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

Homolytic Organosilicon Group Transfer 139
Evidence that the 1,2 transfer of the Me3Si group from silicon to oxygen is an
effective process (Reaction 6.24) has been obtained from a mechanistic investigation
of the reaction of (TMS) 3SiH with nitroalkanes and nitroxides [38,39]. A
lower limit rate constant of 10 7 s 1 at room temperature was obtained for
Reaction (6.24).
Me3Si Me Si
3Si
Me3Si O
SiMe3 Me Si O
3Si Me3Si (6.24)
The 1,2 transfer of the organosilicon group from silicon to oxygen has also
been invoked in the mechanism of the autoxidation of (Me3Si) 3SiH,
(Me3Si) 2MeSiH and poly(hydrosilane)s in general (see Section 8.2) [40,41].
Details of the radical chain autoxidation of (Me3Si) 3SiH and (Me3Si) 2MeSiH,
proved by oxygen-labelling experiments, are shown in Scheme 6.20. Silylperoxyl
radical 89 rearranges to 90 by means of an unusual mechanism followed by
a 1,2 transfer of a Me3Si group to give 91.
Me3Si Me3Si Si O
R Si O
Me3Si Me3SiO Me3SiO R
SiMe3 R Si O
O
89 90 91
Scheme 6.20 Reaction intermediates involved in the autoxidation of silyl substituted silicon
hydrides (R ¼ Me or Me3Si)
The intramolecular migration of the organosilicon groups observed in the
silyl radical adducts of a-diones and ortho-quinones (see Section 5.3.1) are
examples of the 1,4 transfer of a silyl group from oxygen to oxygen. Another
example is shown in Reaction (6.25) for the adduct of 3,6-di-tert-butyl-orthobenzoquinone
with Me3Si: radical. The activation parameters for this symmetrical
fluxional motion are log A=s 1 ¼ 13:2 and Ea ¼ 38:9kJ=mol, which correspond
to a rate constant of 1:8 10 6 s 1 at 208C [42]. It is also observed that
for unsymmetrical a-carbonyl derivatives, where the two adducts are not
equivalent, the equilibrium lies towards the more stable adduct radical [43].
Homolytic 1,5 transfer of an organosilicon group from enoxy oxygen to
alkoxy oxygen has also been observed [44]. As shown in Scheme 6.21, radical
reaction of compound 92 with Bu3SnH afforded compound 93 in a 92 % yield.
O
SiMe3 O
O
O SiMe 3
(6.25)