Ruthenium-catalyzed alkoxylation of a hydrodisilane without Si[sbnd]Si bond cleavage

Article abstract of DOI:10.1016/j.tetlet.2016.11.073

Transition metal-catalyzed synthesis of alkoxydisilanes via dehydrogenative coupling of a hydrodisilane with alcohols is reported. During the reaction, the Si[sbnd]Si bond is preserved effectively when [RuCl₂(p-cymene)]₂is used as a catalyst. Various alcohols can be used in this alkoxylation.

Full text of DOI:10.1016/j.tetlet.2016.11.073

Accepted Manuscript
Ruthenium-catalyzed alkoxylation of a hydrodisilane without Si−Si bond cleav-
age
Ken-ichiro Kanno, Yumi Aikawa, Soichiro Kyushin
PII:
S0040-4039(16)31554-4
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.11.073
TETL 48359
Reference:
Tetrahedron Letters
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14 November 2016
17 November 2016
Please cite this article as: Kanno, K-i., Aikawa, Y., Kyushin, S., Ruthenium-catalyzed alkoxylation of a hydrodisilane
without Si−Si bond cleavage, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.11.073
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Tetrahedron Letters
Ruthenium-catalyzed alkoxylation of a hydrodisilane without Si−Si bond cleavage
Ken-ichiro Kanno*, Yumi Aikawa, Soichiro Kyushin*
aDivision of Molecular Science, Graduate School of Science and Technology, Gunma University, Kiryu, Gunma 376-8515, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Transition metal-catalyzed synthesis of alkoxydisilanes via dehydrogenative coupling of a
hydrodisilane with alcohols is reported. During the reaction, the Si−Si bond is preserved
effectively when [RuCl2(p-cymene)]2 is used as a catalyst. Various alcohols can be used in this
alkoxylation.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Alkoxylation
Dehydrogenative coupling
Hydrooligosilane
Silicon−silicon bond
Hydrosilanes are versatile compounds in organic synthesis.
The Si−H bond can be transformed into a variety of other
functional groups such as alkyl groups, halogen atoms, and
alkoxy groups. Recently, functional group transformation of
hydrooligosilanes is more and more important because
hydrooligosilanes are essential precursors in the synthesis of
organosilicon clusters.1 Furthermore, modification of Si−H
functions of hydrogen-terminated silicon surface2 and silicon
nanosheets3,4 has attracted considerable attentions to improve
their chemical and physical properties. The modification has
expanded their remarkable potentials of application to novel
functional materials.
bond. Realization of this reaction could expand the scope of the
functionalized oligosilane synthesis.15
A series of transition metal catalysts was examined for the
dehydrogenative
alkoxylation
of
1,1,2,2-tetramethyl-1-
phenyldisilane (1) with methanol (Table 1). Among these
catalysts, [RuCl2(p-cymene)]2 gave the best result (entry 1). The
starting material was consumed within 2 h to afford the desired
methoxydisilane 2 in 89% yield along with a small amount of
PhMe2SiOMe (3, 4%).
The effects of solvents and reaction temperature were studied.
Toluene is also a suitable solvent for the reaction, and even at
lower temperature, the reaction proceeded similarly (entries 2 and
3). When the reaction was carried out in THF, a complex mixture
was obtained, and GC analysis showed that only a small amount
of 2 was formed (entry 4). The reactions in hexane and
dichloromethane proceeded smoothly, but the yields of 2 were
lower than that in toluene (entries 5 and 6).
The functional group transformation of hydrooligosilanes is
based on radical reactions,5,6 Lewis acid catalysis,7 and
organomercury-mediated reactions.8,9 Although the functional
group transformation of hydromonosilanes with transition metal
catalysts has extensively been used, examples of
hydrooligosilanes are limited10 because Si−Si bonds have high
reactivity to transition metal complexes to lead to the cleavage of
the Si−Si bonds.11,12 For example, numbers of transition metal
catalysts have so far been found to show activity for the
dehydrogenative coupling of hydromonosilanes with O−H
functions.13,14 However, the reactions of hydrooligosilanes with
alcohols in the presence of transition metal catalysts gave the
degradation products via Si−Si bond cleavage.12 To the best of
our knowledge, there has been no report on the transition metal-
catalyzed alkoxylation of hydrooligosilanes. The most crucial
point to develop functionalization of organosilicon compounds
having Si−Si bonds is how to preserve the Si−Si bonds during the
transformation.
Other ruthenium complexes such as [Cp*RuCl2]n, Ru3(CO)12,
and [Ru(cod)Cl2]n also gave unsatisfactory results (entries 7−9).
In these cases, both of the desired reaction and the undesired
Si−Si bond cleavage occurred simultaneously. When Ru3(CO)12
was used, the reaction was slow, and a large amount of 1
remained (entry 8). These results show that the choice of ligands
is important in this reaction.
Transition metal catalysts other than ruthenium were also
attempted. When CoCl2 or NiCl2 was used, no reaction took place
(entries 10 and 11). The reactions with the transition metal
complexes such as [Rh(cod)2]BF4, PdCl2, and Pd[P(t-Bu)3]2 gave
undesired monosilane 3 as the major product via Si−Si bond
We report herein ruthenium-catalyzed dehydrogenative
alkoxylation of a hydrodisilane with preservation of the Si−Si

2
Tetrahedron Letters
Table 1
Reactions of 1 with methanol in the presence of various transition metal catalysts
Entry
1
Catalysta
Solvent
Benzene
Toluene
Toluene
THF
MeOH (equiv)
Reaction time (h)
1b (%)
0
2b (%)
89
83
77
8
3b (%)
4
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[Cp*RuCl2]n
1.5
1.5
1.4
1.8
1.2
1.1
1.5
1.5
1.5
1.0
0.86
1.0
0.92
1.0
1.5
1.5
2
2
2
3c
3
6
2
8
1
4
2
4
18
2
5
Hexane
CH2Cl2
Toluene
Toluene
Toluene
Benzene
Benzene
Benzene
Benzene
Benzene
Toluene
Toluene
2
17
0
64
62
10
12
33
0
6
7c
8c
9c
2
0
48
24
24
2
5
39
2
Ru3(CO)12
69
22
100
100
0
[Ru(cod)Cl2]n
CoCl2
13
0
10
11
12
13
14
15c
16c
NiCl2
2
0
0
[Rh(cod)2]BF4
PdCl2
48
2
2
42
84
42
2
2
0
Pd[P(t-Bu)3]2
[ReBr(CO)3(thf)]2
[CpMo(CO)3]2
48
24
24
0
0
22
88
40
1
0
a 5 mol% on the employed metal. b Yields were determined by GC. c The reaction was conducted at 0 °C.
cleavage (entries 12−14). The reaction with [ReBr(CO)3(thf)]2
afforded 2 in moderate yield, but the prolonged reaction time
showed no improvement of the yield of 2 (entry 15). The
catalytic activity of [CpMo(CO)3]2 is very weak, and most of 1
remained unreacted (entry 16).
triethylsilane to afford 4 and the mononuclear ruthenium
complex,
(p-cymene)Ru(SiEt3)2H2
(6)
along
with
chlorotriethylsilane.17
Table 2
Thus obtained optimized conditions were applied to
alkoxylation with various alcohols (Table 2). The reactions with
primary alcohols such as ethanol and 1-butanol gave the
corresponding alkoxydisilanes in high yields (entries 2 and 3).
When these reactions were carried out at room temperature, the
yields were decreased due to formation of a large amount of the
alkoxydimethylphenylsilanes. The reaction with 2-propanol was
more sluggish than those with primary alcohols. Therefore, a
large excess amount of 2-propanol was used to reduce the
reaction time, and the desired (isopropoxy)disilane was obtained
in good yield (entry 4). The reaction was applicable to benzyl
alcohol to furnish benzyloxydisilane in high yield (entry 5).
However, the isolated yield was low due to its more susceptible
nature to moisture than the other alkoxydisilanes and difficulty in
separation from benzyl alcohol. The reactions with 2-methyl-2-
propanol and neopentyl alcohol, however, gave no desired
products, probably due to the steric congestion of the employed
alcohols.
Reactions of 1 with various alcohols in the presence of
[RuCl2(p-cymene)]2
Alcohol
(equiv)
Reaction
time (h)
Yielda (%)
63
Product
Entry
1
MeOH
(1.4)
2
24
24
24
24
EtOH
(1.5)
2
3
4
5
78
To obtain information on the reaction mechanism, the
stoichiometric reactions were carried out (Scheme 1). When
[RuCl2(p-cymene)]2 was mixed with 1 in toluene-d8 at room
temperature, the monohydrido-diruthenium complex 4 was
formed in 33% NMR yield along with chlorodisilane 5 (54%)
based on the starting diruthenium complex (maximum 200%
BuOH
(1.5)
87
i-PrOH
(5.6)
80
1
yield). Complex 4 was identified by comparison of the H NMR
spectrum with the reported data.16 The characteristic signal of the
bridged hydride is observed at −10.2 ppm, and two pairs of
aromatic protons and isopropyl methyl protons of the 6-p-
cymene ligands are observed. The formation of 5 was also
confirmed by the 29Si NMR spectrum. The similar results have
been reported in the reaction of [RuCl2(p-cymene)]2 with
PhCH2OH
(1.5)
29 (73)
a Isolated yield. A GC yield was given in parentheses.

This work was partly supported by a Grant-in-Aid for
Scientific Research (No. 26410036) from the Japan Society for
the Promotion of Science, Japan and the Element Innovation
Project of Gunma University, Japan.
References and notes
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Scheme 1. Reaction of [RuCl2(p-cymene)]2 with 1.
The catalytic activity of 4 and 6 (prepared in situ from
[RuCl2(p-cymene)]2 and triethylsilane in toluene according to the
literature17) was examined. The reactions of 1 with methanol in
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a
Si−H -complex intermediate is
included.18,19
5.
6.
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in benzene-d6, no reaction was observed by the 1H NMR
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coupling without Si−Si bond cleavage, other (6-arene)ruthenium
complexes were also examined (Table 3). The benzene complex
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the mesitylene complex was employed, the reaction rate was
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7.
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Reactions of 1 with methanol in the presence of various (6-
arene)ruthenium catalysts
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Reaction
time (h)
Entry
Catalysta
2b (%)
1
2
3
4
5
[RuCl2(benzene)]2
2
2
76
14
85
14
14
[RuCl2(mesitylene)]2
[RuCl2(mesitylene)]2
[RuCl2(C6Me6)]2
72
96
24
RuCl[(S,S)-Tsdpen](p-cymene)
a 5 mol% on the ruthenium atom. b Yields were determined by GC.
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In conclusion, we have developed the efficient transition
metal-catalyzed functionalization of the Si−H bond of 1 with
preservation of the Si−Si bond. Studies on the scope of the
reactions with various types of hydrooligosilanes and alcohols,
and further confirmation of the reaction mechanism are now in
progress.
Acknowledgments

4
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Supplementary data
Supplementary data associated with this article can be found,
in
the
online
version,
at
http://dx.doi.org/10.1016/j.tetlet.2016.xx.xxx.
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Ruthenium-catalyzed alkoxylation of a
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hydrodisilane without Si−Si bond cleavage
Ken-ichiro Kanno, Yumi Aikawa, Soichiro Kyushin

3
Highlights
1) Alkoxylation of a hydrooligosilane has been accomplished without Si−Si bond cleavage.
2) [RuCl2(p-cymene)]2 is the most effective catalyst.
3) Various alcohols can be used in this alkoxylation.