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

152 Consecutive Radical Reactions
The cyclization of d, e-unsaturated acyl radicals has been the research subject
of several groups [27]. The propagation steps for the prototype reaction are
illustrated in Scheme 7.4. The 5-exo:6-endo product ratio varies with the change
of the silane concentration due to the competition of hydrogen abstraction from
the silane with the ring expansion path.
Cl
O
(TMS) 3 Si
O
5-exo
6-endo
O
ring
expansion
O
(TMS) 3 SiH
(TMS) 3 SiH
Scheme 7.4 Propagation steps involving the cyclization of acyl radicals
Cyclization of secondary alkyl radicals can occur with a, b-alkynyl esters,
such as 12, and proceeds with high stereoselectivity to give predominantly
(Z)-exocyclic alkenes at low temperature upon reaction with (TMS) 3SiH
(Reaction 7.17) [28].
CO 2 Me
I
12
Me
n
(TMS) 3SiH Et3B, O2 −78 �C
n = 1
n = 2
CO 2Me
Me
+
MeO 2 C
n
n
(Z) (E)
85%, E:Z = 1:8
82%, E:Z = 1:10
Me
O
O
(7.17)
In the area of stereoselective processes, it is worth mentioning Reaction (7.18)
starting from acyclic precursor 13, where the origin of stereoselectivity could be
found on the transiency of radicals and their ability of reacting before racemization
or conformational changes. This principle is based on the knowledge of
lifetime and reactivity of radicals and is called ‘stereoselection at the steady
state’ [29].
Ph
13
Br
Br
(TMS) 3SiH
AIBN, 65 �C
+
Ph Me Ph Me
99%, trans:cis = 1.4:1
(7.18)