Chemistry Reference
In-Depth Information
X = OH
X=Cl
Single electron transfer
Single electron transfer
•O
2
H
or
•O
-
R
R
R'
OH
R
R
R'
R
R
H
R'
R'
H
R'
R'
H
H
O
2
H
H
NO
O
2
NO
OH
OP
OH
NO
OH
OP
1
O
N
N
O
N
N
O
N
N
O
N
N
O
N
N
O
Ru
IV
Ru
III
Ru
III
Ru
IV
Ru
O
2
OP
2
O
h
u
O
h
u
h
u
h
u
O
O
O
OH
OH
X
Cl
Cl
P = none or H
P
1
= none or H
P
2
= H or none
R'
R
R
R'
O
O
Catalytic
cycle
Catalytic
cycle
R
R
R'
R'
OH
H
H
O
2
OH
R
R
R
R'
R'
R
R'
R'
H
H
OP
O
O
OP
1
N
N
O
N
N
O
O
N
N
O
N
N
O
Ru
III
Ru
III
Ru
III
Ru
III
OP
2
O
O
O
O
•O
2
H
or
•O
-
H
2
O
2
H
2
O
2
OH
OH
Cl
Cl
Hydrogen atom abstraction
Hydrogen atom abstraction
Scheme 11.12.
that the ruthenium complex can be regarded as a mimic of galactose oxidase, although
each metal ion is not the same [23]. Although desymmetrization of
meso -
diols also
traces a similar catalytic cycle, the hydrogen atom is intermolecularly abstracted by
a hydroperoxy or superoxide radical as a consequence of the electron delocalization
onto the substrate oxygen atom instead of the donor oxygen atom, due to the
stabilization of the resultant cation radical by hydrogen bonding with another hydroxy
group.
11.2.4. Vanadium Catalyst
Two elegant reports on vanadium-catalyzed asymmetric aerobic oxidation of
- hydroxy
carbonyl compounds were independently published by the groups of authors [24-26].
Toste and coworkers used a vanadium complex
in situ
prepared from VO(O
i
Pr)
3
and tridentate Schiff base ligand
6
derived from 3,5 - di -
tert
- butylsalicylaldehyde and
(
S
) -
tert
- leucinol, and achieved
k
rel
values ranging from 6 to
α
>
50 in the kinetic resolu-
tion of various
-hydroxy esters (Scheme 11.13). Aliphatic substrates as well as aryl,
alkenyl, and alkynyl ones are effi ciently resolved, although increased reaction times
are required. In addition,
α
-hydroxy amides are also good substrates for the resolution.
The mechanism via a radical species at the carbinol carbon is unlikely for this
vanadium-catalyzed oxidation, because the reaction of an
α
α
- hydroxy ester having
cyclopropyl group at the
α
- position gives no ring - opening product.
