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

Oxygen-Centred Radicals 41
in determining the t-BuO: radical–silane reactivity. The trends outlined above
can be entirely attributed to more favorable thermodynamic factors along the
series.
3.3.2 PEROXYL RADICALS
The kinetics for the reaction of cumylperoxyl radical with a variety of silanes
(Reaction 3.15) have been measured by using inhibited hydrocarbon oxidation
methodology (Table 3.4) [29]. The trends in reactivity for the cumylperoxyl
radical are similar to those observed for t-BuO: radical, although the reactions
are about seven orders of magnitude slower. The rate constants increase along
the two series PhSi(H)Me2 < Ph2Si(H)Me < Ph3SiH and PhSiH3 < Ph2SiH2
< Ph3SiH, when the statistical number of abstracted hydrogens is taken into
account. Furthermore, the rate constants increase about 7 and 660 times on
going from t-BuMe2SiH to Ph3SiH and (Me3Si) 3SiH, respectively.
PhMe2COO: þ R3SiH !PhMe2COOH þ R3Si: (3:15)
The similar reactivity of peroxyl with (CF3) 2NO: radicals towards a large
variety of substrates has been observed previously and attributed to rather
similar thermochemistries and spin density distributions for these two radicals
[30]. The Arrhenius parameters for the reaction of the persistent (CF3) 2NO:
radical with n-Bu3SiH determined by kinetic EPR spectroscopy are
log A=M 1 s 1 ¼ 5:5 and Ea ¼ 32:2 kJ/mol, which corresponds to
k ¼ 4:3M 1 s 1 at 73 8C [31].
3.3.3 ARYLOXYL AND AROYLOXYL RADICALS
The Arrhenius parameters for the reaction of persistent aryloxyl radical 25
with (Me3Si) 3SiH (Reaction 3.16) were measured spectrophotometrically
and found to be log A=M 1 s 1 ¼ 4:3 and Ea ¼ 43:8 kJ/mol [22]. A rate constant
of 4:8 10 4 M 1 s 1 can be calculated at 25 8C. It has been suggested that the
low preexponential factor is due to geometric constraints on the transition state.
Me 3 C
CMe 3
CMe 3
25
O + (Me3Si) 3SiH Me 3 C
CMe 3
CMe 3
OH + (Me3Si) 3Si (3.16)