Biomimetic iron catalysts

Non-heme Iron Catalysts in Biological and Biomimetic Transformations... 

Traylor (38) has also shown that biomimetic iron N-alkylporphyrins themselves are competent catalysts for epoxidation of alkenes with a rate constant of about 104 M-1 s-1. On the basis of these observations and rearrangement reactions of specific alkenes, Traylor has proposed the reaction sequence outlined in Scheme 3 as representative of the oxidation and N-alkylation reactions of the P-450 model systems. In this scheme, the epoxide and the N-alkylated heme are derived from a common, electron-transfer intermediate (caged ferrylporphyrin-alkene cation radical). Collman and co-workers (28, 29) prefer a concerted mechanism (or a short-lived, acyclic intermediate) for epoxidation and N-alkylation reactions. Both authors note that the reactions catalyzed by cytochrome P-450 (and biomimetic reactions) probably can not be ascribed to any single mechanism. 

Costas M, Chen K, Que L (2000) Biomimetic nonheme iron catalysts for alkane hydroxyl-atlon. Coord Chem Rev 200-202 517-544... 

Bakke et al. (1982) have shown how montmorillonite catalyses chlorination and nitration of toluene nitration leads to 56 % para and 41 % ortho derivative compared to approximately 40 % para and 60 % ortho derivatives in the absence of the catalyst. Montmorillonite clays have an acidity comparable to nitric acid / sulphuric acid mixtures and the use of iron-exchanged material (Clayfen) gives a remarkable improvement in the para, ortho ratio in the nitration of phenols. The nitration of estrones, which is relevant in making various estrogenic drugs, can be improved in a remarkable way by using molecular engineered layer structures (MELS), while a reduction in the cost by a factor of six has been indicated. With a Clayfen type catalyst, it seems possible to manipulate the para, ortho ratio drastically for a variety of substrates and this should be useful in the manufacture of fine chemicals. In principle, such catalysts may approach biomimetic chemistry our ability to predict selectivity is very limited. 

Selective Conversion of Hydrocarbons with H202 Using Biomimetic Non-heme Iron and Manganese Oxidation Catalysts... 

A mild aerobic palladium-catalyzed 1,4-diacetoxylation of conjugated dienes has been developed and is based on a multistep electron transfer46. The hydroquinone produced in each cycle of the palladium-catalyzed oxidation is reoxidized by air or molecular oxygen. The latter reoxidation requires a metal macrocycle as catalyst. In the aerobic process there are no side products formed except water, and the stoichiometry of the reaction is given in equation 19. Thus 1,3-cyclohexadiene is oxidized by molecular oxygen to diacetate 39 with the aid of the triple catalytic system Pd(II)—BQ—MLm where MLm is a metal macrocyclic complex such as cobalt tetraphenylporphyrin (Co(TPP)), cobalt salophen (Co(Salophen) or iron phthalocyanine (Fe(Pc)). The principle of this biomimetic aerobic oxidation is outlined in Scheme 8. 

A sterically protected, water-soluble synthetic iron porphyrin could provide a readily available biomimetic catalyst for both basic research and potential industrial applications. Such a synthetic hemin might be superior to the enzyme, in that being a small molecule it could interact, with the polymeric lignin molecule more readily than can ligninase. 

For Further Reading M. Jacoby, Iron-rich zeolite stands up to water, Chemical and Engineering News. September 15, 1997, pp. 8-9. P. T. Tanev and T. J. Pinnavaia, Biomimetic templating of porous lamellar silicas by vesicular surfactant assemblies, Science 271 1267-1269. J. M. Thomas, Solid acid catalysts, Scientific American, April 1992, pp. 112-118. 

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. 

Scheme 4.8 Biomimetic transfer hydrogenation of ketones with iron porphyrin catalysts.

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