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O
O
N
N
c
N
TfO
OTf
Ph
Ph
OTf
Et
O
O
H
Sc cat.
19
(5 mol %)
N
H
N
+
Me
Et
Ph
H
Ph
CH
2
Cl
2
, r.t.
Me
O
Me
O
20a
95% ee (83%)
syn
/
anti
10/1
Me
O
Me
N
Sc cat.
19
(5 mol %)
O
H
N
+
H
Ph
Et
CH
2
Cl
2
, r.t.
Me
Ph
O
Me
Me
O
20b
99% ee (75%)
syn
/
anti
20/1
O
Sc
NHPh
O
H
H
Me
R
H
exo
TS
Scheme 8D.24.
8D.3.4. Chiral Co and Cr Catalysts
Optically active β-ketoiminato cationic Co complexes
21
are designed as the effi cient
catalysts for asymmetric carbonyl-ene reaction of 1,1-disubstituted olefi n and glyoxal
derivatives (Scheme 8D.25) [49]. Similar to the ene reaction using chiral dicationic Pd,
Pt, and Ni complexes, hexafl uoroantimonate as a counter anion is found to be the most
effective. In spite of the low catalyst loading (0.2 mol %), both the isolated yield and
enantioselectivity are maintained in the range of 80-93% and 93-95% ee.
Highly enantioselective carbonyl-ene reactions catalyzed by the salen-Co complexes
bearing a similar structure to β-ketoiminato complexes are also reported (Scheme
8D.26 ) [50] . The salen - cobalt complexes
22b - c
with sterically demanding silyl group are
more effective than salen complex
22a
with the commercially available Jacobsen-type
ligand. The reaction proceeds at room temperature even with 0.1 mol % of complex
22a
to give excellent yield and enantioselectivity (99%, 92% ee). The use of an unactivated
alkene and a monocarbonyl (pyridine-2-carboxaldehyde) in the presense of complex
22c
also facilitates the reaction to afford the enantiopure product.
Ruck and Jacobsen described the use of the interesting chiral tridentate Schiff base
ligands in the Cr - catalyzed asymmetric carbonyl - ene - type reaction with 2 - methoxypro-
pene (Scheme 8D.27) [51]. The Cr complex
23a
smoothly facilitates the reaction in good
enantioselectivity. Electron - defi cient benzaldehyde derivatives bearing
o
- substitution
