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mixture was placed directly on a plug of silica, washed with pentane to
remove toluene and eluted with diethyl ether. The ether was removed in
vacuo to yield the desired carbonyl product. For alcohols with incomplete
oxidation, the desired carbonyl product was isolated via column chro-
matography with mixtures of diethyl ether-hexanes as the eluting solvent.
d
n
4
r
4
n
g
|
6
10.4 Oxidation of Primary Alcohols to Carboxylic
Acids
The oxidation of primary alcohols typically affords the corresponding carb-
oxylic acids when water is used as solvent, since this medium facilitates
aldehyde hydration and therefore its easy oxidation.
Various metals supported on solid matrices have been investigated for this
conversion using water as solvent or co-solvent. Ruthenium supported on
carbon (10% Ru/C) (see also Chapter 3) as a catalyst in toluene (at 90 1C)
under an oxygen atmosphere was able to convert primary aliphatic alcohols
selectively to carboxylic acids when water was added as a co-solvent
(Scheme 10.2).
8
Also concerning heterogeneous catalysis, Hutchings and co-workers
9
showed that gold supported on graphite could oxidize glycerol to glycerate
with 100% selectivity using dioxygen as the oxidant in water, with yields
approaching 60% (see also Chapter 5). It was observed that the selectivity to
glyceric acid and the glycerol conversion were strongly dependent on the
glycerol to-NaOH ratio (Scheme 10.3).
.
Representative procedure for the oxidation of glycerol to glyceric acid.
9
The catalyst (1% Au/graphite) was suspended in a solution of glycerol
(12 mmol) in deionized water (20 mL) containing NaOH (12 mmol) in a
Parr autoclave (50 mL) and the autoclave was pressurized to the required
pressure (3 bar) with oxygen and heated to 60 1C. The reaction mixture
was stirred (1500 rpm) for 3 h, affording 56% glycerol conversion and
100% glyceric acid selectivity.
Finally, the selective oxidation of benzylic alcohols to carboxylic acids or
esters by self-esterification was recently reported in water-soluble Pd and Au
nanoparticles.
10
The important role of water in the selective oxidation of primary alcohols
to the corresponding acids was shown recently with gold nanoparticles
10% Ru/C (5 mol%),
O
2
(1 atm)
90 °C, toluene:H
2
O 4:1
24 h
OH
CH
3
(CH
2
)
8
CH
2
OH
59%
O
Scheme 10.2 Ru/C-catalyzed aerobic oxidation of primary aliphatic alcohols to
carboxylic acids.
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