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

136 Unimolecular Reactions
Br
O Si(SiMe 3 ) 3
O Si
Br
MMe3 Ph Ph
Bu 3SnH (cat.)
AIBN, 80 �C
Bu 3SnH (cat.)
AIBN, 80 �C
M = Si
M = Ge
M = Sn
Si(SiMe3 ) 2
O
91%
SiPh2 O
28%
39%
84%
(6.18)
(6.19)
Interestingly, the corresponding reaction with the silylether group in the
b-position afforded completely different products [23]. Indeed, the reduction
of 79 gave the ortho-silylated phenol 80 in a 50 % yield (Scheme 6.17). The key
step is the SHi reaction of aryl radical 81 at Me3Si silicon atom with formation
of silyl radical 82, confirming the preference for a five-membered transition
state.
Photochemical behaviour of compounds 83–86 [33] in the gas phase has been
reported, in order to distinguish between silyl radical and silylene formation.
Photolysis of the noncyclic precursors 83 and 84 gave products derived from
silyl radicals, which come from a direct Si w Si bond homolysis, with a little
evidence of silylene formation. In contrast, dimethylsilylene (Me2Si:) was observed
as a direct photoproduct from the cyclic precursors 85 and 86. The
reaction sequence including a SHi step shown in Scheme 6.18 for the formation
of dimethylsilylene was proposed to explain the different observations for cyclic
and noncyclic systems.
79
O
SiPh2 SiMe3 Br
O SiPh2
1) slow addition (7h)
Bu 3SnH, AIBN, 80 �C
SiMe 3
2) MeLi
S H i
81 82
O SiPh2
SiMe 3
OH
80, 50%
SiMe 3
Scheme 6.17 SHi reaction at silicon: the preference for a five-membered transition state