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

96 Addition to Unsaturated Bonds
Ph
Ph
O
14
O
+ Ph3SiH OAc
OAc
AcO
AcO
O
SH
13
t-BuONNOBu-t, 60 �C
Ph 3Si
Ph
Ph
*
O
ee 95%
O
(5.12)
Thiol-promoted addition of a variety of silyl radicals to the aromatic moiety
of camptothecin (15) is also reported [32]. The addition occurs predominantly
at C7 and C12 positions depending on temperature. At 105 8C, mixture of
7-silyl (favoured) and 12-silyl camptothecins are formed alongside substantial
amounts of recovered camptothecin. At 160 8C, 12-silyl isomers are formed
preferentially, but the total mass balance is substantially reduced.
12
7
N
Et
OH O
15
The hydrosilylation mechanism and the formation of b-silyl alkyl radical
intermediate have been used for accomplishing other synthetically useful radical
reactions. The reactions of unsubstituted and 2-substituted allyl phenyl sulfides
with (TMS) 3SiH give a facile entry to allyl tris(trimethylsilyl)silanes in high
yields (Reaction 5.13, for X ¼ SPh). In this case, we have the addition of
(TMS) 3Si: radical to the double bond giving rise to a radical intermediate,
but the b-scission with the ejection of a thiyl radical is inserted in the mechanism,
thus affording the transposed double bond. Hydrogen abstraction from
(TMS) 3SiH by PhS: radical completes the cycle of these chain reactions [33]. By
an analogous mechanism, allylsilanes can be obtained from allyl phenyl sulfones,
although in a lower yields (Reaction 5.13, for X ¼ SO2Ph).
Z
X
(TMS) 3SiH
AIBN, 80�C
Z = H, Me, Cl, CN, CO2Et N
X = SPh
X = SO 2 Ph
O
O
(TMS) 3Si
Z
90 - 98%
45 - 82%
+ XH
(5.13)
As we anticipated in Section 5.1.1 the class of silylated cyclohexadienes has
recently been used as radical transfer hydrosilylating agents for some alkenes