Chemistry Reference
In-Depth Information
in mind that they can react not only with the
-allyl intermediate formed but also with
the carbonyl functionality of the leaving group, as far as carboxylates or carbonates are
used. Therefore, in reactions with hard nucleophiles, preferentially, substrates containing
a leaving group without an electrophilic center are applied such as amines [189], ethers
[191], or even alcohols [192].
Consiglio et al. fi rst reported on asymmetric couplings of Grignard reagents with
allylphenyl ethers in the presence of the chiral ligand (
S
,
S
) -
CHIRAPHOS
(
L34
) [193] .
The regio- and diastereoselectivities strongly depend on the Grignard reagent, but not
on the allylic substrate used. The linear and the branched substrate gave the same results
(Table 8B.25), but the aromatic Grignard reagent showed a much higher stereoselectivity.
These results indicate that the reaction proceeds via
π
π
-allyl nickel complexes, and that
these complexes are in a fast equilibration.
By far, the highest selectivity was obtained in the allylation using cyclic substrates.
Herewith, the ethyl Grignard reagent gave the coupling product with excellent 98% ee
(
R
), while the corresponding phenyl reagent reacted completely unselective (Scheme
8B.45 ).
OPh
Ph
E
t
M
B
r
P
h
M
r
NiCl
2
(5 mol %),
(
S
,
S
)-
L34
,
THF, rt, 22 h
NiCl
2
(5 mol %),
(
S
,
S
)-
L34
,
THF, rt, 22 h
98% ee (85%)
6% ee (90%)
Scheme 8B.45.
Nickel-catalyzed allylic alkylations of hard nucleophiles using cyclic allylic
substrates.
TABLE 8B.25. Nickel - Catalyzed Allylic Alkylations Using Linear and Branched
Allylic Substrates
RMgBr
NiCl
2
, (
S
,
S
)-
L34
,
THF, rt
or
+
OPh
R
OPh
R
l
b
l
b
Ph
2
P PPh
2
(
S
,
S
)-
L34
Entry
Substrate
RMgBr
t (h)
Yield (%)
l/b
ee (%)
1
l
EtMgBr
24
80
64:36
22 (
S)
2
l
EtMgBr
24
75
62:38
18 (
S
)
3
b
PhMgBr
120
85
35:65
58 (
R
)
4
b
PhMgBr
60
85
35:65
61 (
R
)
