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

Application to Tandem and Cascade Radical Reactions 175
O
EtO I
+
(Z)- or (E )-isomer
I
+
O
CN
(TMS) 3SiH,
20 atm CO
AIBN, 80 �C
(TMS) 3SiH,
20 atm CO
AIBN, 80 �C
EtO
O
O
45%
O
O
(E)-isomer, 50%
CN
(7.77)
(7.78)
Supercritical carbon dioxide (scCO2) is an emerging reaction medium for
free-radical reactions for a number of reasons, such as the pressure-tunable cage
effect, the absence of radical chain transfer to the medium [88], and unique
properties such as high diffusivity and high miscibility of reactant gases [89].
Since (TMS) 3SiH does not react with CO2 and CO is highly soluble in scCO2,it
is thought that radical carbonylation in scCO2 should be characterized by high
tenability of product distribution [90]. Indeed, the extension of the threecomponents
coupling reaction in this reaction medium gave excellent results
as shown in Reaction (7.79).
I
+ X
(TMS) 3SiH, 50 atm CO
AIBN, 80 �C, 5h
scCO2 (305 atm)
O
X = CN, CO2Me, C(O)Me 90 - 96%
X
(7.79)
The addition of (TMS) 3SiH to the substituted 1,5-dienes in the presence of
CO (Reaction 7.80) is an intramolecular version of the above mentioned
strategy and represents an example of (4 þ 1) radical annulation process [91].
Indeed, the initial addition of silyl radicals to the least substituted terminus is
followed by carbonylation and acyl radical cyclization prior to hydrogen abstraction.
This methodology has been further developed for the preparation of
cyclic ethers (Reaction 7.81). Indeed, the starting compound 73, derived from
the easy regioselective ring opening of appropriate epoxides with benzeneselenolate
followed by hetero-Michael addition to ethyl propiolate, undergoes
radical tandem carbonylation/cyclization reaction in the presence of
(TMS) 3SiH and CO (80 atm) [92].
(TMS) 3 SiH,
50 atm CO
AIBN, 80 �C, 5h
(TMS) 3 Si
O
85%, trans:cis = 7:3
(7.80)