Porphyrin resonance stability

The dianions (P2 ) of the weakly acidic, metal-free porphyrins are generated for spectroscopic observation using metal alkoxides. The pK and pK2 values for etioporphyrin-I (Table 1) have been estimated to be about +16 pK3 for octaalkylporphyrins such as etioporphyrin-I, measured for solubility reasons in 2.5% sodium dodecyl sulfate, is about 5.5, but spectroscopic observation of the monocation is difficult owing to the resonance stabilization available in the symmetrical dication (PH42+). Generally, pK3 and pK4 differ... 

The next macrocycle in the series of bisvinylogous porphyrins 278 to be prepared by the LeGoff group effectively added yet another n-bond between the pyrroles [193]. Unfortunately, both 278 and its diprotonated salt, proved to be unstable decomposing within hours even in the solid state. This high reactivity is consistent with the supposition that there is little or no resonance stabilization in this platyrin 278. However, there is a substantial diamagnetic ring current present... 

The fact that vanadyl tetraphenylporphine was less reactive is not inconsistent with this rationalization although in this instance all four methine carbons have phenyl substituents. It is likely that the formation of the mono-substituted carbonium ion would be inhibited because of steric interactions at the methine position involved. However, a more important consideration is that the benzene ring attached to the methine carbon is not coplanar with the macrocyclic porphyrin ring. The extent of deviation from planarity is enough that a positive charge generated on the methine carbon would receive no resonance stabilization but considerable inductive destabilization as a consequence of the unsaturated substituent. 

The pyrryl carbinols occupy a special place in synthetic pyrrole chemistry it was discovered early that with acid, they can react with another pyrrole nucleus to form the valuable dipyrrylmethane stmcture, which is of interest in the synthesis of porphyrins. Acid leads to the formation of the resonance stabilized pyrrylmethylcarbocation (7.2), which acts as an electrophile by the usual mechanism (Scheme 7.9). 

Fig. 11.1 Relationship among catalytic intermediates of peroxidases. The formal oxidation state of each species is indicated by the numbers +2 to +6. The formal oxidation state of the species directly correlates with the relative energy content of the intermediates. The entry and exit of external electron donors/acceptors is indicated. In spite of its high oxidation state, Compound III is relative inert given the stability provided by the Fe(II) 02, Fe(III) 02 and Fe(IV) 022 resonance forms. Nevertheless, amino acid residues may rescue the free radical of Compound III, restore the iron atom ferric state, and allocate the free radical into a low redox potential site in the protein backbone. When the porphyrin performs as an electron donor, a different reaction occurs, resulting in tetrapyrrole bleaching and iron release...

The substances from the tetrapyrrole class are remarkable both through the very deep colour and through the extreme stability to chemical and physical agents, which naturally go together according to the resonance theory. To this class belong the coloured constituent of the blood haemin and of plants chlorophyll, all derivatives of porphyrin and also the artificial pigments discovered by Linstead, the phthalocya-... 

Oxomanganese(lV) porphyrins identified by resonance raman and infrared spectroscopy Weak bonds and the stability of the half-filled T2g subshell. J. zlm. Chem. Soc. 110,4158—4165. 

In the case of the hemochromogen, cytochrome c, the iron coordinates probably with two imidazole N atoms of histidine so that O2 cannot attach to the iron atom. The imidazole bonds to the iron are relatively weak so that it is necessary to stabilize these bonds. Theorell found that there is a direct chemical thioether linkage from the side chains of the porphyrin to the cytochrome apoprotein this would appear to constrain the two N atoms of imidazoles to lie in a correct spatial configuration for coordination with the iron, when the iron is in the ferrous or in the ferric state. Since the imidazole rings are probably in resonance with the porphyrin ring it would seem likely that electron transfer may occur on collision of a 1-electron donor or acceptor with either a portion of the imidazole ring or a portion of the porphyrin ring. 

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