Dioxygen Complexes as Intermediates in Metal-Catalyzed Oxidation of Organic Substances

Abstract

The direct oxidation of organic substances by metal ions is well known and many systems have been thoroughly investigated. Oxidation of organic substrates by molecular oxygen in the presence of metal ions may occur through the same mechanism, i.e., by direct oxidation of the organic ligand through an inner sphere electron transfer from the ligandto the metal ion followed by reoxidation of the metal ion in solution by molecular oxygen. In certain cases evidence has been found for a concerted reaction involving a metal-ligand dioxygen complex. For this reason it is of interest to explore the evidence for dioxygen complexes as intermediates in metal catalyzed oxidation reactions. In many systems the presence of an oxygen complex as an intermediate has been inferred through a study of reaction kinetics. Such complexes are usually unstable and exist in solution in trace amounts that are undetectable. With the recent advances that have occurred in the synthesis of a wide variety of dioxygen complexes of metal ions (primarily cobalt(II)), it is now possible to study the direct oxidation of organic substrates by di-oxygen complexes. It is of interest to study such reactions as models for oxidase and oxygenase enzyme systems, and for the information that redox reactions of dioxygen complexes would provide on analogous reactions in which a dioxygen complex occurs only as a reactive intermediate at very low concentration.