Porphyrins alkyl-iron

The porphyrin ligand can support oxidation states of iron other than II and III. [Fe(I)Por] complexes are obtained by electrochemical or chemical reduction of iron(II) or iron(III) porphyrins. The anionic complexes react with alkyl hahdes to afford alkyl—iron (III) porphyrin complexes. Iron(IV) porphyrins are formally present in the carbene, RR C—Fe(IV)Por p.-carbido, PorFe(IV)—Fe(IV)Por nitrene, RN—Fe(IV)Por and p.-nittido, PorFe(IV)... 

Figure 3.29b). Unlike the reaction with the iron(O) porphyrin, the electron stoichiometry is of two electrons per molecule. The alkyl iron(III) porphyrin, now formed is indeed easier to reduced than the starting iron(II) porphyrin, thus giving rise to an ECE-DISP mechanism. The rate constant may again be derived from the loss of reversibility or from the positive shift of the wave when it has become totally irreversible, and also, this time, from the passage from a two- to a one-electron stoichiometry upon raising the scan rate (see Section 2.2.2). 

More data were gathered later concerning the reaction of iron(i) and also iron(o) porphyrins with various aliphatic bromides (Lexa et al., 1988). In the case of iron(o) porphyrins, the a-alkyl-iron(ii) complex is obtained directly... 

It should be emphasized that virmaUy all of the above discussion is based on biomimetic chemistry, where the Fe(II) source varies from salts such FeS04 to the more reactive FeCla-THaO as well as heme mimetics (TPP) and ester hematin variants. When heme models are used, since porphyrin alkylation is a favoured process, end-product distributions of products can be very different from when a free ferrous ion source is employed. Furthermore, solvent has been shown to have a profound effect on the rate of reaction and product distributions obtained in iron-mediated endoperoxide degradation. Thus all of these studies are truly only approximate models of the actual events within the malaria parasites. Future work is needed to correlate the results of biomimetic chemistry with the actual situation within the parasite. In general, most workers do accept the role of carbon-centred radicals in mediating the antimalarial activity of the endoperoxides, but the key information defining (a) the chemical mechanism by which these species alkylate proteins and (b) the basis for the high parasite selectivity remains to be unequivocally established. 

The redox potential for the (por)Com/ /(porT)Com/ couple is more negative than the corresponding (por)Fe 1/ /(porT)Fe111/ couple (Table 13.8). In contrast to alkyl-iron porphyrins, (po Co111/ is not reduced by the (por)Co anion. Instead, it is reduced to (porT)CoUI7 anion by a more negative scan (Em, -1.17 to -1.36 V). 

In summary, the electrochemical results indicate that the alkyl-metal bond-formation free energies range from 54 to 146 kJ mol-1 for iron porphyrins and from 84 to 159 kJ mol-1 for cobalt porphyrins. The maximum bond energies are for primary alkyl groups bonded to [(MeO)4TPP]Con and (OEP)Fen porphyrins. The porphyrin dianions [(porT)nFe and (porr)nCo ] facilitate the reduction of C02 to CO via the transient formation of a metal-carbon bond [(por7)M—C(0)0- — AGBF > 50 kJ mol-1 for iron porphyrins]. Thus, iron and cobalt porphyrins are especially effective electrocatalysts for the reduction of C02 ... 

As already mentioned above, iron(I) porphyrins can undergo various reactions. They are of synthetic value for preparation of a-alkyl iron porphyrins, which can be obtained by direct alkylation of the corresponding electrochemically generated iron(I) porphyrins with alkyl halides. The highly reduced species can also be oxidized by molecular oxygen or hydrogen peroxide to yield the iron oxyporphyrin (oxophlorin), which is further oxidized to verdoheme by molecular oxygen, " as discussed in Section 6.1.5. 

Mansuy, D. and J.-P. Battioni (1982). Isolation of sigma-alkyl-iron(Ill) or carbene-iron(II) complexes from reduction of polyhalogenated compounds by iron(Il)-porphyrins The particular case of halothane CFjCHClBr. J. Chem. Soc. Chem. Commun. 638-639. 

It has been shown that these carbenes can insert into Fe-Npor bonds to form vinylidine (A(-alkyl-iron porphyrin metallacycle (N-C-C-Fe)) complexes, " which can then go on to form c/s-bridged vinylidines of the porphyrin, with loss of iron. Carbene equivalents of iodosylbenzene can also react with iron porphyrins to give five-membered Fe-O-C-C-N metallocycles. Iron porphyrin carbenes can also react with amines to produce the isocyanide complexes (equation 58) ... 

The above method has been used for the synthesis of metal alkyl (or aryl) o-bonded porphyrins of iron , cobalt rhodium titanium iridium gallium indium thallium, silicon germanium " " and tin ... 

The reductive electrochemistry of iron-carbene porphyrins has been investigated in aprotic solvents [115]. With the vinylidene complex, there is a 2e -I- H reduction of the ligand leading to the formation of the corresponding iron(II) vinyl complex. The energies required to reduce by two electrons the other carbene complexes are quite similar [115]. The dichlorocarbene complex is an exception because the reduction is facihtated by the extreme instability of the one-electron intermediate. Formation of cr-alkyl iron(III) porphyrins has been confirmed by independent synthesis [116]. The a-alkyl iron(III) porphyrins can then be obtained by a one-electron reoxidation re-... 

Qiu, A. and D.T. Sawyer (1997). The electrochemical evaluation of the metal-carbon bond energies (—AGbf) of alkylated iron and cobalt porphyrins [(por)-M-R]. J. Porphyrins Phthalocyanines 1, 125-134. 

For these last oxnpounds we have shown previously stereochimical effect due to an antiperiplanar preference of foe p-brcmioradical produced after the first reaction step, foe stability of ch in the partial double bond C-C-Br increases for the a-a isomer vis-i-vis foe e-e is(Hner of about 0.20 eV. Thus the last one is more difficult to reduce. For the two series of catalysts this antiperiplanar preference is maintained but is slightly larger by a factor 10 instead of 10 for foe iron and cobalt porphyrins. Alkyl substitution at fimcticMial carbon does not se however to have a large effect. 

Alkyl-iron(III) porphyrins are reduced in two successive reversible one-electron steps ... 

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. ... 

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). ... 

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