Chemistry Reference
In-Depth Information
t-Bu
O
H
O
N
O
V
R
2
O
OCH
3
O
d
n
4
r
4
n
g
|
5
H
5 mol%
CH
3
OH
O
OH
R
1
R'
R'
R'
+
R
R
R
O
2
, TBME, rt
O
O
O
Scheme 6.15 Oxidative kinetic resolution of a-hydroxy ketones catalyzed by chiral
oxidovanadium(
V
) methoxides.
aerobic oxidation catalyzed by chiral vanadium complexes. Chiral oxidova-
nadium(
V
) methoxides prepared from 3,5-diphenyl-N-salicylidene-
L
-tert-
butylglycine (or 3-o-biphenyl-5-nitro-N-salicylidene-
L
-tert-butylglycine) and
vanadyl sulfate in air-saturated MeOH showed the best results for the
aerobic oxidative kinetic resolution of alkyl, aryl and heteroaryl a-hydroxy
ketones with different a-substituents at room temperature in toluene or
tert-butyl methyl ether (TBME). The kinetic resolution selectivity factors
for the aerobic oxidation process were in the range 12 to 41000
(Scheme 6.15).
21
6.2.2 Aerobic Oxidation of Propargylic Alcohols
The aerobic oxidation of propargylic alcohols catalyzed by an oxovanadium
complex was achieved by Uemura and co-workers (Scheme 6.16).
22
Several
V(
V
) and V(
IV
) compounds exhibited excellent catalytic activity, but not V
2
O
5
.
They chose VO(acac)
2
as a suitable catalyst, because it is stable, ease to
handle and inexpensive. The reaction of several propargylic alcohols with
1 mol% of VO(acac)
2
under an oxygen atmosphere in the presence of
molecular sieves 3A in acetonitrile provided the corresponding carbonyl
compounds in moderate to high yields.
A plausible reaction pathway is depicted in Scheme 6.17. First, a V(
IV
)
species (A) reacts with O
2
to form a V(
V
)(B). Species B further reacts with
propargylic alcohol to afford a vanadium alcoholate (C). Next, the product
carbonyl compound and a dihydroxyvanadium species (D) are formed via b-
hydrogen elimination from C. The subsequent dehydration of D by oxidation
with oxygen re-produces species B.
Subsequently, Uemura and co-workers
23
attempted to heterogenize this
vanadium system to allow reuse of the catalyst (for green/sustainable
chemistry) and eventually they discovered that calcium phosphate-vanadate
apatite (CPVAP)
24
could be used as a recyclable catalyst for the aerobic
oxidation of propargylic alcohols (Scheme 6.18). A variety of propargylic
alcohols were oxidized to the corresponding carbonyl compounds in the
presence of a catalytic amount of CPVAP in n-butyronitrile at 100 1C under an
.
Search WWH ::
Custom Search