Metal-free, stereospecific bis-silylation of functionalized alkynes with NHC-supported silylaminosilylene

Article abstract of DOI:10.1021/om3009958

The NHC-supported silylaminosilylene Ar(SiMe₃)N(Cl)Si(IiPr) (1; Ar = 2,6-iPr₂C₆H₃, IiPr = 1,3-diisopropyl-4,5- dimethylimidazol-2-ylidene) is an efficient and stereospecific bis-silylation reagent for a range of functionalized alkynes to yield cis-1,2-bis-silylated alkenes via a 1,4-silyl migration from the amino nitrogen atom to an alkyne carbon atom. The reaction is regio- and stereospecific for terminal alkynes with an electron-withdrawing substituent, thus providing a facile access to 1,2-bis-silylated alkenes with two different silyl groups.

Full text of DOI:10.1021/om3009958

Communication
pubs.acs.org/Organometallics
Metal-Free, Stereospecific Bis-Silylation of Functionalized Alkynes
with NHC-Supported Silylaminosilylene
Haiyan Cui, Bing Ma, and Chunming Cui*
State Key Laboratory of Elemento-organic Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
S
* Supporting Information
ABSTRACT: The NHC-supported silylaminosilylene Ar-
(SiMe3)N(Cl)Si(IiPr) (1; Ar = 2,6-iPr2C6H3, IiPr = 1,3-
diisopropyl-4,5-dimethylimidazol-2-ylidene) is an efficient and
stereospecific bis-silylation reagent for a range of functionalized
alkynes to yield cis-1,2-bis-silylated alkenes via a 1,4-silyl
migration from the amino nitrogen atom to an alkyne carbon
atom. The reaction is regio- and stereospecific for terminal
alkynes with an electron-withdrawing substituent, thus
providing a facile access to 1,2-bis-silylated alkenes with two different silyl groups.
inylsilanes are important alkene derivatives that have been
widely used as synthetic intermediates, monomers for
Reactions of silylenes with alkynes present an alternative
approach for the silylation of alkynes without the aid of
transition-metal catalysts.6 However, previous studies have been
only confined to a few alkynes without functional groups as
models for an understanding of the fundamental chemistry of
silylenes.7 No efforts have been devoted to the employment of
silylenes for the silylation of functionalized alkynes because of
the extremely high reactivity of silylenes toward various
functional groups that may result in the formation of a
complicated mixture. Herein we report on the bis-silylation of
alkynes featuring diverse functional groups with the easily
available NHC-stabilized silylaminosilylene Ar(SiMe3)N(Cl)Si-
(IiPr) (1; Ar = 2,6-iPr2C6H3, IiPr = 1,3-diisopropyl-4,5-
dimethylimidazol-2-ylidene)8 under metal-free conditions
(Scheme 2). Remarkably, the bis-silylation reaction has been
accomplished in a stereospecific manner via a 1,4-silyl migration
from the silylamino nitrogen atom to one of the alkyne central
carbon atoms, representing the first successful approach for the
selective bis-silylation of alkynes with a donor-supported
silylene as the silylation reagent. In addition, the reaction is
V
copolymer plastics, and coupling agents for hybrid silicon-
containing materials.1 Therefore, the development of efficient
and selective silicon reagents for the synthesis of vinylsilanes is
of great interest. Transition-metal-catalyzed hydrosilylation and
bis-silylation of alkynes represent the most straightforward and
atom-economic routes to vinylsilanes.2 1,2-Disilylated alkenes,
stable alkene dianion equivalents,3 have been obtained by
catalytic addition of a Si−Si bond across a C−C triple bond
(Scheme 1). Since the original reports by Kumada and Sakurai
Scheme 1. Group 10 Metal Catalyzed Bis-Silylation of
Alkynes with Disilanes as Bis-Silylation Reagents and the
Possible Four Isomers
Scheme 2. Stereo- and Regioselective Bis-Silylation of
Terminal Alkynes with 1 as the Silylation Reagent under
Metal-Free Conditions
on palladium-catalyzed bis-silylation of alkynes with disilanes,4
stereoselective bis-disilylation catalyzed by group 10 metal
complexes in combination with suitable ligands has been
extensively investigated.5 Despite the successes in the intra-
molecular variants, the selective intermolecular reactions have
been mostly achieved by employments of cyclic disilanes as bis-
silylation reagents. The catalytic intermolecular reaction of
unsymmetrical alkynes with disilanes featuring two different
silicon atoms has met very limited success, due to the difficulty
in controlling the stereo- and regioisomers (Scheme 1) and
their separation. Thus, the development of highly selective bis-
silylation reagents is still highly desirable.
Received: October 23, 2012
© XXXX American Chemical Society
A
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Communication
regiospecific for terminal alkynes with an electron-withdrawing
group (Scheme 2).
min, while those of the other alkynes complete in several hours.
In addition, dialkyl-substituted alkynes led to very slow and
sluggish reactions. For MeO2CCCMe (entry 4) and PhC
CMe (entry 5), regioisomeric mixtures were obtained and
could not be easily separated. It is noteworthy that the reaction
of MeO2CCCPh (entry 6) only gave one isomer, 7a, in high
yield, indicating that it is possible to control the regioselectivity
of the bis-silylation reaction by regulation of the electronic and
steric factors of the alkyne substituents.
Since the reaction of the terminal alkyne PhCCH with 1 is
rapid, terminal alkynes with an electron-withdrawing group
would be ideal choices for the probe of the electronic factors of
alkyne substituents. Thus, the reactions of terminal alkynes
substituted by MeO2C and a series of substituted-phenyl
groups with an electron-withdrawing group including C(O)R,
CO2Me, CN, NO2, Cl, and CF3 groups with 1 have been
studied. Surprisingly, these reactions, in general, are complete
in 10 min and gave only one isomer in good to quantitative
yields on the basis of proton NMR analysis of the crude
products (Table 2), with the exception of carbonyl-substituted
alkynes, which led to the formation of a complicated mixture
that could not be characterized.
Very recently we and others reported the dehydrochlorina-
tion of chlorosilanes by N-heterocyclic carbenes (NHCs) for
the generation of N-heterocyclic silylenes and NHC-stabilized
silylenes.9 We have shown that an NHC-stabilized cyclic
(silole) silylene is highly nucleophilic and could generate an
alkenyl anion intermediate upon treatment with phenyl-
acetylene.10 In order to have an insight into the mechanism
of this type of reaction, the NHC-stabilized aminochlorosily-
lene 1 was employed for the reaction. Stirring a mixture of 1
with phenylacetylene in THF led to a rapid color change from
light green to yellow. The proton NMR analysis indicated the
formation of the two products 2a,b in high yield with a 2a/2b
molar ratio of 1.3/1 (Table 1). They have been separated by
a
Table 1. Bis-Silylation of Internal Alkynes
b
entry
R1
R2
time
regioisomers (a:b); yield (%)
Table 2. Stereo- and Regiospecific Bis-Silylation of Terminal
Alkynes
1
2
3
4
5
6
C6H5
H
10 min
4 h
2a/2b = 1/1.3; 74 (65)
3a = 3b; 100 (71)
4a = 4b; 100 (74)
a
C6H5
C6H5
CO2Me
Me
CO2Me
CO2Me
C6H5
10 min
10 min
6 h
c
5a/5b = 2.3/1; 71
c
Me
6a/6b = 1.8/1; 70
CO2Me
C6H5
10 min
7a; 100 (78)
a
b
Reaction conditions: in THF, room temperature. NMR yield,
c
isolated yields given in parentheses. NMR yield of the mixtures.
b
entry
R
product
yield (%)
7
CO2Me
8
100 (90)
80 (50)
8
o-pyridine
9
extraction with n-hexane. The multinuclear NMR spectroscopic
and X-ray single crystal analysis (see Figures S7 and S8 in the
Supporting Information) of 2a,b disclosed the formation of the
two cis-bis-silylated alkenes. The results prompted us to
investigate the reaction mechanism and the scopes and
limitations of the reaction with various alkynes, as the reaction
is not only mechanistically novel but is also promising for
stereospecific bis-silylation of alkynes under mild conditions in
the absence of metal catalysts.
For simplicity, we first examined three symmetrical alkynes
with phenyl, SiMe3, and MeO2C groups. It was found, by
proton NMR analysis, that phenyl- and MeO2C-substituted
alkynes reacted smoothly with 1 in C6D6 at room temperature
to give cis-bis-silylated alkenes 3 and 4 (Table 1) in quantitative
yield, while the Me3Si-substituted alkyne did not react with 1
under the same conditions, probably due to steric reasons. 3
and 4 have been fully characterized by multinuclear NMR
spectroscopy and elemental analysis. The structure of 3 was
confirmed by X-ray single-crystal analysis (Figure S9 in the
Supporting Information). Heating of 4 in C6D6 at 70 °C
yielded the corresponding bis-silylated maleic anhydride with
the elimination of dimethyl ether, indicating the cis-bis-silylated
structure of 4. These reactions suggested that 1 is a cis-bis-
silylation reagent for alkynes.
9
p-CO2Me-C6H4
p-CN-C6H4
p-NO2-C6H4
p-Cl-C6H4
10
11
12
13
14
100 (76)
100 (79)
100 (62)
76 (65)
10
11
12
13
p-CF3-C6H4
100 (79)
a
b
Reaction conditions: in THF, 10 min, room temperature. NMR
yield, isolated yield given in parentheses.
The bis-silylated products 7a and 8−14 have been fully
characterized by multinuclear NMR spectroscopy, IR spectros-
copy, and elemental analysis. The structures of 7a, 8, and 9
were confirmed by X-ray single-crystal analysis (see Figures
S10−S12 in the Supporting Information). The structures of 10,
11, 13, and 14 were determined by H and 13C NMR and 13C
1
HMBC (heteronuclear multiple bond correlation) experiments,
from which it can be seen that the substituted phenyl group is
on the same vinyl carbon atom as the Me3Si group (see Figures
S1−S4 in the Supporting Information). Compound 12 is not
stable and decomposed in solution within 1 h.
The novel stereoselective bis-silylation reaction apparently
results from the 1,4-SiMe3 migration from the nitrogen atom of
the amino ligand to one of the central alkyne carbon atoms.
The NHC-stabilized silylene 1 can be represented by two
resonance forms, as depicted in Scheme 3. Owing to the highly
nucleophilic nature of the silicon center of 1, it tends to attack
the polarized alkynes to afford the zwitteranionic species A, in
which the carbanion may interact with the SiMe3 group to form
the five-membered-ring intermediate or transition state B with
a hypervalent silicon atom followed by complete cleavage of the
Next, several unsymmetrical internal alkynes were employed
for the investigation of the steric and electronic factors of the
substituents of alkynes on the reaction. It was found that the
alkynes with an ester group significantly accelerated the
reaction (entries 3, 4, and 6 in Table 1). The reactions of
ester-substituted internal alkynes with 1 are complete in 10
B
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alcoholysis of these products might be an ideal choice for these
purposes, as it could generate an activated carbon−Si(OR)3
unit by the cleavage of the SiN and Si−Cl bonds and
transform the NHC to the corresponding imidazolium salt
simultaneously. Indeed, dissolving 10 and 11 in MeOH at room
temperature led to a complete conversion of the NHC-
coordinated silyl group to the Si(OMe)3 group with the
formation of the imidazolium salt, which can be easily separated
by filtration (Scheme 5). The trimethoxylsilyl-substituted
Scheme 3. Proposed 1,4-Silyl Migration Mechanism for the
Selective Bis-Silylation
Scheme 5. Alcoholysis of 10 and 11 for the Recovery of the
NHC
N−Si bond and formation of a Si−C bond to give the bis-
silylated alkenes (Scheme 3). The proposed mechanism is in
line with the observed stereo- and regioselectivity in this work.
Apparently, the existence of an electron-withdrawing group on
the terminal alkynes facilitates the attack of the highly polarized
alkynes by 1 and stabilizes the intermediate A. The exclusive
formation of cis-bis-silylated alkenes is due to the favorable five-
membered silicate intermediate or transition state B. Silyl
migration, such as silyl migration from a carbon to an oxygen
atom (Brook rearrangement), is a ubiquitous process in the
chemical reactions of organosilanes. However, silyl migration
from a nitrogen to a carbon atom has been rarely investigated.11
Because the Lewis acidic site of the silylene 1 is blocked by the
coordinated NHC, it is not easily attacked by other Lewis basic
groups and is thus chemoselective.
Since the NHC-supported silylene 1 has been obtained by
the dehydrochlorination of Ar(SiMe3)NSiHCl2 with 1,3-
diisopropyl-4,5-dimethyl-imidazol-2-ylidene in THF,8a it is
possible that the bis-silylation reaction could be carried out in
one pot without the isolation of the NHC-stabilized silylene 1.
As a representative example, addition of a THF solution of
PhCCPh to a mixture of the silane Ar(SiMe3)NSiHCl2 and
diphenylacetylene in THF from −78 °C to room temperature
indeed resulted in the formation of 3. The bis-silylated product
3 has been isolated in ca. 47% yield after workup (Scheme 4). It
alkenes 10′ and 11′ can be purified by distillation and have
been characterized by standard spectroscopic methods. The
sequence of reactions with these disilylated alkenes might be
invaluable in synthesis.
In summary, we have developed a metal-free route for the
stereo- and regiospecific bis-silylation of typical functionalized
terminal alkynes with the NHC-stabilized silylaminosilylene 1
as the silylation reagent. The silylation reaction is tolerant to a
range of functional groups, demonstrating that NHC-stabilized
silylenes have unique reaction patterns. In addition, the reaction
can be carried out in one pot without the isolation of 1 and the
resulting products can be easily converted to simple
trimethoxylsilyl-substituted alkenes featuring the activated C−
Si bond by alcoholysis with an excess of MeOH, thus providing
a practical route for the bis-silylation of activated alkynes for the
preparation of cis-disilylated alkenes with two different silyl
groups. The bis-silylation reaction is mechanistically distinct
from known catalytic bis-silylation, in which noble-metal
catalysts should be employed for the activation of a Si−Si
bond. Since NHC-stabilized silylaminosilylenes are easily
available, they are potentially promising metal-free bis-silylation
reagents for various unsaturated substrates.
Scheme 4. Representive Example of Bis-Silylation of an
Alkyne in One Pot
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, and tables giving details of the synthesis and
characterization data for the compounds reported in this paper
and CIF files giving crystallographic data for compounds 2a,b,
3, 7a, 8, and 9. This material is available free of charge via the
Internet at http://pubs.acs.org.
can be concluded that the bis-silylation reaction proceeded via
the initial formation of 1. This process is more practical than
that of the direct employment of 1 as the silylation reagent.
However, the almost quantitative reaction of activated alkynes
with 1 as the silylation reagent greatly simplifies the workup
process.
All of the resulting bis-silylated alkenes feature a reactive
silicon center that contains a coordinated NHC, a formal SiN
double bond, and one chloride ligand. The NHC-stabilized
silaimines (SiN double bond) have been previously reported
by us.8b We thought that it would be possible to convert this
silyl group into a simple activated group that may be used for
further transformation such as cross-coupling reactions12 and to
recover the N-heterocyclic carbene. It is envisioned that the
AUTHOR INFORMATION
Corresponding Author
*E-mail: cmcui@nankai.edu.cn.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China and the 973 Program (Grant No. 2012CB821600) for
support of this work.
C
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