Chemistry Reference
In-Depth Information
1-phenylethanol and benzyl alcohol in a ruthenium-catalyzed reaction (see
also Chapter 3).
21
The tetramethylammonium hydroxide-RuCl
2
(PPh
3
)
3
catalyst could be reused after extraction of the product with diethyl ether.
d
n
4
r
4
n
g
|
6
10.7.3 Fluorinated Solvents
Fluorinated solvents offer an interesting alternative to standard organic
solvents in aerobic alcohol oxidation. If the catalyst is suitably tailored with
'fluorous ponytails', it becomes soluble in the fluorous phase and this allows
its recovery and reuse by simple extraction of the products with an organic
solvent at the end of the reaction. Several examples of this technique with
copper-based catalysts have been described.
Knochel and co-workers used a biphasic solvent system composed of
chlorobenzene and perfluorooctyl bromide together with a pyridine ligand
containing fluorinated ponytails for a CuBr-Me
2
S-TEMPO catalytic sys-
tem.
22
The biphasic reaction mixture was stirred at 90 1C while a gentle
stream of oxygen was passing. At the end of the reaction, the mixture was
cooled to 0 1C, the organic layer was decanted and the fluorous phase was
washed with chlorobenzene. The fluorous phase was used directly up to
eight times in further reaction runs with little decrease in activity (see also
Chapter 2, Figure 2.19). Other fluorinated ligands for copper complexes have
been reported in Chapter 2, (Figure 2.23).
10.7.4 Solvent-Free Systems
Chemists must take in account that a solvent is not always necessary, and
several interesting examples of aerobic oxidations of alcohols that do not
need the presence of a solvent have been reported.
The three-component system composed of acetamido-TEMPO-Cu(ClO
4
)
2
-
DMAP described above was also applied under solvent-free conditions (see
also Chapter 2). The three catalyst components were simply recovered by
addition of a non-polar solvent (hexanes) that selectively dissolved the
product aldehydes.
23
In the case of solid alcohols, PEG-200 (not oxidized
under these reaction conditions) was used as solvent.
Gold-based catalysts usually show selectivity towards carboxylic acids
when used in water; employing solvent-free conditions instead results in
good selectivity towards carbonyl compounds (see also Chapter 5 for further
details). For instance, Corma and co-workers showed that Au/CeO
2
catalyst
was active in the selective oxidation of alcohols to aldehydes and ketones
under solvent-free conditions, by using O
2
as oxidant without the require-
ment for the addition of NaOH to achieve high activity.
24
Among solvent-free oxidations, of particular interest are those performed in
the gaseous phase (see also Chapters 5 and 8), which proceed with the same
selectivity and employ silica-supported gold nanoparticles as catalysts.
16
A nice example of iron-based catalysis working in solvent-free conditions
was recently reported by Zhang and co-workers.
25
A task-specific bimagnetic
.
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