Porphins, oxidation

Therefore, it appears that the redox properties of the metallo-porphyrin are required only for the initiation step in these free-radical autoxidations and that the porphyrin is not a stoichiometrically significant catalyst (21, 22, 23). The failure of these simple approaches to a reaction iron-porphine oxide or its equivalent could indicate that the ligation state of iron in the protein, presumably including an axial thiolate, is crucial to the oxygen-transfer properties of P 450. Support... 

In the first step, the fairly acidic proton on CIO of the red biladiene-ac salt 6 is abstracted and, even in solution in polar solvents, the salts are converted into the corresponding yellow bilatriene-u/ic salts 7. With a base such as piperidine, the salts 7 form the green bilatriene-a/>e free base. For further reaction to the porphyrin it is important that the salts 7 are oxidized to the bilatriene enamines 8 which cyclize via the electrophilic carbon of the terminal pyrrole ring by the loss of the leaving group X to 9. Porphin (10) is finally obtained by the loss of... 

Metal complexes of porphine and azaporphine compounds as catalysts of reactions involving oxidation of molecular oxygen. N. S. Enikolopyan, K. A. Bogdanova and K. A. Askarov, Russ. Chem. Rev. (Engl. Transl), 1983, 52,13-26 (161). 

Although oxygen was found to be the only oxidant for conversion of coproporphyrinogen III to protoporphyrin IX, anaerobic systems must obviously exist for the biosynthesis of the latter molecule (43). Porphine itself has not been found in nature but spectral lines identical to those of bis-pyridylmagnesiumtetrabcnzoporphine have been detected in interstellar space (53). 

Manganese nitrosyl porphyrins [215] are considered good models for the iron-nitric oxide analogs, which are relatively unstable but very vital to many biological operations. A six-coordinate manganese nitrosyl porphyrin of the form (por)Mn(NO)(L), where por can be TTP (TTP = tetra(4-methylphenyl)porphine) and L = piperidine, methanol, 1-methyhmidazole, has been prepared [216] in moderate yields by the reductive nitrosylation of the (por)MnCl complex with NO in piperidine. The crystal structures of these compounds give indication of a linear Mn-NO bond [215]. 

See also Metallophthalocyanines absorption spectra during electrochemical oxidation, 321-325 absorption spectra during photolysis, 321-325 Iron porphine... 

N,N, N, iV -Tetrakis(2-benzimidazolylmethyl)-l,3-diaminopropan-2-ol, hydrogen peroxide determination, 637 A,A,A, iV -Tetrakis[2 (l -ethylbenzimidazol-yl)-l, 3-diamino-2-hydroxypropane], transition metal peroxides, 1066 meiO-Tetrakis(4-sulfonatophenyl)porphine, luminol oxidation, 646... 

Dih symmetrical porphyrins such as porphin itself,7 H2TPP and H2OEP9 are conveniently prepared from pyrrole and aldehyde by cyclocondensations (Scheme 1). Addition of Zn11 ion often improves the yield.10 The product is often contaminated with chlorin by-product, which can be converted to porphyrin by oxidants such as DDQ,10 or separated by chromatography.11 Bulky substituents tend to lower the yield, and meso o-substituted phenyl groups give rise to atrope isomers. 

AAA-alumina (75% Si02/25% AI2O3 aluminosilicate gel provided by Davison) was used without further pretreatment. Benzene solutions of either vanadyl porphyrin (5, 10, 15, 20-tetraphenyl-21H, 23H-porphine vanadium oxide) or vanadyl naphthenate were impregnated by incipient wetness onto the EuY or the gel to generate a loading of 1.0 wt % vanadium. Vanadyl naphthenate has been widely utilized as a model V-contaminant and metalloporphyrins have been identified as metallic species in crude oil (29-30). The solvent was removed under... 

We have demonstrated recently that epoxidation and hydroxyl-ation can be achieved with simple iron-porphine catalysts with iodosylbenzene as the oxidant (24). Cyclohexene can be oxidized with iodosylbenzene in the presence of catalytic amounts of Fe(III)TPP-Cl to give cyclohexene oxide and cyclohexenol in 55% and 15% yields, respectively. Likewise, cyclohexane is converted to cyclohexanol under these conditions. Significantly, the alcohols were not oxidized rapidly to ketones under these conditions, a selectivity shared with the enzymic hydroxylations. The distribution of products observed here, particularly the preponderance of epoxide and the lack of ketones, is distinctly different from that observed in an autoxidation reaction or in typical reactions of reagents such as chromates or permanganates (15). 

The free base 5,10,15,20-tetraphenyl-21 A/,23//-porphine (H2tpp) may be prepared from pyrrole and benzaldehyde4 or purchased from Strem Chemicals, P.O. Box 108, Newbury Port, MA 01950 or Aldrich Chemical Co., 940 St. Paul Ave., Milwaukee, WI 53233. The material should be chlorin free, which can be effected either by dry column chromatography using alumina and chloroform or by oxidation of the chlorin to porphyrin using 2,3-dichloro-5,6-dicyano-... 

The enormous amount of work on phthalocyanins1 reflects their prime importance to the dye and pigment industry and is attested to by the numerous patents in the literature. It is our experience, however, that these materials are rarely pure and that their purification, from unknown impurities, is invariably more troublesome than performing the complete synthesis. Phthalocyanins are stronger o-donors than porphines, and the former stabilize higher oxidation states of coordinated metals,2 while the tetrabenzoporphyrins have properties more characteristic of porphines. 

Phthalocyaninato(2-)] iron(II) is a dark blue, thermally stable solid that can be sublimed in vacuo at 300°. It is very soluble in pyridine, giving deep blue solutions of the bis(pyridine) adducts. It also forms an unstable purple hexaaniline adduct when dissolved in aniline. It is soluble in concentrated sulfuric add and dimethyl sulfoxide (slightly) but is insoluble in most other organic solvents. The iron(II) complex, unlike the corresponding iron(II) porphines, is relatively stable toward oxidation to the iron (III) state. The electronic spectrum shows the following absorption bands (1-chloronaphthalene solution) 595 (e = 16,000), 630 (e = 17,000), 658 (e = 63,000) (pyridine solution) 333 (e = 45,000), 415 (e = 15,000), 395 (e = 2000), 658 nm (e = 8000). 

Anodic oxidation of porphin in alkyl nitriles at a platinum electrode takes place in 2 one-electron steps.203 The loss of electrons occurs from the lone pairs on the nitrogen atoms according to Scheme 22. 

Remarkably, a thetic polymer is able to stabilize porphyrins against oxidation. If Mg-octaethylporphin 10) in solid poly(l-vinylimidazole-co-styrene) with more than 20% imidazole units is irradiated in the presence of O2 the jr-cation radical of the ptophyrin is formed quantitatively . With less than 0.1% imidazole units the porphine is oxidized to formylbiliverdin. The results are compared with chlorophyll which is perhaps stabilized in the natural protein against O2 in the same manner. 

The polarographic and controlled potential reduction of porphines (LXXII) has been investigated, and it was shown that porphine was reduced polarographically in two two-electron steps [372]. Reduction of /w o-porphyhyrinedimethyl ester yielded a product that was easily reoxidized by oxygen. From the UV spectrum of the reduced solution, which was reduced in deuterium oxide, it could be shown that the reduction yielded a phlorine (LXXIII) rather than a chlorine (LXXIV). Reduction of a chlorine yielded a chlorine/phlorine. ... 

The formation of the same iron-oxygen covalent bonds from either (1) oxidized iron plus oxy anions via electron-transfer (redox) reactions or (2) radical-radical coupling reactions is summarized in Table 3-11. The valence-electron hybridization for the iron center is included as well as the spin state and estimated covalent bond-formation free energy (AGbf)- A similar set of reactions and data for iron-porphyrin compounds is presented in Table 3-12. Section a emphasizes that, just as the combination of a proton with a hydroxide ion yields a covalent H-OH bond (Table 3-11), (1) the combination of protons and porphyrin dianion (Por -) yields covalent porphine (H2Por), and (2) the addition of Lewis acids (Zn2+ or Fe2+) to porphine (H Por) oxidatively displaces protons to give covalent-bonded ZnilPor and Fe iPor. 

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