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

108 Addition to Unsaturated Bonds
5.4 OTHER CARBON–HETEROATOM MULTIPLE BONDS
Addition of silyl radicals to carbon–nitrogen multiple bonds has mainly been
investigated by EPR spectroscopy [9,66].
Studies on the addition of silyl radicals to compounds containing C
w N
bonds are quite extensive [66]. Silicon-centred radicals add to the C N moiety
w
either at the nitrogen or at the carbon atom depending on the nature of the
substituents (Scheme 5.10). In the majority of cases, the addition at the nitrogen
atom is the preferred one as is expected thermodynamically. Furthermore, it has
been shown by EPR that 1,2-migration of the Me3Si group from carbon to
nitrogen in the Me3SiCH2N(:)R occurs readily (for R ¼ H, the rate constant is
estimated to be 3 103 s 1 at 27 8C), and it is sensitive to the presence of
sterically large groups at the nitrogen atom (for R ¼ t-Bu, the rate constant is
estimated to be 3 101 s 1 at 27 8C) [67].
C N
SiR3 C N
+ R3Si silyl migration
R3Si C N
Scheme 5.10 Reaction paths for the addition of silyl radicals to C N double bonds
w
The adduct of silyl radicals to 4-substituted pyridines and pyrazine monitored
by EPR results from the attack at the nitrogen atom to give radicals 52
and 53, respectively [68,69]. The rate constant for the addition of Et3Si: radical
to pyridine is about three times faster than for benzene (Table 5.3) [24].
N SiR3 N N SiR3 52 53
The addition of the Et3Si: radical to the C N bond of nitrones also occurs
w
readily. A rate constant of 7:1 107 M 1 s 1 at 27 8C has been obtained for
Reaction (5.35), whereas information on the structure of the adduct radicals has
been obtained by EPR spectroscopy [13,70].
O
Ph O
Et3Si + PhCH N
CH N
(5.35)
CMe Si CMe
3
3
The EPR technique has also been employed to investigate the adducts of the
reaction of silyl radicals with various nitrile-N-oxides [71]. As an example, the