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

120 Unimolecular Reactions
1
Si(H)Me 2
Si(H)Me 2
t-BuO
+
t-BuOOBu-t
145 �C
2 3
SiMe 2
Scheme 6.1 Summary from product and EPR studies
SiMe 2
(Scheme 6.2) [2]. Evidence for a five-membered ring product in 1 % yield has
also been obtained. Furthermore, EPR studies of the same reaction allowed for
the identification of the silacyclohexyl radical 6 with two unequivalent bhydrogen
atoms in the temperature range between 73 and 25 8C, as the
only observable intermediate. A kinetic investigation placed the rate constant
for the cyclization of radical 5 between 10 7 < k < 10 9 s 1 [2].
From the above description, it is clear that the cyclization of pent-4-ene
substituted silyl radicals is a fast reaction and that formation of the sixmembered
ring is very strongly favoured. This is of course in antithesis with
the analogous carbon-centered radicals, where the kinetically favored product
has the five-membered ring structure produced by an exo-cyclization (cf.
Section 7.3). The reversal in the regioselectivity of the silyl radicals was initially
explained based on structural factors, such as the longer C w Si bond length and
pyramidal geometry of the silicon centre. Semi-empirical calculations
(MINDO/3) suggested that such behaviour of silyl radicals is due to an enthalpy
factor caused by electronic effects [3]. The six-membered ring formation was
also observed in the reaction of analogous heterosubstituted derivatives 7
(Reaction 6.1) [1].
4
Si(H)Me 2
t-BuO•
t-BuONNOBu-t
C6D6 , 46 �C
Si
6-endo-trig
35%
Si
Si
5 6
Scheme 6.2 Preference for 6-endo-trig cyclization of silyl radicals
+
H a
He H
11%
Si