Chemistry Reference
In-Depth Information
In the Al-catalyzed asymmetric addition of cyanide to aldehydes, asymmetric ampli-
fi cation was reported; highly enantioselective cyanation was achieved using bifunctional-
ized BINOL
6
with moderate ee (Scheme 12.9 ) [18] . Al(III) - tridentate Schiff ' s base
7
of low ee also induced positive NLE in the enantioselective hydrophosphonylation of
benzaldehyde (Scheme 12.10 ) [19] .
Me
2
AlCl (10 mol %)
6
(40% ee, 10 mol %)
HMPA (40 mol %)
OTMS
C
7
H
15
CHO
+
TMSCN
C
7
H
15
CN
MS 4A, Et
2
O, -20
°
C
ca. 80% ee
N
O
OH
OH
O
N
6
Scheme 12.9.
Et
2
AlCl (10 mol %)
i
-Pr
N
OH
t
-Bu
OH
Ad
O
O
7
(ca. 40% ee, 10 mol %)
Ph
P(OEt)
2
OH
+
PhCHO
HP
(OEt)
2
CH
2
Cl
2
, -15
°
C
ca. 80% ee
Ad = adamantyl
Scheme 12.10.
12.2.3. Asymmetric Oxidation
Saito and Katsuki comprehensively studied the highly enantioselective reaction using
metal-salen catalyst. They reported asymmetric amplifi cation in the di-μ - oxo Ti(salen)
complex
8
catalyzed sulfoxidation (Scheme 12.11). In a methanol solution, di-μ - oxo
Ti(salen) was readily dissociated into monomeric species, and it acted as a true catalyst.
The racemic complex of (
R
,
S
) - di - μ-oxo Ti(salen) was probably stable and less soluble,
and positive NLE was achieved [20].
Shibasaki and others disclosed a chiral La complex-catalyzed epoxidation of enones.
The active chiral catalyst was turned out to be 1:1:1 complex generated from La(O-
i
- Pr)
3
,
BINOL, and Ph
3
As =O, and they concluded that preferential formation of heterochiral
