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

32 Hydrogen Donor Abilities of Silicon Hydrides
3.1 CARBON-CENTRED RADICALS
3.1.1 PRIMARY ALKYL RADICALS AND FREE-RADICAL CLOCK
METHODOLOGY
The kinetic data for the reaction of primary alkyl radicals (RCH2:) with a
variety of silanes are numerous and were obtained by applying the free-radical
clock methodology. The term free-radical clock or timing device is used to
describe a unimolecular radical reaction in a competitive study [2–4]. Three
types of unimolecular reactions are used as clocks for the determination of rate
constants for this class of reactions. The neophyl radical rearrangement (Reaction
3.1) has been used for the majority of the kinetic data, but the ring
expansion rearrangement (Reaction 3.2) and the cyclization of 5-hexenyl radical
(Reaction 3.3) have also been employed.
O
2
1
CO 2Me
3
k r
k r
k r
O
CO 2 Me
The concept of this method is illustrated in Scheme 3.1, where the clock
reaction (U: ! R:) is the unimolecular radical rearrangement with a known
rate constant (kr). The rate constant for the H atom abstraction from R3SiH by
a primary alkyl radical U: can be obtained, provided that conditions are found
in which the unrearranged radical U: is partitioned between the two reaction
channels, i.e., the reaction with R3SiH and the rearrangement to R:. At the end
of the reaction, the yields of unrearranged (UH) and rearranged (RH) products
can be determined by GC or NMR analysis. Under pseudo-first-order conditions
of silane concentration, the following relation holds: UH=RH
¼ (kH=kr)[R3SiH]. A number of reviews describe the radical clock approach
in detail [3,4].
(3.1)
(3.2)
(3.3)