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

Carbon-Oxygen Double Bonds 103
cyclohexanone that affords mainly the trans isomer indicating that the axial H
abstraction is favoured [51].
t-Bu
O
(TMS) 3 SiH
AIBN, 80 �C
hν, 20 �C
t-Bu
H
OSi(TMS) 3
trans:cis = 91:9
trans:cis = 98:2
(5.25)
The stereochemical outcome of (TMS) 3SiH addition to chiral ketones can be
predicted by the Felkin–Ahn model. This model, usually applied to the nucleophilic
addition to a carbonyl group, has been extended to the case of the H
abstraction step involving the a-silyloxyalkyl radical [52–54]. Indeed, the
hydrosilylation of ketone 29 with this silane leads predominatly to 30 via the
intermediate radical 32, which is bent in the ground state in accord with EPR
measurements and theoretical calculations [53]. Scheme 5.6 shows for comparison
the classic experiment by Cram for the reduction of ketone by LiAlH4 and
the radical counterpart with (TMS) 3SiH in the presence of thiol as the catalyst,
which furnishes the hydrogen atom. The syn:anti product ratios obtained from
hydride ion (34:35) and hydrogen atom abstraction (37:38) are very similar [52].
In summary, the 1,2-stereoinduction in carbon-centred radicals bearing an asilyloxy
substituent follows the Felkin–Anh rule (syn product).
Me
Ph
O
Me
(TMS) 3 SiH
30 �C
H Me
Me OSi(TMS) 3
Me
OSi(TMS) 3
Me +
Ph
Me
29 30 31
64%, 30:31 = 4:1
Ph
32
Ph
OSi(TMS) 3
Me
(5.26)
It has been reported that (TMS) 3SiCl can be used for the protection of
primary and secondary alcohols [55]. Tris(trimethylsilyl)silyl ethers are stable
to the usual conditions employed in organic synthesis for the deprotection of
other silyl groups and can be deprotected using photolysis at 254 nm, in yields
ranging from 62 to 95 %. Combining this fact with the hydrosilylation of
ketones and aldehydes, a radical pathway can be drawn, which is formally
equivalent to the ionic reduction of carbonyl moieties to the corresponding
alcohols.
Reactions (5.27) and (5.28) provide good evidence for the participation of
radical intermediates in the initial hydrosilylation process [51,56]. Indeed, the