Oxidation with metalloporphyrins

D. Catalytic Oxidation with Metalloporphyrins and Metal Salen... 

Advances in homogeneous oxidation with metalloporphyrins as catalysts 07CJ01039. 

The familiar standard de carbonyl at ion mechanism ( 3, 5) involving a concerted oxidative-addition of aldehyde, CO migration (with subsequent elimination), and reductive-elimination of product, would seem with metalloporphyrins to require coordination numbers higher than six, and in this case Ru(IV) intermediates. Although this is plausible, the data overall strongly suggest a radical mechanism and Ru(III) intermediates. 

At room temperature these polymer-Co(II)porphyrin complex, except CoMb, were irreversibly oxidized to the Co(HI) complexes. Oxidation of the Fe(II)-por-phyrin complexes proceeds more rapidly than that of the Co(II)porphyrin complexes. The next section deals with attempts to make polymer ligands act as inhibitors to the irreversible oxidation of metalloporphyrin complexes. 

The most common reactions involving nucleophiles and porphyrin systems take place on the metalloporphyrin 77-cation radical (i.e. the one-electron oxidized species) rather than on the metalloporphyrin itself. One-electron oxidation can be accomplished electrochemi-cally (Section 3.07.2.4.6) or by using oxidants such as iodine, bromine, ammoniumyl salts, etc. Once formed, the 77-cation radicals (61) react with a variety of nucleophiles such as nitrite, pyridine, imidazole, cyanide, triphenylphosphine, thiocyanate, acetate, trifluoroace-tate and azide, to give the correspondingly substituted porphyrins (62) after simple acid catalyzed demetallation (79JA5953). The species produced by two-electron oxidations of metalloporphyrins (77-dications) are also potent electrophiles and react with nucleophiles to yield similar products. 

Oxometalloporphyrins were taken as models of intermediates in the catalytic cycle of cytochrome P-450 and peroxidases. The oxygen transfer from iodosyl aromatics to sulfides with metalloporphyrins Fe(III) or Mn(III) as catalysts is very clean, giving sulfoxides, The first examples of asymmetric oxidation of sulfides to sulfoxides with significant enantioselectivity were published in 1990 by Naruta et al, who used chiral twin coronet iron porphyrin 27 as the catalyst (Figure 6C.2) [79], This C2 symmetric complex efficiently catalyzed the oxidation... 

On reaction with a stoichiometric amount of hydroperoxide, catalase and horseradish peroxidase are converted to a green colored intermediate. Compound I (5). The chemical nature of Compound I has been extensively debated since its discovery by Theorell 59). Recently, Dolphin et al. 60) have demonstrated that upon one-equivalent oxidation several metalloporphyrins are converted to stable porphyrin jr-cation radicals, the absorption spectra of which possess the spectral characteristics of Compound I, namely, a decreased Soret w-n transition and an appearance of the 620-670-nm absorption bands. Since Moss et al. 61) proposed the presence of Fe(IV) in Compound I of horseradish peroxidase from Mossbauer spectroscopic measurements, it is attractive to describe Compound I as Fe(IV)-P, where P is a porphyrin w-cation radical. Then, Compound I and Compound ES become isoelectronic. Both contain Fe(IV) and a radical the former as a porphyrin radical (P ) and the latter as a protein radical (R ). Then the reaction cycles of horseradish and cytochrome c peroxidases may be compared as shown in Fig. 4. 

Reviews (a) Metalloporphyrins in Catalytic Oxidations (Ed. R. A. Sheldon), Marcel Dekker, New York, 1994 (b) Metalloporphyrins Catalyzed Oxidations (Eds. F. Monta-nari, L. Casella), Kluwer, Dordrecht, 1994 (c) B. Meunier, in Catalytic Oxidations with Hydrogen Peroxide as Oxidant (Ed. G. Strukul), Kluwer, Dordrecht, 1992, p. 153. 

The group of Masui first attempted the direct epoxidation of olefins by using oxygen and NHPI with metalloporphyrins, but they obtained poor results [15]. Ishii and coworkers proposed two different methods. In the first protocol [16,17], the epoxidizing agent is obtained in situ by the aerobic oxidation of a suitable alcoholic (benzhydrol) compound in the presence of catalytic amounts of NHPI. The resulting oxidant, which is not able to promote the epoxidation by itself, is then activated in the presence of an olefin by catalytic amounts of hexafluoroacetone (HFA) (Scheme 6.1). 

The azo dyes used in this study were l-phenylazo-2-naphthol-6-sulfonate (2) and seven derivatives with substituents in the meta or para positions of the phenyl ring (3-9). These were selected as representative l-azo-2-naphthol sulfonate dyes and because the substituents on phenyl ring would allow a systematic study of the mechanism of their oxidation by metalloporphyrin-catalysed systems in aqueous solution. Dye 2 is commercially available (as Acid Orange 12) and was purified by reciystallisation whilst the others (3-9) were prepared by standard diazonium ion/2-naphthol coupling reactions. The purities of all the dyes were checked by TLC, MS and H NMR spectroscopy. Table 1 reports the measured pl values of all the azo dyes used in this study. 

The next important information on the product is derived from the electronic absorption spectrum. This can usually be obtained by electrolysis of a dilute solution of the metalloporphyrin at a constant potential or by oxidometric titrations directly in the absorption cell. Most redox reactions with metalloporphyrins give good isosbestic points when absorption spectra are taken at various stages of oxidation, and they are fully reversible when no chemical addition reaction to the porphyrin ligand has occurred. Ten typical absorption spectra of metalloporphyrins are given below and correlated with the various metalloporphyrin oxidation states. 

The rapid reaction rates of peroxides with metalloporphyrins and the detection of observable oxometalloporphyrin species, especially in the reactions of manganese porphyrins with various oxidants (m-CPBA, NaOCl, KHSOj), have also inspired the use of these porphyrins as detectors... 

Recent results in the field of catalytic oxidations with hydrogen peroxide are reviewed. Most effective catalysts fall into three categories metallorganic compounds, phase-transfer catalysts, redox zeolites. Metalloporphyrins and Pt-phosphine complexes are representative of first category. Mo and W polyoxome-talates and related systems, in association with phase transfer agents, belong to the second one. Titanium silicalite (TS-1) is the most studied redox zeolite. The oxidation of nitrogen and sulphur compounds and Fenton-like reactions are not reviewed. 

A unique property of metalloporphyrinates is the general stability and longevity of Tc-cation radicals as obtained in organic or aqueous solutions by oxidation with iodine, bromine, iV-bromosuccinimide (NBS), iron (III) salts, or cathodic oxidation. Titration of magnesiumporphyrinates in chloroform/methanol with NBS or iodine, for example, gives a clean and quantitative conversion to the 7i-cation rad-... 

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