Selective Palladium-Catalysed Aerobic Oxidation of Alcohols - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 111

surprisingly unreactive towards cinnamyl alcohol (a prototypical allylic
alcohol for aerobic selox) and vanillyl alcohol, with TOFs of 10 and 97,
respectively, although selectivity to the desired aldehydes was 100% for both.
Polyols represent the ultimate challenge for selective oxidation catalysis,
with the coexistence of multiple -OH functions in close proximity providing
numerous competing reaction pathways to navigate in order to yield a single
desired product. In this context, Villa and co-workers 119 highlighted the
potential of Au-Pd bimetallic systems for D -sorbitol (an important sugar
alcohol) and Davis and co-workers 140 similarly for glycerol, with 57 and 80%
selectivity to gluconate and glyceric acid, respectively.
Mono- and bimetallic Pd catalysts are effective for the selective aerobic
oxidation of a broad range of primary and secondary allylic and aromatic
alcohols. Developments in catalyst design and the use of structured support
architectures/stabilizers are enabling the selox of more complex polyols
and sterically demanding alcohols to be achieved, but further selectivity
improvements and energy-ecient routes to the activation of tertiary and
saturated alcohols are still sought in order to enhance the profile and
commercial implementation of palladium in selox.

d n 4 r 4 n g | 2

4.2.9 Concluding Remarks on Heterogeneous Pd Systems
The selective oxidation of complex alcohol substrates can be accomplished
through Pd-mediated heterogeneous catalysis with high turnover and
product selectivity. Application of in situ and operando techniques, such as
in situ X-ray and IR spectroscopy, has helped to elucidate the mechanism
of the oxidative dehydrogenation of alcohols and competing aldehyde
decarbonylation. Surface PdO was identified as the active catalytic species
and deactivation the result of reduction to metallic palladium and con-
comitant self-poisoning by strongly bound CO and carbonaceous residues.
Although this field is still in its relative infancy, breakthroughs in analytical
tools and synthetic approaches to engineering nanoporous supports
and shape/size-controlled nanoparticles have already delivered significant
progress towards improved atom and energy eciency and catalyst stability.

.

4.3 Ligand-Modulated Pd Systems

4.3.1 Introduction
Traditionally, the challenge for Pd( II )-catalysed oxidation reactions was that
the re-oxidation of Pd(0) species by O 2 back to the active Pd( II ) was found to
be slower than the aggregation process. A major breakthrough in the field of
Pd( II ) oxidations was the Wacker process, which was developed for the oxi-
dation of ethylene to acetaldehyde. 141 In this case, Pd(0) is oxidized back to
Pd( II ) using Cu( II ) salts and the resultant Cu( I ) species can be re-oxidized
directly with O 2 . This is an approach that is still exploited today in academic
research; however,

the presence of the copper co-catalysts is often