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

Intramolecular Homolytic Substitution at Silicon 133
R
O
O
CH2Ph Si
62
R
O
O
Si
hν
R = Me or t-Bu
PhCH 2
R
O
O
Si
CH2Ph R O
O Si
+
PhCH 2
R
O
O
Si
O
PhCH 2
63 64
65 66 67
Scheme 6.13 A stepwise pathway via five-membered cyclic intermediate radical 66 in the 1,2
migration of an acyloxy group
6.4 INTRAMOLECULAR HOMOLYTIC SUBSTITUTION AT SILICON
It has been found theoretically that bimolecular homolytic substitution (SH2)
reaction of the methyl radical at the silicon atom in disilane can proceed via both
backside and frontside attack mechanism (Scheme 6.14) [28]. The transition state
for the backside attack (68) is predicted to adopt an exactly collinear arrangement
of the attacking methyl radical and the leaving silyl radical, as it is generally
believed for the majority of SH2 reactions. The transition state for the frontside
attack (69) is predicted to involve an attack angle of around 80 8. At the highest
level of theory, the energy barriers of the two paths are very similar being 47.4
and 48.6 kJ/mol for the backside and frontside attack, respectively. This information
can be easily extrapolated to the intramolecular homolytic substitution
(SHi) reactions for mechanistic consideration, by imagining links between the
attacking carbon-centred radical and one of the two silicon centres.
Reflux of bromide 70 in benzene and in the presence of small amounts of
(TMS) 3SiH and AIBN afforded the silabicycle 71 in a 88 % yield (Reaction
6.14) [29]. On the other hand, the reduction of iodide 72 with (TMS) 3SiH under
standard experimental conditions gave the silacyclopentane 74 in a 24 % yield,
along with the reduced product 73 (Reaction 6.15) [30]. Scheme 6.15 shows the
propagation steps for Reaction (6.14). The key step for this transformation is
the SHi reaction at the central silicon atom via a five-membered transition state
which occurred with a rate constant of 2:4 10 5 s 1 at 80 8C. Similar reactions
are observed in the hydrosilylation of 1,6-dienes with (TMS) 3SiH when the
concentration of reducing agent is kept low by a slow addition in order to avoid
the trapping of a carbon-centred radical [29,30]. Reaction (6.16) shows an
R
O
Si