the crystallization of the a-lithiated oligosilanes:
A solution of the appropriate oligosilane (50 mg) in n-pentane [V(n-pen-
tane)] was combined with a 1.7m solution of tBuLi in n-pentane, which
resulted in immediate cloudiness. The mixture was then stirred for 5 h at
room temperature. After storing each solution at low temperature, the a-
lithiated silanes could be successfully isolated as crystalline solids. Table 3
lists the full reaction conditions, crystallization temperatures, and quanti-
ties of the reactants used, as well as the solvents for the lithiation reac-
tions.
À18.5 ppm (1Si; Si
ACHTUNGTRENNUNG(CH3)3); e.r. >99:1.
Synthesis of (R)-11: The synthesis of (R)-11 was analogous to that of
(R)-8. The crude product was purified by bulb-to-bulb distillation (oven
temperature: 1708C, pressure: 1.0ꢄ10À3 mbar); yield: 705 mg, 1.41 mmol
1
(95%). 1H NMR (500.1 MHz, C6D6): d=0.24 (s, JH,117Sn =158.8 Hz,
1JH,119Sn =166.2 Hz, 9H; Sn
ACHTUNGTRENNUNG
(s, 9H; Ge
G
Synthesis of (R,R)-16: (R,R)-TMCDA [(R,R)-15] (3.90 g, 22.9 mmol) was
dissolved in n-pentane (20 mL) and the solution was cooled to À408C.
At this temperature, a 1.7m solution of tBuLi in n-pentane (17.6 mL,
29.9 mmol) was added, and the reaction mixture was allowed to slowly
warm to room temperature and then stirred for a further 3 h. The mix-
ture was then treated with dimethylphenylchlorosilane (5.12 g,
2
NCCH2C), 2.41, 2.54 (AB system, JAB =14.3 Hz, 2H; SiCH2N), 2.40–2.50
(m, 4H; NCH2CC), 7.30–7.35 (m, 3H; arom. H-m, H-p), 7.60–7.65 ppm
(m, 2H; arom. H-o); {1H}13C NMR (125.8 MHz, C6D6): d=À9.74 (1C;
SiCH2Sn), À7.23 (3C; 1J
Sn =158.8 Hz, 1J
ACHTUNGTRENNUNG(CH3)3), 24.2 (1C; NCCCH2), 26.6 (2C; NCCH2C), 49.8
Sn =166.2 Hz; SnACHTUNRTGEG(NNUN CH3)3),
13C,119
13C,117
À1.95 (3C; Ge
Chem. Eur. J. 2010, 16, 4048 – 4062
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4059

C. Strohmann et al.
Table 3. Reactants, products, reaction conditions, and quantities for the lithiation reactions.
1J
Sn =166.3 Hz; SnCH3), À6.80
13C,119
(SiCH2Sn), À1.64 (SiCH3), 24.7
+
Reactant/m
[mmol])
(reactant) [mg] (n- Product
Equiv. tBuLi (n-
A
V(n-pentane)
[mL]
Crystallization tempera-
ture [8C]
25.2 + 26.1 + 26.2 (CH2, cyclohex-
yl), 40.5 (N(CH3)2), 40.6 (NCH3),
G
AHCTUNGTRENNUNG
(R)-3/50 (0.17)
G
0.2
À78
45.68 (NCH2Si), 64.4 (CHN), 67.0
(CHN), 127.9 (C-m, C6H5), 129.0 (C-
p, C6H5), 134.2 (C-o, C6H5),
140.8 ppm (C-i, C6H5); 29Si NMR
(99.4 MHz, C6D6): d=À3.86 ppm;
119Sn NMR (186.5 MHz, C6D6): d=
6.54 ppm; GC/EI-MS: tR =10.92 min
rac-3/50 (0.17)
(R)-6/50 (0.15)
N
1 (0.17)
1 (0.12)
0.2
0.3
À78
À78
rac-4/50 (0.12)
0.3
À30
[808C
(2 min)–108CminÀ1–2808C
(5 min)]; m/z (%): 468 (1) [M+], 453 (3) [{MÀCH3)+}], 169 (100)
29.9 mmol) at À808C and the trapping reaction was allowed to proceed
for 12 h at RT. The LiCl formed was dissolved by the addition of 2.5m
HCl (20 mL). After the addition of diethyl ether (10 mL), the combined
organic layers were extracted with 2m HCl (3ꢄ20 mL). The aqueous
layers were subsequently adjusted to pH 12 with NaOH and extracted
with diethyl ether (3ꢄ30 mL), and the combined organic layers were
dried over Na2SO4. After removal of all volatile compounds in vacuo, the
crude product was purified by bulb-to-bulb distillation (oven tempera-
ture: 1608C, 2ꢄ10À2 mbar); yield: 4.32 g, 14.2 mmol (62%). 1H NMR
[{MÀPhSi
[{C6H9N
(500.1 MHz, C6D6): d=0.19 (s, JH,117Sn =25.6 Hz, JH,119Sn =26.7 Hz, 9H;
(CH3)CH2SnMe3}+], 135 (25) [{Si(CH3)2Ph}+], 124 (78)
ACHUTGTNRENNUG CAHTUNGTRENNUGN
ACHUTNGRENNUG CAHTUNGTRENNUGN
(CH3)CH2}+], 58 (82) [{N(CH3)2CH2}+]. Minor isomer: H NMR
1
2
2
2
2
SnCH3), 0.34 (s, JH,117Sn =24.8 Hz, JH,119Sn =26.0 Hz, 2H; SnCH2Si), 0.60
(s, 3H; Si(CH3)), 1.06–1.11 (m, 4H; 2CH2, cyclohexyl), 1.72–1.76 (m,
AHCTUNGTRENNUNG
2H; CH2, cyclohexyl), 1.83–1.86 (m, 1H; CH2, cyclohexyl), 1.93–1.97 (m,
1H; CH2, cyclohexyl), 2.36, 2.55 (AB system, 2JAB =14.2 Hz, 1H; N-
AHCUTNGERTN(GNUN CH2)Si), 2.37 (s, 3H; NAHCUTNRTEGNG(UNN CH3)CH2Si), 2.39 (s, 6H; NCATHNUGTREN(NUGN CH3)2), 2.38–2.43
(500.1 MHz, C6D6): d=0.50 (s, 3H; SiACTHNURGTENNUG(CH3)), 0.51 (s, 3H; SiACHTUNTGREN(NUGN CH3)), 1.03–
(m, 2H; CHN), 7.29–7.38 (m, 3H; arom. H), 7.73–7.79 ppm (m, 2H;
arom. H); {1H}13C NMR (125.8 MHz, C6D6): d=À7.92 (1J
=
1.17 (m, 4H; 2CH2, cyclohexyl), 1.70–1.73 (m, 2H; CH2, cyclohexyl),
13C,117Sn
2
158.8 Hz, 1J
Sn =166.4 Hz; SiCH3), À6.84 (SiCH2Sn), À1.79 (SiCH3),
1.84–1.89 (m, 2H; CH2, cyclohexyl), 2.26 (AB system, JAB =14.2 Hz, 1H;
13C,119
N
N
ACHTUGTNRENGUN(CH3)CH2Si), 2.38 (s, 6H; NCAHTUNGTRENNNUG
24.7
+
25.2
+
26.1
+ 26.6 (CH2, cyclohexyl), 40.7 (NACHTUNTGRENUGN(CH3)2), 40.9
AHCTUNGTRENNUNG
(NCH3), 45.70 (NAHCTUNGTRENNUNG
(CH2)Si), 64.5 (CHN), 66.9 (CHN), 128.0 (C-m, C6H5),
129.1 (C-p, C6H5), 134.1 (C-o, C6H5), 140.9 ppm (C-i, C6H5); 29Si NMR
(99.4 MHz, C6D6): d=À3.87 ppm; 119Sn NMR (186.5 MHz, C6D6): d=
À6.33 ppm; GC/EI-MS: as for major isomer.
D2), 7.34–7.36 (m, 3H; arom. H), 7.75–7.77 ppm (m, 2H; arom. H);
{1H}13C NMR (125.8 MHz, C6D6): d=À2.43, À2.40 (Si
ACHTUNGTRENNUNG
(CH2CHN), 25.0 (CH2CHN), 26.1 ((CH2)2CH2), 40.4 (NACHTUNGTRENNUNG
40.5 (NACHTUNGTRENNUNG(CH3)2), 44.2 (NACHTUNGTRENNUNG(CH2)Si), 64.3 (CHN), 67.1 (CHN), 128.0 (C-m,
General procedure for the trapping reaction of 17 with tributyltin chlo-
ride: Silane (R,R)-16 (120 mg, 0.39 mmol) was dissolved in pentane
(0.8 mL) and the solution was cooled to the first temperature indicated
in Table 1. At this temperature, a 1.5m solution of tBuLi in pentane
(0.3 mL, 0.45 mmol) was added and the mixture was slowly allowed to
warm over a period of 24 h to the second temperature indicated in
Table 1. After trapping of the intermediate with tributyltin chloride
(280 mg, 0.85 mmol) and addition of diethyl ether (10 mL), the mixture
was acidified to pH 1 and the organic phase was extracted three times
with water. The combined aqueous phases were then made alkaline to
pH 12, extracted with diethyl ether (5ꢄ30 mL), and the combined organ-
ic layers were dried over Na2SO4. After removal of all volatile com-
C6H5), 129.1 (C-p, C6H5), 134.2 (C-o, C6H5), 139.9 ppm (C-i, C6H5);
29Si NMR (99.4 MHz, C6D6): d=À6.2; [a]2D0 =À15.1 (cyclohexane,
0.7784 g/100 mL); GC/EI-MS: tR =9.71 min [808C (2 min)–108CminÀ1
–
2808C (5 min)]; m/z (%): 304 (6) [M+], 218 (25) [{MÀCH2CH2CH2N-
(CH3)2}+], 169 (85) [{MÀSi
(CH3)2Ph}+], 135 (50) [{Si(CH3)2Ph}+], 124
ACHUTGTNRENNUG CAHTUNGTRENNUGN
(95) [{C6H9NACHTUNGTRENNUNG ACHUTNGTRENNGUN
(CH3)CH2}+], 58 (100) [{N(CH3)2CH2}+]; elemental analysis
(%) calcd for C18H32N2Si: C 71.0, H 10.7, N 9.2; found: C 71.0, H 10.6, N
9.2.
Crystallization of the a-lithiated silane 17: Silane (R,R)-16 (120 mg,
0.39 mmol) was dissolved in pentane (0.8 mL) and the solution was
cooled to À1008C. At this temperature, a 1.5m solution of tBuLi in pen-
tane/hexane (0.3 mL, 0.45 mmol) was added and the mixture was slowly
warmed to À788C. Storage at À788C for 24 h gave colorless crystals of
the diastereomerically pure compound (R,R,SSi)-17. Addition of the al-
kyllithium at À408C and subsequent cooling to À788C yielded crystals of
the 1:1 aggregate of the respective diastereomers (R,R,SSi)-17 and
(R,R,RSi)-17.
pounds in vacuo, the crude product was examined by NMR spectrometry.
Major diastereomer: 1H NMR (500.1 MHz, C6D6): d=0.22 (s, J1-H,117
=
2
Sn
2
4
31.1 Hz, J1-H,119Sn =32.2 Hz, 2H; SiCH2Sn), 0.63 (s, J1-H,117/119Sn =20.0 Hz,
3H; SiCH3), 0.98 (t, 3J=7.33 Hz, 6H; SnCH2), 1.03 (t, 3J=7.27 Hz, 9H;
SnCH2CH2CH2CH3), 0.98–1.10 (m, 4H; 2CH2, cyclohexyl), 1.44–1.48 (m,
6H; SnCH2CH2CH2), 1.57–1.63 (m, 6H; SnCH2CH2), 1.81–1.87 (m, 2H;
CH2, cyclohexyl), 1.93–1.97 (m, 2H; CH2, cyclohexyl), 2.35, 2.62 (AB
Trapping of 17 to form (R,R)-18: (R,R)-16 (500 mg, 1.64 mmol) was dis-
solved in n-pentane (5 mL) and the solution was cooled to À1108C. At
system, 2JAB =14.0 Hz, 2H; N
(s, 6H; N(CH3)2), 2.39–2.43 (m, 2H; CHN), 7.29–7.38 (m, 3H; arom. H),
ACHTUTGNRENNU(G CH2)Si), 2.36 (s, 3H; NACHTUNGTRNEN(UGN CH3)CH2Si), 2.40
AHCTUNGTRENNUNG
this temperature,
a 1.2m solution of tBuLi in pentane (1.4 mL,
7.71–7.80 ppm (m, 2H; arom. H); {1H}13C NMR (125.8 MHz, C6D6): d=
1.68 mmol) was added and the reaction mixture was slowly warmed to
À788C. After storage at this temperature for 24 h, the intermediate was
trapped with trimethyltin chloride (380 mg, 1.91 mmol) and the mixture
was stirred for an additional 16 h at room temperature. Work-up was ac-
complished in the same manner as for compound 16, extracting with
aqueous acid, making the combined aqueous extracts alkaline, and ex-
tracting the aqueous phase with diethyl ether. After removal of all vola-
tile compounds from the combined extracts in vacuo, the crude product
was purified by bulb-to-bulb distillation to afford 18 as a 79:21 mixture
of the two diastereomers as a colorless oil (oven temperature: 2358C, 5ꢄ
10À3 mbar). Yield: 523 mg, 1.12 mmol (68%). Major isomer: 1H NMR
À9.59 (1J
Sn =93.9 Hz, 1J
Sn =98.1 Hz; SiCH2Sn), À1.49 (1J
=
13C,29Si
13C,117
13C,119
3.77 Hz; SiCH3), 10.7 (1J
Sn =155.3 Hz, 1J
Sn =162.6 Hz; SnCH2),
13C,119
13C,117
13.9 (4J
Sn =3.9 Hz; SnCH2CH2CH2CH3), 24.9 + 25.2 + 26.1 + 26.2
13C,117/119
(CH2, cyclohexyl), 27.8 (3J
Sn =28.34 Hz, 3J
Sn =29.64 Hz;
13C,119
13C,117
SnCH2CH2CH2), 29.6 (2J
Sn =9.70 Hz; SnCH2CH2), 40.5 (NACHTUNGTRENNUNG(CH3)2),
13C,117/119
40.7 (NCH3), 46.0 (NCH2Si), 64.49 (CHN), 67.1 (CHN), 127.9 (C-m,
C6H5), 128.9 (C-p, C6H5), 134.20 (C-o, C6H5), 141.0 ppm (C-i, C6H5);
29Si NMR (99.4 MHz, C6D6): d=À3.6 ppm (2J
Sn =11.5 Hz);
29Si,117/119
119Sn NMR (186.5 MHz, C6D6): d=À0.7 ppm. Minor diastereomer:
1H NMR (500.1 MHz, C6D6): d=0.24 (s, J1-H,117 =31.1 Hz, J1-H,119
=
Sn
2
2
Sn
32.2 Hz, 2H; SiCH2Sn), 0.64 (s, 3H; SiCH3), 0.98 (t, 3JHH =7.33 Hz, 6H;
SnCH2), 1.03 (t, 3JHH =7.27 Hz, 9H; SnCH2CH2CH2CH3), 0.98–1.10 (m,
4H; 2CH2, cyclohexyl), 1.44–1.48 (m, 6H; SnCH2CH2CH2), 1.57–1.63 (m,
6H; SnCH2CH2), 1.84–1.87 (m, 2H; CH2, cyclohexyl), 1.93–1.97 (m, 2H;
2
2
(500.1 MHz, C6D6): d=0.19 (s, JH,117Sn =25.6 Hz, JH,119Sn =26.8 Hz, 9H;
2
2
SnCH3), 0.33 (s, JH,117Sn =24.9 Hz, JH,119Sn =26.1 Hz, 2H; SnCH2Si), 0.59
(s, 3H; SiCH3), 1.05–1.10 (m, 4H; 2CH2, cyclohexyl), 1.71–1.75 (m, 2H;
CH2, cyclohexyl), 1.83–1.86 (m, 1H; CH2, cyclohexyl), 1.93–1.97 (m, 1H;
CH2, cyclohexyl), 2.38, 2.60 (AB system, 2JAB =14.0 Hz, 2H; N
G
CH2, cyclohexyl), 2.26, 2.55 (AB system, 2JAB =14.0 Hz, 2H; N
2.35 (s, 3H; N(CH3)CH2Si), 2.38 (s, 6H; N
ACHTUNGTRENNUNG
2.37 (s, 3H; N
(CH3)CH2Si), 2.39 (s, 6H; N
U
G
ACHTUNGTRENNUNG
CHN), 7.28–7.39 (m, 3H; arom. H), 7.74–7.79 ppm (m, 2H; arom. H);
CHN), 7.29–7.38 (m, 3H; arom. H), 7.71–7.80 ppm (m, 2H; arom. H);
{1H}13C NMR (125.8 MHz, C6D6): d=À7.95 (1J
Sn =158.7 Hz,
13C,117
{1H}13C NMR (125.8 MHz, C6D6): d=À9.66 (1J
Sn =98.1 Hz, 1J
=
13C,117Sn
13C,119
4060
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ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 4048 – 4062

Routes to Silicon–Chiral Silanes
FULL PAPER
93.9 Hz; SiCH2Sn), À1.56 (SiCH3), 8.3 (1J
Sn =155.3 Hz, 1J
=
13C,117Sn
d) P. D. Lickiss, C. M. Smith, Coord. Chem. Rev. 1995, 145, 75; e) A.
Sekiguchi, V. Y. Lee, M. Nanjo, Coord. Chem. Rev. 2000, 210, 11.
[2] For example, see: a) B. H. Lipshutz, N. Tanaka, B. R. Taft, C.-T. Lee,
Org. Lett. 2006, 8, 1963; b) B. Zhu, J. S. Panek, Eur. J. Org. Chem.
2001, 1701; c) M. Oestreich, Synlett 2007, 1629.
13C,117
162.6 Hz; SnCH2), 14.1 (4J
Sn =3.9 Hz, SnCH2CH2CH2CH3), 24.7 +
13C,117/119
25.2 + 26.1 + 26.6 (CH2, cyclohexyl), 27.8 (3J
Sn =28.34 Hz, J
=
13C,119Sn
3
13C,117
29.64 Hz; SnCH2CH2CH2), 29.8 (2J
Sn =9.70 Hz; SnCH2CH2), 40.5
13C,117/119
(NACHTUNGTRENNUNG(CH3)2), 40.8 (NCH3), 45.9 (NCAHTUNGTREN(NUGN CH2)Si), 64.54 (CHN), 67.0 (CHN),
128.0 (C-m or C-o, C6H5), 129.0 (C-p, C6H5), 134.16 (C-m or C-o,
[3] For example, see: a) S. Rendler, M. Oestreich, C. P. Butts, G. C.
Lloyd-Jones, J. Am. Chem. Soc. 2007, 129, 502; b) P. J. Stang, A. E.
Learned, J. Org. Chem. 1989, 54, 1779.
[4] G. T. Notte, J. L. Leighton, J. Am. Chem. Soc. 2008, 130, 6676.
[5] S. Shirakawa, P. J. Lombardi, J. L. Leighton, J. Am. Chem. Soc. 2005,
127, 9974.
[6] S. Shirakawa, R. Berger, J. L. Leighton, J. Am. Chem. Soc. 2005,
127, 2858.
[7] a) S. E. Denmark, T. Wynn, G. L. Beutner, J. Am. Chem. Soc. 2002,
124, 13405; b) S. E. Denmark, G. L. Beutner, J. Am. Chem. Soc.
2003, 125, 7800.
[8] S. Rendler, G. Auer, M. Oestreich, Angew. Chem. 2005, 117, 7793;
Angew. Chem. Int. Ed. 2005, 44, 7620.
[9] For one example, see: M. Trzoss, J. Shao, S. Bienz, Tetrahedron:
Asymmetry 2004, 15, 1501.
[10] For one example, see: M. Oestreich, U. K. Schmid, G. Auer, M.
Keller, Synthesis 2003, 2725.
[11] C. Strohmann, J. Hçrnig, D. Auer, J. Chem. Soc. Chem. Commun.
2002, 766.
[12] For example, see: a) C. Strohmann, C. Dꢀschlein, M. Kellert, D.
Auer, Angew. Chem. 2007, 119, 4864; Angew. Chem. Int. Ed. 2007,
46, 4780; b) C. Strohmann, C. Dꢀschlein, Organometallics 2008, 27,
2499; c) C. Dꢀschlein, V. H. Gessner, C. Strohmann, Acta Crystal-
logr. Sect. E 2008, 64, o1950; d) L. H. Sommer, R. Mason, J. Am.
Chem. Soc. 1965, 87, 1619; e) L. H. Sommer, J. E. Lyons, H. Fujimo-
to, J. Am. Chem. Soc. 1969, 91, 7051; f) E. Colomer, R. J. P. Corriu,
J. Chem. Soc. Chem. Commun. 1976, 176; g) E. Colomer, R. J. P.
Corriu, J. Organomet. Chem. 1977, 133, 159.
C6H5), 141.1 ppm (C-i, C6H5); 29Si NMR (99.4 MHz, C6D6): d=À3.6 ppm
(2J
Sn =11.5 Hz); 119Sn NMR (186.5 MHz, C6D6): d=À0.9 ppm.
29Si,117/119
Crystallographic details: Crystallographic data for (R)-7, (R)-10, rac-12,
(R,R,SSi)-17, and the 1:1 aggregate of 17 were collected with a Bruker
APEX-CCD (D8 three-circle goniometer) (Bruker AXS). Cell determi-
nation and refinement were carried out with Smart version 5.622 (Bruker
AXS, 2001), integration with SaintPlus version 6.02 (Bruker AXS, 1999),
and empirical absorption correction with SADABS version 2.01 (Bruker
AXS, 1999). Crystallographic data for rac-7 were collected with a Stoe-
IPDS diffractometer. Cell determination was carried out with Cell (Stoe
& Cie, 1997), cell refinement with Integrate (Stoe & Cie, 1999), and nu-
merical absorption correction with Faceit (Stoe & Cie, 1997). Crystals of
each of the compounds were mounted in an inert oil (perfluoropolyalkyl
ether) at À608C (N2 stream) using the X-TEMP 2 device (T. Kottke, D.
Stalke, J. Appl. Crystallogr. 1993, 26, 615; T. Kottke, R. J. Lagow, D.
Stalke, J. Appl. Crystallogr. 1996, 29, 615; D. Stalke, Chem. Soc. Rev.
1998, 27, 171). The crystal structure determinations were carried out at
À1008C (MoKa radiation, l=0.71073 ꢃ). The structures were solved by
direct and Fourier methods using SHELXS-90 (G. M. Sheldrick, Univer-
sity of Gçttingen, 1990) and SHELXL-97 (G. M. Sheldrick, SHELXL97,
University of Gçttingen, 1997). CCDC-745695 (rac-7), 745696 [(R)-7],
745697 [(R)-10], 745698 (rac-12), 745699 [(R,R,SSi)-17], and 745700 (1:1
aggregate of 17) contain the detailed crystallographic data. Copies of the
data can be obtained free of charge via www.ccdc.cam.ac.uk/data_re-
quest/cif or on application to the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)1223-
336033; email: deposit@ccdc.cam.ac.uk. Further information is available
in the Supporting Information.
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Computational details: All calculations were performed without symme-
try restrictions. Starting coordinates were obtained with Chem3DUltra
10.0. Optimization and additional harmonic vibrational frequency analy-
ses (to establish the nature of stationary points on the potential energy
surface) were performed with the software package Gaussian 03 (Revi-
sion D.01 and Revision E.01) at the same level.[31] The total (SCF) and
zero-point energies (ZPE), all reaction barriers, and the coordinates of
all systems are available in the Supporting Information. Global minima
and vibrational frequency analyses were performed at the B3LYP/6-31+
G(d) and/or M052X/6-31+G(d) level. The vibrational frequency analyses
showed imaginary frequencies for the transition states representing the
corresponding vibration for the deprotonation. For the educts, no imagi-
nary frequencies were obtained. For polar compounds, entropy is crucial-
ly influenced by solvent effects. Additionally calculated Gibbs free ener-
gies seem to be less reliable in such large systems due to very low fre-
quencies, at which the harmonic oscillator model produces significant de-
viations.[32] Thus, only enthalpy values are discussed. Corrections for basis
set superposition errors (BSSE) have not been applied.
[14] For studies on silyl anions, see: a) J. B. Lambert, M. Urdaneta-
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Acknowledgements
We are grateful to the Deutsche Forschungsgemeinschaft and the Fonds
der Chemischen Industrie for financial support and the award of a doc-
toral scholarship (to V.H.G.). C.D. thanks the Studienstiftung des deut-
schen Volkes for a doctoral scholarship. We gratefully acknowledge
Chemetall GmbH and Wacker Chemie AG for providing us with speciali-
ty chemicals.
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functionalized disilane with three equivalents of (R)-mandelic acid
in CDCl3. This resulted in the formation of a diastereomeric salt,
the 1H NMR resonance signals of the trimethylstannyl group in
which could be integrated separately. Comparison of the spectra of
the racemic and enantiomerically pure compounds gave an e.r. of
>99:1 as no signal due to the second possible diastereomer was de-
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[26] A. D. Becke, J. Chem. Phys. 1993, 98, 5648.
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functionalized disilane with an excess of tris[3-(heptafluoropropylhy-
droxymethylene)-(+)-camphorato]europiumACTHNUTRGNEN(UG III) in CDCl3. This re-
sulted in separation of the resonance signals of the SiMe3 group in
the 1H NMR spectrum. Comparison of the spectra of the racemic
and enantiomerically pure compounds gave an e.r. of >99:1 as no
signal due to the second possible diastereomer was detected.
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Received: September 16, 2009
Published online: February 24, 2010
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