Chemistry Reference
In-Depth Information
11.3.7. Niobium Catalyst
More recently, Egami and Katsuki disclosed that niobium(salan) complexes promote
asymmetric epoxidation of allylic alcohols with hydrogen peroxide. The authors
initially employed a chiral
- oxo niobium(salan) complex
30
as catalyst together
with urea·hydrogen peroxide (UHP) as oxidant and found that a range of allylic
alcohols underwent epoxidation with high enantioselectivity (Scheme 11.37). [59].
While known catalysts for the epoxidation of allylic alcohols need alkyl hydroper-
oxides as oxidant, the method can utilize hydrogen peroxide. Subsequently to this,
protocols using the catalyst
in situ
prepared from Nb(O
i
Pr)
5
and the corresponding
salan ligand have been developed. More favorable aqueous hydrogen peroxide is
available as an oxidant under the conditions (H. Egami and T. Katsuki, unpublished
data).
μ
R
3
R
3
O
30
(2 mol
%)
R
1
R
1
OH
OH
+
UHP
(4 equiv.)
Toluene, 40°C, 24 h
H
H
R
2
R
2
O
N
N
Nb
O
O
Ph
Ph
O
OH
O
O
OH
OH
O
OH
2
78%, 80% ee
79%, 83% ee
86%, 72% ee
83%, 68% ee
O
30
Scheme 11.37.
11.3.8. Molybdenum Catalyst
Chiral
C
2
- symmetric bishydroxamic acids
29
are effective auxiliaries for molybdenum-
catalyzed asymmetric epoxidation of unfunctionalized olefi ns as well as the vanadium-
catalyzed epoxidation of allylic alcohols (Scheme 11.38) [60,61]. The suitable choice
of the steric bulkiness of alkyl hydroperoxides leads to achievement of the high enan-
tioselectivity in the asymmetric epoxidation of mono, di-, and tri-substituted olefi ns.
It is worthy to note that the method is stereospecifi c and that the only
cis
- epoxide
O
TBHP
or
CHP
or
THP
MoO
2
(acac)
2
(2 mol
%)
29
(2 mol
%)
CAr
3
O
*
R
1
R
1
*
N
R
3
+
R
3
OH
CH
2
Cl
2
, RT
R
2
R
2
OH
N
50-96% ee
CAr
3
O
29
O
O
O
O
Ar =
R
O
R = Me (
29e
),
t
Bu (
29f
)
94%, 92% ee (
29e
)
92%, 96% ee (
29f
)
95%, 85% ee (
29f
)
95%, 85% ee (
29f
)
Scheme 11.38.
