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

Carbon–Carbon Double Bonds 89
the addition of silyl radicals to double bonds is a remarkably facile process. The
rate constants for the reaction of Et3Si: radical with non-activated olefins are
ca 10 6 M 1 s 1 , whereas olefins having the double bond next to a p-electron
system or to an electron-withdrawing substituent are substantially more reactive.
Kinetic data for silyl radicals other than triethylsilyl are scant. In Table 5.1
these limited data are reported, and the reactivity trend, i.e.,
Et3Si: > Me3SiSi(:)Me2 > (TMS) 3Si:, observed for a group removal from a
particular alkyl halide (cf Chapter 4) seems to hold for the addition to a
particular alkene, too.
Arrhenius parameters are available for the reaction of Et3Si: radicals with
H2C w CH(CH2) 3CH3,H2C w CHPh and H2C w CCl2 [13]. Their preexponential
factors are between 10 9:0 and 10 9:4 M 1 s 1 , which implies that the transition
states for these reactions are fairly ‘loose’. The high reactivity of silyl radicals
towards olefins has been explained in terms of polar effects in some detail [16].
The addition of silyl radicals to alkenes is also a strongly exothermic reaction. It
is estimated from thermochemical background that the addition of Me3Si:
radicals to simple CH2 w CH2 is exothermic by 105 kJ/mol [16]. The addition of
Me3Si: and Me3SiSi(:)Me2 radicals to olefinic monomers using semiempirical
Table 5.1 Rate constants (M 1 s 1 ) for the addition of silyl radicals to alkenes
Substrate Et3Si:a Me3SiSi(:)Me 2 b (TMS) 3Si:c
Cl
CN 1:1 10 9 6:3 10 7
CO 2Me 9:7 10 7
CO 2Me
Cl
4:6 10 8
2:7 10 8
Ph 2:2 10 8 5:9 10 7
O 1:2 10 6
a From Reference [13].
b From Reference [14].
c From Reference [15].
1:4 10 8b 7:6 10 7
4:8 10 6 3:9 10 6
9:6 10 5
9:4 10 5