Chemistry Reference
In-Depth Information
OH
N
t
-Bu
(-)-
1
(11% ee, 2 mol %)
Ph
Et
+
PhCHO
Et
2
Zn
Hexane, -10
C
°
OH
82% ee
NMe
2
OH
(-)-
2
(15% ee, 8 mol %)
Ph
Et
+
PhCHO
Et
2
Zn
Toluene, 0
C
°
OH
95% ee
Scheme 12.4.
R'
2
N
R'
2
N
R'
2
N
*
*
R
R
*
O
*
Zn
Zn
+
ZnR
RZn
O
O
O
Zn
N
R'
2
*
O
*
*
*
O
R'
2
*
*
Zn
R
R
N
R'
2
*
*
Homochiral dimer
(unstable, more dissociable)
Major isomer
(reactive catalyst)
Minor isomer
(reactive catalyst)
Heterochiral dimer
(stable, less dissociable)
Scheme 12.5.
12.4) [8b]. They further performed the precise mechanistic investigation and presented
a model that heterochiral dimer is thermodynamically more stable than homochiral
dimer, and that the enantiomerically enriched remaining monomer operates as a catalyst
[9] (Scheme 12.5). An ab initio molecular orbital study was also demonstrated in a model
reaction between formaldehyde and dimethylzinc using achiral 2-aminoethanol as a
catalyst [10] .
Since the above examples, asymmetric amplifi cation was reported in many reactions
using various chiral catalysts. In the following sections, we describe new entries of asym-
metric amplifi cations, which have been published after the second edition of this topic
[11] .
12.2.2. Asymmetric Alkylation, Conjugate Addition, and Cyanation
Various β-amino alcohols as chiral base catalysts showed positive NLE in asymmetric
1,2-alkylation of aldehydes. Chiral
o
- hydroxyaryldiazaphosphonamide
3
[12] and 1,3 -
diol
4
, possessing bicyclo[2.2.2]octane skeleton, with lower ee [13] also gave the chiral
secondary alcohol with higher ee in the asymmetric alkylation of benzaldehyde using
diethylzinc (Schemes 12.6 and 12.7 ).
