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

126 Unimolecular Reactions
t-Bu
t-Bu SiH
O
N
Boc
R
SePh
slow addition
of R 3 SnH
AIBN, 80 �C
R = Ph
6.1.1 FIVE-MEMBERED RING EXPANSION
t-Bu Bu-t
Si
O Ph
H H
N
Boc
95%
t-Bu Bu-t
Si
O
H
R = SnBu3 N 71%
Boc
The above series of intramolecular hydrosilylations of alkoxysilanes allowed for
the formation of cyclic alkoxysilanes, which are very useful intermediates. For
example, they can be treated with fluoride ion under the conditions of Tamao
oxidation to yield diols [12]. Other established procedures for the preparation of
cyclic alkoxysilanes are the intramolecular hydrosilylation of alkoxysilanes
using transition metal catalysis [13], as well as the radical cyclization of 3-oxa-
4-silahexenyl radicals of type 32 [14–16] that has been successfully applied to the
synthesis of biologically important branched nucleosides [17,18] and C-glycosides
[19]. Interestingly, the cyclization of radical 32 afforded cyclic alkoxysilanes
35 and/or 36, whose relative percentages strongly depend on the
concentration of the reducing agent (Scheme 6.8). For example, at high concentration
of Bu3SnH the five-membered 35 is formed, whereas at a low Bu3SnH
concentration the six-membered 36 predominates. The independent formation
of radical 33 showed that the six-membered ring formation is an authentic ring
enlargement of radical 33 and not a direct cyclization of radical 32 [16].
Scheme 6.9 shows the radical clock methodology approach used for
obtaining the rate constant of the ring expansion (kre). Radical 37 was obtained
by the reaction of the corresponding phenylseleno derivative with (TMS) 3SiH.
A relative rate constant of kH=kre ¼ 20:2M 1 was obtained at 80 8C under firstorder
kinetics. Taking kH ¼ 1:2 10 6 M 1 s 1 at 80 8C, the kre value for the
ring expansion 37 ! 38 was calculated to be 6:1 10 4 s 1 at 80 8C [15].
Extensive mechanistic investigation of the ring expansion 33 ! 34 was
performed in order to differentiate between a ring-opening reaction to give a
silyl radical 39 (path a), followed by the 6-endo cyclization, or a pentavalent
silicon transition state 40 (path b). It was clearly demonstrated that the ring
expansion proceeds via a pentavalent silicon transition state (Scheme 6.10) [16].
(6.7)
(6.8)