Chemistry Reference
In-Depth Information
K
2
CO
3
under O
2
(1 atm) in PhCF
3
as a solvent oxidized primary aliphatic,
allylic and benzylic alcohols to the corresponding aldehydes in quantitative
yields. Primary alcohols even in the presence of secondary alcohols were
selectively oxidized to the corresponding aldehydes in high yield.
Sheldon and co-workers found that a combination of RuCl
2
(PPh
3
)
3
and the
stable nitroxyl radical 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) is a re-
markably effective catalyst for the aerobic oxidation of a variety of primary
and secondary alcohols to give the corresponding aldehydes and ketones,
respectively, with 499% selectivity (Equation 3.3).
33,34
d
n
4
r
4
n
g
|
0
O
RuCl
2
(PPh
3
)
3
H
OH
+
+
N
N
O
R
1
R
2
R
1
R
2
PhCl, 100
o
C
H
OH
(3.3)
TON = ~466
O
2
(O
2
/N
2
= 8/92, 10 bar)
- H
2
O
The Ru/TEMPO system displayed a preference for primary versus sec-
ondary alcohols. Results from Hammett correlation studies (r
¼
0.58) and
the primary kinetic isotope effect (k
H
/k
D
¼
5.1) for the catalytic aerobic oxi-
dation of benzyl alcohol are inconsistent with either an oxoruthenium
(O
¼
Ru)- or an oxoammonium-based mechanism. Accordingly, a (hydrido)-
metal mechanism, involving a 'RuH
2
(PPh
3
)
3
' species as the active catalyst, is
postulated, in which TEMPO acts as a hydrogen-transfer mediator and is
either regenerated by molecular O
2
, under catalytic aerobic conditions, or
converted to TEMPH under stoichiometric anaerobic conditions.
35,36
Grith and co-workers
37
reported the synthesis of the organic soluble
tetra-n-butylammonium perruthenate (TBAP), nBu
4
N
1
RuO
4
, in 1985. They
later found that tetra-n-propylammonium perruthenate (TPAP),
nPr
4
1
RuO
4
, is even easier to prepare, from RuO
4
and nPr
4
NOH in
water.
38,39
TBAP and TPAP are air stable, non-volatile and soluble in a wide
range of organic solvents. The groups of Ley
40
and Marko
41
independently
showed that TPAP-catalyzed the oxidation of alcohols using molecular O
2
as
an oxidant.
As a more convenient catalyst system of high activity, operational
simplicity and low air and moisture sensitivity, a combination of [RuCl
2
-
(p-cymene)]
2
and Cs
2
CO
3
was developed.
42
A related water-soluble
diruthenium complex, [Ru
2
(m-OAc)
3
(m-CO
3
)], is also effective for the aerobic
oxidation of alcohols in water.
43
Using a catalyst system of RuCl
3
/Et
3
N, the
oxidation of benzyl alcohol with molecular O
2
could be achieved with a
turnover frequency (TOF) of 332 h
1
in the absence of solvent.
44
A catalyst
system of RuCl
3
nH
2
O/dicyclohexylamine displayed a preference for the
primary versus secondary benzylic alcohols in both intermolecular and
intramolecular competition experiments.
45
A(m-oxo)tetraruthenium cluster
(Figure 3.3) showed high catalytic activity for a simple and practical aerobic
.
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