Manganese porphyrins catalytic cycle

Figure 6.10 (a) Structure of manganese porphyrin catalyst 14. (b) Catalytic cycle Mn" -porphyrins adsorb to a Au(lll) surface and are reduced to Mn" in the next step one molecule of O2 is homolytically cleaved, distributing the two oxygen atoms over two Mn-porphyrins to generate two Mn —O species these are catalytically active in the epoxidation of added c/s-stilbene to give as... 

One motivation for the characterization of the above compounds has been to more fully understand the involvement of such higher valent manganese porphyrin complexes in model systems which imitate the catalytic activity of monooxygenase cytochrome P-450 and related enzymes. The catalytic cycle of cytochrome P-450 appears to involve the binding and reduction of molecular oxygen at a haem centre followed by the ultimate formation of a reactive iron oxo complex which is responsible for oxidation of the substrate. For example, cytochrome P-450 is able to catalyse alkane hydroxylation with great selectivity. 

There is also debate about the nature of the intermediates involved in the second step in the catalytic cycle illustrated in Fig. 19.1, that is, the transfer of oxygen to the alkene to form the epoxide. No intermediates have been detected experimentally, but five different possibilities have been proposed in the literature for the alkene complexed to the oxidized porphyrin [11,25-29]. The five proposed intermediates are radical, cation, concerted, metallaoxetane, and pi-radical-cation species. The literature is rather complicated due to the lack of direct experimental observation, and it is not clear that conclusions from, say, iron and chromium porphyrins also apply to manganese porphyrins [28]. Arasasingham et al. claim unequivocal evidence for a radical intermediate being involved in the oxidation of alkenes by manganese porphyrins [28]. They also discuss a charge-transfer complex that is similar to the concerted intermediate. Recently, density functional theory (DFT) and quantum mechanics/molecular mechanics (QM/MM) calculations were applied to styrene epoxidation by Mn-porphyrins ... 

Figure 19 (a) Molecular structure of the manganese porphyrin catalyst, (b) Proposed catalytic cycle, (c) Series of STM images of... 

Fig. 37 Catalytic cycle for the fluorination of aliphatic substrates initiated by the manganese porphyrin derivative 107...

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