Chemistry Reference
In-Depth Information
Most Pd catalysts developed to date favor the formation of an achiral linear product
rather than the desired branched isomer [1]. Therefore, the development of highly
regio- and enantioselective Pd catalysts is still a challenge. In contrast to Pd cata-
lytic systems, Ir, Ru, W, and Mo catalysts provide very high selectivity for attack
at the non-terminal carbon to give the chiral product [1, 2].
Since the first enantioselective catalytic process described by Trost in 1977, with
moderate enantioselectivity [3], many catalytic systems have been tested. These
have provided excellent enantiomeric excesses [1]. Unlike asymmetric hydrogena-
tion processes, few diphosphines have provided good enantioselectivities in allylic
substitutions. Though high ees could be obtained in certain cases, for instance
with BINAP and CHIRAPHOS (Figure 10.1), the scope of standard diphosphines
in this process seems limited [1].
One of the most versatile ligands for this process is a diphosphine
1
developed
by Trost (Figure 10.2) [1c, 4]. The remarkable properties of this ligand are related
PPh
2
PPh
2
PPh
2
PPh
2
BINAP CHIRAPHOS
Figure 10.1
BINAP and CHIRAPHOS ligands.
O
O
O
NH
NH
PPh
2
N
PPh
2
Ph
2
P
R
1
R= Ph,
i
Pr,
t
Bu, Bn
Trost's ligand
PHOX ligands
O
Pd
Ph
Ph
P
N
R
R
Pd
R
δ+
Nu
-
R
R
Nu
-
Figure 10.2
Two representative ligands developed for Pd-catalyzed allylic substitution
reactions.
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