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

110 Addition to Unsaturated Bonds
S
X Y
+
R 3 Si
S
X Y
5.5 CUMULENES AND HETERO-CUMULENES
SiR 3
(5.37)
Addition of silyl radicals to cumulenes and their isoelectronic derivatives has
mainly been studied by EPR spectroscopy. The adducts of Me3Si: radical with
the two substituted allenes 54 and 55 have been recorded [73,74]. The attack
occurs at the central atom affording unconjugated allyl-type radicals. In particular
the adduct radical with 55 has been described as a very persistent
‘perpendicular’ allyl radical [74].
Me
Me
C C C
Me
Me
t-Bu
C C C
t-Bu
54 55
A theoretical study of the reactivity of prototype ketene CH2
w C O with
w
several radicals including H3Si: has been reported. Scheme 5.11 shows that
three adducts are possible with the corresponding energy barriers for their
formation calculated at B3LYP/6-31G * level of theory [75]. However, all the
levels of theory used in this study predicted that silyl radical prefers to add to
the carbon terminus of ketene.
E a = 13.8 kJ/mol
H 3SiCH 2C O
H 3 Si
+
26.4
kJ/mol
H 2 C
H 2C C O
SiH 3
Scheme 5.11 Theoretical study of the reactivity of ketene
O
46.9 kJ/mol
H 2 C C
OSiH 3
Trialkylsilyl radicals add to alkyl isocyanate to form imidoyl radicals 56
(Reaction 5.38). Detailed EPR studies established intermediates 56 to be
strongly bent at the carbon bearing the unpaired electron [76]. The absolute
rate constant for the reaction of Et3Si: radical with tert-butyl isocyanate was
found to be 5:5 10 6 M 1 s 1 at 27 8C [13], whereas the relative rate of the
addition of Me3Si: radicals to alkyl isocyanates was found to decrease in the