Metalloporphyrin intermediate, active oxygen

The mechanism of oxygen activation by P-450 has been extensively studied (7c, 5a, b) however, cmcial intermediates in the catalytic cycle are so unstable that it is not possible to examine their structures and reactivities. Accordingly, model systems for P-450 with synthetic metalloporphyrins have illuminated the enzymic reaction mechanism. For instance, the so-called picket fence porphyrin, /ncjo-tetra(a,a,a,a-pivalamidophenyl)porphyrin (TpivPP) Fe(II), prepared by Collman and co-workers, forms a stable complex with O2 6a, b). An X-ray... 

This is similar to the oxenoid species proposed earlier as the active intermediate of enzymatic oxidations. It has been made accessible for mechanistic studies via the reaction of metalloporphyrins (Fe,Mn) with single oxygen donors, such as iodosylbenzene, hypochlorite, etc. This concept helped design efficient and selective epoxidation systems. Selective epoxidizing catalysts utilizing dioxygen are based on ruthenium porphyrins. 

Metalloporphyrin complexes have been shown to catalyze epoxidation of alkenes (Fig. 12), and in this case metal oxo intermediates have been implicated [19-21]. In most cases strong oxidants are required however, it has recently been reported that a ruthenium trans-dioxo complex catalyses direct epoxidation of an alkene with molecular oxygen as the oxidant [22]. It is believed that porphyrinato metal complex-catalyzed epoxidations are related to alkene epoxidations catalyzed by cytochrome P-A50 or other active enzymatic systems. It has recently been shown that creating site-isolation by anchoring (tetra-phenylporphinato)manganese(lll) acetate to a rigid polymer support considerably enhances the rate of epoxidation of cyclohexene by sodium hypochlorite [23]. 

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