Meso-ring position

Fig. 21. Detailed mechanism for HO-1 catalysis. In 1, oxygenation and electron transfer forms the ferric (Fe +)-peroxy complex. Steric factors and H-bonding help to bend the peroxide toward the a-meso-heme position for regio-selective hydroxylation. One proposed mode of forming verdoheme is shown in part 2. A key part of step 2 is the resonance structures between Fe + and Fe +/radical, which enable the porph3rrin ring to be oxygenated. Although the mechanism shown does not require any reducing equivalents (176), there remain experimental inconsistencies on the requirement of an additional electron in step 2. However, reduction of the verdoheme iron is necessary to prepare the substrate for step 3, verdoheme to bihverdin.

Treatment of red, ionic [Au(TPP)]Cl with aqueous sodium hydroxide in DMSO yielded a greenish brown product which was isolated by extraction with n-hexane. It was identified as a product of the nucleophilic addition of hydroxide to a meso (5-) position of the TPP ring, yielding a 5-hydroxy-5-H-porphyrin (hydroxyphlorin) derivative, Au(HOTPP) [303],... 

The numbering schemes of porphyrins and corroles are essentially the same (Fig. 4). Throughout this chapter, the protonated macrocycles are denoted by the addition of Hn to the appropriate abbreviation, where n = the number of protons in the N4 core. The abbreviations cor and por are used to denote the bare correlate and porphyrinate macrocycles, respectively. The 5, 10, 15, and 20 positions on the ring are referred to as meso positions, while the 2, 3, 7, 8, 12, 13, 17, and 18 ring positions are referred to by the shorthand p (they are p-pyrrole carbon atoms). 

Iron Porphyrins. Porphyrias (15—17) are aromatic cycHc compouads that coasist of four pyrrole units linked at the a-positions by methine carbons. The extended TT-systems of these compounds give rise to intense absorption bands in the uv/vis region of the spectmm. The most intense absorption, which is called the Soret band, falls neat 400 nm and has 10. The TT-system is also responsible for the notable ring current effect observed in H-nmr spectra, the preference for planar conformations, the prevalence of electrophilic substitution reactions, and the redox chemistry of these compounds. Porphyrins obtained from natural sources have a variety of peripheral substituents and substitution patterns. Two important types of synthetic porphyrins are the meso-tetraaryl porphyrins, such as 5,10,15,20-tetraphenylporphine [917-23-7] (H2(TPP)) (7) and P-octaalkylporphyrins, such as 2,3,7,8,12,13,17,18-octaethylporphine [2683-82-1] (H2(OEP)) (8). Both types can be prepared by condensation of pyrroles and aldehydes (qv). 

Porphyrins are formally derived from the porphin (1) nucleus by substitution of some or all peripheral positions with various side chains. In the classical system of nomenclature T, introduced by H. Fischer,Sc the peripheral /5-pyrrolic positions are numbered front 1 to 8 and the methine positions (also named meso positions) between the pyrrole rings are designated a, //, y, and 5. The rings are lettered clockwise A, B, C, and D. The classical nomenclature was in the past more and more displaced by a nomenclature which numbers all the carbon... 

Benzoisobacteriochlorin 14 can be prepared from nickel(II) chlorin 12 by vinylogous Vilsmeier reaction to give 13 and subsequent acid-induced cyclization.22 The chlorin 12 is, as expected, alkylated in a meso position adjacent to the reduced pyrrole ring. 

The ligand group can be introduced either on the meso or on the /5-pyrrole position of the porphyrin ring, but the synthesis of the meso-functionalized derivatives is easier and has been more widely exploited. Balch (50-53) reported that the insertion of trivalent ions such as Fe(III) (32) and Mn(III) (33) into octaethyl porphyrins functionalized at one meso position with a hydroxy group (oxophlorins) leads to the formation of a dimeric head-to-tail complex in solution (Fig. 11a) (50,51). An X-ray crystal structure was obtained for the analogous In(III) complex (34), and this confirmed the head-to-tail geometry that the authors inferred for the other dimers in solution (53) (Fig. lib). The dimers are stable in chloroform but open on addition of protic acids or pyridine (52). The Fe(III) octaethyloxophlorin dimer (52) is easily oxidized by silver salts. The one-electron oxidation is more favorable than for the corresponding monomer or p-oxo dimer, presumably because of the close interaction of the 7r-systems in the self-assembled dimer. 

Similar considerations apply to all odd-membered rings, no matter whether the substitutes are 1, 2, 1 3, 1 4 or in any other position relative to each other. It can be illustrated by taking the example of cycloheptane 1, 4-dicarboxylic acid, the ring of which is not planar. The cis form is meso while the trans exists in (+) and (-) forms along with its dl form. 

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