Porphyrin iron monoxide

Formation of the porphyrin iron monoxide (of the Compound I type) as an active oxidizing species in the P-450 system is supported by the successful monoxidation of P 450 by an oxidizing reagent such as iodosobenzene (see Figure 5) (16). This species then effectively hydroxylates an organic substrate. 

Significant characteristics of the porphyrin iron monoxide are seen in the chemical reactivity. Naphthalene is converted initially to the corresponding arene oxide on treatment with P 450 (19), consistent with a molecular mechanism of oxygen transfer from an iron monoxide to the aromatic nucleus. Retention of stereochemistry in the P-450 catalyzed hydroxylation of d ethylbenzene also supports the molecular mechanism. The unusually large kinetic isotope effect observed for the P-450 oxidation of dideutero 1,3-diphenylpropane, kJkD = 11, demonstrates that C—H cleavage is involved in the rate determining step (20), probably in a very unusual environment, not incompatible with a molecular mechanism. 

An explanation of the relative oxygen and carbon monoxide affinities of some iron(II) porphyrin complexes. T. Hashimoto and F. Basolo, Comments Inorg. Chem., 1981,1,199-205 (18). 

J. Hayon, D. Ozer, J. Rishpon, and A. Bettelheim, Spectroscopic and electrochemical response to nitrogen monoxide of a cationic iron porphyrin immobilized in nafion-coated electrodes or membranes. J. Chem. Soc.-Chem. Commun. 619-620 (1994). 

The product is exclusively carbon monoxide, and good turnover numbers are found in preparative-scale electrolysis. Analysis of the reaction orders in CO2 and AH suggests the mechanism depicted in Scheme 4.6. After generation of the iron(O) complex, the first step in the catalytic reaction is the formation of an adduct with one molecule of CO2. Only one form of the resulting complex is shown in the scheme. Other forms may result from the attack of CO2 on the porphyrin, since all the electronic density is not necessarily concentrated on the iron atom [an iron(I) anion radical and an iron(II) di-anion mesomeric forms may mix to some extent with the form shown in the scheme, in which all the electronic density is located on iron]. Addition of a weak Bronsted acid stabilizes the iron(II) carbene-like structure of the adduct, which then produces the carbon monoxide complex after elimination of a water molecule. The formation of carbon monoxide, which is the only electrolysis product, also appears in the cyclic voltammogram. The anodic peak 2a, corresponding to the reoxidation of iron(II) into iron(III) is indeed shifted toward a more negative value, 2a, as it is when CO is added to the solution. 

Other aspects of solvation have included the use of surfactants (SDS, CTAB, Triton X-100), sometimes in pyridine-containing solution, to solubilize and de-aggregate hemes, i.e., to dissolve them in water (see porphyrin complexes, Section 5.4.3.7.2). An example is provided by the solubilization of an iron-copper diporphyrin to permit a study of its reactions with dioxygen and with carbon monoxide in an aqueous environment. Iron complexes have provided the lipophilic and hydrophilic components in the bifunctional phase transfer catalysts [Fe(diimine)2Cl2]Cl and [EtsBzNJpeCU], respectively. 

The kinetics and mechanism for oxygen transfer between 4-cyano-V,V,-dimethylaniline V-oxide and a C2-capped mexo-tetraphenylporphyrinatoiron(III) and mc5 o-tetrakis(pentafiuorophenyl)-porphyrinatoiron(III) have been established. Addition of a copper(II) porphyrin cap to an iron(II)-porphyrin complex has the expected effect of reducing both the affinities and rate constants for addition of dioxygen or carbon monoxide. These systems were studied for tetradecyl-substituted derivatives solubilized by surfactants such as poly(ethylene oxide) octaphenyl ether. ... 

Much of the work on the photoreduction of carbon dioxide centres on the use of transition metal catalysts to produce formic acid and carbon monoxide. A large number of these catalysts are metalloporphyrins and phthalocyanines. These include cobalt porphyrins and iron porphyrins, in which the metal in the porphyrin is first of all photochemically reduced from M(ii) to M(o), the latter reacting rapidly with CO to produce formic acid and CO. ° Because the M(o) is oxidised in the process to M(ii) the process is catalytic with high percentage conversion rates. However, there is a problem with light energy conversion and the major issue of porphyrin stability. 

Iron, as found in the porphyrin derivative hemoglobin, complexes CO to form a stable metal carbonyl. Iron also forms a variety of metal carbon monoxide derivatives such as the homoleptic Fe(CO)5, Fe2(CO)9 and Fe3(CO)i2, the anionic [Fe(CO)4] and its covalent derivative Fe(CO)4Br2, [CpFe(CO)2] and its alkylated covalent derivatives CpFe(CO)2-R with its readily distinguished n (and and a (and / ) iron carbon bonds. By contrast. Mg in its chlorin derivative chlorophyll, which very much resembles porphyrin, forms no such bonds with CO nor is there a rich magnesium carbonyl chemistry (if indeed, there is any at all). 

Carbon monoxide reacts with [Fe(TPP)] to form a five-coordinate complex [Fe(TPP)CO], which can be reduced electrochemically to the corresponding iron(I) species from which, however,245 CO spontaneously dissociates. The Fe—CO interaction is stabilized by the,presence of hydrocarbon chains bound by amide linkages to the ortho position of the TPP phenyl rings. Carbon monoxide adducts of iron(I) complexes of a number of these superstructured porphyrins have been reported.245 The chemistry of these highly reduced species is of relevance to understanding240 the reactions of cytochrome P-450 and the peroxidases. 

Carbon monoxide binds readily to iron(II), but not iron(III), porphyrins to form complexes that are quite distinctive in terms of the spectral properties both of the heme and of the bound CO. Thus, CO has been widely used as a probe of the active site of heme proteins (113,... 

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