Chemistry Reference
In-Depth Information
production, catalytic oxidation using transition metal complexes is the most
reliable approach.
5
Among the various transition metal catalysts, much at-
tention has been focused on the ability of ruthenium catalysts, since they
have often been used for hydrogenation and hydrogen-transfer reactions
6
and in dehydrogenation and oxidation reactions.
7,8
In fact, some success has
been achieved using ruthenium catalysts in the oxidation of alcohols with
NaIO
4
,
9
tert-butyl hydroperoxide,
10,11
chloramine-T,
12
amine-N-oxides,
12
iodosylbenzene,
13
NaBrO
3
14,15
and H
2
O
2
16,17
as oxidants. From both eco-
nomic and environmental viewpoints, there is an urgent demand for greener,
more atom-ecient methods that use molecular O
2
and air as ideal oxidants
and produce only H
2
O as a by-product.
18,19
In combination with transition
metal catalysts, the traditional and environmentally unfriendly oxidation
processes should be replaced with those using the greener oxidants mo-
lecular O
2
and air.
20,21
This section reviews the homogeneous ruthenium-
catalyzed aerobic oxidation of alcohols to the corresponding aldehydes and
ketones, which should stimulate further development in this important field.
d
n
4
r
4
n
g
|
0
3.1.2 Homogeneous Ruthenium-Catalyzed Aerobic Oxidation
of Alcohols
Ruthenium complexes are widely used as catalysts in organic synthesis
22
and
have been studied extensively as catalysts for the aerobic oxidation of alco-
hols.
23
In 1978, Mares and co-workers
24
reported that RuCl
3
nH
2
Ocatalyzes
the aerobic oxidation of secondary alcohols to the corresponding ketones,
albeit in modest yields (Equation 3.1). In 1981, Matsumoto and Ito showed
that RuCl
3
nH
2
OandRuCl
2
(PPh
3
)
3
catalyze the aerobic oxidation of activated
allylic and benzylic alcohols under mild conditions,
25
e.g.,theoxidationof
retinol was performed at room temperature for 48 h to give retinal in 57% yield
(Equation 3.2). Lower aliphatic primary and secondary alcohols were more
eciently oxidized using trinuclear ruthenium carboxylates, Ru
3
O(O
2
CR)
6
L
n
(R
¼
CH
3
,C
2
H
5
;L
¼
H
2
O, PPh
3
; n
¼
0, 1), as catalyst.
26
Their catalytic activity
was about 10 times higher than those of RuCl
3
nH
2
OandRuCl
2
(PPh
3
)
3
.
.
RuCl
3
.
nH
2
O
OH
O
(3.1)
toluene, 100
o
C, 100 h
O
2
(2.5 atm)
44%
RuCl
2
(PPh
3
)
3
OH
ClCH
2
CH
2
Cl, r.t., 48 h
O
2
(1 atm)
retinol
(3.2)
H
O
retinal, 57%
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