Chemistry Reference
In-Depth Information
9.1.1
Diphosphite Ligands
Bidentate phosphites, which were the first carbohydrate-derived ligands applied in
asymmetric hydroformylation, are also the most important and most successful
group of sugar-based tools for this reaction. In 1995 the group of van Leeuwen
introduced bidentate ligands from simple monosaccharides such as d-xylose,
d-mannose, d-glucose, and d-galactose with phosphite donor sites containing a
biphenyl motif (Scheme 9.2,
4-9
) [4]. Further, ligands with mixed phosphite donor
sites, one with a biphenol group and another derived from substituted phenols
were prepared.
catalyst (0.1-0.2 mol%): Rh(acac)(CO)
2
/ligand* (ratio 1:1)
CHO
+
CHO
∗
Me
Ph
Ph
Ph
H
2
, CO, toluene
1
2
3
D-
xylo
D-
manno
D-
manno
O
O
O
O
P
O
P
P
O
O
O
P
O
O
O
O
O
BzO
O
O
O
O
O
O
O
P
O
P
O
O
BzO
O
O
R
2
R
1
OMe
OMe
Me
4a-c
Me
Me
5b,c
6c
Me
O
O
O
=
O
a
b
c
62% (
2
:
3
83:17)
47% (
2
:
3
95:5)
38% (
2
:
3
94:6)
3% ee (
S
)
40% ee (
S
)
50% ee (
S
)
b
c
>99% (
2
:
3
94:6)
67% (
2
:
3
97:3)
42% (
2
:
3
95:5)
31% ee (
R
)
53% ee (
R
)
R
2
R
1
rac
R
1
= R
2
= H
R
1
= R
2
=
t
Bu
a
b
3,6-anhydro-D-
manno
3,6-anhydro-D-
gluco
3,6-anhydro-D-
galacto
c
R
1
=
t
Bu, R
2
= OMe
O
O
O
OMe
OMe
OMe
O
O
O
O
P
O
O
O
O
O
O
P
O
P
P
O
O
O
O
O
O
O
O
P
O
P
O
7c
8c
9c
>99% (
2
:
3
93:3)
7% ee (
S
)
49% (
2
:
3
92:8)
8% ee (
S
)
94% (
2
:
3
93:7)
2% ee (
S
)
Scheme 9.2
First examples of asymmetric hydroformylation of styrene using carbohydrate
diphosphite ligands.
As it turned out, all tested ligands gave good to excellent regioselectivity in favor
of the desired chiral regioisomer. However, both, conversion of the starting mate-
rial and the enantioselectivity, proved to be strongly dependent on the architecture
of the carbohydrate scaffold (Scheme 9.2). The highest stereoselectivities were
observed for
xylo
-configured furanose ligands with 3,5-diphosphite groups (
4
) and
manno
-configured pyranose ligands with phosphites in the 4- and 6-position (
5
),
which led to 50% ee but notably to products with the opposite stereochemistry.
Substitution of the biphenyl moieties with bulky
tert
-butyl in the
ortho
position
and methoxy residues in the
para
position to the phenolic oxygens were a prereq-
uisite for stereoselectivities of around 50% ee, while ligands with unsubstituted
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