Influence of the silyl group on the reactivity of some ortho-lithiated aryl alkyl sulfides

Article abstract of DOI:10.1021/om400236x

The lithiation of brominated aryl (α-dimethylsilyl)alkyl sulfides in diethyl ether produces stable heterocyclic silanes, which were characterized by ¹H, ¹³C, and ²⁹Si NMR spectroscopy and by X-ray crystallography. The reaction involves the intramolecular attack of the phenyl carbanion on the silicon atom with the formation of a pentacoordinated silicon intermediate. The stability of the formed intermediate depends on the solvent. It decomposes easily in THF at -78 C with Si-C bond cleavage; however, it is stable in diethyl ether at room temperature. Addition of water results in the Si-H bond cleavage, while the heterocyclic ring containing the silicon atom is conserved.

Full text of DOI:10.1021/om400236x

Communication
pubs.acs.org/Organometallics
Influence of the Silyl Group on the Reactivity of Some Ortho-
Lithiated Aryl Alkyl Sulfides
Krzysztof Durka,† Tomasz Klis,*,† Janusz Serwatowski,† and Krzysztof Wozniak‡
́
́
†Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warsaw, Poland
‡Warsaw University, Faculty of Chemistry, Pasteura 1, 02-093 Warsaw, Poland
S
* Supporting Information
ABSTRACT: The lithiation of brominated aryl (α-dimethylsilyl)alkyl
sulfides in diethyl ether produces stable heterocyclic silanes, which were
characterized by 1H, 13C, and 29Si NMR spectroscopy and by X-ray
crystallography. The reaction involves the intramolecular attack of the
phenyl carbanion on the silicon atom with the formation of a
pentacoordinated silicon intermediate. The stability of the formed
intermediate depends on the solvent. It decomposes easily in THF at
−78 °C with Si−C bond cleavage; however, it is stable in diethyl ether at room temperature. Addition of water results in the Si−
H bond cleavage, while the heterocyclic ring containing the silicon atom is conserved.
Scheme 1. Influence of the Solvent on the Lithiation of 1
irected lithiation of substituted arenes is widely used for
Dfunctionalization of aromatic systems; however, with the
appropriate substrates, competition between possible reaction
pathways could lead to the formation of a mixture of
products.1−5 We have already demonstrated that the selectivity
of the lithiation of aryl benzyl ethers or sulfides is strongly
influenced by the competitive deprotonation at the benzylic
position.6−8 Benzylic deprotonation in these compounds occurs
easily in THF at −78 °C, and the substituents bonded to the
phenyl ring strongly influence the rate of the reaction. The
effect of the substituent on benzylic deprotonation has been
determined using quantum calculations of proton transfer
reactions as well as kinetic studies of benzylic deprotonation of
sulfides.9,10 The thermodynamic stability of the benzylic anion
may result in the isomerization of the ortho-lithiated aryl benzyl
sulfides to the respective benzyllithium derivatives.11 In this
work we report on the generation of aryllithiums from aryl alkyl
sulfides containing a −SiMe3 or −SiHMe2 group bonded to an
aliphatic carbon atom. As the C−Si bond is polarized toward
the carbon, the silyl group simultaneously exhibits two
properties: the ability to stabilize carbanions on the α carbon
and the susceptibility to be attacked by nucleophiles. We
hypothesize that the presence of the silyl group will significantly
influence the reactivity of the ortho-lithiated sulfides.
in diethyl ether at −78 °C. The reaction in THF was carried
out similarly to that in diethyl ether; however, we used MeI as
the electrophile. After treatment of 1 with t-BuLi (2 equiv) in
THF, we expected facile intramolecular benzylic deprotonation
of the initially formed 1-Li to give 1-Li′. Unexpectedly, the
1
analysis of the H NMR spectrum of the isolated product
revealed the formation of a mixture of 2b and 2c (molar ratio
3.5:1). Whereas the formation of 2b resulted from a hydrogen
[1-4] migration affording 1-Li′ from 1-Li, the formation of 2c
may be attributed to a silicon [1-4] migration with the
formation of the intermediate 1-Li″. We speculate that,
similarly to the Brook rearrangement, the isomerization of 1-
Li to 1-Li″ may occur via a pentacoordinate silicon-containing
intermediate.12,13 However, in contrast to that process, the
driving force in our system is the formation of a stable benzylic
carbanion.
We next turned our attention to a study of the lithiation of 3,
which contains a −SiHMe2 group. In contrast to the lithiation
of 1, the treatment of 3 with t-BuLi (2 equiv) in THF resulted
in a smooth silicon [1-4] migration, affording 2a after addition
We were interested in studying the selectivity of the lithiation
of 1 depending on the solvent. The reaction in diethyl ether
was accomplished using a stepwise protocol which involved
lithiation with t-BuLi at −78 °C followed by quenching of the
reaction mixture with Me2HSiCl (Scheme 1).
The reaction afforded exclusively 2a. Its structure was
1
confirmed by a H NMR spectrum, which showed a septet at
4.63 ppm (Si−H, J = 3.6 Hz) and a singlet at 3.77 ppm (Cbenz
−
H) (1:1) as well as two doublets at 0.46 and 0.45 ppm
(SiH(CH3)2, J = 3.6 Hz) and a singlet at 0.13 ppm (Si(CH3)3).
This result indicates that 1-Li, formed in the first step, is stable
Received: March 20, 2013
Published: May 28, 2013
© 2013 American Chemical Society
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Communication
of Me3SiCl as the electrophile (Scheme 2). On the basis of the
previous results, we initially expected that lithiation of 3 in
should be noted that the formation of 5 from 3 can become an
alternative route for the synthesis of cyclic silanes. Earlier
papers have described the synthesis of this class of compounds
by the reaction of the respective dilithium compounds with
Me2SiCl2.17−21
Scheme 2. Influence of the Solvent on the Lithiation of 3
Considering the possibility of the synthesis of various
cyclosilanes, we decided to evaluate the reactivity of other
sulfides containing the −SiHMe2 group.
In contrast to the reactivity of 3, the lithiation of 6 followed
by hydrolysis with water afforded 7, regardless of the solvent
1
used (Scheme 3). The formation of 7 was confirmed by its H
Scheme 3. Proposed Mechanism of the Formation of 7
diethyl ether, followed by treatment of the reaction mixture
with Me3SiCl, will result in the replacement of the bromine
atom by the −SiMe3 group. Unexpectedly, after hydrolysis of
the reaction mixture with water, we isolated 5 as a crystalline
solid. The 1H NMR spectrum of 5 showed two singlets at 0.04
and 0.45 ppm (Si(CH3)2) and a singlet at 3.92 ppm (Cbenz−H)
(3:3:1, respectively). The formation of 5 was confirmed by
single-crystal X-ray diffraction (Figure 1). It can be assumed
NMR spectrum, which showed a singlet at 4.28 ppm (Cbenz
−
H2) as well as a septet at 4.65 ppm (Si−H, J = 4.0 Hz). We
postulate that the [1-3] migration of the silyl group observed
here is preceded by the formation of the intermediate 6-Li,
containing a strained four-membered ring. The decomposition
of 6-Li to 7-Li is dictated by the lowering of the energy barrier
resulting from ring opening and formation of a stable benzylic
anion.
To obtain more information on the ability of the −SiHMe2
group to undergo intramolecular nucleophilic attack by the
phenyl carbanion, we extended our study to the lithiation of
two more elaborated sulfides, 8 and 10, containing two 2-
bromophenyl groups connected with various sulfur−carbon
linkers with the −SiHMe2 group bonded to the aliphatic carbon
atom. The reactions were carried out in diethyl ether at −78 °C
using 4 equiv of t-BuLi followed by treatment of the reaction
mixtures with Me3SiCl as the electrophile. Interestingly, we did
not observe any incorporation of the −SiMe3 group into the
molecules of the reactants. Lithiation of 8 afforded the eight-
membered heterocyclic compound 9, whose structure was
confirmed by single-crystal X-ray diffraction (Figure 2).
Considering the mechanism of the formation of 9, we assumed
that the dilithiation of 8 should afford 8a followed by
intramolecular five-membered ring closure to give 8b (Scheme
4). A second nucleophilic attack at the silicon atom affords 8c;
Figure 1. Labeling of atoms and estimation of their atomic thermal
motion as anisotropic displacement parameters (50% probability level)
for 5. Selected bond lengths (Å) and angles (deg): Si1−C2 =
1.86(10), Si1−C13 = 1.91(11), S1−C13 = 1.82(10), C1−S1 =
1.77(10); C2−Si1−C13 = 97.2(5), Si1−C13−S1 = 105.4(5), C1−
C2−Si1−C13 = 16.2(9).
that, regardless of the solvent used, the lithiation of 3 involves
Br−Li exchange followed by intramolecular attack of the
carbanion on the silicon atom with the formation of the
hypercoordinated intermediate 3-Li. The formation of stable
pentacoordinated silicon compounds with five carbon ligands is
well-documented, and the structure was confirmed by X-ray
crystallography.14,15 The reactivity of 3-Li depends on the
solvent. Interestingly, the in situ lithiation/silylation of 3 with t-
BuLi/Me3SiCl in THF at −78 °C afforded exclusively 2a,
containing a −SiMe3 group in a benzylic position. This suggests
the rapid formation and decomposition of 3-Li in THF at −78
°C with cleavage of the Si−Cbenzylic bond to give 4-Li. In order
to check the stability of 3-Li, a solution of 3 in diethyl ether was
treated with t-BuLi (2 equiv) at −78 °C. The reaction mixture
was next slowly warmed to room temperature. The obtained
reddish slurry was analyzed by 29Si{1H} NMR spectroscopy
(room temperature, CDCl3 as the external standard). The
29Si{1H} NMR spectrum revealed one peak at −59.9 ppm. This
result was compared with the 29Si{1H} NMR spectrum of the
isolated 5, which showed a resonance at 24.8 ppm. It is
generally accepted that an increase of the silicon coordination
number leads to a strong low-frequency shift.16 This suggests
that 3-Li is stable in diethyl ether solution at room temperature
and the decomposition to 5 occurs after addition of water. It
Figure 2. Labeling of atoms and estimation of their atomic thermal
motion as anisotropy displacement parameters (50% probability level)
for 9. Selected bond lengths (Å) and angles (deg): Si1−C12 =
1.89(27), Si1−C2 = 1.89(29), C12−C7 = 1.42(42), C2−C1 =
1.40(42), C7−S2 = 1.79(30), C1−S1 = 1.79(30), S2−C13 = 1.81(31),
S1−C13 = 1.83(34); C12−Si1−C2 = 116.5(1), C12−C7−S2 =
118.6(2), C2−C1−S1 = 122.2(2), C7−S2−C13 = 98.2(1), C1−S1−
C13 = 101.8(1), C2−Si1−C12−C7 = 65.4(3).
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We hope that the cyclization process presented in this work is
general and can be extended to the synthesis of a broad range
of cyclosilanes. Efforts to expand this new methodology are in
progress.
Scheme 4. Proposed Mechanism of the Formation of 9
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, a table, figures, and CIF files giving experimental
procedures for all synthesized compounds, crystal data, data
1
collection, and refinement parameters for 5, 9, and 11, H and
13C NMR spectra, and crystallographic data for 5, 9, and 11.
This material is available free of charge via the Internet at
http://pubs.acs.org.
however, this intermediate is not reactive toward Me3SiCl.
Addition of water to the reaction mixture results in
decomposition of 8c to 9.
Interestingly, lithiation/silylation of 10 afforded 11 after
hydrolysis of the reaction mixture with water (Scheme 5). The
AUTHOR INFORMATION
■
Corresponding Author
Scheme 5. Proposed Mechanism of the Formation of 11
*T.K.: tel, +48 22 2347575; fax, +48 22 6282741; e-mail,
ktom@ch.pw.edu.pl.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Warsaw University of
Technology. The X-ray measurements of compounds 5, 9,
and 11 were undertaken at the Crystallographic Unit of the
Physical Chemistry Laboratory, Chemistry Department, Uni-
versity of Warsaw. Support from Aldrich Chemical Co.,
Milwaukee, WI, USA, through the donation of chemicals and
equipment is gratefully acknowledged.
structure of 11 was confirmed by single-crystal X-ray diffraction
(Figure 3). To explain this result, we postulate that the initially
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