Chemistry Reference
In-Depth Information
to the bite angle, which is larger than in unstrained Pd-diphosphine complexes.
Consequently, the P-aryl groups generate a chiral cavity, in which the allyl system
is embedded, that provides high ees for several sterically undemanding substrates.
For diphosphines and other homodonor systems, the chiral discrimination is
induced by the
C
2
or
C
1
symmetric backbone of the ligand.
The selection of chiral ligands for highly enantioselective allylic substitution
has focused mainly on the use of mixed bidentate donors such as phosphorus-
nitrogen, phosphorus-sulfur, and sulfur-nitrogen ligands [1, 5]. In this context,
the phosphinooxazoline (PHOX) ligands represent, together with Trost's ligand,
one of the most successful ligands developed for this process (Figure 10.2) [6]. The
efficiency of such hard-soft heterodonor ligands has been mainly attributed to
the different electronic effects of the donor atoms that predominantly produced
the nucleophilic attack at one of the allyl carbon atoms (the one located
trans
to the best
-acceptor).
Other ligands, such as bidentate nitrogen and sulfur donors, have also exhibited
very good catalytic behavior [1, 5].
Carbohydrate ligands have only recently shown their huge potential as a source
of highly effective chiral ligands in Pd-catalyzed asymmetric allylic substitution
reactions. Several types of ligands, mainly heterodonors, have been developed for
this process and some of the results are among the best ever reported [7]. In the
next section we summarize the most relevant catalytic data published for Pd-
catalyzed allylic substitution with carbohydrate ligands.
π
10.2
Ligands
10.2.1
P-Donor Ligands
10.2.1.1
Phosphine Ligands
In 2000,
C
1
-symmetric diphosphine ligands
2
-
4,
with a furanoside backbone, were
applied in Pd-catalyzed asymmetric allylic substitution reactions with moderate
success (Figure 10.3) [8]. These ligands were prepared from d-(
+
)-xylose and
D-
xylo
D-
gluco
L-
talo
5
Ph
2
P
PPh
2
Ph
2
P
PPh
2
PPh
2
PPh
2
O
1
O
O
4
3
2
O
O
O
O
O
O
2
3
4
ee's up to 61% (
S
)
ee's up to 59% (
S
)
ee's up to 61% (
S
)
Figure 10.3
Furanoside diphosphine ligands
2-4
prepared from d-xylose and d-glucose. The
maximum ee values reached are also shown.
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