Selective Palladium-Catalysed Aerobic Oxidation of Alcohols - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 107

d n 4 r 4 n g | 2

Figure 4.12 Proposed mechanisms for crotyl alcohol aerobic selox over Au core-Pd
shell nanoparticles: (left) b-H elimination mechanism; (right) redox
mechanism.
Adapted with permission from Ref. 135. Copyright 2013 American
Chemical Society.

4.2.8 Substrate Scope
The preceding sections have outlined the mechanisms underpinning the
aerobic selox of alcohols over heterogeneous Pd catalysts and associated
structure-function relations. An important consideration for the com-
mercialization of such catalysts is the range of alcohols for which they afford
an economic and speedy synthesis of aldehydes in high yields.
Pd/mesoporous silicas show excellent activity towards primary and
secondary allylic alcohols. 34,38,39 TOFs generally decrease with increasing
molecular mass, likely reflecting slower substrate diffusion into such
mesoporous frameworks rather than intrinsically slower selox kinetics
(Table 4.1). The introduction of 300 nm macropores into SBA-15 led to a
4400% increase in TOF for the long-chain allylic phytol and farnesol (1258
and 1502 h 1 , respectively, with macropores versus 252 and 480 h 1 without)
and still delivering 490% selectivity to the respective aldehydes. 62
Alumina's ability to support sub-2 nm PdO x clusters leads to superior selox
performance compared with silica-supported palladium created via identical
protocols. Similar trends are apparent with regard to the influence of
mesoporosity and increased surface area in conferring faster TOFs for
benzyl, crotyl and cinnamyl alcohols compared with amorphous low surface
area g-alumina analogues. 40,63,137 Despite their higher activities, alumina-
supported palladium nanoparticles exhibit slightly lower selectivity (by
around 10%) towards crotyl and cinnamyl alcohol than over mesoporous
silicas, due to enhanced C ¼ C hydrogenation to saturated products.

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