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

148 Consecutive Radical Reactions
an acyl radical intermediate that, in turn, abstracts hydrogen from the silane to
give the corresponding aldehyde. The successful application of carbonylation
procedures to tandem strategies will be reported in Section 7.7.
Br (TMS) 3SiH AIBN, 80 �C
+
O H
(7.8)
30 atm CO
15 atm CO
16%
29%
80 %
65%
(TMS) 3SiH has also been used as the mediator of C w C bond formation
between an acyl radical and an a, b-unsaturated lactam ester (Reaction 7.9).
The resulting ketone can be envisaged as potentially useful for the synthesis of
2-acylindole alkaloids [17]. Here, the effects of both H-donating ability and
steric hindrance given by the silicon hydride can be seen.
N
Me
O
SePh
+
CO2Bn O
N
CO2Bn (TMS) 3 SiH
AIBN, 80 �C
N
Me
O
72%
CO2Bn O
N
CO2Bn Intermolecular formation of C w C bond can be also coupled with b-elimination
process. In this case, the reaction partners are planned in order to
generate the alkyl radicals R:, which give the addition to unsaturated bonds.
The radical adducts obtained so far do not terminate by H donation from the
silane, but give the b-elimination process with loss of an appropriately designed
group X:. Radical X: in turn abstracts hydrogen from silicon hydride in order
to complete the radical chain. This strategy is carried out with success by the
proper choice of the X group (that can give a stabilized radical species) and the
hydrogen donor. Examples are given in Reactions (7.10) and (7.11), which
formally represent an allylation and a cyanation protocol, respectively. Radical
allylation with (TMS) 3SiH as the mediator and 2-functionalized allyl phenyl
sulfones, have from moderate to good yields, depending on the nature of the
starting materials [18]. Two examples are given in Reaction (7.10). The reaction
proceeded via addition of adamantyl radical to the double bond giving rise to
an intermediate that undergoes b-scission to form PhSO2: radical, which
abstracts hydrogen from the silane to regenerate (TMS) 3Si: radical. The glycosyl
radical cyanation (Reaction 7.11) yielded a-cyanoglycosides in variable
yields, depending on the configuration of sugar: d-galacto (73 %), d-manno
(40 %), d-gluco (71 %) and l-fuco (25 %). Only a-cyano anomers were formed
[19]. The key propagation steps for these transformations are: the addition of
glycosyl radical to the carbon atom of the isocyanide moiety, followed by a
fragmentation yielding a tert-butyl radical and the desired glycosyl cyanide.
tert-Butyl radical abstracts hydrogen from the silane to complete the chain.
(7.9)