Selective catalytic monoreduction of dichlorooligosilanes with Grignard reagents

Article abstract of DOI:10.1016/j.tetlet.2013.10.050

Transition metal-catalyzed monoreduction of dichlorooligosilanes with Grignard reagents is reported. Among the examined catalysts, group 4 metal chlorides such as TiCl₄ and Cp₂TiCl₂ gave the highest reactivity and good selectivity. The reducing power is effectively controlled by changing the catalysts and Grignard reagents to achieve sufficient selectivity depending on the oligosilane substrates.

Full text of DOI:10.1016/j.tetlet.2013.10.050

Tetrahedron Letters 54 (2013) 6940–6943
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Selective catalytic monoreduction of dichlorooligosilanes
with Grignard reagents
⇑
⇑
Ken-ichiro Kanno , Yuka Niwayama, Soichiro Kyushin
Division of Molecular Science, Faculty of Science and Technology, Gunma University, Kiryu, Gunma 376-8515, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Transition metal-catalyzed monoreduction of dichlorooligosilanes with Grignard reagents is reported.
Among the examined catalysts, group 4 metal chlorides such as TiCl4 and Cp2TiCl2 gave the highest reac-
tivity and good selectivity. The reducing power is effectively controlled by changing the catalysts and
Grignard reagents to achieve sufficient selectivity depending on the oligosilane substrates.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 14 August 2013
Revised 5 October 2013
Accepted 10 October 2013
Available online 17 October 2013
Keywords:
Oligosilane
Monoreduction
Grignard reagent
Transition metal catalyst
Studies on oligosilanes with unique properties and structures
are one of the most rapidly developing research areas in organosil-
icon chemistry. For example, linear oligosilanes with fixed confor-
mations,1 push–pull oligosilanes,2 polysilane dendrimers,3 and
organosilicon clusters4–15 have recently been reported. To con-
struct these oligosilanes, the design of appropriate organosilicon
building blocks is very important.
R
R(')
Cl Si Si
R(')
R
R
+ R'2HS iCl path a
or
R'2HSiLi + R2SiCl2
XR2SiLi
path b
H
Cl Si Si Cl
R
R
R
Scheme 1. Methodologies for the synthesis of 1-chloro-2-hydrodisilanes.
1-Chloro-2-hydrodisilanes are simple but versatile organosil-
icon building blocks because two different groups can be intro-
duced to the chlorosilane and hydrosilane moieties. In some
special cases, 1-chloro-2-hydrodisilanes have been reported to be
formed by silylene insertion to chlorohydrosilanes16 and by addi-
tion of hydrogen chloride to disilenes.17,18 More commonly, 1-
chloro-2-hydrodisilanes can be synthesized by the reaction of
(diethylamino)silyllithium with chlorohydrosilane followed by
deaminochlorination19 and the reaction of hydrosilyllithium with
dichlorosilane16a,20 (Scheme 1, path a). This method has usefully
been used in the synthesis of 1-chloro-2-hydrodisilanes with differ-
ent R and R0 groups at the 1,1- and 2,2-positions, respectively.19,20
For the synthesis of more simple 1-chloro-2-hydrodisilanes such
as 1-chloro-2-hydro-1,1,2,2-tetramethyldisilane, selective monore-
duction of readily available 1,2-dichlorodisilanes is a more desirable
way (Scheme 1, path b). However, the desymmetrization of sym-
metrically substituted disilanes is not easy. For example, the reduc-
tion of 1,2-dichloro-1,1,2,2-tetramethyldisilane (1) with lithium
aluminum hydride does not give 1-chloro-2-hydro-1,1,2,2-tetra-
methyldisilane (2) selectively.21 Important progress in solving this
problem was made by Roewer and co-workers.22 They reduced
1,3-dichloro-1,1,2,2,3,3-hexamethyltrisilane
with
tributyltin
hydride in the presence of a catalytic amount of tetrabutylphospho-
nium iodide and obtained 1-chloro-3-hydro-1,1,2,2,3,3-hexameth-
yltrisilane in 67% yield together with the starting compound and
1,3-dihydro-1,1,2,2,3,3-hexamethyltrisilane.22 Improvement of this
method would lead to a practical synthetic way to 1-chloro-2-
hydrodisilanes and their homologs. Another desymmetrization is
the monochlorination of 1,2-dihydrodisilanes. Ishikawa, Kunai,
Ohshita, and co-workers have reported selective monochlorination
of 1,2-dihydrodisilanes with copper salts to give the corresponding
1-chloro-2-hydrodisilanes.23
We report herein the transition metal-catalyzed reduction of
1,2-dichlorodisilanes with Grignard reagents. Although this reduc-
tion method has been reported for several decades,24 the applica-
tion was limited only to monosilanes. We found that the
monoreduction of dichlorodisilanes by this method gives 1-
chloro-2-hydrodisilanes selectively without SiꢀSi bond breaking.
We also applied this method to longer dichlorooligosilanes.
A typical experimental procedure is as follows (Scheme 2). A
solution of BuMgCl (3.2 equiv to disilane 3) in diethyl ether was
added dropwise to a solution of 1,2-dichloro-1,1,2,2-tetraisopropyl-
⇑
Corresponding authors. Tel.: +81 277 30 1292; fax: +81 277 30 1291 (K. Kanno);
tel.: +81 277 30 1290; fax: +81 277 30 1291 (S. Kyushin).
E-mail addresses: kkanno@gunma-u.ac.jp (K. Kanno), kyushin@gunma-u.ac.jp
(S. Kyushin).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.10.050

K. Kanno et al. / Tetrahedron Letters 54 (2013) 6940–6943
6941
Ti Cl4
The results of the monoreduction of 3 with BuMgCl in the pres-
ence of various transition metal catalysts are summarized in
Table 1. Among the examined transition metal catalysts, Cp2TiCl2
showed the highest reactivity. The reaction was completed within
1 h (entry 1), and 4 was selectively formed in 97% yield. The reac-
tivity of TiCl4 and ZrCl4 is lower, but the reactions were completed
in 3 and 6 h, respectively to give 4 in good yields (entries 2 and 3).
The reactivity of group 5 (NbCl5, entry 4) and 6 (CrCl3, entry 5) cat-
alysts are much milder than the group 4 catalysts. The reduction
with CoCl2 (entry 6) or NiCl2 (entry 7) did not proceed well, and
most of the starting material 3 remained unchanged. These results
show that early transition metal catalysts are more effective in this
monoreduction. No reaction took place when the reaction was car-
ried out without any catalysts.
We also examined the effect of Grignard reagents in the pres-
ence of TiCl4 (Table 2). Although the reactivity changes depending
on the employed Grignard reagents, high selectivity was preserved,
and only a trace or small amount of the corresponding dihydrodi-
silane was detected. As shown in entry 2, the amount of BuMgCl
can be reduced to 1.6 equiv without loss of the yield and selectivity
although the prolonged reaction time was needed. The reducing
ability of BuMgCl, i-PrMgCl and i-BuMgCl is almost the same (en-
tries 1, 3 and 4). The reducing ability of t-BuMgCl, however, is sig-
nificantly lower than those of others (entry 5). Probably, this is due
to strong steric hindrance of the tert-butyl group.
i-Pr i-Pr
Cl Si Si Cl
i-Pr i-Pr
i-Pr i-Pr
Cl Si Si
i-Pr i-Pr
(10 mol%)
+
H
+
BuMgCl
3
Et2O
rt, 3 h
3
4 (95%)
(5%)
Scheme 2. Monoreduction of 3 with 3.2 equiv of BuMgCl and a catalytic amount of
TiCl4.
Table 1
Monoreduction of 1,2-dichlorodisilane 3 with 3.2 equiv of BuMgCl in the presence of
various transition metal catalysts
Entry
Catalyst
Reaction time (h)
3a (%)
4a (%)
1
2
3
4
5
6
7
Cp2TiCl2
TiCl4
ZrCl4
NbCl5
CrCl3
CoCl2
NiCl2
1
3
6
1
5
11
0
35
93
90
97
95
89
98
65
7
24
24
24
24
10
a
Yields were determined by GC.
Table 2
Monoreduction of 1,2-dichlorodisilane
presence of TiCl4
3 with various Grignard reagents in the
Entry
RMgCl (equiv)
Reaction time (h)
3a (%)
4a (%)
Selective monoreduction of various dichlorodisilanes was
examined by optimizing the reaction conditions such as the alkyl
group of the Grignard reagent, its amount, and the reaction time
(Table 3). Although Cp2TiCl2 is the best catalyst for the monoreduc-
tion of the isopropyl-substituted dichlorodisilane as shown in
Table 1, its reactivity is too high in the case of the methyl-
substituted dichlorooligosilanes. As a result, dihydrooligosilanes
were formed as by-products, and the selectivity of the monoreduc-
tion decreases. As TiCl4 was found to have suitable reactivity to
control the selectivity, we used TiCl4 in the reduction of the
methyl-substituted dichlorooligosilanes. When 1,3-dichlorotrisi-
1
2
3
4
5
BuMgCl (3.2)
BuMgCl (1.6)
i-PrMgCl (3.2)
i-BuMgCl (3.2)
t-BuMgCl (3.2)
3
21
4.5
4
5
0
0
1
14
95
100
98
97
86
24
a
Yields were determined by GC.
disilane (3) in diethyl ether containing 10 mol % of TiCl4 at 0 °C. The
mixture was stirred at room temperature for 3 h. The GC analysis
showed the formation of 1-chloro-2-hydro-1,1,2,2-tetraisopropyl-
disilane (4) in 95% yield together with 5% of the remaining 3.
Table 3
Monoreduction of various dichlorooligosilanes with Grignard reagents in the presence of TiCl4
Entry
Oligosilane
Me Me Me
RMgCl (equiv)
Reaction time (h)
Product, yielda (%)
Me Me Me
Cl Si Si Si Cl
H
Si Si Si H
1
2
BuMgCl (4.0)
24
24
Me Me Me
Me Me Me
6, 65
5
5
i-PrMgCl (4.0)
6, 90
Me Me Me
Cl Si Si Si
H
3
4
5
i-PrMgCl (2.0)
t-BuMgCl (2.1)
3
Me Me Me
7, 87 (67)
Me Me
Me Me
Cl Si Si Cl
Me Me
Cl Si Si
H
6.5
Me Me
1
2, 100
Me Me Me Me
Cl Si Si Si Si Cl
Me Me Me Me
8
Me Me Me Me
Si Si Si Si
Me Me Me Me
X
H
5
6
i-PrMgCl (2.0)
i-PrMgCl (2.0)
2.5
20
9: X = Cl, 80 (58)
10: X = H, 17
Me2
Si
Me2
Cl
Si
H
Cl
Si
Me2
X
Si
Me2
12: X = Cl, 47b
13: X = H, 46b
11
a
Yields were determined by GC. Isolated yields were given in parentheses.
NMR yields.
b

6942
K. Kanno et al. / Tetrahedron Letters 54 (2013) 6940–6943
Cp2TiCl2
(9 mol%)
Acknowledgments
i-Pr i-Pr
MeO Si Si OMe
i-Pr i-Pr
Si Si H
i-Pr i-Pr
+
X
i-PrMgCl
This work was partly supported by a Grant-in-Aid for Scientific
Research (No. 23550044) from the Japan Society for the Promotion
of Science, Japan and the Element Innovation Project of Gunma
University, Japan. Some of the spectral data of the new compounds
were measured in the Instrumental Analysis Division, Equipment
Management Center, Creative Research Institution, Hokkaido Uni-
versity, Japan.
Et2O
i-Pr i-Pr
rt, 6 h
14
15: X = OMe (73%)
16: X = H (10%)
Scheme 3. Reduction of 14 with 3.0 equiv of i-PrMgCl and a catalytic amount of
Cp2TiCl2.
lane 5 was subjected to the reaction under the standard conditions
for 3, no desired 1-chloro-3-hydrotrisilane was obtained. Instead,
1,3-dihydrotrisilane 6 and the substitution products, BuMe2SiSiM-
e2SiMe2X (X = Cl and H) were obtained (entry 1). The use of
i-PrMgCl as a reducing agent suppressed the formation of the sub-
stitution products because of the bulky isopropyl group, but unde-
sired 1,3-dihydrotrisilane 6 was still afforded in high yield (entry
2). By decreasing both the amount of i-PrMgCl and reaction time,
the desired 1-chloro-3-hydrotrisilane 7 was selectively obtained
in high yield (entry 3). For the monoreduction of 1, t-BuMgCl
was found to be better than i-PrMgCl, 2 was obtained in quantita-
tive yield (entry 4). The reduction of 1,4-dichlorotetrasilane 8
under similar conditions to those in entry 3 gave 1-chloro-4-
hydrotetrasilane 9 in 80% yield as the major product. However,
1,4-dihydrotetrasilane 10 was also formed as the minor product
in 17% yield (entry 5). When 1,2-bis(chlorosilyl)ethane 11 was sub-
jected to reduction, the selectivity was much lower than that of 8
(entry 6). These results suggest the importance of the SiꢀSi linkage
in the selective monoreduction. We also examined the possibility
of the monoreduction of 1,2-dimethoxydisilane 14 (Scheme 3).
1-Hydro-2-methoxydisilane 15 was obtained in 73% yield as the
major product, but 10% of 1,2-dihydrodisilane 16 was formed as
a by-product.
The high selectivity of the monoreduction can be attributed to
difference of the reactivity of dichlorooligosilanes and chlorohy-
drooligosilanes in nucleophilic substitution. The reactivity of the
chlorosilane moiety of dichlorooligosilanes is relatively high be-
cause the electron is withdrawn by two chlorine atoms. The reac-
tivity of the chlorosilane moiety of chlorohydrooligosilanes is
relatively low because of the electron-withdrawing effect of only
one chlorine atom. As a result, the starting dichlorooligosilanes re-
act more rapidly than chlorohydrosilanes. However, the tetrasilane
chain of 8 is too long for the selective monoreduction. The low
selectivity in the monoreduction of 11 compared with that of 8
shows that the electron-withdrawing effect of the chlorine atom
at the opposite end is interrupted by the cleavage of the silicon
chain.
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
10.050.
References and notes
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