Unpredictable reaction of phenyltrichlorosilane with tert-butyllithium

Article abstract of DOI:10.1016/j.inoche.2014.02.039

The reaction of PhSiCl₃ with one equivalent of Li[tBu] at r. t. yielded tBuPhSiCl₂ whereas tBu₂PhSiCl was only a minor product of the reaction of two molar equivalents Li[tBu] with PhSiCl₃ at 60 °C. By contrast,

Full text of DOI:10.1016/j.inoche.2014.02.039

Inorganic Chemistry Communications 44 (2014) 50–52
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Unpredictable reaction of phenyltrichlorosilane with tert-butyllithium
⁎
Stefan Scholz, Inge Sänger, Frauke Schödel, Michael Bolte, Hans-Wolfram Lerner
Institut für Anorganische Chemie, Goethe-Universität Frankfurt am Main, Max-von-Laue-Str. 7, 60438 Frankfurt am Main, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of PhSiCl3 with one equivalent of Li[tBu] at r. t. yielded tBuPhSiCl2 whereas tBu2PhSiCl was only a
minor product of the reaction of two molar equivalents Li[tBu] with PhSiCl3 at 60 °C. By contrast, at r. t. tBuPhSiCl2
was solely formed by this reaction. Treatment of PhSiCl3 with three or more equivalents of Li[tBu] at 98 °C yielded
a mixture of tBu2PhSiH and tBu2(tBuC6H4CMe2CH2)SiH (3). Both silanes could easily be separated from each
other: tBu2PhSiH was distilled from the reaction mixture and the silane 3 could be crystallized from the dissolved
distillation residue. X-ray quality crystals of 3 were grown from a benzene solution at ambient temperature
(monoclinic, P21/c).
Received 19 December 2013
Accepted 28 February 2014
Available online 12 March 2014
Keywords:
tert-Butyllithium
Silanes
© 2014 Elsevier B.V. All rights reserved.
Silenes
X-ray structure analysis
Alkali metal silanides M[SiR3] (e.g. M = Li, Na, K; SiR3 = SitBu3 [1,2],
SiPhtBu2 [2–4], Si(SiMe3)3 [5,6]) have not only become interesting on
account of their molecular structures but have also attracted recent
attention due to their synthetic abilities. It is not at all surprising that
this sort of anion has been used in the synthesis of new main group
element clusters and compounds with elements in low coordination
states e.g. compounds consisting of the heavier group 14 elements
(SiR3 = SitBu3; Si4(SitBu3)4 [7–9], Ge3(SitBu3)4 [7,10], Ge4(SitBu3)4
[7,11], Sn3(SitBu3)4 [7,12], Sn6(SitBu3)6 [7,13], Sn8(SitBu3)6 [7,14], and
Sn9(SitBu3)8 [15]).
In the course of our investigations of silanides which bear steri-
cally demanding substituents we prepared tBu2PhSiH from PhSiCl3
and Li[tBu] by a one-pot procedure [3,16–18]. However, it is worth
to mention that tBu2PhSiH was thereby obtained in low yield
(30%). Therefore the question we asked ourselves was: which prod-
ucts are additionally formed by this reaction? To find out the mech-
anism we performed the reaction of PhSiCl3 with Li[tBu] again and
varied thereby the amount of Li[tBu] as well as the reaction temper-
ature. In this paper we present the products of sequential reactions
of PhSiCl3 with Li[tBu].
Treatment of PhSiCl3 with two molar equivalents of Li[tBu] at room
temperature yielded also tBuPhSiCl2. In the course of this, one equiv-
alent of Li[tBu] still remained. Surprisingly, when the mixture of
tBuPhSiCl2 and Li[tBu] was heated to 60 °C for 4 h, different products
were formed. In the 1H and 13C NMR spectra of the reaction mixture
resonances due to the dichlorosilane tBuPhSiCl2, the silane tBu2-
PhSiH, as well as due to the minor products, the chlorosilane tBu2-
PhSiCl and the silane 3, were obseved. However, we found that
PhSiCl3 reacts with three or more equivalents of Li[tBu] at 98 °C,
forming the silanes tBu2PhSiH and 3. The formation of these silanes
suggested to us the mechanism, as shown in Scheme 1: (i) At first
1,4-addition of Li[tBu] to the phenyl ring of tBu2PhSiCl takes place.
(ii) Then, in a second step the transient 1,4-addition product un-
dergoes LiCl elimination to form the silene 1 [20,21]. (iii) Reaction
of 1 with isobutene produces the [2 + 2] cycloadduct 2. (iv) Rear-
rangement of 2 accompanied by a ring-opening finally gives the si-
lane 3 (cf. hydroxodearylation [22] and dehydrofluorination [23] of
silanes). Apparently the LiH elimination of Li[tBu] must be much
slower than 1,4-addition of Li[tBu] to the phenyl ring of tBu2PhSiCl.
Both silanes, tBu2PhSiH and 3 could be easily separated from each
other. The silane tBu2PhSiH was distilled from the reaction mixture
and 3 could be isolated from the distillation residue by extracting 3
into benzene, filtering, and crystallization. Slowly concentrating the
filtrate led to deposition of X-ray quality crystals of 3.
At first we investigated the equimolar reaction of PhSiCl3 with Li
[tBu]. Treatment of PhSiCl3 with Li[tBu] in 1:1 M ratio in pentane re-
sulted in an immediate reaction at room temperature, and the mix-
ture quickly became heterogeneous. The 1H and 13C NMR spectra
showed that Li[tBu] was completely consumed after stirring the
reaction mixture for 12 h at ambient temperature, and new signals
appeared which could be assigned to tBuPhSiCl2 (cf. Ref. [19]).
The silane 3 shown in Fig. 1 (selected bond lengths and angles in
the figure caption), crystallizes in the monoclinic space group P21/c
[24,25]. The Si(1) atom is coordinated by two tBu groups, one H
atom, and a combined phenylene–isobutylene bridge which links a
third tBu group with the central Si atom. The structural characteris-
tics of the compound reported in this study are consistent with struc-
tural data for other examples of tBu-substituted silyl compounds.
⁎
Corresponding author. Fax: +49 69 798 29260.
E-mail address: lerner@chemie.uni-frankfurt.de (H.-W. Lerner).
http://dx.doi.org/10.1016/j.inoche.2014.02.039
1387-7003/© 2014 Elsevier B.V. All rights reserved.

S. Scholz et al. / Inorganic Chemistry Communications 44 (2014) 50–52
51
Fig. 1. Molecular structure of 3 (50% displacement ellipsoids). Selected bond lengths (Å)
and bond angles (°): Si(1)\C(3) 1.9020(12), Si(1)\C(2) 1.9102(12), Si(1)\C(1)
1.9156(13), Si(1)\H(1) 1.387(15), C(1)\C(12) 1.5360(17), C(1)\C(11) 1.5364(17),
C(1)\C(13) 1.5364(18), C(2)\C(22) 1.5384(18), C(2)\C(21) 1.5392(17), C(2)\C(23)
1.5394(19), C(3)\C(4) 1.5593(15), C(4)\C(42) 1.5344(17), C(4)\C(51)
1.5371(16), C(4)\C(41) 1.5378(18), C(51)\C(56) 1.3800(18), C(51)\C(52)
1.3862(18), C(52)\C(53) 1.390(2), C(53)\C(54) 1.3824(18), C(54)\C(55)
1.3854(17), C(54)\C(6) 1.5322(16), C(55)\C(56) 1.3904(19), C(6)\C(63)
1.5220(18), C(6)\C(62) 1.525(2), C(6)\C(61) 1.5317(19), C(3)\Si(1)\C(2)
109.27(5), C(3)\Si(1)\C(1) 109.32(5), C(2)\Si(1)\C(1) 116.87(6), C(3)\Si(1)\H(1)
110.5(6), C(2)\Si(1)\H(x1) 104.7(6), C(1)\Si(1)\H(1) 106.0(6), C(4)\C(3)\Si(1)
120.54(8).
References
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Scheme 1. Suggested mechanism for silane 3 formation.
However, it is remarkable that the torsion angle about the bond be-
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observed for B(C6F5)3 and (C6H2Me3)2PH [26]. Overall, our study
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Acknowledgments
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We are grateful to the University of Frankfurt for the funding and to
the Rockwood Lithium GmbH for the gift of tert-butyllithium.
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Appendix A. Supplementary materials
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Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2014.02.039.

52
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γ = 90° V = 2181.2(2) Å3, fw = 332.63, Z = 4, and Dc = 1.013 g cm − 3. 31658
reflections were collected in the range of 1.34 to 26.82° at 173 K giving a final resid-
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with a = 12.0089(8) Å, b = 5.9791(2) Å, c = 30.381(2) Å, α = 90° β = 90.820(6)°