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to increase the enantioselectivity than simply by Rh-chiral diphosphine complex without
a chiral diene (Scheme 8D.58) [94]. We have investigated the effect of chiral dienes such
as
C
1
- symmetric dienes
36
in a combination of chiral diphosphine ligands. A remarkable
increase in enantioselectivity is observed by using chiral dienes. When the [RhCl(
36
)]
2
is examined without chiral diphosphine ligand, the product is obtained in 91% yield
within 30 min though with low enantioselectivity (26% ee). However, the combination
of [RhCl(
36
)]
2
with (
R
,
R
)-Me-DUPHOS provides an effective catalyst system to afford
the product in high enantioselectivity and yield (89% ee). The use of (
R
,
R
) - Me - DUPHOS
affords a higher level of enantioselectivity than that obtained with only chiral diene or
the combination of (
R
,
R
) - Me - DUPHOS with achiral [RhCl(cod)]
2
. On the other hand,
the use of (
S
,
S
)-Me-DUPHOS leads to the opposite enantiomer product with lower
enantioselectivity (
9% ee). These results clearly show that both chirality of chiral
dienes and Me-DUPHOS synergistically work in the right combination in the enantio-
discriminating step of the intramolecular [4 + 2] cycloaddition. (
R
,
R
) - Me - DUPHOS is
the matched pair with the chiral dienes
36
, and hence (
S
,
S
) - Me - DUPHOS is the mis-
matched pair. This is the fi rst example of asymmetric synergy between chiral dienes and
diphosphines. While the use of chiral dienes instead of achiral cod or nbd decreased the
catalytic activity, hence, not asymmetric activation, the synergy effect by chiral dienes
and diphosphine leads to a higher enantioselectivity than that by chiral diene itself
without chiral diphosphine.
−
[RhCl(
36
)]
2
(5 mol %)
Diphosphine (11 mol %)
Ph
Ph
Me
O
O
AgSbF
6
(20 mol %)
Me
H
CH
2
Cl
2
, r.t.
-
PPh
3
(
R
,
R
)-Me-Duphos
(
S
,
S
)-Me-Duphos
(
R
,
R
)-Et-Duphos
26% ee (91%)
26% ee (85%)
88% ee (90%)
-9% ee (96%)
95% ee (99%)
Me
OMe
Me
Me
Me
Diene*
36
Scheme 8D.58.
In a possible mechanism (Scheme 8D.59), the substrate-coordinated intermediate
A
initially forms, and then oxidative cyclization of the dieneyne substrate affords the metal-
lacyclopentane. Subsequently, allyl rearrangement and the reductive elimination lead to
the desired cyclic product. It is proposed that chiral dienes coordinate to the Rh center
in monodentate fashion in the enantiodiscriminating step
A
.
Recently, Hayashi et al. reported the use of
C
2
-symmetric chiral diene ligands for a
highly active and enantioselective asymmetric [4 + 2] cycloadditions (Scheme 8D.60)
