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

Heteroatom–Heteroatom Multiple Bonds 113
were obtained that correspond to rate constants of 0.002 and 0:2s 1 , respectively,
at room temperature. Evidence that nitroxide 58 fragments preferentially
at the nitrogen–carbon bond [86] and nitroxide 59 fragments at
the nitrogen–oxygen bond [87] are based on the detection of secondary adducts.
O
t-Bu N OSiPh 3
58
O
Me N OSi(TMS) 3
An Arrhenius expression for the reaction of Me3Si: radical with molecular
oxygen is available from the gas-phase kinetics, from which a rate constant of
ca 1 10 10 M 1 s 1 at room temperature can be calculated [88].
The reaction of molecular oxygen with a variety of silyl radicals
(Me3Si:, Et3Si:, n-Bu3Si:, t-Bu3Si:,Ph2MeSi:, and Ph3Si:) has been investigated
by EPR spectroscopy [89]. Based on 17 O-labelling experiments, it was
found that the structure of t-Bu3SiO2: resembles the structure of alkylperoxyl
radicals, i.e., the two oxygen nuclei are magnetically nonequivalent (Reaction
5.45), and this radical has more p spin density on the terminal oxygen than
alkylperoxyl radicals. Furthermore, trialkylsilylperoxyl radicals exist in equilibrium
with a tetraoxide (Reaction 5.46) at temperatures below 40 8C with
DH8 ¼ 46 8kJ=mol and DS8 < 30 cal K 1 mol 1 .
R3Si: þ O2 !R3Si w O w O : (5:45)
2R3Si w O w O : Ð R3Si w O w O w O w O w SiR3
59
(5:46)
t-BuMe2SiH was found to undergo a radical-initiated oxidation to the corresponding
hydroperoxide and this represents the first case of a silane oxidation
that resembles a hydrocarbon oxidation [90]. Kinetic studies carried out by
following the oxygen uptake allowed the oxidizability of t-BuMe2SiH to be
obtained in the temperature range of 295–350 K. The reaction mechanism is
shown in Scheme 5.13.
A variety of alkyl and/or phenyl substituted silicon hydrides are found to be
oxidized to the corresponding silanols in very good yields in the presence of
molecular oxygen and N-hydroxyphthalimide (60) and Co(II) catalysis (Reaction
5.47) [91, 92]. It is worth mentioning that n-Bu3SiH is essentially inert under
conditions by which Et3SiH affords silanol in an 87 % yield and steric effects are
invoked to play an important role. From a mechanistic point of view, it was
suggested that silylperoxyl radicals, derived from the addition of silyl radicals to
oxygen, abstract rapidly hydrogen from 60 (k 7 10 3 M 1 s 1 at room temperature)
to give the corresponding nitroxide, which in turn abstracts hydrogen
from the starting silane and completes the chain (Reaction 5.48). Since the
DH(O w H) in 60 is 369 kJ/mol [92], Reaction (5.48) is expected to be nearly
thermoneutral for Ph3SiH and strongly endothermic (DHr ¼ 29 kJ=mol)