Selective Si-C bond cleavage as synthetic entry to a functionalized lithiosilane

Article abstract of DOI:10.1021/ja050360s

Treatment of the phenyl-substituted silane 4 with lithium metal afforded the functionalized lithiosilane rac-2 by selective cleavage of one Si-C bond between silicon and a phenyl group. The resulting lithiosilane rac-2 crystallizes as the dimer (2·THF)

Full text of DOI:10.1021/ja050360s

Published on Web 05/13/2005
Selective Si-C Bond Cleavage
as Synthetic Entry to a Functionalized Lithiosilane
Carsten Strohmann,* Daniel Schildbach, and Dominik Auer
Institut fu¨r Anorganische Chemie, UniVersita¨t Wu¨rzburg, Am Hubland, 97074 Wu¨rzburg, Germany
Received January 19, 2005; E-mail: mail@carsten-strohmann.de
Scheme 2
Functionalized lithiosilanes are versatile reagents in organic and
organometallic chemistry.1 They are used for the nucleophilic
introduction of protecting groups and synthesis of silyl-substituted
transition metal complexes1 or silicon-based polymers.2 Of all
known functionalized lithiosilanes especially systems of type
R2(R2N)SiLi (developed by K. Tamao) are used for these purposes.1a,3
In contrast to closely related alkyllithiums, methods of preparation
for lithiosilanes such as Si-Si bond cleavage of disilanes or metal-
metal exchange reactions are limited to specialized starting
compounds and can result in the formation of byproducts. Our
interest in this field concerns an alternative and simple access to
functionalized or even highly enantiomerically enriched lithio-
silanes, which are generally prepared by Si-Si bond cleavage of
suitable disilanes such as highly enantiomerically enriched (R)-1
(cf. Scheme 1).4 In comparison to disilanes, functionalized tetra-
organosilanes are more readily accessible starting materials and
easier to handle.
rotrimethyl- or chloromethyldiphenylsilane at -80 °C result in the
selective formation of disilanes rac-6 or rac-1 and trimethylphen-
ylsilane or methyltriphenylsilane, respectively (cf. Scheme 2). No
significant amounts of other compounds were detected by NMR
and GC/MS analysis of the crude products, indicating a selective
cleavage of the Si-C bond between silicon and one phenyl group.
After workup, the disilanes rac-6 and rac-1 were isolated in yields
of 91 and 90%, respectively. In contrast to the preparation of rac-2
by Si-Si bond cleavage of rac-1 (analogous to the reaction in
Scheme 1), the new Si-C bond cleavage of 4 enables the removal
of most of the byproduct phenyllithium from rac-2 by evaporation
of THF and washing with n-hexane.
Although the Si-C bond cleavage of 4 affords synthetically
utilizable solutions of rac-2, a precise NMR analysis of lithiosilane
rac-2 required purer samples. This was achieved by reacting rac-2
with rac-methoxymethylphenyl(piperidinomethyl)silane10 to give
disilane 7 in good yield.11 NMR analysis indicated that 7 was
formed as a mixture of two diastereomers (d.r. ) 57:43). As for
other aryl-substituted disilanes, subsequent reaction of 7 with lithium
metal results in Si-Si bond cleavage affording rac-2 in the absence
of impurities.
When formed from 7, rac-2 can be isolated in the form of single
crystals suitable for X-ray structural analysis by the removal of
THF and recrystallization from Et2O. Figure 1 shows the molecular
structure of rac-2 in the solid state, which crystallized as the dimer
(2‚THF)2 in the orthorhombic crystal system, space group Pna21.12,13
The asymmetric unit contains two crystallographically independent
dimers, which represent a pair of enantiomers. The central structural
motif of each dimer is a four-membered ring formed by two lithium
centers and one (R)- and one (S)-configured silicon center. This
gives rise to one short [2.708(10)-2.734(12) Å] and one longer
[2.855(11)-2.956(14) Å] Si-Li contact per lithium. The sum of
Si-C bond angles at the Si centers indicates stronger pyramidal-
ization on silicon compared to a tetrahedral arrangement, which is
Scheme 1
Herein we report a new example of reductive Si-C bond
cleavage by lithium resulting in the selective formation of the
lithiosilane rac-2, which should have synthetic potential similar to
that of the systems of K. Tamao. This reaction type was hitherto
only observed as an unwanted side reaction,5 offering in some cases
access to special compounds, which despite their low yields are of
significant interest.5c-e Our focus rests on developing tetraorga-
nosilane precursors for selective Si-C bond cleavage reactions with
lithium metal. In prior work, the Si-CHPh2 bond in (diphenyl-
methyl)-substituted tetraorganosilanes was successfully cleaved with
lithium metal.6 The steric demand of the (diphenylmethyl) group,
however, prevented the preparation of starting material from which
lithiosilane 2 could be accessed.
In our present work, treatment of the phenyl-substituted silane
4 with lithium metal afforded rac-2 by selective cleavage of one
Si-C bond between silicon and a phenyl group (cf. Scheme 2).
Moreover, the solid-state structure of the lithiosilane rac-2 was
determined as the first example of a dimeric organyl-substituted
lithiosilane in the presence of THF.7 In addition, this analysis
supplies vital structural information on the racemic form of the
enantiomerically pure compound 2 (cf. Scheme 1).
The phenyl-substituted silane 4 is available in one step starting
from (chloromethyl)methyldiphenylsilane by amination with pip-
eridine.8 When treating 4 with lithium metal in THF at room
temperature after several minutes a significant change in color is
observed, indicating the start of the Si-C cleavage reaction.9 The
reaction mixture is cooled to -10 °C and stirred for 4.5 h.
Subsequent trapping reactions of the colored solution with chlo-
9
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10.1021/ja050360s CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
methyl-functionalized lithiosilane rac-2, which has major synthetic
potential (e.g., for nucleophilic transfer of silicon moieties).
Currently we are investigating related Si-C bond cleavage reactions
as new synthetic entry to enantiomerically enriched lithiosilanes.
Acknowledgment. This work was supported by the Institut fu¨r
Anorganische Chemie Wu¨rzburg, the DFG, the Graduiertenkolleg
690, and the FCI. D.S. thanks the FCI for a doctoral scholarship.
Supporting Information Available: Crystallographic (CIF) and
experimental details (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
References
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Figure 1. Molecular structure of rac-2 in the crystal; the two crystal-
lographic independent molecules (1M,1R,3M,3S)-(2‚THF)2 and (1P,1R,3P,3S)-
(2‚THF)2 are shown (Schakal plots15). Selected bond lengths (Å) and angles
(deg) of (1M,1R,3M,3S)-(2‚THF)2: Si(1)-C(1) 1.906(7), Si(1)-C(2) 1.925-
(6), Si(1)-C(8) 1.940(9), Si(2)-C(18) 1.902(7), Si(2)-C(19) 1.912(7), Si-
(2)-C(25) 1.958(6), Si(1)-Li(1) 2.933(11), Si(1)-Li(2) 2.711(11), Si(2)-
Li(1) 2.734(12), Si(2)-Li(2) 2.864(11); C(1)-Si(1)-C(2) 102.3(3), C(1)-
Si(1)-C(8) 103.0(3), C(2)-Si(1)-C(8) 101.0(3), C(18)-Si(2)-C(19)
103.8(3), C(18)-Si(2)-C(25) 99.6(3), C(19)-Si(2)-C(25) 106.2(3).
(1P,1R,3P,3S)-(2‚THF)2: Si(3)-C(35) 1.923(7), Si(3)-C(36) 1.960(6), Si-
(3)-C(42) 1.947(6), Si(4)-C(52) 1.939(7), Si(4)-C(53) 1.924(7), Si(4)-
C(59) 1.985(8), Si(3)-Li(3) 2.855(11), Si(3)-Li(4) 2.715(15), Si(4)-Li(3)
2.708(10), Si(4)-Li(4) 2.956(14), C(35)-Si(3)-C(36) 101.6(3), C(35)-
Si(3)-C(42) 99.7(3), C(36)-Si(3)-C(42) 105.1(3), C(52)-Si(4)-C(53)
105.1(3), C(52)-Si(4)-C(59) 103.0(3), C(53)-Si(4)-C(59) 101.9(3).
(7) The preparation of the dimeric silyl-substituted and THF-coordinated
lithiosilane [(Me3Si)SiLi‚THF]2 by distillation of (Me3Si)SiLi‚3THF under
reduced pressure was recently described: Nanjo, M.; Nanjo, E.; Mochida,
K. Eur. J. Inorg. Chem. 2004, 2961.
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3228.
a result of the hybridization defects14 of Si in silyl anions or highly
polar lithiosilanes [306.3° on Si(1), 309.6° on Si(2), 306.4° on Si-
(3), and 308.3° on Si(4)].
Both lithium centers are coordinated by a nitrogen atom and one
THF molecule, respectively. This leads to the formation of a ladder
framework consisting of a central Si-Li-Si-Li and two Si-Li-
N-C four-membered rings. The ligands on the Li centers in each
dimer show formally C2 symmetry (perpendicular to the central
Si-Li-Si-Li ring), while the Si centers show formal inversion
symmetry to each other. By this arrangement, in addition to the
stereogenic silicon centers, two chiral planes are formed, resulting
in four chiral elements per dimer.
(9) This change of color is also known from the reaction of chlorosilanes
under similar conditions. There it is evidence for the formation of the
corresponding lithiosilane, although it is known that the color is not of
the lithiosilane itself, but associated with traces of radical anions formed
by side reactions. See ref 5a and references therein.
(10) The methoxysilane is available by one-pot reaction from chloro(chloro-
In principle, when constructing dimeric aggregates from the
monomeric lithiosilane rac-2, 16 different isomers are imaginable,16
methyl)methylphenylsilane; cf. Supporting Information.
(11) Disilane 7 can also be prepared starting from disilane rac-1. But Si-Si
cleavage of rac-1 and reaction with rac-methoxymethylphenyl(piperidi-
nomethyl)silane results in an inseparable product mixture of rac-1 and 7.
(12) Crystallographic data for (2‚THF)2: C34H56Li2N2O2Si2, M ) 594.87,
orthorhombic, space group Pna21 (no. 33), a ) 19.777(4), b ) 10.400-
(2), c ) 35.503(7) Å, V ) 7302(3) Å3, Z ) 8, Dc ) 1.082 Mg/m3, µ )
0.127 mm-1. 42 344 reflections, 11 040 unique reflections; 7687 with I
> 2σ(I); R1 ) 0.0801 [I > 2σ(I)], wR2(Fo2) ) 0.1829 (all data), absolute
structure parameter 0.05(19). X-ray crystallography data for (2‚THF)2 has
been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC 260574.
(13) The crystal was mounted at -60 °C (N2 stream), using the X-TEMP 2
device (Kottke, T.; Stalke, D. J. Appl. Crystallogr. 1993, 26, 615).
(14) (a) Kutzelnigg, W. Angew. Chem., Int. Ed. Engl. 1984, 23, 272. For a
recent contribution concerning hybridization defects, see: (b) Kaupp, M.;
Riedel, S. Inorg. Chim. Acta 2004, 357, 1865.
(15) Keller, E. Schakal99; University of Freiburg: Freiburg, Germany, 1999.
(16) These isomers include seven pairs of enantiomers and two meso
compounds (for a detailed representation of all isomers cf. Supporting
Information). A thorough discussion of the occurring structural motifs
was given, for example, for 2-lithiobenzylamines, where similar dimeric
aggregates are found. See: (a) Reich, H. J.; Goldenberg, W. S.;
Gudmundsson, B. O.; Sanders, A. W.; Kulicke, K. J.; Simon, K.; Guzei,
I. A. J. Am. Chem. Soc. 2001, 123, 8067 and references therein. (b) Arink,
A. M.; Kronenburg, C. M. P.; Jastrzebski, J. T. B. H.; Lutz, M.; Spek, A.
L.; Gossage, R. A.; Van Koten, G. J. Am. Chem. Soc. 2004, 126, 16249
and references therein.
1
two enantiomers of which were found in the solid state. H, 13C,
and 29Si NMR analyses at -40 °C in toluene-d8 (sample prepared
by dissolving crystals of rac-2) indicate the existence of more than
one isomer in solution. However, no further classification of these
structures was possible. In THF-d8, the situation is simplified as
the observation of a 29Si-7Li coupling at -70 °C (quartet; 1JSiLi
)
48.9 Hz) in the 29Si NMR spectrum clearly supports the existence
of monomeric lithiosilane rac-2.
Although the solid-state structure concerns the racemic lithio-
silane rac-2, one can also expect a dimeric molecular structure for
the enantiomerically pure compound 2. We further emphasize that
this molecular structure has to be different from the structure found
for the racemic lithiosilane rac-2, as each dimer observed in the
crystal structure consists of one (R)- and one (S)-configured unit.
To avoid possible stereochemical problems, all reactions involving
the enantiomerically enriched lithiosilane 2 should be performed
in THF solution.
Starting from simple compound 4, an unanticipated and novel
Si-C bond cleavage reaction permits preparation of the amino-
JA050360S
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