Radical activation of Si-H bonds by organozinc and silylzinc reagents: Synthesis of geminal dizinciosilanes and zinciolithiosilanes

Article abstract of DOI:10.1002/anie.201200126

Si-H bonds can be activated by organozinc and silylzinc compounds in the presence of minute amounts of radical initiators such as tBu₂Hg or AIBN, yielding zinciosilanes in good yields (see scheme). Activation of dihydridosilanes leads to gemina

Full text of DOI:10.1002/anie.201200126

Angewandte
Chemie
DOI: 10.1002/anie.201200126
À
Si H Bond Activation
À
Radical Activation of Si H Bonds by Organozinc and Silylzinc
Reagents: Synthesis of Geminal Dizinciosilanes and
Zinciolithiosilanes**
Roman Dobrovetsky, Yosi Kratish, Boris Tumanskii, Mark Botoshansky,
Dmitry Bravo-Zhivotovskii,* and Yitzhak Apeloig*
Dedicated to Professor Akira Sekiguchi on the occasion of his 60th birthday
Organozinc compounds are very useful reagents in synthesis,
and in analogy to organomagnesium and organolithium
compounds are used mainly as nucleophiles.[1a–c] Some reports
are also available for silylzinc compounds.[1d–g] At the same
time, as Zn and Hg are both Group 12 elements, organozinc
compounds are expected also to exhibit some of the
characteristic reactions of organomercury compounds. For
example, organomercury and silylmercury compounds
undergo radical reactions both thermally and photochemi-
dizinciosilanes. These interesting dimetallic reagents are
potentially very attractive reagents, in a similar fashion to
geminal dizinciomethanes, their organic analogues.[10]
Herein, we present the first examples of radical activation
À
of a Si H bond in R3SiH (R = Me3Si, Me3SiMe2Si) and
Me3SiMe2SiH by R’2Zn (R’ = tBu, Et) and by (tBuMe2Si)2Zn
(1) (for experimental details see Supporting Information).
Using this reaction we synthesized the silylenoid-type com-
pound [Cl(tBuMe2Si)2Si]2Zn (2), which upon lithiation yields
the zinc-bridged bis(silyllithium) [(thf)2Li(tBuMe2Si)2Si]2Zn
(3), the first known compound with lithium and zinc bonded
to the same Group 14 atom. We have also synthesized and
fully characterized the novel tetrametallic bis(zinciosilyl-
lithium)silane [(thf)3Li(tBuMe2Si)2SiZn]2Si(SiMe2tBu)2 (4).
The reactions of Et2Zn, which is commercially available,
and of tBu2Zn with tBuMe2SiH (5), Me3SiMe2SiH (6), and
(Me3Si)3SiH (7) were first studied. The reaction of Et2Zn or of
tBu2Zn with 5 was disappointing; 5 remained unchanged after
3 h at 908C, even in the presence of azobisisobutyronitrile
(AIBN) or tBu2Hg as radical initiators. However, this
situation changes upon silyl substitution of the silane. Thus,
reaction of Me3SiMe2SiH (6) with tBu2Zn for 3 h at 908C
yielded (Me3SiMe2Si)2Zn (8) in 30% yield. When a minute
amount of tBu2Hg, a radical initiator, was added the yield of 8
increased to 95%. In contrast, 6 was recovered unreacted
after reaction with neat Et2Zn and yielded only 10% of 8 in
the presence of tBu2Hg at 908C for 3 h (Scheme 1).
À
cally and activate Si H bonds, leading to silylmercury
compounds.[2–4] Such compounds are also excellent precursors
for the preparation of silyl anions by transmetalation.[2a,3,4a]
The possibility to replace in these reactions the highly toxic
organomercury compounds by the less toxic organozinc
compounds is attractive. The radical chemistry of organozinc
compounds, initiated by dioxygen, is a vigorously developing
field of organic chemistry with many applications in syn-
thesis.[5] In contrast, to the best of our knowledge, there are no
reports on radical reactions of silylzinc compounds.
À
Activation of Si H bonds is a very useful reaction
synthetically and therefore it is intensively studied, both in
academia and in industry.[6–8] There are two common strat-
egies for Si H bond activation: by radical initiators [7] and by
À
transition-metal catalysts.[8] Silylzinc reagents became
recently popular synthons in organic chemistry owing to
their selective and mild nucleophilic nature.[1d–g,9] Therefore,
a single-pot reaction leading to zinciosilanes by activation of
À
Si H bonds has a great synthetic potential. Furthermore, in
analogy to the synthesis of bismercuriosilanes,[2b,3a,b] activa-
tion of dihydridosilanes by diorganozinc or disilylzinc com-
pounds can lead, in a single-step reaction, to new geminal
[*] Dr. R. Dobrovetsky, Y. Kratish, Dr. B. Tumanskii, Dr. M. Botoshansky,
Dr. D. Bravo-Zhivotovskii, Prof. Y. Apeloig
Schulich Faculty of Chemistry and the Lise Meitner-Minerva Center
for Computational Quantum Chemistry
Technion-Israel Institute of Technology, Haifa 32000 (Israel)
E-mail: chrbrzh@tx.technion.ac.il
Scheme 1.
apeloig@technion.ac.il
[**] This research was supported by the Israel Science Foundation the
Fund for the Promotion of Research at the Technion and the
Minerva Foundation in Munich. D.B.-Z., B.T., and M.B. are grateful
to the Ministry of Immigrant Absorption, State of Israel, for a Kamea
fellowship.
With the branched silane (Me3Si)3SiH (7), zincation of the
Si H bond was much smoother. Reaction of 7 with neat
Et2Zn at 608C for 20 min produces [(Me3Si)3Si]2Zn (9) in 30%
yield. In the presence of 0.03 equivalents of AIBN or tBu2Hg
as radical initiators, the yield of 9 increased to 90%. Reaction
of 7 with tBu2Zn under the same conditions gave 9 in 70%
À
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201200126.
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presence of 0.05 equivalent of tBu2Hg yields the desired
product [Cl(tBuMe2Si)2Si]2Zn (2) in 65% yield [Eq. (1)].
Scheme 2.
yield, even without addition of AIBN (Scheme 2). Previously,
9 was obtained by the less-convenient reaction of (Me3Si)3SiLi
with ZnCl2.[11] As both 7 and Et2Zn are commercially
available, 9 becomes now an easily accessible reagent.
Our attempts to carry out the above reactions in the
presence of dry dioxygen, which is effective in initiating
radical reactions with dialkyl zinc,[5] failed, resulting in
complex product mixtures with no traces of the desired
silylzinc compounds. The different reaction course in the
presence of dioxygen is probably due to the extremely high
affinity of intermediate silyl radicals to oxygen, thus termi-
nating the radical chain-propagation step. Thus, for a con-
Compound 2 can be classified as a silylenoid that is geminally
substituted at silicon by an electronegative chlorine atom and
an electropositive zinc atom. Lithiation of 2 with excess
metallic lithium in a 10:1 hexane/THF mixture at room
temperature for 4 h yields the zinc bridged bis(silyllithium)
(3) in 60% yield. Compound 3 was isolated by crystallization
from hexane at room temperature, and its structure, as
determined by X-ray crystallography, is shown in Figure 1.[15]
À
trolled activation of Si H bonds, oxygen must be excluded.
The differences in the yields between the reactions of
tBu2Zn and Et2Zn with (Me3Si)3SiH (7) versus Me3SiMe2SiH
(6) versus tBuMe2SiH (5) probably result from the fact that
tBu2Zn decomposes faster than Et2Zn[12] to yield tBuC radicals,
À
which initiate the radical chain activation of the Si H bond.
Similarly, tBu2Hg is less thermally stable than Et2Hg.[4a]
Furthermore, the ease of hydrogen abstraction follows the
order 7 > 6 > 5, reflecting the thermodynamic stability order
of the corresponding silyl radicals, which become more stable
the higher the number of silyl substituents.[2a,d,7]
To study the effect of the metal on the metalation
reaction, (Me3Si)3SiH was reacted in hexane with tBu2Hg,
tBu2Zn, and tBuLi at room temperature for 24 h. With
tBu2Hg, [(Me3Si)3Si]2Hg (9a) was obtained in 99% yield. With
tBu2Zn, the corresponding disilylzinc 9 was obtained in 50%
yield. In contrast, with tBuLi, 7 remained mostly unreacted,
and only 10% of (Me3Si)3Si(tBu) was obtained (Scheme 3).[13]
Figure 1. ORTEP of the molecular structure of compound 3. Ellipsoids
are set at 10% probability; hydrogen atoms are omitted for clarity.
Selected bond lengths [ꢀ], angles [8], and dihedral angles [8]: Si1–Zn1
2.3620(15), Si4–Zn1 2.3620(15), Si1–Li1 2.531(9), Si4–Li2 2.531(9),
Si1–Si2 2.336(2), Si1–Si3 2.341(2), Si4–Si5 2.336(2), Si4–Si6 2.341(2);
Si1-Zn1-Si4 170.89(7), Zn1-Si1-Li1 116.0(2), Zn1-Si4-Li2 116.0(2), Li1-
Si1-Si4-Li2 54.56(3), Si6-Si4-Si1-Li1-178.16(2), Si6-Si4-Si1-Si2
À50.88(5).
Compound 3 is the first isolated molecule with zinc and
lithium atoms bonded to the same Group 14 atom: either
silicon, carbon, or germanium. Attempts to synthesize com-
pounds having the Li-C-Zn connectivity failed as these
compounds spontaneously yield four-membered (CZn)2
rings.[16] Compound 3 is the second isolated geminal dime-
tallosilane having two different metals (the first is a mercury
analogue of 3[2b]).
Scheme 3.
The above experiments and in particular the higher yields
obtained in the presence of AIBN or tBu2Hg (Scheme 2)
suggest that in analogy to R2Hg,[2b,3a,b] R2Zn reacts with
silanes by a radical mechanism.
Encouraged by these results, we aimed to prepare
functionalized zinciosilanes. Reaction of chlorosilane Cl-
(tBuMe2Si)2SiH (10)[14] with tBu2Zn for 3 h at 1208C in the
The X-ray structure of 3 reveals that the zinc atom in 3 is
not solvated, whereas the lithium atoms are each solvated by
two THF molecules. The Si1-Zn1-Si4 angle of 170.98 is slightly
distored from the ideal 1808 angle previously reported for
disilylzinc.[17a] The bulky silyl substituents and the (thf)2Li
À
groups are nearly staggered in respect to the Si1 Si4 axis. The
À
Si Zn bond lengths in 3 (2.362 ꢀ) are similar to previously
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[11,17]
À
À
reported Si Zn bond lengths (2.356–2.399 ꢀ
). The Si Li
tBu2MeSiLi in hexane at about À408C, persistent R’’Zn-
À
bond lengths (2.531 ꢀ) are similar to r(Si Li) in the
analogous mercury-bridged bis(silyllithium) (2.53 ꢀ[18]).
The 29Si NMR chemical shift of the anionic silicon atoms
(Si1 and Si4) in 3 of À192 ppm is shifted upfield by 80 ppm
from that of Hg[Si(SiR3)2Li]2.[18] This is consistent with the
lower electronegativity of zinc compared with mercury (1.65
vs. 2[19]) and may indicate a higher nucleophilicity of Si1 and
Si4 atoms in 3 compared with those in Hg[Si(SiR3)2Li]2.
(tBuMe2Si)2SiC radicals (13a and 13b) were observed
(Scheme 4).[21] Radicals 13a and 13b were characterized by
the relatively small hyperfine coupling constant with the 29Sia
nuclei (25.3 G and 31.1 G, respectively), which is typical for a-
metal-substituted silyl radicals.[3b,22] 13a and 13b differ in their
R’’ substituents, which were not yet determined.
À
Preliminary studies show that the Si Zn bonds of 3 can be
À
retained while the Si Li bonds react. For example, hydrolysis
À
À
of 3 leads to protonation of the Si Li bonds while the Si Zn
bonds are retained [Eq. (2)]. In contrast, under similar
conditions, (tBuMe2Si)2Zn is hydrolyzed to tBuMe2SiH.
Scheme 4.
Unfortunately, attempts to crystallize 12 failed. However,
lithiation of 12 with dispersed lithium in THF for 4 h at 708C
yields bis(zinciosilyllithium)silane 4 in 70% yield [Eq. (3),
step b]. Compound 4 was crystallized from hexane and the X-
ray molecular structure is shown in Figure 2.[23] The formation
of compound 4 supports the formula proposed for 12.
Attempting to synthesize a geminal bis(zinciosilane),
(tBuMe2Si)2SiH2 (11) was reacted with tBu2Zn in the presence
of 0.05 equivalent of tBu2Hg. After 4 h at 1208C an air-
sensitive material that was insoluble in hydrocarbon solvents
was obtained, but owing to its low solubility we were unable
to characterize the reaction products.
In contrast, reaction of 11 with (tBuMe2Si)2Zn (1) with
0.05 equivalents of tBu2Hg in hexane produced the tetrazin-
ciosilane 12 after 4 h at 1208C in 60% yield [Eq. (3), step a].
This reaction is analogous to the synthesis of silylmercury
oligomers, obtained in the reaction between 11 and (tBu-
Me2Si)2Hg.[3a] The formation of 12 supports a radical mech-
anism for this reaction, similarly to that of the analogous
silylmercury compounds.[20]
Figure 2. ORTEP of the molecular structure of compound 4. Ellipsoids
are set at 20% probability; hydrogen atoms are omitted for clarity.
Selected bond lengths [ꢀ], angles [8], and dihedral angles [8]: Si1–Zn1
2.411(2), Si4–Zn1 2.367(19), Si1–Zn2 2.403(2), Si7–Zn2 2.355(2), Si4–
Li1 2.737(12), Si7–Li2 2.731(12), Si1–Si2 2.348(3), Si1–Si3 2.348(3),
Si7–Si8 2.336(3); Zn1-Si1-Zn2 123.66(6), Si7-Si9 2.322(4), Si4-Si5
2.357(3), Si4-Si6 2.355(3), Si1-Zn1-Si4 174.87(6), Si1-Zn2-Si7
175.51(6), Zn1-Si4-Li1 109.1(3), Zn2-Si7-Li2 115.7(3), Li2-Si7-Si4-Li1
À144.81(3), Si9-Si7-Si4-Si5 À75.99(4), Li1-Si4-Si1-Si7 À86.02(3), Li1-
Si4-Zn1-Si1 À6.98(6), Li1-Si4-Si7-Li2 À144.81(2).
The fact that all the reactions presented above are induced
by radical initiators strongly suggests that these reactions
involve zinc-substituted silyl radicals. Attempts to observe
such radicals by conducting the reactions within the EPR
probe were unsuccessful, which may reflect the short lifetimes
and thus low concentrations of the intermediate radicals.
However, we could show that zincio-substituted silyl radicals
are viable species. Thus, in reactions of 2 or of 12 with
Compound 4 is the first characterized tetrametallosilane
chain, and it has both a geminal dizinciosilyl unit and two
heterozinciolithiosilyl fragments. Thus, 4 contains four active
metallic sites composed of two different metals, which can act
as nucleophilic sites or as radical precursor sites.
The zinc atoms in 4 are not solvated, similar to 3, but each
of the lithium atoms in 4 is solvated by three THF molecules
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À
In conclusion, we have demonstrated that Si H bonds can
be activated by organozinc and silylzinc compounds in the
presence of minute amounts of radical initiators, such as
tBu2Hg or AIBN, yielding zinciosilanes in good yields in
a single-pot reaction. Furthermore, activation of dihydrido-
silanes leads in a single-pot reaction to geminal dizincio-
silanes. We also synthesized the novel zincio-bridged dis-
ilyllithiums 3 and 4, which have three and four metal–silicon
bonds, respectively. We continue to study the scope of the
approach presented herein for the synthesis of other novel
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Received: January 6, 2012
Revised: February 16, 2012
Published online: April 3, 2012
À
Keywords: dialkyl zinc compounds · Si H activation · silicon ·
silyl radicals · zinc
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data), GOF = 0.862. Further crystallographic data of 3 is given in
the Supporting Information. CCDC 858766 (3) contains the
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4176.8(14) ꢀ3, Z = 2, Nonius appa CCD, MoKa radiation
(0.71073 ꢀ), 240 K, 2Vmax = 258, R = 0.0766 (I > 2sI), wR2 =
0.1925 (I > 2sI), Rw = 0.1540 (all data), GOF = 1.004.
CCDC 858767 (4) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
[23] Crystallographic data for 4: C60H138Li2O6Si9Zn2, Mr = 1353.13,
¯
triclinic, space group P1, a = 14.515(3), b = 16.915(3), c =
[24] a) A. Sekiguchi, V. Y. Lee, M. Nanjo, Coord. Chem. Rev. 2000,
210, 11 – 45.
19.109(4) ꢀ, a = 69.30(2), b = 72.12(2), g = 83.58(2)8, V=
Angew. Chem. Int. Ed. 2012, 51, 4671 –4675
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
4675