Catalytic oxidations using cobalt(II) complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 292

5. OXIDATIONS CATALYZED BY COBALT(II)
PORPHYRINS

Metalloporphyrins are versatile oxidation catalysts. The field is
dominated by iron porphyrins, which serve as synthetic models for heme
type oxygenases. They have been extensively studied as witnessed by the
number of review articles and topics on the subject. It is only possible here
to cite a few items of the vast selection 80-88.
Cobalt(II) porphyrins bind dioxygen reversibly and thereby mediate
porphyrin degradation 89,90 , which can be regarded as a catalytic oxidation. In
poorly coordinating solvents (dichloromethane and THF) cobalt(II)
octaethylporphyrin is converted to cobalt(III) octaethyloxaporphyrin
dichloride, a cobalt verdoheme analog, containing an oxidized porphyrin
ring. of a type complex are formed 91,92

(Figure 33).

In an extensive series of studies Lyons and coworkers have shown that
halogenated metalloporphyrins are exceptionally active catalysts for the
selective reaction of alkanes with molecular oxygen 93-100 . The greater the
degree of halogenation of the ring, the greater is the catalytic activity of the
metal complex. Complexes of iron are generally more active than those of
cobalt, manganese, or chromium 101 . The product profile of isobutane
oxidation is characteristic of radical reactions but also sensitive to the nature
of the metal center. The selectivity to tert -butanol is about 90% or better.
Among cobalt complexes catalyzing the oxidation of alkanes (isobutane
or propane), is the most active 101 , surpassing Co(acac) 2 and

Co(BPI)(OAc), or (BPI = bispyridylimino-

isoindoline) 102,103 .