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

34 Hydrogen Donor Abilities of Silicon Hydrides
The rate constants increase along the series Et3SiH < Ph3SiH < (MeS) 3SiH <
(Me3Si) 3SiH with the expected intermediate values for silanes having mixed
substituents. Mechanistic studies have shown that the attack of primary alkyl
radicals on Et3SiH occurs in about 60 % of the cases at the SiH moiety and in
40 % at the ethyl groups at 130 8C. The deuterium kinetic isotope effect (kH=kD)
for the attack on the Si w D bond in Et3SiD have been found to be 2.2 (at 130 8C)
[4]. In the case of (Me3Si) 3SiH the H atom abstraction from the Si w H moiety
amounted to about 95 % of the total reactions [1]. The preexponential factors all
lie in the expected range, and the activation energy is clearly the major factor in
determining the radical–silane reactivity.
Phenyl substitution has only a small effect on the rate constant in contrast
with that on the carbon analogues. The rate constants increase along the series
PhSiH3 < Ph3SiH < Ph2SiH2 for neophyl radical; however, by taking into
account the statistical number of hydrogens abstracted, the order changes as
expected: PhSiH3 < Ph2SiH2 < Ph3SiH. For the discussion on the lack of
resonance stabilization by phenyl groups see Section 1.2. The silanthrane 4 is
one order of magnitude more reactive than Ph2SiH2 towards primary alkyl
radicals (taking into account the statistical number of hydrogen abstracted).
The reason for this enhancement in the reactivity is probably due to the
stabilization of the silyl radical induced either by a transannular interaction
of the vicinal Si substituent or by the quasi planar arrangement of the radical
center [8].
Table 3.1 shows that the rate constant increases substantially with successive
substitution of alkyl or phenyl by thiyl or silyl groups at the Si w H function. In
particular, by replacing a methyl with a Me3Si group, the rate increases by an
order of magnitude, this effect being cumulative. These results are in good
agreement with the thermodynamic data for these silanes, which show the
weakening of the Si w H bond strength on replacing the alkyl substituent with
thiyl or silyl groups (see Section 2.2).
Intramolecular hydrogen abstraction by primary alkyl radicals from the
Si w H moiety has been reported as a key step in several unimolecular chain
transfer reactions [11]. In particular, the 1,5-hydrogen transfer of radicals 8–11
(Reaction 3.4), generated from the corresponding iodides, was studied in competition
with the addition of primary alkyl radicals to the allyltributylstannane
and approximate rate constants for the hydrogen transfer have been obtained.
Values at 80 8C are in the range of (0:4–2) 10 4 s 1 , which correspond to
effective molarities of about 1–2 M.
The competitive kinetics of Scheme 3.1 can also be applied to calibrate the
unimolecular radical reactions provided that kH is a known rate constant. In
particular the reaction of primary alkyl radicals with (Me3Si) 3SiH has been used
to obtain kinetic data for some important unimolecular reactions such as the belimination
of octanethiyl radical from 12 (Reaction 3.5) [12], the ring expansion
of radical 13 (Reaction 3.6) [8] and the 5-endo-trig cyclization of radical 14
(Reaction 3.7) [13]. The relative Arrhenius expressions shown below for the