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

Silicon Hydride / Thiol Mixture 79
X
(Me 3Si) 2Si(H)SSiMe 3
X=Br, I, OC(S)SMe, SePh, CN
4.5 SILICON HYDRIDE / THIOL MIXTURE
AIBN, 85 �C (4.72)
>_ 97%
The low reactivity of alkyl and/or phenyl substituted organosilanes in reduction
processes can be ameliorated in the presence of a catalytic amount of alkanethiols.
The reaction mechanism is reported in Scheme 4.6 and shows that
alkyl radicals abstract hydrogen from thiols and the resulting thiyl radical
abstracts hydrogen from the silane. This procedure, which was termed polarity-reversal
catalysis, has been applied to dehalogenation, deoxygenation and
desulfurization reactions [115].
R + XSH RH + XS
XS + Et3SiH XSH + Et3Si Et3Si + RZ Et3SiZ + R
Scheme 4.6 Propagation steps for polarity-reversal catalysis
For example, 1-bromoadamantane is quantitatively reduced with 2 equiv of
triethylsilane and in the presence of a catalytic amount of tert-dodecanethiol
(Reaction 4.73). Similarly desulfurization Reaction (4.74) occurs readily. Although
generally these reactions do not work in the absence of thiol, the
reduction of xanthate derivatives give similar yields in the presence or absence
of thiol. To account for this result, it was suggested that Et3SiSH is formed in
situ by side reactions, thus obviating the need to add a separate thiol catalyst
[116]. Indeed, a variety of alkyl and/or aryl substituted silanes (R3SiH) react
under normal free radical conditions with carbonyl sulfide (O
w C S), a by-
w
product of Barton–McCombie deoxygenation, to give the corresponding silanethiol
(R3SiSH) (see also Section 5.5) [117].
Br
Et 3 SiH (2 equiv)
t-C 12 H 25 SH (2 mol%)
t-BuONNOBu-t, 70 �C
99%
(4.73)