Porphyrins unsymmetrical

Fig. 18 Self-assembly of a dodecameric supercyclic structure via coordination of a cobalt porphyrin unsymmetrically functionalized with two pyridine ligands...

A mild procedure which does not involve strong adds, has to be used in the synthesis of pure isomers of unsymmetrically substituted porphyrins from dipyrromethanes. The best procedure having been applied, e.g. in unequivocal syntheses of uroporphyrins II, III, and IV (see p. 251f.), is the condensation of 5,5 -diformyldipyrromethanes with 5,5 -unsubstituted dipyrromethanes in a very dilute solution of hydriodic add in acetic acid (A.H. Jackson, 1973). The electron-withdrawing formyl groups disfavor protonation of the pyrrole and therefore isomerization. The porphodimethene that is formed during short reaction times isomerizes only very slowly, since the pyrrole units are part of a dipyrromethene chromophore (see below). Furthermore, it can be oxidized immediately after its synthesis to give stable porphyrins. 

With the catalysis of strong Lewis acids, such as tin(IV) chloride, dipyrromethenes may aiso be alkylated. A very successful porphyrin synthesis involves 5-bromo-S -bromomethyl and 5 -unsubstituted 5-methyl-dipyrromethenes. In the first alkylation step a tetrapyrrolic intermediate is formed which cyclizes to produce the porphyrin in DMSO in the presence of pyridine. This reaction sequence is useful for the synthesis of completely unsymmetrical porphyrins (K.M. Smith, 1975). 

Chlorophyll a (L.P. Vernon, 1966) contains an unsymmetrical porphyrin chromophore with two special features the double bond between C-17 and C-18 is hydrogenated and carhon atoms 13 and 15 hear a carboxylated, isocyclic cyclopentanone ting E. 

The 21//,22//-tautomer 2 with hydrogens at adjacent pyrrole rings is less stable because of penetration of each hydrogen into the van der Waals sphere of the other. However, NMR studies with unsymmetrically substituted porphyrins at low temperature have allowed the observation of both tautomers 1 and 2. The kinetic parameters of tautomerism investigated by NMR measurements at different temperatures are consistent with a two-step process forming 3 from 1 via 2 rather than a concerted two-hydrogen shift which could form 3 from 1 directly. 

In summary, syntheses of porphyrins based on bilenes-ft are flexible, and many unsymmetrical porphyrins have been obtained by these methods. However, the disadvantages are side reactions, sometimes low yields and lack of reactivity. 

Woodward s ingenious synthesis of chlorophyll a is based, in the first part of the synthetic approach, on a classical porphyrin synthesis using two dipyrrylmethanes for the formation of the macrotetracycle. The problem of regioselectivity in the connection of the two unsymmetric pyrrylmethane halves 1 and 2 was solved by the formation of SchifF base 3 between the two halves prior to the condensation so that the macrotetracycle formation occurred by an intramolecular reaction. 

The osmium(VIll) oxide bishydroxylation has been applied to a scries of unsymmetric porphyrins and chlorins bearing different electron-withdrawing or electron-donating groups which... 

Although direct reaction of lanthanide mono-porphyrins with free phthalo-cyanine or its lithium derivatives is generally more efficient than the template synthesis, and gives rise to mixed-ligand complexes, the template strategy can also be applied for synthesis of phthalocyanine-porphyrin complexes, as in the case of unsymmetric bisphthalocyanine complexes (Scheme 8.2, B(b)) [106, 136, 145, 146]. Thus, metallation of free porphyrins with lanthanide salts in TCB or n-octanol leads to single-decker complexes, which then react with phthalonitriles under the action of DBU in alcoholic media to give the desired compounds. 

E.R. Evitt, Catalytica Associates, Inc. Have you prepared polymethylene-linked cofacial porphyrins that contain two different metals in the cofacial unit Is it possible to prepare polymethylene-linked cofacial porphyrins that are unsymmetrical by virtue of two different length bridges in the same molecule ... 

The unnatural porphyrin family still fascinates the imagination. This year, to give a flavor for the diversity, there has appeared hybrid calixphyrins <00OL3103>, doubly A-confused porphyrins , unsymmetrical porphyrins attached to a modified 5,10,15,20-tetra (4-(2-trimethylsilylethynyl)phenyl]-21-23-dithiaporphyrin <00TL3709>,... 

Model Studies. The major difficulty associated with separation and analysis of HPD arises from the fact that there are two hydroxyl and two carboxyl groups positioned unsymmetrically on the porphyrin ring. Statistically, there are 3 ether structural isomers and 4 esters possible between two hematoporphyrin molecules and each of these is a diastereomeric mixture. No doubt, this situation has made the isolation and characterization for a specific structure a nightmare. 

A few current examples are shown below and include unsymmetrically substituted derivatives 14227, the cationic derivative 15228, the neutral derivative 16226 and the classic anionic derivative 17229 230, (tetrabenzoporphyrinato)zinc(II) , an intermediate structure between that of porphyrins and phthalocyanines, and have also been investigated in this respect. However, its quantum yield for singlet oxygen generation (0.023) in liposomes is much lower than that of Photofrin (0.19)177. 

Figure 32. Porphyrin blocking groups offer the possibility of further supramolecular deri-vatization. Amide-linked rotaxane with porphyrin blocking groups 85, its Zn complex 86, and an unsymmetrically capped rotaxane 87.

A porphyrin with a C2 axis perpendicular to its plane may be similarly synthesized by dimerization of a-functionalized dipyrro-methanes or -methenes. If only a half of the molecule is C2 symmetric, the condensation of dipyrrolic intermediates is useful. Unsymmetrical porphyrins are usually synthesized in a stepwise fashion via linear tetrapyrrolic intermediates. 

The porphyrin part of haemoglobin is called haem, and it is an iron(II) complex. It is unsymmetrically substituted with carboxylic acid chains on one side and vinyl groups on the other. 

Gregory A. M. and F. A. Walker. Electronic Effects in Transition Metal Porphyrins. 5. Thermodynamics of Axial Ligand Addition and Spectroscopic Trends of a Series of Symmetrical and Unsymmetrical Derivatives of Tetrephenylporphinatozinc(II). Inorganica chimica Acta. 91 (1984) pp. 95-102. 

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