Chemistry Reference
In-Depth Information
Ojima and others also reported copper-catalyzed asymmetric conjugate addition of
diethylzinc to nitroalkenes [34]. Using a new chiral monodentate phosphoramidite
ligand
34
, high enantioselectivities (up to 99% ee) were obtained (Scheme 8A.24).
Et
*
Cu(OTf)
2
/
34
Toluene, -65°C
NO
2
NO
2
+E
2
Zn
R
R
Up to 99%
ee
R
1
R =
p-
Me-C
6
H
4
,
p-
MeO-C
6
H
4
,
m-
MeO-C
6
H
4
,
o-
MeO-C
6
H
4
,
p-
F-C
6
H
4
,
m-
F-C
6
H
4
,
o-
F-C
6
H
4
,
p-
CF
3
-C
6
H
4
,
o-
CF
3
-C
6
H
4
,
furyl, thienyl, (MeO)
2
CH
Ph
O
O
P N
Ph
34
R
1
Scheme 8A.24.
Chiral
N
-heterocyclic carbenes (NHC) are good ligands for copper-catalyzed conju-
gate addition of dialkylzinc reagents to various Michael acceptors. In 2007, Hoveyda and
coworkers [35] disclosed a chiral NHC for the catalytic asymmetric conjugate additions
of organozinc reagents to cyclic
-
keto esters. These transformations could also give rise
to the enantioselective formation of all-carbon quaternary stereogenic centers bearing
a readily functionalizable carboxylic ester with up to 95% ee (Scheme 8A.25).
γ
Ph
Ph
O
O
S
N
N
O
O
O
2.5 mol % NHC-Ag complex
35
2.5 mol % (CuOTf)
2
•
C
6
H
6
, R
2
Zn, -30
Ag
Ag
R
()
n
CO
2
R
1
()
n
°
C
CO
2
R
1
O
O
N
n =
1 or 2; R
1
= Me,
t
-Bu
R = Me,
i
-Pr, Ph, Et, Me
2
CH(CH
2
)
2
N
S
73-93%
ee
O
Ph
Ph
NHC complex 35
Scheme 8A.25.
P-chiral phosphinophenol and phosphinoanisole could be used as new chiral P/O
hybrid ligands. In 2005, Imamoto and others [36] successfully used this class of ligands
in the Cu-catalyzed asymmetric conjugate addition of diethylzinc to α , β - unsaturated
carbonyl compounds. P-chiral phosphinophenol
36
exhibited high enantioselectivities in
the reactions with various chalcone derivatives with up to 96% ee (Scheme 8A.26).
The TADDOL (2,3 -
O
- isopropylidene - 1,1,4,4 - tetraphenylthreitol) class of chiral tri-
valent phosphorus ligands was also promising for copper-catalyzed enantioselective
conjugate addition. In 2000, Alexakis et al. reported the asymmetric conjugate addi-
tions of diethylzinc to enones with up to 96% ee using this kind of ligands (Scheme
8A.27 [37]). In 2006, Palacios and Vicario reported a Cu-catalyzed asymmetric con-
jugate addition of dialkylzinc to α , β - unsaturated imines by using a TADDOL - derived
