Porphyrin reactivities

Hydrodemetallation pathways for Ni-etioporphyrin and Ni-tetra(3-methylphenyl)porphyrin are shown in Fig. 20. Both are characterized by a sequential hydrogenation-hydrogenolysis global mechanism, but important differences are apparent. Ware and Wei (1985a) rationalized the differences in porphyrin reactivity on the basis of porphyrin molecular structure. Structural differences on the periphery of the metalloporphyrin, in particular the substituent groups at the /3-pyrrolic and methine bridge... 

Webster, I. A., "Catalytic Hydrodemetallation of Nickel Porphyrins Reactivity and Catalyst Surface Studies." Sc.D. thesis, M.I.T., 1984. 

Because of the occurrence of isomers, the synthesis from pyrroles is only useful for porphyrins with eight identical /8-pyrrolic and four identical methine bridge substituents. Famous examples are the syntheses of chloroform-soluble /8-octaethyl-porphyrin and meso-tetraphenylporphyrin which have been used in innumerable studies on porphyrin reactivity (K.M. Smith, 1975). Porphyrins with four long meso-alkyl side-chains can be obtained by use of analogous reactions. These porphyrins have melting points below 100 °C and are readily soluble in petroleum ether. Sulfonation of olefinic double bonds leads to highly charged, water soluble porphyrins (J.-H. Fuhrhop, 197 ). 

The syntheses given are also useful for connecting porphyrins with other chroihophores and reactive groups, e.g., quinoncs. If the reported yields are reproducible, large electron donor-acceptor supramolecules should become accessible on a large scale. 

Chelation itself is sometimes useful in directing the course of synthesis. This is called the template effect (37). The presence of a suitable metal ion facihtates the preparation of the crown ethers, porphyrins, and similar heteroatom macrocycHc compounds. Coordination of the heteroatoms about the metal orients the end groups of the reactants for ring closure. The product is the chelate from which the metal may be removed by a suitable method. In other catalytic effects, reactive centers may be brought into close proximity, charge or bond strain effects may be created, or electron transfers may be made possible. 

Isooctane porphyrin methyl esters reactivation of faded fluorescence benzene and petroleum ether can be employed in the same way [233, 289]... 

The high stability of porphyrins and metalloporphyrins is based on their aromaticity, so that porphyrins are not only most widespread in biological systems but also are found as geoporphyrins in sediments and have even been detected in interstellar space. The stability of the porphyrin ring system can be demonstrated by treatment with strong acids, which leave the macrocycle untouched. The instability of porphyrins occurs in reduction and oxidation reactions especially in the presence of light. The most common chemical reactivity of the porphyrin nucleus is electrophilic substitution which is typical for aromatic compounds. 

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. 

The /)-oxobilanc route to porphyrins 17->20 is more common since here the oxo function does not influence the reactivity of the terminal pyrrole rings at which the cyclization occurs. As with w-oxobilanes the oxo function has to be removed but here it can be done after cyclization at the porphyrin stage. 

This retro-aldol-typc fragmentation is possible because the chlorin chromophore stabilizes the anion formed on loss of the a-oxo acid residue. A related reactivity is observed in the reduction of 3-vinylchlorins, e.g. 24, to 3-ethylporphyrins, e.g. 25, in the presence of hydrogen iodide in acetic acid. The mechanism of this reaction can be represented as a sequence of tautomeric reactions which lead to the completely conjugated porphyrin system.32c-40-54... 

High-valent ruthenium oxides (e. g., Ru04) are powerful oxidants and react readily with olefins, mostly resulting in cleavage of the double bond [132]. If reactions are performed with very short reaction times (0.5 min.) at 0 °C it is possible to control the reactivity better and thereby to obtain ds-diols. On the other hand, the use of less reactive, low-valent ruthenium complexes in combination with various terminal oxidants for the preparation of epoxides from simple olefins has been described [133]. In the more successful earlier cases, ruthenium porphyrins were used as catalysts, especially in combination with N-oxides as terminal oxidants [134, 135, 136]. Two examples are shown in Scheme 6.20, terminal olefins being oxidized in the presence of catalytic amounts of Ru-porphyrins 25 and 26 with the sterically hindered 2,6-dichloropyridine N-oxide (2,6-DCPNO) as oxidant. The use... 

The synthesis, reactivity, spectroscopy, and electrochemi.stry of organometallic iron porphyrins was de.scribed in some detail in the three reviews published in the period from 1986 to I988. Although a brief synopsis of the early chemistry will be given here, this review will focus on more recent developments. 

The chemical reactivity of the organoruthenium and -osmium porphyrin complexes varies considerably, with some complexes (M(Por)R2, M(Por)R and Os(OEP)(NO)R) at least moderately air stable, while most are light sensitive and Stability is improved by handling them in the dark. Chemical transformations directly involving the methyl group have been observed for Ru(TTP) NO)Me, which inserts SO2 to form Ru(TTP)(N0) 0S(0)Me and Ru(OEP)Me which undergoes H- atom abstraction reactions with the radical trap TEMPO in benzene solution to yield Ru(OEP)(CO)(TEMPO). Isotope labeling studies indicate that the carbonyl carbon atom is derived from the methyl carbon atom. "" Reaction of... 

The most significant and widely studied reactivity of the ruthenium and osmium porphyrin carbene complexes is their role in catalyzing both the decomposition of diazoesters to produce alkenes and the cyclopropanation of alkenes by diazoesters. Ethyl diazoacetate is used to prepare the carbene complex 0s(TTP)(=CHC02Et)... 

The chemistry of organorhodium and -iridium porphyrin derivatives will be addressed in a separate section. Much of the exciting chemistry of rhodium (and iridium) porphyrins centers around the reactivity of the M(ll) dimers. M(Por) 2-and the M(III) hydrides, M(Por)H. Neither of these species has a counterpart in cobalt porphyrin chemistry, where the Co(ll) porphyrin complex Co(Por) exists as a monomer, and the hydride Co(Por)H has been implicated but never directly observed. This is still the case, although recent developments are providing firmer evidence for the existence of Co(Por)H as a likely intermediate in a variety of reactions. 

Electronic effects on the reactions of [Rh(Por)h dimers and hydrides were probed by varying the porphyrin macrocycle. OEP and TPP vary considerably in their properties, with OEP being one of the strongest and TPP one of the weakest (7-donors among porphyrin derivatives. However. Rh(Por)]2, Rh(Por)H, and Rh(Por)r showed the same reactivity in a variety of reactions for both OEP and TPP, indicating that electronic effects relating to the porphyrin ligand have... 

Both rhodium and osmium porphyrins are active for the cyclopropanation of alkenes. The higher activity of the rhodium porphyrin catalysts can possibly be attributed to a more reactive, cationic carbene intermediate, which so far has defied isolation. The neutral osmium carbene complexes are less active as catalysts but the mono- and bis-carbene complexes can be isolated as a result. 

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