Chemistry Reference
In-Depth Information
R
R
[RhCl{(
S
,
S
)-Ph-bod*}]
2
(2.5 mol %)
CO
2
Me
CO
2
Me
*
+
AgSbF
6
(10 mol %)
CO
2
Me
CO
2
Me
CH
2
Cl
2
, 25
°
C, 16 h
R = Et
n
Hex
83% ee (72%)
87% ee (48%)
Scheme 8D.61.
[Rh{(
R
)-BINAP}(solvent)]
+
SbF
-
R
R
(10 mol %)
MeO
2
C
MeO
2
C
(CH
2
Cl)
2
MeO
2
C
MeO
2
C
H
50-70°C, 36-48 h
R = Me
R = CH
2
OBn
R = H
>95%
ee
(72%)
>99%
ee
(80%)
52%
ee
(73%)
[Rh{(
R
)-BINAP}(solvent)]
+
SbF
-
(10 mol %)
X
X
(CH
2
Cl)
2
*
r.t. -70°C, 48 h
X = NTs
X = O
56%
ee
(87%)
22%
ee
(95%)
Scheme 8D.62.
8D.7.2. Intermolecular (Hetero) D - A Reaction
Chiral oxazaborolidines
37
(R = H, Me,
n
Bu, and Ar) have been one of the most useful
catalysts for the asymmetric reduction of ketones by using BH
3
· THF or catecholborane
as stoichiometric reductants [98]. Corey et al. extended their study to asymmetric C- C
bond-forming reaction, especially D-A reaction, by a chiral cationic Lewis acid, which
combines a proline-derived oxazaborolidine
37
with a trifl ic acid as an activator (Scheme
8D.63) [99]. In the presence of the catalysts
38
, even at − 95 ° C in dichloromethane,
asymmetric D-A reaction of various dienes, even unreactive dienes such as simple
1,3-butadiene, leads to the corresponding cycloadducts in high yield and enantioselec-
tivity (Table 8D.3 ).
While the catalysts
38
are extremely effi cient for the asymmetric D-A reaction with
α - substituted α , β-enals and various acyclic and cyclic dienes, they are unstable to decom-
pose at or above 0°C, limiting the range of practical application. Ryu and Corey suc-
ceeded in a modifi cation of these catalysts that would increase their stability and catalytic
activity by changing the counterion from OTf to NTf
2
(Scheme 8D.64 ) [100] . The catalyst
39a
obtained is more stable to retain the catalytic effi ciency and functions well even at
20°C. Additionally, they applied the catalyst
39a
to one of the key reactions in the total
