Homogeneous, Heterogeneous and Nanocatalysis - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 31

Cu/nitroxide system is a good example of the crucial points to be followed in
the rational design of new homogeneous catalysts with the aim of broad-
ening the substrate scope and improving reaction conditions. The choice of
the ligand and co-catalyst proved crucial, and in-depth mechanistic in-
vestigations made it possible to shed more light on the properties of the
catalyst to be tailored. Several mono- to tetradentate N,O- or carbene-based
ligands, recently used for the preparation of Cu, Pd, V and Fe catalysts,
proved that, by varying the structure of the ligands, better selectivity and the
possibility of working with green solvents such as water can be tailored by
design. Great effort has also been devoted to the development of one-pot
syntheses toward the preparation of fine chemicals. Elegant tandem
approaches have been reported for the direct synthesis of heterocyclics,
imines and nitriles from alcohols and a nitrogen source.
New heterogeneous catalysts for the aerobic oxidation of alcohols have
attracted much attention in recent years. Monometallic and bimetallic
transition metal-based systems have been successfully applied to the aerobic
oxidation of alcohols to the corresponding carbonyl compounds using O 2 ;
several of them exhibited excellent performance. In particular, the use of
gold-based bimetallic alloys led to significant results for the oxidation of a
variety of molecules, especially significant in the production of chemicals
from biomass sources. Tuning the product selectivity to acid or aldehyde
may be most easily achieved by selecting the solvent. Water facilitates the
formation of acid, while unusual solvents such as scCO 2 may facilitate high
selectivity to aldehyde.
Nanocatalysis is the subject of intense research, and an appreciable in-
crease in the achievement of results in this area may be expected in the
coming years. The synthesis of tuned nanomaterials with controlled
chemical-physical properties may in fact be used to validate theoretical
calculations and to test specific reaction mechanisms while avoiding the
interference of support materials on the active phase.
The use of photochemistry under mild conditions to convert alcohols
selectively to chemicals is an innovative approach. In the near future, inte-
grated systems of MW and photo-catalyst systems may be available; ideal
photo-catalysts for commercial applications, however, should work under
solar light irradiation in aqueous solution and only very few studies on
sunlight use have been reported in the literature.
Although recent advances have shed more light on the mechanisms of the
aerobic oxidation of alcohol and broadened the scope of substrates, select-
ivity and synthetically significant compounds, there is still plenty of room
for further improvements of catalyst performance in the exploration of the
potential usefulness of this system in industrial processes.

d n 4 r 4 n g | 1

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References
1. H. Adolfsson, I. W. C. E. Arends, J.-E. B¨ckvall, M. Beller, A. Beressel,
J. Brinksma, W. R. Browne, J. W. de Boer, B. L. Feringa, M. Girhard,