Aerobic Oxidation with Polymer-Supported Catalysts

3 Aerobic Oxidation with Polymer-Supported Catalysts 

Another way of circumventing the problems linked with organic co-oxidants is the use of molecular oxygen as the co-oxidant [46]. Molecular oxygen presents numerous advantages. It is environmentally friendly, technically attractive as it does not require any removal step, and it enables the use of solid-supported catalyst. 

Notably, the co-oxidant problem also occurs for TEMPO-catalyzed oxidations. For the aerobic oxidation of alcohols with supported TEMPO see the end of Section 4.3.1.2. 

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Pd2+ chelated by a functionalized polymer was reported to be active for the aerobic oxidation of almost all types of alcohols, such as monohydric and polyfunctional alcohols, as well as unsaturated alcohols (331-333). In this work, mono- as well as binuclear catalysts were formed on a polymer support with jS-di- and triketone surface ligands. For many reactants, a large increase in activity is observed in changing from the mono- to the binuclear... 

Oxidation of alcohols. The pernithenate salt is an excellent catalyst for aerobic oxidation of alcohols to aldehydes and ketones in the presence of 4A molecular sieves. The use of a polymer-supported ammonium perruthenat is perhaps an improvement, with good discrimination in the oxidation in favor of primary alcohols. Another versioif specifies a system containing CuCl and 2-aminopyridine also. 

Chiral Co(III)-salen complexes can also serve as efficient catalysts for HKR of terminal epoxides. Polymer-supported chiral salen complexes 156 were prepared from chiral Co complex 154 and ethylene glycol dimethacrylate 155, as shown in Scheme 3.45. The chemical reduction of 156, followed by treatment with acetic acid under aerobic conditions, produced the catalytically active polymer 157, which was used in the HKR of propylene oxide [87]. Some other examples of polymeric salen-Co complexes have also been reported for the same reaction [88, 89]. 

Low molecular weight Schiff base complexes of many metals are well known and in the case of aromatic ligands these tend to have high thermal stability. Polymeric Schiff bases likewise have been well reported, and although many of these have the Schiff base appended as a substituent on a vinyl polymer backbone, others have the Schiff base residue as part of the mainchain. The latter continue to complex metals very well [140, 141] and one early paper reports the use of a Mn(II) polymeric complex in the aerobic oxidation of cumene at 30-100°C [142]. Indeed there is an implication in the paper that the polymer complex is stable to 200°C when complexed O2 tends to be liberated. Wohrle s group have also studied polymeric Schiff bases extensively, again mostly with pendant groups. However, they have reported a mainchain poly Schiff base [143], its complexation with Co(II), Ni(II) and Cu(II), and use of the supported complexes as catalysts in quadricyclane isomerisation to norbornadiene. 

A robust matrix material can also be employed as a support to accommodate active metals, metal oxides, polymers, etc. Thus, three-dimensional graphene-based frameworks were used to support copper phthalocyanines and show improved thermal and chemical stability combined with a competitive overall cost and availability. This catalyst was successfully tested for the selective aerobic oxidation of alcohols to the corresponding carbonyl compounds. The high catalytic activity of the material was related to n-n interactions between benzene moieties of reactants and graphene that favor the interaction of the reagents and catalytic sites. On the other hand, the presence of basic sites on the support also helps to improve the selectivity. 

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