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successfully prepared
pseudo
-enantiomers
27
and
28,
derived from d-galactose and
d-arabinose, respectively (Scheme 10.10) [21b].
The authors found that although three diastereomeric Pd(II) complexes may be
formed from these ligands (one in which
syn
and two
anti
), only one was obtained
in all cases, regardless of the sugar moiety and the protecting groups. This high
diastereoselectivity for the complexation was attributed to the
exo
-anomeric effect
[22]. Accordingly, in the Newman projection of the starting bis-thioglycoside in
the conformation stabilized by the
exo
anomeric effect, it can be seen that only the
coordination of pro-
(S)
lone pair of the sulfur to the Pd leads to a complex that
retains the stabilization of the
exo
-anomeric effect while also being sterically favo-
rable (Scheme 10.11). However, the observed enantiomeric discrimination in the
catalytic reactions seems to be related to the steric shielding exerted by the protect-
ing groups of the carbohydrate scaffolds and not to the high stereocontrol of the
complexation of the sulfur atom directed by the
exo
-anomeric effect.
pro-
S
lone pair
and C1-O
anti
pro-
S
lone pair
and C1-O
syn
pro-
S
lone pair
and C1-O
syn
R
Pd coordination to
pro-
R
lone pair
S
Pd
R
S
C2
O
C2
O
C2
O
S
S
S
Pd
Pd
H1
H1
H1
no
exo
-anomeric effect in operation
exo
-anomeric effect in operation:
favorable n-
s
*(C1-O) interaction with
but sterically unfavorable
S
sterically
favorable
sterically
unfavorable
R
(pro-
R
)
O
S
C2
O
C2
SR
C1
S
pro-
R
lone pair
and C1-O
syn
pro-
R
lone pair
and C1-O
syn
pro-
R
lone pair
and C1-O
anti
SR
H1
R
H1
(pro-
S
)
S
exo
-anomeric effect in operation:
favorable n-
Pd
*(C1-O) interaction
of pro-
S
lone pair
s
R
C2
O
C2
O
C2
O
S
S
S
S
Pd coordination to
pro-
S
lone pair
Pd
Pd
H1
H1
H1
S
exo
-anomeric effect in operation:
favorable n-
s
*(C1-O) interaction
sterically favorable
no
exo
-anomeric effect in operation
R
sterically
unfavorable
sterically
unfavorable
Scheme 10.11
Newman projections illustrating the
exo
-anomeric effect when the Pd is
coordinated to the pro-
(S)
lone electron pair.
10.2.3
Heterodonor Ligands
10.2.3.1
P-S Ligands
Several combinations of
P,S
-donor ligands such as phosphine-thioethers,
phosphinite-thioethers, phosphine-oxathianes, and phosphite-thioethers have
been studied. In particular, the phosphine-thioethers, phosphinite-thioethers,
and phosphine-oxathianes have proven to be effective in enantioselective Pd-
catalyzed allylic substitutions.
The ferrocenylphosphine-thioglucoside ligand
29
(Figure 10.14) with multiple
stereogenic units afforded an ee of 88% in the palladium allylic substitution of
diethyl malonate with 1,3-diphenylprop-2-enyl acetate (
S1
) [23a]. However, when
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