Chemistry Reference
In-Depth Information
formation of the
-allyl complex an irreversible process, in contrast to the corresponding
reaction of an allylic acetate. The method was introduced by Tsuji in 1984, and the
designation “ salt - free ” was coined by Plietker [30,31] . Allylic carbamates can similarly
be used under neutral conditions [32] (Scheme 8B.7).
π
Pd
0
L
n
O
OCH
3
Nu
O
Pd
Nu
O
OCH
3
L
L
O
CH
3
OH
OCH
3
CO
2
H-Nu
Scheme 8B.7.
Allylic alkylation with an allylic carbonate.
A related but less general example is provided by vinyl epoxides, which can also be
reacted with the conjugate acid of a nucleophile without additional base (Scheme 8B.8).
OH
O
O
HNu
[M
]
Nu
+
OH
[M]
Nu
Scheme 8B.8.
Allylic alkylation with a vinyl epoxide.
8B.2.2. Alkylations with Soft Stabilized Carbanions
8B.2.2.1. Enantioselective Reactions of
meso
-Substrates with C-Nucleophiles
The
three known types of
meso
-substrates are described in Figure 8B.2. In allylic substitu-
tions, the oxidative addition of [Pd
0
] constitutes the enantiodiscriminating step. In the
case of the
meso
- compounds,
A1
the Pd fragment coordinates to the alkene from the
face opposite to the leaving groups X, and stereocontrol is equivalent to differentiation
between the two enantiotopic leaving groups. Oxidative addition in case of geminal
diesters
A2
(X = OOCR ′) involves both enantioface differentiating complexation and
ionization in the enantiodiscriminating step [33,34]. Concerning enantioselective vari-
ants, substrate
A3
has so far only been used in amination reactions.
