Chemistry Reference
In-Depth Information
α
,
α
-
trehalose
β
,
β
-
trehalose
HO
HO
HO
O
OH
O
O
O
O
OH
O
O
HO
OH
O
HO
BF
4
BF
4
OH
PPh
2
O
Ph
2
P
OH
PPh
2
Rh
Rh
(cod)
HO
HO
O
(cod)
Ph
2
16a
16b
D-
gluco
+
NMe
3
R
1
O
R
1
O
O
O
O
2X
-
O
Ar
2
P
PAr
2
Rh
(cod)
17
R
1
,R
1
= isopropylidene
18
R
1
= H
a
Ar= Ph
b
Ar= 3,5-Me
2
-C
6
H
3
α
,
α
-
trehalose
α
,
α
-
trehalose
R
1
O
BnO
O
O
O
O
OPAr
2
R
2
O
OBn
O
R
2
O
O
OR
2
OBn
R
1
O
OPAr
2
OR
1
R
1
O
OBn
OR
1
R
2
O
R
2
O
19
R
1
,R
2
= cyclohexylidene
20
R
1
=R
2
= H
21
R
1
= Bn, R
2
= PPh
2
22
R
1
= PPh
2
, R
2
= Bn
a
Ar= Ph
b
Ar= 3,5-Me
2
-C
6
H
3
Figure 8.2
Rh-complexes
16
-
18
and trehalose diphosphinite ligands
19
-
22.
Ligands in which the hydroxyl groups were unprotected were also used to
perform catalytic hydrogenation in water [16]. RajanBabu has prepared a series of
diphosphinites
19
-
22
derived from
-trehalose in which the phosphinite func-
tions were at positions 2,3 of a sugar unit (
19, 20
) or at positions 6,6
α
,
α
′
(
21
) or 4,4
′
(
22
) of both monosaccharide units. The catalytic system Rh/
19
(R
1
,R
2
=
cyclohexy-
lidene) provided an ee of 83% in the hydrogenation of methyl
-acetamidoacrylate
(entry 15, Table 8.1), but it dropped to 49% when the reaction was performed in
water using the catalytic system Rh/
20
(R
1
α
H). One possible explanation
for this decrease in enantioselectivity is the intervention of protonolysis of the
putative Rh-C bond before the final reductive elimination [17]. Later, complex
16b
was modified by replacing the second glucose unit with an ammonium substituent
in the anomeric phenyl group of the ligand, leading to the preparation of rhodium
complexes
17
and
18
[16]. Complex
17
was tested in the hydrogenation of dehy-
droamino acids and ees in organic solvents were slightly lower than those obtained
for the corresponding complexes without the ammonium group (entry 13, Table
8.1). Although the solubility of this complex in water was sufficient to complete
the hydrogenation reaction, it required long reaction times and the ees were very
=
R
2
=
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