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

Other Carbon–Heteroatom Multiple Bonds 109
addition of Et3Si:and Ph3Si: radicals to p-chlorobenzonitrile N-oxide gives
the corresponding persistent iminoxyl radicals in a syn:anti ratio that depends
strongly on the nature of the silyl group (Reaction 5.36). Moreover, the presence
of silyl groups produces a stabilization of syn isomer with respect to the
corresponding species where the silyl group is replaced by a hydrogen atom.
Cl
R 3 Si
+
C
N
O
p-Cl-C6H5 SiR3
C
N
O
+
p-Cl-C6H5 SiR3 C
N
O
syn anti
R = Et
R = Ph
syn:anti = 1:1.7
syn:anti = 1:0.7
(5.36)
The addition of silyl radicals to thiocarbonyl derivatives is a facile process
leading to a-silylthio adducts (Reaction 5.37). This elementary reaction is
the initial step of the radical chain deoxygenation of alcohols or Barton–
McCombie reaction (see Section 4.3.3 for more details). However, rate constants
for the formation of these adducts are limited to the value for the reaction
of (TMS) 3Si: radical with the xanthate c-C6H11OC(S)SMe (Table 5.3), a reaction
that is also found to be reversible [15]. Structural information on the asilylthio
adducts as well as some kinetic data for the decay reactions of these
species have been obtained by EPR spectroscopy [9,72].
Table 5.3 Kinetic data for the addition of silyl radicals to heteroatom-containing
multiple bonds
Substrate Silyl radical T(8C) k=M 1 s 1 Reference
Et3Si: 27 1:3 10
N
6 13
O
PhCH N
Et3Si: 27 7:1 10
CMe3 7 13
RO
S
(TMS) 3Si: 21 1:1 10
MeS
9 15
t-Bu N C O Et3Si: 27 5:5 10 6 13
Me N C O Me3Si: 109 2:3 10 6 77
O C O Me3Si: 109 3:2 104 77
O
Me N
Et3Si: 27 4:3 10
O
7 13
O
t-Bu N
(TMS) 3Si: 21 1:2 10
O
7 15