Chemistry Reference
In-Depth Information
the diene-yne system, the use of DIOP derivative
35a
as a chiral ligand leads to a high
level of enantioselectivity. The effect of the 2-position of 1,3-dioxolane ring is examined
to give the most effi cient yield and enantioselectivity (84%, 73% ee) with ligand
35b
.
Interestingly, the cyclo-products obtained by
35a
and
35c
bear the opposite absolute
confi guration.
Gilbertson et al. also reported that the effi cient asymmetric [4 + 2] cycloaddition is
achieved by using [Rh(diphosphine)(solvent)]
+
complex generated
in situ
from the
hydrogenation of [Rh(diphosphine)(nbd)]
+
complex (Scheme 8D.57) [93]. In this cata-
lytic system, substrates bearing an aromatic ring (R = Ar) on the terminal alkyne cannot
be employed to show low enantioselectivity (
vide infra
) .
P
P
L
: (
S
,
S
)-Me-Duphos
R
[Rh(
L
)(solvent)]
+
SbF
-
Me
R
(6 mol %)
X
X
CH
2
Cl
2
/AcOEt 6/1
Me
H
55°C, 4-11 h
R = H, X = O
87% ee (76%)
88% ee (76%)
91% ee (78%)
R = Et, X = O
R = H, X = C(CO
2
Et)
2
[Rh{(
S
)-BINAP}(solvent)]
+
SbF
-
(6 mol %)
AcOEt
55°C, 72 h
H
Me
Me
O
O
H
>98% ee (64%)
Scheme 8D.57.
The key to effi cient asymmetric catalysis lies in the creation of effective chiral cata-
lysts by a suitable combination of chiral organic ligands and central metals. The asym-
metric catalysts thus prepared can evolve into more activated catalysts with higher
catalytic activity and enantioselectivity by chiral activators to additionally ligate (“asym-
metric activation”) [34]. However, the additional ligation does not necessarily lead to
higher catalytic activity, for which we propose the term “asymmetric synergy (effect)”
leading to higher enantioselectivity without increase in the catalytic activity (even with
decrease).
A number of asymmetric cycloadditions based on Rh catalysts have been developed.
The chiral Rh catalysts are generally prepared from
achiral
diene - Rh precatalysts and
chiral diphosphine ligands, but the effect of the
achiral
dienes on enantioselectivity has
been almost neglected. We have developed a highly effi cient chiral Rh(I) catalysts in
the synergistic combination of
chiral
diene-Rh complexes and chiral diphosphine ligands
