Iron species, generation

The epoxidation of olefins in the presence of ethyl 2-oxocyclopentanecarboxylate as co-substrate by 1/f-imidazole and air using FeCl3 6H20 in MeCN as catalyst was achieved with fair to excellent chemoselectivities. The epoxidation was caused by an active iron species generated by O2, which was activated by the co-substrate. The process involved 4e , instead of 2e , reduction of the O2 molecule in the commonly used peroxides. Aromatic olefins were also oxidized in high yields with excellent chemoselectivity.i2 ... 

Fenton chemistry comprises reactions of H2O2 in the presence of iron species to generate reactive species such as the hydroxyl radical OH. These radicals ( = 2.73 V) lead to a more eflident oxidation chemistry than H2O2 itself (E° = 1.80 V). 

Kim, K., Rhee, S.G., and Stadtman, E.R. (1985) Nonenzymatic cleavage of proteins by reactive oxygen species generated by dithiothreitol and iron./. Biol. Chem. 260, 15394-15397. 

The active species generated when bis(arylimino)pyridine iron (5) and cobalt (6) halides are activated with MAO was, by analogy with metallocene catalysts, initially considered to be a highly reactive mono-methylated cobalt(II) or iron(II) cation of the form LM-Me+ bearing a weakly coordinating counter-anion such as [X-MAO]-(X = halide, Me). To examine this theory a number of spectroscopic investigations have been directed towards identifying the active species (vide infra). 

Iron-centered tetradecker sandwiches, when in contact with oxygen on silica, undergo oxidative fusion with ejection of iron to generate 12-vertex Co2C4B8 clusters.83 The cluster geometry, however, depends on the nature of the substituent X at B(3) in the iron sandwich. When X = H or Cl, the isolated species are of type 62, also obtained directly from C0C2B3 complexes as described in Section 3.01.4.1 64 when X = Me, the cluster produced is 77.84... 

Fig. 14. A mechanism to explain heme modification in the P. vitcde catalase and possibly E. coli HPII. For simplicity, the phenyl ring of T3rr415 is not shown, and only ring III of the heme and the heme iron are shown. Compound I is an oxyferryl species formed, along with water, in the reaction of one H2O2 with the heme. The iron is in a formal Fe oxidation state, but one oxidation equivalent is delocalized on the heme to create the 0x0-Fe" -heme cation, shown as the starting species, compound I. A water on the proximal side of the heme is added to the heme cation species of compound 1 shown in A to generate a radical ion in B. The electron flow toward the oxo-iron would generate the cation shown in (C), leading to the spirolactone product shown in D. In E, an alternate mechanism for the His-Tyr bond formation in HPII is presented that could occur independently of the heme modification reaction. Reprinted with permission of Cambridge University Press from Bravo et al. (93).

Although the epoxidation by nitrous oxide proceeds over non-zeolite catalysts, they also include iron as an active element One may think that in all these cases a special oxygen species generated by N20 plays an important role, similar to the a-oxygen on FeZSM-5. 

Note that no product formation could be obtained in absence of the iron or copper catalyst. Without copper, the iron catalyst generates only stoichiometric amounts (with respect to iron) of an alkenyliron species, which is not reactive towards further transformations such as transmetallation or polymerization. A cuprate, generated by CuBr and ArMgBr, did not undergo carbometallation with the alkyne. These results indicate that the main role of the copper catalyst is most likely to promote the metal... 

Bondy SC, Kirstein S (1996) The promotion of iron-induced generation of reactive oxygen species in nerve tissue by aluminum. Mol Chem Neuropathol 27 185-194... 

Figure 20.8. Iron-mediated generation of reactive oxygen species and generation of singlestrand DNA breaks. (Adapted from Henle et al. J. Biol. Chem. 274, 962-971,1999).

As for iron, there is ample evidence that OH is generated in the Fenton reaction when the latter is carried out at acidic pH, but direct OH generation is often questionable at neutral or basic pH [4,24]. As above mentioned, other hydroxylation mechanisms have been proposed, based on the formation of hyper-valent iron species [100] such as perferryl (Fe=03+) or ferryl (Fe=02+), whose oxidizing powers are smaller than that of OH, for example ... 

It is possible to generate ferryl species by peroxide treatment of ferrous iron ions [18,249], The two-electron oxidation of ferrous (as opposed to ferric) iron does not require the formation of a cation radical, although subsequent reactions may generate hydroxyl radicals. These reactions therefore provide an alternative mechanism to the Fenton reaction for free radical damage associated with low-molecular-weight iron species. In the absence of a protective protein environment, however, such low-molecular-weight ferryl species are unstable and difficult to detect and therefore their existence is controversial [see the review by Koppenol in this volume (Chapter 1)]. 

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