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

90 Addition to Unsaturated Bonds
approaches has been reported [17]. The reaction enthalpy (DHr) at AM1 level of
theory for Me3Si: with acrylonitrile, butyl vinyl ether and styrene was calculated
to be 96:2, 100:4 and 115:9kJ=mol, respectively, whereas the analogous
DHf values for Me3SiSi(:)Me2 were calculated to be 94:6, 70:3 and
106:7kJ=mol, respectively. Transition states for Me3SiSi(:)Me2 attack at the
terminus of acrylonitrile and butyl vinyl ether were calculated to have attack
distances of 3.096 and 3.008 A ˚ , respectively. Due to the high exothermicity of
these reactions, it is not surprising to see that the addition of Me3Si: radicals to
1-hexene is irreversible at temperatures as high as 140 8C [18].
However, (TMS) 3Si: radicals are found to add to a variety of double bonds
reversibly and therefore to isomerize alkenes [19]. An example is shown for the
interconversion of (E)- to (Z)-3-hexen-1-ol and vice versa by (TMS) 3Si: radicals
(Reaction 5.1). Figure 5.1 shows the time profile of this reaction under
standard experimental conditions (AIBN, 80 8C). The equilibration of the two
geometrical isomers is reached in ca 10 h, and the percentage of Z=E ¼ 18=82
after completion corresponds to an equilibrium constant of K ¼ 4:5: The
difference in the stability of the two isomers in 2-butenes, i.e., DG8(E-isomer)
– DG8 (Z-isomer) = 3:1 kJ/mol, corresponds to K ¼ 3:5, since
DG ¼ RT ln K. Therefore, the observed final composition should account
for the difference in the stability of the two isomers [20]. It is worth noting
that care must be taken in synthetic strategies, since isomerization can occur in
situ, while accomplishing other reactions [19].
HOH 2CH 2C
CH 2CH 3
(TMS) 3 Si
HOH 2 CH 2 C
CH 2CH 3
The addition of silyl radicals to double bonds in benzene or substituted
benzenes (Reaction 5.2) is the key step in the mechanism of homolytic aromatic
substitution with silanes [8,9]. The intermediate cyclohexadienyl radical 2 has
been detected by both EPR and optical techniques [21,22]. Similar cyclohexadienyl-type
intermediates have also been detected with heteroaromatics like
furan and thiophene [23].
R 3Si +
2
SiR3 H
The addition of a trialkylsilyl radical to benzene is much less exothermic than
the addition to a non-activated alkene; the DHr of these reactions has been
evaluated to be ca 50 kJ=mol [9,18]. However, the rate constants for the
addition of Et3Si radicals to aromatic and heteroaromatic compounds are
similar to those of non-activated alkenes, i.e., 10 6 M 1 s 1 [24]. Furthermore,
both electron-withdrawing groups and electron-donating groups accelerate the
addition reaction thus indicating that reactivity is determined mainly by the
(5.1)
(5.2)