Chemistry Reference
In-Depth Information
Pro-
R
chiral catalyst
N
OH
(
R
)-
33
(high ee)
N
CHO
t
-Bu
N
Same source
of chirality
N
Opposite
configurations
t
-Bu
32
+
i
-Pr
2
Zn
Pro-
R
chiral catalyst
Achiral catalyst
+
N
OH
(
S
)-
33
(high ee)
N
t
-Bu
R
1
= Ph, R
2
= Me : (1
R
, 2
S
)-DMNE
41
R
1
= Ph, R
2
= H : (
R
)-DMAPE
42
R
1
= H, R
2
= Me : (
S
)-DMA
43
R
3
2
= Me
2
: DMAE
44a
R
3
2
= (CH
2
)
5
: PiE
44b
R
3
2
= Et
2
: DEAE
44c
R
3
2
= Bu
2
: DBAE
44d
R
3
2
= Oct
2
: DOAE
44e
Pro-
R
c
h
a
c
a
l
s
A
c
i
c
a
y
R
1
R
2
NR
3
2
HO
HO
NMe
2
Scheme 12.45.
Ph
HO
NMe
2
HO
NMe
2
(1
R
,2
S
)-DMNE
41
DMAE
44a
N
Zn
O
O
Zn
N
Ph
Mixed dimer
Figure 12.7.
Calculated structure of mixed dimer resulting from the aggregation of isopropylzinc
alkoxides of (1
R
,2
S
) - DMNE
41
and DMAE
44a
.
with (1
S
,2
R
)-DMNE, and achiral DBAE, (
R
) -
33
being obtained. Thus, the enantiofacial
selectivity of the chiral catalyst was reversed by the achiral catalyst
44d
.
Kinetic studies of this reaction with various loadings of catalyst and
ab initio
molecu-
lar orbital calculations indicate that the reversal of the sense of enantioselectivity is due
to the preferential formation of a catalytically active chiral heterodinuclear aggregate
derived from zinc alkoxides of chiral and achiral ligands (Fig. 12.7) [132]. In these reac-
