Chemistry Reference
In-Depth Information
O
O
N
N
N
O
Si
O
Si
O
N
O
N
d
n
4
r
4
n
g
|
2
O
O
O
N
N
N
O
Si
O
Si
N
O
O
N
N
Figure 4.19 Examples of tethered ligand systems for stabilizing Pd nanoparticles:
immobilized guanidine
180f
and bipyridyl and pyridyl variants.
180d
For example, studies show that better/more stable catalysts are produced
when mesoporous silicas such as SBA-15 and SBA-16 are used as the support.
Additionally, although such catalysts can be recycled and reused, they are
really only feasible for activated alcohols; these ligand-stabilized nano-
particles systems have relatively poor performance for unactivated aliphatic
alcohols.
4.4 Concluding Remarks
In the last 15 years, there has been significant progress in the performance
and understanding of aerobic palladium catalyst systems for alcohol oxi-
dation. This applies to heterogeneous and homogeneous/ligand-modulated
systems. At the present time there are still a number of challenges and both
heterogeneous and ligand-modulated systems have their own advantages
and disadvantages. In terms of utilizing Pd-based catalysts on a larger scale,
the use of a heterogeneous catalyst is undoubtedly preferred. They enable
the catalyst to be readily separated from the product and continually re-used
or indeed employed in a continuous flow system. As we have highlighted, at
present, heterogeneous Pd catalysts need improved performance for chal-
lenging substrates such as polyols and unactivated alcohols. In the case of
homogeneous/ligand-modulated systems, the performance of these systems
can have a number of advantages. A number of catalysts have been shown to
oxidize challenging unactivated alcohols effectively and can often operate
under mild conditions. Furthermore, the use of ligands offers the chance to
develop systems that demonstrate excellent selectivity, and we have given
some examples where the catalyst exhibits excellent chemoselectivity. Al-
though we have not discussed this and we have focused on standard alcohol
oxidation, it is worth highlighting that ligands also offer opportunities for
developing chiral catalysts. For example, the groups of Sigman
180
and
Stoltz
181
have (primarily) exploited the use of (-)-sparteine to carry out the
.
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