Chemistry Reference
In-Depth Information
TABLE 8B.29. Asymmetric P t - Catalyzed Allylic Alkylations
CH(CO
2
Me)
2
OAc
NaCH(CO
2
Me)
2
[(allyl)PtCl]
4
(1.25 mol %),
L*
Entry
Ligand
T ( ° C)
t (h)
Conv. (%)
Yield (%)
ee (%)
1
5 mol % (
S
) -
i
- Pr - PHOX
(
L14a
)
20
72
25
—
90
2
5 mol % (
S
) -
i
- Pr - PHOX
(
L14a
)
65
48
81
74
84
3
10 mol % (
S
)
-
i
- Pr - PHOX
(
L14a
)
65
44
100
90
57
4
5 mol % (
S
) -
i
- Pr - PHOX
(
L14a
)/PPh
3
20
16
100
91
2
5
5 mol % (
R
,
R
) -
L34
20
72
39
—
95
6
5 mol % (
R
,
R
) -
L34
56
67
57
—
74
- allylplatinum complexes were
investigated in detail by Williams et al. [203]. In palladium-catalyzed reactions bidentate
ligands are often superior to monodentate ligands, because they favor the formation of
cationic
The factors affecting the reactivity and selectivity of
π
-allyl complexes, which show a higher reactivity than the neutral complexes. In
this case, the nucleophilic attack on the (
π
- allyl)Pd complex is the rate - determining step.
Quite different is the situation for platinum complexes. The complex (dppe)
2
Pt is
a less reactive catalyst than (PPh
3
)
2
Pt-stilbene. Therefore, attempts to carry out asym-
metric allylations in the presence of a bidentate ligand such as the
i
- Pr - PHOX
ligand
(
L14a
) were not very successful if (PPh
3
)
2
Pt-stilbene was used as a platinum source.
For example, 1,3-diphenylallyl acetate gave the substitution product with moderate
28% ee. In contrast, in a PPh
3
-free system where the (
i
- Pr - PHOX
)Pt - complex was
generated from K
2
PtCl
4
, (COD)PtCl
2
, or [(allyl)PtCl]
4
[205], ees up to 90% could be
obtained [206] (Table 8B.29, entry 1). The reaction is very slow and requires refl uxing
in THF for 2 days to go to completion (entry 2). Interestingly, if the ligand was used
in excess, a drop in the enantioselectivity was observed (entry 3). Addition of PPh
3
signifi cantly accelerated the reaction, but resulted in the formation of nearly racemic
material (entry 4).
This clearly indicates that the bidentate ligand/Pt complexes are hemilabile. NMR-
spectroscopic studies indicate that besides the expected complex
A
, the more reactive
monodentate complexes
B
or
C
are formed preferentially in solution, giving rise to
the low ees observed (Fig. 8B.21). Interestingly, also with the bidentate ligand (
R
,
R
)
-
CHIRAPHOS
(
L34
), an excellent 95% ee was obtained at room temperature, but this
reaction was also very slow and the conversion low (Table 8B.5, entries 5 and 6).
π
8B.4. MOLYBDENUM - AND TUNGSTEN - CATALYZED
ENANTIOSELECTIVE ALLYLIC ALKYLATIONS
8B.4.1. Molybdenum - Catalyzed Allylic Alkylations
Although palladium is the most dominant metal in π - allyl chemistry, the analogue
Mo-catalyzed reactions have developed also as powerful synthetic tools in organic syn-
thesis. Molybdenum complexes were one of the fi rst showing different regioselectivities
