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

Oxygen-Centred Radicals 39
Rate constants for the hydrogen abstraction from (Me3Si) 3SiH by diaryl
aminyl radicals 20–24 were determined by using a method in which the corresponding
amines catalyse the reaction of 2,4,6-tri-tert-butylphenoxyl radical
with (Me3Si) 3SiH (see below). At 91 8C, rate constants are 33.5, 7.27, 5.63,
2.45 and 45:8M 1 s 1 for radicals 20, 21, 22, 23 and 24, respectively [22]. Based
N
19
X
N
20 X = Y = H
21 X = Y = CH 3
22 X = H, Y = OCH 3
23 X = Y = OCH 3
24 X = Y = Br
on the analysis of the data, it was suggested that polarized transition states
should be unimportant and the reactivity trend simply reflects the differences in
the bond dissociation enthalpies of amine, DH(Ar2N w H). The Arrhenius
parameters for the reaction of Ph2N: radical with (Me3Si) 3SiH have been
measured (Table 3.3) and fit very well with a reaction in which both substrates
are sterically hindered.
Table 3.3 Rate constants (25 8C) and Arrhenius parameters for the reactions of aminyl
radicals with silicon hydrides [21,22] a
Silane Radical k=M 1 s 1 log A=M 1 s 1 Ea=kJ mol 1
Et3SiH 19 2.7 5.2 27.2
PhSiH3 19 119.2 5.7 28.0
Ph2SiH2 19 32.1 5.8 24.3
Ph3SiH 19 5.3 6.9 27.6
(Me3Si)Me2SiH
(Me3Si) 3SiH
(Me3Si) 3SiH
19
19
10.0
34.4
4.8
4.8
21.8
18.4
b
Ph2N: 4.4 5.2 25.9
a ESR spectroscopy and tert-butylbenzene as the solvent if not otherwise stated.
b Quasi-equilibrium method (see text) and chlorobenzene as the solvent.
3.3 OXYGEN-CENTRED RADICALS
3.3.1 ALKOXYL RADICALS
The reaction of thermally and photochemically generated tert-butoxyl radicals
with silicon hydrides (Reactions 3.13 and 3.14) has been extensively used for the
generation of silyl radicals in EPR studies, time-resolved optical techniques,
and organic synthesis.
Y