Low-temperature addition of organolithiums to functionalized vinylsilanes under formation of secondary α-lithiated alkylsilanes

Article abstract of DOI:10.1021/ja9076167

(Chemical Equation Presented) At -78°C and in apolar solvents, the presence and quality of a coordinating side arm in vinylsilanes dictates both their reactivity toward alkyllithiums and the structural motif in the solid state, deciding whether a defined

Full text of DOI:10.1021/ja9076167

Published on Web 11/11/2009
Low-Temperature Addition of Organolithiums to Functionalized Vinylsilanes
under Formation of Secondary r-Lithiated Alkylsilanes
Christian Unkelbach and Carsten Strohmann*
Anorganische Chemie, Technische UniVersita¨t Dortmund, Otto-Hahn-Strasse 6, 44227 Dortmund, Germany
Received September 8, 2009; E-mail: mail@carsten-strohmann.de
Silylated organolithium compounds are versatile reagents in
organic or organometallic chemistry. They are of fundamental
interest, for example, as intermediates in the Peterson olefination1
or in the synthesis of functionalized alcohols2 via successive Tamao-
type cleavage of the Si-C bond.3 Yet the selective synthesis of
R-silylorganolithium reagents with longer alkyl chains next to the
metalated carbon center is strongly limited or unknown: Selective
R-halogenation or direct deprotonation of alkyl chains other than
methyl groups next to silicon fails. Only the carbolithiation of
vinylsilanes has been established as a tool to obtain the desired
species s these reactions usually proceed in donor solvents at
temperatures around 0 °C or higher.4 Our recent investigations
focused on a mild carbolithiation of vinylsilanes without donor
solvents or additives at low temperatures to avoid side-reactions
like deprotonations or polymerizations. Additional advantages
should include readily available substrates and facile access to
products containing variable chain lengths.
contrast, carbolithiation of vinylsilanes 4a and 4b proceeded smoothly
and rapidly. After 12 h at -78 °C, quenching with Me3SnCl afforded
the stannanes 8a-j in good yields (Scheme 2).
Scheme 2. Carbolithiation of 4a and 4b and Trapping of the
Resulting Lithiated Species 7a-j with Me3SnCl
The relationship between the size of aggregates and reactivity
of the involved species is clearly established;5 diamine or triamine
ligands are usually employed to break down the organolithium
reagents into smaller, sufficiently reactive aggregates. Our approach
was to incorporate commercially available di- or triamine auxiliaries
into the desired vinylsilane systems. For model systems containing
either two or three coordination sites, TMEDA6 (1a) and PMDTA7
(1b) were R-deprotonated with t-BuLi in n-pentane according to
literature procedures, and subsequently reacted with commercially
available chloro-(dimethyl)vinylsilane 3 (Scheme 1). After aqueous
workup and distillation the vinylsilanes 4a,b were isolated in yields
of 55 and 78%, respectively. Vinylsilane 6 served as a model system
with a single coordinating site in its side arm and was synthesized
via amination of (chloromethyl)dimethylvinylsilane with piperidine
in 90% yield.
The addition of n-BuLi, i-PrLi and t-BuLi to 4a and 4b is
favorable even in the absence of coordinating solvents.
Compounds 7a and 7f, the products of the addition of MeLi to
4a and 4b, respectively, can be crystallized from n-pentane/Et2O
at -78 °C (Figure 1).
Scheme 1. Synthesis of the Investigated Vinylsilanes 4a,b and 6
Figure 1. Molecular structures (Schakal representation)8 and drawing of
(7a)2 (left) and 7f (right). Selected bond lengths (Å) and angles (deg): (7a)2
- two slightly different dimers are found in the asymmetric unit, represented
by two sets of bond lengths and angles: Li-Li′ 2.548(6)/2.578(6), C3-Li
2.285(5)/2.277(6), C3-Li′ 2.258(6)/2.249(6), C3-Si 1.806(3)/1.813(3). 7f:
C3-Li 2.180(6), C3-Si 1.799(3).
Both 7a and 7f crystallize in the triclinic crystal system, space
9
j
Carbolithiations of vinylsilanes 4a and 4b and 6 at -78 °C were
examined using commercial solutions of t-BuLi/n-pentane, i-PrLi/
n-pentane, n-BuLi/hexane, PhLi/Bu2O/n-pentane and MeLi/Et2O/
n-pentane and proceeded with variable results. No reaction of
vinylsilane 6 with the latter three occurred over a period of several
months while t-BuLi and i-PrLi afforded amorphous solids after
two weeks which we attributed to an anionic polymerization. In
group P1. The asymmetric unit of 7a contains two molecules which
form two slightly different inversion symmetric dimers in the
crystal. The central structural motif is a planar four-membered ring
formed by the two lithium atoms and two metalated carbon atoms
of each moiety. The bond between silicon and metalated carbon
C(3) is shortened to 1.806(3) and 1.813(3) Å due to the R-carban-
ionic stabilizing effect of silicon.10
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17044 J. AM. CHEM. SOC. 2009, 131, 17044–17045
10.1021/ja9076167  2009 American Chemical Society

C O M M U N I C A T I O N S
In contrast, the additional coordination site provided by the
PMDTA-derived side arm of 4b accounts for the monomeric
carbolithiation product 7f. With a Si-C(3) bond length of 1.799(3)
Å the shortening is more significant than in the dimer.
To support the high reactivity and selectivity of 4a and 4b
observed in the experiment, theoretical studies were performed and
the barriers for the carbolithiation of 4a as the model compound
were computed.11 Additionally, the barrier for the deprotonation
of the methyl groups next to silicon was calculated since similar
deprotonation reactions at low temperatures are known in literature
(cf. Scheme 3).2c,12
deprotonation under kinetic conditions if reacted with alkyllithiums.
Furthermore, a comparison of the geometries of the participating
optimized structures reveals a very small distance (2.16 Å) between
the lithium center and the carbon undergoing metalation which is
very close to the C-Li bond length in the product (2.09 Å). This
tightly packed, product-like transition state brings the coordinating
moiety which surrounds the lithium center proximate to the double
bond. This crowding should account for an efficient differentiation
between the energies of diastereomeric transition states in suitable
chiral substrates similar to the stereoselective carbolithiations of
6-lithio-1-pentenes,17 cinnamyl alcohols18 and ꢀ-methylstyrenes19
already reported.
In summary, two readily synthesized vinylsilanes undergo rapid
carbolithiation by organolithiums at low temperatures. Two or three
internally coordinating amino moieties are required for rapid and
selective reaction at low temperatures. The exceptional reactivity
of these functionalized vinylsilanes prompts us to expand our
investigations to include stereoselective addition steps via chiral
functionalized substrates.
Scheme 3. (Left) Computed Reaction Routes of 7a with
Alkyllithiums: Carbolithiation (A) and R-Deprotonation (B); (Right)
Transition State of the Deprotonation Route (Molekel Plot13
)
Acknowledgment. We are grateful to the Fonds der Chemischen
Industrie and the Deutsche Forschungsgemeinschaft.
Supporting Information Available: Crystallographic (CIF), ex-
perimental and computational data. This material is available free of
charge via the Internet at http://pubs.acs.org.
The computational studies reveal a barrier of 18 kJ/mol for the
addition of MeLi to 4a starting from the monomeric adduct
[4a·MeLi] (Figure 2).14 Whereas the assumption of a monomeric
reaction path in case of the addition of MeLi may be questionable
due to the lack of known comparable complexes the analogous
addition of t-BuLi to 4a, was determined at 12 kJ/mol. In case of
t-BuLi a monomeric complex [4a·t-BuLi] and reaction route can
be justified because structures are known in which bidentate nitrogen
ligands break t-BuLi down to monomers.15
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