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complexes, which compared well with the previously reported N,N-ligands in
the aerobic oxidation of 2-octanol on the gram scale.
10.10 Comparative Tables
Comparative tables are presented in Tables 10.1-10.5.
d
n
4
r
4
n
g
|
6
10.11 Conclusion
In the last 15-20 years, there has been substantial and still increasing
interest in the area of metal-catalyzed aerobic alcohol oxidations. We can
definitely arm that the oxidation of alcohols represents a benchmark re-
action for the development of new catalysts.
Regarding homogeneous alcohol oxidations, several procedures have been
optimized. Based on copper as the active metal, Mark` et al.'s Cu-phen and
Stahl and co-workers' (bpy)Cu(
I
)-TEMPO-NMI systems are among the most
mature procedures in the field, and with palladium as the active metal ex-
cellent reactions have been discovered, Sheldon and co-workers' Pd-(sul-
fonated bathophenanthroline) and Sigman and co-workers' Pd(OAc
2
)-TEA
systems being among the most optimized. Some of these catalytic systems
were also tested on a larger scale to explore their potential utility in indus-
trial processes.
Considerable efforts have also been made regarding sustainability, for
instance, to replace common organic solvents with alternative ones such as
ionic liquids, fluorinated solvents or scCO
2
or to perform the oxidation re-
actions in water or without the use of any solvent. Selective methods to
obtain aldehydes or carboxylic acids from primary alcohols were also de-
veloped, in addition to selective oxidations in which one type of alcohol (e.g.
primary versus secondary or the opposite) can be oxidized in the presence of
the other.
The discovery that Pd and Au nanoparticles (or bimetallic systems) are
effective catalysts for the oxidation of alcohol moieties has further ex-
panded this research field in the search for new heterogeneous systems,
which can allow the recovery and reuse of the metal catalyst and the for-
mation of pure products. Although a tremendous amount of effort has been
applied to the development of new catalysts, many improvements can still
be envisioned. For instance, in order to use these methods in target syn-
thesis, the scope of the individual catalytic systems must be broadened to
include more complex alcohols that are synthetically relevant. Therefore,
many challenges still need to be addressed in terms not only of their
substrate scope, but also regarding their availability at low cost and ro-
bustness in the reaction mixture, with the aim of recycling and reuse in an
industrial perspective.
.
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