Iron -porphyrin coupling with

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. ... 

Two dendrimers based on Fe-porphyrin core carrying peptide-like branches of different sizes have been synthesized in order to have more open and a more densely packed (23) structures [43]. The electrochemical behavior has been examined in CH2C12 and in aqueous solution. In the less polar solvent, the two dendrimers show similar potentials for the Fem/Fen couple, suggesting that the iron porphyrins in both the more open and the more densely packed dendrimers experience similar microenvironments. On the contrary, in water the behavior of the two dendrimers is very different since the reduction from Fem to Fe11 is much easier for the densely packed dendrimer. This result can be explained considering that in the dendrimer with the relatively open structure the aqueous solvation of the iron porphyrin is still possible, whereas in the densely packed one the contact between the heme and the external solvent is signifi-... 

The possibility that substitution results from halogen-atom transfer to the nucleophile, thus generating an alkyl radical that could then couple with its reduced or oxidized form, has been mentioned earlier in the reaction of iron(i) and iron(o) porphyrins with aliphatic halides. This mechanism has been extensively investigated in two cases, namely the oxidative addition of various aliphatic and benzylic halides to cobalt(n) and chromiumfn) complexes. 

The peroxidase reaction is coupled with the formation of free radicals, either directly at the protein backbone or at the porphyrin moiety or both. This topic is borderline to the scope of this review and we will restrict ourselves to those studies which have at least a strong connection with the heme iron situation. We mention however, some relevant articles which have appeared in the period reviewed. Specific interest was given to a bi-functional enzyme from Mycobacterium tuberculosis which has both catalase and peroxidase activity.286-287,288-289 We also mention that a critical role of cations like Ca2+ and K+ has been described.290-291... 

The magnetic properties of the iron porphyrins may be considered in terms of their tetragonal symmetry, which results in the existence of three possible spin states. For the ds Fe111 centre these are the low-spin S = 3 state, the intermediate-spin S = state and the high-spin S = state. Examples of each type are given in Table 15, with information on bond lengths. The S = state can couple to a nearby S = f state to give a new admixed intermediate state (S = , ), which must not be viewed as a spin equilibrium. 

Reduced transition metals with unpaired electrons also undergo autoxidation via radical radical coupling. For example, reduced-iron porphyrins are rapidly converted to dinuclear... 

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. 

Work on the redox chemistry of iron porphyrins and their protein adducts deals mainly with two problems, namely the electron transfer, which is mediated by the FeII/Fe111 couple in cytochromes, and the partial reduction of the oxygen molecule to the superoxide anion or to a peroxo bridge between two iron porphyrins. The work on isolated iron porphyrins will be summarized in this section, whereas the hemoproteins will be covered in Section VI. 

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