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

38 Hydrogen Donor Abilities of Silicon Hydrides
able, on their Arrhenius parameters. The data for the reaction of secondary alkyl
radicals with (Me3Si) 3SiH have been used for the determination of Arrhenius
parameters related to the ring expansion of radical 16 (Reaction 3.10, where
u ¼ 2:3RT kJ/mol) [20]. Trapping of the acyl radical 17 by (Me3Si) 3SiH has been
used as a reference reaction for obtaining the temperature-dependent function of
the acyl radical cyclization (Reaction 3.11) [20]. Reaction (3.12) is an example of
perfluoroalkyl radical clocks, for which Et3SiH is generally found to be the most
appropriate hydrogen donor. Indeed, a kH=kc ¼ 2:0M 1 was obtained at 30 8C
which corresponds to a kc ¼ 3:8 105 s 1 [16].
O
O
k re
16 log k H/k re(M −1) = − 2.2 + 21.3/θ
log k re/s −1 = 0.5 − 39.3/θ
kc O O
17 log k H /k c(M −1 ) = − 1.4 + 10.5/θ
log k c/s −1 = 9.6 − 25.1/θ
3.2 NITROGEN-CENTRED RADICALS
O
18
F 7
k c
O
F 7
(3.10)
(3.11)
(3.12)
The kinetic data of the reactions of dialkylaminyl radicals with silicon hydrides
are limited to the hindered 2,2,6,6,-tetramethylpiperidinyl radical (19) and are
obtained by using EPR spectroscopy [21]. In Table 3.3 are collected the activation
parameters together with the calculated rate constants at 25 8C. Two
remarkable features are the low preexponential factors and the order of reactivity
of phenyl substituted derivatives, where the rate constants increase along the
series: Ph3SiH < Ph2SiH2 < PhSiH3 (taking into account the statistical number
of abstractable hydrogen atoms). This behaviour, which is the opposite to the
analogous reactions of alkyl and alkoxyl radicals, was explained in terms of steric
effects in the transition state that demands a very specific orientation between the
abstracting aminyl radical and the hydrogen–silicon bond in the silane [21].