Silylation reactions of olefins with monosilane and disilane in the presence of a transition metal complex, metal hydride, and radical initiator

Article abstract of DOI:10.1016/S0022-328X(98)00814-6

The catalytic reactions between monosilane or disilane and 1-hexene or 1,5-hexadiene were studied. The reactions with monosilane gave the silylated compounds in the presence of a platinum complex, LiAlH₄ and a radical initiator. In the reactions with disilane, the same silylated compounds were produced in the presence of a platinum complex as in the reaction with monosilane, and disilanylated compounds were obtained using LiAlH₄ and a radical initiator. The reaction mechanisms are discussed.

Full text of DOI:10.1016/S0022-328X(98)00814-6

Journal of Organometallic Chemistry 574 (1999) 241–245
Silylation reactions of olefins with monosilane and disilane in the
presence of a transition metal complex, metal hydride, and radical
initiator
Masayoshi Itoh *, Kenji Iwata, Mineo Kobayashi
Organic Performance Materials Laboratory, Mitsui Chemicals Incorporated, 1190 Kasama-cho, Sakae-ku, Yokohama-city 247-8567, Japan
Received 28 April 1998; received in revised form 18 June 1998
Abstract
The catalytic reactions between monosilane or disilane and 1-hexene or 1,5-hexadiene were studied. The reactions with
monosilane gave the silylated compounds in the presence of a platinum complex, LiAlH4 and a radical initiator. In the reactions
with disilane, the same silylated compounds were produced in the presence of a platinum complex as in the reaction with
monosilane, and disilanylated compounds were obtained using LiAlH4 and a radical initiator. The reaction mechanisms are
discussed. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Silylation; Olefins; Transition metal complex
obtained by the hydrosilylation of olefins with SiH4
in the presence of transition metal complexes [5] and
LiAlH4 [6]. On the other hand, double silylation reac-
tions and dehydrogenative single silylation reactions
of unsaturated hydrocarbons with disilane compounds
(XR2SiSiR2X; X is alkyl or halogen), which were ac-
companied by the cleavage of the Si–Si bond, were
investigated in the presence of transition metal com-
plexes [7]. But reports on the hydrosilylation reactions
and the double silylation reactions of olefins with
Si2H6 are rare.
A large quantity of SiH4 and Si2H6 has recently
been produced and used for polysilicon and amor-
phous silicon. We can now use SiH4 and Si2H6 as
new raw materials in the organosilicon industry. In
this paper, we report the hydrosilylation and double
silylation reactions of olefins with SiH4 and Si2H6 us-
ing transition metal complexes, metal hydrides and
radical initiator as catalysts, and discuss the reaction
mechanisms.
1. Introduction
The hydrosilylations of olefins with HSiCl3,
CH3SiHCl2 or (CH3)3SiH, which are easily obtainable
by Rochow’s direct process, have been extensively in-
vestigated using many transition metal complexes as
catalysts [1]. However the reports on the hydrosilyla-
tions with H2SiCl2, (CH3)2SiH2, C6H5SiH3, SiH4 or
Si2H6, which have some Si–H bonds in the molecule,
are few [2]. The pyrolytic reactions and photo reac-
tions were carried out between monosilane (SiH4) or
disilane (Si2H6) and ethylene or acetylene at an ele-
vated temperature in the gas phase with very low
yields and selectivities [3]. SiH4 was alkylated in the
presence of NaAlR4 (R is alkyl) at an elevated tem-
perature [4]. We have briefly reported that some sily-
lated compounds (RSiH3, R2SiH2) were selectively
* Corresponding author. Fax: +81 458958240.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00814-6

242
M. Itoh et al. / Journal of Organometallic Chemistry 574 (1999) 241–245
Scheme 1. Mechanisms of the hydrosilylation of 1-hexene with (a) SiH4 and (b) Si2H6 in the presence of PtL4.
2. Experimental
SiH3), 1.2–1.5 (m, 8H, CH2–CH2–CH2–CH2–CH2–
CH3), 0.8–1.0 (m, 5H, CH2–CH2–CH2–CH2–CH2–
CH3). 13C-NMR (CDCl3/CDCl3, 77.1 ppm); l 32.5,
31.6, 27.1, 22.7 (CH2–CH2–CH2–CH2–CH2–CH3),
14.2 (CH3), 7.6 (SiH2–CH2). 29Si-NMR (CDCl3/TMS);
l −64.3(t, SiH2), −102.0 (q, SiH3). IR (neat); w(Si–
H) 2150 cm−l, w(C–H) 2970, 2940, 2860 cm−l, l
(Si–H) 919 cm−l. MS (m/z); (EI mode) 146 (M)+, 145
((M–H)+, base peak). (e) Dihexyldisilane (6): 1H-
NMR (CDCl3/CDCl3, 7.26 ppm); l 3.5–3.6 (m, 4H,
SiH2–SiH2), 1.2–1.5 (m, 16H, CH2–CH2–CH2–CH2–
CH2–CH3), 0.8–1.0 (m, 10H, CH2–CH2–CH2–CH2–
CH2–CH3). 13C-NMR (CDCl3/CDCl3, 77.1 ppm); l
32.5, 31.6, 27.4, 22.7 (CH2–CH2–CH2–CH2–CH2–
CH3), 14.2 (CH3), 7.4 (SiH2–CH2). 29Si-NMR (CDCl3/
TMS); l −63.5(t, SiH2). IR (neat); w(Si–H) 2120
cm−l, w(C–H) 2950, 2910, 2850 cm−l, l(Si–H) 938
cm−l. MS (m/z); 230(M)+, 229 ((M–H)+, base peak).
2.1. Reaction procedures
All experiments were performed in an autoclave.
1-Hexene or 1,5-hexadiene and a small amount of
catalyst, that is, PtL4 (L:PPh3), LiAlH4, AIBN (2,2-azo-
bis(isobutyronitrile), or DTBP (di-t-butylperoxide)
were charged into the autoclave. A sample of SiH4 or
Si2H6 was then added under pressure, and the reaction
was carried out under high pressure. The details of the
reaction conditions are shown in Table 1 and Table 2.
After the reaction, all gases in the autoclave were
replaced with nitrogen and passed through a solution of
lithium in ethanol. Unreacted SiH4 and Si2H6, which
were converted into silicon tetraethoxide in the solu-
tion, was analyzed by GC. The liquid products (hexylsi-
lane and dihexylsilane) were separated by distillation
under reduced pressure, and assigned based on the
GC-Mass, IR, and H-, 13C- and 29Si-NMR. They were
1
compared with the spectra of the compounds which
were prepared by the LiAlH4 reduction of hexyl-
trichlorosilane and dihexyldichlorosilane [8]. The
amounts of unreacted olefin and some liquid products
were determined by GC using n-eicosan as an internal
standard.
3. Results and discussion
3.1. The reaction of olefins with SiH4
The results are shown in Table 1. In the presence of
PtL4, hexylsilane (1) and dihexylsilane (2) were ob-
tained by the reaction of SiH4 with 1-hexene, and
1-hexenylsilane (3) and silacycloheptane (4) were ob-
tained by the reaction with 1,5-hexadiene [5].
2.2. Product characterization
(a) Hexylsilane (1) and dihexylsilane (2): see ref. [6].
(b) n-5-hexenylsilane (3): 1H-NMR(CDCl3/TMS); l
5.6–6.0 (m, 1H, CHꢀCH2), 4.8–5.1 (m, 2H, CHꢀCH2),
3.50 (t, 3H, J=4.1 Hz, SiH3), 1.9–2.2 (m, 2H, CH2–
CHꢀCH2), 1.3–1.7 (m, 4H, CH2–CH2–CH2–CH2),
0.6–1.0 (m, 2H, CH2–SiH3). IR (neat); w(Si–H) 2150
cm−1, w(CꢀC) 1641 cm−1, w(C–H) 3075, 2925, 2851
cm−1, l(Si–H) 922 cm−1. MS (m/z); 114 (M)+, 113
((M–H)+, base peak). (c) SilacycloHeptane (4): 1H-
NMR (CDCl3/TMS); l 3.8 (m, 2H, J=3.3 Hz, SiH2),
1.2–1.9 (m, 8H, CH2–CH2–CH2–CH2–CH2–CH2),
0.5–1.1 (m, 4H, CH2–SiH2–CH2). IR (neat); w(Si–H)
2140 cm−1, w(C–H) 2925, 2860 cm−1, l(Si–H) 948,
923 cm−l. MS (m/z); 114 (M)+, 113 ((M–H)+, base
The presumed reaction mechanism is shown in
Scheme 1(a). The reaction rapidly took place during the
first 30 min, but the catalyst was immediately destroyed
as we previously reported [5]. Complete conversion of
SiH4 was not achieved. Therefore, the turnover num-
bers of the reactions (TN) were low compared with
those for HSiCl3, which was completely transformed to
the trichlorosilylated compound and no catalyst poi-
soning was observed under the same reaction condi-
1
peak). (d) Hexyldisilane (5): H-NMR (CDCl3/CDCl3,
7.26 ppm); l 3.6–3.7 (m, 2H, SiH2), 3.1–3.2 (m, 3H,

M. Itoh et al. / Journal of Organometallic Chemistry 574 (1999) 241–245
243

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M. Itoh et al. / Journal of Organometallic Chemistry 574 (1999) 241–245
Scheme 2. Mechanisms of the hydrosilylation of 1-hexene with SiH4
and Si2H6 in the presence of LiAlH4.
The turnover numbers of the reactions (TN) of Si2H6,
as well as SiH4, are much smaller than those of HSiCl3.
The Si–Si bond would be cleaved from the catalyst to
form a H3Si metal bond, and then react with the olefin.
A similar reaction mechanism would be considered for
the silylation reaction (Scheme 1(b)) and the catalyst
poisoning as in the case of SiH4. In the reaction of
XMe2Si–SiMe2X with an olefin (R–CH=CH2) using
PtL4, the double silylation reaction and/or dehydrogena-
tive monosilylation occurred to produce XMe2Si–
C(R)H2–CH2–SiMe2X or R–CHꢀCH–SiR2X ([7]b,
[7]c). In the case of Si2H6, no double silylated com-
pounds, dehydrogenative monosilylated compounds and
disilanylated compounds (RSi2H5) were detected. One of
two SiH3 groups bonded to platinum would easily
decompose to SiH2 and H, then the SiH3 and H would
be attached to the olefin. Perhaps some by-products such
as polysilane would also be produced, though we have
no experimental evidence.
Using LiAlH4, Si–Si bond cleavage did not occur and
hexyldisilane (5) and 1,2-dihexyldisilane (6) were ob-
tained. The hexylaluminum anion would be formed, then
react with Si2H6. Usually the cleavage of the Si–Si bond
and the double silylation reaction occurs between the
disilane compounds and the unsaturated hydrocarbons
using transition metal complexes [7], so IiAlH4 is a new
catalyst for the disilanylation of unsaturated hydrocar-
bons.
tions. Perhaps SiH4 and the produced alkyl and alkenyl-
silanes would decompose to form the Si–metal bond, and
would polymerize involving the intermolecular dehydro-
genative coupling on the catalyst, followed by catalyst
reduction. This presumption is supported by the fact that
the solid compounds, which probably contain platinum
and silicon, are deposited after the reaction. Molnar et
al. reported that alkylhydrosilanes (Et3Si, Pr2SiH2,
AmSiH3) were observed to exert strong poisoning effects
on the hydrogenation of 1-hexene on Cab-O-Sil®-sup-
ported Pt, and the poisoning activity of these silanes
increased with increasing number of Si–H bonds in the
molecules [9]. The TN value of 1,5-hexadiene was lower
than that of 1-hexene (see run nos. 1 and 2). Though there
is no experimental evidence, perhaps it may be would by
the hydrosilylative coupling polymerization of 5-hexenyl-
silane (3), which would induce the catalyst poisoning.
The hydrosilylation reactions also proceeded using
LiAlH4 without a decrease in catalyst reactivity[6]. The
presumed mechanism of this reaction is shown in Scheme
2. It is well known that the alkyl anion is formed by the
interaction between LiAlH4 and olefins (hydroalumina-
tion) above 110°C [10]. The complex SiH4 may react with
these anions to produce the silylated compounds, and
LiAlH4 would be regenerated.
The hydrosilylation reaction also proceeded using a
radical initiator. The silyl radical would easily be pro-
duced by the cleavage of the Si–H bond (Scheme 3).
In the presence of a radical initiator, the disilanyl
compounds are mainly produced. Hexyldisilane (5) and
1,2-dihexyldisilane (6) were produced (Scheme 3). Hsiao
and Waymouth also reported that the hydrosilylation
reaction of olefins with poly(phenylsilane), which con-
tains many Si–Si and Si–H bonds in the molecule,
proceeded with little polymer degradation in the presence
of a radical initiator. These results suggests that the
cleavage of the Si–H bond by a radical would be much
easier than that of the Si–Si bond [11].
3.2. The reaction of olefins with Si2H6
These results are shown in Table 2. In the presence of
PtL4, the cleavage of the Si–Si bond occurred and the
products were quite same as the reaction of SiH4.
Hexylsilane (1) and dihexylsilane (2) were obtained by the
reaction of Si2H6 with 1-hexene, and 5-hexenylsilane (3)
and silacycloheptane (4) were obtained from Si2H6 and
1,5-hexadiene.
Scheme 3. Mechanisms of the hydrosilylation of 1-hexene with SiH4 and Si2H6 in the presence of the radical initiator.

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245
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