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

112 Addition to Unsaturated Bonds
The adducts of azodicarboxylate with Et3Si: or Ph3Si: radicals [79], have
been identified as hydrazyl radicals (Reaction 5.41). The addition of R3Si:
radical to alkyl azides takes place at the terminal nitrogen (Reaction 5.42)
affording the triazenyl s-type radical in which the SOMO is in the NNN
plane (only one canonical structure is shown) [80,81].
R'O2C N N
CO2R' R' N N N
R 3 Si
R 3 Si
R'O2C N N
R3Si CO2R' R' N N N SiR 3
(5.41)
(5.42)
The adducts derived from the reaction of a variety of alkyl or alkoxyl
substituted silyl radicals with aminoboranes have also been recorded [82]. The
silyl radical addition takes place at the boron site to give the aminyl-borane
radical (Reaction 5.43) and structural information for this class of radicals has
been obtained.
N BH 2
R 3 Si
N BH 2 SiR 3 (5.43)
The addition of silyl radicals to the N O bond is very well documented.
w
Indeed, silyl radicals add rapidly to aromatic, heteroaromatic and aliphatic
nitro compounds to give persistent trialkylsilyloxy nitroxide radicals [9].
Rate constants for the addition of Et3Si: radical to MeNO2 [13] and of
(TMS) 3Si: radical to t-BuNO2 [15] are in the range of 107 –108 M 1 s 1 , respectively,
at room temperature (Table 5.3). Structural information about a large
number of adduct radicals as well as some kinetic information associated with
these species have been obtained by EPR spectroscopy [83,84]. For example,
Et3Si: radicals add to 2-nitrothiophene to give the silyloxynitroxide that was
detected both in the cis and trans arrangements (Reaction 5.44). For the
interconversion of the two conformers, an activation energy of 49:3kJ=mol
was obtained [85].
S
N OSiEt 3
O
cis trans
S
N O
OSiEt3 (5.44)
Detailed kinetic studies as a function of temperature have been carried out on
the fate of radicals 58 and 59, derived from the addition of Ph3Si: and
(TMS) 3Si: to t-BuNO2 and MeNO2, respectively [86,87]. Activation parameters
for the first-order decay of radical 58 ( log A=s 1 ¼ 11:1 and
Ea ¼ 78:7kJ=mol) and radical 59 ( log A=s 1 ¼ 13:5 and Ea ¼ 80:5kJ=mol)