Chemistry Reference
In-Depth Information
In 2005, Hou et al. reported on highly stereoselective allylations of acyclic ketones in
the presence of the ferrocene-based bidentate ligand
L32
[157]. The choice of the base
was essential for good results. Lithium bases proved to be more suitable than those
containing potassium or sodium; LiHMDS was found to be the base of choice. Excellent
yields and selectivities were obtained with a wide range of ketones, especially if AgBr
(10 mol %) was added to the reaction mixture. The best enantioselectivities were obtained
with
-alkoxy-substituted ketones, probably because of the formation of a (
Z
) - chelated
enolate (Scheme 8B.37 ).
α
O
OMe
Ph
O
LHMDS
[Pd(
π
-C
3
H
5
)Cl]
2,
L32
,
AgBr, THF, -20°C
Fe
N
N
OAc
Ph
P
Ph
2
Fe
OMe
Ph
2
L32
93% ee (98%)
Scheme 8B.37.
Stereoselective allylic alkylations of acyclic ketone.
In 2007, the same group used ligand
L33
for highly selective reactions using cinnamyl
carbonates [158]. With these, the regioselectivity of the nucleophilic attack also has to
be controlled. Best results were obtained if additional LiCl was added to the lithium
enolates (Table 8B.21). The branched products were formed nearly exclusively (regiose-
lectivity
>
98:2) and with good
anti
- selectivity and excellent ees.
8B.2.3.2. Allylic Alkylations of Ester Enolates
The fi rst example is due to Hegedus
et al. in 1980 [159]. They investigated the allylation of branched ester enolates in the
presence of various ligands. First examples were disappointing, providing only low yields.
The enolate probably added preferentially to the metal rather than the allyl group [160],
TABLE 8B.21. Regio - and Enantioselective Allylic Alkylations of Acyclic Ketones
O
R
1
Fe
R
2
O
LHMDS
[Pd(
OH
O
R
2
OCO
2
Et
P
O
R
1
-C
3
H
5
)Cl]
2
,
(
R
,
R
)-
L33
,LiCl,
DME, -5°C
π
NEt
2
N
(
R
,
R
)-
L33
Entry
R
1
R
2
Yield (%)
anti
:
syn
ee (%)
1
Ph
Ph
83
9:1
98
2
Ph
85
10:1
99
m
- BrC
6
H
4
3
Ph
1 - Naphthyl
83
20:1
99
4
1 - Naphthyl
Ph
89
21:1
93
5
c
- Hex
p
- OMeC
6
H
4
72
5:1
99
