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good yields and high diastereoselectivities and enantioselectivities (er
>
97:3), was also
reported (Scheme 8A.31 ) [45] .
R
2
Li
•
L*
R
1
O
2
N
R
1
R
2
O
2
N
n
-BuLi/(-)-sparteine
39
Toluene, -78
R
1
N
N
°
C
N
Ar
Boc
Ar
Boc
Ar
Boc
R
1
= Ph, Me; R
2
= Ph,
t-
Bu
e
>
9
7
3
Scheme 8A.31.
8A.2.4. Conjugate Addition of Organoborane Reagents
The rhodium-catalyzed asymmetric conjugate addition of organoborane reagents in the
presence of chiral phosphine ligands has been successfully developed by Hayashi et al.
In 1998, these investigators reported the rhodium-catalyzed asymmetric conjugate addi-
tion of arylboronic and alkenylbronic acids to cyclic and acyclic α , β - unsaturated ketones
in the presence of (
S
) - BINAP ligand with high enantioselectivity (Scheme 8A.32 ) [46] .
O
O
Rh(acac)(C
2
H
4
)
2
(S)-binap
40
Dioxane/H
2
O
+ RB(OH)
2
()
n
()
n
R
PPh
2
PPh
2
n
= 0,1,2.
Up to 99% ee
R = aryl, alkenyl
Rh(acac)(C
2
H
4
)
2
(S)-binap
R
1
R
2
R
1
R
2
41
(S)-
binap
+ RB(OH)
2
*
Dioxane/H
2
O
O
R
O
R
1
= Me, R
2
=
i
Pr or n-pent
R = aryl, alkenyl
Up to 97% ee
Scheme 8A.32.
It was shown that the square-planar complex RhPh(PPh
3
)((
S
) - BINAP) was the
key intermediate in the catalytic cycle of asymmetric 1,4-addition of phenylboronic
acid to an
-unsaturated ketones [47], and this reaction proceeded through three
intermediates including phenylrhodium, oxa-
α
,
β
- allylrhodium and hydroxorhodium com-
plexes. The catalytic cycle, as shown in Scheme 8A.33, involved the insertion of an
unsaturated compound into the aryl-rhodium bond followed by hydrolysis, giving the
hydroarylation product and the hydroxorhodium species.
In their subsequent studies, Hayashi et al. extended the scope of substrates from
π
α
- unsaturated ketones to 1 - nitroalkenes [48] and 3 - substituted maleimides [49] . These
reactions afforded the desired products in high yields and enantioselectivities with phos-
phine ligands
40
,
41
, and
42
, as shown in the Scheme 8A.34. Up to 99% ee and 97% ee
,
β
