Convenient access to optically active silyl- and germyllithiums: Synthesis, absolute structure, and reactivity

Article abstract of DOI:10.1016/j.tetlet.2005.10.084

A convenient access to optically active silyl- and germyllithium compounds (^tBuMePhELi?sp, E = Si (1), Ge (2)) was accomplished by only addition of (-)-sparteine (abbr. sp) ligand to racemic silyl- and germyllithiums. Optically active silyllith

Full text of DOI:10.1016/j.tetlet.2005.10.084

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 8945–8947
Convenient access to optically active silyl- and
germyllithiums: synthesis, absolute structure, and reactivity
Masato Nanjo,* Masayuki Maehara, Yuhei Ushida, Yuko Awamura and Kunio Mochida*
Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan
Received 7 September 2005; revised 13 October 2005; accepted 14 October 2005
Available online 2November 2005
Abstract—A convenient access to optically active silyl- and germyllithium compounds (tBuMePhELiÆsp, E = Si (1), Ge (2)) was
accomplished by only addition of (À)-sparteine (abbr. sp) ligand to racemic silyl- and germyllithiums. Optically active silyllithium
(1Æsp) and germyllithium (2Æsp) could be isolated by recrystallization from pentane/toluene mixed solvents. The 1Æsp and 2Æsp have ꢀRꢁ
configuration clearly determined by a single crystal X-ray diffraction. Hydrolysis reactions of 1Æsp and 2Æsp gave the corresponding
t
hydrides, BuMePhEH, in quantitative yields with high enantiomeric excess. The reactions of 2Æsp with some organic halides were
also examined.
Ó 2005 Elsevier Ltd. All rights reserved.
Organic anions, especially those of the group 14 elements
(silicon and germanium), have received considerable
interest in synthetic chemistry and reaction mechanism.1
Despite the large number of reports on silyl- or germyl-
lithium reagents, only a limited number of optically
active silyl- or germyllithium are known to date. In
1963, Brook pioneered the first preparation of optically
active germyllithium compounds by the reaction of enan-
tiomerically enriched hydrogermane with n-butyl-
lithium.2 On the other hand, optically active silyllithium
compounds were first synthesized by Sommer through
the cleavage of an Si–Si bond of an enantiomerically
enriched disilane with lithium metal.3 More recently,
optically active methyl(1-naphthyl)phenylsilyllithium,4,5
n-butylmethylphenylsilyllithium,5 and methylphenyl-
(1-piperidinylmethyl)silyllithium6 have been reported.
Although there is a variety of synthetic routes to these
silyl- and germyllithium compounds, they all require
highly enantiomerically enriched hydrosilanes or hydro-
germanes as starting material.7 Moreover, no reports on
the isolation of optically active pure silyl- or germyl-
lithium have been published so far, because of the prob-
lem of configurational stability of optically active
silyl- or germyllithium, which tends to undergo racemi-
zation at elevated temperatures due to the loss of
enantiomerical purity.8 Herein, we report the direct opti-
cal resolution of silyl- and germyllithium compounds by
using a chiral ligand of (À)-sparteine (abbr. ꢀspꢁ) together
with their absolute structures and some reactivities.
t
A racemic mixture of BuMePhGeLi (2) was prepared
t
t
by the metalation of BuMePhGeH with BuLi in THF
at À40 °C in the presence of half-equivalent of (À)-spar-
teine. In the 1H and 13C NMR spectra of a racemic mix-
ture of 2, single resonances for both tBu and Me groups
7
were observed. However, Li NMR showed two reso-
nances at 0.9 and 2.1 ppm with equal intensities, indicat-
ing the presence of two different lithium atoms existing
in the solution with coordination by THF and (À)-spar-
teine, respectively. As the temperature was raised, these
7
two signals in Li NMR (in mesitylene-d12) broadened
and disappeared at 380 K, and they finally coalesce to
a new signal at 1.5 ppm at 423 K. The temperature-
7
dependent change of the Li NMR signals of 2 may
result from the dynamics of exchange of the solvated
ligand between two lithium atoms. From the variable
temperature 7Li NMR spectra, the value DGꢂ
(380 K) = 76 kJ molÀ1 was estimated by the Arrhenius
equation for the dynamics of sparteine ligand exchange.9
We have succeeded in the optical resolution of mixture 2
by recrystallization from a toluene/hexane mixed sol-
vent. Only germyllithium 2 with a coordination of (À)-
sparteine (2Æsp) was obtained as colorless crystals in
42% isolated yield.10 The specific rotation of 2Æsp
showed +21.8° in toluene and +16.1° in THF. As
Keywords: Silyl- and germyllithium; Sparteine; Optical resolution; X-
ray diffraction.
*
Corresponding authors. Tel.: +81 3 3986 0221; fax: +81 3 5992 1029
(K.M.); e-mail: kunio.mochida@gakushuin.ac.jp
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.10.084

8946
M. Nanjo et al. / Tetrahedron Letters 46 (2005) 8945–8947
tBuMePhGeLi
+
2 THF
tBuLi / THF / (-)-sparteine
-40 oC, 1 h
tBuMePhGeH
tBuMePhGeLi
sp
'optical resolution'
(recrystallization)
(R)- tBuMePhGeLi
sp
sp
2
Scheme 1.
7
expected, the Li NMR spectrum of 2Æsp gave one lith-
ium signal at 2.1 ppm. Compound 2Æsp was stable up
to 150 °C under argon in the solid state or in dried
and degassed hydrocarbon solvents (Scheme 1).
perature by using a chiral ligand of (À)-sparteine was
also successful in a similar method.13 The absolute struc-
ture of 1Æsp was confirmed by X-ray diffraction. The
silicon chiral center has an R-configuration.
The absolute structure of 2Æsp was unequivocally con-
firmed by X-ray diffraction (Fig. 1).11 The crystal struc-
ture possesses a monoclinic crystal system and space
group of P21, which has no symmetry inversion center.
The germanium chiral center has an R-configuration.
The lithium atom is coordinated by two nitrogen atoms
Hydrolysis reactions of 1Æsp and 2Æsp by H2O gave the
corresponding hydrosilane and hydrogermane in quanti-
tative yields with high enantiomeric excess (92% and
95% ee, respectively). The treatment of 1Æsp and 2Æsp
by EtOH gave also the corresponding hydrosilane and
hydrogermane quantitatively with high enantiomeric
excess (93% and 95% ee, respectively). The reactions of
optically active germyllithium 2Æsp with some organic
halides in pentane are summarized in Table 1. The reac-
tion of 2Æsp with Me3SiCl afforded tBuMePhGeSiMe3 in
95% yield. However, the addition of EtBr to 2Æsp pro-
˚
of a (À)-sparteine. The Ge–Li bond length of 2.57(2) A
should be recognized as the shortest one among all
solvated germyllithiums hitherto known (2.598(9)–
2.76(2)).12 The germanium atom has a distorted tetra-
hedral geometry, which was evidenced by the C–Ge–C
bond angles ranging from 99.9(4) to 101.7(3)°. The
geometry around the lithium atom is nearly planar with
the sum of the bond angles around lithium of 355(1)°.
t
duced BuMePhGeEt in a lower yield (70%) than that
found in the reaction of Me3SiCl. Of particular interest
is the fact that the reactions of 2Æsp with a series of benz-
ylic halides were investigated. On treatment of 2Æsp with
benzyl fluoride, substitution reactions occurred to give
the coupling product of tBuMePhGeCH2Ph in 17% yield
with a high enantiomeric excess (96% ee). In this reac-
Optical resolution of
a
silyllithium compound,
tBuMePhSiLi (1), prepared by treatment of disilane,
(tBuMePhSi)2, with lithium metal1 in THF at room tem-
t
tion, the formation of BuMePhGeH (72%, 95% ee)
arising from hydrolysis of unreacted germyllithium
was observed due to the short reaction time. On the
other hand, the reaction of 2Æsp with benzyl chloride in
pentane produced not only the coupling product
t
tBuMePhGeCH2Ph (40%, 68% ee) but also BuMePh-
GeH (36%) and (tBuMePhGe)2 (trace). Benzyl bromide
Table 1. Reactions of 2Æsp with some substrates in pentane
2.sp
+
substrate
product
r.t., 10 min.
Yield (%)a
Substrate
Product
% eeb
H2O
EtOH
EtBr
Me3SiCl
PhCH2F
tBuMePhGeH
100
100
70
95
17
7295
40
36
Trace
17
15
95
95
—
—
96
tBuMePhGeH
tBuMePhGeEt
tBuMePhGeSiMe3
tBuMePhGeCH2Ph
tBuMePhGeH
PhCH2Cl
PhCH2Br
tBuMePhGeCH2Ph
tBuMePhGeH
68
—
—
23
46
—
Figure 1. Molecular structure of 2Æsp. Hydrogen atoms are omitted for
(tBuMePhGe)2
tBuMePhGeCH2Ph
tBuMePhGeH
˚
clarity. Selected bond lengths (A): Ge(1)–C(1) 2.029(8), Ge(1)–C(2)
2.045(7), Ge(1)–C(6) 2.045(8), Ge(1)–Li(1) 2.57(2), N(1)–Li(1) 2.04(2),
N(2)–Li(1) 2.01(2). Selected bond angles (deg): C(1)–Ge(1)–C(2)
99.9(4), C(1)–Ge(1)–C(6) 101.6(4), C(2)–Ge(1)–C(6) 101.7(3), C(1)–
Ge(1)–Li(1) 117.8(4), C(2)–Ge(1)–Li(1) 125.1(4), C(6)–Ge(1)–Li(1)
107.5(4), Ge(1)–Li(1)–N(1) 123.2(5), Ge(1)–Li(1)–N(2) 140.1(6),
N(1)–Li(1)–N(2) 91.6(5).
(tBuMePhGe)2
Trace
a Determined by GC based on the internal standard of n-C20H42
.
b Determined by HPLC using Chiral OJ-H column in hexane/2-pro-
panol (50:1) as an eluent.

M. Nanjo et al. / Tetrahedron Letters 46 (2005) 8945–8947
8947
t
7. Typically, enantiomerically enriched silanes or germanes
can be prepared by the optical resolution of (À)-menth-
oxymethyl(1-naphthyl)phenylsilane or germane, see for
example: (a) Sommer, L. H.; Frye, C. L. J. Am. Chem.
Soc. 1959, 81, 1013; (b) Brook, A. G.; Limburg, W. W. J.
Am. Chem. Soc. 1963, 85, 832; (c) Eaborn, C.; Steward, O.
W. Proc. Chem. Soc. 1965, 521.
was reacted with 2Æsp to give BuMePhGeCH2Ph (17%,
t
23% ee) and BuMePhGeH (15%, 46% ee) with a trace
amount of (tBuMePhGe)2. Although this reaction is
not clean and unidentified products were observed, it
is noted that the resulting tBuMePhGeH has small enan-
tiomeric excess, suggesting that the racemization took
place in the course of the electron transfer reaction.
8. For example, the optical purity of MeNpPhSiLi in THF at
À78 °C keeps 85% ee for 1 h, however, the purity
decreased to 50% ee upon raising the temperature up to
0 °C. See Ref. 5.
We have accomplished a convenient access to optically
active silyl- and germyllithiums compounds by only
addition of (À)-sparteine ligand. The structure and reac-
tivity of optically active silyl- and germyllithium, and
their application to organic synthesis are now under
study.
9. The temperature-dependent NMR experiment was per-
formed in mesitylene-d12 with
a Varian Inova 400
spectrometer (1H NMR at 400.0 MHz and 7Li NMR at
155.4 MHz). No change was observed in the 1H NMR
spectra of 1 at the temperatures ranging from 298 to
423 K.
Experimental procedure and NMR spectral data for 1Æsp
and 2Æsp (PDF). X-ray structural data for 1Æsp and 2Æsp
(CIF). These materials are available free of charge via
the Internet at http://pubs.acs.org.
10. Compound 2Æsp: mp = 160–161 °C (dec); 1H NMR (C6D6,
t
d) 0.68 (s, 3H, Me), 1.42(s, 9H, Bu), 0.5–3.3 (m, 45H, sp),
7.22–7.28 (m, 1H, Ph), 7.40–7.46 (m, 2H, Ph), 7.98–8.02
(m, 2H, Ph); 13C NMR (C6D6, d) 0.4 (Me), 18.1, 22.0
(CMe3), 24.0, 24.6, 25.0, 25.6, 28.2, 30.2, 31.9 (CMe3),
34.8, 35.0, 45.8, 54.3, 57.4, 59.3, 60.6, 66.5, 124.4, 127.0,
7
135.8, 160.6; Li NMR (C6D6, d) 2.11.
Acknowledgments
11. Crystal structure analysis of (R)-2Æsp: Diffraction data
were collected at 120 K on a MacScience DIP2030 image
plate diffractometer employing graphite-monochro-
This work was supported by a Grant-in-Aid for Scien-
tific Research (No. 13740359) from the Ministry of Edu-
cation, Science, Sports and Culture, Japan. We wish to
thank Dr. V. Ya. Lee of University of Tsukuba for help-
ful discussions and for comments on this manuscript.
˚
mated Mo(Ka) radiation (k = 0.71073 A); MF = C26H43-
GeLiN2, MW = 463.15, monoclinic, P21, a = 10.0640(8),
˚
b = 12.9750(9), c = 10.1840(8) A, b = 107.659(4)°, V =
3
1267.17(17) A , Z = 2 ,Dcalcd = 1.214 g cmÀ3. The final R
˚
factor and GOF indicator were 0.0595 (wR2= 0.1793 for
all data) and 1.639, respectively, for 3390 reflections with
I > 2r(I). The absolute structure parameter converged to
0.00(2). CCDC reference number 194094.
References and notes
12. (Me3Si)3GeLi: (a) Freitag, S.; Herbst-Irmer, R.; Lameyer,
L.; Stalke, D. Organometallics 1996, 15, 2839; (b) Heine, A.;
Stalke, D. Angew. Chem., Int. Ed. Engl. 1994, 33, 113;
Germole anions: (c) Hong, J.-H.; Pan, H.; Boudjouk, P.
Angew. Chem., Int. Ed. 1996, 35, 186; (d) West, R.; Sohn,
1. For reviews, see: (a) Tamao, K.; Kawachi, A. Adv.
Organomet. Chem. 1995, 38, 1; (b) Lickiss, P. D.; Smith,
C. M. Coord. Chem. Rev. 1995, 145, 75; (c) Belzner, Z.;
Dehnert, U. In The Chemistry of Organosilicon Com-
pounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chich-
ester, 1998; Vol. 2, p 779; (d) Sekiguchi, A.; Lee, V. Y.;
H.; Powell, D. R.; Muller, T.; Apeloig, Y. Angew. Chem.,
¨
´
Int. Ed. 1996, 35, 1002; Ar3GeLi (Ar = 2-(dimethyl-
amino)phenyl): (e) Kawachi, A.; Tanaka, Y.; Tamao, K.
Eur. J. Inorg. Chem. 1999, 461; Very recently, a nonsolvated
monomeric germyllithium [(tBu2MeSi)3GeLi] has been
reported, and its structure has a short Ge–Li bond length
Nanjo, M. Coord. Chem. Rev. 2000, 210, 11; (e) Riviere,
P.; Castel, A.; Riviere-Baudet, M. In The Chemistry of
Organic Germanium, Tin, and Lead Compounds; Rappo-
port, Z., Ed.; Wiley, 2002; Vol. 2, p 653.
2. Brook, A. G.; Peddle, G. J. D. J. Am. Chem. Soc. 1963, 85,
2338.
˚
of 2.518(7) A, see: (f) Nakamoto, M.; Fukawa, T.; Lee, V.
Ya.; Sekiguchi, A. J. Am. Chem. Soc. 2002, 124, 15160.
3. Sommer, L. H.; Lyons, J. E.; Fujimoto, H. J. Am. Chem.
Soc. 1965, 91, 7051.
4. (a) Colomer, E.; Corriu, R. J. P. J. Chem. Soc., Chem.
Commun. 1976, 176; (b) Colomer, E.; Corriu, R. J. P.
J. Organomet. Chem. 1977, 133, 159.
5. Omote, M.; Tokita, T.; Shimizu, Y.; Imae, I.; Shirakawa,
E.; Kawakami, Y. J. Organomet. Chem. 2000, 611, 2 0.
6. Strohmann, C.; Ho¨rnig, J.; Auer, D. Chem. Commun.
2002, 766.
1
13. Compound 1Æsp: mp = 119–120 °C; H NMR (400 MHz,
C6D6, d) 0.68 (s, 3H, Me), 1.42(s, 9H, tBu), 0.5–3.4 (m,
45H, sp), 7.23–7.28 (m, 1H, Ph), 7.42–7.48 (m, 2H, Ph),
7.97–8.02(m, 2H, Ph); 13C NMR (100 MHz, C6D6, d) 0.9
(Me), 18.0, 19.6 (CMe3), 23.9, 24.5, 25.0, 25.9, 28.1, 30.3,
31.2(C Me3), 34.7, 34.9, 45.8, 54.5, 57.3, 59.3, 60.4, 66.0,
7
124.7, 126.8, 135.8, 156.6; Li NMR (155 MHz, C6D6, d)
1.92.