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

100 Addition to Unsaturated Bonds
5.3 CARBON-OXYGEN DOUBLE BONDS
5.3.1 FORMATION OF SILYL RADICAL ADDUCTS
In the late 1950s it was established that silanes add across the carbonyl moiety
of ketones to give the corresponding silyl ethers either by heating at ca 250 8C
or by UV irradiation [40]. The occurrence of Reaction (5.22) as the key step
in the hydrosilylation of a carbonyl group was substantiated in 1969 when
the EPR spectra of the first adduct radicals were observed for acetone and
trifluoroacetic acid [41]. At present, a large number of EPR parameters for this
class of radicals are available and structural information has been obtained
[42,43].
R 3Si
X
X
+
Y
O
Y
OSiR3 (5.22)
The addition of silyl radicals to a-diones [44,45] or ortho-quinones [46,47]
presents some special features. That is, an intramolecular migration of the silyl
group between the two oxygens occurs in the EPR time scale. Activation
parameters for this fluxional motion have been obtained by simulating the
line width alternation as the temperature changes. The Ea for the benzil adducts
(24,25) ofEt3Si, Ph3Si, and (TMS) 3Si groups are 39.3, 24.7 and 36.8 kJ/mol.
The preexponential factors are between 10 11:6 and 10 12:3 s 1 , which implies
negative activation entropies characterizing these migrations. Therefore, it
was suggested that the reaction coordinates proceed through a cyclic transition
state 26 [44,45].
Ph
Ph
O
O
SiR 3
Ph
Ph
O
O
SiR 3
Ph
Ph
24 25
26
O
O
SiR 3
(5.23)
The addition of silyl radicals to various 2,6-disubstituted quinones takes
place at two different sites, i.e., at the less hindered C w O and at the C w C
double bond, the former being ca 4 times slower (Scheme 5.5) [48]. However,
kinetic studies showed that radical adducts 27 are prone to rearrange to the
thermodynamically more stable isomers 28 via a four-membered transition