Handbook of Functionalized Organometallics Applications in S

N
H
91% (ee:84%)
Scheme 6.31
N
CO 2R
1) All4Sn, SnI4 ClCO2R CH 2Cl 2,-78ºC
2) NaOH, rt
N
COR*
6.4 Radical Reactions of Organotins
N
1)
SnBu 3
ClCO2R CH2Cl2,-78ºC 2) NaOH, rt
6.3.2.3 Catalytic Enantioselective Addition
Organotins are involved in the increasing work related to the catalytic enantioselective
addition to imines [275]. The first example of catalytic, enantioselective allylation
of imines was reported by Yamamoto and coworkers by using 5% of bis
p-allyl palladium complex[276]. Contrary to the BINAP ligand, which was found
to be totally ineffective under these conditions, b-pinene ligands used as nontransferable
allyl ligands gave up to 81% ee. Nevertheless, it was shown that Tol-
BINAP-Cu I catalysts were also efficient for the allylation of N-tosyl imines [277]
giving access to a-amino acids with up to 98% ee. Finally, a polymer-supported
p-allyl palladium catalyst was developed, showing promising results in terms of
stability and reusability, although leading to a moderate ee (13±47%) [278].
6.4
Radical Reactions of Organotins
6.4.1
Introduction
The radical chemistry of organotins is overwhelmed by the tin hydride chemistry.
In the past decades, the knowledge of kinetic parameters authorized the expeditious
construction of complexmolecules by using cascade radical reaction based
on Bu 3Sn methodology. Moreover, these strategies also offered an excellent diastereocontrol,
especially for the construction of polycyclic skeletons. These synthetic
applications of Bu 3SnH, which will not be covered in this chapter, were
reviewed in recent years [279]. In addition to the tin hydride chemistry, there are
several applications of organotins in radical syntheses involving mainly allylstannanes.
6.4.2
Allyltins
6.4.2.1 Mechanistic Overview
Whereas the demonstration of the ability of allyltin reagents to undergo homolytic
cleavage of the carbon±tin bond goes back to the early 1970s [280], it was only ten
years later that Keck and Yates evidenced the synthetic potential of this reaction
N
H
227
N
98% (ee:86%)
CO 2R