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

Intramolecular Formation of Carbon–Carbon Bonds (Cyclizations) 149
(AcO) n
O
Br
I
+
+
X
SO 2 Ph
X = CN, CO 2 Et
t-BuNC
(TMS) 3 SiH
AIBN, 80 �C
(TMS) 3SiH
AIBN, 80 �C
74 - 82%
(AcO) n
O
X
CN
25 - 73%
(7.10)
(7.11)
Heteroaromatic substitution can be successfully achieved by silicon hydrides
or silanes used as the radical mediators with alkyl bromides or iodides [20,21].
Heteroaromatic bases activated by protonation with trifluoroacetic acid react
with alkyl bromides or iodides and (TMS) 3SiH under thermal conditions (in the
presence of 1–2 equiv of AIBN in benzene), affording the desired product in
moderate to good yields. An example is given in Reaction (7.12). Here, the
stoichiometric quantity of AIBN ensured the rearomatization of the stabilized
cyclohexadienyl-type radical intermediate, which reaches a stationary concentration
suitable for intercepting the a-cyanoisopropyl radical, thus leading to the
substitution product. At the same time, the electrophilicity of a-cyanoisopropyl
radical prevents it from adding to the protonated heteroaromatic base, while
completing the course of this nonchain process [20]. This method is the first
report on the alkylation of heteroaromatic bases under non-oxidative conditions.
Br
+
N
H
CF 3 CO 2 −
(TMS) 3 SiH
AIBN (1-2 equiv),
80 �C
N
94%
(7.12)
Similar results can be reached under photochemical conditions (irradiation
with 400 W of visible light in CH2Cl2) using (TMS) 3SiH or (TMS) 4Si [21].
Other silanes are also successful mediators for alkylation of heteroaromatic
compounds. 1,1,2,2-Tetraphenyldisilane also operates under thermal decomposition
of AIBN [11], whereas PhSiH3, Ph2SiH2 and Et3SiH must be coupled
with thermal decomposition of peroxides as radical initiators due to the
stronger Si w H bonds [20].
7.3 INTRAMOLECULAR FORMATION OF CARBON–CARBON
BONDS (CYCLIZATIONS)
Cyclization reactions have boosted the development of free-radical strategies in
organic synthesis. The construction of five- and six-membered ring systems has