Catalytic oxidations using cobalt(II) complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 265

Chapter 6
Catalytic oxidations using cobalt(II) complexes
László I. Simándi
Chemical Research Center, Institute of Chemistry, Hungarian Academy of Sciences, H-1525
Budapest, P.O. Box 17, Hungary
Abstract: The reactivity of cobalt(II) complexes toward dioxygen has long been recognized.
Synthetic oxygen carriers reversibly form mononuclear superoxo and dinuclear
complexes. The bound can be removed by pumping or flushing with an inert gas and this
cycle can be repeated many times over. However, there is always some loss of reversibility,
leading to cobalt- or ligand-centered oxidation. Dioxygen complexes are generally regarded
as the source of activity in cobalt-catalyzed oxidations. The term dioxygen activation is used
to describe the oxidation of added oxidizable substances via interaction with intermediate
dioxygen complexes or their conversion products. Observations of this behavior have
prompted extensive research into the vast area of homogeneous catalytic oxidation using
cobalt complexes. In this review the progress made in the study of various cobalt-based
catalyst systems in the last decade is surveyed. Catalytic oxidations by different classes of
cobalt(II) complexes with salen, porphyrin, phthalocyanin, dioxime, amine, pyridine,
cyclidene, peptide and carboxylato ligands are discussed with specific reference to the
products formed and the underlying reaction mechanisms. For each catalyst type the oxidation
of various substrates is reviewed, including substituted phenols, lignin phenolics, catechols,
anilines, thiols, alcohols, diols, and alkenes. Oxygen insertions, NO oxidation and oxidations
via alkylperoxo complexes are treated. Obviously due to the paramagnetic nature of cobalt(II)
complexes, free-radical mechanisms are predominant in cobalt-catalyzed oxidations,
permitting insight into the nature of reaction intermediates by the ESR technique.
Key words : Catalytic oxidation, homogeneous catalysis, dioxygen activation, dioxygen
complexes, biomimetic oxidation, functional metalloenzyme models, oxidation mechanisms,
oxidative dehydrogenation, oxygen insertion, alkene epoxidation, catecholase reaction