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

50 Reducing Agents
Initiation steps:
Propagation steps:
Termination steps:
R' 3SiH
RH
R' 3 SiH
2 R' 3 Si
R + R' 3 Si
2 R
R•
In
R' 3Si
R' 3Si
RZ
RZSiR' 3
R' 3SiZ
no radical products
Scheme 4.1 The reaction mechanism for the radical chain removal of a functional group by
organosilicon hydrides
following observations have been made [8]: (i) the termination rate constants in
the liquid phase are controlled by diffusion, i.e., close to 10 10 M 1 s 1 (see
below), (ii) in chain reactions radical concentrations are about 10 8 M
(depending on reaction conditions), and (iii) the concentration of substrates is
generally between 0.05 and 0.5 M, indicating that the rate constants for the
chain transfer steps must be higher than 10 3 M 1 s 1 . Using the extensive kinetic
data available on the hydrogen atom abstraction step from silicon hydrides (see
Chapter 3), and on the Z group removal by the silyl radical (see later in this
Chapter), one can choose the conditions for efficient radical reductions. Since
radical chain reactions require a steady supply of radicals, faster propagation
steps (i.e., higher kinetic chain length) require small amounts of radical initiator.
To foresee the reaction efficiency and outcome is a powerful feature of
radical chemistry, provided that synthetic organic chemists acquire a familiarity
with the kinetic data. In my personal experience, the integration of organic