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R
2
O
O
R
1
R
1
O
R
2
O
O
O
O
O
P
P
R
2
O
Pd
R
1
R
1
R
2
Ph
Ph
Nu
-
Figure 10.7
Key Pd-allyl intermediate containing the furanoside diphosphite ligand.
O
O
t
Bu
t
Bu
P
O
P
O
OTBDPS
O
TBDPSO
O
O
O
O
O
=
O
O
OTBDPS
TBDPSO
O
O
t
Bu
t
Bu
O
O
P
O
P
O
17
(2
S
, 3
S
)
(2
R
, 3
S
)
18
Figure 10.8
Diphosphite ligands
17
and
18.
10.2.1.4
Phosphoroamidite Ligands
During recent decades, there has been a huge advance in the use of phosphoroa-
midite ligands for several asymmetric processes [17]. However, to the best our
knowledge, only one family of diphosphoramidite ligands (
19
) based on carbo-
hydrates has been successfully applied in asymmetric catalysis (Scheme 10.8)
[18]. The new diphosphoramidite ligands
19
were synthesized very efficiently
from 3,5-dideoxy-3,5-diamino-1,2-
O
-isopropylidene-ribofuranose by reacting two
equivalents of the desired
in
-
situ
formed phosphorochloridite in the presence of
pyridine. 3,5-Dideoxy-3,5-diamino-1,2-
O
-isopropylidene-ribofuranose was, in turn,
easily prepared on a large scale from inexpensive d-(
)-xylose (Scheme 10.8).
Good-to-excellent activities [TOFs up to 850 mol substrate x (mol Pd x h)
−1
] and
enantioselectivities (up to 95% ee) have been obtained in the Pd-catalyzed allylic
alkylation for several di- (
S1
) and monosubstituted (
S6
) linear and cyclic (
S4
) sub-
strates (Figure 10.9). The results indicate that catalytic performance is greatly
affected by the substituents and the axial chirality of the biaryl moieties of the
ligand. The study of 1,3-diphenyl and cyclohexenyl Pd-
+
-allyl intermediates indi-
cates that the nucleophilic attack takes place predominantly at the allylic terminal
carbon atom located
trans
to the phosphoroamidite moiety attached to C5.
π
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