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

Carbon-Oxygen Double Bonds 101
state. For X ¼ t-Bu, the 1,3 carbon to oxygen shift occurs with
log A=s 1 ¼ 13:8 and Ea ¼ 76:1kJ=mol.
O
X X
O
+
R 3Si
~ 80%
~ 20%
O
X X
X
O
27
O
O SiR3
28
SiR 3
silyl
migration
Scheme 5.5 Reaction paths for the addition of silyl radicals to 2,6-disubstituted quinones
Rate constants and Arrhenius parameters for the reaction of Et3Si: radicals
with various carbonyl compounds are available. Some data are collected in
Table 5.2 [49]. The ease of addition of Et3Si: radicals was found to decrease in
the order: 1,4-benzoquinone cyclic diaryl ketones, benzaldehyde, benzil,
perfluoro propionic anhydride > benzophenone alkyl aryl ketone, alkyl
aldehyde > oxalate > benzoate, trifluoroacetate, anhydride > cyclic dialkyl
ketone > acyclic dialkyl ketone > formate > acetate [49,50]. This order of
reactivity was rationalized in terms of bond energy differences, stabilization of
the radical formed, polar effects, and steric factors. Thus, a phenyl or acyl
group adjacent to the carbonyl will stabilize the radical adduct whereas a
perfluoroalkyl or acyloxy group next to the carbonyl moiety will enhance the
contribution given by the canonical structure with a charge separation to the
transition state (Equation 5.24).
Et3Si: O w CRR0 !Et3Si w O w C : RR 0 !Et3Si þ O w C : RR 0
(5:24)
The data for other silyl radicals are scant. Rate constants for Me3SiSi(:)Me2
radicals with benzil and propionaldehyde are 4:4 10 7 and 2:0 10 7 M 1 s 1 ,
respectively [14], whereas (TMS) 3Si: radicals are at least an order of magnitude
slower than Et3Si: radicals in the analogous additions to duroquinone (1:0
10 8 M 1 s 1 at 25 8C) and 3-pentanone ( 8 10 4 M 1 s 1 at 45 8C) [45].
X