Handbook of Functionalized Organometallics Applications in S

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6 Polyfunctional Tin Organometallics for Organic Synthesis
[281]. This remains nowadays a particularly useful way for introducing various
functionalized allyl groups under mild and neutral conditions. The radical chain
mechanism involving allyltins is a S H2¢ mechanism that can be schematized as
following (Scheme 6.32): the initiation step (i), producing the tributyltin radical, is
done usually by thermolysis of radical initiators or photochemical irradiation of
Bu 6Sn 2. The reaction (ii), forming the initial carbon radical, admits various substrates
such as iodides, bromides, selenides, dithio- and thiocarbonates. Less reactive
substrates such as chlorides, benzoates or phenylthioethers can be used as
well, because the competitive addition of tributyltin radicals to the allyltin reagent
is degenerated due to the reverse b-fragmentation of the resulting radical. By the
way, less reactive substrates such as chlorides, benzoates or phenylthioethers can
be used efficiently as well. The additions of radicals to the allyltin (iv) are approximately
a hundred times slower than the hydride transfer [282], avoiding a premature
quenching of the carbon radicals, so that the evolution of the primarily
formed radical, via several intra- or intermolecular elementary steps (iii), is technically
possible, without using any slow addition or high dilution techniques [283].
This authorizes multicomponent intermolecular coupling reactions, involving
activated olefins [284], or carbon monoxide [285] for instance. Finally, a rapid
b-scission (v) with k f being likely over 10 6 s ±1 occurs to regenerate the tributyltin
radical.
R 2
R 2
Scheme 6.32
SnBu 3
v
SnBu 3
iv
Bu 3Sn
R 2
i
SnBu 3
iii
ii
R 1 -X
R 1
Bu 3SnX
The addition of carbon radicals to allylstannane is not severely affected by their
nature, whenever the electrophilic radicals attack the allyltin p bond more rapidly
than nucleophilic alkyl radicals [286], thus authorizing a wide range of substrates
to react. As a consequence, the radical allylic transfer was applied to the synthesis
of complexstructures such as b-lactams [287], steroids [288], alkaloids [289] or