System porphyrin

Porphyrin system typical of those studied by Hunter and Saunders [Hunter and Saunders 1990],... 

Methyl-coenzyme M reductase participates in the conversion of CO2 to CH4 and contains 6-coordinate nickel(II) in a highly hydrogenated and highly flexible porphyrin system. This flexibility is believed to allow sufficient distortion of the octahedral ligand field to produce low-spin Ni" (Fig. 27.7) which facilitates the formation of a Ni -CHs intermediate. 

Tire macrocyclic core of porphyrin systems 71 is highly conjugated and a number of effective resonance forms can be written. Tliere are nominally 22 TT-electrons but only 18 of these can be included in any one conjugative path (for a modern discussion on this topic, see references 98AGE177 and 99CEJ267). Chlorins (73, dihydroporphyrins), bacteriochlorins (74, tetra-hydroporphyrins), and isobacteriochlorins (75, tetrahydroporphyrins) also have full 18-7r delocalization available, though the number of possible resonance forms is reduced. 

Porphyrin systems therefore obey Hiickel s rule in having An + 2 n = A) TT-electrons in a planar, cyclic, conjugated array. Both major tautomeric forms have delocalization pathways with opposite N-Hs (trails tautomers), as shown in 71a 71b. It is already known (76AHCS1) that tautomers with inner hydrogens adjacent (cis tautomers) are much less stable, playing an important role only in the mechanism of proton transfer in porphyrins and phthalocyanines. 

In this section, reactions in which the porphyrin system is retained are also included. 

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... 

Pentaphyrin(l.l.l.l.l) is the direct homologous congener of the porphyrin system. Formally, but also synthetically, it can be derived when the porphyrin-forming building blocks are extended by a pyrrole unit and a methine bridge or its precursor. 

The observation that addition of imidazoles and carboxylic acids significantly improved the epoxidation reaction resulted in the development of Mn-porphyrin complexes containing these groups covalently linked to the porphyrin platform as attached pendant arms (11) [63]. When these catalysts were employed in the epoxidation of simple olefins with hydrogen peroxide, enhanced oxidation rates were obtained in combination with perfect product selectivity (Table 6.6, Entry 3). In contrast with epoxidations catalyzed by other metals, the Mn-porphyrin system yields products with scrambled stereochemistry the epoxidation of cis-stilbene with Mn(TPP)Cl (TPP = tetraphenylporphyrin) and iodosylbenzene, for example, generated cis- and trans-stilbene oxide in a ratio of 35 65. The low stereospecificity was improved by use of heterocyclic additives such as pyridines or imidazoles. The epoxidation system, with hydrogen peroxide as terminal oxidant, was reported to be stereospecific for ris-olefins, whereas trans-olefins are poor substrates with these catalysts. 

As just mentioned, phosphorus porphyrins have unique photochemical properties. Their photophysics is also interesting. Emitter-quencher assemblies based on porphyrin building blocks have attracted attention due to their potential to serve as models in photosynthetic research (see [90] for an example) or for the development of photoswitches that could be used for the fabrication of molecular electronic/optical devices. In this context, Maiya and coworkers constructed a P(VI) porphyrin system 59b with two switchable azobenzene groups positioned in the apical positions of the pseudo-octahedral phosphorus atom [92]. Photoswitch ability (luminescence on/off) was demonstrated as... 

In classical mechanisms for C—H bond activation either C—H rr-bond donation or cyclic. 4-centered transition states are important, but these are precluded in the porphyrin systems and the mechanism proposed for activation of CH4, toluene, and Hy by the Rh porphyrin radicals is a new mechanistic possibility. 

The scope and limitations for transfer hydrogenation employing either the iron porphyrin system or the combination of iron compound/terpy/PPhs are listed in Table 8. In most cases, the FeCVterpy/PPhs system displays a higher activity. Except for chloromethyl- and cyclopropyl-acetophenone, the desired products were obtained in good to excellent yields. It should be noted that a ring opened product was not observed when cyclopropyl acetophenone was employed. Hence, a radical-type reduction pathway was excluded and a hydride mechanism appeared to be reasonable. 

Similar to porphyrin systems, Ni1 complexes with macrocyclic N4 ligands (in particular of the cyclam type) have been considered particularly instructive with respect to the chemistry of the active site of methyl coenzyme M reductase. In most cases, Ni1 species are produced by... 

A picket fence porphyrin system. Finally, examples of pendant donor groups attached to a porphyrin ring are known. A novel example of such a molecule is meso-a,a,a,a-tetrakis(o-nicotinamidophenyl)porphyrin (111) which is capable of binding two metal ions such that each has a square-planar environment with the square planes orientated coaxially to each other (Gunter et al., 1980). When a kinetically-inert metal ion such... 

A linked porphyrin system. Urea-linked porphyrin rings have also been synthesized (Landrum, Grimmett etal., 1981). For example, imidazolate-... 

It should be noted that Ru(III) porphyrin systems generated elec-trochemically from Ru(II) do not normally give detectable e.s.r. signals (9). 

E. Mechanism Studies with Iron Porphyrin Systems... 

An interesting, pH-dependent mechanistic changeover was reported in the H2A-02-[Fem(TPPS)] (TPPS = 5,10,15,20-tetrakis(p-sulfonatophe-nyl)porphyrinate) system in aqueous solution (23). This water-soluble metalloporphyrin exists as a monomer under slightly acidic conditions and reacts with ascorbic acid on the time-scale of several hours to produce [Fen(TPPS)] in the absence of dioxygen. The formation of the [Fem(TPPS +)] radical was also reported at pH 5, but it is not clear how... 

The distorted octahedral species [10] and [11] are the essential part of the cytochromes acting as redox catalysts. In these, a very specific porphyrin redox potential may have been adjusted by an appropriate choice of the axial ligands exerting a cis effect on the porphyrin system transmitted through the iron atom. In cytochrome c, these axial ligands are the imidazole of a histidine and the thioether function of methionine, as in [10], and in the cytochromes a or b5 they are presumably two imidazoles of histidine, as in [77] (7). 

The associated optical cis effects at the porphyrin system can be understood according to the inductive transmission path shown in Figure 1, Case A Rh,u causes a hypso spectrum via Rh - (P) 7r-donation increasing in the series I < Br < C =CPh < Ph Me for X which is roughly the series of increasing basicity towards the proton, as expected. A common feature (see Sect. 6) is the small effect of the exchange NH3 - HjO in the a-band the Soret band shows a stronger shift. 

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