Cross-metathesis of vinylsilanes carrying electron-withdrawing substituents with olefins in the presence of the second-generation Grubbs catalyst

Article abstract of DOI:10.1016/S0040-4039(03)01831-8

The efficient cross-metathesis of vinylsilanes carrying electron-withdrawing substituents with various olefins is described. High yields and selectivities were obtained when styrene, 1-hexene, and selective functional allyl derivatives were used as the ol

Full text of DOI:10.1016/S0040-4039(03)01831-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 7121–7124
Cross-metathesis of vinylsilanes carrying electron-withdrawing
substituents with olefins in the presence of the second-generation
Grubbs catalyst
Cezary Pietraszuk,a,b Bogdan Marcinieca and Helmut Fischerb,*
aFaculty of Chemistry, Adam Mickiewicz University, 60-780 Poznan, Poland
bFachbereich Chemie, Universita¨t Konstanz, Fach M 727, 78457 Konstanz, Germany
Received 20 June 2003; revised 28 July 2003; accepted 29 July 2003
Abstract—The efficient cross-metathesis of vinylsilanes carrying electron-withdrawing substituents with various olefins is
described. High yields and selectivities were obtained when styrene, 1-hexene, and selective functional allyl derivatives were used
as the olefins.
© 2003 Elsevier Ltd. All rights reserved.
Recently, the olefin metathesis has become an impor-
tant and powerful tool in organic and polymer synthe-
sis 1due to the development of well defined metal
carbene complexes (e.g. compounds I and II) that are
tolerant to functional groups and can act directly as
metathesis initiators.2
and more widely applied in organic synthesis.4,5 There-
fore, we studied the cross-metathesis of terminal vinylsi-
lanes (H2CꢀC(H)SiR3) with olefins with the purpose of
developing new routes to these internal vinylsilanes.
Recently, we showed that the Grubbs catalyst I effec-
tively catalyses the metathesis of vinylsilanes with styre-
nes,6,7 substituted styrenes, alkenes,7 and numerous
allyl derivatives7,8 (Scheme 1) as well as of dienes and
cycloalkenes.9
These metatheses were limited to vinylsilanes contain-
ing three alkoxy or siloxy substituents at silicon. For
organosilicon chemistry, however, the cross-metathesis
of chloro-substituted vinylsilanes10 with alkyl-, aryl-,
and silyl-substituted olefins would be especially attrac-
tive. The resulting chloro-substituted metathesis prod-
ucts constitute very useful starting compounds for a
series of transformations since the chloride at silicon
can easily be replaced by a variety of groups.11 In
addition, the organochlorosilicon products are valuable
(co)monomers for the synthesis of polysiloxanes and
The growing number of reports on the application of
metathesis in organosilicon chemistry proves the large
potential of the reaction for the efficient transformation
of organosilicon compounds.3 Vinylsilanes of the type
R3SiCHꢀC(H)R% constitute a class of compounds more
Scheme 1.
Keywords: vinylsilanes; cross-metathesis; ruthenium; olefins.
* Corresponding author.
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01831-8

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C. Pietraszuk et al. / Tetrahedron Letters 44 (2003) 7121–7124
are intermediates in a great number of organic synthe-
ses, especially in the fast developing field of the palla-
dium-catalysed coupling of vinylsilanes with organic
derivatives.5
sufficient. Since styrene, allyl ethyl ether, and allyl
bromide tend to form olefin self-metathesis products, a
15-fold excess of the vinylsilanes is recommended.
However, even a 15-fold excess of the vinylsilane is not
enough to completely suppress the formation of the
olefin self-metathesis product D (Table 1, entries 3–5
and 8–10).
We now report that the limitations encountered when
using ruthenium catalyst I can be overcome by employ-
ing the second-generation Grubbs catalyst II. In con-
trast to I, complex II effectively catalyses the
cross-metathesis of di- and trichloro-substituted vinyl-
silanes10 with alkyl-, aryl-, and silyl-substituted olefins
at room temperature or at slightly elevated tempera-
tures. Effective stirring and heating of the reaction
mixture in CH2Cl2 to a gentle reflux for 1 h is sufficient
for the reactions to proceed quickly (Scheme 2). Only
very small amounts of the catalyst (0.5 mol% relative to
olefins) are required for a fast and in most cases
complete conversion of the olefins. Since vinylsilanes
are usually inactive to conversion to self-metathesis
products, the vinylsilanes can be used in excess to
minimise the formation of olefin self-metathesis prod-
ucts D. For reactions of 1-hexene with allyltrimethylsi-
lane a 5-fold excess of the vinylsilane appears to be
High yields of cross-metathesis products (Table 1,
entries 1, 2, 6, and 7) were obtained in the reactions of
1-hexene and of allyltrimethylsilane with the vinylsi-
lanes H2CꢀC(H)SiCl3 and H2CꢀC(H)SiCl2Me. The
reactions proceeded highly stereoselectively with a pro-
nounced preference for the formation of the E isomer.12
In contrast, chlorodimethylvinylsilane, and triphenyl-
vinylsilane did not react with 1-hexene (Table 1, entries
11 and 12) suggesting that for efficient cross-metathesis
silyl groups are required that carry strongly electron-
withdrawing substituents at silicon. In accord with this
conclusion displacing triphenylvinylsilane by H2CꢀC
(H)SiR3 (SiR3=Si(C6H4CF3-p)3 or SiCl(C6H4CF3-p)2)
again led to high yield and high stereoselectivity of
cross-metathesis products (Table 1, entries 13 and 14).
Scheme 2.
Table 1. Cross-metathesis of vinylsilanes with olefins (approximate concentrations of B: 0.7 mol l−1 (1-hexene) and 0.4 mol
l−1 (styrene))
Entry
Vinylsilane (A) R3Si=
Olefin (B) R%= Conversion of olefina [%]
Yield of Ca (isolated) [%]
E/Za
Yield of Da
[%]
b
1
2
3
4
5
SiCl3
n-C4H9
CH2SiMe3
CH2OEtc
CH2Brc
Phc
100
100
100
70
100 (90)
100 (92)
92
63
83 (70)
20/1
25/1
8/1
>25/1
>25/1
0
0
8
7
5
c
85
b
6
7
8
9
SiCl2Me
n-C4H9
100
100
100
60
100 (80)
100 (80)
95
55
78 (65)
20/1
22/1
11/1
>25/1
>25/1
0
0
5
5
b
CH2SiMe3
CH2OEtc
CH2Brc
Phc
10
80
Trace
b
11
12
13
14
SiClMe2
SiPh3
Si(C6H4CF3-p)3
SiCl(C6H4CF3-p)2
n-C4H9
n-C4H9
n-C4H9
n-C4H9
Trace
Trace
100
0
0
97
–
–
0
0
b
d,e
>25/1
25/1
Trace
0
b
100
100
b
15
16
Si(OAc)3
n-C4H9
100
100
100 (70)
100 (70)
20/1
25/1
0
0
b
CH2SiMe3
Reaction conditions: CH2Cl2, reflux (or 50°C), 1 h.
a Determined by 1H NMR spectroscopy.
Molar concentrations ratios: b [A]:[B]:[II]=5:1:5×10−3 c [A]:[B]:[II]=15:1:5×10−3 d [A]:[B]:[II]=5:1:2×10−2
; ; .
e Reaction complete after 3 h.

C. Pietraszuk et al. / Tetrahedron Letters 44 (2003) 7121–7124
7123
Scheme 3.
These reactions required a higher concentration of the
catalyst and/or extended reaction times. When 2 mol%
of II were used in the reaction with H2CꢀC(H)Si-
(C6H4CF3-p)3 the complete conversion of the olefin was
observed only after 3 h. Displacement of one 4-trifl-
uoromethylphenyl- by a chloro-substituent led to sub-
stantial increase in cross-metathesis reactivity (Table 1,
entries 13 and 14).
metathesis proceeded highly selectively and only traces
of Cl2MeSi(H)CꢀC(H)SiCl2Me were detected in the
reaction mixture. The formation of the bis(silyl)ethene
establishes that a silylcarbene species is formed as an
intermediate during the metathesis reaction (for a sim-
plified mechanistic scheme of the catalytic cycles see
Scheme 3). The failure to observe the cross-metathesis
product in the system containing H2CꢀC(H)SiClMe2 is
very likely due to rapid decomposition of either the
silylcarbene intermediate [Ru]ꢀC(H)SiClMe2 or the cor-
responding ruthenacyclobutane. The decomposition
presumably follows a mechanism similar to that pro-
posed for the decomposition of I in the presence of
H2CꢀC(H)SiMe3 (b-silyl elimination from a ruthenacy-
clobutane).13 Despite the detection of a self-metathesis
product derived from a silylcarbene intermediate the
majority, if not all, of the cross-metathesis products
seems to be formed by a catalytic cycle involving
[Ru]ꢀCH2 and [Ru]ꢀC(R%)H intermediates (bold cycle in
Scheme 3). All attempts to detect a silylcarbene com-
plex have failed so far.
The crucial role of electron-accepting substituents at
silicon is further supported by comparing the reactivity
of the series H2CꢀC(H)Si(OAc)nMe3−n (n=1–3).
H2CꢀC(H)Si(OAc)3 was found to be highly active in the
reaction with 1-hexene. In contrast, H2CꢀC(H)Si-
(OAc)2Me gave only traces of products and H2CꢀC(H)-
Si(OAc)Me2 did not show any metathesis activity. In
the presence of both of these vinylsilanes the fast
decomposition of carbene complex II by loss of the
carbene moiety was observed.
Cross-metathesis efficiency decreased and small
amounts of olefin self-metathesis products D were
formed when 1-hexene and allyltrimethylsilane were
replaced by styrene, allyl ethyl ether, or allyl bromide.
When allyl ethyl ether was used as the olefin even the
stereoselectivity dropped considerably. Obviously, elec-
tron-withdrawing substituents at the olefin used as co-
reagent tend to decrease cross-metathesis efficiency.
In conclusion, the efficient and highly selective cross-
metathesis of chloro- and acetoxy-substituted vinylsi-
lanes with various olefins in the presence of very small
loadings of [Cl2(PCy3)(IMesH2)Ru(ꢀCHPh)] (II) offers
an attractive route to unsaturated organosilicon com-
pounds. The ability to produce chloro-substituted silyl
derivatives makes the method particularly useful for the
synthesis of functional (co)monomers for silicone chem-
istry and of intermediates in organic synthesis.
Unexpectedly, when a large excess of H2CꢀC(H)Si-
Cl2Me (15 equivalents) was used, the slow formation of
significant amounts of the bis(silyl)ethene Cl2MeSi(H)-
CꢀC(H)SiCl2Me by self-metathesis was observed. Self-
metathesis of vinylsilanes in the presence of well-defined
carbene complexes has not been observed before. The
molar ratio of bis(silyl)ethene to cross-metathesis
product C in the reaction mixtures was 1/8 for the
reaction of H2CꢀC(H)SiCl2Me with allyl bromide, 1/4
with allyl ethyl ether, and 1/3 with styrene, as deter-
mined by NMR spectroscopy. When the excess of
H2CꢀC(H)SiCl2Me in the reactions with 1-hexene and
allyltrimethylsilane was reduced to 5-fold, the cross-
Acknowledgements
C.P. acknowledges a research grant from the Alexander
von Humboldt Foundation. We also thank the
European Commission (contract number JCA1-CT-
2002-70001) and the Fonds der Chemischen Industrie
for the support of these investigations.

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C. Pietraszuk et al. / Tetrahedron Letters 44 (2003) 7121–7124
11. Pawlenko, S. Organosilicon Chemistry; Walter de
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