Synthesis of a highly enantiomerically enriched silagermane and selective cleavage of the Si-Ge bond with lithium

Article abstract of DOI:10.1021/om700848z

(R)-PhMe(CH₂NC₅H₁₀)SiGeMe₃, the first enantiomerically pure silagermane with stereoinformation at the silicon, was synthesized via a lithiosilane with retention of configuration. Further reaction with lithium resulted in unanticipated silicon-germanium bond cleavage to form PhMe(CH₂NC₅H₁₀)SiLi, although the disilane (R)-PhMe(CH₂NC₅H₁₀)SiSiMe₃ undergoes silicon-phenyl bond cleavage. A trapping reaction with pentamethylchlorodisilane occurred with retention of configuration. DFT calculations on the model systems PhH₂SiSiH₃ and PhH ₂SiGeH₃ indicate a stepwise, dissociative electron transfer mechanism for the silicon-element bond cleavage.

Full text of DOI:10.1021/om700848z

Organometallics 2008, 27, 2499–2504
2499
Synthesis of a Highly Enantiomerically Enriched Silagermane and
Selective Cleavage of the Si-Ge Bond with Lithium
Carsten Strohmann* and Christian Da¨schlein
Institut fu¨r Anorganische Chemie, UniVersita¨t Wu¨rzburg, Am Hubland, 97074 Wu¨rzburg, Germany
ReceiVed August 23, 2007
(R)-PhMe(CH2NC5H10)SiGeMe3, the first enantiomerically pure silagermane with stereoinformation
at the silicon, was synthesized via a lithiosilane with retention of configuration. Further reaction with
lithium resulted in unanticipated silicon-germanium bond cleavage to form PhMe(CH2NC5H10)SiLi,
although the disilane (R)-PhMe(CH2NC5H10)SiSiMe3 undergoes silicon-phenyl bond cleavage. A trapping
reaction with pentamethylchlorodisilane occurred with retention of configuration. DFT calculations on
the model systems PhH2SiSiH3 and PhH2SiGeH3 indicate a stepwise, dissociative electron transfer
mechanism for the silicon-element bond cleavage.
extremely limited, only a few authors have reported the synthesis
of such compounds so far (see Figure 1).
Introduction
Functionalized lithiosilanes are versatile reagents in organic
and organometallic chemistry for the nucleophilic introduction
of protecting groups and synthesis of silyl-substituted transition
metal complexes or silicon-based polymers, for example.1–3 In
particular, highly enantiomerically enriched lithiosilanes have
attracted much interest, as they are configurationally stable at
the lithiated silicon center.4 Their inversion process has been
described in review articles and textbooks1c,5 as the inversion
of a free silyl anion, which should be significantly more stable
than the corresponding carbanion.6 Further investigation by our
research group showed that there is another possible mechanism
involving a second solvated lithium cation.7 As the synthetic
pathways to highly enantiomerically enriched lithiosilanes are
All synthetic routes start with enantiomerically enriched
compounds. Enantiomerically enriched lithiosilanes may be
synthesized from chlorosilanes (Figure 1, compound 5),8 by a
cobalt-lithium exchange reaction (see Figure 1, compound
2),2c,d from stannylsilanes (see Figure 1, compound 3),3a or by
silicon-silicon bond cleavage with lithium metal (see Figure
1, compounds 1 and 4),2a,b,3c,l with the latter in particular proving
a very reliable and convenient method. A general prerequisite
for these Si-Si bond cleavages is the presence of at least one
aromatic substituent on the silicon atom. It is hypothesized that
the lowest unoccupied molecular orbital (LUMO) of the disilane
is otherwise too high in energy to accept an electron.9 By
adapting the method by which an optically active silyllithium
was first synthesized by Sommer et al.,2a,b our own research
group has achieved synthesis of the highly enantiomerically
enriched lithiosilane 4 by selective Si-Si cleavage of the
disilane (R)-7.3c Bond cleavage of the lithiosilane 4 and a
subsequent trapping reaction with chlorosilanes occurred with
retention of configuration over the whole reaction sequence (see
Scheme 1).
* Corresponding author. Fax: +49 931 8884605. Tel: +49 931 8884613.
E-mail: mail@carsten-strohmann.de.
(1) (a) Fleming, I.; Roberts, R. S.; Smith, S. C. J. Chem. Soc., Perkin
Trans. 1 1998, 1215. (b) Schubert, U; Schenkel, A. Transition Met. Chem.
1985, 210. (c) Tamao, K.; Kawachi, A. AdV. Organomet. Chem. 1995, 38,
1. (d) Oh, H.-S.; Park, L.-S.; Kawakami, Y. Chirality 2003, 15, 646. (e)
Uhlig, W. J. Organomet. Chem. 2003, 685, 70. (f) Fleming, I. In
Organocopper Reagents; Taylor, R. J. K., Ed.; Oxford University Press:
Oxford, U.K., 1994; p 257.
(2) Selected pioneer works on functionalized lithiosilanes:(a) Sommer,
L. H.; Mason, R. J. Am. Chem. Soc. 1965, 87, 1619. (b) Sommer, L. H.;
Lyons, J. E.; Fujimoto, H. J. Am. Chem. Soc. 1969, 91, 7051. (c) Colomer,
E.; Corriu, R. J. P. J. Chem. Soc., Chem. Commun. 1976, 176. (d) Colomer,
E.; Corriu, R. J. P. J. Organomet. Chem. 1977, 133, 159.
(5) (a) Corriu, R. J. P.; Guerin, C.; Moreau, J. J. E. The Chemistry of
Organo Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: Chich-
ester, 1989; pp 305. (b) Lambert, J. B.; Schulz, W. J. The Chemistry of
Organo Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: Chich-
ester, 1989; pp 1007. (c) Lerner, H.-W. Coord. Chem. ReV. 2005, 249, 781.
(d) Hollemann, A. F.; Wiberg, E. Lehrbuch der Anorganischen Chemie,
101. Auflage Walter de Gruyter: Germany, 1995; p 889. (e) Elschenbroich,
C.; Salzer, A. Organometallics, 4th ed.; VCH: Weinheim, Germany, 1992;
p 112.
(6) (a) Strohmann, C; Lehmen, K.; Wild, K.; Schildbach, D. Organo-
metallics 2002, 21, 3079. (b) Strohmann, C.; Abele, B. C.; Lehmen, K.;
Villafan˜e, F.; Sierra, L.; Mart´ın-Barrios, S.; Schildbach, D. J. Organomet.
Chem. 2002, 661, 149. (c) Strohmann, C.; Buchold, D. H. M.; Seibel, T.;
Wild, K.; Schildbach, D. Eur. J. Inorg. Chem. 2003, 3453. (d) Strohmann,
C.; Abele, B. C.; Lehmen, K.; Schildbach, D. Angew. Chem. 2005, 117,
3196; Angew. Chem., Int. Ed. 2005, 44, 3136. (e) Carstens, A.; Hoppe, D.
Tetrahedron 1994, 50, 6097.
(3) Selected work on lithiosilanes see:(a) Omote, M.; Tokita, T.; Shimizu,
Y.; Imae, I.; Shirakawa, E.; Kawakami, Y. J. Organomet. Chem. 2000,
611, 20. (b) Strohmann, C.; Ulbrich, O.; Auer, D. Eur. J. Inorg. Chem.
2001, 1013. (c) Strohmann, C.; Ho¨rnig, J.; Auer, D. Chem. Commun. 2002,
766. (d) Strohmann, C.; Bindl, M.; Vraass, V. C.; Ho¨rnig, J. Angew. Chem.
2004, 116, 1029; Angew. Chem., Int. Ed. 2004, 43, 1011. (e) Scheschkewitz,
D. Angew. Chem. 2004, 126, 3014; Angew. Chem. Int. Ed. 2004, 43, 2965.
(f) Auer, D.; Kaupp, M.; Strohmann, C. Organometallics 2004, 23, 3647.
(g) Strohmann, C.; Schildbach, D.; Auer, D. J. Am. Chem. Soc. 1995, 127,
7968. (h) Nanjo, M.; Masayuki, M.; Ushida, Y.; Awamura, Y.; Mochida,
K. Tetrahedron Lett. 2005, 46, 8945. (i) Sekiguchi, A.; Nanjo, M.; Kabuto,
C.; Sakurai, H. Organometallics 1995, 14, 2630. (j) Bravo-Zhivotovskii,
D.; Ruderfer, I.; Melamed, S.; Botoshansky, M.; Tumanskii, B.; Apeloig,
Y. Angew. Chem. 2005, 117, 749; Angew. Chem., Int. Ed. 2005, 44, 739.
(k) Strohmann, C.; Da¨schlein, C.; Auer, D. J. Am. Chem. Soc. 2006, 128,
704. (l) Strohmann, C.; Da¨schlein, C.; Kellert, M.; Auer, D. Angew. Chem.
2007, 119, 4864; Angew. Chem., Int. Ed. 2007, 46, 4780.
(7) Auer, D.; Ho¨rnig, J.; Strohmann, C. Organosilicon Chemistry V:
From Molecules to Materials; Auner, N., Weis, J., Eds.; Wiley-VCH:
Weinheim, Germany, 2003; p 167.
(8) Oestreich, M.; Auer, G.; Keller, M. Eur. J. Org. Chem. 2005, 184.
(9) (a) Lickiss, P. D.; Smith, C. M. Coord. Chem. ReV. 1995, 145, 75.
(b) Sekiguchi, A.; Lee, V. Y.; Nanjo, M. Coord. Chem. ReV. 2000, 210,
11.
(4) First determinations of the inversion barriers of silylanions:(a)
Lambert, J. B.; Urdaneta-Perez, M. J.; Sun, H.-N. J. Chem. Soc., Chem.
Commun. 1976, 806. (b) Lambert, J. B.; Urdaneta-Perez, M. J. Am. Chem.
Soc. 1978, 100, 157.
10.1021/om700848z CCC: $40.75
2008 American Chemical Society
Publication on Web 05/06/2008

2500 Organometallics, Vol. 27, No. 11, 2008
Strohmann and Daschlein
Figure 1. Synthesized highly enantiomerically enriched lithiosilanes.
Scheme 3. Synthesis of the Silagermane (R)-10 via the
Highly Enantiomerically Enriched Lithiosilane 4
silagermane (R)-10 an unanticipated, selective cleavage of the
silicon-germanium bond on reaction with lithium. Quantum
chemical studies were used to understand the process of the
involved silicon-element bond cleavage.
Results and Discussion
Figure 2. Molecular structure and numbering scheme of (R)-
10 · MeI in the solid state (ORTEP plot at 50% probability level,
molecule 1 of 4). Selected bond lengths [Å] and angles [deg]:
Si(1)-Ge(1)2.391(3),C(1)-Ge(1)-Si(1)111.0(4),C(11)-Si(1)-Ge(1)
102.8(3), N(1)-C(11)-Si(1) 123.7(6).
Synthesis of the Silagermane (R)-10. The starting compound
for our synthesis was the disilane (R)-7. rac-7 can be obtained
from dichloro(chloromethyl)methylsilane by
a three-step
method.3c The enantiomerically pure disilane (R)-7 (see Scheme
1) was obtained by resolution of the enantiomers with mandelic
acid. (R)-7 crystallized with (R)-mandelic acid from Et2O, while
(S)-7 · (R)-mandelic acid remained as a yellow resin. After five
crystallization cycles (R)-7 could be obtained with an enantio-
meric ratio g99:1.3c
Scheme 1. Selective Cleavage of the Si-Si Bond in the Disi-
lane (R)-7 as a Selective Synthetic Route to the Functionalized
Lithiosilane 4
Treatment of (R)-7 with lithium metal in thf resulted in the
formation of the dark brown solution of the highly enantio-
merically enriched lithiosilane 4 (see Scheme 3). Due to
racemization in solution within hours above 0 °C, the whole
reaction sequence was carried out at -78 °C, as full cleavage
of the silicon-silicon bond and preservation of the stereoin-
formation are guaranteed at this temperature.3c,7 Because the
reaction is heterogeneous, the reaction time for full cleavage of
the Si-Si bond varies between 4 and 6 h, and the concentration
of disilane in the thf solution becomes very important: on one
hand a higher concentration of (R)-7 increases the reaction time,
but on the other hand the high viscosity of the solution makes
it impossible to increase the concentration too high (the reaction
mixture is almost solid at -78 °C). To track the progress of
the reaction, a small amount of the brown solution was treated
with trimethylchlorosilane (as a trapping reagent) and examined
Scheme 2. Selective Cleavage of the Si-Ph Bond in the Disi-
lane (R)-8 as a Selective Synthetic Route to the Functionalized
Lithiosilane 6
Most recently, our group reported the synthesis of the highly
enantiomerically enriched lithiosilane 6 by the novel means of
selective Si-C bond cleavage of a phenyl group from silicon
in the disilane (R)-8 with lithium metal in thf (see Scheme 2).3l
A trapping reaction of the resulting lithiosilane 6 with pentam-
ethylchlorodisilane resulted in the first highly enantiomerically
enriched (er g 98:2) tetrasilane, (R)-9, with stereoinformation
located on the silicon atom (see Scheme 2). In all cases, we
determined the absolute configuration at the stereogenic silicon
center of the compounds involved by X-ray diffraction analysis.
Si-C cleavage and trapping of the lithiosilane 6 occurred with
full retention of stereochemical information (see Scheme 1 and
Scheme 2).
1
by H NMR spectroscopy. After a reaction time of 5.5 h, full
cleavage of the Si-Si bond could be observed, and an excess
of 2.2 equiv of trimethylchlorgermane was added to the dark
brown solution at -78 °C. After aqueous workup and final
distillation, the highly enantiomerically enriched silagermane
(R)-10 was obtained in 83% yield (see Scheme 3). The
determination of the er of the silagermane (R)-10 was performed
with the common method3c in the presence of 3 equiv of (R)-
mandelic acid in C6D6 (for details, see the Supporting Informa-
tion).
As part of our studies on highly enantiomerically enriched
lithiosilanes, we present here the synthesis of the first highly
enantiomerically enriched silagermane with silicon as the
stereogenic center, (R)-10. In contrast to the corresponding
disilane (R)-8, where treatment with elemental lithium causes
silicon-phenyl bond cleavage, we observed in the case of the
In the synthesis of rac-10, silicon-silicon bond cleavage of
rac-7 could be carried out at 0 °C, as preservation of stereoin-
formation was not necessary in this case (although reaction with
trimethylchlorogermane was still carried out at -78 °C). As a
consequence of the higher reaction temperature, a reaction time
of 3.5 h was allowed and rac-10 was isolated in 78% yield.

Highly Enantiomerically Enriched Silagermane
Organometallics, Vol. 27, No. 11, 2008 2501
Scheme 4. Synthesis of (R)-10 · MeI
Figure 3. Suggested mechanisms for ET-induced bond cleavage.
yield after aqueous workup. We found no trace in the NMR
spectra of the trapping product of 11, which would have resulted
from silicon-carbon bond cleavage. The compounds 8 and 10
thus show significantly different reactivity toward lithium metal,
in spite of their similarities.
Treatment of the highly enantiomerically enriched compound
(R)-10 with lithium in thf at -78 °C and trapping the resultant
lithiosilane 4 with pentamethylchlorodisilane resulted in the
formation of (R)-12 with a yield of 78%. (R)-12 is the second
highly enantiomerically enriched trisilane reported.3l We assume
that the synthesis of (R)-12 (shown in Scheme 6) follows the
same stereochemical pathway as the syntheses of 8 and 9
(Schemes 1 and 2, respectively).3c,l The determination of the
er of the silagermane (R)-10 was performed with the common
method3c in the presence of 3 equiv of (R)-mandelic acid in
C6D6 (for details, see the Supporting Information).
Stereochemical Pathway of the Reaction Sequence of
Si-Si Bond Formation and Si-Ge Bond Cleavage. Although
lithiosilanes are expected to react with electrophiles with
retention of the absolute configuration at the stereogenic silicon
center, there are some examples where inversion has been
observed.3d It was therefore important to determine the absolute
configuration of the synthesized silagermane.
The absolute configuration of (R)-10 could be determined as
the R-configuration by X-ray structural analysis of (R)-10 · MeI
(see Scheme 4). Measurements were taken on three different
single crystals of the silagermane (er g 98:2), all of which
showed an R-configuration. Hence, the whole reaction sequence
from (R)-7 to (R)-10 took place with retention of the absolute
configuration at the stereogenic silicon center.
Element-Element Bond Cleavage with Metals: Reac-
tion Behavior of (R)-8 and (R)-10. As described above, the
nearly identical compounds (R)-8 and (R)-10 show different
reaction behavior with lithium. In the disilane (R)-8 the Si-C
bond is cleaved, while in the silagermane (R)-10 the Si-Ge
bond reacts instead. Why does this difference arise? The
cleavage of Si-C, Si-Si, and Si-Ge bonds with lithium should
be strongly exothermic due to the formation of ionic compounds.
For a rate-determing step characterized by a reactant-like
transition state, one expects ease of cleavage to vary as Si-Ge
> Si-Si > Si-C. However, the element-element bond
cleavages here are occurring via electron transfer (ET) processes,
which can effect significant structural and electronic changes
in the species involved and reorganization of the surrounding
solvent molecules.12 Two possible reaction mechanisms have
been discussed for bond cleavage arising from ET from a metal.
For some molecular systems, electron uptake by the acceptor
species (AB) and σ-bond-breaking to yield a radical (A*) and
an anion (B-) occur together in a concerted, one-step process.
More often, the dissociative mechanism is stepwise, involving
the radical anion (AB*-) as a discrete intermediate in a two-
step process (see Figure 3).13,14
(R)-10 · MeI crystallized from i-PrOH/Et2O in the monoclinic
crystal system, space group P21 (see Figure 2; see Table 1 in
the Supporting Information for additional crystal data and
structural refinement details) as colorless needles. The asym-
metric unit contains four molecules of the silagermane. With
an average value of 2.39 Å, the Si-Ge bond lengths are only
slightly shorter than the sum of the covalent radii of silicon
and germanium (2.40 Å)10 and in the range typically displayed
by other achiral silagermanes.11 On close examination, one can
see that all four molecules in the asymmetric unit have different
conformations,arisingfromrotationaroundthesilicon-germanium
and Si-C(11) bonds. Furthermore, the vibrational ellipsoids at
the trimethylgermyl groups indicate that rotation around the
Si-Ge bond is a low-energy process.
Unanticipated Silicon-Germanium Bond Cleavage in
(R)-10. On closer examination of the two highly enantiomeri-
cally enriched compounds (R)-8 and (R)-10, it is apparent that
both compounds are analogues, apart from the fact that the
trimethyl-substituted silicon atom of (R)-8 is replaced by a
trimethylgermyl group in (R)-10. The silicon-germanium bond
in (R)-10 has a similar bond length (2.39 Å) to the silicon-silicon
bond in (R)-8 (2.35 Å),3l and the silicon-germanium bond
(≈200 kJ · mol-1) is almost as stable as the silicon-silicon bond
(≈220 kJ · mol-1).5d,e However, this does not necessarily imply
that (R)-10 and (R)-8 are nearly identical in terms of stability
and reactivity. This may be illustrated by the reactions of the
two compounds with lithium metal. The potential products of
treating 10 with lithium are the lithiosilanes 4 (see Path 2 in
Scheme 5) and 11 (see Path 1 in Scheme 5) or a mixture of the
two; 8 is known to undergo Si-C bond cleavage under these
conditions (Scheme 2).
According to F. Maran et al., cleavage of sulfur-sulfur bonds
in disulfanes follows the stepwise pathway, of which they
described two limiting cases: (i) a more concerted pathway,
where the SOMO of the radical anion intermediate looks almost
the same as the σ*-orbital of the bond to be cleaved (in this
case, the bond length is significantly elongated by the ET); (ii)
(12) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265.
(13) (a) Save´ant, J.-M. AdV. Electron Transfer Chem. 1994, 4, 53. (b)
Eberson, L. Acta Chem. Scand. 1999, 53, 751. (c) Maran, F.; Wayner,
D. D. M.; Workentin, M. S. AdV. Phys. Org. Chem. 2001, 36, 85. (d) Al-
Kaysi, R. O.; Goodman, J. L. J. Am. Chem. Soc. 2005, 127, 1620. (e)
Grobelny, Z. Eur. J. Org. Chem. 2004, 2973. (f) Lorance, E. D.; Kramer,
W. H.; Gould, I. R. J. Am. Chem. Soc. 2004, 126, 14071. (g) Jakobsen, S.;
Jensen, H.; Pedersen, S. U; Daasbjerg, K. J. Phys. Chem. A 1999, 103,
4141. (h) Antonello, S.; Daasbjerg, K.; Jensen, H.; Taddei, F.; Maran, F.
J. Am. Chem. Soc. 2003, 125, 14905. (i) Meneses, A. B.; Antonello, S.;
Are´valo, M. C.; Maran, F. Electrochim. Acta 2005, 50, 1207.
We carried out the reaction by treating rac-10 with lithium
metal in thf at a temperature of 0 °C. In the event, we observed
only selective cleavage of the silicon-germanium bond, to form
the lithiosilane 4. Treatment of 4 with pentamethylchlorodisilane
resulted in the formation of the racemic trisilane rac-12 at 82%
(10) Huheey, J. E. Inorganic Chemistry, 3rd ed.;Harper and Row: New
York, 1983.
(14) Selected examples of element-element bond cleavages:(a) Stro-
hmann, C.; Lu¨dtke, S.; Wack, E. Chem. Ber. 1996, 129, 799. (b) Strohmann,
C. Angew. Chem. 1996, 108, 600; Angew. Chem., Int. Ed. 1996, 35, 528.
(c) Strohmann, C.; Abele, B. C. Angew. Chem. 1996, 108, 2514; Angew.
Chem., Int. Ed. 1996, 35, 2378. (d) Strohmann, C.; Lu¨dtke, S.; Ulbrich, O.
Organometallics 2000, 19, 4223. (e) Strohmann, C.; Wack, E. Z. Natur-
forsch. 2004, 59b, 1570. (f) Peindy, H. N.; Guyon, F.; Jourdain, I.; Knorr,
M.; Schildbach, D.; Strohmann, C. Organometallics 2006, 25, 1472.
(11) Selected articles with structures of achiral silagermanes:(a) Baum-
gartner, J.; Fischer, R.; Fischer, J.; Wallner, A.; Marschner, C; Floerke, U.
Organometallics 2005, 24, 6450. (b) Parkanyi, L.; Hernandez, C.; Pannell,
K. H. J. Organomet. Chem. 1986, 301, 145. (c) Grev, R. S.; Schaefer, H. F.,
III; Baines, K. M. J. Am. Chem. Soc. 1990, 112, 9458. (d) Mallela, S. P.;
Hill, S.; Geanangel, R. A. Inorg. Chem. 1997, 36, 6247.

2502 Organometallics, Vol. 27, No. 11, 2008
Strohmann and Daschlein
Scheme 5. Reactivity Studies of rac-10: Treatment with Lithium Metal in thf at 0 °C Leads to Selective Cleavage of the Silicon-
Gemanium Bond
Scheme 6. Synthesis of the Highly Enantiomerically Enriched
Trisilane (R)-12 by a Trapping Reaction of the Highly Enantio-
merically Enriched Lithiosilane 4 with Pentamethylchloro-
disilane
model systems are that (i) especially Si-Ge and Si-Si bonds
are significantly elongated and (ii) the Si-Ph bond in D•- is
shorter and stronger than the Si-Ph bond in C•-. Nevertheless,
these investigations do not include the effects and possible
location of the nearby lithium cation or the influence of the
solvent. Either of these considerations could conceivably have
significant effects on the reactivities of the different bonds. In
our current investigations, we are attempting to improve our
understanding of the reaction mechanism.
a more stepwise pathway, where the SOMO (single occupied
molecular orbital) and σ*-orbital are quite different and the bond
to be cleaved is only slightly elongated after ET. To the best of
our knowledge, no studies have been performed on these
processes in disilanes. To understand the reaction mechanism
of (R)-8 and (R)-10 with lithium, and the reasons for the different
products in each case, two model systems were studied using
DFT methods [B3LYP/6-31+G(d)]. The chosen model com-
pounds [PhH2SiSiH3 (C) for (R)-8 and PhH2SiGeH3 (D) for (R)-
10] were geometry-optimized both as neutral species and as
Conclusions
The first highly enantiomerically enriched silagermane [(R)-
10] with stereoinformation located at the silicon center has been
synthesized with an er of g98:2. X-ray structural analysis of
the methylated compound (R)-10 · MeI proved that the whole
reaction sequence proceeds with retention of the stereoinfor-
mation at -50 °C. Subsequent treatment of (R)-10 with lithium
resulted in the selective cleavage of the silicon-germanium
bond, yielding the highly enantiomerically enriched trisilane (R)-
11 after trapping reaction (er g98:2). DFT calculations at the
b3lyp/6-31+g(d) level support a stepwise, dissociative electron
transfer mechanism for the silicon-element bond cleavage.
All hitherto conducted experiments concerning silicon-element
bond cleavages (element ) carbon, silicon, and germanium)
proceed under retention of configuration, enabling us to obtain
highly enantiomerically pure compounds by means of this
synthetic route. At the moment we are trying to get a deeper
insight into the single steps of these silicon-element cleavages.
•-
the radical anions PhH2SiXH3 (X ) Si or Ge, respectively).
The ET causes slightly elongated Si-X bonds (C to C•-: Si-Si
2.358 to 2.481 Å; D to D•-: Si-Ge: 2.364 to 2.487 Å).
Furthermore, the Si-Ph bond length in the disilane slightly
increases from a value of 1.886 Å (A) to 1.902 Å (C•-), but is
shortened from 1.880 Å (D) to 1.862 Å (D•-) in the silagermane.
This can be explained both in molecular orbital terms (the
SOMO has more Si-C bonding character in D•- than in C•-
,
as can be seen by inspection of Figure 4)15 and in terms of
ionic bonding: the product of NBO charges on Si (+ve) and C
(-ve) increases on reduction for D (D: -0.350, D•-: -0.364),
indicating stronger electrostatic interactions; for C, on the other
hand, the charge product decreases on reduction (C: -0.338,
Experimental Section
General Procedures. All reactions were carried out under an
argon atmosphere using Schlenk tubes. All solvents, including the
NMR solvents, were dried according to standard procedures. NMR
spectra were measured on a Bruker DRX-300 NMR spectrometer
and on a Bruker-Avance-500 NMR spectrometer with an external
standard of tetramethylsilane (δ ) 0.0). Signal assignments were
supported by DEPT-135 and C,H-COSY experiments and the
relative intensities of the resonance signals. Crystal structure
determination of (R)-10 · MeI was accomplished on a Bruker APEX
diffractometer (D8 three-circle goniometer) (Bruker AXS); data
collection, cell determination, and refinement: Smart version 5.622
(Bruker AXS, 2001); integration: SaintPlus version 6.02 (Bruker
AXS, 1999); empirical absorption correction: SADABS version 2.01
(Bruker AXS, 1999). The structure was solved by applying direct
and Fourier methods, using SHELXS-9017 and SHELXL-97.18 The
non-hydrogen atoms were refined anisotropically. All of the H atoms
were placed in geometrically calculated positions, and each was
assigned a fixed isotropic displacement parameter based on a riding
model. CCDC 658253 contains the detailed crystallographic data
for this publication. This data may be obtained free of charge from
the Cambridge Crystallographic Data Center through www.ccdc.
C
•-: -0.270), indicating a weakening of ionic bonding.
The HOMO and the LUMO of the neutral systems and the
SOMO of the radical anions were then visualized (Figure 4).
The shape of the SOMO shows the location of the unpaired
electron and possible regions where it is stabilized after ET.
Both the HOMO and LUMO differ little between the two
neutral compounds. Even the SOMOs of the radical anion are
similar for the disilane C•- and the silagermane D•-. Crucially,
in both cases the SOMOs differ significantly from the LUMO
of the neutral compound, and therefore the bond cleavage
reaction should follow the more stepwise pathway (ii). This
statement is supported by the only slightly increased Si-X (X
) Ge or Si) distances. The SOMO is a combination of Si-Ph
and Si-X σ-antibonding orbitals; hence, there are two possible
reactive bonds present, and a balance of different effects
determines which of the two will break, and thus which product
will result. The important conclusions we can draw from the
(15) The SOMO is mostly antibonding between Si and C. Looking at
Figure 4, on the underside of the bond, the interaction between the red and
blue areas is antibonding, whereas on the top, some π-bonding interactions
are taking place.
(17) Sheldrick, G. M. SHELXS-90, A Program for the Solution of Crystal
Structures; Universita¨t Go¨ttingen, 1990.
(16) Gaussian 03 (Revision B.04): M. J. Frisch et al., for details see
the Supporting information.
(18) Sheldrick, G. M. SHELXL-97, A Program for Crystal Structure
Refinement; Universita¨t Go¨ttingen, 1997.

Highly Enantiomerically Enriched Silagermane
Organometallics, Vol. 27, No. 11, 2008 2503
Figure 4. HOMOs (left) and LUMOs (middle) of the two neutral model systems PhH2SiSiH3 and PhH2SiGeH3 and SOMOs of the
corresponding radical anions (right).
cam.ac.uk/data_request/cif. Elemental C, H, N analysis was per-
formed on a Leco CHNS 932 elemental analyzer. GC/MS analyses
were performed on a ThermoQuest TRIO-1000 (EI ) 70 eV);
column; Zebron, capillary GC column, ZB-1. Determination of the
optical rotation was performed on a Jasco P-1030 polarimeter.
Synthesis of (R)-10. At a temperature of 0 °C, 1.24 g (2.98
mmol) of (R)-1,2-dimethyl-1,2,2-triphenyl-1-(piperidinomethyl)d-
isilane, (R)-7, was added to 45.5 mg (6.56 mmol) of lithium in 5
mL of thf. At the first sign of color change, the mixture was cooled
to -78 °C and stirred for 5.5 h at this temperature. Afterward the
now-black solution was separated from leftover lithium and added
at -78 °C to a solution of 1.00 g (6.55 mmol) of trimethylchlo-
rogermane in 5 mL of thf, which caused immediate decoloration
of the solution. Afterward the solution was slowly warmed to room
temperature and all volatile compounds were removed under
vacuum. The residue was suspended in 35 mL of 2 M NaOH and
extracted five times with 10 mL of diethyl ether. The combined
organic layers were extracted five times with 10 mL of 2 M HCl
and afterward set to pH 12 with NaOH. Finally the aqueous layer
was extracted five times with 10 mL of diethyl ether, and the
combined organic layers were dried over Na2SO4. After removal
of all volatile compounds under vacuum the crude product was
purified by “Kugelrohr distillation” [oven temperature: 149 °C,
pressure: 1.6 × 10-5 mbar; [R]D20 ) -7.0 (c 0.16 in cyclohexane)].
Yield: 830 mg (2.47 mmol, 83%). Anal. Calcd: C 57.18, H 8.69,
N 4.17. Found: C 57.83, H 8.51, N 4.38. GC/EI-MS: tR ) 8.12
min [80 °C (2 min)-10 °C · min-1-280 °C (5 min)]; m/z [%] 336
(0.8) [(M - H)+], 322 (5) [(M - CH3)+], 218 (80) {[M -
Ge(CH3)3]+}, 98 (100) [H2CdN(C5H10)+]. 1H NMR (500.1 MHz,
C6D6): δ 0.44 [s, 9H; SiGe(CH3)3], 0.59 (s, 3H; SiCH3), 1.33-1.40
(m, 2H; NCCCH2), 1.55-1.62 (m, 4H; NCCH2), 2.38, 2.48 (AB
mL of thf and stirred for 5.0 h at this temperature. Afterward the
now-black solution was separated from leftover lithium and added
at -78 °C to a solution of 1.21 g (7.88 mmol) of trimethylchlo-
rogermane in 5 mL of thf, which caused immediate decoloration
of the solution. Afterward all volatile compounds were removed
under vacuum. The residue was suspended in 35 mL of 2 M NaOH
and extracted five times with 10 mL of diethyl ether. The combined
organic layers were extracted five times with 10 mL of 2 M HCl
and afterward set to pH 12 with NaOH. Finally the aqueous layer
was extracted five times with 10 mL of diethyl ether, and the
combined organic layers were dried over Na2SO4. After removal
of all volatile compounds under vacuum the crude product was
purified by “Kugelrohr distillation” [oven temperature: 138 °C,
pressure: 1.7 × 10-5 mbar; yield: 940 mg (2.80 mmol, 78%)]. The
analytical data comply with the those for (R)-10.
(R)-10 · MeI. To a solution of 50.0 mg (0.15 mmol) of (R)-10
in 2 mL of acetone was added 21.2 mg (0.15 mmol) of methyl
iodide, and the mixture was stirred for 4.5 h at room temperature.
After removal of all volatile compounds under vacuum, the residue
was dissolved again in 3 mL of 2-propanol. After this, a few drops
of Et2O were added to the reaction mixture until the first signs of
precipitation, and the mixture then was stored at -30 °C. After
24 h, a white, crystalline solid was obtained, filtered from the
solvent, and finally washed with a small amount of ice cold
2-propanol/Et2O. Yield: 64.0 mg (0.13 mmol, 90%). Anal. Calcd:
C 42.71, H 6.75, N 2.93. Found: C 42.90, H 6.65, N 3.13. 1H NMR
(500.1 MHz, CDCl3): δ 0.03 (s, 3H; NCH3), 0.3 [s, 9H;
SiGe(CH3)3], 0.75 (s, 3H; SiCH3), 1.62-1.68, 1.69-1.74 (m, 1H
each; NCCCH2), 1.69-1.74, 1.77-1.82 (m, 2H each; NCCH2),
3.72, 3.73 (s, 1H each; SiCH2N), 3.40-3.35 (m, 1H; NCH2C),
3.57-3.62 (m, 2H; NCH2C), 3.65-3.70 (m, 1H; NCH2C), 7.35-7.40
(m, 3H; H-m and H-p), 7.55-7.62 (m, 2H; H-o). {1H}13C NMR
(125.76 MHz, CDCl3): δ -4.1 (SiCH3), -2.3 (3C) [Ge(CH3)3],
0.96 (NCH3), 20.3 (NCCCH2), 20.6, 20.7 (1C each) (NCCH2C),
55.8 (SiCH2N), 64.45, 64.32 (1C each) (NCH2C), 128.9 (2C) (C-
m), 130.5 (2C) (C-p), 134.4 (2C) (C-o), 134.6 (1C) (C-i). {1H}29Si
NMR (99.4 MHz, CDCl3): δ -18.9 (GeSiCH3).
2
system, JAB ) 14.4 Hz, 2H; SiCH2N), 2.39-2.47 (s [br], 4H;
NCH2C), 7.32-7.37 (m, 3H; aromat. H), 7.68-7.72 (m, 2H;
aromat. H). {1H}13C NMR (125.76 MHz, C6D6): δ -4.8 (1C)
(SiCH3), -2.3 (3C) [Ge(CH3)3], 24.2 (1C) (NCCCH2), 26.7 (2C)
(NCCH2C), 49.8 (1C) (SiCH2N), 58.8 (2C) (NCH2C), 128.5 (2C)
(C-m), 129.0 (2C) (C-p), 134.3 (2C) (C-o), 138.5 (1C) (C-i).
20
{1H}29Si NMR (99.4 MHz, C6D6): δ -16.3 (NCSiGe). [R]D
-7.0 (c 0.16 in cyclohexane).
)
rac-12. At a temperature of 0 °C, 1.15 g (3.42 mmol) of (rac)-
10 was added to 52.3 mg (7.53 mmol) of lithium in 5 mL of thf
and stirred for 5.0 h at this temperature. Afterward the now-black
solution was separated from leftover lithium and added at -78 °C
to a solution of 1.26 g (7.53 mmol) of pentamethylchlorodisilane
Synthesis of rac-10. At a temperature of 0 °C, 1.49 g (3.58
mmol) of (R)-1,2-dimethyl-1,2,2-triphenyl-1-(piperidinomethyl)d-
isilane, (R)-7, was added to 54.7 mg (7.88 mmol) of lithium in 5

2504 Organometallics, Vol. 27, No. 11, 2008
Strohmann and Daschlein
in 5 mL of thf, which caused immediate decoloration of the solution.
Afterward all volatile compounds were removed under vacuum.
The residue was suspended in 35 mL of 2 M NaOH and extracted
five times with 10 mL of diethyl ether. The combined organic layers
were extracted five times with 10 mL of 2 M HCl and afterward
set to pH 12 with NaOH. Finally the aqueous layer was extracted
five times with 10 mL of diethyl ether, and the combined organic
layers were dried over Na2SO4. After removal of all volatile
compounds under vacuum the crude product was purified by
“Kugelrohr distillation” [oven temperature: 190 °C, pressure: 1.0
× 10-2 mbar; yield: 981 mg (2.81 mmol, 82%)]. Anal. Calcd: C
61.8, H 10.1, N 4.0. Found: C 61.7, H 9.9, N 3.9. GC/EI-MS: tR )
9.30 min [80 °C (2 min)-10 °C · min-1-280 °C (5 min)]; m/z [%]
334 (1) [(M - CH3)+], 276 (12) {[M - Si(CH3)3]+}, 218 (27)
{[M - Si(CH3)2Si(CH3)3]+}, 98 (100) [(H2CdNC5H10)+], 73 (12)
[(CH3)3Si+]. 1H NMR (300.1 MHz, CDCl3): δ 0.03 [s, 9H;
Si(CH3)3], 0.09 [s, 6H; Si(CH3)2], 0.43 (s, 3H; SiCH3), 1.25-1.40
(m, 2H; NCCCH2), 1.40-1.55 (m, 4H; NCCH2C), 2.15-2.40 (m,
4H, NCH2C), 2.23, 2.31 (AB system, 2JAB ) 14.4 Hz, 2H; SiCH2),
7.25-7.35 (m, 2H; aromat. H), 7.40-7.55 (m, 3H; aromat. H).
{1H}13C NMR (75.5 MHz, CDCl3): δ -6.3 (2C) [Si(CH3)2], -5.0
(SiCH3), -1.5 (3C) [Si(CH3)3], 23.8 (NCCCH2), 26.3 (2C)
(NCCH2C), 49.5 (SiCH2), 58.6 (2C) (NCH2C), 127.6 (2C) (C-m),
128.3 (C-p), 134.0 (2C) (C-o), 139.1 (C-i). {1H}29Si NMR (59.6
MHz, CDCl3): δ -47.8 (SiSiSi), -20.3 (SiCH3), -15.4 [Si(CH3)3].
(R)-12. At a temperature of 0 °C, 0.89 g (2.65 mmol) of (rac)-
10 was added to 40.4 mg (5.83 mmol) of lithium in 3 mL of thf.
At the first sign of color change, the mixture was cooled to -78
°C and stirred for 5.5 h at this temperature. Afterward the now-
black solution was separated from leftover lithium and added at
-78 °C to a solution of 972 mg (5.83 mmol) of pentamethylchlo-
rodisilane in 2 mL of thf, which caused immediate decoloration of
the solution. Afterward all volatile compounds were removed under
vacuum. The residue was suspended in 25 mL of 2 M NaOH and
extracted five times with 5 mL of diethyl ether. The combined
organic layers were extracted five times with 10 mL of 2 M HCl
and afterward set to pH 12 with NaOH. Finally the aqueous layer
was extracted five times with 8 mL of diethyl ether, and the
combined organic layers were dried over Na2SO4. After removal
of all volatile compounds under vacuum the crude product was
purified by “Kugelrohr distillation” [oven temperature: 190 °C,
pressure: 1.0 × 10-2 mbar; yield: 722 mg (2.07 mmol, 78%)]. The
analytical data comply with those of rac-11. [R]D20 ) 12.2 (c 2.40
in cyclohexane).
Acknowledgment. We are grateful to the Institut fu¨r
Anorganische Chemie Wu¨rzburg, the Fonds der Chemischen
Industrie, and the DFG for financial support. Furthermore
we acknowledge the Wacker Chemie AG for providing us
with special chemicals. C.D. thanks the Studienstiftung des
Deutschen Volkes for a doctoral scholarship. Finally, we are
grateful to Dr. James Asher for technical assistance.
Supporting Information Available: NMR studies, X-ray (cif),
and computational data. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM700848Z