Sila-metalation route to hydrido(trialkylsilyl)silyllithiums

Article abstract of DOI:10.1021/ja026705d

The proton abstraction (sila-metalation) of trialkylsilyl-substituted dihydridosilanes with t-BuLi or LDA in THF was found to be a convenient route to the corresponding silyllithiums (RR′SiHLi; 1a, R, R′ = t-BuMe₂Si; 1b, R, R′ = Me₃S

Full text of DOI:10.1021/ja026705d

Published on Web 09/10/2002
Sila-metalation Route to Hydrido(trialkylsilyl)silyllithiums
Takeaki Iwamoto, Junichiro Okita, Chizuko Kabuto, and Mitsuo Kira*
Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Aoba-ku, Sendai 980-8578, Japan
Received April 27, 2002
Although metalation of hydrocarbons has been widely used as a
convenient method for the preparation of corresponding carbanions,1
the deprotonation of hydridosilanes (sila-metalation) forming the
corresponding silyl anions is rather exceptional.2-5 In the reaction
of a hydridosilane with an alkyllithium, nucleophilic attack of the
alkyl anion to silicon usually occurs to give the corresponding
Scheme 1
N
alkylsilane (S 2-Si, path a in Scheme 1), where the hydrido group
serves as a rather good leaving group. An alternative sila-metalation
pathway (path b in Scheme 1) is disfavored because of the hydride
character of the Si-H hydrogen; the bond polarity is Si(δ+)-
H(δ-) in contrast to the polarity of C(δ-)-H(δ+) based on the
Table 1. Reactions of Various Dihydridosilanes with Bases
N
electronegativity differences. The relative reactivity between S 2-
Si (path a) and sila-metalation (path b) should be modified by the
substituents on silicon and the basicity of the attacking reagent.6
yields of products
Trialkylsilyl-substituted hydridosilanes are good candidates as the
substrates for the selective sila-metalation because an electropositive
trialkylsilyl substituent will decrease the electrophilicity of the
silicon center and stabilize the silyl anion formed by the sila-
metalation. In this paper, we report the achievement of the
convenient sila-metalation of trialkylsilyl-substituted dihydridosi-
lanes to give the corresponding silyllithiums, the scope and
limitation of the reaction, and the structure of a hydridosilyllithium
determined by X-ray crystallography.
Typically, to a THF solution (10 mL) of bis(tert-butyldimeth-
ylsilyl)dihydridosilane (2a, 0.500 g, 1.92 mmol) was added a
pentane solution of tert-butyllithium (1.45 mol/L, 1.70 mL, 2.49
mmol) dropwise at -40 °C under argon. After the mixture was
stirred for 1 h at -40 °C, an excess amount of iodomethane (1.00
g, 7.05 mmol) was added to the mixture. The usual workup gave
entry
dihydridosilane 2
base (R′Li)
t-BuLi
3
4
1
2
3
4
5
6
7
8
9
(t-BuMe2Si)2SiH2 (2a)
100 (3a)
88 (3a)
0
100 (3b)
97 (3c)
0
0
2a
2a
i-Pr NLi
2
n-BuLi
t-BuLi
t-BuLi
t-BuLi
i-Pr2NLi
t-BuLi
i-Pr2NLi
t-BuLi
t-BuLi
i-Pr2NLi
100 (4a)
(Me3Si)2SiH2 (2b)
(i-Pr2MeSi)2SiH2 (2c)
(i-Pr3Si)2SiH2 (2d)
2d
0
0
0
0
a
0
0
a
b
(t-BuMe2Si)(Tol)SiH2 (2e)
21 (3e)
97 (3e)
67 (4e)
0
100 (4f)
88 (4g)
80 (4g′)
2e
10
11
12
Pr2SiH2 (2f)
Ph2SiH2 (2g)
2g
0
0
0
a
Hydridosilane 2d was recovered. b Tol ) 4-methylphenyl.
constant of 75 Hz for 1a is much smaller than that for tris(tert-
butyldimethylsilyl)silane (147 Hz),10 indicating that the s-character
of the silicon orbital of the Si-H bond in 1a is much less than that
in 2a due to the electropositive lithium substituent in 1a.
X-ray analysis showed that 1a is dimeric in the solid state,
where two lithium atoms bridge between anionic silicon atoms
forming a parallelogram, and each lithium atom is coordinated by
the corresponding methylhydridosilane 3a (0.526 g, 1.92 mmol,
7
1
00%). The general reaction scheme is shown in eq 1. The results
of the reactions of various dihydridosilanes with several bases are
shown in Table 1. Lithium diisopropylamide (LDA) worked
similarly as a base (entry 2). On the other hand, the reaction of 1a
1
1
with n-BuLi gave the corresponding substitution product, (t-BuMe
2
-
Si) Si(Bu-n)H, quantitatively (entry 3). Whereas the sila-metalation
2
one THF molecule (Figure 1). The Si-H hydrogens are in the plane
of the parallelogram but are disordered.1
3,14
The Si-H- - -Li
of bis(trialkylsilyl)dihydridosilanes 2b and 2c was achieved likewise
by using t-BuLi (entries 4 and 5), the highly sterically hindered
bissilyldihydridosilane 2d reacted with neither t-BuLi nor LDA
distances in 1a are 1.95(5)-1.96(5) Å, which are close to the
1
5
distances calculated for the inverted SiH Li (1.911 Å) and those
3
(
entries 6 and 7). Interestingly, the sila-metalation of silyl(aryl)-
observed in 1,2-dilithio-1,1,2,2-tetrakis(dimethylsilyl)-ethane (2.00-
2.33 Å);16 the agostic interactions are suggested to exist between
lithium and hydrogen atoms in 1a.1
silane 2e occurred satisfactorily using LDA, while the nucleophilic
substitution of 2e was the major pathway when t-BuLi was used
7-19
(
entries 8 and 9). As expected, no sila-metalation of dialkyldihy-
dridosilane 2f or diaryldihydridosilane 2g occurred (entries 10-
2).
Hydridosilylithium 1a was isolated as air- and moisture-sensitive,
The reaction of (t-BuMe Si) GeH with t-BuLi in THF afforded
2
2
2
the corresponding germyllithium, (t-BuMe Si) GeHLi (5), in quan-
2
2
1
titative yield.20,21 A single crystal of 5 was found to have a dimeric
structure similar to that of 1a.
Application of the functional silyl anions prepared by sila-
metalation to organic synthesis is in progress.
8
7
but thermally stable, colorless crystals. The Li resonance in 1a
appeared at 2.0 ppm, suggesting that the lithium ion is not free but
exists as a contact ion-pair in toluene. The J( Si- H) coupling
9
1
29
1
Acknowledgment. This work was supported by the Ministry
of Education, Culture, Sports, Science, and Technology of Japan
*
To whom correspondence should be addressed. E-mail: mkira@si.chem.
tohoku.ac.jp.
1
1604 J. AM. CHEM. SOC. 2002, 124, 11604-11605
9
10.1021/ja026705d CCC: $22.00 © 2002 American Chemical Society

C O MMU N I C A T I O N S
the electrophilicity of the central silicon is reduced and the acidity of the
Si-H hydrogen is increased by the introduction of silyl substituents as R
groups in R
2 2
SiH .
(
7) Even when 2a was treated with 2 equiv of t-BuLi, the corresponding 1,1-
dilithiosilane was not produced.
1
(8) 1a: colorless crystals; H NMR (C
7
D
8
, δ) 0.20 (s, 1H, Si-H), 0.24 (brs,
6
H, SiMe), 0.32 (brs, 6H, SiMe), 1.04 (s, 18H, t-Bu), 1.22-1.31 (m, 4H,
13
THF), 3.50-3.54 (m, 4H, THF); C NMR (C D , δ) 0.3, 1.4 (SiMe),
7 8
29
1
8.2 (C(CH ) ), 25.3 (C(CH ) ), 28.2 (THF), 69.0 (THF); Si NMR (C D ,
3
3
3
3
7
8
7
2 7 8
δ) -188.8 (SiHLi, J(Si-H) ) 75 Hz), 4.5 (t-BuMe Si); Li NMR (C D ,
δ) 2.01 (brs, ν1/2 ) 9.3 Hz).
7
(9) In THF-d
8
, Li resonance of 1a was found at 0.62, suggesting the solvated
7
monomeric structure of 1a. For the solvent effects on the Li resonances,
see: (a) Heine, A.; Herbst-Irmer, R.; Sheldrick, G. M.; Stalke, D. Inorg.
Chem. 1993, 32, 2694. (b) Nanjo, M.; Sekiguchi, A.; Sakurai H. Bull.
Chem. Soc. Jpn. 1998, 71, 741.
(
2 2
10) The smaller J(Si-H) value in Mes SiHLi as compared to that in Mes -
2
SiH has been reported: Roddick, D. M.; Heyn, R.; Tilley, T. D.
Organometallics 1989, 8, 324.
(
11) Recrystallization from hexane gave single crystals of 1a suitable for data
collection. All of the diffraction measurements were carried out on a
Rigaku/MSC Mercury CCD diffractometer with graphite monochromated
Mo KR radiation (λ ) 0.71073 Å). Crystallographic data for 1a: formula,
C
24
H
62Si
6
Li
dimensions 0.30 × 0.35 × 0.40 mm; monoclinic; space group P2
4); cell dimensions, a ) 11.8427(4) Å; b ) 15.943(1) Å; c ) 12.7700-
2
‚(C
4
H
8
O)
2
; formula weight, 677.36; colorless prism; crystal
Figure 1. ORTEP view of hydridosilyllithium 1a. Thermal ellipsoids are
drawn at the 30% probability level. Selected bond lengths (Å) and angles
1
/c (No.
1
(
3
3
4) Å; â ) 107.467(4)°; V ) 2299.9(2) Å ; Z ) 2; Dcalc ) 0.978 g/cm ;
(
deg): Si1-Si2 2.3480(9), Si2-Si3 2.3453(8), Si2-Li1 2.644(4), Si2-
temperature -123.0 °C. A total of 16 981 reflections were collected, of
which 5185 reflections were independent. The number of parameters was
199. The structure was solved by the direct method and refined by the
Li1* 2.667(4), Si2-H 1.44(5), Si2-H′ 1.47(5), H-Li1 1.95(5), H′-Li1*
1
1
.96(5), Si1-Si2-Si3 108.03(3), Li1-Si2-Li1* 67.6(1), Si2-Li1-Si2*
12.3(2).
2
12
full matrix least-squares on F using SHELXL-97. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were included but
not refined. The final R value calculated for 3959 reflections (I > 2σ(I))
was 0.055, and the wR2 for all of the reflections was 0.162. The value of
GOF was 1.039.
(Grants-in-Aid for Scientific Research (B) No. 11440185 (M.K.
and T.I.)) and for the Encouragement of Young Scientists No.
(
12) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refinement;
1
2740336 (T.I.).
Universit a¨ t G o¨ ttingen: Germany, 1997.
(
13) As shown in Figure 1, the disorder is confined to the Si-H hydrogens H
and H′; each of the hydrogens occupies two sites bound to Li1 and Li1*
with a refined occupancy ratio of 48.8(4):51.2(4).
Supporting Information Available: Tables of crystal data, struc-
ture solution, and refinement, atomic coordinates, bond lengths and
angles, anisotropic thermal parameters, and ORTEP drawings for 1a
and 5 (PDF) and experimental details. This material is available free
of charge via the Internet at http://pubs.acs.org.
(14) While no detailed structural parameters are given, Schleyer et al. have
predicted a similar dimeric structure with significant interactions between
hydrogen and lithium atoms in (SiH Li) by theoretical calculations:
3 2
Sannigrahi, A. B.; Kar, T.; Niyogi, B. G.; Hobza, P.; Schleyer, P. v. R.
Chem. ReV. 1990, 90, 1061.
(
(
15) Schleyer, P. v. R.; Clark, T. J. Chem. Soc., Chem. Commun. 1986, 1371.
16) (a) Sekiguchi, A.; Ichinohe, M.; Takahashi, M.; Kabuto, C.; Sakurai, H.
Angew. Chem., Int. Ed. Engl. 1997, 36, 1533. (b) Ichinohe, M.; Sekiguchi,
A.; Takahashi, M.; Sakurai, H. Bull. Chem. Soc. Jpn. 1999, 72, 1905.
References
(
1) Wardwell, J. A. In ComprehensiVe Organometallic Chemistry; Wilkinson,
S. G. W., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press Ltd.: Oxford,
3 2
(17) Very recently, Wiberg et al. reported the X-ray analysis of (t-Bu Si) SiHK-
1
982; Chapter 2, pp 43-120 and references therein.
6 6 2
(C H ) , which exists as a monomer with significant Si-H- - -K interac-
(
2) For recent reviews on silyl anions, see: (a) Kawachi, A.; Tamao, K. AdV.
Organomet. Chem. 1995, 38, 1. (b) Lickiss, P. D.; Smith, C. M. Coord.
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M. J. Organomet. Chem. 2001, 628, 46.
(18) For X-ray analysis of other related hydridosilylmetals with short SiH- - -
metal distances, see: (a) Ring, M. A.; Ritter, D. M. J. Phys. Chem. 1961,
6
5, 182. (b) Pritzkow, H.; Lobreyer, T.; Sundermeyer, W.; van Eikema
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3, 216. (c) Goldfuss, B.; Schleyer, P. v. R.; Handschuh, S.; Hampel, F.;
Bauer, W. Organometallics 1997, 16, 5999 and references therein.
(
3) Metalation of a hydridosilane with potassium hydride has been evidenced
to proceed not by direct deprotonation but via a pentacoordinate silicon
intermediate resulting from the nucleophilic attack of hydride to the silicon
atoms. Corriu, R. J. P.; Guerin, C. J. Chem. Soc., Chem. Commun. 1980,
3
(19) A recent study has shown that close H- - -Li contacts are not necessarily
an indication of the agostic interactions: Scherer, W.; Sirsch, P.;
Shorokhov, D.; McGrandy, G. S.; Mason, S. A.; Gardiner, M. G. Chem.-
Eur. J. 2002, 8, 2324. As suggested by a referee, a more detailed study
of the structure using a combined high-resolution X-ray and neutron
diffraction method is required to elucidate the agostic interactions.
(20) The abstraction of Ge-H hydrogens to give the corresponding germyl
anions has been a well-known process, in contrast to sila-metalation. For
a recent review, see: Armitage, D. A. In ComprehensiVe Organometallic
Chemistry; Wilkinson, S. G. W., Stone, F. G. A., Abel, E. W., Eds;
Pergamon Press Ltd.: Oxford, 1982; Chapter 9.1, p 1 and references
therein.
1
68. Brefort, J. L.; Corriu, R.; Gu e´ rin, C.; Henner, B. J. Organomet. Chem.
1
989, 370, 9.
(
(
(
4) Trichlorosilyl anion was produced by the apparent deprotonation of
trichlorosilane with trialkylamines: (a) Benkeser, R. A.; Foley, K. M.;
Grutzner, J. B.; Smith, W. E. J. Am. Chem. Soc. 1970, 92, 697. (b)
Bernstein, S. C. J. Am. Chem. Soc. 1970, 92, 699. (c) Benkeser, R. A.
Acc. Chem. Res. 1971, 4, 94 and references therein.
5) The reaction of KSiH
Si) SiHK, and (H
deprotonation of (H
3
with (H
Si) SiK has been proposed to be initiated by the
Si) SiH by H SiK. (a) B u¨ rger, H.; Eujen, R.;
3 2 2 3 2
Si) SiH giving a mixture of H SiSiH K,
(H
3
2
3
3
3
2
2
3
Marsmann, H. C. Z. Naturforsch., B: Chem. Sci. 1974, 29B, 149. (b)
Feher, F.; Freund, R. Inorg. Nucl. Chem. Lett. 1974, 10, 561.
1
6 6
(21) 5: H NMR (C D , δ) -0.55 (s, 1H, Ge-H), 0.44 (s, 6H, SiMe), 0.55 (s,
6) Theoretical calculations at the B3LYP/6-31G(d) level have shown that
6H, SiMe), 1.18 (s, 18H, t-Bu), 1.22-1.28 (m, 4H, THF), 3.59-3.63 (m,
H
the charges on the central silicon (RSi) and the Si-H hydrogen (R ) of
4H, THF); 13C NMR (C D , δ) 1.3, 2.5 (SiMe), 18.6 (C(CH ) ), 25.3
6
6
3 3
R
2
SiH
0.056, respectively, and R
respectively, for R ) CH , Ph, and SiH
the SiH group reduces RSi and increases R
2
are strongly dependent on R. RSi values are +0.501, +0.369, and
values are -0.087, -0.084, and -0.046,
. In comparison to CH and Ph,
significantly, indicating that
(C(CH ) ), 28.4 (THF), 68.6 (THF); 29Si NMR (C D , δ) 12.5 (t-BuMe -
3 3 6 6 2
Si). See the Supporting Information for the X-ray data of 5.
+
H
3
3
3
3
H
JA026705D
J. AM. CHEM. SOC.
9
VOL. 124, NO. 39, 2002 11605