Iron compounds heterolytic cleavage

Since H202 is easier to handle than 02, we will focus on the use of the former. Many metals can be used for this transformation [50]. Among them, iron compounds are of interest as mimics of naturally occurring non-heme catalysts such as methane monooxygenase (MMO) [51a] or the non-heme anti-tumor drug bleomycin [51b]. Epoxidation catalysts should meet several requirements in order to be suitable for this transformation [50]. Most importantly they must activate the oxidant without formation of radicals as this would lead to Fenton-type chemistry and catalyst decomposition. Instead, heterolytic cleavage of the 0—0 bond is desired. In some cases, alkene oxidation furnishes not only epoxides but also diols. The latter transformation will be the topic of the following section. 

The catalytic mechanism of heme CATs is a two-step reaction. In the first step, interaction between ferriCAT (Fe +) and hydrogen peroxide leads to the heterolytic cleavage of the 0-0 bond in the substrate molecule and oxygen is bound to the 6th valency of the porphyrin iron. In this reaction Compound I (in which formal oxidation state of Fe is +5), a spectroscopically distinct and enzymatically active form of CAT, is formed. Compound I is an oxyferryl species in which one oxidation equivalent is removed from the iron and one from the porphyrin ring to generate a porphyrin rr-cation radical [206] ... 

After compound P (or K), the heterolytic cleavage of the oxygen-oxygen bond occurs and the two electrons leave together with the formed water. This results in another symmetric oxygen-iron compound, Q, which is probably an iron-bound oxene (31, 63,147). The compound Q has two fewer electrons than the diferric state. Since a perferryl state is not expected to occur as a stable form in a biological compound, Q needs two iron ions. Besides, a mononuclear iron without a second redox partner would probably lead to formation of toxic amino acid... 

Scheme 53 Heterolytic cleavage of Ha at a low-coordinate iron nitrido compound...

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