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

Cyclization Reactions of Silyl Radicals 127
R 2
R 1
O SiPh 2
32
5-exo-trig
R 1
R 1
R 2
R 2
SiPh2 O
SiPh2 O
ring
expansion
R 2
R 1
33 34
Bu 3SnH Bu 3SnH
R 2
R 1
35 36
O SiPh 2
O SiPh 2
Scheme 6.8 Preferred mode of radical 32 cyclization and subsequent ring expansion
k H
H
O
SiPh2 H
(TMS) 3 SiH
H
O
SiPh2 H
k re
37 38
H
H
H
O
SiPh2 (TMS) 3 SiH
H
O
SiPh2 Scheme 6.9 Radical clock methodology approach for measuring the rate constant of ring
expansion (cf. Section 3.1.1)
The above-described ring expansion reaction could play an important role in
the outcome of silyl radical cyclization. Indeed, a cyclic product could derive
first from the exo-mode and then transformed to the endo-mode product via the
ring-expansion pathway (Scheme 6.11). For example, the products reported in
Schemes 6.1 and 6.2 could be interpreted by this cyclization–expansion process,
because of the experimental conditions (the relatively high temperature and
absence of a hydrogen donor). On the other hand, products from both modes of
cyclization are observed in the cyclization of silane 15 (Reaction 6.4). In this
case, although thiols are very good hydrogen atom donors, the catalytic amount
used could have not been enough to suppress completely the ring expansion