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

Intramolecular Formation of Carbon–Carbon Bonds (Cyclizations) 153
Taking advantage of the high reactivity of alkoxyl radicals towards organosilanes,
which is 2–3 orders of magnitude higher than that of primary alkyl
radicals (see Chapter 3), cyclization can start from halocarbonyl compounds
using PhSiH3 (Reaction 7.19) or (TMS) 3SiH [30]. Both 6-exo-trig and 5-exo-trig
cyclizations of alkyl radicals to the carbonyl moieties can be accomplished, with
aldehydes working better than ketones.
O H HO
Br
PhSiH3 (4 equiv)
n n
THF reflux,14h
MeO2C CO2Me MeO2C CO2Me n = 1, 2
85-93%
(7.19)
In Reaction (7.20) is reported the cyclization of a thermally unstable propargyl
bromide cobalt complex mediated by Ph2SiH2 at room temperature and
Et3B=O2 as the radical initiator. However, a mixture of reduced and bromine
atom-transfer products (1:1.8 ratio) are isolated due to the low hydrogen
donation of the employed silane [31].
Ph
(CO) 6 Co 2
Br
CO 2 Me
Ph2SiH2 Et3B, O2 , r.t.
MeO2C Ph H(Br)
(CO) 6 Co 2
70%
(7.20)
Silyl radical adds across the double or triple bonds to generate carboncentred
radicals ready for cyclization reactions. Two examples of hydrosilylation/cyclization
of dienes are given by using Et3SiH or the silylated cyclohexadienes
14 as reagents. Reaction (7.21) shows an example of the bridged bicyclic
formation by initial attack of Et3Si: radical on the terminal alkene [32]. Reaction
(7.22) shows the cyclization of a 1,6-diene and how flexible is this methodology
to introduce any silyl group in the reaction products [33] (cf. Section 4.7).
Yields are moderate to good and the cis:trans ratio of ca 4:1 does not change
substantially with the nature of silylating group as expected. Another potentiality
for the construction of carbocycles is based on the addition of silyl radicals
to the carbonyl group followed by cyclization. Starting from ketoalkene 15, the
formation of a five-membered ring is obtained in good yield and discrete
diastereoselectivity (Reaction 7.23) [34].
O
OMe
CO 2Me
Et 3 SiH
t-BuOOBu-t,
140 �C
O
MeO
CO 2 Me
68%
SiEt 3
(7.21)