Oxidation of Organic Substrates by Metal Ion Complexes

A linear free energy relationship is exhibited in equation (17) 

In this section, owing to the large variety of ligands available, reactions are classified with respect to the metal ion oxidant rather than the reductant. 

Studies of alcohols and organic acids continue to be of interest. Three-electron oxidations, which are a feature of several systems, have been shown to occur in the oxidation of mandelic acid. The rate law is of the form 

In the co-oxidation of 2-hydroxy-2-methylbutyric acid (HMBA) and 2-propanol (/-PrOH), which is a two-stage reaction, the participation of both Cr(VI) and Cr(V) as oxidants has been determined.In the initial reaction 

The kinetics of the oxidation of malonic acid in the presence and absence of Mn(II) have been reinvestigated.The rate law for the uncatalyzed reaction differs from that when Mn(II) is present and where the principal pathway is via reaction of a Mn(II)-malonic acid complex with HCr04T. 

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The concepts of electron and ligand transfer can be applied to the oxidation and reduction of organic substrates by metal complexes,61-64 since one-equivalent changes in the oxidation states of metals in inorganic redox reactions also have analogies in organic chemistry. Thus, the interconversion of the series of species carbonium ion (R+), free radical (R ), and carbanion (R-) results from one-equivalent changes, namely,... 

Redox reactions usually lead, however, to a marked change in the species, as reactions 4-6 indicate. Important reactions involve the oxidation of organic and metalloprotein substrates (reactions 5 and 6) by oxidizing complex ions. Here the substrate often has ligand properties, and the first step in the overall process appears to be complex formation between the metal and substrate species. Redox reactions will often then be phenomenologically associated with substitution. After complex formation, the redox reaction can occur in a variety of ways, of which a direct intramolecular electron transfer within the adduct is the most obvious. 

The coverage in this chapter is not comprehensive and no tabulation of rate data has been attempted. A more extensive account of the material will appear in Volume 9 of the series the presentation of the material also differs slightly from previous volumes. There have been many important developments in this general area. A comprehensive review of one-electron reduction potentials for nonmetallic substrates has appeared. Detailed kinetic studies have been reported for several important reactions where metal ions serve as catalysts in the transformation of organic substrates.Catalytic oxidation reactions involving metal complexes and macrocyclic metal complexes have been reviewed. Continued interest centers on catalysis by metalloporphyrins, and the role of metal complexes in electrocatalytic reductions has been reviewed. ... 

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