Dioxygen activation by iron complexes. The search for reactive intermediates

Three different modes of oxygen activation by iron complexes are described: (1) Oxygen atom transfer by ferric porphyrin peroxo complexes to electron-deficient olefins via a nucleophilic mechanism, (2) oxidative dehydrogenation of a cyclam complex, probably involving a radical intermediate, and (3) stereospecific, non-radical hydroxylation of alkanes by organic peracids catalyzed by iron complexes by an unknown mechanism. These three reactions are presented to remind us that the possible modes of interaction between iron and oxygen are remarkably diverse and that a variety of mechanisms should thus be considered in assessing possible reaction pathways of heme and non-heme iron-containing enzymes.