Hydrosilylation of alkynes catalyzed by ruthenium carbene complexes

Article abstract of DOI:10.1016/j.tetlet.2004.11.025

Hydrosilylation of terminal alkynes with a variety of silanes catalyzed by Cl₂(PCy₃)₂RuCHPh (1) affords mainly the Z-isomer via trans addition in excellent yields. The presence of a hydroxyl group in close proximity to the

Full text of DOI:10.1016/j.tetlet.2004.11.025

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 105–108
Hydrosilylation of alkynes catalyzed by ruthenium
carbene complexes
Sarah V. Maifeld, Michael N. Tran and Daesung Lee*
Department of Chemistry, University of Wisconsin, Madison, WI 53706, USA
Received 10 September 2004; revised 3 November 2004; accepted 4 November 2004
Available online 21 November 2004
Abstract—Hydrosilylation of terminal alkynes with a variety of silanes catalyzed by Cl2(PCy3)2Ru@CHPh (1) affords mainly the Z-
isomer via trans addition in excellent yields. The presence of a hydroxyl group in close proximity to the triple bond was observed to
exert a strong directing effect, resulting in the highly selective formation of the a-isomer. Intramolecular hydrosilylation of a homo-
propargylic silyl ether was demonstrated to give the cis addition product.
Ó 2004 Elsevier Ltd. All rights reserved.
R’3Si
The significant growth in olefin metathesis chemistry can
primarily be attributed to the development of new and
effective metathesis catalysts.1 In particular, GrubbsÕ
ruthenium complexes, Cl2(PCy3)2Ru@CHPh (1)2a–c
and Cl2(PCy3)(NHC)Ru@CHPh (2),2d,e have demon-
strated remarkable reactivity in the presence of a variety
of functional groups. Additionally, complexes 1 and 2
have been employed as catalysts for a variety of trans-
formations,3 including an intramolecular [3 + 2] cyclo-
addition of alkynylidenecyclopropanes4 as well as the
dehydrogenative condensation of alcohols and the hy-
drosilylation of carbonyls.5 Our studies now show that
the utility of 1 can further be extended to include the
stereoselective hydrosilylation of a variety of alkynes
in the presence of various silanes.
SiR’3
HSiR’3
+
+
R
R
SiR’3
catalyst
R
R
Z /
trans addition
E /
cis addition
α
Scheme 1. Isomeric vinylsilanes formed from the hydrosilylation of
alkynes.
Z-vinylsilanes resulting from respective cis and trans
additions across the alkyne (Scheme 1).
To ensure the synthetic utility of alkyne hydrosilylation,
both the regio- and stereoselectivity of this process must
be addressed. A variety of factors including the choice of
substrate, solvent, and temperature have been shown to
affect the regio- and stereoselectivity of alkyne hydrosil-
ylation. In particular, the choice of transition metal cat-
alyst significantly influences the outcome of the reaction.
Rhodium catalysts have been employed for the selective
generation of both E-8 and Z-vinylsilanes.8b,9 The Z-iso-
mer has also been successfully generated from irid-
ium9a,10 and ruthenium11 catalysts. In addition,
ruthenium12 as well as platinum13 catalysts have been
employed for the selective formation of the a-isomer.
Organosilanes play an important and diverse role in or-
ganic chemistry.6 In particular, vinylsilanes have at-
tracted considerable attention due to their versatility in
both laboratory and industrial applications. The transi-
tion metal-catalyzed hydrosilylation of alkynes offers a
simple and direct means of producing vinylsilanes.7
One important issue regards the selectivity of such a
process, since the primary products of the reaction
may contain a mixture of three isomeric vinylsilanes,
including the branched a-isomer as well as the E- and
In previous work from our group in the area of silyl
ether-mediated metathesis reactions,14,15 GrubbsÕ car-
bene catalyst Cl2(PCy3)2Ru@CHPh (1), could effectively
catalyze the activation of silanes to form silyl ethers in
excellent yields via dehydrogenative condensation with
alcohols and hydrosilylation of carbonyls.5 Thus we
anticipated the activation of silanes by 1 could also be
Keywords: Hydrosilylation; Alkyne; Vinylsilane; Ruthenium;
Catalysis.
*
Corresponding author. Tel.: +1 608 265 8431; fax: +1 608 265
4534; e-mail: dlee@chem.wisc.edu
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.11.025

106
S. V. Maifeld et al. / Tetrahedron Letters 46 (2005) 105–108
catalysts 2–4 either contain an imidazolin-2-ylidene2d,e
or imidazol-2-ylidene18 ligand with the benzylidene
group, only catalyst 418c,19 replaces the benzylidene with
a 3-phenylindenylid-1-ene moiety.
Table 1. Hydrosilylation of alkynyl alcohols catalyzed by 1
Entry Alkyne Silane
(Z)/(E)/ Yieldb
T
(°C)/h (a)a
(%)
HO
HO
1
Me2PhSiH 60/13 0:1:3.5
60
2
3
4
5
6
Me2PhSiH 60/4
1.2:0:1
1:2:10
3:1.5:1
80c
NRd
In the reaction between phenylacetylene, triethylsilane,
and 1mol% of catalyst at 65°C, catalysts 1–4 showed
a strong propensity to favor the formation of the Z-iso-
mer 5a, while the a-isomer 5b formed in minor amounts
(Table 2).
α
β
Ph2SiH2
70/12 ––
Me2PhSiH 60/4
82
HO
HO
Ph2SiH2
70/12 ––
NRd
68
Me2PhSiH 60/4
In all cases, the alkyne was completely consumed after
heating for 16h. Hydrosilylation was achieved in 90%
yield when catalyst 1 was employed. Interestingly, reac-
tions catalyzed by 2–4 suffered due to the appearance of
significant amounts of tail-to-tail phenylacetylene dimer
6.20
a Ratio of isomers was determined by 1H NMR.
b Combined % yield of hydrosilylated alcohols.
c Z-Stereochemistry for both regioisomers was determined by NOE.
d Formation of a small amount of silyl ether was noted followed by
significant decomposition of the alkyne in the presence of unreacted
diphenylsilane.
After completing a cursory investigation of catalysts,
further investigation revealed that 1 efficiently catalyzes
the hydrosilylation of various terminal alkynes with a
variety of silanes. In the presence of 1mol% of 1, nearly
all reactions gave complete conversion of the substrate
alkynes to the corresponding vinylsilanes (Table 3).
Hydrosilylation of terminal alkynes generates mainly
linear vinylsilanes with Z-stereochemistry, accompanied
by varying amount of the a-isomer in high combined
yields. The use of more sterically demanding silanes re-
sulted in higher ratios of Z- and E-stereoisomers. Diphen-
yl-(À)-menthoxysilane, derived from the condensation
catalyzed by 1 between diphenylsilane and (À)-menthol,
offered high degrees of Z-isomer selectivity (entries 1
and 7). In particular, hydrosilylation of phenylacetylene
by triphenylsilane resulted exclusively in the Z-isomer at
employed for the hydrosilylation of alkynes. Since the
selectivity between O-silylation and C-silylation remains
unclear in many examples of transition metal catalysis,
we commenced our investigation with molecules con-
taining both alcohol and alkyne functionalities.
In general, alkynyl alcohols, in the presence of silanes
and 1 (1mol%) and in the absence of solvent, underwent
faster hydrosilylation at the triple bond rather than
dehydrogenative condensation at the hydroxyl group
(Table 1).16,17 Hydrosilylation of propargylic and homo-
propargylic alcohols by dimethylphenylsilane favored
formation of the a-isomer, most likely due to the chela-
tion effect of the hydroxyl group.11 This effect was
strongest for propargyl alcohol and 3-butyn-1-ol (entries
1 and 4) but decreased significantly as the distance be-
tween the hydroxyl group and the triple bond increased
(entry 6).
Table 2. Comparison of catalysts 1–4
1 mol % cat.
Hydrosilylation of an internal alkyne yielded two regio-
isomers, both possessing Z-stereochemistry, following
silylation of the a- and b-carbons (entry 2). Surprisingly,
the alkynyl alcohols in entries 3 and 5 were remarkably
resistant toward hydrosilylation by Ph2SiH2 even under
extended reaction times and elevated temperatures,
yielding only small amounts of the corresponding silyl
ethers. This is in stark contrast to our previous observa-
tion that Ph2SiH2 in the presence of 1 is extremely reac-
tive toward the hydroxyl functionality.5
Ph
(1.5 equiv)
+
Et3SiH
(1 equiv)
neat, 65 ˚C, 16 h
Et3Si
Ph
+
+
Ph
SiEt3
Ph
Ph
5a
(Z)/(E)/(a)a
5b
6
Catalyst
% Conversion of 5a,bb
1
2
3
4
1:0:Trace
19:0:1
90c
40
53
31
14.5:0:1
16.7:0:1
With the entries in Table 1 confirming the viability of al-
kyne hydrosilylation promoted by 1, we proceeded to
compare the reactivity of 1 with ruthenium carbene cat-
alysts 2–4 that are similar in structure and also known to
effectively promote metathesis chemistry (Fig. 1). While
a Ratio of isomers was determined by 1H NMR.
b % Conversion was determined by 1H NMR of the crude reaction
mixture.
c Isolated yield.
Mes
N
N
Mes Mes
Ph
N
N
Mes Mes
Ph
N
N Mes
Ph
PCy3
Ru
PCy3
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Ru
PCy3
Ru
PCy3
Ru
PCy3
Ph
1
2
3
4
Figure 1. Ruthenium carbene catalysts.

S. V. Maifeld et al. / Tetrahedron Letters 46 (2005) 105–108
Table 3. Hydrosilylation of terminal alkynes catalyzed by 1a
107
Entry
1
Alkyne
Silane
T (°C)/h
(Z)/(E)/(a)b
Yieldc (%)
OSiPh2H
70/2
20:1:6
>95d,e
n-Bu
2
3
MePh2SiH
Et3SiH
70/2
70/2
14:1:4
9:0:1
75
86
4
5
6
Ph3SiH
MePh2SiH
Me2PhSiH
95/2
70/2
70/2
1:0:0
20:0:1
9:1:1
80
95
72f
Ph
OSiPh2H
7
70/4
10:1:0
>95d,e
8
9
Et3SiH
(EtO)3SiH
70/6
100/12
1:0:0
1.3:1:0
93
40e
10
11
MePh2SiH
Et3SiH
70/3
70/3
21:4:1
26:17:1
70
75
a Alkyne (1.5equiv), silane (1equiv), and 1 (1mol%) were added to a vial and allowed to react at the specified temperature.
b Ratio of isomers determined by 1H NMR.
c Isolated yield.
d Product was unstable to silica gel.
e % Conversion based upon 1H NMR of the crude reaction mixture.
f At 25°C, the trans isomer was generated as the major product.
95°C after 2h, which is comparable to the selectivity
achieved by recent examples employing [RuCl2(p-cym-
Previously studied transition metal-catalyzed hydrosily-
lation reactions suggest that the present hydrosilylation
likely proceeds by initial oxidative addition of the silane
to the metal center; however, characterization of this ini-
ene)]211 and [Cp RhCl ] 8b as catalysts. The use of high-
*
2 2
er temperatures also increased the ratio at which the Z-
isomer was formed (entry 6). Although 1 effectively pro-
moted alkyne hydrosilylation with aryl- and alkylsilanes
at temperatures ranging from 25 to 95°C, a trialkoxysi-
lane such as triethoxysilane was markedly less reactive
even at higher temperatures (entry 9). As in the earlier
cases of alkynyl alcohols, terminal alkynes examined
in this study did not undergo hydrosilylation with
Ph2SiH2 in the presence of 1 at elevated temperatures
and extended reaction times.
1
tial metal hydride complex by H NMR has been elu-
sive.23 The increasing Z-stereoselectivity noted at
higher temperatures and with more sterically demanding
silanes may be explained by the isomerization of an
intermediate vinyl complex, as proposed by Crabtree
and co-workers10b,c and Ojima et al.9c
In conclusion, the commercially available complex 1 has
been shown to be an effective catalyst for the inter- and
intramolecular hydrosilylation of alkynes. A strong
directing effect by the hydroxyl group of alkynyl alco-
hols is suggested to explain the preferential formation
of the a-isomer. Additionally, up to complete selectivity
for the Z-isomer has been accomplished. Further studies
may aid in elucidating the nature of the catalytic species
as well as the mechanistic pathways operating in these
processes.
Intramolecular hydrosilylation of triple bonds has been
shown to be a useful tool in organic synthesis to control
both the regio- and stereochemistry of the resulting dou-
ble bond.21,22 Therefore, we further investigated the pro-
pensity of 1 to internally deliver silanes across tethered
alkynes. Although previous intermolecular hydrosilyl-
ation reactions were run under neat conditions, the
intramolecular hydrosilylation of homopropargylic silyl
ether 7 required a solvent to prevent dehydrogenative
condensation between the hydride-bearing silyl ethers.
Although the use of toluene as a solvent (0.025M)
slowed reaction rates and required higher temperatures
and catalyst loading, the siloxacycle 8 was isolated in
63% yield (Scheme 2).
Acknowledgements
D.L. gratefully acknowledges WARF, NSF, and the
Dreyfus Foundation for financial support as well as Pro-
fessor Steven P. Nolan for the generous donation of cat-
alysts 3 and 4. Support from the NSF and NIH for
NMR and Mass Spectrometry instrumentation is
greatly acknowledged.
i-Pr
5 mol%
Cl2(PCy3)2Ru=CHPh
i-Pr
OSi(i-Pr)2H
Si
O
Supplementary data
80 °C, toluene, 12 h
63%
7
8
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2004.11.025.
Scheme 2. Intramolecular hydrosilylation of
alcohol-derived silyl ether.
a
homopropargylic

108
S. V. Maifeld et al. / Tetrahedron Letters 46 (2005) 105–108
hara, T.; Motegi, R.; Nagai, Y. J. Organomet. Chem. 1977,
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