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

Cyclization Reactions of Silyl Radicals 121
7
Si(H)Me2 X
t-BuOOBu-t
145 �C
X = O
X = SiMe 2
SiMe2 X
24%
23%
The endo-mode of cyclization is found to be the preferred path also in the
lower homologues. Reaction (6.2) shows the reactions of two silanes (8) with
thermally generated t-BuO: radicals to afford the five-membered ring in low
yields via a 5-endo-trig cyclization [1]. EPR spectra recorded from these two
silanes with photogenerated t-BuO: radicals are assigned to secondary alkyl
radical intermediates formed by an intermolecular addition involving the
expected silyl radical and the parent silane [2].
X Si(H)Me 2
8
t-BuOOBu-t
145 �C
X = CH 2
X = SiMe 2
X SiMe 2
18%
13%
Irradiation of trisilane 9 in cyclohexane with 254 nm light generates a very
persistent radical, whose EPR spectra assigned to structure 11 (Scheme 6.3).
The quenching of radical 11 with ethanethiol gave the expected silacyclobutene
12 as a single diastereoisomer, whereas the reaction with molecular oxygen
afforded the lactone 13 [4]. It was suggested that upon homolytic cleavage of
Si w SiMe2Ph in 9, the initially formed silyl radical 10 undergoes a rearrangement
to the more stable silacyclobutenyl radical 11. The proposed rearrangement
can be viewed as a 3-exo-trig cyclization of radical 10 followed by a
specific fission of the internal C w C bond in the constrained bicycle intermediate
to give radical 11.
Allyloxysilanes (14) undergo radical chain cyclization in the presence of ditert-butyl
hyponitrite as radical initiator and thiol as a catalyst at ca 60 8C
(Reaction 6.3) [5]. The thiol promotes the overall abstraction from the Si w H
moiety as shown in Scheme 6.4 and the silyl radical undergoes a rapid 5-endotrig
cyclization. Indeed, EPR studies on the reaction of t-BuO: radical with
silanes 14 detected only spectra from the cyclized radicals even at 100 8C,
which implies that the rate constants for cyclization are 10 3 s 1 at this
temperature.
R
Si(H)Ph2 O
14
t-BuONNOBu-t
R = H t-C12H25SH (cat.), hexane, 60 �C 95%
R = Me Ph3SiSH (cat.), dioxane, 65 �C 95%
R
SiPh2 O
(6.1)
(6.2)
(6.3)