Chemistry Reference
In-Depth Information
ligands were designed to selectively generate a 1:1 vanadium/ligand complex in which
each hydroxyl group coordinates to the vanadium atom. Indeed, the catalytic activity
was kept with more than 1:3 vanadium/ligand ratio.
Trans
- disubstituted and trisubsti-
tuted allylic alcohols underwent epoxidation in high enantiomeric excesses, and the
reaction of
cis
-substituted allylic alcohols, which are substandard substrates for the
Katsuki-Sharpless epoxidation, also proceeded with high enantioselectivity. An intrigu-
ing feature of the catalyst system is the availability of aqueous TBHP as a stoichiometric
oxidant. It is noteworthy that the method is very effective for the asymmetric epoxida-
tion of small allylic alcohols, which has been unsolved for a long time. With the vanadium
catalyst, small epoxy alcohols can be obtained in high yields with high enantioselectivity
more than 90% ee by water-extraction technique. The catalytic system was also applied
to the kinetic resolution of a racemic secondary alcohol, and both the allylic alcohol and
the epoxy alcohol were obtained with high enantiomeric excesses (Scheme 11.34).
The VO(O
i
Pr)
3
/bishydroxamic acid system also promotes asymmetric epoxidation of
homoallylic alcohols. High yields and enantioselectivities were obtained in the epoxida-
tion of both
cis -
and
trans -
substituted homoallylic alcohols (Scheme 11.35) [58]. More-
over, racemic homoallylic alcohols were effectively resolved, and both the recovered
homoallylic alcohols and the epoxides were isolated with high enantiomeric excesses
(Scheme 11.36 ).
VO(O
i
Pr)
3
(1 mol
%)
29a
(2 mol
%)
O
OH
OH
OH
+
aq. TBHP
+
CH
2
Cl
2
, 0°C
Ph
Ph
Ph
51% conv., 95% ee
93% ee
Scheme 11.34.
O
VO(O
i
Pr)
3
(1 mol
%)
29d
(2 mol
%)
Toluene, RT, 24 h
CAr
3
O
N
R
1
R
1
*
*
OH
OH
Ar =
Et
+
CHP
OH
R
2
OH
R
2
N
Et
93-99% ee
CAr
3
O
Et
29d
O
O
O
Ph
*
*
Et
OH
OH
OH
Et
90%, 96% ee
85%, 93% ee
92%, 95% ee
Scheme 11.35.
VO(O
i
Pr)
3
(0.5 mol
%)
29d
(1 mol
%)
O
OH
OH
OH
+
CHP
+
Toluene, RT, 30 h
R
R
R
R = Et
51%, 95% ee
48%, 95% ee
= C
4
H
9
51%, 96% ee
48%, 97% ee
Scheme 11.36.
