Iron tetraarylporphyrins

In the early 1970s it was discovered that P-450 cytochromes are irreversibly inhibited during the metabolism of xenobiotics (1). The formation of a modified heme prosthetic group is associated with enzyme inhibition and subsequent studies have identified these modified complexes as N-alkylated protoporphyrin-IX (2). The chemistry of N-sub-stituted porphyrins was comprehensively reviewed by Lavallee in 1987 (3). Since that time, there have been many significant contributions to this field by several groups. The goal of this chapter is to summarize some of this work as it relates to the mechanism of formation and reactivity of iron N-alkyl porphyrins. Biomimetic model complexes have played an important role in elucidating the chemistry of N-alkyl hemes in much the same way that synthetic iron tetraarylporphyrins have aided... 

NMR spectroscopy is uniquely effective in probing the oxidation state, spin state, and ligation state of iron porphyrins (47, 48) and iron N-alkylporphyrins (22). For iron-tetraarylporphyrins the /3-pyrrole proton resonance is most indicative of the electronic structure of the metal... 

H NMR spectroscopy studies of iron(IIl) a-alkyl and o-aryl porphyrins have been very important in elucidating spin states. Alkyl and most aryl complexes with simple porphyrin ligands (OEP, TPP, or TTP) are low spin, S — I /2 species. NMR spectra for the tetraarylporphyrin derivatives show upheld resonances for the porphyrin pyrrole protons (ca. — 18 to —35 ppm), and alternating upfield and downfield hyperfine shifts for the axial alkyl or aryl resonances. For -alkyl complexes, the a-protons show dramatic downfield shifts (to ca. 600 ppm), upfield shifts for the /3-protons (—25 to — 160 ppm) and downfield shifts for the y-protons (12 ppm). The cr-protons of alkyliron porphyrins are not usually detected as a result of their large downfield shift and broad resonance. These protons were first detected by deuterium NMR in the dcuterated complexes Fe(TPP)CD3 (532 ppm) and Fe(TPP)CD2CDi (562, -117 ppm). ... 

Scheme 1 Schematic structures of iron meio-tetraarylporphyrins and preparation of Fe-porphyrin carbene complexes...

Campagna, S.R., Koronaios, R, Osteryoung, R.A., Cornman, C.R., Spectroscopy and coordination chemistry of iron(lll) tetraarylporphyrins in room-temperature ionic liquids. Book of Abstracts INOR-121, 217th ACS National Meeting, Anaheim, California, 1999. 

From Iron(III) Tetraarylporphyrins and Alkenes. N-alkyl porphyrins are formed via side reactions of the normal catalytic cycle of cytochromes P-450 with terminal alkenes or alkynes. N-alkylpor-phyrins formed from terminal alkenes (with model iron porphyrin catalysts under epoxidation conditions) usually have a covalent bond between the terminal carbon atom of the alkene and a pyrrole nitrogen. The double bond is oxidized selectively to an alcohol at the internal carbon. Mansuy (23) showed that, in isolated examples, terminal alkenes can form N-alkylated products in which the internal carbon is bound to the nitrogen and the terminal carbon is oxidized to the alcohol. Internal alkenes may also form N-alkyl porphyrins (24, 25). 

A good structural and functional model of the heme/nonheme di-iron center of the active site of the prototype NOR has been developed and characterized by Karlin and co-workers.The complexes consist of a binucleating ligand with a tetradentate tris(2-pyridylmethyl)amine moiety tethered to a synthetic tetraarylporphyrin (with three 2,6-difluorophenyl meso substituents see Figure 11). As discussed above, the resonance Raman spectra of the oxidized compound show features in common with NOR supporting the assignment of the active site as oxo-bridged in the oxidized enzyme. 

The first example of FB oxidation of sulfides dates back to 1995 dibenzothiophene and diphenyl stdfide gave the corresponding sulfones in low yields (1.4% and 10%, respectively) upon treatment with O2 at 100 °C in the presence of a not fully characterized perfluorocarbon-soluble iron—phthalocyanine [19]. Following this earlier report, Co(ll)—tetraarylporphyrin Co-5 and Co(I I [—phthalocyanine Co-12 (cf Stmcture) were tested as catalysts for the FB oxidation of methyl phenyl sulfide and para-substituted aryl methyl sulfides with O2 and a sacrificial aldehyde (Table 3) [20]. 

Most of the metalloporphyrin systems used as P450 models are variations on the theme of meso tetraarylporphyrins, with a variety of substituents in the meso-positions. These models manage to combine effective stereospecific oxidation of the products, protection of the iron porphyrin catalyst from... 

One of the problems with much of the work on P450 models is that reactions are performed in organic solvents in which it is not possible to obtain detailed information about the reaction mechanisms involved. This is because the proton activity in organic solvents is not easily determined. It is only in aqueous solution that the conditions necessary for oxygen transfer, such as ionic strength, acidity, and ligand species concentration, are best controlled, and data (e.g. electrochemical and kinetic) are best interpreted. To that end, water-soluble iron and manganese tetraarylporphyrins have been prepared by Bruice et al. and their reactions with hydroperoxides studies. ... 

We describe here a summary of our studies using iron, manganese and chromium /w f -tetraarylporphyrins. An understanding of the mechanisms of these simple cases have begun to provide a conceptual basis for the understanding of the enzymatic pathways. Consideration is given to two problems (1) the mechanisms of oxidation of metalloporphyrins by hydroperoxides in aqueous solution and (2) the mechanism of alkene epoxidation by higher valent metallo-oxo porphyrins. 

A. Robbins, Oxoiron(IV) porphyrins derived from charged iron(III) tetraarylporphyrins and chemical oxidants in aqueous and methanolic solution, J. Chem. Soc. Perkin Trans. 2 549 (1991)... 

The X-ray structure of this complex was reported by Bottomley and coworkers [88]. Remarkably, the electronic structure of such a complex was predicted before its preparation [72], Reduction of trichloromethyltrimethyl-silane by iron(II) tetraarylporphyrins in the presence of a reducing agent also leads to the carbide complex [89]. This surprising result can be explained by the involvement of an unstable a-silylcarbene ferroporphyrin complex. A het-erometallic /i2-carbido complex was isolated from the reaction of a dichloro carbene iron porphyrin and pentacarbonylrhenate (Eq. 13) [90]. The X-ray structure of this trinuclear complex, [(TPP)Fe = C = Re(CO)4Re(CO)5], shows a 1,3-dimetalla-allene system. 

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