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

Cyclization Reactions of Silyl Radicals 125
could be a variety of atoms and substituents. The initial species 29 is a carboncentred
radical of alkyl, a-substituted alkyl, acyl or a-substituted acyl radical
types and its precursor preferably is a phenylseleno derivative, although bromides
and thiocarbonates have also been used [7–11].
t-Bu
t-Bu SiH
O
X Y Z
t-Bu Bu-t
R
H
Z
X
Y
R
5-exo-dig
5-endo-trig
t-Bu
t-Bu SiH R
O
Z
X
Y
29 30
t-Bu
t-Bu Si
O
32 31
H
radical
traslocation
Z
X
Y
Scheme 6.7 Sequential radical reactions including 5-endo-trig cyclization of silyl radical
A few examples are chosen in order to illustrate the potentialities of this
remarkable methodology. In Reaction (6.6) the sequence is initiated by the
removal of the PhSe group and the formation of a carbamoyl radical. It is
worth mentioning that the stereochemical outcome of these cascade reactions is
controlled by the stereochemistry of the oxygen-bearing asymmetric carbon in
29. Indeed, Reactions (6.6) and (6.7) show clearly the stereochemical control.
On the other hand, Reactions (6.7) and (6.8) illustrate the role of R which is
carried as a terminal group in the acetylenic moiety. For R ¼ Ph the last step is
the hydrogen abstraction, whereas for R ¼ SnBu3, the last step is the ejection of
Bu3Sn: radical (cf. Scheme 6.7).
t-Bu
t-Bu SiH
O
H
Ph
SePh
N O
slow addition
of Ph 3 SnH
AIBN, 80 �C
R
t-Bu Bu-t
Si
O
Ph
H H (6.6)
H
N
O
75%