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

General Aspects of Radical Chain Reactions 51
synthesis with physical organic chemistry has been a winning horse for introducing
some novelty in radical-based organic transformations.
Generally, radical chain reactions are carried out in nonpolar solvents although
strong solvent effects on propagation steps are rare. Apart from the
polarity, a much more important criterion for the solvent choice is the solvent’s
inertness towards the chain propagating radicals involved.
4.1.1 RADICAL–RADICAL REACTIONS
The bimolecular rate constant (2kt) for the decay of Me3Si: radical has been
measured by kinetic EPR studies in the liquid phase and found to be 3:0
109 M 1 s 1 at 20 8C [9], whereas a value of 1:5 10 10 M 1 s 1 at 22 8C is
obtained from the time profile of the transient absorption in the gas phase
[10]. The Arrhenius parameters were also measured in the liquid phase and
found to be log A=M 1 s 1 ¼ 9:9 and E ¼ 4:1 kJ/mol [9]. Trimethylsilyl radicals,
generated in the liquid phase by reaction of t-BuO: radicals with Me3SiH,
are shown to react with each other by both combination (Reaction 4.2) and
disproportionation (Reaction 4.3), the former being about 10 times faster
[11,12].
2Me3Si: !Me3SiSiMe3 (4:2)
2Me3Si: !Me3SiH þ CH2
w SiMe (4:3)
2
The bimolecular rate constants for the self-reaction of the
Et3Si:,Me3SiSi(:)Me 2 and (Me3Si) 3Si: radicals have also been obtained in
the liquid phase. A rate constant of 1:4 10 9 M 1 s 1 at 65 8C was measured
for Me3SiSi(:)Me 2 by kinetic EPR spectroscopy [13]. By means of laser flash
photolysis experiments, the Et3Si: and (Me3Si) 3Si: radicals were shown to
decay with second-order kinetics with 2kt=e308nm ¼ 1:1 10 7 cm s 1 and
2kt=e300nm ¼ 5:0 10 6 cm s 1 , respectively, at ambient temperature [14,15]. Estimated
values of appropriate molar absorption coefficients allowed for the
calculation of the values 2kt ¼ 1 10 10 and 5 10 9 M 1 s 1 for Et3Si: and
(Me3Si) 3Si: radicals, respectively [14,16]. While the fate of the reaction between
two Et3Si: radicals is still not known, the termination products of other silyl
radicals have been determined. Pentamethyldisilyl radicals, produced by the
reaction of Me3SiSi(H)Me2 with photogenerated t-BuO: radicals at room
temperature, behave similarly to the Me3Si: radical [17]. That is, products
due to the combination (Reaction 4.4) and disproportionation (Reaction 4.5)
of these radicals were detected in a ratio of 2.1.
: Me2 !Me3SiSiMe2SiMe2SiMe 3 (4:4)
2Me3SiSi
2Me3SiSi
: Me2 !Me3SiSi(H)Me2 þ CH2
w SiMeSiMe3 (4:5)