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

Carbon-Centred Radicals 37
the cyclization reaction at 25 8C [4], which allows the calculation of a value of
kH ¼ 3 10 8 M 1 s 1 at this temperature. Hydrogen abstractions from Ph2SiH2
or Ph3SiH by phenyl radical are more than one order of magnitude slower and
obtained by competing bimolecular processes (Cl atom abstraction from CCl4).
k c
O O
15 log k H/k c(M −1 ) = – 2.6 + 6.7/θ
Rate constants for hydrogen atom abstraction from ((Me3Si) 3SiH by acyl
radicals were measured by using competing decarbonylation reactions as the
radical clock (Reaction 3.9), and the reference data being a-cleavage of the
propanoyl radical in the gas phase [15]. It is worth mentioning that the hydrogen
donation toward RCO: radicals is essentially independent of the nature of
the alkyl substituent R and the Arrhenius parameters are log A=M 1 s 1 ¼ 8:2
and Ea ¼ 29:3 kJ/mol.
O
(3.9)
R
R + CO
The kinetic data for halogenated carbon-centred radicals with silicon hydrides
are also numerous and a few examples are shown in Table 3.2. The
kinetic data for perfluoroalkyl radicals were obtained by competition of the
appropriate silane with the addition to an olefin [16–18]. The kinetic deuterium
isotope effects (kH=kD) on the attack of n-C4F9: on the Si D bond of
w
t-BuMe2SiD and Me3SiSi(D)Me2 have been found to be 3.3 and 2.4, respectively,
at 25 8C [17]. The rate constant for the reaction of Cl3C: radical with
Et3SiH and its temperature dependence were obtained relative to the reaction of
H atom abstraction from cyclohexane. The Arrhenius parameters are
log A=M 1 s 1 ¼ 8:6 and Ea ¼ 33:9 kJ/mol [19].
The trend in reactivity observed for the primary alkyl radical seems also to hold
for the other carbon-centred radicals (Table 3.2), i.e., the rate constants increase
along the series Et3SiH < Ph3SiH < (Me3Si) 3SiH with the expected intermediate
values for silanes having mixed substituents. For triethylsilane, the rate constants
decrease along the series (CF3) 3C: > (CF3) 2CF: > RfCF2CF2: > RCH2: >
R3C: > Cl3C: and cover almost six orders of magnitude. For tris(trimethylsilyl)silane,
the rate constants decrease along the series C6H5: > RfCF2CF2: >
RCH2: R2CH: R3C: > RC(:)O and cover more than four orders of mag-
nitude. The trends outlined above can be entirely attributed to the interplay of
entropic, enthalpic and polar effects, the last effect manifested in the triplet
repulsion of the silicon and carbon atoms of the reacting three-electron system
(see Section 3.7).
The free-radical clock methodology has been also applied to calibrate unimolecular
radical reactions based on the kH values of Table 3.2 and, when avail-
(3.8)