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D-
xylo
D-
ribo
D-
gluco
O
O
O
P
Ph
(RO)
2
PO
(OR)
2
PO
O
O
O
O
P
O
P
O
O
OPh
O
O
O
O
O
O
O
O
P
28
O
29
30
D-
gluco
D-
allo
L-
talo
L-
ido
CH
3
O
O
O
CH
3
CH
3
CH
3
O
O
P
O
P
P
P
O
P
O
O
O
O
O
O
O
O
O
O
O
P
O
O
O
O
O
O
O
O
O
O
O
O
P
P
O
O
31
32
33
34
R
1
R
1
R
2
R
1
=
t
Bu; R
2
=
t
Bu
a
e
f
(
R
)
ax
R
1
= H
(
S
)
ax
R
1
= H
(
R
)
ax
R
1
= SiMe
3
O
=
O
O
R
1
=
t
Bu; R
2
= OMe
b
O
O
O
R
1
= H; R
2
= H
g
h
c
R
1
= SiMe
3
; R
2
= H
d
(
S
)
ax
R
1
= SiMe
3
R
1
R
1
R
2
Figure 8.4
Diphosphite modular ligands
28
-
34.
synthesized very efficiently in one step from the corresponding diols, which were
easily prepared on a large scale from d-(
)-glucose using standard
procedures (Scheme 8.6). Reaction of the corresponding diol with two equivalents
of the desired
in-situ
formed phosphorochloridite in the presence of base afforded
the required ligands as white air-stable solids in moderate-to-good overall yield
(50-67%). The interesting feature of these ligands is their modular character,
which allows sufficient flexibility to fine-tune (i) the different configurations of the
carbohydrate backbone (C3 and C5) and (ii) the steric and electronic properties of
the diphosphite substituents (
33a
-
h
). Excellent enantioselectivities (ee up to 99%)
and activities were achieved in the Rh-catalyzed hydrogenation of dimethyl itaco-
nate (Table 8.3). Systematic variation of stereocenters C3 and C5 at the ligand
backbone showed that enantiomeric excesses depended strongly on the absolute
configuration of C3 and slightly on that of the stereocenter carbon C5. Enantiose-
lectivities were best with ligands
32
with a 6-deoxy-allo scaffold.
Varying the chirality at the axial chiral binaphthyl substituents in ligands
32
showed that the sense of the enantiodiscrimination is predominantly controlled
by the configuration of the biaryls at the phosphite moieties (entries 6 and 7, Table
8.3). Bulky substituents at the
ortho
-positions of the biaryl diphosphite moieties
have a positive effect on enantioselectivity. Enantiomeric excess was highest for
+
)-xylose and d-(
+
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