Oxidative activation Oxygen transfer reactions, catalysis

Despite the fact that synthetic peroxovanadium complexes do not model satisfactorily the active centre of the VHPOs in the sense outlined above, most of them are effective oxidants under mild conditions, or even catalysts for, or active intermediates in the catalysis of, oxygenation reactions using hydrogen peroxide as the oxo transfer agent. Examples of 0X0 transfer reactions to organic substrates carried out via peroxovanadium complexes -the hydroxylation of alkanes, alkenes and aromatic compounds by the picolinato complex 22 (Figure 4.21) - are depicted in Scheme 4.8. 

We will consider both selective and unselective reactions in the next few sections of this chapter. The only criterion will be the nature of the substrate which is catalytically oxidized. The substrates considered will have systems of p or tt electrons by which they are known to react readily with many of the metal complexes used as catalysts. Those instances which are clear examples of coordination catalysis will serve to illustrate principles of catalytic oxygen activation. Those instances in which coordination catalysis plays a less important role may serve to illustrate areas in which future work could provide improvements by means of selective activation and transfer of dioxygen. 

We have considered the oxidation of organic phosphines in some detail because as a group of reactions they constitute the most widely studied examples of coordination catalysis of oxidation. It seems clear that in the presence of many different metal complexes, oxygen is indeed catalytically activated and transferred to the organic substrate within the coordination sphere. For many complexes, phosphine... 

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