Selective Palladium-Catalysed Aerobic Oxidation of Alcohols - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 124

kinetic resolution of secondary alcohols. These enantioselective systems
are a good illustration of the benefits and drawbacks of ligand-modulated
systems. They illustrate the control that can be gained from utilizing ligands,
but at present the performance is not adequate for most industrial appli-
cations. They have demonstrated their utility at an academic level (e.g. in
total synthesis) 181g but the need for 5 mol% Pd and even higher loadings of
an expensive chiral ligand will be prohibitive on a larger scale. The under-
standing that has been developed from their pioneering studies will hope-
fully lead to the development of more ecient chiral catalysts in the future.
The performance of homogeneous Pd( II ) systems for standard alcohol
oxidation has undoubtedly improved substantially over the last 15 years and
their viability on a larger scale would depend on the substrate, for example,
inexpensive fine chemicals (e.g. fragrance molecules) may well need catalyst
loadings even lower than 0.1 mol%. The TONs that are achieved still lag
significantly behind those in other areas of homogeneous catalysis (e.g.
hydrogenation catalysts), and hopefully this can be improved by further
reducing the aggregation of Pd(0) species. An ideal catalyst may be some sort
of solid-supported/covalently tethered Pd( II ) complex that does not aggregate
and exhibits good performance for unactivated alcohols. It is hoped that
continued efforts will result in these remaining challenges being overcome.

d n 4 r 4 n g | 2

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