Iron porphyrins oxidation catalysis

The reduction ofsec-, and /-butyl bromide, of tnins-1,2-dibromocyclohexane and other vicinal dibromides by low oxidation state iron porphyrins has been used as a mechanistic probe for investigating specific details of electron transfer I .v. 5n2 mechanisms, redox catalysis v.v chemical catalysis and inner sphere v.v outer sphere electron transfer processes7 The reaction of reduced iron porphyrins with alkyl-containing supporting electrolytes used in electrochemistry has also been observed, in which the electrolyte (tetraalkyl ammonium ions) can act as the source of the R group in electrogenerated Fe(Por)R. ... 

In relation to enzymic cytochrome P-450 oxidations, catalysis by iron porphyrins has inspired many recent studies.659 663 The use of C6F5IO as oxidant and Fe(TDCPP)Cl as catalyst has resulted in a major improvement in both the yields and the turnover numbers of the epoxidation of alkenes. 59 The Michaelis-Menten kinetic rate, the higher reactivity of alkyl-substituted alkenes compared to that of aryl-substituted alkenes, and the strong inhibition by norbornene in competitive epoxidations suggested that the mechanism shown in Scheme 13 is heterolytic and presumably involves the reversible formation of a four-mernbered Fev-oxametallacyclobutane intermediate.660 Picket-fence porphyrin (TPiVPP)FeCl-imidazole, 02 and [H2+colloidal Pt supported on polyvinylpyrrolidone)] act as an artificial P-450 system in the epoxidation of alkenes.663... 

As stated previously, the silver(III) state is stabilized by the electron-rich nature of the ligand in addition to the cavity size contribution. The silver(III) center may easily obtain an electron from the ligand to form a ligand radical, which satisfies the metal ion s high electrophilicity. This finding may explain why in some silver(lll) complexes the silver(II) and (III) states can switch reversibly. Whether this property of silver can be utilized in oxidation catalysis the way iron and manganese porphyrin systems were used still has to be seen (27-30). 

Mukaiyama and coworkers [1378] have performed such oxidations with molecular oxygen in the presence of pivaldehyde under catalysis with related salen 3.70 Mn(lH) complexes however, lower enantioselectivities were obtained. The use of various manganese or iron porphyrins as catalysts also gives lower enantioselectivities [971, 1379, 1379a], as do the oxidations with chiral oxaziridines in stoichiometric amounts [741],... 

Biomimetic oxidation catalysis has largely focused on complexes with planar tetradentate ligands such as manganese or iron porphyrins and related macrocyclic trans-chelates[5]. There is considerable interest in the synthesis of multinuclear metal complexes since these molecules might be useful as building block for magnetic molecular materials[6] and model compounds for understanding the properties of metalloproteins[7]. 

Epoxidation. With Mn-salen as a catalyst, enol derivatives give 2-hydroxy acetals. Unsaturated acids form lactones during oxidation in the presence of an iron porphyrin. Glycosylation. The action of a PhlO-MejSiOTf combination on thioglycosides consists of oxidation and Lewis acid catalysis, thus allowing the synthesis of disaccharides. 

The initial examples of Fe-oxo species catalyzed C—H bond insertion reactions employed chiral iron porphyrin complexes. As early as the 1980s, Viski and co-workers demonstrated the utility of a chiral iron porphyrin catalyst C20 in the asymmetric hydroxylation reaction (Scheme 1.56). Under the catalysis of C20, benzylic alcohols 145 were prepared in the presence of stoichiometric amounts of oxidant PhlO. At the same time, they also tested a manganese porphyrin complex, which also led to the hydroxylated product, albeit with lower efficiency. 

Tumas and coworkers (238) have investigated the oxidation of cyclohexene using molecular oxygen catalyzed by halogenated iron porphyrins in SCCO2. The authors note that homogeneous oxidation catalysis in SCCO2 remains relatively unexplored, despite the attractive features afforded by this solvent, such... 

At least two systems can be cited as catalysts of peroxide oxidation the first are the iron (III) porphyrins (44) and the second are the Gif reagents (45,46), based on iron salt catalysis in a pyridine/acetic acid solvent with peroxide reagents and other oxidants. The author s opinion is that more than systems for stress testing these are tools useful for the synthesis of impurities, especially epoxides. From another point of view, they are often considered as potential biomimetic systems, predicting drug metabolism. Metabolites are sometimes also degradation impurities, but this is not a general rule, because enzymes and free radicals have different reactivity an example is the metabolic synthesis of arene oxides that never can be obtained by radical oxidation. 

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