"Front Matter". In: Organosilanes in Radical Chemistry - Index of

Oxidation Studies on Silyl-Substituted Silicon Hydrides 189
a g factor of 2.0047, which is assigned to radical 5 obtained by Reaction (8.2).
The observed alternating line width effect with decreasing temperature is indicative
of a restricted rotational motion around the C w Si bond between the
radical centre and the n-hexyl substituent.
HxHxHx
HxHxHx
Si Si Si + t-BuO• Si Si Si +
H H H H H
4 Hx = n-hexyl 5
t-BuOH
Figure 8.2b corresponds to the superimposition of signals from at least two
persistent paramagnetic species. The nature of these species is still unclear even
if the g factors are consistent only with silyl radicals carrying three silyl groups
as in 6. Mechanistic schemes for their formation could be suggested based on
the addition of transient silyl radicals to the transient disilenes generated
photolytically [14].
Si
Si Si Si
8.2 OXIDATION STUDIES ON SILYL-SUBSTITUTED SILICON
HYDRIDES
8.2.1 POLY(HYDROSILANE)S
6
Poly(hydrosilane)s are stable compounds and can be manipulated in the air only
for a short period since they are oxygen sensitive. In order to study the oxidation
products, a xylene solution of poly(phenylhydrosilane) (MW ¼ 2340, MW=Mn ¼
1:72) was refluxed (140 8C) for 12 h in a system exposed to the air [15]. Only minor
changes were observed by GPC analysis whereas FTIR showed characteristic
absorptions due to siloxane-type structures on the polymer backbone. A detailed
NMR analysis, based on 1 H NMR, 29 Si INEPT and 1 H– 29 Si HMQC spectroscopies,
indicated that the oxidized material contains the units 7–10 shown in
Scheme 8.2. In particular, units 7, 8 and 9 þ 10 were present in relative percentages
of 27 %, 54 % and 19 %, respectively, which mean that more than 70 % of the
catenated silicons were altered. It has also been reported that silyl hydroperoxides
and peroxides are not found as products in the autoxidation of poly(phenylhydrosilane)
[16].
Two different kinetic approaches have been used to obtain mechanistic information
[15,16]. Kinetic studies carried out by EPR spectroscopy using fusinite
as a paramagnetic probe of the oxygen concentration allowed the oxidizability
of poly(hydrosilane)s to be obtained [15]. Oxidizability values of 1:8 10 2 and
(8.2)