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

Heteroatom–Heteroatom Multiple Bonds 111
order R ¼ Me > Et > i-Pr > t-Bu at 109 8C, and this trend was proposed to
be steric in origin [76]. In a similar manner, silyl radicals were found to add to
carbon dioxide to give the radical adduct 57 (Reaction 5.39) [77]. The rate
constant for the addition of Me3Si: radical to MeN
w C O is two orders of
w
magnitude faster than the addition to O
w C Oat 109 8C (see Table 5.3).
w
R3Si + R'N C O R'N
R3Si + O C O O
56
57
OSiR 3
OSiR 3
(5.38)
(5.39)
The addition of trisubstituted silanes to carbonyl sulfide has been applied to
the synthesis of the corresponding silanethiol derivatives (Reaction 5.40) [78].
In Scheme 5.12 the mechanism is depicted, starting from the silyl radical attack
to the sulfur atom of O
w C S and ejection of carbon monoxide. The resulting
w
silanethiyl radical abstracts hydrogen from the starting silane, to give the
silanethiol and to generate ‘fresh’ silyl radical (see Section 3.4).
R3SiH + S C O
R = alkyl and/or aryl
O C S
R 3Si
R 3SiSH
t-BuONNOBu-t
60 �C
R 3SiS
O
R 3SiS
R 3SiH
Scheme 5.12 Propagation steps for the formation of silanethiols
R 3SiSH
> 60%
5.6 HETEROATOM–HETEROATOM MULTIPLE BONDS
CO
(5.40)
The addition of silyl radicals to nitrogen–nitrogen and nitrogen–heteroatom
double bonds has mainly been investigated by EPR spectroscopy.