Direct Silyl-Heck Reaction of Chlorosilanes

Article abstract of DOI:10.1021/acs.orglett.8b00847

A nickel complex/Lewis acid combination effectively catalyzed the direct silyl-Heck reaction of chlorosilanes, which are key raw materials in the organosilicon industry, to give synthetically important alkenylsilane products. Trichlorosilanes, dichlorosil

Full text of DOI:10.1021/acs.orglett.8b00847

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Direct Silyl−Heck Reaction of Chlorosilanes
Kazuhiro Matsumoto,‡ Jiadi Huang,‡ Yuki Naganawa, Haiqing Guo, Teruo Beppu, Kazuhiko Sato,
Shigeru Shimada,* and Yumiko Nakajima*
Interdisciplinary Research Center for Catalytic Chemistry (IRC3), National Institute of Advanced Industrial Science and Technology
(AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
S
* Supporting Information
ABSTRACT: A nickel complex/Lewis acid combination effectively
catalyzed the direct silyl−Heck reaction of chlorosilanes, which are key
raw materials in the organosilicon industry, to give synthetically
important alkenylsilane products. Trichlorosilanes, dichlorosilanes, and
monochlorosilanes underwent the silyl−Heck reaction to afford the
corresponding alkenylsilanes in high yields. In the reactions of
dichlorosilanes, a single substitution occurred to give monoalkenylsilanes
in a highly selective manner.
Scheme 1. Silyl−Heck Reactions
hlorosilanes are a key raw material in the organosilicon
industry. The first step for the industrial production of
C
organosilicon compounds is the Muller−Rochow “Direct
̈
Process,” in which silicon metal reacts with RCl, such as
chloromethane and chlorobenzene, to produce chloromethylsi-
lanes or chlorophenylsilanes (e.g., R2SiCl2, RSiCl3, R3SiCl,
HRSiCl2).1 Therefore, a wide variety of chlorosilanes are
commercially available at much lower cost, compared with
other halosilanes and silyl triflates. Thus, the development of
direct transformations of chlorosilanes, especially the incorpo-
ration of organic groups into the silicon center, has attracted
broad attention.
Alkenylsilanes are recognized as useful substrates for several
organic transformations such as Hiyama cross-coupling2 and
Tamao−Fleming oxidation.3 Such compounds are usually
prepared by conventional nucleophilic substitution of halosi-
lanes with alkenylmagnesium or alkenyllithium reagents.
However, the low functional group tolerance of the method
limits the availability of functionalized alkenylsilanes. Alkyne
hydrosilylation is also utilized for alkenylsilane synthesis, and
regioselective and stereoselective reactions have been devel-
oped.4 Considering the wide availability and low cost of
alkenes, as well as chlorosilanes, a preferable route to
alkenylsilanes would be the silyl−Heck reaction of chlorosilanes
(Scheme 1). Pioneering work by Tanaka and co-workers
allowed the palladium-catalyzed reaction of iodotrimethylsilane
with styrenes to give alkenylsilanes.5 Although the original
report was only moderately yielding, Watson and co-workers
recently rediscovered the reaction and a rational design of
palladium and nickel catalysts led to a significant improvement
of the productivity. Under the improved silyl−Heck conditions,
iodosilanes and silyl triflates can be employed as silyl sources.
Chlorosilanes are also usable only in the presence of lithium
iodide, which undergoes halogen exchange in situ with
chlorosilanes to form the corresponding iodosilanes.6−8
Nonetheless, to the best of our knowledge, there is no
precedent for the direct silyl−Heck reaction of chlorosilanes.9
One of the reasons for the lack is the high bond dissociation
energy (BDE) of Si−Cl bonds (Me3Si−Cl: 113 kcal/mol),
which is much higher than the BDE of Si−I bonds (Me3Si−I:
77 kcal/mol) and the BDE of the carbon counterpart, C−Cl
bonds (Me3C−Cl: 80 kcal/mol).10 Although the oxidative
addition of a Si−Cl bond to a metal seems to be quite difficult
because of the extremely high BDE,11 we hypothesized that
nucleophilic metals have a great chance to undergo oxidative
addition via a SN2 pathway,12,13 because Si−Cl bonds are
polarized to a large extent (Pauling electronegativity: Si 1.90, Cl
3.16)14 and nucleophilic substitution of chlorosilanes is the
common protocol in organosilicon synthesis. In this
communication, we present a direct silyl−Heck reaction of
chlorosilanes with styrenes by using a combination of a nickel
catalyst and a Lewis acid.
We first chose electron-rich Ni(cod)2/2PCy3 as a catalyst
and examined the reaction of styrene 1a with Me2SiCl2 2a, the
Received: March 15, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00847
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
silicon center of which is more electrophilic than that of
monochlorosilanes (Table 1).13c−e Me2SiCl2 is the most
Scheme 2. Substrate Scope in the Silyl−Heck Reaction of
Dichlorosilanes
Table 1. Optimization of Reaction Conditions
a
entry
x (mol %)
Lewis acid
y (mol %)
yield (%)
1
2
3
4
5
6
7
8
10
10
10
10
10
10
10
1
none
4
19
5
Zn(OTf)2
ZnCl2
20
20
20
20
20
50
50
50
AlCl3
17
32
58
88
91
90
Me2AlCl
Me3Al
Me3Al
Me3Al
Me3Al
b
9
1
a
1
Determined by H NMR analysis (600 MHz) in the presence of
b
mesitylene as internal standard. NiCl2(PCy3)2 instead of Ni(cod)2/
2PCy3 was used.
abundant organosilicon source, because it is the major product
in the direct process. In the absence of any additional activators,
1
only a trace amount of alkenylsilane 3aa was detected by H
NMR analysis (Table 1, entry 1). To further activate Me2SiCl2
toward nucleophiles, we screened some Lewis acids and
determined that Zn(OTf)2, ZnCl2, and AlCl3 show slight
positive effects, giving the desired product 3aa in 5%−19%
yields (Table 1, entries 2−4).15 Further screening revealed that
alkylaluminum species such as Me2AlCl and Me3Al were more
effective, and alkenylsilane 3aa was obtained in higher yields of
a
4-Methylstyrylsilane (4ba) was obtained in the reaction of 4-
32% and 58%, respectively (Table 1, entries 5 and 6).16
A
chlorostyrene (1f) with 100 mol % Me3Al.
satisfactory yield of 88% was achieved with 50 mol % Me3Al
(Table 1, entry 7), and the catalyst loading of Ni(cod)2 could
be reduced to 1 mol % without reduction in product yield
(Table 1, entry 8). Finally, we found that bench-stable
NiCl2(PCy3)2 was also effective for the silyl−Heck reaction
with no deterioration of the yield (Table 1, entry 9). Notably,
monoalkenylated product 3aa was obtained in a highly selective
manner, supporting our working hypothesis that less-electro-
philic monochlorosilanes are much less reactive than
dichlorosilanes.17 The alkenylchlorosilane product thus ob-
tained is highly useful, because it has both alkenyl and chloro
functionalities.
The NiCl2(PCy3)2/Me3Al system was successfully applied to
the silyl−Heck reaction of various styrene derivatives (Scheme
2). Alkenylchlorosilane products 3 were transformed to the
corresponding alkenylisopropoxysilanes 4 for ease of isolation.
Styrene derivatives with either electron-donating methyl and
tert-butyl groups or an electron-withdrawing fluorine atom at
the para-position successfully underwent the reaction to give
the corresponding alkenylsilanes 4ba, 4ca, and 4ea in good
yields. However, the use of 4-methoxystyrene 1d resulted in a
diminished yield of 31%, probably due to coordination of the
MeO group to Me3Al. In the reaction of 4-chlorostyrene 1f, C−
Cl methylation by nickel-catalyzed cross-coupling with Me3Al
was found to occur preferentially over the desired silyl−Heck
reaction.18 Thus, the reaction with 100 mol % Me3Al afforded
4-methylstyrylsilane 4ba (54%) instead of 4-chlorostyrylsilane
4fa via cross-coupling and silyl−Heck reaction. The ortho-
methyl-substitution is tolerated under the reaction conditions,
affording the corresponding alkenylsilane 4ga in high yield. A
good yield of 60% was observed in the reaction of 2-
vinylnaphthalene 1h. However, either aliphatic olefins or
disubstituted olefins such as 1-octene and trans-β-methylstyrene
did not give the desired alkenylsilanes. Other dichlorosilanes
such as Ph2SiCl2 2b and MePhSiCl2 2c also underwent the
reaction with styrene to give alkenylsilanes 4ab (92%) and 4ac
(66%), respectively.
Not only dichlorosilanes, but also trichlorosilanes are suitable
for this silyl−Heck reaction (Scheme 3). The reaction of
MeSiCl3 5 with 0.9 equiv styrene 1a at lower temperature of 60
°C gave monoalkenylated silane 6 with good selectivity. On the
other hand, when 2 equiv of styrene was used in the reaction at
90 °C, dialkenylated silane 7 was obtained as the major
product.
Although the reaction of Me3SiCl 8a was very sluggish under
the standard conditions, re-examination of the conditions
considerably improved the reaction of monochlorosilanes
(Scheme 4). With 5 mol % NiCl2(PCy3)2 and iPr2NEt instead
of Et3N at 120 °C, MePh2SiCl 8b and Me2PhSiCl 8c as well as
Me3SiCl underwent the silyl−Heck reaction with styrene 1a to
give the corresponding alkenylsilanes 9aa−9ac in 68%−80%
yields.
Based on the reaction mechanism proposed by Watson, and
on our experimental observations, we postulate the following
B
DOI: 10.1021/acs.orglett.8b00847
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
crude reaction mixture (see the SI). Given that the produced
Me2AlCl can also function as a Lewis acid, alkenylsilane can be
produced in sufficient yields, even in the presence of a
substoichiometric amount of Me3Al (50 mol %).
Scheme 3. Silyl−Heck Reaction of Trichlorosilanes
In conclusion, we have developed the first example of a direct
silyl−Heck reaction of chlorosilanes by the combination of
nickel catalyst and aluminum Lewis acid. Under the cooperative
catalysis, trichlorosilanes, dichlorosilanes, and monochlorosi-
lanes can be transformed to the corresponding alkenylsilanes in
high yields with high selectivity. We believe that the cooperative
system offers numerous opportunities for direct and catalytic
transformations of chlorosilanes, which will open up new access
to various organosilicon compounds. Further applications and
mechanistic studies are ongoing in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
Scheme 4. Silyl−Heck Reaction of Monochlorosilanes
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00847.
Experimental procedures and spectral data (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: s-shimada@aist.go.jp.
*E-mail: yumiko-nakajima@aist.go.jp.
ORCID
mechanism for the nickel-catalyzed direct silyl−Heck reaction
of chlorosilanes with styrenes (Scheme 5). First, precatalyst
Yuki Naganawa: 0000-0002-3041-7638
Shigeru Shimada: 0000-0001-7081-6759
Yumiko Nakajima: 0000-0001-6813-8733
Scheme 5. Proposed Catalytic Cycle
Author Contributions
‡These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the “Development of Innovative
Catalytic Processes for Organosilicon Functional Materials”
project (Project Leader: K. Sato) from the New Energy and
Industrial Technology Development Organization (NEDO).
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DOI: 10.1021/acs.orglett.8b00847
Org. Lett. XXXX, XXX, XXX−XXX