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

132 Unimolecular Reactions
rearrangements generally proceed in a nondissociative manner through either
a five-centre/five-electron shift (59) or three-centre/three-electron shift (60),
the latter being a more polarized transition state. All collected data are inconsistent
with a stepwise pathway via a five-membered cyclic intermediate radical
[25].
O
R
O
R O
X Y X Y
O
59 60
Analogous silicon radical reactions are limited to two reports [26,27].
The reaction of photogenerated t-BuO: radical with silane 61 in hexadecane
as solvent containing CCl4 in varying concentrations, provided the two
chlorides shown in Reaction (6.13) [26]. Although no detailed kinetic studies
are reported, the fact that alkyl chloride was the major product even in
the presence of 5 M CCl4 suggests a very fast 1,2 migration of the acetyloxy
group.
H 3C
O
61
O
H
Si
+ CCl 4
H3C CH3 t-BuOOBu-t
O
O
O Cl + Cl O
hν Si
Si
(6.13)
Strong evidence that a cyclic intermediate radical lies on the reaction
coordinate of the 1,2-shift of the acyloxy group in the radical 65, has been
obtained (Scheme 6.13) [27]. Irradiation (10 W low-pressure mercury lamp)
of silane 62 for 4 h at room temperature afforded mainly isomers 63 and
64 together with PhCH2CH2Ph. After hydrolysis compound 63 is quantitatively
transformed in RC(O)CH2Ph. These results are consistent with
Scheme 6.13, where the initial formation of silyl radical 65 is followed by
radical cyclization to radical 66. In turn radical 66 either can be trapped
by benzyl radical to give 63 or further rearranges to radical 67, prior to the
benzyl radical trapping. Deuterium-labelled experiments indicated that the
formation of 63 occurs in the solvent cage whereas 64 arises as both cage and
escape products. EPR spectra recorded during the photolysis of silane 62 at
around 100 8C consisted of the superimposition of radicals 65 and 67, which
suggests a rate constant in the range of 10 2 –10 3 s 1 for the 1,2 migration of
an acyloxy group at this temperature. Therefore, the intramolecular addition
of silyl radical to the ester moiety is a fast process and a few orders of
magnitude faster than the intermolecular addition due to entropic effects (cf.
Chapter 5).