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On the other hand, a tungsten catalyst prepared from (EtCN)
3
W(CO)
3
and bipy
(25 mol % each) provided the secondary monosubstitution product in high yield. Inter-
estingly, if (
Z
)-allyl dicarbonates were used as substrates, substitution occurred with
complete retention of the olefi n geometry, that is, in strong contrast to the result of the
Pd-catalyzed reaction. This observation clearly indicates that
π
-
σ
-
π
isomerization does
not occur under the reaction conditions used.
It was also found that (
Z
)-substrates are more reactive than the corresponding (
E
) -
analogues [241]. Similar selectivities were also obtained with other nucleophiles such as
-keto esters and substituted malonates. In contrast to Mo-catalyzed reactions [209],
sulfone-stabilized anions can be used as well. (C
7
H
8
)W(CO)
3
was found to be a more
convenient catalyst precursor than (MeCN)
3
W(CO
3
), since it is air stable, sublimable,
and readily soluble in THF [239c].
β
8B.4.2.2. Asymmetric W-Catalyzed Allylic Alkylations
An asymmetric version of
this process has been realized fi rst by Lloyd-Jones and Pfaltz using the W-catalyst
A
,
based on the
i-
Pr - PHOX
ligand (
L14a
) (Table 8B.41) [242]. This complex was obtained
from (MeCN)
3
W(CO)
3
and characterized by X-ray crystallography [243]. Alternatively,
this complex can also be prepared
in situ
by heating the ligand with (MeCN)
3
W(CO)
3
or (C
7
H
8
)W(CO)
3
in THF at 60°C for 30 min and then adding the nucleophile and the
allylic substrate.
In an initial experiment, racemic methyl 1-phenyl-allyl carbonate was used as sub-
strate, which gave an acceptable yield (71%) and regioselectivity (
b
/
l
: 78:22) but negli-
gible enantioselectivity (5%). This is in good agreement with the observation that
π
- allyl
W complexes do not undergo (signifi cant)
isomerization. Switching to linear sub-
strates changed the situation dramatically. Although primary allylic carbonates show no
conversion, the corresponding phosphates are more reactive and give the substitution
product in good yield and selectivity (Table 8B.41). In general, good results were
obtained with aryl-substituted substrates (entries 1-5), while (
E
) - 2 - butenylphosphate
π
-
σ
-
π
TABLE 8B.41. Asymmetric W - Catalyzed Allylic Alkylations
CH
3
C
N
O
H
Ph
2
P
N
W
MeO
2
C
CO
2
Me
OC
CO
A
CO
2
Me
CO
+
OPO(OEt)
2
R
R
NaCH(CO
2
Me)
2
,THF,rt
R
CO
2
Me
l
b
Entry
R
t (h)
Yield (%)
Ratio
b
/
l
ee (%)
1
Ph
71
89
74:26
96
2
4 - Me - Ph
136
86
80:20
94
3
4 - Cl - Ph
136
94
78:22
88
4
4 - Ph - Ph
136
98
79:21
91
5
1 - Naphthyl
136
93
95:5
86
6
Me
19
77
27:73
65
