Handbook of Functionalized Organometallics Applications in S

O O
O N
TIPDS
230
O
6 Polyfunctional Tin Organometallics for Organic Synthesis
low temperature with an Et 3B/O 2 initiation process giving interesting diastereoselectivities
(Scheme 6.35) [303].
O O
+ R I + Bu3Sn Et3B/O2 CH2Cl2,-78ºC O N
Scheme 6.35
6.4.3
Other Organotin Reagents
6.4.3.1 Tetraorganotins
R up to 70%
dr up to 3.7:1)
Some related reagent, the 2,4-pentadienyltin, was shown to be reactive as well
[304]. Propargyltin was equally found to be efficient for transferring an allene
group [305]. However, a larger excess of propargyltin is needed, due to the radical
isomerization of the propargyltin to the less reactive allenyltin. This was used in
the synthesis of modified nucleosides [306].
Vinyltins were used for synthetic purpose in radical addition/elimination
sequences. The main limitation comes from the necessity to functionalize the olefin
by suitable groups, such as phenyl [307] or esters [308], in order to stabilize the
transient carbon±centerd radical. This was applied to the stereoselective preparation
of 1¢-C branched nucleosides (Scheme 6.36) [309]. An intramolecular version
was also developed, giving access to methylene cyclopentane units [310].
O
O
O Br
O
O tBu
N NH
Scheme 6.36
O
+
Bu 3Sn
(5 eq)
Ph
(Bu3Sn) 2;hν O O
PhH, rt
O
N
TIPDS
O O
O
tBu
NH
O
Ph
71%
(E:Z = 10:1)
Tin enolates are in metalotropic equilibrium between the O- and C-stannylated
forms, so that the enolate form can be considered as the oxygenated analog of an
allyl tin. Thus, the S H2¢ reaction can be extended to tin enolates, used as electronrich
scavengers for carbon-centerd radicals [311]. A synthetically useful extension
of this reaction proposed the carbostannylation of alkenes with tin enolates [312],
which can be associated to cascade radical process (Scheme 6.37).