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OH
D-
gluco
D-
gluco
D-
gluco
O
HO
O
OPh
Ph
O
Ph
O
O
O
O
O
O
PAr
2
X
-
Ar
2
P
OPh
Ph
2
PO
Ar
2
PO
Rh
OPAr
2
Ph
2
PO
OMe
7
(cod)
8
9
D-
manno
D-
gluco
D-
gluco
OBz
OPPh
2
OTr
OTBDMS
BnO
Ar
2
PO
Ar
2
PO
OBn
O
O
O
O
Ar
2
PO
O
O
Ar
2
PO
Ar
2
PO
Ph
2
PO
Ph
2
PO
Ph
2
PO
Ar
2
PO
Ar
2
PO
Ar
2
PO
BzO
AcNH
13
OMe
OMe
OMe
10
11
14
15
12
a
b
c
d
e
f
g
h
Ar =
Me
3
Si
SiMe
3
Me
Me
F
F
F
3
C
CF
3
OMe
F
CF
3
Figure 8.1
Diphosphinite ligands
7
-
15.
8.3.1
Phosphinite Ligands
Together with the phosphines used by Sinou and Descotes [7] in the late 1970s,
(Section 8.3.3) diphosphinites derived from carbohydrates were the first phospho-
rous ligands used in asymmetric hydrogenation. The first of this type, ligand
diphosphinite
7,
was described by Cullen [8] and Thompson [9] (Figure 8.1). This
ligand provided enantioselectivities up to 80% in the hydrogenation of
α
-acetamidoacrylic acids and their esters (entry 12, Table 8.1). Selke
et al
. system-
atically studied the effect of substituents at the anomeric position, and found
that
configured ligands
8
increased the ee in the hydrogenation of acetamidocin-
namic derivatives
1a
and
1b
in Scheme 8.1 to 96% and 91%, respectively
(entries 1 and 5, Table 8.1) [10]. They also demonstrated the importance of the
all-equatorial arrangement in vicinal diphosphinite hexapyranoside ligands for
high enantioselectivities in rhodium-catalyzed asymmetric hydrogenation. Ligands
8,
derived from d-glucose, were easily obtained by reacting a dichloromethane
solution of phenyl 4,6-
O
-benzylidene-
β
-d-glucopyranose with two equivalents
of the corresponding chlorophosphine in the presence of pyridine as base
(Scheme 8.3).
β
Ph
O
Ph
O
O
ClPAr
2
( 2 equiv)
CH
2
Cl
2
,
Py, DMAP
O
O
HO
O
Ar
2
PO
OPh
OPh
OH
Ar
2
PO
33%-75% yield
8
Scheme 8.3
Synthesis of pyranoside diphosphinite ligands
8.
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