Selective Palladium-Catalysed Aerobic Oxidation of Alcohols - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 122

influence of acid and base chemistry was examined in detail by Sigman and
co-workers for their (IPr-NHC)Pd(OAc) 2 (H 2 O) catalyst system. 162 In their sys-
tem, they added acetic acid for optimal performance. Without the addition of
acetic acid, protonation of the peroxopalladium species can become rate
limiting and reversible formation of Pd(0) could lead to increased catalyst
decomposition. With increasing addition of acetic acid, the b-hydride elim-
ination step becomes more influential until it is rate limiting. They also
suggested that acetic acid leads to more stable catalytic turnover by slowing
the alcohol oxidation step of the reaction, thereby preventing a build-up of
Pd(0), the species thought to be responsible for catalyst decomposition in
most cases. This study highlights the delicate balance in terms of acid-base
chemistry and the challenges in preventing catalyst decomposition, as al-
though Pd(0) is generally not the rate-determining factor in the cycle, it does
have a lifetime and this means that over time it can and does aggregate. 167

d n 4 r 4 n g | 2

4.3.4 Nanoparticles and Ligands
As we have just highlighted, aggregation of Pd(0) species is thought to be a
major pathway of catalyst decomposition in most cases. As we discussed, the
Sigman NHC system utilizes acetic acid to help reduce this pathway, whereas
others have suggested that the use of bulky ligands is a possible method for
preventing aggregation. 174 Mechanistic studies by Stahl and co-workers
found that molecular sieves that were used in the original Uemura/pyridine
system are beneficial as they provide a surface that interacts with the catalyst
and reduces the rate of Pd(0) aggregation. 175 In the first half of this chapter
we focused on heterogeneous catalysts, which are predominantly solid-
supported Pd nanoparticles. The use of molecular sieves in the pyridine
system demonstrates that there is an overlap between 'homogeneous' and
'heterogeneous' systems, and in homogeneous systems the active species
may well not be a defined molecular species. Indeed, Sheldon and co-
workers recently discussed that stabilized nanoparticles may be the active
catalyst in the case of their neocuproine aqueous-DMSO system. 176 As they
pointed out, such catalysts are similar to the ligand-stabilized 'giant pal-
ladium clusters' (before the term 'nanoparticle' was as prevalent) previously
described by others such as Moiseev and co-workers. 177
There is also a link between homogeneous and heterogeneous systems
when we look at the use of ligand-stabilized nanoparticles on solid supports.
For example, in 1999, Kaneda and co-workers reported the use of
1,10-phenanthroline ligand-stabilized 'giant palladium clusters' uniformly
immobilized on a neutral TiO 2 . 178 A number of groups have explored the use
of silica-tethered amine ligands for the stabilization of Pd nanoparticles
(Figure 4.19). 179
The work to-date demonstrates (via TEM) that such systems result in
nanoparticles rather than ligated discrete molecular complexes. From these
studies, there are certainly similar trends to those observed with the types
of heterogeneous systems that we discussed in the first half the chapter.

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