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

140 Unimolecular Reactions
Me3Si O
O
Bu 3SnH(D)
AIBN, 80 �C
O O SiMe 3
92 93, 92%
Me3Si O O O O
Bu3Sn Bu3Sn 94 95
H(D)
SiMe 3
Scheme 6.21 Homolytic 1,5 transfer of Me3Si group from an enoxy oxygen to alkoxy oxygen
When Bu3SnH was replaced with Bu3SnD, the deuterated compound 93 was
isolated in 86 % yield. The intermediate alkoxy radical 94 derived by addition of
Bu3Sn: radical, following by ring opening of the epoxide moiety, undergoes a
fast 1,5 silyl migration to give 95 prior to hydrogen abstraction. A lower limit
rate constant of 10 9 s 1 is estimated for such a unimolecular process. By
replacing Me3Si with t-BuMe2Si group the 1,5 transfer occurred more slowly
since the corresponding alkoxy radical 94 was partially quenched. An analogous
reaction with a chiral organosilicon moiety proceeded with retention of
configuration most likely through a frontside attack mechanism [45]. The 1,5
translocation of SiH3 group between two alkyl carbon atoms or two alkoxy
oxygen atoms (cf. Reaction 6.20) has also been investigated theoretically [46].
Concerted frontside transition states were located with much lower barriers for
the shift between the two oxygens.
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