Oxo Complexes as Homogeneous Oxidation Catalysts

Metal complexes combined with a suitable oxygen atom donor can often act as versatile and selective oxidation catalysts. The oxygen atom donors commonly used are N-methyl morpholine oxide 8.27 (see reaction 8.30), organic hydroperoxides, sodium hypochlorite, hydrogen peroxide, etc. Three such reactions of industrial relevance are shown by 8.30, 8.31, and 8.32. Obviously these reactions are catalytic with respect to the metal complex but stoichiometric with respect to the oxygen atom donor. 

The first reaction, 8.30, is the classical alkene dihydroxylation with osmium tetroxide. In recent years the potential of this reaction has been vastly extended 

The importance of this reaction also lies in the fact that asymmetric epoxidation of alkenes other than allylic alcohols is possible with this catalytic system (see Section 9.3.4). The third reaction relates to catalysts developed by Unilever for improved detergent action in the presence of hydrogen peroxide. The important point to note is that catalytic intermediates with metal-oxo groups play a pivotal role in all these reactions. 

Apart from potential industrial applications, homogeneous catalytic systems with metal-oxo intermediates have direct relevance to certain biological oxidation reactions. The biological reactions involve metalloenzymes such as Cyt P450, which has an iron-porphyrin complex in its active site. The enzyme catalyzes hydroxylation of a hydrocarbon by oxygen. This hydroxylation does not proceed through a radical-chain mechanism. There is sufficient evidence to indicate that a catalytic intermediate with an Fe=0 group is responsible for the hydroxylation reaction. 

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