Chemistry Reference
In-Depth Information
Figure 8B.11.
Allylic substitution with a prochiral nucleophile. (a) Diastereomeric transition states
yielding enantiomeric products using a convex ligand. (b) Concave ligand. (c) Ligand with a coor-
dinating side arm. (d) Stereocontrol by interaction with the nucleophile. (e) Formation of a product
with two stereogenic centers.
8B.2.2.4.1. β - Keto Esters and β - Diketones as Pronucleophiles
These pronucleophiles
have been used successfully with a variety of Pd catalysts since Kagan's pioneering work
in 1978. In 1997, Trost et al. showed that high enantioselectivity can be reached with the
help of the ligand
DPPBA - 1
(
L1a
) [126]. The development up to 1999 has been exhaus-
tively reviewed by Trost and Lee [14].
The early achievements of Trost et al. and more recent results of others are schemati-
cally described in Table 8B.14. The best results were generally obtained with tetralones
(up to 99% ee). Ito et al. described further examples using
BINAP
(
L23a
) as ligand,
which gave rise to enantioselectivities in the range 80-90% ee. The authors described a
few examples of reactions with acyclic β-diketones in addition to the customary cyclic
β-keto esters. Excepting a single example, moderate selectivities were reached with the
ligand
QUIPHOS
(
L24
) [127] .
Hamada et al. achieved excellent results (typically 90-95% ee) with 2-carbomethoxy-
cycloheanone and α-tetralone using the ligand
Ph - DIAPHOX
(
L18
) (cf. Fig. 8B.12).
Essential for success with this unusual ligand was the application of the BSA method
and Zn(OAc)
2
as additive. The authors proposed that the ligand is silylated
in situ
to
give a monodentate ligand, which gives rise to a Pd complex as described in Figure 8B.12.
Thus, the good selectivities are probably the consequence of concave embedding of the
allylic moiety according to Figure 8B.11b or c.
