Chemistry Reference
In-Depth Information
An innovative application of hemilabile P-N ligands was reported through the use
of a chiral ferrocenyl (
R,S
) - PPF - pyrrolidine system in the methoxycarbonylation of
1,2 - dichlorobenzene - Cr(CO)
3
in order to introduce planar chirality in
- complexes
(Scheme 10.10) and achieved up to 95% ee by using the isolated precursor [PdCl
2
(P - N)]
under 1 atm of CO at 60°C in the presence of NEt
3
[72] .
π
N
PdCl
2
Cl
CO
2
Me
Fe
Ph
2
Cl
Cl
Cr
Cr
CO, MeOH, NEt
3
CO
CO
OC
OC
CO
CO
ee up to 95%
Scheme 10.10.
10.3.5. Asymmetric Bis - Alkoxycarbonylation of Vinylarenes
The enantioselective synthesis of optically active butanedioic acid derivatives is of inter-
est, as they are important intermediates of pharmaceuticals [73] and building blocks for
rennin inhibitors [74]. These products are formed by the bis-alkoxycarbonylation of
alkenes (Scheme 10.11), reaction that was fi rst reported by Heck in the early 1970s [75].
However, the fi rst asymmetric version of this reaction was described more than 20 years
later [76] .
R
1
O
[Pd(II)]
R
1
O
+2 CO + 2 R
2
OH
*
2
oxidant
OR
R
2
O
Scheme 10.11.
In this reaction, styrene is usually the substrate, and palladium systems containing
bidentate phosphine ligands are generally used as catalysts. However, the presence of
an oxidant in stoichiometric amount such as benzoquinone is required for this reaction.
The role of the oxidant is to reoxidize the Pd(0) species formed during the process. In
their initial report, Consiglio et al. used a Pd system containing the (
R,R
) - DIOP ligand,
but only low enantioselectivity was achieved [76]. Chiral ligands later used in this reac-
tion are presented in Figure 10.21. The use of atropoisomeric diphosphine ligands
yielded high enantiomeric excess but low chemoselectivity. The selective formation of
the bis-alkoxycarbonylation product is often a problem in this reaction, and by-products
such as methyl - cinnamate, methyl - 2 - phenylpropionate, and methyl - 3 - phenylpropionate
are usually formed during this process.
In 1999, the screening of a series of ligands in the enantioselective bis-methoxycar-
bonylation of alkenes was reported [77]. When styrene was used as a substrate, the cata-
lytic systems containing ligands such as
89
and
90
yielded high enantioselectivities (92%
