Chemistry Reference
In-Depth Information
L80
. In addition, the selectivities obtained with this ligand were further improved
(entries 10 - 14).
8B.8.2.2.3. Allylic Alkylations of Alkylaluminium Reagents
Recently, this copper -
carbene-catalyzed allylation was used in the synthesis of baconipyrone C [371]. Key step
herein was a double asymmetric allylic alkylation using a
C
2
- symmetric substrate. Initial
experiments were carried out with Me
2
Zn and a racemic allyl phosphate to prove if the
allylation is solely controlled by the chiral ligand, or if kinetic resolution of the racemic
substrate occurs, but no reaction was observed with the Ag complexes of
L80
and
L81
(Table 8B.64, entries 1 and 2). Therefore, Hoveyda et al. switched to the more Lewis
acidic and nucleophilic Me
3
Al. Interestingly, with the Ag-
L80
- dimer, a strong kinetic
resolution was observed, and the
anti
-product was formed nearly exclusively with excel-
lent ee in 45% yield (maximum 50%). The ligand
L81
proved to be less effective. For
an application in the investigated synthesis, this kinetic resolution is not required, or
even detrimental. This caused a search for other ligands giving the required product
solely under ligand-controlled conditions. Best results were obtained with the chiral
NHC - sulfonate complex (Ag -
L82
)
2
(entry 5) [372] .
With this catalyst in hand, the
C
2
-symmetric diallylphosphate was subjected to the
same reaction conditions (Scheme 8B.101), giving rise to the methylated product with
excellent enantiomeric excess.
TABLE 8B.64. Asymmetric Allylic Alkylation of Methylzinc and Aluminium Reagents
OBn
OBn
OBn
R
n
M
CuCl
2
•2H
2
O (15 mol %),
L
,THF,
-
15°C, 24 h
OPO(OEt)
2
+
.
anti
syn
Entry
R
n
M
Ligand
Conversion (%)
anti
:
syn
ee (%)
1
Me
2
Zn
(Ag -
L80
)
2
—
2
Me
2
Zn
(Ag -
L81
)
2
—
3
Me
3
Al
(Ag -
L80
)
2
45
20:1
98
4
Me
3
Al
(Ag -
L81
)
2
15
9:1
n.d.
5
Me
3
Al
(Ag -
L82
)
2
95
1.5:1
89
n.d., not determined.
OR
OR
Me
3
Al (4 equiv)
CuCl
2
2 H
2
O (15 mol %),
(Ag-
L82
)
2
(7.5 mol %),
THF,
(EtO)
2
OPO
OPO(OEt)
2
15°C, 16 h
−
>98% ee (61%)
Scheme 8B.101.
Stereoselective allylic alkylation of
meso
- diallyl substrates.
