Disproportionation reactions of organohydrosilanes in the presence of base catalysts

Article abstract of DOI:10.1016/S0022-328X(01)00755-0

Alkoxides, alkyl compounds, amides and hydrides of alkali metals (M) and barium, such as MOR, Ba(OR)₂, n-BuM, PhM, MN(SiMe₃)₂ and MAlH₄ showed high catalytic activities versus the disproportionation reactions of PhSiH₃ to produce SiH₄, Ph₂SiH₂ and Ph₃SiH. A good correlation between the catalyst basicities and the catalytic activities was observed, and a reaction mechanism involving the metal hydride and alkyl metal was proposed. A considerable amount of SiH₄ was produced in the reduction of PhSiCl₃ with LiAlH₄ when over three moles of LiAlH₄ was used.

Full text of DOI:10.1016/S0022-328X(01)00755-0

Journal of Organometallic Chemistry 629 (2001) 1–6
www.elsevier.com/locate/jorganchem
Disproportionation reactions of organohydrosilanes in the presence
of base catalysts
Masayoshi Itoh a,*, Koji Inoue b, Jun-ichi Ishikawa b, Kenji Iwata b
a Chemistry and Biochemistry, Fukushima National College of Technology, 30 Aza Nagao, Taira Kamiarakawa, Iwaki-City,
Fukushima 970-8034, Japan
b Material Science Laboratory, Mitsui Chemicals, Inc., Ad6anced Materials Group, 580-32 Nagaura, Sodegaura-City, Chiba 299-0265, Japan
Received 26 October 2000; received in revised form 14 February 2001; accepted 23 February 2001
Abstract
Alkoxides, alkyl compounds, amides and hydrides of alkali metals (M) and barium, such as MOR, Ba(OR)2, n-BuM, PhM,
MN(SiMe3)2 and MAlH4 showed high catalytic activities versus the disproportionation reactions of PhSiH3 to produce SiH4,
Ph2SiH2 and Ph3SiH. A good correlation between the catalyst basicities and the catalytic activities was observed, and a reaction
mechanism involving the metal hydride and alkyl metal was proposed. A considerable amount of SiH4 was produced in the
reduction of PhSiCl3 with LiAlH4 when over three moles of LiAlH4 was used. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Disproportionation reactions; Phenylsilane; Base catalysts
Catalytic disproportionation reactions of alkoxy
silanes and chlorosilanes have been studied by treating
such silanes with a suitable catalyst under mild condi-
tions shown as follows (Eq. (1)):
1. Introduction
In reactions involving organometallic compounds
such as organosilicon compounds, there have been
many reports on catalytic reactions using transition
metal compounds as catalysts [1,2]. On the other hand,
there are few catalytic reactions reported using com-
pounds of the main group elements as catalysts, which
are generally ionic compounds, even though ionic cata-
lysts have some industrial advantages, for example they
can be easily prepared at low cost and can be separated
from the reacted solution using ion-exchange resins or
zeolites. We have been studying reaction systems in-
volving organosilicon compounds such as hydrosilanes,
using ionic compounds as the catalysts, and have found
high catalytic activities of some compounds of alkali
and alkaline-earth metals toward the hydrosilylation
reactions of hydrosilanes with olefins [3] and dehydro-
genative coupling reactions between hydrosilanes and
alkynyl compounds [4–8]. In this paper, we report a
new catalytic reaction system using ionic compounds as
the catalyst.
(1)
wherein R is a hydrocarbon group or hydrogen, and X
is an alkoxy group or halogen [9]. Base catalysts such as
alkali metal alkoxides have generally been employed.
There have been some reports on the disproportiona-
tion reactions of organohydrosilanes using Lewis acids
such as AlBr3 [10], solid acids such as SiO2–A12O3 [11],
KH, LiAlH4 [12], Red-Al [13] and transition metal
complexes such as (Ph3P)3RhC1 [14], Cp*Fe2(CO)4(m-
CO)(m-PPh2) [15], CP*SmCH(SiMe3)2 [16] or
2
RuCl2[P(C6H4Me-p)3] [17] (Eq. (2)).
(2)
In this paper, we report the disproportionation reac-
tions of arylhydrosilanes in the presence of base cata-
lysts such as alkoxides, alkyl compounds, amides or
hydrides of alkali or alkaline earth metals (Eq. (3)).
* Corresponding author. Tel.: +81-246-460810; fax: +81-246-
460825.
E-mail address: m-itoh@fukushima-nct.ac.jp (M. Itoh).
(3)
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)00755-0

2
M. Itoh et al. / Journal of Organometallic Chemistry 629 (2001) 1–6
2. Experimental
seven ml of THF and 10 mmol of PhSiCl3 were charged
into a 200 ml glass flask with a condenser cooled to
15°C. LiAlH4 (solution of THF, 2.16 mol l−1) of the
specified quantity was then gradually added, while the
temperature was maintained at 40°C or less by cooling.
After 1 h, the reaction temperature was raised to ca.
65°C, and the reaction mixture was refluxed with stir-
ring for an additional 2 h. The products were analyzed
in a manner similar to the disproportionation reaction.
2.1. Reaction procedures
PhSiH3, Ph2SiH2, Ph3SiH, C6H13SiH3 (n-hexylsilane)
and Et3SiH were used as the hydrosilanes for the
disproportionation reactions. These liquid reactants
,
were dried over 3 A molecular sieves prior to use.
Alkali and alkaline-earth metal compounds were
mainly used as the catalysts (see Table 1). The solvents
of diglyme, 1,2-dimethoxyethane (DME), THF, 1,4-
2.2. Product assignment
,
dioxane, hexane and toluene were dried over 3 A
molecular sieves. All experiments were carried out in a
nitrogen atmosphere. A 50 mmol sample of the hy-
drosilane, 2.5 mmol of the catalyst and 10 ml of the
solvent were typically charged into a glass flask which
was connected with a condenser cooled to 15°C. The
reactions were carried out at 50°C for 6 h. The gaseous
products were trapped by reaction with a solution of
LiOEt in ethanol (Li 0.5 wt.%). After the reaction, the
amounts of liquid products in the reaction solution and
the gaseous products trapped as alkoxides in the etha-
nol solution (the amounts of Si(OEt)4 and RnSi-
(OR)4−n) were determined by GC measurement using
n-tetradecane as the internal standard.
The liquid products (PhSiH3, Ph2SiH2, Ph3SiH,
Si(OEt)4) were identified by mass spectroscopy [18] and
GC.
PhSiH3; MS (m/z): (EI mode) 108 (100, M+), 107
(100), 106 (98), 105 (73), 81 (20).
Ph2SiH2; MS (m/z): (EI mode) 106 (100), 105 (50),
184 (37, M+), 78 (33), 53 (17).
Ph3SiH; MS (m/z): (EI mode) 182 (100), 181 (59),
183 (29), 105 (23), 260 (18, M+).
Si(OEt)4; MS (m/z): (EI mode) 193 (100), 149 (86),
163 (70), 79 (57), 208 (11, M+).
The gaseous product (SiH4) was trapped directly in a
cylinder and identified by mass spectroscopy.
SiH4; MS (m/z): (EI mode) 30 (100), 31(86), 29 (15),
32 (11, M+). The data coincided with those in the
literature [18,19].
The reduction of PhSiCl3 to PhSiH3 was carried out
in THF using LiAlH4 with specific mole ratios of
PhSiCl3 to LiAlH4 in a nitrogen atmosphere. Forty
Table 1
a
Disproportionation reactions of PhSiH3
Entry
PhSiH3 (mmol) Catalyst
Amount of
catalyst (mmol)
Reaction
temperature (°C)
Conversion of
PhSiH3
Product yield (%) b
SiH4 Ph2SiH2 Ph3SiH
Ph4Si
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
49.9
50.6
50.2
50.5
49.7
50.7
50.3
50.2
49.8
49.1
50.7
50.2
50.4
50.4
49.4
50.8
49.6
LiOEt
2.5
2.5
2.5
2.5
2.4
2.5
2.5
2.5
2.5
2.6
2.6
2.5
2.5
2.5
2.6
2.5
126
50
50
50
50
50
50
50
50
50
50
100
50
50
50
100
50
50
32.0
58.7
0
99.3
97.5
98.2
0
96.5
97.3
85.0
99.0
95.4
96.3
94.2
98.1
91.0
23.6
9.8
17.0
0
44.7
50.4
25.5
0
43.7
46.6
36.6
53.1
39.2
39.7
47.6
57.2
41.3
1.9
10.3
17.4
0
2.9
6.9
0
0
0
28.0
15.2
15.1
0
2.7
8.4
1.1
21.4
2.0
1.5
13.2
23.8
8.5
0
0
0
0
0.8
7.0
0.1
0
0
0
LiO-i-Pr
LiOPh
NaOEt
KOEt
Ba(OC8H17 2
LiOCOPh
n-BuLi
c
)
23.0
0
41.3
38.2
36.9
16.3
34.1
36.7
25.8
9.3
PhLi
LiN(SiMe3)2
LiN(SiMe3)2
NaN(SiMe3)2
KN(SiMe3)2
LiAlH4
0
0
0.1
Trace
0
0.1
Trace
0
LiAlH4
NaAlH4
30.3
3.3
MgO d
a Reactions were carried out for 6 h. 10 ml of diglyme was used as the solvent unless otherwise noted.
b Determined by GC.
c Barium-2-ethylhexoxide.
d 5.06 g of MgO was used.

M. Itoh et al. / Journal of Organometallic Chemistry 629 (2001) 1–6
3
detected by GC. The first reaction of PhSiH3 proceeded
rapidly during the early stages of the reaction with the
production of SiH4, and Ph2SiH2, and then the reaction
of Ph2SiH2 to produce Ph3SiH gradually proceeded as
shown in Fig. 1. Because SiH4 was identified by conver-
sion to Si(OEt)4, some SiH4 would be lost, especially
during the early reaction stage. When we did not use
the trap containing the ethanol solution of LiOEt,
which was connected to the reaction vessel to convert
SiH4 into Si(OEt)4 with hydrogen gas evolution, smoke
or a flame was observed at the exit of the reaction
vessel.
The alkoxides of the elements belonging to Groups 2
(alkaline earth metals), 11, 13 and 3 of the periodic
table such as Mg(OEt)2, Ca(OEt)2, Sr(OEt)2, Ba(OEt)2,
Cu(OEt)2, Al(OEt)3, Ga(OEt)2 and La(O-i-Pr), which
have higher ionization potential energies [20] and are
weaker bases than the alkali metal alkoxides, were
insoluble in the reaction solution and showed almost no
catalytic activity. Ba(OC8H17)2 (barium-2-ethylhexox-
ide), which is soluble in the solvent, showed a high
catalytic activity.
The order of the catalytic activities of Li compounds
was PhLi\n-BuLi\LiN(SiMe3)2\LiOEtꢀLiOPh,
LiOCOPh (not catalytically active) as shown in Fig. 2.
The order coincides with that of the basicities of the
conjugated acids [21]. Moreover, the catalytic activities
of the alkali metal alkoxides (NaOEtꢁKOEt\UOEt)
correlate with the ionization potentials of the metals
(see Table 1, Fig. 3) [20]. Because the alkali compounds
were soluble in the reaction solution, the basicity of the
catalyst would be one of the factors which determines
the catalytic activity. The addition of 10% of a more
Fig. 1. Disproportionation reaction of PhSiH3 in the presence of
LiAlH4 (entry no.1 in Table 1).
Fig. 2. Disproportionation reaction of PhSiH3 in the presence of Li
compounds.
3. Results and discussion
The reaction results are shown in Table 1. The
disproportionation reactions of PhSiH3 occurred to
produce the monosilane (SiH4), Ph2SiH2, Ph3SiH, and
Ph4Si in the presence of alkoxides, alkyl compounds,
amides or hydrides of alkali metals and barium alkox-
ides, which are strong bases. No other products were
Fig. 3. Disproportionation reaction of PhSiH3 in the presence of
alkali metal alkoxide.

4
M. Itoh et al. / Journal of Organometallic Chemistry 629 (2001) 1–6
Table 2
Effects of solvent and catalyst amount on disproportionation reaction of PhSiH3 in the presence of NaOEt a
Entry
PhSiH3 (mmol)
NaOEt (mmol)
Solvent b
Conversion of PhSiH3
Product yield (%) c
SiH4
Ph2SiH2
Ph3SiH
Ph4Si
1
2
3
4
5
6
7
8
9
50.5
50.0
50.6
50.5
50.5
49.8
50.5
50.5
50.7
50.0
2.5
2.5
1.3
0.3
2.5
2.5
2.5
2.5
2.4
2.5
Diglyme
DME
DME
DME
99.3
99.6
99.7
50.0
99.9
30.5
0
98.0
0
97.4
44.7
53.7
52.1
17.2
53.7
8.7
0
46.0
0
2.9
2.4
4.3
20.9
4.8
9.6
0
28.4
0
33.3
28.0
28.5
28.2
0.1
26.6
0
0
13.5
0
0.9
1.2
0.2
0
0.2
0
0
0
0
0
THF
1,4-Dioxane
Hexane
Hexane–diglyme d
Toluene
10
Toluene–diglyme e
47.1
10.4
a Reactions were carried out at 50°C for 6 h.
b 10 ml of the solvent was used unless otherwise noted.
c Determined by GC.
d 10 ml of hexane and 1 ml of diglyme were used.
e 10 ml of toluene and 1 ml of diglyme were used.
sponding organochlorosilanes [26], which are easily ob-
tained by Rochow’s direct process via the reaction of
RCl with silicon metal.
polar solvent to a non-polar solvent yields results
analogous to those in the polar solvent alone (Table 2).
This might suggest that there is preferential solvation of
the transition state with the more polar solvent.
LiAlH
4
Si+RCl“RnSiCl4−nꢀꢀꢀꢀꢀꢁRnSiH4−n
(4)
The reaction mechanism in which the substituent
moves from the silicon to the lithium is considered
(Si···H···Li, see Fig. 4). When the substituent is a phenyl
group, the activation energy would be low due to the
resonance effect of the benzene ring and the reaction
would proceed easily. C6H13SiH3 was not dispropor-
tionated. There would be no resonance effect in alkyl
substitution. The conversion of Et3SiH was zero and
that of Ph3SiH was much lower than that of PhSiH3,
which would be caused by steric hindrances (Table 3).
As mentioned above, the strong basic catalysts
showed high catalytic activities. We presumed that the
reaction proceeded via an ionic transition state as
shown in Fig. 4. It is well known that alkyl metals [22],
metal amides [23] and metal alkoxides [23,24] react with
PhSiH3 to produce metal hydrides [22]. PhSiH3 would
be activated by the alkyl metal and metal hydride and
a disproportionation reaction would occur. We pro-
posed the four-center-type transition state as shown in
Fig. 4. Five-coordinate silicon hydrides could also be
considered as an intermediate [12,25]. More study must
be done to clarify the reaction mechanism. The above
results indicate that the Si–H bond would be more
easily activated by an ionic catalyst than the C–H
bond, because the C–H bond cannot generally be
activated by base catalysts such as alkoxides, alkyl
compounds, amides or hydrides of alkali metals.
Fig. 5 shows the amount of SiH4 produced during
the reduction of PhSiCl3 with LiAlH4 for various ratios
of LiAlH4 to PhSiCl3. A considerable amount of SiH4
was produced when over 3 mol of LiAlH4 was used.
Fig. 4. Presumed mechanism of the disproportionation reaction of
hydrosilane in the presence of alkali metal compound.
The fact that LiMH4 catalyzed the disproportiona-
tion reaction of organohydrosilane to produce SiH4 is
very important, because almost all organohydrosilanes
are usually synthesized by the reduction of the corre-
Fig. 5. The amount of SiH4 produced in the reduction of PhSiH3 by
LiAlH4.

Table 3
/
Disproportionation reaction of hydrosilanes in the presence of NaOEt a
Entry
Hydrosilane
Amount of hydrosilane (mmol)
NaOEt (mmol)
Solvent b
Conversion of hydrosilane (%)
Product yield (%) c
SiH4
RSiH3
R2SiH2
27.1
R3SiH
R4Si
1
2
3
4
5
6
7
PhSiH3
Ph2SiH2
Ph3SiH
C6H13SiH3
PhSiH3
C6H13SiH3
Et3SiH
50.1
50.0
50.0
48.9
50.5
49.1
49.4
0.5
DME
DME
DME
DME
Diglyme
Diglyme
Diglyme
97.8
82.9
7.9
0
99.3
0
47.5
20.2
0.2
0
44.7
0
13.0
51.7
0
0.50
0.49
0.50
2.5
2.39
2.82
0.6
0
0.1
1.0
0
0.9
0
0
0
2.9
0
0
0
28.0
0
0.7
0
0
0
0
629
a Reactions were carried out at 50°C for 6 h.
b 10 ml of the solvent was used unless otherwise noted.
c Determined by GC.
(

6
M. Itoh et al. / Journal of Organometallic Chemistry 629 (2001) 1–6
The mole ratio of LiAlH4/PhSiCl3, which gave SiH4,
suggests that the partially chlorinated aluminum hy-
drides such as LiAlH3Cl or LiAlH2Cl2 would not be as
active as LiAlH4 for the disproportionation reaction of
PhSiH3.
References
[1] B. Marciniec, J. Gulinski, W. Urbaniak, Z.W. Kornetka, in: B.
Marciniec (Ed.), Comprehensive Handbook on Hydrosilylation,
Pergamon, Tokyo, 1992.
[2] A. Norbert, W. Johann (Eds.), Organosilicon Chemistry III,
Wiley-VCH, Weinheim, 1997.
[3] M. Kobayashi, M. Itoh, Chem. Lett. (1996) 1013.
[4] M. Itoh, M. Mitsuzuka, K. Iwata, K. Inoue, Macromolecules 27
(1994) 7917.
[5] M. Itoh, M. Mitsuzuka, T. Utsumi, K. Iwata, J. Organomet.
Chem. 476 (1994) C30.
PhSiCl3+3LiAlH4“PhSiH3+3LiAlH3Cl
(5)
These results caution us that there is the possibility of
the generation of SiH4 (self-burning gas in air, mini-
mum explosion limit is 0.8 vol%) during the reduction
of PhSiCl3 by LiAlH4. Because the reaction solution is
not always homogeneous, we must take sufficient care,
especially when we use an amount of LiAlH4 larger
than the equivalent amount of an organochlorosilane.
[6] M. Itoh, M. Kobayashi, J. Ishikawa, Organometallics 16 (1997)
3068.
[7] J. Ishikawa, K. Inoue, M. Itoh, J. Organomet. Chem. 552 (1998)
303.
[8] J. Ishikawa, M. Itoh, J. Catal. 185 (1999) 454.
[9] D.L. Bailey, N.Y. Buffalo, U.S. Patent 2723983 (1983); 2745860
(1957); 2723984 (1955).
[10] G.E. Russel, J. Am. Chem. Soc. 81 (1959) 4825.
[11] H. Fujisawa, T. Yamaguchi, Catal. Lett. 17 (1993) 319.
[12] B. Becker, R.J.P. Corriu, C. Guerin, B.J.L. Henner, J.
Organomet. Chem. 369 (1989) 147.
[13] H.G. Woo, S.J. Song, E.J. Cho, I.N. Jung, Bull. Korean Chem.
Soc. 17 (1996) 123.
[14] I. Ojima, S. Inaba, T. Kogure, Y. Nagai, J. Organomet. Chem.
55 (1973) C7.
[15] H. Hashimoto, H. Tobita, H. Ogino, J. Organomet. Chem. 499
(1995) 205.
[16] N.S. Radu, F.J. Hollander, T.D. Tilley, A.L. Rheingold, J.
Chem. Soc. Chem. Commun. (1996) 2459.
[17] T. Sakakura, O. Kumberger, R.P. Tan, M.P. Arther, M.
Tanaka, J. Chem. Soc. Chem. Commun. (1995) 193.
[18] F.W. McLafferty, D.B. Stauffer, The Wiley/NBS Registry of
Mass Spectral Data, Wiley, 1988.
[19] F.E. Saalfeld, H.J. Svec, Inorg. Chem. 2 (1963) 46.
[20] First ionization potential (IP): K (4.34), Na (5.14), Ba (5.21), Li
(5.39), La (5.58), Sr (5.70), Al (5.99), Ga (6.00), Ca (6.11), Mg
(7.65), Cu (7.73).
4. Conclusions
Alkoxides, alkyl compounds, amides and hydrides of
alkali metals (M) and barium, such as MOEt, MO-i-Pr,
Ba(OC8H17)2, n-BuM, PhM, MN(SiMe3)2 and MAlH4
(M; Li, Na, K) showed high catalytic activities for the
disproportionation reactions of PhSiH3 to produce
SiH4, Ph2SiH2 and Ph3SiH. Hydrosilanes having alkyl
substitutions, such as C6H13SiH3 (n-hexylsilane) and
Et3SiH were not disproportionated. The catalytic activ-
ities have good correlation with the ionization poten-
tials of the metals and the pKa of the conjugated acids
of the catalysts. An ionic reaction mechanism involving
the metal hydride and alkyl metal was proposed. A
considerable amount of SiH4 was produced in the
reduction of PhSiCI3 with LiAlH4 when over 3 mol of
LiAlH4 were used.
[21] pKa of the conjugate acids of the lithium compounds: n-BuH
(51), PhH (43), HN(SiMe3)2 (35), EtOH (16), PhOH (10), Ph-
COOH (4).
Acknowledgements
[22] W.H. Nebergall, J. Am. Chem. Soc. 72 (1950) 4702.
[23] R.N. Meals, J. Am. Chem. Soc. 68 (1946) 1880.
[24] H. Gilman, G.N.R. Smart, J. Org. Chem. 19 (1954) 441.
[25] M. Fujita, T. Hiyama, J. Am. Chem. Soc. 106 (1984)
4629.
This work was performed by Mitsui Chemicals, Inc.,
under the management of the Japan Chemical Innova-
tion Institute as part of the Industrial Science and
Technology Frontier Program supported by the New
Energy and Industrial Technology Development
Organization.
[26] S. Tannenbaum, S. Kaye, G.F. Lewenz, J. Am. Chem. Soc. 75
(1953) 3753.
.