Porphyrins iron, autoxidation

Each of the new homoporphyrin derivatives prepared by this alternative approach was demonstrated to form a stable nickel(II) complex when treated with Ni(OAc)2 (e.g., 4.46-4.50). Further, the free-base homoporphyrin 4.41 was shown to form the stable complexes 4.51-4.53 when treated with the appropriate divalent metal salt (M = Cu, Zn, Co, X = OAc). ° Additionally, in analogy to the porphyrins, treatment of the free-base homoporphyrin 4.41 with FeCl2 was found to give, after workup, the iron(III) complex 4.54 (Scheme 4.1.19) it presumably results from normal, porphyrin-like autoxidation at the metal. ... 

Sauer-Masarwa, A., N. Herron, C.M. Fendrick, and D.H. Busch (1993). Kinetics and intermediates in the autoxidation of synthetic, non-porphyrin iron(II) dioxygen carriers. Inorg. Chem. 32, 1086-1094. 

Although each of the metal electrodes has been polished immediately prior to the measurements that are illustrated in Figures 1-3, the initial negative scans often exhibit significant reduction peaks due to autoxidation by O2. The polishing process undoubtedly atomizes some of the M-M surface molecules, which probably initiates the process via a mechanism similar to that for the autoxidation of reduced iron porphyrins (17)... 

Of greatest interest are those compounds that attempt to model hemoglobin directly. Simple iron(II) porphyrins are readily autoxidized first to superoxo species, then to //-peroxo dimers and finally to /x-oxo dimers, as represented in equation (60). Bridge formation must be prevented if carrier properties are to be observed. This has been achieved by the use of low temperature and sterically hindered or immobilized iron(II) porphyrins. Irreversible oxidation is also hindered by the use of hydrophobic environments. In addition, model porphyrins should be five-coordinate to allow the ready binding of 02 this requires that one side should be protected with a hydrophobic structure. Attempts have also been made to investigate the cooperative effect by studying models in which different degrees of strain have been introduced. 

Therefore, it appears that the redox properties of the metallo-porphyrin are required only for the initiation step in these free-radical autoxidations and that the porphyrin is not a stoichiometrically significant catalyst (21, 22, 23). The failure of these simple approaches to a reaction iron-porphine oxide or its equivalent could indicate that the ligation state of iron in the protein, presumably including an axial thiolate, is crucial to the oxygen-transfer properties of P 450. Support... 

In chemistry, it is well known that O2 can be strongly bound to a ferrons iron porphyrin in solvents without any protein matrix however, the oxygenated states of most simple iron porphyrins are irreversibly converted into /r-oxodimers (eqnation3), PFe(III)-0-PFe(III), via peroxo and ferryl intermediates (eqnation 2). The /x-oxodimer is usually very stable in solvents, so it is sometimes called a thermodynamic sink. In addition, autoxidation of PFe(II)-02 to an inert ferric porphyrin easily occurs under aerobic conditions (equation 4). Thus, it is clear that the heme pockets of myoglobin and hemoglobin play an important role in protecting the 02-bound heme from dimerization and autoxidation. 

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

Presence and concentration of hemoproteins and free iron in meats from different species may also influence the rate of lipid autoxidation/degradation during the cooking and subsequent storage periods (13). Thus, development of off-flavors and unpleasant odors referred to as "warmed-over flavor" (2) depends primarily on the degree of unsaturation of lipid components of meats and somewhat on the level of iron-porphyrin materials present in the muscle. 

A second oxidation pathway does not require the bimolecular contact of two iron(II)-porphyrins. Coordinated dioxygen may be released not as O2, as in normal dioxygen transport, but, as noted in Section I.C, as a superoxide radical anion O2 in a process called autoxidation ... 

There already exist examples of significant chemical reactions which demonstrate the unusual reaction characteristic resulting from the presence of two metal centers.Here we examine two, neither of which involves phosphine bridging ligands. The autoxidation of triphenylphosphine catalyzed by four-coordinate iron(II) porphyrins in a noncoordinating solvent like toluene occurs by the mechanism shown in Figure Two... 

Alben et al. (1968) have studied the autoxidation of iron porphyrins, with emphasis on Fe(II) compounds and have discussed mechanisms of autoxidations, which they based on electronic, infrared, and NMR spectra. 

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