Iron , porphyrins

FIGURE 16-4 Electronic Structure of Oxyhemoglobin, (a) The most likely interaction between O2 in the ground state and Fe(II)-heme in the high-spin state x and y axes bisect the angle N — Fe—N. The signs on the oxygen orbitals are appropriate for the it orbitals. 

Other heme compounds are also active biochemically. Cytochrome P-450 catalyzes oxidation reactions in the liver and adrenal cortex, helping to detoxify some substances by adding hydroxyl groups that make the compounds more water-soluble and more susceptible to further reactions. Unfortunately, at times this process has the reverse effect because some relatively safe molecules ai-e converted into potent carcinogens. Peroxidases and catalases are Fe(III)-heme compounds that decompose hydrogen peroxide and organic peroxides. The reactions seem to proceed through Fe(IV) compounds with another unpaired electron on the porphyrin, which becomes a radical cation. Similar intermediates are also known in simpler porphyrin molecules.  

A group of cytochromes (labeled a, b, and c, depending on their spectra) serve as oxidation-reduction agents, converting the energy of the oxidation process into the synthesis of adenosine triphosphate (ATP), which makes the energy more available to other reactions. Copper is also involved in these reactions. The copper cycles between Cu(II) and Cu(I) and the iron cycles between Fe(nl) and Fe(II) during the reactions. Details of the reactions are available in other sources.  

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Iron Porphyrins. Porphyrias (15—17) are aromatic cycHc compouads that coasist of four pyrrole units linked at the a-positions by methine carbons. The extended TT-systems of these compounds give rise to intense absorption bands in the uv/vis region of the spectmm. The most intense absorption, which is called the Soret band, falls neat 400 nm and has 10. The TT-system is also responsible for the notable ring current effect observed in H-nmr spectra, the preference for planar conformations, the prevalence of electrophilic substitution reactions, and the redox chemistry of these compounds. Porphyrins obtained from natural sources have a variety of peripheral substituents and substitution patterns. Two important types of synthetic porphyrins are the meso-tetraaryl porphyrins, such as 5,10,15,20-tetraphenylporphine [917-23-7] (H2(TPP)) (7) and P-octaalkylporphyrins, such as 2,3,7,8,12,13,17,18-octaethylporphine [2683-82-1] (H2(OEP)) (8). Both types can be prepared by condensation of pyrroles and aldehydes (qv). 

N—Fe(IV)Por complexes. Oxo iron(IV) porphyrin cation radical complexes, [O—Fe(IV)Por ], are important intermediates in oxygen atom transfer reactions. Compound I of the enzymes catalase and peroxidase have this formulation, as does the active intermediate in the catalytic cycle of cytochrome P Q. Similar intermediates are invoked in the extensively investigated hydroxylations and epoxidations of hydrocarbon substrates cataly2ed by iron porphyrins in the presence of such oxidizing agents as iodosylbenzene, NaOCl, peroxides, and air. 

Luminol chemiluminescence has also been recommended for measuring bacteria populations (304,305). The luminol—hydrogen peroxide reaction is catalyzed by the iron porphyrins contained in bacteria, and the light intensity is proportional to the bacterial concentration. The method is rapid, especially compared to the two-day period required by the microbiological plate-count method, and it correlates weU with the latter when used to determine bacteria... 

Reinvestigation of iron porphyrins by Mossbauer spectroscopy using synchrotron radiation 98PAC917. 

Heme (C34H3204N4Fe) represents an iron-porphyrin complex that has a protoporphyrin nucleus. Many important proteins contain heme as a prosthetic group. Hemoglobin is the quantitatively most important hemoprotein. Others are cytochromes (present in the mitochondria and the endoplasmic reticulum), catalase and peroxidase (that react with hydrogen peroxide), soluble guanylyl cyclase (that converts guanosine triphosphate, GTP, to the signaling molecule 3, 5 -cyclic GMP) and NO synthases. 

Spin state/stereochemical relationships in iron porphyrins implications for the hemoproteins. W. R. Scheidt and C. A. Reed, Chem. Rev., 1981, 81, 543-555 (111). 

Steady-state kinetics. Consider the interchange of 02 and CO coordinated to myoglobin, an iron porphyrin represented as PFe ... 

Under anaerobic conditions, p,p -DDT is converted to p,p -DDD by reductive dechlorination, a biotransfonnation that occurs postmortem in vertebrate tissues such as liver and muscle and in certain anaerobic microorganisms (Walker and Jefferies 1978). Reductive dechlorination is carried out by reduced iron porphyrins. It is carried out by cytochrome P450 of vertebrate liver microsomes when supplied with NADPH in the absence of oxygen (Walker 1969 Walker and Jefferies 1978). Reductive dechlorination by hepatic microsomal cytochrome P450 can account for the relatively rapid conversion of p,p -DDT to p,p -DDD in avian liver immediately after death, and mirrors the reductive dechlorination of other organochlorine substrates (e.g., CCI4 and halothane) under anaerobic conditions. It is uncertain to what extent, if at all, the reductive dechlorination of DDT occurs in vivo in vertebrates (Walker 1974). 

The synthesis, reactivity, spectroscopy, and electrochemi.stry of organometallic iron porphyrins was de.scribed in some detail in the three reviews published in the period from 1986 to I988. Although a brief synopsis of the early chemistry will be given here, this review will focus on more recent developments. 

Iron porphyrins containing vinyl ligands have also been prepared by hydromet-allation of alkynes with Fe(TPP)CI and NaBH4 in toluene/methanol. Reactions with hex-2-yne and hex-3-yne are shown in Scheme 4. with the former giving two isomers. Insertion of an alkyne into an Fe(III) hydride intermediate, Fe(TPP)H, formed from Fe(TPP)Cl with NaBH4, has been proposed for these reactions. " In superficially similar chemistry, Fe(TPP)CI (present in 10 mol%) catalyzes the reduction of alkenes and alkynes with 200 mol% NaBH4 in anaerobic benzene/ethanol. For example, styrene is reduced to 2,3-diphenylbutane and ethylbenzene. Addition of a radical trap decreases the yield of the coupled product, 2,3-diphenylbutane. Both Fe(lll) and Fe(II) alkyls, Fe(TPP)CH(Me)Ph and [Fe(TPP)CH(Me)Ph] , were propo.sed as intermediates, but were not observed directly. ... 

The reduction ofsec-, and /-butyl bromide, of tnins-1,2-dibromocyclohexane and other vicinal dibromides by low oxidation state iron porphyrins has been used as a mechanistic probe for investigating specific details of electron transfer I .v. 5n2 mechanisms, redox catalysis v.v chemical catalysis and inner sphere v.v outer sphere electron transfer processes7 The reaction of reduced iron porphyrins with alkyl-containing supporting electrolytes used in electrochemistry has also been observed, in which the electrolyte (tetraalkyl ammonium ions) can act as the source of the R group in electrogenerated Fe(Por)R. ... 

Only a handful of rr-bonded iron porphyrin complexes have been structurally characterized, listed in Table HI, and four of these contain porphycene. corrolc. or phthalocyanine ligands rather than porphyrins. " "" Selected data arc given in Table III, and X-ray crystal structures of methyl- and phenyliron porphyrin complexes are shown in Fig. 4. All of the iron(III) porphyrin complexes exhibit... 

Selec ted Data eor Strlctlrally Characterised Iron Porphyrin Complexes... 

Iron porphyrins (containing TPP, picket fence porphyrin, or a basket handle porphyrin) catalyzed the electrochemical reduction of CO2 to CO at the Fe(I)/Fe(0) wave in DMF, although the catalyst was destroyed after a few cycles. Addition of a Lewis acid, for example Mg , dramatically improved the rate, the production of CO, and the stability of the catalyst. The mechanism was proposed to proceed by reaction of the reduced iron porphyrin Fe(Por)] with COi to form a carbene-type intermediate [Fe(Por)=C(0 )2, in which the presence of the Lewis acid facilitates C—O bond breaking. " The addition of a Bronsted acid (CF3CH2OH, n-PrOH or 2-pyrrolidone) also results in improved catalyst efficiency and lifetime, with turnover numbers up to. 750 per hour observed. ... 

The photochemistry of several of the iron porphyrin halocarbene complexes Fe(TPP)(=CXY) (CXY = CCF, CBr2, CCIF. CCl(CN) and the vinylidene complex Fe(TPP)=C=CAr2 (Ar = p-C6Fl4Cl) has been studied in degassed benzene... 

Diazoalkanes are u.seful is precursors to ruthenium and osmium alkylidene porphyrin complexes, and have also been investigated in iron porphyrin chemistry. In an attempt to prepare iron porphyrin carbene complexes containing an oxygen atom on the /(-carbon atom of the carbene, the reaction of the diazoketone PhC(0)C(Ni)CH3 with Fe(TpCIPP) was undertaken. A low spin, diamagnetic carbene complex formulated as Fe(TpCIPP)(=C(CH3)C(0)Ph) was identified by U V-visible and fI NMR spectroscopy and elemental analysis. Addition of CF3CO2H to this rapidly produced the protonated N-alkyl porphyrin, and Bit oxidation in the presence of sodium dithionitc gave the iron(II) N-alkyl porphyrin, both reactions evidence for Fe-to-N migration processes. ... 

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