Manganese Porphyrin Catalysts

Fig. 8 Proposed mechanism of electrocatalytic epoxidation of olefins by manganese-porphyrin catalysts (reprinted with...

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... 

Amination of alkanes (cyclohexane, heptane, adamantane) was achieved with iron and manganese porphyrin catalysts by tosylimidoiodobenzene to yield tosyl-amino derivatives.200 Selective 1-substitution of adamantane (56% yield) and 2-substitution of heptane (66% selectivity) were reported. 

The first reports on iron-catalyzed aziridinations date back to 1984, when Mansuy et al. reported that iron and manganese porphyrin catalysts were able to transfer a nitrene moiety on to alkenes [90]. They used iminoiodinanes PhIN=R (R = tosyl) as the nitrene source. However, yields remained low (up to 55% for styrene aziridination). It was suggested that the active intermediate formed during the reaction was an Fev=NTs complex and that this complex would transfer the NTs moiety to the alkene [91-93]. However, the catalytic performance was hampered by the rapid iron-catalyzed decomposition of PhI=NTs into iodobenzene and sulfonamide. Other reports on aziridination reactions with iron porphyrins or corroles and nitrene sources such as bromamine-T or chloramine-T have been published [94], An asymmetric variant was presented by Marchon and coworkers [95]. Biomimetic systems such as those mentioned above will be dealt with elsewhere. 

It is shown [100] that polyhalide derivatives of iron and manganese porphyrin catalysts applied on Si02 and polymeric matrices via their amino functional group were successfully synthesized. 

Scheme 5.3 Epoxidation of styrene with manganese-porphyrin catalysts...

Scheme 5.22 Oxidation of tetralin with the manganese-porphyrine catalyst 50...

Up to now, only a few catalyst systems based on organic polymers such as molybdenum compounds supported on benzimidazole, polystyrene, or poly(gly-cidyl methacrylate) [9] as well as micelle-incorporated manganese-porphyrin catalysts [66] have been tested in the epoxidation of propene. Molybdenum-doped epoxy resins were also employed in the epoxidation of propene with TBHP and propene oxide yields of up to 88% were obtained [65]. The catalysts were employed repeatedly in up to 10 reactions without significant loss of activity and metal leaching proved to be very low. 

E. Brule, Y. R. de Miguel, Supported manganese porphyrin catalysts as P450 enzyme mimics for alkene epoxidation. Tetrahedron Lett. 43 (2002) 8555. 

Table I shows our results with C1-C3 and cycloCg hydrocarbons and manganese porphyrin catalysts 1 and 2 (Figure 1), with iodosylbenzene as the monooxygen transfer reagent, at room temperature in methylene chloride. It is evident that the supramolecule and open-faced porphyrin catalysts have similar reactivities with the hydrocarbons studied. Also, it is unfortunate that methane is not activated to methanol however, ethane, propane, and cyclohexane are converted to their respective alcohols. Hence, we did not see any special reactivity with the supramolecule catalyst, 1, and rationalize that too much flexibility in the "basket handles" does not provide the shape selectivity that we hoped for to gain a kinetic advantage with the difficult to react methane gas.

POLYMER BOUND MANGANESE PORPHYRIN CATALYSTS FOR OLEFIN EPOXIDATIONS WITH HYDROGEN PEROXIDE )... 

Oxidation of 2,6-DTBP was used as a test reaction for a manganese porphyrin catalyst fixed on a zeolite [104]. 

High yields are obtained with iminoiodinane reagents, however, they cannot be stored and must be used immediately [15]. Moreover, they are tedious to purify, as they form polymeric structures and are insoluble in most organic solvents. Besides tosyl- and nosyl-substituted iminoiodinanes, others are almost impossible to isolate in pure form. It is to overcome these problems that one-pot processes in which the iminoiodinane species is prepared in situ rather than being isolated, have been developed. Che first demonstrated such a reaction for an intermolecular C H insertion (Eq. (5.3)) [31, 32]. The reactive metal nitrene species is generated from a mixture of the requisite amine with diacetoxyiodobenzene, in the presence of manganese porphyrin catalyst. 

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

Closely related to this, other olefins were tested in epoxidation reactions. The heterogeneous epoxidation of cyclooctene with H2O2 in scC02 using a manganese porphyrinate catalyst in the presence of hexafluoroacetone hydrate as the co-catalyst at 40" C and 200 bar, leads to complete transformation into cyclooctene oxide. The co-catalyst forms perhydrates and thereby helps to stabilise the porphyrine catalyst and to solubilise H2O2 in SCCO2. 

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