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In-Depth Information
S
N
2-type TS
S
N
1-type TS
L
L
P
Y
L
L
P
Y
X
X
Pd
Pd
Pd
Pd
X
X
R
R
R
R
Nu
B
C
D
A
Nu
Scheme 8B.22.
Transition states of allylic alkylations.
Ts
O
O
O
P
O
P
O
O
N
M
N
N
(F
5
C
6
)
2
P
N
O
PPh
2
N
t
-Bu
t
-Bu
t
-Bu
Ts
(
R
)-
MeO-MOP
(
L20
)
L19a
L19b
L19c
Figure 8B.10.
Ligands used in the alkylations described in Table 8B.11.
NaCMe(CO
2
Me)
2
Ar
Ar
Ar
CMe (CO
2
Me)
2
Pd
Pd
AcO
L*
AcO
L*
9:1
b
:
l
= 88:12, 90% ee (
S
)
L*
=(
R
)-
MeO-MOP
(
L20
)
Scheme 8B.23.
Stoichiometric reaction of an isolated (π - allyl)Pd complex [109] .
phosphites were used as ligands [95]. Furthermore, steric hindrance caused by one bulky
ligand or a bulky moiety of a chelate ligand is expected to favor the
π
- allyl complex
C
.
In combination with nucleophilic attack at the
- allyl system
trans
to phosphorus [107],
formation of the branched product is expected to be preferred.
Ligands constructed in accordance with the guidelines enumerated above are
described in Figure 8B.10, that is, the
PHOX
ligand
L19a
and the phosphite/oxazolines
L19b
[17]. The best results (Table 8B.11) were obtained with the bis(
N
- tosylamino)
phosphine
L19c
(entries 5 and 6) [108]. The monodentate ligand (
R
) -
MeO - MOP
(
L20
)
[109,110] induced formation of the branched isomer with good regioselectivity if the
starting material was the branched allylic acetate (entries 7 and 8). The ratio phosphine/
Pd did not affect the catalytic activity nor the regioselectivity. A very clear mechanistic
picture emerged by studying the reaction of an isolated
π
- allyl complex (Scheme 8B.23 ).
Selectivities were in excellent agreement with those of the catalyzed reaction given in
Table 8B.11 .
The group of Dai and Hou started with the idea to prepare ligands similar to
L19b
,
but with a ferrocene backbone. In the attempt, the phosphorous amide described in
π
