Chemistry Reference
In-Depth Information
3
-C
3
H
5
)Cl]
2
(0.55 mol%)
7
(1.3 mol%)
THF, BSA, KOAc
[Pd(
η
OAc
CH(COOEt)
2
+ CH
2
(COOEt)
2
Ph
Ph
Ph
Ph
S1
Ligand
%ee
0
16 (
R
)
25 (
R
)
39 (
R
)
17 (
S
)
41 (
S
)
55 (
S
)
18 (
R
)
59 (
R
)
7a
7b
7c
7d
7e
7f
7g
7h
7i
Scheme 10.6
Summary of the enantioselectivities obtained using ligands
7.
In all cases yields
were nearly quantitative. Chemicals yields
>
95% in all cases.
Pd-catalyzed asymmetric allylic alkylation of linear (
S1
) and cyclic (
S4
) substrates
provided low enantioselectivities (up to 31% ee) [12]. The results also showed that
the absolute configuration of the carbon C3 of the ligand controlled the configura-
tion of the allylic alkylation product.
10.2.1.3
Phosphite Ligands
In 2001, the first diphosphite ligand family (
10-16
) applied to Pd-catalyzed asym-
metric allylic substitution reactions was reported (Figure 10.5) [8a, 13]. The new
ligands were 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 10.7) [14]. Therefore, reacting the correspond-
ing diol with two equivalents of the desired
in-situ
formed phosphorochloridite in
the presence of base afforded the desired ligands as white air-stable solids in
moderate to good overall yield (50-67%) [14]. The highly modular construction of
these ligands allows sufficient flexibility to fine-tune (i) the various configurations
of the carbohydrate backbone, (ii) the substituents on C5 (R
+
)-xylose and d-(
+
H, Me, OTBDPS),
and (iii) the steric and electronic properties of the diphosphite substituents (
a-h
).
=
D-
xylo
Ph
Ph
Ph
Ph
Ph
P
O
D-
ribo
P
O
Ph
P
O
O
O
O
O
O
O
O
P
Ph
Ph
8
9
Figure 10.4
Furanoside diphosphinite ligands
8
and
9.
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