Porphyrins and Chlorins

Oxidation of porphyrins and chlorins with osmium(VIII) oxide is an equally valuable method to produce bacteriochlorins, although it also leads to problems of regio- and stereoselectivity. 

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... 

Similiar problems of regioselectivity as in reduction reactions are encountered in oxidation reactions of porphyrins and chlorins. The oxidation of chlorins to isobacteriochlorins can be directed by insertion of zinc(II) or nickel(II) into the macrocycle. Again here, the metal-free chlorins give the bacteriochlorins whereas the metal chlorins, e.g. 1, give isobacteriochlorins, e.g. 3.15a,b I 7... 

The complex with the reduced porphyrin derivative 5,10,15,20-tetramethylisobacteriochlorin, [Ni(TMiBC)] (688), has an -ruffled conformation and a bond length pattern indicative of a reduction of aromaticity, both of which are more pronounced than those found for the corresponding porphyrin and chlorin complexes.1786... 

In an attempt to explain the different behaviour of azomethinic ylides and N-methylnitrones in 1,3-DC reactions with porphyrins and chlorins, a theoretical study has been carried out. The results obtained showed that while in the cycloadditions of porphyrins and chlorins with azomethinic ylides the processes are irreversible and consequently are kinetically controlled, the cycloadditions of such macrocycles involving JV-methylnitrone are clearly reversible, showing that the products from such reactions should be thermodynamically controlled <07MI1>. 

The same approach was subsequently used by Dolphin and co-workers <02CC2622> and also by Robinson and co-workers <03T499> to prepare pyrazoline cycloadducts from porphyrins and chlorins. Scheme 28 shows new chlorin derivatives (81 and 82) which have been obtained in this way. 

The electrochemistry of a range of Ni(n) porphyrins and chlorins has been investigated. All complexes are reduced by a similar one-electron mechanism which appears to involve the formation of anion radicals (Chang, Malinski, Ulman Kadish, 1984). 

This review deals with the synthesis of porphyrinyl-type sugar conjugates developed and biologically accessed, and considers the involvement of macrocycles of the porphyrin and chlorin (dihydroporphyrin) types, with O- and 5-glycosides or ether and ester functions. 

Porphyrins and corrins form part of the prosthetic groups in a large variety of vitally important natural products. As a result of intense interest in the structure, function and mechanistic details of the biological systems containing porphyrins and chlorins, the whole field has developed in a most remarkable way over the past 50 or more years. There can be few other research areas which possess such a rich, eventful and informative historical literature, and yet continue to produce important and exciting revelations in current journals. 

Apart from the obvious use of proton NMR spectroscopy as an aid in structure elucidation, the technique has also been used to probe the phenomena of intermolecular aggregation, apical ligand binding, NH tautomerism, rates of protonation, and a host of theoretical aspects. An extensive review of porphyrin and chlorin NMR spectroscopy has appeared... 

By far the most well known and chemically facile reaction of the porphyrin and chlorin macrocycles is chelation. The porphyrin core is a perfect ligand for a whole host of ions, complexes with most metals in the periodic chart having been prepared and characterized. 

Insertion of metals into the porphyrin and chlorin ring occurs with variable ease (and difficulty) commonly employed metals such as zinc(II), copper(II) and nickel(II) are simply chelated using the corresponding metal acetate in methanol, but others such as mag-nesium(II) and iron(III) require specialized conditions. Since a full description of the best method for any particular case is outside the scope of this Chapter, readers are advised to consult recent reviews (B-75MI30703, B-75MI30704). 

Pyrrole condensation usually gives a mixture of porphyrin and chlorin which are separated by column chromatography." The use of Cu11 as a template is reported to increase the (Chl/Por) ratio while Co11 reduces it.164 The product ratio depends on the reactants and conditions in the Ni11 template condensation (Scheme 43). 

Fe3+ derivatives of porphyrins and chlorins are all high spin in the absence of N-donor axial ligands and in chlorinated solvents [41,42] the low spin state observed in the case of iron corrolate demonstrates that the ligand field strength of corrole is very different from that of other macrocycles. 

Balaban TS. Tailoring porphyrins and chlorins for self-assembly in biomimetic artificial antenna systems. Acc Chem Res 2005 38 612-23. 

Facile oxidation of isobacteriochlorins compared to porphyrins and chlorins has been stated. Extended Hiickel calculations helped to rationalize these differences, indicating likewise that Fe(II) pyridine carbonyl complexes of isobacteriochlorins, unlike those of porphyrins and chlorins, undergo oxidation from the macrocycle rather than the metal to form n radical cations (81PNA2652). 

In syntheses of meso-tetraalkylchlorins, problems arise with contamination by chlorins. A template synthesis was described wherein the ratio of porphyrin and chlorin complexes formed can be controlled by the metal and the amount of the anhydride used (Scheme 5). Demetallation of complexes was... 

Bridged Dimeric Complexes of High-spin Iron(III) Porphyrins and Chlorins... 

Table IV also shows that the quenching rate constants of triplet bacteriochlorins by molecular oxygen approach the value of 4/9 diff> indicative of the formation of CT complexes. The quenching of triplet porphyrins and chlorins is close to l/9 diffi which is consistent with the high energy of excited state CT complexes in these systems 65).

In order to demonstrate the relative importance of age and depth of burial on porphyrin diagenesis, the yields of the porphyrins and chlorins were plotted against depth of burial (Figure la) and geologic age (Figure lb) (4,6-16). In Figure la it is noted that as the depth of burial increases (from ocean floor to 1200 m subbottom) there is a systematic increase in... 

The material obtained from both the Rothemund and Adler et al. approaches is crystalline but crude and usually contains up to 5% of 5,10,15,20-tetraphenyl-2,3-chlorin (38).24 Earlier methodology utilized the chromatographic separation of the porphyrin and chlorin, but a better method involves brief treatment of the crude product with DDQ. In this way (38) is transformed into (2) instead of being laboriously separated from it. 

Oxygen and suitable catalysts can also be used for the conversion of phenol to ben-zoquinone. Thus, irradiation of phenol in the presence of [Cu(bpy)2] or [Cu(l,10-phenanthroline)] + brings about degradation by a path that shows both pH and solvent dependency . Thus in acetonitrile benzoquinone predominates, but in water carbon dioxide is the sole product. Benzoquinone can also be formed from phenol by continuous irradiation in the presence of the catalysts [Crfbpyfs] or less effectively with [Ru(bpz)3] + and [Ru(bpy)3] +. The reaction path involves 02( A ) as the oxidant . Porphyrins such as 5,10,15,20- tetrakis(2,6-dichlorophenyl)porphyrin and chlorins can also be used to convert naphthols and phenols to the corresponding quinones (Scheme 28) . Phthalocyanines immobilized on polymers have also been used as the catalyst system to effect photooxidation. ... 

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