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

Intramolecular Homolytic Substitution at Silicon 135
Me3Si SiMe3 Si
S H i
Me 3 Si
Me 3 Si
SiMe 3
SiMe 3
Si
SiMe3 Me3Si Si SiMe3 Br
Me 3 SiBr
Scheme 6.15 Propagation steps of Reaction (6.14) involving an SHi step via a five-membered
transition state
R
R
Br
Si(SiMe 3) 3
(TMS) 3 SiH
AIBN, 80 �C
R
R
77, R = CO 2 Me 78, 75%
Si(SiMe 3) 2
(6.17)
The SHi reaction at silicon positioning a carbon-centred radical in the
g-position to a silyl ether group, has been introduced as an alternative method
for the preparation of cyclic alkoxysilanes (Scheme 6.16) [31,32]. Reaction
(6.18) shows the extension of the chemistry reported above by using aryl
radicals, which afforded the cyclic alkoxysilane in good yield. However,
by replacing the two SiMe3 groups by phenyls the yield decreased considerably
(Reaction 6.19) in favour of the product derived from 1,5-phenyl migration,
which was obtained in a 55 % yield (cf. Reactions 6.9 and 6.10). Indeed, a better
leaving group like SnMe3 is necessary for increasing the efficiency of cyclic
alkoxysilane. Kinetic data on the SHi reaction at silicon that parallels the
efficiencies observed in Reaction (6.19) have also been obtained.
R 2
R1 R1 Si
O X
R2 R3 O
Si
R1 R1 R3 R4
R5 R4 R5 Scheme 6.16 SHi reaction at silicon: preparation of cyclic alkoxysilanes
+ X