Chemistry Reference
In-Depth Information
11.3.6. Vanadium Catalyst
A combination of VO(acac)
2
and alkyl hydroperoxides is one of the most reliable oxida-
tion systems for hydroxy-directed epoxidation in organic transformation and enables
highly diastereoselective synthesis of epoxy alcohols [50,51]. In 1977, Sharpless and
coworkers identifi ed chiral hydroxamic acids as effective ligands for the vanadium-
catalyzed asymmetric epoxidation of allylic alcohols with TBHP, in which the hydroxamic
acid binds to the vanadium as a bidentate ligand, and moderate to good enantioselectiv-
ity of up to 80% was observed [52]. Although a singly coordinated species has been
proposed as the active catalyst, the formation of inactive doubly or triply coordinated
species reduces the effi ciency of the vanadium-hydroxamic acid system [53]. Neverthe-
less, a variety of chiral hydroxamic acid ligands have been introduced by several groups
in recent years [54]. For example, Yamamoto and coworkers designed hydroxamic acid
27
bearing axially-chiral binaphthyl group for the vanadium-catalyzed asymmetric epoxi-
dation of allylic alcohols, and demonstrated for the fi rst time that high enantioselectivity
of up to 94% ee can be achieved in the vanadium-catalyzed asymmetric epoxidation in
the presence of alkyl hydroperoxide as oxidant (Fig. 11.3) [55]. On the basis of this
fi nding, they further screened
-amino acid-based hydroxamic acids and found that
tert
- leucine - derived
28
is an effective ligand for the reaction. The epoxidation proceeded
effectively with high enantioselectivity in the presence of VO(O
i
Pr)
3
(1 mol %) and
28
(1.5 mol%) [56] .
Eventually, Yamamoto and coworkers identifi ed chiral
C
2
- symmetric bishydroxamic
acids
29
as effi cient ligands for the vanadium-based method (Scheme 11.33) [57]. The
α
HO
O
O
N
N
N
O
O
OH
OMe
27
28
Figure 11.3.
O
R
VO(O
i
Pr)
3
(1 mol
%)
29
(2 mol
%)
R
3
R
3
aq. TBHP
or
CHP
N
O
OH
R
1
R
1
R = CHPh
2
(
29a
),
OH
OH
+
OH
CH
2
CPh
3
(
29b
),
CH(3,5-dimethylphenyl)
2
(
29c
)
R
2
R
2
N
92-97% ee
O
R
29
O
O
O
O
OH
OH
OH
OH
50%, 93% ee (
29a
)
71%, 92% ee (
29b
)
78%, 97% ee (
29c
)
68%, 95% ee (
29a
)
Scheme 11.33.
