C O M M U N I C A T I O N S
corresponds to the isomer found in the crystal (Lis)-4. The
prevalence of this isomersas shown by it being the only one found
in the crystalscould therefore be the result of a thermodynamically
controlled epimerization in solution.
To the best of our knowledge, the title compounds represent the
first crystal structures of enantiomerically pure lithiosilanes. In
future reactions, parameters, such as the coordination number at
the lithium center, the reaction solvent, the reaction temperature,
and the concentration, can be varied. Currently, we are searching
for suitable electrophiles to permit the transfer of stereoinformation
by selective formation of a new stereogenic center.
Acknowledgment. This work was supported by the Deutsche
Forschungsgemeinschaft. We are also grateful to R. Bertermann
and M.-L. Sch a¨ fer for technical assistance.
Figure 3. Molecular structures of Ph2(NEt2)SiLi‚TMEDA (6) in the crystal
hydrogens on THF and TMEDA were omitted for reasons of clarity;
Schakal plot ]. Selected bond lengths (Å) and angles (°) of 6: Si-C(1)
[
1
3
1.949(2), Si-C(7) 1.929(2), Si-N(1) 1.7849(17), Si-Li 2.737(3), Li-N(2)
2.175(4), Li-N(3) 2.173(4), C(7)-Si-N(1) 103.65(9), C(1)-Si-C(7)
101.69(9), C(1)-Si-N(1) 101.67(8).
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.
To clarify the situation in solution, NMR studies of 4 and 5 in
References
toluene-d
8
were conducted, indicating the presence of only one
(
(
(
1) In general, we speak of enantiomerically enriched metal alkyls when we
focus on the stereogenic metalated carbon center. In the real case, these
molecules are almost always diastereomerically enriched metal alkyls.
2) Hoppe, D.; Christoph, G. In Chemistry of Organolithium Compounds;
Rappoport, Z., Marek, I., Eds.; Wiley: Chichester, UK 2004; pp 1055
and references cited therein.
3) (a) Strohmann, C.; Seibel, T.; Strohfeldt, K. Angew. Chem., Int. Ed. 2003,
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Strohfeldt, K.; Schildbach, D.; McGrath, M. J.; O’Brien, P. Organome-
tallics 2004, 23, 5389.
29
diastereomer of 4 in solution at -50 °C. The observation of a Si-
Li coupling (JSiLi ) 47.9 Hz) and two C NMR signals for the
7
13
diastereotopic methyl groups at silicon indicates a distinct Si-Li
contact on the NMR time scale at -50 °C. At room temperature,
no coupling in the 2 Si NMR was observed; however, the methyl
9
groups still emit two 13C NMR signals, showing the strength of
the chiralizing influence of the (-)-sparteine ligand.
8
Further NMR analyses of 5 in toluene-d also indicate one species
(
4) Recent studies on the synthesis of enantiomerically enriched lithiosi-
lanes: (a) Omote, M.; Tokita, T.; Shimizu, Y.; Imae, I.; Shirakawa, E.;
Kawakami, Y. J. Organomet. Chem. 2000, 611, 20. (b) Strohmann, C.;
H o¨ rnig, J.; Auer, D. Chem. Commun. 2002, 766. (c) Oestreich, M.; Auer,
G.; Keller, M. Eur. J. Org. Chem. 2005, 184, and references cited therein.
5) (a) Kottke, T.; Stalke, D. Angew. Chem., Int. Ed. Engl. 1993, 32, 580. (b)
Stey, T.; Stalke, D. In Chemistry of Organolithium Compounds; Rappoport,
Z., Marek, I., Eds.; Wiley: Chichester, UK 2004; pp 47 and references
cited therein.
in solution, which exhibits a 29Si- Li coupling of 65.3 Hz at room
temperature and two sets of signals for the diastereotopic phenyl
groups on silicon. Both observations indicate a (-)-sparteine-Li-
Si contact at room temperature, and that the lithium center is fixed
at silicon on the NMR time scale. Upon cooling of the sample, the
7
(
(
29
signal in the Si NMR broadens as a result of the quadrupole
6) For an overview of typical solid-state structures of lithiosilanes, see results
and references cited in: (a) Lickiss, P. D.; Smith, C. M. Coord. Chem.
ReV. 1995, 145, 75. (b) Tamao, K.; Kawachi, A. AdV. Organomet. Chem.
7
moment of Li. At -90 °C, the coupling is again well resolved
(JSiLi ) 53.3 Hz) as a result of slower relaxation at lower
1
2
995, 38, 1. (c) Sekiguchi, A.; Lee, V. Y.; Nanjo, M. Coord. Chem. ReV.
000, 210, 11. (d) Lerner, H.-W. Coord. Chem. ReV. 2005, 249, 781.
14
temperatures.
To determine the relative energy ratios between the two relevant
(7) (a) Nakamoto, M.; Fukawa, T.; Lee, V. Y.; Sekiguchi, A. J. Am. Chem.
Soc. 2002, 124, 15160. (b) Ichinohe, M.; Kinjo, R.; Sekiguchi, A.
Organometallics 2003, 22, 4621.
diastereomers of 4, DFT calculations were performed on the
B3LYP/6-31+G(d) level.15 Starting from the structural parameters
(8) For examples of dimeric lithiosilanes, see: (a) Klinkhammer, K. W.
Chem.sEur. J. 1997, 3, 1418. (b) Wiberg, N.; Niedermayer, W.; N o¨ th,
H.; Warchhold, M. J. Organomet. Chem. 2001, 628, 46. For tetra- or
hexameric lithiosilanes, see: (c) Sekiguchi, A.; Nanjo, M.; Kabuto, C.;
Sakurai, H. Organometallics 1995, 14, 2630. (d) Schaaf, T. F.; Butler,
W.; Glick, M. D.; Oliver, J. P. J. Am. Chem. Soc. 1974, 96, 7593. (e)
Ilsley, W. H.; Schaaf, T. F.; Glick, M. D.; Oliver, J. P. J. Am. Chem. Soc.
of the solid-state structure, six different isomers were proposed16
and optimized in energy, resulting in the favored isomers (Lis)-4
and (Lir)-4. The energy difference between them is 2.9 kJ/mol,
with (Lis)-4 as the most stable diastereomer (cf. Figure 4).
1
980, 102, 3769.
(
9) Strohmann, C.; Schildbach, D.; Auer, D. J. Am. Chem. Soc. 2005, 127,
7968.
(
10) Lithiosilanes 4 to 6 are crystallized from Et
2
O in the presence of traces
stream),
of THF leftover from their synthesis.
(
11) All crystals of (Lis)-4, 5, and 6 were mounted at -60 °C (N
2
using the X-TEMP 2 device (Kottke, T.; Stalke, D. J. Appl. Crystallogr.
1
993, 26, 615). Furthermore, X-ray crystallography data for (Lis)-4, 5,
and 6 have been deposited with the Cambridge Crystallographic Data
Center as supplementary publication Nos. CCDC 287268 [(Lis)-4], CCDC
2
87269 (5), and CCDC 287270 (6), respectively.
(
12) For free (-)-sparteine, two relevant conformers are known, having an
opposite configuration at one of the nitrogen centers. For quantum
chemical studies on the stability of the conformers, see: Wiberg, K. B.;
Bailey, W. F. J. Mol. Struct. 2000, 556, 239.
(13) Keller, E. Schakal99; University of Freiburg: Freiburg, Germany, 1999.
14) (a) Fraenkel, G.; Chow, A.; Fleischer, R.; Liu, H. J. Am. Chem. Soc. 2004,
(
126, 3983. (b) Johnels, D.; G u¨ nther, H. In The Chemistry of Organolithium
Compounds; Rappoport, Z., Marek, I., Eds.; Wiley: Chichester, UK, 2004;
p 137. (c) Bauer, W.; Winchester, W. R.; Schleyer, P. v. R. Organome-
tallics 1987, 6, 2371.
Figure 4. Energy optimized structures [B3LYP/6-31+G(d)] of (Lis)-4 and
(
15) Gaussian 98, revision A.9; Gaussian, Inc.: Pittsburgh, PA, 1998.
16) Starting from each of the diastereomers, (Lis)-4 and (Lir)-4, three rotamers
were formed by rotation around the Si-Li bond in 120° steps.
17) Portmann, S. Molekel; ETH Z u¨ rich: Z u¨ rich, Switzerland, 2001.
17
(
Lir)-4 (selected hydrogens were omitted for clarity; Molekel plot ).
(
(
Although the small energy difference between both diastereomers
has to be interpreted with caution, the most stable structure
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J. AM. CHEM. SOC.
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