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

88 Addition to Unsaturated Bonds
terminate by radical–radical combination or disproportionation reactions. For
more details on the radical initiators and radical chain reactions see Sections 4.2
and 4.1, respectively. The information in Scheme 5.1 can be extrapolated to any
hydrosilylation reaction of carbon–carbon, carbon–heteroatom, or heteroatom–heteroatom
multiple bonds.
R 3 Si
Y
R 3 SiH
H
R 3 SiH
R 3 Si•
R 3 Si
Scheme 5.1 Initiation and propagation steps for radical-based hydrosilylation
5.1 CARBON–CARBON DOUBLE BONDS
5.1.1 FORMATION OF SILYL RADICAL ADDUCTS
The EPR spectroscopic examination of the adduct radicals obtained by the
addition of silyl radicals to a large number of alkenes has been carried out, and
structural information has been derived [9,10]. b-Trialkylsilyl substituted alkyl
radicals adopt an eclipsed conformation with a sizeable rotational barrier. An
example is given for the adduct 1, obtained by the addition of Me3Si: radical to
acrylonitrile, in which the Arrhenius parameters for the hindered rotation
about the C w C bond is logA=s 1 = 12.5 and Ea ¼ 15:5kJ=mol [11]. Theoretical
studies at the UMP2=DZP þ BF level of theory on the structural properties of a
variety of b substituted ethyl radicals (e.g., H3SiCH2CH2:) have also been
performed [12]. The high population of the eclipsed conformation is a characteristic
of radicals having third-row substituents.
H
H
SiMe 3
1
In
Y
CN
H
In Table 5.1 some representative rate constants for the addition of silyl
radicals to alkenes are reported [13–15]. Inspection of these data reveals that
Y