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

Intermolecular Formation of Carbon–Carbon Bonds 145
similarities with tin reagents (Reactions 7.1 and 7.2) [6,7]. For example, using a
1:1 ratio of cyclohexyl iodide and electron-poor terminal alkenes together with
1.2 equiv of (TMS) 3SiH the desired product was obtained in a good yield. It is
worth pointing out that with iodides the reaction yields are comparable with
those obtained by the tin method, whereas with (TMS) 3SiH the use of alkyl
isocyanides as radical precursors has been possible.
I
NC
+
X
X = CN, CO 2 Me, COMe
+
X
(TMS) 3SiH AIBN, 80 �C
(TMS) 3SiH AIBN, 80 �C
X = CN, CO 2 Me, SO 2 Ph, PO(OEt) 2
85-90%
50-60%
The intermolecular C w C bond formation mediated by (TMS) 3SiH has been
the subject of several synthetically useful investigations, with the scope of biological
testing. A simple example is given in Reaction (7.3) for the synthesis of
bishomolithocholic acid from the corresponding iodo derivative (1), which
needed a modification on the steroid side chain to be introduced [8]. The ease
of isolation and purification of this steroidal analogue allowed a smooth procedure
to the subsequent biological assays. The effect of the bulky (TMS) 3SiH can be
appreciated in the reaction of b- org-substituted a-methylenebutyrolactones
with n-BuI (Reaction 7.4) [9]. The formation of a, b- ora, g-disubstituted lactones
was obtained in good yield and a good diastereoselection is observed when
one of the substituents is a phenyl ring. The prediction of stereochemistry is
simplified by considering the conformation of the cyclic radical intermediate
involved in the reduction step of the reaction sequence. The anti rule can be
successfully applied, as in the majority of the cases involving cyclic radicals, that
is, H donation preferentially occurs in anti fashion to the substituents, in order to
reduce steric interference with bulky silicon hydride. As a matter of fact, for b-or
g-alkyl substituted substrates the same reaction was found to be less selective.
AcO
H
H
1
H
H
I
(TMS) 3SiH H2C=CHCO2Me AIBN, 70 �C
AcO
H
H
H
H
64%
X
X
(7.1)
(7.2)
CO 2 Me
(7.3)