Chemistry Reference
In-Depth Information
were obtained with the 1-nitroalkenes and 3-substituted maleimides as the substrates,
respectively.
Sakuma and Miyaura studied the conjugate addition of arylboronic acids to
-
unsaturated amides in the presence of a chiral rhodium catalyst prepared
in situ
from
Rh(acac)(CH
2
CH
2
)
2
and (
S
)-BINAP [50]. The addition of phenylboronic acid to
N
-
benzyl crotonamide provided (R)-
N
- benzyl - 3 - phenylbutanamide with 93% ee (Scheme
8A.35 ).
α
,
β
R
1
R
1
H
ArB(OR)
2
RhX/(S)-binap
Base, dioxane-H
2
O
100°C
NHR
2
NHR
2
Ar
PPh
2
PPh
2
O
O
Up to 85% yield, 93%
ee
R
1
= Me, Me
2
CH, Ph, C
5
H
11
; R
2
= H, Ph,
c-
C
6
H
11
, CH
2
CH(CH
3
)
2
;
Ar = Ph, 4-CF
3
C
6
H
4
, 4-MeC
6
H
4
, 4-MeOC
6
H
4
; R = H, OC
2
H
4
, OC
2
Me
4
41
(S)-
binap
Scheme 8A.35.
A rhodium-catalyzed enantioselective conjugate addition of organoborane reagents
to enones in the presence of
C
2
-symmetric chiral diene ligand was reported [51]. Hayashi
et al. showed that rhodium-diene complex could be used to effect the asymmetric con-
jugate addition reaction and generate the desired products with high enantioselectivities
(Scheme 8A.36 ).
O
O
[RhCl(C
2
H
4
)
2
]
2
RB(OH)
2
or (RBO)
3
+
Ph
*
R
Ph
R = Ph, 3-MeOC
6
H
4
, 3-ClC
6
H
4
,
4-CF
3
C
6
H
4
, 2-naphthyl,
(E)-n-C
5
H
11
Ch=CH
Up to 94% yield
Up to 99% ee
Scheme 8A.36.
Besides bisphosphines and diene ligands, phosphoramidite ligands were also success-
fully developed. In 2003, Feringa et al. reported that monodentate phosphoramidite
44
was extremely effective in the rhodium-catalyzed conjugate addition of boronic acids
and potassium trifl uoroborates to enones [52,53]. This reaction afforded the desired
products with excellent yields and enantioselectivities (Scheme 8A.37).
Using phosphoramidite ligand
45
, Feringa et al. in 2005 showed the conjugate addi-
tion of arylboroxanes in the preparation of 2-aryl-4-piperidones in high yield and with
excellent enantioselectivity (up to 99% ee) (Scheme 8A.38) [54].
In 2008, Lee and Yun [55] described the asymmetric boration of acyclic
-
unsaturated carbonyl compounds, which provided the ready access to functionalized
chiral organoboron compounds under mild reaction conditions. Using planar chiral
α
,
β
